selectivity - ku leuven

Jia

n L

i P

OR

OU

S IO

N E

XC

HA

NG

E M

EM

BR

AN

ES

WIT

H IM

PR

OV

ED

MO

NO

VA

LE

NT

SE

LE

CT

IVIT

Y

Decem

ber 2

018

ARENBERG DOCTORAL SCHOOL

FACULTY OF ENGINEERING SCIENCE

FACULTY OF ENGINEERING SCIENCE DEPARTMENT OF CHEMICAL ENGINEERING

PROCESS ENGINEERING FOR SUSTAINABLE SYSTEMS Celestijnenlaan 200F BOX 2424 B-3001 HEVERLEE, BELGIUM

tel. + 32 485 632238 [email protected]

www.cit.kuleuven.be

POROUS ION EXCHANGE MEMBRANES WITH IMPROVED MONOVALENT SELECTIVITY

Jian Li

Dissertation presented in partial fulfilment of the requirements for the

degree of Doctor of Engineering Science (PhD): Chemical Engineering

December 2018

Supervisor: Prof. Bart Van der Bruggen

POROUS ION EXCHANGE MEMBRANES

WITH IMPROVED MONOVALENT

SELECTIVITY

Jian Li

Supervisor:

Prof. Bart Van der Bruggen

Members of the Examination Committee:

Prof. Jean Berlamont (Chairman)

Prof. Joos (Joseph) Vandewalle (deputy

chairman)

Prof. Guy Koeckelberghs

Prof. Luc Pinoy

Prof. Kitty Nijmeijer (Eindhoven University of

Technology)

Prof. Yang Zhang (Qingdao Institute of

Bioenergy & Bioprocess Technology,

Chinese Academy of Sciences)

Dissertation presented in

partial fulfilment of the

requirements for the degree

of Doctor of Engineering

Science (PhD):

Chemical Engineering

December 2018

© 2018 KU Leuven, Science, Engineering & Technology

Uitgegeven in eigen beheer, Jian Li, Celestijnenlaan 200F box 2424, B-3001 Leuven (Belgium)

Alle rechten voorbehouden. Niets uit deze uitgave mag worden vermenigvuldigd en/of openbaar gemaakt worden door middel van druk,

fotokopie, microfilm, elektronisch of op welke andere wijze ook zonder voorafgaande schriftelijke toestemming van de uitgever.

All rights reserved. No part of the publication may be reproduced in any form by print, photoprint, microfilm, electronic or any other

means without written permission from the publisher.

Acknowledgements

I

Acknowledgements

Our destiny offers not the cup of despair, but the chalice of opportunity. So let us

seize it, not in fear, but in gladness. ——R.M. Nixon

Thanks a lot for giving me this valuable chance to spend over three years in KU

Leuven to pursue my PhD degree. This is the most important and correct decision in

my life so far and I can still feel all the exciting moments. I vividly remember that

things began in September 2015, when Dr. Xin Li picked me at the Leuven railway

station. Thanks to all the wonderful people I met in Belgium. This is a special moment

to look back on the period just gone. The completion of my doctoral thesis benefits

from valuable supports from many people and I owe them a debt of gratitude.

Firstly, I would like to express my deepest gratitude and most sincere respect to my

promoter Prof. Bart Van der Bruggen for his excellent supervision, advices, research

guidance and numerous supports! This was a journey that I enjoyed very much, and I

will always be thankful for having had you as my promoter. You gave me a wonderful

opportunity to work at this world-class research group in membrane field. In addition

to the academic aspect, I appreciate you a lot for the way you are: work hard and keep

an open mind. I will follow your philosophy to support my future life. I feel very

lucky and proud to be one of your students. Let‘s never say goodbye, because it is just

a start, far from the end at the moment.

Secondly, I would like to thank my colleagues who help me a lot in ProcESS. First of

all, I would like to express my most sincere thanks to the seniors Jiuyang Lin, Ruixin

Zhang, Wenyuan Ye! I learned a lot from your personality on doing research and

optimistic attitude towards life. The biggest thanks to all my friends in ProcESS who

gave me a lot of assistances in my research and life in Leuven: Junyong Zhu, Shushan

Yuan, Jing Wang, Miaomiao Tian, Bin Liu, Xin Li. You are my strongest backing.

Although some of them were graduated, the assistance from all of you made me adapt

myself in our lab and Leuven. I hope you will have a nice future, realize your dreams

and fight for your belief! I also would like to thank my friends and colleagues in this

Acknowledgements

II

fantastic research group: Ruijun, Yan, Yi, Sofie, Ece, Ben, Carlos, Duc, Trang, Indah,

Fred, Mokgadi, Saeed and other members. Thank you for being such a united and

enterprising family. I cherish the international atmosphere we spend together. Bryant

McGill has his befitting quote for you – ―Cooperation is a higher moral principle than

competition.‖ We are trying our best to complement others rather than compete with

others. We are the best.

While I have the chance, I would like to thank Prof. Jean Berlamont from Department

of Civil Engineering and Prof. Joos (Joseph) Vandewalle from Department of

Electrical Engineering for being a chairman/deputy chairman of the jury for my PhD

defence. I also would like to express my utter appreciation to my assessors Prof. Guy

Koeckelberghs (Department of Chemistry, KU Leuven) and Prof. Luc Pinoy

(Sustainable Chemical Process Technology TC, Ghent and Aalst Technology

Campuses) for their kind and useful comments, remarks and engagement during my

entire doctoral years and correcting my thesis. Without your constructive suggestions

and help in my research during the past three years, I could not come to this part of

my PhD. I also express my sincere gratitude to Prof. Kitty Nijmeijer (Department of

Chemical Engineering and Chemistry, TU Eindhoven) and Prof. Prof. Yang Zhang

(Qingdao Institute of Bioenergy and Bioprocess Technology, Chinese Academy of

Science) for agreeing to be my jury members.

I would like to thank for several special persons. Dr. Linfeng Li (Bettergy, US), you

broaden my international horizons and guided me how to be a qualified researcher.

Prof. Jiangnan Shen (Zhejiang University of Technology, China), I have gained a lot

since the first day working with you as a master student. It is hard to imagine my

research career without tremendous supports from you. You are also my introducer to

KU Leuven. Thank you so much for your assistance and time I spent with you. Prof.

Arcadio Sotto Diaz (Rey Juan Carlos University, Spain), I will never forget the

moment you come to my lab and bring me your encouraging words and valuable

suggestions.

Acknowledgements

III

Special thanks to Prof. Laurent Bazinet form Université Laval, Prof. Kang Li form

Imperial College London, Prof. Yong Wang and ShiPeng Sun from Nanjing Tech

University for many interesting discussions and suggestions on membrane technology

during conference.

Furthermore, appreciations should be delivered to Herman Tollet, who took great

effort in helping me to construct the electro-membrane system. Besides, I would also

like to express my sincere gratitude to other important people: Christine, Hanne, and

Michele for your supporting on sample measurements. I am also deeply indebted to

Alena Vaes, Beatrice De Geest and Marie-Claude Deflem for their kindly

administrative help.

I gratefully acknowledge China Scholarship Council (CSC) of the Ministry of

Education, China, for funding me to study in KU Leuven. I benefit a lot from CSC

because it provides me a chance to change my thinking and broaden my horizon

through my stay in KU Leuven. A very special gratitude goes out to all down at

Research Fund for helping and providing the funding for the work.

Last but not least, many thanks also go to all my friends. Thank you very much for the

time spending with me together in KU Leuven. Your accompany and support make

my life in KU Leuven more colorful! You may not know what incredible thing you

did for me, I will cherish that for the rest of my life. Especially, I would like to extend

my gratitude to my girlfriend Yi Huang! Thank you for your accompany,

understanding and support. You are always standing by my side to help me through all

the tough time during my PhD. Moreover, as the only child in my family, I would like

to thank my parents and other family members.

Jian Li

December 2018

Abstract

IV

Abstract

Membranes have gained an important place in the field of chemical technology and

are being used increasingly in a broad range of applications. The key property that is

exploited in every application is the ability of a membrane to control the permeation

of a chemical species in contact with it. Electrodialysis is an electrochemical

separation process in which a gradient in electrical potential is used to separate ions

with charged, ion selective membranes. Considering the importance of ion exchange

membranes for contemporary developments, research efforts have been devoted to

developing novel IEMs or to modifying pristine ion exchange membranes for targeted

activities. Today, new membranes developed by regulating the ionic channels is a

novel direction of research.

In this thesis, ionic channels of the ion exchange membrane were regulated by several

methods concerning the membrane matrix and surface skin layer. From the point of

the membrane matrix, a dry-wet phase-inversion strategy by combining immersion

precipitation and dry-casting was used to control the membrane porosity with the

purpose of improving the physical and electrochemical properties of ion-exchange

membranes. Taking advantage of the porous structure, the desalination ratio reached

95%, and the current efficiency reached 100%. However, during the desalting

procedure, the driving force has two contributions, the electrical field and the

salinity-gradient. As a consequence, the porosity should be controlled to balance the

back diffusion from the concentrate to the diluate with membrane electrical resistance.

It was experimentally shown that a membrane prepared with a 1-h heating time has

more steric hindrance, which can decrease the diffusion of ions, so that a superior

desalination efficiency was obtained. On the other hand, a polyaniline grafted

ultrafiltration membrane was prepared with the purpose to separate monovalent ions

from multivalent ions. Similar with porous ion exchange membranes, transport of ions

by the electrical field was dominant at the beginning of the experiments, while

diffusion dialysis by the salinity gradient plays a larger role in ions transport at the

Abstract

V

end of the experiment. In single salt systems, the polyaniline skin layer can hinder the

transport of multivalent ions due to the electrostatic effect, while no obvious effect on

Na+ ions transport can be observed. In the binary system with Na

+ and Mg

2+ ions, the

value of permselectivity is almost doubled as the flux of Na+ was increased to 12.4×

10-8

mol·cm–2

·s–1

while the flux of Mg2+

was reduced to 3.1×10-8

mol·cm–2

·s–1

.

Furthermore, a facile strategy is reported for fabricating monovalent selective ion

exchange membrane based on the rapid co-deposition of biomimetic adhesive

polydopamine and poly(ethylene imine) by using CuSO4/H2O2 as a trigger. Through

this strategy, the surface properties and the permselectivity of the membranes can be

easily tailored by the addition of PEI and by tuning the PEI molecular weight. The

optimum membranes, with 4 h co-deposition of 60 mg PDA and 120 mg PEI,

permselectivity of SPES-PDA/PEI-2 was 2.5 times higher than that of the SPES

membrane. Especially, the flux of H+ was enhanced by the formation of acid-base

pairs. Remarkably, the PDA/PEI modified ion exchange membrane shows an

excellent operation stability for monovalent separation performance after immersion

in acid and alkaline solution for 7 days. Similarly, MIL(53)-Al with nanochannels was

introduced to the skin layer of the monovalent selective membranes through rapid

codeposition of PDA/PEI followed by a cross-linking reaction. The positive −NH2

allows to reject multivalent cations, while porous Mil(53)-Al can accelerate the

migration of Na+. A mass ratio of 0.2–0.4% (w/v) for Mil(53)-Al yielded a

permselectivity of about 0.3 and an ion flux of about 22.0 and 0.6 mol·cm–2

·s–1

for

Na+ and Mg

2+, respectively. At optimum conditions, the PDA-coated membrane

maintains a high monovalent selectivity with enhanced Na+ flux and an enhanced Na

+

and Mg2+

flux in single salt solutions was obtained. A similar material ZIF-8 was used

to replace MIL(53)-Al for a fabricating monovalent selective ion exchange membrane

via interfacial polymerization. Both Na+ and Mg

2+ exhibited a higher transport

efficiency after introducing the ZIF-8 nanoparticles in single salt solutions. When the

binary mixtures were applied, an enhanced monovalent selectivity and Na+ flux were

obtained.

Abstract

VI

In general, membranes with low resistance and/or selectivity for given ions are critical

in industrial processes. In this thesis, such porous membranes were approved to be

feasible to desalinate and separate monovalent ions. Porous ion exchange membrane

with porosity in the membrane matrix and skin layer by suitable membrane formation

techniques or nanoparticles incorporation can be an efficient way to reduce the

resistance and enhance the ion flux.

Samenvatting

VII

Samenvatting

Membranen hebben een belangrijke plaats verworven op het gebied van chemische

technologie en worden in toenemende mate gebruikt in een breder scala van

toepassingen. Een sleuteleigenschap die in elke toepassing wordt gebruikt, is het

vermogen van een membraan om een chemische soort selectief te transporteren.

Elektrodialyse is een elektrochemisch scheidingsproces waarbij een gradiënt in

elektrische potentiaal wordt gebruikt om ionen te scheiden met geladen, ion

selectieve membranen. Gezien het belang van ionenuitwisselingsmembranen voor

hedendaagse ontwikkelingen, zijn onderzoeksinspanningen gewijd aan het

ontwikkelen van nieuwe ionen uitwisselings membranen (IEMs) of aan het

modificeren van ionenuitwisselingsmembranen voor specifieke toepassingen. Het

reguleren van ionkanalen is hierbij een nieuwe richting van onderzoek.

In deze thesis werden ionische kanalen van een ionenuitwisselingsmembraan

gereguleerd door verscheidene methoden met betrekking tot de membraanmatrix en

de oppervlaktelaag. Voor de membraanmatrix werd een droog-natte

fase-inversiestrategie toegepast, door het combineren van immersieprecipitatie met

droog gieten, om de porositeit van het membraan te reguleren met als doel om de

fysische en elektrochemische eigenschappen van het ionenuitwisselingsmembraan te

verbeteren. Door de voordelen van de poreuze structuur te benutten, bereikte de

efficiëntie van ontzilting 95% en de stroomefficiëntie 100%.

Tijdens het ontziltingsproces heeft de drijvende kracht echter twee bijdragen, namelijk

het elektrisch veld en de gradiënt in saliniteit. Daarom moet de porositeit worden

gecontroleerd om de omgekeerde diffusie van het concentraat naar het diluaat te

compenseren met de elektrische weerstand van het membraan. Experimenteel werd

aangetoond dat het membraan bereid met een verouderingsduur van 1 uur meer

sterische hinder vertoonde. Dit kan de diffusie van ionen verminderen, waardoor een

superieure ontziltingsefficiëntie kan worden verkregen.

Samenvatting

VIII

Anderzijds werd een met polyaniline geënt ultrafiltratiemembraan bereid voor het

scheiden van monovalente ionen en multivalente ionen. Vergelijkbaar met het poreus

ionenuitwisselingsmembraan was het transport van ionen door het elektrisch veld

dominant bij het begin van de experimenten, terwijl diffusiedialyse door de

saliniteitsgradiënt een grotere rol speelt bij het transport van ionen aan het einde van

het experiment. In systemen met slechts één zout kan de PANI-toplaag het transport

van multivalente ionen verhinderen vanwege het elektrostatische effect, terwijl er

geen duidelijk effect op het transport van Na+-ionen wordt waargenomen. In het

binaire systeem met Na+ en Mg

2+ ionen is de waarde van de permselectiviteit bijna

verdubbeld naarmate de flux van Na+

werd verhoogd tot 12.42×10-8

mol·cm–2

·s–1

,

terwijl de flux van Mg2+

werd gereduceerd tot 3.1×10-8

mol·cm–2

·s–1

.

Verder wordt een eenvoudige strategie gerapporteerd voor het fabriceren van een

monovalent ionenuitwisselingsmembraan op basis van de snelle co-depositie van

biomimetisch adhesief polydopamine en poly (ethyleenimine), door gebruik te maken

van CuSO4/H2O2 als trigger. Door deze strategie kunnen de

oppervlakte-eigenschappen en de permselectiviteit van de membranen gemakkelijk

worden aangepast door de toevoeging van PEI en door het PEI-molecuulgewicht in te

stellen. De optimale membranen, met 4 uur co-depositie van 60 mg PDA en 120 mg

PEI, vertoonden een permselectiviteit van SPES-PDA/PEI-2 die 2.5 keer hoger was

dan die van het SPES-membraan. Vooral de flux van H+ werd versterkt door de

zuur-baseparen in de synthese. Opmerkelijk is dat het door PDA/PEI gemodificeerde

ionenuitwisselingsmembraan een uitstekende operationele stabiliteit vertoont voor

monovalente scheidingsprestaties na onderdompeling in een zure en alkalische

oplossing gedurende 7 dagen. Evenzo werd MIL (53)-Al met nanokanalen

geïntroduceerd in de toplaag van de monovalente selectieve membranen door snelle

codepositie van PDA/PEI gevolgd door een verknopingsreactie.

De positieve -NH2 laat toe om multivalente kationen tegen te houden, terwijl poreus

Mil (53) -Al de migratie van Na+ kan versnellen. Een massaverhouding van 0,2-0,4%

Samenvatting

IX

(w/v) voor Mil (53) -Al leverde een permselectiviteit op van ongeveer 0,3 en een

ionenflux van ongeveer 22,0 en 0,6 mol·cm–2

·s–1

voor Na+ en Mg

2+, respectievelijk.

Bij een optimale conditie behoudt het PDA-gecoate membraan een hoge monovalente

selectiviteit met verbeterde Na+ en Mg

2+ flux in oplossingen met één zout. Een

vergelijkbaar materiaal, ZIF-8, werd gebruikt om Mil(53)-Al te vervangen voor het

vervaardigen van een monovalent ionenuitwisselingsmembraan via

grensvlakpolymerisatie. Zowel Na+ als Mg

2+ vertoonden een hogere

transportefficiëntie na introductie van de ZIF-8 nanodeeltjes in oplossingen met één

enkel zout. Wanneer binaire mengsels werden aangebracht, werden een verhoogde

monovalente selectiviteit en Na+ flux verkregen.

Samengevat zijn membranen met lage weerstand en/of goede selectiviteit voor

gegeven ionen van cruciaal belang in industriële processen. In deze thesis werd

aangetoond dat dergelijke poreuze membranen haalbaar zijn om te ontzilten en

monovalente ionen te scheiden. Het gebruik van een poreus

ionenuitwisselingsmembraan met porositeit in de membraanmatrix en toplaag kan

door geschikte technieken voor membraansynthese of incorporatie van nanodeeltjes

een efficiënte manier zijn om de weerstand te verminderen en transport van ionen te

verbeteren.

List of Abbreviations

X


AFM

ED

CED

BMED

RED

FTIR

IEC

NF

UF

PA

RO

SEM

EDAX

TFC

TFN

XPS

ZIF

MF

MOFs

AOPs

SGP

BPMs

PVC

IEMS

MIEMs

PANI

PPY

Atomic force microscopy

Electrodialysis

Conventional Electrodialysis

Bipolar Membrane Electrodialysis

Reverse Electrodialysis

Fourier-transform infrared spectroscopy

Ion exchange capacity

Nanofiltration

Ultrafiltration

Polyamide

Reverse osmosis

Scanning electron microscopy

Energy dispersive spectroscopy

Thin-film composite

Thin-film nanocomposite

X-ray photoelectron spectroscopy

Zeolitic imidazole framework

Microfiltration

Metal organic frameworks

advanced oxidization processes

Salinity gradient power

Bipolar ion exchange membranes

Polyvinyl chloride

Ion exchange membrane

Monovalent selective ion exchange

membrane

Polyaniline

Polypyrrole


XI

PEI

IP

PDA

Ra

Rrms

Rm

MPD

TMC

Zreal

Zimag

I–V

Proton exchange membranes

Polyethyleneimine

Interfacial polymerization

Polydopamine

Average roughness

Root mean square roughness

Maximum vertical difference between the

highest and lowest points

m-phenylenediamine

Trimesoyl chloride

Real impedance

Imaginary impedance

Current–voltage

PEMs

Contents

XII

Contents

1. Introduction ....................................................................................................................... 1

1.1 Background ..................................................................................................................... 1

1.2 Ion exchange membranes ................................................................................................ 7

1.2.1 Preparation of ion exchange membranes ................................................................ 12

1.2.2 Monovalent selective ion exchange membrane ...................................................... 14

1.3 Porous ion exchange membranes .................................................................................. 18

1.4 Motivation and contents of the PhD thesis .................................................................... 20

2. Methods and materials ......................................................................................................... 22

2.1 Chemicals and methods ................................................................................................. 22

2.1.1 Chemicals ............................................................................................................... 22

2.1.2 Porous ion exchange membrane preparation .......................................................... 23

2.1.3 Preparation of monovalent selective ion exchange membrane based on

ultrafiltration membrane .................................................................................................. 24

2.1.4 Preparation of polydopamine/polyethyleneimine modified monovalent selective ion

exchange membrane ........................................................................................................ 25

2.1.5 Preparation of monovalent cation exchange membrane containing hydrophilic

MIL53-(Al) framework ................................................................................................... 27

2.1.6 Preparation of monovalent cation exchange membrane by interfacial

polymerization ................................................................................................................. 28

2.2 Membrane properties and characterization.................................................................... 29

2.2.1 Ion exchange capacity ............................................................................................ 29

2.2.2 Water uptake ........................................................................................................... 30

2.2.3 Water contact angle ................................................................................................ 30

2.2.4 Zeta potential of membrane surfaces ...................................................................... 31

2.2.5 Membrane electrical resistance .............................................................................. 31

Contents

XIII

2.2.6 Diffusion dialysis experiments ............................................................................... 34

2.2.7 Current-voltage and transport number measurements ............................................ 35

2.2.8 Electrodialysis experiments .................................................................................... 39

2.2.9 Structural stability of ion exchange membrane ...................................................... 42

2.2.10 Morphology and structure of membranes ............................................................. 42

2.2.11 Chemical structure and composition of membranes ............................................. 43

2.2.12 Water flux experiments ........................................................................................ 43

3. Cation exchange membranes with controlled porosity in electrodialysis application ..... 44

3.1 Introduction ................................................................................................................... 44

3.2 Results and discussion ................................................................................................... 45

3.2.1 SEM results and water flux .................................................................................... 45

3.2.2 IEC and water uptake ............................................................................................. 47

3.2.3 Contact angle measurements .................................................................................. 48

3.2.4 Membrane resistance and transport number ........................................................... 49

3.2.5 Diffusion dialysis.................................................................................................... 49


3.3 Conclusions ................................................................................................................... 54

4. Charge-assisted ultrafiltration membranes for monovalent ions separation in

electrodialysis .......................................................................................................................... 56

4.1 Introduction ................................................................................................................... 56


4.2.1 SEM results ............................................................................................................ 58

4.2.2 FTIR results ............................................................................................................ 59

4.2.3 IEC, water uptake and contact angle ...................................................................... 60


4.2.5 Desalination parameters during ED: conductivity and pH ..................................... 63

Contents

XIV

4.2.6 Current efficiency ................................................................................................... 67

4.2.7 Monovalent selectivity measurements .................................................................... 69

4.3 Conclusions ................................................................................................................... 70

5. Mussel-inspired modification of ion exchange membrane for monovalent separation ... 71

5.1 Introduction ................................................................................................................... 71


5.2.1 Chemical structure of the membrane surface ......................................................... 74

5.2.2 Morphologies of the membrane .............................................................................. 78

5.2.3 Zeta potential .......................................................................................................... 79

5.2.4 Water contact angle, ion exchange capacity and water uptake .............................. 80

5.2.5 Diffusion experiments ............................................................................................ 82

5.2.6 Electrochemical characterization of the monovalent selective ion exchange

membranes ....................................................................................................................... 83


5.2.8 Stability and effects of molecular weight of PEI .................................................... 86

5.3 Conclusions ................................................................................................................... 89

6. Mussel-inspired monovalent selective cation exchange membranes containing

hydrophilic MIL53(Al) framework for enhanced ion flux ...................................................... 90

6.1 Introduction ................................................................................................................... 90


6.2.1 Surface morphology and chemical structure of the membrane .............................. 91

6.2.2 Contact angle, ion exchange capacity and water uptake ........................................ 94


6.2.4 Electrochemical properties of membranes ............................................................. 97


6.3 Conclusions ................................................................................................................. 102

Contents

XV

7. Thin-Film-Nanocomposite Cation ExchangeMembranes Containing Hydrophobic

Zeolitic Imidazolate Framework for Monovalent Selectivity ............................................... 103

7.1 Introduction ................................................................................................................. 103

7.2 Results and Discussion ................................................................................................ 105

7.2.1 Surface morphology and zeta potential ................................................................ 105

7.2.2 IEC and water uptake ........................................................................................... 109

7.2.3 Diffusion dialysis experiments ............................................................................. 110

7.2.4 Membrane resistance ............................................................................................ 111

7.2.5 Electrodialysis experiments .................................................................................. 112

7.2.6 Monovalent selectivity ......................................................................................... 114

7.3 Conclusions ................................................................................................................. 116

8. Conclusions and recommendations for further research ............................................... 117

8.1 General conclusions..................................................................................................... 117

8.2 Recommendations for further research........................................................................ 121

References ............................................................................................................................. 123

Curriculum Vitae ................................................................................................................... 141

Chapter 1

1

1. Introduction

1.1 Background

The increasing calls for environmental protection have stimulated to manage water

resources more holistically (Tzabiras et al., 2016). However, there is still a struggle to

achieve a sustainable, high quality and a sufficient amount of water supply. During the

Global Risks 2015 Report of the World Economic Forum, water shortage had been

identified as the most serious challenge for humanity in the next few decades (Liu et

al., 2017b). Already today, 50% of the world population is suffering from medium

water shortage while 10% are undergoing extreme water problems (Johnson et al.,

2016). Moreover, it is expected that the global population would grow by nearly 40%

in the next forty years (Pendergast and Hoek, 2011). The increasing demand for water

sources has posed a worldwide threat to water supply systems. More than seventy

percent of the Earth's surface is covered by water, but the available freshwater only

accounts for a tiny fraction of the earth‘s total water supply (Khawaji et al., 2008).

The available drinking water obtained from groundwater and lakes is limited because

much of it is too deep to access or cannot be exploited in a sustainable way.

Furthermore, severe ecosystem damage caused water depletion at a striking rate

across the world (Lattemann and Höpner, 2008). Oceans, containing the most

abundant water resources on the earth, can provide an inexhaustible, continuous and

high-quality water supply without damaging the original freshwater ecosystems

(Elimelech and Phillip, 2011). Thus, developing advanced water treatment

technologies for desalination is imperative.

Several technologies have been developed for sustainable water purification, such as

adsorption, flocculation, distillation, air flotation and advanced oxidization processes

(AOPs) (Zhang et al., 2016c). However, most of the technologies mentioned above

are often energetically, chemically and operationally intensive, and thus require

Chapter 1

2

considerable infusion of capital, engineering expertise and infrastructure, all of which

precludes their use in much of the world (Shannon et al., 2008). Even in highly

industrialized countries, the cost and time needed to develop state-of-the-art

conventional water and wastewater treatment facilities make it arduous to address all

the problems mentioned above. Furthermore, intensive chemical treatments and

residuals resulting from treatment can be other factors that limit industrial

applications. Hence, reducing chemical treatment via developing more effective,

low-cost, robust methods to supply clean water are of paramount importance.

Fortunately, there is much more that science and technology can do to mitigate

environmental impact, and to increase efficiency. Membrane technologies like reverse

osmosis (RO), nanofiltration (NF), ultrafiltration (UF), microfiltration (MF), and

electrodialysis (ED) processes are emerging as effective methods to realize the

removal of contaminants and serving as important technologies in water supply for

different purposes (Hu and Mi, 2013). A semipermeable membrane is required for all

these processes, which can permeate some components with specific

physical/chemical properties while rejecting the others (Hou, 2016; Liu et al., 2016).

Some features of membrane processes include: no chemical additives, no phase

change, low energy consumption, operation simplicity and easy to scale up (Kang and

Cao, 2014).

UF and MF are low pressure filtration membranes which have been extensively used

in particle and natural organic materials removal. The pore size of MF generally spans

from 0.1 µm to 10 µm while the pore size of UF ranges from 0.01 µm to 0.1 µm

(Figure 1.1) (Fiksdal and Leiknes, 2006). RO, which can reject all solutes, is a crucial

technology in desalination. NF is a separation process which has caused widespread

attention. The pore size of a NF membrane is between that of an RO and UF

membrane, which could be potentially applied in separating dissolved organic matter

and divalent ions. Different from traditional pressure driven membranes, rejection

mechanisms for NF membranes should take electrostatic repulsion into consideration.

Chapter 1

3

Nevertheless, the development of membrane technology is largely limited by the

membrane materials, and membrane fouling can be another serious problem (Guo et

al., 2012)

Fig. 1.1 Classification of membranes according to pore size (Mikhaylin and Bazinet, 2016; Xu

and Zhang, 2016)

Electrodialysis (ED), an electric driven process, has been commercially applied for

diverse purposes (Sadrzadeh and Mohammadi, 2008). ED can be mainly operated in

three ways, 1. Conventional Electrodialysis (CED); 2. Bipolar Membrane

Electrodialysis (BMED); 3. Reverse Electrodialysis (RED). A typical CED setup

normally contains alternately arranged cation and anion exchange membranes

between two electrodes. Spacers are incorporated with the gaskets to prevent the

contact of cation and anion exchange membranes. While a direct current is employed

as driving force, cations and anions transport toward the cathode and anode to form a

concentrate and a diluate compartment, respectively. A schematic diagram of an ED

cell is presented in Fig. 1.2. Xu et al. reviewed the industrial applications of CED, this

is summarized in Table 1.1 (Xu, 2005). During ED operation, parameters such as the

desalination ratio, water recovery efficiency, current efficiency, energy consumption

and operating cost should be taken into consideration. It can be concluded from Table

1.1 that the industrialization of CED has been limited due to the high costs. The main

reason is attributed to the high operating cost and membrane cost. In addition, the

Chapter 1

4

issues concerning membrane fouling also lead to high energy consumptions.

Membrane fouling is typically induced by the adsorption of organic matter or the

precipitation of metallic cations. Over the past few years, significant progress in ED

has been made. More promising results were expected by combining ED with other

technologies. By introducing IEMs with extraordinary antifouling and desalination

properties, the disadvantages mentioned above have been resolved to a greater extent.

Fig. 1.2 Schematic diagram of electrodialysis experimental setup (Khan et al., 2016)

Chapter 1

5

Tab

le 1

.1 I

ndust

rial

appli

cati

on o

f co

nven

tional

ele

ctro

dia

lysi

s (E

D)

Lim

itat

ion

s

Co

nce

ntr

atio

n o

f fe

ed a

nd

cost

s

Pro

du

ct w

ater

qu

alit

y a

nd c

ost

s

Mem

bra

ne

pro

per

ties

an

d c

ost

s

Pro

du

ct w

ater

qu

alit

y a

nd c

ost

s

Mem

bra

ne

sele

ctiv

ity

an

d c

ost

s

Co

sts

Co

sts

Sta

tus

of

app

lica

tio

n

Co

mm

erci

al

Co

mm

erci

al

Co

mm

erci

al

Co

mm

erci

al

Com

mer

cial

or

pil

ot

ph

ase

Co

mm

erci

al

Pil

ot

ph

ase

Sta

ck a

nd p

roce

ss d

esig

n

Shee

t fl

ow

, to

rtuous

pat

h s

tack

,

rever

se p

ola

rity

Shee

t fl

ow

, to

rtuous

pat

h s

tack

,

rever

se p

ola

rity

Shee

t fl

ow

sta

ck,

unid

irec

tional

Shee

t fl

ow

, to

rtuous

pat

h s

tack

,

rever

se p

ola

rity

Shee

t fl

ow

, to

rtuous

pat

h s

tack

,

unid

irec

tional

Shee

t fl

ow

, unid

irec

tional

Shee

t fl

ow

, unid

irec

tional

Indust

rial

appli

cati

on

s

Bra

ckis

h w

ater

des

alin

atio

n

Boil

er f

eed w

ater

pro

duct

ion

Was

te a

nd p

roce

ss w

ater

trea

tmen

t

Ult

ra p

ure

wat

er p

roduct

ion

Dem

iner

aliz

atio

n o

f fo

od

pro

duct

s

Tab

le s

alt

pro

duct

ion

Conce

ntr

atio

n o

f re

ver

se o

smosi

s

bri

ne

BMED is a technology that can combine resource recycling, energy conversion, and

environmental protection. Bipolar ion exchange membranes (BPMs) can provide a

Chapter 1

6

continuous H+ and OH

- supply by splitting water molecules between the cation and

anion exchange layer with high efficiency, without the need of introducing any other

salt ions. The most common application of BMED technology is the production of

inorganic acids and bases from the corresponding salts. A number of applications have

been operated at pilot scale, such as the production of mineral acids and bases from

salts, or the generation of acid and base from RO desalination concentrates. Some of

them can even be found in commercial plants, like recycling of HF and HNO3 from

steel picking solutions. Furthermore, the separation of organic acid salts and

acidification of various streams have also been explored. The feasibility has been

proved concerning the conversion of gluconic acid (Alvarez et al., 1997), propionic

acid (Boyaval et al., 1993), lactic acid (Lee et al., 1998), acetic acid (Zhang et al.,

2011), malic acid (Liu et al., 2014a), vitamin C (Yu et al., 2002), formic acid (Ferrer

et al., 2006), lactobionic acid (Gutiérrez et al., 2013), salicylic acid (Liu et al., 2015),

citric acid (Tongwen and Weihua, 2002), and amino acid (Eliseeva et al., 2001).

Due to the concerns arising from energy problems, developing renewable and

sustainable energy conversion and production technologies is crucial. Salinity

gradient power (SGP) can be harvested from mixing water streams of different

salinity. Theoretically, approximately 0.8 kWh is obtainable when 1 m3 of fresh water

flows into the sea, which translates into nearly 2 TW of SGP on the basis of the total

freshwater flow of the major rivers worldwide (Mei and Tang, 2018). RED is an

emerging membrane based technology that captures electricity from controlled

mixing of two water streams of different salinities. The configuration of the setup is

similar to a continuous ED setup, except that a suitable redox couple (the Fe2+

/Fe3+

redox couple or the [Fe(CN)6]4–

/Fe(CN)6]3–

redox couple) is applied. As shown in Fig.

1.3, freshwater channels are between two salt water channels, which are separated by

a CEM on one side and an AEM on the other. For a sodium chloride solution, sodium

ions permeate through the cation exchange membrane in the direction of the cathode,

and chloride ions permeate through the anion exchange membrane in the direction of

the anode. Electroneutrality of the electrode rinse solution is maintained via oxidation

Chapter 1

7

and reduction at the anode surface and cathode surface, respectively. The electrical

current generated is captured directly by an external load.

Fig. 1.3 The schematic representation of a RED system (Vermaas et al., 2014)

1.2 Ion exchange membranes

As discussed above, ED is expected to solve issues related to energy and environment

problems. It is generally accepted that the key of an electrodialysis setup is the ion

exchange membranes. The ion exchange membranes are composed of substrates,

functionalized groups, and movable counter-ions. According to the charge of

functional groups, the ion exchange membrane can be a cation exchange membrane

(CEM), or an anion exchange membrane (AEM). Sulfonic acid, phosphoric acid and

carboxylic acid groups are the most common functional moieties for CEMs, while

guanidinium cations, imidazole cations, and quaternary ammonium cations are the

moieties anchored inside the AEMs matrix. According to the connection of charged

groups to the matrix or their chemical structure, ion exchange membranes can be

further classified into homogeneous and heterogeneous membranes. For

heterogeneous membranes, the charged groups are chemically bonded to or physically

Chapter 1

8

mixed with the membrane matrix. Table 1.2 lists some commercially available

homogeneous and heterogeneous ion exchange membrane membranes. In general, the

thickness of the commercial membrane is around 0.2 mm with a resistance smaller

than 5 Ω·cm2.

Chapter 1

9

Table 1.2 The parameters of commercially available ion exchange membranes

Compamy Membrane type Thickness

(mm)

IEC

(meq/g)

Water

uptake

(%)

Resistance

(Ω·cm2)

Permselectivity

(%)

Shandong

Tianwei

Membrane

Technology CO.,

LTD., China

TWBPI (BPMs)

TWEDAI (AEMs)

TWEDCI (CEMs)

0.18-0.23

0.13-0.16

0.10-0.13

-

-

-

20-30

30-40

20-30

-

≤4

≤4

-

-

-

Tingrun

Membrane Tech,

China

TRJBM (BPMs)

TRJCM (CEMs)

TRJAM (CEMs)

0.16-0.23

0.16-0.23

0.16-0.23

-

1.7-2.0

1.5-1.8

33-40

33-38

22-24

-

2.5-5.5

5.0-8.3

90-95

95-99

90-95

PCA

-Polymerchemie

Altmeier GmbH,

Germany

PC-SK (CEMs)

PC-SA (AEMs)

0.13

0.09-0.13

1

1.3

-

-

0.75-3

1-1.5

96

93

Dupont Co., Inc.

USA

Nafion® 117 (CEMs)

Nafion®

112 (PEMs)

0.183

0.089

-

0.9

-

16

-

1.5

-

97

Fuma-Tech

GmbH, Germany

FKS (CEMs)

FAS (AEMs)

0.09-0.11

0.1-0.12

0.9

1.1

-

-

2-4

2-4

-

-

ASTOM

Corporation,

Japan

CSE (CEMs)

ASE (AEMs)

0.16

0.15

-

-

-

-

1.8

2.6

-

-

Asahi Kasei

Chemicals

Corporation,

Japan

Selemion CMV-SK

(CEMs) 0.13-0.15 2.4 25 2.0-3.5 95

Selemion AMV-SA

(AEMs) 0.11-0.15 1.9 19 1.5-3.0 92

Selemion ASV-SA

(AEMs) 0.11-0.15 - - 2.3-3.5 -

Chapter 1

10

From a historical point of view, the development of IEMs began in the eighteenth

century based on the conception of ―osmosis‖. In 1889, Maigrot and Sabates used a

non-selective separator in an electrochemical process to desalination of a sugar syrup

solution (Paidar et al., 2016). After that, Ostwald began to study the properties of

semipermeable membranes and found that a membrane could reject any electrolyte as

long as the membrane is impermeable either for its cation or its anion. This

phenomenon was confirmed by Donnan. A mathematical equation named ―Donnan

exclusion potential‖ was developed to describe the“membrane potential‖ at the

boundary layer (Donnan, 1911). The term ―electrodialysis‖ was used around 1900,

which was much earlier than the elaboration of the ―Donnan exclusion potential‖.

However, the basic research related to ion exchange membranes lagged behind. Only

after the 1930s, the interest in industrial applications prompted the development of

new methods for synthesis of ion exchange membranes. At that time, anion and

cation-selective membranes were assembled together in one apparatus to form parallel

solution compartments. However, industrial applications were significantly impeded

at that time due to the high electric resistance. With the maturation of membrane

fabrication technology, commercial applications for demineralizing and concentrating

electrolyte solutions were realized in the 1950s. In 1970s, a sulfonated

polytetrafluorethylene based ion exchange membrane with excellent stability was

developed by Dupont as Nafion®. The evolution step resulting in an upsurge in the

application in the chlor-alkali production industry (Grot, 1973). Moreover, bipolar

membranes were created in 1976 by Chlanda et al.. By combining a cation exchange

layer with an anion exchange layer, the application domain of electrodialysis with

bipolar membrane was largely amplified (Chlanda et al., 1978). Nowadays, enhanced

properties of ion exchange membranes have been realized with higher selectivity,

lower electrical resistance and improved mechanical stability. New ion exchange

membrane based technologies such as capacitive deionization, continuous

electrodeionization or diffusion dialysis etc. have gained great interest in water and

waste water treatment, food/drug, chemical process industry as well as biotechnology.

Chapter 1

11

The development history of ion exchange membranes is schematically shown in Fig.

1.4. The related ion exchange membrane based processes and their applications are

summarized in Table 1.3. However, all of today's available electro-membrane

processes and components used in these processes still have technical and commercial

limitations. Despite substantial ongoing developments, it is of critical importance to

explore new methods to improve products and processes.

Table 1.3 Electrodialysis and related processes and their applications (Strathmann, 2010)

Ion exchange membrane based processes Technical applications

Electrodialysis Water desalination and salt pre-concentration

Diffusion dialysis Acid and base recovery from industrial waste waters

Donnan dialysis Water softening, and exchange of ions

Bipolar membrane electrodialysis Production of acids and bases from corresponding

salts

Electrodeionization Production of ultra pure water

Capacitive deionization Water desalination and water softening

Reverse electrodialysis Electrodialytic energy generation

Fig. 1.4 Time line visualization of ion exchange membrane development and their related

processes (Xu, 2005)

Chapter 1

12

1.2.1 Preparation of ion exchange membranes

As mentioned above, according to their structure and preparation procedure, IEMs

can be divided into two major categories: homogeneous and heterogeneous. In

homogeneous IEMs, the fixed charge groups are evenly distributed over the entire

membrane matrix while heterogeneous membranes have distinct macroscopic

uncharged polymer domains of ion exchange resins in the membrane matrix. The

specific properties of ion exchange membranes are all related to the presence of these

charged groups. The distinct of amount, type and distribution of ion exchange groups

determine the most important membrane properties.

Ionic groups can be introduced to the homogeneous ion exchange membrane by three

methods: 1. Polymerization or polycondensation of monomers; at least one of them

must contain a moiety that either is or can be made anionic or cationic, respectively; 2.

Introduction of anionic or cationic moieties into a preformed solid film; 3.

Introduction of anionic or cationic moieties into a polymer, such as polysulfone,

followed by the dissolving of the polymer and casting it into a film.

Fluorinated materials are one of the most widely used materials to fabricate IEMs.

Fluorinated membranes have an extreme chemical and thermal stability in

applications in the chlor-alkali industry and in fuel cell applications. A typical

example for successful exploration of fluorocarbon based ion exchange membranes is

the product developed by DuPont with trade name ‗‗Nafion‘‘. However, the

applications of perfluorinated membranes with high equivalent weights were limited

in fuel cells due to their high cost and fuel loss. Membranes with low equivalent

weights were synthesized by Dow Chemical Company as Dow epoxy in 1998 (Souzy

et al., 2004). This polymeric structure can be described as a Teflon-like backbone with

a side chain attached via an ether group. Styrene-divinylbenzene based IEMs are

Chapter 1

13

involved in many industrial ED applications. Such membranes are synthesized by

polymerization of styrene and divinylbenzene, followed by a sulfonation and

amination reaction after chloridization. Cation exchange membranes are synthesized

by a sulfonation reaction using chlorosulfonic acid or concentrated sulphuric acid

while anion-exchange membranes are obtained by amination after chloromethylation.

The reaction equations for the membrane fabrication are shown in Fig. 1.5.

Fig. 1.5 The preparation of styrene-divinylbenzene based ion-exchange membranes (Nagarale et

al., 2006)

Engineering plastics such as polysulfone and polyethersulfone have been widely used

as a base polymer for ultrafiltration and gas separation because of their excellent

HC CH2

HC CH2

CHH2C

n

SO3H

SO3H

n

a) BPO; 60 ℃

b) RT; H2SO4

n

CH2Cl

CH2Cl

n

BPO; 60 ℃

CH3CH2OCH2Cl

SnCl4; 45 ℃; 4 h

n

n

CH2

NH3C

CH3H3C

CH2

NH3C

CH3H3C

Chapter 1

14

workability and mechanical strength. To date, most of the IEMs consist of polymeric

backbones prepared by functionalization of engineering plastics. However, the

variation of the main chain type and side chain type significantly affects the overall

performance of IEMs. Regarding the chemical structure, IEMs are similar with ion

exchange resins because both of them are bearing functional groups. The major

distinction is primarily related to the mechanical requirements of the membrane

process. Consequently, heterogeneous ion exchange membranes can be fabricated by

mechanical incorporation of powered ion-exchange resin into sheets of rubber,

polyvinyl chloride (PVC), acrylonitrile copolymers or some other extrudable or

moldable matrix (Gohil et al., 2004). By choosing suitable reinforcing materials,

heterogeneous ion exchange membranes can be obtained with controlled

electrochemical properties and mechanical strength. The present fabrication processes

include: a. calendering ion-exchange particles into an inert plastic film; b. dry

moulding of inert film forming polymers and ion exchange particles and then milling

the mould stock; c. resin particles can be dispersed in a solution containing a film

forming binder and then the solvent is evaporated to give ion-exchange membrane.

Comparatively, homogeneous membranes have good electrochemical properties

whereas heterogeneous membranes rather have a very good mechanical strength.

1.2.2 Monovalent selective ion exchange membrane

The separation of monovalent ions from multivalent ions in aqueous solution is a

recurrent problem in industrial applications where water salinity is important. A

typical example is seawater purification, where Ca2+

and Mg2+

should be removed to

prevent scaling. Thus, the optimal drinking water production method for a hard source

water requires the rejection of divalent ions while the monovalent ions should

permeate through the membranes. Further examples can be found for e.g.,

electroplating wastewaters containing heavy metals, which should be selectively

removed. Traditional methods to treat these wastewaters include precipitation,

Chapter 1

15

chelating and chlorination. However, excessive sludge production and slow

precipitation progress with insufficient metal removal limit the industrial application.

Electrodialysis with monovalent selective ion exchange membrane (MIEMs) provides

a method to treat these wastewaters, by separating monovalent and multivalent ions.

Currently, MIEMs are explored in a wide range of applications including seawater

desalination, acid recovery and the removal of specific ions.

The mechanisms to separate a specific ion were proposed by Sata in 1994 (Sata,

1994). The permselectivity between ions is governed by several factors, such as

hydrated radii, affinities with IEMs and migration rates in the membrane phase.

Strategies used for regulating such factors are as follows. The addiction of a

crosslinked skin layer on membrane surface is deemed to be an effective way to

obtain MIEMs. By introducing a dense skin layer, the migration of ions with different

hydrated radii occurs on the basis of a size sieving effect. Hou et al. developed a

highly selective MIEM by forming a polyamide selective layer on the membrane skin

layer. The degree of crosslinking was evaluated based on the surface elemental

composition and the results indicated that membrane with a higher cross-linked

structure lead to an improved mono/divalent cations permselectivity (Hou et al., 2018).

Apart from the widely reported covalent crosslinking, acid-base cross-linking and

crystallization are new emerging technologies to improve the membrane surface

density. An annealing treatment strategy was proposed by Ge et al. (Ge et al., 2014) to

enhance the density of polyvinyl alcohol (PVA)-based CEMs by tuning the structural

crystallinity, which can improve the pore-size sieving effect for ions. Due to the

semi-crystalline nature of PVA, the crystallinity of membranes can be controlled by

adjusting the annealing temperature. The increase of the annealing temperature

dramatically decreases the water uptake. In addition, the ion exchange capacity (IEC)

was maintained, indicating that the membrane intrinsic properties are highly

crystallinity dependent. However, in some cases, crosslinking can decrease the

difference in electrostatic repulsion between the cationic layer and cations with

different valence and weaken the monovalent selectivity of the modified membrane. A

Chapter 1

16

modification method using covalent immobilization and self-crosslinking of chitosan

layer on CEM surface was devised by Wang et al. (Wang et al., 2013). The Zn2+

and

Mg2+

leakage of the membrane samples crosslinked by formaldehyde were increased

by 23% and 69%, respectively, compared to those of membranes only grafted with a

chitosan layer. The reason might be that –NH2 groups of the chitosan layer were

consumed by the crosslinking reaction and the generated secondary amine and tertiary

amine only embraced relatively a weak positive charge due to the solvent effect.

Nevertheless, an increase of the electrical resistance is inevitable for most modified

membranes as the migration of ions is mitigated. To solve this problem, introducing

conducting polymers like polyaniline (PANI) or polypyrrole (PPY) can be an

alternative method to both reject the multivalent ions and reduce the membrane

resistance. Kumar et al. chemically modified organic-inorganic hybrid CEMs by in

situ polymerization of aniline in acidic medium using FeCl3 as an oxidizing agent

(Kumar et al., 2013). The thin layer of PANI grafted on the membrane surface and in

the membrane matrix was thermally stable and dense. The Na+ transport number

across the membranes remained unchanged, whereas the transport number of Zn2+

and

Al3+

decreased after modification with PANI. Sivaraman et al. also reported a method

to modify CEMs by electrochemical deposition (Sivaraman et al., 2007). By this

method of modification, the amount of PANI can be controlled easily. Furthermore,

the presence of a conducting polymer PPY layer on the surface of IEMs resulted in a

slower electro-migration of bivalent ions (Ca2+

, Mg2+

and Cu2+

) in comparison to

monovalent ions, which can serve as an alternative material to PANI for modification

of the IEMs (Gohil et al., 2006a).

Deposition of an oppositely charged layer relative to the charge of bulk IEMs also

enables the preparation of MIEMs. For membranes obtained by this method,

multivalent ions with the same charge as the membrane surface are rejected due to the

relatively large electrostatic repulsion, while monovalent ions are still able to pass

through the skin layer. Polyethyleneimine (PEI), a commercially available cationic

polyamine, is one of the most effective and frequently used polyelectrolytes to

Chapter 1

17

fabricate MIEMs. The combination of PEI with IEMs was realized by adsorption or

electro-deposition (Amara and Kerdjoudj, 2003; Guesmi et al., 2012). However, the

monovalent cation permselectivity of the membranes gradually deteriorates during

electrodialysis because the PEI was desorbed from the membrane surface. Thus,

fixation of the cationic polyelectrolyte on the membrane surface by covalent bonding

has been actively studied. One of the effective methods is to fix PEI on the membrane

surface by acid–amide bonding (Takata et al., 2000). After introducing sulfonyl

chloride groups into the membrane matrix, thin PEI layers were formed on the

membrane surfaces by sulfonyl-amide bonding.

Specific interactions between the ion exchange groups and the mobile ions can be

utilized to regulate the membrane selectivity. By choosing the ionic functional groups

and IEM matrices, selective separation can be achieved. As ionic functional groups

are anchored inside the membrane matrix, the mobility of the multivalent cation is

low in the membrane phase compared to monovalent ions. As a consequence, the

permselectivity of specific ions through the membrane may be changed by altering the

interaction between specific ions, ion exchange groups and the membrane matrix

(Sata, 1994).

Recently, research has proved that the membrane hydrophilicity also impact the

selectivity of the membrane. By decreasing the hydrophilicity, the permselectivity of

MIEMs increases (Gohil et al., 2006b). To improve the monovalent selectivity of

IEMs, the methods mentioned above can also be combined. Although IEMs are

industrially applied in numerous fields, the comprehensive properties of MIEMs still

lag behind to meet the requirements of applications on an industrial scale. More

research attention should be focused on releasing the limitations related to a high

electrical resistance, low permselectivity and insufficient ion flux.

Chapter 1

18

1.3 Porous ion exchange membranes

Advances in membrane technology are related to the development of

high-performance membranes with high permselectivity, low electrical resistance,

high mechanical strength, and high chemical stability. In general, a homogeneous or

heterogeneous ion-exchange membrane is a dense membrane that defines the

transport of ions. Although many strategies have been developed to accelerate ion

transport, these strategies were not economically competitive or resulted in an inferior

electrochemical performance (Malik et al., 2016). Recently, UF membranes were used

in the electrodialysis process to facilitate the migration of ions according to their

charge and molecular size (Bazinet and Moalic, 2011b). Chitosan oligomers are

widely used in biotechnology, pharmaceutical and health food industry because of

their bioactivity (Yamada et al., 2005). During the industrial production, the final

product is a mixture of molecules of different molecular weights and contains

minerals. Thus, fractionation of such mixtures is crucial (Horowitz et al., 1957). Aider

et al. suggested that electrodialysis with ultrafiltration method could be used as a

powerful process for the separation of bioactive chitosan oligomers of interest from

complex feed solutions under mild conditions, and other applications in food,

bio-pharmaceutical, and nutraceutical industries (Aider et al., 2008, 2009). However,

for the application of ultrafiltration membranes in electrodialysis, the separation of

smaller ionic species still remains a challenge, because of the relatively large pore

sizes of these membranes.

Inspired by the methods of performing electrodialysis with ultrafiltration membranes

and preparing pore-filled membranes, porous membranes prepared from charged

polymers with functional groups represent another option. Such membranes with

porous structure might facilitate the migration of ions by reducing the physical steric

hindrance of the surface layer. Thus, the migration resistance could be further

diminished, whereas the functional groups of the membrane matrix could block the

diffusion of ions caused by the concentration gradient.

Chapter 1

19

Porous materials are defined as solids containing empty voids which can host other

molecules. The fundamental feature of these materials is their porosity, the average

size of the pores and the surface area. Typical values for the surface area of porous

materials applied in technological processes range between 2000 and 8000 m2/g. The

most important applications of such materials are the storage of small molecules and

filtering. The framework defining a porous material can consist of either organic or

inorganic materials. With regard to developing new methods to produce novel and

highly flexible functional materials, the loading of porous materials with functional

molecules or objects is attracting an increasing amount of interest (Shekhah et al.,

2011).

More recently, metal-organic frameworks (MOFs) have been explored as novel

nanoporous crystalline solids combining metal ions with organic linkers resulting in

highly porous frameworks, and are emerging as a new family of molecular sieve

materials (Tranchemontagne et al., 2009). Apart from the versatile applications in

ion-exchange, gas separation and storage, optics, drug delivery and catalysis (Kuppler

et al., 2009), their unique properties including uniform pore size, high surface areas

and specific adsorption affinities make MOFs attractive for assembling into

membranes with excellent performance. Li et al. present a new approach to construct

ion nanochannels by in situ assembly of a poly N-vinylimidazolium (ionic liquid) as

the ion carrier within the highly ordered pores of MOFs (Li et al., 2016). At an

application oriented level, the highly designable nature of MOFs allows for

customizing pore morphologies, and the structural diversity of ionic liquids allows a

wide selection of ions. Because the demand for fabricating such porous coatings is

rather obvious, several studies have either demonstrated or proposed new applications

of MOF thin films. However, most of them were applied in gas separation, seldom

with ion selectivity (Bux et al., 2011; Huang et al., 2013). Tailoring nanochannel

morphologies by constructing efficient ion nanochannels with appropriate geometrical

and chemical structures is a feasible strategy for achieving monovalent selectivity. For

Chapter 1

20

instance, zeolitic imidazolate framework-8 (ZIF-8), a subclass of MOFs with a small

aperture with a size of 3.4 Å, can serve as an effective filter to separate hydrated

cations of Mg2+

(4.28 Å) through a size sieving effect. Consequently, ZIF-8 with the

right size makes it a fine candidate for preparing MIEMs. Thus, membranes with a

pore structure would guarantee a high permeance when nanochannels are explored as

a candidate for membrane fabrication and modification.

1.4 Motivation and contents of the PhD thesis

The aim of this thesis is to prepare ion exchange membrane with improved

electrochemical properties by introducing porous structures. The chapters in this

thesis are organized as follows: Chapter 1 provides a brief review about membrane

technology for water treatment. In view of understanding electrodialysis technology,

the basic concept, the separation mechanism, especially the preparation and

modification of the ion exchange membrane, are highlighted. Chapter 3 provides a

dry-wet phase-inversion strategy for the preparation of porous sulfonated poly(ether

sulfone) (SPES) membranes. The objective of this study was to prepare

laboratory-made porous ion exchange membranes with improved physical and

electrochemical properties. The porosity was tuned by altering the time of membrane

exposure to an elevated temperature environment. The membrane resistance and its

application in electrodialysis were conducted.

By introducing PANI on membrane surface, elevated separation properties can be

obtained based on the size of the molecules or ions and their charge. A previous study

reported that electrodialysis with nanofiltration membrane (EDNF) can be carried out

for fractionation of cations (Ge et al., 2016a). By replacing the cation exchange

membrane with NF membranes, the NF membrane can fractionate ions by rejecting

the multivalent ions with larger Stokes radius. Enlightened by EDNF, an efficient

one-step chemical process to graft a thin polyaniline (PANI) layer on the surface and

Chapter 1

21

pores of an ultrafiltration (UF) membrane is reported in Chapter 4. The influence of

the type of UF membrane (with different molecular cutoff values) on the extraction

and selective permeation of cations was investigated, using Na+ and Mg

2+ as the

monovalent and bivalent cations.

In Chapter 5, a one-pot approach to prepare a monovalent selective cation exchange

membrane by polydopamine (PDA)/PEI co-deposition is reported. PDA, known as

―bio-glue‖ can fix PEI molecular chains on the membrane surface while the side PEI

chains serve as repulsive functional groups that reject multivalent ions. With the

assistance of CuSO4/H2O2, the deposition time can be significantly reduced. The

monovalent selectivity and stability of the PDA/PEI modified membranes were

investigated in this part. For Chapter 6, Mil(53)-Al nanoparticles were incorporated

to the skin surface layer to facilitate the ions transport. Theoretically, the hydrated

radius of Na+ is around 3.0 Å, which is smaller than the Mil(53)-Al pores (8.5 Å).

Therefore, the presence of Mil(53)-Al on the thin film composite surface can provide

extra space to enhance the Na+ diffusion process.

Similarly, an interfacial polymerization (IP) method was applied by anchoring zeolitic

imidazolate framework-8 (ZIF-8) to the skin layer of thin film nanocomposite (TFN)

membranes in order to obtain monovalent selectivity in Chapter 7. Monovalent

selectivity can be achieved by interfacial polymerization while the ion flux across the

ion exchange membrane was enhanced by ZIF-8 incorporation. The transfers of

cations during the electrodialysis process were evaluated by both single salt solution

and mixed ions solutions.

Finally, the general conclusions and future perspectives are presented in Chapter 8.

Chapter 2

22

2. Methods and materials

This section outlines the materials and methods used in this dissertation. The first part

summarizes the chemicals and setups required during the experiments. Then a dry-wet

phase inversion method was introduced to explain the preparation process for porous

cation exchange membranes. After that, a method to prepare monovalent selective ion

exchange membranes based on an ultrafiltration membrane was introduced.

Furthermore, monovalent ion exchange membranes with porous surface structure

prepared by dopamine/polyethylenimine co-deposition and interfacial polymerization

were introduced. This section also covers all the characterization methods for the

obtained membranes.

2.1 Chemicals and methods

2.1.1 Chemicals

Poly(ether sulfone) (PES, Ultrason E6020P, 58000 g/mol) was purchased from BASF

(Antwerp, Belgium). Sulfonated poly(ether sulfone) (SPES) was supplied by Zhejiang

University of Technology (Hangzhou, China). Dimethyl sulfoxide (DMSO) of

analytical grade was purchased from Sigma-Aldrich (Overijse, Belgium). Dopamine

hydrochloride, hydrogen peroxide (H2O2, 30% by weight), polyethylenimine (PEI,

branched, Mn = 600, 10000 Da), sodium chloride (NaCl,99%), magnesium chloride

(MgCl2, 98%), sodium sulfate (Na2SO4, 99%), hydrochloric acid (HCl, 1 M),

sodiumhydroxide (NaOH, 98%), sulfuric acid (H2SO4,95–98%), zinc sulfuric

(ZnSO4, 99%), ammonium persulfate (APS), copper sulfate (CuSO4, 99%),

tris(hydroxymethyl) aminomethane (Tris,99.8%), dimethyl sulfoxide (DMSO, ≥

99%), m-phenylenediamine (MPD, ≥99%), trimesoyl chloride (TMC, ≥98%),

hexane (anhydrous, 95%) and aniline were purchased from Sigma-Aldrich (Diegem,

Chapter 2

23

Belgium). Basolite A100 (Mil(53)-Al) and Basolite® Z1200 (ZIF-8) used in

thisexperiment were produced by BASF (Antwerpen, Belgium) and acquired from

Sigma-Aldrich. All reagents and solvents were commercially available and used as

received. Distilled water was used throughout the thesis.

The ultrafiltration membranes used in Chapter 4 were provided by Ultra Water (USA).

Information about the membranes is given in Table 2.1.

Table 2.1 Properties of polyacrylonitrile (PAN) ultrafiltration membranes

Name Water flux (L m-2

·h-1

·bar-1

) Marker rejection (%)

PAN 200 300 80

PAN 350 1000 80

PAN 400 600 75

Pan 450 1200 75

Cation exchange membranes used in Chapter 3 (FKB, thickness = 0.1–0.12 mm; ion

exchange capacity = 1.01 meq/g) were obtained from Fumatech GmbH (Germany).

Ion exchange membranes used in chapter 5 and chapter 6 were commercial anion

exchange membranes (AEM-80045) and cation exchange membranes (CEM-80050)

purchased from Fujifilm Manufacturing Europe B.V (The Netherlands).

2.1.2 Porous ion exchange membrane preparation

In this part, the strategies of immersion precipitation and dry-casting were combined,

to control the membrane porosity with the purpose of improving the physical and

electrochemical properties of ion-exchange membranes. Briefly, 3 g of SPES was

dissolved in 17 g of DMSO at room temperature, and the mixture was stirred until the

SPES was fully dissolved. Subsequently, the polymer solution was degassed by

sonication. The degassed polymer solution was then cast on a glass with a casting

Chapter 2

24

knife to an initial thickness of 250 μm (K4340 Automatic Film Applicator, Elcometer).

The solution on the plate after casting was dried in an oven at 60 °C for different

heating times before being precipitated in deionized water. The formed membranes

were peeled off from the plate and stored in deionized water for further use.

Membranes with different charge densities were prepared by mixing SPES with

various amounts of PES. The preparation method was the same as described above.

The compositions of the prepared membranes and the fabrication conditions are

summarized in Table 2.2.

Table 2.2 Compositions of PES/SPES cation exchange membranes and preparation conditions

Membrane PES (g) SPES (g) DMSO (g) Heating time (h)

PS0 0 3 17 5

PS1 0 3 17 1

PS2 0 3 17 0.5

PS01 0 3 17 0

PS12 1 2 17 0

PS11 1.5 1.5 17 0

PS21 2 1 17 0

PS10 3 0 17 0

2.1.3 Preparation of monovalent selective ion exchange membrane

based on ultrafiltration membrane

Prior to surface modification, the UF membranes were hydrolyzed via immersion into

a 1 M NaOH aqueous solution at ambient temperature for 24 h. The membranes were

subsequently soaked in deionized water for 24 h to create hydrolyzed PAN

membranes. After that, the PAN films were cut into a specific shape and then

Chapter 2

25

immersed in 200 mL 1.0 M HCl aqueous solution containing 2.5 mL of aniline

monomer. The solution was stored in an ice-water bath for 3 h to fully absorb the

aniline monomer into the PAN film. Then, 50 mL of precooled aqueous solution

containing 2.25 g of APS was poured into the above mixture and then shaken well and

stored in the ice-water bath for 24 h. After polymerization, the modified films were

completely washed with 1 M HCl and deionized water.

2.1.4 Preparation of polydopamine/polyethyleneimine modified

monovalent selective ion exchange membrane

The method used to prepare cation exchange membranes was similar to the method

used in Section 2.1.2: 3 g SPES was dissolved in 17 g DMSO at room temperature.

After degassing by ultrasound, the solution was casted on a glass plate, with a

thickness of 250 μm. The assembled membrane was then placed in an oven at 60 °C

for 12 h and then the membrane was peeled off from the glass plate. The SPES

membranes were stored in DI water for further use.

The monovalent selective cation exchange membrane was prepared by a fast

deposition of dopamine/PEI. In detail, PEI and CuSO4 (39.9 mg) were added into

50 mL Tris buffer solution (pH=8.5). Then, dopamine hydrochloride with designed

mass content was mixed into the above solution. After dopamine hydrochloride was

fully dissolved, 0.1 mL of H2O2 was added. Subsequently, the membrane fixed in a

membrane holder was contacted with the dopamine solution for 4 h at room

temperature. After modification, the membranes were rinsed three times with DI

water and stored in water before further use. For comparison, another group of

membranes was soaked in a dopamine solution without adding PEI by using the same

conditions. During the polymerization process, Fenton-like reactions involving

Cu2+

/H2O2 can generate reactive oxygen species (ROS), including OH·, O2-

and H3O+,

which can significantly accelerate the polymerization process (Poyton et al., 2016).

Chapter 2

26

The oxidized quinone forms catechol groups that can react with amino-terminated PEI

via Michael addition/Schiff base reaction (Xu et al., 2010). Besides, Cu2+

ions in the

solution can strongly chelate with dopamine to form stable dopamine-Cu2+

complexes

(Wang et al., 2017a). The detailed mechanism of the co-deposition process is

presented in Fig. 2.1. The assigned labels with different functionalization parameters

of the modified membranes in this work are listed in Table 2.3.

Fig. 2.1 Proposed mechanisms for the reactions between PDA and PEI

Table 2.3 Surface modification parameters corresponding to the assigned membranes

Membrane PEI (mg) H2O2 (mL) Dopamine (mg) Deposition time (h)

SPES 0 0 0 0

SPES-PDA/PEI-0 0 0.1 60 4

SPES-PDA/PEI-1 60 0.1 60 4

SPES-PDA/PEI-2 120 0.1 60 4

SPES-PDA/PEI-3 180 0.1 60 4

Chapter 2

27

2.1.5 Preparation of monovalent cation exchange membrane

containing hydrophilic MIL53-(Al) framework

A CEM was first soaked in deionized water at room temperature until the membrane

was fully hydrolyzed. Afterwards, the membrane was fixed on the lab-made

membrane holder to contact the water solution. The modification process was similar

as described in section 2.1.4. The cation exchange membrane was modified by the

co-deposition of PDA/PEI and Mil(53)-Al using CuSO4/H2O2 as a trigger, followed

by cross-linking with trimesoyl chloride (TMC). PEI (120 mg) was dissolved in a

mixed solution with CuSO4 (39.9 mg) in a Tris buffer solution (50 mM, pH = 8.5).

Then different mass ratios of MIL (53)-Al (0 mg, 10 mg, 20 mg, 30 mg) were added

to the above solution followed by sonication to achieve a uniform dispersion of

nanoparticles. Finally, 60 mg of dopamine hydrochloride was dispersed to the mixture

followed by the addition of H2O2 (0.1 mL). The fresh solution was transferred to the

holder to contact with membrane for 2 h. Subsequently, the aqueous solution on the

membrane surface was replaced by an equal volume of 0.1 wt % TMC/n-hexane

solution to perform the polymerization reaction for 2 min. After the excess organic

solution was drained, the membrane surface was rinsed with water and n-hexane to

remove the remaining chemicals. Finally, the prepared membranes were air-dried for

further use. The fabrication process of monovalent selective CEMs with Mil(53)-Al is

schematically shown in Fig. 2.2.

Chapter 2

28

Fig. 2.2 Schematic diagram of the codeposition process for preparing monovalent selective CEMs

2.1.6 Preparation of monovalent cation exchange membrane by

interfacial polymerization

The pretreatment process of the Fuji-films substrate was the same as described in

Section 2.1.4. The MPD concentration was fixed at 2.0% (w/v). Organic phase

solutions that were used in these experiments were prepared via adding a specific

amount of ZIF-8 to the TMC/n-hexane solution (0.1 wt %) under sonication for 1 h.

The as-prepared membranes with different ZIF-8 loadings (0.00%, 0.02%, 0.04%,

0.06%, and 0.08% in 50 mL n-hexane solution) were designated as M-1, M-2, M-3,

M-4, and M-5. The commercial cation exchange membrane was first immersed into

the aqueous solutions for 5 min to implement the adsorption of MPD. After removing

the remaining aqueous solutions on the membrane surface, the organic phase solution

with a specific amount of ZIF-8 was poured on the surface of the membrane to carry

out the polymerization. The organic solution was removed after 2 min, and the excess

organic solution was drained at the fume hood. The fresh prepared membrane was

washed three times with n-hexane and dried completely to enhance the surface layer

stability.

Chapter 2

29

2.2 Membrane properties and characterization

2.2.1 Ion exchange capacity

The ion exchange capacity (IEC) is the number of fixed charges inside the ion

exchange membrane per unit weight of dry polymer. It is a crucial parameter that

affects almost all other membrane properties. Since the presence of large quantities of

fixed charges promotes membrane swelling, a high IEC is typically accompanied by a

high swelling degree (SD). While a high IEC tends to increase the membrane

permselectivity, a high swelling degree may reduce the effect of the IEC and

adversely affect the permselectivity. Such competing effects call for a compromise

between the IEC and the SD. The IEC is expressed in milli equivalent of fixed groups

per gram of dry membrane (meq/g) and experimentally tested through determining the

number of counter-ions after turning the CEMs into the H+ saturated form and the

AEMs into the Cl− saturated form. The IEC of the prepared (monovalent) cation

exchange membranes was detected by the titration method. The membrane was

immersed in a 1M HCl solution to saturate it with H+. After the membrane was

saturated by H+, the membrane was taken out and the remaining solution on the

membrane surface was removed by a tissue. After that, the membrane was transferred

to a 1 M NaCl solution for 24 h to liberate the H+ ions. The released H

+ ions

concentration was quantified by a 0.01 M NaOH solution. The IEC was calculated

based on the following equation:

𝐼𝐸𝐶 =𝑛𝐻+

𝑊𝑑𝑟𝑦

where 𝑛𝐻+ is the concentration of the released H+ ions and Wdry is the dry membrane

weight (g). A minimum of five measurements was obtained for each membrane to

calculate its average value.

Chapter 2

30

2.2.2 Water uptake

An ion exchange membrane consists of cross-linked polymers and forms a wet

structure when it absorbs water in a solution. The water content is of crucial

importance for the membrane dimensional stability and ionic transport properties. A

high water content implies a loose mechanical structure and often results in a poor

permselectivity, despite its positive effect on the membrane conductivity. The water

content is influenced by the membrane material, fixed charged groups, cross-linking

degree of membrane matrix and surrounding solution conditions. For example, some

AEMs with relatively low cross-linking degrees tend to have a higher water content

than their more cross-linked CEM counterparts. The water uptakes in these

experiments were measured at room temperature based on the water retention inside

the membranes. The membranes were dried at 60 °C in an oven and then weighed

accurately, after which they were immersed in deionized water for at least 24 h to

ensure complete equilibrium and then reweighed after the surface moisture had been

mopped with filter paper. The water uptake (WU) was calculated as:

𝑊𝑈 =𝑚𝑤𝑒𝑡 − 𝑚𝑑𝑟𝑦

𝑚𝑑𝑟𝑦× 100%

where mwet is the weight of IEMs in wet condition and mdry is the weight of membrane

sample in its corresponding dry phase. The results were obtained based on the mean

average value of three measurements.

2.2.3 Water contact angle

Contact-angle measurements have been frequently used to characterize the polarity or

surface energy of polymeric surfaces (Nabe et al., 1997). The water contact angle on

the membrane surface was measured by the sessile drop method using a DATA

Physics System (OCA20, Dataphysics Instruments, Germany) in which a droplet of

water on the surface was imaged by a precision video camera and displayed on a

monitor. A circle fitting analysis software was utilized to record the contact angle. A

minimum of five contact angle measurements was obtained for each membrane to

Chapter 2

31

calculate the average value.

2.2.4 Zeta potential of membrane surfaces

The zeta potential describes the interaction of the electrical surface charges with their

surroundings, although this potential is somewhat different from an actual surface

potential (Mulyati et al., 2013). As the material comes into contact with solutions,

ions from the solution rapidly migrate to the material surface in order to neutralize the

opposite charge from the materials. The charge behavior at the interfaces was

measured by the zeta potential via a Surpass 3 (Anton Paar, Austria) by using 1mM

KCl as the electrolyte solution (pH=6.0).

2.2.5 Membrane electrical resistance

Ion exchange membranes are widely used in ED for the desalination of brackish water,

the production of table salt, recovery of valuable metals from the effluents of

metal-plating industry, and for many other purposes. (Hwang et al., 1999) Membrane

properties and especially the electrical resistance of the membrane, have a significant

impact on the overall ED stack performance. The electrical resistance is directly

related to the maximum power output in reverse electrodialysis and the energy

consumption in electrodialysis. The membrane resistance is determined by the ion

exchange capacity and the mobility of the ions within the membrane matrix.

Furthermore, the operational temperature is another parameter that can impact the

electrical resistance. With increasing temperature, the electrical resistance decreases.

The specific membrane resistance is in principle reported in Ohm/cm. However, more

useful and most often reported in literature is the membrane resistance in Ohm/cm2.

Membrane resistances are often measured under direct current conditions using a

standard 0.5 M NaCl characterization solution. Using Ohm‘s law, the resistance of the

system (membrane + solution) and the membrane, can be determined. However, direct

Chapter 2

32

current measurements are not able to distinguish between the individual membrane

resistance and the additional resistances of the interfacial ionic charge transfer through

the double layer and the diffusion boundary layer. Furthermore, the membrane

resistance measured under direct current conditions becomes strongly concentration

dependent and strongly increases with decreasing concentration at lower

concentrations (Długołęcki et al., 2010a; Długołęcki et al., 2010b).

Electrochemical impedance spectroscopy (EIS) is a powerful technique to provide

valuable information on the electrochemical properties of the membrane system

(Nikonenko and Kozmai, 2011). The resulting voltage drop over the system is

measured as a function of time U(t) and the phase shift φ (◦) relative to the input

signal is determined. The voltage and current variation with time was defined as:

𝑈(𝑡) = 𝑈0 sin 𝑤𝑡 = 𝑈0𝑒𝑗𝑤𝑡

𝐼(𝑡) = 𝐼0 sin(𝑤𝑡 + 𝜑) = 𝐼0𝑒𝑗(𝑤𝑡+𝜑)

Here, U0 and I0 is the voltage and alternating current in phase (A), j is the imaginary

unity (j = √−1). The symbol w is the circular velocity (1 rad/s) which is also referred

to as circular frequency of the alternating current (Zhao et al., 2017).

By using Euler‘s formula:

𝑒𝑗𝑤 = 𝑐𝑜𝑠φ + 𝑗𝑠𝑖𝑛φ

The impedance Z can then in accordance with Ohm‘s law be calculated as:

𝑍 =𝑈𝑡

𝐼𝑡=

𝐼0𝑒𝑗𝑤𝑡

𝐼0𝑒𝑗(𝑤𝑡+𝜑)= |𝑍|𝑒−φ𝑗 = |𝑍|𝑐𝑜𝑠φ − j|𝑍|sinφ

These fixed charged groups attract ions with opposite charge from the salt solution,

which are distributed over the membrane surface and form the electrical double layer.

The interfacial ionic charge transfer from the solution phase through the electrical

double layer to the membrane is referred to as the electrical double layer resistance.

The thickness of this layer is typically in the order of nanometers (Debye length).

(Strathmann, 2004)

Chapter 2

33

When a current passes through an ion exchange membrane, charge is transported

through the membrane by counter ions as a result of Donnan exclusion. In the bulk

solution, current is carried by both positive and negative ions. The difference in ion

transport number between the bulk solution and the membrane results in the building

up of diffusion boundary layers at the membrane surface. (Krol et al., 1999; Sistat et

al., 2008) The concentration decreases at one side of the membrane and increases at

the other side of the membrane; this phenomenon is called concentration polarization

and occurs within the diffusion boundary layer.

EIS allows distinguishing between these different layers because the different layers

respond differently to the applied signal (current) at different frequencies. At the high

frequency range when there is no phase shift between voltage and current and the

Ohmic relation holds, the response of the single membrane can be extracted from the

EIS measurements. (Coster et al., 1996; Park et al., 2006) In principle this resistance

represents the resistance of the membrane containing the solution resistance (RM+S),

but the pure membrane resistance (RM) can be easily extracted by subtracting the

solution resistance (RS) as determined from a blank measurement without a membrane.

When the frequency is decreased, the contribution of the interfacial ionic charge

transfer from the solution phase through the electric double layer to the membrane can

be extracted. Ions start to migrate through the interfacial double layers and a phase

shift is observed. The resistance (RDL) and capacitance (C) of ionic transport through

these double layers becomes visible. This surface charging is similar to what is

observed for a capacitor and the interfacial ionic charge transfer through the double

layer is represented in the equivalent circuit model as a resistor and capacitor in

parallel. When a very low frequency signal (current) is applied, in addition to the

membrane and the ionic transfer through the electrical double layer, concentration

gradients in the diffusion boundary layers adjacent to the membrane become visible.

At these low frequencies, ions are transported through the membrane, the double layer

and the diffusion boundary layer (Fig. 2.3) and the system responds with a certain

Chapter 2

34

delay to the applied signal (Długołęcki et al., 2010b).

Fig. 2.3 Phenomena occurring in the cation exchange membrane and in the layer adjacent to the

membrane (RM is the membrane resistance, RDL is the resistance of the interfacial ionic transfer

from the solution through the double layer into the membrane, C is the capacitance of the

interfacial ionic charge at the membrane surface (double layer), RDBL is the diffusion boundary

layer resistance and Q is the constant phase element representing a non-ideal capacitor of the

diffusion boundary layer.) (Długołęcki et al., 2010b)

The membrane resistance was measured with a Solartron Electrochemical System by

electrochemical impedance spectroscopy (EIS) over a frequency range from 1 kHz to

1 MHz. The conductivity cell was filled with a NaCl or a MgCl2 solution in each

compartment with an effective area of 1 cm2.

2.2.6 Diffusion dialysis experiments

Conductivity-time curves were used to determine the concentration of ions through

diffusion. These curves reflect the ion concentration directly. Here, diffusion dialysis

experiments with a 1 M NaCl diluated compartment cell and an initial concentrated

Chapter 2

35

compartment cell with deionized water were carried out. The concentration gradient

drives ions across the membrane, which has an effective area of 13.84 cm2.

The dialysis coefficient of the membrane for each component is defined as the amount

of the component transported per unit active membrane area, per unit time, per unit

concentration difference of the component and was calculated from the concentration

of species according to the following equation:

𝑈 =M

A · t · ∆C

where M is the amount of the component transported (moles); A is the effective area

(m2); t is the time (h); and ΔC is the logarithmic average concentration between the

two chambers (moles per cubic meter); given by:

∆𝐶 =(𝐶𝑓

𝑜 − 𝐶𝑓𝑡 + 𝐶𝑑

𝑡)

ln𝐶𝑓

0

𝐶𝑓𝑡 − 𝐶𝑑

𝑡

where 𝐶𝑓0 and 𝐶𝑓

𝑡 are the feed concentrations at times 0 and t, respectively, and 𝐶𝑑𝑡

is the dialysate concentration at time t.

2.2.7 Current-voltage and transport number measurements

It has been reported that a cation-exchange membrane modified by formation of a

cationic layer on the membrane surface is preferentially permeable to cations of lower

rather than higher valency and to smaller hydrated cations rather than larger ones. A

method frequently used to characterize the transport properties of cation-exchange

membranes is to study the current-voltage curves corresponding to the membrane

system. The current-voltage curves are associated with the well-known concentration

polarization phenomenon, which is arising at the interface between an ion-exchange

membrane and an electrolyte solution when an electric current passes through the

system. This phenomenon has been widely studied in the last decades with the

purpose of establishing the factors that determine it and resolving the problems that

Chapter 2

36

concentration polarization causes in membrane technology (Mafé et al., 2003;

Rubinstein et al., 1988).

The concentration polarization for cation exchange membranes is due to the fact that

all the charge is carried by cations in the membrane, when in solution the same charge

is carried by cations and anions. In the diluate compartment, the concentration

becomes lower at the membrane surface (Cm) than in the bulk solution (Cb), and in the

concentrate compartment, at the membrane surface, concentration becomes higher

(Cm) (Fig. 2.4). The concentration polarization generates diffusive transport and

creates diffusion boundary layers at membrane surfaces. According to the

concentration polarization theory, the electric current circulating through the

membrane system increases linearly with the voltage increase and eventually reaches

a limiting value, Ilim. In the case of a cation exchange membrane this value is

expressed by:

𝐼𝑙𝑖𝑚 =F · D · 𝐶𝑏

∆𝑡+ · 𝛿

where 𝐶𝑏 is the bulk concentration of the cations, δ is the thickness of the diffusion

boundary layer, D is the diffusion coefficient of the cations, Δt+ is the difference

between the cation transport number in the cation exchange membrane and in the

solution, and F is Faraday‘s constant.

Fig. 2.4 Concentration polarization for cationic exchange membrane (Chamoulaud and Bélanger,

2005)

Chapter 2

37

In practice, the current–voltage curve has a characteristic shape and clearly shows

three regions (Fig. 2.5). First, the Ohmic region that is observed at low current density,

the system resistance could be approximately attributed to ionic transport into the

cation-exchange membrane (Rohm). This region is followed by the current-limiting

region in which current density varies very slowly with the potential to form a

pseudo-plateau. In accordance with the concentration polarization theory, the limiting

current value can give information about the thickness of the diffusion boundary layer,

the diffusion coefficient, or the cation transport number in the membrane. Third, the

current plateau is followed by the electroconvection region, in which the slope of the

current–potential curve increases again (Rec). Those currents larger than the limiting

current are not expected according to the classical theory of concentration polarization

(Barragan and Ruız-Bauzá, 1998).

Fig. 2.5 Typical current-voltage curve for a cation exchange membrane (Chamoulaud and

Bélanger, 2005)

The current-voltage curves of monovalent selective ion exchange membranes were

measured at 25 °C through a two-compartment measuring technique (Fig. 2.6). The

effective monovalent selective ion exchange membrane area was about 13.84 cm2 and

Chapter 2

38

the working electrolytes of electrode were 0.5 mol/L NaCl solutions. The current

across the set-up was supplied by a direct current power system (DF1720SB5A,

Zhejiang Zhongce Electronic Co., Ltd. China). The voltage drop across the membrane

was tested by a voltmeter (ZW1418, Qingdao Qingzhi Instruments Co., China) with

two platinum filaments as electrode probes. The membrane was placed between the

two half-cells, which were equipped with mechanical stirrers in each compartment.

During the experiments, no obvious variation of current voltage curves can be

observed concerning effect of water splitting caused by the electrode reaction.

Fig. 2.6 Schematic diagram of a two-compartment electrolytic cell used for current–voltage

measurement

Membrane transport numbers were determined by measuring membrane potential. A

two-cell apparatus equilibrated a membrane with unequal concentrations (C1 = 0.05

M/C2 =0.01 M) of NaCl solution at both sides were used. The developed potential

across the membrane was measured by multimeter with Ag/AgCl electrodes. During

the experiments, both sections were stirred vigorously to minimize the effects of

boundary layers. The transport number tm was then calculated using the following

equation:

𝐸𝑚 =RT

F(2𝑡𝑚 − 1)𝑙𝑛

𝑎1

𝑎2

where R is the universal gas constant (8.314 J/ (mol·K) ), F is the Faraday constant

Chapter 2

39

(96487 C/mol), T is the absolute temperature (K), and a1 and a2 are the mean activities

of the electrolyte solutions. The mean activity can be expressed as:

𝑎𝑖 = 𝑟𝑖 × 𝑐𝑖

where 𝑐𝑖 is the concentration, and 𝑟𝑖 is the activity coefficient. The mean activities

of the electrolyte can be calculated based on salt concentration. The activity

coefficients were deemed as 1 in the experiments due to the low concentrations.

2.2.8 Electrodialysis experiments

The desalination and selective properties of the membranes were evaluated by ED.

ED is typically operated with a direct current supply, which serves as the driving force

for ions migration. There were three streams in the ED stack: the diluate, the

concentrate, and the electrode rinsing solution. For Chapter 3 and 4, both the

concentrate and diluate compartments were filled with 1 L of a 2 g/L salt solution

while the electrode rinsing solution was 2 L 20 g/L Na2SO4. The experiments were

conducted at constant current conditions and the current applied in Chapter 3 was

0.3 A. The ED stack shown in Fig. 2.7 is applied in the experiments for Chapter 3.

The ED stack applied in Chapter 4 is similar with the setup used in Chapter 3, while

anion exchange membranes in the stack were replaced by cation exchange membranes.

Each compartment had an active membrane area of 64 cm2.

Chapter 2

40

Fig. 2.7 Scheme of a traditional ED setup

For Chapter 5, an ED setup with an active membrane area of 28 cm2 was used. The

concentrate compartment was filled with 150 mL 0.25 M H2SO4 and the diluate

compartment was filled with 150 mL mixed solution (7.5 g ZnSO4 in 1 L 0.25 M

H2SO4 solution). In Chapter 6 and Chapter 7, the diluate and concentrate compartment

were filled with 150 mL 2 g/L salt solutions to analysis the separation performance of

the membranes. For permselectivity experiments, a solution with 0.05 M MgCl2 and

0.5 M NaCl was used in the dilute compartment, and a 0.5 M NaCl was used in the

concentrate compartment. Each compartment has an active membrane area of 19.6

cm2 with an O-ring rubber to avoid leakage during testing. A constant voltage of 20 V

was applied in Chapter 6 for studying desalination properties. The other experiments

were conducted with a constant current of 0.3 A. The electrode rinsing solution was

1 L 20 g/L Na2SO4. The error bar of conductivity-time curves in Chapter 3 and

Chapter 4 indicates an average level of at least three measurements.

The cation flux through the membranes was determined by the concentration change

with time (dCC/dt) in the concentrate compartment according to the following

equation:

𝐽𝑐(𝑚𝑜𝑙/𝑐𝑚2 ∙ 𝑠) =V

𝐴(𝑑𝐶𝐶

𝑑𝑡)

where A is the membrane effective surface area (cm2) and V is the volume (cm

3) of

Chapter 2

41

the concentrate solution.

The desalination efficiency was evaluated in terms of the change in conductivity of

the diluate compartment during the electrodialysis operation. The desalination

efficiency (DE) was calculated using the equation:

𝐷𝐸(%) = 1 −𝜎𝑡

𝜎0

where 𝜎0 is the initial conductivity of the diluate compartment and 𝜎𝑡 is the

conductivity of the diluate compartment at time t.

The permselectivity of a membrane describes the charge selectivity of the ion

exchange membrane. It reflects the ability of the membrane to discriminate between

ions with same charge. The permselectivity of the membranes in Chapter 5 and

Chapter 6 was expressed as follows:

𝑃𝑁+𝑀2+

=𝑡𝑀2+ × 𝐶𝑁+

2𝑡𝑁+ × 𝐶𝑀2+

where 𝑡𝑀2+ and 𝑡𝑁+ are the transport number of multivalent and monovalent ions.

𝐶𝑀2+ and 𝐶𝑁+ are the concentrations of multivalent and monovalent ions in the

diluate compartment. The permselectivity in the other experiments is simply

expressed as the flux ratio of multivalent and monovalent ions.

Current efficiency is an important parameter for assessing the suitability of any

electrochemical process for practical applications. The overall current efficiency (η)

was defined as:

𝜂 =z(𝐶0𝑉0 − 𝐶𝑡𝑉𝑡)F

NIt× 100%

where C0 and Ct are the concentrations of the dilute solution at times 0 and t,

respectively; z is the valence of the ions; V0 and Vt are the volumes of dilute solution

circulated at times 0 and t, respectively; F is the Faraday constant (96500·C·mol−1

); I

is the constant current, N is the number of repeating units; and t is the time allowed

for the electrochemical process.

Chapter 2

42

2.2.9 Structural stability of ion exchange membrane

The stability of the modified layers plays a crucial role in practical applications. The

structural stability was investigated in the light of the performance deterioration. The

membrane was soaked in both strong acid (0.1M HCl) and base environment (0.1M

NaOH) for 7 days. Then, the membranes were taken out and rinsed with DI water to

wipe off residual H+ and OH

-. The permselectivity and ions flux of the membrane

after acid and base treatment were recorded though the aforementioned method.

2.2.10 Morphology and structure of membranes

The cross-sectional and surface morphologies of the resultant membranes were tested

by a field emission scanning electron microscope (Philips XL30-FEG). The samples

for surface imaging were prepared by directly cutting a clean membrane into small

pieces, whereas the samples for cross section were prepared by freezing and breaking

samples in liquid nitrogen. Moreover, the surface morphology and roughness of the

prepared membrane was further analyzed by atomic force microscopy (AFM,

Dimension Icon, Bruker, Germany) with scan areas of 1×1 μm. Approximately 1 cm2

of the prepared membranes was cut and glued on the glass substrate before being

scanned. AFM images were taken in ScanAsyst mode using a FASTSCAN-B probe.

The AFM images were flattened after scanning to remove slope and curvature from

images. After flattening, the root-mean-squared roughness (Rrms) was calculated as:

R𝑟𝑚𝑠 = √∑ (𝑍𝑛 − 𝑍𝑎𝑣𝑒)2𝑁

𝑛=1

𝑁 − 1

where 𝑍𝑎𝑣𝑒 is the arithmetic mean of the height values for all the pixels in the image,

𝑍𝑛 is the height for any given pixel and N is the number of pixels present in the

image. Furthermore, other definitions can also be used to characterize the roughness,

such as the mean roughness (Ra, the mean value of the surface relative to the center

plane) or the peak-to-valley distance (Rm, the distance between the highest data point

Chapter 2

43

and the lowest data point of the surface).

2.2.11 Chemical structure and composition of membranes

The functional groups on the membrane surfaces were verified by Fourier transform

infrared spectroscopy (FTIR, Nicolet Magna-IR 560 Spectrometer). The transmittance

spectra were conducted from 670 to 4000 cm-1

with a resolution of 4 cm-1

at room

temperature. All detections were performed by using air as the background. The

surface chemical compositions of the membranes and the materials were analyzed by

X-ray photoelectron spectroscopy (XPS, AXIS Ultra DLD X-ray photoelectron

spectrometer, Japan) with Mg Kα as the radiation source. The take-off angle of the

photoelectron was set at 90º. Survey spectra of the membranes were collected over a

range of 0 to 1300 eV. The energy dispersive X-ray spectroscopy (EDS)

compositional analysis of membranes surface was performed by a field emission

scanning electron microscope (JEOL Model JSM-6700F, Tokyo, Japan).

2.2.12 Water flux experiments

A dead-end filtration setup was used to study the permeability of the resulting

membranes in Chapter 3. The effective area of the filtration cell was 12.64 cm2. The

water flux was tested at a pressure of 4 bar. The water flux F, used to express the rate

at which the water permeates a membrane, is typically defined as a volume per unit

area per unit time.

F =𝑉

𝐴𝑡

where V is the volume of pure-water permeation, t is the time, and A is the area of the

membrane. A minimum of three measurements was obtained for each membrane to

calculate the average value.

Chapter 3

44

3. Cation exchange membranes with controlled

porosity in electrodialysis application

Adapted from: J. Li, J. Zhu, S. Yuan, J. Lin, J. Shen, B. Van der Bruggen.

Cation-Exchange Membranes with Controlled Porosity in Electrodialysis Application.

Industrial & Engineering Chemistry Research, 28 (2017): 8111-8120.

3.1 Introduction

In general, a homogeneous or heterogeneous ion exchange membrane is a dense

membrane, which defines the transport of ions. Although many strategies have been

developed to accelerate ion transport, these strategies were not economically

competitive or resulted in inferior electrochemical performance (Malik et al., 2016).

Recently, ultrafiltration membranes were used in the electrodialysis process to

facilitate the migration of molecules according to their charge and molecular size.

However, during the application of ultrafiltration membranes in electrodialysis, the

separation of smaller ionic species still remains a challenge due to the relatively large

pore size of these membranes. Pore-filled membranes composed of an ultrafiltration

membrane as the substrate and a polymer with ion-exchange groups can provide both

a high ion conductivity and excellent mechanical properties (Kim et al., 2016) The

application of pore-filled ion exchange membranes has been explored for use in fuel

cells (Lee et al., 2016b), electrochemical energy conversion (Lee et al., 2016c),

vanadium redox flow batteries (Lee et al., 2016c), reverse electrodialysis (Lee et al.,

2017), acid recovery by diffusion dialysis (Lin et al., 2017) and pharmaceutical

preparations (Åkerman et al., 1998). In spite of this, pore filling may have some

restrictions in practice for the following reasons: it is accepted that the introduction of

ion exchange groups onto a membrane is the most effective and practical method for

the preparation of ion exchange membranes; however, the presence of the uncharged

Chapter 3

45

porous substrate would hinder the ion transport through the membranes. Inspired by

methods of electrodialysis with ultrafiltration membranes and preparing pore-filled

membranes, porous membranes prepared from charged polymers with functional

groups represent another option. A porous ion exchange membrane can be defined as

a charged membrane with functional groups and charges. Such membranes with

porous structures might facilitate the migration of ions by reducing the physical steric

hindrance of the surface layer. Thus, the migration resistance could be further

diminished, whereas the functional groups of the membrane matrix could block the

diffusion of ions caused by the concentration gradient. However, the fabrication of

porous ion exchange membranes is still a major challenge limiting the optimization of

the membrane performance referring to pore size, porosity and resistance.

3.2 Results and discussion

3.2.1 SEM results and water flux

The properties of the membrane, such as IEC, water uptake, resistance and

mechanical stability, can be modified by varying preparation conditions. Fig. 3.1

shows the morphologies of the top surface and cross section of membranes prepared

with different heating times. No obvious differences can be seen on the membrane

surface. However, the difference in the cross section is more expressed. With

increasing heating time, membrane surfaces were found to become denser. A

membrane prepared by solvent evaporation generally has a dense surface structure

without any pores, as shown in Fig. 3.2(a1). On the other hand, the membranes

prepared by phase inversion in DI water without curing have a relatively large pore

size, as shown in Fig. 3.2(a4). From the images of samples Fig. 3.2(a2 and a3), it can

be seen that membranes fabricated with the dry-wet phase inversion method have

three layers: two nanoporous layers with a macroporous layer in between. The only

difference is that the membrane with 1 h heating time had a comparatively dense inner

Chapter 3

46

layer that was layered in shape, whereas the membrane with 0.5 h heating time had an

irregular porous inner layer. This is because the polymer concentration in the surface

layer increased as the solvent evaporated, thus leading to a dense surface layer. This

layer on the membrane surface would inhibit the exchange solvent and non-solvent

during the immersion process and facilitates the formation of a thick skin layer with

smaller pores (Buonomenna et al., 2007; Jansen et al., 2005).

Fig. 3.1 SEM images of membranes prepared with different heating time (a1 – a4: cross section of

membranes prepared by heating time of 5 h, 1 h, 0.5 h and 0 h, respectively; b1 – b4: surface

images of membranes prepared by heating time of 5 h, 1 h, 0.5 h and 0 h, respectively)

The water flux is an important parameter in the practical application of polymeric

porous membranes. It is mainly influenced by structural parameters such as pore size

and porosity, with greater pore size and porosity resulting in an enhanced water flux

Fig. 3.2 shows the water flux of porous ion exchange membranes prepared with

different heating times. The results reflect the pore size and porosity to some extent.

As can be seen in Fig. 3.2, the water flux was negligible for dense membranes. This

can be explained by the fact that a dense membrane is not capable of providing

pathways for water transport. For the membrane prepared by the dry-wet phase

inversion method, the water flux was enhanced by the porous interlayer. The structure

a1

a4a3

a2b1

b2

b3

b4

Chapter 3

47

of this interlayer explains the differences in water flux for membranes prepared with

heating times of 0.5 and 1 h. For the membranes prepared by phase inversion, the

highest water flux (of about 43.0 L/(h·m2)) was obtained, which was due to the

increased pore size and porosity throughout the membrane matrix. These water fluxes

are consistent with SEM images of membranes prepared with different heating times.

Fig. 3.2 Effect of heating time on water flux for porous ion exchange membranes

3.2.2 IEC and water uptake

The content of charged functional groups in an ion exchange membrane plays a key

role in providing a hydrophilic and electro-static environment for ion transport. The

ion-exchange capacity, an important factor related to the conductivity and transport

properties, is used to identify the charge density of the membranes. The IEC values of

the as-prepared membranes are shown in Fig. 3.3a. Compared to the membrane

directly immersed in water, membranes prepared by dry-wet phase inversion had a

smaller IEC. This is mainly due to the following reason. The pores inside the porous

ion exchange membrane could stock some acid solutions inside the membrane matrix.

The membrane with higher porosity tends to increase the released H+ concentration,

thus a higher IEC value can be obtained. Incorporating SPES to the porous membrane

Chapter 3

48

is expected to improve the IEC and water uptake. Water uptake is known to have a

profound effect on the membrane conductivity and flux. The water uptake is

significantly improved after the addition of charged SPES (see Fig. 3.3b). The

elevated water uptake is ascribed to the increase of the SPES content in the PES

membrane matrix. The sulfonate groups of SPES are hydrophilic, and an increase of

the IEC from 0.92mmol/g to 1.79 mmol/g with increasing SPES content resulted in

ahigher water content. The water uptake for SPES membrane without PES is around

164%, which is much higher than PES membrane without SPES (132%).

Fig. 3.3 IEC and water uptake change with heating time and radio of PES/SPES

3.2.3 Contact angle measurements

The IEC and water uptake were reduced with increasing heating time because of the

decreased porosity in the polymer matrix. This is consistent with the results of contact

angle measurements, as can be seen in Fig. 3.4, a membrane with high porosity tends

to have a higher surface hydrophilicity. Moreover, the component of membrane

matrix would also have an impact on surface hydrophilicity. By comparing PS01with

PS10, contact angle reduced from 75° to 53°, indicating that a higher surface

hydrophilicity was obtained.

Chapter 3

49

Fig. 3.4 Contact angle change with heating time and ratio of PES/SPES

3.2.4 Membrane resistance and transport number

Compared to porous membranes, dense membranes result in a higher electrical

resistance. On the contrary, membranes prepared by immersion and precipitation have

the greatest pore size and porosities, thereby giving rise to a higher transport number

(0.99). For membranes with different heating time in this work, the order of the

porosity was found to be as follows: 0 h > 0.5 h > 1 h > 5 h. In this case, higher

porosities with higher transport numbers followed the order of 0 h > 0.5 h > 1 h > 5 h.

The resistance of the membranes followed the order of 0 h < 0.5 h < 1 h < 5 h, which

conformed that membranes with higher porosity tended to have a lower resistance.

Table 3.1 Resistance and transport number of membrane with different heating time

PS0 PS1 PS2 PS01

Membrane resistance (Ω cm2) 1.92 1.64 1.27 0.97

Transport number (tm) 0.89 0.92 0.98 0.99

3.2.5 Diffusion dialysis

In diffusion dialysis, solutes pass through an ion exchange membrane from the high to

the low concentration side. Fig. 3.5 shows that the conductivity in the concentrated

cell increases with time during the diffusion process; this was caused by salt diffusion

Chapter 3

50

from the concentrate chamber (1 M NaCl) to the diluate chamber (distilled water).

However, the migration rates for the membranes prepared with different heating times

were quite different. Only a few salts migrated though the dense membrane, whereas

migration through the porous membranes was evident. Dialysis coefficients were

calculated based on changes in concentration in both compartments of the dialysis cell.

The dialysis coefficient of membrane directly immersed in water was found to be

108 mol/(h·m2) at room temperature. As the heating time during membrane

preparation increased, the dialysis coefficient decreased. The dialysis coefficient of a

dense membrane reached as low as 1.4 mol/(h·m2), much smaller than that of the

membrane directly immersed in water. It can be concluded from the slope of the

conductivity–time curves that a porous membrane with smaller pore size and

comparatively dense surface tended to have a lower diffusion of NaCl. This is due to

the steric hindrance effect of the dense surface. The dialysis coefficients of the

different membranes were 1.4, 4.9, 8.6, 108 for dry, 1 h heating time, 0.5 h heating

time, and 0 h heating time membrane, respectively, which are consistent with the

diffusion dialysis results.

Fig. 3.5 Conductivity change of concentrated compartment during diffusion progress for

membrane with different heating times

0 10 20 30 40 50 60

0

1

2

3

4

5

6

T (min)

Co

nd

uctivity (

ms/c

m)

5 h

1 h

0.5 h

0 h

Chapter 3

51


As presented in Fig. 3.6, the conductivity of the diluate compartment decreased during

the experiments, but the demineralization rates were different. For the membrane

dried directly, the conductivity change was much faster than for the porous ion

exchange membrane in the first stage. An increasing heating time led to a membrane

with lower porosity and dense structure, so that salt diffusion from the concentrate to

the diluate compartment driven by the concentration gradient was mitigated, and as a

result, the conductivity change was enhanced. For an heating time of 1 h, the

conductivity change during the first 100 min was similar to that for the membrane

dried directly. However, after that, it is found that the demineralization rate of the

dense membrane was surpassed by that of the membrane prepared with 1 h heating

time. In this case, the increased demineralization rate can be explained by the

comparatively high transport number for ions. For the membrane prepared with 0.5 h

and 0 h heating time, the porosities were larger. In this case, the effect of diffusion by

the salt gradient plays a much more important role, thus, the conductivity changes

were smaller. The trend of the conductivity-time curves for the membrane prepared

with 0.5 h and 0 h heating time was similar to that of the membranes prepared with 1

h and 5 h heating time. The conductivity change was similar when the conductivity

was greater than 1.6 mS/cm. After that, the desalination efficiency of the membrane

with 0.5 h heating time became higher. This can be explained by the fact that the

membrane with 0.5 h heating time had a smaller porosity; as a result, steric hindrance

plays an integral part in hindering the salt diffusion.

Chapter 3

52

Fig. 3.6 Conductivity change of diluate compartment during ED process for membrane with

various heating times

The SPES content was also found to have a great influence on the overall properties

of the porous ion exchange membrane. For the membrane prepared with different

SPES content, the changes in conductivity of the SPES/PES composite membranes

are shown in Fig. 3.7. It can be concluded that the desalination efficiency was

enhanced with increasing SPES content. This can be explained by the fact that an

increase in the SPES content enhanced the IEC and water content. As a result, ion

transport was facilitated and the conductivity reduction of diluate compartment

became much more apparent.

0 50 100 150 200 250 300

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

Time (min)

Dry

1 h

0.5 h

0 h

Con

du

ctivity (

ms/c

m)

Chapter 3

53

Fig. 3.7 Conductivity change of diluate compartment during ED process for membrane with

different SPES content

To evaluate the performance of the membranes prepared with different heating time

for brine recovery, the current efficiency of the ED stack was calculated for the case

when the conductivity of the diluate compartment reached 1.0 mS/cm, as shown in

Fig. 3.8 (right figure). The current efficiency increased first from 90% to 100%

because of the formation of pores inside the membrane that facilitated the transport of

ions; however, as heating time reduced, the porosity increased further, and the current

efficiency decreased again. This can be explained by the fact that a further increase of

the porosity would enhance the diffusion resulting from the gradient of salt

concentration. Furthermore, the transport of chlorides also contributed to the decrease

of current efficiency. To investigate the influence of the SPES content, the current

efficiency was calculated when the desalination process was conducted for 180 min. It

was found that the SPES content had a profound effect on the current efficiency. With

the increase of SPES, the IEC and was increased. The enhanced IEC could hinder the

transfer of NaCl and Cl- from the concentrate to dilute compartment, thus resulting in

an enhanced current efficiency.

Chapter 3

54

Fig. 3.8 Current efficiency of ED process at different heating times and SPES content

3.3 Conclusions

The combination of immersion precipitation and dry-casting to control the

membrane porosity is an effective method for improving the physical and

electrochemical properties of the resulting membrane. The membrane resistance can

be largely reduced by introducing pores into the membrane matrix. The modified

membranes show an improved IEC and water uptake and a decreased contact angle.

The current-voltage curve and diffusion experiments confirmed that the resistances of

membranes with higher porosities were reduced, and thus, the diffusion of salts

through membranes was also enhanced. A compromise between the membrane

resistance and diffusion should be made to optimize the mechanical stability and

separation capability of the membrane. During desalination by ED, changes in

conductivity were obvious for the membrane with 1 h heating time and for the dense

membrane. However, for the membrane directly immersed in water and for the

membrane prepared with a 0.5 h heating time, the desalination efficiencies were lower;

this can be explained by the enhanced salt diffusion of membrane with high porosities.

Compared with the dense membrane, the membrane prepared with 1 h heating time

had a higher desalination efficiency. This trend was similar to the trend in the current

efficiency for the membranes prepared in this work. The desalination efficiency

reached 95% and the current efficiency was around 100% under optimized conditions.

Chapter 3

55

This chapter provides new insights into how to develop porous ion exchange

membranes and lays a foundation for further research on low resistance membranes.

Chapter 4

56

4. Charge-assisted ultrafiltration membranes for

monovalent ions separation in electrodialysis

Adapted from: J. Li, J. Zhu, J. Wang, S. Yuan, J. Lin, J. Shen, B. Van der Bruggen.

Charge-assisted ultrafiltration membranes for monovalent ions separation in

electrodialysis. Journal of Applied Polymer Science. 135(24), 2018:45692.

4.1 Introduction

As mentioned in Chapter 1, monovalent selective ion selective membranes have the

capability to separate monovalent ions from a solution containing both monovalent

and multivalent ions. Different approaches for the fabrication of anion exchange

membranes with permselectivity for monovalent ions have been reported, including

the adjustment of the crosslinking degree. Nevertheless, increasing the degree of

crosslinking would reduce the process efficiency. For this reason, crosslinking is not a

suitable approach for the fabrication of monovalent selective ion exchange

membranes (Kumar et al., 2013). Sieving of ions by the formation of a highly

crosslinked surface layer was found effective in changing the permselectivity between

monovalent and divalent ions (Sata, 1994). The thickness of this layer should be

optimized to balance the tradeoff between the monovalent-ion selectivity and the

electrical resistance (Güler et al., 2014). Recently, electrodialysis with nanofiltration

(EDNF), a process combining electrodialysis with nanofiltration was used for the

separation of multivalent ions from monovalent ions (Ge et al., 2016a). By replacing

the cation exchange membrane with composite ion exchange membrane, the porous

composite membrane can fractionate ions by rejecting the multivalent ions with larger

Stokes radius. In addition, the permselectivity and ion flux were enhanced due to the

porous structure of the nanofiltration membrane.

Chapter 4

57

It should be noted that a similar technology named electrodialysis with ultrafiltration

(EDUF) has been used for selective isolation of proteins (Doyen et al., 2014) and

peptides (Doyen et al., 2011a, b; Firdaous et al., 2009). In this process, a conventional

electrodialysis cell is used, in which some ion exchange membranes are replaced by

ultrafiltration membranes. The common characteristics of these two technologies are

that an electrical field is used as the driving force to separate molecules on the basis of

their electrical charge and size. However, due to the lower molecular cut-off of NF

membranes, UF membranes have a lower resistance compared to NF membranes,

which accelerates the migration of ions (Bazinet and Moalic, 2011a). Ultrafiltration

(UF) membranes are a type of porous and asymmetrical membranes prepared via

phase inversion, having a very thin and nanoporous skin layer and a thick and

macroporous supporting layer (Guillen et al., 2011). The typical molecular weight

cut-off (MWCO) of UF membranes is above 1 kDa, making it extremely challenging

to remove monovalent salts (Mukherjee et al., 2015). By modifying monomers on the

surface or within the pores of a robust microporous host membrane, the modified

porous membrane provides the mechanical strength to mitigate the impact of osmotic

forces. Lin et al. (Lin et al., 2016) modified asymmetrical porous ultrafiltration

membranes by blocking the pores at the top surface. The membranes exhibited a high

acid permeability coefficient, of about 0.041-0.062 m h-1

and a separation factor

(H+/Fe

2+) of 30.4-84.4, which is superior to most reported membranes. With the

difference in ion transport mechanism in the membrane caused by the special

membrane microstructure, the results suggest a significant improvement in salt

diffusion properties; however, such superior properties were never explored in

electrodialysis (Lin et al., 2016).

Polyaniline (PANI), a low cost material with high conductivity in partially oxidized

state, and a high chemical stability in acid medium (Zhu et al., 2016), is facile to be

constructed to different structures such as foams (Xie et al., 2016) and nanorods (Fu et

al., 2016). With PANI, elevated separation properties can be obtained based on the

size of the molecules or ions and their charge. This has been widely used in composite

Chapter 4

58

membranes and surface modification of ion exchange membranes. The influence of

polymerization time, surface charge density and doping agents (Farrokhzad et al.,

2015; Nagarale et al., 2004; Sata et al., 1999) on the electrochemical properties of

membranes has been comprehensively investigated by previous studies. In this

Chapter, a facile and inexpensive method by modifying PANI on the surface of UF

membrane is reported. PANI grown along the network of a UF membrane can

enhance transport, hinder dialysis and ensure an efficacious utilization of energy. With

the inclusion of PANI, separation properties based on the size of the molecules and

ion charge were investigated in electrodialysis for desalination of NaCl and MgCl2

solutions. The influence of the type of UF membrane (with different molecular cut-off

values) on the extraction and selective permeation of cations was investigated, using

Na+ and Mg

2+ as the monovalent and bivalent cations.


4.2.1 SEM results

SEM images of the membrane surface were taken to study the morphology changes of

the membrane surface caused by aniline modification. Fig. 4.1 shows the SEM images

of primary and modified PAN 350 ultrafiltration membranes. It can be seen that

before treatment, the pores of PAN 350 are uniformly distributed on the surface. After

the polymerization reaction, the color of the membrane turned from white to dark blue,

which indicates that the surface of PAN 350 was fully covered by polyaniline. In the

SEM images, it can be seen that most PANI is uniformly grown on the surface of PAN

350 membranes. PANI was found to have a unique cluster morphology, and appeared

to be growing on a substrate. Although there were defects on the surface, where the

surface was not uniform after modification, all the pores were occupied. The presence

of polyaniline can greatly shift the transport properties of the membranes, which will

be further discussed.

Chapter 4

59

Fig. 4.1 Surface morphology of PAN 350 (Left) and modified PAN 350 membranes (Right)

4.2.2 FTIR results

Theoretically, PANI has the potential to form hydrogen bonds with polymers

containing carbonyl groups. Because of its H-donating imine groups, PANI has been

found to be compatible or at least partially compatible with some polymer counterpart

such as polycarbonate and nylon 6 (Ouyang et al., 2008; Pan et al., 2005). FTIR

spectroscopy is an extremely useful and convenient technique to detect the formation

of hydrogen bonding. PAN 350 membranes often have a peak at 1666 cm−1

, which is

assigned to the vibration of the C=O bonds derived from the hydroxylation of PAN (Ji

and Zhang, 2008). After modification, the PANI composite shows two peaks of

carbonyl groups at 1693 and 1669 cm-1

, respectively. This can be explained by the

hydrogen bonding between PANI and the hydrolyzed PAN 350 membranes. Among

the two peaks, the carbonyl band at the lower wavenumber of 1669 cm-1

is caused by

the hydrogen bonding between N-H of PANI and C=O of PAN; the band at 1693 cm-1

is the reflection of free carbonyl groups (Jeon et al., 1999). The absorption peaks at

2937, 2243, 1452 and 1354 cm-1

are related to the stretching vibration of methylene (–

CH2–), nitrile groups (–CN–), methylene and hydrocarbon chains, respectively (Li et

al., 2014b; Zhang et al., 2016b). The band at 3323 cm-1

in PAN 350 membrane

corresponds to the stretching vibration of the –OH groups and indicates some water

molecules remaining in the PAN network. These two bands are also observed in the

membrane after modification with polyaniline. After the modification, new peaks at

Chapter 4

60

1609 cm−1

and 1506 cm−1

attributed to the C−C stretching of the quinonoid (Q) and

benzenoid (N) rings of PANI are clearly observed in the curve of the modified PAN

350 membranes. The C-N vibration of PANI was found located at 1248 cm−1

(Bayramoğlu et al., 2010). The FTIR observation (Fig. 4.2) confirms the presence of

PANI on the surface of the membranes.

Fig. 4.2 FTIR of PAN 350 membranes before and after modification

4.2.3 IEC, water uptake and contact angle

For a salt solution, the ion exchange membrane can greatly reject the permeation of

that salt due to its repulsion of one specific ion. Porous UF membranes can serve as

migration media to consolidate the migration of various kinds of ions; however,

during that process, back diffusion from the concentrate to the diluate is a problem.

By introducing PANI to the stereo hierarchical porous composite structure, functional

groups from PANI were added to the modified PAN 350 membranes, which can

greatly elevate its selectivity and slow down penetration during the desalting process.

Considering the porous structure and loose support layer, as well as the low selectivity

toward ions with small hydrated diameter, the IEC of hydrolyzed PAN 350 membrane

is significantly more dependent on the space in membrane matrix rather than on the

total amount of exchangeable anions. It would be pointless here to determine the IEC

Chapter 4

61

of a hydrolyzed PAN 350 membrane. In this chapter, the IEC of modified anion

exchange membranes was optimized to 1.50 mmol/g, while the water uptake and

contact angle were also changed. The water uptake has a profound effect on the

transport behavior of ions across the membrane, as well as the dimensional and

mechanical stability of membranes. In principle, a high water uptake of the ion

exchange membranes would be beneficial to the ion transport. The primary PAN 350

membranes have a water uptake of 123%. After modification, it decreases to 111%.

Such a high water uptake, superior to 100%, can also be explained by the space in

membrane matrix. Fig. 4.3 shows that the contact angle rapidly increases from 22.3°

for the primary membrane to 43.1° for the modified PAN 350 membrane, because of

the highly hydrophilic hydrolyzed PAN and the hydrophobic PANI. Introducing PANI

decreases the water uptake while it increases the contact angle. In addition, the

occupation of PANI in the pore space can also contribute to the decline of the water

uptake.

Fig. 4.3 IEC, water uptake and contact angle of the PAN 350 and the modified PAN 350

membrane

Chapter 4

62


In Fig 4.4, it can be seen that the conductivity of the concentrated cell increases

during the experiments, which is caused by the diffusion of salt from the concentrate

chamber (1 M NaCl) to the diluate chamber (distilled water). Dialysis coefficients

were calculated based on changes in concentration in both compartments of the

dialysis cell. The dialysis coefficient of the primary PAN 350 membrane is 108

mol/(h·m2) at room temperature. After the addition of PANI, it is noticeable that the

modified membrane impedes the diffusion of NaCl. The dialysis coefficient of the

modified PAN 350 membrane decreases to 9.1 mol/(h·m2). The PAN 350 membrane

with PANI on its surface has a lower diffusion of NaCl compared to the primary

membrane due to the positive charge of PANI, which hinders the transport of NaCl

across the membrane. Furthermore, the effect of sterical hindrance due to the presence

of PANI may play a role.

Fig. 4.4 Conductivity–time curves of diffusion progress

Chapter 4

63

4.2.5 Desalination parameters during ED: conductivity and pH

The evolution of conductivity during ED treatment in different currents is presented in

Fig. 4.5. According to the experimental results, regardless of the current density, the

conductivity of the brine salts solution in the diluate compartment decreases during

the experiments. Fig. 4.5 shows that the current density affects the transport of ions.

When the current is 0.2 A, the conductivity of the diluate compartment decreased

gradually from 3.5 and 4.0 mS/cm to 2.7 and 3.1 mS/cm in the first 3 hours for NaCl

and MgCl2, respectively. At a higher current (0.4 A), the conductivity of the diluate

compartment dropped sharply at the beginning of the experiments and then continues

to decrease until reaching a steady state. At higher current, the transport of ions

accelerated, which led to a dramatic change of the conductivity of the diluate

compartment. Regardless, the conductivity change was not so obvious after 100 min.

It should be pointed out that during the desalting procedure, the driving force is a

compromise between the electrical field and the salinity-gradient power. At the

beginning of the experiments, transport of ions by the electrical field would be

dominant, however, at the end of the experiment at 0.4 A, the conductivity of the

diluate compartment increased again, which implies that ion transport by the electrical

field is surpassed by salinity gradient. The change of pH in the diluate compartment

could be another reason affecting the conductivity change. Although the pH change

was not recorded during the experiment, at 0.3 A the possibility of water splitting at a

higher current for this system could occur (Fig. 4.8). For the experiment at 0.3 A, the

slope of the conductivity-time curve is surrounded by the curve of 0.2 and 0.4 A at the

beginning of the experiment. It can be concluded that for PAN 350 ultrafiltration

membranes used in electrodialysis for desalination, the enhanced current can increase

the migration of ions, however, if the current becomes too high, a reducing

desalination efficiency was obtained due to the back diffusion and water splitting of

electrolyte across the membrane.

Chapter 4

64

Fig. 4.5 Conductivity curves of the diluate compartment at different current density

Conversely, the conductivity of the concentrate increased during the experiments (Fig.

4.6). However, there was a discrepancy with the changes of the diluate compartment

to some extent. Both NaCl and MgCl2 experiments present an increase in

conductivities in the concentrate department; however, the slope of the

conductivity-time curve is different. Table 4.1 lists the conductivity change of the

different compartments at the first 100 min. It was observed that the conductivity

variation of the concentrate compartment is always higher than that of the diluate

compartment, especially at larger current, which suggests that the current efficiency

was decreased. Comparing with NaCl, the variation of the MgCl2 concentration is

much larger, which can be explained by the different transport rates of Na+ and Mg

2+

in cation exchange membranes and PAN 350 membranes. The Na+ from the

electrolyte is able to permeate from the anode to the concentrate compartment and the

diluate compartment, and ultimately to the electrolyte again. Once the Na+ enters the

diluate compartment, it replaces the Mg2+

migration to support the current, so that the

current efficiency decreases. Besides, the protons generated by the electrode reaction

could enter the concentrate compartment, giving rise to a pH change in the

concentrate compartment. Considering the possible water splitting that may occur at

the UF membrane surface, both electrode reactions and water splitting can contribute

Chapter 4

65

to the increase of the conductivity.

Fig. 4.6 Conductivity curves of the concentrate compartment at different current density.

Table 4.1 Conductivity and concentration change of diluate and concentrate compartment

Conductivity change NaCl

0.2 A

NaCl

0.3 A

NaCl

0.4 A

MgCl2

0.2 A

MgCl2

0.3A

MgCl2

0.4A

Diluate compartment (mS/cm) 0.8 1.4 1.5 0.9 0.9 0.7

Concentrate compartment (mS/cm) 0.8 1.6 1.7 1.4 2.2 1.7

Desalination using the modified PAN 350 is summarized in Fig. 4.7. In the presence

of polyaniline, there is no obvious effect on the removal of NaCl. Moreover, while the

current is fixed at 0.2 A, for MgCl2, the conductivity changes in the diluate and

concentrate compartment for the primary PAN 350 membrane is 0.9 and 1.4 mS/cm,

which is almost the same as the modified PAN 350 membranes (1.0 and 1.4 mS/cm,

respectively). However, at 0.4 A, the conductivity changes in the diluate compartment

for primary PAN 350 membrane is 0.7 mS/cm, and it reaches its minimum after 100

min. After this, the conductivity slightly increases again. The main reason expected to

contribute to this could be the water splitting phenomenon. The modified PAN 350

membrane can greatly increase the salt removal from 4.0 mS/cm to 2.1 mS/cm after

Chapter 4

66

180 min desalination. The maximum removal rate was increased from 17.4% to

48.1%, which is almost a factor 3. The presence of polyaniline can hinder the Mg2+

transport from the concentrate to the diluate compartment because of the electrostatic

effect, while there is no obvious effect on Na+ ions. This effect has been applied in

preparation of monovalent selective exchange membrane by Kumar et al. (Kumar et

al., 2013). After the surface modification with PANI, the Na+ transport number across

the membranes was unchanged, whereas the Zn2+

and Al3+

transport numbers

decreased (Kumar et al., 2013). Thus, at lower current there is no obvious change

between the primary and modified PAN 350 membranes; however, at high current, the

desalination ratio significantly increases.

Fig. 4.7 Conductivity change of modified PAN 350 membranes in NaCl and MgCl2 solutions [(a)

NaCl experiments; (b) MgCl2 experiments].

The pH of the diluate and concentrate solution changed differently over the process.

Considering the transport rate and desalination efficiency, a current of 0.3 A is

considered. As shown in Fig. 4.8, in the concentrate compartment, the general trend is

that the pH decreased continuously as the electroseparation process progressed. A

decrease in the pH could be expected as the positive charges of PANI on the surface

of UF membranes could release protons and acidify the solution. The acidification in

the concentrate compartment was caused by the H+ ions migrating from the diluate

Chapter 4

67

compartment. The pH of the diluate solution remained constant for the unmodified

membrane, however, for the modified membranes, the pH slightly changed over the

180 min of electrodialysis. For this reason, the acidification of the concentrate

compartment and comparatively neutral diluate compartment, conductivity

discrepancies of the two compartments can be explained.

Fig. 4.8 pH change of the solutions during NaCl and MgCl2 desalination process using modified

and unmodified UF membranes (U. unmodified membrane M. modified PAN 350 membrane)

4.2.6 Current efficiency

In order to evaluate the performance of the modified PAN 350 membrane in a stack

for brine recovery, the current efficiency of the ED stack at different currents is shown

in Fig. 4.9. For NaCl, the highest current efficiency is at 0.3 A for the primary and for

the modified PAN 350 membrane. An increase in current density to a certain extent

reinforces the migration of ions and shortens the operating time simultaneously, which

can result in an increase of the current efficiency (Shen et al., 2014). Subsequently,

the current efficiency of the ED stack decreases with the current density. Although the

transfer of Na+ and Cl

− ions through the ion exchange membrane was accelerated as

the current density increases, several factors may suppress the current efficiency. With

Chapter 4

68

elevated current density, the splitting of water on ion exchange membranes intensified,

the produced protons and OH- would compromise the current to some content. Co-ion

leakage and permeation through the ion exchange membranes results from the high

concentration gradient, which decreases the current efficiency again (Shen et al.,

2013). However, for MgCl2, it is a totally different condition. For the PAN 350

membrane, the current efficiency decreases with the current density due to the

enhanced transport of Na+ than the Mg

2+ and the diffusion though the membrane.

However, after modification by polyaniline, the modified PAN 350 membrane has an

elevated current efficiency. This can be explained by the observation that the

introduction of polyaniline can greatly reduce the diffusion by an electrostatic

repulsion force between polyaniline and the ions as well as the shrunk pores of the

modified PAN 350 membrane. Fig. 4.9 shows that the current efficiency can be

greatly improved, especially at 0.4 A. The current efficiency in the NaCl concentrate

experiment increased from 31.4% to 36.5%, which is 16.4% higher than before. For

MgCl2, this reached to 52.6%, much higher than that of NaCl. Therefore, it can be

concluded that modified PAN 350 membranes offer a high potential for the

application in electrodialysis aiming at green production.

Fig. 4.9 Current efficiency of the electrodialysis stack at different currents

Chapter 4

69

4.2.7 Monovalent selectivity measurements

The permselectivity and ion flux values were obtained by measuring the Na+ and

Mg2+

ions concentration change in the diluate compartment. The ion flux of Na+ and

Mg2+

was 9.7×10-8

mol·cm–2

·s–1

and 4.5×10-8

mol·cm–2

·s–1

before modification. After

modification, the flux of Na+ was slightly increased to 12.4×10

-8 mol·cm

–2·s

–1 while

the flux of Mg2+

was reduced to 3.1×10-8

mol·cm–2

·s–1

. As a result, the

permselectivity increased from 2.15 to 3.98, which is almost a factor 2. It can be

explained by the fact that the dense layer can reject the divalent ions, with larger

Stokes radius, more effectively.

To further understand the separation process of the experiments, PAN based

ultrafiltration membranes with different molecular cutoff values were measured to

determine whether the mean pore size could influence the permselectivity. The flux

and permselectivity of different membranes are summarized in Table 4.2.

Table 4.2 The flux and permselectivity of different PAN based membranes

(B: before modification A: after modification)

PAN200 PAN350 PAN400 PAN450

B A B A B A B A

Na+ 11.3 13.0 9.7 12.4 12.1 13.3 11.1 15.0

Mg2+

7.1 4.2 4.5 3.1 4.2 3.8 5.4 5.6

Permselectivity 1.60 3.07 2.15 3.98 2.85 2.85 2.05 2.67

Results showed that the permselectivity of ultrafiltration membranes with lower

molecular cut-off is higher. It can be concluded that ultrafiltration membranes with

smaller pore size are more suitable for preparing membranes for monovalent ions

separation in electrodialysis. However, a higher perm-selectivity value of 7 and a Na+

flux of around 2.2×10−7

mol cm−2

s−1

were obtained for NFM in comparison with

those of PANI modified UF membrane. The PANI modified UF membrane still

Chapter 4

70

exhibits an enhanced perm-selectivity for the Na+/Mg

2+ system. To conclude, this

method provides new routes for preparing membranes based on ultrafiltration

membranes for efficient separation of monovalent ions by electro-driven separation

techniques.

4.3 Conclusions

Newly developed PANI modified membranes were successfully applied to produce

concentrated brines. The resulting membranes were evaluated in terms of ion

exchange capacity, water uptake and diffusion dialysis performance. In addition, the

obtained membranes exhibit enhanced properties toward desalination of brine salts by

electrodialysis. This novel technique has advantages not only regarding desalination

but also in terms of the enhanced current efficiency that was observed. This method

provides the possibilities of preparing porous ion exchange membranes to enhance

transport properties of anion exchange membranes, and suggests new routes to

develop monovalent selective membranes with low resistance and high transport

efficiency.

Chapter 5

71

5. Mussel-inspired modification of ion exchange

membrane for monovalent separation

Adapted from: J. Li, S. Yuan, J. Wang, J. Zhu, J. Shen, B. Van der Bruggen.

Mussel-inspired modification of ion exchange membrane for monovalent separation.

Journal of Membrane Science. 553(2018):139-150.

5.1 Introduction

As reviewed in Chapter 1, electrodialysis (ED) is one of the most economic and

advanced separation processes which enables the continuous separation and

concentration of brine water, especially when the concentration of the feed solution is

below 5 g/L. Compared to other technologies such as multistage flash (MSF)

evaporation and reverse osmosis (RO), ED has intrinsic advantages including

selective desalination and enhanced water recovery (Vaselbehagh et al., 2014).

Furthermore, the reduced chemicals usage and low energy consumption can

effectively diminish the cost of production. To date, electrodialysis has been applied

in various applications like biorefinery effluents (Luiz et al., 2017; Luiz et al., 2018),

rare earth elements recycling (Sadyrbaeva, 2015) organic acid recovery (Eliseeva et

al., 2009; Luo et al., 2002), and brackish water/wastewater treatment (Ghaffour et al.,

2013). Nevertheless, in some cases, precipitation caused by scaling compounds (Ca2+

,

Mg2+

, SO42-

, CO32-

) inevitably occurs in the concentrated compartment, which gives

rise to a deleterious effect on the desalination performance (Asraf-Snir et al., 2016).

For a standard ion exchange membrane, the presence of multivalent ions can suppress

the migration of monovalent ions by occupying the ion exchange transfer sites of the

membrane (Firdaous et al., 2007; Liu et al., 2017a). Therefore, the development of

IEMs with the ability to separate multivalent ions from monovalent ions is required.

Furthermore, IEMs with monovalent selectivity could potentially expand the

Chapter 5

72

application scope of ED to disciplines like metallurgy (Reig et al., 2017), sodium

chloride production (Zhang et al., 2017b) and reverse electrodialysis (Güler et al.,

2014).

In view of exploiting the differences in ion valances and hydrated ionic radii, many

approaches have been proposed to design selective cation exchange membranes

(CEMs) capable of separating multivalent ions from a mixed solution. Deposition of a

thin charged surface layer is considered a promising way to prepare monovalent

selective ion exchange membranes because the charged skin surface can increase the

repulsion forces towards multivalent ions. Abdu et al. (Abdu et al., 2014) reported a

layer-by-layer assembly of poly(ethyleneimine) (PEI)/poly(styrenesulfonate) (PSS)

bilayers on commercial CEMs (Astom, Japan); the perm-selectivity that was obtained

is comparable with commercial monovalent CEMs while simultaneously a lower

energy consumption is maintained. Wang et al. (Wang et al., 2013) fabricated a

monovalent CEM by preparation of a photo-induced self-polymerized chitosan layer.

An enhanced monovalent selectivity was obtained for both H+/Zn

2+ and Na

+/Mg

2+

system during the ED process. The leakage of Zn2+

and Mg2+

were reduced by 27.4%

and 62.4%, respectively. Nevertheless, membranes prepared by surface modification

still have disadvantages. Firstly, detachment of the surface functional layer from the

primary membrane may occur due to their weak electrostatic attraction (Wang et al.,

2013). Accordingly, attempts to bind covalently a cation charged layer on CEMs have

been tried to enhance the membrane stability. However, sulfonyl chloride acid

introduced to the reaction is not environmentally friendly. In addition, the

uncontrollable thickness of the modified layer and the reduced functional groups are

still important issues. Thus, developing new methods to fabricate monovalent

selective ion exchange membranes with high selectivity and low resistance in an

economically and environmentally friendly manner is imperative.

Enlightened by mussel-inspired surface chemistry, various attempts have been made

to modify membrane surfaces by this method due to the simplicity, controllability and

Chapter 5

73

extensive applicability (Yang et al., 2014a). Dopamine, a low-molecular-weight

catecholamine, is known as ‗bio-glue‘ derived from mussel adhesive proteins. (Li et

al., 2014a; Liu et al., 2014b) Unlike traditional coating protocols, the inherently

robust adhesion virtues of the catechol structure triggered by self-polymerization has

been broadly utilized in membrane surface functionalization for various purposes such

as structural stability enhancement (Li et al., 2015), antifouling properties

improvement (Li et al., 2014c) and optimization of separation properties (Yang et al.,

2017; Zhao et al., 2016). The self-polymerization of dopamine is a complex process

which involves covalent polymerization and non-covalent interactions (Choi et al.,

2014; Lee et al., 2016a; Wang et al., 2016). Although this strategy is successful in

constructing multifunctional coatings, the air-oxidized formation of PDA takes a long

time (Gao et al., 2013; Zhang et al., 2017a). Besides, aggregation of PDA oligomers

based on non-covalent interactions possibly causes uneven and unstable coating layers

after a long time of deposition (Zhang et al., 2016a). Recently, two main strategies, i.e.

rapid deposition of dopamine triggered by CuSO4/H2O2 and PDA/PEI co-deposition,

were proposed to overcome the above mentioned disadvantages (Wang et al., 2017b).

These two measures can greatly shorten the deposition time and promote a

homogeneous and robust PDA coating, thereby laying the foundation for the potential

development of such bio-inspired deposition process.

In this study, a one-pot approach is reported to prepare a monovalent selective cation

exchange membrane by PDA/PEI co-deposition. PEI is an amorphous polymer with

high thermal and chemical stability, which has been widely applied in water

desalination and membrane modification (Al-Maythalony et al., 2017; Guo et al.,

2016). PDA, known as ―bio-glue‖ can fix PEI molecular chains on the membrane

surface while the side PEI chains serve as repulsive functional groups that reject

multivalent ions. With the assistance of CuSO4/H2O2, the deposition time can be

significantly reduced as CuSO4/H2O2 produces a large amount of reactive oxygen

species (ROS), which is except to accelerate the polymerization of dopamine.

Chapter 5

74


5.2.1 Chemical structure of the membrane surface

During the process of dopamine polymerization, interactions between the PDA

coatings and the supporting layer are composed of covalent and non-covalent

interactions, including H-bonding, π-π and electrostatic interactions (Jiang et al.,

2011). Thus, a tightly adherent facial layer is formed on the SPES membrane surface

after being soaked in PDA/PEI aqueous solution for a specific time (Han et al., 2012).

By comparing SEM images for membrane prepared with different PDA/PEI ratios

(Fig. 5.1), a distinct change was observed. For SPES-PDA/PEI-0 membrane, the

surface was covered with numerous nanostructured papillae. Normally, PDA

depositions consist of spherical and linked agglomerates with a size ranging from ca.

100 to 500 nm (Jiang et al., 2011). After introducing PEI, the surface roughness of

PDA-coated membrane was found to be reduced. The PEI/PDA complex can be

formed on the membrane surface because PEI with nucleophilic amine groups can

crosslink with dopamine and other reactive intermediates through Michael addition or

Schiff base reactions (Huang et al., 2015; Lv et al., 2015; Zhang et al., 2014; Zhao et

al., 2015). At this condition, the reduction of non-covalent interactions effectively

diminished the PDA aggregates. When the PEI/dopamine ratio reaches 3, the

membrane surface becomes smooth and no protrusions are observed.

Chapter 5

75

Fig. 5.1 Surface SEM images of SPES and modified SPES membranes (a. Primary SPES

membrane; b. SPES-PDA/PEI-0; c. SPES-PDA/PEI-1; d. SPES-PDA/PEI-2; e. SPES-PDA/PEI-3)

Furthermore, EDAX was applied to identify the element composition of the

membrane surface. The EDAX spectra of different membrane are shown in Fig. 5.2

and the elemental composition of the membrane surface is summarized in Table 5.1. It

was evident that the original SPES substrate contains C and O while no N and Cu can

be detected. Compared with the SPES membrane, a nitrogen peak was found for the

PDA-modified membrane. The atomic content for nitrogen was 33.4%, which is

ascribed to the amine groups of the PDA coatings. After introducing PEI to the

modified procedure, the atomic content of N increased from 33.4% for the

SPES-PDA/PEI-0 membrane to 39.6% for the SPES-PDA/PEI-1 membrane,

demonstrating that PEI was successfully anchored on the membrane surface. The N

ratio further increases for the SPES-PDA/PEI-2 membrane, resulting from the

increased content in PDA/PEI composites. However, when the PEI/dopamine ratio

reached 3, the atom percentage of nitrogen was reduced to 45.6%, which indicated

that the adhesive properties were affected when the concentration of PEI is too high.

Furthermore, with increasing PEI concentration, the Cu2+

concentration reduces

continuously. The catechol groups of PDA can chelate Cu2+

ions from the solution,

which leads to an enhanced stability. Furthermore, catechol/quinone groups present in

PDA are able to covalently couple to nucleophilic amines of PEI (Chien et al., 2012).

Chapter 5

76

With more PEI incorporated to the surface, a reduced copper content would be

obtained. All the changes of element concentration confirm the presence of the PDA

or PDA/PEI coatings on the SPES membrane.

Fig. 5.2 EDAX results of SPES and modified SPES membrane (a. Primary SPES membrane; b.

SPES-PDA/PEI-0; c. SPES-PDA/PEI-2)

Table 5.1 EDAX mapping determined elemental composition (in at %) of membrane surface

C N O Cu

SPES 60 - 40 -

SPES-PDA/PEI-0 32.7 33.4 32.0 1.9

SPES-PDA/PEI-1 28.8 39.6 31.0 0.6

SPES-PDA/PEI-2 25.3 47.8 26.2 0.7

SPES-PDA/PEI-3 28.2 45.6 26.1 0.1

Apart from the electrostatic effect by the positive functional groups of PEI, the

thickness of the modified layer could also impact selectivity due to the steric

Chapter 5

77

hindrance effect. Fig. 5.3(a-d) shows the cross-section morphologies of the composite

membranes. It is obvious that the membrane is covered by a modified layer. The

thickness of the modified layer is typically more than 80 nm, however, it could be

easily adjusted by altering the reaction time or the mass ratio between dopamine and

PEI (Yang et al., 2014b). As a rubber is used to fix the membrane during the

co-deposition process, a boundary is formed on the membrane with and without

dopamine modification. By collecting the height profile along a line across the

boundary, the thickness of the modification layer is obtained. As shown in Fig. 5.3e,

the thickness is obtained from the AFM smooth surface morphology profile along the

line. The thickness firstly increases when PEI was incorporated to the co-deposition

process and then decreases when the mass ratio of PDA/PEI further reduced to 1:3

while the concentration of dopamine is constant. The increased thickness of the

modified layer is due to the accelerating deposition of PDA and PEI composites

generated by Michael addition or Schiff base reaction. Because excessive PEI could

terminate the polymerization of dopamine, at this condition, PEI cannot attach onto

the membrane surface. Thereby, the modified coatings are thinner when the

concentration of PEI is too high (Lv et al., 2015).

Fig. 5.3 The cross-section images of modified SPES membranes (a. SPES-PDA/PEI-0; b.

SPES-PDA/PEI-1; c. SPES-PDA/PEI-2; d. SPES-PDA/PEI-3) and (i. the thickness of the

modified layer observed by AFM smooth surface morphology)

Chapter 5

78

5.2.2 Morphologies of the membrane

The three-dimensional AFM surface topography pictures for the unmodified SPES

and modified SPES membranes are presented in Fig. 5.4. Similar with the results from

SEM, the primary SPES membrane was observed to be relatively smooth; the

morphology was changed after modification. The surface roughness values of the

membranes, including Ra, Rrms and Rm, are listed in Table 5.2. The SPES membrane

has a smoother surface than the SPES-PDA/PEI-0 and SPES-PDA/PEI-1 membranes,

as confirmed by the surface roughness (Ra) results. This is possibly due to the

adhesion and agglomeration of the PDA nanoaggregates. However, Ra and Rrms

remarkably decrease from 0.66 and 1.17 nm to 0.38 and 0.52 nm for

SPES-PDA/PEI-2 membrane, indicating that PEI can be a good candidate to

smoothen the surface structure of the membranes. Nonetheless, a further increase of

the content of PEI has no obvious effect on Ra and Rrms, which indicates that further

increasing the content of PEI may reduce the adhesive properties of PDA (Lv et al.,

2015). The roughness change for the modified membrane was consistent with the

previously discussed SEM results, confirming that the co-deposition of PDA/PEI can

effectually alter the surface structure and morphology of the membranes. The grafted

PEI molecules, of which the chains are rather flexible, endow the membranes with a

potential for monovalent selectivity.

Chapter 5

79

Fig. 5.4 AFM topography of SPES membrane and modified SPES membranes (a. Primary SPES

membrane; b. SPES-PDA/PEI-0; c. SPES-PDA/PEI-1; d. SPES-PDA/PEI-2; e. SPES-PDA/PEI-3)

Table 5.2 AFM surface roughness parameters of the pristine and modified membranes: Ra

(average roughness), Rrms (root mean square roughness), and Rm (maximum vertical difference

between the highest and lowest points)

Membrane Ra (nm) Rrms (nm) Rm (nm)

SPES 0.66 1.17 16.5

SPES-PDA/PEI-0 3.21 4.88 46.8

SPES-PDA/PEI-1 1.25 1.85 16.8

SPES-PDA/PEI-2 0.38 0.52 8.09

SPES-PDA/PEI-3 0.76 1.08 14.4

5.2.3 Zeta potential

Since the permselectivity of monovalent cations can be tuned by adjusting surface

charge characteristics, the zeta potential for modified membranes is indicated in Table

5.3. Both the SPES membrane and SPES-PDA/PEI-0 membranes are negatively

charged, with zeta potentials of -3.9 and -30.1 mV, respectively. Conversely, after

grafting of PEI to the modified layer, the zeta potential of SPES-PDA/PEI-1

membranes shifted to 7.4 mV, greatly increasing the zeta potential. Thus, a positively

charged surface could hinder the transport of multivalent ions through electrostatic

Chapter 5

80

effects. Furthermore, the zeta potential of the modified membranes reaches up to

13.4 mV with increasing PEI content. As previously discussed, the increase of PEI

content on the PDA/PEI coatings can lead to an elevated positive charge density for

the modified membrane. By incorporating PEI to the PDA layer, the increased

positive charge density may result in an improved monovalent selectivity. However,

the resultant SPES-PDA/PEI-3 membranes with a more neutral charge gave rise to a

reduced electro-static effect. The reduced zeta potential is consistent with the reduced

N content results obtained from EDAX.

Table 5.3 Zeta potential of SPES and PDA/PEI modified monovalent cation exchange membranes

Membrane SPES SPES-PDA/PEI-0 SPES-PDA/PEI-1 SPES-PDA/PEI-2 SPES-PDA/PEI-3

Zeta

potential

(mV)

-3.9 -30.1 7.4 13.4 8.3

5.2.4 Water contact angle, ion exchange capacity and water uptake

The SPES membrane exhibits the best hydrophilicity with a water contact angle

decrease from 40° to 31° in 60 s (Fig. 5.5), owing the presence of hydrophilic

functional groups in the membrane matrix. The initial contact angle sharply increases

from 40° to 62° after introducing a PDA layer, which is in agreement with the original

analysis of the hydrophilicity of the PDA layer (Luo et al., 2013). When hydrophilic

groups are covered by PDA/PEI composites, the increase of the PEI mass ratio with

inherent abundance of hydrophilic amine groups can substantially increase the

wettability of the membranes. Furthermore, a high PEI content would impact the

polymerization/deposition rate of dopamine. Therefore, the reduction of thickness for

the coating layer with reduced contact angle can be another factor contributing to an

enhanced hydrophilicity.

Chapter 5

81

Fig. 5.5 Dynamic water contact angles of SPES, SPES-PDA/PEI-0, SPES-PDA/PEI-1,

SPES-PDA/PEI-2 and SPES-PDA/PEI-3 membranes in 1 minute

It can be seen from Table 5.4 that the IEC of SPES was 1.76 mmol/g. It could be

expected that the negative dopamine coating layer caused by the deprotonation of

phenolic hydroxyl groups at neutral pH could increase the IEC. However, the

thickness of the SPES membrane matrix is far more than that of the coating layer, thus

the IEC remains constant (Kim et al., 2014). With a further increase of the PEI ratio, a

reduction trend has been identified concerning the IEC of the modified membrane,

although the variation is not obvious. The reduction of the IEC value is due to the fact

that some –SO3H groups on the membrane surface are neutralized by the electrostatic

effect between -NH2 groups and –SO3H groups. The water uptake has an obvious

effect on the membrane conductivity because water molecules serve as ‗‗vehicles‘‘ for

the transportation of the ions from the anode to the cathode (Muthumeenal et al.,

2014). In the SPES membrane, the sulfonate ions in membrane matrix are hydrated

with absorbed water molecules. The increase in water uptake of SPES-PDA/PEI-0

suggests that the increase in negative charge density attracted more water molecules

inside the membrane matrix. A similar trend for SPES-PDA/PEI-1, SPES-PDA/PEI-2

and SPES-PDA/PEI-3 membranes is caused by the incorporation of hydrophilic PEI

0 10 20 30 40 50 6020

30

40

50

60

70

80

90

Wa

ter

con

tact a

ng

le (

o)

Time (s)

SPES

SPES-PDA/PEI-0

SPES-PDA/PEI-1

SPES-PDA/PEI-2

SPES-PDA/PEI-3

Chapter 5

82

in the membrane matrix.

Table 5.4 Ion exchange capacity and water uptake of the membrane before and after co-deposition

Membrane IEC (mmol/g) Water uptake (%)

SPES 1.76 28.5

SPES-PDA/PEI-0 1.77 30.8




5.2.5 Diffusion experiments

Diffusion dialysis performances, represented by the dialysis coefficient, are

investigated to evaluate mass transport though ion exchange membranes. The

diffusion of salts leads to an increased conductivity in the concentrated compartments

during the experiments. However, the migration rate is quite different. By introducing

a PDA layer to the SPES membrane surface, the IEC increases, but due to the denser

and more compact coating layer, the diffusion dialysis slows down (see Fig. 5.6a) and

thus, the dialysis coefficient was reduced from 0.49 mol/(h·m2) to 0.32 mol/(h·m

2)

(Fig. 5.6b). The transport of NaCl further decreases by introducing PEI in the

co-deposition process, indicating that PEI coated on the modified layer is able to

decelerate the transport of NaCl. Thus, the dialysis coefficient is significantly

decreased. A pronounced increase in dialysis coefficient was observed with a further

increase of the PEI content. The SPES-PDA/PEI-3 membrane has a faster diffusion of

NaCl compared to SPES-PDA/PEI-2 and SPES-PDA/PEI-1 membrane due to the

increased PEI concentration. In this case, the addition of PEI reduces the adhesive

properties, so that PDA/PEI composites fail to attach to membrane substrates. The

enhanced electrostatic interaction of –NH2 with Cl- ions could be another factor to

enhance the dialysis coefficient. Thus, with increased PEI content, the dialysis

Chapter 5

83

coefficients follow the order SPES-PDA/PEI-3 > SPES-PDA/PEI-2 >

SPES-PDA/PEI-1.

Fig. 5.6 Diffusion experiment (a) conductivity variation in concentrate compartment of diffusion

cells employing monovalent selective ion exchange membranes (b) dialysis coefficients

5.2.6 Electrochemical characterization of the monovalent selective

ion exchange membranes

The current voltage curves of the prepared membranes with different PEI content are

shown in Fig. 5.7. The SPES membrane has the lowest limiting current density. With

reduced transport number, the limiting current density increases. The limiting current

density of the I-V curves indicates that the transport number follows the order SPES >

SPES-PDA/PEI-0 > SPES-PDA/PEI-1 > SPES-PDA/PEI-2 > SPES-PDA/PEI-3. This

trend is different from the transport process calculated as dialysis coefficients, which

can be explained by the dominant effect of positive charge density at the existence of

electric field. With respect to the limiting current density, no significant variation can

be observed in terms of the limiting current density. A similar phenomenon was

observed by Belashova et al. and Pismenskaya et al., which can be explained by the

compensation between the effects of the increased hydrophobicity and the conducting

heterogeneity caused by dopamine aggregates (Belashova et al., 2012; Pismenskaya et

Chapter 5

84

al., 2012). The higher surface hydrophobicity, the higher limiting current density,

while the conducting heterogeneities decrease the limiting current density (Volodina

et al., 2005).

The impedance spectra of the monovalent selective ion exchange membranes are

shown by the Nyquist diagram, in which the real impedance (Zreal) is plotted against

the imaginary impedance (-Zimag). The Nyquist plots show that the Ohmic resistance

of SPES membranes was 31.6 Ω. As can be observed, introducing a functional layer

results in a higher area resistance of the membrane (Luo et al., 2008). For the PDA

decorated membrane, the Ohmic resistance increases to 32.8 Ω. The Ohmic resistance

of the modified membranes further increased after incorporating PEI in the coatings.

For the SPES-PDA/PEI-1, SPES-PDA/PEI-2 and SPES-PDA/PEI-3 membranes, the

Ohmic resistances were 33.9 Ω, 35.1 Ω and 35.6 Ω, respectively. This increased

Ohmic resistance is attributed to both the compact coating layer and enhanced

positive charge of PEI. In addition, it would be expected that the monovalent

selectivity would increase by the PDA/PEI co-deposition coating layer, because the

monovalent selectivity is increased with the increase of the total amount of positive

charges on the membrane surface.

Fig. 5.7 (a) Current–voltages for different SPES based monovalent selective ion exchange

membrane (b) Nyquist plot showing the impedance spectra of different membranes

Chapter 5

85


In order to investigate the monovalent selectivity of the PDA/PEI modified membrane,

electrodialysis experiments were conducted. The monovalent selectivity of a

membrane was determined by calculating bulk transport numbers of H+ and Zn

2+

based on ionic fluxes under constant direct current density of 10.6 mA/cm2. For the

SPES membrane, the leakage of Zn2+

was 12.7%. It can be observed from Fig. 5.8a

that the Zn2+

leakage decreases in the order SPES-PDA/PEI-0, SPES-PDA/PEI-1, and

SPES-PDA/PEI-2. For SPES-PDA/PEI-0 membrane, the dense surface structure

increased the steric resistance for Zn2+

. In comparison, a slight increase of H+ flux can

be observed, which can be explained by the increased negative charge density and the

smaller Stokes radius of H+. With the increase of the PEI content, Zn

2+ leakage further

decreases, which is ascribed to the increased positive charge density. The flux for H+

increases for SPES-PDA/PEI-1, which is caused by the formation of H+ transfer

channels. The compact acid–base pairs formed by hydroxyl groups and amino groups,

can effectively block Zn2+

, while the transport of H+ is facilitated (Ge et al., 2015).

Based on the Grotthuss mechanism and the pore-size sieving effect, the increased H+

flux can be explained. The reduced H+ flux for SPES-PDA/PEI-2 membrane is due to

the enhanced positive charge density, which hinders the H+ transport through the

electrostatic effect. When the PEI/PDA ratio was 3 during the co-deposition process,

although the positive charge density was increased, both the leakage of Zn2+

and the

flux of H+ increase. The reason can be the reduced attachment, which reduces the

thickness of the functional layer. Thus, dopamine based monovalent selective ion

exchange membranes exhibit a lower Zn2+

leakage compared to an SPES membrane.

Besides, PDA/PEI composites exhibit excellent properties to facilitate H+ transport.

The 𝑃𝐻+𝑍𝑛2+

was greatly reduced after modification.

Chapter 5

86

Fig. 5.8 (a) Zn2+

leakage and perm-selectivity (b) fluxes of H+ of the SPES membrane and the

monovalent selective ion exchange membranes prepared by co-deposition

5.2.8 Stability and effects of molecular weight of PEI

The stability of the modified layer is of significant importance for the practical

applications of ED technology. It has been proven that a PDA layer has a superior

stability in neutral and weak acidic/basic solutions (Yang et al., 2014a). However, the

PDA coatings may become unstable in strong acid and base conditions because the

non-covalent connections disintegrate (Jiang et al., 2013; Wei et al., 2013). To

investigate the stability of membrane modified by PDA/PEI codeposition in harsh

environment, the membrane was immersed in 0.1 M HCl or 0.1 M NaOH for 7 days.

Fig. 5.9 indicates that no significant changes of the perm-selectivity and ion flux take

place when the PDA/PEI modified membranes were rinsed in acid solutions.

Comparatively, the modified membranes show slightly variation after treatment with

alkaline solutions. The slight increase of perm-selectivity and reduction of H+ flux

may be attributed to the loss of some non-covalently bonded components. At this

condition, reduced functional groups and acid-base pairs could cause a variation of the

perm-selectivity and H+ flux. However, the intrinsically robust adhesion of the PDA

catechol structure with the membrane matrix can still maintain the separation

properties in harsh environments. In addition, the residual copper ions on the surface

of modified layer could serve as cross-linking sites by chelation with the amine/imine

Chapter 5

87

groups of PDA. In summary, due to the covalent crosslinking between PDA and PEI,

the affinity properties and structural stability of the co-decorated membranes can be

greatly enhanced.

Fig. 5.9 Permselectivity and H+ flux of the SPES-PDA/PEI-2 membrane. a. before and after acid

treatment; b. before and after alkaline treatment; c. with different PEI molecular weight

The molecular weight of PEI could also have an effect on the membrane

perm-selectivity and H+ flux. In order to ensure the same amount of –CH2CH2NH-,

the mass ratio of dopamine to PEI was fixed at 1:2. Typically, PEI with a higher

molecular weight will resulted in a more hydrophobic surface because large PEI

molecules can impede the deposition process. As a result, a higher selectivity is

obtained (Ran et al., 2017; Yang et al., 2016). In addition, by elongating amine side

chains, the electrostatic interactions between amine groups and Zn2+

are enhanced (Ge

et al., 2017). However, the deposition amount reduces with increasing PEI molecular

weight due to the unfavorable crosslinking between dopamine and PEI, which in turn

leads to a poor separation performance. With increasing mobility of the side chains,

hydrophilic ―channels‖ were constructed by self-assembly of ionic side chains, which

Chapter 5

88

can facilitate the migration of protons (Choi et al., 2005; Ge et al., 2016b; Li and

Guiver, 2014). As a consequence, a slight increase of the proton flux is observed.

In this study, the obtained results demonstrate that the PEI/PDA modified membrane

greatly reduces the Zn2+

leakage. While comparing to monovalent membranes

prepared by other methods (Table 5.5 and Table 5.6), the perm-selectivity of the

prepared membranes was comparable to most membranes. However, this method still

showed a higher Zn2+

leakage compare to monovalent membranes prepared by

annealing treatment. By comparing the modified membrane with commercial

monovalent cation exchange membrane, the modified membrane has a higher H+ flux

while maintaining almost the same Zn2+

flux. To conclude, PDA/PEI codeposition

could be a promising way to prepare monovalent cation exchange membranes.

Table 5.5 Some typical examples about monovalent cation selectivity - Zn2+

leakage

Method Zn

2+ leakage of primary

membrane

Zn2+

leakage of modified

membrane Reference

Quaternized chitosan 8.5% 1.01% (Hu et al.,

2008)

Photo-induced covalent

immobilization of

chitosan

27.4% 4.5% (Wang et

al., 2013)

Annealing treatment 0.3% 0.01% (Ge et al.,

2014)

Chemical modification by

polyquaternium-7 22% 14.2%

-

CSO - 9.1%

This work 12.5% 5.7% -

Chapter 5

89

Table 5.6 Some typical examples about monovalent cation selectivity - Permselectivity

Method Perm-selectivity*

Modified membrane

Reference Zn2+

flux

(mol·cm–2

·s–1

)

H+ flux

(mol·cm–2

·s–1

)

EDNF 354 6.7·10-6

2.3·10-3

(Ge et al.,

2016a) CSO 15 67.9·10-6

1.7·10-3

This work 52 14.1·10-6

1.8·10-7

-

* Perm-selectivity is simply calculated as the ratio of monovalent ion and divalent ion fluxes

5.3 Conclusions

Monovalent selective ion exchange membranes have been prepared by PDA/PEI

co-deposition with the assistance of CuSO4/H2O2. A series of characterizations for the

modified membranes by SEM and EDAX demonstrated the formation of a functional

layer on the membrane surface. With increasing PEI content, the water uptake of the

membranes increased, while the IEC of the membranes slightly decreased. AFM

images indicated that at optimized conditions (SPES-PDA/PEI-2), the surface of the

modified membrane was covered uniformly by the PDA/PEI layer. In addition, the

thickness of the modified layer was first increased and then decreased with the

increase elevation of PEI mass ratio. Membrane transport properties were measured

by diffusion experiments and current–voltage curves. The electrical resistance was

increased after surface modification. Electrodialysis experiments for H+/Zn

2+ system

exhibited that a superior monovalent selectivity could be obtained after introducing

the PDA/PEI layer. After 7 days of acid/alkaline treatment, the modified membrane

shows an excellent stability in terms of permselectivity and H+ flux. With increased

PEI molecular weight, both the permselectivity and H+ flux were increased.

Considering the increased permselectivity, high H+ flux, high hydrophilicity and

long-term stability, PDA/PEI co-deposition can be a promising method to enhance the

monovalent selectivity of a membrane.

Chapter 6

90

6. Mussel-inspired monovalent selective cation

exchange membranes containing hydrophilic

MIL53(Al) framework for enhanced ion flux

Adapted from: J. Li, J. Zhu, S. Yuan, X. Li, Z. Zhao, Y. Liu, Y. Zhao, A. Volodine, J.

Li, J. Shen, B. Van der Bruggen. Mussel-inspired monovalent selective cation

exchange membranes containing hydrophilic mil53 (Al) framework for enhanced ion

flux. Industrial & Engineering Chemistry Research. 57(2018): 6275-6283.

6.1 Introduction

As indicated in Chapter 5, it is of interest to discover novel materials and

methodologies to prepare monovalent CEMs. The method of rapid co-deposition of

PDA and PEI by using CuSO4/H2O2 as trigger can form a thin selective layer. As a

consequence, the surface properties and the monovalent selectivity of the resultant

membranes can be easily tailored. The optimum membranes, with 4 h co-deposition

of 60 mg PDA and 120 mg PEI, exhibited a 2.5 times higher permselectivity than the

primary membrane. Especially, the flux of H+ was enhanced in the binary system of

H+/Zn

2+. Meantime, the PDA/PEI modified ion exchange membrane shows excellent

operation stability for monovalent separation performance in both acid and alkaline

situation. However, the selectivity of the as-prepared membrane in other systems,

especially Na+/Mg

2+ system, is unknown.

To obtain a promising monovalent selective ion exchange membrane, the monovalent

selectively is not the only factor to be taken into consideration. The membrane

resistance, current efficiency, and energy consumption are the key parameters to

evaluate an electrodialysis process. MIL-53(Al), a sub-branch of metal-organic

frameworks (MOFs), contains 1D diamond-shaped channels with pores of nanometer

Chapter 6

91

dimensions (Ramsahye et al., 2007). The mesoporous structure enables a high mass

transfer efficiency, and the hydrophilic characteristics facilitate a uniform dispersion

of MIL-53(Al) in aqueous solution (Pashley et al., 2005). These superiorities open up

the possibility of preparing monovalent selective CEMs with enhanced ion flux by

incorporating MIL-53(Al). In this study, novel organic-inorganic thin film composite

(TFC) monovalent CEMs were fabricated by a fast co-deposition of PDA/PEI

composites with MIL-53(Al). MIL-53(Al) was adopted as mesoporous component to

maintain a high ion flux while the positively charged PDA/PEI coatings ensure the

rejection of multivalent ions.


6.2.1 Surface morphology and chemical structure of the membrane

The surface morphology of PDA/PEI based thin film nanocomposite (TFN)

monovalent selective CEMs with different contents of MIL (53)-Al (0%, 0.2%, 0.4%

and 0.6%) are shown in Fig. 6.1. It can be observed that co-deposition with a

PDA/PEI skin layer has no significant change on the surface morphology. After the

addition of MIL (53)-Al, small rougher dots can be observed, indicating an

inhomogeneous decoration of MIL (53)-Al. With increasing Mil(53)-Al loading to 0.4%

w/v, many nodules were found. The nodules on the membrane surface are attributed

to the aggregation of Mil(53)-Al. Despite the aggregation, the coverage becomes

higher for the membrane with higher Mil(53)-Al content. However, too much

Mil(53)-Al will result in a serious aggregation, which is difficult to deposit on the

substrate surface. As a consequence, defects on the membrane surface can be found as

indicated by the red circle in Fig. 2 (PDA-mil-30), while no obvious

Mil(53)-Alaggregration can be found in the blue circle. Therefore, the fabrication of

PDA/PEI modified membrane with suitable Mil(53)-Al nanoparticles incorporation is

particularly important.

Chapter 6

92

Fig. 6.1 SEM images of (M-0) unmodified ion exchange membrane and TFC monovalent

selective membrane surfaces. Nanoparticle loadings for PDA-0, PDA-mil-10, PDA-mil-20 and

PDA-mil-30 are 0.0%, 0.2%, 0.4% and 0.6% (w/v), respectively (The scale bar represents 2 µm)

During the modification process, a tight skin facial layer is formed on the membrane

surface through interactions such as π-π interaction, hydrogen bonding interaction,

and electrostatic interaction. According to the three-dimensional AFM surface

topography pictures presented in Fig. 6.2, the PDA/PEI decorated membrane has no

obvious changes on the surface morphology, which is in agreement with the SEM

results. The surface root mean square height (Rsq) was similar after Mil(53)-Al

decoration, whereas the maxium height (Rsz) increased from 15.4 nm to 23.0 nm

(Table 6.1). The nanoparticles on the membrane surface improve the Rsz; meantime,

the Rsq is stable due to the low loading degree. A higher Mil(53)-Al coverage with

more nanoparticles incorporation causes a further increase of the surface roughness.

When the amount of decorated Mil(53)-Al reached 0.6% (w/v), a distinct increase of

roughness was detected. This is in accordance with the aggregation of nanoparticles

found in the SEM images.

Chapter 6

93

Fig. 6.2 AFM topography of primary membrane and modified membranes (a. M-0; b. PDA-0; c.

PDA-mil-10; d. PDA-mil-20; e. PDA-mil-30)

Table 6.1 AFM surface roughness parameters of the pristine and modified membranes

Membrane type M-0 PDA-0 PDA-mil-10 DA-mil-20 DA-mil-30

Root mean square height (Rsq) 1.81 nm 1.86 nm 1.87 nm 1.99 nm 3.77 nm

Maximum height

(Rsz) 15.7 nm 15.4 nm 23.0 nm 24.6 nm 31.6 nm

Typical XPS spectra of M-0, PDA-0, PDA-mil-20 are shown in Fig. 6.3. For all the

membranes, C 1s, N 1s and O 1s characteristic peaks were obtained. In comparison

with M-0 membranes, the emission peak for N 1s is more intensive for the modified

membrane, which is assigned to the increased nitrogen content of PEI. For the

PDA-mil-20 membrane, although the Al and Cu concentration is too low to obtain Al

2s, Al 2p, Cu 2p1/2 and Cu 2p3/2 peaks, the elemental concentration given by Table 6.2

confirms the presence of Mil (53)-Al and Cu. For the method of PDA/PEI

co-deposition, the thickness of the modified function layer is generally around 100 nm

(Lv et al., 2015, 2016), which means a limited anchor effect while the size of the

nanoparticles is large enough. As indicated by SEM and AFM results, no obvious

aggregates with large size were observed. As a consequence, Mil (53)-Al

nanoparticles with large size failed to be fixed on the membrane surface, ultimately

leading to a low Al concentration.

Chapter 6

94

Fig. 6.3 XPS spectra of M-0, PDA-0 and PDA-mil-20 membranes

Table 6.2 Atomic concentrations of C, N, O and Al for M-0, PDA-0 and PDA-mil-20 membranes

Membrane

Element atomic concentrations (%)

C N O Al

M-0 73.5 4.0 19.7 0.0

PDA-0 69.0 6.2 23.2 0.0

PDA-mil-20 74.3 4.8 19.6 0.3

6.2.2 Contact angle, ion exchange capacity and water uptake

The water contact angle, ion exchange capacity and water uptake of the newly

developed membranes were studied to explore the effects of Mil (53)-Al nanoparticles.

The commercial cation exchange membranes showed an initial water contact angle of

31°, which indicates the high hydrophilic properties of the primary CEMs (Fig. 6.4).

After decoration by PDA/PEI composites, the membrane surface became more

hydrophobic as water contact angles increased to 77°. The decreased hydrophilicity

can be explained by the more hydrophilic nature of the primary CEMs and the

depletion of amine groups in PEI to react with acryl chloride groups in TMC (Yang et

Chapter 6

95

al.). The addition of hydrophilic Mil (53)-Al nanoparticles to TFC membranes greatly

enhances the surface hydrophilicity. However, no obvious variation was observed

with further increase the decorated Mil (53)-Al concentration.

Fig. 6.4 Contact angle, IEC and water uptake of PDA/PEI modified membranes with Mil(53)-Al

at different loadings (a. contact angle, b. IEC and water uptake)

The ion exchange capacity is responsible for the ionic conductivity of the membranes,

while the water uptake can affect the transport behavior of ions across the membrane.

After surface modification, both the IEC and water uptake exhibited a slight increase

from 1.34 mmol/g to 1.38 mmol/g and 33.4% to 37.6%, respectively. Since PDA/PEI

composites are positive charged via a synergetic effect of -NH2, -OH and -COOH

groups, the change of IEC is not as obvious as the water uptake. For the M-0 and

PDA-0 membranes, the surface composition was uniform while the surface of

PDA-mil-10, PDA-mil-20, and PDA-mil-30 was heterogeneous, the strong affinity of

Mil(53)-Al for water would give rise to hydrophilic regions on the membrane surface.

These hydrophilic regions formed around the cluster of chains lead to absorption of

water and attraction of protons. Furthermore, the established pores of Mil(53)-Al can

accommodate water molecules due to their relatively large sizes. The prepared

membrane with Mil(53)-Al shows an enhanced hydrophilicity, and a more positive

charge density, indicating a strong potential use for separation of monovalent ions.

Chapter 6

96


Diffusion dialysis experiments were undertaken to understand the diffusional

ion-transport process, and particularly the effect of structural parameters with

different Mil(53)-Al content. The diffusion of salts from the concentrate chamber to

the diluate chamber caused an increase of the conductivity in the concentrated cell

(Fig. 6.5). For the diffusion experiments of the primary membrane M-0, after 1 h

self-diffusion, the conductivity of the concentrated compartment changed from

15.8 µS/cm to 258.8 µS/cm. The pristine interfacial polymerization between PDA/PEI

and TMC limits the diffusion process. As a result, the diffusion of NaCl for the

PDA-0 membrane becomes slower, and thus the conductivity change was reduced.

Theoretically, the hydrated radius of Na+ is around 3.0 Å and radius of Cl

- is 1.8 Å

(Kang et al., 2014; Tansel et al., 2006), which is smaller than the Mil(53)-Al pores

(8.5 Å) (Yang et al., 2013). Therefore, the presence of Mil(53)-Al on the TFC surface

can provide extra space to enhance the salts diffusion process. Furthermore, the

improvement of IEC and water uptake could form ionic transfer pathways on the

functional layer of the membrane surface, and facilitate the transport of salts.

However, the diffusion was mitigated when the concentration increased to 0.4% (w/v).

At this condition, ionic pathways on the membrane surface are occupied by the

increase of additive particles and narrowed as space limiting factors (Nemati et al.,

2015). When the Mil(53)-Al incorporation reaches 0.6% (w/v), the high concentration

of Mil(53)-Al particles tends to agglomerate to form larger particles, thus large filler

clusters are formed and salts can be easily transported. Although the conductivity

variation in the concentrate compartment was not obvious during the diffusion

experiments, the performance of the membranes after modification changed

significantly, which were confirmed by the following characterizations.

Chapter 6

97

Fig. 6.5 NaCl diffusion process on the surface of TFC membranes with different Mil(53)-Al

loadings

6.2.4 Electrochemical properties of membranes

Electrochemical impedance spectroscopy (EIS) is an important tool to enlighten

electrochemical phenomena related to membranes, allowing to quantify the resistance

of the membrane matrix. EIS results obtained in both NaCl and MgCl2 solutions are

shown in Fig. 6.6. However, the phenomena are very different. For the experiments

conducted in NaCl solution, the increased resistance indicates that the presence of the

PDA/PEI layer atop the CEMs hinders the ionic transport. A reduction of the

membrane resistance was observed after introduction of Mil(53)-Al, because porous

structures greatly facilitate the Na+ mitigation. As a consequence, Na

+ permeating

through the CEMs becomes easier and the membrane resistance is reduced. An

increase in electrical resistance arising from the increasing Mil(53)-Al loadings

implies that a hybrid membrane containing proper inorganic materials can

significantly improve the membrane performance, but an excessive proportion of the

inorganic materials leads to a high resistance (Mistry et al., 2008). Such a variability

in conductivity is in good agreement with the previous results of diffusion dialysis,

confirming that an excess of Mil(53)-Al disrupts the ion transfer pathways in this

particular system (Mistry et al., 2008). For the EIS results conducted in MgCl2

0 10 20 30 40 50 60

0

50

100

150

200

250

300

Co

nd

uctivity (s/c

m2)

T (min)

M-0

PDA-0

PDA-10

PDA-20

PDA-30

Chapter 6

98

solutions, two arcs appeared on the impedance spectrum. Typically, the geometric

arch at low frequencies is determined by the ionic migration while the high

frequencies of the Nyquist plot are the sum of the membrane resistance and the

solution resistance (Zhao et al., 2018). The total Ohmic resistance of the membrane

with different Mil(53)-Al loadings was in this order: PDA-mil-30 < PDA-0 <

PDA-mil-10 < PDA-mil-20 < M-0. It is remarkable that the resistance of M-0 is

higher than PDA-0. The lower resistance in these circumstances may be related to the

enhanced shielding effect of double layer compression between the positive PDA/PEI

modified layer and the negative CEMs, which reduced the electrostatic repulsion

between the modified layer and Mg2+

(Mo et al., 2008). In addition, the great affinity

of Mg2+

for the functional groups of the CEMs gives the membrane a high

electroconductance (Rodzik, 2005). The only difference of the geometric arch at high

frequencies is the diameter of the additional arch, which indicates the formed

functional layer on membrane surface can restrict the ions transport (Zhao et al.,

2017). With insight into the second diffusional arc at low frequencies, despite no

significant difference can be observed concerning the diameter of arch, the slight

reduction trend for membrane after modification confirms the restriction of the ion

migration through the diffusion layer, electrical double layer and the CEMs.

Fig. 6.6 Mil(53)-Al effect on EIS of membrane for NaCl and MgCl2 solutions

The I–V curves of membranes with different Mil(53)-Al loadings are shown in Fig.

6.7. For the M-0 membrane, the limiting current density did not clearly appear within

Chapter 6

99

measuring conditions. A membrane with low resistance could facilitate the transfer of

ions, therefore, concentration polarization can be effectively avoided. By this means,

the increased limiting current density ensured the application of ED under higher

current density and thus an increased efficiency (Ge et al., 2016a). After modification

by PDA/PEI, a plateau appears. This behavior corresponds to the formation of the

modified layer, which enhances the concentration polarization. Moreover, the

enhanced surface hydrophobicity can be another factor contributing to the increase of

the plateau length (Balster et al., 2007; Güler et al., 2014). For the Mil(53)-Al

incorporated membrane, no apparent plateau can be found, which confirms the

promotion effect of Mil(53)-Al for ion migration.

Fig. 6.7 Current voltage of M-0, PDA-0 and PDA-mil-20 membranes


The desalination performance of the membranes was first investigated by

electrodialysis using single salt solutions (NaCl system and MgCl2 system,

respectively). In these experiments, a constant-voltage strategy was applied. When the

desalination process continues, the conductivity of the diluate compartment was

Chapter 6

100

recorded as a function of the desalination time. During the experiments, the volume

was stable for the diluate and concentrate compartments, thus the water flow across

the membrane can be neglected. The flux of Na+ and Mg

2+ reduced significantly after

the interfacial polymerization of PDA/PEI, nevertheless, no distinct effect on total

desalination ratio can be observed. Since the current density tends to decrease with the

increasing resistance of the diluate compartment, the concentration variation in the

diluate compartment would be expected to be minimized. To better understand this

specific process, the concentrations of Na+ and Mg

2+ at 30 min were used to calculate

the ion flux due to the large variations of concentrations in the first 30 min. As shown

in Fig. 6.8, higher Na+ and Mg

2+ fluxes were obtained after introducing the porous

Mil(53)-Al nanoparticles. The steric hindrance effect becomes much more obvious

with increasing the Mil(53)-Al content, thus the flux of Na+ and Mg

2+ was reduced.

However, the reduction of the flux of Mg2+

was not obvious while the flux of Na+ was

significantly reduced, which suggests a more obvious effect of the Mil(53)-Al content

on Na+ with smaller hydrated radius.

Fig. 6.8 Conductivity change of diluate compartment for different membranes at a. NaCl, b.MgCl2

systems and c. ion flux

Chapter 6

101

For the separation of monovalent from divalent cations, a comparison of monovalent

and multivalent fluxes is presented in Fig. 6.9. Likewise, PDA/PEI modified

membranes exhibited a much lower Mg2+

flux than the untreated membrane,

demonstrating the improvement of the steric hindrance effect. In contrast, the flux of

Na+ is comparatively higher to compensate for the reduction in the migration current

and the permselectivity of the modified membrane decreased from 1.26 to 0.37.

Incorporating Mil(53)-Al nanoparticles slightly enhances the flux of Na+, while the

Mg2+

flux is sustained. Improving the selectivity is typically at the expense of the flux

of monovalent ions; Mil(53)-Al nanoparticles incorporation can be an alternative way

to solve this problem. A further increase of the Mil(53)-Al content has no obvious

effect on the permselectivity; however, in this case, both the Na+ and Mg

2+ permeance

increased. This proves the desalination contribution from Mil(53)-Al, indicating a

higher current efficiency. For PDA-mil-30, the transfer resistance was further

decreased and the voids facilitated the Mg2+

transfer from the interior of the

membrane to the solution. In this case, Mg2+

with higher electrostatic interaction

occupied the ion exchange transfer sites, resulting in a lower Na+ flux and a higher

permselectivity. Furthermore, aggregates resulted in an uneven surface, so that the

difference in PDA/PEI polymer thickness also contributed to the lower selectivity.

Fig. 6.9 The ion flux and permselectivity of Na+/Mg

2+ system during ED

Chapter 6

102

Table 6.3 lists the ion selectivity values of commercial monovalent ion selective

CEMs (Luo et al., 2018). The ionic radii for Mg2+

and Ca2+

are 1.0 Å and 0.72 Å,

respectively. Thus, separation of Na+ with Ca

2+ tend to be easier than the separation of

Na+ with Mg

2+. By comparing the selectivity of the membrane modified by dopamine

and a commercial CSO membrane, it is obvious that the membrane prepared by

dopamine has a high selectivity.

Table 6.3 Reported selectivity of commercial monovalent ion selective IEMs

CMX CMS CSO

𝑃𝑁𝑎+𝐶𝑎2+

1.56 0.81 0.58

6.3 Conclusions

In conclusion, novel monovalent selective ion exchange membranes were fabricated,

with potential for large scale application. Moreover, the usage of PDA/PEI solution

can be performed in an economic and environmentally friendly way (Yang et al.,

2014a). Because of the high cationic charge density of PEI, an ultrathin PDA/PEI

selective layer was constructed to reject multivalent ions while Mil(53)-Al

nanoparticles would serve as a porous additive to enhance the ion flux. In particular,

the selectivity is maintained at a high level, demonstrating an excellent monovalent

selectivity. This study can provide new insights into utilizing mussel-inspired

materials for creating ion channels for various promising applications.

Chapter 7

103

7. Thin-Film-Nanocomposite Cation

ExchangeMembranes Containing Hydrophobic

Zeolitic Imidazolate Framework for Monovalent

Selectivity

Adapted from: J. Li, Z. Zhao, S. Yuan, J. Zhu, B. Van der Bruggen. High-Performance

Thin-Film-Nanocomposite Cation Exchange Membranes Containing Hydrophobic

Zeolitic Imidazolate Framework for Monovalent Selectivity. Applied Sciences.

135(24), 2018:45692.

7.1 Introduction

As mentioned in Chapter 5 and Chapter 6, monovalent selectivity is governed by the

affinity towards the fixed charge groups and the migration speed within the membrane

matrix (Ge et al., 2017). Based on these effects, IEMs with selectivity for specific ions

have been explored by including a thin charged skin layer or by generating a compact

functional layer on the surface of IEMs. Thin film composite (TFC) membranes,

comprising an ultrathin separating barrier layer prepared by interfacial polymerization

on top of a membrane, could be used in electrodialysis for the purpose of separating

multivalent ions from a mixed solution containing monovalent and multivalent ions

(Ge et al., 2016a). However, the inevitable higher area resistance caused by a surficial

functional layer increases the energy consumption at the same time. In recent years,

thin film nanocomposite (TFN) membrane emerged as a new type of composite

membranes, and have been widely studied and industrially applied (Jeong et al., 2007).

Nanoparticles are incorporated within the interfacial layer of the TFC membrane, with

the aim of enhancing the properties of the surface layer such as hydrophilicity,

permeability, selectivity, stability and surface charge density (Lau et al., 2015).

Currently, TFNs are widely used in forward osmosis (FO), reverse osmosis (RO) and

Chapter 7

104

nanofiltration (NF) (Amini et al., 2013; Peyravi et al., 2014; Safarpour et al., 2015).

However, they are seldom applied in electromembrane processes.

Metal-organic frameworks (MOFs), as a class of hybrid inorganic-organic solid

compounds, have gained interest due to their structural and functional tunability

(Kitagawa, 2014). They can serve as porous materials similar to zeolites while having a

better affinity for the polymeric chains (Sorribas et al., 2013). In addition, the flexibility

in pore size of MOFs can be controlled by choosing appropriate organic ligands and

inorganic secondary building units, which significantly broadens their application in

molecular sieving. Zeolitic imidazolate frameworks (ZIFs) are a subclass of MOFs

with large surface areas and pore volumes. Notably, ZIF-8, obtained by the reaction of

Zn2+

with 2-methylimidazole as linker, has gained great interest in membrane

utilization due to its high chemical and thermal stability (Sorribas et al., 2013; Van

Goethem et al., 2016; Wee et al., 2013). Duan et al. added 0.4 w/v% ZIF-8

nanoparticles to a TFN membrane with an 162% permeance increase while maintaining

a high salt rejection (Duan et al., 2015). However, the interfacial nanogaps present in

the functional layer of the decorated membrane cannot be fully avoided (Wang et al.,

2015b). ZIF-8 has a small aperture with a size of 3.4 Å and a comparatively large cavity

with a size of 11.6 Å (Shi et al., 2012). The small aperture of ZIF-8 can serve as an

effective filter to separate hydrated cations of Mg2+

(4.28 Å) through a size sieving

effect (Bazinet and Moalic, 2011a; Kang et al., 2014). Although there is a significant

variation on the reported size of hydrated Na+ (between 2.99 and 3.58 Å), in general,

the hydrated cations with larger crystal radii have weaker hydration shells, so that the

detachment of the hydration shell would occur when ions pass the solution-membrane

interface (Kotov et al., 2002; Tansel et al., 2006). Consequently, ZIF-8 has the right

size to separate dehydrated Na+ (0.95 Å) and hydrated Mg

2+ (Firdaous et al., 2007; Han

et al., 2015).

Different from the previously reported strategy to form a dense cationic charged layer

by chemical modification, this chapter presents an interfacial polymerization strategy

with nanoparticles to separate monovalent and multivalent ions. Porous ZIF-8 was used

as nanofiller underneath the surficial functional layer to separate monovalent ions. The

influence of the ZIF-8 content on the desalination performance was explored to

determine the optimal preparation parameters.

Chapter 7

105

7.2 Results and Discussion

7.2.1 Surface morphology and zeta potential

The surface morphology of the prepared membranes with different ZIF-8 contents is

displayed in Fig. 7.1. The surface of the primary Fujifilm membrane was flat, whereas

that of a MPD/TMC membrane (M-1) exhibited the typical ridge-and-valley feature.

This phenomenon resulted from the violent reaction between the small molecular

amine and acyl chloride. With increasing ZIF-8 loading to the surface layer, the

membrane showed a more net-like structure with denser and smoother zones. At higher

ZIF-8 loadings, especially at 0.08%, more ―cubic‖ structures are visible, which

represents that ZIF-8 nanoparticles are covered by polyamide. No direct relation

between the morphology and ZIF-8 concentration was observed, indicating that

nanoparticles were covered by a continuous PA film. It is believed that interfacial

polymerization is a diffusion controlled process. MPD adheres on the membrane

surface, diffuses and reacts with TMC in the organic phase. When no MPD permeates

across the barrier layer, the reaction stops. For the monovalent selective ion exchange

membrane that was prepared with ZIF-8 nanoparticles, ZIF-8 deposits on the

membrane first and then embedded under the PA layer, leading to similar SEM images

(Van Goethem et al., 2016). The elemental weight distribution on the membrane

surface was determined by EDAX to confirm the presence of ZIF-8. In Table 7.1, a low

Zn content can be observed. With increasing concentration of ZIF-8 nanoparticles, a

higher Zn content can be achieved. The EDAX mapping (Fig. 7.2) demonstrated a

uniform distribution of ZIF-8 on the membrane surface. However, the variation of the

Zn concentration is not obvious. Thus, the results of SEM and EDAX suggest a

successful encapsulation of ZIF-8 nanoparticles. The PA surface structures with

various crosslinking degree can be explored by further analyzing the element ratios

between O and N. For the modified membrane without ZIF-8 nanoparticles, the O/N

ratio is about 1.75. It was obvious that no distinct difference can be seen in the O/N

ratio with the increase of ZIF-8 concentration. This indicates that the crosslinking

degree of MPD/TMC copolymer was maintained stable when the ZIF-8 concentration

is blow 0.08%. The zeta potential of the as-prepared membranes is listed in Table 7.2.

Both the primary ion exchange membrane and the MPD/TMC modified membrane

Chapter 7

106

are negatively charged, with zeta potentials of −16.4 and −20.3 mV, respectively.

After introducing ZIF-8 into the selective layer, the zeta potential of the modified

membrane shifted to −22.3 mV. With a further increasing ZIF-8 content, the zeta

potential continued to decrease. It should be noticed that the slight reduction of zeta

potential is not consistent with the O/N ratio results that were obtained from EDAX,

which can be explained by the detection depth of EDAX. A detection depth around 1

μm for EDAX with complex background can reduce the impact of carboxylic acid

groups on O/N ratio. Thus, the ZIF-8 loading inside the MPD/TMC surface layer

could generate more carboxylic acid groups.

Fig. 7.1 SEM images of the unmodified and modified ion exchange membrane.

Nanoparticle loadings are (M-1) 0.00%, (M-2) 0.02%, (M-3) 0.04%, (M-4) 0.06%, and

(M-5) 0.08% (w/v).

Chapter 7

107

Fig. 7.2 EDAX mapping for the membrane after ZIF-8 incorporation (Light dots

are of Zn)

Table 7.1 Atomic concentrations of C, N, O and Zn obtained by EDAX results

C (%) N (%) O (%) Zn (%)

M-0 56.80 14.31 28.89 -

M-1 59.35 14.78 25.87 -

M-2 57.59 16.60 25.78 0.03

M-3 57.77 16.14 26.01 0.08

M-4 58.30 16.92 24.70 0.08

M-5 59.07 15.49 25.32 0.12

Table 7.2 Zeta potential results of the primary and modified membranes

Membrane M-0 M-1 M-2 M-3 M-4 M-5

Zeta potential (mV) -16.4 -20.7 -22.3 -23.7 -23.8 -24.5

The water contact angle measurements also confirmed the presence of ZIF-8 under the

PA film (Fig. 7.3). After modification, contact angles increased from 27.6° to 43.7°.

This indicates that the primary commercial cation exchange membranes are highly

hydrophilic; by introducing the MPD/TMC layer, the surface becomes more

hydrophobic. The contact angle increased from 43.7° to 78.9° for M-2 after

incorporation ZIF-8 nanoparticles. The addition of a certain amount of ZIF-8

nanoparticles to TFN membranes greatly reduces the surface hydrophilicity, which is in

accordance with previous research (Lind et al., 2009). Assuming that some ZIF-8

nanoparticles are bared on the modified layer, water contact angles would increase with

the increase of the ZIF-8 nanoparticles content. However, a comparatively more

Chapter 7

108

hydrophilic surface was obtained with increasing ZIF-8 nanoparticles content, which

means that ZIF-8 nanoparticles tend to be covered by the polyamide layer (Duan et al.,

2015; Lind et al., 2009). As was proved by previous zeta potential results, increased

ZIF-8 loadings suggest an increased surface charge density of the MPD/TMC surface

layer, and thus increased carboxylic acid groups lead to an increased hydrophilicity.

Furthermore, For hydrophilic materials, the increased roughness could also contribute

to the enhancement of the hydrophilicity (Wang et al., 2015a). However, the slight

change of the zeta potential as well as the comparative poor hydrophilicity of the

membrane surface finally caused a small variation of the contact angle. As shown in

Fig. 7.4 and Table 7.3, the pristine membrane has a smooth surface, with a roughness of

1.0 nm. After modification, the Rsq significantly increases to 36.3 nm. By raising the

initial concentration of ZIF-8 to 0.08%, the surface roughness increased to 70.8 nm,

with the contact angles decreasing from 78° to 71° (Erbil et al., 2003; Nabe et al., 1997).

As a consequence, a further increase of ZIF-8 nanoparticles loadings tends to reduce

the contact angles.

Fig. 7.3 Contact angle of the modified ion exchange membranes

0

10

20

30

40

50

60

70

80

90

100

M-5

M-4M-3M-2

M-1

M-0

Co

nta

ct

an

gle

()

Chapter 7

109

Fig. 7.4 AFM topography of the membrane with and without modification

Table 7.3 AFM surface roughness parameters of the pristine and modified membranes

Membrane type M-0 M-1 M-2 M-3 M-4 M-5

Root mean square height (Rsq) 1.0 nm 36.3 nm 43.4 nm 51.5 nm 65.2 nm 70.8 nm

Maximum height (Rsz) 4.4 nm 210 nm 230 nm 274 nm 347 nm 335 nm

7.2.2 IEC and water uptake

The ion-exchange capacity yields the ionic conductivity of the membranes, while the

water uptake can affect the transport behavior of ions across the membrane. The IEC

and water uptake as a function of ZIF-8 content for all the prepared membranes are

presented in Fig. 7.5. Changes of the IEC and water uptake can be observed after

surface modification of the MPD/TMC layer from 1.34 mmol/g to 1.42 mmol/g and

33.4% to 30.6%, respectively. Since MPD/TMC composites are negatively charged

(Kang and Cao, 2012), the IEC increases after introducing more negative functional

groups. The dense structure of the MPD/TMC layer is more hydrophobic, which

indicates the reduced water molecular accessibility to the surface matrix. Therefore, an

Chapter 7

110

increased IEC and a reduced water uptake for the MPD/TMC modified membrane were

obtained. Furthermore, all the membranes exhibited an increase in IEC and water

uptake with increasing ZIF-8 content. Besides, the large cavity of ZIF-8 can

accommodate water molecules inside the nanoparticles, which can be another factor

contributed to the higher water uptake. Consequently, the prepared PA/ZIF-8

membrane surface has an enhanced water uptake, and a more negative charge density

with increased ZIF-8 content.

Fig. 7.5 IEC and water uptake of the membranes with different ZIF-8 loadings


Diffusion dialysis using the synthesized membranes with different ZIF-8 content was

carried out to study the diffusional transport process. In Fig. 7.6, it can be seen that the

conductivity of the permeate side increases during the experiments, which is caused by

the diffusion of salts from the high concentration chamber to the low concentration

chamber. However, the conductivity change rate is different. After 1 h self-diffusion,

the conductivity of the M-0 membrane changed from 10 µS/cm to 275 µS/cm. The

interfacial polymerization between MPD and TMC limited the diffusion process; as a

consequence, the diffusion of NaCl for the M-1 membrane becomes slower, and the

1.0

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

IEC

(m

mo

l/g

)

28

30

32

34

36

38

40

M-0 M-5M-4M-3M-1 M-2

Wa

ter u

pta

ke

(%)

Chapter 7

111

conductivity change reduced. Theoretically, the radius of Cl- is around 1.8 Å (Kang et

al., 2014; Tansel et al., 2006), which is smaller than the ZIF-8 pores (3.4 Å). Therefore,

adding a small amount of ZIF-8 to the TFN increased the NaCl flux. With a higher

ZIF-8 loading in the surface selective layer, the salt flux further increases, which can be

explained by the following two reasons. First, increased ZIF-8 loadings introduce more

free space that can facilitate the salt migration. More importantly, the structural

changes with ZIF-8 incorporation and the possible voids between the organic/inorganic

interphase could facilitate the NaCl permeance (Chung et al., 2007; Duan et al., 2015).

Fig. 7.6 Conductivity change of diluate compartment during diffusion process

7.2.4 Membrane resistance

As can be observed in Fig. 7.7, for the M-0 membrane, the membrane resistance

increased by a factor 1.5 for MgCl2 solution compared to the NaCl solution. The

increased resistance for NaCl solutions indicates that the presence of the MPD/TMC

layer near the cation-exchange membrane hinders the ionic transport. Since the similar

IEC and water uptake yield a relative constant resistance, the transport of Na+ is not

much affected by the ZIF-8 incorporation. In the case of Mg2+

, a reduction of the

membrane resistance was observed, because carboxylic acids are week acids so that the

interaction with Mg2+

was greatly mitigated. As a consequence, the Mg2+

permeation

through the cation exchange membrane becomes easier and the membrane resistance is

0 10 20 30 40 50 600

50

100

150

200

250

300

350

400

Co

nd

uctivity (s/c

m)

T (min)

M-0

M-1

M-2

M-3

M-4

M-5

Chapter 7

112

reduced. The variation of resistance with ZIF-8 content on the membrane surface was

large. Since incorporation of ZIF-8 would increase the water uptake, in this regard, a

further enhancement of membrane conductivity is obtained. However, with the increase

of ZIF-8 loading, the membrane resistances continue to increase. In this condition, the

steric hindrance effect of ZIF-8 and the enhanced crosslinking play a much more

important role, rather than the electrostatic effect. In contrast, while the ZIF-8 loading

reached 0.8%, a higher filler concentration caused unselective voids with a significant

drop in rejection. As a result, the membrane resistance in MgCl2 solution drops again.

Fig. 7.7 ZIF-8 effect on EIS of membrane for NaCl and MgCl2 solutions


The electrochemical behavior of the modified membranes was investigated by

electrodialysis using single NaCl and MgCl2 systems. During the NaCl desalination

experiment, 2 g/L NaCl solutions were used as diluate and concentrate compartment,

respectively; while they were replaced by 2 g/L MgCl2 solutions during the MgCl2

purification experiment. A Na2SO4 solution with a concentration of 20 g/L was used as

the electrode rinsing solution while the current density was maintained at 15.3 mA

cm−2

. The conductivity of the diluate compartment decreased with the purification time

for both systems (Fig. 7.8). No obvious variation on desalination performance can be

observed after surface modification and the incorporation of nanoparticles. For ED,

constant-current or constant-voltage can be applied as operating mode. In previous

studies using a constant-voltage strategy, the conductivity of the diluate compartment

Chapter 7

113

continually decreased. The elevated system resistance, and thus lower current density,

retards the transfer of ions through the ion exchange membrane. Since a

constant-current system could maintain a stable current, the effect of a variation of the

current density on the concentration change of the diluate compartment would be

expected to be minimized. To obtain a better understanding of this specific process, the

concentrations of Na+ and Mg

2+ at 15 min were considered to calculate the cation flux,

due to the fact that the voltage of the system would exceed the maximum voltage of the

power supply after 15 min. It can be seen from Fig. 7.8c that a reduction of the Na+ and

Mg2+

ion flux is obtained after surface modification. Conversely, a higher Na+ transport

and a lower Mg2+

transport were found after introducing the ZIF-8 nanoparticles.

Generally, the increased IEC could facilitate the transport of Na+ and Mg

2+, however,

ZIF-8 under the PA surface layer could hinder the Mg2+

transport. The results were

different from previous results, which were carried out by incorporating Mil53-(Al)

nanoparticles in Chapter 6. The free diameter of Mil53-(Al) is close to 0.85 nm, which

makes it easier to transfer Na+ and Mg

2+ than ZIF-8. With more ZIF-8 incorporated into

the membrane matrix, both the Na+ and Mg

2+ transport increased, which can be

explained by the higher IEC and the formation of unselective voids. When compared

with the membrane with Mil53-(Al), the interfacial polymerization with ZIF-8 method

provides an enhanced IEC and diffusion ability with a lower water uptake. Furthermore,

a higher electro-resistance to Mg2+

ions could impose the membrane with the

possibility to separate monovalent ions. The energy consumption that was required

during the ED process was also considered (Fig. 7.8d). The energy consumption was

found to decrease with increasing ZIF-8 loadings, which confirmed the facilitated

migration of cations.

Chapter 7

114

Fig. 7.8 a. Conductivity change of diluate compartment for NaCl system; b.

Conductivity change of diluate compartment for MgCl2 system; c. Flux of ions in 15 min

and d. Energy consumption for different systems.

7.2.6 Monovalent selectivity

For the separation of monovalent and divalent cations, a comparison of monovalent and

multivalent cations fluxes is presented in Fig. 7.9. In the binary mixtures, MPD/TMC

modified membranes show a much lower Na+ and Mg

2+ flux than the untreated

membrane, demonstrating the improvement of the steric hindrance effect.

Simultaneously, the monovalent selectivity of the modified membrane increased from

1.77 to 3.66. Particularly, with incorporating ZIF-8 nanoparticles, the monovalent

selectivity notably increased to 4.03. However, the flux of Na+ reduces, which is

different from the observations in single salt desalination. The larger affinity of Mg2+

for the ion exchange groups inside the membrane matrix would allow them to occupy

more ions exchange transfer sites; as a consequence, a strong suppression was imposed

on the transfer of Na+ ions. Furthermore, the more hydrophobic membrane surface

could reduce the permeation of strongly hydrated cations, while facilitating the less

hydrated ones (Sata, 2000). Combining the contribution of the dense MPD/TMC layer

as well as the size sieving effect of ZIF-8, the monovalent selectivity was greatly

enhanced. Further increasing the ZIF-8 content has no obvious effect on the

Chapter 7

115

monovalent selectivity; however, in this case, both the Na+ and Mg

2+ permeance

increased. At this condition, the increased amount of ZIF-8 reduced the thickness of the

dense surface, which would also contribute to the increase of the ion flux. For M-5,

although the transfer resistance was further decreased, the voids facilitated both the Na+

and Mg2+

migration from the membrane matrix to the solution. In this case, a lower

monovalent selectivity was obtained.

Fig. 7.9 The ion flux and monovalent selectivity of Na+/Mg

2+ system during ED

The results clearly demonstrated that porous structure of the surface skin layer

decreased the transfer resistance of ions and improved the flux of monovalent ions.

The dense layer could reject the divalent ions with larger Stokes radius effectively.

The selectivity of the modified membrane is similar to commercial CSO

monovalent ion exchange membranes, however, the flux is restricted due to the

dense polyamide layer (Fig. 7.10).

Mg2+

Flux

Na+ flux

0

1

2

3

4

5

6

7

8

9

10

0

1

2

3

4

5

6

7

8

9

10

M-5M-4M-3M-2M-1M-0

Monovalent

selectivity

Flu

x 1

08 (

mo

lcm

-2s

-1)

Pe

rm-s

ele

ctiv

ity

Chapter 7

116

Fig. 7.10 The ion flux and perm-selectivity of Na+/Mg

2+ system (Ge et al., 2016a)

7.3 Conclusions

ZIF-8 was successfully anchored under the skin layer of commercial ion exchange

membranes by interfacial polymerization. The monovalent selectivity of the modified

membrane increased from 1.77 to 4.03, which is more than a twofold increase.

Furthermore, during the separation process, the Na+ flux maintained a similar level

compared to the primary membrane, while the Mg2+

flux was significantly reduced.

When single salt solution purification experiments were conducted, the ZIF-8

incorporation could decrease the energy consumption by facilitating the ion transport.

Considering the increased monovalent selectivity and the enhanced Na+ flux,

introducing nanoparticles to the surface functional layer could be a promising way to

enhance the ion flux in monovalent selective ion exchange membrane applications.

Chapter 8

117

8. Conclusions and recommendations for further

research

8.1 General conclusions

Separation processes play an extraordinary role in modern industries (where they

comprise 40-70% of both capital and operational costs) due to their three primary

functions: concentration, fractionation, and purification. The ultimate goal of

separation technologies is to achieve the precise and rapid separation of different

molecules from aqueous solutions, organic solutions and gas mixtures. These features

are necessary and significant for modern industries, as well as for the environment in

view of resource saving and recovery, and sustainable development. Simultaneously,

the pursuit of advanced separation technologies is never-ending.

Membrane-based separation technology, emerging as a promising tool to fulfill

separations, has aroused huge interests in recent years. ED, a type of technology

which arranges ion-exchange membranes alternately in a direct current field, has been

widely used to demineralize, concentrate and/or convert salt-containing solutions.

Over the past decade, the development of IEMs has attracted a large amount of

research attention in fields of materials, preparation and applications, due to their

academic and industrial values. The properties of ion-exchange membranes are

determined by different parameters, such as the density of the polymer network, the

hydrophobic or hydrophilic character of the matrix polymer, the type and

concentration of the fixed charges in the polymer, and the morphology of the

membrane itself. The most desired properties of IEMs are as follows: high

permselectivity, low electrical resistance, good mechanical and form stability, high

chemical and thermal stability and low production costs. Many of today's available

membranes meet most of the required properties mentioned above, however, more

attention should be paid to the development of ion-exchange membranes with higher

Chapter 8

118

permselectivity, lower electrical resistance and better chemical and thermal stability at

lower cost. To date, most of the IEMs consist of polymeric backbones prepared by

either post-functionalization of pre-existing polymers or direct polymerization of

functionalized monomers. Particularly, with the rapid progress in nano-science, the

regulation and control of polymer structures allow for the formation of ionic channels,

which is a new development in this field.

In this thesis, new preparation methods and rationally designed nanomaterials are

explored to effectively fabricate porous ion exchange membranes and monovalent

selective ion exchange membrane, aiming at reducing the limitations of low

permselectivity, in-sufficient ion flux, and high electrical resistance. First, a dry-wet

phase-inversion strategy by combining immersion precipitation and dry-casting was

used to control the membrane porosity with the purpose of improving the physical and

electrochemical properties of ion-exchange membranes. In addition to control the

porosity to balance the membrane electrical resistance with the diffusion caused by

the concentration gradient, it was experimentally shown that the porosity can

influence the IEC and water uptake of the membrane and, thus, further affect the

resistance. As demonstrated by experimental data for desalination by electrodialysis, it

was found that a membrane dried at 60 °C for 1 h had the highest desalination

efficiency. This is mainly because porous membranes facilitate the transport of ions.

The membrane prepared with 1-h heating time had more steric hindrance, which can

decrease the diffusion of ions, so that a superior desalination efficiency can be

obtained. Furthermore, a membrane with higher density of functional groups was

found to have a higher desalination efficiency, because of the electrostatic effect of the

membrane. Remarkably, under optimal membrane preparation conditions, the

desalination efficiency reached 95%, and the current efficiency reached 100%. It was

concluded that the performance of a porous membrane with controllable porosity can

enhance the ED process with respect to energy efficiency and desalination efficiency.

New methods of fabricating membranes with pores such as immersion precipitation

and dry-casting are thought to be potential routes to decreasing the electrical

Chapter 8

119

resistance.

On the other hand, an efficient one-step chemical process to graft a thin PANI layer on

the surface and pores of an UF membrane is reported. During the desalting procedure,

the driving force has two contributions, namely the electrical field and the

salinity-gradient. Initially, transport of ions by the electrical field is dominant, while at

the end of the experiment, diffusion dialysis by the salinity gradient plays a larger role

in ion transport. In single salt solutions, the presence of PANI can hinder Mg2+

transport from the concentrate to the diluate compartment because of the electrostatic

effect, while there is no obvious effect on Na+ ions. In the binary system, the flux of

Na+ slightly increased to 12.4×10

-8 mol·cm

–2·s

–1 while the flux of Mg

2+ reduced to

3.1×10-8

mol·cm–2

·s–1

, so that the permselectivity is almost doubled. It can be

concluded that this method is suitable for preparing membranes based on UF

membranes for efficient separation of monovalent ions by electro-driven separation

techniques.

Furthermore, a simple method is proposed for fabricating monovalent selective ion

exchange membranes based on the rapid co-deposition of biomimetic adhesive

polydopamine and poly(ethylene imine) by using CuSO4/H2O2 as a trigger. Through

this strategy, the surface properties and the permselectivity of the membranes can be

easily tailored by the addition of PEI and by tuning the PEI molecular weight. Surface

characterization revealed that overall enhanced surface properties including low

roughness, favorable hydrophilicity, and enhanced positive charge can be achieved

after the addition of PEI. The optimum membranes, with 4 h co-deposition of 60 mg

PDA and 120 mg PEI in 50 mL Tris buffer solution, permselectivity of

SPES-PDA/PEI-2 was 2.5 times higher than that of the SPES membrane, especially,

the flux of H+ was enhanced. In addition, the PDA/PEI modified ion exchange

membrane shows an excellent operational stability for monovalent separation

performance after immersion in acid and alkaline solution for 7 days. Comparing

membranes prepared with different molecular weights of PEI, results revealed that

Chapter 8

120

modification with a lower molecular weight PEI yields a higher selectivity. This facile

strategy may provide new opportunities not only to develop monovalent selective ion

exchange membranes but also to engineer the surface of numerous materials in energy

and environmental applications.

Similarly, taking advantage of the nanochannels of MIL(53)-Al, monovalent selective

membranes were prepared through rapid codeposition of PDA/PEI and Mil(53)-Al,

followed by cross-linking with TMC. The positive −NH2 allows to reject multivalent

cat ions, while porous Mil(53)-Al can accelerate the migration of Na+. The effects of

the embedded nanoparticles on the physicochemical properties of the prepared

membranes, and on the monovalent selective performance were investigated. A mass

ratio of 0.2–0.4% (w/v) for Mil(53)-Al is the optimum protocol, yielding a membrane

with a permselectivity of about 0.3 and an ion flux of about 22.0 and 0.6 mol·cm–2

·s–1

for Na+ and Mg

2+, respectively. At this condition, the PDA-coated membrane

maintains a high monovalent selectivity with enhanced Na+ and Mg

2+ flux in single

salt solutions.

In addition, a similar material ZIF-8 was used to replace MIL(53)-Al for fabricating

monovalent selective ion exchange membrane via interfacial polymerization. No

significant changes of the surface structure of the PA/ZIF-8 based membranes were

observed. Nevertheless, the presence of ZIF-8 under the PA layer plays a key role in

the separation process. For single salt solutions that were applied in electrodialysis

(ED), faster transport of Na+ and Mg

2+ was obtained after introducing the ZIF-8

nanoparticles, however, the desalination efficiency remained constant. When the

hybrid membranes were applied to electrodialysis for binary mixtures containing Na+

as well as Mg2+

, it was demonstrated that the monovalent selectivity and Na+ flux

were enhanced by a higher ZIF-8 loading. Considering the superior performance

derived from ZIF-8 hybrid surface layer, a promising future of ZIF-8 based

nanocomposites as a surface functional agent for versatile applications is anticipated.

Chapter 8

121

In conclusion, ED is an economic process using ion exchange membranes for

producing drinking water when the salinity of target water is below 5 g/L. In order to

generate high quality water to meet the requirements of specific industrial processes,

membranes with low resistance and/or selectivity for given ions are required.

Introducing the porosity to the membrane matrix and skin layer by suitable membrane

formation techniques or nanoparticles incorporation can be an efficient way to reduce

the resistance and enhance the flux of ions.

8.2 Recommendations for further research

In view of the experiments and conclusions obtained in this thesis, some

recommendations can be made for further research.

1. In Chapter 3, a dry-wet phase-inversion method was adapted to fabricate porous

ion exchange membranes with the purpose of improving the physical and

electrochemical properties of ion-exchange membranes. However, the pore size

and the size distribution of porous ion exchange membrane should also be

intensively evaluated. In addition, nanoparticles incorporation could be another

option to preciously control the porosity.

2. In Chapter 4, a monovalent selective ion exchange membrane was prepared based

on a UF membrane. If the UF membrane was replaced by a porous ion exchange

membrane with good properties, the results may be more promising. Such as the

membrane used in Chapter 3 or the membrane prepared with other methods to

control the ionic channels.

3. In Chapter 6 and Chapter 7, beyond the effect of the nanocomposite amount, the

size and the size distribution of the nanocomposite should also be intensively

evaluated as they could have a large influence on the resultant membrane.

Chapter 8

122

4. Except ZIF-8 and MIL(53)-Al, the potential of other nanoparticles such as MOFs

and carbon nanotubes should also be investigated. Two-dimensional (2D)

materials have emerged as nano-building blocks to develop high-performance

separation membranes that feature unique nanopores and/or nanochannels. Other

two-dimensional (2D) materials such as porous graphene, 2D MOFs,

molybdenum disulfide (MoS2), MXene and C3N4 should also be studied as

options for monovalent selective ion exchange membrane fabrication.

5. Nanoparticles with different pore size can be chosen to separate specific ions.

6. Since the membranes were prepared on lab scale, the membranes should be

investigated in view of industrial applications.

7. In this thesis, the binary mixture solution were H+/Zn

2+ or Na

+/Mg

2+ solutions.

However, real waste water is much more complicated. Therefore, the applicability

of monovalent selective ion exchange membranes in real industrial waste water

should be evaluated.

Through the invention of new methods, configurations and processes as well as by the

improvement of membrane performances, as presented here, it can be expected that

electrodialysis with novel ion exchange membrane will play a more important role in

waste water purification/reclamation.

References

123

References

Abdu, S., Martí-Calatayud, M.-C.s., Wong, J.E., García-Gabaldon, M., Wessling, M. (2014)

Layer-by-layer modification of cation exchange membranes controls ion selectivity and water splitting.

ACS Appl Mater Interfaces 6, 1843-1854.

Aider, M., Brunet, S., Bazinet, L. (2008) Electroseparation of chitosan oligomers by electrodialysis

with ultrafiltration membrane (EDUF) and impact on electrodialytic parameters. Journal of Membrane

Science 309, 222-232.

Aider, M., Brunet, S., Bazinet, L. (2009) Effect of solution flow velocity and electric field strength on

chitosan oligomer electromigration kinetics and their separation in an electrodialysis with ultrafiltration

membrane (EDUF) system. Separation and Purification Technology 69, 63-70.

Åkerman, S., Viinikka, P., Svarfvar, B., Järvinen, K., Kontturi, K., Näsman, J., Urtti, A., Paronen, P.

(1998) Transport of drugs across porous ion exchange membranes. Journal of controlled release 50,

153-166.

Al-Maythalony, B.A., Alloush, A.M., Faizan, M., Dafallah, H., Elgzoly, M.A., Seliman, A.A.,

Al-Ahmed, A., Yamani, Z.H., Habib, M.A., Cordova, K.E. (2017) Tuning the Interplay between

Selectivity and Permeability of ZIF-7 Mixed Matrix Membranes. ACS Appl Mater Interfaces.

Alvarez, F., Alvarez, R., Coca, J., Sandeaux, J., Sandeaux, R., Gavach, C. (1997) Salicylic acid

production by electrodialysis with bipolar membranes. Journal of Membrane Science 123, 61-69.

Amara, M., Kerdjoudj, H. (2003) Modification of cation-exchange membrane properties by

electro-adsorption of polyethyleneimine. Desalination 155, 79-87.

Amini, M., Jahanshahi, M., Rahimpour, A. (2013) Synthesis of novel thin film nanocomposite (TFN)

forward osmosis membranes using functionalized multi-walled carbon nanotubes. Journal of

Membrane Science 435, 233-241.

Asraf-Snir, M., Gilron, J., Oren, Y. (2016) Gypsum scaling of anion exchange membranes in

electrodialysis. Journal of Membrane Science 520, 176-186.

Balster, J., Yildirim, M., Stamatialis, D., Ibanez, R., Lammertink, R.G., Jordan, V., Wessling, M. (2007)

Morphology and microtopology of cation-exchange polymers and the origin of the overlimiting current.

The Journal of Physical Chemistry B 111, 2152-2165.

References

124

Barragan, V., Ruız-Bauzá, C. (1998) Current–voltage curves for ion-exchange membranes: a method

for determining the limiting current density. J Colloid Interface Sci 205, 365-373.

Bayramoğlu, G., Metin, A.Ü., Altıntas, B., Arıca, M.Y. (2010) Reversible immobilization of glucose

oxidase on polyaniline grafted polyacrylonitrile conductive composite membrane. Bioresour Technol

101, 6881-6887.

Bazinet, L., Moalic, M. (2011a) Coupling of porous filtration and ion-exchange membranes in an

electrodialysis stack and impact on cation selectivity: A novel approach for sea water demineralization

and the production of physiological water. Desalination 277, 356-363.

Bazinet, L., Moalic, M. (2011b) Coupling of porous filtration and ion-exchange membranes in an

electrodialysis stack and impact on cation selectivity: A novel approach for sea water demineralization

and the production of physiological water. Desalination 277, 356-363.

Belashova, E., Melnik, N., Pismenskaya, N., Shevtsova, K., Nebavsky, A., Lebedev, K., Nikonenko, V.

(2012) Overlimiting mass transfer through cation-exchange membranes modified by Nafion film and

carbon nanotubes. Electrochimica Acta 59, 412-423.

Boyaval, P., Seta, J., Gavach, C. (1993) Concentrated propionic acid production by electrodialysis.

Enzyme and microbial technology 15, 683-686.

Buonomenna, M., Macchi, P., Davoli, M., Drioli, E. (2007) Poly (vinylidene fluoride) membranes by

phase inversion: the role the casting and coagulation conditions play in their morphology, crystalline

structure and properties. European Polymer Journal 43, 1557-1572.

Bux, H., Chmelik, C., Krishna, R., Caro, J. (2011) Ethene/ethane separation by the MOF membrane

ZIF-8: molecular correlation of permeation, adsorption, diffusion. Journal of Membrane Science 369,

284-289.

Chamoulaud, G., Bélanger, D. (2005) Modification of ion-exchange membrane used for separation of

protons and metallic cations and characterization of the membrane by current–voltage curves. J Colloid

Interface Sci 281, 179-187.

Chien, H.-W., Kuo, W.-H., Wang, M.-J., Tsai, S.-W., Tsai, W.-B. (2012) Tunable micropatterned

substrates based on poly (dopamine) deposition via microcontact printing. Langmuir 28, 5775-5782.

Chlanda, F.P., Lee, L.T., Liu, K.-J., (1978) Bipolar membranes and method of making same. Google

Patents.

Choi, P., Jalani, N.H., Datta, R. (2005) Thermodynamics and proton transport in Nafion I. Membrane

References

125

swelling, sorption, and ion-exchange equilibrium. Journal of the Electrochemical Society 152,

E84-E89.

Choi, Y.-S., Kang, H., Kim, D.-G., Cha, S.-H., Lee, J.-C. (2014) Mussel-inspired dopamine-and

plant-based cardanol-containing polymer coatings for multifunctional filtration membranes. ACS Appl

Mater Interfaces 6, 21297-21307.

Chung, T.-S., Jiang, L.Y., Li, Y., Kulprathipanja, S. (2007) Mixed matrix membranes (MMMs)

comprising organic polymers with dispersed inorganic fillers for gas separation. Progress in Polymer

Science 32, 483-507.

Coster, H.G., Chilcott, T.C., Coster, A.C. (1996) Impedance spectroscopy of interfaces, membranes and

ultrastructures. Bioelectrochemistry and Bioenergetics 40, 79-98.

Długołęcki, P., Anet, B., Metz, S.J., Nijmeijer, K., Wessling, M. (2010a) Transport limitations in ion

exchange membranes at low salt concentrations. Journal of Membrane Science 346, 163-171.

Długołęcki, P., Ogonowski, P., Metz, S.J., Saakes, M., Nijmeijer, K., Wessling, M. (2010b) On the

resistances of membrane, diffusion boundary layer and double layer in ion exchange membrane

transport. Journal of Membrane Science 349, 369-379.

Donnan, F.G. (1911) Theorie der Membrangleichgewichte und Membranpotentiale bei Vorhandensein

von nicht dialysierenden Elektrolyten. Ein Beitrag zur physikalisch‐chemischen Physiologie. Berichte

der Bunsengesellschaft für physikalische Chemie 17, 572-581.

Doyen, A., Beaulieu, L., Saucier, L., Pouliot, Y., Bazinet, L. (2011a) Demonstration of in vitro

anticancer properties of peptide fractions from a snow crab by-products hydrolysate after separation by

electrodialysis with ultrafiltration membranes. Separation and purification technology 78, 321-329.

Doyen, A., Beaulieu, L., Saucier, L., Pouliot, Y., Bazinet, L. (2011b) Impact of ultrafiltration membrane

material on peptide separation from a snow crab byproduct hydrolysate by electrodialysis with

ultrafiltration membranes. Journal of agricultural and food chemistry 59, 1784-1792.

Doyen, A., Udenigwe, C.C., Mitchell, P.L., Marette, A., Aluko, R.E., Bazinet, L. (2014) Anti-diabetic

and antihypertensive activities of two flaxseed protein hydrolysate fractions revealed following their

simultaneous separation by electrodialysis with ultrafiltration membranes. Food chemistry 145, 66-76.

Duan, J., Pan, Y., Pacheco, F., Litwiller, E., Lai, Z., Pinnau, I. (2015) High-performance polyamide

thin-film-nanocomposite reverse osmosis membranes containing hydrophobic zeolitic imidazolate

framework-8. Journal of Membrane Science 476, 303-310.

References

126

Elimelech, M., Phillip, W.A. (2011) The future of seawater desalination: energy, technology, and the

environment. Science 333, 712-717.

Eliseeva, T., Shaposhnik, V., Krisilova, E., Bukhovets, A. (2009) Transport of basic amino acids

through the ion-exchange membranes and their recovery by electrodialysis. Desalination 241, 86-90.

Eliseeva, T., Tekuchev, A.Y., Shaposhnik, V., Lushchik, I. (2001) Electrodialysis of amino acid

solutions with bipolar ion-exchange membranes. Russian Journal of Electrochemistry 37, 423-426.

Erbil, H.Y., Demirel, A.L., Avcı, Y., Mert, O. (2003) Transformation of a simple plastic into a

superhydrophobic surface. Science 299, 1377-1380.

Farrokhzad, H., Moghbeli, M., Van Gerven, T., Van der Bruggen, B. (2015) Surface modification of

composite ion exchange membranes by polyaniline. Reactive and Functional Polymers 86, 161-167.

Ferrer, J.J., Laborie, S., Durand, G., Rakib, M. (2006) Formic acid regeneration by electromembrane

processes. Journal of Membrane Science 280, 509-516.

Fiksdal, L., Leiknes, T. (2006) The effect of coagulation with MF/UF membrane filtration for the

removal of virus in drinking water. Journal of Membrane Science 279, 364-371.

Firdaous, L., Dhulster, P., Amiot, J., Gaudreau, A., Lecouturier, D., Kapel, R., Lutin, F., Vézina, L.-P.,

Bazinet, L. (2009) Concentration and selective separation of bioactive peptides from an alfalfa white

protein hydrolysate by electrodialysis with ultrafiltration membranes. Journal of membrane science 329,

60-67.

Firdaous, L., Malériat, J.P., Schlumpf, J.P., Quéméneur, F. (2007) Transfer of monovalent and divalent

cations in salt solutions by electrodialysis. Separation Science and Technology 42, 931-948.

Fu, X., Wang, S., Xia, Z., Li, Y., Jiang, L., Sun, G. (2016) Aligned polyaniline nanorods in situ grown

on gas diffusion layer and their application in polymer electrolyte membrane fuel cells. International

Journal of Hydrogen Energy 41, 3655-3663.

Güler, E., van Baak, W., Saakes, M., Nijmeijer, K. (2014) Monovalent-ion-selective membranes for

reverse electrodialysis. Journal of Membrane Science 455, 254-270.

Gao, H., Sun, Y., Zhou, J., Xu, R., Duan, H. (2013) Mussel-inspired synthesis of

polydopamine-functionalized graphene hydrogel as reusable adsorbents for water purification. ACS

Appl Mater Interfaces 5, 425-432.

Ge, L., Liu, X., Wang, G., Wu, B., Wu, L., Bakangura, E., Xu, T. (2015) Preparation of proton selective

membranes through constructing H+ transfer channels by acid–base pairs. Journal of Membrane

References

127

Science 475, 273-280.

Ge, L., Wu, B., Li, Q., Wang, Y., Yu, D., Wu, L., Pan, J., Miao, J., Xu, T. (2016a) Electrodialysis with

nanofiltration membrane (EDNF) for high-efficiency cations fractionation. Journal of Membrane

Science 498, 192-200.

Ge, L., Wu, B., Yu, D., Mondal, A.N., Hou, L., Afsar, N.U., Li, Q., Xu, T., Miao, J., Xu, T. (2017)

Monovalent cation perm-selective membranes (MCPMs): New developments and perspectives.

Chinese Journal of Chemical Engineering.

Ge, L., Wu, L., Wu, B., Wang, G., Xu, T. (2014) Preparation of monovalent cation selective membranes

through annealing treatment. Journal of Membrane Science 459, 217-222.

Ge, X., He, Y., Guiver, M.D., Wu, L., Ran, J., Yang, Z., Xu, T. (2016b) Alkaline Anion‐Exchange

Membranes Containing Mobile Ion Shuttles. Adv Mater 28, 3467-3472.

Ghaffour, N., Missimer, T.M., Amy, G.L. (2013) Technical review and evaluation of the economics of

water desalination: current and future challenges for better water supply sustainability. Desalination

309, 197-207.

Gohil, G., Binsu, V., Shahi, V.K. (2006a) Preparation and characterization of mono-valent ion selective

polypyrrole composite ion-exchange membranes. Journal of Membrane Science 280, 210-218.

Gohil, G., Nagarale, R., Binsu, V., Shahi, V.K. (2006b) Preparation and characterization of monovalent

cation selective sulfonated poly (ether ether ketone) and poly (ether sulfone) composite membranes. J

Colloid Interface Sci 298, 845-853.

Gohil, G., Shahi, V.K., Rangarajan, R. (2004) Comparative studies on electrochemical characterization

of homogeneous and heterogeneous type of ion-exchange membranes. Journal of Membrane Science

240, 211-219.

Grot, W., (1973) Laminates of support material and fluorinated polymer containing pendant side chains

containing sulfonyl groups. Google Patents.

Guesmi, F., Hannachi, C., Hamrouni, B. (2012) Selectivity of anion exchange membrane modified with

polyethyleneimine. Ionics 18, 711-717.

Guillen, G.R., Pan, Y., Li, M., Hoek, E.M. (2011) Preparation and characterization of membranes

formed by nonsolvent induced phase separation: a review. Industrial & Engineering Chemistry

Research 50, 3798-3817.

Guo, H., Ma, Y., Qin, Z., Gu, Z., Cui, S., Zhang, G. (2016) One-Step Transformation from

References

128

Hierarchical-Structured Superhydrophilic NF Membrane into Superhydrophobic OSN Membrane with

Improved Antifouling Effect. ACS Appl Mater Interfaces 8, 23379-23388.

Guo, W., Ngo, H.-H., Li, J. (2012) A mini-review on membrane fouling. Bioresour Technol 122, 27-34.

Gutiérrez, L.-F., Bazinet, L., Hamoudi, S., Belkacemi, K. (2013) Production of lactobionic acid by

means of a process comprising the catalytic oxidation of lactose and bipolar membrane electrodialysis.

Separation and Purification Technology 109, 23-32.

Han, G., Zhang, S., Li, X., Widjojo, N., Chung, T.-S. (2012) Thin film composite forward osmosis

membranes based on polydopamine modified polysulfone substrates with enhancements in both water

flux and salt rejection. Chemical Engineering Science 80, 219-231.

Han, L., Galier, S., Roux-de Balmann, H. (2015) Ion hydration number and electro-osmosis during

electrodialysis of mixed salt solution. Desalination 373, 38-46.

Horowitz, S.T., Roseman, S., Blumenthal, H.J. (1957) The Preparation of glucosamine

oligosaccharides. I. Separation1, 2. J Am Chem Soc 79, 5046-5049.

Hou, L., Wu, B., Yu, D., Wang, S., Shehzad, M.A., Fu, R., Liu, Z., Li, Q., He, Y., Afsar, N.U. (2018)

Asymmetric porous monovalent cation perm-selective membranes with an ultrathin polyamide

selective layer for cations separation. Journal of Membrane Science 557, 49-57.

Hou, X. (2016) Smart gating multi ‐ scale pore/channel‐ based membranes. Adv Mater 28,

7049-7064.

Hu, M., Mi, B. (2013) Enabling graphene oxide nanosheets as water separation membranes. Environ

Sci Technol 47, 3715-3723.

Hu, Y., Wang, M., Wang, D., Gao, X., Gao, C. (2008) Feasibility study on surface modification of

cation exchange membranes by quaternized chitosan for improving its selectivity. Journal of Membrane

Science 319, 5-9.

Huang, K., Liu, S., Li, Q., Jin, W. (2013) Preparation of novel metal-carboxylate system MOF

membrane for gas separation. Separation and Purification Technology 119, 94-101.

Huang, N., Zhang, S., Yang, L., Liu, M., Li, H., Zhang, Y., Yao, S. (2015) Multifunctional

electrochemical platforms based on the michael addition/schiff base reaction of polydopamine

modified reduced graphene oxide: Construction and application. ACS Appl Mater Interfaces 7,

17935-17946.

Hwang, G.-J., Ohya, H., Nagai, T. (1999) Ion exchange membrane based on block copolymers. Part III:

References

129

preparation of cation exchange membrane. Journal of Membrane Science 156, 61-65.

Jansen, J.C., Macchione, M., Oliviero, C., Mendichi, R., Ranieri, G.A., Drioli, E. (2005) Rheological

evaluation of the influence of polymer concentration and molar mass distribution on the formation and

performance of asymmetric gas separation membranes prepared by dry phase inversion. Polymer 46,

11366-11379.

Jeon, B.H., Kim, S., Choi, M.H., Chung, I.J. (1999) Synthesis and characterization of polyaniline–

polycarbonate composites prepared by an emulsion polymerization. Synthetic Metals 104, 95-100.

Jeong, B.-H., Hoek, E.M., Yan, Y., Subramani, A., Huang, X., Hurwitz, G., Ghosh, A.K., Jawor, A.

(2007) Interfacial polymerization of thin film nanocomposites: a new concept for reverse osmosis

membranes. Journal of Membrane Science 294, 1-7.

Ji, L., Zhang, X. (2008) Ultrafine polyacrylonitrile/silica composite fibers via electrospinning.

Materials Letters 62, 2161-2164.

Jiang, J., Zhu, L., Zhu, L., Zhang, H., Zhu, B., Xu, Y. (2013) Antifouling and antimicrobial polymer

membranes based on bioinspired polydopamine and strong hydrogen-bonded poly (N-vinyl

pyrrolidone). ACS Appl Mater Interfaces 5, 12895-12904.

Jiang, J., Zhu, L., Zhu, L., Zhu, B., Xu, Y. (2011) Surface characteristics of a self-polymerized

dopamine coating deposited on hydrophobic polymer films. Langmuir 27, 14180-14187.

Johnson, R.J., Stenvinkel, P., Jensen, T., Lanaspa, M.A., Roncal, C., Song, Z., Bankir, L.,

Sánchez-Lozada, L.G. (2016) Metabolic and kidney diseases in the setting of climate change, water

shortage, and survival factors. Journal of the American Society of Nephrology 27, 2247-2256.

Kang, G.-d., Cao, Y.-m. (2012) Development of antifouling reverse osmosis membranes for water

treatment: a review. Water Res 46, 584-600.

Kang, G.-d., Cao, Y.-m. (2014) Application and modification of poly (vinylidene fluoride)(PVDF)

membranes–A review. Journal of Membrane Science 463, 145-165.

Kang, K.C., Linga, P., Park, K.-n., Choi, S.-J., Lee, J.D. (2014) Seawater desalination by gas hydrate

process and removal characteristics of dissolved ions (Na+, K+, Mg 2+, Ca 2+, B 3+, Cl−, SO 4 2−).

Desalination 353, 84-90.

Khan, M.I., Luque, R., Akhtar, S., Shaheen, A., Mehmood, A., Idress, S., Buzdar, S.A. (2016) Design

of anion exchange membranes and electrodialysis studies for water desalination. Materials 9, 365.

Khawaji, A.D., Kutubkhanah, I.K., Wie, J.-M. (2008) Advances in seawater desalination technologies.

References

130


Kim, D.-H., Park, J.-S., Choun, M., Lee, J., Kang, M.-S. (2016) Pore-filled anion-exchange membranes

for electrochemical energy conversion applications. Electrochimica Acta 222, 212-220.

Kim, K.-Y., Yang, E., Lee, M.-Y., Chae, K.-J., Kim, C.-M., Kim, I.S. (2014) Polydopamine coating

effects on ultrafiltration membrane to enhance power density and mitigate biofouling of ultrafiltration

microbial fuel cells (UF-MFCs). Water Res 54, 62-68.

Kitagawa, S. (2014) Metal–organic frameworks (MOFs). Chem Soc Rev 43, 5415-5418.

Kotov, V., Peregonchaya, O., Selemenev, V. (2002) Electrodialysis of binary electrolyte mixtures with

membranes modified by organic species. Russian Journal of Electrochemistry 38, 927-929.

Krol, J., Wessling, M., Strathmann, H. (1999) Concentration polarization with monopolar ion exchange

membranes: current–voltage curves and water dissociation. Journal of Membrane Science 162,

145-154.

Kumar, M., Khan, M.A., AlOthman, Z.A., Siddiqui, M.R. (2013) Polyaniline modified organic–

inorganic hybrid cation-exchange membranes for the separation of monovalent and multivalent ions.


Kuppler, R.J., Timmons, D.J., Fang, Q.-R., Li, J.-R., Makal, T.A., Young, M.D., Yuan, D., Zhao, D.,

Zhuang, W., Zhou, H.-C. (2009) Potential applications of metal-organic frameworks. Coordination

Chemistry Reviews 253, 3042-3066.

Lattemann, S., Höpner, T. (2008) Environmental impact and impact assessment of seawater

desalination. Desalination 220, 1-15.

Lau, W., Gray, S., Matsuura, T., Emadzadeh, D., Chen, J.P., Ismail, A. (2015) A review on polyamide

thin film nanocomposite (TFN) membranes: history, applications, challenges and approaches. Water

Res 80, 306-324.

Lee, E.G., Moon, S.-H., Chang, Y.K., Yoo, I.-K., Chang, H.N. (1998) Lactic acid recovery using

two-stage electrodialysis and its modelling. Journal of Membrane Science 145, 53-66.

Lee, H., Han, T., Cho, K.Y., Ryou, M.-H., Lee, Y.M. (2016a) Dopamine as a novel electrolyte additive

for high-voltage lithium-ion batteries. ACS Appl Mater Interfaces 8, 21366-21372.

Lee, J.-H., Lee, J.-Y., Kim, J.-H., Joo, J., Maurya, S., Choun, M., Lee, J., Moon, S.-H. (2016b) SPPO

pore-filled composite membranes with electrically aligned ion channels via a lab-scale continuous

caster for fuel cells: An optimal DC electric field strength-IEC relationship. Journal of Membrane

References

131

Science 501, 15-23.

Lee, M., Kang, H., Jeon, J., Choi, Y., Yoon, Y. (2016c) A novel amphoteric ion-exchange membrane

prepared by the pore-filling technique for vanadium redox flow batteries. Rsc Advances 6,

63023-63029.

Lee, M.S., Kim, H.K., Kim, C.S., Suh, H.Y., Nahm, K.S., Choi, Y.W. (2017) Thin Pore‐Filled Ion

Exchange Membranes for High Power Density in Reverse Electrodialysis: Effects of Structure on

Resistance, Stability, and Ion Selectivity. ChemistrySelect 2, 1974-1978.

Li, M., Xu, J., Chang, C.-Y., Feng, C., Zhang, L., Tang, Y., Gao, C. (2014a) Bioinspired fabrication of

composite nanofiltration membrane based on the formation of DA/PEI layer followed by cross-linking.

Journal of Membrane Science 459, 62-71.

Li, N., Guiver, M.D. (2014) Ion transport by nanochannels in ion-containing aromatic copolymers.

Macromolecules 47, 2175-2198.

Li, W., Meng, Q., Li, X., Zhang, C., Fan, Z., Zhang, G. (2014b) Non-activation ZnO array as a

buffering layer to fabricate strongly adhesive metal–organic framework/PVDF hollow fiber membranes.

Chem Commun (Camb) 50, 9711-9713.

Li, Y., Su, Y., Li, J., Zhao, X., Zhang, R., Fan, X., Zhu, J., Ma, Y., Liu, Y., Jiang, Z. (2015) Preparation

of thin film composite nanofiltration membrane with improved structural stability through the

mediation of polydopamine. Journal of Membrane Science 476, 10-19.

Li, Y., Su, Y., Zhao, X., He, X., Zhang, R., Zhao, J., Fan, X., Jiang, Z. (2014c) Antifouling, high-flux

nanofiltration membranes enabled by dual functional polydopamine. ACS Appl Mater Interfaces 6,

5548-5557.

Li, Z., Wang, W., Chen, Y., Xiong, C., He, G., Cao, Y., Wu, H., Guiver, M.D., Jiang, Z. (2016)

Constructing efficient ion nanochannels in alkaline anion exchange membranes by the in situ assembly

of a poly (ionic liquid) in metal–organic frameworks. Journal of Materials Chemistry A 4, 2340-2348.

Lin, X., Kim, S., Zhu, D.M., Shamsaei, E., Xu, T., Fang, X., Wang, H. (2017) Preparation of porous

diffusion dialysis membranes by functionalization of polysulfone for acid recovery. Journal of


Lin, X., Shamsaei, E., Kong, B., Liu, J.Z., Zhao, D., Xu, T., Xie, Z., Easton, C.D., Wang, H. (2016)

Asymmetrically porous anion exchange membranes with an ultrathin selective layer for rapid acid

recovery. Journal of Membrane Science 510, 437-446.

References

132

Lind, M.L., Ghosh, A.K., Jawor, A., Huang, X., Hou, W., Yang, Y., Hoek, E.M. (2009) Influence of

zeolite crystal size on zeolite-polyamide thin film nanocomposite membranes. Langmuir 25,

10139-10145.

Liu, G., Luo, H., Wang, H., Wang, B., Zhang, R., Chen, S. (2014a) Malic acid production using a

biological electrodialysis with bipolar membrane. Journal of Membrane Science 471, 179-184.

Liu, H., Ruan, H., Zhao, Y., Pan, J., Sotto, A., Gao, C., Van der Bruggen, B., Shen, J. (2017a) A facile

avenue to modify polyelectrolyte multilayers on anion exchange membranes to enhance monovalent

selectivity and durability simultaneously. Journal of Membrane Science 543, 310-318.

Liu, J., Yang, H., Gosling, S.N., Kummu, M., Flörke, M., Pfister, S., Hanasaki, N., Wada, Y., Zhang, X.,

Zheng, C. (2017b) Water scarcity assessments in the past, present, and future. Earth's Future 5,

545-559.

Liu, X., Li, Q., Jiang, C., Lin, X., Xu, T. (2015) Bipolar membrane electrodialysis in aqua–ethanol

medium: Production of salicylic acid. Journal of Membrane Science 482, 76-82.

Liu, Y., Meng, H., Konst, S., Sarmiento, R., Rajachar, R., Lee, B.P. (2014b) Injectable

dopamine-modified poly (ethylene glycol) nanocomposite hydrogel with enhanced adhesive property

and bioactivity. ACS Appl Mater Interfaces 6, 16982-16992.

Liu, Z., Wang, W., Xie, R., Ju, X.-J., Chu, L.-Y. (2016) Stimuli-responsive smart gating membranes.

Chem Soc Rev 45, 460-475.

Luiz, A., McClure, D.D., Lim, K., Leslie, G., Coster, H.G., Barton, G.W., Kavanagh, J.M. (2017)

Potential upgrading of bio-refinery streams by electrodialysis. Desalination 415, 20-28.

Luiz, A., Spencer, E., McClure, D.D., Coster, H.G., Barton, G.W., Kavanagh, J.M. (2018) Membrane

selection for the desalination of bio-refinery effluents using electrodialysis. Desalination 428, 1-11.

Luo, G., Pan, S., Liu, J. (2002) Use of the electrodialysis process to concentrate a formic acid solution.


Luo, Q., Zhang, H., Chen, J., Qian, P., Zhai, Y. (2008) Modification of Nafion membrane using

interfacial polymerization for vanadium redox flow battery applications. Journal of Membrane Science

311, 98-103.

Luo, R., Tang, L., Zhong, S., Yang, Z., Wang, J., Weng, Y., Tu, Q., Jiang, C., Huang, N. (2013) In vitro

investigation of enhanced hemocompatibility and endothelial cell proliferation associated with

quinone-rich polydopamine coating. ACS Appl Mater Interfaces 5, 1704-1714.

References

133

Luo, T., Abdu, S., Wessling, M. (2018) Selectivity of ion exchange membranes: A review. Journal of

Membrane Science.

Lv, Y., Yang, H.-C., Liang, H.-Q., Wan, L.-S., Xu, Z.-K. (2015) Nanofiltration membranes via

co-deposition of polydopamine/polyethylenimine followed by cross-linking. Journal of Membrane

Science 476, 50-58.

Lv, Y., Yang, H.-C., Liang, H.-Q., Wan, L.-S., Xu, Z.-K. (2016) Novel nanofiltration membrane with

ultrathin zirconia film as selective layer. Journal of Membrane Science 500, 265-271.

Mafé, S., Manzanares, J.A., Ramirez, P. (2003) Modeling of surface vs. bulk ionic conductivity in fixed

charge membranes. Physical Chemistry Chemical Physics 5, 376-383.

Malik, M.S., Qaiser, A.A., Arif, M.A. (2016) Structural and electrochemical studies of heterogeneous

ion exchange membranes based on polyaniline-coated cation exchange resin particles. Rsc Advances 6,

115046-115054.

Mei, Y., Tang, C.Y. (2018) Recent developments and future perspectives of reverse electrodialysis

technology: A review. Desalination 425, 156-174.

Mikhaylin, S., Bazinet, L. (2016) Fouling on ion-exchange membranes: classification, characterization

and strategies of prevention and control. Adv Colloid Interface Sci 229, 34-56.

Mistry, M.K., Choudhury, N.R., Dutta, N.K., Knott, R., Shi, Z., Holdcroft, S. (2008) Novel organic−

inorganic hybrids with increased water retention for elevated temperature proton exchange membrane

application. Chemistry of Materials 20, 6857-6870.

Mo, H., Tay, K.G., Ng, H.Y. (2008) Fouling of reverse osmosis membrane by protein (BSA): effects of

pH, calcium, magnesium, ionic strength and temperature. Journal of Membrane Science 315, 28-35.

Mukherjee, R., Sharma, R., Saini, P., De, S. (2015) Nanostructured polyaniline incorporated

ultrafiltration membrane for desalination of brackish water. Environmental Science: Water Research &

Technology 1, 893-904.

Mulyati, S., Takagi, R., Fujii, A., Ohmukai, Y., Matsuyama, H. (2013) Simultaneous improvement of

the monovalent anion selectivity and antifouling properties of an anion exchange membrane in an

electrodialysis process, using polyelectrolyte multilayer deposition. Journal of Membrane Science 431,

113-120.

Muthumeenal, A., Neelakandan, S., Rana, D., Matsuura, T., Kanagaraj, P., Nagendran, A. (2014)

Sulfonated polyethersulfone (spes)–charged surface modifying macromolecules (csmms) blends as a

References

134

cation selective membrane for fuel cells. Fuel cells 14, 853-861.

Nabe, A., Staude, E., Belfort, G. (1997) Surface modification of polysulfone ultrafiltration membranes

and fouling by BSA solutions. Journal of Membrane Science 133, 57-72.

Nagarale, R., Gohil, G., Shahi, V.K. (2006) Recent developments on ion-exchange membranes and

electro-membrane processes. Adv Colloid Interface Sci 119, 97-130.

Nagarale, R., Gohil, G., Shahi, V.K., Trivedi, G., Rangarajan, R. (2004) Preparation and

electrochemical characterization of cation-and anion-exchange/polyaniline composite membranes.

Journal of colloid and interface science 277, 162-171.

Nemati, M., Hosseini, S., Bagheripour, E., Madaeni, S. (2015) Electrodialysis Heterogeneous Anion

Exchange Membranes Filled with TiO2 Nanoparticles: Membranes' Fabrication and Characterization.

Journal of Membrane Science and Research 1, 135-140.

Nikonenko, V.V., Kozmai, A.E. (2011) Electrical equivalent circuit of an ion-exchange membrane

system. Electrochimica Acta 56, 1262-1269.

Ouyang, Q., Cheng, L., Wang, H., Li, K. (2008) Mechanism and kinetics of the stabilization reactions

of itaconic acid-modified polyacrylonitrile. Polymer Degradation and Stability 93, 1415-1421.

Paidar, M., Fateev, V., Bouzek, K. (2016) Membrane electrolysis—History, current status and

perspective. Electrochimica Acta 209, 737-756.

Pan, W., Yang, S.L., Li, G., Jiang, J.M. (2005) Electrical and structural analysis of conductive

polyaniline/polyacrylonitrile composites. European Polymer Journal 41, 2127-2133.

Park, J.-S., Choi, J.-H., Woo, J.-J., Moon, S.-H. (2006) An electrical impedance spectroscopic (EIS)

study on transport characteristics of ion-exchange membrane systems. J Colloid Interface Sci 300,

655-662.

Pashley, R., Rzechowicz, M., Pashley, L., Francis, M. (2005) De-gassed water is a better cleaning agent.

The Journal of Physical Chemistry B 109, 1231-1238.

Pendergast, M.M., Hoek, E.M. (2011) A review of water treatment membrane nanotechnologies.

Energy & Environmental Science 4, 1946-1971.

Peyravi, M., Jahanshahi, M., Rahimpour, A., Javadi, A., Hajavi, S. (2014) Novel thin film

nanocomposite membranes incorporated with functionalized TiO 2 nanoparticles for organic solvent

nanofiltration. Chemical Engineering Journal 241, 155-166.

Pismenskaya, N.D., Nikonenko, V.V., Melnik, N.A., Shevtsova, K.A., Belova, E.I., Pourcelly, G., Cot,

References

135

D., Dammak, L., Larchet, C. (2012) Evolution with time of hydrophobicity and microrelief of a

cation-exchange membrane surface and its impact on overlimiting mass transfer. The Journal of

Physical Chemistry B 116, 2145-2161.

Poyton, M.F., Sendecki, A.M., Cong, X., Cremer, P.S. (2016) Cu2+ binds to phosphatidylethanolamine

and increases oxidation in lipid membranes. J Am Chem Soc 138, 1584-1590.

Ramsahye, N.A., Maurin, G., Bourrelly, S., Llewellyn, P.L., Loiseau, T., Serre, C., Férey, G. (2007) On

the breathing effect of a metal–organic framework upon CO 2 adsorption: monte carlo compared to

microcalorimetry experiments. Chem Commun (Camb), 3261-3263.

Ran, J., Wu, L., He, Y., Yang, Z., Wang, Y., Jiang, C., Ge, L., Bakangura, E., Xu, T. (2017) Ion

exchange membranes: New developments and applications. Journal of Membrane Science 522,

267-291.

Reig, M., Vecino, X., Valderrama, C., Gibert, O., Cortina, J. (2017) Application of selectrodialysis for

the removal of As from metallurgical process waters: recovery of Cu and Zn. Separation and

Purification Technology.

Rodzik, I. (2005) Selectivity of Transport of Sodium, Magnesium, and Calcium Ions through a

Sulfo-Cationite Membrane in Mixtures of Solutions of Their Chlorides. Russian Journal of

Electrochemistry 41, 888-891.

Rubinstein, I., Staude, E., Kedem, O. (1988) Role of the membrane surface in concentration

polarization at ion-exchange membrane. Desalination 69, 101-114.

Sadrzadeh, M., Mohammadi, T. (2008) Sea water desalination using electrodialysis. Desalination 221,

440-447.

Sadyrbaeva, T.Z. (2015) Separation of cobalt (II) from nickel (II) by a hybrid liquid membrane–

electrodialysis process using anion exchange carriers. Desalination 365, 167-175.

Safarpour, M., Khataee, A., Vatanpour, V. (2015) Thin film nanocomposite reverse osmosis membrane

modified by reduced graphene oxide/TiO 2 with improved desalination performance. Journal of


Sata, T. (1994) Studies on ion exchange membranes with permselectivity for specific ions in

electrodialysis. Journal of Membrane Science 93, 117-135.

Sata, T. (2000) Studies on anion exchange membranes having permselectivity for specific anions in

electrodialysis—effect of hydrophilicity of anion exchange membranes on permselectivity of anions.

References

136


Sata, T., Ishii, Y., Kawamura, K., Matsusaki, K. (1999) Composite membranes prepared from cation

exchange membranes and polyaniline and their transport properties in electrodialysis. Journal of the

Electrochemical Society 146, 585-591.

Shannon, M.A., Bohn, P.W., Elimelech, M., Georgiadis, J.G., Marinas, B.J., Mayes, A.M. (2008)

Science and technology for water purification in the coming decades. Nature 452, 301.

Shekhah, O., Liu, J., Fischer, R., Wöll, C. (2011) MOF thin films: existing and future applications.

Chem Soc Rev 40, 1081-1106.

Shen, J., Huang, J., Liu, L., Ye, W., Lin, J., Van der Bruggen, B. (2013) The use of BMED for

glyphosate recovery from glyphosate neutralization liquor in view of zero discharge. Journal of

hazardous materials 260, 660-667.

Shen, J., Yu, J., Liu, L., Lin, J., Van der Bruggen, B. (2014) Synthesis of quaternary ammonium

hydroxide from its halide salt by bipolar membrane electrodialysis (BMED): effect of molecular

structure of ammonium compounds on the process performance. Journal of Chemical Technology and

Biotechnology 89, 841-850.

Shi, G.M., Yang, T., Chung, T.S. (2012) Polybenzimidazole (PBI)/zeolitic imidazolate frameworks

(ZIF-8) mixed matrix membranes for pervaporation dehydration of alcohols. Journal of Membrane

Science 415, 577-586.

Sistat, P., Kozmai, A., Pismenskaya, N., Larchet, C., Pourcelly, G., Nikonenko, V. (2008)

Low-frequency impedance of an ion-exchange membrane system. Electrochimica Acta 53, 6380-6390.

Sivaraman, P., Chavan, J., Thakur, A., Hande, V., Samui, A. (2007) Electrochemical modification of

cation exchange membrane with polyaniline for improvement in permselectivity. Electrochimica Acta

52, 5046-5052.

Sorribas, S., Gorgojo, P., Tellez, C., Coronas, J., Livingston, A.G. (2013) High flux thin film

nanocomposite membranes based on metal–organic frameworks for organic solvent nanofiltration. J

Am Chem Soc 135, 15201-15208.

Souzy, R., Ameduri, B., Boutevin, B. (2004) Synthesis and (co) polymerization of monofluoro, difluoro,

trifluorostyrene and ((trifluorovinyl) oxy) benzene. Progress in Polymer Science 29, 75-106.

Strathmann, H. (2004) Ion-exchange membrane separation processes. Elsevier.

Strathmann, H. (2010) Electrodialysis, a mature technology with a multitude of new applications.

References

137


Takata, K., Yamamoto, Y., Sata, T. (2000) Modification of transport properties of ion exchange

membranes: XIV. Effect of molecular weight of polyethyleneimine bonded to the surface of cation

exchange membranes by acid–amide bonding on electrochemical properties of the membranes. Journal

of Membrane Science 179, 101-107.

Tansel, B., Sager, J., Rector, T., Garland, J., Strayer, R.F., Levine, L., Roberts, M., Hummerick, M.,

Bauer, J. (2006) Significance of hydrated radius and hydration shells on ionic permeability during

nanofiltration in dead end and cross flow modes. Separation and Purification Technology 51, 40-47.

Tongwen, X., Weihua, Y. (2002) Citric acid production by electrodialysis with bipolar membranes.

Chemical Engineering and Processing: Process Intensification 41, 519-524.

Tranchemontagne, D.J., Mendoza-Cortes, J.L., O‘Keeffe, M., Yaghi, O.M. (2009) Secondary building

units, nets and bonding in the chemistry of metal–organic frameworks. Chem Soc Rev 38, 1257-1283.

Tzabiras, J., Vasiliades, L., Sidiropoulos, P., Loukas, A., Mylopoulos, N. (2016) Evaluation of water

resources management strategies to overturn climate change impacts on Lake Karla watershed. Water

Resources Management 30, 5819-5844.

Van Goethem, C., Verbeke, R., Hermans, S., Bernstein, R., Vankelecom, I. (2016) Controlled

positioning of MOFs in interfacially polymerized thin-film nanocomposites. Journal of Materials

Chemistry A 4, 16368-16376.

Vaselbehagh, M., Karkhanechi, H., Mulyati, S., Takagi, R., Matsuyama, H. (2014) Improved

antifouling of anion-exchange membrane by polydopamine coating in electrodialysis process.


Vermaas, D.A., Saakes, M., Nijmeijer, K. (2014) Early detection of preferential channeling in reverse

electrodialysis. Electrochimica Acta 117, 9-17.

Volodina, E., Pismenskaya, N., Nikonenko, V., Larchet, C., Pourcelly, G. (2005) Ion transfer across

ion-exchange membranes with homogeneous and heterogeneous surfaces. J Colloid Interface Sci 285,

247-258.

Wang, H.-B., Li, Y., Dong, G.-L., Gan, T., Liu, Y.-M. (2017a) A convenient and label-free colorimetric

assay for dopamine detection based on the inhibition of the Cu (ii)-catalyzed oxidation of a 3, 3′, 5,

5′-tetramethylbenzidine–H 2 O 2 system. New Journal of Chemistry 41, 14364-14369.

Wang, J., Zhu, J., Tsehaye, M.T., Li, J., Dong, G., Yuan, S., Li, X., Zhang, Y., Liu, J., Van der Bruggen,

References

138

B. (2017b) High flux electroneutral loose nanofiltration membranes based on rapid deposition of

polydopamine/polyethyleneimine. Journal of Materials Chemistry A 5, 14847-14857.

Wang, L., Fang, M., Liu, J., He, J., Deng, L., Li, J., Lei, J. (2015a) The influence of dispersed phases

on polyamide/ZIF-8 nanofiltration membranes for dye removal from water. Rsc Advances 5,

50942-50954.

Wang, L., Fang, M., Liu, J., He, J., Li, J., Lei, J. (2015b) Layer-by-layer fabrication of

high-performance polyamide/ZIF-8 nanocomposite membrane for nanofiltration applications. ACS

Appl Mater Interfaces 7, 24082-24093.

Wang, M., Jia, Y.-x., Yao, T.-t., Wang, K.-k. (2013) The endowment of monovalent selectivity to cation

exchange membrane by photo-induced covalent immobilization and self-crosslinking of chitosan.


Wang, T., Qiblawey, H., Sivaniah, E., Mohammadian, A. (2016) Novel methodology for facile

fabrication of nanofiltration membranes based on nucleophilic nature of polydopamine. Journal of


Wee, L.H., Lescouet, T., Ethiraj, J., Bonino, F., Vidruk, R., Garrier, E., Packet, D., Bordiga, S.,

Farrusseng, D., Herskowitz, M. (2013) Hierarchical Zeolitic Imidazolate Framework‐8 Catalyst for

Monoglyceride Synthesis. ChemCatChem 5, 3562-3566.

Wei, H., Ren, J., Han, B., Xu, L., Han, L., Jia, L. (2013) Stability of polydopamine and poly (DOPA)

melanin-like films on the surface of polymer membranes under strongly acidic and alkaline conditions.

Colloids and Surfaces B: Biointerfaces 110, 22-28.

Xie, P., Rong, M.Z., Zhang, M.Q. (2016) Moisture Battery Formed by Direct Contact of Magnesium

with Foamed Polyaniline. Angewandte Chemie International Edition 55, 1805-1809.

Xu, L.Q., Yang, W.J., Neoh, K.-G., Kang, E.-T., Fu, G.D. (2010) Dopamine-induced reduction and

functionalization of graphene oxide nanosheets. Macromolecules 43, 8336-8339.

Xu, Q., Zhang, W., (2016) Next-Generation Graphene-Based Membranes for Gas Separation and Water

Purifications, Advances in Carbon Nanostructures. InTech.

Xu, T. (2005) Ion exchange membranes: state of their development and perspective. Journal of


Yamada, K., Odomi, M., Okada, N., Fujita, T., Yamamoto, A. (2005) Chitosan oligomers as potential

and safe absorption enhancers for improving the pulmonary absorption of interferon‐α in rats. Journal

References

139

of pharmaceutical sciences 94, 2432-2440.

Yang, C.-X., Ren, H.-B., Yan, X.-P. (2013) Fluorescent metal–organic framework MIL-53 (Al) for

highly selective and sensitive detection of Fe3+ in aqueous solution. Analytical chemistry 85,

7441-7446.

Yang, H.-C., Liao, K.-J., Huang, H., Wu, Q.-Y., Wan, L.-S., Xu, Z.-K. (2014a) Mussel-inspired

modification of a polymer membrane for ultra-high water permeability and oil-in-water emulsion

separation. Journal of Materials Chemistry A 2, 10225-10230.

Yang, H.-C., Xu, W., Du, Y., Wu, J., Xu, Z.-K. (2014b) Composite free-standing films of

polydopamine/polyethyleneimine grown at the air/water interface. Rsc Advances 4, 45415-45418.

Yang, H.C., Wu, M.B., Li, Y.J., Chen, Y.F., Wan, L.S., Xu, Z.K. (2016) Effects of polyethyleneimine

molecular weight and proportion on the membrane hydrophilization by codepositing with dopamine.

Journal of Applied Polymer Science 133.

Yang, X., Du, Y., Zhang, X., He, A., Xu, Z.-K. (2017) Nanofiltration Membrane with a Mussel-Inspired

Interlayer for Improved Permeation Performance. Langmuir 33, 2318-2324.

Yu, L., Lin, A., Zhang, L., Jiang, W. (2002) Large scale experiment on the preparation of vitamin C

from sodium ascorbate using bipolar membrane electrodialysis. Chemical Engineering

Communications 189, 237-246.

Zhang, C., Ma, M.-Q., Chen, T.-T., Zhang, H., Hu, D.-F., Wu, B.-H., Ji, J., Xu, Z.-K. (2017a)

Dopamine-Triggered One-Step Polymerization and Codeposition of Acrylate Monomers for Functional

Coatings. ACS Appl Mater Interfaces 9, 34356-34366.

Zhang, C., Ou, Y., Lei, W.X., Wan, L.S., Ji, J., Xu, Z.K. (2016a) CuSO4/H2O2‐Induced Rapid

Deposition of Polydopamine Coatings with High Uniformity and Enhanced Stability. Angewandte

Chemie 128, 3106-3109.

Zhang, F., Gao, S., Zhu, Y., Jin, J. (2016b) Alkaline-induced superhydrophilic/underwater

superoleophobic polyacrylonitrile membranes with ultralow oil-adhesion for high-efficient oil/water

separation. Journal of Membrane Science 513, 67-73.

Zhang, R., Liu, Y., He, M., Su, Y., Zhao, X., Elimelech, M., Jiang, Z. (2016c) Antifouling membranes

for sustainable water purification: strategies and mechanisms. Chem Soc Rev 45, 5888-5924.

Zhang, R., Su, Y., Zhao, X., Li, Y., Zhao, J., Jiang, Z. (2014) A novel positively charged composite

nanofiltration membrane prepared by bio-inspired adhesion of polydopamine and surface grafting of

References

140

poly (ethylene imine). Journal of Membrane Science 470, 9-17.

Zhang, W., Miao, M., Pan, J., Sotto, A., Shen, J., Gao, C., Van der Bruggen, B. (2017b) Separation of

divalent ions from seawater concentrate to enhance the purity of coarse salt by electrodialysis with

monovalent-selective membranes. Desalination 411, 28-37.

Zhang, X., Li, C., Wang, Y., Luo, J., Xu, T. (2011) Recovery of acetic acid from simulated acetaldehyde

wastewaters: Bipolar membrane electrodialysis processes and membrane selection. Journal of


Zhao, F.-Y., Ji, Y.-L., Weng, X.-D., Mi, Y.-F., Ye, C.-C., An, Q.-F., Gao, C.-J. (2016) High-flux

positively charged nanocomposite nanofiltration membranes filled with poly (dopamine) modified

multiwall carbon nanotubes. ACS Appl Mater Interfaces 8, 6693-6700.

Zhao, J., Fang, C., Zhu, Y., He, G., Pan, F., Jiang, Z., Zhang, P., Cao, X., Wang, B. (2015) Manipulating

the interfacial interactions of composite membranes via a mussel-inspired approach for enhanced

separation selectivity. Journal of Materials Chemistry A 3, 19980-19988.

Zhao, Z., Shi, S., Cao, H., Li, Y. (2017) Electrochemical impedance spectroscopy and surface

properties characterization of anion exchange membrane fouled by sodium dodecyl sulfate. Journal of


Zhao, Z., Shi, S., Cao, H., Shan, B., Sheng, Y. (2018) Property characterization and mechanism

analysis on organic fouling of structurally different anion exchange membranes in electrodialysis.


Zhu, K., Gao, Y., Tan, X., Chen, C. (2016) Polyaniline-Modified Mg/Al Layered Double Hydroxide

Composites and Their Application in Efficient Removal of Cr (VI). ACS Sustainable Chemistry &

Engineering 4, 4361-4369.

Curriculum Vitae

141

Curriculum Vitae

Jian Li

PERSONAL INFORMATION

Date of Birth: July 25th

, 1990

Nationality: Chinese

Gender: Male

CONTACT

Process Engineering for Sustainable Systems (ProcESS)

Department of Chemical Enigineering

KU Leuven Chem & Tech

Celestijnenlaan 200F

3001 Heverlee, Leuven, Belgium

Tel: (+32) 485632238

Email: [email protected]; [email protected]

EDUCATION

Sept. 2015-Present: Ph.D in Chemical Engineering

KU Leuven, Belgium

Sept. 2012-Jun. 2015 Master degree in Chemical Engineering

Zhejiang University of Technology, China

mailto:[email protected]

mailto:[email protected]

Curriculum Vitae

142

Sept. 2008-Jun. 2012 Bachelor in Chemical Engineering

Yancheng Institude of Technology, China

Publications

143

PUBLICATIONS

1. J Li, S Yuan, J Wang, J Zhu, J Shen, B Van der Bruggen. Mussel-inspired

modification of ion exchange membrane for monovalent separations. Journal of

Membrane Science. 553 (2018): 139-150.

2. J Li, J Zhu, S Yuan, J Wang, J Shen, B Van der Bruggen. Mussel-inspired

monovalent cation exchange membranes containing hydrophilic MIL53(Al)

framework for enhanced ion flux. Industrial & Engineering Chemistry

Research 57. 18(2018):6275-6283.

3. J Li, Z Zhao, S Yuan, J Zhu, B Van der Bruggen. High-performance

thin-film-nanocomposite cation exchange membranes containing hydrophobic

zeolitic imidazolate framework for monovalent selectivity. Applied sciences.

8.5(2018): 759

4. J Li, J Zhu, J Wang, S Yuan, J Lin, J Shen, B Van der Bruggen. Charge‐assisted

ultrafiltration membranes for monovalent ions separation in electrodialysis.

Journal of Applied Polymer Science 135.24 (2018): 45692.

5. J Li, J Zhu, S Yuan, J Lin, J Shen, B Van der Bruggen. Cation-Exchange

Membranes with Controlled Porosity in Electrodialysis Application. Industrial &

Engineering Chemistry Research 56.28 (2017): 8111-8120.

6. J Li, ST Morthensen, J Zhu, S Yuan, J Wang, A Volodine, J Lin, J Shen, B van

der Bruggen. Exfoliated MoS 2 nanosheets loaded on bipolar exchange

membranes interfaces as advanced catalysts for water dissociation. Separation

and Purification Technolog 194 (2018): 416-424.

7. J Li, Y Xu, M Hu, J Shen, C Gao, B van der Bruggen. Enhanced conductivity of

monovalent cation exchange membranes with chitosan/PANI composite

modification. RSC Advances 5.110 (2015): 90969-90975.

8. J Li, M Zhou, J Lin, W Ye, Y Xu, J Shen, C Gao, B Van der Bruggen.

Mono-valent cation selective membranes for electrodialysis by introducing

polyquaternium-7 in a commercial cation exchange membrane. Journal of

Membrane Science 486 (2015): 89-96.

Publications

144

9. S Yuan, J Li, J Zhu, A Volodine, J Li, G Zhang, P Van Puyvelde, B Van der

Bruggen. Hydrophilic nanofiltration membranes with reduced humic acid fouling

fabricated from copolymers designed by introducing carboxyl groups in the

pendant benzene ring. Journal of Membrane Science. 563(2018): 655-663

10. J Lin, C Y. Tang, C Huang, Y Tang, W Ye, J Li, J Shen, R Van den Broeck, J

Van Impe, A Volodin, C Van Haesendonck, A Sotto, P Luis, B Van der Bruggen.

A comprehensive physico-chemical characterization of superhydrophilic loose

nanofiltration membranes. Journal of Membrane Science. 501 (2016): 1-14.

11. L Hao, J Liao, Y Jiang, J Zhu, J Li, Y Zhao, B Van der Bruggen, A Sotto, J Shen.

Sandwich‖-like structure modified anion exchange membrane with enhanced

monovalent selectivity and fouling resistant. Journal of Membrane Science,

556(2018): 98-106.

12. B Han, J Pan, S Yang, M Zhou, J Li, B Van der Bruggen, A. Sotto, C Gao, J Shen.

Novel composite anion exchange membranes based on quaternized

polyepichlorohydrin for electromembrane application. Industrial & Engineering

Chemistry Research. 2016, 55(26): 7171-7178.

13. J Wang, J Zhu, M T Tsehaye, J Li, S Yuan, G Dong, X Li, Y. Zhang, J Liu, B

Van der Bruggen. High flux electroneutral loose nanofiltration membranes based

on rapid deposition of polydopamine/polyethyleneimine. Journal of Materials

Chemistry A, 2017, 5(28): 14847-14857.

14. J Zhu, J Hou, R Zhang, S Yuan, J Li, M Tian, P Wang, Y Zhang, A Volodin, B

Van der Bruggen. Rapid water transport through controllable, ultrathin polyamide

nanofilms for high-performance nanofiltration[J]. Journal of Materials Chemistry

A, 2018,6, 15701-15709

15. Y Jiang, J Liao, S Yang, J Li, Y. Xu, H. Ruan, A. Sotto, B Van der Bruggen, J

Shen. Stable cycloaliphatic quaternary ammonium-tethered anion exchange

membranes for electrodialysis. Reactive and Functional Polymers, 130(2018):

61-69

Publications

145

INTERNATIONAL CONFERENCES

J Li, B Van der Bruggen. Mussel-inspired modification of ion exchange membrane

for monovalent separations. MELPRO 2018, May 13-16, 2018. Prague, Czech

Republic. (Poster)

selectivity - ku leuven

Documents