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Micellar-Enhanced Ultrafiltration of Palladium and Platinum

Anions

BY

Gwicana Sakumzi

A dissertation submitted in fulfilment of the requirements for the degree of

MASTER OF TECHNOLOGY: CHEMISTRY

in the Faculty of Science at the

NELSON MANDELA METROPOLITAN UNIVERSITY

January 2007

Supervisor: Dr N.M. Vorster



TABLE OF CONTENTS

Acknowledgements.................................................................................. i

Summary….............................................................................................. ii

Abbreviations ........................................................................................... iv

List of Figures........................................................................................... v

List of Tables............................................................................................ viii

CHAPTER ONE

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

1.1. Background to waste treatment .................................................... 1

1.2. Definition of a hazardous waste.................................................... 2

1.3. Overview of platinum group metals............................................... 2

1.3.1. Occurrence and Origin........................................................... 3

1.3.2. Abundance and Distribution................................................... 3

1.3.3. Chemical properties............................................................... 5

1.3.4. Basic coordination chemistry of platinum and palladium....... 5

1.3.5. Industrial applications of PGMs ............................................. 6

1.3.5.1. Catalysis......................................................................... 6

1.3.5.2. Jewellery ........................................................................ 7

1.3.5.3. Electronics...................................................................... 7

1.4. Surfactants.................................................................................... 8

1.4.1. Overview................................................................................ 8

1.4.2. Classification of surfactants ................................................... 12

1.4.3. Industrial applications of surfactants...................................... 12

1.5. Membranes................................................................................... 13

1.5.1. Overview................................................................................ 13

1.5.2. Typical membrane properties ................................................ 15

1.5.3. Classification of membranes.................................................. 15

1.5.4. Membrane modules............................................................... 18

1.5.5. Pressure-driven membrane filtration processes .................... 19

1.5.6. Ultrafiltration system .............................................................. 20

1.5.6.1. Micellar-enhanced ultrafiltration system ......................... 21



1.6. Research objectives...................................................................... 23

CHAPTER TWO

2. Experimental and Key Performance Aspects...................................... 24

2.1. Materials ....................................................................................... 24

2.2. Synthetic procedures .................................................................... 25

2.2.1. Preparation of surfactant solutions ........................................ 25

2.2.2. Preparation of surfactant/metal ion solutions......................... 25

2.2.3. Membrane module preparation.............................................. 25

2.3. Apparatus...................................................................................... 27

2.3.1. Ultrafiltration system set-up ................................................... 27

2.3.2. Major components of ultrafiltration system ............................ 29

2.3.2.1. Membrane module ......................................................... 29

2.3.2.2. Peristaltic pump.............................................................. 30

2.3.2.3. Controlled temperature bath........................................... 30

2.3.2.4. Operation of MEUF......................................................... 30

2.4. Analytical techniques.................................................................... 31

2.4.1. Ultraviolet-visible spectrophotometer (UV-VIS) ..................... 31

2.4.1.1. Cetylpyridinium chloride calibration curve ...................... 31

2.4.2. Inductively Coupled Plasma Mass Spectrometry (ICP-MS)... 32

2.4.3. Conductometer ...................................................................... 33

2.5. Key Performance Aspects of the Ultrafiltration system................. 33

2.5.1. Principle and Operation of MEUF system.............................. 33

2.5.2. Assessment and monitoring of the membrane condition....... 34

2.5.3. Regeneration of an ultrafiltration membrane ......................... 35

2.5.4. Fouling in membrane processes............................................ 36

2.5.4.1. Factors affecting fouling ................................................. 36

2.5.4.2. Common fouling problems.............................................. 36

2.5.5. Membrane cleaning ............................................................... 37

CHAPTER THREE

3. Preliminary investigation of metal ion and surfactant retention............ 39

3.1. Definitions ..................................................................................... 39

3.2. Influence of various parameters.................................................... 40

3.2.1. Effect of pressure variation .................................................... 40

3.2.2. Effect of temperature variation............................................... 42



3.2.3. Effect of cetylpyridinium chloride concentration variation ...... 44

3.3. Micellar-enhanced ultrafiltration of Pt (lV) and Pd (ll) anions ........ 45

3.3.1. Micellar-enhanced ultrafiltration of Pt (lV) and Pd (ll) anions

in acidic medium ................................................................... 45

3.3.2. Micellar-enhanced ultrafiltration of Pt (lV) and Pd (ll) anions

in neutral medium.................................................................. 48

3.3.3. Micellar-enhanced ultrafiltration of a mixture of Pt (lV) and

Pd (ll) anions in neutral medium............................................ 50

3.3.4. Effect of an electrolyte on metal ion and surfactant retention 52

3.4. Membrane interaction ................................................................... 53

3.5. Summary of preliminary investigations ......................................... 55

CHAPTER FOUR

4. Investigation of cetylpyridinium chloride retention................................ 56

4.1. Overview....................................................................................... 56

4.2. Investigation of the effects of an electrolyte.................................. 56

4.2.1. Effects of hydrochloric acid concentration variation............... 57

4.2.2. Effects of nitric acid concentration variation .......................... 60

4.2.3. Effects of sodium chloride concentration variation ................ 62

4.3. Summary of CPC retention investigations .................................... 64

CHAPTER FIVE

5. The concept of micellisation and conductivity investigations ............... 65

5.1. The concept of micellisation.......................................................... 65

5.2. Conductivity study......................................................................... 66

5.2.1. Experimental determination of the critical micelle

concentration of cetylpyridinium chloride ............................... 66

5.2.2. Experimental procedure......................................................... 66

5.2.3. Determination of the degree of ionization .............................. 68

5.3. Influence of an electrolyte on the cmc value and degree of

ionisation ............................................................................... 69

5.3.1. Determination of cmc in the presence of HCl ........................ 69

5.3.2. Determination of cmc in the presence of Pd (ll) anions and

aqua regia .............................................................................. 71

5.3.3. Determination of cmc in the presence of Pt (lV) anions and

aqua regia .............................................................................. 72



5.5. Summary of the conductivity investigations.................................. 74

CHAPTER SIX

6. Improved conditions for the retention of metal ion and surfactant........ 75

6.1. Micellar-enhanced ultrafiltration of individual metal ions............... 75

6.1.1. Ultrafiltration of platinum (lV) anions...................................... 75

6.1.2. Ultrafiltration of palladium (ll) anions...................................... 77

6.2. Micellar-enhanced ultrafiltration of Pt (lV)-Pd (ll) mixture.............. 79

6.3. Membrane response during Pt/Pd mixture ultrafiltration............... 82

6.3.1. CPC retention ........................................................................ 82

6.3.2. Flux variation ......................................................................... 83

6.4. Summary of the investigation of improved conditions……………..84

CHAPTER SEVEN

7. Conclusive remarks.............................................................................. 85

7.1. Conclusion.................................................................................... 85

7.2. Recommendations........................................................................ 86

References .............................................................................................. 87

APPENDIX A ........................................................................................... 89

APPENDIX B ........................................................................................... 97

APPENDIX C ........................................................................................... 99

APPENDIX D ........................................................................................... 101



i

ACKNOWLEDGEMENTS

I wish to express my sincere thanks to:

• Dr Nicole Vorster for her wonderful support, guidance and motivation.

• Prof EP Jacobs and Prof P. Loyson for their support.

• Dr E. Hosten for his assistance.

• A special tribute to my friends, Lwandile Mcingana, Akhona, Sivuyile,

Chanda and Mtheza for their assistance and support throughout this study.

• My fellow students and co-workers at NMMU, and the entire PETCRU

personnel for their assistance.

• Dr C. Viviers and Mr A. Joubert and co-workers at HCSA for their

understanding and support.

• CCETSA, NRF and PET for their financial support.

• My mom and the entire family for their encouragement and support tocarry out this work during the trying times.

The “Almighty” for making everything possible.



ii

EXECUTIVE SUMMARY

The project was concerned with studying the capability of a micellar-enhanced

ultrafiltration system (MEUF) to remove platinum group metal ions namely Pt

(lV) and Pd (ll) chloro anions from aqueous industrial waste effluents. South

Africa has the world’s largest reserves of platinum group metals (PGMs) and

other valuable metals such as manganese, chrome ores, titanium minerals

etc. which are required for new automotive and other technologies, including

fuel cells, catalytic converters and lighter components. The consistent loss

with the industrial waste stream and the toxicological effects of these precious

metals led to the need to develop new and effective methods to recover them

from industrial waste effluents.

With such a wide variety of fields where these PGMs are used and the failure

of the traditional techniques namely sedimentation, fermentation etc. to

effectively reduce or recover these highly toxic and precious metal ions prior

to discharging industrial waste effluents, it is necessary to explore other

techniques such as membrane technology that can be used to recover these

valuable species from industrial waste streams.

The present study involved the use of a cationic surfactant, viz cetylpyridinium

chloride, which was introduced into an aqueous solution containing palladium

and platinum metal anions. The surfactant forms charged micelles above a

certain critical concentration value. The metal anions adsorb onto the

available oppositely charged sites on the micelle surfaces and are then able

to be retained by a suitable membrane. Hollow fibre ultrafiltration membranes

with the MWCO of +/- 10 kD and +/-30nm pore size were used as a filter

component in this study. For this MEUF system to be effective, it was vital thatthe anionic metal ion species adsorbed sufficiently onto the available

oppositely charged sites of the micelles and that the micelles were retained

efficiently by the membrane.



iii

Results obtained during the investigation made it possible to make certain

predictions about the micellisation process. It was also found that, it was not

only the metal ion: surfactant (M:S) ratio that was critical, but the presence of

other electrolytes in the aqueous stream proved to have a huge impact on the

capability of the MEUF system.

Findings of this research study showed that the MEUF system using

cetylpyridinium chloride (CPC) can be used to recover or retain Pt (lV) and Pd

(ll) anions from industrial waste effluents. It was also found that PtCl62-, due to

its greater adsorption capabilities onto the micelle surface than PdCl42- or

PdCl3(H2O)-, was preferentially retained in neutral medium. This may be

exploited as a possible means of separating the two metal ions.

The developed system offers the following advantages over some traditional

and current methods: simplified unit operation line flow process, smaller

amounts of chemical usage and no solid toxic sludge to be disposed of.

Applications of this work could be of vital importance in catalytic converter

recycling, especially in Port Elizabeth where extensive automobile parts

manufacturing occurs.

KEYWORDS:

Platinum, palladium, micellar-enhanced ultrafiltration, surfactant,

micellisation, cetylpyridinium chloride



iv

ABBREVIATIONS

MEUF Micellar-Enhanced Ultrafiltration

mM Millimolar

CPC Cetylpyridinium chloride

PGMs Platinum group metals

cmc Critical micelle concentration

NaCl Sodium chloride

HCl Hydrochloric acid

HNO3 Nitric acidICP-MS Inductively Coupled Plasma Mass Spectrophotometry

UV-VIS Ultraviolet visible



v

LIST OF FIGURES

Figure 1.1: Worldwide distribution of PGMs 4

Figure 1.2: Micelle formation in dilute solution 9

Figure 1.3: Schematic diagram of a micelle 9

Figure 1.4: Schematic diagram showing the occurrence of cmc with 10

changing solution parameters

Figure 1.5: A typical diagram showing the occurrence of a cmc value 11

at the Kraft point with changing solution parameters

Figure 1.6: Cetylpyridinium chloride structure 12

Figure 1.7: Hollow fibre module 18

Figure 1.8: Tubular module 18

Figure 1.9: Pressure-driven membrane separation processes 19

Figure 1.10: A mechanistic scheme showing the adsorption of an 22

anionic metal species such as Pt (lV) onto a cationic micelle

Figure 2.1: SEM image of a cross section of the polysulfone hollow fibre 26

membrane

Figure 2.2: SEM image of a part of the outer surface of the polysulfone 26

hollow fibre membrane

Figure 2.3: Photo of a laboratory scale MEUF system 28

Figure 2.4: Schematic diagram of an ultrafiltration system 29

Figure 2.5: CPC calibration curve 32

Figure 2.6: A typical layout of an ultrafiltration system 34

Figure 2.7: Pure water flux measurements taken after various 35

experiments

Figure 3.1: Effects of pressure variation at 30°C and 10 mM CPC 41

Figure 3.2: Effects of temperature variation 150 kPa and 10 mM CPC 42Figure 3.3: Variation of CPC concentration at 30°C and 150 kPa 44

Figure 3.4: Investigation of Pt and Pd anions retention at 150 kPa and 46

30°C in acidic medium as a function of CPC concentration

Figure 3.5: Investigation of Pt and Pd anions retention at 150 kPa and 48

30°C in neutral medium as a function of CPC concentration



vi

Figure 3.6: Species distribution curves of Pd (ll)/chloride system 49

Figure 3.7: Investigation of Pt and Pd anions separation in acidic 51

medium at 150 kPa, 30°C as a function of CPC concentration

Figure 3.8: Effects of sodium chloride on Pt and Pd anions retention 52

Acidic medium at 150 kPa and 30°C using 40 mM CPC

Figure 3.9: Effects of various species on flux variation at 150 kPa and 54

30°C in neutral medium

Figure 4.1: Variation of hydrochloric acid concentration at 30°C, 58

150 kPa and 10 mM CPC

Figure 4.2: Variation of nitric acid concentration at 30°C,150 kPa and 59

10 mM CPC

Figure 4.3: Study of variation of sodium chloride concentration at 61

30°C, 150 kPa and 10 mM CPC

Figure 4.4: Surface tension of a 10 mM CPC solution as a function of 62

HNO3 concentration

Figure 4.5: Study of variation of sodium chloride acid concentration 63

at 30°C, 150 kPa and 10 mM CPC

Figure 5.1: Determination of the cmc of CPC and its degree of 67

ionisation

Figure 5.2: Plot of the conductivity vs CPC concentration in the 70presence of HCl

Figure 5.3: Plot of the conductivity vs CPC concentration in the 71

presence of Pd (ll) anions in aqua regia

Figure 5.4: Plot of the conductivity vs CPC concentration in the 73

presence of Pt (lV) anions in aqua regia

Figure 6.1: MEUF of Pt (lV) anions at 40 mM CPC, 30°C and 150 kPa 76

in acidic medium

Figure 6.2: MEUF of Pd (lV) anions at 40 mM CPC, 30°C and 150 kPa 78

in acidic medium

Figure 6.3: MEUF of a mixture of Pd (lV)/Pt (lV) anions at 40mM CPC, 80

30°C and 150 kPa in acidic medium

Figure 6.4: MEUF of a mixture of Pt (lV)/Pd (lV) anions at 40 mM CPC, 81

30°C and 150 kPa in acidic medium



vii

Figure 6.5: Effects of metal ion ratio on CPC retention 82

Figure 6.6: Flux variation during Pt (lV)/Pd (lV) anions ultrafiltration 84



viii

LIST OF TABLES

Table 1.1: Examples of common surfactants 12

Table 1.2: Description of different types of membranes 16

Table 1.3: Summary of membrane separation processes 20

Table 2.1: List of Chemicals used for synthesis and analysis 24

Table 2.2: Major components of a laboratory scale MEUF system 28

Table 2.3: Membrane module specifications 29

Table 2.4: Peristaltic pump specifications 30

Table 2.5: ICP-MS settings 33

Table 2.6: Membrane foulants and control measures 37Table 6.1: Summary of varying metal ion concentration ratios 79



1

CHAPTER ONE

INTRODUCTION

In this chapter, a brief overview on waste treatment, the chemistry and

industrial applications of platinum group metals, surfactants and

membranes will be given. The scope of this research study will also be

outlined.

1.1 Background to waste treatment

In recent years, waste treatment has become a necessary activity of

various industries, most importantly, of chemical industries. The major

purpose of wastewater treatment is to remove as much of the

suspended solids and hazardous materials present in waste streams

prior to their disposal back to the environment. The emphasis on

wastewater treatment has not only been generated by the negative

impact that wastewater constituents impose on the community due to

their toxic and harmful effects, but also by the huge profits that can be

enjoyed by chemical industries from the recovery of waste materials. For

some chemical industries, the recovered components can be

reprocessed, thus leading to higher yields of their products.

Stringent regulations after the introduction of an Environmental

Management System (EMS) standard, namely the ISO 14001 series,

require all industries to comply with the regulatory systems and to

continuously improve wastewater treatment activities.1, 2 Since the costs

and constraints of treatment systems can be unbearable, the necessity

to reduce the content of these highly toxic as well as non-biodegradable

constituents prior to discharging the waste stream into sewage is of vital

importance. 3



2

It has been noted that the use of traditional techniques, namely lime

precipitation, ion exchange, activated carbon adsorption, electrolytic

processes etc. are ineffective in reducing the levels of carcinogenic

components to the required levels as stipulated in the Environmental

Management System. 2, 4 It has also been noted that the use of

membrane separation processes in wastewater treatment is an

attractive, as well as a more preferred technique, especially in the

chemical industry, than the previously mentioned methods.

1.2 Definition of a hazardous waste

Hazardous waste is defined as an organic or inorganic element or

compound that due to its chemical, physical and toxicological properties

may be deemed harmful and/or brings chronic impacts to human health

and the environment.1 It can be generated from a variety of agricultural,

commercial, domestic and industrial activities and may be in the form of

a liquid, sludge or a solid. Within this hazardous waste there are

inorganic pollutants such as platinum group metals that are considered

to be highly toxic, non-biodegradable and which have definite

carcinogenic effects. 3 These features contribute not only to the degree

of hazard, but are also of great importance in the ultimate choice of a

safe and environmentally reasonable method of disposal or treatment.

1.3 Overview of platinum group metals (PGMs) 5, 6

Platinum group metals form part of the transition metal series. The group

is made up of the following metals, namely, palladium, platinum, iridium,

osmium, rhodium and ruthenium. Theoretically, platinum group metal

ores are assumed to contain 20% of each of palladium, platinum,

osmium and ruthenium and 6% of iridium and rhodium.



3

1.3.1 Occurrence and origin

The platinum group metals in the lithosphere have been transferred from

the earth’s interior. Tectonic movements of the earth’s crust followed by

the eruption of magma have led to the presence of PGMs in regions

closer to the surface. Chemical interaction with silicate layers such as

sulphides, arsenides, antimonide etc. at high temperatures has played a

major role in the origination of platinum group metals. However, the

PGMs later separated from ultrabasic magmas, hence the ores are

sulphide free. Platinum and palladium, and sometimes together with

nickel or copper, have undergone some hydrothermal reactions with

chlorides in the earth’s interior which have led to the formation of the

primary deposits of the platinum-bearing rock. Activities in those primary

deposits depend on the following factors, namely, concentration of

platinum metals present, accessibility, size of deposits, their value and

the economic potential of the accompanying materials.

The platinum group metals occur mainly in ore deposits in a large

number of minerals. Workable deposits usually contain mainly sperrylite

(PtAs2), cooperite (PtS), ferroplatinum (Fe-Pt), polyxene (Fe-Pt-other

PGMs) etc. These minerals are often associated with specific carrier

materials such as iron pyrites, nickel iron pyrites, or chrome iron pyrites.

These minerals as well as the platinum group metals are rarely found in

exact stoichiometric ratios. 7, 8, 9 The platinum group metals, as already

stated, often occur in a very complex ore body in which they are bonded

mainly to the so-called soft ligands such as sulphides or polysulphides,

arsenides and selenides.

1.3.2 Abundance and Distribution

The abundance of platinum group metals, which occupy an intermediate

position, based on their atomic number and atomic mass would be

expected to be 10-4 ppm based on the manner of formation of their

atomic nuclei. 8 A global distribution diagram of the platinum group



4

metals is shown in Fig. 1.1. South Africa is a leading producer of

platinum group metals because it has the world’s largest reserves of

PGMs. The worldwide distribution of the PGMs shows that there are only

two countries, namely, South Africa and Russia, that produce a

significant amount of these PGMs. 8 The main ore bodies in South Africa

are owned by two companies, namely, Anglo American and Impala

Platinum. The mining, refining and processing of these PGMs in South

Africa does not only serve as a source of income for the country, it also

brings about economic stability. 2

0.0%

10.0%

20.0%

30.0%

40.0%

50.0%

60.0%

South Africa Russia Finland Zimbabwe

USA Canada China Colombia

Figure 1.1: Worldwide distribution of platinum group metals

It is also said 6, 9 that, in South Africa, PGMs together with other by-

products namely gold, silver, nickel, copper and cobalt occur in

economic concentrations in three extensive layered reefs associated

with the mafic rocks of the Rustenburg layered suite of the Bushveld

Complex. They are the Merensky reef, the UG2 Chromitite layer and the

Platreef. Small quantities of the PGMs are also produced from golddeposits of the Witwatersrand Basin and from the copper ores of the

Phalaborwa Complex. South Africa’s mineral wealth and the increasing

demands of the platinum group metal products throughout the world,

makes the process of hydrometallurgy vital. Also, the chemistry related

to the dissolution of the ore of the PGMs, their successive separation



5

and ultimate production of the pure individual PGMs are thus very

important for the economy of this country.

1.3.3 Chemical properties

The chemical properties of the platinum group metals resemble those of

the late 4d and 5d transition metals as listed below:

• Platinum group metals have multiple oxidation states viz. Pt (ll), Pt

(lV), Pd (ll), Pd (lV) etc.

• They show high covalent character in bonding.

• They have slow ligand exchange due to kinetic stability of their

complexes.

• In their metallic form, the metals are inert and show great stability.

These above-mentioned properties greatly contribute towards the

problematic nature of the dissolution, isolation and processing of PGMs.

Also, these same characteristics make these metals suitable for a variety

of applications, namely, catalysis, synthesis, etc.7, 8, 10, 11

1.3.4 Basic coordination chemistry of platinum and palladium 7, 8

Palladium is a member of the 4d transition metal series while platinum

falls within the 5d series. Palladium and platinum have the same number

of electrons in their valence shell, viz. ten electrons in excess of the

preceding noble gas (s0d10 for Pd and s1d9 for Pt). These metals have

similar electron configurations since they belong in the sub-group that

constitutes nickel, palladium and platinum. The most significant

characteristic properties of the two metals are the kinetic stability of their

complexes and the high covalent character of their bonding. These

metals are known to prefer the “soft” donor ligands with covalent

properties as opposed to “hard” donor ligands such as ammonia and

aliphatic amines, and thus they show a greater preference for pi

acceptor ligands like chloride, sulphur, arsenic and phosphorous donors.



6

Furthermore, the divalent metal complexes of these two metals are

almost, without exception, spin-paired square planar d8 systems, while

their tetravalent compounds are spin-paired octahedral d6 systems.

Based on the previously mentioned identical behaviours of these two

metals, it can be stated that these metals have similar preferences with

respect to ligand selectivity and symmetry of their coordination

compounds; and have similar electronic as well as magnetic properties.

1.3.5 Industrial applications of platinum group metals 8, 12

1.3.5.1 Catalysis

Platinum group metals are renowned for contributing towards a pollution

free environment. They are extensively used in automobile catalytic

converters where they catalyse the detoxification of nitrates and

reduction of hydrocarbons in automobile exhaust gas emissions to

acceptable low levels. They are also used in various complex catalytic

processes such as aromatization, cracking, cyclisation, desulphurisation,

hydrocracking, hydroprocessing and reforming in the petroleum industry

to manufacture a wide variety of products.

Nitric acid that is used in the production of nitrogenous fertilizers (78%),

and explosives (12%), is produced by catalytic oxidation of ammonia

over gauzes woven from very fine platinum (90%)-rhodium/palladium (5-

10%) alloy wire. 9

Similar catalysts are employed in the Andrussen and Degussa

processes for the manufacturing of hydrogen cyanide from methane and

ammonia. These plants produce nitrous oxides that are reduced by a

platinum-impregnated catalyst contained either in a ceramic honeycomb,

alumina-pellet or nickel-chromium alloy ribbon support.

PGMs are also used in processes involved in the manufacture of liquid-

phase chemicals which include:



7

• Highly specific hydrogenation catalysts.

• Complex catalysts using polymeric supports to catalyze a whole

spectrum of organic reactions.

• Metal-cluster catalysts in which organo-metallic compounds form

cluster structures that can model biological catalysts.

• Oil from coal processes.

1.3.5.2 Jewellery 8, 12

In jewellery making processes, platinum offers the following advantages:

• It is harder (4.3 on mohs scale) than gold and does not nick or

scratch easily.

• It does not tarnish like silver.

• It holds gemstones better because of its superior strength and

hardness.

However, it has one disadvantage in the form of a high melting point and

thus has to be alloyed with rhodium in order to be suitable for jewellery

setting.

1.3.5.3 Electronics 5, 12

Platinum is employed in electrical contacts that are used under severe

conditions such as heavy-duty relays, switches, thermostats, voltage

regulators and slip-ring assemblies. It is also used in making thin-film

circuits, resistance elements, multilayer capacitors and oxygen sensors.Thermocouples employed in the steel and glass smelting industry use a

fine-wire platinum/rhodium alloy for the positive electrode and a pure

platinum wire for the negative electrode to provide a long life span and

the highest degree of accuracy.



8

Palladium is used in capacitors as contact materials applying to any

electrical contacts operating at low currents and voltages under low

contact forces due to the reasons listed below:

• Freedom from oxide films and tarnish.

• High melting temperature which results in a high resistance to arc

erosion and to the welding of contact surfaces such as telephone

switching relays, electrical and electronic apparatus, resistors, gas

turbine engines and atomic reactors, spinnerets to produce rayon

fibre, heating pads, high voltage regulators, relays, thermostats,

sliding contacts, magnetos, vibrators and signs.

1.4. Surfactants

1.4.1 Overview

Surfactants are defined as molecules that are usually surface active in

aqueous solutions. 13 These molecules adsorb strongly at the water-air

interface and subsequently lower the surface energy of water (However,

inorganic electrolytes that are desorbed at the water air interface behave

differently as they tend to slightly increase the surface energy of water).

Surfactant molecules are amphiphilic in nature, that is, they have a

charged head group that is hydrophilic and a hydrophobic hydrocarbon

tail. It is this unique feature that makes surfactants adsorb so effectively

at a surface interface. In dilute solutions, surfactant molecules are in the

form of monomers. The number of monomers adsorbed at the surface

increases with increasing surfactant concentration until the surface

becomes saturated. At this saturation point called the critical micelle

concentration, the monomers in the bulk solution aggregate to form

micelles in order to minimize the exposure of the hydrophobic tail to the

hydrophilic environment while maintaining the maximum interaction of

the charged head group with the water. The resulting micelles are

usually spherical in shape and range from 50 to 100 monomers as



9

shown in Fig. 1.2 below. An increase in surfactant concentration tends to

increase the number of micelles per unit volume.

Figure 1.2: Micelle formation in dilute solution

For a particular spherical micelle, the head (hydrophilic) groups are not

closely packed on the surface of the micelle and calculations indicate

that the head groups occupy not more than a third of the micelle surface

(see Fig. 1.3). 13

Figure 1.3: Schematic diagram of a micelle 13

+ + + + + + + +

+ +

+ + +

+ +

+ +

+ + + +

+



10

Repulsion between the head groups tends to increase the surface area

hence some of the counter ions remain associated with the micelles so it

is not fully ionized.

Above the critical micelle concentration (cmc) some of the surfactant

solution properties such as conductivity, surface tension and viscosity

suddenly change. This abrupt change in solution properties can be used

as a means of determining the cmc (see Fig. 1.4).

Figure 1.4: Schematic diagram showing the occurrence of the cmc with

changing solution parameters

Micelles are soluble in water but only above a certain temperature called

the Kraft temperature. This is the triple point on the phase diagram of an

ionic surfactant (see Fig. 1.5 below). Below this temperature and above

the cmc the micelles form gels. The solubility of a surfactant generally

depends on how easily it dissociates in water and how well the head

group is solvated. The solubility generally increases with increasing

temperature.

Surface tension

Electricalconductivity

S o l u t i o n p a r a m e t e r

Concentration of surfactant

cmc



11

Figure 1.5: A typical phase diagram of an ionic surfactant showing the

cmc value at the Kraft point

There are a number of possible micelles shapes, namely, spherical, rod-

shaped or disk-shaped that can result when micellisation takes place.

The shape of a micelle is determined mostly by the ratio of the area

requirement of the head group of the surfactant under the prevailing

conditions to the volume of the hydrophobic tail group. If this ratio is

large, as in the case of n-alkyl surfactants, spherical surfactants are

formed, whereas small ratios, such as in the case of nonionic alkyl

polyglycol ethers, lead to the formation of rod-shaped micelles. Rod-

shaped micelles can also be formed by the addition of an electrolyte into

ionic surfactant solutions that leads to the screening of the head groups’

electrostatic repulsive forces which result in a reduction of the head-

group area. Spherical micelles can also transform at higher surfactant

concentrations into rod-shaped or disk-shaped micelles and this change

is indicated by a second critical micelle concentration where there is a

rapid change of colligative properties of a surfactant.13

Gel

Micellar solution

cmc

Monomersolution

T

c

Kraftpoint



12

1.4.2 Classification of surfactants

Surfactants can be classified into the following groups, namely, anionic

(negatively charged head groups), cationic (positively charged head

groups) and nonionic (no surface charge). Examples of each type are

listed in Table 1.1 below.

Table 1.1: Examples of common surfactants

Type Examples Chemical Formula

Sodium dodecyl sulphate CH3 (CH2)11SO4-Na+

Anionic Sodium dodecyl benzene

sulphonate (SDS)CH3 (CH2)11C6H4SO3

-Na+

Cetyltrimethylammonium

bromide (CTAB)CH3 (CH2)15N(CH3) 3

+Br- Cationic

Cetylpyridinium chloride C21H38NCl

Nonionic Polyethylene oxide CH3 (CH2)7(O.CH2CH2)8OH

The structure of the cationic surfactant, namely, cetylpyridinium chloride

that was used for this research study is shown in Fig 1.6 below.

Figure 1.6: Cetylpyridinium chloride structure

1.4.3 Industrial applications of surfactants 14

Surfactants are considered to be of significant importance in various

industries, namely, catalysis, detergent, emulsification, lubrication, oil

recovery industry and the pharmaceutical industry. Some applications

are listed below:

N + C H 3 Cl -



13

• They have been used as solubilizing agents and as probes for the

hydrophobic binding sites in the investigation of molecular properties

of membrane proteins and lipoproteins.

• Surfactants have been used in the investigation of the denaturation

and thermal stability experiments of bacteriorhodopsin.

• They are also employed to promote dissociation of proteins from

nucleic acids on extraction from biological material.

• Surfactants have been used as useful reagents in analytical chemistry

techniques, namely, chromatography and luminescence

spectroscopy.

1.5 Membranes

1.5.1 Overview

Industrial and scientific development during the last 200 years has

brought about a variety of industrial-scale separation techniques such as

distillation, precipitation, crystallization, extraction, adsorption and ion

exchange. However, processes that employ semi-permeable

membranes as separation barriers have recently supplemented these

traditional techniques. Although membranes and membrane processes

were initially introduced as an analytical tool in chemical and biomedical

laboratories, they later developed into industrial products and methods

with greater technical and commercial impact.15,16 A membrane is

defined as an interphase that influences the transport of chemical

species in a particular manner. A membrane structure has lateral

dimensions much greater than its thickness, through which mass

transfer may occur under a variety of driving forces.

17

There are several kinds of membranes that are currently in use, namely:

16



14

• Membranes that are made from synthetic polymers (also copolymers

and blends).

• Inorganic membranes that use inorganic porous supports and

inorganic colloids such as zirconium oxide (ZrO2) or alumina with

appropriate binders.

• Melt-spun “thermal inversion” membranes like hollow fibre

membranes and track-etched membranes.

The great majority of analytically important ultrafiltration membranes

belong to the first type, that is, synthetic polymers. These membranes

are usually made of polycarbonate, cellulose (esters), polyamide, and

polysulphone. The greater interest recently in membrane separation

processes originates from the simplicity of the systems, affordability in

terms of cost, as well as their application in various fields namely,

chemical, food, pharmaceutical as well as wastewater treatment

industries.18 There are unique characteristics that influence the

membrane performance namely: selectivity, speed (flow rate with which

the feed solution passes through the membrane) and the stability that

relies mainly on the components that were used for the manufacturing of

the membrane. Membranes are formed by various methods from

numerous materials both of which profoundly influence the morphology

and structure of the resulting product. Some of the industrial applications

of membranes are listed below. 16

• Membranes (e.g. ultrafiltration (UF), nanofiltration (NF), microfiltration

(MF) and reverse osmosis (RO)) are used to treat industrial waste

effluents to recover valuable constituents.

• They are used to concentrate, purify and/or fractionatemacromolecular solutions in various industries e.g. (UF and MF).

• They are used to produce potable water from seawater.

• They are used to remove urea and some other toxins from the

bloodstream in the medical field (RO).



15

1.5.2 Typical membrane properties

• A membrane can be heterogeneous or homogeneous and/or

asymmetric or symmetric in structure.

• It may be liquid or solid, and can carry neutral, negative or positive

charges.

• A membrane can have varying functional groups with different

binding or complexing capabilities.

• Membrane thicknesses range from 100 nm to more than 1 cm.

• The electrical resistance of a membrane may vary from thousands of

mega ohms to a fraction of an ohm.

1.5.3 Classification of membranes 15, 16

Membranes can be classified into five groups, namely, asymmetric,

electrically charged barriers, homogeneous, liquid films with selective

carriers and microporous membranes as shown in Table 1.2.

Membrane transport can be achieved by diffusion of individual molecules

or through convection induced by the following variables, namely,

concentration, pressure, temperature, or electrical potential gradient.16, 18



16

Table 1.2: Description of different types of membranes

Membrane

type

Structure

description

Separation

process

Preparation

techniques

Microporous Solid matrix

Pore diameter:

(20 µm – 1 nm)

Materials used:

Ceramics, Metals,

and Polymers

Sieving mechanism

determined by pore

diameter and

particle size.

Sintering of

powders,

stretching of films,

irradiation and

etching of films

and phase

inversion

techniques.

Homogeneous Dense film

through which a

mixture of

chemical species

is transported

under the driving

force of

concentration,

pressure, etc.

Mass transport

occurs strictly by

diffusion.

Prepared from

polymers, metals,

or metal alloys by

film-forming

techniques.

Asymmetric Thin skin layer

(0.1 – 1 µm) on a

highly thick

porous

substructure (100

– 200 µm)

Separation

performance

determined by the

nature of the

membrane

material or pore

size

Phase inversion

process that

leads to an

integral structure

deposition of a

thin polymer film

on a microporous

substructure



17

Table 1.2 contd: Description of different types of membranes

Membrane typeStructure

description

Separation

process

Preparation

techniques

Ion exchange Highly swollen

gels carrying

fixed positive or

negative

charges.

Different organic

polymer

matrices and

functional

groups

determine the

ion exchange

properties.

Matrix that

consists of a

hydrophobic

polymer such as

polystyrene,

polyethylene, or

polysulphone.

Fixed ionic

moiety can be

one of the

following: SO3, -

COO, AsO3, -

NH3, -NH2, -NH,

-N

Liquid films Comprise a thin

oil film

separating two

phases

consisting of

aqueous

solutions or gas

mixtures.

Use carriers to

selectively

transport

components

such as metal

ions at a

relatively high

rate across the

membrane

interphase.

The pores of a

microporous

membrane are

filled with the

selective liquid

barrier material

or the

membrane is

stabilised as a

thin oil film by a

surfactant.



18

1.5.4 Membrane modules 19

Membranes can be formed into different configurations e.g. flat sheet,

capillaries or hollow fibre. The advantage of a hollow fibre configuration

is increased surface area. A number of capillary or hollow fibres can be

bundled together to form a module. Fig. 1.7 and Fig. 1.8 show two types

of membrane modules.

Figure 1.7: Hollow fibre module

Figure 1.8: Tubular module



19

1.5.4 Pressure-driven membrane filtration processes

Membrane separation processes can differ significantly with respect to

membrane type, driving forces, areas of application and industrial or

economic relevance. Ultrafiltration, microfiltration and reverse osmosis

are the most important pressure-driven separation processes. The

performance of a membrane in a pressure-driven separation process is

determined by its filtration rate (i.e. transmembrane flux at certain

hydrostatic pressure) and its mass separation properties (i.e. retention

capability).

A summarized schematic diagram of pressure-driven membrane

separation processes and examples of species that can be retained

using those processes is shown in Fig. 1.9 below.

Figure 1.9: Pressure driven membrane separation processes 19

Membrane filtration processes can be applied to the treatment of

industrial waste containing inorganic constituents, such as metal ions, at

low concentrations.



20

The different membrane filtration processes are classified according to

the type of chemical species that is to be retained, the corresponding

membrane pore size that is required to retain that species and the

applicable driving forces as shown in Table 1.3 below which lists the

membrane processes in order of the type of species to be retained.

Table 1.3: Summary of pressure-driven membrane separation

processes 16

ProcessMembrane

type

Driving

force

Separation

methodApplication

Microfiltration Symmetric

microporous

membrane,

0.1 to 10

µm pore

radius

Hydrostatic

pressure

difference,

10 to 500

kPa

Sieving

mechanism

due to pore

radius and

adsorption

Sterile

filtration,

clarification

Ultrafiltration Asymmetric,

microporous

membrane,

1 to 10 nm

pore radius

Hydrostatic

pressure

difference,

0.1 to 1

MPa

Sieving

mechanism

Separation

of macro-

molecular

solutions

Reverse

osmosis

Asymmetric

skin-type

membrane

Hydrostatic

pressure

difference,

2 to 10

MPa

Solution

diffusion

mechanism

Separation

of salts and

microsolutes

from

solutions

1.5.6 Ultrafiltration

Ultrafiltration is one of the several pressure-driven membrane

technologies. It can be used to separate small colloids and large



21

molecules from water and other aqueous media. The system is similar to

microfiltration and differs only in the size of the separated particles and

the membranes used. The ultrafiltration process falls between reverse

osmosis and microfiltration in terms of the size of the particles removed,

with ultrafiltration removing particles in the 0.002 to 0.1 micron range,

and typically rejecting organics over 1000 molecular weight while the

ions and smaller organics are allowed to pass through. A feed solution

containing a mixture of components of different sizes is brought to the

surface of a semi- permeable membrane. Under the driving force of a

hydrostatic pressure gradient, solvent or small solutes permeate the

membrane as filtrate while the larger particles or molecules and unbound

ions are retained by the membrane and concentrated in the retentate.

The separation is based solely on a sieving effect and particles are

separated solely according to their dimensions. This process is regarded

as an alternative to conventional and other traditional clarification

techniques like flocculation, sedimentation, etc.20

1.5.6.1 Micellar-enhanced ultrafiltration for metal ion retention

Supported liquid membranes, where a complexing agent is added to an

organic solvent contained in a porous hydrophobic membrane support

have been applied to metal ion separation, but the use of these methods

on an industrial scale is limited by the inherent instability of such

membranes. Other techniques such as reverse osmosis and

nanofiltration membrane systems that are able to retain metal ions can

also be applied but have limitations that are mainly due to their high

costs which result from high transmembrane pressures and product

fluxes which tend to be generally low.3

Micellar-enhanced ultrafiltration has been shown to be an effective

method for the removal of low levels of toxic heavy metal ions and

organic compounds from industrial waste effluents.4



22

In micellar-enhanced ultrafiltration, a surfactant is added to an aqueous

solution containing the solutes of interest (metal ions) at a concentration

higher than its critical micelle concentration (cmc) in order to capture

ionic solutes. Once in solution, the surfactant forms large amphiphilic

aggregate micelles with charged surfaces, ranging from 50 to 100

surfactant molecules. This phenomenon of micelle formation is called

micellisation. The metal ions of opposite charge adsorb onto the charged

micelle surface. The micelles increase the hydrodynamic size of the

metal ion solutes that brings about their retention by the ultrafiltration

membranes. See Fig 1.10 below.

Figure 1.10: A mechanistic scheme showing the adsorption of an anionic

metal species such as Pt (lV) onto a cationic micelle

Among the advantages of this method are the low-energy requirements

of the ultrafiltration process and its high removal efficiency owing to the

effective trapping of the metal ion solutes by the micelles. MEUF has

been used to separate organic pollutants, heavy metals, chromates,

nitrates and sulphates using both cationic and anionic surfactants as well

as polyelectrolytes as additives.4, 21

Cationic surfactant monomer

Retentate

PtCl62-

Permeate

_

_

_

+

+

+

+++

++

+

+ +++ +_

_

_

_

_

+ Cationic surfactant monomer

Retentate

PtCl62-

Permeate

_

_

_

+

+

+

+++

++

+

+ +++ +_

_

_

_

_

+



23

1.6 Research objectives

In this research project, the ability of a membrane ultrafiltration system

that is commonly used for the separation as well as the recovery of

dissolved molecules or colloids in aqueous medium on the basis of

molecular size was investigated for the retention of platinum group

metals, namely, palladium and platinum. Within this membrane

ultrafiltration system, a suitable cationic surfactant, namely,

cetylpyridinium chloride (CPC), was introduced in order to form colloidal

aggregates that would increase the hydrodynamic size of the metal ions

thus improving their retention by the system. The resulting process is

referred to as micellar-enhanced ultrafiltration (MEUF). Variables such

as pressure, temperature, pH, surfactant: metal ratio (S:M) and the effect

of using electrolytes like sodium chloride were investigated in order to

establish the conditions that can achieve the highest retention of

platinum and palladium anions as well as surfactant from industrial

waste streams.



24

CHAPTER TWO

EXPERIMENTAL AND KEY PERFORMANCE ASPECTS

In this chapter, the reagents, apparatus or equipment as well as

analytical and synthetic procedures that were used during this research

study will be described. The key performance aspects of the membrane

module will also be explained.

2.1 MaterialsAll materials that were used in the preparation of synthetic solutions with

their sources and respective grades are listed in Table 2.1 and were

used without purification.

Table 2.1: List of chemicals used for synthesis and analysis

CHEMICAL NAME FORMULA SOURCE GRADE

Cetylpyridinium chloride

(98%)

C21H38ClN.H2O Aldrich AR

Dichloromethane CH2Cl2 Saarchem AR

Sodium hydroxide NaOH Aldrich AR

Hydrochloric acid (32%) HCl Saarchem AR

Nitric acid (55%) HNO3 Saarchem AR

Sodium

hexachloroplatinate (lV)

Na2PtCl6 Aldrich CP

Potassium

hexachoropalladate (IV)

K2PdCl6 Aldrich CP

Sodium hypochlorite NaOCl Saarchem AR

Potassium chloride KCl Saarchem AR

Sodium chloride NaCl Saarchem AR

AR: Analytical reagent

CP: Chemically pure



25

2.2 Synthetic procedures

2.2.1 Preparation of surfactant solutions

A calculated amount of cetylpyridinium chloride salt was weighed and

dissolved in deionized water to make a 100 mM surfactant stock

solution. Aliquot amounts were taken from the prepared stock solution to

prepare surfactant solutions of different concentrations for ultrafiltration

experiments.

2.2.2 Preparation of surfactant/metal solutions

1.0 mM metal stock solutions were prepared by dissolving calculated

amounts of either Na2PtCl6 or K2PdCl6 salt in 5.0 ml aqua regia solution

(vol. ratio HCl:HNO3 = 3:1) or other acid mixture in a small beaker and

transferring into a 500 ml volumetric flask. These prepared stock

solutions were used to prepare surfactant/metal solutions by taking

aliquots of the metal stock solutions and adding them to various amounts

of cetylpyridinium chloride. Other experimental variables such as pH,

electrolyte concentration and temperature etc. were adjusted as required

by the reaction conditions prior to the ultrafiltration process.

2.2.3 Membrane module preparation

A bundle of double skinned polysulphone hollow fibre membranes

(twelve per unit) of approximately 30 nm pore sizes, 10 kD MWCO

manufactured at the Institute of Polymer Science at Stellenbosch

University was prepared and properly fitted into a stainless steel

membrane tube. A quickset epoxy resin was used to seal the

membranes into the tube in order to ensure that there were no leaks of

metal feed solution during an ultrafiltration experiment. SEM images of

the cross-section and surface of one of the hollow fibres were obtained



26

using the electron microscope in the Physics Department of the NMMU.

These images are found in Figs. 2.1 and 2.2 respectively.

Figure 2.1: SEM image of a cross-section of the polysulphone hollow

fibre membrane

Figure 2.2: SEM image of a part of the outer surface of the polysulphone

hollow fibre membrane



27

2.3 Apparatus

A laboratory scale membrane ultrafiltration system was used in all

ultrafiltration experiments carried out in this research study.

2.3.1 Ultrafiltration system set-up

The design of a membrane filtration system is very important as it

determines the rates of physical transport phenomena associated with

the chemical process or reaction, which in turn determines the outcome

of the chemical reaction.22, 23 To have an ideal and uniform micellisation

process occurring during the ultrafiltration experiments, the feed

reservoir should have a sufficient volume to contain a certain amount of

matter for a certain period of time. It should also offer the possibility for

sufficient contact between the reactants. The feed metal ion solution was

contained in a 300 cm3 beaker that was housed in a controlled

temperature water bath and mechanically agitated. A four-bladed stirrer

was used for agitation at approximately 300 rpm. A peristaltic pump was

used to pump the feed solution through the membrane module, with the

feed, retentate and permeate samples taken at specified intervals. A

photograph of the laboratory scale ultrafiltration unit is shown in Fig. 2.3

and a diagrammatic representation is depicted in Fig. 2.4. Table 2.2 lists

the major components of the system.



28

90

Figure 2.3: Photo of a laboratory scale MEUF system

Table 2.2: Major components of lab scale MEUF system

Component Name

A Feed vessel

B Peristaltic pump

C Pressure gauge

D Membrane module

E Permeate vessel

F Pressure release valve

G Overhead stirrer

H Thermostat bath

B

A

C D

E

F

G

H



29

PermeateFeed

solution

Retentate

Pressure

gauge

Membrane (hollow fibre) in S/S casing

Pressure

control

valve

Thermostat bath

Stirrer

Figure 2.4: Schematic diagram of MEUF system

2.3.2 Major components of Ultrafiltration system

2.3.2.1 Membrane module (D)

This unit is a stainless steel pipe that was used to house the

polysulphone membranes used during the ultrafiltration process. It is

about 38 cm long and is held firmly at a slight angle to the horizontal to

allow continuous and uniform flow of the feed solution through the

membrane and also to prevent the membranes from folding inside it and

subsequently being damaged. A list of membrane filter unit

specifications is shown in Table 2.3.

Table 2.3: Membrane module specifications

Variable Value (units)

No of fibres 12

Length 38 cm

Outside diameter 2.0 mm

Membrane Area 0.024 m2

Peristaltic pump



30

2.3.2.2 Peristaltic pump (B)

The pump was used to pump the feed solution through the membranes

from the inside to the outside of each hollow fibre during the ultrafiltration

process. It was driven at a constant speed of 45 rpm in all the

ultrafiltration experiments. The pump played an important role as it

helped to maintain the consistent flow of the feed solution in all the

experiments. Peristaltic pump specifications are shown in Table 2.4

below.

Table 2.4: Peristaltic pump specifications

Variable Value (units)

Model Gilson’s Minipulse 3

Speed 0 - 48 rpm

Operating Pressure Range 0 - 600 kPa

2.3.2.3 Controlled temperature bath (H)

A stainless steel controlled temperature water bath was used to control

and maintain a constant temperature in all the ultrafiltration experiments.

2.3.2.4 Operation of MEUF

The feed solution was pumped axially across the membrane surface to

ensure tangential flow that reduces the concentration polarization

phenomenon near the membrane surface. Permeate and retentate

streams were recycled into the feed reservoir to maintain steady state

conditions. Initial feed, retentate and permeate samples were collected

after the completion of each ultrafiltration run and analysed for metal ion

and surfactant content by Inductively Coupled Plasma Mass



31

Spectrometry (ICP-MS) and Ultraviolet-visible spectrophotometry

respectively. Every UF run was performed in duplicate, and the results

averaged.

2.4 Analytical techniques

The methods that were employed for the analyses of the surfactant and

platinum group metals together with the parameters and the instrumental

settings are described below.

2.4.1 Ultraviolet visible spectrophotometry (UV-VIS)

UV-visible analysis of feed and permeate samples were performed on a

Beckman DU-650 spectrophotometer to determine the surfactant

content. A 0.1 cm quartz cell was used. 24, 25 The surfactant,

cetylpyridinium chloride absorbs in the UV region. The wavelength of

maximum absorption occurs at 260 nm and this was thus chosen as the

best wavelength for spectral analysis. Feed samples (20 ml) were taken

from the untreated solution containing platinum group metals, prior to an

ultrafiltration run while the permeate samples were taken at the

permeate outlet at the end of each ultrafiltration run.

2.4.1.1 CPC calibration curve

Fig. 2.5 below shows the CPC calibration curve that was used

throughout this study for the conversion of CPC absorbance data to

concentration in units of mM. Calculated amounts of CPC salt were

weighed and dissolved in deionised water to make up standards and

analysed for CPC content by UV-VIS spectrophotometry at 260 nm

against water as blank. The data pertaining to this experiment can be

found in Appendix A, Table A1.



32

y = 4.219x

R2 = 0.9992

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

0 0.05 0.1 0.15 0.2 0.25

CPC concentration (mM)

a b s

Figure 2.5: CPC calibration curve

2.4.2 Inductively coupled plasma mass spectrometry (ICP-MS)

Due to its sensitivity and low detection limit 8, 9, 26, a Perkin-Elmer Sciex

6100 ICP-MS with a Perkin-Elmer AS 90 autosampler was chosen asthe best instrument to perform analysis of the PGMs investigated in this

research. A “spectrascan” certified multi element standard Au, Ir, Os, Pd,

Pt, Re, Rh and Ru at 100 µg/ml was used to prepare the standards

ranging from 5.0 to 1000 µg/L in 1% (v/v) of hydrochloric acid. Also, an

In 1000 µg/ml standard solution was used to prepare a 100 µg/L In

solution in 1% (v/v) hydrochloric acid. This In standard solution was

mixed online and the In was used as an internal standard to counter the

matrix effect. The instrumental settings of the Perkin Elmer ICP-MS are

listed in Table 2.5 below.



33

Table 2.5: ICP-MS settings

Parameter Value (units)

Nebuliser Flow rate 0.71 L/min

RF power 1400 W

Detection limit 10 – 20 µg/L

All the metal ion/surfactant samples were diluted either 100 or 200 times

in order to fall within the detectable range of the ICP-MS for metal

analysis and a second set diluted 40 or 100 times for UV-VIS surfactant

analysis.

2.4.3 Conductometer

In this study, a Metrohm 660 conductometer with conductivity cell (model

no. 6.0908.110) coupled with a platinum thermocouple electrode was

used for conductivity measurements of the surfactant solutions with

various additives.

2.5 Key Performance Aspects of the Ultrafiltration system

2.5.1 Principle and Operation of MEUF system

An untreated solution containing the micelles and the solutes of interest

(PGMs) is forced against a semi-permeable membrane. The feed

solution flows through the hollow fibre ultrafiltration membrane with pore

sizes small enough to block the passage of micelles. The rejection of

micelles brings about the removal of the solutes of interest while the

unbound ions and surfactant monomers are able to pass through the

ultrafiltration membrane to the permeate outlet. The operation of the

MEUF system was performed in batch mode of 20 minute runs. Fig. 2.6

shows a typical batch mode operation.



34

Figure 2.6: A typical layout of a batch Ultrafiltration system 32

2.5.2 Assessment and monitoring of the membrane condition

It is vital that the membrane condition is kept the same for all

ultrafiltration experiments, thus the membrane performance is measured

prior and after each experiment by taking pure water flux (PWF)

measurements. The pure water flux measurements before and after

various ultrafiltration runs and after back flushing at 100 kPa are plotted

against time as shown in Fig. 2.7 below. The pure water flux after each

run with either Pt, Pd or CPC alone was significantly reduced indicating

fouling of the membrane surface. However, after back flushing the

membrane for about 15 minutes, the original pure water flux was

restored. This indicates that the fouling is reversible.



35

0

20

40

60

80

100

0 20 40 60 80 100 120

Time (minutes)

P e r m e a t e F l u x ( L / M 2 h )

Original PWF After Pd run After Pt run

After backflushing After CPC run

Figure 2.7: Pure water flux measurements at 100 kPa taken after various

ultrafiltration experiments

2.5.3 Regeneration of an ultrafiltration membrane 18, 30

During the ultrafiltration process, a membrane can either be permanently

or temporarily damaged depending on the nature of the species that is

being purified or recovered. This damage of the membrane can be due

to membrane pore clogging which occurs as a result of a cake or gel

layer formation on the membrane surface. However, a temporary gel

layer due to concentration polarization onto the membrane surface is

sometimes desirable as it helps to improve the retention of the targeted

species. Although a temporary gel layer on the membrane surface can

be reversed by back-flushing the membrane, a permanent cake or gel

layer is not desired as it reduces the membrane performance.



36

2.5.4 Fouling in membrane processes

Fouling is the result of irreversible deposition or adsorption of solutes

onto the membrane surface or pores. It has been established that an

irreversible fouling may lead to the following:

• Flux decline

• Change in membrane selectivity and / or retention

2.5.4.1 Factors affecting fouling

The following factors can lead to fouling of a membrane:

• Unfavourable hydrodynamics

• Unfavourable membrane-solute interaction

• Membrane surface topography

• Hydrophilicity of the membrane

• Pore morphology and size distribution

• Membrane surface modification

2.5.4.2 Common fouling problems

A number of foulants that have been found to have a major impact in

membrane fouling in various ultrafiltration processes are listed in Table

2.6 below.



37

Table 2.6: Membrane foulants and control measures

Process Foulants Control measures

Generic

Improve

hydrodynamics.

Shear back flush. Air

scouring. Controlled

flux operation.

Hydrocarbon surfactants Limit concentration

Proteins

Control ionic

environment, pH, use

porous membranes

Ultrafiltration and

Microfiltration

Biological solids, bacteria,

Flux control

Prefiltration

Use hydrophilic and

more porous

membranes

2.5.5 Membrane cleaning 18

Membrane cleaning is an essential requirement in membrane process

applications. The frequency of cleaning entirely depends on the rate of

fouling and operating protocol used, hence the cleaning may either be

necessary after each ultrafiltration process or after months of continuous

use of the membranes. The condition and performance of an

ultrafiltration membrane are maintained by closely monitoring the pure

water flux measurements. This can be approached in several ways, with

the main aim being to minimize the adsorption of substances onto the

membrane surface, thereby reducing the risk of shortening the lifespan

of an ultrafiltration membrane, the amount of a cleansing reagent

required, as well as the frequency of cleaning. Studies have shown that

there is no specific technique that can prevent certain substances, such



38

as proteins, lipids, and phenolic substances from adsorbing onto the

membrane surface.

Pure water flux measurements were taken prior to ultrafiltration

experiments and used as a reference and also after each experimental

run to monitor the condition of the ultrafiltration membrane. Upon

observing that the pure water flux measurements deviated significantly

from what they were prior to ultrafiltration experiments, necessary steps

that involved the following were taken, namely:

• Linear flush that can be used, for any membrane module for loosely

bound species and is a useful step before applying chemical

cleaning processes.

• Backflushing the entire system with pure water with, or without,

applied pressure.

• Using 0.1 mM of sodium hydroxide followed by 1.4 mM of

hypochlorite and flushing with 1 mM of hydrochloric acid.

The procedure used for cleaning was found to be effective as it was able

to restore the pure water flux measurements to their original and normal

values that were used as reference measurements throughout the

research studies.



39

CHAPTER THREE

PRELIMINARY INVESTIGATION OF SURFACTANT AND

METAL ION RETENTION

This chapter involves the preliminary investigation of the factors which

influence the retention of cetylpyridinium chloride by a specific

ultrafiltration membrane. The capability of the micellar-enhanced

ultrafiltration (MEUF) system to retain palladium (ll) and platinum (lV)

anionic species with the aid of a cationic surfactant, namely,

cetylpyridinium chloride, from synthetic aqueous solutions was also

investigated as well as the possibility of separating these two metal ions

from a synthetic mixture typical of the industrial waste effluent. The

results are presented here in the form of graphs, with the data tables

pertaining to each graph contained in Appendix A.

3.1 Definitions 18

Some of the key concepts or terms that will be used for discussion in this

research study are defined below:

• A membrane is a thin barrier or film between two phases that allows

preferential transport of some species over others.

• Concentration polarization is the build-up of concentration of the

species to be rejected near the membrane surface (Cm) leading to a

higher concentration of the species near the membrane surface, (Cm)

compared to the species concentration in the bulk of the solution,

(Cb). This can lead to the formation of a temporary cake layer or in

the worst case the formation of a permanent cake layer on the

membrane surface.



40

• Flux is the throughput of solution per unit area and is defined as

follows:

Jv = permeability x driving force, and is expressed as

Litres/m2h (LMH)

• Retention (rejection) = (1 – Cp /Cf) x 100%

Cp: permeate concentration

Cf : feed concentration

• Molecular weight cut off (MWCO) is the molecular mass of

solute with 90% retention by the membrane.

3.2 Influence of various parameters

In this section, an investigation of the factors influencing the retention of

the surfactant was done. Among the factors that were investigated were:

pressure variation, temperature variation and the effect of increasing

surfactant concentration.

3.2.1 Effect of pressure variation

This experiment was done in order to determine the pressure at which

the best compromise between retention of the surfactant and permeate

flux could be obtained. A set of cetylpyridinium chloride solutions of

constant concentration (10 mM) was prepared by dissolving a calculated

amount of CPC salt in water in a 250 ml volumetric flask and then

transferring it into a suitable feed container for the ultrafiltration

experiments. Ultrafiltration runs were done at various pressures ranging

from 50 to 250 kPa. The temperature was kept constant at 30°C in order

to prevent any precipitation of the surfactant. Each ultrafiltration

experiment was run for 20 minutes and at the end of each experiment,

20 ml permeate (P) and feed (F) samples were collected for CPC

analysis by UV-VIS spectrophotometry. The results for this investigation

are shown in Fig. 3.1 below with the data in Table A2 in Appendix A.



41

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0

100.0

0 50 100 150 200 250 300 Pressure (kPa)

C P C r e t e n t i o n ( %

)

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0

P e r m e a t e F l u x ( L / m

2 h )

CPC retention Permeate flux Figure 3.1: Effect of pressure variation at 10 mM CPC and 30°C

From Fig. 3.1 it can be seen that cetylpyridinium chloride retention

decreased slightly initially and then more dramatically beyond 150 kPa

while the permeate flux increased continuously with increasing pressure

up to 200 kPa after which it decreased. The decreasing retention with

increasing pressure is a consequence of greater mass transport at

higher pressures allowing more surfactant to pass through the

membrane. This is also reflected by the increasing flux measurements

with increasing pressure, up to 200 kPa. The sharp decline in flux at

pressures greater than 200 kPa is due to concentration polarization near

the membrane surface. A pressure of 150 kPa was chosen for all further

experiments as this was seen to be the best compromise between

permeate flux and retention.



42

3.2.2 Effect of temperature variation

This investigation was done in order to establish a suitable temperature

that would enable complete micellisation of CPC which would in turn

lead to optimum retention of the surfactant. A set of cetylpyridinium

chloride solutions of constant concentration (10 mM) was prepared by

dissolving a suitable amount of CPC salt in deionised water in a 250 ml

volumetric flask and then transferred into a suitable container that was

used for ultrafiltration experiments. Ultrafiltration runs were done at

various temperatures ranging from 10 to 50°C. The pressure was kept

constant at 150 kPa. Each ultrafiltration experiment was run for 20

minutes and at the end of each experiment, 20 ml permeate and

retentate samples were collected for CPC analysis by UV-VIS

spectrophotometry. Results for this investigation are shown in Fig. 3.2

below with the data in Table A3 in Appendix A.

Figure 3.2: Effect of temperature variation at 10 mM CPC and 150 kPa

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

0.0 10.0 20.0 30.0 40.0 50.0 60.0

Temperature (degrees)

C P C

r e t e n t i o n ( % )

0.0

20.0

40.0

60.0

80.0

100.0

120.0

140.0

160.0

180.0

200.0

P e r m e a t e F l u x ( L / m 2 h )

CPC retention Permeate flux



43

Looking at Fig 3.2 above, it can be seen that at temperatures below

20°C, cetylpyridinium chloride retention is very low, despite choosing a

concentration value (10 mM) that is well above the critical micelle

concentration that is required for this surfactant to form micelles. The low

retention in this temperature region (0°C to 20°C) can be attributed to

the fact that ionic surfactants only form micelles when the hydrocarbon

chains are sufficiently fluid at temperature above the chain melting

temperature. This temperature region is thus below the Kraft

temperature, the point below which a surfactant becomes insoluble.

A significant increase in retention occurred at temperatures above 20°C,

with maximum retention, greater than 80%, achieved at around 30°C.

The higher retention observed at this point can be attributed to complete

micellisation assumed to have occurred. However, beyond 30°C a

significant decrease in CPC retention was observed and this might be

due to greater mass transport as a result of higher temperatures that

permits the passage of the monomers and some micelles through the

membrane pores.

However, while the CPC retention showed three different behaviours

with increasing temperature, the flux measurements showed a

continuous decrease with increasing temperature with a gradual

levelling-off at higher temperatures. It can be assumed that the initial flux

decline was due to increased numbers of micelles in solution and

concentration polarization at the membrane surface which correlates

with the increasing retention of surfactant. The latter levelling-off effect

could be a result of partial membrane pore clogging due to the presence

of micelles in the pores.



44

3.2.3 Effect of cetylpyridinium chloride concentration variation

A study to determine the surfactant concentration at which sufficient

micellisation would occur was done. A set of cetylpyridinium chloride

solutions of varying concentrations ranging from 0 to 45 mM was

prepared by weighing suitable amounts of CPC salt. The respective salts

were dissolved in deionised water in 250 ml volumetric flasks and then

transferred into a suitable container that was used for ultrafiltration

experiments. Ultrafiltration runs were done at a constant temperature

(30°C) and at a pressure of 150 kPa. A temperature of 30°C was chosen

as this was the temperature at which maximum retention of CPC

occurred. Each ultrafiltration experiment was run for 20 minutes and at

the end of each experiment, 20 ml permeate and retentate samples were

collected for CPC analysis by UV-VIS spectrophotometry. The results for

this investigation are shown in Fig. 3.3 below with the data in Table A4 in

Appendix A.

0.0 10.0 20.0 30.0 40.0 50.0 60.0 70.0 80.0 90.0

100.0

0.0 10.0 20.0 30.0 40.0 50.0 CPC concentration (mM)

C P C r e t e n t i o

n ( % )

0.0 20.0 40.0 60.0 80.0 100.0 120.0 140.0 160.0


CPC retention) Permeate flux

Figure 3.3: Effect of CPC concentration variation at 30

C and 150 kPa



45

From Fig. 3.3 above, it is quite clear that an increase in cetylpyridinium

chloride concentration led to a continuous gradual increase in

cetylpyridinium chloride retention. This consistent increase started at

above 75% cetylpyridinium chloride retention in the lower concentration

range of cetylpyridinium chloride (0 to 10 mM) with the maximum

retention observed just above 95% at 45 mM cetylpyridinium chloride.

The high cetylpyridinium chloride retention at greater CPC concentration

can be ascribed to two factors, namely, concentration polarization and

sufficient micellisation. As has been stated earlier, concentration

polarization on the membrane surface is sometimes beneficial as it helps

to enhance the retention of species of particular interest. It can also be

said that the higher retention of cetylpyridinium chloride confirms that the

very small number of monomers present in solution is due to fairly

complete micellisation that is achieved with increasing concentration of

cetylpyridinium chloride. While it was possible to achieve the desired

high retention of cetylpyridinium chloride of more than 95% at 45 mM of

cetylpyridinium chloride, it was also observed that, the flux

measurements decreased sharply with increasing surfactant

concentration and levelled at high CPC concentration. This product flux

decline can be attributed initially to the increased number of micelles in

solution as a result of an increased CPC concentration and finally to the

concentration polarization phenomenon close to the membrane surface.

3.3 Micellar-enhanced ultrafiltration of Pt (lV) and Pd (ll) anions

Several experiments were performed in order to establish the effect of

parameters such as the surfactant: metal ion ratio (S : M), the pH of the

solution and the presence of an electrolyte on the retention of both the

metal ions and the surfactant.

3.3.1 MEUF of Pt (lV) and Pd (ll) anions in acidic medium

This experiment was done in order to study the potential of the system

to remove the metal anions of interest, viz, PtCl62- and PdCl4

2- from a



46

synthetic aqueous solution. The synthetic solutions were prepared by

dissolving a suitable pre-weighed PGM chloride salt in aqua regia (vol

ratio of HNO3 : HCl = 1: 3) solution and introducing various amounts of

CPC solution as required per ultrafiltration run. For these experiments,

the metal ion concentration was kept constant at 0.1 mM while the CPC

concentration was varied from 0 to 40 mM. The temperature and

pressure were kept constant at 30°C and 150 kPa respectively. Each

ultrafiltration experiment was done in duplicate and the samples were

analysed as stated in section 2.4. The analysis data was averaged and

used to construct the curves as shown in Fig. 3.4 below. The data can

be obtained in Tables A5 and A6 in Appendix A.

0

20

40

60

80

100

120

0 5 10 15 20 25 30 35 40 45


M e t a l i o n

r e t e n t i o n

( %

)

Pt retention CPC retention in Pt (lV)

Pd retention CPC retention in Pd (lV)

Figure 3.4: Investigation of Pt and Pd anions retention at 150 kPa and

30°C in acidic medium as a function of CPC concentration

Looking at the curves in Fig. 3.4 above, it is evident that, the MEUF

system is able to retain the platinum group metal ions namely Pt (lV) and

Pd (ll) from a synthetic solution similar to that of an industrial waste

effluent. The retention of the two metal ions was very similar with a slight

decrease in retention with increasing CPC concentration being observed



47

in both cases. The retention values for Pt (lV) decreased from 98.9% to

94.4% and for Pd (ll) from 98.8% to 95.5% as CPC values increased

from 1 to 40 mM. However, although the MEUF system was able to

achieve such high retentions for both metal ions, the retention of the

surfactant (CPC) in the presence of each metal ion decreased

significantly with the highest retention obtained at only 60% and the

lowest retention well below 20%. This dramatic decline in cetylpyridinium

chloride retention may be due to the presence of HCl and HNO3 in the

solution. These acids are electrolytes and as stated in section 1.4.1 on p

11 electrolytes may affect the micellisation process and might cause the

micelles to change their shape from a spherical shape to a rod-like

structure with larger surface area but smaller diameter.13 These rods

might be small enough to be able to pass through the membrane pores.

The decline in CPC retention was greater in the Pt (lV) solution than in

the Pd (ll) solution.

A possible explanation for the fact that the metal ion retention was close

to 100% even though the surfactant retention was very low could be that

the larger metal anions are partly responsible for the change in micelle

shape due to their need for a larger surface area. Since the metal ion

concentration is very low compared to the surfactant concentration, and

their tendency to adsorb onto the metal surface is greater than that of

supporting electrolyte, practically all metal ions are adsorbed from the

solution. The hydrodynamic size of a micelle containing PdCl42- or PtCl6

2-

anions is larger than that of a micelle containing only chloride or nitrate

anions, thus the metal-containing micelles are retained by the membrane

while the remainder of the possibly rod-shaped micelles and

unaggregated monomers pass through the pores. Another possible

explanation for the fact that the metal ions are retained so well could be

the interaction of the metal anions with the charged head groups of

surfactant molecules adsorbed at the membrane surface.3,4,21

The significant decline in cetylpyridinium chloride retention in the

presence of the metal ions, led to a further investigation of the influence



48

of the different electrolytes on the retention of CPC. This will be dealt

with in Chapter 4.

3.3.2 MEUF of Pt (lV) and Pd (ll) anions in neutral medium

Ultrafiltration of the PGMs in neutral medium was done in order to study

the effect of pH on metal ion retention. The ultrafiltration solutions were

prepared in the same manner as in 3.3.1 above except that the pH of the

solutions was adjusted to a value of 7 +/- 0.02 using a 10% sodium

hydroxide solution prior to starting each ultrafiltration experiment. The

ultrafiltration runs were performed in duplicate and the data obtained

from sample analysis were averaged and used to construct the curves

as shown in Fig. 3.5 below. The data can be found in Tables A7 to A9 in

Appendix A.

88.0

90.0

92.0

94.0

96.0

98.0

100.0

0 20 40 60 80 100 120


m

e t a l i o n

r e t e

n t i o n (

%

)

Pt retention Pd retention

Figure 3.5: Investigation of Pt and Pd anions retention at 150 kPa and

30°C in neutral medium as a function of CPC concentration



49

0

10

20

30

40

50

60

70

80

90

100

0 1 2 3 4 5 6 7 8

p Cl

% o

f P

d ( I I )

It can be seen in Fig. 3.5 that in neutral medium there is a difference in

the retention behaviour between Pd (ll) and Pt (lV), with Pt (lV) being

retained to a greater extent than Pd (ll). This difference becomes more

significant at higher CPC concentrations. However, both metal ions

experience a decrease in retention with increasing CPC concentration.

This could be related once again to the electrolyte effect, which has a

bigger effect on retention values, in this case due the added Na+ ions.

Since it is well known that Pd (lV) immediately reduces to Pd (ll) in

solution due to the instability of Pd (lV) solutions, species distribution

curves of the Pd (ll) / Cl – system as shown in Fig 3.6 below will be used

to predict and explain the observations that occurred during the

ultrafiltration of a mixture of Pt (lV) and Pd (ll) anions in aqueous

solutions.

Figure 3.6: Species distribution curves of Pd/Chloride system (generated

by IUPAC Stability Constant Database speciation software¡

IUPAC

and Academic Software 2000)

Pd2+

PdCl+

PdCl2

PdCl3-

PdCl42-



50

From the species distribution curve, it can be seen that at pCl = 1

(chloride range in which solutions are prepared) Pd (ll) is present as

PdCl42- and PdCl3(H2O)-. In neutral medium, OH activity is greater than

in acidic medium, thus the possibility of forming hydrated mixed

chloro/hydroxo species is greater. Also, ligand exchange of Pd (II) is

105 times faster than that of Pt (IV) since exchange can take place via

an associative mechanism with a 5-coordinate intermediate for square-

planar Pd (II), which is 4-coordinated, whereas Pt (IV) tends to be

octahedral (6-coordinated). Thus it is more likely that hydrated species

are formed for Pd (II) rather than Pt (IV). Hydrated species are more

hydrophilic and interact with water by hydrogen bonding and thus have a

smaller tendency to associate with a micelle. This could account for the

lower retention of Pd (ll) in neutral medium. 8,24,26

3.3.3 MEUF of a mixture of Pt (lV) and Pd (ll) anions in neutral

medium

The experimental solutions were prepared by mixing equal quantities

(0.1 mM each) of the two metal ions and varying the CPC concentration

in the range of 0 to 100 mM. The pH of the ultrafiltration solutions was

adjusted to a pH value of 7± 0.2. Other experimental variables, namely,

the pressure and temperature were kept at 150 kPa and 30°C

respectively. Samples were drawn at the end of each ultrafiltration run

and prepared for the analysis of both metal ions (Pt and Pd) and the

surfactant (CPC) as described in section 2.4. The results are presented

in Fig. 3.7 below. The data can be found in Table A10 in Appendix A.



51

0.0

20.0

40.0

60.0

80.0

100.0

120.0

0 20 40 60 80 100 120

[CPC] mM

m e t a l i o n r e t e n t i o

n ( % )

Pd retention Pt retention

Figure 3.7: Investigation of Pt and Pd anions separation in neutral

medium at 150 kPa, 30°C as a function of CPC concentration

From a closer look at Fig. 3.7 above, it is evident that the system can be

used to partially separate the two metal ions in a mixture at high CPC

concentrations. High retention for Pt (lV) that was close to 100%

throughout the studied CPC concentration range was obtained, while Pd(ll) was retained similarly at CPC concentration up to 20 mM with a

significant decrease in its retention after 20 mM CPC concentration to

reach a minimum value of around 80% at CPC concentration of 100 mM.

Also, it can be noted that there was a maximum of about 20% difference

in terms of the retention of the two metal ions, and this difference

indicates a competition for adsorption on the available oppositely

charged sites on the micelle surface with PtCl62- anions being

preferentially adsorbed onto the micelles. This would be due to the

reasons discussed in section 3.3.2.



52

3.3.4 Effect of an electrolyte on metal ion and surfactant retention

The experimental solutions were prepared as in section 3.3.1 above

except that the CPC concentration was kept constant at 40 mM and

varying amounts ranging from 0 to 100 mM of an electrolyte namely,

sodium chloride were introduced into the metal ion (0.1 mM) and

surfactant solutions prior to diluting them to the mark. The pH of the

solutions was not adjusted and ranged between 4.6 and 5.3. Duplicate

ultrafiltration runs were carried out as usual. Samples were taken and

prepared for metal ion and CPC analysis. Sample analysis data was

compiled accordingly and the averaged values were used to construct

the curves shown in Fig. 3.8 below. The corresponding data tables are

Table A11 (1) and Table A11 (4) in Appendix A.

0

20

40

60

80

100

120

0.00 20.00 40.00 60.00 80.00 100.00 120.00

[NaCl] (mM)

m e t a l i o n / C P

C

r e t e n t i o n ( % )

Pd retention Pt retention CPC with Pd CPC with Pt

Figure 3.8: Effects of sodium chloride on retention of Pt and Pd anions

in acidic medium at 150 kPa and 30°C using 40 mM CPC

It is well known that the industrial waste streams often contain some

electrolytes that are used during chemical processing. Also, it is well

known that the presence of electrolytes can influence the properties of a



53

surfactant13 and may affect the removal of the metal ions. It has been

reported in literature,3,4,21 that the introduction of an electrolyte reduces

the critical micelle concentration at which micelles form thereby

increasing the number of micelles in solution and thus the micelle

retention. Increased micelle retention was not observed here as can be

seen in the curves in Fig. 3.8 above. The retention of the two metal ions

remained fairly constant at above 90% throughout the sodium chloride

concentration range. However, the retention of the surfactant was greatly

reduced in the presence of sodium chloride especially during the

ultrafiltration of platinum as it significantly decreased from just above

40% to just below 10%. This dramatic decrease in cetylpyridinium

chloride retention cannot be attributed to the disruption of micellisation

since the metal ion retention was still high in the presence of an

electrolyte but might be due to a change of shape of the micelles from

spherical to rod-like as it has been noted in section 3.3.1. Although

during the ultrafiltration of palladium, the cetylpyridinium chloride

retention was still higher at lower sodium chloride concentration range (0

to 30 mM) than in the ultrafiltration of platinum, it showed a consistent

decrease after 40 mM sodium chloride concentration from just above

80% to close to 40%. It is clear that the Pt (lV) anion has a much greater

effect on the retention of the CPC micelles than the Pd (ll) anions. This is

possibly due to the double charge of the PtCl62- compared with

PdCl3(H2O)-; the PtCl62- having a greater screening effect than

PdCl3(H2O)- and thus might more readily promote a change of shape of

the micelle.

3.4 Membrane interaction

Membrane interaction with a variety of species was also studied by care-

fully monitoring the variation of the permeate flux measurements during

all the ultrafiltration runs. Flux data was compiled from duplicate

measurements and the averaged values were used to construct the

curves shown in Fig. 3.9 below. The data is found in Table A12.



54

0 20 40 60 80

100 120 140

0.0 20.0 40.0 60.0 80.0 100.0 120.0 [CPC] mM

P e r m e a t e F l u x ( L

/ M 2 h r )

CPC alone CPC + 0.1 mM Pd CPC + 0.1 mM Pt CPC + 0.1 mM Pd/Pt mixture

Figure 3.9: Effect of various species on flux at 150 kPa and 30°C

Flux decline followed the usual trend with the decrease in permeate flux

observed with increasing surfactant concentration (from 0 to 100 mM).

The initial steep flux decline can be attributed to an increased number of

micelles in solution with increasing CPC concentration and the

subsequent flatter portion of the curves is due to concentration

polarization near the membrane surface. Based on these findings, it can

be seen that working in a lower CPC concentration range up to 20 mM is

more advantageous in order to achieve higher fluxes. Also, after each

experimental run the membranes can be backwashed and/or cleaned

with an alkali mixture in order to restore the membrane condition.



55

3.5 Summary of the preliminary investigations

• CPC retention was linearly dependent on pressure up to 150 kPa

after which the retention became less dependent on pressure

increases.

• It was found that it is important to maintain a temperature of 30 oC in

order to enhance the micellisation process.

• CPC retention increased with increasing initial surfactant

concentration and it was found that retention close to 100 % can be

achieved above 40 mM surfactant concentration.

• The system was able to achieve a high retention of both Pd (ll) and

Pt (lV) metal anions in acid medium.

• Pt (lV) retention in neutral medium was slightly greater than that of

Pd (ll) and this was attributed to its greater adsorption onto the

micelle surface than Pd (ll) due to Pd (ll) species being partially

hydrated.

• Introduction of an electrolyte, namely, sodium chloride failed to

improve the retention of both the metal ion and the surfactant, but in

fact dramatically worsened the retention of CPC.

• The membrane responded differently in various mediums. This wasshown by the flux variations of the various species.

• Although the system showed a great potential to retain both Pd (ll)

and Pt (lV) anions from an aqueous solution, it failed to achieve

optimum retention of the surfactant. This led to the investigation of

conditions which affect the retention of CPC and the establishment of

improved experimental conditions that would achieve higher

retention of both the metal ion and surfactant. This investigation is

described in the following chapter.



56

CHAPTER FOUR

INVESTIGATION OF CETYLPYRIDINIUM CHLORIDE

RETENTION

4.1 Overview

This chapter focuses on an investigation of the effects of different

electrolytes on cetylpyridinium chloride retention. This investigation was

necessitated by the realization that cetylpyridinium chloride retention by

the polysulphone membranes used during the ultrafiltration experiments

was negatively affected by the presence of electrolytes and metal ions

during preliminary investigations of both the palladium (ll) and platinum

(lV) synthetic solutions. Though, it had already been observed that

cetylpyridinium chloride was able to adsorb and effectively increase the

hydrodynamic size of the Pt (lV) and Pd (ll) ions in aqueous solution as

required during the micellisation process, it was also critical to keep the

amount of the surfactant going to the waste stream at a minimal level in

order to avoid further pollution of the waste stream by the surfactant. A

series of experiments was conducted where the effect of electrolytes

such as nitric acid, hydrochloric acid and sodium chloride were carefully

studied in order to establish the optimum conditions required in order to

achieve maximum retention of the surfactant.

4.2 Investigation of the effects of an electrolyte

In this section, an investigation of the cause of the deviating retention of

the cetylpyridinium chloride in the presence of electrolytes such as

hydrochloric acid, nitric acid and sodium chloride in synthetic solutions of

the surfactant will be done. The content of cetylpyridinium chloride



57

solutions was kept constant at 10 mM in all the ultrafiltration experiments

while increasing amounts of different electrolytes were added to the

surfactant solutions. The choice of the concentration was based on the

findings that were obtained in section 3.2.3. The results will be shown

here in graphical form while the data can be found in tables in Appendix

B.

4.2.1 Effect of hydrochloric acid concentration variation

The influence of hydrochloric acid addition on the CPC retention was

investigated. This investigation was necessitated by the realisation in

preliminary experiments in Chapter Three that CPC retention deviated

significantly in the presence of metal ion solutions containing

electrolytes. A set of cetylpyridinium chloride solutions of constant

concentration (10 mM) was prepared by dissolving a suitable amount of

CPC salt in deionised water in a 250 ml volumetric flask, with varying

amounts of hydrochloric acid (ranging from 0 to 20 mM) added.

Ultrafiltration runs were done at constant temperature (30oC) and

pressure (150 kPa). Each ultrafiltration experiment was run for 20


retentate samples were taken for CPC analysis by UV-VIS

spectrophotometry. The results for this investigation are shown in Fig.

4.1 below with the data in Table B1 in Appendix B.



58

0.0

20.0

40.0

60.0

80.0

100.0

0.0 5.0 10.0 15.0 20.0 25.0 Hydrochloric acid concentration (mM)

C P C r e t e n t i o n ( % )

0.0

20.0

40.0

60.0

80.0

100.0


CPC retention Permeate flux Figure 4.1: Variation of hydrochloric acid concentration at 30°C, 150

kPa and 10 mM CPC

From the curves in Fig. 4.1 above, it can be seen that cetylpyridinium

chloride retention decreased dramatically from just above 80% to less

than 20% with increasing electrolyte (HCl) concentration ranging from 0

mM to 20 mM. The flux measurements showed a slight increase from

approximately 60 L/m2h to just above 80 L/m2h at 0 mM to 5 mM acidic

medium and remained fairly constant with increasing hydrochloric acid

concentration.

In order to determine what changes in solution properties occur upon

addition of hydrochloric acid, surface tension and viscosity

measurements were taken of solutions prepared in the same way as

those used in the ultrafiltration runs and covering the acid concentration

range used. Surface tension measurements were made manually using

a tensiometer with torsion ring while viscosity measurements were made



59

with an Ostwald viscometer. This data is plotted as a function of acid

concentration in Fig. 4.2.

0.04

0.041

0.042

0.043

0.044

0.045

0 5 10 15 20 25[HCl] mM

s u r f a c e t e n s i o n ( N / m )

6.9

6.95

7

7.05

7.1

7.15

7.2

v i s c o s i t y ( N / m )

surface tension

viscosity

Figure 4.2: Surface tension and viscosity of a 10 mM CPC solution as a

function of HCl concentration

From the curves in Fig. 4.2 above, it can be seen that both

cetylpyridinium chloride surface tension and viscosity decreased

dramatically with the addition of hydrochloric acid and then reached a

minimum value after 5 mM HCl was added. This can be explained interms of surface effects, in the case of surface tension data, and effects

in the bulk of the solution, in the case of viscosity measurements.

At 10 mM CPC the solution surface is already saturated with adsorbed

monomers since the concentration of CPC exceeds the cmc value.

Addition of chloride ions causes a screening effect of these ions on the

positively charged head groups of the monomers. This allows the head

groups which normally repel each other, to come closer together thus

creating additional space at the surface for the adsorption of more

monomers from the solution thereby decreasing the surface tension

further, until a new saturation point is reached (in this case, around 5

mM HCl). The screening effect of the additional chloride ions on the

micelles in the bulk of the solution is similar, allowing the head groups on



60

the micelle surface to come closer together thereby decreasing the ratio

of the head group area to tail volume. If this effect occurs to a very large

extent, that is, the ratio becomes lower than a certain critical value, the

micelle can change its shape from a spherical to a more rod-like

structure. This change is reflected in the abrupt viscosity change at

around 5 mM HCl.

The abrupt changes in both the surface tension and viscosity at around 5

mM HCl could thus be indicating a saturation point and a change in

micelle structure and could be the reason for the decreased retention of

the surfactant solution. This abrupt change also corresponds with the

point at which permeate flux reaches a maximum constant value (see

Fig. 4.1).

4.2.2 Effect of nitric acid concentration variation

The influence of nitric acid addition on CPC retention was investigated. A

set of cetylpyridinium chloride solutions of constant concentration (10

mM) was prepared by dissolving a suitable amount of CPC salt in

deionised water in a 250 ml volumetric flask, with varying amounts of

nitric acid (ranging from 0 to 20 mM) added. Ultrafiltration runs were

done at constant temperature (30oC) and pressure (150 kPa). Each

ultrafiltration experiment was run for 20 minutes and at the end of each

experiment, 20 ml permeate and retentate samples were taken for CPC

analysis by UV-VIS spectrophotometry. The results are shown in Fig. 4.3

with the data in Table B2 in Appendix B.



61

0.0

20.0

40.0

60.0

80.0

100.0

120.0

0.0 5.0 10.0 15.0 20.0 25.0

Nitric acid concentration (mM)

C P C

r e t e n t i o n

( %

)

0.0

20.0

40.0

60.0

80.0

100.0

120.0

P e r m

e a t e

F l u x

( L / M

2 h )


Figure 4.3: Variation of nitric acid concentration at 30°C, 150 kPa and 10

mM CPC

A closer look at the curves in Fig. 4.3 show that hydrochloric acid and

nitric acid behaved differently with respect to retention of cetylpyridinium

chloride. With nitric acid variation, it can be seen that the retention of the

surfactant eventually increased after an initial decrease, from nearly 80%

to above 90% with increasing acid concentration, whereas flux

measurements showed a dramatic decline from approximately 100

L/m2h to just above 40 L/m2h. It is evident that nitric acid concentration

did not affect the micellisation process as much as hydrochloric acid,

since the system was still able to achieve a high enough retention of the

surfactant.

Surface tension measurements (see Fig. 4.4) show that compared with

HCl, surface tension decreased continually with increasing HNO3

concentration and there was no abrupt change in the measurements

over the concentration range measured. This could possibly be



62

explained as follows: although nitrate and chloride ions are both

monovalent the nitrate ion has more electron lone pairs than the chloride

ion thus providing a larger screening effect of the surfactant head groups

which means that the surface saturation point of the additional surfactant

molecules being adsorbed is reached at higher nitrate concentration.

0.034

0.035

0.036

0.037

0.038

0.039

0.04

0.041

0.042

0.043

0.044

0.045

0 5 10 15 20 25[HNO3] mM

s u r f a c e

t e n s i o n ( N / m )

HNO3

HCl

Figure 4.4: Surface tension of a 10 mM CPC solution as a function of

HNO3 concentration

4.2.3 Effect of sodium chloride concentration variation

The influence of sodium chloride addition on CPC retention was

investigated. A set of cetylpyridinium chloride solutions of constant

concentration (10 mM) was prepared by dissolving a suitable amount of

CPC salt in deionised water in a 250 ml volumetric flask, with varying

amounts of sodium chloride (ranging from 0 to 20 mM) added.

Ultrafiltration runs were done at constant temperature (30oC) and

pressure (150 kPa). Each ultrafiltration experiment was run for 20


retentate samples were taken for CPC analysis by UV-VIS

spectrophotometry. The results are shown in Fig. 4.5 below.



63

0.0

10.0

20.0

30.0

40.0

50.0

60.0

70.0

80.0

90.0

0.0 5.0 10.0 15.0 20.0 25.0

Sodium chloride concentration (mM)

C P C

r e t e n t i o n

( %

)

0.0

20.0

40.0

60.0

80.0

100.0

120.0

P e r m

e a t e

F l u x

( L / M

2 h )


Figure 4.5: Study of variation of sodium chloride acid concentration at

30°C, 150 kPa and 10 mM CPC

It is evident from Fig. 4.5 above that introduction of an electrolyte like

sodium chloride failed to improve the retention of a surfactant, but in fact

decreased it dramatically; a similar effect that was observed with

hydrochloric acid addition. These results do not agree with the work of

other researchers 3 who claim that introducing sodium chloride to

surfactant solutions reduces the critical micelle concentration leading to

increased micellisation and increased retention. This is not the case

here, and the results can be explained in the same way as for

hydrochloric acid addition (section 4.2.1).



64

4.3 Summary of CPC retention investigations

• Introduction of chloride ions reduces the surfactant retention

significantly and the effect has been shown by viscosity and surface

tension measurements to be probably due to a change in micelle

structure.

• Introduction of nitrate ions did not have a significant negative impact

on the surfactant retention. This might be explained in terms of a

second change in micelle structure .

• It is concluded that the use of HCl to dissolve the metal salts should

be kept to a minimum, thus the volume ratio of HNO3:HCl should be

increased in the acid mixture used to dissolve Pt and Pd salts.

• In order to shed further light on the reduced surfactant retention in

the presence of electrolytes, the following chapter investigates the

determination of the cmc value of cetylpyridinium chloride under

various experimental conditions.



65

CHAPTER FIVE

THE CONCEPT OF MICELLISATION AND CONDUCTIVITY

INVESTIGATIONS

In this chapter the focus will be on studying the extent to which

micellisation occurred during the ultrafiltration experiments in the

previous chapters. Conductivity studies will be used to confirm the

theoretical cmc value of cetylpyridinium chloride and to determine the

degree of ionisation of CPC in different media. The effects of various

electrolytes namely, aqua regia, hydrochloric acid and nitric acid on the

conductivity measurements of CPC solutions will also be investigated.

5.1 Micellisation

It is important that certain conditions namely, critical micelle

concentration (cmc), Kraft temperature point and the solubility of the

surfactant are known in order to maximise the micellisation effect. The

critical micelle concentration of a surfactant can be experimentally

determined by employing a number of methods including, conductivity,

surface tension measurements and capillary electrophoresis (CE). 13, 14,

29 These techniques can help to determine the extent or the degree to

which micellisation has occurred. It has been reported that the addition

of an electrolyte such as sodium chloride to an aqueous solution

containing the surfactant facilitates the reduction of the critical micelle

concentration that in turn increases the micellisation process. 4



66

5.2 Conductivity study

In this study, a Metrohm 660 conductometer with conductivity cell (model

no. 6.0908.110) coupled with a platinum thermocouple electrode was

used to confirm and establish the influence of the following factors as

listed below:

• Determination and confirmation of the cmc value of CPC.

• Addition of varying amounts of different electrolyte(s) and

synthetic mixtures containing either Na2PtCl6 or K2PdCl6 to

cetylpyridinium chloride solutions to observe the effect of these

on conductivity measurements.

5.2.1 Experimental determination of the cmc of CPC

This experiment was done in order to confirm the theoretical cmc value

(0.9 mM) of cetylpyridinium chloride obtained from the literature. 3, 13 Of

the various methods that can be employed namely, conductivity, surface

tension and capillary electrophoresis, conductivity was chosen for this

study due to the accessibility and simplicity of the equipment.

5.2.2 Experimental procedure

• The conductometer was calibrated with 0.1 M KCl solution.

• After calibration, the cell constant of the conductivity cell at a

specific temperature was calculated and other parameters and

applicable settings of the conductometer were set prior to taking

the conductivity measurements.

Cetylpyridium chloride solutions of varying concentrations (0.2 mM to 2

mM) were prepared by dissolving a calculated amount of CPC salt in

deionized water and diluting this stock solution to the required

concentration.



67

The conductivity measurements of the prepared CPC solutions were

recorded as shown in Table C1, in Appendix C, and a plot showing the

relationship between conductivity and CPC concentration is shown in

Fig. 5.1 below:

R2

= 0.9956

R2

= 0.9966

0

20

40

60

80

100

120

140

160

180

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

[CPC] mM

c o n d u c t i v i t y ( u S / c m

Slope = 94

Slope = 43.36

Figure 5.1: Determination of the cmc of CPC and its degree of ionisation

The formation of the micelles leads to an abrupt change in conductivity

of solution due to removal of ions from solution since the counter ions

are adsorbed onto the micelle surface; thus the point at which the slope

of the conductivity curve changes can be regarded as the onset of

micellisation. Therefore the point of intersection of the two straight line

portions of the plot in Fig. 5.1 is taken as the cmc value.

Looking at the curve in Fig. 5.1 above, it can be seen that the obtained

cmc value is in agreement with the reported literature value (0.9 mM), 3,

13 thus the chosen technique and the methodology followed were

correct. The slopes of each plot in Fig. 5.1 above were also used to

determine the degree of ionisation of the micelles as explained in

section 5.2.3 below.



68

5.2.3 Determination of the degree of ionisation

The determination of the degree of ionisation of CPC micelles was done

in order to investigate the available adsorption sites on the micelle

surface. The degree of ionisation of a micelle was determined by

measuring the change in slope of the solution electrical conductivity (¢

)

vs total concentration (C) as the solution goes through the cmc 13, 28, 29,

assuming the following conditions:

• ¢

is only due to free chloride ions in solution

• ¢

due to CP+ and micelles is negligible, thus¢

£

CCl

• When C < cmc:¢

= ACCl , since CPC monomer is fully ionised,

where A = constant

• When C > cmc, CCl = cmc +¤

(C – cmc)

where¤

is the degree of ionisation of a micelle.

• Thus¢

= A (cmc +¤

(C – cmc))

• Gradients above and below the cmc can be calculated as

follows:

dk /dC ( C > cmc ) =¤

A and dk /dC ( C < cmc ) = A

• Thus the ratio of these slopes gives¤

, which is the degree of

ionisation.

A closer look at Fig. 5.1 above also shows that the ratio of the two

slopes, viz, 43.36 and 94.00, respectively, gives a 46% degree of

ionisation. This means that 54% of the chloride counter ions are

associated with the micelle surface or adsorbed onto the micelle

surface.



69

5.3 Influence of electrolytes on the cmc value and degree of

ionisation

Conductivity data is not useful when large amounts of acid are present in

the surfactant solution because then the conductivity due to the acid is

too high and small changes due to CPC are not observed, thus only

small amounts of acid, to give a pH of around 3, were added to the CPC

solutions. The results pertaining to this investigation are presented in

graphical form below.

5.3.1 Determination of cmc in the presence of HCl

Surfactant solutions of various concentrations were prepared by

dissolving calculated amounts of CPC salt in deionised water. HCl was

added in the same volumetric flask (100 ml) prior to filling up to the mark

with deionised water, to give a final concentration of 0.5 mM. The

concentrations of the CPC solutions ranged from 0 mM to 2.2 mM.

Conductivity measurements were made using a pre-calibrated (see

section 5.2.1) Metrohm 660 conductivity meter. The conductivity meter

measurements were plotted against CPC concentration in Fig. 5.2 and

the data is given in Table C2 in Appendix C. The ratio of the two slopes

was used to predict the degree of ionisation and the cmc value was also

determined.



70

0.1

0.2

0.3

0.4

0.5

0 0.2 0.4 0.6 0.8 1 1.2 1.4 1.6 1.8 2 2.2

[CPC] mM

c o n d u c t i v i t y ( m S / c m

Slope = 0.2286

Slope = 0.098

Slope = 0.0369

Figure 5.2: Plot of the conductivity vs CPC concentration in the presence

of 0.5 mM HCl

A closer look at Figure 5.2 above shows that when a fixed amount of HCl

is added to each CPC solution, conductivity measurements show three

abrupt changes with the first cmc value occurring at around 0.25 mM

CPC. This is in agreement with the accepted theory that electrolytes

lower the cmc value of surfactants by reducing the head group area due

to electrostatic screening of the head group charges by the additional

counter ions. As a result of this, micellisation occurs sooner than it

normally occurs at lower surfactant concentrations. A second cmc value

occurs at around 1.2 mM CPC. This could indicate the point at which the

spherical micelles transform into rod-shaped micelles with increasing

surface area and corresponding greater degree of counter anion

adsorption.

The degree of ionisation (obtained from the ratio of the two slopes in Fig.

5.1) is around 46% for CPC alone, but is reduced to 43% when HCl is

added. The excess Cl ions in solution are now attracted to the charged

micelles causing them to be removed from solution, thus leading to the

abrupt change in conductivity, and the lower ionisation value. At the



71

second cmc value, the degree of ionisation is further reduced to 37.6%

which shows that even more ions are associated with the micelle

structure which now has a larger surface area.

5.3.2 Determination of cmc in the presence of Pd (ll) anions and

aqua regia

CPC solutions were prepared in the same manner as described in 5.3.1

except that aliquots of the 1 mM Pd (ll) stock solution used for the

preparation of the MEUF solutions were added to each solution, to give

0.01 mM concentration of Pd (ll), prior to making up to the mark with

deionised water in the volumetric flasks (100 ml). The plot of solution

conductivity against CPC concentration is shown in Fig. 5.3. The data is

in Table C3 in Appendix C.

0.6

0.8

1

1.2

1.4

1.6

1.8

2

0 1 2 3 4 5 6 7 8 9

[CPC] mM

c o n d u c t i v i t y ( m S / c m )

Slope = 0.0164

Slope = 0.5424

Slope = 0.1209

Figure 5.3: Plot of the Conductivity vs CPC concentration in the

presence of 0.01 mM Pd (ll) anions in aqua regia

In the presence of Pd with aqua regia, the conductivity of increasing

amounts of CPC showed interesting results. The first cmc value is not

shown in Fig. 5.3 since it is out of the concentration range studied. (In

acid medium we have seen that it occurs at around 0.2 mM CPC.) It can



72

be seen that there is a flattening off of the conductivity between 0.2 and

2 mM CPC indicating the adsorption of counter-ions onto the micelle

surfaces. This is followed by an abrupt change in conductivity indicating

a cmc value of around 2 mM. This could be the second cmc, the point at

which the micelles could be changing shape. Between 2 and 3 mM CPC

there is a sharp increase in conductivity. This could indicate a transition

state where there is a change of structure. At around 3 mM there is

another cmc value. The degree of ionisation i.e. the ratio of the two

slopes around the cmc value of 3 mM is 22%. This value is considerably

less than that of the micelles in the pure surfactant solution (46%)

indicating that the palladium ions are adsorbed onto the micelle surface.

5.3.3 CMC determination in the presence of Pt (lV) anions and

aqua regia

CPC solutions were prepared in the same manner as described in 5.3.1

except that aliquots of the 1 mM Pt (lV) stock solution used for the

preparation of the MEUF solutions were added to each solution, to give

0.01 mM concentration of Pt (lV), prior to making up to the mark with

deionised water in the volumetric flasks (100 ml). The plot of solution

conductivity against CPC concentration is shown in Fig. 5.4. The data is

in Table C4 in Appendix C.



73

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

0 1 2 3 4 5 6 7 8 9

[CPC] mM

c o n d u c t i v i t y ( m S

/ c m )

Slope = 0.0376

Slope = 0.34

Slope = 0.0535

Figure 5.4: Plot of the conductivity vs CPC concentration in the presence

of 0.01 mM Pt (lV) anions in aqua regia.

Looking at Fig 5.4 above, it can be seen that as was the case with Pd,

two cmc values are observed. The first cmc value is close to 2 mM while

the second cmc value is around 3 mM. The ratio of the two slopes

around 3 mM CPC (0.0535/0.34) gives 15.7%. This value is less than

that obtained for Pd (ll) (22%), indicating that there are fewer ions in

solution i.e. more counter ions are associated with the micelle surface as

can be observed from the flatter slope. This can mean that PtCl62- ions

promote the transformation of the micelles to rod-shapes to a greater

extent than PdCl42- ions with perhaps greater overall surface area

leading to more elongated or longer rods.



74

5.4 Summary of the conductivity investigations

• Findings on this research show that the presence of electrolytes

lowers the initial cmc value of the surfactant and creates a second

cmc value which possibly indicates a change in micelle shape or

structure.

• Also, it was found that the presence of the metal ions brings about

a further change in conductivity through a third cmc value which

could indicate a further possible change of micelle structure.

• The ratios of the slopes around the cmc values give an indication of

the degree of ionisation of the micelles and thus the amount of ions

associated with the micelle surface.

• The values of the slopes show that the Pt (IV) ions are absorbed to

a greater extent than the Pd (II) ions onto the micelle surface.



75

CHAPTER SIX

IMPROVED CONDITIONS FOR THE RETENTION OF

METAL ION AND SURFACTANT

In this chapter, a complete study of the optimum retention of both metal

ions (Pt (lV) and Pd (ll)) and surfactant (CPC) will be investigated. Also

the possibility of separating Pt (lV) from Pd (ll) will be explored. These

investigations will be based on the findings obtained in the previous

chapters that focused on establishing the reasons for the reduced

retention of surfactant. The results for this chapter are illustrated in the

form of graphs with the data contained in tables in Appendix D.

6.1 Micellar-enhanced ultrafiltration of individual metal ions

6.1.1 Ultrafiltration of platinum (lV) anions

Ultrafiltration of platinum metal ions alone as a function of metal ion

concentration was done in order to determine the capability of the MEUF

system to retain different amounts of metal from the synthetic solution.

The synthetic solutions used for these experiments were prepared as

follows:

• A platinum stock solution was prepared by weighing a suitable

amount of the metal ion salt as mentioned previously.

• The weighed salt was dissolved with a mixture of 2.5 mM HCl and 5

mM HNO3 and deionized water in a 500 ml volumetric flask.

• 2.5 mM HCl was the minimum amount of HCl that was necessary to

dissolve PdCl62- salt and is considerably less than the amount in

aqua regia.



76

• Varying amounts of the metal solution were taken from the prepared

stock solution to prepare the solutions for the ultrafiltration

experiments and the appropriate amount of CPC was added as

required by each ultrafiltration experiment.

Fig. 6.1 shows the retention of both Pt (lV) and CPC as well as the

permeate flux as a function of metal ion concentration for experiments

performed at 150 kPa, 30°C and using 40 mM CPC. The data is found in

Tables D1 and D2 in Appendix D.

92.0

93.0

94.0

95.0

96.0

97.0

98.0

99.0

100.0

0.00 0.10 0.20 0.30 0.40 0.50

Pt concentration (mM)

R e t e n t i o n

%

0.0

20.0

40.0

60.0

80.0

100.0

P e

r m

e a t e

F l u x

L / m

2 h r

Pt retention CPC retention Flux variation

Figure 6.1: MEUF of Pt (lV) anions at 40 mM CPC, 30°C, and 150 kPa in

acidic medium

From the curves in Fig. 6.1 above, it can be seen that the retention of Pt

(lV) was between 98% and 100% with a slight increase observed as

initial Pt (lV) concentration in the solution increased. The CPC retention

was 99% at low metal concentration and decreased to around 93% at



77

higher metal concentration. The higher retention can be attributed to

sufficiently large micelles which form at 40 mM CPC and a low enough

electrolyte concentration to prevent the electrolyte effect that was

discussed in the previous chapter. However, the slight decrease in CPC

retention observed with increasing Pt (lV) concentration can be attributed

once again to a slight electrolyte effect, since the higher metal

concentration solutions also contained proportionately more electrolytes.

The flux measurements decreased slightly from just above 45 L/m2h and

remained fairly constant with increasing metal ion concentration (0 to 0.4

mM) at constant surfactant concentration (40 mM). This slight decline in

flux measurements can be attributed to concentration polarization on

the membrane surface that occurs increasingly with increasing metal ion

concentration.

6.1.2 Ultrafiltration of Palladium (ll) anions

Ultrafiltration of palladium metal ion alone was done in order to

determine the capability of the MEUF system to retain the metal from the

synthetic solution with increasing palladium concentration. Palladium

synthetic solutions used in these ultrafiltration experiments were

prepared exactly in the same manner as described in section 6.1.1

above.

Fig. 6.2 shows the retention of both Pd (ll) and CPC, as well as the

permeate flux as a function of metal ion concentration for experiments

performed at 40 mM CPC, 150 kPa and 30°C. The data is found in

Tables D3 and D4 in Appendix D.



78

90.0

92.0

94.0

96.0

98.0

100.0

102.0

0.00 0.10 0.20 0.30 0.40 0.50

Pd concentration (mM)

m

e t a l i o n r

e t e n t i o n

( %

)

0.0

20.0

40.0

60.0

80.0

100.0

P e r m

e a t e

F l u x ( L

/ M

2 h )

Pd retention CPC retention Permeate flux

Figure 6.2: MEUF of Pd (ll) anions at 40 mM CPC, 30 °C, and 150 kPa in

acidic medium

Looking at the curves of the palladium-cetylpyridinium chloride system in

Fig. 6.2, it is evident that Pd (II) was retained at 100% irrespective of the

initial Pd (II) concentration. However, it was also observed that the

retention of cetylpyridinium chloride gradually decreased from 98 to 90%

with increasing metal ion concentration. Although the overall surfactant

retention was high, the slight decrease would be due to increasing

electrolyte concentration as metal ion concentration increases. Also, it

was found that flux measurements decreased slightly initially and later

remained fairly constant with increasing palladium concentration from 0

to 0.4 mM, as was found with Pt (lV) ultrafiltration experiments.



79

6.2 Micellar-enhanced ultrafiltration Pt (lV)-Pd (ll) mixture

The possibility of separating the two metal ions in an acidic synthetic

mixture containing varying ratios (1:1 to 1:6) of the two metal ions, was

investigated. The total metal concentration of the synthetic mixture was

kept constant at 1 mM throughout, while the ratios of the metal ions were

varied. Table 6.1 shows the exact metal ion ratios that were used in

these ultrafiltration experiments.

Table 6.1: Summary of varying metal ion concentration ratios

Metal ion

ratio

Exact *MA

concentration

(mM)

Exact *MB

concentration

(mM)

1:1 0.05 0.05

1:2 0.0333 0.0666

1:3 0.025 0.075

1:4 0.02 0.08

1:6 0.015 0.085

*Can be any of the two metal ions, that is Pt or Pd

Fig. 6.3 shows the micellar-enhanced ultrafiltration of Pt (lV)/Pd (ll)

mixture in acidic medium when Pd (ll) concentration ratio is kept

constant. The data is found in Table D7 in Appendix D.



80

92.0

93.0

94.0

95.0

96.0

97.0

98.0

99.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

metal ion ratio (1 : x)

m e t a l i o n r e t e n t i o

n ( % )

Pt re te ntion Pd re te ntion

(1 = constant Pd concentration ratio, x = changing Pt concentration ratio)

Figure 6.3: MEUF of a mixture of Pd (ll) - Pt (lV) anions at 40 mM CPC,

30°C, and 150 kPa in acidic medium

Pt (lV) anion retention increases sharply initially until it reaches 98% at

Pd:Pt ratio of 1:2 and later decreases to below 96% at Pd:Pt ratio of 1:4

ratios and increases again to 98% at Pd:Pt ratio of 1:6. Pd (ll) retention

follows the same trend as well but at an overall lower retention. This

difference in metal ion retention shows that there is competition between

the two metal ions for available sites on the surfactant surface with the

platinum ions being preferentially adsorbed onto the micelles. In each of

these solutions except the 1 : 1 ratio, the concentration of platinum ions

is greater than that of palladium ions, hence the separation factor

increases with increased platinum concentration.

Fig. 6.4 shows the micellar-enhanced ultrafiltration of Pt (lV)/Pd (ll)

mixture in acidic medium when Pt (lV) concentration ratio is kept

constant and the amount of palladium relative to platinum is increasing.

The data is found in Table D8 in Appendix D.



81

95.0

96.0

97.0

98.0

99.0

100.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

metal ion ratio (1 : x)

M

e t a l i o n

r e t e n t i o n

%

Pt retention Pd retention

(1 = constant Pt concentration ratio, x = changing Pd concentration ratio)

Figure 6.4: MEUF of a mixture of Pt (lV)-Pd (ll) anions at 40 mM CPC,

30°C, and 150 kPa in acidic medium

Looking at Fig. 6.4 above, it is evident that palladium retention increases

steadily from just above 96% to nearly 99% with increasing metal ion

concentration ratio (Pt: Pd), that is, with more Pd (ll) present in the

system compared to Pt (lV) , the Pd (ll) is increasingly retained whilst

platinum retention decreases slightly from 98% to 97% in the same

metal ion concentration ratio region (1:1 to 1:3). Also, it was observed

that the retention of the two metal ions increased slightly again beyond1:4 Pt:Pd concentration ratio with platinum achieving more than 99.5%

while palladium retention remained below 98%. The variation in retention

of the two metal ions can be attributed to the differing adsorbing

capabilities that they have towards the available charged adsorbing sites

on the surfactant surface. However, the differences remain very small.



82

6.3 Membrane response during Pt/Pd mixture ultrafiltration

Membrane response during the micellar-enhanced ultrafiltration of the

mixture of Pt (lV) and Pd (ll) was carefully monitored by monitoring the

cetylpyridinium chloride retention and permeate flux variation.

6.3.1 CPC retention

The curves in Fig. 6.5 below were constructed from the sample analysis

data that was obtained during the ultrafiltration experiments of the Pt/Pd

synthetic mixtures. The samples were analysed for CPC content by UV-

VIS spectrophotometry. The data pertaining to this investigation can be

obtained in Tables D5 and D6 in Appendix D.

88.0

89.0

90.0

91.0

92.0

93.0

94.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0metal ion ratio

C P C r e t e n t i o n ( % )

CPC retention when Pt ratio is constant

CPC retention when Pd ratio is constant

Figure 6.5: Effects of metal ion ratio on CPC retention in acidic medium

in the presence of various mol ratios of Pd and Pt



83

In the presence of fixed Pd (ll) in the metal ion concentration range of 1:1

to 1:4 Pd : Pt, it can be seen that CPC retention decreased sharply from

just above 92% to just above 88%, while in the same region of metal ion

ratio in the presence of constant Pt (lV), CPC retention slightly increased

initially from just above 92% to close to 94% and later decreased and

followed the same trend as in the case where Pd (ll) concentration was

kept constant. For the two metal ions above 1:4 metal ratios, CPC

retention increased sharply from as low as 88% to nearly 92%. However,

though the CPC retention was affected by the changing metal ion ratio,

the system where Pt (lV) ratio was kept constant was the most affected

as it only achieved a maximum of just more than 92% while in the

system where Pd (ll) ratio was kept constant close to 94% retention of

CPC was achieved. The electrolyte content is constant in these

experiments since the total metal ion concentration was held constant.

The only change is the relative amounts of Pd (ll) and Pt (lV). It is then

evident that the metal ions themselves influence the shapes of the

micelles due their large sizes and need for a larger surface area. The

PtCl62- anion has a larger effect on the micelle structure than the PdCl4

2-

as can be seen from the lower retention of CPC when there are more Pt

(lV) ions relative to Pd (ll) ions.

6.3.2 Flux variation

The curves in Fig. 6.6 are constructed from the sample analysis data

that was obtained during the ultrafiltration runs of Pt/Pd synthetic

mixtures. The data is found in Tables D7 and D8 in Appendix D.

A closer look at the curves in Fig. 6.6 shows that for both systems, thus

when either Pt : Pd = 1 : x or Pd : Pt = 1 : x, the flux measurements

continually decreased throughout. However, although the flux variations

in both systems followed the same trend, flux measurements for the Pd :

Pt =1 : x system were still higher than in the system of Pt : Pd = 1 : x.



84

0.0

10.0

20.0

30.0

40.0

50.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

metal ion ratio (mM)

P e r m

e a t e

F l u x

v a r i a t i o n

( L / M

2 h )

Permeate flux when Pt concentration ratio is constant

Permeate flux when Pd concentration ratio is constant

Figure 6.6: Flux variation during Pt(lV)/Pd (lV) ultrafiltration

The differences in flux measurements can be attributed to the manner in

which these two metal ions adsorb on the available oppositely charged

sites of the surfactant surface. The slight decrease in flux measurementscan also be due to different ratios of the metal ion and the way that they

influence a micelle shape which influences flux.

6.4 Summary of the investigation of improved conditions

• The conditions for improved micellisation to occur in this

investigation lead to the use of 40 mM of the CPC surfactant, a

pressure of 150 kPa, and a temperature of 30°C.

• It was also found that controlling or minimizing the electrolyte

content, especially chloride content, led to optimum retention of

both the metal ion(s) and the surfactant.

• Quantitative separation of the two metal ions under these

conditions could, however, not be achieved.



85

CHAPTER SEVEN

CONCLUSIONS

7.1 Conclusion

Based on the findings of this study, it can be concluded that the MEUF

system using cetylpyridinium chloride (CPC) can be used to recover or

retain Pt (lV) and Pd (ll) anions from industrial waste effluents. It can also

be said that PtCl62-

due to its greater adsorption capabilities onto themicelle surface than PdCl4

2- or PdCl3 (H2O)- was preferentially retained in

neutral medium. This may be exploited as a possible means of

separating the two metal ions.

However, although the system was able to achieve optimum retention of

both metal ions, the CPC retention was very poor during initial

investigations. This was attributed to the electrolyte effect which

influences the micellisation process, possibly changing the shape or

structure of the micelles. These findings necessitated the investigation of

suitable experimental conditions that would permit the optimum retention

of both the metal ion(s) and the surfactant. The study considered the

influence of different electrolytes on the physical properties of CPC

solutions to establish the conditions under which surfactant retention

was a maximum.

The conditions which were found for optimum micellisation to occur in

this investigation lead to the use of 40 mM of the CPC surfactant, 150

kPa, and 30°C. It was also found that controlling or minimizing the

electrolyte content led to optimum retention of both the metal ion(s) and

the surfactant.



86

7.2 Recommendations

Continued investigation of this project could be carried out, viz.:

• Investigation of the capability of a different membrane that would

provide different specifications namely MWCO and membrane pore

size.

• Using a different cationic surfactant with different properties that may

not be as sensitive to electrolyte effects.

• Investigating the retention of other platinum group metals.

• Focusing on the separation of Pt (lV) and Pd (ll) by incorporating into

the surfactant micelles a specific separating agent such as an

alkylpyrazole, for example, which is known to be Pd (ll) specific.



87

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Elizabeth. (pp 12-14)



89

APPENDIX A

Preliminary investigations of metal ion and surfactant retention

Table A1: CPC calibration curve

CPC

(mM)Absorbance

0.00098 0.0048

0.00196 0.01

0.00392 0.0191

0.0049 0.0232

0.0098 0.0452

0.0196 0.0948

0.0392 0.175

0.098 0.424

0.196 0.8182

Table A2: Effect of pressure variation

Table A2 (1): CPC data

CPC concentration (mM)Experiment

number

[CPC]I

(mM)

Pressure

(kPa)Feed Permeate

#1 10 50 9.755 0.549

10 100 9.958 0.854

10 150 9.921 1.287

10 200 9.949 2.763

10 250 9.919 4.543

#2 10 50 9.968 0.521

10 100 10.086 0.823

10 150 9.769 1.194

10 200 9.846 2.99010 250 9.824 4.733



90

Table A2 (2): Averaged data: Effect of pressure variation

Flux (L/m2h) % CPC Retention[CPC]

(mM)

Pressure

(kPa) # 1 # 2 Av # 1 # 2 Av

10 50 69.28 69.03 69.16 94.37 94.78 94.58

10 100 99.03 95.17 97.09 91.42 91.84 91.63

10 150 120.82 119.39 120.10 87.03 87.78 87.41

10 200 141.07 138.78 139.92 72.23 69.63 70.93

10 250 107.04 106.02 106.53 54.20 51.82 53.01

Table A3: Effect of temperature variation

CPC concentration (mM)[CPC]

(mM)

Temperature

°°°°C

Flux

(L/m2

h) Feed Permeate

Retention

%

10 10 184.04 9.96 7.81 21.61

10 20 160.98 10.10 7.40 26.79

10 30 107.05 9.91 1.73 82.60

10 40 93.98 10.02 3.50 65.08

10 50 83.12 9.48 3.98 57.97



91

Table A4: CPC concentration variation

CPC concentration

(mM)Experiment

number

[CPC]i

(mM)

Flux

(L/m2h)

Feed Permeate

Retention

%

0 103.58 10.178 1.754 82.77

2.5 146.28 2.448 0.618 74.77

5.0 146.28 4.950 1.029 79.22

7.5 146.28 7.565 0.832 89.00

10 136.08 10.163 1.075 89.42

12.5 80.67 13.875 1.413 89.82

15.0 66.79 14.909 1.562 89.52

25 53.83 23.830 1.238 94.80

#1

45 49.90 45.054 1.554 96.55

0 103.58 10.178 1.754 82.77

2.5 147.91 2.662 0.872 67.24

5.0 98.01 5.400 1.255 76.76

7.5 85.82 7.769 1.311 83.12

10 82.35 10.001 1.628 83.72

12.5 80.67 13.875 1.413 89.82

15.0 66.28 15.734 1.282 91.85

25 53.829 23.830 1.238 94.80

#2

45 49.900 45.054 1.554 96.55



92

Table A5: Pd ultrafiltration in varying CPC concentration in acidic

medium

Pd concentration (ppm)

Experiment # 1


Experiment # 2[Metal]

mM

[CPC]

mMF* R* P* F* R* P*

0.1 1 7.307 1.015 0.089 7.307 1.015 0.089

0.1 2 7.587 2.302 0.255 7.587 2.302 0.255

0.1 3 6.236 1.469 0.075 6.236 1.469 0.075

0.1 4 7.726 2.746 0.177 7.726 2.746 0.177

0.1 5 8.109 1.99 0.735 8.109 1.99 0.735

0.1 10 7.806 4.304 3.623 7.813 9.146 0.347

0.1 20 7.826 4.509 1.812 8.898 10.147 0.257

0.1 40 7.378 7.853 1.419 9.569 9.888 0.014

0.1 60 9.641 8.452 0.509 10.026 16.576 1.051

0.1 80 8.111 8.806 1.011 8.834 10.23 0.945

0.1 100 9.774 9.459 0.726 7.307 1.0156 0.089

*F = Feed concentration, R = Retentate concentration, P = Permeate concentration

Table A6: Averaged data of Pt and Pd ultrafiltration with varying

CPC concentration in acidic medium

Platinum-CPC system

% Retention

Palladium-CPC system

% Retention

[Metal]

mM

[CPC]

mMPt CPC Pd CPC

0.1 1 98.85 59.43 98.78 56.28

0.1 2 98.79 41.86 98.46 36.30

0.1 3 98.87 31.94 99.53 33.63

0.1 4 98.56 24.64 97.98 31.10

0.1 5 98.25 21.60 97.10 35.28

0.1 10 98.06 18.26 97.77 30.60

0.1 15 97.59 15.14 96.38 29.61

0.1 20 95.84 18.21 97.45 43.680.1 30 95.66 18.75 96.13 35.21

0.1 40 94.41 7.63 95.52 42.16



93

Table A7: Pt ultrafiltration with varying CPC concentration in

neutral medium

Pt concentration (ppm)

Experiment # 1

Pt concentration (ppm)


mM

[CPC]

mMF R P F R P

0.1 1 14.23 4.34 0.133 14.23 4.343 0.132

0.1 2 14.69 3.62 0.201 14.69 3.623 0.201

0.1 3 16.85 4.17 0.226 16.85 4.171 0.226

0.1 4 17.85 4.74 0.289 17.85 4.74 0.289

0.1 5 17.59 6.54 0.297 17.59 6.54 0.297

0.1 10 23.99 0.27 0.766 22.61 15.72 0.534

0.1 20 24.51 3.06 0.196 21.75 13.77 0.741

0.1 40 19.16 16.19 0.658 21.22 13.84 0.689

0.1 60 18.39 13.34 0.704 19.77 10.87 1.291

0.1 80 19.31 14.98 0.650 20.7 9.497 1.078

0.1 100 18.25 17.11 0.683 14.23 4.343 0.132

Table A8: Pd ultrafiltration with varying CPC concentration in

neutral medium


Experiment # 1



mM

[CPC]

mM

F R P F R P

0.1 1 7.307 1.016 0.089 7.307 1.016 0.089

0.1 2 7.587 2.302 0.256 7.587 2.302 0.256

0.1 3 6.236 1.469 0.076 6.236 1.469 0.076

0.1 4 7.726 2.746 0.177 7.726 2.746 0.177

0.1 5 8.109 1.990 0.736 8.109 1.990 0.736

0.1 10 7.806 4.304 3.623 7.813 9.146 0.348

0.1 20 7.826 4.509 1.812 8.898 10.147 0.258

0.1 40 7.378 7.853 1.419 9.569 9.888 0.014

0.1 60 9.641 8.452 0.509 10.026 16.576 1.0510.1 80 8.111 8.806 1.011 8.834 10.230 0.945

0.1 100 9.774 9.459 0.726 7.307 1.016 0.089



94

Table A9: Averaged data of Pt and Pd ultrafiltration with varying

CPC concentration in neutral medium

Metal ion- CPC sytem

% Retention[Metal]

(mM)

[CPC]

(mM)Pt Pd

0.1 1 99.07 98.88

0.1 2 98.63 98.79

0.1 4 98.38 97.70

0.1 5 98.31 95.55

0.1 10 97.26 90.31

0.1 40 96.66 89.52

0.1 60 96.17 89.30

0.1 80 95.52 98.88

0.1 100 95.05 98.79

Table A10: Separation study of Pt/Pd mixture in acidic medium

Pd concentration

(ppm)

Pt concentration

(ppm)% Retention[Metal]

mM

[CPC]

mMF R P F R P Pt Pd

0.1 10 11.14 2.08 0.44 20.64 4.24 0.44 97.86 96.03

0.1 20 10.10 5.83 0.53 19.91 12.33 0.42 97.88 94.76

0.1 40 10.58 6.49 2.09 20.60 11.40 0.66 96.77 80.280.1 80 10.25 8.10 2.61 18.55 13.42 0.99 94.65 74.55

0.1 100 10.53 8.47 2.35 20.28 14.29 0.58 97.12 77.72



95

Table A11: Effect of an electrolyte in metal ion and surfactant

retention

Palladium-cetylpyridinium chloride-sodium chloride system

[CPC]

mM

Palladium concentration

(ppm)

% Retention[Metal]

mM

[NaCl]

mMF P F R P Pd CPC

0.1 0 - - - - - 90.31 93.34

0.1 10.00 41.13 4.61 8.307 6.724 0.357 85.79 88.79

0.1 25.00 36.03 7.18 6.482 5.139 0.437 84.95 80.07

0.1 50.00 39.64 8.89 4.849 4.288 0.752 84.46 77.58

0.1 100.00 38.68 21.83 6.710 3.627 1.044 84.20 43.56

Platinum-cetylpyridinium chloride-sodium chloride system

[CPC]

mM

Platinum concentration

(ppm)% Retention[Metal]

mM

[NaCl]

mMF P F R P Pt CPC

0.1 0 - - - - - 96.66 38.37

0.1 10.00 37.64 27.51 15.21 11.69 0.69 95.05 27.59

0.1 25.00 40.70 35.83 15.27 12.43 0.57 95.86 12.03

0.1 50.00 44.23 41.70 16.15 8.12 0.76 95.24 5.73

0.1 100.00 44.13 41.66 13.40 4.90 0.77 94.15 5.55



96

Table A 12: Flux variation with CPC concentration in various

experimental conditions (Membrane response)

Flux (L/m2h)

Experiment CPC (mM)Batch # 1 Batch # 2 Average

10 62.75

20 57.91

40 39.64

80 27.7

CPC alone

100 27.43

10 96.05 89.99 93.02

20 48.11 49.54 48.83

40 37.62 36.48 37.05

80 25.70 29.95 27.83

CPC + Pd

100 25.45 28.72 27.09

10 126.90 122.05 124.48

20 60.41 63.22 61.82

40 54.49 47.29 50.89

80 33.88 34.28 34.08

CPC + Pt

100 27.95 24.59 26.27

10 60.03

20 58.01

40 51.89

80 38.87

Pd + Pt

100 36.33

Table A 13: Flux measurements after ultrafiltration of various

species

Flux (L/m2h)

Time

(minutes) Original After CPCAfter

Pd

After

Pt

After

backflush

20 94.14 74.19 64.19 70.21 94.1940 94.14 74.16 64.20 70.20 94.19

60 94.14 74.15 64.20 70.20 94.19

80 94.14 74.13 64.20 70.20 94.19

100 94.14 74.12 64.20 70.20 94.19



97

APPENDIX B

Investigation of cetylpyridinium chloride retention

Table B1: Variation of hydrochloric acid concentration

CPC concentration

(mM)Experiment

number

[HCL]

mM

Flux

(L/m2h)

Feed Permeate

Retention

%

0 66.839 10.178 1.754 82.77

5 86.330 10.123 2.981 70.55

10 86.636 9.999 5.060 49.39#1

15 86.687 10.039 7.078 29.49

20 86.738 10.118 8.210 18.86

0 66.890 10.178 1.754 82.77

5 86.228 10.316 4.183 59.45

10 86.330 10.236 5.693 44.38

15 86.432 10.300 6.525 36.65

#2

20 86.636 10.241 8.241 19.53

Table B2: Variation of nitric acid concentration

CPC concentration

(mM)Experiment

number

[HNO3]

mM

Flux

(L/m2h)Feed Permeate

Retention

%

0 103.576 10.178 1.754 82.77

5 84.850 9.795 4.131 57.83

10 70.819 10.261 3.097 69.81#1

15 70.666 10.254 1.691 83.50

20 49.951 10.297 1.497 85.46

0 106.484 10.178 1.754 82.77

5 88.830 10.427 3.780 63.75

10 76.279 10.591 3.368 68.20

15 69.646 10.304 1.812 82.42

#2

20 49.237 10.147 1.537 84.85

0 106.484 10.178 1.754 82.77

5 84.29 9.518 3.886 59.169

10 71.43 9.461 2.078 78.034

15 67.40 9.654 1.567 83.767#3

20 50.10 9.759 1.236 87.331



98

Table B3: Variation of sodium chloride concentration

CPC concentration

(mM)Experiment

number

[NaCl]

mM

Flux

(L/m2h)

Feed Permeate

Retention

%

0 102.20 11.293 1.754 84.47

2.5 55.51 9.849 6.917 29.77

5.0 53.01 9.653 7.058 26.89

7.5 51.84 9.792 7.093 27.57

10.0 51.18 9.806 7.500 23.51

15 50.61 10.503 8.034 23.51

#1

20 65.77 9.666 6.710 30.58

0 107.30 10.519 1.799 82.89

2.5 60.67 10.560 4.418 58.16

5.0 56.84 10.305 4.618 55.18

7.5 52.50 10.528 5.014 52.37

10.0 51.69 10.464 5.467 47.76

15 51.43 10.503 7.845 25.31

#2

20 65.77 9.666 6.710 30.58

0 107.30 10.519 1.799 82.89

2.5 64.75 10.570 5.565 47.35

5.0 63.73 9.849 4.217 57.18

7.5 88.37 9.789 4.969 49.24

10.0 63.32 9.797 7.066 27.87

15 62.71 9.796 4.126 57.88

#3

20 65.77 9.666 6.710 30.58

0 107.30 10.519 1.799 82.89

2.5 98.07 9.844 3.121 68.29

5.0 88.83 9.773 5.551 43.20

7.5 88.37 9.789 4.969 49.24

10.0 84.49 9.525 5.788 39.24

15 82.40 9.637 7.402 23.20

#4

20 80.82 9.581 7.465 22.09



99

APPENDIX C

Conductivity studies

Table C1: Determination of the cmc of CPC

cell constant = 0.9365

[CPC] conductivity Temp

mM¥

S/cm °°°°C

0.2 37.3 28.4

0.4 55.7 28.4

0.6 77.2 28.5

0.8 92.8 28.5

1 109.6 28.4

1.2 121 28.4

1.4 129.6 28.2

1.8 145 28.3

2 154.4 28.2

degree of ionisation = 0.461

Table C2: Determination of the cmc of CPC in the presence of 0.5

mM HCl

[CPC]mM

ConductivitymS/cm

0 0.294

0.16 0.33

0.24 0.349

0.32 0.357

0.4 0.3645

0.48 0.3725

0.56 0.3815

0.64 0.391

0.8 0.4025

1 0.42251.2 0.446

1.4 0.462

1.6 0.4565

1.8 0.4705

2 0.491



100

Table C3: Determination of cmc of CPC in the presence of Pd and

aqua regia

[CPC]mM

conductivitymS/cm

0.2 1.04

0.4 0.812

0.6 0.82

0.8 0.83

1 0.83

1.2 0.83

1.4 0.84

1.8 0.84

2 0.84

2.4 1.19

2.8 1.34

3 1.39

3.5 1.49

4 1.44

4.5 1.57

5 1.65

6 1.9

7 1.93

8 1.92

Table C4: Determination of cmc of CPC in the presence of Pt and

aqua regia

[CPC]

mM

conductivity

mS/cm

0.2 1.333

0.4 0.72

0.6 0.819

0.8 0.867

1 0.869

1.2 0.87

1.4 0.873

1.8 0.879

2 0.832

2.4 0.975

2.8 1.1043 1.114

3.5 1.125

4 1.129

4.5 1.136

5 1.237

6 1.252

7 1.339

8 1.345



101

APPENDIX D

Improved experimental conditions for the metal ion and surfactant

retention

MEUF of Pt (lV) anions in acidic medium

Table D1: CPC data

Averaged absorbanceCPC concentration

(mM)Sample no.

Feed Permeate Feed Permeate

Retention

%

1 0.9709 0.0201 46.025 0.477 98.96

2 0.9805 0.0208 46.480 0.493 98.94

3 0.9939 0.0787 47.115 1.865 96.04

4 1.0895 0.1310 51.647 3.105 93.99

5 1.0261 0.1416 48.642 3.357 93.10

Table D2: Metal ion data

Metal ion concentration

(mM)Sample no.

Feed Permeate

Flux

(L/m2h)

Retention

%

1 0.0332 0.0008 48.60 97.73

2 0.0869 0.0014 43.70 98.38

3 0.1354 0.0021 38.90 98.48

4 0.1902 0.0012 36.24 99.40

5 0.2557 0.0012 35.63 99.55

MEUF of Pd (ll) anions in acidic medium

Table D3: CPC data


(mM)Sample no.

Feed Permeate Feed Permeate

Retention

%

1 1.0293 0.0394 48.794 0.933 98.092 0.9945 0.0493 47.144 1.168 97.52

3 0.9927 0.0721 47.059 1.708 96.37

4 0.9923 0.0914 47.040 2.165 95.40

5 0.9242 0.1755 43.811 4.160 90.51



102

Table D4: Metal ion data

Metal ion concentration

(mM)Sample no.

Feed Permeate

Flux

(L/m2h)

Retention

%

1 0.0237 3.4E-05 45.18 99.86

2 0.0354 4.6E-05 31.90 99.87

3 0.0350 9.8E-06 31.14 99.97

4 0.0636 1.6E-05 30.07 99.97

5 0.1712 2.4E-05 29.56 99.99

Pt/Pd mixture ultrafiltration in acidic medium

Table D5: CPC data obtained when [Pt] : [Pd] = 1 : x

Averaged absorbance CPC concentration

(mM)

Metal ion

ratio

Pt : Pd Feed Permeate Feed Permeate

Retention

%

1 : 1 0.9963 0.1564 47.229 3.707 92.15

1 : 2 0.9863 0.1312 46.755 3.110 93.35

1 : 3 1.0023 0.1353 47.514 3.207 93.25

1 : 4 0.9819 0.1753 46.547 4.155 91.07

1 : 6 0.9919 0.1603 47.021 3.799 91.92

Table D6: CPC data obtained when [Pd] : [Pt] = 1 : x


(mM)

Metal ion

ratio

Pd : Pt Feed Permeate Feed Permeate

Retention

%

1 : 1 1.0212 0.1582 48.410 3.750 92.25

1 : 2 1.0166 0.1627 48.192 3.856 92.00

1 : 3 1.0062 0.1847 47.699 4.378 90.82

1 : 4 0.9202 0.2031 43.622 4.814 88.96

1 : 6 1.0302 0.1672 48.836 3.963 91.89



103

Table D7: Metal ion data when [Pd] : [Pt] = 1 : x

Table D8: Metal ion data when [Pt] : [Pd] = 1 : x

[Platinum]

(mM)

[Palladium]

(mM)

Retention

%RatioFlux

(L/m2h)

F P F P Pt Pd

1 : 1 28.84 0.05 4.8E-04 0.05 9.5E-04 96.6 93.3

1 : 2 27.77 0.0666 3.8E-04 0.0333 3.5E-04 98.2 96.9

1 : 3 25.27 0.075 5.6E-04 0.025 2.9E-04 97.3 95.1

1 : 4 22.15 0.08 9.8E-04 0.02 5.3E-04 95.8 92.4

1 : 6 20.67 0.085 5.7E-04 0.015 5.2E-04 98.3 94.1

[Platinum]

(mM)

[Palladium]

(mM)

Retention

%RatioFlux

(L/m2h)

F P F P Pt Pd

1 : 1 40.17 0.05 4.6E-04 0.05 4.7E-04 98.1 96.3

1 : 2 38.44 0.033 3.5E-04 0.0666 2.5E-04 97.8 98.3

1 : 3 35.48 0.025 3.1E-04 0.075 2.0E-04 97.1 98.8

1 : 4 29.66 0.02 3.6E-04 0.08 5.0E-04 96.2 97.0

1 : 6 25.42 0.015 4.3E-04 0.085 4.3E-04 99.4 97.8

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