1

Instituto Superior Técnico (December 2016)

Extended Abstract to obtain the Master of Science Degree in Integrated Master in Chemical Engineering

New Asymmetric Mixed Matrix Membranes of Cellulose Acetate and Silica

Gonçalo Diogo Gomes Mendes

Keywords

Cellulose acetate

Silica

Mixed matrix membranes

Sol-Gel

Abstract

The industrial growth of the membrane processes has led to an intense investigation

and optimization of these processes and synthesis of new membranes. Thus,

traditional membranes, such as cellulose acetate membranes, prepared by the

phase inversion method, are subjected to structural modifications, such as the mixed

matrix membranes of cellulose acetate and silica obtained by condensation, in acidic

medium and in the polymer solution, of silanol groups and hydroxyl groups of

cellulose acetate. In this work, amorphous silica contents of 0%, 5%, 10%, 15%,

20%, 30% and 40% were tested. It was observed a linear decrease in the hydraulic

permeability with the silica content, as evidenced by the increased membrane

density observed in FESEM analysis. In the assays of the molecular weight cut-off

and salt rejections the rejection coefficient increases with the silica content.

The characterization by ATR-FTIR and EDS proves the presence of silica in the

active layer of the membrane through the band, at 1080 cm-1, attributed SI-O-C.

1 INTRODUCTION

The last century has witnessed a much higher population

growth than the recorded in previous centuries, about 4,6

billion people in a total of 7 billion [1]. This excessive growth

led to an increase in consumption, which in turn resulted in

an increase of waste released and exaggerated use of

natural resources. To solve this problem, technology has

been developed to separate and treat the waste produced by

human beings, such as deodorization, distillation and

filtration. In the last 50 years, it is in this field that the use of

membranes, using a pressure gradient as driving force for

transport, has been gaining emphasis. This type of

membranes is characterized by separation mechanism or

application.

The conventional membranes can be produced from organic

or inorganic materials and may be structurally divided into

four groups: microporous, integral asymmetric, composite or

homogenous asymmetric [2]. The study of these structures is

extremely important nowadays since these membranes have

gained a strong interest due to the development of new

technologies, as is the case of nanotechnology, which have

improved their performance and removed unwanted

characteristics to the industry. In the last decade,

nanotechnology passed quickly from an academic concept to

a commercial reality. Nowadays, technology at nanoscale is

already available and being used in membranes to modify

and improve the performance, permeability, catalytic

reactivity and fouling resistance. This area of study allowed

the appearance of different types of membranes, such as

ceramic nano-structured membranes, biological membranes

and the organic-inorganic membranes [3]. Mixed matrix

membranes (MMM), included in the last group, were first

introduced in 1990 by Zimmerman et al., where the

characteristics of the polymeric membrane are

complemented with the benefits of inorganic compounds, in

particular the increase in selectivity, targeted functionalities,

and improvement in thermal, chemical and mechanical

stability [4]. These membranes have a huge area of

application, from gas permeation to pervaporation, and more

recently the health care industry (brest cancer treatment and

artificial kidney) [5,6].

2 EXPERIMENTAL PROCEDURE

2.1 MATERIALS

The casting solutions were prepared using cellulose acetate

(CA) from Sigma-Aldrich (~30.000 g/mol) as the polymer for

the membranes, acetone from LabChem and formamide

2

from Sigma-Aldrich as the solvents system, tetraethyl

orthosilicate (TEOS) from Sigma-Aldrich as the silica

precursor of silica and HNO3 from Panreac to obtain an

acidic pH. To obtain the rejection coefficients, four solutions

were prepared using sodium chloride (VWR), sodium

sulphate (Scharlau), magnesium chloride (Riedel-de Haën)

and magnesium sulphate (Merck). To obtain the molecular

weight cut-off, polymers with different molecular weights

were used: anhydrous glucose D(+) from Panreac, PEG

1.000 and PEG 10.000 from Merck, Dextran 70.000 and

Dextran 110.000 from Pharmacia. To dry the membranes, 2-

propanol and n-hexane from Prolabo were used.

2.2 MANUFACTURING OF THE MEMBRANES

2.2.1 PREPARATION OF THE CASTING SOLUTIONS

In this work, two types of membranes were prepared:

asymmetric pure cellulose acetate membrane and mixed

matrix membranes with cellulose acetate and silica. To

manufacture the membranes, a casting solution consisting of

a polymer (cellulose acetate) and a solvent system

(formamide as the weak solvent and acetone as the strong

solvent) had to be prepared. The preparation followed the

experimental protocol of the research group of Professor

Maria Norberta de Pinho [7]. In the casting solutions of the

membranes with silica, the previous method was modified,

with the addition of TEOS (silica precursor), distilled water

and HNO3 to the starting mixture. The proportions are shown

in

Table 2-2. The quantities of cellulose acetate, acetone and

formamide were kept so that the pore size did not change.

The homogenization of the mixtures was performed in a P

Selecta Vibromatic mechanical stirrer at 550 rpm for 24

hours.

2.2.2 CASTING OF THE MEMBRANES

The membranes were fabricated by the wet phase inversion

method described by Kunst and Sourirajan [8]. The

homogeneous casting solution mixture is placed in a glass

dish and, with the aid of a casting knife, spread evenly over

the plate. During the evaporation time (30 seconds), the

acetone evaporates from the surface of the membrane,

increasing the concentration of polymer and giving rise to the

active layer, located in the contact area with the air. After

this, the glass support with the membrane is placed in a

coagulation bath (deionized water and ice) that will promote

the exchange of non-solvent between the membrane matrix

and the bath.

Table 2-1 – Abbreviations used for each membrane with

different content in silica.

Membrane Silica content (%)

A0 0

A1 5

A2 10

A3 15

A4 20

A5 30

A6 40

2.3 CHARACTERIZATION

2.3.1 MORPHOLOGICAL STUDIES

To study the behaviour of silica in cellulose acetate

membranes, FESEM (Field Emission Scanning Electron

Microscopy) was used to assess the morphology and size of

the membrane. Before conducting the analysis, the

membranes were dried and then placed in liquid nitrogen so

that the cross section was uniform throughout the

membrane. Finally, a gold and palladium film was deposited

on the sample so that it conducts electricity due to the use of

the SEM electron beam. The experiments were made using

a JE-7001F FEG-SEM (JEOL), operating at 15,0kV with a

magnification of 750×, 5.000× and 10.000×.

2.3.2 SURFACE CHARACTERIZATION

To characterize both surfaces of the membranes, two types

of analysis were used: ATR/FTIR spectroscopy and Energy-

Dispersive X-ray Spectroscopy (EDS). The ATR/FTIR

spectroscopy was performed in a Nicolet 5700 FT-IR,

running two analyses per membrane. The spectra were

obtained with 256 scans between 4.000 and 650 cm-1, with a

0,5 cm-1 resolution and a gain of 8. The EDS analyses were

made using an equipment adjacent to the one used for

FESEM, a signal processing unit from Bruker. This

equipment allowed an analysis up to 15 μm in depth.

2.3.3 ZETA POTENTIAL

To obtain the zeta potential of the membranes, an Electro

Kinetic Analyser from Anton Paar was used. The zeta

potential was determined in a pH range of 4 to 9 using a

0,001 M KCl electrolyte and a pressure of 500 mbar. The pH

value was changed manually using HCl and NaOH 0,1 M

solutions. Initially the equipment circulates the fluid through

the cell for 60 seconds at 300 mbar and then a bypass is

done for 30 seconds at 300 mbar. After this process the zeta

potential measurement began and the pressure was raised

3

Table 2-2 – Composition of the casting solutions.

Chemical

% w/w A0 A1 A2 A3 A4 A5 A6

Cellulose Acetate 17,0 16,4 15,8 15,2 14,5 13,1 11,6

Formamide 30,0 29,1 27,9 26,7 25,6 23,1 20,5

Acetone 53,0 51,1 51,1 47,2 45,2 40,9 36,3

TEOS - 3,0 6,1 9,3 12,6 19,5 26,9

H2O - 0,5 1,0 1,6 2,1 3,4 4,7

HNO3 4 drops (pH~2)

From 0 to 500mbar for 120 seconds, two times in each

direction. The potential is given by the Equation 2.1 and is

automatically obtained by the software (𝐸𝑠 – streaming

potential; Δp – pressure variation along the capillary; η –

viscosity of the liquid; κ – conductivity of the liquid; 𝜀𝑅 and 𝜀0

– permittivity of the liquid and the empty space, respectively;

𝜆𝑠 – surface conductivity; r – radius of the capillary) [9].

𝜁 =𝐸𝑠∆𝑝

×𝜂

𝜀𝑅 × 𝜀0× (𝜅 +

2 × 𝜆𝑠𝑟

) (2.1)

2.4 EVALUATION OF THE PERMEATION PERFORMANCE

Since the permeation characteristics of the manufactured

membranes were unknown, the characterization was carried

out in ultrafiltration and nanofiltration installations.

2.4.1 NF AND UF CHARACTERIZATION

The nanofiltration installation is composed by six permeation

cells in series, a pump, a pressure control valve, a flow meter

and two pressure gauges. A Hydra-Cell G-03 diaphragm

pump was used to feed the cells. This pump was used up to

40 bar and has a maximum pressure of 70 bar. For the

pressure and flow control, a valve was used at the exit of the

installation, which was monitored with a manometer and a

flowmeter.

The membranes were also characterized in the ultrafiltration

plant, with a pressure gradient of less than 4 bar. The

ultrafiltration plant consists of an ISGEV AS71B4 pump,

which was placed at a maximum pressure of 6 bar, pressure

and flow control are performed using a throttle valve, placed

at the outlet of the concentrate. The pressure and flow

readings are carried out, respectively, on the pressure gauge

and the rotameter, placed near the outlet of the concentrate.

The cells, in which the membranes are found, are the same

as the cells used in the nanofiltration installation and have a

permeation area of 13.2 cm2. These cells were developed at

IST following the original drawing by Dr. Matsuura and Dr.

Souririjan of the National Council of Research in Ottawa,

Canada.

2.4.2 COMPACTION OF MEMBRANES

In order to reach the steady state in the permeation tests, the

membranes were compacted for 2 hours at a pressure 20%

higher than the maximum used in the experiments (40 bar

and 6 bar, respectively, in the nano and ultrafiltration

installations).

2.4.3 HYDRAULIC PERMEABILITY

The hydraulic permeability (Lp), quotient between the pure

water permeate flow of the membrane (Jpw) and the imposed

pressure difference (ΔP) (Equation 2.2), was measured for

the manufactured membranes in both facilities. In

ultrafiltration conditions, the experiment was performed at 1,

1,5, 2, 2,5, 3, 3,5 and 4 bar at a flow rate of 180 L/h. In

nanofiltration conditions, the experiment was performed at

2,5, 5, 7, 10, 15, 20, 25 and 30 bar at a flow rate of 6 L/min.

𝐿𝑝 =𝐽𝑝𝑤

∆𝑃 (2.2)

2.4.4 REJECTION COEFFICIENTS

The rejection coefficient (R) to the salts was calculated from

the concentration differences presented in the feed (𝐶𝑎) and

in the permeates (𝐶𝑝). The measurement of the salt

concentration in the permeate was performed by conductivity

measurement in a Crinson GLP 32 conductimeter, which has

a reproducibility of ± 0,1% and a reading error of ≤0,5% [10].

𝑅 =𝐶𝑎 − 𝐶𝑝

𝐶𝑎 (2.3)

2.4.5 MOLECULAR WEIGHT CUT-OFF

In the case of the ultrafiltration plant, the molecular cut-off

was also studied in order to observe at what particle size the

membrane can effectively separate the polymers.

3 RESULTS AND DISCUSSION

3.1 MORPHOLOGICAL STUDIES

As seen in the figures below (Figure 3-1 – Figure 3-8),

increasing the amount of silica in the cellulose acetate

4

membranes does not cause a significant difference in

membrane structure, in visual terms. The most visible

difference is the compression of the membranes with silica

compared with the membrane with pure cellulose acetate,

where the structure appears to be less dense. That is, in

pure cellulose acetate membranes there appears to be a

greater free space between the polymer chains, which has

effect in the permeation tests which are further forward.

Another important parameter observed is the good

dispersion of silica on the membrane proved by the shown

structure that does not differ in different places of the

membrane. Finally, when compared for each membrane, the

two surfaces present a different appearance proving that

these membranes are asymmetric (Figure 3-7 and Figure

3-8).

The following figures are presented in a crescent order of

silica, starting by the one with no silica and ending in the

membrane with 40% of silica (Figure 3-1–Figure 3-6). The

first three represent an active layer magnification and the last

three the surface of the membranes. Only for the membrane

with no silica was possible to measure the active layer. So,

five measures of the active layer were taken in five different

spots. The results are presented in Table 3-1.

Figure 3-1 – Cross section of the active layer (A0; ×10.000).

Figure 3-2 – Cross section of the active layer (A4; ×10.000).

Figure 3-3 – Cross section of the active layer (A6; ×10.000).

Figure 3-4 – Surface of the active layer (A0; ×15.000).

Figure 3-5 – Surface of the active layer (A4; ×15.000).

Figure 3-6 – Surface of the active layer (A6; ×15.000).

5

Figure 3-7 – Surface of the active layer (A5; ×30.000).

Figure 3-8 – Surface of the porous layer (A5; ×30.000).

Table 3-1 – Membrane and active layer thickness for each

membrane.

Membrane Membrane

thickness (𝜇𝑚)

Active layer

thickness (𝜇𝑚)

A0 59,6 4,8±1,0

A1 52,5 -

A2 51,3 -

A3 66,8 -

A4 50,6 -

A5 54,8 -

A6 58,0 -

3.2 SURFACE CHARACTERIZATION

3.2.1 EDS

Beginning by analysing the active layer of the membranes

(Figure 3-9-Figure 3-11), five peaks of different elements can

be observed: oxygen and carbon, which are present in the

cellulose acetate, silica, that has been added through its

precursor (TEOS), gold and palladium, which were deposited

at the time of the analysis in the form of a film, to make the

sample conductive to electricity. As for the peaks of carbon

and oxygen, there is a decrease in intensity as more silica is

added compared to the gold and palladium peaks which are

approximately constant. This decrease is due to the fact that

there is less cellulose acetate, in percentage, relative to the

membranes with greater amount of silica. In membranes A5

and A6 the carbon and oxygen peaks become lower than the

silica peak. The silica peak starts by not existing in the first

two membranes, it appears in membranes A3 and A4 with

values similar to those of the film used and assumes

significant values in the two membranes with the highest

amount of silica (A5 and A6).

Figure 3-9 – Active layer EDS graphs of the 0% silica

membranes.

Figure 3-10 – Active layer EDS graphs of the 20% silica

membranes.

Figure 3-11 – Active layer EDS graphs of the 40% silica

membranes

When analysed, the results of the porous layer (Figure 3-12

and Figure 3-13) present different conclusions from those of

the active layer. As for oxygen and carbon peaks, the

behaviour remains, with the exception of A5 membrane

6

which has the highest values. However, the peaks of silica

exhibit a different behaviour than that of the active layer,

since increasing the amount of silica does not correspond to

an increase of the silica peak in the results, except for the

membrane A6 which has the highest peak. This result

indicates that the silica that is placed in the casting solution

migrates mostly to the active layer, which is responsible for

the permeation.

Figure 3-12 – Porous layer EDS graphs of the 20% silica

membranes.

Figure 3-13 – Porous layer EDS graphs of the 40% silica

membranes.

3.2.2 ATR/FTIR

The spectra were read with the membranes wet, removing

only the surface water. By analysing the spectrum in parts

and using the peaks already described in the literature, the

main peaks of the spectrum (Figure 3-14) were identified

according to known connections in the structure of cellulose

acetate. Starting at the highest wavelengths, between the

3000-3800 cm-1 is the -OH band (stretching) composed of all

the connections established between these two elements, in

which water is included. Since the stronger bonds are

observed at smaller wavelengths, it can be concluded from

spectrum analysis that there is a greater number of -OH to

establish hydrogen bonds than to establish another type of

bond, given the higher intensity of the band for shorter

wavelengths.

At 2960 cm-1 and between 2850-2910 cm-1 are the

characteristic stretching bands of the C-H bond, for -CH3 and

-CH2 present in the molecule, respectively. In the case of the

band identified at 2400 cm-1, it identifies the connection of

CO2 and is often missing from the spectrum since it is at any

reading due to the CO2 existing in the air. The carbonyl band

(C=O) is found at 1740 cm-1 and at 1635 cm-1 the band

relative to the bending of the H-O-H bond from water. At

1368 cm-1 lies the bending band of the -CH group. In the

zone of smaller wavelengths, it is found one of the most

important bands in the characterization of the manufactured

membranes. Between 1030-1260 cm-1 there are two bands

presented: the first, at 1225 cm-1, related to the stretching of

the C-O-C bond of cellulose acetate, and the second

between the 1000-1100 cm-1 (Figure 3-15) composed of

several bonds. The behaviour of the previous band was

observed with increasing silica. It was concluded that the

shape and intensity of that band depended on the increase

of silica, so the band was de-convulsed in order to better

understand its behaviour.

Figure 3-14 – Full spectrum of the fabricated membranes.

Figure 3-15 – ATR/FTIR band between 1000 and 1100 cm-1.

3.2.3 DECONVOLUTION OF BAND 1000-1100 CM-1

Using the Origin® program, it was possible to de-convolute

the band between 1000 and 1100 cm-1, consisting of several

peaks as shown in Figure 3-16 and Figure 3-17. In this

7

program the most significant peaks are found through the

second derivative of the points obtained in the spectrum

between 1000-1100 cm-1 and then added these points so

that the sum corresponds to the value of the initial band. In

this case three peaks were identified and marked with three

different colours. The areas of each peak were obtained so

that, with increasing silica, it could be established a

comparison between the values for the area of each of the

identified peaks. The deconvolution shows the difference

between the membrane with no silica and the membrane

with 40% of silica.

Figure 3-16 – Deconvolution of the band between 1000-1100

cm-1 for membrane A0.

Figure 3-17 – Deconvolution of the band between 1000-1100

cm-1 for membrane A6.

By observing the graphs it can be seen that as the silica

content increases, there is a decrease in the dark blue peak

and an increase in the pink peak. It is concluded that the

dark blue peak corresponds to the C-O-C bond of the

cellulose acetate [7] and that the pink peak corresponds to

the Si-O-C and Si-H bonds [11]. In this way it was possible to

confirm the existence of the connection between silica and

the polymeric structure of the cellulose acetate membrane.

3.3 ZETA-POTENTIAL

Observing the Figure 3-18, it can be concluded that as the

pH increases, the zeta potential becomes more negative.

There is a greater dependence of zeta potential on pH in the

range of 4 and 6,5, where the greatest variations occur. Also

in this range, membranes with a silica content higher than

20% (A4, A5, A6) are those with the greatest variation in zeta

potential (between 3 and 5 mV), with a similar slope,

whereas membranes with a lower or no content of silica (A0,

A2, A3) are less dependent on pH variation. In the pH range

between 6,5 and 9,5 there is less dependence of the zeta

potential on pH, where there is a stabilization and the value

does not vary more than 1 mV. It can also be seen that

increasing the silica content in the membrane makes the

zeta potential value even more negative. This value would be

expected for the cellulose acetate membrane (without silica),

since the zeta potential is already negative and silica is

negatively charged (due to oxygen). This behaviour makes

the membrane more reactive with water, which in turn will

increase the bonding by hydrogen bonds near the surface of

the membrane and thus increase the amount of gel formed

by the bonds that are established as more water circulates

through the membrane. This behaviour will reinforce the

conclusion already established by the analysis of the -OH

band, in which the increase of silica in the membrane

increases the amount of gel formed at the surface, which

acts as a barrier to the passage of water, resulting in a

reduction of the hydraulic permeability.

Figure 3-18 – Zeta potential in function of pH.

-10

-9

-8

-7

-6

-5

-4

-3

-2

3,5 4,5 5,5 6,5 7,5 8,5 9,5 10,5

Ze

ta p

ote

nti

al (m

V)

pH

A0

A2

A3

A4

A5

A6

8

3.4 HYDRAULIC PERMEABILITY

3.4.1 NANOFILTRATION

By observing the pure water flows as function of the pressure

difference it is concluded that there is a linear behaviour up

to the pressure of 7 bar. At this point there is a stabilization

of the flow value with the increase of pressure. The values

obtained for the hydraulic permeability show no tendency:

the membranes with the lowest and the highest Lp are the

membrane A3 with 15% silica (Figure 3-19) and A2 with 10%

silica, respectively (Figure 3-20). This inconsistent behaviour

may be due to the fact that the membranes do not have

nanofiltration characteristics and/or the results obtained are

withdrawn at a pressure of less than 10 bar at which the

pump damper cannot stabilize.

Figure 3-19 – Hydraulic permeability of the 15% silica

membrane in nanofiltration.

Figure 3-20 – Hydraulic permeability of the membranes in

nanofiltration.

3.4.2 ULTRAFILTRATION

For ultrafiltration, a decrease in the hydraulic permeability

with the increase of silica is observed, with a similar value for

the membranes A3 and A4. A probable cause for this

behaviour may be that silica is introduced into the membrane

structure, decreasing the free space which, in turn,

decreases the flow of pure water through the membrane.

When compared to the cellulose acetate membrane without

silica prepared by Figueiredo [7], with a Lp of 32,1

kg/(h.m2.bar), it was verified that for the membranes with

smaller amounts of silica (A2, A3, A4) the displayed values

of hydraulic permeability are much higher. As for membranes

with higher silica contents, an opposite behaviour can be

observed, in which the hydraulic permeability of the A5

membrane (with 30% silica) is the one that most resembles

Figueiredo's membrane.

Figure 3-21 – Hydraulic permeability of the 15% silica

membrane in ultrafiltration.

Figure 3-22 – Hydraulic permeability of the membranes in

ultrafiltration.

3.5 REJECTION COEFFICIENTS

3.5.1 NANOFILTRATION

For nanofiltration conditions, the salt rejection coefficients

present values lower than 7% for all the membranes used.

This value is low and indicates that under these conditions

the salt rejection is negligible.

3.5.2 ULTRAFILTRATION

The A2 membrane is the membrane with the lowest salt

rejection value, with a maximum rejection of 34,1% for NaCl

and a minimum rejection of 13,3% for Na2SO4. The

membrane A6 is the membrane with the highest salt

rejection value, between two and three times higher than

those presented by the membrane with less silica (A2). In

this membrane the maximum rejection value is 75,8% and

corresponds to NaCl, while a minimum rejection value of

0

50

100

150

200

250

300

350

400

0 5 10 15 20 25 30

Flu

x [

kg

/(h

. m2)]

ΔP (bar)

A3

30,330,9

26,9

28,7

29,8

26,9

24

25

26

27

28

29

30

31

32

A1 A2 A3 A4 A5 A6

Lp

[kg

/(h

. m2. b

ar)

]

y = 56,554xR² = 0,9829

0

50

100

150

200

250

300

0 1 2 3 4 5

Flu

x [

kg

/(h

. m2)]

ΔP (bar)

A4

80,8

58,8 56,6

33,0

9,9

0

10

20

30

40

50

60

70

80

90

A2 A3 A4 A5 A6

Lp

[kg

/(h

. m2

. ba

r)]

9

45% to Na2SO4. In ultrafiltration there is an increase in the

salts rejection, with the increase of the incorporated silica.

This behaviour can be explained since the increase in silica

implies a decrease in flux, followed by an increase

membrane selectivity [12]. With a difference of 30% in silica

content, from the membrane with the least amount to the

membrane with most silica, the rejection of NaCl, Na2SO4,

MgCl2 and MgSO4 increased 122%, 238%, 193% and 212%,

respectively. By comparing the rejection coefficients to the

salts for the different membranes, the following trend can be

verified: 𝑅𝑁𝑎𝐶𝑙 > 𝑅𝑀𝑔𝐶𝑙2 > 𝑅𝑀𝑔𝑆𝑂4 > 𝑅𝑁𝑎2𝑆𝑂4.

Figure 3-23 – Salt rejection of the membranes for the different

salts.

3.6 MOLECULAR WEIGHT CUF-OFF

It can be concluded that for compounds with molecular

weights less than 1.000 Da the rejection rates are reduced

for any of the membranes. The minimum rejection obtained

for PEG 1.000 was for the membrane A4, with a rejection of

7,7% and the maximum for the membrane A6 with 63,9%

rejection. For the molecular weight of 10.000 Da the rejection

increases considerably, reaching minimum value of 50,2%

for the A2 membrane and maximum value of 94,5% for the

A6 membrane. The highest molecular weights present

results close to 100%, with rejection rates varying between

90-96,1% and 95-99,3% for PEG 70.000 and Dextran

110.000, respectively. As molecular weights increase,

rejection rates also increase, as would be expected

since in ultrafiltration the separation is done also by size

exclusion. On the other hand, it is observable that with

increasing silica content, the rejection rate increases for

almost all the polymers analysed, except for the A4

membrane where, for polymers with molecular weights less

than 10 kDa, the rate of rejection value is lower relative to

the membrane with a lower content of silica (A3, 15% silica)

4 CONCLUSIONS

When compared to the results obtained in ultrafiltration for

the 0% silica membrane, the permeability increased in the

lowest silica added membranes (10%, 15% and 20%)

respectively 152%, 84% and 77%. The membrane with 30%

silica has a membrane permeability similar to the A0

membrane, with a deviation of 3%, while in the membrane

with 40% of silica content the hydraulic permeability

decreased by 69%. In nanofiltration, the values obtained for

permeability were not explanatory of the increase of silica,

with variations without an observable tendency. It can be

explained by the collection of data being carried at the

stabilization pressures of the installation.

The most rejected salt was NaCl followed by MgCl2, MgSO4

and Na2SO4, which indicates that the size of the salt is not

preponderant in its rejection, since the NaCl is the salt that

presents the smaller size, however presenting the highest

rejection coefficient. In nanofiltration conditions, the rejection

to the salts presented results lower than 8%, which indicates

that in this range of work the permeation to the salts is

negligible.

In the molecular cut-off there is a trend identified in the

assays. Once silica content is increased there is a decrease

in the minimum molecular weight rejection to a 95%

confidence level. However, there is an exception: in the

membrane with 15% silica and with a confidence interval of

95%, the minimum molecular weight rejected was higher

than the one rejected by the membrane with 10% silica,

contrary to the trend observed previously.

Through the morphological analysis by FESEM it was

concluded that increasing the silica content in the membrane

causes an increase in the density of the structure, since the

silica bound to the cellulose acetate decreases the free

space around the polymer. As regards to the membrane

surfaces, it is also possible to conclude that the increase in

silica content makes it difficult to distinguish between the

active membrane layer and porous layer when observed in

cross section. On the other hand, when the same

membranes are observed, attending on the surfaces, it is

concluded that the active layer has a different structure of the

porous layer. The active layer has a denser surface while in

the porous layer one can easily distinguish the pores of the

polymer, which proves the asymmetry of the membranes.

By the EDS analysis it was possible to confirm the presence

of silica in the membranes and to study its dispersion at the

same time. It was found that the increase of the silica content

in the active layer corresponded, with the exception of the

membrane A4, to the increase of the silica peak, thus

proving its existence in the membrane structure. On the

0,0

10,0

20,0

30,0

40,0

50,0

60,0

70,0

80,0

90,0

100,0

A2 A3 A4 A5 A6

Reje

cti

on

co

eff

icie

nt

(%)

NaCl Na2SO4 MgCl2.6H2O MgSO4.7H2O

10

other hand, once the two surfaces of the membrane were

analysed, it was possible to conclude that there is a

preference of migration of silica to the active layer, since in

the porous layer an increase in the content of silica did not

correspond to an increase in its peak.

About the ATR/FTIR spectroscopy, the binding between

silica and cellulose acetate was detected through the band

identified at 1080 cm-1, characteristic of the Si-O-C bond,

after the de-convulsion. By analysing the previous band, and

once those analyses were performed on both surfaces, it

was possible to observe the different distribution of silica in

the two layers of the membrane, where there is a preferential

migration to the active layer.

Analysing the result obtained for the zeta potential it can be

concluded that the addition of silica to the membrane matrix

decreases the value of the zeta potential, making it more

negative and increasing the reactivity with water. So, the

increasing silica content increases the affinity with water,

promoting the formation of a gel, characteristic of the

hydrogen bonds established by the oxygen groups of silica

and water.

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