Role of charge effects during membrane filtrationlib.ugent.be/fulltxt/RUG01/002/063/522/RUG01-002063522_2013_0001_AC.pdfRole of charge effects during membrane filtration Nguyen Phuong

Faculty of Bioscience Engineering

Academic year 2012 – 2013

Role of charge effects

during membrane filtration

Nguyen Phuong Tu

Promotors : Prof. Dr. Ir. Paul Van der Meeren

Prof. Dr. Ir. Arne Verliefde

Tutor :Ir. Arnout D’Haese

Master’s dissertation submitted in partial fulfillment of the

requirements for the degree of

Master in Environmental Sanitation

COPYRIGHT

‘The author and the promoters authorize consultation and partial reproduction of this thesis

for personal use. Any other reproduction or use is subject to copyright protection. Citation

should clearly mention the reference of this work.’

‘De auteur en de promoters geven toelating deze thesis voor consultatie beschikbaar te stellen

en delen ervan te kopieren voor persoonlijk gebruik. Elke ander gebruik valt onder de

beperkingen van het auteursrecht, in het bijzonder met betrekking tot de verplichting de bron

te vermelden bij het aanhalen van resultaten uit deze scriptie.’

Gent, August 2013

The promoters

De promotoren

Prof. Dr. Ir. Paul Van der Meeren

Prof. Dr. Ir. Arne Verliefde

The author

De auteur

Nguyen Phuong Tu

i

ACKNOWLEDGEMENTS

I wish to express my deepest gratitude to, first and foremost, my promoter Prof. Dr. Ir.

Paul Van der Meeren for giving me the opportunity to work in the Laboratory of the

Department of Applied Analytical and Physical Chemistry and guiding my thesis, for his

patient in correcting, giving the useful comments and explanation to complete my

experiments and thesis manuscript. Without his persistent help, this dissertation would not

have been possible.

My sincere thanks also go to my co-promoter Prof. Dr. Ir. Arne Veliefde for his support for

my work in the laboratory. His enthusiasm in instruction and discussion of the results of

filtration experiments encouraged me so much to finish my experiments in the last weeks

of work. He is also the first person that led me to the interest in membrane through the

course of Analysis and Abatement of Water Pollution.

I am most grateful to my tutor Arnout D’Haese for his kindness, friendly assistance during

the whole thesis process, especially at the beginning of my lab work. Also I thank to Eric

Gillis, Quenton Denon, all other staffs and colleagues in the lab for their professional

information and warmly instruction.

In addition, I am thankful to Prof. M. Van der Heede, Prof. Peter Goethals, Coordinators of

Centre for Environmental Sanitation for their help in both academic and personal issues

throughout two years of my master course.

Special thanks to Belgian Technical Cooperation (BTC) for their financial support for my

studying, I cannot forget the aids that they gave to me when I first time came to Belgium.

The most lovely thanks, I would like to give to my family, my friends, especially, my

husband and my son who are always beside me, bring me the meaningful and colorful life.

Finally, I wish to thank to Belgium with very nice people, thank to the IC train between

Liege and Gent which has really become my “close friend” for two years. For that, I thank

all of them.

Nguyen Phuong Tu

August, 2013

ii

LIST OF ABBREVIATIONS AND SYMBOLS

ABBREVIATIONS DESCRIPTION

DI water Deionized water

EDL Electrical double layer

IEP Isoelectric point

MWCO Molecular weight cut-off

NF Nanofiltration

UF Ultrafiltration

RO Reverse Osmosis

SP Streaming potential

SPC Streaming potential coefficient

TMP Transmembrane pressure

ULPRO Ultra-low pressure reverse osmosis

LATIN SYMBOLS DESCRIPTION

ci Molar concentration of ion (mol/l)

Cp,i Permeate concentration of uncharged solute (mM)

Cb,i Feed concentration of uncharged solute (mM)

Cm Charged solute concentration at membrane surface (mM)

Cb Charged solute concentration at bulk fluid (mM)

e Electron charge (1.602x10-19C)

F Faraday’s constant (96487 Cmol-1)

I Ionic strength (mol/l)

k Boltzmann constant (1.38x10-23 JK-1)

logKow Logarithm of the octanol-water partitioning coefficient (-)

pKa Logarithm of the acid dissociation constant (-)

pKb Logarithm of the base dissociation constant (-)

R Universal gas constant (8.3145 J.mol-1K-1)

Runcharge Rejection of uncharged solute (-)

T Absolute temperature

Zm Ionic valence (-)

z Charge number (-)

iii

V Membrane potential (V)

∆Gsurf Surface affinity for the solute

∆Gsolv Solvent affinity for the solute

∆Gads Free energy of adsorption

ΔE Streaming potential (V)

ΔP Applied pressure (N/m2)

GREEK LETTERS DESCRIPTION

βcharge Charge concentration polarization (-)

ε0 Permitivity of free space (8.854 x 10-12 F/m)

εr Dielectric constant for the streaming solution (-)

ζ Zeta potential (V)

η Viscosity of the streaming solution (Pa.s)

κ-1 Debye length (m)

λ0 System conductivity (S/m)

σe Surface charge density (Cm-2)

iv

TABLE OF CONTENTS

Chapter 1 INTRODUCTION ............................................................................................. 1

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

1.2 Goal of the thesis .......................................................................................................... 2

Chapter 2 LITERATURE REVIEW .................................................................................. 3

2.1 Overview of membrane ................................................................................................ 3

2.1.1 Introduction of membrane technology ............................................................... 3

2.1.2 Nanofiltration membrane .................................................................................... 4

2.2 Role of charge effects during membrane process ......................................................... 5

2.2.1 The role of membrane charge on the retention of ionic solutes ......................... 5

2.2.2 Effects of solute charge on the retention of ionic solutes ................................... 6

2.3 Parameters affecting charge .......................................................................................... 8

2.3.1 Parameters affecting the membrane charge ........................................................ 8

2.3.2 Parameters affecting the solute charge ............................................................. 13

2.3.3 Interaction between charged membrane and charged solutes ........................... 15

2.4 Measurements and equations to be used ..................................................................... 15

2.4.1 Measurement of charge properties ................................................................... 15

2.4.2 Measurement of the rejection of charged organic solute .................................. 20

Chapter 3 MATERIALS AND METHODS ..................................................................... 22

3.1 Materials ..................................................................................................................... 22

3.1.1 Membrane ......................................................................................................... 22

3.1.2 Chemicals ......................................................................................................... 23

3.1.3 Other equipment ............................................................................................... 24

3.2 Measurement method .................................................................................................. 24

3.2.1 Tangential streaming potential procedure ........................................................ 24

3.2.2 Membrane filtration procedure ......................................................................... 31

Chapter 4 RESULTS AND DISCUSSION ...................................................................... 35

v

4.1 Streaming potential, zeta potential and membrane surface charge density ................ 35

4.1.1 Tangential streaming potential measurement on the surface membrane .......... 35

4.1.2 Calculation of zeta potential ............................................................................. 37

4.1.3 Calculation of membrane surface charge density ............................................. 38

4.1.4 Limitation of streaming potential and zeta potential determination ................. 38

4.2 Influence of parameters .............................................................................................. 39

4.2.1 Influence of the pressure drop on streaming potential ..................................... 39

4.2.2 Influence of the ionic strength on membrane charge ....................................... 40

4.2.3 Influence of the adsorption of charged particles on the membrane charge ...... 43

4.2.4 Influence of pH on the membrane charge ........................................................ 45

4.2.5 Influence of electrolyte type on the membrane charge ..................................... 47

4.2.6 Influence of flow rate on streaming potential ................................................... 48

4.2.7 Rejection of pharmaceuticals as a function of pH ............................................ 49

Chapter 5 CONCLUSIONS AND RECOMMENDATIONS ........................................ 54

5.1 Conclusions ................................................................................................................. 54

5.2 Recommendations for future works ............................................................................ 55

vi

LIST OF TABLES

Table 2-1: Retentions for four different salts for the nanofiltration membrane with the

effective membrane pore radius of 1nm at the feed concentration of 50 mol.m-3 (Schaep et

al., 2001) ................................................................................................................................ 7

Table 3-1: Characteristics of membrane given by manufacturer ........................................ 22

Table 3-2: Electric conductivity and pH of solutions at (25 ± 2) 0C .................................. 27

Table 3-3: Electric conductivity and pH of solutions at 25 ± 20C ...................................... 29

Table 3-4: Electric conductivity and pH of solutions at 25 ± 20C ...................................... 30

Table 4-1: The zeta potential values (expressed in mV) as a function of KCl concentration

at different pressure drops ................................................................................................... 37

Table 4-2: The streaming potential values (expressed in mV) as a function of KCl

concentration at different pressure drops. ............................................................................ 40

Table 4-3: Streaming potential coefficient, zeta potential and surface charge density of NF-

270 membrane for electrolyte solutions at different pressure drops ................................... 43

Table 4-4: Streaming potential and zeta-potential for a NF-270 membrane measured with

various electrolyte solutions at pressure drop of 0.3 bar ..................................................... 44

Table 4-5: Streaming potentials, zeta potentials and surface charge density of NF-270

membrane as a function of different electrolyte solutions at applied pressure of 0.2 bar ... 47

Table 4-6: The streaming potential values measured for a NF-270 membrane with different

flow rate of 2 mM KCl solution at 0.2 bar .......................................................................... 48

Table 4-7: pH and sodium salicylate concentration (expressed in mM) obtained after four

filtration experiments with NF-270 membrane. The feed solution contained 5 mM KCl and

0.5 mM sodium salicylate .................................................................................................... 50

Table 4-8: Rejection values of sodium salicylate as a function of feed solution pH with

NF-270 membranes ............................................................................................................. 51

Table 4-9: Some physico-chemical characteristics of sodium salicylate (Log Kow = 1.5 and

pKa = 3) ................................................................................................................................ 53

vii

LIST OF FIGURES

Figure 2-1: Schematic representation of the separation possibility of nanofiltration membrane (Boussu, 2007) ..................................................................................................... 4 Figure 2-2: Effect of solute type on the separation dependency on pH (Xu and Lebrun, 1999) ...................................................................................................................................... 8 Figure 2-3: Zeta potential of NTR-729 membrane dependent on pH in simple electrolyte solution. The legend in the figure shows the different concentrations of NaCl solution (expressed in mM) (Tay et al., 2002) .................................................................................. 10 Figure 2-4: Surface charge density of NTR-729 membrane dependent on pH in simple electrolyte solution. The legend in the figure shows the concentration of NaCl solution (expressed in mM) (Tay et al., 2002) .................................................................................. 10 Figure 2-5: Zeta potential values for the NFT50 membrane in the pH range 4.0 - 8.3 in the presence of monovalent cation K+ and divalent cations Ca2+ and Mg2+ (Teixeira et al., 2005). ................................................................................................................................... 11 Figure 2-6: Effective charge density as a function of the salt concentration for the CA 30 membrane (Schaep et al., 2001). ......................................................................................... 12 Figure 2-7: Rejection of organic acids with Trisep TS80 in Milli-Q as a function of molecular weight and feed water pH (Verliefde et al., 2008). ............................................ 13 Figure 2-8: Effect of pH on the rejection of organic compounds using NTR-729HF membrane (Kim et al., 2006) ............................................................................................... 14 Figure 2-9: Schematic presentation of possible charge distribution at a membrane-aqueous solution interface. (1) EDL model of membrane; (2) simplified EDL of membrane (Tay et al., 2002) .............................................................................................................................. 17 Figure 2-10: Schematic representation of the development of the streaming potential at the solid/liquid interface along a membrane pore under pressure driven liquid flow (Mockel et al., 1998). ............................................................................................................................. 19 Figure 2-11: Schematic diagram of the experimental set-up to measure the streaming potential: 1 – Electrolyte solution. 2- Pump. 3- Measuring cell.4- Upper membrane.5- Pt electrode. 6- Lower membrane. 7- Potential difference measurement (Tay et al., 2002) ... 20 Figure 2-12: Concentration profiles for positively (+), negatively (-) and uncharged (0) solutes in the vicinity of a negatively charged membrane surface (Verliefde, 2008) ......... 21 Figure 3-1: Chemical structure of the crosslinked aromatic polyamide top layer (NF-270) (Boussu, 2007) ..................................................................................................................... 23 Figure 3-2: Schematic illustration of the streaming potential system (Peeters et al., 1999) ............................................................................................................................................. 25

viii

Figure 3-3: Schematic representation of the streaming potential measurement with KCl solution ................................................................................................................................ 26 Figure 3-4: Schematic representation of the streaming potential measurement with KCl and KCl + Al2O3 solutions .................................................................................................. 29 Figure 3-5: Maximum absorption peak in UV spectrum for a 0.5 mM sodium salicylate solution measured at pH 3, 4, 6.5 and 10 between wavelengths ranging from 200 to 500 nm ............................................................................................................................................. 32 Figure 3-6: Calibration curve for sodium salicylate at pH 3, 4, 6.5 and 10 at ambient temperature. The UV absorbance was integrated within the wavelength range from 200 nm to 500 nm ............................................................................................................................. 32 Figure 3-7: Nanofiltration set-up for rejection experiments of sodium salicylate solution with NF-270 membrane ....................................................................................................... 33 Figure 4-1: Electrical potential over the NF-270 membrane at a pressure drop of 0.2 bar with a 2 mM KCl solution at room temperature. ................................................................. 36 Figure 4-2: Electrical potential difference over the NF-270 membrane at a pressure drop of 0.3 bar with a 5 mM NaOH solution at room temperature. ................................................. 37 Figure 4-3: Electrical potential over the NF-270 membrane at a pressure drop of 0.2 bar with a 2 mM acetic acid (C2H4O2) solution at room temperature. ...................................... 38 Figure 4-4: Streaming potential for a NF-270 membrane measured with 2 mM KCl solution at different pressure drops of 0.15, 0.2 and 0.3 bar ............................................... 40 Figure 4-5: Streaming potential values for NF-270 membrane as a function of applied pressure for different KCl solution concentrations .............................................................. 41 Figure 4-6: Zeta potential for a NF-270 membrane as a function KCl solution concentrations ...................................................................................................................... 42 Figure 4-7: Representation of the structure of piperazine (a), protonated piperazine (b), carboxy (c) and deprotonated carboxyl (d) .......................................................................... 46 Figure 4-8: Zeta potential for NF-270 membrane as a function of pH with 2 mM acetic acid (C2H4O2), 1 mM acetic acid (C2H4O2) + 1 mM sodium acetate (C2H3NaO2), 2 mM sodium acetate (C2H3NaO2), and 2 mM sodium hydroxide (NaOH) .................................. 46 Figure 4-9: Permeate flux as function of time with sodium salicylate for NF-270 membrane ............................................................................................................................ 50 Figure 4-10: Rejection of sodium salicylate as a function of feed solution pH with NF-270 membrane ................................................................................................................................

ix

ABSTRACT

In this dissertation, the charged solute rejection of a thin-film composite polyamide

nanofiltration membrane and its relation to membrane surface charge characteristics were

investigated. Efficient removal of ionic solutes by nanofiltration membranes largely relies

on exclusion by charge effects. On the other hand, the charge properties of the

nanofiltration membrane surface may become affected by solute sorption, pH of electrolyte

solutions. Streaming potential measurements of negatively charged NF-270 nanofiltration

membranes were carried out for several solution chemistries, including the inorganic

solutes KCl, NaOH, CaCl2 as well as FeCl3. In addition, their filtration performance was

investigated as a function of pH using sodium salicylate.

Tangential streaming potential measurements were conducted to investigate the alteration

of the membrane charge value under the influence of pH, ionic strength and solute type.

This alteration was investigated through the consideration of zeta potentials that could be

calculated using streaming potential values. The experimental results reported that the zeta

potential decreased when increasing the solute ionic strength and became more negative by

increasing the pH. With these experiments the iso-electric point (IEP) of NF-270

membranes was obtained simultaneously. The determination of zeta potentials also

demonstrates the implications of adsorption of ions and charged particles on the membrane

surface charge.

A filtration experiment with sodium salicylate was performed to examine the effect of

electrostatic interaction between the charged membrane and charged pharmaceuticals. The

results obviously demonstrate the significant effects of electrostatic interaction on the

retention of charged organic solute by NF membranes. In addition, the significant role of

pH during the membrane process was also revealed in this study.

Introduction

Role of charge effects during membrane filtration 1

Chapter 1 INTRODUCTION Equation Chapter (Next) Section 1

1.1 Background

The development of high performance and innovative processes is crucial for a sustainable

industrial growth. Membrane science and technology is expected to play an increasingly

important role in the future for various industrial sectors because of its numerous

advantages compared to conventional treatment technology; particularly, separations with

membranes have become increasingly important (Boussu, 2007).

The heart of every membrane process is formed by the membrane itself, which can be

considered as a thin film interposed between two fluid phases, the selective permeation

through which is governed by particle or molecular size, chemical affinity to the membrane

material and/or the mobility of the permeating species within the membrane (Van der

Meeren and Verliefde, 2012).

Based on the membrane separation processes, the membranes can be divided into

microfiltration, ultrafiltration, nanofiltration and reverse osmosis membranes. Among of

them, nanofiltration which has been largely developed and commercialized over the past

decade, is one of promising technologies for separation of neutral and charged solutes in

aqueous solutions (Wang et al., 2009). It has two remarkable features: one is the MWCO,

which is intermediate between RO membranes and UF membranes and ranges from 200 to

2000 Da; the other is the separation of electrolytes due to the materials containing charged

groups (Wang et al., 2009). The combination of these two features creates the specific

advantages for nanofiltration membranes. For instance, the charged thin-film composite

membrane can reject ions much smaller than the membrane pore radii (Xu and Lebrun,

1999). In order to increase the advantages of nanofiltration in process applications, it is

important to improve the characteristics of the NF membrane. The ampholytic behavior of

the material is used in the development of NF membranes. If the ampholytic polymer

contains weak basic and/or weak acidic groups, the charge of the membranes is affected by

the pH. Therefore, we can use the ampholytic property of the membrane material to make

it negatively or positively charged or neutral by adjusting the pH value of the solution to be

treated (Xu and Lebrun, 1999).

Introduction


1.2 Goal of the thesis

The objective of this study is investigation of charge effect on the nanofiltration membrane

during its performance. NF-270 was chosen as the model membrane because it is the

typical nanofiltration membrane with applications in the drinking water production

(Boussu et al., 2006). Firstly, the behavior of the ampholytic NF-270 membrane was

revealed with measurement of zeta potential in different electrolyte solutions as a function

of pH value, ionic strength and electrolyte type. At the same time, with these experiments,

the dramatic effects of charged solutes, as well as charged particles on the membrane

surface charge through the adsorption mechanism were also considered. On the other hand,

because the membrane is ampholytic and has the charge, inherently, there is electrostatic

interaction between charged solutes and the charged membrane surface, and this

interaction will affect the retention capacity of the nanofiltration membrane. For this

reason, filtration experiments using NF-270 membranes were conducted with sodium

salicylate as the negatively charged form of salicylic acid in the range of tested pH values.

The salt rejection percentage as a function of pH was calculated to determine the effects of

charge interaction on the retention of charged solutes by nanofiltration membranes, in turn,

investigating whether the electrostatic interaction of charged membrane and charged

solutes increases the filtration ability of NF membranes.

Literature Review


Chapter 2 LITERATURE REVIEW Equation Chapter (Next) Section 1

2.1 Overview of membrane

2.1.1 Introduction of membrane technology

Membrane operations in the last years have shown their potentialities in the rationalization

of production systems. Their intrinsic characteristics of efficiency, operational simplicity

and flexibility, relatively high selectivity and permeability for the transport of specific

components, low energy requirements, good stability under wide spectrum of operating

conditions, environment compatibility, easy control and scale-up have been confirmed in a

large variety of applications and operations, as molecular separation, fractionation,

concentrations, purifications, clarifications, emulsifications, crystallizations, etc., in both

liquid and gas phases and in wide spectrum of operating parameters such as pH,

temperature, pressure, etc. Therefore, today, membrane science and technology is

considered to consist the basic aspects that satisfy the requirements of process

intensification, which is the most interesting strategy for realizing sustainable growth

(Strathmann et al., 2006, Drioli and Fontananova, 2004).

Membrane technologies, compared to conventional technologies, respond more efficiently

to the requirements of process intensification strategy because they permit drastic

improvements in industrial production, substantially decreasing the equipment-size/

production-capacity ratio, energy consumption, and/or waste production so resulting in

cheaper, sustainable technical solutions (Drioli and Fontananova, 2004).

The limitations still existing today to the large-scale industrial applicability of some

membrane operations can be attributed to their important disadvantages such as membrane

fouling (resulting in a flux decline and possibly deterioration of the separation properties),

low chemical and/or thermal resistance of the membranes, a limited life-time of the

membranes and high cost for installation, operating and maintenance (Van der Meeren and

Verliefde, 2012).

Due to the limitations, membrane engineering is probably still at its infancy (Strathmann et

al., 2006). However, many efforts on new module configuration designs and on the

implementation of more efficient strategies for concentration polarization and fouling

control are showing growing possibilities. A continuous research effort, on fundamental

Literature Review


aspects of transport phenomena in the various membrane operations already existing and in

the new ones under investigation, is still necessary (Drioli and Fontananova, 2004).

2.1.2 Nanofiltration membrane

Nanofiltration (NF) is used in a wide range of drinking water, wastewater, and industrial

applications (e.g. water softening, removal of colorants and organic matter) (Szoke et al.,

2003). The NF membranes display separation characteristics in the intermediate range

between reverse osmosis (RO) and ultrafiltration (UF). Compared to RO membranes, NF

membranes have a loose structure and enable higher permeate fluxes and lower operating

pressures. Compared to UF membranes, NF membranes have a tighter structure and are

therefore able to reject small organic molecules having molecular weights as low as 200 to

300 Dalton (Levenstein et al., 1996, Hilal et al., 2003). Another feature of these

membranes is that most of them are electrically charged in aqueous media due to their

materials or adsorption of charged species. Hence, their separation mechanisms involve not

only the ordinary “sieve effect” but also the “charge effect”. Therefore, they can reject

charged solutes of much smaller size than the dimensions of the pores. This charge effect

can be particularly used to remove ions from wastewater, or whey, and also separate ions

according to their ionic valences (Fievet et al., 2002), the rejection of divalent ions is over

97%, whereas monovalent ion rejection is only 20 to 80% (Van der Meeren and Verliefde,

2012). The flux is typically 1000l/m2/day at 5 bar. Most currently used NF membranes are

aliphatic polyamide thin film composite (PA TFC). The largest application of NF is water

softening. Besides, NF may be used to remove organic color compounds and

trihalomethaneprecurors such as humic acids (Van der Meeren and Verliefde, 2012).

Figure 2-1 illustrates the separation capability of nanofiltration processes.

Figure 2-1: Schematic representation of the separation possibility of nanofiltration

membrane (Boussu, 2007)

Literature Review


2.2 Role of charge effects during membrane process

The separation mechanism of NF is normally explained in terms of charge and/or size

effects. Transport of uncharged solutes takes place by convection due to a pressure

difference and by diffusion due to a concentration gradient across the membrane. A sieving

mechanism is responsible for the retention of uncharged solutes. For charged components

an electrostatic interaction takes place between the component and the membrane, as most

nanofiltration membranes are charged (mostly negatively) (Schaep et al., 1998). The

charge effect plays an important role in filtration performance. Due to the special feature of

the NF membrane surface (mostly having fixed negative surface charge), the capacity of

separation is influenced by the steric effect (due to small pore diameter) and the charge on

the surface of the pore (Donnan exclusion phenomena) (Seidel et al., 2004). This explains

why these membranes exhibit ion-selectivity. At low ionic environment (i.e., low

concentration of ionic solute), the multivalent negative ions are separated by the NF

membrane to a higher degree than monovalent ions, the latter can pass more freely through

pore of the membrane (Orecki et al., 2004).

On the other hand, the electrokinetic phenomenon involved in the nanofiltration process is

remarkably affected by the physico-chemistry of the solution to be filtered (Elimelech et

al., 1994, Childress and Elimelech, 1996, Nyström et al., 1994, Szymczyk et al., 1999,

Nyström and Zhu, 1997). Hence, by altering the physico-chemistry of the aqueous

environment, optimum conditions for charge interaction between the particles and

membrane may lead to a high efficiency of the process (Tay et al., 2002).

Although the charge effect plays a crucial role in transmission of substances across the

membrane, research work has not been carried out comprehensively (Elimelech et al.,

1994, Nyström et al., 1989). Most of the research work has been focusing on the

measurement of membrane charge and the effects of solution physico-chemistry on the

electrokinetic potential – zeta potential or streaming potential (Childress and Elimelech,

1996, Elimelech et al., 1994, Nyström et al., 1994, Nyström and Zhu, 1997, Ricq et al.,

1996, Mullet et al., 1997, Lettmann et al., 1999).

2.2.1 The role of membrane charge on the retention of ionic solutes

The charge of the membrane is significant to membrane performance because charge

affects the electrostatic repulsion between the ions or charged molecules and the membrane

surface (Childress and Elimelech, 2000). The origin of a membrane charge is clear. When

Literature Review


brought into contact with an aqueous electrolyte solution, membranes do acquire an

electric charge through several possible mechanisms. These mechanisms may include

dissociation of functional groups, adsorption of ions from solution, and adsorption of

polyelectrolytes, ionic surfactants and charged macromolecules. This charging mechanism

can take place as well on the exterior membrane surface as on the interior pore surface of

the membrane. These surface charges have an influence on the distribution of ions in the

solution due to the requirement of the electroneutrality of the system. This leads to the

formation of an electrical double layer, so that we have a charged surface and a

neutralizing excess of counter-ions in the adjacent solution (Schaep et al., 2001). When a

charged membrane is placed in a salt solution, an equilibrium occurs between the

membrane and the solution. Because of the presence of a fixed membrane charge, the ionic

concentrations in the membrane are not equal to those in the solution. The counter-ion

(opposite sign of charge to the fixed charge in the membrane) concentration is higher in the

membrane phase than in the bulk solution, while the co-ions (same sign of charge as the

fixed membrane charge) concentration is lower in the membrane phase. A potential

difference at the interphase, called the Donnan potential, is created to counteract the

transport of counter-ions to the solution phase and of co-ions to the membrane phase.

When a pressure gradient across the membrane is applied, water is transported through the

membrane. The effect of the Donnan potential is then to repel the co-ion from the

membrane. Because of electroneutrality requirements the counter-ion is also rejected and

salt retention occurs (Schaep et al., 1998).

2.2.2 Effects of solute charge on the retention of ionic solutes

Previous studies on the rejection of organics by NF and ULPRO membranes reported that

the retention of solutes depends upon both solute properties and membrane properties.

According to Ozaki and Li (2002) and Berg et al. (1997), the rejection of organic acids

increased with increasing pH relative to the solute’s dissociation constant (pKa). Most NF

membranes are negatively charged at neutral pH due to the dissociation of acidic

functional groups on the membrane surface. Therefore, electrostatic interactions between

charged organic solutes and the charged membranes surface can also play a role in the

rejection of organic micropollutants. Most studies on electrostatic interactions have

reported an increase in rejection of negatively charged organic solutes due to electrostatic

repulsion between the negatively charged membrane and the negatively charged organic

solute. This high rejection, however, is dependent on feed water pH, since both membrane

Literature Review


surface charge and organic solute charge vary according to pH (through the dissociation of

the functional groups as a function of the pKa) (Bellona and Drewes, 2005).

The effect of solute charge on the ionic retention also can be reflected in the influence of

ion valence. At low ionic environment (i.e., low concentration of ionic solute), the

multivalent negative ions are separated by the NF membrane to a higher degree than

monovalent ions, the latter can pass more freely through the pore of the membrane.

Generally speaking, nanofiltration membranes repulse divalent ions having the same

charge as those at the surface of the pore (Wai Lin et al., 2007). Schaep et al. (2001) found

the same result. Table 2-1 shows that the differences in ion valence are very much

influencing ion retention. Calculations were carried out for electrolyte solutions with the

same concentration in equivalents per m3. The findings of about the influence of ion

valence on the salt retention are in agreement with the qualitative aspects of the well-

known Donnan exclusion mechanism: the higher the valence of the co-ion, the higher the

salt retention; the higher the valence of counter-ion, the lower salt retention. For MgCl2 a

negative retention is calculated, as consequence of a high charge interaction (i.e.,

attraction) between the divalent magnesium ion and whether negative retentions in single

salt solutions are possible or not, but it has not yet been experimentally observed.

Calculations show smaller membrane charges and also for smaller membrane pore radii the

MgCl2 retention increases and becomes positive again. This means that in situations where

size effects become more important, relative to charge effects, the retention for MgCl2

increase (Schaep et al., 2001).

Table 2-1: Retentions for four different salts for the nanofiltration membrane with the

effective membrane pore radius of 1nm at the feed concentration of 50 mol.m-3 (Schaep et

al., 2001)

NaCl Na2SO4 MgCl2 MgSO4

1-1 1-2 2-1 2-2

19% 92% -2% 62%

Several authors have found similar conclusions as above (Tsuru et al., 1991, Chowdhury et

al., 1994, Tsuru et al., 1998, Abrabri et al., 1998). However, Xu and Lebrun (1999)

reported that this conclusion about the retention according to the ionic valence is only true

for the quite negatively charged membrane. As shown in Figure 2-2.

+ when pH > 9.0, Na2SO4 > NaCl > MgCl2;

Literature Review


+ when pH < 5.5, MgCl2 > Na2SO4 > NaCl;

+ when 5.5 < pH < 9.0, Na2SO4 > MgCl2 > NaCl.

The separation of MgCl2 can be greater than that of Na2SO4 and NaCl, which is also

observed by Rios et al. (1996) when they worked with positively charged membrane. So,

the dependency of the separation on the electrolyte valence types is determined by the

charged of the membrane, and the salt rejection by charge NF membrane is mainly due to

the surface interaction between the ions and the membrane (Xu and Lebrun, 1999).

Figure 2-2: Effect of solute type on the separation dependency on pH (Xu and Lebrun,

1999)

2.3 Parameters affecting charge

2.3.1 Parameters affecting the membrane charge

2.3.1.1 Effect of pH on the membrane charge

The membrane skin, for most TFC membranes, carries a negative charge to minimize the

adsorption of negatively charged foulants present in membrane feed waters and increase

the rejection of dissolved salts. The negative charge on the membrane surface is usually

caused by sulfonic and/or carboxylic acid groups that are deprotonated at neutral pH.

Membrane surface charge is usually quantified by zeta potential measurements. Studies

have determined that pH had an effect upon the charge of a membrane due to the

disassociation of functional groups. Zeta potentials for most membranes have been

observed in many studies to become increasingly more negative as pH is increased and

functional groups deprotonated (Bellona et al., 2004). A polymeric NF membrane typically

consists of hydrophobic functional groups (alkyl or aromatic chains) which alternate to

Literature Review


hydrophilic functional groups (-CONH2, -COOH, -NH2, -SO3-, -R3N+,etc) whose acidic

characteristics are obviously different; the extent of hydrophilic functional groups

dissociation is indeed strongly pH dependent. The active part of the polymer (which is

involved in charge determination) is assumed as the sum of two different group sites:

hydrophilic sites is indicated as R1H and they are considered to give rise to the pH

dependent “fixed” charge whose sign is the result of the prevailing mechanism among the

concomitant “protonation” and “de-protonation” of functional groups. The group of

hydrophobic sites is indicated as RH and they are considered to give rise only to

competitive adsorption (Bandini, 2005).

At low pH, there is assumed to have high proton concentration in the solution, leading to

the protonation of R1H, making the charge of membrane becomes positive. At high pH,

due to the low proton concentration in the solution, leading to the de-protonation of these

hydrophilic sites, releasing R1-, making the membrane charge becomes negative. The

synthetic representation of acid/base equilibrium reactions was given by two following

equations:

R1H + H+ = R1H2+→ protonation

R1H = R1- + H+→deprotonation

Figure 2-3 shows the effect of pH on the membrane zeta potential in a simple electrolyte

(NaCl) solution. The dramatic change of zeta potential can be seen under the influence of

pH, not only the magnitude of the value but also the sign of the potential. In the pH range

around 2.5 – 11.5, the zeta potential of NTR-729 drops from +20mV down to nearly -

65mV. Accordingly, the membrane charge density varies from + 1µC/cm2 to -5µC/cm2, as

shown in Figure 2-3 (Tay et al., 2002).

Responses to the variation of pH are different between zeta potential and surface charge

density. As shown in Figure 2-3, for the pH range of 2.5-9, the value of zeta potential

decreases significantly with the increase of solution pH. After that, the absolute value of

zeta potential might tend to grow in smaller steps (NaCl concentration < 5 mM). In

contrast, the membrane surface charge density is linearly correlated to the pH value of the

solution. That is solution which can be illustrated in Figure 2-4, surface charge density

changes proportionally with the pH value. In general, with the increase of pH value, the

membrane surface charge density tends to decrease from positive to negative value,

regardless of the ionic strength or any kind of impurities present in the solution (Tay et al.,

2002).

Literature Review


Figure 2-3: Zeta potential of NTR-729 membrane dependent on pH in simple electrolyte

solution. The legend in the figure shows the different concentrations of NaCl solution

(expressed in mM) (Tay et al., 2002)

Figure 2-4: Surface charge density of NTR-729 membrane dependent on pH in simple

electrolyte solution. The legend in the figure shows the concentration of NaCl solution

(expressed in mM) (Tay et al., 2002)

2.3.1.2 Effect of solution ionic strength on the membrane charge

The ionic effect on the membrane surface charge can be described in terms of ionic

strength. Both the concentration of ions and the valence of ions contribute to such effect.

The ion adsorption on the membrane can cause the change in the membrane charge, thus

affecting the retention of the solute. Ions in the solution may take different roles in the

formation of EDL. Some ions, like multi-valence ions, may form complexes with the

membrane and change the property of the membrane. Others may be subject to the

electrical field of the membrane charge and distribute adjacent to the membranes surface.

These ions form the diffuse layer and are regarded as “indifferent ions”. Due to these

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reasons, multi-valence ions are more effective in influencing the membrane charge

property (Tay et al., 2002). Teixeira et al. (2005) conducted the experiments for calculating

the zeta potential of NFT50 membrane with different electrolytes and received the results

which were performed in Figure 2-5.

Figure 2-5: Zeta potential values for the NFT50 membrane in the pH range 4.0 - 8.3 in the

presence of monovalent cation K+ and divalent cations Ca2+ and Mg2+ (Teixeira et al.,

2005).

Figure 2-5 showed that in the presence of divalent cation (Ca2+), the membrane is more

positively charged over the entire pH range and the IEP shifts from 4.2 to 5.6. Since the

NFT50 membrane is negatively charged above pH 4.2, complex formation of the Ca2+ with

the membrane is electrostatically favourable. The Ca2+ adsorption on the membrane surface

reduces its negative charge, yielding a net positive charge for pH below 5.6. On the other

hand, when both divalent cations (Mg2+) and anions (SO42-) are present in the solution, the

effect of the divalent anion is opposite to the effect of the divalent cation (Childress and

Elimelech, 1996), Mg2+ adsorbs less than Ca2+, therefore the zeta potential curve is

intermediate between the curves obtained for KCl and CaCl2. In fact, the ionic strength

increases from KCl solution to both MgSO4 and CaCl2 electrolytes. The increase of the

ionic strength produces a shielding effect responsible for the decrease of the membrane

negative charge along the surface (Teixeira et al., 2005). This conclusion was confirmed by

(Tay et al., 2002), it is clear from Figure 2-3, when increasing the concentration of NaCl,

the zeta potential will decrease.

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The membrane charge density also depends very much on the type of salt and on the salt

concentration but in the complicated way. The data obtained from the experiments of

Schaep et al. (2001) with CA 30 membrane at various salt concentrations are shown in

Figure 2-6

Figure 2-6: Effective charge density as a function of the salt concentration for the CA 30

membrane (Schaep et al., 2001).

It can be seen from Figure 2-6 that the charge density is not constant but depends on the

ionic strength of the solution and can be described by linear isotherms. This phenomenon

was found before and was ascribed to ion adsorption on the membrane material (Bowen et

al., 1997, Peeters, 1997, Wang et al., 1995). In the case of, e.g., NaCl, adsorption of

chloride ions would then lead to a more negative membrane charge at higher electrolyte

concentrations. Figure 2-6 also shows that the membrane charge becomes positive for the

two magnesium salts. This suggests that each individual ion could have its individual

contribution to the membrane charge by means of adsorption. In that way the presence of

magnesium could alter the sign of the membrane charge so that the membranes become

positively charged. This could explain the fact that MgCl2 is better retained than NaCl:

magnesium is the co-ion (i.e the ion with the same sign of charge as the membrane charge)

in the case of MgCl2 and has a higher valence than chloride, which is the co-ion in the case

of NaCl. On the basis of Donnan-exclusion, a higher co-ion valence causes a higher salt

retention. Further experiments with a solution of LaCl3 enabled to compare the retention of

NaCl, MgCl2 and LaCl3. For CA 30, the retention sequence was found to be LaCl3 > MgCl2

> NaCl (Schaep et al., 2001).

Literature Review


In general, the effect of solution ionic strength on the membrane surface charge is in

different aspects. Decrease the zeta potential by high ionic strength is regarded as the

reduction of electrostatic force. However, results obtained from the membrane surface

charge density suggested that the conclusion is different. The surface charge model reveals

that the amount of membrane surface charge will be increased by raised ionic

concentration. This is an important fact for membrane filtration. The principle of

membrane filtration is to retain particles in water by the membrane. If the particles easily

adhere to the membrane, they may be more able to pass through the membrane, or deposit

on the membrane leading to membrane fouling. The electrostatic force is catalogued as the

repulsive force. If dosage of high valence ions into the solution might strengthen the

electrostatic force between the particles and the membrane, it would be much easier for the

membrane to reject the particles and prevent membrane from fouling (Tay et al., 2002).

2.3.2 Parameters affecting the solute charge

2.3.2.1 Effect of pH on the charged solute dissociation

The solute charge is usually expressed through the acid or base dissociation coefficient,

pKa or pKb of the solute (Bellona et al., 2004). Therefore, it is expected to depend on the

pH levels of the solution. Verliefde et al. (2008) carried out some experiments for rejection

of organic acids with the Trisep TS80 membrane and got the result as given in Figure 2-7:

Figure 2-7: Rejection of organic acids with Trisep TS80 in Milli-Q as a function of

molecular weight and feed water pH (Verliefde et al., 2008).

The rejection of all acids in Milli-Q water at pH 8 is above 93%. This high rejection is due

to electrostatic repulsion between the negatively charged membrane and the completely

dissociated organic acid (Verliefde et al., 2008). At pH 8, all acids are completely

Literature Review


dissociated and thus rejection is independent of the degree of dissociation. At pH 5,

however, dissociation is incomplete (except for malonic acid), and rejection of all organic

acids is lower than at pH 8. The dissociation of organic matters influenced by pH and pKa

in rejection by nanofiltration was also reported by Kim et al. (2006).

Figure 2-8: Effect of pH on the rejection of organic compounds using NTR-729HF

membrane (Kim et al., 2006)

Figure 2-8 shows the effect of pH of the feed solution on the rejection of typical organics.

The NTR-729HF membrane was used for a series of experiments. The rejection of MCPA,

whose pKa value is 3.1, increased with an increase of pH value of feed solution. In

addition, the rejections of p-nitrophenol (pKa = 7.1) and 2,4,5-trichlorophenol (pKa = 6.7)

increased markedly with pH when the pH was roughly above the pKa. Theoretically,

almost all dissociated organic compounds are in a state of ionic form when the solution pH

is larger than pKa. In the case of MCPA, p-nitrophenol and 2,4,5-trichlorophenol, the

fraction in the state of negatively charged ions of those compounds increases with pH. On

the other hand, the membrane used is negatively charged when the pH of the solution is

more than the iso-electric point value of the membrane, and is positively charged when less

than the iso-electric point value. Hence, it is considered that the increase of rejections of

those compounds with pH, especially in the alkaline region, resulted from an increase of

mutual repulsion between the compounds dissociated into ions and membrane.

Furthermore, the decrease of rejection of MCPA below pH 5 would be caused by the

attraction between the dissociated MCPA and the membrane. The rejection of acetic acid

(pKa = 4.7) was similar to those of compounds such as MCPA. Phenol rejection was

extremely low over the range of pH tested, probably because phenol was in a state of

Literature Review


molecular form derived from its large pKa. Low rejection was also obtained for ethyl

alcohol of undissociated compounds. From these results, it appears that the rejection of

organic compounds by NF membrane is dependent on pKa (Kim et al., 2006).

2.3.3 Interaction between charged membrane and charged solutes

The charge of membrane surface can be changed by adsorption of polyelectrolytes, ionic

surfactants, and charged macromolecules. This interaction was mentioned in an article of

(Elimelech et al., 1994). The free energy of adsorption, ∆Gads, of a solute is given by

ads surf solvG G GΔ = Δ −Δ (2.1)

Where ∆Gsurf represents the surface affinity for the solute and ∆Gsolv is the solvent affinity

for the solute. Thus, a solute that is hydrophobic in character will readily adsorb onto a

solid surface. For polyelectrolytes adsorption arises from London-van der Waals

interactions, hydrophobic bonding of nonpolar segments, hydrogen bonding, electrostatic

attraction, and chemical reaction with surface functional groups. If the polyelectrolyte and

the membrane surface are similarly charged, adsorption occurs if the non-electrostatic

attraction is greater than the electrostatic repulsion. In this case adsorption is enhanced by

increasing the ionic strength or by the presence of polyvalent counterions. If the

polyelectrolyte and surface are oppositely charged, then adsorption is dominated by

electrostatic attraction (Elimelech et al., 1994).

2.4 Measurements and equations to be used

2.4.1 Measurement of charge properties

Different methods have been investigated to determine and interpret charge properties of

membrane surface, most of them work on the measurement of electrokinetic phenomena

(Huisman et al., 2000) and zeta potential is a useful parameter for this. Several techniques

are available that can be used to determine the zeta potential of surfaces. Among of these

streaming potential technique is most suitable for membrane surfaces (Elimelech et al.,

1994). Streaming potential measurements have been carried out to calculate the zeta

potential and the kinetic surface charge density of the membranes.

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2.4.1.1 Surface charge density and zeta potential

2.4.1.1.1 The concept of surface charge density

The plane surface of the membrane has an electrical charge. The quantity of the surface

charge is termed as “membrane charge density”, which can be defined as the charge

quantity per unit area on the membrane surface (Tay et al., 2002). Surface charge density

was found to be an important surface parameter in predicting the separation effect in NF

membrane due to the influence the solution physic-chemistry (Tiraferri and Elimelech,

2012, Tay et al., 2002). The surface charge density can be calculated from the

determination of zeta potential.

2.4.1.1.2 The concept of zeta potential

The concept of zeta potential is derived from the theory of EDL, which describes the EDL

as consisting of the Stern layer and the diffuse layer from the solid-water interface (Tay et

al., 2002).

In the case of a charged membrane immersed in aqueous solution, the EDL associated with

the membrane-solution interface can be schematically summarized in Figure 2-9(1). The

membrane surface at X = 0 has a potential of Ψs, which is related to the membrane

property. The charge can be obtained originally by dissociation of the functional group,

adsorption of charged particles, or even chemical bounding. The compact Stern layer

(0<X<X1) is formed by immobile ions under the influence of the membrane charge. And

the outer layer (X1<X<X3) is termed as diffuse layer.

To simplify the EDL model in describing the electrokinetic phenomenon of the

membranes, the following assumptions are made:

There is an imaginary “plane” (at X = X1) to separate the compact layer (0<X<X1) and the

diffuse layer (X1<X<X3). The plane has an electrical charge. The quantity of the surface

charge is termed as “membrane surface charge density”, which can be defined as the

charge quantity per unit area on the membrane surface. The Stern layer is much thinner

than the diffuse layer, even less than the distance of X1X2. The shear plane (at X = X2), to

which zeta potential ζ is related, is supposed to be located in diffuse layer. The simplified

EDL model can be depicted in Figure 2-9(2) (Tay et al., 2002)

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Figure 2-9: Schematic presentation of possible charge distribution at a membrane-aqueous

solution interface. (1) EDL model of membrane; (2) simplified EDL of membrane (Tay et

al., 2002)

2.4.1.1.3 Calculation of zeta potential and membrane surface charge density

In this work, the zeta potential is determined by measuring the streaming potential. The

measurements are made by flow over the exterior surface of the membranes, which means

that only the charge density on the exterior surface of the membrane will be determined.

The relationship between the measurable streaming potential and the zeta potential is given

by the well-known Helmholtz-Smoluchowski equation:

0

0 r

EP

η λζε ε

Δ × ×=Δ × ×

(2.2)

Where ΔE is the streaming potential (V), ΔP is the applied pressure that causes the

hydrodynamic flow (N/m2), η the viscosity of the streaming solution (Pa.s), λ0 the system

conductivity (S/m), ε0 the permitivity of free space (8.854 x 10-12 F/m), and εr the dielectric

constant for the streaming solution.

Literature Review


This equation was already simplified for surfaces with low surface conduction by

eleminating the surface conductivity (Mockel et al., 1998)

Although zeta potential is the membrane/electrolyte interface parameter more widely used

for electrokinetic characterization, the electrokinetic surface charged density (σe), or

surface charge density at the shear plane, can be easily obtained from zeta potential data by

the Gouy-Chapman equation (Ariza and Benavente, 2001):

2 sinh2e

kT zeze kTεκ ζσ ⎛ ⎞= ⎜ ⎟⎝ ⎠

(2.3)

Where k is the Boltzmann constant (1.38 x 10-23 J/K), e the electron charge (1.602 x 10-19

C), κ-1 the Debye length, z the charge number and T the absolute temperature (298K)

The Debye-length 1κ − can be calculated from Equation (2.4):

κ −1 = εRT

2F 2I (2.4)

I is the ionic strength and F is Faraday’s constant

212 i i

iI z c= ∑ (2.5)

In which zi is the charge number of ion I and ci represents the molar concentration of that

ion (mol/l)

2.4.1.2 Streaming potential

2.4.1.2.1 The concept of streaming potential

The streaming potential remains the most widely used technique for determining the ζ-

potential of membranes (Fievet et al., 2003). Streaming potential is the potential difference

at zero electric current, caused by the flow of liquid under a pressure gradient through a

capillary, plug, diaphragm, or membrane. The difference is measured across the plug or

between the ends of the capillary. Streaming potentials are created by charge accumulation

caused by the flow of counter-charges inside capillaries or pores (Delgado et al., 2005).

Figure 2-10 shows the development of streaming potential at the liquid/solid interface

along the membrane pore under the pressure driven liquid flow. The solid surfaces are

negatively charged. Liquid, driven by a pressure difference from inlet to outlet, flows

laminarly with a parabolic profile between the plates. The liquid sweeps along the mobile

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charges of the diffuse part of the electrical double layer (in this case positive), associated

with the surface. The flow of mobile charge is an electric current arising solely from the

flow of liquid, called “surface current”. If there is no net flow of charge from the solid

surfaces to the solution, a bulk current carried by neutral electrolyte must flow back

through the liquid to complete the electrical circuit. The bulk current flows back to a

negative pole at the inlet and completes the electrical circuit. The reference electrodes

placed at either end of the flow channel to detect the streaming potential.

Figure 2-10: Schematic representation of the development of the streaming potential at the

solid/liquid interface along a membrane pore under pressure driven liquid flow (Mockel et

al., 1998).

2.4.1.2.2 Tangential streaming potential measurements

In the case of asymmetric or composite membranes, the ζ-potential is now frequently

determined from tangential flow streaming potential measurements (Fievet et al., 2003).

This technique consists in applying a pressure difference across a thin channel formed by

two identical substrates facing one another and separated by a spacer (Fievet et al., 2003,

Sbaï et al., 2003).

Literature Review


Figure 2-11: Schematic diagram of the experimental set-up to measure the streaming

potential: 1 – Electrolyte solution. 2- Pump. 3- Measuring cell. 4- Upper membrane. 5- Pt

electrode. 6- Lower membrane. 7- Potential difference measurement (Tay et al., 2002)

2.4.2 Measurement of the rejection of charged organic solute

At pH around 4 – 5, the NF-270 membrane is uncharged (or has a very low surface

charge). Therefore, the rejection values of both solutes at this pH will be regarded as the

rejection values in the absence of electrostatic interactions. Thus, according to Verliefde

(2008), the rejection of solutes at low pH can be calculated as follows:

,arg

,

1 p iunch e

b i

CR

C= − (2.6)

Where i is the solute of interest, and Runcharge, Cp,i and Cb,i are respectively the rejection, the

permeate concentration and the feed concentration of the solute.

Verliefde (2008) also reported the equations for the calculation of the rejection at high pH

values. At higher pH, the membrane assumes a negative surface charge and charge

interactions will affect the rejection of the solutes. Therefore, the rejection differs from the

rejection at low pH. For positively charged solute, the rejection decreases due to charge

attraction (charge concentration polarization > 1). For the negatively charged solute, the

rejection increases because the concentration of the solutes at the membrane surface

decreases due to charge repulsion (charge concentration polarization < 1).

Concentration profiles of charged solutes in the vicinity of a charged surface can be

calculated using the Gouy-Chapman theory for colloidal solutes and extrapolating this

theory to charged solutes approaching flat surfaces. This leads to the following Boltzmann

distribution for charged organic solutes approaching the membrane surface:

arg. .exp.

m mch e

b

C Z V FC RT

β −⎛ ⎞= = ⎜ ⎟⎝ ⎠ (2.7)

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Where βcharge is the charge concentration polarization, Zm is the organic ion valence, V is

the membrane surface potential (in V), F is Faraday’s constant, R is the universal gas

constant, T is the temperature (in K) and Cm and Cb are the charged solute concentrations

at the membrane surface and in the bulk fluid, respectively. This is schematically

represented in Figure 2-13

.

Figure 2-12: Concentration profiles for positively (+), negatively (-) and uncharged (0)

solutes in the vicinity of a negatively charged membrane surface (Verliefde, 2008)

The charge concentration polarization is the concentration in- or decrease of charged

solutes in the vicinity of the charged membrane surface, due to electrostatic interactions.

As the first approach, the membrane zeta potential is used instead of the membrane surface

potential in the calculation of Equation (2.7)

For the charged solutes, the rejection can be calculated as follows:

arg arg arg arg1 1 1 (1 )p pch e ch e ch e unch e

b m

C CR R

C Cβ β= − ≈ − = − − (2.8)

With Cp/Cm substituted by (1-Runcharged). Of course, this substitution can only be made if the

hydrodynamic concentration polarization is assumed to be equal to one. Otherwise, p

m

CC

has

to be substituted by arg1 unch eRβ

−, where β is the hydrodynamic concentration polarization.

Thus, if Runcharged is known, and βcharge can be calculated according to Equation (2.8), the

theoretical rejection of a charged solute can be predicted.

Materials and Methods


Chapter 3 MATERIALS AND METHODS Equation Chapter (Next) Section 1

The tangential streaming potential experiments were carried out for determination of zeta

potential and membrane surface charge density at various pressure drops, solute

concentrations and electrolyte types with NF-270 membranes in order to investigate the

dependency of membrane charge on the pH, ionic strength and electrolyte type.

In addition, filtration experiments with aqueous sodium salicylate solutions were also

conducted with the NF-270 membranes at different pH values of the feed and then, the

retention results were determined based on the ratio of concentration of the permeate and

the feed.

3.1 Materials

3.1.1 Membrane

Nanofiltration membrane

All the experiments in this work were carried out with commercial polymeric

nanofiltration membranes. Flat polyamide thin film composite NF-270 membranes were

used which were produced by FILMTEC. Some characteristics of the membrane as

presented by the manufacturer are summarized in Table 3-1 and the chemical structure is

shown in Figure 3-1

Table 3-1: Characteristics of membrane given by manufacturer

Membrane Type Polyamide Thin-Film composite Average pore diameter 0.84 nm Maximum Operating Temperature 450C Maximum Operating Pressure 41 bar Maximum Pressure Drop 1.0 bar MWCO 200 -300 Da pH range, continuous operation 3-10 pH range, short-term cleaning (30 min) 1-12 Maximum Feed Flow 15.9 m3/h Maximum Feed Silt Density Index SDI5 Free Chlorine Tolerance <0.1ppm CaCl2 rejection (%) 40 – 60 MgSO4 rejection (%) >97 Charge (neutral pH) Negative

(Source:http://msdssearch.dow.com/PublishedLiteratureDOWCOM/dh_0074/0901b803800749e1.

pdf?filepath=liquidseps/pdfs/noreg/609-00519.pdf&fromPage=GetDoc, Lin et al. (2007))



Figure 3-1: Chemical structure of the crosslinked aromatic polyamide top layer (NF-270)

(Boussu, 2007)

The membranes were kept in a refrigerator at about 50C. Before use, all membranes were

rinsed with de-ionized (DI) water for 24h to get rid of preservation liquids present in the

membrane. The membranes to be used were cut to fit the cells and glued on the glass plate

and were placed by facing their active layers to the incoming solution.

3.1.2 Chemicals

A series of different electrolyte solutions at various concentrations were used for

experiments with the nanofiltration membrane for studying the influence of the parameters

on the membrane charge. They were prepared by dissolving the amount of solute to the DI

water. Most of chemicals used for these experiments are manufactured by VWR

International S.A.S.

Sodium salicylate (Chemical formula: HOC6H4COONa; Molar mass: 160.10 g/mol) is a

sodium salt of salicylic acid. It is used in medicine as an analgesic and antipyretic. The

sodium salicylate used in the experiments was manufactured by Merck – Germany and of

pro analysis purity. According to the manufacturer, solubility amounts to 660 g/l (200C),

the partitioning coefficient (log Kow) is 1.5 and the pKa is 3 (Jaskari et al., 2000).

For ionizable solutes, the hydrophobicity is dependent on the percentage of uncharged

versus charged species, and thus it is pH-dependent (Verliefde et al., 2008). Based on the

data about the pKa and log Kow, the dependence of hydrophobicity of sodium salicylate on

pH can be determined according the following equation:

( )( )log log log(1 10 )apH pKpH owD K −= − + (3.1)

Where D is the ratio of the equilibrium concentrations of all species (unionized and

ionized) of a molecule in octanol to the same species in the water phase; logKow is the



logarithm of the octanol-water partitioning coefficient and pKa is the logarithm of the acid

dissociation constant.

3.1.3 Other equipment

The electric conductivity of these solutions was measured with the Consort multi-

parameter analyser C3020 and Electrochemical analyser C6010 in the laboratory by

immersing the probe into the solutions at different concentrations.

The pH values of the solutions were measured by using HANNA HI 4222, consisting of an

electrode for measuring the pH and another for measuring the temperature of solution.

Before measuring the pH of samples, the equipment needs to be calibrated.

A heat controller equipment KAC-MAG HS7 was used to maintain the constant

temperature for solutions.

The absorbance of samples was determined with an Ultrospec 1000 UV/Visible

Spectrophotometer from Pharmacia Biotech.

The pump which was used for filtration set-up was Sigma from Prominent company.

3.2 Measurement method

3.2.1 Tangential streaming potential procedure

3.2.1.1 Tangential streaming potential module

Figure 3-2 shows a schematic drawing of the cell which is used to support the membrane

samples for the streaming potential measurements. The membrane samples are glued upon

glass plates (microsope slides). The dimensions of the membrane samples are 76 x 26 mm.

The cell consists of two plexiglass parts containing sample channels that hold the glass

slides with the membranes glued on top. Clamps hold both cell parts together. Under the

glass slides silicon rubber sheets can be put to prevent leakage at the interface between cell

and glass slide. The cell parts are separated by a spacer which serves two functions:

spacing between sample plates and flow direction. Platinum electrodes are inserted into the

chambers at both ends of the cell (Peeters et al., 1999). The cell, housed in the Faraday

cage was connected to the earth to reduce the outside effects that interferes the

measurement results. The flow inlet and outlet are in parallel with the channel axis and

only one flow direction is possible (Ariza and Benavente, 2001). The positive and negative

poles were connected with the outlet of the flow (from the membrane to the waste



container) and the inlet of the flow (from the sample container to the membrane),

respectively.

Figure 3-2: Schematic illustration of the streaming potential system (Peeters et al., 1999)

3.2.1.2 Basic tangential streaming potential procedure

The set-up of tangential streaming potential allows the streaming potential to be measured

along the surface, rather than through the surface (Peeters et al., 1999). For determination

of the influence of pressure drop, ionic strength on the streaming potential, several

electrolyte solutions with different concentrations were used.

Before the start of each measurement, the membrane cell was fed with the electrolyte

solution for a period of about 3 minutes to remove the entrapped air bubbles in the

measuring system. During each measurement, the testing solution was sucked from feeding

solution reservoir by applying the driving pressure. The applied pressure drop was

controlled by the suction of a mechanical vacuum pump and varied in the range of 0.15 to

0.3 bar and was monitored with an accuracy of 0.01 bar. The mass of the feeding solution

that flows into the waste container (g) and the measurement duration (minute) were

recorded and then converted to the flow rate using the density of the solution. This flow

rate was kept at 0.35 – 0.5l/minute during the entire run with checking the flow rate after

each measurement. If the flow rate was out of the range above, the closing-opening valve

controlling was checked and the distance between sample plates was adjusted to ensure the

pressure in the cell. For instance, the flow rate for pumping 2 mM potassium chloride

(KCl) through the cell at 0.2 bar can be calculated as follows:

Initial solution mass - Final solution mass 3275.0 1919.2 451.93 0.451Measurment duration 3min min min

g g g lQ −= = = =



The density of the solution was considered as the density of pure water due to the small

solution concentration ρ = 1 g/ml.

The measurements were conducted from low to high salt concentration for each applied

pressure with the same membrane to avoid the contamination from previous solution. Data

acquisition took place with a computer. When the valve was closed, the solution stopped

flowing through the cell (non-flow mode), the flow started again when the valve was

opened (flow mode). The interval for each time of closing and opening is 8s. The

difference in the potential between flow and non-flow mode is set equal to the streaming

potential. Ten potential difference values were calculated corresponding to five times of

opening-closing the valve. Then, a mean value was obtained, indicating its standard

deviation as error. The streaming potential was measured using a digital multimeter and

recorded by the computer in the system with a LABVIEW program.

All the measurements were carried out at room temperature of (25±2)0C. All solutions

were used at natural pH of the solutions without any pH adjustment.

The schematic representation of the streaming potential measurement with KCl solution is

illustrated in Figure 3-3:

Figure 3-3: Schematic representation of the streaming potential measurement with KCl

solution

Pressure gauge

Vacuumpump

Multimeter

Computer

Membrane Cellsolution

Opening- ClosingValve

KClWaste container

Electrodes



3.2.1.3 Experimental design for determining the effect of electrolyte concentration on the

membrane charge and effect of pressure drop on streaming potential

The experimental procedure for determining the alteration of membrane charge depending

on electrolyte concentration and the influence of pressure drop on streaming potential

value was carried out in accordance with the basic tangential streaming potential

measurement that was represented in 3.2.1.3. In this experiment, potassium chloride (KCl)

solutions were prepared with concentrations of 1 mM, 2 mM and 5 mM. These solutions

were introduced to the membrane cell in succession as function of the applied pressure

drops of 0.15, 0.2 and 0.3 bar. Based on the results obtained from this experiment, both the

effects of potassium chloride (KCl) concentration and pressure drop on streaming potential

were assessed, then, the zeta potential could be defined. For the calculation of zeta

potential from streaming potential, the electric conductivities of solutions were measured

and reported in Table 3-2.

Table 3-2: Electric conductivity and pH of solutions at (25 ± 2) 0C

Solution Electric conductivity (S/m) pH

1 mM KCl 0.01 6.13

2 mM KCl 0.03 6.22

5 mM KCl 0.06 6.93

1 mM KCl + Al2O3 0.01 5.39

5 mM NaOH 0.09 11.10

3.2.1.4 Experimental design for determining the effect of charged particles adsorption on

membrane charge

For investigation of the effect of positively charged particles on the membrane charge, a

30% (w/w) stock suspension of alumina (Al2O3) (Evonik Degussa GmbH) was added into

the 1mM potassium chloride (KCl) solutions with the concentration of 100 mg/l. The

specific density of the alumina suspension is 1.163 g/ml.

The experiments with the mixture of potassium chloride and alumina (KCl +Al2O3) for

determination of streaming potential with the presence of positively charged particles were

conducted with the same procedure as that for the experiment with potassium chloride

(KCl) solutions. However, in this case, the three-way valve was applied so that one after



another, the potentials of potassium chloride (KCl) solution and mixed solution were

obtained.

Firstly, potassium chloride (KCl) solution was allowed to pass through the cell for 3

minutes, during this time, all the bubbles were removed from the system, and afterwards,

the potential for this solution was recorded. Then, the three-way valve was changed so that

the potassium chloride (KCl) solution was stopped passing through the cell and potassium

chloride and alumina (KCl +Al2O3) solution was introduced. After the first potential value

had been obtained, the membrane was soaked into the potassium chloride and alumina

(KCl +Al2O3) solution by stopping the flow during 30 minutes. Subsequently, the

measurement was repeated with potassium chloride and alumina (KCl +Al2O3) solution.

This procedure was reproduced 5 times. Then, the three- way valve was changed again to

allow the passing of potassium chloride (KCl) solution. Due to the adsorption of alumina

particles on the membrane surface, the membrane charge was altered into positive.

Therefore, in order to remove the alumina particles from the membrane surface, 5 mM

sodium hydroxide (NaOH) solution was pumped through the cell and steeped there for 16

hours, then, the potentials were observed and streaming potentials were calculated. The

sodium hydroxide (NaOH) solution was applied to the membrane until the membrane

charge changes to negative again. Finally, the 1 mM potassium chloride (KCl) solution

was fed to the membrane cell for checking the value of streaming potential.

All the values were recorded by the computer. The measurements were carried out at 0.3

bar. Figure 3-4 demonstrates the schematic representation of streaming potential

measurement with the mixed solution.



Pressure gauge

Vacuumpump

Opening- ClosingValve

Multimeter

Computer

KCl+Al Osolution

2 3 KClsolutionWaste container Membrane Cell

Electrodes

Figure 3-4: Schematic representation of the streaming potential measurement with KCl

and KCl + Al2O3 solutions

3.2.1.5 Experimental design for determining the effect of pH on membrane charge

The solutions tested in the pH effect experiments were chosen to represent the different pH

values. The pH and electric conductivity of these solutions were measured and summarized

in Table 3-3.

Table 3-3: Electric conductivity and pH of solutions at 25 ± 20C


2 mM C2H4O2 0.01 3.81

1 mM C2H4O2 + 1 mM C2H3NaO2 0.01 5.01

2 mM C2H3NaO2 0.02 7.27

2 mM NaOH 0.04 10.52

The streaming potential experiment was performed in turn with 2 mM acetic acid

(C2H4O2), 1 mM acid acetic (C2H4O2) + 1 mM sodium acetate (C2H3NaO2), 2 mM sodium

acetate (C2H3NaO2), and 2 mM sodium hydroxide (NaOH). Prior to a new measurement,

the membrane was soaked with the next measured solution for half an hour without



pressure and the measurement with each solution was repeated three times to ensure the

activities of electrolyte and membrane can occur fully. The streaming potential was

recorded at a pressure drop of 0.2 bar. After the streaming potential was determined, the

zeta potential could be calculated.

3.2.1.6 Experimental design for determining the effect of electrolyte type on membrane

charge

This experiment was carried out with three salts of different valence ions as 2 mM

potassium chloride (KCl), 2 mM calcium chloride (CaCl2) and 2 mM iron chloride (FeCl3).

Measurements were carried out firstly with 2 mM potassium chloride (KCl), then the

calcium chloride (CaCl2) was continued to be introduced through the membrane cell for

three times. A next measurement was run with 2 mM potassium chloride (KCl) during the

current condition of the membrane. After three times of running with 2 mM potassium

chloride (KCl), the streaming potential of 2 mM Iron(III) chloride (FeCl3) was measured

by the system. At the end of the experiment, before withdrawing a given membrane from

the analyzer cell, once again, the run was conducted with 2 mM potassium chloride (KCl)

and its streaming potential was recorded.

This assay was implemented at a pressure drop of 0.2 bar.

For calculation of the zeta potential from the streaming potential and other analyses, the

pH and electric conductivity of these feed solutions were measured and given in Table 3-4.

Table 3-4: Electric conductivity and pH of solutions at 25 ± 20C


2 mM CaCl2 0.04 6.99

2 mM FeCl3 0.10 3.08

3.2.1.7 Experimental design for determining the effect of flow rate on streaming potential

This influence was investigated with the measurement of streaming potential for 2 mM

potassium chloride (KCl) solution at a fixed pressure drop of 0.2 bar. For this experiment,

in order to change the flow rate through the channel, the channel height was modified by

controlling six screw fittings. In this case, two measurements were implemented for two

states of the screwing fittings: tightening the screws and loosening the screws until there

was leakage of water from the cell. The procedure for streaming potential measurement



was completed as presented in 3.2.1.2. Three streaming potential data series were collected

for each position of screws. Besides, a planar channel was assumed for this case where the

height (h) of the channel is very small compared to the width (w) (h<<w), the channel

height can be calculated according to Poiseuille:

33

8 8L LQP Q hh w P wη ηΔ = → =

Δ ×

In which ΔP represents the pressure drop between the two ends of the channel (N/m2), L is

the total length of the channel (m), Q is the volumetric flow rate (m3/s) and η is the

dynamic viscosity of the streaming solution which was approximated as the dynamic

viscosity of pure water (η = 0.89x10-3 Pa.s).

3.2.2 Membrane filtration procedure

3.2.2.1 Analysis of the feed and permeate solution

Sodium salicylate (C7H5NaO3) concentrations in the samples of the feed and permeate

solution were determined through the measurement of UV absorbance of each sample with

a calibration curve which was constructed on the basis of the absorbance of sodium

salicylate at various concentrations and pH values (Figure 3-6). The upper limit of

absorbance in the UV spectrophotometer is 3. Therefore, all solutions were prepared in

concentrations that corresponded to an absorbance less than 3. In this UV spectrum test, a

series of 0.1 mM, 0.2 mM, 0.3 mM, 0.4 mM and 0.5 mM of sodium salicylate was

prepared at pH values that were used in the filtration experiment, such as pH = 3 (adding

0.1 M hydrochloric acid solution for adjustment), pH = 4 (adding 0.1 M hydrochloric acid

solution for adjustment), pH = 6.5 (only mixing with DI water), and pH =10 (adding 0.1 M

sodium hydroxide solution for adjustment). At each pH level, samples were taken to

measure the absorbance of the solution three times. For each solution at a fixed pH value,

the absorbance spectrum was scanned to find the wavelength with the maximum

absorption peak and this value of sodium salicylate was read from the computer at a

specific wavelength. Then, the average absorbance values were calculated. Figure 3-5

represents the absorption spectra for 0.5 mM solution as a function of pH.



Figure 3-5: Maximum absorption peak in UV spectrum for a 0.5 mM sodium salicylate

solution measured at pH 3, 4, 6.5 and 10 between wavelengths ranging from 200 to 500 nm

Figure 3-6: Calibration curve for sodium salicylate at pH 3, 4, 6.5 and 10 at ambient

temperature. The UV absorbance was integrated within the wavelength range from 200 nm

to 500 nm

3.2.2.2 Cross-flow filtration set up

Cross-flow filtration experiment was carried out to investigate the retention of charged

organic solute by nanofiltration membrane. The schematic diagram of the cross-flow used

nanofiltration membrane is shown in Figure 3-7. The installation consists of membrane cell

0

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

260 270 280 290 300 310 320

Abs

orba

nce

Wavelength (nm)pH 3 pH 4 pH 6.5 pH 10

y = 3.201xR² = 0.995

y = 3.365xR² = 0.990

y = 3.210xR² = 0.996

y = 3.214xR² = 0.992

0.00

0.20

0.40

0.60

0.80

1.00

1.20

1.40

1.60

1.80

0 0.1 0.2 0.3 0.4 0.5 0.6

Ab

sorb

ance

Concentration (mM)

pH 10 - 296nm

pH 6.5 - 296nm

pH 4 - 297nm

pH 3 - 299nm



in which the NF-270 membrane is packed. The dimensions of membrane sample are 200 x

50 mm. The feed solution was delivered to the membrane cell from the feed tank (10 l) by

a piston pump. A heat controller equipment was used to keep the constant temperature for

feed solution at (25 ± 2) 0C. The concentrate was recirculated back to the feed tank during

the filtration run and its flow rate was monitored by a rotameter and controlled at 30 l/h,

the frequency of the pump was 90 cycles/min. The applied transmembrane pressure is

regulated using a valve in the concentrate stream; the transmembrane pressure was

measured with a precision manometer. The permeate was collected in a volumetric flask

and timed to calculate the permeate flux afterwards. After each measurement duration, the

permeate volume was allowed to be recycled to the feed tank.

Figure 3-7: Nanofiltration set-up for rejection experiments of sodium salicylate solution

with NF-270 membrane

3.2.2.3 Filtration protocol

The rejection experiments were carried out with aqueous solutions of the pharmaceutical

sodium salicylate. Before the experiment, the NF-270 membrane was equilibrated by

immersing it into the solution of 0.5 mM sodium salicylate and 5 mM sodium chloride

(KCl) and putting in the fridge for 24 hours. Prior to the experiments, the membrane was

stabilized at 5 bar using DI water for 24 hours to remove the remaining preservatives and

to ensure the compaction of the membrane. Then, the mixed solution containing 0.5 mM

sodium salicylate and 5 mM potassium chloride (KCl) (added as background electrolyte)

which was adjusted at pH 10 by adding sodium hydroxide (NaOH) was pumped through

Membrane module

25o

Feedsolution

PumpTemperature control

Pressure gauge

Permeate solution

Valve

Rotameter

Concentrate



the membrane cell with a flow rate of 30 l/h and the pressure was controlled at 3 bar. The

duration for this experiment was 12 hours. In the next stage, the feed solution was adjusted

to pH 6 by adding hydrochloric acid (HCl) into the feed reservoir. This solution was

introduced to the membrane for 10 hours at 3 bar. The feed solution pH was subsequently

reduced to a value of 4 and this filtration experiment was run for 10 hours. Finally, a

solution of 0.5 mM sodium salicylate and 5 mM potassium chloride (KCl) at pH 3 was

passed over the membrane during 10 hours. After each measurement, a permeate sample

was taken and the pH as well as UV absorbance of the feed and permeate were measured.

Throughout the filtration run, the permeate flow rate was observed every hour and in turn,

the permeate flux was calculated according to this formula: mem

QJA

= , in which J is the

permeate flux (m3/m2.hour); Q is the permeate flow rate (m3/hour) and Amem is the area of

the membrane (Amem = 200 x 50 = 10000 mm2 = 0.01 m2)

Results and Discussion


Chapter 4 RESULTS AND DISCUSSION

The tangential streaming potential experiments with electrolyte solutions were conducted

to determine the streaming potential of the membrane. From streaming potential values,

zeta potential and membrane surface charge density can be calculated. Hereby, the

ampholytic behavior of nanofiltration membrane under the influence of pH, ionic strength

and solute type were evaluated and discussed.

Besides, the rejection value of sodium salicylate ions by NF-270 membrane as a function

of pH was also calculated by performing the filtration experiment. Based on the retention

results, the electrostatic interaction between charged solute and charged membrane surface

was assessed whether this interaction affects the filtration efficiency of the membrane.

4.1 Streaming potential, zeta potential and membrane surface charge density

4.1.1 Tangential streaming potential measurement on the surface membrane

The tangential streaming potential measurement was run with various solutions. Based on

the data obtained from the program on the computer, there are two ways for receiving the

streaming potential values.

Case 1: The solution of 2 mM potassium chloride (KCl) was pumped through the cell by

the suction of vacuum pump; the applied pressure difference was set at 0.2 bar. The

streaming potential was measured continuously by two Pt-electrodes. The individual

streaming potential values acquisition took place automatically with a computer using

LABVIEW program (Yu, 2010). Measurements were performed at room temperature.

Figure 4-1 shows a typical graph for a streaming potential measurement. The difference in

the electrical potential between flow (lower line in Figure 4-1) and non-flow (upper line in

Figure 4-1) mode is set equal to the streaming potential. The data of electrical potential are

given in Table 6.1 in Appendix. The individual values of the streaming potential (-2.58, -

2.69, -2.52, -2.75, -2.80, -2.90, -2.74, -2.66, -2.67, -2.71 mV) were obtained by the

difference between the average electrical potential values of flow and non-flow at each

time. A mean value of -2.70 mV was calculated with a standard deviation of 0.10 mV. The

negative sign of the streaming potential value is associated with the negative charge of the

membrane.



Figure 4-1: Electrical potential over the NF-270 membrane at a pressure drop of 0.2 bar

with a 2 mM KCl solution at room temperature.

Case 2: The procedure for measurement of streaming potential of 5 mM sodium hydroxide

(NaOH) solution was as same as the previous measurement with 2 mM potassium chloride

(KCl) solution and the applied pressure difference was set at 0.3 bar. Figure 4-2 illustrates

a graph for a streaming potential measurement. The data of electrical potential are given in

Table 6.2 in Appendix. For this case, the individual streaming potential (-5.84, -3.61, -5.73,

-4.78, -4.33, -5, -4.41, -5.14, -5.01, -5.86 mV) values were obtained by the difference

between electrical potential values of flow and non-flow at the transition point. A mean

value of -4.97 mV was calculated with a standard deviation of 0.73 mV. The negative sign

of the streaming potential value is associated with the negative charge of the membrane.

The reason for application of this calculation method for such curves is that the calculation

as case 1 produced a high standard deviation.

0

2

4

6

8

10

12

14

0 20 40 60 80 100 120 140 160

Ele

ctri

c po

tent

ial (

mV

)

Time (s)



Figure 4-2: Electrical potential difference over the NF-270 membrane at a pressure drop

of 0.3 bar with a 5 mM NaOH solution at room temperature.

4.1.2 Calculation of zeta potential

The streaming potential values obtained allow to calculate the zeta potential by the use of

Equation (2.2)

0

0 r

EP

η λζε ε

Δ × ×=Δ × ×

In which: ΔE is the streaming potential (V), ΔP the applied pressure that causes the

hydrodynamic flow (N/m2), ε0 the permittivity of free space, (i.e. ε0 = 8.854x10-12 F/m), εr

the dielectric constant for the electrolyte solution and η the viscosity of the electrolyte

solution. They were approximated by the dielectric constant and viscosity of pure water

(i.e. εr = 78.54 and η = 0.89x10-3 Pa.s), λ0 represents the electric conductivity of the

electrolyte solution (S/m) at 25± 20C and ζ is the zeta potential (V).

Using Equation (2.2), the values of zeta potential were obtained as shown in Table 4-1:

Table 4-1: The zeta potential values (expressed in mV) as a function of KCl concentration

at different pressure drops

KCl

concentration

(mM)

Zeta potential (mV)

Pressure drop

= 0.15 bar

Pressure drop

= 0.2 bar

Pressure drop

= 0.3 bar

Average value

1 -5.04± 0.25 -6.89± 1.21 -6.11± 1.18 -6.01± 0.76

2 -4.65± 0.39 -4.67± 0.17 -4.54± 0.53 -4.67± 0.11

5 -4.33 ± 0.76 -4.45 ± 0.31 -3.98± 0.42 -4.26± 0.20

50

55

60

65

70

75

80

85

90

95

0 20 40 60 80 100 120 140 160

Ele

ctri

c po

tent

ial (

mV

)

Time (s)



4.1.3 Calculation of membrane surface charge density

After the zeta potential was obtained, the membrane surface charge density has been

determined according to Equation (2.3), (2.4) and (2.5)

Taking the 1 mM potassium chloride (KCl) solution at 250C and ζ = -6.01 mV, the Debye

length could be calculated by:

10

1 92

43

6.95 10 8.31 298. 9.61 10

2 9.65 10 1

C J KVm mol K m

C molmol m

κ−

− −× × ×

= = ×⎛ ⎞× × ×⎜ ⎟⎝ ⎠

The surface charge density was obtained by:

( )10 3

49 2

6.95 10 6.01 104.35 10

9.61 10e

C V CVmm m

σ− −

−−

× × − ×= = − ×

×

Similarly, the membrane surface charge density with other zeta potentials could be

calculated and the results of streaming potential coefficient (SPC), zeta potential (ζ) and

surface charge density (σe) for various electrolyte solutions are listed in Table 4-3.

4.1.4 Limitation of streaming potential and zeta potential determination

4.1.4.1 Limitation of streaming potential measurement

The most important limitation of streaming potential is the unstable results. The tangential

streaming potential measurement is easily affected by the surrounding factors and causing

the fluctuated readings. One of error curves is demonstrated in Figure 4-3.

Figure 4-3: Electrical potential over the NF-270 membrane at a pressure drop of 0.2 bar

with a 2 mM acetic acid (C2H4O2) solution at room temperature.

46

48

50

52

54

56

58

60

0 20 40 60 80 100 120 140 160

Str

eam

ing

pot

enti

al (

mV

)

Time (s)



The reason for this error can be explained as either the influence of factors surrounding the

streaming potential set-up in the laboratory or the presence of bubbles during the

measurement although all the bubbles were tried to be removed before each measurement.

This error will lead to the lack of reproducibility of the experiment, in turn, affecting the

accuracy of the measurement values and therefore, the evaluation of parameters influence

on the membrane charge can be less precise.

4.1.4.2 Limitation of zeta potential determination

Tangential streaming potential measurement to be used for calculation of zeta potential is

interpreted by classical Helmholtz-Smoluchowski (H-S) equation. Nevertheless, some

authors have recently shown that, the membrane porous body can contribute to the

conduction phenomenon and the basic H-S relation is no more applicable in these

conditions and the values of the zeta potential can be strongly underestimated if the

membrane body conductance is not considered and the values calculated by the H-S

equation could be up to 5 – 10 times lower than those calculated by coupling streaming

potential and cell electric conductance measurement (Déon et al., 2012).

4.2 Influence of parameters

4.2.1 Influence of the pressure drop on streaming potential

The streaming potential values of a 2 mM potassium chloride (KCl) solution are

represented in Figure 4-4 as a function of pressure drop. Linear regression asserts that the

streaming potential is linearly proportional to the applied pressure drop and it negatively

increased when increasing the applied pressure. From this point, the streaming potential

coefficient can be calculated as ESPCP

Δ=Δ

which shows the development of streaming

potential as a result of an applied pressure gradient. This value is nearly constant for each

solution concentration in the measured pressure range. Therefore, for other experiments

henceforth, streaming potential determination at one single pressure drop is sufficient for

investigation of the effect of charged ions on membrane charge.



Figure 4-4: Streaming potential for a NF-270 membrane measured with 2 mM KCl

solution at different pressure drops of 0.15, 0.2 and 0.3 bar

4.2.2 Influence of the ionic strength on membrane charge

To determine the influence of ionic strength on streaming potential, zeta potential and

membrane surface charge density were calculated on the basis of streaming potential

values obtained from the tangential streaming potential procedure which was performed

with several potassium chloride (KCl) solutions of 1 mM, 2 mM and 5 mM at applied

pressure of 0.15, 0.2 and 0.3 bar. The streaming potential values of these experiments were

summarized in Table 4-2:

Table 4-2: The streaming potential values (expressed in mV) as a function of KCl

concentration at different pressure drops.

KCl concentration

(mM)

Streaming potential (mV)

Pressure 0.15 bar Pressure 0.2 bar Pressure 0.3 bar

1 -4.54 ± 0.22 -8.28 ± 1.38 -11.02 ± 2.12

2 -2.02 ± 0.17 -2.70± 0.10 -3.94 ± 0.46

5 -0.81 ± 0.14 -1.10 ± 0.2 -1.48 ± 0.16

DI water was not used because the ionic strength was not defined. In addition, the electric

conductivity may be largely affected by small amounts of electrolyte, and there is also

minor quantity of ions in DI water. Hence, streaming potential measurements may show

fluctuations. The conversion to zeta-potential is troublesome and the physical

interpretation of the zeta-potential is hampered. To overcome these problems, a higher

y = -13.27xR² = 0.999

-‐4.5

-‐4

-‐3.5

-‐3

-‐2.5

-‐2

-‐1.5

-‐1

-‐0.5

0

0 0.05 0.1 0.15 0.2 0.25 0.3

Stre

amin

g po

tent

ial (

mV

)

ΔP (bar)

KCl 2mM



electrolyte concentration was preferred (Yu, 2010). The streaming potential results which

are expressed in mV as a function of pressure drop are given in Figure 4-4.

Figure 4-5: Streaming potential values for NF-270 membrane as a function of applied

pressure for different KCl solution concentrations

From Figure 4-5, it is clear that the streaming potentials of the membrane decreased whilst

increasing the concentration of the potassium chloride (KCl) solution. This result was also

affirmed in the experiments of Peeters (Peeters et al., 1999). The relationship between

streaming potential of the membrane and concentration in electrolyte solution is explained

by increasing the ionic strength, the dispersion layer of EDL becomes thinner, which

causes more opposite counter-ions to enter the solvent layer, neutralizing part of negative

charge on the surface. Moreover, the high ionic strength may make the charge of

membrane surface shielded, which results in the decrease of streaming potentials (Qiu et

al., 2009).

The relation linking the streaming potential and zeta potential was already mentioned in

(Szymczyk et al., 1997), at the same applied pressure, the zeta potential of membrane for a

solution is proportional to streaming potential and therefore, with the increasing the ionic

strength of the solution, the zeta potential is also reduced. The decrease of zeta potential

with higher concentration of potassium chloride (KCl) solution is illustrated in Figure 4-6.

With 1 mM potassium chloride (KCl) solution, the zeta potential value was -6.01 mV and

y = -37.00xR² = 0.973

y = -13.27xR² = 0.999

y = -5.150xR² = 0.991

-‐14

-‐12

-‐10

-‐8

-‐6

-‐4

-‐2

0

0 0.05 0.1 0.15 0.2 0.25 0.3

Stre

amin

g po

tent

ial (

mV

)

ΔP (bar)

KCl 1mM

KCl 2mM

KCl 5mM



this decreased to -4.26 mV at 5 mM potassium chloride (KCl) solution. This result is

reasoned by reducing the thickness of dispersion layer, the surface of shear gets closer to

the membrane surface, thus zeta potential set at fixed distance becomes lower (Afonso,

2006).

Figure 4-6: Zeta potential for a NF-270 membrane as a function KCl solution

concentrations

Although the zeta potential is considered as important indicator of membrane surface

charge density, the calculation of membrane surface charge density obtained in this

experiment is in contrast to zeta potential behavior as function of ionic strength.

Table 4-3 shows the effect of ionic strength on the zeta potential and surface charge

density in simple ion solution (potassium chloride solution). When the concentration of

potassium chloride (KCl) rose from 1 mM to 5 mM, the zeta potential values decreased but

the result for the surface charge density was different. These values increased and

dramatically increased (in absolute values) if there was the presence of solution with high

valence ions (referred to Table 4-5). Especially, when membrane was contacted with

Iron(III) Chloride (FeCl3), the surface charge density became positive. This result was the

same in the experiment of (Escoda et al., 2010, Tay et al., 2002, Schaep et al., 2001) and is

explained probably due to progressive adsorption of ions from the solution onto the

membrane material (Escoda et al., 2010). It can be stated another reason for the increase of

surface charge density based on Equation (2.3). This equation shows that although the zeta

potential decreases, the Debye length also reduces, leading the increase of surface charge

density.

-‐7

-‐6

-‐5

-‐4

-‐3

-‐2

-‐1

0

0 1 2 3 4 5

Zeta

pot

entia

l (m

V)

KCl concentration (mM)



Table 4-3: Streaming potential coefficient, zeta potential and surface charge density of NF-

270 membrane for electrolyte solutions at different pressure drops

Solution concentration

(mM)

SPC

(mV/bar) Zeta potential

(mV)

Surface charge density

(mC/m2)

1 mM KCl -37.00 ± 4.57 -6.01 ± 0.93 -0.43 ± 0.05

2 mM KCl -13.38 ± 0.31 -4.62 ± 0.07 -0.48 ± 0.01

5 mM KCl -5.15 ± 0.25 -4.26 ± 0.24 -0.53 ± 0.02

4.2.3 Influence of the adsorption of charged particles on the membrane charge

To investigate the effect of charged particles adsorption on the membrane charge, the

different electrolyte solutions were used including: 1 mM potassium chloride (KCl)

solution, 1 mM potassium chloride and alumina (KCl solution + 100 mg/l Al2O3 30%) and

5 mM sodium hydroxide (NaOH) solution. The procedure of the assay was carried out in

accordance with the process that was described in 3.2.1.4. The results of streaming

potential and zeta potential are summarized in Table 4-4.



Table 4-4: Streaming potential and zeta-potential for a NF-270 membrane measured with

various electrolyte solutions at pressure drop of 0.3 bar

Order Solution Streaming potential (mV)

Zeta potential (mV)

1 1 mM KCl -8.23± 0.55 -4.57 ± 0.30 1 1 mM KCl+Al2O3 30% -6.94± 0.47 -3.85 ± 0.26 2 1 mM KCl+Al2O3 30% -4.46± 0.28 -2.48 ± 0.15 3 1 mM KCl+Al2O3 30% 6.03± 0.62 3.35 ± 0.34 4 1 mM KCl+Al2O3 30% 5.89± 0.56 3.26 ± 0.31 5 1 mM KCl+Al2O3 30% 6.28± 0.43 3.48 ± 0.24 1 1 mM KCl 4.80± 0.74 2.67 ± 0.41 2 1 mM KCl 2.68± 0.17 1.49 ± 0.09 3 1 mM KCl 4.54± 0.77 2.52 ± 0.43 4 1 mM KCl 3.27± 0.25 1.81 ± 0.14 5 1 mM KCl 2.17± 0.19 1.20 ± 0.11 1 5 mM NaOH -3.20± 0.26 -11.73 ± 0.94 2 5 mM NaOH -5.76± 1.58 -21.14 ± 5.80 3 5 mM NaOH -4.97± 0.73 -18.24±2.67 4 5 mM NaOH -6.63± 0.70 -24.31 ± 2.58 5 5 mM NaOH -5.02± 0.63 -18.41 ± 2.30 1 1 mM KCl -7.87± 0.53 -4.36 ± 0.29 2 1 mM KCl -7.63± 0.55 -4.23 ± 0.31 3 1 mM KCl -9.46± 0.84 -5.25 ± 0.46 4 1 mM KCl -9.18± 0.77 -5.09 ± 0.43 5 1 mM KCl -9.08± 1.36 -5.03 ± 0.75

The experimental results in Table 4-4 reveals the reduction of the negative zeta potential in

the presence of alumina (Al2O3). From the third measurement with mixed solution, the

membrane charge value became positive (ζ = 3.35 ± 0.34 mV). After that, although a 1

mM potassium chloride (KCl) solution was fed to the membrane, the membrane surface

charge could not get back to its original (negative) charge. A probable explanation for this

phenomenon might be attributed to the adsorption of alumina particles on to the

nanofiltration membrane. The positive particles could be considered as the foulant and

bound to the negative membrane. From the experiment, it is clear that the membrane

became less negative after introduction of positive particles and retained this charge over

the fouling period. Afterwards, 5 mM sodium hydroxide (NaOH) solution with a pH of

11.10 was applied as cleaning agent to bring the membrane to its original charge. As the

average diameter of alumina particles within the pH range of 2 to 6 is 235 nm

(Hakimhashemi, 2012) which is much bigger than the average pore diameter of the NF-270

membranes of 0.84 nm (Nghiem and Hawkes, 2007), the solid particles mainly created a



layer on the membrane surface and they could be displaced by sodium hydroxide (NaOH)

solution. The reason for this removal is attributed to the interaction between Al2O3

particles and charged membrane. The sodium hydroxide (NaOH) solution has a higher pH

(≈ 10) than the IEP of the NF-270 membrane (≈4-5), thus, it will help increasing the

negative surface charge of the membrane, at the same time, the IEP of Al2O3 particles is

around 8.9 and hence particles became negatively charged at a pH above 8.9 (Chen et al.,

2007). These things caused the re-dispersion in the aqueous media due to electrostatic

repulsion (Hakimhashemi, 2012) between alumina and membrane surface. Another

possible explanation for removal of Al2O3 particles from the membrane by sodium

hydroxide (NaOH) solution is the reaction between NaOH and Al2O3 to form NaAl(OH)4

which is soluble in the water in accordance with the equation:

Al2O3(s) + 3H2O(l) + 2NaOH(aq)→ 2NaAl(OH)4(aq)

Table 4-4 represents the zeta potential of 1 mM potassium chloride (KCl) solution; both

before applying the mixed solution of 1 mM potassium chloride and alumina (KCl +Al2O3)

and after cleaning with sodium hydroxide (NaOH). The zeta potential values at two times

are almost similar, suggesting that the membrane surface is restored close to its original

condition after being fouled and cleaned (Lawrence et al., 2006).

4.2.4 Influence of pH on the membrane charge

Figure 4-8 illustrates the zeta potentials as function of pH for NF-270 membrane

determined using 2 mM acetic acid (C2H4O2), 1 mM acetic acid (C2H4O2) + 1 mM sodium

acetate (C2H3NaO2), 2 mM sodium acetate (C2H3NaO2), and 2 mM sodium hydroxide

(NaOH) as the electrolytes. There was the decrease of zeta potential when increasing the

pH from 3.81 corresponding to 2 mM acetic acid (C2H4O2) to pH 10.52 corresponding to 2

mM sodium hydroxide (NaOH). Especially, at pH 5.01, a reversal of the sign of the

membrane surface charge was found, the zeta potential value of the membrane at pH 5.01

was -0.41 mM, demonstrating the alteration of membrane charge from positive to negative.

Then, the zeta potential became negative and increased in absolute value as a function of

pH. This result is consistent with the literature review regarding the effect of pH on the

membrane charge and was also confirmed by many experiments of others (Childress and

Elimelech, 2000, Bellona and Drewes, 2005). The result of this experiment can be

explained by the reaction of carboxyl and piperazine functional groups that would be

expected on the membrane surface. The positive surface charge below the IEP would result



from the protonation of the piperazine functional groups at low pH value, and the negative

charge above the IEP would result from deprotonation of the carboxyl groups (Childress

and Elimelech, 2000). The protonation of piperazine group and deprotonation of carboxyl

group are illustrated in Figure 4-7 (Bishnoi and Rochelle, 2000)

(a) (b)

(c) (d)

Figure 4-7: Representation of the structure of piperazine (a), protonated piperazine (b),

carboxy (c) and deprotonated carboxyl (d)

Figure 4-8: Zeta potential for NF-270 membrane as a function of pH with 2 mM acetic

acid (C2H4O2), 1 mM acetic acid (C2H4O2) + 1 mM sodium acetate (C2H3NaO2), 2 mM

sodium acetate (C2H3NaO2), and 2 mM sodium hydroxide (NaOH)

According to Figure 4-8, the IEP of NF-270 membrane is around 4.8. This value also

complies with the findings in the study of Ozaki and Li (2002) and Wai Lin et al. (2007)

which showed that the IEP of NF-270 membrane was around 5.3. This value is also in the

range of the IEP for polyamide based NF membranes that is 3.5 – 6 (Ramaswamy et al.,

2013)

-‐30.00

-‐20.00

-‐10.00

0.00

10.00

20.00

30.00

0 2 4 6 8 10 12

Zeta

pot

entia

l (m

V)

pH



4.2.5 Influence of electrolyte type on the membrane charge

Through the experimental procedure with monovalent salt (KCl) and multivalent salts

(CaCl2, FeCl3), which was stated in 3.2.1.6, the influence of various electrolyte types was

observed. From the assessment of the change in zeta potential values, the effect of cation

adsorption on the membrane surface was revealed. At the same time, the determination of

membrane surface density also provided the evidence about the adsorption effect of ions

on the membrane surface. All the data are provided in Table 4-5.

Table 4-5: Streaming potentials, zeta potentials and surface charge density of NF-270

membrane as a function of different electrolyte solutions at applied pressure of 0.2 bar

Order Solution pH Streaming potential

(mV)

Zeta potential (mV)

Surface charge density (mC/m2)

1 2mM KCl 6.22 -2.52± 0.35 -4.36± 0.61 -0.45± 0.06 1 2mM CaCl2

6.99 -1.54± 0.29 -4.14± 0.79 -0.73±0.14

2 2mM CaCl2 -1.02± 0.08 -2.74± 0.22 -0.49±0.04 3 2mM CaCl2 -1.17± 0.38 -3.15± 1.03 -0.56±0.18 1 2mM KCl

6.22 -1.81± 0.47 -3.13± 0.82 -0.32±0.08

2 2mM KCl -1.76± 0.17 -3.03± 0.29 -0.31±0.03 3 2mM KCl -1.67± 0.30 -2.89± 0.53 -0.29±0.05 1 2mM FeCl3

3.08 3.77± 0.86 22.90± 5.21 5.96±1.46

2 2mM FeCl3 6.39± 0.38 38.86± 2.28 10.67±0.75 3 2mM FeCl3 5.95± 0.36 36.15± 2.17 9.83±0.68 1 2mM KCl

6.22

3.13± 0.59 5.40± 0.97 0.55±0.09 2 2mM KCl 3.23± 0.52 5.58± 0.89 0.57±0.09 3 2mM KCl 4.21± 0.57 7.27± 0.99 0.75±0.10 4 2mM KCl 2.51± 0.26 4.34± 0.45 0.44±0.04 5 2mM KCl 2.28± 0.13 3.94± 0.22 0.40±0.02

This experiment recognized that the membrane charge showed a different behavior

depending on the type of the background electrolyte which it was put in contact with. The

data in Table 4-5 generally shows an slight decrease in negative value of zeta potential

when calcium chloride (CaCl2) was used as electrolyte and this in turn indicates the

importance of the role of positive calcium ions binding to the negatively charged

membrane (Lawrence et al., 2006). At pH value is around 6-7, the membrane is negatively

charged which leads to the adsorption of Ca2+ ions on the membrane surface and hence,

reduces its negative charge. This result is in agreement with the observation made by

Teixeira et al. (2005) as well as Lawrence et al. (2006) and others.



After the experiment with calcium chloride (CaCl2), 2 mM potassium chloride (KCl)

solution was introduced again to the membrane cell to remove the Ca2+ ions. However, the

zeta-potential values could not become similar to the initial values, probably because an

amount of Ca2+ ions still stayed on the membrane surface.

The zeta potential values of NF-270 membrane with 2 mM Iron(III) chloride solution

(FeCl3) demonstrated that the membrane became positively charged when contacting with

this solution. Firstly, this result may be related to the influence of pH on the membrane

charge, since the pH of 2 mM Iron(III) chloride (FeCl3) solution is only 3. This value is

below the IEP of the membrane and thus makes the membrane charge becoming positive.

On the other hand, the zeta potential of the NF-270 membrane measured with 2 mM FeCl3

solution is much higher than the one with 2 mM acid acetic solution although its pH is near

the pH value of 2 mM acetic acid (C2H4O2) solution. The explanation of this phenomenon

could be attributed to the adsorption of Fe3+ ion to the membrane surface. Due to the

influence of Fe3+ binding, the membrane had a highly positive value which could not be

returned to negative value even if the 2 mM potassium chloride (KCl) solution was passed

through the cell for five times. Hence, the adsorption of Fe3+ is irreversible.

4.2.6 Influence of flow rate on streaming potential

As described in the 3.2.1.7 of Chapter 3, six data series of streaming potential and channel

height were obtained and given in Table 4-6

Table 4-6: The streaming potential values measured for a NF-270 membrane with different

flow rate of 2 mM KCl solution at 0.2 bar

Order Flow rate (l/m)

Streaming potential (mV)

Channel height (mm)

1 0.43 -2.21 0.65 2 0.42 -3.44 0.64 3 0.47 -3.71 0.70 1 0.42 -2.26 0.63 2 0.48 -2.31 0.72 3 0.52 -3.36 0.78

Based on these data, Spotfire S+ program was used for investigating the relation between

the flow rate and the streaming potential value. The following results were obtained

*** Linear Model ***

Call: lm(formula = sp ~ fr, data = SDF9, na.action = na.exclude)



Residuals:

1 2 3 4 5 6

0.5154 -0.7653 -0.7629 0.3894 0.7068 -0.08347

Coefficients:

Value Std. Error t value Pr(>|t|)

(Intercept) 0.0112 3.8327 0.0029 0.9978

fr -6.3347 8.3678 -0.7570 0.4912

Residual standard error: 0.7231 on 4 degrees of freedom

Multiple R-Squared: 0.1253 Adjusted R-squared: -0.09335

F-statistic: 0.5731 on 1 and 4 degrees of freedom, the p-value is 0.4912

95% confidence interval on the regression parameter:

> -6.3347 + 8.3678 * qt(0.975, 4)

[1] 16.89804

> -6.3347 - 8.3678 * qt(0.975, 4)

[1] -29.56744

The Multiple R-squared is only 0.1253 and 95% confidence interval of the slope is in the

range of [-29.57; 16.90]. This range contains the value 0, therefore, it can be concluded

that there is no significant effect of flow rate on streaming potential value.

4.2.7 Rejection of pharmaceuticals as a function of pH

4.2.7.1 The permeate flux of membrane

The flux of the membrane is the volume of permeate passing through the membrane per

unit area and time (Figure 4-9). In general, the permeate flux of the applied solution during

the filtration was quite stable. Since there was only a slight reduction of the permeate flux

from 29 liter /m2.hour to 25 liter /m2.hour. Therefore, it can be suggested that there is no

effect of fouling on the membrane during the filtration process. This gentle reduction can

be explained as the compaction of membrane under high pressure. Moreover, the increase

of osmotic pressure might play a role for the decrease of permeate flux during filtration

process at each pH. During the filtration, there was the growth of feed solution

concentration due to the extract of permeate and recirculation of concentrate, causing the

higher osmotic pressure and resulting in flux decline. However, this effect is indeed minor

and reversible; the main reason for flux reduction is considered as the membrane

compaction consequence. The phenomenon of flux decline for clean membrane has also

been mentioned in previous study (Bui, 2012).



Figure 4-9: Permeate flux as function of time with sodium salicylate for NF-270

membrane

4.2.7.2 Rejection of sodium salicylate as the function of pH

The rejection values of sodium salicylate are described in this section. The concentration

and the pH of the feed solution and of the permeate solution received after the filtration

experiments with NF-270 membranes as a function of pH are illustrated in Table 4-7:

Table 4-7: pH and sodium salicylate concentration (expressed in mM) obtained after four

filtration experiments with NF-270 membrane. The feed solution contained 5 mM KCl and

0.5 mM sodium salicylate

Experiment 1 Experiment 2 Experiment 3 Experiment 4

pH C

(mM)

pH C

(mM)

pH C

(mM)

pH C

(mM)

Feed before 10.00 0.50 6.50 0.51 4.00 0.53 3.00 0.54

Permeate 10.00 0.09 6.50 0.21 4.00 0.32 3.00 0.44

Feed after 10.00 0.64 6.50 0.60 4.00 0.68 3.00 0.59

Feed+ Permeate 10.00 0.52 6.50 0.52 4.00 0.54

The data in Table 4-7 were applied in Equation (2.8) for calculation of the rejection of

sodium salicylate, in which the zeta potential values could be taken from the zeta potential

0

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0 2 4 6 8 10 12 14 16 18 20 22 24 26 28 30 32 34 36 38 40 42

Perm

eate

flux

(m3 /m

2 .h

our)

Time (hour)

pH6.5 pH4 pH3pH10



values as function of pH that were calculated in 4.2.3 and 4.2.4. The summary of the

calculations is described in Table 4-8.

Table 4-8: Rejection values of sodium salicylate as a function of feed solution pH with

NF-270 membranes

pH Ion valence Zeta potential (mV) Rejection (%)

10 -1 -24 93%

6.5 -1 -5 66%

4 -1/0 (91/9) 1 37%

3 -1/0 (50/50) 2 14%

These results are schematized in Figure 4-10

Figure 4-10: Rejection of sodium salicylate as a function of feed solution pH with NF-270

membrane

The rejection values of sodium salicylate demonstrated that the rejection of this salt by the

NF-270 membrane increased with increasing the pH. A first probable reason for this

phenomenon could be attributed to the electrostatic interaction between the charged

membrane surface and charged pharmaceuticals. As the pKa of sodium salicylate is around

3 (Jaskari et al., 2000), this salt can be considered as a negatively charged solute in the

range of pH from 4 to 10. The electrostatic interaction is dependent on the change of

0

10

20

30

40

50

60

70

80

90

100

0 2 4 6 8 10

Salt

reje

ctio

n (%

)

pH



membrane charge as a function of feed solution pH. As the pH increased, the membrane

became more negatively charged and hence the salicylate anion was increasingly repelled

from the membrane surface, which largely improved the rejection.

At pH 10, the membrane was highly negatively charged, so the rejection that was observed

was high (approximately 93%), due to electrostatic repulsion between negatively charged

solutes and the negatively charged membrane surface. At pH 6.5, as the pH decreased, the

membrane became less negatively charged, making the electrostatic repulsion turned to be

declined. Although the rejection was less than that at pH 10, the surface charge was

apparently still sufficiently high to maintain electrostatic repulsion and caused the rather

high rejection values (Verliefde et al., 2008).

At pH 4 which was near the IEP of the membrane, the membrane charge was either equal

to zero or slightly positive while the solute charge was negative (90% was negatively

charged ions and 10% was neutral). The solutes could act as counterions of the membrane

surface charge, therefore instead of electrostatic repulsion, a weak attractive interaction

between the membrane and the solutes started occurring, resulting in a lower rejection

capacity of the membrane. Finally, when the pH decreased to 3, the membrane charge was

more positive, and the solute was present in both the negatively charged and neutral form

(50% was negatively charged ions and 50% was neutral), leading to stronger attraction

between the membrane surface and negatively charged solutes, hence, the rejection value

at this pH was very low which was only 14%. In addition, the reason for the low rejection

of sodium salicylate at pH 3 and 4 is also explained more details with the concept of

“charge concentration polarization” by Verliefde et al. (2008). Because of solutes were

attracted towards the oppositely charged membrane, the concentration of negatively charge

solutes at the membrane surface increased compared to the bulk solution, which resulted in

lower observed rejection values.

Moreover, pH 4 and pH 3 are two special pH values in which pH 4 is closed to the IEP of

the NF-270 membrane and pH 3 is the pKa of sodium salicylate. At these two pH values,

the membrane and pharmaceutical solutes could have the positive (negative) charge or

neutral. For these cases, beside the effect of electrostatic interaction, the sieving effect also

played a role in filtration of the membrane. However, because the molar mass of sodium

salicylate (160 g/mol) is less than the MWCO of the NF-270 membrane, thus, it could give



the low rejection ability for the membrane and it is also logical that a low rejection ability

for the membrane was observed in the case of attractive electrostatic interactions.

Besides the factors that affect the retention of sodium salicylate as mentioned above, the

hydrophobic interaction also played an important role for this acidic pharmaceutical. The

hydrophobicity of sodium salicylate at various pH values was calculated in accordance

with Equation (3.1) and gave the results as presented in Table 4-9.

Table 4-9: Some physico-chemical characteristics of sodium salicylate (Log Kow = 1.5 and

pKa = 3)

pH Charge at pH value Log D(pH)

10 -1 -5.50

6.5 -1 -2.00

4 -1/0 (91/9) 0.45

3 -1/0 (50/50) 1.20

It is clear that there was a dramatic increase in the hydrophobicity of this solute whilst the

pH value decreased, leading to the increase in binding of sodium salicylate to the

hydrophobic groups of the NF-270 membrane and hence reduced the affinity of this solute

to the solvent. This phenomenon also contributed to the reduction in rejection of sodium

salicylate by NF membrane.

Conclusions and Recommendations


Chapter 5 CONCLUSIONS AND RECOMMENDATIONS

5.1 Conclusions

The aim of this dissertation was to study the electrokinetic characteristics of thin film

composite nanofiltration membrane. The well-known commercial NF-270 nanofiltration

membrane was applied for these experiments.

In the first part of this work, a series of experiments with tangential streaming potential

measurements was carried out for evaluation of the membrane surface charge

characteristics under the influence of parameters such as pH, ionic strength and type of

solute. The streaming potential results obtained through the experiments were used for

calculation of the zeta potential, which is frequently used as indicative parameter to assess

the membrane surface charge. As expected, it can be found in all experiments that the zeta

potential values decreased when ionic strength increased and increased with the increasing

of pH. The dependence of zeta potential on ionic strength can be explained based on the

EDL theory and the dependence on pH values can be attributed to the

protonation/deprotonation of functional groups on the membrane surface. Interestingly, the

membrane surface charge density calculated in the experiments with mono-valent and

multi-valent ions increased in absolute value with the growth of the ionic strength even

though the zeta potential declined. Besides, the reported results also provided evidence that

the adsorption of positively charged ions and particles such as Fe3+ and Al2O3, respectively

on the membrane surface can cause the zeta potential less negative. Surprisingly, the

binding of Fe3+ on membrane surface was significant in alteration of the membrane charge,

whereby the membrane became positively charged and remained this charge even though

there was the sweeping of potassium chloride (KCl) solution through the membrane.

Regarding the IEP of NF-270 membrane, the experiments in this work detected that this

value was 4.8. Although this result was in agreement with other studies, it was slightly

higher than that in some other experiments with the same solution.

The second part of this work focused on filtration experiments with NF-270 membranes

using sodium salicylate as solute at different pH values. As expected, the rejection values

were reduced at lower pH values. The high rejection percentage of NF at higher pH seems

to be dependent on the electrostatic repulsion between the organic anions and the



negatively charged membrane surface. At lower pH, the effect of electrostatic repulsion

vanished and was replaced by an electrostatic attraction which lead to a reduced rejection

efficiency of NF. In addition, steric hindrance and hydrophobicity of the organic solute

might also contribute to the influence on rejection value.

Through the experiments done in this thesis, it can be concluded that electrostatic

interaction between the charged membrane surface and a charged solute is an inherent

characteristic of nanofiltration membrane, which gives significant effects during the

filtration process, and increases the filtration efficiency of the membrane. This property

becomes more important since the most common application of NF membranes is water

softening and removal of dissolved organic matter.

5.2 Recommendations for future works

This thesis has investigated and observed an important influence of charge during the

membrane performance. However, this work still contains limitations that need to be

considered in further studies.

Firstly, the electrokinetic properties of the membrane were determined by using tangential

streaming potential measurements. However, due to the limitations of this method stated in

this dissertation, another way which can be suggested for checking the membrane charge

properties is transmembrane streaming potential measurements. This method can reflect

the global composite membrane charge properties, which include the charge properties of

the thin membrane skin, as well as of the support layer and the walls of the support pores

(Chen et al., 2007b). Hence, this set-up shall give more exact results on the overall charge

of composite membranes.

Secondly, the IEP of NF-270 observed in this experiment was slightly higher than in other

articles, even though the experiment was duplicated. Therefore, further research should be

done to verify the IEP.

Moreover, regarding the calculation of βcharge according to Equation (2.7), the ζ- potential

was used instead of membrane surface potential. However, ζ- potential does not always

accurately represent the real membrane potential that is needed for this calculation

(Verliefde, 2008). Therefore, better methods to describe the membrane potential should be

studied so that the prediction of the rejection of charged organic solutes will be more

accurate.



In addition, the filtration experiments here were only carried out with sodium salicylate

which was negatively charged at all values of tested pH. In the future, more experiments

with various types of negatively charged, neutral and positively charged organic solutes

should be conducted to have a clearer investigation of the influence of electrostatic

interaction on the retention efficiency of NF membrane.

REFERENCES

Abrabri, M., Larbot, A., Persin, M., Sarrazin, J., Rafiq, M. & Cot, L. 1998. Potassium

titanyl phosphate membranes: surface properties and application to ionic solution filtration.

Journal of membrane science, 139, 275-283.

Afonso, M. D. 2006. Surface charge on loose nanofiltration membranes. Desalination, 191,

262-272.

Ariza, M. J. & Benavente, J. 2001. Streaming potential along the surface of polysulfone

membranes: a comparative study between two different experimental systems and

determination of electrokinetic and adsorption parameters. Journal of membrane science,

190, 119-132.

Bandini, S. 2005. Modelling the mechanism of charge formation in NF membranes:

Theory and application. Journal of membrane science, 264, 75-86.

Bellona, C. & Drewes, J. E. 2005. The role of membrane surface charge and solute

physico-chemical properties in the rejection of organic acids by NF membranes. Journal of

membrane science, 249, 227-234.

Bellona, C., Drewes, J. E., Xu, P. & Amy, G. 2004. Factors affecting the rejection of

organic solutes during NF/RO treatment—a literature review. Water Research, 38, 2795 -

2809.

Berg, P., Hagmeyer, G. & Gimbel, R. 1997. Removal of pesticides and other

micropollutants by nanofiltration. Desalination, 113, 205-208.

Bishnoi, S. & Rochelle, G. T. 2000. Absorption of carbon dioxide into aqueous piperazine:

reaction kinetics, mass transfer and solubility. Chemical Engineering Science, 55, 5531-

5543.

Boussu, K. 2007. Influence of membrane characteristics on flux decline and retention in

nanofiltration. PhD dissertation, Katholieke Universiteit Leuven.

Boussu, K., Zhang, Y., Cocquyt, J., Van der Meeren, P., Volodin, A., Van Haesendonck,

C., Martens, J. A. & Van der Bruggen, B. 2006. Characterization of polymeric

nanofiltration membranes for systematic analysis of membrane performance. Journal of

Membrane Science, 278, 418-427.

Bowen, W. R., Mohammad, A. W. & Hilal, N. 1997. Characterisation of nanofiltration

membranes for predictive purposes — use of salts, uncharged solutes and atomic force

microscopy. Journal of membrane science, 126, 91-105.

Bui, T. N. 2012. Influence of membrane fouling on the removal of pharmaceutical. Master

thesis, Ghent university.

Chen, H. Y. T., Wei, W. C. J., Hsu, K. C. & Chen, C. S. 2007. Adsorption of PAA on the

α-Al2O3 Surface. Journal of the American Ceramic Society, 90, 1709-1716.

Childress, A. E. & Elimelech, M. 1996. Effect of solution chemistry on the surface charge

of polymeric reverse osmosis and nanofiltration membranes. Journal of Membrane Science

119, 253 - 268.

Childress, A. E. & Elimelech, M. 2000. Relating nanofiltration membrane performance to

membrane charge (electrokinetic) characteristics. Environmental Science and Technology,

34, 3710 - 3716.

Chowdhury, G., Matsuura, T. & Sourirajan, S. 1994. A study of reverse osmosis separation

and permeation rate for sulfonated poly(2,6-dimethyl-1,4-phenylene oxide) membranes in

different cationic forms. Journal of Applied Polymer Science, 51, 1071-1075.

Delgado, A. V., Gonzalez-Caballero, F., Hunter, R. J., Koopal, L. K. & Lyklema, J. 2005.

Measurement and Interpretation of Electrokinetic phenomena (IUPAC Technical Report).

Pure and Applied Chemistry, 77, 1753 – 1805.

Déon, S., Fievet, P. & Osman Doubad, C. 2012. Tangential streaming potential/current

measurements for the characterization of composite membranes. Journal of Membrane

Science, 423–424, 413-421.

Drioli, E. & Fontananova, E. 2004. bbMembrane technology and sustainable growth.

Chemical Engineering Research and Design, 82, 1557-1562.

Elimelech, M., Chen, W. H. & Waypa, J. J. 1994. Measuring the zeta (electrokinetic)

potential of reverse osmosis membranes by a streaming potential analyzer. Desalination,

95, 269 - 286.

Escoda, A., Fievet, P., Lakard, S., Szymczyk, A. & Déon, S. 2010. Influence of salts on the

rejection of polyethyleneglycol by an NF organic membrane: Pore swelling and salting-out

effects. Journal of membrane science, 347, 174-182.

Fievet, P., Labbez, C., Szymczyk, A., Vidonne, A., Foissy, A. & Pagetti, J. 2002.

Electrolyte transport through amphoteric nanofiltration membranes. Chemical Engineering

Science, 57, 2921-2931.

Fievet, P., Sbaï, M., Szymczyk, A. & Vidonne, A. 2003. Determining the ζ-potential of

plane membranes from tangential streaming potential measurements: Effect of the

membrane body conductance. Journal of Membrane Science, 226, 227 - 236.

Hakimhashemi, M. 2012. Liposomal-Enhanced Electro-Ultrafiltration for Organic

Pollutant Removal from Aqueous media. PhD dissertation, Ghent University.

Hilal, N., Mohammad, A. W., Atkin, B. & Darwish, N. A. 2003. Using atomic force

microscopy towards improvement in nanofiltration membranes properties for desalination

pre-treatment: a review. Desalination, 157, 137-144.

Huisman, I. H., Prádanos, P. & Hernández, A. 2000. Electrokinetic characterisation of

ultrafiltration membranes by streaming potential, electroviscous effect, and salt retention.

Journal of membrane science, 178, 55-64.

Jaskari, T., Vuorio, M., Kontturi, K., Urtti, A., Manzanares, J. A. & Hirvonen, J. 2000.

Controlled transdermal iontophoresis by ion-exchange fiber. Journal of Controlled

Release, 67, 179-190.

Kim, S., Ozaki, H. & Kim, J. 2006. Effect of pH on the rejection of inorganic salts and

organic compound using nanofiltration membrane. Korean Journal Chemical Engineering,

23(1), 28 - 33.

Lawrence, N. D., Perera, J. M., Iyer, M., Hickey, M. W. & Stevens, G. W. 2006. The use

of streaming potential measurements to study the fouling and cleaning of ultrafiltration

membranes. Separation and Purification Technology, 48, 106-112.

Lettmann, C., Mockel, D. & Staude, E. 1999. Permeation and tangential flow streaming

potential measurements for electrokinetic characterization of track-etched microfiltration

membranes. Journal of Membrane Science 59, 243 - 51.

Levenstein, R., Hasson, D. & Semiat, R. 1996. Utilization of the Donnan effect for

improving electrolyte separation with nanofiltration membranes. Journal of Membrance

Science, 116, 77-92.

Lin, Y.-L., Chiang, P.-C. & Chang, E. E. 2007. Removal of small trihalomethane

precursors from aqueous solution by nanofiltration. Journal of Hazardous Materials, 146,

20-29.

Mockel, D., Staude, E., Dal-Cin, M., Darcovich, K. & Guiver, M. 1998. Tangential flow

straming potential measurements: hydrodynamic cell characterization and zeta potentials

of carboxylated polysulfone membranes. Journal of Membrance Science, 145, 211-222.

Mullet, M., Fievet, P., Reggiani, J. C. & Pagetti, J. 1997. Surface electrochemical

properties of mixed oxide ceramic membranes: Zeta-potential and surface charge density.

Journal of Membrane Science, 123, 155-165.

Nghiem, L. D. & Hawkes, S. 2007. Effects of membrane fouling on the nanofiltration of

pharmaceutically active compounds (PhACs): Mechanisms and role of membrane pore

size. Separation and Purification Technology, 57, 176-184.

Nyström, M., Lindström, M. & Matthiasson, E. 1989. Streaming potential as a tool in the

characterization of ultrafiltration membranes. Colloids and Surfaces, 36, 297-312.

Nyström, M., Pihlajamäki, A. & Ehsani, N. 1994. Characterization of ultrafiltration

membranes by simultaneous streaming potential and flux measurements. Journal of

Membrane Science, 87, 245-256.

Nyström, M. & Zhu, H. 1997. Characterization of cleaning results using combined flux and

streaming potential methods. Journal of Membrane Science, 131, 195-205.

Orecki, A., Tomaszewska, M., Karakulski, K. & Morawski, A. W. 2004. Surface water

treatment by the nanofiltration method. Desalination, 162, 47-54.

Ozaki, H. & Li, H. 2002. Rejection of organic compounds by ultra-low pressure reverse

osmosis membrane. Water Research, 36, 123-130.

Peeters, J. M. M. 1997. Characterization of nanofiltration membranes. PhD dissertation,

University of Twente.

Peeters, J. M. M., Mulder, M. H. V. & Strathmann, H. 1999. Streaming potential

measurements as a characterization method for nanofiltration membranes. Colloids and

Surfaces A: Physicochemical and Engineering Aspects, 150, 247-259.

Qiu, Y.-r., Miao, C. & Ye, H.-q. 2009. Characterization of polysulfone hollow-fiber

ultrafiltration membrane and its cleaning efficiency by streaming potential and flux

method. Journal of Central South University of Technology, 16, 195-200.

Ramaswamy, S., Huang, H. J. & Ramarao, B. V. 2013. Separation and Purification

Technologies in Biorefineries, Wiley.

Ricq, L., Pierre, A., Reggiani, J.-C., Zaragoza-Piqueras, S., Pagetti, J. & Daufin, G. 1996.

Effects of proteins on electrokinetic properties of inorganic membranes during ultra- and

micro-filtration. Journal of Membrance Science 114, 27 - 38.

Rios, G. M., Joulie, R., Sarrade, S. J. & Carlès, M. 1996. Investigation of ion separation by

microporous nanofiltration membranes. AIChE Journal, 42, 2521-2528.

Sbaï, M., Szymczyk, A., Fievet, P., Sorin, A., Vidonne, A., Pellet-Rostaing, S., Favre-

Reguillon, A. & Lemaire, M. 2003. Influence of the Membrane Pore Conductance on

Tangential Streaming Potential. Langmuir, 19, 8867-8871.

Schaep, J., Van der Bruggen, B., Vandecasteele, C. & Wilms, D. 1998. Influence of ion

size and charge in nanofiltration. Separation and Purification Technology, 14, 155-162.

Schaep, J., Vandecasteele, C., Wahab Mohammad, A. & Richard Bowen, W. 2001.

Modelling the retention of ionic components for different nanofiltration membranes.

Separation and Purification Technology, 22-23, 169-179.

Seidel, A., Waypa, J. J. & Elimelech, M. 2004. Role of charge (Donnan) exclusion in

removal of arsenic from water by a negatively charged porous nanofiltration membrane.

Environmental Engineering Science 18, 105 - 113.

Strathmann, H., Giorno, L. & Drioli, E. 2006. An Introduction to Membrane Science and

Technology, Wiley.

Szoke, S., Patzay, G. & Weiser, L. 2003. Characteristics of thin-film nanofiltration

membranes at various pH-values. Desalination, 151, 123-129.

Szymczyk, A., Fievet, P., Aoubiza, B., Simon, C. & Pagetti, J. 1999. An application of the

space charge model to the electrolyte conductivity inside a charged microporous

membrane. Journal of Membrane Science, 161, 275-285.

Szymczyk, A., Pierre, A., Reggiani, J. C. & Pagetti, J. 1997. Characterisation of the

electrokinetic properties of plane inorganic membranes using streaming potential

measurements. Journal of Membrane Science, 134, 59-66.

Tay, J. W., Liu, J. & Sun, D. D. 2002. Effect of solution physico-chemistry on the charge

property of nanofiltration membranes. Water Research, 36, 585 - 598.

Teixeira, M. R., Rosa, M. J. & Nyström, M. 2005. The role of membrane charge on

nanofiltration performance. Journal of membrane science, 265, 160-166.

Tiraferri, A. & Elimelech, M. 2012. Direct quantification of negatively charged functional

groups on the membrane surfaces. Journal of Membrance Science, 389, 499 - 508.

Tsuru, T., Takezoe, H. & Asaeda, M. 1998. Ion separation by porous silica-zirconia

nanofiltration membranes. AIChE Journal, 44, 765-768.

Tsuru, T., Urairi, M. & Nakao, S. I. 1991. Reverse osmosis of single and mixed

electrolytes with charged membranes: experiment and analysis. Journal of Chemical

Engineering of Japan, 24, 518 - 524.

Van der Meeren, P. & Verliefde, A. R. D. 2012. Membrane processes in Environmental

Technology, Ghent University.

Verliefde, A. R. D. 2008. Rejection of organic micropollutants by high pressure

membranes (NF/RO). PhD dissertation, Katholieke Universiteit Leuven.

Verliefde, A. R. D., Cornelissen, E. R., Heijman, S. G. J., Verberk, J. Q. J. C., Amy, G. L.,

Van der Bruggen, B. & van Dijk, J. C. 2008. The role of electrostatic interactions on the

rejection of organic solutes in aqueous solutions with nanofiltration. Journal of Membrane

Science, 322, 52-66.

Wai Lin, S., Pérez Sicairos, S. & Félix Navarro, R. M. 2007. Preparation, characterization

and salt rejection of negatively charged polyamide nanofiltration membranes. Journal of

the Mexican Chemical Society, 51, 129-135.

Wang, X.-L., Shang, W.-J., Wang, D.-X., Wu, L. & Tu, C.-H. 2009. Characterization and

applications of nanofiltration membranes: State of the art. Desalination, 236, 316-326.

Wang, X. L., Tsuru, T. & Togoh, M. 1995. Evaluation of pore structure and electrical

properties of nanofiltration membranes. Journal of Chemical Engineering of Japan, 28,

186 - 192.

Xu, Y. & Lebrun, R. E. 1999. Investigation of the solute separation by charged

nanofiltration membrane: effect of pH, ionic strength and solute type. Journal of

membrane science, 158, 93-104.

Yu, B. 2010. Quantification of surface charge properties of nanofiltration and

ultrafiltration membranes by streaming potential measurement. Master thesis, Ghent

university.

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