- Email: [email protected]
Study on Interfacial Electrical Phenomena of Sulfonated Polyethersulfone Nanofiltration Membrane by Electrokinetic Method
CHINESE JOURNAL OF ANALYTICAL CHEMISTRY Volume 38, Issue 4, April 2010 Online English edition of the Chinese language journal
Cite this article as: Chin J Anal Chem, 2010, 38(4), 547–550.
RESEARCH PAPER
Study on Interfacial Electrical Phenomena of Sulfonated Polyethersulfone Nanofiltration Membrane by Electrokinetic Method MA Zhun, GAO Xue-Li, WANG Meng, WANG Duo, GAO Cong-Jie* Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China
Abstract:
Experimental investigations on interfacial electrical phenomena of NTR-7450 nanofiltration membrane that has
dissociated functional groups were performed by electrokinetic method. To obtain the accurate zeta potential, the corresponding measures were used; for examples, the determination of the total conductance of the system including the surface conductance, and the membrane body conductance by means of the electrochemical workstation and the alteration of the channel heights. Furthermore, the surface charge density was estimated by the Gouy-Chapmann double-electric layers theory, and the influences of the ionic strength and type of ions (KCl, K2SO4 and K3PO4) on the membrane surface charge were evaluated. On the basis of the preliminary experimental results, the mechanism of charge formation of NTR-7450 membrane was explored. That is, its charge formation was attributed to the dissociation of the functional group in electrolyte solution with a relatively low concentration (0.1–0.5 mol·m–3). However, at the relatively high concentration, the charge formation of NTR-7450 nanofiltration membrane should be ascribed to the specific adsorption of ions. And, the relationship between the volume charge densities, X, and the feed solution concentration, C, agreed well with Freundlich absorption isotherms. That is, for KCl solution, ln|X| (mol·m–3) = 2.337 + 0.722lnC (mol·m–3); for K2SO4 solution, ln|X| (mol·m–3) = 3.584 + 1.119lnC (mol·m–3); for K3PO4 solution, ln|X| (mol·m–3) = 2.988 + 1.067lnC (mol·m–3). Key Words:
1
Polyethersulfone; Nanofiltration membrane; Interfacial charged phenomena; Electro-kinetic method; Charge densities
Introduction
The interfacial electrical phenomena had significant influence on the properties of membrane such as the flux, the rejection, the anti-fouling ability and so on. It has been widely accepted that the nanofiltration membrane would acquire charges in aqueous phase resulting from specific adsorption and/or functional group dissociation[1–3]. In our previous work[4], the charged characteristic of polyamide nanofiltration membrane was explored by means of the trans-membrane streaming potential mode. It was concluded that the charge formation was attributed to the specific adsorption of anions according to the overall charge behavior including the skin layer, the transition layer and the supported layer.
In this study, the objective was focused on the study of the charged characteristic of nanofiltration membrane with dissociated functional groups. Therefore, the commercial nanofiltration membrane, NTR-7450, was selected, on which the sulfonated polyethersulfone was coated. To accurately obtain the zeta potential, the corresponding measures were used, such as the determination of the total conductance of the system including the surface conductance and the membrane body conductance by means of the electrochemical workstation and the alteration of the channel heights. Furthermore, on the basis of the Gouy-Chapmann doubleelectric layers theory, the influences of the concentration of electrolyte solution, type and the valence of ions on the charged properties of the membrane surface were investigated.
Received 9 July 2009; accepted 21 October 2009 * Corresponding author. Email: [email protected] This work was supported by the National Science foundation of China (Nos. 20706050, 20636050) and the National Basic Research Program of China (No. 2009CB623402). Copyright © 2010, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(09)60037-2
MA Zhun et al. / Chinese Journal of Analytical Chemistry, 2010, 38(4): 547–550
2
Theory section
A series of electrokinetic phenomena occurs because of the interaction of two phases when a single electrolyte solution is forced to flow along a membrane by an external pressure, among which an electrical potential difference is generated between the two ends of the membrane. In the steady state, the electrical potential difference is the so-called streaming potential[5]. Most often, the measurement of streaming potential is used to estimate the zeta potential that can not be measured directly. The zeta potential, defined as electrokinetic potential at the hydrodynamic plane of shear between surface and solution where there is relative motion according to the electrical double layer theory, is an important and reliable indicator for the distribution of the membrane surface charge. Generally, the zeta potential can be simulated with the streaming potential through the Yaroshchuke equation[6] (¨E/¨P)Gt = (İ0İr/Ș)(2hLc/l)ȗ (1) where, ¨E/¨P is the streaming potential coefficient (V/MPa); Gt is the system total conductance including the slit channel conductance, the surface conductance and the membrane body conductance; İr is the dielectric constant of water (78.5 at 25 ºC); İo is the vacuum permittivity (8.854 × 10–12 C m–1·V); Ș is the solution viscosity (kg m–1·s–1); 2h is the height of the slit channel (m); l is the width of slit channel (m); Lc is the length of slit channel (m) and ȗ is zeta potential (V). The streaming potential and total conductance were measured at various channel heights, different electrolyte solution concentrations and types by means of the tangential streaming potential measurement and the electrochemical workstation. The accurate zeta potential values were evaluated according to the Eq.(1). Furthermore, the membrane surface charge density was determined on the basis of Gouy-Chapmann double-electric layers theory[7] and converted into the volume charge density (X)[8,9]. ı = {2İ0İrțBT ȈCiNA[exp(–zieȗ/țBT) – 1]}0.5 (2) X = 2ı/rpF (3) where, ı is the electrical charge density on the membrane/ electrolyte interface (C·m–2); țB is the Boltzmann constant (1.3806 × 10–23 J K–1); Ci is the concentration of ion i in the salt solution (mol·m–3); NA is the Avogadro constant (6.022 × 1023 mol–1); zi is the valence of ion i (± eq mol–1); e is the electronic charge (1.6022 × 10–19 C); T is the absolute temperature (K); rp is the effective membrane pore radius (nm) and F is the Faraday’s constant (96487 C·mol–1).
3 3.1
K3PO4 (pure analytical grade), and the electrolyte solution conductance was measured by a conductivity meter (DDS-11A Conductive meter, Shanghai Rex Instrument Factory). The electrolyte solution was forced through the slit channel using the nitrogen gas (purity of 99%). The self-made Ag/AgCl electrodes linked by a multimeter (VC890D multimeter, Shenzhen Victor Hi-Thch Co., Ltd.) were used to obtain the streaming potential by means of the self-made tangential streaming potential measurement system. The pH of the electrolyte bulk solution was measured by a pH meter (Delta 320 pH meter, Mettler-Toledo Instruments Ltd, Shanghai). The total conductance was measured by the IM6ex electrochemical workstation (Zahner Elektrik, Germany). 3.2
Experimental equipment and procedure
Figure 1 was a schematic view of the experimental setup that was used for the streaming potential measurement. The membranes were soaked with the testing solution overnight to equilibrate the membranes with the electrolyte solution. After that, two identical membranes facing their active layers each other was partitioned by a Teflon spacer. The Teflon spacer had a slit channel with a size of 10 mm × 176 mm as the testing channel. The electrical potential difference (¨E) developing along the slit channel was measured by two self-made Ag/AgCl reference electrodes, which were placed on each side of membrane sample and linked with a high input impendence multimeter when the electrolyte solution was forced to flow along the tangential streaming potential cell. The pressure difference between the measuring cell ends was controlled by changing the flow of nitrogen gas, which was varied from 0 to 0.07 MPa. As the pressure and the streaming potential reached a stable value, the measurement was commenced. The streaming potential measurement of NTR-7450 nanofiltration membrane was carried out at various channel heights, concentrations of electrolyte solution and types.
Experimental section Apparatus and chemicals
The experimental work was performed with the sulfonated polyethersulfone nanofiltration membrane (NTR-7450, Nitto Denko). The electrolyte solutions were KCl, K2SO4 and
Fig.1
Schematic representation of tangential streaming potential measurement system
1. Electrolyte solution container; 2. Ag/AgCl electrodes; 3. Polytetrafluoroethylete shims
MA Zhun et al. / Chinese Journal of Analytical Chemistry, 2010, 38(4): 547–550
On the basis of our previous work[10], the total conductance of the system was measured using the electrochemical workstation. In this work, the two-electrode mode was adopted. That is, the constant current was injected into the testing pool by the electrochemical workstation in the case of no fluid flow in the system. And then, the corresponding potential drops were acquired at various channel heights, different electrolyte solution concentrations and types. Furthermore, the total conductance of testing cell was estimated according to the Ohm’s law. The concentrations of electrolyte solution varied from 0.10 mol·m–3 to 10 mol·m–3. Experiments were carried out at constant temperature (20 ± 0.5) ºC and at natural pH value (6.5 ± 0.1).
4 4.1
Result and discussion Zeta potential in different systems
The streaming potential was measured at various channel heights using the tangential streaming potential system. As shown in Fig.2, the streaming potential was measured in the channel height of 300 ȝm. It could be concluded that the membrane was deemed to be negatively charged in electrolyte solutions. The streaming potential (¨E) decreased with the increase of electrolyte solution concentration. Moreover, it was also found that the streaming potential coefficient decreased and approached to zero with the increase of electrolyte solution concentration. In the case of low electrolyte solution concentration or higher membrane surface charge, the surface conductance played a non-negligible role in the determination of zeta potential. Furthermore, the contribution of membrane body conductance was also non-negligible, or else it would lead to an underestimated zeta potential. Consequently, the determination of zeta potential was evaluated from the Eq.(1) by a series of measurements in the case of various channel heights, different electrolyte solution concentrations and species. Figure 3 showed a comparison of the values of zeta potential at various electrolyte solution concentrations and types. As could be seen, the concentrations of electrolyte
solution and anion species had no influence on the magnitude of zeta potential under lower concentration. It was reasonable to conclude that the charge formation of the NTR-7450 nanofiltration membrane should have resulted from the dissociation of the sulfonic group under the investigating concentration range. On the one hand, the shielding effect of counter-ions was weak. On the other hand, the electrostatic repulsion had significant influence on the adsorption of the co-ion on the surface of membrane. As the bulk concentration increased, the zeta potential dramatically changed with the anionic species and anionic strength. This was because of the screening effect of counter-ions that became stronger, which led to the increase of the adsorption concentration of co-ion in the compact layer. Thus, it was obvious that the mechanism of charge formation of nanofiltration membrane should be attributed to the specific adsorption in the range of higher concentration. In the case of three testing electrolyte solutions, the magnitude of the zeta potential followed the order: K2SO4 > K3PO4 > KCl. It was possible that the phenomenon stepped from the competition between the steric and electrostatic repulsion effects. 4.2
Membrane surface charge density and volume charge density in different systems
The membrane surface charge densities of the nanofiltration membrane could be inferred from the zeta potential by means of the well-known relation Eq.(2) derived from the Gouy-Chapmann double-electric layers theory. As could be seen from the Fig.4, the membrane surface charge density kept constant with the increase of the electrolyte solution concentration and varied little with the anionic nature in the case of lower concentration. It was due to the weak screening effect of the counter-ion, which resulted from the low concentration of the counter-ion on the membrane, and the electrostatic repulsion hindered the adsorption of the co-ion on the membrane surface. Therefore, the mechanism of the charge formation of NTR-7450 nanofiltration membrane should owe to the dissociation of the sulfonic group under the low concentration. In the case of higher concentration, the
Fig.2 Streaming potentials of NTR-7450 nanofiltration membrane as a function of applied pressures measured in different systems at 300 ȝm of double channel height Ŷ, 0.1 mol·m–3; Ɣ, 0.3 mol·m–3; Ÿ, 0.5 mol·m–3; ź, 1.0 mol·m–3; ƹ, 5.0 mol·m–3; Ÿ, 10 mol·m–3
MA Zhun et al. / Chinese Journal of Analytical Chemistry, 2010, 38(4): 547–550
C/mM
Fig.3
Zeta potential of NTR-7450 nanofiltration membrane in different systems
C/mM
Fig.4 Surface charge density of NTR-7450 nanofiltration membrane in different systems
charge density increased with the increase of electrolyte solution concentration and varied significantly with anionic species. It was likely that the anions might easily adsorb onto the membrane surface because of the stronger screening effect of counter-ions. Therefore, the mechanism of membrane charge formation should be attributed to the anionic specific adsorption. In the case of measuring electrolyte solution, the ranking of the membrane surface charge density coincided with the order of the zeta potential. The membrane surface charge density was further converted into the membrane volume charge density (X) through Eq.(4) in the range of electrolyte solution concentration from 1.0 mol·m-3 to 10.0 mol·m-3. The plot of X versus C showed a curvilinear shape, and the straight lines can be obtained by plotting these data in an ln-ln graphic. Thus, it appeared that the membrane volume charge density could be correlated with the feed concentration in terms of a Freundlich isotherm: ln|X| = a + blnC (4) Moreover, the slope of the straight line, b, is predicted by the equation found in b = (șCZc)Zc/2, where zc is the cation valence (eq mol–1) and șC is the number of cation moles per mole of dissociated salt (mol/mol). Bowen et al[11,12] explored the transport of electrolyte solutions with single components, such as NaCl and Na2SO4, in several feed concentrations for six nanofiltration membranes, and reported the existence of a linear logarithmic correlation between the membrane surface charge and the feed solution concentration. They obtained identical straight lines
for both single solutions of NaCl and Na2SO4, the slope of which ranged from 0.499 to 0.875 for several membranes. Afonso[7,13] assessed the electrokinetic characteristic of two 5-nanofiltration membranes in several electrolyte solutions by tangential streaming potential and found the existence of a relationship of linear logarithmic correlation between the membrane surface charge and the feed solution concentration (NaCl), and achieved the straight lines with slopes of 0.475 and 0.527. As mentioned earlier, these works all investigated the charged behavior of polyamide nanofiltration membrane and assumed that only the specific adsorption on the membrane surface played a crucial role in the mechanism of charge formation. In this work, the charge formation of the sulfonated polyethersulfone nanofiltration membrane (NTR-7450) that had functional group dissociation was performed by electrokinetic method together with the determination of the total conductance. It was confirmed that the charge formation of NTR-7450 nanofiltration membrane should be ascribed to functional group dissociation in the case of lower concentration. The charge formation of NTR-7450 nanofiltration membrane should be attributed to the specific adsorption in the range of higher concentration. Moreover, the anionic adsorption sequence on the membrane surface was SO42– > PO43– > Cl–, which accorded with the order of zeta potential and the membrane surface charge density. The parameter a, b and the correlation coefficient R could be seen from Table 1.
5
Conclusions
In this study, an investigation on the charged behavior of the NTR-7450 sulfonated polyethersulfone nanofiltration membranes by means of tangential streaming potential measurements system and the determination of total conductance was carried out. From the preliminary experimental results, it was concluded that the mechanism of charge formation of NTR-7450 membrane resulted from functional group dissociation under lower concentration (0.1–0.5 mol·m –3 ). Under higher concentration (1.0–10 mol·m–3), the charge formation of NTR-7450 nanofiltration membrane should owe to the specific adsorption. Furthermore, the relationship between the membrane volume charge densities, X, and the feed solution concentration, C, agreed well with Freundlich absorption isotherms. That is, for KCl solution, ln|X| (mol·m–3) = 2.337 + 0.722lnC (mol·m–3); for K2SO4 solution, ln|X| (mol·m–3) = 3.584 + 1.119lnC (mol·m–3); Table 1 Parameters of Freundlich adsorption isotherms in electrolyte solution for NTR-7450 nanofiltration membrane Electrolyte solution
a
b
R
KCl K2SO4 K3PO4
2.337 3.584 2.988
0.722 1.119 1.067
0.993 0.999 0.969
MA Zhun et al. / Chinese Journal of Analytical Chemistry, 2010, 38(4): 547–550
for K3PO4 solution, ln|X| (mol·m–3) = 2.988 + 1.067lnC (mol·m–3). The concentrations of electrolyte solution, the type and the valence of ions had important influences on the absolute value of zeta potential and the charge density. In the case of higher concentration, the absolute magnitude of zeta potential and charge density followed the order: K2SO4 > K3PO4 > KCl.
[5]
Wang J, Wang X L. Journal of Chemical Engineering of
[6]
Andfiy Y, Volker R. Langmuir, 2002, l8: 2036–2038
Chinese Universities, 2003, 17(4): 372–376 [7] [8] [9]
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Bowen W R, Mukhtar H. J. Membrane Sci., 1996, 112:
Anal. Chem., 2006, 34(10): 1507–1510 263–274
Hagmeyer G, Gimbel R. Desalination, 1998, 117: 247–256 Wang M, An Q F, Wu L G, Mo J X, Gao C J. Chinese J. Anal.
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Ma Z, Wang M, Wang D, Su B W, Gao C J. Chinese J. Anal. Chem., 2008, 36(12):1707–1710
Bowen W R, Cao X W. J. Membrane Sci., 1998, 140: 267–273
Chem., 2007, 35(4): 605–610 [4]
Garcia-Aleman J, Dickson J M. J. Membr. Sci., 2004, 235(1-2): 1–13
Bowen W R, Mohammad A W. Desalination, 1998, 117: 257–264
Palmeri J, Blanc P, Larbot A, David P. J. Membr. Sci., 1999, 160(2): 141–170
References [1]
Afonso M D, Hagmeyer G, Gimbel R. Separation and Purification Technology, 2001, 22: 529–541
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Cite this article as: Chin J Anal Chem, 2010, 38(4), 547–550.
RESEARCH PAPER
Study on Interfacial Electrical Phenomena of Sulfonated Polyethersulfone Nanofiltration Membrane by Electrokinetic Method MA Zhun, GAO Xue-Li, WANG Meng, WANG Duo, GAO Cong-Jie* Key Laboratory of Marine Chemistry Theory and Technology, Ministry of Education, College of Chemistry and Chemical Engineering, Ocean University of China, Qingdao 266100, China
Abstract:
Experimental investigations on interfacial electrical phenomena of NTR-7450 nanofiltration membrane that has
dissociated functional groups were performed by electrokinetic method. To obtain the accurate zeta potential, the corresponding measures were used; for examples, the determination of the total conductance of the system including the surface conductance, and the membrane body conductance by means of the electrochemical workstation and the alteration of the channel heights. Furthermore, the surface charge density was estimated by the Gouy-Chapmann double-electric layers theory, and the influences of the ionic strength and type of ions (KCl, K2SO4 and K3PO4) on the membrane surface charge were evaluated. On the basis of the preliminary experimental results, the mechanism of charge formation of NTR-7450 membrane was explored. That is, its charge formation was attributed to the dissociation of the functional group in electrolyte solution with a relatively low concentration (0.1–0.5 mol·m–3). However, at the relatively high concentration, the charge formation of NTR-7450 nanofiltration membrane should be ascribed to the specific adsorption of ions. And, the relationship between the volume charge densities, X, and the feed solution concentration, C, agreed well with Freundlich absorption isotherms. That is, for KCl solution, ln|X| (mol·m–3) = 2.337 + 0.722lnC (mol·m–3); for K2SO4 solution, ln|X| (mol·m–3) = 3.584 + 1.119lnC (mol·m–3); for K3PO4 solution, ln|X| (mol·m–3) = 2.988 + 1.067lnC (mol·m–3). Key Words:
1
Polyethersulfone; Nanofiltration membrane; Interfacial charged phenomena; Electro-kinetic method; Charge densities
Introduction
The interfacial electrical phenomena had significant influence on the properties of membrane such as the flux, the rejection, the anti-fouling ability and so on. It has been widely accepted that the nanofiltration membrane would acquire charges in aqueous phase resulting from specific adsorption and/or functional group dissociation[1–3]. In our previous work[4], the charged characteristic of polyamide nanofiltration membrane was explored by means of the trans-membrane streaming potential mode. It was concluded that the charge formation was attributed to the specific adsorption of anions according to the overall charge behavior including the skin layer, the transition layer and the supported layer.
In this study, the objective was focused on the study of the charged characteristic of nanofiltration membrane with dissociated functional groups. Therefore, the commercial nanofiltration membrane, NTR-7450, was selected, on which the sulfonated polyethersulfone was coated. To accurately obtain the zeta potential, the corresponding measures were used, such as the determination of the total conductance of the system including the surface conductance and the membrane body conductance by means of the electrochemical workstation and the alteration of the channel heights. Furthermore, on the basis of the Gouy-Chapmann doubleelectric layers theory, the influences of the concentration of electrolyte solution, type and the valence of ions on the charged properties of the membrane surface were investigated.
Received 9 July 2009; accepted 21 October 2009 * Corresponding author. Email: [email protected] This work was supported by the National Science foundation of China (Nos. 20706050, 20636050) and the National Basic Research Program of China (No. 2009CB623402). Copyright © 2010, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences. Published by Elsevier Limited. All rights reserved. DOI: 10.1016/S1872-2040(09)60037-2
MA Zhun et al. / Chinese Journal of Analytical Chemistry, 2010, 38(4): 547–550
2
Theory section
A series of electrokinetic phenomena occurs because of the interaction of two phases when a single electrolyte solution is forced to flow along a membrane by an external pressure, among which an electrical potential difference is generated between the two ends of the membrane. In the steady state, the electrical potential difference is the so-called streaming potential[5]. Most often, the measurement of streaming potential is used to estimate the zeta potential that can not be measured directly. The zeta potential, defined as electrokinetic potential at the hydrodynamic plane of shear between surface and solution where there is relative motion according to the electrical double layer theory, is an important and reliable indicator for the distribution of the membrane surface charge. Generally, the zeta potential can be simulated with the streaming potential through the Yaroshchuke equation[6] (¨E/¨P)Gt = (İ0İr/Ș)(2hLc/l)ȗ (1) where, ¨E/¨P is the streaming potential coefficient (V/MPa); Gt is the system total conductance including the slit channel conductance, the surface conductance and the membrane body conductance; İr is the dielectric constant of water (78.5 at 25 ºC); İo is the vacuum permittivity (8.854 × 10–12 C m–1·V); Ș is the solution viscosity (kg m–1·s–1); 2h is the height of the slit channel (m); l is the width of slit channel (m); Lc is the length of slit channel (m) and ȗ is zeta potential (V). The streaming potential and total conductance were measured at various channel heights, different electrolyte solution concentrations and types by means of the tangential streaming potential measurement and the electrochemical workstation. The accurate zeta potential values were evaluated according to the Eq.(1). Furthermore, the membrane surface charge density was determined on the basis of Gouy-Chapmann double-electric layers theory[7] and converted into the volume charge density (X)[8,9]. ı = {2İ0İrțBT ȈCiNA[exp(–zieȗ/țBT) – 1]}0.5 (2) X = 2ı/rpF (3) where, ı is the electrical charge density on the membrane/ electrolyte interface (C·m–2); țB is the Boltzmann constant (1.3806 × 10–23 J K–1); Ci is the concentration of ion i in the salt solution (mol·m–3); NA is the Avogadro constant (6.022 × 1023 mol–1); zi is the valence of ion i (± eq mol–1); e is the electronic charge (1.6022 × 10–19 C); T is the absolute temperature (K); rp is the effective membrane pore radius (nm) and F is the Faraday’s constant (96487 C·mol–1).
3 3.1
K3PO4 (pure analytical grade), and the electrolyte solution conductance was measured by a conductivity meter (DDS-11A Conductive meter, Shanghai Rex Instrument Factory). The electrolyte solution was forced through the slit channel using the nitrogen gas (purity of 99%). The self-made Ag/AgCl electrodes linked by a multimeter (VC890D multimeter, Shenzhen Victor Hi-Thch Co., Ltd.) were used to obtain the streaming potential by means of the self-made tangential streaming potential measurement system. The pH of the electrolyte bulk solution was measured by a pH meter (Delta 320 pH meter, Mettler-Toledo Instruments Ltd, Shanghai). The total conductance was measured by the IM6ex electrochemical workstation (Zahner Elektrik, Germany). 3.2
Experimental equipment and procedure
Figure 1 was a schematic view of the experimental setup that was used for the streaming potential measurement. The membranes were soaked with the testing solution overnight to equilibrate the membranes with the electrolyte solution. After that, two identical membranes facing their active layers each other was partitioned by a Teflon spacer. The Teflon spacer had a slit channel with a size of 10 mm × 176 mm as the testing channel. The electrical potential difference (¨E) developing along the slit channel was measured by two self-made Ag/AgCl reference electrodes, which were placed on each side of membrane sample and linked with a high input impendence multimeter when the electrolyte solution was forced to flow along the tangential streaming potential cell. The pressure difference between the measuring cell ends was controlled by changing the flow of nitrogen gas, which was varied from 0 to 0.07 MPa. As the pressure and the streaming potential reached a stable value, the measurement was commenced. The streaming potential measurement of NTR-7450 nanofiltration membrane was carried out at various channel heights, concentrations of electrolyte solution and types.
Experimental section Apparatus and chemicals
The experimental work was performed with the sulfonated polyethersulfone nanofiltration membrane (NTR-7450, Nitto Denko). The electrolyte solutions were KCl, K2SO4 and
Fig.1
Schematic representation of tangential streaming potential measurement system
1. Electrolyte solution container; 2. Ag/AgCl electrodes; 3. Polytetrafluoroethylete shims
MA Zhun et al. / Chinese Journal of Analytical Chemistry, 2010, 38(4): 547–550
On the basis of our previous work[10], the total conductance of the system was measured using the electrochemical workstation. In this work, the two-electrode mode was adopted. That is, the constant current was injected into the testing pool by the electrochemical workstation in the case of no fluid flow in the system. And then, the corresponding potential drops were acquired at various channel heights, different electrolyte solution concentrations and types. Furthermore, the total conductance of testing cell was estimated according to the Ohm’s law. The concentrations of electrolyte solution varied from 0.10 mol·m–3 to 10 mol·m–3. Experiments were carried out at constant temperature (20 ± 0.5) ºC and at natural pH value (6.5 ± 0.1).
4 4.1
Result and discussion Zeta potential in different systems
The streaming potential was measured at various channel heights using the tangential streaming potential system. As shown in Fig.2, the streaming potential was measured in the channel height of 300 ȝm. It could be concluded that the membrane was deemed to be negatively charged in electrolyte solutions. The streaming potential (¨E) decreased with the increase of electrolyte solution concentration. Moreover, it was also found that the streaming potential coefficient decreased and approached to zero with the increase of electrolyte solution concentration. In the case of low electrolyte solution concentration or higher membrane surface charge, the surface conductance played a non-negligible role in the determination of zeta potential. Furthermore, the contribution of membrane body conductance was also non-negligible, or else it would lead to an underestimated zeta potential. Consequently, the determination of zeta potential was evaluated from the Eq.(1) by a series of measurements in the case of various channel heights, different electrolyte solution concentrations and species. Figure 3 showed a comparison of the values of zeta potential at various electrolyte solution concentrations and types. As could be seen, the concentrations of electrolyte
solution and anion species had no influence on the magnitude of zeta potential under lower concentration. It was reasonable to conclude that the charge formation of the NTR-7450 nanofiltration membrane should have resulted from the dissociation of the sulfonic group under the investigating concentration range. On the one hand, the shielding effect of counter-ions was weak. On the other hand, the electrostatic repulsion had significant influence on the adsorption of the co-ion on the surface of membrane. As the bulk concentration increased, the zeta potential dramatically changed with the anionic species and anionic strength. This was because of the screening effect of counter-ions that became stronger, which led to the increase of the adsorption concentration of co-ion in the compact layer. Thus, it was obvious that the mechanism of charge formation of nanofiltration membrane should be attributed to the specific adsorption in the range of higher concentration. In the case of three testing electrolyte solutions, the magnitude of the zeta potential followed the order: K2SO4 > K3PO4 > KCl. It was possible that the phenomenon stepped from the competition between the steric and electrostatic repulsion effects. 4.2
Membrane surface charge density and volume charge density in different systems
The membrane surface charge densities of the nanofiltration membrane could be inferred from the zeta potential by means of the well-known relation Eq.(2) derived from the Gouy-Chapmann double-electric layers theory. As could be seen from the Fig.4, the membrane surface charge density kept constant with the increase of the electrolyte solution concentration and varied little with the anionic nature in the case of lower concentration. It was due to the weak screening effect of the counter-ion, which resulted from the low concentration of the counter-ion on the membrane, and the electrostatic repulsion hindered the adsorption of the co-ion on the membrane surface. Therefore, the mechanism of the charge formation of NTR-7450 nanofiltration membrane should owe to the dissociation of the sulfonic group under the low concentration. In the case of higher concentration, the
Fig.2 Streaming potentials of NTR-7450 nanofiltration membrane as a function of applied pressures measured in different systems at 300 ȝm of double channel height Ŷ, 0.1 mol·m–3; Ɣ, 0.3 mol·m–3; Ÿ, 0.5 mol·m–3; ź, 1.0 mol·m–3; ƹ, 5.0 mol·m–3; Ÿ, 10 mol·m–3
MA Zhun et al. / Chinese Journal of Analytical Chemistry, 2010, 38(4): 547–550
C/mM
Fig.3
Zeta potential of NTR-7450 nanofiltration membrane in different systems
C/mM
Fig.4 Surface charge density of NTR-7450 nanofiltration membrane in different systems
charge density increased with the increase of electrolyte solution concentration and varied significantly with anionic species. It was likely that the anions might easily adsorb onto the membrane surface because of the stronger screening effect of counter-ions. Therefore, the mechanism of membrane charge formation should be attributed to the anionic specific adsorption. In the case of measuring electrolyte solution, the ranking of the membrane surface charge density coincided with the order of the zeta potential. The membrane surface charge density was further converted into the membrane volume charge density (X) through Eq.(4) in the range of electrolyte solution concentration from 1.0 mol·m-3 to 10.0 mol·m-3. The plot of X versus C showed a curvilinear shape, and the straight lines can be obtained by plotting these data in an ln-ln graphic. Thus, it appeared that the membrane volume charge density could be correlated with the feed concentration in terms of a Freundlich isotherm: ln|X| = a + blnC (4) Moreover, the slope of the straight line, b, is predicted by the equation found in b = (șCZc)Zc/2, where zc is the cation valence (eq mol–1) and șC is the number of cation moles per mole of dissociated salt (mol/mol). Bowen et al[11,12] explored the transport of electrolyte solutions with single components, such as NaCl and Na2SO4, in several feed concentrations for six nanofiltration membranes, and reported the existence of a linear logarithmic correlation between the membrane surface charge and the feed solution concentration. They obtained identical straight lines
for both single solutions of NaCl and Na2SO4, the slope of which ranged from 0.499 to 0.875 for several membranes. Afonso[7,13] assessed the electrokinetic characteristic of two 5-nanofiltration membranes in several electrolyte solutions by tangential streaming potential and found the existence of a relationship of linear logarithmic correlation between the membrane surface charge and the feed solution concentration (NaCl), and achieved the straight lines with slopes of 0.475 and 0.527. As mentioned earlier, these works all investigated the charged behavior of polyamide nanofiltration membrane and assumed that only the specific adsorption on the membrane surface played a crucial role in the mechanism of charge formation. In this work, the charge formation of the sulfonated polyethersulfone nanofiltration membrane (NTR-7450) that had functional group dissociation was performed by electrokinetic method together with the determination of the total conductance. It was confirmed that the charge formation of NTR-7450 nanofiltration membrane should be ascribed to functional group dissociation in the case of lower concentration. The charge formation of NTR-7450 nanofiltration membrane should be attributed to the specific adsorption in the range of higher concentration. Moreover, the anionic adsorption sequence on the membrane surface was SO42– > PO43– > Cl–, which accorded with the order of zeta potential and the membrane surface charge density. The parameter a, b and the correlation coefficient R could be seen from Table 1.
5
Conclusions
In this study, an investigation on the charged behavior of the NTR-7450 sulfonated polyethersulfone nanofiltration membranes by means of tangential streaming potential measurements system and the determination of total conductance was carried out. From the preliminary experimental results, it was concluded that the mechanism of charge formation of NTR-7450 membrane resulted from functional group dissociation under lower concentration (0.1–0.5 mol·m –3 ). Under higher concentration (1.0–10 mol·m–3), the charge formation of NTR-7450 nanofiltration membrane should owe to the specific adsorption. Furthermore, the relationship between the membrane volume charge densities, X, and the feed solution concentration, C, agreed well with Freundlich absorption isotherms. That is, for KCl solution, ln|X| (mol·m–3) = 2.337 + 0.722lnC (mol·m–3); for K2SO4 solution, ln|X| (mol·m–3) = 3.584 + 1.119lnC (mol·m–3); Table 1 Parameters of Freundlich adsorption isotherms in electrolyte solution for NTR-7450 nanofiltration membrane Electrolyte solution
a
b
R
KCl K2SO4 K3PO4
2.337 3.584 2.988
0.722 1.119 1.067
0.993 0.999 0.969
MA Zhun et al. / Chinese Journal of Analytical Chemistry, 2010, 38(4): 547–550
for K3PO4 solution, ln|X| (mol·m–3) = 2.988 + 1.067lnC (mol·m–3). The concentrations of electrolyte solution, the type and the valence of ions had important influences on the absolute value of zeta potential and the charge density. In the case of higher concentration, the absolute magnitude of zeta potential and charge density followed the order: K2SO4 > K3PO4 > KCl.
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