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Influences of colloidal stability and electrokinetic property on electrodialysis performance in the presence of silica sol
Journal of Colloid and Interface Science 270 (2004) 406–412 www.elsevier.com/locate/jcis
Influences of colloidal stability and electrokinetic property on electrodialysis performance in the presence of silica sol Hong-Joo Lee a,∗ and Seung-Hyeon Moon b a Nuclear Power Laboratory, Korea Electric Power Research Institute (KEPRI), 103-16 Munji-Dong, Yuseong-Gu, Daejeon 305-380, South Korea b Department of Environmental Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), 1 Oryong-Dong, Buk-Gu,
Gwangju 500-712, South Korea Received 21 April 2003; accepted 25 August 2003
Abstract Negatively charged silica sol is known to lead to fouling of anion exchange membranes during electrodialysis (ED) as a result of its deposition on the membrane surface. It is known that the fouling potential is related to the physical and electrochemical properties of the silica particles as well as those of the anion exchange membranes. In this study, the properties of the silica sol were characterized in terms of its particle size, turbidity, and zeta potential in order to predict their effects on the electrodialysis performance. In the stability of colloidal particles, the critical coagulation concentrations of silica sol were determined as functions of ionic strength, cation species, and solution pH. In the electrodialysis of NaCl solution containing silica sol with various concentrations of CaCl2 , the colloidal behavior related to deposition and transport was examined during and after electrodialysis. The electrodialysis experiments clearly showed that the deposition and transport of silica sol during electrodialysis were related to the colloidal stability of dispersion. 2003 Elsevier Inc. All rights reserved. Keywords: Characterization; Silica sol; Electrodialysis; Zeta potential
1. Introduction Electrodialysis (ED) is an electrochemical separation process that employs electrically charged ion exchange membranes with an electrical potential difference as a driving force. ED applications can be found in the environmental and biochemical industries as well as in the production of table salt and the desalination of seawater [1,2]. Fouling of ion exchange membranes is considered to be one of the most important limitations on the design and operation of electrodialysis processes [3–7]. It is generally accepted that fouling occurs in membrane processes due to deposition of foulants (organic substances, colloidal particles, proteins, etc.). Most colloidal particles existing in natural waters and in many industrial effluent streams have negative surface charges, depending on the solution pH. The surface charge * Corresponding author. Current address: Membrane Separation Processes Group, Research Institute for Green Technology, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. E-mail address: [email protected] (H.-J. Lee).
0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.08.042
of aquatic colloids reflects their surface chemical properties and the chemical composition of natural waters. Examples of aquatic colloids are clay minerals, colloidal silica particles, iron oxides, aluminum oxides, manganese oxides, organic colloids, suspended matters, and calcium carbonate precipitates [8]. It is generally accepted that colloidal fouling is caused by the accumulation of colloidal particles on the membrane surface, followed by coagulation. The deposition of silica sol significantly decreases the performance of the pressure-driven membrane processes (ultrafiltration, nanofiltration, and reverse osmosis) by plugging into the pore structures [9–11]. Electrodialysis performance is also affected by deposition of colloidal particles, which is related to the electrochemical properties of the particles [12,13]. Colloidal particles having negative charges move toward anion exchange membranes in the electric field during electrodialysis. Then the colloidal particles deposit on the membrane surface due to electrostatic attraction between the positively charged anion exchange membrane and the negatively changed colloidal particles [13]. The coagulation of colloidal particles and electrokinetic properties (zeta potential) are accepted to be two of the main factors affecting electrodialysis perfor-
H.-J. Lee, S.-H. Moon / Journal of Colloid and Interface Science 270 (2004) 406–412
mance, and these two values give useful information for the prediction of fouling potentials [14–16]. It is known that the rate of coagulation depends mainly on the stability of the colloidal particles and that the salt concentration and the solution pH are the two main factors affecting the colloidal stability [9–11]. The stability of a dispersion (the ability of the dispersion to resist coagulation) may be related to its kinetic stability, which in turn depends on the force barrier preventing collision between the particles and thus preventing their coagulation [17,18]. Coagulation occurs when the interaction energy barrier is higher than other applied energy barriers of the colloidal particles. When there are no repulsive interactions between the particles, the dispersion will be unstable to coagulation due to the electrostatic stability of the colloidal particles [18]. The electrolyte concentration at which slow coagulation turns into rapid coagulation can be determined accurately in colloidal stability experiments. The resulting value is considered as one of the important characteristic values of colloidal particles, called the critical coagulation concentration (CCC). The CCC depends mainly on the following parameters: (i) the elapsed time, (ii) the uniformity of the sample, (iii) the potential at the surface, and (iv) the valence of the ions [18]. The objectives of this study are (i) to characterize colloidal particles in terms of electrochemical properties for the colloidal stability and (ii) to investigate colloidal coagulation effects on electrodialysis performance and colloidal behaviors (mainly deposition and transport) during electrodialysis.
2. Materials and methods The colloidal particles of silica sol were characterized in terms of particle size, turbidity, and zeta potential. The intensities of the colloidal particles in a solution were measured at 330 nm with a PCS 4700 particle size analyzer (Malvern, England) [17]. In addition, the turbidity of the silica sol was measured with a DRT-15 CE portable turbidimeter (HF Scientific Inc., USA) using a nephelometric turbidity unit (NTU). An ICP-AES (Jovin Yvon 38 Plus, France) was used at a selected wavelength of 251.611 nm to measure the Si concentration. The zeta potentials were estimated using the Smoluchowski equation from electrophoretic mobility measurements using an ELS-8000 electrophoretic apparatus (Otsuka Electronics, Japan) in order to investigate the influence of cation species on the electrokinetic properties [11,19–21]. The influence of ionic strength and solution pH on the zeta potential was also considered in this study. A TS-1 electrodialyzer (Tokuyama Corp., Japan), was used with two cell pairs, consisting of a cation-exchange membrane (NEOSEPTA CMX) and an anion-exchange membrane (NEOSEPTA AMX) (Tokuyama Corp., Japan), in a concentration of 0.4 wt% silica sol, Ludox HS-40 (Aldrich, USA) [3,22]. The characteristics of CMX and AMX membranes were shown elsewhere [12,23]. Five liters of 0.1 M NaCl solution containing silica sol was circulated
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as a dilute solution and 800 ml of 3.0 wt% of Na2 SO4 as an electrode rinse solution. During electrodialysis experiments, 5 l of 0.05 M NaCl solution was used as a concentrated solution. The pH values of the dilute and concentrated solutions remained between pH 7 and 8 without pH control. The flow rates of the dilute and concentrated solutions were set at 0.2–0.3 l/min. Coagulation and particle size distribution were controlled by varying the concentration of CaCl2 in a background electrolyte solution (0.1 M NaCl). During electrodialysis experiments, a constant direct current of 0.6 A (current density 6.0 mA/cm2 ) was supplied. After electrodialysis experiments, 500 ml of 1.0 wt% NaOH and distilled water were circulated for 20 min in the dilute and concentrated solution compartments for cleaning-in-place of membranes and compartments.
3. Results and discussion 3.1. Colloidal coagulation of silica sol by turbidity measurements Of the characterized properties of silica sol, the particle size distribution of 1.0 wt% silica sol in 0.01 M KCl showed a monodisperse distribution having a mean particle size of 12.3 nm [12,22]. The influence of the ionic species was observed in electrolyte solutions containing monovalent cations (K+ and Na+ ) and divalent cations (Ca2+ and Mg2+ ). Considering the mean particle size and turbidity of silica sol in different electrolytes in Table 1, little difference in the particle size distributions of silica sol was observed between KCl and NaCl. However, it is noteworthy that the size distributions of silica sol in CaCl2 and MgCl2 ·6H2 O showed significantly higher values due to the coagulation of colloidal particles. This colloidal coagulation was clearly confirmed by the turbidity measurements. The variation caused by the changes in the valence and the nature of the cation species is explained by the Shultz–Hardy rule, which states that the coagulation effectiveness of these ions markedly increases [18]. In order to investigate the effect of different valences on colloidal stability the critical coagulation concentrations (CCC) of monovalent and divalent cations were measured by the turbidity. The values of NTU with respect to electrolyte solutions were measured in KCl and NaCl solution for monovalent cations and the result is shown in Fig. 1a. In the case of divalent cations, the turbidities of silica sol were Table 1 Particle size and turbidity of silica sol in different ionic species Electrolyte solution containing 1.0 wt% silica sol 0.01 M KCl 0.01 M NaCl 0.01 M CaCl2 0.01 M MgCl2 ·6H2 O
Mean particle size (nm)
Turbidity (NTU)
12 14 1145 621
7.7 7.6 1377 832
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(a)
Fig. 2. Influence of high-ionic-strength electrolyte on the coagulation of silica sol.
(b) Fig. 1. The critical coagulation concentrations of silica sol with different cation species. (a) Monovalent cations (symbols: circles for KCl and squares for NaCl). (b) Divalent cations (symbols: circles for CaCl2 and squares for MgCl2 ·6H2 O).
measured in CaCl2 and MgCl2 ·6H2 O (see Fig. 1b). The values of CCC in the electrolytes showed little difference for the ions with the same valence, implying that the colloidal stability depends mainly on the valence rather than on the ionic species. As seen in the figure, the CCC of the monovalent cations was measured to be 0.45 M and that of the divalent cations 0.0015 M, the ratio of the two values being 300:1. It is reported that the molar concentration of CCC is inversely proportional to the sixth power of the valence (z) of the ions in the solution. Therefore, the CCCs for monovalent, divalent, and trivalent cations would be expected to be in the proportion of 1, 2−6 , 3−6 (or 100:6.7:0.8) [24,25]. The determined values of CCC in electrolyte solutions from the coagulation experiments in this study show a stronger dependence on z than expected and reflect the increased specific adsorption of the counterions in the Stern layer with increasing valences. It is well known that colloidal coagulation and stability are affected by ionic strength in a background electrolyte solution as well as by the nature of the ion species. The coagulation of silica sol occurred when the molecular concentration of CaCl2 was 0.0015 M in the absence of NaCl.
Fig. 3. Influence of cation species on the zeta potentials of silica sol (concentration of electrolyte solution: 0.01 M).
However, silica sol began to coagulate at 0.008 M of CaCl2 (approximately six times as great) under the condition of 0.1 M NaCl (see Fig. 2). 3.2. Colloidal coagulation of silica sol by the zeta potential measurements The electrochemical properties of colloidal particles can be characterized also by the electrokinetic property (or the zeta potential) related to the surface potential of the particles, which is estimated by measuring the electrophoretic mobilities in a microelectrophoresis cell [19]. Figure 3 presents the zeta potentials of silica sol in electrolytes with monovalent and divalent cations, showing that monovalent cations had higher values than divalent cations. The lower zeta potentials of silica sol in the electrolyte solutions of divalent cations (Ca2+ and Mg2+ ) imply a higher sorption capacity on the silica sol due to the larger ionic radius and the higher positive charges, compared with the zeta potentials in the solutions of monovalents (K+ and Na+ ) [18,25]. The zeta potentials of silica sol in the electrolytes showed little difference for the ions of the same valence, depending mainly on the valence rather than on the ionic species, as discussed in the previous section.
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Fig. 4. Zeta potential as a function of the solution pH (electrolyte solution: 0.01 M KCl).
It is known that the electrokinetic properties of colloidal particles are affected by the solution pH values, which is observed in electrophoretic mobility measurements. The charge density of the silica particles decreases, and thus the surface potential becomes more positive due to the attractive van der Waals force as the pH values decrease [9]. The zeta potential determined by the electrophoretic mobility of silica sol as a function of solution pH value is shown in Fig. 4. The isoelectric point (IEP) of colloidal silica sol was found to be pH 3 [8,12]. In the solution at pH values higher than IEP, the surface potential of silica sol becomes highly negative, and then the repulsive electrostatic forces dominate. Because the pH values of the dilute and concentrated solutions in electrodialysis experiments remained between 7 and 8, silica particles are negatively charged during the experiments. Since the AMX membrane is positively charged with anion-exchangeable functional groups as Cl− form, the colloidal particles deposit on the anion exchange membrane surface due to electrostatic attraction [12,26]. 3.3. Influence of coagulation on the ED performance in the presence of silica sol As shown in Fig. 2, the coagulation of silica sol occurred at 0.008 M of CaCl2 in the solution with high ionic strength (0.1 M NaCl). Therefore, the concentrations of CaCl2 in the feed solutions were varied between 0 M, 0.001 M, and 0.005 M (below the CCC) and 0.01 M (above the CCC). The performances of the desalting experiments performed with various concentrations of CaCl2 in the feed solution containing 0.4 wt% silica sol in 0.1 M NaCl were compared in terms of changes of cell resistance and the NaCl concentration in the dilute solution. Considering the changes of NaCl concentration in dilute solutions in Fig. 5, the transport rates of NaCl were somewhat higher below the CCC (0.01 M and 0.05 M CaCl2 ). The transport rates of NaCl even in the presence of silica sol were slightly increased, as shown in Fig. 5. The particles of silica sol deposited on the anion exchange membrane surface, followed by formation of a loosely packed layer for ions to transport, which was reported in the previous study [12]. In the electrodialysis of the feed solution having a concentration of 0.01 M CaCl2 , the transport rate of
Fig. 5. Influence of Ca2+ concentrations on the removal rate of NaCl in electrodialysis experiments (feed solution: 0.1 M NaCl with 0.4 wt% silica sol).
Fig. 6. Influence of Ca2+ concentration on cell resistance in electrodialysis experiments (feed solution: 0.1 M NaCl with 0.4 wt% of silica sol).
NaCl decreased substantially due to coagulation of the silica sols in the electrolyte of above the CCC. Figure 6 shows the effect of the divalent cation on the cell resistance changes. The cell resistances in electrodialysis of a solution containing CaCl2 showed higher values from the beginning to 200 min than that of the reference experiment without CaCl2 , mainly due to low electric mobility of the divalent cation. Meanwhile, the resistance decreased with time in the experiments at 0.005 and 0.01 M CaCl2 after an operation time of 500 min since more divalent ions existed. The flux and the current efficiency of NaCl in the electrodialysis of different concentrations of CaCl2 in the feed solutions are illustrated in Table 2. In the experiments with CaCl2 concentrations below CCC, the flux of NaCl and the current efficiency remained at 2.14–2.18 mol/m2 h and above 89%, respectively. Meanwhile, the electrodialysis performances decreased in the case of CaCl2 concentrations above the CCC. It is thought that the reduced electrodialysis performances at levels above the CCC are due to the presence of the divalent cation with its low electric mobil-
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Table 2 Flux and current efficiency of NaCl with different concentrations of CaCl2 in the dilute solutions Concentration of CaCl2 in dilute solutiona 0M 0.001 M 0.005 M 0.01 M
Flux of NaCl (mol/m2 h)
Current efficiency of NaCl (%)
Remarks
2.14 2.19 2.18 2.05
95.8 96.0 89.3 83.2
Below the CCCb
Above the CCCb
a Dilute solution: 0.4 wt% of silica sol in 0.1 M NaCl. b The value of CCC was determined to be 0.008 M in Fig. 2.
Fig. 8. Amount of Si concentrations remaining in the dilute solutions according to time.
Fig. 7. Amount of Si concentrations transported into the concentrated solutions according to time.
ity and the resulting increased repulsion forces between the ions and the membrane in the electric field.
Fig. 9. Time courses of Ca2+ concentration and mean particle sizes of silica sol in the dilute solutions (0.01 M CaCl2 ).
3.4. Influence of the coagulation on deposition and transport of silica sol
tion increased with time. At a concentration higher than the CCC, the net attractive force dominates and coagulation of the colloidal particles occurs in the bulk solution due to high ionic strength [9]. During the early period of electrodialysis it is possible for coagulated silica sols to adsorb easily on the membrane surface, thus decreasing the concentration of silica sol in the bulk solution at an ionic strength higher than CCC. As the applied current increased with elapsed time, cation concentration of the electrolyte solution decreased down to CCC in the presence of silica sol, thus increasing concentration of the dispersed silica sol in the bulk solution (see Fig. 9). It is expected that deposition of the colloidal particles is affected by colloidal coagulation. When the concentration of CaCl2 was lower than the CCC, the amount of Si deposited on the anion exchange membrane surface increased with time, as illustrated in Fig. 10. It is thought that silica particles moved toward the surface as the ionic strength decreased, subsequently accumulating on the membrane surface. In the case of a bulk solution having a high ionic strength (0.01 M CaCl2 ), the coagulation of silica sol increased the amount of accumulated silica sol on the membrane surface at the early period. In the following period, the deposited amount decreased as the ionic strength of the bulk solution decreased. It is thought that colloidal particles with negative surface
In order to investigate the effect of coagulation on the transport of silica sol, the concentrations of silica sol in the concentrated solution were measured based on the Si concentration analysis in the electrodialysis experiments with 0.005 M CaCl2 (below the CCC) and 0.01 M CaCl2 (above the CCC). Figure 7 shows the time course of Si in the concentrated solutions (0.005 M and 0.01 M CaCl2 ) in the electrodialysis of 0.1 M NaCl containing 0.4 wt% silica sol with CaCl2 . In the electrodialysis experiment with 0.01 M CaCl2 , a smaller amount of silica sol transported through the AMX membranes than in the experiment with 0.005 M CaCl2 . Compared with the transport rates of silica sol, the rates were evaluated as 157.3 mg Si/m2 h for 0.005 M CaCl2 and 63.5 mg Si/m2 h for 0.01 M CaCl2 . It is suggested that the low transport rate of silica sol in the solution above CCC is due to the coagulation of the colloidal particles. Figure 8 shows the time courses of Si concentration for 0.005 and 0.01 M CaCl2 in the dilute solutions. The remaining Si concentration in the dilute concentrations showed different aspects in the experiments for electrolyte solutions lower and higher than the CCC. In the case of 0.01 M CaCl2 (above the CCC), the concentration of Si in the dilute solu-
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preventing it from transporting through the anion exchange membranes and thus accumulating on the membrane surface [12,26].
4. Summary
Fig. 10. Amount of Si deposited on the membrane surfaces according to time. Table 3 Influence of Ca2+ concentrations on the silica sol behaviors Feed solutionsa
0.005 M CaCl2
0.01 M CaCl2
Mean particle size at initial, dilute solution (nm)
133
1771
Mean particle size at final, dilute solution (nm)
20
258
Amount of transported Si into concentrated solution (mg/l)
15.1
6.1
Amount of deposited Si on the membrane surfaceb (mg/cm2 )
3.38
2.73
Below the CCC
Above the CCC
Remark
a Each feed solution: 0.4 wt% of silica sol in 0.1 M NaCl. b Estimated from the mass balance.
charges adsorbed easily on the anion exchange membrane surface (see Fig. 4). As the ionic strength decreased during electrodialysis, however, the deposited silica particles desorbed from the anion exchange membrane surface. The result implies that the behavior of colloidal particles is related to their electrochemical properties rather than to the properties of the membranes. The electrodialysis performances with different concentrations of Ca2+ are compared in Table 3 in terms of the removal efficiencies of Ca2+ in the dilute compartment, the amount of silica sol transported into the concentrated compartment, and the amount of silica sol deposited on the membrane surface. The particle sizes of silica sol in the feed and dilute solutions decreased to 20 nm when the CaCl2 concentrations were below the CCC. Even when Ca2+ was almost removed from the dilute solution in CaCl2 concentrations above the CCC, however, the particle size decreased only to 258 nm. The coagulation of silica sol in the electrodialysis above the CCC reduced the electrodialysis performances as seen in Table 2. The behavior of silica sol showed somewhat different aspects due to changes in the Ca2+ concentration in the feed solution considering the amounts of deposited and transported silica particles, whose results were shown in Fig. 10. The amount of deposited silica sol above the CCC was less than that below the CCC. It is likely that the increased repulsive force constituted a barrier for the silica sol,
In this study, the colloidal stability and electrochemical properties of commercial silica sol having negative surface charges were examined in terms of its role as a foulant in electrodialysis, with the expectation that it would foul anion exchange membranes by deposition, followed by the formation of a layer on the membrane surface. In a stability test of colloidal particles, the critical coagulation concentration (CCC) in the presence of silica sol was measured as a functions of ionic strength, the cation species, and the solution pH. Electrodialysis performance was examined in the electrodialysis of NaCl solution containing silica sol with various concentrations of CaCl2 after the CCC value was determined. The electrodialysis performance, evaluated by the NaCl flux and current efficiency in the presence of silica sol, decreased in the experiments at CaCl2 concentrations above the CCC as a result of coagulation. The results showed that the behavior of silica sol is related to the colloidal stability, depending on the concentration of CaCl2 . The amount of silica sol transported into the concentrated solution decreased with increasing concentration of Ca2+ in the feed solutions, and then the amount of deposited silica sol decreased during electrodialysis experiments. It was clearly shown through this study that the electrodialysis performance and the behavior of colloidal particles (mainly deposition and transport) during electrodialysis experiments were affected by their stability, depending on the electrochemical properties of the electrolyte solutions and the colloidal particles.
Acknowledgment This work was supported by the National Research Laboratory (NRL) Program of the Korean Institute of Science and Technology Evaluation and Planning (KISTEP) (Project 2000-N-NL-01-C-185).
References [1] H. Strathmann, in: W.S. Winston Ho, K.K. Sirkar (Eds.), Membrane Handbook, 1990. [2] H.J. Lee, S.H. Moon, S.P. Tsai, Sep. Purif. Technol. 27 (2002) 89. [3] H.J. Lee, J.H. Choi, J. Cho, S.H. Moon, J. Membrane Sci. 203 (2002) 115. [4] V. Lindstrand, A. Jönsson, G. Sundström, Desalination 130 (2000) 73. [5] V. Lindstrand, G. Sundström, A. Jönsson, Desalination 128 (2000) 91. [6] H.J. Lee, S.J. Oh, S.H. Moon, J. Chem. Technol. Biotechnol. 77 (9) (2002) 1005. [7] H.J. Lee, S.J. Oh, S.H. Moon, Water Res. 37 (2003) 109. [8] X. Zhu, M. Elimelech, Environ. Sci. Technol. 31 (1997) 3654.
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[9] A.S. Jönsson, B. Jönsson, Sep. Sci. Technol. 31 (19) (1996) 2611. [10] G. Belfort, R.H. Davis, A.L. Zydney, J. Membrane Sci. 96 (1994) 1. [11] L. Ricq, A. Pierre, J. Reggiani, J. Pagetti, A. Foissy, Colloids Surf. A Physicochem. Eng. Aspects 138 (1998) 301. [12] H.J. Lee, J.S. Park, M.S. Kang, S.H. Moon, Korean J. Chem. Eng., in press (2003). [13] J.J. Schoeman, M.A. Thompson, in: J. Mallevialle, P.E. Odendaal, M.R. Wiesner (Eds.), Water Treatment Membrane Processes, McGraw–Hill, New York, 1996. [14] J. Cho, G. Amy, J. Pellagrino, Y. Yoon, Desalination 118 (1998) 101. [15] H.J. Lee, D.H. Kim, J. Cho, S.H. Moon, Desalination 151 (2002) 43. [16] D. Möckel, E. Staude, M. Dal-Cin, K. Darcovich, M. Guiver, J. Membrane Sci. 145 (1998) 211. [17] R.J. Hunter, Introduction to Modern Colloid Science, Oxford Univ. Press, Oxford, 1993. [18] P.C. Hiemenz, R. Rajagopalan, Principles of Colloid and Surface Chemistry, 3rd ed., Dekker, New York, 1997.
[19] K. Oka, K. Furusawa, in: H. Ohshima, K. Furusawa (Eds.), Electrical Phenomena at Interface—Fundamentals, Measurements, and Applications, 2nd ed., Dekker, New York, 1998. [20] M. Elimelech, C.R. O’Melia, Colloids Surf. 44 (1990) 165. [21] Y. Shim, H.J. Lee, S. Lee, S.H. Moon, J. Cho, Environ Sci. Technol. 36 (2002) 3864. [22] Dupont, available at: http://www.dupont.com/silica, 1999. [23] Tokuyama Corporation, NEOSEPTA-Ion Exchange Membranes, 1993. [24] D.J. Shaw, Introduction to Colloid and Surface Chemistry, 4th ed., Butterworth–Heinemann, 1992. [25] W. Stumm, Chemistry of the Solid–Water Interface—Processes at the Mineral–Water and Particle–Water Interface in Natural Systems, Wiley, New York, 1992. [26] H.J. Lee, Studies on Fouling of Anion Exchange Membranes and Fouling Mitigation in Electrodialysis, Ph.D. dissertation, Kwangju Institute of Science and Technology (K-JIST), 2002.
Influences of colloidal stability and electrokinetic property on electrodialysis performance in the presence of silica sol Hong-Joo Lee a,∗ and Seung-Hyeon Moon b a Nuclear Power Laboratory, Korea Electric Power Research Institute (KEPRI), 103-16 Munji-Dong, Yuseong-Gu, Daejeon 305-380, South Korea b Department of Environmental Science and Engineering, Kwangju Institute of Science and Technology (K-JIST), 1 Oryong-Dong, Buk-Gu,
Gwangju 500-712, South Korea Received 21 April 2003; accepted 25 August 2003
Abstract Negatively charged silica sol is known to lead to fouling of anion exchange membranes during electrodialysis (ED) as a result of its deposition on the membrane surface. It is known that the fouling potential is related to the physical and electrochemical properties of the silica particles as well as those of the anion exchange membranes. In this study, the properties of the silica sol were characterized in terms of its particle size, turbidity, and zeta potential in order to predict their effects on the electrodialysis performance. In the stability of colloidal particles, the critical coagulation concentrations of silica sol were determined as functions of ionic strength, cation species, and solution pH. In the electrodialysis of NaCl solution containing silica sol with various concentrations of CaCl2 , the colloidal behavior related to deposition and transport was examined during and after electrodialysis. The electrodialysis experiments clearly showed that the deposition and transport of silica sol during electrodialysis were related to the colloidal stability of dispersion. 2003 Elsevier Inc. All rights reserved. Keywords: Characterization; Silica sol; Electrodialysis; Zeta potential
1. Introduction Electrodialysis (ED) is an electrochemical separation process that employs electrically charged ion exchange membranes with an electrical potential difference as a driving force. ED applications can be found in the environmental and biochemical industries as well as in the production of table salt and the desalination of seawater [1,2]. Fouling of ion exchange membranes is considered to be one of the most important limitations on the design and operation of electrodialysis processes [3–7]. It is generally accepted that fouling occurs in membrane processes due to deposition of foulants (organic substances, colloidal particles, proteins, etc.). Most colloidal particles existing in natural waters and in many industrial effluent streams have negative surface charges, depending on the solution pH. The surface charge * Corresponding author. Current address: Membrane Separation Processes Group, Research Institute for Green Technology, National Institute of Advanced Industrial Science and Technology (AIST), 1-1-1 Higashi, Tsukuba, Ibaraki 305-8565, Japan. E-mail address: [email protected] (H.-J. Lee).
0021-9797/$ – see front matter 2003 Elsevier Inc. All rights reserved. doi:10.1016/j.jcis.2003.08.042
of aquatic colloids reflects their surface chemical properties and the chemical composition of natural waters. Examples of aquatic colloids are clay minerals, colloidal silica particles, iron oxides, aluminum oxides, manganese oxides, organic colloids, suspended matters, and calcium carbonate precipitates [8]. It is generally accepted that colloidal fouling is caused by the accumulation of colloidal particles on the membrane surface, followed by coagulation. The deposition of silica sol significantly decreases the performance of the pressure-driven membrane processes (ultrafiltration, nanofiltration, and reverse osmosis) by plugging into the pore structures [9–11]. Electrodialysis performance is also affected by deposition of colloidal particles, which is related to the electrochemical properties of the particles [12,13]. Colloidal particles having negative charges move toward anion exchange membranes in the electric field during electrodialysis. Then the colloidal particles deposit on the membrane surface due to electrostatic attraction between the positively charged anion exchange membrane and the negatively changed colloidal particles [13]. The coagulation of colloidal particles and electrokinetic properties (zeta potential) are accepted to be two of the main factors affecting electrodialysis perfor-
H.-J. Lee, S.-H. Moon / Journal of Colloid and Interface Science 270 (2004) 406–412
mance, and these two values give useful information for the prediction of fouling potentials [14–16]. It is known that the rate of coagulation depends mainly on the stability of the colloidal particles and that the salt concentration and the solution pH are the two main factors affecting the colloidal stability [9–11]. The stability of a dispersion (the ability of the dispersion to resist coagulation) may be related to its kinetic stability, which in turn depends on the force barrier preventing collision between the particles and thus preventing their coagulation [17,18]. Coagulation occurs when the interaction energy barrier is higher than other applied energy barriers of the colloidal particles. When there are no repulsive interactions between the particles, the dispersion will be unstable to coagulation due to the electrostatic stability of the colloidal particles [18]. The electrolyte concentration at which slow coagulation turns into rapid coagulation can be determined accurately in colloidal stability experiments. The resulting value is considered as one of the important characteristic values of colloidal particles, called the critical coagulation concentration (CCC). The CCC depends mainly on the following parameters: (i) the elapsed time, (ii) the uniformity of the sample, (iii) the potential at the surface, and (iv) the valence of the ions [18]. The objectives of this study are (i) to characterize colloidal particles in terms of electrochemical properties for the colloidal stability and (ii) to investigate colloidal coagulation effects on electrodialysis performance and colloidal behaviors (mainly deposition and transport) during electrodialysis.
2. Materials and methods The colloidal particles of silica sol were characterized in terms of particle size, turbidity, and zeta potential. The intensities of the colloidal particles in a solution were measured at 330 nm with a PCS 4700 particle size analyzer (Malvern, England) [17]. In addition, the turbidity of the silica sol was measured with a DRT-15 CE portable turbidimeter (HF Scientific Inc., USA) using a nephelometric turbidity unit (NTU). An ICP-AES (Jovin Yvon 38 Plus, France) was used at a selected wavelength of 251.611 nm to measure the Si concentration. The zeta potentials were estimated using the Smoluchowski equation from electrophoretic mobility measurements using an ELS-8000 electrophoretic apparatus (Otsuka Electronics, Japan) in order to investigate the influence of cation species on the electrokinetic properties [11,19–21]. The influence of ionic strength and solution pH on the zeta potential was also considered in this study. A TS-1 electrodialyzer (Tokuyama Corp., Japan), was used with two cell pairs, consisting of a cation-exchange membrane (NEOSEPTA CMX) and an anion-exchange membrane (NEOSEPTA AMX) (Tokuyama Corp., Japan), in a concentration of 0.4 wt% silica sol, Ludox HS-40 (Aldrich, USA) [3,22]. The characteristics of CMX and AMX membranes were shown elsewhere [12,23]. Five liters of 0.1 M NaCl solution containing silica sol was circulated
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as a dilute solution and 800 ml of 3.0 wt% of Na2 SO4 as an electrode rinse solution. During electrodialysis experiments, 5 l of 0.05 M NaCl solution was used as a concentrated solution. The pH values of the dilute and concentrated solutions remained between pH 7 and 8 without pH control. The flow rates of the dilute and concentrated solutions were set at 0.2–0.3 l/min. Coagulation and particle size distribution were controlled by varying the concentration of CaCl2 in a background electrolyte solution (0.1 M NaCl). During electrodialysis experiments, a constant direct current of 0.6 A (current density 6.0 mA/cm2 ) was supplied. After electrodialysis experiments, 500 ml of 1.0 wt% NaOH and distilled water were circulated for 20 min in the dilute and concentrated solution compartments for cleaning-in-place of membranes and compartments.
3. Results and discussion 3.1. Colloidal coagulation of silica sol by turbidity measurements Of the characterized properties of silica sol, the particle size distribution of 1.0 wt% silica sol in 0.01 M KCl showed a monodisperse distribution having a mean particle size of 12.3 nm [12,22]. The influence of the ionic species was observed in electrolyte solutions containing monovalent cations (K+ and Na+ ) and divalent cations (Ca2+ and Mg2+ ). Considering the mean particle size and turbidity of silica sol in different electrolytes in Table 1, little difference in the particle size distributions of silica sol was observed between KCl and NaCl. However, it is noteworthy that the size distributions of silica sol in CaCl2 and MgCl2 ·6H2 O showed significantly higher values due to the coagulation of colloidal particles. This colloidal coagulation was clearly confirmed by the turbidity measurements. The variation caused by the changes in the valence and the nature of the cation species is explained by the Shultz–Hardy rule, which states that the coagulation effectiveness of these ions markedly increases [18]. In order to investigate the effect of different valences on colloidal stability the critical coagulation concentrations (CCC) of monovalent and divalent cations were measured by the turbidity. The values of NTU with respect to electrolyte solutions were measured in KCl and NaCl solution for monovalent cations and the result is shown in Fig. 1a. In the case of divalent cations, the turbidities of silica sol were Table 1 Particle size and turbidity of silica sol in different ionic species Electrolyte solution containing 1.0 wt% silica sol 0.01 M KCl 0.01 M NaCl 0.01 M CaCl2 0.01 M MgCl2 ·6H2 O
Mean particle size (nm)
Turbidity (NTU)
12 14 1145 621
7.7 7.6 1377 832
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(a)
Fig. 2. Influence of high-ionic-strength electrolyte on the coagulation of silica sol.
(b) Fig. 1. The critical coagulation concentrations of silica sol with different cation species. (a) Monovalent cations (symbols: circles for KCl and squares for NaCl). (b) Divalent cations (symbols: circles for CaCl2 and squares for MgCl2 ·6H2 O).
measured in CaCl2 and MgCl2 ·6H2 O (see Fig. 1b). The values of CCC in the electrolytes showed little difference for the ions with the same valence, implying that the colloidal stability depends mainly on the valence rather than on the ionic species. As seen in the figure, the CCC of the monovalent cations was measured to be 0.45 M and that of the divalent cations 0.0015 M, the ratio of the two values being 300:1. It is reported that the molar concentration of CCC is inversely proportional to the sixth power of the valence (z) of the ions in the solution. Therefore, the CCCs for monovalent, divalent, and trivalent cations would be expected to be in the proportion of 1, 2−6 , 3−6 (or 100:6.7:0.8) [24,25]. The determined values of CCC in electrolyte solutions from the coagulation experiments in this study show a stronger dependence on z than expected and reflect the increased specific adsorption of the counterions in the Stern layer with increasing valences. It is well known that colloidal coagulation and stability are affected by ionic strength in a background electrolyte solution as well as by the nature of the ion species. The coagulation of silica sol occurred when the molecular concentration of CaCl2 was 0.0015 M in the absence of NaCl.
Fig. 3. Influence of cation species on the zeta potentials of silica sol (concentration of electrolyte solution: 0.01 M).
However, silica sol began to coagulate at 0.008 M of CaCl2 (approximately six times as great) under the condition of 0.1 M NaCl (see Fig. 2). 3.2. Colloidal coagulation of silica sol by the zeta potential measurements The electrochemical properties of colloidal particles can be characterized also by the electrokinetic property (or the zeta potential) related to the surface potential of the particles, which is estimated by measuring the electrophoretic mobilities in a microelectrophoresis cell [19]. Figure 3 presents the zeta potentials of silica sol in electrolytes with monovalent and divalent cations, showing that monovalent cations had higher values than divalent cations. The lower zeta potentials of silica sol in the electrolyte solutions of divalent cations (Ca2+ and Mg2+ ) imply a higher sorption capacity on the silica sol due to the larger ionic radius and the higher positive charges, compared with the zeta potentials in the solutions of monovalents (K+ and Na+ ) [18,25]. The zeta potentials of silica sol in the electrolytes showed little difference for the ions of the same valence, depending mainly on the valence rather than on the ionic species, as discussed in the previous section.
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Fig. 4. Zeta potential as a function of the solution pH (electrolyte solution: 0.01 M KCl).
It is known that the electrokinetic properties of colloidal particles are affected by the solution pH values, which is observed in electrophoretic mobility measurements. The charge density of the silica particles decreases, and thus the surface potential becomes more positive due to the attractive van der Waals force as the pH values decrease [9]. The zeta potential determined by the electrophoretic mobility of silica sol as a function of solution pH value is shown in Fig. 4. The isoelectric point (IEP) of colloidal silica sol was found to be pH 3 [8,12]. In the solution at pH values higher than IEP, the surface potential of silica sol becomes highly negative, and then the repulsive electrostatic forces dominate. Because the pH values of the dilute and concentrated solutions in electrodialysis experiments remained between 7 and 8, silica particles are negatively charged during the experiments. Since the AMX membrane is positively charged with anion-exchangeable functional groups as Cl− form, the colloidal particles deposit on the anion exchange membrane surface due to electrostatic attraction [12,26]. 3.3. Influence of coagulation on the ED performance in the presence of silica sol As shown in Fig. 2, the coagulation of silica sol occurred at 0.008 M of CaCl2 in the solution with high ionic strength (0.1 M NaCl). Therefore, the concentrations of CaCl2 in the feed solutions were varied between 0 M, 0.001 M, and 0.005 M (below the CCC) and 0.01 M (above the CCC). The performances of the desalting experiments performed with various concentrations of CaCl2 in the feed solution containing 0.4 wt% silica sol in 0.1 M NaCl were compared in terms of changes of cell resistance and the NaCl concentration in the dilute solution. Considering the changes of NaCl concentration in dilute solutions in Fig. 5, the transport rates of NaCl were somewhat higher below the CCC (0.01 M and 0.05 M CaCl2 ). The transport rates of NaCl even in the presence of silica sol were slightly increased, as shown in Fig. 5. The particles of silica sol deposited on the anion exchange membrane surface, followed by formation of a loosely packed layer for ions to transport, which was reported in the previous study [12]. In the electrodialysis of the feed solution having a concentration of 0.01 M CaCl2 , the transport rate of
Fig. 5. Influence of Ca2+ concentrations on the removal rate of NaCl in electrodialysis experiments (feed solution: 0.1 M NaCl with 0.4 wt% silica sol).
Fig. 6. Influence of Ca2+ concentration on cell resistance in electrodialysis experiments (feed solution: 0.1 M NaCl with 0.4 wt% of silica sol).
NaCl decreased substantially due to coagulation of the silica sols in the electrolyte of above the CCC. Figure 6 shows the effect of the divalent cation on the cell resistance changes. The cell resistances in electrodialysis of a solution containing CaCl2 showed higher values from the beginning to 200 min than that of the reference experiment without CaCl2 , mainly due to low electric mobility of the divalent cation. Meanwhile, the resistance decreased with time in the experiments at 0.005 and 0.01 M CaCl2 after an operation time of 500 min since more divalent ions existed. The flux and the current efficiency of NaCl in the electrodialysis of different concentrations of CaCl2 in the feed solutions are illustrated in Table 2. In the experiments with CaCl2 concentrations below CCC, the flux of NaCl and the current efficiency remained at 2.14–2.18 mol/m2 h and above 89%, respectively. Meanwhile, the electrodialysis performances decreased in the case of CaCl2 concentrations above the CCC. It is thought that the reduced electrodialysis performances at levels above the CCC are due to the presence of the divalent cation with its low electric mobil-
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Table 2 Flux and current efficiency of NaCl with different concentrations of CaCl2 in the dilute solutions Concentration of CaCl2 in dilute solutiona 0M 0.001 M 0.005 M 0.01 M
Flux of NaCl (mol/m2 h)
Current efficiency of NaCl (%)
Remarks
2.14 2.19 2.18 2.05
95.8 96.0 89.3 83.2
Below the CCCb
Above the CCCb
a Dilute solution: 0.4 wt% of silica sol in 0.1 M NaCl. b The value of CCC was determined to be 0.008 M in Fig. 2.
Fig. 8. Amount of Si concentrations remaining in the dilute solutions according to time.
Fig. 7. Amount of Si concentrations transported into the concentrated solutions according to time.
ity and the resulting increased repulsion forces between the ions and the membrane in the electric field.
Fig. 9. Time courses of Ca2+ concentration and mean particle sizes of silica sol in the dilute solutions (0.01 M CaCl2 ).
3.4. Influence of the coagulation on deposition and transport of silica sol
tion increased with time. At a concentration higher than the CCC, the net attractive force dominates and coagulation of the colloidal particles occurs in the bulk solution due to high ionic strength [9]. During the early period of electrodialysis it is possible for coagulated silica sols to adsorb easily on the membrane surface, thus decreasing the concentration of silica sol in the bulk solution at an ionic strength higher than CCC. As the applied current increased with elapsed time, cation concentration of the electrolyte solution decreased down to CCC in the presence of silica sol, thus increasing concentration of the dispersed silica sol in the bulk solution (see Fig. 9). It is expected that deposition of the colloidal particles is affected by colloidal coagulation. When the concentration of CaCl2 was lower than the CCC, the amount of Si deposited on the anion exchange membrane surface increased with time, as illustrated in Fig. 10. It is thought that silica particles moved toward the surface as the ionic strength decreased, subsequently accumulating on the membrane surface. In the case of a bulk solution having a high ionic strength (0.01 M CaCl2 ), the coagulation of silica sol increased the amount of accumulated silica sol on the membrane surface at the early period. In the following period, the deposited amount decreased as the ionic strength of the bulk solution decreased. It is thought that colloidal particles with negative surface
In order to investigate the effect of coagulation on the transport of silica sol, the concentrations of silica sol in the concentrated solution were measured based on the Si concentration analysis in the electrodialysis experiments with 0.005 M CaCl2 (below the CCC) and 0.01 M CaCl2 (above the CCC). Figure 7 shows the time course of Si in the concentrated solutions (0.005 M and 0.01 M CaCl2 ) in the electrodialysis of 0.1 M NaCl containing 0.4 wt% silica sol with CaCl2 . In the electrodialysis experiment with 0.01 M CaCl2 , a smaller amount of silica sol transported through the AMX membranes than in the experiment with 0.005 M CaCl2 . Compared with the transport rates of silica sol, the rates were evaluated as 157.3 mg Si/m2 h for 0.005 M CaCl2 and 63.5 mg Si/m2 h for 0.01 M CaCl2 . It is suggested that the low transport rate of silica sol in the solution above CCC is due to the coagulation of the colloidal particles. Figure 8 shows the time courses of Si concentration for 0.005 and 0.01 M CaCl2 in the dilute solutions. The remaining Si concentration in the dilute concentrations showed different aspects in the experiments for electrolyte solutions lower and higher than the CCC. In the case of 0.01 M CaCl2 (above the CCC), the concentration of Si in the dilute solu-
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preventing it from transporting through the anion exchange membranes and thus accumulating on the membrane surface [12,26].
4. Summary
Fig. 10. Amount of Si deposited on the membrane surfaces according to time. Table 3 Influence of Ca2+ concentrations on the silica sol behaviors Feed solutionsa
0.005 M CaCl2
0.01 M CaCl2
Mean particle size at initial, dilute solution (nm)
133
1771
Mean particle size at final, dilute solution (nm)
20
258
Amount of transported Si into concentrated solution (mg/l)
15.1
6.1
Amount of deposited Si on the membrane surfaceb (mg/cm2 )
3.38
2.73
Below the CCC
Above the CCC
Remark
a Each feed solution: 0.4 wt% of silica sol in 0.1 M NaCl. b Estimated from the mass balance.
charges adsorbed easily on the anion exchange membrane surface (see Fig. 4). As the ionic strength decreased during electrodialysis, however, the deposited silica particles desorbed from the anion exchange membrane surface. The result implies that the behavior of colloidal particles is related to their electrochemical properties rather than to the properties of the membranes. The electrodialysis performances with different concentrations of Ca2+ are compared in Table 3 in terms of the removal efficiencies of Ca2+ in the dilute compartment, the amount of silica sol transported into the concentrated compartment, and the amount of silica sol deposited on the membrane surface. The particle sizes of silica sol in the feed and dilute solutions decreased to 20 nm when the CaCl2 concentrations were below the CCC. Even when Ca2+ was almost removed from the dilute solution in CaCl2 concentrations above the CCC, however, the particle size decreased only to 258 nm. The coagulation of silica sol in the electrodialysis above the CCC reduced the electrodialysis performances as seen in Table 2. The behavior of silica sol showed somewhat different aspects due to changes in the Ca2+ concentration in the feed solution considering the amounts of deposited and transported silica particles, whose results were shown in Fig. 10. The amount of deposited silica sol above the CCC was less than that below the CCC. It is likely that the increased repulsive force constituted a barrier for the silica sol,
In this study, the colloidal stability and electrochemical properties of commercial silica sol having negative surface charges were examined in terms of its role as a foulant in electrodialysis, with the expectation that it would foul anion exchange membranes by deposition, followed by the formation of a layer on the membrane surface. In a stability test of colloidal particles, the critical coagulation concentration (CCC) in the presence of silica sol was measured as a functions of ionic strength, the cation species, and the solution pH. Electrodialysis performance was examined in the electrodialysis of NaCl solution containing silica sol with various concentrations of CaCl2 after the CCC value was determined. The electrodialysis performance, evaluated by the NaCl flux and current efficiency in the presence of silica sol, decreased in the experiments at CaCl2 concentrations above the CCC as a result of coagulation. The results showed that the behavior of silica sol is related to the colloidal stability, depending on the concentration of CaCl2 . The amount of silica sol transported into the concentrated solution decreased with increasing concentration of Ca2+ in the feed solutions, and then the amount of deposited silica sol decreased during electrodialysis experiments. It was clearly shown through this study that the electrodialysis performance and the behavior of colloidal particles (mainly deposition and transport) during electrodialysis experiments were affected by their stability, depending on the electrochemical properties of the electrolyte solutions and the colloidal particles.
Acknowledgment This work was supported by the National Research Laboratory (NRL) Program of the Korean Institute of Science and Technology Evaluation and Planning (KISTEP) (Project 2000-N-NL-01-C-185).
References [1] H. Strathmann, in: W.S. Winston Ho, K.K. Sirkar (Eds.), Membrane Handbook, 1990. [2] H.J. Lee, S.H. Moon, S.P. Tsai, Sep. Purif. Technol. 27 (2002) 89. [3] H.J. Lee, J.H. Choi, J. Cho, S.H. Moon, J. Membrane Sci. 203 (2002) 115. [4] V. Lindstrand, A. Jönsson, G. Sundström, Desalination 130 (2000) 73. [5] V. Lindstrand, G. Sundström, A. Jönsson, Desalination 128 (2000) 91. [6] H.J. Lee, S.J. Oh, S.H. Moon, J. Chem. Technol. Biotechnol. 77 (9) (2002) 1005. [7] H.J. Lee, S.J. Oh, S.H. Moon, Water Res. 37 (2003) 109. [8] X. Zhu, M. Elimelech, Environ. Sci. Technol. 31 (1997) 3654.
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[9] A.S. Jönsson, B. Jönsson, Sep. Sci. Technol. 31 (19) (1996) 2611. [10] G. Belfort, R.H. Davis, A.L. Zydney, J. Membrane Sci. 96 (1994) 1. [11] L. Ricq, A. Pierre, J. Reggiani, J. Pagetti, A. Foissy, Colloids Surf. A Physicochem. Eng. Aspects 138 (1998) 301. [12] H.J. Lee, J.S. Park, M.S. Kang, S.H. Moon, Korean J. Chem. Eng., in press (2003). [13] J.J. Schoeman, M.A. Thompson, in: J. Mallevialle, P.E. Odendaal, M.R. Wiesner (Eds.), Water Treatment Membrane Processes, McGraw–Hill, New York, 1996. [14] J. Cho, G. Amy, J. Pellagrino, Y. Yoon, Desalination 118 (1998) 101. [15] H.J. Lee, D.H. Kim, J. Cho, S.H. Moon, Desalination 151 (2002) 43. [16] D. Möckel, E. Staude, M. Dal-Cin, K. Darcovich, M. Guiver, J. Membrane Sci. 145 (1998) 211. [17] R.J. Hunter, Introduction to Modern Colloid Science, Oxford Univ. Press, Oxford, 1993. [18] P.C. Hiemenz, R. Rajagopalan, Principles of Colloid and Surface Chemistry, 3rd ed., Dekker, New York, 1997.
[19] K. Oka, K. Furusawa, in: H. Ohshima, K. Furusawa (Eds.), Electrical Phenomena at Interface—Fundamentals, Measurements, and Applications, 2nd ed., Dekker, New York, 1998. [20] M. Elimelech, C.R. O’Melia, Colloids Surf. 44 (1990) 165. [21] Y. Shim, H.J. Lee, S. Lee, S.H. Moon, J. Cho, Environ Sci. Technol. 36 (2002) 3864. [22] Dupont, available at: http://www.dupont.com/silica, 1999. [23] Tokuyama Corporation, NEOSEPTA-Ion Exchange Membranes, 1993. [24] D.J. Shaw, Introduction to Colloid and Surface Chemistry, 4th ed., Butterworth–Heinemann, 1992. [25] W. Stumm, Chemistry of the Solid–Water Interface—Processes at the Mineral–Water and Particle–Water Interface in Natural Systems, Wiley, New York, 1992. [26] H.J. Lee, Studies on Fouling of Anion Exchange Membranes and Fouling Mitigation in Electrodialysis, Ph.D. dissertation, Kwangju Institute of Science and Technology (K-JIST), 2002.