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Determining the zeta-potential of ceramic microfiltration membranes using the electroviscous effect
Journal of Membrane Science 147 (1998) 187±194
Determining the zeta-potential of ceramic micro®ltration membranes using the electroviscous effect Ingmar H. Huismana,*, Gun TraÈgaÊrdha, Christian TraÈgaÊrdha, Arto PihlajamaÈkib a
Food Engineering, Lund University, PO Box 124, 221 00 Lund, Sweden Department of Chemical Technology, Lappeenranta University of Technology, PO Box 20, 53851 Lappeenranta, Finland
b
Received 25 September 1997
Abstract The possibility of measuring the zeta-potentials of porous membranes using the electroviscous effect was investigated. The 1 zeta-potential of Membralox ceramic micro®ltration membranes was determined both with the newly developed electroviscous technique and by streaming potential measurements. It was found that the electroviscous technique provided a simple means of obtaining accurate values of zeta-potential, especially for higher zeta-potentials. The streaming potential measurements were found to be more suitable for the determination of the iso-electric point, i.e. the pH at which the zetapotential is zero. The iso-electric points of new a-alumina, zirconia, and titania membranes were found to be 8.5, 8.0, and 6.3, respectively. Upon using the membranes and cleaning them with a detergent, the iso-electric point of the a-alumina membrane decreased to 6.5, and that of the zirconia membrane decreased to 5.2, while the iso-electric point of the titania membrane stayed virtually constant. Cleaning these membranes with a strong acid or base could not reverse the observed decreases in iso-electric point. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Zeta-potential; Electroviscosity; Micro®ltration; Ceramic membranes
1. Introduction The main factor limiting the application of cross¯ow micro®ltration is fouling of the membranes, which reduces the ¯ux and impairs the separation properties. The surface characteristics of the membrane material, such as zeta-potential, surface charge density, and hydrophobicity, are among the factors that in¯uence membrane fouling [1].
*Corresponding author. Tel.: +46 46 222 9820; fax: +46 46 222 4622; e-mail: [email protected] 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00135-5
The zeta-potential is an important measure of particle±particle and particle±surface interactions. These interactions have been shown to be of importance to membrane performance. Both the amount of fouling [1±3] and the reversibility of fouling [4] are dependent on the zeta-potentials of the feed suspension particles and of the membrane surface. For example, it has been shown that negatively charged paints foul negatively charged membranes less than positively charged paints do [5]. It is clear from this example that the sign of the zeta-potential (positive or negative) is often more important than its absolute value. Many membranes have a positive zeta-potential at low pH and a
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negative zeta-potential at high pH. The pH at which the transition from positive to negative occurs is called the iso-electric point. The iso-electric point is in general obtained from zeta-potential measurements at different pH. Although the zeta-potential of particles can be determined easily and accurately by electrophoretic techniques, there is, as yet, no easy and accurate way to determine the zeta-potential of membranes. The traditional way of determining the zeta-potential for ultra®ltration and micro®ltration membranes is to measure the streaming potential E created by a pressure drop over the membrane (transmembrane pressure, TMP), and to calculate the zeta-potential using the Helmholtz±Smoluchowski equation E "0 "r ; TMP K
(1)
where "0 is the permittivity of free space, "r the relative permittivity of the solvent, the viscosity of the solution, K the conductivity of the solution, and is the zeta-potential of the membrane [6]. Eq. (1) is valid only as long as the Debye length of the solution is small compared to the radius of the pores [7,8]. This condition is ful®lled for the micro®ltration membranes and ionic strengths considered in the current paper. The values that are to be inserted into Eq. (1) for the three solution properties, "r, , and K, are the effective local values in the membrane pore. These values are generally dif®cult to determine. Most authors use the values as observed for the bulk solution. For "r this is acceptable, since "r near surfaces hardly differs from "r in the bulk phase [9]. However, the values of and K near surfaces may be signi®cantly different from those observed in the bulk solution [7,8]. If an electrolyte solution is pressed through a capillary with charged surfaces, ions are moved away from their preferred positions in the double layer. This costs extra energy and can be described as an increase in the apparent viscosity. This phenomenon is referred to as the electroviscous effect. The conductivity of a solution is strongly dependent on the concentration of ions. Since the local ionic concentration in the pores is higher than that in the bulk solution, the conductivity within the pores increases. The surface of the pores may also contribute to the total conductivity. The difference between the effective conductivity in the
pore and the bulk conductivity is referred to as surface conductivity. Although the in¯uence of electroviscosity can be calculated [10], no general method is available to determine the in¯uence of surface conductivity on the zeta-potential. Therefore zeta-potentials obtained from Eq. (1) only have a qualitative value (the zeta-potential is generally underpredicted). Another disadvantage of the streaming potential method is that the electrodes used are often intolerant towards aggressive chemicals. For example: Ag/AgCl electrodes, which are commonly used, are not stable at pH values higher than 8. An alternative method of determining the zetapotential of membranes is to perform electro-osmotic measurements [11]. These measurements are based on a principle similar to that of streaming potential measurements. While for streaming potential measurements the electric voltage induced by a given pressure is measured, in electro-osmosis, the electrolyte ¯ow through the pores induced by an electrical ®eld is measured. Using electro-osmosis to measure the membrane zeta-potential has the same disadvantages as using streaming potential measurements. A third method of determining the zeta-potential of membranes is to perform electrophoretic measurements on particles made of the same material as the membrane [2]. Although electrophoretic measurements on particles give a more reliable value of the zeta-potential than do streaming potential or electroosmosis measurements, the disadvantage of the electrophoretic technique is that measurements are not made on the membrane itself; the characteristics of the particles may be different from those of the membrane. Even if the particles are obtained by grinding a membrane, it cannot be guaranteed that the surface properties of these particles are representatives of the surface of the active layer of the membrane [5]. Recently, a new method has been developed for the determination of the zeta-potential of membranes, employing the electroviscous effect [12]. Levine et al. [10] showed that the apparent viscosity in a cylindrical pore is related to the zeta-potential of the pore surface as a 0
1ÿ
8 e=kT2 1 ÿ GF kr2
!ÿ1 ;
(2)
I.H. Huisman et al. / Journal of Membrane Science 147 (1998) 187±194
where a is the apparent viscosity, 0 the bulk viscosity of the electrolyte solution, the zeta-potential of the capillary surface, the Debye constant, and r is the capillary radius. G and F are dependent on and on r, and can be determined from graphs given in [10]. For large values of r, G0 and F1. is a dimensionless parameter describing the properties of the electrolyte [10,12]. For aqueous monovalent salt solutions at 208C, 35.8/ (where denotes the molar conductivity in ÿ1 cm2 molÿ1). The validity of the theory presented by Levine et al. (including Eq. (2)) has recently been demonstrated through an exact numerical solution of the governing equations [13]. The apparent viscosity is easily obtained from water ¯ux measurements at constant transmembrane pressure, the Debye constant is calculated from the salt concentration, the conductivity can be measured, and the pore radius can either be measured or is known from information supplied by the manufacturer. Thus all variables in Eq. (2) are known, except for , and G and F as they depend on . The zeta-potential can then be calculated from Eq. (2) by an iterative method. The great advantage of this method is that no special equipment is needed other than a membrane rig that allows measurements of transmembrane pressure and ¯ux. Another advantage is that surface conductivity does not in¯uence the results, so the calculated zetapotentials do not suffer from systematic errors. A clear disadvantage of the technique is that knowledge of the membrane pore size is necessary for a quantitative calculation of the zeta-potential. However, knowledge of the pore-size is not necessary to determine the isoelectric point, the pH at which the zeta-potential is zero. ÿ1 is a measure for the thickness of the electrical double layer. It is shown in [10] that the electroviscous effect is greatest if the pore radius r2.5 ÿ1; the effect decreases for both smaller and larger values of r. A measurable electroviscous effect is obtained for r roughly in the interval 0.3±30 [10]. For concentrations of 10ÿ3 M monovalent salt, this corresponds to pore diameters (2r) in the range 6±600 nm, which is in the ultra®ltration and micro®ltration range. The aim of the current study is to show that electroviscosity measurements provide a simple method of determining accurate values of zeta-potential for porous membranes. Zeta-potentials of various ceramic
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micro®ltration membranes were determined by both the electroviscous method and the well-established streaming potential method, and the results compared. Different zeta-potentials were obtained for new membranes, as supplied by the manufacturer, and membranes that had been used and cleaned with a detergent. Reasons for these differences are discussed. 2. Materials and methods The micro®ltration membranes used were from the Membralox series produced by SCT (Bazet, France). The active layer of the membranes consisted of aalumina, zirconia, or titania. The membranes were tubular, had an inner diameter of 6.85 mm, a ®lter length of 0.23 m, and a pore size of 0.2 mm. All electrolyte solutions used were prepared from ROtreated and MilliQ-®ltered water, and reactant grade chemicals. Streaming potential measurements were performed with the equipment described by NystroÈm et al. [14]. This set-up contains two electrodes: one is placed in the centre of the tubular membrane, and the other outside the tube. The potential difference across the membrane was measured at different values of the transmembrane pressure, at a constant cross¯ow velocity of 0.14 m sÿ1 (Re2400). Measurements were performed at 258C. pH and salinity were adjusted by adding KOH and HCl, and KCl. Apparent zeta-potentials were calculated from measured values of E/ TMP using Eq. (1). Electroviscous effects were determined by measuring the water ¯ux at different values of pH and salt concentration. This was done in a cross¯ow micro®ltration rig described in detail elsewhere [15]. A uniform transmembrane pressure (i.e. TMP independent of the position along the membrane) was guaranteed by a circulating ¯ow on the permeate side, which created a pressure drop equal to the pressure drop on the feed side. Measurements were performed at 208C, the cross¯ow velocity was 5 m sÿ1, and the transmembrane pressure about 20 kPa. pH and salinity were adjusted by adding NaOH and HCl, and NaCl. Some experiments were performed with potassium salts (KOH, KCl), but the type of salt (sodium or potassium) was found to have little effect on the observed zeta-potentials.
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Fig. 1. Flux of water containing different salt concentrations at pH5 for an a-alumina membrane.
Fig. 2. Relative apparent viscosity versus time at different pH values, for a new zirconia membrane.
The electroviscous measurements are very sensitive to small temperature ¯uctuations, as viscosity depends strongly on temperature. The temperature was controlled within 0.58C, but recorded with a higher accuracy. Measured ¯uxes were then corrected for variation in viscosity caused by the observed temperature ¯uctuations. During some of the measurements, the pH was kept constant while the salt concentration was increased stepwise from 110ÿ4 to 0.1 M. Increasing the salinity initially led to an increase in ¯ux (see Fig. 1). At higher salt concentrations the ¯ux levelled off, and became independent of the salinity. This is in agreement with Eq. (2), which predicts that for 0.2 mm pores, at concentrations above about 0.01 M, the apparent viscosity is essentially equal to the bulk viscosity. The physical explanation of the observed phenomena is as follows. Upon increasing the salt concentration, the Debye length (ÿ1) decreases. Therefore the `effective' pore size increases and approaches the real pore size; the water ¯ux increases accordingly. A similar explanation can be given, using Eq. (2). Increasing the salt concentration increases the dimensionless parameter r, so that the right-hand side of Eq. (2) approaches unity. Therefore the apparent viscosity decreases and approaches the bulk viscosity. At very high salt concentrations (0.1 M) the ¯ux decreased again, since high salt concentrations cause notable increases in the bulk viscosity. Using Darcy's law, the membrane resistance can be determined from the high-salinity measurements; this resistance can
then be used to calculate the apparent viscosities at lower salt concentrations. Zeta-potentials are then calculated using Eq. (2), which results in absolute values without a sign. Determining zeta-potentials at increasing pH will ®rst result in a decrease in the absolute value to zero (the iso-electric point) and then an increase in the absolute value. A negative sign is then added to the values obtained above the isoelectric point. Another series of measurements was performed at constant salt concentration (110ÿ3 M) while increasing the pH from about 3.5 to about 10. Typical results are shown in Fig. 2. The observed drift in apparent viscosity for pH8.5 and pH9.2 was caused by a drift in pH at these pH values. From Eq. (2) it is clear that the electroviscous effect causes a/0 to be greater than 1.00 at all pH values except for pH near the iso-electric point, where a/01.00. This corresponds well with the results shown in Fig. 2. The iso-electric point can then be estimated from Fig. 2 to be roughly 7.6. Both the streaming potential measurements and the electroviscosity measurements were performed on two membranes of each material. The reproducibility of the data was good. Results from only one membrane of each material are shown here. 3. Results and discussion Zeta-potentials were determined for a-alumina, zirconia, and titania membranes, both for `virgin'
I.H. Huisman et al. / Journal of Membrane Science 147 (1998) 187±194
Fig. 3. Zeta-potentials obtained from streaming potential measurements for an a-alumina membrane and a zirconia membrane, both before using (virgin state) and after using and cleaning (used membrane). The salt concentration is 0.001 M.
(new) membranes and for `used' membranes. The virgin membranes had not been used for ®ltration, nor had they been cleaned with detergent prior to determining the zeta-potential. The only pre-treatment they had received was overnight conditioning in MilliQ water or in a KCl solution. After determining the streaming potentials for these membranes, they were used for several ®ltration experiments with silica particles and they were cleaned several times with 1% Ultrasil 10 (Henkel, Germany) at 558C. After several months the zeta-potentials of these membranes, now designated `used' membranes, were determined again. The used membranes were cleaned, and thoroughly rinsed with MilliQ water prior to zeta-potential measurements. Zeta-potentials derived from streaming potential measurements are shown in Fig. 3, both for virgin and for used a-alumina and zirconia membranes. It can be seen that the membranes in their virgin state have high iso-electric points. After use, the iso-electric points of the membranes decreased. Experiments on virgin and used titania membranes did not show any signi®cant decrease in iso-electric point. The Ag/AgCl electrodes used for the streaming potential measurements cannot be used at pH8. For the new a-alumina and zirconia membranes no iso-electric points could therefore be determined. For a fair comparison between the electroviscosity method and the streaming potential method, membranes should be used for which both methods can at least measure the iso-electric point. Zeta-potentials of the
191
Fig. 4. Zeta-potentials of a used a-alumina membrane at a salt concentration of 0.001 M.
Fig. 5. Zeta-potential of a used zirconia membrane at a salt concentration of 0.001 M.
`used' membranes, which have iso-electric points of around 6, were therefore used to compare the two methods. Zeta-potentials of used a-alumina, zirconia, and titania membranes are shown for a salt concentration of 10ÿ3 M as a function of pH in Figs. 4±6, respectively. In each of these ®gures, the results calculated from both streaming potential measurements and electroviscosity measurements are shown. The obtained zeta-potentials decreased with increasing pH as would be expected for a surface with acid±base groups. Comparing the results obtained from the two methods, it can be seen that the zeta-potential curves are similar, and that the iso-electric points are very similar. However, the absolute values of the zetapotentials differ strongly; the streaming potential measurements giving the lower values as expected.
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I.H. Huisman et al. / Journal of Membrane Science 147 (1998) 187±194 Table 1 Iso-electric point obtained with two methods, for virgin (new) membranes, and for used and cleaned membranes Membrane a-Alumina Zirconia Titania
Fig. 6. Zeta-potential of a used titania membrane at a salt concentration of 0.001 M.
It is well known that zeta-potentials obtained from streaming potential measurements through membrane pores are generally underpredicted [6,14,16]. The electroviscosity measurements performed with the titania membranes show a pH range in which the zeta-potential is zero, instead of one distinct isoelectric point (Fig. 6). This is due to the fact that electroviscosity measurements are not very sensitive at low zeta-potentials. This low sensitivity at low zetapotentials is inherent in the method, and is caused by the non-linear dependence of the apparent viscosity on the zeta-potential (see Eq. (2)). This means in practice that there is a threshold value for zeta-potential measurements, which in our case is about 12 mV. Zetapotentials below this do not give a measurable electroviscous effect. The iso-electric pH range observed in Fig. 6 should therefore be seen as a range in which the zeta-potentials are lower than 12 mV. A consequence of the low accuracy at low zeta-potentials is that the iso-electric point can only be estimated with limited accuracy. For zeta-potentials greater than the threshold value the accuracy is much higher. The inaccuracy is estimated to be 30% of which the major part is caused by inaccuracy in the pore size. Table 1 shows the iso-electric points obtained from streaming potential measurements and from the electroviscous technique. This table illustrates the good agreement between the two different methods, which strengthens our con®dence in the newly developed electroviscous technique. Observed values of iso-electric point are thus 8.5 for the a-alumina membranes, 8.0 for the zirconia membranes, and 6.3 for the titania
New Used New Used New Used
Streaming potential (0.2)
Electroviscous (0.4)
>8 6.4 8.0 5.2 6.3 6.2
8.5 6.0 7.8 5.4 6.0 5.8
membranes. All of these values are realistic when compared to respective literature values [17], [18,19], [17,20] respectively. All these obtained isoelectric points are slightly higher than the values provided by the manufacturer [21]. Table 1 also shows the decrease in iso-electric points observed for the a-alumina and the zirconia membrane, but not for the titania membrane. It was suspected that this decrease in iso-electric point was caused by the cleaning agent, as it has been observed by others that cleaning polymeric ultra®ltration membranes with Ultrasil 10 decreases the iso-electric point [22]. Ultrasil 10 is an alkaline cleaning agent containing surfactants. The decrease in iso-electric point could be caused by a chemical alteration of the surface, induced by the high pH and high temperatures; it could also be caused by adsorption of surfactants onto the membrane surface, changing its properties. In order to establish which of these factors caused the observed phenomena, the following series of experiments was carried out. The iso-electric point was determined for a virgin zirconia membrane. This membrane then underwent different `cleaning' procedures. After every procedure the iso-electric point of the membrane was determined by electroviscous measurements. The membrane was ®rst treated with 1% NaOH at 558C for 1 h. It was then `cleaned' with 1% Ultrasil at 558C for 1 h. Subsequently, the membrane was treated with 1% HNO3 at 558C for 2 h, and ®nally it was treated with 1% NaOH at 558C for 1 h. Results are given in Table 2. Treatment with NaOH did not change the iso-electric point of the membrane, whereas subsequent treatment with Ultrasil 10 had a considerable effect. This supports the hypothesis that the change in iso-electric point is caused by adsorption of surfactants onto the
I.H. Huisman et al. / Journal of Membrane Science 147 (1998) 187±194 Table 2 Iso-electric point for the same zirconia membrane after different treatments Treatment
Iso-electric point (0.4)
Virgin membrane 1% NaOH 1% Ultrasil 10 1% HNO3 1% NaOH
7.8 8.0 5.8 6.2 6.2
membrane surface. This adsorption seems to be irreversible, as neither cleaning with HNO3, nor cleaning with NaOH could restore the iso-electric point to its original value. A conclusion that can be drawn from these ®ndings is that iso-electric points provided by membrane manufacturers are often of limited value, as in most micro®ltration applications cleaning agents containing surfactants are used, which are likely to irreversibly change the properties of the membrane surface. 4. Conclusions Combining electroviscosity theory with water ¯ux measurements at different values of pH and salt concentration provides a simple and fast method of determining zeta-potentials for micro®ltration membranes. 1 Zeta-potentials of various 0.2 mm Membralox ceramic micro®ltration membranes were determined both by the electroviscous method and by streaming potential measurements. The iso-electric points obtained with the two methods were similar. However, zeta-potentials obtained with the electroviscous technique were higher than those obtained with streaming potential measurements, which agrees with expectations, since streaming potential measurements are known to result in systematic underprediction of the zeta-potential. Streaming potential measurements, however, show low scatter and are therefore very suitable for relative measurements and for determining the iso-electric point. Electroviscosity measurements show greater scatter, especially at zetapotentials below 25 mV. This makes them less suitable for the exact determination of the iso-electric point. For pH values away from the iso-electric point, the
193
zeta-potential values obtained from electroviscosity measurements are probably the most accurate. Cleaning the membranes with Ultrasil 10 resulted in an irreversible decrease in the iso-electric point for aalumina and zirconia membranes, whereas the isoelectric point of titania membranes remained virtually constant. This decrease was probably caused by adsorption of surfactants onto the membrane, as no decrease was observed when the membranes were cleaned with NaOH solutions. 5. Notation E e F G J K k r T TMP "0 "r 0 a
potential (V) electron charge (C) parameter in Eq. (2) (dimensionless) parameter in Eq. (2) (dimensionless) permeate flux (m sÿ1) conductivity ( ÿ1 mÿ1) Boltzmann constant (J Kÿ1) pore radius (m) temperature (K) transmembrane pressure (Pa) electrolyte parameter (dimensionless) permittivity of free space (C2 Jÿ1 mÿ1) relative permittivity (dimensionless) viscosity (Pa s) bulk viscosity of electrolyte solution (Pa s) apparent viscosity of electrolyte solution in pore (Pa s) Debye constant (mÿ1) zeta-potential (V)
Acknowledgements This work was ®nancially supported by the Swedish Foundation for Membrane Technology. The streaming potential measurements presented were performed at the department of Chemical Technology at Lappeenranta University of Technology (Finland). This was made possible by the Nordic Academy for Advanced Study (NorFA) through a travel grant to I.H. Huisman within the Network for Membrane Technology. The authors would like to express their gratitude to Prof. Marianne NystroÈm for her hospitality and fruitful discussions.
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References [1] K. Rodemann, E. Staude, Electrokinetic characterization of porous membranes made from epoxidized polysulfone, J. Membr. Sci. 104 (1995) 147±155. [2] D. Elzo, I.H. Huisman, E. Middelink, V. Gekas, Charge effects on inorganic membrane performance in a crossflow microfiltration process, Colloid Surf. B 138 (1998) 145±159. [3] W.R. Bowen, D.T. Hughes, Properties of microfiltration membranes. Part 2. Adsorption of bovine serum albumin at aluminium oxide membranes, J. Membr. Sci. 51 (1990) 189± 194. [4] I.H. Huisman, D. Elzo, E. Middelink, C. TraÈgaÊrdh, Properties of the cake layer formed during crossflow microfiltration, Colloid Surf. B 138 (1998) 265±281. [5] J.A. Howell, M. NystroÈm, Fouling Phenomena, in: J.A. Howell, R. Field, V. Sanchez (Eds.), Membranes in Bioprocessing: Theory and Application, Chapman and Hall, London, 1993, pp. 203±241. [6] M. NystroÈm, M. LindstroÈm, E. Matthiasson, Streaming potential as a tool in the characterization of ultrafiltration membranes, Colloids Surfaces 36 (1989) 297±312. [7] B. Oldham, F.J. Young, J.F. Osterle, Streaming potential in small capillaries, J. Colloid Interface Sci. 18 (1963) 328±336. [8] C.C. Christoforou, G.B. Westermann-Clark, J.L. Anderson, The streaming potential and inadequacies of the Helmholtz equation, J. Colloid Interface Sci. 106 (1985) 1±11. [9] S.S. Dukhin, B.V. Derjaguin, Electrokinetic Phenomena, in: E. Matijevic (Ed.), Surface and Colloid Science, vol. 7, Wiley, New York, 1974. [10] S. Levine, J.R. Marriott, G. Neale, N. Epstein, Theory of electrokinetic flow in fine cylindrical capillaries at high zetapotentials, J. Colloid Interface Sci. 52 (1975) 136±149. [11] W.R. Bowen, R.J. Cooke, Properties of microfiltration membranes: Computer automated determination of the electrokinetic properties of polycarbonate membranes, J. Colloid Interface Sci. 141 (1991) 280±287.
[12] I.H. Huisman, B. DutreÂ, K.M. Persson, G. TraÈgaÊrdh, Water permeability in ultrafiltration and microfiltration: Viscous and electroviscous effects, Desalination 113 (1997) 95± 103. [13] W.R. Bowen, F. Jenner, Electroviscous effects in charged capillaries, J. Colloid Interface Sci. 173 (1995) 388±395. [14] M. NystroÈm, A. PihlajamaÈki, N. Ehsani, Characterization of ultrafiltration membranes by simultaneous streaming potential and flux measurements, J. Membr. Sci. 87 (1994) 245±256. [15] I.H. Huisman, D. Johansson, G. TraÈgaÊrdh, C. TraÈgaÊrdh, Design of a crossflow microfiltration unit for studies of flux and particle transport, Trans. IChemE. 75 A (1997) 508-512. [16] K.J. Kim, A.G. Fane, M. NystroÈm, A. PihlajamaÈki, W.R. Bowen, H. Mukhtar, Evaluation of electroosmosis and streaming potential for measurement of electric charges of polymeric membranes, J. Membr. Sci. 116 (1996) 149± 159. [17] M. Mullet, P. Fievet, J.C. Reggiani, J. Pagetti, Surface electrochemical properties of mixed oxide ceramic membranes: Zeta-potential and surface charge density, J. Membr. Sci. 123 (1997) 255±265. [18] M. Prica, S. Biggs, F. Grieser, T.W. Healy, Effect of calcination temperature on the electrokinetic properties of colloidal zirconia, Colloid Surf. A 119 (1996) 205±213. [19] J. Randon, Influence de l'interface oxyde-solution sur les performances des membranes d'ultrafiltration en zircone, Ph.D. Thesis, Universite de Montpellier II (1991). [20] H. Yotsumoto, R.-H. Yoon, Application of extended DLVO theory, J. Colloid Interface Sci. 157 (1993) 426±433. [21] P. Chanaud, R. Soria, Streaming potential study of surface 1 modified Membralox ceramic membranes, Euromembrane '97, Twente, Netherlands, 1997. [22] M. NystroÈm, H. Zhu, Characterization of cleaning results using combined flux and streaming potential methods, J. Membr. Sci. 131 (1997) 195±205.
Determining the zeta-potential of ceramic micro®ltration membranes using the electroviscous effect Ingmar H. Huismana,*, Gun TraÈgaÊrdha, Christian TraÈgaÊrdha, Arto PihlajamaÈkib a
Food Engineering, Lund University, PO Box 124, 221 00 Lund, Sweden Department of Chemical Technology, Lappeenranta University of Technology, PO Box 20, 53851 Lappeenranta, Finland
b
Received 25 September 1997
Abstract The possibility of measuring the zeta-potentials of porous membranes using the electroviscous effect was investigated. The 1 zeta-potential of Membralox ceramic micro®ltration membranes was determined both with the newly developed electroviscous technique and by streaming potential measurements. It was found that the electroviscous technique provided a simple means of obtaining accurate values of zeta-potential, especially for higher zeta-potentials. The streaming potential measurements were found to be more suitable for the determination of the iso-electric point, i.e. the pH at which the zetapotential is zero. The iso-electric points of new a-alumina, zirconia, and titania membranes were found to be 8.5, 8.0, and 6.3, respectively. Upon using the membranes and cleaning them with a detergent, the iso-electric point of the a-alumina membrane decreased to 6.5, and that of the zirconia membrane decreased to 5.2, while the iso-electric point of the titania membrane stayed virtually constant. Cleaning these membranes with a strong acid or base could not reverse the observed decreases in iso-electric point. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Zeta-potential; Electroviscosity; Micro®ltration; Ceramic membranes
1. Introduction The main factor limiting the application of cross¯ow micro®ltration is fouling of the membranes, which reduces the ¯ux and impairs the separation properties. The surface characteristics of the membrane material, such as zeta-potential, surface charge density, and hydrophobicity, are among the factors that in¯uence membrane fouling [1].
*Corresponding author. Tel.: +46 46 222 9820; fax: +46 46 222 4622; e-mail: [email protected] 0376-7388/98/$19.00 # 1998 Elsevier Science B.V. All rights reserved. PII: S0376-7388(98)00135-5
The zeta-potential is an important measure of particle±particle and particle±surface interactions. These interactions have been shown to be of importance to membrane performance. Both the amount of fouling [1±3] and the reversibility of fouling [4] are dependent on the zeta-potentials of the feed suspension particles and of the membrane surface. For example, it has been shown that negatively charged paints foul negatively charged membranes less than positively charged paints do [5]. It is clear from this example that the sign of the zeta-potential (positive or negative) is often more important than its absolute value. Many membranes have a positive zeta-potential at low pH and a
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negative zeta-potential at high pH. The pH at which the transition from positive to negative occurs is called the iso-electric point. The iso-electric point is in general obtained from zeta-potential measurements at different pH. Although the zeta-potential of particles can be determined easily and accurately by electrophoretic techniques, there is, as yet, no easy and accurate way to determine the zeta-potential of membranes. The traditional way of determining the zeta-potential for ultra®ltration and micro®ltration membranes is to measure the streaming potential E created by a pressure drop over the membrane (transmembrane pressure, TMP), and to calculate the zeta-potential using the Helmholtz±Smoluchowski equation E "0 "r ; TMP K
(1)
where "0 is the permittivity of free space, "r the relative permittivity of the solvent, the viscosity of the solution, K the conductivity of the solution, and is the zeta-potential of the membrane [6]. Eq. (1) is valid only as long as the Debye length of the solution is small compared to the radius of the pores [7,8]. This condition is ful®lled for the micro®ltration membranes and ionic strengths considered in the current paper. The values that are to be inserted into Eq. (1) for the three solution properties, "r, , and K, are the effective local values in the membrane pore. These values are generally dif®cult to determine. Most authors use the values as observed for the bulk solution. For "r this is acceptable, since "r near surfaces hardly differs from "r in the bulk phase [9]. However, the values of and K near surfaces may be signi®cantly different from those observed in the bulk solution [7,8]. If an electrolyte solution is pressed through a capillary with charged surfaces, ions are moved away from their preferred positions in the double layer. This costs extra energy and can be described as an increase in the apparent viscosity. This phenomenon is referred to as the electroviscous effect. The conductivity of a solution is strongly dependent on the concentration of ions. Since the local ionic concentration in the pores is higher than that in the bulk solution, the conductivity within the pores increases. The surface of the pores may also contribute to the total conductivity. The difference between the effective conductivity in the
pore and the bulk conductivity is referred to as surface conductivity. Although the in¯uence of electroviscosity can be calculated [10], no general method is available to determine the in¯uence of surface conductivity on the zeta-potential. Therefore zeta-potentials obtained from Eq. (1) only have a qualitative value (the zeta-potential is generally underpredicted). Another disadvantage of the streaming potential method is that the electrodes used are often intolerant towards aggressive chemicals. For example: Ag/AgCl electrodes, which are commonly used, are not stable at pH values higher than 8. An alternative method of determining the zetapotential of membranes is to perform electro-osmotic measurements [11]. These measurements are based on a principle similar to that of streaming potential measurements. While for streaming potential measurements the electric voltage induced by a given pressure is measured, in electro-osmosis, the electrolyte ¯ow through the pores induced by an electrical ®eld is measured. Using electro-osmosis to measure the membrane zeta-potential has the same disadvantages as using streaming potential measurements. A third method of determining the zeta-potential of membranes is to perform electrophoretic measurements on particles made of the same material as the membrane [2]. Although electrophoretic measurements on particles give a more reliable value of the zeta-potential than do streaming potential or electroosmosis measurements, the disadvantage of the electrophoretic technique is that measurements are not made on the membrane itself; the characteristics of the particles may be different from those of the membrane. Even if the particles are obtained by grinding a membrane, it cannot be guaranteed that the surface properties of these particles are representatives of the surface of the active layer of the membrane [5]. Recently, a new method has been developed for the determination of the zeta-potential of membranes, employing the electroviscous effect [12]. Levine et al. [10] showed that the apparent viscosity in a cylindrical pore is related to the zeta-potential of the pore surface as a 0
1ÿ
8 e=kT2 1 ÿ GF kr2
!ÿ1 ;
(2)
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where a is the apparent viscosity, 0 the bulk viscosity of the electrolyte solution, the zeta-potential of the capillary surface, the Debye constant, and r is the capillary radius. G and F are dependent on and on r, and can be determined from graphs given in [10]. For large values of r, G0 and F1. is a dimensionless parameter describing the properties of the electrolyte [10,12]. For aqueous monovalent salt solutions at 208C, 35.8/ (where denotes the molar conductivity in ÿ1 cm2 molÿ1). The validity of the theory presented by Levine et al. (including Eq. (2)) has recently been demonstrated through an exact numerical solution of the governing equations [13]. The apparent viscosity is easily obtained from water ¯ux measurements at constant transmembrane pressure, the Debye constant is calculated from the salt concentration, the conductivity can be measured, and the pore radius can either be measured or is known from information supplied by the manufacturer. Thus all variables in Eq. (2) are known, except for , and G and F as they depend on . The zeta-potential can then be calculated from Eq. (2) by an iterative method. The great advantage of this method is that no special equipment is needed other than a membrane rig that allows measurements of transmembrane pressure and ¯ux. Another advantage is that surface conductivity does not in¯uence the results, so the calculated zetapotentials do not suffer from systematic errors. A clear disadvantage of the technique is that knowledge of the membrane pore size is necessary for a quantitative calculation of the zeta-potential. However, knowledge of the pore-size is not necessary to determine the isoelectric point, the pH at which the zeta-potential is zero. ÿ1 is a measure for the thickness of the electrical double layer. It is shown in [10] that the electroviscous effect is greatest if the pore radius r2.5 ÿ1; the effect decreases for both smaller and larger values of r. A measurable electroviscous effect is obtained for r roughly in the interval 0.3±30 [10]. For concentrations of 10ÿ3 M monovalent salt, this corresponds to pore diameters (2r) in the range 6±600 nm, which is in the ultra®ltration and micro®ltration range. The aim of the current study is to show that electroviscosity measurements provide a simple method of determining accurate values of zeta-potential for porous membranes. Zeta-potentials of various ceramic
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micro®ltration membranes were determined by both the electroviscous method and the well-established streaming potential method, and the results compared. Different zeta-potentials were obtained for new membranes, as supplied by the manufacturer, and membranes that had been used and cleaned with a detergent. Reasons for these differences are discussed. 2. Materials and methods The micro®ltration membranes used were from the Membralox series produced by SCT (Bazet, France). The active layer of the membranes consisted of aalumina, zirconia, or titania. The membranes were tubular, had an inner diameter of 6.85 mm, a ®lter length of 0.23 m, and a pore size of 0.2 mm. All electrolyte solutions used were prepared from ROtreated and MilliQ-®ltered water, and reactant grade chemicals. Streaming potential measurements were performed with the equipment described by NystroÈm et al. [14]. This set-up contains two electrodes: one is placed in the centre of the tubular membrane, and the other outside the tube. The potential difference across the membrane was measured at different values of the transmembrane pressure, at a constant cross¯ow velocity of 0.14 m sÿ1 (Re2400). Measurements were performed at 258C. pH and salinity were adjusted by adding KOH and HCl, and KCl. Apparent zeta-potentials were calculated from measured values of E/ TMP using Eq. (1). Electroviscous effects were determined by measuring the water ¯ux at different values of pH and salt concentration. This was done in a cross¯ow micro®ltration rig described in detail elsewhere [15]. A uniform transmembrane pressure (i.e. TMP independent of the position along the membrane) was guaranteed by a circulating ¯ow on the permeate side, which created a pressure drop equal to the pressure drop on the feed side. Measurements were performed at 208C, the cross¯ow velocity was 5 m sÿ1, and the transmembrane pressure about 20 kPa. pH and salinity were adjusted by adding NaOH and HCl, and NaCl. Some experiments were performed with potassium salts (KOH, KCl), but the type of salt (sodium or potassium) was found to have little effect on the observed zeta-potentials.
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Fig. 1. Flux of water containing different salt concentrations at pH5 for an a-alumina membrane.
Fig. 2. Relative apparent viscosity versus time at different pH values, for a new zirconia membrane.
The electroviscous measurements are very sensitive to small temperature ¯uctuations, as viscosity depends strongly on temperature. The temperature was controlled within 0.58C, but recorded with a higher accuracy. Measured ¯uxes were then corrected for variation in viscosity caused by the observed temperature ¯uctuations. During some of the measurements, the pH was kept constant while the salt concentration was increased stepwise from 110ÿ4 to 0.1 M. Increasing the salinity initially led to an increase in ¯ux (see Fig. 1). At higher salt concentrations the ¯ux levelled off, and became independent of the salinity. This is in agreement with Eq. (2), which predicts that for 0.2 mm pores, at concentrations above about 0.01 M, the apparent viscosity is essentially equal to the bulk viscosity. The physical explanation of the observed phenomena is as follows. Upon increasing the salt concentration, the Debye length (ÿ1) decreases. Therefore the `effective' pore size increases and approaches the real pore size; the water ¯ux increases accordingly. A similar explanation can be given, using Eq. (2). Increasing the salt concentration increases the dimensionless parameter r, so that the right-hand side of Eq. (2) approaches unity. Therefore the apparent viscosity decreases and approaches the bulk viscosity. At very high salt concentrations (0.1 M) the ¯ux decreased again, since high salt concentrations cause notable increases in the bulk viscosity. Using Darcy's law, the membrane resistance can be determined from the high-salinity measurements; this resistance can
then be used to calculate the apparent viscosities at lower salt concentrations. Zeta-potentials are then calculated using Eq. (2), which results in absolute values without a sign. Determining zeta-potentials at increasing pH will ®rst result in a decrease in the absolute value to zero (the iso-electric point) and then an increase in the absolute value. A negative sign is then added to the values obtained above the isoelectric point. Another series of measurements was performed at constant salt concentration (110ÿ3 M) while increasing the pH from about 3.5 to about 10. Typical results are shown in Fig. 2. The observed drift in apparent viscosity for pH8.5 and pH9.2 was caused by a drift in pH at these pH values. From Eq. (2) it is clear that the electroviscous effect causes a/0 to be greater than 1.00 at all pH values except for pH near the iso-electric point, where a/01.00. This corresponds well with the results shown in Fig. 2. The iso-electric point can then be estimated from Fig. 2 to be roughly 7.6. Both the streaming potential measurements and the electroviscosity measurements were performed on two membranes of each material. The reproducibility of the data was good. Results from only one membrane of each material are shown here. 3. Results and discussion Zeta-potentials were determined for a-alumina, zirconia, and titania membranes, both for `virgin'
I.H. Huisman et al. / Journal of Membrane Science 147 (1998) 187±194
Fig. 3. Zeta-potentials obtained from streaming potential measurements for an a-alumina membrane and a zirconia membrane, both before using (virgin state) and after using and cleaning (used membrane). The salt concentration is 0.001 M.
(new) membranes and for `used' membranes. The virgin membranes had not been used for ®ltration, nor had they been cleaned with detergent prior to determining the zeta-potential. The only pre-treatment they had received was overnight conditioning in MilliQ water or in a KCl solution. After determining the streaming potentials for these membranes, they were used for several ®ltration experiments with silica particles and they were cleaned several times with 1% Ultrasil 10 (Henkel, Germany) at 558C. After several months the zeta-potentials of these membranes, now designated `used' membranes, were determined again. The used membranes were cleaned, and thoroughly rinsed with MilliQ water prior to zeta-potential measurements. Zeta-potentials derived from streaming potential measurements are shown in Fig. 3, both for virgin and for used a-alumina and zirconia membranes. It can be seen that the membranes in their virgin state have high iso-electric points. After use, the iso-electric points of the membranes decreased. Experiments on virgin and used titania membranes did not show any signi®cant decrease in iso-electric point. The Ag/AgCl electrodes used for the streaming potential measurements cannot be used at pH8. For the new a-alumina and zirconia membranes no iso-electric points could therefore be determined. For a fair comparison between the electroviscosity method and the streaming potential method, membranes should be used for which both methods can at least measure the iso-electric point. Zeta-potentials of the
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Fig. 4. Zeta-potentials of a used a-alumina membrane at a salt concentration of 0.001 M.
Fig. 5. Zeta-potential of a used zirconia membrane at a salt concentration of 0.001 M.
`used' membranes, which have iso-electric points of around 6, were therefore used to compare the two methods. Zeta-potentials of used a-alumina, zirconia, and titania membranes are shown for a salt concentration of 10ÿ3 M as a function of pH in Figs. 4±6, respectively. In each of these ®gures, the results calculated from both streaming potential measurements and electroviscosity measurements are shown. The obtained zeta-potentials decreased with increasing pH as would be expected for a surface with acid±base groups. Comparing the results obtained from the two methods, it can be seen that the zeta-potential curves are similar, and that the iso-electric points are very similar. However, the absolute values of the zetapotentials differ strongly; the streaming potential measurements giving the lower values as expected.
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I.H. Huisman et al. / Journal of Membrane Science 147 (1998) 187±194 Table 1 Iso-electric point obtained with two methods, for virgin (new) membranes, and for used and cleaned membranes Membrane a-Alumina Zirconia Titania
Fig. 6. Zeta-potential of a used titania membrane at a salt concentration of 0.001 M.
It is well known that zeta-potentials obtained from streaming potential measurements through membrane pores are generally underpredicted [6,14,16]. The electroviscosity measurements performed with the titania membranes show a pH range in which the zeta-potential is zero, instead of one distinct isoelectric point (Fig. 6). This is due to the fact that electroviscosity measurements are not very sensitive at low zeta-potentials. This low sensitivity at low zetapotentials is inherent in the method, and is caused by the non-linear dependence of the apparent viscosity on the zeta-potential (see Eq. (2)). This means in practice that there is a threshold value for zeta-potential measurements, which in our case is about 12 mV. Zetapotentials below this do not give a measurable electroviscous effect. The iso-electric pH range observed in Fig. 6 should therefore be seen as a range in which the zeta-potentials are lower than 12 mV. A consequence of the low accuracy at low zeta-potentials is that the iso-electric point can only be estimated with limited accuracy. For zeta-potentials greater than the threshold value the accuracy is much higher. The inaccuracy is estimated to be 30% of which the major part is caused by inaccuracy in the pore size. Table 1 shows the iso-electric points obtained from streaming potential measurements and from the electroviscous technique. This table illustrates the good agreement between the two different methods, which strengthens our con®dence in the newly developed electroviscous technique. Observed values of iso-electric point are thus 8.5 for the a-alumina membranes, 8.0 for the zirconia membranes, and 6.3 for the titania
New Used New Used New Used
Streaming potential (0.2)
Electroviscous (0.4)
>8 6.4 8.0 5.2 6.3 6.2
8.5 6.0 7.8 5.4 6.0 5.8
membranes. All of these values are realistic when compared to respective literature values [17], [18,19], [17,20] respectively. All these obtained isoelectric points are slightly higher than the values provided by the manufacturer [21]. Table 1 also shows the decrease in iso-electric points observed for the a-alumina and the zirconia membrane, but not for the titania membrane. It was suspected that this decrease in iso-electric point was caused by the cleaning agent, as it has been observed by others that cleaning polymeric ultra®ltration membranes with Ultrasil 10 decreases the iso-electric point [22]. Ultrasil 10 is an alkaline cleaning agent containing surfactants. The decrease in iso-electric point could be caused by a chemical alteration of the surface, induced by the high pH and high temperatures; it could also be caused by adsorption of surfactants onto the membrane surface, changing its properties. In order to establish which of these factors caused the observed phenomena, the following series of experiments was carried out. The iso-electric point was determined for a virgin zirconia membrane. This membrane then underwent different `cleaning' procedures. After every procedure the iso-electric point of the membrane was determined by electroviscous measurements. The membrane was ®rst treated with 1% NaOH at 558C for 1 h. It was then `cleaned' with 1% Ultrasil at 558C for 1 h. Subsequently, the membrane was treated with 1% HNO3 at 558C for 2 h, and ®nally it was treated with 1% NaOH at 558C for 1 h. Results are given in Table 2. Treatment with NaOH did not change the iso-electric point of the membrane, whereas subsequent treatment with Ultrasil 10 had a considerable effect. This supports the hypothesis that the change in iso-electric point is caused by adsorption of surfactants onto the
I.H. Huisman et al. / Journal of Membrane Science 147 (1998) 187±194 Table 2 Iso-electric point for the same zirconia membrane after different treatments Treatment
Iso-electric point (0.4)
Virgin membrane 1% NaOH 1% Ultrasil 10 1% HNO3 1% NaOH
7.8 8.0 5.8 6.2 6.2
membrane surface. This adsorption seems to be irreversible, as neither cleaning with HNO3, nor cleaning with NaOH could restore the iso-electric point to its original value. A conclusion that can be drawn from these ®ndings is that iso-electric points provided by membrane manufacturers are often of limited value, as in most micro®ltration applications cleaning agents containing surfactants are used, which are likely to irreversibly change the properties of the membrane surface. 4. Conclusions Combining electroviscosity theory with water ¯ux measurements at different values of pH and salt concentration provides a simple and fast method of determining zeta-potentials for micro®ltration membranes. 1 Zeta-potentials of various 0.2 mm Membralox ceramic micro®ltration membranes were determined both by the electroviscous method and by streaming potential measurements. The iso-electric points obtained with the two methods were similar. However, zeta-potentials obtained with the electroviscous technique were higher than those obtained with streaming potential measurements, which agrees with expectations, since streaming potential measurements are known to result in systematic underprediction of the zeta-potential. Streaming potential measurements, however, show low scatter and are therefore very suitable for relative measurements and for determining the iso-electric point. Electroviscosity measurements show greater scatter, especially at zetapotentials below 25 mV. This makes them less suitable for the exact determination of the iso-electric point. For pH values away from the iso-electric point, the
193
zeta-potential values obtained from electroviscosity measurements are probably the most accurate. Cleaning the membranes with Ultrasil 10 resulted in an irreversible decrease in the iso-electric point for aalumina and zirconia membranes, whereas the isoelectric point of titania membranes remained virtually constant. This decrease was probably caused by adsorption of surfactants onto the membrane, as no decrease was observed when the membranes were cleaned with NaOH solutions. 5. Notation E e F G J K k r T TMP "0 "r 0 a
potential (V) electron charge (C) parameter in Eq. (2) (dimensionless) parameter in Eq. (2) (dimensionless) permeate flux (m sÿ1) conductivity ( ÿ1 mÿ1) Boltzmann constant (J Kÿ1) pore radius (m) temperature (K) transmembrane pressure (Pa) electrolyte parameter (dimensionless) permittivity of free space (C2 Jÿ1 mÿ1) relative permittivity (dimensionless) viscosity (Pa s) bulk viscosity of electrolyte solution (Pa s) apparent viscosity of electrolyte solution in pore (Pa s) Debye constant (mÿ1) zeta-potential (V)
Acknowledgements This work was ®nancially supported by the Swedish Foundation for Membrane Technology. The streaming potential measurements presented were performed at the department of Chemical Technology at Lappeenranta University of Technology (Finland). This was made possible by the Nordic Academy for Advanced Study (NorFA) through a travel grant to I.H. Huisman within the Network for Membrane Technology. The authors would like to express their gratitude to Prof. Marianne NystroÈm for her hospitality and fruitful discussions.
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