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Zeta-potential and rejection rates of a polyethersulfone nanofiltration membrane in single salt solutions
Journal of Membrane Science 165 (2000) 251–259
Zeta-potential and rejection rates of a polyethersulfone nanofiltration membrane in single salt solutions Mathias Ernst a , Alexander Bismarck b , Jürgen Springer b , Martin Jekel a,∗ a
b
Technische Universität Berlin, Institut für Technischen Umweltschutz, Sekr. KF 4, Straße des 17 Juni 135, D-10623 Berlin, Germany Technische Universität Berlin, Institut für Technische Chemie, Fachgebiet Makromolekulare Chemie, Sekr. TC6, Straße des 17 Juni 135, D-10623 Berlin, Germany Received 15 April 1999; received in revised form 15 July 1999; accepted 15 July 1999
Abstract The zeta-potentials (ζ -potentials) of a polyethersulfone nanofiltration (NF) membrane (PES 10, Celgard GmbH) were determined in single salt solutions (Na2 SO4 and KCl-electrolytes) at different concentrations and pH values and results were compared with measured rejections rates of the electrolytes in the same aqueous medium. With increasing Na2 SO4 concentration the rejection rate reaches a maximum at 8 × 10−5 mol/l, which corresponds to the negative maximum of the ζ -potential at rising electrolyte concentrations. Selective adsorption of the SO2− 4 anions were found to be responsible for increasing rejection rate in the lower concentration range (up to 1 × 10−3 mol/l). However, the absolute value of the ζ -potential not directly relates to the determined rejection rate of Na2 SO4 . Increasing the Na2 SO4 concentration results in a sign reversal of the measured ζ -potential, which indicates the specific adsorption of Na+ . This was also proved by the shift of the isoelectric point (i.e.p.) to higher pH-values with increasing Na2 SO4 concentration. At the membrane’s i.e.p. the observed rejection rates were not minimal. For a KCl solution it was shown that there is no preferential adsorption of the ions, neither Cl− nor K+ , on the membrane surface, which leads to the conclusion that in this case the electrostatic properties of the membrane are only affected by the dissociation of the surface sulfonic acids. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Nanofiltration; Salt rejection; Charged membranes; Zeta-potential; Specific adsorption
1. Introduction In the past 10 years nanofiltration (NF) has become a widely used membrane technology often replacing reverse osmosis (RO) because of lower energy consumption and higher flux rates, which is usually attributed to larger pore sizes of the material [1]. ∗ Corresponding author. Tel.: +49-30-314-25058/23339; fax: +49-30-314- 23850. E-mail address: [email protected] (M. Jekel).
With a molecular weight cut-off (MWCO) range between 200 and 10000 g/mol NF lies between the classical non-porous RO membranes, where separation is mainly a function of solution diffusion coefficient of the dissolved organic and/or inorganic species and the ultrafiltration (UF), which is generally regarded as a pore filtration technology. However, alongside the molecular sieve effect, NF membranes have a second separation mechanism, originating in charge effects by the dissociation of surface groups such as sulfonated or carboxyl acids [2].
0376-7388/00/$ – see front matter ©2000 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 6 - 7 3 8 8 ( 9 9 ) 0 0 2 3 8 - 0
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Depending on the pH and concentration of the feed solution the surface charge forms an electrostatic potential that interacts with the ions in solution. The membrane surface is usually negatively charged which produces a repulsive force on anions and on cations because of the maintenance of electroneutrality. This special electrostatic interaction is the reason for better rejection rates of divalent anions, while single-charged ions can usually permeate the NF membranes [3,4]. Although NF-membranes are finding increased applications in various fields, their transport mechanism is not yet well understood. Several studies have used different models to understand the rejection behaviour of NF membranes as a function of pore size distribution, diffusion coefficients, concentration polarisation and surface charge densities and it was shown that predictions for single electrolyte solutions in most cases were accurate, however, the accuracy decreases rapidly for mixtures [5–7]. In these studies the Donnan equation and the extended Nernst–Planck equation have often been applied to the model of the microporous membrane to describe the influence of the electrical properties of the membrane surface and to calculate the transport of ions to the membrane’s surface and through the pores according to the strength of the electrostatic field [8]. The model considers the contribution of electrostatic repulsive force on ion transport by a the effective volume charge density 8X of the membranes surface. 8X (and another parameter needed for rejection calculation: the ratio of membrane porosity to membrane thickness /δ) can be determined by fitting the rejection rates of a membrane in dependence of the concentration of a single electrolyte (e.g. NaCl) [9]. The relationship between 8X and the electrolyte concentration is usually assumed to be linear in a ln–ln diagram and calculated by means of an Freundlich isotherm [10]. Calculated Freundlich parameters are generalised subsequently to other single electrolytes (Na2 (SO4 )) to model the rejection rate in the different aqueous solution with good agreement [7,10]. However, Hagmeyer and Gimbel showed that the fitting parameter 8X is not only a function of electrolyte concentration but also of the pH-value of the solution. They determined 8X via the surface charge σ by measuring the ζ -potential of the membrane surface in dependence of pH and used the parameter successfully for rejection calculations [11].
The aim of the presented study was to measure the ζ -potential of a commercial NF membrane (PES 10, Celgard GmbH, Wiesbaden) with varying pH and electrolyte concentration and to compare results directly with rejection experiments performed in the same aqueous solution. Assuming that the volume charge density will correlate with the determined ζ -potential, it is possible to check the influence of the electrostatic properties of the membrane surface on the rejection characteristics directly. By using this practical approach, the contribution of the charge density to the overall rejection performance of the investigated NF-membrane can be determined. However, it should be mentioned that the electrical properties of a membrane depend not only on the ζ -potential of membranes surface groups, but also on the pore charge distribution, which is difficult to determine directly, especially in the narrow pore range of NF-membranes [12]. In a first approach, it can be assumed that the effective volume charge density inside the membrane pores do not differ greatly from the surface one [6] and therefore, the measured ζ -potential of the membrane surface can be equated to the ζ -potential inside the pores. However, the differences of electrical ‘pore properties’ and electrical ‘surface properties’ are a function of pore radius. The electrochemical double layer overlapping which occurs in narrow pores and the resulting higher solution concentrations in the pores in comparison to the bulk one are the reason for this behaviour [13].
2. Experimental Salt rejection experiments were carried out in a single stage 2540 element spiral wound (SW) module pilot plant (membranes surface: Amembr = 1.6 m2 ) as well as a flat sheet testing cell (196 cm2 ) plant, both operating in a recycling loop (Fig. 1). The height of the feed spacer for all membranes was 1.2 mm (47 mil, diamond). De-ionised water (average conductivity <200 S/m) was used as feed water, KCl and Na2 SO4 (for analysis ACS, Merck KgaA, Darmstadt) were added in desired concentrations. Both systems were operated at a trans-membrane pressure of 1p = 0.7 MPa (approximately 100 psi) and with a feed flow rate of Vfeed = 1.1 m3 /h (SW-configuration)
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Fig. 1. Flow diagram of NF plant for rejection experiments.
or 0.1 m3 /h (flat sheet testing cell), which corresponds to an effective cross-flow velocity of CFVeff = 0.23 m/s for both configurations (SW or flat sheet testing cell). Flow rates were measured by either rota-meter or scale. The temperature was controlled by a cooler (Haake Kryothermat 140, TP1) and kept at 20◦ C. Concentrations and rejection rates of ionic species were determined by conductivity (WTW LF 95) and pH was adjusted by addition of 0.1 M HCl, KOH (KCl-experiments) or NaOH, H2 SO4 (Na2 SO4 -experiments). The flat sheet membranes of NADIR® -NF-PES 10 were supplied by Celgard GmbH, Wiesbaden and the SW-module was purchased from the same company. The active layer of this membrane material was made of polyethersulfone with polyvinylpromi-done as co-polymer on polypropylene carrier. Celgard publishes a water permeability of 75 l/(m2 h) and a MWCO in the range of 1000 g/mol at a pressure of 0.7 MPa [14]. Bowen calculated the rejection of the PES 10 membrane in 10−3 mol/l NaCl electrolyte by assuming a pore radius of 1 nm. He also fitted the pore radius rp of the PES 5, that shows slightly higher rejection rates than PES 10 to rp = 0.72 nm by rejection experiments in Na2 SO4 solution by considering the hindered transport in the pores [15]. According to his results the pore radius of PES 10 should be in
the range of 0.7–1 nm. Prior to the experiments a new membrane was operated for at least 24 h to achieve a constant permeate pure water flux; the permeate was discharged for the first 6 h of this period. After adjusting the pH or electrolyte concentration in the experiment, the aqueous solution was filtrated for 15 min at constant permeate flux rate to establish constant chemical conditions on the membrane’s surface before taking samples. The ζ -potentials were determined with the electrokinetic analyser (EKA, Anton Paar KG, Graz, Austria) based on the streaming potential method using flat sheets of the membrane’s material in a commercial plate or foil measuring cell. Details of the ζ -potential measuring technique and the calculation of the ζ -potential have been reported elsewhere [16–18]. Measuring the pH dependence of the ζ -potential the acidity or basicity of solid surfaces can be determined qualitatively. From the ζ = f(pH) graph curve it can be determined whether adsorption or dissociation processes predominate. Assuming that the formation of the electrical double layer is caused by the dissociation of the acidic functionalities (which can be expected for a polyethersulfone membrane), the pH dependence of the ζ -potential reaches a negative plateau in the alkaline range (Fig. 3). The plateau is caused by the complete dissociation of these acidic
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surface groups. If there is a change of the sign of the ζ -potential in the acidic range this is caused firstly by the repression of the dissociation of the acidic surface groups and secondly by the adsorption of the ions determining the surface potential (like protons). The pH value of the isoelectric point (i.e.p.), where ζ = 0 mV, is also a measure of the acidity (leading to low i.e.p.) or basicity (resulting in high i.e.p.) of a solid surface. Furthermore if the i.e.p. is a function of electrolyte concentration this is a proof for specific adsorption of anions on the membrane surface [19]. Generally it is assumed, that the correction in surface conductivity for a given pH value at a given electrolyte concentration only changes the magnitude of the ζ -potential value but does not affect the position of the i.e.p. nor the overall shape of the function ζ = f (pH). Therefore, it is not necessary to correct the surface conductivity for ζ -potential measurements in order to compare the samples qualitatively [20]. Since the reproducibility of electrokinetic measurements depends strongly on the sample quality, every measurement of the pH-dependence needs a well defined starting point. Therefore, all investigated membrane samples were rinsed 24 h before use. All measurements were performed at 20◦ C ± 1◦ C. The pH-dependence of the ζ -potential was determined at constant electrolyte concentration. An electrolyte KCl was used as well as Na2 SO4 . In order to hold the ion strength constant, we altered the pH value in the interval of pH 3–9 by addition of 0.1 M HCl or KOH solution in the case of a KCl-electrolyte or H2 SO4 and NaOH for Na2 SO4 . The streaming potential was measured with Ag/AgCl-electrodes (SE 4.2, Senortechnik Meinsberg, Waldheim/Sa., Germany) for the KCl electrolyte and platinum electrodes (MC 4.2, Senortechnik Meinsberg, Waldheim/Sa., Germany) for the Na2 SO4 electrolyte. In order to measure the ζ -potential as a function of the electrolyte concentration the analyser was filled with de-ionised water (Millipore, pH 4.6–5.1) and the measuring cell was rinsed several times until the conductivity in the measuring system fell below 300 S/m. First the water value of the ζ -potential was measured, subsequently the KCl or Na2 SO4 -concentration was raised using a digital burette (Brand, Wertheim, Germany). The system including the measuring cell was rinsed in both measuring directions before measurements.
All ζ -potentials presented in this study were not corrected in surface conductivity, since this correction (in particular if the plate/foil measuring cell is used [21]) does not take into account the often unsatisfactory results [22]. However, it is likely to suppress differences between and common features of the membranes, which can better be derived from the plots of the uncorrected data [12].
3. Results and discussion Fig. 2 displays the influence of increasing Na2 SO4 concentration on the rejection rate in the single electrolyte solution at a pH-value of 5.2. Starting at a Na2 SO4 concentration of 1 × 10−5 mol/l the rejection rate rises slightly and reaches a maximum at 8 × 10−5 mol/l. With increasing concentration of the electrolyte the rejection rate decreases continually to less than 20% at 6 × 10−3 mol/l. The experiment was performed in a SW configuration but results with a flat sheet configuration were similar. The permeability of the membrane was 2.77 × 10−11 m/(s Pa) [10 l/(m2 h bar)]. Regarding the measured Na2 SO4 concentration dependence of the ζ -potential, which equals the adsorption isotherm of the dissolved ions on the polymeric membrane [23], one can first observe, that the negative ζ -potential decreases slowly due to the excess adsorption of anions up to a Na2 SO4 -concentration of 4 × 10−4 mol/l to increase drastically to the maximum negative ζ -potential (ζ max ) at a concentration of 1 × 10−3 mol/l. Afterwards, the ζ -potential decreases again with increasing Na2 SO4 -concentration to reach the point of charge reversal (p.c.r.) at 3 × 10−3 mol/l, where the ζ -potential goes through 0 mV, to increase drastically (the positive ζ -potential) with further increase of the Na2 SO4 -concentration. This reversal of the sign of the ζ -potential is due to overcompensation by counter charge adsorption in the Stern-layer, and clearly it cannot occur if Na+ is not specifically adsorbed [24]. Due to the hydrophilic character of the PES-membrane the measured ζ -potential in distilled water is quite small (−0.5 ± 0.2 mV) [25]. Since the ζ -potential–concentration curve reflects the ion adsorption onto the membrane surface and, therefore, the negative surface charge increase in the lower Na2 SO4 concentration range up to a maximal
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Fig. 2. Rejection and ζ -potentials as function of the Na2 SO4 concentration at pH 5.2.
value (ζ max ), the repulsive electrostatic forces might also increase. Due to the selective SO2− 4 adsorption in the lower concentration range the rejection rate of Na2 SO4 increases too, and then decreases again at higher Na2 SO4 concentrations. However, the rejection rate becomes not minimal at the point of ζ -potential reversal (p.c.r.) and will also not increase again with increasing positive ζ -potential. The maximum of the rejection rate (Rmax ) depending on the Na2 SO4 concentration compared to ζ max is shifted by approximately one log unit. The shift of the maximal values (Rmax and max ) might be explained by the different experimental conditions. In the SW-module the configuration was 1p = 0.7 MPa and the water was filtered through the membrane (i.e. the flow is directed perpendicular to the membrane surface), whereas the configuration in the EKA-measuring cell was 1p = 0.015 MPa without any filtration of water (flow direction is horizontal to the membrane surface). Due to the horizontal flow direction during the ζ -potential measurements the pore-structure influence is completely excluded [12], and therefore, the measured ζ -potential does not consider the electrokinetic properties inside the pores. Regarding the structure of an asymmetric composite membrane, the obtained values from streaming potential measurements give only qualitative evidence.
With the transport of feed solution through the membrane while filtration, the concentration of ions on the surface of membrane (concentration polarisation) and especially inside the membrane pores will be higher in comparison to the bulk solution [13]. According to the EKA measurement therefore the ζ -potential in the membranes pores will be shifted to higher solution concentrations. At Na2 SO4 bulk concentrations below 1 × 10−3 mol/l this will lead to higher (more negative) ζ -potential in the pores in comparison to the surface ζ -potential measured in bulk solution concentration. As a consequence the resulting repulsive force on anions is higher and the maximum of Na2 SO4 rejection will be reached earlier (Fig. 2). The plot of Na2 SO4 rejection as function of the pH-value of the solution at different concentrations (Fig. 3) displays a strong dependence of the separation characteristics on the pH-value. With rising electrolyte concentration the rejection always decreases notably, but the decrease is more pronounced in the acidic pH range. The rejection curve seems to be shifted to higher pH values with higher concentrations. Above pH 8 the rejection rate reaches 80–90% even at higher Na2 SO4 concentrations of 1 × 10−3 mol/l. The slight decrease of rejection rates at pH 9 for 5.7 × 10−5 and
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Fig. 3. Rejection and ζ -potentials as function of pH at varying electrolyte (Na2 SO4 ) concentration.
2.6 × 10−4 mol/l can be explained by the increasing influence of the OH− ions content in the feed water on the measuring error of the concentration of Na2 SO4 by electrical conductivity. As indicated by the pH-dependence of the ζ -potential measured in the Na2 SO4 electrolyte, the PES-membrane contains a significant amount of dissociable acidic surface groups (–SO3 H) causing low i.e.p.-values. Measuring the function ζ = f(pH) at varying Na2 SO4 -electrolyte concentrations (6 × 10−5 , 3 × 10−4 and 8 × 10−4 mol/l) indicates the ‘unexpected’ preferentially (specifically) adsorption behaviour of the Na+ cations on the PES membrane. It becomes obvious, since the i.e.p. is a function of the electrolyte concentration (Fig. 3) and with increasing Na2 SO4 concentration the i.e.p. increases. If Na+ adsorbs specifically, this renders the ζ -potential more positive and the i.e.p. is restored by letting more OH− ions adsorb, i.e. by increasing the pH [24]. However, this observed behaviour coincide with the measured Na2 SO4 concentration dependence, where a sign reversal of the ζ -potential was observed at higher salt concentrations. This is an unexpected behaviour, since for the most investigated membrane materials the SO2− 4 anions are preferentially adsorbed at the membrane surface [19,26,27]. In this case the i.e.p. does not equal the point of zero charge (p.z.c.) [28]. As
can be seen from Fig. 2 the ζ plateau -values of higher Na2 SO4 -concentrations are in correspondence to the ζ -potential value obtained from the concentration dependence. The ζ -potential plateau area at lower pH values is also established earlier in comparison to higher Na2 SO4 -concentrations. As can be clearly seen, the rejection rate does not depend directly on the value of the negative ζ -potential and even at the i.e.p. the rejection rates does not reach a minimum, at positive ζ -potentials the rejection rate decreases further. The rejection rate reaches an early plateau value if the corresponding ζ -potential reaches an early plateau. The membrane filtration of KCl electrolyte solutions at different pH values shows a different behaviour than that of Na2 SO4 relative to the varied concentrations. The rejection rate decreases continuously with increasing KCl concentration (Fig. 4) over the whole range of pH, showing highest rejection rates in the alkaline pH range. In this case the coherence between ζ -potential and rejection is restricted to the fact that the highest rejection of each KCl concentration is reached in the range of highest (most negative) ζ -potential at the same electrolyte concentration. No stable plateau value of the rejection rate can be related to the relatively stable ζ -potential plateau value of −26 mV at KCl concentrations of 6 × 10−4 or 3 × 10−3 mol/l, respectively.
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Fig. 4. Rejection and ζ -potentials as function of pH at varying electrolyte (KCl) concentration.
In analogy to Na2 SO4 solutions the absolute magnitude of the measured ζ -potential cannot explain the magnitude of rejection directly. However, the i.e.p. for KCl solution (i.e.p. KCl = 3.37 ± 0.1) is lower than the i.e.p.-values observed for Na2 SO4 electrolyte and the KCl rejection for each concentration finds a minimum in the same range. At low KCl concentrations (6.4 × 10−5 ) and a pH-value of 9, one can again see the influence of OH− permeation on the measurement of electrical conductivity. Looking at the function of ζ -potential in dependence of KCl concentration at a pH of 4.9 (Fig. 5) reveals another adsorption behaviour of the dissolved K+ and Cl− ions in comparison to the Na2 SO4 electrolyte. The negative ζ -potential increases with increasing KCl-concentration, but the ζ -potential reaches a plateau from 1 × 10−4 to 1 × 10−3 mol/l, to decrease again with further increase of the KCl-concentration due to the adsorption of Cl− ions. Comparing these results with the pH-dependence of the ζ -potential, determined at varying KCl-electrolyte concentrations (6 × 10−5 , 6 × 10−4 and 3 × 10−3 mol/l) (Fig. 4) one can see that the i.e.p. is not a function of the electrolyte concentration (i.e.p. = 3.37 ± 0.1), which would mean that the dissolved K+ (surprisingly, since Na+ is specifically adsorbed) and Cl− ions do no
interact specifically with the PES surface or nearly identical adsorption of both co- and counterions, or a noticeable affinity for one ion ([27], and footnotes therein). In fact, if the affinity of the adsorbed ion is very high, the surface may be saturated at very low electrolyte concentrations and, therefore, the i.e.p. appears not to change with varying ionic strength [27]. In this case the dependence of ζ -potential on rising KCl concentration does not explain the characteristics of KCl rejection rates. Higher KCl concentrations lead to higher negative ζ -potentials but also to decreasing rejection rates. The reason of decreasing rejection rate might be found in the non-preferential adsorption of chloride (no shift of i.e.p.), which therefore, do not contribute to the repulsive electrostatic force of the charged membrane. However, it should be noticed that single charged ions are usually not strongly affected by the electrostatic repulsive force of the membrane’s surface and therefore, permeate preferentially [5].
4. Conclusions The NF membrane PES 10 shows a maximum rejection rate at a Na2 SO4 concentration in the range of 8 × 10−5 mol/l, which is in correspon-
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Fig. 5. Rejection and ζ -potentials as function of the KCl concentration at pH 4.9.
dence with a negative maximum of ζ -potential at a Na2 SO4 -concentration of 1 × 10−3 mol/l. Selective anions in a lower concentraadsorption of SO2− 4 tion range up to 1 × 10−3 mol/l onto the membrane surface was found to be responsible for increasing ζ -potential and rising rejection rates with increasing Na2 SO4 concentrations. However, an increasing Na2 SO4 -concentration leads to a reversal of the sign of the ζ -potential (p.r.z.), which indicates an specific adsorption of Na+ . This was also proved by the shift of the i.e.p. towards higher pH-values at higher Na2 SO4 -concentrations. In this case the ζ -potential of the membranes surface is not only a function of the dissociation of the acidic surface groups (SO3 -H), but it is also influenced by the adsorption of the dissolved ions. Yet the rate of Na2 SO4 rejection is not directly determined by the absolute value of ζ -potential and even at the i.e.p.-value of the PES 10 membrane (or at the p.r.z.) the rejection rate for Na2 SO4 is not minimal. It was further shown that in the alkaline pH range the rejection rate reaches its maximum value in accordance to the -potential reaching its plateau value. No specific adsorption on the membrane surface was found for KCl solutions. The i.e.p. is not a function of KCl concentration in the ζ -pH plot. However, the ζ -potential increases with increasing electrolyte
concentration due to excess adsorption of Cl− . As a consequence the rejection rate decreases continually with increasing KCl concentration. It should be kept in mind that because of smaller molecular diameter and single valence the Cl− anion permeates more easily than the SO2− 4 anion, even if the membrane surface were affected by electrostatic interactions of the anions. Further studies on the dependence of ζ -potential and rejection rates in single electrolyte solutions with regard to smaller pore sizes of the membrane material and specific/non-preferential adsorption are in progress.
Acknowledgements We gratefully acknowledge funding through the research grant 02 WA 95425 of the Federal Ministry of Education and Research of Germany and the supervision by the Research Center Karlsruhe as well as the kind donation of the PES 10 membrane material by Celgrad GmbH, Wiesbaden. The authors wish to thank Dr. G. Hagmeyer of the IWW Rhenish-Westphalian Institute for Water Research for his helpful discussion and suggestions. Many thanks to Elmar Rother and Astrid Lembke for performing some ζ -potential measurements.
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Zeta-potential and rejection rates of a polyethersulfone nanofiltration membrane in single salt solutions Mathias Ernst a , Alexander Bismarck b , Jürgen Springer b , Martin Jekel a,∗ a
b
Technische Universität Berlin, Institut für Technischen Umweltschutz, Sekr. KF 4, Straße des 17 Juni 135, D-10623 Berlin, Germany Technische Universität Berlin, Institut für Technische Chemie, Fachgebiet Makromolekulare Chemie, Sekr. TC6, Straße des 17 Juni 135, D-10623 Berlin, Germany Received 15 April 1999; received in revised form 15 July 1999; accepted 15 July 1999
Abstract The zeta-potentials (ζ -potentials) of a polyethersulfone nanofiltration (NF) membrane (PES 10, Celgard GmbH) were determined in single salt solutions (Na2 SO4 and KCl-electrolytes) at different concentrations and pH values and results were compared with measured rejections rates of the electrolytes in the same aqueous medium. With increasing Na2 SO4 concentration the rejection rate reaches a maximum at 8 × 10−5 mol/l, which corresponds to the negative maximum of the ζ -potential at rising electrolyte concentrations. Selective adsorption of the SO2− 4 anions were found to be responsible for increasing rejection rate in the lower concentration range (up to 1 × 10−3 mol/l). However, the absolute value of the ζ -potential not directly relates to the determined rejection rate of Na2 SO4 . Increasing the Na2 SO4 concentration results in a sign reversal of the measured ζ -potential, which indicates the specific adsorption of Na+ . This was also proved by the shift of the isoelectric point (i.e.p.) to higher pH-values with increasing Na2 SO4 concentration. At the membrane’s i.e.p. the observed rejection rates were not minimal. For a KCl solution it was shown that there is no preferential adsorption of the ions, neither Cl− nor K+ , on the membrane surface, which leads to the conclusion that in this case the electrostatic properties of the membrane are only affected by the dissociation of the surface sulfonic acids. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Nanofiltration; Salt rejection; Charged membranes; Zeta-potential; Specific adsorption
1. Introduction In the past 10 years nanofiltration (NF) has become a widely used membrane technology often replacing reverse osmosis (RO) because of lower energy consumption and higher flux rates, which is usually attributed to larger pore sizes of the material [1]. ∗ Corresponding author. Tel.: +49-30-314-25058/23339; fax: +49-30-314- 23850. E-mail address: [email protected] (M. Jekel).
With a molecular weight cut-off (MWCO) range between 200 and 10000 g/mol NF lies between the classical non-porous RO membranes, where separation is mainly a function of solution diffusion coefficient of the dissolved organic and/or inorganic species and the ultrafiltration (UF), which is generally regarded as a pore filtration technology. However, alongside the molecular sieve effect, NF membranes have a second separation mechanism, originating in charge effects by the dissociation of surface groups such as sulfonated or carboxyl acids [2].
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Depending on the pH and concentration of the feed solution the surface charge forms an electrostatic potential that interacts with the ions in solution. The membrane surface is usually negatively charged which produces a repulsive force on anions and on cations because of the maintenance of electroneutrality. This special electrostatic interaction is the reason for better rejection rates of divalent anions, while single-charged ions can usually permeate the NF membranes [3,4]. Although NF-membranes are finding increased applications in various fields, their transport mechanism is not yet well understood. Several studies have used different models to understand the rejection behaviour of NF membranes as a function of pore size distribution, diffusion coefficients, concentration polarisation and surface charge densities and it was shown that predictions for single electrolyte solutions in most cases were accurate, however, the accuracy decreases rapidly for mixtures [5–7]. In these studies the Donnan equation and the extended Nernst–Planck equation have often been applied to the model of the microporous membrane to describe the influence of the electrical properties of the membrane surface and to calculate the transport of ions to the membrane’s surface and through the pores according to the strength of the electrostatic field [8]. The model considers the contribution of electrostatic repulsive force on ion transport by a the effective volume charge density 8X of the membranes surface. 8X (and another parameter needed for rejection calculation: the ratio of membrane porosity to membrane thickness /δ) can be determined by fitting the rejection rates of a membrane in dependence of the concentration of a single electrolyte (e.g. NaCl) [9]. The relationship between 8X and the electrolyte concentration is usually assumed to be linear in a ln–ln diagram and calculated by means of an Freundlich isotherm [10]. Calculated Freundlich parameters are generalised subsequently to other single electrolytes (Na2 (SO4 )) to model the rejection rate in the different aqueous solution with good agreement [7,10]. However, Hagmeyer and Gimbel showed that the fitting parameter 8X is not only a function of electrolyte concentration but also of the pH-value of the solution. They determined 8X via the surface charge σ by measuring the ζ -potential of the membrane surface in dependence of pH and used the parameter successfully for rejection calculations [11].
The aim of the presented study was to measure the ζ -potential of a commercial NF membrane (PES 10, Celgard GmbH, Wiesbaden) with varying pH and electrolyte concentration and to compare results directly with rejection experiments performed in the same aqueous solution. Assuming that the volume charge density will correlate with the determined ζ -potential, it is possible to check the influence of the electrostatic properties of the membrane surface on the rejection characteristics directly. By using this practical approach, the contribution of the charge density to the overall rejection performance of the investigated NF-membrane can be determined. However, it should be mentioned that the electrical properties of a membrane depend not only on the ζ -potential of membranes surface groups, but also on the pore charge distribution, which is difficult to determine directly, especially in the narrow pore range of NF-membranes [12]. In a first approach, it can be assumed that the effective volume charge density inside the membrane pores do not differ greatly from the surface one [6] and therefore, the measured ζ -potential of the membrane surface can be equated to the ζ -potential inside the pores. However, the differences of electrical ‘pore properties’ and electrical ‘surface properties’ are a function of pore radius. The electrochemical double layer overlapping which occurs in narrow pores and the resulting higher solution concentrations in the pores in comparison to the bulk one are the reason for this behaviour [13].
2. Experimental Salt rejection experiments were carried out in a single stage 2540 element spiral wound (SW) module pilot plant (membranes surface: Amembr = 1.6 m2 ) as well as a flat sheet testing cell (196 cm2 ) plant, both operating in a recycling loop (Fig. 1). The height of the feed spacer for all membranes was 1.2 mm (47 mil, diamond). De-ionised water (average conductivity <200 S/m) was used as feed water, KCl and Na2 SO4 (for analysis ACS, Merck KgaA, Darmstadt) were added in desired concentrations. Both systems were operated at a trans-membrane pressure of 1p = 0.7 MPa (approximately 100 psi) and with a feed flow rate of Vfeed = 1.1 m3 /h (SW-configuration)
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Fig. 1. Flow diagram of NF plant for rejection experiments.
or 0.1 m3 /h (flat sheet testing cell), which corresponds to an effective cross-flow velocity of CFVeff = 0.23 m/s for both configurations (SW or flat sheet testing cell). Flow rates were measured by either rota-meter or scale. The temperature was controlled by a cooler (Haake Kryothermat 140, TP1) and kept at 20◦ C. Concentrations and rejection rates of ionic species were determined by conductivity (WTW LF 95) and pH was adjusted by addition of 0.1 M HCl, KOH (KCl-experiments) or NaOH, H2 SO4 (Na2 SO4 -experiments). The flat sheet membranes of NADIR® -NF-PES 10 were supplied by Celgard GmbH, Wiesbaden and the SW-module was purchased from the same company. The active layer of this membrane material was made of polyethersulfone with polyvinylpromi-done as co-polymer on polypropylene carrier. Celgard publishes a water permeability of 75 l/(m2 h) and a MWCO in the range of 1000 g/mol at a pressure of 0.7 MPa [14]. Bowen calculated the rejection of the PES 10 membrane in 10−3 mol/l NaCl electrolyte by assuming a pore radius of 1 nm. He also fitted the pore radius rp of the PES 5, that shows slightly higher rejection rates than PES 10 to rp = 0.72 nm by rejection experiments in Na2 SO4 solution by considering the hindered transport in the pores [15]. According to his results the pore radius of PES 10 should be in
the range of 0.7–1 nm. Prior to the experiments a new membrane was operated for at least 24 h to achieve a constant permeate pure water flux; the permeate was discharged for the first 6 h of this period. After adjusting the pH or electrolyte concentration in the experiment, the aqueous solution was filtrated for 15 min at constant permeate flux rate to establish constant chemical conditions on the membrane’s surface before taking samples. The ζ -potentials were determined with the electrokinetic analyser (EKA, Anton Paar KG, Graz, Austria) based on the streaming potential method using flat sheets of the membrane’s material in a commercial plate or foil measuring cell. Details of the ζ -potential measuring technique and the calculation of the ζ -potential have been reported elsewhere [16–18]. Measuring the pH dependence of the ζ -potential the acidity or basicity of solid surfaces can be determined qualitatively. From the ζ = f(pH) graph curve it can be determined whether adsorption or dissociation processes predominate. Assuming that the formation of the electrical double layer is caused by the dissociation of the acidic functionalities (which can be expected for a polyethersulfone membrane), the pH dependence of the ζ -potential reaches a negative plateau in the alkaline range (Fig. 3). The plateau is caused by the complete dissociation of these acidic
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surface groups. If there is a change of the sign of the ζ -potential in the acidic range this is caused firstly by the repression of the dissociation of the acidic surface groups and secondly by the adsorption of the ions determining the surface potential (like protons). The pH value of the isoelectric point (i.e.p.), where ζ = 0 mV, is also a measure of the acidity (leading to low i.e.p.) or basicity (resulting in high i.e.p.) of a solid surface. Furthermore if the i.e.p. is a function of electrolyte concentration this is a proof for specific adsorption of anions on the membrane surface [19]. Generally it is assumed, that the correction in surface conductivity for a given pH value at a given electrolyte concentration only changes the magnitude of the ζ -potential value but does not affect the position of the i.e.p. nor the overall shape of the function ζ = f (pH). Therefore, it is not necessary to correct the surface conductivity for ζ -potential measurements in order to compare the samples qualitatively [20]. Since the reproducibility of electrokinetic measurements depends strongly on the sample quality, every measurement of the pH-dependence needs a well defined starting point. Therefore, all investigated membrane samples were rinsed 24 h before use. All measurements were performed at 20◦ C ± 1◦ C. The pH-dependence of the ζ -potential was determined at constant electrolyte concentration. An electrolyte KCl was used as well as Na2 SO4 . In order to hold the ion strength constant, we altered the pH value in the interval of pH 3–9 by addition of 0.1 M HCl or KOH solution in the case of a KCl-electrolyte or H2 SO4 and NaOH for Na2 SO4 . The streaming potential was measured with Ag/AgCl-electrodes (SE 4.2, Senortechnik Meinsberg, Waldheim/Sa., Germany) for the KCl electrolyte and platinum electrodes (MC 4.2, Senortechnik Meinsberg, Waldheim/Sa., Germany) for the Na2 SO4 electrolyte. In order to measure the ζ -potential as a function of the electrolyte concentration the analyser was filled with de-ionised water (Millipore, pH 4.6–5.1) and the measuring cell was rinsed several times until the conductivity in the measuring system fell below 300 S/m. First the water value of the ζ -potential was measured, subsequently the KCl or Na2 SO4 -concentration was raised using a digital burette (Brand, Wertheim, Germany). The system including the measuring cell was rinsed in both measuring directions before measurements.
All ζ -potentials presented in this study were not corrected in surface conductivity, since this correction (in particular if the plate/foil measuring cell is used [21]) does not take into account the often unsatisfactory results [22]. However, it is likely to suppress differences between and common features of the membranes, which can better be derived from the plots of the uncorrected data [12].
3. Results and discussion Fig. 2 displays the influence of increasing Na2 SO4 concentration on the rejection rate in the single electrolyte solution at a pH-value of 5.2. Starting at a Na2 SO4 concentration of 1 × 10−5 mol/l the rejection rate rises slightly and reaches a maximum at 8 × 10−5 mol/l. With increasing concentration of the electrolyte the rejection rate decreases continually to less than 20% at 6 × 10−3 mol/l. The experiment was performed in a SW configuration but results with a flat sheet configuration were similar. The permeability of the membrane was 2.77 × 10−11 m/(s Pa) [10 l/(m2 h bar)]. Regarding the measured Na2 SO4 concentration dependence of the ζ -potential, which equals the adsorption isotherm of the dissolved ions on the polymeric membrane [23], one can first observe, that the negative ζ -potential decreases slowly due to the excess adsorption of anions up to a Na2 SO4 -concentration of 4 × 10−4 mol/l to increase drastically to the maximum negative ζ -potential (ζ max ) at a concentration of 1 × 10−3 mol/l. Afterwards, the ζ -potential decreases again with increasing Na2 SO4 -concentration to reach the point of charge reversal (p.c.r.) at 3 × 10−3 mol/l, where the ζ -potential goes through 0 mV, to increase drastically (the positive ζ -potential) with further increase of the Na2 SO4 -concentration. This reversal of the sign of the ζ -potential is due to overcompensation by counter charge adsorption in the Stern-layer, and clearly it cannot occur if Na+ is not specifically adsorbed [24]. Due to the hydrophilic character of the PES-membrane the measured ζ -potential in distilled water is quite small (−0.5 ± 0.2 mV) [25]. Since the ζ -potential–concentration curve reflects the ion adsorption onto the membrane surface and, therefore, the negative surface charge increase in the lower Na2 SO4 concentration range up to a maximal
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Fig. 2. Rejection and ζ -potentials as function of the Na2 SO4 concentration at pH 5.2.
value (ζ max ), the repulsive electrostatic forces might also increase. Due to the selective SO2− 4 adsorption in the lower concentration range the rejection rate of Na2 SO4 increases too, and then decreases again at higher Na2 SO4 concentrations. However, the rejection rate becomes not minimal at the point of ζ -potential reversal (p.c.r.) and will also not increase again with increasing positive ζ -potential. The maximum of the rejection rate (Rmax ) depending on the Na2 SO4 concentration compared to ζ max is shifted by approximately one log unit. The shift of the maximal values (Rmax and max ) might be explained by the different experimental conditions. In the SW-module the configuration was 1p = 0.7 MPa and the water was filtered through the membrane (i.e. the flow is directed perpendicular to the membrane surface), whereas the configuration in the EKA-measuring cell was 1p = 0.015 MPa without any filtration of water (flow direction is horizontal to the membrane surface). Due to the horizontal flow direction during the ζ -potential measurements the pore-structure influence is completely excluded [12], and therefore, the measured ζ -potential does not consider the electrokinetic properties inside the pores. Regarding the structure of an asymmetric composite membrane, the obtained values from streaming potential measurements give only qualitative evidence.
With the transport of feed solution through the membrane while filtration, the concentration of ions on the surface of membrane (concentration polarisation) and especially inside the membrane pores will be higher in comparison to the bulk solution [13]. According to the EKA measurement therefore the ζ -potential in the membranes pores will be shifted to higher solution concentrations. At Na2 SO4 bulk concentrations below 1 × 10−3 mol/l this will lead to higher (more negative) ζ -potential in the pores in comparison to the surface ζ -potential measured in bulk solution concentration. As a consequence the resulting repulsive force on anions is higher and the maximum of Na2 SO4 rejection will be reached earlier (Fig. 2). The plot of Na2 SO4 rejection as function of the pH-value of the solution at different concentrations (Fig. 3) displays a strong dependence of the separation characteristics on the pH-value. With rising electrolyte concentration the rejection always decreases notably, but the decrease is more pronounced in the acidic pH range. The rejection curve seems to be shifted to higher pH values with higher concentrations. Above pH 8 the rejection rate reaches 80–90% even at higher Na2 SO4 concentrations of 1 × 10−3 mol/l. The slight decrease of rejection rates at pH 9 for 5.7 × 10−5 and
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Fig. 3. Rejection and ζ -potentials as function of pH at varying electrolyte (Na2 SO4 ) concentration.
2.6 × 10−4 mol/l can be explained by the increasing influence of the OH− ions content in the feed water on the measuring error of the concentration of Na2 SO4 by electrical conductivity. As indicated by the pH-dependence of the ζ -potential measured in the Na2 SO4 electrolyte, the PES-membrane contains a significant amount of dissociable acidic surface groups (–SO3 H) causing low i.e.p.-values. Measuring the function ζ = f(pH) at varying Na2 SO4 -electrolyte concentrations (6 × 10−5 , 3 × 10−4 and 8 × 10−4 mol/l) indicates the ‘unexpected’ preferentially (specifically) adsorption behaviour of the Na+ cations on the PES membrane. It becomes obvious, since the i.e.p. is a function of the electrolyte concentration (Fig. 3) and with increasing Na2 SO4 concentration the i.e.p. increases. If Na+ adsorbs specifically, this renders the ζ -potential more positive and the i.e.p. is restored by letting more OH− ions adsorb, i.e. by increasing the pH [24]. However, this observed behaviour coincide with the measured Na2 SO4 concentration dependence, where a sign reversal of the ζ -potential was observed at higher salt concentrations. This is an unexpected behaviour, since for the most investigated membrane materials the SO2− 4 anions are preferentially adsorbed at the membrane surface [19,26,27]. In this case the i.e.p. does not equal the point of zero charge (p.z.c.) [28]. As
can be seen from Fig. 2 the ζ plateau -values of higher Na2 SO4 -concentrations are in correspondence to the ζ -potential value obtained from the concentration dependence. The ζ -potential plateau area at lower pH values is also established earlier in comparison to higher Na2 SO4 -concentrations. As can be clearly seen, the rejection rate does not depend directly on the value of the negative ζ -potential and even at the i.e.p. the rejection rates does not reach a minimum, at positive ζ -potentials the rejection rate decreases further. The rejection rate reaches an early plateau value if the corresponding ζ -potential reaches an early plateau. The membrane filtration of KCl electrolyte solutions at different pH values shows a different behaviour than that of Na2 SO4 relative to the varied concentrations. The rejection rate decreases continuously with increasing KCl concentration (Fig. 4) over the whole range of pH, showing highest rejection rates in the alkaline pH range. In this case the coherence between ζ -potential and rejection is restricted to the fact that the highest rejection of each KCl concentration is reached in the range of highest (most negative) ζ -potential at the same electrolyte concentration. No stable plateau value of the rejection rate can be related to the relatively stable ζ -potential plateau value of −26 mV at KCl concentrations of 6 × 10−4 or 3 × 10−3 mol/l, respectively.
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Fig. 4. Rejection and ζ -potentials as function of pH at varying electrolyte (KCl) concentration.
In analogy to Na2 SO4 solutions the absolute magnitude of the measured ζ -potential cannot explain the magnitude of rejection directly. However, the i.e.p. for KCl solution (i.e.p. KCl = 3.37 ± 0.1) is lower than the i.e.p.-values observed for Na2 SO4 electrolyte and the KCl rejection for each concentration finds a minimum in the same range. At low KCl concentrations (6.4 × 10−5 ) and a pH-value of 9, one can again see the influence of OH− permeation on the measurement of electrical conductivity. Looking at the function of ζ -potential in dependence of KCl concentration at a pH of 4.9 (Fig. 5) reveals another adsorption behaviour of the dissolved K+ and Cl− ions in comparison to the Na2 SO4 electrolyte. The negative ζ -potential increases with increasing KCl-concentration, but the ζ -potential reaches a plateau from 1 × 10−4 to 1 × 10−3 mol/l, to decrease again with further increase of the KCl-concentration due to the adsorption of Cl− ions. Comparing these results with the pH-dependence of the ζ -potential, determined at varying KCl-electrolyte concentrations (6 × 10−5 , 6 × 10−4 and 3 × 10−3 mol/l) (Fig. 4) one can see that the i.e.p. is not a function of the electrolyte concentration (i.e.p. = 3.37 ± 0.1), which would mean that the dissolved K+ (surprisingly, since Na+ is specifically adsorbed) and Cl− ions do no
interact specifically with the PES surface or nearly identical adsorption of both co- and counterions, or a noticeable affinity for one ion ([27], and footnotes therein). In fact, if the affinity of the adsorbed ion is very high, the surface may be saturated at very low electrolyte concentrations and, therefore, the i.e.p. appears not to change with varying ionic strength [27]. In this case the dependence of ζ -potential on rising KCl concentration does not explain the characteristics of KCl rejection rates. Higher KCl concentrations lead to higher negative ζ -potentials but also to decreasing rejection rates. The reason of decreasing rejection rate might be found in the non-preferential adsorption of chloride (no shift of i.e.p.), which therefore, do not contribute to the repulsive electrostatic force of the charged membrane. However, it should be noticed that single charged ions are usually not strongly affected by the electrostatic repulsive force of the membrane’s surface and therefore, permeate preferentially [5].
4. Conclusions The NF membrane PES 10 shows a maximum rejection rate at a Na2 SO4 concentration in the range of 8 × 10−5 mol/l, which is in correspon-
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Fig. 5. Rejection and ζ -potentials as function of the KCl concentration at pH 4.9.
dence with a negative maximum of ζ -potential at a Na2 SO4 -concentration of 1 × 10−3 mol/l. Selective anions in a lower concentraadsorption of SO2− 4 tion range up to 1 × 10−3 mol/l onto the membrane surface was found to be responsible for increasing ζ -potential and rising rejection rates with increasing Na2 SO4 concentrations. However, an increasing Na2 SO4 -concentration leads to a reversal of the sign of the ζ -potential (p.r.z.), which indicates an specific adsorption of Na+ . This was also proved by the shift of the i.e.p. towards higher pH-values at higher Na2 SO4 -concentrations. In this case the ζ -potential of the membranes surface is not only a function of the dissociation of the acidic surface groups (SO3 -H), but it is also influenced by the adsorption of the dissolved ions. Yet the rate of Na2 SO4 rejection is not directly determined by the absolute value of ζ -potential and even at the i.e.p.-value of the PES 10 membrane (or at the p.r.z.) the rejection rate for Na2 SO4 is not minimal. It was further shown that in the alkaline pH range the rejection rate reaches its maximum value in accordance to the -potential reaching its plateau value. No specific adsorption on the membrane surface was found for KCl solutions. The i.e.p. is not a function of KCl concentration in the ζ -pH plot. However, the ζ -potential increases with increasing electrolyte
concentration due to excess adsorption of Cl− . As a consequence the rejection rate decreases continually with increasing KCl concentration. It should be kept in mind that because of smaller molecular diameter and single valence the Cl− anion permeates more easily than the SO2− 4 anion, even if the membrane surface were affected by electrostatic interactions of the anions. Further studies on the dependence of ζ -potential and rejection rates in single electrolyte solutions with regard to smaller pore sizes of the membrane material and specific/non-preferential adsorption are in progress.
Acknowledgements We gratefully acknowledge funding through the research grant 02 WA 95425 of the Federal Ministry of Education and Research of Germany and the supervision by the Research Center Karlsruhe as well as the kind donation of the PES 10 membrane material by Celgrad GmbH, Wiesbaden. The authors wish to thank Dr. G. Hagmeyer of the IWW Rhenish-Westphalian Institute for Water Research for his helpful discussion and suggestions. Many thanks to Elmar Rother and Astrid Lembke for performing some ζ -potential measurements.
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