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Use of Polyelectrolyte Layer-by-Layer Assemblies as Nanofiltration and Reverse Osmosis Membranes Wanqin Jin, Ali Toutianoush, and Bernd Tieke* Institut fu¨ r Physikalische Chemie, Universita¨ t zu Ko¨ ln, Luxemburgerstrasse 116, D-50939 Ko¨ ln, Germany Received November 21, 2002. In Final Form: February 10, 2003 Measurements of ion transport and water flux across ultrathin multilayered membranes of polyelectrolytes were carried out under nanofiltration and reverse osmosis conditions. The polyelectrolyte membranes were prepared upon alternating electrostatic layer-by-layer adsorption of polyvinylamine (PVA) and polyvinyl sulfate (PVS) on porous supports. The pressure-driven transport of aqueous electrolyte solutions containing NaCl, Na2SO4, MgCl2, and MgSO4 in 1 and 10 mM concentration was investigated. For MgCl2 and MgSO4, a complete rejection was observed independently from the concentration of the feed solution and the pressure applied. For NaCl and Na2SO4, the rejections were 84 and 96% at 5 bar, and 93.5 and 98.5% at 40 bar, respectively. The hydraulic permeability of the composite membrane was 113.7 mL/(m2 h bar). It was only little affected by the presence of salt. At low and moderate pressure the membranes are suitable for water softening applications, while at pressures of 40 bar or higher they can be used for water desalination. Effects of the stirring of the feed solution on the membrane characteristics are also discussed.
1. Introduction The alternate electrostatic adsorption of cationic and anionic polyelectrolytes represents an easy, environmentally sound method for preparation of polymer films with thickness in the nanometer range.1-3 As recently reviewed, the layer-by-layer adsorption of polyelectrolytes can also be used for the coating of porous supporting membranes with an ultrathin separation layer.4,5 In this way, a new type of composite membrane becomes accessible, which is excellently suited for separation of alcohol/water mixtures under pervaporation conditions6-8 and separation of monoand divalent inorganic cations9,10 and anions.9-11 The selective ion transport originates from the multibipolar structure of the separating membrane. The membrane consists of positively and negatively charged polyelectrolyte layers in alternating sequence. The charged layers strongly interact with the permeating ions by rejecting the equally charged ions and attracting the oppositely charged ions. The strength of the interactions is proportional to the charge density of the permeating ions and the polyelectrolytes constituting the membrane.9,10 As a consequence, the permeability of di- and multivalent ions is much lower than that for monovalent ions, and a highly selective transport is observed.9-11 * To whom correspondence should be addressed. (1) Decher, G. Science 1997, 277, 1232. (2) Bertrand, P.; Jonas, A.; Laschewsky, A.; Legras, R. Macromol. Rapid Commun. 2000, 21, 319. (3) Hammond, P. Curr. Opin. Colloid Interface Sci. 2000, 4, 430. (4) Tieke, B.; van Ackern, F.; Krasemann, L.; Toutianoush, A. Eur. Phys. J. E 2001, 5, 29. (5) Tieke, B. In Handbook of Polyelectrolytes and Their Applications; Tripathy, S. K., Kumar, J., Nalwa, H. S., Eds.; Am. Sci. Publ.: 2002; p 115. (6) Krasemann, L.; Toutianoush, A.; Tieke, B. J. Membr. Sci. 2001, 181, 221. (7) Toutianoush, A.; Krasemann, L.; Tieke, B. Colloids Surf., A 2001, 198-200, 881. (8) Meier-Haack, J.; Lenk, W.; Lehmann, D.; Lunkwitz, K. J. Membr. Sci. 2001, 184, 233. (9) Krasemann, L.; Tieke, B. Langmuir 2000, 16, 287. (10) Toutianoush, A.; Tieke, B. In Novel Methods to Study Interfacial Layers; Mo¨bius, D., Miller, R., Eds.; Elsevier: Amsterdam, 2001; p 416. (11) Harris, J. J.; Stair, J. L.; Bruening, M. L. Chem. Mater. 2000, 12, 1941.
Methods to further improve the selectivity have also been described.12,13 However, up to now very few studies were concerned with pressure-driven transport of ions across polyelectrolyte multilayer assemblies. Pressure-driven ion transport is applied in technically important separation processes such as nanofiltration (NF) and reverse osmosis (RO).14-16 While NF refers to a separation process carried out at a transmembrane pressure difference below 30 bar, RO is carried out at higher pressure.15 Under NF conditions, the rejection of sodium chloride is up to 60%, while, under RO conditions, it is much higher. Introduction of membrane-bound charges into NF membranes improves the ion rejection, because the transport of di- and multivalent ions is slowed by electrostatic interactions. The rejection behavior allows us to use NF membranes for water softening applications, for example, while RO membranes are suitable for water desalination. Since the polyelectrolyte multilayer membranes exhibit a dense network structure and contain cationic as well as anionic charges, it was of interest to study the ion transport across these membranes under NF and RO conditions. Up to the present, only a very short, preliminary study on the use of the layer-by-layer assemblies as NF membranes has been reported.17 The purpose of this communication is to present a first comprehensive study on the pressure-driven transport of ions across selfassembled polyelectrolyte multilayers. It is demonstrated that the membranes are suitable for NF applications, such as water softening at low pressure, and for RO applications such as water desalination at high pressure. (12) Stair, J. L.; Harris, J. J.; Bruening, M. L. Chem. Mater. 2001, 13, 2641. (13) Dai, J.; Balachandra, A. M.; Lee, J. I.; Bruening, M. L. Macromolecules 2002, 35, 3164. (14) Yaroshchuck, A.; Staude, E. Desalination 1992, 86, 115. (15) Rautenbach, R. Membranverfahren-Grundlagen der Modul- und Anlagenauslegung; Springer: Berlin, 1996; Chapters 8 and 9, p 142. (16) Tsuru, T.; Urairi, M.; Nakao, S. I.; Kimura, S. J. Chem. Eng. Jpn. 1991, 24 (4), 518. (17) Bruening, M. L.; Harris, J. J.; Sullivan, D. M. Polym. Mater. Sci. Eng. 2001, 85, 525.
10.1021/la020926f CCC: $25.00 © 2003 American Chemical Society Published on Web 03/08/2003
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2. Experimental Section Materials. Poly(vinylamine) (PVA, molecular weight 100 000) was kindly supplied by BASF, Ludwigshafen, and poly(vinyl sulfate potassium salt) (PVS, molecular weight 350 000) was purchased from Acros. NaCl, Na2SO4, MgCl2, and MgSO4 (analytical grade) were purchased from Fluka. All chemicals were used without further purification. Milli-Q water was used as solvent. PAN/PET supporting membranes treated with oxygen plasma were provided by Dr. A. Hu¨bner, Sulzer Chemtech GmbH, Neunkirchen. Methods. Preparation of Polyelectrolyte Multilayers. Polyelectrolytes were dissolved in 1 M aqueous NaCl solution in a concentration of 10-2 monomoles L-1 (monomole ) mole of monomer unit), with the water being acidified with aqueous HCl to pH 1.7. For adsorption of the polyelectrolyte layers, the supporting membrane was immersed (a) in the solution of the cationic polyelectrolyte, (b) in pure water, (c) in the solution of the anionic polyelectrolyte, and (d) in water again. The sequence of steps a-d was carried out 60 times. The immersion time for the individual steps was 20-30 min; the temperature was about 20 °C. For the dipping procedure, a home-built computerized apparatus was used. The size of the membranes was 12 × 12 cm2. Nanofiltration and Reverse Osmosis. Measurements were carried out using a homemade apparatus working under stirred and nonstirred dead end conditions. The apparatus consists of a temperature-controlled pressure chamber made of stainless steel (volume V ) 1.8 L; maximum pressure ) 40 bar), a stirrer (rotating speed ) 700 rpm), and the actual cell containing the membrane (area A ) 36.8 cm2). At the bottom, the cell is equipped with an outlet for the permeate solution (Figure 1). For each electrolyte salt, a new membrane was used. In the first step, the new membrane was conditioned with pure water at 5 bar for 3 h in order to remove residual salt, which was eventually left in the membrane from the preparation process. For this purpose the chamber was filled with 1.75 L of water and the applied pressure p was adjusted with nitrogen gas from a pressurized bottle. As the second step, the pure water flux Jw was determined at 10, 25, and 40 bar by collecting the permeated water in a beaker over a time period of 2-3 h. In the third step, the pure water was replaced by the electrolyte solution of low concentration (10-3 mol/L). After a 2-h prerunning was made at 5 bar for steady state, the permeate solutions were collected at different pressures over a period of 2-3 h, respectively. Three samples were collected at each pressure. If the pressure was changed, again a 1-h prerunning was made. In step four, the membrane was washed with pure water for 2 h in order to regenerate the original Jw value. In the next step, the electrolyte solution of higher concentration (10-2 mol/L) was investigated as described in step three. Finally, the membrane was regenerated upon washing with pure water for 2 h and Jw was measured again as described in step two. If not especially noted, all experiments were carried out at room temperature under stirring. Determination of Flux and Ion Content in Permeate. The permeation flux J was calculated from the measured volume V of the permeate, which was flowing across the membrane of area A in the time period ∆t,
J)
V [L/(m2 h)] A∆t
The salt rejection R was calculated using the equation
R ) (1 - cp/cf) × 100% with cp and cf being the salt concentrations in permeate and feed solution. The concentration of the permeate was measured using a computerized Knauer high-pressure liquid chromatograph consisting of a pump K1001, a thermostated column, a refractometer Wellchrom K-2301, and a conductivity detector LFD 550101. The cations were detected using a 100 mm × 4.6 mm cation column with a cation precolumn-kit. The operating temperature was 35.0 ( 0.1 °C. Degassed aqueous methanesulfonic acid (3 mM) was used as mobile phase. The ion concentration was determined from the corresponding peak area
Figure 1. Schematic representation of the RO/NF apparatus. after equilibration with a set of standard electrolyte solutions of different concentration. cp values represent average values of at least two chromatographic measurements from one permeate solution. R and J values represent average values from three measurements under identical NF or RO conditions.
3. Results and Discussion For our studies, the polyelectrolyte multilayer membranes were adsorbed on porous PAN/PET supports as described in the Experimental Part. All studies were carried out using separating membranes of 60 PVA/PVS bilayers in thickness. The relatively high number of 60 bilayers was chosen in order to ensure that the separating membrane is dense and free of defects. In Figure 2a, permeation flux and rejection of two aqueous NaCl feed solutions of 1 and 10 mM concentration are plotted as a function of the operating pressure ∆p. The operating pressure represents the difference between the applied pressure p and the osmotic pressure π of the feed solution. The pressure dependence of the flux Jw of pure water across the membrane is also indicated. As expected, Jw increases linearly with the pressure, with the hydraulic permeability being 113.7 ( 5.7 mL/(m2 h bar). The corresponding value for the bare substrate is 237 ( 20 L/(m2 h bar), which means the hydraulic permeability is reduced by a factor of about 2 × 103 upon the coating with the 60 polyelectrolyte bilayers. It can also be seen that the solution permeation flux J of both NaCl solutions increases linearly with pressure, with the slope being practically equal to the pure water permeation flux. For the highest salt concentration, J deviated from Jw by less than 5%. At operating pressures from 5 to 20 bar, the Na+ rejection increased from 84 to 92%, while at an operating pressure of 40 bar, a rejection of 93.5% was reached. For both salt concentrations, the same Na+ rejection was found within the experimental error. In Figure 2b, permeation flux and rejection of aqueous MgCl2 solutions of 1 and 10 mM concentration are plotted versus ∆p of the feed solution. Since a new membrane was used, the water flux Jw was slightly different. As in the case of the NaCl solution, the flux increases linearly with the operating pressure, with the increase being slightly more dependent on the salt concentration than is the case for the NaCl solutions. For the two differently concentrated solutions, the Mg2+ rejection is practically complete over the whole pressure range. The strong rejection can be ascribed to the presence of the positively charged ammonium groups of PVA in the membrane hindering the permeation of the divalent magnesium ions strongly. The permeation behavior of aqueous sodium sulfate is shown in Figure 2c. For a 1 mM feed solution at 5 bar, the rejection of the sodium ions was 89%. This
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Figure 3. Plot of salt rejection and permeation flux J of 1 and 10 mM aqueous solutions of sodium chloride (a) and magnesium chloride (b) as a function of the operative pressure ∆p. Room temperature; feed solution not stirred. Jw and J′w indicate the flux of pure water before and after the measurement of the electrolyte solution, respectively.
Figure 2. Plot of salt rejection and permeation flux J of 10-3 and 10-2 M aqueous solutions of sodium chloride (a), magnesium chloride (b), sodium sulfate (c), and magnesium sulfate (d) as a function of the operative pressure ∆p. Room temperature; feed solution stirred at 700 rpm.
is considerably higher than that for the sodium ions of a sodium chloride solution, because the simultaneously permeating, divalent sulfate counterions are more strongly rejected by the polymer-bound sulfate groups than the chloride ions of NaCl. Different from the case of the other electrolyte solutions, the rejection was influenced by the salt concentration of the feed solution. For a 10 times higher concentration, we found a sodium ion rejection of 96.4% at 5 bar, which increased to 98.5% at 40 bar. We ascribe the concentration effect to sorption of the permeating sulfate ions at the membrane matrix so that the charge density in the membrane becomes higher, and the permeation rate of the ions decreases. Sorption of various di- and multivalent ions by a polyelectrolyte multilayer membrane has already been reported previously.18 The permeation behavior of magnesium sulfate is very similar to that of magnesium chloride. Again the magnesium ions are nearly completely rejected independent of the pressure applied (Figure 2d). In a further set of experiments, the ion transport was studied under nonstirred conditions. Aqueous solutions of NaCl and MgCl2, both in concentrations of 1 and 10 mM, were investigated. The results are shown in Figure 3. As usually observed, the flux J of the electrolyte solutions increased linearly with the operating pressure. The slope was slightly smaller than that for the pure water flux Jw and decreased for higher salt concentration. After (18) Toutianoush, A.; Tieke, B. Mater. Sci. Eng. C 2002, 22, 135.
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the permeation of the electrolyte solutions, the pure water flux J′w was measured, and the value of J′w turned out to be 10% smaller than that for Jw. We speculate that either the membrane underwent a structural change and became more dense or some of the permeating ions were irreversibly sorbed by the membrane so that the free volume determining the permeation flux decreased. The latter seems more likely because, after prolonged water permeation of several hours, the original water flux Jw could be nearly restored. The salt rejection was quite different from those of the experiments with stirring. Let us first discuss the Na+ rejection. Two main features were observed: First, the rejection strongly decreased with increasing operating pressure; for the 1 mM NaCl solution, for example, the rejection decreased from about 80% at 10 bar to nearly 40% at 40 bar. Second, there was a strong decrease in Na+ rejection upon increasing the salt concentration in the feed solution. For example, if the salt concentration was increased from 1 to 10 mM and the operating pressure was 10 bar, the Na+ rejection decreased from 80 to 64%. The effect can be ascribed to concentration polarization at the feed membrane surface. When the water permeates across the membrane, the electrolyte is left behind in the feed solution. Especially at the membrane surface, the ion concentration increases strongly and a solute concentration profile develops from the membrane surface to the bulk of the feed solution. Moreover, the higher solute concentration at the membrane surface causes a higher ion concentration in the membrane, which leads to a shielding of the polymer-bound charges so that the ion rejection is reduced. The shielding effect increases, if either the concentration of the permeating salt solution is increased or the operating pressure for the salt solution is increased. For the rejection of the Mg2+ ions, essentially the same behavior was found as that for the sodium ions. Again, due to concentration polarization at the feed membrane surface, the rejection decreased with increasing operating pressure and increasing solute concentration of the feed solution. For the 1 mM solution, Mg2+ rejection decreased from 100% at 10 bar to 75% at 40 bar, and for an operating pressure of 40 bar, the rejection decreased from 75 to 60% on increasing the electrolyte salt concentration from 1 to 10 mM. This study shows that efficient stirring of the feed solution is very important and strongly affects the ion rejection of the membrane. It is comparable with previous studies showing that the circulation velocity of the feed solution is an important parameter in NF and RO experiments under cross-flow conditions.19,20 (19) Alfonso, M. D.; de Pinho, M. N. Ind. Eng. Chem. Res. 1998, 37, 4118. (20) Alfonso, M. D.; de Pinho, M. N. J. Membr. Sci. 2000, 179, 137.
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4. Summary and Conclusions Our studies indicate that ultrathin polyelectrolyte layerby-layer assemblies are capable of rejecting ions from aqueous electrolyte solutions, if a pressure is applied to the feed side of the membrane. At low pressure, the membranes exhibit a rejection of 1,1-electrolytes much stronger than that for commercial nanofiltration membranes. With increasing pressure, a further increase in ion rejection is found until, at 40 bar, 93.5 and 98.5% are reached for sodium chloride and sulfate, respectively. At this pressure, the ion rejection is comparable with those of commercial membrane modules, in which multiple separation occurs,15,21 while in our apparatus only a single separation step takes place. Even better values can be expected for the multilayer membranes, if the pressure is further increased. The salt rejection increases in the series 1,1- < 1,2- < 2,1- e 2,2-electrolyte, because the permeating divalent ions exhibit stronger electrostatic interactions with the membrane-bound charged groups than the monovalent ones. Sodium sulfate is less strongly rejected than magnesium chloride, because the magnesium ions have a higher affinity to the sulfate ions of PVS than the permeating sulfate ions to the ammonium ions of PVA. Our studies also indicate that an efficient stirring of the feed solution is very important in order to avoid a concentration polarization at the membrane surface, which immediately decreases the ion rejection. In conclusion, it is demonstrated that the polyelectrolyte multilayer assemblies represent multipurpose membranes not only useful for separation of alcohol/water mixtures and dehydration of organic solvents4-8 but also suited for water softening and desalination applications under NF and RO conditions, respectively. While the ion rejection is comparable with those of commercial membranes, the flux is lower by a factor of approximately 10. Thus, in further work, the thickness of the separating layer will be varied and the effect on flux and ion rejection will be tested. Moreover, the content of both the cations and anions in the permeate will be directly analyzed in order to study Donnan effects on the ion transport, and the effect of pH of the feed solution on the salt rejection will be investigated. Acknowledgment. Dr. A. Hu¨bner, Sulzer Chemtech, Neunkirchen, is thanked for kindly supplying plasmatreated PAN/PET supporting membranes. One of us (W.J.) thanks the Alexander von Humboldt foundation for a research fellowship. LA020926F (21) Scott, K. Handbook of Industrial Membranes, 2nd ed.; Elsevier Adv. Technol.: Oxford, 1998.
