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Effect of the ionic characteristics of charged membranes on the permeation of anionic solutes in reverse osmosis
Journal of Membrane Science 169 (2000) 237–247
Effect of the ionic characteristics of charged membranes on the permeation of anionic solutes in reverse osmosis C.K. Yeom ∗ , S.H. Lee, J.M. Lee Chemical Process and Engineering Center, Applied and Engineering Chemistry Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-606, South Korea Received 24 May 1999; received in revised form 20 October 1999; accepted 25 October 1999
Abstract In the separation of an anionic surfactant, ammonium perfluoroalkyl carboxylates from the residual aqueous solution in poly(tetrafluoroethylene) (PTFE) emulsion polymerization, reverse osmosis (RO) process was carried out by using three kinds of charged membranes, that is, anionic, cationic and nonionic membranes. Sodium alginate (SA), chitosan (CS) and poly(vinyl alcohol) (PVA) were prepared as the anionic, cationic and nonionic membranes, respectively. The permeation behaviors were investigated with the charge characteristics of the membranes in terms of electrostatic interaction between permeant and membrane, namely, Donnan potential. Also, the effects of operating condition, such as, feed pressure, feed concentration and permeation rate, have been discussed in terms of membrane fouling and concentration polarization. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Charged membranes; Reverse osmosis; Anionic surfactant; Membrane fouling; Sodium alginate; Poly(vinyl alcohol); Chitosan
1. Introduction In reverse osmosis (RO) processes employing ion-charged membranes, one of the most important features of the membranes is their ability to separate ion solutes from water. Possible mechanisms for the separation of electrolytes are (1) sieving, (2) electrostatic interactions between the membrane and the ions or between the ions mutually and (3) difference in diffusivity and solubility or a combination of these [1]. Their separation potentials for ionic solutes in aqueous solutions can be explained with the chemical potential gradient and the electrical potential gradient as driving force. Thus, the transport of the ions ∗ Corresponding author. E-mail address: [email protected] (C.K. Yeom).
across the charged membrane is well described by the extended Nernst–Planck flux equation in terms of diffusion and migration terms, as a result of concentration and electrical potential gradient [2,3]. In the charged membrane in contact with an electrolyte solution, the concentration of co-ions, i.e., ions with the same charge as the membrane, in the membrane will be lower than that in solution, whereas the counter-ions, which have the opposite charge, have a higher concentration in the membrane than in the solution. On account of this concentration difference of the ions, a potential difference is generated at the interface between the membrane and the solution to maintain electrochemical equilibrium between solution and membrane. By this potential, which is called the Donnan potential, co-ions are repelled by the membrane, whereas counter-ions are attracted. The
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Donnan equilibrium is dependent on factors viz. feed concentration, fixed charge density in the membrane and valence of the co-ion or counter-ion. One of the factors influencing the separation in case of single ionic solute solutions is the distribution of the co-ions between membrane and solution. In both charged and uncharged membranes, the distribution coefficient can be influenced by the interaction between the ions and the membrane material. In case of charged membranes, the equilibrium distribution of ions between membrane and solution is influenced by the presence of charged groups at the membrane as well. In the previous work [4–6], after an emulsion polymerization in which an anionic fluorochemical surfactant was used as an emulsifier in the polymerization of fluorinated monomers, the recovery of the anionic surfactant from the residual liquid by utilizing RO process has also been investigated. It would be beneficial from the point of view of recycling the expensive materials as well as protecting the environment. For the separation of the anionic surfactant from the residual liquid in the emulsion polymerization, the selection of an anionic polymer as membrane material could be rationalized in terms of minimizing concentration polarization and membrane fouling because the anionic membrane would repel the anionic surfactant molecules from its surface by electrostatic repulsion. When the charged membrane was used in these processes, the separation of the ionic compound could be affected by the intrinsic characteristics of the membranes as well as the electrostatic interaction between the ionic permeant molecules and the charged membrane. In this study, in the separation of an anionic surfactant, ammonium perfluoroalkyl carboxylates from the residual aqueous solution in polytetrafluoroethylene (PTFE) emulsion polymerization, RO process was carried out by using three kinds of charged membranes, that is, anionic, cationic and nonionic membranes. Sodium alginate (SA), chitosan (CS) and poly(vinyl alcohol) (PVA) were employed as the anionic, cationic and nonionic membranes, respectively. The permeation behaviors were investigated with the charge characteristic of the membrane in terms of electrostatic interaction between permeant and membrane. Also, the effects of operating condition, such as feed pressure, feed concentration and permeation rate, have been discussed in terms of membrane fouling and concentration polarization.
2. Experimental 2.1. Materials SA and CS (extrapure grades) were purchased from Showa Chemical (Tokyo, Japan), calcium chloride (extrapure grade) from Oriental Chemical (Seoul, Korea), hydrochloric acid (35% content guaranteed reagent) and glutaraldehyde (GA) (25% content in water, pure grade) from Junsei Chemical (Tokyo, Japan). PVA was purchased from Aldrich. The average molecular weight and saponification of the PVA were 30,000 and 99%, respectively. An anionic surfactant, ammonium perfluoroalkyl carboxylates (FluoradTM Fluorochemical surfactant FC-143, hereafter called FC-143) was purchased from 3M Company (USA). Calcium chloride and GA were used as cross-linking agents of SA and PVA, respectively, and HCl was used as a catalyst in the cross-linking reaction between GA and PVA. A commercial polysulfone (PS) ultrafiltration membrane was used as a support substrate. The support membrane was generously provided by Saehan (Seoul, Korea). The membrane has an asymmetric structure supported by nonwoven polyester fabric. The molecular cut-off was 30,000 for SH membrane. Ultrapure deionized water was used. All chemicals were used without any further purification. 2.2. Membrane preparation 2.2.1. Homogeneous membranes 2.5 wt.% of SA casting solution was prepared by dissolving SA in distilled water. All solutions were filtered before use to remove undissolved particles. The casting solution was cast onto a glass plate with the aid of a Gardner casting knife. The cast films were immersed in a calcium chloride–CS aqueous solution for preparing the cross-linked SA membrane for 30 min. The concentrations of CaCl2 and CS in the aqueous solution were fixed at 1.5 and 0.1 wt.%, respectively, as suggested in the previous work [4]. For the preparation of the CS membrane, 2% content of CS aqueous solution containing 1.5 vol% acetic acid was cast on a glass plate. After being dried in a fume hood, the membrane was soaked in 2 vol% H2 SO4 solution (water/ethanol = 50/50 volume ratio) for 10 min and then washed with distilled water several times. All the dry membranes fabricated were 11–13 m in thickness. A
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Fig. 1. Sequence of fabrication of thin film composite membranes.
detail on the preparation of the SA and CS membranes is described elsewhere [4]. A PVA casting solution was prepared by dissolving PVA in distilled water at about 80◦ C. Polymer content in the solution was 10 wt.%. The casting solution was cast onto a glass plate with the aid of a Gardner casting knife and dried at room temperature in a fume hood for 1 day. The dry membrane was peeled off the glass plate and immersed at 30◦ C for 48 h in a reaction solution which contained 5 vol% of GA and 0.05 vol% of HCl in acetone. After the cross-linking reaction, the membrane was taken out of the reaction solution, washed out several times with pure methanol to eliminate any possible residual HCl and GA, and then dried under vacuum for 24 h. A detail on the preparation of the PVA membrane is described elsewhere [7]. Membranes prepared ranged from 12 to 15 m in thickness. 2.2.2. Fabrication of thin film composite membranes In an earlier work [5,6], a new coating technique had been established to fabricate thin film composite membranes, by which hydrophilic polymers could be coated in thin film on hydrophobic support membranes. In this study, the three hydrophilic polymers, SA, CS, and PVA, were coated on the PS support membrane in thin film composite membrane, respectively, by using the established technique. The fabrication of the thin film composite membrane is depicted schematically in Fig. 1. First, the PS support membrane was pretreated with a reactant to the cast polymer to increase its surface energy or to increase its wettability by dipping in its aqueous solution. Thereby,
the reactant molecules were dispersed in the pores and the surface of the support so that the polymer could be coated uniformly thin on the support surface by an interfacial reaction between the reactant and the hydrophilic polymer. The respective cross-linking agents to the cast polymers were used as the reactant. The detail of the coating procedure is well described elsewhere [5,6]. The prepared composite membranes were 0.3–0.7 m in coating thickness. 2.3. Measurements of the swelling ratio of membrane and pH of feed solution with its concentration Swelling measurements of the ionic membranes were performed to determine the amount of the anionic surfactant aqueous solution absorbed into the membranes in electrochemical equilibrium between the solution and the membranes. Dry membrane strips were immersed in the aqueous solutions with various contents thermostated at 30◦ C for 48 h to allow the strips to reach equilibrium sorption. The dimension of a strip was about 7 cm × 1.5 cm. After measuring the swollen length, l, of a strip at equilibrium sorption, the strip were dried for 30 h at room temperature under vacuum and then the dry length, l0 was measured. The swelling ratio, R, for an isotropic material is defined as R=
l − l0 l0
(1)
All measurements were repeated four or five times and the resulting data had a standard deviation of
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Fig. 2. A schematic representation of a reverse osmosis test apparatus.
±6%. pH of the aqueous solution was measured by a pH meter (686 Titroprocessor, Metrohm, Swiss) at surfactant concentrations from 0 to 2000 ppm and at room temperature. 2.4. Reverse osmosis experiments A schematic representation of the reverse osmosis test apparatus is shown in Fig. 2. It consists of four flat sheet membrane cells, and a closed loop recycle system in which the piping, fittings and the cells were made of SS-316. A diaphragm pump (Hydro-cell Model-13, Wanner Engineering, MN, USA) equipped with a pressure regulating valve (Model C46) was used for recycling the anionic surfactant solution through the system at a pressure. The membrane cell was designed to allow high fluid velocity parallel to the
membrane surface. The effective membrane area in the membrane cell was 19.63 cm2 . Feed from the feed tank was passed through a heat exchanger and four membrane cells subsequently and into a back-pressure regulator (TESCOM Corporation, MN, USA) through stainless steel tubing. The low pressure retentate travelled from the back-pressure regulator through a rotameter (Brooks Instrument Division, Emerson Electric, MN, USA) and was returned to the feed tank. Temperature was monitored in the feed line and in the retentate line by K-type thermocouples using a digital thermometer (Han Young Model DX7, Seoul, Korea). The membranes were rinsed with deionized water before using. The feed solutions were aqueous FC-143 solutions with solute contents from 0 to 2000 ppm. Pure water was pressurized and circulated over the surface of the membranes at 50 bar for
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at least 4 h before any measurements of membrane performances were made to minimize any effect of membrane compacting. After pressuring the membrane and then adjusting feed pressure to a desired value, pure water fluxes (PWFs) were measured. Subsequently, solution fluxes started to be measured with permeating time when an amount of concentrated solution determined to set a given feed concentration was introduced into the feed tank containing pure water. The feed flow through the system was maintained at 1.0 l/min and the feed temperature was kept constant at 30 ± 0.1◦ C. Samples of the permeate were collected at various time intervals to determine the permeate flux through the membrane. The conductivities of the permeate and feed solution were measured by a Conductance–Resistance Meter (YSI Model 34, YSI Scientific, OH, USA) and compared with a calibration curve to give solute concentrations in the permeate and the feed solution, respectively. The rejection exhibited on a membrane was defined as CP 100 (2) Rejection% = 1 − CF
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Fig. 3. Plot of pH with solute concentration in feed solution at 30◦ C.
where CP and CF are surfactant concentrations in bulk permeate and feed, respectively. The data on fluxes and rejections were determined by multiple replications and standard deviations of the observations were determined to be 5% for flux and 0.5% for rejection.
3. Results and discussions 3.1. Swelling behavior with solute concentration in feed Fig. 3 presents pH change of feed solution with its solute concentration. The solution gave the pH value that decreased rapidly and then slightly with increase in the former’s concentration, showing a more acidic characteristic. Fig. 4 demonstrates the corresponding sorption property of each membrane with solute concentration in feed. PVA membrane with nonionic characteristics had the constant value of swelling ratio independent of solute concentration. For the anionic SA membrane, the swelling ratio increased with feed concentration because the anionic membrane tends to be more ionized and thereby characterized as being more hydrophilic under a more acidic circumstance.
Fig. 4. Swelling ratios of SA, CS and PVA membranes with feed composition, respectively, at 30◦ C.
On the contrary, the cationic CS membrane had a dramatic decrease in swelling ratio with increasing solute content. It could be explained in terms of the neutralization by ionic complexation between anionic solute molecules and cationic sites in the membrane by which the resulting membrane loses its hydrophilicity and then is less swollen in the aqueous solution. According to the solution–diffusion mechanism [8], such sorption behaviors of the ionic membranes would
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Fig. 5. Fluxes of homogeneous membranes with permeation time at 30◦ C: feed pressure = 40 bar, solute concentration in feed = 1000 ppm.
affect differently the permeation performance of the anionic surfactant solution through the respective membrane, which will be discussed later. 3.2. Effect of permeating time on permeation properties of the ionic solute 3.2.1. Homogeneous membranes Fig. 5 illustrates the fluxes of the anionic surfactant solution through the three homogeneous membranes at 40 bar of feed pressure. Here, the initial flux through the respective membrane at time = 0 was its PWF. Solution fluxes were observed to be increased or decreased with permeating time, depending on the charge characteristic of the membrane. Solution flux through the nonionic PVA membrane was little changed and it kept as much as the PWF along with operating time. However, both the anionic and cationic membranes underwent remarkable flux changes which were opposite: they had very rapid and significant incline and decline in fluxes with permeating time in the beginning, respectively, and then they attained gradually their steady state values. The fluxes through the membranes were normalized by dividing them by the respective PWFs to explain the flux change relative to PWF. The flux normalized with re-
Fig. 6. Flux ratios and rejection% of homogeneous membranes with permeation time at 30◦ C: feed pressure = 40 bar, solute concentration in feed = 1000 ppm, J = solution flux, J0 = pure water flux (PWF).
spect to PWF, that is, flux ratios of solution flux to PWF were plotted against permeating time in Fig. 6. The solution flux through SA membrane was enhanced by more than two times, in comparison with its PWF. It is associated with an increase in the hydrophilicity of the membrane by ionizing the charged groups in the membrane to a great extent under such an acidic circumstance in the feed solution as discussed in terms of the sorption behaviors. Also, the high rejection characteristic of the SA membrane is attributed to the repulsive interaction between the anionic solute and the anionic membrane. In the case of CS membrane, PWF was the highest in value among the membranes, but the solution flux decreased rapidly to less than 10% of PWF in
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the initial stage and then decreased slightly further with permeating time. Basically, the flux-decline results in the concentration polarization of solutes and membrane fouling which are caused by selective permeation. However, when the cationic membrane was involved, ionic complexation took place between the ionic surface of the membrane and the counter-ions so that the anionic solute molecules could be adsorbed on the surface, causing the membrane fouling. In addition, electrostatic attraction developed between the membrane and the counter-ion solute could facilitate the concentration polarization at the adjacent feed. Thus, in turn, these phenomena presumably increased both adsorbed layer resistance and osmotic pressure which resulted in the significant flux-decline. Also, the cationic membrane gave poor rejection of the anionic solute because the counter-ion solute could permeate to some extent due to the attractive interaction. On the other hand, the nonionic PVA membrane kept a constant permeation performance along with permeating time, showing no relationship between membrane performance and the electrostatic interaction like the charged membranes.
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the composite membrane were much greater than those of the homogeneous membranes. The CS membrane is observed to undergo a rapid and significant flux decline as in the homogeneous membrane, but the SA and PVA composite membranes had flux changes different from those of the corresponding homogeneous membranes. Looking at the flux ratios in Fig. 8, all of the ratio values in composite membranes for SA and PVA at a time were smaller than those in the homogeneous membranes. The ratio for SA membrane increases rapidly to maximum initially and then decreases slightly with time, and that for PVA membrane decreases continuously with time. Decreasing the ratio for both the membranes might have something to do with the high flux of the composite membranes. In
3.2.2. Thin film composite membranes Fig. 7 exhibits the solution fluxes with time through the composite membranes, respectively. The PWFs of
Fig. 7. Fluxes of composite membranes with permeation time at 30◦ C: feed pressure = 40 bar, solute concentration in feed = 1000 ppm.
Fig. 8. Flux ratios and rejection% of composite membranes with permeation time at 30◦ C: feed pressure = 40 bar, solute concentration in feed = 1000 ppm, J = solution flux, J0 = pure water flux (PWF).
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principle, as flux through the membrane increases, the concentration polarization of solute or membrane fouling occurs significantly, resulting in suppression of the permeation. From this point of view, decrease in the ratio is obviously due to the concentration polarization or membrane fouling induced by high solution flux through the composite membranes. As a result, it is summarized that the decrease in the flux ratio is related to the membrane fouling or concentration polarization which is presumably caused by high solution flux rather than the electrostatic interaction between solute and membrane for both PVA and SA membranes, whereas the electrostatic interaction played a crucial part in the membrane fouling and concentration polarization for the CS membrane.
3.3. Effect of feed pressure
Fig. 9. Flux ratios and rejection% of homogeneous membranes with feed pressure at 30◦ C: solute concentration in feed = 1000 ppm.
Fig. 10. Flux ratios and rejection% of composite membranes with feed pressure at 30◦ C: solute concentration in feed = 1000 ppm.
Figs. 9 and 10 present the permeation performances of pure water and solution with feed pressure through the homogeneous and composite membranes, respectively. Solution flux and rejection% were measured 30 min after permeation started. Similarly, as discussed previously, solution flux through SA membrane was enhanced, having a higher value than PWF, while solution fluxes in CS and PVA membranes were suppressed, giving lower values than PWF. It is found from these figures that the ratio of solution flux to PWF decreases with increasing feed pressure for all the membranes regardless of the type of membrane. Also, the ratio decrease was found to be more sig-
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Fig. 11. Flux ratio and rejection% of homogeneous SA membrane with feed pressure at 30◦ C and different solute concentrations in feed.
nificant in composite membrane rather than in homogeneous membrane. In principle, flux is proportional to feed pressure and inversely proportional to membrane thickness in reverse osmosis. Thus, it could be rationalized that when an increase in solution flux was achieved by either increasing feed pressure or decreasing membrane thickness in this study, membrane fouling and concentration polarization became significant, causing the suppression of the permeation. From the observations, it could be confirmed again that increasing flux by increasing the feed pressure or by using composite membrane negatively affects the flux ratio. Rejection was observed to be increased for SA membrane while it decreased for both CS and PVA membranes with increasing feed pressure. 3.4. Effect of feed composition Figs. 11–13 present membrane performance with feed composition for the charged and noncharged
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Fig. 12. Flux ratio and rejection% of CS homogeneous membrane with feed pressure at 30◦ C and different solute concentrations in feed.
membranes. The membrane shows a completely different tendency of membrane performance with feed composition, depending on the charged characteristic of the membrane. In the case of the anionic SA membrane, the flux ratio increased with increasing solute concentration in feed. In other words, the permeation of solution could be enhanced more at a higher solute concentration. As discussed in Figs. 3 and 4, when the solute concentration was higher, the resulting solution was characterized as being more acidic and the charged groups of the anionic membrane in contact with the solution were ionized more, and thereby, the membrane could be more hydrophilic and there could be greater permeation of the solution. Some evidence for the greater ionization at higher solute concentration could be found from the rejection% data. At higher solute concentration, rejection% of SA membrane is observed
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solute molecules. As a result of the ionic complexation between the cationic membrane and counter-ion molecules, the CS membrane would have an adsorption layer formed on its surface and a less hydrophilic characteristic which could suppress the permeation of the water component of the solution, being in favor of the permeation of solute instead. Therefore, with increasing feed composition, solution flux is reduced and the rejection of solute could be deteriorated by the ionic complexation. The membrane performance of the nonionic PVA membrane seems to be independent of feed composition similar to what is shown as regards sorption behavior in Fig. 4. From the observations given above, it can be seen that the permeation of the anionic surfactant solution through the charged membranes is affected considerably by the sorption properties.
4. Conclusions
Fig. 13. Flux ratio and rejection% of PVA homogeneous membrane with feed pressure at 30◦ C and different solute concentration in feed.
to be higher as well. More ionized SA membrane in higher content feed would produce stronger repulsive interaction between the anionic solute molecules and the membrane. Therefore, as a result of greater ionization of the charged groups of the SA membrane at higher solute content, higher rejection% could be accomplished by the stronger repulsive force developed between the membrane and the solute. A decrease in the flux ratio could be observed with increasing feed pressure at 2000 ppm of solute concentration, which is because of high flux as mentioned above. On the other hand, the cationic CS membrane exhibits a opposite trend of membrane performance with feed composition: the higher the solute concentration in feed, the lower both the flux ratio and the rejection% are. When the solute concentration was higher, more charged groups of the membrane took part in neutralization by ionic complexation with the counter-ionic
For the separation of an anionic surfactant, ammonium perfluoroalkyl carboxylates from the residual aqueous solution in PTFE emulsion polymerization, RO process was carried out by using three kinds of charged membranes, that is, anionic, cationic and nonionic membranes. SA, CS and PVA were employed as the anionic, cationic and nonionic membranes, respectively. The permeating ionic solute behaved differently in permeation through these three membranes due to different electrostatic interactions with the membrane. The effect of the charge characteristic of the membrane has been discussed in terms of the permeation and separation of the ionic solute in RO process. Permeation of the surfactant solution was enhanced for the anionic SA membrane by greater ionization of the charged groups of the membrane, thereby characterizing the membrane as being more hydrophilic when in contact with the solution rather than with pure water. For the cationic CS membrane, significant ionic complexation took place between the membrane and the counter-ion solute, resulting in a less hydrophilic characteristic of the membrane and the formation of an adsorption layer, which made the permeation performance of the CS membrane so poor. The nonionic PVA membrane showed membrane performance independent of solute electrostatic property. Also, increas-
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ing flux is found to accelerate membrane fouling and concentration polarization. [5]
References [1] T. Tsuru, M. Urairi, S.I. Nakao, S. Kimura, Reverse osmosis with single and mixed electrolytes with charged membranes: experiment and analysis, J. Chem. Eng. Jpn. 24 (1991) 518. [2] W.T. Bowen, H. Mukhtar, Characterization and prediction of separation performance of nanofiltration membranes, J. Membr. Sci. 112 (1996) 263–274. [3] J.M.M. Peeters, J.P. Boom, M.H.V. Mulder, H. Strathmann, Retention measurements of nanofiltration membranes with electrolyte solutions, J. Membr. Sci. 145 (1998) 199–209. [4] C.K. Yeom, C.U. Kim, B.S. Kim, K.J. Kim, J.M. Lee, Recovery of anionic surfactant by reverse osmosis membrane
[6]
[7]
[8]
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process Part I. Preparation of polyelectrolyte-complexed anionic membranes, J. Membr. Sci. 143 (1998) 207–218. C.K. Yeom, C.U. Kim, B.S. Kim, K.J. Kim, J.M. Lee, Recovery of anionic surfactant from waste water by reverse osmosis membrane process Part II. Fabrication of thin film composite membranes of by interfacial reaction, J. Membr. Sci. 156 (1999) 192–206. C.K. Yeom, C.U. Kim, B.S. Kim, K.J. Kim, J.M. Lee, Stable polyion complex membranes with a double structure, Korean Patent, KR 97-40677 (1997) and PCT International Patent, PCT/KR 98/00321 (1998). C.K. Yeom, K.-H. Lee, Pervaporation separation of water–acetic acid mixtures through poly(vinyl alcohol) membranes crosslinked with glutaraldehyde, J. Membr. Sci. 109 (1996) 257–265. E.A. Mason, Statistical-mechanical theory of membrane transport, J. Membr. Sci. 51 (1990) 1–81.
Effect of the ionic characteristics of charged membranes on the permeation of anionic solutes in reverse osmosis C.K. Yeom ∗ , S.H. Lee, J.M. Lee Chemical Process and Engineering Center, Applied and Engineering Chemistry Division, Korea Research Institute of Chemical Technology, P.O. Box 107, Yusong, Taejon 305-606, South Korea Received 24 May 1999; received in revised form 20 October 1999; accepted 25 October 1999
Abstract In the separation of an anionic surfactant, ammonium perfluoroalkyl carboxylates from the residual aqueous solution in poly(tetrafluoroethylene) (PTFE) emulsion polymerization, reverse osmosis (RO) process was carried out by using three kinds of charged membranes, that is, anionic, cationic and nonionic membranes. Sodium alginate (SA), chitosan (CS) and poly(vinyl alcohol) (PVA) were prepared as the anionic, cationic and nonionic membranes, respectively. The permeation behaviors were investigated with the charge characteristics of the membranes in terms of electrostatic interaction between permeant and membrane, namely, Donnan potential. Also, the effects of operating condition, such as, feed pressure, feed concentration and permeation rate, have been discussed in terms of membrane fouling and concentration polarization. ©2000 Elsevier Science B.V. All rights reserved. Keywords: Charged membranes; Reverse osmosis; Anionic surfactant; Membrane fouling; Sodium alginate; Poly(vinyl alcohol); Chitosan
1. Introduction In reverse osmosis (RO) processes employing ion-charged membranes, one of the most important features of the membranes is their ability to separate ion solutes from water. Possible mechanisms for the separation of electrolytes are (1) sieving, (2) electrostatic interactions between the membrane and the ions or between the ions mutually and (3) difference in diffusivity and solubility or a combination of these [1]. Their separation potentials for ionic solutes in aqueous solutions can be explained with the chemical potential gradient and the electrical potential gradient as driving force. Thus, the transport of the ions ∗ Corresponding author. E-mail address: [email protected] (C.K. Yeom).
across the charged membrane is well described by the extended Nernst–Planck flux equation in terms of diffusion and migration terms, as a result of concentration and electrical potential gradient [2,3]. In the charged membrane in contact with an electrolyte solution, the concentration of co-ions, i.e., ions with the same charge as the membrane, in the membrane will be lower than that in solution, whereas the counter-ions, which have the opposite charge, have a higher concentration in the membrane than in the solution. On account of this concentration difference of the ions, a potential difference is generated at the interface between the membrane and the solution to maintain electrochemical equilibrium between solution and membrane. By this potential, which is called the Donnan potential, co-ions are repelled by the membrane, whereas counter-ions are attracted. The
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Donnan equilibrium is dependent on factors viz. feed concentration, fixed charge density in the membrane and valence of the co-ion or counter-ion. One of the factors influencing the separation in case of single ionic solute solutions is the distribution of the co-ions between membrane and solution. In both charged and uncharged membranes, the distribution coefficient can be influenced by the interaction between the ions and the membrane material. In case of charged membranes, the equilibrium distribution of ions between membrane and solution is influenced by the presence of charged groups at the membrane as well. In the previous work [4–6], after an emulsion polymerization in which an anionic fluorochemical surfactant was used as an emulsifier in the polymerization of fluorinated monomers, the recovery of the anionic surfactant from the residual liquid by utilizing RO process has also been investigated. It would be beneficial from the point of view of recycling the expensive materials as well as protecting the environment. For the separation of the anionic surfactant from the residual liquid in the emulsion polymerization, the selection of an anionic polymer as membrane material could be rationalized in terms of minimizing concentration polarization and membrane fouling because the anionic membrane would repel the anionic surfactant molecules from its surface by electrostatic repulsion. When the charged membrane was used in these processes, the separation of the ionic compound could be affected by the intrinsic characteristics of the membranes as well as the electrostatic interaction between the ionic permeant molecules and the charged membrane. In this study, in the separation of an anionic surfactant, ammonium perfluoroalkyl carboxylates from the residual aqueous solution in polytetrafluoroethylene (PTFE) emulsion polymerization, RO process was carried out by using three kinds of charged membranes, that is, anionic, cationic and nonionic membranes. Sodium alginate (SA), chitosan (CS) and poly(vinyl alcohol) (PVA) were employed as the anionic, cationic and nonionic membranes, respectively. The permeation behaviors were investigated with the charge characteristic of the membrane in terms of electrostatic interaction between permeant and membrane. Also, the effects of operating condition, such as feed pressure, feed concentration and permeation rate, have been discussed in terms of membrane fouling and concentration polarization.
2. Experimental 2.1. Materials SA and CS (extrapure grades) were purchased from Showa Chemical (Tokyo, Japan), calcium chloride (extrapure grade) from Oriental Chemical (Seoul, Korea), hydrochloric acid (35% content guaranteed reagent) and glutaraldehyde (GA) (25% content in water, pure grade) from Junsei Chemical (Tokyo, Japan). PVA was purchased from Aldrich. The average molecular weight and saponification of the PVA were 30,000 and 99%, respectively. An anionic surfactant, ammonium perfluoroalkyl carboxylates (FluoradTM Fluorochemical surfactant FC-143, hereafter called FC-143) was purchased from 3M Company (USA). Calcium chloride and GA were used as cross-linking agents of SA and PVA, respectively, and HCl was used as a catalyst in the cross-linking reaction between GA and PVA. A commercial polysulfone (PS) ultrafiltration membrane was used as a support substrate. The support membrane was generously provided by Saehan (Seoul, Korea). The membrane has an asymmetric structure supported by nonwoven polyester fabric. The molecular cut-off was 30,000 for SH membrane. Ultrapure deionized water was used. All chemicals were used without any further purification. 2.2. Membrane preparation 2.2.1. Homogeneous membranes 2.5 wt.% of SA casting solution was prepared by dissolving SA in distilled water. All solutions were filtered before use to remove undissolved particles. The casting solution was cast onto a glass plate with the aid of a Gardner casting knife. The cast films were immersed in a calcium chloride–CS aqueous solution for preparing the cross-linked SA membrane for 30 min. The concentrations of CaCl2 and CS in the aqueous solution were fixed at 1.5 and 0.1 wt.%, respectively, as suggested in the previous work [4]. For the preparation of the CS membrane, 2% content of CS aqueous solution containing 1.5 vol% acetic acid was cast on a glass plate. After being dried in a fume hood, the membrane was soaked in 2 vol% H2 SO4 solution (water/ethanol = 50/50 volume ratio) for 10 min and then washed with distilled water several times. All the dry membranes fabricated were 11–13 m in thickness. A
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Fig. 1. Sequence of fabrication of thin film composite membranes.
detail on the preparation of the SA and CS membranes is described elsewhere [4]. A PVA casting solution was prepared by dissolving PVA in distilled water at about 80◦ C. Polymer content in the solution was 10 wt.%. The casting solution was cast onto a glass plate with the aid of a Gardner casting knife and dried at room temperature in a fume hood for 1 day. The dry membrane was peeled off the glass plate and immersed at 30◦ C for 48 h in a reaction solution which contained 5 vol% of GA and 0.05 vol% of HCl in acetone. After the cross-linking reaction, the membrane was taken out of the reaction solution, washed out several times with pure methanol to eliminate any possible residual HCl and GA, and then dried under vacuum for 24 h. A detail on the preparation of the PVA membrane is described elsewhere [7]. Membranes prepared ranged from 12 to 15 m in thickness. 2.2.2. Fabrication of thin film composite membranes In an earlier work [5,6], a new coating technique had been established to fabricate thin film composite membranes, by which hydrophilic polymers could be coated in thin film on hydrophobic support membranes. In this study, the three hydrophilic polymers, SA, CS, and PVA, were coated on the PS support membrane in thin film composite membrane, respectively, by using the established technique. The fabrication of the thin film composite membrane is depicted schematically in Fig. 1. First, the PS support membrane was pretreated with a reactant to the cast polymer to increase its surface energy or to increase its wettability by dipping in its aqueous solution. Thereby,
the reactant molecules were dispersed in the pores and the surface of the support so that the polymer could be coated uniformly thin on the support surface by an interfacial reaction between the reactant and the hydrophilic polymer. The respective cross-linking agents to the cast polymers were used as the reactant. The detail of the coating procedure is well described elsewhere [5,6]. The prepared composite membranes were 0.3–0.7 m in coating thickness. 2.3. Measurements of the swelling ratio of membrane and pH of feed solution with its concentration Swelling measurements of the ionic membranes were performed to determine the amount of the anionic surfactant aqueous solution absorbed into the membranes in electrochemical equilibrium between the solution and the membranes. Dry membrane strips were immersed in the aqueous solutions with various contents thermostated at 30◦ C for 48 h to allow the strips to reach equilibrium sorption. The dimension of a strip was about 7 cm × 1.5 cm. After measuring the swollen length, l, of a strip at equilibrium sorption, the strip were dried for 30 h at room temperature under vacuum and then the dry length, l0 was measured. The swelling ratio, R, for an isotropic material is defined as R=
l − l0 l0
(1)
All measurements were repeated four or five times and the resulting data had a standard deviation of
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Fig. 2. A schematic representation of a reverse osmosis test apparatus.
±6%. pH of the aqueous solution was measured by a pH meter (686 Titroprocessor, Metrohm, Swiss) at surfactant concentrations from 0 to 2000 ppm and at room temperature. 2.4. Reverse osmosis experiments A schematic representation of the reverse osmosis test apparatus is shown in Fig. 2. It consists of four flat sheet membrane cells, and a closed loop recycle system in which the piping, fittings and the cells were made of SS-316. A diaphragm pump (Hydro-cell Model-13, Wanner Engineering, MN, USA) equipped with a pressure regulating valve (Model C46) was used for recycling the anionic surfactant solution through the system at a pressure. The membrane cell was designed to allow high fluid velocity parallel to the
membrane surface. The effective membrane area in the membrane cell was 19.63 cm2 . Feed from the feed tank was passed through a heat exchanger and four membrane cells subsequently and into a back-pressure regulator (TESCOM Corporation, MN, USA) through stainless steel tubing. The low pressure retentate travelled from the back-pressure regulator through a rotameter (Brooks Instrument Division, Emerson Electric, MN, USA) and was returned to the feed tank. Temperature was monitored in the feed line and in the retentate line by K-type thermocouples using a digital thermometer (Han Young Model DX7, Seoul, Korea). The membranes were rinsed with deionized water before using. The feed solutions were aqueous FC-143 solutions with solute contents from 0 to 2000 ppm. Pure water was pressurized and circulated over the surface of the membranes at 50 bar for
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at least 4 h before any measurements of membrane performances were made to minimize any effect of membrane compacting. After pressuring the membrane and then adjusting feed pressure to a desired value, pure water fluxes (PWFs) were measured. Subsequently, solution fluxes started to be measured with permeating time when an amount of concentrated solution determined to set a given feed concentration was introduced into the feed tank containing pure water. The feed flow through the system was maintained at 1.0 l/min and the feed temperature was kept constant at 30 ± 0.1◦ C. Samples of the permeate were collected at various time intervals to determine the permeate flux through the membrane. The conductivities of the permeate and feed solution were measured by a Conductance–Resistance Meter (YSI Model 34, YSI Scientific, OH, USA) and compared with a calibration curve to give solute concentrations in the permeate and the feed solution, respectively. The rejection exhibited on a membrane was defined as CP 100 (2) Rejection% = 1 − CF
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Fig. 3. Plot of pH with solute concentration in feed solution at 30◦ C.
where CP and CF are surfactant concentrations in bulk permeate and feed, respectively. The data on fluxes and rejections were determined by multiple replications and standard deviations of the observations were determined to be 5% for flux and 0.5% for rejection.
3. Results and discussions 3.1. Swelling behavior with solute concentration in feed Fig. 3 presents pH change of feed solution with its solute concentration. The solution gave the pH value that decreased rapidly and then slightly with increase in the former’s concentration, showing a more acidic characteristic. Fig. 4 demonstrates the corresponding sorption property of each membrane with solute concentration in feed. PVA membrane with nonionic characteristics had the constant value of swelling ratio independent of solute concentration. For the anionic SA membrane, the swelling ratio increased with feed concentration because the anionic membrane tends to be more ionized and thereby characterized as being more hydrophilic under a more acidic circumstance.
Fig. 4. Swelling ratios of SA, CS and PVA membranes with feed composition, respectively, at 30◦ C.
On the contrary, the cationic CS membrane had a dramatic decrease in swelling ratio with increasing solute content. It could be explained in terms of the neutralization by ionic complexation between anionic solute molecules and cationic sites in the membrane by which the resulting membrane loses its hydrophilicity and then is less swollen in the aqueous solution. According to the solution–diffusion mechanism [8], such sorption behaviors of the ionic membranes would
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Fig. 5. Fluxes of homogeneous membranes with permeation time at 30◦ C: feed pressure = 40 bar, solute concentration in feed = 1000 ppm.
affect differently the permeation performance of the anionic surfactant solution through the respective membrane, which will be discussed later. 3.2. Effect of permeating time on permeation properties of the ionic solute 3.2.1. Homogeneous membranes Fig. 5 illustrates the fluxes of the anionic surfactant solution through the three homogeneous membranes at 40 bar of feed pressure. Here, the initial flux through the respective membrane at time = 0 was its PWF. Solution fluxes were observed to be increased or decreased with permeating time, depending on the charge characteristic of the membrane. Solution flux through the nonionic PVA membrane was little changed and it kept as much as the PWF along with operating time. However, both the anionic and cationic membranes underwent remarkable flux changes which were opposite: they had very rapid and significant incline and decline in fluxes with permeating time in the beginning, respectively, and then they attained gradually their steady state values. The fluxes through the membranes were normalized by dividing them by the respective PWFs to explain the flux change relative to PWF. The flux normalized with re-
Fig. 6. Flux ratios and rejection% of homogeneous membranes with permeation time at 30◦ C: feed pressure = 40 bar, solute concentration in feed = 1000 ppm, J = solution flux, J0 = pure water flux (PWF).
spect to PWF, that is, flux ratios of solution flux to PWF were plotted against permeating time in Fig. 6. The solution flux through SA membrane was enhanced by more than two times, in comparison with its PWF. It is associated with an increase in the hydrophilicity of the membrane by ionizing the charged groups in the membrane to a great extent under such an acidic circumstance in the feed solution as discussed in terms of the sorption behaviors. Also, the high rejection characteristic of the SA membrane is attributed to the repulsive interaction between the anionic solute and the anionic membrane. In the case of CS membrane, PWF was the highest in value among the membranes, but the solution flux decreased rapidly to less than 10% of PWF in
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the initial stage and then decreased slightly further with permeating time. Basically, the flux-decline results in the concentration polarization of solutes and membrane fouling which are caused by selective permeation. However, when the cationic membrane was involved, ionic complexation took place between the ionic surface of the membrane and the counter-ions so that the anionic solute molecules could be adsorbed on the surface, causing the membrane fouling. In addition, electrostatic attraction developed between the membrane and the counter-ion solute could facilitate the concentration polarization at the adjacent feed. Thus, in turn, these phenomena presumably increased both adsorbed layer resistance and osmotic pressure which resulted in the significant flux-decline. Also, the cationic membrane gave poor rejection of the anionic solute because the counter-ion solute could permeate to some extent due to the attractive interaction. On the other hand, the nonionic PVA membrane kept a constant permeation performance along with permeating time, showing no relationship between membrane performance and the electrostatic interaction like the charged membranes.
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the composite membrane were much greater than those of the homogeneous membranes. The CS membrane is observed to undergo a rapid and significant flux decline as in the homogeneous membrane, but the SA and PVA composite membranes had flux changes different from those of the corresponding homogeneous membranes. Looking at the flux ratios in Fig. 8, all of the ratio values in composite membranes for SA and PVA at a time were smaller than those in the homogeneous membranes. The ratio for SA membrane increases rapidly to maximum initially and then decreases slightly with time, and that for PVA membrane decreases continuously with time. Decreasing the ratio for both the membranes might have something to do with the high flux of the composite membranes. In
3.2.2. Thin film composite membranes Fig. 7 exhibits the solution fluxes with time through the composite membranes, respectively. The PWFs of
Fig. 7. Fluxes of composite membranes with permeation time at 30◦ C: feed pressure = 40 bar, solute concentration in feed = 1000 ppm.
Fig. 8. Flux ratios and rejection% of composite membranes with permeation time at 30◦ C: feed pressure = 40 bar, solute concentration in feed = 1000 ppm, J = solution flux, J0 = pure water flux (PWF).
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principle, as flux through the membrane increases, the concentration polarization of solute or membrane fouling occurs significantly, resulting in suppression of the permeation. From this point of view, decrease in the ratio is obviously due to the concentration polarization or membrane fouling induced by high solution flux through the composite membranes. As a result, it is summarized that the decrease in the flux ratio is related to the membrane fouling or concentration polarization which is presumably caused by high solution flux rather than the electrostatic interaction between solute and membrane for both PVA and SA membranes, whereas the electrostatic interaction played a crucial part in the membrane fouling and concentration polarization for the CS membrane.
3.3. Effect of feed pressure
Fig. 9. Flux ratios and rejection% of homogeneous membranes with feed pressure at 30◦ C: solute concentration in feed = 1000 ppm.
Fig. 10. Flux ratios and rejection% of composite membranes with feed pressure at 30◦ C: solute concentration in feed = 1000 ppm.
Figs. 9 and 10 present the permeation performances of pure water and solution with feed pressure through the homogeneous and composite membranes, respectively. Solution flux and rejection% were measured 30 min after permeation started. Similarly, as discussed previously, solution flux through SA membrane was enhanced, having a higher value than PWF, while solution fluxes in CS and PVA membranes were suppressed, giving lower values than PWF. It is found from these figures that the ratio of solution flux to PWF decreases with increasing feed pressure for all the membranes regardless of the type of membrane. Also, the ratio decrease was found to be more sig-
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Fig. 11. Flux ratio and rejection% of homogeneous SA membrane with feed pressure at 30◦ C and different solute concentrations in feed.
nificant in composite membrane rather than in homogeneous membrane. In principle, flux is proportional to feed pressure and inversely proportional to membrane thickness in reverse osmosis. Thus, it could be rationalized that when an increase in solution flux was achieved by either increasing feed pressure or decreasing membrane thickness in this study, membrane fouling and concentration polarization became significant, causing the suppression of the permeation. From the observations, it could be confirmed again that increasing flux by increasing the feed pressure or by using composite membrane negatively affects the flux ratio. Rejection was observed to be increased for SA membrane while it decreased for both CS and PVA membranes with increasing feed pressure. 3.4. Effect of feed composition Figs. 11–13 present membrane performance with feed composition for the charged and noncharged
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Fig. 12. Flux ratio and rejection% of CS homogeneous membrane with feed pressure at 30◦ C and different solute concentrations in feed.
membranes. The membrane shows a completely different tendency of membrane performance with feed composition, depending on the charged characteristic of the membrane. In the case of the anionic SA membrane, the flux ratio increased with increasing solute concentration in feed. In other words, the permeation of solution could be enhanced more at a higher solute concentration. As discussed in Figs. 3 and 4, when the solute concentration was higher, the resulting solution was characterized as being more acidic and the charged groups of the anionic membrane in contact with the solution were ionized more, and thereby, the membrane could be more hydrophilic and there could be greater permeation of the solution. Some evidence for the greater ionization at higher solute concentration could be found from the rejection% data. At higher solute concentration, rejection% of SA membrane is observed
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solute molecules. As a result of the ionic complexation between the cationic membrane and counter-ion molecules, the CS membrane would have an adsorption layer formed on its surface and a less hydrophilic characteristic which could suppress the permeation of the water component of the solution, being in favor of the permeation of solute instead. Therefore, with increasing feed composition, solution flux is reduced and the rejection of solute could be deteriorated by the ionic complexation. The membrane performance of the nonionic PVA membrane seems to be independent of feed composition similar to what is shown as regards sorption behavior in Fig. 4. From the observations given above, it can be seen that the permeation of the anionic surfactant solution through the charged membranes is affected considerably by the sorption properties.
4. Conclusions
Fig. 13. Flux ratio and rejection% of PVA homogeneous membrane with feed pressure at 30◦ C and different solute concentration in feed.
to be higher as well. More ionized SA membrane in higher content feed would produce stronger repulsive interaction between the anionic solute molecules and the membrane. Therefore, as a result of greater ionization of the charged groups of the SA membrane at higher solute content, higher rejection% could be accomplished by the stronger repulsive force developed between the membrane and the solute. A decrease in the flux ratio could be observed with increasing feed pressure at 2000 ppm of solute concentration, which is because of high flux as mentioned above. On the other hand, the cationic CS membrane exhibits a opposite trend of membrane performance with feed composition: the higher the solute concentration in feed, the lower both the flux ratio and the rejection% are. When the solute concentration was higher, more charged groups of the membrane took part in neutralization by ionic complexation with the counter-ionic
For the separation of an anionic surfactant, ammonium perfluoroalkyl carboxylates from the residual aqueous solution in PTFE emulsion polymerization, RO process was carried out by using three kinds of charged membranes, that is, anionic, cationic and nonionic membranes. SA, CS and PVA were employed as the anionic, cationic and nonionic membranes, respectively. The permeating ionic solute behaved differently in permeation through these three membranes due to different electrostatic interactions with the membrane. The effect of the charge characteristic of the membrane has been discussed in terms of the permeation and separation of the ionic solute in RO process. Permeation of the surfactant solution was enhanced for the anionic SA membrane by greater ionization of the charged groups of the membrane, thereby characterizing the membrane as being more hydrophilic when in contact with the solution rather than with pure water. For the cationic CS membrane, significant ionic complexation took place between the membrane and the counter-ion solute, resulting in a less hydrophilic characteristic of the membrane and the formation of an adsorption layer, which made the permeation performance of the CS membrane so poor. The nonionic PVA membrane showed membrane performance independent of solute electrostatic property. Also, increas-
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ing flux is found to accelerate membrane fouling and concentration polarization. [5]
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