Cellulose Acetate and Sulfonated Polysulfone Blend Ultrafiltration

Membrane Laboratory, Department of Chemical Engineering, Anna University, Chennai-600 025, India. Ultrafiltration membranes are largely being applied ...
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Ind. Eng. Chem. Res. 2001, 40, 4815-4820

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Cellulose Acetate and Sulfonated Polysulfone Blend Ultrafiltration Membranes. Part III. Application Studies Malaisamy Ramamoorthy and Mohan Doraiswamy Raju* Membrane Laboratory, Department of Chemical Engineering, Anna University, Chennai-600 025, India

Ultrafiltration membranes are largely being applied for macromolecular and heavy metal ion separations from aqueous streams. Cellulose acetate and sulfonated polysulfone-based membranes prepared in the absence and presence of the polymeric additive poly(ethylene glycol) 600 in various compositions were subjected to the separation of macromolecular proteins such as bovine serum albumin, egg albumin, pepsin, and trypsin. Toxic heavy metal ions such as copper, nickel, cadmium, and zinc were subjected to separation by the blend membranes by complexing them with the polymeric ligand polyethyleneimine. The effect of the polymer blend compositions and additive concentrations on the rejection and permeate flux of both proteins and metal ions is discussed. The separation and permeate flux efficiencies of the blend membranes are compared with those of pure cellulose acetate, sulfonated polysulfone, and polysulfone membranes. Introduction Conventional separation techniques such as precipitation, ozonation, adsorption on activated carbon, and biological treatment suffer from drawbacks such as time, chemical, and manpower consumption; energy costs; and disposal problems coupled with environmental hazards. The nonconventional technique membrane ultrafiltration (UF) was found to be more effective in the separation of soluble proteins from aqueous solutions.1 The separation of proteins by membrane was found to be advantageous owing to the nondestructivity and limiting denaturation of proteins in the process.2 Intensive research has been carried out by several researchers on the transmission and rejection of BSA using cellulose acetate and polysulfone membranes, and they have ascertained that membrane ultrafiltration is a reliable process for macromolecular separations.3,4 Modified and unmodified polysulfone ultrafiltration membranes have been used for the fractionation of egg protein solution.5 The modified membranes had increased water flux because of their hydrophilic carboxyl and sulfonic groups. Cellulose acetate-polyurethane blend ultrafiltration membranes using PVP as an additive have been prepared recently and applied to the separation of proteins such as bovine serum albumin (BSA), egg albumin (EA), pepsin, and trypsin, achieving more than 90% separation.6,7 Similarly, several chemical, electronic, electro-coating, metal refining, and finishing industries face severe problems in terms of disposal of their waste streams when highly toxic or valuable constituents such as heavy metal ions are present. From these waste streams, heavy metals such as Cu, Ni, Zn, and Co could be separated and concentrated through binding of the target metal ions in a polyelectrolyte with water-soluble macromolecular compounds and subsequent ultrafiltration of the bound metals from the unbound components.8 Thus, toxic heavy metals could be eliminated from waste streams, and the pre* Corresponding author. E-mail: [email protected] and [email protected].

cious metals could be recovered and reused. The separation of Cu2+ and Ni2+ from Fe3+ ions by complexation with alginic acid has been attempted.9 Other researchers have studied the effect of operating parameters on the selective separation of heavy metals from binary mixtures via polymer-enhanced ultrafiltration.10 Cellulose acetate/mycel cellulose secondary acetate blend membranes have also been prepared and used to separate copper from aqueous feeds with 1000-3000 ppm of copper.11 Polyethyleneimine (PEI) was used as the chelating agent for the metal ion. Cellulose acetate (CA) was also blended with polyurethane (PU), and the blend membrane was used for the separation of Cu2+, Ni2+, Zn2+, and Cd2+.7 The present work is one of our series of investigations into the preparation of cellulose acetate/sulfonated polysulfone (CA/SPS) blend ultrafiltration membranes and their characterization and applications. The objective of the present paper is to study the effects of the polymer blend composition of the ultrafiltration membranes and the concentration of the polymeric watersoluble additive poly(ethylene glycol) 600 (PEG 600) on the rejection and product rate of proteins such as bovine serum albumin, egg albumin, pepsin, and trypsin and toxic heavy metal ions such as copper, nickel, cadmium, and zinc. We also expect that, because polysulfone or modified polysulfone has a wide range of pH, mechanical, and chemical resistance, the blend membranes made up of SPS will also exhibit pH tolerance and mechanical and chemical resistance. Experimental Section Materials. Polyethyleneimine (Mw ) 600 0001 000 000) 50% aqueous solution was procured from Fluka Chemie AG, Steinheim, Germany, and used as a 1 wt % aqueous solution for the metal complexation studies. Sodium phosphate monobasic anhydrous and sodium phosphate dibasic heptahydrate were procured from CDH Chemicals Ltd., Mumbai, India, and used for the preparation of phosphate buffer solutions. Proteins. Proteins, viz., bovine serum albumin (BSA), Mw ) 69kDa; pepsin, Mw ) 35 kDa; and trypsin, Mw )

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20 kDa, were purchased from SRL Chemicals Ltd., Mumbai, India, and used as received. Egg albumin (EA), Mw ) 45 kDa, was obtained from CSIR Bio Chemical Centre, New Delhi, India. Metal Salts. Copper sulfate (AR), nickel sulfate (AR), and zinc sulfate (AR) were procured from Merck (I) Ltd., Mumbai, India, and used as such for the preparation of aqueous metal ion solutions. Cadmium chloride (AR) was procured from Qualigens Fine Chemicals Ltd., Mumbai, India, and used as such. Deionized and distilled water was employed for the preparation of metal, protein, dextran, and 1 wt % polyethyleneimine (PEI) aqueous solutions and was also used for the preparation of the gelation bath. Rejection Studies. The ultrafiltration experiments were carried out using pure CA membranes; CA/SPS blend membranes of 95/5%, 85/15%, and 75/25% compositions as representatives of the blend membrane system; and pure SPS and PSf membranes at a transmembrane pressure of 345 kPa. Protein Rejection. Proteins such as BSA, EA, pepsin, and trypsin were dissolved (0.1 wt %) in phosphate buffer (0.5 M, pH 7.2) and used as standard solutions. For all experiments, the concentration of the feed solution was kept constant. After the membrane was mounted in the UF cell, the chamber was filled with individual protein solution and immediately pressurized under nitrogen atmosphere to the desired level (345 kPa), which was maintained constant throughout the run. Permeate was collected over measured time intervals in graduated tubes, and the tube contents were analyzed for protein content by spectrophotometry (Hitachi, model U-2000) at λmax ) 280 nm. The percent protein separation was calculated from the concentrations of the feed and permeate using the equation

% SR ) 1 -

()

Cp × 100 Cf

Upon completion of a run, the ultrafiltration cell was emptied, and the membrane was removed and washed gently with pure water to remove adherent protein solution and then reinserted in the clean cell for remeasurement of its pure-water flux. Metal Ion Rejection. Aqueous solutions of Cu(II), Ni(II), Zn(II), and Cd(II) with approximate concentrations of 1000 ppm in 1 wt % solution of PEI in deionized water were prepared. The pH’s of these aqueous solutions were adjusted to 6 ( 0.25 by adding small amounts of either 0.1 M HCl or 0.1 M NaOH. Solutions containing PEI and individual metal ions were thoroughly mixed and left standing for 5 days to complete binding.12,13 During ultrafiltration, for each run, the first few milliliters of the permeate was discarded. For the presetting of all of the membranes and the maintenance of constant flux, each metal ion-PEI chelate solution was run in the UF kit at 345 kPa (with compressed air). The permeate flux and percent separation were determined by analyzing the concentrations of the feed and permeate. The concentration of each metal ion in the permeate and feed was measured with an atomic absorption spectrophotometer (Perkin-Elmer 2380). The pH’s of the feed and permeate solutions were measured with an Elico pH meter. In the absence of metal ions, the concentration of PEI was also confirmed by UV-visible

spectrometry (Hitachi, model U-2000 spectrophotometer) at λmax ) 269 nm. Results and Discussion Protein Rejection Studies. The CA/SPS blend membranes of 95/5%, 85/15%, and 75/25% compositions in the absence and in the presence of various additive concentrations were subjected to protein separation under a nitrogen atmosphere, and the results were compared with the separation and product rate of pure CA, SPS, and PSf membranes. The pH of the protein solution was maintained constant at 7.2, as the permeation and rejection of proteins depend on the feed pH and the possible membrane solute interactions at various pH’s.14 A protein of low molecular weight, trypsin, was used for the ultrafiltration experiments, because it was expected that ultrafiltration of a high-molecular-weight protein at the beginning would cause fouling of membrane, which would spoil the originality of the pores for the separation and comparison of low-molecular-weight proteins. Thus, the separation was performed in the order trypsin, pepsin, egg albumin, and bovine serum albumin, and the permeate fluxes were also simultaneously measured. Role of Polymer Blend Composition. Cellulose acetate in the absence of any additive when subjected to the separation of BSA, EA, pepsin, and trypsin offered high separations of 95, 94, 84, and 75% respectively. For CA/SPS blend membranes in the absence of additive, as the SPS content was increased, the separation decreased for all proteins. Thus, for the 95/5% CA/ SPS blend membrane, BSA exhibited a rejection of 93%, which was reduced to 92% for 75/25% blend membrane. A similar trend was observed for other proteins with lower rejection values. Pure SPS membranes in the absence of additive, on the other hand, exhibited a lower separation of 88% for BSA. The rejection decreased in the order EA > pepsin > trypsin. The rejection results for the CA/SPS membranes in the absence of additive are comparable with those for the PSf membranes, which are 93 and 64% for BSA and trypsin, respectively. For all of the above membranes, BSA exhibited, a higher separation and trypsin exhibited a lower separation, which is due to the higher molecular weight of 69 kDa and lower molecular weight of 20 kDa of the respective proteins. Thus, the size of the solute played a major role in the separation performance. For the experiments, the range of accuracy fell between (0.5% of the corresponding rejection values. Role of Additive Concentration. The presence of additive in the casting solutions for CA, CA/SPS blend, and SPS membranes had a considerable effect on the separation efficiency. For CA (100%) membranes at 2.5 wt % additive, the BSA rejection was found to be 94%, which decreased to 76% upon increase of the additive to 10 wt %. Similar results were also observed for the other proteins, with varying magnitudes. This might be due to the leaching of the additive, PEG 600, from the membrane during gelation, thus creating pores on the membrane surface. This occurs because PEG 600 is hygroscopic and watersoluble in nature and, hence, will be dissolved in nonsolvent water and will leave the membrane surface during gelation.

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For CA/SPS blend membranes also, for a given polymer composition, when the additive was increased from 2.5 wt %, the separation efficiency decreased. All of the blend membranes with various additive concentrations showed similar trends for all of the protein molecules. Thus, the 95/5% CA/SPS membrane resulted in 93% rejection for BSA, which was reduced to 74% at a 10 wt % PEG 600 concentration. The other proteins also exhibited similar rejection behavior upon increase of additive. However, for all additive concentrations, the increase in SPS composition in the blend resulted in a decrease in the rejection value. Similarly, for a given additive concentration of 2.5 wt % and for a given protein molecule, the separation first decreased and then increased when the SPS composition in the blend membrane was increased. For example, for EA, when the SPS composition in the blend membrane was increased from 5 to 15 wt %, the separation decreased from 90 to 86%. However, when the SPS content was increased from 15 to 25%, the separation increased unexpectedly to 88%. Similar trends were observed for 7.5 and 10 wt % additive when the SPS composition in the blend was increased from 5 to 25 wt %. However, for 5 wt % additive, the above trend could have been observed had the study been carried out for 80/20% CA/SPS blend membranes. This is because of the interaction between the additive and the SPS in the presence of CA, which results in the formation of aggregate pores through slow precipitation and the formation of a “sponge”-type structure of membranes, as reported for PAN membranes by Xiuli and Coworkers.15 Similar trends were observed for all additive concentrations. The 95/5% blend membrane with 2.5 wt % additive exhibited higher rejections of 90, 70, and 64% for EA, pepsin, and trypsin, respectively, and lower separations of 60, 50, and 46% were observed for the 75/25% CA/SPS membrane with 10 wt % additive. Pure SPS membranes, being more hydrophilic in nature, showed significantly lower rejection values than their corresponding blend membranes. Thus, for a given protein molecule, when the additive in the membrane was increased from 2.5 to 10 wt %, the rejection value decreased from 83 to 55% for BSA, from 76 to 49% for EA, from 58 to 40% for pepsin, and from 56 to 36% for trypsin. The rejection values for the CA/SPS blend membranes in the presence of additive are, however, comparable to those of the PSf membranes, whose values fall in the ranges 91-85%, 89-84%, 75-67%, and 62-56% for BSA, EA, pepsin, and trypsin, respectively. For all of the membranes, BSA had higher separation because of its bigger size, and trypsin had lower separation because of its smallest molecular size of all. The range of accuracy fell between (0.5% for all of the corresponding rejection values. Protein Solution Product Rate Studies. The protein solution product rate values for the CA, CA/SPS, and SPS membranes both in the absence and in the presence of additive were measured, and the results are discussed. The flux values observed also represent the product rate efficiencies of the membranes. Role of Polymer Blend Composition. Pure CA (100%) membranes, in the absence of additive, showed the lowest permeate flux of 9.56 L m-2 h-1 for BSA. The other proteins had comparatively higher fluxes for pure CA membranes, as shown in Figure 1. For the CA/SPS blend membrane of 95/5% composition without additive, for a given protein molecule, for

Figure 1. Effect of PEG 600 concentration on flux of proteinss CA (100%) membranes.

Figure 2. Effect of PEG 600 concentration on flux of proteinss CA/SPS 95/5% blend membranes.

Figure 3. Effect of PEG 600 concentration on flux of proteinss CA/SPS 85/15% blend membranes.

example BSA, when the SPS content in the blend was increased from 5 to 25%, the flux also increased from 11.68 to 34.12 L m-2 h-1. Similar increases in flux values were observed for the other proteins and also for all other blend compositions (Figures 2-4). Pure SPS membranes in the absence of additive exhibited a higher flux value of 64.41 L m-2 h-1 for BSA than the respective blend membranes. All other proteins also had higher flux values for the pure SPS membranes as expected and depicted in Figure 5. The product rate

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Figure 4. Effect of PEG 600 concentration on flux of proteinss CA/SPS 75/25% blend membranes.

Figure 5. Effect of PEG 600 concentration on flux of proteinss SPS (100%) membranes.

values of CA/SPS blend membranes with high SPS contents are higher than those of PSf membranes, whose values were found to be 12.46, 16.62, 17.66, and 19.74 L m-2 h-1 for BSA, EA, pepsin, and Trypsin, respectively. In all of the above product rate studies, the magnitude of the flux of trypsin was higher than that of pepsin, which, in turn, was higher than that of EA. The lowest flux value for BSA might be due to the greater solute size (Einstein-Stokes radius) and to fouling because of its higher molecular weight. Further, for all SPS compositions, the product rate values of the membranes did not deviate to a larger extent from their pure-water flux values, indicating low fouling. Role of Additive Concentration. All of the membranes prepared in the presence of additive had higher fluxes than those prepared in the absence of additive. Thus, in pure CA (100%) membranes, a given protein molecule had enhanced flux when the additive amount was increased from 2.5 to 10 wt %. Thus, BSA had fluxes of 32.70 L m-2 h-1 for 2.5 wt % PEG in CA (100%) membranes and 72.52 L m-2 h-1 for 10 wt % PEG. Similar trends were observed for the other proteins also, as shown in Figure 1. For the CA/SPS blend membrane of 95/5% composition, an increase of the additive amount from 0 to 10 wt % increased the protein solution product rate from 11.68 to 90.18 L m-2 h-1 for BSA, as shown in Figure 2. All of the other blend compositions also exhibited similar behavior when the additive was increased, as depicted in Figures 3 and 4. Similar trends were also shown by all other proteins. This might be due to the

formation of macrovoids in the membrane, because of the faster rate of leaching of additive during gelation from the membrane matrix leading to higher flux values in proportion to the concentration of additive. This has been evidenced by SEM data from our previous studies. On the other hand, an increase of SPS up to 25% increased the flux linearly only for the additive concentration of 5 wt %; at other additive concentrations, the flux declined for 25% SPS membranes. Trends similar to those of the blend membranes with 2.5, 7.5, and 10 wt % additive concentrations could have also been observed for a 5 wt % additive concentration had the blend membrane of 80/20% CA/SPS blend composition been subjected to this study. This effect was exhibited by all of the protein molecules and might be due to the interactions between SPS and PEG in the presence of CA, as discussed earlier. Pure SPS membranes exhibited product rate values for all of the proteins that were higher than those of any blend membranes with corresponding additive concentrations. From Figure 5, it is observed that, at 10 wt % of additive, BSA showed a maximum flux of 180.77 L m-2 h-1, and trypsin had a maximum flux of 197.40 L m-2 h-1. The product rate value for the Psf membrane upon an increase of the additive amount from 2.5 to 10 wt % ranged from 36.62 to 75.32 L m-2 h-1 for BSA, from 39.74 to 80.54 L m-2 h-1 for EA, from 42.85 to 81.63 L m-2 h-1 for pepsin, and from 43.97 to 83.83 L m-2 h-1 for trypsin. The trends are similar to those shown in Figure 1. For all of the membranes, irrespective of additive concentration and polymer composition, the order of product rate was found to be trypsin > pepsin > EA > BSA. This trend can be explained by the fact that the flux value of a protein is inversely proportional to its size. Metal Ion Rejection Studies. In this investigation, aqueous feed solutions containing toxic heavy metal ions, such as Cu2+, Ni2+, Zn2+, and Cd2+, were prepared at a metal ion concentration of 1000 ppm and were complexed with 1 wt % polyethylenimine solution. To study the effect of PEI on rejection, experiments were carried out for metallic salt solutions in the absence of PEI, and it was observed that all metal ions of sulfate and chloride solutions completely passed through membranes over the entire range of acidic pH values investigated. Because all metal ions precipitate as insoluble hydroxides beyond pH 7, the pH was maintained at 6 ( 0.25 when the experiments were carried out for the metal-PEI complexes. At this pH, strong protonation of the metal chelates, along with a relatively larger extent of stretching of the complex, takes place.13 The roles of the polymer blend composition and additive concentration in the rejection behavior of metals ions were carried investigated, and the results are discussed below. Role of Polymer Blend Composition. The separation of metal ions was found to be influenced by the polymer composition. Thus, the pure cellulose acetate membranes in the absence of additive yielded the highest separation of 99.61% for Cu2+ ion. The other metal ions Ni2+, Zn2+, and Cd2+ also exhibited their highest separations of 96.02, 90.74, and 90.08%, respectively. The CA/SPS blend membranes, on the other hand, showed decreasing separations from 97.47 to 92.78% for Cu2+ metal ion when the SPS content was increased from 5 to 25% in the blend in the absence of additive. Similar trends were observed for all other metal ions.

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This decreasing order of separation with increasing SPS content in blend might be due to the inhomogeneity arising as a result of the higher SPS content creating voids in the blend membranes. The pure SPS membrane in the absence of additive yielded 84% separation for Cu2+ ion. The separation values for other metal ions Ni, Zn, and Cd by the SPS membrane were lower than those for the corresponding blend membranes, with values of 82, 61, and 59%, respectively. The rejection values of the CA/SPS membranes are comparable to those of PSf membranes. In all of the CA, CA/SPS, and SPS membranes, Cd2+ exhibited a lower percent separation than Zn2+, which, in turn, was lower than that of Ni2+. Copper exhibited higher separation because of its stronger complex formation with the polymeric ligand PEI.8 Also the trend coincides with the dependence of molecular or complex size on the rejection behavior of the metals ions. Role of Additive Concentration. The effects of the additive, PEG 600, concentration on the metal ion rejection behavior of pure CA, CA/SPS, and pure SPS membranes were determined, and it was observed that pure CA membranes exhibited a decrease in rejection behavior from 97.56 to 90% when the additive concentration was increased from 2.5 to 10 wt % for copper ions. For CA/SPS blend membranes of 95/5% composition, for Cu2+ metal ion, as the additive was increased from 2.5 to 10 wt %, the rejection decreased linearly from 94.15 to 83.95%. This trend was exhibited by all of the blend membranes for all of the metal ions studied in this investigation. The decrease in rejection value upon increase in additive content might be due to the leaching of additive during gelation creating larger pores. Further, for a given additive concentration, for example 2.5 wt %, when the SPS content in the blend was increased from 5% to 15% the rejection of BSA decreased from 94.15 to 89.57%. However, at 25% SPS, the rejection increased again to 91.16%. Similar trends were observed for all additive concentrations except 5 wt % of additive concentration, for the reason explained in the section on protein rejection studies. This unusual behavior of decreasing and then increasing rejection is due to the interaction between PEG and SPS in the presence of CA in the formation of pores, as explained earlier. Similar trends were also observed for all other additive concentrations. Pure SPS membranes exhibited a linear decrease in rejection for copper ion with increasing additive concentration. The rejection values were comparable with those of PSf membranes, whose rejection ranged from 91 to 54%. For all of the membranes, irrespective of their polymer composition and additive content, Cd2+ showed the lowest rejection, whereas Cu2+ exhibited higher separations than the other metal ions. This highest separation of Cu2+ might be due to the formation of stronger complexes with PEI through stable short bonds, in accord with the Jahn-Teller effect.16 Metal Ion Solution Product Rate Studies. The product rate of metal ions is essential to predicting the economics of the membrane separation process. Thus, the metal ion product rate values for all of the CA,CA/ SPS and SPS membranes were measured. The polymer blend composition and additive concentration have significant effects on the metal ion product rate behavior of the pure and blend membranes, as discussed in detail in the following two sections.

Role of Polymer Blend Composition. The pure CA membrane in the absence of additive offered a product rate value of 7.79 L m-2 h-1 for Cu2+ ion. The other metal ions also had lower flux values because the smaller size and number of pores of cellulose acetate. The values were found to be 10.12, 10.90, and 11.68 L m-2 h-1 for Ni2+, Zn2+, and Cd2+, respectively. For CA/SPS blend membranes, for Cu2+ metal ion, an increase of SPS in the blend from 5 to 25% increased the flux from 9.74 to 19.87 L m-2 h-1. Similarly, Ni2+ exhibited an increase of flux from 13.76 to 23.76 L m-2 h-1, Zn2+ from 14.02 to 25.71 L m-2 h-1, and Cd2+ from 15.58 to 26.49 L m-2 h-1. The flux trend for the 95/5% Ca/SPS composition membrane was similar to that shown in Figure 3, and the trends for the 85/15% and 75/25% CA/SPS membrane are similar to that shown in Figure 1. All of the metal ions showed similar trends. These results can be accounted for by the hydrophilic character of SPS. Pure SPS membranes exhibited a higher flux value of 66.49 L m-2 h-1 for Cu2+ than expected when compared with the other blend membranes. The flux trends of the SPS membrane for the other metal ions are similar to that shown in Figure 5. The product rate values for the CA/SPS blend membranes with higher SPS contents in the absence of additive were higher than those of the PSf membranes, whose values ranged from 14.54 to 29.74 L m-2 h-1 for Cu2+ to Cd2+. The lowest flux of Cu2+ might be due to the Cu-PEI complex having the greatest stability and largest size compared to the PEI complexes with Ni2+, Zn2+, and Cd2+.15 Cd2+ exhibited the highest flux for the above membranes because of its smaller size. Role of Additive Concentration. As the additive concentration was increased in the CA membrane from 2.5 to 10 wt %, the Cu2+ solution product rate also increased linearly from 20.25 to 71.68 L m-2 h-1. The flux values, however, were higher for the other metal ions, and the trend was found to be similar to that of Figure 5. In the CA/SPS blend membrane of 95/5% composition, an increase in additive concentration from 2.5 to 10 wt % increased the flux value from 31.16 to 77.92 L m-2 h-1 for the Cu2+ metal ion. This enhancement of the flux might be due to the higher rate of leaching of PEG from the membrane, leading to larger pore formation, as reported already.17 The product rate values for the 85/ 15% and 75/15% CA/SPS composition membranes were found to fall in the ranges of 45.19-118.44 and 42.07115.32 L m-2 h-1, respectively, for Cu2+ metal ion when the additive concentration was varied from 2.5 to 10 wt %. The other metal ion solutions exhibited relatively higher flux values, with the trends for the 95/5% blend membranes being similar to that of Figure 3 and the trends for the 85/15% and 75/25% blends being similar to that of Figure 1. On the other hand, for a given additive concentration, for example 2.5 wt %, when the SPS content was increased in the blend from 5 to 15%, the flux increased from 31.16 to 45.19 L m-2 h-1; however, at 25% SPS, the flux decreased to 42.07 L m-2 h-1. This trend was observed for all additive concentrations except for 5 wt % and can be explained on the basis of our earlier rejection results. This unusual trend might be due to solvent-nonsolvent exchange during gelation at higher PEG and SPS contents in the presence of CA and aggregation of SPS and PEG polymer matrixes, leading to smaller pore formation.18 The results also exhibit good

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agreement with our previous scanning electron microscopic observations and indicate that the experiments were carried out at identical and standard conditions. The trends in the permeate fluxes for the pure SPS and PSf membranes with various additive concentration were similar to that of Figure 5. Conclusion The preparation of membranes by blending cellulose acetate with sulfonated polysulfone offers a membrane system with a high hydrophilic nature. The additive PEG 600 was found to be compatible with the blend polymers. The rejection studies of CA/SPS blend membranes indicated a maximum of 95% for BSA, and the permeate flux was found to be higher than that obtained with membranes prepared from the individual polymers. Among the toxic metal ions studied, copper exhibited more than 97% separation. The percent rejection and product rate of the proteins and metal ions are comparable with those of the pure polymeric membranes. We also show that the incorporation of the sulfonic group in polysulfone, the blend composition, and the presence hydrophilic additive in the membrane casting solution play major roles in determining the separation and product rate efficiencies of the resulting membranes. Acknowledgment M.R. thanks CSIR, New Delhi, India, for the award of a senior research fellowship to him. Thanks are also given to the University Grants Commission, New Delhi, India, for financial assistance. Literature Cited (1) Kroner, K. H.; Schutte, H.; Hustedt, H.; Kula, M. R. Crossflow filtration in the down stream processing of enzymes. Process Biochem. 1984, April, 67. (2) Medda, D. A.; Nguyen, Q.; Dellaucherie, E. Biospecific ultrafiltration: A promising purification technique for proteins. J. Membr. Sci. 1981, 9, 337. (3) Nakatsuka, S.; Michaels, A. S. Transport and separation of proteins by ultrafiltration through sorptive and nonsorptive membranes. J. Membr. Sci. 1992, 69, 189. (4) Opong, W. S.; Zydney, A. L. Hydraulic permeability of protein layers deposited during ultrafiltration. J. Colloid Interface. Sci. 1991, 142, 41.

(5) Ehsani, N.; Parkkinen, S.; Nystrom, M. Fractionation of natural and model egg-white protein solutions with modified and unmodified polysulfone UF membranes. J. Membr. Sci. 1997, 123, 105. (6) Sivakumar, M.; Malaisamy, R.; Sajitha, C. J.; Mohan, D.; Mohan, V.; Rangarajan, R. Ultrafiltration application of cellulose acetate-polyurethane blend membranes. Eur. Polym. J. 1999, 35, 1649. (7) Sivakumar, M.; Malaisamy, R.; Sajitha, C. J.; Mohan, D.; Mohan, V.; Rangarajan, R. Preparation and performance of cellulose acetate polyurethane blend membranes and their applications II. J. Membr. Sci. 2000, 169 (2), 215. (8) Volchek, K.; Krentsel, E.; Zhilin, Y.; Shtereva, G.; Dytnersky, Y. Polymer binding/ultrafiltration as a method for concentration and separation of metals. J. Membr. Sci. 1993, 79, 253. (9) Solpan, D.; Sahan, M. The separation of Cu2+ and Ni2+ from Fe3+ ions by complexation with alginic acid and using a suitable membrane. Sep. Sci. Technol. 1998, 33 (6), 909. (10) Muslehiddinoglu, J.; Uludag, Y.; Ozbelge, H. O.; Yilmaz, L. Effect of operating parameters on selective separation of heavy metals from binary mixtures via polymer enhanced ultrafiltration. J. Membr. Sci. 1998, 140, 251. (11) Sivakumar, M.; Malaisamy, R.; Sajitha, C. J.; Mohan, D.; Mohan, V. Preparation and performance evaluation of polysulfone-cellulose acetate blend membranes for ultrafiltration. In Proceedings of the 4th National Symposium on Progress in Materials Research; IMRE/NUS: Singapore, 1998; pp 250-254. (12) Juang, R. S.; Chen, M. N. Measurement of binding constants of poly(ethyleneimine) with metal ions and metal chelates in aqueous media by ultrafiltration. Ind. Eng. Chem. Res. 1996, 35, 1935. (13) Jarvis, N. V.; Wagener, J. M. Mechanistic studies of metal ion binding to water-soluble polymers using potentiometry. Talanta 1995, 42 (2), 219. (14) Musale, D. A.; Kulkarni, S. S. Effect of membrane-solute interactions on ultrafiltration performance. J. Macromol. Sci., Rev. Macromol. Chem. Phys. 1998, C38 (4), 615. (15) Xiuli, Y.; Hongbin, C.; Xiu, W.; Yongxin, Y. Morphology and properties of hollow fibre membrane made by PAN mixing with small amount of PVDF. J. Membr. Sci. 1998, 146, 179. (16) Huheey, J. E. Inorganic Chemistry, 3rd ed.; Harper International Editions; Harper and Row: New York, 1983. (17) Young, T. H.; Wang, D. M.; Hsieh, C. C.; Chen, L. W. The effect of second phase inversion on microstructures in phase inversion EVAL membranes. J. Membr. Sci. 1998, 146, 169.

Received for review February 12, 2001 Revised manuscript received July 31, 2001 Accepted July 31, 2001 IE010150Y