Dynamically formed membranes prepared from aluminum ion

Dynamically Formed Membranes Prepared from Aluminum Ion E. D R I O L I , 1 H. K. L O N S D A L E , 2 AND W. P U S C H Max-Planck-Institut fiir Biophysik, 6 Frankfurt am Main, West Germany

Received January 21, 1974 Dynamically formed membranes have been prepared from dilute solutions of aluminum chloride during hyperfiltration experiments with partially annealed asymmetric cellulose acetate membranes. The formation of the membranes is reversible and is strongly influenced by the concentration (or pH) of the solution, as well as by the apparent porosity of the substrate membrane. In this note we describe certain phenomena we have observed in hyperfiltration experiments (1), which suggest that dynamic membrane formation m a y be a more general phenomenon than heretofore recognized; the observations m a y also have relevance to the flux decline or "compaction" phenomena commonly observed in both ultrafiltration and hyperfiltration.

were made with an atomic absorption spectrophotometer. RESULTS AND DISCUSSION

EXPERIMENTAL Hyperfiltration experiments were performed in an apparatus described previously (2). The membranes used were a series of asymmetric cellulose acetate membranes prepared in our laboratory according to the procedure described by Manjikian, Loeb, and McCutchan (3). The membranes were annealed in water at temperatures of 60 ° to 90°C prior to use. All experiments were performed at 25°C and at 40 atm applied pressure, with a feed brine velocity sufficient to minimize boundary layer effects (1). The area of the hyperfiltration cells was 12 cm 2. The chemicals used, LiC1 and AIC13.6H20, were reagent grade. Analyses of feed and permeate solutions for Li and A1 i Permanent address: Istituto di Principi di Ingegneria Chimica, Universita di Napoli, Naples, Italy. 2Permanent address: Route 3 Box 1352, Bend, Oregon 97701.

Shown in Fig. 1 is the volume flux Q, through three membranes as a function of time. The membranes are designated according to their annealing temperature, e.g. the " C A 60" membrane was annealed at 60°C. Prior to the zero time shown in Fig. 1, the volume flux was measured for several hours with 10-4 M LiC1 feed and found to be stable at the initial values shown in the figure. At time = 0, the feed was changed to a mixture of 10-4 M LiC1 and 3 X 10-4 M A1C13 and there was initially a very sharp decline in the rejection of Li + exhibited by the CA 60 and CA 75 membranes. Over a longer time period, there was a decline in volume flux for these same membranes, as shown in the figure, with an accompanying increase in the rejection of Li +. The membrane properties stabilized after about one day at which time the feed solution concentration was increased to 10-2 M LiCl and 3 X 10-2 M AIC13, with a return to essentially the initial volume flux rates, as shown in the figure, but with a rejection of Li + again much lower than the original value. The rejection of Li + during a similar series of tests is shown in Fig. 2. Here, the rejection

355 Copyright ~ 1975 by Academic Press, Inc. All rights of reproduction in any form reserved.

Journal of Colloid and Interface Science, Vol. 51, No. 3, June 1975

356

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magnitude was exhibited by the CA 75 membrane at both concentrations. When the concentration of the feed was increased to 10-2 M LiC1 anJ 3 X 10-2 M A1C13, the Li + rejection dropped sharply for the CA 60 and CA 75 membranes (curve D), and the volume fluxes for all membranes returned essentially to their initial values. Prior to the discussion of the experimental findings the following remarks should be made on the properties of cellulose acetate membranes with regard to small electrolyte concentrations. As was shown recently (4, 5), cellulose acet~ te membranes have a low fixed charge capacity of about 10-3 m E q / g wet membrane because of negatively charged

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DYNAMICALLY FORMED MEMBRANES carboxyl groups (R-COO-). Therefore, a cellulose acetate membrane will behave like a cation exchange membrane in the presence of dilute electrolyte solutions. Thus, in the range of electrolyte concentrations used, the properties of the asymmetric cellulose acetate membranes may be better described by ion exchange and pore mechanisms than by a solution-diffusion mechanism. Because of these low fixed charge capacities the asymmetric cellulose acetate membranes used should behave as cation exchange membranes at electrolyte concentrations of the order of mxgnitude of the fixed charge concentration. This effect is demonstrated by similar experiments with NaC1, the results of which are shown in Fig. 3. There the dependence of NaCI rejection on NaCI concentration is shown for two different situations, viz NaC1 alone and NaC1 plus a constant concentration of A1CIs. When only NaC1 is present, the NaCI rejection increases with decreasing NaCI concentration in agreement with a Donnan exclusion mechanism. On the other hand, in the presence of A1C13, the NaC1 rejection is nearly independent of the NaC1 concentration (Fig. 3) and stays at the low value of rejection which it approaches at higher NaC1 concentrations if no A1CI3 is present. In this range of concentration Donnan effects are already ineffective because of the large electrolyte concentration compared to the low fixed charge concentration. Thus, one could conclude from the experimental findings for LiC1 in the presence of A1Cl3 that the fixed charges of the low-annealed membranes are neutralized. This could be due to (1) A13+ ions which are taken up by the membranes in preference to Li+ ions, (2) H + ions which are present as hydrolysis products of A1C13 and will interact with the R-COO- groups of cellulose acetate, and/or (3) complexes of aluminum hydroxides such as ALIA13(OH)-]~+3 which should be present as will be discussed later. By these ions or molecules adsorbed on the pore walls, the rejection forces acting on the dissolved ions can change (6) as well as the mechanical

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FIo. 3. NaC1 rejection, rNacl, as a function of NaC1 brine concentration, c8', for pure NaCI feed solutions (O C)) and NaC1 feed solutions containing 3 X 10-4 M/1 A1C13(O 0) at 50 atm and 25°C, using an asymmetric cellulose acetate membrane annealed at 60°C. permeability due to a significant constriction of the pores. We believe the results are best understood in terms of a reversible, dynamic membrane created in the pores of the skin of the asymmetric membranes by some hydrolysis product of aluminum in the feed solution. The dynamic membrane apparently requires both certain conditions in the solution (pH and/or concentration) and a suitable fine porous substrate in which to form. The pH of the solution during the series of experiments described in Fig. 2 changed from 4.1 (at 3 X 10- 8 M A1C13) to 4.2 and then to 3.5 (at 3 X 10-2 M A1CI~). From these pH values and the complex aqueous chemistry of aluminum (7, 8, 9, 10), we hypothesize that a complex ion, such as AI[A18(OH)8]~ "+3, is responsible for the observed phenomena. The fact that no abrupt changes occurred in the flux or rejection exhibited by the membranes annealed at temperatures of 80°C or above suggests that the dynamic membrane formation is also dependent on the size of the pores in the cellulose acetate membrane. The decrease in Li + rejection in the presence of aluminum, which occurred with all the membranes initially (i.e., before "dynamic membrane formation") and again after the dynamic membrane was apparently destroyed, Journal of Colloid and Interface Science, Vol. 51, No. 3, June 1975

358

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FIG. 4. Rejection of Li+ vs annealing temperature, showing the effect of the order in which the solutions were used. requires comment. This may be due to a change of the fixed charge concentration (Donnan effect) as discussed before if only a Donnan exclusion mechanism is assumed to be responsible for the LiCI rejection in the range of LiC1 concentrations used. On the other hand, this is apparently the result of a special "Donnan membrane effect" in which the rejection of an ion can be markedly decreased, even to negative values, by the presence of an excess of an ion of the same sign which is highly rejected by the membrane. This was demonstrated by Lonsdale et al. (11) with NaCI-Na2SO4 mixtures, and is the subject of a more thorough, current work (12). In the present experiments, the rejection of aluminum was essentially 100% for all the membranes throughout the testing sequence, so that the condition necessary for the Donnan membrane effect is fulfilled. We believe that the initial decrease in Li + rejection observed when AI ion was first introduced (curves A to B in Fig. 2) or when the dynamic membrane is apparently destroyed (curves C to D) is mainly the result of this effect. The steady state reduction in Li + rejection observed with the membranes annealed at higher temperatures, where no dynamic membrane formation was apparent, is in reasonable agreement with a quantitative treatment of this phenomenon recently proposed (12). According to this Journal of Colloid and Interface Science, Vol. 51, No. 3, J u n e 197.5

treatment, the decrease in rejection of the membrane-permeable ion (Li + in this case) depends only on the ratio of the concentration of impermeable-to-permeable ion. In the present experiments this ratio was maintained constant for all test conditions (3 AI: 1 Li) and the changes in Li + rejection with A1 ion concentration appear to be the result of dynamic membrane formation. A similar set of results, obtained in somewhat different order, is presented in Fig. 4. A comparison of Fig. 2 with Fig. 4 shows that the order in which the solutions were tested is important. From Fig. 4 it appears that the effect produced at 3 X 10.4 M A1 persists when the concentration is raised to 3 X 10.3 M A1 but is lost at 3 N 10.2 M A1. However, from Fig. 2 it appears that this effect is not produced if the concentration is initially set at 3 X 10. ~ M AI. In each case, the final reduction in Li + rejection (curve D in both figures) was accompanied by a sharp rise in volume flux, apparently signalling the destruction of the dynamic membrane at 3 X 10.2 M A1. Dynamic membrane formation was first reported by Marcinkowsky, Kraus, and coworkers at Oak Ridge National Laboratories about 10 years ago (13, 14). In those studies, substrates with relatively large pores (0.1-10 ~m) were used. The size of the pores in the cellulose acetate membranes used in our

DYNAMICALLY FORMED MEMBRANES

studies is too small to be accurately determined, but has been estimated by several groups of workers to be 10-20 A diameter for membranes annealed at 60° or 75°C. From the present results, it appears that dynamic membrane formation may be a fairly general phenomenon, requiring only (1) a suitable matching of sizes of the species in solution and the pores in the substrate, and (2) an attachment mechanism. Very recently, Tanny and Jagur-Grodzinski (15) have reported dynamic membrane formation with polyelectrolytes using partially cured cellulose acetate membranes. In their work, the sharp drop in volume flux observed here was not reported, and it appeared that the size-matching requirement favored membranes annealed at 70°-77°C. The flux decline observed here may be related to the compaction effect commonly observed in hyperfiltration. Small concentrations of soluble iron are present in virtually all hyperfiltration systems and the strong similarity between the aqueous chemistry of iron and aluminum suggests that effects similar to those we have reported here may be important even with highly annealed membranes over long time periods. The fact that these dynamic membranes are apparently reversibly formed suggests that they may have practical utility. More work is required before a complete understanding of this phenomenon is at hand. In particular, the kinetic aspects, the effect of pH, the correlation of the size of the species in solution with the pore size of the substrate, and the attachment mechanism require further study. This work is in progress. REFERENCES 1. MERTEN, U., (Ed.), "Desalination by Reverse Osmosis," The MIT Press, Cambridge, Mass., 1966.

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2. GR6PL, R., ANn Puscn, W., Desalination 8, 277 (1970). 3. MANJIKIAN,S., LO~B, S., AI'~YDMcCuTcHAN, J. W., "Proc. First Int. Desalination Syrup.," Paper SWD/2, Washington D.C., 1965. 4. MmmNG, C. P., Am) SPmGLER, K. S., "Proc. Nato Advanced Study Inst. 'Polyelectrolytes II,' Forges les Eaux, 1973" (E. Sdldgny, Ed.), Reidel, Dordrecht, Holland, in preparation. 5. Puscn, W., "Proc. Nato Advanced Study Inst. 'Polyelectrolytes II,' Forges les Eaux, 1973" (E. Sddgny, Ed.), Reidel, Dordrecht, Holland, in preparation. 6. JACAZlO,G., PROBSTEIN, R. F., SONIN, A. A., AND YUNG, D., "Porous Materials for Reverse Osmosis Membranes: Theory and Experiment," Fluid Mechanics Laboratory, MIT Cambridge, Mass. 02139; R. F. PROBST~IN,A. A. Som~, AND D. YUNG, Desalination 15, 303 (1973). 7. "Gmelins Handbuch der anorganischen Chemie, 8." Auflage, System Nr. 35, A1, Tell A, Abtlg. 1, S. 426, Verlag Chemie G.m.b.H., Berlin 1935. 8. "Handbook of Geochemistry" (K. H. Wedepohl, Ed.), Vol. II-2, Chap. 13-G-1 to G-6 and 13-H-1 to H-9. 9. Committee Report "State of the Art of Coagulation" presented at the annual Conference of the Jour. A.W.W.A., June 23, 1970. 10. LAITn'~N, H. A., "Chemical Analysis," McGrawHill, New York, 1960, p. 111. 11. LONSDALE,H. K., RILEY, R. L., MIr.ST~AD, C. E., LAGRANGE, L. D., DOUGLAS, A. S., AND SACHS, S. B., Office of Saline Water Research and Development Progress Report No. 577, U.S. Government Printing Office,Washington D. C., 1970, pp. 118-129. 12. LONSDALE, H. K., PUSCH, W., AND WALCH, A., Donnan-membrane effects in hyperfiltration of ternary systems, Trans. For. Soc. 71, 501 (1975). 13. MARCINKOWKSY,A. E., KRAUS, K. A., PHILLIPS~ H. O., JOrINSON,J. S., AND SIJOR, A. J., J. Amer. Chem. Soc. 88, 5744 (1966). 14. KRAUS,K. A., SnOR, A. J., ANDJOHNSON,J. S., JR., Desalination 2, 243 (1967). 15. TANNY, G., AND JAGUR-GRODZlNSKI,J., Desalination 13, 53 (1973).

Journal of Colloid and Interface Science, Vol. 51, No. 3, June 1975