Desalination, 29 (1979) 239-246 0 Eisevier Scientific Publishing Company, Amsterdam - Printed in The Netherlands
OSMOSlS BY COMPOSITE
HIROSHI NOMURA, TAKE0 YAMABE
(Received August 15,19‘79)
The chaxged metimes were prepared from the s~onation and an&&ion of SBR resins, and their reverse osmosis performances were investiiated. The sulfonated membranes show high salt rejection. This result is in accord with the result of membrane potentials. Relatively excelIent membranes were ohtained by amination of the sulfonated membranes. This is probably owing to the formation of a dense charged region near the membrane surface; the composite charged membranes of sandwich-type are obtained by this method. The reverse osmosis performances of these membranes from various materials were made clear_
In recent years, prodigious deveIopment has been marked for the preparation of reverse osmosis membranes from synthetic polymers which are superior in resistances to chemical agents, heat, and bacteria. On the other hand, various effects of the fixed charge density of fhe membrane have been discussed; e.g. T&W model by Teorell+ Meyer, and Sievers, and so on [I, Z]. The electrodialysis process is widely applied to the separation of inorganic ions by the chzuged membranes, and their appIication for reverse osmosis also has been discussed; for exampfe, theoretical approaches to reverse osmosis separation by charged membrane have been undewn by Simons et al. IS] and Dresner . The dynamically formed membrane, which arouses wide interest, is one of the cmed membranes [5-71, Furthermore, by constructii the differently charged domains, the composite charged membrane is used for the piezodiaIysis process f8-111, In the present work, we examin ed reverse osmosis properties of the membranes which were prepared by the iu&oduction of s&fox&e groups aud quaternary ammonium groups as cation and anion exchange groups, respect-
ively, into poly(styrenecohutadiene) resin Moreover, some sandwich-type charged membranes were prepared by the introduction of quaternary groups onto the surface of the su.Lfonatedmembrane, and the membrane performances were evaluated.
The membrane materials used were three hinds of poly(styreneco-butadiene) resins.(SBR) which contain styrene content greater than 40%. Before membrane preparation, the materials were purified by the following method. At first, a 10% SBR tetrahydrofuran solution was prepared, and the gelated part was removed by filtration. The solution was dropped into methanol in order to coagulate and recover the polymer material. The material was removed from methanol by filtration through a glass filter and dried in vacuum The membranes were prepared by the casting method at room temperature from the solution as listed in Table I. The thickness of the casting solution was 500 pm, and the evaporation time was 5 min. The membranes formed by evaporation were immersed in methanol for 2 h. The membrane was crosslinked or cycled by the reaction with anhydrous stank chloride vapor for 2 days at room temperature, and washed with methanol.
TABLE I POLY (BUTADLENE-CO-STYRENE)
RESINS AS MEMBRANE
COMPOSITION OF CASTING SOLUTIONS
Styrene content (monomer %)
Sample 1 (Random) Tafuden 4003 (Block) Solprene 303 (Partial Block)
46.0 40.0 48.0
*Furnished by Asahi Kasei Co.
Composition of casting solution (wt%) (SBR : THF : Dioxane)
30 25 30
: 40 : 43 : 40
: : :
30 32 30
The membrane was sulfonated by a tetrachloroethane solution of chlorosulfonic acid of given concentration. The reaction time was 1 h. After the reaction was stopped by adding glacial acetic acid, the chlorosuifouyl group was hydrolyzed to sulfonic acid group by treatment with a 2N sodhnn hydroxide aqueous solution. For introduction of quaternary ammonium group, at fW the chloromethylation was carried out by the tsedxnent with chloromethyl ether for 4 h in the presence of zinc chloride as a FriedelCrafts catalyst (10 g per 100 ml chloromethyl ether), Next, the treated membrane was aminated in a mixed solution of benzene and methanol (1:l) saturated with trimethyhunine by passing the gas through the sofution, at 40°C for 4 h, After the blow of trhnethylamine gas was st;opped, the membrane was immersed in the solution for ore night. Composite charged membranes were prepared by ~~odu~~ quatemary Arnold groups into sulfonated membranes. Measurementsof membraneperformances In oxxierto discuss the membrane performances, two experiments ware carried out; one was measurements of membrane potential, and another was reverse osmosis experiments. The membrane potential was measured by using the following system; Hg f HgzCX2 / KC1 (sat.) / O,lN KCf f/ Membrane fl UBlN KCI I KC1 (sat.) / E&Cl,
In order to reduce the concentration polarization near the membrane surface, the feed solution was circulated at a flow rate of 1.0 ml/m.in.AU the measurements were carried out at room temperature. For reverse osmosis experiments, a usual flow-type permeation cell was used . The experiments were carried out at 25*C, and the operating pressure was 100 kg/cm2. The salt concentrations were determined by an electroconductivity monitorThe s&t rejection, Rej, (%), is determined from the relation: Rej (%)=
salt ~oncent~tion in feed - salt concent~tion
in product x loo
ssit concentration in feed (1)
For the membrane made of Solprene 303 resin, the relative solute permewas measured. The relative solute permeation r&i0 is deation ratio, I$ol. fined by the following relation:
solute permeation ratio of a given solute, Ps solute permeation ratio of NaCI (reference), P~scl
where solute permeation ratio is defined by the fuHowing equation,
solute concentration in product
soiute concentration in feed
The circulation velocity of the feed solution was 24 I/h. Since it is much larger than the total product rate (0.18 l/h or less), the concentration polarization effect could be substantially ignored.
RESULTS AND DISCUSSION
Measurements of membmne potential The membrane potential and the transport number of the membrane made of Tafuden 4003 resin were shown in Table II. The transport numbers of sulfonated membranes (TS membranes), which were prepared by treatment with chlorosulfonic acid of concentrations higher than LO%, have a nearly constant value, which is rather lower than that (0.94 or higher) of the commercial cation-exchange membranes, whereas the transport number of quaternary ammonium membrane (TA membrane) showed a high value of
Membrane potential E (obs)/E (calcd) E (obs)
Transport number C, or C_
0.73 0.81 0.79 0.81
0.86 0.90 0.90 0.90
0.93 -50.1 TA _-______-_-__-____-_~~~~-~--~~-~-~~~~~~~-~-~~-~
0.67 0.68 0.69
0.83 0.84 0.84
39.0 TS-05 43.3 TS-10 43.7 TS-15 43.3 TS-20 -_--_-------___-_-_-_-----_-----_-----_-~-~----
TSA10 TSA-15 TSA - 20
36.5 36.8 36.9
Name of the membrane means; T: Tafuden 4003, S: Suifonated, A: Aminated. Figures designate the concentration of chlorosnlfonic acid; 05, 10, . . . mean 0.5%. 1.0% and so on. Membrane potentials were measured for System: O.lN-O.OlN KC1 aq. solutions.
RO BY COMPOSlTE
0.97, which is comparable with that of the commercial anion-exchange membrane. The composite charged membranes (TSA membranes) show the membrane potential of the sign characteristic of sulfonated membrane, but the value somewhat decreases on introducing the quaternary ammonium groups. The ammonium groups were introduced only on the surface region of the sulfonated membrane; the membrane is supposed to have a sandwich-type structure as shown schematically in Fig. 1.
Fig. 1. Schematic representation of composite
SANDWICH - TYPE
Effect of preparation conditions on membrane performances of reuerse osmosis For the TS membranes made of Tafuden 4003 resin, the effect of the concentration of chlorosulfonic acid used in the membrane performances of reverse osmosis is shown in Fig. 2. With an increase in the concentration of chlorosulfonic acid, the product rate steeply increases, and the salt rejection decreases. The membrane prepared by treatment with 2.0% chlorosulfonic acid shows substantially no salt rejection. This is owing to an increasing swelling, as the higher product rate indicates. On the other hand, the membrane potential remains nearly constant above the concentration of 1.0% as Table II shows. Because the membrane potential is primarily governed by the fixed charge density, these results indicate that the increase in chlorosulfonic acid concentration results in the increase in the degree of sulfonation together with the increase in the degree of swelling_
Reverse osmosis performances of various type membranes The performances of various type membranes prepared in this work are shown in Table III. Generally speaking, sulfonated membranes have properties of relatively low salt rejection and high product rate, whereas aminated membranes have performances of high rejection and low product rate. This result agrees with the data of membrane potential represented already. The high rejection of aminated membranes is probably owing to a high charge density near the surface region of membrane, for the amination takes place select ively in the surface region. The low salt rejection of sulfouated membranes is
0 Rodut Rate
Fig. 2 Effect of concentration of chlorosulfonic acid on membrane performance (Tafuden 4003). TABLE
REVERSE OSMOSIS PERFORMANCES (2500 em)
Membrane material (SBR Resin)
Type of membrane
Product rate (m3/m2. day)
62.5 80.0 86.5
5.20 x 10-l 0.73 x 10-l 1.38 x 10-l
S A SA
88.2 94.5 88.9
5.00 x 1o-2 1.02 x lo-2 2.09 x lo-=
1.36 X lo-=
SA ______--------------------~---------------~---Tafuden 4003
The concentration 303: 1.0%.
acid; Sample 1: 0.5%, Tafuden
4003 and Solprete
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improved by the amination; that is, the SA membranes show high rejection, while the product rate somewhat decreases This improvement is due to the formation of a dense charged region which is formed by stmrig interaction between differentIy charged groups. The performances of the SA membranes are different according to the kind of SBR resins as membrane materials; the membrane from Solprene 303 shows the highest rejection. For the SA membranes prepared from Sample 1 and Solprene 303 resins, the effect of the feed concentration on the membrane performances were examin ed and the results are shown in Figs. 3 and 4, respectively. The salt rejection of the membrane from Sample 1 is linearly dependent of the feed concentration as expected, but the extent of decrease is very large, while the rejection of the membrane from Solprene 303 .remains relatively high value up to the high feed concentration of 10,000 ppm NaCl. As for the product rate, the SA membrane of Sample 1 is superior to that of Solprene 303. This is the difference in the chemical structure of the membrane material: Solprene 303 has a partial block structure, whereas Sample 1 has a random one. The membrane of block copolymer could retain the tightly dense structure.
0 Product Rate
Fig. 3. Effect of feed concentration on membrane performance (Sample 1). Fig. 4. Effect of feed concentration on membage
Relative permeation charged, membrane
ratio of some solutes through composite
The permselectivity of the SA membrane (Solprene 303) for some salts is shown in terms of the relative solute permeation ratio in Table Iv. The permeability of cations is in the order; A13+ < Mg*+ < Na’, while the permeability of anions is in the order; SOi- < Cl-. The strong exclusion of ions of higher valencies is expected from the ion exclusion mechanism of charged membranes. TABLE IV RELATIVE PERMEATION COMPOSITE
0.02 0.02 0.02 0.02
M M M M
NaCl Na2S04 MgC12 AIC13
RATIO OF SOME SOLUTES THROUGH
93.8 96.8 94.9 95.9
1.22 1.04 1.06 1.64
x x X X
10” 10-2 lo-* lo-’
1.00 0.51 0.95 0.66
From the above discussion, excellent membranes for reverse osmosis could be obtained by amination of sulfonated membranes. The composite charged membranes of sandwich-type obtained by this method showed good reverse osmosis performance. Moreover, it could be expected that membranes made of SBR resin are superior in resistance to chemical agents, heat, and bacteria.
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K. H. Meyer and J. F. Sievers. Helv. Chim Acta., 19 (1936) 649.665.987. K. H. Meyer and W. Straus. Helv. Chim Acta., 19 (1936) 795. R. Simons and 0. Kedem, Desalination, 13 (1973) 1. L. Dresner, Desalination, 15 (1973) 1. K. A. Kraus and A. E. Marcinkowsky, Science, 151 (1966) 194. D. G. Thomas and W. R. Mixon, Desalination, 15 (1974) 287. hl. Igawa, M. Send. H. Takahashi, and T. Yamabe, J. Appl. Polym. Sci., 22 (1978) 1607. F. B. Leitz, Desalination 13 (1973) 373. J. Shorr and F. B. Leitz, Desalination, 14 (1974) 11. C. R. Gardner, J. N. Weinstein, and S. R. Caplan, Desalination, 12 (1973) 19. T. Yamabe, K. Umezawa, Sb. Yosbida, and N. Takai, Desalination, 15 (1974) 127. H. Nomura, Sh. Yoshida, M. Send. H. Takashashi, and T. Yamabe, J. Appl. Polym. Sci., 22 (1978) 2609.