Development of membranes and resins for piezodialysis

Desalinafion. @

14 (1974)

1 i -20

Elsevicr Scientific Publishing Company,

DEVELOPMENT

Amsterdam

OF MEMBRANES

AND

- Printed in The Netherlands

RESINS

FOR

PlEZODlALYSIS* J. SHORR

AS0

F. B. LEITZ

fonrcs. Inc.. Watcffonn. (Received

Novcnlbr

Mass..

U.S.A.

IO. 1973)

SUMMARY

For piezodialysis to become a useful water desalination process, the most critical requirement is the development of appropriate membranes. The membrane presently being sought is a mosaic of cation-passing and anion-passing resins, each resin having a high degree of coupling between the mobile counterions and water. Of a wide variety of possible fabrication techniques, three are of particular interest: phase separation. pattern molding and latex-po:yelectrolyte fabrication. The latexpoiyelectrolyte fabrication technique has produced membranes which bring piezodialysis close to practical reality. INTRODUCTION

Piezodialysis requires membranes which have very high permeabilities for salt. A group of membr;czes which may have this property are charge mosaic

membranes. In the ideal case such a membrane consists of segments of ion-exchange resins which extend evenly from one face of the membrane to the other. The resins should be so structured that a high degree of coupling exists between the mobile ions and the gel water of the membrane. The size of the resin segments must be sufficiently smdl that the voltage drop caused by the required passage of ions

through

they external

solutions

be negligible. The coupling factor and the resin paper in this journal (I). Finally, although it is not of primary consideration at the moment, the membrane must be capable of manufacture at a reasonable price. In this paper we use “resin” to refer to the monofunctional ion exchange components which make up a bifunctional mosaic membrane and “membrane” to refer to a fully reacted film which contains anion- and cation-passing regions. The factors which must be considered in any membrane fabrication fall into segment size are discussed in a previous

l Presented at the Fourth Symposium on Fresh Water from the Sea. Heidelberg, September 9-14, 1973.

J. SHORR AND F. 6. LEITZ

12

three general classifications: transport properties, geometrical properties and physical properties. Transport properties are primarily functions of the resin. Requirements, some typical values and a discussion of how these properties relate to the variables of fabrication of the resins are given in the section on resin studies below. The principal geometric factors are characteristic length of the mosaic pattern. area ratio of anion to cation and thickness of the membrane. These are determined by the method of making mosaic membranes, a number of which are discuss& stability

below.

Required

physical

properties

include

physical

strength,

chemical

and physical stability. PSysical strength must be sufficient to permit the membranes to be handled and to permit the application of as much as several hundred atmospheres across the membrane. Sufficient chemical stability is required that already formed sites not be destroyed when a subsequent reaction is carried out. The amount of swelling of some charge carrying polymers is influenced by the external Fait concentration. Without suficient physical stability. a composite membrane can be literally torn apart. A satisfactory piezodialysis membrane has at [east some minimum value of each of these properties. RESIN

STL’DIES

The most useful starting point for the development of a piezodialysis membrzme is the coupling ratio of the resins. Analysis of the process suggests that unless a value of about 0.4 or higher is obtained for the overall coupling factor. the process is not likely to yield economic desalting. This is not to say that both resins must have a high coupling factor since a very good resin can pull along a mediocre counterpart. The derivation of the tests for the three transport coefficients and experimental methods for determining these are given in reference (2). Briefly, the Rux coefficients are calculated from three dynamic properties of the resin: conductivity, electroosmotic water transport and water permeabih;y. A high coupling ratio requires that water transport and conductivity be high and that permeability be low. A group of commercial ionexchange membranes were tested to determine the transport properties of their resins. Results appear in Table I. These materials are of particular interest because of their relative familiarity. Included in the table are the three flux coefficients: L, ,. LIZ, and L2,; the water transport factor, f; and the coupling factor, 6. The range of properties in this group of resins is very wide. At this point we can only speculate on the question of what sort of membrane morphology would produce high coupling. To simplify the discussion we will consider,

pores.

for the moment,

that the membrane

consists

of a bundle

of capillary

If the pores are straight and of relatively constant cross-section, then the coupling between water and ions should be high and independent of pore diameter. provided that the pore diameter is large compared to the size of the ions. If the pores are tortuous and have large changes in cross-section, then there should

hfEh¶BRANES AND

13

RESXNS FOR PiJZODLALYStS

TABLE I TRAX3-ORT ORDER

PROPERTtES

OF DECRWlXi

OF

RESlhS

COUPLING

tN SOME

- ___.-___--

la&s AMF Ionics AMF Tokuymm soda Tokuyam So&a lonics Ionics ionsc IOIli3C

l

EXCHANGE

MELWIRANES

ARRAYGED

-_.--.-

IN

... ..-- _.___._ ._...._,___ _._

L’ll x IO” LE x IO’1 Lrc?xIO” f _ __.__..--_..-__...- - __.._. -___-.___

61GWG** t lffKWL 6tAZLc1v3I rv.xa_ A103** cL.2.sT** AV!%T 61DYG1IVDYG MA3148 MC3142-

_ ______._.. .“_. .___..._- _.__.-_ “_ l

ION

-...- -. .. . ..--_- .---

DesigMfion

ioflics fonics

CUhVWERCIAL

RATIO

. .._

5500.

100.

2-t

8550. 570. 86. 1.502. 3t9. 270. 893. 110. 376. 409. m.

102. IS. II.

1.81 0.79 3.3 1.42 2.49 0.73 4.8 0.66 0.88 0.65 1.1

____.I____..._.

Zi.1 10.2 *::: 25 4.05 3-26 2.3 -

._._.-.

_.

._.._

_

48. 56. 19. 3.3 15. 4.I 6.2 4.2 3.8 4.6 5.0 2.1

___.

b

0.86 0.67 0.50 V-43 0.21 0.13 0.10 0.097 0.086 0.050 0.040 0.016

.._..

In gfw/cm3sec jou!e &w-l

* Cation resin

be significant velocity gradients in the solution passing through the membrane and a tower degree of coupling. This is somewhat borne out by -severai of the data in Table 1. The Ionics 61GWG and I 1OKWL resins contain larger and less tortuous pores than the other Lonics membranes and have considerably higher coupling factors. The charge density affects the water transport factor (which decreases as the charge density increases) but should leave the coupling factor unaffected unless the fixed charge sites are sufkiently fat apart that the mobile ions must, as it were, “leap” from site to site. which would certainly decrease the coupling factor. A further factor to consider is the uniformity of the charge density. Localized regions of low charge density should have much the same effect as a membrane of low overall charge density. This may explain the very low coupling factors observed with the Ionac membranes which contain a non-ionic binder. A demonstration of the degree to which the transport properties of a resin can be tailored was performed using aminatcd pofyvinyi chloride (PVC). This is described more fully in reference (2). Aminated PVC is prepared by the reaction of an amine with PVC, followed by quaternization with a methylating agent. The result is an anion exchange resin with a high exchange capacity and a low electrical resistance. Two procedures can be used. The heterogenous procedure involves suspending a commercial film of PVC, usually containing plasticizer, in the amine at a temperature between 40” and 80°C for several hours. The film is then suspended in a quaternization agent. The homogeneous procedure invokes addition of amme to PVC dissolved in a solvent. After several hours, a film is cast and the solvent is evaporated. The dry film is quaternized as before. The transport properties of this

3.

14 ?ype of resin are affected by the choice whether the aminated Slm is quaternized. and temperature

of the various

SHORR ANI3 F. B, LEITZ

of procedure. the type of amine used. the quaternization agent used and time

reactions.

ibfETtiODS OF SfOSMC hfEhlBRANE FABRICATION

Each

required for a good piezodialysis membrane are ion-exchange resins. The problem is to devise a chemical proce&re which puts all of them together in a single membrane. A sariety of methods have been considered for the purpose of making charge-mosaic membranes. These are briefly reviewed below. A description of the chemiwi approach is inc!uded in each case. For those methods which faited separately

of the properties

availabie

to produce

in existing

a salt-enriching

membrane,

the critical step or typical

mode of failure

is specified where possible. Three methods of mosaic membrane fabrication have appeared in the literature which are more of historical interest than significant as practical methods of mosaic membrane fabrication. Kollsman (3) claims to have observed salt enrichment with KCI, He used

two semi-circular pieces of membrane placed side by side. The pieces were compos& of ground-up Amberlite IR-120 and IRA400 resins held together with polystyrene binder. The characteristic length in his apparatus was about 1.9 cm. At 16.4 atm this “membrane” produced an average salt enrichment of 2.3 with an initial feed concentration of 0.025/V. It is not possibk from the data given to calculate the permeation rate, however, it must have been very low. A membrane which was partly anionic and partly cationic was produced by deKi3riisy and Short by partial immersion in a reagent bath (4). A suifochiorinated polyethylene film was partly hydrolyzed to yield the cation exchange function and subsequently aminated to yield the anion exchange function. With this membrane a high rate of diffusion of salt from KCi solution to water was observed near the boundary (5). Weinstein and Caplan (6) prepared mosaic membranes by embedding ion-exchange beads in a silicone rubber matrix. The heads, half cation and half anion, were arranged in checkerboard fashion with tweezers. These membranes, while

unable

to withstand

pressure

heads,

were

used

for investigating

a wide

variety of coupied fiow phenomena. Phse seporittion Two poiymers can be brought into solution using a mixture of two solvents. As the more volatite sotvent evaporates from a cast film of this solution, the less soluble polymer begins to form aggregates. On dryness, a film is obtained which is a mosaic memb~ne precursor. For a given pair of polymers the domain size can be controiied by altering the polymer ratio, the solvents and the solvent ratio. This technique was used with several polymers.

MEMBRANES

A

AND

mixture

formamide

15

RESINS FOR PIEZODIALYSIS

of

(DMF)

3076 polystyrene

(molecular

weight

20,000)

in dimethyl-

and 107; PVC

in 1:I DMF-tetrahydrofuran (THF) was cast as a film and dried. On dryness, well-defined polystyrene granules dispersed in the PVC were observed. Sulfonation of the polystyrene in concentrated sulfuric acid containing 0.1 oL Ag2S04 followed by hydrolysis in IO?/, NaOH produced the cation

resin. The

PVC.

ethylene pentamine

unaffected

by the first reaction,

was aminated

in tetra-

(TEPA)

at 80°C for two hours. The membrane was washed in water and quaternized in a methanol solution of dimethyl sulfate. The result was a mosaic structure clearly observable to the eye. However. none of the numerous variations of this type of membrane produced salt enrichment. Approximately the same procedure was used employing an already formed

cation

resin. A

109~ solution

of sulfonated

polystyrene.

having

an exchange

capacity

of !.5 meq/dry g. in DM F was mixed with IO?: PVC in I:1 DMF-THF. After film casting and drying, the PVC was aminated with tetraethylene pentamine and quatcrnized v\ ith 20 “/Adtmethyl sulfate in methanol. The difference in swelling of the two resins when immersed in water was occasionally sufficient to cause rupture of the membrane in the sulfonated areas. The generally high water fluxes observed with this type of membrane indicated that even apparently sound films

contained microscopic cracks and hoies. Films were cast from a mixture of sulfonatcd polystyrene,

having a molecular

weight of 250.000 and an exchange capacity of I.2 to 1.8 meq/dry g, and polyvinylbenzyl chloride (PVEC) in methanol-acetone_ The PVBC was aminated with trimethyl amine. The final exchange capacity was about 0.6 meq/dry g of composite membrane for each resin. Visual examination showed a clear phase separation. Le., islands of aminated PVBC in a film of sulfonated polystyrene. as well as fairly numerous pinholes. in a attempt to improve the integrity of the film a block copolymer of styrene and butadiene

was added.

Films cast from methanol-toluene

solution were tough

and flexible.

However, and developed

during amination the films lost much of their original strength leaks. Attempts to improve film strength by cross-linking with AlCI, reduced swelling somewhat but did not permit sound films to be formed. In spite of the wide variety of polymer mixtures. solvent combinations and other

variations

which

were

studied,

leak

tight films of sufficient mechanical

strength

for pressure testing could not be obtained. Salt enrichment did not occur in any of the systems. This technique solves one of the fundamental problems of mosaic membrane fabrication (that of obtaining resin domains of appropriate size) so nicely that it still remains attractive.

iUotding

of pattern

In this technique a mosaic is produced by casting the membrane in a mold. A glass plate is covered with a paraffin film, about 0.01 cm thick. The paraffin is cut into a striped or checkerboard pattern with a characteristic dimension of 0.1 to

16

J. SHORR AND F. B. LEITZ

0.3 cm.

Alternate stripes or squares of paraffin are peeled off. A resin solution is cast into these areas and dried. The remaining paraffin is removed and the spanks are filled with a second resin solution. For this method it is necessary to have ionexchange resins. or precursors, which are soluble in some solvent. The cation resin used was partially sulfonated polystyrene. With an exchange capacity below 2 meqldry g, this IS insoluble in water but soluble in methanol. The anion resin used was aminated PVC in DMF solution. After formatton of the mosaic film. the cation resin was hydrolyzed in IO‘?
This method was developed to take advantage of the very desirable physical properties of PVC film. some of which are retained when PVC is converted to ionexchange resins. The sulfonation conditions for PVC are fairly severe and the product is inso’luble in any solvent that we used. Consequently. we attempted to introduce the sulfonated PVC into the membrane in the form of a fine powder. Sulfonated PVC was made by immersing PVC in a 1:l mixture of concentrated sulfuric acid and chlorsulfonic acid at 6040°C for two hours. After hydrolysis in 10% NaOH the resin had an exchange capacity of 3 meq/dry gram. Fine particles of this resin were suspended in a 10 9; solution of PVC in 1: I DMF-THF. After a film was cast and dried it was aminated in TEPA. The resulting film was strong and coherent to the eye. A group of such membranes were made. These showed 0 to 5% salt enrichment and very high water permeability. This permeability, higher by an order of magnitude than was obtained from aminated PVC without the suspended particles, indicated leakage which was probably due to insufficient bonding between the two resin phases. The small salt enrichment, under these circumstances suggests that this type of membrane might work well if a better bond between the phases could bc obtained. Thermal iamination

The first membrane which successfully demonstrated salt enrichment in our laboratory was composed of styrenized polyethyiene resins. A large number of resin sheets are stacked alternately cation and anion and laminated under heat and pressure into a b&k. Slices O.COScm to 0.01 cm thick are cut perpendicular to the resin interfaces, giving membranes which have alternate stripes of cation and anion resins.

17

MEMBRANES AND RESINS FOR PIEZODIALYSIS

These membranes were used to demonstrate that piezodialysis could in Salt enrichments of up to I.4 were principle be used for water desalination. observed. Membranes of this type were also used to demonstrate that the mathematical model was valid and that the effect of characteristic length was approximately as predicted (2). Because of the low coupling factors of the resins. high enrichments were not obtained. We had some success in improving the transport properties of this type of resin by increasing the amount of polystyrene sorbed into the polyethylene film (7). This, however. led to failure of the membranes to bond thermally. Attempts to glue the resin sheets with an adhesive were not successful.

In the preparation of mosaic membranes by selective activation or masking, a portion of an existing film is exposed to one set of reagents through a mask and then either the unreacted portion or the whole film is exposed to other reagents. This method has the advantage that polymer chains cross resin segment boundaries so the finished membrane can be almost as strong as an unreacted polymtr film. A great deal of work has been done using crosslinked polystyrene fiIms as a basis (2, 7). it was determined that sulfonated areas were not materially affected by subsequent chloromethylation and amination. but that the reverse procedure could not be successfully

used.

Consequently,

sulfonaticn

was performed

first for this

group of membranes. Two problems were never successfully solved. First, despite the wide variety of waxes, paints, metals and rubbers that were used as masking materials, there was always some degradation of the masking material by the sulfonation reagent. Second, while the sulfonation reagent diffuses through the film. it also diffuses laterally so that the reacted areas are trapezoidal rather than rectangular in crosssection. This could be overcome only by making the segments large compared to the membrane thickness, which is undesirable. Recently. Schindler (8) has developed an ingenious method of masking using a photochemical reaction to graft two types of monomers to a polyolefin film. By use of a reaction sequence which involves only a single application of the mask, charge mosaic membranes of the ideal structure with segment sizes smaller than the film thickness may be made. If the resius so formed have reasonably high coupling factors, then very good piezodialysis membranes may emerge. This work is still in progress. Block copoijmerirrcl lion

Under proper conditions, similar segments of ABA block copolymers segregate themselves during film casting to form domains of considerable size. The attractiveness of this pr&ess as a means of obtaining both resin separation and strong films inspired work both by Schindier and Yasuda (ZU) and Lopatin ef al. (II). The basic ditiicuity lay in making domains of proper size and morphology.

18

I.

SHORR AND F. B. LEIT2

A relatively new method is the use of ionotropic gels as templates for mosaic membrane fabrication (9). An ionotropic gel can be made as a lilm having *. large void fraction of almost cylindrical pcres. By filling the gel volume with one resin and the pores with a second. which potentially can be accomplished through a variety of procedures, a piezodiatysis membrane very ciase to the ideal model. can be obtained_ A particularly attractive feature of this procedure is that the pore diameters. and hence the resin segment size, can apparently be made small compared to the membrane thickness.

The best performance observed to date has been frum the group of mcmThis ;s the most signifkant develop‘t?ranes described as “tatex-potyelectrolytes”. ment of the present membrane development program. With the presently obtainabte performancq piezodiaiysis is OR the verge of being a pS-acticaidesalting process_ This type of membrane is basicafty a cross-linked synthetic rubber film, which has been comerted to an anion exchange resin, which contains a relatively continuous network of a %crYloose cation resin. The Iatex used contains Soof;; solids of equal weight fraction styrene and

MEMBRANES AND RESINS FOR PIEZODIALYSIS

mixture consists of latex. sodium polystyrene hydroxide. A cast film is dried, crosslinked

butadiene. The casting sodium or ammonium chloromethylated backing materials

19 sulfonate and with ALCI,,

These membranes have been prepared without to overcome the low tear strength of the

and aminated. and with cloth

backing

unbacked films. An extensive

investigation

has been

made

into

the effects

of fabrication

variables on the physical properties and on the transport properties of this type of membrane (12). The result of a compilation of local optimization procedures is the membrane described below. Further improvement in performance appears highly likely. There are substantial interactions between fabrication variables which have not yet been explored. We are only beginning an investigation of the morphology of this type of membrane which may have significant impiications.

Performance of latex-polyelectrolyte

membrane

Seven high

pressure runs were made with membrane 95-70-2-I with initial charge concentrations between 02M and 0.01 M NaCI at 1500 psi (102 atm). These

tests were run batchwise.

the high pressure solution

concentration

slowly de-

creasing as the run proceeded. The salt enrichment and water flux rate are plotted versus high pressure solution concentration in Figs. 1 and 2, respectively. Characters of different shapes indicate the different runs. The sequence of runs was with initial concentrations of: 0.05, 0.1. 0.08. 0.03, 0.2, 0.1 and 0.02M. With this type of membrane it is typical that the first data point will have a low salt enrichment.

.

ol

I

I

I

.,.,I

I

t

*,.,.I

1

0.1

0.01

0.001 nigh

prusuc

.olucicm

c.xxencr.rton

C’

Fig. 2. Comparison of predicted and measured water flux. Membrane: 95-70-2-I Presure: 102 atm.

20

J.

SHORR AND F. B. LEITZ

If we exclude the first data point, the rest of the data are well correlated by the in Fig. I, except for the set indicated by the triangles. This was the last pressure run and we believe that the membrane was beginning to leak. The solid line on Fig. 1 is the prediction of salt enrichment from flux para-

dashed line

meters for the separate resins. Techniques

for measuring

the flux parameters

are

detailed

in reference (12). The agreement between the predicted curve and the data curve is astonishingly good. when we consider the great divergence of any likely membrane morphology from the model. The agreement between the predicted curve (solid line) and data values for water th~x, shown in Fig. 2, is also good. Here, however, there is a fair amount of scatter both within data sets and between them. The agreement in this case is largely attributable to our having used one of these data points to determine the composite water permeability coefficient. ACKNOWLEDCEhfENTS Tke sponsorship of this work by the Office of Saline Water. U.S. Department of the Interior. under Contracts 14-01-0001-611 and -2333 is gratefully acknowledged.

REFERENCES

1. 2. 3. *4. 5. 6. 7.

F. F. P. F. F.

B. Lmz, Dedinulion. I3 (1973) 373. 8. LEIIZ AND f. SHORR, Ofice of Saline IVaIcr, Res. Develop. Progr. Rept. No. 775, 1972. KOUMAN. U.S. Parent 2.987.d72. 1961. DEK~R~SY AND 1. SHORR, DECHEMA Monogmph, 47 (1%2) 477. DE#~R&SY. Nafure. 197 (1963) 635. J. N. Wuxmm~ AND S. R. CAPLAN. Science, 167 (1%8) 70. F. B. LEIIZ. S. S. ALEMNDER AND A. S. DOUGLAS, Ofice of Saline &Varer, Res. Develop.

Progr. Rep). No. 45-7. 1969. Quarterly Report No. I on Contract 14-30-2747, Office of Saline Water. 1973. 8. A. SCHMDLER, 9. R. E. KESI-IXC. .I. F. DIITER AND A. J. GOTCHER, Quurferfy Report No. Z on Contact 14-W3 i 28. Oflice of Saline Water. 1972. 10. A. S~HINDLER ANO H. YASUDA. Ofice of Saline &Vater. Res. Develop. Progr. Rept. No. 689. II. IZ

1971. G. LCIPATIH. H. A. NEWEY. E. T. BISHOP. W. P. CYNEILL AND A. B. KREWIF;GHALS. Ofice of Saline Water, I&s. Develop. Progr. Repr. No. 690, 1971_ F. B. LEITZ et al.. Piemfiolysb-Fourfh Reporf, Status Report on Contract 14-014001-2333, Of&c of Salins Water. 1973.