Thin-Film Composite Reverse-Osmosis Membranes - American

21 Thin-Film Composite Reverse-Osmosis Membranes: Origin, Development, and Recent Advances JOHN E. CADOTTE and ROBERT J. PETERSEN

Downloaded by UNIV LAVAL on June 22, 2014 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch021

FilmTec Corporation, 15305 Minnetonka Boulevard, Minnetonka, MN 55343 The original Loeb-Sourirajan membrane consisted of an asymmetric film having an ultrathin, dense, surface barrier layer integrally supported by a thick, porous, spongy underlayer. An approach to the improvement of such membranes was to separately fabricate these two layers, each maximized for performance, then join them together as laminates. The final laminates would serve as high-performance thin-film composite reverse osmosis membranes. The origin and development of this concept, as carried out at North Star Research and Development Institute and more recently at FilmTec Corporation by the authors, will be briefly described, followed by a description of recent membrane advances in this area. Examples reviewed are ultrathin cellulose acetate membranes, the invention of microporous polysulfone support films, and the development of NS-100 and NS-200 membranes. Two new membranes of this type, designated as NS-300 and FT-30, will be described. Both are chlorine-resistant, non-polysaccharide thin-film composite membranes. The NS-300 membrane has brackish water desalination characteristics, very high fluxes, and potential applications in brackish and waste water treatment processes. The FT-30 membrane possesses high flux and seawater rejection characteristics, and is finding use in single-pass seawater desalination for potable water production. The origin of thin-film-composite reverse osmosis membranes began with a newly formed research institute and one of its first employees, Peter S. Francis. North Star Research and Development Institute was formed in Minneapolis during 1963 to fill a need for a nonprofit contract research institute in the Upper Midwest. Francis was given the mission of developing the chemistry division through support, in part, by federal research contracts. At this time the initial discoveries by Reid and Breton (1) on the desalination capability of dense cellulose acetate membranes and by Loeb and Sourirajan (2) on asymmetric cellulose acetate membranes had recently been published. Francis speculated that improved membrane performance could be achieved, if the ultrathin, dense barrier layer and the porous substructure of the asymmetric 0097-6156/81/0153-0305$05.50/0 © 1981 American Chemical Society

In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981.


306

SYNTHETIC

MEMBRANES:

DESALINATION

membrane were separately f a b r i c a t e d , then laminated together. Each of these two l a y e r s could then be i n d i v i d u a l l y optimized for maximum performance. In 1964 F r a n c i s f a b r i c a t e d the f i r s t thin-film-composite c e l l u l o s e acetate reverse osmosis membrane. The u l t r a t h i n barr i e r l a y e r was made by f l o a t - c a s t i n g a l i q u i d f i l m of c e l l u l o s e acetate s o l u t i o n i n cyclohexanone onto a water surface ( 3 ) . M i g r a t i o n of the cyclohexanone from the organic phase i n t o the aqueous phase l e f t behind a f l o a t i n g s k i n of c e l l u l o s e a c e t a t e on the water. The u l t r a t h i n polymer f i l m could be laminated to a support l a y e r by s l i d i n g an appropriate microporous support f i l m under the water surface and b r i n g i n g i t underneath the f l o a t i n g f i l m . The thickness of the dense b a r r i e r f i l m could be c o n t r o l l e d to w i t h i n ± 15 percent over the range of 200 to 5,000 angstroms. T h i s achievement l e d to an i n i t i a l research contract with the O f f i c e of S a l i n e Water, U.S. Department of the I n t e r i o r , which e v e n t u a l l y expanded i n t o a 13 year, broadly based membrane research e f f o r t at North Star (now merged into Midwest Research Institute). The f i r s t thin-film-composite membranes used microporous c e l l u l o s e acetate f i l m s as porous supports f o r the b a r r i e r l a y e r membranes ( 3 ) . These support l a y e r s were asymmetric LoebS o u r i r a j a n membranes themselves, but were cast under c o n d i t i o n s that promoted p o r o s i t y and high water f l u x , i n excess of 250 g a l lons per square foot per day ( g f d ) . These composite membranes behaved w e l l except f o r a problem of low f l u x , which was about 2-3 gfd a t 1500 p s i pressure i n simulated seawater t e s t s . This was traced to both compaction and r e l a t i v e l y low surface p o r o s i t y i n the asymmetric c e l l u l o s i c support l a y e r d e s p i t e i t s i n i t i a l l y high f l u x . B e t t e r r e s u l t s were achieved using M i l l i p o r e VSWP m i c r o f i l t r a t i o n membranes as porous supports; r e s u l t i n g f l u x e s rose to about 5 gfd (A). These were a l s o c e l l u l o s i c i n nature, however, and compaction appeared to be a problem i n wet, long term, high pressure t e s t s . In 1966, Cadotte developed a method f o r c a s t i n g microporous support f i l m from p o l y s u l f o n e , polycarbonate, and polyphenylene oxide p l a s t i c s ( 4 ) . Of these, p o l y s u l f o n e (Union Carbide Corporat i o n , Udel P-3500) proved to have the best combination o f compact i o n r e s i s t a n c e and surface m i c r o p o r o s i t y . Use of the microporous sheet as a support f o r u l t r a t h i n c e l l u l o s e acetate membranes produced f l u x e s of 10 to 15 gfd, an increase of about f i v e - f o l d over that of the o r i g i n a l microporous asymmetric c e l l u l o s e acet a t e support. Since that time, microporous p o l y s u l f o n e has been widely adopted as the m a t e r i a l of choice f o r the support f i l m i n composite membranes, while f i n d i n g use i t s e l f i n many u l t r a f i l t r a t i o n processes. A s i g n i f i c a n t advance was made i n the a r t of thin-film-comp o s i t e membranes by Cadotte i n 1970 w i t h the advent of the NS-100 membrane ( 5 ) . T h i s reverse osmosis membrane contained an u l t r a t h i n a r y l - a l k y l polyurea formed i n s i t u on a microporous p o l y s u l -


21.

CADOTTE AND PETERSEN

Thin-Film

Composite

RO Membranes

3 0 7

fone support. This membrane was f u l l y n o n c e l l u l o s i c , having no c e l l u l o s e ester polymers i n e i t h e r the b a r r i e r zone or the porous support zone. Two important c h a r a c t e r i s t i c s that r e s u l t e d were n o n b i o d e g r a d a b i l i t y and no compaction under sustained high pressure. The membrane, furthermore, demonstrated s i n g l e - p a s s seawater d e s a l i n a t i o n q u a l i t i e s . Most thin-film-composite membranes since that time have been n o n c e l l u l o s i c .


P r e p a r a t i v e Routes to Thin-Film-Composite Membranes A schematic diagram of a t y p i c a l , commercial q u a l i t y t h i n film-composite membrane i s presented i n Figure 1. The microporous p o l y s u l f o n e support f i l m i s cast on a woven or non-woven f i b e r backing m a t e r i a l , u s u a l l y made from p o l y e s t e r f i b e r s . The p o l y sulfone support i s approximately 50 urn (two mils) i n thickness and about h a l f of i t penetrates i n t o the p o l y e s t e r backing material. This p o l y e s t e r web-polysulfone s t r u c t u r e i s u s u a l l y employed without drying f o r a p p l i c a t i o n of the reagents used to form the t h i n b a r r i e r l a y e r . A f t e r d e p o s i t i o n of the b a r r i e r l a y e r , the composite membrane i s subsequently d r i e d and/or heat-cured to complete the membrane p r e p a r a t i o n . In instances where the t h i n b a r r i e r l a y e r i s not i n t e g r a l l y attached to the support surface, drying may form an adequate bond. Generally i t i s p r e f e r r e d that the t h i n b a r r i e r l a y e r should be w e l l bonded to the support surface to prevent damage or delamination during use. Table 1 l i s t s f i v e approaches that have been used f o r forming the b a r r i e r l a y e r of a composite membrane. The f i r s t method l i s t e d i s the forming of the u l t r a t h i n membrane separately on a d i f f e r e n t surface such as by f l o a t - c a s t i n g on water or by slowly withdrawing a c l e a n , f l a t g l a s s p l a t e from a d i l u t e s o l u t i o n of the polymer (the C a r n e l l - C a s s i d y technique) . This u l t r a t h i n f i l m i s then t r a n s f e r r e d to a microporous support f i l m . Adhesion between the t h i n b a r r i e r f i l m and the support surface was sometimes a problem i n t h i s approach. This method has been used mostly f o r c e l l u l o s e acetate composite membranes, although i t i s a general method that can be used with many polymers. For the remaining methods l i s t e d i n Table 1 the t h i n b a r r i e r l a y e r i s formed i n s i t u , i . e . , d i r e c t l y on the support s u r f a c e . Methods B and C, i n v o l v i n g d i p - c o a t i n g of the porous support i n a s o l u t i o n of a polymer or a r e a c t i v e monomer, appear to be a l o g i c a l and simple approach f o r forming thin-film-composite membranes. However, few s u c c e s s f u l reverse osmosis membranes have been made by these techniques. One problem has been the l i m i t e d solvent r e s i s t a n c e of p o l y s u l f o n e support f i l m . Only water, lower a l c o hols and a l i p h a t i c hydrocarbon solvents can be used as s o l v e n t s . A second problem with Methods B and C i s that the coating s o l u t i o n must be d i l u t e to produce a t h i n b a r r i e r l a y e r . D i l u t e , low v i s c o s i t y s o l u t i o n s tend to migrate upon drying to produce d e f e c t i v e or discontinuous c o a t i n g s . Lonsdale and coworkers (9) have f a b r i c a t e d c e l l u l o s i c t h i n -


SYNTHETIC


308

Figure 1.

Table 1.

MEMBRANES:

DESALINATION

Schematic of a thin-film-composite membrane

General Methods Used f o r F a b r i c a t i n g Composite Reverse Osmosis Membranes

A)

Cast the u l t r a t h i n b a r r i e r membrane s e p a r a t e l y , then to a porous support.

B)

Dip-coat a polymer s o l u t i o n onto a support f i l m , followed by drying.

C)

Dip-coat a r e a c t i v e monomer or prepolymer s o l u t i o n onto a support followed by heat or r a d i a t i o n c u r i n g .

D)

Deposit a b a r r i e r f i l m from a gaseous phase monomer plasma onto the support.

E)

I n t e r f a c i a l l y polymerize a r e a c t i v e s e t of monomers a t the surface of the support.


laminate


21.


Thin-Film

Composite

RO Membranes

309

film-composite membranes by Method B, using a c e l l u l o s e n i t r a t e c e l l u l o s e a c e t a t e porous support and a c e l l u l o s e acetate b a r r i e r l a y e r . A p o l y a c r y l i c a c i d c o a t i n g was f i r s t a p p l i e d to the microporous support t o prevent solvent i n t r u s i o n i n t o the micropores. The b a r r i e r l a y e r was then a p p l i e d by d i p - c o a t i n g techniques. Later under reverse osmosis c o n d i t i o n s the p o l y a c r y l i c a c i d i n t e r l a y e r was washed out through the support. A notable example of Method C i s the NS-200 membrane, which was f i r s t discovered at North Star i n 1972 (10,11). The s a l t b a r r i e r i n t h i s case was a sulfonated polyfurane r e s i n formed i n place a t 125 t o 140°C. T h i s membrane was p r e f e r e n t i a l l y formed by f i r s t impregnating a p o l y s u l f o n e support with an aqueous s o l u t i o n c o n t a i n i n g ( i n weight percentages) 20 isopropanol, 1 high molecular weight polyethylene g l y c o l (Union Carbide Corporation, Carbowax 20M), 2 f u r f u r y l a l c o h o l , and 2 s u l f u r i c a c i d . After excess s o l u t i o n was drained away, the coated f i l m was placed d i r e c t l y i n an oven, and formation of a b l a c k b a r r i e r r e s i n occurred very r a p i d l y . T y p i c a l oven cure time was 15 minutes or l e s s . Laboratory-produced NS-200 membranes f r e q u e n t l y e x h i b i t e d seawater s a l t r e j e c t i o n s as high as 99.9 percent a t 20 gfd (tested a t 1000 p s i and 25°C). In s p i t e of the e x c e l l e n t i n i t i a l p r o p e r t i e s of the membrane, development e f f o r t s have not been s u c c e s s f u l because of a problem of long term i n s t a b i l i t y of the membrane. Elemental a n a l y s i s of the heat-cured membrane showed that a l a r g e p r o p o r t i o n of the s u l f u r i c a c i d c a t a l y s t was incorporated i n t o the membrane, probably as s u l f o n i c a c i d and s u l f a t e e s t e r groups. The high i o n i c charge on the membrane tended to produce excessive s w e l l i n g i n sodium c h l o r i d e s o l u t i o n , but t h i s could be counteracted with post-treatment of the membrane i n 0.1 percent barium hydroxide. The a c t u a l cause of membrane i n s t a b i l i t y , whether due to an uns t a b l e chemical bond or t o a gradual, i r r e v e r s i b l e s w e l l i n g of the s t r u c t u r e , has not been determined. Method D i n Table 1 represents a case where d r y support f i l m s were always used because of the need to employ a vacuum and because of the very nature of plasma d e p o s i t i o n processes. Yasuda (12) showed that a wide v a r i e t y of gas phase r e a c t a n t s could be used i n t h i s technique. Not only conventional v i n y l monomers were used but a l s o any organic compounds with adequate vapor pressure. Further, copolymers could be prepared by i n t r o d u c t i o n of a second r e a c t a n t such as n i t r o g e n . Wydeven and coworkers (13,14) showed the u t i l i t y of t h i s method i n preparing reverse osmosis membranes from an a l l y l a m i n e plasma. P o l y s u l f o n e supports a r e w e l l s u i t e d f o r the f i f t h method l i s t e d i n Table 1. In t h i s approach, Method E, the support f i l m i s saturated with a water s o l u t i o n c o n t a i n i n g diamines, polyamines or diphenols, plus other a d d i t i v e s such as a c i d acceptors and s u r f a c t a n t s . The saturated f i l m i s contacted with a nonmiscible solvent c o n t a i n i n g d i - or t r i a c y l c h l o r i d e r e a c t a n t s . A condens a t i o n polymer forms at the i n t e r f a c e . The f i l m i s d r i e d to bond the t h i n i n t e r f a c i a l f i l m to the support s u r f a c e . In some


SYNTHETIC

310

MEMBRANES:

DESALINATION

instances an a d d i t i o n a l heat-cure i s r e q u i r e d . The p o l y s u l f o n e support t o l e r a t e s the a l k a l i n e c o n d i t i o n s of the r e a c t i o n as w e l l as the d r y i n g and heat-cure steps i n t h i s process. Two e x c e l l e n t examples of t h i s membrane system have been developed, NS-100 and PA-300 ( 5 1 5 ) . The NS-100 membrane was made by impregnating a p o l y s u l f o n e support with a 0.67 percent aqueous s o l u t i o n of polyethylenimine, d r a i n i n g away excess reagent, then c o n t a c t i n g the f i l m with a 0.1 percent s o l u t i o n of t o l u e n e d i i s o c y anate i n hexane. An u l t r a t h i n polyurea b a r r i e r l a y e r formed at the i n t e r f a c e . T h i s membrane was then heat-cured at 110°C. A l a t e r v e r s i o n of t h i s membrane was developed (designated NS-101), which used i s o p h t h a l o y l c h l o r i d e i n place of t o l u e n e d i i s o c y a n a t e , producing a polyamide (16). With e i t h e r type of membrane, s a l t r e j e c t i o n s i n simulated seawater t e s t s at 1000 p s i exceeded 99 percent. Two types of r e a c t i o n s took place i n t h i s process that were b e l i e v e d to be important f o r c o n t r o l l i n g membrane p r o p e r t i e s . The f i r s t r e a c t i o n , which took place a t the i n t e r f a c e and which involved the primary and secondary amine groups of p o l y e t h y l e n i mine (PEI) with the d i f u n c t i o n a l reactant i n hexane, proceeded very r a p i d l y a t room temperature to produce, i n the case of isopht h a l o y l c h l o r i d e , a polyamide surface s k i n (see Reaction I ) . The second r e a c t i o n took place during d r y i n g of the membrane at 110°C. The r e s i d u a l polyethylenimine under the polyamide surface s k i n was c r o s s l i n k e d by e l i m i n a t i o n of ammonia between adjacent amine groups (see Reaction I I ) . The r e a c t i o n s produced a membrane having three d i s t i n c t zones of i n c r e a s i n g p o r o s i t y : 1) the microporous p o l y s u l f o n e support f i l m , 2) a t h i n , c r o s s l i n k e d polyethylenimine zone of intermediate p o r o s i t y and moderate s a l t r e j e c t i o n , and 3) the dense polyamide (or polyurea) surface s k i n which acted as the high r e t e n t i o n barrier. The PA-300 membrane was commercially developed by R i l e y and coworkers (15), and i s s i m i l a r t o the NS-101 membrane i n s t r u c t u r e and f a b r i c a t i o n method. The p r i n c i p a l d i f f e r e n c e i s the s u b s t i t u t i o n of a polyetheramine, the adduct of p o l y e p i c h l o r o h y d r i n with 1,2-ethanediamine, i n place of polyethylenimine. Use of the polyetheramine was s i g n i f i c a n t improvement i n that c o n s i d e r a b l y higher membrane f l u x e s were p o s s i b l e a t s a l t r e j e c t i o n s equivalent to the NS-100 membrane system. The a c t u a l b a r r i e r l a y e r i n the PA-300 membrane i s a polyamide formed by i n t e r f a c i a l r e a c t i o n of i s o p h t h a l o y l c h l o r i d e with the polyetheramine. Considerable a c t i v i t y has been generated on composite reverse osmosis membranes by Japanese r e s e a r c h e r s . Patent a p p l i c a t i o n s were r e c e n t l y published, f o r example, covering research at T e i j i n L t d . on i n t e r f a c i a l l y formed membranes prepared from p o l y d i a l l y l a m i n e s (17) and from amine adducts of t r i s - ( g l y c i d y l ) isocyanurate (18). Both types of membranes were formed on microporous p o l y s u l f o n e supports. Kurihara and coworkers have developed a composite membrane, designated PEC-1000, which i s formed by an


?


21.


Thin-Film

Composite

RO

Membranes

311

a c i d - c a t a l y z e d polymerization process on the surface of a microporous polysulfone support (19). The chemical composition was not s p e c i f i e d , but the method of f a b r i c a t i o n and the r e s u l t i n g membrane p r o p e r t i e s are r e m i n i s c i e n t of the NS-200 example. Recent New Advances


In 1977 the North Star membrane research group was spun o f f by Midwest Research I n s t i t u t e , forming FilmTec Corporation. Two new thin-film-composite reverse osmosis membranes have been under development a t FilmTec Corporation since that time, the NS-300 and the FT-30 membranes. NS-300 Membrane. The NS-300 membrane evolved from an e f f o r t at North Star to form an i n t e r f a c i a l p o l y ( p i p e r a z i n e i s o p h t h a l a mide) membrane. C r e d a l i and coworkers had demonstrated c h l o r i n e r e s i s t a n t poly(piperazineamide) membranes i n the asymmetric form (20). The NS-100, NS-200, and PA-300 membranes were a l l r e a d i l y attacked by low l e v e l s of c h l o r i n e i n reverse osmosis feedwaters. In the p u r s u i t of a c h l o r i n e - r e s i s t a n t , nonbiodegradable thin-film-composite membrane, our e f f o r t s to develop i n t e r f a c i a l l y formed p i p e r a z i n e isophthalamide and terephthalamide membranes were p a r t i a l l y s u c c e s s f u l i n that membranes were made with s a l t r e j e c t i o n s as high as 98 percent i n seawater t e s t s . However, the v a r i a b i l i t y of these membranes was extremely high i n regards to s a l t r e j e c t i o n , and the membranes g e n e r a l l y e x h i b i t e d low f l u x . A v a r i a n t of t h i s membrane was then made by r e p l a c i n g the i s o p h t h a l o y l c h l o r i d e with i t s t r i a c y l c h l o r i d e analog, t r i m e s o y l chloride (benzene-l,3,5-tricarboxylic acid chloride)(21,22). This membrane demonstrated a v a s t l y improved f l u x compared with the p o l y ( p i p e r a z i n e isophthalamide) membrane, but i t s seawater s a l t r e j e c t i o n was low — i n the range of 60 to 70 percent. A reverse osmosis t e s t with a magnesium s u l f a t e feedwater showed greater than 99 percent s a l t r e t e n t i o n , however, d i s p e l l i n g the p o s s i b i l i t y that low sodium c h l o r i d e r e j e c t i o n s were due to d e f e c t s i n the polyamide b a r r i e r l a y e r . The p i p e r a z i n e polyamide was soon concluded to have the f o l l o w i n g s t r u c t u r e (see Reaction I I I ) . Two of the a c y l c h l o r i d e groups of t r i m e s o y l c h l o r i d e are shown to be involved i n the r a p i d i n t e r f a c i a l polymerization with p i p e r a z i n e to produce a polyamide which, most l i k e l y , i s n e a r l y l i n e a r i n c o n f i g u r a t i o n . The t h i r d a c y l c h l o r i d e group would then hydrolyze i n the aqueous environment to a carboxylate group, although some of these l a t t e r groups probably a l s o react with p i p e r a z i n e to produce branching and c r o s s l i n k i n g . The t r i m e s o y l c h l o r i d e could be mixed with i s o p h t h a l o y l c h l o r i d e to produce copolyamide b a r r i e r l a y e r s . S a l t r e j e c t i o n s toward s y n t h e t i c seawater improved as the isophthalamide content of the b a r r i e r l a y e r increased. S u r p r i s i n g l y , membrane f l u x passed through a peak r a t h e r than simply d e c l i n i n g as a f u n c t i o n


SYNTHETIC

MEMBRANES:

DESALINATION


312



21.

CADOTTE A N D PETERSEN

Thin-Film

Composite

RO

Membranes

313

of i n c r e a s i n g isophthalamide content. T h i s i s i l l u s t r a t e d by the data i n Table 2. Maximum water p e r m e a b i l i t y c h a r a c t e r i s t i c s were found a t an approximate copolymer r a t i o of 67 percent i s o p h t h a l i c and 33 percent t r i m e s i c groups. The d i f f e r e n c e s i n the magnesium s u l f a t e versus sodium c h l o r i d e r e j e c t i o n appear to be due to the charged nature of the membrane b a r r i e r l a y e r , s i n c e i t i s r i c h i n carboxylate groups. Table 3 i l l u s t r a t e d t h i s phenomenon, wherein a s i n g l e t e s t s p e c i ment (made with the p i p e r a z i n e trimesamide homopolymer) was s e q u e n t i a l l y exposed to feed s o l u t i o n s of sodium c h l o r i d e , magnesium c h l o r i d e , sodium s u l f a t e , and magnesium s u l f a t e . The c h l o r i d e s a l t s were both p o o r l y r e t a i n e d while r e t e n t i o n of the s u l f a t e s a l t s was e x c e l l e n t . Thus, s a l t r e t e n t i o n i n the carboxyl a t e - r i c h NS-300 membrane was c o n t r o l l e d by the anion s i z e and charge. This membrane could not d i s t i n g u i s h between the u n i v a l e n t sodium i o n and the d i v a l e n t magnesium i o n , which i s quite the opposite of the behavior observed f o r asymmetric c e l l u l o s e acetate membranes. S a l t passage through the NS-300 membrane may be described as a n i o n - c o n t r o l l e d . The performance of t h i s membrane system towards v a r i o u s feedwaters i n the l a b o r a t o r y t r i a l s i s shown i n Table 4, again with a s i n g l e set of t e s t specimens exposed to the d i f f e r e n t feedwaters. I n i t i a l s a l t r e j e c t i o n s of the NS-300 membrane specimens were poorer than average i n t h i s study, but the compara t i v e r e s u l t s are nevertheless i n f o r m a t i v e . The 90:10 i s o p h t h a l i c : t r i m e s i c copolyamide membrane showed s u f f i c i e n t f l u x and s a l t r e j e c t i o n to be u s e f u l i n the reverse osmosis s o f t e n i n g of a "hard" w e l l water. In t h i s case, the w e l l water contained about 500 ppm of calcium and magnesium bicarbonates. As might be a n t i c i p a t e d , the 67:33 copolyamide composite could not r e t a i n the monovalent bicarbonate i o n , such that hardness r e j e c t i o n was only 55 to 65 percent. Magnesium s u l f a t e , sodium c h l o r i d e , and s y n t h e t i c seawater r e j e c t i o n s , while below average i n t h i s set of membranes, followed the p r e d i c t e d p a t t e r n . I t should be noted that seawater r e j e c t i o n s were always higher than d i l u t e sodium c h l o r i d e s o l u t i o n r e j e c t i o n s . The h y d r o p h i l i c b a r r i e r l a y e r apparently t i g h t e n s up i n contact with concentrated s a l t s o l u t i o n s , and b a r r i e r l a y e r compaction at high pressures may be a c o n t r i b u tive factor. Tests were a l s o run with simulated b r a c k i s h a g r i c u l t u r a l drainage water, as i l l u s t r a t e d i n Table 4. A feedwater composition c o n t a i n i n g sodium, calcium, c h l o r i d e , s u l f a t e , and bicarbonate ions was prepared i n such a way as to d u p l i c a t e the water i n the Mohawk-Wellton drainage canal at Yuma, A r i z o n a . S a l t r e j e c t i o n s were r e l a t i v e l y poor toward t h i s s y n t h e t i c feedwater, but when t h i s water was l i n e - s o f t e n e d and a c i d i f i e d to pH 5.5 with s u l f u r i c a c i d , s a l t r e j e c t i o n of the 90:10 copolyamide improved markedly. However, the membrane's water f l u x d e c l i n e d by n e a r l y 50 percent. S a l t r e j e c t i o n and f l u x were found i n t h i s and other examples to be markedly dependent on pH. As the pH approached the pKa of


314

SYNTHETIC

MEMBRANES:

DESALINATION

Table 2. Effect of the Isophthaloyl:Trimesoyl Chloride Ratio on the Performance of NS-300 Membranes in Reverse Osmosis Tests Acid Chloride Ratio

3

Reverse Osmosis Test Results 0.5% MgSO^ 200 psig


Trimesoyl

a

Isophthaloyl

Flux Salt Rej. (gfd) (percent)

3.5% Synthetic Seawater 1500 psig Flux (gfd)

Salt Rej. (percent)

100

0

26

99.3

80

68

75

25

31

99.3

96

64

33

67

77

99.9

94

65

25

75

58

99.6

73

78

10

90

18

99.9

33

96

0

100

4

99.0

20

98

Aqueous phase contained 1% piperazine, 1% Na^PO^, 0.5% dodecyl sodium sulfate; hexane phase contained 1% (w/v) of acyl chlorides.

Table 3.

E f f e c t of Cation and Anion Valence on S a l t R e j e c t i o n P r o p e r t i e s of NS-300 Membranes

Solute

Flux (gfd)

a

Salt Rejection (percent) 3

NaCl

42

50

MgCl

32

46

41

97.8

32

97.9

2

Na S0 2

MgS0

a

4

4

Reverse osmosis t e s t c o n d i t i o n s : 0.5% s a l t c o n c e n t r a t i o n , 200 p s i , 25°C, p o l y ( p i p e r a z i n e trimesamide) membrane.



0

600 600

s y n t h e t i c r i v e r water

s y n t h e t i c lime-softened a g r i c u l t u r e d drainage w a t e r

(a) (b) (c)

250

synthetic a g r i c u l t u r a l drai.nage water*

(Psig)

95.5

44 23

66 85.5

13

94.5

95 78

15 25

91

17

82

18

9.5

—

47

30

82

56

53

58

28.6

—

76

76

63

56

90

65

55

Reverse Osmosis Test Results 90:10 IPC:TMC 67:33 IPC.-TMC Flux Salt Rejection Flux Salt Rejection (gfd) (percent) (percent) (gfd)

c a . 500 ppm calcium and magnesium bicarbonates. 3390 ppm TDS 2880 ppm TDS

3

1000

200

3.5% s y n t h e t i c seawater

4

200

0.5% NaCl

0.5% MgS0

90 200

a

Pressure

Performance of NS-300 Membranes on Various Feedwat ers at D i f f e r e n t Pressures

hard w e l l water

hard w e l l w a t e r

Feedwater

Table 4.



316

SYNTHETIC

MEMBRANES:

DESALINATION

the membrane's carboxylate groups, the b a r r i e r l a y e r tightened. This same e f f e c t was s i n c e demonstrated i n spiral-wound NS-300 membrane elements placed on t e s t toward b r a c k i s h water at Roswell Test F a c i l i t y operated by the O f f i c e of Water Research and Technology, U.S. Department of the I n t e r i o r . E f f o r t s to f a b r i c a t e the NS-300 thin-film-composite membrane by continuous machine c a s t i n g at FilmTec Corporation have been only p a r t i a l l y s u c c e s s f u l . A severe problem of membrane v a r i a b i l i t y was experienced, which was due i n part o s t e n s i b l y to minor v a r i a t i o n s i n machine-made p o l y s u l f o n e support f i l m s . This was studied, and i t was postulated that, s i n c e there was no intermediate p o r o s i t y zone as the c r o s s l i n k e d polyethylenimine l a y e r i n the NS-100 membrane, the poly(piperazineamide) membranes would be more s e n s i t i v e to the d e f e c t s i n the underlying p o l y s u l f o n e support (22). An approach to overcome t h i s problem would be to form p i p e r azine-terminated oligomers to r e p l a c e p i p e r a z i n e i n the i n t e r f a c i a l r e a c t i o n . These oligomers could p o s s i b l y generate a l i g h t l y c r o s s l i n k e d intermediate zone between the surface b a r r i e r l a y e r and the microporous p o l y s u l f o n e s u b s t r a t e . Oligomers were synthes i z e d by r e a c t i o n of a c y l h a l i d e s with an excess of p i p e r a z i n e i n 1,2-dichloroethane. The amine-terminated polyamide oligomers had poor s o l u b i l i t y i n t h i s solvent system, and p r e c i p i t a t e d out almost i n s t a n t l y upon formation. This served to l i m i t the degree of polymerization of the oligomers to l e s s than ten monomer u n i t s . Even so, p o r t i o n s of the products were i n s o l u b l e i n water and were removed by f i l t r a t i o n during the p r e p a r a t i o n of the aqueous o l i g o meric amine s o l u t i o n s . Table 5 l i s t s the best performance data obtained f o r p i p e r a z i n e oligomer membranes i n t e r f a c i a l l y reacted with i s o p h t h a l o y l c h l o r i d e . The o b j e c t i v e of these t e s t s was to achieve single-pass seawater d e s a l i n a t i o n membranes. As such, the presence of f r e e carboxylate groups was avoided; use was made of the t r i m e s o y l c h l o r i d e or a l t e r n a t e t r i a c y l h a l i d e s i n the oligomer formation step. A few examples of seawater d e s a l i n a t i o n membranes were obtained. Best r e s u l t s were seen f o r piperazine-cyanurate prepolymers i n t e r f a c i a l l y c r o s s l i n k e d by i s o p h t h a l o y l c h l o r i d e , but f l u x e s were low i n view of the operating t e s t pressure of 1500 p s i . A l s o , i n d i v i d u a l membrane r e s u l t s with p i p e r a z i n e oligomers were e q u a l l y as e r r a t i c as was experienced f o r p i p e r a z i n e d i r e c t l y . The only notable advantage of the p i p e r a z i n e oligomer approach was the a b i l i t y to i n c o r p o r a t e cyanurate r i n g s into the membrane s t r u c t u r e . Cyanuric c h l o r i d e was too prone to h y d r o l y s i s to provide good i n t e r f a c i a l membranes with p i p e r a z i n e , otherwise. In summary, the NS-300 membrane system a c t u a l l y comprises a f a m i l y of membranes, with reverse osmosis p r o p e r t i e s determined by the i s o p h t h a l i c : t r i m e s i c r a t i o . E x c e p t i o n a l l y high f l u x e s are p o s s i b l e at high r e t e n t i v i t y l e v e l s f o r d i s s o l v e d s a l t s c o n t a i n i n g p o l y v a l e n t anions. This membrane type may f i n d a p p l i c a t i o n s i n the d e s a l i n a t i o n of b r a c k i s h s u l f a t e ground waters or i n d u s t r i a l


In Synthetic Membranes:; Turbak, A.; ACS Symposium Series; American Chemical Society: Washington, DC, 1981. NaOH

* Twenty to 24 hour t e s t s , 1500 p s i , 25°C, 2.5% s y n t h e t i c seawater

cyanuric chloride/6:1 piperazine:morpholine

triethylamine

1:1 t r i m e s o y l : i s o p h t h a l o y l chloride/piperazine

triethylamine NjN'-dimethylpiperazine

chloride/piperazine

cyanuric

NaOH

triethylamine

NaOH

Acid Acceptor

Membranes Formed Using P i p e r a z i n e Oligomers and I s o p h t h a l o y l C h l o r i d e

phosphorus o x y c h l o r i d e / piperazine

chloride/piperazine

chloride/piperazine

cyanuric

trimesoyl

trimesoyl chloride/ piperazine

Composition of Oligomer

Table 5.

8.9

33.9

45

23.9

13.9

58

12.5

99.0

92.4

93.9

98.0

99.2

93.8

99.0

Reverse Osmosis Test Data Flux (gfd) S a l t R e j e c t i o n (%)


318

SYNTHETIC


process waters, and may have u t i l i t y sucrose and l a c t o s e c o n c e n t r a t i o n .

MEMBRANES:

DESALINATION

i n food a p p l i c a t i o n s such as

FT-30 Membrane. FT-30 i s a new thin-film-composite membrane discovered and developed by FilmTec. I n i t i a l data on FT-30 membranes were presented elsewhere (23). I t was r e c e n t l y i n t r o duced i n the form of spiral-wound elements 12 inches long and 2 to 4 inches i n diameter (24). The b a r r i e r l a y e r of FT-30 i s of p r o p r i e t a r y composition and cannot be revealed at t h i s time pending r e s o l u t i o n of p a t e n t a b i l i t y matters. The membrane shares some of the p r o p e r t i e s of the p r e v i o u s l y described "NS" s e r i e s of membranes, e x h i b i t i n g high f l u x , e x c e l l e n t s a l t r e j e c t i o n , and nonb i o d e g r a d a b i l i t y . However, the response of the FT-30 membrane d i f f e r s s i g n i f i c a n t l y from other n o n c e l l u l o s i c thin-film-composite membranes i n regard to v a r i o u s feedwater c o n d i t i o n s such as pH, temperature, and the e f f e c t of c h l o r i n e . Table 6 l i s t s s e v e r a l of the s a l i e n t p r o p e r t i e s of t h i s new composite membrane. When s a l t r e j e c t i o n was evaluated at d i f f e r e n t pressures i n simulated seawater t r i a l s , potable water ( c o n t a i n i n g l e s s than 500 ppm d i s s o l v e d s a l t s ) was generated at as low as 600 p s i , with very good f l u x (12 gfd) a t that pressure. In s p i r a l wound membrane element t r i a l s on a c t u a l 33,000 ppm seawater, potable water was obtained even at 500 p s i , a l b e i t at low f l u x . These r e s u l t s surpass by f a r the c a p a b i l i t i e s of any of the "NS" s e r i e s of membranes. The FT-30 membrane was found to be r e s i s t a n t to s w e l l i n g or s a l t r e j e c t i o n l o s s e s at high feedwater temperatures. In simulated seawater t e s t s , the membrane had s t a b i l i z e d at about 99 percent s a l t r e j e c t i o n a t temperatures of 40°C and higher. The membrane has been s u c c e s s f u l l y evaluated f o r sugar concentrat i o n a t 95°C. In t r i a l s at d i f f e r e n t feedwater c o n c e n t r a t i o n s , the FT-30 membrane showed s i n g l e - p a s s seawater d e s a l t i n g c a p a b i l i t i e s at up to 6.0 percent s y n t h e t i c seawater. B a s i c a l l y , any combination of pressure and b r i n e c o n c e n t r a t i o n at room temperature that gave a membrane f l u x of 15 gfd a l s o r e s u l t e d i n a 99 percent l e v e l of salt rejection. Concerning the e f f e c t of pH, over a range of 5 to 11 the FT-30 membrane e x h i b i t e d 99 percent or greater s a l t r e j e c t i o n towards s y n t h e t i c seawater at 1000 p s i . Below pH 5, s a l t r e j e c t i o n s as measured by conductimetric techniques gave e r r a t i c v a l u e s . I t i s now b e l i e v e d that t h i s r e f l e c t e d a c i d i t y t r a n s p o r t through the membrane r a t h e r than s a l t passage. At pH 12, s a l t r e j e c t i o n s f e l l below 98 percent due probably to membrane s w e l l i n g . Some membrane l o t s showed t h i s lower s a l t r e j e c t i o n a t pH 12; others d i d not. The FT-30 membrane w i l l withstand exposure to a pH range of 1 to 12 f o r c l e a n i n g purposes. Both a c i d i c and a l k a l i n e membrane c l e a n i n g reagents can be employed, i n c l u d i n g , f o r example, 0.1 percent phosphoric a c i d or 0.5 percent t r i s o d i u m phosphate combined with an a n i o n i c s u r f a c t a n t . Nonionic s u r f a c -


CADOTTE

A N DPETERSEN


T a b l e 6.

Feedwater

3.5%

SSW

Thin-Film

Composite

RO

Membranes

F l u x and S a l t R e j e c t i o n o f FT-30 Membranes a s a F u n c t i o n o f Temperature, P r e s s u r e , and B r i n e C o n c e n t r a t i o n . Pressure. (psi)

Temperature (°C)

400

25

a

3.5% SSW

Flux (gfd) 4.3

Salt Rej. (percent) 95.5

600

12

98.8

800

20

99.3

1000

30

99.4

20

23

99.5

1000

30

35

99.2

40

55

99.0

50

65

98.9

60

72

99.0

54

99.5

2.0

43

99.4

4.0

25

99.4

6.0

16

1.0% SSW

1000

7.5

8.0 a

SSW = s y n t h e t i c

25

99.0 97.8

seawater.



320

SYNTHETIC

MEMBRANES:

DESALINATION

tants have been found to i n t e r a c t n e g a t i v e l y with the FT-30 membrane, reducing f l u x . This thin-film-composite membrane has been found to have appreciable r e s i s t a n c e to degradation by c h l o r i n e i n the f e e d water. Figure 2 i l l u s t r a t e s the e f f e c t of c h l o r i n e i n tap water at d i f f e r e n t pH v a l u e s . Chlorine (100 ppm) was added to the tap water i n the form of sodium h y p o c h l o r i t e (two equivalents of h y p o c h l o r i t e ion per stated equivalent of c h l o r i n e ) . Membrane exposure to c h l o r i n e was by the s o - c a l l e d " s t a t i c " method, i n which membrane specimens were immersed i n the aqueous media i n s i d e c l o s e d , dark g l a s s j a r s f o r known periods. Specimens were then removed and tested i n a reverse osmosis loop under seawater t e s t c o n d i t i o n s . At a l k a l i n e pH v a l u e s , the FT-30 membrane showed e f f e c t s of c h l o r i n e attack w i t h i n four to f i v e days. In a c i d i c s o l u t i o n s (pH 1 and 5), c h l o r i n e attack was f a r slower. Only a one to two percent d e c l i n e i n s a l t r e j e c t i o n was noted, f o r example, a f t e r 20 days exposure to 100 ppm c h l o r i n e i n water at pH 5. The FT-30 t e s t s at pH 1 were n e c e s s a r i l y terminated a f t e r the f o u r t h day of exposure because the microporous p o l y s u l fone substrate had i t s e l f become t o t a l l y embrittled by c h l o r i n e attack. In a r e l a t e d case, FT-30 membrane elements were placed on c h l o r i n a t e d seawater feed at OWRT's W r i g h t s v i l l e Beach Test Facility. Flux and s a l t r e j e c t i o n were s t a b l e f o r 2000 hours at 0.5 to 1.0 ppm c h l o r i n e exposure. C h l o r i n e a t t a c k d i d become n o t i c e a b l e a f t e r 2000 hours, and s a l t r e j e c t i o n had dropped to 97 percent at 2500 hours while f l u x increased s i g n i f i c a n t l y . Long term l a b o r a t o r y t r i a l s at d i f f e r e n t c h l o r i n e l e v e l s l e d to the c o n c l u s i o n that the membrane w i l l withstand 0.2 ppm c h l o r i n e i n sodium c h l o r i d e s o l u t i o n s at pH 7 f o r more than a year of continuous exposure. In summary, the FT-30 membrane i s a s i g n i f i c a n t improvement i n the a r t of thin-film-composite membranes, o f f e r i n g major improvements i n f l u x , pH r e s i s t a n c e , and c h l o r i n e r e s i s t a n c e . S a l t r e j e c t i o n s c o n s i s t e n t with s i n g l e - p a s s production of potable water from seawater can be obtained and held under a wide v a r i e t y of operating c o n d i t i o n s (ph, temperature, pressure, and b r i n e c o n c e n t r a t i o n ) . This membrane comes c l o s e to being the i d e a l membrane f o r seawater d e s a l i n a t i o n i n terms of p r o d u c t i v i t y , chemical s t a b i l i t y , and n o n b i o d e g r a d a b i l i t y . Scanning E l e c t r o n Microscopy

Studies

Various n o n c e l l u l o s i c thin-film-composite membranes were examined by scanning e l e c t r o n microscopy (SEM). Figure 3 i l l u s t r a t e s the type of surface s t r u c t u r e and c r o s s - s e c t i o n s that e x i s t i n these membranes. Figure 3a shows the surface m i c r o p o r o s i t y of p o l y s u l f o n e support f i l m s . Micropores i n the f i l m were measured by both SEM and TEM; t y p i c a l l y pore r a d i i averaged 330 A. Figure 3b i s a photomicrograph of a c r o s s - s e c t i o n of a NS-100 membrane.


Thin-Film



Composite

RO

Membranes

U

\\

80

^ 6 0

V\

0

2

4

PRESSURE-800 PSI

^

FEEDWATER

g

SEAWATER

TEMPERATURE • 25°C

6

8 IMMERSION

10

12

TIME

[DAYS]

14

16

^

18

jj 1

2

20

Figure 2. Exposure of FT-30 membranes to 100 ppm chlorine in water at different pH levels. Effect on salt refection in simulated seawater reverse osmosis tests: (0) pH 1; O PH 5; (O) pH 8; (A) pH 12.


SYNTHETIC


322

MEMBRANES:

DESALINATION

Figure 3a. SEM photomicrographs of composite membranes: surface structure of microporous polysulfone support material.

Figure 3b. SEM photomicrograph of composite membranes: cross-section of a NS-100 composite membrane showing the porous polysulfone substructure.

Figure 3c. SEM photomicrograph of composite membranes: surface view of the NS-100 membrane.




Thin-Film

Composite

RO Membranes

323

Figure 3d. SEM photomicrograph of composite membranes: surface view of the NS-200 membrane.

Figure 3e. SEM photomicrograph of composite membranes: surface view of the poly (piperazine trimesamide) version of the NS-300 membrane.

Figure 3f. SEM photomicrograph of composite membranes: surface view of the FT-30 membrane.



324

SYNTHETIC

MEMBRANES:

DESALINATION

A smooth top surface corresponding to the dense b a r r i e r l a y e r i s evident. The porous, spongy p o l y s u l f o n e matrix i s evident below t h i s surface l a y e r . Although not evident i n t h i s photomicrograph, the thickness of the b a r r i e r l a y e r and c r o s s l i n k e d polyethylenimine intermediate l a y e r , taken together, i s approximately 2000 A. Figure 3c i s a high m a g n i f i c a t i o n view of an NS-100 membrane s u r face, and shows a f e a t u r e l e s s p l a i n punctuated by o c c a s i o n a l a r t i f a c t s (loose p o l y s u l f o n e microbeads). Figure 3d i l l u s t r a t e s the surface of a NS-200 membrane. The surface appears to c o n t a i n nodules of the sulfonated polyfurane r e s i n , which apparently were present i n the aqueous coating before heat-curing, or formed during e a r l y stages of the heatcuring o p e r a t i o n . Figure 3e contains a photomicrograph of the surface of a p o l y ( p i p e r a z i n e trimesamide) b a r r i e r l a y e r i n t e r f a c i a l l y formed on a p o l y s u l f o n e support. Swelling of the membrane apparently occurred concurrent with i t s formation to produce the type of s t r u c t u r e seen i n Figure 3e. Reasons f o r t h i s surface s t r u c t u r e are described elsewhere (22). Figure 3f shows the surface of an FT-30 membrane. A f a i r l y rough topography i s present. I t can be seen from these SEM photomicrographs that the surface of thin-film-composite membranes can vary s u b s t a n t i a l l y from one type to another. In f a c t , i t i s p l a u s i b l e that some of these membranes can be i d e n t i f i e d by t h e i r c h a r a c t e r i s t i c surface topography through examination by SEM. Literature Cited

1. 2. 3. 4. 5.

6. 7. 8. 9.

Reid, C.E.; Breton, E . J . ; J. Appl. Polymer Sci., 1959, 1, 133. Loeb, S.; Sourirajan, S.; Advan. Chem. Ser., 1962, 38, 117. Francis, P.S.; "Fabrication and Evaluation of New Ultrathin Reverse Osmosis Membranes," National Technical Information Service, Springfield, VA, Report No. PB-177083, 1966. Rozelle, L.T.; Cadotte, J . E . ; Corneliussen, R.D.; Erickson, E.E.; "Development of New Reverse Osmosis Membranes for Desalination," ibid., Report No. PB-206329, 1967. Rozelle, L.T.; Kopp, C.V.,Jr.; Cadotte, J . E . ; Kobian, K.E.; "Nonpolysaccharide Membranes for Reverse Osmosis: NS-100 Membranes," in "Reverse Osmosis and Synthetic Membranes," Sourirajan, S., Ed., National Research Council Canada, Ottawa, 1977, p.249. Riley, R.L.; Lonsdale, H.K.; Lyons, C.R.; Merten, U.; J. Appl. Polymer Sci., 1967, 11, 2143. Carnell, P.H.; Cassidy, H.G.; J . Polymer Sci., 1961, 55, 233. Carnell, P.H.; J . Appl. Polymer Sci., 1965, 9, 1963. Lonsdale, H.K.; Riley, R.L.; Lyons, C.R.; Carosella, D.P., Jr.; "Transport in Composite Reverse Osmosis Membranes," in "Membrane Processes in Industry and Biomedicine," Bier, M., Ed, Plenum Press, 1971, p.101.


21.

10.

11.

12.


13. 14. 15. 16. 17. 18. 19. 20. 21.

22.

23. 24.

CADOTTE

AND

PETERSEN

Thin-Film

Composite

RO

Membranes

325

Cadotte, J . E . ; Kopp, C.V., J r . ; Cobian, K.E.; Rozelle, L . T . ; "In Situ-Formed Condensation Polymers for Reverse Osmosis Membranes: Second Phase," National Technical Information Service, Springfield, VA, Report No. PB-234198, 1974, p.6. Cadotte, J . E . ; Cobian, K.E.; Forester, R.H.; Petersen, R . J . ; "Continued Evaluation of In Situ-Formed Condensation Polymers for Reverse Osmosis Membranes," ibid., Report No. PB-253193, 1976, p.32. Yasuda, H.; "Composite Reverse Osmosis Membranes Prepared by Plasma Polymerization," in "Reverse Osmosis and Synthetic Membranes," Sourirajan, S., Ed., National Research Council, Canada, Ottawa, 1977, p.263. Bell, A.T.; Wydeven, T.; Johnson, C.C.; J . Appl. Polymer Sci., 1975, 19, 1911. Hollahan, J.R.; Wydeven, T.; J . Appl. Polymer Sci., 1977, 21, 923. Riley, R.L.; Fox, R.L.; Lyons, C.R.; Milstead, C.E.; Seroy, M.W.; Tagami, M.; Desalination, 1976, 19, 113. Cadotte, J . E . ; U.S. 4,039,440 (1977). Sasaki, H.; Hayashi, Y.; Hara, S.; Kawaguchi, T.; Minematsu, H.; Brit. UK Pat. Appl. 2,027,614 (1980); Chem. Abstr., 1980, 92, 77403r. Kawaguchi, T.; Hayashi, Y.; Taketani, Y.; Mori, Y.; Ono, T.; Fr. Demande 78-15546 (1978); Chem. Abstr., 1980, 92, 59897a. Kurihara, M.; Kanamaru, N.; Harumiya, N.; Yoshimura, K.; Hagiwara, S.; Desalination, 1980, 32., 13. Credali, L . ; Chiolle, A.; Parinni, P.; Desalination, 1974, 14, 137. Cadotte, J . E . ; Steuck, M.J.; Petersen, R . J . ; "Research on In Situ-Formed Condensation Polymer for Reverse Osmosis Membranes," National Technical Information Service, Springfield, VA, Report No. PB-288387, 1978, p.10. Cadotte, J . E . ; King, R.S.; Majerle, R . J . ; Petersen, R . J . ; "Interfacial Synthesis in the Preparation of Reverse Osmosis Membranes," paper presented at 179th Ann. Amer. Chem. Soc. Meeting, Houston, TX, March 23-28, 1980; Marcel Dekker, in press. Cadotte, J . E . ; Petersen, R . J . ; Larson, R.E.; Erickson, E . E . ; Desalination, 1980, 32, 25. Petersen, R . J . ; Larson R.E.; Majerle, R . J . ; "Development of the FT-30 Thin-Film Composite Membrane for Desalting Applications," Technical Proceedings, 8th Ann. Conf. National Water Supply Improvement Assn., San Francisco, CA, July 6-10, 1980.

Acknowledgements The authors are indebted to the O f f i c e of Water Research and Technology and the former O f f i c e of S a l i n e Water, U.S. Department of the I n t e r i o r , f o r t h e i r support of t h i s work over the past


326

SYNTHETIC

MEMBRANES:

DESALINATION

s e v e r a l years. P o r t i o n s of the research on NS-300 and FT-30 were supported by OWRT under Contracts 14-34-0001-6512, 14-34-0001-8512, and 14-34-0001-8547. December 4, 1980.


RECEIVED


Thin-Film Composite Reverse-Osmosis Membranes - American

Recommend Documents