Membranes for Reverse Osmosis - American Chemical Society

25 Poly(vinyl alcohol) Membranes for Reverse Osmosis 1

MOSHE G. KATZ and THEODORE WYDEVEN, JR. Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch025

NASA—Ames Research Center, Moffett Field, CA 94035

Polyvinyl alcohol(PVA) has been shown at very early stages to have very poor salt selectivity (1). Without treatment it has salt rejections in the range between 20-50%. Being a material with very good film forming properties and high water permeability, i t always seemed an attractive material for high water flux,reverse osmosis(RO) membranes. Chemical crosslinking is known to improve considerably the selective permeability to water and salts of the PVA. Thus PVA membranes crosslinked with formaldehyde were shown (2) to have salt rejections in the range of 4-8x10 cm /s. Even better results were obtained by crosslinking the PVA membranes using tolylene diisocyanate (3). By this method PVA membranes with salt rejections of up to 99 2% and water permeability coefficients in the range of 2-5x10 cm /s were reported. Another attractive quality of the PVA is its outstanding chemical stability. Peter and Stefan (4) investigated the chemical stability of PVA vis a vis a series of solvents and reagents and some of their results are summarized in Table I. This table presents a comparison between the chemical resistances of PVA, polyvinyl butyral(PVB), polyvinyl acetate(PVAc), and cellulose acetate(CA). It is evident from this table that the PVA is considerably more stable than the rest of the tested materials. In this article, results of studies of the water and salt transport properties of PVA membranes and a method for preparing thin skinned, high flux PVA membranes are reported. -8

2

-7

2

Transport P r o p e r t i e s of PVA Membranes The transport p r o p e r t i e s s t u d i e s were made i n the framework of the e v a l u a t i o n of the e f f e c t of r a d i a t i o n c r o s s l i n k i n g and heat treatment on the PVA, and the p o s s i b i l i t y to improve the RO performance of these membranes using these methods. The water 1

Current address: Pure and A p p l i e d R a d i a t i o n Chemistry ment, Soreq Nuclear Research Center, Yavne, I s r a e l .

0097-6156/81/0153-0383$05.00/0 © 1981 American Chemical Society

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

Depart-

SYNTHETIC

Downloaded by UNIV OF CALIFORNIA SAN DIEGO on June 2, 2015 | http://pubs.acs.org Publication Date: May 21, 1981 | doi: 10.1021/bk-1981-0153.ch025

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MEMBRANES:

DESALINATION

Table I Chemical s t a b i l i t y of PVA a t 20°C ( from P e t e r et a l . (4)) Membrane m a t e r i a l

Concentration Reagent HN0

3

H S0 2

4

PVB

PVAc

CA

3

0

0

0

3

0

0

0 0

PH

PVA

12

0

48

0

(weight %)

7

0

3

0

0

NH^OH

25

13

3

0

0

0

NaOH

10

14

3

0

0

0

HC1

Phenol

0.1

3

3

0

0

0

Phenol

1.0

3

3

0

0

0

Phenol

2.0

3

3

0

0

0

Ethanol

100

7

3

0

0

0

DMF

100

7

3

0

0

0

DMSO

100

7

3

0

0

0

Formamide

100

7

3

0

0

0

S t a b i l i t y degrees: 0=Destruction of membrane l=Strong s w e l l i n g , decrease of membrane p r o p e r t i e s 2 = L i t t l e s w e l l i n g , membrane s t i l l usable 3=Stable, no a l t e r a t i o n o f membrane p r o p e r t i e s


25.

KATZ AND WYDEVEN

Poly(vinyl

alcohol) Membranes

385

and s a l t p e r m e a b i l i t i e s through r a d i a t i o n c r o s s l i n k e d and heat treated PVA were measured a t v a r i o u s a p p l i e d pressures, temperatures , and s a l t c o n c e n t r a t i o n s . For the a n a l y s i s of the e x p e r i mental r e s u l t s , the Lonsdale s o l u t i o n d i f f u s i o n model was employed,where the water f l u x ( F ) , s a l t f l u x ( F ) , and s a l t r e j e c t i o n ( R ) are given by the f o l l o w i n g equations: S

/ ~\ \ -.-I D


U

)

F

(2)

w

c

V

AP-ATT

_ wm wm w RT X

Ac F =- D K - r ^ s sm X D

K RT c"

wm wm w

,

c'-c" s

where D and c are the d i f f u s i o n c o e f f i c i e n t of water and the water concentration i n the membrane, r e s p e c t i v e l y ; V^ i s the part i a l molar volume of water i n the membrane; AP and ATT are the d i f f e r e n c e s i n a p p l i e d pressure and osmotic pressure across the membrane; R and T a r e the gas constant and the absolute temperature r e s p e c t i v e l y ; X i s the e f f e c t i v e membrane t h i c k n e s s ; D and K a r e the d i f f u s i o n c o e f f i c i e n t of s a l t and the s a l t d i s t r i b u t i o n c o e f f i c i e n t , r e s p e c t i v e l y ; and Ac =c'-c" i s the d i f f e r e n c e between the s a l t c o n c e n t r a t i o n on the fiign* pressure s i d e and the s a l t c o n c e n t r a t i o n on the low pressure s i d e of the membrane. R a d i a t i o n C r o s s l i n k e d PVA Membranes. The water and s a l t p e r m e a b i l i t y c o e f f i c i e n t s of the r a d i a t i o n c r o s s l i n k e d PVA membranes, obtained by using equations 1 and 2 were found to decrease w i t h i n c r e a s i n g the a p p l i e d pressure. A l i n e a r c o r r e l a t i o n was found between the r e c i p r o c a l of the water p e r m e a b i l i t y c o e f f i c i e n t and the pressure, as shown i n Figures 1 and 2, at v a r i o u s temperatures. T h i s l i n e a r c o r r e l a t i o n can be expressed by the f o l l o w i n g equation:

(4)

— = a + bAP u c wm wm

S u b s t i t u t i n g equation 4 i n t o the water f l u x equation(equation 1) y i e l d s a c o r r e l a t i o n between the water f l u x ( F ) and pressure, which i s i d e n t i c a l with the e m p i r i c a l r e l a t i o n s h i p suggested earl i e r by P a u l ( 5 ) , f o r s o l v e n t permeation through swollen hydrogels;

(5)

a RT XF

RT AP ^

V



386 SYNTHETIC MEMBRANES: DESALINATION


25.

KATZ AND WYDEVEN


( 6 )

F

w

=

ART

Polyvinyl alcohol)

a +

Membranes

387

bAP

Ebra-Lima and Paul a l s o s t u d i e d the water t r a n s p o r t through terephtalaldehyde c r o s s l i n k e d PVA membranes ( 6 ) . From the dependence of the water f l u x on p r e s s u r e , reported by them, and shown i n F i g u r e 3, we were able to evaluate the corresponding permeability c o e f f i c i e n t s at the v a r i o u s p r e s s u r e s . These data are a l so p l o t t e d along with our data i n F i g u r e 2, showing good agreement. T h i s agreement between the two s e t s of data can a l s o be v i s u a l i z e d by a p l o t of the r e c i p r o c a l of the water f l u x ( F ) vs. the r e c i p r o c a l of the pressure drop(AP), as shown i n F i g u r e 4, where the f l u x e s were normalized f o r the membrane thickness X. The temperature dependence of the water p e r m e a b i l i t y of r a d i a t i o n c r o s s l i n k e d PVA membranes i s shown i n F i g u r e 5. From t h i s dependence an a c t i v a t i o n energy of 6.0±0.2 kcal/mole can be d e r i v e d , i n good agreement w i t h data reported e a r l i e r by P e t e r and M i t t e l s t a d t (7) and by P a u l ( 6 ) . The s a l t r e j e c t i o n of the r a d i a t i o n c r o s s l i n k e d PVA membranes was very low, around 20-50%, i . e . i n the same range as found e a r l i e r f o r untreated PVA. No systematic change of the s a l t r e j e c t i o n w i t h temperature was observed i n the temperature range b e t ween 24-65°C, suggesting that the a c t i v a t i o n energy of the s a l t p e r m e a b i l i t y was i d e n t i c a l with that of the water, and t h e r e f o r e , the s a l t f l u x was i n c r e a s i n g p r o p o r t i o n a l l y w i t h the water f l u x at h i g h e r temperatures. However, the r e j e c t i o n was found to be s t r o n g l y c o n c e n t r a t i o n dependent, as shown i n F i g u r e 6. The very low s a l t r e j e c t i o n s , i . e . the high s a l t f l u x e s and the i d e n t i c a l temperature dependence of the water and s a l t permea b i l i t i e s may be considered as suggesting a coupled water and s a l t t r a n s p o r t . However, i t i s evident from the s a l t r e j e c t i o n data i n F i g u r e 6 that the s a l t r e j e c t i o n i s i n c r e a s i n g w i t h p r e s s u r e , as expected from the s o l u t i o n - d i f f u s i o n model. The p l o t s of the r e c i p r o c a l of the s a l t r e j e c t i o n ( R ) vs. the r e c i p r o c a l of the pressure drop(AP),shown i n F i g u r e are l i n e a r and i n t e r c e p t i n g at 1.0 or very c l o s e to i t . This i s i n agreement with the Lonsdale equation f o r s a l t r e j e c t i o n (equation 3) by sol u t i o n - d i f f u s i o n membranes, and suggests uncoupled s a l t and water flows. D i f f u s i v e S a l t P e r m e a b i l i t y Through PVA Membranes. In order to v e r i f y the interdependence between the water and s a l t t r a n s p o r t through PVA, the d i f f u s i v e p e r m e a b i l i t y of s a l t through an unp r e s s u r i z e d PVA membrane was measured. The d i f f u s i o n system employed i n t h i s experiment i s d e s c r i b e d s c h e m a t i c a l l y i n F i g u r e 8. The d i f f u s i o n c e l l c o n s i s t s of an upper c e l l and a lower c e l l , separated by the t e s t e d membrane, a 150 micron t h i c k , untreated PVA membrane. I n i t i a l l y , the lower c e l l containes d i s t i l l e d wat e r , which i s c i r c u l a t e d through a c o n d u c t i v i t y c e l l , and the


SYNTHETIC

MEMBRANES:

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388

Figure 4. Pressure dependence of water fluxes through PVA membranes as determined by Ebra-Lima and Paul (6) and in this work: ([J) Ebra-Lima and Paul's data at 24°C (\ = 35fim); (O) data for radiation cross-linked, 115,000 mol wt, 100% hydrolyzed PVA (k = 4.0 ^m).


KATZ AND WYDEVEN


25.

Polyvinyl

alcohol)

Membranes

389

E

Q5

2.8

2.9

3.0

3.1

3.2

3.3

3.4

1

1000/T,°K~

2 4 6 10 20 FEED CONCENTRATION, mM

3.5

Figure 5. Temperature dependence of the water permeability of radiation crosslinked PVA membranes under various pressure differentials: ([J) AP = 200 psi; (A) AP = 600 psi; (O) AP = 1000 psi. Open symbols refer to 100% hydrolyzed, 115,000 mol wt PVA; solid symbols refer to 100% hydrolyzed, 86,000 mol wt PVA. The lines are the best fit to the data points evaluated by linear regression analysis.

Figure 6. Feed concentration dependence of the salt rejection of radiation cross-linked PVA membranes under various pressure differentials: (•) AP = 200 psi; (A) AP = 600 psi; (O) AP = 1000 psi. Open symbols refer to 100% hydrolyzed, 115,000 mol wt PVA; solid symbols refer to 100% hydrolyzed, 86,000 mol wt PVA.


SYNTHETIC


390

MEMBRANES:

DESALINATION

Figure 8. Diffusive permeability measurement system: (1) thermostated bath; (2) diffusion cell; (3) membrane; (4) thermometer; (5) stirrer; (6) circulation pump; (7) conductivity cell; (8) conductivity meter; (9) recorder.


25.

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Polyvinyl alcohol)

Membranes

391

c o n d u c t i v i t y i s recorded c o n t i n u o u s l y . The upper c e l l i s f i l l e d w i t h a s a l t s o l u t i o n of known c o n c e n t r a t i o n and the r a t e of change of the s a l t c o n c e n t r a t i o n i n the lower c e l l i s determined. Some of the r e s u l t s obtained by these measurements are p r e sented i n Table I I . These data are comparable w i t h corresponding


Table I I D i f f u s i v e s a l t p e r m e a b i l i t y through untreated^PVA membranes measured at atmospheric pressure ( cm /s x 10 )

^ v J J a l t concentration i n ^^x. upper c e l l Temperature\J (°C)

20

0.1

0.025

w e i g h t % )

25 37 50

A c t i v a t i o n energy (kcal/mole)

1.16 1.54 2.21

1.31 1.87 2.70

1.82 2.41 3.52

5.0+0.4

5.5+0.5

5.1±0.5

Table I I I S a l t p e r m e a b i l i t y of r a d i a t i o n c r o s s l i n k e d gVA determined by reverse osmosis experiments ( cm^/s x 10 ) ^^§alt

c o n c e n t r a t i o n of feed s o l u t i o n

Temperatu7>O (°C)

\ 24 37 55

A c t i v a t i o n energy (kcal/mole)

0.125

0.025

1.69 2.52 4.08

0.62 0.92 1.49

w e i 8 h t % )

.

6.0±0.2

r e s u l t s d e r i v e d from RO experiments w i t h r a d i a t i o n membranes, l i s t e d i n Table I I I , above.

6.0±0.2

crosslinked

Heat Treated PVA Membranes. The concept of independent water and s a l t t r a n s p o r t through PVA i s f u r t h e r supported by the permea b i l i t y p r o p e r t i e s of heat t r e a t e d PVA membranes. I t was found that by s u b j e c t i n g the PVA membranes to heat treatment, a sharp decrease of the water and s a l t p e r m e a b i l i t i e s i s caused. The



392

SYNTHETIC

MEMBRANES:

DESALINATION

e f f e c t of the heat treatment d u r a t i o n at 175°C on the water and s a l t p e r m e a b i l i t i e s through PVA i s presented i n F i g u r e 9. I t i s evident i n t h i s f i g u r e that the decrease i n s a l t p e r m e a b i l i t y i s much steeper than the decrease i n water p e r m e a b i l i t y as a r e s u l t of the heat treatment. Study of the temperature dependence of the s a l t and water p e r m e a b i l i t i e s through heat t r e a t e d PVA membranes i n d i c a t e s that the heat treatment e f f e c t i s expressed mainly i n changes i n perm e a b i l i t y a c t i v a t i o n e n e r g i e s . The a c t i v a t i o n parameters f o r wat e r and s a l t p e r m e a b i l i t y of PVA membranes f o l l o w i n g v a r i o u s treatments are presented i n Table IV. I t i s evident that the s a l t p e r m e a b i l i t y a c t i v a t i o n energy i s i n c r e a s i n g about 2-3 times as Table IV Temperature dependence of water and s a l t p e r m e a b i l i t i e s of heat t r e a t e d PVA membranes -E /RT P=P e a

Heat treatment

Temp. (°C)

Duration (min)

Untreated 120 80 160 70 175 35 175 70 Radiation crosslinked

Water p e r m e a b i l i t y S a l t p e r m e a b i l i t y Feed concentration P .kcal. Vole ;

00 2

(cm /s)

E ,kcalx Mnole'

P oo

(mM)

2

(cm /s)

6.39±0.16 0.18±0.04 6.63±0.21

1916

15.8

6.00±0.61 6.79±0.39 8.87±0.28 9.5210.20

1319 313 1114

16.6 25.0 1.10 1.70

6.03±0.07 0.1010.01 6.0310.07

312

6.12±0.23 7.54±0.26 7.86±0.27 7.53±0.14

0.14±0.04 0.27±0.10 0.24±0.08 0.13±0.03

914

2.00

f a s t as the a c t i v a t i o n energy f o r water permeation as a r e s u l t of the changes induced by the heat treatment i n the PVA membranes. The d i f f e r e n t a c t i v a t i o n energies of s a l t and water transport through the heat t r e a t e d membranes a l s o suggest uncoupled t r a n s port of s a l t and water. We may conclude that our f i n d i n g s support independent water and s a l t permeation processes, and suggest that the s a l t permeat i o n i s governed by a s o l u t i o n - d i f f u s i o n transport mechanism. P r e p a r a t i o n of T h i n Skinned, Asymmetric PVA Membranes In order to o b t a i n t h i n skinned, high f l u x membranes from PVA, s e v e r a l approaches were t r i e d . The method presented i n t h i s a r t i c l e i s reminiscent of the c l a s s i c a l phase i n v e r s i o n method, which i s widely a p p l i e d i n c a s t i n g of asymmetric RO membranes„ However, i n s t e a d of using a g e l l i n g bath composed of a nonsolvent



25.

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Poly(vinyl

alcohol)

393

Membranes

f o r the membrane m a t e r i a l and m i s c i b l e w i t h the s o l v e n t from which the membrane i s c a s t , a "complexing" bath was used, which was a s o l u t i o n of a complexing agent i n water, i . e . water was the only l i q u i d i n v o l v e d i n the c a s t i n g procedure. The c a s t i n g procedure c o n s i s t e d of drawing an aqueous s o l u t i o n of PVA to a t h i n l a y e r , and a f t e r an evaporation p e r i o d , immersing i n the complexing bath. The complexing bath used i n our study was b a s i c a l l y a saturated CuS04 s o l u t i o n , w i t h or without a s e r i e s of p o s s i b l e a d d i t i v e s . A f t e r a p e r i o d of e q u i l i b r a t i o n of over 24 hours, the membranes were d r i e d and subjected to dry heat treatment,in order to s t a b i l i z e the asymmetric s t r u c t u r e obt a i n e d . The p r e p a r a t i o n c o n d i t i o n s of s e v e r a l membranes prepared by t h i s method are shown i n Table V. Table V P r e p a r a t i o n c o n d i t i o n s of asymmetric p o l y v i n y l a l c o h o l membranes*

Membrane

Drying time (sec)

Time i n saturated CUSO4

(hr) AS 3 AS 4 AS 5 AS 23

90 90 30 480

24 185 160 119

Heat treatment Temperatui:e Time (min) (°C) 175 175 175 175

10 10 13 10

*The membranes were cast from a 9% aqueous s o l u t i o n of 100% hydrolyzed, 115,000 MW p o l y v i n y l a l c o h o l .

The s a l t and water p e r m e a b i l i t i e s of these membranes were compared w i t h those of homogeneous PVA membranes heat t r e a t e d under s i m i l a r c o n d i t i o n s . The s a l t r e j e c t i o n s of s e v e r a l homogeneous and asymmetric membranes are presented i n F i g u r e 10. The s o l i d l i n e s i n t h i s f i g u r e are based on r e s u l t s obtained from homogeneous membranes, w h i l e the data p o i n t s r e f e r to the asymm e t r i c membranes. I t i s evident from t h i s f i g u r e that the s a l t r e j e c t i o n s of the homogeneous and asymmetric membranes are i n the same range. Comparison of the the water f l u x e s through these membranes, given i n Table V I , r e v e a l s however, that w h i l e through 6 micron t h i c k homogeneous membranes only f l u x e s below 1 GFD were obtained, the f l u x e s through the asymmetric membranes were higher by more than an order of magnitude. A l s o a c l e a r c o r r e l a t i o n between the water f l u x and the drying p e r i o d b e f o r e "quenching" i n the complexing s o l u t i o n i s evident from the data l i s t e d i n t h i s t a b l e . Thus f o r 30 seconds d r y i n g p e r i o d , membranes w i t h f l u x e s of 38 to 50 GFD were obtained, while longer drying periods are followed by a gradual decrease i n the water f l u x . These f i n d i n g s were i n t e r p r e t e d as i n d i c a t i n g that a t h i n skinned, asymmetric s t r u c t u r e was obtained. In order to v e r i f y



394

SYNTHETIC

MEMBRANES:

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Figure 9. Effect of the duration of heat treatment at 175°C on salt and water permeabilities of PVA at 30°C: (A) salt permeability coefficients; (O) water permeability coefficients. Open symbols refer to 100% hydrolyzed, 86,000 mol wt PVA; solid symbols refer to 100% hydrolyzed, 115,000 mol wt PVA.

100

r

40 -

FEED CONCENTRATION, mM

Figure 10. Salt rejection of heat-treated asymmetric and homogeneous PVA membranes as a function of feed concentration at AP = 1000 psi and t = 30°C: (0) AS5; O AS23; (O) AS4; (%) AS3. The solid lines represent the data obtained for the heat-treated homogeneous PVA membranes.


25.

KATZ AND WYDEVEN

Polyvinyl alcohol)

Membranes

395

Table VI Comparison of water f l u x e s through heat t r e a t e d asymmetric and homogeneous p o l y v i n y l a l c o h o l membranes*


Membrane

HT175-70 HT175-35 AS 5 AS 3 AS 4 AS 23

Thickness (microns)

Drying time (sec)

6.0 6.0 30 90 90 480

Water f l u x (GFD)

0.6-0.7 0.8-1.1 38-50 19-25 24-30 14-16

*Measured at AP=1000 p s i , 30-40°C t h i s , the dense and the porous s i d e s of these membranes were inspected by a scanning e l e c t r o n microscop(SEM). As expected, the backside of the membranes were found to have a c h a r a c t e r i s t i c porous s t r u c t u r e with pore s i z e s i n the range of 2 to 3 micrones. Figure 11(a)and(b)presents the SEM p i c t u r e s of the dense and porous sides of the inspected membrane at a m a g n i f i c a t i o n of x3000. Other SEM p i c t u r e s were a l s o obtained at m a g n i f i c a t i o n s of up to x5000. At these m a g n i f i c a t i o n s there i s no evidence of p o r o s i t y on the s k i n s i d e . In order to a s c e r t a i n that the p i c t u r e i s f o cused at the membrane s u r f a c e , a segment covered with f i n e p a r t i c l e s , which are v i s i b l e i n the micrographs,was s e l e c t e d . So f a r , the s e l e c t i v i t y of these membranes was t e s t e d only with regard to s o l u t i o n s of s a l t i n water. There are i n d i c a t i o n s however, from v a r i o u s sources, l i k e the work of P e t e r and Stefan (4_) , that these membranes may be v a l u a b l e f o r other separations. In p r i n c i p l e , i t i s a l s o p o s s i b l e to improve and modify the s e l e c t i v i t y p r o p e r t i e s of these membranes by using other methods f o r c r o s s l i n k i n g them, such as f o r m a l i z a t i o n or r e a c t i o n w i t h t o l y l e n e d i i s o c y a n a t e , mentioned e a r l i e r , or by s e l e c t i n g other complexing agents f o r the c a s t i n g procedure. Abstract The r e s u l t s of a reverse osmosis study of r a d i a t i o n c r o s s l i n k ed and heat t r e a t e d p o l y v i n y l alcohol (PVA) membranes are reported. In the framework of t h i s study the p e r m e a b i l i t y of water and s a l t through these membranes was i n v e s t i g a t e d . In parallel, the d i f f u s i v e transport of s a l t through PVA was a l s o s t u d i e d . The r e s u l t s suggest that the transport of s a l t and water through PVA i s uncoupled. The s a l t transport data can be r a t i o n a l i z e d i n terms of a modified s o l u t i o n - d i f f u s i o n model. A new procedure f o r c a s t i n g t h i n skinned asymmetric PVA mem-


SYNTHETIC

MEMBRANES:

DESALINATION


396

Figure 11. SEMs of asymmetric PVA membranes at X3000 magnification: (a) the dense side (skin) of the membrane; (b) the porous side of the membrane.


25.

KATZ AND WYDEVEN

Polyvinyl alcohol)

Membranes

397

branes was developed. The c h a r a c t e r i s t i c s of these membranes were found to be c l o s e l y c o n t r o l l e d by the c a s t i n g c o n d i t i o n s and by the posttreatments. Data on the reverse osmosis performance and s t r u c t u r e of some of these membranes are r e p o r t e d . L i s t of Symbols a b CA c


1

c^ c^ c" c wm Ac s DMF DMSO D^ D wm E^ F s F w GFD K MW P

i n t e r c e p t i n equation 4 slope i n equation 4 c e l l u l o s e acetate s a l t c o n c e n t r a t i o n i n the s o l u t i o n on the h i g h pressure side of the membrane s a l t c o n c e n t r a t i o n i n the s o l u t i o n on the low pressure s i d e o f the membrane water concentration i n the s o l u t i o n on the high pressure s i d e o f the membrane water concentration i n the s o l u t i o n on the low pressure s i d e o f the membrane water concentration i n the membrane under the c o n d i t i o n s of the RO t e s t . = c -c" s s dimethyl formamide dimethyl s u l f o x i d e d i f f u s i o n c o e f f i c i e n t o f s a l t i n the membrane under the c o n d i t i o n s o f the RO t e s t d i f f u s i o n c o e f f i c i e n t o f water i n the membrane under f

the c o n d i t i o n s o f the RO t e s t a c t i v a t i o n energy salt flux water f l u x

psi PVA PVAc PVB AP R R

gallons/square f o o t , day salt distribution coefficient molecular weight p e r m e a b i l i t y c o e f f i c i e n t (the product o f d i f f u s i o n c o e f f i c i e n t and the c o n c e n t r a t i o n of the permeating species i n the membrane) preexponential f a c t o r i n the Arrhenius r e p r e s e n t a t i o n of the p e r m e a b i l i t y c o e f f i c i e n t pounds/square i n c h polyvinyl alcohol p o l y v i n y l acetate polyvinyl butyral h y d r o s t a t i c pressure d i f f e r e n t i a l across the membrane gas constant salt rejection

RO SEM T

reverse osmosis scanning e l e c t r o n microscope(microscopy) absolute temperature

P^

g


SYNTHETIC MEMBRANES:

398

V w X ATT

DESALINATION

p a r t i a l molar volume of water i n the membrane membrane thickness osmotic pressure d i f f e r e n t i a l across the membrane

Acknowledgement. T h i s study was done i n the framework of a N a t i o n a l Research C o u n c i l , P o s t d o c t o r a l Resident A s s o c i a t e s h i p awarded to M.G.K.


Literature Cited

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

Michelsen, D.L.; Harriott P., Appl.Polym.Symp., 1970, 13, 27. Chen, C.T.; Chang, Y . T . ; Chen, M.C.; Tobolsky, A.V.; J.Appl. Polym.Sci., 1973, 17, 789. Dick, R.; Nicolas, L.; Desalination, 1975, 17, 239. Peter, S; Hese, N.; Stefan, R.; Desalination, 1976, 19, 161. Paul, D.R.; Ebra-Lima, O.M.; J.Appl.Polym.Sci., 1970, 14, 2201 Ebra-Lima,O.M.; Paul, D.R.; J.Appl.Polym.Sci., 1975, 19, 1381 Peter, S.; Mittelstadt, D.; Kolloid-Z. u Z. Polymere, 1973, 251, 225.

RECEIVED

January 2,

1981.


Membranes for Reverse Osmosis - American Chemical Society

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