DESALINATION

ELSEVIER Desalination 128 (2000) 5746 www.elsevier.com/locate/desal

Synthesis and characterization of low molecular weight cut off ultrafiltration membranes from cellulose propionate polymer

S. Khan a, A.K. Ghosh b, V. Ramachandhran b, J. Bellare a, M.S. Hanra b*, M.K. Trivedi a, B.M. Misra b

"DeDartment of Chemical Engineering, Indian Institute of Technology, Mumbai, 400 076, "Desalination Division, Bhabha Atomic Research Centre, Mumbai, 400 085, India

Fax i 91 22 5560750

Received 25 May 1999; accepted 24 November 1999

Abstract

This paper describes the synthesis and characterization of low molecular weight cut-off ultrafiltration (UF) membranes from cellulose propionate (CP) polymer using dimethyl acetamide solvent. The casting conditions are studied with reference to changes in the nature of additive and additive to solvent ratio. The membranes are characterized in terms of product permeation rate, solute retention for different electrolytes, as a function of feed concentration and pressure. The retention of different dyes are also included. Molecular weight cut off profiles using polyethylene glycol solutes of different molecular weights are presented. The separation behaviour of oil- water and milk protein systems are presented in order to highlight the fouling resistance of CP membranes.

Kevwords: Ultrafiltration membranes; Cellulose propionate

1. Introduct ion esters, polyamides, polysulfone, polyacrylonitrile and polyvinylidene difluoride are some of the

Ultrafiltration (UF) is a membrane filtration polymers used for making ultrafiltration

technology widely used in diverse fields such as water treatment, dairy industry, paint recovery, membranes by the classical wet phase inversion pulp and paper industry and biotechnology [1]. method [2,3]. The various control parameters

involved in the wet phase inversion technique The mechanism of separation in ultrafiltration

affecting the membrane performance is well involves not only the size differences among various solute and solvent molecules under understood andextensivelyreported.

The factors which affect the pore size of consideration, but also adsorption and surface charge characteristicsofmembranes. Cellulosic polyacrylonitrile membrane are reported by

Congjie [4]. The properties of ultrafiltration

*Corresponding author, membranes which make them susceptible to

0011-9164/00/$- See front matter © 2000 Elsevier Science B.V. All rights reserved

58 S. Khan et al. /Desalination 128 (2000) 57 66

fouling, causing flux decline are discussed by 2.2. Preparation o f membrane

Fane [5]. Hvid [6] reported that hydrophilic In an air tight bottle, a specified quantity of surfaces are less prone to protein fouling and cellulose propionate was mixed with a definite suggested methods to make the membrane amount of purified DMAc solvent. The solution surface more hydrophilic. Composite UF mem- was thoroughly mixed in a mechanical shaker for branes from blended polymers have also been 24 h at room temperature (25°C) till complete reported by several authors. Polyvinylidene dissolution of the polymer took place. The difluoride and polymethyl methacrylate blended additives were subsequently added and the UF membranes are reported by Suzane and solution was mechanically mixed for homo- Klauss [7]. Polyurea/polyurethane UF mem- genisation for another 24 h at room temperature. branes over macroporous polysulfone substrate The solution viscosity was measured using a are also reported [6]. Resistance to colloidal

standard Brookfield viscometer. fouling, chlorine tolerance, temperature and pH The casting solution thus obtained was stability and the ability to control the molecular manually spread over a smooth glass plate under weight cut-off are some of the important aspects controlled temperature and humidity as specified to be considered during the synthesis of UF

in the subsequent parts of the text. The thickness membranes. Casting and characterization of of the membrane was maintained by using side cellulose acetate butyrate UF membranes has

runner tapes. A single tape has a thickness of been reported from this laboratory recently [8]. 40 p. Desired number of tapes are fixed on edges Among the cellulosic esters, cellulose propionate to control the casting thickness. The manual appears to have not received adequate attention casting speed is around 4 cm/s. No deliberate and studies were initiated on the suitability of

solvent evaporation was allowed. The glass plate cellulose propionate as a possible material for was quickly immersed in a demineralised water making UF membranes. The casting and

bath maintained at 25°C temperature. Im- characterization of UF membranes from

mediately, phase inversion started and after cellulose propionate are reported in this paper, sometime, thin membranes separated from the

glass. After 15 min in gelling medium, the membrane was thoroughly rinsed free of solvent

2. Materials and methods and additive before use. Then the actual thick- ness of the membranes were measured by using

2.1. Materials" a micrometer. The membrane was always wet stored in 0.25% formaldehyde solution. Cellulose propionate polymer (average

molecular weight 200,000, degree of substitution 2.3. Characterization o f membrane with respect to propionyl group 2.66 and with

respect to acetyl group 0.138) was procured from Membranes were characterized in terms of M/s Aldrich Chemical Co., USA. Maleic acid solute retention data and product permeate flux was procured from M/s Lancaster, England. for different solute systems at 500 kPa pressure. Other solvents used in this work are locally A constant recirculation type UF cell was used procured. N,N-dimethyl acetamide (DMAc) was wherein feed solution is pumped across a given distilled under reduced pressure and stored over membrane specimen (area 14.5 cm 2) at a flow molecular sieve ~A. 1,4-Dioxane, acetone, rate of 3 l/m using a reciprocating pump. The formamide and methanol were purified by the desired pressure was set using a back pressure usual distillation method, regulating valve. The reject and product streams

S. Khan et al. /Desalination 128 (2000) 57-66 59

are recycled back to the tank to maintain the feed lower boiling point of acetone among the three concentration. Temperature of the feed was swelling additives, resulting in rapid loss of maintained at 25°C. Sodium sulfate and other acetone during the short time interval between electrolytes in feed and permeate samples were casting and gelling. Maleic acid was observed to analysed by conductance measurements. The give the highest permeate flux as well as sodium dyes were analysed spectrophotometrically. The sulfate retention. The results indicate that the analytical method for determining PEG is nature of the additive has a strong effect on the already reported in our previous paper [8]. structure of the casting solution which in turn

affects the membrane performance. It appears that the acidic nature of the additive probably

3. Results and discussions solvates the cellulose propionate polymer by

3.1. Effect of casting parameters on membrane hydrogen bonding with the carbonyl carbon as performance well as react with the amide solvent altering the

solvent power [9]. This could lead to an 3.1.1. Nature of:he additive optimum swelling of the polymer with desirable

The effect of various additives on membrane distribution of the size of the super molecular performance was studied using methanol, maleic polymer aggregates and the degree of polymer acid, oxalic acid, acetone, 1,4-dioxane and network within the aggregates. It is significant to formamide. In all these cases the composition, note that the casting solution viscosity is higher casting conditions and the membrane test in the case of acidic additives. The n-electrons conditions were maintained identical. The associated with the double bond of the maleic amount of additive in the casting formulation in acid may also contribute to the dipole-dipole each case was kept identical to the amount of interaction between additive and the polymer. It polymer. The membranes were designated as is also to be noted that when no additive is CP1 to CP7. The results are given in Table 1. present in the casting solution (CPI), the The casting solution viscosity and total thickness permeate flux is rather low and is comparable to of the membrane are also included in the same what is obtained with neutral or basic additives. table. The viscosity of the casting solution is Further experiments were carried out using found to be more when the additive is present. In maleic acid as the additive. the case of acetone and 1,4-dioxane which are swelling agents for the polymer, the viscosity of

3.1.2. Additive~solvent ratio casting solution was observed to be lower as compared to acidic additives. No significant The effect of increase in the maleic acid variation of thickness is noticed for all the content in the casting solution was studied membranes. Among the additives tested, oxalic keeping the total polymer concentration acid, maleic acid and methanol were found to unaltered. The maleic acid in the dope solutions give higher permeate fluxes and significant were varied from 5% (w/w) to 15% (w/w). The sodium sulfate retention, whereas 1,4-dioxane, DMAc content in the casting solution was acetone and formamide were found to give lower progressively reduced from 82.5% (w/w) to permeate fluxes. Among the latter set of addi- 72.5% (w/w) in these cases to maintain the total tives, acetone was found to give a slightly higher polymer concentration. Higher maleic acid sodium sulfate separation though the permeate concentration makes the casting solution too flux was low. This could be due to the relatively viscous for membrane casting. With an increase

60 s. Khan et a{/Desalination 128 (2000) 57 66

Table 1 Effect of various additives on membrane performance

Membrane Additive Casting solution Total membrane Performance

viscosity, ccn t i thickness, gm Flux, L.m-2.h -~ Solute retention poise

CP1 Nil 1840 182 3.2 0.292 CP2 1,4-Dioxane 2138 173 6.5 0.049 CP3 Methanol 2432 197 33.6 0.135 CP4 Oxalic acid 2879 164 51.5 0.192 CP5 Fomlamide 2642 179 6.94 0.197 CP6 Acetone 2021 187 8.92 0.297 CP7 Maleic acid 2867 195 68.7 0.394

Casting solution composition (w/w): polymer 12.5%, DMAc = 75%, additive 12.5%; Casting atmosphere: humidity - 72% R.H., temperature 22°C; Testing conditions: pressure = 500 kPa, feed concentration 2000 ppm Na2S()4

Table 2 EffEct ofmaleic acid/DMAc ratio on CP membrane performance

Membrane Maleic acid/ Casting solution Total membrane Perlbrmance DMAc ratio viscosity, ccnti poise thickness, gm

Hux, L.m 2. h-~ Solute retention

CP8 0.06 2145 171 18.6 0.362 CP9 0.13 2539 179 24.2 0.345 CP 10 0.17 2867 164 70.4 0.323 CP 11 0.21 3742 187 90.4 0.3 ! 3

Casting solution composition (w/w): polymer = 12.5%, 87.5% consisting ofmaleic acid and DMAc in the weight ratio given in table; Casting atmosphere: humidity 72% R.H., temperature 22°C: Testing conditons: pressure 500 kPa, teed concentration

2000 ppm Na2SO 4

in the maleic acid content, the water permeation stage and hence there is no significant change in rate was found to increase. The solute retention final membrane thickness with increasing casting with respect to sodium sulfate marginally solution viscosity. The results indicate that

declines. The results are given in Table 2 higher additive to solvent ratio improves wherein the performance of four different productivity of membranes without significant membranes CP8 to C P I I are given along with loss in separation behaviour. the ratio o f maleic acid to N,N-dimethyl acetamide, the casting solution viscosity and total membrane thickness. It can be seen that the 3.1.3. Membrane thickness

viscosity o f the casting solution gradually The effects o f overall thickness o f the mere- increases with increase in the maleic acid branes were studied by increasing the casting concentration but the change in total thickness is thickness for CP7 membrane and the results arc not significant. The additive and the solvent are given in Table 3. It was found that the actual leached out o f the membrane during gelling thickness o f the membranes after it was formed

S. Khan et al./Desalination 128 (2000) 5~66 61

Table 3 Table 4 Effect of total membrane thickness on membrane Separation of various electrolyte by CP7 membrane performance

S1. No. Electrolyte Solute retention SI. No. Cast thick- T o t a l Perlbrmance 1 NaC1 0.152

ness, ~tm membrane thickness, Flux, Solute 2 CaC12 0.094 lam E.m-2.h -I retention 3 Na2SO 4 0.352

1 160 82 193.2 0.312 4 MgSO4 0.212 2 240 114 147.0 0.352 5 KzCrO 4 0.272 3 320 176 85.2 0.352 6 K2Cr207 0.242 4 360 198 69.2 0.377 5 600 352 23.0 0.382 7 K3Fe(CN)6 0.572

8 K4Fe(CN)6 0.624 Casting solution composition (w/w): polymer = 12.5%, DIVIAc-75%, additive = 12.5%: Casting atmosphere: Casting solution composition (w/w): polymer 12.5%, humidity=72% R.H., temperature 22°C; Testing con- DMAc 75%, additive 12.5%; Casting atmosphere: ditions: pressure = 500 kPa, feed concentration- 2000 ppm humidi ty 72% R.H.. temperature 22°C; Testing condi- Na:SO.~ tions: pressure = 500 kPa, feed concentration = 2000 pm

Na2SO4

was nearly half o f the casting thickness. This to NaC1 and that o f NaCI was higher as indicates densification o f the polymer network compared to CaC12. This shows that these during the gelling process with the leaching out membranes exhibit better retention behaviour for o f the additive and solvent. The pure water electrolytes with multivalent anions. This permeability o f the membranes obtained behaviour suggests the possible existence o f decreased drastically with increase in actual negative charges on membrane surface which thickness o f the membranes. For nearly four fold excludes multivalent anions from the membrane increase in membrane thickness, there was nine pores by Donnan exclusion. The poor separation fold decrease in pure water flux. This could ofmult ivalent cations could be due to favourable indicate two possibilities: one, that there could electrostatic attraction. Similarly, a comparison be a large number o f blind pores and the other, of retention data between Na2SO4 and MgSO4 the increase in skin thickness which could be indicate that the overall solute retention is different for different membranes. Solute dictated by the increasing valancy o f the cations, separation is however found to remain steady for with the valancy o f the anions remaining the membranes with different total thickness, same. Retention data of K2CrO4 and K2Cr207 are

lower than that o f Na2SO4, possibly because o f

3.2. Effect o f process parameters on membrane lower surface charge density o f their respective performance anions due to their large size. Comparison o f

retention data o f K4Fe(CN)6 and K3Fe(CN)6 with 3.2.1. Performance for different electrolyte K2CrO4 and K2Cr207 again indicate the higher

systems retention behaviour for higher valancy anions.

The performance o f cellulose propionate membranes (CP7) obtained for different

3.2.2. Effect o f solute concentration electrolytes were evaluated under identical test conditions. The results are given in Table 4. It The effect of feed solute concentration with can be seen that the retention behaviour o f the respect to different electrolytes, namely NaCI, membranes for Na2SO4 was higher as compared Na2SO4, K4Fe(CN)6 and K3Fe(CN)6 on the

62 S. Khan et al. /Desalination 128 (2000) 57-66

retention behaviour for CP7 membrane was 10 , / / , , , ' i

studied and the results are given in Fig. 1. It can °9- _- , be seen that as the feed concentration increases 98- ,-s ~:/-:::: .... .-A.

J~ -" -. \V ~ • / from 50 ppm to 2000 ppm, the observed solute ~_ 07-. , . . . . . . . \ ...._-_ , retention goes through a maximum for all the ~ 06- °,. --; • electrolytes. The maximum solute retention is ~ o6- " " ' . K3~°~ON~6 observed at around 500 ppm. The drop in solute ~ 04- i--.

- Na2SO 4

retention beyond 500 ppm appears less drastic 93-_ for NaCI and steeper for higher valancy anions. 92- _ . . . . . . . . . • -- . . o c

typi . . . . . . . . . The cal maxima in solute retention observed 01 88 "8°o 1096 1898 20c8 for cellulose propionate membranes is similar to Feed concentration ( mg./L ) what is previously reported for sulfonated polycarbonate [10] and cellulose acetate [11] Fig. 1. Dependence of solute retention on feed membranes. At higher feed concentrations the concentration for CP7 membrane.

solute diffuses through a near neutral membrane whereas at lower feed concentration (-10 -2 mole/l) ~° ' ~° the action of fixed charge of membrane in-

7C 0 35 fluences the mobility of ions within the membrane. The residual negative charge of -~E ~o- ~ o~o~

cellulose propionate could be due to the presence ~ ° ~_ of carboxylic group (-COOH) which could have ~_ ~0- .... been formed by the surface oxidation of . --{ hydroxyl groups. ~0- 0~

The action of the fixed charges of the ~;~ ~;o L, &

membrane is believed to affect the mobility of P~e ..... (kPa)

solute ions in the membrane phase. When the external feed concentration becomes comparable Fig. 2. Variation of flux and solute retention with pressure

for CP7 membrane. to the fixed charge of the membrane, a Donnan equilibrium will exit between the solution and

the operating pressure though not the membrane phase which excludes coions from the membrane resulting in high solute retention, proportionately. The solute retention increases

with increase in pressure and declines at higher When the external solute concentration is in excess o f the fixed charge concentration, the pressures. It is seen that the permeate flux sorption of colon into the membrane phase is through these membranes declines initially at less hindered and could lead to lower solute any given operating pressure, even with dilute retention, electrolyte solutions and stabilises subsequently.

The time required to attain a stable flux at a

3.2.3. E f f e c t o f o p e r a t i n g p r e s s u r e given pressure is recorded and all the reported data are taken after the flux has stabilised. The

The effect of increasing operating pressure on less than expected increase in the flux with the membrane performance for CP7 membrane proportionate increase in the pressure indicate with respect to Na2SO4 feed solution was studied some morphological changes occurring in the and the results are given in Fig. 2. It can be seen membrane and use of high pressure does not seem that the permeate flux increases with increase in to give any of the expected process advantage.

S. Khan et al. /Desalination 128 (2000) 57 66 63

~ 0 ~ , , n ' i ' n ' i q ' , ' , ' 1 0 ~ , n ' i ' n ' e ' t ' T

0 9 • • 0 9 •

0 8 -

w c~ / i 0 7 - • ~ ,

0 6 /

06

05 , i r , i , i , i ' i , i , i ' , ] , i , i , i , ~ , i , ~ ' i

500 1000 1500 2000 2500 300C 3500 4000 4500 500 1000 1500 2000 2500 3300 3500 4000 4500

M o l e c u l a r w e i g h t ( DaL tons ) M o l e c u l a r w e i g h t ( Da l t on }

Fig. 3. Molecular weight Cut-offprofile of CP8 membrane. Fig. 4. Molecular weight cut-offprofile of CP9 membrane.

10 ~ T I ' * ' ~ ' , ' r , l o , i , i , ~ , t , F , i , ~ , , '

09 00 - • - •

,/

g

~ - / } '

~J 0 7 - ' ~ "

o6j ' • 0 6 - •

05 F I ' I u ' q ' i ' i ' ~ ' I I ' I ' I ~ ' i ' I ' F

C 500 1000 1500 2000 2500 3000 3500 4000 4500 500 1000 "500 2000 2500 3000 3500 4000 4500

M o l e c u l a r w e i g h t ( D a L t o n ) M o l e c u l a r w e i g h t ( Da l t on )

Fig. 5. Molecular weight cut-offprofile of CPl0 membrane. Fig. 6. Molecular weight cut-offprofile ofCP1 l membrane.

The drop in solute retention at higher pressures is samples, namely, CP8, CP9, CP10 and CP1 I are typical of what is already reported [11] for given in Figs. 3 to 6. All the membrane samples porous membranes from our laboratory, give a MWCO of around 1000 Daltons. The

It is possibly due to higher permeate profiles obtained for all the four membranes with withdrawal through the membrane at high different permeate flux but with comparable pressures resulting in the build-up of solute retention characteristics are similar to each other. concentration at the boundary layer adjacent to The profiles also do not appear to be sharp, the membrane at a rate faster than the effective probably because the size differences of mass transfer coefficient of solute on the high polyethylene glycol solutes in the dissolved state pressure side. do not vary proportionately with molecular

weight in low molecular weight region. 3.2.4. Molecular weight cut-oil (MWCO)

profile 3.2.5. Separation of dyes

The molecular weight cut-off (MWCO) Membrane performance for four different profiles for four cellulose propionate membrane dyes for CP7 membrane is given in Table 5. It can

64 S. Khan et al./Desalination 128 (2000) 57 66

Table 5 i F J Separation behaviour of CP7 membrane for difl'erent dyes ~0-

• • • | . | ~ 1 o o

SI. No. Dye Molecular Solute 65- 093 weight, retention ~7 6c- • 0 B6

~ E

Daltons ~ ~ c70

1 methyl red 269.31 0.623 ~ j 5C " 0 71 R

2 methylene blue 373.90 0.392 ~ "J 45 " " 054

3 rhodamine B 479.02 0.452 ~ F . . . . . . . . ~ m L o~, ~o lo~ DM . . . . . 40 0 57

4 bromophenol blue 669.98 0.794 3 5 - - - r i ~ ~ i I 0 5 0

2C0 30O 4O0 500 600 7O0

Casting solution composition (w/w): polymer = 12.5%, ~ . . . . . . . ~p, DMAc = 75%. additive = 12.5%; Casting atmosphere: humidity = 72% R.H., temperature = 22°C; Testing con- Fig. 7. Separation behaviour of CP7 membrane lbr cutting ditions: pressure = 500 kPa, feed concentration = 50 mg/L oil water system as a functkm of pressure.

be seen that the retention o f dyes does not i i q

increase with the increase in the molecular ~0- . . . , : ,l,--,00 weight and the retention depends more on the ~ • , .... molecular structure and the ionic nature o f the ~- ~o o0~

E J

dyes. The separation of methyl red with ~ ~.~_ 0~0~ molecular weight o f 269.31 and bromophenol ~ .... 5C ~ 07 !

E blue with the molecular weight o f 669.98 are e 45 . . . . : 6 4

higher, whereas the retention o f methylene blue "-I . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . / . . . . . . . ~, . . . . . 4 0 - '3 5~ '

with molecular weight 373.9 and rhodamine B ° p J i , i F

with molecular weight o f 479.02 are lower. The ~ 2 0 0 . . . . . . . 0 0 6 0 . . . . . C 5 0

retention o f lower molecular weight methyl red ............ ' is higher than the retention o f higher molecular l:ig. 8. Separation behaviour of CP7 membrane lbr milk weight rhodamine B dye. The higher retention o f protein as a function of pressure. methyl red and bromophenol blue could be due to the anionic nature o f the dye, whereas the

lower retention o f the other two could be due to the cationic nature o f the dye. membranes give a high retention for cutting oil

and milk proteins. The permeate flux is found to increase with increase in pressure for both the

3.2. 6. Fouling susceptibility studies systems and levels o f f at higher pressures

The susceptibility o f cellulose propionate indicating the formation o f gel layer on membranes for adso@tive fouling is studied. The membrane surface. The limiting flux is more or observed charged characteristics o f the less identical for both the systems and occurs at membranes are expected to confer some degree nearly the same operating pressure. After the o f resistance for adsorptive fouling. The testing, the membrane sample was washed permeate flux and retention behaviour o f cell- thoroughly with running water and the pure ulose propionate membrane (CP7) were studied water permeation rates were measured. The for cutting o i l -water system as well as for milk fouling susceptibility was measured by proteins as a function o f pressure and the results comparing the pure water permeation rate o f the are given in Figs. 7 and 8. It can be seen that the membrane sample before (PWP °) and after the

S. Khan et al. ,,'Desalination 128 (2000) 5 ~ 6 6 65

membrane is used under severe fouling improves the water permeation rate with no conditions (PWPF). The results are given in drastic effect on solute retention. Table 6. It can be seen that the pure water per- 3. The water permeation rate was found to meation rates after testing were lower than the decrease drastically with an increase in the untested membranes. For sake of comparison, total thickness of membranes under identical polysulfone and aromatic polyamide ultra- casting conditions. filtration membranes (prepared in our laboratory) 4. Cellulose propionate membranes prepared, were also tested under similar conditions. It can exhibit improved selectivity towards be seen that the drop in pure water permeation multivalent anions and the retention rates with respect to cellulose propionate is behaviour of membranes show a typical lo~'er and comparable to that ofpolysulfone than maxima as previously reported. More aromatic polyamide membranes, detailed studies need to be caried out.

5. The retention behaviour o f cellulose propionate membranes improves marginally

Table 6 with applied pressure but declines at higher Comparative lbuling susceptibility of CP7 membranes pressures. The permeation rates through

Cutting oil-water feed Milk protein feed membranes do not increase proportionately Membrane pWp 0, pwp v, pwp °, pwp F, with increase in pressures indicating some

l"m-2h-I Lm-2h-1 l"mZ'h-I L ' m - 2 h - I morphological changes with increase in Cellulose 72.4 64.3 71.7 65.1 pressures. propionate 6. The membrane samples give satisfactory (cp7) retention behaviour for anionic dyes Polysulfone 122 105 120.4 109.2 whereas cationic dyes are rejected to a

Aromatic 84 62.1 87.2 6 7 . 4 comparatively lower extent. This suggests polyamide existence of fixed anionic groups.

Casting solution composition (w/~.): polymer= 12.5%, 7. The fouling resistance of the membrane for DMAc = 75%, additive= 12.5%: Casting atmosphere: oil and milk proteins appear to be humidity=72% R.tt.. temperature=22°C; Testing c o n - satisfactory. ditions: pressure = 500 kPa, time of operation: 0 zero time, F final time

References

4. Conclusions [1] S. Sourirajan and T. Matsuura, Reverse Osmosis and Ultrafiltration Process Principles. National

1. Ultrafiltration membranes with a MWCO of Research Council, Canada, Ottawa, 1985. -1000 were prepared from cellulose [2] R.F. Kesting, Synthetic Polymeric Membranes,

McGraw Hill Book Co., New York, 1971. propionate polymer with N,N-dimethyl [3] M. Cheryan, UItrafiltration Hand Book, acetamide as the solvent. Technomic Publishing Co., Inc., London, Old

2. The effect of different additives in the Basel, 1986. casting dope on the membrane performance [4] G. Congjie, Desalination, 62 (1987) 89. was studied. Among the additives studied [5] A.G. Fane and C.J.D. Fell, Desalination, 62 (1987) maleic acid was found to be the most 117. suitable. Increase in maleic acid to N,N- [6] K.B. Hvid, J. Memb.Sci., 53 (1990)189.

[7] P. Suzane and V.P. Klaus, J. Memb. Sci., 109 dimethyl acetamide ratio in the casting dope (1992) 165.

66 S. Khan et al./Desalination 128 (2000) 57.66

[8] A.D. Sabde, M.K. Trivedi, V. Ramachandhran, [10] A.K. Ghosh, V. Ramachandhran, M.S. Hanra and M.S. Hanra and B.M. Misra, Desalination, 114 B.M. Misra, J Polymer Materials, 15 (1998) 279. (1997)223. [11] V. Ramachandhran and B.M. Misra, J. Applied

[9] B. Kunst and Z. Vajnaht, in: Synthetic Membranes, Polymer Science, 28 (1983) 1641 Vol. 1, A.F. Turbak, ed., ACS Symposium Set., 153 (1981) 235.