preparation, characterization and performance of polyethersulfone ultrafiltration membranes

DESALINATION

ELSEVIER Desalination 133 (2001) 3 140 www.elsevier.com/locate/desal

Preparation, characterization and performance of polyethersulfone ultrafiltration membranes

B.K. Chaturvedi”, AK. Ghoshb, V. Ramachandhranb, M.K. Trivedi”, M.S. HanTab*, B.M. Misrab

aDepartment of Chemical Engineering, Indian Institute of Technology, Mumbai 400 076, India bDesalination Division, Bhabha Atomic Research Centre, Mumbai 400085, India

Tel. +91 (22) 550-5050 Ext. 24756; Fax +91 (22) 550-5151; e-mail: [email protected]

Received 15 May 2000; accepted 23 October 2000

Abstract

Asymmetric ultrafiltration (UF) membranes were prepared horn locally made wholly aromatic polyethersulfone (PES) polymer, similar to Vitrex type, using aprotic solvents and organic additives by phase inversion method. The effect of nature of additive, solvent, ambient humidity during membrane casting and gelling medium on membrane performance were studied. The membranes were characterized in terms of separation behaviour for polyethylene glycol-5000 solute, molecular weight cut off (MWCO) profile and direct electron microscopic observations. The performance of UF membranes for sodium sulfate, rhodamineS dye and oligomers of starch hydrolyzate are reported as a function of pressure.

Keywords: Ultrafiltration; Polyethylene glycol; Separation; Polyethersulfone; Semipermeable membranes

1. Introduction

Over the past three decades, membrane processes have been adopted to perform a variety of separation operations by different industries. The efficiency as well as the economics of the various industrial processes can be greatly improved if the membrane processes are suitably integrated in the exiting process, particularly, where fractionation of a set of components is

*Corresponding author.

more desired than total conversion or separation. Ultrafiltration is a process of separating extremely small suspended particles and dissolved macro- molecules from fluids using asymmetric membranes of surface pore size in the range of 50 to 1 nm. The primary basis of separation is molecular size although secondary factors such as molecular shape and charge can play a significant role. UF membranes are ofien operated in a tangential flow mode where the feed stream sweeps tangentially across the upstream surface of membranes as filtration occurs, thereby

001 l-9164/01/$- See front matter Q 2001 Elsevier Science B.V. All rights reserved PII: SO01 l-9164(01)00080-7

32 B.K. Chaturvedi et al. /Desalination 133 (2001) 31-40

maximizing flux rates and membrane life. Though a large number of research papers have appeared on the characterization [ I-91, industrial application [ 10,l l] and fouling and transport modelling [ 12-l 51 of UF membranes, preparation of UF membranes is not widely reported. Though the membrane preparation techniques are well known, the precise membrane casting procedure outlining choice of solvent, additive, concentration and other relevant details are not available for several polymer candidates. The conventional phase separation method for synthesizing semipermeable membrane is to dissolve a given polymer candidate in a suitable solvent and a swelling agent to form a viscous dope, spreading the homogenized casting dope over a smooth clean substrate, allowing the partial evaporation of solvent and gelling the nascent film in a suitable precipitating medium. This technique provides sufficient flexibility for tailor making UF membranes to suit a given objective. There are several polymers and ceramic materials, used for the preparation of UF membranes. Very little is known about the precise formulation of the casting dopes and the optimum conditions of membrane preparation, except perhaps for cellulose acetate. Preparation and performance characterization of cellulose acetate butyrate and cellulose propionate based UF membranes are reported [ 16,171 from our laboratory. Aromatic polysulfone family of polymers is extensively used for UF membrane preparation due to their wide temperature, pH and chlorine tolerance. Aromatic polysulfone class of polymers essen- tially comprises ether, sulfone and substituted or unsubstituted methylene groups connected through aromatic groups in para-para orientation, in varying proportions. Polysulfone of Udel type is extensively studied whereas polyethersulfone is not widely reported for aqueous based sepa- rations. This paper reports preparation, characterization and performance behaviour of polyethersulfone UF membranes.

2. Materials and methods

2. I. Materials

Polyethersulfone (PES) in powder form was obtained from M/s. Gharda Chemicals Company, India. The inherent viscosity (Ed.) of the polymer in N-methyl pyrrolidone solvent and the equilib- rium moisture uptake at ambient conditions (82% RH and 26°C) are 1.27 dl/g and 2.07% respectively. Maleic acid was procured from M/s. Lancaster, England. The solvents used for membrane making are N,N-dimethyl formamide (DMF) and N-methyl pyrrolidone (NMP) and were of reagent grade. Polyethylene glycol (PEG) of different molecular weights and piperazine (AR grade) were locally procured and used as additives. Rhodamine-B was procured from M/s. E. Merck. India, Ltd. Starch hydrolyzate was locally procured and used as such.

2.2. Preparation of membrane

In airtight glass bottle, a specified quantity of polymer was taken and then a known quantity of the solvent was added. The solution was kept agitated for several hours for complete dissolu- tion. Calculated quantities of additives were subsequently added and the solution was homogenized and kept for deairation. The solution viscosity was measured using a standard Brookfield viscometer.

The dope solution thus obtained was spread over a smooth glass plate with the help of a knife edge. The thickness of the membranes was controlled by varying the thickness of adhesive tapes at the sides of the glass plate. The glass plate was kept in an environment of controlled temperature and humidity during membrane casting. No deliberate solvent evaporation period was allowed. The glass plate was subsequently immersed in a gelling bath, which is generally demineralized water maintained at a known temperature otherwise as mentioned in the text. Immediately phase inversion starts and after few

B.K. Chaturvedi et al. /Desalination 133 (2001) 3140 33

minutes thin polymeric film separated out from the glass. It was repeatedly washed with demineralized water and wet stored. The actual thickness of the membranes was measured using a micro- meter.

2.3. Characterization of membrane

was analysed spectrophotometrically at wave- length of 555 nm. Oligomers of starch hydrolyzate were analyzed by standard Benedict’s test, the details of which are outlined elsewhere [19]. The surface structure morphology was obtained by SEM after coating with 40% gold and 60% palladium film of the order of 60 mn thickness.

Membranes were characterized in terms of transmembrane flux and solute separation of different systems in a tangential flow type test cell offering a membrane area of 15 .4cm2 at 5 kg/cm* pressure. The feed water is pumped across a given membrane specimen at a flow rate of 3 l/m using a reciprocating pump. The experimental set up is described elsewhere [ 181. Flux was calculated by taking the average of three readings taken at a regular interval by noting the time taken to collect lOm1 of permeate for each conditions. Three sets of membrane samples were made for each casting condition specified in this paper and the average flux and solute separation data are reported. After each run the whole cell was rinsed thoroughly with demineralized water and membrane was washed to remove any deposition.

3. Results and discussions

3.1. Efect of casting parameters on membrane performance

3.1. I. Nature of solvent and additive

The molecular weight cut off (MWCO) was constructed by measuring the separation values of polyethylene glycol solutes of varying molecular weights. The analytical method for deter- mining the concentration of polyethylene glycol was given previously [ 161. The rhodamine-B dye

Five UF membrane samples, designated as PES 1 to PES 5 were prepared using different solvents and additives as given below. PES 1 and PES 2 membranes were prepared using polymer and solvent namely, dimethyl formamide (DMF) and N-methyl pyrrolidone (NMP), respectively. PES 3 to PES 5 membranes were prepared using three different additives, namely, maleic acid, piperazine and PEG-l 2000 with dimethyl formamide as the solvent. The additives were selected specifically to study the effects of acidic or basic nature (of the additive) on the membrane performance. The composition, casting solution viscosity, total membrane thickness and pure water permeation rate are given in Table 1.

Table 1

Effect of solvent and additive on pure water permeability of UF membranes

Membrane Composition of casting solution, Solution Total membrane Pure water wt. % viscosity, thickness, permeability,

centipoise pm L.m-z.h-’ P S N

PES 1 15 85 (DMF) - 19200 212 45 PES 2 15 85 (NMP) - 17120 198 8.7 PES 3 15 70 (DMF) 15 (MA) 21214 221 121 PES 4 15 70 (DMF) 15 (PP) 20792 214 28 PES 5 15 70 (DMF) 15 (PEG) 21484 209 113

P, polymer; S, solvent; N, additive; Membrane area = 15.4 cm2; Applied pressure = 5 kg/cm2; Temperature = 24’C; Relative humidity = 55%; Gelling bath - demineralized water at 22OC


The cast thickness of all the five membranes, before gelling, based on the thickness of side runner tapes is 400pm and the final membrane thickness is found to be nearly half the cast thickness indicating densification of polymer network during gelling process. The viscosity of the casting solution is found to be slightly higher with DMF solvent as compared to NMP solvent. Similarly, presence of additive improves the solution viscosity. The pure water permeation rate is found to be significantly higher for PES 1 membrane as compared to PES 2 membrane. Incorporation of additives in the casting solution generally increases the water permeation rate. The results indicate that the pure water permeability values are significantly affected by the nature of the additives. The basic additive, namely, piperazine is found to give a lower pure water permeability as against acidic additive namely, maleic acid. Use of higher molecular weight additive namely, polyethylene glycol- 12000, gives membranes of higher pure water permeation rate.

As mentioned above, the viscosity of the casting solution with DMF as the solvent is higher as compared to the casting solution with NMP as the solvent. The higher viscosity in the former case denotes the presence of large size super molecular polymer aggregates in solution owing to the poor solvating power of DMF. The lower viscosity in the later case indicates relatively smaller size polymer aggregates in solution owing to the higher solvating power of NMP. The time taken for the completion of the gelling process was noted by following the time taken for the appearance of the totally opaque film in both cases and it was found that gelation is slower (23 s for 15cmx 15cmx400um as cast membrane dimension) in the case of latter whereas gelation was faster (11 s for similar dimension) in the former case. Faster gelation signifies rapid solidification of the polymer aggregates forming aggregate pores in the final

membrane leading to more open pore channels. Slower gelation signifies more gradual solidification of the polymer aggregates forming closer aggregate pores leading to narrow pore channels. The size of the polymer aggregates, as qualita- tively indicated by viscosity data, decides to a significant extent the ultimate pore structure of the membranes.

3.1.2. E#ect of ambient humidity

The effect of ambient humidity during membrane casting was studied by preparing membranes at two different relative humidities (RH) namely, 55% and 90% RH, respectively, for the set of five different compositions as mentioned in the previous sections. The pure water permeability data obtained are given in Table 2. It can be seen that exposure to higher ambient humidity gives more porous membranes with higher water permeation rates in all the cases. Exposure to 90% RH leads to partial phase separation during membrane casting, prior to gelation, for casting solutions with DMF as the solvent. To assess the relative stability of PES 1 and PES 2 casting composition, calculated quantities of water were added to vigorously mixed casting solutions and the point of phase separation was followed with the amount of water relative to solvent. It was found that barely 1% of water (relative to mole fraction of solvent) was sufficient to cause phase separation for PES 1 whereas PES 2 solution accommodates up to 5% water indicating that PES 1 composition is closer to phase boundary curve as compared to PES 2 composition.

3.1.3. E#ect of concentration of additive

The effect of concentration of additive in the casting solution on membrane performance was studied by changing the amount of maleic acid in PES 3 composition, relative to DMF concentra-

B.K. Chaturvedi et al. /Desalination 133 (2001) 3140 35

Table 2 Effect of ambient humidity on membrane performance

Membrane Relative humidity, Pure water % permeability,

L.m-*.h-’

PES 1 55 44 90 59.3

PES 2 55 8.4 90 13

PES 3 55 121 90 135.3

PES 4 55 28.8 90 32.7

PES 5 55 111.8 90 121.2

Membrane area = 15.4 cm2; Applied pressure = 5 kg/cm$ Temperature = 24OC; Gelling bath - demineralized water ai 22OC

tion without changing the polymer concentration in the dope. Maleic acid concentration varied from 5 to 20% and maleic acid of higher concentration could not be added as the solution turned to be highly unstable with polymer precipitating out. The pure water permeation rate for four different membranes is given in Table 3. It can be seen that with increase in additive to solvent

ratio in the casting solution, the pure water permeability rate increases almost linearly.

3. I. 4. Efect of gelling medium

The effect of gelling medium on the membrane performance was studied by using demineralized water maintained at three different temperatures as well as using demineralized water containing 2% (w/w) DMP as gelling medium. The pure water permeation rate for all the four membranes are given in Table 4. It can be seen that with increase in temperature of demineralised water, the pure water permeation rate increases. Presence of even small quantity of solvent in the gelling medium is found to lower the pure water permeation rate. The higher temperature of the gelling medium facilitates faster diffusion of nonsolvent into the membrane leading to more open pore channels. Presence of solvents in the gelling bath reduces the mole fraction of the nonsolvent which lowers its thermodynamic activity. Hence the diffusion rate of nonsolvent is reduced to allow gradual solidification of polymer network at the interface.

Table 3

Effect of concentration of maleic acid on membrane performance

Membrane Composition of casting solution,

wt. %

Solution viscosity, Total membrane Pure water

centipoise thickness, permeability,

um L.m-*.h-’

P S N PES 3a 15 80 5 20812 209 73.7 PES 3b 15 75 10 21294 215 100.5 PES 3c 15 70 15 21214 221 121 PES 3d 15 65 20 214767 239 131.1

P, polymer; S, solvent; N, additive; Membrane area = 15.4 cmz; Applied pressure = 5 kg/cm? Temperature = 24°C;

Relative humidity = 55%; Gelling bath - demineralized water at 22°C

36 B. K. Chaturvedi et al. /Desalination I33 (2001) 31-40

Table 4

Effect of gelling medium on UF membrane performance

(PES 3)

Sl. no. Gelling medium Pure water permeability, L.m-*. h-’

Demineralized water at 5°C 59.9



Demineralized water 94.4 containing 2% DMF at 25°C

Membrane area = 15.4 cmz; Applied pressure = 5 kg/cm2;

Temperature = 24°C; Relative humidity = 55%

3.2. Membrane characterization

3.2.1. Separation data for PEG -5000 solute

The separation data for the five UF membrane samples namely, PES 1 to PES 5 for PEG-5000 solute is presented in Table 5. The results indicate that the membranes give varying separation for PEG-5000 with the lowest separation (12.8%) for PES 5 membrane and the highest separation (44.9%) for PES 1 membrane. Incorporation of additives in the casting solution not only has increased permeate flux but also lowered the solute separation. PES 2 membrane is characterized by both lower flux and solute separation as compared to PES 1 indicating the suitability of DMF as polymer solvent for this polymer as compared to NMP, in terms of both permeate flux and solute separation. Use of high molecular weight neutral additive, PEG- 12000, gives membranes with high flux but poor solute separation.

The effect of pressure on the separation performance with respect to PEG-5000 solute for three different membranes namely, PES 1, PES 3 and PES 4 is given in Figs. 1 and 2. It can be seen that the permeate flux as well as solute

Table 5

Separation data for PEG-5000 for different UF mem-

branes

Membrane Product flux, L.m-‘.h-’

Solute separation

PES 1 35.5 0.449

PES 2 6.6 0.343

PBS 3 109.4 0.182

PES 4 22.8 0.317

PES 5 108.2 0.128

Feed = 200 ppm PEG-5000; Membrane area = 15.4 cmz;

Applied pressure = 5 kg/cm2; Temperature = 24’C

2750 , . ( , , . ,

_ 2500 -

_; 9 2250-

“E zoo01 6 1760- /

*H--O

-.-PESI -

-

5 15GO-

. -.- PES3

CF / -.- PES4 -

a, 126O- iij . 0 lOOO- g 750- m------m

a” 500- ¤/~m--r_-

250- *AH 0

0 1 2 3 4 5 6

Applied pressure (kg/cm2)

Fig. 1. Variation of permeate flux for PEG-5000 for PES

membranes as a functior of pressure.

0.45 - -.- PES 1

.A

.ym .

0.40 - -.- PES 3

.E 0.35- z

b 0.30-

-.- PES 4 /

@ :

3 0.25-

,--‘:--.--‘--’

s 5 0.20- .N

v)

.fl*---*H

l -M. .

0.15- . O.lO* , . , ( , ,

1 Apilied preshe (kglcrfi2) 5

Fig. 2. Separation of PEG-5000 for PES membranes as a

function of pressure.

B.K. Chaturvedi et al. /Desalination 133 (2001) 31-40 37

separation increase with increase in applied pressure within the pressure range studied. However the permeate flux and solute separation levels off at higher pressures. As the pressure increases, more solvent permeates through the membranes leaving behind partial accumulation of retained solute on the high-pressure side of the membrane. This accumulated solute is however likely to be washed away tangentially depending upon the flow velocity of the feed. However due to the stagnant layer formation and irreversible adsorption of solute on membrane surface, the flow resistance of membranes to the permeating solvent increases. The build up of flow resistance of membranes as a result of adsorptive fouling is more at higher pressures, thereby causes a gradual leveling of permeate flux. The formation of fouling layer on membrane surface, lowers the permeate flux but does not appear to significantly improve the solute separation behaviour of membranes. It is also to be noted that the extent of improvement of solute separation as a result of increase in applied pressure depends on the intrinsic separation behaviour of membranes for a given solute. Membranes exhibiting poor solute separation (PES 3 and PES 4) do not show any significant improvement in separation behaviour with increase in pressure. On the other hand membranes exhibiting moderate solute separation (PES 1) show significant improvement in separation behaviour with increase in pressure.

3.2.2. Molecular weight cut o_fl’proJile

The molecular weight cut off (MWCO) profiles were constructed for three UF membrane samples namely, PES 1, PES 3 and PES 4 by measuring solute separation for various PEG solutes of molecular weight ranging from 5000 to 35000 Dalton. The results are given in Fig. 3. It can be seen that the separation increases with increase in molecular weight of PEG solute and the profiles are not sharp but are gradual and

20000 25000 3Jcmo 35ooa

Molecular weight of PEG solute (Dalton)

Fig. 3. Molecular weight cut off profiles of the membranes.

diffused for the three membrane samples. The MWCO for PES 1 and PES 4 are measured to be around 12000 Dalton whereas for PES 3 membrane, the cut off is found to be at around 30000 Dalton With higher molecular weight PEG, the solute separation tends to become asymptotic.

3.2.3. Microscopic observations

The SEM photographs of skin surface of PES 1, PES 3 and PES 4 membrane specimens are taken under identical magnification and are given in Figs. 4, 6 and 7. The porous surface of PES 1 (the surface in contact with the glass plate at the time of gelling) membrane sample is given in Fig. 5. Comparison of the skin and porous surface, SEM pictures clearly show the asymmetric nature of these membranes. The skin surface SEM pictures indicate smooth structure devoid of any macroscopic voids for PES 1 (where DMF was used as the solvent in the absence of additive) membrane. PES 3 (where DMF was used as the solvent with maleic acid as the additive) membrane specimen reveals the presence of circular voids of more or less uniform dimensions on skin surface. Some of the voids appear to taper in with depth. The surface structure of PES 4 (where DMF as the solvent and piperazine as the additive) membrane appears to

B. K. Chaturvedi et al. /Desalination 133 (2001) 31-40

Fig. 4. SEM photograph for skin surface of PES 1 membrane.

Fig. 5. SEM photograph for porous surface of PES 1

membrane.

Fig. 6. SEM photograph for skin surface of PES 3 Fig. 7. SEM photograph for skin surface of PES 4

membrane. membrane.

be intermediate between PES 1 and PES 3 with rough skin surface devoid of any macroscopic voids.

3.2.4. Separation performance for other solute systems

The separation performance with respect to Na$O,, rhodamine-B dye and oligomers of starch hydrolyzate are evaluated for the three different UF membrane samples namely, PES 1, PES 3 and PES 4 at different pressures. The results are given in Figs. S-10. The separation of NazS04 is found to be lower; in general, as

compared to separation of PEG-5000 solutes. However separation marginally improves for all the membranes with increase in pressure and levels off at higher pressures. Separation of Na,S04 follows the separation behaviour observed for PEG-5000 solutes with PES 1 membrane showing higher separation and PES 3 membrane showing lower separation. Separation of rhodamine-B dye (50 ppm) was studied in the presence of a surfactant, namely, sodium lauryl sulfate (250ppm). It was reported [6] from our laboratory that the presence of sodium lauryl sulfate improves the separation of rhodamine-B dye and higher concentration of sodium lauryl

B. K. Chaturvedi et al. /Desalination 133 (2001) 31-40 39

5 0.20 -

._ i5 & 0.16-

Q a O.lZ-

5 $I 0.06-

0.04 - Feed 2000 ppm Na,SO, -

1 2 3 4 5

Applied pressure (kg/cm2)

Fig. 8. Separation performance of UF membranes at

different pressure for Na2S04.

0.6 I ’ r

0.1 Feed 50 ppm rhodamine 6 dye with 250 ppm sodium lauryl sulfate

( 1 1 2 3 4 5

Pressure (kg/cm’)

Fig. 9. Rhodamine-B dye separation performance of UF

membranes for different pressures.

0.8 I

1 1 0.7 -

,E 0.8- Tii

/ . z OS-

ry

8 - g 0.4-

N : s . :+:

$3 0.3- ./ //y-• -.-PESI -

““: .A

--t PES 3

-.- PES 4 .

0.1. , , . . , , , . . 1 2 3 4 5

Pressure (kg/cm*)

Fig. 10. Separation of oligomers of starch hydrolyzate by

UF membranes at different pressures.

sulfate increases dye separation for cellulose acetate butyrate UP membranes. The separation of rhodamine-B dye is found to be similar to what is observed for PEG-5000 solutes. The comparable separation behaviour of high molecular weight PEG solute (M.wt. = 5000) and low molecular weight rhodamine-B dye (polar organic solute, M.wt. = 442.57) indicates the role of the surfactant in modifying membrane solution interface as well as the interaction of polar organic solute with the membrane interface. The separation of rhodamine-B is found to increase with increase in pressure and levels off at higher pressures. The order of separation for different membranes is also similar to what is observed for other solutes. The separation behaviour for oligomers of starch hydrolyzate was studied using 2% starch solution, hydrolysed by boiling for 1 h in 0.165% HCl solution and analysing the oligimers in feed and permeate samples as glucose. Separation of oligomers are observed to be better than PEG-5000 solute, especially at higher pressures. The separation is found to increase with increase in pressures as observed in other solutes.

4. Conclusions

1. Ultrafiltration membranes with MWCO in the range of 12000-30000 Dalton can be prepared from polyethersulfone polymer with proper adjustment of polymer, solvent and additive. The casting conditions are outlined in detail.

2. The nature of solvent is found to play a significant role in the UF membrane performance. The acidic/basic nature of the additive affects the viscosity of the casting solution as well as the water permeation rate of the resultant membranes. The higher ambient humidity during membrane casting also contributes to higher water permeation rate of resultant membranes.

3. Water permeation rate of UF membranes is also controlled by the composition and temperature of gelling bath.


4. Direct SEM photographs of either surface of membranes show differential porous structure indicating asymmetric nature of these membranes.

5. Separation of Na2S04, rhodamine-B dye and oligomers of starch hydrolyzate increases up to 5 kg/cm2 with increasing pressures.

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preparation, characterization and performance of polyethersulfone ultrafiltration membranes

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