Journal of Membrane Science 214 (2003) 211–221

Surface modification of ultrafiltration membranes bypreadsorption of a negatively charged polymer

I. Permeation of water soluble polymers and inorganicsalt solutions and fouling resistance properties

A.V.R. Reddy∗, D.J. Mohan, A. Bhattacharya, V.J. Shah, P.K. GhoshCentral Salt and Marine Chemicals Research Institute, Gijubhai Badheka Marg, Bhavnagar 364002, Gujarat, India

Received 5 April 2002; received in revised form 30 October 2002; accepted 13 November 2002

Abstract

Polyethersulfone ultrafiltration membranes (MWCO: 9000, 35,000 and 85,000) were surface modified by preadsorption ofpoly(sodium 4-styrenesulfonate) (PSS) upon the permeation of aqueous solution of the polymer for about 100 min. Membraneswith lower MWCO values were modified primarily on the top surface, whereas high MWCO membranes were modified bothon the surface and pore walls as well. The IR spectra of surface modified membranes show absorption bands at 1040 cm−1 dueto symmetric vibration of SO3− group, and at 2925 and 2852 cm−1 corresponding to asymmetric and symmetric stretchings,respectively, of aliphatic (backbone CH2 and CH) unit present in PSS. Surface modified membranes have shown separationsof 28–56% for Na2SO4, 11–19% for NaCl and<5% for CaCl2 with water permeation rates of 35–102 lm h at 2 kg/cm2 whentested for 500 ppm feed solution. The rejection of Na2SO4 decreases in an exponential manner when the concentration ofNa2SO4 in the feed solution was increased from 500 to 2500 ppm, which is a characteristic of negatively charged membranes.The antifouling nature of unmodified and surface modified membranes were compared by ultrafiltration of aqueous solutionsof different molecular weight polyethylene glycols and dextrans. Surface modified membranes have shown better antifoulingproperties compared to unmodified membranes, as indicated by flux decline with operating time.© 2002 Elsevier Science B.V. All rights reserved.

Keywords:PES ultrafiltration membranes; Poly(sodium 4-styrenesulfonate); Surface modification; Infrared spectra; Antifouling properties

1. Introduction

In recent years, ultrafiltration technology has re-ceived tremendous importance for concentration,purification and fractionation of various products indiverse fields such as food, medical, and biotechno-logical industries, paper industry, dairy industry, etc.Most of the commercial ultrafiltration membranes are

∗ Corresponding author.E-mail address:salt@csir.res.in (A.V.R. Reddy).

made from hydrophobic polymers like polysulfone,polyethersulfone, polypropylene, polyethylene andpolyvinylidenefluoride due to their excellent chemi-cal resistance and thermal and mechanical properties.But, hydrophobic membranes are easily susceptiblefor fouling, i.e. non-specific adsorption of feed so-lutes on the membrane surface and pores resultingin severe decline in permeate flux with operationtime [1–3]. It has been generally acknowledged thatmembranes with hydrophilic surface are less suscep-tible for fouling and often reversible[4–10]. Thus,

0376-7388/02/$ – see front matter © 2002 Elsevier Science B.V. All rights reserved.doi:10.1016/S0376-7388(02)00547-1

212 A.V.R. Reddy et al. / Journal of Membrane Science 214 (2003) 211–221

the ideal membrane would combine the superior bulkproperties of hydrophobic polymers with the surfacechemistry of hydrophilic materials for a variety ofseparation applications.

Surface modification of membranes is an attrac-tive approach to change the surface properties ofthe membrane in a defined selective way while pre-serving its macroporous structure. Several techniqueshave been employed to impart surface hydrophilicityto conventional hydrophobic membranes to improvetheir fouling resistance properties. These techniquesinclude: (1) radiation and photochemical grafting ofhydrophilic polymers[11–15], (2) glow dischargelow temperature plasma treatments[16–20], (3) co-valent attachment of hydrophilic polymers and func-tional monomers[21–23], and (4) coating of a thinlayer of hydrophilic polymer on membrane surfaceby physical adsorption[24–27]. We have earlier re-ported polysulfone ultrafiltration membranes coatedwith a polysaccharide (carrageenan) crosslinked withisophthaloyl chloride and trimesoyl chloride[28] andpolysulfone membranes containing carboxylic acidgroup[29] for the separation of biopolymers.

The present paper reports surface modification ofpolyethersulfone ultrafiltration membranes by phys-ical adsorption of poly(sodium 4-styrenesulfonate)(PSS), and their characterization by IR spectra, per-meation of inorganic solutes like NaCl, Na2SO4, andCaCl2, and aqueous solutions of different molecu-lar weight polyethylene glycols and dextrans. Theobjective of this investigation is to throw light onthe mechanism of adsorption of negatively chargedpolymer on the surface of ultrafiltration membraneshaving same chemistry but different pore sizes toimpart antifouling properties.

2. Experimental

2.1. Materials

Polyethersulfone with inherent viscosity of 1.27 dl/gin N-methylpyrrolidone was procured from GardhaChemicals, India. Dextrans (molecular weight: 148,77 and 41 kDa) and polyethylene glycols (MW: 35,20, and 6 kDa) and PSS (MW: 70 kDa) were obtainedfrom Aldrich, USA. All the other chemicals were ofanalytical grade and used without further purification.

2.2. Membrane modification

Flat sheet polyethersulfone ultrafiltration mem-branes were made by traditional phase-inversionmethod on Holytex non-woven polyester fabric fromdimethylformamide solutions of the polymer using amotor driven membrane casting machine as reportedelsewhere[29]. A discussion on casting conditions ofPES ultrafiltration membranes falls beyond the scopeof this paper.Table 1shows the characteristics of PESultrafiltration membranes used in this investigation.

Surface modification of PES membranes was car-ried out by permeating the solution of poly(sodium4-styrenesulfonate) (2000 pm) in water using Amicondead-end type stirred ultrafiltration cell (400 ml). Af-ter fixing the membrane in the test cell, initially purewater was permeated at 4 kg/cm2 for about 45 min andthen measured the pure water flux of the membraneat 2 kg/cm2. Then the solution of PSS was passedthrough the membrane for 100 min at 2.0 kg/cm2. Thepermeate flux was measured at different time inter-vals and the data are given inFig. 1. After passingPSS solution, the membrane was washed thoroughlywith water and measured its pure water permeabil-ity. The surface modified membranes are designatedas PES-9sm, PES-35sm and PES-85sm. They werecharacterized by IR spectra, permeation of aqueoussolutions of different molecular weight polyethy-lene glycols and dextrans, and solutions of inorganicsolutes like NaCl, Na2SO4 and CaCl2 in water.

2.3. Ultrafiltration of PEG/dextran solution

All the ultrafiltration experiments were carriedout using the same membrane circle, unless other-wise mentioned, using Amicon ultrafiltration cell.After the measurement of pure water flux (PWF0) at

Table 1Characteristics of polyethersulfone ultrafiltration membranes

Membrane MWCO (kDaa) PWF (lm h) Mean poresizeb (nm)

PES-9 9,000 37.5 2.7PES-35 35,000 78.2 5.6PES-85 85,000 119.4 9.3

MWCO values are determined from solute transport data of PEGand dextrans.

a Kilodaltons.b Based on solute transport data.

A.V.R. Reddy et al. / Journal of Membrane Science 214 (2003) 211–221 213

Fig. 1. Time vs. relative flux in the permeation of PSS solution.

2.0 kg/cm2, 500 ppm aqueous solutions of differentmolecular weight polyethylene glycols and dextranswere permeated in the following order: PEG: 35, 20and 6 kDa; dextrans: 148, 77 and 41 kDa. After thepermeation of about 45 min of each solution, mea-sured the flux, washed the membrane with distilledwater and recorded pure water permeability (PWFuf )at 2.0 kg/cm2. The concentration of PEG/dextrans inthe feed and permeate solutions were determined bysize exclusion chromatography. The solute rejectionwas calculated using the following expression:

solute rejection(R, %) =(

1 − Cp

Cf

)× 100 (1)

whereCf and Cp are concentrations of solute in thefeed and permeate, respectively.

2.4. Permeation of inorganic salt solutions

Aqueous solutions of NaCl and CaCl2 (500 ppm)and Na2SO4 (500, 1000, 1500, 2000, 2500 and3000 ppm) were permeated at 2.0 kg/cm2 in ultrafil-tration test cell. The concentrations of solute in thefeed and permeate were determined by conductivitymeasurements, and then calculated the rejection ofthe solute according toEq. (1).

2.5. Methods of calculation

Flux recovery ratio (FRR), which is assumed to bea measure for fouling resistance nature of membrane,was calculated using the following expression:

FRR(%) =(

PWFuf

PWF0

)× 100

where PWF0 and PWFuf are pure water fluxes of themembrane before and after ultrafiltration, respectively.

The relative flux (RF) was calculated by comparingthe stabilized flux (PF) of PSS solution with the purewater flux before ultrafiltration as follows:

RF(%) =(

PF

PWF0

)× 100

2.6. Size exclusion chromatography

Water’s HPLC equipped with 515 pump, 2410 RIdetector, three ultrastyragel columns and millineum-32software was used for the determination of the con-centration of PEG and dextrans in feed and permeatesolutions.

2.7. Infrared spectra

Infrared spectra of the membrane circles in dry con-dition were obtained with Perkin-Elmer Spectrum GXFT-IR spectrophotometer.

3. Results and discussion

3.1. Surface modification and characterizationof membranes

Polyethersulfone ultrafiltration membranes (MWC0:9000, 35,000 and 85,000 Da) were surface modified

214 A.V.R. Reddy et al. / Journal of Membrane Science 214 (2003) 211–221

by the permeation of aqueous solution of poly(sodium4-styrenesulfonate) for about 100 min. Permeationof PSS solution results in the physical adsorption ofpolymer on the surface and in the pore walls of themembrane. The adsorption of PSS in the pore wallsdepends on the MWCO value of membrane, i.e. mem-brane pore size. Generally, the adsorption of poly-mers on materials arises from electrostatic attraction,hydrophobic binding of non-polar segments, hydro-gen bonding, and chemical reactions with functionalgroups on the surface[30].

The rejections of PSS were >99.5% for PES-9,95.7% for PES-35 and 83.8% for PES-85 which wereconsistent with the molecular weight cut off valuesof the membranes. The relative flux (RF) with time(Fig. 1) and pure water flux recovery ratio (FRR) ofthe membranes in the permeation of PSS solution aredependent on the MWCO values, i.e. pore size of themembranes. In the case of PES-9, which has a lowmolecular weight cut off (9000 Da), i.e. smaller poresize than the permeating PSS molecules, steady-stateflux was reached just at the beginning of the perme-ation of PSS solution with a relative flux of about 90%.Moreover, the initial pure water flux has been fullyrecovered at the end of PSS filtration. The adsorptionof PSS in the pore walls of PES-9 is regarded as un-likely because the molecular weight of the polymer(MW: 70,000 Da) is much larger than the MWCO ofthe membrane and thus will not be able to enter insidethe membrane pores. This situation makes it possiblethat the adsorption of PSS molecules takes place onlyon the surface of the PES-9 membrane as depicted inFig. 2a.

In cases of PES-35 and PES-85 membranes, about15–17% flux decline was observed at the beginning

Fig. 2. Schematic representation of mechanism of PSS adsorptionon membranes having different pore sizes: (a) narrow pore, (b)medium pore and (c) large pore.

of the permeation of PSS solution and the steady-stateflux was reached within 15 min with a relative fluxof about 80%. The flux recovery ratio values are 93and 95% for PES-85 and PES-35, respectively. Bytaking into consideration the MWCO values of thetwo membranes and the molecular weight of PSS,the above observations may be explained by assum-ing that some of the pores of these two membranesare of the same order of magnitude as that of PSSmolecules, and the remaining pores are smaller inPES-35 for the PSS molecules to enter inside, andlarger in PES-85 to retain the polymer molecules.When the size of the macromolecule in the feed isof the same order of magnitude of membrane pores,some of the smaller molecules of the feed could lodgein the pores without necessarily going through them.This physical blockage of the pores will cause a dropin flux in the first few minutes of operation. The PSSmolecules can easily pass through the relatively largerpores of PES-85 thus making it plausible the inter-nal surface of the membrane is completely adsorbedwith thin layer of polymer. Hence, in case of PES-35and PES-85 the permeation of PSS solution results inthe adsorption of the solute on the membrane surfaceand partially in the pore walls as well as shown inFig. 2b and c.

According to the Hagen–Poiseuille equation, thewater flux through the porous membranes can be ex-pressed as:

J0 = Nπd4 �P

128ηδ(2)

whereN is the number of pores (per unit area) hav-ing diameterd, δ the length of the pores (membranethickness),η the water viscosity and�P the pressuredifference across the pores. As perEq. (2), narrow-ing of membrane pores due to the adsorption of PSSchains should cause a decrease in the water perme-ability of the membranes. However, the pure waterfluxes of both neat and PSS treated membranes are al-most identical. This could be a result of increased hy-drophilicity of the membrane surface due to adsorbedPSS, like that described by Childress and Deshmukh[31]. It is possible that hydrophobic parts of PSS bindonto the hydrophobic parts of the membrane, and thehydrophilic sulfonic acid (SO3H) groups are directedtowards the solution. As a result the membrane wouldbecome more hydrophilic and more wettable to water

A.V.R. Reddy et al. / Journal of Membrane Science 214 (2003) 211–221 215

Fig. 3. Infrared spectra of (a) PES-35 and (b) PES-35sm membranes.

as well. It may be appropriate to mention here that hy-drophobic membranes are non-wettable by water anda pressure gradient is needed in order to pass waterthrough the membrane pores.

3.2. Infrared spectra

Fig. 3 shows the IR spectra of clear and PSS mod-ified membranes. On comparison of the spectra, itis very clear that the modified membranes exhibitfour new absorption bands at 1040, 2852, 2925 and3500 cm−1 which correspond to PSS unit. The peakat 1040 cm−1 is assignable to symmetric stretch-ings of SO3

− group [32,33], and those at 2925 and2852 cm−1 are due to asymmetric and symmetricstretching, respectively, of backbone aliphatic (CH2and CH) group of PSS moiety[34–36]. The broadband at 3500 cm−1 arises due to hydroxyl (O–H)moiety of sulfonic acid group. The bands at 1579and 1486 cm−1 are assignable to the aromatic groups,and the 1322/1295 cm−1 doublet and the band at

1152 cm−1 are due to aromatic sulfone chromophoreof poyethersulfone[37].

3.3. Rejection of inorganic solutes

Table 2gives the rejections of NaCl, Na2SO4 andCaCl2 by surface modified membranes. Since all thethree membranes have very large pores to reject in-organic solutes based on the solute size, the observedrejections for these solutes can be explained exclu-sively based on the negative charge that was created on

Table 2Rejections of inorganic solutes by modified membranes

Membrane Rejection (%)

Na2SO4 NaCl CaCl2

PES-9sm 56 19 5PES-35sm 39 13 <5PRS-85sm 28 11 <5

216 A.V.R. Reddy et al. / Journal of Membrane Science 214 (2003) 211–221

Fig. 4. Variation in Na2SO4 rejection of membranes with feed concentration: (a) PES-9sm, (b) PES-35sm and (c) PES-85sm.

the membrane surface by physical adsorption of PSSwhich has sulfonic acid (SO3H) group. It may be ap-propriate to mention here that sulfonated polysulfonemembrane, which has a negative charge, rejected inor-ganic solutes by means of electric effect even thoughthe size of membrane pores are much larger than thesize of salts, namely a fractional molecular weight cutoff of about 10,000 Da[38].

For all the membranes, the rejection of saltsdecreases in the following order:R(Na2SO4) >

R(NaCl) > R(CaCl2). They exhibited rejections ofabout 11–19% for NaCl, 28–56% for Na2SO4 and<5% for CaCl2 when tested with 500 ppm feed so-lution. Higher rejection was observed for PES-9smand minimum rejection was shown by PES-85sm. Inthe case of PES-9sm with low MWCO (smaller porediameter), the permeation of PSS solution may resultin the dynamic formation of a thin layer of PSS onthe membrane surface which fully covers the pore en-trances with the negatively charged group. However,there is a relatively low degree of pore narrowing of

the comparatively larger pores of PES-85 membraneby the adsorption of PSS molecules. This results inthe presence of lesser charge density at the entranceof the pores of this membrane. The higher the chargedensity on the membrane surface and pore walls thehigher is the rejection for salt solutes. In order to verifythe stability of the adsorbed macromolecular species,the rejections of inorganic solutes were measured af-ter permeating water for about 10 h. It was observedthat the rejections were remained almost same thusconfirming the stability of the adsorbed PSS layer.

Fig. 4clearly indicates that for all the surface mod-ified membranes the rejection of Na2SO4 decreaseswith the increase of its concentration in the feed solu-tion. The rejections of Na2SO4 were decreased from56 to 25% for PES-9sm, 37 to 10% for PES-35smand 28 to 6% for PES-85sm when the concentrationof the solute in the feed was increased from 500 to2500 ppm. The decrease in the rejection of Na2SO4 inall cases appears to be in an exponential pattern underthe experimental conditions.

A.V.R. Reddy et al. / Journal of Membrane Science 214 (2003) 211–221 217

The higher rejection for divalent anionic salt(Na2SO4) compared to monovalent anionic salt (NaCland CaCl2), and the decrease in Na2SO4 rejectionwith feed concentration clearly suggests the opera-tion of Donnan’s ion exclusion phenomenon in whichelectrostatic interactions plays an important role. Thepresence of negative charge on the membrane surfaceis believed to affect the mobility of solute ions in thefeed solution. When the concentration of feed solu-tion becomes comparable to the fixed charge on themembrane, a Donnan equilibrium will exist betweenthe solution and the membrane phase which excludesco-ions from the membrane resulting in high soluterejection. However, when the concentration of feedsolution is in excess of the concentration of fixedcharge on the membrane, the sorption of co-ionsinto the membrane phase is less hindered and couldlead to lower solute rejection. The rejection sequenceof R(Na2SO4) > R(NaCl) > R(CaCl2) has beenreported by several authors and is attributed to thestrong negative charge of the membrane[39,40].

3.4. Ultrafiltration of PEG/dextrans

Concentration polarization and fouling of mem-branes in the ultrafiltration of PEG and dextransolutions[41] have been reported. Antifouling char-acteristics of native and surface modified polyether-sulfone membranes were measured by ultrafiltrationof aqueous solutions of different molecular weightpolyethylene glycols and dextrans at constant pres-sure. The data on the rejection of PEGs by clear andsurface modified membranes before and after ultra-filtration are presented inTable 3. For all the solutes,surface modified membranes exhibited somewhat

Table 3Rejections (R, %) of polyethylene glycols before and after ultra-filtration

Membrane PEG-6000 PEG-35000

Initial Final Initial Final

PES-9 85.8 98.3 97.8 99.8PES-35 42.6 92.0 94.7 99.7PES-85 34.7 73.8 78.3 94.8PES-9sm 95.7 96.5 99.1 99.5PES-35sm 88.4 95.2 99.5 99.8PES-85sm 63.7 78.2 92.5 96.9

R (%) were calculated by size exclusion chromatography.

higher rejections than neat membranes. The magni-tude of increase in the rejection depends on the initialmolecular weight cut off of the membrane and molec-ular weight of feed solute as well, and it was more forlow molecular weight solute (PEG-6000) for all thethree membranes. For example, PES-9sm has shownabout 96% rejection for PEG-6000 which is about10% higher than that of PES-9 (MWCO: 9000 Da) forthe same solute. However, the difference (∼1%) inthe rejections of PES-9 and PES-9sm for PEG-35000and PEG-20000 solutes is almost negligible. Thismay be due to the fact that the neat PES-9 mem-brane shows rejections of about 98% for PEG-35000and PEG-20000 solutes, and hence, leaves a verylittle scope for further increase in the rejection bysurface modification. High molecular weight cut offPES-85 membrane has initially shown the rejectionsof about 35 and 78% for PEG-6000 and PEG-35000,respectively. PES-85sm exhibited about 28 and 20%higher rejections compared to PES-85 for PEG-6000and PEG-35000, respectively. Similarly, PES-35smhas shown about 45% increase in the rejection forPEG-6000 than the corresponding neat PES-35 mem-brane. The higher rejections for PEG solutes clearlyindicates pore narrowing due to the adsorption of PSSmolecules.

Both unmodified and PSS modified membranesexhibited higher rejections for PEG solutes after thecompletion of ultrafiltration experiments, and the dif-ference was very less in the case of surface modifiedmembranes. The relative size of the membrane poresand the solute molecule in the feed are very impor-tant factors which affects the magnitude of increasein the rejections. For PEG-6000 solute, the rejectionhas been increased from about 86 to 98% for PES-9,from 43 to 92% for PES-35 and from 35 to 74% forPES-85. PES-9sm has shown almost same rejectionfor PEG-6000 in the beginning and at the end of ul-trafiltration experiments thus suggesting that there isnot much change in the porous structure of the mem-brane. In the case of PES-85sm, an increase in therejection of about 14% (64 to 78%) was observed forPEG-6000. However, except for PES-85, the differ-ence in the rejection of the membranes for PEG-35000is very less. The results strongly suggest that a mem-brane with an initially low rejection for a solute of lowmolecular weight polymer can become fouled veryeasily due to ‘internal fouling’ consistent with either

218 A.V.R. Reddy et al. / Journal of Membrane Science 214 (2003) 211–221

Fig. 5. Time vs. flux recovery ratios of the membranes in the ultrafiltration of PEG and dextran solutions.

pore blockage or pore constriction followed by sometype of ‘external fouling’. Membrane fouling resultsin the decrease of pore radius that can be expressedfrom the Poieseulle’s law as follows:

J

Jf=

(r0

rf

)4

whereJ andJf are water fluxes before and after foul-ing, andr0 and rf are membrane pore radius beforeand after fouling. Hence, the observed increase in therejection of the membranes after ultrafiltration couldbe explained by assuming the phenomenon of porenarrowing due to the adsorption of feed solutes bothon the membrane surface and pore walls as well.

Fig. 5 gives the pure water flux recovery ratio(FRR) for neat and surface modified membranes in thepermeation of aqueous solutions of different molec-ular weight PEGs and dextrans for a period of about3 h. It is very clear fromFig. 5 that initially both theunmodified and surface modified membranes exhibitalmost similar flux recovery ratio values of about85–90%. Then afterwards, however, the flux declinein unmodified membranes was very rapid as indicatedby the very lower FRR values when compared tosurface modified membranes. The flux decline wasmore in the case of membranes having lower molec-ular weight cut off value. This may be due to the factthat the adsorption of macromolecular solute on the

A.V.R. Reddy et al. / Journal of Membrane Science 214 (2003) 211–221 219

membrane surface results in the blocking of poreswhich eventually results in decrease in the permeateflux. After the permeation of PEG/dextran solutionsfor about 3 h, the FRR values for unmodified mem-branes were 55–65%. However, the values for surfacemodified membranes were about 85–90%.

The increase in rejection of PEG and the decreasein flux recovery ratios strongly suggest the fouling ofmembranes which results in pore blocking and nar-rowing. SEM micrographs of fresh and PEG/dextransolution passed membranes (Fig. 6) confirm theformation of a thin polymer layer on the top of

Fig. 6. SEM photographs of (a) fresh and (b) fouled membranes (PES-35).

membrane. According to sieve slit model[42,43] thesolute rejection ‘R’ relates to the solute size and thepore size as follows:

R = rs

rp

where rs is the solute radius andrp the pore radiusof the membrane. This model has been adopted forultrafiltration membranes which have a rejection ofmore than 90% for a macromolecular solute. The poreradius of the membranes in this investigation werecalculated from Stokes radius of polyethylene glycols

220 A.V.R. Reddy et al. / Journal of Membrane Science 214 (2003) 211–221

Table 4Pore sizes of the membranes before and after ultrafiltration

Membrane r in (nm)a ruf (nm)a

PES-9 2.27 2.13PES-35 5.60 2.13PES-85 9.30 5.60PES-9sm 2.13 2.13PES-35sm 2.27 2.13PES-85sm 5.60 5.60

a Based on >90% separation for PEG solutes (PEG 6000 and35,000 Da);r in and ruf are pore radius before and after ultrafiltra-tion.

using the following expression[44]:

Rs = 16.73× 10−10M0.557

where M is the molecular weight of polyethyleneglycol. Table 4compares the pore radius of the mem-branes before and after ultrafiltration of PEG/dextransolutions. Minimum pore narrowing (∼0.14 nm) wasobserved for membranes with low MWCO (PES-9)as compared to membranes with high MWCO. Incases of PES-35 and PES-85, a decrease of about3.47–3.7 nm in pore size was occurred. But, the de-crease in the pore sizes of PSS modified membranesis comparatively very less. However, the overall de-crease in the pore sizes of the membranes due to PSSmodification and ultrafiltration of macromolecularsolutes is comparable to the decrease of unmodifiedmembranes due to ultrafiltration of PEG/dextran so-lutions only. The major difference between modifiedand unmodified membranes lies in their flux recovery.Flux recovery ratios of >90% were obtained for themodified membranes compared to∼55% for unmod-ified membranes. The higher FRR values stronglyindicate that the surface modified membranes haveexcellent ‘cleanability’ and ‘antifouling’ characteris-tics when compared to unmodified membranes overa few cycles under the experimental conditions. TheFRR is generally taken as a measure of the foulingresistance nature of the membrane, and the higher theFRR the higher the fouling resistance[45].

4. Conclusion

Ultrafiltration membranes containing negativelycharged sulfonic acid group on the surface wereobtained when aqueous solution of poly(sodium

4-styrenesulfonate) was permeated through the mem-branes. The extent and nature of surface modifica-tion of membranes by preadsorption of polymersare strongly dependent on the sizes of membranepores. Membranes with lower cut off values are pri-marily modified only on the top surface because themacromolecules are not able to enter inside the pore.However, membranes with higher MWCO, PSS per-meation results in the formation of charged groupsboth on the surface and pore walls of the membrane.These membranes reject inorganic solutes like NaCl,Na2SO4, etc. due to the electrostatic repulsion. Ultra-filtration of aqueous solutions of different molecularweight polyethylene glycols and dextrans have shownthat the surface modified membranes possess excel-lent ‘cleanability’ and antifouling characteristics overa few cycles under the experimental conditions.

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