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Journal of Membrane Science 169 (2000) 215228

Preparation and performance of cellulose acetatepolyurethane blendmembranes and their applications II

M. Sivakumar a, R. Malaisamy b, C.J. Sajitha b, D. Mohan b,, V. Mohan c, R. Rangarajan ca Department of Chemical and Environmental Engineering, National University of Singapore, Singapore, Singaporeb Department of Chemical Engineering, Alagappa College of Technology, Anna University, Chennai 600 025, India

c Central Salt and Marine Chemicals Research Institute, Bhavnagar 364002, India

Received 3 May 1999; received in revised form 21 October 1999; accepted 25 October 1999

Abstract

Characterization and application of ultrafiltration membranes are of great interest today, both as a tool in choosing theproper membrane for the filtration system used and in the development of new and better membranes. Cellulose acetate wasblended with polyurethane in a polar solvent in the presence of polyvinylpyrrolidone as an additive. The effects of polymercomposition and additive concentration on membrane compaction, pure water flux, water content, membrane hydraulicresistance and morphological studies were discussed. Measurement of transmembrane flux and appropriate macro soluterejection during stirred ultrafiltration of aqueous solutions of proteins and metal ions chelated with polyethyleneimine werecarried out individually using CA/PU blend membranes. 2000 Elsevier Science B.V. All rights reserved.

Keywords: Ultrafiltration; Polymeric membranes; Polymer blend; Metal ion removal; Protein separation

1. Introduction

The development of synthetic polymeric mem-branes with better temperature, pH and solvent resis-tance, makes it possible to use membrane technologyto solve concentration, purification and separationproblems in the chemical industry. Ultrafiltration (UF)and reverse osmosis (RO) have attracted increasingattention for the treatment of wastewater and the re-covery of valuable products from industrial effluents[1].

Ultrafiltration is now widely employed in thebiotechnology industry for separating proteins andpeptide drugs from fermentation broths [2], due to

Corresponding author. Fax: +91-44-235-0240.E-mail address: [email protected] (D. Mohan).

the following advantages compared to other separa-

tion processes: it involves no change of phase andis relatively non-destructive to the easily denaturableproteins, requires low hydrostatic pressure and it canbe performed at ambient or relatively low temperatureand thereby denaturation of protein molecules is elim-inated [3]. However, its use has been confined mainlyto the concentration and diafiltration of dilute pro-tein solutions. Pre-formed ultrafiltration membraneswhich serve as barriers to the diffusive transport ofmacromolecules, have been reported as a practicalsystem for the purification and concentration of diluteprotein solutions [4]. Michaels [5] has discussed thechemical constitution of these polymer membranes

and their potential biomedical applications.Apart from protein separations, ultrafiltration mem-

branes are also gaining importance in abating indus-

0376-7388/00/$ see front matter 2000 Elsevier Science B.V. All rights reserved.PII: S0 376-7388 (99)0033 9-7


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216 M. Sivakumar et al. / Journal of Membrane Science 169 (2000) 215228

trial pollution. Industrial effluents are known to be themajor source of heavy metal pollution. In industrialwastewater, metals are found in different forms suchas, soluble, insoluble, organic, reduced, oxidised, freemetal, precipitated, adsorbed and complex. The pres-ence of these metals, even in trace quantities, leads todetrimental effects on the ecological system.

Conventional methods of removal of heavy metalssuch as solvent extraction, activated sludge processand air flotation technique, do not result in 100%removal of heavy metal ions. However, these draw-backs are overcome by a non-conventional membraneseparation technique [6]. Hence, attempts are beingfocussed to separate these ions from industrial wastew-ater using membrane processes. It may, however, benoted that the ultrafiltration technique alone may notbe sufficient for ionic level separations. Hence, to en-hance the rejection of metal ions, chelating polymerslike polyethyleneimine (PEI) etc., are used for the

complexation of metal ions in aqueous feed solutionswhich is then subsequently separated by ultrafiltrationmembranes. The complexation of ions results in an in-crease in the size of the permeating species renderingultrafiltration to be applicable, which otherwise wouldhave had to be separated by RO membranes at higherpressure. Therefore, complexation helps in reducingcost.

In order to prepare membranes with improved per-formance for separation of proteins like bovine serumalbumin, egg albumin, trypsin, pepsin etc., and alsofor the removal of toxic heavy metal ions such as Cu,Ni, Zn, Cd etc., an investigation on the blending of

cellulose acetate (CA) with polyurethane (PU) in thepresence of additive, polyvinylpyrrolidone (PVP) in apolar medium was made to obtain a CAPU blend UFmembrane. The effect of compaction time on pure wa-ter flux for different compositions of blend membraneswas studied. Further, the effects of polymer composi-tion as well as non-solvent swelling agent PVP, on purewater flux, hydraulic resistance, water content, molec-ular weight cut-off, rejection of proteins and heavymetal ions were investigated and discussed.

2. Experimental

MYCEL cellulose diacetate (CA), 5770 (acetylcontent 39.99 wt%) was procured from Mysore Ac-

etate and Chemicals (India) and was used, afterrecrystallization(Tg=219C and Mw=115 kDa) fromacetone. Commercial grade polyurethane (Tg=23Cand Mw=160 kDa) Grade no 58311, obtained fromChemplast (India) was used as such. Polyvinylpyrroli-done (PVP) K30 (Mw=40 kDa) was procured fromCDH Limited and used as a non-solvent swellingagent. Polyethyleneimine (PEI), Mw=3040kDa,50wt% aqueous solution from Fluka AG (France)was used as received.

Analar grade N,N-dimethyl formamide (DMF) fromSD Fine Chemicals (India) wasdistilled in vacuum andsieved through molecular sieves (4 ) for removingmoisture and stored in a dry condition. Other solventsof analar grade such as acetone and methanol from SDFine Chemicals were used as received. Copper sul-phate (AR), nickel sulphate (AR); zinc sulphate (AR)and cadmium sulphate (AR) procured from Merck (In-dia) were used as such. Deionized water was used for

the preparation of 1% PEI solution, which was subse-quently used for the preparation of all the metal ionsolutions in this study.

2.1. Preparation of membranes

2.1.1. Solution blending of polymers

The casting solution of pure CA, PU and blend poly-mers of CA/PU (17.5 wt%) were prepared by blend-ing different compositions of polymers under constantstirring for 4h at room temperature using DMF asa solvent. The concentration of the additive PVP, inthe casting solution was varied from 0 to 7.5wt%.

Depending on the additive concentration, the solventcomposition varied from 82.5 to 75 wt%.

2.1.2. Membrane preparation

During membrane casting the relative humidity wasmaintained at 605% and the temperature was kept at231C. The membranes were cast [7,8] using a cast-ing blade on a glass plate and were allowed to evapo-rate for 30 s. The membranes were gelled at 10C. Af-ter 30 min of gelation, the membranes were removedfrom the gelation bath, washed thoroughly with dis-tilled water to remove the solvent and surfactant andstored in distilled water containing 0.1% of forma-

lin solution to prevent microbial attack. A uniformthickness of 0.220.02 mm was maintained for all themembranes in the study.


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M. Sivakumar et al. / Journal of Membrane Science 169 (2000) 215228 217

2.2. Characterization of membranes

All the membranes were compacted for 5 h at414kPa in Spectrum (USA) ultrafiltration kit. Allthese compacted membranes were characterized interms of their pure water flux, water content, hydraulicresistance and morphological studies [9] as follows.

2.2.1. Compaction

The thoroughly washed membranes were loaded inthe UF test cell and initial flux was taken, 20s afterpressurization at a transmembrane pressure of 414kPa.The water flux was measured at every 1h and theflux generally declined sharply in the earlier hours andleveled off after 34 h.

2.2.2. Pure water flux (PWF)

Membranes after compaction were subjected to atransmembrane pressure of 345 kPa to measure thewater flux. The flux was measured under steady-stateflow. From the observed values, the PWF was deter-mined from the expression,

Jw =Q

AT

where, Jw is the water flux (l/m2h), Q the quantity ofwater permeated (l), T the sampling time (h) and Ais the membrane area (m2).

2.2.3. Water content

For the determination of water content, membraneswere soaked in distilled water for 24h. The soaked

membranes were mopped with blotting paper andweighed accurately using 0.0001g accuracy elec-tronic balance. The dry weights were measured afterthe wet membrane samples were placed in a dryer at75C for 48 h and cooled it to room temperature ina dessicator. From the two values, the percent watercontent was determined as follows.

% water content

=wet sample weight dry sample weight

Wet sample weight 100

2.2.4. Membrane hydraulic resistance (Rm)

The hydraulic resistance of the membranes was de-termined by measuring the PWF at different trans-membrane pressures (P) such as 69, 138, 207, 276

and 345 kPa. The membrane hydraulic resistance wasevaluated from the slope of pure water flux vs. trans-membrane pressure difference (P) using the equa-tion

Jw =P

Rm

where, Jw is the water flux (lm2 h1), P is trans-membrane pressure difference and Rm is the mem-brane hydraulic resistance

2.2.5. Morphological studies

The top surface and cross-sectional morphology ofthe membranes were studied using Scanning ElectronMicroscopy (LEICA STEREOSCAN S440, Philips,UK). Scanning electron micrographs were takenat various magnifications for the top surface andcross-sectional views of the membranes.

2.2.6. Molecular weight cut-offMolecular weight cut-off of the membrane was de-termined by identifying an inert solute which has thelowest molecular weight and has solute rejection of80100% in steady-state UF experiments. Thus theprotein, BSA, was chosen and its percent rejectionthrough the blend membranes was analyzed usingUV-spectrophotometry at max of 280nm.

2.2.7. Protein rejection

After mounting the membrane in the UF cell, thechamber was filled with individual protein solutionand immediately pressurised to the desired level

(345kPa) and maintained constant throughout therun. Proteins like BSA, EA, pepsin and trypsin weredissolved (0.1%) in phosphate buffer (0.05M, pH 7.2)and used as standard solutions. For all experiments,the concentration of the feed solution was kept con-stant. The permeate was collected over measured timeintervals in graduated tubes and the contents wereanalyzed for protein content by spectrophotometry(Hitachi-2000) at max 280 nm. Protein separation wascalculated from concentrations of feed and permeateusing the equation:

% SR = 1Cp

Cf

100

where, % SR is the % solute rejection, Cp and Cf areconcentrations of permeate and feed, respectively.


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2.2.8. Metal ion rejection

Aqueous solutions of Cu(II), Ni(II), Zn(II) andCd(II) with an approximate concentration of 1000 ppmin one wt% solution of PEI in deionized water wereprepared. The pH of these solutions were adjusted to6.25 by adding a small amount of either 0.1 M HClor 0.1 M NaOH. Solutions containing PEI and metalions or metal chelates were thoroughly mixed and leftstanding for 5 days to complete binding.

For each run, the first few ml of the permeate werediscarded. For the pre-setting of all the membranesand to maintain a constant flux, each metal ion-PEIchelate solute was run in the UF kit at 345 kPa. Thepermeate flux and % separation were determined byanalysing the feed and the permeate. The concen-trations of each metal ion in the permeate and feedwere measured by an atomic absorption spectropho-tometer (PerkinElmer 2380). The pH values in thefeed and the permeate were measured with Elico pH

meter.

3. Results and discussion

The solution blending of the two hydrophilic poly-mers viz., cellulose acetate and polyurethane carriedout in this investigation resulted in ultrafiltrationmembranes with improved performance with respectto water flux, membrane resistance etc. comparedto the membranes made from pure CA. During thisstudy, the roles played by the compositions of thepolymer blend and additive in the casting solution, on

membrane performance were investigated. The appli-cability of the membranes for toxic heavy metal ionremoval from the aqueous phase in the presence ofa macromolecular complexing agent was attempted.Due to the stronger tendency of branched PEI toform complexes with divalent metal ions [10,11]through amino groups, PEI has been chosen as themacromolecular chelating agent instead of otherbinding/complexing agents such as polyacrylic acid,polyvinyl alcohol, polydiallyldimethylammonium-chloride, EDTA etc. Further, the effects of polymercomposition as well as non-solvent swelling agentPVP, on pure water flux (PWF), hydraulic resistance,

water content, molecular weight cut off (MWCO),morphology and separation of proteins and heavymetal ions were investigated and discussed.

3.1. Effect of additive on compaction studies

The effect of polymeric additive, PVP in the castingsolution at different compositions of CA/PU in theblend on PWF during compaction is shown in Table 1.From the table, it is observed that as the concentrationof PU in the blend increases, at a constant compositionof 2.5wt% additive PVP, the flux increases linearly.For all PVP concentrations such as 5.0 and 7.5 wt%,the flux which is high at the initial hour, reduces oncompaction and attains a steady state after 45h.

As the concentration of PVP in the casting solutionincreases to 2.5 wt%, due to the leaching out of theadditive, the pore density and pore size of the mem-branes increase which leads to higher flux.

However, as the PVP concentration increases to5wt%, the increase in PU composition from 5 to25 wt%, reduces the flux and the values are also com-paratively low. This is due to PUPVP interaction

[12] which leads to InterPenetratingNetwork whichin turn reflects the reduction in flux due to decreasein the leaching out of PVP.

However, when the PVP concentration increasesfrom 5.0 to 7.5 wt%, the increase in PU compositionfrom 5 to 25 wt%, reduces the flux but the values arecomparatively higher than those with a 5 wt% addi-tive. This trend may be due to the enhanced leachingrate of excess PVP, which may predominate to higheradditive concentration, which leads to higher and big-ger pores, resulting in an increase of flux.

3.2. Effect of additive on pure water flux

After compaction at 414 kPa for 4 h, all the abovemembranes were analysed for PWF at a constant trans-membrane pressure, i.e at 345 kPa. Pure water fluxeswere measured after 3060 min of pressurization at345kPa.

The role of additive (PVP) concentration in a mem-brane casting solution, on flux of CA membranesprepared both in the presence and absence of PU isshown in Fig. 1. A linear increase in PWF from 14 to80.1lm2 h1 with increasing PVP content from 0 to7.5 wt% in the casting solution is observed for 100%CA membranes (Fig. 1a). An increase in PVP concen-

tration in casting solution, favours membranes with amore open network structure than that would occur inthe absence of PVP [13] and hence increases the flux.


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Table 1Variation of pure water flux of CA/PU blend membranes with time

Composition (17.5wt%) PVP (wt%) Pure water flux (lm2 h1)

CA (%) PU (%) Time (h)

0 1 2 3 4 5

100 0 0 26.8 20.3 16.1 16.1 16.1 16.1

100 0 2.5 50.6 49.5 48.6 48.6 48.6 48.695 5 2.5 70.5 68.5 66.5 66.0 66.0 66.090 10 2.5 85.1 83.2 81.2 80.0 80.0 80.085 15 2.5 143.2 138.5 135.5 134.4 134.4 134.480 20 2.5 190.0 185.2 175.2 175.2 175.2 175.275 25 2.5 215.3 211.5 195.8 190.0 190.0 190.0

100 0 5 83.1 82.0 73.2 68.5 68.5 68.595 5 5 28.9 27.5 27.2 24.5 24.5 24.090 10 5 27.5 26.0 25.0 23.1 23.0 23.085 15 5 25.6 24.3 23.5 22.5 22.0 22.080 20 5 24.3 22.8 21.9 21.5 21.5 21.575 25 5 22.8 22.0 21.5 20.3 20.3 20.3

100 0 7.5 106.5 90.9 90.0 84.9 83.1 83.195 5 7.5 29.7 28.2 28.0 27.5 27.5 27.5

90 10 7.5 28.0 26.5 26.0 25.0 24.5 24.585 15 7.5 26.4 25.4 24.1 23.7 23.7 23.780 20 7.5 25.5 25.0 24.0 23.0 23.0 23.075 25 7.5 24.3 23.7 23.8 22.0 22.0 22.0

Fig. 1. Pure water flux of CA/PU blend membranes with differentconcentrations of PVP. CA/PU compositions: a=100/0; b=95/15;c=90/10; d=85/15; e=80/20; f=75/25.

Further, for the membranes prepared in the pres-ence of PU ranging from 5 to 25 wt%, an increase inPVP content from 0 to 2.5 wt% increases the flux (Fig.1bf). This is due to the leaching out of PVP from thecast nascent membrane during gelation thereby creat-ing bigger pores. However, a further increase in PVP

concentration beyond 2.5wt% upto 7.5 wt%, the PWFdecreased drastically in the case of membranes with ahigh PU content of 25% (Fig. 1f) and may be due tothe following.

The decline in flux beyond 2.5 wt% PVP for CA/PUblend membranes may be due to closer segmental ar-rangements of the two different polymers, PU andPVP, thereby reducing the pore size in the resultantmembrane. This is evidenced by scanning electronmicrographs of blend membranes (Fig. 6). In addi-tion, swelling of PVP predominantly takes place dueto its hydrophilic nature rather than leaching out ofPVP during gelation. This in turn blocks the pores to

a greater extent and consequently reduces flux. Thisphenomenon is especially accelerated at high PVPconcentrations in the casting solution.


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Fig. 2. Water content of CA/PU blend membranes with differentconcentrations of PVP. CA/PU compositions: a=100/0; b=95/15;c=90/10; d=85/15; e=80/20; f=75/25.

3.3. Effect of additive on water content

Fig. 2 represents the effect of concentration of poreformer, PVP, on the water content of blend membraneswith different compositions of CA/PU. For all CA/PUcompositions, the % water content of the membraneincreased upto a PVP concentration of 2.5 wt%. Thismay be due to the higher leachability of PVP at itslower concentration, i.e. at 2.5 wt%. However, beyond2.5 wt% of PVP, i.e. at 5 and 7.5 wt%, the water con-tent of all blend membranes (Fig. 2bf), except for100% cellulose acetate (Fig. 2a), decreases due to thefact that swelling takes place to a greater extent whichdoes not allow the PVP to get leached out, resultingin reduction in pore size and hence the water contentat higher PVP concentrations [14].

3.4. Effect of additive on hydraulic resistance

The effect of concentration of PVP (i.e. 2.5, 5.0and 7.5 wt%) in casting solutions on PWF at different

transmembrane pressures, viz. 69, 138, 207, 276, 345and 414 kPa of blend membranes are shown in Figs.3 to 5. It is evident from the figures that an increase

in transmembrane pressure (TMP) increases PWF at alinear rate. However, when PVP content increased be-yond 2.5 wt%, the increase in flux is relatively smallerthan that with 2.5wt% PVP at various TMPs. At ahigher PVP concentration, swelling of PVP takes placeat a higher rate thereby blocking the pores. As a result,transport resistance increases resulting in lowered fluxand higher membrane resistance (Rm) at higher PVPconcentration (Table 2). The linear relation of flux toapplied pressure can directly be connected to transportresistance [15].

3.5. Morphological studies

The Scanning Electron Microscope (SEM) is apowerful tool to investigate the structure of polymericintegrally skinned membrane [16]. To develop highperformance polymeric membranes, it is essential todesign the molecular and morphological structures of

the membranes for their specific applications. SEM

Fig. 3. Pure water flux of CA/PU blend membranes at differenttransmembrane pressures (2.5wt% PVP). CA/PU compositions:a=100/0; b=95/15; c=90/10; d=85/15; e=80/20; f=75/25.


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Fig. 4. Pure water flux of CA/PU blend membranes at differ-ent transmembrane pressure (5 wt% PVP) CA/PU compositions:a=100/0; b=95/15; c=90/10; d=85/15; e=80/20; f=75/25.

photo micrographs of membranes prepared at variousconcentrations of PVP in CA/PU blend casting so-lutions are presented in Fig. 6af . The micrographsshow typical finger-like voids beneath the skin layer.The shape and structure were related to the concen-tration and composition of additive in the castingsolution. The top surface and cross-sections of the

Table 2Hydraulic resistance (Rm) of CA/PU blend membranes

Composition (wt%) Rm (kPa/lm2 h1)

CA (%) PU (%) PVP concentration (wt%)

0 2.5 5 7.5

100 0 22.52 8.33 5.22 4.2895 5 18.75 5.62 15.60 12.5090 10 16.60 4.25 16.20 14.16

85 15 6.80 2.50 17.50 15.0080 20 5.50 2.08 20.83 16.6075 25 5.25 1.50 33.33 20.80

Fig. 5. Pure water flux of CA/PU blend membranes at differenttransmembrane pressures (7.5wt% PVP). CA/PU compositions:a=100/0; b=95/15; c=90/10; d=85/15; e=80/20; f=75/25.

membranes clearly show their asymmetric nature inall cases. The membranes exhibit classical asymmet-ric structure with macrocavities beneath the top skinlayer. The cavities are separated by walls having aporous honey-comb structure as reported [17].

The effect of the addition of PVP to the CA/PU

membrane casting solution, on the structure of themembrane is also depicted in Fig. 6ac. As the PVPconcentration increases in the casting solution, from2.5 to 7.5 wt%, the number of voids increases in theporous layer (Fig. 6cf). A similar observation is alsoreported in the case of PSf/PVP [18]. A more pro-nounced trend with smaller size cavities and morethick skin layer is also observed by Kinzer et al. [17].

The pores are non-uniform in size rather thanuniform in the case of membranes made from theCA/PU blend casting solution prepared in the pres-ence of 2.5, 5.0 and 7.5 wt% PVP and are shown inFig. 6ac, respectively. Further, it is evident from the

above figures that the pore size of membranes kept ondecreasing with increasing PVP content in the blendsolution [19].The smaller pores of blend membranes


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Fig. 6. SEM micrographs of CA/PU (75/25) blend membranes with different concentrations of additive, PVP. Top surface (500): (a)2.5wt% PVP; (b) 5 wt% PVP (c) 7.5 wt% PVP; Cross Section (750): (d) 2.5wt% PVP; (e) 5 wt% PVP; (f) 7.5 wt% PVP.

(Fig. 6b,c) obtained by SEM at higher PVP, i.e. 5and 7.5wt% in CA/PU blend solution establishedour earlier finding of lower flux due to the reducedleachability of PVP during gelation in view of stronginteractions between PU and PVP.

Fig. 6df explain the cross-sectional view of mem-

branes prepared in the presence of 2.5, 5 and 7.5 wt%PVP. The formation of pores and the thickness of thewall may be due to the following: according to the

interstitial void model, the polymer chain exists asrandom coils, which swell to accommodate solventmolecules. A swollen linear polymer molecule in solu-tion occupies a space which is generally quite spheri-cal with a larger radius. Hence a large sphere will forma larger void (pore). Further, when three (CA/PU/PVP)

such spheres contact each other a void is formed.The empty space within each polymer coil forms smallpores in a membrane. The space between aggregates


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of such coils forms the larger pores. Since, the CA/PUblend is highly hydrophilic, CA/PU molecules wouldswell upon by sorption of water, both during the courseof membrane making and during its use in an aqueousenvironment. Such a scenario is also reported in thecase of the carboxylated polysulfone/PSf blend systemand aminated PSf/PSf and sulfonated PSf/PSf blendmembranes [20,21].

When the concentration of the additive was low, thefinger-like voids were not well defined. When addi-tive concentration was increased, the number of voidsdecreased while the size and thickness of the wall be-tween voids increased relatively.

The reduction in PWF could have been the result ofeither smaller pore sizes or lower porosity, which arealso clearly visualized by SEM micrographs in surfacepores as well as a cross-section of Fig. 6af. Similarobservations were made by Dal-Cin et al. [22].

In order to confirm the smaller pore size of PU

membranes in the presence of additive PVP, in thecasting solution, the SEM of pure PU (100%) in theabsence as well the presence of 5 and 7.5 wt% PVPwere taken and are shown in Fig. 7ac, respectively.The micrograph (Fig. 7a) shows the PU elastomerwith non-uniform pores. The sponge-like structuresin the PU membranes were similar to those observedby Koenhen et al. [23] in the PU/DMF/water system.However, in the presence of PVP at 5 wt% concen-tration(Fig. 7b), the solution mixture appears to formMillar IPN [12], due to (i) hydrophilic nature of bothPU and PVP and (ii) swelling of PVP in the gelationbath. Further, at 7.5 wt% PVP concentration, slightly

open pores are shown in the Fig. 7c due to leachingout of excess PVP.

3.6. Molecular weight cut-off (MWCO)

The MWCO of all the membranes of the CA/PUblend with various concentrations of PVP, i.e. 0, 2.5,5.0 and 7.5 wt% were determined individually basedon % rejection of proteins and shown in Table 3. Theblend membrane (15wt% PU and 85wt% CA with0 % PVP) showed 45 kDa MWCO. However, an in-crease in the additive PVP concentration to 2.5 wt%increases the MWCO beyond 69 kDa due to the for-

mation of larger sized pores [9]. A further, increasein the PVP concentration to 5 wt% creates membraneswith a MWCO of 20kDa, due to the reduced pore

Fig. 7. SEM micrographs of PU (100%) membranes with differentconcentrations of additive, PVP. Top surface (250): (a) 0wt%PVP; (b) 5wt% PVP; (c) 7.5wt% PVP.

size which arises from the PUPVP interaction inthe CA/PU blend composition. However, a further in-crease in PVP beyond 5 wt% enhances the leachingof PVP and hence results in membranes with higherpores than those with 5 wt% PVP. Further, the leach-

ing of PVP is not as effective as in 2.5 wt% PVP dueto PU/PVP interaction and this in turn results in mem-branes with lower MWCO (35kDa) in the case of


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Table 3Rejection of proteins and MWCO of CA/PU blend membranes

Composit ion (%) PVP (wt%) MWCO (kDa) Percent rejection

CA PU BSA (69 kDa) EA (45 kDa) Pepsin (35 kDa) Trypsin (20 kDa)

100 0 0 20 95.0 92.9 90.9 84.285 15 0 45 84.5 80.4 77.8 75.075 25 0 69 79.8 76.6 71.2 65.6

100 0 2.5 45 91.8 83.4 72.5 65.085 15 2.5 > 69 66.1 64.0 61.5 58.575 25 2.5 > 69 63.4 60.3 58.0 53.0

100 0 5.0 > 69 88.0 75.0 68.5 61.085 15 5.0 20 88.5 86.0 83.6 81.075 25 5.0 20 92.4 90.0 87.1 80.9

100 0 7.5 > 69 84.4 64.0 62.5 60.085 15 7.5 35 86.0 84.0 81.0 78.875 25 7.5 45 83.0 80.5 77.4 76.0

7.5 wt% PVP compared to 2.5 wt% PVP. An increasein the PU content to 25 wt% in CA blend membranes

with all the concentrations of PVP showed the sametrend observed earlier, i.e. higher MWCO (69 kDa)for 0wt% PVP and further increase in PVP from 5to 7.5 wt%, gave a MWCO of 20 and 45 kDa, respec-tively.

3.7. Protein rejection

The rejection of the proteins, viz. BSA, EA, pepsinand trypsin were carried out using CA/PU blend mem-branes with two different PU concentrations, i.e. at 15and 25 wt%. The role of additive PVP in the castingsolution of blend membrane on rejection of proteins

was also attempted. The rejections and fluxes of allprotein solutions, as a function of PVP compositionare illustrated in Figs. 8 to 11. When PVP was addedto the casting solution, upto 2.5 wt% the percent re-

jection of BSA was found to reduce to 66.1. However,the flux increases to 40.5 lm2 h1. This may be dueto the fact that the presence of PVP in the casting so-lution favours the formation of larger sized pores onthe skin layer, during the gelation process of the mem-brane through leaching [24].

However, further increase in PVP concentration to5 wt% not only increases the PU/PVP interaction, butreduces the leachability of PVP to a greater extent

during the gelation process. This in turn reduces thepore size and consequently increases rejection with re-duced flux. Further, at higher concentrations of PVP,

i.e. beyond 5 wt%, the rate of leaching takes a pre-dominant role by overcoming PU/PVP interaction and

this in turn increases the pore size. Hence, a lower %rejection with higher flux for BSA was observed at7.5 wt% PVP content in blend solution. Further, ourfindings are confirmed by higher pores for 7.5 wt%PVP by SEM (Fig. 6) compared to 5 wt% PVP.

Fig. 8. Rejection of proteins using CA/PU (85/15) blend mem-branes with different PVP concentrations.


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Fig. 9. Experimental permeate flux of proteins using CA/PU(85/15) blend membranes with varied PVP concentrations.

Fig. 10. Rejection of proteins using CA/PU (75/25) blend mem-branes with different PVP concentrations.

Fig. 11. Experimental permeate flux of proteins using CA/PU(75/25) blend membranes with different PVP concentrations.

Among the proteins studied, BSA was found tohave higher rejection. The order of % rejection are;EA>pepsin>trypsin. These trends may be due to thelower molecular weights of EA, pepsin and trypsinwhich are 45, 35 and 20 kDa, respectively. Further,due to higher MWCO of new blend membranes, allthese proteins of lower molecular weight are found topass through the membranes [25].

A similar trend was also observed when the PU

content was increased to 25 wt% in blend membranes.However, % rejection of BSA was much lower thanthat of 15wt% of PU content as expected due tobigger pore size on the surface due to segmental gapbetween the CA/PU blend membrane at a higherpercentage of PU content (25 wt%). The above trendis reversed with increased concentrations of additive(beyond 2.5 wt% PVP). This is due to reduced poresize in case of CA/PU blend membranes at higherPVP concentrations influenced by PU/PVP interac-tions and reduced leaching of additive.

3.8. Metal ion rejection

The effect of additive concentration, from 0 to7.5 wt% in the above blend membranes on the rejec-


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Fig. 12. Rejection metal chelates using CA/PU (85/15) blendmembranes with different concentrations.

Fig. 13. Experimental permeate flux of metal chelates using CA/PU(85/15) blend membranes with different PVP concentrations.

Fig. 14. Rejection of metal chelates using CA/PU (75/25) blendmembranes with different PVP concentrations.

tion and flux of Cu(II), Ni(II), Zn(II) and Cd(II) wasalso studied and the results are depicted in Figs. 12 to15. It is seen that Cu(II) has more % rejection (93%)for 0% PVP for 15 wt% PU blend membranes. Furtherincrease in PVP concentration to 2.5 wt%, reducesthe rejection of Cu(II) to 88.5%. The increase in PVPenhances the aggregate pore size due to longer poly-mer segmental distance [13]. This, in turn reduces therejection. However, a further increase in PVP results

in higher interaction between PU and PVP and thisreduces the leachability of PVP during the gelationprocess. Consequently, the pore size reduces and thisin turn increases rejection as reflected by 5% PVP inblend. Further, at an increased PVP content, beyondthis optimum level, the rate of leaching of PVP mayoutweigh the PU/PVP interaction. This again resultsin higher pore size as evidenced by SEM (Fig. 6). Thisin turn reduces the % rejection and increases the flux.

Copper metal ion exhibits a stronger binding capac-ity with PEI than other metals in the following order:

Cu(II) > Ni(II) > Zn(II) > Cd(II).

This may be due to the fact that PEI has a strongertendency to form complexes with Cu(II) at a pH of 6,thereby increasing its rejection.


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Fig. 15. Experimental permeate flux of metal chelates using CA/PU(75/25) blend membranes with different PVP concentrations.

Further, the complexing capacity depends on thenumber of functional groups in the macromolecu-lar complex and the atomic weight of the metal andhence the observed trend. Similar observations havebeen reported by Mandel and Leyte [26] for thepolymethacrylic acid system. The same trend was ob-served for other ions, i.e Ni(II), Zn(II) and Cd(II) butcomparatively lower rejection than Cu(II) ion. Blendmembranes with 25 wt% PU exhibited lower rejection

and higher flux as expected (Figs. 13 and 14). In allthe cases, metal ions chelated to PEI showed betterrejections as compared to pure metal ion solutions asfeed, due to complex formation with PEI, based onthe John Tellar distortion effect [27].

4. Conclusions

Cellulose acetatepolyurethane blend membraneswere effectively used for aqueous separation of metalions and proteins by ultrafiltration. The non-solventswelling agent, polyvinylpyrrolidone, plays a key role

in controlling the pore size and miscibility of the aboveblends. It was also observed to have optimised thehydrophilic nature and permeability/permselectivity

balance. These novel blend membranes showed betterresults for the separation of proteins and metal ions.

Acknowledgements

The authors are thankful to AICTE for financialassistance, Chemplast(I) Ltd, Chennai for supplyingthe polyurethane sample for this study. The authors(MS, RM and CJS) thank CSIR, New Delhi, India forthe grant of Senior Research Fellowship.

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