Desalination 333 (2014) 45–51

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Desalination

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PBI-BuI and PAN-PSSALi based UF membranes: Effects of solute andmembrane surface interactions on rejection and flux

Deepti Bhagat 1, Bhavana Mule 1, Neeraj Mandlekar 1, Kiran Pandare 1, Ulhas Kharul ⁎Polymer Science and Engineering Division, CSIR-National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, India

H I G H L I G H T S

• Ultrafiltration type of porosity generated in PBI-BuI based membranes• H-bonding interactions between PEG and PBI led to considerable fouling and flux reduction.• Presence of PEG on PBI membrane surface was established by different tools.• PAN-PSSALi membrane surface showed lower PEG-membrane interactions.

⁎ Corresponding author. Tel.: +91 20 2590 3015; fax: +E-mail address: uk.kharul@ncl.res.in (U. Kharul).

1 Tel.: +91 20 2590 3015; fax: +91 20 2590 2615.

0011-9164/$ – see front matter © 2013 Elsevier B.V. All rihttp://dx.doi.org/10.1016/j.desal.2013.11.036

a b s t r a c t

a r t i c l e i n f o

Article history:Received 21 August 2013Received in revised form 16 November 2013Accepted 19 November 2013Available online 15 December 2013

Keywords:UltrafiltrationPolybenzimidazoleRejectionGel permeation chromatographySolute adsorption

Ultrafiltration membrane using tert-butylpolybenzimidazole (PBI-BuI) was prepared and characterized for fluxand rejection performance usingGel Permeation Chromatography (GPC). Polyethylene glycol (PEG) and polyeth-ylene oxide (PEO)with different molecular weights were used as the solutes. While using feed solution contain-ing mixture of PEGs, higher rejection was observed than using individual PEG. The water flux of PBI-BuImembrane after passing individual PEG solutions showed considerable (~36%) reduction, which could be attrib-utable to the PEG adsorption on themembranepore surface. PEG adsorptionwas further substantiated by SEM, IRand TGA. The amphoteric nature of PBI-BuI could cause H-bonding between membrane surface and PEG mole-cules, leading to PEG adsorption on the membrane and pore surface. To ascertain this postulation, a study withPAN-PSSALi (which does not contain H-bonding) based UF membrane containing negatively charged \SO3−

groupwas done. It was found that PEG adsorption in this case is not as predominant as in earlier case. Thismem-brane showed marginal reduction in water flux of 8%, vis-à-vis 36% reduction shown by PBI-BuI based mem-brane. This indicated that H-bonding present in PBI-BuI is mainly responsible for the PEG adsorption on itsmembrane and pore surface, in spite of PEG being a neutral molecule.

© 2013 Elsevier B.V. All rights reserved.

1. Introduction

In porousmembranes, especially of ultrafiltration and nanofiltrationtype, flux and rejection properties are primarily governed by pore size,their density and surface morphology. Chemical nature of polymerused for making the membrane plays a key role in governing these pa-rameters. It has been well documented in the literature that water-soluble macromolecules adsorb readily on the polymeric ultrafiltrationmembranes [1]. Polybenzimidazole (PBI) to be used as amembranema-terial is attracting considerable attention due to its excellent thermo-chemical and mechanical stability [2–4]. Although polybenzimidazolebased membranes are known in the literature, they are mostlynanofiltration (NF) type [2,5,6]. Rejection analysis of these membraneswas carried out using neutral solutes with the help of Total Organic

91 20 2590 2615.

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Carbon (TOC) analysis as an analytical tool [2,5,6]. Recently, Xing et al.could obtain PBI-based ultrafiltration membrane while casting mem-brane at elevated temperature. The rejection analysis of these mem-branes was done using TOC [7].

The determination of rejection/molecular weight cut off (MWCO) isone of the popular methods to characterize the membrane pore size.The MWCO is obtained by plotting rejection of selected solutes versustheir molar mass [8]. The solutes have to satisfy certain criteria such asminimal interactions with the membrane surface, availability of incre-mental molar masses, good solubility in the solvent used for membranecharacterization (usuallywater) and narrowmolecularweight distribu-tion. Various solutes andmethods have been suggested for the determi-nation of MWCO of porous membranes [9]. These need to be employedunder different analytical conditions, depending upon the nature of thesolutes used. Commonly used solutes are polyethylene glycols (PEGs)[8], dextrans [9], proteins [10], cephalexin [2] etc.; while majorly usedanalytical tools are UV spectroscopy [10], Gel Permeation Chromatogra-phy (GPC) [9,11], HPLC with evaporative light scattering detection [12],

46 D. Bhagat et al. / Desalination 333 (2014) 45–51

TOC [2,6,7,9], conductivity [5], etc. GPC has been extensively used fordetermining the solute concentration and thereby MWCO to evaluateporosity of membranes prepared using various polymers such as PS,PES, PVDF, cellulose acetate [9], etc.

Platt et al. [8] and Tam et al. [11] have reported two methods to de-termine the solute rejection. The first method involved successive per-meation of several solutions, each containing a single solute. Thistechnique is time consuming. The second method involved passing amixture of solutes (having different molar masses) in a single solution.The advantage of this method is that a single experiment is sufficient todetermine the solute rejection of varying molecular weights. However,differences have been reported between these methods [8,11]. The ad-vantage offered by the latter (mixed solute method) being faster wasmasked due to the measured cut-off value being under-estimated andvalues obtained by the first method are always lower in comparison tothose obtained by the second method [8]. Authors have attributed thisbehavior to the increased solute concentration on the membrane sur-face leading to observed retention values being lower than the actualones. When the mixtures were injected, the concentration polarizationaffected the selectivity. The higher molecular weight solutes areretained completely and this resulted in higher rejection of the lowmo-lecular weight solutes [8].

In view of potential of PBI as a porous membrane material [2,5–7],present study focuses on investigating adsorption behavior of PEG on anew membrane based on polybenzimidazole containing t-butyl group(PBI-BuI), porous membranes of which are not reported in the litera-ture. One of its monomers, dicarboxylic acid possesses t-butyl groupand exhibits better solvent solubility than the widely used PBI basedon isophthalic acid [13]. While analyzing rejection performance, PBI-BuI based membrane was found to exhibit interactions even withthe neutral PEG molecules. This was validated by using different ana-lytical tools (SEM, FTIR and TGA). In order to further substantiate thisfinding, flux and rejection analysis of another membrane material,which does not exhibit H-bonding in its chemical structure; viz.,PAN-PSSALi; was chosen. It exhibits distinct ionic character (negativecharge by virtue of SO3

− group) without any native hydrogen capableof H-bonding. The membranes were made by phase inversion methodand analyzed by using PEG and PEO possessing different molecularweights.

2. Materials and methods

2.1. Materials

3,3′-Diaminobenzidine (DAB), 5-tert-butylisophthalic acid (BuI), poly-ethylene glycols (PEGs) of differentweight averagemolecularweight (0.6to 35 kDa), polyethylene oxide (MWof 100 and 200 kDa), lithium salt ofpoly(4-styrenesulfonic acid) (PSSALi), N,N-dimethylacetamide (DMAc),N-methyl-2-pyrrolidinone (NMP) and sodium nitrate (NaNO3) wereprocured from Aldrich Chemicals. Polyacrylonitrile (PAN) was receivedfrom Indian Petroleum Corporation Limited (IPCL) (Vadodara, India;viscosity averaged molecular weight = 41,000 g/mol). Nonwovenfabrics used as support for membrane making were based on poly-propylene (FO-2470, procured from Freudenberg) and polyester(Hollytex-3324, procured from Ahlstrom USA). Polyphosphoric acid

Scheme 1. Structures of m

(PPA) was obtained from Alfa Aesar. Conc. H2SO4 (98%) and NaOHwere procured from Thomas Baker. Sodium hydrogen carbonate(NaHCO3) was procured from Merck. Lithium chloride (LiCl) was pro-cured from S. D. Fine Chemicals (India); while zinc chloride (ZnCl2,GR grade) was procured from M/s. Loba Chemie. All these chemicalswere of AR/LR grade and used without further purification.

2.2. Synthesis of PBI-BuI

Polybenzimidazole (PBI-BuI) based on DAB and BuI was synthesizedby solution polycondensation as reported earlier [13] (Scheme 1).

Typically, a three-necked round bottom flask equipped with me-chanical stirrer, N2 inlet and an outlet guarded with CaCl2 drying tubewere charged with 2400 g of PPA and heated to 120 °C under constantflow of N2. An 80 g (0.3734 mol) of DAB was added while stirring.After its dissolution, 82.98 g (0.3734 mol) of BuI was added; tempera-ture was raised to 170 °C and maintained for 5 h. The temperaturewas further raised to 210 °C and maintained for further 14 h. The poly-mer was obtained by precipitating the reaction mixture into stirredwater. The precipitated polymer was crushed and washed with water,aq. NaHCO3 and again water; till the filtrate was neutral to pH. Obtainedpolymer was soaked in acetone for 16 h, collected by filtration and wasdried at 100 °C, for 3 days. It was further purified by dissolving in DMAc(2.5 wt.%), centrifugation (at 2800 rpm for 3 h) and reprecipitation instirredwater. Itwasdried at 100 °Cunder vacuum for aweek and storedin the desiccator until use.

2.3. Membrane preparation

2.3.1. Preparation of membrane based on PBI-BuIThe dope solution of PBI-BuI was prepared using 64 g of dried

polymer, 400 ml of NMP as the solvent and 1.28 g of LiCl as an addi-tive. The solution was stirred under dry atmosphere at 100 °C for48 h. It was then allowed to cool at room temperature and degassedto remove entrapped air. The solution was centrifuged at 12000 rpmfor 30 min. The membrane was casted on a nonwoven support fabric(FO-2470) using a pilot scale continuous membrane casting facility at6 °C gelation temperature and 65 °C curing temperature. The knifegap was adjusted to 350 μm and casting speed was set to 0.5 m/min.This membrane was kept in water for overnight in order to removeexcess of the solvent and then stored with 0.5% formalin solution at4 °C until use.

2.3.2. Preparation of membrane based on PAN-PSSALiA solution containing 24.64 g of ZnCl2, 535.36 g NMP and 56 g

PSSA-Li was prepared while stirring for 24 h. To this solution, vacuumdried PAN powder (184 g) was added and stirring continued forfurther 48 h under dry conditions. The formed dope solution wasdegassed and then centrifuged at 2800 rpm for 2 h. The membranewas casted on a moving nonwoven support fabric (Hollytex-3324)with a knife gap of 350 μm using the pilot scale continuous mem-brane casting facility at 20 °C gelation temperature and 40 °C curingtemperature. This membrane was kept in water for overnight in orderto remove solvent and then stored with 0.5% formalin solution at 4 °Cuntil use.

onomers and PBI-BuI.

Fig. 1.GPC chromatogram of PEGmixture and single component PEGs using (a) single col-umn and (b) three columns in series.

47D. Bhagat et al. / Desalination 333 (2014) 45–51

2.4. Membrane characterization

2.4.1. Measurement of water fluxWater flux was measured using a dead-end stirred cell assembly

possessing active area of 14 cm2 at 2 bar upstream pressure, whilemaintaining the permeate side at ambient. Initially, 50 ml of distilledwater was allowed to permeate through the membrane in order to re-move the formalin (with which the membrane was stored) and thenwater flux was measured while maintaining 600 rpm as the stirringspeed. To check the reproducibility, 5 membrane samples were ana-lyzed under identical conditions.

2.4.2. Rejection analysis of PEG using Gel Permeation Chromatography(GPC)

The rejection analysis of membranes was performed using PEG andPEO of differentmolecular weights. These solutes with theweight aver-agemolecularweight of 0.6, 2, 4, 10, 20, 35, 100 and 200 kDawere used.Two types of feed solutions were prepared with a total concentration of1 g/l in DI water; as this concentration is commonly being used for therejection analysis [8,9]. In the first type, a solution containing mixtureof PEGs was used, while in the second type, solution of only one PEGwas used at a time.

A 75 ml of PEG solution was charged to a dead end stirred cell as-sembly and the permeate was collected at 2 bar upstream pressurewhilemaintaining 600 rpm as the stirring speed. Initial 0.5 ml of the fil-trate was discarded and 3 ml was collected for the analysis. Beforecharging the stirred cell by another PEG solution, membrane waswashed with 0.1 M NaOH solution in order to minimize the fouling.This was performed by charging the cell with 75 ml of NaOH solutionand stirring for 5 min at ambient pressure, followed by collecting10 ml of the permeate at 2 bar upstream pressure; so that the PEGadsorbed on the membrane as well as on the pore surface can be re-moved. This was followed by water wash in a stirred cell itself, till thepermeate was neutral to pH.

The concentration of the PEG in the feed/permeate sample was ana-lyzed by GPC (Waters, equipped with autosampler, interfaced to a Wa-ters 410 RI detector driven by Empower software). GPC columns usedwere Ultrahydrogel 250 (dimensions: 7.8 × 300 mm; 500 Å pores;10 μm particles), 120 column (dimensions: 7.8 × 300 mm; 120 Åpores; 6 μmparticles) and single Ultrahydrogel linear. The feed and per-meate samples to be analyzed were filtered through 0.22 μm syringetype filter and then charged to the autosampler of GPC. All the analyseswere carried out at 35 °C, using 0.1 MNaNO3 solution in DIwater as themobile phase with a flow rate of 1 ml/min. Area under the peak wasused to determine the concentration of feed and permeate solutions.The percent rejection (%R) was calculated by using following equation,

%R ¼ 1− CP

C f

!" #� 100

where, Cp is the concentration of permeate, while Cf is the feed concen-tration. In order to check the reproducibility, 5membrane sampleswereanalyzed under identical conditions and variation in the rejection isshown in Figs. 2 and 6. The rejection analyzed using mixture of PEGs(Set I) and single PEG (Set II) as a solute in the feed solution is shownin Fig. 2. Themolecular weight cut-off (MWCO) of amembranewas de-duced from the characteristic rejection at 90%.

2.4.3. Scanning Electron Microscopy (SEM), FTIR and TGACross-section of membranes was analyzed by SEM (Leica,

Stereoscan, 440). The samples were prepared by fracturing at liquid ni-trogen temperature and then vacuum dried at ambient temperature for24 h. These samples were gold sputtered using vacuum electricsputtering device (AMITEC) before being mounted onto the samplestud. The accelerating voltage used was 20 KeV. The IR images of the

parent and PEG fouled membranes (before and after washing) weretaken in ATR mode (PerkinElmer UATR).

Thermogravimetric analysis (TGA) of PBI-BuI, PAN-PSSALi andupper layer of the membrane fouled by PEG was performed using thePerkinElmer TGA model ST 5000 under N2 atmosphere at temperature50–900 °C with a heating rate of 10 °C/min. The polymer was scrapedfrom the membrane coupon using silicon carbide abrasive rod in sucha way that it was free of base material (nonwoven support). For com-parison, TGA of the pure PEG (35 kDa) was also performed.

3. Results and discussion

3.1. Analysis of PBI-BuI based membranes

3.1.1. Water fluxThe average water flux at 2 bar upstream pressure was found to be

135 (±12) l·m−2·h−1. PBI-based nanofiltration type ofmembranes re-ported in the literature exhibited water flux of 1.7–3.58 l·m−2·h−1 at1 bar [2,14]. While assuming the linear relation between the waterflux and applied pressure, flux of PBI-BuI membrane is considerablyhigher than the reported flux of PBI membranes. This indicated thatthe present membrane falls under ‘ultrafiltration’ category and not theNF. In order to further determine the molecular weight cut off(MWCO) of this membrane, PEG is chosen as a solute, which is widelyreported in the literature for analyzing rejection of membranes pre-pared using different polymers such as PS, PES, PVDF, cellulose acetate,

Fig. 2. Comparison of rejection using single and mixture of PEGs in feed for PBI-BuI mem-brane and reduction in water flux (Jw) with molecular weight of PEG.

48 D. Bhagat et al. / Desalination 333 (2014) 45–51

etc. [9]. PEGs are usually chosen to characterize membranes, since theyare easily water soluble, available readily with fairly narrow molecularweight distributions, neutral in nature and thus are anticipated tohave low adsorption on the polymeric membrane surface [15]. Inorder to determine the rejection of present membranes, GPC methodwas needed to be developed for the quantification of PEG present infeed and permeate samples, as discussed below.

3.1.2. Method developmentThe behavior of columns towards analyzing PEG solution is given

in Fig. 1. Initially, PEG sample (1 g/l) was injected on a singleUltrahydrogel GPC column and the retention time for individual PEGwas determined (Fig. 1a). In order to save the time of experimentationand minimize the membrane fouling, it is advisable that the feed solu-tion contain mixture of PEGs of varying molecular weights, ratherthan analyzing individual PEG solutions [8,9]. Hence, a mixture contain-ing equal proportion of PEGs (viz., 35, 20, 10, 4, 2 and 0.6 kDa) with atotal concentration of 1 g/l was injected.

As could be seen from Fig. 1a, the chromatogram (i) containingmix-ture of PEGs was poorly resolvable, where peaks were highly merged.This chromatogram corresponding to the mixture of six PEGs of differ-ent molecular weights showed only three intermixed peaks instead ofanticipated six peaks. In order to obtain better resolution, the mixturewas further splitted into two sets, such that their molecular weights re-mainwell apart fromeach other. A chromatogram (ii) of Set I containinga mixture of PEGs possessing molecular weight of 35, 10 and 2 kDa(well apart from each other) when compared with chromatograms ofindividual PEGs (iii–v); it was found that though peaks could be evidentin chromatogram (ii) and better resolvable than in case of chromato-gram (i); they were shifted from the peak positions of individualPEGs. Similar behavior was observed with set II comprising mixture of

Fig. 3. SEM of a) parent PBI-BuI membrane, b) PEG fouled memb

PEGs possessing molecular weight of 20, 4 and 0.6 kDa (not shown inthe Figure due to similar complexity). Similar complex behavior was re-ported by Christel et al. [9]; who have used TSK G-4000PW column andfound that the retention times of PEGswere inconsistentwhen themix-ture of PEGswas injected. One of the reasons for this peak shifting couldbe the association of PEGs in the solution, whichmay be beyond resolu-tion capacity of a single column.

In order to increase the resolution further, instead of using single col-umn for analysis, three columnswere connected in series (ultrahydrogelcolumns, as given in Section 2.4.2). When the PEG mixture (Set II) wasanalyzed, PEG retention times (chromatogram ‘i’ in Fig. 1b)were similarto that of single PEG component (chromatograms ii, iii and iv in Fig. 1b).Moreover, it was better resolvable than in the single column (chromato-gram ‘ii’ in Fig. 1a) and similar retention times were obtained as thatof the single component PEG. Hence, this protocol was followed forthe rejection analysis.

3.1.3. Rejection and MWCO analysisResults obtained after passing PEG mixture (Set I and II, sequential-

ly) through the membrane and subsequent analysis by a series of threecolumns are shown in Fig. 2.

It was found that there is no correlation between the values of PEGrejection with its molecular weight when solution containing mixtureof PEGs was used. For example, rejection of PEG with molecular weightof 2 kDawas found to be higher than that of PEGwithmolecularweightof 35 kDa. This is unacceptable in view of their large difference in themolecular weights. This observation could be attributable to the multi-ple reasons, such as (i) solute–solute interactionwhen PEGmixturewasfed to the membrane, (ii) fouling of the membrane by PEG (which iswell known in the literature) [8,11], etc. This fouling could also begoverned by themolecularweight of PEG. This observation necessitatedthe use of single PEG in the feed for analyzing the rejection of a mem-brane, than using themixture of PEGs in a single solution. Thus, a singlecolumnwas used instead of three columns in series, tominimize time ofanalysis. This variation in mode of operation has resulted in acceptablerejection values of PEGs,which now can bewell correlatedwith themo-lecular weight of respective PEG used in the feed (Fig. 2).

It was also observed that the rejection of a particular PEGwas higherwhen passed in a mixture than that of single component (with an ex-ception of PEG with molecular weight of 20 kDa, which could be anartifact). Such higher rejection in case of PEG mixtures used in feed; incomparison to the single PEG in the feed is reported in the literaturefor other polymermembranes aswell andwas attributed to the adsorp-tion phenomenon [9,16]. The MWCO of the present PBI-BuI membranewas found to be ~26 kDa.

Although in between two PEG solutions, a membrane washing pro-tocol (0.1 M NaOH, followed by water wash; as given in Section 2.4.2)was employed; a regular decline inwater flux of themembranewas ob-served (blue colored curve in Fig. 2), while analyzing PEG in the orderof their decreasing molecular weight. After passing five individual PEGsolutions (single component), the initial flux of the membrane was

rane and c) PEG fouled membrane at higher magnification.

Fig. 5. TGA spectra of a) PBI-BuI, b) PEG and c) scratched PBI-BuI sample from PEG fouledmembrane.

49D. Bhagat et al. / Desalination 333 (2014) 45–51

reduced by ~36%. This conveys that in spite of washing with NaOH andwater after every PEG; considerable fouling of the membrane pore sur-face had occurred. This decrease in water flux could be the effect of thebuilt-up of PEG within the pores. Observed reduction in the water fluxwas considerably higher than the similar reduction reported in the liter-ature for other polymeric membrane based on regenerated cellulose,polyethersulfone, cellulose acetate, polysulfone and polyvinyl difluoride[8,9]. In order to investigate the reasoning behind this observed largedecrease in water flux, the fouled membrane samples were subjectedto SEM, FTIR and TGA characterizations and the findings are describedbelow.

3.1.4. Analysis of fouled PBI membrane by SEM, FTIR and TGAFig. 3a shows SEM images of PBI-BuI membrane which is not ex-

posed to PEG, while Fig. 3b and c are SEMs of PEG fouled membrane atdifferent magnification.

The SEM image of parent membrane (Fig. 3a, which is unexposed toPEG) is cleaner than the later two images of PEG-fouled membrane.Since the sample preparation for SEM analysis was done under identicalcondition, it could be speculated that the observed difference could beattributable to PEG deposited on the membrane surface. Since SEManalysis may not be conclusive enough, FTIR analysis of the membranesurface was performed as given in Fig. 4.

Fig. 4a shows the FTIR spectrum of parent PBI membrane (beforepassing PEG solution) resembles with the spectra of PBI-BuI reportedearlier [13]. Fig. 4b shows spectrum of PEG-35 kDa; inwhich prominentbroad band at ~3654 cm−1 could be attributable to the\OH stretching(as well as due to the absorbed moisture, if any); a band at 2899 couldbe ascribed to the C\H stretching; while a band at 1105 cm−1 is attrib-utable to the C\O stretching vibrations. The spectra of membrane afterpassing PEG-35 kDa (Fig. 4c) showed broad band at the C\H vibrationregion (2842 to 3035 cm−1) and a prominent band at 1100 cm−1,owing to C\O vibrations. Since these bands were absent in the case ofparent PBImembrane (Fig. 4a), it could be concluded that they are orig-inating from the PEG adsorbed on the membrane/pore surface. Thespectrum of membrane after washing with NaOH followed by water(Fig. 4d) showed that the intensity of aliphatic C\H vibration band(2969 cm−1) is reduced; but is still higher than that for the parentPBI. In addition, the band at 1108 cm−1 is also seen; intensity of

Fig. 4. FTIR spectra of a) parent PBI-BuI membrane, b) PEG, c) PEG fouled membrane andd) membrane after washing.

which is again found to be reduced than in case of Fig. 4c. This infersthat though NaOH followed by water wash could help in removingPEG, it was not completely removed. This remaining PEG could just bephysically occluded.

Since FTIR analysis could predominantly offer surface analysis, inorder to further support presence of PEG in the membrane pores, TGAof membrane samples (which is truly a bulk analysis) was performed.Fig. 5 shows TGA of parent PBI-BuI (Fig. 5a), PEG (Fig. 5b) and that ofscratched surface sample using Si–C rods from the PEG fouled mem-brane surface (Fig. 5c).

The degradation pattern of (a) and (b) is in agreementwith the deg-radation temperature of PBI and PEG, respectively, as reported in theliterature [13,17]. TGA of PEG fouled sample (c) showed onset of initialweight loss at ~200 °C attributable to the absorbed water. Such waterabsorption by PBI is known earlier [13,17], which is also seen in thepresent case, though to a small extent (since PBI-BuI sample used foranalysis was vacuum dried). In the spectra 5c, the next degradation be-ginning at ~ 400 °C could be attributable to the degradation of adsorbedPEG present on the membrane surface. This is in accordance with thedegradation temperature of PEG (curve b in Fig. 5). The weight loss atthis temperature is about ~9%, which indicates that a substantialamount of PEGwas adsorbed on thePBI-BuImembrane and its pore sur-face. The degradation of this sample beginning at ~550 °C resembleswith the degradation of PBI, as could be seen in case of parent PBI-BuIsample as well (Fig. 5a).

Fig. 6. PEG and PEO rejection for PAN-PSSALi membrane and reduction in water flux (Jw)with molecular weight of PEG.

Fig. 7. SEM of a) parent PAN-PSSALi membrane, b) PEG fouled membrane and c) PEG fouled membrane at higher magnification.

50 D. Bhagat et al. / Desalination 333 (2014) 45–51

The large amount of PEG adsorbed on the PBImembrane and its poresurface (~9%), which remained even after washing with NaOH andwater (supported by IR spectra) could be a reason for the large reductionin water flux as seen from Fig. 2. This behavior is significantly differentthan that of membranes prepared from PS, PES PVDF, cellulose acetateetc. [9], where such significant PEG adsorptionwas not reported. The ad-sorption of PEG on the membrane and pore surface could be explainedon the basis of H-bonding between PBI-BuI and PEG. Amphoteric natureof PBI due to N–H functionality of imidazole group belonging to its re-peat unit is well known [2]. This peculiarity of polybenzimidazolescould be a predominant reason for the observed fouling of itsmembraneand its pore surface by PEG; as reflected by a large reduction in thewaterflux. The pore surface fouling would reduce the pore size and thus thewater flux.

In order to validate the hypothesis of membrane fouling due to H-bonding of PBI-BuI with PEG, a membrane with negatively charged sur-face by virtue of \SO3Li functionality in PAN-PSSALi based membranewas analyzed as given below. Since this membrane is negativelycharged and there is no hydrogen (as like in PBI-BuI case), H-bondingon its surface is anticipated to be absent.

3.1.5. Analysis of PAN-PSSALi membraneWater flux of 5 samples of PAN-PSSALi membrane was measured

using dead-end stirred cell assembly at 2 bar upstreampressure, similarto the PBI-BuImembrane case, as explained in Section 2.4.2. The averagewater flux of this membrane was 24 (±1) l·m−2·h−1. The rejection ofPEG and PEO for this membrane is shown in Fig. 6.

Fig. 8. FTIR spectra of a) parent PAN-PSSALi membrane, b) PEG, c) PEG fouled membraneand d) membrane after washing.

The MWCO for this membrane was found to be 113 kDa. The varia-tion in rejection among different membrane samples was representedby error bars. In comparison to the PBI-BuI based membrane, PAN-PSSALi membrane showed smaller decrease in water flux after passingPEGs during MWCO determination (Fig. 6, blue curve). After exposureto all these PEGs, reduction in the water flux was just 8%; while itwas ~ 36% in the case of PBI-BuI basedmembrane (Fig. 2). This indicatesthat the fouling by PEG in PAN-PSSALimembrane ismuch lesser as com-pared to the PBI-BuI based membrane. This could be ascribed to thepresence of negative charge present on the PAN-PSSALi based mem-brane surface, which is anticipated to remain neutral, if not possess re-pulsive interactions with PEG/PEO; vis-a-vis the H-bonding basedfouling in the case of PBI-BuI based membrane by PEG.

The fouling observed in the case of PAN-PSSALi membrane could bephysical in nature; where solutes are occluded due to physical entrap-ment in the pores and lead to observed reduction in the water flux. Un-like in PBI-BuI case, SEMof PAN-PSSALi basedmembrane (Fig. 7) did notshow significant deposition of the solute on the membrane surface.

The IR spectra of fouled (Fig. 8c) membrane did not show any bandoriginating from adsorbed PEG (1105 cm−1 and 2899 cm−1; as seenfrom Fig. 8b). It may be recalled that IR spectra of fouled PBI-BuI mem-brane (Fig. 4) had shown prominent bands due to adsorbed PEG. Thespectrum of PAN-PSSALi membrane that had undergone washing treat-ment (Fig. 8d) indicates that themembrane surface chemistry is similarto the parent untreated membrane (Fig. 8a).

The TGApattern of fouled and parent PAN-PSSALimembrane (Fig. 9)is similar to each other. These two analyses are supportive to the factthat PAN-PSSALi membrane does not exhibit substantial fouling; asthat was observed for PBI-BuI based membrane.

Fig. 9. TGA spectra of a) PAN-PSSALi and b) scratched PAN-PSSALi sample fromPEG fouledmembrane surface.

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4. Conclusions

A GPC method was needed to be developed for the accurate estima-tion of PEG concentration in feed and permeate samples used for deter-mining rejection of membranes based on PBI-BuI and PAN-PSSALi. Therejection performance of the PBI-BuI based membrane obtained usingsingle PEG in the feed solution was found to better correlate with themolecularweight of PEG used; than in the case of feed solution contain-ing mixture of PEGs. The sequential passage of PEGs through PBI-BuImembrane significantly lowered the water flux and indicated substan-tial membrane fouling. Adsorption of PEGs on the PBI-BuI membranesurface was assessed by SEM, FTIR and TGA and was ascribed to H-bonding. To ascertain this, PAN-PSSALi membrane possessing negative-ly charged sulfonic acid functionality was analyzed.While doing succes-sive PEG analysis, considerable difference in behavior of reduction inwater fluxwas observed than that for PBI-BuI basedmembrane. This re-vealed that membrane material interaction with solute molecules is re-sponsible for fouling. Although the use of PEGs as a neutral solute foranalyzing rejection is well accepted, our findings revealed that a differ-ent behaviorwas observedwhen PEGwas used as a solute for analyzingrejection performance of the PBI-BuI based membrane.

Acknowledgment

Financial assistance from the Council of Scientific and Industrial Re-search, Ministry of Science and Technology (CSC 0122) and the Depart-ment of DrinkingWater Supply,Ministry of Rural Development, Govt. ofIndia (No. W-11035/18/2008-R&D) is duly acknowledged.

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