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Effect of PVP Molecular Weights on the Properties ofPVDF-TiO2 Composite Membrane for Oily WastewaterTreatment ProcessC. S. Ongab, W. J. Lauab, P. S. Gohab, B. C. Nga, T. Matsuuraac & A. F. Ismailab

a Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia,Johor, Malaysiab Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia,Johor, Malaysiac Industrial Membrane Research Laboratory, Department of Chemical and BiologicalEngineering, University of Ottawa, Ottawa, ON, CanadaAccepted author version posted online: 28 Jul 2014.Published online: 30 Sep 2014.

To cite this article: C. S. Ong, W. J. Lau, P. S. Goh, B. C. Ng, T. Matsuura & A. F. Ismail (2014) Effect of PVP MolecularWeights on the Properties of PVDF-TiO2 Composite Membrane for Oily Wastewater Treatment Process, Separation Science andTechnology, 49:15, 2303-2314, DOI: 10.1080/01496395.2014.928323

To link to this article: http://dx.doi.org/10.1080/01496395.2014.928323

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Separation Science and Technology, 49: 2303–2314, 2014Copyright © Taylor & Francis Group, LLCISSN: 0149-6395 print / 1520-5754 onlineDOI: 10.1080/01496395.2014.928323

Effect of PVP Molecular Weights on the Properties of PVDF-TiO2Composite Membrane for Oily Wastewater Treatment Process

C. S. Ong,1,2 W. J. Lau,1,2 P. S. Goh,1,2 B. C. Ng,1 T. Matsuura,1,3 and A. F. Ismail1,2

1Advanced Membrane Technology Research Centre (AMTEC), Universiti Teknologi Malaysia, Johor, Malaysia2Faculty of Petroleum and Renewable Energy Engineering, Universiti Teknologi Malaysia, Johor, Malaysia3Industrial Membrane Research Laboratory, Department of Chemical and Biological Engineering, University of Ottawa,Ottawa, ON, Canada

Polyvinylidene fluoride (PVDF) hollow fiber ultrafiltrationmembranes consisted of TiO2 and different molecular weight (Mw)of polyvinylpyrrolidone (PVP) (i.e., 10, 24, 40, and 360 kDa) wereprepared to treat synthesized oily wastewater. The membraneperformances were characterized in terms of pure water flux,permeate flux, and oil rejection while their morphological prop-erties were studied using SEM, AFM, and tensile tester. Resultsshow that the PVDF-TiO2 composite membrane prepared fromPVP40k was the best performing membrane owing to its promis-ing water flux (72.2 L/m2.h) coupled with good rejection of oil(94%) when tested with 250 ppm oily solution under submergedcondition. It is also found that with increasing PVP Mw, themembrane tended to exhibit higher PVP and protein rejection,greater mechanical strength, smaller porosity, and a smoothersurface layer. Regarding the effect of pH, the permeate fluxof the PVDF-PVP40k membrane was reported to increase withincreasing pH from 4 to 7, followed by decrease when the pHwas further increased to 10. Increasing oil concentration in thefeed solution was reported to negatively affect the water flux ofPVDF-PVP40k membrane, owing to the formation of thicker oillayer on the membrane surface which increased water transportresistance. A simple backflushing process on the other hand couldretrieve approximately 60% of the membrane original flux with-out affecting the oil separation efficiency. Based on the findings,the PVDF-TiO2 membrane prepared from PVP40k can be poten-tially considered for oily wastewater treatment process due to itsgood combination of permeability and selectivity and reasonablyhigh water recovery rate.

Keywords polyvinylidene fluoride; polyvinylpyrrolidone; ultrafiltra-tion; oily solution; hydrophilicity; submerged membrane;titanium dioxide

Received 7 January 2014; accepted 22 May 2014.Address correspondence to W. J. Lau, Advanced Membrane

Technology Research Centre (AMTEC), Universiti TeknologiMalaysia, Johor, Malaysia. E-mail: lwoeijye@utm.my or lau_woeijye@yahoo.com

Color versions of one or more of the figures in the article can befound online at www.tandfonline.com/lsst.

INTRODUCTIONOily wastewater possesses potential risk to the water envi-

ronment due to the existence of many hazardous hydrocarbonmixture, chemical components, and heavy metals. To pre-vent the severe pollution in water environment, a stringent oildischarge standard is established in different countries. Forinstance, in Malaysia, the effluent discharged from industrialsectors should comply with the national primary regulatory ofdischarged standard as stipulated in the Environmental QualityAct (EQA), 1974. Figure 1 compares the maximum oil dis-charged concentration set by different countries. As can beseen, the maximum oil concentration of effluent discharge isvaried from one country to another and this could be due tothe geographical condition and the characteristics of the origi-nal wastewater. Compared to other countries, Malaysia is foundto have much lower tolerance value for the oil discharge limitconcentration, i.e., 10 ppm in comparison to 40 ppm found inthe United States and Russia and 100 ppm in Indonesia, SaudiArabia, and Qatar. Thus, more exploration in oily wastewatertreatment is required to meet the stringent local discharged limitand to deal with increasing global oil demand.

As documented in literature, there are many oil separatingmethods reported so far concerning the efficiency of the oilrefinery wastewater treatment process. These include gravitysettling, centrifugation, air flotation, fibrous/packed bed coa-lescence, and membrane-based technologies (1–7). However,the increasing global oil demand has made these treatment pro-cesses very challenging. In particular, conventional treatmentmethods are mostly useful for free oil solution with oil dropletssize > 150 µm or unstable oil-water emulsion with droplet sizesbetween 20 and 150 µm. In order to separate much smaller oildroplets from stable oil-water emulsion, membrane technolo-gies are generally considered as attractive technique to sub-stitute the existing conventional treatment. Furthermore, it hasbeen previously reported that for oil and gas produced treatmentprocess, membrane technologies require significantly lowerenergy consumption and operating cost (i.e., $0.08–0.34/bbl)than those of conventional technologies ($0.1–5/bbl) (8, 9).

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FIG. 1. Maximum effluent discharge oil concentration for produced water in different countries [26].

The low energy consumption of membrane technologies, in par-ticular ultrafiltration (UF), is mainly attributed to low operatingpressure (0.5–1 bar) applied throughout the operation period(10–13). Even though the operating pressure of UF is remark-ably lower than that of nanofiltration (NF) and reverse osmosis(RO) membranes, its water permeability achieved is still verypromising for industrial application, i.e., 25–350 L/m2.h.bar(10–13). However, some argued that small contaminants andstable oil-water emulsion might be difficult to remove usingmicroporous UF membrane compared to NF and RO mem-branes. In order to extend the uses of UF membrane on theseparation of stable oil-water emulsion, organic and inorganicadditives were used in this work and added into dope solutionwith the aims of producing desired properties of UF mem-brane for this particular process. A literature search revealedthat polyvinylpyrrolidone (PVP) is favored over other organicadditives, mainly due to its excellent pore forming ability andwide range of molecular weight (Mw) (14). Table 1 summarizessome of the previous studies on the use of PVP as additive toenhance the water permeability of pristine membranes made ofvarious polymeric materials (15–22). Usually, the enhancementin membrane water flux upon PVP addition is caused by theformation of finger-like or sponge-like structure in the mem-brane sublayer. It has been previously reported by researchersthat the higher the Mw of PVP used, the significant the sup-pression of macrovoids formation and the denser the skin layer,which leads to lower water flux (16, 21, 23). Nevertheless,considering the importance of both thermodynamic and kineticeffects during the phase inversion process, one should be care-ful about the concentration and Mw of PVP used, which aredependent on the polymer and solvent type used for dopepreparation.

To the best of our knowledge, the effect of four types of PVPwith different Mw (i.e., 10, 24, 40, and 360 kDa) on the mor-phology and performance of polyvinylidene fluoride (PVDF)membrane incorporated with titanium dioxide (TiO2) nanopar-ticles has not been reported so far. In view of this, an attempt

is made to investigate the effect of a wide range of Mw ofPVP (10–360 kDa) as additive on the properties of PVDF-TiO2 composite membrane for oily wastewater treatment undersubmerged conditions. In the selection of membrane-basedpolymer and other additives, PVDF blending with 2 wt% TiO2

was favorable due to its good combination of water flux andoil rejection as reported in our previous research work (25).In the present work, the changes in the morphological structuresand separation performances of the PVDF-TiO2 membranes arefurther investigated by varying Mw of PVP added. The bestperforming membrane is further evaluated for oily wastewaterprocess by varying the oil concentration and pH value of thesynthesized feed solution. A simple backflushing cleaning pro-cess is also performed on the membrane in order to investigateits flux recovery and oil rejection after the cleaning process.

EXPERIMENTALMaterials

PVDF (Kynar®760) pellets purchased from Arkema Inc.,

Philadelphia, USA were used as the main membrane form-ing material. N,N-dimethylacetamide (DMAc) (Merck, > 99%)was used as solvent to dissolve polymer without further purifi-cation. Polyvinylpyrrolidone (PVP) (10, 24, 40, and 360 kDa)purchased from Sigma Aldrich and titanium dioxide (TiO2)(Degussa P25, average particle size ∼21 nm) from Evonikwere used as additives to enhance PVDF membrane proper-ties. Bovine serum albumin (BSA, 67 kDa), egg albumin (EA,43 kDa), and trypsin (23 kDa) were purchased from SigmaAldrich. The cutting oil obtained from Ridge Tool Company,Ohio, USA was used to synthesize oily solution of different oilconcentrations.

Preparation of Membrane and Membrane Module18 wt% of PVDF was added into pre-weighed DMAc sol-

vent after being dried for 24 h in oven at 50◦C. The solutionwas then mechanically stirred at 600 rpm until all the polymeric

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EFFECT OF PVP MOLECULAR WEIGHTS FOR OILY WASTEWATER TREATMENT PROCESS 2305

TABLE 1Overview on the previous studies on the effect of different Mw of PVP on membrane properties

ReferenceMW of PVP

(kDa) aPolymer bSolvent Results on membrane properties

Jung et al. (17) 10-360 PAN DMSO Water flux was improved with addition ofPVP but it was decreased by increasingMw of PVP from 10 to 360 kDa.

Chakrabarty et al.(16)

24-360 PSf NMP,DMAc

Increasing Mw of PVP from 24 to 360 kDacould lead to lower flux due to thereduction of pore size caused by theswelling of PVP on the membranesurface layer.

Ochoa et al. (24) 40 and 360 PES DMF, THF Permeabilities were increased by addingPVP40 and 360 kDa into PES dopesolution.

Zhang et al. (22) 10-1300 PSf DMAc Both the porosity and pure water flux ofmembrane were increased by increasingthe Mw of PVP from 10 to 58 kDa butboth tended to decrease with furtherincrease in the MW of PVP.

Basri et al. (15) 10, 40 and 360 PES NMP PES membrane became more hydrophilicbut the pore size and porosity were notsignificantly affected upon PVP addition.

Lang et al. (19) 10, 24 and 58 PVB NMP The size and number of membrane poresdecreased with increasing Mw of PVP.

Sadrzadeh andBhattacharjee(21)

10 and 1300 PES NMP Applying PVP1300kDa in the PES/NMPsolution could result in formation ofdenser membrane skin layer which led todecrease in water flux.

Matsuyama et al.(20)

10-1300 PSf DMAc Addition of PVP with high Mw suppressedthe formation of macrovoids and tendedto increase the sponge layer-like structurein membrane sublayer.

aPAN = Polyacrylonitrile, PSf = Polysulfone, PES = Polyethersulfone, PVB = Polyvinyl butyral.bDMSO = Dimethyl sulfoxide, NMP = N-methyl-2-pyrrolidone, DMAc = Dimethylacetamide, DMF = Dimethylformamide, THF =

Tetrahydrofuran.

pellets were completely dissolved. It was followed by the addi-tion of 2wt% of TiO2 nanoparticles and 5 wt% of PVP into themixture (see Table 2). The dope solution was then ultrasoni-cated to remove any air bubbles trapped in the solution prior tothe spinning process. The viscosity of the dope solutions wasmeasured by a basic viscometer (Model: EW-98965-40, ColeParmer).

By using the solutions prepared, PVDF membranes of dif-ferent properties were fabricated using dry-jet wet spinningmethod as described elsewhere (27). The as-spun hollow fiberswere immersed into water bath for 2 days to remove the resid-ual solvent. Prior to air drying, the fibers were post-treated by

TABLE 2The dope formulation of PVDF-TiO2 hollow fiber

membrane with and without PVP additive

PVDF PVP TiO2 DMAc ViscosityMembrane (wt%) (wt%) (wt%) (wt%) (mPa.s)

PVDF (control) 18 − 2 80 5,375PVDF-PVP10k 18 5 2 75 10,804PVDF-PVP24k 18 5 2 75 11,986PVDF-PVP40k 18 5 2 75 17,118PVDF-PVP360k 18 5 2 75 27,882

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2306 C. S. ONG ET AL.

10 wt% glycerol aqueous solution for 1 day to minimize fibershrinkage and pore collapse. At last, hollow fibers were dried atroom temperature for 3 days before module fabrication.

A bundle of 60 hollow fibers with the length of approxi-mately 28 cm (total effective membrane area: 607 cm2) waspotted into a polyvinyl chloride (PVC) tube using epoxy resin(E-30CL Loctite

®Corporation, USA). The membrane module

was then left at room temperature for hardening before its pro-truding parts were cut and fixed into a PVC adaptor to completethe module preparation.

Preparation of Synthetic Oily SolutionThe synthetic oily solution was prepared by mixing distilled

water with commercial cutting oil (RIDGID Nu-Clear CuttingOil, #70835, Ridge Tool Company). The emulsion was pre-pared by keeping the oil-water mixture in a blender (Model:BL 310AW, Khind) for 2 min with an agitation speed of 50Hz atroom temperature. The size of oil droplets was measured usingZetasizer Nano ZS (Malvern Instrument Inc., Southborough,MA), with a refractive index of 1.47 and 1.333 for the oildroplets and dispersant (water), respectively. The oil dropletsize distribution of the synthesized solution was in the rangeof 0.4–2.6 µm with a mean particle diameter of 1.08 µm (±0.16), as shown in Fig. 2.

Filtration ExperimentThe separation performance was assessed by placing two U-

shaped hollow fiber membrane modules at the bottom of a sub-merged tank containing approximately 14 L of oily wastewater.To minimize the fouling effect, a constant air flow rate of5 L/min generated by an air compressor (Model: 2HP singlecylinder 24L tank, Orimas) was used to generate air bubbles

FIG. 2. Size distributions of oil droplets in synthetic oily solution (oil concen-tration = 250 ppm).

within the submerged tank through air diffuser installed under-neath the membrane modules. Water permeate was producedusing a peristaltic pump (Model: 77200-60, Masterflex L/S,Cole Parmer) by creating a vacuum condition on the permeateside. Both the vacuum pressure and pump flow rate were keptconstant at –15 inHg and 15 mL/min, respectively, through-out the experimental period. A 10 mL sample was taken fromthe permeate for sample analysis and the remaining perme-ate was recycled back to the tank. Three measurements weremade for each sample and then the average value was reportedtogether with its standard deviation (based on 95% confidencelevel). The membrane water flux (J) was determined accordingto Eq. (1).

J = Q

At(1)

where J is the water flux (L/m2h), Q is the quantity of the per-meate (L), A is the effective membrane area (m2), and t is thetime (h) to obtain the quantity of Q.

The performance of membranes was evaluated using differ-ent feed solutions by changing the oil concentration (250 ppm,500 ppm, and 1000 ppm) in the feed. The membrane oilrejection was then calculated using the following equation.

R =(

1 − Cp

CF

)× 100 (2)

where R is the oil rejection (%), Cp and CF are the concentrationof oil in the permeate (ppm) and the feed (ppm), respectively.The oil concentration in the permeate and feed was deter-mined using a UV-vis spectrophotometer (Model: DR5000,Hach) measured at a wavelength of 294 nm at which the max-imum absorption occurs. On the other hand, the PVP andprotein rejection of membrane were assessed using a lab-scalecross-flow permeation system as described elsewhere (28). Theexperiment was conducted at 1 bar using feed aqueous solutioncontaining either 1000 ppm BSA, EA, or trypsin. The proteinconcentrations in the feed and permeate were then measuredusing an UV-vis spectrophotometer at a wavelength of 280 nmto determine the membrane rejection efficiency. With respect toPVP rejection four different Mw of PVP (i.e., 10, 24, 40, and360 kDa) were prepared in the concentration of 1000 ppm. TheTOC concentrations in the feed and permeate were measured bya TOC analyzer (Model: TOC-LCPN, Shimadzu) to determinethe membrane rejection of PVP.

In addition, the antifouling property of the resulted mem-brane was also studied in this work. First, the best performingmembrane with balanced performance of permeability andselectivity was selected to treat the oily solution for 180 min.After 180 min of experimental run, the feed solution wasreplaced with deionized water and the membrane was cleanedby reversing the direction of the permeate flow for 60 min (alsoknown as backflushing). Bubbling of air controlled at flow rate

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EFFECT OF PVP MOLECULAR WEIGHTS FOR OILY WASTEWATER TREATMENT PROCESS 2307

of 5 L/min was also employed to assist the cleaning process byremoving the oil droplets attached on the membrane surface.

Membrane CharacterizationContact Angle Goniometer

The contact angle of membranes was determined by the ses-sile drop technique using a contact angle goniometer (Model:OCA 15EC, Dataphysics) with deionized water as the liquid.At least 10 locations were arbitrarily chosen on the membranesurface in order to yield an average value.

Scanning Electron Microscope (SEM)The outer surface and cross-sectional morphology of mem-

branes was observed by tabletop SEM (Model: TM 3000,Hitachi). Prior to the analysis, the hollow fiber was immersedinto liquid nitrogen for a few minutes followed by freeze-fracturing to obtain a perfect cut structure. The fiber was thenplaced onto a carbon-tape aluminum holder and coated withplatinum under vacuum.

Atomic Force Microscope (AFM)The membrane surface roughness and mean pore sizes were

investigated by AFM (Model: SPA-300HV, Seiko). A smallpiece of fiber was cut and adhered on a 1 cm2 square papercard using double-sided adhesive tape. The membrane surfacewas scanned in the size of 5 µm × 5 µm. The surface rough-ness of the membrane was expressed in terms of mean surfaceroughness (Ra). The pore size on the membrane outer sur-face was measured by visual inspection on the obtained lineprofiles from AFM images of the same membrane at differ-ent areas. To obtain the pore size, cross-sectional line profileswere selected to traverse the micron scan surface areas of theAFM images. The pore diameter was measured by inspectingline profiles of different high peaks and low valleys on thesame AFM images. The pore sizes were reported based on theaverage of 60 measurements.

In order to investigate the pore size distribution, 60 darkspots from the AFM images are measured and a graph was plot-ted according to the method described by Khayet et al. (29).Based on this method, the measured pore sizes are arranged inan ascending order. Median ranks are calculated using Eq. (3).

Median or 50 % rank =[

j − 0.3

n + 0.4

]× 100% (3)

where j is the order number of the pores when arranged inascending order and n is the total number of pores measured.

If pore sizes have log-normal distribution, the median rankversus ascending pore size would be a straight line on thelog-normal plot. From the graph, the mean pore size (μp) andgeometric standard deviation (σp) can be determined. The meanpore size will correspond to 50% of cumulative number of poresand the geometric standard deviation can be calculated from the

ratio of 84.13% of cumulative number of pores to that of 50%.From the mean pore sizes and geometric standard deviation, thepore size distribution can be determined using Eq. (4).

df (dp)

d(dp)= 1

dp ln σp(2π)1/2 exp

(−(ln dp − ln μp

)2

2(ln σp

)2

)(4)

where dp is the membrane pore size (nm), σp is the geometricstandard deviation, and μp is the mean pore size (nm).

Mechanical Tensile TesterThe fiber tensile test was performed at room temperature

on a tensile tester (Model: LRX 2.5KN, LLYOD). The gaugelength of membrane sample was fixed at 50 mm and the gaugerunning speed was set at 10 mm/min. The mechanical proper-ties of the membrane were then evaluated with respect to tensilestrength and elongation using NEXTGEN software.

Membrane PorosityThe membrane porosity, ε, is defined as the volume of the

pores per the total volume of the porous membrane as follows:

ε =(Wwet − Wdry)

ρw

(Wwet − Wdry)ρw

+ Wdryρp

× 100 (5)

where ε is the membrane porosity (%), wwet is the weight of thewet membrane (g), wdry is the weight of the dry membrane (g),ρp is the density of the polymer (g/cm3), and ρw is the densityof water (g/cm3).

RESULTS AND DISCUSSIONEffect of Molecular Weight of PVP on MembraneStructural Properties

Figures 3 and 4 show the SEM cross-sectional images and3D AFM surface images of the hollow fiber membranes pre-pared with different Mw of PVP, respectively. It was clearlyobserved that the membrane with the addition of PVP haslarger macrovoids in the sublayer in comparison to the neatPVDF-TiO2 membrane. However, with the increase of thePVP Mw in the dope solution, the size of macrovoids wasdecreased while the sponge layer became larger. The finger-like macrovoids gradually developed at the external and internalsurface layer with increasing Mw of PVP. The morphologicalchange with the PVP Mw observed by SEM can be explained bythe change in the ratio (nonsolvent inflow to the spun polymersolution/solvent outflow out from the spun polymer solution)during the phase inversion process and the change in the viscos-ity of the polymer solution. When PVP is added to the spinningdope, the nonsolvent (water) flow into the spun polymer solu-tion is enhanced by the presence of highly hydrophilic PVPin the solution, resulting in the formation of large macrovoids.This explains the increase in the size of macrovoids from PVDF

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(a) (b)

(e)

(d) (c)

FIG. 3. SEM cross-sectional images of PVDF-TiO2 membranes prepared from different Mw of PVP, (a) PVDF, (b) PVDF-PVP10k, (c) PVDF-PVP24k,(d) PVDF-PVP40k and (e) PVDF-PVP360k.

to PVDF-PVP10k. However, the viscosity of the spinning dopeincreases as the Mw of PVP increases (see Table 2), which sup-presses the rate of nonsolvent inflow. As a result, less wateris drawn into the polymer solution, leading to a smaller sizeof the macrovoids. Thus, the maximum of the macrovoid sizefound at PVDF-PVP10k is the result of the interplay of theeffects of the PVP Mw on nonsolvent/solvent flow rate ratioand the nonsolvent/solvent exchange rate. Figure 5 shows thepore size distribution of PVDF membrane incorporated withdifferent Mw of PVP. The narrower pore size distribution andsmaller pore size were observed as the Mw of PVP increasedfrom 10 kDa to 360 kDa.

Table 3 presents several important properties of the mem-brane prepared from different Mw of PVP. As can be seen, an

abrupt change is noticed from PVDF to PVDF-PVP10k, whichis followed by a gradual change in the reverse direction as theMw of PVP increases from 10k to 360k. For example, the poros-ity increased from PVDF (81.95%) to PVDF-PVP10k (88.59%)and then gradually decreased to PVDF-PVP360k (82.27%).This change reflects the macrovoid size that also increasedfrom PVDF to PVDF-PVP10k and then decreased graduallyto PVDF-PVP360k. As for the properties of the membranesurface, both the pore size and the roughness parameter (seeFig. 4) increased from PVDF to PVDF-PVP10k, followed bythe decrease from PVDF-PVP10k to PVDF-PVP360k. Thisshows a parallel relationship between the surface pore size andthe bulk macrovoid size. As more nonsolvent water is drawninto the spun polymer solution, the larger pores and the larger

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EFFECT OF PVP MOLECULAR WEIGHTS FOR OILY WASTEWATER TREATMENT PROCESS 2309

(c)

(a) (b)

(e)

Ra = 17.58nm

(d)

Ra = 26.50nm

Ra = 16.37nm Ra = 19.15nm

Ra = 15.35nm

FIG. 4. 3D AFM images (outer surface) of PVDF-TiO2 membranes prepared from different Mw of PVP, (a) PVDF, (b) PVDF-PVP10k, (c) PVDF-PVP24k,(d) PVDF-PVP40k and (e) PVDF-PVP360k.

FIG. 5. Probability density function curve generated from the pore sizesmeasured by AFM for PVDF membrane prepared at different Mw of PVP.

macrovoids are formed at the membrane surface as well asin the membrane bulk, respectively. With an increase in sur-face pore size, the surface roughness parameter also increases.On the other hand, the contact angle decreases from PVDF toPVDF-PVP10k, which is likely due to the high hydrophilic-ity of PVP used. Then, the contact angle keeps increasing asthe PVP Mw increases from 10 k to 360 k. This change is dueto the effect of decreasing roughness as the Wenzel equationpredicts.

To further understand the effect of membrane pore sizetowards the separation efficiency, the rejection of three pro-teins (BSA, EA, and Trypsin) and different Mw of PVP bythe studied membranes are presented in Figs. 6 and 7, respec-tively. Figure 6 shows that protein rejection is in the decreasingorder of BSA > EA > trypsin except for PVDF-PVP10k mem-brane where the BSA rejection is lower than EA. The aboveorder is exactly the order of decreasing Mw (or molecular size)

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TABLE 3Effect of different Mw of PVP on the PVDF-TiO2 membrane properties with respect to porosity, pore size

and contact angle

Membrane Porosity (%) Pore size (nm) Contact angle (◦)

PVDF (control) 81.95 ± 1.22 121.85 ± 1.54 75.84 ± 2.99PVDF-PVP10k 88.59 ± 3.67 134.14 ± 1.46 59.95 ± 1.34PVDF-PVP24k 85.36 ± 6.69 119.05 ± 1.73 66.88 ± 0.79PVDF-PVP40k 85.15 ± 3.04 114.39 ± 1.61 69.05 ± 0.96PVDF-PVP360k 82.27 ± 8.09 106.47 ± 1.53 85.99 ± 3.48

FIG. 6. Rejection of BSA, EA and trypsin with different Mw of PVP on thePVDF-TiO2 membrane (Operating conditions: temperature = 25◦C, proteinconcentration = 1000 ppm, pressure = 1 bar and duration = 60 min).

FIG. 7. Rejection of different Mw of PVP on the PVDF-TiO2 membrane pre-pared with different Mw of PVP (Operating conditions: temperature = 25◦C,PVP solution concentration = 1000 ppm, pressure = 1 bar and duration =60 min).

of the proteins. Furthermore, the protein rejection decreasedfrom PVDF to PVDF-PVP10k membrane and increased grad-ually from PVDF-PVP10k to PVDF-PVP360k. This trend isexactly the same as the trend observed by the surface pore size.

In other words, the protein rejection is controlled by the poreat the membrane surface. Figure 7 illustrates the performanceof membranes in rejecting PVP of different Mw. It is to beexpected that the rejection of different Mw of PVP shows anincreasing order, which is PVP10k < PVP24k < PVP40k <

PVP360k. This order is exactly the order of increasing molecu-lar size of the solute following an increase in Mw. Similar to thetrend observed in the protein rejection, PVP rejection of mem-brane decreased from PVDF to PVDF-PVP10k membrane andincreased gradually from PVDF-PVP10k to PVDF-PVP360k.The rejection profile against protein and PVP revealed that theseparation efficiency of the UF membrane was mainly governedby the surface pore of the membrane itself.

Table 4 illustrates the mechanical properties of PVDF-TiO2

membrane with different Mw of PVP. Upon addition of PVPregardless of their Mw, all PVP containing membranes demon-strated decrease in the tensile strength and elongation-at-break.The deterioration in mechanical strength can be reasonablyexplained by the formation of macrovoids in the membrane sub-layers as evidenced in the SEM cross-section images. Althoughthe mechanisms involved have not been clearly established, butit has been proposed that the formation of macrovoid would actas weak points of the whole membrane integrity and result inhigh tendency of membrane structure collapse as compared toneat PVDF membrane (14, 30). However, it is worth mention-ing that the deterioration in the membrane mechanical strengthhas negligible effect on our present study as the submerged UFmembrane system only applied very low vacuum pressure.

TABLE 4Effect of different Mw of PVP on the PVDF-TiO2

membrane properties with respect to mechanical strength

MembraneTensile strength

(MPa)Elongation-at-break

(%)

PVDF (control) 2.77 ± 0.16 351.50 ± 110.69PVDF-PVP10k 2.05 ± 0.16 300.34 ± 111.59PVDF-PVP24k 2.14 ± 0.34 310.36 ± 135.12PVDF-PVP40k 2.18 ± 0.19 312.00 ± 103.08PVDF-PVP360k 2.57 ± 0.10 344.50 ± 136.49

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EFFECT OF PVP MOLECULAR WEIGHTS FOR OILY WASTEWATER TREATMENT PROCESS 2311

Effect of Different Molecular Weight of PVP onMembrane Flux and Oil Rejection

Figure 8 shows the results from the experiments con-ducted using pure water and synthesized oily wastewater. FromFig. 8(a), the pure water flux (Jw1) was reported to increasefrom PVDF to PVDF-PVP10k and gradually decreased fromPVDF-PVP10k to PVDF-PVP360k. This pattern is exactly thesame as those observed in the surface pore size and the bulkporosity. It seems natural that Jw1 increases as both surface poresize and bulk porosity increase. The pattern observed in Jw1 isalso exactly the same as the pattern observed in hydrophilicity,i.e., the hydrophilicity increased, shown as the decrease in con-tact angle (see Table 3) from PVDF (control) to PVDF-PVP10kmembrane and decreased gradually from PVDF-PVP10k toPVDF-PVP360k. Hence the increase in surface hydrophilicityis also contributing to greater Jw1.

FIG. 8. (a) Pure water flux (Jw1) and permeate flux (Jw2) (b) oil rejection(%) and ratio of Jw2/ Jw1 of PVDF-TiO2 membranes prepared at differentMw of PVP (Operating conditions: temperature = 25◦C, oil concentration =250 ppm, air bubble flow rate = 5 L/min and vacuum pressure = –15 inHg,Mw = 0 means PVDF-TiO2 membrane without PVP).

The permeate flux of the oil/water mixture (Jw2) was slightlylower than Jw1. Interestingly, the flux ratio of Jw2/Jw1 illus-trated in Fig. 8(b) shows the pattern of PVDF < PVDF-PVP10k> PVDF-PVP24k > PVDF-PVP40k > PVDF-PVP360k. Thedecrease of the Jw2/Jw1 ratio with increasing Mw of PVP canbe attributed to the rapid formation of oil layer on the mem-brane surface, which has led to the increase in water transportresistance (30). On the other hand, a reverse pattern of PVDF >

PVDF-PVP10k < PVDF-PVP24k < PVDF-PVP40k < PVDF-PVP360k, was observed in oil rejection. It could be explainedby the increase of membrane hydrophobicity with the increas-ing of PVP Mw (as shown in Table 3) which has facilitatedstronger interaction between the membrane surface and the oillayer. Thus, more oil particles are adsorbed and attached onthe membrane surface, resulting in thicker oil layer formationand higher oil rejection. Overall, it can be said that PVDF-PVP40k is the best performing among the studied membranesdue to its good combination of water permeability and oilselectivity.

Effect of pH on Membrane PerformanceTo further understand the performance of the PVDF-

PVP40k membrane, the effect of feed pH on the membranewater flux and rejection during oily wastewater treatmentwas evaluated by varying the feed pH in the range of 4–10.As shown in Fig. 9, the membrane achieved the lowest fluxat pH 4 (41.58 L/m2.h) followed by pH 10 (53.50 L/m2.h)and pH 7 (72.15 L/m2.h). Interestingly, the trend observed inpermeate flux is reverse of the pattern found in membrane sep-aration efficiency, i.e., the lowest the permeate flux the highestthe oil rejection, and vice versa. In comparison to the neutralcondition, the decrease in membrane permeate flux at acidicand alkali environment is most likely due to the rapid fouling

FIG. 9. Effect of feed pH on the permeate flux and oil rejection of PVDF-PVP40k membrane (Operating conditions: temperature = 25◦C, oil concen-tration = 250 ppm, air bubble flow rate = 5 L/min and vacuum pressure =–15 inHg).

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2312 C. S. ONG ET AL.

resulted from greater oil rejection efficiency. This phenomenonis mainly due to the existence of charges on membrane surfaceat pH 4 and 10. Furthermore, the surface charge of membraneat pH 7 is at the lowest level since the isoelectric point (IEP) ofP25 TiO2 (which embedded in membrane matrix) is at betweenpH 6.2 and 6.9 as reported elsewhere (31, 32). Therefore, themembrane would carry no net surface charge at pH near to7 which allows more oil particles passing through the mem-brane and further results in lower rejection as experienced inthis work. Similar findings were also reported by Hua et al.(33) in which they found that membrane achieved the lowestflux at pH 3.8 in comparison to other pHs ranging from 4 to10. However, it must be pointed out that they attributed the lowpermeability of membrane to the better stability of oil dropletsat acidic environment instead of to severe fouling problem aselucidated by us.

Effect of Oil Concentration on Membrane Flux and OilRejection

To further investigate the separation performance of PVDF-PVP40k membrane, the oil concentration of the oily solutionwas varied between 250 ppm and 1000 ppm. The normalizedwater flux and oil rejection of the membrane as a function oftime are shown in Fig. 10. It is found that at low oil concen-tration the decline of membrane flux is not as severe as thatobserved at high oil concentration. For 250 ppm oil solution,the membrane flux declined around 19% in 180 min comparedto ∼30% and ∼33% recorded for 500 ppm and 1000 ppm oilysolution, respectively. The increase in flux decline rate can beexplained by a thicker oil layer formed on the membrane sur-face at high concentration of the oily solution which causeswater transport resistance to increase and water flux to decrease(30). Additionally, the hydrophobicity of the membrane hasalso increased as indicated by the increase in contact angle. Thishas further enhanced the interaction between the membranesurface and the oil layer, causing more oil particles attachedand absorbed on the membrane surface and contributing tomembrane flux deterioration.

Despite the flux decline, the increase in oil rejection with theincreasing oil concentration is observed in Fig. 10(b). This phe-nomenon is expected as oil droplets are likely to be adsorbedand attached on the membrane surface, resulting in thicker oillayer formation as the oil concentration increases, which leadsto higher oil rejection. At 250 ppm, the initial oil rejection wasaround 70%, but was slowly increased with time and even-tually achieved 97% removal of oil. Similar increasing trendwas also observed in the oil concentration of 500 ppm and1000 ppm, although higher oil concentration could achieverelatively higher oil rejections at the early stage of the filtrationprocess.

Effect of Cleaning Process on Membrane Water Flux andOil Rejection

In order to assess the extent of fouling on the mem-brane performance with respect to water flux and rejection,

FIG. 10. Filtration experiments of PVDF-PVP40k membrane at different oilconcentrations (a) Normalized flux ratio and (b) Oil rejection (Operating con-ditions: temperature = 25◦C, air bubble flow rate = 5 L/min and vacuumpressure = –15 inHg).

PVDF-PVP40K membrane was subject to backflushing clean-ing process using deionized water for 60 min after being usedfor treating oily solution of 250 ppm for up to 3 h. Figure 11compares the performances of the membrane as a function oftime before and after cleaning process. The results revealed thatthe approximately 60% of the initial flux of membrane couldbe retrieved by a simple backflushing cleaning process withoutinvolving any chemical agents and optimized cleaning condi-tions. Further increase in membrane flux recovery rate can bepractically conducted, provided the chemical cleaning solutionused is mild to the membrane structural properties. Even thoughthe membrane water flux was not able to be completely recov-ered by backflushing process, it is interesting to note that nosignificant difference was experienced in separation efficiencyas the membrane tended to achieve a promising rejection rate

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EFFECT OF PVP MOLECULAR WEIGHTS FOR OILY WASTEWATER TREATMENT PROCESS 2313

FIG. 11. Comparison the performances of PVDF-PVP40k membrane as afunction of time before and after cleaning process, (a) permeate flux and fluxrecovery and (b) oil rejection (Operating conditions: temperature = 25◦C,oil concentration = 250 ppm, air bubble flow rate = 5 L/min and vacuumpressure = –15 inHg).

against oil particles (> 95%) after 150 min of the filtrationprocess.

CONCLUSIONSThe permeation performance of PVDF-TiO2 membranes

with different Mw of PVP was evaluated for the treatment ofaqueous protein solution and oil/water mixture. The resultsobtained showed that the membrane properties were affectedwith the use of different Mw of PVP as additive during mem-brane preparation. Based on the results obtained from this study,the addition of PVP to PVDF membrane could improve thehydrophilicity, pore size, and porosity of the PVDF membrane.Among the PVDF-PVP membranes studied, it can be con-cluded that PVDF-PVP40k is the best performing membranedue to its high permeability and selectivity achieved during oilfiltration experiment. Experimental results also revealed that themembrane flux reduction during filtration is mainly due to the

reversible fouling as the flux could be significantly retrieved bysimple water backflushing process. With respect to oil rejection,it is found that the excellent separation efficiency of membraneafter cleaning process is slightly affected (less than 2% reduc-tion). Based on the results obtained in this work, it is showedthat PVP is a very good additive in improving the propertiesof PVDF-TiO2 composite membrane by enhancing not onlymembrane permeability but also selectivity, making the mod-ified composite membrane a good candidate for sustainabledevelopment of oily wastewater treatment.

ACKNOWLEDGEMENTSThe authors would like to thank the management sup-

port from the Research Management Centre (RMC), UniversitiTeknologi Malaysia.

FUNDINGThe authors gratefully acknowledge the financial support

from the European Commission FP7 - LIMPID (Project num-ber: 310177).

SUPPLEMENTAL MATERIALSupplemental data for this article can be accessed on the

publisher’s website.

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