Effect of nano-sized Al2O3-particle addition on PVDF ultrafiltration

Journal of Membrane Science276(2006)162–167Effect of nano-sized Al2O3-particle addition on PVDF ultrafiltrationmembrane performanceLu Yan a,b,Yu Shui Li a,∗,Chai Bao Xiang a,Shun Xianda ca Department of Municipal and Environmental Engineering,Harbin Institute of Technology,Harbin150090,Chinab Department of Chemistry,Daqing Petroleum Institute,Daqing163318,Chinac Daqing OilField E&P Research Institute,Daqing631712,ChinaReceived2April2005;received in revised form19September2005;accepted20September2005Available online19October2005AbstractA polyvinylidenefluoride(PVDF)ultrafiltration(UF)membrane was modified by dispersing nano-sized alumina(Al2O3)particles uniformly in a PVDF solution(19%polymer weight).Membranes were prepared by a phase-inversion process and using UF experiments comparing waterflux and, molecular-weight cut-offs for the wet membranes.The contact angle between water and the membrane surface was measured in order to quantify the hydrophilicity changes of the membrane surface.Membranes surface morphology,surface and cross-sectional structures,and nanometer-particle distribution on the membrane surface were examined by atomic-force microscopy(AFM),scanning electron microscopy(SEM),and confocal laser-scanning microscopy(CLSM),respectively.Thermal analysis(DSC)was performed in order to investigate the interactions between membrane components.The effects of the nanometer Al2O3-particles concentration in the polymer dope on the permeation properties,membrane strength, and anti-fouling performance were examined.The experimental results indicated that Al2O3–PVDF composite membranes exhibit significant differences in surface properties and intrinsic properties due to nanometer-particles addition.©2005Elsevier B.V.All rights reserved.Keywords:Membrane fouling;Nano-sized Al2O3particles;Polyvinylidenefluoride;Ultrafiltration membrane1.IntroductionUltrafiltration has been used extensively in many membrane-separation processes,especially in the wastewater treatment field,including oil–water and protein effluent separation[1–7]. The hydrophilicity of the membrane and its porous structure play important roles in membrane-separation processes.A suit-able porous membrane must have high permeability,good hydrophilicity,and excellent chemical resistance to the feed streams.In order to obtain high permeability,membranes should have high surface porosity and good pore structure.An asymmet-ric membrane is ideal for this purpose.Polyvinylidenefluoride (PVDF)is one material that can form,such asymmetric mem-branes.This polymer is thermally stable and resistant to corro-sion by most chemicals and organic compounds.PVDF-based membranes show outstanding anti-oxidation activities,strong ∗Corresponding author.Tel.:+8645186282101.E-mail address:yushuili163@(Y.S.Li).thermal and hydrolytic stabilities,and good mechanical andfilm-forming properties.PVDF membranes can thus be used in many ultrafiltration processes through various modifications.Many studies have attempted to improve the performance of PVDF membranes using various techniques,including physi-cal blending,chemical grafting,and surface modifications.The blending of polymers has the advantage of easy preparation using phase inversion.The addition of hydrophilic materials to the dope solution increases the water permeability of a mem-brane with similar pore size and pore distribution,due to an increase in pore density as well as in the hydrophilicity of the membrane surface and inside the pores[8,9].Polymethyl methacrylate(PMMA)is an organic material that is commonly blended with PVDF.The addition of hydrophilic PMMA not only improves the membrane pore size distribution and pore structure,but also enhances its permeation performance without a loss of retention[10,11].Additional organic materials that can improve PVDF membrane properties include polymethy acry-late(PMA)[12,13],polyvinyl acetate(PV AC)[14],and cellu-lose acetate(CA)[15].The addition of organic hydrophilic mate-0376-7388/$–see front matter©2005Elsevier B.V.All rights reserved. doi:10.1016/j.memsci.2005.09.044L.Yan et al./Journal of Membrane Science276(2006)162–167163rials can improve some properties of membranes,but reduces membrane strength.Recent studies of PVDF-blending modifications have focused on blending the polymer with inorganic materials.The addition of inorganicfillers has led to increased membrane per-meability and improved control of membrane-surface properties [16–18].Inorganic materials that can be blended with PVDF include silica[18],zirconium dioxide(ZrO2)[19],and some small molecule inorganic salts,such as lithium salts[20].The common feature of these modifications is the addition of a higher proportion of inorganic materials.SiO2–PVDF,ZrO2–PVDF, and LiClO4–PVDF have ratios of inorganic particles to PVDF of 0.2,1.0,and0.128,respectively[18–20].The PVDF-membrane morphology and elasticity are both significantly affected by the mass of inorganic materials added.In the present study,PVDF UF membranes were modified by inorganic nano-sized Al2O3particles.This research aimed to prepare Al2O3–PVDF composite membranes using the phase-inversion method by including a small proportion of Al2O3 particles but effectively improving the membrane performance. The effects of the Al2O3-particle concentration in the casting solution on the membrane hydrophilicity,permeationflux,mor-phology,mechanical properties,and anti-fouling performance were examined.2.Experimental2.1.MaterialsThe PVDF used was a commercial product(FR904). Dimethylacetamide(DMAC;>99%,reagent)was employed as the solvent.Alumina(Al2O3)particles(10nm in size)were added to PVDF solutions.The other additives were sodium hexad-phosphate and polyvinyl pyrrolidone(PVP).A mixture of distilled water and ethanol was used as the nonsolvents for the polymer precipitation.2.2.Membrane preparationAl2O3–PVDF composite membranes were prepared by the phase-inversion method.Casting dopes were prepared by dis-solving the PVDF in the solvent at room temperature and adding nano-sized Al2O3particles and other additives to the casting dopes under stirring.In order to obtain optimal dispersions of the particles in the polymer solutions,agitation was required for at least24h.The casting solutions were then kept in the dark for at least24h to remove air bubbles.An appropriate amount of the dope was dispersed uni-formly on a glass plate.After30s exposure to air,the glass plate was immersed in a bathfilled with distilled water and ethanol.Different Al2O3-particle concentrations(0–4%,by weight of PVDF)of polymer dopes consisting of PVDF(19%,by weight of the solution),DMAC(78%,by weight of the solution),and additives(1%sodium hexad-phosphate,by weight of PVDF; 3%PVP,by weight of the solution)were prepared;these were labeled as PVDF-0,PVDF-1,PVDF-2,PVDF-3,and PVDF-4,respectively.The membranes formed were washed with,and soaked in,distilled water until they were used as samples for characterization.2.3.Characterization of membraneThe membrane-permeation properties were tested in a UF laboratory unit(effective area=32.3cm2)fed with distilled water at different transmembrane pressures(ranging from0.1 to0.4MPa).The same unit was fed with proteins of dif-ferent molecular weights that is,cell pigment C(molecu-lar weight=12,400),pepsin(molecular weight=35,000),and bovine serum albumin(molecular weight=67,000)in order to obtain the molecular-weight cut-off values of membranes. The membrane porosities were measured using dry–wet weight method.The contact angle between water and the membrane surface was measured with a contact-angle measurement appa-ratus(DSA10,American,needlepoint size=1.5mm)accord-ing to the sessile-drop method.Briefly,a water droplet was deposited on aflat homogeneous membrane surface and the contact angle of the droplet with the surface was measured. The value was observed until there was no change in con-tact angle during the short measurement period.Each con-tact angle was measuredfive times atfive different points of each membrane sample and an average value was calcu-lated.The tensile strength and elongation-at-break of the mem-branes were determined using a universal electronic strength measurement instrument(W-56).The measurements were carried out at room temperature and the rate of pull was 2mm/min.The surface and cross-sectional structures of the membranes were examined by scanning electron microscopy(SEM;S-4700,Japan).Cross-sections were prepared by fracturing the membranes at the temperature of liquid nitrogen.All spec-imens were coated with a thin layer of gold before being observed using SEM.The membrane surface morphology,in terms of the mean surface roughness(R a),was studied by atomic-force microscopy(BioScope TM,USA)using the tap-ping mode.The mean roughness was defined as the average value of the surface relative to the center plane for which the volumes enclosed by the images above and below the plane were equal.This parameter was calculated using the following equation[21]:R a=1L x L yLxLy|f(x,y)|d x d yHere,f(x,y)is the surface relative to the central plane,and L x and L y are the dimensions of the surface.The nanometer-particle distribution in the modified mem-brane was analyzed based on the differential coefficient-interference pattern identified using confocal laser-scanning microscopy(TCS SP,Germany).Thermal analysis was car-ried out using a differential scanning calorimeter(Pyris,Amer-ica)in order to observe the effect of inorganic particles on the polymer membrane.Membrane fouling was tested in a dead-end ultrafiltration cell fed with1%(weight)␣-amylase solution.164L.Yan et al./Journal of Membrane Science 276(2006)162–1673.Results and discussion3.1.Effect of the addition of nano-sized Al 2O 3particles on membrane fluxMembrane fluxes were affected by the addition of nano-sized Al 2O 3particles.Fig.1shows that increased Al 2O 3concen-trations led to increased water permeate fluxes,although this trend cease when the quantity of the nano-sized Al 2O 3particles reached a certain level.This finding can be interpreted as fol-lows.PVDF is a hydrophobic polymer.Its hydrophilicity can be improved significantly by the addition of nano-sized Al 2O 3particles,which have some favorable characteristics,such as hydrophilicity and higher ratio surface areas.Thus,the water fluxes are increased.However,with casting drops containing more than a certain concentration of nano-sized Al 2O 3particles,the flux cannot be improved further as the nano-sized Al 2O 3par-ticles coalesce.In terms of both economy and practicality,2%(weight)of nano-sized particles in the casting solution are ideal.It can also be seen from Fig.1that the water fluxes improved when the pressure increased for all membranes;this finding was specific to asymmetric microfiltration and ultrafiltration mem-branes.Water fluxes were improved by increasing the driving force.However,an excessive driving force would increase the operating cost of a facility and affect the life expectancy of membranes.In the current experiment,a transmembrane pres-sure of 0.1MPa was employed.Under this pressure differential,the water flux of the PVDF-2membrane was approximately 100L/(m 2h)higher than that of the PVDF-0membrane.3.2.Contact angle,porosity,rejection,and molecular-weight cut-off values of membranesThe contact angle is an important parameter for measur-ing surface hydrophilicity [22,23].In general,a smaller contact angle corresponds to a more hydrophilic material.As can be seen from Table 1,increasing the Al 2O 3-particle concentration caused a decrease of the contact angle.However,the porosity,rejection,and molecular-weight cut-off values were not affectedFig.1.Distilled water fluxes of membranes prepared using dopes with different nano-sized Al 2O 3particles.Table 1Membrane contact angles,porosities,rejection values,and molecular-weight cut-offs Membrane PVDF-0PVDF-1PVDF-2PVDF-3PVDF-4Contact angle (◦)83.6467.8757.4258.6258.97Porosity (%)54.9954.7255.3153.0955.82Rejection (%)95.5096.3196.597.0596.45Molecular-weight cut-off3500035000350003500035000by the Al 2O 3-particle concentration.These results demonstrated that adding Al 2O 3particles to PVDF polymer could improve its hydrophilicity,but did not affect the pores size or the number of pores.As a result,the flux through the composite membranes increased significantly.3.3.Mechanical propertiesThe tensile strength and elongation-at-break of the mem-branes are shown in Table 2.Both values,especially the former,were improved.The elongation-at-break initially increased with the addition of nano-sized particles,reached a peak when the Al 2O 3-particle concentration was 2%(weight)and then declined as the Al 2O 3-particle concentration was further increased.These behaviors indicate that adding an appropriate amount of nano-sized Al 2O 3particles to a PVDF solution can improve the mem-brane’s mechanical properties.However,an excessive amount of inorganic Al 2O 3particles in the cast solution can cause the membrane elasticity to decline,thereby leading to a decrease in the membrane’s elongation-at-break value.Nonetheless,for a membrane consisting of 2%(weight)Al 2O 3particles,the values of both the tensile strength and the elongation-at-break values were increased by more than 50%.3.4.Microstructures of membranesSEM and atomic-force microscopy (AFM)analyses were per-formed to compare the morphological changes between modi-fied and unmodified membranes.Fig.2shows SEM micrographs of the surface,cross-section,and inner porous-surface structures of the membranes PVDF-0and PVDF-2.Micropores were dis-tributed on both membrane surfaces and there were no apparent differences between the modified and unmodified membranes.Both membranes showed typical asymmetric morphology with finger-like pores linked by sponge walls;the latter contained large numbers of micropores that allowed the finger-like pores to communicate with each other.These findings indicate that theTable 2Mechanical properties of membranesTensile strength (N)Elongation-at-break (%)PVDF-013.2031 4.72PVDF-118.3593 5.89PVDF-219.92187.34PVDF-320.2343 6.67PVDF-420.53125.58L.Yan et al./Journal of Membrane Science 276(2006)162–167165Fig.2.SEM micrographs of the membrane surface,cross-section,and inner porous structures:(a,c,and e)PVDF-0and (b,d,and f)PVDF-2.addition of nano-sized Al 2O 3particles did not affect the struc-tures of the surface,cross-section,and inner pores.Therefore,the mechanism of PVDF membrane-structure formation was not altered by the addition of inorganic nano-sized Al 2O 3.Fig.3displays three-dimensional AFM images of the mem-brane surfaces.The surface roughness of the modified membrane was apparently higher than that of the unmodified membrane.In the range of the scan areas 10␮m ×10␮m,the R a of the modified and unmodified membranes were 114.0and 64.4nm,respectively.Higher roughness commonly led to two changes in the modified membrane;an increase of efficient filtration area and a decrease of the anti-fouling performance.Because Al 2O 3particles are porous and ceramic,water molecules can permeate to them.Their accumulation on the membrane surface can increase,but not reduce,the effective filtration area of the membrane.The enlarged efficient filtration area increases the membrane flux directly.The membrane-fouling trend increases with roughness owing to contaminants accumulating in the “val-leys”of the rough membrane surface;however,if the increase in roughness is caused by the accumulation of hydrophilic Al 2O 3particles on the membrane surface,it improves the membrane surface hydrophilicity significantly,and reduces the interaction between the contaminants and the membrane surface.Further-more the increased membrane hydrophilicity can enhance the anti-fouling and performance and the permeate flux.These results show that increased membrane-surface roughness does not have a negative effect on membrane performance;rather,it effectively improves the permeating flux and anti-fouling prop-erties.3.5.Al 2O 3-particle distribution in the membraneThe performance of the modified membrane was influenced by the distribution of nano-sized particles.Sodium hexad-phosphate,an anionic surfactant,is used as a dispersant and a crosslinking reagent in order to make inorganic particles dis-perse uniformly in the cast dopes.Fig.4shows a differential coefficient-interference pattern produced using confocal laser-scanning microscopy (CLSM)for the Al 2O 3-particle distribu-tion in the membrane.Fig.4illustrates that most Al 2O 3particles were dispersed uniformly in the membrane,with the exception of a few large clusters that might have resulted from particlers overlapping or from nano-sized particles coalescing in the mem-brane.Fig.3.AFM three-dimensional surface images of membranes:(a)PVDF-0and (b)PVDF-2.166L.Yan et al./Journal of Membrane Science276(2006)162–167Fig.4.Differential coefficient interference pattern of Al2O3-particle distribution in the modified membrane.3.6.Thermal analysisThermal analysis(DSC)was performed on the PVDF-0 and PVDF-2membranes in order to investigatethe interaction between the polymer and the inorganic particles.Thermograms were recorded during cooling and heating at a controlled rate to evaluate their thermodynamic properties.Each sample was heated from50.00to200.00◦C at10.00◦C/min,held for1min at 200.00◦C and then cooled from200.00to50◦C at10.00◦C/min. Fig.5shows that the addition of nano-sized Al2O3particles has no effect on either the melting point or the enthalpy of the crystal-lizations.However,the enthalpy of fusion and the crystallization point declined slightly in the modified membrane compared with the unmodified membrane.This indicated that the addition of Al2O3particles had no effect on crystal formation,although it had a minor effect on crystal perfection during the formation of PVDF membranes[20].Fig.5.DSC thermograms of membranes PVDF-0and PVDF-2.For PVDF-0,the enthalpy of fusion was59.102and the enthalpy of crystallization was −36.173.For PVDF-2,the enthalpy of fusion was50.768J/g and the enthalpy of crystallization was−36.779J/g.Fig.6.Permeate-flux decline with increasing time for the different membranes measured at0.1MPa transmembrane pressure.3.7.Anti-fouling performanceMembrane fouling caused the permeationflux to decline drastically.The mechanism that produces membrane fouling is complex.However,membrane hydrophilicity is the main factor affecting the surface-adsorption properties of the membranes. Fig.6shows that the permeationfluxes of␣-amylase solution for the membranes declined as thefiltration time increased,followed by a long period with a steady value.The rate offlux decline, which was defined as theflux of the membrane at the time of consistentflux divided by the initialflux of the clear mem-brane,was no more than27.4%for the modified membranes;the lowest value was18.2%compared with38.5%for the unmod-ified membrane.This shows that the composite membranes modified by Al2O3particles had a more favorable anti-fouling performance.4.ConclusionsAl2O3–PVDF composite membranes were synthesized using the phase-inversion process.The addition of nano-sized Al2O3 particles did not affect the pore size,pore numbers,or crystal formation of the PVDF membranes.Although the membrane-surface morphology was altered by increased of surface rough-ness,it had no negative effects on membrane permeability or anti-fouling performance.The permeation-flux increase of the modified membrane was attributed to increases in the surface hydrophilicity and the efficientfiltration area due to the addition of hydrophilic inorganic nano-sized Al2O3particles.The tensile strength and break elongate ratio values of the membrane were improved more than50%for the membrane that consisted of 2%(weight)Al2O3particles.The improved hydrophilicity of the composite membrane also enhanced the anti-fouling perfor-mance.Furthermore,the rate offlux decline was only18.2%for the2%(weight)Al2O3-particle membrane at a transmembrane pressure of0.1MPa.L.Yan et al./Journal of Membrane Science276(2006)162–167167AcknowledgementsThe paper was supported by the973National Basic Research Program,the Teaching and Research Award Program for Out-standing Young Teachers in Higher Education Institutions of the Ministry of Education,and the Daqing Science and Technology Bureau,China.References[1]B.Tansel,J.Regula,R.Shalewitz,Treatment of fuel oil and crude oilcontaminated waters by ultrafiltration 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