聚醚砜膜改性 国外

Hydrophilic modification of poly(ether sulfone)ultrafiltration membrane surface by self-assembly of TiO 2nanoparticlesMing-Liang Luo a ,*,Jian-Qing Zhao a ,Wu Tang b ,Chun-Sheng Pu caCollege of Materials Science and Engineering,South China University of Technology,Guangzhou 510640,ChinabLASMIS,University de Technologie de Troyes,Troyes Cedex 10010,France cCollege of Petroleum Engineering,Xi’an Shiyou University,Xi’an 710065,ChinaReceived 1June 2004;received in revised form 17November 2004;accepted 17November 2004Available online 25December 2004AbstractMembrane fouling is one of the major obstacles for reaching the ultimate goal,which realizes high flux over a prolonged period of ultrafiltration (UF)operation.In this paper,TiO 2nanoparticles of a quantum size (40nm or less)in anatase crystal structure were prepared from the controlled hydrolysis of titanium tetraisopropoxide and characterized by X-ray diffraction (XRD)analysis and transmission electron microscopy (TEM).The hydrophilic modification of poly(ether sulfone)UF membrane was performed by self-assembly of the hydroxyl group of TiO 2nanoparticle surface and the sulfone group and ether bond in poly(ether sulfone)structure through coordination and hydrogen bond interaction,which was ascertained by X-ray photoelectron spectroscopy (XPS).The morphology and hydrophilicity were characterized by scanning electron microscopy (SEM)and contact angle test,respectively.The composite UF membrane was also characterized in terms of separation behavior for polyethylene glycol-5000solute.The experimental results show that the composite UF membrane has good separation performance and offers a strong potential for possible use as a new type of anti-fouling UF membrane.#2004Elsevier B.V .All rights reserved.Keywords:Ultrafiltration;Self-assembly;Anti-fouling membrane;TiO 2nanoparticles;Hydrophilicity1.IntroductionIn recent years,more and more attention in ultrafiltration (UF)membrane has been attracted for a variety of applications in waste-water treatment,substance separation,solute concentration,and so on.The major drawback in the extensive use of membranes includes membrane fouling,which results in flux decline during operation [1].Several types of fouling could occur in the membrane system,e.g.crystalline fouling,organic fouling,particulate and colloidal fouling,and microbial fouling,etc.[2].Many approaches to control membrane fouling have been performed,which generally involve pretreatment of the feed solution,surface modification of the membrane (like hydrophobic or hydrophilic and/locate/apsusc*Corresponding author.Tel.:+862087113576;fax:+862087113576.E-mail address:yfsailing_wxg@ (M.-L.Luo).0169-4332/$–see front matter #2004Elsevier B.V .All rights reserved.doi:10.1016/j.apsusc.2004.11.054electronegative or electropositive modification),opti-mization of module arrangement and process condi-tions,and periodic cleaning[3,4].Even after the development for decades,particulate and colloidal fouling still remains the main reason forflux decline in the process of industrial wastewater treatment[3,5].Poly(ether sulfone)(PES)is a kind of special engineering plastics with good performances.The structure of PES used in the paper was shown in Fig.1. It can be imagined that PES has crystalline to some degree because of harder benzene ring and softer ether bond existed in the structure.PES possesses many good performances such as high mechanical property and heat distortion temperature,good heat-aging resistance,environmental endurance and processing. It has become one of important separation membrane materials,but its hydrophobicity controlled by PES structure leads to low membraneflux and fouling easily,which has greatly effect on applicationfield and usage life of separation membrane.It is necessary to modify PES membrane surface by physical and chemical methods and improve its hydrophilicity. Recently,modification methods of UF membrane involve ultraviolet irradiation[6],graft polymeriza-tion[7,8],glow discharge[9],and ozone[1,10,11]on membrane surface,and so on.The hydrophilicity and anti-fouling capability of UF membrane increase to some degree,but some of them are complicated and it is difficult to control the hydrophilicity of membrane, others may make some performances of UF membrane loss.A new approach to modify PES ultrafiltration membrane by self-assembly of TiO2nanoparticles is presented in this paper.Titanium dioxide(TiO2)has been the focus of numerous investigations in recent years,because its high hydrophilicity[12,13],stable chemical property, innocuity and low cost,etc.Most of researches carried out in thisfield have focused on the use of TiO2 powder suspended in the water as a catalyst[14].The method of TiO2self-assembly on the surface with the terminal functional groups(for example,single-crystal silicon,quartz,and glass substrates)has been used to fabricate multilayer ultrathinfilms[15–17]. The self-assembly behavior of TiO2on the surface of polymer with COOH,SO2OH,sulfone group and ether bond may be explained by two different adsorption schemes.One scheme is that TiO2was bound with oxygen atoms of these groups via coordination to Ti4+cations.The other scheme is to form a hydrogen bond between these groups and the hydroxyl group of TiO2surface[18].Thus,it is probable to self-assemble the TiO2nanoparticles on membrane sur-face.In this paper,TiO2nanoparticles were prepared from the controlled hydrolysis of titanium tetraiso-propoxide.The particle structure and size were characterized by X-ray diffraction(XRD)analysis and transmission electron microscopy(TEM).The composite UF membrane was prepared by self-assembly of TiO2on the membrane surface.X-ray photoelectron spectroscopy(XPS)was performed with the UF-tested membrane after the actual UF operation conditions.The morphology and hydrophilicity of composite UF membrane were characterized by scanning electron microscopy(SEM)and contact angle test,respectively.The composite ultrafiltration membrane was also characterized in terms of separa-tion behavior for polyethylene glycol-5000solute.The anti-fouling and fouling mitigation of the composite UF membrane was examined and verified.2.Experimental2.1.Preparation of the nanosized TiO2particlesTiO2nanoparticles were prepared from the con-trolled hydrolysis of titanium tetraisopropoxide at acidic condition[19].A 1.25ml sample of Ti(OCH(CH3)2)4(AR;Chengdu Unite-chemcial Company,China)dissolved in25ml of absolute ethanol was dropped to250ml of distilled water (48C),adjusted to pH1.5with hydrochloric acid under vigorous stirring.After this mixture was stirred overnight,a transparent colloidal suspension was obtained.Powdered sample was obtained by evapor-ating(358C)using a rotavapor and by drying(508C) under vacuum.The gel powder was annealed in muffle furnace at5008C and the nanosized TiO2particles were obtained.M.-L.Luo et al./Applied Surface Science249(2005)76–8477 Fig.1.Molecular structure of the PES.2.2.Preparation of PES/TiO2compositeultrafiltration membranesThe PES ultrafiltration membrane was made via phase inversion method[20].The UF membrane was rinsed in a sodium carbonate solution(0.2wt.%)and then washed with distilled water.The neat PES membrane with an area of38.5cm2was dipped in the transparent TiO2colloidal solution,stirred for1min by ultrasonic method and placed for1h to deposit TiO2nanoparticles on the membrane surface and then washed with distilled water.2.3.Characterization2.3.1.Characterization of the nanosized TiO2 particlesThe crystal structure of TiO2nanoparticle was characterized by X-ray diffraction(XRD).XRD analysis was performed on TiO2powder samples with a PHI-5400X-ray diffractometer using18kW Cu K a(l=0.15418nm)radiation.The particle size was determined by a JEOL transmission electron micro-scope(TEM,JEOL JEM-200CX)at120kV.For the TEM observation,TiO2powder in distilled water solution(0.5g lÀ1)was dropped on a carbon-coated grid and then dried at room temperature.2.3.2.Characterization of the PES/TiO2composite UF membraneThe surface topologies of the PES membrane containing TiO2nanoparticle were investigated with a JSM-5800scanning electron microscope(SEM).The surface morphology of the neat PES membrane was also examined and was compared with that of the TiO2 self-assembled version.For the SEM observation,the membrane samples were cut into appropriate sizes and the surfaces were coated with platinum or gold by a sputter coating machine.The pore size distribution of composite membrane was measured by mercury displacement method with Autopore9220II aperture-testing meter.X-ray photoelectron spectroscopy(XPS)was performed on the surface of composite membrane with a PHI-5400spectrometer using Mg K a X-ray (1253.6eV).The X-ray gun was operated at10kVand 1mA.The spectra were taken at the takeoff angle (defined as the angle between the detected photoelec-tron beam and the membrane surfaces)of458to give a sampling depth of ca.2.3nm.The sensitivity factors of individual elements for quantitative analyses were taken with the values from the standard vision library provided by the manufacturer,which were based on a combination of photoelectric cross-section,transmis-sion function,and inelastic mean free path.Static contact angles of the composite membrane surface were measured using the captive air bubble technique.Membranes were inverted in deionized water and air bubbles were placed in contact with the surface. The static angle was measured using an Eromag-1 contact angle testing apparatus.The contact angle was determined from the average value of10-times measurements and the measurement error wasÆ38.2.3.3.Separation performance of the PES/TiO2 composite UF membraneThe mass transfer characteristics of UF membrane for200ppm polyethylene glycol(PEG-5000)aqueous solution were determined in laboratory at0.2MPa, 258C for30min with the apparatus of a continuous flow type.The waterflux was calculated by direct measurement of the mass of the permeateflow:J¼V=At(1) where J is the membraneflux(L mÀ2hÀ1),V the permeate volume(L),A the membrane area(m2) and t the ultrafiltration time(h).The solute rejection was defined as follows:R¼ð1Àc p=c fÞÂ100%(2) where R is the solute rejection,c f the feed concentra-tion,c p the permeate concentration.3.Results and discussion3.1.Crystal structure and size of synthesized TiO2 nanoparticleFig.2shows the X-ray diffraction spectrum of TiO2 particles.The synthesized particles are composed entirely of anatase compared with those reported for rutile(110)(2u of27.458)and anatase(101) (25.248)[21].The particle size is determined by transmission electron microscopy(TEM)explicitly. The particles size is about5–42nm(Fig.3).M.-L.Luo et al./Applied Surface Science249(2005)76–84 783.2.Microstructure and pore size distribution of the composite membraneThe microstructure of the composite membrane is shown in Fig.4.As shown in Fig.4a (SEM of the composite membrane cross-section),the sectional structure of composite membrane has asymmetry and many smaller pores,which shape looks like the finger,are filled in the membrane.When the dope solution is coated on the smooth glass plate,the pore size formed towards the air is smaller due to the solution evaporation,but the pore size formed towards the glass plate is larger because the exchange happened quickly between the solvent phase and the non-solvent phase and immediately phase inversion starts and after few minutes thin polymeric composite film separated out from the glass.Fig.4b and c are the SEM graphs of the membrane surface before and after treated by TiO 2colloidal solution,respectively.The neat PES mem-brane has the typical surface morphology of a characteristic ridge-and-valley structure (Fig.4b),but the ridge-and-valley structure is not so visible.Fig.4c displays the surface morphology of the TiO 2self-assembled composite membrane,where TiO 2nanoparticles appear to exist as nodular shapes of ca.40nm or less on the surfaces of the ridges and the membrane surface has clear ridge-and-valley struc-ture.The pore size distribution of the composite membrane is shown in Fig.5.As shown in Fig.5,the composite membrane has narrow pore size distribution,small pore size and average pore size 27.1nm.3.3.Surface characterization of the composite membraneTo con firm the self-assembly TiO 2nanoparticles on the composite membrane surface and further to estimate the abrasive resistance of the membrane surface,X-ray photoelectron spectroscopic (XPS)analyses were carried out for the PES/TiO 2composite membrane treated under various conditions.The constituent elements of the composite membrane surface are hydrogen,carbon,oxygen,sulfur,chlorine,and titanium.Thus,XPS analyses were performed on the elements of carbon,oxygen,sulfur,chlorine,and titanium,but not on hydrogen because its photoelec-tron cross-section was too small to be characterized by XPS.The core-electron binding energies of the constituent elements are typically 287eV (C 1s),537eV (O 1s),23eV (O 2s),229eV (S 2s),270eV (Cl 2s),199eV (Cl 2p)and 458eV (Ti 2p)[22].Fig.6shows the resulting spectrum,in which all the photoelectron peaks appear at positions similar to the above values and the presence of Ti peaks.The results provide evidence of TiO 2self-assembly on the composite membrane surface.On the basis of the observed photoelectron peaks and corresponding sensitivity factors,the relative atomic concentrations of the individual elements can be calculated:C i ¼A i =S iP mj A j =S j(3)where A i is the photoelectron peak area of the element i ,S i the sensitivity factor for the element i ,and m theM.-L.Luo et al./Applied Surface Science 249(2005)76–8479Fig.2.XRD spectrum of the synthesized TiO 2.Fig.3.TEM micrograph of the TiO 2nanoparticles.number of the elements in the sample.In Table 1,the elemental compositions determined by an angle-resolved XPS analysis were summarized for the com-posite membranes with different washing conditions and UF operation time.There was an initial drop in the relative atomic concentration of Ti element after washing the composite membrane,which had been just formed from dipping into the TiO 2colloidal solution.An additional loss of TiO 2nanoparticles was observed upon further UF operation with run time of 3h.The UF process was operated in the cross-flow mode where the feed solution was pumped across the composite membrane parallel to its surface.It was found that TiO 2particles were wiped out after 3h UFM.-L.Luo et al./Applied Surface Science 249(2005)76–8480Fig.4.SEM micrographs of the UF membrane:(a)cross-section;(b)neat surface;(c)surface treated by TiO 2colloidalsolution.operation and thought the loosely bound TiO2parti-cles cannot overcome the shear force.However,the TiO2loss did not continue to progress as the UF operational time increased more,and the amount of TiO2leveled-off after10h of UF operation.This result indicates that a considerably substantial amount of TiO2nanoparticles remains tightly bound on the surface of the membrane under actual UF running conditions,which is expected to improve the hydro-philicity of PES membrane and prevent from the membrane fouling.3.4.Design of TiO2nanoparticle self-assembled composite membraneTiO2nanoparticle in the anatase form is very hydrophilic,photoactive and practical for the wide-spread environmental applications such as water purification,wastewater treatment,hazardous waste control,air purification,and water disinfection [19,23,24].In this research,composite UF membrane was devised by the self-assembly between TiO2 nanoparticle and polymer with the ether bond and sulfone group(Fig.7)because of the strong electronegative of oxygen in the ether bond and sulfone group of the PES.As UF process was operated in the cross-flow mode under high pressure,simply adsorbed particles may be detached from membrane surface.XPS results in Table1indicate that some TiO2 particles in composite membrane have a sufficient binding strength for the actual operation,which agree with other researches on the interaction behavior of TiO2nanoparticle[18,25].It is concluded that a novel organic–inorganic membrane is successfully prepared by self-assembly process.3.5.Hydrophilicity and UF performance of the composite membraneThe hydrophilic and separation performance of the membrane surface untreated and treated by TiO2 colloidal solution are presented in Table2.The area of UF membrane is38.5cm2,applied pressure 0.2MPa,operational temperature258C and feed concentration200ppm(PEG-5000)in this experi-ment.As shown in Table2,the contact angle of the membrane treated by TiO2colloidal solution become small and theflux and the retention increase greatly.M.-L.Luo et al./Applied Surface Science249(2005)76–8481Fig.6.XPS spectra of the elements on the composite membrane. Table1Elements compositions of the PES/TiO2composite membrane undervarious washing conditions and UF operational timeSample a Takeoff angle(8)Relative atomic concentration(%)C O S Ti Cl14543.9530.8810.77 5.728.6824541.8032.8410.77 5.169.4334540.9534.5210.97 3.589.9844540.9534.5410.97 3.569.98a Analyses were performed for the TiO2self-assembled UFmembranes(1)just after preparation,(2)after washing withflowingwater,(3)after UF operation of3.0h,(4)after UF operation ofanother10.0h.The initial structure of the UF membrane is changed due to the self-assembly of TiO 2nanoparticles and the ridge-and-valley structure on the membrane surface is more clear.The roughness of the membrane surface increases.Generally,the hydrophilicity of membrane surface increases with the increase of roughness [26].On the other hand,the hydroxylcontent of membrane surface increases due to incorporate TiO 2nanoparticles into membrane sur-face.The hydroxyl is polarity and can interact well with water molecules through van der Waals ’force and hydrogen bond.So the probability of water permeated through the composite membrane enhances and the probability of PEG-5000permeated through the composite membrane drops off.Fig.8shows that the fluxes of the neat PES membrane and the treated PES membrane vary with time.As seen in Fig.8,the PES membrane treated by TiO 2colloidal solution always keeps higher flux.This experimental result shows that the anti-fouling performance of the PES ultra filtration membrane is improved remarkably due to TiO 2nanoparticles self-assembly.A strong potential for possible use as a new type of anti-fouling UF membrane is offered.M.-L.Luo et al./Applied Surface Science 249(2005)76–8482Fig.7.Mechanism of self-assembly of TiO 2nanoparticles:(I)by a coordination of sulfone group and ether bond to Ti 4+;(II)by a H-bond between sulfone group and ether bond and surface hydroxyl group of TiO 2.Table 2Contact angle and UF performance of membrane Sample a Contact angle (8)Flux (L m À2h À1)Retention (%)539.670.221.9619.2102.934.5aAnalyses were performed for (5)the neat and (6)the TiO 2self-assembled UF membranes.4.ConclusionsMembrane fouling by hydrophobic substances has been known to be the main cause to deteriorate the UF performance of PES membranes.It was developed that a new type of composite membrane as an approach to solve fouling problem.Its anti-fouling effect was characterized.The TiO2nanoparticle of a quantum size(40nm or less)in anatase form was prepared by the controlled hydrolysis of titanium tetraisopropoxide.The particle structure and size were characterized by X-ray diffraction(XRD)analysis and transmission electron microscopy(TEM).TiO2 nanoparticles were incorporated onto the poly(ether sulfone)membrane surface by self-assembly.X-ray photoelectron spectroscopy(XPS)demonstrated quantitatively that TiO2particles were tightly self-assembled with a sufficient bonding strength for the actual UF process.The contact angle test of the composite membrane shows that the hydrophilicity of the membrane surface improves remarkably.The fouling experiment verified a substantial prevention of the composite membrane against the hydrophobic substances fouling,suggesting a possible use as a new type of anti-fouling composite membrane. AcknowledgementsThe authors wish to acknowledge thefinancial support from the National Basic Research Program-2001CB-2091grant for this project.Thanks are also due to Environmental Science Research Institute, Chinese Academy of Sciences for supplying us with the PES membrane and Xi’an Modern Chemistry Research Institute for access to the XRD,XPS,TEM and SEM equipments.We also would like to thank Prof.Fengji LU for many valuable discussions. 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