磺化聚醚砜_纳米TiO_2复合超滤膜制备及其抗污染机理_英文_

SEPARATION SCIENCE AND ENGINEERINGChinese Journal of Chemical Engineering, 19(1) 45—51 (2011)Fabrication of SPES/Nano-TiO2 Composite Ultrafiltration Membrane and Its Anti-fouling Mechanism*LUO Mingliang (罗明良)**, WEN Qingzhi (温庆志), LIU Jialin (刘佳林), LIU Hongjian (刘洪见) and JIA Zilong (贾自龙)College of Petroleum Engineering, China University of Petroleum (East China), Qingdao 266555, ChinaAbstract Membrane fouling is one of the major obstacles for reaching a high flux over a prolonged period of ul-trafiltration (UF) process. In this study, a sulfonated-polyethersulfone (SPES)/nano-TiO2 composite UF membrane with good anti-fouling performance was fabricated by phase inversion and self-assembly methods. The TiO2 nanoparticle self-assembly on the SPES membrane surface was confirmed by X-ray photoelectron spectroscopy (XPS) and FT-IR spectrometer. The morphology and hydrophilicity were characterized by scanning electron mi-croscopy (SEM), atomic force microscopy (AFM) and contact angle goniometer, respectively. The anti-fouling mechanism of composite UF membrane was discussed through the analysis of the micro-structure and component of UF membrane surface. The results showed that the TiO2 content and the micro-structure of the composite UF membrane surface had great influence on the separation and anti-fouling performance.Keywords anti-fouling, ultrafiltration membrane, sulfonated-polyethersulfone, TiO2 nanoparticle, phase inversion, self-assembly1 INTRODUCTIONIn recent years, more and more attention in ul-trafiltration (UF) membrane has been attracted for a variety of applications in wastewater treatment, sub-stance separation, solute concentration, and so on. The major drawback in the extensive use of membranes includes membrane fouling, which results in flux de-cline during operation [1]. Polyethersulfone (PES) is a special engineering plastics. It possesses many good characteristics such as high mechanical property and heat distortion temperature, good heat-aging resistance, environmental endurance as well as easy processing. It has become an important membrane material, but its hydrophobicity controlled by PES molecular structure leads to low membrane flux and poor anti-fouling performance [2]. Recently, modification methods of UF membrane involve ultraviolet irradiation [3], graft polymerization [4, 5], glow discharge [6], ozone [7, 8], and so on in order to improve PES hydrophilicity. Among these methods, blending with inorganic materials, es-pecially nanoparticles, has attracted much interest due to their convenient operation and mild conditions.Nanoparticles used to modify organic membranes include SiO2, TiO2, Al2O3, and so on [9], among which TiO2 receives most attention because of its stability, availability, and hydrophilicity [10]. Molinari et al. prepared TiO2/polymer composite membranes in order to develop photocatalytic membrane reactors for waste-water treatment [11-13]. Kwak and coworkers studied the ability of a TiO2/polymer thin film composite (TFC) reverse osmosis membrane under ultraviolet (UV) radiation to mitigate biofouling by a photobactericidal effect [14, 15], focusing on the photocatalytic or photo-bactericidal effects of TiO2 nanoparticles under UV radiation. Recently, Luo et al. [16] and Yang et al. [17] prepared PES/nano-TiO2 and polysulfone(PSF)/nano- TiO2 composite UF membrane, respectively. The hy-drophilicity, mechanistic performance and thermal sta-bility of the composite UF membrane were improved greatly, but in the UF operation, the low washing re-sistance of nanoparticles on the membrane surface affects the anti-fouling period and service perform-ance due to the weak interaction between PES (or PSF) and nano-TiO2 resulted from the less active groups.In this study, sulfonated-polyethersulfone (SPES) is used as the membrane material due to its good per-formance as well as PES, the active site and functional group in SPES. The SPES/nano-TiO2 composite UF membrane is prepared by phase inversion and self-assembly methods, which is expected to present good anti-fouling and washing resistance performance. The morphology and hydrophilicity of UF membrane are characterized by scanning electron microscopy (SEM), atomic force microscopy (AFM) and contact angle goniometer, respectively. X-ray photoelectron spectroscopy (XPS) and FT-IR spectrometer are em-ployed to analyze the mechanism of nano-TiO2 self-assembly on the SPES membrane surface. To examine the fouling mitigation ability of membranes, a filtration experiment is carried out and the anti-fouling mechanism is discussed through the analysis of the micro-structure and composition of UF membrane surface.2 EXPERIMENTAL2.1 MaterialsThe SPES powder was obtained from JilinReceived 2009-12-08, accepted 2010-09-20.* Supported by the Natural Science Foundation of Shandong Province (Q2007B01). ** To whom correspondence should be addressed. E-mail: yfsailing_wxg@Chin. J. Chem. Eng., Vol. 19, No. 1, February 2011 46University, China. The inherent viscosity (ηinh) of the polymer in N-methyl pyrrolidone solvent and the equilibrium moisture uptake under ambient condition (relative humidity of 82% and 26 °C) were 1.27 dl·g−1 and 2.07%, respectively. The solvent N,N-dimethyl acetamide (DMAc) was in AR grade, and TiO2 col-loidal suspension was prepared by the sol-gel method in our laboratory [2].2.2 Fabrication of SPES/TiO2 composite ultrafil-tration membranesThe SPES ultrafiltration membrane was fabri-cated by phase inversion method. N,N-dimethyl acetamide (DMAc) solvent was used to dissolve the SPES for several hours under continuous agitation for complete dissolution and elimination of bubbles. The uniform transparent solution was then cast on a smooth glass plate with a knife edge. The thickness of the membrane was controlled by varying the thickness of adhesive tapes on the sides of the glass plate. The glass plate was kept in an environment with controlled tem-perature and humidity during membrane casting. After casting, the glass plate was immediately immersed in a gelling bath, which is generally demineralized water maintained at certain temperature, and the phase in-version started. After a few minutes, the thin polymeric film was taken from the glass. The film was washed repeatedly with demineralized water and stored wetted.The wet UF membrane was rinsed in a sodium carbonate solution [0.2% (by mass)] and then washed with demineralized water. The neat SPES membrane with an area of 38.5 cm2 was dipped in the transparent TiO2 colloidal solution, stirred for 1 min by ultrasonic method and immersed for 1 h, and then washed with demineralized water.2.3 Characterization2.3.1Characterization of the SPES/TiO2 composite UF membraneThe size of TiO2 nanoparticles was determined by a JEOL transmission electron microscope (TEM, JEOL JEM-200CX) at 120 kV. The surface morphol-ogy, roughness and mean pore size of the neat SPES membrane and the composite membrane were ob-served with a JSM-5800 scanning electron microscope (SEM) and a DualSope TM atomic force microscope (AFM). The pore sizes were measured by inspecting line profiles of different low valleys (i.e. pores) and high peaks (i.e. nodules) on the AFM images at differ-ent locations of a membrane surface. Then the mean pore size of membrane was calculated.The contact angle of membrane surface was the average value of 10 measurements by Eromag-1 contact angle goniometer and the measurement error was ±3°.A FT-IR spectrum of the composite was recorded on a Perkin-Elmer RXI over the range of 400-4000 cm−1. The spectral resolution was 4 cm−1. X-ray pho-toelectron spectroscopy (XPS) was employed to ana-lyze the component of the composite membrane sur-face with a PHI-5400 spectrometer. The spectra were taken at the takeoff angle (defined as the angle between the detected photoelectron beam and the membrane surfaces) of 45° to give a sampling depth of ca. 2.3 nm.2.3.2Separation performance of the SPES/TiO2 composite UF membraneThe mass transfer characteristics of UF membrane for 0.02% (by mass) polyethylene glycol (PEG-5000) aqueous solution were determined in an apparatus with a continuous flow at 0.2 MPa and 25 °C for 30 min. The water flux was calculated by direct meas-urement of the mass of the permeate flow:/J V A t=⋅(1) where J is the membrane flux (L·m−2·h−1), V is the permeate volume (L), A is the membrane area (m2), and t is ultrafiltration time (h).The solute rejection was defined aspf1100%cRc⎛⎞=−×⎜⎟⎝⎠(2)where R is the solute rejection, c f is the feed concen-tration, and c p is the permeate concentration.3 RESULTS AND DISCUSSION3.1 Morphology, roughness and pore size of the composite membrane surfaceThe TiO2 nanoparticle size assembled on the SPES membrane is determined by TEM, as shown in Fig. 1. The particle size is about 5-42 nm. The SEM graphs of the surface morphology before and after treated by TiO2 colloidal solution are shown in Fig. 2. The neat SPES membrane has the typical surface morphology of a ridge-and-valley structure [Fig. 2 (a)], but the structure is not apparent. Fig. 2 (b) displays the surface morphology of the TiO2 self-assembled com-posite membrane, where TiO2 nanoparticles appear as nodular shapes on the ridges so that the membranesurface has clear ridge-and-valley structure. Fig. 3Figure 1 TEM micrograph of TiO2 nanoparticlesChin. J. Chem. Eng., Vol. 19, No. 1, February 2011 47demonstrates the 3D AFM image of membrane sur-face for the neat SPES membrane and the SPES/TiO 2 composite membrane at a scan size of 5 µm×5 µm. The brightest area presents the highest point of the membrane surface and the dark regions indicate valley or membrane pores. The surface morphology is greatly changed due to TiO 2 nanoparticles assembled on SPES membrane and the ridge-and-valley structure on the composite membrane is distinct.The average pore size and roughness of mem-brane surface were obtained from AFM images using Danish Micro Engineering Scanning Probe Micro-scopes (DME SPM) software. The size of 30 pores in 1 µm×1 µm area of membrane surface was measured from height profile of two-dimensional AFM images using SPM software and the average value was re-ported. The results are given in Table 1. The mean pore size of the composite membrane surface decreases slightly due to the self-assembly of TiO 2 nanoparticles on the SPES membrane surface. The surface rough-ness parameters of the membrane, expressed in terms of the mean roughness (S a ), the root mean square of the Z data (S q ) and the mean difference between the highest peaks and lowest valleys (S z ), were calculated by DME SPM software in 10 µm×10 µm scan sizeand are presented in Table 1. The roughness parame-ters for the composite membranes increase remarkably. Since the roughness parameters depend on the Z -value, which is the vertical distance that the piezoelectric scanner moves, this relationship is expected. When the surface includes deep depressions (pores) and high peaks (nodules), the tip moves up and down over a wide range and the roughness parameter of surface is high. 3.2 XPS and FT-IR analyses of the composite membrane surfaceTo confirm the self-assembly TiO 2 nanoparticleson the composite membrane surface and further to(a) Neat SPES (b) SPES/TiO 2Figure 2SEM micrograph of UF membrane surface(a) Neat SPES (b) SPES/TiO 2Figure 3 AFM micrographs of UF membraneTable 1 Mean pore size and roughness of the neat SPESand SPES/TiO 2 membrane surfaceRoughness/nm SampleMean pore size/nmS aS qS zSPES 98 7.86 9.64 69.5 SPES/TiO 2 85 55.2 67.3 325Chin. J. Chem. Eng., Vol. 19, No. 1, February 201148 estimate the abrasive resistance of the membrane sur-face, FT-IR method was employed to analyze the in-teraction between nano-TiO 2 and SPES. X-ray photo-electron spectroscopic (XPS) was carried out for in-vestigating the change of membrane surface elements under various UF conditions.The FT-IR spectra of the neat SPES polymer and SPES/TiO 2 composites are shown in Fig. 4. Curve a is the FT-IR spectrum of the neat SPES. The absorption peaks at 1300 cm −1 and 1151 cm −1 attribute to the asymmetrical and symmetrical vibrations of the sul-fone group, respectively [18]. The absorption peak at 1243 cm −1 attributes to the stretching vibration of the ether C O C bond in the SPES polymer [19]. The absorption peak at 1028 cm −1 attributes to the sym-metrically stretching vibration of O S O in the sul-fonic group SO 3OH. The absorption peaks at 1465 cm −1 and 737 cm −1 are produced by substituent num-ber from 2 to 3 due to the incorporation of sulfonic group to the benzene ring [20]. Curve b is the FT-IR spectrum of the SPES/TiO 2 composite. In comparison to curve a, the positions of the peaks at 1243 cm −1 and 968 cm −1 shift to 1239 cm −1 and 975 cm −1, respec-tively, due to the self-assembly of TiO 2 nanoparticles in the SPES. Some new absorption peaks appear at 3435 cm −1, 1638 cm −1 and 1357 cm −1 due to the asso-ciated hydroxyl O H stretching vibration, C O in amide group characteristic peak [21] and Ti O bond vibration [22]. The existence of C O in amide group characteristic peak indicates the presence of a little DMAc in the composites. The results show that the hydrolysis and condensation of a tetra-n -butyl titanate (TBT) leads the formation of TiO 2 sol, and partially cross-linking reactions occur in the ultrasonic wave processing. It reveals that there is hydrogen bonding and bidentate coordination interaction between Ti OH or Ti OCH 2CH 3 and sulfonic group SO 2OH, ether CO C bond (or sulfone SO 2 group). The poten-tial chemical bond structure model of the SPES/TiO 2composite is shown in Fig. 5.Figure 5 Chemical bond structure models of SPES/TiO 2 compositeThe constituent elements of the composite mem-brane surface are hydrogen, carbon, oxygen, sulfur, chlorine, and titanium. XPS analyses were performed on the elements of carbon, oxygen, sulfur, chlorine, and titanium, but not on hydrogen because its photo-electron cross-section is too small to be characterized by XPS. The core-electron binding energies of the constituent elements are typically 287eV (C1s), 537eV (O1s), 23eV (O2s), 229eV (S2s), 270eV (Cl2s), 199eV (Cl2p) and 458eV (Ti2p) [23]. Fig. 6 shows the result-ing spectrum, in which all the photoelectron peaks appear at positions similar to the above values and Ti peaks appear. The results provide the evidence of TiO 2 self-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://i ii mj jjA S C A S =∑(3)where A i is the photoelectron peak area of element i , S i is the sensitivity factor for element i , and m is theFigure 4 FT-IR spectra of pure SPES and SPES/TiO 2 composites a —SPES; b —SPES/TiO 2Chin. J. Chem. Eng., Vol. 19, No. 1, February 2011 49number of the elements in the sample. In Table 2, the elemental compositions determined by an angle-resolved XPS analysis are summarized for the composite membranes with different washing conditions and UF operation time. There is an initial drop in the relative atomic concentration of Ti element after washing the composite membrane, which attaches to the surface when dipping into the TiO 2 colloidal solution. An ad-ditional loss of TiO 2 nanoparticles is observed in the UF operation for 5 h. The UF process was operated in the cross-flow mode where the feed solution was pumped across the composite membrane parallel to its surface. Some TiO 2 particles are wiped out and the loosely bound TiO 2 nanoparticles cannot overcome the shear force. However, the TiO 2 loss does not con-tinue as the UF operation time increases, and the amount of TiO 2 changes little after 15 h of UF opera-tion as shown by the samples No. 4 and No. 5. This result indicates that a considerable amount of TiO 2 nanoparticles remains tightly bound on the membrane surface under actual UF operation conditions, which is expected to improve the hydrophilicity of SPES mem-brane and prevent the membrane from fouling. The results of samples No. 4 and No. 6 indicate that the washing resistance of the SPES/TiO 2 composite mem-brane increases remarkably compared to the PES/TiO 2 composite membrane [16]. Thus the increase of active site, functional group and electronegativity on the SPES membrane surface enhances the interaction be-tween SPES and nano-TiO 2 based on the FT-IR and XPS analyses.3.3 Anti-fouling mechanism and separation per-formance of the composite membraneTiO 2 nanoparticles in the anatase form are very hydrophilic, photoactive and practical for wide envi-ronmental applications such as water purification, wastewater treatment, hazardous waste control, air purification, and water disinfection [24]. In this work, the composite UF membrane is devised by the self-assembly between TiO 2 nanoparticle and SPES with the ether bond, sulfuryl group and sulfonic group (as shown in Fig. 5) because of the strong electro-negative oxygen in the ether bond, sulfuryl group and sulfonic group of the SPES. As the UF process was operated in the cross-flow mode under high pressure, simply adsorbed particles may be detached from the membrane surface. XPS results in Table 2 indicate that some TiO 2 particles in the composite membrane have sufficient binding strength for the actual operation, which agrees with other researches on the interaction behavior of TiO 2 nanoparticles [25]. It is concluded that a novel organic-inorganic membrane is success-fully prepared by self-assembly process.The hydrophilic and separation performance ofFigure 6 XPS spectra of the elements on the composite membraneTable 2 Elements compositions of the SPES/TiO 2composite membrane under various washingconditions and UF timeRelative atomic concentration/% Sample ①Takeoff angle/(°) C O S Ti Cl 1 45 43.08 31.26 12.56 6.52 6.582 45 40.41 33.64 12.56 5.867.533 45 40.18 34.32 12.24 5.587.684 45 39.63 35.06 12.14 5.227.955 45 39.65 35.02 12.15 5.217.9764540.95 34.64 10.97 3.469.98① Analyses for the TiO 2 self-assembled SPES UF membranes(1) just after preparation, (2) after washing with flowing water,(3) after UF operation for 5.0 h, (4) after UF operation for an-other 15.0 h, and (5) after UF operation for another 45.0 h; (6) analysis for the TiO 2 self-assembled PES UF membranes after the same UF operation time as No. 4 sample [16].Chin. J. Chem. Eng., Vol. 19, No. 1, February 2011 50the membrane surface untreated and treated by TiO2 colloidal solution are presented in Table 3. The area of UF membrane is 38.5 cm2, the applied pressure is 0.2 MPa, operation temperature is 25 °C, and the feed concentration is 0.02% (by mass) (PEG-5000) in this test. As shown in Table 3, the contact angle of the UF membrane treated by TiO2 colloidal solution is smaller, but the flux and retention increase to some degree. Combined to the results of FT-IR and XPS analysis, it is shown that the hydrophilicity of the PES itself is im-proved through sulfonation and nano-TiO2 self-assembly on the SPES membrane surface and the antifouling performance is better. On the other hand, the SEM and AFM photographs show that the microstructure of SPES membrane surface is changed due to nano-TiO2 self-assembly. The ridge-and-valley structure with micro- or nano-scale is distinct on the SPES membrane surface (Fig. 3) and the surface roughness is changed (Table 1). Generally, the contact angle of hydrophilic surface decreases with the increases of roughness [26], so the contact angle of the composite UF membrane declines remarkably. Since the hydroxyl is rich on the nano-TiO2 surface and TiO2 nanoparticles have high hydrophilicity and large specific surface, the hydroxyl content on the composite membrane is increased greatly by the self-assembly of nanoparticles on the membrane surface and the membrane hydrophilicity is higher. The water molecules are easy to permeate through the membrane and the flux increases signifi-cantly. At the same time, the pore structure is changed with TiO2 incorporated into membrane. The pore sizes become more uniform and the surface becomes more compact, so the retention is improved.4 CONCLUSIONSMembrane fouling by hydrophobic substances is the main cause to deteriorate the ultrafiltration (UF) performance of polyethersulfone (PES)-type mem-branes. A new type of composite membrane is devel-oped as an approach to solve the fouling problem. TiO2 nanoparticles are incorporated onto the sulfonated polyethersulfone membrane surface by self-assembly. The micro- or nano-scale ridge-and-valley structure of the composite UF membrane is examined with scan-ning electron microscopy (SEM) and atomic force microscopy (AFM) and the roughness of membrane surface is determined. The FT-IR and X-ray photo-electron spectroscopy (XPS) demonstrate that TiO2 particles are tightly self-assembled with sufficient bonding strength for the actual UF process. The con-tact angle test of the composite membrane shows that the hydrophilicity of the membrane surface is improved remarkably. 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