| In view of the membrane fouling caused by photocatalysts in suspended photocatalysis coupling with membrane process, TiO2 photocatalyst with spherical (P25), tubular (TNT) and wirelike (TNW) morphology were considered in this thesis. On basis of the investigation of colloidal and interfacial properties, structural morphologies and photocatalystic activities of photocatalysts, the factors affecting aggregation in water, and deposition on membrane and particle size variation in photodegradation were firstly studied, aiming to provide reference for the selection of suitable photocatalyst and operating condition in alleviating the membrane fouling in the coupling process. Secondly, the fouling mechanism of low-pressure hollow fiber membrane caused by commercially available TiO2 particles (P25) were studied based on the viewpoint of morphological observation, theoretical calculation and colloidal interaction between photocatalyst and pollutant, which can provide useful guidance for the selection of membrane and design of coupling process in suspended photocatalysis coupling with membrane process.The coagulation and sedimentation behavior of P25, TNT and TNW were investigated by determining surface charge density, analyzing coagulates, performing sedimentation experiments and calculating interaction energy via DLVO theory. The effect of solution pH and morphology of photocatalyst were considered. The results indicated that morphology could change surface charge density but could not shift the point of zero charge (PZC). The PZC of the three applied photocatalysts were located at around pH=6.0. The stability of photocatalyst in suspension was dependent on solution pH and morphology of photocatalyst. Median size do.s, fractal dimension df and initial setting velocity v0 of coagulation were found to increase with pH at first and then decrease, all three of them can be up to maximum at pH=6.0. The order of d0.5 and df were TNW>TNT>P25 and TNT>P25>TNW, respectively. The calculation of potential energy by the aid of DLVO theory suggested the potential barrier between two particles was the fateful factors that dominated the stability of photocatalyst with different morphology. The linear relationship between initial setting velocity v0 and potential barrier indicated the promising application of potential barrier in predicating the stability of TiO2 in suspension.Flat sheet ultrafiltration (UF) membrane made of polyethersulfone (PES) was applied in separating P25, TNT and TNW from their suspension. Effect of the state of coagulation and operating pressure on membrane fouling were investigated via scanning electronic microscope (SEM) observation, specific cake resistance calculation and compressibility comparison. The morphological effect of photocatalyst on their deposition was considered. The aggregation of TiO2 was benefit to alleviate membrane fouling, for the cake made of highly aggregated nanoparticles, the slippage and rotation of zero-dimensional P25 in cake formation resulted in cake compress along with the increase of applied pressure. Two stages of compression with compressibility n of 0.238 and 0.351 were observed. These motions were totally and partially inhibited by using one-dimensional TNT and TNW photocatalysts respectively. While for cake made of relatively dispersed TiO2, SEM observation confirmed that orientation behavior of these rigid dispersed aggregates overwhelmed applied pressure and controlled the specific cake resistance at high pressure. On the whole, the order of membrane fouling caused by photocatalyst was P25>TNT>TNW.The photocatalytic activities of the three kinds of photocatalysts were evaluated via degradation of dye active brilliant red X-3B (X-3B). The intermediates in photodegradation of dye were analyzed and their effect on particle size was studied. Two types of flat sheet UF membrane made of Polyacrylonitrile (PAN) and PES, respectively, were used to separate photocatalyst and photodegraded product from their mixture, the influence of photodegradation of dye on membrane fouling and permeate quality were mainly considered. The order of photocatalytic activity in discoloration was P25>TNT>TNW but in minalization the order was TNT>P25>TNW. The low molecular acid and inorganic ions produced in photodegradation of dye made the solution conductivity increase and solution pH decrease. However, low molecular acid dominated the aggregation of photocatalyst and resulted in the decrease of d0.5 from 10.239,12.667 and 22.178 um to 3.341,7.012 and 13.572?m, for P25, TNT and TNW, respectively. Depended on the relative strength of membrane fouling caused by dye and photocatalyst, photodegradation of dye decreased the membrane fouling of PAN membrane but increased the flux decline of PES membrane. UF membrane was capable of separating photocatalyst from reacted suspension, but its effect on enhancing permeate quality was poor.With consideration of large specific surface area, wide application in industrial scale and easy to realize continuous operation, low-pressure hollow fiber membranes were applied to separate the commercially available TiO2 (P25). The separation efficiency, membrane fouling and backflushing efficiency was considered. The fouling mechanism of membranes with different materials and pore structures were investigated in detail by fitting flux decline, observing cake structure with the help of SEM, qualitative analyzing cross-section of membrane by the aid of X-ray energy dispersive spectrometer (EDS), calculating filtration resistance and force exerted on photocatalysts. The cross-flow filtration experiments were performed on PAN UF membrane with MWCO 60 kDa, PES UF membrane with MWCO 65 kDa, and other two kinds of PES microfiltration (MF) membrane with pore size 0.2 ?m and 0.4 ?m. The rejections of P25 by the four kinds of membrane were up to 99.9%. Hydrodynamic backflush was unable to completely recover TiO2 catalysts and eliminate the filtration resistance due to the adhesion of nanosized TiO2 catalysts in/on the membranes. Rapid flux decline happened after TiO2 particles entered into membrane, blocked internal pores and formed cake layer on the top of membrane. The fouling behaviors of membranes were explored and associated with membrane pore size as well as the interaction between TiO2 particles and membrane. The fundamental relationship was set up between the maximum force Fmax imposed on photocatalyst and the characteristics of cake layer. It was found that cake layer made of TiO2 aggregates presented the ability to resistant external force. This property resulted in the cake layer with dense bottom layer and loose top layer, and the porosity of TiO2 cake increased along the height of the cake in the presence of flux decline. Fmax played the dominated roles in affecting membrane flux and cake structure.Filtration performances of membrane were investigated in a photocatalysis coupling with PAN hollow fiber UF membrane process. X-3B dye and P25 was adopted as target pollutant and photocatalyst, respectively. Special attentions were focused on the dye-TiO2 interactions and their effect on membrane flux and dye rejection. Solution pH was proved to be the predominant force that controlled the interactions by changing the surface charge characteristics of TiO2 and altering the size and fractal dimension of TiO2 aggregates. Those behaviors not only determined the property and structure of deposit layer but also resulted in the alteration of surface hydrophilicity. In dye-TiO2 interaction region, photodegradation of dye enhanced the membrane flux, but this effect became insignificant in non-adsorptive regions. Aggregate size and fractal dimension were applied in the calculation of the specific cake resistance by the aid of Carman-Kozeny equation. The theoretical specific cake resistance was close to the experimental specific cake resistance for the cake made of single-component TiO2, while due to the dye-TiO2 interaction, the model was invalid in predicting the specific cake resistance in filtration of dye-TiO2 mixture. The rejection of dye in the presence of TiO2 was found to decrease markedly due to the deposition of TiO2 particles on membrane interface. Bridging effect of TiO2 between membrane interface and dyes produced by electronic interaction, coordination and hydrogen bonding was responsible for the decrease of dye rejection. |
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