Research For Preparation And Application Of SiO₂/TiO₂ Composite In Micro-polluted Water

The increasing of organic micro-pollutants in source water has brought serious challenges for drinking water treatment. Micro-polluted raw water treated by regular water purification process can't guarantee the security of water quality. Therefore it needs to add depth process on the basis of the previous technology. In recent years, TiO2 has been attended in environmental treatment and energy exploitation due to thoroughly mineralization of organic matter, good bactericidal effect and inhibition of viral activity. However, the practical application of TiO2 has been very limited because the e--h+ pairs produced by TiO2 in the illumination are easy to recombine and TiO2 doesn't fully utilize solar energy with a larger band gap. There fore, modifications of TiO2 become a research focus to improve its photocatalytic activity.In this paper, the SiO2/TiO2 composites were prepared by two-step sol-gel method. The degradation of micro-organic pollutant in water used to choose higher photocatalytic activity of SiO2/TiO2. And further to modify SiO2/TiO2 to obtain more efficient photocatalysts. The structures of the catalysts were characterized using XRD, TG-DTA, FT-IR, UV-vis, XPS and TEM methods.As well as, their mechanisms were studied in detail. In this experiment, the main contents of study were as follows: (1) SiO2/TiO2 composites were prepared with tetraethoxysilane and tetrabutyl titanate as silicon source and titanium source respectively, anhydrous ethanol as solvent, acetyl acetone as inhibitor and hydrochloric acid as catalyst for hydrolysis of TEOS. The optimum preparation conditions of the SiO2/TiO2 were determined by photocatalytic degradation effects of micro-polluted humic acid in water and the characterizations of SiO2/TiO2 were analyzed. Results showed that the introduction of silicon inhibited phase transformation and particle size increase of TiO2 under the same calcining temperature. The best calcining temperature was 800℃when silicon doping concentration was 10% for SiO2/TiO2.(2) The calcining temperature of SiO2/TiO2 was determined by silicon doping concentration. Water content was greatly impacted on the photocatalytic activity of SiO2/TiO2. In experiment,5% silicon doping concentration for SiO2/TiO2 was prepared at 600℃and was applied to degradation of trace methyl orange in water. Analysis's of SiO2/TiO2 characterized by XPS before and after etching showed that the Si atom was diffused to powder particle surface during heat treatment process, which leading to appear Ti4+at the outer surface, at the same time, Ti3+and oxygen at the near surface of TiO2. The key reasons for higher activity of SiO2/TiO2 were the combined effects of Ti3+ and oxygen vacancies with the co-existence of Ti4+and Ti3+could effectively separate charge.(3) The loaded TS2.5-550 films were prepared by thermal deposition. Their degradation of HA in micro-polluted water and inactivation of E.coli were studied. The result showed that degradation rate of HA was faster than the inactivation rate of E.coli by TS25-550 films during photocatalytic process. The mechanism was that e--h+pairs produced on film under UV light illuminate had redox ability and lots of active species such as·OH and·O2- were generated when e- and h+pairs reacted with dissolved oxygen in water and water respectively. HA might be oxidized to nontoxic substances such as CO2 and H2O and E.coli cell might be damaged by these active species.(4) Gd3+ doped SiO2/TiO2 photocatalytic materials were prepared using sol-gel method and their photocatalytic activities were studied by the mineralization of HA in slightly polluted water (pH=6.5). The results showed that HA in water was thoroughly degraded and the mineralization rate of HA was 97% by Gd-SiO2/TiO2 photocatalyst after 150 min. This was an increase nearly 38% to the value obtained for TiO2 and of 30% for that of SiO2/TiO2 when measured in parallel. We believed that the reasons for the higher activity of Gd-SiO2/TiO2 composites could be explained as follows:1) Gd3+doping inhibited the phase transformation of TiO2 from anatase to rutile, as well as grain growth. Under the same conditions, the average crystal grain of Gd-SiO2/TiO2, SiO2/TiO2 and TiO2, respectively, were 25.5 nm,35.2 nm and 37.8 nm; 2) Gd3+ doping not only enhanced the absorption light intensity within the wavelength 200 to 800 nm, but also moved the absorption band edge to shorter wavelengths, thereby broadening the band gap of the catalyst semiconductor. Under the same conditions, TiO2, SiO2/TiO2 and Gd-SiO2/TiO2 band gap was 2.57 eV,2.62 eV and 2.69 eV; 3) the amount of surface defect Ti3+ and lattice oxygen on the Gd-SiO2/TiO2 catalyst were increased. These would help to improve the photocatalytic activity.(5) Fe doped SiO2/TiO2 photocatalysts responded to visible light were prepared and the degradation of HA in micro-polluted water was selected to investigate the photocatalytic activity of Fe-SiO2/TiO2. Characterization results showed that Fe doping not only made the absorption edge of catalyst red shift and increased the absorption intensity of visible light for the catalyst. In addition, the Fe3+/Fe2+ redox pairs were prepared on catalyst surface which were useful for separation of the e--h+ pairs. Therefore the visible light activity of Fe-SiO2/TiO2 was significantly higher than that of SiO2/TiO2 and TiO2.