| Arsenic is one of the most toxic environmental pollutants. It presents mainly in the form of trivalent arsenic (As(Ⅲ)) and pentavalent arsenic (As(Ⅴ)) in the natural environment. Compared to As(Ⅴ), As(Ⅲ) is more toxic and more mobile. So, As(Ⅲ) is generally oxidized to As(Ⅴ) and then is removed by conventional water treatment techniques. But the techniques are generally expensive and even may cause secondary-pollution.Semiconductor photocatalysis has the merits of low cost, simple equipment and process, and non-secondary-pollution. It can achieve removal of environmental pollutants, solar hydrogen production, photoelectrochemical corrosion and other applications by solar light. But the currently reported efficiency is still rather low. So the aim of this thesis is to improve the efficiency of photocatalytic oxidation of As (Ⅲ) under visible light. The following three-part work was carried out.Metal-containing dye such as bipyridine ruthenium photosensitized TiO2can oxidize As(III), but the cost is high. In part one, with a typical and cheap non-metallic dye-Eosin Y (C20H8Br4O5) sensitizer, the kinetics and mechanism of photocatalytic oxidation As(III) over TiO2films under visible light were investigated. The results showed that although the photooxidation rate of As(Ⅲ) was rather slow in aerated aqueous solutions (pH=3.0) and at open-circuit, addition of I" could significantly increase the rate at both open-circuit and an anodic potential bias. As(III) could be highly efficiently oxidized with a Faraday efficiency of～100%under anodic bias, in the presence of I-but without oxygen. In the dark, with a cathode bias and air, As(Ⅲ) could be oxidized by the superoxide radical (O2·-) with a Faraday efficiency of～100%as well. The dye was regenerable and consequently photostable. Mechanistic studies suggest that the dye cation could not directly and efficiently oxidize As(III), but could react with I" to generate I2·-which enabled As(III) to be highyly oxidized.Hematite (α-Fe2O3) is an ideal visible light catalyst used in photoelectrochemical oxidation water, particularly for Ti-doped α-Fe2O3, but the photocatalytic oxidation of As(III) has not been reported. In part two, electrochemical reduction processing was used, Ti-doped α-Fe2O3films electrodes as working electrodes, a platinum mesh as counter electrodes, a saturated calomel electrodes as reference electrodes,1M NaOH solutions as electrolyte. The effect of photooxidation of As(III) on Ti-doped α-Fe2O3films electrodes without and with electrochemical reduction processing was investigated. The results showed that photooxidation rate of As(III) could be improved by electrochemical reduction processing. Mechanistic studies suggested that As(III) could be directly oxidized to formate As(V) by the occurrence of two-electron reaction.To further improve photocatalysis activity of Ti-doped α-Fe2O3films electrodes, In part three, electrochemical deposition method was used in0.05M Fe(III),0.08M TEA and2M NaOH solutions, Ti-doped α-Fe2O3films electrodes as working electrodes, a platinum mesh as counter electrodes, a saturated calomel electrodes as reference electrodes. Fe3O4surface-modified Ti-doped α-Fe2O3films electrodes was photocatalysts and the effect of deposition potential and deposition time was studied. Mechanistic studies suggested that the increase of reaction rate was not determined by the change of light absorption, but due to accelerating the rate of interfacial charge-transfer and/or decreasing recombination of the photo-generated carriers. |
PDF Full Text Request