| Arsenic is widely distributed in the natural water environment, and 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 water. As (Ⅲ) is more toxic and more mobile than As (V), conventional water treatment technologies are efficient for the removal of As (V) but less for As (Ⅲ). Accordingly, in order to improve the efficiency of the removal of arsenic, it is necessary to oxidize As(III) to ease of handling As (V).Semiconductor photocatalysis can achieve energy clean production and environmental pollutants treatment. It has the merits of non-secondary pollution, low cost and simple process, and etc., showing an attractive prospect. TiO2photocatalysis can oxidize As (III) to As (V). However, its drawbacks are that the efficiency is low and that it can only absorb the UV light accounted for-5%of sunlight. The research in this thesis is dedicated to overcome the above two shortcomings, and the study is divided into two parts and5chapters. In Part one, fluorine and iodine ions are employed to improve the photocatalytic oxidation rate of As (Ⅲ) over ultraviolet irradiated nanostructured TiO2thin films, respectively, and its respective mechanism is investigated as well. In Part two, regenerated dye photosensitization is used to oxidize As (Ⅲ) under visible light, iodide ions are employed to further enhance the oxidation rate and the relevant reaction mechanism is discussed. Main results are as follows:Addition of fluoride ions in aqueous solutions can improve the rate of TiO2photocatalytic oxidation of As (Ⅲ) but the mechanism is not clear enough. In Chapter3, the influence of fluoride ion concentration, external applied potential, film thickness and other parameters on the separation efficiency of photocarriers in nanostructured TiO2thin films in the absence of oxygen, was systematically studied by a photoelectrochemical method. The results show that fluoride ions could significantly improve the separation efficiency or interface transfer rate of photoholes, consequently improving the oxidation rate of As (III) via photoholes. By the measurements of electron density, Mott-Schottky curve, transient photovoltage decay, fluorescence spectra, and etc, it was demonstrated that fluoride ions could decrease the electron density, negatively shift the flatband potential and reduce the surface recombination rate of photocarriers, which are the main reasons for the improved charge separation efficiency.Considered that TiO2photocatalytic oxidation rate of As (Ⅲ) is low and that suspended catalysts are difficult to recover, in Chapter4, the effect of iodine ion concentration, applied potential, electron acceptor, light intensity and etc on the nanostructured TiO2photocatalytic oxidation of As (III) was systematically studied. It was found that under illumination and in the presence of nitrogen, iodide could efficiently scanvenge photoholes to produce active iodine species and depress the recombination of photocarriers, thus enhancing the oxidation of As (III); in the dark and in the presence of air, iodide could promote the reduction of oxygen to produce more active oxygen species, therefore improved the dark oxidation rate of As (III) whereas it is not more than40%of theoretical expectation, indicating iodide deactivated a large amount of active oxygen species; Under illumination, open-circuit and in the presence of air, although iodide increased the As (Ⅲ) oxidation via the photooxidative pathway, it decreased that via the photoreduction pathway due to its reaction with active oxygen species, thus the promotion of iodide is not very significant under these situations.In Chapter5, with typical renewable dye, N719bipyridine ruthenium as a sensitizer, the effect of As (Ⅲ) concentration, potential, O2, light intensity and other factors on the oxidation of As(Ⅲ) over nanostructured TiO2thin films under visible light, was studied. The results show that As (Ⅲ) could be highly efficiently oxidized under visible light at open-circuit and in the presence of air. Mechanistic studies show that under the conditions of deoxygenating, illumination and anodic bias potentials (thus no superoxide radical, O2·-the Faraday efficiency of As (Ⅲ) oxidation by dye cations (S+) was close to100%, suggesting that S+is able to highly efficiently oxidize As (Ⅲ); while in the dark, with air and at cathodic bias potentials (thus without S+) the Faraday efficiency of O2·-was close to100%as well. Under the conditions of open-circuit, air and visible illumination, the relative contribution of S+and O2·-to the oxidation of As (Ⅲ) is33%and67%, respectively.To further enhance the oxidation of As (Ⅲ) and the stability of the catalyst for the regenerated N719dye photosensitization system, the influence of iodide ions on the photosensitization oxidation of As (Ⅲ) was studied in Chapter6. The results show that addition of iodine ions could significantly increase the oxidation of As (Ⅲ) under both open-circuit and bias condition. When the initial As (Ⅲ) concentration is greater than0.3mM, the Faraday efficiency is close to100%. At0.4V (versus a saturated calomel electrode) and in the presence of nitrogen, the oxidation rate was improved by a factor of7-fold. In the dark and with air, although iodide ions could promote the reduction of oxygen, both the formation rate of As (Ⅴ) and the Faraday efficiency were reduced; under open circuit conditions, the role of iodide ions in promoting the oxidation of As (Ⅲ) was not apparent as that under bias and in the presence of nitrogen.In Chapter7, the mechanism of the rate of dye photosensitized oxidation of As(III) promoted by iodine ions was studied. Thermodynamics data and experimental results support that I2·-is the major intermediates in the photosensitized oxidation process. Under illumination and in the presence of N2, I" promoted the regeneration of S+by a one-electron reaction. The efficiency of I2·-oxidizing As(III) is approximate to100%. In the dark and with air, although I-accelerated the production rate of electron-initiated reactive oxygen species(EIROS), it decreased the formation of As(V) because EIROS reacted with I-to generate I2·-whose oxidative capacity is weaker. Under OC and with I-, the ratio of formation of As(V) through S+was increased from33%to71.1%, while that via EIROS was decreased from67%to28.9%.These results provide a foundation for the full use of sunlight to achieve the oxidation of As (III) and the adopted photoelectrochemical methods give an important reference for the mechanism research of photocatalytic and dye photosensitization oxidation of other pollutants. |
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