Mechanism And Kinetics Of Degrading Organic Pollutants By Photocatalytic Ozonation Process

Photocatalysis could mineralize organics completely to CO₂, water or inorganic ions,but the recombination rate between holes and photo-induced electrons is very high,leading to low efficiency in removing pollutants. Ozonation is facile to degrade organics,but the reaction is selective and mineralization ability is low. Combining photocatalysiswith ozonation （photocatalytic ozonation） is a feasible way to make up each other. Onone hand, the presence of ozone would restrain the recombination of electron-hole pairsand improve the photocatalytic activity. On the other hand, photocatalysis could removeozone inert pollutants. Therefore, photocatalytic ozonation has revealed wide prospectfor water treatment.Taking the H-acid as the pollutant model, the degradation of it was investigated inphotocatalysis （TiO₂/UV）, ozonation （O₃/UV） and photocatalytic ozonation （TiO₂/O₃/UV）processes. The effect of three reactive species: holes, hydroxyl radicals and superoxideradicals, was systematically studied. The intermediates in TiO₂/UV, O₃/UV andTiO₂/O₃/UV processes were detected by HPLC/MS and the degradation pathway ofH-acid in TiO₂/O₃/UV process was analyzed. The kinetics of H-acid degradation inTiO₂/O₃/UV process was studied based on the Langmuir-Hinshewood model and pseudofirst order kinetic equation. Further, a visible light reactive photocatalyst, Cu（Ⅱ）-TiO₂,was prepared by grafting Cu ions on the surface of TiO₂. This Cu（Ⅱ）-TiO₂wascombinated with ozone to degrade oxalic acid under visible light. The mechanism of thissystem and the stability of Cu（Ⅱ）-TiO₂were investigated. The results of this thesis wereshown as followed:After reaction for10min,97%of H-acid was degraded in TiO₂/O₃/UV processmuch higher that in TiO₂/UV （12%） and O₃/UV （70%） processes. When iodide ions wereadded to H-acid solution in TiO₂/UV process, the degradation rate of H-acid decreased from82%to15%after reaction for120min. When benquine and t-butanol were added toH-acid solution in TiO₂/O₃/UV process individually, the influence to degradation rate ofH-acid is weak. The results indicated that H-acid was basically oxidized by holes on thesurface of TiO₂and ozone and the effect of hydroxyl radicas and superoxide radicals wasminor. Fluorescenence emission spectra revealed that the emission inte nsity of TiO₂became low in the presence of ozone. After bubbling ozone to TiO₂suspension for40min, the intensity decreased to65%of the original one, indicading that ozone couldeffectively restrain the recombination between photo-induced holes and electrons andimprove the efficiency of photocatalysis.Based on the results of HPLC/MS, H-acid was firstly oxidized to mutil-hydroxyaromatic compounds, unsaturated benzene carboxylic acid and quinones. After furtherreaction, the aromaticity of intermediates was totally removed and reached completelymineralization through Photo-Kolbe reactions.Based on the Langmuir-Hinshewood model and pseudo first order kinetic equation,the kinetic equatin of degrading H-acid in TiO₂/O₃/UV process was obtained. Thehomogenerous reaction rate constant k, heterogenerous reation rate constant k’ozoneandadsorption equilibrium constant were turned out to be0.293min-1、14.302ppm/minand0.464ppm-1respectively. According to the constants, the photocatalytic heterogenerousreaction rate and ozonic homogenerous reaction rate were higher than that in TiO₂/UVand O₃/UV respectively. According to the values of k and k’ozone, the Hatta number wascalculated to be0.13in TiO₂/O₃/UV process. The value of Hatta number lower thancritical value0.3, indicating that ozone could reach the surface of TiO₂and achievesynergetic effect.By grafting Cu ions on the surface of TiO₂, visible light reactive Cu（Ⅱ）-TiO₂wasprepared. In the degradation experiments for3h, oxalic acid was almost not degraded byozonation;30%of oxalic acid was removed in Cu（Ⅱ）-TiO₂/vis; almost all oxalic acid was removed in Cu（Ⅱ）-TiO₂/O₃/vis, much more efficient than other processes. When iodideions were added to oxalic acid solution, the degradation rate of oxalic acid inCu（Ⅱ）-TiO₂/vis decreased from30%to14%. When t-butanol was added to the solution,the degradation rate in Cu（Ⅱ）-TiO₂/O₃/vis changed little. It could be induced that oxalicacid was basically oxidized by holes and the effect of hydroxyl radical was minor inCu（Ⅱ）-TiO₂/O₃/vis. Experiments were taken to investigated the stability of Cu（Ⅱ）-TiO₂inphotocatalytic ozonation reaction and it turned out that Cu（Ⅱ）-TiO₂would not deactiveafter several reaction cycyles. Accordingly, this photocatalytic ozonation system is anefficient and stable advance oxidation process.