Catalytic Ozonation By Metal Doped-TiO₂ For Degradation Of Organic Pollutants In Water

With the rapid development of industry, the human being are facing ever increasing problem of water crisis. The traditional water treatment process is difficult to remove toxic organic compounds from water. Having many advantages, such as: quick response, high efficiency for the degardation of stabel organic pollutants, advanced oxidation processes are widely investigated. In addition to being used as a photocatalyst, TiO2 also showed good catalytic activity in the field of catalytic ozonation. In this paper, nanosized rutile TiO2 and metal doped TiO2 were prepared by sol-gel method, and investigated for their effectiveness in degrading organic pollutants in catalytic ozonation.The best way of catalyst doping were selected and the preparation conditions were optimized, using nitrobenzene and oxalic acid as the target respectively. The surface properties of the catalysts were characterized. The mechanism of catalytic ozonation was speculated.In this paper, nano-sized TiO2 was propared by sol-gel method, and nitrobenzene was used as the target for catalytic ozonation experiment. Different crystal structure of TiO2 have different catalytic properties. Anatase TiO2 almost has no ability for catalytic ozonation, while rutile TiO2 has good catalytic properties. In the rutile-type TiO2 catalytic ozonation reaction, the nitrobenzene removal rate increases respectively with the increase in the amount of catalyst, ozone concentration, reaction temperature. The removal rate of nitrobenzene increases with the increase of solution pH. When the solution pH value is 10, the degradation rate of nitobenzene is highest. In different water background, the removal rate of nitrobenzene were almost the same. Rutile TiO2 catalyzed ozonation is the dominant reaction mechanism of hydroxyl radical. Degradation intermediates of nitrobenenze generated during catalytic ozonation is basically the same with those generated in ozonation alone: That is: carboxylic acids, aldehydes and ketones, phenols.Cobalt, iron, nickel, manganese and zinc were chosen for doping TiO2. The presence of Mn/TiO2,Ni/TiO2,Co/TiO2,Fe/TiO2,Zn/TiO2 are significantly increased the removal rate of nitrobenzene by ozonation comparing with the case of ozone oxidation alone. Rutile TiO2 catalytic effect is not significantly different from the five types of catalyst. Doping is not able to increase the effect of catalytic ozonation of nitrobenzene. Trace metal ion was released from the catalyst, but the concentrations of metal ion dissolved are far below the national standards.The presence of Mn/TiO2 and Co/TiO2 significantly improved the removal rate of TOC. By the analysis of intermediate products, Mn/TiO2 and Co/TiO2 can further oxidize nitrobenzene intermediates, such as oxalic acid which is difficult to be directly oxidized by ozone molecules.Using oxalic acid as a target, the catalytic activity of catalysts were examined. Mn/TiO2 and Co/TiO2 have good catalytic ability, and can significantly enhance the ability of ozone to remove oxalic acid. TiO2,Ni/TiO2,Fe/TiO2,Zn/TiO2 have poor capacity to remove oxalic acid by catalytic ozonation. The calcination temperature and ratio of metal ion doping were optimized. Changing the calcination temperature and doping ratio almost had no effect on Ni/TiO2,Fe/TiO2,Zn/TiO2 for improving the catalytic activity. The optimal calcination temperature of Mn/TiO2 is 500℃, with a mixed phase of anatase and rutile. The best molar doping ratio of Mn/TiO2 is 1/20. The optimal calcination temperature of Co/TiO2 is 500℃, with a mixed phase of anatase and rutile. The best molar doping ratio of Co/TiO2 is 1/30.The influencing factors on the degradation of oxalic acid in catalytic ozonation process were studied respectively with Mn/TiO2 and Co/TiO2 as the catalyst. Within the ozone flux in this experiment, the changing of ozone flux almost had no effect on oxalic acid removal rate. Oxalic acid removal rate decreased with the increase of initial concentration of oxalic acid. Oxalic acid removal rate increased with the increase of the amount of catalyst used and the water temperature respectively, and almost reached completely removal. The removal rate of oxalic acid reacting at 10℃, was much lower than those achieved at 20℃, 30℃, 40℃. The charge of positive and negative values of Mn/TiO2 surface is affected by pH value, and the ionization degree of oxalic acid and by pH values. The pH value significantly affected the removal rate of oxalic acid. At lower initial pH values, oxalic acid removal rate is faster.Measured by XRD, the doping of Mn and Co make the transition temperature of anatase to rutile phase increase, and the size of catalyst increases. Obeserved by means of XRF and XPS, the proportion of Mn and Co on the catalyst surface is higher than the proportion of internal side of the catalyst. Mn and Co were combined with the titanium and went into the TiO2 lattice. Mn and Co in the catalyst were in the form of MnO2 and TiCoO3. There were Ti3+ in the surface of the catalyst, and oxygen vacancy and lattice defects were formed. In the catalyst surface, Ti mainly exists in the form of Ti4+. In the catalytic ozonation, oxalic acid was mineralizad and converted directly into CO2 and H2O. Tert-butyl alcohol addition and ESR experiments showed the generation of hydroxyl radical in catalytic ozonation. Hydroxyl radical is the oxidant formed in this reaction, but not the only oxidant. It is speculated that oxalic acid was absorbed or complexed in the catalyst surface in the first step. Oxalic acid in the surface of catalyst was oxidized by hydroxyl radical and ozone molecule. The adsorption of oxalic acid is the control steps in the whole reaction.