Photocatalytic Reduction Of Uranyl Ions Over Nanostructured TiO₂

Uranium is naturally a radioactive element, which can seriously affect biologicalgrowth and even lead to various diseases after it enters organisms. Due to thedevelopment of current nuclear industry, there is an increasing amount of waste watercontaining radioactive uranium, which was harmful to natural environment andhuman being. Thus, the appropriate treatment is necessary before discharge. Thetraditional processes have many disadvantages, such as, trivial operation, high cost,secondary contamination, and so on. In recent years, titanium dioxide （TiO₂）-basedphotocatalysis has received considerable attention as an alternative for treating andrecovery of heavy metal ions from wastewater.Titania （TiO₂） has been intensively studied as a semiconductor photocatalyst forvarious applications owing to its high activity, chemical stability and nontoxicity.Among the three polymorph phases of TiO₂, i.e. anatase, rutile and brookite, only thefirst two have been widely studied and their photoactivity was tested and compared.On the contrary, only a few recent papers dealing with the photoreactivity of purebrookite owing to the difficulties of obtaining its pure form. It was generally acceptedthat the photoreactivity of anatase was better than that of rutile. But contraryconclusions were usually drawn in the literatures on the relative photoreactivity of thetwo types TiO₂. The disagreement of the results may lie in the intervening effect ofvarious coexisting factors, such as crystal size, specific surface area, crystalmorphology, and preparation methods, or in the way the activity is expressed.The photocatalytic reduction of U（VI） over TiO₂（Degussa P25） catalysts underultraviolet （UV） light irradiation was investigated in the second part of the thesis.Operating parameters such as the pH of solution, dissolved oxygen levels, and thehole scavenger were examined in detail to determine the optimal experimental conditions. Results showed that the process follow the pseudo-first-order reaction inkinetics. Under the tested conditions, reduction is more efficient at near-neutral pH.The photocatalytic of U（VI） was obviously promoted by methanol. Kineticinvestigations with different methanol concentrations demonstrated that U（VI）photoreduction was dependent on methanol concentration; the more methanol, thefaster the uranyl photoreduction rate. The presence of dissolved oxygen will preventthe process and will result in rapid reoxidation of any deposited photoreduced uranyl.In the third part of the thesis, rutile, anatase and brookite were prepared throughhydrothermal treatment at certain temperature. The effect of the crystalline phase ofTiO₂on its photocatalytic activity has been investigated by testing a series of bothhome-made and commercial TiO₂photocatalysts. Results revealed that the relativephotoreactivity of the TiO₂samples was as follows: RR> BR> AR APFurthermore the effects of crystal morphology on the photoreactivity of theanatase were also studied in the forth part of the thesis. It was found that themorphological difference between the anatase does not exert significant influence ontheir performance in photocatalytic reduction of uranyl ions.