Adsorption And Photocatalytic Degradation Of Organic Pollutants In POM- And Fluoride-Modified TiO₂ Suspensions

Photocatalytic technology based on TiO₂ has provided an effective and promising means for the remediation of environmental pollutants in air and water. However, a vital problem, i.e., low quantum efficiency (-4%), hampered its widespread practical applications. One effort of the research in this field is to modify the surface properties of TiO₂ particles. In this thesis, the adsorption and photocatalytic degradation of organic pollutants, anonic dye X3B and phenol, in an aerated aqueous suspensions of polyoxometalate- and fluoride-modified TiO₂ under UV light were investigated. The thesis consists of three sections as follows.Section I deals with the critical impact of adsorption on the rate of organic degradation by TiO₂ photocatalysis. The photocatalytic degradation (PCD) of organic contaminant in water over supported or unsupported TiO₂ is generally monitored by the concentration change of target organics in the liquid phase. In this work, we show that such apparent initial rate of PCD is much smaller than the total initial rate of PCD, determined by total loss of X3B, an anionic azo-dye, both from the solution and on the TiO₂ surface. For a safety reason, such factor of adsorption should be considered into the routine assessment about the relative photoactivity among different highly sorptive catalysts, and about the kinetic analysis as well.Section II has examined the effects of polyoxometalate and fluoride on adsorption and photocatalytic degradation of X3B on TiO₂. It has been reported that addition of polyoxometalates (POM) or fluoride anions into the TiO₂ dispersions can significantly enhance the photocatalytic degradation of weakly adsorptive organic pollutants in water. However, we observed that the degradation rate of X3B was significantly decreased in the presence of POM, whereas the rate was greatly increased in the presence of fluoride. It was noted that the adsorption of X3B on TiO₂ was markedly decreased in the presence of either POM or fluoride. It is proposed that the surface occupation of POM on TiO₂ accelerates the production of surface-bound -OH radicals, whereas the fluoride replacement of surface hydroxyl groups of TiO₂ increases the production of free hydroxyl radicals in solution, It is also suggested that the reactive species following the relative order of free hydroxyl radicals > subsurface holes > surface-bound hydroxyl radicals.Section III has made a deep investigation into the effect of fluoride on the photocatalytic degradation of phenol on TiO₂ prepared under different conditions. It was found that the presence of fluoride only enhanced the degradation of phenol on TiO₂ with anatase phase, while it retarded the phenol degradation on TiO₂ with pure rutile phase. The intermediates of phenol degradation were investigated and the reaction mechanism was also discussed.