Mechanistic Features of the TiO2 Heterogeneous Photocatalysis of Arsenic and Uranyl Nitrate in Aqueous Suspensions Studied by the Stopped-Flow Technique

Abstract

The dynamics of the transfer of electrons stored in TiO2 nanoparticles to AsIII, AsV, and uranyl nitrate in water was investigated by using the stopped-flow technique. Suspensions of TiO2 nanoparticles with stored trapped electrons (etrap-) were mixed with solutions of acceptor species to evaluate the reactivity by following the temporal evolution of etrap- by the decrease in the absorbance at λ=600 nm. The results indicate that AsV and AsIII cannot be reduced by etrap- under the reaction conditions. In addition, it was observed that the presence of AsV and AsIII strongly modified the reaction rate between O2 and etrap-: an increase in the rate was observed if AsV was present and a decrease in the rate was observed in the presence of AsIII. In contrast with the As system, UVI was observed to react easily with etrap- and UIV formation was observed spectroscopically at λ=650 nm. The possible competence of UVI and NO3- for their reduction by etrap- was analyzed. The inhibition of the UVI photocatalytic reduction by O2 could be attributed to the fast oxidation of UV and/or UIV.