Kinetics of the laser-induced photoreduction of U(VI) in aqueous suspensions of TiO2 particles

The reduction of U(VI) to U(IV) in aqueous suspensions of TiO2 (pH 4.5-7) was investigated by time-resolved laser-induced fluorescence (TRLIF). The laser irradiation was used both to provide the energy necessary for the photogeneration of charge carriers and to monitor the kinetics of the oxidation state change directly in the measurement cuvette by exciting the U(VI) fluorescence. The U(VI) photoreduction was found to involve only adsorbed uranium species. The TRLIF measurements of U(VI) fluorescence lifetimes and individual emission fluorescence spectra allowed a selective identification of the Various U(VI) hydro complexes present in the aqueous-phase and adsorbed on the mineral surface. A reaction mechanism is proposed based on the kinetics results obtained at pH 7 where adsorption of the hydroxocomplex (UO2)(3)(OH)(5)(-) on TiO2 is complete.