Photochemical Reactions of Uranium(VI) in Aqueous Phosphoric Acid Solutions

Abstract

Photochemical reactions of U(VI) ion with halogen and pseudohalogen anions. I−, Br−, Cl− and NCS−, were studied in aqueous phosphoric acid solutions under irradiation of nitrogen laser. The formation of U(IV) was observed in the reactions with I−, Br− and NCS−, but not with CI−. The yield of U(IV) increased in the order Br−<NCS−<I−. This order was the same as the quenching rate constants of the excited U (VI) ion with these anions, but the reverse of their standard redox potentials. These facts are consistent with the electron transfer mechanism between excited U (VI) and these anions. The rates of the formation of U (IV) in the presence of Br− were measured by the spectrophotometric method. It was found that the rate equation was first order in both U (VI) and Br−. The results were reasonably interpreted by a series of reaction processes involving U(V) and Br radical. The photo-oxidation of tris(1, 10-phenanthroline)-Fe (II) ion by U (VI) was also studied in the same procedures. Based on the rate law, we proposed a one-electron transfer mechanism involving U(V) as an intermediate.