Heterogeneous photocatalytic removal of U(VI) in the presence of formic acid: U(III) formation

The efficiency of TiO2 heterogeneous photocatalysis under UV–visible light for removal of uranyl (UO22+) (0.25 mM, pH 3) in the presence of formic acid (HCOOH) was investigated. The effect of the counterion of the uranium salt (perchlorate and acetate–nitrate) and the use of quartz and glass photoreactors were analyzed. The transformation of U(VI) in the presence of HCOOH was significantly high and the best removal conditions have been established. The highest efficiency was observed for uranyl perchlorate in the quartz photoreactor with 0.001 M HCOOH, while uranyl acetate yielded a lower removal. HCOOH concentrations >0.01 M provoked uranium reoxidation. U(VI) photocatalytic transformation was also studied by direct spectrophotometry using TiO2 nanoparticles in the presence of 1 M HCOOH at pH < 2. U(V), U(IV) and U(III) were detected. U(III) formation was also apparent in the absence of the photocatalyst. This is the first time that formation of U(III) in a photochemical U(VI) system is reported.