Photochemical reduction of U(VI) in aqueous solution in the presence of 2-propanol

• UV–vis photochemical reduction of uranyl is revisited.
• Mechanism depends on the presence of anions and irradiation conditions.
• Role of nitrate as an intermediate in uranium reduction not previously reported.
• Nitrate/short UV–vis light is the best system for U(VI) removal.

The efficiency of the photochemical reduction of uranyl (UO22+) (0.25 mM, pH 3) was analyzed in the absence and in the presence of 2-propanol (2-PrOH), evaluating the effect of anions (nitrate, acetate and perchlorate) and the use of quartz and glass photoreactors. A higher removal was observed under all conditions with the quartz photoreactor; the best system was that starting from uranyl nitrate, where a 90% UO22+ removal was attained after 15 min irradiation. In the absence of 2-PrOH, the acetate system was the only one where some U(VI) removal was observed (15% after 15 min in the quartz photoreactor) but, in the presence of the alcohol, the acetate system was the least reactive. Under irradiation, nitrate and acetate were consumed during the reaction, causing important effects on the uranyl conversion, while perchlorate was inactive. Uranium was removed from the system as a solid residue in the case of nitrate- (as U(IV)) and perchlorate-containing systems (as U(VI)), while it remained in solution in the case of the acetate systems. Mechanisms are proposed for the different conditions. The importance of nitrate as a reducing agent of uranium species has not been previously reported. The use of uranyl nitrate and short-near UV–vis light seems to be the most advisable way of removing uranyl from solutions, in the form of a stable precipitate.

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