Approaching zero discharge in uranium reprocessing: photochemical reduction of uranyl.

We have studied the photochemical reduction of uranyl to generate UO2 without hydrogen reduction. Formate and oxalate were examined as potential reductants that only lead to CO2 production as a side product. Despite the similar nature of the two reductants, the mechanism for quenching the uranyl excited-state changes drastically and leads to dramatically different chemistry at low pH. Oxalate quenches by unimolecular electron transfer and formate quenches by H-atom abstraction. Because of the change in mechanism, photochemical reduction of uranyl with formate works with high efficiency at low pH while photolysis in the presence of oxalate leads to the generation of CO and no net uranyl reduction. Photochemical reduction of uranyl with formate at low pH leads to U(IV) in solution that can then be precipitated as UO2 by simply raising the pH with yields as high as 99.992%.