Approaching zero discharge in uranium reprocessing: Photochemical reduction of uranyl

We have studied the photochemical reduction of uranyl to generate UO2 witho ut hydrogen reduction. Formate and oxalate were examined as potential reduc tants that only lead to CO2 production as a side product. Despite the simil ar nature of the two reductants, the mechanism for quenching the uranyl exc ited-state changes drastically and leads to dramatically different chemistr y at low pH. Oxalate quenches by unimolecular electron transfer and formate quenches by fl-atom abstraction. Because of the change in mechanism, photo chemical 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 acid no net uranyl reduction. Photochemical reduction of uranyl with for mate 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%.