Approaching Zero Discharge in Uranium Reprocessing:  Photochemical Reduction of Uranyl

Abstract

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%.

Photoluminescence of Uranium(VI): Quenching Mechanism and Role of Uranium(V)

“…The quenching of uranium(VI) luminescence by halide ions was studied by DFT calculations on monohalide 1:1 complexes with saturated first shell (UO 2 …”

“…In the F À system (no quenching), spin density is localized almost exclusively on the uranyl unit (spin density 1.93 on UO 2 ), indicating that the singletto-triplet excitation occurs by excitation within the uranyl Table 1. The structural parameters (uranium to halogen interatomic distance) in the ground state and the lowest-lying triplet state of uranyl(VI) monohalide UO 2 …”

“…The forth column gives the vertical transition energy from the lowest-lying triplet state to the ground state. + is 1.27 and 0.74 for UO 2 and Br entities, respectively. This trend may be predictable from the electronegativity of the halide ions, but the extension to organic compounds, such as methanol, requires a more detailed quantum chemical description as performed here based on DFT.…”

“…Here, TD-DFT is again used in a comparative manner, but is now applied to singlet-to-singlet transitions in UO 2 F + , UO 2 Cl + , and UO 2 I + . The five low-lying singlet excited states of UO 2 F + , UO 2 Cl + , and UO 2 I + and their major MO contributions are given in Table 2.…”

“…Its application to nuclear fuel reprocessing by selective reduction of uranium(VI) to uranium(IV) had been a subject of a number of investigations in the 1970s and 1980s, and uraA C H T U N G T R E N N U N G nium(VI) photoreduction has also been a subject of investigation during the last ten years. [2][3][4][5][6] Uranyl(VI) photochemistry further forms the basis for uranium(VI) detection and determination of its speciation by laser-induced luminescence spectroscopy. [7] Photoreduction and photoinduced luminescence of uranium(VI) are closely related to each other, because in both cases the first process is photoexcitation of uranium(VI) to higher excited states.…”