Electrochemical and spectrochemical analysis of U(VI) reduction in nitric acid solutions

The reduction behaviors of U(VI) ions in nitric acid media were investigated electrochemically by cyclic voltammetrys (CVs) and square wave voltammetrys (SWVs). The electrochemical redox reactions of uranium between different oxidation states was found to proceed quasi-reversibly at 295 K. At low nitric acid concentrations and in the absence of additional chemical reductant, the reduction of U(VI) was found to be diffusion controlled. The diffusion coefficient of U(VI) was determined to be 3.30–8.83 × 10 −6  cm s −1 and the activation energy for the diffusion of U(VI) was calculated as 23 kJ mol −1 by cyclic voltammetry at different temperatures. The reduction of U(VI) at relatively high nitric acid concentrations was further studied by chemical reduction (hydroxylamine, N,N-diethylhydroxylamine and acetaldoxime), potentiostatic/galvanostatic electrolysis, as well as the two ways combined (electrolysis in the presence of chemical reductants). No obvious reduction of U(VI) was observed by absorption spectroscopy when only electrolysis or chemical reductants were used. In contrast, U(VI) could be obviously reduced to U(IV) by electrolysis in the presence of chemical reductants. Moreover, hydroxylamine was found to be the best choice among the three chemical reductants used as holding reductant to ensure efficient reduction of U(VI) by electrolysis. Temperature has mixed effect on the electrolytic reduction rate as evaluated by absorption spectra.