Reduction of uranium(Ⅵ) at titanium electrode in nitric acid hydrazine media

Abstract

This work has investigated the electrochemical properties and kinetics on the reduction of uranium(VI) at titanium electrode in a simulated solution of nitric acid and hydrazine. The analysis of a system with the electro-reduction and the mass transfer of uranium(VI) from organic phase was simulated by a model. The reduction of uranium(VI) needed a very high activation overpotential at non-pretreated titanium electrode in nitric acid and non-neutralized hydrazine media. It was affected by the ratio of hydrazine mono-hydrate to nitric acid rather than by only absolute amount of hydrazine mono-hydrate. When the above ratio exceeded 0.5, the reduction current decreased rapidly due to decrease of solution conductivity and increase of iR drop which were caused by consuming proton in the solution by the hydrazine mono-hydrate. There is less effect on the uranium reduction by the hydrazine mono-hydrate when the ratio is less than 0.5. The reduction of uranium(VI) was governed by charge-transfer controlled mechanism. It also was totally irreversible below potential showing a limiting current in current-potential curves. The experiment was performed for the neutralization of hydrazine mono-hydrate by nitric acid at pH 4.25. The reduction peaks of uranium(VI) of the voltammograms measured by electrochemical non-pretreated titanium electrode was not clear enough to be used for the kinetic analysis of the reduction of uranium(VI). The existence of oxide film at the titanium electrode without non-electrochemical pretreatment was confirmed by EPMA and a chronoamperometry method. The oxide film was effectively removed by a cathodic polarization method. The time for the removal of the oxide film, $t_{pr}$, depended only on a supplying electrolytic potential, $E_{pr}$. $$t_{pr} = 15.8E_{pr}\,^{-3.1}(-0.55\le{E_{pr}}\le{-0.9}\;V)$$ The electrode pretreated by the cathodic polarization gave good reduction peaks of uranium(VI) in the voltammogram by which kinetic analysis could be...