An in-situ X-ray absorption spectroelectrochemical study of the electroreduction of uranium ions in HCl, HNO3, and Na2CO3 solutions

Abstract

A spectroelectrochemical cell was fabricated for in-situ X-ray absorption spectroscopy (in-situ XAS). The XAS spectra of the uranium LIII edge were monitored in electrolyte solutions during the electrochemical reduction. Tetravalent uranium, U4+, in 1 mol dm-3 (M) hydrochloric acid (HCl) was electrochemically prepared from hexavalent uranium, UO22+, by constant current electrolysis, and the extended X-ray absorption fine structure (EXAFS) was analyzed. The concentration ratio of UO22+ and U4+, which were formed via the disproportionation of pentavalent uranium, UO2+, during the electrolysis, were calculated based on the intensity of the signal for the two axial oxygen atoms in the linear UO22+ unit, the U-Oax, bond that had a radial structural function. The apparent redox potential of the UO22+/U4+ couple in 1 M HCl was determined based on the Nernst equation using the concentrations of UO22+ and U4+. The electrode potential was shown to be close to the formal potential of the UO22+/UO2+ couple as reported previously. This result indicates that the UO2+ that was formed electrochemically at the electrode disproportionated to form UO22+ and U4+ in the bulk solution. The in-situ XAS of UO22+ in 0.1 M nitric acid was also performed. The U4+ that formed was partially re-oxidized to UO22+ by the NO3- present in the solution. The formation of the UO2+ carbonato complex was observed by in-situ XAS in a 1 M sodium carbonate solution during the bulk electrolysis. The edge jump of the X-ray absorption near edge structure (XANES) spectrum shifted from 17.164 to 17.163 keV, and the bond distances of U-Oax and U-O for CO32- increased from 1.78 to 1.88 Å and from 2.42 to 2.53 Å, respectively, because of the reduction of the UO22+ to the UO2+ carbonato complex.