Propagation of U(V)-reduction in the presence of U(IV) aggregate in a weakly acidic solution

Highlights: • Reduction product from UO₂⁺ forms UO₂ aggregate in a weakly acidic solution. • The aggregates formed in a bulk solution accelerates disproportionation of UO₂⁺. • Deposition of UO₂ on the electrode surface promotes electrode reduction of UO₂⁺ - Abstract: Reduction processes of UO₂²⁺ in weakly acidic solutions were investigated based on electrochemical and spectrophotometric measurements. A reversible one-electron reduction of UO₂²⁺ to UO₂⁺ and a further irreversible reduction of UO₂⁺ were observed voltammetrically at a gold microdisk electrode in weakly acidic perchlorate solutions of pH from 2.0 to 3.5. Aggregates of U(IV) were formed as a deposit on the electrode and a colloid in the bulk solution, when the controlled-potential bulk electrolysis was carried out at a gold gauze electrode, even though the potential applied was that available for the first one-electron reduction wave of UO₂²⁺ observed at a gold microdisk electrode. It was elucidated that the aggregate was produced by the combination of the one-electron reduction to UO₂⁺ and the disproportionation of UO₂⁺ producing U(IV) and UO₂²⁺. The aggregate enhanced the rate of the disproportionation of UO₂⁺, and hence the reduction current of UO₂²⁺ increased abruptly when a definite amount of aggregate was formed on the electrode, in the solution, or both. The aggregate of a two-electron reduction product of UO₂²⁺ was determined to be poorly crystallized UO₂ of fluorite-type structure, and the characteristics of the aggregate were different from the crystalline UO₂