Autocatalytic reaction of uranium reduction and formation of colloidal particles in a weakly acidic solution

Kitatsuji, Yoshihiro  ; Otobe, Haruyoshi  ; Kihara, Sorin*

We have studied electrolytic redox behaviours of U ions under weak acidic conditions. UO is reduced to UO reversibly. The UO ion disproportionates to produce U and UO in an acid solution, and the reaction rate of the disproportionation is lower in weaker acidic solution up to pH ca. 2.1. It was found that reduction current of UO increased abruptly after the induction period of several minutes. Acceleration of UO reduction coincides to adsorption of reduction product on the electrode surface which was observed by the electrochemical quartz crystal microbalance. As a result of electrolytic reduction of UO to U(IV), black fine particles were formed both on the surface of the electrode and in the solution. The size of the U(IV)-containing particles formed in a solution of pH 2.5 was analysed to be ca. 10 nm. The particles in the colloid solution were filtered after aging the colloidal particles, and filtrate was analysed by X-ray diffraction measurement system. Characteristic diffraction peaks of XRD pattern of the U-containing particles can be assigned to be poorly crystallized UO of fluorite structure. These facts suggest that the aggregated U(IV) work as catalyst for the electrolytic reduction and the disproportionation of UO.