Chemical and electrochemical behavior of uranium(VI) in oxide salt melts: The effect of the nature of the salt-solvent’s cation and anion

Abstract

The effect of the electrolyte composition, deposition potential, temperature, and the salt-solvent’s cation on the oxygen coefficient (atomic ratio [O]/[U]) of uranium oxides is studied. The oxides are obtained by potentiostatic electrolysis of the M₂EO₄-M₂E₂O₇-UO₂EO₄ melts (M = Li, Na, K, Cs; E = W, Mo, S). The oxygen coefficient of the cathodic product is found to increase with the melt temperature. Shifting the deposition potential toward more electronegative values, raising the M₂E₂O₇ concentration, and lowering the concentration of UO₂EO₄ in the oxide salt electrolytes promotes the formation of uranium oxides at the cathode with smaller values of the oxygen coefficient. All other conditions being equal, the latter diminishes with increasing radius of the salt-solvent’s cation in a series of alkali metals. The electrolytes on the basis of Li₂WO₄ and Li₂MoO₄ are discovered to behave abnormally. Replacing the tungstate electrolytes by the molybdate ones with the same cationic composition (at constant $c_{M_2 E_2 O_7 }$, $c_{UO_2 EO_4 }$, and T) amplifies the oxygen coefficient of the cathodic product. The results of the study are satisfactorily explained in the framework of a model for the ionic composition of uranyl-containing oxide salt electrolytes, which is based on the notions concerning the complexing and stepwise solvolysis of the uranyl ions.