Redox Processes in Solid‐State Uranyl (Oxy)hydroxide Minerals

Abstract

AbstractThe correlation between the structure of a series of uranyl minerals of the type [(UO2)8O2(OH)12] and M[(UO2)xOy(OH)z] (M=Na, x=4, y=2, z=5; M=K2, x=6, y=4, z=6; M=Ca, x=6, y=4, z=6), and their electrochemical properties have been investigated by using solid‐state cyclic voltammetry. Solid‐state electrochemical investigations at near‐neutral pH values reveal that all minerals undergo redox reactions in the potential range from −0.72 to −0.91 V vs. Ag/AgCl. This process is assigned to the U(VI)/U(V) redox couple and a further reduction, observed between −0.95 and −1.20 V, is associated with the U(V)/U(IV) reduction. The nature of the reduced products has been probed by using vibrational spectroscopy and SEM−EDX, and was found to be UO2+x. The mechanism of reduction has been elucidated and shown to be dominated by two independent one‐electron transfer reactions with disproportionation playing at most a minor role. The rate of charge propagation through the solid deposits, characterized by the homogeneous charge‐transport diffusion coefficient, DCT, is much lower in these minerals than structurally related systems with ion, rather than electron, transfer being rate limiting. Overall, the insights into the nature and rate of redox conversion suggest that the uranyl minerals formed by the oxidative corrosion of spent nuclear fuels are not electrochemically innocent.