The aquatic thermodynamics of uranium is an important tool for evaluating the environmental risks of radioactive waste repositories. Lately, it has also become an indispensable tool in designing effective strategies for acid recuperation of uranium. Presently, there are two main methods for obtaining the standard Gibbs free energies of uranium species and precise values of ion activities. The first approach is obtaining them from redox potentials, and the second approach is obtaining them from half-wave potentials by performing voltammetries using inert noble metal electrodes immersed in solutions with redox species.
Effective application of these two approaches requires a thorough understanding of the kinetics of the interfacial reaction, particularly for multi-electron processes involving adsorption. Moreover, there are limited mechanistic studies on inert electrodes for acidic conditions, especially for reducing UO22+ to U4+. This is important because it plays a vital role in ensuring the preservation of UO22+ and U4+ as initial and final species, respectively. To date, there is a growing body of knowledge on uranyl reduction in acidic conditions. Most of these studies are based on polarography where successive electron transfer steps are assigned to different waves.
There are, however, several discrepancies involving the interpretation and analysis of the reduction waves. Recently, disproportionation reaction has been used to describe the effects of acid concentration on the diffusion currents for the initial polarographic wave. Interestingly, this reaction was observed to follow a widely accepted disproportion rate equation and the reaction can be retarded when a U(V)-U(VI) complex is formed.
Inspired by the previous findings, Andrés G. Muñoz and Stephan Weiss from Gesellschaft für Anlagen-und Reaktorsicherheit (GRS) GmbH and Helmholtz Zentrum Dresden-Rossendorf investigated the kinetics of electrochemical reduction of uranyl on gold in hydrochloric acid (HCl) solution of pH< 1. The proposed reaction mechanism was validated through numerical modeling of the voltammetric waves based on the reaction model to determine the kinetic parameters. This reaction model consisted of a first quasi-reversible electron transfer and a second interfacial U(V) intermediate species reduction with the adsorption of U(IV) products generated during the process. The role of chloride in the electron transfer as well as the implication of the disproportionation reaction during the interfacial process, was examined. Their work is currently published in the Journal of the Electrochemical Society.
The researchers showed the strong dependence of kinetic parameters on the [HCl], suggesting the occurrence of electron transfer through the inner sphere mechanism due to the chloride ligand adsorption. Through surface oversaturation of U(IV), it was possible to form micro-crystalline UO2. The electron transfer steps were closely followed by chemical dissolution assisted by a large proton concentration that facilitated the desorption and transformation of unstable uranyl in U(H2O)94+. The accumulated chloride strongly repelled the complexed uranyl, preventing the deposition of uranyl complexes at [HCl] of approximately 0.5 mol l-1. This was the cause of the significant reduction of the electron transfer rate.
In summary, the study reported the kinetics of electrochemical reduction of UO22+ to U4+ in a hydrochloric acid medium. Results showed that using a reduction mechanism that comprised a quasi-reversible first electron transfer and subsequent irreversible second electron transfer was highly effective for reproducing the current-potential curves of the voltammetric-based experiments. In a statement to Advances in Engineering, Dr. Andrés Muñoz said that the presented mechanism involving numerical reproduction of voltammetric waves is a promising strategy for determining kinetic parameters for multi-step electrochemical reactions.
Muñoz, A., & Weiss, S. (2022). Kinetic Aspects of the Electrochemical Reduction of Uranyl in HCl Solutions. Journal of the Electrochemical Society, 169(1), 016510.