Redox Stability of Uranium in High Ionic Strength Media: An Electrochemical and Spectroscopic Study
Authors: Lehmann Susanne , Bok Frank , Ikeda-Ohno Atsushi , Munoz Andres Gabriel 
Published: February 2020

Knowledge of redox behavior of uranium in concentrated solutions of the hexary oceanic salt system is essential for the transport modeling of radioactive nuclides in repositories for the long-term disposal of radioactive wastes in rock salt formations. Especially critical is the behavior of the couple U(IV)/U(VI) in the surrounding high ionic strength saline milieu, which regulates the release of mobile U(VI) species at given redox conditions defined by the presence of trace oxygen. The redox potential (Eredox) and the solubility in brines in particular implicate the activity of the different ligand- and hydroxo-complexes (ai): Eredox = f(mi, ai, βj), where βj is the complexation constant. Thus, the prediction of transport behavior of uranium at given chemical environments needs appropriate complexation and thermodynamic models assuming the ionic activities beyond the limited Debye-Hückel theory. This latter is supplied by the Pitzer formulation1, where deviations from the limit Debye-Hückel behavior are given by specific interaction parameters among the constituting ionic species, which have to be determined experimentally.
For the determination of the redox potential, an approach consisting in unfolding the redox system in two electrochemical reactions is pursued: (i) U(VI) → U(V) and (ii) U(IV) → U(V), where the kinetics of each partial reaction is studied systematically at different concentrations of the ground electrolyte. Thus ir i (Eredox) = ia ii (Eredox). In this work, we focus on the aqueous chloride system.
The kinetics of the electrochemical reduction and oxidation reactions was studied in a three-electrodes type cell using a Au working electrode, Pt wire counter electrode and a Ag/AgCl in3 M KCl reference electrode. Experiments were performed under an oxygen-free atmosphere in a glove-box. Cyclic voltammetry (CV) and normal pulse voltammetry (NPV) were applied to investigate the influence of chloride concentrations in the kinetics parameters, such as reaction rate, k, and the diffusion coefficient in solutions containing U(VI) or U(IV) in HCl solutions from 0.1 to 1.3 M. The solution speciation of uranium was investigated by UV/vis absorption spectroscopy.
CV and NPV experiments show that the reduction of U(VI) occurs in two steps. The first wave appearing between -0.2 V and -0.3 V vs ref. can be ascribed to the reaction U(VI) àU(V), in line with previous reports2,3,4. The second cathodic wave occurs with a simultaneous gas generation. The cathodic process involved in the second wave can be described in terms of an acceleration of hydrogen evolution reaction by adsorbed intermediary species. According to thermodynamic calculations using the extended Debye-Hückel equation as a rough ionic activity approximation and NEA-data base5, the predominating species in the systems under study are UO2
2+ and UO2Cl+. The concentration of the chloride complex increases with increasing HCl concentration, as indicated by the spectral changes on UV-vis absorption data. Spectroscopic measurements with U(IV), on the other hand, do not indicate a strong complexation effect. According to the results of voltammetric experiments, chloride complexes seem to influence the second electron transfer step. A reaction mechanism will be discussed in terms of electrochemical, spectroscopic and modeling results.
References

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