Electrolytic reduction of Eu(III) in acidic chloride solutions

Abstract

The method of electrolytic reduction is deemed to be well-suited for the selective reduction of Eu(III) to Eu(II) and subsequent precipitation of highly pure EuSO₄. To more fully understand fundamental aspects of the electrolytic reduction of Eu(III), cathodic polarization characteristics of Eu(III) on a titanium electrode were examined in aqueous solutions of EuCl₃ and EuCl₃−HCl. The effects of cathode potential, catholyte agitation speed, EuCl₃ concentration, temperature, etc. were investigated. The reduction of Eu(III) started at a cathode potential of about −0.6 V vs SHE. The plateau current for the reduction of Eu(III) was reached at around −1.2 V vs SHE. A further decrease in cathodic potential resulted in a decrease in the reduction current. The plateau current shown by the cathodic polarization curve exhibited a half order dependence on the stirring speed of electrolyte and first order dependence on the EuCl₃ concentration. The apparent activation energy was 15.7 kJ-mol⁻¹. These results suggest that the plateau current given by the cathodic polarization curve is diffusionlimiting in the electrolytic reduction of Eu(III).

Batch-type electrolytic reduction of Eu(III) was investigated using a bipolar electrolytic cell, which consisted of a titanium cathode, a platinum anode and an anion exchange membrane. The effects of pH, reduction current, catholyte flow rate, temperature and Eu(III) concentration on the reduction rate of Eu(III) and on current efficiency were investigated. It was found that a current efficiency of over 0.6 could be achieved by keeping the pH of the electrolyte constant at 2, in order to avoid hydrolysis of Eu(III).