Transferring from Eu 3+ to Eu 2+ in Eu 2 O 3 -LiCl-KCl molten salt: an optical and electrochemical investigation

Europium is an important element in the nuclear industry, which is usually applied in the high-temperature melt system. Therefore, due to relatively limited progress in understanding europium among all lanthanide elements, this study aims to disclose the important behavior of Eu3+ (Eu2O3) in the LiCl-KCl molten salt system employing optical and electrochemical strategies and to provide a theoretical basis for the subsequent electrolytic separation of Eu from other metals in nuclear waste. The optical behavior and electrochemical behavior of Eu2O3 in LiCl-KCl molten salt were tested by Raman, UV-vis diffuse reflectance spectra and CV, SWV, CP and other electrochemical methods. Through spectroscopic studies, it was found that Eu2O3 formed a new phase EuOCl in LiCl-KCl melt at 973 K. With the increase of Eu2O3 ratio, the band gap of products first increased and then decreased. Through electrochemical studies, it was found that Eu3+ underwent a single electron exchange process in LiCl-KCl melt, corresponding to the conversion between Eu(III) and Eu(II). The process was a quasi-reversible diffusion-controlled process. A series of electrochemical tests were carried out to determine the diffusion coefficient D of Eu (III) and Eu (II) as (1.94 ± 0.05) × 10−5 cm2 s−1 and (1.95 ± 0.05) × 10−5 cm2 s−1, respectively. This work reveals the electrolytic process of Eu3+ in molten system, which may influence on the spent fuel treatment.