Electrochemical separation of UO22+ and Ni2+ using formed WO3 film electrode in LiCl-KCl eutectic

Abstract

To separate UO₂²⁺ from the impurity element Ni²⁺, WO₃ film electrode was chosen as cathode to study the electrochemical behaviors of UO₂²⁺ and Ni²⁺. First, WO₃ film was prepared at applied voltage of 25 V in NH₄F-(CH₂OH)₂ solution. The formed WO₃ film exhibited a porous structure and preferentially grew along the (020) crystal direction. Then, the electroreduction mechanisms of Ni²⁺ and UO₂²⁺, as well as the kinetic properties of UO₂²⁺/UO₂ couple and nucleation mode of UO₂ were studied on WO₃ film electrode before and after the addition of the Ni²⁺ using various electrochemical techniques. The electrode reactions of Ni²⁺ and UO₂²⁺ were found to be one-step two-electron transfer process and two-step one-electron transfer process, respectively. The deposition potential of UO₂⁺/UO₂ was found to be more negative than that of Ni²⁺/Ni, indicating that UO₂ and Ni could be co-deposited. The nucleation mode of UO₂ and the exchange current density(j₀) of UO₂²⁺ /UO₂ couple were measured on formed WO₃ film electrode by CA and LP techniques. It was found that in the presence of Ni²⁺, the nucleation mode of UO₂ unchanged, and the value of j₀ became bigger, showing that the presence of Ni²⁺ facilitates the reduction of UO₂²⁺ to UO₂. The electrochemical separation was carried out on W and WO₃/W film electrode through constant potential electrolysis for 1 h. The deposition products were characterized by XRD, SEM-EDS and AFM which were composed of dendritic Ni and octahedral UO₂ on W electrode and octahedral UO₂ and polyhedral Ni on WO₃/W film electrode. Notably, after 6 h electrochemical separation, the obtained product was only UO₂. ICP-AES analysis result indicated that as the electrolysis proceeded, the concentration of UO₂²⁺ decreased, while the concentration of Ni²⁺ first decreased and then increased, and finally returned to its initial value, showing the complete separation of Ni²⁺ and UO₂²⁺. The separation factor (SF) of UO₂²⁺/Ni²⁺ was estimated to be 240 ± 5.