Direct dissolution of UO 2 in carboxyl-functionalized ionic liquids in the presence or absence of Fe-containing ionic liquids

• Published in: Dalton transactions : an international journal of inorganic chemistry 2020-11, Vol.49 (42), p.14881-14890
• Main Authors: Yao, Aining, Xiong, Xiaogen, Kang, Mingliang, Guo, Yanan, Chen, Chong, Chu, Taiwei
• Format: Article
• Language: English
• ISSN: 1477-9226; 1477-9234
• DOI: 10.1039/D0DT02740E

Dissolution of UO 2 is a prerequisite for the reprocessing of spent nuclear fuel. This study showed that UO 2 could be directly dissolved in a single carboxyl-functionalized ionic liquid (IL), [HOOCMmim][Tf 2 N] 1-carboxymethyl-3-methylimidazolium bistriflimide, or [HOOCEtmim][Tf 2 N] 1-carboxyethyl-3-methylimidazolium bistriflimide. The addition of an extra Fe-containing IL, [Emim][FeCl 4 ] (Emim, 1-ethyl-3-methylimidazolium) or [Bmim][FeCl 4 ] (Bmim, 1-butyl-3-methylimidazolium) could significantly improve the dissolution kinetics. Results demonstrated that the dissolution process in the early stage could be described by using the pseudo first-order rate law. The apparent activation energy for UO 2 dissolution in the mixture of the Fe-containing IL and carboxyl-functionalized IL was calculated to be ∼67 kJ mol −1 , implying that the reaction was mainly controlled by a chemical process. Nevertheless, the influence of the diffusion process is non-negligible since the IL has a relatively high viscosity that can retard the diffusion of the formed uranyl species from the UO 2 surface. Spectroscopic studies and density functional theory calculations indicated that the uranyl ion coordinated with carboxylate groups is the predominant product for UO 2 dissolution in the single carboxyl-functionalized IL, while uranyl chloride complexes would also form in the mixed ILs. The dissolved uranyl species can be successfully recovered from the ILs by extraction. The success of UO 2 dissolution in the carboxyl-functionalized IL with or without the Fe-containing IL indicates that the Fe-containing IL and oxygen can serve as an effective catalyst and oxidant for the dissolution of UO 2 , respectively.