New approach for efficient separation of U and Mo using a task-specific ionic liquid from hydrochloric acid medium: extraction and electrochemical investigation

Abstract

This investigation reports the extraction of UO₂Cl₄²⁻ and MoO₂²⁺via neutral (R₄N)₂UO₂Cl₄ and cationic MoO₂(Ph)⁺ complexes (following ‘solvation’ and ‘cation exchange’ mechanisms), using a task-specific ionic liquid (TSIL). The processes were found to be spontaneous and endothermic in nature. The formation of an inner sphere complex resulted in the release of several water molecules during complexation, making the extractive mass transfer entropically driven. The ionic liquid was found to be highly radiolytically stable. Multiple contacts with aqueous carbonate solution were required for back extraction of U and Mo from the ionic liquid phase. Cyclic voltammmetry analysis was used to probe mechanistic insight into the extraction process and to understand the nature of complexation of the ionic liquid with U ions. An interesting observation related to a change in the redox physicochemical parameters of U in ionic liquid was observed compared to that in aqueous medium, indicating the complexation of the U species with the TSIL. The shift in redox potential of U after complexation evidenced the difference in its speciation during mass transfer. The redox thermodynamic (peak and formal potential), diffusion coefficient, and heterogeneous electron transfer kinetic parameters (αn and k⁰) of U in the TSIL were reported. The higher viscosity of ionic liquid was found to slow down the rate of mass transfer during extraction, which can be optimized by proper dilution. A trade-off between extraction efficiency and extraction kinetics was evidenced.