Dissolution of uranium oxides and electrochemical behavior of U(VI) in task specific ionic liquid

Abstract

A task-specific ionic liquid, protonated 1-carboxy-N,N,N-trimethylmethanaminium bis(trifluoromethylsulfonyl)imide trivially known as protonated betaine bis(trifluoromethylsulfonyl)-imide ([Hbet][NTf2]) was prepared and the dissolution of uranium oxides, UO3, UO2 and U3O8 in it was studied. Dissolution of UO3 in [Hbet][NTf2] was very rapid and the saturation solubility of uranium was found to be 15 wt. % at 373 K. In contrast, dissolution of UO2 was sluggish and it was facilitated only by the oxidation of UO2 to UO2 2+. U3O8 was insoluble up to 453 K. A new procedure was developed for the individual separation of uranium oxides using [Hbet][NTf2] based on differences in solubilities. The electrochemical behavior of U(VI) in the resultant solution was investigated by cyclic voltammetry at glassy carbon working electrode at 373 K. A surge in the cathodic peak current at -0.48 V (vs . Fc/Fc+) was due to the reduction of U(VI) to U(V) and the corresponding anodic peak current was observed at a potential of 0.64 V. Increasing the potential sweeping rate increases the peak current and shifts the peak potential negatively indicating the irreversible electroreduction of U(VI) in [Hbet][NTf2]. The diffusion coefficient of U(VI) in [Hbet][NTf2] was determined to be of the order of ∼10−8 cm2/s.