Synergistic extraction of uranium(VI) with TODGA and hydrophobic ionic liquid mixtures into molecular diluent

Abstract

The extraction of uranium(VI) from aqueous nitric acid solutions with a neutral extractant, N,N,N′,N′-tetra(n-octyl)diglycolamide (TODGA), and with the mixtures of TODGA and a hydrophobic ionic liquid, 1-butyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([C4mim][Tf2N]), into a molecular diluent, 1,2-dichloroethane, has been systematically investigated. The extraction efficiency of U(VI) ions was greatly enhanced by addition of a small amount of ionic liquid to an organic phase containing TODGA. The synergistic effect comes from the higher hydrophobicity of U(VI) extracted species formed by TODGA and weakly coordinating Tf2N− anions as compared to those formed by TODGA and NO3− anions as counterions in the conventional extraction system. Based on our experimental results, we concluded that the partition of Tf2N− anions between the two liquid phases is the dominant factor governing the extractability of uranium(VI) with the mixture of TODGA and ionic liquid as extractant. We showed that the extraction of U(VI) from aqueous nitric acid solutions both by TODGA alone and its mixtures with [C4mim][Tf2N] into 1,2-dichloroethane can be quantitatively described on the basis of the solvation extraction mechanism. However, in the extraction system with added hydrophobic ionic liquid, the partition of Tf2N− anions between the two immiscible phases and the interaction between bis(trifluoromethylsulfonyl)imide acid, HTf2N, and TODGA molecules in the organic phase should be taken into account.