A highly efficient solvent system containing functionalized diglycolamides and an ionic liquid for americium recovery from radioactive wastes

Abstract

Three room temperature ionic liquids (RTILs), viz. C₄mim⁺·PF₆⁻, C₆mim⁺·PF₆⁻ and C₈mim⁺·PF₆⁻, were evaluated as diluents for the extraction of Am(III) by N,N,N′,N′-tetraoctyl diglycolamide (TODGA). At 3 M HNO₃, the D_Am-values by 0.01 M TODGA were found to be 102, 34 and 74 for C₄mim⁺·PF₆⁻, C₆mim⁺·PF₆⁻ and C₈mim⁺·PF₆⁻, respectively. The extraction of Am(III) decreased with increasing feed acidity for all three diluents, indicating an ion exchange mechanism for the extraction. The stoichiometry of the extracted species suggested that two TODGA molecules were associated with Am(III) during the extraction for all three RTILs and the conditional extraction constants have been determined. The D_M-values for different metal ions followed the order: 75 (Am(III)) > 30.7 (Pu(IV)) > 3.9 (Np(IV)) > 1.19 (Pu(VI)) > 0.52 (U(VI)) > 0.12 (Cs(I)) > 0.024 (Sr(II)). The distribution behaviour of Am(III) was also studied with a recently synthesized calix[4]arene-4DGA (C4DGA) extractant dissolved in C₈mim⁺·PF₆⁻. Using this extractant diluent combination, the D_Am-value was 194 at 3 M HNO₃ using 5 × 10⁻⁵ M C4DGA, suggesting a very high distribution coefficient at very low extractant concentrations. The stoichiometry of the extracted species containing Am was found to be 1 : 2 (M : L) in C₈mim⁺·PF₆⁻. The thermodynamics of the extraction was also studied for both extractants in C₈mim⁺·PF₆⁻. The use of RTILs gives rise to significantly improved extraction properties than the commonly used n-dodecane and an unusual increase in separation factor values was seen for the first time which can lead to selective separation of Am from wastes containing a mixture of U, Pu and Am.