A remarkable enhancement in Am3+/Eu3+selectivity by an ionic liquid based solvent containing bis-1,2,4-triazinyl pyridine derivatives: DFT validation of experimental results

Abstract

Mutual separation of trivalent actinide (An³⁺) and lanthanide (Ln³⁺) using several soft (N) donor ligands (bis(5,6-dialkyl-1,2,4-triazinyl)pyridine (R-BTP)) is attempted for the first time in room temperature ionic liquid (RTIL) medium. The results indicate a spectacular enhancement in the selectivity as compared to that in molecular diluents with a separation factor (S.F.) of >3000 for Am³⁺ over Eu³⁺ using the methyl derivative (Me-BTP) in RTIL medium using [C_nmim]·[NTf₂] as the diluents (where n = 2, 3, 4, 6 or 8). Such a high S.F. value has never been reported before with any of the R-BTP derivatives in molecular diluents. An opposite trend in the distribution ratio values of both Am³⁺ and Eu³⁺ with the increasing size of the alkyl (R) group is observed in RTIL medium when compared with that in molecular diluents. The differences in the extraction behaviour of R-BTPs in RTILs vis-à-vis molecular diluents are explained on the basis of the difference in the nature of complexes extracted in these two distinctly different media as supported by the time resolved fluorescence (TRFS) study. An unusually high extractability and selectivity for Am³⁺ over Eu³⁺ with Me-BTP was attributed to the formation of a 1 : 4 complex for Am³⁺, which was never reported earlier with any of the R-BTP derivatives in molecular diluents. DFT studies indicated higher metal ‘d’ and ‘f’ orbital participation (covalence) in the bonding with R-BTP in the case of Am³⁺ complexes as compared to that in the case of Eu³⁺ complexes, which resulted in the selectivity of these classes of ligands. The observed results may have a great significance in the radioactive waste management involving the partitioning and transmutation strategy.