Benzene-centred tripodal diglycolamides for the sequestration of trivalent actinides: metal ion extraction and luminescence spectroscopic investigations in a room temperature ionic liquid

Abstract

Three benzene-centred tripodal diglycolamide (Bz-T-DGA) ligands, where the diglycolamide (DGA) moieties are attached to a central benzene ring through ethylene spacers (L_I), amide groups (L_II) or ether linkages (L_III), were evaluated for their extraction behaviour towards trivalent actinide and lanthanide ions in a room temperature ionic liquid (RTIL), viz. 1-butyl-3-methylimidazolium bis(trifluoromethane)sulfonimide ([C₄mim][Tf₂N]). The extraction behaviour of these ligands in [C₄mim][Tf₂N] medium was compared with that obtained in the molecular solvent n-dodecane showing an opposite selectivity of L_III > L_II > L_Ivs. L_I > L_II > L_III. In contrast to the n-dodecane medium, where a solvation extraction mechanism prevailed, a cation exchange mechanism was found to be operative in the RTIL medium. The stoichiometry of the extracted Am³⁺complex was found to be 1 : 2 (metal/ligand) and a nitrate ion was absent in the extracted complex. The luminescence spectroscopy of the Eu³⁺/ligand extracted complexes in the [C₄mim][Tf₂N] phase confirmed the absence of water molecules and that all the primary coordination sites of the metal ion are occupied by the ligands. The ligands display very large Pu/U, Am/U and Eu/U separation factor in the RTIL medium.