Mechanism of trivalent actinide/lanthanide separation using bis (2,4,4-trimethylpentyl) dithiophosphinic acid (cyanex 301) and neutral O-bearing Co-extractant synergistic mixtures
Résumé
The mechanism of M(III) extraction (M = Am, Eu) using mixtures of purified Cyanex 301 with neutral O-bearing co-extractants is investigated from the standpoint of theoretical chemistry. The strong coordination of M(III) to soft donor S atoms of Cyanex 301 is shown to be caused by a covalent effect that is significantly greater for Am(III)─S than for Eu(III)─S bonds. The M(III) extractabilities are correlated with the calculated effective charge on O atom in neutral organophosphorous co-extractant molecules. Synergism is explained by the formation of mixed ligand complexes which changes the nature of the M─S bonds due to O → M electron density transfer. Co-extractants which are weak donors strengthen M─S bonds but strong donors weaken them. The 31P NMR chemical shifts of the neutral O-bearing organophosphorous co-extractants, which are correlated with the M(III) extractabilities, confirm the synergism mechanism. Shifts in charge transfer energy in UV-visible absorption spectra of Eu(III) synergistic mixed complexes compared with Eu(III)-Cyanex 301 complexes are related to the Eu–S distances in the extracted complexes. The Eu–S distances in the Eu(III) extracted complexes were also found to be related to the intensity of the synergistic effect and to the Am(III)/Eu(III) extraction selectivity.