Mechanism of trivalent actinide/lanthanide separation using bis(2,4,4-trimethylpentyl) dithiophosphinic acid (Cyanex 301) and neutral O-bearing co-extractant synergistic mixtures

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 P-31 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.