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 Cy anex 301 with neutral O-bearing co-extractants is investigated from the sta ndpoint 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 tha t is significantly greater for Am(III)-S than for Eu(III)-S bonds. The M(II I) extractabilities are correlated with the calculated effective charge on O atom in neutral organophosphorous co-extractant molecules. Synergism is e xplained by the formation of mixed ligand complexes which changes the natur e of the M-S bonds due to O-->M electron density transfer. Co-extractants w hich are weak donors strengthen M-S bonds but strong donors weaken them. Th e P-31 NMR chemical shifts of the neutral O-bearing organophosphorous co-ex tractants, which are correlated with the M(III) extractabilities, confirm t he synergism mechanism. Shifts in charge transfer energy in UV-visible abso rption spectra of Eu(III) synergistic mixed complexes compared with Eu(III) -Cyanex 301 complexes are related to the Eu-S distances in the extracted co mplexes. The Eu-S distances in the Eu(III) extracted complexes were also fo und to be related to the intensity of the synergistic effect and to the Am( III)/Eu(III) extraction selectivity.