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 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 ...