Mechanism of trivalent actinide/lanthanide separation using synergistic mixtures of di(chlorophenyl)dithiophosphinic acid and neutral O-bearing co-extractants

The mechanism of synergistic selective extraction of Am(III) over Eu(III) f rom aqueous nitric acid solutions using di(chlorophenyl)dithiophosphinic ac id [(Cl Phi)(2)PSSH] and neutral O-bearing organophosphorus co-extractants (B) has been investigated. (Cl Phi)(2)PSSH forms weaker complexes with Am(I II) than R2PSSH (where R = alkyl group). However, the Am(III)/(Cl Phi)(2)PS SH complexes are stronger than the corresponding Eu(III)/(Cl Phi)(2)PSSH co mplexes, although the difference is not as large as in the case of the corr esponding complexes formed with R2PSSH. The donor properties of co-extracta nts are also discussed. Chemical shifts in the P-31 NMR spectra of the orga nophosphorus co-extractants correlate qualitatively with the extractability of Am(III) and Eu(III) in (Cl Phi)(2)PSSH + B synergistic mixtures. The sy nergistic effect determines the selectivity not only qualitatively but also quantitatively. The high selectivity observed using tri-2-ethylhexyl phosp hate (T2EHP) is caused by a synergistic bonding effect for the Am(III) comp lex and an antagonistic effect for Eu(III). A donor-acceptor mechanism is shown to promote Am(III) extraction for almos t all B co-extractants. A contribution of entropy terms for extracted Eu(II I) extraction is assumed to be significant. Interatomic distances REu-S in the Eu(III) complexes are calculated. The softness of S-bearing extractants XYPSSH (with X and Y = R, RO or ClC6H 4) is discussed from the point of view of the effective charge carried by t he S atoms. The hardness of Eu(III) and Am(III) is evaluated.