Highly Selective Separation of Actinides from Lanthanides by Dithiophosphinic Acids: An in-Depth Investigation on Extraction, Complexation, and DFT Calculations.

To further reveal the extraction model for selective separation of trivalent actinides over lanthanides by dithiophosphinic acids (DPAHs), five representative DPAH ligands with different substituent groups have been synthesized, and their extraction and complexation behaviors toward Am/Eu have been investigated both experimentally and theoretically. The introduction of electron-withdrawing group -CF into DPAH ligands is beneficial to their extractability among the five ligands. Slope analyses show that both Am and Eu were extracted as tetra-associated species with DPAH ligands. In addition, the results obtained from luminescence spectroscopy, Raman spectroscopy, and ESI-MS suggest that all of the five DPAHs coordinate with Eu mainly in the form of ML(HL)(HO) (L represents deprotonated DPAH). Density functional theory (DFT) calculations on the thermodynamic parameters illustrate that the extractability of DPAHs is dominated by the deprotonation property of these ligands. Meanwhile, molecular orbital analysis indicates that the unoccupied valence orbitals of Am display a stronger affinity to the sulfur lone electron pair than those of Eu, which should be one of the key factors contributing to the excellent selectivity of Am over Eu by DPAH ligands.