Complexation of trivalent lanthanides and actinides with diethylenetriaminepentaacetic acid: Theoretical unraveling of bond covalency

Separation of trivalent actinides (An(III)) and lanthanides (Ln(III)) is a pivotal step with respect to the treatment of nuclear waste based on the partitioning and transmutation (P&T) strategy, which is also extraordinarily challenging due to their similar coordination chemistry. The diethylenetriaminepentaacetate (DTPA) ligand has been demonstrated to possess selective separation ability toward An(III) over Ln(III) in aqueous media. Nevertheless, the extracted complexes of An(III)/Ln(III) with DTPA, and the origin of selectivity for An(III) are still not well deciphered. In this work, from theoretical perspective and at the molecular level, the geometrical structures, bonding nature as well as thermodynamic behaviors of possible complexes of An(III) (An = Am, Cm, Cf) and Ln(III) (Ln = Nd, Eu) with DTPA in the aqueous phase have been systematically studied using scalar relativistic density functional theory (DFT). All bonding analyses indicate that the metal-ligand bonds possess weak but non-negligible covalent interactions, and An(III)-DTPA species demonstrate more covalency compared to Ln(III) analogues. In addition, the covalency of the metal-ligand bonding of An(III)-DTPA increases across the actinide series from Am to Cf, which stems from the increased orbital degeneracy of the 5f orbitals of actinides and the 2p orbitals of the ligands. According to thermodynamic analysis, the anhydrous species [Cf(HDTPA)](-) is the most likely species for Cf(III) in acidic solution, whereas [M(HDTPA)(H2O)](-) are more favorable for Am(III) and Cm (III), probably due to the different bonding nature of the transplutonium series. This work affords new theoretical insights into the coordination chemistry of An(III)/Ln(III)-DTPA complexes, and paves the way to further design efficient DTPA-based ligands for An(III)/Ln(III) separation. Moreover, the DTPA ligand may also be applied for in-group separation of trivalent actinides, which is quite meaningful for the extraction of Cf(III). (C) 2019 Elsevier B.V. All rights reserved.