Uncovering the inverse relationship between Am/Eu separation capability and softness of N-heterocyclic carboxylate ligands

Abstract

In the advanced spent fuel wet reprocessing process, mutual separation of trivalent actinide ions (An³⁺) and lanthanide ions (Ln³⁺) is extremely challenging. The development of back-extraction separation ligands is considered to be a viable alternative to realized efficient An/Ln separation. Using density functional theory (DFT) calculations, we have studied the back-extraction behaviours and Am/Eu separation capabilities of three ethylenediamine N-heterocyclic carboxylate ligands including N, N, N’-tris (2-pyridylmethyl)-N’-(ethylacetate) ethylenediamine (HL^3py), N, N, N’-tris (2-pyrazinylmethyl)-N’-(ethylacetate) ethylenediamine (HL^3pz), and N, N, N’-tris (2-triazinylmethy)-N’-(ethylacetate) ethylenediamine (HL^3tz). Although HL^3pz is softer than HL^3py, HL^3py shows slightly better Am³⁺ selectivity over Eu³⁺compared to HL^3pz. This inverse relationship between ligands’ softness and their Am/Eu separation capabilities was explored through bonding nature analyses and back-extraction reactions. Though small in magnitude, Am-N_ring and Eu-N_ring bonds in the studied extraction complexes possess different extend of covalent component, and this difference may be the key mechanism of these back extractants for Am/Eu separation. Due to the hydrogen bonding and intermolecular interactions, the Am, Eu ions and extractants usually assemble in organic diluent forming supramolecular complexes. In this work, the thermodynamic properties of back-extraction from Am/Eu-DMDOHEMA and Am/Eu-HDEHP supramolecular species have been explored for the first time through quantum chemical calculations, which well reproduced the relative differences in selectivity Am³⁺ back extraction with HL^3py and HL^3pz, offering an explanation for the inverse relationship between ligand's softness and its Am³⁺ preference.