Accurate quantum chemical modelling of the separation of Eu3+ from Am3+/Cm3+ by liquid–liquid extraction with Cyanex272

Abstract

The experimentally observed extraction complexes of trivalent lanthanide Eu³⁺ and actinide Am³⁺/Cm³⁺ cations with Cyanex272 [bis(2,4,4-trimethylpentyl) phosphinic acid, denoted as HC272] and Cyanex301 [bis(2,4,4-trimethylpentyl) dithiophosphinic acid, denoted as HC301] have been studied by using relativistic energy-consistent 4f- and 5f-in-core pseudopotentials for trivalent f elements, combined with density functional theory and a continuum solvation model. It has been found that, as a result of hydrogen bonding, HC272 exists primarily as a self-associated species, whereas HC301 is preferably a monomer. The calculations show that in case of all three M³⁺ (M = Eu, Am, Cm) ions for HC272 the extraction complexes M[H(C272)₂]₃ are formed prior to M(C272)₃, whereas for HC301 the extraction complexes M(C301)₃ have priority over M[H(C301)₂]₃. The calculated M–O and M–S bond lengths and the M–P distances of these preferred extraction complexes agree very well with the available experimental data. The obtained changes of the Gibbs free energies in the liquid–liquid extraction reactions (1): M_aqu³⁺ + 3(HC272)_2,org → M[H(C272)₂]_3,org + 3H_aqu⁺ and (2): M_aqu³⁺ + 3HC301_org → M(C301)_3,org + 3H_aqu⁺ agree with the experimentally observed thermodynamical priorities of HC272 and HC301, i.e., HC272 prefers Eu³⁺ over Am³⁺/Cm³⁺ and HC301 prefers Am³⁺/Cm³⁺ over Eu³⁺. The obtained changes of the Gibbs free energies in reaction (2) (Eu, 68.1 kJ mol⁻¹; Am, 46.5 kJ mol⁻¹) agree quite well with the experimental findings (Eu, 63.3 kJ mol⁻¹; Am, 44.1 kJ mol⁻¹).