Computational study of the interactions of tetravalent actinides (An = Th–Pu) with the α-Fe13 Keggin cluster

Abstract

In recent years, evidence has emerged that actinide (An) uptake at the enhanced actinide removal plant (EARP) at the UK's Sellafield nuclear site occurs via An interactions with an α-Fe₁₃ Keggin molecular cluster during ferrihydrite formation. We here study theoretically the substitution of aquo complexes of the actinides Th–Pu onto a Na-decorated α-Fe₁₃ Keggin cluster using DFT at the PBE0 level. The optimised Pu–O and Pu–Fe distances are in good agreement with experiment and Na/An substitutions are significantly favourable energetically, becoming more so across the early 5f series, with the smallest and largest Δ_rG° being for Th and Pu at −335.7 kJ mol⁻¹ and −396.0 kJ mol⁻¹ respectively. There is strong correlation between the substitution reaction energy and the ionic radii of the actinides (Δ_rε₀R² = 0.97 and Δ_rG° R² = 0.91), suggesting that the principal An-Keggin binding mode is ionic. Notwithstanding this result, Mulliken and natural population analyses reveal that covalency increases from Th–Pu in these systems, supported by analysis of the occupied Kohn–Sham molecular orbitals where enhanced An(5f)–O(2p) overlap is observed in the Np and Pu systems. By contrast, quantum theory of atoms in molecules analysis shows that U-Keggin binding is the most covalent among the five actinides, in keeping with previous studies.