Computational Study of the Interactions of Tetravalent Actinides (An = Th – Pu) with the α-Fe13 Keggin Cluster

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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 ΔrGo being for Th and Pu at -335.7 kJ mol-1and -396.0 kJ mol-1 respectively. There is strong correlation between the substitution reaction energy and the ionic radii of the actinides rε0 R2 = 0.97 and ΔrGo 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.
Original languageEnglish
Pages (from-to)5947-5956
JournalDalton Transactions
Publication statusPublished - 4 Mar 2024


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