Multimodal X-ray microanalysis of a UFeO4: evidence for the environmental stability of ternary U(v) oxides from depleted uranium munitions testing

Abstract

An environmentally aged radioactive particle of UFeO₄ recovered from soil contaminated with munitions depleted uranium (DU) was characterised by microbeam synchrotron X-ray analysis. Imaging of uranium speciation by spatially resolved X-ray diffraction (μ-XRD) and X-ray absorption spectroscopy (μ-XAS) was used to localise UFeO₄ in the particle, which was coincident with a distribution of U(V). The U oxidation state was confirmed using X-ray Absorption Near Edge Structure (μ-XANES) spectroscopy as +4.9 ± 0.15. Le-Bail fitting of the particle powder XRD pattern confirmed the presence of UFeO₄ and a minor alteration product identified as chernikovite (H₃O)(UO₂)(PO₄)·3H₂O. Refined unit cell parameters for UFeO₄ were in good agreement with previously published values. Uranium–oxygen interatomic distances in the first co-ordination sphere were determined by fitting of Extended X-ray Absorption Fine Structure (μ-EXAFS) spectroscopy. The average first shell U–O distance was 2.148 ± 0.012 Å, corresponding to a U valence of +4.96 ± 0.13 using bond valence sum analysis. Using bond distances from the published structure of UFeO₄, U and Fe bond valence sums were calculated as +5.00 and +2.83 respectively, supporting the spectroscopic analysis and confirming the presence of a U(V)/Fe(III) pair. Overall this investigation provides important evidence for the stability of U(V) ternary oxides, in oxic, variably moist surface environment conditions for at least 25 years.