Uranium Redox Transformations after U(VI) Coprecipitation with Magnetite Nanoparticles

Abstract

Uranium redox states and speciation in magnetite nanoparticles coprecipitated with U(VI) for uranium loadings varying from 1000 to 10 000 ppm are investigated by X-ray absorption spectroscopy (XAS). It is demonstrated that the U M high energy resolution X-ray absorption near edge structure (HR-XANES) method is capable to clearly characterize U(IV), U(V), and U(VI) existing simultaneously in the same sample. The contributions of the three different uranium redox states are quantified with the iterative transformation factor analysis (ITFA) method. U L XAS and transmission electron microscopy (TEM) reveal that initially sorbed U(VI) species recrystallize to nonstoichiometric UO nanoparticles within 147 days when stored under anoxic conditions. These U(IV) species oxidize again when exposed to air. U M HR-XANES data demonstrate strong contribution of U(V) at day 10 and that U(V) remains stable over 142 days under ambient conditions as shown for magnetite nanoparticles containing 1000 ppm U. U L XAS indicates that this U(V) species is protected from oxidation likely incorporated into octahedral magnetite sites. XAS results are supported by density functional theory (DFT) calculations. Further characterization of the samples include powder X-ray diffraction (pXRD), scanning electron microscopy (SEM) and Fe 2p X-ray photoelectron spectroscopy (XPS).

Redox Processes in Solid‐State Uranyl (Oxy)hydroxide Minerals

“…In contrast, the {E E} mechanism, i. e., two successive one‐electron transfers, U(VI)→U(V)→U(IV) without disproportionation, provides a superior fit especially on the oxidative branch. Uranyl(V) species are, in solution,, prone to disproportionation but some evidence has been reported upon their relative stability in environmental studies with iron (oxyhydr)oxide phases . The only stable uranium(V) in nature is the mineral Wyartite, Ca[U 5+ (UO 2 ) 2 (CO 3 )O 4 (OH)] ⋅ 7H 2 O …”

A Novel Metastable Pentavalent Plutonium Solid Phase on the Pathway from Aqueous Plutonium(VI) to PuO2 Nanoparticles

“…The chemical shift of the intermediate Pu phase is clearly resolved in the HERFD data reported in Figure and indicates the presence of the Pu V oxidation state, in agreement with the Pu M 4 HERFD results. However, for actinide systems, HERFD at the L 3 edge is not as sensitive as the M 4 edge HERFD to the presence of minor contributions (<10 %) from different oxidation states . HERFD at the L 3 edge is, however, extremely sensitive to the local structure around the absorber, which results in specific post‐edge features …”

Differential uranyl(v) oxo-group bonding between the uranium and metal cations from groups 1, 2, 4, and 12; a high energy resolution X-ray absorption, computational, and synthetic study

“…Experimentally, High Energy Resolution X-ray Absorption Near Edge Structure (HR-XANES) and Resonant Inelastic X-ray Scattering (RIXS) have recently emerged as very sensitive tools for characterisation of oxidation states and electronic structures of actinide elements. [28][29][30][31][32][33][34][35][36][37][38] In particular, since they directly probe the An 5f valence unoccupied orbitals, they can distinguish small variations in the chemical bonding at the metal for uranyl(VI) and uranyl(V) compounds. These experimental spectroscopic techniques are most readily interpreted in combination with computational analyses.…”