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

Uranium redox states and speciation in magnetite nanoparticles co-precipitated with U(VI) for uranium loadings varying from 1,000 to 10,000 ppm are investigated by X-ray absorption spectroscopy (XAS). It is demonstrated that the U M4 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 present in the samples are quantified with the iterative transformation factor analysis (ITFA) method. U(V) incorporated in octahedral magnetite sites remains stable over 226 days under ambient conditions as unambiguously shown for the magnetite nanoparticles containing 1,000 ppm uranium. U L3 XAS and transmission electron microscopy (TEM) studies reveal that initially precipitated U(VI) phases recrystallize to non-stoichiometric UO2+x nanoclusters within 147 days when stored under anoxic conditions. 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 X-ray photoelectron spectroscopy (XPS).