Oxidation state and coordination environment of uranium in sodium iron aluminophosphate glasses

Abstract

An analysis of the X-ray absorption near edge structure (XANES) and the extended X-ray absorption fine structure (EXAFS) of uranium determined the oxidation state and coordination environment of uranium atoms in glasses containing 40 mol % Na2O, 10 mol % Al2O3, 10 mol % Fe2O3, and 40 mol % P2O5 to which uranium oxides were added to a concentration of 50 wt % (above 100%). If the added amount of UO2 was small, uranium occurred as U(IV) in a near-octahedral oxygen environment with an average U–O distance in the first coordination sphere of 2.25 A. At higher concentrations of uranium oxides introduced both as UO2 and as UO3, uranium occurred as U(V) and U(VI); the first coordination sphere is split; shorter (~1.7–1.8 A) and longer (2.2–2.3 A) distances were observed, which corresponded to the axial and equatorial U–O bonds in uranyl ions, respectively; and the redox equilibrium shifted toward U(VI). The glass with the maximal (~33 wt %) UO3 concentration contained mainly U(VI). The existence of low-valence uranium species can be related to the presence of Fe(II) in glasses. The second coordination sphere of uranium manifests itself only at high concentrations of uranium oxides.