Competitive Reaction of Neptunium(V) and Uranium(VI) in Potassium–Sodium Carbonate-Rich Aqueous Media: Speciation Study with a Focus on High-Resolution X‑ray Spectroscopy
journal contributionposted on 13.12.2019, 19:21 authored by Tonya Vitova, Ivan Pidchenko, Dieter Schild, Tim Prüßmann, Vanessa Montoya, David Fellhauer, Xavier Gaona, Elke Bohnert, Jörg Rothe, Robert J. Baker, Horst Geckeis
Neptunium(V) and uranium(VI) are precipitated from an aqueous potassium–sodium-containing carbonate-rich solution, and the solid phases are investigated. U/Np M4,5-edge high-energy resolution X-ray absorption near edge structure (HR-XANES) spectroscopy and Np 3d4f resonant inelastic X-ray scattering (3d4f RIXS) are applied in combination with thermodynamic calculations, U/Np L3-edge XANES, and extended X-ray absorption fine structure (EXAFS) studies to analyze the local atomic coordination and oxidation states of uranium and neptunium. The XANES/HR-XANES analyses are supported by ab initio quantum-chemical computations with the finite difference method near-edge structure code (FDMNES). The solid precipitates are also investigated with powder X-ray diffraction, scanning electron microscopy–energy dispersive X-ray spectroscopy, and Raman spectroscopy. The results strongly suggest that K[NpVO2CO3](cr), K3[NpVO2(CO3)2](cr), and K3Na[UVIO2(CO3)3](cr) are the predominant neptunium and uranium solid phases formed. Despite the 100 times lower initial neptunium(V) concentration at pH 10.5 and oxic conditions, neptunium(V)-rich phases predominately precipitate. The prevailing formation of neptunium(V) over uranium(VI) solids demonstrates the high structural stability of neptunium(V) carbonates containing potassium. It is illustrated that the Np M5-edge HR-XANES spectra are sensitive to changes of the Np–O axial bond length for neptunyl(V/VI).