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

Neptunium­(V) and uranium­(VI) are precipitated from an aqueous potassium–sodium-containing carbonate-rich solution, and the solid phases are investigated. U/Np M_4,5-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[Np^VO₂CO₃]_(cr), K₃[Np^VO₂(CO₃)₂]_(cr), and K₃Na­[U^VIO₂(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).