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

Tonya Vitova; Ivan Pidchenko; Dieter Schild; Tim Prüßmann; Vanessa Montoya; David Fellhauer; Xavier Gaona; Elke Bohnert; Jörg Rothe; Robert J Baker; Horst Geckeis

doi:10.1021/acs.inorgchem.9b02463

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

Inorg Chem. 2020 Jan 6;59(1):8-22. doi: 10.1021/acs.inorgchem.9b02463. Epub 2019 Dec 13.

Authors

Affiliations

¹ Institute for Nuclear Waste Disposal (INE) , Karlsruhe Institute of Technology , P.O. 3640, D-76021 Karlsruhe , Germany.
² School of Chemistry , University of Dublin, Trinity College , Dublin 2 , Ireland.

PMID: 31834788
DOI: 10.1021/acs.inorgchem.9b02463

Abstract

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).