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

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 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).

High-energy resolution X-ray spectroscopy at actinide M4,5 and ligand K edges: what we know, what we want to know, and what we can know

“…4a shows the HERFD data at the U M 4 edge of the UO 2 , UO 3 and KUO 3 , published by Leinders and co-workers 76 as a representative example of the oxidations state detection in tetravalent (UO 2 ), hexavalent (UO 3 ) and pentavalent (KUO 3 ) uranium systems. Similar trends were observed later for the plutonium [61][62][63][64] and neptunium 71 systems in different chemical states.…”

“…The actinide M 4,5 HERFD-XANES method gives the opportunity to fingerprint the oxidation state of the actinidecontaining materials. The first U M 4 experiment has been done on the mixed uranium oxides 3 , which was followed by several other Please do not adjust margins Please do not adjust margins studies on the chemical state verification in actinide systems 3,6,9,12,[60][61][62][63][64][65][66][67][68][69][70][71][72][73][74][75][76][77] . Fig.…”

Probing the Local Coordination of Hexavalent Uranium and the Splitting of 5f Orbitals Induced by Chemical Bonding

“…The possibility of accurately reproducing with DFT calculations the trends correlated to local coordination offers a powerful tool for U M 4,5 HERFD XANES analysis, which can be extended to actinides with empty and singly occupied 5fshells. 10,24 ■ EXPERIMENTAL AND COMPUTATIONAL DETAILS Details on sample preparation and characterization can be found in ref 25 for CaUO 4 , Sr 3 UO 6 , and SrUO 4 and ref 3 for Cs 2 UO 2 Cl 4 . X-ray diffraction (XRD) showed that each compound was single-phase and well crystallized.…”

“…In a recent work, good agreement was obtained neglecting the CH. 24 In other cases, the ground-state f-DOS from DFT did not reproduce the correct separation between A and C, probably because of the absence of the CH effect. 3,23 Figure S1 reports the inclusion of spin orbit and CH in two steps.…”

X-ray absorption spectra of f-element complexes: insight from relativistic multiconfigurational wavefunction theory

“…[46][47][48][49][50][51] XANES at the metal L and M edges is also used in Ln and An chemistry research. 26,36,[52][53][54][55][56][57][58][59][60][61][62][63][64][65] The L edge probes dipole-allowed 2p → nd transitions and also provides information about the fshell occupation. The (relative) intensity between the spectral features, and the energy of the main edge, are very sensitive to the metal oxidation state and f-electron count.…”