铀酰离子在Na2CO3-H2O2溶液体系中的配位行为研究OA

Alkaline-based spent fuel reprocessing,which uses carbonate solutions instead of conventional nitric acid media,has emerged as a promising strategy due to its lower corrosion,enhanced safety,reduced secondary waste generation and avoidance of nitrogen oxide emissions.The coordination behavior of uranyl ions in a Na2CO3-H2O2 solution system was investigated in this study,which is essential for understanding the oxidative dissolution mechanism of uranium oxides and optimizing subsequent separation processes.In particular,the main soluble uranyl species and elucidating their coordination interactions under different pH and ligand concentration conditions were identified.A series of experiments were conducted to examine the speciation and structural characteristics of uranyl complexes in alkaline peroxide media.Na2CO3/NaHCO3 buffer solutions(total carbonate concentration of 0.5 mol/L)were prepared and supplemented with 0.5 mol/L H2O2.UO2 was dissolved in these solutions under ambient conditions,and the resulting uranyl species were analyzed using Raman spectroscopy and UV-vis titration.Raman spectra reveal the presence of several uranyl complexes,including[UO2(CO3)3]4-and[UO2O2(CO3)2]4-,whose relative abundance varies significantly with pH.UV-vis titration further confirms the transformation of uranyl-carbonato complexes to peroxo-carbonato uranyl complexes as peroxide concentration increases,with[UO2O2(CO3)2]4-identified as the dominant species at high O22-/U ratios.To gain a deeper understanding of the coordination interactions,quantum chemical calculations were performed using ORCA 5.0.4 and Multiwfn 3.8 software.Geometry optimization and frequency analysis were carried out with the PBE0 functional and D3(BJ)dispersion correction,along with appropriate basis sets and relativistic effective core potentials for uranium.Theoretical analyses,including atoms-in-molecules(AIM)topological analysis,Mayer and Laplacian bond order evaluations,independent gradient model based on Hirshfeld partition(IGMH),and localized orbital locator(LOL)mapping,were employed to probe the nature and strength of U-O bonding.Results demonstrate that the U-Oper bonds exhibit higher electron density,shorter bond lengths,and greater covalent character than U-Ocarb bonds,although both types of interactions are mainly electrostatic in nature.The formation of stronger U-Oper bonds also induces a weakening of U-Ocarb interactions,reflecting competitive coordination behavior.In conclusion,the study identifies[UO2O2(CO3)2]4-as the predominant uranyl complex in carbonate-peroxide solutions under alkaline conditions and confirms that peroxo ligands form stronger coordination bonds with uranyl ions than carbonate ligands.These findings provide essential insights into the oxidative dissolution mechanisms of uranium oxides in alkaline media and support the development of simplified,efficient,and selective SNF(spent nuclear fuel)reprocessing technologies based on non-nitric acid systems.