Coordination Behavior of Uranyl Ions in Na₂CO₃-H₂O₂ Solution

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)₃⁴⁻ and UO₂O₂(CO₃)₂⁴⁻, 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)₂⁴⁻ identified as the dominant species at high \mathrmO_2^2- /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-O_per bonds exhibit higher electron density, shorter bond lengths, and greater covalent character than U-O_carb bonds, although both types of interactions are mainly electrostatic in nature. The formation of stronger U-O_per bonds also induces a weakening of U-O_carb interactions, reflecting competitive coordination behavior. In conclusion, the study identifies UO₂O₂(CO₃)₂⁴⁻ 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.