Investigation of Configurations of \begin{document}${ \left[{\bf{UO}}_{\bf{2}}({\bf{OH}})_{\boldsymbol{m}}({\bf{H}}_{\bf{2}}{\bf{O}})_{\boldsymbol{n}}\right]^{{\bf{2}}-{\boldsymbol{m}}}} $\end{document} in Uranium Wastewater

Abstract

Uranium-containing wastewater from the spent nuclear fuel are radioactive wastes as well as energy sources. It is meaningful to investigate the configurations of aqueous U（Ⅵ） for its environmental migrations and configuration conversions. Herein the uranyl complexes of \begin{document}$ \left[\mathrm{UO}_2(\mathrm{OH})_m(\mathrm{H}_2\mathrm{O})_n^{ }\right]^{2-m} $\end{document} (m=1-4, n=1-2) were investigated by density functional theory at the PBE0 level, in combination with ECP80MWB_AVQZ + 2f basis set for uranium and 6-311+G** basis set for hydrogen and oxygen. The optimized geometries, energies, formation processes and thermodynamic properties of the complexes were predicted. In the six coordinated complexes that are spontaneously produced, the ligand numbers of OH‒ and H2O are 4 and 2, respectively. There exist either linear or “V” configurations, sorting by the angles of Oyl-U-Oyl, when a linear uranyl dication coordinated with H2O and hydroxyl. The “V” type complexes with Oyl-U-Oyl angle being about 40° are found only when the coordinated number of OH‒ is more than 2. The total energy of “V” type [UO2(OH)3]‒ is lower than that of its linear isomer by 131.46 kJ/mol at the MP2 level. The complexes with U-OH2 bond and “V” type Oyl-U-Oyl are the most stable. The complexes tend to have a “V” configuration when the coordinated hydroxyl number is 3 or 4. The lengths of U-Oyl bond increase as the angles of Oyl-U-Oyl decrease. The complexes tend to be cage-like as more hydroxyl and H2O are coordinated. When \begin{document}${\mathrm{UO}}_2^{2+} $\end{document} coordinates with OH‒ one by one, a large amount of energy is released initially. As the coordinated number increases, the released energy dramatically decreases or even changes to an energy adsorbing process. When \begin{document}${\mathrm{UO}}_2^{2+} $\end{document} coordinates with H2O one by one, the processes release a small amount of energy continuously. The increase of H2O ligand is beneficial to the stability of the complexes. The formations of [UO2(OH)m]2‒m (m＜4) are spontaneous processes. However, for the formation of [UO2(OH)4]2‒, only the process of linear [UO2(OH)3]‒ converting to “V” type [UO2(OH)4]2‒ is spontaneous, indicating the complexes tend to form “V” configuration when the number of OH‒ is 4. The spontaneous pathways and energy changes in the stepwise coordination processes were speculated on the basis of energy analysis. The above characteristics of [UO2(OH)m(H2O)n]2‒m play an important role on the ion activity, solubility and ion exchange of uranyl in alkaline solutions. OH‒ and H2O compete with each other in coordinating, which influences the concentration and species distribution of [UO2(OH)m(H2O)n]2‒m configurations. The results herein provide theoretical basis for understanding and controlling the behaviors and reactions of uranyl aqueous solutions.

Keywords

• spent nuclear fuel
• uranium reprocessing
• uranyl ion
• density functional theory
• thermodynamic