Existence State And Complexation Behavior Of Uranyl Ion In Ionic Liquids

The potential application of the“green solvent”ionic liquids?ILs?in the reprocessing of spent nuclear fuel has attracted increasing attentions in recent years.A comprehensive knowledge of the fundamental complexation behavior of metal ions in ILs could help better design and develop ILs-based separation system.In this thesis,the complexation of U?VI?with a series of ligands in a typical ionic liquid,C₄minNTf₂,was studied by means of UV-vis absorption spectrophotometry,isothermal titration calorimetry and DFT calculations.The speciation of complexes was identified and thermodynamic parameters such as the stability constants,enthalpy and entropy were obtained.For the complexation of U?VI?with nitrate in ILs,spectrophotometric titration suggested that three complexes,UO₂?NO₃?⁺,UO₂?NO₃?₂ and UO₂?NO₃?₃^-,could be formed in the solution.The stepwise stability constants were determined to be logk₁=4.5,logk₂=3.7,and logk₃=3,respectively.The optimized coordination configuration of UO₂?NO₃?₃^-was calculated by the density function theory?DFT?.The reaction of forming complexes is exothermic in ILs with low water content,while that is endothermic in ILs with relatively high water content.For the complexation of U?VI?with crown ethers?18C6,DCH18C6,and DtBuCH18C6?,1:1 complexes UO₂?18C6?²⁺,UO₂?DCH18C6?²⁺and UO₂?DtBuCH18C6?²⁺were formed for all the three crown ethers,with the stability constants were determined to be4.00,4.10 and 4.16,respectively.DFT calculations indicate that the coordination configuration are inner inclusion complexes.The complexation is endothermic in ILs with a water content of 150?g/g.In addition,the stability constant of UO₂?18C6?²⁺decreases with the increase of water content in the ionic liquid.For the complexation of U?VI?with CMPO,spectrophotometric titration suggests that three successive complexes,UO₂L_j²⁺?j=1-3?,formed both in“dry”?water content<250?g/g?and“wet”?water content¹2500?g/g?ionic liquid.However,the thermodynamic parameters are distinctly different in the two ILs.In“dry”IL,the complexation strength between CMPO and U?VI?is much stronger,with stability constants of the respective complexes more than one order of magnitude higher than that in“wet”IL.Energetically,the complexation of U?VI?with CMPO in“dry”IL is mainly driven by negative enthalpies.In contrast,the complexation in“wet”IL is overwhelmingly driven by highly positive entropies as a result of the release of a large amount of water molecules from the solvation sphere of U?VI?.Moreover,comparisons between the fitted absorption spectra of complexes in“wet”IL and that of extractive samples from solvent extraction have identified the speciation involved in the extraction of U?VI?by CMPO in ionic liquid.The structure of extraction species is UO₂L₃²⁺to a large extent and the extraction reaction is endothermic,elevating temperature favors the progress of extraction reaction.