Ionic Liquids-Based Extraction: A Promising Strategy for theAdvanced Nuclear Fuel Cycle Xiaoqi Sun, Huimin Luo, and Sheng Dai* Chemical Sciences Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Energy and Transportation Science Division, Oak Ridge National Laboratory, Oak Ridge, Tennessee 37831, United States Department of Chemistry, University of Tennessee, Knoxville, Tennessee 37916, United States State Key Laboratory of Rare Earth Resource Utilization, Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun 130022, P. R. China

Ionic Liquids-Based Extraction: A Promising Strategy for the Advanced Nuclear Fuel Cycle

Ionic liquid chemistry has developed into a tremendously popular field of chemistry in the last decade. It has been realized that ionic liquids show features that cannot be realized in conventional molecular solvents. The widely tuneable class of ionic liquids has become important for lanthanide and actinide coordination chemistry, f-element spectroscopy, f-element electrochemistry and electrodeposition, organic synthesis and catalysis as well as inorganic nanomaterial synthesis. For example, ionic liquids offer the possibility to synthesize and crystallize quite uncommon lanthanide compounds. Ionic liquids can also be designed in such a way that they become excellent media to study the optical properties of lanthanide compounds. And even ionic liquids based on f-elements can be made. Hydrophobic ionic liquids may replace organic solvents in liquid/liquid f-element extraction processes, not only as a safer extraction phase but also as a smart extractant. Their wide electrochemical window and large ion conductivity make ionic liquids interesting solvents to study the electrodeposition of the very electropositive lanthanide elements. For f-element-catalyzed reactions, higher reaction rates, enhanced selectivity, better immobilization of the catalyst and an easy product recovery have often been observed in certain ionic liquids.

Ionic Liquids for Lanthanide and Actinide Chemistry

To assess the nature of chemical bonds in uranyl(VI) complexes with Lewis base ligands, such as F(-), Cl(-), OH(-), CO(3)(2-), and O(2)(2-), we have used quantum chemical observables, such as the bond distances, the internal symmetric/asymmetric uranyl stretch frequencies, and the electron density with its topology analyzed using the quantum theory of atoms-in-molecules. This analysis confirms that complex formation induces a weakening of the uranium-axial oxygen bond, reflected by the longer U-O(yl) bond distance and reduced uranyl-stretching frequencies. The strength of the ligand-induced effect increases in the order H(2)O < Cl(-) < F(-) < OH(-) < CO(3)(2-) < O(2)(2-). In-depth analysis reveals that the trend across the series does not always reflect an increasing covalent character of the uranyl-ligand bond. By using a point-charge model for the uranyl tetra-fluoride and tetra-chloride complexes, we show that a significant part of the uranyl bond destabilization arises from purely electrostatic interactions, the remaining part corresponding either to charge-transfer from the negatively charged ligands to the uranyl unit or a covalent interaction. The charge-transfer and the covalent interaction are qualitatively different due to the absence of a charge build up in the uranyl-halide bond region in the latter case. In all the charged complexes, the uranyl-ligand bond is best described as an ionic interaction. However, there are covalent contributions in the very stable peroxide complex and, to some extent, also in the carbonate complex. This study demonstrates that it is possible to describe the nature of chemical bond by observables rather than by ad hoc quantities such as atomic populations or molecular orbitals.

Probing the Nature of Chemical Bonding in Uranyl(VI) Complexes with Quantum Chemical Methods.

Uranyl complexes dissolved in room-temperature ionic liquids have diagnostic absorption and emission spectra which reflect the molecular symmetry and geometry In particular, the characteristic vibrational fine structure of the absorption spectra allows identification of the molecular symmetry of a uranyl complex The concept of speciation of uranyl complexes is illustrated for the hydrated uranyl ion, the tetrachloro complex [UO2Cl4]2-, the trinitrato complex [UO2(NO3)3]-, the triacetato complex [UO2(CH3COO)3]-, and the crown ether complex [UO2(18-crown-6)]2+ in imidazolium and pyrrolidinium bis(trifluoromethylsulfonyl)imide ionic liquids The competition between 18-crown-6 and small inorganic ligands for coordination to the uranyl ion was investigated The crystal structures of the hydrolysis product [(UO2)2(μ2-OH)2(H2O)6] [UO2Br4](18-crown-6)4 and imidazolium salt [C6mim]2[UO2Br4] are described

Speciation of uranyl complexes in ionic liquids by optical spectroscopy.

Synchrotron radiation (SR) based techniques have been utilized with increasing frequency in the past decade to explore the brilliant and challenging sciences of actinide-based materials. This trend is partially driven by the basic needs for multi-scale actinide speciation and bonding information and also the realistic needs for nuclear energy research. In this review, recent research progresses on actinide related materials by means of various SR techniques were selectively highlighted and summarized, with the emphasis on X-ray absorption spectroscopy, X-ray diffraction and scattering spectroscopy, which are powerful tools to characterize actinide materials. In addition, advanced SR techniques for exploring future advanced nuclear fuel cycles dealing with actinides are illustrated as well.

Exploring actinide materials through synchrotron radiation techniques

The first coordination sphere of the uranyl cation in room-temperature ionic liquids (ILs) results from the competition between its initially bound counterions, the IL anions, and other anions (e.g., present as impurities or added to the solution). We present a joined spectroscopic (UV−visible and extended X-ray absorption fine structure)-simulation study of the coordination of uranyl initially introduced either as UO2X2 salts (X- = nitrate NO3-, triflate TfO-, perchlorate ClO4-) or as UO2(SO4) in a series of imidazolium-based ILs (C4mimA, A- = PF6-, Tf2N-, BF4- and C4mim = 1-methyl-3-butyl-imidazolium) as well as in the Me3NBuTf2N IL. The solubility and dissociation of the uranyl salts are found to depend on the nature of X- and A-. The addition of Cl- anions promotes the solubilization of the nitrate and triflate salts in the C4mimPF6 and the C4mimBF4 ILs via the formation of chloro complexes, also formed with other salts. The first coordination sphere of uranyl is further investigated by molecular dyna...

Uranyl Coordination in Ionic Liquids: The Competition between Ionic Liquid Anions, Uranyl Counterions, and Cl- Anions Investigated by Extended X-ray Absorption Fine Structure and UV−Visible Spectroscopies and Molecular Dynamics Simulations

and A. Ouadi

Papers

Uranyl Coordination in Ionic Liquids: The Competition between Ionic Liquid Anions, Uranyl Counterions, and Cl- Anions Investigated by Extended X-ray Absorption Fine Structure and UV−Visible Spectroscopies and Molecular Dynamics Simulations