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Different inner-sphere coordination environments are observed for the uranyl nitrate complexes formed with octyl-phenyl-N,N-diisobutylcarbamoylmethylphosphine oxide and tributyl phosphate in dodecane and in the hydrophobic ionic liquids (ILs) [C(4)mim][PF(6)] and [C(8)mim][N(SO(2)CF(3))(2)]. Qualitative differences in the coordination environment of the extracted uranyl species are implied by changes in peak intensity patterns and locations for uranyl UV-visible spectral bands when the solvent is changed. EXAFS data for uranyl complexes in dodecane solutions is consistent with hexagonal bipyramidal coordination and the existence of UO(2)(NO(3))(2)(CMPO)(2). In contrast, the complexes formed when uranyl is transferred from aqueous nitric acid solutions into the ILs exhibit an average equatorial coordination number of approximately 4.5. Liquid/liquid extraction results for uranyl in both ILs indicate a net stoichiometry of UO(2)(NO(3))(CMPO)(+). The concentration of the IL cation in the aqueous phase increases in proportion to the amount of UO(2)(NO(3))(CMPO)(+) in the IL phase, supporting a predominantly cation exchange mechanism for partitioning in the IL systems.

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To better understand the bonding in complexes of f-elements by polydentate N-donor ligands, the complexation of americium(III) and lanthanide(III) cations by 2-amino-4,6-di-(pyridin-2-yl)-1,3,5-triazine (ADPTZ) was studied using a thermodynamic approach. The stability constants of the 1:1 complexes in a methanol/water mixture (75/25 vol %) were determined by UV-visible spectrophotometry for every lanthanide(III) ion (except promethium), and yttrium(III) and americium(III) cations. The thermodynamic parameters (DeltaH degrees , DeltaS degrees) of complexation were determined from the temperature dependence of the stability constants and by microcalorimetry. The trends of the variations of DeltaG degrees , DeltaH degrees , and DeltaS degrees across the lanthanide series are compared with published results for other tridentate ligands and confirm strongly ionic bonding in the lanthanide-ADPTZ complexes. Comparison of the thermodynamic properties between the Am- and Ln-ADPTZ complexes highlights an increase in stability of the complexes by a factor of 20 in favor of the americium cation. This difference arises from a more exothermic reaction enthalpy in the case of Am, which is correlated with a greater degree of covalency in the americium-nitrogen bonds. Quantum chemistry calculations performed on a series of trivalent actinide and lanthanide-ADPTZ complexes support the experimental results, showing a slightly greater covalence in the actinide-ligand bonds that originates from a charge transfer from the ligand sigma orbitals to the 5f and 6d orbitals of the actinide ion.

The TALSPEAK process is an established option for lanthanide/minor actinide separations using solvent extraction. In this process, selective extraction of lanthanides is achieved by contacting a water-soluble aminopolycarboxylate complexant in a concentrated carboxylic acid buffer with a liquid cation exchanging extractant in an immiscible organic diluent. Although TALSPEAK process development has been successful on several levels, studies of the detailed fundamental chemistry have revealed undesirable complex interactions between aqueous and organic solute species. These complications threaten to impair process modeling and could impact engineered operations. In the present work, results are reported describing equilibrium partitioning and phase transfer kinetics trends for trivalent lanthanide ions and americium into bis-2-ethyl(hexyl) phosphoric acid (HDEHP) or structural analog 2-ethyl(hexyl) phosphonic acid mono-2-ethylhexyl ester (HEH[EHP]) organic phases from aqueous lactate solutions containing diethylenetriamine-N,N,N′,N′′,N′′-pentaacetic acid (DTPA), triethylenetetramine-N,N,N′,N′′,N′′′,N′′′-hexaacetic acid (TTHA), or N-(2-hydroxyethyl)ethylenediamine-N,N′,N′-triacetic acid (HEDTA). The undesirable partitioning of Na+, lactic acid, and water into the organic phase is greatly reduced when HEH[EHP] replaces HDEHP as the extractant. TTHA appears to offer little advantage over DTPA in conventional TALSPEAK, but both DTPA and TTHA are too strong for use in combination with HEH[EHP]. The combination of HEDTA with HEH[EHP] achieves good balance and exhibits a nearly flat pH dependence between 2.5 and 4.5, in contrast with conventional TALSPEAK. The latter combination demonstrates more predictable performance than is seen in conventional TALSPEAK, while providing acceptable americium/lanthanide separation factors. The HEDTA/HEH[EHP] combination offers the additional advantage of more rapid phase transfer kinetics for the heavier lanthanides without the need for high concentrations of a lactate buffer.