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This study investigates uranium solvent extraction under AMEX process conditions. The use of pure extractants without diluents or phase modifiers allows us not only to reduce the use of volatile organic compounds but also to provide higher extraction yields without third-phase formation. Pure extractants are protonated amines or quaternary ammoniums with suitable counter ions, which act at the interface between ion pairs and protic ionic liquids. The mixture of sulphates anion (SO42−) and bis(trifluoromethanesulfonyl)imide anion (NTf2−) revealed unexpected nonlinear extraction behaviors, which appear highly important to rationalize for optimized application. A spectroscopic analysis (NMR, UV-vis, FT-IR, and EXAFS) showed that uranium extraction occurs via a protonated amine and three sulphates. A nonlinear extraction could further be interpreted by considering a water and acid transfer between the two phases: at lower sulphate ratios, the release of acid from the organic phase into the aqueous phase was shown to influence the number of protonated amines in the organic phase, affecting uranium extraction before its enhancement. Furthermore, the extraction loss at higher sulphate ratios was assigned to the destabilization of bidentate uranium–sulphate complexes due to a competition between water and sulphates. [ABSTRACT FROM AUTHOR]

Extraction mechanisms of two bifunctional extractants which differ only by the grafting of an alkyl chain between their two functions were investigated at molecular and supramolecular scales to investigate the origin of their very different separation factors toward uranium and zirconium. Investigation of the complex structure with spectroscopic analysis (Fourier Transform Infra-Red (FTIR), ElectroSpray Ionization Mass Spectroscopy (ESI-MS) and Extended X-Ray Absorption Fine Structure (EXAFS)) demonstrated that alkylation does not affect the chelation mechanism of uranium and zirconium. Stoichiometries of complexes remain identical for both extractants: UO2L2(NO3)2 and zirconium polynuclear complexes are formed after extraction into the organic phase. The origin of selectivity was therefore investigated by considering the supramolecular self-assembly of the two bifunctional molecules. Small-Angle X ray and Neutrons Scattering (SAXS and SANS) showed that the highest separation factors between U and Zr are obtained when smaller aggregates are formed. The results of this study therefore suggests that the selectivity is controlled by the supramolecular self-assembly of the two extractant molecules. Thanks to a smaller packing parameter, the non-alkylated molecule forms bigger aggregates analogous to reverse micelles that can extract additional polar species in their polar core through a solubilization effect, thus decreasing the separation factor between uranium and zirconium. [ABSTRACT FROM AUTHOR]

Among the fission products present in the spent nuclear fuel, technetium exhibits a singular behavior in reprocessing operations performed by solvent extraction. Indeed, this strong acid readily dissociates to form the oxo-anion TcO4− that may interfere with uranium(VI), plutonium(IV), and zirconium(IV) in the extraction cycles of the PUREX process. This paper focuses on the uranium-technetium complex with TBP and on its non-radioactive rhenium surrogate. Despite the large set of distribution data available for rhenium and technetium extraction with TBP, the structures of the co-extracted complexes remain largely unknown. However, it is important to understand clearly the extraction mechanism of technetium with TBP in the PUREX process to optimize the separation process and to model its behavior during the extraction steps. Based on distribution data available in the literature, a thermodynamic model was developed for the extraction of technetium with TBP for a large excess of uranium(VI) in organic phase. A good representation of uranium and technetium distribution data was thus obtained when considering the formation of (HTcO4)(TBP)n complexes, as well as mixed UO2(NO3)(TcO4)(HNO3)x(TBP)n, complexes. In the complex UO2(NO3)2(HNO3)x(TBP)n., one pertechnetate anion replaces one nitrate in the uranium coordination sphere. Combination of complementary spectroscopic techniques (FT-IR and X-ray absorption) supported by theoretical calculations (density functional theory) with organic phases containing a large excess of technetium(VII) or rhenium(VII) enabled full characterization of the limit mixed uranium−technetium species and also of mixed uranium-rhenium species. Details on the coordination of the uranium-technetium complex are provided with the help of DFT calculations and XAS measurements. [ABSTRACT FROM AUTHOR]

Knowledge of the complex microstructure in solvent extraction phases is mandatory for a full comprehension of ionic separation. Coupling EXAFS with MD simulations for uranyl extraction in sulfuric media with tertiary amine extractants enabled unravelling of the unprecedented uranyl tri-sulfate structure. [ABSTRACT FROM AUTHOR]

In industrial nuclear fuel reprocessing, small amounts of ruthenium are extracted by tri-n-butyl-phosphate (TBP) at the same time as uranium and plutonium. This behavior increases solvent radiolysis and requires secondary extraction cycles to minimize the residual ruthenium content in uranium and plutonium products. However, the solvent ruthenium extraction mechanism remains largely unexplored. This study addresses the speciation of ruthenium in solvent extraction conditions by complementary infrared and X-ray absorption spectroscopy. First, spectroscopic result interpretation is supported by a single crystal X-ray diffraction study on reference compounds to unambiguously demonstrate that the ruthenium extraction mechanism is driven by a weak outer-sphere Ru-TBP interaction. Second, the ruthenium coordination sphere is quantitatively characterized. Ruthenium speciation in the organic phase depends on the initial aqueous phase, and both monomeric ruthenium nitrosyl trinitrate complexes and a hydrolyzed dimeric ruthenium nitrosyl complex are shown. Average coordination numbers for nitrate, hydroxide, and aquo ligands are accurately determined in both phases, by applying a constrained EXAFS fit approach. [ABSTRACT FROM AUTHOR]