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The use of tri-alkylcarbamides (TrAC) as novel extractants for the separation of uranium(VI) and plutonium(IV) by solvent extraction from spent nuclear fuels is investigated. The comparison between extraction properties of different tri-alkylcarbamides and tetra-alkylcarbamides indicates their higher ability to co-extract uranium(VI) and plutonium(IV). Moreover, tri-alkylcarbamides present very high separation factors SFU/Pu (up to 100) at lower nitric acid concentration showing that they are good candidates for U(VI)/Pu(IV) separation without redox chemistry. The addition of branched 2-ethylhexyl chains induces a decrease of distribution ratios and U/Pu separation factor. Due to the formation of intermolecular hydrogen bond between carbonyl and -NH functions, the viscosity of tri-alkylcarbamides is higher compared to tetra-alkylcarbamides. Nevertheless, the introduction of branched alkyl chains decreases the viscosity of the solvent and increases the limit organic concentration (LOC) for uranium(VI), preventing from third phase formation. [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]

The use of tetra-alkylcarbamides as novel extractants for the separation of uranium(VI) and plutonium(IV) by solvent extraction from spent nuclear fuels is investigated in this study. Batch extraction experiments show that tetra-alkylcarbamides strongly extract U(VI) with high distribution ratios. Plutonium(IV) can be co-extracted with U(VI) at high nitric acid concentration, while high U(VI)/Pu(IV) selectivities can be reached at lower acidity. Loading capacity experiments with high uranium concentrations show that alkyl chains longer than butyl are necessary to avoid third phase formation. Nevertheless, the viscosity of uranium-loaded solvents gets too high with alkyl chains longer than pentyl. Overall, this study shows that with TPU extractant (with four pentyl chains), an efficient co-extraction of uranium and plutonium can be reached (DU,Pu > 1) for a concentration of nitric acid higher than 4 mol•L−1, while the partition between uranium(VI) and plutonium(IV) could be operated even at 2 mol•L−1 nitric acid without redox chemistry. [ABSTRACT FROM AUTHOR]

Butyl-N,N-di(2-ethylhexyl)carbamoyl-nonylphosphonate (DEHCNPB) is a new amido-phosphonic acid bifunctional ligand with a high uranium extraction efficiency from wet phosphoric acid. In the present work, the DEHCNPB coordination structure toward uranium and its aggregation properties have been fully characterized using experimental and theoretical approaches. DEHCNPB forms associated species, and its aggregation properties such as aggregation number or CAC have been determined from SAXS and SANS measurements. The preferred site for bonding of DEHCNPB toward U(VI) has been determined from IR, EXAFS, and DFT calculations. [ABSTRACT FROM AUTHOR]

In a solvent extraction process, the compositions of the phases in thermodynamic equilibrium (described as a Winsor-II regime) must be determined to obtain the extraction isotherms of ions as well as co-extracted water. By comparing the extractions of a series of acids by the malonamide DMDOHEMA (N,N’-dimethyl-N,N’-dioctyl hexylethoxy malonamide) in n-heptane, the specific anion effects regarding third phase formation and the strength of the acid-extractant interaction were investigated. It is shown that third phase formation is driven by hydration enthalpy of acid, while the polar core microstructure is controlled by the pKa of the acids. Upon acid extraction, the promotion of third phase formation follows the series H2SO4≈ H3PO4≈ HClO4> HNO3> HCl > HCOOH, which correlates to hydration enthalpy of acid in the case of monoacids. The combination of IR spectroscopy and DFT calculations revealed two different modes of acid extraction, either by hydrogen bonding (extraction of non-dissociated acid: HA) or by protonation of the extractant (extraction of dissociated acid: H+A−). The strength of the amide-acid interaction (protonation vs. hydrogen bonding) is correlated to the pKa of the acid and is responsible for the microstructure of the solution. [ABSTRACT FROM PUBLISHER]