Christian Sorel

The N,N-dialkylamide DEHiBA (N,N-di-2-ethylhexyl-isobutyramide) is a promising alternative extractant to TBP (tri-n-butylphosphate) to selectively extract uranium(VI) from plutonium(IV) and spent nuclear fuel fission products. Extraction of technetium, present as pertechnetic acid (HTcO₄) in the spent fuel solution, by DEHiBA was studied for different nitric acid and uranium concentrations. The uranium(VI) and technetium(VII) coextraction mechanism with DEHiBA was investigated to better understand the behavior of technetium in the solvent extraction process. Uranium and technetium distribution ratios were first determined from batch experiments. On the basis of these data, a thermodynamic model was developed. This model takes into account deviations from ideality in the aqueous phase using the simple solution concept. A good representation of uranium and technetium distribution data was obtained when considering the formation of (DEHiBA)_i(HNO₃)_j(HTcO₄)_k complexes, as well as mixed (DEHiBA)₂(UO₂)(NO₃)(TcO₄) and (DEHiBA)₃(UO₂)(NO₃)(TcO₄)(HNO₃) complexes, where one pertechnetate anion replaces one nitrate in the uranium coordination sphere in the two complexes (DEHiBA)₂(UO₂)(NO₃)₂ and (DEHiBA)₃(UO₂)(NO₃)₂(HNO₃). Combination of complementary spectroscopic techniques (FT-IR and X-ray absorption) supported by theoretical calculations (density functional theory) enabled full characterization of the formation of mixed uranium–technetium species (DEHiBA)₂(UO₂)(NO₃)(TcO₄) in the organic phase for the first time. The structural parameters of this complex are reported in the paper and lead to the conclusion that the pertechnetate group coordinates the uranyl cation in a monodentate fashion in the inner coordination sphere. This study shows how combining a macroscopic approach (distribution data acquisition and modeling) with molecular-scale investigations (FT-IR and X-ray absorption analysis supported by theoretical calculations) can provide a new insight into the description of a solvent extraction mechanism.

The promising new extractant molecule butyl-1-[N,N-bis(2-ethylhexyl)carbamoyl]nonyl phosphonic acid (DEHCNPB) was designed and used to develop a new solvent extraction process for the selective recovery of uranium from phosphoric acid. This bifunctional extracting molecule shows high affinity and selectivity for uranium(VI) versus iron(III) and the other elements present in phosphoric acid (Al, Ti, V, etc.). Batch equilibrium experiments were first carried out to determine the stoichiometries of the different complexes formed with uranium(VI) and iron(III) in organic phase and to optimize the different steps of the process at laboratory scale. These experimental data were then used to develop a chemical model to simulate uranium(VI) and iron(III) extraction from phosphoric acid, which was implemented in the PAREX simulation code. A flowsheet was calculated and tested in laboratory-scale mixer-settlers on a genuine phosphoric acid industrial solution. The continuous counter-current test was very successful and showed the possibility to recover more than 95% of uranium decontaminated from the main impurities. A Fe/U ratio of 0.03% was measured in the uranium product, which confirms the high potential of this new solvent for the further production of nuclear-grade uranium from phosphate ores.

This paper presents the methodology developed in order to thoroughly characterise a solvent extraction system containing high solute concentrations. The chemical system selected is N,N-(2-ethylhexyl)isobutyramide (DEHiBA) diluted in one alkane with increasing concentration of uranium(VI). Combining experiments with theoretical calculations allowed a deeper understanding of the extraction mechanism. A thermodynamic study was performed by the classical van't Hoff method and also by direct calorimetry to provide the enthalpies of extraction and specific heats. Dedicated methods like vapour pressure osmometry and electrospray ionisation mass spectrometry analysis provide information about the stoichiometry of the extracted species. Spectroscopic investigations with ultraviolet–visible and Fourier transform infrared probed the uranium coordination. Finally, a combination of molecular dynamics simulations, and small and wide-angle X-ray scattering experiments investigated the organisation in the organic phase beyond the molecular scale. It was shown that the high concentrations of uranium extracted have no influence on the stoichiometry of the complexes and the coordination of uranium in the inner sphere. The thermodynamic properties related to the extraction process and obtained with a fine consideration of the activity coefficients showed to be the same as those found with trace concentration. However, an unexpected organisation beyond the molecular scale was observed with an important role of nitrates as bridging ligands which could explain some physico-chemical properties. This approach could be applied to other chemical systems (other N,N-dialkylamides or other cations) to identify the origin of the different affinities between ligands and the difference of selectivity between cations.

In this study, a thermodynamic model based on Mikulin and Sergievskii–Dannus equations to calculate activity coefficients in both organic and aqueous phases was used to model plutonium(IV), nitric acid and water extraction by 30% TBP in dodecane. The speciation of Pu(IV) in the organic phase was determined via extraction isotherms modeling. The best agreement between experimental and calculated plutonium distribution ratios was obtained by considering two Pu(IV) complexes in the organic phase: Pu(NO₃)₄(TBP)₂ and Pu(NO₃)₄(TBP)₃.