Understanding the structure of actinide ions in organic solution is of particular interest in the development of effective solvent extraction separations for waste remediation in nuclear fuel cycles. In complex organic phases, in which actinide-ligand bonds are diverse and versatile, several complexes can coexist and quantitative structural information is often lacking because of the difficulty to interpret experimental data. Quantum chemistry associated with experimental data can improve this analysis and more particularly help in the EXAFS measurement interpretation. In the present study, we have combined extended EXAFS with XRD, infrared spectroscopy and theoretical calculations to develop an efficient method to solve polydispersity in organic solvent. The crystal data are used as reference structural models. EXAFS spectra were measured in the solid state and in solution. To help EXAFS analysis, ab initio EXAFS spectra were computed from structural parameters and Debye-Waller factors derived from quantum chemistry without using fitting parameters. It was then applied to elucidate the structures of plutonium(IV) and uranium(VI) ions with a series of N,N-dialkyl amide, as well as co-extracted uranium/technetium by the same N,N-dialkyl amide ligands. For U(VI), EXAFS analysis combined with XRD shows that the uranyl structure is identical in the solution and in the solid state and is independent of the N,N-dialkyl amide alkyl chains. For plutonium(IV) complexes, it was not possible to resolve unambiguously plutonium coordination structures in solution from EXAFS measurements and the coordination structure was established from the comparison between experimental and ab initio EXAFS spectra. With linear alkyl chain amides, Pu(IV) adopts identical structures in the solid state and in solution while a different coordination structure is found in solution with branched alkyl chain-amides. Finally, it is shown that EXAFS spectra can be reproduced from ab initio calculations without using fitting parameters and support structural characterization in complex solvent extraction systems.

How x-ray absorption spectroscopy merge with theoretical models to solve actinides and technetium structure in solvent extraction

256th National Meeting (2018)

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