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Investigation of metal ion extraction and aggregate formation combining acidic and neutral organophosphorus reagents


Advanced nuclear fuel cycles are dependent on successful chemical separation of various elements present in the used fuel. Numerous solvent extraction systems have been developed for this purpose. In these solvent extraction systems, aggregation phenomena can interfere with the efficiencies of the processes and decrease the successfulness of the separation. The predictability of these phenomena have been challenging due to the lack of a fundamental understanding of the mechanism that drives these aggregate formations. Previous research has shown that these aggregates may be linked to a synergistic effect, whereby the extraction of metal ion into the organic phase is enhanced with mixtures of extraction reagents compared to each reagent used alone. To investigate the aggregation more closely and to probe the possibility of a link between aggregation and synergy, combined solutions of tri-n-butyl phosphate (TBP) and dibutyl phosphoric acid (HDBP) in n-dodecane were chosen as the organic phase to be contacted with an aqueous phase containing a single metal ion from the lanthanide series in nitric acid. Distribution ratios of the metal ions between the phases as well as water uptake and nitric acid extraction into the organic phase were used to investigate this aggregation phenomenon more closely and to understand the extraction behavior of this binary extractant system. Additionally, X-ray and neutron scattering and X-ray absorption techniques were used to investigate the macromolecular phase behaviors of the organic phase, with scattering yielding information regarding aggregate size and shape and absorption yielding information on the complex environment of the extracted metal ions. The combination of these techniques provide a more complete picture of the organic phase and may shed light on the link to synergy and mixed complexes. Our studies show that the complexes resulting from extraction of lanthanides into TBP:HDBP system differ from those that have been previously explored in these types of systems. This finding coupled with the formation of elongated rod-like aggregate structures formed in the organic phase may suggest that the complexes formed by the extraction of lanthanides by TBP:HDBP may be incorporated into the micellar structures and indicate a link between synergy and aggregate formation.

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