Doctor of Philosophy (PhD)
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Chemistry, Radiochemistry, Actinide, Lanthanides, Mercury, Dipicolinamides, Dithiopicolinamides
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Spent nuclear fuel (SNF) – or used nuclear fuel (UNF) – contains long-lived minor actinides such as 241Am, 245Cm, and 237Np, together with fission products that include lanthanides. Minor actinides are responsible for much of the radiotoxicity and heat generation that limits the capacity of geological repositories. Thus, removing minor actinides from UNF can reduce storage time required for decay to natural levels of activity by several orders of magnitude. Actinide(An)/Lanthanide(Ln) separation processes via solvent extraction by selective complexation with organic or aqueous ligands have to overcome difficulties due to similarities in their ionic radii for the +3 oxidation state. Actinide valence orbitals (5f) allow for a stronger covalent component in metal-ligand interactions with soft-donor ligands, as compared to the 4f orbitals in lanthanides. Therefore, we have synthesized ligands with soft-donor sites that can take advantage of slight differences in hardness between An(III) and Ln(III) for selective An3+ separations.
We have investigated the binding and extraction properties of An and Ln with ligands that contain the C=O vs. the C=S group, specifically dipicolinamides vs. dithiopicolinamides. The S-donor thioamide ligand did not show strong binding towards Ln(III) in the UV-visible and NMR spectra in CH3CN, yet it was shown to extract Am(III) over Ln(III) from highly acidic solutions. Gas-phase studies and theoretical DFT calculations both showed stronger binding of An(III) vs. Ln(III) for the thioamide vs. the amide ligand in agreement with extraction results. Moreover, a dipyridine-dipicolinamide ligand was also used on an aqueous environment as a holdback reagent that keeps Am(III) selectively in the aqueous phase while HDEHP complexes Ln(III) in the organic phase, taking advantage of the difference on hardness between An(III) and Ln(III).
Our dipicolinamide and dithiopicolinamide results open new possibilities for efficient waste transmutation processes and for minor actinide recycling that can increase uranium utilization. Aside from SNF, dithiopicolinamide ligands also showed promise for addressing the presence of mercury in cold war nuclear waste. As Hg is present in the nuclear waste tanks at the Savannah River Site in several forms, including organic Hg, the mercury problem has been of concern.
Lehman-Andino, Ingrid, "Dipicolinamide Ligands and Analogs for Lanthanides (Ln), Actinides (An) and Mercury (Hg) for Application to Nuclear Fuel Cycle Separations and Waste Management" (2020). FIU Electronic Theses and Dissertations. 4501.
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