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Coordination chemistry of lanthanides and actinides with dithiophosphates and its radiolysis products
Gomez Martinez, Daniela
Gomez Martinez, Daniela
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2025
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Abstract
With increased energy demands and heavy reliance on depleting fossil fuels, there has been a renewed interest in nuclear energy because of its low CO2 emissions and high energy density. The use of nuclear energy involves fission reactions of low-enriched uranium that produces considerable amount of products that lead to issues with the treatment of used nuclear fuel (UNF). They decrease the fission efficiency and require replenishment of the fuel. Reprocessing the used nuclear fuel requires the separation of neutron capture products from the lanthanide fission products. Stable lanthanide isotopes are roughly 33% of the fission products, and some of these lanthanides such as neodymium-143, samarium-149, and gadolinium-157, have large neutron capture cross sections and are regarded as neutron poisons.
The separation of trivalent lanthanides from the actinides is challenging because chemically, they behave similarly. They have similar sizes, oxidation states and are both considered hard Lewis acids. Fortunately, actinides have a slight preference for soft donor atoms compared to the lanthanides and so ligands containing soft-donor atoms such as nitrogen and sulfur are attractive candidates for separation processes. The ligand bis(2,4,4 trimethylpentyl)dithiophosphinic acid (Cyanex 301) is preferred among sulfur-donor ligands because of its remarkably high separation factor (SFAm/Eu = 5900). Unfortunately, Cyanex 301 radiolytically degrades in these recycling processes. It degrades from a dithiophosphinic acids to a monothiophosphinic acids and then to an phosphinic acids. During this conversion from a soft donor ligand to a hard-donor ligand selectivity of the actinides over the lanthanides is lost.
This thesis will explore how the coordination environment of the f-element complexes changes as Cyanex 301 degrades in these radioactive extractions. This work uses ligands analogous to the radiolytically degraded products formed in these separations. Lanthanide and iii actinide complexes are synthesized and characterized with single crystal XRD and other techniques such as optical spectroscopy.
The work done in this thesis shows the notable differences in bond lengths between the lanthanide and actinide complexes isolated, that affects the selectivity of dithiophosphinic acids in these extractions. The key finding is that there is a significant difference in the bond lengths between the actinide and lanthanide complexes isolated in one of the systems. The knowledge obtained from this research is crucial for bridging the gap between fundamental studies and applications.
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