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Evaluating the impact of phase modifiers on trivalent f-element coordination and organic phase composition in solvent extraction
Peroutka, Allison Ann
Peroutka, Allison Ann
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2024
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Abstract
Efforts to address climate change depend on enhancing separations of f-elements, but challenges emerge because of their comparable physical and chemical properties. Intra-group separations (within Ln or An) can be achieved by utilizing size-based separations that exploit the decrease in ionic radii across the period. Meanwhile, successful inter-group separations (between An and Ln) have been achieved by employing ligands containing softer nitrogen or sulfur donor atoms. Different extractants are typically chosen based on whether the targeted separation is intra-group or inter-group, but additional methods to enhance f-element separations remain the same. Selectivity in solvent extraction is determined by both inner-sphere coordination (choice of extractant) and long-range dispersion interactions (choice of solvent). Traditionally, modifications to extractant design involve adjusting the functional group responsible for metal coordination or the non-polar alkyl tails to improve separations. Since selectivity depends on both inner-sphere coordination and long-range dispersion interactions, adding a phase modifier to the diluent – an organic molecule slightly more polar than the diluent – may offer a simpler way to enhance f-element separations.
This work examines how phase modifiers alter the composition of the organic phase and affect the complexation of f-elements, shedding light on the structural changes induced by these modifiers. The impact of alcohol alkyl chain length on the extraction of Ln, H2O, and HNO3 in TODGA in n-dodecane is explored, alongside the influence of these alcohols on vibrational spectra. Furthermore, the implications of alcohol structure and inorganic acids on extraction and Ln inner-sphere complexation in TODGA in n-dodecane are investigated. The methodologies developed to study the role of phase modifiers in TODGA systems are applied to ADAAM-EH systems to investigate the effects of phase modifiers on f-element selectivity, inner-sphere complexation, and organic phase composition. The key finding of this work is that the inclusion of phase modifiers impacts f-element selectivity due to their role in the organic phase. The role of phase modifiers is dependent upon the flexibility of the metal-extractant complex in the absence of phase modifiers. This work demonstrates that solvent modifications can be leveraged for f-element separations by influencing either inner-sphere or outer-sphere coordination.
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