PENTAVALENT ACTINYL CATION-CATION COMPLEXES IN AQUEOUS AND NON-AQUEOUS MEDIA

John Freiderich

A primary component of advanced nuclear fuel cycles is the separation of trivalent lanthanides from trivalent minor actinides. Due to similar chemistries of these two groups this separation is a challenging task. Redox-based separation schemes remain an attractive option for reprocessing efforts and can be highly effective in separating cations in solution based on changes in oxidation state. Unlike lanthanides, americium (Am) may exist in several higher oxidation states (Am4+, AmO₂+, AmO₂2+). However, stabilization of higher oxidation states of americium (+5, +6) is challenging in solution media, however, complexation of cations can improve redox stability. The cation-cation complex is one such complexation event that may be able to satisfactorily stabilize the upper oxidation states of Am. In addition, the solution conditions (i.e. high ionic strengths and high metal ion concentrations) for reprocessing of used nuclear fuel are such that cation-cation complexation is thermodynamically favorable and understanding the role of these complexes remains an important avenue of research. The actinyl-actinyl complex was first reported more than 50 years ago and since then has seen a little more than two dozen publications. These publications have primarily focused on proving the existence of the complex and determining its structural features with the goal of possibly understanding the mechanism of complexation. There have been both high ionic strength aqueous and non-aqueous reports of complexes, with little work done to determine the details of complex formation. The focus of this dissertation was to identify components and conditions that promote cation-cation complex formation in the solution phase. A series of experiments were devised to probe various aspects of this solution chemistry and to ascertain the role each of these aspects may play in complex formation. The aspects considered included water activity, ionic strength, water structure, complexing media, and the identity of the actinyl ligand and the metal center. Electrochemical, spectrophotometric, and computational techniques were used to probe the aspects mentioned above. As an overall note, the cation-cation complexation arises from the combined effect of many contributions to free energy and this phenomenon appears to be more complicated than a purely electrostatic interaction.

PENTAVALENT ACTINYL CATION-CATION COMPLEXES IN AQUEOUS AND NON-AQUEOUS MEDIA

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