Cation-Mediated Interactions of Uranyl Peroxide Nanoclusters

<p>This dissertation covers three related research projects investigating the nanoscale interactions of uranyl peroxide nanoclusters with monovalent and divalent cations. Each project utilizes complementary experimental techniques, such as ultra-small angle X-ray scattering (USAXS), small-angle X-ray scattering (SAXS), dynamic light scattering (DLS), single crystal X-ray diffraction (SC-XRD), and ultraviolet-visible near-infrared (UV-vis-NIR) spectroscopy, to study the behavior of the nanoclusters in aqueous systems. The first project investigated the nanoscale interactions between U₆₀Ox₃₀, [((UO₂)(O₂))₆₀(C₂O₄)₃₀]^60-, and neptunium(V) as a function of neptunium concentration. Our findings showed that neptunium induces aggregation of U₆₀Ox₃₀ when the concentration was ≤ 10 mM Np, while (NpO₂)₂C₂O₄6H₂O(cr) and studtite (((UO₂)(O₂)(H₂O)₂2H₂O(s)) formed at 15–25 mM Np. These results suggest that neptunium coordinates with the bridging oxalate ligands in U₆₀Ox₃₀, leaving metastable uranyl peroxide species in solution.</p><p>The second project explored the nanoscale interaction between U₆₀[(UO₂)(O₂)(OH)]₆₀and U₆₀Ox₃₀ with plutonium, which was added as Pu(VI). Our results showed that Pu(VI) was reduced to Pu(V) in the presence of U₆₀ and a mix of Pu(IV) and Pu(V) in the presence of U₆₀Ox₃₀ over a two-week period. Pu(V) subsequently promoted the aggregation of U₆₀ in the form of blackberries and U₆₀ macro-aggregation in the form of U₆₀ blackberry brambles. The latter represents a new structure not previously identified in the literature. In the U₆₀Ox₃₀ system, Pu(IV/V) promoted aggregation in the form of large mass fractals. All aggregates became more compact with increasing plutonium concentration, suggesting possible encapsulation of plutonium within U₆₀ and U₆₀Ox₃₀.</p><p> The third project investigated the aggregation of U₆₀nanoclusters as a function of alkali and alkaline earth metal concentrations. Our results showed that counterion-mediated attraction was the primary driver for U₆₀ aggregation. Other factors, such as cation concentration, charge, and hydration radii, also influence the size and type of aggregates. We observed a distinct trend in aggregate size triggered by the addition of alkali and alkaline earth metal cations: Na⁺ > K⁺ > Rb⁺ > Cs⁺ and Mg²⁺ > Ca²⁺ > Sr²⁺ > Ba²⁺. Tertiary structures were most prevalent among alkali metal cations, specifically for systems containing 9.5 mM Rb⁺ or 9.5 mM K⁺, but were also observed in the system containing 2.5 mM Ba²⁺. The compactness of the aggregates played an important role in overall size, with cations that had larger atomic radii and smaller hydration spheres producing the most compact aggregates. Further research is necessary to deepen our understanding of the mechanisms underlying aggregate formation and the behavior of other uranyl peroxide nanoclusters.</p><p>These studies provide insights into the nanoscale interactions of uranyl peroxide nanoclusters with actinides elements, alkali metals, and alkaline earth metals. Our findings provide the basis for the application of uranyl peroxide nanoclusters throughout the nuclear fuel cycle, perhaps most specifically in the separation of used nuclear fuels.</p>