The Behavior of Uranyl Peroxide Nanoclusters at the Mineral-Water Interface

This dissertation pioneers the macroscopic characterization of uranyl peroxide nanocluster interactions with different mineral surfaces. Batch sorption experiments were conducted in order to determine the effects of time, pH, ionic strength, sorbate concentration, and sorbent concentration on the sorption of [(UO₂)(O₂)(OH)]₆₀^60- (U₆₀) and [(UO₂)₂₄(O₂)₂₄(P₂O₇)₁₂]^48- (U₂₄Pp₁₂) to hematite (α-Fe₂O₃), goethite (α-FeOOH), montmorillonite, quartz (SiO₂), and anorthite.

This work reveals that U₆₀ persists in the presence of Fe(III) minerals, montmorillonite, and common alkali ions for months. The sorption behavior of U₆₀ in the presence of Fe(III) minerals mimics that of a discrete anionic species whose contacts at the mineral-water interface are driven by the formation of electrostatic, outer-sphere surface complexes. U₆₀ also survives the sorption-desorption process. In clay mineral suspensions, the counter-cations associated with the cluster undergo cation exchange reactions with exchangeable cations associated with the clay. The type of cation (i.e., monovalent or divalent) that is exchanged and the amount present in the system largely dictates the removal of U₆₀ from solution via surface precipitation to form large agglomerates of clusters. The sorption of U₆₀ to minerals that have a low point of zero charge, (i.e., quartz and anorthite) is not favorable under alkaline conditions due to repulsion of the negatively-charged mineral surface and the negatively-charged uranyl peroxide cage.

The incorporation of the pyrophosphate ligand (P₂O₇^4-) into the framework of uranyl peroxide nanoclusters leads to the formation of functionalized uranyl peroxide nanoclusters. The pyrophosphate-functionalized nanocluster U₂₄Pp₁₂ may form specific interactions with hematite and goethite surfaces through the terminal oxygen atoms of the pyrophosphate unit.

The removal of U₆₀ and U₂₄Pp₁₂ are not due to surface-mediated reduction of U(VI) to U(IV) by interactions with minor reducing components in goethite, hematite and montmorillonite. The governing rate laws describing U₆₀ and U₂₄Pp₁₂ interactions with goethite and hematite were determined through the use of a kinetic analysis derived as a part of this work. The rate of uptake and the amount of U₂₄Pp₁₂ removed in systems containing goethite was greater than compared to U₆₀. These results demonstrate that the rate of uptake of particular uranium nanoclusters may be largely dependent on the charge density of the clusters as well as the functionality of the linkages connecting uranyl peroxide polyhedral units.