URANYL PEROXIDE NANOCLUSTER (U60) IN THE PRESENCE OF NA-MONTMORILLONITE, ANORTHITE, AND QUARTZ

The mobility of uranium in the subsurface environment has been partially addressed by sorption experiments of the uranyl ion (UO₂²⁺) to various minerals in diverse geochemical systems. However, research regarding the behavior of uranyl peroxide nanoclusters, such as [(UO₂)(O₂)(OH)]₆₀^60- (U₆₀), in the environment is relatively scarce. These clusters are predicted to form in aqueous environments where alpha radiolysis produces sufficient quantities of hydrogen peroxide under alkaline conditions (e.g., legacy nuclear waste tanks, nuclear reactor accidents, used nuclear fuel cooling pools, geologic repositories) and their formation may significantly impact our current understanding of the fate and transport of U(VI) if they behave differently than (UO₂²⁺). Batch sorption experiments were used to examine the sorption of U₆₀ to Na-montmorillonite (SWy-2), anorthite (CaAl₂Si₂O₈), and quartz (SiO₂) as a function of U₆₀ concentration, mineral concentration, and pH. Relatively rapid and quantitative removal of U₆₀ from solution was observed in the presence of Na-montmorillonite (SWy-2); however, much less U₆₀ was removed after partial purification of SWy-2. The removal mechanism appears to be promoted by the exchange of Li⁺ counter ions, normally weakly associated with U₆₀ in solution, for Ca²⁺ ions naturally present in the clay. In solution, Ca²⁺ induces aggregation and precipitation of U₆₀. On the contrary, U₆₀ is not removed by anorthite and quartz and persists in the presence of these minerals for at least one month. This is attributed to the low point to zero charge of these minerals, which leads to a negatively-charged mineral surface and repulsion of the negatively charged uranyl peroxide cage. Scanning electron microscopy with energy dispersive X-ray spectroscopy (SEM/EDS) and Raman spectroscopy were used to analyze the mineral phases before and after reactions with U₆₀. SEM/EDS identified U-rich precipitates with a noticeable wt% of Ca²⁺ and Raman spectra revealed signals from the symmetric stretch of U=O and O-O vibrations of peroxo groups associated with the clay surface. This research demonstrates that uranyl peroxide clusters like U₆₀ may be more mobile in environmental systems than U(VI), which has the potential to alter our current understanding of the fate and transport of uranium in the geosphere.