{U60} NANOCLUSTER ADSORPTION ONTO CLAY MINERAL SURFACES

In aqueous solution, uranyl ions (UO₂²⁺) can readily react with hydrogen peroxide (H₂O₂) and base cations (e.g., Li and K) to form various uranyl peroxo nanocluster species. These topologically complex, highly soluble species typically measure on the order of nanometers in diameter and have been shown to persist intact in alkaline environments for extended periods of time. Were groundwater to contact spent nuclear fuel, for example in a subsurface waste repository setting, the subsequent production of hydrogen peroxide via alpha-radiolysis could potentially result in the formation of these clusters. Such species could thus significantly influence the mobility of uranium in the subsurface environment. This work presents an update on research investigating the interaction of the {U₆₀} cluster (Li₄₀K₂₀[(UO₂)(O₂)(OH)]₆₀(H₂O)₂₁₄) with surfaces of phyllosilicate minerals commonly found in the subsurface such as muscovite [KAl₂(AlSi₃O₁₀)(OH)₂ and kaolinite [Al₂Si₂O₅(OH)₄]. For kaolinite, the adsorption behaviour of the {U₆₀} clusters over several weeks is assessed using batch-type experiments by carrying both cluster concentration and solid-cluster ratio. Non-linear behavior is observed. For muscovite, in situ atomic force microscopy (AFM) and synchrotron-based X-ray reflectivity techniques (crystal truncation rod/resonant anomalous X-ray reflectivity – CTR/RAXR) assist in elucidating certain details of the attachment mechanism on the mineral surface. Results presented here will show the complex nature of surface-cluster interface interactions, and how these are a function of bulk solution composition as well as the chemistries and topologies of both nanoclusters and solid surfaces.