Calcium-Facilitated Aggregation and Precipitation of the Uranyl Peroxide Nanocluster U60 in the Presence of Na-Montmorillonite

The unique and diverse features of uranyl peroxide nanoclusters may contribute to the enhanced mobility of uranium in the environment. This study examines the sorption of the uranyl peroxide nanocluster [UO2(O2)(OH)]6060‑ (U60) to Na-montmorillonite (SWy-2), plagioclase (anorthite), and quartz (SiO2) as a function of time, U60 concentration, and mineral concentration. SWy-2 was studied in both its untreated form as well as after two different pretreatments, denoted as partially treated SWy-2 and fully treated SWy-2. U60 was removed (∼99%) from solution in the presence of untreated and partially treated SWy-2. However, U60 was not removed from suspensions containing anorthite, quartz, or fully treated SWy-2, even after several months. The removal of U60 from suspensions containing untreated SWy-2 is promoted in part 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+ ions induce the aggregation of nanoclusters, which precipitate on the surface of SWy-2. Ca-rich U60 aggregates associated with SWy-2 were identified and characterized by scanning electron microscopy with energy dispersive spectroscopy, Raman spectroscopy, and X-ray photoelectron spectroscopy. This research enhances our understanding of the molecular-scale processes controlling U60 behavior at the mineral−water interface.