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 [UO 2 (O 2 )(OH)] 60 60- (U 60 ) to Na-montmorillonite (SWy-2), plagioclase (anorthite), and quartz (SiO 2 ) as a function of time, U 60 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. U 60 was removed (∼99%) from solution in the presence of untreated and partially treated SWy-2. However, U 60 was not removed from suspensions containing anorthite, quartz, or fully treated SWy-2, even after several months. The removal of U 60 from suspensions containing untreated SWy-2 is promoted in part by the exchange of Li + counter-ions, normally weakly associated with U 60 in solution, for Ca 2+ ions naturally present in the clay. In solution, Ca 2+ ions induce the aggregation of nanoclusters, which precipitate on the surface of SWy-2. Ca-rich U 60 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 U 60 behavior at the mineral-water interface.