Kinetics of Uranyl Peroxide Nanocluster (U60) Sorption to Goethite.

Abstract

The unique properties of uranium-based nanomaterials may significantly impact our current understanding of the fate and transport of U(VI) in environmental systems. Sorption of the uranyl peroxide nanocluster [(UO2)(O2)(OH)]6060- (U60) to goethite (α-FeOOH) was studied using batch sorption experiments as a function of U60 concentration (0.5-2 g·L-1), mineral concentration (100-500 m2·L-1), and pH (8-10). The resulting rate law describing U60 interactions with goethite at pH 9 was R = - krxn[U60]0.29±0.02[goethite]1.2±0.1 where krxn = (6.7 ± 2.0) × 10-4 (g·L-1)0.71±0.02(m2·L-1)-1.2±0.1(day-1). The largest fraction of U60 removed from solution was at pH 8, which is below the isoelectric point of the goethite used in this study. Site density calculations suggest that U60 may exist on the goethite surface at a center-to-center distance of 5.4-6.5 nm, depending upon pH, which mirrors the center-to-center distance observed in the aqueous phase near the U60 solubility limit. At pH 10, approximately 20% uranium was desorbed within 3 days. Analysis of the reacted mineral surface using X-ray photoelectron spectroscopy confirmed the presence of a single U(VI) species on the mineral surface, and electrospray ionization mass spectrometry revealed that U60 remains intact during the sorption and desorption processes. These results demonstrate that the behavior of U60 at the goethite-water interface is similar to that of discrete U(VI) but is governed by different sorption mechanisms and reaction kinetics, which has the potential to alter our current understanding of the fate and transport of uranium species in the environment.