Kunaljeet S. Tanwar

A multi-scale assessment of As(V)-aluminum oxyhydroxide interactions

Hydrated aluminum oxides (HAO) have been identified as major sinks for contaminants, such as arsenic, in aquatic and soil ecosystems. This research focused on the assessment of As(V)-HAO interactions by combining macroscopic studies with x-ray absorption spectroscopy (XAS). Sorption of As(V) onto HAO was studied as a function of reaction time, sorbate loading, and the presence of phosphate. The short-term isotherm studies indicated that HAO has a high sorption capacity for As(V) (approximately 1.9 x 10 –3 mol As(V) g –1 HAO) and the uptake of As(V) was not significantly affected by equimolar amounts of phosphate at pH 4.5. The results of long-term kinetic studies, where a continuous influx of As(V) was maintained, revealed rapid initial sorption to the external surface of HAO within less than four hours, followed by slow uptake over several weeks, which could be attributed to intraparticle diffusion. Complementary XAS analyses suggested that As(V)-HAO interactions can be described by one average reaction mechanism, resulting in bidentate inner-sphere complexes. This mechanism was independent of time, sorbate loading, and the presence of phosphate under the reaction conditions employed in this study. Overall, the results of this research emphasized that microporous oxyhydroxides of aluminum are much larger sinks for oxyanions, such as As(V), than presently understood.