SIMULTANEOUS INNER- AND OUTER-SPHERE ARSENATE ADSORPTION ON IRON AND ALUMINUM OXIDE SURFACES

Adsorption of toxic elements onto mineral surfaces reduces their bioavailability and potential for transport. A fundamental understanding of adsorption processes is desired in order to predict the fate of contaminants in complexes systems and to generalize about the behavior of these elements in the environment. The adsorption of arsenate (AsO₄^3-) at pH 5 on the (012) surfaces of corundum and hematite was studied using resonant anomalous X-ray reflectivity (RAXR). Two distinct sorbed arsenate species were observed in roughly equal proportions: an inner-sphere species consistent with a bridging bidentate complex, and an outer-sphere species, presumably adsorbed via hydrogen bonding. The relative fraction of arsenate adsorbed in the outer-sphere complex was generally independent of the arsenate concentration in solution. These results suggest that outer-sphere arsenate adsorption may be a significant sequestration mechanism that has been largely overlooked in past studies and whose impacts on the fate and bioavailability of arsenic need further evaluation.

This work is supported by the Geosciences Research Program of the Office of Basic Energy Sciences, U.S. Department of Energy, through contract W-31-109-ENG-38 to Argonne National Laboratory.