INTERFACIAL WATER ORDERING AND COMPLEX OXOANION ADSORPTION ON HEMATITE AND CORUNDUM SURFACES

Development of thermodynamic and kinetic models to predict the behavior of chemical species in environmental and geological systems benefit from a fundamental understanding of mineral-water interface structure and reactivity. Our work in recent years has sought to provide new insight into metal oxide-water interfaces in two areas: the ordering of water and the mechanisms of oxoanion adsorption at these interfaces. We have explored the structure of interfacial water on a number of hematite (α-Fe₂O₃) and corundum (α-Al₂O₃) surfaces. While ordering in this water appears to be a general phenomenon, with the degree of ordering declining away from the surface on the length scale of approximately 1-2 nanometers, quantitative differences in water structure and positional disorder exist among the different surfaces examined. We have also recently investigated arsenate (AsO₄^3-) adsorption on the (012) surfaces of these minerals and have demonstrated that the accepted conceptual model for this process has been missing a key mechanism: outer-sphere adsorption. We observed that arsenate simultaneously forms inner- and outer-sphere surface complexes at pH 5 and 0.01 M ionic strength on both oxides. We have begun to investigate the dependence of this unexpectedly complex adsorption behavior on pH, ionic strength, and the crystallographic orientation of the sorbent mineral phase. These studies explore how variations in chemical conditions and interfacial water ordering affect arsenate adsorption behavior. Our observations suggest a connection between the molecular-scale arrangement of water near a mineral surface and the chemical properties of that surface.