Interfacial water structure on the (0 1 2) surface of hematite: Ordering and reactivity in comparison with corundum

Many geochemical reactions that control the composition of natural waters, contaminant fate and transport, and biogeochemical element cycling take place at the interface between minerals and aqueous solutions. A fundamental understanding of these important processes requires knowledge of the structure of mineral-water interfaces. High-resolution specular X-ray reflectivity was used to determine the structure of the hematite (0 1 2)-water interface. Relaxation of the surface was observed to be minor, and water was found to order near the hematite surface. Two sites of adsorbed water are inferred to be ordered laterally, one bridging between triply coordinated functional groups and the other bridging between the singly coordinated functional groups on the surface, as steric constraints limit the possible arrangements of water molecules occurring at the observed heights above the hematite surface. Relaxations of the hematite and corundum (0 1 2) surfaces, which are isostructural, are similar and limited primarily to the top most layer of the structures. No significant changes to the interfacial stoichiometry (i.e., partial occupancy of surface species) are observed in either case. The structure of interfacial water is similar on the hematite and corundum (0 1 2) surfaces as well, although water appeared to be less well ordered on the hematite surface. This may be due to expected differences in the oxygen exchange rates from surface functional groups or the apparent better matching of the corundum oxygen lattice to the natural structural ordering in water, and suggests that the dielectric constant gradients of interfacial water may differ on the two surfaces. Similar charging behavior is expected for these surfaces as similar types of surface functional groups are exposed. Although generally similar, subtle differences in the reactivity of hematite and corundum (0 1 2) surfaces to arsenate adsorption, and possibly the adsorption of other species, may be related to the difference in ordering of interfacial water observed in this study.