Water Ordering and Redox Process at Hematite-Water Interfaces

Reactions at hematite-water interfaces affect contaminant fate and transport, partially control biogeochemical iron cycling, and fractionate iron isotopes. The ability to predict and quantify these and other important processes in nature requires a fundamental understanding of the structure of hematite-water interfaces and the reactions that occur here. The ordering of water at hematite surfaces and the structural transformations these surfaces undergo as a result of reaction with aqueous Fe(II) have been characterized. Water takes on an ordered arrangement near the hematite (012) and (110) surfaces, with the degree of ordering declining away from the surface on the length scale of approximately 1-2 nanometers. Fe(II) induces orientation-dependent structural changes to hematite surfaces that suggest growth occurs on (001) and dissolution on (110) and (012). This behavior occurs under both neutral (pH 7) and acidic (pH 3) conditions. Fe(II) presumably serves a catalytic role, transporting Fe between different surfaces through solution with electrons conducting back through the hematite structure in the reverse direction. This dynamic interfacial redox process may be responsible for the recent observations of electron transfer and isotope fractionation in Fe(II)-iron oxide systems.