Abstract:
Reactions on naturally abundant hematite (alpha-Fe2O 3) surfaces significantly influence the transport and bio-availability of a number of important nutrients and contaminants. The surface reactivity of alpha-Fe2O3 is dependent on the surface structure, i.e. the identity and coordination of chemical moieties exposed at the surface. The surface structure is strongly influenced by the presence of water and common aqueous species such as Fe(II). Therefore, it is important to understand how the surface structure evolves in the presence of water and aqueous species (e.g. Fe(II)) in order to model the surface reactivity of hematite in natural aquatic systems. The current study provides a detailed experimental investigation of the surface structure of two predominant natural faces of alpha-Fe2O 3, the (1102) and (0001) surfaces under hydrated conditions in absence and presence of aqueous Fe(II). The surface structure of hydrated alpha-Fe2O3(1102) prepared via a room-temperature wet chemical and mechanical polishing (CMP) procedure is consistent with a surface termination where the top layer of iron atoms is absent compared to the stoichiometric bulk termination. The annealing of CMP prepared alpha-Fe2O3(1 102) in air at 773 K results in transformation of the surface to a structure consistent with the stoichiometric termination. For CMP prepared alpha-Fe2O3(0001), the experimental results show a co-existence of two distinct structural domains on the surface. The first domain corresponds to hydroxylation of surface Fe atoms, and the second domain is formed by complete removal of the surface Fe cation leading to an exposed oxygen layer on the surface. The exposure of CMP prepared alpha-Fe2O3(1 102) and (0001) to aqueous Fe(II) results in structural modification of both surfaces due to adsorption of Fe(II) at crystallographic lattice sites followed by oxidation to Fe(III). Preliminary research conducted to identify the effect of Fe(II) induced surface modification on reactivity using Pb(II) as a reactive probe indicates that the clean and Fe(II)-modified surfaces exhibit significantly different reactivity towards Pb(II). Overall, the systematic structural characterization of hydrated and Fe(II)-modified alpha-Fe 2O3 surfaces presented in the current study will provide a basis to elucidate surface structure-reactivity relationships for hematite and will aid in developing models of mineral-water interfacial reactivity.
