APPLICATIONS OF CRYSTAL CHEMISTRY TO MINERAL SURFACE GEOCHEMISTRY

Mineral surface geochemistry deals with the interaction of biological organisms and chemical species, including water, with mineral surfaces. The interaction of mineral surfaces with heavy metals and metalloids in aqueous solutions is of major interest because of the possibility they can sorb on mineral surfaces, which can effectively sequester them in specific pH ranges. XAFS spectroscopy studies have yielded quantitative information about the structures and modes of binding of cations and oxoanions at mineral/water interfaces. However, what is not known in most cases is the structure of the mineral surface in contact with water and how protons contribute to the stability of metal sorption complexes. Crystal chemical principles, including Pauling’s bond valence principle, can be used to help constrain the types of surface sites to which metal(loid)s can bind, as well as the number of protons that can bind to a surface oxygen that is also bonded to a metal(loid) ion. In an extension of this approach, we have carried out grazing-incidence EXAFS studies of Pb(II) sorption on single crystal hematite surfaces in contact with water. Pb(II) was found to bind dominantly in an inner-sphere, bidentate fashion to FeO6 octahedra on the a-Fe2O3 (0001) and (1-102) surfaces, which is consistent with our earlier study of Pb(II) binding to a-Al2O3 (1-102) (J. Colloid Interface Sci. 1997, 85, 473). In contrast, we found that Pb(II) forms dominantly outer sphere complexes at the a-Al2O3 (0001)/water interface (Geochim. Cosmochim. Acta 1996, 60, 3541). In order to understand these differences in reactivity, we have recently carried out crystal truncation rod diffraction studies of these hydrated surfaces. This CTR work found significant structural differences between the (0001) surfaces of a-Fe2O3 and a-Al2O3, which helps explain their differences in reactivity to Pb(II). Application of crystal chemical principles to these metal/metal oxide systems, as well as to those in which AsO43- and SeO42- are aqueous sorbents, helps constrain their sorption behavior as well as proton release.