ABSTRACT
We have characterized the adsorption of Co(II) on the (0001) and (1102) surfaces of alpha-Al(2)O(3) single crystals under ambient conditions using polarization-dependent grazing-incidence X-ray absorption fine structure spectroscopy, in combination with bond valence modeling. Co(II) ions were found to be adsorbed on both surfaces in an inner-sphere fashion. Adions were found to adsorb dominantly in a tridentate fashion (i.e., bonded to three surface oxygens) on the (0001) surface and dominantly in a tetradentate fashion on the (1102) surface. Based on EXAFS results and bond valence analysis, plausible surface complexation reactions for Co(II) sorption on these two surfaces can be written as represent surface water molecules, hydroxyl groups, and oxygens bonded to one, two, and three Al cations, respectively. Copyright 1999 Academic Press.
ABSTRACT
Surface precipitation is an important process in many areas of science and technology, including modeling contaminant segregation from groundwater to solid phases and dispersion of active phases on catalyst supports. XAFS, TEM, and XPS measurements of Co(II) sorbed on Al2O3 demonstrate that surface precipitates have formed from solutions that are undersaturated with respect to any known bulk solid phase. The precipitates have a structure similar to that of Co(OH)2(s), but are disordered and have a high concentration of Co vacancies. The data plus thermodynamic reasoning have been used to analyze the plausibility of various models for surface precipitation and to show that for Co(II)/Al2O3 it occurs by forming a double-hydroxide phase containing substrate-derived Al(III) ions. This idea was corroborated by mixing aqueous solutions of Al(III) and Co(II) at the pH and concentration of the sorption samples, forming a stable colloidal precipitate that is less soluble than either Al(OH)3 or Co(OH)2. The Co XAFS of this material was similar to that of the sorption samples. Successful quantitative models of metal ion transport in groundwater need to include the possibility of forming ternary and higher order precipitates that include ions derived from sparingly soluble solids. For catalyst impregnation, surface coprecipitation can prevent production of a well-dispersed precursor material.
ABSTRACT
The structures and compositions of Pb(II) adsorption complexes and surface binding sites on alpha-Al2O3 (0001) and (1&1macr;02) surfaces were investigated in the presence of water using grazing-incidence X-ray absorption fine structure (GI-XAFS) spectroscopy. Pb(II) ions were found to adsorb in an inner-sphere mode on alpha-Al2O3 (1&1macr;02) but as outer-sphere complexes on alpha-Al2O3 (0001). The distance between the outer-sphere complexes and the surface places useful constraints on double-layer properties of water. A bond-valence model is described that relates the reactivities of surface functional groups and adsorption complexes to their molecular structures and compositions, and places constraints on the stoichiometries of adsorption reactions, including proton release. The EXAFS and modeling results suggest that Pb(II) and Co(II) ions bond to [AlAlAl-->O-1/2--> and [Al-OH+1/22] surface functional groups. In contrast, [AlAl > OH] groups complex Co(II) but not Pb(II). The results indicate the importance of using structurally defined surface sites to describe reactions at oxide-water interfaces.