Sorption of Co(II) on metal oxide surfaces II.: Identification of Co(II)(aq) adsorption sites on the (0001) and (1(1)over-bar02) surfaces of α-Al2O3 by grazing-incidence XAFS spectroscopy

We have characterized the adsorption of Co(II) on the (0001) and (<1(1)over bar 02>) surfaces of alpha-Al2O3 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 (<1(1)over bar 02>) 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 [GRAPHICS] where (Al-OH2), [GRAPHICS] and [GRAPHICS] represent surface water molecules, hydroxyl groups, and oxygens bonded to one, two, and three Al cations, respectively. (C) 1999 Academic Press.