X-ray scattering and X-ray absorption spectroscopy studies of the structure and reactivity of aluminum oxide surfaces

Sorption reactions at mineral surfaces have a significant influence on the transport and chemical speciation of trace metals and metalloids in aquatic systems. The extent of solute partitioning and stability of sorbed species are related to the mode of sorption, which is strongly influenced by the structure and composition of the reactive substrate surface. The work presented here is intended to give some insight into the reactivity of aluminum oxide surfaces based on the structures of sorption complexes, surface structures, and in-situ determinations of sorption affinity of Pb(II) and Se(VI) on single crystal surfaces.; X-ray absorption fine structure (XAFS) spectroscopy was used to elucidate the sorption modes and local structure of Zn(II) sorption complexes on powdered and single crystal aluminum oxide substrates. Zn(II) was found to form predominantly inner-sphere bidentate surface complexes with AlO₆ polyhedra. At sorption densities greater than ∼1.5 μmole/m2 on the powdered substrates the XAFS spectra indicate the formation of a three-dimensional mixed-metal coprecipitate, with a hydrotalcite-like local structure.; The influence of surface structure on the overall reactivity was examined using long period x-ray standing wave measurements. Direct in-situ determination of Pb(II) and Pb(II)/Se(VI) uptake on the (0001) and (1–102) surfaces of α-Al₂O₃ showed that the (1–102) surface is roughly a factor of two more reactive than the (0001) surface. The interpretation of the reactivity differences is based on the local structure of exposed surface functional groups on the two surfaces. The structural models used in this interpretation are based on the results of crystal truncation rod (CTR) diffraction measurements of the hydrated surface structures. These measurements show that the hydrated α-Al₂O ₃ (0001) surface is oxygen/hydroxyl terminated, with a 53% contracted double Al layer directly below and the surface oxygens are in two-fold coordination with the underlying aluminum. In contrast the best model for the α-Al ₂O₃ (1–102) surface suggest that the surface is terminated by oxygens which are in single-, double- and triple-coordination with underlying aluminum.