Effects of surface structures on surface charging and anion adsorption behaviors of aluminum and iron oxides

Aluminum and iron oxides are two of the most important surface-reactive minerals at the earth surface. The chemical reactions that occur at the oxide surfaces not only play an important role in the geochemistry of many elements but also determine the fate of many human-introduced chemical compounds such as fertilizers, pesticides, and pharmaceuticals. Despite their importance, the surface chemistry of Al and Fe oxides is not fully understood. The major obstacle is that surface potential at the oxide surface can neither be measured nor predicted by an equation like the Nernst equation. This difficulty has made it a challenging task to relate a surface structure to a specific surface charging or anion adsorption behavior of Al and Fe oxides, or vice versa.;In this study, using surface complexation theory, we attempt to explain the three macroscopic surface behaviors of Al and Fe oxides in terms of their surface structures: (1) the shift in pHpac and symmetry of surface charging curves with the hematite samples prepared under different conditions; (2) the maximum of phosphate adsorption at pH 4.0 that appears on Al oxides but not on Fe oxides; and (3) the maximum of ibuprofen adsorption that appears at pH 4.5 for Al oxides, at pH 2.5 for hematite, but does not on kaolinite.;Our results indicate that (1) the variation in surface charging behaviors with differently prepared hematite samples is due to the difference in surface structure between dominant crystal faces; (2) the maximum of phosphate adsorption on Al oxides results from the absence of proton-reactive triply coordinated surface hydroxyls at the transformed surfaces of Al oxides; and (3) the maximum of ibuprofen adsorption arises from the replacement of singly coordinated surface hydroxyls at the surface of Al and Fe oxides, but the pH of maximum adsorption is affected by whether the triply coordinated surface hydroxyls are present at the crystal faces of oxides.;The approaches developed in this study to relate macroscopic behaviors to surface structures are not only of theoretical significance but also of practical application in designing oxides-based water filtering system.