Response of interfacial water to arsenate adsorption on corundum (0 0 1) surfaces: Effects of pH and adsorbate surface coverage

The interaction of water with metal oxide mineral surfaces affects chemical reactions that are important to many natural processes. Previous studies demonstrate that the various functional groups on mineral surfaces control ion adsorption as well as the ordering of interfacial water, but the effect of water structure on the reactivity and dynamics of interfacial reactions has not been systematically investigated. In this study, surface X-ray scattering measurements on corundum (0 0 1) surfaces with and without arsenate over a range of pH conditions have been used to determine the response of interfacial water structure to arsenate adsorption. In the absence of arsenate, the structure of interfacial water near this surface varies little over the pH range of 5-9, suggesting that surface charging from protonation-deprotonation under the conditions studied is inadequate to induce extensive restructuring. In contrast, interfacial water undergoes substantial restructuring upon arsenate adsorption, indicating that charged surface complexes substantially perturb the arrangement and order of interfacial water on this surface. The overall interfacial water structure also varies proportionally with arsenate surface coverage, with adsorbed water layers moving closer to the surface and the extended layering of interfacial water showing reduced positional disorder as arsenate surface coverage increases. Systematic variations in interfacial water properties with increasing arsenate concentration at each pH value are consistent with the coexistence of two distinct water structures (in the absence and presence of adsorbed arsenate) that vary in proportion with adsorbate surface coverage. These observations demonstrate that the adsorption of arsenate alters the structure of interfacial water near corundum (0 0 1) surfaces, possibly through the modification of the charge state of surface sites or by providing new sites to which water may hydrogen bond. Such adsorbate-induced restructuring of interfacial water may contribute to the energetics of chemical reactions at mineral-water interfaces.