Comparative response of interfacial water structure to pH variations and arsenate adsorption on corundum (0 1 2) and (0 0 1) surfaces

The atomic-scale structure of the interface between metal oxide minerals and aqueous solutions regulates chemical reactions that are fundamental to many geological and environmental processes. Reactions occur at surface sites having multiple possible coordination states, each of which display distinct pH-dependent charging behavior that affects interactions with adsorbates and interfacial water. Our previous work has shown that corundum (0 0 1) surfaces induce weak spatial ordering of interfacial water that varies little between pH 5 and 9 but is substantially altered by the adsorption of arsenate. The (0 0 1) surface is dominated by doubly coordinated functional groups that are largely uncharged over the pH range examined, but other surfaces have functional groups in different coordination states with distinct charging behavior. Corundum (0 1 2) surfaces are known to induce stronger interfacial water ordering than (0 0 1) surfaces, but how this is affected by pH changes or ion adsorption has not been investigated. In this study, surface X-ray scattering measurements have been used to determine the response of interfacial water structure on corundum (0 1 2) surfaces to pH variations and the surface coverage of adsorbed arsenate. Comparison of interfacial water properties on (0 1 2) surfaces to those observed for (0 0 1) surfaces is then used to systematically investigate how surface site coordination state affects ion adsorption mechanisms, interfacial water structure, and the feedback between them. In the absence of arsenate, interfacial water displays little variation in its arrangement near the (0 1 2) surface between pH 5 and 9. This general invariance is also observed for the (0 0 1) surface, suggesting that over the pH range of most natural waters, surface site protonation-deprotonation appears inadequate to induce extensive restructuring of interfacial water. The adsorption of arsenate weakly perturbs interfacial water structure near the (0 1 2) surface, with both the spatial arrangement and ordering of the adsorbed water sites independent of arsenate surface coverage, in contrast to the substantial restructuring of interfacial water near the (0 0 1) surface. This suggest that on surfaces with initially weak water ordering [i.e., corundum (0 0 1) surfaces], the addition of a charged adsorbate species may induce water restructuring, but on surfaces with initially strong water ordering [i.e., corundum (0 1 2) surfaces], water structure is generally unperturbed. As arsenate forms coexisting inner- and outer-sphere complexes on both surfaces, adsorption mechanisms do not appear to control the resulting restructuring of interfacial water. Instead, the different surface functional groups present on the (0 1 2) and (0 0 1) surfaces, with their distinct charging behaviors, likely drive the response of interfacial water to arsenate adsorption. These observations suggest that on surfaces where water restructures upon ion adsorption, the dielectric constant, capacitances, and electrostatic activity corrections will all differ from the pristine surface. In contrast, on surfaces where water structure is unaltered by ion adsorption these parameters will be unperturbed. This should be considered during future refinements of surface complexation models to better predict adsorption behavior at environmental interfaces.