Chemical reactions at oxide mineral-water interfaces affect contaminant fate and transport, nutrient availability, carbon cycling and sequestration, and nanoparticle mobilization. These reactions occur at surface sites having multiple possible coordination states that interact with both adsorbates and interfacial water.  Surface X-ray scattering measurements have been used to investigate how surface site coordination state dictates ion adsorption mechanisms and interfacial water structure, and the feedback between these, on aluminum and iron oxide surfaces.  Surfaces of corundum and hematite dominated by singly coordinated functional groups induce strong ordering of interfacial water.  In contrast, (001) surfaces of these materials, which are dominated by doubly coordinated functional group, induce weak water ordering that is largely unaffected by pH. This suggests that the charge state of functional groups plays a major role in inducing interfacial water ordering.  Functional group coordination state also affects the mechanism of oxoanion adsorption, with arsenate forming bridging bidentate surface complexes on most hematite and corundum surfaces but taking on a tridentate coordination on corundum (001).  This latter configuration is highly unexpected as doubly coordinated oxygen functional groups undergo slow ligand exchange and appear to be overbonded after the surface complex forms.  Arsenate adsorption also induces structural changes in interfacial water on all surfaces.  The most substantial water restructuring occurs on (001) surfaces, with arsenate adsorption inducing strong positional ordering and a substantial rearrangement of near-surface water molecules.  These observations demonstrate the fundamental role of surface sites in controlling interfacial structure and suggest that water restructuring may contribute to the energetics of interfacial reactions.

Connection between surface site coordination, interfacial water structure, and ion adsorption mechanisms

247th National Meeting (2014)

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.