Metal Oxide Surfaces and Their Interactions with Aqueous Solutions and Microbial Organisms

During the past decade, interest in chemical reactions occurring at metal oxide-aqueous solution interfaces has increased significantly because of their importance in a variety of fields, including atmospheric chemistry, heterogeneous catalysis and photocatalysis, chemical sensing, corrosion science, environmental chemistry and geochemistry, metallurgy and ore beneficiation, metal oxide crystal growth, soil science, semiconductor manufacturing and cleaning, and tribology. The metal oxide-aqueous solution interface is reactive due to acid-base, ligand-exchange, and/or redox chemistry involving protons (hydronium ions), hydroxyl groups, aqueous metal ions, and aqueous organic species and also complexes among these species. Interfacial localization of those species (adsorption) may result from electrostatic, chemical complexation, and hydrophobic interactions chemistry and geochemistry. Indeed, hydrous oxides of Al, Fe, Mn, and aluminosilicates such as clays are ubiquitous in the natural environment, and their surface chemical properties control such important phenomena as nutrient and contaminant element release and uptake, pH buffering, water quality, and soil rheological properties. These surfaces function as templates for the growth of other solid phases and as a matrix for microflora. Polyvalent metal ions [e.g., Fe(III) and Mn(IV)] in the metal (hydr)oxide-aqueous solution interfacial region serve as terminal electron acceptors in the respiratory cycle of microorganisms common to soil, aquatic and marine sediments, and groundwater, a process of central importance to the global biogeochemical cycling of C, N, P, and other elements. Given the importance of metal oxide surfaces in these processes, surprisingly little is known about their atomic-scale structure and chemical reactivity, particularly in aqueous environments.