The partitioning of ions at water/solid oxide and water/clay interfaces is poorly understood at the molecular level, yet detailed structural information about surface complexes is essential for quantitative description of many interfacial processes. These processes play fundamental roles in many fields, including geochemistry, oceanography, and environmental engineering. The proposed work will utilize high-intensity, wavelength-tunable synchrotran radiation and X-ray Absorption Spectroscopy (XAS), specifically Extended X-Ray Absorption Fine Structure (EXAFS) and X-ray Absorption Near Edge Structure (XANES), to obtain direct, in-situ measurements of nearest- neighbor, and where possible, next-neighbor distances and identities, coordination numbers, and in favorable systems, distances between adsorbate ions and cations in the solid surface. This information will be used to assist in the definition of composition and structure of surface complexes and to distinguish among adsorption, absorption, precipitation, and coprecipitation. Ten systems will be studied: involving Pb2+, Sr2+, CO2+, Cu2+, SeO4/2-, and SeO3/2- as adsorbates, and geothite, gamma-Al2O3, rutile, and kaolinite as adsorbents. The proposed study will provide detailed compositional and structural information about adsorption complexes, and an opportunity to compare results obtained by several methods. It will also provide an opportunity to test and refine current theoretical models for adsorption processes.
