Investigations of the Structural Properties of Aqueous Geochemical Interfaces by the Use of X-ray Diffraction, Standing Waves and Spectroscopy

P. J. Eng,^1,2 T. P. Trainor,¹ G. A. Waychunas,³ J. G. Catalano,⁴ M. Newville,¹ and G. E. Brown, Jr.⁴

¹Consortium for Advanced Radiation Sources, University of Chicago, Chicago, IL 60637 USA
²The James Franck Institute, University of Chicago, Chicago, IL 60637 USA
³Earth Sciences Division, Lawrence Berkeley National Laboratory, Berkeley, CA 94720, USA
⁴Department of Geological & Environmental Sciences, Stanford University, Stanford, CA 94305 USA

The interaction of liquid water with the surfaces of natural solids is one of the most fundamental chemical reactions occurring in nature. Its importance extends from geochemical processes such as mineral dissolution and sorption/desorption reactions -- which play major roles in the growth of crystals, chemical weathering, contamination of ground waters, environmental restoration, and the biogeochemical cycling of elements -- to the reaction of water with sulfate aerosol particles in the troposphere. I will present the results of our measurements of the structure of metal oxide surfaces under ambient or in-situ conditions (i.e., in the presence of bulk solution at ambient temperature). Utilizing x-rays from the Advanced Photon Source at GeoSoilEnviroCARS sector 13 we employ surface and interface sensitive x-ray methods in our studies, such as grazing incidence XAFS, crystal-truncation-rod x-ray scattering and x-ray standing wave. These probes are unrivaled in their ability to provide us with physical and chemical information about surfaces under natural conditions, and especially in the presence of bulk water.