MSA AWARD LECTURE: STRUCTURE AND REACTIVITY OF GEOCHEMICAL INTERFACES

Chemical reactions occurring at the mineral-fluid interface play a central role in dictating the geochemistry of natural waters. A great deal of effort has been devoted to developing geochemical models that incorporate a mechanistic framework for describing interfacial processes. However, the development and testing of such models has proven difficult due to the challenges involved in the analysis of heterogeneous interface systems under relevant geochemical conditions. Among the most productive experimental approaches to studying mineral-fluid interfaces has been the application of synchrotron based x-ray scattering and spectroscopy. These tools have enabled a number of advances in our understanding of the mineral-water interface. In particular, surface x-ray scattering techniques have allowed systematic study of interface structure and the structural modifications that result from changes in (bio)geochemical conditions. Such studies are essential to furthering the development of a structure based approach to understanding the reactivity of mineral-fluid interfaces, and hence to improving both conceptual and quantitative models of geochemical pathways.

This lecture will present an overview of recent work focused on determining the structure and reactivity of low index faces of common iron-(hydr)oxide phases. Iron-oxides are of particular interest due to their widespread occurrence, typically high specific surface area, and high surface reactivity, making them important scavengers of aqueous trace metals, and substrates that support the heterogeneous transformation of aqueous contaminants. Specifically, we will focus on the changes in surface structure associated with variations in primary variables such as temperature and redox potential, and how these changes influence surface reactivity with respect to adsorption processes. The experimental models will be discussed in comparison with the results of periodic density functional theory and ab initio thermodynamic calculations in order to provide a detailed interpretation of the energetics of the systems under investigation. Finally, the implications for understanding macroscopic reactivity based on the molecular scale models will be discussed, as well as avenues for future investigation.