Inner-sphere adsorption geometry of Se(IV) at the hematite (100)–water interface

Abstract

The 3-dimensional structure of adsorption complexes on mineral surfaces provides insight into the fundamental mechanisms controlling sorption processes. This is important to the development of a general understanding of the behavior of contaminants such as selenite in the environment. The adsorption of selenite (SeO 2− 3) on the hematite (100) surface was studied using X-ray standing wave (XSW) measurements. Inner-sphere bidentate surface complexes bridging between adjacent singly-coordinated oxygen sites were identified as the primary adsorption site. The lack of binding to doubly-coordinated oxygen sites that were also exposed on the surface was likely due to differences in the reactivity or exchange kinetics of these sites or cation–cation repulsion, although the latter appears to be a secondary effect based on past observations. While these bridging bidentate geometries are similar to those inferred in past spectroscopic studies, the Se–Fe distances are such that these species might be misidentified as edge-sharing complexes if studied by EXAFS spectroscopy, highlighting the need for a fundamental understanding of mineral surface structure.