Application of EXAFS spectroscopy to the study of abiotic redox processes in soils and sediments

The past decade has witnessed explosive development in the application of non-invasive spectroscopic techniques, and specifically of synchrotron-based EXAFS spectroscopy, to determine the local structure of poorly-crystallized solids, the crystal chemistry of substituted elements, and the sorption mechanism of metals in contact with the major contituants of soils and sediments (clays, Fe and Mn oxides, phosphates, carbonates). Relatively few structural studies have been conducted on redox systems and, consequently, the nature of mineral transformations at the oxic-anoxic boundary so that the molecular-level mechanism of surface-induced redox reactions of trace elements are lacking. The capacity of EXAFS to determine the oxidation state and the structural environment of redox sensible elements like Fe, Mn, Co and Cr will be illustrated with four case studies.