Synchrotron-Based Techniques for Monitoring Metal Transformations

Abstract

X rays are a powerful probe for investigating metal and radionuclide transformations in soils, sediments, and groundwaters. In particular, synchrotron-based X-ray investigations can identify the changes in an element’s valence state and chemical speciation that often result from microbially mediated electron transfer. This chapter describes some of the synchrotron-based X-ray techniques (X-ray absorption spectroscopy, X-ray fluorescence, and X-ray microscopy) that can be used to improve understanding of metal transformations. The X-ray absorption near-edge structure (XANES) technique provides an in situ probe of an element’s oxidation state and clearly can contribute significantly to an understanding of the fate of elements in the environment, in both solid and solution phases. In addition to its utility, XANES is relatively easy to implement and has been perhaps the most commonly used synchrotron-based X-ray technique for monitoring metal transformations in environmental studies. XANES spectroscopy focuses on the energy range near an element's absorption edge, which is related to the element's valence state. Extended X-ray absorption fine-structure (EXAFS) focuses on the energy region well above the absorption edge and yields information on the local chemical environment of the absorbing element. Investigators can initiate the use of synchrotron radiation in their research in a number of ways.