AN OVERVIEW OF SYNCHROTRON RADIATION APPLICATIONS TO LOW TEMPERATURE GEOCHEMISTRY AND ENVIRONMENTAL SCIENCE

Abstract

The availability of synchrotron radiation (SR) to the scientific community has revolutionized the way X-ray science is done in many disciplines, including low temperature geochemistry and environmental science. The key reason is that SR provides continuum vacuum ultraviolet (VUV) and X-ray radiation five to ten orders of magnitude brighter than that from standard sealed or rotating anode X-ray tubes. This increase in brightness has resulted in many experimental studies at the molecular scale that simply could not have been done using conventional X-ray sources and in a number of new discoveries about chemical and biological processes that determine the behavior and fate of trace elements, plant nutrients, and environmental pollutants in complex natural systems. A national/international community of geochemists as well as environmental scientists from a variety of disciplines have utilized the four existing SR facilities in the U.S. (the ALS, the APS, the NSLS, and SSRL) in a variety of applications including (1) X-ray absorption spectroscopy (XAS) studies of liquid water, which resulted in the discovery of a new defect structure of water; (2) XAS studies of chemical reactions at solid-aqueous solution interfaces, which resulted in the first detailed molecular-scale information on the structure and bonding of sorbates at environmental interfaces; (3) X-ray reflectivity and X-ray standing wave (XSW) studies of the structure of hydrated mineral surfaces and of the electrical double layer at mineral/solution interfaces, which has resulted in some of the first direct information on these structures under in situ conditions; (4) X-ray diffraction (XRD) studies of the structure of single crystals materials such as kaolinite that are too small for such studies using conventional X-ray sources; (5) photoemission spectroscopy studies of the reaction of water with minerals, which provided some of the first information on hydroxylation of mineral surfaces that could be correlated with high-level quantum chemical calculations; (6) X-ray fluorescence microprobe studies of the speciation, distribution, and phase association of different chemical species of a given element in complex environmental matrices at spatial scales of a few micrometers; (7) X-ray fluorescence microtomography studies of the interaction of metal ions with plant roots, which has provided insights about how plants sequester heavy metals such as Pb and Zn; (8) XAS and XSW studies of the interaction of metal ions with bacterial surfaces, which has provided new insights about the impact of microbial biofilms on mineral surface reactivity and biomineralization processes that result in the growth of nanometer-scale minerals in and on microbial cells; (9) X-ray microscopy studies of natural organic matter (NOM), which provided the first images of the macromolecular structure of NOM under in situ conditions and the changes it undergoes as a function of solution conditions; and (10) XAS, micro-XAS, and micro-XRD studies of heavy metal and metalloid pollutants in complex environmental samples, which have revealed their molecular-level speciation and an improved understanding of their sequestration mechanisms and potential bioavailability. Selected examples from this partial list of applications will be presented. In addition, the U.S. SR facilities available to the geochemistry-environmental science communities will be summarized.