Synchrotron Techniques

Molecular-scale insights arising from synchrotron (SR) studies have significantly enlightened our understanding of processes controlling natural and engineered remediation of contaminants and have accelerated closure of costly large-scale DOE legacy site operations such as the Rocky Flats Environmental Technology Site¹. The SLAC SFA is working to leverage SR studies to hasten remediation and closure of other large DOE legacy and nuclear-materials-related contamination sites.

Synchrotron (SR)-based techniques provide powerful probes of the structures of complex materials and the speciation of sorbed and solute metal ions, and hence play key roles in elucidating fundamental biogeochemical processes that underpin modern environmental remediation science. High intensity collimated SR sources allow noninvasive in-situ analysis of dilute, hydrated, and radioactive samples. SR x-rays can be focused to submicron spots, allowing for x-ray fluorescence (XRF) elemental- and oxidation-state imaging of microstructures, chemical microgradients, and microenvironments in natural samples. X-ray absorption spectroscopy (XAS) includes x-ray absorption near-edge structure spectroscopy (XANES) near-edge x-ray absorption fine structure (NEXAFS), which is similar to XANES in concept but is used to describe measurements at ca <1 keV, and extended x-ray absorption fine structure (EXAFS). XAS techniques provide element-specific information about the oxidation states and local molecular-structures of metal ions in complex environmental materials, including vacancy defects and structural distortions. SR x-ray scattering (SR-XS) techniques are myriad and notably include in-situ powder diffraction (SR-PD) and grazing-incidence powder diffraction (GI-PD), from which unit cell parameters, atomic positions, and particle dimensions can be extracted. Other important SR-XS techniques include x-ray total scattering-pair distribution function (XPDF), which provides information on atom pair correlations in crystalline and x-ray amorphous materials, crystal truncation rod (CTR) surface x-ray scattering, which interrogates the molecular-scale structures of mineral-water interfaces, and small-angle x-ray scattering (SAXS), which provides average nanoparticle diameters and morphologies. SR-PD and SAXS can be measured in real time during mineral transformation reactions to directly and unambiguously measure the kinetics of these processes, which are otherwise difficult to obtain. Both XAS and SR-PD can be performed on the micron scale. Long-period x-ray standing wave fluorescence yield (XSW-FY) spectroscopy can be used to evaluate the relative binding affinities of aqueous ions in microbial biofilm coatings on minerals vs. the mineral substrate.

An excellent reference for synchrotron-based techniques ².