The SSRL-BER Environmental Remediation Science program supports researchers in NABIR, EMSP, and BER-EMSI programs, funded by the DOE Office of Biological and Environmental Sciences (BER). We provide advice and direct support for planning and conducting experiments at SSRL and analyzing results. BER-ERSD supports similar programs at ALS, APS, and NSLS.

SFA Website

Scientists who are interested in information about experimental capabilities and/or conducting measurements at SSRL should contact Sam Webb or John Bargar. Additional information about SSRL is provided at the SSRL home page.

XAS and X-RAY SCATTERING TECHNIQUES and EXPERIMENTAL CAPABILITIES at SSRL.

SSRL BEAM LINE INFORMATION.

HOW TO GET BEAM TIME AT SSRL.

This program is supported by DOE-BER, Environmental Remediation Sciences Division. Participants are asked acknowledge BER and other agencies’ support for SSRL on their publications.

A PRIMER ON SYNCHROTRON TECHNIQUES FOR ENVIRONMENTAL REMEDIATION SCIENCE.

Synchrotron radiation (SR)-based techniques provide unique capabilities to determine metal ion and organic speciation and to characterize complex inorganic and organic environmental solids in subsurface and waste materials. The high intensity of SR sources and x-ray photon-in/photon-out detection allow noninvasive in-situ analysis of dilute, hydrated, and radioactive samples, including soils, microbial organisms, plant roots, and HLW materials. SR x-rays can be focused to beams of micron and sub-micron dimension, which allows the study of microstructures, chemical microgradients, and microenvironments that often fundamentally control the behavior of contaminants in the environment, in waste forms, and in contaminated vessels. SR techniques have thus emerged as key tools in environmental remediation science. Key experimental characterization techniques used in environmental remediation research at SSRL are as follows:

XANES Spectroscopy X-ray absorption near-edge structure (XANES) spectroscopy is frequently used to quantitatively measure metal oxidation states in contaminated sediments, at solid-water interfaces, in tank wastes, in HLW waste forms, in in-situ remediation technologies, and in bacteria-mineral assemblages (figure 1). See October 2004 and March 2004 science highlights for examples of using XANES to quantitatively determine technetium and chromium oxidation states in Hanford sediments. When applied to low-Z elements such as oxygen, the term XANES is frequently replaced by the term NEXAFS (near-edge x-ray absorption fine structure). Frequently, XANES or NEXAFS spectroscopy can be used to identify the host phase or molecular identity of an ion by using a “fingerprint” approach. XANES spectroscopy is particularly useful for following changes in metal ion or organic speciation as a function of time under in-situ conditions. At SSRL, XANES measurements for elements with Z>20 are performed at hard x-ray (i.e. > 4 KeV) beam lines listed in table 1. Sulfur, chlorine, and phosphorous NEXAFS can be measured at SSRL BL 6-2, and lower-Z elements can be measured at the soft x-ray beam lines listed in table 1.

Elements that can be studied: Atomic Number ~20-95 (hard x-ray range), Atomic Number ~13-19 (intermediate x- ray range), Atomic Number ~4-12 (soft x-ray range). Concentration range: ≥ 5 ppm, element of interest.

Figure 1. X-Ray Absorption spectroscopy (XAS). The basic XAS experiment is illustrated at the top: a monoenergetic x-ray beam is passed through a sample. The incident x-ray photon flux is counted by an ion chamber detector "upstream" of the sample. Another detector (labeled "I1") measures the flux of x-rays that have passed through the sample. The absorbance spectrum is calculated from the logarithm of the ratio of these x-ray flux values and is illustrated in the center of the figure. To collect a spectrum, the energy of the incident x-ray beam is scanned through the binding energy of a particular core electron (typically 1s or 2p electrons). This process is illustrated for the element nickel, for which the 1s binding energy is 8333 eV. Below this energy, most of the incident x-rays (60 to 70%) pass through the sample, and the spectrum is a featureless, sloping line. X-ray absorption by the sample jumps rapidly at the nickel 1s binding energy, and the corresponding spectral feature is called the absorption edge. The spectrum contains significant structure in the vicinity of the absorption edge, and the detailed spectra in this region are referred to as XANES (x-ray absorption near-edge structure) or NEXAFS (near edge x-ray absorption fine structure) spectra. The EXAFS region (extended x-ray absorption fine structure) starts at about 10 eV above the absorption edge and also contains significant structure.

The inset at the bottom left shows chromium K-edge XANES spectra for the (III) and (VI) oxidation states of chromium. As can be seen from the inset figure, the XANES spectra are significanly different for these two oxidation states, and this difference allows quantitative oxidation state determination.

EXAFS spectra can be fit to determine the local atomic structure around atoms in almost any type of material. The inset on the bottom right illustrates this for uranium(VI) (uranyl) bonded to the surfaces of hematite (iron oxide) in water. Uranium LIII-edge EXAFS spectra allow the specific molecular structure of these interfacial complexes to be determined, as illustrated (see reference for Bargar et al, Geochimica Cosmochimica Acta V 64, in the year 2000 MEIS publication reference list).

Synchrotron TECHNIQUES Primer, CONTINUED

EXAFS Spectroscopy. Extended x-ray absorption fine structure (EXAFS) spectroscopy is used to quantitatively determine the local atomic structure around metal ions, including the distances to neighboring atoms (up to about 6 Å), the number of these neighbors, and their chemical identities (Figure 1.). A key advantage of this technique is that it can be used to study amorphous and highly disordered materials, as well as ordered materials. Thus, as with XANES, these measurements can be performed on metals dissolved in solutions, at solid-solution interfaces, in poorly crystalline environmental solids, contaminated sediments, in tank wastes, in HLW waste forms, in in-situ remediation technologies, and in bacteria-mineral assemblages. Frequently, EXAFS spectroscopy is used to identify the host phase or molecular identity of an ion by using a “fingerprint” approach. See the October 2004 and November 2003 science highlights for examples of the use of EXAFS in environmental remediation science.

Elements that can be studied: Atomic Number ~20-95 (hard x-ray range), Atomic Number ~13-19 (intermediate x-ray range)..
Concentration range: ≥ 50 ppm, element of interest.

SR X-ray Diffraction. SR-XRD or WAXS (wide-angle x-ray scattering) can be used to identify dilute and poorly crystalline phases such as mineral coatings and bacteriogenic precipitates in complex materials such as subsurface sediments and wet bacteria-mineral assemblages. SR-XRD is also extremely useful for studying long range order and disorder in natural materials, and for studying the structures of solid and liquid surfaces at interfaces. Because of the high flux of high-energy photons available at synchrotron sources, SR-XRD can be performed routinely on wet materials and on highly radioactive samples that must be sealed in containers. SR-XRD provides information on the long-range (>6 Å) atomic structure of materials, which is highly complementary to the oxidation state and short-range atomic structure information derived from XANES and EXAFS spectroscopy. SR-XRD measurements are often an important complement to EXAFS for unambiguously assigning crystalline phase identities in complex multi-phase mixtures. See the November 2003 science highlight for an example of the use of SR-XRD to environmental remediation science.

Concentration range: ≥ 200 ppm, phase of interest.

SSRL BEAMLINES FOR MEIS RESEARCH

HOW TO GET BEAM TIME AT SSRL

Interested scientists can access SSRL facilities in a variety of ways.

1. Research Programs. The most common methods used to obtain beam time are the single-experiment proposal, which provides beam time for two calendar years, and the program proposal, which can be renewed once to provide four years of beam time. Users can also submit a rapid turn around proposal for feasibility measurements (see paragraph 2., below) or a letter of intent proposal to obtain limited beam time for highly novel experiments. Additional information on SSRL proposals can be found at http://www-ssrl.slac.stanford.edu/users/user_admin/guide.html

2. Feasibility Measurements. It is often necessary for prospective and new users to conduct initial feasibility measurements to determine the suitability of a technique for their scientific needs. The SSRL-BER Environmental Remediation Science Program facilitates this crucial step by providing advice to users and assisting with rapid access for feasibility measurements. Interested scientists should contact Sam Webb or John Bargar to discuss their experimental needs.