1 / 25

An Overview of Synchrotron Techniques for Studying Environmental Processes

An Overview of Synchrotron Techniques for Studying Environmental Processes. Paul Northrup Brookhaven National Laboratory Environmental Sciences Department Environmental Research & Technology Division. “Nothing is so difficult but that it may be found out by seeking.”

richard-rivas
Télécharger la présentation

An Overview of Synchrotron Techniques for Studying Environmental Processes

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. An Overview of Synchrotron Techniques for Studying Environmental Processes Paul Northrup Brookhaven National Laboratory Environmental Sciences Department Environmental Research & Technology Division “Nothing is so difficult but that it may be found out by seeking.” -Terence (ca. 150 BC) NCSS July 18, 2006

  2. The National Synchrotron Light Source • User facility • 300mA, 2.8 GeV • IR >>>> 100KeV

  3. Techniques: • X-ray absorption: • absorption spectroscopy (XAS) • fluorescence (XRF) • microscopy • X-ray scattering: • diffraction • IR spectroscopy/microscopy

  4. Applications of Synchrotron Techniques • Complex environmental systems and how contaminants interact • Imaging/mapping elemental distributions • A probe of chemical and structural state: • Oxidation state and chemical bonding • Local and long-range structures • Reactivity, mobility, bioavailability (toxicity) • Biological & geochemical processes • Element-specific and Non-destructive • Trace or major components, processes

  5. Xray Absorption Spectroscopy: Each edge of each element has a characteristic binding energy M L K Sulfur K edge Absorption occurs when the energy of the incident photon is sufficient to eject the electron.

  6. XAS measurement • Direct: transmission through sample. • Absorption = ln(Io/It)

  7. XAS measurement • Indirect: X-ray fluorescence produced as electron “hole” is filled. • Characteristic energy for each element. • Proportional to absorption.

  8. Three components of XAS: • Edge step • Electron transitions • Extended • oscillations • Each carries • different • information.

  9. Absorption edge step Eo indicates oxidation state, by small shifts. Absorption (step height) is proportional to concentration. <<<<<reduced oxidized>>>>>

  10. Electron transitions • Promotion of electron to available (unfilled) level of absorbing atom -- or neighbor. • Peak energy differs from edge energy. • Sensitive to electronic configuration and bonding. • Rules: Allowed: s-p p-s p-d d-p d-f Forbidden: s-s p-p d-d

  11. Uranium L3 and M5 edges • Importance: U6+ highly soluble, U4+ relatively immobile • L3 absorption edge indicates oxidation state • M5 edge dominated by 3d > 5f transition

  12. Fe K absorption edge • - Standards and sediments: • Hematite: • Fe3+ oxide • Vivianite: • Fe2+ phosphate • - Indicative of redox • processes Fe2+ Fe3+

  13. S K edge • 2 edge steps (oxidation states) • 1s to 3p electron transition: • 1: sulfide/thiol (R-S-R/R-SH), 2: thiophene, • 3: sulfoxide (R-(SO)-R), • 4: sulfite/sulfone • (R-OSO2-/R-(SO2)-R), • 5: sulfonate (R-SO3-), • 6: sulfate (R-OSO3-) 1 2 3 4 5 6

  14. Organic S species • Sulfur in sediments • Sulfate (bio)reduction • Sulfur in plant roots • Physiological response to toxin (Zn)

  15. EXAFS • Extended oscillations due to backscatter of electron from neighboring atoms • Interference pattern: • Distance • What element (size) • Coordination number • U incorporation into a • mineral

  16. EXAFS data analysis S-Zn 4@2.35Å S in ZnS structure S-S 12@3.83Å S-Zn2 12@4.49Å S-Zn S-S

  17. P K edge: • P interacts with U • Oxidation state • Organic/inorganic species • 1s to 3p transition

  18. Phosphate in solution • Structural response to pH • Degree of protonation induces shift in peak: (PO4)3- vs. H3PO4 • Transformation of organic phosphate ester to free phosphate • Action of microbial phosphatase • Uranium

  19. Identifying phosphates: • (1) Presence of Ca bound to phosphate oxygen creates new transition • (2) Uranium phosphate • (3) Fe phosphate

  20. Phase identification: • Sometime quick “fingerprinting” is possible • Calcite vs aragonite -- both CaCO3

  21. Microbeam XRF, XAS Fe U • Map concentration of major and trace elements • Analyze species and structure at isolated points • U association with Fe oxides • U incorporation into calcite • Pu with Mn oxides • U reduction at Fe(II)/Fe(III) surfaces • ID minor components, precipitates • Interactions of contaminants with plants, microbes

  22. “Soft” X-rays: • STXM • C, N, O edges, very low energy • Spectral analysis to image distribution of different organic compounds and oxides • Resolution ~30 nm • Image distribution of contaminants within/around single cells: • bioreduction, metabolism, toxicity

  23. IR microscopy/spectroscopy: • Vibrations rather than electronic effects • Organic functional groups, ID and distribution • Correlations with metal distribution

  24. X-ray Diffraction: • Planes of atoms in a crystalline solid diffract X-rays • Diffraction angle depends on spacing between layers • Crystal structures have unique diffraction patterns • Identify crystalline phases: minerals, precipitates • Two types: • Powder diffraction: ID major and minor components in bulk samples • Microdiffraction: ID individual grains

  25. Summary: • Several synchrotron tools are useful to study molecular-scale and bulk chemical and processes in the environment • Most questions are best addressed using a combination of techniques

More Related