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. Absorption = ln(Io/It)
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: - Indicative of redox
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-)

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 in ZnS structure S-Zn 4@2.35Å S-S 12@3.83Å S-Zn2
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 Map concentration of major and trace elementsFe 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