An Overview of Synchrotron Techniques for Studying Environmental Processes
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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
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The National Synchrotron Light Source - User facility- 300mA, 2.8 GeV- IR >>>> 100KeV
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Techniques:
• X-ray absorption:
– absorption spectroscopy (XAS)
– fluorescence (XRF)
– microscopy
• X-ray scattering:– diffraction
• IR spectroscopy/microscopy
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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
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Xray Absorption Spectroscopy: Each edge of each element has a characteristic binding energy
Sulfur K edge
M
L
K
Absorption occurs when the energy of the incident photon is sufficient to eject the electron.
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XAS measurement
• Direct: transmission through sample.
Absorption = ln(Io/It)
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XAS measurement
• Indirect: X-ray fluorescence produced as electron “hole” is filled.• Characteristic energy for each element. • Proportional to absorption.
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Three components of XAS:
• Edge step• Electron transitions• Extended
oscillations• Each carries different information.
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Absorption edge step
Absorption (step height) is proportional to concentration.
Eo indicates oxidation state, by small shifts.
<<<<<reduced oxidized>>>>>
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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-pp-sp-dd-pd-f
Forbidden:s-sp-pd-d
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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
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Fe K absorption edge
Fe3+
- Standards and sediments: - Hematite:
Fe3+ oxide- Vivianite:
Fe2+ phosphate
- Indicative of redox
processes
Fe2+
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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
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Organic S species
• Sulfur in sediments • Sulfate (bio)reduction
• Sulfur in plant roots • Physiological response to toxin (Zn)
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EXAFS• Extended oscillations due to backscatter of electron from neighboring atoms • Interference pattern: • Distance • What element (size)• Coordination number • U incorporation into a
mineral
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EXAFS data analysis
S in ZnS structure
S-Zn [email protected]Å
S-S [email protected]Å
S-Zn2 [email protected]Å
S-S
S-Zn
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P K edge: • P interacts with U• Oxidation state • Organic/inorganic species• 1s to 3p transition
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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
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Identifying phosphates:
• (1) Presence of Ca bound to phosphate oxygen creates new transition• (2) Uranium phosphate• (3) Fe phosphate
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Phase identification:
• Sometime quick “fingerprinting” is possible• Calcite vs aragonite -- both CaCO3
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Microbeam XRF, XAS• 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
Fe
U
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“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
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IR microscopy/spectroscopy:
• Vibrations rather than electronic effects• Organic functional groups, ID and distribution• Correlations with metal distribution
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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
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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