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

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

Techniques:

• X-ray absorption:

– absorption spectroscopy (XAS)

– fluorescence (XRF)

– microscopy

• X-ray scattering:– diffraction

• IR spectroscopy/microscopy

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

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.

XAS measurement

• Direct: transmission through sample.

Absorption = ln(Io/It)

XAS measurement

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

Three components of XAS:

• Edge step• Electron transitions• Extended

oscillations• Each carries different information.

Absorption edge step

Absorption (step height) is proportional to concentration.

Eo indicates oxidation state, by small shifts.

<<<<<reduced oxidized>>>>>

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

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

Fe K absorption edge

Fe3+

- Standards and sediments: - Hematite:

Fe3+ oxide- Vivianite:

Fe2+ phosphate

- Indicative of redox

processes

Fe2+

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

Organic S species

• Sulfur in sediments • Sulfate (bio)reduction

• Sulfur in plant roots • Physiological response to toxin (Zn)

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

mineral

EXAFS data analysis

S in ZnS structure

S-Zn 4@2.35Å

S-S 12@3.83Å

S-Zn2 12@4.49Å

S-S

S-Zn

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

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

Identifying phosphates:

• (1) Presence of Ca bound to phosphate oxygen creates new transition• (2) Uranium phosphate• (3) Fe phosphate

Phase identification:

• Sometime quick “fingerprinting” is possible• Calcite vs aragonite -- both CaCO3

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

“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

IR microscopy/spectroscopy:

• Vibrations rather than electronic effects• Organic functional groups, ID and distribution• Correlations with metal distribution

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

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