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.” -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 M L K Sulfur K edge 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 Eo indicates oxidation state, by small shifts. Absorption (step height) is proportional to concentration. <<<<<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-p p-s p-d d-p d-f Forbidden: s-s p-p d-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 • - Standards and sediments: • Hematite: • Fe3+ oxide • Vivianite: • Fe2+ phosphate • - Indicative of redox • processes Fe2+ Fe3+

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-Zn 4@2.35Å S in ZnS structure S-S 12@3.83Å S-Zn2 12@4.49Å S-Zn S-S

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 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

“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

An Overview of Synchrotron Techniques for Studying Environmental Processes