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.

Absorption = ln(Io/It) 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: - Indicative of redox 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 in ZnS structure S-Zn 4@2.35Å 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 Map concentration of major and trace elements 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