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Environmental Science at the APS
Matt Newville, Univ of Chicago / GeoSoilEnviroCARS (sector 13)
What x-ray techniques are used for enviromental science?
– x-ray fluorescence and imaging
– x-ray absorption spectroscopy (EXAFS, XANES)
– x-ray diffraction and scattering (including surfaces)
What kinds of enviromental science questions can be asked at the APS?
– Where are the different elements (Z > 15) in a sample?
– What valence state are different elements in?
– How are the elements bonded to one another?
XCITE Workshop: Environmental Science
Synchrotrons and Environmental Science
What can x-rays do for environmental science?
–Trace element (heavy element) mapping and speciation in a wide range of
samples: soils, minerals, plant roots, mine tailings, microbes and biofilms.
–Fundamental studies of sorption and interface structures: how metals stick
to mineral surfaces.
XCITE Workshop: Environmental Science
X-ray Fluorescence: What elements are here?
Experiment: Measure characteristic x-ray lines from electronic core levels for
each atom.
Element Specific: all elements (with Z>15 or
so) can be seen at the APS, and it is usually
easy to distinguish different elements.
Quantitative: relative abundances of elements
can be made with high precision and accuracy.
Low Concentration: concentrations down to a
few ppm can be seen.
Natural Samples: samples can be in solution,
liquids, amorphous solids, soils, aggregrates,
plant roots, surfaces, etc.
Small Spot Size: measurements can be made
on samples down to microns in size...
XCITE Workshop: Environmental Science
X-ray Fluorescence Maps: Cs in Mica
Using a small x-ray beam (~5x5mm),
fluorescence maps can be made to show
where selected elements are enriched in a
sample.
Here is a map of Cs concentration in a mica
sample from Pacific Northwest National
Labs, that was cut across the cleavage
planes of the mineral. The Cs signal was
measured by monitoring the Cs La line.
The maximum Cs concentration was
~10ppm, and was seen to be between the
mica layers.
XCITE Workshop: Environmental Science
100x100mm image of Cs in mica,
using a 5x5mm beam, and taking
3mm steps. Each point was
collected for 30s. The incident x-ray
energy was 10KeV.
X-ray Absorption: What physical/chemical state?
Experiment: Measure x-ray absorption coefficient m as a function of x-ray energy
around an x-ray absorption edge of a selected element. That is, measure how the
fluorescence peak height varies as you scan energy over a core electron energy.
Element Specific: as with x-ray fluorescence
Low Concentration: chosen element can be as
low as ~10 ppm
XANES = X-ray Absorption Near-Edge Spectroscopy
EXAFS = Extended X-ray Absorption Fine-Structure
Natural Samples: crystallinity is not required
-- samples can be liquids, amorphous solids,
soils, aggregrates, and surfaces.
Local Structure Information: EXAFS gives
atomic species, distance, and number of nearneighbor atoms around selected element
Valence Probe: XANES gives chemical state
and formal valence of selected element
Small Spot Size: measurements can be made
on samples down to microns in size.
XCITE Workshop: Environmental Science
X-ray Absorption: What physical/chemical state?
X-ray Absorption Spectroscopy is one of the only available techniques that gives a
direct measurement of the chemical state (valence state) of an element. In many
envirornmentally relevant cases, the valence state is as important as the total
concentration of an element.
Cr(VI) is highly carcinogenic and
highly mobile in ground water.
XCITE Workshop: Environmental Science
Cr(III) is not carcinogenic or very toxic,
and is not mobile in ground water.
Plutonium sorbed onto Yucca Mountain Soil
M Duff, D Hunter, P Bertsch (Savannah River Ecology Lab, U Georgia)
M Newville, S Sutton, P Eng, M Rivers (Univ of Chicago)
A natural soil from the proposed Nuclear Waste
Repository at Yucca Mountain, NV, was exposed
(in a lab!) to an aqueous solution of 239Pu (~1mM).
Fluorescence Maps of 150mm X 150mm areas
were made with a 4x7mm x-ray beam. Mn, Fe,
As, Pb, Sr, Y, and Pu fluorescence peaks were
measured simultaneously at each point.
The Pu was seen to be correlated with Mn-rich
minerals, not with the zeolites, quartz, or Fe-rich
minerals. This tells us that Pu
X-ray absorption measurements were made at the
Pu LIII edge of “hot spots” A1 and A2, and
showed a mixture of Pu4+ or Pu5+ but not Pu6+.
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Plutonium sorbed onto Yucca Mountain Soil: EXAFS
XANES features showed the Pu to be in
either Pu4+ or Pu5+ (or a mixture of the
2) but not Pu6+. Since the initial Pu
solution had Pu5+ and since the
The Extended XAFS (ie, the isolated
wiggles showed Pu coordinated by 6--8
oxygens at ~2.26Angtroms, consistent
with Pu4+ or Pu5+ (but again not Pu6+).
No “second neighbor” could be seen
from this data, probably indicating that
the Pu is weakly bound to the disordered
Mn minerals.
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Sr in coral (Porites lobata) and seawater temperature
Nicola Allison, Adrian Finch (Univ of Brighton, Univ of Hertfordshire, UK)
Matt Newville, Steve Sutton (Univ of Chicago)
Sr abundance in aragonite (CaCO3) formed by
corals has been used to estimate temperature and
composition of seawater.
X-ray Fluorescence maps of a coral section (right)
made using a 5 x 5mm beam from the GSECARS
microprobe and a 5mm step size shows incomplete
correlation between Sr and Ca. The relative Sr
abundance therefore varies substantially on this
small length scale, although the aragonite must
have been formed at constant temperature.
Ca
Sr
The Sr XAFS was measured at a spot with high Sr
concentration -- above the solubility limit of Sr in
aragonite.
300mm
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EXAFS of Sr in coral (Porites lobata)
Nicola Allison, Adrian Finch (Univ of Brighton, Univ of Hertfordshire, UK)
Matt Newville, Steve Sutton (Univ of Chicago)
Since Sr is just above solubility limit (~1%)
in aragonite, will Sr precipitate out into
strontianite (SrCO3: structural analog of
aragonite) ?
First shell EXAFS is same for both cases
(strontianite, aragonite): 9 Sr-O bonds at
~2.5A, 6 Sr-C at ~3.0A.
Second shell EXAFS clearly shows Sr-Ca
(not Sr-Sr) dominating, as shown at left by
contrast to SrCO3 data, and by comparison
to a simulated EXAFS spectrum of Sr
substituted into aragonite.
The coral is able to trap Sr in aragonite at a
non-equilibrium concentration.
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High Resolution X-ray Fluorescence and EXAFS
Matt Newville, Steve Sutton, Mark Rivers, Ian Steele (U Chicago) , Mark Antonio
(ANL), Louis Cabri (NRC Canada), Robert Gordon, Daryl Crozier (Simon Fraser)
A complication in measuring fluorescence
and EXAFS in natural and heterogeneous
samples is the prescence of fluorescence
lines from other elements near the line of
interest.
This Wavelength Dispersive Spectrometer
has much better resolution (~10eV) than a
solid state detector (~150eV). It uses a
Rowland circle geometry, not electronics,
to select energies of interest. It makes an
excellent complement to Ge multi-element
solid-state detectors.
The WDS allows us to measure the
fluorescence spectra and even EXAFS
(for the first time anywhere!) on these
systems with overlapping lines.
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Grazing Incidence XAFS: Surface Spectroscopy
Tom Trainor, Gordon Brown Jr, (Stanford), Glenn Waychunas (LBNL)
Peter Eng, Matt Newville, Steve Sutton (Univ of Chicago)
A basic characterization of the bonding of ions
to mineral-water interfaces in the presence of
water is vital for understanding how metals
interact with natural surfaces.
X-ray Reflectivity and Grazing Incidence EXAFS
give unique information about sorbed species on
surfaces, and can be measured in the presence
of a water layer.
The high collimation of the APS source and the
GSECARS General Purpose Diffractometer
greatly enhance the ability to look at sorption on
natural mineral surfaces
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