Environmental Science at the APS

1
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 gt 15) in a
  sample?

• What valence state are different elements in?

• How are the elements bonded to one another?

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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.

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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 Zgt15 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...
4
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.
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.
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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
Natural Samples crystallinity is not required
-- samples can be liquids, amorphous solids,
soils, aggregrates, and surfaces.
EXAFS Extended X-ray Absorption Fine-Structure
Local Structure Information EXAFS gives atomic
species, distance, and number of near-neighbor
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.
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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.
Cr(III) is not carcinogenic or very toxic, and is
not mobile in ground water.
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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.
9
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)
Ca
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. The Sr XAFS was measured
at a spot with high Sr concentration -- above the
solubility limit of Sr in aragonite.
Sr
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