XCITE Workshop: Environmental Science

Environmental Science at the APSMatt Newville, Univ of Chicago / GeoSoilEnviroCARS (sector 13)

What x-ray techniques are used for enviromental science?

– Where are the different elements (Z > 15) in a sample?

What kinds of enviromental science questions can be asked at the APS?

– x-ray absorption spectroscopy (EXAFS, XANES)

– x-ray diffraction and scattering (including surfaces)

– x-ray fluorescence and imaging

– 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?

Element Specific: all elements (with Z>15 or so) can be seen at the APS, and it is usually easy to distinguish different elements.

Experiment: Measure characteristic x-ray lines from electronic core levels for each atom.

Natural Samples: samples can be in solution, liquids, amorphous solids, soils, aggregrates, plant roots, surfaces, etc.

Low Concentration: concentrations down to a few ppm can be seen.

Quantitative: relative abundances of elements can be made with high precision and accuracy.

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 (~5x5m), 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 L line.

The maximum Cs concentration was ~10ppm, and was seen to be between the mica layers. 100x100m image of Cs in mica,

using a 5x5m beam, and taking 3m steps. Each point was collected for 30s. The incident x-ray energy was 10KeV.

XCITE Workshop: Environmental Science

X-ray Absorption: What physical/chemical state?

Element Specific: as with x-ray fluorescence

Experiment: Measure x-ray absorption coefficient 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.

Natural Samples: crystallinity is not required -- samples can be liquids, amorphous solids, soils, aggregrates, and surfaces.

Low Concentration: chosen element can be as low as ~10 ppm

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

XANES = X-ray Absorption Near-Edge SpectroscopyEXAFS = Extended X-ray Absorption Fine-Structure

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.

Cr(III) is not carcinogenic or very toxic, and is not mobile in ground water.

XCITE Workshop: Environmental Science

Plutonium sorbed onto Yucca Mountain SoilM 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 (~1M).

Fluorescence Maps of 150m X 150m areas were made with a 4x7m x-ray beam. Mn, Fe, As, Pb, Sr, Y, and Pu fluorescence peaks were measured simultaneously at each point.

The Pu was seen to becorrelated 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+.

XCITE Workshop: Environmental Science

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.

Plutonium sorbed onto Yucca Mountain Soil: EXAFS

XCITE Workshop: Environmental Science

Sr in coral (Porites lobata) and seawater temperatureNicola 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 5m beam from the GSECARS microprobe and a 5m 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.

300m

Ca

Sr

XCITE Workshop: Environmental Science

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.

XCITE Workshop: Environmental Science

High Resolution X-ray Fluorescence and EXAFSMatt 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.

XCITE Workshop: Environmental Science

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.

Grazing Incidence XAFS: Surface SpectroscopyTom Trainor, Gordon Brown Jr, (Stanford), Glenn Waychunas (LBNL) Peter Eng, Matt Newville, Steve Sutton (Univ of Chicago)

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