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 > 15) in a sample? • What valence state are different elements in? • How are the elements bonded to one another?

2. 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.

3. 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...

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.

5. X-ray Absorption: What physical/chemical state? Experiment: Measure x-ray absorption coefficient mas 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.

6. 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.

7. 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 Mapsof 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 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+.

8. 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

10. 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.

11. 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.

12. 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