GeoSoilEnviroCARSGeoSoilEnviroCARS The University of Chicago

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M. Newville, S. R. Sutton, M. L. Rivers, and P. J. Eng, Consortium for Advanced Radiation Sources, University of ChicagoC. Hansel, S. Fendorf, Department of Geological and Environmental Sciences, Stanford UniversityN. Keon, D. Brabander, Department of Civil and Environmental Engineering, MIT

Element-specific Microtomographic Imaging of Metals in Environmental Systems

The Distribution of Toxic Metals in Roots

Oxidation State Tomograms: As3+ / As5+

The Problem with Self-Absorption

Fluorescence Tomography

Horizontal andVerticalKirkpatrick-Baez focusing mirrors

Typical spot size: 3 – 5mFluorescence

detector:multi-element Ge detector

Sample x-y-z- stage: 1m step sizes

Optical microscope (10x to 50x) with video system

Sample on a silica fiber on standard goniometer head

For assessing As contamination in roots, knowing the total elemental concentration is not enough: the oxidation state is also desired. By selecting the incident x-ray energy, we can preferentially detect As3+ or As5+.

x

Fluorescence Sinograms for Zn, Fe, and As collected simultaneously for a section of contaminated root (see below): x: 300m in 5m steps : 180 in 3 steps

AsFeZn

The role of root-borne carbonate nodules in the attenuation of contaminant metals in aquatic plants has been investigated with EXAFS, SEM and x-ray fluorescence imaging.

A tomogram gives cross-sectional information about a sample without having to slice it physically.

Fluorescence x-ray tomography is done by scanning a pencil-beam across the sample. The sample is rotated around and translated in x and the x-ray fluorescence intensity from selected elements are measured, making an x- intensity map or sinogram. This can be reconstructed by software into a virtual slice through the sample. Transmission x-rays are measured as well to give an overall density tomogram.

data collection is relatively slow can be complicated by self-absorption

can collect multiple fluorescence lines

thin x-ray beam

rotation stage

Sample

Fluorescence

detector

fluoresced x-rays

transmitted x-rays

Transmission detector

xtranslation stage

G.F. Rust, and J. Weigelt IEEE TRANSACTIONS ON NUCLEAR SCIENCE, 75, pp 14 (1998)

A. Simionovici, et al. in Developments in X-Ray Tomography II, SPIE Proceedings 3772, 304-310 (1999)

A. Simionovici, et al, Nuclear Instruments and Methods in Physics Research A, 467-468, pp 889-892 (2001)

C. G. Schroer, Applied Physics Letters, 79 (12), 1912-1914 (2001)

Formally, the fluorescence intensity for a tomogram is complex:

Where is the atomic densityfor element i (which is what we really want to know). Here

is the absorption of the incident beam along the incident beam-path, and

is the absorption of the fluoresced beam along the path to the detector.

The self-absorption problem is difficult to solve in general, requiring a self-consistent solution for . The root samples have low self-absorption, so we can set g = 1, and applystandard tomographic methods to convert the measured sinograms to .

detector

incident

beam

In a similar project, this group is studying a Superfund site (Wells G+H wetland) that gained notoriety in A Civil Action, with a reservoir of approximately 10 tons of arsenic within the upper 50 cm of the sediment profile.

Most of the arsenic is sequestered in the wetland peat sediments with relatively little getting in the groundwater.

In contrast, riverbed sediments near the wetland (2m away) have higher concentrations of aqueous (mobile) arsenic despite lower solid phase concentrations.

The metabolic activity of the wetland plants may help to explain the sequestration of arsenic in the wetland.

Wells G&H Typha root 2

Cu

As

Fe

Pb

Zn

300 m

Experimental Setup: x-ray microprobe

A standard x-ray microprobe station with an undulator, fluorescence detector, and a rotation stage is all that is needed for fluorescence tomography measurements!

The GSECARS x-ray microprobe station: APS sector 13-ID.

E3

ET

E5

Weighted redox: As3+=43%; As5+=57%. As3+/ As5+ is generally heterogeneous (boxed areas) and there is a tendency for As5+ to be on the exterior (circled area).

As3+ “As5+ ”

As total

Fluorescence Tomography Features:

Selected Further Reading:

Fluorescence tomograms were made with incident energy tuned to the As3+ white-line, the As5+ white-line, and well above the As K-edge.

The differences in the resulting tomograms tells the differences in the distribution of As3+ and As5+.

Colleen Hansel, Scott Fendorf (Stanford University)

20 Fe 180

2 Pb 20

Nicole Keon, Daniel Brabander (MIT)

Nicole Keon,Daniel Brabander (MIT)

These sinograms are reconstructed to cross-sectional images of the 300 m root (Phalaris arundinacea), showing Fe and Pb uniformly distributed in the root epidermis while Zn and Mn are correlated with nodules. Arsenic is poorly correlated with the epidermis, suggesting a non-precipitation incorporation.

The raw fluorescence tomograms consists of elemental fluorescence (uncorrected for self-absorption) as a function of position and angle: a sinogram. This data is reconstructed as a virtual slice through the sample by a coordinate transformation of (x,) (x, y). The process could be repeated for different slices to give three-dimensional information.

Getting the elemental distribution and correlations across the root cross-section is very important for determining the phases of metals in and on the root. Physically slicing the root would do enough damage that elemental maps would be compromised.

Photograph of root and reconstructed slices of fluorescent x-ray CT for selected elements (color bars in fg/m3).

5 Ca 20

0.4 Zn 2

0.5 Mn 2

GeoSoilEnviroCARS is supported by:NSF EAR-9906456 DOE DE-FG02-94ER14466.