Photon Factory Activity Report 2004 #22 Part B (2005)
Chemistry

16A1/2004P007

Speciation of chromium in a polyacrylamide hydrogel by µ-XANES imaging
Gerald SCHMIDT1, Michael KERSTEN*1, Mari MIZUSAWA2 and Kenji SAKURAI2
1Gutenberg University, D-55099 Mainz, Germany
2National Institute for Materials Science, Sengen, Tsukuba, Ibaraki 305-0047, Japan
Introduction
Chromium has become an important soil contaminant
at many sites, and facilitating in situ reduction of toxicCr(VI) to nontoxic Cr(III) is becoming an attractive
remediation strategy. A variety of natural substances havebeen tested for this purpose, including Mn(II) and Fe(II)as potential reductants in presence of organic carbon, butthe complex reaction kinetics are not yet fully understood.
Experimental
A PMMA hydrogel was first soaked in a K2CrO4
solution and transported to the Photon Factory in a sealed
Petri dish. The role of the PMMA gel is to allow diffusivetransport of the reacting components. The experiment wasstarted by applying a drop of Mn(II) solution to the centerof the gel. A couple of hours later, XANES imaging
experiments were conducted at BL-16A1 in fluorescencemode. The XRF microscope described elsewhere [1] wasused to collect the image data. The monochromator wasrun through an energy range from 5968 to 6101 eV in 221steps. At each individual energy step the CCD recorded afluorescence image at a spatial resolution of 20 µm per
pixel. Exposure times were between 1.5 and 10 seconds.
I0 data were recorded by an ionization chamber.
Absorption coefficients (I1/I0) were calculated for eachpixel in each image. Remote sensing software was used toprocess the large amount of spectral image data [2].
Classes of identical XANES spectra were selected byapplying a Minimum Noise Fraction (MNF)
transformation [3] and selecting specific areas in a scatter
ndrd
plot of the 2 and 3 MNF eigenimages.

Results and Discussion
Two classes of XANES spectra were identified (Fig. 1).
The spatial distribution of pixels selected in the scatterndplot (b) is shown in the 2 eigenimage map (a). Pixels
assigned to class 2 (green) are to be found not only at thecenter of the sample, but mostly at compositional
oscillation halos comprising of the reaction product MnO2.
The remaining sample area is assigned to the dominatingclass 1 (red). The normalized XANES plots (c) show adifference in the amplitude of the pre-edge peak for bothareas. A significant reduction in the pre-edge heightindicates partial reduction of the Cr(IV) to Cr(III) in theoscillating halos. Development of the oscillatory zoning isdriven by a complex interplay between the relatively slow
kinetics of the redox reaction, and buildup of a diffusional
front of Mn(IV)/Cr(III) reactants until super-saturation ofthe Mn(IV) reactant is reached at some distance, andsubsequent adsorption of the Cr(III) reactant onto the thus
formed halo of colloidal MnO2 precipitates within thehydrogel matrix.
References[1] K. Sakurai and H. Eba, Anal. Chem. 75, 355 (2003).
[2] M. Sarzynski et al., HASYLAB Annual Report 1999,(2000).
[3] A.A. Green et al., IEEE Transactions on Geoscienceand Remote Sensing 26, 65 (1988).
* michael.kersten@uni-mainz.de10.8

Class 1

Absorption [a.u.]Class 20.6

0.4

0.205970a)

b)

5990

6010

6030

-0.2
Energy [eV]c)
ndndrdFigure 1: a) 2 eigenimage of the MNF transformation, b) scatter plot of the 2 and 3 eigenimages, and c) averaged
XANES spectra of the classes 1 (red) and 2 (green) selected from the scatter plot. The sample map (a) shows the spatial
distribution of chromium phases that are assigned to the classes 1 and 2.

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