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Sensitivity of Soil Manganese Oxides

XANES Spectroscopy May Cause Reduction

^a Dep. of Plant and Soil Sciences, Hills Bldg., Univ. of Vermont, Burlington, VT 05405-0082
^b Univ. of Chicago/CARS, National Synchrotron Light Source, Brookhaven National Lab., Upton, NY 11973-5000

View larger version (20K): [in a new window]   Fig. 1. Repeated Mn XANES scans of a Supersoil sample. The first series of scans were run successively on the same 310- by 300-µm spot. After energy calibration, the second series was run on a different spot, 1.4 mm offset from the initial scans. The energy is relative to the pre-edge peak of a Mn(VII) standard

View larger version (18K): [in a new window]   Fig. 2. Repeated Mn XANES scans of a Lordstown-I sample run successively on the same 77- by 90-µm spot. After energy calibration, the sixth scan was taken on a different spot, 1.0 cm offset from the original. The energy is relative to the pre-edge peak of a Mn(VII) standard

View larger version (17K): [in a new window]   Fig. 3. Repeated Mn XANES scans of a Lordstown-II sample run successively on the same 26- by 27-µm spot. The fifth scan was taken at the same spot 15 h after the fourth scan (some error in the relocation of the sample is possible). The energy is relative to the pre-edge peak of a Mn(VII) standard

View larger version (18K): [in a new window]   Fig. 4. Short-cycle repeated Mn XANES scans of a Supersoil sample. Each scan of a 310- by 300-µm spot lasted 8.0 min. The energy position was calibrated after the sixth run. The energy is relative to the pre-edge peak of a Mn(VII) standard

View larger version (29K): [in a new window]   Fig. 5. X-ray absorption near-edge structure (XANES) spectra and modeled fits with Mn(II) from the repeated-scan studies. The spectra shown are the first and last in each series. Dotted lines show the model fit to the first scan and the combination of this model with Mn(II) that best fits the last scan. A spectra of aqueous Mn(II) is given in (a) for comparison

View larger version (24K): [in a new window]   Fig. 6. Change in oxidation state vs. time of exposure to the x-ray beam for all the repeated-scan experiments. The elapsed time was measured from the beginning of each scan. The Lordstown-II + Mn(II) sample had 20 mmol kg-1 Mn(II) added just before the first scan. Only the first 4 of the 21 scans in this series are shown

View larger version (22K): [in a new window]   Fig. 7. Successive Mn XANES scans of a Lordstown-II sample that had been pretreated with Mn(II). Scans are averaged into groups of four and offset by 0.05 relative intensity (scan 5 was interrupted by a beam dump and is not shown). The soil had 80 mmol kg-1 of Mn(II) added in two doses 3 and 4 wk before analysis, and 20 mmol kg-1 of Mn(II) added just before analysis

View larger version (26K): [in a new window]   Fig. 8. Comparisons of Mn XANES spectra of moist and air-dried soils. Different subsamples of each soil were analyzed. A spectra of synthetic birnessite is given in (a) for comparison