Sensitivity of Soil Manganese Oxides

The examination of soils by x‐ray absorption near‐edge structure (XANES) spectroscopy can provide valuable clues for determining the structure and composition of Mn oxides. XANES experiments can be conducted at ambient temperatures using field‐moist samples. This feature is attractive in the study of soil Mn because of the well‐documented effects of sample drying on reactivity. The present study was undertaken to determine whether the analysis of field‐moist soils by XANES spectroscopy has an effect on the oxidation state of the Mn. Repeated scans were taken on the same spot using seven medium‐ to high‐Mn soils. In all 10 experiments with untreated soils, the main absorption edge dropped in relative energy in successive scans, indicating reduction of Mn. The change in eV was as high as 1.7 eV after 150 min of exposure time. After a series of scans showing reduction, small movement of the sample in the x‐ray beam and reexamination showed no reduction, indicating that the effect was limited to the x‐ray spot. Modeling the change with Mn(II) was successful and showed increases in reduced Mn between 6 and 20% after three to five repeated scans. The slopes of eV change vs. exposure time were remarkably similar in most experiments, even with large differences in the density of photons between runs. Comparisons of initial moist scans with those of recently air‐dried samples found little difference, suggesting minimal reduction in a single XANES scan under our experimental conditions.