Oxidation of Added Mn(II) in Soils Observed by XANES Spectroscopy and Cr(III) Oxidation

Abstract

Soil manganese oxides are highly reactive and can strongly influence soil chemical transformations even when present in relatively minor concentrations. The continued reactivity of these oxides is dependent on regeneration, yet the mechanisms of Mn(II) oxidation in soils are not completely understood. We examined oxidation of added Mn(II) in four soils with a reducible Mn content ranging from 9.1 to 319 mmol kg⁻¹ (0.05–1.75%). The Mn in these soils was concentrated in manganiferous oxide coatings or concretions ∼50–100 µm in diameter. The progress of oxidation of Mn(II) was measured by both Mn K-edge XANES spectroscopy and the ability of the soil to oxidize added Cr(III)Cl₃ to Cr(VI) (Standard Net Cr Oxidation Test). Addition of Mn(II) to a soil sample caused an initial decrease in the energy position of the main absorption edge in XANES spectra collected from areas of high Mn concentration, showing sorption of the added Mn(II). Repeated scans over 30–50 h showed movement of the main edge to a higher energy, close to that of the soil before treatment, demonstrating oxidation. Chromium oxidation initially decreased due to blockage of reactive sites by sorbed Mn(II). It then increased linearly over the same time period to values higher than the untreated soil, confirming the oxidation of the added Mn(II) and an increase in overall Mn oxide reactive sites. Oxidation continued even after exposure to strongly ionizing X-rays, suggesting a coupled biotic/abiotic pathway.