Abstract

Fe(II) oxidation by O2 is an important process generating Fe (oxyhydr)oxides, which play sorptive, structural and electron-transfer roles in soils. Here we explored how native minerals and organic matter (OM) affect the rate of Fe(II) oxidation and resulting de novo Fe(III) minerals in soil slurries. A topsoil were collected from Luquillo tropical forest, and a topsoil and subsoil were collected from Calhoun cultivated site. We oxidized 57Fe(II) in these soils either untreated or with OM and/or Fe (oxyhydr)oxides removed. We measured Fe oxidation kinetics by tracking the loss of Fe(II) and characterized the de novo Fe(III) solids using 57Fe Mössbauer spectroscopy. We find that OM retarded Fe(II) oxidation, while pre-existing Fe (oxyhydr)oxides played a significant role in catalyzing Fe(II) oxidation. The non-extractable (residual) soil minerals (i.e.phyllosilicates and quartz) after removing Fe (oxyhydr)oxides, had a minor effect on oxidation rates. In the topsoils, OM resulted in a lower-crystallinity Fe(III) minerals, including nanogoethite and highly-disordered Fe phases, relative to OM-removal treatment. Goethite with varying crystallinity was promoted by the pre-existing Fe (oxyhydr)oxides in all soils, in contrast to homogenous oxidation where lepidocrocite was formed. Compared to the topsoils, Fe(II) oxidation resulted in large-particle goethite with the highest crystallinity in the Calhoun subsoil that was more enriched in native crystalline Fe-(oxyhydr)oxides and more depleted in OM. Crystalline hematite, was also formed in the Calhoun subsoil, most likely due to a templating effect of pre-existing hematite. These findings suggest that the nature of resulting Fe minerals in soils and sediments may, besides other environmental factors, depends on strongly on resident soil OM and Fe phases.

Citation

Chen, Chunmei, and Aaron Thompson (2020): The Influence of Native Soil Organic Matter and Minerals on Ferrous Iron Oxidation. ASA-CSSA-SSSA International Annual Meeting, 9-13 November 2020.

This Paper/Book acknowledges NSF CZO grant support.