Ferrous Iron Oxidation under Varying pO₂ Levels: The Effect of Fe(III)/Al(III) Oxide Minerals and Organic Matter

Abiotic Fe(II) oxidation by O₂ commonly occurs in the presence of mineral sorbents and organic matter (OM) in soils and sediments; however, this tertiary system has rarely been studied. Therefore, we examined the impacts of mineral surfaces (goethite and γ-Al₂O₃) and organic matter [Suwannee River fulvic acid (SRFA)] on Fe(II) oxidation rates and the resulting Fe(III) (oxyhydr)oxides under 21 and 1% pO₂ at pH 6. We tracked Fe dynamics by adding ⁵⁷Fe(II) to ⁵⁶Fe-labeled goethite and γ-Al₂O₃ and characterized the resulting solids using ⁵⁷Fe Mössbauer spectroscopy. We found Fe(II) oxidation was slower at low pO₂ and resulted in higher-crystallinity Fe(III) phases. Relative to oxidation of Fe(II)(aq) alone, both goethite and γ-Al₂O₃ surfaces increased Fe(II) oxidation rates regardless of pO₂ levels, with goethite being the stronger catalyst. Goethite surfaces promoted the formation of crystalline goethite, while γ-Al₂O₃ favored nano/small particle or disordered goethite and some lepidocrocite; oxidation of Fe(II)aq alone favored lepidocrocite. SRFA reduced oxidation rates in all treatments except the mineral-free systems at 21% pO₂, and SRFA decreased Fe(III) phase crystallinity, facilitating low-crystalline ferrihydrite in the absence of mineral sorbents, low-crystalline lepidocrocite in the presence of γ-Al₂O₃, but either crystalline goethite or ferrihydrite when goethite was present. This work highlights that the oxidation rate, the types of mineral surfaces, and OM control Fe(III) precipitate composition.