Submicroscale and Molecular Insights into Labile Organic Matter Transformation and Sequestration at Newly Formed Iron Minerals–Water Interfaces during O2-Mediated Fe(II) Oxidation

Abstract Abiotic Fe(II) oxidation and subsequent precipitation are key pathways for controlling labile organic matter (OM) dynamics in O2-perturbed subsurface environments, yet the underlying coupling mechanisms remain poorly understood. Herein, OM transformation and sequestration during O2-mediated heterogeneous Fe(II) oxidation were evaluated at neutral pH across various C/Fe ratios. Increasing C/Fe ratios impeded Fe(II) oxidation but enhanced the level of reactive oxygen species (ROS) generation. Despite efficient sequestration, the OM immobilization efficiency varied with C/Fe ratios, with a minor fraction of OM being oxidized by ROS. Higher C/Fe ratios promoted ferrihydrite formation but inhibited its transformation into lepidocrocite, forming mineral-organic composites constituted by Fe–O cores and edge-/corner-sharing Fe–Fe shells. A direct visualization at submicroscale revealed heterogeneous spatial distributions of different organo functional groups on newly formed Fe mineral surfaces, reflecting OM selective immobilization. Newly formed Fe minerals preferentially immobilized high-molecular-weight compounds and those with more oxygenated or highly unsaturated structures, with stronger molecular fractionation at higher C/Fe ratios. Molecular-level analysis revealed that a lower C/Fe ratio promoted the conversion of phenols to quinones, aromatic ring opening, and carboxylation of aromatic substituents but inhibited polymerization of phenolic substances. Collectively, these findings deepen our knowledge of heterogeneous Fe mineral formation coupled with OM sequestration and transformation, advancing our understanding of Fe and C geochemical cycling in oxic soils and sediments.