Organic Carbon Sequestration by Secondary Fe–MnComplex Minerals via the Anoxic Redox Reaction of Fe(II) and Birnessite

Organic matter (OM) sequestrated by iron (Fe) and manganese (Mn) complex minerals may be an effective way to achieve organic carbon (OC) persistence in sediments and soils. However, the impact of Fe–Mn complex minerals from a redox reaction of Fe(II) and birnessite on OC dynamics remains unexplored. Herein, the immobilization and molecular transformation of fulvic acid (FA), mediated by a redox reaction of Fe(II) and birnessite, were investigated at pH 4.0–8.0 with 1–5 mM Fe(II). Higher pH and Fe(II) addition promoted Fe(II) oxidation coupled with the reductive transformation of birnessite. The coupled adsorption–oxidation of FA occurred, and FA sequestration efficiency increased with a decrease in pH and increasing Fe(II) concentrations. Fe(II) oxidation primarily formed ferrihydrite, and a small portion of lepidocrocite and feitknechtite was detected under higher pH and Fe(II) addition conditions. In mineral–organo composites, Fe/Mn–O, Fe–C, edge- and corner-sharing Fe–Fe, and Mn–Mn bonds constituted the coordination environment. FA was associated with Fe–Mn complex minerals to improve its stability, while C in FA distributed within nanopores and thin coaggregates exhibited a higher oxidation state. Compounds with high molecular weight, O/C, and aromatic and unsaturated structures, predominantly composed of lignin, tannin, and condensed hydrocarbons, exhibited stronger binding to Fe–Mn complex minerals under lower pH and higher Fe(II) addition conditions. These findings provide a fundamental understanding of heterogeneous Fe–Mn complex mineral formation coupled with OM sequestration, which has significant implications for understanding the long-term cycling of Fe, Mn, and C in natural settings.