Silicate passivation and organic ligand functionality steer ferrihydrite recrystallization and MAOM retention

Ferrihydrite (Fh) is a ubiquitous, metastable Fe oxyhydroxide that strongly influences the cycling and persistence of mineral-associated organic matter (MAOM) in redox-dynamic soils. Under anoxic conditions, thiol-bearing organics can promote reductive dissolution and generate surface Fe(II) that catalyzes transformation to more crystalline Fe phases, yet how silicates and organic functionality jointly regulate these processes remains unclear. Here, we combine batch incubations of Si–Fh coprecipitates (Si/Fe = 0–0.18) with comparative experiments using representative low-molecular-weight ligands (glutathione, cystine, and glutamic acid) to determine how Si and molecular structure govern adsorption, Fe(II) availability, transformation kinetics, and mineral products. Increasing Si/Fe suppressed aqueous Fe(II) release and increased retention of surface-associated Fe(II)/Fe(III), stabilizing Fh for weeks at Si/Fe = 0.18 and shifting transformation pathways at lower Si to mixtures of lepidocrocite and a porous goethite phase enriched in Si and OM. In parallel, ligand adsorption was highly mineral-phase specific (Fh ≫ lepidocrocite) and depended on size and functional-group richness, which in turn modulated Fe(II)-catalyzed transformation selectivity: cystine most strongly suppressed progression beyond lepidocrocite, whereas glutathione and glutamic acid permitted hematite formation under higher Fe(II), consistent with coupled effects of surface passivation and aqueous complexation of reactive Fe species. Electron microscopy and elemental mapping revealed preferential OM retention in amorphous Fh and selective incorporation of Si and OM into porous goethite, while lepidocrocite showed limited OM association. Together, these results identify Si/Fe and ligand functionality as geochemical “switches” that control Fe mineral evolution and determine whether MAOM is excluded, stabilized, or redistributed during anoxic transformation, with direct implications for carbon persistence in ferruginous soils.