Coprecipitated Silica Controls the Divergent Pathways of Cysteine-Induced Ferrihydrite Transformation

The reductive transformation of ferrihydrite (Fh) is a key process controlling iron mineral evolution and mineral-associated organic matter (MAOM) cycling in soils and sediments, yet the regulating role of coprecipitated silica remains poorly constrained. Here, we investigated the influence of coprecipitated silica on cysteine-induced Fh transformation under anoxic conditions using Fh and Si-Fh coprecipitates with Si/Fe molar ratios of 0.05 and 0.1. Mineral evolution and Fe speciation were monitored using X-ray diffraction (XRD), Mössbauer spectroscopy, X-ray photoelectron spectroscopy (XPS), transmission electron microscopy coupled with energy-dispersive X-ray analysis (TEM-EDX), and selective chemical extractions. In silica-free Fh, cysteine induced rapid transformation into lepidocrocite through reductive dissolution and recrystallization pathways that largely excluded silica and MAOM from the newly formed mineral phase. In contrast, coprecipitated silica altered the extent, rate, and pathway of mineral evolution. At a Si/Fe ratio of 0.05, crystalline phase formation was delayed and redirected toward a porous-like goethite phase that retained substantial silica and organic associations. At a Si/Fe ratio of 0.1, formation of crystalline secondary phases was strongly suppressed, coinciding with accumulation of labile surface-associated Fe(III). These findings demonstrate that coprecipitated silica can decouple reductive electron transfer from mineral recrystallization and alter the fate of MAOM during Fh transformation under anoxic conditions.