Influence of Coprecipitated Organic Matter on Fe2+(aq) -Catalyzed Transformation of Ferrihydrite: Implications for Carbon Dynamics
The poorly crystalline Fe(III) hydroxide ferrihydrite is an important sink for organic matter (OM), nutrients and contaminants in soils and sediments. Aqueous Fe(II) is known to catalyze the transformation of ferrihydrite to more crystalline and thus less reactive phases. While coprecipitation of OM with ferrihydrite could be a common process in many environments due to changes in pH, redox potential or ionic strength, little is known about the impacts of coprecipitated OM on Fe(II)-catalyzed ferrihydrite transformation and its consequences for C dynamics. Accordingly, we explored the extent and pathways of Fe(II)-induced transformation of OM-ferrihydrite coprecipitates and subsequent C mobility. Mössbauer spectroscopic results indicated that the coprecipitated OM within ferrihydrite weakened the inter-particle magnetic interactions and decreased average particle size. The coprecipitated OM resulted in diminished Fe(II)-induced ferrihydrite transformation and thus preservation of ferrihydrite. The secondary mineral profiles upon Fe(II) reaction with ferrihydrite were a function of OM content and Fe(II) concentration. At low Fe(II) levels, OM completely inhibited goethite formation and stimulated lepidocrocite formation. At high Fe(II) levels, whereas goethite was formed in the presence of OM, OM reduced the amount of goethite and magnetite formation and increased the formation of lepidocrcocite. The solid-phase C content remained unchanged after reaction, while OM desorpability by H2PO4- was enhanced following reaction of OM-ferrihydrites with aqueous Fe(II). These findings provide insights into the reactivity of natural ferrihydrite containing OM in soils and sediments and the subsequent impact on mineral evolution and C dynamics.