Ferrous iron-induced secondary mineral formation regulates organic carbon bioavailability and methane recovery from iron-enhanced primary sludge.

Iron salt-based chemically enhanced primary treatment effectively captures organic carbon from municipal wastewater, providing a concentrated substrate for methane recovery through anaerobic digestion. However, methane production from iron salt-based chemically enhanced primary sedimentation (Fe-CEPS) sludge remains substantially lower than expected, and the mechanisms responsible for this reduction remain poorly understood. Here, the relationship between iron-bound organic carbon (Fe-OC) formation and methane production was investigated by combining anaerobic digestion experiments with iron mineral-glucose model systems. Increasing Fe dosage reduced methane yield from 221.1 to 74.7 mL CH4 g-1 VS while promoting Fe-OC accumulation. A strong negative correlation was observed between Fe-OC content and methane production. Fresh Fe-CEPS sludge contained only limited Fe-OC (<10% of total organic carbon), whereas Fe-OC accounted for approximately 26% of total organic carbon after anaerobic digestion, indicating that Fe-OC formed predominantly during digestion rather than during coagulation. Iron speciation analyses revealed extensive Fe(III) reduction accompanied by secondary iron mineral formation. Model experiments further demonstrated that Fe(II)-induced mineral transformation increased mineral crystallinity, enhanced organic carbon stabilization, reduced carbon bioavailability, and consequently suppressed methane production. Scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy further revealed progressively stronger spatial associations between organic carbon and iron minerals as mineral crystallinity increased. These findings reveal a previously unrecognized mechanism whereby Fe(II)-induced secondary iron mineral formation promotes Fe-OC accumulation and reduces organic carbon bioavailability during anaerobic digestion. Regulating iron mineral transformation may therefore represent an effective strategy for improving methane recovery from iron-enhanced primary sludge.