Iron redox cycling drives enhanced methanogenesis in magnetic biochar-mediated anaerobic digestion of waste-activated sludge

Anaerobic digestion provides an essential pathway for reducing organic waste while simultaneously recovering bioenergy. To enhance this process, magnetic biochars are frequently employed as conductive additives to promote direct interspecies electron transfer (DIET) among syntrophic microorganisms. However, the fundamental mechanisms regarding how iron species leached from these materials influence iron transformation and electron flux remain poorly understood. Here we show that the leaching of iron species from magnetic biochar establishes a stable Fe(Ⅲ)/Fe(Ⅱ) redox cycle that accelerates the hydrolysis, acidogenesis, and methanogenesis of waste-activated sludge. We find that cumulative methane production increases by 17% as leached Fe(Ⅲ) facilitates dissimilatory iron reduction, followed by secondary mineralization into high-crystalline iron species. This process selectively enriches electroactive taxa, including Geobacter and Methanothrix, and transitioned the dominant electron transfer mechanism from cytochrome c-dependent pathways to a Fe(Ⅲ)/Fe(Ⅱ) redox-driven DIET. These mechanisms advance our understanding of conductive material-mediated AD, offering strategies to optimize energy recovery from waste-activated sludge and support sustainable sludge management in wastewater treatment.