Comparing the effects and mechanisms of granular activated carbon and magnetite nanoparticles in dry anaerobic digestion: Focusing on microbial electron transfer and metagenomic evidence

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Enhancing direct interspecies electron transfer (DIET) via granular activated carbon (GAC) and magnetite nanoparticles (MNPs) to promote methanation performance in dry anaerobic digestion (AD) systems has been extensively studied in recent years. However, the underlying mechanisms, particularly how GAC and MNPs influence DIET and extracellular polymeric substances (EPSs), remain poorly understood. In this study, the different effects and mechanisms of GAC and MNPs on dry AD fed with pig manure (PM) and corn straw were compared, with additions of 10, 20, and 50% of the volatile solid (VS) of PM. The results indicated that GAC improved the cumulative specific methane yields (CSMYs) by 18.8–20.9%, while a high dose of MNPs (50% of the VSPM) decreased the CSMY and hydrolysis rate constant by 27.0 and 75.4%, respectively. GAC provided microorganism attachment sites and functional groups for electron transfer, improving hydrolysis by enriching fermenters such as Fastidiosipila and Rikenellaceae. Nevertheless, MNPs stimulated intense dissimilatory iron reduction by Clostridium sp. in the early stage, competing with methanogens for substrates. Low and medium GAC doses (10 and 20% of the VSPM) increased the utilization of redox-active substances (e.g., humic substances and proteins) in EPS, while the high MNP dose stimulated their secretion due to microbial self-protection, impairing electron transfer. Metagenomic evidence showed that the medium GAC dose improved four methanogenic pathways and DIET mediated by microbial nanowires, while a high MNP dose impeded the final step of methanogenesis. These results provide deeper insights into the mechanisms of different conductive materials on dry AD.