Unlocking methanogenesis at 45°C: Synergistic roles of Zero-Valent iron and magnetite in Restoring cellulose anaerobic digestion

Anaerobic digestion (AD) operated at transitional temperature is a promising technology for treating de-oiled food waste (FW). However, under such conditions, the metabolic activity of methanogens will be significantly negatively affected compared to conventional mesophilic or thermophilic conditions, which limits its practical application. In this study, the AD of cellulose at 45 °C were investigated, and the synergistic effects of zero-valent iron (ZVI) and magnetite (Fe₃O₄) on enhancing methanogenesis were explored. The results indicated that the combination of ZVI and Fe₃O₄ played a crucial role in promoting cellulose methanogenesis during AD at 45 °C. Specifically, the methane yield reached 300.1 mL/g VS, representing a 58 % increase compared to the control group. Further analysis revealed that ZVI + Fe₃O₄ enhanced the levels of coenzyme F420 and acetate kinase. Firmicutes and Methanosarcina were the dominant microbial groups involved in normal methanogenesis at 45 °C, and a positive correlation between these two groups was observed. Metagenomic sequencing further demonstrated that the combination of ZVI and Fe₃O₄ synergistically increased the abundance of genes associated with both acetate cleavage and CO₂ reduction methanogenic pathways. The dominant pathways achieved a dynamic equilibrium, functioning according to the available substrates at different stages within the reactor. Life Cycle Assessment (LCA) results indicated that coupling ZVI and Fe₃O₄ could effectively reduce carbon emissions and fossil resource depletion. The study provided theoretical evidence for the application of AD in treating cellulose-based substrates at transitional temperatures.