Nano-Fe₃O₄ Enhances the Biological Removal of Low-Concentration Methane during Long-Term Operation

Mitigating low-concentration methane (CH₄) emissions remains a significant challenge in climate change mitigation. Utilizing methane-oxidizing bacteria (MOB) to remove low-concentration CH₄ offers a sustainable approach, but its performance is limited by the slow growth of MOB and the poor mass transfer of hydrophobic CH₄. Here, we propose a strategy to enhance CH₄ removal in biofilters by incorporating nano-Fe₃O₄ to construct an MOB-Fe₃O₄ hybrid system. With stable nano-Fe₃O₄ stimulation, the biofilter achieved a 1.8- to 3.0-fold increase in the CH₄ elimination rate under various operational conditions. Specifically, carbon derived from CH₄ was preferentially redirected toward the synthesis of hydrophobic biomass, improving interfacial CH₄ transfer, while MOB also exhibited enhanced chemotactic potential. Consistently, the transcription of genes encoding both particulate methane monooxygenase (pMMO) and soluble methane monooxygenase (sMMO) was enhanced, indicating an overall reinforcement of the methane oxidation metabolism. Consequently, dominant MOB taxa increased by more than 2.0-fold in abundance, while the net CO₂ yield decreased from 78.3% to 45.8%, accompanied by a 2.6-fold increase in the community-level intracellular ATP concentration. Overall, this study demonstrates that nano-Fe₃O₄ improves the performance and stability of biofilters for low-concentration CH₄ removal, offering a practical and effective strategy for methane mitigation.