Nano-Fe ₃ O ₄ 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.