Integrated methane microbial fuel cell system for concurrent nitrogen and sulfur removal through autotrophic cathodic bioreduction

The cathodic reduction of nitrate and sulfate by autotrophic organisms in microbial fuel cells (MFC) has received much attention. However, systematic investigations into the feasibility of methane-driven nitrate and sulfate reduction remain limited. In this study, an MFC integrating an autotrophic biocathode with dissolved methane (CH₄) as the electron donor was developed to couple methane-driven reduction of nitrate and sulfate with electricity generation. The biology reductive properties, microbial characteristics and functional metabolic mechanisms were investigated under single electron acceptor (NO₃⁻/SO₄²⁻) and mixed electron acceptor (NO₃⁻ + SO₄²⁻) working conditions. The results showed that with nitrate and sulfate acting as electron acceptors alone, MFC achieved the maximum removal rates of nitrate and sulfate of 89.1 % and 26 %, respectively. With mixed electron acceptors provided, the removal rates of nitrate and sulfate decreased by 62.5 % and 14.2 %, respectively. However, the anaerobic oxidation of methane (AOM) was promoted, and its output voltage reached a maximum. The anode chambers of all methane autotrophic denitrification‑sulfur removing MFCs shared similar microbial structures, with dominant functional genera including Methylocystis, Hyphomicrobium and Methylomonas, and the dominant bacteria in the cathode chamber were Pseudomonas, Nitrospira, Desulfovibrio, Hyphomicrobium and Acidovorax. The genes coding for methane metabolism were upregulated when nitrate and sulfate coexisted, while the genes related to sulfur metabolism and denitrification metabolism were downregulated. These findings provide novel insights into the application of AOM-MFC systems for the treatment of wastewater with nitrogen and sulfur contaminants.