Understanding nitrogen removal mechanisms of microbial fuel cells with different air-cathodes: pathways and microbial...

Single-chamber air-cathode microbial fuel cells (SA-MFCs) are an aeration-free, energy-positive technology for nitrogen removal, which is critical for environmental protection. However, existing studies on nitrogen removal mechanisms in SA-MFCs are conflicting, hindering further development. Focusing on removal mechanisms, this study comprehensively investigated three potential nitrogen removal pathways (ammonia volatilisation, electrochemical oxidation and biological conversion) using both conventional hand-made and 3D-printed air cathodes. Circuit current controls, nitrification inhibitor assays and complete nitrogen speciation (NH₄⁺, NO₂^-, NO₃^- and total nitrogen) were used to distinguish the pathways. Dual-electrode 16S rRNA sequencing, analysed with BugBase, FAPROTAX and PICRUSt2, linked microbial taxa to functional steps and confirmed spatial segregation of nitrifiers and denitrifiers. Across all conditions, only 23-30 % of NH₄⁺ was lost via volatilisation, 70-77 % was removed through simultaneous nitrification and denitrification while electrochemical oxidation was negligible. This study establishes a unified nitrogen removal mechanism that supports the advancement of SA-MFC technology. TAKE HOME MESSAGE: This study reveals that in SA-MFCs, SND is the primary pathway for nitrogen removal, while ammonia volatilization occurs but is not essential.