Data underlying the publication: Nitrous Oxide as a Major Product of Anoxic Bioanode Ammonium Oxidation: Pathway Insights and Implications

Anoxic ammonium (NH₄⁺) oxidation at bioanodes offers a feasible alternative to conventional biological nitrogen removal (BNR), eliminating the need for aeration and enabling direct nitrogen gas (N₂) production. However, the extent of nitrous oxide (N₂O) emissions in this process remains unclear. This study examines N₂O emissions during anoxic NH₄⁺ oxidation at a polarized bioanode (+0.55 V vs Ag/AgCl), focusing on the roles of key intermediates—hydroxylamine (NH₂OH) and nitrite (NO₂^-)—in N₂O formation, and microbial functional groups involved in N₂O production, aiming to elucidate the underlying N₂O production pathways during bioanodic NH₄⁺ oxidation. Results show that up to 40% of oxidized NH₄⁺ was converted to N₂O, while NO₂^- and NO₃^- remained at relatively low levels (2.1%–11.1%). NH₂OH oxidation was identified as the dominant N₂O production pathway, likely involving nitric oxide (NO) as an intermediate. NO₂^- reduction also contributed to N₂O formation, albeit to a minor extent, while in situ N₂O reduction—considered to be mediated by denitrifiers—offers potential for mitigating its emissions. In parallel, an electro-anammox-like pathway, which does not involve nitrifiers, facilitated direct NH₄⁺ conversion to N₂ without N₂O formation. These findings highlight the potential for substantial N₂O emissions from bioanode-based systems and underscore the importance of pathway control to mitigate greenhouse gas release.