Abiotic\nNitrous Oxide (N<sub>2</sub>O) Production Is Strongly pH Dependent,\nbut Contributes Little to Overall N<sub>2</sub>O Emissions in Biological\nNitrogen Removal Systems

Hydroxylamine\n(NH₂OH) and nitrite (NO₂^–),\nintermediates during the nitritation process, can engage in chemical\n(abiotic) reactions that lead to nitrous oxide (N₂O) generation.\nHere, we quantify the kinetics and stoichiometry of the relevant abiotic\nreactions in a series of batch tests under different and relevant\nconditions, including pH, absence/presence of oxygen, and reactant\nconcentrations. The highest N₂O production rates were measured\nfrom NH₂OH reaction with HNO₂, followed by HNO₂ reduction by Fe²⁺, NH₂OH oxidation\nby Fe³⁺, and finally NH₂OH disproportionation\nplus oxidation by O₂. Compared to other examined factors,\npH had the strongest effect on N₂O formation rates. Acidic\npH enhanced N₂O production from the reaction of NH₂OH with HNO₂ indicating that HNO₂ instead\nof NO₂^– was the reactant. In departure\nfrom previous studies, we estimate that abiotic N₂O production\ncontributes little (< 3% of total N₂O production) to\ntotal N₂O emissions in typical nitritation reactor systems\nbetween pH 6.5 and 8. Abiotic contributions would only become important\nat acidic pH (≤ 5). In consideration of pH effects on both\nabiotic and biotic N₂O production pathways, circumneutral\npH set-points are suggested to minimize overall N₂O emissions\nfrom nitritation systems.