Abiotic Nitrous Oxide (N₂O) Production Is Strongly pH Dependent, but Contributes Little to Overall N₂O Emissions in Biological Nitrogen Removal Systems

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