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

Hydroxylamine (NH 2 OH) and nitrite (NO 2 - ), intermediates during the nitritation process, can engage in chemical (abiotic) reactions that lead to nitrous oxide (N 2 O) 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 N 2 O production rates were measured from NH 2 OH reaction with HNO 2 , followed by HNO 2 reduction by Fe 2+ , NH 2 OH oxidation by Fe 3+ , and finally NH 2 OH disproportionation plus oxidation by O 2 . Compared to other examined factors, pH had the strongest effect on N 2 O formation rates. Acidic pH enhanced N 2 O production from the reaction of NH 2 OH with HNO 2 indicating that HNO 2 instead of NO 2 - was the reactant. In departure from previous studies, we estimate that abiotic N 2 O production contributes little (< 3% of total N 2 O production) to total N 2 O 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 N 2 O production pathways, circumneutral pH set-points are suggested to minimize overall N 2 O emissions from nitritation systems.