Insights into chronic zinc oxide nanoparticle stress responses of biological nitrogen removal system with nitrous oxide emission and its recovery potential

The nitrogen transformation performances and greenhouse gas nitrous oxide (N₂O) emissions in a sequencing batch reactor under chronic exposure to zinc oxide nanoparticles (ZnO NPs) were quantified and the system's self-recovery potentials were assessed. ZnO NPs posed a dose-dependent depression effect on the removal efficiencies of ammonia nitrogen (NH₄⁺-N) and total nitrogen (TN), and the N₂O emissions. The suppressed N₂O emissions had a positive relationship with the activity ratios of nitrite/NO reductases and N₂O reductase, and were expected to be caused by the inhibited heterotrophic denitrification process. The inhibition of glucose metabolism key enzymes and electron transport chain activities would be responsible for the heterotrophic denitrification performances deterioration. Furthermore, the removal efficiencies of NH₄⁺-N and TN were recovered to control levels through the nitrite-shunt. However, the N₂O emission increased significantly above the control during the recovery period mainly due to the irreversibility of the depressed nitrite oxidation activities.