Transient disturbance of CeO2 nanoparticles enhances N2O emissions during biological wastewater treatment

Although the negative effects of CeO 2 nanoparticles (CeO 2 NPs) on wastewater treatment processes have been confirmed, their impacts on the greenhouse gas nitrous oxide (N 2 O) emissions during biological nitrogen removal (BNR) remain unclear. Herein, the response of BNR and associated N 2 O emissions in a sequencing batch reactor (SBR) to different loads (1–50 mg/L) of CeO 2 NPs was investigated. Although CeO 2 NPs induced the dose-dependent inhibition on NH 4 + oxidation rate, the NH 4 + -N and total nitrogen (TN) removal efficiencies did not change. The N 2 O emission flux was enhanced along with the increase in the CeO 2 NP dose, and there was a positive correlation between N 2 O emission and nitrite accumulation. In addition, NH 4 + oxidation activity indicators ammonia monooxygenase (AMO) activity, specific ammonia oxidation rate (SAOR), and specific oxygen uptake rate (SOUR) were suppressed under the 25 and 50 mg/L CeO 2 NP stresses. The increase in reactive oxygen species (ROS) levels induced by NP stress resulted in the downregulation of the amoA and nosZ gene expression. Furthermore, the CeO 2 NP shock load caused the severe suppression of NH 4 + oxidizing bacteria-related Nitrosomonas activities while promoted the denitrifying glycogen-accumulating organism (potential N 2 O producer)-related Dechloromonas activities. In summary, this study proves evidence that the transient disturbance of CeO 2 NPs would negatively affect the biological nitrogen transformation to release more N 2 O. • CeO 2 NPs inhibited NH 4 + -N oxidations but not affect the NH 4 + -N and TN removal efficiencies. • N 2 O emission was enhanced by CeO 2 NPs along with nitrite accumulation. • CeO 2 NPs down-regulated the NH 4 + oxidation and N 2 O reduction associated genes’ expressions. • CeO 2 NPs suppressed Nitrosomonas activities while promoted Dechloromonas ones.