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

Abstract

Although the negative effects of CeO2 nanoparticles (CeO2 NPs) on wastewater treatment processes have been confirmed, their impacts on the greenhouse gas nitrous oxide (N2O) emissions during biological nitrogen removal (BNR) remain unclear. Herein, the response of BNR and associated N2O emissions in a sequencing batch reactor (SBR) to different loads (1–50 mg/L) of CeO2 NPs was investigated. Although CeO2 NPs induced the dose-dependent inhibition on NH4+ oxidation rate, the NH4+-N and total nitrogen (TN) removal efficiencies did not change. The N2O emission flux was enhanced along with the increase in the CeO2 NP dose, and there was a positive correlation between N2O emission and nitrite accumulation. In addition, NH4+ 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 CeO2 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 CeO2 NP shock load caused the severe suppression of NH4+ oxidizing bacteria-related Nitrosomonas activities while promoted the denitrifying glycogen-accumulating organism (potential N2O producer)-related Dechloromonas activities. In summary, this study proves evidence that the transient disturbance of CeO2 NPs would negatively affect the biological nitrogen transformation to release more N2O.