Degradation of Naproxen by UV-irradiation in the presence of nitrate: Efficiency, mechanism, products, and toxicity change

Nitrate photolysis can produce reactive species that may affect the fate of coexisting micropollutants in the aquatic environment. In this study, the photo-degradation of naproxen (NPX) in the presence of nitrate was investigated. The observed pseudo-first-order rate constant (k(obs)) increased linearly with the nitrate dosage increasing from 1 mM to 10 mM. According to the EPR analysis and the quenching experiments, hydroxyl radicals (HO center dot) and reactive nitrogen species (RNS, mainly NO2 center dot) were found to play major roles in NPX degradation at pH 7.0 with the contribution rates of 34.3% and 40.7%, respectively. The k(obs) decreased with increasing pH due to the lower contribution of HO center dot and RNS. Moreover, water matrix components impacted the kinetics of NPX decomposition differently. Natural organic matter (NOM) had an inhibitory effect on the degradation of NPX, while bicarbonate and chloride ions slightly promoted the degradation. Furthermore, the degradation products in the system were identified by UPLC Q-TOF-MS analysis, and the transformation of NPX was shown to be mainly side-chain cleavage, hydroxylation, ring-opening, and nitration, indicating the role of HO center dot as well as RNS. According to the cytotoxicity assay and the toxicity of identified products predicted by ECOSAR program, the formation of several toxic products with aldehyde and nitro groups led to the increased toxicity during NPX degradation. Our study provides insights into the transformation mechanisms and the toxicity evolution of NPX in the photolysis system.