Degradation of metronidazole during the UV365-LED /chlorine process: Kinetics, influencing factors, and halonitromethanes formation

Abstract

Effective removal of refractory metronidazole (MNZ) from the aquatic environment is important for the safeguarding of human health and ecological safety. Herein, the combination of ultraviolet light-emitting diode at 365 nm wavelength and free chlorine (UV365-LED/chlorine) was used for the removal of MNZ. The UV365-LED/chlorine presented higher MNZ degradation efficiency (90.6%) compared to UV365-LED photolysis (29.96%) and chlorination alone (negligible). The degradation of MNZ by the UV365-LED photolysis and UV365-LED/chlorine process followed pseudo-first-order kinetics, and the corresponding degradation rate constants were 8.23 × 10–3 and 5.505 × 10–2 min–1, respectively. Quenching experiments indicated that hydroxyl radicals (·OH), ozone (O3), and reactive chlorine species (RCS) were responsible for MNZ removal during the UV365-LED/chlorine process. Meanwhile, probe experiments found that·OH and RCS were the major reactive species for MNZ elimination. Increasing chlorine dosage, raising UV power, and lowering pH of solution were beneficial to the MNZ degradation and corresponding halonitromethanes (HNMs) formation. HCO3–, NH4+, and Cl– presented inhibitory effects on MNZ degradation to different degrees, while Br– slightly improved MNZ degradation rate constants. Based on the analysis of MNZ degradation intermediates, the possible MNZ degradation pathways were proposed. Besides, energy consumption of UV365-LED/chlorine process decreased by 85.05% compared with UV365-LED photolysis. These findings contribute to the practical application of UV365-LED/chlorine process for MNZ removal in water systems and wastewater.