Degradation of tetracycline by UV activated monochloramine process: Kinetics, degradation pathway, DBPs formation and toxicity assessment

Abstract

The combination of UV and monochloramine (UV/NH2Cl) was investigated to eliminate tetracycline (TC) in water. The results indicated that TC degradation by UV/NH2Cl oxidation far exceeded that by UV photolysis and dark chloramination and these processes well fitted pseudo-first-order kinetics. With the increase of NH2Cl dosage, the pseudo-first-order rate constant (kobs) value increased linearly during TC chloramination, while the growth rate of kobs presented a gradual downward trend in UV/NH2Cl process. TC degradation by UV photolysis, dark chloramination and UV/NH2Cl oxidation displayed highly pH-dependent. As solution pH increased from 5.0 to 9.0, the TC removal increased during UV photolysis, while decreased during dark chloramination. For UV/NH2Cl process, the TC degradation showed a decreasing trend with increasing pH from 5.0 to 7.0, but further increasing pH to 9.0 presented an increasing trend. Reactive chlorine species acted as the major contributors to TC degradation in pH range of 6.0–8.0. TC degradation was not apparently affected after the introduction of HA and chloride, but was substantially accelerated in the presence of HCO3−. The proposed degradation pathways involved the hydroxylation, demethylation, dehydration and chlorine substitution processes. Compared with dark chloramination, the N-Nitrosodimethylamine (NDMA) generation was enhanced during TC oxidation by UV/NH2Cl process. The NDMA concentration increased with increasing solution pH from 6.0 to 8.0 and also elevated in the presences of humic acid (HA), nitrite, nitrate and ammonium during TC oxidation by chloramination and UV/NH2Cl. The results of acute toxicity test demonstrated that the toxicity of the reacted solution exhibited similar trends during dark chloramination and UV/NH2Cl treatment and was found to be obviously higher than UV photolysis.