Efficient degradation of dimetridazole during the UV/chlorine process: Kinetics, pathways, and halonitromethanes formation

Abstract

Dimetridazole (DMZ) is widely used as a veterinary antibiotic, but it is recalcitrant in the water environment. The residual of DMZ has raised widespread concern because of its harmful effects on human health and environmental safety. This work aimed to investigate the degradation of DMZ and the corresponding formation of halonitromethanes (HNMs) during the ultraviolet (UV)/chlorine process. The UV/chlorine process presented an excellent performance in DMZ degradation.The observed degradation rate constant of DMZ by UV/chlorine was 1.4 × 10−1 min−1. The hydroxyl radicals (·OH) and chlorine radicals (·Cl) were the predominant reactive species for DMZ degradation, and corresponding contributions were 79.8% and 13.7%, respectively. With the increase of free chlorine dosage, the degradation efficiency of DMZ firstly increased and then slightly decreased, while the formation of HNMs increased after 40 min UV/chlorine treatment. The higher degradation efficiency of DMZ was observed at acid circumstances, while higher HNMs formation was found at alkaline conditions due to base-catalyzed hydrolysis and less degradation of HNMs. The presences of Cl–, Br–, HCO3– and humic acid (HA) inhabited the degradation of DMZ. Furthermore, the possible degradation pathways of DMZ were proposed based on five intermediate products identified by UPLC/MS. Overall, the UV/chlorine was an efficient method for DMZ removal, but the formation of HNMs from DMZ should be evaluated before using UV/chlorine.