Synergy between UV light and trichloroisocyanuric acid on methylisothiazolinone degradation: Performance, kinetics and degradation pathway.

Methylisothiazolinone (MIT) is widely used in daily chemicals, fungicides, and other fields and its toxicity has posed a threat to water system and human health. In this study, ultraviolet (UV)/trichloroisocyanuric acid (TCCA), which belongs to advanced oxidation processes (AOP), was adopted to degrade MIT. Total chlorine attenuation detection proved that TCCA has medium UV absorption and a strong quantum yield (0.49 mol E). At a pH of 7.0, 93.5% of MIT had been decontaminated after 60 min in UV/TCCA system (k = 4.4 × 10 min, R = 0.978), which was much higher than that in the UV alone system and TCCA alone system, at 65% (1.7 × 10 min, R = 0.995) and 10% (1.8 × 10 s, R = 0.915), respectively. This system also behaved well in degrading other five kinds of contaminants. Tert-butanol (TBA) and carbonate (CO) were separately used in quenching experiments, and the degradation efficiency of MIT decreased by 39.5% and 46.5% respectively, which confirmed that HO• and reactive chlorine species (RCS) were dominant oxidants in UV/TCCA system. With TCCA dosage increasing in a relatively low concentration range (0.02-0.2 mM) and pH decreasing, the effectiveness of this AOP system would be strengthened. The influences of coexisting substances (Cl, SO, CO, NO and NO) were explored. MIT degradation pathways were proposed and sulfur atom oxidation and carboxylation were considered as the dominant removal mechanisms of MIT. Frontier orbital theory and Fukui indexes of MIT were employed to further explore the degradation mechanism.