Degradation of carbaryl by UV/chlorine process: Optimal degradation conditions, reaction intermediates, and mechanisms

Carbaryl, a widely used carbamate insecticide, is toxic to various organisms and humans, persisting in the water cycle despite its relatively rapid environmental degradation. This study thoroughly investigates carbaryl degradation using UV/chlorine treatment, aiming to identify optimal degradation conditions and understand reaction mechanisms. The effects of oxidant concentration, solution pH values, natural organic matter (NOM), and alkalinity on carbaryl degradation by UV/chlorine were examined. The UV/chlorine treatment exhibited significantly enhanced carbaryl degradation compared to UV treatment alone or dark chlorination (i.e., chlorination without UV). Carbaryl was effectively decomposed below pH 7.0 (<pKa of HOCl, which is 7.5); at higher UV dosages, carbaryl was completely removed regardless of solution pH. While NOM and alkalinity reduced degradation efficiency by scavenging reactive species, this effect could be mitigated by increasing UV dosages. Furthermore, residual NaOCl in the internal circulating water of an odor deodorization tower was effectively repurposed for carbaryl degradation, representing an innovative, practical application of our laboratory findings. The degradation pathway of carbaryl was also elucidated, revealing the formation of various intermediates through reactions with hydroxyl radicals (HO·) and reaction chlorine species (RCS). These findings contribute to developing advanced water treatment technologies for mitigating persistent organic pollutants derived from agricultural practices.