Identification, Formation, and Toxicity of Haloimidazoles as Emerging Nitrogenous Aromatic Disinfection Byproducts in Drinking Water

Identification of unknown disinfection byproducts (DBPs) remains a significant challenge in the supply of safe drinking water, as identified DBPs do not fully account for DBP-related adverse effects on human health. In this study, gas chromatography coupled with high-resolution mass spectrometry, along with multiple identification using the electron ionization and chemical ionization sources, was employed to identify six haloimidazoles as newly nitrogenous aromatic DBPs in drinking water. These compounds include 4-chloroimidazole, 4-bromoimidazole, 2,4-dibromoimidazole, 4,5-dibromoimidazole, 2,4,5-tribromoimidazole, 4-iodoimidazole, and 4,5-diiodoimidazole. A 13.6% yield of haloimidazoles was observed during chlorination of the amino acid histidine, and formation pathways are proposed. The detection frequency of the individual haloimidazoles ranged from 50 to 100% in eight drinking water samples, and their concentrations ranged from below the detection limit to 28.1 ng/L, with a median value of 4.4 ng/L. The cytotoxicity of the six haloimidazoles was evaluated by using Chinese hamster ovary cells. Notably, 2,4,5-tribromoimidazole exhibited the highest cytotoxicity with an EC₅₀ value of 53.2 μM, 138-252 times more toxic than regulated trichloromethane and dichloroacetic acid. These findings suggest that haloimidazoles are important nitrogenous aromatic DBPs in drinking water, and further investigation is needed to elucidate their potential long-term importance to human health.