From Micropollutant to DBP Driver: The Unexpected Reactivity of 1-Chlorobenzotriazole in Water Disinfection

The increasing adoption of potable wastewater reuse is challenged by persistent micropollutants─such as benzotriazole─which are poorly removed by conventional treatment and may produce toxic disinfection byproducts (DBPs). This study investigated benzotriazole chlorination and identified 1-chlorobenzotriazole as the primary product, formed preferentially under near-neutral to acidic pH and excess chlorine─conditions typical in disinfected water. The DBP formation potential of 1-chlorobenzotriazole was evaluated using two organic matter standards and secondary wastewater effluents and compared against chlorine and monochloramine. Surprisingly, 1-chlorobenzotriazole formed DBP levels comparable to or greater than those from chlorine, including trihalomethanes, haloaldehydes, haloketones, haloacetonitriles, and halonitromethanes. This reactivity is attributed to 1-chlorobenzotriazole's ability to function as a free chlorine reservoir, sustaining chlorination reactions and promoting continued DBP formation─unlike inorganic <i>N</i>-halamines. DBP speciation was strongly pH-dependent and mirrored chlorine behavior, supporting the chlorine reservoir effect. Additionally, 1-chlorobenzotriazole exhibited precursor- and matrix-dependent reactivity, especially with complex matrices like secondary wastewater effluents, where it acted as both a chlorine source and a direct DBP precursor. Overall, this work provides the first detailed evaluation of 1-chlorobenzotriazole DBP formation potential, revealing an overlooked pathway for halogenated DBP production in water disinfection, and emphasizes the importance of considering benzotriazole transformation products in advanced reuse systems.