Overlooked roles of ClO and Cl in micropollutant abatement and DBP formation by chlorination.

This study investigated the roles of diverse free available chlorine (FAC) species including HOCl/OCl, HOCl, ClO, and Cl in the degradation of micropollutants. The degradation of 5 micropollutants was significantly affected by pH, FAC dosage, and chloride (Cl) concentration. The reaction orders in FAC (n) of 5 micropollutants (acetaminophen, carbamazepine, naproxen, gemfibrozil, and mecoprop) ranged from 1.4 ± 0.2 to 2.1 ± 0.3 at pH 3 - 5, evidencing the importance of ClO and Cl for micropollutant abatement. A simplified method for the determination of second-order rate constants (k) of specific FAC species with micropollutants was developed. Herein, the k for neutral/dissociated forms of 5 micropollutants with Cl and ClO were determined in the ranges of 9.3 (± 0.2) × 10 ∼ 2.9 (± 0.2) × 10 M s and 1.8 (± 0.1) × 10 ∼ 3.7 (± 0.6) × 10 M s, respectively. They were 4 - 7 orders of magnitude higher than those of HOCl, whereas those of OCl and HOCl were negligible. By using kinetic modeling, Cl was more important under acidic conditions and higher Cl levels with contributions of 37.9 - 99.2% at pH 5 in pure water. ClO played a dominant role in micropollutant degradation in pure water (56.4 - 87.3%) under neutral conditions. Furthermore, both Cl and ClO played vital roles in the formation of disinfection byproducts (DBPs) during chlorination of carbamazepine and natural organic matter. This study highlights the overlooked roles of ClO and Cl in micropollutant abatement and DBP formation during chlorination.