Overlooked roles of Cl₂O and Cl₂ in micropollutant abatement and DBP formation by chlorination

This study investigated the roles of diverse free available chlorine (FAC) species including HOCl/OCl-, H2OCl+, Cl2O, and Cl-2 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 micro -pollutants (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 Cl2O and Cl-2 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-2 and Cl2O were determined in the ranges of 9.3 (+/- 0.2) x 10(2) - 2.9 (+/- 0.2) x 109 M-1 s(-1) and 1.8 (+/- 0.1) x 10(4) - 3.7 (+/- 0.6) x 109 M-1 s(-1), respectively. They were 4 -7 orders of magnitude higher than those of HOCl, whereas those of OCl- and H2OCl+ were negligible. By using kinetic modeling, Cl-2 was more important under acidic conditions and higher Cl- levels with contributions of 37.9 - 99.2% at pH 5 in pure water. Cl2O played a dominant role in micropollutant degradation in pure water (56.4 -87.3%) under neutral conditions. Furthermore, both Cl-2 and Cl2O 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 Cl2O and Cl-2 in micropollutant abatement and DBP formation during chlorination.