Kinetics and mechanism of the reaction of hydrogen peroxide with hypochlorous acid: Implication on electrochemical water treatment

Abstract

Reduction of HOCl to Cl− by in-situ electrochemical synthesis or ex-situ addition of H2O2 is a feasible method to minimize Cl-DBPs and ClOx− (x = 2, 3, and 4) formation in electrochemical oxidative water treatment systems. This work has investigated the kinetics and mechanism of the reaction between H2O2 and HOCl. The kinetics study showed the species-specific second order rate constants for HOCl with H2O2 (k1), HOCl with HO2− (k2) and OCl− with H2O2 (k3) are 195.5 ± 3.3 M−1s−1, 4.0 × 107 M−1s−1 and 3.5 × 103 M−1s−1, respectively. The density functional theory calculation showed k2 is the most advantageous thermodynamically pathway because it does not need to overcome a high energy barrier. The yields of 1O2 generation from the reaction of H2O2 with HOCl were reinvestigated by using furfuryl alcohol (FFA) as a probe, and an average of 92.3% of 1O2 yields was obtained at pH 7–12. The second order rate constants of the reaction of 1O2 with 13 phenolates were determined by using the H2O2/HOCl system as a quantitative 1O2 production source. To establish a quantitative structure activity relationship, quantum chemical descriptors were more satisfactory than empirical Hammett constants. The potential implications in electrochemical oxidative water treatment were discussed at the end.