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

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 (k(1)), HOCl with HO2 (k(2)) and OCl with H2O2 (k(3)) are 195.5 +/- 3.3 M 1s 1, 4.0 x 10(7) M(-1)s(-1) and 3.5 x 10(3) M 1s 1, respectively. The density functional theory calculation showed k(2) is the most advantageous thermodynamically pathway because it does not need to overcome a high energy barrier. The yields of O-1(2) 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 O-1(2) yields was obtained at pH 7-12. The second order rate constants of the reaction of O-1(2) with 13 phenolates were determined by using the H2O2/HOCl system as a quantitative O-1(2) 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.