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 ClO_x⁻ (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.