Kinetics of the reaction between hydrogen peroxide and aqueous iodine: Implications for technical and natural aquatic...

Oxidative treatment of iodide-containing waters can lead to a formation of potentially toxic iodinated disinfection byproducts (I-DBPs). Iodide (I^-) is easily oxidized to HOI by various oxidation processes and its reaction with dissolved organic matter (DOM) can produce I-DBPs. Hydrogen peroxide (H₂O₂) plays a key role in minimizing the formation of I-DBPs by reduction of HOI during H₂O₂-based advanced oxidation processes or water treatment based on peracetic acid or ferrate(VI). To assess the importance of these reactions, second order rate constants for the reaction of HOI with H₂O₂ were determined in the pH range of 4.0-12.0. H₂O₂ showed considerable reactivity with HOI near neutral pH (k_app = 9.8 × 10³ and 6.3 × 10⁴ M^-1s^-1 at pH 7.1 and 8.0, respectively). The species-specific second order rate constants for the reactions of H₂O₂ with HOI, HO₂^- with HOI, and HO₂^- with OI^- were determined as k_H2O2+HOI = 29 ± 5.2 M^-1s^-1, k_HO2^-_+HOI = (3.1 ± 0.3) × 10⁸ M^-1s^-1, and k_HO2^-_+OI^- = (6.4 ± 1.4) × 10⁷ M^-1s^-1, respectively. The activation energy for the reaction between HOI and H₂O₂ was determined to be E_a = 34 kJ mol^-1. The effect of buffer types (phosphate, acetate, and borate) and their concentrations was also investigated. Phosphate and acetate buffers significantly increased the rate of the H₂O₂-HOI reaction at pH 7.3 and 4.7, respectively, whereas the effect of borate was moderate. It could be demonstrated, that the formation of iodophenols from phenol as a model for I-DBPs formation was significantly reduced by the addition of H₂O₂ to HOI- and phenol-containing solutions. During water treatment with the O₃/H₂O₂ process or peracetic acid in the presence of I^-, O₃ and peracetic acid will be consumed by a catalytic oxidation of I^- due to the fast reduction of HOI by H₂O₂. The O₃ deposition on the ocean surface may also be influenced by the presence of H₂O₂, which leads to a catalytic consumption of O₃ by I^-.