● H₂O₂ quenching rates by Cl/S-based chemicals were measured

Residual H₂O₂ from UV/H₂O₂ treatment can be quenched by thiosulfate, bisulfite, and chlorine, but the kinetics of these reactions have not been reported under the full range of practical conditions. In this study, the rates of H₂O₂ quenching by these compounds were compared in different water matrices, temperatures, pH, and when using different forms of bisulfite and chlorine. In general, it was confirmed that thiosulfate would be too slow to serve as a quenching agent in most practical scenarios. At pH 7–8.5, chlorine tends to quench H₂O₂ more than 20 times faster than bisulfite in the various conditions tested. An important observation was that in lightly-buffered water (e.g., alkalinity of 20 mg/L as CaCO₃), the form of chlorine can have a large impact on quenching rate, with gaseous chlorine slowing the reaction due to its lowering of the pH, and hypochlorite having the opposite effect. These impacts will become less significant when water buffer capacity (i.e., alkalinity) increases (e.g., to 80 mg/L as CaCO₃). In addition, water temperature should be considered as the time required to quench H₂O₂ by chlorine at 4 °C is up to 3 times longer than at 20 °C.