Chlorine is preferred over bisulfite for H₂O₂ quenching following UV-AOP drinking water treatment

Drinking water treatment using UV/H₂O₂ advanced oxidation typically results in residual H₂O₂ that requires quenching to minimize its interference with downstream processes. Chemical quenching using chlorine or bisulfite are options, but there is some uncertainty in the literature about the kinetics of the bisulfite reaction, with some reports quoting the reaction as fast, and others as slow. Part of the contradictory information may be due to interference in H₂O₂ analysis by bisulfite. An analytical method was developed to avoid this interference, in which monochloramine first selectively quenched bisulfite, and then H₂O₂ was measured spectrometrically using titanium(IV) oxysulfate for color development. The confirmatory experiments suggested that the bisulfite reaction with H₂O₂ is actually relatively slow, with a half-life in the order of hours to days depending on the pH and the reagent concentrations. As a result, within the typical pH range of drinking water treatment (e.g., 6-9), chlorine is preferred over bisulfite as the H₂O₂ quenching agent on the basis of reaction kinetics. However, a decrease in pH will lead to an increase in the bisulfite-H₂O₂ reaction rate along with a decrease in the Cl₂-H₂O₂ reaction rate, such that at pH < 5.7 bisulfite is the faster reagent. Both bisulfite and chlorine were observed to react with H₂O₂ following a stoichiometric ratio of 1:1 in the natural water matrix tested.