Kinetics of Molecular Chlorine Release during Photochemical Ozonation of Acidic Chloride Solutions

Abstract

The kinetics of the release of molecular chlorine during the oxidation of the chloride ion in acidified aqueous solutions (pH ≤2) under the action of ozone and UV radiation with a wavelength of 254 nm has been studied. In contrast to the dark reaction of O₃ with Cl^–(aq), the photochemical reaction gives significant yields of Cl₂ relative to the starting O₃, reaching 70% in weakly acidic solutions at pH 2. At increased acidity (pH ≤0.1), the Cl₂ yields during the photochemical and dark ozonation are closer. The combined action of ozone and UV irradiation on the oxidation of Cl^–(aq) is achieved due to the generation of free hydroxyl OH radicals and hydrogen peroxide H₂O₂ during the photolysis of O₃ in an aqueous medium. Due to the interaction with OH radicals, the oxidation of chloride ions is accelerated, and in the “O₃–Cl^–(aq)–hν (254 nm)” system there is an additional source of active free radicals capable of participating in the oxidation of Cl^–(aq), the nature of which cannot be explained based on the known mechanisms of chemical processes in similar systems. Hydrogen peroxide inhibits the oxidation of Cl^–(aq), since it effectively interacts with Cl₂ and other chlorine compounds in the nonnegative oxidation states and reduces them to the chloride ion.