Perchlorate Formation by Ozone Oxidation of Aqueous Chlorine/Oxy-Chlorine Species: Role of ClxOy Radicals

The environmental occurrence of perchlorate (ClO4-) can be related to either natural or anthropogenic sources. Recent studies highlighted the ubiquitous occurrence of natural ClO4- in the environment including wet deposition in the United States. Limited studies have investigated potential mechanisms responsible for natural ClO4- production in the environment These studies have neither addressed the influence of relevant reaction conditions nor have they evaluated the rates of ClO4- production. The purpose of this study was to determine the comparative yields and rates of ClO4- production from O-3 mediated oxidation of Cl-, OCl-, ClO2-, ClO3-, and ClO2. The influence of reactant (O-3 and ClOx-) concentration and pH were evaluated. The comparative rate and efficiency of ClO4- production is generally greater for higher oxidation states of Cl (2.7 to 0.5% for ClO2-/ClO2 and 0.02 to 0.005% for OCl-/HOCl oxidation) with the notable exception of ClO3- which does not react with O-3. The very slow rate of ClO4- production from Cl- (similar to 20 x 10(-9) mM min(-1)) even at elevated O-3 and Cl- concentrations implies negligible potential for anthropogenic ClO4- formation in process units of water/wastewater systems that use O-3 for treatment Based on results of ClO4- formation from tested Cl species and available literature, we propose a potential formation pathway for ClO4- from Cl- with emphasis on the role of ClO2 and higher oxy-chlorine radicals/intermediates (e.g., Cl2O6) in its formation.