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

Abstract

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 O3 mediated oxidation of Cl(-), OCl(-), ClO2(-), ClO3(-), and ClO2. The influence of reactant (O3 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 O3. The very slow rate of ClO4(-) production from Cl(-) ( approximately 20 x 10(-9) mM min(-1)) even at elevated O3 and Cl(-) concentrations implies negligible potential for anthropogenic ClO4(-) formation in process units of water/wastewater systems that use O3 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.