In Situ Formation of Free Chlorine During ClO₂Treatment: Implications on the Formation of Disinfection Byproducts

Abstract

Chlorine dioxide (ClO₂) is commonly used as an alternative disinfectant to chlorine in drinking water treatment because it produces limited concentrations of halogenated organic disinfection byproducts. During drinking water treatment, the primary ClO₂ byproducts are the chlorite (50-70%) and the chlorate ions (0-30%). However, a significant portion of the ClO₂ remains unaccounted for. This study demonstrates that when ClO₂ was reacting with phenol, one mole of free available chlorine (FAC) was produced per two moles of consumed ClO₂. The in situ formed FAC completed the mass balance on Cl for inorganic ClO₂ byproducts (FAC + ClO₂^- + ClO₃^-). When reacting with organic matter extracts at near neutral conditions (pH 6.5-8.1), ClO₂ also yielded a significant amount of FAC (up to 25%). Up to 27% of this in situ formed FAC was incorporated in organic matter forming adsorbable organic chlorine, which accounted for up to 7% of the initial ClO₂ dose. Only low concentrations of regulated trihalomethanes were produced because of an efficient mitigation of their precursors by ClO₂ oxidation. Conversely, dichloroacetonitrile formation from ClO₂-induced generation of FAC was higher than from addition of FAC in absence of ClO₂. Overall, these findings provide important information on the formation of FAC and disinfection byproducts during drinking water treatment with ClO₂.