The multiple roles of chlorite on the concentrations of radicals and ozone and formation of chlorate during UV photolysis of free chlorine

Abstract

Chlorine dioxide (ClO2) has emerged as a promising alternative to free chlorine for water disinfection and/or pre-oxidation due to its reduced yields of chlorinated disinfection byproducts. ClO2 decomposes to form chlorite (ClO2−), which influences the following advanced oxidation processes (AOPs) for micropollutant abatement in drinking water. This study aims at investigating the effects of ClO2− on the concentrations of reactive species (e.g., radicals and ozone) and on the formation of chlorate in the UV/chlorine AOP. Results showed that the concentration of ClO· in the UV/chlorine process remarkably decreased by 98.20–100.00% in the presence of ClO2− at concentration of 0.1–1.0 mg·L−1 as NaClO2. The concentrations of HO· and ozone decreased by 42.71–65.42% and by 22.02–64.31%, respectively, while the concentration of Cl· was less affected (i.e., 31.00–36.21% reduction). The overall concentrations of the reactive species were differentially impacted by ClO2−’s multiple roles in the process. UV photolysis of ClO2− generated HO· but not Cl·, ClO· or ozone under the drinking water relevant conditions. ClO2− also competed with chlorine for UV photons but this effect was minor (< 1.0%). The radicals/ozone scavenging by ClO2− outcompeted the above two to lead to the overall decreasing concentrations of the reactive species, in consistency with the kinetic model predicted trends. ClO2− reacted with radicals and ozone to form chlorate (ClO3−) but not perchlorate (ClO4−). HO· played a dominant role in ClO3− formation. The findings improved the fundamental understanding on micropollutant abatement and inorganic byproduct formation by the UV/chlorine process and other AOPs in ClO2−-containing water.