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

Chlorine dioxide (ClO) 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. ClO decomposes to form chlorite (ClO), which influences the following advanced oxidation processes (AOPs) for micropollutant abatement in drinking water. This study aims at investigating the effects of ClO 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 ClO at concentration of 0.1-1.0 mg·L as NaClO. 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 ClO's multiple roles in the process. UV photolysis of ClO generated HO but not Cl, ClO or ozone under the drinking water relevant conditions. ClO also competed with chlorine for UV photons but this effect was minor (< 1.0%). The radicals/ozone scavenging by ClO outcompeted the above two to lead to the overall decreasing concentrations of the reactive species, in consistency with the kinetic model predicted trends. ClO reacted with radicals and ozone to form chlorate (ClO) but not perchlorate (ClO). HO played a dominant role in ClO formation. The findings improved the fundamental understanding on micropollutant abatement and inorganic byproduct formation by the UV/chlorine process and other AOPs in ClO-containing water.