The Roles of Reactive Species in Micropollutant Degradation in the UV/Free Chlorine System

The UV/free chlorine process forms reactive species such as hydroxyl radicals (HO center dot), chlorine atoms (Cl-center dot), Cl-2(center dot-), and O center dot-. The specific roles of these reactive species in aqueous micropollutant degradation in the UV/chlorine process under different conditions were investigated using a steady-state kinetic model. Benzoic acid (BA) was chosen as the model micropollutant. The steady-state kinetic model developed fitted the experimental data well. The results showed that HO center dot and Cl-center dot contributed substantially to BA degradation, while the roles of the other reactive species such as Cl-2(center dot-) and O center dot- were negligible. The overall degradation rate of BA decreased as the pH increased from 6 to 9. In particular, the relative contributions of HO center dot and Cl-center dot to the degradation changed from 34.7% and 65.3% respectively at pH 6 to 37.9% and 62% respectively at pH 9 under the conditions evaluated. Their relative contributions also changed slightly with variations in chlorine dosage, BA concentration and chloride concentration. The scavenging effect of natural organic matter (NOM) on Cl-center dot was relatively small compared to that on HO center dot, while bicarbonate preferentially reduced the contribution of Cl-center dot. This study is the first to demonstrate the contributions of different reactive species to the micropollutant degradation in the UV/chlorine system under environmentally relevant conditions.