Concentration-dependent chloride effect on radical distribution and micropollutant degradation in the sulfate radical-based AOPs

This study quantitatively evaluated the effect of chloride (Cl-) on the radical distribution and micropollutant degradation in the UV/peroxydisulfate AOP using both experimental and modeling approaches. Results showed that SO4•- was significantly scavenged by Cl- at environmentally relevant concentrations (1-5000 mg/L). With increasing Cl- concentrations from 1 to 5000 mg/L, Cl- transformed SO4•- to HO• and then to Cl2•-. The critical role of Cl2•- as a precursor of HO• in the radical transformation was highlighted. The inhibitory effects of bicarbonate and dissolved organic matter (DOM) on micropollutant degradation was more significant in the presence of Cl- than that in the absence of Cl-, mainly due to the consumption of Cl2•- by bicarbonate and DOM. Using the model-predicted radical concentrations in the UV/peroxydisulfate process in the presence of different concentrations of Cl-, the degradation rate constants of 34 micropollutants and the contributions of each radical to the degradation were predicted and compared. The findings improved the fundamental understanding of the Cl- effect on radical transformation and micropollutant degradation in the SO4•--based AOPs. The model enables to foresee whether a SO4•--based AOP is effective for the degradation of a certain micropollutant in the water with known concentrations of Cl-.