Modelling study on the effects of chloride on the degradation of bezafibrate and carbamazepine in sulfate radical-based advanced oxidation processes: Conversion of reactive radicals

The degradation rates of pharmaceuticals contaminants (such as bezafibrate and carbamazepine) in the UV/S 2 O 8 2− process at different Cl − concentrations (0.01, 0.1, 1.0, 10, and 100 mM) and pH values (3, 5, 7, 9 and 11) were investigated in this study. The degradation rate of bezafibrate showed an upward trend before the critical point (Cl −  = 1.0 mM), the maximum enhancement was 51%, then the degradation rate decreased. The degradation rate of carbamazepine increased continuously, and the maximum enhancement was 66% when the Cl − concentration was 100 mM. The degradation rate of bezafibrate showed the maximum value at pH = 7 while the most rapid degradation rate of carbamazepine was observed at pH = 3. To explain these interesting results, a kinetic model was used to investigate the conversion of reactive radicals (such as SO 4 − , OH , Cl , and Cl 2 − ) at different Cl − concentrations and pH values. Simulation results showed that low concentrations (<1.0 mM) of Cl − transformed SO 4 − into OH and Cl . With the further increased concentration of Cl − from 1.0 mM to 100 mM, OH and Cl can be converted into Cl 2 − . Furthermore, Cl 2 − was the dominant reactive species when pH < 7, whereas OH dominated when pH > 7. Few organic chlorinated products were observed in the UV/S 2 O 8 2− /Cl − process, indicating that the reactions between bezafibrate/carbamazepine and the reactive chlorine radicals Cl /Cl 2 − occurred mainly through electron transfer and H-extraction, whereas the substitution reaction leading to the formation of organic chlorinated products was not dominated. This study positively demonstrated the absence of chlorinated product formation during SR-AOPs application.