Generation mechanisms of active free radicals during ciprofloxacin degradation in the ultrasonic/K2S2O8 system

Ciprofloxacin (CIP) removal efficiency in aqueous solutions in the ultrasonic (US), K2S2O8, and US/K2S2O8 systems was investigated. The free radical generation and action ratio were studied based on variations of K2S2O8 concentration, ultrasonic power, pH, and the addition of isopropanol (ISP) or tert-butyl alcohol (TBA) in the US/K2S2O8 system. The results showed that under conditions of 20 mg·L−1 CIP concentration, 20 mmol·L−1 K2S2O8 concentration, an ultrasonic power of 360 W and pH = 7, CIP removal efficiency in the US/K2S2O8 system was 92.20% after 180 min. The reaction in the US/K2S2O8 system was explicitly divided into two stages: free radical generation and pollutants degradation. The ultrasonic and chain reaction facilitated enhanced generation of SO4−• and HO•. The presence of K2S2O8 can promote HO• generation and K2S2O8 concentration also exerted a significant effect on SO4−• generation, however, high concentrations were not beneficial to the reaction. Quenching reactions occurred under high concentrations of HO• and SO4−•. During the initial stage of the reaction, HO• played a more prominent role than SO4−•, however, the role of SO4−• gradually increased as the reaction proceeded and eventually surpassed HO•.