Degradation of Tetracyclines by Ultrasonic Catalyzed Calcium Peroxide Process: Roles of Reactive Species, and Oxidation Mechanism

Tetracyclines (TC) are frequently detected in aqueous environment due to its resistance to traditional biological treatment. This study proposed an innovative synergistic technology coupling ultrasound (US) and calcium peroxide (CaO2) to abate TC in wastewater. The results demonstrated that 99.2% of TC was degraded in US/CaO2 system, but only about 30% and 4.5% of TC was removed by alone CaO2 and US process, respectively. The analysis of specific quenchers and electron paramagnetic resonance (EPR) experiments indicated that the generated reactive oxidizing species in US/CaO2 system included hydroxyl radicals (●OH), superoxide radicals (O2-●), and single oxygen (1O2), whereas ●OH and 1O2 were mainly responsible for the degradation of TC. The removal of TC in US/CaO2 system has close association with the ultrasonic frequency, dosage of CaO2 as well as TC, and the initial pH. The degradation pathway of TC in US/CaO2 process was proposed based on the detected oxidation products. Common inorganic anions including chloridion (Cl-), nitrate ion (NO3-), sulfate ion (SO42-) showed negligible influences on the removal of TC in US/CaO2 system, and US/CaO2 process showed satisfactory degradation of TC in natural wastewater. Overall, this work demonstrated that multiple reactive oxidizing species (●OH, 1O2 and O2-●) were formed in US/CaO2 system, which was remarkable for the application of US/CaO2 technology and understanding the mechanisms of CaO2-based oxidation process.