Degradation of tetracyclines via calcium peroxide activation by ultrasonic: Roles of reactive species, oxidation mechanism and toxicity evaluation

Tetracyclines (TC) frequently detected in the aqueous environment pose threats to humans and ecosystems. The synergistic technology coupling ultrasound (US) and calcium peroxide (CaO2) has a great potential to abate TC in wastewater. However, the degradation efficiency and detailed mechanism of TC removal in the US/CaO2 system is unclear. This work was carried out to assess the performance and mechanism of TC removal in the US/CaO2 system. The results demonstrated that 99.2% of TC was degraded by the combination of 15 mM CaO2 with ultrasonic power of 400 W (20 kHz), but only about 30% and 4.5% of TC was removed by CaO2 (15 mM) or US (400 W) alone process, respectively. Experiments using specific quenchers and electron paramagnetic resonance (EPR) analysis indicated that the generation of hydroxyl radicals (•OH), superoxide radicals (O2-•), and single oxygen (1O2) in the process, whereas •OH and 1O2 were mainly responsible for the degradation of TC. The removal of TC in the US/CaO2 system has a close relationship with the ultrasonic power, the dosage of CaO2 and TC, and the initial pH. The degradation pathway of TC in the US/CaO2 process was proposed based on the detected oxidation products, and it mainly included N,N-dedimethylation, hydroxylation, and ring-opening reactions. The presence of 10 mM common inorganic anions including chloridion (Cl-), nitrate ion (NO3-), sulfate ion (SO42-), and bicarbonate ion (HCO3-) showed negligible influences on the removal of TC in the US/CaO2 system. The US/CaO2 process could efficiently remove TC in real wastewater. Overall, this work firstly demonstrated that •OH and 1O2 mainly contributed to the removal of pollutants in the US/CaO2 system, which was remarkable for understanding the mechanisms of CaO2-based oxidation process and its future application.