Effects and mechanism of diclofenac degradation in aqueous solution by US/Zn0

A system of ultrasound radiation coupled with Zn0 was applied to degrade diclofenac. The effects of initial pH, dosage of Zn0 and ultrasound density were investigated. To further explore the mechanism of the microcosmic reaction, the fresh and used Zn0 powders were characterized by SEM, XRD and XPS. Radical scavengers were used to determine the oxidation performance of strong oxidizing free radicals on diclofenac, including hydroxyl radicals and superoxide radicals. The results showed that the optimum removal of diclofenac reached to over 85% at pH of 2.0 in 15 min, with Zn0 dosage of 0.1 g/L and ultrasound density of 0.6 W/cm3. TOC removal of 72.6% in 15 min and dechlorination efficiency of diclofenac reached 70% in 30 min. Characterization results showed that a ZnO membrane was generated on the surface of Zn particles after use. According to the mass spectrometry results, several possible pathways of diclofenac degradation were proposed, and most diclofenac was turned into micro-molecules or CO2 finally. The synergistic effect of US/Zn0 in the reactions led to a proposed degradation mechanism in which zinc could directly attack the target contaminant diclofenac because of its good reducibility with the auxiliary functions of ultrasonic irradiation, mechanical shearing and free radical oxidation.