Highly Efficient Tetracycline Degradation under Simultaneous Hydrodynamic Cavitation and Electrical Discharge Plasma in Flow

Abstract

Tetracyclines are widely used antibiotics that are often carelessly released into the environment, posing a potential threat to the ecosystem. Due to the lack of efficient methods to remove tetracyclines from wastewater, a remarkable research effort on the remediation of tetracyclines has been undertaken. The synergistic effect of hydrodynamic cavitation (HC) and electrical discharge (ED) plasma on the degradation of a number of antibiotics in water has been studied in this work. Catalyst- and oxidant-free processes have been carried out using a new pilot-scale hybrid device (HC/ED plasma) working in flow-through mode (330 L/h). Tetracycline hydrochloride (TC), doxycycline hyclate (DC) and oxytetracycline dihydrate (OC) were selected as the model compounds. Antibiotic degradation tests were performed using a 5 L water solution at various antibiotic concentrations (10, 25, 50, 75, and 100 mg/L). All experiments were performed over 15 min, and samples were either collected using the flow-through method, or every 5 min when using the loop configuration. The temperature was kept constant at 30 °C, with fluctuations of ±2 °C. The influence of applied HC input pressure (45, 60, and 70 bar), applied ED amplitude frequency (10 and 48 kHz) and the pH values of the initial solutions (2 and 11) on antibiotic degradation rate have been investigated. Near quantitative TC (>98%), DC (98%) and OC (95%) degradation was documented after only 15 min of combined HC/ED plasma treatment at 70 bar and 48 kHz. To better understand the synergistic effect of coupled HC/ED treatment on antibiotic degradation, a dosimetry assay was performed to quantify the oxidizing compounds generated by this technology. Specifically, the coupled HC/ED plasma treatment was able to linearly increase the amount of oxidants in water as a function of time, reaching a maximum concentration of 13.426 mmol/L after 15 min, which is more than 85-times higher than that of HC alone (0.153 mmol/L). This study demonstrates the impressive efficiency of hybrid HC/ED plasma technology in degrading recalcitrant antibiotics in wastewater without the need for catalysts and oxidants.