Ultrasonic-Cavitation-Enhanced Biodegradation of Ciprofloxacin: Mechanisms and Efficiency

Ciprofloxacin (CIP), a persistent fluoroquinolone antibiotic, poses serious environmental concerns due to its low biodegradability and widespread presence in aquatic ecosystems. This study investigates the synergistic application of low-frequency ultrasonic cavitation and biological treatment to enhance CIP removal efficiency. Experiments have shown that under the optimal biological treatment conditions (6 g/L sludge concentration, pH 8), single biological treatment for 48 h can only remove 41.9% CIP and 24.9% total organic carbon (TOC). Ultrasonic pretreatment was conducted under varying frequencies and pH conditions to determine optimal cavitation parameters, while biodegradation performance was evaluated at different sludge concentrations and pH levels. Results indicated that in 10 mg/L CIP wastewater under alkaline conditions (pH 9.0), CIP and TOC removal efficiencies reached 58.9% and 35.2%, respectively, within 30 min using 15 kHz ultrasound irradiation. When ultrasonic pretreatment was followed by biological treatment, overall removal rates increased to 96.3% for CIP and 90.4% for TOC, significantly outperforming either method alone. LC-MS analysis identified several degradation intermediates during ultrasonic pretreatment, revealing key transformation pathways such as piperazine ring cleavage, hydroxylation, and defluorination. Furthermore, toxicity evaluation using the T.E.S.T. model confirmed a substantial reduction in ecological risk after ultrasonic treatment. Overall, the combined ultrasonic–biological process offers a cost-effective and environmentally sustainable strategy for the efficient removal of fluoroquinolone antibiotics from wastewater.