Sustainable removals of antibiotics via biochar-enhanced ultrasound cavitation effect: synergy of carbon nanotube bonded biochar@Fe3C composite and low frequency energy efficiency

Abstract The recalcitrant antibiotics of enrofloxacin (ENT) and amoxicillin (AMT) were difficult to remove by conventional sonication. To address this challenge, a new type of carbon nanotube covalently bonded biochar@Fe 3 C composite (BCM@Fe) was first designed by calcination and employed as a solid cavitation material (SCM) under low-frequency ultrasound (US) conditions to accelerate the removals of ENT and AMT. Compared to conventional carbon nanotube@Fe 3 C composites, BCM@Fe demonstrated significantly improved removal performance, achieving 15.5-fold and 3.50-fold higher removal rates for ENT and AMT, respectively. The removal efficiencies increased by 32.1–32.3% compared with a conventional shake system. Mechanistic studies revealed a dual removal mechanism involving simultaneous adsorption and degradation. The coupling of low-frequency ultrasound with BCM@Fe had synergistic effects; the US promoted the dispersion of the composites and inhibited H 2 O-induced oxidation by generating surface-localized cavitation bubbles. Notably, BC in BCM@Fe was found to amplify cavitation effect with performance strongly correlated with material characteristics such as pH, carbonization degree, aromaticity, hydrophobicity, and graphitization. Degradation differed between antibiotics: the degradation of ENT predominantly occurred at the material surface, while that of AMT took place in the liquid phase. Overall, the successful access to low-cost SCM integrating with low-frequency ultrasound made the possible for potential application in antibiotic wastewater. Graphical Abstract