Ball milling boosted magnetic cotton husk-derived biochar adsorptive removal of oxytetracycline and ciprofloxacin from water

Oxytetracycline (OTC) and ciprofloxacin (CIP) contamination have caused serious risks to human health, and modified biochar is considered as a potential adsorbent for their removal. The typical agricultural waste of cotton husk was used as the feedstock, then was combined with γ-Fe₂O₃ and ball milling to innovatively synthesize nano zero-valent iron (nZVI) supporting magnetic cotton hush-derived biochar (Fe₂O₃@BMBC) by reductive calcination. Fe₂O₃@BMBC performed excellent adsorption performance with a maximum adsorption capacity for OTC (266.7 mg·g⁻¹) and CIP (83.36 mg·g⁻¹), and its adsorption capacity was 1.6 and 2.3 times that of cotton husk biochar (BC). Characterization analysis showed that Fe₂O₃@BMBC contained abundant oxygenated functional groups (e.g., -OH, C = O and Fe–O) and its surface was covered by diverse iron oxides. The high magnetization sensitivity of Fe₂O₃@BMBC guaranteed that it was easily to be separated by a magnet. Oxygenated functional groups (e.g., -OH, C = O and Fe–O) participated in the adsorption process, and solution pH significantly affected the adsorption behaviour, and pseudo-second-order model and Freundlich model better fitting the kinetics and isotherms data. These results confirmed that π-π conjugation, H-bonding, Fe–O complexation and electrostatic interactions contributed to the greater adsorption capacity of Fe₂O₃@BMBC. Ethanol + ultrasound could efficiently regenerate the used Fe₂O₃@BMBC and maintain its sustainable adsorption performance for OTC and CIP. Additionally, Fe₂O₃@BMBC performed a good environmental security in a wide pH range (from 3 to 11) in view of the low leaching risk of Fe.