Research on efficient removal of ciprofloxacin through sequential rice straw biochar modification via alkali activation and manganese oxides

Biochar is considered an environmentally friendly and cost-effective option for removing antibiotics from aqueous solutions. In this study, biochar derived from rice straw (RBC) was sequentially modified by KMnO4 and NaOH (Mn/Na-RBC) to achieve ciprofloxacin (CIP) removal. To explore the adsorption mechanisms between Mn/Na-RBC and CIP, the characterization analyses, adsorption isotherms, thermodynamics, and kinetics were investigated. The results revealed that Mn/Na-RBC significantly increased the specific surface area (154.12 m2·g–1), carbonization, and the number of oxygen-containing functional groups compared with pristine biochar, thereby substantially enhancing the CIP adsorption capacity. Through fitting of the adsorption model, it was found that the highest adsorption capacity was 32.25 mg·g–1 under acidic conditions (pH=3), and the pseudo-second-order and Freundlich model fit the adsorption behavior well. The main mechanisms of the adsorption process were pore filling, electrostatic interaction, H-bonding, π–π conjugation, n–π interaction and surface complexation. Moreover, the Mn/Na-RBC still exhibited a remarkable capacity to selectively adsorb CIP even in the presence of high ionic strengths. Thus, Mn/Na-RBC is a promising adsorbent for removing emerging contaminants from aquatic environments.