Enhanced adsorption of ciprofloxacin by biochar with acid and alkali modification: Performance comparison and mechanism analysis

The widespread presence of antibiotic residues in aquatic environments, resulting from the extensive use of antibiotics in livestock and aquaculture, has driven the search for effective remediation methods. Adsorption, particularly using biochar, has emerged as a promising approach. In this study, rice straw-derived biochar (denoted as B5) modified with phosphoric acid (H 3 PO 4 -modified B5, PB5) and potassium hydroxide (KOH-modified B5, KB5) at the same concentration was investigated for ciprofloxacin (CIP) adsorption. PB5 exhibited a maximum Langmuir adsorption capacity of 25.6 mg/g for CIP, surpassing KOH-modified (8.77 mg/g) and unmodified biochar (3.71 mg/g). The optimized pH for adsorption was 3.0–7.0, where electrostatic interactions and hydrogen bonding dominated. Characterization revealed that acid-base modification altered the surface morphology, pore structure, and functional groups of biochar. PB5 exhibited a rougher surface, increased porosity, and enhanced oxygen-containing functional groups, while KB5 had a more pronounced increase in large specific surface area (SSA). Adsorption experiments indicated a multistage process involving chemisorption, external diffusion, and intraparticle diffusion. The pseudo-second-order model and the Freundlich isotherm best described the adsorption behavior, indicating chemisorption-dominated non-homogeneous adsorption. Density Functional Theory (DFT) calculations and X-ray photoelectron spectroscopy (XPS) analysis identified the -COOH group on biochar as the main active site for CIP adsorption, with a strong dual-force interaction. pH and anion studies showed that pH affected adsorption via electrostatic interactions, and divalent anions, especially SO 4 2− , inhibited adsorption, more so for PB5 due to its larger pore size. This study elucidates the distinct adsorption mechanisms of acid and alkali-modified biochar for CIP, providing valuable insights for the design and optimization of biochar for antibiotic removal in water treatment, Further research should explore diverse antibiotic models and advanced research tools to address real-world environmental challenges. • Preparation and structural characterization of acid/base-modified biochar. • Comparison of acid/base modification effects on ciprofloxacin adsorption performance. • Analysis of adsorption kinetics and heterogeneous isotherm models. • Investigation of pH and anion effects on adsorption inhibition/promotion mechanisms. • DFT-based identification of carboxyl groups as active adsorption sites.