Enhanced adsorption of ciprofloxacin from water using raw and phosphoric acid-modified cauliflower-leaves derived biochar: response surface optimization

Abstract

Ciprofloxacin (CIP), a widely prescribed fluoroquinolone antibiotic, has emerged as an environmental concern because of its persistence in aquatic systems, limited removal by conventional treatment processes, and role in the spread of antibiotic resistance. While biochar adsorption has gained attention for pharmaceutical removal, the use of cauliflower-leaf-derived biochar, particularly after chemical modification, has received limited investigation. Furthermore, studies integrating optimization of both biochar preparation and adsorption performance remain scarce. In the present study, raw cauliflower-leaf biochar (CLB10) and phosphoric-acid-modified biochar (PAM-CLB1.5) were evaluated for CIP removal from water using Response Surface Methodology based on a Box–Behnken experimental design. The optimized preparation conditions, i.e., pyrolysis time, temperature, and particle size for CLB10 were identified as 90 min, 475 °C, and 272.5 μm, respectively. Under optimized adsorption conditions, CLB10 achieved 78.43% CIP removal, whereas PAM-CLB1.5 exhibited substantially higher removal efficiency (97.54%), demonstrating the beneficial effect of phosphoric acid modification. Surface characterization showed that acid treatment enhanced pore development, increased specific surface area from 15.03 to 24.65 m2 g-1, and introduced phosphorus-containing functional groups that promoted stronger adsorbent–adsorbate interactions. CIP adsorption data were better described by the Langmuir isotherm model for both CLB10 and PAM-CLB1.5, suggesting predominant monolayer-type adsorption behavior under the investigated conditions. Kinetic data followed the pseudo-second-order model, indicating that adsorption was strongly influenced by surface-site-controlled interactions. Higher removal efficiency was attributed to the combined effects of pore filling, hydrogen bonding, electrostatic attraction, and π–π interactions. Regeneration experiments demonstrated satisfactory reusability, with the modified biochar retaining approximately 87% removal efficiency after five adsorption–desorption cycles. It was concluded that PAM-CLB1.5 could be used as an efficient adsorbent for the removal of CIP from water.