Adsorption of Ciprofloxacin onto Lotus Stem-Derived Biochar: Effects of Solution pH, Temperature, and Coexisting Cations

Abstract

The discharge of ciprofloxacin (CFX) into aquatic environments poses a threat to ecosystem health. In this study, biochar derived from lotus stem (Nelumbo nucifera) was evaluated as a low-cost adsorbent for CFX removal from aqueous solution. The biochar was characterized by SEM/EDX, BET surface area, FTIR, and point of zero charge (pH_pzc). Batch sorption experiments were conducted to examine the effects of solution pH (3, 7, and 10), temperature (20, 26, and 32 °C), and coexisting cations. The kinetics of sorption could be explained using a two-stage intraparticle diffusion model, and the equilibrium isotherms were fitted to the Langmuir model. The maximum adsorption capacity (Q_max) was 22.96 mmol kg⁻¹ at pH 7 and 26 °C. Solution pH influenced the sorption capacity, yet substantial sorption was maintained across the entire pH range studied. Thermodynamic analysis revealed an endothermic, entropy-driven and spontaneous process at all temperatures. Divalent Ca²⁺ suppressed Q_max by 49.0% through competition with CFX for adsorption sites. The sorption process under pH 7 was facilitated by electrostatic attraction, hydrogen bonding, and π–π stacking interactions. Overall, unmodified lotus stem biochar proved to be a promising and cost-effective adsorbent for CFX removal from water.