Adsorption of Ciprofloxacin onto Lotus Stem-Derived Biochar: Effects of Solution pH, Temperature, and Coexisting Cations
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 (pHpzc). 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 (Qmax) 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 Qmax 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.