Ciprofloxacin sorption by non-activated and activated biochar derived from millet husks and water hyacinth

Ciprofloxacin (CIP) is one of the antibiotics detected in significant amounts in the environment, and poses adverse toxicity risk to aquatic and soil organisms. The study objectives were to prepare and characterize activated and non-activated biochar materials, establish the optimum conditions for batch sorption experiments, and determine the sorption capacity and efficiency of the biochars for CIP. Water hyacinth and millet husks were used to prepare the biochars by slow pyrolysis at 350 °C and 500 °C, and KOH was used for activation. The biochar materials were characterized using Boehm titration, Fourier Transform-Infra Red Spectroscopy (FT-IR) and Scanning Electron Microscopy (SEM). Moisture content, ash content, pH and bulk density of the biochar were also determined. Batch equilibrium sorption experiments were used to study the sorption process and efficacy of CIP by the biochars. The results showed the percentage (%) yield of water hyacinth biochar (WBC) and millet husks biochar (MBC) prepared at 350 and 500 °C were WBC350 (48.78%) > WBC500 (42.28%) > MBC350 (38.25%) > MBC500 (36.13%). The optimum conditions for batch sorption experiments were 24 h equilibration time, solution pH of 8, biochar dosage of 0.4 g L-1, and CIP concentration of 5 mg L-1. Sorption data was modeled into Langmuir and Freundlich models for sorption isotherms; and pseudo-first order and pseudo-second order kinetic model for sorption kinetics, while the intraparticle diffusion model was used to describe the diffusion mechanisms. Activated biochar prepared at 500 °C showed higher removal efficiency (83–88%) compared to non-activated biochar prepared at both 500 °C and 350 °C (50–70%). Sorption kinetics was best described by pseudo-second-order kinetics (R2 > 0.99), while the sorption process could be best described by the Freundlich isotherm ((R2 > 0.97), and intraparticle diffusion models. Therefore, multilayer sorption of CIP on heterogeneous biochar surfaces was the most plausible sorption mechanism. It could be concluded that biochar derived from water-hyacinth and millet husks can be used as adsorbents for ciprofloxacin removal in contaminated water.