This study reports the synthesis of calcium oxide nanoparticles supported activated carbon from the algal–bacterial activated sludge by using eco-friendly and non-toxic reagents. The prepared activated carbon was employed as competent material for the antibiotic compound ciprofloxacin (CIP) removal from the liquid phase. The synthesized activated carbon has been characterized by using Brunauer–Emmett–Teller, scanning electron microscopy–energy-dispersive X-ray analysis, Fourier transform infrared spectroscopy, X-ray photoelectron spectroscopy, and X-ray powder diffraction analysis. The sol–gel method was used to synthesize calcium oxide nanoparticles utilizing white chicken eggshells as the calcium source. Neem (Azadirachta indica) leaf extract was used as the promoter during the synthesis. The adsorption studies show that the pH of the solution has a substantial impact on the adsorption of CIP. Maximum CIP removal of 89.5% was obtained at pH 4.0. The Langmuir adsorption isotherm was found to be well fitted to the experimental results. It was observed that the maximum adsorption capacity was 8.1 mg/g. The adsorption kinetics were best described by a pseudo-second-order kinetic model. The most plausible CIP removal mechanisms involved cation exchange, cation–π interaction, hydrophobic effect, and hydrogen bonding. © 2022, The Author(s) under exclusive licence to Iranian Society of Environmentalists (IRSEN) and Science and Research Branch, Islamic Azad University.