Adsorption of tetracycline onto a composite of avocado seed–derived activated carbon and Al-pillared Moroccan clay: kinetic, isotherm, and mechanistic study

Abstract

Adsorption of tetracycline from aqueous solution was investigated using a hybrid adsorbent composed of avocado seed-derived activated carbon (AC) and aluminum-pillared Moroccan bentonite (Al-PILC). Structural characterization by XRF, XRD, and FTIR confirmed successful clay pillaring and the development of oxygen-containing functional groups on the activated carbon surface. Batch adsorption experiments were performed under varying operating conditions to evaluate the influence of adsorbent dosage, pH, contact time, and initial tetracycline concentration. The optimum adsorption conditions were pH 6.5, adsorbent dosage of 100 mg, contact time of 180 min, and temperature of 30 °C. Under these conditions, the 50:50 composite (PC4) achieved a tetracycline removal efficiency of 94.2 ± 1.3%, compared with 98.5 ± 0.8% for AC and 52.3 ± 1.5% for Al-PILC. Adsorption kinetics were best described by the pseudo-second order model (R² > 0.99), indicating the predominance of chemisorption-related interactions. Equilibrium data followed the Langmuir isotherm model, with a maximum adsorption capacity of 87.4 mg g⁻¹ for PC4. The adsorption process involved pore filling, π–π interactions, hydrogen bonding, electrostatic attraction, and surface complexation. Although pure activated carbon exhibited the highest adsorption capacity, the composite provided improved structural stability while reducing activated carbon consumption. These results demonstrate that the Al-PILC/AC composite is an effective and sustainable adsorbent for tetracycline removal from contaminated water.