Effective Composites Based on Chitosan-Coated Activated Bentonite for Tetracycline Removal from Wastewater

In this study, a composite adsorbent (CS-ABnt) based on activated bentonite and chitosan was prepared using a simple coating method in the form of beads designed to combine high adsorption performance with the ease of solid–liquid separation typical of bead-type materials, serving as a low-cost tetracycline adsorbent. The composite was characterized by FTIR, EDX, and zeta potential measurements, which confirmed successful chitosan coating of the bentonite surface and a marked shift in the point of zero charge (from pH 5.54 for chitosan to pH 7.32 for CS-ABnt). Batch adsorption experiments showed that tetracycline removal was strongly pH-dependent, with an optimum near pH 6, and that the CS-ABnt composite consistently outperformed its individual components (chitosan and activated bentonite) across all tested doses, contact times, and temperatures. Equilibrium data were best described by the Langmuir isotherm, yielding a maximum adsorption capacity of 49.11 mg·g−1, while kinetic data followed the pseudo-second-order model, revealing that the rate-limiting step is chemisorption. The thermodynamic parameters (ΔG° < 0, ΔH° > 0, ΔS° > 0) indicated a spontaneous, endothermic adsorption process. Taken together, these results enabled a 95% removal efficiency of tetracycline to be achieved.