Adsorption of tetracycline from contaminated water using ZnO, montmorillonite, and ZnO/montmorillonite composites: adsorption kinetics and the role of pH zcp in adsorption capacity

Abstract The increasing pollution of water bodies by tetracycline (TC) has emerged as a looming threat to both environmental sustainability and human health, and the development of novel and effective remediation techniques is essential. The purpose of the present research was to explore the potential of montmorillonite (Mnt) and ZnO/Mnt composites as cost-effective and eco-friendly adsorbents for the removal of TC from polluted water sources. Batch adsorption experiments were carried out under controlled laboratory conditions, where adsorption isotherms, kinetic studies, and zero-charge point (pH zcp ) determinations were performed systematically to evaluate the performance of ZnO, Mnt, and ZnO/Mnt composites. The results highlighted the underlying importance of surface charge to adsorption by establishing pH zcp for ZnO, Mnt, and the ZnO/Mnt composite. The effects of pH on the surface charge of adsorbents (ZnO, Mnt, and the ZnO/Mnt) and the equilibrium structure of TC were measured systematically and trends that are imperative for understanding the dynamics of adsorption were identified. The removal efficiencies of TC at the optimal pH of 5 were 100% for Mnt, 70% for ZnO/Mnt, and 4% for ZnO. Mnt exhibited the greatest adsorption capacity (125 mg g –1 ), particularly effective within the pH range of 3–7, demonstrating its strong potential for pollutant removal. However, the ZnO/Mnt composite, although showing a lower adsorption capacity (72 mg g –1 ), offers additional advantages due to the photocatalytic properties of ZnO. Under light irradiation, ZnO promotes the mineralization of adsorbed TC into harmless products such as CO₂ and H₂O, thereby reducing the risk of secondary pollution. While Mnt alone efficiently captures TC, the lack of degradation may pose environmental challenges. By integrating adsorption with photocatalysis, the ZnO/Mnt composite provides a more sustainable, dual-functional approach, highlighting the significance of coupling pollutant capture with degradation for effective and eco-friendly water treatment.