CuBTC–clay composites with tunable ratios for antibiotic removal: unraveling isotherm, kinetic, and thermodynamic study

Abstract

The growing contamination of water bodies with persistent antibiotics, such as tetracycline, presents a critical environmental challenge, demanding urgent and effective remediation strategies. The present investigation introduces a novel adsorbent, a hybrid composite of CuBTC and HNT clay, engineered for the highly efficient removal of tetracycline (TC) from wastewater. The CuBTC–HNT composite was synthesized in different ratios (1 : 1, 1 : 3, 3 : 1, and 1 : 5) and underwent extensive characterization using FESEM, EDS, FTIR, XPS, XRD, HRTEM, TGA, and BET surface area analysis. The adsorption process was carefully optimised using the synthesised hybrid composite as an adsorbent by adjusting crucial variables like dose, contaminant concentration, pH, temperature, stirring speed, and contact duration. Under optimized conditions, the composite demonstrated an outstanding adsorption efficiency of 94% of 25 ppm TC in 35 minutes within the pH range of 5–10. Moreover, reusability tests showed a consistent adsorption performance of 82% even after multiple cycles, reinforcing its sustainability and practical feasibility. Six different equilibrium isotherm models were employed: Freundlich, Temkin, Harkins–Jura, Halsey, Dubinin-Radushkevich, and Langmuir. Among these, the Langmuir model showed the best fit with a high correlation coefficient (R² = 0.9963), confirming monolayer adsorption primarily governed by physiosorption (adsorption energy: 6.13 kJ mol⁻¹). Mechanistic insights from after-adsorption characterization (XRD, FTIR, and FESEM-EDS) revealed key interactions, including π–π stacking, hydrogen bonding, electrostatic attractions, and pore filling. Kinetic studies were conducted using five models—pseudo-first-order, pseudo-second-order, elovich, intraparticle diffusion, and liquid film model—where the pseudo-second-order model best described the adsorption process (R² = 0.997), while thermodynamic analysis indicated that the process was endothermic, spontaneous, and entropically favourable (ΔH = 34.73423 kJ mol⁻¹, ΔG = −0.49777 kJ mol⁻¹, and ΔS = 0.109077 kJ mol⁻¹ K⁻¹). This research delivers a comprehensive and in-depth evaluation of an advanced adsorption system, bridging fundamental adsorption science with practical environmental applications which is an urgent global need for cleaner and safer water resources.