ZIF-67/CQD nanohybrids for combined adsorptive and photocatalytic removal of tetracycline: kinetic, isotherm, and mechanistic insights

Abstract

The uncontrolled dissemination of antibiotic residues in aquatic environments necessitates the development of multifunctional materials capable of efficiently eliminating such pollutants through synergistic mechanisms. In this investigation, a series of zeolitic imidazolate framework (ZIF-67)/carbon quantum dot (CQD) nanohybrids were fabricated via a facile solvothermal strategy by modulating the CQD loading (5%, 10%, and 15% w/w) to optimize their adsorption and photocatalytic efficiencies toward tetracycline remediation. Structural, morphological, and physicochemical attributes were meticulously elucidated through X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, field-emission scanning electron microscopy (FESEM), energy-dispersive X-ray spectroscopy (EDS), Brunauer–Emmett–Teller (BET) surface area analysis, photoluminescence spectroscopy (PL), ultraviolet-visible diffuse reflectance spectroscopy (UV-vis DRS), and X-ray photoelectron spectroscopy (XPS), confirming the high crystallinity, uniform CQD dispersion, enlarged surface area, and improved charge-transfer dynamics of the composites. Among the synthesized samples, the 10ZQ (10% w/w CQD-loaded ZIF-67) composite exhibited the most pronounced performance, achieving 99% removal of 50 ppm tetracycline within 50 min under sunlight irradiation, corresponding to an apparent rate constant of k = 0.0057 min⁻¹. Adsorptive behavior was evaluated through comprehensive modeling using Langmuir, Freundlich, Temkin, Harkins–Jura, Halsey, and Dubinin–Radushkevich isotherms, along with kinetic models, including pseudo-first-order, pseudo-second-order, Elovich, liquid-film diffusion and intraparticle diffusion equations, and degradation kinetics, including zero-order, pseudo-first-order, and pseudo-second-order kinetics. The adsorption data fit well in the Langmuir model (R² = 0.9916) and Elovich kinetics model (R² = 0.9977), indicating homogeneous monolayer chemisorption. Photocatalytic degradation kinetics also closely followed the zero-order model (R² = 0.9932). Reactive species trapping experiments revealed that superoxide radicals (_˙O₂⁻) were the dominant oxidants in tetracycline oxidation. Furthermore, mineralisation analysis showed 76% TOC removal and 78% COD removal, confirming the substantial oxidation of tetracycline into harmless inorganic products. HRMS analysis unveiled the evolution of various intermediates via ring-cleavage, demethylation, and hydroxylation reactions, allowing a detailed mechanistic pathway to be proposed. Overall, this work introduces a robust dual-functional ZIF-67/CQD heterostructure exhibiting exceptional stability, recyclability, and activity for antibiotic removal, thereby establishing a promising platform for sustainable wastewater purification.