Eco-Friendly Tetracycline Remediation Using Robust and Highly Reusable ZIF-67/g-C <sub>3</sub> N <sub>4</sub> Nanocomposites with Coupled Adsorption and Photocatalytic Pathways: A Deep Dive into Isotherms, Kinetics, Thermodynamics, and Degradation Pathways
Palkaran Sethi, Sanghamitra Barman, Soumen Basu
Abstract
High Resolution Image Download MS PowerPoint Slide The growing prevalence of tetracycline (TC) in aquatic environments necessitates the development of multifunctional materials that can effectively perform both adsorption and photocatalytic degradation. In the current investigation, zeolite imidazole framework (ZIF-67)/graphitic carbon nitride (g-C 3 N 4 ) nanocomposites were synthesized via an in-situ approach at varying their weight ratios (1:1, 1:3, and 3:1), and evaluated for their performance in removing TC from water. Structural and morphological characterizations (X-ray diffraction (XRD), Fourier transform infrared (FTIR), field emission scanning electron microscopy (FESEM), high resolution transmission electron microscopy (HRTEM), Mott Schottky, and Brunauer–Emmett–Teller (BET)) confirmed successful formation of a heterojunction, with enhanced surface area and crystallinity. Ultraviolet–visible diffuse reflectance spectroscopy (UV–vis DRS) analysis revealed enhanced visible light absorption and a narrowing of the band gap to 1.93 eV in the optimal composite. Photoluminescence (PL) spectra with Time-Resolved PL showed reduced electron–hole recombination, with the 3:1 composite exhibiting the lowest emission intensity and increased average lifetime. The influence of key operational factors─pH, catalyst loading, initial pollutant concentration, reaction time, and light─was systematically investigated. Among the prepared composites, the ZC31 sample exhibited the best performance, achieving 98.1% degradation of 25 ppm TC within 120 min under visible light, under the optimized conditions with a high-rate constant of 0.00174 min –1 . Adsorption behaviour was meticulously assessed through six isotherm models (Freundlich, Temkin, Langmuir, Harkins–Jura, Dubinin–Radushkevich, and Halsey) and five kinetic models (pseudo-second-order, Elovich, pseudo-first-order, liquid film diffusion, and intraparticle diffusion). Furthermore, the kinetics of photocatalytic degradation were analyzed through three models: pseudo-first-order, pseudo-second-order, and zero-order kinetics. The adsorption behaviour followed the Langmuir isotherm ( R 2 = 0.9908) and intraparticle diffusion kinetics ( R 2 = 0.99398). Photocatalytic degradation under visible light followed zero-order kinetics ( R 2 = 0.99255), with the best-performing composite achieving 98.1% TC degradation in 120 min. Thermodynamic evaluation demonstrated the process to be spontaneous and endothermic, with Δ G 0 = −4.48881, Δ H 0 =15.63032, and Δ S 0 = 0.0623. Reusability tests demonstrated stable photocatalytic performance over six cycles, whereas post-degradation characterizations (XRD and FESEM-EDS) confirmed structural stability. Mineralization was validated through significant TOC and COD reductions of 68.9% and 69.4%, respectively. Moreover, HRMS analysis identified key intermediates and proposed a plausible degradation pathway involving hydroxyl ( • OH) radicals as revealed by scavenger studies. This work emphasises the potential of ZF/CN nanocomposites as a robust, reusable, and visible-light-active platform for eco-friendly remediation of antibiotic-contaminated water.