Enhanced photocatalytic degradation of methylene blue dye via valorization of a polyethylene terephthalate plastic waste-derived metal–organic framework-based ZnO@Co-BDC composite catalyst

Abstract

The growth of industrialization contributes to the pollution of natural water bodies. Among these pollutants, organic dyes and plastic waste materials account for the majority of contaminants and are associated with health risks. This research explores a metal–organic framework (MOF)-based composite catalyst, ZnO@Co-BDC, for the degradation of methylene blue (MB). We developed a Co-BDC MOF synthesized through a solvothermal route using terephthalic acid (BDC) as a linker, which was extracted from polyethylene terephthalate plastic waste via alkaline hydrolysis, and using cobalt nitrate hexahydrate as a cobalt source. The ZnO@Co-BDC composite catalyst was synthesized through a solvothermal route using cobalt, BDC and zinc precursors. The obtained products were characterized using powder X-ray diffraction, transmission electron microscopy, Fourier transform infrared spectroscopy, and ultraviolet diffuse reflectance spectroscopy (UV-DRS). The performance of ZnO@Co-BDC was assessed for the degradation of MB and showed 87.5% under visible light irradiation for 80 min, surpassing pristine ZnO (74%) and Co-BDC MOF (39%) under the same conditions. The kinetic study indicated that the degradation followed first-order kinetics with a rate constant of 2.501 × 10⁻² min⁻¹. Furthermore, the effects of catalyst dose, irradiation time, pH, and MB concentration were optimized for the efficient composite catalyst ZnO@Co-BDC. The photodegradation mechanism was also investigated through UV-DRS and quenching experiments in the presence of different scavengers. Meanwhile, the developed composite demonstrated excellent recovery and reuse capabilities for up to six cycles under optimal conditions. The developed MOF-based composite catalyst enabled the simultaneous valorization of plastic waste and remediation of environmental pollutants by converting waste to wealth.