Comparative study of the degradation of the 2,4-dichlorophenoxyacetic acid herbicide using zinc oxide and zinc oxide/graphene oxide photocatalysts: efficiency, stability, toxicity, kinetics and mechanism insights

This study investigated the degradation of the herbicide 2,4-dichlorophenoxyacetic acid (2,4-D) in aqueous solution through heterogeneous photocatalysis under simulated solar irradiation. For this purpose, zinc oxide (ZnO), graphene oxide (GO), and a ZnO/GO composite were synthesized and characterized using Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), dynamic light scattering (DLS), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), diffuse reflectance spectroscopy (DRS), and photoluminescence (PL) analyses. Compared with pure ZnO, the ZnO/GO composite exhibited enhanced structural, morphological, and optical properties owing to the incorporation of GO. These improvements included a reduction in particle size (from 15.565 to 9.528 µm), a more homogeneous dispersion of ZnO particles on the GO sheets, enhanced visible-light absorption, and more efficient separation of photogenerated charge carriers. As a result, the composite demonstrated superior photocatalytic performance, achieving 96.6% degradation of a 2,4-D aqueous solution (10 mg L −1 ) within 150 min using a photocatalyst dosage of 0.25 g L −1 . Under the same experimental conditions, pure ZnO achieved a degradation efficiency of 88.5%. Furthermore, the ZnO/GO composite showed a greater mineralization capacity, reaching 84.82% total organic carbon (TOC) removal, whereas ZnO achieved 73.60% TOC removal. The chemical stability of both photocatalysts was confirmed over three consecutive reuse cycles. In addition, the treated solutions obtained after photocatalytic degradation using either ZnO or ZnO/GO did not exhibit phytotoxic effects toward Deva lettuce seeds ( Lactuca sativa ). However, the relative growth index (RGI) obtained for the solution treated with the ZnO/GO composite was approximately 3.2 times higher than that observed for ZnO. Overall, the ZnO/GO composite outperformed pure ZnO in all evaluated aspects, demonstrating that the incorporation of GO enhances the intrinsic properties of ZnO and contributes to the development of a more efficient photocatalytic material for the degradation of emerging contaminants in water.