ZnO/Bentonite composites for environmental remediation: The influence of preparation method on composite structural characteristics and dye removal effectivity

This study aims to evaluate the effect of various ZnO/Bentonite (ZnO/B) composite preparation techniques on their structural characteristics and photocatalytic activity. ZnO/B composites were synthesized using three different methods, namely impregnation (ZnO/B-Imp), ion exchange (ZnO/B-Ion), and doping (ZnO/B-Dop). Each composite was characterized by Fourier-transform infrared spectroscopy (FTIR), X-ray diffraction (XRD), UV–vis diffuse reflectance spectroscopy (UV-Vis DRS), and its photocatalytic activity was tested against methylene blue (MB) under xenon lamp irradiation. FTIR results showed the successful dispersion of ZnO into the bentonite matrix, as indicated by the characteristic Zn-O vibration band observed at 449–438 cm −1 . XRD analysis revealed that the ZnO/B-Ion composite had a crystallinity of 50 %, which was higher compared to that of pure bentonite (40 %). The crystallite size of the ZnO phase in this composite was 38.63 nm. In contrast, ZnO/B-Dop showed damage to the Si-O-Al framework due to excessive doping, resulting in a smaller crystallite size of 33.14 nm with a higher crystallinity of 56 %. The ZnO/B-Imp composite had a crystallinity of 37 %, but no clear peaks of ZnO were detected, suggesting that ZnO was dispersed to the surface with small crystalline domains. The band gap energy of bentonite (4.20 eV) decreased to 3.22 eV and 3.78 eV in the ZnO/B-Imp and ZnO/B-Ion, respectively, while it increased to 5.45 eV in ZnO/B-Dop due to structural damage. Photocatalytic performance tests showed that ZnO/B-Ion provided the highest activity with 98 % MB removal in 60 min, surpassing ZnO/B-Imp (59 %) and ZnO/B-Dop (32 %). These results indicate that the ion exchange method produces ZnO/B composites with improved structural and functionality properties, which is critical to the development of effective photocatalysts for dye wastewater treatment.