Waste fly ash–ZnO as a novel sunlight-responsive photocatalyst for dye discoloration

Treating waste with a waste material using freely available solar energy is the most effective way towards sustainable future. In this study, a novel photocatalyst, partly derived from waste material from the coal industry, was developed. Fly ash hybridized with ZnO (FA–Zn) was synthesized as a potential photocatalyst for dye discoloration. The synthesized photocatalyst was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, and ultraviolet–visible/near infra-red spectroscopy. The photocatalytic activity was examined with the discoloration of methylene blue used as synthetic dye wastewater. All the experiments were performed in direct sunlight. The photocatalytic performance of FA–Zn was found to be better than that of ZnO and the conventionally popular TiO₂. The Langmuir–Hinshelwood model rate constant values of ZnO, TiO₂, and FA–Zn were found to be 0.016 min⁻¹, 0.017 min⁻¹, and 0.020 min⁻¹, respectively. There were two reasons for this: (1) FA–Zn was able to utilize both ultraviolet and visible parts of the solar spectrum, and (2) its Brunauer–Emmett–Teller surface area and porosity were significantly enhanced. This led to increased photon absorption and dye adsorption, thus exhibiting an energy-efficient performance. Therefore, FA–Zn, partly derived from waste, can serve as a suitable material for environmental remediation and practical solar energy applications.