Fabrication of visible-light-active ZnCr mixed metal oxide/fly ash for photocatalytic activity toward pharmaceutical waste ciprofloxacin

Visible active photocatalysts have attracted interest as a potentially efficient and sustainable approach for pollutant degradation treatment. In this work, ZnCr mixed metal oxide/fly ash (ZnCr-MMO/FA) composites were prepared using fly ash (FA) from China coal mining power plants to improve photocatalytic performance. The formation of the ZnCr-MMO/FA composites was proved by XRD results which showed the two-phases combination of FA and MMO. Also, the presence of FA in the ZnCr-MMO/FA composite provided a good arrangement of small particles of ZnCr-MMO on the surface of FA and higher light absorption ability with a lower energy bandgap compared with the pure ZnCr-MMO. The photocatalytic performance of the prepared ZnCr-MMO and ZnCr-MMO/FA composites in different FA contents was evaluated by degradation of ciprofloxacin (CIP), which is an antibiotic and often found in pharmaceutical wastes, under visible light irradiation. Clearly, the ZnCr-MMO/FA composites showed higher photocatalytic decomposition of CIP than pristine ZnCr-MMO, and the highest degradation performance for CIP was obtained from the ZnCr-MMO/FA sample with 0.5 mg FA, which has a reaction rate constant approximately four times greater than that of pure ZnCr-MMO. The improved CIP degradation activity of the ZnCr-MMO/FA composite might not only be due to the good arrangement of ZnCr-MMO particles on the FA surface but also high separation and less recombination of photogenerated electrons-holes pairs, proven by the photoluminescence spectra, photocurrent density, and electrochemical impedance spectroscopy results. The lower recombination of the charge carriers of the ZnCr-MMO/FA composite might be caused by the heterojunction between the ZnCr-MMO as a main photocatalyst and FA. Thus, the proposed ZnCr-MMO/FA composite can be used as an effective photocatalyst for wastewater treatment, including the decomposition of organic pollutants, such as discarded pharmaceuticals.