Emerging applications of waste fly ash for remediation of environmental contaminants: a high-value and sustainable approach towards utilization of waste materials

Fly ash is a greyish-black powdery substance easily available and produced as a side product of burning any organic substance. A huge amount of fly ash is generated worldwide, and today’s world faces a severe problem of its disposal. Owing to its various applications, researchers began utilizing fly ash as a catalyst to solve environmental problems. Due to the presence of metal oxides in fly ash, it acts as a good adsorbent. It can be used to support semiconductor photocatalysts in degrading pollutants from wastewater under light irradiation. The current review discusses the preparation methods of fly ash-based materials such as impregnation, wet chemical synthesis, electrospinning, modified meta-organic decomposition, impregnation, sol–gel, layer-by-layer assembly, precipitation/co-precipitation and hydrothermal/solvothermal technique. The application of fly ash and related materials for photocatalytic degradation of organic pollutants and the factors that affect the photodegradation, mechanism, kinetics of photodegradation, and approaches to make photocatalytic degradation more efficient have been reviewed. The techniques used to characterize fly ash nanocomposites, like powdered X-ray diffraction technique, Fourier transform infrared spectroscopy, and scanning electron microscopy, have been discussed to understand the topic better. Furthermore, the applicability of the fly ash and modified fly ash for the sorption of volatile organics, SOx, NOx, and mercury from flue gases has also been summarized. Moreover, the review also provides a scope for further research in the field of photodegradation and air remediation using fly ash to develop robust and low-cost materials for environmental remediation.