Transforming wastes into functional photocatalysts for effluent treatment: advancing circular economy practices

Abstract The concept of circular economy (CE) strongly advances sustainability by promoting the reuse of waste materials, thereby extending their operational lifetime. It would be an innovative idea to use these waste materials for waste treatment using freely available solar energy. This can be achieved through photocatalysis, which uses semiconductor materials to generate powerful radicals that can oxidize and/or reduce pollutants. Conventional photocatalysts must be synthesized, which incurs raw-material costs. Waste-derived photocatalysts, however, not only serve as suitable functional materials but also facilitate trash reduction, reuse, and recycling- aligning with fundamental waste management principles and environmental sustainability. This article reviews the performance of such semiconductor materials derived from wastes to realize and implement CE principles into practice, thereby contributing to SDGs 6, 7, 9, and 12. The first part of this article includes an indepth discussion on the basics of photocatalysis, methods to enhance photocatalytic activity and minimise recombination, synthesis techniques, and the influence and optimization of key parameters affecting photocatalytic performance. The second part provides a critical review of waste materials that can be transformed into photocatalysts. Finally, directions for industrialization, SWOC analysis, and future perspectives are outlined. This review offers a comprehensive reference on developing efficient photocatalysts from waste materials, addressing existing gaps and supporting future research and commercialisation.