Mechanistic Insights into Photocatalytic Degradation of Organic Pollutantsby Semiconductor-based Inorganic Catalysts: A Systematic Review

More organic pollutants in water sources are now a major global environmental challenge. Photocatalysis using semiconductor materials is now being seen as a great way to get rid of persistent pollutants with mild methods. The way photocatalysis works is affected by several mechanisms, including generating electron-hole pairs, creating reactive oxygen species (ROS) and how those charges move. To use these systems in real applications, it is important to understand how they work. This systematic review aims to examine how semiconductorbased catalysts degrade organic pollutants during photocatalysis and what affects their efficiency. A goal of the review is to find holes in prior studies and detail the benefits of understanding the mechanisms for the practical use of photocatalysis in wastewater treatment. A search of the literature was done in Scopus, Web of Science, ScienceDirect, PubMed and Google Scholar. Research from the last 10-15 years was analysed, looking at original studies involving TiO₂, ZnO, CdS as photocatalysts and the degradation of organic pollutants. Only studies about photocatalysis, experimental settings, and catalysts were included, while studies that were reviews, written outside of English, or not connected to the subject were excluded. A total of 20 studies were selected using our screening and quality assessment process. The process of photocatalytic degradation was described in the review, and the main steps found include photogeneration of electron-hole pairs in the semiconductor and the formation of ROS. Morphology, crystallinity, and doping of the catalyst strongly affected how effective the photocatalysis was. The intensity of light, the temperature, and pH all strongly affected how fast the chemicals degraded. Photocatalysis may solve many of the problems from organic water pollution, yet there are still difficulties with keeping catalysts stable and effective. Research measuring improvements in catalysts and in the way we operate photocatalytic systems is necessary to use them on a larger environmental scale.