A review on ZnO and its modifications for photocatalytic degradation of prominent textile effluents: Synthesis, mechanisms, and future directions

Effective and eco-friendly approaches are essential for dealing with organic pollutants in industrial waste. Textile dyes are toxic and carcinogenic, and releasing them into the environment leads to water pollution, posing significant health risks to all living beings and endangering aquatic life. The use of semiconducting metal oxides for photocatalytic pollutant degradation has emerged as an advanced wastewater treatment strategy. The multifunctional nature of ZnO, stemming from its affordability, environmentally friendly characteristics, structure-dependent properties, and ability to fully mineralize pollutants, positions it as a superior photocatalyst compared to other materials. However, its primary drawback is poor performance in visible light. This review delves into various synthesis techniques and their effects on morphology, structure, and band gap, and its subsequent influence on the photocatalytic performance. The review highlights the effect of doping ZnO with various alkaline earth metals, transition metals, noble metals, rare earth metals, and non-metals, along with studies reported on ZnO-metal oxide nanocomposites for photocatalytic dye degradation, by providing insights into the photodegradation mechanism involved. Furthermore, this review extensively investigates the strategies to improve ZnO's efficiency in degrading textile dyes, namely cationic and anionic dyes, under visible light conditions. For thorough understanding, the influence of various operational parameters such as catalyst loading, irradiation source, dye concentration, reaction time, and reaction kinetics on photocatalytic efficiency have been investigated in detail. The review also addresses existing challenges and potential future directions, providing some insights for possible development in this field.