An insight into photo-catalytic degradation mechanism of persistent pollutants with transition metal oxides and their composites: Photocatalysis mechanism, rate affecting parameters, and removal pathways

Photocatalysis process in pollution control applications has concerned considerable attention given the increasing seriousness of environmental challenges including water, air, and soil. The present review of state-of-the-art research focuses on the current research trends in the development and application of transition metal oxides [TMOs]-based photocatalysis systems to improve the removal trends of organic pollutants from wastewater and photocatalytic mechanisms. For easy understanding, the up-to-date progress in the construction and design of heterojunction systems such as Type I/Type II heterojunction, p-n heterojunction, p-n-p heterojunction, Z-scheme heterojunction, and C-scheme heterojunction have been defined in detail into four categories based on the TMOs composition and mode of application. This review focuses on the detailed assessment of supported and suspended catalysts, their specific need, methodologies for performance evaluation, and the main factors that influence the removal mechanism. In addition, all the possible advanced and fundamental mechanisms of dye degradation, especially azo dye, triphenylmethane, and indigo dye with different categories of catalysts, are summarized. The current challenges and researcher’s approaches on how they successfully solve the drawbacks as well as future research trends in TMOs-based photocatalytic systems are discussed.