A critical review of different types of biochar-based catalysts and mechanisms in advanced oxidation processes for organic contaminants removal

Biochar (BC)-assisted advanced oxidation processes (AOPs) offer an unprecedented opportunity for organic contaminant remediation because of their high efficiency and environmental friendliness. However, the catalytic mechanisms for different BC-assisted AOPs need to be further explored, considering that each type of BC catalyst possesses unique active sites and physicochemical properties, which influence associated catalytic mechanisms. Most existing reviews have focused on individual types of BC or AOPs, especially in relevance to mechanism. Furthermore, there is a lack of detailed understanding of the catalytic mechanisms derived from the various active sites in BC catalysts which restrict the ease of evolving design strategy for ideal catalytic structures. Therefore, this review evaluates and presents the current knowledge on the relevant active sites of BC-based catalysts, feasibility for generation of reactive oxygen species, and the associated catalytic mechanisms involved in AOPs for the four most known types of BC, which are pristine BC (P-BC), non-metallic heteroatom-doped BC (NM-BC), metal BC composite (M-BC), and metal and heteroatom co-doped BC (MHC-BC) catalysts. The factors influencing the catalytic properties of BC materials are systematically discussed, while highlighting the relationships with associated mechanisms and synthetic conditions. Finally, the current challenges and prospects associated with BC-AOPs are outlined. Overall, this review aims to inspire the rational design of more advanced BC-based catalysts for AOPs in the remediation of organic contaminants in the environment.