New notion of biochar: A review on the mechanism of biochar applications in advannced oxidation processes

As an economical and readily available carbonaceous material, biochar has gradually grown into an alternative candidate in catalysis field. A general cognition has been repeatedly reported that the functional structures on biochar were the active sites in heterogeneous catalytic systems (including oxygen-containing groups, defects, persistent free radicals (PFRs), etc.). Moreover, loaded metal component and doped heteroatoms could not only could tune the inherent properties of biochar, but also act be denoted as active sites in catalysis system. However, the detailed action mechanism of each active sites has rarely been investigated. To this end, the roles of biochar in persulfate activation system, Fenton like system, and photocatalytic system were summarized. The review concluded that 1) the oxygen-containing groups and PFRs on original biochar could favor the catalytic process through generating radicals; 2) Once metal was loaded on biochar, the loaded metal would grow as the prominent active site; 3) Doped heteroatom could work as active sites or induce other Lewis base site on biochar to promote the catalysis process. Furthermore, the doped heteroatom was usually associated with electron transfer process; 4) The increased graphitization degree and specific surface area were able to enhance the adsorption ability of biochar, which ultimately facilitated the catalytic process. Up to now, a myriad of studies has explored the effect of synthetic parameters on the functional structures of biochar, which provided sufficient basic data for the oriented synthesis of biochar. This review is devoted to bridge the gap between mechanism and synthesis of biochar and promote the application of biochar in the field of catalysis.