“…3.0) and formation of iron sludge that interrupts the Fe(II)/Fe(III) catalytic cycle. − Consequently, tremendous efforts have been devoted to developing various heterogeneous Fenton systems that overcome the abovementioned limitations. − Compared to the homogeneous version, the heterogeneous Fenton reaction involves complicated solid–liquid interface processes and mechanisms. − Elucidating the oxidation mechanism and degradation pathway of organic pollutants in these systems is of paramount importance, not only for a deeper understanding from a fundamental perspective but also for the design of more efficient heterogeneous Fenton systems from a practical perspective. A well-accepted protocol is to examine the nature of the reactive species (e.g., HO · , high-valent metal-oxo species) that are derived from the heterogeneous metal–H 2 O 2 complex (M–OOH, M is the active metal site) ,, and then to evaluate the contribution of each reactive species to the degradation of one or several specific compounds, assuming that the activation of H 2 O 2 and the subsequent pollutant degradation are separate processes. , Some recent studies on other advanced oxidation processes have indicated that when the organic compounds are able to form strong interaction with the catalyst, both processes, i.e., the oxidant activation and the pollutant degradation, might entangle with each other, resulting in more abundant oxidation mechanisms and degradation pathways of the target molecules. , However, such a possible role of the interaction between the organic compounds and the catalyst in the oxidation mechanism/pathway has been scarcely considered in classic H 2 O 2 -based heterogeneous Fenton systems. The elucidation of the mechanisms involved in the heterogeneous Fenton reaction that considers the interaction between the pollutant and the catalyst could better reflect the real water treatment scenarios in which the pollutants are diverse in molecular structures, some of which could form strong interaction with the catalyst.…”