Advanced oxidation processes (AOPs) have attracted much attention in the field of water or wastewater treatment. As one of the most investigated AOPs, Fenton reagents have notably advantages of convenient operation, mild condition and high performance of degradation. Iron oxide catalyzed Fenton-like reaction, which can be operated effectively in wide range of pH values with convenient catalyst separation and reutilization, has been intensively investigated as one of the most promising developments of Fenton-like reaction during the past two decades. However, when compared with traditional Fenton reagents, these iron oxides initiated Fenton-like processes often encounter much more complicated reaction steps. The main mechanisms are reviewed, including radical mechanism, oxygen vacancies mechanism and high-valent iron species mechanism. The rate of Fe(Ⅱ) generation or Fe(Ⅲ) reduction in pure iron oxides is greatly limited and the circulation of Fe(Ⅲ)/Fe(Ⅱ) has demonstrated to be the rate-controlling steps of Fenton-like reaction. In order to accelerate this limiting step and improve the reactivity of Fenton-like reaction, modified catalysts including multivalent iron mixtures and transition metal doped iron oxides are developed. Before describing the catalytic performance, effects of modification on the morphology, structure and element composition of iron oxides are examined profoundly. Results show that the speciation of iron, the specific surface area of catalysts and the electron transfer between H₂O₂ and iron oxide are all playing an important role in the reactivity of Fenton-like reaction. The future development and investigations of heterogeneous catalysts are also discussed. Contents
1 Introduction
2 Mechanism and kinetics for the Fenton-like reaction catalyzed by iron oxide
2.1 Radical mechanism
2.2 Active site mechanism
2.3 Oxygen vacancies mechanism
2.4 High-valent iron species mechanism
3 Production or regeneration of Fe(Ⅱ)_surf-the rate-determining step
4 Modification of iron oxides
4.1 Multivalent iron mixture
4.2 Transition metal doped iron oxide
5 Conclusion and outlook