Review on the synthesis and activity of iron-based catalyst in catalytic oxidation of refractory organic pollutants in wastewater

Recently, an increasing number of publications concerned on the catalytic oxidation of refractory organic pollutants in wastewater dependent on the iron-based catalysts. Until now, the iron-based catalysts are diversity including Fe2+/Fe3+, ZVI, iron oxides (FeOOH, Fe2O3, Fe3O4, FeO), iron sulfides (FexSy) and Fe-bimetallic due to various valences and phases. A comprehensive understanding on the synthesis and applications of them as catalysts were little concerned. In this work, the methods for synthesis and control of the size, morphology, crystal phase and chemical valent properties of the iron-based catalysts were reviewed by classifying as Fe2+/Fe3+, ZVI, iron oxides, pyrite, Fe-MOF and Fe-bimetallic. Their applications in Fenton/Fenton-like oxidation, ozonation, wet air oxidation, PMS/PS activation, photocatalytic oxidation etc., for refractory organic pollutants oxidation were summarized. Generation of free radicals for oxidation of refractory organic pollutants in different systems was regarded as the main catalytic mechanisms. Refining controlled growth of size, morphology, crystal phase and chemical valence of iron-based catalysts will be prospective and challenge in synthesis of iron-based catalysts. Co-catalytic reaction will be promising for accelerating the electron transfer for generation of radicals. Quantitative analysis of the free radical is remained the challenge to understand the catalytic oxidation mechanism. We look forward to the fact that this review could inspire the readers to refine the synthesis of iron-based catalysts with special catalytic property for constructing the catalytic oxidation process and to understand the mechanism of degradation of refractory organic pollutants in wastewater.