The effect of transition metal substitution on the catalytic activity of magnetite in heterogeneous Fenton reaction: In interfacial view

Recently, growing interest has been evoked in the application of transition metal substituted magnetite as heterogeneous Fenton catalyst in the degradation of organic pollutants. In the present study, the effect of substitution on the heterogeneous Fenton catalytic activity of magnetite was investigated in interfacial view. The substitution of transition metals improved the degradation of organic pollutants by accelerating the OH free radical generation and enhancing the adsorption of pollutant and H2O2. As the octahedral sites were exclusively exposed at magnetite surface, V3+ and Cr3+ occupying the octahedral sites not only participated in H2O2 decomposition to produce OH, but also quickened the electron transfer in the inverse spinel structure to reduce Fe3+ to Fe2+ with higher Fenton activity. Ti4+ , V3+ and Cr3+ increased the specific surface area and surface hydroxyl amount of magnetite, resulting in larger adsorption of methylene blue (MB) and H2O2 and accordingly faster degradation. In Ti-V co-doped magnetite, though Ti4+ was thermodynamically unfavorable for OH generation, Ti4+ showed a more prominent effect than V3+ on improving catalytic activity of magnetite, ascribed to its more significant effect on the increase of specific surface area and surface hydroxyl amount of magnetite. The catalytic efficiency was closely related to not only the nature of substituting metals, but also the pollutant characters. Due to the different adsorption behaviors, the degradation MB and acid orange II followed different degradation mechanism and was also described by different kinetics. The above results will be benefit for the application of transition metal substituted magnetite in environmental engineering.