Niobium substituted magnetite as a strong heterogeneous Fenton catalyst for wastewater treatment

In this study, a series of Nb substituted magnetites; Fe3−xNbxO4 (x = 0.0, 0.022, 0.049, 0.099, and 0.19) were prepared and characterized by XRD, BET surface area, TEM, VSM, XPS, and chemical experiments. The magnetite inverse spinel structure and magnetic property were maintained in all the synthetized samples. A significant decrease in crystal size (≈two times) and increase in specific surface area (≈three times) were observed with increased Nb content, resulting in higher adsorption capacity of the samples. In addition, the reactivity of the synthetized samples was examined through degradation of methylene blue solution using Fenton-like reaction. It was found that the incorporation of niobium significantly improved the degradation of methylene blue of which total MB removal was achieved within 180 min at higher molar ratios of Nb (x = 0.19). This could be attributed to the generated oxygen vacancies on the surface of catalysts, the contribution of the introduced Nb cations in Fenton oxidation cycle for regeneration of Fe2+ cations, and increase in adsorption capacity of the samples due to larger surface area. The MB degradation through Fe2.79Nb0.19O4/H2O2 system was well described by the pseudo-first-order equation in kinetics. All samples showed good stability under the studied pH conditions. The amount of niobium leached was not detectable in neutral and basic solutions and the samples could be reused in oxidation process for several times without a significant decrease in their catalytic efficiency. The results proved that incorporation of niobium into magnetite significantly improved the characteristics and effectiveness of the heterogeneous catalyst for Fenton treatment of recalcitrant effluents.