Converting Fe-rich magnetic wastes into active photocatalysts for environmental remediation processes

Abstract Hematite/magnetite photocatalysts were prepared by thermal treatment at 500 and 1000 °C of a Fe-rich waste obtained from a niobium mining. Rietveld analysis of XRD data and 57 Fe Mossbauer spectroscopy indicated that during the heat treatment magnetite is partially converted into hematite, which was the active photocatalytic phase in the samples. The remaining magnetite in the composites was useful to retard the fast electron-hole recombination in hematite. Zeta potential measurements revealed that the point of zero charge (pzc) of the iron oxides increased due to the thermal treatment, which favored the adsorption of negatively charged dye molecules. Diffuse reflectance measurements indicated that the band gap energies of iron oxides increased after the heat treatment due to a higher amount of hematite in those samples. Moreover, the valence band edges of hematite were more positive after the treatment of samples, which increased its dye photo-oxidation power. The photo-oxidation of neat methylene blue and Congo red in the presence of all samples was more efficient than that of neat methyl orange. On the other hand, it was verified from the competitive studies using the three dyes simultaneously that the photo-oxidation of methyl orange was favored, while the methylene blue and Congo red photo-oxidation decreased due to the methyl orange is replacing Congo red in the active photocatalytic sites of hematite. The proposed mechanism of dyes degradation was based on two main processes: (i) direct electron transfer from the dyes to the valence band of hematite, and (ii) dye oxidation by photogenerated OOH radicals on the hematite surface.