Key role of FeO in the reduction of Cr(VI) by magnetic biochar synthesised using steel pickling waste liquor and sugarcane bagasse

Magnetic biochars with different iron content were synthesised and used for Cr(VI) removal. The species of iron oxides in that magnetic materials were Fe3O4, Fe2O3 and FeO, which were identified by various instruments. Scanning electron microscope indicated that the iron oxides distributed into the pores or on the surface of the biochar matrix, meanwhile, transmission electron microscopy illustrated that the exposed crystal planes of iron oxides in the magnetic biochar were difference. The maximum adsorption capacity of Cr(VI) by magnetic biochar was up to 71.04 mg/g and the adsorption capacity of Cr(VI) was positively correlated with iron content. Meanwhile, a series of experiments demonstrated that the FeO in the magnetic biochar was the key active component for the removal and reduction of Cr(VI). Finally, the mechanism analysis showed that the removal of Cr(VI) was attributed to the reduction, electrostatic adsorption and complexation, and reduction occupied a leading position. This study provides new insight into the role of iron oxides in magnetic biochar for the removal of Cr(VI). •Magnetic biochars with different iron content were synthesised using steel pickling waste liquor.•Structure and reactivity of magnetic biochar were related to the species and content of iron oxides.•A majority of hexavalent chromium was reduced by magnetic biochar.•The active components in the magnetic biochar responsible for the reactivity were identified.