Removal of Cr(VI) ions from wastewater by Fe3O4-loaded porous sludge biochar

In this work, porous sludge biochar (PSBC) was prepared by molten salt-assisted pyrolysis of municipal sludge, and PSBC loaded with Fe3O4 (Fe3O4@PSBC) was synthesized by chemical precipitation. The effects of pH (2.0-10.0), sorbent dosage (0.1-2 g/L), coexisting ions (Ca2+, Mg2+, Cu2+, Pb2+, Cl-, SiO32-, NO3-, CO32-, SO42-, and PO43-), adsorption temperature (288, 298, and 308 K), initial Cr(VI) ion concentrations (50-150 mg/L), and adsorption time (5-300 min) on the removal of Cr(VI) ions by the sorbent were investigated. The mechanism of the removal of Cr(VI) ions was characterized by scanning electron microscope (SEM), Fourier transform infrared (FTIR), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). The result showed that the removal of Cr(VI) ions on PSBC and Fe3O4@PSBC had a strong dependence on the pH of solution. The maximum adsorption capacity of Cr(VI) ions by PSBC and Fe3O4@PSBC was 162 and 209 mg/g, respectively, at a dosage of 0.4 g/L, pH of 3, and temperature of 298 K. The removal process of Cr(VI) ions could be fitted by the Langmuir isotherm model and pseudo-second-order kinetic model. The breakthrough curves were in good agreement with the theoretical values of the Thomas model. The mechanism of the removal of Cr(VI) ions by Fe3O4@PSBC mainly contain complexation, reduction, and electrostatic interaction. This work proposes a new removal material for Cr(VI)-containing wastewater.