Removal of chromium(VI) by MnFe2O4 and ferrous ion: synergetic effects and reaction mechanism

MnFe2O4 was a magnetic material that can be used to adsorb contaminants in the wastewater. Fe(II) could act as a reductant to transfer Cr(VI) into Cr(III). In this paper, mesoporous MnFe2O4 prepared by the coprecipitation method was incorporated with Fe(II) to remove Cr(VI). The samples before and after reaction were characterized by means of X-ray diffraction, scanning electron microscopy, transmission electron microscopy, Fourier transform infrared spectroscopy, and X-ray photoelectron spectroscopy. A total of 98~100% of Cr(VI) in solution was removed by MnFe2O4/Fe(II) hybrid system within a wide pH range (pH 3.0–9.0), which is due to the synergetic effects of adsorption from MnFe2O4 and reduction from Fe(II). Cr(VI) was reduced to Cr(III) by ≡Mn(II) on MnFe2O4 and Fe(II). Cr(III) and Fe(III) produced during reaction formed Cr(III)–Fe(III) hydroxides/oxyhydroxides and deposited on MnFe2O4. The inhibiting action of phosphate on the removal of Cr(VI) was greater than that of sulfate. Cr(VI) removal rate retained 94.5% at the fourth recycle test, which showed excellent re-usability of MnFe2O4.