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

MnFe 2 O 4 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 MnFe 2 O 4 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 MnFe 2 O 4 /Fe(II) hybrid system within a wide pH range (pH 3.0–9.0), which is due to the synergetic effects of adsorption from MnFe 2 O 4 and reduction from Fe(II). Cr(VI) was reduced to Cr(III) by ≡Mn(II) on MnFe 2 O 4 and Fe(II). Cr(III) and Fe(III) produced during reaction formed Cr(III)–Fe(III) hydroxides/oxyhydroxides and deposited on MnFe 2 O 4 . 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 MnFe 2 O 4 .