The preparation of a novel iron/manganese binary oxide for the efficient removal of hexavalent chromium [Cr(<scp>vi</scp>)] from aqueous solutions

Abstract

To remove hexavalent chromium Cr(vi) efficiently, a novel Fe-Mn binary oxide adsorbent was prepared <i>via</i> a "two-step method" combined with a co-precipitation method and hydrothermal method. The as-prepared Fe-Mn binary oxide absorbent was characterized <i>via</i> transmission electron microscopy (TEM), scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier-transform infrared spectra (FTIR), thermogravimetric analysis (TGA), zeta potential, BET and X-ray photoelectron spectroscopy (XPS). The results indicated that the morphology of the adsorbent was rod-like with length of about 100 nm and width of about 50-60 nm, specific surface area was 63.297 m² g^-1, has the composition of α-Fe₂O₃, β-MnO₂ and MnFe₂O₄ and isoelectric point was observed at pH value of 4.81. The removal of Cr(vi) was chosen as a model reaction to evaluate the adsorption capacity of the Fe-Mn binary oxide adsorbent, indicating that the Fe-Mn binary oxide adsorbent showed high adsorption performance (removal rate = 99%) and excellent adsorption stability (removal rate > 90% after six rounds of adsorption). The adsorption behavior of the Fe-Mn binary oxide was better represented by the Freundlich model (adsorption isotherm) and the pseudo-second-order model (adsorption kinetic), suggesting that the adsorption process was multi-molecular layer chemical adsorption. The possible adsorption mechanism of the Fe-Mn binary oxide for the removal of Cr(vi) included the protonation process and the electrostatic attraction interactions.