Cr(VI) adsorption from aqueous solution and its reactions behavior on the surfaces of granular Fe–Mn binary oxides

Abstract

A granular Fe–Mn binary oxide (GFMO) was prepared for Cr(VI) removal and characterized by its Brunauer–Emmett–Teller surface area, scanning electron microscopy, X‐ray photoelectron spectroscopy, X‐ray diffraction, and Fourier transform infrared spectroscopy. Scanning electron microscopy images of the GFMO showed it had a rough surface and heterogeneous porous structure. The Brunauer–Emmett–Teller surface area was 57.64 m2/g. X‐ray photoelectron spectra showed that manganese and iron existed mainly in the +IV and + III oxidation states, respectively. Adsorption of Cr(VI) by the GFMO was investigated. Kinetic data showed that the Cr(VI) adsorption onto the GFMO followed a pseudo‐second‐order model, indicating that the adsorption process was chemisorption. Equilibrium data were analyzed by both the Freundlich and Langmuir models, and the Langmuir isotherm model fitted better with the maximum adsorption capacity of 16.79 mg/g at 318 K. The Cr(VI) adsorption on GFMO was endothermic and nonspontaneous in nature. It was also strongly pH dependent, with higher Cr(VI) removal efficiency occurring under acidic conditions and a sharp decrease in adsorption as the pH increased. Coexisting anions competed with Cr(VI) for adsorption on the GFMO in the order silicate > sulfate > nitrate. Cr(VI) adsorption on the GFMO mainly occurred via anion exchange, surface complexation, and electrostatic attraction.Novelty or Significance: Combined adsorbent with iron oxides and manganese oxides was usually synthesized for heavy metal removal and showed an excellent adsorption capacity. However, the Fe–Mn binary oxide adsorbent mentioned above were mostly exhibited in particles, limiting its engineering application due to the dispersion state in aqueous solutions. Therefore, novel granular Fe–Mn oxide adsorbents, with good stability and high adsorption capacity, are highly desired. A novel granular Fe–Mn oxide (GFMO) adsorbent with the polyvinyl alcohol (PVA) as the adhesive was synthesized to investigate its adsorption behaviors for Cr(VI) for further used as filler in most water treatment equipment. © 2018 American Institute of Chemical Engineers Environ Prog, 38: S176–S184, 2019