Sorption of Cr(VI) on Mg-Al-Fe layered double hydroxides synthesized by a mechanochemical method

Mg-Al-Fe layered double hydroxides (LDHs) with a constant Mg2+/(Fe3+ + Al3+) molar ratio but varying Fe3+/(Al3+ + Fe3+) molar ratios (R-Fe) from 0-1 were synthesized by a mechanochemical method. Sorption of Cr(VI) on the LDHs in aqueous solutions was investigated by a batch technique. The sorption process obeyed pseudo-second-order kinetics, and the sorption equilibrium could be well described with the Freundlich isotherm. The saturated sorption amount increased with increasing R-Fe, indicating that the replacement of Al by Fe in LDHs is favorable for Cr(VI) sorption. The mechanisms of Cr(VI) sorption included the intercalation of Cr(VI) oxyanions into the LDH gallery, and the surface complexation between dichromate anions and hydroxyl groups of the LDHs. The driving forces of Cr(VI) sorption on LDHs included physical binding (or electrostatic attraction) and chemical binding. The physically sorbed amount of Cr(VI) on the LDHs was almost independent of R-Fe, while the chemically sorbed amount obviously increased with increasing R-Fe. That is, the increase of R-Fe causes the chemical activity of surface sorption sites of the LDHs to increase, resulting in their sorption capacity for Cr(VI) increasing. In addition, the sorption capacity of the LDHs synthesized by the mechanochemical method is comparable with those of LDHs synthesized by coprecipitation and hydrothermal methods. Thus, Mg-Fe LDHs are promising sorbents for treating Cr(VI)-containing wastewater, and the mechanochemical method can be used to synthesize LDH sorbents.