Adsorption and redox conversion behaviors of Cr(VI) on goethite/carbon microspheres and akaganeite/carbon microspheres composites

In this study, novel mesoporous composites of goethite/carbon microspheres (α-FeOOH/CMSs) and akaganeite/carbon microspheres (β-FeOOH/CMSs) were successfully prepared and employed to remove Cr(VI). Scanning electron microscopy and transmission electron microscopy of α-FeOOH/CMSs and β-FeOOH/CMSs composites indicated α-FeOOH and β-FeOOH crystals were dispersed on the surface of CMSs. From Brunauer–Emmett–Teller surface area measurement, the average pore widths of α-FeOOH/CMSs and β-FeOOH/CMSs composites were 6.1 and 2.4 nm, respectively, indicating that α-FeOOH/CMSs and β-FeOOH/CMSs were mesoporous materials. The results of batch experiments showed that the performance of α-FeOOH/CMSs in Cr(VI) removal was better than β-FeOOH/CMSs. The removal efficiencies of Cr(VI) by α-FeOOH/CMSs were all over 90.0% after 180 min in the pH range from 3.0 to 9.0. The adsorption of Cr(VI) onto α-FeOOH/CMSs and β-FeOOH/CMSs composites followed the Freundlich isotherm model and pseudo-second-order kinetic model. The maximum adsorption capacities of Cr(VI) by α-FeOOH/CMSs and β-FeOOH/CMSs composites are 55.37 and 29.37 mg/g, respectively. The coexisting Cl−, Cu2+, and Ni2+ had no significant effect on Cr(VI) removal, whereas SO42− and PO43− hindered the removal of Cr(VI). Through X–ray photoelectron spectroscopy, Cr(VI) can not only be directly adsorbed on α-FeOOH/CMSs and β-FeOOH/CMSs composites, but also be reduced to less toxic Cr(III). The α-FeOOH/CMSs exhibited great potential on the removal of Cr(VI) from wastewater.