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 (alpha-FeOOH/CMSs) and akaganeite/carbon microspheres (beta-FeOOH/CMSs) were successfully prepared and employed to remove Cr(VI). Scanning electron microscopy and transmission electron microscopy of alpha-FeOOH/CMSs and beta-FeOOH/CMSs composites indicated alpha-FeOOH and beta-FeOOH crystals were dispersed on the surface of CMSs. From Brunauer-Emmett-Teller surface area measurement, the average pore widths of alpha-FeOOH/CMSs and beta-FeOOH/CMSs composites were 6.1 and 2.4 nm, respectively, indicating that alpha-FeOOH/CMSs and beta-FeOOH/CMSs were mesoporous materials. The results of batch experiments showed that the performance of alpha-FeOOH/CMSs in Cr (VI) removal was better than beta-FeOOH/CMSs. The removal efficiencies of Cr(VI) by alpha-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 alpha-FeOOH/CMSs and beta-FeOOH/CMSs composites followed the Freundlich isotherm model and pseudo-second-order kinetic model. The maximum adsorption capacities of Cr(VI) by alpha-FeOOH/CMSs and beta-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 alpha-FeOOH/CMSs and beta-FeOOH/CMSs composites, but also be reduced to less toxic Cr(III). The alpha-FeOOH/CMSs exhibited great potential on the removal of Cr(VI) from wastewater.