Reduction removal of hexavalent chromium by zinc-substituted magnetite coupled with aqueous Fe(II) at neutral pH value

In this study, Cr(VI) removal by Zn-substituted magnetite coupled with aqueous Fe(II) through adsorption and reduction was investigated. A series of Zn-substituted magnetites (Fe3−xZnxO4, x = 0, 0.25, 0.49, 0.74 and 0.99) were synthesized by a coprecipitation method, followed by systematic characterization, e.g., X-ray diffraction (XRD), thermogravimetry (TG), specific surface area and surface acid/base titration. The characterization results show that all samples had a spinel structure without an obvious alternation by Zn substitution. The BET surface area and surface site density gradually increased with Zn substitution. During Cr removal under neutral pH value, Cr(VI) was first adsorbed on the magnetite surface and then reduced to Cr(III) by Fe3−xZnxO4and adsorbed Fe(II); finally, it was immobilized on the Fe3−xZnxO4surface, which was verified via X-ray photoelectron spectroscopy (XPS) and X-ray absorption near edge spectroscopy (XANES) analyses and desorption experiment. With the increase in the Zn content in magnetite, the Cr removal efficiency initially decreased; then, it improved, which was ascribed to the decreasing Cr(VI) reduction and increasing Cr(VI) adsorption. This variation in activity by Zn substitution was discussed in terms of the reaction mechanism as well as the cationic microstructure and surface properties of Fe3−xZnxO4.