Highly efficient removal of hexavalent chromium from electroplating wastewater by ferrous/ferric hydroxide complex: From lab-scale to pilot-scale study

• FHC(SO 4 2- ) shows better Cr(VI) removal performance than FHC(Cl - ) and FHC(CO 3 2- ). • Surface structural Fe(II) and surface -OH are the key factors of Cr(VI) removal. • Removal mechanisms are electrostatic attraction, reduction and co-precipitation. • Chemical cost and sludge yield are reduced by 71.3% and 54.1% in pilot-scale study. • Effective Fe-based materials to remediate electroplating wastewater are proposed. Low electron-transfer efficiency and cumbersome processes are widely known to negatively affect the application of Fe-based materials for treating electroplating wastewater. In this study, ferrous/ferric hydroxide complex (FHC) was successfully synthesized to greatly improve the electron-transfer efficiency for Cr(VI) removal; meanwhile, the effect of the interlayer type on Cr(VI) removal and the possible removal mechanisms were well investigated. The sulfate interlayer of FHC was shown to strongly promote Cr(VI) removal, with 10.4% and 29.0% improvements over those achieved by chloride and carbonate interlayers, respectively, in close association with the amounts of surface structural Fe(II), surface −OH, and adsorbed water. The initial pH value was demonstrated to exert almost no effect on Cr(VI) removal by FHC(SO 4 2− ) owing to the buffering effect of the surface –OH. These results were different from some previously reported Cr(VI) removal of Fe-based materials. Moreover, the results of characterization and lab-scale experiments indicated that most of Cr(VI) binds rapidly with the surface −OH owing to electrostatic attraction. The peak at 618 cm −1 in the FT-IR spectra shifted to 549 cm −1 and new peaks at 576.79 and 586.39 eV in the XPS spectra were observed after the reaction, which was attributed to the formation of CrOOH and FeCr 2 O 4 owing to reduction and coprecipitation. Pilot-scale experiments revealed that the chemical cost and sludge yield during the FHC process decreased by 71.3% and 54.1%, respectively, compared with those in the NaHSO 3 process. These findings provide new insight for utilizing different interlayers to improve the reactivity of FHCs in remediating electroplating wastewater containing Cr(VI).