Oxalate Modification Dramatically Promoted Cr(VI) Removal with Zero-Valent Iron

In this study, we demonstrated that oxalate modification could promote the Cr(VI) removal performance of zerovalent iron (ZVI) and increase the removal rate constant by about 20 times. This dramatic enhancement of Cr(VI) removal was attributed to the binding of oxalate groups on the surface of ZVI in a monodentate mononuclear configuration. The surface structure of oxalate-modified zerovalent iron (OA-ZVI) and the Cr(VI) adsorption behaviors on ZVI and OA-ZVI were carefully investigated. The theoretical results revealed that this oxalate modification favored Cr(VI) adsorption on the ZVI surface with a much lower adsorption energy and also accelerated electrons transfer from the iron core to the surface, which could produce more surface Fe(II) to easily reduce Cr(VI) via a single electron transfer pathway, greatly promoting the reduction removal of Cr(VI). More importantly, OA-ZVI still remained at a high Cr(VI) removal activity even after six cycles of use for both simulated and real Cr(VI)-containing wastewaters, revealing its excellent continuous Cr(VI) removal ability for practical application. This study provides an environmental-benignity surface modification method of ZVI and also highlights the importance of surface functional groups on the Cr(VI) removal property of ZVI.