Enhanced Cr(VI) removal of zero-valent iron with high proton conductive FeC2O4·2H2O shell

In this study, we coated a high proton conductive shell on the zero-valent iron (ZVI) surface by mechanically ball-milling ZVI with oxalic acid dihydrate (OX-ZVI), and demonstrated that the generated FeC 2 O 4 ·2H 2 O shell dramatically improved the Cr(VI) removal rate of ZVI by about 15–80 times. Owing to a higher proton conductivity of FeC 2 O 4 ·2H 2 O shell than that of Fe 2 O 3 shell, proton could easily transfer through FeC 2 O 4 ·2H 2 O shell into iron core and be reduced to H, accompanying with fast surface-bound Fe 2+ generation, resulting in high efficiency of Cr(VI) removal in a wide pH range. Meanwhile, the removed Cr(VI) was deposited on OX-ZVI surface in the formation of Fe x Cr 1-x (OH) 3 composites, accompanied by the appearance of typical hollow structure derived from iron core dissolution. This study clarifies the significance of proton transfer on the reactivity of zero-valent iron, and also provides a new strategy to prepare highly active zero-valent iron for Cr(VI) removal.