Liquid Nitrogen Activation of Zero-Valent Iron and Its Enhanced Cr(VI) Removal Performance

In this study, we report that liquid nitrogen treatment is a promising zero-valent iron activation method that does not remove the iron oxide shell; this can improve the apparent Cr(VI) removal rate constant of zero-valent iron by about 4–120 times, depending on the particle sizes and the suppliers of zero-valent iron. It was found that liquid nitrogen, with its low temperature of 77 K, could crack the iron oxide shell of zero-valent iron to produce abundant fractures because of the different thermal expansion coefficients of iron oxide and iron. These fractures provided suitable mass transfer channels for the inward transfer of water/oxygen molecules to the iron core and the subsequent in situ generation of Fe(II) for the reduction of Cr(VI) to Cr(III). More importantly, systematic characterizations confirmed the generation of an Fe(III)/Cr(III)/Cr(VI) composite on the surface of zero-valent iron during the removal, suggesting its environmental benignancy. This study provides a novel physical zero-valent iron activation method, sheds light on the importance of the iron oxide shell of zero-valent iron on Cr(VI) removal, and clarifies the intrinsic Cr(VI) removal mechanism of zero-valent iron.