Fate and Mechanistic Insights into the Transformation of Aged Nanoscale Zerovalent Iron (nZVI^A) Reacted with Cr(VI): Impact of Aging Time in Oxic Water

Abstract

Dynamic transformation of mineralogical phases and chemical compositions of nanoscale zerovalent iron (nZVI) influence its reactivity as well as its affinity toward contaminants in water solution, because corrosion of nZVI under different geochemical conditions alters intermediate products and the core–shell structure. In this study, nZVI particles aged in oxic water (nZVIA) over experimental time scales of 2, 6, 12, 24, 48, and 72 h are reacted with CrO42– [Cr(VI)] in solution at pHinitial 4.4. Transmission electron microscopy (TEM) and X-ray diffraction (XRD) analyses demonstrate that the mineralogical phases of nZVIA after reacted with Cr(VI) significantly depend upon aging time. X-ray photoelectron spectroscopy (XPS) analysis testify that the Cr(VI) and Cr(III) species coexist on the surface of the nZVIA samples, proving the dual function of mixed-valent iron oxides, adsorption and reduction. The existence of chromium oxide and/or chromite (FeCr2O4) accompanied by magnetite/maghemite and lepidocrocite in the products proves the presence of structural Fe(II) in the corrosion products during the aging period. The decrease of reactivity observed for nZVIA over long time scales ascribes to a decline in reduction ability. This manuscript provides insight into the aging effect and removal capacity of nZVI for heavy metals and promotes the understanding of the fate of nZVI–Cr(VI) reaction products in the natural environment.