Transformation and composition evolution of nanoscale zero valent iron (nZVI) synthesized by borohydride reduction in static water

[Display omitted] •Aging effects of nZVI prepared by a borohydride reduction method are investigated over a period of 90d in static water.•Both the structures and the compositions of the corrosion products change with the process of aging.•nZVI is firstly oxidized to hollow-out spherical shapes, and eventually transformed to lath-like and acicular-shaped crystals.•The corrosion products are mainly γ-FeOOH mixed with small amounts of Fe3O4 and γ-Fe2O3 when aged up to 90d. The reactivity of nanoscale zero valent iron (nZVI) toward targeted contaminants is affected by the initial nZVI composition and the iron oxides formed during the aging process in aquatic systems. In this paper, the aging effects of nZVI, prepared using a borohydride reduction method in static water over a period of 90days (d), are investigated. X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), and Raman spectroscopy are used to characterize the corrosion products of nZVI. Results show that both the structures and the compositions of the corrosion products change with the process of aging. The products of nZVI aged for 5d in static water media are mainly magnetite (Fe3O4) and maghemite (γ-Fe2O3), accompanied by lepidocrocite (γ-FeOOH). For products aged 10d, XRD data show the formation of ferrihydrite and lepidocrocite. When aged up to 90d, the products are mainly γ-FeOOH mixed with small amounts of Fe3O4 and γ-Fe2O3. Transmission electronic microscopy (TEM) images show that the core–shell structure forms into a hollow spherical shape after 30d of aging in aquatic media. The results indicate first that iron ions in the Fe(0) core diffuse outwardly toward the shell, and hollowed-out iron oxide shells emerge. Then, the iron oxide shell collapses and becomes a flaky, acicular-shaped structure. The type and the crystal phase of second iron oxide minerals are vastly different at various aging times. This study helps to explain the patterns of occurrence of specific iron oxides in different natural conditions.