Formation-of ferrihydrite and associated iron corrosion products in permeable reactive barriers of zero-valent iron

Ferrihydrite, which, is known to form in the presence of oxygen and to be stabilized by the adsorption of Si, PO₄ and SO₄, is ubiquitous in the fine-grained fractions of permeable reactive barrier (PRB) samples from the U.S. Coast Guard Support Center (Elizabeth City, NC) and the Denver Federal Center (Lakewood, CO) studied by high-resolution transmission electron microscopy and selected area electron diffraction. The concurrent energy-dispersive X-ray data indicate a strong association between ferrihydrite and metals such as Si, Ca, and Cr. Magnetite, green rust 1, aragonite, calcite, mackinawite, greigite and lepidocrocite were also present, indicative of a geochemical environment that is temporally and spatially heterogeneous. Whereas magnetite, which is known to form due to anaerobic Fe⁰ corrosion, passivates the Fe⁰ surface, ferrihydrite precipitation occurs away from the immediate Fe⁰ surface forming small (<0. μm) discrete clusters. Consequently, Fe⁰-PRBs may remain effective for a longer period of time in slightly oxidized groundwater systems where ferrihydrite formation occurs compared to oxygen-depleted systems where magnetite passivation occurs. The ubiquitous presence of ferrihydrite suggests that the use of Fe⁰-PRBs may be extended to applications that require contaminant adsorption rather than, or in addition to redox-promoted contaminant degradation.