Removal of hexavalent chromium from groundwater using zero-valent iron media

Groundwater contaminated by hexavalent chromium [Cr(VI)] is a worldwide problem resulting from the widespread industrial use of chromium. Zerovalent iron (Fe⁰) and Fe(II)-containing minerals have been found to be effective at removing dissolved Cr(VI) from water via reductive precipitation of trivalent chromium [Cr(III)] compounds. Permeable reactive barriers (PRBs) containing Fe⁰ filings can be effective for removing Cr(VI) from groundwater for concentrations up to ˜10,000 μg/L. However, both field and laboratory studies indicate that higher Cr(VI) concentrations result in passivation of the Fe⁰ surfaces and make Fe⁰ ineffective for long-term Cr(VI) removal. Results from a field test in Elizabeth City, North Carolina indicate that Fe⁰ barriers can remove Cr(VI) to levels below 1 μg/L for more than 8 years with no loss in performance, and only minor amounts of pore clogging by precipitated minerals. Spectroscopic evidence indicates that Cr(III) forms solid solutions with a variety of Fe(II) and Fe(III) containing oxides, such as carbonate green rust, siderite, and ferric hydroxide. Microbiological activity that results in precipitation of sulfide minerals, such as mackinawite, also appears to contribute to Cr(VI) removal.