Ind. Eng. Chem. Res. 2008, 47, 2131-2139
2131
APPLIED CHEMISTRY Stoichiometry of Cr(VI) Immobilization Using Nanoscale Zerovalent Iron (nZVI): A Study with High-Resolution X-Ray Photoelectron Spectroscopy (HR-XPS) Xiao-qin Li, Jiasheng Cao, and Wei-xian Zhang* Department of CiVil and EnVironmental Engineering, Lehigh UniVersity, 13 East Packer AVenue, Bethlehem, PennsylVania 18015
Remediation of chromium-contaminated sites presents both technological and economic challenges as conventional methods are often too expensive in removing chromium in the soil matrix such as chromium ore process residue (COPR). In this work, reduction and precipitation of hexavalent chromium [Cr(VI)] by nanoscale zerovalent iron (nZVI) are evaluated. Cr(VI) is rapidly reduced and immobilized on the iron nanoparticle surface. In the pH range of 4 to 8, the nZVI has a chromium removal capacity ranging from 180 to 50 mg Cr/g nZVI. Under similar conditions, microscale iron particles (100 mesh) typically have a capacity of less than 4 mg Cr/g Fe. Characterizations with high-resolution X-ray photoelectron spectroscopy (HRXPS) indicate that Cr(VI) is reduced to Cr(III), which is subsequently incorporated into the iron oxyhydroxide shell of nZVI and form alloy-like Cr-Fe hydroxides with a representative formula approximating (Cr0.67Fe0.33)(OH)3 or Cr0.67Fe0.33OOH. The Cr-Fe hydroxide shell is relatively stable and serves as a sink for Cr(VI). Because of the fast reaction kinetics and high chromium removal capacity, nZVI has the potential to become an effective remedial agent for in situ immobilization of chromium-contaminated soil and groundwater. Introduction Chromium is a metal commonly used in industrial processes such as electroplating, leather tanning and mineral extraction, and as a common ingredient used in protective coatings, pigments, and stainless steel.1-4 It is estimated that the U.S. alone has a chromium stockpile of over 5 million metric tons.1 Because of accidental and sometimes intentional release into the environment, chromium has become one of most commonly detected environmental contaminants, especially in soil and groundwater. For example, it was reported that, in Jersey City, New Jersey, there are at least 120 sites contaminated with chromium with over 20 million tons of chromium ore process residue (COPR), which typically contain over 1000-10 000 mg Cr/kg.5 Chromium is a potential carcinogen and is highly toxic to humans, animals, and plants. The U.S. EPA maximum contaminant level (MCL) for total chromium in drinking water is set at 0.1 mg/L.6-8 Chromium in natural water exists in two stable oxidation states, hexavalent (Cr(VI)) and trivalent (Cr(III)). Cr(VI) species such as chromate (CrO42-, HCrO4-) and dichromate (Cr2O72-) are highly soluble and mobile in aquatic systems and are of great environmental concern. On the other hand, Cr(III) species are relatively stable and have low solubility (
