Stoichiometry of Cr(VI) Immobilization Using Nanoscale Zerovalent Iron (nZVI): A Study with High-Resolution X-Ray Photoelectron Spectroscopy (HR-XPS)

Abstract

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 (HR-XPS) 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 (Cr 0.67 Fe 0.33 )(OH) 3 or Cr 0.67 Fe 0.33 OOH. 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.