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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.

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Nanoscale zerovalent iron (nZVI) has increasingly been used for environmental remediation and in toxic waste treatment. Most applications exploit its large surface area and high reactivity, the latter being a function of zerovalent iron content. In this work, temperature programmed reduction was applied to measure oxygen in nZVI. Iron oxides in nZVI were reduced by hydrogen to form metallic iron and water, which was then measured with an online mass spectrometer to determine oxygen content of the sample. For fresh nZVI prepared by sodium borohydride reduction of iron salts, average oxygen content was 8.21%. Total iron content was approximately 90.35% by the method of acid digestion; Fe(III) content was estimated at 14.37%, and that of zerovalent iron [Fe(0)] at 75.98%. The oxygen content quickly increased to 26.14% after purging with oxygen for four hours. Several other techniques were also used to characterize the iron nanoparticles. High resolution TEM provided direct evidence of the oxide shell structure and indicated that the shell thickness was predominantly in the range of 2-4 nm. The surface elemental composition was determined from high-resolution X-ray photoelectron spectroscopy. The nZVI oxygen content results fill a knowledge gap on nZVI composition.

Hollow and nanoporous particles of zero-valent iron (ZVI) were prepared with template-directed synthesis. Polymer resin beads (0.4 mm diameter) were coated with nanoscale iron particles by reductive precipitation of ferrous iron [Fe(II)] with sodium borohydride. The resin was calcinated at 400 °C to produce hollow and nanoporous iron spheres. The nanoporous iron oxides were then reduced to metallic iron by hydrogen at 500 °C. Scanning electron microscope images of the reduced iron spheres showed that the particles were hollow. The shell thickness was approximately 5 μm and highly porous. Brunauer–Emmett–Teller specific surface area was 2100 m2/kg. In comparison, the theoretical specific surface area of solid iron particles of the same size is just 1.9 m2/kg. Batch tests showed that the surface area normalized reactivity of the porous particles were 14–31% higher than microscale iron particles with similar surface areas for the transformation of hexavalent chromium [Cr(VI)], azo dye Orange II {4-[(2-hydroxyl-1-naphthalenyl)azo]-benzenesulfonic acid monosodium}, and trichloroethene. The combined performance enhancement (larger surface area and higher surface activity) is significant (>1200 times).