Zero-valent iron (ZVI) nanoparticles: The core-shell structure and surface chemistry

Nanoscale zero-valent iron (nZVI) has been applied increasingly in the treatment of toxic and hazardous wastes and remediation of contaminated soil and groundwater. More than 30 field applications have been reported since 2001. Thus far most of the research and field applications have been on the reduction and dechlorination of chlorinated organic pollutants such as tetrachloroethene, trichloroethene, and to a lesser degree chlorinated benzene and polychlorinated biphenyls (PCBs). Research reported in this dissertation contributes to the development of the nZVI technology in two aspects: (1) systematic characterization of the nZVI with X-ray photoelectron spectroscopy (XPS), and (2) evaluation of the nZVI for separation, transformation, and sequestration of inorganic pollutants such as heavy metal ions and sulfide.;A detailed knowledge of the surface properties is vital to the understanding of the salient reaction mechanisms, kinetics and intermediate and final product profiles. The transport, distribution and fate of nanoparticles in the environment also depend on these surface properties. Surface characterization of nZVI is the focus of this study. The nZVI particles are nearly spherical and have sizes generally less than 100 nm. X-ray photoelectron spectroscopy (XPS) analysis confirms that the nZVI particles had a core-shell structure with FeOOH as the shell and metallic Fe at the core. The aging or oxidation dehydrates the iron oxide shell and forms Fe2O3. Results further suggest that pH plays a minor role except that the Fe oxidation rate increases slightly at both higher and lower pH.;Except a few studies on the reduction of hexavalent chromium, relatively little work has been published on nZVI for treatment of inorganic pollutants. The removal mechanisms of Cr(VI) and Ni(II) are investigated in this study. Both Cr(VI) and Ni(II) can be rapidly immobilized at nZVI surface. The removal capacity ranges from 180 to 50 mg/g nZVI for Cr(VI) and is approximately 130 mg/g nZVI for Ni(II) sequestration. This capacity is significantly higher than common absorbents such as clay and zeolite. XPS studies indicate that Cr(VI) is completely reduced to Cr(III), which is subsequently incorporated into the iron oxyhydroxide shell of nZVI and form stable alloy-like Cr-Fe hydroxides. We have for the first time determined the surface Cr-Fe structure as (Cr0.67Fe0.33)(OH)3 or Cr0.67Fe 0.33OOH. Similarly Ni(II) quickly forms surface hydroxide complex, part of which is subsequently reduced to metallic nickel on the nanoparticle surface. We have reported for the first time that both Ni(0) and Ni(II) exist at the surface.;We have also examined the immobilization of a number of heavy metal ions, including Zn(II), Cd(II), Pb(II), Cu(II), Ag(I), and Hg(II). It further demonstrates the potential of nZVI. We showed that the unique core-shell structure can explain the concurrent sorption and reductive precipitation of metal ions, which results in the high removal capacities. XPS survey suggests that for metal ions such as Zn(II) and Cd(II) with standard potential E0 very close to or more negative than that of iron (-0.41 V), the removal mechanism is sorption/surface complex formation. For metals with E0 greatly more positive than iron, for instance Cu(II), Ag(I), Hg(II), and Cr(VI), the removal mechanism is predominantly reduction. Meanwhile, metals with E 0 slightly more positive than iron for example NI(II) and Pb(II) can be immobilized at the nanoparticle surface by both sorption and reduction. The dual sorption and reduction mechanisms on top of the large surface of nano-sized particles produce rapid reaction and high removal efficiency, and offer nZVI as an efficient material for treatment and immobilization of toxic heavy metals.;The lack of cost-effective technology for odor control has long been a vexing problem facing numerous municipalities and industries. In this research, nZVI particles are explored for scavenge of volatile sulfur compounds, which are the main malodorous compounds in biosoilds. Both hydrogen sulfide and dimethyl disulfide can be quickly removed by nZVI. The XPS scans of both iron and sulfide on the nanoparticle surface provide conclusive evidence that sulfide is immobilized at the nZVI surface in the form of both FeS and FeS2, which are stable and essentially insoluble in water. The reactions of iron with sulfur are directly related to the surface area of the iron particles. As such, the small size, large surface area, and high surface reactivity make nanoparticles potentially ideal reagents for treatment of large volumes of solid and liquid wastes.