There are reports that nano-sized zero-valent iron (Fe0) exhibits greater reactivity than micro-sized particles of Fe0, and it has been suggested that the higher reactivity of nano-Fe0 may impart advantages for groundwater remediation or other environmental applications. However, most of these reports are preliminary in that they leave a host of potentially significant (and often challenging) material or process variables either uncontrolled or unresolved. In an effort to better understand the reactivity of nano-Fe0, we have used a variety of complementary techniques to characterize two widely studied nano-Fe0 preparations:  one synthesized by reduction of goethite with heat and H2 (FeH2) and the other by reductive precipitation with borohydride (FeBH). FeH2 is a two-phase material consisting of 40 nm α-Fe0 (made up of crystals approximately the size of the particles) and Fe3O4 particles of similar size or larger containing reduced sulfur; whereas FeBH is mostly 20−80 nm metallic Fe particles (aggregates ...

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Characterization and properties of metallic iron nanoparticles: Spectroscopy, electrochemistry, and kinetics

Using the isotope specificity of 57Fe Mossbauer spectroscopy, we report spectroscopic observations of Fe(II) reacted with oxide surfaces under conditions typical of natural environments (i.e., wet, anoxic, circumneutral pH, and about 1% Fe(II)). Mossbauer spectra of Fe(II) adsorbed to rutile (TiO2) and aluminum oxide (Al2O3) show only Fe(II) species, whereas spectra of Fe(II) reacted with goethite (α-FeOOH), hematite (α-Fe2O3), and ferrihydrite (Fe5HO8) demonstrate electron transfer between the adsorbed Fe(II) and the underlying iron(III) oxide. Electron-transfer induces growth of an Fe(III) layer on the oxide surface that is similar to the bulk oxide. The resulting oxide is capable of reducing nitrobenzene (as expected based on previous studies), but interestingly, the oxide is only reactive when aqueous Fe(II) is present. This finding suggests a novel pathway for the biogeochemical cycling of Fe and also raises important questions regarding the mechanism of contaminant reduction by Fe(II) in the presenc...

Spectroscopic Evidence for Fe(II)−Fe(III) Electron Transfer at the Iron Oxide−Water Interface

Fast transformation of iron oxyhydroxides by the catalytic action of aqueous Fe(II)

Magnetite nanoparticles (MNPs) are ubiquitous components of the subsurface environment, and increasing attention has been paid to MNPs due to their highly reductive and heterogeneous catalysis reactivity for the degradation of organic contaminants. However, most previous research studies neglected the generation of reactive oxygen species (ROS) by MNPs, which plays an important role in the transformation of contaminants. In this paper, we investigated the activation of persulfate (PS) by MNPs for the degradation of 2,4,4′-CB (PCB28), a selected model compound, and the underlying mechanism was elucidated. The results indicated that the PS can be activated by MNPs efficiently for the degradation of PCB28 at neutral pH. Electron paramagnetic resonance (EPR) technique was used to detect and identify the radical species in these processes. The mechanism of the activation of PS by MNPs was that superoxide radical anion (O2 −) generated by MNPs could activate the PS to produce more sulfate radicals (SO4 −), which favored the degradation of PCB28. The conclusion was further confirmed by quenching studies with the addition of superoxide dismutase (SOD). The effects of Fe(II) and pH on the degradation of PCB28 by PS/MNPs as well as the generation of ROS by MNPs were also studied. Both sorbed Fe(II) on MNPs surface and increased pH led to production of more O2 −, which activated the PS to give more SO4 − to degrade PCB28. In addition, increasing the oxygen concentration in the reaction solution favored the generation of O2 − as well as the degradation of PCB28. The findings of this study provide new insights into the mechanism of heterogeneous catalysis based on MNPs and the reactivity of MNPs toward environmental contaminants.

Superoxide radical driving the activation of persulfate by magnetite nanoparticles: Implications for the degradation of PCBs

Due to their special properties, people have been increasingly interested in studying applications of nanoscale metal materials in environmental engineering. Literature about the current research on the synthesis, properties, and environmental applications of nanoscale iron-based materials is reviewed and summarized in this article. Different physical and chemical methods used for synthesizing nano-iron-based particles with desired size, structure, and surface properties are described. We also emphasize important properties of nano-iron-based particles, including the density and intrinsic reactivity of surface sites. These properties directly influence the chemical behavior of such particles and, consequently, affect their applications in water and wastewater treatment and in air pollution control. Environmental applications of nano-iron particles are discussed in detail, including removal of chlorinated organics, heavy metals, and inorganics.

Synthesis, properties, and environmental applications of nanoscale iron-based materials: A review

Uptake of ferrous iron from aqueous solution by iron oxides results in the formation of a variety of reactive surface species capable of reducing polyhalogenated methanes (PHMs). Pseudo-first-order reaction rate constants, kobs, of PHMs increased in the order CHBrCl2 < CHBr2Cl < CHBr3 < CCl4 < CFBr3 < CBrCl3 < CBr2Cl2. The kobs values increased with the exposure time, teq, of Fe(II) to suspended iron oxides which was attributed to the rearrangement of initially sorbed Fe(II) species to more reactive surface species with time. At pH 7.2, the kobs values of PHMs also increased with the concentration of surface-bound ferrous iron, Fe(II)sorb, particularly when Fe(II)tot was increased to concentrations where surface precipitation becomes likely. At fixed total Fe(II) concentrations, kobs values increased exponentially with pH. The highest reactivities were associated with pH conditions where surface precipitation of Fe(II) is expected. Fe(II)sorb and pH, however, had opposite effects on the product formation ...

Klaus Pecher

Papers

Reduction of Polyhalogenated Methanes by Surface-Bound Fe(II) in Aqueous Suspensions of Iron Oxides