| Abstract
| - There are reports that nano-sized zero-valent iron (Fe0)exhibits greater reactivity than micro-sized particles ofFe0, and it has been suggested that the higher reactivityof nano-Fe0 may impart advantages for groundwaterremediation or other environmental applications. However,most of these reports are preliminary in that they leavea host of potentially significant (and often challenging) materialor process variables either uncontrolled or unresolved.In an effort to better understand the reactivity of nano-Fe0, we have used a variety of complementary techniquesto characterize two widely studied nano-Fe0 preparations: one synthesized by reduction of goethite with heat and H2(FeH2) and the other by reductive precipitation withborohydride (FeBH). FeH2 is a two-phase material consistingof 40 nm α-Fe0 (made up of crystals approximately thesize of the particles) and Fe3O4 particles of similar size orlarger containing reduced sulfur; whereas FeBH is mostly20−80 nm metallic Fe particles (aggregates of <1.5 nm grains)with an oxide shell/coating that is high in oxidizedboron. The FeBH particles further aggregate into chains.Both materials exhibit corrosion potentials that are morenegative than nano-sized Fe2O3, Fe3O4, micro-sized Fe0, or asolid Fe0 disk, which is consistent with their rapid reductionof oxygen, benzoquinone, and carbon tetrachloride.Benzoquinonewhich presumably probes inner-spheresurface reactionsreacts more rapidly with FeBH than FeH2,whereas carbon tetrachloride reacts at similar rateswith FeBH and FeH2, presumably by outer-sphere electrontransfer. Both types of nano-Fe0 react more rapidly than micro-sized Fe0 based on mass-normalized rate constants, butsurface area-normalized rate constants do not show asignificant nano-size effect. The distribution of productsfrom reduction of carbon tetrachloride is more favorable withFeH2, which produces less chloroform than reaction withFeBH.
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