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

There are reports that nano-sized zero-valent iron (Fe-0) exhibits greater reactivity than micro-sized particles of Fe-0, and it has been suggested that the higher reactivity of nano-Fe-0 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-Fe-0, we have used a variety of complementary techniques to characterize two widely studied nano-Fe-0 preparations: one synthesized by reduction of goethite with heat and H-2 (Fe-H2) and the other by reductive precipitation with borohydride (Fe BH). Fe H2 is a two-phase material consisting of 40 nm alpha-Fe-0 (made up of crystals approximately the size of the particles) and Fe3O4 particles of similar size or larger containing reduced sulfur; whereas Fe-BH is Mostly 20-80 nm metallic Fe particles (aggregates of < 1.5 nm grains) with an oxide shell/coating that is high in oxidized boron. The FeBH particles further aggregate into chains. Both materials exhibit corrosion potentials that are more negative than nano-sized Fe2O3, Fe3O4, micro-sized Fell, or a solid Fe-0 disk, which is consistent with their rapid reduction of oxygen, benzoquinone, and carbon tetrachloride. Benzoquinone-which presumably probes inner-sphere surface reactions-reacts more rapidly with Fe EIH than Fe-H2, whereas carbon tetrachloride reacts at similar rates with Fe BH and Fe-H2, presumably by outer-sphere electron transfer. Both types of nano-Fe-0 react more rapidly than mic rosized Fe-0 based on mass-normalized rate constants, but surface area-normalized rate constants do not show a significant nano-size effect. The distribution of products from reduction of carbon tetrachloride is more favorable with Fe-H2, which produces less chloroform than reaction with Fe-BH.