PNNL

Abstract

There are reports that nano-sized zero-valent iron (Fe°) exhibits greater reactivity than micro-sized particles of Fe°, which 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. To better understand the reactivity of iron nanoparticles, we have used a variety of complementary techniques to characterize two widely studied nano Fe° preparations: one synthesized by heat-reduction of goethite under H2 (FeH2) and the other by reductive precipitation with borohydride (FeBH). X-ray diffraction (XRD), transmission electron microscopy (STXM) showed particles of similar size (40-80 nm), but surface area measurements varied widely with method of measurement (4-60 m² g-¹). FeH2 is a two-phase material consisting of ?-Fe° and Fe3O4, doped with reduced sulfur, whereas FeBH is mostly metallic Fe with an oxide shell that is high in boron. Both materials exhibit corrosion potentials that are more negative than nano-sized Fe2O3, Fe3O4, micro-sized Fe°, or a solid Fe° disk, consistent with their rapid reduction of oxygen, benzoquinone, and carbon tetrachloride. Benzoquinone-which presumably probes inner-sphere surface reactions-reacts more rapidly with FeBH than with FeH2, whereas with carbon tetrachloride, FeBH and FeH2 react at similar rates, presumably by outer-sphere electron transfer. Whether either material reacts more rapidly with the probes than micro-sized Fe° is unclear due to uncertainties in the appropriate specific surface areas. The distribution of products from reduction of carbon tetrachloride is more favorable with FeH2, which produces less chloroform than reaction with FeBH.