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 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 Fe0, or a solid Fe0 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 FeBH than FeH2, whereas carbon tetrachloride reacts at similar rates with FeBH and FeH2, presumably by outer-sphere electron transfer. Both types of nano-Fe0 react more rapidly than micro-sized Fe0 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 FeH2, which produces less chloroform than reaction with FeBH.

Aging (or longevity) is one of the most important and potentially limiting factors in the use of nano-Fe 0 to reduce groundwater contaminants. We investigated the aging of Fe H2 (Toda RNIP-10DS) in water with a focus on changes in (i) the composition and structure of the particles (by XRD, TEM, XPS, and bulk Fe 0 content) and (ii) the reactivity of the particles (by carbon tetrachloride reaction kinetics, electrochemical corrosion potentials, and H 2 production rates). Our results show that Fe H2 becomes more reactive between 0 and ∼2 days exposure to water and then gradually loses reactivity over the next few hundred days. These changes in reactivity correlate with evidence for rapid destruction of the original Fe(III) oxide film on Fe H2 during immersion and the subsequent formation of a new passivating mixed-valence Fe(II)-Fe(III) oxide shell. The effect of aging on the rate of carbon tetrachloride reduction was best described by the corrosion potential of Fe H2 , whereas the yield of chloroform from this reaction correlated best with the rate of H 2 production. The behavior of unaged nano-Fe 0 in the laboratory may be similar to that in field-scale applications for source-zone treatment due to the short reaction times involved. Long-term aged Fe H2 acquires properties that are relatively stable over weeks or even months.