Aging of iron nanoparticles in aqueous solution: Effects on structure and reactivity

Aging (or longevity) is one of the most important and potentially limiting factors in the use of nano-Fe⁰ 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 Fe⁰ content) and (ii) the reactivity of the particles (by carbon tetrachloride reaction kinetics, electrochemical corrosion potentials, and H₂ 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 H₂ production. The behavior of unaged nano-Fe⁰ 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.