Aging of Iron Nanoparticles in Aqueous Solution:  Effects on Structure and Reactivity

Abstract

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.

Activation process of air stable nanoscale zero-valent iron particles

“…It is clear that there is an increase of the reactivity of both air-stable nanoparticles with water aging, which is in line with some previous studies [18,28,31]. This enhancement takes place basically after an activation time of 36-48 hours, and times longer than 48…”

“…The detail of shell morphology for STAR 197 nanoparticles before and after activation process is shown in Figure 4. This shell detachment has been already observed after one-day reaction with deionized water [18,31]. There was very little evidence of oxides forming during water aging.…”

“…An increase in reactivity after 1-2 days of water immersion has been reported for carbon tetrachloride, trichloroethane and 1,1,1,2-tetrachloroethane [18,28,31], but other studies have shown little changes on reactivity during the first days of aging [13,32]. In the cases in which an increase of reactivity was detected, different mechanisms were proposed to explain reactivity recovery: (I) The dissolution of the anionic hydroxo species from a spontaneously-formed oxide/hydroxide shell, such as Fe(OH)y 2-y and Fe(OH)x 3-x known to be soluble and unstable in alkaline media [11].…”

“…(II) The volumetric expansion of the nZVI core and the intermediate oxides formed during corrosion, which leads the oxide layers to flake off and expose metallic iron to the contaminants [18,31]. The expansion of the corrosion products is being studied due to expected clogging problems in the aquifer matrix during nZVI treatment [33].…”

Characterization challenges for nanomaterials

“…In recent work on Fe H2 particles, we have examined the time dependence of the reaction rate, the branching ratio, and hydrogen evolution, as well as the XRD and TEM data noted here. [23] …”