Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 83

particle sizes also yield slightly different morphologies; thus some of the variation
in catalytic activity attributed to size effects might actually be due to differences in
particle shape. Hence, future studies of catalytic behaviour should include more
characterization of morphology. Also, from study to study, and even within studies,
the surface coatings vary, which may strongly affect the catalytic properties. More
systematic comparisons of the catalytic properties of nanoparticles with different
coatings would be very helpful. It should be noted that the effect of size upon a
redox reaction will be dependent upon the particular energetics of that system.

3.3.2.2 Zero - Valent Iron and Iron Oxides

Iron
For years, zero-valent iron (ZVI), or metallic iron, has been studied for the remedia-
tion of contaminated areas (Lo et al. , 2007). As the standard reduction potential for
the (Fe/Fe 2+ ) redox reaction is

0.44V (Atkins, 1998), metallic iron is able to reduce
and transform many substances, including Cr 6+ , Pb 2+ and halogenated organic pol-
lutants. More recently, nanoparticulate ZVI or nZVI (including mixtures of nZVI
plus an activating metal, e.g. palladium) have been investigated for use in environ-
mental clean-up efforts (Zhang, 2003; Zhang et al. , 1998 ; Schrick et al. , 2002 ; Tratnyek
and Johnson, 2006). nZVI is attractive for this application due to its higher specifi c
surface area, as ZVI reductive transformation rates have been shown to be propor-
tional to surface area (Johnson et al. , 1996). Also, nZVI can be directly injected into
contaminated sites, allowing for fl exibility in its application (Li et al. , 2006a, 2006c ).
Those interested in optimizing the performance of nZVI have begun to examine
how physical characteristics such as nanoparticle size affect reactivity. Results have
varied. For example, nZVI has been shown to reduce polychlorinated biphenyls
(Lowry and Johnson, 2004), while microscale ZVI cannot. On the other hand, in
another study on the reduction of nitrate, surface area normalized rate constants
for 9.5 nm and 45 nm nanoparticles did not even vary by an order of magnitude
(Liou et al. , 2006). Interestingly enough, various nZVI preparations have been
shown to alter which products result from the reductive transformations of halo-
genated organics (Liu et al. , 2005a, 2005b; Nurmi et al. , 2005). In the reduction of
carbon tetrachloride, two different preparations of nZVI and microscale ZVI pro-
duced different amounts of chloroform, an undesirable product (Nurmi et al. , 2005 ).
Such fi ndings are signifi cant, as they mean that the chemical behaviour of nZVI
might be controlled by merely altering the preparation used.
The origins of nZVI chemical behaviour have not yet been exactly determined,
although various studies indicate differences in crystallinity (Liu et al. , 2005a ) and
the amount of oxides or other elements present (Liu et al. , 2005b ; Nurmi et al. , 2005 )
could be critical. The iron nanoparticle sizes used are far too large to have electronic
structures signifi cantly different from the bulk, so this is an unlikely cause of their
behaviour (Wang et al. , 2000). One possible factor infl uencing the reactivity of nZVI
would be nanoparticle shape and surface bonding coordination. Future studies of
nZVI could utilize samples of higher uniformity in shape and size.
It should be noted that nZVI has proved not only to be successful at decompos-
ing pollutants in the laboratory. nZVI can effectively decompose pollutants in situ,

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