Size/Shape – Property Relationships of Non - Carbonaceous Inorganic Nanoparticles and their Environmental Implications - Environmental and Human Health Impacts of Nanotechnology - page 84

as demonstrated with multiple fi eld tests in which nZVI was directly introduced to
the environment (Li et al. , 2006c; Zhang, 2003; Elliott and Zhang, 2001; Quinn
et al. , 2005). Given this success, use of nZVI for remediation may become more
widespread. Thus, it is particularly important to consider its potential environ-
mental effects. A recent study showed that humic acids can sorb strongly onto nZVI
and even react with it, removing humic acid from solution (Giasuddin et al. , 2007 ).
While nZVI may be used with the best of intentions for remediation, it may have
unintended consequences.

Iron o xides
It is already well established that iron oxides, including hydroxides and oxyhydrox-
ides, play an important role in the environment. Both macro and nanoscaled iron
oxides are naturally present in the environment and are involved in multiple chemi-
cal and transport processes (Davison and De Vitre, 1992; Cornell and Schwertmann,
2003 ; Brown et al. , 1999; Dzombak and Morel, 1990). It is therefore likely that
synthetic iron oxides could infl uence the environment as well, if released in
suffi cient quantities.
Iron oxides are of great interest for various nanotechnologies, not only because
of their intrinsic properties but also because of their low cost and low toxicity.
Magnetic iron oxides have been studied for applications such as magnetic reso-
nance imaging contrast enhancement (Lee et al. , 2006) and high density data
storage (White et al. , 1997). Other potential nanotechnology applications include
hydrogen generation (Vayssieres et al. , 2005) and catalysis (Tsodikov et al. , 2005 ;
Halim et al. , 2007 ; Liu et al. , 2007). In addition to being synthesized for applications
in their own right, nanoscaled iron oxides are also present in preparations of nZVI
(Martin et al. , 2008 ; Li et al. , 2006a , Nurmi et al. , 2005 ; Liu et al. , 2005a ).
As with their bulk counterparts, nanoscale iron oxides are known to be redox
active. In particular, there are multiple examples of nanoscale iron (III) oxide reduc-
tion by organic molecules (Roden, 2003; Torrent et al. , 1987 ; Houben, 2003 ; Larsen
and Postma, 2001). This includes molecules such as hydroquinone, a synthetic analog
of biological electron transfer molecules (Anschutz and Penn, 2005). Iron (III)
oxides can also participate in redox biochemistry as electron receptors for bacteria
respiring under anaerobic conditions (Roden, 2003; Roden and Zachara, 1996).
As well as participating directly in redox reactions, nanoscale iron oxides can act
as catalysts in low-temperature systems. One reaction that multiple types of iron
oxide nanoparticles (Fe 3 O 4 , ferrihydrite,

- Fe 2 O 3 ) can catalyze is the decomposition
of hydrogen peroxide, which can be useful for oxidizing and degrading organic
pollutants in water (Filip et al. , 2007 ; Hermanek et al. , 2007; Zelmanov and Semiat,
2008 ; Gao et al. , 2007). This catalytic activity may have signifi cance in biological
systems, as it mimics the enzyme activity of peroxidases (Gao et al. , 2007 ). Another
reaction of environmental importance that can be catalyzed (photocatalyzed) is the
oxidation of sulfi te (Faust et al. , 1989 ).
The effect of size upon the redox reactivity of iron oxides is still not well estab-
lished. While a number of studies have examined the effect of varying particle
surface area (or size) on reaction rates, results from studies vary (Roden, 2003; Liu
et al. , 2006 ; Gao et al. , 2007 ; Schwertmann et al. , 1985; Larsen and Postma, 2001;

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