Page 321 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

306 Environmental Applications of Nanomaterials

for most chlorinated solvents, which is optimal. The exception is carbon
tetrachloride (CCl ), where reduction by nanoiron results in chloroform
4
(CHCl ) as a long-lived reactive intermediate or product. Chloroform is
3
more mobile and toxic than carbon tetrachloride, and reactive particles
that can promote the complete conversion of carbon tetrachloride to
methane (CH ) or to carbon dioxide (CO ) is most desirable.
4
2
For reactive nanoparticles, where the particle itself is the reductant
and therefore has a finite reducing power, it is important to understand
0
the factors influencing the Fe utilization efficiency. The iron utilization
efficiency is defined as the mass of target contaminant degraded per unit
mass of nanoiron added. The degradation products formed can strongly
influence the nanoiron mass required to degrade a given mass of con-
0
taminant to innocuous products. For example, using Fe as the reduc-
tant TCE can dechlorinate to partially or fully saturated dechlorination
products (Eq. 4), where the values of the coefficients a and b are deter-
mined by the products formed (e.g., a 4, b 3 for ethane, and a 2,
b = 3 for acetylene). Eq. 4 is the net reaction derived from the two half-
reactions shown in Eqs. 1 and 2.

1
0
aFe 1 TCE 1 s2a 2 bdH S prod 1 aFe 21 1 bCl 2 (4)
For TCE, removing all of the chlorines to form the corresponding hydro-
carbon makes it nontoxic. For nanoiron made from borohydride reduc-
tion of dissolved iron, the dominant reaction product is ethane (C H ) (Liu
6
2
et al. 2005a). For another type of nanoiron, the dominant reaction prod-
uct is acetylene (C H ) (Liu et al. 2005b). Both are nonchlorinated and
2
2
are equally nontoxic, however, the carbon in ethane is highly reduced (C
average oxidation state is –III), while the carbon in acetylene is not as
reduced (C average oxidation state in acetylene is –I). The average oxi-
dation state of C in the parent compound (TCE) is I, so reduction of TCE
0
to ethane requires twice as much Fe as for reduction of TCE to acety-
lene. This is also evident in the value of a for ethane (a 4) versus
acetylene (a 2). Reactive nanoparticles can potentially be designed for
optimal efficiency by selecting for products that are nontoxic but require
the smallest possible redox swing.

Fate and lifetime
Nanoparticles have great potential to benefit the environment by
improving groundwater remediation, but before their widespread release
it is prudent to evaluate the potential risks associated with their use.
This requires a fundamental understanding of their long-term fate and
lifetime. Most nanoiron is comprised of primary particles ranging in
0
size from ~40 to 100nm that have an Fe core and Fe-oxide shell that is