Page 317 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

302 Environmental Applications of Nanomaterials

remediation with any type of nanoparticle, it is important to know which
groundwater contaminants will respond to the treatment and which will
not. It is also important to know how long the reactive or catalytic par-
ticles will remain active as this will determine important operation deci-
sions such as how much to inject and when reinjection may be necessary.
Degradation of halogenated hydrocarbons, particularly chlorinated sol-
0
vents, occurs via a reductive process. The Fe in the nanoiron is oxidized
by the chlorinated solvent, which is subsequently reduced. For chlorinated
hydrocarbons, the reduction typically results in the replacement of a chlo-
rine atom with a hydrogen atom. For heavy metals, the metal, such as
Pb(II) or Cr(VI), is reduced to its zerovalent form on the nanoiron surface,
or forms mixed (Fe-Metal) precipitates that are highly insoluble (Ponder
et al. 2000). The general half-reactions for the oxidation of iron and the
reduction of chlorinated organic compounds (COC) or heavy metals are
given in Eqs. 1 to 3, where Me is a metal ion of charge a.

# 2
0
Fe S Fe 21 1 2 e (1)
#
#
1
2
COC 1 n e 1 m H S products 1 3Cl 2 (2)
#
2
Me a1 1 b e S Me a2b (3)
0
In the case of nanoiron, or Fe -based bimetallics, the reduction of the
contaminant is surface-mediated, and the particle itself is the reductant.
The attractiveness of nanoiron is that the particles have a high surface-
to-volume ratio and therefore have high reactivity with the target
contaminants. The following generalizations can be made about the
reactivity and lifetime of all nanoparticulate remedial agents that are
themselves the reactive material—that is, not true catalysts according
to the formal definition of a catalyst:
■ Any process that affects the surface properties of the particles (e.g.,
formation of an Fe-oxide on the surface) can affect their reactivity.

■ Any oxidant (e.g., O or NO ) competing with the target contaminant
2 3
will utilize electrons and may lower the rate and efficiency of the
nanoiron treatment for the target contaminants.
■ Reactive nanoparticles that serve as a reactant rather than a cata-
lyst will have a finite lifetime, the length of which depends on the
concentration of the target contaminant, the presence of competing
oxidants, and the selectivity of the particles for the desired reaction.

Nanoiron reactivity
Since 1997, there have been many laboratory studies conducted to
determine the range of contaminants that are amenable to reductive