Page 318 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials for Groundwater Remediation 303

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dechlorination by nanoiron or bimetallics such as Fe /Pd nanoparti-
cles. A summary of the reactive surface area normalized pseudo-first-
order reaction rate constants for each type of contaminant and iron
is given in Table 8.1. For some forms of nanoiron, the reported sur-
face area-normalized reaction rate constants are similar to those
reported for iron filings, which have been used successfully as in situ
reactive barriers for groundwater remediation in shallow aquifers
(Wilkin et al. 2005), suggesting that there is not a quantum nanosize
effect for these particles (Liu et al. 2005b; Nurmi et al. 2005). Rather,
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the high surface area of nanoiron (10–50 m /g) compared to iron fil-
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ings (0.01–1 m /g) makes them attractive for in situ remediation of
groundwater because of the higher reaction rates that they can pro-
vide. This is exemplified in the reaction with perchlorate (Cao et al.
2005); perchlorate reduction by nanoiron was rapid enough to make
it useful for in situ perchlorate reduction, whereas iron filings reduced
perchlorate far too slowly to be useful (Moore et al. 2003). There are
instances where nanosize effects may be evident. Nanoiron made from
the borohydride reduction of dissolved Fe has exhibited behavior that
may be attributable to small particle size (i.e., a nanosize effect).
First, nanoiron made from reduction of dissolved Fe by sodium boro-
hydride has the ability to activate H 2 gas and use it in the reduction
of chlorinated ethenes (Liu et al. 2005a; Liu et al. 2005b). This abil-
ity was attributed to the amorphous structure and the nanosized Fe 0
crystallites (~1 nm) in these particles. Micron-scale iron filings and
other forms of nanoiron with larger crystallites cannot active H 2 .
Second, this nanoiron that does not have any noble metal catalyst can
dechlorinate polychlorinated biphenyl compounds (PCBs) at ambient
temperature and pressure (Lowry and Johnson 2004; Wang and Zhang
1997). Iron filings and other types of nanoiron without an added cat-
alyst cannot dechlorinate PCBs under these conditions. Novel nano-
materials that leverage the nanosize effects may degrade highly
recalcitrant compounds that are not amenable to degradation by cur-
rently available materials.
Laboratory investigations (Table 8.1) indicate that the reactivity
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afforded by nanoiron and Fe /Pd bimetallic nanoparticles is sufficient
to make it useful for in situ remediation of groundwater. It is important
to note, however, that most of these studies use fresh nanoiron particles
and do not observe reactivity over the lifetime of the particles so they
should be considered as initial rates. There have been few investigations
on the effect of aging (oxidation) or precipitation of Fe-containing min-
erals (e.g., FeCO 3 ) on nanoiron reactivity throughout its lifetime. It is
likely that many of the conclusions regarding geochemical effects on the
reactivity of iron filings will be applicable to nanoiron systems, but this
has not yet been demonstrated. If reactivity of nanoiron is found to be