Page 343 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

328 Environmental Applications of Nanomaterials

removed, thereby removing the contaminants (Tungittiplakorn et al.
2004). The potential to remove the contaminants from the nanoparticles
and reuse them was also demonstrated.
Environmental systems are complex and contain a wide variety of
organic and inorganic constituents that can compete for reactive sites
regardless of the contaminant of choice. For example, polymeric
hydrophobic nanoparticles designed to sequester PAHs or PCBs might
also sequester hydrophobic soil organic matter and block access to the
contaminants. In the case of DNAPL targeting, the available surface
area of DNAPL could be small in comparison to the total mass (e.g.,
pools), thereby limiting the mass of nanoiron that could be delivered to
the DNAPL/water interface. It may be sufficient to use less specific meth-
ods to control the travel distance such that reactive groundwater reagents
can simply be placed near the vicinity of the source and remain there.
This avoids the need for highly complex surface chemistry to provide
selectivity, which will likely be expensive. Most metal-oxide nanoparti-
cles will be relatively immobile in saturated porous media without suf-
ficient coatings to make them mobile. As discussed previously, the
transport distance for a specific coating type will vary depending on the
modifier type and on the geochemical conditions at the site (pH, ionic
strength, and ionic composition). This implies the potential to match the
modifier type to the geochemical conditions at the site to achieve a spec-
ified transport distance. With this approach, remedial agents could be
delivered to specific regions within or near the source zone. This could
also be done by using surface coatings that desorb at a specific rate such
that the nanoparticles travel a specific distance and then stop.
Nanoparticle surface modifications, coupled with innovative engineered
delivery schemes, will undoubtedly be necessary to achieve adequate
targeting of nanoparticulate reagents for groundwater remediation.

Challenges
Subsurface heterogeneity and complex NAPL architecture make reme-
diation difficult (Dai et al. 2001; Daus et al. 2001; Illangasekare et al.
1995). These issues also pose significant challenges to accurately deliv-
ering nanoparticulate remedial agents to subsurface contaminants.
Nanoparticles will tend to travel along zones of high hydraulic conduc-
tivity, which may or may not be the desired placement area. For
nanoparticles to reach the NAPL/water interface, particles may be
required to diffuse across flow lines as they travel through the porous
media. Even 10-nm particles may have prohibitively low diffusion coef-
ficients. For example, a micromodel study by Baumann et al. (2005)
investigated the targeting ability of a triblock copolymer-modified nano-
iron that had been shown to partition to the NAPL/water interface ex
situ (Saleh et al. 2007). At approach velocities of 2 to 13 m/d (residence