Page 339 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

324 Environmental Applications of Nanomaterials

repulsions provided by the polyelectrolyte used here are much more
effective than for surfactants that rely solely on electrosteric repulsions,
or natural polymers or surfactants (e.g., alginate), and should be con-
sidered a best-case scenario (i.e., most transportable). Even under these
conditions, only transport distances of a few 10s of meters are expected
since these cations are present in most surface and subsurface waters.
Thus even nanomaterials that are engineered to be highly mobile in the
subsurface are not expected to be exceptionally mobile in the subsurface.
This poses challenges from a standpoint of delivering the particles in situ,
but comes as a relief from the standpoint of potential risks posed by
releasing these particles into the environment. Further, this implies
that the incidental release of engineered nanomaterials that inadver-
tently contaminant our current water-treatment infrastructure should
be easily removed as these processes employ ionic strength increases and
addition of divalent cations to destabilize particles to remove them.

Targeting
The ability to target specific contaminants is important in any in situ
groundwater remediation scheme. Specificity is advantageous for sev-
eral reasons:

■ Higher contaminant transformation rates. Since most reactions
and sorption processes are first order with respect to the contaminant
of interest, placing reactive or sorptive particles in the sources area
where contaminant concentrations are highest will provide the most
rapid transformations
■ Higher efficiency. Placing reactive particles at the contaminant
source where the contaminant concentrations are highest will allow
the preferred process (degradation or sorption of the contaminant)
to out-compete the competitive processes (e.g., H evolution from
2
nanoiron).
■ Ability to maintain the reactive particles in the area of con-
tamination and minimize unwanted exposures. If the reactive
particles migrate with the natural groundwater gradient and do not
have any affinity for the contaminant of interest, they may move
through the source area before they are fully utilized. Targeting nano-
materials to the contaminant will maximize the efficiency of the reme-
diation process and minimize the mass of nanoparticles needed.
Further, the ability to maintain the reactive nanoparticles in the con-
taminant source zone will minimize the potential for unwanted migra-
tion and exposure of sensitive biological targets in wetlands, lakes, or
streams where groundwater may be discharging.