Page 346 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials for Groundwater Remediation 331

and attachment to aquifer media grains (particle-media grain interac-
tions). The geochemistry at each site (e.g., pH, ionic strength, and ionic
composition) will dramatically affect the mobility of engineered nano-
materials in the subsurface and must be considered in their design and
application. The ability to tailor the surface coatings to site-specific geo-
chemical conditions for well-controlled placement of engineered nano-
materials in the subsurface appears to be obtainable in the near term and
will enhance the cost-effectiveness of this remedial approach. A higher
mobility of nanomaterials in the environment due to the use of surface
coatings, however, implies a greater potential for exposure as nanoma-
terials are dispersed over greater distances and their effective persistence
in the environment increases. The trade-off between enhanced mobility
and effectiveness and potential exposure and risks must be considered on
a case-by-case basis.
The ability to target specific contaminants or to concentrate the reac-
tive nanomaterials in the contaminant source zone will be required to
make in situ groundwater remediation cost-effective. The use of surface
coatings to enhance target specificity is very promising and remains an
active area of research. Despite the ability to develop nanomaterials with
target specificity, there remain significant challenges for in situ delivery
of nanomaterials in the subsurface due to unfavorable hydrodynamics in
many cases. Even at low approach velocities typical of groundwater flow,
the diffusion rates of particles that are 10 to 100 nm in diameter across
flow lines to adsorbed contaminants may be prohibitively slow to allow
them to diffuse across flow lines to reach their targets. Methods to tailor
the surface chemistry of nanomaterials to concentrate reactive materials
in specific regions in the subsurface appear promising, and may be a
more obtainable near-term goal to achieve well-controlled placement of
nanomaterials in contaminant source zones.

List of Acronyms and Symbols
aq: aqueous
CMC: carboxy methylcellulose
COC: chlorinated organic compounds
DCA: dichloroethane
DCE: dichloroethene
DIRB: dissimilatory iron reducing bacteria
DLVO: Deraguin-Landau-Verwey-Overbeek
DNAPL: dense nonaqueous phase liquid
d p : particle diameter (L)

e : electron