Page 345 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

330 Environmental Applications of Nanomaterials

In principle, nanoparticles that have a strong affinity and selectiv-
ity for the target contaminant could be injected into the subsurface
and allowed to migrate nonspecifically toward the source area where
they will locate and degrade or sequester the target contaminant. This
is analogous to targeted drug delivery where drugs are introduced into
the body and migrate toward the target (diseased tissue). If this is not
achievable, delivering nanoparticles near the source area may be
acceptable. This will, however, require that the source area be very
well characterized in terms of the contaminant masses and locations,
and that the hydrogeology at the site be well known. Without this char-
acterization, it will be difficult to engineer and implement an effective
delivery system. Source zone characterization is difficult, however, so
techniques to effectively characterize these areas are needed.
Partitioning tracers (e.g., alcohols) and interfacial tracers (e.g., sur-
factants) have shown some promise for characterizing DNAPL source
zones (Annable et al. 1998). Methods to characterize sources of heavy
metals such as Cr(VI) are less well developed.

Summary and Research Needs
Unique and inexpensive reactive or highly sorptive engineered nano-
materials are becoming commercially available. The use of these novel
engineered nanomaterials for improved in situ remediation of ground-
water contaminants is promising, but engineering nanomaterials that
are highly selective for the reactions of interest and that have long life-
times such that they are cost-effective for in situ remediation of ground-
water contaminants remain active areas of research. Potential concerns
about the exposure and human health effects of these engineered nano-
materials must also be addressed to ensure the safe deployment of these
materials. Future research developing novel nanomaterials for ground-
water remediation will have to consider the reactivity, selectivity, and
longevity of materials as well and the potential exposure and human
health effects of the materials.
Even though relatively inexpensive commercially produced reactive
nanomaterials for in situ groundwater remediation (e.g., nanoiron) are
available, methods to deliver these materials to subsurface contaminants
remain a challenge. Intuitively, the assumption is often made that
nanoparticles will be more mobile in porous media due to their small
size. However, all other factors being equal, smaller particles are less
mobile due to their relatively large diffusivity that produces more frequent
contacts with the surfaces of aquifer porous media. The use of surface coat-
ings to enhance subsurface transport and to encourage selectivity toward
specific groundwater contaminants will be needed for effective deployment
of engineered nanomaterials in groundwater remediation. These coat-
ings can be used to control aggregation (particle-particle interactions)