Page 326 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanomaterials for Groundwater Remediation 311

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The two electrons from Fe oxidation can be used to dechlorinate TCE
(Eq. 4), or can be used to produce H 2 (Eq. 5) . The mass of TCE dechlo-
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rinated per mass of Fe will depend on the relative rates of each of these
reactions, and on the value of n in Eq. 4, which is a function of the
dechlorination products formed. The products of TCE degradation using
nanoiron vary from acetylene (using RNIP) to ethane (using iron made
from the borohydride reduction of dissolved iron). Acetylene is the best-
case scenario as this pathway only requires 4 moles of electrons per mole
of TCE (n = 4). It requires twice as many to transform TCE to ethane
(n = 8). Assuming nanoiron costs $50/kg and that all injected zerovalent
iron is used to transform TCE, the cost of treatment ranges from $44/kg
TCE (for acetylene) to $88/kg TCE (for ethane). If oxidants other than

TCE were present (e.g., NO 3 or dissolved oxygen), the process efficiency
may decrease and the cost for iron would increase. For example, if only
10 percent of the electron from iron oxidation were used to transform
TCE, the cost would increase by a factor of 10. Careful consideration of
the site geochemistry and appropriate bench scale feasibility testing
are necessary to assess the economic feasibility of reactive nanoparti-
cles at a particular site.

Delivery and Transport Issues
The use of reactive nanomaterials for groundwater remediation is prom-
ising considering the availability and effectiveness of many types of
nanomaterials for degrading or sequestering environmental contami-
nants of concern. The attractiveness of nanomaterials is their potential
to be used in situ, directly degrading the contaminants in the subsur-
face without the need to excavate them or pump them out of the ground.
Realizing the potential of nanomaterials will require the ability to inject
them into the subsurface and transport them to the contaminant source
zone where they rapidly degrade the contaminants. If particles cannot
be delivered to and remain at the source of contamination, they will not
be utilized efficiently.

Injection methods and delivery vehicles
Reactive nanoparticles can be injected into a well and allowed to trans-
port down gradient from the injection site to the contaminated area.
Typically this is done in existing wells, or in wells drilled specifically for
the remediation process. Drilling and packing a well, even a small diam-
eter (3-inch) well is quite expensive. Direct push wells (Figure 8.7) are
a lower cost alternative to drilled wells, and are the most often used
delivery tool for remediation with nanoiron. These are hydraulic devices
mounted on a truck or tractor. They can create wells that are 2 to 3 inches