Page 258 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanoparticle Transport, Aggregation, and Deposition 243

(or equivalently, mobility) will have the effect of reducing concentra-
tions in environmental systems. Aggregation may occur between
nanoparticles or with other materials (heteroaggregation). In either
case, a reduction in stability associated with an increase in aggrega-
tion rate has direct consequences for the removal of nanoparticles
from air and water in engineered systems (e.g., water and wastewater
treatment systems) as well as in the environment.
The role of aggregation in determining toxicity may be less evident.
Reductions in active surface area that occur during aggregation and an
increased proximity of particle surface area within aggregates may
fundamentally reduce reactivity compared with nanoparticles in an
unaggregated state. Recent evidence suggests that destabilized
nanoparticles experience a decreased ability to produce reactive oxygen
species [23] and this may have implications regarding their toxicity [24].
Aggregation may also alter the bioavailability of the nanomaterial if
larger materials are less capable of entering cells. Conversely, aggre-
gation may be at the heart of the toxic response as in the case of carbon
nanotube agglomeration within lungs, which has been observed to lead
to suffocation in laboratory animals where these materials were inten-
tionally introduced [25, 26]. It is possible that heteroaggregation of
nanoparticles with organic molecules or other materials may essentially
imbed the nanomaterial in another functionality or may alter the avail-
ability of the nanomaterial due to changes in size or chemistry.
Aggregation will be equally important in environmental engineering
applications of nanomaterials. The proposed use of nanoparticles for
groundwater remediation is one example where nanoparticle aggrega-
tion may decrease the effective surface area of the particles, which
reduces their ability to effectively oxidize or react with other com-
pounds. Aggregation may affect the mobility of the nanomaterial and
therefore the ability to deliver nanoparticles to a desired location in the
subsurface. In summary, the environmental applications, exposures,
and effects of nanoparticles are therefore likely to depend on the con-
ditions under which nanoparticles remain as discrete units or aggre-
gate into clusters.
Some nanomaterials have an inherently small affinity for water.
For example, the fullerene C 60 is negligibly soluble in water. Such
nanomaterials may be modified to make them compatible with a given
end-use or processing requirements in aqueous systems. Nanoparticles
may be further modified to maintain a stable suspension. Particle
suspensions are commonly stabilized through changes in solution
chemistry or modification of particle surface chemistry [27]. How-
ever, functionalizing nanoparticles surfaces may compromise the
characteristics of the nanoparticles that make them desirable in the
first place.