Page 303 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

288 Principles and Methods

result in higher particle aggregation or growth rates [113]. For exam-
ple, higher particle concentrations result in higher particle growth rates
(60 nm/day in the winter and 103 nm/day in the summer) as a result of
higher collision frequencies.

Summary
For many nanoparticles larger than several tens of nanometers, many
traditional relationships and models used for colloidal systems may be
used for describing nanoparticle behavior in aqueous systems. However,
when particles are smaller than approximately 20 nm, particle behav-
ior increasingly resembles that of a molecular solute and intermolecu-
lar forces play a greater role in determining the transport, aggregation,
and deposition of these materials. Heterogeneities of the surfaces with
which nanoscale particles may interact will also play an increasingly
important role, and the characterization of these surfaces is therefore
important in predicting nanoparticle behavior.
Nanoparticle transport at the mesoscopic scale in aqueous systems is
dominated by their characteristically high diffusion coefficients as a
result of their small size. While this confers a high mobility to nanopar-
ticles in a liquid or gas, it also results in them having high contact effi-
ciencies with potential collector surfaces, making them relatively
immobile even when they possess low attachment efficiencies ( < 0.1).
The stability of nanoparticle dispersions will largely determine how
long nanoparticles are likely to remain in the nano-domain.
Nanoparticles often aggregate to form clusters both with and without
the presence of destabilizing agents or changing chemical conditions. It
is therefore necessary to consider the transport of nanomaterials both
as nanoparticles and as materials that may transition into the colloidal
domain or larger, where they may be subject to transport mechanisms
such as gravitational settling. Aggregation will therefore likely reduce
the persistence of these materials in the environment and possibly their
bioavailability.
The deposition of nanoparticles on surfaces ranging from sand grains
in aquifers to leaves on trees will also reduce their persistence.
Nanoparticles can be entrapped by myriad media ranging from mineral
formations to biofilms. The interaction between nanoparticles and sur-
faces may be assessed within the context of various established interfa-
cial models, such as the DLVO model discussed here.
According to the DLVO theory and its extensions, the stability of par-
ticle dispersions results from the sum of repulsive and attractive forces
at their interface with neighboring solid interfaces. Generally, repulsive
electrostatic interactions between like charged particles controls dis-
persion stability, though the nature of these and other interactions