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Nanomaterials for Groundwater Remediation 315

models is that the bed is clean—that is, only particle-media grain inter-
actions are explicitly accounted for. This may not be the case for
groundwater remediation using engineered nanoparticles where high
concentration slurries of polydisperse particles will be injected to
achieve a 0.1 wt% to 0.5 wt% concentration of particles in the aquifer.
Under these conditions, particle-particle interactions are important,
and filter-ripening models may be more appropriate than clean bed fil-
tration models. Regardless of the model type (clean bed or filter ripen-
ing), model inputs will include particle size, attachment coefficients,
flow velocity, ionic strength, and ionic composition as discussed below.
Aggregation and surface modification, and how these affect the mobil-
ity or engineered nanomaterials and the ability to target specific regions
in the subsurface, are discussed.

Effect of aggregation. If aggregation is rapid, and creates particles that
are greater than a few microns in diameter, there is potential for this
aggregation to limit their transportability. For many nanoparticles, aggre-
gation will likely be a mechanism that significantly limits their transport
in the environment, and makes it possible to remove them using standard
water-treatment practices such as flocculation/sedimentation or mem-
brane filtration. The rate of particle aggregation and the size and mor-
phology of the aggregates formed depends on both the collision frequency
(transport) and the collision/attachment efficiency (i.e., the magnitude of
the attractive and repulsive forces between the particles).
Experience shows that many nanoparticle suspensions are typically
highly unstable and rapidly aggregate (Saleh et al. 2005a), making
them difficult to handle. The reason for the rapid aggregation is that
their small size (~100 nm) gives them high diffusion coefficients (trans-
port rates) and an exceptionally high number of collisions, so even
dilute suspensions of nanoparticles with relatively low sticking coef-
ficients would rapidly aggregate and not remain as individual nanopar-
ticles in suspension under normal environmental conditions. Moreover,
the presence of cations and anions, particularly divalent cations such
2+ 2+
as Mg and Ca , further destabilizes nanoparticles and increases
their rates of aggregation. For nanoiron, 100-nm particles present at
5
a volume fraction ( ) as low as = 10 rapidly aggregate into 5-micron
particles in just a few minutes (Figure 8.9). This problem is exacer-
bated for nanoparticles such as nanoiron that are magnetic and there-
fore subject to non-DLVO magnetic attractive forces. The time scale of
colloid dispersion stability is determined by the magnitude of the
energy barrier between particles. According to classical DLVO theory,
the major attractive energy is van der Waals energy (V vdW ) while the
major repulsive energy is electrostatic interaction energy (V ) (de
ES
Vicente 2000; Elimelech et al. 1995; Evans 1999; Heimenz 1997;