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318 Environmental Applications of Nanomaterials

followed by sedimentation and filtration are commonly used to treat
drinking water (Viessman and Hammer 1998), the tendency of nanopar-
ticles to rapidly aggregate suggests that they will be easily removed in
traditionally used water-treatment systems.
The rapid aggregation of most types of nanoparticles has led to the
use of coatings to modify their surface chemistry in a way that minimizes
aggregation and increases the stability of aqueous dispersions.
Surfactants and natural and synthetic polymers have been proposed as
a means to stabilize nanoiron suspensions in order to make them trans-
port effectively in the subsurface (Saleh et al, 2005, 2007). This will
affect their fate and transport characteristics in the environment, and
may also affect their ability to be removed from water supplies or during
treatment, so the impact of surface coatings on the environmental fate
and transport should be considered in the design process for the coat-
ings. For example, coatings could be used to deliver particles in the sub-
surface, but then biodegrade, transform, or desorb after some time so
that the particles are no longer mobile and the potential for exposure is
minimized.

Surface modification. Effective use of many of the new nanoparticles
being developed will require that particle aggregation be limited. For
example, metal-containing nanoparticles being considered as MRI con-
trast agents will have to be designed to be dispersible in aqueous envi-
ronments with high osmolarity such as blood (Sitharaman et al. 2004;
Veiseh et al. 2005). Surface functionalization is a common tool for min-
imizing or controlling nanoparticle aggregation. This niche of nan-
otechnology has the potential to create nanoparticles that could be
widely distributed and mobile in the environment, easily assimilated
into people or other biological organisms, and difficult to remove by
remediation and treatment if needed. The uncertainties surrounding the
effect of surface coatings on the fate, transport, and potential toxicity
of engineered nanomaterials makes this a rich area for current and
future research on nanotechnology. Nanoparticles used for in situ
groundwater remediation will require surface coatings for their intended
application (Saleh et al. 2005a; Saleh et al. 2007). There are three classes
of typical surface coatings used, including polymers, polyelectrolytes,
and surfactants. These coatings can impart charge to the particles (pos-
itive or negative) and can provide three modes of colloidal stabilization
that make them mobile in the environment: electrostatic, steric, or elec-
trosteric (Figure 8.11). Both natural and synthetic varieties of each type
of modifier are widely available and used (Table 8.2).
In general, high molecular weight polymers (synthetic or natural)
provide steric repulsions that may limit nanoparticle-bacteria inter-
actions (Figure 8.11). Polyelectrolytes are large polymers containing