Page 22 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

8 Nanotechnology as a Tool for Sustainability

Nanoparticles are nearly all surface. As an approximation, a 4 nm
diameter solid particle has more than 50 percent of its atoms at surface.
One gram of single-walled carbon nanotubes (SWNTs) has over 10 m 2
of available surface area. This results in nanoparticles being highly sur-
face reactive and implies that their behavior will, to a great degree, be
mediated by interfacial chemical interactions. These interactions should
therefore be governed by the characteristics of these surfaces as reflected
in adsorption energies, the change in the surface energy heterogeneity
due to the change of lattice parameters, the distortion of the bonds, and
the increase of the surface pressure.
Because atoms at interfaces behave differently, nanomaterials are
likely to have unique properties compared to larger bulk materials of
the same general composition. Also, as the size of particles of a given
material approaches the nanoscale, material properties such as electrical
conductivity, color, strength, and reactivity may change. These changes
are in turn related to underlying effects of size that include quantum
confinement in semiconductor particles, surface plasmon resonance in
some metal particles, and superparamagnetism in magnetic materials.
Greater reactivity, and the ability of some nanoparticles to act as elec-
tron shuttles or, in other cases, as photocatalysts, holds particular inter-
est in environmental applications. The photocatalytic properties of
mineral and fullerene nanomaterials are presented in Chapter 5.
The ability of some nanomaterials to photocatalytically produce reac-
tive oxygen species (ROS) that may be used to oxidize contaminants or
inactivate microorganisms may also present a risk of toxicity to organ-
isms. A methodology for assessing nanoparticle toxicity is presented in
Chapter 6. While the toxicity of some nanomaterials may be related to
ROS production, there may be other possible mechanisms as discussed
in Chapters 11 and 12.
For nanomaterials to present a risk, there must be both a hazard,
such as toxicity, and potential for exposure to these materials. The
interfacial chemical properties of nanoparticles in aqueous media affect
particle aggregation and deposition processes that in turn affect expo-
sure. Nanoparticle stability and transport are important in determin-
ing whether these materials can be removed by water treatment
technologies, or whether nanoparticles have a high potential for mobil-
ity in the environment. The mobility of submicron particles and the fac-
tors that control particle transport, aggregation, and deposition in
aqueous systems have been explored extensively, particularly for cases
such as silica and latex suspensions [14–16]. However, despite a large
number of publications describing procedures for producing nanopar-
ticles of specific size, shape, and composition displaying unique prop-
erties of reactivity and mobility compared with better-known bulk
phases, there has been little theoretical consideration of the special