Page 288 - Environmental Nanotechnology Applications and Impacts of Nanomaterials

Nanoparticle Transport, Aggregation, and Deposition 273

the actual contact area leading to higher mobilization rates. However, the
opposite scenario is likely for nanoparticles as their size will allow them
to fit in between even tightly spaced surface asperities. In other words,
if the particle diameter is greater than the distance between any two
asperities, adhesion will tend to be low, as the contact area is low [80].
However, if the particle diameter is less than this distance, the adhesion
will be higher, due to a higher contact area. Thus, roughness should tend
to increase the contact area between the collector and the depositing
nanoparticle [78] and resuspension should be reduced.

Nanoparticle Behavior in Heterogeneous
Systems
Environmental systems are characterized by a number of naturally
occurring colloidal materials, such as clays, products from mineral
weathering, bacterial polysaccharides, and other organic macromole-
cules. Natural organic matter (NOM) [83] is a broad class of organic
macromolecules and is particularly relevant as it is ubiquitous to nearly
all soil and water systems. Here we will consider cases where NOM
and inorganic materials may interact with nanomaterials and alter
their subsequent behavior in aqueous systems.
In laboratory analyses, particle properties are often characterized in
relatively simple model systems such as in solutions of one or two back-
ground electrolytes added to purified and deionized water. The large
number of components present in natural waters may include mono- and
multi-valent salts, NOM, clays, microorganisms, and other colloidal
materials. Each of these materials, either individually or in concert,
can alter the characteristics of particles [84].
For example, the size and highly reactive nature of NOM confer on
these molecules a high interfacial reactivity that influences physico-
chemical interactions in rhizospheric and aquatic environments [85].
Nanoparticles may have similar properties. The diversity of nanoscale
materials and the complex composition of soil suggests a nearly infinite
number of interactions. Two likely possibilities that merit consideration
in assessing the role of organics in determining the state of nanoparti-
cles in aqueous systems and exposures to living organisms are: 1)
nanoparticles adsorbed into organo-mineral complexes, and 2) nanopar-
ticles diffusing through (or being entrapped within) extracellular mate-
rials and biofilms.
Particles in natural waters, regardless of size, may associate with
organic macromolecules that subsequently alter the particle’s interfa-
cial and physical characteristics (e.g., charge, reactivity, size) [86]. For
example, association of NOM with particles can alter their effective
charge, reactivity, and hydrodynamic radius [84]. Nanoparticles in