Environmental Risks of Nanomaterials - Nanotechnologies for Water Environment Applications - page 602

pharmaceutical products, disposal of screen monitors, computer boards, automobile tires,
clothing and cosmetics). Differences in NP mobility in porous media may be a function
of surface chemistry, particle size, and conditions such as high ionic strength, divalent
ions (Brant et al., 2005a).
In the air, several factors affect the fate of airborne NMs, such as the length of
time during which the particles remain airborne, the nature of NMs' interaction with
other airborne particles or molecules, and the distance that NMs may travel prior to
deposition. The processes related to atmospheric transport of NMs include diffusion,
agglomeration, wet and dry deposition, and gravitational settling. The settling rate is
proportional to the particle diameter, while the rate of diffusion is inversely proportional
to the particle diameter. Deposited or agglomerated NPs are not easily resuspended in
the air (Aitken et al., 2004). In order to be easily dispersed, NMs typically are
functionalized on their surfaces. In the air, NPs have been found to act synergistically
with other pollutants (e.g., O 3 , NO x ) (Elder et al., 2000). The interactions between
particles and gaseous components may complicate the assessment of the NP effects and
their associated risks.
In soil, the fate of NMs depends upon the physical and chemical properties of the
NMs. NMs released to soil can be strongly sorbed to the soil due to their high surface
areas, while NMs are small enough to enter into smaller spaces between soil particles
and travel farther than larger particles before being trapped in the soil matrix. Studies
have demonstrated the differences in mobility of a variety of insoluble NMs in a porous
medium (Lecoanet and Wiesner, 2004; Lecoanet et al., 2004). In addition, the properties
of the soil and environment can affect NM mobility. For example, the mobility of
mineral colloids in soil and sediments is strongly affected by their charges.
In water, the fate of NMs is controlled by aqueous solubility or dispersability,
particle diameter, interactions between a NM and chemicals in the system, and
biological and abiotic processes. NMs can be retained in the water column due to
diffusion and dispersion. Waterborne NPs settle more slowly than larger particles of the
same material but can be removed from water by agglomeration, adsorption,
biodegradation or photocatalyzed reactions. In the case of abiotic processes, both
chemical and photoactivated reactions in particle/water systems are likely involved in
NM transformations. Light-induced photoreactions often are important in determining
environmental fate of chemical substances. For example, heterogeneous photoreactions
on surfaces of metal oxide NMs (such as titanium dioxide and zinc oxide) are
increasingly being used as a method for drinking water, wastewater and groundwater
treatment.
Many NMs frequently form much larger colloidal aggregates. For example, 20-
nm anatase particles easily form aggregates with a stable diameter of ~200 nm.