Environmental Engineering Reference
In-Depth Information
and 5), and their transformation, degradation and persistency. In addition, the
fate of nanomaterials in the environment is likely to vary with the physical and
chemical characteristics of the nanomaterials and the containing medium and the
interaction of nanomaterials and other environmental contaminants. Understanding
the fate and behaviour of natural colloids (nanoparticles) in water and soil (Chapter
4 ) and ultrafi ne particles in air (Chapter 5) will improve our understanding and
ability to predict the fate and behaviour of manufactured nanomaterials in these
systems.
1.13.1
Fate in Air
Natural and particularly adventitious (e.g. from fossil fuel combustion) nanomateri-
als are very abundant in the atmosphere and are present in the vicinity of any
combustion process (see details in Chapter 5). Their concentration is highly ele-
vated above the unpolluted background concentration in areas with heavy human
occupancy. Atmospheric nanomaterials have three main sources: (i) primary emis-
sion, refers to those that are directly emitted from road traffi c exhaust and indus-
trial combustion; (ii) secondary emission, refers to those that are formed within
the atmosphere from the condensation of low volatility vapours formed from the
oxidation of atmospheric gases; and (iii) formation during diesel exhaust dilution.
However, there are few, if any, data sets on manufactured nanomaterials in the
atmosphere in terms of sources and concentrations. This lack of data about engi-
neered nanomaterials in the atmosphere is due to the absence of methods capable
of discriminating engineered nanomaterials from the background concentration
from other sources (Harrison, 2007), which is similar to the situation in aquatic and
terrestrial environments (Chapter 4).
From what is known from fi ne and ultrafi ne particles, nanomaterials can undergo
several processes in the atmosphere. Some nanomaterials can grow by condensa-
tion of low volatility compounds, or shrink by evaporation of adsorbed water or
other volatiles, resulting in the variation in particle size distribution but not the
overall number concentration (Zhang et al. , 2005). Further, atmospheric nanoma-
terials can aggregate, resulting in an increase in particle size with a decrease in the
number concentration (Gidhagen et al. , 2004). They can be lost from the atmo-
sphere by dry and wet deposition, both of which are effi cient for very small particles
of natural origin and so presumably also for engineered nanomaterials. This results
in a reduction in particle number concentration and a shift in particle size distribu-
tion to larger sizes (Clarke et al. , 2004). They can be diluted in mixing with cleaner
air. For example, particles from traffi c mix upward with less polluted air, leading
to a reduction in the number concentration and generally an increase in particle
size distribution simply because the diluted air contains a distribution of larger
particle sizes (Zhang et al. , 2005). These processes are discussed in detail in Chapter
5 for adventitious nanoparticles.
1.13.2
Fate in Water
The potential fate and behaviour of engineered nanomaterials, once they are
released into the aquatic environment, can be understood in the light of the existing
Search WWH ::




Custom Search