Overview of Nanoscience in the Environment - Environmental and Human Health Impacts of Nanotechnology - page 14

systems, colloids are defi ned (Chapter 4) as materials naturally produced by pro-
cesses such as erosion and degradation, within the size range 1- 1000 nm (Lead and
Wilkinson, 2006). In the atmosphere, particles (Chapter 5) are mainly classifi ed
according to their size (PM 10 , PM 2.5 and PM 0.1 ). Thus, nanoparticles correspond to
PM 0.1 (these are usually called ultrafi nes in this fi eld) and overlap with the small
fraction ( < 100 nm) of aquatic colloids.

1.9.4 State

Nanomaterials can be present in different states, either free, fi xed or aggregates.
Free nanomaterials are single individual nanomaterials. Fixed nanomaterials are
those incorporated in products such as computer chips, rackets, tennis balls and so
on. Aggregates are associations of nanomaterials in a network-like structure that
forms due to the effect of physicochemical properties of the media in which they
are contained.

1.10 Sources of Nanomaterials in the Environment

An overview of potential sources and pathways of nanomaterials in the environ-
ment is provided in Figure 1.4. As contaminants, engineered nanomaterials may be
released from a point source, such as a production facility, landfi ll and wastewater
treatment plant, or a non-point source, such as wear from material containing
nanomaterials. They may be accidentally released during production and transport
via a leakage from improper sealing or intentionally released, such as the use of
zero-valent iron nanoparticles for soil remediation. Further, nanomaterials can be
released into the environment either as single nanoparticles or as embedded in a
matrix, from where release of nanomaterials will occur through the degradation of
the matrix material (Kohler et al. , 2007). Currently, the main source of adventitious
nanomaterials is the incomplete oxidation of anthropogenic compounds by human
activities, such as wood burning, fuel burning in diesel engines or cars with defective
catalytic converters. Sources and pathways of natural aquatic nanoparticles are
discussed in more detail in Chapter 4 and those of atmospheric nanoparticles are
discussed in Chapter 5 .

1.11 Properties of Nanomaterials

At the nanoscale, material properties such as mechanical, electronic, magnetic,
optical, chemical, biological and others may differ signifi cantly from the properties
of micrometre-sized or bulk counterparts. For instance, they can be more chemi-
cally reactive and catalytic, have greater strength or conduct electricity more effec-
tively. This difference in materials behaviour at nanoscale level can be explained
by high specifi c surface area per unit mass, the increase of proportion surface atoms
with the decrease in size (Oberdorster et al. , 2005a), the presence of undercoordi-
nated bonds, greater disorder at the surface or other mechanisms. Properties that