Environmental Engineering Reference
In-Depth Information
(Buffl e and van Leeuwen, 1992; Wilkinson and Lead, 2007). In addition to these
fi elds of environmental science, the pure and applied physics (nanoscience) and
physical chemistry (colloid and surface chemistry) can also contribute, to form the
basis for the development of analytical and characterization methodologies for
manufactured NPs in the environment. However, the instruments and methods
often used in nanoscience and colloid chemistry rely on the assumption that samples
are very simple systems with often only one type of NP, no other impurities, narrow
size distributions (low polydispersity) and high concentrations. Most of these
assumptions do not hold for environmental samples or ecotoxicological tests and
caution should be taken when applying methods optimized for different operating
conditions, even for the same type of manufactured NP. It is appropriate here to
discuss briefl y defi nitions of the terms NP, colloids and other terms from a measure-
ment perspective; general defi nitions of NPs are discussed in Chapter 1.
The fi elds of nanoscience and nanotechnology are young and rapidly developing.
Therefore, rigorous defi nitions have not yet been formulated but extensive work
are ongoing in various international and national standardization organizations, for
example ISO, CEN, BSI, ANSI, and so on. Defi nitions for nanotechnology are that
it applies to nanomaterials that have at least one dimension in the size range
1-100 nm plus that the material shall have an added functionality due to its size or
that the nanotechnology includes manipulations of the materials at this length scale.
But defi nitions of nanomaterials as such are often defi ned based only on their sizes.
For example, a nanoparticle is defi ned to having all three dimensions in the range
1- 100 nm (ISO, 2008 ). This is the defi nition of NP that will be applied here.
Colloids or colloidal particles, were fi rst defi ned in 1861 as particles ' being
immune to sedimentation' (Graham, 1861). Colloids have been defi ned as being one
continuous phase inside another continuous phase. One trivial example of a col-
loidal system is milk, where the fat droplets and proteins are dispersed in the water
phase. The upper size range of colloids is controlled by the particle's properties of
being small enough to have Brownian motion (diffusion) that dominates over sedi-
mentation and that holds up to about 1
m (depending on density) for non-aggre-
gating NPs. IUPAC has operationally defi ned colloids as particles having at least
one dimension in the size range 1-1000 nm (Lyklema and van Olphen, 1979), and
that is probably the most widely accepted defi nition (Lead and Wilkinson, 2006).
However, other defi nitions are used. For instance, Gustafsson and Gschwend dis-
cussed a concept of a more functional defi nition of colloids (Gustafsson and
Gschwend, 1997) in natural aquatic systems. In their ' chemcentric ' view, colloids are
defi ned as particles or macromolecules that can provide a molecular milieu which
chemicals can be sorbed into and onto from the bulk phase. Such a milieu can be
signifi cantly different from the bulk phase, for example in terms of ionic composi-
tion, pH or charges. A colloid particle can be large enough to have an electrical
double layer (EDL) around the particle with charges on the particle surface.
However, this charge potential drops off exponentially as the distance from the
surface increases and in the bulk phase. The chemcentric view on colloids is, there-
fore, not primarily based on size. Another important distinction/highlight of the
chemcentric view is that colloids that fulfi l the colloidal requirements in one water
system may not do so in another, where water chemistry or physics are different (e.g
different size cut-offs for sedimentation in a stagnant and turbulent water body).
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