Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 119

(Alasonati et al. , 2007 ; Wilkinson et al. , 1999). Far less work on these polysaccharides
has been performed when compared to HS, primarily for this reason.

4.4 Intrinsic Properties of Environmental Colloidal Particles

This section focuses on the colloidal properties that are relevant to their environ-
mental behaviour and characterization principles, especially as they might enable
us to understand the fate and behaviour of manufactured nanoparticles.

4.4.1 Size

Size is the primary means of defi ning colloids in natural systems (Section 4.2) and
is a useful parameter, as other physical and chemical parameters relevant to col-
loidal behaviour, for example diffusion coeffi cient, are infl uenced by, or correlate
with, size. Therefore, potentially their behaviour and role in the biogeochemical
cycling of trace pollutants and other processes may be understood in terms of size.
Colloids, that is particles smaller than 1 m, tend to stay in suspension, while larger

particles tend to sediment (Buffl e and Leppard, 1995) and so can be thought of as
fundamentally different from particles, even if they are composed identically in
chemical terms. As noted, at sizes

µ

10 -25 nm, environmental properties such as
metal binding, zeta potential and redox properties may change radically in com-
parison to the bulk or larger phases of the same composition (Madden et al. , 2006 ;
Madden and Hochella, 2005).
The size distribution of natural particles (Figure 4.1a) depends on the source and
nature of particles, physical, chemical and biological processes, such as erosion,
degradation, aggregation, disaggregation and ageing, and the physicochemical
parameters of the system, such as pH, ionic strength and redox potential. Typical
TEM images of natural colloidal particles from a River and a lake (Buffl e et al. ,
1998) are shown in Figures 4.1b and 4.1c, and it is clear that it is hard to defi ne an
exact particle size due to the inherent differences in colloidal shape and to (possibly
reversible) aggregation.

<

4.4.2 Surface Charge

There are two types of surface charge in colloids (Figure 4.4). The fi rst is a perma-
nent charge which arises from the isomorphous substitution of cations within the
colloid, for example substitution of silicon(IV) by aluminium(III) in kaolinite. The
second is a variable charge originates from chemical reactions at the colloidal
surface: (i) ionization or dissociation of the surface functional groups (e.g. the dis-
sociation of protons from carboxylic groups); (ii) dissolution of ionic solids (e.g.
AgI); or (iii) specifi c sorption of charged species, for example simple ions such as
Ca 2+ , surfactant ions and polyelectrolyte chains such as humic substances or syn-
thetic surfactants. The total charge is the sum of permanent and variable charges.
At pH values of natural conditions, most colloidal particles are negatively
charged. The exception to this is iron oxides, which have a point of zero charge (pzc,
the pH at which the overall charge equal zero) of about 7-9 depending on their