Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 122

of surface functional groups on sorbed NOM (Hunter and Liss, 1982; Loder and
Liss, 1985). This adsorbed surface layer is likely to dominate the surface properties
of colloids such as charge. Therefore, a useful approximation in terms of surface
charge and aggregation may be to treat colloids as a single class of colloidal materi-
als, irrespective of their nature, (Filella and Buffl e, 1993 ; O ' Melia, 1980 ). However,
these surface coatings may be patchy (Gibson et al. , 2007), depending on the nature
of the underlying substrate, the NOM type and the solution conditions, meaning
that this assumption must be tested in most circumstances.
The presence of NOM surface coating on environmental colloids was fi rst shown
using surface charge measurements by electrophoresis. The use of TEM, AFM and
fi eld fl ow fractionation have given further insight into the thickness and nature of
such a surface coating. The formation of surface coating on a mica surface from
IHSS Suwannee River FA is shown in Figure 4.6 (Gibson et al. , 2007 ). The thickness
of fi lm found was of about 0.4-5 nm. It has been shown that humic substances sorbs
to iron oxide colloids (Baalousha et al. , 2008), resulting in the formation of nanoscale
surface coating. The thickness of this surface coating was found to be of the order
of 0.8 nm on iron oxide particles in the presence of 25 mg l

1 humic acid, although
aggregation was also increased at these concentrations due to bridging and charge
neutralisation.
Surface coating of colloids by NOM is likely to affect aggregation behaviour
resulting in reduced aggregation through charge stabilization (Jekel, 1986) and
steric stabilization mechanisms (Tipping and Higgins, 1982) or enhanced aggrega-
tion through charge neutralization and bridging mechanisms caused by fi brillar
attachment (Buffl e et al. , 1998 ).

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4.4.4 Fractal Dimension

Aggregation of natural colloids results in the formation of fractal aggregate struc-
tures. A fractal object has a self-similar structure at all levels of magnifi cation, that
is it can be sub-divided into parts, each of which is a reduced-size copy to the whole
structure. There are three types of fractal structures: exact self-similar, quasi self-
similar and statistical self-similar. The latter is the weakest type of self similarity, in
which the fractal has a statistical numerical measure which is preserved across dif-
ferent scales. Natural colloidal aggregates generally fall under this type. The fractal
dimension, D , can be defi ned as a statistical quantity that gives an indication of how
a fractal structure appears to fi ll space. The fractal dimension can be described by
a geometric power law scaling each dimensional geometry (volume (v) or mass (m)
for three dimensions D 3 , projected area (A) for two dimensions D 2 , or perimeter
(P) for one dimension D 1 ) and characteristic length scales (L) of the aggregate (Lee
and Kramer, 2004 ). D 1 provides information about the morphology of the aggregate
related to the irregularity of the aggregate boundary or perimeter. D 2 provides
information about the projected area of an aggregate and D 3 provides information
about the mass distribution within the aggregate.

morv ∝ L

D

A ∝ L

D

P ∝ L

D

1

(4.1)

3 2