Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 135

interactions. For example, biopolymers can bridge natural colloidal particles
and induce their aggregation (Chen et al. , 2006 ) (Section 4.5.4.4 ).

In environmental systems, the size distribution of organic and inorganic colloids
is thought to follow a Pareto-type law in the nanometre to micrometre range
(Filella and Buffl e, 1993). This size distribution represents that of individual colloids
and their aggregates. Quantitative physico-chemical aggregation theory (DLVO
theory, Section 4.5.1) exists only for identical, compact, spherical particles (homoag-
gregation). However, there is no such general theory for aggregation of a mixture
of different particles (heterocoaggregation), in particular for aggregation involving
polymers. The section below summarizes the present understanding of aggregation
properties of the major aquatic colloids. Surface fi lm formation may reduce the
problem to one of homoaggregation in some situations.

4.6.1.2 Role of Natural Organic Matter

Some work has investigated the effect of the different fractions of NOM (humic
substances and biopolymers) on colloid stability. The available literature suggests
that NOM can have different effects on natural colloids, as described in Figure 4.13.
Humic substances form a surface coating on inorganic colloids, in general enhanc-
ing colloidal surface charge and, therefore, their stability. However, in the presence
of a high concentration of cations, Humic substances can enhance aggregation via
bridging mechanisms. The net effect will depend on surface coverage and the
degree of charge alteration. For model compounds, it has been shown that the
adsorption of negatively charged humic substances to positively charged iron oxide
will result in destabilization only for low surface coverage (Baalousha et al. , 2008 ;
Ferretti et al. , 1997; Stumm, 1992). Steric repulsion has also been suggested as a
mechanism for enhanced stability caused by Humic substances.
Biopolymers infl uence colloid stability either by being adsorbed to colloidal
surfaces (Dickinson and Eriksson, 1991) or by being expelled from the area between
the particles in case of non-adsorbing polymers (depletion layer) (Jenkins and
Snowden, 1996 ; Tuinier et al. , 2003). Aggregation by a depletion mechanism is not
a common process in environmental colloids. The adsorption of small quantities of
polymers leads to colloidal aggregation by charge neutralization or colloid bridging,
whereas the adsorption of larger quantities may stabilize the colloidal suspension
via steric stabilization mechanism.
Analysis by microscopy of freshwater colloids, often shows small inorganic col-
loids embedded in networks of fi brillar materials (Figure 4.1b). Interaction of
inorganic colloids with biopolymers is likely due to the minimal electrostatic repul-
sion because of low surface charge density of biopolymers (Section 4.3.2.2). In such
a situation a highly stable colloidal suspension might produce large aggregates in
the presence of biopolymers. Because biopolymers are very long in comparison
with the colloid diameter, they can serve as long bridges between colloids. The
attached colloid may interact with another polymer, leading to the formation of
loose aggregate networks extending to a large dimension. Further, humic sub-
stances may aggregate as small spheroids along the fi bril of biopolymers (Buffl e
and Leppard, 1995) suggesting that humic substances might interact with fi brils