Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 109

(McCarthy and Zachara, 1989). Colloid-facilitated transport is now a well recog-
nized process in porous media such as soils and aquifers. Small colloids compete
with the solid, immobile phase for trace contaminant sorption (e.g. metals (Chen
et al. , 2005), organic pollutants (White et al. , 2005) and nutrients (Heathwaite et al. ,
2005)) and increase the distances travelled by pollutants with respect to those
predicted from non-colloidally bound components (Kaplan et al. , 1995 ; Laegdsmand
et al. , 1999 ; McCarthy, 1998 ).
The bioavailability of both metals and hydrophobic organic contaminants
(HOCs) is affected by colloids. In the metals area, there are well defi ned equilib-
rium chemistry based models (biotic ligand model, BLM, and free ion activity
model, FIAM), which are standard in the literature, implicating colloids in (usually)
a reduction of toxicity (Campbell, 1995; Paquin et al. , 2002 ). Dynamic models
accounting for solution chemical kinetics and mass transport phenomena are being
used more extensively (van Leeuwen et al. , 2005). The BLM is now a regulatory
tool in the Unites States and in parts of the EuropeanUnion. Although less well
developed theoretically, HOCs toxicity is often related to octanol- water partition-
ing coeffi cient. Toxicity also depends on the presence of humic acids (Galle et al. ,
2005) and particulate or dissolved organic matter (Hodge et al. , 1993 ). Humic sub-
stances have been shown to affect cell permeability, cell charge and nutrient avail-
ability (Kola and Wilkinson, 2005) and this may be a mechanism for their impacts
on toxicity, in addition to their effect on solution speciation.
From the above discussion, it is clear that colloids are important components in
the environment. They can control pollutant chemistry, that is pollutant speciation,
and, consequently, infl uence their transport and bioavailability (Doucet et al. , 2006 ;
Lead et al. , 1999). Although advances have been made in understanding the behav-
iour and role of colloids in environmental systems in the last few decades, much is
still unknown due to the intrinsic complexity of natural colloids, the lack of appro-
priate experimental techniques and the signifi cant gap between coagulation theory,
which was developed to describe simple systems of identical, spherical, non-living
particles and the reality of natural systems that contain heterogeneous mixture of
particles (Buffl e, 1988). The ongoing introduction (accidentally or deliberately) of
manufactured nanoparticles (NPs) will likely be controlled to a large extent by
these natural colloids, especially given the likely difference in concentrations;
natural colloids present at mg l − 1 and NPs at g l − 1 , typically in freshwaters. Thus,

natural colloids are important, intrinsically, in considering the literature to better
understand the likely fate and behaviour of NPs, and because they will interact
directly with NPs, altering their fate and behaviour. Recent studies suggest that the
introduction of manufactured NPs may have a potential harmful effect on the
environment due to their potential toxicity (Lovern and Klaper, 2006; Oberdorster
et al. , 2006) and other indirect environmental effects (Zhang, 2003). The specifi c
fate and behaviour of engineered nanoparticles in environmental systems, as far as
it is known, was discussed in Chapter 1. In this chapter colloidal behaviour will be
reviewed, as well as the parameters which determine their fate and behaviour in
the aquatic and terrestrial environments. This knowledge will help to improve our
somewhat sketchy understanding of the likely fate and behaviour of manufactured
nanoparticles in these environments.

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