Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 110

This chapter begins by looking at the major types of colloidal particles and their
properties, which are related to environmental processes. This is followed by a
discussion of interaction forces (both DLVO and non-DLVO) between colloidal
particles, and the fate and behaviour of colloidal particles in aquatic and terrestrial
systems. The discussion on natural aquatic and terrestrial colloids is limited to the
extent to which it will aid in appreciation of the likely behaviour of manufactured
nanoparticles. For a fuller discussion of natural colloids, the reader is directed else-
where (Lead and Wilkinson, 2006a).

4.2 Defi nition

Traditionally, a colloidal system has been defi ned as a dispersion of one phase in
another, where the dispersed phase is between 1 nm and 1 m in one dimension

(IUPAC, 2002). Accordingly, natural aquatic colloids are formally defi ned as mate-
rial with one dimension between 1 nm and 1

µ

µ m (Figure 4.1), while particles are

larger than 1 m (Hofmann et al. , 2003 ; Lead and Wilkinson, 2006b ). Alternatively,

colloids can be defi ned as organic or inorganic entities small enough to be domi-
nated by aggregation and to remain in the water column over reasonable times-
cales, but large enough to have supramolecular structure and properties, for
example electrical surface charge (Lead and Wilkinson, 2006a, 2006c). Particles are
large enough (

µ

m) to be dominated by sedimentation, rather than aggregation
(Buffl e and Leppard, 1995). This defi nition was developed and somewhat extended
by Gustafsson and Gschwend (1997), where an aquatic colloid can be defi ned as
any constituent that provides a molecular milieu into and onto which chemicals can
escape from the bulk aqueous solution, while its vertical movement (in water) is
not signifi cantly affected by gravitational settling over reasonable timescales. In
practice, in much of the literature, colloids are defi ned as materials which permeate
a fi lter (pore size between 0.1 and 1.0

> 1 µ

m, often with little standardization in the
literature data) while being retained by an ultrafi lter (1- 100 kDa, nominal pore
size). It is clear that the formal, mechanistic and practical defi nitions do not entirely
mesh and some work on standardization is required.
Within this colloidal fraction it is useful to defi ne a nanoscale fraction (Lead and
Wilkinson, 2006c ; Wigginton et al. , 2007), which may be thought of as between 1
and 100 nm, as with manufactured nanoparticles (Chapter 2). However, the size
range below about 10-25 nm may be the range in which environmental properties
such as metal binding, zeta potential and redox properties change radically com-
pared with the bulk or larger-sized phases (Lyven et al. , 2003 ; Madden et al. , 2006 ;
Madden and Hochella, 2005).

µ

4.3 Major Types of Environmental Colloids

Aquatic and terrestrial colloids are highly heterogeneous in size, shape, structure,
chemical composition and other properties. These colloids include organic and
inorganic compounds and even biota with proportions dependent on the nature
of the inputs, within media processes and outputs (Bertsch and Seaman, 1999;