Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 139

which is more rapid. In surface erosion, small particles are separated from the
surface of the aggregate while in large scale fragmentation the aggregates split into
pieces of comparable sizes (Jarvis et al. , 2005). Highly branched aggregates with a
small fractal dimension break by this fragmentation mechanism, while compact
aggregates with a large fractal dimension favour a surface erosion mechanism
(Yeung and Pelton, 1996). Cleary more research is needed to investigate the pos-
sible disaggregation of natural environmental aggregates and the role played by
natural organic molecules. The fi ndings have clear implications for the environmen-
tal fate and behaviour of manufactured NPs.

4.6.3 Sedimentation Behaviour

The settling behaviour of a hard, non-permeable sphere can be described in a rela-
tively straightforward manner by Stokes' law. However, aggregation of colloidal
particles in natural waters results in the formation of large, fractal and permeable
aggregates (Section 4.4.4) (Johnson et al. , 1996a ; Lartiges et al. , 2001 ). Thus, Stokes '
law is not suitable to describe the settling behaviour of natural colloidal aggregate.
The settling behaviour of such aggregates is dependent on the drag force and per-
meability of the solvent through the porous aggregates.
Fractal aggregates in natural waters have a heterogeneous mass distribution and
porosity, resulting from the coagulation of small and more densely packed clusters
into larger and overall less dense aggregates (Johnson et al. , 1996a ; Lartiges et al. ,
2001). Pores formed within the fractal aggregate will permit greater interior fl ow
through the aggregate, resulting in a faster settling velocity. It has been demon-
strated that fractal aggregates (with heterogeneous pore sizes) settle faster than
predicted by Stokes' law (Johnson et al. , 1996a; Logan and Hunt, 1987), indicating
that intra - aggregate fl ow reduces the drag for aggregates compared to that for the
equivalent impermeable particles. As the fractal dimension increases, the permea-
bility decreases and the fl uid mechanics resembles more closely that of an isolated
impermeable sphere (Chellam and Wiesner, 1993).
The settling behaviour of fractal aggregates is not well understood since these
aggregates do not behave as spheres with constant density. The settling behaviour
of fractal aggregates depends on many properties, including porosity, size, perme-
ability and buoyant density, which need be determined to predict fractal aggregate
sedimentation. Several models have been developed to predict the sedimentation
behaviour of fractal aggregates (Tang et al. , 2002; Tang and Raper, 2002). It has
been found diffi cult to describe mathematically the non-homogeneous distribution
of pores within fractal aggregates, although this non-homogeneity has been mod-
elled by assuming that porosity varies radially from the centre of gyration
(Veerapaneni and Wiesner, 1996 ).

4.7 Fate and Behaviour of Colloids and Nanoparticles in Porous Media

The transport of colloids in porous media is dominated by inter alia size, shape,
surface properties, the physical-chemical properties of the porous medium (grain
size, surface properties) and the fl uid (velocity, ionic composition, density and