Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 108

lead). However, in many cases metals were distributed to some extent among all
phases due to iron-carbon interactions. Other phases such as manganese oxides
and sulfi des may also be important in trace element binding (Baalousha et al. ,
2006a ).
As with inorganic contaminants, colloids may signifi cantly infl uence the distribu-
tion and fate and behaviour of organic contaminants. For instance, the majority of
polycyclic hydrocarbons (PAHs) were found to be present in large ( m) fl ocs

(Leppard et al. , 1998), which were essentially aggregates of small colloids. Marvin
et al. , (2004) showed that PAHs were primarily associated with particles less than
2

> 20 µ

m in diameter. The majority of these particles were found to be fractal aggregates
of humic substance. In marine systems, the majority of polychlorinated biphenyls
(PCBs) were found to be associated with particulate matter (

µ

> 1.2 µ m), although in

the fraction m, colloidal binding (40-80%) was dominant (Burgess et al. ,

1996). Up to 93% of polychlorinated biphenyls were found to be associated with
colloids in a coastal sea area (Totten et al. , 2001). The interaction of selected phar-
maceuticals (Maskaoui et al. , 2007) and endocrine disrupting chemicals (EDCs)
(Liu et al. , 2005 ; Zhou et al. , 2007) with natural colloids has also been more recently
investigated. While the more hydrophobic pharmaceuticals showed a linear depen-
dency of the K coc (colloidal organic carbon sorption coeffi cient) and the K ow
(octanol-water partition coeffi cient), the K coc of the more hydrophilic EDCs were
independent of the K ow , highlighting the importance of different binding mecha-
nisms. Polychlorinated dibenzo- p -dioxins and dibenzofurans (PCDD/Fs) were
found to be relocated from soil to groundwater associated with colloids (Hofmann
and Wendelborn, 2007 ).
The behaviour of colloidal particles is dominated by aggregation/disaggregation
and sedimentation in aquatic systems (Buffl e et al. , 1998) and attachment to sur-
faces in porous media (McDowell-Boyer et al. , 1986). These processes are highly
infl uenced by solution physico-chemistry (e.g. pH and cation types and concentra-
tions) and behaviour of natural organic molecules (Wilkinson et al. , 1997a, 1997b ).
Aggregation of colloids results in the formation of large structures, which sediment
in the water body or attach to surfaces in soils which results in their loss, thus
eliminating the chemicals from a water body in a processes known as colloidal
pumping (Honeyman and Santschi, 1992). Colloids are often porous and form
fractal-like aggregate structures; this depends greatly on solution conditions such
as pH and ionic strength (Baalousha et al. , 2006b; Chen and Eisma, 1995; Senesi et
al. , 1996). Such porosity and conformation of colloids and their aggregates may
result in sorption-desorption of chemical compounds (e.g. pollutants and nutrients)
and possibly a permanent retention within the structure of colloids/aggregates (Kan
et al. , 1994). Further, the fractal nature of colloidal aggregates infl uences their sedi-
mentation behaviour.
It is now well known that contaminant and nutrient transport processes in marine
and freshwater systems are dominated by the transport of particles and substances
associated with them (Benedetti et al. , 2002 ; Dai et al. , 1995 ; Santschi et al. , 1997 ).
For decades, processes of contaminant relocation in soil and groundwater were
believed to occur predominantly in a two-phase system: the mobile liquid phase
and the immobile solid phase, a potentially mobile solid phase was neglected

< 1.2 µ