Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 115

Silica colloids are present in all natural waters, and thus are expected to play an
important role in environmental systems. They can play an important role in metals
speciation (Osthols, 1995; Schindler et al. , 1976), limit the primary production in
estuaries and oceans (Conley and Malone, 1992; Dugdale et al. , 1995 ), alter the
species composition of phytoplankton in oceans (Offi cer and Ryther, 1980; Turner
et al. , 1998) and, ultimately, may have important consequences on the level of
carbon dioxide in the atmosphere and, therefore, the global climate (Harrison, 2000;
Treguer and Pondaven, 2000).

4.3.1.3 Sources of Inorganic Colloids

Inorganic colloidal particles can be produced by a range of different processes. In
situ mobilization of particles in soil and subsurface water can be a signifi cant source,
as can chemical precipitation from supersaturated solutions, biotic processes and
sources such as glacial erosion in glacial lakes and rivers, or waste disposal.
The main sources of mineral colloids in surface waters are detachment, from soil
surfaces (pedogenic) or by sediment resuspension, formation of solid phases by
chemical precipitation, or biogenic, from primarily microbial origin. The formation
of solid phases by chemical precipitation is a common process. Calcite precipitates
in lakes and oceans (Stabel, 1986), metal sulfi des precipitate in anoxic waters (Ben-
Yaakov, 1999; Morse and Luther, 1999), iron and manganese oxyhydroxides form
in redox gradients in sediment porewaters (v.d.Kammer et al. , 2003) and in transi-
tion layers of eutrophic lakes and sediments (Davison, 1993).
In soils and groundwater, the main source of colloidal material is their detach-
ment from the soil surface due to changes in solution chemistry (Kaplan et al. ,
1996), rainfall or soil irrigation (Bertrand and Sor, 1962). The main processes of
colloid translocation, mobilization and generation in a groundwater environment
are shown schematically in Figure 4.3. Colloid release is more important at high
pH and low ionic strength and high fl ow velocity (Kaplan et al. , 1993 ; Laegdsmand
et al. , 1999; Roy and Dzombak, 1996). Human activities such as waste disposal,
groundwater pumping and artifi cial recharge can enhance the formation and mobi-
lization of colloids by disturbing groundwater and soil solution chemistry (Gschwend
and Reynolds, 1987; Hofmann and Schö ttler, 1998 ; Liang et al. , 1993 ). Redox gra-
dients in groundwaters may produce iron and manganese oxide colloids due to
reoxidation of anoxic groundwaters (Christensen et al. , 2001 ; Hofmann, 2002 ;
Wolthoorn et al. , 2004). The hydrolysis of radionuclides, such as actinides may also
result in the formation of colloidal particles (Bates et al. , 1992 ).

4.3.2 Organic Macromolecules

Most dissolved, colloidal and particulate organic matter (DOM, COM and POM)
in soil, sediments and natural water are still poorly characterized at the molecular
level, due to their inherent chemical and structural complexity (Leenheer and
Croué, 2003), although their origins, reactions and fates have been extensively
studied (Dignac et al. , 2000 ; Hedges et al. , 2000 ; Wakeham et al. , 1997 ; Williams
and Druffel, 1988). Natural organic matter (NOM) in the aquatic or terrestrial
environment can be divided into two classes of compounds: non-humic material