Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 112

of surface freshwater colloidal material of various sizes, as observed by transmission
electron microscopy. Here, three types of colloids that are recognized as the major
colloidal components in environmental systems are presented, namely inorganic
colloids, humic substances and biopolymers (Buffl e et al. , 1998). The later two types
will be presented under the same category of organic macromolecules. For more
details, the reader is referred to a recent review of this area (Filella, 2006).

4.3.1 Inorganic Colloids

There are two main types of inorganic colloidal particles in oxygenated terrestrial
and aquatic environments, which are aluminium phyllosilicates (e.g. clay, mica,
chlorite) and oxides and hydrous oxides of iron (e.g. haematite and magnetite),
manganese (e.g. pyrolusite) and silicon (e.g. SiO 2 ). Other inorganic colloids can also
be found, but they are usually minor components (e.g. other groups of silicates) or
are primarily present in anoxic waters (e.g. FeS, FeS 2 , MnS). Sulfi des have also been
found to be a potentially important minor species in oxic waters. Calcium carbonate
can be found in signifi cant amount in freshwaters but is mostly in particulate form
(Sigg, 1994) with weak metal binding.

4.3.1.1 Aluminium Phyllosilicates

Phyllosilicates are a subgroup of silicates, an extensive group of minerals which
are derived from silica (SiO 2 ). All clay minerals, that is aluminium phyllosilicates,
belong to this group. They are phyllosilicates which form parallel tetrahedral
silicon sheets and octahedral aluminium sheets. The most common clay minerals
within the phyllosilicates are kaolinite, illite, vermiculite and the smectite/montmo-
rillonite group. Clay minerals from these four groups are the most abundant
inorganic colloids in aquatic and terrestrial systems and are generally weathering
products from soils and rocks (Berner and Holdren, 1977; Helgeson et al. ,
1984; Murphy and Helgeson, 1987). They usually have an irregular shape and
a range of crystalline structures and sizes, covering both the colloidal and particu-
late range. Clay minerals control the composition of natural waters and contribute
to the formation of secondary solids as many clay minerals. They have a high
cation exchange capacity and different types of surface charge (permanent
and variable, i.e. pH dependent). Clay particle aggregation is complicated as they
have different modes of aggregation (plane - to - plane, plane - to - edge and edge - to -
edge) (Lagaly and Ziesmer, 2003). The stability of clays in natural water is mainly
determined by charge and charge heterogeneity (Aurell and Wistrom, 2000; Chang
and Sposito, 1996). As with all standard or reference materials, the application of
results obtained on isolated clays in laboratory studies to their behaviour in natural
environments should be performed cautiously for several reasons, given the
heterogeneity, complexity and spatial and temporal variability of natural samples.
Isolated and pure clays may have different surface properties compared to natu-
rally occurring clays (Schulthess and Huang, 1990; Schulthess and Sparks, 1989),
larger particle sizes are often used compared to natural colloids (Aldahan et al. ,
1999 ; Arnold et al. , 2001) and given the extensive use of particle pretreatment such
as drying, grinding and saturation with sodium ions, leading to the removal of