Natural Colloids and Nanoparticles in Aquatic and Terrestrial Environments - Environmental and Human Health Impacts of Nanotechnology - page 117

4.3.2.1 Humic Substances

Humic substances are a heterogeneous mixture of low to high molar mass organic
acids that are defi ned operationally by the standard technique used for their extrac-
tion and isolation. Humic substances are generally divided operationally into three
classes of materials: fulvic acids (FA) are materials soluble in water under all pH
conditions, while humic acids (HA) are soluble at pH

2 and humins are insoluble
at all pHs (Thurman and Malcolm, 1981). They cannot be fully defi ned as yet by
structure or function, which are complex, temporally and spatially variable and
highly infl uenced by solution conditions (Hayes et al. , 1989). New technical devel-
opments have given more insight to the structures of humic substances, which have
been explained in different ways including macromolecules (Swift, 1989), supramo-
lecular association of small molecules (Piccolo, 2001), micelles (Thurman et al. ,
1982; Wershaw, 1999) and soft, semi-permeable spheres (Duval et al. , 2005 ), although
these structures are usually technique dependent (Lead and Wilkinson, 2006c).
However, it is generally agreed that they are structurally complex macromolecules,
strong adsorbers in the UV-Visible range (rich in chromophores) and are weak
polyfunctional acids. Humic acids are generally terrestrial, while aquatic humic
substances are dominated by FA, showing that the procedural defi nition has some
use in interpreting geochemical behaviour.
As with inorganic colloids, humic substances have different sources in different
environments. The principle source of humic substances in terrestrial and fresh-
water environments is the degradation of higher plants, whereas the main source
in marine systems is the degradation of plankton (Aiken et al. , 1996 ; Kristensen,
1990). Terrestrial, freshwater and marine humic substances have signifi cantly dif-
ferent chemical characteristics, with a signifi cant contribution from autochthonous
sources in freshwater. Freshwater humic substances, have a high C/N ratio (40 to
50), are rich in aliphatic carbon, have strong absorption in the near UV and are
often depicted with a highly condensed, cyclic molecular structure. In contrast,
marine humic substances have a low C/N ratio (15 to 20) (C/N ratio of fresh micro-
bial material is between 5 and 10 to 1, for comparison), are rich in aromatic carbon,
have weak absorption in the near UV and are often depicted with a more open,
linear molecular structure. These differences arise from differences in the organic
matter sources and formation processes of these two environments (Hedges et al. ,
1997 ).
Humic substances have important environmental functions. They have an impor-
tant role in regulating the chemical reactivity, speciation (Tipping, 2002), bioavail-
ability and toxicity of metal ions in the natural environment (Koukal et al. , 2003 ;
McGeer et al. , 2002 ; Van Ginneken et al. , 2001). They also regulate the speciation
(De Paolis and Kukkonen, 1997; Khan and Schnitzer, 1972), bioavailability and
toxicity of organic pollutants (Haitzer et al. , 1998). They play an important role
in stabilizing inorganic and other colloids (Tipping and Higgins, 1982; Tipping
and Ohnstad, 1984) through surface sorption and charge and steric stabilization,
and have also been shown to be toxic themselves (Bernacchi et al. , 1996 ; Qi et al. ,
2008 ).

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