Page 98 - Soil and water contamination, 2nd edition

Solid phase constituents 85

Negatively charged surface

H H
O
OH
6642 6642 6642
R

Figure 4.9 Schematic diagram of a hydrogen bond between a hydroxyl group and a mineral surface.
The sorption of organic molecules to mineral surfaces occurs via various mechanisms.
Positively charged organic solutes, such as for example amino acids (at pH values below PZC )

and polypeptides, are readily removed from the dissolved phase by cation exchange . The
active functional hydroxyl or carboxyl groups have a dipole character like water molecules,
which means that the centres of positive and negative charge do not coincide. The H atoms
of the functional groups and water molecules possess a significant amount of positive charge
and the O atoms a negative charge. This leads to the formation of so-called hydrogen bond s
between the H atoms and the negatively charged mineral surface, often via H O molecules
2
(see Figure 4.9). Large humic molecules are often attached to mineral surfaces by multiple
hydrogen bonds.
The sorption of organic compounds changes the sorption properties of the mineral
surface. If the organic molecule sorbs in a position that leaves its active functional groups or

chelating sites exposed, it will tend to take cations from solution to the particulate phase . If
it sorbs to the mineral phase in such a way that its reactive sites are obstructed, it becomes
less effective at capturing cations. Besides the polar regions as explained above, large organic
molecules often also have non-polar regions. If these large molecules bind to minerals, the
mineral surface becomes less polar. This makes the mineral surface a better sorbent for non-
polar or hydrophobic organic pollutants. This sorption mechanism will be discussed further
in Chapter 9.

The presence of these organic coatings is particularly significant because it enhances the
cohesion between mineral particles. This leads to the formation of stable aggregates in soil.
In surface water, the organic–mineral complexes promote the flocculation process (Johnson
et al., 1994). Note too that living microorganisms also contribute actively to the process of
coagulation of soil or sediment through feeding. The aggregates thus formed take the form
of faecal pellets. In groundwater and on lake and river beds, microorganisms also form
so-called biofilms that consist of a consortium of bacteria, algae , and fungi, embedded in
an extracellular polysaccharide matrix that acts as glue (Lock, 1994). Biofilms form slimy
coatings on coarse materials such as sand, gravel, and even pebbles, and may contain
inclusions of inorganic particles. The biofilm may be eroded from the lake or river bed and
so form fragile organic aggregates. While these flocs are settling, they continually collide with
and capture smaller particles and DOM, and therefore grow bigger (Van Leussen, 1988).
4.3.4 Decomposition of organic matter

Organic matter is decomposed through oxidation, which involves numerous microorganisms
that break down the organic molecules through multiple enzymatic reactions (Van Cappellen
and Wang, 1995). It was shown above that these reactions produce a number of intermediate
organic compounds (e.g. humic and fulvic acids ) but ultimately, the organic matter is fully
decomposed or mineralised to carbon dioxide and water, plus some other inorganic minerals.

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