Page 85 - Soil and water contamination, 2nd edition

72 Soil and Water Contamination

of the colloidal particles are smaller than 0.45 μm and so may also be classed as dissolved
matter.
Depending on their physical and chemical properties, solids (including colloidal

particles) interact with the dissolved phase via sorption and precipitation–dissolution
reactions. Almost all pollutants and other chemicals are subject to sorption or precipitation–
dissolution reactions, or both; the rate and extent of these reactions is often referred to as the

reactivity of the solid phase (e.g. sediment, organic matter). A key factor that determines the
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reactivity of the solid phase is the specific surface area, expressed in m g . Coarse materials
with a small specific surface area are much less reactive than fine paricles or nanomaterials
with a large specific surface area; this is particularly the case for sorption reactions. Usually
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a specific surface area of 10 m g is adopted as the criterion to divide materials that are
of no importance for sorption reactions from materials that interact significantly with the
liquid phase (Bolt and Bruggenwert, 1978). With regard to dissolution reactions, inorganic
materials with a large specific surface area almost always have a low solubility, since very
small particles of a readily soluble mineral would normally quickly dissolve and disappear.

In the subsurface environment, i.e. soil and groundwater, the solid phase does not
generally change position and so may retain nutrients, metals , and pollutants that are being
carried in solution; alternatively, if these compounds are present in the solid phase , they

may be released into the flowing groundwater. In surface waters, solid components may be
transported by the flowing water as bed load or suspended sediment (synonyms: suspended
matter, suspended solids (SS)). Solid particles in surface water are often transported at
a slower rate than dissolved constituents, since during transport the particulate matter
may settle due to gravity. The rate of settling is positively related to the specific density
and the diameter of the particles. Suspended particulate matter may also partly consist of
colloidal matter with particle sizes of less than 10 μm, which hardly settles in surface water.
Accordingly, colloidal matter is relatively mobile and in combination with its large reactivity ,
may be an important carrier of pollutants – in both soil and groundwater
Note that suspended solids in surface water are not only a carrier of pollutants;
sometimes they may also be considered as pollutants themselves. When concentrations of
suspended solids (including living algae ) are high, the water clarity is considerably reduced,
which affects the photosynthesis and therefore the growth of aquatic plants and algae.
Moreover, if suspended solids contain considerable amounts of easily degradable organic
matter , the oxidation of organic matter depletes the dissolved oxygen concentration

in surface water, which, in turn, may lead to fish mortality. Given the role of solid phase
constituents as pollutants and carriers of pollutants, it is clearly important to understand
the physical and chemical properties and processes involved. This will shed light on
the environmental transport and fate phenomena of pollutants. Below, therefore, the
composition and main properties of solid components will be discussed first, prior to other
chemical constituents in soil and water.

4.2 INORGANIC COMPONENTS

4.2.1 Composition and formation

Inorganic components occur mainly in a limited number of compounds with a definite
crystalline structure, called minerals. Table 4.1 lists some common types of minerals found
in soil and grouped according to the anionic constituents. The inorganic components
originate from physical and chemical weathering of bedrock materials and can be grouped
into primary and secondary minerals. Primary minerals may only have undergone physical
weathering and have not changed chemically since their formation in the Earth’s crust.

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