2.7). These atoms may form an integral part of the molecule, alternatively they
may be present in functional groups (see Section 2.7.1). Functional groups impart
specific chemical properties on a molecule. Hydroxyl (-OH) and carboxyl
(-COOH) functional groups increase polarity, making a molecule more soluble
(Box 4.14), while -COOH also makes the molecule acidic due to dissociation of
H+(see Section 2.7.1). The various structural forms of organic molecules, for
example saturated and unsaturated chains and rings (see Section 2.7), result in a
diverse range of organic contaminants (Fig. 4.26).
Once in the soil environment organic contaminants may move in, or interact
with, the soil atmosphere, soil water, mineral fractions and organic matter.
Ultimately, however, the organic contaminants will either dissipate or persist
(Fig. 4.27). Compounds persist if they are of low volatility, low solubility (Box
4.14) or have a molecular structure that resists degradation. Conversely, if com-
pounds are highly volatile, highly soluble or are easily degraded, they will be120 Chapter Four
Box 4.13 Radon gas: a natural environmental hazardRadon gas (Rn) is a radioactive decay product
of uranium (U), an element present in crustal
oxides (e.g. uraninite—UO 2 ), silicates (e.g.
zircon—ZrSiO 4 ) and phosphates (e.g. apatite—
Ca 5 (PO 4 ) 3 (OH, F, Cl)). These minerals are
common in granitic rocks, but are also to be
found in other rocks, sediments and soils.
Uranium decays to radium (Ra), which in turn
decays to radon (Rn) (see Section 2.8). The
isotope^222 Rn exists for just a few days before
it also decays, but, if surface rocks and soils
are permeable, this gas has time to migrate
into caves, mines and houses. Here, radon or
its radioactive decay products may be inhaled
by humans. The initial decay products,
isotopes of polonium,^218 Po and^216 Po, are
non-gaseous and stick to particles in the air.
When inhaled they lodge in the lungs’
bronchi, where they decay—ultimately to
stable isotopes of lead (Pb)—by ejecting a
radiation particles (see Section 2.8) in all
directions, including into the cells lining the
bronchi. This radiation causes cell mutation
and ultimately lung cancer. Having said this,
radon is estimated to cause only about one in
20 cases of lung cancer in Britain, smoking
being a much more serious cause.
Radon gas is invisible, odourless and
tasteless. It is therefore difficult to detect andits danger is worsened by containment in
buildings. Radon is responsible for about half
the annual radiation dose to people in
England, compared with less than 1% from
fallout, occupational exposures and
discharges from nuclear power stations.
In England, about 100 000 homes are
above the government-adopted ‘action level’
of 200 becquerels m-^3. Various relatively low-
cost steps can be taken to minimize home
radon levels, including better underfloor
sealing and/or ventilation. Building homes in
low-radon areas remains an obvious long-
term strategy, but such simple solutions are
not always applicable, because of either
geographic or economic constraints. For
example, bauxite processing in Jamaica
produces large amounts of waste red mud.
This material binds together strongly when
dry, and is readily available as a cheap
building material. Unfortunately, the red
mud also contains higher levels of^238 U than
most local soils. These cheap bricks are thus
radioactive from the decay of^238 U and a
potential source of radon. Only careful
consideration of the health risks in
comparison with the economic benefits
can decide whether red mud will be used as
a building material.