Page 172 - Soil and water contamination, 2nd edition

Radionuclides 159
Iodine-131
Iodine-131 is only short-lived (half-life 8.04 d) and is therefore highly radioactive, but only
significantly so in the first weeks after a release of radionuclides into the environment. Iodide
(I ) is the primary species of I in soil except under exceptional arid and alkaline conditions.
-
Iodide sorbs poorly to most soil materials, but can be taken up by some clays, organic
131
matter, and metal sulphides (Zhang et al., 2002). The main exposure pathway of I is from
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cow’s milk. Cows that have ingested I deposited on the leaves of grass transfer it rapidly
to their milk. Radioiodine is easily accumulated in the thyroid gland, especially in humans
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who suffer iodine shortage, and so may induce thyroid cancer. The biological half-life of I
is approximately 100 days in the thyroid, 14 days in bone, and 7 days in the kidney and
reproductive organs.
Caesium-137
+
Caesium is chemically similar to potassium and occurs as Cs cation in soil and water. It is
very soluble but also readily adsorbed by clay minerals . Directly after atmospheric deposition
137
of radiocaesium, it is very mobile in runoff water, but the bulk of Cs is adsorbed by soil
materials in the topsoil. Fixation of caesium by illitic clay minerals causes the mobility and
bioavailability to decrease by a factor of ten during the first five years after initial deposition.
After this period, the decrease of bioavailability follows radioactive decay (half-life 30.17 y)
(Smith et al., 2000). The gamma radiation emitted by decay product 137m Ba (see Table 8.6)
is of most concern from a health perspective. Caesium-137 uptake by crops is the main
pathway of Cs transfer into the animal and human food chains. Since Cs competes with
potassium, 137 Cs uptake is greater in soils with low potassium content. Like potassium, Cs
tends to be absorbed in neural and muscle tissues. The average biological half-life of 137 Cs
is about 45 days. Furthermore, radiation from soil contaminated by 137 Cs contributes
significantly to external radiation exposure.

Strontium-90
137
90
Like Cs, Sr has a relative long radioactive half-life (28.64 y) and is readily absorbed by
2+
plants in the form of Sr . It is chemically similar to calcium and therefore accumulates in
bone and bone marrow. Since Sr is not specifically adsorbed by clay minerals , it is more
mobile than Cs in soil and water.
137
Plutonium
Plutonium and other transuranic elements do not exist naturally, with the exception of the
small amounts of Pu generated by natural fission. Natural fission is very rare and is only
found in a few locations: for example, the Oklo natural reactor in Gabon. In general,
plutonium exhibits a complex and diverse geochemistry. It may change its oxidation state
due to slight changes in redox potential and often two or more of its oxidation states co-exist
(Runde, 2002). At the low concentrations in which they occur under natural conditions in
soil and water, the major process that determines their environmental behaviour is sorption
onto particulates and clay mineral surfaces. Surface contamination, rather than uptake by
plant roots, is the main pathway of Pu accumulation in plants and crops (Zhang et al.,
2002). Although there is some controversy about whether Pu is one of the most dangerous
substances known, there is no doubt that it is very toxic. Therefore, the toxicity of Pu as a
heavy metal accumulating in the bone and liver is likely to be greater than its radiotoxicity,
which is relatively low due to the relatively long physical half-lives. Nevertheless, most Pu
isotopes are alpha emitters and may induce lung cancer when inhaled: for example, due to
airborne resuspension of contaminated soil particles by wind erosion .

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