115
Anthropogenic sources of environmental pollution by
238
U, including agricultural ecosystems, are:
• Extraction of uranium in mines and a formed uranium trace
• The atomic and nuclear power industry
• Agriculture (mineral fertilisers, primarily phosphoric, may contain
238
U in high
concentrations) and uranium content in irrigation water (Drichko 1983; Alexakhin
1982)
• The use of radioactive projectiles in military conflicts
With the systematic use of waste (phosphogypsum) and complex phosphoric
fertilisers, the accumulation of gross and mobile forms of heavy natural radionuclides can occur in soils. It can reach the levels exceeding background concentrations by 1.5–2.0 times (Drichko 1983).
The nature and direction of the soil formation processes determine the distribution of uranium in the soils of certain landscape zone. These factors result in the
great variety of uranium content in soils. Uranium is accumulated mainly in the sod
and the upper arable soil horizon.
2 Behaviour and Forms of
238
U in Soils of Agricultural Lands
An important factor determining the behaviour of the
238
U, which enters the land
cover, is its high mass concentration in the soils (Alexakhin and Korneyev 1992).
The state and fixation (sorption) of
238
U in soils depends on many factors
(Alexakhin et al. 1990). The degree of oxidation of
238
U in soils is (IV) and (VI). In
a highly reducing medium,
238
U is predominantly in 4-valence and, in oxidative, in
6-valence forms.
Sorption of
238
U in soils depends on the chemical properties, the physical and
chemical state and concentration of elements, the granulometric and mineralogical
composition of soils, the content of organic matter, the presence of certain ions in
the solution, migratory-capable colloids, complexing agents, etc. (Drichko 1983;
Alexakhin et al. 1990; Alexakhin and Korneyev 1992).
Soils with pH from 5 to 6 contain
238
U mainly in the form of well-sorbed hydrolysed forms of uranyl, and for this reason, the transition of the radionuclide into
water extract from these soils is negligible. At a small (about 1 mM) content of
carbonates in soil solution, hydrolysis forms of uranyl prevail in the pH range of
2–12 (Iskra and Bakhurov 1981). Therefore, even at a high pH of the solutions, the
recovery of
238
U from the soils does not increase. Thus, at pH 11, the fraction of
radionuclide that has passed from leached chernozem to water extract is only 1.5%
(Iskra and Bakhurov 1981). In neutral and alkaline media, carbonate and phosphate
compounds predominate (Sultanbaev and Grigoryev 1979), and this can reduce and
weaken the absorption of
238
U in the soil (Iskra and Bakhurov 1981). At pH above
10, the equilibrium shifts again towards the formation of hydrolysis forms, and the
recovery of
238
U decreases. In media, rich with CaCO 3 and MgCO 3 , the absorption
The Behaviour of Uranium in Soils and the Mechanisms of Its Accumulation…
Anthropogenic sources of environmental pollution by
238
U, including agricultural ecosystems, are:
• Extraction of uranium in mines and a formed uranium trace
• The atomic and nuclear power industry
• Agriculture (mineral fertilisers, primarily phosphoric, may contain
238
U in high
concentrations) and uranium content in irrigation water (Drichko 1983; Alexakhin
1982)
• The use of radioactive projectiles in military conflicts
With the systematic use of waste (phosphogypsum) and complex phosphoric
fertilisers, the accumulation of gross and mobile forms of heavy natural radionuclides can occur in soils. It can reach the levels exceeding background concentrations by 1.5–2.0 times (Drichko 1983).
The nature and direction of the soil formation processes determine the distribution of uranium in the soils of certain landscape zone. These factors result in the
great variety of uranium content in soils. Uranium is accumulated mainly in the sod
and the upper arable soil horizon.
2 Behaviour and Forms of
238
U in Soils of Agricultural Lands
An important factor determining the behaviour of the
238
U, which enters the land
cover, is its high mass concentration in the soils (Alexakhin and Korneyev 1992).
The state and fixation (sorption) of
238
U in soils depends on many factors
(Alexakhin et al. 1990). The degree of oxidation of
238
U in soils is (IV) and (VI). In
a highly reducing medium,
238
U is predominantly in 4-valence and, in oxidative, in
6-valence forms.
Sorption of
238
U in soils depends on the chemical properties, the physical and
chemical state and concentration of elements, the granulometric and mineralogical
composition of soils, the content of organic matter, the presence of certain ions in
the solution, migratory-capable colloids, complexing agents, etc. (Drichko 1983;
Alexakhin et al. 1990; Alexakhin and Korneyev 1992).
Soils with pH from 5 to 6 contain
238
U mainly in the form of well-sorbed hydrolysed forms of uranyl, and for this reason, the transition of the radionuclide into
water extract from these soils is negligible. At a small (about 1 mM) content of
carbonates in soil solution, hydrolysis forms of uranyl prevail in the pH range of
2–12 (Iskra and Bakhurov 1981). Therefore, even at a high pH of the solutions, the
recovery of
238
U from the soils does not increase. Thus, at pH 11, the fraction of
radionuclide that has passed from leached chernozem to water extract is only 1.5%
(Iskra and Bakhurov 1981). In neutral and alkaline media, carbonate and phosphate
compounds predominate (Sultanbaev and Grigoryev 1979), and this can reduce and
weaken the absorption of
238
U in the soil (Iskra and Bakhurov 1981). At pH above
10, the equilibrium shifts again towards the formation of hydrolysis forms, and the
recovery of
238
U decreases. In media, rich with CaCO 3 and MgCO 3 , the absorption
The Behaviour of Uranium in Soils and the Mechanisms of Its Accumulation…