116
strength can increase due to the precipitation of Ca 2 (UO 2 (CO 3 ) 3 and Ca,
Mg(UO 2 (CO 3 ) 3 ). The minimum absorption of
238
U by sod horizons of sod-podzolic
and sod-meadow soils was found at pH 2–3, maximum (97–98%) at pH 5–8
(Ovchenkov et al. 1974).
A significant concentration of humus, as a rule, increases the sorption of radionuclides in the soil (Rusanova 1971; Sultanbaev and Grigoryev 1979; Sultanbaev
and Kinkalova 1979). Organic substances of the soil solution bind
238
U to complex
compounds, differing in their solubility and stability (Manskaya et al. 1956; Evseeva
et al. 1974). For example, the strength of the element complexes with humic acids
is 1.5–2 times higher on average than with fulvic acids (Kichigin and Nosova 2004).
Under reducing conditions at pH 5, humic acids and, at pH 6.0–6.6, fulvic acids
precipitate
238
U, and in oxidative conditions, soluble compounds are formed. The
carbonate concentration of 1 M × 10
−3
almost eliminates the formation of soluble
hydrolysis products and prevents the humate binding at the content of the humic
acids lower than 0.1 M × 10
−6
. At carbonate concentrations between 5 M × 10
−8
and
5 M × 10
−4
, the formation of uranium humates dominates (Rachkova et al. 2010).
The effect of organic substances on the processes of absorption in the soil is due to
the fact that they are themselves collectors of radioactive elements, or modify the
absorption properties of other sorbents, and also form mobile compounds with
radionuclides. Mechanisms of sorption and fixation of
238
U by soils have been
poorly studied. Absorption process includes adsorption, ion exchange and precipitation, particularly due to oxidation-reduction reactions. Some authors (Manskaya
et al. 1956; Evseeva et al. 1974) believe that the last of the above mechanisms is the
most important in the process of sorption of uranium in the soil. At the same time,
it was experimentally established that hexavalent uranium retains its valence in
complexes with natural humic acid (Teterin et al. 2001). The mechanism of sorption
consists of the exchange of hydrogen cations and uranyl UO
2+
; that is why the
capacity and rate of absorption depend on the acidity of the medium. Fe
3+
ions
inhibit the formation of uranyl humates due to sorption on the surface of particles of
colloidal humic acid (Teterin et al. 2001) that prevents the migration of
238
U in the
form of absorbed or molecular solutions of uranyl humates. At the same time, the
addition of humic acid to the hematite suspension reduces the absorption of the
radionuclide; the degree of reduction depends on the pH of the medium and the
amount of humic acid. In addition, the dominant factor is the fraction of
238
U bound
to humates. In the presence of fulvates, the sorption of the radionuclide by the
clayey soil components is completely inhibited (Kornilovich et al. 2001).
Sorption of
238
U in soil is related to the size and specific surface area of soil particles (Nikiforova 1967; Baranov and Morozova 1971; Rubtsov and Pravdina 1971;
Bykova 1973; Ishchenko 1988; Ishchenko and Butnik 1990). For soil with a high
content of physical clay and silt, the high values of the distribution coefficients were
set (Ranson 1973; Sheppard et al. 1980; Barnett et al. 2000). The content of
238
U in
the solid phase of clay soils is 200 − 7.9 × 10
5
times higher than in the equilibrium
solution. For desert-sandy, meadow, meadow-swamp soils and light and typical
sierozems, a direct dependence of the absorption and mobility of
238
U on the content
of humus, potassium, calcium and physical clay has been established (Ishchenko
A. N. Ratnikov et al.
strength can increase due to the precipitation of Ca 2 (UO 2 (CO 3 ) 3 and Ca,
Mg(UO 2 (CO 3 ) 3 ). The minimum absorption of
238
U by sod horizons of sod-podzolic
and sod-meadow soils was found at pH 2–3, maximum (97–98%) at pH 5–8
(Ovchenkov et al. 1974).
A significant concentration of humus, as a rule, increases the sorption of radionuclides in the soil (Rusanova 1971; Sultanbaev and Grigoryev 1979; Sultanbaev
and Kinkalova 1979). Organic substances of the soil solution bind
238
U to complex
compounds, differing in their solubility and stability (Manskaya et al. 1956; Evseeva
et al. 1974). For example, the strength of the element complexes with humic acids
is 1.5–2 times higher on average than with fulvic acids (Kichigin and Nosova 2004).
Under reducing conditions at pH 5, humic acids and, at pH 6.0–6.6, fulvic acids
precipitate
238
U, and in oxidative conditions, soluble compounds are formed. The
carbonate concentration of 1 M × 10
−3
almost eliminates the formation of soluble
hydrolysis products and prevents the humate binding at the content of the humic
acids lower than 0.1 M × 10
−6
. At carbonate concentrations between 5 M × 10
−8
and
5 M × 10
−4
, the formation of uranium humates dominates (Rachkova et al. 2010).
The effect of organic substances on the processes of absorption in the soil is due to
the fact that they are themselves collectors of radioactive elements, or modify the
absorption properties of other sorbents, and also form mobile compounds with
radionuclides. Mechanisms of sorption and fixation of
238
U by soils have been
poorly studied. Absorption process includes adsorption, ion exchange and precipitation, particularly due to oxidation-reduction reactions. Some authors (Manskaya
et al. 1956; Evseeva et al. 1974) believe that the last of the above mechanisms is the
most important in the process of sorption of uranium in the soil. At the same time,
it was experimentally established that hexavalent uranium retains its valence in
complexes with natural humic acid (Teterin et al. 2001). The mechanism of sorption
consists of the exchange of hydrogen cations and uranyl UO
2+
; that is why the
capacity and rate of absorption depend on the acidity of the medium. Fe
3+
ions
inhibit the formation of uranyl humates due to sorption on the surface of particles of
colloidal humic acid (Teterin et al. 2001) that prevents the migration of
238
U in the
form of absorbed or molecular solutions of uranyl humates. At the same time, the
addition of humic acid to the hematite suspension reduces the absorption of the
radionuclide; the degree of reduction depends on the pH of the medium and the
amount of humic acid. In addition, the dominant factor is the fraction of
238
U bound
to humates. In the presence of fulvates, the sorption of the radionuclide by the
clayey soil components is completely inhibited (Kornilovich et al. 2001).
Sorption of
238
U in soil is related to the size and specific surface area of soil particles (Nikiforova 1967; Baranov and Morozova 1971; Rubtsov and Pravdina 1971;
Bykova 1973; Ishchenko 1988; Ishchenko and Butnik 1990). For soil with a high
content of physical clay and silt, the high values of the distribution coefficients were
set (Ranson 1973; Sheppard et al. 1980; Barnett et al. 2000). The content of
238
U in
the solid phase of clay soils is 200 − 7.9 × 10
5
times higher than in the equilibrium
solution. For desert-sandy, meadow, meadow-swamp soils and light and typical
sierozems, a direct dependence of the absorption and mobility of
238
U on the content
of humus, potassium, calcium and physical clay has been established (Ishchenko
A. N. Ratnikov et al.