64
relatively low-molecular humic acids and anions of mineral acids, for example,
[UO 2 Cl]
+
, [UO 2 Cl 2 ]
0
, [UO 2 SO 4 ]
0
, etc. U mob can also be represented by the simplest
hydrolyzed forms of uranyl ([UO 2 OH]
+
, [UO 2 (OH) 2 ]
0
, [UO 2 (OH) 3 ]
−
) and their soluble complexes with soil components. In an acidic environment, the dominant form
of uranium (VI) is UO 2
2+
, complexes of hydroxides and phosphates are found in a
neutral medium, and at a pH of 7.5 or more, uranium can also enter the soil waters
in the form of bi- and tricarbonate uranyl complexes [UO 2 (CO 3 ) 3 ]
4−
and
[UO 2 (CO 3 ) 2 ]
2−
(Sparovek et al. 2002; Diemann 2012).
As can be seen from the data presented in Fig. 10, for all the temperature regimes
studied, the content of U mob in the soil increased with an increase in soil moisture
from 5.5 to 60% of the WC. A subsequent increase in soil moisture from 60 to 100%
resulted in a decrease in the U mob content, and the levels reached were practically
preserved with a subsequent increase in humidity to 140% of the WC.
An increase in U mob content with an increase in soil moisture from 5.5% to 60%
could be due to an increase in the portion of water-soluble carbonate uranyl complexes that should be present in the soil under analysis (рН Н2О —7.9). According to
Chernykh and Prasanna (2000), an increase in soil moisture at a temperature above
(10–15) °C leads to an increase in pH, which facilitates the dissolution of atmospheric carbon dioxide in the soil solution and the formation of water-soluble carbonate complexes. Reduction of U mob content with increasing the soil moisture from
60% to 100% of the WC could be due to a change in oxidation-reduction conditions
as a result of a decrease in the oxygen content when water filled the soil pore space.
This led to the reduction of U (VI) to U (IV). Since U (IV) compounds have a lower
solubility in water than U (VI) compounds, this could be the reason for the decrease
in the content of U mob with increasing soil moisture (Katz et al. 1991).
In addition, an increase in the pH of the soil medium with an increase in soil
moisture from 60% to 100–140% of the WC increased the degree of hydrolysis of
uranium compounds, which could contribute to the sorption of hydrolysis products
by the components of the solid phase of the soil.
It should be noted that the content of U mob in the soil samples subjected to freezing
was higher than in the samples that were hold at the temperature of +15 °C. This was
observed at all levels of soil moisture, most notably for soil samples with a moisture
content of 100% and 140% of the WC. From the data obtained, it follows that freezing
the wet soil during the vegetation period can lead to an increase in U mob content, which
contributes to the accumulation of uranium by terrestrial ecosystem vegetation.
8 Conclusions
As a result analysis of the literature and our own experimental data, it was established that at present, there is uranium of natural and technogenic origin at the territory of Belarus. Technogenic uranium is mainly uranium of Chernobyl origin with
an admixture of uranium of global fallout. Uranium of Chernobyl origin concentrated predominantly in the Polesie State Radiation Ecological Reserve. In the rest
of the territory, there is mainly natural uranium, but use of mineral fertilizers,
G. A. Sokolik et al.
relatively low-molecular humic acids and anions of mineral acids, for example,
[UO 2 Cl]
+
, [UO 2 Cl 2 ]
0
, [UO 2 SO 4 ]
0
, etc. U mob can also be represented by the simplest
hydrolyzed forms of uranyl ([UO 2 OH]
+
, [UO 2 (OH) 2 ]
0
, [UO 2 (OH) 3 ]
−
) and their soluble complexes with soil components. In an acidic environment, the dominant form
of uranium (VI) is UO 2
2+
, complexes of hydroxides and phosphates are found in a
neutral medium, and at a pH of 7.5 or more, uranium can also enter the soil waters
in the form of bi- and tricarbonate uranyl complexes [UO 2 (CO 3 ) 3 ]
4−
and
[UO 2 (CO 3 ) 2 ]
2−
(Sparovek et al. 2002; Diemann 2012).
As can be seen from the data presented in Fig. 10, for all the temperature regimes
studied, the content of U mob in the soil increased with an increase in soil moisture
from 5.5 to 60% of the WC. A subsequent increase in soil moisture from 60 to 100%
resulted in a decrease in the U mob content, and the levels reached were practically
preserved with a subsequent increase in humidity to 140% of the WC.
An increase in U mob content with an increase in soil moisture from 5.5% to 60%
could be due to an increase in the portion of water-soluble carbonate uranyl complexes that should be present in the soil under analysis (рН Н2О —7.9). According to
Chernykh and Prasanna (2000), an increase in soil moisture at a temperature above
(10–15) °C leads to an increase in pH, which facilitates the dissolution of atmospheric carbon dioxide in the soil solution and the formation of water-soluble carbonate complexes. Reduction of U mob content with increasing the soil moisture from
60% to 100% of the WC could be due to a change in oxidation-reduction conditions
as a result of a decrease in the oxygen content when water filled the soil pore space.
This led to the reduction of U (VI) to U (IV). Since U (IV) compounds have a lower
solubility in water than U (VI) compounds, this could be the reason for the decrease
in the content of U mob with increasing soil moisture (Katz et al. 1991).
In addition, an increase in the pH of the soil medium with an increase in soil
moisture from 60% to 100–140% of the WC increased the degree of hydrolysis of
uranium compounds, which could contribute to the sorption of hydrolysis products
by the components of the solid phase of the soil.
It should be noted that the content of U mob in the soil samples subjected to freezing
was higher than in the samples that were hold at the temperature of +15 °C. This was
observed at all levels of soil moisture, most notably for soil samples with a moisture
content of 100% and 140% of the WC. From the data obtained, it follows that freezing
the wet soil during the vegetation period can lead to an increase in U mob content, which
contributes to the accumulation of uranium by terrestrial ecosystem vegetation.
8 Conclusions
As a result analysis of the literature and our own experimental data, it was established that at present, there is uranium of natural and technogenic origin at the territory of Belarus. Technogenic uranium is mainly uranium of Chernobyl origin with
an admixture of uranium of global fallout. Uranium of Chernobyl origin concentrated predominantly in the Polesie State Radiation Ecological Reserve. In the rest
of the territory, there is mainly natural uranium, but use of mineral fertilizers,
G. A. Sokolik et al.