52
5.2 Reversibly Bound (Exchangeable) and Mobile
(Conditionally Biologically Available) Uranium in the Soils
Uranium reversibly associated with components of the sorption complex of soil is in
potentially mobile (exchangeable) form. These can be cationic, anionic, and neutral
(molecular) uranium complexes with organic and inorganic components of the soil,
as well as soluble hydrolysis products that are capable of entering soil solution. The
composition and ratio of these forms of uranium depends on the redox conditions,
the pH of the medium, and the presence of complexing addends (Sheppard and
Campbell 1980; Katz et al. 1991; Rachkova et al. 2010). In the upper well-aerated
horizons of soils, as a rule, an oxidizing environment is formed; therefore, uranium,
as a rule, is in the degree of oxidation +6. In this case, reversibly bound uranium
could be presented of UO 2
2+
, its complex compounds, and hydrolytic forms of U
(VI), (UO 2 OH
+
, UO 2 (OH) 2
0
UO 2 (OH) 3
−
, UO 2 (OH) 4
2−
, etc.), which can be found in
the soil waters (Katz et al. 1991; Sparovek et al. 2002; Rachkova et al. 2010).
Reversibly bound (exchangeable) uranium is distributed between the solid phase
and the pore solution of the soil until interphase equilibrium is established. However,
the bulk of reversibly bound uranium remains in the solid phase of the soil. In redistribution of uranium along the soil profile and the assimilation by vegetation through
the root systems, its content in the soil pore solution decreases, and the equilibrium
distribution of the element between the solid phase and the pore solution of the soil
is disturbed. Restoration of the disturbed equilibrium is carried out by transition from
a solid phase to a solution of uranium, reversibly connected with components of the
soil complex (exchangeable form). The reserve of uranium in pore solutions at the
soil boundary with the root system of the plant can be supplemented additionally
by uranium more firmly associated with the components of the soil complex and
entering the aqueous phase of the soil under conditions of increased acidity. Such
conditions are formed by the plant in the process of assimilation of nutrients
(Grodzinsky 1989). Elements entering the soil solution at the border with the root
system of plants are in a form that can be called mobile or conditionally biologically
available.
The relative quantity of uranium associated with reversibly bound fraction in the
soil samples is presented in Fig. 4.
In the soil samples studied, the uranium content in the exchange form (U exch ) was
within the range of 0.1–2.9% of the total uranium content in the soil. The minimum
portion of U exch (0.10–0.33%) is found in loamy soils and the maximum in sandy
soils (1.5–2.9%). Sandy loam soils contain an intermediate position between them
in the content of exchangeable uranium (0.8–1.6%). In organic soils portion of
exchangeable uranium was within the range of 0.7–2.6%. The minimum content of
U exch is characteristic of peat-bog soil with relatively low mass fraction OM sol
(~0.2%, P4 plot), and the maximum content of U exch was found in peat-bog soil with
a relatively high mass fraction of OM sol (2.1%, P3 plot).
The relative quantity of uranium associated with mobile (conditionally biologically available) fraction (U mob ) in the soil samples is presented in Fig. 5.
G. A. Sokolik et al.
5.2 Reversibly Bound (Exchangeable) and Mobile
(Conditionally Biologically Available) Uranium in the Soils
Uranium reversibly associated with components of the sorption complex of soil is in
potentially mobile (exchangeable) form. These can be cationic, anionic, and neutral
(molecular) uranium complexes with organic and inorganic components of the soil,
as well as soluble hydrolysis products that are capable of entering soil solution. The
composition and ratio of these forms of uranium depends on the redox conditions,
the pH of the medium, and the presence of complexing addends (Sheppard and
Campbell 1980; Katz et al. 1991; Rachkova et al. 2010). In the upper well-aerated
horizons of soils, as a rule, an oxidizing environment is formed; therefore, uranium,
as a rule, is in the degree of oxidation +6. In this case, reversibly bound uranium
could be presented of UO 2
2+
, its complex compounds, and hydrolytic forms of U
(VI), (UO 2 OH
+
, UO 2 (OH) 2
0
UO 2 (OH) 3
−
, UO 2 (OH) 4
2−
, etc.), which can be found in
the soil waters (Katz et al. 1991; Sparovek et al. 2002; Rachkova et al. 2010).
Reversibly bound (exchangeable) uranium is distributed between the solid phase
and the pore solution of the soil until interphase equilibrium is established. However,
the bulk of reversibly bound uranium remains in the solid phase of the soil. In redistribution of uranium along the soil profile and the assimilation by vegetation through
the root systems, its content in the soil pore solution decreases, and the equilibrium
distribution of the element between the solid phase and the pore solution of the soil
is disturbed. Restoration of the disturbed equilibrium is carried out by transition from
a solid phase to a solution of uranium, reversibly connected with components of the
soil complex (exchangeable form). The reserve of uranium in pore solutions at the
soil boundary with the root system of the plant can be supplemented additionally
by uranium more firmly associated with the components of the soil complex and
entering the aqueous phase of the soil under conditions of increased acidity. Such
conditions are formed by the plant in the process of assimilation of nutrients
(Grodzinsky 1989). Elements entering the soil solution at the border with the root
system of plants are in a form that can be called mobile or conditionally biologically
available.
The relative quantity of uranium associated with reversibly bound fraction in the
soil samples is presented in Fig. 4.
In the soil samples studied, the uranium content in the exchange form (U exch ) was
within the range of 0.1–2.9% of the total uranium content in the soil. The minimum
portion of U exch (0.10–0.33%) is found in loamy soils and the maximum in sandy
soils (1.5–2.9%). Sandy loam soils contain an intermediate position between them
in the content of exchangeable uranium (0.8–1.6%). In organic soils portion of
exchangeable uranium was within the range of 0.7–2.6%. The minimum content of
U exch is characteristic of peat-bog soil with relatively low mass fraction OM sol
(~0.2%, P4 plot), and the maximum content of U exch was found in peat-bog soil with
a relatively high mass fraction of OM sol (2.1%, P3 plot).
The relative quantity of uranium associated with mobile (conditionally biologically available) fraction (U mob ) in the soil samples is presented in Fig. 5.
G. A. Sokolik et al.