INTRODUCTION
In the terrestrial environment, soils represent the principal repository of
the transuranic elements over geologic time.
The major factor governing
transuranic availability to plants in soils will be the solubility of the
transuranics associated with the solid phase, since in order for root
uptake to occur a soluble species must exist adjacent to the root membrane
for some finite period. The form of this soluble species will have a
strong influence on its stability in soil solution and on the rate and
extent of uptake, and perhaps, mobility and toxicity in the plant.
Furthermore, it is the solubility and form of the element which largely governs
mobility in soil.
Thus, any assessment of the long-term behavior of the
transuranics in the terrestrial environment must be based on determination
of the factors influencing solubility and form of soluble species in soil.
These factors are illustrated in Fig. 1 and include the concentration and
chemical form of the element entering soil, soil properties, aa these
influence the elemental distribution between the solid and liquid phase,
and soil processes, such as microbial activity, as these influence the
kinetics of sorption reactions, transuranic conceptration, and the form of
soluble and tnsoluble chemical spectfes.
PLANTS
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The transuranic elements of principal importance in the nuclear fuel cycle
(Pu, Am, Cm, Np) may enter the soil through several avenues (Vaughan et al.,
1976) including (1) fallout from atmospheric testing, (2) possible airborne
particulates passing a filter and liquid effluents, both during reprocessing
of spent fuels and fuel fabrication and (3) leaching from waste storage
facilities. The major sources of the transuranics may be classified accord-
ing to expected initial solubility in soil (Fig. 2).
PROPERTIES »>
SOIL
SOLUTION
(SOLUBLE Pu)
<
SOIL
PROCESSES
Particulate oxides of
the transuranics initially may be expected to be largely insoluble in the
soil solution.
Ultimately, solubility is expected to be a function of the
composition, configuration and equivalent diameter of the particle as well
as soil properties and processes.
Oxide particles of the highest specific
activity and containing the highest concentrations of impurities in the
crystal lattice may exhibit greatest solubility.
The combination of configuration and equivalent diameter as reflected in surface area exposed to
solution will be the other main factor governing oxide solubility.
Once
solubilized, the transuranic elements will be subject to the chemical reac-—
tions governing soluble salts.
Hydrolyzable transuranics entering the soil
in acid solutions sufficiently concentrated to maintain soluble fons may be
expected to be rapidly insolubilized due to hydrolysis on dilution and
subsequent precipitation on particle surfaces. These include Pu(EiI, IV,
and VI), Am (III, EV}, Cm (111, IV), and Np (111, IV, VI). Conversely,
transuranics not subject to marked hydrolysis may be initially more soluble.
These,include Pu (¥) and Np (V).
Immobilization of these chemical species
{Pu0; or NpO2") may occur through cation exchange reactions with particulate surfaces.
Complicating this situation, disproportionation and complexation reactions may occur concurrently.
Transuranics entering the soil as stable organocomplexes as might occur in
the vicinity of a spent fuel separation facility may be initially highly
Soluble (Wildung and Garland, 1975).
The duration of solubility and
mobility in the soil will be a function of the stability of the complex to
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SOIL
4
SOURCE
Fig,
1.
the terrestrial
Factors influencing transuranic behavior in
environment.