Ob
Table 7,
Stability of DTPA Complexes with the Transuranic Elements
(Hafez, 1969).
Complex
Pp
;
Neptunium
47
NO CIV) DTPA
tk (IV) LSDTPA
Pp
2
3
,
Stability
constant
ae
1024
rd
biological uptake (plants, microorganisms) of the elements and subsequent
release on decomposition.
Several studies have demonatrated plant uptake
of Pu and Am and incorporation into above ground tisaue.
Stable
PH Range
.
recent studies (Wildung and Garland, 1974) have indicated that barley roots
(uncontaminated with soil particles), contained 3-8 times more Pu than the
shoots, The roots of plante are in intimate contact with the aoil and may
be expected to decompose rapidly (weeks) under appropriate conditiona of
temperature and moisture, even in arid regions (Wildung et al., 1975}.
Relatedly, microorganisms, due to their distribution in soil and large
absorptive surface, compete efficiently with plants for fiona in soil
(Alexander, 1961).
Studies described in a previous section demonstrated
the association of Pu with microbial cella.
Growth of microbial cells, a
significant portion of the soil biomass, may therefore represent an
important mechanism for biological incorporation of the transuranic
elements. Decomposition of microbial cells generally proceeds at a more
on
_
0.5 - 5.8
>5.8
Plutonium
Pu(IV) DTPA
[Pu(Iv)] ,DTPA,
PulIV} DTPA,
.
102%
1 0
10/18
101%
5.8
5 8 - 8.5
>8 5
1970
1023
1.8 - 6
>6
Americium
{[Am(III)] DTPA
Am(III) DTPA
These tissues,
deposited on soil either through litter fall or agricultural incorporation
of crop residues will be subject to microbial decomposition. Furthermore,
rapid rate than plant tissues.
Little ig known of the form of the transuranics in plant or microbial
tissues; of the form, rate, and extent of the transuranics released on
decomposition of these tissues; or of the chemical reactiona governing
transuranic solubility after decomposition. However, conaidering the
known products of microbial metaboliem of organic subatances, including a
number of atrong complexing agents (previous section), and the susceptibility of a number of the transuranic elements to complexation (previous
section), it may be concluded that the tranauranics, initially immobilized
through biological uptake, may be at least as soluble and perhaps more
solubie on decomposition.
* Curium may be expected to form complexes of stabilities
similar to americium.
**Unstable in oxygenated solutions.
In preliminary studies (Wildung and Garland, unpublished), Pu-amended soil
‘
162
containing largely undecomposed roots from a previous barley crop was
leached with water and Pu solubility compared to a fallow soil containing
Pu at similar levels. The resulta indicated that soluble Pu was initially
immobilized by incorporation into roots, decreasing by a factor of 10
after root growth. Root decomposition studies are in progress. Previously
observed (Romney et al., 1970) increases in Pu uptake from soils by plants
with increased time, generally attributed to increased root development,
may have been due to increased availability through a recycling procesa on
decomposition of plant rocta. The importance of the process will be dependent upon transuranic availability to different plants and microorganisms,
the turnover rate of this tissue in soils under different conditions and
the atability, chemistry and biological availability of transuranic metabolites. Until this information is developed, the long-term effects of
recycling processes will remain unknown.