Table 1.
Solubilities of Plutonium in Water Extracts of a Ritzville
Silt Loam as Determined by Filtration with Membranes
of
Different Pore Sizes (Garland and Wildung, 1977).
Membrane
Pore Size,
Plutonium
Solubility,
vm
pg/g*
5
0.45
0.01
0.0015
6.0012
0.9010
60,009
20,000
4,000
1,000
300
50
“Plutonium added at a level of 620,000 pg/q of soil,
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from particulate matter, it is evident that Pu in these filtrates may be in
colleodial forms. The Pu in the 0.0010 vy filtrate appeared soluble, was
stable in solution, and approximated the quantity of Pu taken up by plants
(Wildung and Garland, 1974).
Of the soluble Pu forms likely to enter soils
(previous section), Pu({NO3), and Pu-DTPA likely represent, in their respective chemistries, the Tange in soil behavior likely to occur. The water
solubility (< 0.01 p) of 7?%pu and 73%Pu amended to a Ritzville silt loam
(organic C content 0.7%, pH 6.2) in the Pu(NO3),y and Pu-DTPA forms differs
markedly (Wildung and Garland, 1975). The DTPA complexes of both isotopes
were water-soluble in soil and appeared to be etable over the first 40 days
of incubation (Fig. 4).
After 7 days of incubation, the 7°*Pu-DTPA appeared
to be slightly less soluble then the 7**pu-DIPA. After 95 days of incubation, both isotopes, initially added as the complex, appeared to decrease in
solubility; perhaps as a result of microbial degradation of the organic
mofety and the development of new chemical equilibria.
Equilibrium concentrations of soluble Pu added as the nitrate were not
obtained until 7-10 days. The solubility of 7?*Pu and 7°°Pu added to the
soil as nitrates was much lower than the DTPA complexes, likely reflecting
hydrolysis to the largely insoluble hydrated oxide,
It is clear that
organic ligands may have a pronounced effect on Pu solubility in soil.
The
tate of decrease in solubility of each isotope added as the nitrate was
similar. However, in contrast to the slightly lower solubility of the
? 98 Ppu-DTPA compared to the 735 pu-DIPA, 24%pu added ag the nitrate was a
consistent factor of 2-3 times more soluble than *?°Pu initially added as
the nitrate. This difference probably resulted from the formation of larger
hydrated oxide particles at the higher Pu concentration Cpu), but it may
also have reflected the presence of soil components such as organic ligands,
which stabilized Pu in solution but were present in Limited concentrations
and became important only at lower Pu concentrations.
The water solubility of 738pu, when incorporated in relatively large Pu
oxide particles (*1 yy), would be expected to be greater than the solubility
ef 73°pu oxide particles of similar size due to crystal damage and radiolysis arising from the greater specific activity of the 239pu (an approximate
factor of 270).
However, the behavior of the two isotopes in soil on solubilization of the oxide might be expected to follow a course similar to that
exhibited by the nitrates (Fig. 4).
Equilibrium solubility after 6 days of incubation (Garland et al., 1976) of
Pu, added as Pu(NO3)}4, in soils of different properties occurred after
approximately 20 hours (Fig. 5).
The quantities of Pu soluble at equilibrium in water and 0.01 HMCaCl, differed with soil type.
In the CaCl,
solution, Bolubility was lowest in the Muscatine soil which exhibited higher
silt and clay content than the other soils.
Importantly, at equilibrium
there was more Pu extracted by water than 0,01 M CaCl. in the Muscatine
soil, The Hesson and Ritzville soils did not exhibit this property.
This
may be related to a difference in the dispersibility of fine colloids in
this soil and/or the presence of higher concentrations of stabilizing
ligands. However, the lack of a proportional dilution effect (not shown in
Fig. 5) in the water extractability of Pu at lower solution to soil ratios
in thia soil as compared to the Ritzville and Hesson soils, provided
135