of Pu after only 4 days of incubation, suggesting the development of a
microbial population particularly capable of alteration of Pu solubility.
Again, there was no change in the < 0.01 u fraction which amounted to
approximately 10% of the Pu present in the < 0.45 u fraction.
At least under the conditions of this study, the evidence strongly suggested
that the solubility of Pu in soil was influenced by the activity of the
soil microflora.
The potential mechanisms effecting the change in solubility include mechanisms (1) and (2) described above, i.e. indirectly through
the production of organic acids that may complex Pu or the alteration of
the solution pH and/or Eh near the soil colloid (or Pu polymer) without
measureable effects on the overall acil pH; or directly through the transformation of Pu. Microbial effects may have occurred at the colloidal
level since detectable increases in solubility did not occur in the
< 0.01 » filtrates.
This appears to implicate the indirect mechanisms
since direct changes generally occur at the molecular level and these
would be reflected in changes in Pu concentration in the < 0.01
filtrates
provided polymerization did not occur during extraction.
Of course, a
combination of the mechanisms is possible, i.e., alteration of affinity
for organic ligands through a change in valence state.
If the mechaniam
of solubilization was indirect, the results might be applicable to other
transuranic elements, e.g., from consideration of the aqueous chemistry
described in a previous section, a reduction in pH would be expected to
increase the solubility of the other transuranic elements as well as Pu.
Increased water solubility of Pu on incubation under optimum conditions
for microbial activity may be expected to increase Pu uptake by plants
provided the limiting factor is not discrimination at the root membrane.
In order to determine if the increased solubility on incubation resulted
in increased Pi uptake by plants, the soila, incubated as previously
described, were planted to barley and cultured using a split-root technique
which allowed measurement of the uptake, sites of deposition and chemical
forms of Pu in plant shoots and roots (Wildung and Garland, 1974), The
results were compared to the results of similar plant studies in which the
soils had not been incubated.
Prior incubation, which in microbial studies was shown to increase the
solubility of Pu in soil, increased Pu uptake by shoots compared to the
unincubated controls.
The effect was greatly accentuated in the case of
the soil-free roote and incubation increased the soil to plant concentra~
tion ratios by up to 37 times relative to the umincubated control, depending
upon Pu soil concentration level.
Thus, plant uptake measurements tend to
verify measurements of increased Pu solubility in the < 0.45 uy fraction in
the incubated soil.
However, if, as previously discussed, Pu in particulate
form (i.e., > 0,01 UW) wag not available to plants, and if increases in the
concentration of Pu in the 0.01 te 0.45 wy fraction resulting from microbial
activity were not due to procedural artifacts arising from polymerization,
the colloidal particles may be serving as a reservoir of plant—available
Pu manifested only when the equilibrium concentration of Pu in solution is
reduced by plant uptake,
146
Effect on the Soil Microflora
Soil organisms may be expected to be present at highest levels in the
immediate vicinity of soil colloids (Alexander, 1961).
From the aqueous
chemistry of the transuranics and on the basis of recent information on
transuranic chemistry in soil (previous section), the transuranic elements
in soil are likely to be associated mainly with colloids.
Thus, soil
microorganisms may be exposed to relatively high concentrations even
when total transuranic seil concentration is low.
It is therefore
necessary to determine the toxicity of the transuranic elements to soil
Microorganisms, as microorganisms exhibiting resistance to the chemical
effects of the transuranics may have the highest potential for participating
in alteration of transuranic form. However, the transuranic series does
not contain stable isotopes and organisms chemically resistant to these
elements must exhibit a degree of radfation resistance which is dependent,
in large part, upon the radiochemistry of the isotope. Resistance to the
chemical effects of transuranics may occur by three general mechanisms
including (1} tnability of the transuranics to produce a toxic effect on
cell metabolism at the cytoplasmic or exocytoplasmic levels (2) tnability
of organisms to transport the transuranics or (3) ability of the organisms
to convert transuranics, by the direct and indirect mechanisms discussed
in a previous section, to a form that is either less capable of entering
It is che latter mechanism which is
the cell or is not toxic to the cell,
likely most important in alteration of transuranic form in soil.
Effect on Microbial Types, Numbers and Activity. The effect of soil Pu
concentration on the soil microflora has been measured as a function of
changes in microbial types and numbers and soil respiration rate (Wildung
et al., 1973, 1974a). A noncalcareous Ritzville silt loam (pH 6.7) was
amended with 7?°Pu(NO3), at levels of 0.05, 0.5 and 10 wCi/g and with
Subsamples of
starch, N and water to provide optimal microbial activity.
soil were periodically removed to determine the changes in types and °
During this period, soil respiration
numbers of soil microflora with time.
rate was monitored by continuous collection of sotl-evolved CU>.
The growth curve of fungi (Fig. 7) was generally typical of the growth
response for other classes of microorganisms. Total microbial numbers
were compared at the end of logarithmic growth. The organisms generally
Growth rates were
reached this stage after 8 to 14 days of incubation.
compared over the intervals of maximum microbial growth for each organism
The results are summarized in Table 3}.
at each Pu concentration.
The Pu did not generally affect the rate of growth but decreased the total
numbers of all classes of microorganisms at levels as low as 0.05 uci/e
or 7 ug/g. The fungt were the exception, differing from the controls only
Thus, the Pu did not
at a Pu concentration of 10 uCi/g or 144 ug/g.
affect maximum generation rate but rather affected the lag period or onset
of the stationary phase, limiting microb{al numbers.
The accumulative CO2 curve generally corresponded to the growth curve of
In the case of the other classes of organisms, maximum logthe fungi.
arithmic growth occurred before the rate of C02 evolution reached minimum
147