inte microbial tissues, and (4) release of metals on decomposition of
organic residues.
All of these mechanisms may be operational in transformatione of transuranic elements in soils. However, on the basia of present knowledge, it
is not possible to draw conclusions as te their relative importance in
affecting the long-term behavior of the transuranic elements. Since there
ie a paucity of information available, rather than discuss these mechaniems
individually they will be addressed around a framework of current information, limited principally to Pu.
Microbial Alteration of Solubility in Soil
1500
ACCUMULATIVE
CARBON
DIOXIDE, mg
To provide a preliminary assessment of the potential for microbial alteration of Pu solubility in soil under aerobic conditions, Wildung et al.
(1973, 1974) measured soil reapiration rate (an index of soil microbial
activity), microbial types and numbers and Pu water solubility in sterile
(gamma irradiation) and nonsterile soils, which contained 10 Ci Pu/g of
Carbon dioxide evolution waa utilized aa a
soil [added as Pu(NO3),].
measure of soil respiration rate. To measure Pu solubility, the soil was
subsampled at intervals during incubation over a 30 day period, and the
subsamples (1g) suspended in 1 liter of distilled water. After a 4 hr
equilibration period, an aliquot of the soil suspension was filtered
through 5, 0.45 and 0.01 » millipore filters.
The Pu in the 0.5 and
0.01 »p filtrates was designated water soluble although it was recognized
that Pu Likely was present as fine colloids (previous section).
which represented a Pu level leas than 0.2% of that applied, did not
change significantly with treatment,
o
0.0
a 100
fF
STERILE
TREATMENTS
\
0
PLUTONIUM WATER
SOLUBILITY (< 0.45u)
IN SOIL, %
Changes in the soil respiration rate and Pu solubility during the initial
30 day incubation are shown in Fig. 6.
The concentration of Pu in the
0.45 p filtrate during the incubation period ranged from approximately 0.5
to 1.5% of the Pu initially applied.
Solubility was initially higher in
the sterile soil than in the nonsaterile soil but was relatively constant
with time in the sterile soil. The initial increase in solubility in the
sterile soil was anticipated in view of the known increases in soluble
organic matter which result from gamma {irradiation of soil.
The Pu solubility (< 0.45 y) in the nonsterile soil, while initially
lower, increased by a factor of 3 with incubation time to 14 days and
remained significantly higher than the sterile soil during the incubation
period.
This increase generally followed the accumulative CO2 curve, and
maximum solubility occurred at the end of logarithmic growth for all
classes of organisms. The concentration of Pu in the 0.01 u filtrate,
1000
PLUTONIUM, uCi/g
1.0
0.5
0
Fig. 6,
STERILE soll
x
0
NONSTERILE SOIL
!
]
10
20
INCUBATION TIME, DAYS
'
30
Changes in soil respiration rate and solubility of appt
i with time of soil incubation
1
.
In an ancillary experiment, incubation was continued for 65 days until the
COz evolution rate reached a constant level.
The Pu-containing sterile soli
was then inoculated with the Pu-treated nonsterile soil and the respiration
rate and solubility of Pu in the inoculated soil measured for a period of
30 days, When the sterile soils were inoculated with nonaterile soil, COQz
evolution increased at a much more rapid rate without a lag phase, and
thia was accompanied by a factor of 2 increase in water solubility (< 0.45 yu)
144
145
(Wildung Qu
akh.y
,