diameter.
Tamura (1974, 1975, 1976a) also studied soil samples from safety
shot areas in NTS.
These studies of particle size association included separation of different silt and clay sizes and showed that the association was
primarily with che coarse silt (50 - 20 um) and fine sand (125 - 50 um) sizes.
have been extracted.
Since the citrate does not reduce the iron and manganese
in the samples under the extraction conditions, the citrate-extractable plutonium may not be bound with those elements,
lLron and manganese form surface
oxide coatings on particles; thus the dithionite treatment coupled with citrate
would serve to remove the surface coatings and possibly remove surface-sorbed
plutonium as well.
The particle size segregation of Pu-contaminated soils by Tamura (1974) was
made without benefit of chemical or physical treatment to induce particle
dispersion. This approach was taken in order to evaluate the degree of assoclation of plutonium as it exists in the soil.
Later data were reported on size
association of the plutonium with and without ultrasonic treatment for dispersion (Tamura, 1976b).
In addition to comparing the effect of ultrasonic
treatment on two NTS soils, data was included on a soil from the floodplain in
Oak Ridge (ORNL) and a segment of bottom sediment from the canal at Mound
Laboratory (ML).
The same data are reproduced in Tables 2 and 3.
Bondietti et al. (1976) evaluated the amount of organically associated
plutonium in a floodplain soil in Oak Ridge.
They used a resin technique and
estimated that about 15% of the plutonium in the soil may have been associated
with soil organic material.
Sodium-resin, prebuffered at the soil pH, was
used to maintain constant pH and decalcify the soil organic matter.
This is
in contrast to sodium hydroxide extraction of organic matter which raises the
pH.
Destruction of organic matter with sodium hypochlorite released 82% of
the plutonium. They concluded that since the alkaline hypochlorite treatment
would not dissolve the iron oxides, most of the plutonium was surface sorbed
rather than intimately assoctated with iron.
Since the fioodplain soil and
the Lake Michigan sediments represent different matrices, the mode of association could be different as postulated by these investigators.
It would be
informative if the different treatments employed by the investigators could be
applied in a similar manner to both samples so that deductions regarding the
association could be evaluated.
The ultrasonic treatment increased the plutonium contribution of the finer
size fractions of the total soil (Table 3).
In general, the increase in the
plutonium contribution in the finer resuspendible sizes was also accompanied
by an increase in mass contribution in these finer sizes (Table 2).
This
suggests that the plutonium contribution in the finer sizes was primarily due
to plutonium-soil particle disaggregation and not necessarily to plutonium
detachment from the soil particle.
Considering the less than 5 um sizes, the
ultrasonic treatment increased the percentage contribution by two to five
times, although the total percentage contribution in the NTS samples was still
less than 10%.
In the sample from Mound Laboratory, the ultrasonic treatment
increased the Pu in the less than 5 ym size from 34% to 81%.
More significantly,
The high Kd's of over 100,000 assoctated with aged (years) sources described
above, as well as with the groundwater particulates at Enewetak Atoll and the
sediment from the canal near Mound Laboratory, compared to the lower Kd of
about 1,000 for shorter term (days) simulated tests in the laboratory
suggest
a biogeochemical process which increases retention.
The Process of increasing
retention has implications in reduction of uptake by plants as well
as in the
resuspension process, both in terms of the size of the particles being
resuspended and the depositional patrern in the lungs.
the Pu in the less
than 2
pm size
fraction in Mound
Laboratory sample
increased
from 17 to 70%.
The importance of the association of the plutonium with the
fine size of soil particles will be discussed further in the later section of
this paper.
Muller and Sprugel (1977) determined the distribution of plutonium in three
soils located within 50 miles of Mound Laboratory in Ohio.
Two of the sofls
In discussing plutontum particle size, two tmmediate considerat
ions become
apparent.
First, when Pu releases occur from stacks, the particles
to be
considered are pure metal or metal oxides.
When these particles deposit on
the ground surfare, interactions with the soil particles
occur.
Depending on
the character of the initial source, the intensity
of the association with
soil particles may be different.
This paper is concerned with the plutonium
in soils and sediments, hence it does not consider
the initially released
particles.
That plutonium attaches itself to host soil particles
has been verified directly
by microscopic and alpha-track measurement techniques
(Hayden, 1976; Nathans
and Holland, 1971: Nathans et a7., 1976).
Mork (1970) studied the size assoc tation of plutonium in Yucca Flat on the Nevada
Test Site (NTS) and showed that
most of the plutonium was aasoctated with
soil particles greater than 44 um
100
-:
were “background samples reflecting fallout levels; the third was a soil
PLUTONIUM SIZE CHARACTERISTICS
sample taken one mile east of the Laboratory, containing sufficient ??3py
originating from the operations to differentiate it from fallout levels.
The
samples were treated with a carbonate-oxalate dispersant and the fractions
were analyzed for plutonium.
Between 60-75% of the plutonium was assoc {lated
with particles less than 4 pm size, and about 907 of the activity was assoctated
with particles less than 45 um (silt and clay}.
For comparison, a bottom
sediment sample taken in the canal at Mound Laboratory showed very similar
distribution with 81% of the Pu in the less than 5 um and 100% in the combined
silt and clay fraction (Tamura, 1976b).
Plutonium associations with different particle sizes in soiis of Rocky Flats
have been reported by Little et al. (1973}.
The mode of segregation involved
sieves with only mechanical shaking of oven-dried samples. The particle size
distribution showed that the less than 104 um size particles in the soil
contributed 3.74 in one sample and 11.2% another.
The 3.7% and 11.2% weight
fractions reported by Little et al. (1973) contributed 11 and 377, respectively,
to the total soil activity.
The low amount of fine materials likely caused by
the oven drying which would tend to "cement" particles, the short shaking
101