NERC-LV-539-33
RETENTION OF ELEMENTAL AND PARTICULATE
RADIOIODINE ON ALFALFA
by
B. J. Mason
K. W. Brown
H. W. Hop
J. C. McFarlane
National Environmental Research Center-Las Vegas
U. S. ENVIRONMENTAL PROTECTION AGENCY
Las Vegas, NV 89114
Published July 1974
This research performed under a Memorandum
of Understanding No. AT(26-l)-539
for the
U. S. ATOMIC ENERGY COMMISSION
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This report was prepared as an account of work sponsored by the United
States Government. Neither the United States nor the United States
Atomic Energy Commission, nor any of their contractors, subcontractors,
or their employees, makes any warranty, expressed or implied, or assumes
any legal liability or responsibility for the accuracy, completeness or
usefulness of any information, apparatus, product or process disclosed,
or represents that its use would not infringe privately-owned rights.
Available from the National Technical Information Service,
U. S. Department of Commerce,
Springfield, VA 22151
Price: paper copy $4.00, microfiche $1.45
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NERC-LV-539-33
RETENTION OF ELEMENTAL AND PARTICULATE
RADIOIODINE ON ALFALFA
by
B. 0. Mason
K. W. Brov/n
H. W. Hop
J. C. McFarlane
National Environmental Research Center-Las Vegas
U. S. ENVIRONMENTAL PROTECTION AGENCY
Las Vegas, NV 89114
Published July 1974
This research performed under a Memorandum
of Understanding No. AT(26-l)-539
for the
U. S. ATOMIC ENERGY COMMISSION
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ABSTRACT
131
Synthetic participate and gaseous I contaminants were deposited
on a three-week-old stand of alfalfa. One portion of the samples
collected during the test was washed with a detergent solution. The
analysis of these samples revealed that a fraction of the I could
be removed by this method but that the plants would have to be "cleaned"
during a very short period of time following contamination in order to
have any appreciable benefit. The retention of the iodine on the
vegetation appears to be particle size dependent during early time
periods with the larger particles having a shorter effective half-life;
131
however, during the second phase of the loss of I from the plants
131
there was no effect of particle size. The elemental I contaminant
showed essentially no rapid early loss but decayed with an effective
half-life close to the physical decay rate.
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INTRODUCTION
The transfer of radioiodine from source to man occurs primarily
through the forage-cow-mil k-man food chain, as reported by
CHAMBERLAIN and CHADWICK^5) Although there are other pathways, the
route through milk appears to be a greater hazard to the critical
(?]
population, described by the FEDERAL RADIATION COUNCILV ' as being
the human child. Of the various radioisotopes of iodine, BOLLES and
BALLOIP ' consider only I to be a major radiological health
129
problem. However, I may become a significant problem due to its
long half-life and, therefore,its persistence in the environment. Radio-
active iodine has historically been formed primarily during nuclear
detonations and continues to be a pollutant from reactor operations and
fuel reprocessing.
Studies carried out in this laboratory have indicated that the
retention of radioiodine by plants may be dependent upon the size of the
particle with which the iodine is associated. The experiment presented
in this report was designed to determine the effect of particle sizes
131
and form of the contaminant upon the retention of I by alfalfa
(Medicago sativa). Two aerosols with different sized particles were
used in this study; comparison was made between these aerosols and
a gaseous contaminant.
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METHODS
Previous tests conducted by the National Environmental Research
Center, Las Vegas (NERC-LV) utilized diatomaceous earth, sized to yield
particles with a designated count median diameter, to provide a source
of particulate contamination. The detailed methods for preparing these
aerosols were outlined by STANLEY ejb aj_. . Basically, diatomaceous
earth (DE) was milled in a standard ball mill, sieved, and mixed with
a solution of ethyl alcohol and NaOH. To this mixture, 0.15 mCi of
131
carrier-free I was added, stirred, and air dried. The aerosol was
released by blowing the particles from a round-bottom flask through a
generator nozzle with compressed air.
I Ol
The gaseous contaminant ( I?) was prepared similarily to that of
HAWLEY^ . The following solutiore were added in sequence to a generating
flask. The first contained 1.5 mg of Nal carrier, 0.5 ml H-PO^j and
131
10 ml of distilled water along with 0.2 mCi of Na I. The second solution
contained 130 ml of 2N H?SO 4. The third was used to initiate the reaction
and consisted of 30 mg of NaNO? and 10 ml of distilled water. The post-
generation reducing solution (to stop the I~ generation) consisted of
5 ml of 30% H^PO? and 5 ml distilled water. To disperse I?, the acidic
solution of Na I was oxidized and the I2 was sparged from the solution
with nitrogen gas. Generation of the gaseous materials was regulated to
last for approximately 30 minutes.
The distribution and transport of both particulate and gaseous con-
taminants over the field were accomplished by the katabatic or drainage
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winds. The generators were arranged in a fashion similar to those des-
crihed by .Stanley, et_ a]_.'10^
Three, four-meter by five-meter experimental plots were established
in a uniform stand of alfalfa. The alfalfa was approximately three
weeks old and measured about 45 cm in height. Each plot was subdivided
into 80, one-half-meter square subplots. The three plots were contaminated
131
with I as follows: one plot received a particulate aerosol of 0.43 ym
count median diameter (CMD), another received an aerosol of 0.15 ym CMD,
and the third plot received gaseous I?. Sampling times and treatments
were randomly assigned to the various plots.
The retention of I depends upon how tightly the contaminant
becomes attached to the vegetation. As a means of evaluating the
retention of the contaminants on alfalfa, a number of samples collected
from each plot were washed with a nonionic detergent solution. Although
131
chemical factors such as solubility of the I, polarity of the
detergent, and pH may influence the effectiveness of the washing, the
authors assumed that the material left after washing was effectively
"fixed" to the vegetation.
Samples were collected at predetermined intervals following the
aerosol release. To collect a relatively uniform area of vegetation,
2
a ring (0.075 m ) was placed in the center of the subplot. All stems
of alfalfa which originated within this ring were cut one cm above
the ground and placed in a plastic bag.
Samples that were to be washed were placed in six liters of a
0.1 percent solution of Joy®a commercial nonionic liquid detergent,
to which had been added about 0.1 ml of Antifoam A®, a silicone defoamer.
3
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After two minutes of soaking in the wash water, the samples were
vigorously agitated and the detergent was drained from the sample.
The vegetation was twice rinsed by submerging and agitating the
sample in six liters of clean water. After rinsing, the samples were
air dried and placed in clean plastic bags. The alfalfa in the
sample bag was pressed into a 500-ml plastic counting container. The
plastic container was taped shut and placed in a clean plastic bag.
The samples were counted on a 4- by 4-inch sodium iodide (Tl) crystal
mated with a single channel analyzer. After counting, the samples
were dried for 24 hours at 70°C in a forced draft oven, and weighed.
The dry weight was used to determine the initial moisture content
which averaged 89.7 ±1.2 percent.
/o\
Data from previous experiments by MCNELIS et a]_.v ' and STANLEY
et aj_.' ' have indicated that the deposition of the aerosol on
alfalfa frequently exhibited "hot soots" which were thought to be
caused by the micrometeorology over the vegetation. To account for
these variations, 4-inch planchets coated on one surface with a
nonsetting alkyd resin were placed at 2-meter intervals throughout
the plot. The planchets were placed on top of wooden stakes which
were driven into the ground so that the planchets were even with the
top of the forage.
The amount of I deposited on the planchets was used to normalize
the deposition on the plants. Normalization was done by dividing the
131
activity value of the planchet into the concentration of I deposited
1 3-1
on adjacent plants. The value obtained is the amount of I per gram
of dry tissue per unit of deposited activity.
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RESULTS AND DISCUSSION
Release of the radioactivity occurred at 11:00 p.m. when the nighttime
drainage winds stabilized at about 1 mph. First samples were collected 1
hour after the end of the release. Deposition on the three major
plots is shown in Table 1. Deposition activity was based on planchets
located throughout the plots.
Previous experiments by MCFARLANE and MASON^ and STANELY et. al_. ^10^
indicated that the retention of particulate radioiodine on vegetation would
follow a two-phase curve. Figures 1, 2, and 3 present results of the three
different contaminants. Each point on each graph represents the average
of four replicates. The pattern of the control (unwashed) retention
curves of the particulate contaminants follows a two-phase curve, where
initial loss rates were faster than at later times.
Linear regression was used to determine the effective half-life of
the two components of the control sample data and of Phase II of the washed
sample data. Table 2 presents the computed half-lives. The slopes of
the lines were significantly different from zero at the 95% confidence
level with correlation coefficients ranging from 0.61 to 0.95. The poorer
fits were on the Phase I components of the curves where the greatest
effects of the early loss of the contaminant were observed.
It is believed that iodine lost from the nonwashed samples during
Phase I as shown in Figures 2 and 3 was attached to the larger particles
found in the distribution of particle size used to make up the two-tagged
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131
Table 1. I Deposited on Experimental Plots
1 "31
Plot Contaminant I Activity
(nCi/m2)
1 Gaseous - I2 17 ± 3
2 0.16 urn CMD-DE* 19 ± 7
3 0.43 m CMD-DE 30 ± 10
*
CMD = count median diameter
DE = diatomaceous earth
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< o
2 a
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z
D.U
5.0
4.0
3.0
2.0
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I I I I I
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^ UNWASHED
°XS WASHED
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TIME AFTER RELEASE, days
Figure 1. Retention of 131I on alfalfa contaminated with131l-tagged
gaseous elemental iodine.
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TIME AFTER RELEASE, days
Figure 2. Retention of 131I on alfalfa contaminated with 131l-tagged
0.16 Mm CMD diatomaceous earth.
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4.0
3.0
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WASHED -a-
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TIME AFTER RELEASE, days
8
Figure 3. Retention of 131I on alfalfa contaminated with 131l-tagged
0.43 urn CMD diatomaceous earth.
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Table 2. Effective Half-life of 131I on Alfalfa
Contaminated with Various Synthetic Contaminants
Plot Contaminant Effective Half-life (days)
Phase I Phase II
Control Treatments
1 Gaseous I,, 3.5 ± 1.0 6.6 ± 0.8
2 0.16 ym CMD-DE 1.5 ±0.4 5.3 + 0.5
3 0.43 ym CMD-DE 0.8 ± 0.2 5.1 ± 0.7
Wash Treatments
1 Gaseous \2 * 6.7 ± 0.7
2 0.16 ym CMD-DE * 6'4 ± 1>8
3 0.43 ym CMD-DE * 5.5 ± 0.6
*
Not calculated.
10
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diatomaceous earth contaminants. The data in Table 3 show that the material
with the larger count median diameter (0.42 pm) contained more particles
in the 1 pm range. These larger particles would be more likely influenced
by the shaking action of the wind. Although handling acts in a similar
manner, this is a common factor with all samples and should only influence
the scatter of data points around the mean.
In the National Air Pollution Control Administration (NAPCA) document,
AIR QUALITY CRITERIA FOR PARTICIPATE MATTER^1 \ the following statement
is made:
"All available evidence suggests that solid oarticles
with diameters less than one micron, always adhere
when they collide with each other or with a larger surface."
A study was made of the particle size and activity distribution of the
particles used in this experiment. The data shown in Table 3 indicate
that the percent of the total activity that was lost in Phase I closely
equals the percentage of particles greater than one micrometer. If
one can assume uniform distribution of activity and can assume that
one micron limit mentioned in the NAPCA quote above is valid, then one
can assume that the loss of activity in Phase I is due to the loss of
particles by some mechanical means such as shaking. This corresponds
to the findings of MARTINA ' who noted that most of the activity found
on desert plants five days after Project Sedan* was associated with
particles less than 5 um in diameter. He suggests that most of the
particles larger than 5 um had been shaken from the plants by wind.
Although these data in no way prove that adhesion is the factor which
causes particles less than one micrometer to be retained, they do
*A nuclear explosive cratering experiment conducted by the U. S. Atomic
Energy Commission in July 1962.
11
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Table 3. Particle Size and Activity Distribution
for the Particulate Contaminants
Particle Size
CMD
pro
0.16
0.43
Particle Size Distribution
Percent
38.6
60.0
61.4
40.0
Activity Lost
Phase I Phase II
Percent
38.9 61.1
59.7 40.3
12
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indicate that some force appears to "bind" the smaller particles to
the vegetative tissues, and they also suggest that these "bound"
particles contain the activity found on the vegetation after the initial
loss of radioactivity.
The gaseous elemental iodine contaminant behaved quite differently
from the particulate materials. Since there were no known particles in
the gaseous material, a Phase I component for the control (unwashed) was
not expected. Figure 1 shows that this was essentially correct. The
slight Phase I component may be due to either a slight vaporization of
i 11
the 1 M or to a loss of contaminated dust particles. Statistically
there was no difference in the slope of the retention curves in the 0- to
2-day period when compared to the slope in the period from 2 to 9 days.
The washed sample activity curve indicates that a constant amount
of the gaseous activity was removed by the washing technique. This
result suggests that one portion of the radioiodine was either fixed or
inaccessible to the washing solution, while the other portion was readily
accessible to washing. On the other hand, washing the particulate
contaminated samples appears to have removed approximately the same
amount of activity as was lost by the speculated mechanical action.
After the first day, washing removed no iodine from the particulate
contaminated samples. This suggests that particles within a particular
size range may be readily bound to the tissues and can be removed only with
more vigorous methods than were used in this study. The fact that there
appears to have been a change in the ease with which the particles could
be influenced by washing supports this conclusion.
13
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The "peak" in the activity in all three wash curves cannot be
explained; however, it has been seen a number of times in other experiments
/o\
conducted by MCFARLANE and MASON ' where the washing technique was used.
Sunrise occurred just prior to the third sample collection. It is
possible that sunlight may have altered the chemical form of the iodine
contaminant or the relationship between the iodine and the constituents
of the surface of the leaf tissues. Also, light caused stomates to open
for the first time after contamination which may have affected the
retention.
The slopes of the Phase II curves in the washed sample data' are
similar and are not significantly different from Phase II of the cor-
responding control two-phase curves. The fact that the
i "31
loss of activity was less tnan the half-life of I
(8.08 days) is probably due to the dilution of the activity produced
by the growth of the forage. Based on the data in Table 2, it was
determined that the growth effect would have to produce an apparent loss
of activity with a half-time equal to about 25 days to account for the
difference. Evaluation of growth data for the alfalfa stand used in
this study produced a half-time value ranging from 16-28 days for the
stage of maturity at the time of the study. It therefore seems probable
that plant growth accounted for the discrepancy between the approximately
6-day half-life observed in this experiment and the expected half-life
of 8 days due to radioactive decay.
14
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CONCLUSION
No correlation existed between the participate size and the retention
of I tagged aerosols deposited on alfalfa after the first few days.
The particulate treatments had a two-component behavior pattern. The
131
first component consisted of a rapid loss of I which was attributed
to mechanical action affecting the larger particles. The second component
131
consisted of a gradual decrease in I content and is attributed primarily
to radioactive decay and to the dilution by plant growth. Of secondary
importance is the slow loss of the smaller sized particles containing iodine.
131
The loss of gaseous iodine ( 1^) from plant surfaces is a more
complex phenomena. Although there is a fixation of I?, the mechanism
of this reaction is not understood.
The data obtained from the wash studies indicated that any decontamination
measures should be taken within the first few hours after the deposition
131
of I to have any appreciable effect upon the removal of this contaminant
(o)
from forage. This supports the findings of BARTH et a_l_. , they observed
131
that rainfall failed to remove I from growing alfalfa following its
deposition from the Pin Stripe Event*
*An underground nuclear test conducted by the U. S. Atomic Energy
Commission in April, 1966.
15
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REFERENCES
1. Air quality for participate matter. (1969) National Air Pollution
Control Administration Publ. No. AP-49. U. S. Department of Health,
Education and Welfare, Washington, DC.
2. Background material for the development of radiation protection
standards. (1961) Federal Radiation Council Staff Report No. 2,
Federal Radiation Council, Washington, DC.
3. BARTH D. S., ENGEL R. E., BLACK S. C. and SHIMODA W. S. (1966)
Dairy farm radioiodine studies following the Pin Stripe Event of
April 25, 1966. SWRHL-41r. U. S. Department of Health, Education
and Welfare, Southwestern Radiological Health Laboratory, Las Vegas, NV.
4. BOLLES R. C. and BALLOU N. E. (1956) Calculated activities and
9-3C
abundances of U fission products. USNRDL-456, U. S. Naval
Radiological Defense Laboratory, San Francisco, CA.
5. CHAMBERLAIN A. C. and CHADWICK R. C. (1953) Deposition of airborne
radioiodine vapor. Nucleonics J3, 22-25.
6. HAWLEY C. A., JR., SELL C. W., VOLELZ 6. L. and ISLITZER N. F. (1964)
Controlled environmental radioiodine test at the national reactor
testing station. IDO-12035. Idaho Operations Office.
7. MARTIN W. E. (1965) Interception and retention of radioactive
fallout by desert shrubs in the Sedan fallout field. PNE-238F.
University of California, Los Angeles, CA.
8. MCFARLANE 0. C. and MASON B. J. (1970) Plant radioiodine relation-
ships: A review. SWRHL-90r. U. S. Department of Health, Education
and Welfare, Southwestern Radiological Health Laboratory, Las Vegas, NV.
16
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REFERENCES (Continued)
9. MCNELIS D. N., BLACK S. C. and WHITTAKER E. L. (1971) Radioiodine
field studies with synthetic aerosols. SWRHL-103r. U. S. Environmental
Protection Agency, National Environmental Research Center. Las Vegas, NV.
10. STANLEY R. E., BLACK S. C. and BARTH D. S. (1969) 131I dairy cow
studies using a dry aerosol. (Project Alfalfa) SWRHL-42r.
U. S. Department of Health, Education and Welfare, Southwestern
Radiological Health Laboratory, Las Vegas, NV.
17
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