ITR-6
SOUTHWESTERN RADIOLOGICAL HEALTH LABORATORY
INTRALABORATORY TECHNICAL REPORT
October 21, 1965
AN EVALUATION OF AN ACTIVATED CHARCOAL FILTER
FOR THE COLLECTION OF GASEOUS AIRBORNE IODINE
Earl L. Whittaker
Ronald D, Heck
SUMMARY
A tabulation is given of the results of a number of laboratory experi-
ments performed to determine the collection efficiency of an activated
charcoal filter (Gelman type AC-1) for molecular iodine and methyl
iodide, the two predominant forms of environmental gaseous airborne
iodine.
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PROCEDURE
A0 Molecular Iodine
Gaseous molecular iodine was generated in an all=glass generator
system heated by an external electric furnace. Molecular iodine
was produced in the generator by the oxidation of sodium iodide
with sodium dichromate and heat. The generator was heated
slowly up to approximately 400 C.
The generator was connected to the charcoal filter holder by all-glass
tubing except for a very short connecting piece of Tygon tubing, A
Gelman Bantam Air Filter unit was used to provide air flow through
the apparatus. Humidity was measured by an inline, wet-bulb,
dry-bulb chamber,, located at the inlet side of the generator.
In experiments where less than room humidity was needed, the hu-
midity was lowered by passing the air through dry ice cold traps.
Higher than room air humidity was provided by two household
cool=vapor humidifier units operated in parallel-
Iodine-131 as sodium iodide was used in all of the molecular iodine
experiments, A 3" x 3" diameter Nal(Tl) crystal detector and a
400 channel gamma spectrometer were used for measuring the
I activity. The quantity of iodine used in each experiment was
a trace amount, micromole or less.
The molecular iodine was generated over a period of approximately
four hours. The filter was then gamma counted and returned to the
apparatus for a two hour air flush at a flow of 31 liters per minute.
The filter was then gamma counted again and the loss of I activity
was determined. This was the procedure for each of the experiments.
The air flow rate during the generating period was controlled through
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each experiment but was different for different experiments. The
air flush in each experiment was at 31 liters per minute for two hours.
The humidity was controlled within a few percent through the gener-
ating and flushing periods of each experiment but was different for
different experiments .
B. Methyl Iodide
Essentially the same apparatus was used for generating and collecting
methyl iodide. However p because of the high vapor pressure of
methyl iodide, air was not passed through the generator for most of
the generating period. The outlet of the generator was connected to
the air flow system and the inlet was capped,, thus the generator was
simply a reservoir above which air was flowed. AlsoB the generator
was immersed in an ice bath for about one=third of the generating
time (one=half hour) to prolong the generating time, A flush period
was not done in the methyl iodide experiments because so little of the
methyl iodide was collected by the activated charcoal filter.
The amount of the methyl iodide used in these experiments was 50
microliters when the specific activity was low (3 experiments) and
20 microliters when the specific activity was high (11 experiments).
One microliter of methyl iodide contains 20 02 mg of iodine.
In methyl iodide experiments in which the flow rate was 15 liters per
minute or gr eater s the humidity was measured and is shown in
Table 2. At flow rates of 10 liters per minute or less,, humidity
measurements were not made but the humidity was probably 15% or
less because the air was dried through the same cold traps as for
the other experiments.
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The activated charcoal filters were dried at 100 C before using,, This
was done because a number of investigators have reported moisture
interference in the collection of methyl iodide by activated charcoal.
Because early results showed such low collection efficiency for
methyl iodide0 and because of the possibility of moisture interference
(pre-dried charcoal adsorbs moisture readily)0 an internal resistance
heater was added to the apparatus to heat the air to 100°C or higher.
The heater was located at the outlet side of the generator„ A thermo-
couple was also added to the apparatus, located downstream from
the heater and ahead of the activated charcoal filter. The heater
was adjusted so that the air temperature, before passing through the
activated char coal „ was 125°C0 as indicated by the thermocouple.
Experiments with less than 10 liters per minute air flow and heated
air showed a sharp increase in the collection efficiency. Three ex-
periments were conducted to determine if the iodine being collected
in the heated air experiments was methyl iodide,, It was determined
that all or nearly all of the iodine collected was molecular iodine and
was formed by the methyl iodide oxidation when passing over the hot
wire of the resistance heater. This conversion of the methyl iodide
to molecular iodine was quantitative only at the low flow rate of one
liter per minute. A higher resistance heater temperature would
probably give quantitative conversion at higher flow rates.
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DISCUSSION AND CONCLUSION
Table la shows a significant trend of decreasing collection efficiency
with increasing flow rate for molecular iodine. Table Ib shows a
slightly higher average collection efficiency for molecular iodine in
air above 40% humidity than in air below 40% humidity. The range
of low to high results in the collection efficiencies of Table 1 and
suggests non-uniformity in the activated charcoal filters. However,,
at flow rates of 23 LPM or less,, this activated charcoal filter (Gel-
man AC-1) does have a high collection efficiency for gaseous air-
borne molecular iodine0
Table 2 shows that the filter evaluated in these experiments e^diibits
a very low collection efficiency for gaseous,, airborne methyl iodide.
Since methyl iodide can be a significant component of gaseous air-
borne radioiodine in the environments the use of this filter to deter-
mine total radioiodine in the air environment is questionable.
The incorporation of a hot wire heater into an air sampler for the
purpose of oxidizing the methyl iodide and then collecting the result-
ing molecular iodine shows promise but the low flow rate required
is a serious limitation, A spark chamber is much more efficient in
releasing iodine from methyl iodide and can be used at much higher
flow rates.
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Table 1
Molecular Iodine Experiments Data
Experiment Generating Period
No. Air Flow (LPM)
30
34
35
36
32
33
47
48
49
50
31
37
38
39
40
41
42
43
44
45
46
13
13
13
13
23
23
30
30
30
30
31
31
31
31
31
31
31
31
31
31
31
Percent of Relative
Humidity
41
83
81
83
63
63
ZO
8
20
12
36
37
81
46
82
56
52
14
16
10
16
Percent of Iodine
Collected on Filter
95
99
93
99
97
92
76
75
93
75
97
71
81
77
93
67
71
77
80
81
87
Percent of Loss
during Air Flush*
9
5
0
1
0
1
8
0
0
2
9
6
6
0
1
1
4
0
2
1
1
-Percent of the iodine collected on the filter that was flushed off during the air flush period of
two hours at a flow rate of 31 LPMo
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Table la
Percent of Iodine Collected vs. Air Flow during Generating Period
Generating Period
Air Flow (LPM)
13
13
30,31
Average Percent of
Iodine Collected
96 (low 93, high 99)
94 (low 92, high 97)
80 (low 67, high 97)
Number of Exper-
iments Averaged
2
15
Table lb
Percent of Iodine Collected vs. Percent of Relative Humidity of Air
Percent of Relative
Humidity
Below 40
Above 40
Average Percent of
Iodine Collected
81 (low 71. high 97)
88 (low 67 0 high 99)
Number of Exper-
iments Averaged
10
11
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Table 2
Methyl Iodide Experiments Data
Experiment Air Flow Percent of Relative Percent of Iodine
No. (LPM)** Humidity Collected on Filter
51* 30 3S 3
52* 15 45 3
53* 15 18 7
61 3 See Procedure 1
56 2 " " 1
59 1 " " 3
*In these experiments a different lot of methyl iodide with a lower
specific activity was used.
**Air temperature was approximately room temperature.
Table 3
In Line Heater Methyl Iodide Experiments Data
Experiment Air Flow Percent of Relative Percent of Iodine
No. (LPM) Humidity Collected on Filter
54 15 15 Less than 1
64 10 See Procedure 4
63 5 11 • 28
60 3 " " 61
55 2 MI,- 66
57 1 " " 100
58 1 '• '• 100
62 1 '• I: 100
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LIST OF INTRALABORATORY REPORTS
R e po rt numbe r
ITR-1
ITR-2
Title
Author(s)
Flame photometric analysis: Eval=
uation of reversed oxyacetylene
flame modification.
™* .4.- f B(*C j 90- .
Determination of or and or in
whole milk: precipitation and
separation of milk protein by
trichloroacetic acid*
Raws on and
Dillon
Stevenson
ITR-3
ITR-4
Construction of a thermometric
titrator.
Thermometric studies of selected
calcium and strontium chelateso
Bretthauer9
Williams and
Fumagalli
Bretthauer
ITR-5
Preliminary studies on the devel-
opment of an airborne iodine
sampler.
Whittaker9
Bretthauer0
Griffins Worford,
and Raws on
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