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Coagulation and scavenging of radioactive aerosols
JOURNAL OF COLLOID SCIENCE 17, 703-716 (1962)
COAGULATION AND SCAVENGING OF RADIOACTIVE AEROSOLS J. Rosinski, D. Werle, and C. T. Nagamoto Armour Research Foundation of Illinois Institute of Technology, Technology Center, Chicago 16, Illinois Received July 28, 1961 ABSTRACT The coagulation constant of nonradioactive and radioactive metallic aerosols produced by an exploding-wire technique was determined experimentally. At early stages of coagulation the coagulation constant of radioactive gold aerosols (2 to 3.5 c./g.) was approximately twenty times the mean value of slightly radioactive aerosols (50 to 900 mc./g.). The values of the coagulation constant were corrected for deposition on vertical wall surfaces by means of a derived equation. Scavenging of radioactive aerosols was divided into three groups based upon the mechanism of coalescence: (I) dry particulate m a t t e r mixed with an aerosol, (2) dry particulate m a t t e r formed in the aerosol atmosphere, and (3) hygroscopic and liquid particulate m a t t e r formed in the aerosol atmosphere. Brownian motion in the presence of a water vapor concentration gradient around condensing droplets was found to be the most effective scavenging mechanism for slightly radioactive aerosols. INTRODUCTION
There are two general methods of producing solid aerosols: (a) mechanical reduction and subsequent dispersion of solids initially present as relatively coarse pieces; (b) formation of a solid dispersed phase from vapor through the process of nucleation, and subsequent growth of particles due to condensation and coagulation. Formation of aerosols by the exploding-wire technique is a method of the second group. The large amount of electrical energy stored in the capacitor bank produces almost instantaneous heating of the fine metal wire to above boiling temperature of the metal. Aerosols formed by the exploding-wire technique are reproducible. Therefore, the technique was used in this comparative study of nonradioactive and radioactive aerosol behavior. METHODS AND APPARATUS
A simplified sketch of the electrical system used for exploding wire is shown in Fig. 1. Two 10-kv., 52-~f., 0.5-~H. capacitors connected in parallel are charged t o 6 kv. The capacitors are discharged through a 2.5-cm. length of metal wire by actuating relay switches A and B. Relay A closes first, 703
704
ROSINSKI, WERLE~ AND NAGA~IOTO I l O VhC
I' F~G. 1. Electrical system for exploding wire. (1) 10-kv. 52-/zf. capacitor bank; (2) 10-kv, 5-ma. power pack; (3) powerstat; (4) relay A; (5) firing switch; (6) test wire; (7) relay B; (8) 175,000-ohm bleeder.
followed by relay B, consisting of an aluminum cylinder which completes the circuit as it falls past the opposing electrodes in the wall of a 1-in. polystyrene tube. Mter completing the circuit, the metal cylinder continues to fall until contact is made with a 175,000-ohm circuit, which bleeds off the residual charge in the capacitor bank. The wire was mounted between two insulated pins in the aerosol chainber, an 82-liter cylinder. The pins were held by nuts, which were made of the same metal as the wire to prevent contamination of the aerosol. The pressure in the chamber was maintained below atmospheric up to the time of explosion. The turbulence from the jet of filtered air used to bring the pressure to atmospheric immediately after the explosion produced thorough mixing. The outer and inner walls of the chamber and all parts of the apparatus were treated with an antistatic material to eliminate surface charge effects on aerosol behavior. After the explosion and subsequent introduction of the scavenging system, the chamber was thermally isolated to minimize thermal effects. The scavenging test apparatus is shown in Fig. 2. When activated carbon was used as a scavenging agent, it was placed in an inverted crucible cover 4 cm. below the wire and the carbon was dispersed by the shock wave of the explosion.. In experiments conducted with water-saturated air, the air was bubbled through a water bath before it was filtered. The bottom of the chamber was wetted with 40 ml. of water, and partial evacuation of the chamber was limited to 35-50 ram. of mercury below atmospheric pressure in order to preclude condensation.
COAGULATION AND SCAVENGING OF RADIOACTIVE AEROSOLS
705
Fro. 2. Scavenging test apparatus. (1) Aerosol chamber; (2) air filter; (3) thermal precipitator; (4) manometer; (5) air filters; (6) gas bottles; (7) test wire; (8) capacitor bank; (9) air pump; (10) lead shielding; (11) brass slit; (12) scintillation probes; (13) seintillation spectrometer and readout system; a, b, c, d, and e are positions of thermometers. TABLE I Temperature Distribution in the Aerosol Chamber Time after explosion OJzin.) 3 6 15 30 60 90 120 150 180 360 30-see. lamp exposure 60-sec. lamp exposure
Temperature (°F.) a
b
Position c
d
e
73.3 73.2 73.2 73.2 73.2 73.1 73.1 73.1 73.1 72.5 72.5 72.6
73.2 73.1 73.2 73.2 73.1 73.1 73.1 73.1 73.1 72.5 72.6 72.7
73.4 73.3 73.3 73.3 73.3 73.3 73.3 73.3 73.3 72.7 72.7 72.8
73.4 73.4 73.4 73.4 73.4 73.2 73.3 73.3 73.3 72.7 72.7 72.8
73.2 73.1 73.1 73.1 73.1 73.1 73.1 73.1 73.1 72.5 72.6 72.6
T e m p e r a t u r e a n d t e m p e r a t u r e g r a d i e n t were m e a s u r e d in t h e c h a m b e r a t 5 points, a t h r o u g h e (Fig. 2), w i t h precision m e r c u r y t h e r m o m e t e r s g r a d u ated to 0.2°F. T h e t h e r m o m e t e r s were lowered i n t o position j u s t after explosive e v a p o r a t i o n . T h e results are given in T a b l e I. T h e coagulation c o n s t a n t was n o t d e t e r m i n e d d u r i n g these t e m p e r a t u r e m e a s u r e m e n t s .
706
ROSINSKI, ~VERLE, AND NAGAMOTO
The aerosol was sampled periodically with a Strong-Ficklen oscillating thermal precipitator (1). Samples were collected on microscope slides and on Formvar screens for dark-field and electron microscope examination. The duration and the rate of sampling were adjusted to minimize overlapping of particles over the sampling area. Short duration samples withdrawn from the aerosol chamber at the end of each experiment showed the presence of large agglomerates as well as small particles, indicating that the agglomerates were present in the aerosol atmosphere and were not formed in the thermal precipitator or on the Formvar screens during sampling. The air in the chamber was filtered prior to the explosive evaporation until the number of nuclei present was equal to 4.5 < 103/cm. ~ ± 10 %. Considering the diameter of the wire, the length of the wire, the amount of electrical energy to be supplied, and other experimental variables, the reproducibility of the determined coagulation constants was generally better than 4-20 %. THEORY Calculation of the temperature of a metal vapor during explosive evaporation is impossible because of lack of: information on energy losses in the electrical circuit (2). The temperature calculated from spectroscopic data, assuming black-body emission, was found to be somewhere between 7000 ° and 8000°]~. The average velocity of the shock wave at atmospheric pressure was 1.0-1.5 km./sec. The linear velocity of vapor traveling behind the shock front was approximately 1 km./sec. Whytlaw-Gray (3) has shown that disappearance of aerosol particles by coagulation depends on the square of the number of particles present and the coagulation constant, so that -
~
~ =
k n ~,
[1]
where k is the coagulation constant, in cm.~/number-sec. The rate of disappearance of particles in the chamber is due not only to coagulation but also to loss of particles to the wall surface. This loss is proportional to the aerosol concentration. The equation is: -
~
~ = #n,
[2]
where fl is the wall surface loss constant. The simultaneous solution of Eqs. [1] and [2] for the total disappearance of particles is given by Green and Lane (4), who considered fl as a constant. Formation of aerosol was observed immediately after explosion. Turbulent motion in the chamber ceased practically completely in 2 rain. Changes
COAGULATION
AND
SCAVENGING
OF
RADIOACTIVE
AEROSOLS
707
in size distribution due to coagulation were followed from zero time, i.e., 3 rain. after explosive evaporation. As coagulation proceeds, larger particles form which settle faster along the direction of the gravitational field. They scavenge smaller particles during sedimentation. Coagulation was studied for the time period in which the last mechanism of particle removal was negligible. It was found t h a t the mean rate of deposition on the vertical wall surface was 2.6 × 10-la g./cm.~-sec., and the mean value of ~ for radioactive aerosols was 3.43 × 10-5 see. -~. Deposition on the vertical wall surface was practically not detectable after 120 rain. The constant ¢~ is a function of particle size and therefore time. Under the experimental conditions, for a time period of 120 min. (to = 3 rain. after aerosol generation) it was found that ~ can be a~mroximated by the equation for a straight line.
[J = a - - bt,
[3]
where a = 6.86 X 10-5 sec. -1, b = 9.53 X 10-9 sec. -2, and t is time in seconds. The equation for the change in number of particles is: _
d n = icn2 + (a -- bl)n dt
[4]
where
K = $1iexp(--at-i-bt2/2)n
- - e x p ( - - a t ° +l b tn° 2 /o2 )
'
[5]
and t
4) ---=
ft
e -at+bt212 d t .
[6]
o
The general expression for the coagulation constant of polydispersed aerosols in fact is: ¢~ Qo
where f(rl, t) is a function of particle size distribution versus time and lc(r~, r2) is the coagulation constant of particles with radii r~ and r2. The coagulation constant of monodispersed aerosol particles decreases with increasing particle size for particles above 0.054 ~. For coagulating aerosols, however, the increase in the average particle size is compensated by the increase in polydispersion, and the over-all coagulation constant becomes constant or m a y even increase, as shown by Deryagin and Vlasenko (5). Muller (6) points out that the probability of collision of non-
708
ROSINSKT, W E R L E
AND NAGAMOTO
C9
~v
~C
"~
~
~
~
~
~
~
~ L ~
:n ©
©
o
COAGULATION AND SCAVENGING OF RADIOACTIVE AEROSOLS
709
spherical particles is always greater than that of spheres. Elongated agglomerates eventually form spongelike structures which do not settle readily. Such agglomerates progressively sweep a large volume of aerosol, and the agglomeration rate, now similar to the rate of a catalytic reaction, increases. It is important to realize that coagulation depends on n 2, but the rate of loss to the vertical wall surface depends on n. In the later stages of coagulation, loss to the wall should be of greater relative importance, but at the same time ¢/approaches zero for stationary aerosols and deposition on the vertical surface actually ceases. The values for the coagulation constant given in Table II are corrected for deposition on vertical wall surfaces by means of Eq. [5]. COAGULATION
Differences in the behavior of radioactive and nonradioactive aerosols can be expressed by means of the coagulation constant (Table II). The coagulation of nonradioactive cadmium aerosols was found to be dependent on exposure to visible light. Exposure to a collimated light beam during coagulation gave a wide range of values for the coagulation constant (k = 0.56 to 1.30 X 10-9 cm.3/sec.). The net electrostatic charge, determined at the same time, showed variations not only in charge but also in sign. Therefore electrostatic forces ~eem to be predominant during this type of coagulation. Thermal forces due to exposure to light ~re negligible. The changes in the temperature inside the chamber are given in Table I. The aerosols which were shielded from intense light did not show rapid changes in coagulation constant (lc = 0.51 X 10-9 cm.3/sec.). The behavior of silver aerosols was similar to that of cadmium. The coagulation constant of nonradioactive gold aerosols was found to be about five times larger than that of slightly radioactive gold aerosols (50 to 900 mc./g.). The difference is due to the presence of electrostatic charge on nonradioactive particles. The net negative charge was 0.94 e/particle for k = 2.70 X 10-9 cm.~/number-sec. It can be assumed that there is no net electrostatic charge on radioactive gold aerosol particles. Therefore such aerosols can be regarded as electrostatically neutral. This does not mean, of course, that a particle does not possess an electrostatic charge at any time. The residence time of such a charge, however, is small compared to the disintegration rate. As the radioactivity increased to 29 me. per test aerosol (2 to 3.5 c./g.), an unusual increase in coagulation constant at early stages of coagulation was observed. The constant was approximately twenty times the mean value determined for slightly radioactive systems. Coagulation may therefore be enhanced by the presence of highly ionized gas. Ionization pro-
710
ROSINSKI, WERLE. AND NAGAMOTO
FIG. 3. Gold-carbon agglomerates 12 min. after evaporation.
duced by radioactive decay does not influence coagulation of larger particles. At later stages of coagulation the coagulation constant of radioactive aerosols is equal to that of nonradioactive aerosols. The system is too complex to try to explain this type of coagulation on a quantitative basis. Coagulation of an aerosol composed of a mixture of gold and activated carbon particles did not deviate from coagulation of a nonradioactive gold aerosol. A photomicrograph of gold-carbon agglomerates is shown in Fig. 3. A photomicrograph of radioactive gold particles is shown in Fig. 4. The growth of agglomerates in the form of heavy particles imbedded in the spongelike structure was observed during all the experiments with radioactive aerosols. SCAVENGING OF RADIOACTIVE AEROSOLS
Scavenging agents can be divided into three basic groups: (1) dry powders or liquid droplets which are mixed with the radioactive aerosol, (2) dry particulate matter formed in the aerosol atmosphere, and (3) hygroscopic and liquid particulate matter formed in the aerosol atmosphere.
COAGULATION
AND
SCAVENGING
OF R A D I O A C T I V E
AEROSOLS
711
/z
a~
FIG. 4. Radioactive gold agglomerates 5 hr. after evaporation. The results of experiments on scavenging, each representing the average of two tests, are tabulated in Table III, to which all experiment numbers refer in the text. The reproducibility of measurements of the remaining radioactivity in the chamber versus time was better than :t:4 %. The first group has the least powerful scavengers. Water droplets falling through an aerosol, however, can scavenge some aerosol particles. Results with coarse and fine water sprays are given in experiments 4 and 5. Radioactive aerosols can be removed to a small extent by dispersing a dry powder in the aerosol. The scavenging particles cannot be too large because they will fall through the aerosol atmosphere without removing the radioactive particles. They cannot be too small because they will remain suspended for too long a period of time. Dry nonhygroscopic chemical smokes formed in the aerosol atmosphere belong to the second group. Particles of scavenger attach themselves to radioactive aerosol particles during rapid coagulation due to Brownian motion at the time of formation. Larger agglomerates produced in this way settle or can be removed by other means. The higher the concentration
712
ROSINSKI~ WERLE~ AND NAGAMOTO TABLE III
Scavenging of Radioactive Gold Aerosols Expt. no.
Conditions
Scavenger (vol./vol. %)
Calculated d~article ameter (u)
1-me. radioactivity, 55% r.h., 76°F., 19.3-g./cm. 8 density
None
0.38 0.32 0.28 0.25 0.22 0.20 0.17 0.15 0.12
1-mc. radioactivity, 31% r.h., 79°F., 1.0-g./cm) density
0.153 ethylene oxide 0.153 boron trifluoride
2.45 2.20 2.02 1.95 1.87 1.80
1.72 1.63 1.40
1-mc.radioactivity, 32% r.h., 80°F., 1.5-g./cm. 3 density
0.153 ammonia 0.153 hydrogen chloride
1.90
1.78 1.68 1.60 1.53
1.45 1.35 1.23
0.96 1-me. radioactivity, Coarse water spray 90-100% r.h., 81°F., 19.3-g./cm. 3 density
l-inc, radioactivity, 9{)-100% r.h., 77°F., 19.3-g./cm? density
Fine water spray
0.52 0.45 0.38 0.33 0.30 0.27 0.24 0.21 0.17 0.64 0.48 0.42 0.39 0.36 0.34 0.30 0.27 0.22
713
COAGULATION AND SCAVENGING OF RADIOACTIVE AEROSOLS TABLE III--Continued
Expt. no.
Conditions
Scavenger
(vcl./vol.%)
Remaining radioactivity (%)
Time (rain.)
CalcuSettling lated velocity particle (cm,/min.) [diameter (~)
i-1-me. r a d i o a c t i v i t y , 60-100% r.h., 81°F., 1.0-g./em. 3 density
0.153 ammonia excess formic acid vapor
90 80 70 60 50 40 30 20 10
8.99 11.26 12.98 13.99 16.03 18.75 25.00 37.13 53.00
4.17 4.70 3.33 4.20 2.89 3.90 2.68 3.75 2.34 3.50 2.00 i 3 . 2 0 1.50 [ 2.80 I 1.01 i 2.30 0.708 1.90
1-me. r a d i o a c t i v i t y , 40% r.h., 75°F., l,O-g./cm. 3 density
0.153 silicon t e t r a fluoride 0.153 ammonia
90 80 70 60 50 40 30 20 10
23 26 28 32 35 41 48 60 85
1.63 1.44 1.34 1.19 1.07 0.915 0.782 0.623 0.441
2.95 2.75 2.65 2.50 2.35 2.20 2.00 1.80 1.50
1-me. radioactivity, 0.044 silicon t e t r a 90-100% r.h., fluoride 67°F., 1.O-g./cm. s , density
90 80 70 60 50 40 30 20 10
7.00 8.60 10.19 11.98 13.99 16.03 19.23 24.51 37.13
5.36 4.36 3,68 3.13 2.68 2.34 1.95 1.53 1.01
5.40 4.80 4.40 4.10 3.75 3.50 3.20 2.70 2.30
9-10
1-me. activity, 90100% r.h., 73°F. (expt. 9), 64°F. (expt. 10), 1.0 g./ cm? density
0.153 silicon t e t r a fluoride
90 80 70 60 50 40 30 20 10
4.00 6.00 7.00 7.18 7.54 8.30 10.50 15.50 30.99
9.38 6.25 5.36 5.22 4.97 4.52 3.57 2.42 1.21
7.20 5.80 5.40 5.30 5.20 4.90 4.40 3.60 2.52
11
1-me. r a d i o a c t i v i t y , 90-100% r.h., 82°F., 1.0-g./cm. 3 density
Coarse w a t e r spray 0.044 silicon t e t r a fluoride
90 80 70 60 50 40 30 20 10
3.50 4.99 6.20 7.20 8.50 10.50 12.21 14.48 24.51
10.72 7.51 6.05 5.21 4.41 3.57 3.07 2.59 1.53
7.70 6.40 5.70 5.30 4.90 4.40 4.00 3.70 2.80
714
ROSINSKI,
W E R L E , AND NAGAMOTO
TABLE III--Concluded Expt. no.
12
Conditions
Scavenger(vol./'Jol.%)
1-mc. radioactivity, Fine w a t e r s p r a y 90-100% r.h., 0.044 silicon t e t r a 78°F., 1.0-g./cm. 3 fluoride density
Remain-' ing radio-] Time activity I (min.) (%)
~
Calculated particle (cm./mirt.) diameter Settling velocity
(~)
90
80 70 60 50 40 30 20 10
4.99 5.70 6.20 6.50 6.70 7.50 8.99 13.49
7.51 6.58 6.05 5.77 5.60 5.00 4.17 2.78
6.40 6.00 5.70 5.60 5.50 5.20 4.70 3.85
of scavenger~ the larger the particles formed. However, agglomeration is influenced more by turbulence during mixing than by changes in concentration. The results of scavenging with ammonia-hydrogen chloride and boron trichloride-ethylene oxide are given in experiments 2 and 3. Scavenging with the third category is more effective than with the first two groups because of an additional mechanism. Removal of the radioactive aerosol particles is enhanced b y the presence of molecular motion around condensing droplets due to the condensation of water vapor (Stefan's flow (7)). The growth of water droplets of hygroscopic particles in the presence of water vapor seems to be the most effective factor in scavenging of slightly radioactive aerosols. Aerosol particles do not necessarily serve as condensation nuclei during scavenging. The results with hygroscopic particles are given in experiments 6 through 10 and are shown in Fig. 5. The pronounced influence of water vapor in enhancing the scavenging effectiveness of silicon tetrafluoride led to tests with water sprays to improve the scavenging rate and to produce particles separable by standard equipment, e.g., the cyclone separator. A Bete PB-3 spray nozzle was used to evaluate the scavenging effectiveness of a fine water spray. When operated at 58 p.s.i.g., drops from 6 to 110 ~ were produced. In experiment 5 the nozzle was operated for a 30-see. period 6 rain. after the explosion, and 76 cm. 3 of fine water spray resulted. In 56 rain. the radioactivity was reduced to 50 %. In comparison, 124 rain. was required to reduce the activity to 50 % without the water spray. When the fine water spray was used for 30 see. 3 rain. after the explosion in combination with 0.044 % by volume of silicon tetrafluoride (experiment 12), only 7 rain. was needed to lower the radioactivity to 50 %. When 0.044 % silicon tetrafluoride was used to scavenge radioactive particles from saturated air (experiment 8), a 50 % reduction in activity required 14 min. Even when 0.153 % silicon tetrafluoride was used with saturated air at room temperature (experiments 9 and 10), the scavenging effectiveness
COAGULATION lO0
I
AND
i
0
SCAVENGING I
12
24
36
I
I
I
OF RADIOACTIVE l
i
I
I
I
I
l
48 60 ?2 84 TIME AFTER EXPLOSION, MIN.
715
AEROSOLS I
96
I
I
108
120
FIG. 5. Scavenging of radioactive gold particles by scavengers formed in the aerosol atmosphere. (Numbers correspond to experiment number in Table III.) I00
I
I
I 108
1
8O
160
I
o
0
I 12
l
l 24
I
I 36
1 TIME
I P 4B AFTER
I I I I 60 72 EXPLOSION, MIN.
I 84
I
I 96
I
]20
FIG. 6. Scavenging of radioactive gold particles with ~vater sprays. (Numbers correspond to experiment number in Table III.) did not match that of experiment 12, in which a much smaller smount of silicon tetrafluoride was used. This indicates that the amount of moisture available in saturated air at room temperature is not sufficient to grow the larger drops which are available when using a fine water spray.
716
ROSINSKI~ WERLE, AND NAGAMOTO
A Bete PT-20 spray nozzle operated at 20 p.s.i.g, produced water drops ranging in size from 40 to 860 t~ (experiments 4 and 11). In 30 sec. of operation 6 rain. after explosion, 87 cm. ~ of spray was produced. This coarse spray was less effective than the smaller volume of spray (experiment 5), as 76 rain. was needed to reduce radioactivity by 50% (experiment 4). When the coarse water spray was used for 20 sec. 3 rain. after the explosion in combination with 0.044% silicon tetrafluoride (experiment 11), 8 rain. was needed to settle 50 % of the radioactive aerosol. The rapid settling of the larger water drops reduces the effectiveness of the coarse spray as an available source of water. The results of scavenging tests with silicon tetrafluoride and water sprays are given in Fig. 6. Scavenging of slightly radioactive aerosols can be accomplished by means of a combination of capture mechanisms. Brownian motion in the presence of a water vapor concentration gradient around condensing water droplets was found to be the most effective scavenging mechanism. It is assumed here that electrostatic forces are negligible in the presence of ionizing radiation. The scavenging mechanism of highly radioactive aerosols and of nonradioactive aerosols will differ appreciably. Electrostatic forces usually predominate in the scavenging of nonradioactive aerosols. Scavenging of highly radioactive aerosols is predicted to be influenced predominantly by the same factors which increase their coagulation. ACKNOWLEDGMENTS This work was sponsored by the United States Atomic Energy Commission, Washington, D. C., under Contracts AT(11-1)-586 and AT(11-1)-578, and by Armour Research Foundation, Project, No. C 905. The authors wish to thank J. Pierrard for many valuable hours of discussion. REFERENCES
I. YAFFE, C. D., BYERS, D. H., AND HOSEY, A. D. "Encyclopedia of Instrumentation for Industrial Hygiene." University of Michigan, Ann Arbor, Michigan, 1956. 2. CHASE, W. G., AND CULLINGTON, E. H., Instrumentation for Geophys. Research No. 7, AFCRC-TR-57-235, 1957. 3. WI-IYTLAW-GRAY,R., AND PATTERSON,H. S. "Smoke." Arnold, London, 1932. 4. GREEN, H. L., AND LANE, W. ]=~. "Particulate Clouds." Van Nostrand, London, 1957. 5. I n FucHs, N. A., "The Mechanics of Aerosols," p. 343. Chemical Warfare Laboratories Special Publication 4-12, 1955. 6. MULLER,H., Kolloidehem. Beih. 27,223 (1928). 7. STEFAN,G., Wien. Bet. 83,943 (1881).
COAGULATION AND SCAVENGING OF RADIOACTIVE AEROSOLS J. Rosinski, D. Werle, and C. T. Nagamoto Armour Research Foundation of Illinois Institute of Technology, Technology Center, Chicago 16, Illinois Received July 28, 1961 ABSTRACT The coagulation constant of nonradioactive and radioactive metallic aerosols produced by an exploding-wire technique was determined experimentally. At early stages of coagulation the coagulation constant of radioactive gold aerosols (2 to 3.5 c./g.) was approximately twenty times the mean value of slightly radioactive aerosols (50 to 900 mc./g.). The values of the coagulation constant were corrected for deposition on vertical wall surfaces by means of a derived equation. Scavenging of radioactive aerosols was divided into three groups based upon the mechanism of coalescence: (I) dry particulate m a t t e r mixed with an aerosol, (2) dry particulate m a t t e r formed in the aerosol atmosphere, and (3) hygroscopic and liquid particulate m a t t e r formed in the aerosol atmosphere. Brownian motion in the presence of a water vapor concentration gradient around condensing droplets was found to be the most effective scavenging mechanism for slightly radioactive aerosols. INTRODUCTION
There are two general methods of producing solid aerosols: (a) mechanical reduction and subsequent dispersion of solids initially present as relatively coarse pieces; (b) formation of a solid dispersed phase from vapor through the process of nucleation, and subsequent growth of particles due to condensation and coagulation. Formation of aerosols by the exploding-wire technique is a method of the second group. The large amount of electrical energy stored in the capacitor bank produces almost instantaneous heating of the fine metal wire to above boiling temperature of the metal. Aerosols formed by the exploding-wire technique are reproducible. Therefore, the technique was used in this comparative study of nonradioactive and radioactive aerosol behavior. METHODS AND APPARATUS
A simplified sketch of the electrical system used for exploding wire is shown in Fig. 1. Two 10-kv., 52-~f., 0.5-~H. capacitors connected in parallel are charged t o 6 kv. The capacitors are discharged through a 2.5-cm. length of metal wire by actuating relay switches A and B. Relay A closes first, 703
704
ROSINSKI, WERLE~ AND NAGA~IOTO I l O VhC
I' F~G. 1. Electrical system for exploding wire. (1) 10-kv. 52-/zf. capacitor bank; (2) 10-kv, 5-ma. power pack; (3) powerstat; (4) relay A; (5) firing switch; (6) test wire; (7) relay B; (8) 175,000-ohm bleeder.
followed by relay B, consisting of an aluminum cylinder which completes the circuit as it falls past the opposing electrodes in the wall of a 1-in. polystyrene tube. Mter completing the circuit, the metal cylinder continues to fall until contact is made with a 175,000-ohm circuit, which bleeds off the residual charge in the capacitor bank. The wire was mounted between two insulated pins in the aerosol chainber, an 82-liter cylinder. The pins were held by nuts, which were made of the same metal as the wire to prevent contamination of the aerosol. The pressure in the chamber was maintained below atmospheric up to the time of explosion. The turbulence from the jet of filtered air used to bring the pressure to atmospheric immediately after the explosion produced thorough mixing. The outer and inner walls of the chamber and all parts of the apparatus were treated with an antistatic material to eliminate surface charge effects on aerosol behavior. After the explosion and subsequent introduction of the scavenging system, the chamber was thermally isolated to minimize thermal effects. The scavenging test apparatus is shown in Fig. 2. When activated carbon was used as a scavenging agent, it was placed in an inverted crucible cover 4 cm. below the wire and the carbon was dispersed by the shock wave of the explosion.. In experiments conducted with water-saturated air, the air was bubbled through a water bath before it was filtered. The bottom of the chamber was wetted with 40 ml. of water, and partial evacuation of the chamber was limited to 35-50 ram. of mercury below atmospheric pressure in order to preclude condensation.
COAGULATION AND SCAVENGING OF RADIOACTIVE AEROSOLS
705
Fro. 2. Scavenging test apparatus. (1) Aerosol chamber; (2) air filter; (3) thermal precipitator; (4) manometer; (5) air filters; (6) gas bottles; (7) test wire; (8) capacitor bank; (9) air pump; (10) lead shielding; (11) brass slit; (12) scintillation probes; (13) seintillation spectrometer and readout system; a, b, c, d, and e are positions of thermometers. TABLE I Temperature Distribution in the Aerosol Chamber Time after explosion OJzin.) 3 6 15 30 60 90 120 150 180 360 30-see. lamp exposure 60-sec. lamp exposure
Temperature (°F.) a
b
Position c
d
e
73.3 73.2 73.2 73.2 73.2 73.1 73.1 73.1 73.1 72.5 72.5 72.6
73.2 73.1 73.2 73.2 73.1 73.1 73.1 73.1 73.1 72.5 72.6 72.7
73.4 73.3 73.3 73.3 73.3 73.3 73.3 73.3 73.3 72.7 72.7 72.8
73.4 73.4 73.4 73.4 73.4 73.2 73.3 73.3 73.3 72.7 72.7 72.8
73.2 73.1 73.1 73.1 73.1 73.1 73.1 73.1 73.1 72.5 72.6 72.6
T e m p e r a t u r e a n d t e m p e r a t u r e g r a d i e n t were m e a s u r e d in t h e c h a m b e r a t 5 points, a t h r o u g h e (Fig. 2), w i t h precision m e r c u r y t h e r m o m e t e r s g r a d u ated to 0.2°F. T h e t h e r m o m e t e r s were lowered i n t o position j u s t after explosive e v a p o r a t i o n . T h e results are given in T a b l e I. T h e coagulation c o n s t a n t was n o t d e t e r m i n e d d u r i n g these t e m p e r a t u r e m e a s u r e m e n t s .
706
ROSINSKI, ~VERLE, AND NAGAMOTO
The aerosol was sampled periodically with a Strong-Ficklen oscillating thermal precipitator (1). Samples were collected on microscope slides and on Formvar screens for dark-field and electron microscope examination. The duration and the rate of sampling were adjusted to minimize overlapping of particles over the sampling area. Short duration samples withdrawn from the aerosol chamber at the end of each experiment showed the presence of large agglomerates as well as small particles, indicating that the agglomerates were present in the aerosol atmosphere and were not formed in the thermal precipitator or on the Formvar screens during sampling. The air in the chamber was filtered prior to the explosive evaporation until the number of nuclei present was equal to 4.5 < 103/cm. ~ ± 10 %. Considering the diameter of the wire, the length of the wire, the amount of electrical energy to be supplied, and other experimental variables, the reproducibility of the determined coagulation constants was generally better than 4-20 %. THEORY Calculation of the temperature of a metal vapor during explosive evaporation is impossible because of lack of: information on energy losses in the electrical circuit (2). The temperature calculated from spectroscopic data, assuming black-body emission, was found to be somewhere between 7000 ° and 8000°]~. The average velocity of the shock wave at atmospheric pressure was 1.0-1.5 km./sec. The linear velocity of vapor traveling behind the shock front was approximately 1 km./sec. Whytlaw-Gray (3) has shown that disappearance of aerosol particles by coagulation depends on the square of the number of particles present and the coagulation constant, so that -
~
~ =
k n ~,
[1]
where k is the coagulation constant, in cm.~/number-sec. The rate of disappearance of particles in the chamber is due not only to coagulation but also to loss of particles to the wall surface. This loss is proportional to the aerosol concentration. The equation is: -
~
~ = #n,
[2]
where fl is the wall surface loss constant. The simultaneous solution of Eqs. [1] and [2] for the total disappearance of particles is given by Green and Lane (4), who considered fl as a constant. Formation of aerosol was observed immediately after explosion. Turbulent motion in the chamber ceased practically completely in 2 rain. Changes
COAGULATION
AND
SCAVENGING
OF
RADIOACTIVE
AEROSOLS
707
in size distribution due to coagulation were followed from zero time, i.e., 3 rain. after explosive evaporation. As coagulation proceeds, larger particles form which settle faster along the direction of the gravitational field. They scavenge smaller particles during sedimentation. Coagulation was studied for the time period in which the last mechanism of particle removal was negligible. It was found t h a t the mean rate of deposition on the vertical wall surface was 2.6 × 10-la g./cm.~-sec., and the mean value of ~ for radioactive aerosols was 3.43 × 10-5 see. -~. Deposition on the vertical wall surface was practically not detectable after 120 rain. The constant ¢~ is a function of particle size and therefore time. Under the experimental conditions, for a time period of 120 min. (to = 3 rain. after aerosol generation) it was found that ~ can be a~mroximated by the equation for a straight line.
[J = a - - bt,
[3]
where a = 6.86 X 10-5 sec. -1, b = 9.53 X 10-9 sec. -2, and t is time in seconds. The equation for the change in number of particles is: _
d n = icn2 + (a -- bl)n dt
[4]
where
K = $1iexp(--at-i-bt2/2)n
- - e x p ( - - a t ° +l b tn° 2 /o2 )
'
[5]
and t
4) ---=
ft
e -at+bt212 d t .
[6]
o
The general expression for the coagulation constant of polydispersed aerosols in fact is: ¢~ Qo
where f(rl, t) is a function of particle size distribution versus time and lc(r~, r2) is the coagulation constant of particles with radii r~ and r2. The coagulation constant of monodispersed aerosol particles decreases with increasing particle size for particles above 0.054 ~. For coagulating aerosols, however, the increase in the average particle size is compensated by the increase in polydispersion, and the over-all coagulation constant becomes constant or m a y even increase, as shown by Deryagin and Vlasenko (5). Muller (6) points out that the probability of collision of non-
708
ROSINSKT, W E R L E
AND NAGAMOTO
C9
~v
~C
"~
~
~
~
~
~
~
~ L ~
:n ©
©
o
COAGULATION AND SCAVENGING OF RADIOACTIVE AEROSOLS
709
spherical particles is always greater than that of spheres. Elongated agglomerates eventually form spongelike structures which do not settle readily. Such agglomerates progressively sweep a large volume of aerosol, and the agglomeration rate, now similar to the rate of a catalytic reaction, increases. It is important to realize that coagulation depends on n 2, but the rate of loss to the vertical wall surface depends on n. In the later stages of coagulation, loss to the wall should be of greater relative importance, but at the same time ¢/approaches zero for stationary aerosols and deposition on the vertical surface actually ceases. The values for the coagulation constant given in Table II are corrected for deposition on vertical wall surfaces by means of Eq. [5]. COAGULATION
Differences in the behavior of radioactive and nonradioactive aerosols can be expressed by means of the coagulation constant (Table II). The coagulation of nonradioactive cadmium aerosols was found to be dependent on exposure to visible light. Exposure to a collimated light beam during coagulation gave a wide range of values for the coagulation constant (k = 0.56 to 1.30 X 10-9 cm.3/sec.). The net electrostatic charge, determined at the same time, showed variations not only in charge but also in sign. Therefore electrostatic forces ~eem to be predominant during this type of coagulation. Thermal forces due to exposure to light ~re negligible. The changes in the temperature inside the chamber are given in Table I. The aerosols which were shielded from intense light did not show rapid changes in coagulation constant (lc = 0.51 X 10-9 cm.3/sec.). The behavior of silver aerosols was similar to that of cadmium. The coagulation constant of nonradioactive gold aerosols was found to be about five times larger than that of slightly radioactive gold aerosols (50 to 900 mc./g.). The difference is due to the presence of electrostatic charge on nonradioactive particles. The net negative charge was 0.94 e/particle for k = 2.70 X 10-9 cm.~/number-sec. It can be assumed that there is no net electrostatic charge on radioactive gold aerosol particles. Therefore such aerosols can be regarded as electrostatically neutral. This does not mean, of course, that a particle does not possess an electrostatic charge at any time. The residence time of such a charge, however, is small compared to the disintegration rate. As the radioactivity increased to 29 me. per test aerosol (2 to 3.5 c./g.), an unusual increase in coagulation constant at early stages of coagulation was observed. The constant was approximately twenty times the mean value determined for slightly radioactive systems. Coagulation may therefore be enhanced by the presence of highly ionized gas. Ionization pro-
710
ROSINSKI, WERLE. AND NAGAMOTO
FIG. 3. Gold-carbon agglomerates 12 min. after evaporation.
duced by radioactive decay does not influence coagulation of larger particles. At later stages of coagulation the coagulation constant of radioactive aerosols is equal to that of nonradioactive aerosols. The system is too complex to try to explain this type of coagulation on a quantitative basis. Coagulation of an aerosol composed of a mixture of gold and activated carbon particles did not deviate from coagulation of a nonradioactive gold aerosol. A photomicrograph of gold-carbon agglomerates is shown in Fig. 3. A photomicrograph of radioactive gold particles is shown in Fig. 4. The growth of agglomerates in the form of heavy particles imbedded in the spongelike structure was observed during all the experiments with radioactive aerosols. SCAVENGING OF RADIOACTIVE AEROSOLS
Scavenging agents can be divided into three basic groups: (1) dry powders or liquid droplets which are mixed with the radioactive aerosol, (2) dry particulate matter formed in the aerosol atmosphere, and (3) hygroscopic and liquid particulate matter formed in the aerosol atmosphere.
COAGULATION
AND
SCAVENGING
OF R A D I O A C T I V E
AEROSOLS
711
/z
a~
FIG. 4. Radioactive gold agglomerates 5 hr. after evaporation. The results of experiments on scavenging, each representing the average of two tests, are tabulated in Table III, to which all experiment numbers refer in the text. The reproducibility of measurements of the remaining radioactivity in the chamber versus time was better than :t:4 %. The first group has the least powerful scavengers. Water droplets falling through an aerosol, however, can scavenge some aerosol particles. Results with coarse and fine water sprays are given in experiments 4 and 5. Radioactive aerosols can be removed to a small extent by dispersing a dry powder in the aerosol. The scavenging particles cannot be too large because they will fall through the aerosol atmosphere without removing the radioactive particles. They cannot be too small because they will remain suspended for too long a period of time. Dry nonhygroscopic chemical smokes formed in the aerosol atmosphere belong to the second group. Particles of scavenger attach themselves to radioactive aerosol particles during rapid coagulation due to Brownian motion at the time of formation. Larger agglomerates produced in this way settle or can be removed by other means. The higher the concentration
712
ROSINSKI~ WERLE~ AND NAGAMOTO TABLE III
Scavenging of Radioactive Gold Aerosols Expt. no.
Conditions
Scavenger (vol./vol. %)
Calculated d~article ameter (u)
1-me. radioactivity, 55% r.h., 76°F., 19.3-g./cm. 8 density
None
0.38 0.32 0.28 0.25 0.22 0.20 0.17 0.15 0.12
1-mc. radioactivity, 31% r.h., 79°F., 1.0-g./cm) density
0.153 ethylene oxide 0.153 boron trifluoride
2.45 2.20 2.02 1.95 1.87 1.80
1.72 1.63 1.40
1-mc.radioactivity, 32% r.h., 80°F., 1.5-g./cm. 3 density
0.153 ammonia 0.153 hydrogen chloride
1.90
1.78 1.68 1.60 1.53
1.45 1.35 1.23
0.96 1-me. radioactivity, Coarse water spray 90-100% r.h., 81°F., 19.3-g./cm. 3 density
l-inc, radioactivity, 9{)-100% r.h., 77°F., 19.3-g./cm? density
Fine water spray
0.52 0.45 0.38 0.33 0.30 0.27 0.24 0.21 0.17 0.64 0.48 0.42 0.39 0.36 0.34 0.30 0.27 0.22
713
COAGULATION AND SCAVENGING OF RADIOACTIVE AEROSOLS TABLE III--Continued
Expt. no.
Conditions
Scavenger
(vcl./vol.%)
Remaining radioactivity (%)
Time (rain.)
CalcuSettling lated velocity particle (cm,/min.) [diameter (~)
i-1-me. r a d i o a c t i v i t y , 60-100% r.h., 81°F., 1.0-g./em. 3 density
0.153 ammonia excess formic acid vapor
90 80 70 60 50 40 30 20 10
8.99 11.26 12.98 13.99 16.03 18.75 25.00 37.13 53.00
4.17 4.70 3.33 4.20 2.89 3.90 2.68 3.75 2.34 3.50 2.00 i 3 . 2 0 1.50 [ 2.80 I 1.01 i 2.30 0.708 1.90
1-me. r a d i o a c t i v i t y , 40% r.h., 75°F., l,O-g./cm. 3 density
0.153 silicon t e t r a fluoride 0.153 ammonia
90 80 70 60 50 40 30 20 10
23 26 28 32 35 41 48 60 85
1.63 1.44 1.34 1.19 1.07 0.915 0.782 0.623 0.441
2.95 2.75 2.65 2.50 2.35 2.20 2.00 1.80 1.50
1-me. radioactivity, 0.044 silicon t e t r a 90-100% r.h., fluoride 67°F., 1.O-g./cm. s , density
90 80 70 60 50 40 30 20 10
7.00 8.60 10.19 11.98 13.99 16.03 19.23 24.51 37.13
5.36 4.36 3,68 3.13 2.68 2.34 1.95 1.53 1.01
5.40 4.80 4.40 4.10 3.75 3.50 3.20 2.70 2.30
9-10
1-me. activity, 90100% r.h., 73°F. (expt. 9), 64°F. (expt. 10), 1.0 g./ cm? density
0.153 silicon t e t r a fluoride
90 80 70 60 50 40 30 20 10
4.00 6.00 7.00 7.18 7.54 8.30 10.50 15.50 30.99
9.38 6.25 5.36 5.22 4.97 4.52 3.57 2.42 1.21
7.20 5.80 5.40 5.30 5.20 4.90 4.40 3.60 2.52
11
1-me. r a d i o a c t i v i t y , 90-100% r.h., 82°F., 1.0-g./cm. 3 density
Coarse w a t e r spray 0.044 silicon t e t r a fluoride
90 80 70 60 50 40 30 20 10
3.50 4.99 6.20 7.20 8.50 10.50 12.21 14.48 24.51
10.72 7.51 6.05 5.21 4.41 3.57 3.07 2.59 1.53
7.70 6.40 5.70 5.30 4.90 4.40 4.00 3.70 2.80
714
ROSINSKI,
W E R L E , AND NAGAMOTO
TABLE III--Concluded Expt. no.
12
Conditions
Scavenger(vol./'Jol.%)
1-mc. radioactivity, Fine w a t e r s p r a y 90-100% r.h., 0.044 silicon t e t r a 78°F., 1.0-g./cm. 3 fluoride density
Remain-' ing radio-] Time activity I (min.) (%)
~
Calculated particle (cm./mirt.) diameter Settling velocity
(~)
90
80 70 60 50 40 30 20 10
4.99 5.70 6.20 6.50 6.70 7.50 8.99 13.49
7.51 6.58 6.05 5.77 5.60 5.00 4.17 2.78
6.40 6.00 5.70 5.60 5.50 5.20 4.70 3.85
of scavenger~ the larger the particles formed. However, agglomeration is influenced more by turbulence during mixing than by changes in concentration. The results of scavenging with ammonia-hydrogen chloride and boron trichloride-ethylene oxide are given in experiments 2 and 3. Scavenging with the third category is more effective than with the first two groups because of an additional mechanism. Removal of the radioactive aerosol particles is enhanced b y the presence of molecular motion around condensing droplets due to the condensation of water vapor (Stefan's flow (7)). The growth of water droplets of hygroscopic particles in the presence of water vapor seems to be the most effective factor in scavenging of slightly radioactive aerosols. Aerosol particles do not necessarily serve as condensation nuclei during scavenging. The results with hygroscopic particles are given in experiments 6 through 10 and are shown in Fig. 5. The pronounced influence of water vapor in enhancing the scavenging effectiveness of silicon tetrafluoride led to tests with water sprays to improve the scavenging rate and to produce particles separable by standard equipment, e.g., the cyclone separator. A Bete PB-3 spray nozzle was used to evaluate the scavenging effectiveness of a fine water spray. When operated at 58 p.s.i.g., drops from 6 to 110 ~ were produced. In experiment 5 the nozzle was operated for a 30-see. period 6 rain. after the explosion, and 76 cm. 3 of fine water spray resulted. In 56 rain. the radioactivity was reduced to 50 %. In comparison, 124 rain. was required to reduce the activity to 50 % without the water spray. When the fine water spray was used for 30 see. 3 rain. after the explosion in combination with 0.044 % by volume of silicon tetrafluoride (experiment 12), only 7 rain. was needed to lower the radioactivity to 50 %. When 0.044 % silicon tetrafluoride was used to scavenge radioactive particles from saturated air (experiment 8), a 50 % reduction in activity required 14 min. Even when 0.153 % silicon tetrafluoride was used with saturated air at room temperature (experiments 9 and 10), the scavenging effectiveness
COAGULATION lO0
I
AND
i
0
SCAVENGING I
12
24
36
I
I
I
OF RADIOACTIVE l
i
I
I
I
I
l
48 60 ?2 84 TIME AFTER EXPLOSION, MIN.
715
AEROSOLS I
96
I
I
108
120
FIG. 5. Scavenging of radioactive gold particles by scavengers formed in the aerosol atmosphere. (Numbers correspond to experiment number in Table III.) I00
I
I
I 108
1
8O
160
I
o
0
I 12
l
l 24
I
I 36
1 TIME
I P 4B AFTER
I I I I 60 72 EXPLOSION, MIN.
I 84
I
I 96
I
]20
FIG. 6. Scavenging of radioactive gold particles with ~vater sprays. (Numbers correspond to experiment number in Table III.) did not match that of experiment 12, in which a much smaller smount of silicon tetrafluoride was used. This indicates that the amount of moisture available in saturated air at room temperature is not sufficient to grow the larger drops which are available when using a fine water spray.
716
ROSINSKI~ WERLE, AND NAGAMOTO
A Bete PT-20 spray nozzle operated at 20 p.s.i.g, produced water drops ranging in size from 40 to 860 t~ (experiments 4 and 11). In 30 sec. of operation 6 rain. after explosion, 87 cm. ~ of spray was produced. This coarse spray was less effective than the smaller volume of spray (experiment 5), as 76 rain. was needed to reduce radioactivity by 50% (experiment 4). When the coarse water spray was used for 20 sec. 3 rain. after the explosion in combination with 0.044% silicon tetrafluoride (experiment 11), 8 rain. was needed to settle 50 % of the radioactive aerosol. The rapid settling of the larger water drops reduces the effectiveness of the coarse spray as an available source of water. The results of scavenging tests with silicon tetrafluoride and water sprays are given in Fig. 6. Scavenging of slightly radioactive aerosols can be accomplished by means of a combination of capture mechanisms. Brownian motion in the presence of a water vapor concentration gradient around condensing water droplets was found to be the most effective scavenging mechanism. It is assumed here that electrostatic forces are negligible in the presence of ionizing radiation. The scavenging mechanism of highly radioactive aerosols and of nonradioactive aerosols will differ appreciably. Electrostatic forces usually predominate in the scavenging of nonradioactive aerosols. Scavenging of highly radioactive aerosols is predicted to be influenced predominantly by the same factors which increase their coagulation. ACKNOWLEDGMENTS This work was sponsored by the United States Atomic Energy Commission, Washington, D. C., under Contracts AT(11-1)-586 and AT(11-1)-578, and by Armour Research Foundation, Project, No. C 905. The authors wish to thank J. Pierrard for many valuable hours of discussion. REFERENCES
I. YAFFE, C. D., BYERS, D. H., AND HOSEY, A. D. "Encyclopedia of Instrumentation for Industrial Hygiene." University of Michigan, Ann Arbor, Michigan, 1956. 2. CHASE, W. G., AND CULLINGTON, E. H., Instrumentation for Geophys. Research No. 7, AFCRC-TR-57-235, 1957. 3. WI-IYTLAW-GRAY,R., AND PATTERSON,H. S. "Smoke." Arnold, London, 1932. 4. GREEN, H. L., AND LANE, W. ]=~. "Particulate Clouds." Van Nostrand, London, 1957. 5. I n FucHs, N. A., "The Mechanics of Aerosols," p. 343. Chemical Warfare Laboratories Special Publication 4-12, 1955. 6. MULLER,H., Kolloidehem. Beih. 27,223 (1928). 7. STEFAN,G., Wien. Bet. 83,943 (1881).