Envlron. Sci. Technol. 1988, 22, 711-717
plished by connecting in series a sulfur-impregnated active alumina bed and sulfur-impregnated activated carbon bed.
Conclusions Through the experiments on the adsorption of mercury vapor on sulfur-impregnated adsorbents, the following conclusions are obtained. (1)Impregnation of sulfur increases the adsorption capacity of active alumina and zeolite by several orders of magnitude, and the equilibrium adsorbed mass is equal to the stoichiometrical value obtained from Hg + S HgS. (2) Sulfur-impregnated silica-based supporting materials have unusual concave breakthrough curves. (3) The deficiency of the sulfur-impregnated active alumina and zeolite beds (nonzero initial outlet concentration) can be covered by combining them with the sulfur-impregnated activated carbon bed. Registry NO, Hg, 7439-97-6;S, 7704-34-9;C, 7440-44-0;A1203,
Literature Cited (1) Otani, Y.; Kanaoka, C.; Usui, C.; Matusi, S.; Emi, H. Environ. Sci. Technol. 1986, 20, 735-738. (2) Kato, T. “Nihon Gaishi Research Report-Kankyo Souchi Tokushu”; NGK Insulators Ltd.: Japan, 1976; p 22-27. (3) Suzuki, M.; Kubota, H.; Kanaya, K. Kougai to Taisaku 1985,21, 69-76. (4) Coolidge, A. S. J . Am. Chem. SOC.1927, 49, 1949-1952. (5) Kobayashi, Y.; Hori, M.; Tomita, M.; Yamaoka, T. Anzen Kogaku 1981,20, 30-34. (6) Barrer, R. M.; Whitemann, J. L. J. Chem. SOC.A 1967,59, 19-25. (7) Lovett, W. D.; Cunniff, F. T. Chern. Eng. Prog. 1974, 70, 43-47. (8) Steijns, M.; Peppelebos,A.; Mars, P. J. Colloid Interface Sci. 1976, 57, 181-186. (9) Sinha, R. K.; Walker, P. L., Jr. Carbon 1972,10,754-758.
-
1344-28-1.
Received for review August 3, 1987. Accepted January 4, 1988.
Neutralization Kinetics for Polonium-218 Kal-Dee Chut and Phlllp K. Hopke” Department of Nuclear Engineering and Institute for Environmental Studies, University of Illlnois, 1005 W. Western Avenue, Urbana, Illinois 6 1801
In a well-defined experimental system the neutralization of polonium-218 ions was investigated as a function of the physical and chemical properties of the controlled composition atmosphere. The diffusion coefficient of polonium-218 under various concentrations of trace gas NOz in nitrogen was measured. The mobilities of Po+ and PoOz+are determined by combining experimental results with a computer model of the system. Three neutralization mechanisms were individually studied. The small-ion recombination rate has been found to be proportional to the square root of radon concentration. The electronscavenging mechanism is responsible for the neutralization of Po+ in NOz or HzO in nitrogen. When PoOz+is formed, the electron-transfer mechanism dominates the neutralization process. The electron is transferred to PoOz+from molecules with lower ionization potentials. The ionization potential of PoOz+is also determined to be 10.44 f 0.05 eV.
Introduction It has been known for a long time that exposure to radon and its decay products represents an increased risk of lung cancer. Significant numbers of uranium and other underground miners have shown large increases in lung cancer incidence when exposed to the higher levels of radioactivity permitted in the past. Recent s w e y s of indoor air have led the National Council on Radiation Protection to increase its estimates of dose to the bronchial epithelium from 450 mrem in 1975 (1) to 3000 mrem in 1984 (2). Recently, there have been findings of individual houses with extremely high airborne radioactivity levels, and there is increasing concern that indoor radon and radon decay products represent a significant threat to public health. There are still, however, many uncertainties as to the fundamental chemical and physical properties of the radon decay products. +Presentaddress: P.O. Box 1526, Laurel, MD 20707-0953. 0013-936X/88/0922-0711$01.50/0
Radon is an inert gas, but its radioactive decay products are chemically active. Polonium-218 (RaA) is formed from the a decay of radon-222. It has been found to be formed as a singly charged positive ion 88% of the time (3). The neutral species occurs the remaining 12% of the time. The neutralization of polonium-218 is of interest for several reasons. There are a number of radon-monitoring systems that depend on electrostatic collection of the charged decay products (4). These systems have shown sensitivity to water vapor and other atmospheric conditions that can prove to be a problem in making reliable radon concentration measurements. In addition, it has been suggested that the polonium ion initiates ion nucleation leading to the formation of clusters (5). Thus, the nature of the processes leading to neutralization and their rates is important to the understanding of the environmental effects of radon decay products. Porstendorfer (6)found that the diffusion coefficient can be a good indicator of the neutralization behavior of radon daughters. Neutral species have a diffusion coefficient on the order of 0.071 cmz/s, while the mixture of charged and neutral ions have a value of approximately 0.035 cmz/s (7). Various methods have been used to measure the diffusion coefficient of polonium-218in atmospheres containing different trace gases such as water vapor or nitrogen dioxide. Raabe (8)used a technique outlined by Townsend (9)to determine the diffusion coefficient of zlsPin various relative humidities. This technique consists of drawing the radon daughters from a large chamber through cylindrical tubes of various lengths and observing the fractions of polonium atoms penetrating through the tube after correcting for the production of radon daughters during the passage of radon gas through the tubes. When neutralization of ions occurs in a diffusion tube, the diffusion coefficient will increase resulting in an increase of particle deposition on the tube wall and less radioactivity penetrating through the tube. Raabe observed that the diffusion coefficient of 218Pdecreased with increasing humidity such that it was 0.047 cmz/s at a partial pressure of 0.49
0 1988 American Chemical Society
Environ. Sci. Technol., Vol. 22, No. 6, 1988 711
kPa of water vapor or a relative humidity of 15% and 0.035 cmz/s at a partial pressure of 0.92 kPa of water vapor or a relative humidity of 35 % . However, using the two-filter method where the decay products are removed by the entrance filter, Thomas and LeClare (10) found an increasing diffusion coefficient with increasing humidity up to 20% relative humidity. These contradictory results can be explained by comparing the age of the polonium species in Raabe’s and Thomas’ experiments. Raabe’s work reported that the ratio of zlaPo atoms to 214Pbatoms was about 2.4:1, indicating that the age of his 21EPowas on the order of a few minutes. In the two-filter method, the inlet filter assures that the only source of 21EPoin the tube is through radon decay within the tube. Thus, the age of 21EPoin the diffusion tube is very much younger. Since clusters grow with time and the concentrations of the trace gases, it is likely that Raabe found smaller diffusion coefficients at higher humidities due to the growth of the clusters. It is, therefore, necessary to have good control of the age of radon progeny if consistent results are desired. In their study, Porstendorfer and Mercer (11)used a method similar to Thomas and LeClare. However, Porstendorfer and Mercer designed their system to collect activity on a central electrode in the diffusion tube allowing the analysis of the charged ions. They found the neutral zlzPbspecies had diffusion coefficient of 0.068 cm2/s independent of the relative humidity. The diffusion coefficient of the charged species was estimated as 0.024 cm2/s in dry air and increased with increasing humidity. It was found that at relative humidities between 30 and 90% the diffusion coefficient increased to 0.068 cm2/s, the value observed in the neutral case. When radon decays, free electrons are produced along the a track and the polonium recoil path. Charge equilibrium is reached by small-ion recombination (12). Busigin et al. (13) measured the rate for small-ion recombination to be 10.06-12.34 s-l in argon and 8.05-16.17 s-l in air. Thus, the exact rates for small-ion recombination are not well known. Frey et al. (7) studied the diffusion coefficient of 21EPo in various experimental gases and estimated the neutral polonium diffusion coefficient to be 0.079 cmz/s. They found that polonium was completely neutralized at relative humidities of 20 and 80% in nitrogen and in 10 ppm NO2 in dry nitrogen. They could not explain this latter phenomenon. It was postulated that at least two mechanisms existed for neutralization. If oxygen is present, a polonium dioxide ion will be formed with a high enough ionization potential to be neutralized by removing electrons from lower ionization potential molecules. The neutralization of polonium ions in nitrogen with water vapor cannot be explained by this mechanism since the oxide cannot form and the ionization potential of H20 is too high for electron transfer. Frey et al. suggested that water molecules scavenge the electrons along the a track and then diffuse to polonium ions to neutralize them. Goldstein and Hopke (14),using the two-filter method, measured the diffusion coefficient of 218Poin pure nitrogen to be 0.0340 cmz/s. They suggested that since nitrogen dioxide has an electron affinity of 2.32 eV (15),it can form negative ions through scavenging electrons from the recoil path of polonium. The negative ions then diffuse to the positive polonium ion and yield neutralization. Thus, if this electron-scavenging mechanism holds, the diffusion coefficient of polonium should increase with NO2 concentration as confirmed by their experimental results in the concentration range from 50 to 700 ppb. Considering 712
Environ. Sci. Technol., Vol. 22, No. 6, 1988
HIGH VOLTAGE TERMINAL (ON BOTH P L A T E S ) SILICON 0ET:CTOR
LUCITE
\
STAINLESS STEEL PLATE
TEFLON
Flgure 1. Schematic diagram of the diffusion chamber.
such a mechanism for water, a calculation (16) on negative ions of nonionic polar molecules showed that H20 cannot bind an electron. Since the hydroxyl radical has an electron affinity of 1.83 eV, OH produced by water radiolysis was considered the possible neutralizing agent. They confirmed the electron-transfer mechanism and found the ionization potential of polonium dioxide ions to be in the range of 10.35-10.53 eV. Their work clearly demonstrated the existence of the two separate neuttalization mechanisms. However, they could not narrow this range further since the obvious candidate gases with appropriate ionization potentials in this range are reactive compounds (C2H, and H2S) that yielded an intermediate diffusion coefficient of zlEPo(0.055 cm2/s). Radiolysis or chemical combination may occur with these gases to yield this intermediate value for the molecular diffusion coefficient of zlePo. Thus, different and even contradictory results have been obtained for the diffusion coefficient of polonium-218 under various conditions. The major influential factors that can affect the experimental results are as follows: (1) the neutralization rate of polonium or polonium dioxide ions, (2) the contents of trace gases, and (3) the age of the species when the measurements are being taken. Our purpose is to investigate the kinetics of the neutralization of polonium and polonium dioxide ions under various, well-controlled environmental conditions. In our experiments, a new device is used to determine the ionization potential of polonium dioxide and confirm the results of Goldstein and Hopke. All three neutralization mechanisms are carefully investigated and their neutralization rates are measured in well-controlled atmospheres.
Experimental System A diffusion chamber (Figure 1)was designed to perform these series of experiments. It is composed of two parallel, stainless steel plates and two Teflon plates that form a rectangular chamber. Both ends of the chamber are designed to hold filters to remove the decay products from the inlet stream and cause a homogeneous flow in the chamber. The stainless steel plates are connected to a DC high voltage, yielding a uniform electric field. A custom fabricated rectangular silicon detector (Enertec) with a flat surface at ground potential is mounted in the grounded plate so that a continuous monitoring process is feasible while maintaining a uniform electric field. The radon source (Pylon Model RN-1025) can produce 3400 pCi/min in nitrogen controlled by a mass flow controller. The test gas (NO2or H 2 0 in Nz, for example) is mixed with the radon-laden gas and directed through the diffusion chamber, Humidities ranging from 200 to 4000 ppm can be produced by a diffusion cell designed by Miguel and Natusch (17). Humidities higher than 4000 ppm are produced by a bubbler and measured by a dew point hygrometer (General Eastern Model 1100DP). The electric field permits the examination of the neutralization rate by moving the ions until they are neutralized. When the ions are drawn away from or pushed
toward the detector, the total counts in the 21sPopeak will decrease or increase, respectively. As the polonium atoms reach the zone observed by the detedor, there will be both charged and neutral atoms present, and the total counts will be a function of the neutralizing conditions. Thus, as the neutralization rate increases, the effective diffusion coefficient increases, and the number of atoms penetrating to the detecting zone decreases. A simple one-dimensionalmass balance model has been previously described (18). This analysis permits the estimation of the effective distance from the detector that an emitted a can be detected. Since the a particle will lose energy as it passes through the air, the a’s emitted farther from the detector will have less energy upon reaching it. Because a lower energy bound is get in order to integrate the a spectral peak from polonium-218, there is then a sharp boundary between those a particles able to be counted and those that will be excluded from the count. This critical distance will be denoted by D,. A three-dimensional analysis of the system was made by solving the equations for the distributions of both charged and neutral species (19). After the radon gas entered the chamber, both charged and neutral polonium atoms are formed. They diffuse, deposit, and decay along the path in the chamber. In the interim, the ions are either neutralized or drawn away by the electric field. The complex relations can be expressed by the following two mass balance equations:
aN+ =
0 LL
.04
z
v) 0
021 A
3 LL
0
5
Chu and Hopke Goldstein and Hopke
!
-
n 0
(3)
+ +
where a = fQ/v, b = (A p,, KE/D,)/v, c = D+/u, h’= h - KE/pn,and 0 = b + c[(rn.rr/h12+ ( n . r r / ~ ) ~and ],
IJ-
where a’ = (1 - f)Q/v + fQpn/(p + p,)u, b’ = X/u, c’ = fQpn/(X p,)v, d’= D/v, A,, = cl[sin (rn - h/h?7r]/[2( m / h- l / h ? r ] - [sin (rn + h/h?a1/[2(rn/h + l/h?a]]Dn, Dn = [sin (n - l)a]/[2(n - 1)7r/w] - [sin (n + l).rr]/[2(n l)a/w] (n # I), D, = w/2 (n = 11, and B,, = b’+ dl[(rna/h)z ( n . r r / ~ ) ~ ] .
+
with boundary conditions
N+(O,y,z)= N+(h- KE/p,,y,z) = N+(x,O,Z)= N+(x,w,z) = 0 N+(x,y,z)= 0 for x 1 h - KE/pn and
N(O,y,z) = N(h,y,z) = N(x,O,z) = N(x,w,z)= 0 N+(x,y,0)= N(x,y,O) = 0 where Nand N+ are the concentrations of polonium atoms and ions, f is the fraction of polonium-218 born charged, u is the velocity of the gas flow, X is the decay constant of polonium-218, pn is the neutralization rate of polonium ions, Q is the 218Poproduction rate that equals the decay constant of radon times the concentration of radon gas, K is the mobility of polonium ions, E is the strength of the electric field, D and D+ are the diffusion coefficients of neutral and charged polonium-218, and h and w are the height and width of the diffusion chamber, respectively. The (KE/D,)N+term represents the reduction of ions in the detecting zone due to the effect of the potential field E attracting the ion of mobility K. KE is the ion velocity and Dc is the distance to be traversed in order not to be counted. The KE/bnterm in the boundary conditions is the distance the ion can move before neutralization since l / p n is the mean time to neutralize an ion. Equations 1and 2 can be solved to obtain the concentrations of charged and neutral polonium-218. The solutions to these equations are given by Chu (19) under the
+
+
Results and Discussion Diffusion Coefficient of Polonium-218. After the gas mixture (radon nitrogen test gas) entered the diffusion chamber, polonium atoms are formed. Some of them diffuse to the walls, some decay, and the rest remaining in the gas stream will reach the detecting area. According to electron-scavenging mechanism, the neutralization of polonium ions should increase with the increasing concentration of NOz in gas. The variation of the polonium diffusion coefficient with NOz concentrations ranging from 50 to 900 ppb was studied. The detailed calculation of the diffusion coefficient for these conditions has been previously published (18). The diffusion coefficient of polonium-218 increased with the concentration of NOz. Complete neutralization was observed when the concentration of NOz in N2was greater than 700 ppb (Figure 2). As shown in this figure, the results determined by the continuous-monitoringmethod are in excellent agreement with the results from the twofilter method (14). The accuracy of these results and the ability to obtain better statistical precision were improved by the continuous-monitoring method because of the well-defined conditions and the extended sampling and counting time. These results indicate that the chamber is producing reliable results for the polonium diffusion coefficients.
+
+
Environ. Scl. Technol., Vol. 22, No. 6, 1988
713
Table I. Neutralization Rate of Polonium Ions in Argon, Air, and Nitrogen radon concn, atoms/cm3 1.07 X 5.30 X 2.26 x 4.52 x 9.05 x 1.36 x 1.70 x 2.71 x 0
11
\i IADON
50
?I
100
CONCBNTPATION' (pcill)'''
Mobility of Polonium Ions. To calculate the neutralization rates, the mobility K must be known. Using a proportionality relation, Busigin et al. (13) estimated the mobility of 218Po+to be 2.04 cm2/(s.V) in nitrogen or air. They were not able to estimate the mobility of Po02+by proportionality because the molecular radius would be substantially different. Bricard et al. (20) studied the mobility of thoron decay products in air and found two aged cluster groups with mobilities 0.56 and 0.86 cm2/(s.V), respectively, and a freshly formed ion group with mobility 1.85 cm2/(s-V). Since the distributions of polonium atoms and ions have been expressed in eq 3 and 4, an integration over the detecting zone can be made to compare the measured value X,. The mobilities of the Po+ and PoOzt were thus extracted to be 1.87 f 0.10 and 1.86 f 0.11 cm2/(s.V), respectively. The neutralization rates are calculated on the basis of these values. Neutralization Rate Studies. (A) Small-Ion Recombination. The polonium ion recombination rate is measured under various radon concentrations from 1274 to 15283 pCi/L in pure nitrogen. Figure 3 shows the increase of neutralization rate from 0.85 to 7.51 as the radon concentration increases. Busigin et al. (13) in their formulation of small-ion recombination coefficients predict that the recombination rate should be proportional to the square root of radon concentration. It is assumed that neutralization of 21sPotspecies occurs primarily through recombination with negative small ions, then the neutralization rate constant should be proportional to the negative small-ion concentration. Busigin et al. (13) have shown that if it is assumed that a steady state exists and that Ct = C-,then (5)
where T is the rate constant for small ion production and a is the recombination coefficient for negative and positive small ions. The neutralization rate of 21sPot species, R , should be proportional to C- and hence to the square root of radon concentration: R =A
~ T c ~ , / / ~
(6)
From Figure 3, there is a minimum dose required before the neutralization behavior can be observed. Nolan (21) in his discussion on weak ionization behavior concluded that when systems are small, the diffusion and plateout of the ions cannot be ignored. Our system is even smaller than his definition of "small". Thus, for very low concentrations of radon, most ions are lost by diffusion before they can be detected. Using higher radon concentrations, Busigin et al. (13) measured the neutralization rate of polonium ions in argon 714
Envlron. Scl. Technol., Vol.
22,
No. 6, 1988
lo6
12.34 12.03 0.85 h 0.05 2.27 h 0.06 3.85 f 0.09 4.82 f 0.11 5.64 f 0.12 7.51 f 0.16
lo6 104 104 104 105 105 105
125
Figure 3. Small-ion recombination rate of 2'8Po+ In nitrogen versus the square root of the concentration of radon.
c.. = d T C R n / "
neutralization rate, 5-l
and in air. Table I compares their experimental results with the measurements from our similar experiments in nitrogen. The linear relation shown in Figure 3 is found to be
R = 0 . 0 1 7 6 6 - 1.613
(7)
where the radon concentration CRnis in atoms/cm3. To compare the experimental results in argon and nitrogen, the neutralization rate of polonium ions in nitrogen can be calculated from eq 7 for the radon concentration of 1.07 X lo6 atoms/cm3, and the calculated value is 16.60 s-'. The ionization potential of argon (15.76 eV) is slightly higher than nitrogen (15.58 eV). Therefore, more electrons will be found along the polonium recoil path and the a track in nitrogen than in argon, and a higher neutralization rate could be expected. Luhr (22) discussed the recombination coefficient a in air and found the value to be 1.4 X lo4 cm3/s. Since there are approximately 2 X lo6 ion pairs produced in air when one radon atom decays (23) and the decay constant for radon is 2.097 X lo4 s-l, the small-ion production coefficient T in eq 6 can be calculated to be 0.4194 [ion pairs/s]. Comparing eq 6 and 7, the proportionality constant A can This is an order of magnitude be evaluated as 3 X estimate because the ion production and recombination coefficients are measured in air instead of nitrogen. (B) Electron Scavenging Mechanism. It has been observed that polonium ions can be neutralized by water vapor or NOz in nitrogen (14) as well as in air. This neutralization was attributed to be caused by the transfer of electrons from the polonium recoil path to the ion by trace gas molecules. By adjusting the concentration of the electron scavenger, it is possible to control the rate of neutralization. Miller (24) suggested that the neutralization of polonium by water vapor occurs in a time frame of a few milliseconds. A measurement of the rate of this process would provide a better understanding of this mechanism. Neutralization rates were measured in various concentrations of NO2 and H 2 0 in pure nitrogen. Figures 4 and 5 show the neutralization rate of polonium ions as a function of the concentration of NO2 and H20, respectively. However, the same neutralization mechanism might be expected to give a similar pattern between the trace gas concentration and the neutralization rate. The hydroxyl radicals from water vapor radiolysis have been suggested as the electron scavengers (14). This process can be expressed as H 2 0 H' *OH (8)
-
'OH 'OH-
--
+e
+ Po+
+
'OH-
Po
+ 'OH
(9) (10)
This hypothesis was tested by adding a radical scavenger
7 400
A / -A
i t
0
IO0
200
300
500
400
800
-1
I
n
700 800 900
IOU0 1100
NO, CONCENTRHT ION ( pph) Flgure 4. Neutrallzation rate of *'ePo+ in nitrogen for various concentratlons of NO2.
Flgure 8. Neutralization rate of Po+ In nitrogen versus the square root of the concentration of H,O.
q
Lu
t
3000
I
'
d
/"/ / /
I
.
1
01
2000
IO00
0
H,
4000
300U
MOLECULRR CUNCENTRRT ION i 1 U'zl;m.3)
0 CONCENTRRTI ON ( ppm)
Flgure 5. Neutralizatlon rate of 216Po+in nitrogen for various concentrations of H,O.
Table 11. Neutralization Rate of Po+in Humidified Nitrogen with and without Ethanol
[HZ019PPm
[C2H60],ppm
neutralization rate, s-l
3035 3976 5980 5980
0 0 1.30 3.51
47.9 1.1 47.9 1.1 3.21 0.08 3.21 0.08
* * *
(ethanol). Processes 9 and 10 are then dominated by the preferred reaction: HR
+ 'OH
-
RO'
+ HzO
(11)
The neutralization rate was then reduced to the small-ion recombination range (Table 11). The concentration of hydroxyl radicals can be evaluated by this equation: d[OH]/dt = Q[H,O] - k[H][OH]
(12)
where Q is the disintegration constant of water molecule and k is the recombination constant. When steady state is obtained, and [HI = [OH], the concentration of hydroxyl radicals can be expressed as
Since the hydroxyl radical is the electron scavenger and ita concentration is proportional to the square root of the concentration of water vapor, a plot of the neutralization
Flgure 7. Neutrallzationrate of PoO,' versus the molecular concentration of the electron donor assuming one polonium atom per cm3.
rate versus the square root of the concentration of water vapor should reveal the same pattern as is in the NO2 case. By comparing Figures 4 and 6, similar patterns of neutralization rates are obtained. It was found that 50 ppb NOz yields a neutralization rate of 103.7 s-l. For 20 ppb NO2, a neutralization rate of 40 s-l will be obtained. Disregarding the differences in electron affinity (1.83 eV for OH and 2.32 eV for NOz)and diffusion coefficient between OH and NO2 (15, 25), the same concentration of hydroxyl radicals should provide a neutralization rate in the same range. Since 1056 ppm H 2 0 produced a neutralization rate of 38 s-l, we are then able to calculate the production of hydroxyl radical to be 20 ppb OH = 1.9 x 10-2 ppb OH (14) 1056 ppm H 2 0 PPm H2O This result suggests that for 1 ppm of water vapor, the polonium recoil produces on the order of 1.9 X ppb of hydroxyl radicals in the immediate vicinity of the polonium recoil path. (C) Electron-TransferMechanism. When oxygen is present, PoO2+is rapidly formed as the polonium ion attains thermal velocity. With an ionization potential between 10.35 and 10.53 eV (14),it can remove an electron from molecules with lower ionization potentials (e.g., NO2 with IP = 9.79 eV). Figure 7 shows the neutralization rates of PoO2+in ppb range of NOz in air. This neutralization is faster than that due to the electron-scavenging mechanism since the electrons can be transferred directly by a Environ. Scl. Technol., Vol. 22, No. 6, 1988 715
n
1000 1
I
IONIZATION POTENTIAL
(eV)
Figure 8. Proportionality between the ionization potential of the trace gas and the logarithm of electron-transfer rate constant.
single collision with a donor molecule. The collision rate between polonium dioxide ions and NO2 molecules was calculated as (26) 21, = 7 r d l d 2 u N l N z / ~ 2
(15)
where Z12is the collision rate, dl and d z are the diameters of NOz and Po', respectively, u is the average speed of NO2 molecules, and Nl and N2 are the concentrations of NO2 and Po'. For NOz concentrations of 100-1000 ppb and a radon concentration of 7642 pCi/L, the collision rate By. comparison with the ranges from 123 to 1230 s-l ~ m - ~ experimental neutralization rates, it appears that about 90% of the collisions resulted in an electron transfer. Figure 7 also shows the results of the same experiments for NH3 and n-C5HlZin air, respectively. However, the collision efficiencies are much lower; 3% for NH, and 1% for n-CSH,,. Since the ionization potentials are higher (10.2 eV for NH3 and 10.35 eV for n-C5HI2),there is less energy available for these molecules to transfer an electron to the polonium oxide ion. (D) Ionization Potential of PoOz'. Due to the limitation of the two-filter method, Goldstein and Hopke (14) were only able to determine that the ionization potential of Po02+was within a 10.35-10.53 eV range. The current experiments providing the rate constants allow us to estimate the ionization potential. The logarithmic plot of the rate constants in Figure 8 shows a linear relation as a function of the ionization potential of the trace gas. Assuming the potential barrier to transfer an electron is constant and the energy available to transfer the electron then decreases with increasing ionization potential of the electron donor, the reaction rate constant and the ionization potential should have the relation (27,28)
R= (7r / v% (ri + r,)2 [87rk T(1/mi
+ 1/ m,) ] 1/2e-r(Ex'Ea-Ei)lkT (16)
where R is the rate constant, ri and r, are the radii of polonium dioxide and the trace gas, respectively, mi and m, are the molecular mass of polonium dioxide and the trace gas, respectively, Ei and E , are the ionization potentials of polonium dioxide and the trace gas, respectively, and E, is the activation energy of the electron-transfer reaction. I' = y(kT/q), et Ei - E, + E , + (5/2)kT, and I.L can be obtained from the relation EC1
exp(-w / et,) 716
-
EC2
exp(-w/
4
Envlron. Scl. Technol., Vol. 22, No. 6, 1988
(17)
where e E , + (3/2)kT, ecl and cc2 are the collision efficiencies for different trace gases. From eq 17, an average value of y can be obtained. Thus I? can also be evaluated. With an initial approximation of Ei,the ionization potential of PoOz' can be obtained through iterations for each activation energy E,. The value of activation energy is found to be 0.85 f 0.02 eV. The ionization potential of polonium dioxide is determined to be 10.440 f 0.054 eV. Conclusions A well-defined experimental system is now available to investigate the neutralization of polonium as well as its other physical and chemical properties. The diffusion coefficient of polonium-218 was found to increase with the concentration of NO2in nitrogen in agreement with earlier studies. The mobilities of Po' and PoO2+have been determined to be 1.87 f 0.10 and 1.86 0.11 cmz/(s.V), respectively. The neutralization rates for the three mechanisms have been determined. The small-ion recombination rate has been found to be proportional to the square root of radon concentration. The neutralization of Po+ by NO2 or H20 in nitrogen is confirmed by the electron-scavenging mechanism. Furthermore, the hydroxyl radicals from water vapor radiolysis have been proven to be the electron scavengers that result in the neutralization of polonium ions. When PoOz+can be formed, the electron-transfer mechanism dominates the neutralization process. The electrons are transferred by collisions between PoO2+and electron contributors at rates that are close to the binary collision frequency for low ionization potential gases. The ionization potential of polonium dioxide has been experimentally determined to be 10.44 k 0.05 eV. There are generally 10-200 ppb NO, in a typical house (29, 30). In addition there are a few thousand ppm of organic compounds. Humidity varies according to season, locations, and whether a humidifier or dehumidifier is being used. In any case, neutralizing agents exist in most indoor atmospheres. When radon decays in air, PoO2+is formed and will be promptly neutralized by stripping the orbital electron from available trace gases including NO, and low ionization potential organic compounds. Thus, the neutralization of the Po+ ion is rapid in the typical indoor environment. Many aerosol formation or particle clustering calculations (5) have been made assuming Po' or PoO2+to be the nucleus of coagulation since the electrostatic force of the ion enhances the formation of clusters. However, these results show that Po' and PoO2+will be neutralized before they can serve as nucleating centers. In an atmosphere with very few neutralizing agents, nucleation can occur but may be followed by an evaporation process when the charge is neutralized. These results thus indicate that a modification of the model used in the radiolytic nuclei formation or particle clustering calculations will be needed. Since many positive ions and free electrons are produced along the polonium recoil path as well as the CY track, the molecules with lowest ionization potential will be left as ions that cannot be neutralized except through the slow process of small-ion recombination. These ions can serve as the nuclei for particle formation.
*
Registry No. 218Po', 93862-53-4; Po', 56797-55-8; PoOz', 113600-30-9; NOz, 10102-44-0; HzO, 7732-18-5.
Literature Cited (1) National Council on Radiation Protection and Measurements "Report No. NCRP-45"; 1975. (2) National Council on Radiation Protection and Measurements "Report No. NCRP-77"; 1984.
Environ. Sci. Technol. 1988, 22, 7 17-720
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Received for review June 15,1987, Accepted December 30, 1987. We thank the US.Department of Energy for their support of the project under ContractsDE AC02 83ER60186 and DE FG02 87ER60546.
NOTES Chlorinated Pesticides In Indoor Air David J. Anderson and Ronald A. Hites" School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
Indoor concentrations of chlorinated pesticides were measured in the air of 12 homes and found to be elevated with respect to typical outdoor concentrations. For example, indoor to outdoor concentration ratios for ychlordane were as high as 60 in the living area of one home and as high as 1000 in the basement of this home. Indoor sources for these chemicals are implied.
Introduction The average person spends about 90% of her or his time indoors (1). Thus, it is ironic that the setting of air quality standards for the protection of human health is based only on outdoor pollutant concentrations (1-5). Clearly, an evaluation of total human exposure to air pollutants should include effects of both outdoor and indoor air. For this reason, indoor air pollution has recently received a great deal of comment from both the mass media and the scientific community. From a scientific viewpoint, the emphasis has been on knowing the identities and concentrations of indoor air pollutants. The majority of the early research on indoor air dealt with the measurement of gaseous pollutants such as carbon monoxide (CO), nitrogen oxides (NO,), ozone (OJ, and sulfur oxides (SO,) (5). A chronological summary 0013-936X/88/0922-0717$01.50/0
of major U S . indoor air research reveals the neglect of organic pollutants in early indoor air research (5). There has been some research on volatile organic compounds (6-12), but there are few data on pesticides and related compounds (13-15). Our research focuses on the identification and quantification of semivolatile organic compounds (vapor presmmHg) in the indoor air of single-family sures, 1dwellings. This general category of indoor pollutants includes chlorinated pesticides. We measured the indoor concentrations of some of these compounds in the air of 12 homes and compared these concentrations to typical outdoor concentrations. Possible sources of these chemicals are proposed for some of the compounds.
Experimental Section Sampling. Homeowners were solicited by a news release that appeared in the local newspaper; 62 responses were received. Questionnaires .were used to select a subset of 12 homes for sampling. The questions requested general information about the size and age of the home and about possible sources of organic chemicals. The majority of the homes were located in the Bloomington, IN, area and were sampled during November of 1985 to October of 1986.
0 1988 American Chemical Society
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