Photochemical Decomposition of Lead Halides from Automobile Exhaust John M. Pierrard Department of Civil Engineering, University of Washington, Seattle, Wash. 98105
rn One of the inorganic compounds exhausted by automobiles burning leaded fuel with dibromoethane and dichloroethane as scavengers is the mixed halide, lead bromochloride. The rates of production of chlorine and bromine by photodecomposition of this compound were determined and used to estimate halogen production in a model freeway environment. The further involvement of chlorine in chemical reactions after its conversion to a peroxide free radical is briefly discussed.
M
easurements of halides in atmospheric aerosols show that there is a distinct increase of the bromide ion concentration in urban areas over the background found in unpolluted regions. Aerosol samples collected by cascade impactor (Lininger. Duce. et nl., 1966) in Cambridge, Mass., and by filter in Washington, D. C.. (Gordon and Larson, 1964) showed bromide concentrations up to an order of magnitude higher than those measured by Duce, Winchester, et al. (1965) in unpolluted air over the island of Hawaii. Apparently the measured bromide excess over the natural background is introduced into the atmosphere by exhaust from automobiles burning leaded fuel. Commercial antiknock fluids contain alkyl lead compounds and employ halogen compounds to scavenge lead deposits from the engine combustion chamber. The most commonly used commercial lead antiknock fluid contains ethylene dichloride and ethylene dibromide in the mole ratio 2 to 1, and in such amount that the scavenger is in 50 excess over the stoichiometric amount required to react with all the lead present in the fluid (Working Group On Lead Contamination, 1965). According to Hirschler, Gilbert, et a/. (1957), the chief constituents of the inorganic lead compounds leaving the exhaust system are PbCIBr, two forms of NH,Cl. ZPbClBr, and 2NH4Cl.PbC1Br. The species PbClBr (lead bromochloride) appears to be an ordered structure which is stable at ordinary temperatures and isomorphous with PbCI? and PbBr, (Calingaert, Lamb, et nl., 1949). Both lead chloride and lead bromide darken on exposure to sunlight (Norris. 1895; Renz, 1921; Sanyal and Dhar, 1923), presumably releasing free halogen. The structural similarity between these lead halides and lead bromochloride suggests that photolysis of the latter should also be expected. Experimental
Lead bromide, which is white, darkens perceptibly after only a few minutes' exposure to bright sunlight. To verify that bromine is indeed released during photolysis, 1 mmole of 48
Environmental Science & Technology
lead bromide powder ground to pass a 325-mesh sieve was placed in the bottom of a sealed borosilicate glass tube containing a 2 - ~ m area . ~ of sublimed anthracene. but out of direct contact with the anthracene. Irradiation of the sealed air-filled tube for 24 hours by two 15-watt black light fluorescent tubes (F15T8BL) at a distance of approximately 15 cm. from the lead bromide resulted in the conversion of some of the anthracene to a yellow compound. This test was also conducted with lead chloride and lead bromochloride. The lead bromochloride was prepared by heating a mixture of equimolar amounts of finely ground lead chloride and lead bromide at 250" C. for several hours (Lamb and Niebylski, 1951). Its composition was verified by x-ray diffraction, using the lattice constants reported by Julien and Ogilvie (1960). In all cases, anthracene was converted to a yellow product. Since irradiation of anthracene alone under the same conditions did not result in a color change, these observations suggested that all three of the lead halides released halogen during photolysis, forming haloanthracenes. Another series of experiments was made in which 5-mm.wide strips of filter paper impregnated with the disodium salt of fluorescein were suspended in closed air-filled borosilicate glass tubes containing samples of the halIdes. The halides were irradiated as described above, with the result that in 30 minutes there was conversion of fluorescein to eosin in the tube containing lead bromide. as evidenced by a color change from yellow to pink. This was confirmed by moistening the paper and exposing it to ammonia vapors, which produced the red color of ammonium eosin (Oelke, 1950). After 2 hours of irradiation the filter paper in the tube containing lead bromochloride had changed to salmon color, which could not be accounted for by bleaching of the fluorescein. Comparison of this color with the color of samples of the reaction products of chlorine and bromine with fluorescein suggested that the color change was due to release of both bromine and chlorine from the mixed halide. This sample also gave a positive test for eosin. The lead chloride sample was irradiated for 18 hours, at the end of which time the color of the fluorescein-impregnated filter paper had changed to pale yellow, indicative of release of chlorine during photolysis. Because there are important differences in the reactivity of chlorine and bromine, measurements were made of the rate of photolytic production of the two halogens from lead bromochloride. Samples of approximately 160 mg. of powdered, sieved lead bromochloride in 5 ml. of carbon tetrachloride were made up in stoppered borosilicate glass flasks and irradiated by four 15-watt black light fluorescent tubes, while being agitated by a mechanical shaker to keep the powder in suspension. Periodically, a sample was removed and analyzed spectrophotometrically for chlorine and bromine.
Table I. Production of Bromine and Chlorine during Photolysis of Lead Bromochloride in Carbon Tetrachloride Bromine Bromine Chlorine Chloride Bromine Chlorine Concn . Concn. Concn. Duration of Mass of Released, Released, pmole/L. pmole/L. pmole/L. PbCIBr, Irradiation, pmole pmole of CCI, of cc14 of cc1, Hr. Sample Mg. 1 . 1 0.6 110 200 7 . 8 1 162.0 9.75 2.7 1.6 270 480 46 22.5 2 164.1 3.0 2.3 410 76 520 28.8 3 154.2 3.1 2.4 425 86 570 33.4 4 158.5 3.8 2.5 435 120 690 47.0 5 161.4
Since a mixture of chlorine and bromine in carbon tetrachloride forms the interhalogen bromine chloride, allowance for this factor was made in the analysis; the molar absorptivity values used for BrCl were those of Popov and Mannion (1952) (Table I). The size distribution of the lead bromochloride powder was determined by measurements of photomicrographs using a Zeiss particle size analyzer. Assuming the value 6 grams ~ m . - ~ for the density of the lead bromochloride particles, based on the unit cell volume given by Calingaert, Lamb, et al. (1949) and the fact that the unit cell contains four molecules, the ratio of cross-sectional area to mass found for the powder was 70.5 cm.? gram-'. The approximate distribution of radiant energy incident on the lead bromochloride samples is compared in Figure 1 with the energy distribution for the Los Angeles area (General Motors Research Lab., 1962) with the sun at 20" zenith angle. The energy distribution for the experiments was calculated from measurements of losses by reflection and absorption due to the borosilicate glass flasks and data for the spectrum of the fluorescent tubes employed (Illuminating Engineering Society, 1966). Disciission The mechanism of release of gaseous halogens from the lead halides can be visualized in simplest terms as involving absorption of a photon which releases a mobile electron and a hole. These mobile defects then diffuse to trapping sites within the crystal or on its surface. Neutralization of trapped electrons by lead ions creates photolytic lead, which has been studied in lead chloride by Verwey (1966), and in lead iodide by Dawood and Forty (1962, 1963) and Forty, Dawood, et al. (1964). Neutral halogen atoms created at the surface as a result of diffusion of holes from the crystal interior can react with other halogen atoms and escape into the surroundings or enter into reactions with other gas phase constituents before desorption. Also, two halogen atoms can combine within the volume of the crystal to form a halogen molecule. The observation that lead bromochloride releases both bromine and chlorine can be explained in terms of this mechanism by considering the history of a hole as it diffuses through the crystal toward the surface. Lead chloride and lead bromide have layer lattices in which each lead atom is surrounded by six halogen atoms in distorted octahedral arrangements (Pauling, 1960). There are two inequivalent lattice positions available for the halide ions, and from their observations of the rate of expansion of the unit cell with changing composition in the lead chloride-lead bromide system, Calingaert, Lamb, et al. (1949) concluded that the bromide ions in lead bromochloride preferentially occupy those halide positions
300
350
40 0
W avelsngth, n m
Figure 1. Radiant energy distribution
having the larger space volume around them. Thus, the bromide and chloride ions are regularly distributed through the crystal lattice and either a chloride or a bromide ion has a chance of losing an electron to a hole at an adjacent halide site. The fact that 35 to 44% of the total halogen released in these experiments (Table I) was chlorine indicates that the probability of a chloride ion losing a n electron to a hole is nearly as high as that for a bromide ion, at least for those holes which reach the surface and combine with others to produce halogen molecules which escape. This experimentally observed ratio of chlorine to bromine released is used in the calculation of the overall efficiency of photolysis which follows. The upper wavelength limit of optical absorption of lead bromide is approximately 400 nm. (Verwey, 1966); that for lead chloride is approximately 340 nm. (Fesefeldt, 1930; Best, 1961). As a measure of the energy available for photolytic decomposition of lead bromochloride, the energy associated with photons of wavelength less than 400 nm. was deterVolume 3, Number 1, January 1969 49
Table 11. Average Production Rates of Bromine and Chlorine by Photolysis of Lead Bromochloride Rate of Release Rate of Release of Bromine. of Chlorine, Sample pmole Mw.-' Set.-' pmole Mw.-l Set.-' 1 2 3 4 5
1.7 X 1.8 x 1.7 x 1.4 x 1.2x
10-6 10F 10P
10-6
0.9 X 1 . 1 x 10-6 1 . 3 x 10-6 1 . 1 x 10-6 0.8 X
mined from the curves of Figure 1. For the Los Angeles area sunlight curve, the total radiant energy below 400 nm. is 5.7 mw. cm.-2. The corresponding value for the radiation reaching the laboratory samples is 1.6 mw. cm.-2. Although a calculation of the quantum efficiency of production of halogen by photolysis of lead bromochloride cannot yet be made because the optical absorption is unknown, it is possible t o compute the average rate of release of halogen based on the assumption that all radiation below 400 nm. is photochemically active. If M is the number of moles of halogen released from a mass, m. of lead bromochloride particles, with cross sectional area A cm.? per unit mass, during irradiation for a period, t , by light of intensity I , the rate of release of halogen is MInvltZ (Table 11). The rate of release of bromine ranged from 1.2 X 10-6 to 1.8 X pmole cm.-2 set.-' per mw. cm.-* while that of chlorine varied from 0.8 X to 1.3 X pmole cm.-2 set.-' per mw. cm.-2. The arithmetic means of the over-all efficiencies of chlorine and bromine production as determined by these experiments are 1 X 10-6 and 1.6 X 10-6 pmole mw.-' sec.-I, respectively. These values are used below in making an estimate of the contribution of this source to the atmospheric halogen concentration. Photolj.sis of Lead Halides and Urban Atmospheric Chemistry
It was hoped that the data of Lininger, Duce, et al. (1966) could be used to relate solar radiation to the ratio of bromide to lead ions, but unfortunately, no radiation data were obtained during their sampling periods (Newell, 1968). However, some of their data, which show a trend toward increase of the ratio of bromide to lead ions with decreasing visibility, may be explained on the basis of photolytic decomposition of parent lead halides. While horizontal visibility and solar radiation a t the ground are not necessarily related, it is reasonable in this case to assume that reduction of the shorter wavelength radiation which is active for photolysis was associated with lower visibilities. Lininger's data show that haze was detected on four of the six occasions when visibility was restricted, indicating that reduction by haze of the short wavelength component of sunlight was associated with lower visibility. Therefore, during the low visibility periods, less photolytic decomposition would have occurred, resulting in larger ratios of bromide to lead ions than observed during sunny sampling periods. In the present context, we are, however, more concerned with the fact that this implies increased release of halogens to the atmosphere during sunny periods. It is of interest to calculate the potential production of halogens by photolysis of lead halides from automobile exhaust in a n urban environment. Although the experimental data presented here refer only to photolysis of one of the lead halide components of exhaust, a rough estimate of the order of 50 Environmental Science & Technolog:
magnitude of the contribution of this source to atmospheric halogen content can still be made. A study of the size distribution and lead content of fine particles in automobile exhaust (Mueller, Helwig, et al., 1964) showed that from 62 to 80 wt. of the lead-containing particles exhausted under cruise conditions were smaller than 2 microns in diameter, and a t least 68 wt. of these were smaller than 0.3 micron in diameter. The work of Hirschler, Gilbert, et al. (1957) suggests that it is reasonable to assume that the precursor of one-half the lead is lead bromochloride. Atmospheric concentrations of lead in traffic in the Los Angeles area in 1961-62 were as high as 54.3 p g . m . (Working ~~ Group on Lead Contamination, 1965). As a representative value for lead bromochloride concentration in the vicinity of a heavily travelled freeway during rush hours. we accordingly take 30 ~ g . m . - ~Then, . with the conservative assumption that all the particles are 0.3 micron in diameter, the cross sectional area of lead bromochloride is 2.5 X lop4 cm.2 per liter of air. Using the average production rates of halogen determined from the individual values listed in Table I1 and assuming 5.7 mw. cm.-2 of photochemically active radiation, we calculate that bromine and chlorine respectively are produced at the rates 0.2 and 0.1 p.p.b. hour-'. Junge (1956, 1957) measured the gaseous halogen component of the atmosphere by a method which did not distinguish between chlorine and bromine. He found average values of halogen expressed as chlorine ranging from 0.26 p.p.b. in Florida to 1.4 p.p.b. in Massachusetts. Georgii and Weber (1960) found up to 2.9 p.p.b. of halogen expressed as chlorine in Frankfurt am Main. The mechanisms heretofore suggested for the production of halogen involve reaction of sea salt with ozone (Cauer, 1951) or with sulfuric acid droplets (Eriksson, 1959, 1960). The above estimate of the production rate of halogen by the photolytic process discussed here suggests that this mechanism may well play an important role in establishment of the halogen content of urban atmospheres. The fate of the halogen produced by this or other mechanisms is open to question. Since bromine and chlorine are both photodissociated by visible light, they can readily participate in a great variety of reactions with other atmospheric constituents. However, the overwhelming probability for a reactive encounter by a chlorine atom is via the fast reversible reaction C1 O2 M e ClOO M (Porter, 1950; Porter and Wright, 1953; Benson and Buss, 1957). This reaction will dominate not only because of its very high speed but also because oxygen is many times more abundant than any other possible reaction partner in polluted air. The analogous reaction for bromine occurs only to a limited extent (Porter, 1950; Zeelenberg, 1958; Burns and Norrish, 1963). Using the estimate of Benson and Buss (1957) that the equilibrium constant for the reaction as written above is within a n order of magnitude of 0.05 atm.-', the ratio (ClOO)/(Cl) lies between 0.1 and 10%. The free radical C100, not to be confused with the isolatable species chlorine dioxide, can itself probably enter into numerous reactions with both organic and inorganic atmospheric contaminants. For example, a possible chain mechanism for the oxidation of nitric oxide can be postulated:
+
+ +
+ hv +M c1 + ClOO + NO C10 + N O Clz
-+
0 2
-+
+
2C1 ClOO
(1)
+M
+ NO2 Cl + NO2 C10
(2) (3)
(4)
2c10
c1
+
c1
+ ClOO
2C1
+M
+ ClOO Cln + Cln + M
+
+
0 2
(5)
Literature Cited
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Benson, S. W., Buss, J. H., J. Chem. Phys. 27,1382-4 (1957). Best, K.-J.,2.Physik 163, 309-20 (1961). Burns, G., Norrish, R . G . W., Proc. Roy. SOC.271A, 289-95 (1963). Calingaert, G., Lamb, F. W., Meyer, F., J. Am. Chem. SOC. 71. 3709-20 (1949). Cauer, H., in “Compendium of Meteorology,” T. F. Malone, Ed., 1126-36, Boston Am. Meteorol. SOC.,Boston, Mass., 1951. Clyne, M. A. A., Coxon, J. A., Trans. Faraday SOC.62,1175-89 (1966). Dawood, R. I.. Forty, A. J., Phil. Mag. 7, 1633-51 (1962). Dawood, R. I., Forty, A. J., Phil. Mag. 8, 1003-8 (1963). Duce, R. A., Winchester, J. W., Van Nahl, T., J. Geophys. Res. 70,1175-9 (1965). Eriksson, E., Tellus 11,375-403 (1959); 12,63-109 (1960). Fesefeldt, H., 2.Physik 64, 741-8 (1930). Forty. A. J., Dawood, R. I., Tubbs, M. R., J . Sci. Instr. 41, 274-6 (1964). General Motors Research Labs.. “Search.” Seot. 1962: Cf. Korth, M., U. S. Public Health Ser. Publ. Nb. 999-AP-20, 1966. Georgii, H.-W., Weber, E., Air Force Cambridge Res. Labs. Tech. Note 60-827,1960. Gordon, C. M., Larson, R. E., J . Geophys. Res. 69, 2881-5, (1964). Hirschler, D. A., Gilbert, L. F., Lamb, F. W., Niebylski, L. M., Ind. Eng. Chem. 49, 1131-42 (1957). Illuminating Engineering Society, “IES Lighting Handbook,” pp. 8-84, 4th ed., New York, N. Y . , 1966. Julien, H. P., Ogilvie, R. E., J . Am. Chem. Soc. 82, 293-5 (1960). Junge. C. E., Tellus 8, 127-39 (1956). Junge; C. E., Tellus 9; 528-37 (1957). Lamb. F. W.. Niebvlski. . , L. M.. Anal. Chem. 23. 1388-97 (1951). Lininger, R. L., Duce, R. A., Winchester, J. W., Matson, W. R., J . Geophys. Res. 71,2457-63 (1966). Mueller, P. K., Helwig, H. L., Alcocer, A. E., Gong. W. K., Jones, E., Am. SOC. Testing Materials, Special Tech. Publ. No. 352,1964. Newell, R. E., Massachusetts Institute of Technology, Cambridge, Mass., personal communication, 1968. Norris, R. S., Am. Chem. J . 17, 189-91 (1895). Oelke, W. C., “Semimicro Qualitative Analysis,” pp. 162-3, Heath, Boston, Mass., 1950. Pauling, L., “The Nature of the Chemical Bond,” pp. 407-9, Cornell Univ. Press, Ithaca, N. Y . , 1960. Popov, A. I., Mannion, J. J., J. Am. Chem. SOC.74, 222-4 (1952). Porter, G., Disc. Faraday SOC.9, 60-69 (1950). Porter, G., Wright, F. J., Disc. Faraday SOC.14, 23-34 (1953). Renz, E., 2. Anorg. Chem. 116, 62-70 (1921). Sanyal, A. K., Dhar, N. R., Z . Anorg. Chem. 128, 212-17 (1923). Urone, P., Lutsep, H., Noyes, C. M., Parcher, J. F., EWIRON. SCI.TECHNOL., 2 (8), 611-8 (1968). Verwey, J. F., J. Phys. Chem. Solids 27,468-71 (1966). Working Group on Lead Contamination, U. S. Public Health Ser. Publ. No. 999-AP-12, 1965. Zeelenberg, A. P., Nature 181,42 (1958).
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The atom transfer Reaction 4 is rapid and stoichiometric (Clyne and Coxon, 1966). However, the rates of Reaction 3 and the thermodynamically favored competitive reaction ClOO NO + ClNO Oz (which would be followed by NO) are not known, so that no conclusion ClNO !.% C1 can be drawn about the importance of this mechanism without further kinetic data. As mentioned above, reactions between photochemically generated halogen atoms and exhausted hydrocarbons are also possible. It is provocative to ask whether there might not be significant production of the potent lachrymatory haloarylketones in sunny areas heavily polluted by automotive sources.
+
+
+
Conclusions
The estimate given for the rate of production of halogen in a polluted region was based on the assumption that the experimental data for photolysis of lead bromochloride in a slurry in carbon tetrachloride can be applied to the process when it occurs in air. Although there is no evident reason why this should not be so, direct measurements of halogen release in air are planned to test the validity of this assumption and to obtain more complete data on the dependence of photolysis on the wavelength of the incident radiation. If this is accepted, the estimate of halogen production rate given here is only a lower limit, since it was based on the assumption that all particles considered had 0.3-micron diameter and that only lead bromochloride was photolyzed. Further work on exhaust samples from an engine burning fuel of known antiknock fluid content is planned to establish the degree and rate of photolytic halogen production by exhaust particles as affected by their chemical composition. The fact of chlorine production by this mechanism holds special significance because of the very rapid reaction between atomic chlorine and molecular oxygen to produce the free radical C100, which may subsequently become involved in the oxidation of such pollutants as CO, NO, and SOz, perhaps by chain mechanisms. In addition, it is expected that the ultimate product of photolysis of lead bromochloride in the atmosphere is PbO or other lead oxide. These lead oxides may themselves catalyze or photosensitize the oxidation of gaseous pollutants (Urone, Lutsep, et al., 1968). A more thorough discussion of these aspects of the presence in urban atmospheres of automotive lead compounds is beyond the scope of this paper. Hopefully, this communication will stimulate interest in the possible catalytic role played by these photoactive particles, and in their atmospheric residence time. Acknowledgment
The author thanks R. J. Charlson, who suggested the anthracene test.
Received f o r review June 24, 1968. Accepted October 25, 1968. Financial support was procided under Public Health Seroice Special Fellowship IF3 AP37, 293-01.
Volume 3, Number 1, January 1969 51
