Environ. Sci. Technol. 1086, 20, 138-145
chlorinated 2-oxo-3-pentenoic acids. They are not formed in any detectable amounts in the E-stage spent liquors which indicates that they are alkali labile and therefore probably decompose relatively fast already at neutral pH, probably forming chlorobutenoic acids found in the total bleaching effluents where the pH is higher, about pH 4. A likely mechanism for the formation of these analogues has not yet been proposed. To our knowledge these particular compounds have not previously been reported. As early as 1890,however, Zincke et al. reported the formation of very similar compounds in their study of chlorination of catechols (26). The environmental effects of all these compounds (Table I) are unknown as far as we know. At a pH of about 7 they should occur as salts readily soluble in water and will therefore probably not be retained in the organisms for a longer period but should pass quickly back into the surrounding aquatic environment. Possibly, trichloroacetic acid may decompose or metabolize to chloroform and carbon dioxide. Conclusions
(1)The C- and E-stage spent liquors contain about 300 and 30 g/ton of pulp of chlorinated low molecular and ether-extractable acids, respectively (calculated as OgC1). (2) About 30% (100 g/ton) of the ether-extractable acidic OgCl in the C-stage and about 60% (18 g/ton) in the E-stage liquors have been identified. (3) Chlorinated acetic acids are by far the most predominating chloro organic acids so far identified. (4) A new series of chlorinated keto acids has been identified. (5) Totally 31 carboxylic acids have tentatively been identified. Registry No. 1,79-11-8;2,79-43-6; 3,76-03-9;416,26952-44-3; 6, 99165-89-6;7, 2257-35-4;8/9, 99165-90-9; 10, 99165-97-6; 11, 99165-94-3; 12113,99165-91-0; 14, 99165-95-4; 15, 144-62-7; 16, 141-82-2; 17, 600-33-9; 18/19,6915-18-0; 20/21, 19071-21-7;22, 43180-81-0; 23,65-85-0; 24b, mas-95-7; 25b, 69845-51-8;2611, 9916596-5;27,8&14-2;28,527-72-0;29,99165-92-1;30,29010-86-4; 31, 99165-93-2.
Coal Combustion Aerosols and SO,:
Literature Cited (1) Kringstad, K. P.; Lindstrom, K. Enuiron. Sci. Technol. 1984, 18,236~. (2) Lindstrom, K.; Nordin, J.; Osterberg, F. In “Advances in The Identification and Analysis of Organic Pollutants in Water”; Keith, L. H., Ed.; Ann Arbor Science: An Arbor, MI, 1981; voi. 2, p 1059. (3) Ota, M.; Durst, W. B.; Dence, C. W. Tuppi 1973,56, 139. (4) Leach, J. M.; Thakore, A. A. J. Fish Res. Board Can. 1975, 32, 1249. (5) Shimada, K. Kumi Pa Gikyoshi 1977, 31, 97. (6) Kamazava, K.; Hosoya, S.; Nakamo, J. Kumi Pa Gikyoshi 1977, 31, 399. (7) Kachi, S.; Yonese, N.; Yoned, Y. Pulp Pup. Can. 1980,81, 105. ( 8 ) Shimada, K. Mokuzui Gukkaishi 1981, 27, 470. (9) Pfister, K.; Sjostrdm, E. Sven. Pupperstidin. 1981,81,195. (10) Pfister, K.; Sjostrom, E. Pup. Puu 1979, 61, 220. (11) Pfister, K.; Sjostrom, E. Pup. Puu 1979, 61, 367. (12) Pfister, K.; Sjostrom, E. Pup. Puu 1979, 61, 449. (13) Pfister, K.; Sjostrom, E. Pup. Puu 1979, 61, 525. (14) Pfister, K.; Sjostrom, E. Pup. Puu 1979, 61, 619. (15) Sjostrom, L.; Rhdestrom, R.; Lindstrom, K. Sven. Papperstidin. 1982, 85, R7. (16) Eriksson, B.; Sjostrom, L. Sven. Pupperstidn. 1976, 79,570. (17) Schoniger, W. Miltrochim. Acta 1955, 123. (18) Schlenk, H.; Gellerman, J. Anal. Chem. 1960, 32, 1412. (19) Johnson, J. D.; Christman, R. F.; Norwood, D. L.; Millington, D. s. Environ. Health Perspect. 1982, 46, 63. (20) Rook, J. J. In “Water Chlorination. Environmental Impact and Health Effects”; Jolley, R. L.; Brungs, W. A.; Cumming, R. B., Eds.; Ann Arbor Science: Ann Arbor, MI, 1980; Vol. 2, p 85. (21) Moye, C. J. J . Chem. Soc., Chem. Commun. 1967, 196. (22) Zincke, T.; Rubinowitsch, S. Chem. Ber. 1890, 23, 3766. (23) Gierer, J.; Sundholm, L. Sven. Pupperstidn. 1971, 74, 345. (24) Simson, B.; Ayers, J.; Schwab, G.; Galley, M.; Dence, C. Tuppi 1978, 61, 41. (25) Gess, J. M.; Dence, C. Tuppi 1971, 54, 1114. (26) Zincke, T.; Kiister, F. Chem. Ber. 1890, 23, 812. Received for review November 15,1984. Accepted August 6,1985. This work was supported by Nordisk Zndustrifond.
An Interdisciplinary Analysis
Mary 0. Amdur,” Adel F. Sarofim, Matthew Neville, Rlchard J. Quann, John F. McCarthy, John F. Elliott, Hua Fuan Lam, Adrianne E. Rogers, and Mlchael W. Conner Energy Laboratory, Massachusetts Institute of Technology, Cambridge, Massachusetts 02 139
Coal combustion produces both SOzand submicrometer metal oxides that can react to form irritant aerosols with potential effects on health. The overall problem is multifaceted. This article provides an example of the contribution chemical engineers, metallurgists, toxicologists, and pathologists workihg closely together can make. Experimental coal combustion studies defined conditions leading to the formation of submicrometer fume and characterized its composition. Effects of temperature and humidity during mixing on the reaction products of SO2 and ZnO, which occur in the fume, were determined. These atmospheres produced altered pulmonary function and morphology in guinea pigs. Functional and morphologic changes were correlated. Introduction
Reports outlining research needs on health effects of fossil fuel combustion (1,2)identify a variety of gaps in 138
Environ. Sci. Technol., Vol. 20, No. 2, 1986
our knowledge that can be filled only by the integration of data from several disciplines. One major gap is in knowledge of the interaction of inorganic particles and SOz produced by coal combustion. The submicrometer fraction of the combustion aerosol is of particular importance. Fine particles, which can penetrate deep into the lung, are the most difficult to remove from combustion effluents and hence are likely to be emitted to the atmosphere where they have a long residence time. They are enriched in trace metals that can react with SOz to produce irritant aerosols. To delineate potential health effects of coal combustion, data are needed on mechanisms leading to the formation of submicrometerfume, on how combustion conditions and coal characteristics affect composition and size of particles in the fume, and on forms in which sulfur-containing particles are emitted to the atmosphere. Data are also needed on the interaction of SO2 with individual metal oxides present in the fume. Little is known of the nature of particulate sulfur-containing species
0013-936X/86/0920-0138$0 1.50/0
0 1986 American Chemical Society
formed under various conditions of temperature and humidity during mixing. Pure metal oxides generated in laboratory furnaces can be mixed with SO2 under independently controlled conditions of temperature and humidity. The furnaces are suitable generation systems for experimental inhalation exposure chambers. Data on the toxicological response of animals to metal oxide-S02 combinations can be interpreted rationally only when the chemical nature of the interaction products is known. Aerosols used in earlier animal studies of SO2-aerosol interaction were formed by nebulization of aqueous solutions of soluble salts. All chemical reactions occurred at ambient temperatures. Although neither the nature of the particles nor the temperature history of their formation and mixing with SO2 was analogous to combustion conditions, these background data provided guidance for planning studies reported here. The Amdur-Mead technique (3) for measuring mechanical behavior of the lungs in guinea pigs is a sensitive tool for evaluation of respiratory tract irritants. These materials cause constriction of the airways; the resultant increase in resistance to airflow can be measured quantitatively. When a variety of aerosols, which alone do not increase resistance, are added to SO2the response is greater than that produced by the same concentration of SOz alone. Potentiation is the toxicological term used to describe this greater response. Data so obtained provide some insight into the physical and chemical factors that affect the degree of biological response to gas-aerosol mixtures. The formation of irritant, sulfur-containing aerosols is the mechanism underlying the potentiation of SO2 by particles. Studies using aerosols of sodium chloride demonstrated that a decrease in particle size (4),an increase in aerosol concentration (4), or increased relative humidity (5) can augment the degree of potentiation of the irritant response to SO2. Aerosols of soluble salts of manganese, vanadium, and ferrous iron (6) gave a strong potentiation because they promoted the conversion of SO2to sulfuric acid (7,8). The amount of sulfuric acid present on the aerosols of these salts was measured (8). Guinea pigs exposed to that amount of sulfuric acid added to SO2 showed the same increase in airway resistance as that produced by the SO2-aerosol mixtures (9). The response to SOz gas alone returns rapidly to preexposure values, but SO2-areosol mixtures resemble irritant aerosols, such as sulfuric acid or irritant sulfates, in that the pulmonary flow resistance remains elevated for an hour or more after the end of a 1-h exposure (4). The magnitude of this prolonged response is related to the total dose of aerosol (6). We chose ZnO as the prototype metal oxide aerosol for our initial toxicological studies. Zinc is enriched in the submicrometer fraction of field-collected samples of fly ash (IO)and on the surface of atmospheric particles (11). Our experimental coal combustion studies indicated an enrichment of the submicrometer-size fume in Zn. ZnO interacts with SOz,and humidity plays a role in the nature of the reaction (12,13). Zinc sulfate, a possible product of the reaction, is an irritant (14). We examined the effect of temperature and humidity on the reaction products of ZnO and SOz and correlated these data with toxicological responses evaluated by several criteria. In this article we present data that (1)indicate a possible mechanism for the formation of the submicrometer fume related to combustion conditions and coal composition, (2) delineate the nature of the sulfur reaction products and validate the relevance of our model furnaces to coal com-
VOLATILE INORGANIC INTERNAL MINERAL
[No
HETEROGENEOUS CONDENSATION OR
Zn,SOx]
REDUCING
CHAR PARTICLE COAL PARTICLE IIOprnI
DURING COMBUSTION
-
DECREASING
CHAR BURNOUT
RESIDUAL FLY ASH
(l-zoprn)
TEMPERATURE c=3
Flgure 1. Schematic representation of events during combustion of pulverized coal particles.
bustion processes, and (3) correlate the chemical and toxicological interaction of ZnO and SO2. Our criteria of toxicological response permit correlation of functional impairment and morphologic damage to lungs of test animals. Taken together, our data indicate the potential of an integrated interdisciplinary approach to the investigation of health-related effects of burning fossil fuels.
Submicrometer Aerosols Produced during Coal Combustion Fly ash generated by combustion of pulverized coal in both laboratory (15,16) and field (17,18) studies exhibits a bimodal size distribution. Most of the ash is in the 1-20-pm range; a small amount, approximately 1% ' by mass, is in the submicrometer range. We have characterized the submicrometer aerosol with particular emphasis on the composition of surface layers, which have the greatest potential for interaction with sulfur oxides. The two size modes are formed by different mechanisms. This can be appreciated by a consideration of the history of a burning coal particle, shown schematically in Figure 1. The mineral matter associated with a coal particle is composed of micrometer-size crystallites of common minerals (e.g., clays, carbonates, and pyrites) embedded in the carbonaceous matrix and atomically dispersed elements chemically bound to carboxylic or other functional groups in the coal. As coal particles burn, included mineral matter decomposes, fuses, and adheres to the carbonaceous surface. The particles are drawn together and coalesce as the carbon surface recedes and yield primarily residual ash particles in the size range of 1-20 pm. In parallel with this process, a small portion of the mineral matter is vaporized and condenses to form submicrometer particles. In the boiler of an electric power generating plant these particles grow to about 0.05 pm, distinctly smaller than the smallest residual ash particles. Because the two modes of the size distribution are formed by different mechanisms, they differ in composition (15-1 7). The composition of the submicrometer aerosol and its dependence on coal type (16,19) have been studied systematically in a laboratory furnace fully described elsewhere (19). Pulverized coal of a size representing the mean diameter used in commercial units (50 pm) was burned in 20% oxygen at a furnace temperature of 1750 K, yielding peak particle temperatures of about 2200 K. The conditions and percentage of ash in the submicrometer aerosol (0.2-2.2%) were similar to those found in large boilers (18). Gaseous and solid products of combustion were withdrawn and quenched in a collection probe. An Andersen cascade impactor was positioned immediately downstream of the collection probe for recovery and on-line classification of ash particles (16,19). The submicrometer aerosol particles passed through the impactor and either were collected on 0.2-pm Fluoropore membrane filters for purposes of mass determination and neutron activation Environ. Scl. Technol., Vol. 20, No. 2, 1986
139
Table I. Amount and Composition of Submicrometer Aerosol
coala
Illinois No. 6 Montana lignite bituminous
A1203
NazO pZo6
Mn
v
Cr Zn
co As cs
1.6 28.7 0.6 1.1
57.2 1.5 10.8
2.8 3470 1880 962 3900 2350 6490 63
'Burned in 20% Os:furnace temDerature. 1750 K.
analysis or on silver membrane filters for Auger spectroscopy and electron spectroscopy for chemical analysis (ESCA) or were deposited on transmission electron microscope grids using a TSI electrostatic precipitator. In some experiments, the submicrometer aerosol was classified into four size ranges using a TSI mobility classifier. The composition of each size cut was determined by neutron activation analysis. The adequacy of the sampling technique was verified by closure of material balances on the ash constituents (19). The amount and composition of the submicrometer aerosol produced by a Western lignite and two Eastern bituminous coals are shown in Table I. The dominant elements are Mg, Ca, and Fe for the lignite and Si, Fe, and Na for the bituminous coals, reflecting the differences in the composition of the mineral matter in coals of different origin. Organically bound Mg and Ca in the Western lignite are responsible for the high concentrations of these elements found in the submicrometer aerosols. The volatization of the major elements is augmented by the chemical reduction of their stable refractory oxides to the more volatile suboxides, e.g., SiO, or metal, e.g., Mg. This occurs because the oxides are exposed to a locally reducing atmosphere within the coal particle. When the reduced vapors diffuse out and away from the burning coal particles, they encounter a greater oxygen potential, reoxidize, and nucleate to form fine particles that subsequently grow by collision and coalescence (20). Particle size is wellpredicted by coagulation theory (16,20). Metals such as As, Sb, Se, and Zn, which are also volatile at combustion temperatures, vaporize in parallel with the refractory oxides. These elements and their salts will condense out at lower temperatures on surfaces of either larger residual ash particles or of the submicrometer aerosol (21). The deposition of volatile species will be distributed between the submicrometer aerosol and the residual ash in amounts that depend on relative surface area and kinetics of deposition (21). The amounts of volatile elements in the submicrometer aerosol generally correlate well with the concentrations of the element in the parent coal. For example, the Zn in the submicrometer aerosol formed by the combustion of 14 coals is approximately 20% of the Zn in the parent coal. 140
Environ. Sci. Technol., Vol. 20, No. 2, 1986
DIAMETER 20
nm 10
4
-n
c,
Major Oxides, wt. % 3.6 48.7 58.4 3.3 10.8 1.6 12.1 33.8 0.6 1.3 5.7 11.4 6.8 6.7 Trace Elements, ppm 9730 3710 100 2330 360 3490 1140 17100 140 130 375 834 29 110
40
Alabama Rosa bituminous
Amount in Total Ash, % 2.9 0.9 SiOz MgO CaO FeO
1021
z
: : $ loia I-z
W U
z
0 U
- Na v - zn x 1 0 ' A
YV A
f 01001 0.00
0.03
0.06
AS
XlO'
Sb X l O '
0.09
i 0.12
INVERSE DIAMETER nm-' Flgure 2. Distribution of elements in four size fractions of submicron aerosol separated with mobility classifler.
Compounds that condense late concentrate on the surface layers of particles, and the concentrations increase with decreasing particle size. These two effects have been found previously (10, 11) in field studies of residual ash particles. More recent studies on field-collected ash have determined both the anionic and cationic composition of the surface layers (22)and have shown the concentration of sulfur at the ash surfaces (23)primarily in the form of sulfates (22). Our studies show that the surface enrichment behavior extends to the submicrometer aerosol. The results of Auger spectroscopy on the submicrometer aerosols produced from a Montana lignite (20)showed that the concentrations of Na, K, and S decrease as the surface layers are removed by sputtering with an ion beam, whereas Fe and Mg increase. These data imply that the submicrometer aerosol consists of a core, the refractory metal oxides precipitated in the high-temperature combustion of the furnace, coated by the more volatile species that condense only in the downstream cooler zones of the furnace. Neutron activation analysis of four narrow-size cuts of the aerosol obtained by use of the mobility classifier provides supporting evidence as shown in Figure 2. Oxides of Fe, Ca, and Mg, which are precipitated first and form the core of the aerosol particles, show no strong size dependence. The volatile elements Na, As, Zn, and Sb show a concentration proportional to the reciprocal of particle diameter, as would be predicted (21)from a mass transfer controlled deposition rate in the free molecular regime (the aerosol particle diameter is significantly smaller than a mean free path of about 0.6 pm for the condensing vapors at combustion temperatures). The significance of the results for potential health effects is that the outer surface of the aerosol particles is composed primarily of the volatile species, predominantly the alkali metals and sulfur. Of the trace elements, Zn is particularly important, accounting for as much as 1.7% by weight of the total aerosol mass and a higher concentration in the surface coating. SO, is the sulfur oxide favored thermodynamically at combustion temperatures. Although some SOs is formed in the flame zone by the reaction of supraequilibrium concentrations of atomic oxygen with SO2,most of the SO3 is produced at the lower temperatures in the convective
ambient surroundings (26). A similar behavior has also been observed for fly ash collected in the field (27). The field studies were motivated by the measurement of resistivities of fly ash for purposes of evaluating the performance of electrostatic precipitators. It was concluded that t@euse of recovered or washed fly ash did not provide valid results because a sulfuric acid coating, which greatly reduces the resistivity of particles, does not survive collection and storage. For similar reasons, resuspension of collected, stored fly ash particles would be unsuitable for the dethion of potential irritant effeects related to sulfuric acid; as will be indicated later, we find the layer of sulfuric acid to be the toxicologically important factor. It is imperative therefore that particles for inhalation studies be freshly generated and exposed to SO2 and water vapor at temperatures such that the surface-catalyzed production of sulfuric acid can be simulated. The following section describes a generation system that meets these specifications.
45-
4.01
Time (sec)
Flgure 3. Change in pH produced by addition of freshly formed ash from combustion of 1 g of Illlnols No. 6 coal (4.5% S) to 200 mL of water.
passes of a furnace where its formation is favored thermodynamically. The homogeneous oxidation of SO2 to SO3 is slow, and the rate of SO3 formation is therefore strongly dependent upon surface reactions as will be demonstrated in the next section on the system involving ZnO particles. Results of ESCA analysis indicate that most of the sulfur is present as sulfate. Some of the SO3 formed will react with the vaporized alkali metals to form Na2SOl and K2S04. Some will form sulfuric acid, which can either homogeneously nucleate to form an acid mist or condense on the surface of the aerosol. From studies of the change in size of the humidified aerosols made with a mobility chromatograph (24) it was determined that the sulfuric acid was present either as ash-free particles smaller than the submicron ash or as a surface coating on the submicrometer ash. The amount of sulfuric acid formed is found to be strongly dependent on coal type and temperature history of particles in a furnace. For six coals studied, the amount of sulfuric acid is found to range from negligible amounts for a Montana lignite with a high alkali content to an amount equivalent to 9% of the sulfur content of the cod, these variations are attributed to differences in the ability of the ash produced to both catalyze the oxidation of SO2 and neutralize the resulting acid. The amount of sulfuric acid was also found to be strongly dependent upon the postcombustion conditions. The highest acid production was observed when the furnace temperature was 1000 K. This is consistent with the observation by Wickert (25) that the catalysis of SO2 to SO3 by a coal ash passes through a maximum at about 1000 K. A measure of acid formation is provided by the change in pH when freshly collected areosol is suspended in a known amount of water. Resulta of adding freshly formed ash from the combustion of 1g of an Illinois No. 6 coal (containing 4.5% by weight sulfur) to 200 mL of water is shown in Figure 3. The pH drops rapidly as the acid on the particle surface is dissolved, followed by a slower neutralization of the acid by the basic oxides (CaO and MgO) in the core of the particles. Analogous observations have been made for a copper smelter that was found to have a sulfuric acid content corresponding to about 2.6% of the total sulfer emissions at the stack but which decreased in the plume as a consequence of neutralization of the acid by particles entrained into the plume from the
Interaction of ZnO and SO2 We studied also the interaction of pure metal oxides with SO2. Information is needed about the effect of specific metals on the conversion of SO2 to sulfuric acid and the variables that affect such reactions. As the initial metal oxide we chose ZnO, which is present in high concentrations on the surface of the ultrafine aerosols formed in our coal combustion studies as well as in the submicrometer fraction and on the surface of field-collected samples. The interaction of ZnO and SO2 in the presence of oxygen and water vapor is expected to form zinc sulfite, zinc sulfate, or because of the surface catalyzed oxidation of SO2 to SO3, sulfuric acid. The parameters that favor the formation of the different products were investigated in a small laboratory furnace designed to produce ZnO and mix it with SO2 and water vapor under carefully controlled conditions. The theory and operation of the furnace used to generate ZnO aerosol have been described elsewhere (28). Powdered Zn is heated in a crucible to approximately 500 "C. The Zn vapor is carried into the reaction zone by a stream of deoxygenated purified argon where it is contacted downstream in the furnace by purified air. Zn is oxidized to form a supersaturated ZnO, which condenses forming an aerosol. The effluent from the furnace is mixed with cool, humidity-controlled air to provide the atmosphere for a dynamic animal exposure chamber. The apparatus is designed so that SO2 and H 2 0 separately can be added either upstream of the furnace, in which case contact with ZnO occurs at 500 "C, or to cool diluting air where contact occurs at 24 "C. The PHz0was 9.1 X and 2.4 X atm for the conditions of 30 and 80% chamber relative humidities, respectively. Variation in operating conditions of the furnace produced ZnO particles of differing shapes (28). The conditions used here produced spherical particles that tended to form chain aggregates. The size distribution of the aerosol was determined by collection on precoated carbon grids with an electrostatic precipitator, followed by electron microscopic examination. The mean particle size, expressed as projected area diameter of the aggregates, was 0.05 pm with a geometric standard deviation of 2.0. The particles are in the size range of the ultrafine particles produced during coal combustion. The chemical state of the sulfur associated with the aerosol was determined by ESCA, using methods similar to those used to characterize atmospheric aerosols (29). Without addition of water vapor to the system, no sulfur Environ. Sci. Technol., Vol. 20, No. 2, 1986
141
\
I 0 . 1
17i7 1697 1672 Binding Energy, eV c(
c(cI
so, so; so;, Flgure 4. ESCA spectra of SO,-ZnO reaction products: (I) from sample generated by mixlng at 500 O C with water vapor added to the furnace; (11) from sample generated by mixing at 500 O C but with water vapor added at 24 O C .
was detected on the ZnO aerosol. Representative ESCA spectra of particles collected on a silver membrane and stored in an argon atmosphere are shown in Figure 4 for two experimental conditions in which water vapor was added either at furnace temperature or at room temperature. Spectrum I is from particles generated in a hot zone of the furnace (500 "C) when both the SO2 and water vapor were introduced upstream of the furnace. Three oxidized sulfur species are present. The major peak is centered at 169.7 eV and corresponds to SO-.: Two shoulders are also observed, one at a lower binding energy of 167.2 eV, attributable to S032-and one at a higher oxidation state of 172.7 eV, which may correspond to SO3 adsorbed on the particle surface. Spectrum I1 was from samples prepared by admitting filtered humidified air (25 "C) to the furnace exit stream, which contained ZnO and SO2. The resultant chamber relative humidity was 80%. The final concentration of SO2 was 1.5 ppm. The center of the spectrum's intensity shifted to the lower binding energies and is located at 167.2 eV, which corresponds to Sot-. A slight shoulder observed at 169.7 eV correlates with the location of S042-. It is well-known that the heterogeneous oxidation of SO2 and SO3 over catalysts such as vanadium pentoxide increases with increasing temperature at these temperature levels. We attribute the formation of S042-in our system to oxidation on the ZnO surfaces, since the homogeneous gas phase oxidation of SO2 to SO3is slow and we anticipate a similar type of temperature-dependent rate-controlling mechanism. The SO3formed can react with water vapor to form sulfuric acid or with the ZnO to form ZnS04. In parallel with these reactions SO2may also react with ZnO to form ZnS03, but the presence of water vapor was essential for the reaction, i.e., with low water vapor concentration little or no ZnS03 was formed. Lowell et al. (30)evaluated 47 metal oxides and reported that ZnO efficiently removed SO2from flue gas. At temperatures greater than 200 "C ZnSOl is the predominant species formed, while below this temperature a greater proportion of ZnSO, is formed. This is in good agreement with our findings. Much of the literature on SO2-to-SO3conversion is for dry systems of interest in the commercial process for sulfuric acid production. The strong influence of water vapor on this reaction is consistent with studies (13) on the rate of removal of SO2 from a gas as it passed over solids selected to represent urban aerosols. Humidity was also found to influence the sulfate/sulfite ratio in an aerosol produced by mixing a submicrometer aerosol of Fe203with a humidified S02/air mixture that had been 142
Environ. Sci. Technol., Vol. 20, No. 2, 1986
5 5-
50' 100
' '"l"'i
IO'
'
' " 1 1 1 "
IO2
' '
"1111'
10)
' ''t''J
IO'
Time (sec)
Figure 5. Drop in pH produced by suspension of ZnO particles generated wlth addltion of SO2 and water vapor to furnace.
passed over a vanadium pentoxide-alumina catalyst bed maintained at 400 OC (30). We sputtered samples showing strong SO?- signals with an argon ion beam to remove surface layers. After sputtering, all traces of sulfur disappeared leaving only the spectrum of a clean, virtually sulfur-free ZnO surface. All of the sulfur was located in the upper 10-20 A of the surface of particles, paralleling the results on the coalgenerated aerosol. As shown in Figure 5, the pH dropped rapidly to 5.8 when ZnO particles generated in the presence of SO2 and water vapor (conditions of spectrum I, Figure 4) were suspended in deionized water. The ZnO particles, like the coal combustion areosols, were coated with a sulfuric acid layer. The pH remained depressed because the ZnO did not act to neutralize the surface layer of acid as occurred with the coal particles. The presence of sulfur species in different chemical states on the surface of particles and the importance of surface reactions resemble data on atmospheric aerosols; sulfate and sulfite species have been associated with soot particles and atmospheric aerosols (29). Aerosols produced photochemically in the presence of various concentrations of SO2contained sulfate as well as chemisorbed SO2and SO3 (32). Catalytic oxidation of SO2 to sulfate on the surface of soot and graphite and the promotion of this conversion by water vapor (33)have been reported. As will be indicated in the next section, the chemical state of the sulfur present on the surface of the ZnO particles determines the magnitude and duration of the irritant response observed in our exposed animals.
Toxicological Evaluation The initial assessment of irritant response used the Amdur-Mead technique for measuring mechanics of respiration in unanesthetized guinea pigs (2). Airway resistance, the resistance to flow of gas in and out of the lungs, is measured. Irritants such as SO2and sulfuric acid cause a constriction of bronchial smooth muscle that results in a measurable increase in airway resistance that is related to the dose of irritant. Measurements are made every 5 min during a 30-min preexposure control, a 1-h exposure, and a 1-h postexposure period. Sufficient data are available from exposures to nonirritant materials, such as sodium chloride aerosol ( 4 ) or humidified air (34),to indicate that the physiological preparation per se is stable over the time periods used.
UExposure
Mix 24°C RH 30%
48OoC RH 30%
Mix
Mix 480 "C RH 80%
Mix Humid 480°C RH 30%
i
4 N.20
N=19
N=IO
NE8
Figure 6. Changes in alrway resistance produced by 1 ppm SO2 and 1-2 mg/m3 ZnO mixed under different conditions of temperature and humidity. Exposure and postexposure periods were 1 h. N is the number of animals per group. An asterisk indicates statistically significant difference.
We examined the response to combinations of 1 ppm SO2and 1-2 mg/m3 ZnO mixed under different conditions of temperature and humidity. The complete respiratory data have been reported elsewhere (34). Exposure to 1 ppm SO2alone produces an increase in airway resistance of l0-12% above control values. This response is readily reversible by 1-h postexposure. The question being asked is therefore whether the addition of ZnO, which alone does not cause elevated airway resistance (35),has altered the response expected from SOz exposure in magnitude and/or duration. The increase in airway resistance produced by the 1-h exposure and the residual response 1 h after exposure are shown in Figure 6. SO2and ZnO were mixed at chamber temperature (24 "C) or at furnace temperature (480 OC) without the addition of water vapor to the system. The relative humidity in the chamber was 30%. The resistance increases of 8% for mixing at room temperature and 12% for mixing at furnace temperature returned to preexposure values by an hour after the end of exposure. A readily reversible increase of this magnitude is the anticipated response to 1 ppm sulfur dioxide (34). The animal data thus give no evidence that a significant amount of irritant sulfur aerosol was formed under these conditions of mixing and are in agreement with our ESCA analysis, which gave no indication of sulfur on the ZnO aerosol under dry conditions of mixing. SO2 and ZnO were next mixed in the furnace with water vapor added to the diluting air to increase the chamber relative humidity to 80% at 24 OC. Guinea pigs exposed to the mixture showed a resistance increase of 29%,which indicates the formation of a different, more irritant material. An increase of this order of magnitude is not produced by 1 ppm SOz, and ZnO alone does not increase resistance (35). The resistance returned to control values during the postexposure hour. The rapid reversibility suggested that the irritant material was not sulfuric acid or zinc sulfate because the increased resistance caused by these materials continues during the postexposure hour (14,36).Our animal data are in agreement with our ESCA data (Figure 4, spectrum 11),which indicate that there was sulfur present on the zinc oxide aerosol and that its predominant form was sulfite not sulfate. When water vapor was added to the furnace during mixing, the animal exposure data again indicated the formation of a new irritant. The response, however, differed from that observed when water vapor was added at 24 OC. The resistance increased and remained elevated during the postexposure hour. The residual response could be explained if SO2had been converted on the aerosol to
sulfuric acid and/or zinc sulfate. Our ESCA data (Figure 4, spectrum I) showed the predominant sulfur species under these conditions of mixing to be 502- and indicated that some 50-: and perhaps adsorbed SO3 were also present. The pH decrease produced when the aerosol was added to water (Figure 5) indicated the presence of sulfuric acid. The animal data are again consistent with chemical data. Because of the rapid reversibility of the response to exposure systems shown to contain predominantly sulfite, no further studies of these atmospheres were done. Studies using additional measurements of pulmonary function following exposure were made on animals exposed to atmospheres shown to contain sulfuric acid coated particles. To examine further the prolonged response to this mixture, we measured various lung volumes and carbon monoxide diffusion capacities (DLCO) in guinea pigs after 3-h exposures to 1 ppm SO2and 1,3, or 6 mg/m3 ZnO with water vapor present in the furnace. Groups of animals exposed for 3 h to purified air serve as controls. Measurements were completed within 2 h after exposure. Detailed methodology and results are reported elsewhere (37). Reductions in the various compartments of lung volume and in DLco provide indicators of impaired pulmonary function. These parameters were not affected by exposure to either 1 ppm SOz or 6 mg/m3 ZnO alone. Compared to air, the SO2-ZnO mixture produced a decrease in lung volumes that was related to the concentration of ZnO. At 1 mg/m3 ZnO, only vital capacity was reduced. At 3 mg/m3 functional residual capacity and residual volume also decreased; at 6 mg/m3 all lung volumes, including total lung capacity, decreased. SO2-ZnO mixtures caused a decrease in DLco that was related to the concentration of ZnO. Values were 5, 20, or 55% below control values after exposure to 1, 3, or 6 mg/m3 of ZnO, respectively. A statistically significant reduction in alveolar volume occurred only at 6 mg/m3 ZnO. The diffusing capacity of lungs provides an index o$ the dimensions of the pulmonary capillary bed and the integrity of the alveolar-capillary membrane. A decrease in DLco indicates an impediment to the transfer of oxygen from inspired air to arterial blood. In association with reduced alveolar volume, the decrease in DLco suggests a restriction defect, probably involving a loss of capillary bed as well as of alveoli. These changes in conjunction with decreased lung volumes, as observed in these experiments, indicate maldistribution of ventilation due to alveolar duct constriction. Functional changes produced by 3-h exposures to 1 ppm SO2and 5 mg/m3 ZnO were slow to reverse. As indicated in Figure 7, total lung capacity remained depressed for 24 h and vital capacity and functional residual capacity for 48 h. All lung volumes had returned to control values 72 h after exposure. DLco was still below control values 72 h after exposure. Lung weight/body weight ratios were above normal only at 24 h after exposure. To identify structural abnormalities that might correlate with the functional changes, morphologic studies, measurement of respiratory tract permeability to a large molecule (horseradish peroxidase, HRP),and labeling of epithelial cells with 3H-thymidine to evaluate damage and repair were performed. Detailed results are reported elsewhere (38). In the lungs of guinea pigs exposed for 3 h to 5 mg/m3 ZnO and 1 ppm SO2 mixed in a humid furnace there was distension of the perivascular and peribronchial connective tissues, which was interpreted as pulmonary edema and Environ. Sci. Technol., Vol. 20, No. 2, 1986
143
Table 11. Response to 1 ppm SO2 plus ZnO-3-h Exposure 1
'"A-4
ZnO, mg/ma DLco, %' horseradish peroxidase, %" angiotensin converting enzyme, %"
I
2.5
-25 +62 +20
5 -54 +146 +58
'% difference from air controls. 0
12
24
36
40
60 72
- 0
Time oiler exposure, hours
Figure 7. Postexposure alterations in total lung capacity (TLC), vltai capacity (VC), functional residual capacity (FRC), and CO diffusing capacity (Dk,) produced by a single 3-h exposure to 1 ppm SO2 and 5 mg/m3 ZnO mixed In a humid furnace. Control animals (16) were exposed for 3 h to air. Other group sizes included the followlng: 1 h, 8; 24 h, 10; 48 h, 8; 72 h, 7. An asterlsk indicates statistically significant difference.
1 I70
2
160
c
c
s 140
L
0
8 120 100 Lung w l / B o d y wt
Bronchus
Terminal Bronchiole
Alveolus
Figure 8. Postexposure alterations in lung weightlbody weight ratio and In localized labeling with 3H-thymldine following a single 3-h exposure to 1 ppm SO2 and 25 mg/m3 ZnO mixed in a humid furnace. Controls were exposed for 3 h to air. Eight animals were used per group.
was confirmed by electron micrographs and an increase in lung weight. The alveolar interstitium also appeared distended (38). The changes were present in 83% of animals studied immediately or 12 h after exposure but were not seen by 48 h after exposure. Morphology of tracheal and bronchial epithelium and numbers of secretory cells were normal. There was a significant increase in the permeability of the respiratory tract to HRP 1h after exposure. By 24 h the permeability had returned to normal. The 3H-thymidinelabeling index was twice normal in bronchiolar epithelium 48 h after exposure. These findings support and help explain functional changes after exposure to the same atmospheres. As noted above, lung volumes and DLco were still depressed at 48 h. To examine the effect of increased aerosol concentration, animals were exposed for 3 h to 25 mg/m3 ZnO and 1ppm SO2. The abnormalities were qualitatively similar but much more marked. Morphological studies showed prolonged edema and inflammation, which progressed in severity from 24 to 48 h with extension of infiltrates from the peribronchial region to the alveolar ducts and avleoli. As shown in Figure 8 the lung weight/body weight ratios continued to increase from 24 to 48 h; the epithelial labeling index peaked in the terminal bronchioles at 24 h 144
Environ. Sci. Technol., Vol. 20, No. 2, 1986
but rose progressively in the alveoli from 24 to 48 h. These observations are consistent with the time course of the morphologic changes. The edema may be the result of the release of vasoactive mediators from pulmonary mast cells or alteration of sympathetic tone exerted on small vessels or direct damage to endothelial cells (39). One measure of endothelial cell damage is an elevation of angiotensin converting enzyme in the plasma. Our observation of such increases in exposed animals suggested that endothelial cell damage is involved as one mechanism of edema production. Damage to airway epithelial cells was indicated by the morphologic, autoradiographic, and HRP studies. Altered permeability of the damaged cells or release by them or by inflammatory cells of enzymes or chemical mediators may have contributed to the edema. The susceptibility of terminal and respiratory bronchioles to damage by SO2-ZnO mixtures appears similar to their sensitivity to damage by NO2, 03, and cadmium salts (40-42). Single 3-h exposures to 1 ppm SO2 and 5 mg/m3 ZnO produced transient, but functionally important, alterations in the lung. Threshold limit values for occupational exposure are 2 ppm SO2 and 5 mg/m3 ZnO. Our concentrations are thus not unrealistic but fall in the range within which exposure of man occurs. At a constant SO2 concentration of 1ppm the observed response increases with increasing concentrations of ZnO. Table I1 shows the alterations in DLco, permeability of the epithelium to horseradish peroxidase, and plasma angiotensin converting enzyme resulting from damage to endothelial cells produced by 1 ppm SO2plus either 2.5 or 5 mg/m3 ZnO. Doubling the aerosol concentration approximately doubles the response. This is in agreement with earlier animal studies ( 4 , 6 ) . When DLc, was measured in animals exposed for 3 h to 0.5 ppm and 5 mg/m3 ZnO the value was 48% below control, Le., the same order of magnitude as that produced by 1ppm SO2and 5 mg/m3 ZnO. These data are consistent with the mechanism of the SO2-ZnO interaction discussed above. The amount of sulfuric acid formed depends upon the number of active sites available; this is a function of the aerosol concentration. The partial pressure of SOz is not the limiting factor in the reaction. The combined toxicological and chemical data point clearly to the need for control of emission of ultrafine particles. This may be as important as,or perhaps even more important than the control of SO2 per se in efforts to reduce atmospheric sulfuric acid. Our pH measurements suggest a sulfuric acid concentration of about 40 hg/m3. The fact that it is present as a surface layer on ultrafine particles that deposit in the alveolar region of the lungs accounts for the magnitude of the changes produced by this very low concentration. The presence of such a surface layer of sulfuric acid on our ultrafine coal combustion aerosols makes our toxicological results of practical significance. Registry No. SiOz,7631-86-9;MgO, 1309-48-4;CaO, 1305-78-8; FeO, 1345-25-1;A1203, 1344-28-1;Na20,1313-59-3;P&, 1314-56-3; H2S04,7664-93-9; ZnS04, 7733-02-0; ZnS03, 13597-44-9; Mn, 7439-96-5;V, 7440-62-2; Cr, 7440-47-3;Zn, 7440-66-6;co, 7440-48-4;
Environ. Scl. Technol. 1986, 20, 145-149
(23) Cabaniss, G. E.; Linton, R. W. Environ. Sci. Technol. 1984, 18, 271-275. (24) Liu, B. Y. H.; Pui, D. Y. H.; Whitby, K. T.; Kittelson, D. B.; Kousaka, Y.; McKenzie, R. L. Atmos. Enuiron. 1978, 12, 99-104. (25) Wickert, K. BWK 1959,11, 266-279. (26) Eatough, D. J.; Christensen, J. J.; Eatough, N. L.; Hill, M. W.; Major, T. D.; Mangelson, N. F.; Post, M. E.; Ryder, J. F.; Hansen, L. D.; Meisenheimer, R. G.; Fischer, J. W. Atmos. Environ. 1982,16, 1001-1015. (27) Potter, E. In “Proceedings of the 4th International Clean Air Congress”; Kasuga, S., Ed.; Japanese Union Air Pollution Assoc.: Tokyo, 1977; p 808. (28) McCarthy, J. F.; Yurek, G. J.; Elliott, J. F.; Amdur, M. 0. Am. Znd. Hyg. Assoc. J. 1982, 43, 880-886. (29) Craig, N. L.; Harker, A. B.; Novakov, T. Atmos. Environ. 1974; 8, 15-21. (30) Schlesinger, R. B.; Gurman, J. L.; Chen, L.-C. Atmos. Environ. 1980, 14, 1279-1287. (31) Lowell, P. S.; Schwitzgebel, K.; Parsons, T. B.; Sladek, K. J. Znd. Eng. Chem. Process Des. Dev. 1971, 10, 384-390. (32) Clark, W. E.; Landis, D. A.; Harker, A. B. Atmos. Environ. 1976,10,637-644. (33) Novakov, T.; Chang, S. G.; Harker, A. B. Science 1974,186, 259-261. (34) Amdur, M. 0.;McCarthy, J. F.; Gill, M. W. Am. Znd. Hyg. ASSOC.J. 1983, 44, 7-13. (35) Amdur, M. 0.;McCarthy, J. F.; Gill, M. W. Am. Znd. Hyg. ASSOC.J. 1982, 43, 887-889. (36) Amdur, M. 0.;Dubriel, M.; Creasia, D. A. Enuiron. Res. 1978,15, 418-423. (37) Lam, H. F.; Peisch, R.; Amdur, M. 0. Toxicol. Appl. Pharmacol. 1982, 66, 427-433. (38) Conner, M. W.; Rogers, A. E.; Amdur, M. 0. Toxicol. Appl. Pharmacol. 1982,66, 434-442. (39) Cross, C. E.; Parsons, G. H.; Gorin, A. B.; Last, J. A. In “Mechanisms in Respiratory Toxicology”; Witschi, H., Nettesheim, P., Eds.; C.R.C.: Boca Raton, FL, 1982; pp 219-246. (40) Castleman, W. L.; Dungworth, D. L.; Schwa&, L. W.; Tyler, W. S. Am. J. Pathol. 1980, 98, 811-840. (41) Hayes, J. A.; Snider, G. L.; Palmer, K. C. Am. Rev. Respir. Dis. 1976,113, 121-130. (42) Stephens, R. J.; Freeman, G.; Crane, S. C.; Furiosi, N. J. Exp. Mol. Pathol. 1971, 14, 1-19.
As, 7440-38-2; Cs, 7440-46-2; angiotensin converting enzyme, 9015-82-1; horseradish peroxidase, 62628-26-6.
Literature Cited (1) Comar, C. L.; Nelson, N. EHP, Environ. Health Perspect. 1975,12, 149-170. (2) National Research Council, Committee on Research Needs on Health Effects of Fossil Fuel Combustion Products, Final Report; National Academy of Sciences: Washington, D.C., 1980; pp 1-73. (3) Amdur, M. 0.;Mead, J. Am. J.Physiol. 1958,192,364-368. (4) Amdur, M. 0. In “Inhaled Particles and Vapors”; Davies, C. N., Ed.; Pergamon: Oxford, 1961; pp 281-292. (5) McJilton, C.; Frank, N. R.; Charlson, R. E. Science 1973, 182, 503-504. (6) Amdur, M. 0.;Underhill, D. W. Arch. Enuiron. Health 1968, 16,460-468. (7) Johnstone, H. F. In “Inhaled Particles and Vapors”; Davies, C. N., Ed.; Pergamon: Oxford, 1961; pp 95-108. (8) Amdur, M. 0. “Proceedings of the Conference on Health Effects of Air Pollutants”; National Academy of Sciences, U.S. Government Printing Office: Washington, D.C., 1973; pp 175-205. (9) Amdur, M. 0. Am. Znd. Hyg. Assoc. J. 1974,35,589-597. (10) Davidson, R. L.; Natusch, D. F. S.; Wallace, J. R.; Evans, C. A. Environ. Sci. Technol. 1974,8, 1107-1113. (11) Linton, R. W.; Loh, A.; Natusch, D. F. S.; Evans, C. A.; Williams, P. Science 1976, 191, 852-854. (12) Dyson, W. L.; Quon, J. E. Environ. Sci. Technol. 1976,10, 476-481. (13) Judeikis, H. S.; Stewart, T. B.; Wren, A. G. Atmos. Environ. 1978,12, 1633-1641. (14) Amdur, M. 0.;Corn, M. Am. Ind. Hyg. Assoc. J. 1963,24, 326-333. (15) Flagan, R. C.; Taylor, D. D. In “18th Symposium (International) on Combustion”; The Combustion Institute: Pittsburgh, PA, 1981; pp 1227-1235. (16) Neville, M.; Quann, R. J.; Haynes, B. S.; Sarofim, A. F. In “18th Symposium (International) on Combustion; The Combustion Institute Pittsburgh, PA, 1981; pp 1267-1274. (17) Ondov, J. M.; Ragaini, R. C.; Biermmn, A. H. Environ. Sci. Technol. 1979,13,946-953. (18) McElroy, M. W.; Carr, R. C.; Ensor, D. S.; Markowski, G. R. Science 1982,215,13-19. (19) Quann, R. J.; Neville, M.; Janghorbani, M.; Mims, C.; Sarofim, A. F. Enuiron. Sci. Technol. 1982,16, 776-781. (20) Neville, M.; Sarofim, A. F. In “19th Symposium (International) on Combustion”; The Combustion Institute: Pittsburgh, PA, 1982; pp 1441-1449. (21) Haynes, B. S.; Neville, M.; Quann, R. J.; Sarofim, A. F. J. Colloid Interface Sci. 1982, 87, 266-278. (22) Hansen, L. D.; Silberman, D.; Fisher, G. L.; Eatough, D. J. Environ. Sci. Technol. 1984, 18, 271-275.
Received for review November 26,1984. Accepted July 29,1985. This research was supported by Program Project Grant ES02429 from the National Institute of Environmental Health Sciences and by Grant R8-0910410 from the Environmental Protection Agency; it was begun under Contract RPlll2 from Electric Power Research Institute.
Synthesis of Mutagenic Compounds in Crankcase Oils Mohamed Abdeinasser, Mark Hyland, and Neil D. Jespersen* Chemistry Department, St. John’s University, Jamaica, New York 11439
rn Motor oils become mutagenic after use in internal combustion engines. This work has shown that the major factor involved in the production of these mutagens is nitrogen dioxide. Sulfur dioxide and other gases do not seem to cause the production of mutagens. These results may be related to the mutagenicity of diesel exhaust particulates and some synthetic fuels.
Introduction In 1978 Payne et al. (1) demonstrated that dimethyl sulfoxide (Me,SO) extracts of used crankcase oils were mutagenic. The mutagenic activity reported was not due 0013-936X/86/0920-0145$01.50/0
to benzo[a]pyrene or benzanthracene, both of which are known minor constituents of motor oil (2). A 1980 review of the carcinogenic potential of petroleum hydrocarbons, sponsored by the American Petroleum Institute, stated “The possibility of carcinogenic potential should be thus considered in planning for either the recycling or the disposal of used motor oils” (3). Since Me430 extracts of refined motor oils show no mutagenic activity (1) in the Ames test ( 4 ) and extracts of used motor oils are mutagenic, this work attempts to identify the factor(s) that contribute to the synthesis of these mutagenic substances. To do this, virgin motor oil was subjected to various conditions thought to be present
0 1986 American Chemical Society
Environ. Sci. Technol., Vol. 20, No. 2, 1986 145
