Environ. Sci. Technol. 1985, 19, 935-942
(11) Ware, E. Chem. Rev. 1950, 46, 403-470. (12) Willson, W. G.; Hendrikson, J. G.; Mann, M. D.; Mayer,
G. G.; Olson, E. S. Twelfth Biennial Lignite Symposium, University of North Dakota, Grand Forks, ND, May 1983. Received for review February 21, 1984. Revised manuscript
received November 13,1984. Accepted April 19,1985. This work was supported by the Assistant Secretary for the Environment, Office of Environmental S a f e t y Engineering, Environmental Control Technology Branch, U.S.Department of Energy, and by the Fossil Energy Division through the Morgantown Energy Technology Center, under Contract DE-AC03- 76SF00098.
Ozone-Cyclohexene Reaction in Air: Quantitative Analysis of Particulate Products and the Reaction Mechanism Shiro Hatakeyama,* Takeshi Tanonaka,+ Jlan-hua Weng,s Hiroshi Bandow, Hlroo Takagi, and Hajime Akimoto
Division of Atmospheric Environment, The National Institute for Environmental Studies, P.O. Tsukuba-gakuen, Ibaraki 305, Japan Both gaseous and particulate products of the cyclohexene-ozone reaction were analyzed. Major gaseous products were aldehydes that consist of adipaldehyde (CHO(CH,),CHO), glutaraldehyde (CHO(CH,),CHO), and pentanal (CH,(CH,),CHO). The s u m of the primary yields of aldehydes reaches as high as 50%. In addition to aldehydes, formic acid, CO, and C 0 2 were produced, but formaldehyde was not detected. Main particulate products were adipaldehyde, 6-oxohexanoic acid (CHO(CH2)&OOH), adipic acid (HOOC(CH,),COOH), glutaraldehyde, 5-oxopentanoic acid (CHO(CH,)&OOH), and glutaric acid (HOOC(CH,),COOH). All these compounds were analyzed quantitatively, and the fraction of initial cyclohexene converted to aerosol organic carbon was estimated to be 13 f 3% as the value extrapolated to a ppm concentration range of reactants. Although the reaction mechanism is in general explainable in terms of the Criegee mechanism, the reaction pathway to form formic acid is quite unique in this reaction system. The entire mechanism was discussed on the basis of the quantitative product analysis data.
Introduction Atmospheric oxidation of cycloalkenes has been recognized (1,2) as a major source of difunctional compounds such as dicarboxylic acids, dialdehydes, oxo carboxylic acids, oxo alcohols, etc., which constitute a significant fraction of organic aerosol associated with photochemical smog (3-6). Grosjean and Friedlander (2) studied the photooxidation of cyclohexene-NO,-air and cyclopentene-NO,-air systems in a smog chamber as "model" reactions to yield such organic aerosols and identified difunctional products qualitatively on the basis of mass spectra. The reaction sequences to give the products were proposed in terms of both the ozone reaction and the OH-initiated oxidation. On the other hand, gaseous products of cyclohexene-ozone reactions have been reported early by Vrbaski and Cvetanovic (7) and recently by Niki et al. (8) semiquantitatively. While rate constants of cycloalkene-ozone reactions have been reported (9), quantitative product analysis has not been performed, and the reaction mechanism is not well understood in comparison with that of alkenes. In the present study, gas-phase reactions of cyclohexene and ozone were studied in detail in order to reveal the t Present address: Environmental Pollution Control Center Co. Ltd., Higashikojiya, Tokyo 144,Japan. Present address: Chinese Research Academy of Environmental Sciences, Beijing, China.
*
0013-936X/85/0919-0935$01.50/0
reaction mechanism. Quantitative analyses of gaseous products by Fourier transform infrared spectroscopy (FT-IR) and of aerosol products by GC and gas chromatography/mass spectrometry (GC/MS) all based on authentic samples were made for the first time, and their formation mechanism was assessed.
Experimental Section Reaction and Analyses. Three types of reactors were used (I) an evacuable and bakable smog chamber (6 m3), (11) a cylindrical quartz vessel (11 L) equipped with multireflection mirrors for FT-IR analyses, and (111)Pyrex bulbs (4 L). The detail of the smog chamber has already been reported (10). Briefly, the chamber wall was coated with PFA (tetrafuluoroethylene-perfluoroalkylvinyl ether copolymer) and was temperature controlled at 30 f 1 "C. Multireflection mirrors for FT-IR analyses were equipped inside. Quantitative analyses of gaseous products were made mainly in the smog chamber by means of FT-IR (Block Engineering-JASCO International, FTS-496s; path length, 221.5 m; resolution, 1cm-l; scan times, 64 or 128, -2.5 and 5 min, respectively) with triangular apodization function and no zero filling. The absorptivities (base 10, t o r i 1 m-l at 30 "C) used are as follows. HCOOH, 1.24 (1105 cm-l, peak to valley); pentanal, 0.272 (2715 cm-l, from the base line); glutaraldehyde, 0.584 (2715 cm-l, from the base line); adipaldehyde, 0.576 (2715 cm-', from the base line). The concentrations of CO (2177 cm-l) and CO, (2362 cm-l) were obtained by use of calibration curves that have already been reported from this laboratory (11). Typical initial concentrations of reactants for the smog chamber experiments were 1-2 ppm of cyclohexene and 1.5-3 ppm of ozone. The quartz vessel was used for the runs with 1803,and analyses of gases were carried out by means of FT-IR(path length 40 m; resolution 1cm-l; scan times 32 or 64, 1and 2.5 min, respectively). Gas chromatographic (Shimadzu GC-6A with FID) analyses of gaseous products were also performed by use of 4-L bulbs as reactors. The typical concentration of cyclohexene and ozone employed for this experiment was 180 and 210 ppm, respectively. A 2-m column of Porapack Q was used for analysis at a constant temperature (190 "C). OH radical initiated oxidation of cyclohexenewas carried out in order to check whether the gaseous products produced in ozone-cyclohexene reactions can also be produced in OH-cyclohexene reactions. OH radicals were generated by the photolysis of methyl nitrite (CH,ONO) in air (ex-
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-
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0 1985 American Chemlcal Society
Environ. Sci. Technol., Vol. 19,
No. 10, 1985 935
perimental detail is the same as that reported in our previous paper (12)).
-
CH30N0 + hv CH30 + O2 HOz
+ NO
CH30 + NO
HCHO
-+
OH
+ H02
+ NO2
Photolysis of azomethane (CH3N=NCH3) in the presence of cyclohexene under 1atm of air was also performed to check the contribution of side reactions brought about by alkyl, alkylperoxyl, alkoxyl, and HOz radicals. The experimental condition of photolysis was the same as that for the photolysis of methyl nitrite. Particulate products were analyzed by means of GC (column Chemipack A, 3 m a t 100 “C) and GC/MS (NEVA, Model TE-600; column Silicone OV-101 1.5% on Chromosorb, 5 ft). After the introduction of cyclohexene (13-187 ppm) and ozone (16-224 ppm) in a 4-L bulb, the reaction mixture was allowed to stand for a certain period of time to let aerosols deposit on the wall. Gases left in the bulb were then purged with pure nitrogen, and the bulb was washed with 50 cm3of ether solution of diazomethane. The resulting solution was concentrated to -5 cm3, treated with diazomethane again for complete esterification, diluted to a constant volume, and subjected to GC or GC/ MS analysis. Experiments under 1 atm of pure nitrogen were also carried out. Photolysis of gaseous cyclohexene ozonide was performed in the 11-L quartz cell at 26 f 2 “C by use of six sun lamps (Toshiba FL 20 SE; 260 IX I400 nm, A,, = 310 nm) surrounding the reactor coaxially. The light intensity as measured by the NOz photodissociation rate, kl, was 0.3 min-l. Materials. Cyclohexene, glutaric acid, adipic acid, and pentanal were commercially availabIe from Wako Pure Chemical Industry, Ltd., and used after trap-to-trap distillation. Pure glutaraldehyde was prepared by the extraction with ether from 70% aqueous solution of glutaraldehyde (Wako) followed by the evaporation of the solvent and repeated trap-to-trap distillation. No impurity was detected by gas chromatographic analysis. Diazomethane was prepared by decomposition of N-methyl-Nnitroso-p-toluenesulfonamide (Tokyo Kasei) in carbitol with KOH. Adipaldehyde was prepared by oxidative cleavage of 1,2-cyclohexanediol(Aldrich) with lead tetraacetate (13). 6-Oxohexanoic acid (adipaldehydic acid, CHO(CHJ4COOH) was synthesized by oxidative cleavage of 2-hydroxycyclohexanone (14) (prepared by the hydrolysis of 2-chlorocyclohexanone(15)(Aldrich)) with lead tetraacetate. Methyl 5-oxopentanoate (methyl ester of glutaraldehydic acid, CHO(CH2)3COOCH3) was prepared according to the method described by Huckstep et al. (16). Methyl esters of 5-hydroxypentanoic acid (HO(CHz)4COOH) and 6-hydroxyhexanoicacid (HO(CHz)4COOH)were prepared by the methylation of sodium salts of the acids (17,18)with dimethyl sulfate. 5-Hydroxypentanal (HO(CHZ).&HO) was synthesized according to Schniepp and Geller (19) by the hydrolysis of dihydropyran. It is noteworthy that this compound is reported to exist in a cyclic form (2-hydroxytetrahydropyran)in the proportion of 95% a t equilibrium (19).We found that the gas-phase IR spectrum of the distilled material showed an 0-H stretching band and no carbonyl stretching band. This fact confirms that the compound takes the cyclic form. Isotope-labeled ozone 1803was prepared with le02 (Nippon Sanso; atomic purity -99%) by silent discharge. Cyclohexene ozonide was obtained by passing ozone through hexane solution of cyclohexene a t 10 “C (20). 938
Environ. Sci. Technol., Vol. 19, No. 10, 1985
3000
1O O L
20’00 Wavenurnber/cni’
Flgure 1. I R spectrum of gaseous products of cyclohexene-ozone reaction (A) and I R spectra of aldehydes. (B) Pentanal, (C) glutaraldehyde, and (D)adipaldehyde.
Methyl nitrite and azomethane were prepared according to Hartung and Crossley (21)and Renaud and Leitch (22), respectively, and used after trap-to-trap distillation.
Results and Discussion Gaseous Products and Their Yields. Figure 1A shows the IR spectrum of the products obtained in the cyclohexene (2 ppm)-ozone (1.5 ppm) reaction in the smog chamber. Aldehydes, CO, C02, and HCOOH were observed as gaseous products. Identification of the aldehydes were made by GC analysis of the gaseous products obtained in the Pyrex cell runs. Pentanal (C4H,CHO), glutaraldehyde (CHO(CHZ),CHO),and adipaldehyde (CHO(CHJ,CHO) were identified as gaseous aldehydes. Neither formaldehyde nor acetaldehyde was detected by means of IR spectroscopy. C3 and C4 aldehydes were negligible on the basis of GC analysis. Pentanal was detected for the first time in this reaction system. The yield was determined to be 17.2 f 1.7% of consumed cyclohexene based on the GC analysis. This yield is an average for eight runs in the concentration range 35-175 ppm of cyclohexene in the presence of excess ozone (initial concentration ratio of ozone to cyclohexene was 1.2). No dependence of the yield on initial concentration was observed under the experimental conditions employed. Glutaraldehyde and adipaldehyde were detected both as gaseous and as particulate products by means of GC analysis in the Pyrex cell runs. Since these dialdehydes have much higher vapor pressure than oxo acids and dicarboxylic acids, they are expected to exist mostly in the gas phase in the smog chamber runs a t the ppm concentration range. Parts B-D of Figure 1depict the reference IR spectra of pentanal, glutaraldehyde, and adipaldehyde in the gas phase, respectively. Due to the similarity of the IR spectra for all the mono- and dialdehydes as shown in Figure 1, IR spectra alone do not allow the identification and quantitative analysis of each product aldehyde in this reaction system. The IR absorption coefficients of adipaldehyde (0.576 torr-l m-l) and glutaraldehyde (0.584 torr-’ m-l) for the band a t 2715 cm-l are about twice as large as that of pentanal (0.272 torr-l m-l). Therefore, assuming that an average absorption coefficient of the 2715-cm-l band is 0.28 torr-’ m-l per each -CHO group, we can estimate the total yield of “formyls” in terms of [Apentanal + 2(Aglutaraldehyde) + 2(Aadipaldehyde)]/ (-Acyclohexene) to be 80
-
1
Table I. Molar Yield of Gaseous Products in Cyclohexene-Ozone Reactions in the Smog Chamber
[ c ~ H ~ [~ oI ~ ~ ,I ~ , ppm ppm formyls,0 % HCOOH, % CO, % COz, % 2.00 1.99
1.50 2.25
1.02 1.00 1.00
1.75
94
2.29 2.68
average
17 18 19 18 19 18f2
10 12 13 12 11 12f1
I7 81 72 I7 80 f 17
"Yield of formyls is [Apentanal ( Aadiua1dehvde)l/ (-Acvclohexene).
+
40 39 43
45 42f6
B(Aglutara1dehyde) + 2HCOOH
l
o
20
40
5
60 80 TOO 120 140 160
5
Time / min Flgure 3. Typlcal time profile of reactants and products of cyclohexene (2.0 ppm)-ozone (1.5 ppm) reaction in the smog chamber.
o
io
3a Ti me I min
20
40
50
-
Flgure 2. Time prafile of the yield of gaseous dialdehydes produced in the high concentration runs. [C8H8], = 180 ppm; [O,], = -210 ppm. (0)Glutaraldehyde; (0)adipaldehyde.
f 17 % for the ppm concentration runs as shown in Table I. Since the yield of pentanal is 17.2 f 1.7% as described above, the sum of the yield of glutaraldehyde and adipaldehyde is calculated to be 31 f 9%. Thus, the total yield of gaseous aldehyde is 48 f 10% . On the other hand, GC measurements for higher concentration reactions ([CeHlolo = -180 ppm) revealed that the yields of gaseous glutaraldehyde and adipaldehyde a t a short reaction time were 6.7% and 1.9%,respectively, as shown in Figure 2. Yields of particulate glutaraldehyde and adipaldehyde were 3.7% and 1.5% a t less than 10 min of reaction time as will be discussed later (see Table 11). If we regard OPA (2.3%) and the half of OHA (1.6%) as oxidation products of glutaraldehyde and adipaldehyde (the other half of OHA is regarded as a direct product), respectively, the sum of the total yield of dialdehydes is 18% with the estimated overall error of ca. f 5 % . Thus, the yield of dialdehydes estimated from the FT-IR measurements for the low concentration runs and from the GC measurements for the high concentration runs agrees barely within the error limits. The lower average yield estimated from the results of the higher concentration runs may be due to the loss of dialdehydes during the concentration process of samples. If we assume that the ratio of the primary yield of glutaraldehyde to adipaldehyde, 2.5 f 1.0 as estimated above for the high concentration runs, daes not depend on the reactant concentrations significantly, the yields of gaseous glutaraldehyde and adipaldehyde in the low concentration runs are estimated to be 22 and 9%, respectively. A typical time profile of the reactants and gaseous products for the smog chamber run is shown in Figure 3. The decay of formyls after reaching a maximum in contrast t o the other gaseous products should be due to gae to particle conversion, deposition on the wall, and sequential oxidation of dialdehydes to oxo acids and dicarboxylic acids as will be described later. Table I summarizes the yields of gaseous products in the smog chamber runs a t ppm concentration range. In addition to the products given in Table I, ethylene was detected by GC analysis of
%
6ow 20
100
50
100 B
60 'ID
20
6ow 50
100
50
100
*I* 20
mle
Flgure 4. Mass spectra of authentic samples of adlpaldehyde (A) and methyl esters of B-oxopentanoic acid (B) and 6-oxohexanoic acid (C).
the higher concentration runs, but the yield was less than 2%. Formation of formic acid and ethylene in the cyclohexene-ozone reactions in the gas phase has been reported by Vrbaski and Cvetanovic (7) and Niki et al. (8),although the formation mechanism was not discussed. The latter authors also noted the absence of formaldehyde in the product IR spectrum. These results agree well with those in the present study. Since neither pentanal nor ethylene was detected in OH radical initiated oxidation of cyclohexene,these compounds should be the primary products of ozone-cyclohexene reactions. Particulate Products and Their Yields. Particulate products identified conclusively in the present study are glutaraldehyde, 5-oxopentanoic acid (CHO(CH2)3COOH, hereafter referred to as QPA), glutaric acid (HOOC(CH,),COOH), adipaldehyde, 6-oxohexanoic acid (CHO(CH2)&OOH, hereafter referred to as OHA), adipic acid (HOOC(CH,),COOH), and succinic acid (HOOC(CH2)2Environ. Scl. Technol., Voi. 19, No. 10, 1985
937
Table 11. Yield of Aerosols from Cyclohexene-Ozone Reactions in 1 atm of Airn
min
ppm
total total succ adi OHA, adi molar carbon glu OPA, glu c5 c6 % acid, % total, % ald. % % acid, ?4 total, o/c yield, % conv, 70 PPm acid, % ald, %
2 3 3 5 7 8 10 10 10 20 22 30 30 30 30 60 60 60 60 60 60 60 120 120 180 180 180 980 1320 5760
180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180 180
210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210 210
0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.80 0.96 0.00 1.02 0.00 0.86 1.12 0.41 1.17 0.97 1.21 1.12 1.10 0.90 0.93 1.00 1.00 1.17 1.16 1.24 1.22 1.34 1.17
3.54 3.69 3.21 4.10 3.61 3.81 3.05 2.83 3.08 3.31 2.44 3.87 2.44 2.79 2.46 1.28 1.36 2.60 2.60 2.13 2.28 1.98 1.95 1.93 1.47 1.79 1.50 0.36 0.69 0.50
2.20 1.84 1.78 2.66 2.37 3.04 3.34 3.12 3.40 3.52 3.98 3.66 4.15 4.43 3.76 5.10 4.49 5.10 5.26 4.63 4.75 4.57 5.22 4.71 5.15 5.12 5.01 4.58 4.21 2.92
17.8 17.6 38.4 106.4 106.6 115.0 245.3
21.4 21.4 49.9 129.5 127.9 138.0 294.3
0.00 1.79 1.05 4.22 0.65
0.00 0.00 0.00 1.44 0.00 1.23 1.27
1.92 2.54 2.20 0.35 3.01 4.89 5.57
60 60 60 60 60 60 60
1.85 1.32
0.00 0.00 0.00 0.00 0.00 0.00 1.16 1.69 1.39 1.52 1.76 1.76 1.76 2.43 1.86 4.76 3.07 2.62 2.54 2.64 2.24 2.76 3.04 3.09 3.90 4.09 3.59 5.32 6.34 6.89
5.74 5.53 4.99 6.76 5.98 6.85 7.55 7.64 7.87 8.35 8.18 9.29 8.35 9.65 8.08 11.14 8.92 10.32 10.40 9.40 9.27 9.31 10.21 9.73 10.52 11.00 10.10 10.26 11.24 10.31
1.33 1.54 1.36 1.87 1.41 1.49 1.31 1.58 1.75 1.20 1.36 1.47 1.33 1.60 1.22 0.82 0.92 1.36 1.50 1.25 1.10 1.00 0.99 1.08 0.91 1.31 1.04 0.24 0.62 0.65
2.77 2.42 2.46 3.93 3.38 4.37 4.99 4.64 4.32 4.29 5.14 4.73 5.04 5.46 4.68 4.14 4.33 5.37 5.51 4.70 4.71 4.26 4.57 4.13 3.89 4.58 3.95 2.91 2.63 1.76
0.83 0.56 0.64 1.18 1.10 1.71 2.23 2.10 1.72 2.09 2.55 2.54 3.05 3.15 2.94 5.43 4.21 3.85 4.11 3.69 3.77 4.08 4.97 4.19 5.39 6.40 5.05 7.66 8.87 10.07
4.93 4.52 4.46 6.98 5.89 7.57 8.53 8.32 7.79 7.58 9.05 8.74 9.42 10.21 8.84 10.39 9.46 10.58 11.12 9.64 9.58 9.34 10.53 9.40 10.19 12.29 10.04 10.81 12.12 12.48
10.67 10.05 9.45 13.74 11.87 14.42 16.08 16.76 16.62 15.93 18.25 18.03 18.63 20.98 17.33 22.70 19.35 22.11 22.64 20.14 19.75 19.58 21.74 20.13 21.88 24.45 21.38 22.29 24.70 23.96
9.71 9.13 8.62 12.61 10.87 13.28 14.82 15.22 14.99 14.54 16.55 16.48 16.95 19.00 15.85 20.45 17.54 19.99 20.53 18.21 17.91 17.72 19.71 18.18 19.74 22.23 19.28 20.17 22.38 21.85
4.79 5.84 3.92 3.92 1.64 4.58 4.19
6.71 8.38 6.12 5.71 4.65 10.70 11.03
0.00 0.00 0.96 1.36 0.71 0.00 0.35
2.74 3.58 2.57 1.58 3.28 4.54 4.40
4.70 5.17 3.14 5.42 3.58 3.84 4.80
7.44 8.75 6.67 8.36 7.57 8.38 9.55
14.15 18.92 13.84 18.29 12.87 20.93 21.90
13.03 16.93 12.47 15.93 11.88 18.53 19.62
c5/c6
1.16 1.22 1.12 0.97 1.02 0.90 0.89 0.92 1.01 1.10 0.90 1.06 0.89 0.95 0.91 1.07 0.94 0.98 0.94 0.98 0.97 1.00 0.97 1.04 1.03 0.90 1.01 0.95 0.93 0.83 0.98 f 0.18* 0.90 0.96 0.92 0.68 0.61 1.28 1.15
a Abbreviations: succ acid, succinic acid; glu acid, glutaric acid; glu ald, glutaraldehyde; adi ald, adipaldehyde; adi acid, adipic acid. Average.
COOH). 5-Hydroxypentanal (HO(CH,),CHO), 5hydroxypentanoic acid (HO(CH,),COOH), and 6hydroxyhexanoic acid (HO(CH,),COOH) were not detected. All the products were identified by means of GC and GC/MS by comparison with authentic samples (acids were identified after methylation). Figure 4 shows the mass spectra of authentic samples of adipaldehyde and methyl esters of OPA and OHA, for which literature data were not available to our knowledge. Among these products, glutaraldehyde, glutaric acid, and adipic acid have been identified mass spectrometrically by comparison with authentic samples by Grosjean and Friedlander (2) in the photochemical oxidation reaction of cyclohexene-NO,-air system in an outdoor smog chamber. The presence of half aldehydes (OPA and OHA) has been assessed (2) by interpretation of their mass spectra, and adipaldehyde has been postulated (2) in the reaction mechanism without experimental evidence. The present study established all of these products proposed by Grosjean and Friedlander (2). Although 5-hydroxypentanal, 5-hydroxypentanoicacid, and 6-hydroxyhexanoic acid has been detected in the sampled atmospheric aerosols ( 4 ) ,and the latter two compounds were also reported in the photooxidation of cyclohexene (Z), they are not the major products in the ozone-cyclohexene reaction on the basis of the present results. These w-hydroxy aldehyde and acids might be formed via hydrolysis of w-nitrato aldehyde and acids observed in the ambient aerosol (4) or 938
Environ. Sci. Technol., Vol. 19, No. 10, 1985
1
lot
Time / min
Flgure 5. Yields of difunctional C5 compounds as functions of time. (0) glutaraldehyde; (A)5-oxopentanoic acid; (0)glutaric acid.
in the cyclohexene-NO,-air system (2). Quantitative analyses for the particulate products were made for the f i s t time after sensitivity calibration for each compound. Table I1 summarizes the yields of particulate products under different conditions. Figures 5 and 6 show the yield of each particulate product as a function of elapsed time after the reactants were mixed. In this series of runs, the initial concentrations of cyclohexene and ozone were fixed at 180 and 210 ppm, respectively. For these runs, excess ozone was used so as to bring complete consumption of cyclohexene. Since the gas-phase reaction between the reactants is completed
Table 111. Yield of Aerosols under 1 atm of Air and 1 atm of Nza
a
time, min
[C&IO]O, mtorr
mtorr
buff gas
du ald, %
OPA, %
60 60 60 60 60 60 60 60 60 60
135 135 135 135 135 135 135 135 135 135
160 160 160 160 160 160 160 160 160 160
air air air air air air air NZ N2 N2
1.28 1.36 2.60 2.60 2.13 2.28 1.98 2.02 1.78 1.87
5.10 4.49 5.10 5.26 4.63 4.75 4.57 0.66 0.83 1.69
[0310,
glu acid, %
adi ald, %
OHA, %
adi acid, %
4.76 3.07 2.62 2.54 2.64 2.24 2.76
0.82 0.92 1.36 1.50 1.25 1.10 1.00 1.57 1.45 1.20
4.14 4.33 5.37 5.51 4.70 4.71 4.26 2.09 2.54 3.27
5.43 4.21 3.85 4.11 3.69 3.77 4.08 0.44 0.57 0.74
0.00 0.00 0.00
See footnote a of Table I1 for abbreviations. I
I
I
I
--I
~
10
1
103
102 Time /
1 04
min
Flgure 8. Yields of difunctional Ce compounds as functlons of time. (0)Adlpaldehyde; (A) 6-oxohexanoic acid; (0)adipic acid.
within a few minutes under this experimental condition, slow variation of the yield of each particulate products as depicted in these figures should be due to secondary oxidation. Figure 5 clearly demonstrates that glutaraldehyde is the major primary product among the C6 difunctional compounds and the sequential oxidation process of CHO(CHJ3CHO CHO(CH2)3COOH -* HOOC(CH2)3COOH forms OPA and glutaric acid successively. As for the C, difunctional compounds both adipaldehyde and OHA seem to be the primary products, and the similar sequential oxidation process of CHO(CH2)4CH0 CHO(CH2)4COOH HOOC(CH2)&OOH is responsible for the adipic acid and secondary formation of OHA. As listed in Table 111, formation of OPA, glutaric acid, and adipic acid was remarkably suppressed in the nitrogen system, while that of glutaraldehyde, adipaldehyde, and OHA was not suppressed significantly. This evidence strongly supports the above conclusion that glutaraldehyde is the sole primary C5 product and both adipaldehyde and OHA are the primary c6 products. Nonlinear least-squares calculation for the sequential processes was performed for the data obtained after 1h when the wall deposition is thought to be completed. Thus, the half-lives of the oxidative decay of glutaraldehyde, OPA, adipaldehyde, and OHA are obtained to be about 60,90,40 and 40 h, respectively, under our experimental conditions. Figure 7 shows the total molar yield of the particulate products and total aerosol organic carbon (AOC) yield in the cyclohexene (180 ppm)-ozone (210 ppm) reactions in the 4-L cell as a function of reaction time. Here, the total AOC yield is defined by lOOC [ (Aparticulate product/mol) X (carbon number of product/6)] / (-Acyclohexene/mol). Since the gas-phase reaction is completed within a few minutes as noted before, an increase of the total aerosol yield should be due to sedimentation of aerosols. Thus, the constancy of the yield after about 1h implies that the
-
-
-
10
1
102
103
Time /min
Flgure 7. Time profile of the total molar yield and the total aerosol organic carbon yield. (0and solld line) Molar yield; (0and broken line) aerosol organic carbon yield.
\
-u2
>
-
10-
._ ( 010
I
I
100
2 00
/
mTorr
Flgure 8. Total molar yield and the total aerosol organic carbon yield as a function of lnltlal concentration of cyclohexene. (0and solid line) Molar yield; (0 and broken line) aerosol organic carbon yield.
deposition has been completed within this period under our experimental conditions and that the sequential oxidation of c5and c6 difunctional compounds as seen in Figures 5 and 6, respectively, takes place mainly on the wall after deposition. Since the ratio of the total C5 compounds to the total c6 compounds in particulate products is constant (0.98 f 0.18) and independent of the reaction time as shown in Table 11,degradation of c6 compounds does not take place in the course of heterogeneous oxidation. Figure 8 depicts the dependence of the yield of total aerosols on the initial concentration of reactants. In these runs, the ratio of the initial concentration of cyclohexene to ozone was kept constant at -0.85, and the concentration of cyclohexene was changed from 17 to 245 ppm (see also Table 11). As shown in Figure 8, total yield of particulate products as measured a t 1 h increased slightly as the Environ. Scl. Technol., Vol. 19, No. 10, 1985
939
Scheme I
B
A
The reaction would be analogous to the CHZO2reaction forming formic acid after rearrangement followed by stabilization. As for the formation mechanism of another primary C6 product, adipaldehyde, one possible pathway of the reaction of the Criegee intermediate, CHO(CH2)4CHO0 CHO(CH2)4CHO0+ O2 CHO(CH,),CHO O3 (12)
C
-
F
G
H
concentration increased. From the linear least-squares treatment, the total molar yield and total organic carbon yield of deposited aerosol a t the intercept (low concentration limit) are estimated to be 15 f 4% and 13 f 3%, respectively. The latter value is consistent with the value of 5-17% of aerosol organic carbon yield reported by Grosjean and Friedlander (2) for collected samples of suspended aerosol in the photooxidation of cyclohexeneNO,-air system. Mechanism of Cyclohexene-Ozone Reaction. While the mechanisms of ozone-alkene reactions have been studied extensively (23-25), studies on the mechanism of ozonecycloalkene reactions are very limited. Grosjean and Friedlander (2) proposed a reaction sequence for cyclohexene based on the primary Criegee split analogous to ozone-alkene reactions. The characteristic of the ozonecyclohexene reaction is that, after the Criegee split of primary ozonide, the initial number of carbon atoms of cycloalkenes is conserved in the intermediate biradical, whose further reactions lead to difunctional products. (1) Formation of Cs Compounds. The major initial products composed of five carbon atoms are pentanal and glutaraldehyde. The formation mechanism of these products is proposed in Scheme I. Thus, steps 6 and 7 are envisioned as molecular and radical mechanism of C02 elimination corresponding to the reactions of the simplest Criegee intermediate, CH200, giving C 0 2 + H2 and C02 + 2H (26). Glutaraldehyde is proposed to be formed via the CHO(CH2)3CH20radical which may be produced by the radical-radical reactions of CHO(CH2),CH200. In the photochemical reaction of cyclohexene-NO, system, glutaraldehyde would be formed (2) via the same radical produced by the CHO(CH2),CH200+ NO CHO(CH2)3CH20+ NO2 reaction. Successive heterogeneous oxidation reactions of glutaraldehyde on the wall should give OPA and glutaric acid as already described. By analogy with the reaction of the CHzOO to give CO + H20 (26), intermediate B may yield 5-hydroxypentanal (HO(CH2)FHO)+ CO. However, neither this compound nor its possible oxidation product, 5-hydroxypentanoic acid, was detected in this study. Therefore, it is concluded that this reaction path is not important in the ozone-cyclohexene reaction. This conclusion is consistent with the fact that the alkyl-substituted Criegee intermediates such as CH3CHOO do not give ROH CO (27). (2) Formation of c6 Compounds. The major initial c 6 product, OHA, is thought to be formed by the collisional stabilization of the Criegee intermediate after rearrangement to E in Scheme I as was proposed by Grosjean and Friedlander (2).
-
+
ELC
COOH
CHO
940
Environ. Sci. Technol., Vol. 19, No. 10, 1985
(11)
+
was checked in the run by using isotope-labeled 1803.If the above reaction takes place, 1801602 should be generated in the cy~lohexene-~~O, reaction in air containing unlabeled 02.However, 1801602 was not observed by means of FT-IR in the cyclohexene (11.7 ppm)-1803 (28.4 ppm)-160,/N2 (1 atm) run; spectral subtraction of I8O3 from the spectrum of reaction mixture gave no peak around 1100-1000 cm-l, although 1801602 was expected to show a characteristic band between 1055 cm-l, which corresponds to 1603, and 995 cm-l, which corresponds to 1803. This result negates the possibility of reaction 1 2 as an adipaldehyde-forming step. It should be noted in Figure 3 that the stoichiometry of the consumption of cyclohexene and ozone is not unity, and more cyclohexene molecules are consumed than ozone molecules reacted under our experimental conditions. This fact strongly suggests that some active intermediate species produced during the course of the reaction reacts with cyclohexene to result in the excess consumption. Several active intermediates can be formed in this reaction system such as alkyl radical like F in Scheme I, alkylperoxyl radical like G, alkoxyl radical like H, H 0 2 from the H atom produced in the step 7, and OH radical. We found that the photolysis of azomethane in air in the presence of cyclohexene gave no significant consumption of cyclohexene, even though the photolysis gives several kinds of radicals according to the following reactions: CH3N=NCH3 + hv 2CH3 + N2 (13) CH3 + O2 C H 3 0 0 (14)
-- + +
O2 (15) (16) CH30 + O2 H 0 2 HCHO Thus, it can be concluded that alkyl, alkylperoxyl, alkoxyl, and hydroperoxyl radicals have negligible reactivity toward cyclohexene at least under 1atm of air. Therefore, the OH radical seems to be the most probable candidate for the species that consumes extra cyclohexene. However, a simple H02-OH chain reaction involving a reaction step HOz + O3 OH + 2 0 2 cannot explain the excess consumption of cyclohexene over ozone. A possibility of a reaction sequence to regenerate OH radical, e.g. 2CH300
2CH30
-
0
+
OH
a::. a'' - c::: -..-
OOH
--+
+
OH
OOH
may be conceived. Such a reaction sequence can also be a source of adipaldehyde. Further study is now under way, and the results will be published elsewhere. (3) Formation of Formic Acid, Ethylene, and Other Products. As for the gaseous products of ozone-cyclohexene reactions, no discussion on the mechanism has been made so far. Since formaldehyde is not observed in the reaction mixture, formic acid is not the secondary product due to the oxidation of the aldehyde. The primary formation of HCOOH has been established by the results of the runs with isotope-labeled ozone. Thus, in the 1803-
Scheme I1
r
Scheme IV H
l*
H 0.17 ( 0.26)
[CEF]*
Scheme I11 D+
Ct,; i
co
HCOOH
0
H
L, HCOOH,CO, etc. stabilization
[‘-.*2C2H4
cyclohexene-air reaction, formic acid formed was HC180180Hexclusively, and no isotopically mixed species was observed (28). Similarly, almost all CO and C02 produced in this run were labeled with l80,though a smaller amount of C160180was detected. These results clearly indicate that the C1 species observed in the cyclohexene-ozone reaction are mainly due to the primary fragmentation of the initially formed Criegee intermediate or its isomers. Two possible pathways can be conceived as the formation mechanism of formic acid and ethylene. One is unimolecular decomposition of vibrationally excited ozonide, which could be formed by intramolecular reactions of the biradical (B in Scheme I) (Scheme 11). The other is the route via CH2(CH2),C0 biradical as depicted in Scheme 111. In both the routes tetramethylene (CH2CH2CH2CH2) is the most probable candidate of the precursor of ethylene in the cyclohexene-ozone reaction since low-pressure pyI A rolysis of tetrahydropyridazine (N=N(CH2)&H2) is reported (29) to give ethylene and cyclobutane a t a yield ratio of 83:17. In order to check the possibility of the mechanism, the photolysis of cyclohexene ozonide (16 ppm) in air (1atm) was carried out. The products observed were formic anhydride, formic acid, ethylene, CO, and COz. Since it is known (30,31)that thermal decomposition of ozonide of cyclic olefin proceeds via the same pathway as that for photolysis, formic anhydride should be produced if the reaction of Scheme I1 takes place. Although the formation of formic acid and ethylene is in accord with the above mechanism, a much higher yield of formic anhydride than other products in the photolysis of cyclohexene ozonide is in marked contrast to the cyclohexene-ozone reaction where formic anhydride was not observed. Therefore, this route is not plausible. So far the reaction pathway as described in Scheme I11 is the most promising mechanism. The same biradical (CH2(CH2),CO)is supposed as an intermediate to give ethylene and CO in the photolysis of cyclopentanone (32). The reaction of the tetramethylene biradical with O2 may be the source of succinic acid. (4) Branching Ratio of Each Reaction Pathway. From the results of the present study it became clear that the major pathway of the reaction of ozone with cyclohexene followed the Criegee mechanism analogous to that of simple alkenes. Contribution of each reaction pathway is determined on the basis of product analysis data given in Scheme IV. Here, the initial yield of gaseous glutaraldehyde, 0.22, in the ppm concentration runs and the yields of gaseous and particulate C6 compounds, 0.12, in the higher concentration runs are taken to be the upper and lower limits of the fraction of the reaction forming C5 compounds. Since adipaldehyde is not a direct product of the 0,-cyclohexene reaction, it should be excluded in
t
Lc5 aerosol
I
I
+ C02
+
CE;gH
-
CS
0.12 -0.2 2 (0.18 -0.3 3 1 > 0.12(>0.18)
aerosol
the scheme. Since the yield of OHA under low reactant concentration was not determined, only the lower limit estimated by the results of high concentration runs is quoted in Scheme IV. The value is the difference of the final yield of the C6 aerosol and the initial yield of adipaldehyde. The fraction of the remaining pathways are based on the pentanal and HCOOH yields. Summation of the above yields gives the carbon balance of up to 60%. However, Figure 3 shows that the consumption of 0,is only about 60% of the cyclohexene reacted (average 66% for five runs). Therefore, if we assign the above yields to the stoichiometric ozone reaction assuming that the excess consumption of cyclohexene is due to side reactions and does not contribute to the selected products, the fraction of each reaction channel (the values in parentheses) sums up to nearly unity.
Literature Cited (1) Grosjean, D. In “Ozoneand Other Photochemical Oxidants”; National Academy of Sciences-National Research Council: Washington DC, 1977; Chapter 3, pp 45-125. (2) Grosjean, D.; Friedlander, S. K. Adv. Environ. Sci. Technol. 1980,9,435-473. (3) Schuetzle,D.; Crittenden, A. L.; Charson, R. J. J. Air Pollut. Control Assoc. 1973,23, 704-709. (4) Schuetzle, D.; Cronn, D.; Crittenden, A. L.; Charlson, R. J. Environ. Sci. Technol. 1975, 9, 838-845. (5) Hidy, G. M.; Appel, B. R.; Charlson, R. J.; Clark, W. E.;
Friedlander, S. K.; Hutchson, D. H.; Smith, T. B.; Suder, J.; Wesolowski, J. J.; Whitby, K. T. J. Air Pollut. Control ASSOC. 1975,25, 1106-1114. (6) Grosjean, D.; Cauwenberghe, K. V.; Schmid, J. P.; Kelley, P. E.; Pitts, J. N., Jr. Enuiron. Sci. Technol. 1978, 12, 313-317. (7) Vrbaski, T.; Cvetanovic, R. J. Can. J. Chem. 1960, 38, 1063-1069. ( 8 ) Niki, H.; Maker, P. D.; Savage, C. M.; Breitenbach, L. P. Environ. Sci. Technol. 1983,17, 312A-322A. (9) Atkinson, R.; Aschmann, S. M.; Carter, W. P. L.; Pitts, J. N., Jr. Int. J . Chem. Kinet. 1983, 15, 721-731. (10) Akimoto, H.; Hoshino, M.; Inoue, G.; Sakamaki, F.; Washida, N.; Okuda, M. Environ. Sci. Technol. 1979, 13, 471-475. (11) Akimoto, H.; Bandow, H.; Sakamaki, F.; Inoue, G.; Hoshino, M.; Okuda, M. Environ. Sci. Technol. 1980, 14, 172-179. Hatakeyama, S.; Akimoto, H. J . Phys. Chem. 1983, 87, 2387-2395. English, J., Jr.; Barber, G. W. J. Am. Chem. SOC.1949, 71, 3310-3313. Baer, E. J. Am. Chem. SOC.1942, 64, 1416-1421. Bartlett, P. D.; Woods, G. F. J . Am. Chem. SOC.1940,62, 2933-2938. Huckstep, M.; Taylor, J. K.; Caton, M. P. L. Synthesis 1982, 881-882. Marvel, C. S.; Birkheimer,E. R. J. Am. Chem. SOC.1929, 51, 260-261. Fox, S. W.; Polak, E. H.; Bullock, M. W.; Kobayashi, Y. J . Am. Chem. SOC.1951, 73,4979-4980. 1
Environ. Sci. Technol., Vol. 19. No. 10, 1985
941
Environ. Sci. Technol. 1985, 79, 942-946
Schniepp, L. E.; Geller, H. H. J. Am. Chem. SOC.1946,68, 1646-1648. Blust, G.; Lohaus, G. Justus Liebigs Ann. Chem. 1953,583, 2-6. Hartung, W. H.; Crossley, F. “Organic Syntheses”, Collect. Vol. 11; Wiley: New York, 1943; pp 363-364. Renaud, R.; Leitch, L. C. Can. J. Chem. 1954,32,545-549. Martinez, R. I.; Herron, J. T.; Huie, R. E. J . Am. Chem. SOC.1981, 103, 3807-3820, and references cited therein. Herron, J. T.; Martinez, R. I.; Huie, R. E. Znt. J . Chem. Kinet. 1982, 14, 201-224. Bailey, P. S. “Ozonation in Organic Chemistry”; Academic Press: New York, 1978 (Vol. l ) , 1982 (Vol. 2). Herron, J. T.; Huie, R. E. J. Am. Chem. SOC.1977, 99, 5430-5435. Herron, J. T.; Huie, R. E.; Int. J . Chem. Kinet. 1978, 10, 1019-1041.
(28) The assignment of HC1s0180Hin IR spectrum was based on the data in the following: Hatakeyama, S.; Bandow, H.; Okuda, M.; Akimoto, H. J . Phys. Chem. 1981, 85, 2249-2254. (29) Santilli, D. S.; Dervan, P. B. J. Am. Chem. SOC.1979,101, 3663-3664. (30) Story, P. R.; Morrison, W. H., 111; Hall, T. K.; Farine, J.-C.; Bishop, C. E. Tetrahedron Lett. 1968, 3291-3294. (31) Story, P. R.; Hall, T. K.; Morrison, W. H., 111; Farine, J. C. Tetrahedron Lett. 1968, 5397-5400. (32) Srinivasan, R. In “Advances in Photochemistry”; Noyes, W. A., Jr., Ed.; Interscience Publishers: New York, 1963; pp 83-113, and references cited therein.
Received for reuiew June 20,1984. Revised manuscript receiued January 30, 1985. Accepted April 22, 1985.
Polychlorinated Biphenyl Emissions to the Atmosphere in the Great Lakes Region. Municipal Landfills and Incinerators Thomas J. Murphy,” Leo J. Formanski, Bruce Brownawell, and Joseph A. Meyer Chemistry Department, DePaul University, Chicago, Illinois 606 14
In an effort to identify sources of polychlorinated biphenyls (PCBs) to the atmosphere, the concentration of PCBs in emissions from several municipal sanitary landfills and refuse and sewage sludge incinerators in the Midwest was determined. Sanitary landfills continuously emit the gaseous products of anaerobic fermentation along with other volatile materials to the atmosphere. Thus, they can be continuing sources of vapor-phase contaminants to the atmosphere. A projection, based on the amount of methane generated annually from landfills and a PCB to methane ratio of 0.3 pg of PCBs/m3 of CH4 found from the landfills sampled, indicates that the annual PCB emissions from sanitary landfills in the U.S. is on the order of 10-100 kg/year. The concentrations of PCBs from the incinerator stacks sampled ranged from 0.3 to 3 pg/m3, and the annual emissions per stack sampled were 0.25 kg/year. The emission rates found here are small compared to the 900 000 kg/year of PCBs estimated to cycle through the atmosphere over the U.S. annually.
Introduction The presence of measurable concentrations of polychlorinated biphenyls (PCBs) in the atmosphere throughout the northern and southern hemispheres is now well established (1-5). The fact that the PCBs in the atmosphere can exert significant deleterious effects has been demonstrated in the Great Lakes region where it has been shown that the atmosphere is presently a major source of PCB inputs to Lakes Michigan, Superior, and Huron (5-10). Bioaccumulation by the biota in Lake Michigan has led to levels of PCBs in adult sports fish (11) above the FDA limit for interstate commerce of 2 ppm (mg/kg) (12). Levels in adult fish of all species in all of the Great Lakes are above the International Joint Commission criteria of 0.1 mg/kg, and adverse health effects due to their presence have been demonstrated (13). The amount of PCBs transported by the atmosphere is quite large. Concentrations of about 7 ng/m3 of PCBs have been reported in cities and towns of the Midwest (6, 14-16), and concentrations of 0.5-2 ng/m3 have been reported in rural and remote areas (1-5,17). On the basis an estimate of 0.05 ng/m3 in rural areas, 5 ng/m3 urban 942
Environ. Sci. Technol., Vol. 19, No. 10, 1985
areas, and a mixed height in the atmosphere of 2 km, it was calculated that the air over the U.S. a t any time contains about 18000 kg of PCBs (18). This estimate is conservative due to the low concentration assumed for rural areas. If an average residence time in the atmosphere is assumed to be 1week, about 900 000 kg/year of PCBs annually cycle through the atmosphere over the U.S. This works out to an average input to, or deposition from, the atmosphere of about 60 g/(km2.year). This deposition rate is in reasonable agreement with that found to be coming into Lake Michigan from the atmosphere (7). Unfortunately, there is little information available on the sources to the atmosphere of this 900 000 kg/year of PCBs. Probable sources include the following: the evaporation of PCBs used in the past for such open uses as paints, wood preservatives, plasticizers, etc.; the evaporation of spilled or leaked PCBs from transformers, large capacitors, hydraulic systems, and equipment containing large volumes of PCBs and still in service or in storage; the evaporation from landfills or incinerators of PCBs from materials disposed of in municipal refuse; the evaporation of PCBs improperly disposed of to open areas such as the use of waste PCB fluids to oil roads etq emissions of PCBs from engines and furnaces burning liquid or gaseous fuels containing or contaminated with PCBs; the reevaporation of PCBs from land areas where they have been deposited by wet and dry deposition from the atmosphere. In the past, PCBs were included in the manufacture of a variety of materials that could end up in municipal waste. Some of these materials are still permitted to be disposed of in municipal waste. This includes carbonless carbon paper and most of the billions of small, PCB-containing capacitors that have been manufactured. Large numbers of these capacitors have been used in the ballasts on fluorescent light fixtures, in consumer electronics, and as the starting capacitor on motors in refrigerators, washing machines, air conditioners, etc. There are still no restrictions on the disposal of these capacitors. It has been estimated that, by 1978,140 x lo6 kg of PCBs had been disposed of in landfills (19). With respect to sanitary landfills, since they continuously generate CHI and C 0 2by the anerobic decomposition
0013-936X/85/0919-0942$01.50/0
0 1985 American Chemical Society
