Environ. Sei. Technol. 1983, 17, 352-357
McMurry, P. H.; Wilson, J. C. J . Geophys. Res., in press. Dydek, S. T. Ph.D. Dissertation, University of North Carolina, Chapel Hill, NC, 1982. Robbins, R. C.; Cadle, R. D. J. Phys. Chem. 1958, 62, 469-471. Tang, I. N.; Munkelwitz, H. R.; Davis, J. G. J. Aerosol Sci. 1978, 9, 505-511. Raabe, 0. G. In “Fine Particles”;Liu, B. Y. H., Ed.; Academic Press: New York, 1976; pp 57-112. Hodgeson, J. A. Toxicol. Environ. Chem. Rev. 1974, 2, 81-97. Liu, B. Y. H.; Pui, D. Y. H. J . Colloid Interface Sci. 1974, 47, 155-171. Knutson, E. 0.; Whitby, K. T. J . Aerosol Sci. 1975, 6 , 443-451. Liu, B. Y. H.; Pui, D. Y. H.; Whitby, K. T.; Kittelson, D.
B.; Kousaka, Y.; McKenzie, R. L. Atmos. Environ. 1978, 12, 99-104. Received for review October 14, 1982. Accepted February 17, 1983. This research was supported by Grant ATM-8113156 from the National Science Foundation. Early phases of the research were supported i n part by Contract EY-76-S-02-1248 f r o m the Energy Research and Development Administration and Grant R 803851 f r o m the United States Environmental Protection Agency. Although the research described i n this article has been supported in part by the United States Environmental Protection Agency, it has not been subjected to the Agency’s required peer and policy reviews and therefore does not necessarily reflect the views of the agency, and no official endorsement should be inferred.
Infrared Spectroscopic Study of Peroxyacetyl Nitrate (PAN) and Its Decomposition Products Peter W. Bruckmann” Landesanstalt fur Immissionsschutz, D-4300 Essen 1, West Germany
Helge Willner Institut fur Anorganische Chemi der Universitat Hannover, D-3000 Hannover 1, West Germany
Infrared and Raman spectra of peroxyacetyl nitrate (PAN) in the gas phase as well as IR spectra in an Ar matrix were recorded. Out of 27 fundamentals, 23 vibrations could be assigned. The IR absorptivities of some key bands were reevaluated and compared with literature data. The first steps of the thermal decompositionof pure PAN were examined by flash pyrolysis with subsequent trapping and analysis of the products. The detection of the acetylperoxy radical proved the homolytic cleavage of PAN. The thermal decay of PAN and the stoichiometry of the products were further studied in glass vessels and in an IR gas cell. The disappearance rate of PAN decreases considerably with falling partial pressures. The implications of this pressure dependence and of the observed stoichiometry for the decay mechanism are discussed.
Introduction Following the pioneering work of Stephens ( l ) peroxy, acetyl nitrate (PAN) has been measured as one of the main constituents of photochemical smog at many places all over the world (2-5). In view of this long measurement period it may be surprising that some features of PAN, e.g., its vibrational spectra and its decomposition mechanisms,are still somewhat uncertain. Though the original infrared spectrum (6) has been confirmed by several authors (7, 8), as have spectra obtained in an O2 matrix (9),it has recently been disputed (IO). The absorptions at 1300, 1430, and 1055 cm-l have been reported to be due to impurities (10). This would make the quantitative determinations of PAN in ambient air questionable, since most of them depend on the absorptivities given by Stephens ( 1 ) in establishing the concentration of their calibration mixtures. The difficulties seem to originate from the reported thermal instability of PAN ( I ) , which makes an isolation in pure form difficult. To dispel1 these ambiguities, we decided to undertake thorough examinations of the vibrational spectra of pure PAN and to check the IR absorptivities of some key bands in the gas phase. 352
Environ. Sci. Technol., Vol. 17, No. 6, 1983
The second point that needs clarification is the thermal and photochemical decompositionof PAN. Stephens and co-workers ( 1 ) were the first to study the composition of diluted PAN stored in N2 at room temperature. They proposed a cyclic intramolecular decomposition mechanism via a six-membered transition state. Later, it was shown (11, 12) that the first step of the decomposition consists in the equilibrium 1 and not in a cyclic rearrangement. CH3C002N02
* CH3C002. + NO2
(1)
The observed products (C02, CH3N03, CH3NOz ( 1 ) ) were assumed to originate from radical reactions, starting with the disproportionation of the peroxy radical (eq 2). 2 CH&(O)OO*
-+
2CH,C02* + 0
2
(2)
Because of the lack of experimentaldata, this scheme could not be verified. Additional products like CO, 02, and CHI were detected by Kacmarek et al. (13),who followed the decomposition of pure PAN in an infrared gas cell. It could also be shown that thermal decomposition did not obey first-order kinetics, so that extrapolations from experiments performed in the 10-torr range (13)to ambient conditions with partial pressures of PAN in the lO*-torr range (13) might be misleading. The stoichiometry of the decomposition, however, one prerequisite for the discussion of the mechanism, could not be derived from the data. We therefore undertook a reexamination of the thermal decay in glass vessels and gas cells, in order to establish the stoichiometry. In addition, PAN was flash pyrolyzed s and partial pressures in the with travel times 10-4-torr range, followed by trapping the products in an Ar matrix. This technique offers the opportunity to identify primary products by IR spectroscopy under conditions disfavoring reactions of peroxy radicals with itself because of their great dilution, as in the atmosphere. The results can then be compared with product distributions obtained at higher concentrations. The matrix technique is also suited to analyze products of the photochemical decomposition of PAN, which have
00 13-936X/83/09 17-0352$01.50/0
0 1983 American Chemical Society
Figure 1. Rotatable cryostat with pyrolysis pipe: (A) cooled matrix support (CsIwlndow); (B) shield agalnst heat radiation: (C)transfer line for PAN/Ar gas mlxture; (D) displacable resistance heating.
not been described so far. In this work, we report on the results of the aforementioned studies. Finally, we have checked some properties of PAN, as the triple point and vapor pressure equations, which have been determined only once (13). Experimental Section PAN was made by photolyzing a mixture of acetaldehyde (200 torr), NO2 (200 torr), C12 (200 torr), and 02 (600 torr) within a 2-L Pyrex bulb for 1h (14).The light source consisted of 16 RPR 3500-A wavelength lamps (Southern New England Ultraviolet Co.). The gaseous reaction mixture was partly condensed in a glass trap at -110 "C under continuous pumping to remove the more volatile compounds. Subsequently, the product was transferred into another trap, which was filled with molecular sieve (5 A) and kept at -196 "C. Further purification was achieved by trap-to-trap condensation under dynamic v a c u q . The three traps used were cooled to -40, -80, and -196 "C. The purity of PAN was >99% as determined by IR spectroscopy. PAN was quite stable in our high-vacuum glass system, exhibiting an estimated half-life of 10 days at 1.5 torr and 25 "C. The IR spectra were recorded in the range 200-4000 cm-l with a spectral resolution of 1cm-l by an IR grating spectrometer (Perkin-Elmer 325). Raman spectra were obtained with a Varian Cary 82 spectrometer (resolution ca. 4 cm-') taking an Arf ion laser (514.5 nm, 100 mW) as light source. The frequency accuracy was f l cm-l. Details of the IR matrix equipment and the attached high-vacuum apparatus are given elsewhere (15). Spectra of pure COP, NO2, CO, CH3N03, CH3N02,and CH300CH3were recorded in the gas phase as well as in Ar matrix, in order to estimate their concentrations in the pyrolysis experiments. For the measurement of the absorptivities, pure liquid PAN (>99%) was allowed to evaporate into an evacuated gas cell (10 cm long). The resulting pressure (in the range 1-5 torr) was measured by means of a high-precision gauge (300-BH100 MKS-Baratron), and the concentration of PAN within the cell was calculated, assuming ideal gas behavior. IR spectra were recorded as described above. The decay of pure PAN (3.7-0.8 torr) at 25-34 OC was studied within a 10-cm IR gas cell over several weeks, and the products were detected by IR spectroscopy. Likewise, the decomposition of PAN in Ar (1:500), stored within the high-vacuum apparatus, was followed at room temperature. In a third experiment, PAN (30 and 1.5 torr) was pyrolyzed at 50 "C in sealed glass bulbs, and the products were analyzed at the end of the experiments. In order to detect primary decomposition products, PAN in Ar (1:500) was flash pyrolyzed by means of a heated quartz tube (6 X 1 mm), placed 5 cm in front of a CsI window, cooled down to 10 K (Figure 1). The experimental conditions (pressures 300 nm. Below 300
Table VI. Frequencies (cm-' ), Intensities, and Assignment of the Products from the UV Photolysis of the Acetylperoxy Radical in an Ar Matrix
Table IV. Product Ratio from the Flash Pyrolysis Experiments pyrolysis temp, "C
PAN reacted, %
CO, :NO,
U
int
assgnt
v
int
assgnt
250 350 400 4 50
20 50 90 95
0.4 :1 0.5:l 0.4:1 0.6:l
2482 2343 2143.3 2138.5 2128.5
VW M VW VW VW
HCO. CO, COa CO Cob
1862 1625 1595 1090 659
W-M VW VVW W
HCO. H,O
Table V. Frequencies (cm-' ), Intensities, and Assignment of the Pyrolysis Products of PAN, Isolated in an Ar Matrix'
a
U
int
3710 D 3605D 2905D 2898 2343 D 2280D 2138.5 1947 1875 1853 1843 1835 1781 1769 1744 1732 1655 1645 1625 1611 D 1499 1440 1425 1420 1367
VW VVW VW VVW VS VW VW VW W W S W W VW W M VW VW VW VS VVW VVW W W M
assgnt CO, CO, CO; C CO, B NO
B A PAN B
B C PAN CH,NO, C,NO, B,NO, NO,
c
C A A A
V
1299 1278 1234 1181 1169 1152 1142 1138 1099 1094 1082 1078 1030 978 971 931 853 809 789 749 736 731 663 641
int
assgnt
W VW VW VW VW M VW W
PAN A B B A A
M
A C
W VW VW W VW M VVW VW VVW M VW M W S W
? ?
C PAN NO2 A ?
CO,
Symbols A-C are explained in the text.
nm, it was photolyzed quite efficiently. Observed products were C02 (main product), CO, NO, CH3N03, H20, and NO2. Vacuum Flash Pyrolysis. To get insight into the first steps of the decomposition mechanism of PAN, mixtures of PAN in Ar were pyrolyzed within s, as described in the Experimental Section. The experiments were designed to trap primary products with lifetimes Z10-3 s on the cooled CsI window (Figure 1). In addition, PAN was diluted to lo4 torr in order to reduce bimolecular reactions as far as possible, analogous to the atmosphere. At 250 OC, first decomposition products could be observed in the IR matrix spectra, the thermal decomposition being complete at 450 "C (Table IV). Figure 3 shows a typical matrix spectrum of the products and compares it to a matrix spectrum of pure PAN. In Table V the frequencies of the observed produds are given. Besides PAN, C02,and NO2,minor amounts of CO, NO, and CH3N03could be identified. Additionally, several new bands appeared, which were tentatively assigned to products A-C (see below). In all experiments, C 0 2 and NO2 were formed in an approximately fixed ratio of 1:2 (Table IV). The small amount of NO originated from further decomposition of NO2,as could be shown by blank experiments, pyrolyzing NO2 under the same experimental conditions. The traces of CH3N03(300 nm. Frequencies as well as intensities of the bands marked with A are very similar to the vibrations of the CH3C(0)02 moiety of the PAN molecule (Figure 3, Table V). This suggests that compound A is the CH3C(0)02radical, formed in the first step of the homolytic dissociation process (1).This assignment is confirmed by an analysis of the products of the UV photolysis (Table VI). Only COz, CO, H20, and the HCO radical (20) were observed, in agreement with the proposed structure for A (eq 3). The simultaneous presence of H2,02,and the CH3. CH,C(O)OO.
3OC-400 nm
CH&(O)OO*
300-400 nm
300-400 nm
CH,C(O)OO*
___*
COZ
+ HCO + H2
+ 02 + CH3 CO + HCO+ H2O CO
(3)
radical in the matrix cages was indicated by characteristic shifts relative to the pure, matrix isolated compounds. These shifts were in accordance with literature data for the CO/H20 system (21). The other absorptions in Table V (marked B and C) were stable under photolysis with wavelengths greater than 300 nm. Because possible products like C2Hs,CH3N03, CH300CH3,etc., can be excluded on the basis of reference spectra, they must belong to yet unknown decomposition products. Taking into account their frequencies and their low intensities, it is not unreasonable to assume them to originate from the minor side reaction 4, which has been
-
CH3C(0)OON02
fast
NO3. + CH3C(0)0. CH,-O-C=O
(4)
postulated on the basis of INDO calculations (22). Unfortunately, IR spectra of these radicals are missing, so that the structures of B and C are open for speculation. Returning to the main products, one can derive the stoichiometry of the main decomposition path from the observed product distribution (Table IV): 2PAN
-
2N02
+ C02 + CH3C(O)OO. + X*
(5) This can be conceived as a combination of the homolytic dissociation process 1and the partial decarboxylation of the peroxy radical, either directly (eq 6) or via intermediate
--
CH3C(0)O0. 2CH,C(O)OO. CH3C02.
-+
CH30. + C02
(6)
2CH3C02. + 02
(2)
+ COZ
(7)
CH3.
Environ. Sci. Technol., Vol. 17, No. 6. 1983 355
steps (eq 2 and 7). According to eq 6 and 7, X. should be the CH30. or the CH3. radical. Unfortunately, none of these radicals was clearly identified, and the tautomer to CH30., the CH20H radical (23),could not be detected. Thus, it is not possible to decide what is happening with the missing part of the PAN molecule. The direct IR spectroscopic observation of the CH3CO02.radical proves that reaction 1is indeed the first step in the thermal decomposition (11,12). At higher temperatures, the cleavage reaction 4 may exist as minor mode. The further decomposition depends on the fate of the acetylperoxy radical. Under the experimental conditions of the flash pyrolysis, the lifetime of the acetylperoxy radical seems to be approximately twice as long as that of PAN. Thermal Decomposition. As stated in the Experimental Section, the thermal decay of pure PAN was studied in IR gas cells, in sealed glass tubes, and in a high-vacuum apparatus (15)made of glass. The lifetime of PAN depended on temperature, pressure, and the nature of the vessel (I). The surfaces of CsI windows, e.g., were oxidized within several minutes, probably by the arising NOz. On the other hand, KBr windows were stable for several days. The decay of PAN (1.5 torr in an IR gas cell) was strongly accelerated by rising temperatures. The half-lives- were 60 h a t 34 OC and 400 h at 25 OC, respectively. This behavior is in accordance with the temperature dependence of the first decomposition step (1)(12).A new feature of the thermal decomposition is its marked pressure dependence. Observed half-lives spanned from 500 h at 0.8 torr to 15 h at 3.7 torr (IR gas cell, 25 "C). The order of the decomposition was more closely examined in the experiments performed in the sealed glass tubes (50 "C, 1.5 and 30 torr of PAN, Figure 4). From the graphs it can be concluded that the decomposition did not follow simple first- or second-order kinetics. The semilogarithmic plot suggests that the reaction starts with higher than first order kinetics and changes after several hours to approximately first order. The following products of the thermal decomposition in the IR gas cell as well as in the sealed glass tubes could be detected: C02, CH3N03,CH3N02(nitromethane), 02, and CO. In order to derive the stoichiometry of the decomposition, the partial pressures of the products (except O2and CO) were quantified by IR spectroscopy. O2 and CO were separated at -196 "C from the other compounds and measured by mass spectrometry. Contrary to reports in the literature where the product distribution was found to be erratic (1,13), the products, and approximately also their distributions, turned out to be identical. The exact stoichiometry, however, was slightly pressure dependent: lOPAN lOCO2 + 6CH3N03 + 3CH3N02 + 0 2 + 0.3CO + ? (30 torr) (8)
-
lOPAN --* lOCO2
+ 7.5CH3N03 + 2CH3N02 + 0.502
+ 0.2c0 + ? (1.5 torr)
(9)
In the experiment performed at 1.5 torr, the concentration of nitromethane did not increase further after 10 h, which coincided with the observed change in the order of the decomposition. Equations 8 and 9 indicate that the NO2 liberated in the first step of the decomposition 1 is so efficiently trapped by CH3 and CH30 radicals under the experimental conditions applied that it could not be detected. In the vacuum glass apparatus, however, where PAN was stored at lower pressures (0.1 torr, 25 OC) in the presence of 100 torr of Ar, NO2 was also observed, the C02/N02ratio being 1:2, as in the pyrolysis experiments. The experimental findings can now be compared with 356
Environ. Sci. Technol., Vol. 17, No. 6, 1983
.
0
5
10
40
30
20
50 t i h ]
Figure 4. Thermal decay of pure PAN in sealed glass vessels at 50 O C with initial pressures Po = 1.48 (bottom) and 32.4 torr (top). The uncertainties of the pressure measurements are indicated by emor bars.
reaction schemes of the decomposition mechanism. After the equilibrium 1,following steps of eq 2,7, and 10-12 were 2 CH,C(0)02* 2CH3C02. + 0 2 (2)
-- + + + + -
CH3C02.
CH3.
CH3. + CH3C(0)02. CH30. CH3.
NO2
NO2
COZ
CH30.
(7)
CH,C02.
(10)
CHJVO,
(11)
CHJVO2
(12)
proposed by Cox et al. (12)(products identified in the present,study are shown in italics). The observed product distribution is in accordance with this scheme, if it is assumed that the methyl radical predominantly reacts via reaction 10 instead of being trapped by NO2. Also the intermediate behavior between first- and second-order kinetics of the thermal decomposition and the change to approximately first order in the course of the reaction can nicely be explained. At the beginning of the decay, when PAN partial pressures are relatively high, the bimolecular self reaction (2) (25,26)predominates. Later, the peroxyacetyl radical is principally destroyed by reaction 10, the CH3radical being recycled by reaction 7 (first order in CH3C002.). The observed stoichiometry as well as the reaction order can thus be regarded as experimental confirmation of the proposed reaction scheme (12). This sequence is mainly important for the decay of calibration mixtures of PAN in the laboratory. It cannot be transferred directly to the real atmosphere. In ambient air, O2and NO will react with the radicals involved in the reaction sequence and the PAN decomposition will be controlled by temperature and the NO/N02 ratio (12).
Acknowledgments The experiments were performed a t the Ruhr Universitat Bochum. We return thanks to A. Haas for providing the laboratory facilities.
Environ. Sci. Technol. 1983, 17,357-361
Registry No. PAN, 2278-22-0; CH&OOy, 36709-10-1.
Literature Cited Stephens,E. R. In “Advancesin Environmental Sciences”; Pitts, J. N., Metcalf, R. L., Eds.; Wiley-Interscience: New York, 1969; Vol. 1, pp 119-146. Darley, E. F.; Kettner, K. A.; Stephens,E. R. Anal. Chem. 1963, 35, 589-591. Niebcer, H.; van Ham, J. Atmos. Environ. 1976,10, 115-120. Penkett, S.; Sand&, F. J.;Lovelock,J. E. Atmos. Environ. 1975, 9, 139-140.
(14) Gay, B. W., Jr.; Noonan, R. C.; Bufalini, J. J.; Hanst, P. L., Environ. Sci. Technol. 1976, 10, 82-85. (15) Willner. H. Z. Anorg. A l k . Chem. 1981. 481. 117-125. (16) Shimanouchi, T. Phis. Chem. Ref. Data 1977, 6 ,
993-1102. (17) Siebert,H. “Anwendungen der Schwingungsspektroskopie in der Anorganischen Chemie”;Springer Verlag: Berlin, 1966; p 96. (18) Siebert,H. “Anwendungender Schwingungsspektroskopie in der Anorganischen Chemie”; Springer Verlag: Berlin, 1966; p 93.
(19) Stephens,E. R. Statewide Air Pollution Research Center, University of California,Riverside,personal communication,
Bruckmann, P.; Mulder, W. Schriftenr.Landesanst. Immissionsschutz Landes Nordrhein-Westfalen, Essen 1979,
1979. (20) Milligan, D. E.; Jacox, M. E. J. Chem. Phys. 1964, 42,
47, 30-41.
Stephens, E. R. Anal. Chem. 1964,36, 928-929. Nicksic, S. W.; Harkins, J.; Mueller, P. K. Atmos. Environ.
3032-3036. (21) Dubost, H. Chem. Phys. 1976,12, 139-151. (22) Ohkubo, K.; Sato, H. Bull. Chem. SOC.Jpn. 1979, 52,
1967, I , 11-18.
Penkett, S. A.; Sandalls, F. J.; Jones, B. M. R. In “Ozon und Begleitsubstanzen im photochemischen Smog”;VDI Report, Diisseldorf, 1977, pp 47-54. Varetti, E. L.; Pimentel, G. C. Spectrochim.Acta 1974,30A, 1069-1072. Adamson, P.; Gunthard, H. H. Spectrochim. Acta 1980, 36A, 473-475. Hendry, D. G.; Kenley, R. A. J. Am. Chem. SOC.1977,99, 3198-3199. Cox, R. A.; Roffey, M. J. Environ. Sci. Technol. 1977, 11, 900-906. Kacmarek, A. J.; Solomon,I. J.; Lustig, M. J. Inorg. Nucl. Chem. 1978,40, 574-576.
1525-1526. (23) Jacox, M. E. Chem. Phys. 1981, 59, 213-230. (24) Viossat, V.; Chamboux, J. Bull. SOC.Chim. Fr. 1972, 1699-1702. (25) Bennett, J. E. J. Am. Chem. SOC.1973, 95, 4008-4010. (26) Weaver, J.; Meagher, J.; Shortridge, R.; Heicklen, J. J. Photochem. 1975,4,341-360. Received for review October 22, 1982. Accepted February 28, 1983. H.W. acknowledges generous support by the Fonds der Chemie.
Seasonal Variation of Cadmium Toxicity toward the Alga Selenastrum capricornutum Printz in Two Lakes with Different Humus Content Marit Laegreid,* Joroif AlstadVtDag Klaveness,’ and Hans Martln Selpt University of Oslo, Department of Chemistry, Blindern, Oslo 3, Norway, and university of Oslo, Department of Limnology, Blindern, Oslo 3, Norway
w The alga Selenastrum capricornutum Printz is used to investigate the potential of natural lake water to reduce cadmium toxicity. The two lakes involved differ in trophic status and in concentration and composition of dissolved organic matter, one being a typical dystrophic bog lake, the other a less humus influenced, eutrophic lake. In the dystrophic lake, the toxic effect is determined mainly by the free cadmium activity. In the eutrophic, less humus influenced lake, however, the toxic effect shows considerable seasonal variations with a toxicity far exceeding what would be expected according to the estimated free ion activity during summer. It is hypothesized that qualitative changes in the composition of the dissolved organic matter during the production period are responsible for this effect. Introduction The availability of metals to living organisms depends on the organism in question, on the speciation of the metals, i.e., if they exist as free ions, as complexes with organic or inorganic ligands or adsorbed to particles, and also on the presence of other anions and cations. Most natural waters have shown a certain capacity to ameliorate metal toxicity toward living organisms. The complexation capacity depends on concentration and composition of Department of Chemistry.
* Department of Limnology. 0013-936X/83/0917-0357$01.50/0
inorganic and organic components in the water, which are made available and regulated through physical, chemical, and biological processes in the catchment area and in the lake itself. A number of authors have proposed that mainly free metal ions are toxic to phytoplankton and that all compounds able to reduce the free ion activity thus reduce the metal toxicity. This has been demonstrated for copper in synthetic seawater by using the chelator tris(hydroxymethy1)aminomethane in the concentration range 1-10 mM (1,2) and ethylenediaminetetraacetic acid (EDTA) in the concentration 0.1 mM (2). It has also been shown for copper in natural seawater ( 3 ) . Several studies of natural freshwater containing large amounts of humic matter have also been carried out. It has been found that increasing humic content causes decreasing metal toxicity toward phytoplankton (4-8). Sunda and Lewis (5) also showed that for copper toxicity in the alga Pavlova lutheri, by measuring the copper ion activity with an ion-selective electrode, the toxicity was a function of free copper activity. Complexation capacity in most natural waters seems associated mainly with dissolved organic matter (9, 10). Dissolved organic matter in lakes is derived from two main sources: allochthonous, i.e., derived from the catchment area; autochthonous, i.e., produced in the lake itself. The concentration and composition of dissolved organic matter may both vary between different lakes and vary seasonally in a single lake, as a result of variation in runoff
0 1983 American Chemical Society
Environ. Sci. Technol., Vol. 17,No. 6, 1983 357
