Environ. Sci. Techno/. 1984, 18, 157-163
Jackson, R. E.; Inch, K. J. Sci. Ser.-Can. Inland Waters Dir. 1980, 104, NHRI Pap. No. 7. Champ, D. R.; Young, J. L. Atomic Energy of Canada Limited, 1979, Report AECL-6404. Bauer, F. P.; Rieck, H. S., Jr. Battelle Pacific Northwest Laboratories, Jan 1973, Report BNWL-SA-4478. Alberts, J. J.; Wahlgren, M. A. Environ. Sci. Technol. 1981, 15, 94. Buffle, J.; Deladocy, P.; Haerdi, W. Anal. Chim. Acta 1978, 100, 339. Florence, T. M.; Batley, C. E. Talanta 1977, 24, 151. Cassidy, R. M.; Elchuk, S.; McHugh, J. 0. Anal. Chem. 1982, 54, 727. VanOlphen, J. H. "An Introduction to Clay Colloid Chemistry";Wiley-Interscience: New York, 1977.
(16) Means, J. L.; Crerar, D. A.; Duguid, J. 0. Science (Washington, D.C.) 1978, 200, 1477. (17) Suarez, D. L.; Langmuir, D. Geochim. Cosmochim. Acta 1976, 40, 589. (18) Patterson, R. J.; Spoel, T. Water Resour. Res. 1981,17,513. (19) Jackson, R. E.; Inch, K. J. Environ. Sci. Technol. 1983,17, 231. (20) Cooper, E. L.; McHugh, J. 0. Atomic Energy of Canada Limited, 1981, Report AECL-7513.
Received for review December 22, 1982. Revised manuscript received July 19,1983. Accepted August 8,1983. J.0.McHugh's contribution was supported by funding from a National Sciences and Engineering Research Council postdoctoral fellowship.
Reactivity of Polycyclic Aromatic Hydrocarbons toward Nitrating Species Torben Nielsen Chemistry Department, Ris0 National Laboratory, DK-4000 Roskllde, Denmark
The transformation rates of 25 polycyclic aromatic hydrocarbons (PAH) and 4 derivatives of anthracene are investigated in diluted solutions containing dinitrogen tetraoxide and nitric and nitrous acid under conditions which are relevant for determining the relative reactivities of PAH and their derivatives toward nitrogen compounds in the atmosphere. The reactivities of some of the PAH are several orders of magnitude higher than that of benzene. The reactions are apparently electrophilic, as electron-donating substituents, e.g., methyl, enhance the reactivity of anthracenes, and electron-withdrawing substituents, e.g., nitro, diminish it. The reactivities of PAH correlate with spectroscopical constants, and it seems possible to relate the differences in reactivity to differences in the chemical structure. A classification of the reactivity of PAH in electrophilic reactions in the environment under dark and weak lighting conditions is set up. The presence of polycyclic aromatic hydrocarbons (PAH) in stack and exhaust gases and in the atmosphere has been thoroughly investigated (1-3). Recently, nitro derivatives of PAH have been demonstrated to be present in diesel exhaust gases (4-6)and in ambient air (7-11). In addition, biological and chemical tests have demonstrated that nitroarenes constitute a significant part of the socalled directly acting mutagens present in extracts of airborne particulate matter (12,13). Some nitro-PAH are strongly mutagenic in Salmonella tests (12, 13), and nitro-PAH have been demonstrated to be carcinogenic in animal experiments (14) as well as mutagenic in test systems using mammalian cells (15). Furthermore, transformation of some PAH to nitro-PAH has been observed in experiments using relatively low concentrations of nitrogen dioxide and nitric acid (10, 16-19). Thus, nitroarenes in exhaust and stack gases and nitroarenes formed in the atmosphere presumably invoke health hazards. In contrast to benzene and its derivatives, the nitration of PAH has been studied only to a limited extent (20-22). It would hardly be possible to make reliable predictions on the fate of PAH and other types of polycyclic organic materials (POM) in the atmosphere without a thorough knowledge of the fundamental processes (18, 19). This work is part of a study investigating the nitration reactions of PAH. The purpose of the work is to determine the relative reactivities of PAH and PAH derivatives in reactions with nitrating species. The transformation rates 0013-936X/84/0918-0157$01.50/0
of 25 PAH and 4 derivatives of anthracenes are investigated in diluted solutions of dinitrogen tetraoxide and nitric and nitrous acid. The correlations of the transformation rates with spectroscopic constants and a simple theoretical aromaticity index strongly suggest that the reactivity of the single compounds depends upon their chemical structure. On the basis of the results attained, a classification of the reactivity of PAH is suggested. Experimental Setup and Techniques The decomposition of the PAH (original concentration 0.1-3 ppm) was investigated in a mixture of watermethanol-dioxane, 40:36:24 (v/v), in most cases at 24.9 f 0.1 "C. The use of organic solvents is necessary to get a reasonable solubility of the PAH. The use of a mixture of methanol and dioxane at a ratio of 3:2 reflects only that the PAH standards were dissolved in this mixture in advance. The concentrations of the added nitric acid (fuming; p.a. Merck) and sodium nitrite were 0.16 and 0.016 M, respectively. In addition, diluted fuming nitric acid contains minute amounts of dinitrogen tetraoxide (dimer of nitrogen dioxide). The concentration of N204was not determined, but it has been the same in all the reported experiments, as the same bottle of fuming nitric acid was used in each case, and in each case fresh solutions were made. By means of the UV absorption spectrum (on Cary 16 UV spectrophotometer) (300-400 nm) of a freshly prepared solution of 0.16 M HNOB(fuming) and 0.016 M NaN02 in water-methanol-dioxane, 40:36:24, and the extinction coefficients of the major bands of N204and HNOz in vapor phase (23),it was evaluated afterward that the concentration of N204had been about 3 X M. In all cases, the reaction mixtures were stored in the dark to avoid photochemical reactions. The analysis of the reaction mixtures was made by high-performance liquid chromatography (HPLC). The equipment consisted of a Waters pump 6000 A, a Rheodyne 7120 sample injector with a 20-pL loop, and a Perkin-Elmer LC 1000 fluorescence detector. The chromatogram was displayed on a Kipp-Zonen BD 41 recorder. The selected excitation and emission wavelengths varied in the single cases, dependent upon the investigated PAH. The column used was Zorbax ODs, Nucleosil Cls, or Lichrosorb RP 18, all 5 pm and 25 cm X 4.6 mm. The reaction mixtures contained acids and oxidants, which caused degradation of the column material. The use of a short precolumn, Nucleosil5 C18 6 cm X 4.6
0 1984 American Chemical Society
Environ. Sci. Technol., Vol. 18, No. 3, 1984 157
mm, inhibited the degradation of the main column. The precolumn was changed for every 100-200 analyses. The eluent was either methanol-water (mainly 81) or methanol-acetonitrile-water (mainly 6:2:1) at a flow rate of 1.0 mL/min. The reaction mixtures were analyzed directly without any treatment, and the quantification was performed by comparing the peak heights of the investigated compounds with those of standard mixtures. A more thorough discussion of the use of HPLC for the analysis of PAH has been presented elsewhere (24). Results and Discussion Relevance of the Model System. In stack and exhaust gases nitrogen monoxide is the dominant nitrogen species. It is relatively quickly oxidized to nitrogen dioxide during the mixing of the plume with ambient air and during the transport (25). By means of a complex sequence of reactions in gas phase (25,26),reactions in liquid aerosols (27), and reactions on particle surfaces (28),nitrogen dioxide is further transformed. After a relatively short period nitric acid and nitrate are the most abundant nitrogen compounds associated with particles and are the only eventual products. However, nitrous acid (29) and peroxyacetyl nitrate (PAN) (30) may also be present in the atmosphere in abundant concentrations. Recently, nitrogen trioxide has been identified in ambient air (31). Alkyl nitrates and nitrites may be formed by esterification of nitric and nitrous acid, respectively, with alcohols and by reactions between alkoxy radicals and nitrogen oxides (32,33). Dinitrogen tetraoxide is perhaps formed in minor amounts by the uptake of nitrogen dioxide in the liquid phase on aerosols and by adsorption of nitrogen dioxide on particle surfaces. Thus, particle-associated PAH will be exposed to a variety of nitrogen-containing compounds during their transport in the atmosphere. However only nitrate, nitric acid, nitrogen dioxide, dinitrogen tetraoxide, nitrous acid, alkyl nitrites, and alkyl nitrates are assumed to be involved in nitration reactions of PAH. So far, there are no indications suggesting that nitrogen monoxide should cause transformation of PAH (17). PAN causes transformation of reactive PAH acting as an oxidant, but not as a nitrating species (16). Nitrogen trioxide appears to possess low reactivity toward unsubstituted aromatic hydrocarbons (34). Nitrous acid has been demonstrated to catalyze the nitration of reactive PAH in strong acidic solutions (22),and recently dinitrogen tetraoxide also has been demonstrated to act as a catalyst in weak acidic solutions (35). Furthermore, benzo[a]pyrene and perylene adsorbed on glass fiber filters were transformed only when they were exposed to a mixture of nitrogen dioxide and nitric acid (16). Thus, an appropriate model system for the "real world" situation should contain N(II1) and N(1V) as well as N(V) compounds, the latter being the most abundant in solution experiments. The reaction system used in this investigation, therefore, appears to be a reasonable model system for determinihg the relative reactivities of PAH and derivatives of PAH. The use of a mixture of polar solvents should be a reasonable model for "wet" particles. If water alone was used as solvent, the pH in the system would have been 1.05 f 0.05, a value which is probably somewhat lower than that being typical for the liquid phase on airborne wet particulates (36). However, the difference is not serious as the purpose of the investigation is to determine the relative reactivities of PAH. Chemistry of the Reaction System. The original composition of the nitrogen compounds in the reaction mixture does not represent a stable system. Consequently, 158 Environ. Sci. Technol., Vol. 18, No. 3, 1984
changes in the composition will occur with time. The dissolved dinitrogen tetraoxide will be in equilibrium with its monomer, nitrogen dioxide, although in polar solvents the concentration of this will be very small (37). In addition, N204will dissociate into NO+ and NO,- (38). Most of the NO+ produced will be transformed relatively quickly to nitrous acid and methyl nitrite by reaction with water and methanol (39,40). This is caused by the strong displacement to the left of the equilibriums HN02 H+ + NO+ + H20and CH30N0 + H+ + NO+ CH30H, as the pH is relatively high (41). Hence, during the initial stage of reaction the concentration of NO+ is predominantly determined by the concentrations of dinitrogen tetraoxide and nitrogen dioxide. The concentrations of NO2, N204, and NO+ decrease during the initial period, when after some time their concentrations will be very low and are mainly determined by the concentrations of H+, HN02, and CH30N0. To some extent this corresponds with the real world situation, where NO2 is transformed to nitrous acid and nitric acidlnitrate by reactions in the liquid phase on particulates (42). The transformation of Nz04was studied by means of the UV absorbance in the range 310-330 nm of the reaction mixture (without any PAH added) at different times. The transformation reaction was neither a simple first-order nor second-order reaction with respect to N204,and the rate decreased with time. After 6.5 days the concentration of N204 was 50% of the concentration at the start. It appears that N204is much more stable in solutions containing organic solvents than in pure water (42). It seems most plausible that the catalytic effect of nitrous acid (22)in strong acidic solutions is caused by NO+. It is impossible to decide at present whether the catalytic effect of dinitrogen tetraoxide (35)or nitrogen dioxide (16) is caused by N2O4, NO2,or NO+. With regard to solution experiments recent experimental data on the influence of the nitrate content on the transformation rate of anthracene indicate perhaps that N204is the real catalytic species (35). Relative reactivities of PAH. In most of the experiments where decomposition of the PAH was observed, the decomposition rate (-dc/dt) showed a linear dependency on the concentration, c, of the PAH, -dc/dt = kc, or in other words In c = -kt + In co (1) where k is the apparent rate constant for the reaction, t is the time, and co is the concentration of the PAH at t = 0. In Figure 1the variation of the concentration of benzo[ghi]peryleneduring the experiment is shown. The value of k was obtained by estimating the slope by a leastsquares procedure. The reproducibilities of the determined values of k are on the order of 20%. The half-lives, tip, of the PAH were obtained from tl12 = (In 2)/k (2)
+
+
The half-lives and the relative reactivities of 25 PAH are given in Table I. For 10 of these no significant decrease in the concentration was observed after 1month or more. In two cases, pyrene and picene, significant decreases in the rate constants with time were observed. In the case of picene, this seems to be caused by the formation of a fluorescent compound with almost the same retention time as picene. If the transformation rate of a PAH is much faster than that of the catalyzing species, one should expect a linear dependency as in eq 1. Correspondingly, if the catalyzed transformation rate of the PAH is much slower than the transformation rate of the catalyzing species, one should
Table 11. Calculated Relative Reactivities and Half-Lives of 10 PAHa relative nitrarelative tion compound rateb reactivityC half-lifed anthanthrene 78000 30000 perylene 77000 30000 benzo[a]pyrene 27000 2000 pyrene 17000 600 coronene 3450 9 triphenylene 1600 1 200 days 660 0.1 4 years phenanthrene naphthalene 520 0.06 7 years 1.5 1x 4 x l o 7 years benzene a Conditions: see Table I. From ref 20. Conditions: see Table I. Calculated from In (relative reactivity) = 2.62 In (relative nitration rate) - 19.2 (r = 0 . 9 3 ; ~< 0.01). Calculated from t , , , (min) = 2.2 X 105/relativereactivity.
0
5
x)
15
20
25
Figure 1. Graph showlng the correlation between the relative concentration of benzo[gh/]perylene and the time. The decomposkion rate constant, k , = 8.8 X lo-' mln-' (f = 0.9991; p < 0,001).
Table I. Half-Lives and Relative Reactivities of 25 PAHa
compound
half-life
relative nitrarelative tion reactivity rateb
anthanthrene 2.2 min 1000OOc 78000 perylene -3min 73000 77000 9,lOdmethylanthracene 11 rnin 21000 9-methylanthracene 34 min 6500 picene 100 min 2100d 2-methylanthracene 140 min 1600 anthracene 190 min 1200 benzo[a]pyrene 200 min 1100 27000 1-methylanthracene 260 min 860 benzo [ghilperylene 5.5 days 28 pyrene 5.7 days 27d 17000 coronene 23 days 6.8 3450 chrysene 24 days 6.3 1750 dibenz [a$ ]anthracene 29 days 5.3 benz[a]anthracene 34 days 4.5 benzo[ elpyrene > 100 days 200 days 200 days 300 days 300 days 300 days 70 days 400 days 800 days 800 days
