Nitration and Photonitration of Naphthalene in Aqueous Systems

Jan 19, 2005 - Abstract. The nitration of naphthalene was studied in aqueous solution to gain insight into the processes leading to the nitration of a...
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Environ. Sci. Technol. 2005, 39, 1101-1110

Nitration and Photonitration of Naphthalene in Aqueous Systems DAVIDE VIONE,* VALTER MAURINO, CLAUDIO MINERO, AND EZIO PELIZZETTI Dipartimento di Chimica Analitica, Universita` di Torino, Via P. Giuria 5, 10125 Torino, Italy

The nitration of naphthalene was studied in aqueous solution to gain insight into the processes leading to the nitration of aromatic compounds in atmospheric hydrometeors. Reactants used were nitric acid, nitrogen dioxide and peroxynitrous acid in the dark, nitrate, and nitrite/nitrous acid under illumination. Naphthalene nitration can lead to two possible isomers, 1- and 2-nitronaphthalene. The former nitrocompound preferentially forms upon electrophilic processes and in the presence of nitrogen dioxide. Electrophilic nitration of naphthalene takes place in the presence of concentrated nitric acid, but nitration with nitric acid and oxidants (charge-transfer nitration) occurs under much milder conditions than with nitric acid alone. Charge-transfer nitration may have some environmental significance in particular cases, e.g. in acidic aerosols in the presence of HNO3 and oxidants. Nitrogen dioxide is thought to have a role in PAH nitration in the Antarctic particulate matter. In previous papers we have found that nitration induced by peroxynitrous acid, HOONO, can follow two pathways, the former electrophilic (leading for instance to the formation of nitrophenols from phenol) and the latter probably involving HOONO itself (accounting for the formation of nitrobenzene from benzene). In the case of naphthalene and HOONO the electrophilic pathway mainly leads to 1-nitronaphthalene, while the other one preferentially yields 2-nitronaphthalene. The nitration of naphthalene in the presence of nitrite/nitrous acid under irradiation leads to both nitroisomers in similar ratios, and the process is not inhibited by hydroxyl scavengers. This excludes nitrogen dioxide as reactive species for nitration and marks a difference with phenol photonitration and a similarity with the behavior of benzene under comparable conditions. Nitrite photochemistry (and nitrite-induced photonitration as well) is expected to be relevant in fog and cloudwater in polluted areas. An important difference with the gas-phase nitration is that the radicals •OH and •NO are unlikely to play a relevant role in the nitration of 3 naphthalene in aqueous solution.

Introduction The presence of aromatic nitroderivatives, and nitro-PAHs in particular, on atmospheric particulate is a matter of concern for the possible impact of these compounds on human health (1-3), mostly due to their high direct mutagenic activity (4, 5). * Corresponding author phone: ++39-011-6707633; fax: ++39011-6707615; e-mail: [email protected] www.abcrg.it. 10.1021/es048855p CCC: $30.25 Published on Web 01/19/2005

 2005 American Chemical Society

The formation of atmospheric nitro-PAHs is thought to take place by at least four mechanisms (2, 6): (a) hightemperature electrophilic nitration of PAHs during combustion processes; (b) daytime •OH-mediated nitration in the gas phase; (c) nighttime •NO3-mediated nitration in the gas phase; (d) liquid-phase nitration. Each process gives different nitrated isomers or different proportions of the same isomers. In some cases from the concentration and temporal evolution data of nitrated PAHs in atmospheric particulate it is possible to gain indications on the nitration mechanism and possibly the sources (2). The main obstacle in this context is represented by the little amount of information available on the liquid-phase nitration. In fact, while mechanisms (a), (b), and (c) have been extensively studied, comparatively less information has been gathered on mechanism (d). Liquid-phase nitration of aromatic compounds can take place both in the dark, in the presence of HNO3 and/or • NO2/N2O4 (7-11), and under illumination, due to the photolysis of nitrate and nitrite (12-14). Using phenol as a model aromatic molecule, we have recently identified various conditions in which aromatic nitration in solution can take place (15-20). They include nitrate and nitrite UV irradiation and dark nitration by HNO3 and HNO2. Moreover, nitrous acid oxidation in the dark as well as the reaction between nitrous acid and hydrogen peroxide can lead to enhanced nitrophenol formation (21). Also the photooxidation of NO2and HNO2, induced by photoactive sources of •OH and other oxidants, can enhance phenol nitration (22, 23). Such photoactive sources are nitrate, dissolved Fe(III), and semiconductor oxides. In many cases phenol nitration was linked with the generation of nitrogen dioxide. Quite interestingly, a recent study on the nitration and photonitration of benzene in the presence of nitrite/nitrous acid showed that the pathways are different from the ones followed in the nitration of phenol (24). In this work we have used naphthalene as a model aromatic molecule. It is the lightest PAH, and the study of its behavior can give more reliable information about the reactions of this class of compounds than studies on phenol or benzene. Moreover, the water solubility of naphthalene is still sufficient to enable homogeneous phase studies without the need of supporting this compound on (supposed) inert substrates such as silica or alumina. The comparison between the nitration pathways of phenol and benzene under similar conditions has yielded interesting information on the role that the molecular structure has on reactivity toward nitration (24). Data concerning naphthalene would certainly be of help for a better understanding of this issue. The nitration of naphthalene in the gas phase has been extensively studied (25-27). Under such conditions the formation of nitronaphthalenes occurs via reaction between naphthalene and •OH or •NO3 (homolytic addition to the aromatic ring), followed by reaction between the radical addition intermediate and •NO2 to yield the nitroderivatives upon elimination of H2O or HNO3. Some processes leading to the thermal nitration of naphthalene in aqueous solution are known, though not all of them are environmentally significant (28). On the contrary, very little work has been done on the photonitration of naphthalene in aqueous solution. In a paper on azaarene nitration a hint was given about the possibility of naphthalene nitration upon nitrate photolysis (29), but very qualitative information was reported. VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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CB lowbleed/MS, 60 m length, 0.32 mm i.d.). The conditions employed were as follows: carrier flow 1 mL min-1 (He), injector temperature 300 °C, oven temperature 40 °C (3 min), then at 300 °C at 15 °C min-1, stay at 300 °C for 20 min, manual splitless injection (1 µL). Further experimental details are reported in refs 21 and 22.

Results and Discussion

FIGURE 1. Absorption spectra of naphthalene (N), nitrate, nitrite, and nitrous acid. Irradiance spectra of the TL 01 and TL K05 lamps. Hereafter in the text: irradiation under the TL K05 lamps ) UVA irradiation; irradiation under the TL 01 lamp ) 313 nm irradiation.

Experimental Section Naphthalene (purity grade 98%), 1-nitronaphthalene (1NN, 99%), 2-nitronaphthalene (2NN, 85%), 1-naphthol (>99%), 2-naphthol (99%), and H2SO4 (96-98%) were purchased from Aldrich, NaNO3 (>99%), NaH2PO4‚H2O (>99%), Na2HPO4‚ 2H2O (>99.5%), H2O2 (35%), HNO3 (65%), HClO4 (70%), acetonitrile (LiChrosolv gradient grade), and 2-propanol (LiChrosolv gradient grade) from Merck, NaNO2 (>97%) and (NH4)2Ce(NO3)6 (>99%) from Carlo Erba, nitrogen dioxide (>99.5%) from Messer Greisheim. All reagents were used as received without further purification. Water used was of Milli Q quality. Irradiation was carried out in magnetically stirred, cylindrical Pyrex glass cells (diameter 4.0 cm, height 2.3 cm), containing 5 mL of aqueous solution. The used UV sources were an array of three 40 W Philips TL K05 lamps (emission maximum 360 nm, total photon flux in the cells 3.6 × 10-7 Ein s-1, actinometrically determined (30)) and a 100 W Philips TL 01 lamp (emission maximum 313 nm, total photon flux in the cells 1.3 × 10-7 Ein s-1). The emission spectra of the lamps, together with the absorption spectra of nitrate, nitrite, nitrous acid, and naphthalene, are shown in Figure 1. The emission spectra were measured with an Ocean Optics SD2000 CCD spectrophotometer, the absorption ones with a Varian Cary “100 Scan” UV-vis spectrophotometer. Considering that the Philips TL K05 lamps emit radiation in a wide wavelength interval, centered in the UVA region, experiments carried out under these lamps will hereafter be indicated as “UVA irradiation”. Runs in the dark were carried out in magnetically stirred vials, wrapped with aluminum foil. Analysis was executed with a Merck-Hitachi HPLC. The column used was a RP-C18 LichroCART (Merck, length 125 mm, diameter 4 mm), packed with LiChrospher 100 RP-18 (5 µm diameter). Isocratic elution was performed with a 50/50 mixture of acetonitrile/aqueous NaH2PO4 (0.050 M) at a flow rate of 1.0 mL min-1. The retention times were (min) as follows: naphthalene (10.7), 1-naphthol (4.5), 2-naphthol (3.9), 1-nitronaphthalene (8.0), 2-nitronaphthalene (9.0). The column dead time was 0.9 min, detection wavelength 210 nm. Samples for GC-MS analysis were salted with NaCl and extracted with CH2Cl2, and the extract was then dried with anhydrous Na2SO4 and concentrated under a gentle stream of high-purity nitrogen. The concentrated extract was injected into a Thermo-Finnigan GC-MS, equipped with a Trace GC 2000 gas chromatograph, a GCQ plus ion trap mass spectrometer, and a Varian WCOT fused silica column (CP-Sil 8 1102

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Identified intermediates of the transformation of naphthalene were 1- and 2-nitronaphthalene (1NN, 2NN). In the presence of HOONO and of nitrate and nitrite under irradiation also 1- and 2-naphthol were detected. In some GC-MS runs, small amounts of 1,2- and 1,4-naphthoquinone were also found. The present paper will however focus on the processes leading to the formation of 1NN and 2NN. Nitration of Naphthalene with Nitric Acid. The mixture HNO3 + H2SO4 is a long-known nitrating agent, acting via protonation of nitric acid and subsequent formation of NO2+ upon water elimination. Protonation of HNO3 by H2SO4 is more effective than self-protonation of nitric acid. Electrophilic naphthalene nitration by NO2+ preferentially yields 1NN (31). See also Table 1, entries #1-2, for the kinetics of such a process. Aromatic nitration in the liquid phase is unlikely to occur via this pathway in the vast majority of environmental cases. However, sulfuric and nitric acids are present for instance at elevated concentration in the Antarctic aerosol (32, 33). In the Antarctica, nitric acid is in part of marine and in part of stratospheric origin, where it is produced by cosmic rays and subsequently transported downward by the polar vortex (34). Antarctic sulfuric acid is of biogenic origin, mainly arising from the oxidation of dimethyl sulfide (35, 36). The gassolid partitioning of sulfuric, nitric, and hydrochloric acids is thought to have a deep impact on the anionic composition of the Antarctic snow (37). Nitration of Naphthalene with Nitrogen Dioxide. The nitration of naphthalene by nitrogen dioxide has been studied in dichloromethane (8). Under such conditions, the yield of 1NN was about 20 times higher than the yield of 2NN. We carried out a similar experiment in aqueous solution, adding gaseous •NO2/N2O4 (partial pressure 0.3 atm) to a 2.0 × 10-4 M naphthalene solution. Nitrogen dioxide is not stable in aqueous solution, as it undergoes fast dimerization and hydrolysis (38):

2•NO2 a N2O4

[k1 ) 4.5 × 108 M-1 s-1; k-1 ) 6.9 × 103 s-1] (1)

N2O4 (+H2O) f HNO2 + NO3- + H+

[k2 ) 1.0 × 103 s-1] (2)

The solutions were buffered with a mixture of NaH2PO4 and Na2HPO4 to avoid acidification due to formation of nitric acid upon hydrolysis of N2O4. The final pH was about 7. As a consequence of water solubilization followed by reactions 1 and 2, gaseous nitrogen dioxide disappears a few seconds after its addition to the system. After disappearance of nitrogen dioxide, 21.5% of the initial naphthalene is transformed. The yield in 1NN is about 13% of transformed naphthalene, while the yield in 2NN is much lower (0.7% of transformed naphthalene). Nitrogen dioxide thus mainly yields 1NN. Interestingly, the yield ratio of nitroisomers in water is very similar to the one reported for naphthalene nitration in CH2Cl2 (8). Nitration of Naphthalene in the Presence of (NH4)2Ce(NO3)6. The 360 nm photolysis of cerium ammonium nitrate, (NH4)2Ce(NO3)6, in the presence of HNO3 yields the •NO3

TABLE 1. Initial Degradation Rate of Naphthalene and Initial Formation Rates of 1NN and 2NNa no.

components of the system (other than naphthalene)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26

6.5% (1.44 M) HNO3 6.5% (1.44 M) HNO3 + 9.6% (1.80 M) H2SO4 10-3 M (NH4)2Ce(NO3)6 + 0.20 M HNO3 0.20 M HNO3 10-3 M (NH4)2Ce(NO3)6 + 0.20 M HNO3 0.20 M HNO3 10-3 M KMnO4 + 0.20 M HNO3 NaNO2 1.0 × 10-3 M NaNO2 3.0 × 10-3 M NaNO2 0.10 M 0.002 M NaNO2 + 0.002 M H2O2 0.002 M NaNO2 + 0.002 M H2O2 0.002 M NaNO2 + 0.002 M H2O2 0.002 M NaNO2 + 0.002 M H2O2 0.002 M NaNO2 + 0.002 M H2O2 0.002 M NaNO2 + 0.002 M H2O2 0.002 M NaNO2 + 0.002 M H2O2 0.002 M NaNO2 + 0.002 M H2O2 0.002 M NaNO2 + 0.002 M H2O2 + 0.25 M 2-prop. 0.002 M NaNO2 + 0.002 M H2O2 + 0.25 M 2-prop. 0.002 M NaNO2 + 0.002 M H2O2 + 0.25 M 2-prop. 0.002 M NaNO2 + 0.002 M H2O2 + 0.25 M 2-prop. 0.002 M NaNO2 + 0.002 M H2O2 + 0.25 M 2-prop. 0.002 M NaNO2 + 0.002 M H2O2 + 0.25 M 2-prop. 0.002 M NaNO2 + 0.002 M H2O2 + 0.25 M 2-prop. 0.002 M NaNO2 + 0.002 M H2O2 + 0.25 M 2-prop.

irrad. λ

pH

dark 2. Both the pH trend and the effect of 2-propanol under such conditions bear similarity with the nitration of benzene in the presence of HOONO (24). In that case nitration was not electrophilic, and candidate reactive species were HOONO, •OH + •NO2, and •NO2/N2O4. However, the •OH + •NO2 pathway would be inhibited in the presence of 2-propanol (21, 24), while nitrogen dioxide preferentially yields 1NN, as reported earlier in this paper, and cannot therefore account for Rate2NN > Rate1NN at pH > 2. Accordingly, nitration upon reaction with HOONO seems more likely. Interestingly the same conclusion has been reached in the case of benzene nitration, based on different arguments (24). We will now give a tentative explanation of the regiospecificity of the reaction between naphthalene and HOONO, yielding 2NN (see also Scheme 1, pathway C).

HOONO might preferentially attack the position 1 of naphthalene with its oxygen atom bound to hydrogen. The position 1 is actually more electron-rich and is the preferential addition site for many species (e.g. NO2+ and NO+ (42)). The interaction between HOONO and the position 1 of naphthalene might induce a homolytic cleavage of HOONO, somewhat analogous to reaction 12. The hydroxyl group would then remain on the position 1, and the nitro group might add to the adjacent position 2, yielding 1-hydroxy-2-nitro-1,2-dihydronaphthalene. This unstable intermediate might then evolve into 2NN upon water elimination, a pathway commonly described in the context of gas-phase nitration (26, 27). In summary, two different nitration pathways appear to be operational in the presence of HOONO: (i) electrophilic nitration initiated by species originating from HOONO protonation; this pathway prevalently yields 1NN but also affects the formation rate of 2NN at pH < 2. (ii) Nitration by HOONO, preferentially yielding 2NN. Pathway (i) also leads to the formation of nitrophenols from phenol (21), while pathway (ii) also yields nitrobenzene from benzene (24). Naphthalene nitration in the presence of HNO2 + H2O2 cannot be accounted for by the formation of HNO3 from peroxynitrous acid (reaction 14). Actually, naphthalene nitration by HNO3 only occurs in the presence of more concentrated nitric acid at a lower pH (see Table 1, entries #1 and 6). It is interesting to observe that naphthalene also produces 1- and 2-naphthol, in similar yields, in the presence of HOONO. Naphthol production is inhibited in the presence of 2-propanol, as can be expected from a hydroxyl-mediated process. Naphthalene Photonitration upon Nitrate Irradiation. Nitrate photolysis is an important source of reactive species in both natural waters and atmospheric hydrometeors (12). It yields •OH, •NO2, nitrite, and peroxynitrous acid (reactions 18-20, 1, 2, and 9) (55-57):

NO3- + hν f •NO2 + •O- [Φ18305nm ) 0.010] (18) •

O- + H+ a •OH

[pKa,19 ) 11.9]

NO3- + hν f NO2- +O

(19)

[Φ20 ) 0.001]

(20)

Naphthalene was irradiated in the presence of nitrate under the TL 01 lamp (emission maximum 313 nm, near the nitrate absorption maximum, see Figure 1). Figure 3 reports the time evolution of naphthalene and 2NN upon irradiation of VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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TABLE 2. Initial Degradation Rate of Naphthalene and Initial Formation Rates of 1NN and 2NNa no.

components of the system (other than naphthalene)

irrad. λ

pH

naphthalene degr. rate (M s-1)

1NN form. rate (M s-1)

2NN form. rate (M s-1)

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

0.10 M NaNO3 0.10 M NaNO3 + 2-propanol 0.10 M 0.10 M HNO3 0.01 M NaNO2 0.02 M NaNO2 0.03 M NaNO2 0.04 M NaNO2 0.05 M NaNO2 0.07 M NaNO2 0.10 M NaNO2 0.01 M NaNO2 + 1.00 M 2-propanol 0.002 M NaNO2 0.002 M NaNO2 0.002 M NaNO2 0.002 M NaNO2 0.002 M NaNO2 0.002 M NaNO2 0.002 M NaNO2 0.002 M NaNO2

313 nm 313 nm 313 nm UVA UVA UVA UVA UVA UVA UVA UVA UVA UVA UVA UVA UVA UVA UVA UVA

6.0 6.0 1.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 7.0 1.0 2.0 2.5 3.0 3.5 4.0 5.0 6.0

(2.6 ( 0.2) × 10-8 (1.6 ( 0.1) × 10-9 (2.4 ( 0.3) × 10-8 (4.1 ( 0.4) × 10-9 (3.5 ( 0.3) × 10-9 (3.7 ( 0.2) × 10-9 (3.7 ( 0.1) × 10-9 (2.9 ( 0.2) × 10-9 (3.4 ( 0.4) × 10-9 (3.2 ( 0.6) × 10-9 (6.7 ( 0.1) × 10-10 (1.5 ( 0.1) × 10-7 (2.5 ( 0.1) × 10-7 (1.6 ( 0.1) × 10-7 (1.2 ( 0.1) × 10-7 (6.4 ( 0.2) × 10-8 (3.6 ( 0.2) × 10-8 (2.3 ( 0.1) × 10-8 (7.2 ( 0.4) × 10-9

Lb Lb (1.3 ( 0.2) × 10-9 (4.4 ( 0.8) × 10-12 (7.0 ( 2.6) × 10-12 (8.5 ( 0.1) × 10-12 (1.0 ( 0.2) × 10-11 (1.7 ( 0.2) × 10-11 (1.7 ( 0.2) × 10-11 (2.4 ( 0.3) × 10-11 (5.3 ( 0.2) × 10-12 (2.2 ( 0.3) × 10-10 (2.2 ( 0.7) × 10-10 (1.8 ( 0.4) × 10-10 (1.3 ( 0.2) × 10-10 (8.6 ( 0.7) × 10-11 (5.0 ( 1.7) × 10-11 Lb Lb

(3.0 ( 0.1) × 10-11 (2.5 ( 0.1) × 10-11 2 (see Figure 2 and Table 1, entries #11-18), which could account for the formation of 2NN upon irradiation of nitrate at pH 6.0 (Figure 3). Furthermore, from the observed pH trend in the presence of naphthalene and HOONO (see Figure 2), one can foresee that 1NN should become the prevailing nitroisomer at pH < 2. Coherently, the 313 nm irradiation of naphthalene (initial concentration 1.0 × 10-4 M) in the presence of 0.10 M HNO3 (pH 1.0) mainly yields 1NN (see Table 2, entry #3). Another argument in favor of peroxynitrous acid is that the formation of 2NN in the presence of HOONO at pH > 2 is affected by 2-propanol at a very limited extent, as is 2NN formation upon nitrate photolysis (see Figure 2B and compare with Figure 3). Turning again to Figure 3 it is interesting to observe that, in the presence of the alcohol, 2NN reaches higher concentration values for long irradiation times. The time evolution curves are consistent with a lower degradation rate of 2NN in the presence of 2-propanol. Also the initial degradation rate of naphthalene is much lower with 2-propanol (see Table 2, entries #1 and 2). As already introduced, considering that 2-propanol scavenges over 99% of the hydroxyl radicals in the system, and that it does not absorb radiation above 300 nm, it can be inferred that hydroxyl plays a relevant role in the degradation of both naphthalene and 2NN. 2NN is also known to undergo direct photolysis under UV irradiation (2), but no effect of 2-propanol on the photolysis rate of 2NN at 313 nm would be expected. Irradiation of naphthalene and NaNO3 also gives 1- and 2-naphthol with comparable yields. Their formation is most likely due to reaction between naphthalene and hydroxyl. Naphthalene Photonitration upon Nitrite Irradiation. The photolysis of nitrite is a source of reactive species in the environment in a similar way as nitrate photolysis, the lower concentration of nitrite in many environmental compartments being counterbalanced by its higher photochemical activity (12, 63). Furthermore, the formation of nitrous acid

FIGURE 4. Initial formation rates of NN (1NN + 2NN) and nitrite absorptance, rNO2-, as a function of [NO2-]. Initial naphthalene concentration 1.0 × 10-4 M, UVA irradiation, pH 7. Note: 5.0E-11 is a compact form for 5.0 × 10-11. on carbonaceous surfaces in the presence of water vapor is at the same time an important sink for atmospheric nitrogen dioxide and a source of nitrite (64). The potentially relevant role that nitrite can play in the atmospheric aqueous phase is confirmed by the fact that nitrite photolysis accounted for 47-100% of hydroxyl formed upon irradiation of fogwater from California’s Central Valley (65). Nitrite photolysis primarily generates •OH and •NO (66) but also induces a complex series of radical reactions. The set of reactions can however be simplified when focusing on the processes controlling the steady-state [•NO2], [N2O4], and [N2O3] (16). The relevant reactions are thus 21-27 plus 1, 2, and 19 (quantum yields and rate constants are taken from ref 66):

NO2- + hν f •NO + •O-

[Φ21360nm ≈ 0.025] (21)

•

OH + NO2- f OH- + •NO2 [k22 ) 1.0 × 1010 M-1 s-1] (22) NO2- + hν f •NO2 + e-aq •

NO + •NO2 f N2O3

[Φ23 e 0.076 Φ21] (23)

[k24 ) 1.1 × 109 M-1 s-1]

(24)

N2O3 (+ H2O) f 2NO2- + 2H+ [k25 ) 5.3 × 102 s-1] (25) O2 + e-aq f •O2-

[k26 ) 1.9 × 1010 M-1 s-1] (26)

O2- + NO2- (+ 2H+) f H2O2 + •NO2

•

[k27 ) 5 × 106 M-1 s-1] (27)

Irradiation was carried out under the TL K05 lamps (emission maximum at 360 nm, UVA irradiation), near the absorption maximum of nitrite (see Figure 1). Figure 4 shows the initial formation rate of nitronaphthalenes (NN ) 1NN + 2NN) as a function of nitrite concentration (the rates of the single compounds are reported in Table 2, entries #4-10). The table indicates that 1NN and 2NN show comparable initial formation rates upon nitrite photolysis. Figure 4 also reports the absorptance of nitrite under UVA irradiation, and the comparison between the reported data indicates that the initial formation rates of nitronaphthalenes are proportional to nitrite absorptance RNO2- (the fraction of incident radiation absorbed by nitrite (67)). Among the species formed upon nitrite photolysis, the steady-state concentrations [N2O4] and [N2O3] are proportional to RNO2-, while [•NO2] and [•NO] are proportional to RNO2-1/2 (16, 18).

FIGURE 5. Time evolution of naphthalene (N, initial concentration 1.0 × 10-4 M), 1NN, and 2NN in the presence of 0.01 M NaNO2, with and without 1.0 M 2-propanol. UVA irradiation, pH 7 in both cases. Note separate concentration scales.

This means that, for instance, the nitration of naphthalene is unlikely to occur upon reaction with •NO2 or •NO as the rate-determining step. However, also N2O4 and N2O3 can be ruled out as reactive species for nitration. The formation of 1NN and 2NN with similar rates is not compatible with nitration by nitrogen dioxide (•NO2 or N2O4), either in the dark or under irradiation: see refs 8 and 61 and the data presented earlier in this paper. Nitration of naphthalene by N2O3 can be ruled out by the observation that no nitronaphthalenes at all form in the presence of naphthalene and HNO2, the thermal decomposition of which yields N2O3 (see Table 1, entries #8-10, and reactions 16 and 17). The trend of nitronaphthalene formation as a function of [NaNO2] (Figure 4) also constitutes evidence against hydroxyl-mediated nitration. Such a process would in fact proceed at a rate proportional to the steady-state [•OH]. Due to the photochemical hydroxyl formation upon nitrite photolysis and to the consumption reactions of •OH with nitrite itself and with 1.0 × 10-4 M naphthalene (17, 60), both [•OH] and nitronaphthalene formation should be maximum for 0.005 M NaNO2, which is not compatible with the data reported in Figure 4. The time evolution of naphthalene, 1NN, and 2NN upon UVA irradiation of 1.0 × 10-4 M naphthalene and 0.01 M NaNO2 is reported in Figure 5 (see also Table 2, entry #4). Nitronaphthalenes form at comparable rates. The formation rate of both nitronaphthalenes is increased upon addition of 1.0 M 2-propanol to the system, as reported in Figure 5 and in Table 2, entry #11. The addition of the alcohol can be expected to consume hydroxyl and thus inhibit the formation of nitrogen dioxide (reaction 22). The fact that the addition of 2-propanol does not inhibit the formation of nitronaphthalenes constitutes additional evidence against naphthalene nitration involving nitrogen dioxide. These findings mark a difference with the behavior of phenol under comparable conditions: nitration of phenol upon nitrite photolysis most likely occurs via reaction with nitrogen dioxide and is inhibited by the addition of 2-propanol (16). On the contrary, the nitration of naphthalene upon nitrite irradiation is very similar to the formation of nitrobenzene from benzene under the same conditions: proportionality to RNO2- and lack of inhibition in the presence of hydroxyl scavengers (24). Accordingly, it can be hypothesized that the nitration processes of both benzene and naphthalene upon nitrite irradiation follow a similar pathway, different from the one involving phenol. A tentative explanation, advanced in the case of benzene (24), is that the nitration process might involve reaction with light-excited species (NO2-*), which VOL. 39, NO. 4, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 6. Initial formation rates of 1NN and 2NN as a function of pH. Initial conditions: 1.0 × 10-4 M naphthalene, 0.002 M NaNO2, UVA irradiation, pH adjusted by addition of HClO4. Note: 4.0E-10 is a compact form for 4.0 × 10-10. would account for the proportionality to RNO2- and for the lack of inhibition by 2-propanol. Figure 6 reports the initial formation rates of 1NN and 2NN in the presence of 1.0 × 10-4 M naphthalene and 0.002 M NaNO2 as a function of pH (see also Table 2, entries #1219). The pH trend indicates the presence of an acid-base equilibrium, which most likely involves the acidic dissociation of HNO2 (pKa ≈ 3.3 (68)). Irradiation of HNO2 induces naphthalene nitration at a far higher rate than irradiation of nitrite, as already observed in the case of benzene (24). Differently from the case of phenol (17), and similarly with benzene (24), the nitration of naphthalene upon irradiation of HNO2 is a photoinduced process because HNO2 does not nitrate naphthalene in the dark. Upon request by the reviewers, and using their comments and proposals as a starting point, we would now try to propose a tentative pathway for the reaction between naphthalene and radiation-excited nitrite and nitrous acid. The experimental data presented so far indicate HNO2* and NO2-* as possible reactive species, with HNO2* probably having a higher reactivity. These light-excited species might be able to oxidize naphthalene. On the contrary, oxidation of naphthalene by nitrite/nitrous acid in the dark is likely to be a very slow process, if it takes place at all (see Table 1, entries #8-10). Reaction between HNO2*/ NO2-* and naphthalene might result in electron transfer, thus yielding naphthalene radical cation. Electron-transfer nitration does however yield 1NN as the main nitroderivative (see Scheme 1, pathway A) and is therefore not consistent with the comparable amounts of 1NN and 2NN formed upon irradiation of nitrite/nitrous acid. Indeed, all the naphthalene nitration pathways involving positively charged species (naphthalene radical cation, NO2+, NO+) preferentially yield 1NN. Accordingly, our proposal is that the reaction between naphthalene and HNO2*/ NO2-* does not involve cationic intermediates. HNO2*/ NO2-* might react by hydrogen abstraction, yielding the naphthyl radicals. These radicals might then react with nitrogen dioxide to yield 1NN and 2NN (see Scheme 1, pathway D). Note that in Scheme 1 the reaction is shown for HNO2*, but NO2-* can be expected to give a similar process. Our proposal is based on the assumption that hydrogen abstraction gives both naphthyl isomers, in analogy with the radical addition of •OH on naphthalene that takes place on both positions, 1 and 2, yielding 1- and 2-naphthol in similar amount. In Scheme 1, pathway D, we still require nitrogen dioxide to take part to the reaction, but there is a huge difference if the reaction is initiated by nitrogen dioxide or if this species is involved at a later stage. In the very reasonable hypothesis 1108

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that the first reaction stage is the rate-determining one, a reaction initiated by nitrogen dioxide would be strongly inhibited upon •NO2 depletion. The same would not necessarily be true if •NO2 is involved at a later, non ratedetermining, stage. Furthermore, the addition of the alcohols as hydroxyl scavengers does certainly deplete nitrogen dioxide, which is for instance reflected in the inhibition of phenol nitration (16), but does not necessarily eliminate • NO2 from the solution. Indeed, under the studied conditions, some nitrogen dioxide might be produced in secondary processes, e.g. reactions 23 and 27, which are not inhibited by the addition of hydroxyl scavengers (66). Kinetic simulations we have run in a previous paper indicate that reaction 23 might have a quantum yield very near its upper limit (16). Finally, it is interesting to observe that the addition of 2-propanol, apart from the lack of inhibition, does actually increase the formation rate of 1NN and 2NN (see Figure 5 and Table 2, entries #4 and 11). This effect can be accounted for in the context of a nitration process initiated by hydrogen abstraction. Hydroxyl scavenging by 2-propanol takes place upon hydrogen abstraction to yield the propanoyl radical (47, 55, 59). This radical might then be able to abstract a hydrogen atom from naphthalene to yield naphthyl and 2-propanol, thus enhancing naphthalene nitration (see Scheme 1, pathway D). Mechanistic Implications. A comparison between the nitration processes of phenol, benzene, and naphthalene under similar conditions is now possible. Many features of the nitration of naphthalene are similar to the ones of benzene (no nitration by HNO2 in the dark, nitration not initiated by nitrogen dioxide upon nitrite photolysis). Phenol behaves differently and the difference is most likely due to the presence of the OH group, which for instance should be the site for the attack by HNO2 in the dark (54). The kinetics of nitrophenol formation in the presence of phenol and nitrous acid in the dark is in fact fully compatible with a direct reaction between phenol and HNO2 (69). The main difference between naphthalene and benzene is that the former is more activated to electrophilic attack, mainly leading to the formation of 1NN. This fact accounts for the partially different reactivity of naphthalene and benzene in the presence of HOONO: benzene only reacts with HOONO, while naphthalene also reacts with electrophilic species. Another interesting feature is that the nitration of naphthalene in aqueous solution is not initiated by •OH + •NO2 or •NO3 + •NO2, differently from gas-phase nitration (25-27). Environmental Significance. In the present work, various pathways for the nitration and photonitration of naphthalene in solution are presented. Naphthalene has been used here as a model Polycyclic Aromatic Hydrocarbon. All the studied pathways have a potential role in the environment, under appropriate conditions. For instance, electrophilic nitration may play some role in acidic aerosols, possibly in the Antarctica. This paper does however show that electrophilic nitration requires rather extreme conditions in the absence of oxidants. The addition of oxidizing compounds, which favor electron-transfer processes (see Scheme 1, pathway A), allows nitration to take place under milder conditions. Another pathway, nitration by nitrogen dioxide (in the presence of HNO3 traces) is a possible explanation to account for the occurrence of 1-nitropyrene, 9-nitroanthracene, and 2-nitrofluoranthene on the Antarctic particulate matter (70, 71). This work presents evidence for the formation of peroxynitrous acid, HOONO, upon nitrate UV irradiation. Considering that sodium and ammonium nitrate are major components of atmospheric particulate matter (72) and that photoformed HOONO is likely to account for aromatic nitration by irradiated nitrate under gas-solid conditions in

the presence of oxygen (62), it can be concluded that HOONO plays a potentially relevant role in the chemistry of atmospheric aerosols. Finally, it is very important to observe that the photonitration of naphthalene (and of benzene (24)) upon irradiation of HNO2/NO2- is not inhibited by hydroxyl scavengers. The environmental importance of nitrite photochemistry is usually decreased by the presence of natural scavenging agents (12), but this seems not to be the case for the photonitration of nonphenolic aromatic compounds. Nitrite can reach relevant concentration values (up to 10-4 M (65)) in fogwater in polluted areas. Aromatic nitration upon nitrite irradiation can thus be relevant in the presence of elevated nitrite levels, in sunlit hydrometeors (24).

Acknowledgments Financial support by CNR, PNRA - Progetto Antartide, Interuniversity Consortium “Chemistry for the Environment” (INCA) and Universita` di Torino - Ricerca Locale is gratefully acknowledged.

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Received for review July 23, 2004. Revised manuscript received November 16, 2004. Accepted November 24, 2004. ES048855P