Environ. Sci. Technol. 2005, 39, 3646-3655
Photochemical Transformations of Benzo[e]pyrene in Solution and Adsorbed on Silica Gel and Alumina Surfaces S I L V I N A F I O R E S S I † A N D R A F A E L A R C E * ,‡ School of Science and Technology, Universidad del Turabo, P.O. Box 3030, Gurabo, Puerto Rico 00778-3030, and Department of Chemistry, University of Puerto Rico, P.O. Box 23346, San Juan, Puerto Rico 00931-3346
The photodegradation of benzo[e]pyrene (BeP), a ubiquitous polycyclic aromatic hydrocarbon (PAH) contaminant, was investigated in solution and adsorbed on surfaces modeling the atmospheric particulate matter to provide fundamental information that could help to clarify its fate in the atmosphere. Diones, diols, and hydroxy derivatives were identified as the major photoproducts of BeP irradiated under simulated atmospheric conditions. The relative distribution of the products and the photodegradation rates of BeP were affected by the average pore size of the surface. Major photoproducts characterized in samples adsorbed on silica gel and alumina surfaces were not observed in irradiated solutions of BeP in hexane. In acetonitrile, the photodegradation rate was faster than in hexane, and one of the diones was observed. Different photoreaction pathways seem to take place in polar versus nonpolar microenvironments.
could be taking place in the photodestruction process of adsorbed PAHs. For example, the photochemical oxidation of anthracene adsorbed on silica gel occurs entirely by a singlet oxygen-mediated mechanism (5), while pyrene photodegrades through a radical-mediated mechanism (6). The photodegradation reactions of 1-methoxy-naphthalene (7) and perylene (8) are examples of mixed mechanisms in which both types of pathways contribute to the photodestruction of the PAH. Thermal reactions with nitrate and hydroxy radicals in heterogeneous phase are also a common degradation pathway of PAHs in the atmosphere (9). We have reported recently that BeP reacts photochemically through a radical cation-mediated mechanism (10). Further reactions of the BeP radical with water or oxygen give origin to stable photoproducts. Diones, diols, and monohydroxy derivatives have been generally found as the major photoproducts of PAHs adsorbed on silica gel and alumina surfaces (5-11). Evidence for photochemical transformation under laboratory conditions has indicated that photolytic half-lives for particlebound PAHs are highly dependent on the substrate to which these are adsorbed (12, 13). We have found that the yield of BeP radical cation formation varied widely with surface composition and was much lower in alumina than in silica (10, 14). The surface physical properties and the PAH loading both influenced the radical yield and the radical decay rate. Therefore, it is expected that if the intermediate species involved in the photodegradation mechanism are affected the final products will also vary. In this work, we investigate the photodegradation of BeP in solution and adsorbed on models for inorganic atmospheric particulate matter. We characterized the BeP’s photoproducts to provide fundamental information that could clarify its fate in the atmosphere. The effects of the PAH microenvironment on the photodegradation rate and on the nature and yield of the photoproducts were also studied.
Materials and Methods Introduction Polycyclic aromatic hydrocarbons (PAHs) are a group of priority pollutants found in the atmosphere adsorbed principally on the particulate matter. They are formed during the combustion of organic matter under oxygen-deficient conditions (1, 2). The photochemistry of PAHs adsorbed on surfaces modeling the atmospheric particles is an area of intense research because several PAHs have shown to be cancer promoters (3). However, it has been demonstrated that the PAHs themselves are not the actual carcinogenic agent because metabolic activation is required to initiate the formation of tumors. The direct mutagenic agents are usually oxidized derivatives of PAHs such as epoxides, diols, and diones produced through enzymatic reactions (4). These derivatives of PAHs can be also produced by photochemical reactions of the parent PAH in the atmosphere. Therefore, the study of the interaction of PAHs with light can contribute to a better understanding of their behavior in the atmosphere and help to establish a more realistic evaluation of toxicity and risk. The photodegradation of adsorbed PAHs occurs mainly via two mechanisms: (1) through a singlet oxygen-mediated pathway, and (2) through a radical intermediate mechanism. One of these reaction pathways or a combination of both * Corresponding author phone: (787)764-2433; fax: (787)756-8242; e-mail:
[email protected]. † Universidad del Turabo. ‡ University of Puerto Rico. 3646
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Chemicals. BeP (Sigma Chemical Co.) was used as received. Its purity was verified to be higher than 99% by HPLC. The solvents used were HPLC grade (Optima, Fisher Scientific). Nonactivated silica gel of 25, 60, and 150 Å average pore size (with BET surfaces areas of 437, 409, and 315 m2/g, respectively) and alumina type F-20 (Sigma Chemical Co.) were used as adsorbents. Standards of some BeP metabolites (benzo[e]pyrene-4,5-dione, benzo[e]pyrene-trans-9,10-dihydrodiol, 3-hydroxybenzo[e]pyrene, and 9-hydroxybenzo[e]pyrene) were purchased from Midwest Research Institute and used to confirm the photoproducts identification. Sample Preparation and Irradiation Procedures. The adsorbed samples were prepared by adding a measured volume of standard solution of BeP in hexane to a weighed amount of adsorbent. The solvent was evaporated under a N2 gas flow with continuous stirring. The effect of coadsorbed water was studied by saturating the adsorbent with deionized water before the addition of the standard solution. To prepare samples without physisorbed water, the silica gels were dried in an oven at 130-150 °C for 40 h and cooled to room temperature under vacuum prior to adding the BeP stock solution. The content of water of the surfaces was determined gravimetrically giving 9.0% for 25 Å, 7.7% for 60 Å, and 4.5% for 150 Å pore size. All samples were irradiated with an Oriel 1000 W Xe(Hg) lamp. A Corning 7-54 glass filter was used to isolate the wavelength range between 250 and 350 nm. In addition, a water filter was placed between the lamp and the sample cell to prevent overheating caused by infrared radiation incident on the sample. Two types of cells were 10.1021/es049192e CCC: $30.25
2005 American Chemical Society Published on Web 04/15/2005
FIGURE 1. Photodegradation of BeP 1.2 × 10-7 mol/g adsorbed on silica (25 Å pore size) under an air atmosphere using a static cell. Irradiation time: (--) 0 min; (‚‚‚‚‚) 0.5 min; (- - -) 3 min; (bold --) 8 min; (****) 30 min. used: a static quartz cell of 2 mm of path length and a Pyrex rotary cell. The static cell was used to perform the spectroscopic studies. The rotary cell was used to perform photodegradation studies that required large quantities of sample, as was the case of the analysis of photoproducts by HPLC. Photodegradation rate constants, valid only under the stated experimental conditions, were determined by fitting the fluorescence intensity of the samples as a function of the irradiation time to the first-order rate equation:
ln(F0/Ft) ) krt
(1)
where F0 and Ft are the fluorescence intensities at the monitoring wavelength at time zero and t, respectively, and kr is the apparent rate constant (see inset in Figure 1). Chromatographic Analysis. The photoproducts formed by the irradiation of BeP were analyzed by extracting, from 0.3 g of sample with 2.0 mL of methanol in an ice bath to prevent thermal degradation of some unstable photoproducts, stirred for 2 min, and filtered using a syringe with a Millipore disposable filter disk. Twenty microliters of the resultant solution were injected in a Waters HPLC instrument with a 966 diode array UV-vis detector and a SupelcosilLC-PAH reverse phase column (Supelco, 5 µm particle size, 4.6 mm diameter, 25 cm length). A 1 mL/min gradient of methanol and water, starting with 80% and increasing to 100% of methanol in 15 min, was used as mobile phase to analyze the photoproducts distribution. The major chromatographic peaks were fraction collected and further analyzed by fluorescence and mass spectrometry using electrospray ionization to characterize the BeP photoproducts. Some products that resulted labile in the methanolwater eluent were separated using 100% acetonitrile as mobile phase to avoid decomposition during the fraction collection and further analysis. The degree of conversion of BeP into the identified photoproducts was estimated using the molar absorption coefficients of the products and the response factor from the HPLC chromatograms. The response factor f was calculated using the following equation:
area ) fC
(2)
where the area represents the integrated area of the photoproduct peak in the HPLC chromatogram, is the molar absorption coefficient, and C is the amount (in moles) of compound in the volume injected (100 µL). The proportionality factor f was calculated by injecting known amounts of a standard solution of the compound of interest.
FIGURE 2. Effect of the surface nature and loading on the photodegradation rate of adsorbed BeP. The photolysis experiments were carried out in a rotary cell, and the BeP remaining in the sample was determined by HPLC. The uncertainties of the measurements are in the range of 4-10% of the absolute values. Photoproducts Analysis. A Cary 1E UV-visible spectrophotometer was used to record the absorption spectra of the fractions. The excitation and emission fluorescence spectra were measured in a 4800 SLM spectrofluorometer. A Quattro II quadrupole mass spectrometer with electrospray ionization was used to analyze the photoproducts. Each collected fraction was injected into the electrospray using a Hamilton syringe pump at a flow of 600 µL/min. The cone voltage was varied between 40 and 80 V, and the temperature was selected at 90 °C. At higher temperatures, the signal intensity was found to decrease, possibly due to thermal decomposition of the compounds.
Results and Discussion Photodegradation of Adsorbed BeP. Photodegradation studies of BeP adsorbed on alumina and silica gel of different average pore diameters, in the presence and absence of oxygen, and with or without coadsorbed water were carried out to establish the effect of these variables on the photodegradation rate and on the distribution of the photoproducts. The sample loading used was approximately 1 × 10-7 mol/g (0.03% of a formal monolayer) to ensure that it was in the linear range of the curve of fluorescence against concentration. This guaranteed that the BeP monomer was the irradiated species and allowed us to directly relate fluorescence and absorption intensities to the BeP loading at the surface. The photoirradiation of BeP adsorbed on silica gel or alumina surfaces exposed to air resulted in a decrease in the intensity of the absorption and emission bands (Figure 1), demonstrating the photodegradation of the PAH. Moreover, the unirradiated adsorbed sample was originally white but turned light brown during the photolysis. In a static cell under laboratory conditions, a half-life of 4 min was determined for BeP adsorbed on silica gel, and of 9 min for BeP adsorbed on alumina. The process followed a first-order kinetics (eq 1) with apparent rate constants of 0.173 min-1 for BeP adsorbed on silica and 0.089 min-1 for BeP adsorbed on alumina. Samples adsorbed on silica gel of 25, 60, and 150 Å were also irradiated in a rotary cell to study the effect of the pore size on the photodegradation rate. The rotary cell held up to 10 g of sample, but the photolysis is less efficient due to the loss of incident radiation, making the photodegradation much slower than in the static cell. On silica gels of 25 and 60 Å average pore diameters, similar photodegradation rates were determined, while a slightly faster rate was observed for the 150 Å silica gel (Figure 2). Nevertheless, the rates on VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the three silica gels were not very different, although they were 2 times faster than the photodegradation of BeP on alumina. Behymer and Hites (12) previously observed differences in the photostability of BeP adsorbed on alumina and silica gel. They determined that the photolytic half-life of BeP on activated alumina was 60% greater than on silica. Other five-rings PAHs, including benzo[a]pyrene and perylene, have shown the opposite tendency, being much more photoreactive when adsorbed on activated alumina than on silica gel. Nevertheless, studies made in our laboratory have also shown that perylene adsorbed on unactivated alumina photodegradates more slowly than on unactivated silica (15). The lower photodegradation rates reported for BeP on alumina were explained (10) in terms of a lower yield of formation and a faster decay of the BeP radical cation on this surface. This suggests that the PAH structure and the particular substrate and its pretreatment all can affect the photodegradation rate. The irradiation of BeP adsorbed on silica gel in the presence of oxygen generated a broad absorption band in the visible region with maximum at 415 nm. A low intensity band with maximum at 415 nm and a broad one around 500 nm appeared in the emission spectra (Figure 1). The intensity of these bands increased with irradiation time, suggesting that they corresponded to the formation of adsorbed photoproducts. Samples irradiated under an argon atmosphere presented photodegradation rates similar to those exposed to air. However, the emission bands at 415 and 500 nm did not appear, suggesting that the products formed under an inert atmosphere were different from those formed in the presence of O2. No change in the photodegradation rate was observed in the presence of coadsorbed water, although the rate of formation of the photoproducts decreased. Samples adsorbed on alumina irradiated in the presence of oxygen showed the formation of the 500 nm band, but did not present the emission at 415 nm, suggesting that the photoproducts formed were different. The reverse phase HPLC chromatograms of the extracts of irradiated samples of adsorbed BeP showed at least nine peaks in addition to that corresponding to BeP (Figure 3). The area under these peaks increased continuously with the irradiation time, providing evidence that these were from primary photoproducts. The retention times of the products were smaller than those of BeP, indicating that these compounds were more polar than BeP, possibly due to the incorporation of oxygen functionalities to the aromatic system. The absorption spectra of the major photoproducts separated by HPLC are shown in Figure 4. Photoproducts Characterization. The photodegradation product with a retention time of 13 min (P5) was one of the products that presented the largest peak area in the chromatograms of all of the adsorbed samples analyzed and was identified as benzo[e]pyrene-4,5-dione (Figure 5a). The UVvis spectrum of the BeP-4,5-dione in methanol (photoproduct P5 in Figure 4) presented two overlapping maxima at 255 and 261 nm and a broad absorption band of low intensity that extended from 360 to 500 nm. This compound has the aromatic skeleton of triphenylene, and its absorption spectrum was very similar to the absorption spectrum of this PAH of four rings (16). The fluorescence excitation spectrum showed two bands at 300 and 345 nm, and the emission spectra consisted of two bands at 390 and 415 nm. In addition, the electrospray mass spectrum of benzo[e]pyrene-4,5-dione presented a molecular ion with a m/z value of 283.3 (M + H)-, and two additional peaks at 255.3 and 227.2 m/z, corresponding to the loss of one and two CO groups, respectively. The assignment of the MS signals was confirmed by adding a standard. Because the electrospray ionization method used to analyze this photoproduct was in the negative ion mode, it was expected that the parent ion corresponded 3648
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to the (M - H)- ion. However, some compounds with high electron affinity can form doubly negative charged ions during the ionization processes in the presence of electron-donating solvents. Because aromatic quinones are good electron acceptors capable of forming M2- ions (17), the subsequent proton abstraction from the solvent should give the observed signal for a (M2- + H+) species, with a total charge of -1. In irradiated samples at a high surface loading corresponding to a coverage of 2% of a monolayer (4.5 × 10-6 mol/g), one minor product which eluted with a retention time of 15 min was identified as 3-hydroxybenzo[e]pyrene (Figure 5b). Its electrospray mass spectrum (Figure 6a) presented only one peak at 267.3 m/z corresponding to the (M - H)- ion. The absorption and emission spectra depicted vibronic structure similar to the spectrum of BeP, although blue shifted. The identity of this compound was verified by co-injection of a standard solution of 3-hydroxybenzo[e]pyrene and an extract of photolyzed BeP. The absorption spectra and the retention time of the peak P6 were similar to those of the 3-hydroxybenzo[e]pyrene, suggesting that this product could be also a hydroxy-BeP. Its electrospray mass spectrum (in the negative ionization mode) presented a peak at 267.3 m/z, indicating that it corresponded to a BeP phenol. Ultraviolet absorption data for only a few hydroxy derivatives of BeP are available in the literature (18). The absorption maxima and the relative intensities of the bands corresponding to the photoproduct P6 recorded in a methanol solution were comparable to those reported for 1hydroxybenzo[e]pyrene (Table 1). The differences in the absorption maxima of the spectrum of P6 in methanol and those reported for 1-hydroxybenzo[e]pyrene in ethanol are less than 5 nm and may be due to the different solvents used. A possible mechanism for the formation of 1- and 3-hydroxybenzo[e]pyrene is presented in Scheme 1. The photoproduct P7 presented a relatively large retention time of 19 min suggesting that it was a compound of relatively low polarity, whereas the electrospray mass spectrum (parent ion at 283.3 m/z) indicated that it was a dione. The only two known diones of BeP are the 4,5-BeP-dione and its 9,10 isomer. The UV-vis absorption spectrum of P7 (Figure 4) was very different from that of the 9,10-BeP-dione; the extracted photoproduct presented absorption maxima at 445, 380, 305, and 232 nm, whereas the 9,10-BeP-dione showed absorption maxima at 503, 424, 332, 319, and 272 nm (19). The fluorescence emission spectra of P7 presented two main bands at 400 and 425 nm and two weaker broad bands at 475 and 510 nm. Comparing the spectral data of this dione with the available spectral information of pyrene and benzo[a]pyrene (20) diones suggested that the BeP photoproduct might be a quinone with the two carbonyl functionalities located distantly in the BeP molecule (Figure 5e). The absorption spectra of the PAH’s diones with adjacent carbonyls generally present a strong and sharp absorption band around 260 nm and a weaker band at longer wavelengths (in the 350-400 nm region). The diones of PAHs with the carbonyl groups positioned separately commonly present several absorption bands of similar intensities extending from 200 to 450 nm. Moreover, the nonadjacent diones exhibit fluorescence emission at longer wavelengths than the adjacent diones (21), supporting the suggestion that photoproduct P7 was a nonadjacent dione. This product was observed as a major product only in samples of BeP adsorbed on silica gel of average pore size of 25 Å. In samples irradiated under a nitrogen atmosphere, it appeared although in smaller amounts. This dione was very labile in solution, specially in protic solvents such as water and methanol, but was very stable when adsorbed on the silica gel surface and could last several weeks without significant degradation. Because the PAHs and its degradation products are found in the atmosphere mainly adsorbed on the particulate matter, the
FIGURE 3. HPLC chromatogram of extracts of photolyzed samples of BeP adsorbed on silica gel of different pore sizes under air atmosphere. Irradiation time: 12 h. possible harmful effects of this compound could be important in the environment despite its low stability in solution. The photoproduct with retention time of 9 min (P3) presented a UV-visible spectrum with maximum absorption at 261 nm (Figure 4). Because of its rather short retention time, it was expected to correspond to a relatively polar product. The absorption spectrum of the 4,5-BeP-dihydrodiol reported in the literature (22) is identical to that recorded for the photoproduct P3. The recorded electrospray mass spectrum corresponding to the chromatographic peak P3 was of very low intensity and was noisy. The MS in the positive mode presented a signal at 287.0 m/z that might be attributable to the (M + H)+ ion (Figure 6a). However, the major peaks at 297 and 315 m/z were difficult to assign due in part to the instability and tendency of aromatic diols to form adducts with the solvent during the ionization process
(23). Because the compound was so labile, it was not possible to isolate it. Even when the collection was done in an ice bath and the fraction kept under an N2 atmosphere, the compound decomposed. Nonetheless, photoproduct P3, identified as the 4,5-BeP-dihydrodiol, was a primary photoproduct of BeP. Thus, its formation did not proceed from the degradation of the 4,5-BeP-dione. Moreover, the dione was also a primary photoproduct, implying that it was not formed by oxidation of the diol. Therefore, both compounds must be produced independently, the diol being a possible product of the incomplete oxidation of the BeP ring. A plausible reaction mechanism for the formation of the 4,5-BeP-dione and the diol is depicted in Scheme 2. The proposed mechanism initiates with the photoionization of the BeP molecule to form the radical cation. Further reaction of the cation with water and oxygen coadsorbed on the surface VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Absorption spectra of the major photoproducts of BeP adsorbed on silica gel of 25 Å pore size. The spectra were recorded at the exit of the HPLC column with a UV-vis diode array detector.
FIGURE 5. Molecular structure of some photoproducts of BeP. yields a peroxide radical, which gives the 4,5-BeP-dihydrodiol and dione through a disproportionation reaction. The photoproduct P1 exhibited an absorption spectrum similar to that of the dione P7, although shifted to the blue. This peak (P1) appeared at a very short retention time (2.5 min), suggesting that this photoproduct is highly polar, probably a tetraol or a tetrahydro-tetraol derivative of BeP. The photoproduct P1 was very unstable in solution. Even adsorbed on silica gel, it decomposed in a few hours if it was not stored under an inert atmosphere at 4 °C. The photoproducts P2 and P4 exhibited UV-vis spectra comparable to those of P3 and P5. These may correspond to derivatives of BeP substituted in the positions 4 and 5 but containing functionalities other than hydroxyl and carbonyl groups. The photoproduct P8 presented an absorption spectrum with a highly defined vibronic structure, similar to that of BeP. Assuming that this product has a molar coefficient comparable to that of BeP, it was present in significant amounts only in photolyzed samples at high loadings. The large retention time of P8 (24 min) suggests that it was a relatively nonpolar compound. Because only the 4,5-BeP dione and the 3-hydroxy-BeP standards were available, and the of the dione P7 (with Rt ) 19 min) was unknown, the yield of this compound could not be determined exactly. The yield was estimated assuming a molar absorption coefficient for P7 of 2 × 104 M-1 cm-1 at 254 nm, which is in the order of magnitude of the of the
nonadjacent diones of BaP and pyrene. The of the 4,5BeP-diol had been reported in the literature as 5.97 × 104 M-1 cm-1 at 254 nm in methanol (22). Assuming that the response factor f (as defined in eq 1) is independent of the compound under study, and using an average of those determined for 3-OH-BeP and 4,5-BeP-dione, the amount of the 4,5-BeP-dihydrodiol produced during the photolysis was also estimated. The results for a sample of BeP adsorbed on silica gel 25 Å pore size at an initial loading of 2.3 × 10-7 mol/g (less than 0.1% of a monolayer) photolyzed for 4 and 8 h are shown in Table 2. The efficiency of the extraction process was taken into consideration in these determinations by using a recovery percentage of 40% for BeP as well as for the photoproducts. The percent of conversion was determined with respect to the amount of BeP degraded during the irradiation period. For samples photolyzed for 8 h under an air atmosphere, approximately 19 % of the destroyed BeP was converted to the 4,5-BeP-dione. Under a nitrogen atmosphere, only 4% of the BeP was converted to the 4,5BeP-dione after 8 h of irradiation. The yield of 3-hydroxyBeP was still very low, only a 3% of conversion in samples with a high initial BeP coverage (2% of a monolayer). The yield of the 1-hydroxy-BeP was not determined because its molar absorption coefficient is not available in the literature. Effect of the Surface, Coadsorbed Species, and BeP Loading on the Photodegradation. The relative distribution of the photoproducts, as well as the photodegradation rate, was affected by the average pore size of the surface. For samples adsorbed on silica gel of 25 Å pore size, the diones were the major photoproducts (assuming similar ), while for BeP adsorbed on silica gel of 60 and 150 Å, the nonadjacent dione (P7) was a minor product. For samples adsorbed on silica gel of 150 Å pore size, the number of photoproducts increases considerably. The distribution of photoproducts for samples of BeP adsorbed on alumina was also different from that for silica gel of 25 Å at similar surface loadings. The relative distribution of photoproducts did not change significantly with an increasing loading concentration of benzo[e]pyrene. However, at high surface coverage, the relative intensities of the peaks varied, and several minor products, which at low loadings were part of experimental noise, could be detected (Supporting Information). For example, the area of the peak corresponding to the nonadjacent dione P7 was twice as large as the area of peaks P1 VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 6. Electrospray mass spectra of 3-hydroxybenzo[e]pyrene (a) and chromatographic peak P3 (b).
TABLE 1. Absorbance Maxima (in nm) of 1-OH-BeP in Ethanol (from Ref 18) and Those of the Photoproduct P6 in Methanola
a
1-hydroxy-BeP
P6
225 (1.0) 230 (0.83) 263 (0.81) 280 (0.77) 295 (0.90) 350 (0.45)
225 (1.0) 234 (0.70) 265 (0.70) 283 (0.72) 291 (0.75) 346 (0.40)
The numbers in parentheses are the relative intensities of the bands.
and P5 (4,5-BeP-dione) in the chromatograms of samples of coverage of 2% of a monolayer, whereas in the samples with coverage of approximately 0.1%, the areas of these three peaks were similar. In addition, the peaks P2, P6 (1-hydroxy-BeP), and P8 were at the noise level in the chromatograms of low loading samples, but in samples of high loading, they were 3652
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well defined and with areas comparable to those of peaks P1 and P5. The photodestruction rate was significantly affected by an increase in surface loading. Only a 30% destruction of BeP was achieved in samples of BeP at an initial coverage of 4.5 × 10-6 mol/g irradiated during 8 h, whereas for samples that were 15 times less concentrated an almost total destruction of BeP was reached in the same time. Irradiation of the high loading samples for additional time did not lead to significant further destruction of BeP, as can be noted in Figure 2 from the plateau reached at long irradiation times. At this high loading, the presence of aggregates becomes important (14), and it is possible that these aggregates were less photoreactive than the monomers that dominate at lower loadings. If aggregates are reactive, one expects that dimers of BeP or photoproducts of BeP dimers to be detected as products of the photodegradation. Toluene and methylene chloride are generally used to extract PAH’s dimers from the surfaces (24). However, the extraction of irradiated samples
SCHEME 1. Possible Reaction Pathway for the Formation of 1-Hydroxy-BeP and 3-Hydroxy-BeP
TABLE 2. Percent of Conversion of Some Photoproducts of BeP Adsorbed on Silica Gel (Initial Surface Loading: 2.3 × 10-7 mol/g) and Photolyzed under an Air Atmosphere photoproduct 4,5-BeP-dione P7 4,5-BeP-dihydrodiol 3-hydroxy-BeP
SCHEME 2. Proposed Reaction Mechanisms for the Formation of 4,5-BeP-dihydrodiol and 4,5-BeP-dione
of BeP at high loading with these solvents did not reveal the presence of dimers, supporting the hypothesis of low photoreactive aggregates. When samples of a low loading (0.1% of a monolayer) were irradiated under a nitrogen atmosphere, the nonadjacent dione (P7) was not formed. Under these conditions, the photolysis took more than 8 h to produce an 80% of photodestruction of BeP. It is possible that the sample lost part of the adsorbed water because a continuous flow of nitrogen was passed through the sample cell during the irradiation to maintain the inert atmosphere. Oxygen was also removed from the cell by evacuation of the sample at a pressure of less than 1 Torr with a mechanical pump for
% of conversion in 4 h
% of conversion in 8 h
14 61 3
19 49 3 3
2 h. The irradiation of this sample resulted in the formation of the same two major photoproducts, the 4,5-BeP-dione and the nonadjacent dione. This suggested that the oxygen in the gas phase did not have a significant effect on their formation mechanism, although adsorbed water on the surface could have participated in these processes. To study the effect of the adsorbed water, a sample of BeP adsorbed on dry silica gel was irradiated in a closed cell under vacuum for 8 h. The HPLC chromatograms of the extracts of the irradiated sample did not show any peaks besides that of BeP. However, the area of the BeP’s peak decreased 80%, demonstrating its photodegradation. It is possible that the photoproducts formed on the dry surface were very polar and thus were strongly adsorbed to the surface, or that photoexcited BeP reacted with the silanol groups on the silica gel during the irradiation, resulting in products chemically bound to the surface. Several organic solvents such as acetonitrile, acetone, methylene chloride, hexane, and toluene were used in the solvent extraction process without success to extract the photoproducts. Considering the possibility that very polar derivatives of BeP containing acidic or basic functionalities were formed, water, acidic, and basic aqueous solutions were also tried as extraction solvents. None of these efforts were effective in removing the adsorbed products from the surface even when the samples were sonicated for 30 min. Although the photoproducts could not be extracted, it is evident that adsorbed water plays a determinant role in the photodegradation mechanism of BeP on the surface. Photodegradation of BeP in Solution. Photodegradation studies of BeP in solution were also performed to compare the photochemical behavior of the PAH adsorbed on surfaces and in solution. Because it has been reported that diesel soot particles are covered by a thin liquid organic phase (25-27), organic solvents such as hexane, hexadecane, and toluene have been used in several studies as models of the aerosols formed during the diesel combustion. Therefore, photodegradation studies of PAHs in organic solvents could provide information about the fate of these compounds in diesel exhaust aerosols. Irradiation of solutions of BeP in hexane in a static cell of 1 cm of path length under an air atmosphere led to its almost complete degradation in 6 h. These results were very different from those obtained by McDow et al. (27), who found that BeP in a hexadecane solution did not degrade even after 6 h of irradiation with a 450 W mercury lamp and a borosilicate filter. The absorption spectra of photolyzed solutions of BeP in hexane showed a broad absorption band of low intensity that appeared between 200 and 430 nm (Figure 7). The intensity of this band increased with the irradiation time, implying that it corresponded to the formation of photoproducts absorbing at these wavelengths. The fluorescence emission spectra of irradiated solutions of BeP in hexane (λex ) 330 nm) presented a broad band that extended from 400 to 550 nm. These bands appeared at shorter wavelengths than those seen during the irradiation of BeP adsorbed on silica gel. The HPLC analysis of the irradiated solutions showed that the main photoproducts of irradiated BeP in hexane appeared at retention times smaller than 12 min. The UV-vis absorption spectra VOL. 39, NO. 10, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 7. Absorption spectra of BeP in hexane at different irradiation times under air atmosphere. Initial concentration: 4.5 × 10-5 M. of these compounds were different from those for products with similar retention times from adsorbed samples. Furthermore, the major photoproducts found in samples of BeP adsorbed on silica gel (the BeP’s diones) were not observed in the irradiated solutions of BeP in hexane. These results suggested that BeP in nonpolar solvents photodegrades through a different reaction pathway than when adsorbed on surfaces. As mentioned above, the photoreaction mechanism of adsorbed BeP is proposed to occur through a radical cation-mediated pathway. Laser flash photolysis studies of BeP in solution have shown that BeP in polar solvents readily photoionizes to produce a radical cation, whereas in hexane solutions the BeP’s radical cation absorption bands were not observed probably due to a poor stabilization of the radical ions (10). Thus, other radical species could be participating in the photodegradation mechanism. The photodegradation of BeP in acetonitrile was much faster than the photolysis in hexane solutions. The complete destruction of BeP was achieved in less than 90 min. The UV-vis spectra of the samples irradiated for at least 10 min showed a band at 260 nm, whose intensity increased with irradiation time up to 30 min of photolysis. At longer irradiation times, this band begun to disappear due to the degradation of the primary photoproducts absorbing at this wavelength. Additionally, a broad band that extended from 300 to 450 nm with low intensity was formed during the photodegradation. The HPLC chromatograms of the photolyzed solutions of BeP in acetonitrile showed the presence of the 4,5-BeP-dione as observed in adsorbed samples. This compound was the major photoproduct formed during the first 10 min of irradiation. At irradiation times of 20 min or longer, a peak with retention time of 23 min became the main product. This compound presented an absorption spectrum similar in shape to that of the 4,5-BeP-dione, although blue-shifted with absorption maximum at 240 nm. It is possible that this photoproduct corresponded to a secondary product formed by photodegradation of the dione. Several unresolved peaks with retention times less than 7 min were also observed in the photolyzed solutions. However, these products were in small amounts and their absorption spectra were very noisy. The photoproducts observed in acetonitrile solutions were different from those observed during the irradiation of BeP in hexane, suggesting that a different photodegradation process occurred in these two solvents. Because acetonitrile is a polar solvent, the reaction mechanism is thought to occur through a radical cationmediated process, as is the case of BeP adsorbed on silica gel. The photodegradation of pyrene in polar solvents (water and methanol) also occurs through a radical cation-mediated mechanism (28) similar to the photodegradation of pyrene 3654
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adsorbed on silica gel. However, the photodegradation of pyrene and other PAHs in cyclohexane solutions has been proposed to occur through homolysis of an excited PAH molecule to form a PAH neutral radical and a hydrogen radical, producing PAHs-solvent adducts as photoproducts (29). In summary, the mechanism of photodegradation of BeP in solution was influenced by the microenvironment polarity in the same way that has been observed for other PAHs such as pyrene, perylene, and acenaphthene. BeP adsorbed on inorganic oxides photodegraded slowly in comparison with other PAHs such as pyrene, benzo[a]pyrene, perylene, and anthracene. One of the major photoproducts of BeP in the adsorbed samples and in acetonitrile solutions was identified as 4,5-BeP-dione. A 4,5-BeP-dihydrodiol (P3) was also found in samples of BeP adsorbed on silica gel. Two phenols were identified as minor products, 3-hydroxy-BeP and 1-hydroxyBeP (P6), and one of the photoproducts detected in irradiated samples of BeP adsorbed on silica gel of 25 Å pore size was isolated and partially characterized as a dione. Some of the isolated photoproducts were unstable in solution. This suggests that in the atmospheric aerosols containing a liquid organic or water phase, these unstable products could decompose having shorter lifetimes in the atmosphere than when adsorbed on inorganic surfaces. These results stress the importance of studying the photodegradation of organic pollutants in different environments to help determine the possible sinks and fate of these contaminants. The 4,5-BePdihydrodiol and the 3-hydroxy-BeP have been identified as BeP metabolites with no carcinogenic activity (30). However, there is no information on the toxicity and mutagenic activity of BeP’s diones or of the other photoproducts of BeP. It is known that aromatic radicals and diones are very reactive species capable of producing extensive DNA damage (31). Therefore, the photochemical degradation of BeP seems be a process that could increase the toxicity of the particulate matter instead of an appropriate remediation process to remove this PAH from the environment.
Acknowledgments We acknowledge the financial support of EPA-USA (Grant R-823328-01-0) and the BRIN-PR program (Grant P20 RR16470, NCRR-NIH). We also acknowledge the National Cancer Research (Midwest Research Institute) for providing us with BeP derivatives used as standards.
Supporting Information Available Two figures. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) (a) Ramdahl, T. Retene-a molecular marker of wood combustion in ambient air. Nature 1983, 306, 580. (b) Nielsen, T. Traffic contributions of polycyclic aromatic hydrocarbons in the center of a large city. Atmos. Environ. 1996, 30, 3481. (c) Westerholm, R. N.; Alsberg, T.; Frommelin, A. B.; Strandell, M. E.; Rannug, U.; Winquist, L.; Grigoriadis, V. Egeback, K.-E. Effect of fuel polycyclic aromatic hydrocarbon content on the emissions of polycyclic aromatic hydrocarbons and other mutagenic substances from a gasoline-fueled automobile. Environ. Sci. Technol. 1988, 22, 925. (2) Baek, S. O.; Field, R. A.; Goldstone, M. E.; Kirk, P. W.; Lester, J. N.; Perry, R. A review of atmospheric polycyclic aromatic hydrocarbons; Sources, fate and behavior. Water, Air, Soil Pollut. 1991, 60, 279. (3) (a) Pitts, J. N., Jr.; Grosjean, D.; Mischke, T. M.; Simon, V. F.; Poole, D. Mutagenic activity of airborne particulate organic pollutants. Toxicol. Lett. 1997, 1, 65. (b) U.S. National Academy of Sciences, Particulate Polycyclic Organic Matter; Committee on Biological Effects of Atmospheric Pollution, National Research Council, National Academy Press: Washington, DC, 1972.
(4) Miller, E. C.; Miller, J. A. Searches for ultimate chemical carcinogens and their reactions with cellular macromolecules. Cancer 1981, 47, 2327. (5) Dabestani, R.; Ellis, K. J.; Sigman, M. E. Photodecomposition of anthracene on dry surfaces: products and mechanism. J. Photochem. Photobiol., A 1995, 86, 231. (6) Reyes, C.; Medina, M.; Crespo-Hernandez, C.; Ceden ˜ o, M.; Arce, R.; Rosario, O.; Steffenson, D. M.; Ivanov, I. N.; Sigman, M. E.; Dabestani, R. Photochemistry of pyrene on unactivated and activated silica surfaces. Environ. Sci. Technol. 2000, 34, 415. (7) Sigman, M. E.; Barbas, J. T.; Chevis, E. A.; Dabestani, R. Spectroscopy and photochemistry of 1-methoxy naphthalene on silica. New J. Chem. 1996, 20, 243. (8) Sotero, P.; Arce, R. Surface and adsorbates effects on the photochemistry and photophysics of adsorbed perylene on unactivated silica gel and alumina. J. Photochem. Photobiol., A 2004, 167, 191. (9) (a) Atkinson, R.; Arey, J.; Zielinska, B.; Aschmann, S. M. Kinetics and nitro-products of gas-phase OH and NO3 radical-initiated reactions of naphthalene, fluoranthene and pyrene. Int. J. Chem. Kinet. 1990, 22, 999. (b) Calvert, J. G.; Atkinson, R.; Becker, K. H.; Kamens, R. M.; Seinfeld, J. H.; Wallington, T. J.; Yarwood, G. The Mechanisms of Atmospheric Oxidation of Aromatic Hydrocarbons; Oxford University Press: New York, 2002. (10) Fioressi, S.; Arce R. Excited states and intermediate species of benzo[e]pyrene photolyzed in solution and adsorbed on surfaces. J. Phys. Chem. A 2003, 107, 5968. (11) Lane, D. A.; Katz, M. In Fate of Pollutants in the air and water environment, Part 2; Suffet, I., Ed.; Wiley-Interscience: New York, 1977; pp 137-154. (12) Behymer, T. D.; Hites, R. A. Photolysis of polycyclic aromatic hydrocarbons adsorbed on fly ash. Environ. Sci. Technol. 1988, 22, 1311. (13) Korfmacher, W. A.; Wehry, E. L.; Mamantov, G.; Natusch, D. F. S. Resistance to photochemical decomposition of polycyclic aromatic hydrocarbons vapor-adsorbed on coal fly ash. Environ. Sci. Technol. 1980, 14, 1094. (14) Fioressi, S.; Arce, R. The effect of physical and chemical properties of the surface on the photochemistry and spectroscopy of adsorbed benzo[e]pyrene. Polycyclic Aromat. Compd. 1999, 14-15, 285. (15) Sotero, P.; Arce, R. Studies on the phototransformations of perylene adsorbed on nonactivated silica gel and alumina as models of the atmospheric particulate matter. Polycyclic Aromat. Compd. 1999, 14-15, 295. (16) Sadtler Research Laboratories. Sadtler Standard Spectra; Sadtler Research Laboratories: Philadelphia, PA, 1974-1976. (17) Van Berkel, G. J.; McLuckey, S. A.; Glish, G. L. Preforming ions in solution via charge-transfer complexation for analysis by electrospray ionization mass spectrometry. Anal. Chem. 1991, 63, 2064. (18) Lee, H.; Shyamasundar, N.; Harvey, R. G. Isomeric phenols of benzo[e]pyrene. J. Org. Chem. 1981, 46, 2889.
(19) Platt, K. L.; Oesch, F. Synthesis of non-K-region ortho-quinones of polycyclic aromatic hydrocarbons from cyclic ketones. Tetrahedron Lett. 1982, 23, 163. (20) 20. Handbook of Analytical end Spectral Data for PAH; Midwest Research Institute: Kansas City, MO, 1993; Vol. I. (21) Becker, R. S.; Natarajan, L. V. Comprehensive absorption, photophysical/chemical, and theoretical study of 2-5 ring aromatic hydrocarbon diones. J. Phys. Chem. 1993, 97, 344. (22) Lehr, R. E.; Taylor, C. W.; Kumar, S. Synthesis of the non-Kregion and K-region trans-dihydrodiols of benzo[e]pyrene. J. Org. Chem. 1978, 43, 3462. (23) Koeber, R.; Niessner, R.; Bayona J. M. Comparison of liquid chromatography-mass spectrometry interfaces for the analysis of polar metabolites of benzo[a]pyrene. Fresenius’ J. Anal. Chem. 1997, 359, 267. (24) Dabestani, R. Photophysical and photochemical behavior of polycyclic aromatic hydrocarbons on silica surfaces. Inter-Am. Photochem. Soc. Newsl. 1997, 20, 24. (25) McDow, S. R.; Vartiainen M.; Sun, Q.; Hong, Y.; Yao, R.; Kamens, R. M. Combustion aerosol water content and its effect on polycyclic aromatic hydrocarbon reactivity. Atmos. Environ. 1995, 29, 791. (26) Odum, J.; McDow, S. R.; Kamens, R. M. Mechanistic and kinetic tudies of the photodegradation of benz[a]anthracene in the presence of methoxy phenols. Environ. Sci. Technol. 1994, 28, 1285. (27) McDow, S. R.; Sun, Q.; Vartiainen, M.; Hong, Y.; Fister, T.; Yao, R.; Kamens, R. M. The effect of organic composition on polycyclic aromatic hydrocarbons decay in atmospheric aerosols. Environ. Sci. Technol. 1994, 28, 2147. (28) Kubat, P.; Civis, S.; Muck, A.; Barek, J.; Zima, J. Degradation of pyrene by UV radiation. J. Photochem. Photobiol. 2000, 132, 33. (29) Liu, A.; Loffredo, D. M.; Trifunac, A. D. Photoionization and ensuring ion-molecule reactions of polycyclic aromatic hydrocarbons in alkane and alcohol solutions. J. Phys. Chem. 1993, 97, 3791. (30) (a) Katz, M.; Chan, C.; Tosine, H.; Sakuma, T. In Polynuclear Aromatic Hydrocarbons; Jones, P. W., Leber, P., Eds.; Ann Arbor Science: Ann Arbor, MI, 1979; pp 171-189. (b) Pierce, R. C.; Katz, M. Chromatographic isolation and spectral analysis of polycyclic quinines. Applications to air pollution analysis. Environ. Sci. Technol. 1976, 10, 45. (31) Flowers-Geary, L.; Harvey, R. G.; Penning, T. M. Cytotoxicity of polycyclic aromatic hydrocarbon o-quinones in rat and human hepatoma cells. Chem. Res. Toxicol. 1993, 6, 252.
Received for review May 31, 2004. Revised manuscript received December 20, 2004. Accepted February 3, 2005. ES049192E
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