Environ. Sci. Technol. 1994, 28, 1285-1 290
Mechanistic and Kinetic Studies of the Photodegradation of Benz[ alanthracene in the Presence of Methoxyphenols Jay R. Odum, Stephen R. McDow,' and Richard M. Kamens
Department of Environmental Sciences and Engineering, School of Public Health, University of North Carolina at Chapel Hill, Chapel Hill, North Carolina 27599-7400 Kinetic studies were employed to assess an empirical rate law describing the rate of photodegradation of polycyclic aromatic hydrocarbons (PAH) in the presence of substituted methoxyphenols. A solution of benz[alanthracene (BaA) and vanillin in toluene was chosen as the model system. A rate law and corresponding rate constants were determined for this system: -d[BaAl/dt = k~[BaAl+ k~[BaAl[vanillin], where k~ = 0.024 f 0.005 h-I and k B = 81.17 f 3.19 M-l h-*. Further experiments using structure-reactivity relationships were applied to the model system to investigate the mechanism for BaA photodegradation. Data from these experiments suggest that the rate-determining step in the mechanism is hydrogen abstraction of the phenolic hydrogen from vanillin. The photochemical reaction between PAH and methoxyphenols is most likely a very important degradation pathway for PAH associated with wood combustion aerosol. Introduction Photochemical reaction is an important removal process for atmospheric polycyclic aromatic hydrocarbons (PAH) (1,2).PAH photochemical reactivity in organic solvents has been studied extensively for over 40 years, and singlet oxygen has been determined as the most important oxidant (3-6). There has also been a considerable amount of research conducted on gas-phase PAH reactivity, and it is known that hydroxyl attack is the major atmospheric oxidation pathway (7, 8). However, many of the most mutagenic PAH in the atmosphere are primarily associated with combustion source aerosols (9). PAH associated with combustion source particles are also photochemically reactive (10,11),but there has been very little work done in determining mechanisms of photodegradation for particulate-bound PAH because information on particle composition is generally incomplete and varies considerably from source to source. Reported photolysis rates for particulate-bound PAH vary widely (12-14). I t has been suggested that one reason such variations exist is that the rates are strongly influenced by the chemical and physical surface properties of the associated particles (15). These properties vary significantlydepending on the source,fuel, and combustion conditions under which the particles are generated. For example, 30-60 % of the carbon associated with diesel soot is elemental (16).The rest is comprised mostly of nonpolar aliphatic and aromatic organics ( I 7).In contrast, as much as 90 % of the carbon associated with wood smoke particles is organic (18). This organic carbon is much more polar than that associated with diesel soot and contains large amounts (120-300 pg/mg of particulate carbon) of methoxylated phenols (19). Wood soot particles also form a
*
To whom correspondenceshould be addressed.
0013-936X/94/0928-1285$04.50/0
0 1994 American Chemical Society
viscous liquid when collected by impaction, and it has been suggested that PAH might be dissolved in a liquid layer comprised of these organics on the particle (20,21). It was previously reported by members of this group that methoxyphenols greatly enhance the photodegradation of PAH in solution (20). Since methoxyphenols comprise such a large fraction of the organic material associated with wood smoke (19),it seems reasonable that they may participate in the degradation of PAH associated with wood soot. Therefore, it is desirable to try to gain an understanding of the mechanism by which methoxyphenols participate in this reaction and to formulate a method to obtain a rate law and rate constants for several PAH and methoxyphenols. In this paper, we will discuss the experiments used to develop this methodology and present a rate law, rate constants, and possible mechanism for a model system-benz[alanthracene (BaA)and vanillin in a toluene solution. Experimental Section All photodegradation experiments were carried out in a merry-go-round reactor (Ace Glassware, Vineland, NJ) equipped witha 450-W medium pressure mercury arc lamp in a quartz immersion well and a 366-nm filter. The entire apparatus was submerged in an 18 gal water bath that was maintained at a constant temperature of 16.5 f 0.5 "C by pumping the water (flow rate = 75 mL/min) through exterior copper coils that were submerged in a 20 gal ice bath. Samples were irradiated in 13mm X 100mm quartz test tubes (Ace Glassware). Light intensity was periodically measured with a ferrioxalate actinometer (20) and averaged around 2.2 f 0.3 x 1016 photons/s. All quantitative analyses were conducted on a HewlettPackard 5890 gas chromatograph (GC) equipped with a J&W 30-m, 0.32 mm i.d., DB-5 column with a 0.25-pm film thickness and interfaced to a Hewlett-Packard 5971A mass selective detector (MSD). All injections were performed on column, and the acquisition mode was selected ion monitoring. The temperature program was as follows: 110 "C for 2 min, 110-220 "C at 12 "C/min, 220-270 "C at 6 "C/min, 270-300 "C at 20 "C/min, hold a t 300 "C for 1 min. Experiments designed to assess structure-reactivity relationships were performed by preparing four sets of samples simultaneously. Each set consisted of four samples and one standard. All solutions were prepared in optima-grade toluene (Fisher T291-4). The concentration of benz[alanthracene (Aldrich B220-9) in all sets was 1X M. The final volume of each sample was 5 mL. Each set contained a different para-substituted guaiacol. Set 1contained 1 X M acetovanillone (Aldrich A1,080-9). Set 2 contained 1 X 10-3 M 4-hydroxy-3-methoxybenzonitrile (Aldrich 16,260-4). Set 3 contained 2 x lo4 M 4-nitroguaiacol (Aldrich 32,682-8). Set 4 contained 1X 10-3M vanillin (Aldrich V110-4). Four 5-mL samples from each set were irradiated simultaneously. The rate Environ. Scl. Technol.. Vol. 28, No. 7, 1994 1285
~~ _
Table 1. Correlation between Vanillin Concentration and kob.
[BaA](M)
set no.
[vanillin] (M)
1 2 3 4
2 x 10-3
l X l P
1x103
1 x 104
5X10-4 2x104
lXl0d lXl0d
kobs 0 - l )
0.176 rt 0.017O 0.097 rt 0.003 0.058 f 0.006 0.044 i 0.004
Equals 1 standard error of slope. 0.19,
,
1
h
r
L
G d
9
[Vanillin] ( ~ 1 0 ~ ~ )
Figure 1. Correlation between observed, pseudo-first-order rate constants for BaA photodegradation and vanillin concentration.
of decay of BaA was monitored by removing one sample from each set from the reactor every hour for 4 h and quantitating on the GC/MSD. Rate law experiments were conducted by preparing four sets of samples simultaneously. All solutions were prepared in optima grade toluene. The concentration of BaA in all samples was 1X lo4 M. The concentration of vanillin M, in set 2 was 1 X 103M, in set 3 in set 1 was 2 X was 5 X lo4 M, and in set 4 was 2 X lo4 M. The rate of decay of BaA was monitored by removing one sample from each set from the reactor every hour for 4 h and quantitating on the GC/MSD. Results and Discussion
It was previously determined by this group that the photodegradation of benz[a]anthracene (BaA) exhibits pseudo-first order rate behavior when samples have a vanillin concentration that is at least 20 times greater than BaA (20). The rate law that describes this behavior is -d[BaAl/dt = kob,[BaAl
(1)
To elucidate the form Of kobs, an experiment was performed to examine the relationship between vanillin concentration and hobs. Four sets of samples, each containing different concentrations of vanillin, were irradiated in the photoreactor simultaneously. The data is listed in Table 1. A plot of kobs versus vanillin concentration gives a straight line (r2= 0.995) with a slope of 81.17 f 3.19 M-' h-' and a y-intercept of 0.024 f 0.005 h-1 (Figure 1). Since this plot yields a straight line with a positive intercept, the rate equation for BaA photodegradation in the presence of vanillin is
1286 Envlron. Sci. Technol., Vol. 28, No. 7, 1994
_
where k A = y-intercept and k~ = slope. This type of treatment requires that vanillin concentration be relatively constant over the course of the experiment. In all four sets, vanillin degradation was less than 15%. The form of this rate law implies that there are at least two mechanisms responsible for BaA photodegradation in this system. Considering that PAH are known sensitizers of singlet oxygen and that attack of singlet oxygen is the major photodegradation pathway for PAH in organic solvents (3-6),it seems reasonable to assume that the term independent of vanillin concentration is due to singlet oxygen attack on BaA. Furthermore, benz[alanthracene7,12-dione, which is a known product of singlet oxygen addition to BaA, has been identified as a reaction product by matching its mass spectrum to a standard. However, a singlet oxygen mechanism of this type generally produces second-order kinetics (3-6). Thus, further study is needed to confirm the mechanism that is responsible for the first term in the rate law. Given the above rate expression, an attempt was made to determine the mechanism responsible for the second term in the rate law. In 1972, Matsuura et al. documented that triplet sensitizers were capable of abstracting phenolic hydrogen from catechol and hydroquinone derivatives (22). This precedence suggested that in our model system BaA may be acting as a triplet sensitizer and abstracting the phenolic hydrogen of vanillin to create a phenoxy radical and a BaA radical. In order to test this hypothesis, experiments using the principles of structure-reactivity relationships were employed. In 1991, Bordwell and Cheng showed that electrondonating para-substituents (EDPS) and electron-withdrawing para-substituents (EWPS) were able to stabilize phenoxy radicals (23). They found that a linear free energy relationship existed between the stabilizing effects of electron-donating substituents and their u+ parameters. The u+ parameters are defined as the log of the ratio of the acid dissociation constants for different para-substituted tert-cumyl chlorides to the acid dissociation constant for unsubstituted tert-cumyl chloride. They are a measure of a substituent's ability to stabilize an electron-deficient reaction site that is para to the substituent, by coming into direct resonance with the reaction site. The relationship that Bordwell and Cheng found suggests that d parameters are a measure of the stability of phenoxy radicals with EDPS. Thus, in order to determine if BaA was abstracting hydrogen from vanillin to create phenoxy radicals, an experiment was performed to see if BaA rates of photodegradation, in the presence of various parasubstituted methoxyphenols, could be correlated with the u parameters corresponding to those para-substituents. However, due to the commercial unavailability of a sufficient number of guaiacols with EDPS,guaiacols with EWPS were used. It was reasoned that o- values would provide a measure of the stability of phenoxy radicals with EWPS, because o- parameters are a measure of an EWPS ability to stabilize an electron-rich reaction site by coming into direct resonance with that site. Four different sets of samples were irradiated in the photoreactor simultaneously. Each set contained BaA and a different para-substituted guaiacol. A rate constant (kB) was obtained for each set (see Table 2). A Hammett-style plot of log(kB) versus the o- parameters corresponding to the para-substituted groups yields a straight line (r2 =
Table 2. Correlation between BaA Photodegradation in Presence of Various para-Substituted Methosyphenols and the U- Parameters Corresponding to Those para-Substituents set no. methoxyphenol uk~ (M-1 h-l) acetovanillone 4-hydroxy-3-methoxybenzonitrile 4-nitroguaiacol vanillin
1 2
3
3
0.84 0.88 1.24 1.03
43 *6= 32 h 3 465h 25 83 f 6
Equals 1standard error of slope.
-0.7
-0.5 -0.6
-0.7
-9 5-
-0.6 -0.9
-1 -1.1
-
-
-
-1.4 -1.5 -1.6-1.2
-1.3
-”.-0.5
-0.4
-
-1.7
I
1
0
2
4
6
-
-0,s h
Ym
-1.1
v
-
0 0
-
-
-1.3
0.7
0.Q
1.3
1.1
6 Figure 2. Hammeti plot for para-substituted guaicols.
@ -d“0
@H3
CH II HCCH,
-
@Ch
-
-d“0
-@ CH ~ C H ,
d,
4--------t
0
@ CH I HCCHl
Figure 3. Resonance stabilization of phenoxy radicals by electronwithdrawing (NO2) and electron donating (HC=CHCHd groups.
0.96) (Figure 2). As stated earlier, the o- parameters can be interpreted as a relative gauge of the amount of stability that the corresponding para-substituents yield to a phenoxy radical. For example, NO2 has a relatively high oparameter (6= 1.24). The stability provided by NO2 is due to its strong ability to delocalize the odd electron in the corresponding phenoxy radical by coming into direct resonance with the reaction site (Figure 3). This delocalization stabilizes the radical product and thus decreases the free energy of the reaction compared to that of an unsubstituted guaiacol. This decrease in free energy is related to the rate through the Hammett equation:
where ACT is the free energy difference between the ground-state complex and the transition state, kNOz and k H are the second order rate constants for BaA photo-
degradation (Le., kg) in the presence of 4-nitroguaiacol and guaiacol, respectively, R is the ideal gas constant, and T i s the absolute temperature. This type of relationship shows that the rate increases with u-. As stated earlier, Bordwell and Cheng observed that a correlation existed between phenoxy radical stability and a+ parameters correspondingto electron-donatingpara-substituents (23). Thus, whether a group is electron-donating or electronwithdrawing does not seem to be the important factor in the stabilization of these radicals. It is the relative ability of the group to come into direct resonancewith the reaction site and delocalize the odd electron that seems to exert the largest influence on the phenoxy radical stability (see Figure 3). Bordwelland Cheng also calculateda type of a parameter specificallyfor phenoxy radicals called radical stabilization energies (ARSE) (23). These ARSE were calculated from the following equation:
where Eox(A-),dCd is the oxidation potential of the disassociated acid (i.e., phenoxy anion) calculatedfrom the bond disassociation energy and equilibrium constant of the undissociated acid (Le., phenol), and Eox(A-)obsd is the experimentally observed value of the oxidation potential of the substituted phenoxy anion. The difference in the predicted and observed oxidation potentials is due to the stability that a para-substituent provides to the phenoxy radical. The log of the rate constants obtained in this experiment (Le., log(kB)) were also plotted against these ARSE parameters (Figure 4). A point for vanillin was not included in this plot because Bordwell and Cheng did not calculate a ARSE value for the aldehyde (CHO) group. However, the data obtained in this experiment suggest that ARSE for an aldehyde para-substituent is 2.65. The fit obtained using the ARSE parameters (r2= 0.999) is even better than that obtained using the o- parameters for the cyano (CN) and aceto (COCH3 points. This is not surprising considering that the ARSE parameters were calculated specifically for phenoxy radicals while oparameters were calculated for phenol itself. Therefore, it seems that the most likely interpretation of the linearity of these Hammett plots is that hydrogen abstraction of Environ. Scl. Technol., Vol. 28. No. 7, 1994 1287
the phenolic hydrogen of vanillin is the rate-determining step of the mechanism. One might suppose that the type of behavior exhibited in these Hammett plots could be explained equally well if one assumed that the rate-limiting step was absorption of light by vanillin followed by loss of the phenoxy hydrogen to create a phenoxy radical. One might then assume that o- parameters could be related to the quantum yield of radical formation. Then if these radicals reacted with BaA, rates of BaA photodegradation might show a correlation with u- parameters. However, as shown in eq 3, r- is related to the log of the rate constant of the ratedetermining step. In the above scenario, the rate constant is not simply related to the quantum yield of formation. It is related to the product of the quantum yield of radical formation and the extinction coefficient (e). Therefore, the explanation mentioned above might be plausible if the extinction coefficients (e) of all the substituted guaiacols were the same. However, this is not the case. While acetovanillone, 4-cyanoguaiacol, and vanillin have similar extinction coefficients at 366 nm (e = 15M-l cm-l), 4-nitroguaiacolhas an extinction coefficientthat is 2 orders of magnitude larger than the other three. This difference in the extinction coefficients would add an extra 2 orders of magnitude to the kg rate constant for the 4-nitroguaiacol/BaA reaction. Therefore, the nitro point would not fit on the Hammett line if the rate-determining steps were absorption of light by the methoxyphenol followed by phenoxy radical formation. Previous experiments performed by this group also discount the above mechanistic hypothesis (20). In these experiments, benzo[a]pyrene (BaP) was irradiated in the presence of vanillin with both 366 nm light and the full mercury spectrum above 300 nm. When BaP was irradiated with the full mercury spectrum above 300 nm, the rate of BaP photodegradation was very similar to that of BaA when it was irradiated with the same spectrum. Yet at 366 nm, the rate of photodegradation of BaP is a factor Of 5 faster than that of BaA. This can be explained by the fact that at 366 nm the extinction coefficient of BaP is 8 times larger than that for BaA. Yet the sum of the extinction coefficients at the wavelengths of the full mercury spectrum above 300 nm for the two compounds are similar. Thus, the absorption of light by the PAH is clearly the important absorption step in the mechanism. Therefore, in light of the above explanations, it seems clear that the abstraction of hydrogen from vanillin is the rate-determining step in the mechanism and that an excited state of BaA is essential for this reaction to occur. In the model system, there are only two species capable of abstracting the phenolic hydrogen of vanillin, excited triplet BaA and singlet oxygen. A mechanism in which triplet BaA abstracts the phenolic hydrogen of vanillin is shown in Figure 5. In this mechanism, # is a term relating the incident light intensity to the total light that is absorbed in the reaction cell. As long as absorbance is low (Le., A
