Environ. Sci. Technol. 1985, 19, 1004-1006
NOTES Photolysis of Polycyclic Aromatic Hydrocarbons Adsorbed on Simulated Atmospheric Particulates Thomas D. Behymer and Ronald A. Hltes" School of Public and Environmental Affairs and Department of Chemistry, Indiana University, Bloomington, Indiana 47405
w Fifteen polycyclic aromatic hydrocarbons (PAH) adsorbed on silica gel, alumina, fly ash, and carbon black were photolyzed in order to study their atmospheric fate. Photolytic half-lives for these particle-bound PAH were found to be highly dependent on the substrate to which they were adsorbed. On silica gel and alumina, PAH showed a wide range of photolytic half-lives, indicating a relationship between structure and photochemical reactivity. However, PAH on environmental substrates such as fly ash and carbon black show similar half-lives for most PAH, indicating a photolytic process that is independent of structure and dependent on the physical and chemical nature of the substrate. In fact, fly ash and carbon black appear to stabilize some PAH; this would facilitate their transport from combustion sources through the atmosphere. Introduction The atmospheric fate of polycyclic aromatic hydrocarbons (PAH) has received considerable attention (1-6). Studies of marine and lacustrine sediments, the ultimate sinks of PAH, have shown relative abundances of individual PAH that are similar to those detected in the atmosphere; this suggests that PAH are stable in the atmosphere (7). This is, however, not an unchallenged conclusion. Laboratory studies have found that some PAH photodegrade in solution and also when adsorbed to various substrates, including soot and fly ash; in both cases, photolysis lifetimes can be as short as a few hours (8-15). Other studies have suggested that PAH, when adsorbed to coal fly ash, may be resistant to photochemical degradation (16-18). Clearly, more work is required to understand these conflicting observations. Our study utilizes fluidized-bed and rotary photoreactors to simulate environmental conditions encountered by particle-bound PAH. Fifteen common PAH were adsorbed on silica gel, alumina, and coal fly ash and then photolyzed in order to study the photochemical behavior of these compounds. In addition, a carbon black, which naturally contains relatively high concentrations of PAH, was studied. Ultimately, our goal is to relate the half-lives of PAH determined from these studies under laboratory conditions to processes occurring in the environment. Experimental Section Two reactors were used in this study: a fluidized-bed photoreactor based on a design described by Daisey et al. (15) and a rotary photoreactor similar to that described by Korfmacher et al. (16). Both reactors were made from Pyrex. The light source was a 450-W medium-pressure mercury vapor lamp (Canrad-Hanovia Inc.), and it was 1004
Environ. Sci. Technol., Vol. 19, No. 10, 1985
placed 16 cm from the center of the photoreactors during all experiments. In the spectral region in which most PAH absorb, 300-410 nm, this source had an irradiance of 17.6 f 1.4 W/m2 ( N = 3) measured in the rotary photoreactor by chemical actinometry with o-nitrobenzaldehyde (19,20). This irradiance is lower than the 36 W/m2 reported by Daisey et al. for a 200-W medium-pressure mercury vapor lamp (15). However, in our study, the light source was placed in a water immersion well for cooling, and a reflector was not used. The PAH to be studied were adsorbed at a level of about 25 pglg of substrate by evaporating a solution (benzene or methylene chloride solvent) onto either silica gel (100-200 mesh, Fisher Scientific Co.), neutral alumina (Brockmann Activity I, 80-200 mesh, Fisher Scientific Co.), or unextracted fly ash (collected at a coal-fired power plant in Bloomington, IN). Each substrate was sized prior to PAH adsorption with a series of four sieves (openings of 335, 250, 180, and 125 pm, respectively). Fly ash in the 180-250-pm range was studied while silica gel and alumina in the range less than 125 pm were studied. A 260-nm carbon black (Cabot Corp.) known to contain high PAH concentrationswas also studied (21). These four substrates were analyzed (Particle Data Laboratories, Ltd., Elmhurst, IL) by three-point nitrogen BET adsorption to determine particle surface area. The silica gel had a surface area of 637 m2/g, the alumina, 224 m2/g, the fly ash, 5.10 m2/g, and the carbon black, 11.6 m2/g. With these surface areas and approximate PAH concentrations of 25 pg/g of substrate for each PAH, all experiments were carried out at less than one monolayer coverage. In a typical experiment, a 1-5-g sample of particles was added to the photoreactor. Duplicate samples (25-100 mg) were collected at timed intervals and placed in preweighed amber vials to prevent further photodegradation following sampling. Duplicate experiments were also run in the dark as a control; over a 25-h period, no concentration changes were observed. All the samples were weighed, and a known quantity of a deuterated PAH mixture was added for later quantitation. This internal standard consisted of fluorene-dlo,dibenzothiophene-d8, phenanthrene-dIo,pyrenedlo, chrysene-dlz,perylene-dlz,and benzo[ghi]perylene-dlz (Merck Sharp + Dohme, Canada). With the exception of the carbon black samples, which were Soxhlet extracted, all samples were immediately extracted by loading the PAH-coated particulates and internal standard into a Pasteur pipet (plugged with glass wool). Nanograde methylene chloride (50 mL) was then passed over the particulates in this miniature chromatography column to extract the adsorbed PAH, internal standard, and any products of the photolysis. The volume of the extract was reduced by rotary evaporation and placed in an amber vial. Final volume was adjusted with a stream of dry nitrogen.
0013-936X/85/09 19-1004$01.50/0
0 1985 American Chemical Society
5 :: L
120
..
90
--
Table I. Half-Lives (in Hours) for the Photolysis of PAH on Different Substrates Determined in the Rotary Photoreactor (Approximately 25 pg of Each PAH/g of Substrate Except for the Carbon Black)
> o
2
maa:
Fluor
0 0 2
II
6o
4HCP,
--
0 HALF-LIFE
20 OBSERVED
40
silica gel
PAH
,
60
80
100
IN ROTARY PHOTOREACTOR
acenaphthylene acenaphthene fluorene dibenzothiophene phenanthrene anthracene 4H-cyclopenta[deflphenanthrene fluoranthene pyrene benz [a]anthracene chrysene benzo[e]pyrene benzo[a]pyrene perylene benzo[ghi]perylene
120 (HOURS)
Flgure 1. Comparison of half-life measurements made in the fluldked-bed and rotary photoreactors (70 pg of each PAH/g of silica gel). The slope of the llne (1.190) is not statistically different than unity at a significance level of 0.1 YO. Fir, fluorene; Phen, phenanthrene; BeP, benzo[e] pyrene; Fluor, fluoranthene; 4HCP, 4H-cyclopenta[def]phenanthrene; BghiP, benzo[ghi]peryiene; Pyr, pyrene; Dibenz, dlbenzothlophene.
The samples were qualitatively and quantitatively analyzed by using gas chromatographic mass spectrometry (GC/MS) on a Hewlett-Packard 5985B GC/MS instrument. This instrument was fitted with a 30 m X 0.25 mm i.d. fused silica capillary column coated with a bonded silicone (J & W, DB-5, equivalent to SE-54) stationary phase; helium was used as the carrier gas. Both full scan and selected ion monitoring modes were used with an electron energy of 70 eV. For each run, 1p L of the extract was loaded splitless at 50 OC for 1min. The temperature was then ramped at 4 OC/min to a final temperature of 290 "C where it was held for 12 min. Qualitative identification of reaction products was performed in the electron impact mode. Compound identification was based on comparison of retention times and mass spectra to those for commercially available standards as well as on mass spectral interpretation. Quantitative analysis of the PAH was performed by using selected ion monitoring. To increase reproducibility and sensitivity of quantitation, five ions at 0.1-amu increments were measured for each ion of interest. For example, the mass range of 177.9-178.3 amu was measured for phenanthrene (M, 178) in increments of 0.1 amu. Dwell times of 25 ms were used to get 15-20 measurements per ion across each eluting peak. Quantitation was performed by mass chromatogram integration of the molecular ion for each PAH followed by the use of the proper response factor vs. the appropriate deuterated PAH internal standard. All response factors were calculated experimentally by using commercially available standards. All concentrations reported are in concentration of PAH per gram of substrate. Half-lives were then calculated on the basis of the change in PAH concentration following 25 h of irradiation. Results and Discussion Of the two reactors, the fluidized-bed photoreactor is best suited for mimicking the environment. Atmospheric processes are simulated by suspending the particles in a gas flow, thus allowing for three-dimensional irradiation. This photoreactor, however, is plagued with the sticking of particles caused by static electricity and with the loss of particulates from the reactor. These problems increase with decreasing particle size, and thus, studies involving very small particulates are difficult to perform. In addition, the setup and sampling of the fluidized bed of particles is tedious. The rotary photoreactor is a simpler device. Particles of any size can be rotated in the cell which is transversally irradiated by the light source. Furthermore, setup and sampling are convenient. Al-
0.7 2.0 110 18 150 2.9 28 74 21 4.0
100 70 4.7 3.9 7
fly ash
carbon blacka
0.5
44 44 37 49 49 48
170 b >loo0 >loo0 >loo0 310
6.5
42
>lo00
23 31 2.0 78 110
44 46 38 38 35 31 33 29
>loo0 >lo00 650 690 >loo0 570 870 >lo00
alumina 2.2 2.2
62 45 45
1.4 1.2 22
aFor quantitative analysis of carbon black, see ref 17. *Acenaphthene was not originally present on the carbon black we studied. u W
2.0
m
z 3 Z
1.5
1.0
2 3 0.5
p I 4 o
r = 0.912
l 20
40
60
eo
HALF-LIFE
100
120
140
1130
(HOURS)
Flgure 2. Comparison of Dewar reactivity numbers with the observed half-lives for the photolysis of PAH on silica gel. Chry, chrysene; BaA, benz[a ]anthracene; Pery, perylene; Anth, anthracene: BaP, benzo[a Ipyrene (see Figure l caption for additional PAH abbreviations).
though this reactor does not simulate the environment as well as the fluidized-bed photoreactor, half-lives for PAH determined in both photoreactors under identical conditions are equivalent (see Figure 1). Thus, the rotary photoreactor has been used in our studies to simulate environmental processes. The half-lives for the photolysis of 15 PAH (25 pg of each PAH/g of substrate) adsorbed on silica gel, alumina, and a coal fly ash are shown in Table I. (All experiments carried out in the dark showed negligible PAH degradation.) On silica gel and alumina, PAH show a wide range of photolytic half-lives, indicating a relationship between structure and photochemical reactivity. Acenaphthylene, acenaphthene, anthracene, benz [a]anthracene, benzo [a]pyrene, and perylene were the most reactive while fluorene, phenanthrene, fluoranthene, chrysene, and benzo[e]pyrene were the most stable. The observed reactivity of some PAH adsorbed on silica gel can be explained by structure-reactivity relationships such as Dewar reactivity numbers. These are energies required for the removal of a a electron at a specific carbon center from the remaining a system (22). Figure 2 is a plot of our observed photolytic half-lives vs. the Dewar reactivity number for eight PAH. There is a good correlation between the Dewar number and the half-life. On fly ash, all PAH showed similar half-lives (see Table I); this indicates that the photolytic process is independent Environ. Sci. Technol., Vol. 19, No. 10, 1985
1005
of structure and is dependent on the physical and chemical nature of the substrate. Studies on another substrate, carbon black, which contains relatively high concentrations of PAH, also indicate that photolytic half-lives are almost independent of structure (see Table I). In addition, these half-lives are generally very long (greater than 1000 h). Therefore, we conclude that the photolytic half-lives of PAH appear to be highly dependent on the physical and chemical properties of the substrate to which they are adsorbed. These data seem to support those of Korfmacher et al. and Dlugi and Gusten, who suggested that fly ash can stabilize the photochemical degradation of PAH in the adsorbed state (16-18). This may be due to the carbon content of the fly ash. We have shown that PAH adsorbed to a carbon substrate, carbon black, have remarkably long half-lives. This agrees with the data of Falk et al. (23))who demonstrated that both pyrene and benzo[a]pyrene underwent little photodegradation when adsorbed to soot (1%loss for pyrene and a 10% loss for benzo[a]pyrene after 48 h of irradiation). These substrate-dependent differences in PAH stability may be due to differences in the physical characteristics of the substrates. The greater photochemical reactivity for some PAH on silica gel and alumina may be due to the large surface areas of these substrates. Their specific surface areas are much greater than those for fly ash and carbon black. This would allow for greater PAH dispersion and may tend to enhance the rate of photodegradation. One other major difference between substrates is their color. Silica gel and alumina are both white while the carbon black and the fly ash are black. As suggested by Korfmacher et al. (16))there may be an “inner-filterneffect: the darker substrates absorb more of the incident light which would reduce the rate of PAH degradation on fly ash and carbon black. Differences in the physical properties of the substrates certainly would account for some of the observed photochemical behavior of particle-bound PAH. However, it is likely that complex, chemical interactions between an adsorbed PAH and a substrate’s surface could also affect the ultimate stability of the PAH. Thus, further study of these interactions is required to fully understand the photochemistry of adsorbed PAH. When considering the atmospheric fate of PAH in the environment, one should consider not only the rates of degradation but also any products formed during irradiation. In some cases, it is possible that degradation products may be more potent carcinogens than the parent PAH. This alone warrants concern about the implications of these transformations. To date, we have identified two of the several degradation products which were produced in these experiments: anthracene-9,lO-dione and benz[a]anthracene-7,12-dione which result from the photolysis of anthracene and benz[a]anthracene, respectively.
mental PAH behaved as they do on silica gel and alumina, several PAH including benzo[a]pyrene would not be observed in environmental sinks such as marine and lacustrine sediments. Since this is not true, we conclude that coal fly ash and carbon black more closely simulate environmental substrates and can stabilize PAH; this facilitates their transport from combustion sources through the atmosphere. Acknowledgments
We thank P. Hoppe for clerical assistance. Registry No. Flr, 86-73-7; Dibenz, 132-65-0; Phen, 85-01-8; Anth, 120-12-7;4HCP, 203-64-5; Fluor, 206-44-0; Pyr, 129-00-0; BaA, 56-55-3; Chry, 218-01-9; BeP, 192-97-2;BaP, 50-32-8;Pery, 198-55-0; BghiP, 191-24-2; acenaphthylene, 208-96-8; acenaphthene, 83-32-9; alumina, 1344-28-1.
Literature Cited National Research Council “Polycyclic Aromatic Hydrocarbons: Evaluation of Sources and Effects”; National Academy Press: Washington, DC, 1983. Edwards, N. T. J.Environ. Qual. 1983,12, 427-441. Nielsen, T.; Ramdahl, T.; Bjerrseth, A. Environ. Health Perspect. 1983, 47, 103-114. Pith, J. N., Jr. Enuiron. Health Perspect. 1983,47,115-140. Van Cauwenberghe, K.; Van Vaeck, L. Mutat. Res. 1983, 116, 1-20. Nikolaou, K.; Masclet, P.; Mouvier, G. Sci. Total Environ. 1984, 32, 103-132. Laflamme, R. E.; Hites, R. A. Geochim. Cosmochim.Acta 1978, 42, 289-303. Bowen, E. J. Adv. Photochem. 1963, I , 23-42. Inscoe, M. N. Anal. Chem. 1964,36, 2505-2506. Seifert, B. J. Chromatogr. 1977, 131, 417-421. Issag, H. J.; Andrews, A. W.; Janini, G. M.; Barr, E. W. J. Liq. Chromatogr. 1979,2, 319-325. 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. Thomas, J. F.; Mukai, M.; Tebbens, B. D. Environ. Sci. Technol. 1968, 2, 33-39. Fox, M. A,; Olive, S. Science (Washington,D.C.) 1979,205, 582-583. Daisey, J. M.; Lewandowski, C. G.; i o r i , M. Environ. Sci. Technol. 1982,16, 857-861. Korfmacher, W. A,; Wehry, E. L.; Mamantov, G.; Natusch, D. F. S. Environ. Sci. Technol. 1980, 14, 1094-1099. Korfmacher, W. A,; Natusch, D. F. S.; Taylor, D. R.; Mamantov, G.; Wehry, E. L. Science (Washington D.C.) 1980, 207, 763-765. Dlugi, R.; Gusten, H. Atmos. Environ. 1983,17, 1765-1771. Leighton, P. A.; Lucy, F. A. J. Chem. Phys. 1934,2,756-759. Pitts, J. N., Jr.; Cowell, G. W.; Burley, D. R. Enuiron. Sci. Technol. 1968, 2, 435-437. Simonsick, W. J., Jr.; Hites, R. A. Anal. Chem. 1984, 56, 2749-2754. Dewar, M. J. S. J. Am. Chem. SOC. 1952, 74, 3357-3363. Falk, H. L.; Markul, I.; Kotin, P. AMA Arch. Ind. Health 1956,13, 13-17.
Conclusions
From Table I, we can conclude that adsorbed PAH can and do undergo photodegradation and that the rates of degradation are dependent on the substrate. If environ-
1006
Environ. Sci. Technol., Voi. 19, No. 10, 1985
Received for review February 2,1985. Accepted April 19,1985. This work was supported by the US.Department of Energy (Grant 8OEV-10449).
