Environ. Sci. Technol. 2008, 42, 2432–2438
Photochemical Degradation of Polycyclic Aromatic Hydrocarbons in Oil Films ´ E L . P L A T A , * ,† DESIRE CHARLES M. SHARPLESS,‡ AND CHRISTOPHER M. REDDY† Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Woods Hole, Massachusetts 02543, and Department of Chemistry, University of Mary Washington, Fredericksburg, Virginia 22401
Received September 21, 2007. Revised manuscript received December 4, 2007. Accepted January 7, 2008.
Photochemical processes affect the fate of spilled oil in the environment, but the relative contribution and kinetics of these degradation pathways are not fully constrained. To address this problem, we followed the weathering of No. 6 fuel oil by periodically sampling rocks covered with a film of oil from Buzzards Bay, MA after the April 2003 Bouchard 120 oil spill. Two sets of polycyclic aromatic hydrocarbon (PAH) isomers, benzo[a]pyrene (BAP) and benzo[e]pyrene (BEP), and benz[a]anthracene (BAA) and chrysene (CHR), were found to have very different disappearance rates in spite of their close structural similarity (kBAA/kCHR ∼ 2.0, kBAP/kBEP ∼ 2.2). This welldocumented phenomenon is suspected to arise from differing capacity for direct photoreaction in the oil film. To investigate the validity of this assumption, we developed a model to estimate the contribution of direct photolysis to the loss of these PAHs from the oil. Newly determined PAH quantum yields demonstrate that the efficiency of phototransformation in hydrophobic media are 2 orders of magnitude lower (Φ′ ∼ 10-5) than in aqueous systems, and the thickness and lightattenuating properties of the oil film reduce the potential for photoreaction by up to 2 orders of magnitude. Given these limiting factors, direct photolysis cannot account for the complete removal of these PAHs (except BAP). Additional results suggest that singlet oxygen photodegradation pathways are not favored in hydrophobic media, as they are in some mineralassociated and aqueous systems. Our results indicate that photomediated reactions with other compounds in the oil mixture were responsible for PAH photodegradation in the oil film.
Introduction Polycyclic aromatic hydrocarbons (PAHs) and their photooxidation products are often considered responsible for the acute toxicity of oil spills in the marine environment (1–3), yet the relative rates of phototransformations in oil are not well-constrained. As a result, we are unable to predict accurately the fate of PAHs in oil-impacted coastal zones and gauge the relative contribution of photochemical processes to the weathering of spilled oils. While the physical * Corresponding author fax: 617-288-8850; e-mail: dplata@ whoi.edu. † Woods Hole Oceanographic Institution. ‡ University of Mary Washington. 2432
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heterogeneity of field and laboratory samples makes it difficult to draw comparisons between studies, it is generally observed that select PAHs are resistant to photolytic transformation compared to their close structural isomers. For example, the linearly annelated PAHs, benzo[a]pyrene (BAP) and benz[a]anthracene (BAA), are rapidly degraded compared to their isomers benzo[e] pyrene (BEP) and chrysene (CHR), respectively (4, 5). Some researchers have employed relative isomer compositions to diagnose the occurrence of photodegradation in environmental samples (6, 7). However, isomer ratios are subject to a number of natural transformation processes and environmental conditions, and such analyses are unreliable and certainly not quantitative (4, 8). Attempts to quantify photolytic processes are complicated by the diversity of possible chemical transformations (e.g., with singlet oxygen (1O2) or other excited-state species, H-atom abstraction, and free radical chain reactions 5, 8–14). These photoreactions are generally classified as either (1) direct photolysis, in which the molecule of interest absorbs light and subsequently degrades, or (2) indirect photolysis, in which a different chemical absorbs light and then reacts to degrade the molecule of interest. In dilute aqueous systems, direct photolysis is principally responsible for the degradation of PAHs, as reactive oxygen species are not generated efficiently and are consumed rapidly in most natural waters (5, 15, 16). In hydrophobic oil films, some have suggested that the abundance of photosensitizers and the extended lifetime (τ) of 1O2 in hydrophobic solvents (e.g., τtoluene ∼ 26 µs, τwater ∼ 2–4 µs) could allow 1O2 to reach critical levels and becomeanimportantpathwayforPAHdegradation(10,17–19). Larson and Hunt (10) showed that chemically quenching 1O limited the formation of photooxidized products, but 2 Lichtenthaler et al. (12) suggested that 1O2 was not a significant source of radicals compared to direct photolysis in crude oil films. In aqueous systems, phototransformations can proceed via a direct mechanism, where excited-state PAHs react with triplet-state O2 to yield oxidized products (11). The efficiency of these light-mediated reactions (i.e., the quantum yield) exhibits a strong dependence on oxygen concentration that has only recently been realized (20). Quantum yields are also influenced by solvent polarity (5), and the majority of published PAH quantum yields have been determined in water. Unfortunately, these are not applicable in hydrophobic media. In oil-impacted beaches, photoreactions in oil films may be an important degradation pathway for toxic PAHs, such as BAP. To examine the role of direct photolysis to PAH removal from coastal areas, we studied the weathering of oil coated on rocks from Nyes Neck beach in North Falmouth, MA. These rocks were covered with a film of No. 6 fuel oil following the Bouchard 120 oil spill on April 27, 2003. An estimated 375,000 L was released into Buzzards Bay, covering approximately 150 km of coastline (21). For 5 months following the spill, we measured the PAH content on discreet oilcovered samples and calculated pseudo-first-order disappearance rates, kobs (eq 1), for each compound. We then compared that rate to the maximum-possible rates of direct photodegradation, kdir (eq 2), which we calculated from a well-established photochemical model (15). This model required that we track the time-evolving light attenuation and thickness of the oil films. In addition, we determined the quantum yields of direct photoreaction in toluene to simulate a hydrophobic environment. As a result of the experimental design, our definition of “direct photolysis” includes reactions of excited-state PAHs with dissolved oxygen, self-sensitized reactions of singlet oxygen with PAHs, and reactions of 10.1021/es702384f CCC: $40.75
2008 American Chemical Society
Published on Web 02/28/2008
excited-state PAHs with themselves. By measuring the PAH composition of the oil over time and comparing it to the modeled degradation due to direct photolysis, we evaluated the contribution of this loss mechanism to the overall removal of select PAHs from the oil-impacted beach. -d[PAH]oil film ) kobs[PAH]oil film dt
(1)
Experimental Procedures Sample Preparation and GC Analysis. Oil-covered rocks were selected from the impacted area from May to October 2003. On the 16th day following the spill, we divided the shore into two zones; the surf zone (SZ; tidally washed twice daily) and the dry zone (DZ; washed only during spring tides). The oil was extracted twice with 90:10 dichloromethane:methanol (DCM:MeOH) and dried over activated sodium sulfate. A fraction of this extract was used to determine the mass of the total soluble extractable material (TSEM). Another fraction was transferred to an activated silica gel column, eluted with DCM, and analyzed by gas chromatography-mass spectrometry (GC-MS; Agilent 6890 GC interfaced to an Agilent 5973 MSD). Internal standard (dodecahydrotriphenylene) recoveries ranged from 70 to 100%, and laboratory blanks were free of the analyzed PAHs. In early samples (May-June) with higher chrysene abundance, chrysene and triphenylene were not baseline-resolved, and their integrated sum was reported for the chrysene values. This chromatographic limitation results in a higher-than-actual observed degradation rate for chrysene. Observed PAH Loss. The mass of each PAH was normalized to the TSEM mass (µg of PAH (mg of TSEM)-1), and the observed degradation constant, kobs, was taken as the slope of the regression of natural log of the normalized concentration (C) on time (i.e., ln(C [µg of PAH (mg of TSEM)-1]) vs time). Photochemical Model for PAH Loss. The rate constant for direct photolytic degradation of a chemical (kdir, eq 2) depends on (1) the specific rate of light absorption by the compound (ka,oilfilm) and (2) the efficiency of phototranformation per absorbed photon (Φ′, where the prime denotes the dependence on oxygen concentration (20)). In the case where incident light absorption is complete (e.g., an opaque oil film), the specific rate at which a compound absorbs light (eq 3) is a function of the incident light intensity (Wλ), the attenuation of light by the reaction medium (Rλ through the reactive layer (of thickness z), and the ability of the compound to absorb light (ελ) (15). kdir ) ka,oil filmΦ’ ka,oil film )
Wλελ
∑ Rz λ
(2) (3)
λ
We employed the American Society for Testing and Materials light fluxes (Wλ) (37° spectrum (22)). This spectrum overestimates the actual sunlight intensity at the latitude of our site (41.5°). The absorption spectra, quantum yields, attenuation coefficients, and mixed layer thickness were determined experimentally (as described later) and affect the model as follows. Absorption spectra of these PAHs are fairly consistent between different hydrophobic media, and we expect our measurements in toluene to reflect accurately the compounds’ absorptivities in the oil film. The quantum yields (Φ′) depend on the oxygen concentration, and we assume that the concentration in the film is roughly equivalent to the oxygen concentration in our experimental determination in toluene. Oxygen solubility varies by about a factor of 2 between many hydrophobic solvents, including toluene and olive oil (23–25). A doubling in [O2] results in 1.5- to 2-fold
increases in quantum yield (20), so our estimates of photolytic degradation rates should be accurate within a factor of 2. The experimentally determined attenuation of the oil film introduces relatively little error (
