Environ. Sci. Technol. 2000, 34, 1934-1937
Pyrogenic Polycyclic Aromatic Hydrocarbons in Oil Burn Residues ROBERT M. GARRETT,† C H A N T A L C . G U EÄ N E T T E , ‡ , § COPPER E. HAITH,† AND R O G E R C . P R I N C E * ,† ExxonMobil Research and Engineering Company, Annandale, New Jersey 08801, and SINTEF Applied Chemistry, N-7034 Trondheim, Norway
Recent field trials and tanker accidents have shown that burning crude oil at sea can be an effective response for marine oil spills. Nevertheless, there is concern that the residue may have elevated levels of potentially toxic pyrogenic polycyclic aromatic hydrocarbons. We have simulated a marine oil slick burn using Statfjord crude oil, a light paraffinic North Sea crude. The burn was over seawater to an efficiency of 85%, typical of efficiencies achieved in the field. We have used gas chromatography coupled with mass spectrometry to examine the polycyclic aromatic hydrocarbons present in the crude oil burn residue and used hopane as a conserved internal marker in the oil to allow us to quantify the generation of pyrogenic compounds. The concentrations of several of the pyrogenic aromatic compounds were somewhat enriched in the residue, but these increases were outweighed by the mass of oil consumed in the burn. In situ burning substantially reduced the total amount of polycyclic aromatic hydrocarbons left on the water surface after the spill.
Introduction The most widely used options for responding to marine oil spills are mechanical recovery and the application of dispersants (1-4). One promising additional approach is to burn the oil in situ (5). This is particularly valuable in broken ice (6-10), but it has also been used on a small scale following spills on open water (e.g., ref 11). Some major tanker accidents have been accompanied by fire, and significant amounts of oil have been removed from the environment by combustion; the Aegean Sea (12) and Haven (13) are recent examples. One problem associated with burning oil on water is that the oil layer must be thick enough that it can act as a thermal insulator to allow the surface to be at combustion temperatures despite the proximity of the cold sea. In heavily iceinfested waters, the ice acts to keep the oil from spreading, but on the open sea it is usually necessary to collect and contain the oil in booms, preferably fire-resistant ones. In any case, the burning oil layer eventually becomes so thin that it extinguishes, and there is always some residual unburned oil. Even though in situ burning can be a very effective tool, there are several environmental concerns that may limit its * Corresponding author phone: (908)730-2134; fax: (908)730-3042; e-mail:
[email protected]. † ExxonMobil Research and Engineering Company. ‡ SINTEF Applied Chemistry. § Present address: International Tankers Owners Pollution Federation, Staple Hall, Stonehouse Court, 87-90 Houndsditch, EC3A 7AX, U.K. 1934
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use. One is the black smoke that accompanies the combustion, although this can be partially overcome by adding smoke suppressants to the burning oil (14, 15). The potential environmental impacts of the smoke and soot have been studied in some detail (16-21), and the impact of the smoke and soot is generally limited to the immediate area downwind of the burn, usually less than 1.5 km. The other concern that has sometimes been expressed is that the residual oil remaining after combustion may pose an environmental hazard because of increased levels of pyrogenic polycyclic hydrocarbons generated during the burn. Some of the pyrogenic polycyclic aromatic hydrocarbons are on the U.S. EPA list of priority pollutants (22). We have examined the chemical composition of the residuum left after the combustion of Statfjord crude oil, with and without ferrocene to suppress smoke. We expected the burn to have several effects. Obviously a lot of oil was burned and was thereby completely removed from the environment and converted to carbon dioxide and water. But when the fire self-extinguishes, there will still be some evaporation of the residual oil as it cools to ambient temperatures. It is possible that pyrogenic compounds, made during the burn, will have accumulated in the residue; this is the main focus of this work. We find that the residual oil remaining after combustion is not very different from that which would be generated by extensive weathering at sea without combustion, although in much lower amounts because of the combustion. While there was a small increase in the concentrations of polycyclic aromatic hydrocarbons in the oil residue, this increase was offset by the total amount consumed in the burn, so that the net effect of the burn was to remove these compounds from the environment.
Materials and Methods Crude Oil. Statfjord crude oil, a light paraffinic oil from the North Sea with an API gravity of 37.8, was evaporated to lose approximately 10 wt %. This value was chosen to represent the weathering that might happen before a spill response could begin. Small-scale burns were carried out in two open steel drums cut down to a height of 30 cm. A total of 1.25 L of oil was placed over 20 cm of seawater and then ignited with gelled gasoline. The tests were conducted outdoors at an ambient temperature of about 5 °C. The oil layer, initially 5 mm thick, was allowed to burn until it self-extinguished. Crude oil slicks typically burn down to a thickness of approximately 0.8 mm, at which point the oil is too thin to sustain burning (8). The total burn efficiency was approximately 85%, which is consistent with results from earlier experiments (23) with this oil on a similar scale and under similar conditions. In one experiment, ferrocene (2 wt %) was premixed into the oil prior to placing it in the drum. Moir et al. (15) report that this is the amount at which maximum soot reduction is achieved, although smaller amounts also significantly reduce soot emissions. The burns were carried out simultaneously. Very little black smoke was produced from the burning oil containing ferrocene, while a copious black smoke plume emanated from the burning crude oil that did not contain ferrocene. Oil residues after the burn extinguished itself were rather viscous and could be scooped from the surface with a spatula. They did not sink. Gas Chromatography/Mass Spectroscopy. Analysis of the oil by gas chromatography/mass spectroscopy (GC/MS) essentially followed published procedures (24). Separation was performed on a Hewlett-Packard HP 5890 gas chromatograph fitted with a 30 m × 0.25 mm fused silica capillary 10.1021/es991255j CCC: $19.00
2000 American Chemical Society Published on Web 04/12/2000
FIGURE 1. Total ion chromatograms of Statfjord crude oil before and after combustion without and with ferrocene added as a soot suppresser. Note that the initial oil has lost some of the lighter components due to evaporation; the first major peak in the Statfjord crude is nonane. column with 5% cross-linked phenyl methyl silicone as the stationary phase. Helium was used as the carrier gas at a flow rate of 1 mL/min. Oil residues and samples of the initial oil were dissolved at a concentration of 20-30 mg/mL in methylene chloride, and 1 µL of these dilutions was injected automatically by a HP 6890 injector. The column temperature was set to 45 °C for the first 4 min, increased 8 °C/min to a temperature of 270 °C, then increased 5 °C/min to 31 °C, and maintained at 310 °C for 5 min. Mass spectral data were obtained with a Hewlett-Packard 5972 mass selective detector at an electron energy of 70 eV over a mass range of 35-500 amu in the total ion mode to characterize total hydrocarbon, and in selected ion mode to examine the depletion or production of selected species including the 16 U.S. EPA priority polycyclic aromatic hydrocarbon pollutants. Methylsubstituted aromatic compounds were quantified together in this study without regard to the position of the methyl group. Likewise, the dimethyl- and ethyl-substituted compounds are grouped as the C2 compounds, and the trimethyl-, methyl ethyl-, and propyl-substituted compounds are grouped together as C3 compounds (24). The 16 U.S. EPA polycyclic aromatic hydrocarbon priority pollutants were quantified against standards purchased from Sigma-Aldrich (St. Louis, MO). Spectral tuning with perfluorotributylamine followed U.S. EPA Method 8270C. Thin-Layer Chromatography. Analysis of samples by thin-layer chromatography was conducted using 0.9 mm Chromarod quartz rods sintered with silica gel (Iatron Laboratories, Tokyo) (25). Oil residues and samples of the initial oil were dissolved at a concentration of 20-30 mg/mL in methylene chloride, and 1 µL was applied and chromatographed in n-heptane for 35 min, in toluene for 15 min, and in 95% methylene chloride-5% methanol for 2 min. Chromatographs were analyzed on an Iatroscan MK-5 flame ionization detector (Iatron Laboratories).
Results and Discussion Figure 1 shows the total ion chromatograms of the initial oil and burn residues. The prominent, regular peaks represent the normal alkanes, each differing from the one before by the addition of a single carbon. The burn residue is depleted in the smaller alkanes while the larger alkanes appear unchanged, consistent with some distillation of the unburned oil during the burning process or as the oil cooled after it extinguished. Figure 2 shows the single ion chromatogram for ions of m/z ) 191, which detects the triterpanes including the hopanes (26). Quantitation of the various individual com-
FIGURE 2. m/z ) 191 chromatograms of Statfjord crude oil before and after combustion without and with ferrocene added as a soot suppressor. The largest peak, at approximately 37.4 min, is 17r(H),21β(H)-hopane.
FIGURE 3. Depletion of paraffins and isoparaffins in the burn residue oil as compared with the initial oil. ponents in this study was by integration of total peak area normalized to hopane, and thus it is important to note that the hopane distribution was unaffected by burning under these conditions. Hopane is quite resistant to many biological and chemical processes, and the use of hopane as an internal GC/MS standard is well-established (27-30). Figure 3 shows the percent depletion of some selected saturated hydrocarbons in the burn residue relative to the initial oil, using hopane as a conserved internal standard, and Figure 4 shows a similar figure for some three- and fourring polycyclic aromatic hydrocarbons. These losses are clearly correlated with molecular weight, and we attribute them to distillation during and after the burn. The almost complete conservation of the C30 alkane, which boils at 450 °C, shows that the residue cannot have reached this temperature, even though the temperature of the flame was probably 900-1200 °C (5). Figure 5 shows how these losses only slightly alter the overall chemical composition of the residual oil, as measured by thin-layer chromatography. It is important to bear in mind that this technique effectively “weathers” the oil as it is being analyzed. All components with less than about 15 carbons are lost during the chromatography and do not contribute to the fractions shown in the figure. The gas chromatography data of Figure 1 lead us to expect that the initial oil will become weathered during the thin-layer chromatography analysis, while the two burn residue samples have already lost the volatile components and are probably unaffected by the analysis. VOL. 34, NO. 10, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 4. Depletion of three- and four-ring aromatics in the burn residue oil as compared with the initial oil.
FIGURE 7. m/z ) 228 chromatograms of the original oil and burn residues.
FIGURE 5. Composition of the original oil and burn residues, determined by thin-layer chromatography.
FIGURE 8. m/z ) 252 chromatograms of the original oil and burn residues.
FIGURE 6. m/z ) 202 chromatograms of the original oil and burn residues. The data presented thus far show that the residual oils after the burn have the appearance of highly weathered oils that have undergone substantial evaporation during the combustion and subsequent cooling. In the following figures, we address whether the residual oils have gained any pyrogenic polycyclic aromatic hydrocarbons. Figure 6 shows the m/z ) 202 ion chromatograms of the oils. Fluoranthene and pyrene, present at about 16 and 10 ppm in the initial oil, increased to about 40 ppm each in the burn residue. It seems likely that these have been generated during the combustion. 1936
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Figure 7 shows the m/z ) 228 ion chromatogram indicating that chrysene, present at about 32 ppm in the initial oil, did not change significantly (to 36 ppm), whereas benz[a]anthracene, present at about 9 ppm in the initial oil, approximately doubled to 18 ppm, which we attribute to generation during combustion. Figure 8 shows the m/z ) 252 ion chromatogram. Benzo[b]fluoranthene and benzo[k]fluoranthene are only poorly resolved on this column, but it is clear that the levels of benzo[b]fluoranthene are similar in the initial oil and burn residues, while benzo[k]fluoranthene is increased in the burn residues. Benzo[a]pyrene increased from approximately 4 ppm in the initial oil to about 14 ppm in the burn residue. Thus, several pyrogenic compounds were found at higher concentrations in the residual oil than in the initial oil. Nevertheless, these increases were relatively small and, in all cases, were outweighed by the absolute loss of these compounds during the combustion. Thus, the total amount of all of the polycyclic aromatic hydrocarbons on the U.S. EPA Priority Pollutant list in the initial oil decreased during the burn, as shown in Figure 9. For example, even though the concentration of pyrene in the residual oil was approximately 4-fold higher than in the initial oil, the total amount in the “spill” decreased by about 40% along with the 85% reduction in total oil during the burn. We found no significant differences in the burn residues with and without ferrocene for any of the EPA Priority Pollutant polycyclic aromatic hydrocarbons.
FIGURE 9. Loss of polycyclic aromatic hydrocarbons in Statfjord crude oil following in situ burning. The data are the average of burns with and without ferrocene as a soot suppressant. Acenaphthylene, acenaphthene, and anthracene, were undetectable in either the original oil or the burn residue. Naphthalene was present in the initial oil but was absent from the burn residues, presumably due to evaporation. Wang et al. (21) have recently reported an extensive study of both the residue and the soot generated during a largescale test burn of diesel fuel. Diesel is much more flammable than crude oil, and the residue left when the fire extinguished was only 0.1-0.3% of the initial volume as compared to 15% for the experiment reported here. They also saw an increase in four- to six-ring polycyclic aromatic hydrocarbons. While their approach to quantitation did not use a conserved internal marker (hopanes are absent in diesel), they also concluded that the generation of polycyclic aromatic hydrocarbons, and their presence in the residue, was substantially outweighed by the combustion process. Our results are very consistent with those of Benner et al. (15), who burnt Alberta Sweet crude under carefully controlled laboratory conditions. Since Benner et al. (15) did not use conserved internal species in their analyses, their estimates of the extent of generation of the pyrogenic compounds are complicated by the evaporation that also occurred. Nevertheless, they also concluded that, although burning spilled oil generates five-ring and larger aromatic compounds, the overall effect of a successful burn is to remove polycyclic aromatic hydrocarbons from the environment. Our findings reinforce their conclusion and that of Wang et al. (21), that a successful in situ burn will substantially reduce the potential local environmental impact of polycyclic hydrocarbons in spilled oil.
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Received for review November 8, 1999. Revised manuscript received February 14, 2000. Accepted February 29, 2000. ES991255J
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