Environ. Sci. Technol. 2000, 34, 4348-4353
Vaporization of Polycyclic Aromatic Hydrocarbons (PAHs) from Sediments at Ambient Conditions STEVEN B. HAWTHORNE* AND CAROL B. GRABANSKI Energy and Environmental Research Center, University of North Dakota, P.O. Box 9018, Grand Forks, North Dakota 58202
Vaporization of PAHs from historically contaminated sediments was determined by exposing two different sediments to air and outdoor sunlight in glass flasks for 100 days and by shaking both sediments with water in the laboratory for 50 days. Vaporized PAHs were collected on polyurethane foam (PUF) sorbent plugs placed in the flask necks. Sediment temperatures in the outdoor flasks were similar to those of surface soil temperatures in the surrounding area (up to 60 °C), demonstrating that “greenhouse” heating in the flask was not significant. For both sediments exposed to outdoor sunlight, losses of the more volatile PAHs (up to three-ring compounds) ranged from ∼10 to 90%, and small amounts of even the fourand five-ring PAHs were found in the vapor samples. The fractions of total PAHs vaporized (collected on the PUF sorbents) were 51% and 28% for sediments with total PAH concentrations of 34000 mg/kg and 17000 mg/kg, respectively. The quantity of PAHs collected from the vapor phase plus the quantity remaining on the sediment agreed with the initial mass of PAHs on the fresh sediments, demonstrating that PAH degradation was not significant for the airdried sediments. Vaporization of total PAHs from the sediments shaken with water for 50 days accounted for 23% and 8% of the total PAHs from the two sediments, even though both sediments showed large numbers of PAH-degrading bacteria. These results suggest that vaporization of PAHs can contribute significantly to the reductions in PAH concentrations that are reported for biological treatment.
Introduction Polycyclic aromatic hydrocarbon (PAH)-contaminated soils and sediments have been found at many manufactured gas plant (MGP) sites throughout the United States and worldwide (1). Interest in remediating these sites has led to many studies on bioremediation. Unless the PAHs have become highly sequestered (i.e., bound to the soil or sediment matrix), and, therefore, unavailable to microorganisms (2, 3), bioremediation typically results in much higher removal of low molecular weight PAHs than higher molecular weight PAHs (4-13). In general, PAHs with 4 rings show only low removals, while larger PAHs are largely unaffected by bioremediation (4-13). Recently, Bosma et al. concluded that, “Mass transferand not the intrinsic microbial activitysis in most cases the critical factor in bioremediation” (3), and the importance of * Corresponding author phone: (701)777-5256; fax: (701)777-5181; e-mail:
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mass transfer in the biotreatment of MGP sites has been reviewed (14). These observations parallel the fact that lower molecular weight PAHs have substantially higher water solubilities and vapor pressures than higher molecular weight PAHs. The majority of bioremediation studies are conducted under aerobic conditions and, therefore, offer the possibility of vaporizing PAHs during the treatment. Unfortunately, this possible loss mechanism is rarely quantitated, and any reduction in PAH concentrations is ascribed to biologically mediated degradation (4-13). It might seem that, based on their high boiling points (e.g., 218 °C for naphthalene, 340 °C for phenanthrene and anthracene, ∼390 to 440 °C for four-ring PAHs), vaporization of PAHs is unlikely. However, it is well-known that these same PAHs in urban air exist substantially in the vapor phase rather than associated with air particulate matter. For example, in several recent studies to determine whether PAHs in ambient air are in the vapor phase or associated with air particulate matter, naphthalene was found only in the vapor phase, phenanthrene and anthracene (3-ring PAHs) were found >95% in the vapor phase, and 4-ring PAHs were typically found from ∼50 to 90% in the vapor phase (15-17). Even benzo[a]pyrene (which has a boiling point of 496 °C) was reported to average ∼10% in the vapor phase in air samples collected in July (17). When bioremediation is performed under field conditions such as landfarming, soils and sediments are typically tilled on a frequent basis and various nutrients and water are added. These treatments cause frequent exposure of new portions of the soil to surface air, and to solar heating. A limited survey of local agricultural fields we conducted on a ∼32 °C summer day demonstrated that surface soil temperatures often reach >60 °C (which is not a surprise to anyone who has walked barefoot on tilled fields on hot sunny days). In addition, the strong smell of coal tar from treated wood (e.g., railroad ties) on summer days demonstrates the volatilization of PAH compounds. In fact, an initial sampling we performed showed PAHs as large as four rings collected from the vapor phase above railroad tracks, in agreement with laboratory studies on the vaporization of PAHs from creosote-treated wood (18). Clearly, the potential for PAHs to partition into the vapor phase from soil and sediments undergoing treatment exists, even though bioremediation studies typically assume that all removed PAHs have been degraded. Based on these preliminary observations, we exposed two historically contaminated sediments impacted by former MGP activities to air and sunlight at ambient conditions and collected the PAHs which were vaporized. After 100 days, the quantity of PAHs collected from the vapor phase and the quantities remaining on the sediments were compared to the initial mass of PAHs on the fresh sediments to determine if vaporization or degradation was primarily responsible for the decrease in sediment PAH concentrations. Similarly, the PAHs vaporized from the same two sediments in sediment/ water slurries exposed to air at room temperature for 50 days were collected and determined.
Experimental Section Sediment Samples. Two sediment samples were collected near former MGP facilities. Both were air-dried (to ∼2% moisture), homogenized, sieved to 65c
>84c 89 90 54 29 26 6 4 2 3 2 2 2 6 2 1 2 51
96 94 99 108 109 99 96 84 100 105 97 86 88 102 111 106 >89c
a Concentrations and standard deviations are based on triplicate Soxhlet extractions before and after exposure of the sediment to evaporation for 100 days. b Mass balance is based on the sum of PAHs collected from the vapor phase and remaining in the sediment after 100 days compared to their concentrations in Day 0 (untreated) sediments. c Significant naphthalene was lost from the PUF sorbents from breakthrough during collection; therefore, these values are minimums.
TABLE 2. Vaporization of PAHs from Sediment “B” after 100 Days Exposure to Ambient Sunlight concn, mg/kg ( SDa
naphthalene 2-methylnaphthalene 1-methylnaphthalene fluorene dibenzothiophene phenanthrene fluoranthene pyrene benz[a]anthracene chrysene benzo[b,k]fluoranthene benzo[e]pyrene benzo[a]pyrene perylene indeno[1,2,3-cd]pyrene dibenzo[a,h]anthracene benzo[ghi]perylene total PAHs
day 0
day 100
% mass balanceb
% collected from vapor
443 ( 85 563 ( 70 458 ( 47 282 ( 46 156 ( 16 784 ( 81 334 ( 32 368 ( 40 196 ( 25 222 ( 28 282 ( 46 114 ( 23 189 ( 39 38 ( 7 75 ( 5 34 ( 2 79 ( 2 16600 ( 1900
170 ( 18 180 ( 19 113 ( 17 190 ( 10 138 ( 13 748 ( 71 355 ( 31 371 ( 32 203 ( 13 236 ( 16 324 ( 23 132 ( 14 196 ( 13 39 ( 2 83 ( 8 34 ( 3 84 ( 9 12400 ( 740
106 101 95 91 99 105 108 102 104 107 115 117 104 104 112 102 108 105
64 68 74 26 11 9 2 1 1 1 0 1 1 2 1 0 1 28
a Concentrations and standard deviations are based on triplicate Soxhlet extractions before and after exposure of the sediment to evaporation for 100 days. b Mass balance is based on the sum of PAHs collected from the vapor phase and remaining in the sediment after 100 days compared to their concentrations in Day 0 (untreated) sediments.
features of the curves are interesting to note. First, the general shapes of the vaporization curves are the same as those typically reported for other PAH loss studies such as biodegradation and water desorption, i.e., the PAHs exhibit a rapid release phase followed by a slower rate of desorption typical of mass transfer limitations (3, 14) (Figures 1 and 2). Second, the amounts of PAHs emitted in the first day and first week are quite large. For example, 35% (sediment “A”) and 64% (sediment “B”) of the total naphthalene vaporized over 100 days was vaporized in the first day, and 85% of the total naphthalene vaporized from both sediments was vaporized in the first week. For the total PAHs vaporized over 100 days, 40% of mass collected in the PUF sorbents was vaporized in the first week for sediment “A,” and 55% of the total mass collected in the PUF sorbents was vaporized in the first week for sediment “B”. Since acclimation of microorganisms (and subsequent degradation of PAHs) can require several days after a bioremediation treatment begins
(22-24), these results show that large amounts of PAHs can be emitted from field treated soils and sediments before microbiological activity becomes significant. Vaporization from Sediment/Water Slurries. Quantities of PAHs vaporized from the sediment/water slurries over 50 days were not as large as from the dry sediments exposed to ambient sunlight for 100 days, as might be expected simply because PAH molecules must presumably desorb into bulk water before partitioning to the air can occur. However, vaporization still had a significant effect on PAH removals, especially for sediment “A”. Total PAHs vaporized (i.e., collected in the PUF sorbent) accounted for 23% of the total PAH mass of sediment “A” and ∼8% of the total PAH mass for sediment “B” (Table 3). As before, the concentrations of higher molecular weight PAHs remained unchanged (within analytical error) in the sediments extracted after the 50-day exposure, and the PAH losses were restricted primarily to the two- and three-ring PAHs. However, in contrast to the VOL. 34, NO. 20, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 3: Vaporization and Degradation of PAHs in Sediment/Water Slurries after 50 Days sediment “A”
naphthalene 2-methylnaphthalene 1-methylnaphthalene acenaphthylene acenaphthene fluorene dibenzothiophene phenanthrene larger PAHs total PAHs
sediment “B”
% vaporizeda
% degradedb
% vaporizeda
% degradedb
36 ( 6 54 ( 9 87 ( 14 39 ( 6 33 ( 5 14 ( 2 4.3 ( 0.5 3.2 ( 0.4 NCc 23 ( 3
60 ( 10 43 ( 7 6.3 ( 1.0 6.5 ( 1.0 6.3 ( 1.0 1.3 ( 0.2
