Enhanced Sorption of PAHs in Natural-Fire-Impacted Sediments from

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Enhanced Sorption of PAHs in Natural-Fire-Impacted Sediments from Oriole Lake, California Julia Sullivan,† Kevyn Bollinger,† Anthony Caprio,‡ Mark Cantwell,§ Peter Appleby,|| John King,† Bertrand Ligouis,^ and Rainer Lohmann*,† †

Graduate School of Oceanography, University of Rhode Island, Narragansett, Rhode Island 02882, United States Sequoia & Kings Canyon National Parks, National Park Service, Three Rivers, California 93271, United States § ORD\NHEERL-Atlantic Ecology Division, U.S. Environmental Protection Agency, Narragansett, Rhode Island 02882, United States Dept of Mathematical Sciences, University of Liverpool, Liverpool L69 3BX, U.K. ^ Laboratory of Applied Organic Petrology, Universit€at T€ubingen, 72070 T€ubingen, Germany

)

‡

bS Supporting Information ABSTRACT: Surface sediment cores from Oriole Lake (CA) were analyzed for organic carbon (OC), black carbon (BC), and their δ13C isotope ratios. Sediments displayed high OC (20-25%) and increasing BC concentrations from ∼0.40% (in 1800 C.E.) to ∼0.60% dry weight (in 2000 C.E.). Petrographic analysis confirmed the presence of fire-derived carbonaceous particles/BC at ∼2% of total OC. Natural fires were the most likely cause of both elevated polycyclic aromatic hydrocarbon (PAH) concentrations and enhanced sorption in Oriole Lake sediments prior to 1850, consistent with their tree-ring-based fire history. In contrast to other PAHs, retene and perylene displayed decreasing concentrations during periods with natural fires, questioning their use as fire tracers. The occurrence of natural fires, however, did not result in elevated concentrations of black carbon or chars in the sediments. Only the 1912-2007 sediment layer contained anthropogenic particles, such as soot BC. In this layer, combining OC absorption with adsorption to soot BC (using a Freundlich coefficient n = 0.7) explained the observed sorption well. In the older layers, n needed to be 0.3 and 0.5 to explain the enhanced sorption to the sediments, indicating the importance of natural chars/inertinites in sorbing PAHs. For phenanthrene, values of n differed significantly between sorption to natural chars (0.1-0.4) and sorption to anthropogenic black carbon (>0.5), suggesting it could serve as an in situ probe of sorbents.

’ INTRODUCTION The fate of hydrophobic organic compounds (HOCs), such as polycyclic aromatic hydrocarbons (PAHs), in sediments and soils depends largely on their sorption to solid phases.1,2 In urban/industrial sediments and soils, sorption is often dominated by anthropogenically derived geosorbents, such as soot, tar pitch, coal, or nonaqueous phase liquids (NAPL).2-5 Soot black carbon (BC) is formed by the condensation of small aromatic moieties in the gas phase of high-temperature combustion processes.6 Much less attention has been paid to the effect naturally produced charcoals and chars have on the sorption of HOCs in the field. Natural fires affect huge areas and have major impacts on land cover, atmospheric chemistry, and regional and global climate.7-9 For example, in the U.S. alone, close to 4 million hectares have been subject to prescribed and wildland fires annually.10 The deposition of charred biomass and charcoal from r 2011 American Chemical Society

these fires changes the geochemistry6 and physical properties of soils and sediments.11,12 Highly reflecting organic particles with porous char morphology are classified as natural char.13 They are formed under the influence of heat from fire on gelified organic matter (for example, burning of gelified organic matter in the organic horizon of a forest soil, occurring during a ground fire). Fusinite particles (charcoal), which form by burning of fresh or more or less unaltered plant tissues, differ from natural char by their typical well-preserved cellular structures and the absence of degassing pores.14 In this field study, we took sediment samples from Oriole Lake (latitude, 36.4602226 N; longitude, 118.7370406 W), a small 6.8 m Received: November 12, 2010 Accepted: February 11, 2011 Revised: February 7, 2011 Published: March 15, 2011 2626

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Environmental Science & Technology deep pond less than 1 ha in area at 1727 m elevation on a SW-facing slope in the Mineral King drainage in Sequoia and Kings Canyon National Park (SEKI; Three Rivers, CA 93271) ( Figure 1 of the Supporting Information). Fire chronologies for Oriole Lake derived from fire scarred trees extend back to ∼1600. Natural fires occurred every 5-20 years until ca. 1870, when Euro-American settlement resulted in changes to the fire regime.15 Previous work estimated chars to be abundant at ca. 5%-10% (dry weight), but dropped from 10% to 7% at 37 cm depth, probably because of fire exclusion since 1900 C.E.16 This suggested the presence of pyrogenic particles, which could be used to trace past fires and establish their impact on the in situ sorption of PAHs in Oriole Lake sediments. We focused on PAHs, as controlled burning experiments suggest that they are emitted at much higher concentrations than other HOCs.17 Depending on fire conditions, charcoal and char could be important contributors to sorption of HOCs and control parts of the carbon cycle in regions most affected by frequent fires. In previous work, we demonstrated the importance of soot BC for the in situ sorption of different HOCs by incubating sediments with passive polyethylene (PE) samplers in tumbling experiments.18 The PE sampler accumulates HOCs until phase equilibrium in the sediment-water slurry is achieved.19 Knowledge of the HOC’s PE-water partition coefficients allowed us to quantify the concentrations of the dissolved HOCs (Cw) in equilibrium with the PE sampler and the sediments. In summary, we hypothesized that we could identify the sedimentary record of natural fires via (i) the presence of soot and charcoal; (ii) the presence of elevated PAH concentrations, characteristic ratios, and the presence of retene; and (iii) enhanced in situ sorption of PAHs in layers affected by natural fires, presumably due to the presence of soot and charcoal.

’ MATERIALS AND METHODS Site Location and Sampling. The sediment at its maximum depth of around 5.00-5.10 m was dated to be 3375 ( 150 14C yr B.P.16 Five biological cores (ca. 1 m length each in polypropylene tubes, diameter 14 cm) were collected from Oriole Lake in July 2007. Each core was sampled using a piston to create a seal above the sediment water interface. The cores were capped and shipped upright back to the University of Rhode Island. They were extruded vertically in a clean laboratory, sliced in 1 cm intervals, and stored in amber glass jars. Sediment Dating. Dried samples from sediment core OL07 BC-1 and BC-3 were analyzed for 210Pb, 226Ra, and 137Cs by direct γ-assay at the Liverpool University Environmental Radioactivity Laboratory using Ortec HPGe GWL series well-type coaxial low background intrinsic germanium detectors20 (for more details, see Supporting Information). PAH Extractions. Sediment-bound PAHs were extracted using a modification of the Accelerated Solvent Extraction (ASE) method of ref 21. Portions [1-5 g, dry weight (dw)] of sediment from each incubation were centrifuged to remove excess water, mixed with clean sand, and extracted using a Dionex ASE 350. Each sample was extracted using a 1:1 ratio of acetone and n-hexane with six static cycles (10 min each) at 150 C and 104 kPa. The extracts were dried using clean sodium sulfate. Prior to ASE extraction, deuterium-labeled PAH internal standards were added to the packed ASE cells. Porewater Incubations. A nondepletive, polyethylene (PE) passive sampling technique was employed to measure the freely

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dissolved concentrations of PAHs, following the method of ref 18. As the initial cm-resolution sediment analysis revealed only minor changes from sample to sample, we homogenized each of the three layers of the core, corresponding to 1912-2007 C.E. (1-16 cm), 1836-1912 C.E. (17-28 cm) and ∼17801836 C.E. (28-36 cm). These sediment layers were incubated in triplicate with different masses (ranging from 0.15 to 1.0 g) of polyethylene (PE) samples to measure porewater concentrations of PAHs and to deduce the importance of the different carbonaceous phases for the sorption of PAHs in situ. Incubations were agitated on a shaker table for 3-4 weeks each. In general, the extractions of different sized PEs and sediments agreed within 25% of each other, indicating that equilibrium had been reached (only two PEs were analyzed from the 1780-1836 incubations, and only two sediments from the 1836-1912 incubation). Instrument Analyses and QC. Analysis for PAHs was conducted on an Agilent 6800 gas chromatograph (GC) coupled to an Agilent 5973N mass spectrometer (MS) operated in the negative electronic ionization mode (for more details, see the Supporting Information). Selected Physicochemical Properties. To obtain a full set of polyethylene-water partitioning coefficients (KPE-Ws) for PAHs, an updated correlation versus octanol-water partitioning coefficients (Kows) was used from ref 22 (Table 1 of the Supporting Information ). We chose internally consistent Kow for PAHs from Ma et al.23 OC-water distribution coefficients (Kocs) and activity coefficient in aqueous solution (γwsat) for PAHs were calculated relative to Kows as described in ref 24. BC-water distribution coefficients (KBCs) were estimated on the basis of the correlation with the PAH’s γwsat from ref 18. OC and BC Analysis. Approximately 2 mg of sediment was placed in ceramic crucibles, dried at 70 C for 12 h, ground with a mortar and pestle, and sieved. TOC and BC are measured directly, with OC accounting for the difference (or OC = TOC - BC). BC was quantified by oxidizing the non-BC organic carbon at 375 C for 24 h (chemothermal oxidation, CTO-375) according to the method of ref 25 and then analyzed for TOC. Sediment samples were analyzed for %C and δ13C using a Carlo Erba 1500 elemental analyzer coupled to a VG Optima stable isotope mass spectrometer as detailed in ref 26. Several BC reference standards27 were routinely included in BC runs: SRM 1941b (marine sediment), risotto char, chestnut char, melanoidin, and muffled sand as a blank. BC was only detected in SRM 1941b (0.71% dw ( 010%; δ13C = -25.9% ( 0.58%), but not detected in the chars, melanoidin, or sand, confirming the selectivity of CTO-375 for sootlike BC. Petrographic Analysis. Approximately 50-100 g of sediment from the incubations was analyzed by petrography following the procedures detailed in refs 28 and 29 at the Laboratory for Organic Petrology at the University of T€ubingen. Polished organic concentrates of the sediment subsamples were investigated by microscopic analysis using reflected light and blue light/ UV fluorescence, and different carbonaceous particles were identified, such as anthropogenic particles and recent and fossil organic particles (see Supporting Information and Table 5 of the Supporting Information for examples). Sorption Models. Porewater concentrations, Cdiss (ng/mL), were calculated as Cdiss ¼ CPE =KPE-w 2627

ð1Þ

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Figure 1. Depth profiles of organic carbon (OC, in % dry weight) and black carbon (BC, in % dry weight  10) and their δ13C values (in %).

where CPE (ng/mLPE) is the PAH concentration in PE normalized to PE’s density (0.92 g/cm3). OC-Absorption Model. In the OC-absorption model, the measured sediment-water distribution of PAHs, Kd, was explained according to Kd ¼ foc Koc

ð2Þ

with Koc being the PAH’s OC-water partitioning constants taken from the literature and foc the sediments’ OC fraction. Combined OC- and BC-Adsorption Model. Alternatively, the overall partitioning of PAHs was attributed to OC absorption and adsorption onto the BC fraction (fBC)3 via Kd ¼ foc Koc þ fBC KBC Cw n - 1

ð3Þ

with KBC being the PAH’s soot BC-water adsorption constant, n the Freundlich coefficient, and Cw the dissolved concentration in μg/L. We chose n = 0.730 and KBCs from harbor sediment equilibrations.18

’ RESULTS AND DISCUSSION Full Core Analysis. Core Dating. The two cores sent out for radiodating, BC-1 and BC-3, yielded very different results. Although both cores had similar surface concentrations, unsupported 210Pb in BC-1 declined much more rapidly with depth, reaching values below the limit of detection at a depth of less than 20 cm compared to 80 cm in BC-3. 137Cs in BC-1 was detected only in the topmost sample (0-1 cm). Because of the much higher accumulation rate in BC-3, its entire 1 m length covered a period of no more than 150 years. The detailed results (Tables 2 and 3 of the Supporting Information) suggest that this core is from a site subject to sediment focusing. As we were interested in the effects of natural fires over the last ∼250 years, our work focused on BC-1, which appears to span a much longer time period. Although the amount of 210Pb data for BC-1 is limited, it does seem likely that sedimentation at this site was relatively uniform. A best estimate of the mean sedimentation rate is 0.016 g cm-2 y-1 (0.16 cm y-1) (for a chronology, see Table 3 of the Supporting Information). Downcore Profiles of BC, OC, and δ13C. Our analysis of the top 60 cm of the core BC-1 showed minor fluctuations in BC content (around 0.5%) and a very high OC content of mostly

between 20% and 25% (Figure 1). δ13C values of BC were consistently heavier by 2%-3% (i.e., more enriched in 13C) than δ13C values of the OC fraction (Figure 1). The OC fraction displayed a significant change in δ13C values from 1630 to 2007, resulting in more depleted δ13C values with depth (Figure 1 and Table 4 of the Supporting Information). Over the last ∼200 years (0-30 cm), BC concentrations declined significantly, from ∼0.60% to ∼0.40% dw (Figure 3 of the Supporting Information). During the same period, OC concentrations also declined, albeit with a weaker correlation (Table 4 of the Supporting Information). For both OC and BC, δ13C values became significantly more depleted in 13C (i.e., lighter) over the last 200 years (0-30 cm), but not in preindustrial sediments (39-60 cm) (Table 4 of the Supporting Information). Overall, the OC and BC profiles suggest that there were two distinct periods captured in the sediments of Oriole Lake. There was an almost constant background signal from OC and BC prior to 1800, in terms of their δ13C values (Figure 1). Both OC and BC displayed a sharp decrease at 50 cm, probably indicating dilution by a mineral phase. Since 1800, there was evidence for increasing (atmospheric) deposition of BC and OC during the industrialization of the northern hemisphere, leading to enhanced C deposition to Oriole Lake (Figure 3 of the Supporting Information). Presumably, the high background concentrations of OC in Oriole Lake masked the impact of C-deposition, resulting in weaker correlations. Additional evidence for longterm changes of carbon residing in Oriole Lake can be gleaned from the carbon fractions’ δ13C values. The overall δ13C values of OC and BC were consistent with C3-vegetation (-23% to 28%), dominating C-influx into Oriole Lake,31 in-line with the vegetation present in its watershed. The enrichment of 13C values of OC and BC in the recent sediment (Figure 1) could be due to the deposition of carbon from fossil fuel emissions.32 With the high amount of OC present, there is potential of charring, leading to erroneously high values of BC. There was a significant intercorrelation of OC and BC concentrations from 1630 to 2007 (r2 = 0.58) over the whole time span analyzed, but a much weaker correlation was found over last 200 years (r2 = 0.19). Conversely, the correlation between OC and BC was highly significant for the deepest 39-60 cm section (r2 = 0.87) (Table 4 of the Supporting Information). As reported above, δ13C values were offset between the two carbon pools, suggesting that different fractions of carbon were involved. Density plots (Figure 2 of the Supporting Information) suggested that the influx of OC and BC into Oriole Lake might have been coupled. Nevertheless, in the preindustrial time horizon it seems possible that charring of OC might have inflated BC values. Bulked Sediment Layers. Sediment PAH Concentrations. The three sediment layers displayed vastly different PAH concentrations and profiles (Figure 2, Table 4 of the Supporting Information). All PAHs were summed and reported as ∑18PAH, except for perylene and retene, which can have a biological origin (see below). The depth distribution of ∑18PAHs was consistent with natural fires having influenced the middle (2000 ng/g dw) and deep layer (300 ng/g dw), but not the top layer. Even though there was an increase of BC in the top sediment layer, it displayed the lowest PAH concentrations, at 50 ng/g dw. This suggested that natural fires constituted a more important source of PAHs to Oriole Lake than other local sources or their long-range transport to the lake. We investigated the distribution of different PAHs (“molecular ratios”) to assess whether the PAHs stemmed from petroleum spills or the combustion of fossil fuels or wood.33 2628

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Figure 2. Concentrations of sum PAHs, retene, and perylene in different sediment incubations from Oriole Lake core BC-1 in (a) ng/g dw sediment and (b) ng/g PE.

Table 1. Concentrations and δ13C Values of OC and BC in Sediments Based on Chemical and Petrographic Analysisa chemical analysis (BC via CTO-375) %OC

δ C 13

%BC

δ C 13

BC as %OC

petrographic analysis (% of total organic matter) huminites

liptinites

inertinites

natural char

coke

hard coal/soot

visible BC

1912-2007

23.4

29.2

0.55

26.0

2.3

93.6

4.0

2.2



0.2



2.2

1836-1912 ∼1780-1836

21.5 22.7

30.1 29.8

0.46 0.44

27.3 27.6

2.1 1.9

93.8 92.8

4.4 5.2

1.6 1.8

0.2 0.2

nd nd

nd nd

1.8 2.0

a

Huminite, humified plant fragments (i.e., plant fragments derived from soft tissues (cellulose) and woody tissues (cellulose and lignin); liptinite, lipidrich plant fragments (i.e., oil secretions from leaves, algae, spores, pollen); inertinite, carbon-rich plant materials (i.e., charred or oxidized: tissues, plant secretions, humic gels); visible BC, sum of inertinite, natural char, anthropogenic particles (coke, hard coal, soot); funginites (fungal remains) are excluded from BC calculation; nd, not detected; , detected, but not quantified.

Wood combustion should be characterized by ratios of fluoranthene over fluoranthene and pyrene (fl/[fl þ pyr]) > 0.5, anthracene over anthracene and phenanthrene (an/[an þ phen]) > 0.1 and indeno[1,2,3-c,d]pyrene over indeno[1,2,3-c,d]pyrene and benzo[g,h, i]perylene (ip/[ip þ b(ghi)p]) > 0.5.33 A petrogenic source would be indicated by elevated methylphenanthrenes relative to phenanthrene and by an/[an þ phen] < 0.1. All PAH ratios established here suggested a pyrogenic origin of the PAHs (fl/[fl þ pyr] ratio 0.5-0.65; an/[anþphen] ratio 0.17-0.5; ip/[ipþb(ghi)p] ratio 0.47 -0.55; methylphenanthrenes < phenanthrene). The fl/flþpyr and ip/[ipþbghip] ratios suggested grass/wood/coal combustion as sources. Unexpectedly, retene and perylene concentrations displayed inverse trends with total PAHs (Figure 2). Retene (and perylene) concentrations were highest in the top layer, at 830 ng/g (520 ng/g), elevated in the deepest layer, at 410 ng/g (280 ng/ g), but very low in the middle layer, at merely 63 ng/g (91 ng/g). Retene was proposed as a marker molecule for coniferous (soft) wood combustion in air,34 but its presence in sediments could also be due to biological degradation of the wood tissue.35 The presence of perylene has been attributed to numerous sources, including pyrogenic and diagenetic origin.35 The inverse relationship between ∑18PAHs and retene/perylene abundance in the sediments suggests that both might originate from the same source, but via different pathways. When natural fires consumed woody tissue, they probably precluded the in situ formation of retene and perylene in the sediments (middle layer). Conversely, in the absence of fires, woody tissue reached Oriole Lake, from which retene and perylene were formed (upper layer). According to dendrochronological (tree-ring) fire history reconstructions, natural fires have been absent from Oriole Lake since ∼1850, but occurred regularly prior to 1850.15 Major fire

events in the Oriole Lake area were recorded in 1822, 1827, 1847, and 1872. This further supports our interpretation that the lower and middle sediment layers contained records of natural fires, but not the most recent layer. Porewater PAH Concentrations. ∑18PAHs was highest in the deepest layer (410 ng/gPE) and at 150-190 ng/gPE in the upper layers (Figure 2). Retene also displayed highest concentrations in the deepest layer (530 ng/gPE), lowest in the middle layer (56 ng/gPE), and at 290 ng/gPE in the top layer. In contrast, perylene was highest in the top layer (210 ng/gPE), but displayed similar concentrations of ∼110-130 ng/gPE in the other layers (Figure 2). Petrographical Analysis of Sediments. The three samples were rich in organic matter (OM) (Table 5 of the Supporting Information) and were dominated by humified plant fragments (Table 1; Huminite: 93-94 vol %) (Figure 3a). Lipid-rich plant fragments (liptinites; see Table 1) represented about 4.5 vol % (Figure 3b). Black carbon was present at 2.2 vol % in the top layer and 1.8 vol % in the deeper sections. BC related to forest fire was present in similar quantity in all samples (charcoal þ natural char) (Figure 3c,d). However, the top sample contained additional anthropogenic particles (hard coal, coke, char from fossil fuel combustion, and soot) (Figure 3e,f). These particles were not encountered in the lower samples, confirming the general dating of the core. The petrographic results (Table 5 of the Supporting Information) were based on the analysis of particles >2 μm (size limitation due to magnification 500-1000). Consequently, it is possible that the soot BC was slightly underestimated.12 In addition, the samples contained a great quantity of detrohuminite (fine humic fragments) that could have masked tiny fragments of soot. There was excellent agreement between the results of BC analysis by CTO-375 (increasing from 1.9% of OC in the deepest 2629

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Figure 3. Microscopic examples of organic and black carbon particles.

layer, 2.1% in the middle, to 2.3% in the top layer) with results by petrography (Table 1). Natural chars formed at low temperatures are generally oxidized during CTO-375. We included risotto char, chestnut char, and melanoidin in the CTO-375 runs, all of which were completely removed. Most likely, chars formed during natural fires in Oriole Lake were resistant to CTO375, possibly due to their formation at higher temperatures. It is also conceivable that the very high OC content of the sediments resulted in preferential oxidation of huminites and liptinites first, resulting in the preservation of natural chars, charcoals, and soots. Sedimentary Evidence for the Presence of Natural Fires in Oriole Lake. While PAH results suggested natural fires affected the deeper layers, there was no indication of increases in natural chars in these layers. Presumably, as indicated by their constant concentrations among all three layers, there is a continuous flux of these particles into the lake. A major fire would leave topsoil vulnerable to erosion during strong rains. This could possibly explain unchanged char and charcoal concentrations after a natural fire, as the fire signal would be diluted with soil richer in minerals, but lower in OC and BC. Indeed, there was a major change in magnetic susceptibility from ca. 15-25 cm, coupled with a gentler increase in sediment density (Figure 2 of the Supporting Information). In fact, there are also smaller increases apparent in the lowest (28-36 cm) layer, supporting the existence of a smaller scale natural fire recorded in the deepest

sediment layer analyzed. This agrees with the tree-based fire history of numerous natural fires in Oriole Lake Grove prior to 1850.15 Sorption Models. Concentrations of the PAHs in the PEs that were incubated with the different sediment layers displayed a much narrower range of concentrations compared to the sediments (Table 4 of the Supporting Information). This reflects upon different availabilities of the PAHs or the extent to which sorption was holding PAHs in the sediments, as discussed below. Concentrations of ∑18PAHs in PE did not mirror its sediment profile, while retene’s PE concentrations changed with depth similar to its sediment profile (Figure 2). OC-Absorption Model. Values of Koc taken from the literature (Table 1 of the Supporting Information) have an uncertainty of a factor of (2-3.24 The OC-absorption model strongly underestimates the partitioning of PAHs (Figure 4a), by ∼1 order of magnitude for the deepest layer and by ∼2-3 orders of magnitude for the upper layers. These results imply that additional sorption to the sediments is occurring, most likely to reduced carbonaceous geosorbents such as soot black carbon and chars, as detected both by petrography and CTO-375 in all layers. Combined OC-Adsorption and OC-Absorption Model. The combined OC-Absorption and BC-Adsorption model explained the observed distributions well for the most recent sediment layer, with most values within a factor of 3 of predictions (Figure 4b). In contrast, using n = 0.7, Kd,meas of the deepest layer were still underpredicted by factors of 5-10, while the middle layer was 2630

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Figure 4. Measured sediment-porewater distribution (Kd,meas) of PAHs versus predictions based on (a) absorption into OC and (b) combined OC and BC adsorption.

Table 2. Comparison of Freundlich Coefficients n and Freundlich Sorption Constants KF [μg/kg char/(μg/L)n] for Phenanthrene Sorption onto Natural Chars and Black Carbon natural char/charcoal Freundlich n

black carbon log KF

Freundlich n

ref

log KF

ref

0.40 ( 0.009 (natural wood char)

4.6

36

0.52 ( 0.01 (hexane soot)

4.4

37

0.39 ( 0.002 (extracted natural wood char)

4.7

36

0.41 (SRM 2975 diesel soot)

4.3

37

0.60 (SRM 1650b diesel soot)

45.47

37

0.13 - 0.42 (artificial wood chars at 300-820 C)

5.04-6.11

39

0.58-0.64 (black carbon substrates)

5.6-6.3

38

0.24-0.60 (natural wood chars) 0.30-0.35 (charred kerogen at 450 C)

5.6-5.7

40

0.54-1.00 (kerogens at 200-400 C)

4.6-6.5

40

0.24-0.32 (charred kerogen at 500 C) 0.47 (artificial wood char, unknown temperature)

5.8 6.1

41

0.51-0.64 (natural kerogens)

5.0-5.4

41

0.42-0.47 (charred sawdust at 400 C)

7.1-8.0

42

0.21-0.38 (presumably natural charcoal)

5.0

this work

0.55 (presumably black carbon, top layer)

5.0

this work

underpredicted 10-1000 times (Figure 4 of the Supporting Information). Sootlike black carbon seems to be able to account for additional sorption present in the most recent sediment, but is insufficient to explain sorption in the deeper layers. If soot BC values were artificially elevated due to charring, real sorption due to BC would be further underestimated. These results strongly suggest the presence of additional sorption beyond the phases considered above, most likely due to the presence of natural charcoals throughout the sediment core. Explaining the Observed Sorption of PAHs in Oriole Lake Sediments. We investigated whether the addition of a separate lipid-phase (based on the presence of liptinites at around 5% of TOC) could account for the additional sorption. Yet partitioning to lipids (assuming log Klipid-w = 0.91 log Kow þ 0.50)24 is not enough to account for the missing sorption. Any further subdividing of the BC phases (into chars, charcoals, coke, coals, or soot) would not help either, as soot is thought to be among the strongest natural geosorbents (i.e., excluding activated carbon or NAPL phases).38 Enhanced PAH Sorption to Chars. The only way to explain the observed enhanced sorption of PAHs in the deeper sediment layers affected by natural fires is to use a different value for the Freundlich coefficient n. The value of 0.7 is based on sorption isotherms derived for soot black carbon in urban/industrialized sediments, with strong competition from other hydrophobic organic contaminants. Furthermore, these anthropogenically affected sediments typically

contain PAHs at much higher concentrations than encountered in Oriole Lake. A Freundlich coefficient of n = 0.7 gave a good fit for the most recent sediment layer. In contrast, we needed to use n = 0.3 for the middle layer and 0.5 for the deepest layer to reconcile predicted and measured Kd values (Figure 4b). Freundlich Coefficient of Phenanthrene Sorption to Chars. Phenanthrene is the PAH most commonly used for sorption isotherms; hence, we compared sorption results from Oriole Lake with literature values (Table 2). We obtained n = 0.54 (top), 0.21 (middle), and 0.38 (deepest layer) for phenanthrene, using a log KF = 5.0. Other sorption studies using natural wood chars resulted in similar n-values (Table 2). Sorption to artificial wood chars and charred kerogens also yielded n-values 0.5.38 The difference in Freundlich coefficients could represent a means of distinguishing between sorption to soots and charcoals in situ.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information as discussed in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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’ AUTHOR INFORMATION Corresponding Author

*E-mail: [email protected].

’ ACKNOWLEDGMENT This work was supported by grant PRF 46099-G2 from the American Chemical Society-Petroleum Research Fund, and task agreement J8558100500 from the Joint Fire Science Project. We thank Chip Heil, Nate Vinhanteiro (URI), and SEKI staff for field support and Pam Luey (URI) for laboratory help. ’ REFERENCES (1) Luthy, R. G.; Aiken, G. R.; Brusseau, M. L.; Cunningham, S. D.; Gschwend, P. M.; Pignatello, J. J.; Reinhard, M.; Traina, S. J.; Weber, W. J.; Westall, J. C. Sequestration of hydrophobic organic contaminants by geosorbents. Environ. Sci. Technol. 1997, 31 (12), 3341–3347. (2) Allen-King, R. M.; Grathwohl, P.; Ball, W. P. New modeling paradigms for the sorption of hydrophobic organic chemicals to heterogeneous carbonaceous matter in soils, sediments, and rocks. Adv. Water Resources 2002, 25 (8-12), 985–1016. (3) Accardi-Dey, A.; Gschwend, P. M. Assessing the combined roles of natural organic matter and black carbon as sorbents in sediments. Environ. Sci. Technol. 2002, 36 (1), 21–29. (4) Ghosh, U.; Gillette, J. S.; Luthy, R. G.; Zare, R. N. Microscale location, characterization, and association of polycyclic aromatic hydrocarbons on harbor sediment particles. Environ. Sci. Technol. 2000, 34 (9), 1729–1736. (5) Cornelissen, G.; Gustafsson, O. Importance of unburned coal carbon, black carbon, and amorphous organic carbon to phenanthrene sorption in sediments. Environ. Sci. Technol. 2005, 39 (3), 764–769. (6) Masiello, C. A. New directions in black carbon organic geochemistry. Mar. Chem. 2004, 92 (1-4), 201–213. (7) Kuhlbusch, T. A.; Crutzen, P. J. Toward a global estimate of black carbon in residues of vegetation fires representing a sink of atmospheric CO2 and a source of O2. Global Biogeochem. Cycl. 1995, 9, 491–501. (8) Ito, A.; Penner, J. E., Historical emissions of carbonaceous aerosols from biomass and fossil fuel burning for the period 1870-2000. Global Biogeochem. Cycle 2005, 19, (2). (9) Cooke, W. F.; Ramaswamy, V.; Kasibhatla, P. A general circulation model study of the global carbonaceous aerosol distribution. J. Geophys. Res.-Atmos. 2002, 107, D16. (10) NIFC, National Interagency Fire Center, http://www.nifc. gov/index.html. (11) Schmidt, M. W. I.; Skjemstad, J. O.; Jaeger, C. Carbon isotope geochemistry and nanomorphology of soil black carbon: Black chernozemic soils in central Europe originate from ancient biomass burning. Global Biogeochem. Cycl. 2002, 16, 70–71to78. (12) Fernandes, M. B.; Skjemstad, J. O.; Johnson, B. B.; Wells, J. D.; Brooks, P. Characterization of carbonaceous combustion residues. I. Morphological, elemental and spectroscopic features. Chemosphere 2003, 51 (8), 785–795. (13) Kwiecinska, B.; Petersen, H. I. Graphite, semi-graphite, natural coke, and natural char classification-ICCP system. Int. J. Coal Geol. 2004, 57, 99–116. (14) ICCP, The new inertinite classification (ICCP System 1994). Fuel 1994, 2001, (80), 459-471. (15) Caprio, A. C. Temporal and spatial dynamics of pre-EuroAmerican fire at a watershed scale, Sequoia and Kings Canyon National Parks. Assoc. Fire Ecol. Misc. Publ. 2004, No. No. 2, 107–125. (16) Davis, M. B. Fire History of Sequoia Park—Feasibility Study. Report to the U.S. Park Service, Sequoia and Kings Canyon National Parks, 1985, 14 pp. (17) Lee, R. G. M.; Coleman, P.; Jones, J. L.; Jones, K. C.; Lohmann, R. Emission factors and importance of PCDD/Fs, PCBs, PCNs, PAHs,

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