NMR Characterization of 13C-Benzene Sorbed to ... - ACS Publications

A demonstration of the high variability of chars produced from wood in bushfires. Anna V. McBeath ... Australian Journal of Soil Research 2010 48 (7),...
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Environ. Sci. Technol. 2006, 40, 1764-1769

NMR Characterization of 13C-Benzene Sorbed to Natural and Prepared Charcoals R O N A L D J . S M E R N I K , * ,† RAI S. KOOKANA,‡ AND JAN O. SKJEMSTAD‡ Soil and Land Systems, School of Earth and Environmental Sciences, The University of Adelaide, Waite Campus, Urrbrae, South Australia, 5064, Australia, and CSIRO, Land and Water, PMB 2, Glen Osmond, SA 5064 Australia

We investigated how the NMR properties of uniformly 13C-labeled benzene molecules are influenced by sorption to charcoals produced in the laboratory and collected from the field following wildfires. Uniformly 13C-labeled benzene was sorbed to two charcoals produced in the laboratory at 450 and 850 °C. The chemical shift of benzene sorbed to the higher-temperature charcoal was 5-6 ppm lower than that of benzene sorbed to the lowertemperature charcoal. This difference was attributed to stronger diamagnetic ring currents (which cause a shift to lower ppm values) in the more condensed or “graphitic” hightemperature charcoal. The chemical shift of benzene sorbed to two charcoals collected from the field following wildfires indicated a degree of charcoal graphitization intermediate between that of the two laboratory-prepared charcoals. Variable contact time and dipolar dephasing experiments showed that the molecular mobility of sorbed benzene molecules increased with increasing charcoal graphitization, and also increased with increasing benzene concentration. We propose that the chemical shift displacement of molecules sorbed to charcoal could be used to identify molecules sorbed to black carbon in heterogeneous matrixes such as soils and sediments, and to establish how condensed or “graphitic” the black carbon is.

Introduction The behavior and fate of organic molecules in soils and sediments are in large part controlled by sorption to the solid phase, and for hydrophobic organic compounds (HOCs) the most important sorbent is organic matter. Numerous researchers have proposed that organic matter consists of various components, or contains various domains with different sorption properties (1-7), resulting in complex sorption behavior. Furthermore, highly aromatic materials produced during incomplete combustionsvariously referred to as black carbon (8-10), charcoal (9, 11, 12), soot (9, 13), and high surface area carbonaceous material (3, 5, 14)shave been identified as quantitatively important high-affinity sorbents in soils and sediments. The strong affinity for HOCs of soot and charcoal has been demonstrated (15-18), however, quantifying the influ* Corresponding author phone: +61 8 8303 6511; fax: +61 8 8303 7436; e-mail: [email protected]. † The University of Adelaide. ‡ CSIRO, Land and Water. 1764

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ence on sorption of such materials in soils and sediments is complicated by difficulties in quantifying black carbon. A number of techniques have been proposed, but agreement among them is poor (19). Part of the problem is that black carbon is not a single material, but rather a continuum of materials varying from partially charred materials through to graphite (19, 20). Different techniques measure different portions of this continuum, and hence produce widely varying black carbon “contents” for soils and sediments that contain materials across the black carbon range. The situation is further complicated by the fact that the sorption affinities of different categories of black carbon vary by several orders of magnitude (15). There have been a number of attempts to quantify the influence of black carbon on sorption of HOCs to soils and sediments. Gustafsson et al. (13) proposed a framework for understanding the sorption properties of sediments containing black carbonsa two-phase model consisting of a lowaffinity “normal” organic matter phase and a high-affinity black carbon phasesand also proposed a method to quantify the black carbon fraction. Using partitioning constants derived from established Koc-Kow relationships for the organic matter phase and from activated carbon for the black carbon phase, they reported good agreement between measured and calculated sorption affinities for Boston Harbor sediments. Accardi-Dey and Gschwend (8) developed this model further by introducing a nonlinear sorption isotherm for the black carbon component, and by directly determining the sorption properties of the isolated black carbon fraction. However, it has since been shown that the technique they used to isolate the black carbon fraction can greatly increase its sorption affinity (9). It has also been suggested that the method used to isolate black carbon in these two studies is subject to artifacts (21). Karapanagioti et al. (12) also attempted to quantify the influence of black carbon on sorption of HOCs, but used a different method (organic petrography) to quantify black carbon, and determined the sorption coefficient for the black carbon component through fits to sorption data at a range of sorbate concentrations. All of these studies rely on indirect evidence to infer the distribution of sorbate molecules between organic matter and black carbon domains. A clearer picture may emerge were it possible to determine directly the distribution of sorbate molecules in heterogeneous sorbent matrixes, without having to physically separate different organic components. Nuclear magnetic resonance (NMR) spectroscopy has the potential to achieve this goal, since the NMR properties of magnetically active atomic nuclei (e.g., 13C and 1H) are modulated by their chemical environment at the molecular scale. These properties include not only the resonant frequency, but also peak broadness and a range of NMR relaxation rates. Thus, the NMR properties of sorbed molecules may reflect where and how they are bound to the matrix. We recently described the first implementation of this approach, which involved the application of the spectral editing technique proton spin relaxation editing (PSRE) to soil and sediment samples containing sorbed HOCs (22, 23). PSRE enables differentiation and quantification of organic domains on the basis of differences in their T1H relaxation rates (24). The distinctively rapid relaxation rate of black carbon (24) makes it ideally suited to this technique. Since sorbed molecules “inherit” the T1H relaxation rate of the matrix to which they are sorbed (22), PSRE on samples containing sorbed molecules enables direct quantification of sorbate concentrations in each organic matter domain. 10.1021/es051895o CCC: $33.50

 2006 American Chemical Society Published on Web 02/11/2006

High sensitivity was achieved by using 13C-labeled sorbate molecules. In this paper we investigate how other NMR properties of uniformly 13C-labeled benzene molecules are influenced by sorption to charcoal. In particular, we investigate whether sorption to charcoal affects the chemical shift and relaxation rates of benzene sufficiently for them to serve as a “signature” of sorption to charcoal and hence provide a means to quantify benzene sorbed to charcoal in heterogeneous environmental matrixes.

Experimental Section Preparation and Collection of Charcoals. The two laboratory-prepared charcoals were produced by placing red gum (Eucalyptus camaldulensis) wood chips in closed porcelain crucibles and heating in a muffle furnace. Charcoal L-450 was produced by heating the furnace to 450 °C in 1 h, holding at this temperature for 2 h, then allowing the crucibles to cool to room temperature. Charcoal L-850 was produced by heating the furnace to 850 °C in 1 h, holding at this temperature for 1 h, then allowing the crucibles to cool to room temperature. The C contents of charcoal L-450 and L-850 were 65.1% and 86.1%, respectively. Two charcoals were collected from the Tanami desert, Australia, within 2 days of a wildfire in November 1998. Charcoal F-W (field-wood) was sampled from a small burnt log (probably from a species of Eucalyptus). Charcoal F-G (field-grass) was sampled from under a burnt clump of Spinifex grass. The C contents of charcoal F-W and F-G were 77.3% and 67.7%, respectively. Sorption Experiments. Aliquots of charcoal (135 mg) were placed in 150 mL glass bottles with PTFE-lined lids and shaken with water (132.3 mL) for 42 h to ensure hydration of the charcoal. Calcium chloride (1.35 mL, 1 M) was added to aid flocculation. Uniformly 13C-labeled benzene (Sigma-Aldrich, St. Louis, MO) was added as a solution in methanol (1.35 mL) at an appropriate concentration to give initial overall 13C-benzene concentrations of 1, 10, and 100 mg L-1. The methanol concentration (1%) was considered insufficient to produce significant solvent effects. The suspensions were shaken for 24 h, then centrifuged. Solution concentrations of 13C-benzene were determined by gas chromatography on 5 mL of the supernatant extracted with 2 mL of dichloromethane. Propyl-benzene (0.5 mL, 10 mg L-1) was added as an internal standard. The charcoals containing sorbed 13Cbenzene were isolated on glass microfiber filters (Whatman, Brentford, England) under suction and stored in sealed glass vials. Benzene concentrations in solution were determined on a Perkin-Elmer Auto System gas chromatograph using a flame ionization detector (FID) and a DB-5 column (30 m length, 0.25 mm i.d., 0.25 µm film thickness). The injection temperature was 200 °C, with 1 µL of sample injected. Helium was used as the carrier gas with a constant pressure of 22 psi. The detector was heated to 250 °C. The initial temperature of the oven was set to 35 °C. It was held for 3 min and then ramped to 80 °C at 5 °C min-1. The detection limit of the method was 0.1 mg L-1. The concentrations of benzene sorbed to the charcoal were calculated by difference. Single-point partition coefficients (Kd) were determined as

Kd )

Concentration of benzene sorbed to charcoal Concentration of benzene in solution

NMR Spectroscopy. Solid-state 13C magic angle spinning (MAS) NMR spectra were obtained at a frequency of 50.3 MHz on a Varian Unity 200 spectrometer (Varian, Palo Alto, CA). Samples were spun at 5000 ( 100 Hz. Chemical shifts were externally referenced to the methyl resonance of

hexamethylbenzene at 17.36 ppm. Cross polarization (CP) and dipolar dephasing (DD) spectra were acquired using a 1-ms contact time and a 1-s recycle delay. Direct polarization (DP) spectra were acquired using a 90-s recycle delay. Spin counting was carried out using the method of Smernik et al. (25).

Results 13C

NMR Characterization of the Charcoal Samples. The C cross polarization (CP) and direct polarization (DP) NMR spectra of the four charcoals are shown in Figure 1. The CP technique is often preferred in solid-state 13C NMR studies of organic matter due to its greater sensitivity, however this can come at the expense of quantitative detection, especially for charcoals (26-28). All spectra are, as expected, dominated by a signal in the region 110-140 ppm, which can be assigned to aromatic carbon. Spinning sidebands (SSBs) appear at chemical shifts 100 ppm removed from the central aromatic signal. Some small differences between the charcoal spectra are apparent, especially for the laboratory-prepared charcoal produced at 450 °C (L-450), which contains a shoulder at 150-160 ppm that can be assigned to O-substituted aromatic C, and small signals in the O-alkyl and alkyl regions (0-100 ppm). The presence of these signals indicates that the charring process for L-450 was incomplete. For each charcoal, corresponding CP and DP spectra exhibit a similar distribution of signal. There are small differences in the chemical shift of the aromatic peak, both between chars and between corresponding CP and DP spectra (Table 1). Spin counting was applied to all of the spectra shown in Figure 1, and shows that DP observabilities (59-87%, Table 1) were higher than corresponding CP observabilities (19-38%). 13C NMR CP and DP Spectra of Charcoal Samples with Sorbed 13C-Benzene. Uniformly 13C-labeled benzene was sorbed to the laboratory-prepared charcoals L-450 and L-850 by exposing the charcoal to aqueous solutions containing three different concentrations of 13C-benzenes 1, 10, and 100 mgL-1sfor 24 h. Table 2 shows the concentration of benzene remaining in solution, the concentration of benzene sorbed to the charcoal (determined by difference), and the single-point partition coefficients (Kd) calculated from these. The Kd values are much higher for the charcoal produced at 850 °C (L-850) than for the charcoal produced at 450 °C (L-450). For charcoal L-450 there is also a trend of decreasing Kd with increasing benzene loading. This is consistent with previous studies of HOC sorption to black carbon (8, 16-18). It was not possible to determine the effect of benzene loading on Kd for charcoal L-850 because the concentration of benzene remaining in solution at the lower two loadings was close to the detection limit. The charcoals with sorbed 13C-benzene were isolated by filtration and minimal air-drying, so as to limit loss of benzene through volatilization (see below). These charcoal samples will be referred to according to the initial concentration of 13C-benzene in the batch sorption, e.g. L-450-100 refers to the charcoal produced in the laboratory at 450 °C that was exposed to a 100 mg L-1 solution of 13C-benzene. 13C CP NMR spectra were acquired, and the contribution of the charcoal carbon was subtracted to generate the “difference spectra”swhich show only the signal from the sorbed 13Cbenzenespresented in Figure 2. Only the aromatic region is shown, to highlight the differences in chemical shift of the 13C-benzene sorbed to the different charcoals. 13C DP NMR spectra (not shown) were also acquired, and these were very similar in appearance to the corresponding CP spectra. Figure 2 shows that the chemical shift of the sorbed 13Cbenzene is around 5-6 ppm lower when sorbed to L-850 than when sorbed to L-450. There is a weaker trend of decreasing chemical shift with decreasing benzene concen13

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FIGURE 1.

13C

cross polarization (CP) and direct polarization (DP) NMR spectra of the charcoals.

TABLE 1. Chemical Shift of Peak Maxima, and Percent NMR Observabilities for CP and DP 13C NMR Spectra of the Charcoals

L-450 L-850 F-W F-G

chemical shift at maximum signal intensity (ppm)

percent (%) NMR observability of C (Cobs)

CP

DP

CP

DP

128.0 125.3 126.7 127.1

125.5 122.6 125.1 128.7

38 19 29 31

78 59 84 87

tration. There is also a clear trend of decreasing line width with increasing 13C-benzene concentration. Spin counting was used to quantify the relative contribution of charcoal and benzene to the spectra. Table 3 shows that the proportion of signal derived from sorbed benzene varied between 16% and 42% for CP spectra of L-450, and between 37% and 94% for CP spectra of L-850. The proportion of signal derived from sorbed benzene was generally lower for the DP spectra. This is due to the higher observability of 1766

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TABLE 2. Results from Batch Sorption initial concn concn of benzene concn of benzene of benzene remaining in sorbed to (mg L-1) solution (mg L-1) charcoal (mg g-1) L-450 L-450 L-450 L-850 L-850 L-850

1 10 100 1 10 100

0.48 6.26 72.92 0.12a 0.14a 2.13

0.52 3.74 27.08 0.88 9.86 97.87

Kd (g L-1) 1070 598 371 7130b 71300b 45900

a Close to the detection limit. b Large degree of uncertainty in K d value due to the final benzene solution concentration being close to the detection limit (0.1 mg L-1).

charcoal C in the DP spectra. The concentration of 13Cbenzene in each charcoal was calculated from the DP spin counting results, and is also presented in Table 3. For these calculations it was assumed that the sorbed 13C-benzene was detected quantitatively in the 13C DP spectra. The concentrations of sorbed benzene determined by NMR (Table 3) are similar to those determined in the batch sorption experiments (Table 2) for the charcoals with the lowest

FIGURE 3. Aromatic region of the 13C CP NMR spectra of all four charcoals exposed to 100 mg L-1 of 13C-benzene.

FIGURE 2. Aromatic region of the 13C CP NMR spectra of 13C-benzene sorbed to the laboratory-prepared charcoals. These were generated by subtracting the 13C CP spectra of the corresponding charcoal from the 13C CP NMR spectra of the charcoals containing sorbed 13C-benzene.

TABLE 3. Proportion of Total NMR Signal Derived from 13 C-Benzene, and Concentration of 13C-Benzene in Charcoals Containing Sorbed 13C-Benzene proportion of total signal derived from 13C-benzene CP DP L-450-1 L-450-10 L-450-100 L-850-1 L-850-10 L-850-100 F-W-100 F-G-100

16 23 42 37 73 94 79 65

11 19 46 17 54 90 71 65

concn of benzene sorbed to charcoal solution as determined from DP spectrum (mg g-1) 0.49 0.90 3.3 0.78 4.5 35.6 12.1 8.6

benzene concentrations (L-450-1 and L-850-1). However, the NMR-determined concentrations are much lower than the corresponding batch sorption concentrations for the charcoals treated with 10 and 100 mg L-1 of 13C-benzene. This most likely reflects substantial losses of benzene due to volatilization during the isolation of the charcoals. These losses were proportionately greater at the higher concentration, and were also greater for charcoal L-450 than for charcoal L-850. Uniformly 13C-labeled benzene was sorbed to the two charcoals collected from the field following a wild fire (F-W and F-G), using the same methodology as used for the laboratory-prepared charcoals. The field charcoals were prepared using only the highest benzene concentration of 100 mg L-1. The 13C CP NMR spectra of the two charcoal samples prepared in this way (F-W-100 and F-G-100) show that the chemical shift of the 13C-benzene resonance for each of the field charcoals falls between those of the two laboratoryprepared charcoals (Figure 3). Variable Contact Time (VCT) and Dipolar Dephasing (DD) Experiments. The strength of 13C-1H dipolar coupling is dependent on C-H interatomic distances and the degree

FIGURE 4. (a) Signal intensity versus contact time for 13C-benzene sorbed to the charcoals. (b) Signal intensity versus dephasing delay for 13C-benzene sorbed to the charcoals. of molecular motion. Since the C-H interatomic distances of sorbed 13C-benzene molecules are all identical, 13C-1H dipolar coupling provides a direct gauge of the molecular motion of the benzene molecules. The strength of 13C-1H dipolar coupling was assessed by measuring the rate of polarization transfer during the “contact time” of a cross polarization experiment (29, 30), and by measuring the rate at which signal decays in the absence of 1H decoupling, i.e., the rate of dipolar dephasing (29, 31). Figure 4a shows how NMR signal intensity varies with contact time for 13C-benzene in five charcoals containing sorbed 13C-benzene. The contact time at which maximum signal intensity is reached increases in the order L-450-100, F-G-100, F-W-100, L-850-100, among the samples treated with 100 mg L-1 13C-benzene. Since slow signal build-up indicates reduced 13C-1H dipolar coupling, and increased molecular motion, the degree of molecular motion of sorbed benzene molecules must also increase in this order. Also, the rate of signal build up is faster for L-850-10 than for L-850-100, indicating reduced molecular mobility of 13Cbenzene at the lower concentration. Figure 4b shows how NMR signal intensity decreases with “dephasing delay” (i.e., the period during which the 1H decoupler is turned off) in the dipolar dephasing (DD) VOL. 40, NO. 6, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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FIGURE 5. 13C CP NMR spectra of mixtures of charcoal L-450 and charcoal L-850 exposed to 100 mg L-1 of 13C-benzene. experiment. The rate of signal decay decreases in the order L-450-100, F-G-100, F-W-100, L-850-100. Since reduced rates of signal decay indicate reduced 13C-1H dipolar coupling, and increased molecular motion, this finding confirms the results from the contact time experiments that this is the order of increasing molecular motion of sorbed benzene. Comparison of the dipolar dephasing curves for L-850-100 and L-850-10 confirm that molecular motion of benzene decreases with decreasing concentration for charcoal L-850. In all cases, the rate of dipolar dephasing is lower for the sorbed benzene than for methyl groups in soils and model compounds (31), indicating that the molecular motion of the sorbed benzene molecules is greater than for unhindered methyl group rotation. Sorption of 13C-Benzene to Mixtures of Charcoals L-450 and L-850. Uniformly 13C-labeled benzene was sorbed to four mixtures of charcoals L-450 and L-850, using the same methodology as used for the other charcoals, and an initial benzene concentration of 100 mg L-1. The ratio of L-450 to L-850 was varied from 1:1 to 20:1. The 13C CP NMR spectra of the mixed charcoal samples prepared in this way are shown in Figure 5. For the 1:1 mixture, a signal at 121.6 ppm is observed, the same chemical shift found for 13C-benzene sorbed to L-850. For the 4:1 mixture, the main resonance is at 121.6 ppm, but a weak shoulder can also be seen at the chemical shift found for 13C-benzene sorbed to L-450. Increasing the proportion of L-450 increases the relative size of the downfield (higher ppm) resonancesit appears as a stronger shoulder in the 9:1 mixture, and is seen as a separate peak in the 20:1 mixture. For the 20:1 mixture, signal derived from each of the resonances was quantified using the PSRE technique, and found to be nearly equal (data not shown). This suggests that the affinity of charcoal L-850 is about 20 times greater than that of charcoal L-450, or around 5 times lower than may be expected from the ratio of Kd values of 123 calculated from the batch sorption results (Table 2).

Discussion The key finding of this study is that the chemical shift of the 13C-benzene resonance varied over a range of 5.5 ppm when 1768

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sorbed to the four different charcoals (Figure 3). The greatest difference was between the two laboratory-prepared charcoals L-450 (produced at a temperature of 450 °C) and L-850 (produced at a temperature of 850 °C). We attribute these differences in chemical shift to diamagnetic currents produced by delocalized π-electrons in extended aromatic structures or graphite-like microcrystallites, which produce an overall shielding effect or displacement to lower ppm values (26). The strength of these currents is directly related to the average diameter of the graphite-like microcrystallites (26). Freitas and co-workers attributed the decreasing chemical shift of charcoal resonances with increasing heattreatment temperature to this effect (26, 27, 32). We contend that these currents affect the chemical shift of the 13C nuclei in benzene molecules sorbed to the surface of charcoal, as well as the 13C nuclei in the actual charcoal structures. The difference in chemical shift (Table 1, CP values) between L-450 (128.0 ppm) and L-850 (125.3 ppm) is consistent with this explanation. Note that the chemical shift displacement for the sorbed benzene molecules is about twice as large as that for the charcoals themselves (Table 1). This effect appears to provide a way of not only differentiating between molecules sorbed to black carbon from those sorbed to other types of organic matter, but also provides a gauge of how condensed or “graphitized” is the immediate environment of the sorbed molecule. This effect on sorbate chemical shift is local, as evidenced by the separate resonances produced in samples containing mixtures of L-450 and L-850 (Figure 5). The chemical shift of 13C-benzene sorbed to charcoals collected in the field suggests that the degree of graphitization in these materials is intermediate between those of charcoals produced at 450 and 850 °C, and that the charcoal produced from grass (F-G) is less graphitic than that produced from wood (F-W). This may reflect differences in the temperature or duration of the fire that produced these charcoals, or may reflect differences in the nature of the plant residues (18, 26). The concentration of sorbate also had a small but consistent effect on the chemical shift of the sorbed 13Cbenzene (Figure 2). At lower concentrations the chemical shift was lower. This may be because the highest affinity sites are in the most graphitic regions. Another possibility is that at higher concentrations there are regions of greater than monolayer coverage, and that sorbate molecules that are further removed from the surface are less affected by the currents in the charcoal. The variable contact time (VCT) and dipolar dephasing (DD) experiments provided further information about the nature of the sorption interaction and, in particular, the molecular mobility of the sorbate molecules. There was an increase in molecular mobility with increasing graphitization of the matrix (as determined by the chemical shift of the 13C-benzene), despite the concomitant increase in the strength of sorption (Figure 4). In other words, sorption to more graphitic surfaces is stronger, yet the sorbate molecules are more mobile. This may be due to facile rotation of benzene molecules sorbed face-on to planar aromatic surfaces. Greater than monolayer coverage, especially for the more graphitized charcoals, is another possible explanation. Molecular mobility was also higher at higher concentrations (cf VCT and DD curves for L-850-100 and L-850-10 in Figure 4). Again this may reflect greater than monolayer coverage at the higher concentrations. Unfortunately, it was not possible to obtain meaningful VCT or DD data for the samples with the lowest concentrations of sorbed benzene (L-450-1, L-450-10, and L-850-1), due to a combination of lower signal from the 13Cbenzene and a greater proportion of signal coming from the charcoal itself. The parameter most sensitive to sorbate concentration was the 13C-benzene resonance line width, which decreased substantially with increasing sorbate con-

centration for both laboratory-prepared charcoals (Figure 2). This suggests that these charcoals contain a variety of sorption sites and that the highest affinity sites are the most variable. The low recovery of 13C-benzene for some of the charcoals is a limitation of the technique described here. Comparison of sorbed benzene concentrations determined from batch sorption (Table 2) and NMR (Table 3) shows that while 13Cbenzene losses were only around 10% for the 1 mg L-1 treatments, these losses increased with increasing concentration, up to 88% for L-450-100. Losses were 2-3 times greater for L-450 than for L-850 at the two higher benzene concentrations. The likely cause of these losses is volatilization of benzene during isolation of the charcoals. Differences in the recoveries are consistent with sorption being stronger for L-850 than for L-450, and also stronger at the lower benzene concentrations. This limitation could be overcome by using less volatile sorbatesswe are currently pursuing this line of study. We also look forward to using the techniques described here to investigate sorption of HOCs to soils and sediments that contain black carbon along with other types of organic matter. We believe the ability to directly identify and quantify molecules sorbed to black carbon, and the ability to determine the degree of aromatic condensation of the black carbon, represent powerful new tools for better understanding HOC sorption to soils and sediments.

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Acknowledgments We thank Ludger Bornemann and Natasha Waller (CSIRO Land and Water) for technical support in sorption experiments, and Dr. Dean Graetz (CSIRO Atmospheric Research, Canberra, Australia) for providing charcoals F-W and F-G.

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Received for review September 26, 2005. Revised manuscript received January 10, 2006. Accepted January 18, 2006. ES051895O VOL. 40, NO. 6, 2006 / ENVIRONMENTAL SCIENCE & TECHNOLOGY

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