Role of Black Carbon in the Sorption of ... - ACS Publications

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Role of Black Carbon in the Sorption of Polychlorinated Dibenzop-dioxins and Dibenzofurans at the Diamond Alkali Superfund Site, Newark Bay, New Jersey Matthew K. Lambert,† Carey Friedman,‡ Pamela Luey,† and Rainer Lohmann*,† † ‡

Graduate School of Oceanography, University of Rhode Island, 215 South Ferry Road, Narragansett, Rhode Island 02882, United States Massachusetts Institute of Technology, Center for Global Change Science, 54-1810, 77 Massachusetts Avenue, Cambridge, MA 02139, United States

bS Supporting Information ABSTRACT: The sorption of polychlorinated dibenzo-pdioxins and dibenzofurans (PCDD/Fs) to organic carbon (OC) and black carbon (BC) was measured in two sediment cores taken near the Diamond Alkali superfund site (DA) in the Passaic River and Newark Bay, New Jersey (U.S.A.). An OC partitioning model and a BC-inclusive, Freundlich distribution model were used to interpret measurements of freely dissolved PCDD/Fs using passive samplers in sediment incubations, together with measured sedimentary concentrations of OC, BC, and PCDD/Fs. Samples were also analyzed for polycyclic aromatic hydrocarbons (PAHs) as controls on the two distribution models. The OC partitioning model underpredicted the distribution of PAHs and PCDD/Fs by 10
100-fold. The Freundlich model predicted the distribution of PAHs at the DA to within a factor of 2
3 of observations. Black carbon
water partition coefficients (KiBC) for PCDD/Fs, derived from literature results of both field and laboratory studies differed up to 1000-fold from values derived from this study. Contrary to expectations, PCDDs displayed stronger sorption than either PCDFs or PAHs relative to their subcooled liquid aqueous solubilities. Even though the presence of BC in the sediments reduced the overall bioavailability of PCDD/Fs by >90%, the sediments at 2 m depth continue to display the highest pore water activities of PCDD/Fs.

’ INTRODUCTION Polychlorinated dibenzo-p-dioxins and polychlorinated dibenzofurans (PCDD/Fs) are a particularly problematic group of pollutants, not only because of their high level of toxicity at low concentrations but also because of their persistence in the environment and their tendency to bioaccumulate.1
3 For these reasons, PCDD/Fs remain a hazard to human and ecosystem health long after their original release to a particular environment. The transport and fate of hydrophobic organic contaminants (HOCs), a group which includes PCDD/Fs, are strongly controlled by sorption to particles due to their hydrophobicity.4 However, it is their freely dissolved concentration (Ciw) that is important for uptake by aquatic organisms.3
5 Improvements in passive sampling technology have made it possible to measure Ciws accurately, but such methods require many weeks of sampling time for trace contaminants like PCDD/Fs. This fact has historically led to an interest in being able to predict the Ciws based on the sediment-bound concentration. The classic paradigm for the sorption of HOCs in complex matrixes composed of minerals and organic matter is that it is r 2011 American Chemical Society

dominated by absorption into organic carbon (OC):6
8 KiD ¼

Cisolid ¼ fOC KiOC Cipw

ð1Þ

where KiD is the observed distribution of a chemical i between sediment and pore water (mL/g), Cisolid and Cipw are the sediment-bound (ng/g) and freely dissolved pore water concentrations (ng/mL), subcooled liquid respectively, fOC is the OC fraction in the sediment (g OC/g sediment, dry weight), and KiOC is the equilibrium partitioning coefficient of chemical i between OC and water (mL/g). Field observations often indicate greater sorption of HOCs to sediments than can be explained by absorption into OC only.7,9,10 Preferential adsorption of HOCs onto black carbon (BC), a highly reduced (i.e., increased C/O ratio relative to OC) and aromatic form of OC, can increase the distribution onto the solid phase by 1
3 orders of Received: November 25, 2010 Accepted: March 27, 2011 Revised: March 20, 2011 Published: April 19, 2011 4331

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magnitude.11,12 The following Freundlich distribution model has been proposed to account for the effect of BC:9 KiD ¼

Cisolid ¼ fOC KiOC þ fBC KiBC Cnipw
1 Cipw

ð2Þ

where fBC is the BC fraction in the sediment (g BC/g sediment, dry weight), KiBC is the equilibrium distribution constant of chemical i between BC and water ((ng/g)/(ng/mL)n), and n is the Freundlich exponent for adsorption onto BC. KiBC represents an adsorptive process and is nonlinearly dependent on the Cipw term. Chiou and Kile,13 working with peat and mineral soils, found that adsorption to glassy OC (defined functionally, similar to BC) becomes saturated when the ratio of Cipw/Csat iw (where Csat iw is the HOC’s aqueous solubility at saturation) rises above 0.010
0.015. At Ciws of around 1 fg/L, assuming fOC = 0.1 and fBC = 0.01, the predicted distribution of pyrene, a polycyclic aromatic hydrocarbon (PAH), using eq 2 is 3 orders of magnitude higher than that predicted by eq 1, whereas at Ciws of around 1 mg/L, BC plays no role in the sorption of pyrene.11 The OC term in eq 2 contributes little to the total predicted distribution at environmentally relevant concentrations of PCDD/Fs.9 Sorption mechanisms for HOCs to BC are complex and vary according to type of BC and the structure of the particular HOC.10,14 Laboratory-derived KiBC values for PCDD/Fs measured sorption to clean, highly reduced soot but did not account for variation in the quality of the available BC or for competitive sorption between PCDD/Fs, HOCs, or other organic compounds.15 KiBC values derived from studies of in situ contaminated sediments are often limited to measuring a small number of congeners over the existing range of concentrations. These factors contributed to a wide range of literature KiBC values for PCDD/Fs and cause a large degree of uncertainty in predicting Cipw values. This study seeks to address these challenges by deriving KiBC values for the entire size range of PCDD/Fs, i.e., from mono- to octachlorinated congeners, from measurements of the distribution of PCDD/Fs in in situ contaminated estuarine sediment. The Diamond Alkali superfund site (DA), in the Passaic River and Newark Bay, New Jersey (U.S.A.), provided ideal environmental conditions for this study. Much of the PCDD/F input to the Passaic River and Newark Bay has been linked to the activities of the Diamond Alkali Chemical Company (DACC).16 The DACC manufactured a variety of organic chemicals, such as pesticides and herbicides (including Agent Orange), and PCDD/ Fs as byproducts. The resulting chemical waste was released into the Passiac River from 1951 until 1969, resulting in high PCDD/F concentrations in the sediments. A maximum concentration of the most toxic congener, 2,3,7,8 tetrachlorinated dibenzo-p-dioxin (2,3,7,8-TCDD), was measured at 21 ppb (ng/g) in the Passaic River.17 Concentrations of PCDD/Fs in the sediment decrease above and below the 1950
1960s sediment horizon, with the dominant congeners from the DA site being 2,3,7,8TCDD and octachlorinated dibenzo-p-dioxin (OCDD).18 Both the fBC and the type of BC are likely to vary with depth in the cores due to recorded changes in the urban energy sources and municipal waste burning practices over the last 100 years.19 These ranges in Cipw and fBC are expected to produce KiBC values that are broadly applicable to other contaminated sediments. In light of the planned remediation, improving our understanding of the sorptive capacity of contaminated sediments is a timely endeavor. The removal of 150 000 m3 of

contaminated sediment from the Passaic River, adjacent to the DACC, is scheduled for summer, 2011.20 The Freundlich coefficient, n, is poorly known for PCDD/Fs, yet it strongly affects the calculations of the Freundlich distribution model (eq 2). The sorption of PAHs to BC is well-studied and typically exhibits n ≈ 0.7.7,9 PAHs and PCDD/Fs have similar degrees of hydrophobicity, are capable of forming a planar configuration, and are capable of interacting with BC via overlapping π-bonds. When exposed to the same sedimentary OC and BC, they should exhibit similar distributions to BC. Therefore, PAH concentrations at the DA were quantified to assess the appropriateness of the assigned Freundlich coefficient for PCDD/F sorption to BC. Our research was thus aimed at providing better understanding of the in situ distribution and availability of sedimentary PCDD/Fs. In more detail, we hypothesized that (i) adsorption to BC has significantly reduced the pore water concentrations of PCDD/Fs and PAHs in the sediments and (ii) laboratory-derived KiBCs will overestimate sorption due to competition with other HOCs for BC sorption in the real environment.

’ MATERIALS AND METHODS Sediment Site and Collection. The sediments used in this study were collected from the Passaic River, adjacent to the DACC property, and from 4 km downstream in Newark Bay, using 2 m sediment push-cores (Supporting Information Figure SI5). Both sites are located within the DA and experience daily tidal currents. The sediments were divided into 2 cm intervals and stored in clean glass jars with aluminum foil lined lids at
10 °C. Approximate dates of deposition were obtained by comparing results from standard 137Cs and 210Pbexcess methods (see the Supporting Information, p 9). A high degree of uncertainty was assigned to these dates due to the complex downcore profiles of 137 Cs and 210Pb. Total Organic Carbon and Black Carbon Analysis. Sedimentary total organic carbon (TOC) and BC concentrations, with their stable carbon isotope ratios, were analyzed using a Carlo Erba 1500 elemental analyzer coupled to a VG Optima stable isotope mass spectrometer. BC was isolated by oxidizing the non-BC organic carbon at 375 °C for 24 h according to the method of Gustafsson et al. and then analyzed for TOC.21 OC was defined as TOC minus BC. Pore Water and Sediment Concentrations. A nondepletive, polyethylene (PE) passive sampling technique was employed to measure the freely dissolved concentrations of PCDD/Fs and PAHs in pore water, following Lohmann et al.22 Nondepletive passive sampling methods measure only the freely dissolved fraction of HOC. Therefore, it is not necessary to account for the effects of sorption to dissolved OC. This method determined freely dissolved concentrations of HOCs by equilibrating them with PE (25 μm thick), sediments, and water, such that the PE sorbs less than 5% of the combined sorption capacity of the sediment, dissolved OC, and water. This is considered to be the maximum amount a passive sampler can remove without measurably influencing the equilibrium pore water concentrations.23 Equilibrium distribution constants between PE and water (KiPEw) for PAHs and PCDD/Fs were derived from Adams et al. (log KiPEw = (1.13)(log KOW)
0.86, R2 = 0.89) and were used to calculate Cipw from the PE concentrations.24 This linear free energy relationship (LFER) was developed from PAH, PCDD/F, and PCB data. Since the desorption of HOCs from 4332

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the sediment is the rate-limiting step of this method, we used a different mass of passive sampler in each replicate as an indicator of equilibrium.25 Higher Cipws measured by smaller samplers would indicate that equilibrium had not been reached. Prior to measuring Cipws, an equilibrium experiment was set up to verify the length of time needed and the size range of PEs necessary to give a meaningful indication of equilibrium. For this experiment, sediments from the Passaic River core, 74
136 cm deep, were homogenized by hand mixing. Using these sediments (∼50 g dry weight per sample), distilled water, and clean PE samplers, 12 sediment
water
PE slurries were set up following Lohmann et al.22 The PE sheets used were 0.11, 0.54, 1.1, and 2.2 mL in volume, with three replicate slurries of each PE size. Blanks, consisting of a PE sheet and distilled water, were also prepared. Sodium azide (0.43 μmol/mL) was used as a biocide to prevent algal growth on the sampler or biodegradation of the HOCs. This concentration is low enough that it did not affect the partitioning of the HOCs to the PE.26 The slurries were agitated on a shaker table at 20 ( 1 °C. At 3 week intervals, one replicate slurry of each PE size was removed and analyzed for pore water and sediment-sorbed concentrations of PCDD/Fs to establish an equilibrium time series. Sediment
water
PE slurries for Cipw measurements were prepared identically to those in the equilibrium experiment for samples in the Passaic River core at 4
10, 102
108, and 196
202 cm and in the Newark Bay core at 4
10, 24
30, 42
48, and 54
60 cm depth. These depths were chosen to capture the most contaminated sediments based on the estimated date of deposition. PAH data are not presented for depths 7 and 105 cm in the Passaic River core as technical difficulties with the mass spectrometer raised doubts on the quality of our PAH measurements at these depths. On the basis of the results of the equilibrium experiment (see below), Cipw measurements of PCDD/Fs and PAHs were equilibrated on the shaker table for ∼9 weeks, with triplicate measurements using 0.11, 0.54, and 1.1 mL PEs. Sediment-bound PCDD/Fs and PAHs were extracted from the slurry sediments using a modification of the accelerated solvent extraction (ASE) method of Kiguchi et al.27 (see the Supporting Information for details). PE sampler and sediment extractions were cleaned up using carbon and silica columns (see the Supporting Information for details). Samples were analyzed for PAHs via GC
MS and PCDD/Fs by GC
MS/MS (see the Supporting Information for details). Details on quality control (surrogate standard recoveries, blank concentrations, detection limits, etc.) are given in the Supporting Information. Distribution Models. The KiOC values were derived from Xia,28 for both PCDD/Fs and PAHs, using a LFER with their octanol
water partitioning coefficients (KiOW): log K iOC ¼ ðlog K iOW Þð0:97Þ
0:12

ð3Þ

Two sets of KiBC values were considered: (i) derived from Lohmann et al.,22 using a LFER: log K iBC ¼ ðlog K iOW Þð0:59Þ þ 2:3

ð4Þ

and (ii) derived from B€arring et al.15 B€arring et al. did not account for the nonlinearity of adsorption onto soot; hence, the Kisoot values from B€arring et al. are not directly equivalent to KiBC values. The Kisoot values were corrected by normalizing them to Cn
1 ipw , using n = 0.7 (Supporting Information, p 13). A LFER using B€arring et al.15 KiBC values and Csat iw (L) values from Aberg

et al.29 was used to extrapolate KiBCs to other congeners: log K iBC ¼ ð
log Csat iw ðLÞÞð0:41Þ þ 4:18 ðn ¼ 5Þ

for PCDDs

ð5Þ

and log K iBC ¼ ð
log Csat iw ðLÞÞð0:51Þ þ 4:23 ðn ¼ 5Þ

for PCDDs

ð6Þ Predicted KiDs were calculated from eqs 1 and 2 using measured fOC, fBC, Cn
1 ipw (assuming n = 0.7) and distribution constants from eqs 3
6. The KiOC values from Xia were assumed to accurately account for partitioning to OC.28 On the basis of earlier sensitivity analysis, it is likely that any error attributable to the LFER from Xia would have a negligible effect on the modeled partitioning.9 These predicted KiD values were then compared to the observed distributions.

’ RESULTS AND DISCUSSION Equilibrium Pore Water Concentrations. On the basis of the PE equilibrium experiment, the sorptive capacity of the PE samplers was less than 5% for all congeners measured and less than 1% for most congeners, including 2,3,7,8-TCDD, given 50 g dry weight of sediment in each slurry. After 9 weeks, 2,3,7,8TCDD was in equilibrium in the 0.11, 0.54, and 1.1 mL samplers (Supporting Information Figure SI4). PCDD/F congeners smaller than hexachlorinated dibenzo-p-dioxin (HxCDD) also showed similar results (Supporting Information Table SI1). Pore water concentrations of heptachlorinated dibenzo-p-dioxin (HpCDD) and dibenzofuran (HpCDF) and octachlorinated dibenzofuran (OCDF) were below detection limits. Measured pore water concentrations of OCDD varied by 50% (0.019 ( 0.009 pg/L) after 9 weeks (Supporting Information Figure SI4). However, the 1.1 mL sampler recorded a higher Cipw than the 0.54 mL sampler, which remained constant over time. This indicates that OCDD had reached equilibrium in the three smaller samplers by 9 weeks, but the analytical variability for this congener is higher than for the smaller congeners. The 2.2 mL PE samplers generally did not reach equilibrium during this experiment and were discarded. PAHs have been shown to equilibrate faster than PCDD/Fs; therefore, 9 weeks should be enough time for PAHs to equilibrate.25 Due to poor internal standard recoveries of the carbon-labeled 2,7-dichlorinated dibenzo-p-dioxin standard, we were only able to quantify congeners 2,4,8-trichlorinated dibenzofuran and larger. 2,3,7,8-TCDD displayed both the highest and lowest Cipws in the Passaic River core (3.5 ( 0.5 pg/L at 199 cm; 0.027 pg/L at 7 cm) (Supporting Information Table SI2). The other PCDD/Fs congeners followed a similar pattern in the Passaic River core, with maximum Cipws at 199 cm and minimum at 7 cm. Cipws of 2,3,7,8-TCDD at the Newark Bay site ranged from 0.020 ( 0.006 pg/L at 27 cm to 0.07 ( 0.03 pg/L at 45 cm (Supporting Information Table SI3). The Cipws of other PCDD/ F congeners varied with depth by less than an order of magnitude at the Newark Bay site. At both sites, the Cipw/Csat iw ratio was ,0.01 (∼10
8), suggesting that BC adsorption sites were not saturated.13 Cipws of PAHs were generally highest at 199 cm depth in the Passaic River core. Overall, pore water concentrations of PAHs in both cores were similar (Supporting Information Tables SI2 and SI3). Pyrene had a concentration of 0.13 ( 0.08 μg/L at 199 cm in the Passaic River core compared to 0.12 ( 0.09 μg/L at 57 cm 4333

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Figure 1. Observed partitioning vs the predicted partitioning, based on eqs 1 and 2, in both the Newark Bay and the Passaic River cores. The black line represents the 1:1 log/log relationship. Values for KiOC were derived from Xia (ref 28): log KiOC = (0.97)(log KiOW)
0.12. Panel a represents predicted partitioning based on eq 1. Panel b represents predicted partitioning based on eq 2 and KiBC values derived from Lohmann et al. (ref 22). Panel c represents predicted partitioning based on eq 2 and KiBC values derived from B€arring et al. (ref 15). The values displayed are the averages of triplicate samples. Error bars are the standard deviation of the triplicates. Samples with no error bars represent a measurement in which the congener was detected in only one of the replicates. Error bars were not displayed for the x-axis because the standard deviations were generally smaller than the displayed data point.

in the Newark Bay core (Supporting Information Table SI3). PAH concentrations varied less than an order of magnitude with depth.

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Sediment-Sorbed Concentrations. The Passaic River core displayed both the highest and lowest Cisolids of 2,3,7,8-TCDD (170 ( 40 ng/g at 199 cm; 0.24 ( 0.04 ng/g at 7 cm) (Supporting Information Table SI4), matching the pore water distribution of 2,3,7,8-TCDD. Bopp et al.18 reported a sedimentsorbed 2,3,7,8-TCDD concentration of 7.6 ppb (ng/g) dated to the mid-1960s and a peak in 2,3,7,8-TCDD of 21 ppb, below the mid-1960s horizon, in a sediment core adjacent to the DACC. The Passaic River core dates to the mid-1970s at 199 cm depth and exhibited higher sediment-sorbed concentrations. All PCDD/F congeners measured were 1
2 orders of magnitude more concentrated at 199 cm than at 7 cm in the Passaic River. In Newark Bay, Cisolids of 2,3,7,8-TCDD were much lower, ranging from 0.208 ( 0.009 ng/g at 7 cm to 0.6 ( 0.2 ng/g at 45 cm (Supporting Information Table SI5). Other PCDD/F congeners showed less downcore variation in Cisolids at the Newark Bay site than the Passaic River site. The highest Cisolids of PAHs occurred in the Passaic River core at 199 cm. However, Cisolids at both sites varied by less than an order of magnitude for PAHs (Supporting Information Tables SI4 and SI5). Pyrene concentrations ranged from 1930 ( 50 to 6000 ( 400 ng/g, and benzo[ghi]perylene concentrations ranged from 450 ( 30 to 1000 ( 100 ng/g. Organic Carbon and Black Carbon. The Passaic River core fOC ranged from 4.1% to 8.6%, generally increasing with depth (Supporting Information Figure SI6), with a δ13C range of
27.1% to
25.6% (mean
26.2%). fOC in Newark Bay sediments varied greatly with depth (0.4
5.4%), with δ13C ranging from
27.5% to
21.8% (mean
24.5%) (Supporting Information Figure SI7). These δ13C values were consistent with the hypothesis that terrestrial OC dominated the Passaic River core and terrestrial and marine-derived OC influenced Newark Bay sediments.30 At the Passaic River, fBC generally increased from 0.6% at the surface to 2.1% toward the bottom of the core (Supporting Information Figure SI6), with δ13C ranging from
27.6% to
21.0% (average
23.5%). fBC in Newark Bay showed a distinct peak at 46 cm (1.7%) with concentrations above this depth decreasing quickly to ∼0.5% and concentrations below gradually decreasing to ∼0.1% (Supporting Information Figure SI7). The δ13C ranged from
28.6% to
23.4% (average
25.1%). Both δ13C ranges were similar to those reported for pyrogenic PAHs.31 Distribution Models. Equation 1 underpredicted the distribution of PAHs, PCDFs, and PCDDs at the DA 10
100-fold for all compound classes similar to the findings of Cornelissen et al. (Figure 1a).7 Equation 2 predicted the distribution of PAHs to within a factor of 2
3 of the observed values, using n = 0.7 and KiBC values from Lohmann et al. (Figure 1b).22 Applying the same KiBC values to PCDD/Fs resulted in predictions that were within a factor of 10 of the observed distribution for congeners smaller than 2,3,7,8-TCDF. The distributions of the more hydrophobic congeners (2,3,7,8-TCDD to OCDD) were underpredicted 10
100-fold by eq 2. Using the B€arring et al.15 KiBC values, the PCDF distributions were overpredicted by 1
2 orders of magnitude (Figure 1c). The predicted PCDD distributions had a slope greater than 1, resulting in the distributions of 1,2,3,4,6,7,8-HpCDD and OCDD being underpredicted by ∼10fold, whereas the distributions of 2,3,7-triCDD, 2,3,7,8-TCDD, and 1,2,3,7,8-PnCDD varied around the 1:1 line. The observed distributions of each compound class in Figure 1a and for PCDD/Fs in Figure 1, parts b and c, have slopes 4334

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Table 1. Log KiBC Values Derived from Newark Bay and Passaic River PCDD/F Data and Calculated Using Freundlich Coefficients of n = 0.6, 0.7, and 0.8a log KiOC congener

Xia (ref 28)

log KiBC, this study

log KiBC

n = 0.6

SD

n = 0.7

SD

n = 0.8

SD

Lohmann et al. (ref 22) B€arring et al. (ref 15) Cornelissen et al. (ref 35)

2,4,7-triCDD

5.79

6.45

0.21

6.98

0.19

7.52

0.17

5.92

6.88

2,3,7,8-TCDD

6.55

6.61

0.13

7.22

0.11

7.84

0.09

6.38

7.07

9.04

1,2,3,7,8-PnCDD 1,2,3,4,6,7,8-HpCDD

6.68 7.48

7.33 7.50

0.16 0.07

8.15 8.40

0.16 0.08

8.97 9.29

0.16 0.09

6.46 6.94

7.41 7.41

9.32 10.20

OCDD

7.95

8.39

0.10

9.24

0.12

10.10

0.14

7.23

7.72

10.65

2,4,8-triCDF

5.44

5.66

0.08

6.19

0.08

6.74

0.08

5.71

7.33

2,3,7,8-TCDF

5.90

5.56

0.09

6.18

0.07

6.82

0.06

5.98

7.68

1,2,3,7,8-PnCDF

6.27

6.10

0.14

6.80

0.12

7.53

0.12

6.21

7.94

9.49

1,2,3,4,7,8-HxCDF

6.71

6.94

0.19

7.61

0.19

8.28

0.18

6.47

8.44

9.76

9.36

1,2,3,4,6,7,8-HpCDF

6.83

7.18

0.10

7.99

0.10

10.07

0.34

6.55

8.78

10.43

OCDF

7.66

8.09

0.34

8.93

0.34

9.77

0.33

7.05

9.28

9.76

a

Log KiBC values presented from this study are the mean ( the standard deviation of the mean (n ranges from 2 to 25). Log KiOC values presented are derived from Xia (ref 28): log KiOC = (0.97)(log KiOW)
0.12. Other log KiBC values are derived from Lohmann et al. (ref 22) (log KiBC = (0.59)(log KiOW) þ 2.3), B€arring et al. (ref 15) (corrected for Cipw and an n = 0.7), and Cornelissen et al. (ref 35) values were measured from partitioning in clean Baltic Sea sediments. KiBC units are (ng/g)/((ng/mL)n), and KiOC units are mL/g.

that are greater than one, indicating the model deviance increases with increasing hydrophobicity. The wide range of predicted distributions for PCDD/Fs from eq 2 was due to either KiBC values and/or a Freundlich coefficient, n, that was inappropriate for the BC in a given sample. The KiOC values of PAHs and PCDD/Fs have been measured for several decades, with good agreement between studies.8,28,32 Also, a study by Accardi-Dey and Gschwend indicated that eq 2 was insensitive to the KiOC value.9 The analytical variability in the methods used by this study ranged from 5% for the OC analysis to ∼15% for the BC and Cipw analyses. It is unlikely that these factors contributed significantly to the observed discrepancies. The Freundlich coefficient is thought to be controlled by the sorption site geometries of BC, as well as the hydrophobicity of the sorbate.33,34 This makes it difficult to know n a priori. Laboratory studies typically use sediment at a wide range of HOC concentrations and fit n to the resulting data. We attempted to derive both n and KiBC directly from plots of log Cipw versus BC-normalized Cisolid, but correlations were either not significant (PCDD/Fs) or resulting n values were extremely small (PAHs). This suggests that beyond a wide range of PCDD/F and PAH concentrations in sediment and pore water, there was also significant variability in the quality of carbonaceous sorbents present. Interpretation of field-derived data hence generally rely on n values from the literature or the sorption characteristics of similar contaminants. In our case, we used PAH sorption characteristics as an in situ check on the chosen n values. The Csat iw (L), an inverse measure of hydrophobicity, for the PAHs measured in this study ranged from
log Csat iw (L) = 4.94
8.78 and from
log Csat iw (L) = 6.03
9.83 for PCDD/Fs. Given that PAHs and PCDD/Fs were exposed to the same BC, this suggested n = 0.7 should be appropriate for PCDD/Fs, too. It is more likely that the different model predictions resulted from KiBC values that did not accurately describe the in situ distributions at the DA. The Lohmann et al.22 KiBC values were derived using an LFER based on PAH distributions. It is difficult to explain the underprediction of PCDD/Fs based on these values. Field and laboratory studies often report lower KiBC values for PCDD/Fs than for PAHs with comparable Csat iw (L)s.

The Lohmann et al. values ought to have resulted in an overprediction of PCDD/F partitioning.10,22 The B€arring et al.15 KiBC values were measured using a pure soot, which is typically a stronger sorbent than other, less reduced types of BC and likely causes the overprediction of modeled PCDF distribution based on the B€arring et al.15 values. It is also likely that competition from other HOCs or dissolved OC for BC sorption sites lowered the apparent KiBC of PCDFs at the DA. Cornelissen et al.7 noted that sorption of phenanthrene to natural BC can be up to 1 order of magnitude lower than that to laboratory-produced soot due to OC molecules competing for or blocking BC sorption sites. In contrast to the overpredicted PCDF distribution, eq 2 with the B€arring et al.15 KiBC values generally underpredicted PCDD sorption, though predictions for the smaller PCDDs were close to the observed sorption. This is also difficult to explain. B€arring et al.15 did not measure any congeners larger than 1,3,6,8-TCDD; hence, we extrapolated KiBC values to larger congeners (see above). Although this relationship worked for 2,3,7,8-TCDD and smaller congeners, it might be inappropriate to extrapolate KiBC values beyond the size range of congeners measured by B€arring et al.15 In Situ KiBC Values. Field-derived KiBC values were calculated using Freundlich coefficients of n = 0.6, 0.7, and 0.8 (Table 1). The following LFERs were developed for the calculated KiBC values, with n = 0.7 (Figure 2a): log K iBC ¼ ð
log Csat iw ðLÞÞð0:52Þ þ 3:34

ðR 2 ¼ 0:950, n ¼ 5Þ

PAHs ð7Þ

log K iBC ¼ ð
log Csat iw ðLÞÞð1:15Þ
0:65

ðR 2 ¼ 0:976, n ¼ 5Þ

PCDDs ð8Þ

log K iBC ¼ ð
log Csat iw ðLÞÞð0:76Þ þ 1:36

ðR 2 ¼ 0:973, n ¼ 6Þ

PCDDs ð9Þ

4335

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Figure 2. Calculated log KiBC values for PCDFs, PCDDs, and PAHs (n = 0.7) vs their
log Csat iw (L). The dotted line represents the LFER for PAHs 2 (log KiBC = (
log Csat iw )(L)(0.52) þ 3.34, R = 0.950, n = 6). The dashed line represents the LFER for PCDFs (log KiBC = (
log Csat iw )(L)(0.76) þ 1.36, R2 = 0.973, n = 6). The solid line represents the LFER for PCDDs (log 2 KiBC = (
log Csat iw )(L)(1.15)
0.65, R = 0.976, n = 5). Panel a is a direct comparison of log KiBC values to the
log Csat iw (L). Panel b compares the sat log[(KiBC)(0.001)(Cn
1 ipw )] (n = 0.7) to the
log Ciw (L) to compare the KiBC values at the same assumed chemical activity. Each data point is the average KiBC of the congener measured in all samples, and the error bars are the standard error.

On the basis of Csat iw (L), the PCDD KiBC values were higher than those of PAHs and PCDFs, indicating that PCDDs had the strongest adsorption to BC at the DA (Figure 2a). Surprisingly, and contrary to previous results (for example, refs 34 and 36), our results indicated that PCDDs displayed stronger sorption than PCDFs and PAHs. The reasons are not clear. The direct comparison of KiBCs in Figure 2a implies that all HOCs display Cipw of 1 μg/mL. We replotted the figure assuming that all compounds share the same chemical activity (0.001Csat iw (L)) in the pore water to account for the nonlinear effect of Cipw (Figure 2b). PCDD shows stronger sorption to BC in Figure 2b as well. Although it is possible that the enhanced sorption of PCDDs relative to PCDFs is linked to their higher abundance at the site, this would not explain the higher sorption of PCDDs relative to PAHs. When using their log KiOW values, PAH values show greater sorption to BC than PCDD/Fs (Supporting Information Figure SI9). This is surprising since KiOW and Csat iw (L) values are closely related characteristics of HOCs. Nevertheless, using Csat iw (L) is the correct way of comparing adsorption constants, rather

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than a partitioning-based approach such as KiOW. It is noteworthy that, contrary to many other sites, PCDDs were spilled as an impurity from chemical manufacturing and this may have contributed to their enhanced sorption, though the exact mechanism remains elusive. Comparisons of KiBC Values for PCDD/Fs. The calculated KiBC values for PCDD/Fs were up to 100-fold larger than those derived from Lohmann et al.22 KiBC values from Cornelissen et al.35 for PCDD/Fs, included in Table 1, were calculated using measurements of the sediment distribution in a pristine environment (using n = 0.61) in where atmospheric transportation of soot was thought to be the dominant transportation mechanism of BC. These values were 1.5
4 orders of magnitude higher than the KiBC values calculated here for n = 0.6. This may reflect a lack of competitive sorption from other HOCs and OC, as the Baltic Sea sampling site was much less contaminated than the DA superfund site. It is also possible that the increased sorption to soot over other types of BC plays a roll in this large difference. At the DA site it is likely that other types of BC were present in the sediments given the heavy industrial and urban land use. The standard deviation of the mean for the log KiBC values of individual congeners ranged from 0.06 to 0.34 log units, indicating a small degree of variability in the samples. This amount of variability is typically attributed to variations in the type or quality of OC and BC as well as varying amounts of competition from other HOCs for sorption sites.34 The two sets of literature KiBC values used in eq 2 resulted in widely varying predictions for PCDD/Fs. Armitage et al.37 indicated that the B€arring et al.15 KiBC values resulted in overpredicted PCDD/F distribution. Persson et al.36 showed that the distribution of PCDD/Fs in water column particles and surface sediments of a contaminated fjord were positively correlated with the distribution of BC. However, a later study by the same authors38 investigated the predictive ability of eqs 1 and 2 in the same fjord and concluded that eq 1 provided the best estimates of distribution. Equation 2 overpredicted the distribution, using the B€arring et al.15 KiBC values. It is also important to note that Persson et al.38 used traditional methods to measure Cipw, which involved centrifugation and filtration to separate pore water and required a correction for sorption to dissolved organic matter, rather than the passive sampling methods used in this study. A review by Burkhard39 indicated that the partitioning coefficients for HOCs between dissolved organic carbon (DOC) and water (KiDOC) can vary over 2 orders of magnitude due to differences in the quality of OC and the methods used to measure OC. It is likely that Cipw values measured by these traditional methods are not comparable to passive sampling methods and may explain the differences we observed. Despite differences in the quantity and source of OC and BC, and the range of freely dissolved concentrations of PCDD/Fs (2
3 orders of magnitude), all calculated PCDD/F KiBC values have standard deviations of the mean less than 0.35 log units. These KiBC values are indicative of the equilibrium distribution to BC as it occurs in contaminated estuaries. Although these values showed some spatial variation, they improved on laboratory measurements by accounting for different types of BC and competitive sorption to BC. They differed greatly from the field derived KiBC values of Cornelissen et al.35 indicating that any KiBC values are unlikely to be universally applicable. However, the KiBC values presented here were specific to PCDD/Fs, were calculated under conditions common to contaminated coastal sites, and provided LFERs based on congeners ranging in size from trichlorinated to octachlorinated. Implications. At the Passaic River site, the majority of PCDD/ Fs are buried to around 200 cm. Equation 2 implies an average of 97% and 93% of 2,3,7,8-TCDD is sorbed to BC in the Newark 4336

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Environmental Science & Technology Bay and Passaic River cores, respectively. This strongly supports our hypothesis that BC has significantly reduced the pore water concentrations of PCDD/Fs and PAHs in the sediments. This represents a large reduction in the bioavailability of PCDD/Fs at this site and suggests a large sorptive capacity for Passaic River sediments. Our results imply that laboratory-derived KiBC values overpredict partitioning for PCDFs but not for PCDDs. We note that the observed Cipws directly lead to a much reduced diffusivity within the sediments, as opposed to models assuming absorption to OC or BC inclusive sorption. As a consequence, diffusion within the sediment has not resulted in similar pore water concentrations between the cores. The depths affected by the chemical spills continue to display highly elevated concentrations of PCDD/Fs both in sediment and pore water.

’ ASSOCIATED CONTENT

bS

Supporting Information. Additional information on the sample site, analytical methods, OC, BC, and PCDD/F concentrations for the samples. This material is available free of charge via the Internet at http://pubs.acs.org.

’ AUTHOR INFORMATION Corresponding Author

*Phone: (401) 874-6612; fax: (401) 874-6811; e-mail: lohmann@ gso.uri.edu.

’ ACKNOWLEDGMENT This research was supported by the Hudson River Foundation (Grant 007/07a). We thank the three anonymous reviewers for their constructive comments, Dr. Mark Cantwell (U.S. EPA, Atlantic Ecology Division) and Julia Sullivan (URI) for advice and help measuring OC and BC, Drs. John King and Chip Heil (URI) for help taking the sediment cores, and Drs. Art Spivack and Anne Veeger (URI) for advice and guidance throughout the research process. ’ REFERENCES (1) Birnbaum, L. S. The mechanism of dioxin toxicity: relationship to risk assessment. Environ. Health Perspect. 1994, 102 (Suppl 9), 157–167. (2) Sinkkonen, S.; Paasivirta, J. Degradation half-life times of PCDDs, PCDFs and PCBs for environmental fate modeling. Chemosphere 2000, 40 (9
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