Environ. Sci. Technol. 2000, 34, 1620-1626
Preferential Sorption of Planar Contaminants in Sediments from Lake Ketelmeer, The Netherlands M I C H I E L T . O . J O N K E R †,‡ A N D F O P P E S M E D E S * ,† Ministry of Transport, Public Works and Water Management, National Institute for Coastal and Marine Management, P.O. Box 207, 9750 AE Haren, The Netherlands, and Aquatic Ecology and Water Quality Management Group, Department of Environmental Sciences, Wageningen Agricultural University, P.O. Box 8080, 6700 DD Wageningen, The Netherlands
Organic carbon normalized sediment-water distribution coefficients (KOC) of a series of in situ and (mass-labeled) added chlorobenzenes, PCBs, and PAHs were determined in an aged and a more recently deposited sediment layer from Lake Ketelmeer, The Netherlands, using the cosolvent method. The resulting log KOC’s for both in situ and added compounds are rather high compared to literature data (up to 9.3 for benzo[ghi]perylene), with the ones in the recently deposited layer being generally higher than those in the aged layer (up to 1.6 log units). The results do not show the well-established effects due to increased contact time of contaminants with sediments (i.e. “aging” or “slow sorption”) and indicate that after only short equilibrium times (11 days) KOC’s can even be higher than after several decades of contact time in the field. In addition, distribution coefficients show a remarkable dependence on sorbate planarity, manifested by significantly different log KOC-log KOW relationships for planar (chlorobenzenes, PAHs, and mono-ortho-substituted PCBs) and nonplanar (multiple-ortho-substituted PCBs) contaminants in both sediment layers. The confirmed presence of sootlike material for which planar aromatic compounds probably have strong affinity is brought up to explain these observations. Differences in KOC’s between both layers are discussed in relation to contact time and analyzed soot fractions.
matrix (5, 6), the presence of specific soot-like organic carbon fractions (7), and contact time of the contaminant with the sorbent (8) may all contribute more or less to a variation in the observed partition coefficient. Variations are generally limited to a factor of 10, but the presence of sootlike material and differences in contact time may cause variations even up to more than 2 orders of magnitude (7, 9). Longer contact times are reported to result in higher KOC’s, the so-called “aging” phenomenon. Contaminants probably diffuse slowly into the sorbent particles, becoming sequestered in micropores in the inorganic matrix (10, 11) or nanovoids located in hard glassy organic matter (12, 13). This makes them less or slower available for partitioning to the aqueous phase. Therefore, aging is currently referred to as “resistant” (based on the desorption process) or “slow sorption” (14, 15). Sorption behavior may also change due to compositional changes of organic matter in time (i.e. mineralization) (16, 17). Full equilibration may therefore take many months or years to complete. The effects of slow sorption on the KOC have extensively been demonstrated using freshly vs “historically” spiked sediments and soils, both in laboratory (18, 19) and in field studies (20, 21) and also by comparing freshly spiked with field contaminated sediments and soils (9, 21, 22). However, KOC data showing possible effects within one field situation (e.g. between sediment layers of different age) are scarce. Nevertheless, this kind of data is needed in order to (a) describe (vertical) mobility of contaminants and (b) assess bioavailability for sediment or soil dwelling organisms living at different depths. Although a few researchers demonstrated variable KOC values with depth (9, 23, 24), to our knowledge, no significant trends have been found until now. In the present study, the in situ KOC’s of a series of chlorobenzenes, PCBs, and PAHs (comprising a log KOW range of 3.4-7.5) were determined in an aged and a more recently deposited sediment layer from Lake Ketelmeer, a sedimentation area of the Rhine River in The Netherlands. In addition, KOC’s of several added compounds were determined to gain insight into short-term (ad)sorption. Possible differences in sorption behavior between both sediment layers for these compounds cannot be attributed to contact time factors (i.e. slow sorption) but are the result of dissimilarities in affinity for the sorbents (caused by e.g. compositional differences between the organic matrices). Among the added compounds some mass-labeled analogues of in situ contaminants were present to be able to verify equilibrium conditions.
Experimental Design Introduction The organic carbon normalized partition coefficient (KOC) describes the equilibrium distribution of organic compounds over organic carbon present in sediments or soils and the aqueous phase. Therefore, it determines the mobility and bioavailability of the compounds and might be regarded as one of the most important input parameters in environmental fate models. The KOC was introduced as a compound specific constant, being independent of sediment or soil type (1). However, research during the last two decades has revealed that factors such as solids concentration (2), salinity (3), experimental procedure (4), composition of the organic * Corresponding author phone: 31-50-5331306; fax: 31-505340772; e-mail:
[email protected]. † National Institute for Coastal and Marine Management. ‡ Wageningen Agricultural University. 1620
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All partition coefficients were determined using the so-called cosolvent method (25, 26). This method is based on the solubility enhancing properties of polar organic solvents (e.g. methanol) and requires KOC determinations at different cosolvent-water mixtures. The underlying “cosolvency theory” is described elsewhere (25, 27). The advantage of this method is that the frequently occurring artifacts due to the presence of dissolved organic carbon (i.e. underestimation of the KOC, dependence on solids concentration) are circumvented (25). The contribution of the fraction of contaminant sorbed to dissolved organic carbon (DOC) to the total mass of contaminant in the liquid phase is negligibly small in mixtures containing >20 vol % methanol (25). Extrapolating the >20% methanol part of the linear curve of log KOC vs methanol percentage to 0% methanol therefore gives the actual coefficient for the distribution of the contaminant over the sorbent and pure water. The cosolvent method has successfully been applied in several studies 10.1021/es9906251 CCC: $19.00
2000 American Chemical Society Published on Web 03/23/2000
regarding sorption of organic compounds to sediments and soils (e.g. refs 25, 26, 28). In situ partition coefficients were determined for all chlorobenzenes (except for mono-chlorobenzene), PCBs with the IUPAC no. 28, 31, 52, 101, 118, 138, 180 and 187, and for the PAHs phenanthrene, anthracene, fluoranthene, pyrene, benzo[a]anthracene, chrysene, benzo[e]pyrene, benzo[b]fluoranthene, benzo[k]fluoranthene, benzo[a]pyrene, benzo[g,h,i]perylene, and indeno[1,2,3-c,d]pyrene. Mass-labeled compounds added were as follows: 1,4-dichlorobenzeneD4, 1,2,4-trichlorobenzene-D3, hexachlorobenzene-13C6, and anthracene-D10. The added amounts of these chemicals were equal to the amounts of their unlabeled analogues present in the systems (sediments). Unlabeled chemicals added were dibenzo[a,h]anthracene and PCBs with the IUPAC no. 18, 44, 105, 156, 170, and 204. The last compound was not originally present in the sediments. The other ones existed only in low concentrations, and they were added in such amounts that final concentrations in the sediments were 5-20 times native concentrations.
Material and Methods Chemicals. Test compounds, organic solvents, and remaining chemicals were obtained from various suppliers and all had a declared purity of g 98% (solvents were of ultra-resianalyzed grade). Purity of all chlorobenzenes and PCBs, several PAHs, acetone, and hexane was confirmed prior to use. All chemicals were used without further purification. Sediments. A sediment core was taken from Lake Ketelmeer, The Netherlands, using a Vrij-Wit corer. From the core, the upper layer (0-30 cm) and a deeper layer (40120 cm) were collected. These layers were deposited in the 1980s-1990s and 1950s-1970s, respectively (29). Sediments were sieved (500 µm), intensively mechanically homogenized (>40 min), and stored at 5 °C. Organic carbon contents were determined to be respectively 3.75 and 6.48% for the upper and deeper layer by means of catalytic incineration at 1020 °C followed by katarometric detection of produced CO2 with a Fisons Na1500 NC analyzer. Prior to the analysis, inorganic carbon was removed with HCl. Sum concentrations of native chlorobenzenes, PCBs, and PAHs used in the experiments were respectively 1.5, 0.4, and 14 mg/kg dw for the upper layer and 4.8, 1.9, and 30 mg/kg dw for the deeper layer. Cosolvent Experiments. For both sediment layers, eight stainless steel (SS) pressure tanks of 10 or 20 L equipped with a metal magnetic stirrer were almost completely filled with water-methanol mixtures of 0, 8, 16, 22, 28, 34, 40, and 48% (w/w) methanol. While stirring, sediments were added, obtaining suspensions of 10 g dw/L. Subsequently, CaCl2 (0.005 mol/L), NaN3 (25 mg/L), and 25 µL/L of the spike solution (cocktail of all spike compounds in acetonitrile) were added. Then, all vessels were equilibrated (stirred) for 11 days at 20 ( 1 °C. Following this period, the sediments were allowed to settle during 40 h after which the supernatant was filtered. One atm of N2 was used to press the liquid phase of each pressure tank through a 2 mm id SS pipe over a glassfiber filter (Schleicher & Schuell, 14.2 cm L, pore size 1-1.5 µm, acetone-washed, kiln-fired at 180 °C) which was located in an SS filter holder. The first liter of the filtrate served to equilibrate the filter and was discarded. The remainder of the liquid phase was filtered in two fractions for the duplicate analyses of the micropollutants. Via an SS pipe, the filtrates were transported directly from the filter into 10 L extraction bottles, where they were collected underneath a hexane layer (400 mL), preventing evaporation of the volatile compounds. Filtrates were acidified with 10 mL of 35% nitric acid for conservation purposes and to prevent formation of emulsions during extraction. This extraction was carried out directly in the 10 L bottles using the continuous batch liquid-liquid extraction technique
FIGURE 1. Examples of cosolvent curves for 1,2,4-trichlorobenzeneD3 (diamonds), PCB-101 (squares), and benzo[g,h,i]perylene (triangles) in the upper sediment layer. Solid symbols represent data used for determination of KOCext’s (dotted lines). described by Hermans et al. (30) with 100 mL of hexane as extractant. Extraction time was 36 h. Sediments were recovered from the vessels by transferring the remaining suspensions to 800 mL jars and centrifuging at 1500 rpm and 20 °C for 10 min. Subsamples were dried with Na2SO4 (kilnfired for 2 h at 550 °C) and Soxhlet-extracted for 16 h with hexane/acetone (3:1). All extracts were split up. One part was cleaned-up over an Al2O3 column and used for PAH analysis, which was carried out on an HP 1050 HPLC equipped with two Jasco FP-920 fluorescence detectors. The other part was cleaned-up using an Al2O3/silica column, desulferized with Cu-powder, and used for PCB and chlorobenzene analysis. PCBs were measured using a double-column Perkin-Elmer gas chromatograph with two 63Ni Electron Capture Detectors. Chlorobenzene analysis was performed on an HP 5890 II gas chromatograph equipped with an HP 5989A quadrupole mass spectrometer for detection. Quality Assurance. Quality of the analytical work was assured by several provisions. These included extensive cleaning of glass and SS equipment, the use of dark or covered glassware to minimize PAH photolysis, and frequent analysis of an internal reference sediment. Furthermore, the efficiency of the liquid-liquid extraction method was checked and approved by several post-extractions, and all samples were corrected for blanks and cleanup recoveries (64-97%, dependent on the chemical) on the basis of numerous procedure blank and recovery measurements. In addition, recoveries were monitored by adding recovery compounds (1,2-dichlorobenzene-D4, PCB-29, PCB-155, and peryleneD12) to each sample prior to extraction. Mass balances for all equilibration vessels were calculated. Averaged mass balances measured 99 ( 11 and 96 ( 10% for the upper and deeper sediment layer, respectively.
Results and Discussion Relationship between Partition Coefficients and Cosolvent Fractions. All contaminants in both sediment layers behaved according to the cosolvency theory. Log-linear curves are created when KOC’s are plotted against methanol fractions. Some examples of “cosolvent curves” are given in Figure 1. Log KOC’s obtained by extrapolation of the linear part of these curves (16-40 or 8-40% methanol) to 0% methanol (log VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Log KOCext Values with 95% Confidence Intervals for Chlorobenzenes, PCBs, and PAHs in Two Sediment Layers from Lake Ketelmeerc log KOCext compound 1,2-dichlorobenzene 1,3-dichlorobenzene 1,4-dichlorobenzene 1,4-dichlorobenzene-D4 1,2,3-trichlorobenzene 1,2,4-trichlorobenzene 1,2,4-trichlorobenzene-D3 1,3,5-trichlorobenzene 1,2,3,4-tetrachlorobenzene 1,2,3,5- + 1,2,4,5tetrachlorobenzeneb pentachlorobenzene hexachlorobenzene hexachlorobenzene-13C6 PCB-18 PCB-28 PCB-31 PCB-44 PCB-52 PCB-101 PCB-105 PCB-118 PCB-138 PCB-153 PCB-156 PCB-170 PCB-180 PCB-187 PCB-204 phenanthrene anthracene anthracene-D10 fluoranthene pyrene benzo[a]anthracene chrysene benzo[e]pyrene benzo[b]fluoranthene benzo[k]fluoranthene benzo[a]pyrene benzo[g,h,i]perylene dibenzo[a,h]anthracene indeno[1,2,3-c,d]pyrene
0-30 cm layer
40-120 cm layer
log KOWa
4.88 ( 0.29 5.13 ( 0.30 5.18 ( 0.29 4.99 ( 0.13 5.79 ( 0.25 6.12 ( 0.14 5.71 ( 0.10 5.96 ( 0.13 6.74 ( 0.42 6.87 ( 0.35
4.95 ( 0.11 5.11 ( 0.20 5.21 ( 0.13 4.08 ( 0.17 5.56 ( 0.16 5.50 ( 0.13 4.39 ( 0.31 5.36 ( 0.12 5.91 ( 0.13 5.91 ( 0.19
3.4 3.5 3.4 3.4 4.1 4.1 4.1 4.2 4.6 4.6
6.72 ( 0.22 6.79 ( 0.33 6.42 ( 0.36 6.50 ( 0.09 7.97 ( 0.16 7.76 ( 0.14 6.96 ( 0.19 6.88 ( 0.17 7.16 ( 0.10 8.08 ( 0.26 8.08 ( 0.47 7.77 ( 0.50 7.60 ( 0.25 8.25 ( 0.32 8.20 ( 0.18 7.88 ( 0.50 7.90 ( 0.16 8.30 ( 0.22 6.72 ( 0.28 6.90 ( 0.20 6.66 ( 0.39 6.86 ( 0.24 6.80 ( 0.33 8.01 ( 0.37 7.89 ( 0.25 8.34 ( 0.30 8.39 ( 0.31 8.49 ( 0.25 8.97 ( 0.33 9.32 ( 0.28 8.47 ( 0.41 9.20 ( 1.14
6.25 ( 0.49 6.57 ( 0.17 5.56 ( 0.89 5.38 ( 0.51 6.39 ( 0.81 6.22 ( 0.82 6.00 ( 0.51 6.14 ( 0.31 6.73 ( 0.31 6.93 ( 0.60 7.08 ( 0.57 7.33 ( 0.25 7.32 ( 0.28 7.38 ( 0.34 7.80 ( 0.41 7.84 ( 0.29 7.68 ( 0.25 8.18 ( 0.17 5.58 ( 0.43 5.58 ( 0.30 5.05 ( 0.60 5.88 ( 0.43 5.95 ( 0.26 6.74 ( 0.65 6.86 ( 0.52 7.54 ( 0.34 7.59 ( 0.33 7.82 ( 0.17 7.76 ( 0.34 8.69 ( 0.27 7.64 ( 0.56 8.63 ( 0.28
5.2 5.7 5.7 5.6 5.7 5.7 5.9 6.0 6.4 6.7 6.6 6.8 6.9 7.2 7.0 7.1 6.9 7.5 4.6 4.6 4.6 5.2 5.2 5.9 5.8 6.4 5.8 6.2 6.0 6.9 7.0 7.0
a log K OW values for chlorobenzenes are from ref 53 and for PCBs from refs 54 and 55 (sometimes averaged). Values for PAHs are adopted from ref 56. b Due to low concentrations of 1,2,3,5-tetrachlorobenzene, these two isomers could not be separated chromatographically. Log KOC’s concern the sum of both compounds. c Bold values represent added compounds.
KOCext) along with accompanying 95% confidence intervals and log KOW’s are presented in Table 1. Deviations from curve linearity are observed for the more hydrophobic compounds at 0, 8, and sometimes 16% methanol. The downward curvatures to lower, apparent KOC’s (KOCapp’s) are the result of association of the chemicals with DOC. Log KOCapp values show a cutoff at approximately 7.58, whereas log KOCext’s increase to 9.3. Apparently, binding to DOC significantly contributes to the speciation of these compounds in the liquid phase at low cosolvent fractions. Since it is known that mainly the more hydrophobic chemicals tend to sorb to DOC (31), it is not surprising that curvatures were observed to be more pronounced as the chemicals’ hydrophobicity increased. In addition to these downward curvatures, upward curvatures are observed at 48% methanol for all compounds in both sediment layers (see Figure 1). Showing a nearly 1622
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constant deviation from the regression lines of 0.3 log units (for in situ as well as added compounds and for contaminants predominantly present in the liquid as well as sediment phase), this phenomenon seems to be independent of chemical and sediment properties and might be an effect of the cosolvent. Because organic solvents are thought to have swelling effects on the sediment organic matter (32, 33) high methanol fractions may alter the characteristics of this material, influencing the partition behavior of contaminants (34). However, it was demonstrated that water also shows swelling effects to approximately the same extent as methanol does (35). With respect to the statement that methanol might extract organic matter from sediments (36) it should be noted that sediment organic carbon contents in the present experiments did not decrease with increasing methanol fraction (3.55 ( 0.04 and 6.44 ( 0.11% for the upper and deeper layer, respectively). Verification of Equilibrium Conditions. Accuracy of KOC values greatly depends on whether equilibrium is attained in the experiments or not, a fact which is often overlooked or not verified. In the present study an estimate concerning the extent to which this condition occurred was made on the basis of the comparison between added and in situ contaminant analogues (mass-labeled 1,4-dichlorobenzene, 1,2,4-trichlorobenzene, hexachlorobenzene, and anthracene vs unlabeled native analogues, and PCB-44, -105, -170 and dibenzo[a,h]anthracene vs their closest congeners: PCB-52, -118, -180 and benzo[g,h,i]perylene, respectively). Curves of all PCBs couples in both layers coincide (data not shown), and KOCext values are equal or comparable. In contrast, curves of the PAH couple dibenzo[a,h]anthracene/benzo[g,h,i]perylene and all mass-labeled/in situ analogue couples do not coincide, principally indicating that the equilibrium state is not attained for these compounds. However, in the upper sediment layer KOCext values for the latter (with the exception of 1,2,4-trichlorobenzene) are very close to each other (Table 1), and 95% confidence intervals overlap (whereas in the deeper layer curves and KOCext values are much further apart). Based on the PCB data and the fact that both sediment layers sometimes give almost identical results (e.g. PCB-180, -204, in situ dichlorobenzenes) it can be concluded that the experiments were performed adequately and that the systems were at least (very) close to equilibrium. The differences between mass-labeled and their in situ analogues are therefore probably caused by dissimilarities in sediment or contaminant properties. This will be discussed later. Comparison of the Two Sediment Layers. The results given in Table 1 demonstrate that the two sediment layers considerably differ in sorption behavior. This is emphasized by the fact that the slopes of the cosolvent curves are usually steeper in the upper layer. All KOCext values of in situ as well as added compounds in this layer are higher than their corresponding ones in the deeper layer (up to 1.6 log units) with the exception of the in situ dichlorobenzenes and some PCBs which are (approximately) the same. This is quite contrary to what could be expected on the basis of slow sorption kinetics, by which higher partition coefficients are obtained as contact time of compounds with sorbents increases. In the present experiments slow sorption should have given rise to the following KOC sequence for a certain compound: deeper layer (in situ) > upper layer (in situ) > deeper layer (added) g upper layer (added). In general, Table 1 however shows the following order: upper layer (in situ) > upper layer (added) g deeper layer (in situ) > deeper layer (added). From this it can be concluded that (a) in the deeper layer short-term sorption clearly yields lower KOC’s than longterm sorption (in the field), which is in accordance with the slow sorption concept and observations by others (9, 21, 22); (b) short-term sorption affinity for the upper layer is greater than for the deeper layer, not showing the reported influence
FIGURE 2. Comparison of logKOCext values for hexachlorobenzene (diamonds), PCBs (squares), and PAHs (triangles) in the upper (open symbols) and deeper (solid symbols) layer with previously determined values in sediment from Rotterdam Harbor (38). Dotted line represents the 1:1 relationship. of organic matter aging on the KOC (16, 17); (c) no effect of slow sorption on the KOC is found in the aged layer in relation to the recently deposited layer (in fact, the opposite is found); and (d) after only 11 days of equilibrium KOC’s can be comparable to and sometimes even higher than those obtained after several decades of contact time in the field. Sorption Dependence on Sorbate Planarity. KOCext values determined in both sediment layers are rather high in comparison with values usually reported in the literature. Differences up to 3.0 log units for, e.g. benzo[a]pyrene in the upper layer are found (37). These large KOC values are caused by possible special sorption characteristics of the sediments and/or the different determination method used, which is not influenced by third phase effects. Which of these two (mainly) determines the difference can be sorted out by comparing the KOCext values with KOC’s determined in the same sediments with other methods or with the same method in other sediments. With respect to the former, Ten Hulscher et al. (9) found an average in situ log KOC for 1,4-dichlorobenzene in a sediment core from the same area of 5.1 (using centrifugation as a separation technique) which is almost equal to the values measured in the present study. Values for other di- and trichlorobenzenes were somewhat lower though. The latter possibility is checked in Figure 2 by comparing data for hexachlorobenzene, PCBs (except 156 and 204), and PAHs from both layers with previously obtained data for these compounds in sediments from Rotterdam Harbor (average of three) using the cosolvent method (no other chlorobenzenes were tested in the latter sediments) (38). From this figure it appears that all log KOCext values in the deeper layer pretty well coincide with their corresponding ones in Rotterdam Harbor sediments, whereas values from the upper layer are usually higher. All in all it might be concluded that in particular in the upper layer the extreme high partition coefficients are probably the result of specific sediment sorption properties. A closer look at Figure 2 reveals that the contaminants in the upper layer which do not deviate from the Rotterdam Harbor data are the PCBs 101, 138, 153, 170, 180, and 187.
Interestingly, these are all multiple-ortho-PCBs, which differ from the other compounds in not having or being able to adapt a planar molecular configuration. In addition to this, when the log KOCext values of both sediment layers are plotted against their log KOW’s (see Figure 3) a clear influence of sorbate planarity on sorption behavior becomes visible, also for the deeper layer, though less pronounced. Among the compounds tested, planar contaminants are chlorobenzenes, PAHs, and mono-ortho-PCBs (i.e. PCB-28, 31, 105, 118, and 156), whereas nonplanar compounds are PCBs 18, 44, 52, 101, 138, 153, 170, and 180 (di-ortho’s), PCB-187 (tri-ortho), and PCB-204 (tetra-ortho). In both layers native chlorobenzenes, PAHs, and PCBs 28 and 31 display abnormal (strong) sorption behavior compared to the remaining PCBs. In the upper layer this conduct also applies to added chlorobenzenes (except for hexachlorobenzene), PAHs, and PCB-105 and -118, while in the deeper layer these compounds side with the multiple-ortho-PCBs. Here, this combined group shows a nearly one on one relationship with the KOW, reflecting sorption based on hydrophobic properties only. For both “planar” and “nonplanar” contaminant groups in both layers significantly different (F-test: P < 0.01) log KOClog KOW relationships are applicable which are given in Table 2. The large positive intercepts of in particular the lines of the planar contaminants indicate that hydrophobicity alone fails to explain the sorption behavior of these compounds. The abnormal behavior of planar compounds is a wellestablished phenomenon in toxicology (39) where planar (i.e. non-ortho-substituted) PCBs show higher (dioxin-like) toxicity because of their greater affinity for the Ah receptor. Furthermore, coplanar PCBs appear to bioaccumulate to a larger extent (40) and show increased sorption to aerosols (41, 42) compared with ortho-substituted congeners. Increased bioaccumulation potential can however partly be ascribed to higher KOW’s for these compounds (see Table 1) and partly to the absence of steric hindrance during transport over biomembranes (40). Similarly, increased sorption of coplanar congeners to urban aerosols was shown to be accounted for by the higher octanol-air partition coefficients of these congeners (42). Data on the influence of sorbate planarity on sorption to sediments and soils are scarce. PayaPerez et al. (43) and Booij et al. (44) reported preferential sorption of coplanar over multiple-ortho-substituted PCBs (both (much) less pronounced than in the present study) but gave no explanation. The slightly increased KOC’s observed by Paya-Perez et al. (43) can fully be traced back to higher accompanying KOW’s. Soot as Preferential Partition Medium for Planar Compounds. Preferential sorption of coplanar PCBs in the two sediment layers from Lake Ketelmeer cannot be explained by the congeners’ KOW’s. After all, in Figure 2 which shows the phenomenon KOCext values are directly compared with other ones (determined by the same cosolvent method), eliminating any influence of KOW’s. Therefore, the sediments probably contain matrices to which specifically coplanar compounds can sorb preferentially. From the analytical chemistry several of these matrices are known. Charcoal and porous graphite but mostly activated carbon (45) are used to separate PCBs with different degrees of ortho-substitution (i.e. planarity). It is therefore very plausible to suppose that both sediments contain activated-carbon-like material such as soot, (char)coal, or ash, possessing a highly condensed carbon structure. Hereafter, we colloquially will refer to this material as “soot”. The presence of soot has been suggested to explain extreme high distribution coefficients for PAHs (e.g. refs 7, 23, 46-48) since it appears to sorb these compounds significantly stronger per mass unit of carbon than natural organic matter does (7, 47, 48). Most of these KOC’s are still lower than the ones determined in the present study. VOL. 34, NO. 9, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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FIGURE 3. Log KOCext against log KOW values for chlorobenzenes (diamonds), PCBs (squares), and PAHs (triangles) in the upper (a) and deeper (b) layer. Solid symbols represent added compounds. Dashed lines are the relationships log KOC ) log KOW. Dotted lines separate planar and nonplanar contaminant groups.
TABLE 2. Log KOC-Log KOW Relationships for Planar and Nonplanar Contaminant Groups in Both Sediment Layers sediment layer (cm)
relationship
r2
compounds
0-30
log KOCext ) 1.09 log KOW + 1.48 log KOCext ) 1.00 log KOW + 0.92 log KOCext ) 0.97 log KOW + 1.42 log KOCext ) 1.08 log KOW - 0.06
0.92 0.95 0.87 0.93
planar (all CBsa (except HCBsb) and PAHs, PCBs 28, 31, 105, 118) nonplanar (HCBs, remaining PCBs) “planar” (in situ CBs and PAHs, PCBs 28, 31) “nonplanar” (added CBs and PAHs, remaining PCBs)
40-120 a
Chlorobenzenes.
b
Hexachlorobenzene and hexachlorobenzene-13C6.
McGroddy et al. (23) explained the high PAH KOC’s they observed by supposing the PAHs to be sequestered or irreversibly bound in the soot matrix as a result of their pyrogenic origin. As a consequence the compounds would only be partially available for partitioning to the aqueous phase (quantified by an available for equilibrium partitioning (AEP)-fraction) on a time scale of 30-50 years. Chemicals such as PCBs which are introduced to the soot matrix through the aqueous phase would be fully available (46). However, in the light of the present results the data of McGroddy et al. can also be interpreted differently, all the more since their (and our) PAHs were completely solvent-extractable. Possibly, PAHs are not unavailable for partitioning, but simply have soot-water distribution coefficients that are extremely high. After all, KOCext values for added chemicals in the upper sediment layer from Lake Ketelmeer were found to be almost equal to those for native compounds. If McGroddy et al. had tested coplanar PCBs (instead of di-ortho congeners) they might have come to the same conclusion. Sorption of planar aromatic compounds to soot is probably exceptionally strong because of the ability of the compounds to approach the flat sorption surface very closely, making favorable π-cloud overlap possible (7) or in that the chemicals exactly fit into the soot’s pores. Gustafsson et al. (47) developed an analytical method to quantify the sedimentary soot phase, based on removal of nonsoot organic matter by thermal oxidation at 375 °C. We applied this exact same method to both our sediment layers which resulted in carbon residues of 0.4 ( 0.07% (n ) 3) for the upper layer and 0.8 ( 0.2% (n ) 7) for the deeper layer, proving the existence of soot in both cases. The measured soot contents fall within the range known for other sediments, i.e., 0.01-1.8% based on sediment dry weight and 2-38% based on organic carbon content (49). Considering the results from Table 1 and Figures 2-3 one would however expect the 1624
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soot contents in our sediments to be the other way round since the “soot-effects” are much more pronounced in the upper layer. The fact that more soot in the deeper layer causes less soot-related effects may possibly be accounted for by (a combination of) three hypothetical explanations. (1) It is possible that the two layers contain different types of “soot”, each having their own specific affinity for planar compounds. PAH contamination in the deeper layer is most likely dominated by input dating from the 1950s till mid 1970s, whereas the upper layer consists of more recently (1980s/ 1990s) deposited material (29). PAHs in the former layer therefore largely originate from combustion of coal and in the latter layer from combustion of gasoline and diesel, next to other sources. Combustion of different fuels leads to formation of different types of soot (50, 51) possibly having different sorption (pores or surface) characteristics. Next to this, aging processes might result in alteration of the soot matrix and its sorption properties. The observation of the KOCext values for in situ dichlorobenzenes being identical in both layers is however not explained by this hypothesis. (2) The presence of material that preferentially sorbs planar contaminants in the upper layer may be underestimated since the soot quantification method does not include matrices such as coal (47). On the other hand, the soot content in the deeper layer may be overestimated due to the presence of pollen (47). Of course, both possibilities also apply to the opposite layer. It should be noted that the first two hypotheses do not fully elucidate the difference between layers in sorption behavior of added compounds (Table 1). (3) Considering the high KOCext values of (planar) added compounds in the upper layer, it might be assumed that part of the sorption sites on/in the soot matrix was available, resulting in similar sootlike sorption for both native and added contaminants. In the deeper layer KOCext values for planar in situ compounds (with the exception of dichlorobenzenes)
are lower than their corresponding ones in the upper layer, whereas those for added contaminants are even much more reduced. Relating this to the fact that the total analyzed contaminant concentrations are 2.3 times higher than in the upper layer, we hypothesize that the sorption sites on soot in this layer were completely or at least largely occupied. As a result only the possibility of hydrophobic partitioning in the organic matrix remained for part of the planar in situ and almost the entire mass of added compounds which reduced the partition coefficients. Analyzed compounds are of course only a fraction of the total sum of contaminants. A study regarding sediment cores from the same area showed that concentrations of several other planar compounds (i.e. dioxins and furans) in layers from the 1960s/1970s were generally 1 order of magnitude higher than in recently (1990) deposited layers (29). Moreover, also natural organic matter might sorb to soot, in particular the highly aromatic fraction which is generally elevated in aged sediments due to mineralization processes (5, 6). The KOCext values of native dichlorobenzenes being equal to those in the upper layer might be explained by the fact that these chemicals can fill up small spaces left open between larger sorbed compounds because of their small molecular dimensions. The third hypothesis might also account for the differences in slopes of the cosolvent curves between the layers. For the abnormal behavior of hexachlorobenzene and PCB 156 in the upper layer we have no explanation. Gustafsson et al. (47) proposed an equation for normalizing distribution coefficients (Kd’s) to soot fraction (fSC) next to organic carbon content (fOC):
Kd ) fOCKOC + fSCKSC
Transport, Public Works and Water Management, The Netherlands (RWS-BD/WRO-B-96048). To effectuate the paper additional financial support was given by TNO-MEP, The Netherlands. The authors would like to thank Erik H. G. Evers for initiating and managing the project and A. A. Koelmans for his helpful comments on the paper. Grietje Nummerdor, Jos Hermans, and Karin Koning are acknowledged for their indispensable practical assistance.
Literature Cited (1) (2) (3) (4) (5) (6) (7) (8) (9) (10) (11) (12) (13) (14) (15)
(1)
(16)
where KOC and KSC are respectively the organic carbon and soot carbon normalized partition coefficients. The abovementioned third hypothesis implies that soot has a finite sorption capacity (similar to activated carbon (52)) which will be revealed in terms of Langmuir-type sorption. Equation 1 may therefore only be valid at “low” (planar) contaminant concentrations (i.e. , soot’s sorption capacity). Gustafsson et al. (7) estimated KSC values for PAHs assuming the sorption process to be thermodynamically similar to the association of PAHs with pure PAH crystals. Their results are almost identical to our KOCext values for these compounds in the upper layer. The KOCext values are however normalized to the sum of the organic plus soot carbon fractions. Since the soot fraction is approximately one tenth of the total carbon fraction, application of eq 1 for the upper layer (assuming KOC ≈ KOW) leads to KSC values that are roughly one log unit higher than those estimated by Gustafsson et al. (7). For the deeper layer, KSC values similar to the theoretical estimates of Gustafsson et al. can be calculated. The present data do not show effects related to the contact time of the contaminants with the sediments (except for the case added vs in situ compounds in the deeper layer). Any possible effects of slow sorption occurring within or between the sediment layers are overruled by the effects due to the presence of soot. This is of course no conclusive evidence for the absence of slow sorption because this process and sorption to soot can exist next to one another. Our results however demonstrate that when soot is involved, distribution of planar contaminants might be fully dominated by sorption to this material. Enhanced sorption to soot appears therefore not to be limited to PAHs as was assumed by most researchers. Moreover, equilibrium conditions may already be attained within several days.
(17) (18) (19) (20) (21) (22) (23) (24) (25) (26) (27) (28) (29) (30) (31) (32) (33) (34) (35) (36) (37)
Acknowledgments The research described in this paper was financially supported by the Civil Engineering Division of the Ministry of
(38) (39)
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Received for review June 2, 1999. Revised manuscript received January 12, 2000. Accepted January 21, 2000. ES9906251
