Research Communications Stable Chlorine Isotopic Compositions of Aroclors and Aroclor-Contaminated Sediments C H R I S T O P H E R M . R E D D Y , * ,† LINNEA J. HERATY,‡ BEN D. HOLT,‡ NEIL C. STURCHIO,‡ TIMOTHY I. EGLINTON,† NICHOLAS J. DRENZEK,† LI XU,† JAMES L. LAKE,§ AND KEITH A. MARUYA# Department of Marine Chemistry and Geochemistry, Woods Hole Oceanographic Institution, Falmouth, Massachusetts 02543, Environmental Research Division, Argonne National Laboratory, Argonne, Illinois 60439, Atlantic Ecology Division, National Health and Effects Research Laboratory, USEPA, Rhode Island 02882, and Skidaway Institute of Oceanography, Savannah, Georgia 31411
Introduction Polychlorinated biphenyls (PCBs) are toxic and persistent organic compounds that have pervasively contaminated the environment. Understanding the transport and fate of PCBs is a fundamental, yet often challenging, task, especially when attempting to identify responsible parties, to calculate mass balances, to determine whether natural attenuation is occurring, and to develop and evaluate remediation plans. Historically, these issues have been approached using the molecular signatures of the contaminant. This approach is often hindered by complex PCB distributions that result from multiple contributions and by modifications to the original signature as a result of transport and postdepositional processes. The use of stable chlorine isotopic ratio compositions of PCBs as an alternative means of constraining inputs and the processes that determine their fate has not been explored to date. There are two naturally occurring stable isotopes of chlorine, 35Cl and 37Cl. The relative abundance of each isotope is nominally 76 and 24%, respectively. This ratio is expressed as
δ37Cl (‰) ) ((Rsample/Rstandard) - 1)*1000
An exploratory investigation was conducted to evaluate if stable chlorine isotopic ratios of polychlorinated biphenyls (PCBs) could be useful in studying the processes that determine their transport and fate in the environment. First, we determined the variability of δ37Cl in the source materials for PCBs. Second, we determined if the δ37Cl values of contaminated environmental samples fell outside the range in source variability. The isotopic variability among the source materials (Aroclors) was rather small; δ37Cl values ranged from -3.37 to -2.11‰ (mean and standard deviation, -2.78 ( 0.39‰, n ) 12). There was no correlation between the δ37Cl values and percentages of chlorine in the mixtures. We also found very similar values in several Clophen mixtures and one Phenoclor. The δ37Cl values in the total PCBs isolated from Aroclorcontaminated sediments from the Hudson River, New Bedford Harbor, and Turtle River Estuary ranged from -4.54 to -2.25% (n ) 19). While most of the δ37Cl values were within 2 standard deviations of the mean Aroclor value (our assumed estimate for overall source variability), two of the PCB contaminated sediment samples from New Bedford Harbor did appear to be isotopically distinct. The PCBs in these sediments had lower amounts of less chlorinated congeners (when compared to the source material) and were likely isotopically affected by alteration processes that preferentially removed these congeners. Compound specific measurements of two congeners in Aroclor 1268 suggest that there are no large congener-specific differences in the stable chlorine isotope ratios in Aroclors. This study shows that the δ37Cl values of PCBs may be a potentially useful diagnostic tool in studying the transport and fate of PCBs and indicates that additional research is warranted.
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where Rsample is the 37Cl/35Cl of the 37Cl/35Cl of standard mean ocean
sample and Rstandard is the chloride (SMOC). Due to mass-dependent effects, where one isotope may react faster or distribute itself in one phase preferentially over another, there can be measurable differences in the ratio of these isotopes (1). Geochemists have exploited these small variations in the stable isotopic compositions of elements such as carbon in order to study natural processes (2). This approach is becoming increasingly popular for the study of organic contaminants (3). For example, Heraty et al. (4) showed that when microbes in laboratory experiments aerobically degrade dichloromethane, the δ37Cl values of the residual dichloromethane increased as a function of the amount degraded. To evaluate the utility of chlorine isotopes as tools to study the environmental biogeochemistry of PCBs, two important steps need to be performed: (I) determine the variability of Cl isotopic compositions of the source materials and (II) compare the δ37Cl variability of the source material to PCBs found in environmental samples that show clear evidence of loss or alteration. If there are no significant isotopic differences between the source material and the samples, this approach will be futile. However, if there are significant differences, then this may indicate that some biological, chemical, and/or physical process has acted on the PCBs and affected their isotopic compositions. If a characteristic isotope effect can be determined for each process, stable chlorine isotope ratios of PCBs may prove to be a very useful indicator. In this exploratory study, we measured the δ37Cl values of a variety of Aroclors (source materials), which are mixtures of PCBs that were produced and sold in the United States by Monsanto. We then extracted and measured the δ37Cl values of total PCB fractions from sediments that have been subject to chronic Aroclor contamination over several decades.
* Corresponding author phone: (508)289-2316; fax: (508)457-2164; e-mail:
[email protected]. † Woods Hole Oceanographic Institution. ‡ Argonne National Laboratory. § USEPA. # Skidaway Institute of Oceanography. 10.1021/es9908220 CCC: $19.00
2000 American Chemical Society Published on Web 05/25/2000
Sediment samples were collected from the Hudson River, New York (HR), New Bedford Harbor, Massachusetts (NBH), and the Turtle River Estuary, Georgia (TRE).
Methods Aroclors. Aroclors 1016, 1242, 1254, 1260, and 1268 were purchased from three environmental standard suppliers: Accustandard (New Haven, CT), Chem Service (West Chester, PA), and Ultra Scientific (North Kingstown, RI). Dr. Jan Boon of The Netherlands Institute of Sea Research also kindly donated several Clophens (A30, A50, and A60) and one Phenoclor (DP-6). These PCB mixtures were produced by Bayer (Germany) and Caffaro (Italy), respectively. Stock solutions of each mixture were prepared in pentane, and the δ37Cl values were determined without any other prior treatment. Hudson River (HR) Sediments. A sediment core (0-10 cm depth) was collected in July 1999 at the 193 mile marker in the HR. Mostly Aroclor 1242 from two General Electric capacitor plants contaminated this area (5, 6). These samples show extensive and significant evidence of reductive dehalogenation. Sediments in the top 4 cm have total PCBs concentrations of ∼7 µg g-1 and are composed of mono-, di-, tri-, and tetrachlorobiphenyls. Sediments in the deeper sections (4-10 cm) have PCBs concentrations of ∼2 µg g-1 and are composed mostly of the final products of reductive dehalogenation, 2-chlorobiphenyl and 2,2′- and 2,6-dichlorobiphenyl (as one peak) (5). New Bedford Harbor (NBH) Sediments. Various sediment cores were collected in upper and lower NBH by piston corer in July 1988 and January 1991. These sediments were contaminated with Aroclors 1016, 1242, and 1254 by a former electrical capacitor plant and exhibit a range of PCBs concentrations (40-3000 µg g-1) that reflect varying degrees of environmental alteration (7). The more concentrated samples, which were from upper NBH, show strong evidence of reductive dehalogenation. The less concentrated samples, which are from lower NBH or a marsh in upper NBH, appear to have lost some of their lower chlorinated congeners to the air or water and show no evidence of reductive dehalogenation. One additional sample was obtained from material dredged from 1994 to 1995 from upper NBH. This material is now contained in a facility adjacent to NBH, and the sample was collected in April 1999. The concentration of PCBs was 300 µg g-1 in the dredged material, and the congener distribution was similar to the other samples from upper NBH. Turtle River Estuary (TRE) Sediments. One sediment core (0-2, 2-6, 6-10, and 10-18 cm) and one surface grab sample (0-5 cm) were collected in the Purvis Creek marsh of the TRE in April 1998 and May 1997, respectively. This site is adjacent to the LCP Chemicals Superfund site where a former chloro-alkali plant used and discharged a large amount of Aroclor 1268 (8). These samples are highly contaminated with PCBs (380-1490 µg g-1) and show little or no evidence of any type of environmental alteration. Isolation of PCBs from Marine Sediments. PCBs were extracted from the sediments by pressurized fluid extraction with a 50/50 hexane/acetone solution. The extracts were then partitioned against water. The resulting hexane extracts were treated with concentrated sulfuric acid, activated copper, and chromatographed on a silica gel column with hexane as the eluent to isolate a PCB fraction. The purified PCB fraction was then partitioned against deionized water, rotaryevaporated to near dryness, and redissolved in pentane. To check the purity of the extract, a small portion of the extract was analyzed with a gas chromatograph interfaced to a mass spectrometer (GC-MS), and the remaining extract was analyzed for δ37Cl. GC-MS analysis revealed that the PCBs were the main source of chlorinated compounds.
TABLE 1. The δ37Cl Values of PCBs mixture
ca. % Cl
suppliera
δ37Clb (‰)
Aroclor 1016 Aroclor 1016 Aroclor 1242 Aroclor 1242 Aroclor 1242 Aroclor 1254 Aroclor 1254 Aroclor 1254 Aroclor 1260 Aroclor 1268 Aroclor 1268 Aroclor 1268 Clophen A30 Clophen A50 Clophen A60 Phenoclor DP-6
41c 41c 42 42 42 54 54 54 60 68 68 68 42 54 60 60
ChemService AccuStandard ChemService Ultra Scientific AccuStandard ChemService Ultra Scientific AccuStandard Ultra Scientific ChemService Ultra Scientific AccuStandard
-3.07 -2.60 -2.89 -2.20 -2.11 -3.38 -2.93 -2.68 -2.96 -3.22 -2.83 -2.44 -2.49 -3.26 -3.31 -3.17
a Name of company that supplied the standards. The Clophens and Phenoclor DP-6 were donated by Dr. Jan Boon of The Netherlands Institute for Sea Research. b The estimated precision for δ37Cl measurements is 0.13‰. c Calculated from the average molecular distribution of mono-, di-, tri-, and tetrachloro congeners in Aroclor 1016 from (11).
Analysis of δ37Cl. The Aroclors and PCB samples isolated from sediments (in pentane) were transferred to precombusted 9-in. Pyrex tubes in amounts to yield generally 3-20 µmol of chlorine. The pentane was removed under a stream of nitrogen. Precombusted copper oxide (1-2 g) was added to each tube. The δ37Cl analyses were then performed according to Holt et al. (10). Quality Control. To test the precision of this method, the δ37Cl values of several neat samples were analyzed in triplicate. The standard deviations ranged from 0.10 to 0.16‰, respectively. To determine the precision of δ37Cl values for PCBs isolated from sediment matrices, one sediment sample from NBH was divided into three aliquots, and the isolated PCB fraction for each aliquot was analyzed. The standard deviation was 0.12‰. Based on these results, we estimate the precision for δ37Cl measurements to be 0.13‰ for all samples. To check that the process of extracting and isolating PCBs from marine sediments did not affect the δ37Cl values, a pristine sediment sample was spiked with Aroclor 1260 (Ultra) and processed. The δ37Cl of the PCB fraction isolated from the spiked sediment (-2.98‰) agreed well with the δ37Cl value of the neat unprocessed Aroclor 1260 (-2.96‰). Preparative Capillary Gas Chromatography (PCGC). Two individual compounds were isolated with repeated injections (∼100) on a PCGC system, as described in Eglinton et al. (9). They were separated with a 60-m SGE BPX-5 fused silica column (0.5 µm film thickness; 0.53 mm I.D.) and collected in cryogenically cooled u-tubes (0 °C). The purity of each isolated compound was determined by GC and in each case was 99+%. To test that the process of isolating compounds by PCGC does not significantly fractionate their δ37Cl values, we added IUPAC-103 (2,2′,4,5′,6-pentachlorobiphenyl) to a solution of Aroclor 1268 and then isolated IUPAC-103 from the mixture. The δ37Cl values of IUPAC-103 before (-4.61 ‰) and after being trapped on the PCGC (-4.68 ‰) were very similar and indicate that the PCGC can be used to isolate PCB congeners.
Results and Discussion δ37Cl Values of Aroclors (Source Materials). The δ37Cl values of the 12 different Aroclors spanned a relatively narrow range from -3.37 to -2.11‰ (Table 1; Figure 1) compared to a much larger range measured in other chemically manufactured chlorinated organic compounds, -6.82 to +4.08‰ (4, 10, 12-14). Surprisingly, there did appear to be some VOL. 34, NO. 13, 2000 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. δ37Cl Values of PCBs Extracted from Aroclor-Contaminated Sediments sediment core B; 0-1 cm core B; 1-2 cm core B; 2-3 cm core B; 3-4 cm core B; 4-10 cm
concn of total PCBs (µg g-1) 6.3 7.7 6.4 6.4 1.7
δ37Cl of PCBsa (‰)
date of collection and location of sampling
Hudson River -3.24 July 1999; location H7 in ref 6 at a water depth of 1.5 m -3.18 same as above -3.23 same as above -2.86 same as above -2.35 same as above
core C25; 2.5-5 cm core C36; 5-7.5 cm core C36; 7.5-15 cm core I14; 5-7.5 cm core C23; 0-2.5 cm dredged material core I14; 15-17.5 cm core C23; 7.5-15 cm core I11; 15-17.5 cm
47 69 98 1200 722 300 1720 3400 2960
New Bedford Harbor -4.54 January 1991; 41°40′03′′N, 70°54′48′′W -4.12 January 1991; 41°38′50′′N, 70°55′18′′W -3.64 January 1991; 41°38′50′′N, 70°55′18′′W -3.50 June 1988; See Figure 1 in ref 7. -3.19 January 1991; 41°41′13′′N, 70°55′04′′W -3.05 April 1999; See text for details. -3.02 June 1988; See Figure 1 in ref 7. -2.76 January 1991; 41°41′13′′N, 70°55′04′′W -2.53 June 1988; See Figure 1 in ref 7.
grab 0-5 cm core MSL 0-2 cm core MSL 2-6 cm core MSL 6-10 cm core MSL 10-18 cm
420 380 500 970 1490
Turtle River Estuary -2.84 May 1997; location “MCL” in Figure 1 of ref 8. -2.70 April 1998; location “MCL” in Figure 1 of ref 8. -2.44 same as above -2.30 same as above -2.25 same as above
a
The estimated precision for δ37Cl measurements is 0.13‰.
FIGURE 1. Plot of δ37Cl values in the commercial PCBs mixtures. The legend shows the supplier of the Aroclor mixtures. The shaded box is the range in δ37Cl values measured in other chlorinated organic compounds. The estimated precision is ∼0.13‰. systematic bias among the suppliers of the Aroclors. Generally, those purchased from Chem Service were the most depleted, and those purchased from Accustandard were the most enriched in 37Cl. There was no obvious correlation between the mass percent chlorine and the δ37Cl values in the Aroclors, indicating that the δ37Cl did not simply depend on the congener composition of the mixture but rather on a more general factor associated with the manufacturing of Aroclors. Based on this finding, the source material variability can be calculated from all 12 mixtures, rather than for each specific Aroclor mixture. The mean and standard deviation of the δ37Cl values from the 12 Aroclor mixtures was -2.78 ( 0.39‰ (n ) 12). At this time, we can neither explain why the δ37Cl values of the Aroclors are depleted relative to SMOC nor explain their overall variability. The variability is likely due to differences in the starting materials, the synthesis, purification, or postproduction storage and handling of the products, or some combination of these factors. It worth noting that the three Clophen mixtures and the one Phenoclor DP-6 mixture had very similar δ37Cl values to those of the Aroclors (Table 1 and Figure 1), implying that the isotopic composition of commercial PCBs mixtures is not specific to the manufacturer or country of production. This similarity among different manufacturers of PCBs may be useful when investigating the transport and fate of PCBs in environmental 2868
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FIGURE 2. Plot of δ37Cl values of total PCBs versus concentration of total PCBs in the HR, NBH, and TRE sediments. The solid line is the mean δ37Cl value from the analysis of the 12 Aroclor mixtures (source). The dashed lines represent 1 and 2 standard deviations from the mean Aroclor δ37Cl values (1σ and 2σ, respectively). Error bars are based on the estimated precision of ∼0.13‰. samples impacted by PCBs from more than one manufacturer or country (e.g., global distillation). For this initial study, we assume that these 12 samples are representative of any Aroclor mixture manufactured by Monsanto that would have contaminated the HR, NBH, and TRE sediments. Ideally samples of the actual Aroclors that contaminated these sites would be used, but they are not available. However, based on the observed variability within available commercial mixtures, we hypothesize that any δ37Cl value for PCBs from sediments more than 2 standard deviations away from the mean Aroclor δ37Cl value is outside the variability of the source material. Based on this premise, we infer that samples exhibiting δ37Cl values beyond this range have been isotopically affected by some biological, chemical, and/or physical process. δ37Cl of PCBs Extracted from Sediments. The δ37Cl values of the total PCBs extracted from the sediment samples are given in Table 2 and compared to the source materials in Figure 2. The δ37Cl values of the PCBs from the HR samples range from -3.24 to -2.35‰ (n ) 5) and fall within 2 standard deviations of the δ37Cl values of the source material.
In the TRE sediments, which were contaminated with Aroclor 1268, the δ37Cl values ranged from -2.84 to -2.25‰ and are also within the 2 standard deviations of the mean δ37Cl values of Aroclors (Figure 2). For the NBH sediments, the δ37Cl values of the total PCBs ranged from -4.54 to -2.53‰; n ) 9 (Figure 2). Five samples yielded δ37Cl values within the source variability (-3.19 to -2.53‰, n ) 5); these tended to have the highest PCB concentrations and were collected in close proximity to the electrical capacitor plant that was the likely point source of these Aroclors in upper NBH. However, two samples clearly lie outside the range of the source variability (-4.54 and -4.12‰). These two samples were from the marsh in upper NBH (-4.54‰) and from lower NBH (-4.21‰) and have the lowest concentration of total PCBs (47 and 69 µg g-1, respectively). Both of these samples have similar congener profiles that show extensive losses of the lower chlorinated congeners, perhaps resulting from one or several alteration processes that preferentially acts on them. These results suggest that alteration processes likely induced a significant change in the δ37Cl value of the total PCBs found in the NBH sediments. Two other samples also yielded δ37Cl values with error bars that impinge on the source variability range (-3.64‰ and -3.50‰) but which appear to fall along a continuum of δ37Cl values (and PCB concentrations) for samples from NBH. The sample with a δ37Cl of -3.64‰ was from lower NBH and had a concentration of 98 µg g-1. The other sample (-3.50‰) was from upper NBH, with a total PCBs concentration of 1200 µg g-1. There are several other interesting trends in the total PCBs δ37Cl values for the different sediments. While the δ37Cl values are mostly encompassed within the limits of the apparent source variability (and hence, any explanations are speculative), these trends are beyond the precision of the measurement. For example, the δ37Cl values are clearly correlated with PCB concentration in the NBH and TRE sediments. While most of the sediment samples did fall within our estimate of the Aroclor isotopic variability, two of the NBH samples did appear to be isotopically distinct, indicating that environmental processes impart measurable differences in the δ37Cl values of PCBs. There are at least two explanations for why the two samples, which had lost the lower chlorinated congeners, were outside the Aroclor isotopic variability. The first possibility is that the environmental processes that preferentially removed the lower chlorinated congeners induced an isotopic fractionation effect that is manifested in the δ37Cl values of the remaining PCBs. The other possibility is that there may be compound-specific δ37Cl differences between individual Aroclor congeners as there are for carbon isotopes. Jarman et al. (15) showed that for any Aroclor mixture the δ13C values of each congener generally decreased with the amount of chlorines per biphenyl. That is, the least chlorinated congener has the most enriched δ13C value, and the most chlorinated congener in a mixture has the most depleted δ13C value. If a similar phenomena occurs for chlorine isotopes (i.e., the most chlorinated congeners are the most depleted in 37Cl), then the data for the 2 NBH samples could be simply explained by loss of the less chlorinated, isotopically enriched congeners. This would cause the residual total PCBs δ37Cl value to be depleted [as observed]. Hence, the change in the δ37Cl values in NBH would not be due to a true isotope effect but rather a change in the isotopic mass balance. To test this hypothesis, we isolated two individual congeners IUPAC-187 (2,2′,3,4′,5,5′,6-heptachlorobiphenyl) and IUPAC-206 (2,2′,3,3′,4,4′,5,5′,6-nonachlorobiphenyl) from Aroclor 1268 (Ultra) by PCGC (9). We chose to isolate individual compounds from Aroclor 1268 because its contains highly chlorinated congeners and hence the least amount of PCGC runs were needed to isolate sufficient material for isotopic analysis. These compounds were found
to exhibit the same δ13C trend observed by Jarman et al. (15). The δ13C values for IUPAC-103 and IUPAC-206 are -24.08 ( 0.28‰ and -27.51 ( 0.05‰, respectively (16). The δ37Cl values of the trapped IUPAC-187 and IUPAC-206 were -3.20‰ and -2.97‰, respectively. These values are essentially the same based on the precision of the measurement and are also very close to the δ37Cl value of the bulk Aroclor 1268 (-2.83‰). These results indicate that, in contrast to stable carbon isotopic compositions (15), there are no large congener-specific differences in stable chlorine isotope ratios in Aroclors. We conclude, therefore, that the δ37Cl values observed in the two NBH samples are not due to a change in the isotopic mass balance but more likely to result from an isotope effect associated with sample alteration. In summary, we have measured the δ37Cl values of several commercial mixtures of PCBs (Aroclors, Clophens, and Phenoclor) and PCBs isolated from Aroclor-contaminated sediments. The δ37Cl values of the Aroclors fall within a narrow range from -3.4 to -2.1‰ (mean and standard deviation, -2.78 ( 0.39‰, n ) 12) and yielded no correlation between the mass percent chlorine and the δ37Cl values. The δ37Cl values of PCBs extracted from the sediments ranged from -4.54 to -2.25‰ and most were within 2 standard deviations of the mean Aroclor value. Only two sediment samples from NBH were outside this range. The PCBs in these sediments were depleted in the less chlorinated congeners (when compared to the source material), and the isotope values likely reflect the alteration processes that acted upon these compounds. These isotopic variations suggest that δ37Cl could be a useful diagnostic tool for tracing the sources and fate of PCBs and indicate that additional research is warranted. Compound-specific δ37Cl measurements of two congeners in Aroclor 1268 suggest that there are no large intercongener differences in the stable chlorine isotopic composition of Aroclors. Since most chemical and physical processes act on a congener-specific basis, we suggest that future research should be directed at a molecular level. This would likely uncover signals carried by individual congeners that may be otherwise attenuated in measurements of total PCBs. Laboratory experiments designed to determine the chlorine isotope effects associated with volatilization, solubilization, photodegradation, biological uptake, and aerobic and anaerobic microbial degradation are needed. Such experiments should provide a framework for assessing the behavior of PCBs in the environment from an isotopic standpoint.
Acknowledgments We wish to thank Mr. Sean Sylva and Mr. Alex Sessions for their laboratory assistance and Drs. John Hayes and John Farrington for helpful discussions. Thanks are extended to Dr. John Brown of General Electric for his help with the Hudson River sediments. We also thank Dr. Jan Boon (NIOZ) for donating the Clophens and Phenoclor mixtures. This work was funded by WHOI NSF Grant OCE-9708478, Rinehart Coastal Research Center grant and the Environmental Management Science Program of the U.S. DOE, under contract W-31-109-Eng-38 to Argonne National Laboratory. This is WHOI contribution no. 10194.
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(13) van Warmerdam, Frape, S. K.; Aravena, R.; Drimmie, R. J.; Flatt, H.; Cherry, J. A. Appl. Geochem. 1995, 10, 547-552. (14) Jendrzejewski, N.; Eggenkamp, H. G. M.; Coleman, M. L. Anal. Chem. 1997, 69, 4259-4266. (15) Jarman, W. M.; Hilkert, A.; Bacon, C. E.; Collister, J. W.; Ballschmitter, K.; Risebrough, R. Environ. Sci. Technol. 1998, 33, 833-836. (16) Reddy, C. M. Woods Hole Oceanographic Institution, unpublished results.
Received for review July 21, 1999. Revised manuscript received April 4, 2000. Accepted April 18, 2000. ES9908220
