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Response to Comment on “Reevaluation of Air-Water Exchange Fluxes of PCBs in Green Bay and Southern Lake Michigan” We thank Goss et al. (1) for their interest in our published research detailing PCB Henry’s law constants (2, 3) and their application to modeling net air-water exchange fluxes in Lake Michigan (4). We agree that determining accurate and precise values of physical constants for environmentally relevant chemicals is both challenging and often neglected. Measurements are difficult, and care must be taken to ensure the validity of the results. Compiling and evaluating data and deriving consensus values are also extremely important activities and require a comparable level of diligence. We address the specific points raised by Goss et al. (1) below and provide more detail in the Supporting Information. In short, we believe that their comments result from an incomplete analysis of the available literature, inexact application of structure-activity relationships to highly hydrophobic chemicals, and a mischaracterization of our experimental design and resulting data. The thorough re-analysis of our previous studies prompted by the Goss et al. critique (1) did not result in any substantial changes to our original conclusions. (a) Comparing Our PCB Congener Henry’s Law Constants with the Literature. We agree that our KH values for the higher chlorinated congeners are larger than those reported by Murphy et al. (5) and Brunner et al. (6) but note that they are consistent with others in the literature (7). The differences are greatest for hepta-, octa-, and nonachlorobiphenyls [Murphy et al. (5) do not report KH values for congeners with eight or more chlorines]. These extremely hydrophobic chemicals are difficult to study experimentally and are generally handled poorly by computational methods; their physical properties, whether measured or modeled, are likely to be the least certain. Nonetheless, our values are quite consistent with those of Burkhard et al. (7), who predicted KH values for each PCB congener from the ratio of the liquid (or subcooled liquid) vapor pressure and aqueous solubility (see Table 2 in the Supporting Information for ref 3). The Burkhard et al. data were calculated using thermodynamic models of liquid vapor pressures and aqueous solubilities, and one might interpret the agreement between their values and our measurements as a form of the “thermodynamic compatibility” sought by Goss et al. We are not sure why the Burkhard et al. (7) enthalpy data were used by Wania’s group in their review paper (8), but the corresponding KH values were not used. While our measured KH values are different than some in the earlier literature, after careful analysis of potential measurement errors and bias and comprehensive checks of our data for internal consistency, we conclude that this difference does not indicate problems with our data. (b) Potential Experimental Artifacts. We agree with Goss et al. (1) that laboratory experiments using aqueous solutions of extremely hydrophobic chemicals are difficult and prone to artifacts. During the development of our experimental protocol for measuring KH values, we carefully considered a wide range of potential artifacts. Goss et al. (1) suggest that adsorption of PCB congeners on the surface of individual bubbles rising through the water column enhances the resulting gas-phase PCB congener measurement. We disagree on both theoretical and empirical grounds. Assuming PCB enrichment at the bubble interface [which has to our 10.1021/es030710f CCC: $27.50 Published on Web 01/27/2004
2004 American Chemical Society
knowledge not been directly measured experimentally but rather inferred from studies that adsorb gas-phase organic compounds onto thin water layers (e.g., ref 9)], at equilibrium the fugacity of the congener is equal in the dissolved, interfacial, and gas phases. Since the presence of an interface does not alter the fugacity capacity of either the dissolved or gas phases, the equilibrium gas and dissolved PCB concentrations are identical to those in the absence of interfacial sorption. Therefore, the gas-phase PCB concentration in the rising bubble at the bulk air-water surface in our apparatus reflects Henry’s law partitioning. What happens to the PCBs that are enriched on the water side of the bubble surface when the bubble breaks the surface? Ample evidence demonstrates that breaking bubbles generate aerosols enriched with surface-active materials. We are aware of no mechanism by which PCB molecules organized on the water side of the bubble interface can be transported to the overlying gas phase upon bubble breaking. In the absence of experimental data, it seems likely that those PCBs associated at the bubble interface would return to the bulk dissolved phase upon bubble bursting. Production of aerosol particles by breaking bubbles strongly depends on the presence of salt or surfactants in the water, a process exhaustively characterized in the marine chemistry literature (e.g., ref 10). In pure water such as used in our experiments, aerosol generation is limited to relatively large water droplets, and nearly all of these droplets quickly fell to the water surface and were mixed into the bulk solution. An extremely small fraction of the droplets transported with the air stream was prevented from reaching the polyurethane foam (PUF) trap by a glass impactor. Tests with dilute chloride solutions confirmed that the impactor prevented water droplets from reaching the PUF trap. Therefore, PCBs associated with these droplets were not inadvertently quantified as part of the gaseous PCB concentration. A very conservative estimate of potential artifacts due to PCB adsorption to droplets shows that the maximum possible positive bias in the gas-phase measurement (and therefore, Henry’s law constant) is on the order of 17% (see Supporting Information for calculation). We conclude, therefore, that neither theory nor experimental data support the contention by Goss et al. (1) that our measured Henry’s law constants are biased by factors of 2-70. (c) Variable Enthalpies among PCB Congeners. Goss et al. (1) suggest that the enthalpies of air-water exchange (∆HH) reported in our paper have unreasonably large variability among congeners and that the range of ∆HH is not plausible. We disagree based both upon the interactions of extremely hydrophobic molecules in aqueous solutions and upon a comparison of our data with those in the literature. Indirect estimates of ∆HH for the homologous series of chlorobiphenyls were calculated by Burkhard et al. (7) using the predicted temperature dependence of vapor pressures and solubilities. Seventeen out of the 26 ∆HH values for the PCB congeners estimated by Burkhard et al. (7) were higher than those that we measured experimentally. Only 7 PCB congeners in our study have ∆HH values much greater than 80 kJ/mol. In fact, our ∆HH values are similar and sometimes lower than those previously reported. The only other literature data of experimentally measured ∆HH for PCB congeners by ten Hulscher et al. (11) agree well with our data for the two PCB congeners common to both studies (50 vs 33 kJ/mol for PCB-28 and 52 vs 31 kJ/mol for PCB-52). The measured variation of ∆HH within each homologue group is not random but rather depends strongly upon the number of ortho chlorines (Figure 3 in ref 3). The position of the chlorine on VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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the biphenyl ring causes quantitative differences among the heats of phase transfer among the congeners within a homologue group. Falconer and Bidleman (12) found that congeners with fewer ortho-substituted chlorines have lower vapor pressures and greater dependencies on temperature (higher enthalpy of vaporization) within a homologue group. This effect is exactly what we observe in our ∆HH data. PCB congeners with ∆HH above 80 kJ/mol in our current study are hexachlorobiphenyls and octachlorobiphenyls. Goss et al. (1) estimate the enthalpy of vaporization (∆HVAP) for hexachlorobiphenyls as 80-90 kJ/mol. If ∆HVAP and the enthalpy of dissolution (∆HSOL) are known for a compound, ∆HH can be estimated from the difference between ∆HVAP and ∆HSOL. For many organic compounds, ∆HSOL is usually positive. As compounds become increasingly more polar, ∆HSOL approaches zero or may even be slightly negative. Therefore, for organic nonpolar compounds such as the PCB congeners, ∆HH values are typically less than or of similar magnitude as their corresponding ∆HVAP values. There are only a few literature values for ∆HVAP and even fewer for ∆HSOL For some of the hexachlorobiphenyls, ∆HVAP is estimated to be around 80 kJ/mol; however, ∆HVAP has also been estimated to be as high as 106 kJ/mol for hexachlorobiphenyls (7). In a review of the temperature dependence data for PCBs, Shiu and Ma (13) recommend ∆HVAP for PCB congeners between the hexachlorobiphenyl group and decachlorobiphenyl between 105 and 222 kJ/mol. Hence, a ∆HH greater than 80 kJ/mol is not unprecedented. Of the 26 measured temperature dependencies in our paper, only five are over 100 kJ/mol. For these five congeners, few experimental ∆HVAP are available, and there are no experimental ∆HSOL data. For PCB congeners 77 and 101, where ∆HVAP (13) and ∆HSOL (15) are available in the literature, the difference in these values compares very well with our experimental data:
PCB congener 77: ∆HVAP ) 87.2 kJ/mol (12) ∆HSOL ) 50.7 kJ/mol (14) ∆HH ) 39.8 kJ/mol (our study, 3) PCB congener 101: ∆HVAP ) 86.4 kJ/mol (12) ∆HSOL ) 31.9 kJ/mol (14) ∆HH ) 29.7 kJ/mol (our study, 3) Comparing ∆HH and ∆HVAP assumes that the energetics of phase change of a PCB vaporizing from a pure liquid is equivalent to a PCB partitioning between dissolved and vapor phases. Due to the dominating influence of the interactions (or lack thereof) between PCB and water molecules, the factors controlling ∆HH differ from those controlling ∆HVAP. Using ∆HH and ∆HSOL can give relatively accurate estimations of the ∆HH for various organic compounds if the entropy term is a relatively constant and minor contributorto the overall energy oftransfer. However, the total energy and all molecular scale factors that contribute to the overall free energy should be included, as these values cause the molecule to distribute between the phases. Both ∆H and ∆S interactions determine the magnitude of a compound’s phase distribution. For many organic compounds, ∆H values are compared because the transfer of an organic compound from one phase to another phase is governed primarily by enthalpy rather than entropy. For many organic compounds that do not exhibit strong intra- or intermolecular forces, ∆HVAP divided by the boiling point yields a ∆SVAP value between 80 and 85 J/mol‚K. However, organic compounds such as PCBs do exhibit intra- and intermolecular interactions, and the entropy term is nontrivial. Falconer and Bidleman (12) 1630
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obtained ∆HVAP and ∆SVAP from gas chromatographic retention data for 32 PCB congeners. The ∆SVAP for all 32 congeners were higher than those predicted by Trouton’s Rule and ranged between 96 and 110 J/mol‚K. Because of the substantial differences in electronegativity of hydrogen and chlorine atoms, PCB congeners can likely form hydrogen bonds. Relative to the gas phase, hydrogen-bonded solubilized compounds in liquids are more ordered; therefore, the transition from the liquid to the gas phase will likely result in a greater increase in entropy. For most of the PCB congeners measured in our work, ∆HVAP is the dominant factor in controlling the overall energy of transfer from the pure liquid to the gas phase. However, both ∆HH and ∆SH are important in the total energy of transfer from dissolved to gas phases due to substantial solvent-solute interactions. Therefore, both ∆H and ∆S need to be considered in order to compare the energies between vaporization, dissolution, and Henry’s law for PCB congeners. ∆HH and ∆SH are predominantly driven by interactions of the PCB congeners with surrounding water molecules. More energy is required to break and make bonds and to create cavity space for organic compounds in water, particularly for the larger molecules. These additional energies may cause ∆HH and ∆SH to be higher than ∆HVAP. Since the difference in energy is a combination of enthalpic and entropic contributions, both ∆H and ∆S interactions contribute significantly to the overall energy of phase transfer between vapor and solution for PCB congeners. Therefore, to assess the differences in energy between vaporization, dissolution, and Henry’s law, comparing the total free energy is more appropriate. Comparing the available ∆H, ∆S, and ∆G at 25 °C for 15 PCB congeners (see Supporting Information), the total free energy of KH is less than that of the total free energy of vaporization even for compounds where ∆HH is higher than ∆HVAP. This is due to the nontrivial entropic contributions to dissolved gas partitioning of these hydrophobic chemicals. While we appreciate the desire to develop rules constraining physical constants, we caution against extrapolating relationships developed with smaller, more polar molecules to constrain the aqueous behaviors of highly hydrophobic chemicals. Evaluating new measurements using empirical structure-activity relationships derived from prior measurements is particularly prone to bias. (d) Enthalpy-Entropy Compensation Effect. We agree with Goss et al. (1) that ∆HH can only be predicted from a single experimental ln KH value if a correlation between ln KH and ∆HH exists for a compound class. We measured KH for 26 PCB congeners at five temperatures, calculated ∆HH using the Gibbs-Helmholtz equation, and estimated compound-specific errors in both terms (3). In their Figure 2, Goss et al. (1) combined data from our paper across homologue groups, masking the within homologue correlations, and erroneously concluded that ln KH and ∆HH are not correlated. As described in our paper (3) and shown in Figure 1 here, significant correlations exist between ln KH and ∆HH within each PCB homologue group. This is clear evidence of enthalpy-entropy compensation and forms the basis for our model. The “specific numerical situation” referred to by Goss et al. (1) is not due to happenstance but rather to the enthalpyentropy compensation effect, a phenomenon that is welldocumented in the physical chemistry literature (e.g., ref 15). “Compensation” refers to the dampening in the variance in ∆G (and therefore KH) within a class of chemicals due to the covariance of ∆H and ∆S. For example, incremental changes in the thermodynamic functions of hydration of alkanes upon addition of CH groups demonstrate that ∆H - T∆S is almost completely compensated at both low (i.e., 273 K) and high (around 570 K) temperatures (ref 16, p 43). Hence, the increment in ∆GH for CH2 is nearly equal to zero,
FIGURE 1. Our data plotted by Goss et al. (their Figure 2) grouped by PCB homologue group, demonstrating the enthalpy-entropy compensation effect.
resulting in large and favorable hydration enthalpies and large and unfavorable entropies. Gallicchio et al. (17) prove this further for free energies, enthalpies, and entropies of hydration of the alkanes. For a series of alkanes, ∆H and T∆S show substantial systematic variation with surface area. Because of the compensation between ∆H and -T∆S, ∆G shows little change. This is similar to our results for PCB congeners. In both cases, changes in ∆G are fairly small relative to changes in ∆H and ∆S. Grunwald (15) discusses the potential error associated with this effect and proves that the compensation effect can be rationalized thermodynamically beyond “error” for processes in liquid solutions. The compensation effect is ubiquitous and observed for many classes of organic chemicals, including alkanes, alcohols, carbonyls, and nitroalkanes (see Supporting Information for examples and additional references). The Henry’s Law partitioning behavior that we measured within each PCB homologue group is consistent with the physical-chemical literature. Finally, Goss et al. (1) ask whether our calculation of the net air-water exchange fluxes of PCB congeners in Lake Michigan (4) fully considers all sources of uncertainty. First, we note that much of the discussion above about Henry’s law constants concerns congeners that contain six or more chlorines. These are present in relatively low concentrations dissolved in Lake Michigan water and in the overlying gas phase; therefore, they comprise only a small portion of the overall flux. Excluding these congeners changes the net t-PCB air-water flux by only 4% in Green Bay. The diffusional gradient in Green Bay is much larger than the uncertainty in Henry’s law constants, and net flux is clearly positive
(volatilization). In southern Lake Michigan, which is closer to equilibrium with respect to air-water exchange, excluding these same congeners has a larger impact on the t-PCB fluxes on a percentage basis because the fluxes are smaller, but the net flux (sum of all congeners) is positive for all sampling periods. We agree that these air-water exchange calculations benefit from an error analysis, as we have done in our previous publications (18, 19). We used the assumptions outlined by Hoff (20), Hoff et al. (21), and Bruhn et al. (22) and assumed a 0.3 coefficient of variation for H, consistent with the measurement errors reported by Bamford et al. for PCB congeners containing 1-5 chlorines (2). Excluding the high molecular weight congeners from the analysis, we can state with a high degree of confidence that the net flux of t-PCBs in Green Bay is positive in all samples. In southern Lake Michigan, the error analysis reveals that for about 70% of the measurements the concentration gradient was different from zero, with the net flux being positive in 98% of these instances. Thus we stand by the major conclusion of our paper (4) that the net flux of t-PCBs in Lake Michigan is predominantly positive and that water to air fluxes are more important for the removal of PCBs from both Green Bay and Lake Michigan than previously recognized. In Bamford et al. (4), we concluded that “the results presented in our study are the most comprehensive list of ... KH values and their dependency on temperature for all 209 PCB congeners”. While we welcome examination of our work and encourage others to explore our findings through additional experiments, we stand by these original conclusions. VOL. 38, NO. 5, 2004 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Supporting Information Available Text, table, and figure. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Goss, K. U.; Wania, F.; McLachlan, M. S.; Mackay, D.; Schwarzenbach, R. P. Environ. Sci. Technol. 2004, 38, 16261628. (2) Bamford, H. A.; Poster, D. L.; Baker, J. E. J. Chem. Eng. Data 2000, 49, 1069-1074. (3) Bamford, H. A.; Poster, D. L.; Huie, R.; Baker, J. E. Environ. Sci. Technol. 2002, 36, 4395-4402. (4) Totten, L. A.; Gigliotti, C. L.; Offenberg, J. H.; Baker, J. E.; Eisenreich, S. J. Environ. Sci. Technol. 2003, 37 (9), 1739-1743. (5) Murphy, T. J.; Mullin, M. D.; Meyer, J. A. Environ. Sci. Technol. 1987, 21, 155-162. (6) Brunner, S.; Hornung, E.; Santl, H.; Wolff, E.; Piringer, O. G.; Altschuh, J.; Bru ¨ ggemann, R. Environ. Sci. Technol. 1990, 24, 1751-1754. (7) Burkhard, L. P.; Armstrong, D. E.; Andren, A. W. Environ. Sci. Technol. 1985, 19, 590-596. (8) Li, N.; Wania, F.; Lei, Y. D.; Daly, G. L. J. Phys. Chem. Ref Data. (in press). (9) Raja, S.; Yaccone, F. S.; Ravikrishna, R.; Valsaraj, K. T. J. Chem. Eng. Data 2002, 47, 1213-1219. (10) Martensson, E. M.; Nilsson, E. D.; de Leeuw, G.; Cohen, L. H.; Hansson, H. C. J. Geophys. Res. Lett. 2003, 108 (D9), art. no. 4297. (11) ten Hulscher, T. E. M.; vander Velde, L. E.; Bruggeman, W. A. Environ. Toxicol. Chem. 1992, 11, 1595-1603. (12) Falconer, R. L.; Bidleman, T. F. Atmos. Environ. 1994, 28, 547554. (13) Shiu, W. Y.; Ma, K. C. J. Phys. Chem. Ref Data 2000, 29, 387462. (14) Dickhut, R. M.; Andren, A. W.; Armstrong, D. E. Environ. Sci. Technol. 1986, 20, 807-810. (15) Grunwald, E. Thermodynamics of Molecular Species; WileyInterscience: New York, 1997; Chapter 6. (16) Belouso, V. P.; Iu Panov, M. Thermodynamic Properties of Aqueous Solutions of Organic Substances; CRC Press: Boca Raton, FL, 1994; 386 pp. (17) Gallicchio, E.; Kubo, M. M.; Levy, R. M. J. Phys. Chem. B 2000, 104 (26), 6271-6285. (18) Nelson, E. D.; McConnell, L. L.; Baker, J. E. Environ. Sci. Technol. 1998, 32, 912-919. (19) Bamford, H. A.; Ko, F. C.; Baker, J. E. Environ. Sci. Technol. 2002, 36, 4245-4252. (20) Hoff, R. M. J. Great Lakes Res. 1994, 20, 229-239.
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(21) Hoff, R. M.; Strachan, W. M. J.; Sweet, C. W.; Chan, C. H.; Shackleton, M.; Bidleman, T. F.; Brice, K. A.; Burniston, D. A.; Cussion, S.; Gatz, D. F.; Harlin, K.; Schroeder, W. H. Atmos. Environ. 1996, 30, 3505-3527. (22) Bruhn, R.; Lakaschus, S.; McLachlan, M. S. Atmos. Environ. 2003, 37, 3445-3454.
Joel E. Baker Chesapeake Biological Laboratory University of Maryland Solomons, Maryland 20688-0038
Lisa A. Totten, Cari L. Gigliotti, and John H. Offenberg Department of Environmental Sciences Rutgers University New Brunswick, New Jersey 08903
Steven J. Eisenreich* JRC Institute for Environment and Sustainability I-21020 Ispra, Italy
Holly A. Bamford Office of Oceanic and Atmospheric Research National Oceanic and Atmospheric Administration 1315 East-West Highway Silver Spring, Maryland 20910
Robert E. Huie Physical and Chemical Properties Division National Institute of Standards and Technology 100 Bureau Drive, Stop 8381 Gaithersburg, Maryland 20899-8381
Dianne L. Poster Analytical Chemistry Division National Institute of Standards and Technology 100 Bureau Drive, Stop 8392 Gaithersburg, Maryland 20899-8393 ES030710F
