Environ. Sci. Technol. 2004, 38, 1624-1625
Response to the Comment on “Influence of Soot Carbon on the Soil-Air Partitioning of Polycyclic Aromatic Hydrocarbons”
TABLE 1. Comparison of KSA Values Predicted from Equations 1 and 2
Goss raises concern on the suitability of estimating the soot-air partition constant (KSA, L kg-1) for a given compound from its measured soot-water partition constant (KSW, L kg-1) and dimensionless Henry’s law constant (H′) by
log KSA ) log KSW/H′
No experimental measurements of KSA have so far been reported for PAHs. Therefore in our two recent papers, KSA values were estimated from eq 1 as the best available procedure, as similarly done by other authors when assessing the influence of soot carbon on gas-particle partitioning (3-5). However, after the submission date of our two papers, van Noort reported a thermodynamically based model to estimate KSA values for PAHs (6). Following van Noort (6) and introducing the appropriate unit conversion factors, KSA can be estimated from the subcooled vapor pressure (pL, Pa) and the soot-specific surface area (AEC, m2 g-1) by
(2)
Measurements of AEC of carbon black and those soot types contributing to aerosols, and thus atmospherically influenced soils, have been reported in several studies (6-8) and range from 59.1 to 370 m2 g-1. Table 1 compares the KSA values used by Ribes et al. (1) with those predicted by eq 2 using 59.1 and 370 m2 g-1 as an estimation for AEC. For phenanthrene and pyrene, the KSA values used in our recent papers are within the range of KSA values given by eq 2. For fluoranthene and chrysene, the used KSA are only slightly above and below those predicted from eq 2. Therefore, until experimental measurements of KSA are available, prediction of KSA by either eq 1 or eq 2 provide reasonably similar results, and both estimation methods seem feasible for assessing the influence of soot on soil-air and particlegas partitioning. In the example reported by Goss, application of eq 1 is not adequate for the estimation of the surface-air partition constant because water molecules interact strongly with the R-Al2O3 surface, and this results in an important contribution to the adsorption free energy. * Corresponding author e-mail:
[email protected]. 1624
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log KSA (L kg-1)b
log KSA (L kg-1)c
log KSA (L kg-1)d
0.069 0.0067 0.0049 0.000004
9.13 10.58 10.23 12.27
8.70 9.56 9.67 12.3
9.50 10.36 10.47 13.09
a From ref 13. b Predicted from eq 1 in ref 1. c Predicted from eq 2 assuming that AEC ) 59.1 m2 g-1. d Predicted from eq 2 assuming that AEC ) 370 m2 g-1.
(1)
Besides the introductory examples given by Goss, which will not be commented on since they are not specifically relevant to our two recent papers (1, 2), an example is given of the potential error in the determination of the surface-air partition constant if it is calculated from the surface-water partition constant and H′ for the particular case of naphthalene adsorption onto R-Al2O3. In Goss’ comment, alternative procedures to estimate KSA are not suggested, nor is there a specific discussion on the applicability of eq 1 in the case of adsorption of polycyclic aromatic hydrocarbons (PAHs) onto soot carbon.
log KSA ) -0.85 log pL + 8.94 - log(998/AEC)
phenanthrene fluoranthene pyrene chrysene
pL (Pa)a
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 38, NO. 5, 2004
However, soot has an hydrophobic character, and it interacts significantly with water only after aging due to the interactions of polar compounds adsorbed on the surface rather than to the soot itself (9). Thus, water molecules removed from the soot surface when a PAH adsorbs are likely to not contribute significantly to the overall adsorption free energy, which is dominated by the strong soot-PAH interactions. In addition to a weaker interaction of water with the soot surface, PAH affinity for the soot surface is several orders of magnitude higher than for R-Al2O3 (8, 10, 11). Further evidence of the potential suitability of eq 1 in the case of the PAH-soot system is given by the assessment of our field data. Goss and Schwarzenbach (12) have shown that comparison of absolute partition constants is not always the best method to assess sorption processes. Conversely, the assessment of the slope when plotting two partition coefficients for compounds with different physical-chemical properties can provide more reliable information about the similarity of two given sorbents (12). Figure 4 of one of our recent papers (2) shows that the plot of the predicted gas-particle partition constants (KP, m3 µg-1) from KSA versus field-derived KP for samples from the Baltimore and Chesapeake Bay atmosphere results in slopes not statistically different than one, suggesting that the predicted KSA from eq 1 reflect the mechanisms driving the field gas-particle partitioning of PAHs. Nevertheless, we agree that there is a need for experimental measurements to determine KSA for the different types of soot carbon, for a number of different organic compounds, and for different relative humidities. These measurements would allow scientists to validate further the different estimation models of KSA and gain a more detailed knowledge on the processes driving the environmental partitioning of PAHs and other organic pollutants.
Literature Cited (1) Ribes, S.; Van Drooge, B.; Dachs, J.; Gustafsson, O.; Grimalt, J. O. Environ. Sci. Technol. 2003, 37, 2675-2680. (2) Dachs, J.; Eisenreich, S. J. Environ. Sci. Technol. 2000, 34, 36903697. (3) Mader, B. T.; Pankow, J. F. Environ. Sci. Technol. 2002, 36, 5218-5228. (4) Ferna´ndez, P.; Grimalt, J. O.; Vilanova, R. M. Environ. Sci. Technol. 2002, 36, 1162-1168. 10.1021/es0307118 CCC: $27.50
2004 American Chemical Society Published on Web 01/24/2004
(5) Ngabe´, B.; Poissant, L. Environ. Sci. Technol. 2003, 37, 20942099. (6) van Noort, P. C. M. Environ. Toxicol. Chem. 2003, 22, 11791188. (7) Ludwig, S.; Schmidt, H. D. J. Chromatogr. 1990, 520, 69-74. (8) Bucheli, T. D.; Gustafsson, O ¨ . Environ. Sci. Technol. 2000, 34, 5144-5151. (9) Weingartner, E.; Burtscher, H.; Baltensperger, U. Atmos. Environ. 1997, 15, 2311-2327. (10) Jonker, M. T. O.; Koelmans, A. A. Environ. Sci. Technol. 2002, 36, 3725-3734. (11) Mader, B. T.; Goss, K.-U.; Eisenreich, S. J. Environ. Sci. Technol. 1997, 31, 1079-1086. (12) Goss, K.-U.; Schwarzenbach, R. P. Environ. Sci. Technol. 1998, 32, 2025-2032. (13) Mackay, D.; Shiu, W. Y.; Ma, K. C. Physical-chemical properties and Environmental fate handbook; CRC: Boca Raton, FL, 2000.
Jordi Dachs,* Sandra Ribes, Barend van Drooge, and Joan Grimalt Department of Environmental Chemistry (IIQAB-CSIC) Jordi Girona 18-26 Barcelona E-08034, Catalunya, Spain
Steven J. Eisenreich Institute for Environment and Sustainability Joint Research Centre Ispra I-20120, Ispra, Italy
Ørjan Gustafsson Institute of Applied Environmental Research Stockholm University Stockholm, Sweden ES0307118
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