Correspondence Response to Comment on “Comparative Assessment of the Global Fate and Transport Pathways of Long-Chain Perfluorocarboxylic Acids (PFCAs) and Perfluorocarboxylates (PFCs) Emitted from Direct Sources” In response to the Comment regarding our recent study (1), we first note that nearly all of the criticisms made by Raynes and Forest are related to property data estimated with SPARC from Arp et al. (2), while the results and discussion in the main text of ref 1 are primarily based on model simulations conducted with property data estimated using COSMOtherm v2.1. Model outputs using property data generated by SPARC were used to evaluate uncertainty in our model assessment due to uncertainty in partitioning properties and are presented mainly in the Supporting Information of ref 1. Selected Air-water Partitioning Coefficients (KAW). Raynes and Forest cite empirical log KAW values of -2.99 ( 0.04 from Li et al. (3) and -3.61 ( 0.07 (assuming pKa ) 2.8) and -3.90 ( 0.02 (assuming pKa ) 1.3) calculated from data in Kutsuna and Hori (4) as evidence for substantial bias in the values for PFOA published in ref 2. However, the Henry’s law constant (KH) values reported in ref 4 are 9.9 ( 1.5 mol dm-3 atm-1 (assuming pKa ) 2.8) and 5.0 ( 0.2 mol dm-3 atm-1 (assuming pKa ) 1.3) which, when converted to log KAW, actually correspond to values of approximately -2.4 and -2.1 respectively. The favorable agreement between these experimental values and the COSMOtherm estimate (-2.37) was explicitly discussed in ref 4. Our interpretation remains that the COSMOtherm estimate for PFOA selected in ref 1 is acceptable and does not require adjustment. The SPARC value (-1.69) deviates more substantially from available data but is still reasonably consistent with ref 4. Furthermore, the COSMOtherm and SPARC output provide a complete and self-consistent set of the physicochemical properties required as input to model the neutral species. Selectively adjusting only some of the property values generated by the software is undesirable because consistency within and between each homologue could be compromised. Selected Octanol-water Partition Coefficients (KOW). For reasons discussed in the Supporting Information, we are not convinced that comparisons between the data in Jing et al. (5) and SPARC log D calculations are appropriate with respect to the absolute values. Regardless, assuming the comparisons presented by Raynes and Forest in their Comment are valid, a more obvious response to the claim that the log KOW estimates from ref 2 have substantial positive biases is to recognize that the original SPARC and COSMOtherm values are approximately 1.5-2.5 and 2-5 orders of magnitude lower than the current SPARC estimates respectively and would presumably yield much lower log D values. “New” versus “Old” Property Estimates. Raynes and Forest characterize the physicochemical property values from SPARC used in ref 1 as “out-of-date” and “obsolete” and imply that these values can therefore be discredited. A better approach to evaluating the reliability of any softwaregenerated property estimates is to compare the calculated values to available empirical data. Unfortunately, such an exercise for these compounds is hindered by the paucity of measurements, particularly for the C9-C13 homologues. However, as demonstrated by Raynes and Forest in their Comment, the log KAW for the linear C8 homologue generated 10.1021/es902229g CCC: $40.75
Published on Web 08/12/2009
2009 American Chemical Society
by New SPARC (-1.36) is in poorest agreement with the empirical data in comparison to the original COSMOtherm and SPARC estimates (2). In response to the suggestion by Raynes and Forest to confirm the “validity” of the original COSMOtherm estimates, we have obtained new estimates of log KAW and KOW for several PFCAs (Goss, K-U, Pers. commun; see the Supporting Information). The old values (2) are within 0.3 log units of the new values and we consider both sets of property values to be equally acceptable. We also note that Jing et al. (5) reported an experimentally derived fragmental contribution of the CF2 group to the calculated formal partition coefficient between octanol and water of ∼0.6 log units. This value yields information on the relative partitioning behavior of the homologues as a function of chain length. As discussed in ref 5, the corresponding values from COSMOtherm and SPARC output presented in ref 2 are ∼0.5 and 0.8, respectively. Both values are reasonably close to the experimental value but arguably COSMOtherm is in better agreement (as is the case for log KAW). Assuming that comparisons between the experimental and computational approaches are valid, the data in ref 5 provide support for our selected property values with respect to the influence of chain length on partitioning behavior. Linear versus Branched Isomer Property Estimates. While no distinction was made between branched and linear isomers with respect to physicochemical properties, it was not our intention to suggest that there actually are no differences. For example, the soil and sediment-water partitioning coefficients for linear and branched C1-C8 homologues calculated by Raynes and Forest (6) using ALOGPS and SPARC-derived property estimates have differences up to 0.3 log units (minor) and 2 log units (substantial) respectively. But, as also discussed in ref 6, empirically derived partitioning data for branched isomers are currently unavailable, preventing a rigorous evaluation of these estimates. Besides the lack of empirical measurements, the decision to make no distinction between isomers was based on the available data regarding the isomer profiles of commercial APFO and APFN. Commercial APFN is manufactured using a process which results exclusively in linear isomers whereas the historic production process for APFO results in a mixture containing ∼80% linear and ∼20% branched isomers (see refs 1 and 7). While branched C9-C13 isomers were present in very low amounts in commercial APFO produced historically (7), the information available to us suggests that the overwhelming majority of direct emissions of C9-C13 result from the use of commercial APFN, justifying our focus on linear isomers for these homologues. With respect to C8, the fact that ∼ 80% of emissions related to the production and use of APFO were of the linear isomer limits the potential bias introduced by our simplifying assumption. In summary, we believe that the specific criticisms raised by Rayne and Forest in their Comment are largely without merit. The broader point is that there are large uncertainties in the physicochemical property data available for PFCAs (and other highly fluorinated compounds). We agree entirely but nevertheless contend that the modeling results from ref 1 provide valuable insights into the potential behavior of these substances in the environment.
Acknowledgments We thank Kai-Uwe Goss for generating new physicochemical property estimates using COSMOtherm v2.1 and both KaiVOL. 43, NO. 18, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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Uwe Goss and Hans Peter Arp for valuable discussions related to these estimates and available empirical data.
Supporting Information Available Additional details including two tables. This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) Armitage, J. M.; Macleod, M.; Cousins, I. T. Comparative assessment of the global fate and transport pathways of longchain perfluorocarboxylic acids (PFCAs) and perfluorocarboxylates (PFCs) emitted from direct sources. Environ. Sci. Technol. 2009, DOI: 10.1021/es900753y. (2) Arp, H. P. H.; Niederer, C.; Goss, K. U. Predicting the partitioning behavior of various highly fluorinated compounds. Environ. Sci. Technol. 2006, 40, 7298–7304. (3) Li, H.; Ellis, D.; Mackay, D. Measurement of low air-water partition coefficients of organic acids by evaporation from a water surface. J. Chem. Eng. Data 2007, 52, 1580–1584. (4) Kutsuna, S.; Hori, H. Experimental determination of Henry’s law constant of perfluorooctanoic acid (PFOA) at 298 K by means of an inert-gas stripping method with a helical plate. Atmos. Environ. 2008, 42, 8883–8892.
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(5) Jing, P.; Rodgers, P. J.; Ameniya, S. High lipophilicity of perfluoroalkyl carboxylate and sulfonate: Implications for their membrane permeability. J. Am. Chem. Soc. 2009, 131, 2290– 2296. (6) Raynes, S.; Forest, K. Congener specific organic carbon normalized soil and sediment-water partitioning coefficients for the C1 through C8 perfluoroalkyl carboxylic and sulfonic acids. Nat. Precedings, 2009, DOI: 10.1038/npre.2009.3011.2. (7) Reagen, W. K.; Lindstrom, K. R.; Jacoby, C. B.; Purcell, R. G.; Kestner, T. A.; Payfer, R. M.; Miller, J. W. Environmental characterization of 3M electrochemical fluorination derived perfluorooctanoate and perfluorooctanesulfonate. Presented at SETAC North America 28th Annual Meeting, Milwaukee, 2007 (Available on request to J.M.A.).
James M. Armitage, Matthew MacLeod, and Ian T. Cousins Department of Applied Environmental Science (ITM), Stockholm University, SE-10691 Stockholm, Sweden, and Institute for Chemical and Bioengineering, Swiss Federal Institute of Technology, ETH Zurich, HCI G129, CH-8093 Zu ¨ rich, Switzerland ES902229G
