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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 their article, Armitage et al. apply the BETR Global multispecies mass balance model toward the fate and transport of C8 through C13 perfluorinated carboxylic acids (PFCAs) (1). The authors acknowledge that “[t]he fate of PFC(A)s in the environment is determined by their partitioning properties and speciation... [k]ey environmental partitioning properties include... Kaw... Kow... Koa.” These researchers then erroneously claim that “[h]ere we use what data are available in the literature.” Well before the submission date for this manuscript, Li et al. (2) and Kutsuna and Hori (3) had published experimentally determined Kaw values for the straight-chain perfluorooctanoic acid (n-PFOA), and Jing et al. (4) had published experimental octanol-water distribution coefficients (log D) for the straight-chain C4 through C10 PFCAs. Armitage et al. (1) fail to cite these studies or integrate the findings into their manuscript. Li et al. (2) found a dimensionless log Kaw of -2.99 ( 0.04 for n-PFOA, in reasonable agreement with the corresponding values of -3.61 ( 0.07 and -3.90 ( 0.02 reported by Kutsuna and Hori (3) at their assumed pKa values of 2.8 and 1.3, respectively. These Kaw values differ by up to 2.2 orders of magnitude compared to the SPARC (log Kaw)-1.69) and COSMOtherm (log Kaw)-2.37) estimates used by Armitage et al. (1). In addition, Armitage et al. (1) use the SPARC Kaw, Kow, and Koa values reported in 2006 by Arp and Goss (5) for the C6 through C11 PFCAs, and then perform linear extrapolation on this historical data to obtain corresponding values for the C12 and C13 PFCAs. As was already observed by Arp and Goss (5) in 2006, SPARC physicochemical property estimates were changing over time as this program is continuously being updated. Given the ease with which values can be calculated (http://ibmlc2.chem.uga.edu/sparc/), it is unclear why Armitage et al. chose not to verify these estimates as the 2006 SPARC physicochemical property estimates for PFCAs are now well out-of-date. Using the current version of SPARC (August 2007 release w4.0.1219-s4.0.1219), the Kaw, Kow, and Koa estimates (values reported as Armitage et al.fcurrent SPARC) for the straight chain C6 through C13 PFCAs are as follows: log Kaw, C6 (-2.48f-2.58), C7 (-2.15f-2.03), C8 (-1.69f-1.36), C9 (-1.12f-0.58), C10 (-0.45f0.30), C11 (0.33f1.29), C12 (0.95f2.39), and C13 (1.62f3.59); log Kow, C6 (3.12f4.50), C7 (3.82f5.36), C8 (4.59f6.26), C9 (5.45f7.23), C10 (6.38f8.26), C11 (7.40f9.35), C12 (8.30f10.50), and C13 (9.23f11.73); and log Koa, C6 (5.60f7.08), C7 (5.97f7.39), C8 (6.28f7.62), C9 (6.57f7.81), C10 (6.83f7.96), C11 (7.07f8.06), C12 (7.35f8.11), and C13 (7.61f8.14). The substantial differences in these SPARC estimates calls into question the relevance of any modeling results based on obsolete values. As well, the authors should use a current version of COSMOtherm to confirm the estimates from this program published in 2006 are still valid. Furthermore, the experimental log D data of Jing et al. (4) are not in good agreement with the current SPARC log D estimates at pH 6-7 (values reported as Jing et al.fcurrent SPARC): C6 ) 0.70f2.53, C7 ) 1.31f3.41, C8 10.1021/es9020469 CCC: $40.75
Published on Web 08/12/2009
2009 American Chemical Society
) 1.92f4.33, C9 ) 2.57f5.32, and C10 ) 2.90f6.36. The poor agreement between the experimental and SPARC log D values for these compounds strongly suggests that this program does not adequately model the octanol-water partitioning behavior for these compounds, and that the corresponding log P estimates (i.e., log Kow in the terminology employed by Armitage et al., which involves the partitioning of the purely molecular form of PFCAs at pH values much below the pKa values) by both SPARC and COSMOtherm have positive biases ranging up to several log units. The authors also state that “no distinction has been made between linear and branched isomers” with regard to all physicochemical properties and biological partitioning behavior used in their modeling approach. This assumption is clearly in sharp contradiction to work performed to date by the research community (6-13), and will have resulted in substantial modeling errors. As a result, with the out-of-date physicochemical property estimates by SPARC (and potentially COSMOtherm) and failure to include and compensate for the known Kaw and octanol-water partitioning behavior available for some straight-chain PFCAs, it is difficult to assess the accuracy or utility of the modeling results as currently presented in ref (1) without additional efforts by these authors to determine if their findings are still valid.
Literature Cited (1) Armitage, J. M.; Macleod, M.; Cousins, I. T. Comparative assessment of the global fate and transport pathways of long-chain perfluorocarboxylic acids (PFCAs) and perfluorocarboxylates (PFCs) emitted from direct sources. Environ. Sci. Technol. 2009, 43, 5830–5836. (2) 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. (3) 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. (4) Jing, P.; Rodgers, P. J.; Amemiya, S. High lipophilicity of perfluoroalkyl carboxylate and sulfonate: Implications for their membrane permeability. J. Am. Chem. Soc. 2009, 131, 2290–2296. (5) 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. (6) Rayne, S.; Forest, K.; Friesen, K. J. Computational approaches may underestimate pKa values of longer chain perfluorinated carboxylic acids: Implications for assessing environmental and biological effects. J. Environ. Sci. Health A 2009, 44, 317-326. (7) Rayne, S.; Forest, K.; Friesen, K. J. Estimated bioconcentration factors (BCFs) for the C4 through C8 perfluorinated alkylsulfonic acid (PFSA) and alkylcarboxylic acid (PFCA) congeners. J. Environ. Sci. Health A 2009, 44, 598-604. (8) Rayne, 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, http://dx.doi.org/10.1038/ npre.2009.3011.2. (9) Rayne, S.; Forest, K. An assessment of organic solvent based equilibrium partitioning methods for predicting the bioconcentration behavior of perfluorinated sulfonic acids, carboxylic acids, and sulfonamides. Nat. Precedings 2009, http:// hdl.handle.net/10101/npre.2009.3256.1. (10) Rayne, S.; Forest, K. A. comparative assessment of octanolwater partitioning and distribution constant estimation methods for perfluoroalkyl carboxylates and sulfonates. Nat. Precedings 2009, http://dx.doi.org/10.1038/npre.2009.3282.2. (11) De Silva, A. O.; Tseng, P. J.; Mabury, S. A. Toxicokinetics of perfluorocarboxylate isomers in rainbow trout. Environ. Toxicol. Chem. 2009, 28, 330–337. VOL. 43, NO. 18, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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(12) Benskin, J. P.; De Silva, A. O.; Martin, L. J.; Arsenault, G.; McCrindle, R.; Riddell, N.; Mabury, S. A.; Martin, J. W. Disposition of perfluorinated acid isomers in SpragueDawley rats; Part 1: Single dose. Environ. Toxicol. Chem. 2009, 28, 542–554. (13) De Silva, A. O.; Benskin, J. P.; Martin, L. J.; Arsenault, G.; McCrindle, R.; Riddell, N.; Martin, J. W.; Mabury, S. A. Disposition of perfluorinated acid isomers in SpragueDawley rats; Part 2: Subchronic dose. Environ. Toxicol. Chem. 2009, 28, 542–554.
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Sierra Rayne and Kaya Forest Ecologica Research, Penticton, British Columbia, Canada, V2A 8J3, and Department of Chemistry, Okanagan College, Penticton, British Columbia, Canada, V2A 8E1
ES9020469
