Environ. Sci. Technol. 2009, 43, 6112
Comment on “Perfluoroalkyl Contaminants in an Arctic Marine Food Web: Trophic Magnification and Wildlife Exposure” Kelly et al. (1) examine the trophic level magnification and wildlife exposure for a suite of halogenated contaminants, including a number of individual straight chain perfluoroalkyl compounds. The authors cite a 2006 study by Arp et al. (2) as the source of SPARC calculated log Kow and log Koa partitioning constants, and personal communication with Dr. David Ellis at Trent University as evidence that SPARC overestimates the log Kow values of PFCAs by one unit. In their Table S3, Kelly et al. quote their “adjusted” log Kow for PFTA at 8.8, only 0.2 units below their SPARC reported value from Arp et al. of 9.0, while all other ∆log Kow values between the reported SPARC values by Arp et al. and the “adjusted” log Kow values by Kelly et al. are one unit. This difference in log Kow value adjustment pattern for PFTA needs clarification. In addition, despite the use of the superscripts “c” and “d” in Table S3, there are no corresponding footnotes to clearly indicate where the log Koa values came from or how the Kpw and Kpa partition constants were calculated. The log Koa values in Table S3 for PFHpA through PFUnA appear to be the “New SPARC” values reported by Arp et al., but Arp et al. do not report either log Kow or log Koa values for PFDoA or PFTA. The log Koa values for PFOS and PFOSA (7.8 and 8.4) do not match those reported by Arp et al. (6.2 and 4.3). Kelly et al. appear to be using the COSMOtherm log Koa values for PFOS and PFOSA from Arp et al., without explaining whether this is intentional or in error. Kelly et al. then proceed to use the values presented in Table S3 to make some important conclusions regarding the environmental behavior of these compounds, such that PFOA, PFNA, and PFOS are “low Kow-high Koa compounds (Kow < 105, Koa > 106)...expectedtoonlybiomagnifyinair-breathingwildlife”.Given the ease of computing log10 Kow and Koa values with SPARC using the SMILES molecular language input, it is unclear why Kelly et al.quotethe2006SPARCcalculatedvaluesfortheninecompounds ofinterestfromArpetal.(althoughonlysevenofthesecompounds actuallyhavedatainArpetal.),ratherthanexaminethecorrelations between their experimental data and the current SPARC version. Even in 2006, Arp et al. clearly noted that SPARC calculated properties changed dramatically between the two versions of the program that existed over the course of their study (“spring 2005” versus “February 2006”). SPARC has been updated again since the work of Arp et al. Using the current version of SPARC (August 2007releasew4.0.1219-s4.0.1219),wehavecalculatedthefollowing log Kow and log Koa values for the PFCAs and PFSAs discussed in Table S3 by Kelly et al. (data presented as “Feb 2006”f“Apr 2009” SPARC values): log Kow, PFHpA (3.82f5.36), PFOA (4.59f6.26), PFNA (5.45f7.23), PFDeA (6.38f8.26), PFUnA (7.40f9.35), and PFOS (5.26f4.67); log Koa, PFHpA (5.93f7.39), PFOA (6.25f7.62), PFNA (6.55f7.81), PFDeA (6.82f7.96), PFUnA (7.07f8.06), and PFOS (6.17f6.02). There are substantial changes in the SPARC predictions using the historical 2006 data from Arp et al. and the 2007 SPARC release. The new SPARC data indicate, even assuming a one unit reduction in log Kow from the estimates, that PFOA and PFNA could be classified as high Kow-high Koa compounds, not “low Kow-high Koa compounds” as Kelly et al. state. Furthermore, PFOS could be classified under the approach of Kelly et al. as a potentially low Kow-low Koa compound, not an unequivocally low Kow-high Koa compound. Furthermore, PFOSA is erroneously described by Kelly et al. as a “neutral lipophilic chemical (log Kow ) 6.3, log Koa ) 8.4)”. These authors fail to recognize that PFOSA has acidic amide protons. 6112
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 15, 2009
ThisfactwaslikelyfirstrecognizedbySteinle-DarlingandReinhard (3) who quoted, at the time in 2008, a SPARC calculated pKa of 6.52 for this amide group. The current version of SPARC gives a pKa of 6.24 for PFOSA. In our broader study on the acidity of primary and secondary amide groups in both linear and branched perfluoroalkylsulfonamides, we have validated the SPARC pKa estimation method (4), suggesting that PFOSA will be substantially ionized in relevant fresh and marine waters and in physiological fluids such as the blood. Work published nearly three decades ago has clearlylinkedsulfonamideaciditytobiologicaluptakeandtransfer patterns in aquatic organisms (5), a body of literature ignored by Kelly et al. Consequently, PFOSA is not a “neutral lipophilic chemical”, and any such analyses based on this assumption are incorrect. Kelly et al. also make the statement that there are problems with “using a Kow based (hydrophobicity) approach for evaluating bioaccumulation potential of PFAs.” One must not confuse lipophilicity, a subset of hydrophobicity, with the more general concept of hydrophobicity. For example, it is widely stated that perfluoroalkyl groups are oleophobic and hydrophobic, meaning that hydrophobic compounds are not necessary lipophilic. As a number of studies have shown, increasing perfluoroalkyl chain length correspondingly increases the partitioning of PFAs onto/into biological materials. While the electrostatic interaction between the carboxylate or sulfonate headgroup and proteinaceous materials clearly plays a major role in the partitioning behavior of PFAs, so does the hydrophobic driving force from an increasing perfluoroalkyl chain length. Although differences in the perfluoroalkyl chain length may be important determinants for variation in the electrostatic character of the headgroup at short chain lengths, it does not appear reasonable that increasing the perfluoroalkyl chain length from C8 to C9 would affect the electrostatic character of the headgroup in such a manner and magnitude to explain the increase in biological partitioning behavior between these two compounds. Thus, the likely explanation for the difference in the biological partitioning behavior of longer chain PFAs is most probably a hydrophobic rationale. Namely, the electrostatic contribution of the headgroup to protein partitioning is essentially constant for all longer chain PFAs, and the increasing hydrophobicity of the perfluoroalkyl chain with increasing chain length is driving the corresponding increases in partitioning constants.
Literature Cited (1) Kelly, B. C.; Ikonomou, M. G.; Blair, J. D.; Surridge, B.; Hoover, D.; Grace, R.; Gobas, F. A. P. C. Perfluoroalkyl contaminants in an Arctic marine food web: Trophic magnification and wildlife exposure. Environ. Sci. Technol. 2009, 43 (11), 4037–4043. (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) Steinle-Darling, E.; Reinhard, M. Nanofiltration for trace organic contaminant removal: Structure, solution, and membrane fouling effects on the rejection of perfluorochemicals. Environ. Sci. Technol. 2008, 42, 5292–5297. (4) Rayne, S.; Forest, K. A new class of perfluorinated acid contaminants: Primary and secondary substituted perfluoroalkyl sulfonamides are acidic at environmentally and toxicologically relevant pH values. J. Env. Sci. Health A 2009, in press. (5) Lo, I. H.; Hayton, W. L. Effects of pH on the accumulation of sulfonamides by fish. J. Pharmacokinet. Biopharm. 1981, 9, 443–459.
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 ES9013079 10.1021/es9013079 CCC: $40.75
2009 American Chemical Society
Published on Web 07/09/2009
