Environ Sci Technol 43, 2009, 6112 .pdf
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Title: Comment on “Perfluoroalkyl Contaminants in an Arctic Marine Food Web: Trophic Magnification and Wildlife Exposure”
Author: Sierra Rayne† and Kaya Forest‡
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Environ. Sci. Technol. 2009, 43, 6112
Downloaded by CKRN CNSLP MASTER on July 31, 2009
Published on July 9, 2009 on http://pubs.acs.org | doi: 10.1021/es9013079
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 >
the ease of computing log10 Kow and Koa values with SPARC using
the SMILES molecular language input, it is unclear why Kelly et
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
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.
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 15, 2009
(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
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
(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,
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
10.1021/es9013079 CCC: $40.75
2009 American Chemical Society
Published on Web 07/09/2009