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Comment on “Degradability of an Acrylate-Linked, Fluorotelomer Polymer in Soil” Washington et al. (1) recently reported their results from a biodegradation study of an acrylate fluorotelomer polymer (AFTP) in a synthetic soil. In their manuscript, numerous references were made to a previous biodegradation study that we conducted for another AFTP in four field soils (2) following the OECD guideline for aerobic soil studies (3). We would like to correct a key misunderstanding of Washington et al. concerning our experimental results. Washington et al. characterized the ACN/water soil extraction used in our work as “ineffective” and suggested that incomplete removal of degradation products from soil resulted in calculated polymer degradation half-lives that “most likely do not characterize AFTP degradation rates”. We respectfully disagree with this assessment. In our study, the ACN/water extraction method that was used to remove fluorinated residuals and degradation products from the polymer/soil matrix had excellent recoveries. When our polymer was added to soil, the recovery of residual 8-2 FTOH was 137 ( 11% (slightly high but consistent), whereas the recovery of PFOA was 123 ( 40%. Although these recovery figures were not explicitly included in our publication, they can be readily calculated from Tables S3 and S16 in the Supporting Information provided with our study. These recovery values affirm that the ACN/water extraction methodology quantitatively extracted PFO from the polymer/soil matrix over time and provided valid experimental data to support our subsequent mathematical modeling. It should also be pointed out that high recoveries result in shorter calculated values of polymer half-life values, implying that the polymer half-life of 1200-1700 years that we reported may be conservative. The loss of perfluorocarboxylic acids (PFCAs) reported by Washington et al. and the computed degradation halflives of 16-130 days contradict a substantial body of published research that supports the understanding that PFCAs do not degrade biotically or abiotically in soil (ref 4 and references therein). PFCA degradation has only been reported under extreme conditions (e.g., incineration, strong
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UV irradiation, and high ultrasonic energy) (4). In our AFTP study, we did not observe a loss of PFCAs with time based on recoveries from aged soil. It should be noted that the PFCA degradation half-lives computed by Washington et al. are calculated from only two data points (at day 497 and day 546) and each of the two data points is based on a different extraction method. As a rule, valid degradation kinetic determinations should be based on at least six time points with uniform extraction and analysis at each time point (3). The study of Washington et al. provides further understanding of the substantial challenges confronted in studying the biodegradation potential of fluorotelomer-based polymers. Based on valid extraction results and kinetic modeling of the extensive set of biodegradation data in our study (four field soils, each with 12 time points extending over a period of 2 years), we reaffirm our published conclusion that the half-life of a commercial fluorotelomer-based acrylate polymer is between 1200 and 1700 years and that this source represents a small potential contributor to levels of PFOA in the environment.
Literature Cited (1) Washington, J. W.; Ellington, J. J.; Jenkins, T. M.; Evans, J. J.; Yoo, H.; Hafner, S. C., Degradability of an acrylate-linked, fluorotelomer polymer in soil. Environ. Sci. Technol. 2009, 43 (17), 6617-6623. (2) Russell, M. H.; Berti, W. R.; Szostek, B.; Buck, R. C. Investigation of the biodegradation potential of a fluoroacrylate polymer product in aerobic soils. Environ. Sci. Technol. 2008, 42 (3), 800–807. (3) OECD Guideline for the Testing of Chemicals. Aerobic and Anaerobic Transformation in Soil, Guideline 307; Organisation for Economic Cooperation and Development: Paris, 2002. (4) Vecitis, C. D.; Park, H.; Cheng, J.; Mader, B. T.; Hoffmann, M. R. Treatment technologies for aqueous perfluorooctanesulfonate (PFOS) and perfluorooctanoate (PFOA). Front. Environ. Sci. Engin. China 2009, 3 (2), 129–151.
Mark H. Russell*, Ning Wang, William R. Berti, Bogdan Szostek, and Robert C. Buck E. I. duPont de Nemours and Co., Inc., Newark, Delaware 19714 ES902348W
10.1021/es902348w
2010 American Chemical Society
Published on Web 12/11/2009
