Environ. Sci. Technol. 2009, 43, 1234–1235
Response to Comment on “Aerosol Enrichment of the Surfactant PFO and Mediation of the Water-Air Transport of Gaseous PFOA” We thank Dr. Mader for his comments and critical analysis of our recently published paper (1). Within the original manuscript we stated that, “Three separate methods were used to detect the presence of gas phase PFOA”. We agree that this was worded incorrectly and that the sentence should have read, “Three separate methods were used to collect gas phase PFOA”. The methods that were employed are well documented standard techniques for trapping volatile organics, including organic acids, from the gas phase (2-6). Mader points out that the use of LC/ MS/MS in the analysis of PFOA, by necessity, converts it to PFO, thus the structural identity of original compound must be inferred based on the method that was used for its preanalysis collection (note that this is the case for all neutral analytes measured by LC/MS/MS). Therefore, as the methods of collection were key to interpreting the analytes’ gas phase identity three independent gas collection techniques were employed. Furthermore, the PFO anion cannot move from the aqueous aerosol to the air to be collected by XAD. On these bases, it was indeed inferred that the original identity of the species was the gaseous neutral acid, PFOA. Despite Mader’s claim, the collection efficiency of the aqueous aerosols was indeed evaluated and reported in the original publication, “The volume of water in the system remained constant ruling out enhancement due to droplet evaporation” (1). Specifically, the volume of aerosols collected was equivalent to the volume of water lost from the bulk, i.e., total volume remained constant. The techniques and instrumentation suggested by Mader are used for solid particle aerosols, hence are not applicable to the measurement of the aqueous aerosols in our system. Although the reported experimental evidence (1) suggested that sonochemical degradation was insignificant (see Supporting Information), it was decided for the purpose of this response to seek definitive experimental proof that degradation of PFO under these conditions was not significant. A solution of PFO at 200 ppb was prepared at pH 7.5. The solution was sonicated for 24 h with the same piezoelectric device as before although no headspace was provided, and thus no aerosols were generated. Aqueous aliquots were removed hourly and analyzed for PFO concentration by LC/ MS/MS. No significant loss of PFO was observed. Hence, the mechanisms for loss of PFO from the bulk water required the formation of aerosols in our system. Sonochemical degradation was insignificant. We agree with Mader that a more thorough quantitative assessment of the differential abiotic transport potential of branched and linear PFCAs is warranted. For such a quantitative model, key physical properties for both the linear and branch species would need to be measured. This was outside the scope and purpose of the material presented within the published paper. However, from the literature (7, 8), and as inferred by Mader, there is a difference in the physical properties of linear and branched isomers. Therefore, from a qualitative standpoint they must have different transport potentials, as this is based upon their physical properties. Although, given the lack of published physical properties, it is not possible to conduct a full quantitative assessment of the magnitude of isomer fractionation between 1234
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temperate and arctic regions it can be shown, quantitatively, that the linear to branched ratio in rainwater is expected to differ significantly when considering only one of the pertinent governing physical properties: the pKa. For example, the predicted difference in the pKa for the linear PFOA and a branched isomer (2,3,3,4,4,5,5,6,6,6-decafluoro-2-(pentafluoroethyl)-hexanoic acid) is 0.97 (9) with the linear isomer having the higher value. For PFO(A) released as both linear and branched isomers in a ratio of 78:22, using the measured pKa of the linear isomer (pKa ) 3.8) and an estimated pKa for the branched isomer (pKa ) 2.8), in rainwater with a pH of 4.2 various fractions can be estimated. For the linear isomer 71.5% will be the anion while for the branched isomer 96.2% will be the anion. If, as we contend, only the neutral species is available to move into the gas phase, the isomeric ratio in the gas phase is will be a reflection not only of the released ratio but also the neutral-to-anionic ratio. Thus, the gas phase isomeric ratio is estimated to be approximately 96:4 for rainwater with a pH of 4.2. If it is assumed that only the fraction in the gas phase will be transported out of the immediate vicinity of the manufacturing and use, the isomeric ratio “released” to remote regions via transport of PFOA will be 96:4. In these remote regions, for rainwater with a pH of 5, the isomeric ratio in the gas phase is estimated to be 99.6: 0.4. Clearly, the difference in the physical property of pKa alone is sufficient to account for a substantial change in the isomeric ratio available to accumulate in Greenland polar bears and will be strongly influenced by the pH of the rain, and one would expect that a greater proportion of branched isomers would be observed in temperate rainwater than in rainwater in remote regions. Further, we note Regan et al. of 3M indicate that, “environmental samples [soil and water] associated with ECF production showed exclusively highly branched PFOA” whereas the production material lots contained 78% linear isomers (10), suggesting that major isomeric discrimination is occurring locally, contrary to the claim of Mader. Mader has attempted to model the magnitude of gas phase transfer of PFO from an aerosol reported in our study and to compare it with field observations (11). Unfortunately his model is excessively simple and fundamentally flawed in several key areas. (i) Our study was neither at equilibrium nor even at steady-state. Hence, in order to simulate it a dynamic model is and was required (1). Mader’s model assumes equilibrium. (ii) PFO is an established surface active agent. It is well documented that PFCAs dissociate at the water surface to form a surface active anion, that is to say, the parent acid is not a surfactant, but upon dissociation forms a surface active anion (12). Further, it is reported that, even at low concentrations, the anions effectively lower the surface tension of a medium by selective adsorption to the surface (12). This important physical property is not incorporated within Mader’s model where all water, whether bulk or aerosol droplet, is considered to be homogeneous in the concentration of PFO. This is not expected theoretically (7, 12) nor is it experimentally observed in the environment (13). (iii) It was assumed in Mader’s model that the pH of the bulk water is the same as the pH of the water at the air-water surface interface, this is reported not to be the case (14, 15). This would have a major impact on the species distribution of any acid-surfactant anion pair because the surfactant adsorbs preferentially to the air-water interface. (iv) And finally, a theoretical pKa for PFOA of 0 was employed in Mader’s model. Numerous experimental studies report that 10.1021/es8033654 CCC: $40.75
2009 American Chemical Society
Published on Web 01/21/2009
the pKa values of PFOA and other long chain PFCAs are much higher than zero (16-22). The recent work of Arp and Goss (23) cited by Mader on the use of fiber filters for PFCA air sampling in the environment is not relevant to our study as there was no particulate matter. Finally, we agree that aerosol-mediated water-air transfer is unlikely to be solely responsible for the PFO(A) observed in the atmosphere. However, we remain convinced that, as we stated in the original manuscript, aqueous aerosol generation is a source of PFOA to the atmosphere.
Supporting Information Available This material is available free of charge via the Internet at http://pubs.acs.org.
Literature Cited (1) McMurdo, C. J.; Ellis, D. A.; Webster, E.; Butler, J.; Christensen, R. D.; Reid, L. K. Aerosol enrichment of the surfactant PFO and mediation of the water-air transport of gaseous PFOA. Environ. Sci. Technol. 2008, 42, 3969–3974. (2) Gallego, E.; Roca, F. J.; Perales, J. F.; Guardino, X.; Berenguer, M. J. VOCs and PAHs emissions from creosote-treated wood in a field storage area. Sci. Total Environ. 2008, 402, 130–138. (3) Wei, M. C.; Chang, W. T.; Jen, J. F. Monitoring of PAHs in air by collection on XAD-2 adsorbent then microwave-assisted thermal desorption coupled with headspace solid-phase microextraction and gas chromatography with mass spectrometric detection. Anal. Bioanal. Chem. 2007, 387, 999– 1005. (4) Jonsson, B. A. G.; Welinder, H.; Pfaffli, P. Determination of cyclic organic acid anhydrides in air using gas chromatography.1. A review. Analyst 1996, 121, 1279–1284. (5) Yamamoto, M.; Kurihara, N.; Uchiyama, K.; Hobo, T. Determination of trace trimethylamine in ambient air by headspace gas chromatography-surface ionization detector. Bunseki Kagaku 2007, 56, 573–577. (6) Andersen, M. P. S.; Toft, A.; Nielsen, O. J.; Hurley, M. D.; Wallington, T. J.; Chishima, H.; Tonokura, K.; Mabury, S. A.; Martin, J. W.; Ellis, D. A. Atmospheric chemistry of perfluorinated aldehyde hydrates (n-CxF2x+1CH(OH) 2, x ) 1, 3, 4): Hydration, dehydration, and kinetics and mechanism of Cl atom and OH radical initiated oxidation. J. Phys. Chem. A. 2006, 110, 9854–9860. (7) Bernett, M. K.; Zisman, W. A. Surface properties of perfluoro acids as affected by terminal branching and chlorine substitution. J. Phys. Chem. 1967, 71, 2075–2082. (8) Kissa, E. Fluorinated Surfactants and Repellents, 2nd ed.; Dekker: New York, 2001. (9) Calculated using Advanced Chemistry Development (ACD/ Labs) Software V8.14 for Solaris (1994-2008 ACD/Labs). (10) Reagen, W. K.; Lindstrom, K. R.; Jacoby, C. B.; Purcell, R. G.; Kestner, T. A.; Payfer, R. M.; Miller, J. W. InSETAC North America 28th Annual Meeting, Milwaukee, 2007.
(11) Jahnke, A.; Berger, U.; Ebinghaus, R.; Temme, C. Latitudinal gradient of airborne polyfluorinated alkyl substances in the marine atmosphere between Germany and South Africa (53° N-33° S). Environ. Sci. Technol. 2007, 41, 3055–3061. (12) Kissa, E. Determination of Organofluorine in Air. Environ. Sci. Technol. 1986, 20, 1254–1257. (13) Ju, X. D.; Jin, Y. H.; Sasaki, K.; Saito, N. Perfluorinated surfactants in surface, subsurface water and microlayer from Dalian Coastal waters in China. Environ. Sci. Technol. 2008, 42, 3538–3542. (14) Vacha, R.; Buch, V.; Milet, A.; Devlin, P.; Jungwirth, P. Autoionization at the surface of neat water: is the top layer pH neutral, basic, or acidic? Phys. Chem. Chem. Phys. 2007, 9, 4736–4747. (15) Buch, V.; Milet, A.; Vacha, R.; Jungwirth, P.; Devlin, J. P. Water surface is acidic. Proc. Natl. Acad. Sci., U. S. A. 2007, 104, 7342–7347. (16) Burns, D. G.; Ellis, D. A.; Li, H.; McMurdo, C. J.; Webster, E. Experimental pKa Determination for Perfluorooctanoic Acid (PFOA) and the Potential Impact of pKa Concentration Dependence on Laboratory-Measured Partitioning Phenomena and Environmental Modeling. Environ. Sci. Technol. 2008, 42 (24), 9283–9288. (17) Ylinen, M.; Kojo, A.; Hanhijarvi, H.; Peura, P. Disposition Of Perfluorooctanoic Acid In The Rat After Single And Subchronic Administration. Bull. Environ. Contam. Toxicol. 1990, 44, 46– 53. (18) Moroi, Y.; Yano, H.; Shibata, O.; Yonemitsu, T. Determination of acidity constants of perfluoroalkanoic acids. Bull. Chem. Soc. Jpn. 2001, 74, 667–672. (19) Guo, W.; Brown, T. A.; Fung, B. M. Micelles And Aggregates Of Fluorinated Surfactants. J. Phys. Chem. 1991, 95, 1829– 1836. (20) Lopez-Fontan, J. L.; Sarmiento, F.; Schulz, P. C. The aggregation of sodium perfluorooctanoate in water. Colloid Polym. Sci. 2005, 283, 862–871. (21) Brace, N. O. Long Chain Alkanoic and Alkenoic Acids with Perfluoroalkyl Terminal Segments. J. Org. Chem. 1962, 27, 4491–4498. (22) Igarashi, S.; Yotsuyanagi, T. Homogeneous Liquid-LiquidExtraction By Ph Dependent Phase-Separation With A Fluorocarbon Ionic Surfactant And Its Application To The Preconcentration Of Porphyrin Compounds. Mikrochim. Acta 1992, 106, 37–44. (23) Arp, H. P. H.; Goss, K. U. InSETAC North America 29th Annual Meeting, Tampa, 2008.
David A. Ellis* and Eva Webster Department of Chemistry and Centre for Environmental Modelling and Chemistry, Trent University, 1600 West Bank Drive, Peterborough, Ontario, K9J 7B8 Canada * Corresponding author e-mail:
[email protected]; tel: (705) 748 1011; fax: (705) 748 1625.
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