Article pubs.acs.org/est
Atmospheric Oxidation of Polyfluorinated Amides: Historical Source of Perfluorinated Carboxylic Acids to the Environment Derek A. Jackson,† Timothy J. Wallington,‡ and Scott A. Mabury†,* †
Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, ON, Canada M5S 3H6 Ford Motor Company, Mail Drop SRL-3083, Dearborn, Michigan 48121 United States
‡
S Supporting Information *
ABSTRACT: Polyfluorinated amides (PFAMs) are a class of fluorinated compounds which were produced as unintentional byproducts in the electrochemical fluorination process used for polyfluorinated sulfonamide synthesis in 1947−2002. To investigate the historical potential of PFAMs as an atmospheric perfluorinated acid (PFCA) source we studied Nethylperfluorobutyramide (EtFBA) as a surrogate for longer chained PFAMs. Smog chamber relative rate techniques were used to measure bimolecular rate coefficients for reactions of EtFBA with chlorine atoms and hydroxyl radicals. It was found kCl = (2.08 ± 0.15) × 10−11 cm3 molecule−1 s−1 and kOH = (2.65 ± 0.50) × 10−12 cm3 molecule−1 s−1 and the atmospheric lifetime of EtFBA with respect to reaction with OH was estimated to be approximately 4.4 days. Offline sampling with both GC-MS and LC-MS/MS techniques was used to determine the products and hence a plausible pathway of atmospheric oxidation of EtFBA. Three primary oxidation products were observed by GC-MS, the N-dealkylation product C3F7C(O)NH2 and two carbonyl products, probably C3F7C(O)N(H)C(O)CH3 and C3F7C(O)N(H)CH2CHO. These primary products react further to give perfluorocarboxylic acids (PFCAs) as detected by LC-MS/MS, suggesting that eight carbon PFAMs were a historical source of PFCAs to remote regions, including the Canadian Arctic.
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INTRODUCTION From 1947 to 2002, the major production method for perfluorooctanoic acid (PFOA) was by the electrochemical fluorination (ECF) process with 3M being the dominant manufacturer.1 The ECF process used to make PFOA produces multiple byproducts, the most studied of which are branched constitutional isomers of PFOA. A multitude of monitoring studies have established isomeric PFOA as a worldwide human blood contaminant2−5 that does not degrade under environmentally relevant conditions. Therefore, understanding the sources of branched PFOA isomers to the environment, including human blood, has been a significant area of study. Recently, the polyfluorinated amides (PFAMs) have been discovered as historical environmental contaminants.6 These compounds were historically produced as unintentional byproducts of polyfluorinated sulfonamide synthesis by ECF7 and can serve as precursors of perfluorocarboxylic acids (PFCAs) including PFOA. The PFAMs have the general structure CxF2x+1C(O)N(H)(R) where x is an integer, typically 7 for historical materials and R is an alkyl group, either methyl (MeFOA when x = 7) or ethyl (EtFOA when x = 7). Such compounds could have served as historical PFOA precursors, both linear and branched, by biological hydrolysis of the amide bond.6 Since PFAMs are predicted to be more volatile than their sulfonamide analogs, partitioning to the atmosphere is © 2013 American Chemical Society
expected to be significant. Once in the atmosphere, they would be capable of undergoing oxidation reactions initiated by hydroxyl radicals (OH). The amide compounds MeFOA and EtFOA are analogs of the sulfonamides MeFOSA and EtFOSA, respectively. Previous studies to determine the atmospheric reactions of polyfluorinated sulfonamides have used the photochemical smog chamber at the Ford Motor Company.8,9 In the previous studies, a more volatile four carbon analog of the eight carbon material was studied to facilitate introduction into the smog chamber. It was found that EtFBSA (C4F9SO2N(H)CH2CH3) produces a homologous series of PFCAs as final degradation products by reaction with chlorine atoms.8 In the present study, a four carbon version of a historical PFAM compound (EtFBA, C3F7C(O)N(H)CH2CH3) was synthesized and the kinetics and pathway of atmospheric oxidation initiated by reaction with Cl atoms and OH radicals were studied. Specifically, our focus was to understand whether eight carbon PFAMs could have served as historical Received: Revised: Accepted: Published: 4317
February 6, 2013 April 5, 2013 April 15, 2013 April 15, 2013 dx.doi.org/10.1021/es400617v | Environ. Sci. Technol. 2013, 47, 4317−4324
Environmental Science & Technology
Article
⎛ [EtFBA]0 ⎞ k 5 ⎛ [reference]0 ⎞ ln⎜ ln⎜ ⎟ ⎟= ⎝ [EtFBA]t ⎠ k 7 ⎝ [reference]t ⎠
atmospheric precursors of PFOA and/or shorter chained PFCA homologues.
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EXPERIMENTAL DETAILS Chemicals. For information on the syntheses performed in the present study, including EtFBA, please refer to the Supporting Information (SI). Relative Rate Kinetic Experiments. Atmospheric oxidation experiments to determine the rates of reaction of EtFBA with Cl and OH were carried out at 296 K at the Ford Motor Company (Dearborn, MI) using a 140 L Pyrex photochemical smog chamber.10 Before use EtFBA was subjected by freeze− pump−thaw cycling to remove volatile impurities. Initial concentrations of EtFBA in the chamber experiments were 4.7−7.2 mTorr. The reference compounds ethylene (C2H4), acetylene (C2H2) or ethyl chloride (C2H5Cl) were present at an initial concentration of 4−10 mTorr. For oxidations using Cl atoms, chlorine gas (Cl2) was added to an approximate concentration of 120 mTorr. For oxidations initiated by OH radical, methyl nitrite (CH 3 ONO) was added to an approximate concentration of 100 mTorr. For reactions performed in the presence of NOx, nitric oxide (NO) was added to a concentration of 8 mTorr. Diluent air (ultra pure) was added to achieve a pressure inside the smog chamber of 700 Torr. Photochemical oxidations were initiated using 22 fluorescent blacklamps (GE F15T80BL, 365 nm) surrounding the chamber. Chlorine atoms were generated by the photolysis of molecular chlorine: Cl 2 + hν → 2Cl
where k5 is the desired rate coefficient for the reaction of Cl with EtFBA and k7 is the rate coefficient for the reference reaction. The slope of a plot of the decay of EtFBA versus the reference compound gives the rate coefficient ratio from which the rate of reaction with EtFBA can be determined. Typical UV irradiation times ranged from 5 to 10 s and were followed by acquisition of an infrared spectrum from which the loss of EtFBA and the reference compound were determined. The uncertainties reported for k5 and k6 include both two standard deviations in the slopes of the regression analyses and our estimate of potential systematic uncertainties in the spectral subtraction process which contributed approximately 5% uncertainty to the measure rate coefficient ratio. Control experiments were performed that showed there was no discernible (
