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Comment on “Indirect Photolysis of Perfluorochemicals: Hydroxyl Radical-Initiated Oxidation of N-Ethyl Perfluorooctane Sulfonamido Acetate (N-EtFOSAA) and Other Perfluoroalkanesulfonamides” Plumlee et al. (1) investigate the hydroxyl radical initiated indirect photolysis of various perfluoroalkyl sulfonamides. A substantial portion of this manuscript is dedicated to an analysis of pathways and mechanisms for the oxidation of these perfluorochemicals in aquatic systems. Unfortunately, the authors have neglected the acidity of primary and secondary amidic protons on a number of compounds under consideration. In Figure 2 of their article (1), the authors provide proposed pathways for the aqueous indirect photolysis of several well-known straight chain perfluorooctane sulfonamides, including N-ethylperfluorooctane sulfonamidoethanol (N-EtFOSE), N-ethylperfluorooctane sulfonamidoacetate (N-EtFOSAA), N-ethylperfluorooctane sulfonamide (N-EtFOSA), perfluorooctane sulfonamidoacetate (FOSAA), and perfluorooctane sulfonamide (FOSA) as well as two intermediate aldehydes designated “Aldehyde 1” (Nethylperfluorooctane sulfonamidoacetaldehyde; N-EtFOS-
AcetAld) and “Aldehyde 2” (perfluorooctane sulfonamidoacetaldehyde; FOSAcetAld). In addition to the carboxylate groups on N-EtFOSAA and FOSAA, which the SPARC software program (http://ibmlc2.chem.uga.edu/sparc/; v.4.2 August 2007 release w4.0.1219-s4.0.1219) estimates to have pKa values of about 3.9, the amidic protons on the following compounds are also expected to yield environmentally relevant acidity constants (pKa values estimated by SPARC (2)): N-EtFOSA, pKa ) 9.0; FOSAcetAld (“Aldehyde 2”), pKa ) 7.0; FOSAA, pKa ) 7.4; and FOSA, pKa ) 6.2. The authors did not specify the pH values for their experiments; thus, it is unclear which substrate species were dominant in solution. At an assumed pH of 7 (Plumlee et al. (1) state they are working in “Milli-Q” or “HPLC-grade” water), the amidic protons of N-EtFOSA, FOSAcetAld (“Aldehyde 2”), FOSAA, and FOSA would be about 1%, 50%, 29%, and 86% dissociated, respectively. Consequently, some of the starting and intermediate compounds in their study would have had significant potential reactivity contributions via the amidic anions, affecting the degradation pathways shown in Figure 2 of ref 1 as well as the mechanisms given in Figures 3 and 4 of ref 1, all of which only consider reactivity from the undissociated amidic forms. As an example, we provide in Figure 1 herein a revised version of Figure 2 from ref 1, whereby the possible speciation for each compound along
FIGURE 1. Revised proposed pathways for the aqueous indirect photolysis of perfluoroalkyl sulfonamides via reaction with hydroxy radicals. Modified from ref 1 to include environmentally relevant speciation of acidic moieties. 10.1021/es9022464 CCC: $40.75
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each degradation pathway proposed in ref 1 is shown [note: other reactants and products for each net reaction (e.g., O2, H2O, · OOH, RCHO, RCOOH, etc.) are not given for brevity as these would differ for the molecular and ionized compounds at each step. Readers are referred to Figure 2 in ref 1 for further detail on additional proposed reactants and products for the undissociated forms; pKa range for PFOA from ref 3]. It is also important to note that the molecular form of the carboxylate groups in N-EtFOSAA and FOSAA cannot be ignored in some natural aquatic and atmospheric systems. Even at pH values of 5.5, about 2.5% of the carboxylate groups are undissociated. The absorbance spectra for the perfluoroalkyl sulfonamides given in Figure 1 of ref 1 were obtained in pure methanol, which would significantly suppress amidic proton ionization (e.g., the pKa of FOSA in methanol is estimated by SPARC at 7.9, about 1.7 units higher than estimated in water). Previous work by the groups of Arnold and McNeill on nonfluorinated sulfa drugs (4) suggests significant (i.e., up to tens of nanometers) hypsochromic shifts in moving from the neutral to anionic amidic moiety. If this effect can be extrapolated to the perfluoroalkyl sulfonamides, it would imply these compounds absorb even less solar radiation (and also at the water/wastewater treatment relevant about 254 nm wavelengths) than Figure 1 in ref 1 suggests. Conversely, if the perfluoroalkyl sulfonamides exhibit bathochromic shifts upon amidic ionization, these compounds could absorb significantly more radiation at environmentally relevant wavelengths and be subject to direct photolysis. At present, there remains ambiguity in this regard. If solubility issues preclude acquisition of perfluoroalkyl sulfonamide absorbance spectra in pure water, spectra could be obtained in progressively more aqueous methanolic solutions and any solvatochromic effects extrapolated (with appropriate caveats) to those expected in natural aquatic systems. In addition, the pseudo first-order decay rate constants for reactions of these perfluoroalkyl sulfonamides with hydroxyl radicals (kOH) given in Table 1 of ref 1 are likely pH dependent, meaning the results given by Plumlee et al. represent composite values of the molecular and dissociated forms for their currently unspecified solution pH values. Natural waters contain a range of pH values from 8 in marine systems and some saline lakes. The data given in ref 1 cannot reliably and generally extrapolated to these matrices without additional studies to determine the specific mechanisms, rates,
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and product distributions for the molecular and ionized forms of each compound, whose reactivities toward hydroxyl radicals may vary substantially.
Literature Cited (1) Plumlee, M. H.; McNeill, K.; Reinhard, M. Indirect photolysis of perfluorochemicals: Hydroxyl radical-initiated oxidation of N-ethyl perfluorooctane sulfonamido acetate (N-EtFOSAA) and other perfluoroalkanesulfonamides. Environ. Sci. Technol. 2009, 43, 3662–3668. (2) (a) Hilal, S. H.; Karickhoff, S. W.; Carreira, L. A. A rigorous test for SPARC’s chemical reactivity models: Estimation of more than 4300 ionization pKas. Quant. Struc. Act. Relat. 1995, 14, 348–355. (b) 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. (c) 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, 13, in press. (3) (a) Goss, K. U. The pKa values of PFOA and other highly fluorinated carboxylic acids. Environ. Sci. Technol. 2008, 42, 456–458. (b) Burns, D. C.; 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, 9283–9288. (c) Rayne, S.; Forest, K.; Friesen, K. J. Computational approaches may underestimate pKa values of longer-chain perfluorinated carboxylic acids: Implications for assessing environmental and biological effects. J. Env. Sci. Health A 2009, 44, 317–326. (d) Cheng, J.; Psillakis, E.; Hoffmann, M. R.; Colussi, A. J. Acid dissociation versus molecular association of perfluoroalkyl oxoacids: Environmental implications. J. Phys. Chem. A 2009, 113, 8152–8156. (4) Boreen, A. L.; Arnold, W. A.; McNeill, K. Photochemical fate of sulfa drugs in the aquatic environment: Sulfa drugs containing five-membered heterocyclic groups. Environ. Sci. Technol. 2004, 38, 3933–3940.
Sierra Rayne Ecologica Research, Penticton, British Columbia, Canada V2A 8J3
Kaya Forest Department of Chemistry, Okanagan College, Penticton, British Columbia, Canada V2A 8E1 ES9022464
