Correspondence Comment on “Aerosol Enrichment of the Surfactant PFO and Mediation of the Water-Air Transport of Gaseous PFOA” McMurdo et al. (1) address the hypothesis that aerosolmediated transport of perfluorooctanoate (PFO) is a pathway by which PFO can move from water bodies to the gas phase and undergo long-range atmospheric transport. Serious questions arise as to whether the data reported in the manuscript support this hypothesis. Despite the authors claim that three separate methods were used to detect the presence of gas-phase PFOA, PFOA was never measured directly in any of the three techniques. In each method air was passed through either an alkaline solution or an XAD bed. The collection medium was then either analyzed directly using liquid chromatography/mass spectroscopy (LC/MS), or an extract from the medium was analyzed using LC/MS. In either case the Micromass Quattro mass spectrometer used in the LC/MS method detects the PFO anion, or fragment(s) of this species; the instrument does not distinguish between PFOA and PFO. Therefore, these measurements do not confirm that gaseous PFOA was present in the exhaust from the system as PFOA was never measured directly in this study, moreover PFO present on aerosols would also be collected by the media used and detected by LC/MS. A direct measurement technique, such as Fourier Transform Infrared Spectroscopy, should have been used to confirm the presence of PFOA in the gas phase. The efficiency of the aerosol collection system used was not evaluated. The authors state that, “the absence of water from the aerosol breakthrough traps indicated the effective collection of gas-phase material.” An alternative, and more likely, explanation for this observation is that the breakthrough trap does not collect aerosol with significant efficiency. (The system used here is atypical for aerosol collection. Common examples of typical aerosol collection systems are filtration or inertial impaction devices.) No quantitative data are presented to document the collection efficiency of the atypical system used in this study. A commercially available optical particle counter, aerodynamic particle sizer, condensation nucleus counter, or inertial impactor could have been used to directly establish the efficiency of the aerosol collection system and confirm that the mass of PFO escaping the collection vessel on aerosols is minimal. Moreover, there is strong reason to believe that the aerosol collection system used may not have effectively removed aerosols from the gas stream. The “aerosol condensation” technique used is ostensibly based on thermophoretic deposition of aerosols from the warm gas stream to the cold vessel surface. Thermophoretic precipitators typically operate at temperature gradients of 1000 to 10,000 K cm-1 and at flow rates of a few mL min-1 (2). Based on the Supporting Information provided with the manuscript, the temperature gradient was likely far less than optimal as only an ice bath was used to cool the essentially ambient temperature gas stream, and the flow rate was between 50 and 100 mL min-1 which exceed the rates typically considered effective for aerosol collection. To generate aerosol the authors placed a piezoelectric ultrasonic aerosol-generating device into an aqueous solution containing PFO. Vecitis et al. (3) have shown that PFO is degraded to primarily CO2 and F- in aqueous solutions using 1232
9
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 43, NO. 4, 2009
such devices, with degradation rates as high as 2% per minute. The system used by McMurdo et al. (1) operated at 50 kHz. Figure 1 shows the first-order rate constant for PFO degradation when exposed to piezoelectric ultrasonic devices operated at frequencies between 20 and 1078 kHz. In light of these data, it is likely that some portion of the loss of PFO was a result of the ultrasonic-induced degradation of PFO. The authors report a 0.8% per hour loss of PFO from bulk solution when aerosol was generated and recycled back into solution. Vecitis et al. (3) also operated their system as a closed system, and aerosols were allowed to return to the bulk solution. A mole balance of F- was observed by Vecitis et al. (3) at all time points; an appropriate increase in moles of F- measured in the solution corresponded to the decrease in moles of PFO (Figure 3b in reference (3)). McMurdo et al. (1) should have measured F- during the experiment using a method with detection limits appropriate for the concentration of PFO in the system to evaluate the extent of ultrasonic-induced degradation. (Insufficient information was provided to evaluate the power density as well as the manufacturer of the ultrasonic system used so as to estimate the PFO degradation rate using the data of Vecitis et al. (3).) The authors suggest that differences in the physical/ chemical properties of branched versus linear PFO isomers would result in a significant increase in the ratio of linear to branched isomers in the environment. The authors do not state the magnitude of any differences nor do they show any data that such differences are large enough to explain observations such as the 95:5 ratio of linear to branched PFO isomers in Greenland polar bears (4). In ambient samples taken from a location near a 3M fluorochemical production facility, the linear-to-branched ratio was identical among the raw PFOA product and samples collected in river sediments downstream of the production location (5). It is unfortunate that the authors did not carry out a similar quantitative analysis to evaluate if the magnitude of their proposed linear/branched partitioning discrimination mechanism can explain the 95:5 ratio observed. Without such an evaluation, inferring that partitioning discrimination could transform the linear/branched ratio of PFOA originating from the electrochemical fluorination process from 78:22 to the 95:5 ratio observed in the Greenland samples would be unfounded.
FIGURE 1. Half-life of PFO in an aqueous solution as a function of the frequency of the piezoelectric ultrasonic device. All studies done at power density of 250 W/L using the experimental and analytical methods described in ref (3). 10.1021/es803031k CCC: $40.75
2009 American Chemical Society
Published on Web 01/21/2009
The magnitude of the marine aerosol to gas phase transfer process can be evaluated using data and recent measurements available in the literature. From acid/base chemistry the fraction RHA of an undissociated acid is equal to 1/(1 + 10pH-pKa). An RPFOA value of 3 × 10-8 is calculated using McMurdo et al.’s reported ocean pH of 7.5 and the thoroughly reviewed pKa of 0 for PFOA (6, 7). Such a low value of RPFOA is not surprising given that PFOA is a very strong acid. The concentration of PFOA in the open ocean, [PFOA]aq, is calculated by multiplying RPFOA by the average open ocean PFO concentration, [PFO]aq, of 144 pg L-1 from Yamashita et al. (9). This is valid since RPFOA ) 3 × 10-8 and [PFO]aq ≈ [PFO]aq + [PFOA]aq. The gas-phase concentration of PFOA in equilibrium with the open ocean, [PFOA]g, is calculated using [PFOA]aq and the measured Henry’s Law value for PFOA of 0.025 atm-L mol-1 (10). Thus the calculated value of [PFOA]g based on actual field data is 5 × 10-6 pg m-3. If the aerosol enrichment factor suggested by McMurdo (1) is correct then [PFOA]aerosol ≈ 55.7 [PFOA]aq and the maximum calculated value of [PFOA]g would be estimated to be 3 × 10-4 pg m-3. Clearly the equilibrium concentration of gaseous PFOA predicted from the marine aerosol to gas phase exchange process is very low. This value can be compared to the average total gas plus particle phase PFOA/PFO concentration of 0.9 pg m-3 measured by Jahnke et al. (11) in the marine boundary layer of the open ocean. (Note: In light of the recent work of Arp and Goss (8, 12) this value is likely dominated by gas adsorption artifacts, and therefore represents both gas and particle-phase PFOA/PFO.) Therefore a larger source of atmospheric PFOA/PFO exists than can be explained by the aerosol to gas phase mediated route described by McMurdo et al. (1).
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) Hinds, W. C. Aerosol Technology: Properties, Behavior, and Measurements of Airborne Particles, 2nd Ed.; John Wiley & Sons, Inc: New York, 1999; p 176.
(3) Vecitis, C. D.; Park, H.; Cheng, J.; Mader, B. T.; Hoffman, M. Kinetics and Mechanism of the Sonolytic Conversion of the Aqueous Perfluorinated Surfactants, Perfluorooctanoate (PFOA), and Perfluorooctane Sulfonate (PFOS) into Inorganic Products. J. Phys. Chem A. 2008, 112, 4261–4270. (4) De Silva, A. O.; Mabury, S. A. Isolating Isomers of Perfluorocarboxylates in Polar Bears (Ursus maritimus) from Two Geographical Locations. Environ. Sci. Technol. 2004, 38, 6538– 6545. (5) Reagen, W. K.; Lindstrom, K. R.; Jacoby, C. B.; Purcell, R. G.; Kestner, T. A.; Payfer, R. M.; Miller, J. W. Environmental Characterization of 3M Electrochemical Fluorination Derived Perfluorooctanoate and Perfluorooctanesulfonate. Platform no. 455, Milwaukee SETAC North America 28th Annual Meeting, November 2007. (6) Goss, K. U. The pKa values of PFOA and other highly fluorinated carboxylic acids. Environ. Sci. Technol. 2008, 42, 456–458. (7) Goss, K. U. Correction: The pKa values of PFOA and other highly fluorinated carboxylic acids. Environ. Sci. Technol. 2008, 42 (13), 5032. (8) Arp, H. P. H.; Goss, K. U. Irreversible sorption of trace concentration of perfluorocarboxylic acids to fiber filters used for air sampling. Atmos. Environ. 2008, 42, 6869–6872. (9) Yamashita, N.; Kannan, K.; Taniyasu, S.; Horii, Y.; Okazawa, T.; Petrick, G.; Gamo, T. Analysis of Perfluorinated Acids at Parts-Per-Quadrillion Levels in Seawater Using Liquid Chromatography-Tandem Mass Spectrometry. Environ. Sci. Technol. 2004, 38, 5522–5528. (10) Li, H. X.; Ellis, D.; Mackay, D. Measurements of low airwater partition coefficients of organic acids by evaporation from a water surface. J. Chem. Eng. Data 2007, 52, 1580–1584. (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° C). Environ. Sci. Technol. 2007, 41 (9), 3055–3061. (12) Arp, H. P. H.; Goss K. U. The Gas/Particle Partitioning Behavior of Perfluorocarboxylic Acids in Terrestrial Atmosphere. Platform no. 396, Tampa Bay SETAC North America 29th Annual Meeting, November 2008.
Brian T. Mader 3M Company, Environmental Laboratory, Building 260-5N-17, St. Paul, Minnesota, 55144 ES803031K
VOL. 43, NO. 4, 2009 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
9
1233
