Sci. Environ. Technol. 2003, 37, 3816-3820
Atmospheric Lifetime of Fluorotelomer Alcohols
D. A. ELLIS, J. W. MARTIN, AND S. A. MABURY Department of Chemistry, University of Toronto, 80 St. George Street, Toronto, Ontario, Canada M5S 3H6 M. D. HURLEY, M. P. SULBAEK ANDERSEN, AND T. J. WALLINGTON* Ford Motor Company, SRL-3083, P.O. Box 2053, Dearborn, Michigan 48121-2053
Relative rate techniques were used to study the kinetics of the reactions of Cl atoms and OH radicals with a series of fluorotelomer alcohols, F(CF2CF2)nCH2CH2OH (n ) 2, 3, 4), in 700 Torr of N2 or air, diluent at 296 ( 2K. The length of the F(CF2CF2)n- group had no discernible impact on the reactivity of the molecule. For n ) 2, 3, or 4, k(Cl + F(CF2CF2)nCH2CH2OH) ) (1.61 ( 0.49) 10-11 and k(OH + F(CF2CF2)nCH2CH2OH) ) (1.07 ( 0.22) 10-12 cm3 molecule-1 s-1. Consideration of the likely rates of other possible atmospheric loss mechanisms leads to the conclusion that the atmospheric lifetime of F(CF2CF2)nCH2CH2OH (n g 2) is determined by reaction with OH radicals and is approximately 20 d.
Introduction
Perfluorooctane sulfonate (PFOS) is a globally distributed, persistent, and bioaccumulative (1, 2) contaminant readily found in human blood and the tissues of many different species of marine animals and birds (3-5). Similarly, longchain perfluoroalkyl carboxylates (PFCAs with notation (CF3(CF2)xCOOH where x ) 6-12) have recently been observed in fish (6, 7) and mammals (8). Neither PFOS nor the longer chain PFCAs have generally been used directly in consumer or industrial materials other than in aqueous film-forming foams (AFFF) or as polymerization aids in fluoropolymer manufacture (9) and presumably are degradation products of precursor chemicals. The most likely precursors to PFOS are the electrochemically produced polyfluorinated sulfonamidoethanols that were widely used to covalently incorporate the PFOS backbone into polymeric materials (10) and have been identified in recent air sampling campaigns (11, 12). Concern over possible ecological and human health impacts associated with PFOS has led to the phase out of this compound and related chemistries in industrial processes and products. The source for the PFCAs observed is currently not known, although a potential source would include the polyfluorinated alcohols. The fluorotelomer alcohols (FTOHs) are linear chain polyfluorinated alcohols that are used in analogous applications as the sulfonamidoethanols where the ethanol moiety serves as the means to link the perfluorinated alkyl "tail" to various polymers. Fluorotelomer alcohols are currently used as intermediates in the manufacture of a diverse number of products that have a multitude of uses, including graphic
* Corresponding author phone: [phone]; fax: [phone]; e-mail: [e-mail].
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arts, paints, coatings, polymers, adhesives, waxes, polishes, metals, electronics, and caulks (10). The worldwide production (2000-2002) has been estimated at 5 106 kg yr-1, of which 40% is produced in North America. It is estimated that 80% of FTOHs are incorporated into polymeric materials with the remaining 20% used in nonpolymeric applications (13). The generic structure of the FTOHs used in the current study is F(CF2CF2)nCH2CH2OH (n ) 2-4). Fluorotelomer alcohols (FTOHs) are named according to the number of fluorinated carbons within the chain as compared with the number of hydrogenated carbon atoms (e.g., 4:2 FTOH ) F(CF2CF2)2CH2CH2OH). As a result of the telomerization manufacturing process, the fluoroalkyl chains tend to have even-numbered carbon chain lengths (e.g., C4, C6, ..., C16). 6:2 FTOH, 8:2 FTOH, and 10:2 FTOH have been measured in the troposphere in two recent studies. Martin et al. reported tropospheric concentrations in the range of 17-87 pg m-3 (11). Stock et al. observed levels up to 135 pg m-3 (12). The mechanism by which FTOHs gain entry into the environment is unknown at present. It has been speculated that release into the atmosphere occurs following biotic and/or abiotic degradation of polymer and nonpolymeric materials that incorporate FTOHs. Alternatively, release into the atmosphere may occur due to the presence of residual amounts of the fluorotelomer alcohol that was not covalently linked to the polymer. Fluorotelomer alcohols have low water solubility and high vapor pressure and are expected to partition into the atmosphere following release to the environment (14). In light of their substantial industrial use, FTOHs would be expected to be observed in the atmospheric gas phase. Environmental monitoring studies (11, 12) indicate that the compounds are indeed widely disseminated in the troposphere with higher concentrations at urban sites relative to rural areas. An assessment of the environmental impact of FTOHs requires information concerning their atmospheric persistence and degradation products. Although the atmospheric rate constant for the reaction of OH with primary alcohols is relatively well-established, there does not appear to be any information regarding the effect of a -(CF2CF2)nF group on the reactivity of OH radicals. The goal of the present investigation was to determine the reactivity of OH and Cl toward a series of FTOHs, estimate the atmospheric lifetime of FTOHs, and provide insight into the likely environmental fate of these fluorotelomer alcohols.
Experimental Section
Experiments were performed in a 140-L Pyrex reactor interfaced to a Mattson Sirus 100 FTIR spectrometer (15). The reactor was surrounded by 22 fluorescent blacklamps (GE F15T8-BL), which were used to photochemically initiate the experiments. Chlorine atoms were produced by photolysis of molecular chlorine:
Cl2 + h f Cl + Cl
(1)
OH radicals were produced by the photolysis of CH3ONO in the presence of NO in air:
CH3ONO + h f CH3O + NO CH3O + O2 f HO2 + HCHO HO2 + NO f OH + NO2
(2) (3) (4)
Relative rate techniques were used to measure the rate constant of interest relative to a reference reaction whose rate constant has been established previously. To study OH
10.1021/es034136j CCC: $25.00 2003 American Chemical Society Published on Web 07/12/2003
ENVIRONMENTAL SCIENCE & TECHNOLOGY / VOL. 37, NO. 17, 2003
radical kinetics, CH3ONO/NO/reactant/reference/air mixtures were irradiated using the UV fluorescent blacklamps. The relevant reactions in the system were (eqs 2-6):
OH + reactant f products OH + reference f products
(5) (6)
In such experiments, the loss of reactant and reference are given by
-d[reactant] ) k5[OH][reactant] dt -d[reference] ) k6[OH][reference] dt
Integration gives
ln
(
[reactant]t0 [reactant]t
)
) k5[OH]t
ln
(
[reference]t0 [reference]t
) )
) k6[OH]t
FIGURE 1. Loss of F(CF2CF2)nCH2CH2OH (squares, n ) 2; circles, n ) 3; triangles, n ) 4) vs C2H4 and C2H5Cl following exposure to Cl atoms in 700 Torr of either N2 (open symbols) or air (filled symbols) diluent at 296 K. Reagents were obtained from commercial sources at purities >99% and were subjected to repeated freeze-pumpthaw cycling before use. In smog chamber experiments, it is important to check for unwanted loss of reactants and products via photolysis, dark chemistry, and heterogeneous reactions. Control experiments were performed in which mixtures of reactants (except Cl2 or CH3ONO) in N2 were subjected to UV irradiation for 15-30 min (durations exceeding those used in the experiments described below) and product mixtures obtained after the UV irradiation of reactant mixtures were allowed to stand in the dark for 15 min. There was no observable loss of reactants or reference compounds, suggesting that photolysis and heterogeneous reactions are not a significant complication in the present work. Quoted uncertainties are two standard deviations from least squares regressions.
where [reactant]t0, [reactant]t, [reference]t0, and [reference]t are the concentrations of reactant and reference at times t0 and t; k5 and k6 are the rate constants for reactions 5 and 6; and [OH] is the OH radical concentration. The reactant and reference have equal exposure to OH radicals, hence:
ln
(
[reactant]t0 [reactant]t
)
)
[reference]t0 k5 ln k6 [reference]t
(
(I)
Plots of ln([reactant]t0/[reactant]t) versus ln([reference]t0/ [reference]t) should be linear, pass through the origin, and have a slope of k5/k6. The loss of F(CF2CF2)nCH2CH2OH (n ) 2, 3, or 4) and the reference compounds were monitored by FTIR spectroscopy using an infrared path length of 27 m and a resolution of 0.25 cm-1. Infrared spectra were derived from 32 co-added interferograms. F(CF2CF2)2CH2CH2OH and F(CF2CF2)3CH2CH2OH are liquids (vapor pressures of 1514 and 802 Pa at 298 K) and were introduced into the chamber by transferring the vapor above the liquid into calibrated volumes. The contents of the calibrated volumes were swept into the chamber using N2 or diluent air. F(CF2CF2)4CH2CH2OH is a solid (vapor pressure of 250 Pa at 298 K) and was introduced into the chamber by passing a fraction of the diluent gas over the solid sample. The concentrations of F(CF2CF2)2CH2CH2OH and F(CF2CF2)3CH2CH2OH in the chamber were calculated using the measured pressures of their vapor in the calibrated volumes. The concentration of F(CF2CF2)4CH2CH2OH in the chamber was estimated by comparison of its integrated absorption feature at 3550-3650 cm-1 (O-H stretch) with those of F(CF2CF2)2CH2CH2OH and F(CF2CF2)3CH2CH2OH, which have integrated band strengths of (3.5 ( 0.5) 10-18 cm molecule-1. Reactant and reference compounds were monitored using integrated absorption features over the following wavenumber ranges (cm-1): F(CF2CF2)nCH2CH2OH, 3550-3650; C2H4, 850-1050; C2H5Cl, 640-1350; and C2H2, 670-800. Initial concentrations of the gas mixtures for Cl atom relative rate experiments were 5-10 mTorr of F(CF2CF2)nCH2CH2OH, 5-15 mTorr of the reference compounds (C2H5Cl or C2H4), and 50-100 mTorr of Cl2 in 700 Torr of either N2 or diluent air. In the OH radical experiments, the initial reagent concentrations were 5-10 mTorr of F(CF2CF2)nCH2CH2OH, 5-8 mTorr of the reference compounds (C2H2 or C2H4), 100-300 mTorr of CH3ONO, and 0-28 mTorr NO in 700 Torr of diluent air. All experiments were performed at 296 K.
Results
Relative Rate Study of the Reaction of Cl Atoms with F(CF2CF2)nCH2CH2OH. The kinetics of reaction 7 were measured relative to reactions 8 and 9:
Cl + F(CF2CF2)nCH2CH2OH f products Cl + C2H4 f products Cl + C2H5Cl f products
(7) (8) (9)
Figure 1 shows the loss of F(CF2CF2)nCH2CH2OH versus loss of the reference compounds following the UV irradiation of F(CF2CF2)nCH2CH2OH /Cl2/reference mixtures in air or N2, diluent. Experiments were performed in which the reactivity of Cl atoms with F(CF2CF2)2CH2CH2OH, F(CF2CF2)3CH2CH2OH, and F(CF2CF2)4CH2CH2OH were measured relative to the reactivity of C2H5Cl. As seen in Figure 1, there was no discernible difference in the reactivity of the three different fluoro alcohols. The reactivity of F(CF2CF2)2CH2CH2OH was measured relative to that of C2H4 in 700 Torr of either N2 or air, diluent. As seen in Figure 1, there was no discernible effect of diluent gas on the kinetic data. The lines through the data in Figure 1 are linear least-squares fits to the combined data sets, which give values of k7/k8 ) 0.193 ( 0.033 and k7/k9 ) 1.78 ( 0.21. Using k8 ) 9.29 10-11 (16) and k9 ) 8.04 10-12 (17), we derive k7 ) (1.80 ( 0.31) 10-11 and (1.43 ( 0.17) 10-11 cm3 molecule-1 s-1. The kinetics
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of reaction 8 are dependent on total pressure, the value of k8 used is appropriate for 700 Torr (16). Results obtained using the two different reference compounds were, within the experimental uncertainties, indistinguishable. The fact that consistent values of k7 were derived from experiments using different reference compounds and diluent gas suggests the absence of significant systematic errors in the present work. We choose to cite a final value for k7 that is the average of the two determinations together with error limits that encompass the extremes of the individual determinations. Hence, k7 ) (1.61 ( 0.49) 10-11 cm3 molecule-1 s-1. We estimate that systematic uncertainties associated with the reference rate constants contribute an additional 10% uncertainty to k7. There are no previous data for reactions of Cl atoms with F(CF2CF2)nCH2CH2OH with which we can compare our results. We find that F(CF2CF2)2CH2CH2OH, F(CF2CF2)3CH2CH2OH, and F(CF2CF2)4CH2CH2OH have indistinguishable reactivities toward Cl atoms. This seems reasonable based upon expectations that the F(CF2CF2)n- group will not react with Cl atoms and that its influence on the reactivity of the two -CH2- groups will not change markedly upon increasing "n" from 2 to 4. Hence, the results presented herein can be generalized to k(Cl + F(CF2CF2)nCH2CH2OH) ) (1.61 ( 0.49) 10-11 cm3 molecule-1 s-1 for n g 2. Kinetic data are available for reactions of Cl atoms with several nonfluorinated alcohols. At ambient temperature, the reaction of Cl atoms with C2H5OH proceeds with a rate constant of (9.5 ( 1.9) 10-11 cm3 molecule-1 s-1 with 93 ( 2% of reaction occurring via attack on the -CH2- group (18). Reaction of Cl atoms with n-C3H5OH occurs with a rate constant of 1.5 10-10 cm3 molecule-1 s-1 (19). It is clear that the "fluorinated tail" of the F(CF2CF2)nCH2CH2OH molecule causes a substantial deactivation of the reactivity of the -CH2groups toward Cl atoms. This effect presumably reflects an increase in C-H bond strength caused by fluorination of the molecule. This observation is consistent with the body of literature data showing that Cl atoms react with hydrofluorocarbons at rates that are generally much lower than the rates of reaction with corresponding alkanes (19). Kelly and Sidebottom (20) have reported values of k(Cl + CF3CH2OH) ) (7 ( 1) 10-13 and k(Cl + CF3CH2CH2OH) ) (2.22 ( 0.08) 10-11 cm3 molecule-1 s-1 at 298 K. Papadimitriou et al. (21) have reported k(Cl + CF3CH2OH) ) (6.3 ( 0.9) 10-13 cm3 molecule-1 s-1 at 303 K. Comparison with the results of the present work suggest that the deactivating effect of the CF3- and F(CF2CF2)n- groups are very similar and that the majority of the reaction of Cl atoms with F(CF2CF2)nCH2CH2OH proceeds via attack on the -CH2- group R to the alcohol function. Relative Rate Study of the Reaction of OH Radicals with F(CF2CF2)nCH2CH2OH. The reactivity of OH radicals toward F(CF2CF2)2CH2CH2OH was studied relative to reactions 11 and 12. The reactivity of F(CF2CF2)3CH2CH2OH and F(CF2CF2)4CH2CH2OH were studied relative to reaction 11:
FIGURE 2. Loss of F(CF2CF2)nCH2CH2OH (squares, n ) 2; circles, n ) 3; triangles, n ) 4) vs C2H2 and C2H4 following exposure to OH radicals in 700 Torr of air diluent at 296 K. k11 ) 8.5 10-13 (22) and k12 ) 8.66 10-12 (23), we derive k10 ) (1.00 ( 0.13) 10-12 and (1.13 ( 0.16) 10-12 cm3 molecule-1 s-1. Within the experimental uncertainties, consistent results were obtained in experiments conducted using the two different reference compounds. choose We to cite a final value for k10 that is the average of the two determinations together with error limits that encompass the extremes of the individual determinations. Hence, k10 ) (1.07 ( 0.22) 10-12 cm3 molecule-1 s-1. We estimate that systematic uncertainties associated with the reference rate constants contribute an additional 10% uncertainty to k10. As was the case for the Cl atom kinetics, F(CF2CF2)2CH2CH2OH, F(CF2CF2)3CH2CH2OH, and F(CF2CF2)4CH2CH2OH have indistinguishable reactivities toward OH radicals. Using the logic presented in the previous section, it seems reasonable to generalize our result; for n g 2, k(OH + F(CF2CF2)nCH2CH2OH) ) (1.07 ( 0.22) 10-12 cm3 molecule-1 s-1. There have been no previous studies of the reactions of OH atoms with F(CF2CF2)nCH2CH2OH. However, our result can be compared with k(OH + CF3CH2OH) ) 1.0 10-13 (20, 24), k(OH + CF3CH2CH2OH) ) 1.1 10-12 (20), k(OH + CF3CF2CH2OH) ) 1.0 10-13 (25, 26), k(OH + CH3CH2OH) ) 3.2 10-12 (19), and k(OH + CH3CH2CH2OH) ) 5.5 10-12 cm3 molecule-1 s-1 (19). The presence of fluorinated substituents deactivate -CH2- groups toward reaction with OH radicals. In contrast, the -OH functionality provides an activating influence (27). Hence, the values of k(OH + CF3CH2OH) ) 1.0 10-13 and k(OH + CF3CF2CH2OH) ) 1.0 10-13 provide an estimate for the upper limit of the reactivity of the -CH2group to the alcohol function in F(CF2CF2)nCH2CH2OH. Combining this with our measured value of the overall reactivity of the molecule, k(OH + F(CF2CF2)nCH2CH2OH) ) (1.07 ( 0.22) 10-12 cm3 molecule-1 s-1, it can be concluded that the bulk (>90%) of reaction with OH is likely to occur at the R -CH2- group. Finally, it is of interest to compare the reactivity of OH radicals toward F(CF2CF2)nCH2CH2OH) measured herein with that predicted using the widely used structure-activity relationship (SAR) proposed by Kwok and Atkinson (27). The SAR method predicts k10 ) 4.2 10-12 cm3 molecule-1 s-1, which is a factor of approximately 4 higher than the measured value. As noted previously (25, 27), the SAR method has difficulty in predicting the reactivity of OH radicals with highly fluorinated compounds.
OH + F(CF2CF2)nCH2CH2OH f products OH + C2H2 f products OH + C2H4 f products
(10) (11) (12)
Figure 2 shows the loss of F(CF2CF2)nCH2CH2OH versus loss of the reference compounds on exposure to OH radicals. It can be seen from Figure 2 that there is no discernible difference in the reactivity of F(CF2CF2)2CH2CH2OH, F(CF2CF2)3CH2CH2OH, and F(CF2CF2)4CH2CH2OH relative to C2H2 reference. The lines through the data in Figure 2 are linear least-squares fits to the combined data sets, which give values of k10/k11 ) 1.18 ( 0.15 and k10/k12 ) 0.131 ( 0.018. Using
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Atmospheric Implications
The value of k10 measured here can be used to provide an estimate of the atmospheric lifetime of F(CF2CF2)nCH2CH2OH with respect to reaction with OH radicals. Assuming an atmospheric lifetime for CH3CCl3 with respect to reaction with OH radicals of 5.7 yr (28) and a rate constant for the CH3CCl3 + OH reaction of 1.0 10-14 cm3 molecule-1 s-1 (29) leads to an estimate of the atmospheric lifetime of F(CF2CF2)nCH2CH2OH of (1.0 10-14)/(1.1 10-12) 5.7 365 20 d. The optimal temperature for such a scaling analysis is 272 K (30) (rather than 296 K used here), but we do not have any data for k10 at 272 K. By analogy to other fluorinated organics (28), the temperature dependence of reaction 1 is expected to be similar to that for reaction of OH radicals with CH3CCl3. Hence, the use of 296 K rather than 272 K is not expected to have any material impact on the estimated atmospheric lifetime. The approximate nature of the atmospheric lifetime estimate provided here should be stressed. The average daily concentration of OH radicals in the atmosphere varies significantly with both location and season (31). The atmospheric lifetime of F(CF2CF2)nCH2CH2OH with respect to reaction with OH radicals will be substantially longer at high latitudes in winter than at low latitudes in summer. The lifetime derived herein is an estimate of the global average lifetime with respect to reaction with OH radicals. In addition to reaction with OH radicals, organic compounds are removed from the atmosphere via photolysis, wet deposition, dry deposition, and reaction with NO3 radicals, Cl atoms, and O3. For saturated compounds such as F(CF2CF2)nCH2CH2OH, reaction with NO3 radicals and O3 are typically too slow to be of importance. The average concentration of Cl atoms in the troposphere is several orders of magnitude less than that of OH radicals (32). In the present study, we observe that Cl atoms are 15 times more reactive than OH radicals toward F(CF2CF2)nCH2CH2OH. Loss via reaction with Cl atoms will not be a significant atmospheric loss mechanism for F(CF2CF2)nCH2CH2OH. Alcohols do not absorb at UV wavelengths >200 nm (33), and fluorination causes a blue shift in their UV spectra (34). Hence, photolysis of F(CF2CF2)nCH2CH2OH will not be important in the troposphere. The rate of wet deposition can be calculated for the 8:2 FTOH using the method first described by Brimblecombe and Dawson (35) and further refined by Shepson et al. (36). The method involves the assumption that wet deposition for the water-soluble species is a simple first-order loss process. We assume that the telomer is removed by wet deposition resulting from equilibration with water in precipitating stratus for mid-latitudes. The calculated air/water dimensionless Henry's law constant (H*), using a measured water solubility (Cw) of 148 g/L and a vapor pressure (Vp) of 250 Pa at 298 K, was found to be 316 (37). The first-order removal constant (kWD) was calculated using expression II:
length of the hydrophobic fluoro tail in the molecule decreases, which results in an increase in both water solubility and vapor pressure. In terms of Henry's law constant, the changes in water solubility and vapor pressure will approximately offset each other. Therefore, the Henry's law constants for the 4:2 and 6:2 FTOHs are expected to be comparable to that for 8:2 FTOH. In light of the very long (2.5 106 yr) estimated lifetime of 8:2 FTOH with respect to wet deposition, it seems reasonable to conclude that wet deposition is a negligible loss mechanism for F(CF2CF2)nCH2CH2OH. An estimation of the dry deposition velocity of the 8:2 alcohol to a body of water was conducted in a fashion similar to that outlined by van Pul et al. (38) using expression III:
d ) 1/((ra + rb + rs)
(III)
kWD ) (RrEe(-z/Zx))/(Zx(H* + )
(II)
where Rr is the annual rainfall rate, taken to be 3.2 10-8 m/s; E is the enhancement factor due to evaporation of falling rain droplets, taken to be 1.33; z is the characteristic height of stratus clouds (3.5 km); Zx is the scale height for the 8:2 FTOH (2.3 km), and is the fraction of air volume occupied by liquid water content (1 10-6). Using this method, the rainout constant was calculated to be 1.28 10-14 s-1. Thus, the expected lifetime (WD ) 1/kWD) of 8:2 FTOH with respect to wet deposition is estimated to be 2.5 106 yr. Relative to OH as a tropospheric sink, wet deposition is therefore expected to be an insignificant loss mechanism. Moving from 8:2 to the 6:2 and 4:2 FTOHs, the
where ra and rb are the resistances to transport for the layer, taken to be 192 and 67 s m-1, respectively. The rs constant for the surface resistance of a gaseous substance can be further defined as H*rw, in which rw is 105 s m-1. The dry deposition velocity of a compound to water is typically greater than that to land. As such, this can be taken as an upper limit to the significance of dry deposition occurring. Using this method, the dry deposition velocity (d) was calculated to be 8.69 10-6 m s-1, and thus the dry deposition rate (kDD ) d/Zx) can be estimated to be 3.78 10-9 s-1 and the expected lifetime due to this process to be in the order of =8.4 yr. As was the case for wet deposition, dry deposition is not expected to be a significant atmospheric loss mechanism for F(CF2CF2)nCH2CH2OH. The atmospheric lifetime of F(CF2CF2)nCH2CH2OH is determined by its reaction with OH radicals and is approximately 20 d. Several important conclusions can be drawn from this lifetime estimate. First, the combination of this lifetime with the observed atmospheric concentrations can be used to estimate the flux of F(CF2CF2)nCH2CH2OH into the atmosphere. Hence, assuming that these compounds are restricted to the Northern Hemisphere and are well-mixed in the troposphere (0-10 km), it can be calculated that a flux on the order of 100-1000 t yr-1 is needed to maintain the observed atmospheric concentrations (11). It should be stressed that this is a crude calculation and that more extensive spatial sampling of the FTOHs is required to refine this estimate. Second, given its relatively short atmospheric lifetime, F(CF2CF2)nCH2CH2OH will have a negligible global warming potential. Third...