Environ. Sci. Technol. 2007, 41, 5357-5362
Effect of Fluorotelomer Alcohol Chain Length on Aqueous Solubility and Sorption by Soils JINXIA LIU AND LINDA S. LEE* Department of Agronomy, Purdue University, West Lafayette, Indiana 47907-2054
Fluorotelomer alcohols (FTOHs) are a group of polyfluorinated alkyl chemicals that have been widely studied as precursors to perfluorocarboxylates such as perfluorooctanoic acid and for which knowledge on their fate in soils is sparse. The solubility and sorption by soil of the homologous 4:2 to 10:2 FTOHs were measured in water or cosolvent/ water solutions. For the smaller 4:2 and 6:2 FTOHs, solubility and sorption could be measured adequately in aqueous systems although transformation was apparent even in γ-irradiated and autoclaved systems. Sorption coefficients estimated by measuring both sorbed and solution-phase concentrations were not significantly affected by the biotransformation process. The use of cosolvents was employed for probing the behavior of the longer-chain FTOHs with limited aqueous solubility. A single log-linear correlation between aqueous solubility and modified McGowan molar volumes resulted for the n-alkanols and FTOHs. Soil organic carbon (OC) consistently appeared to be the key soil property influencing sorption of the FTOHs while the perfluorocarbon chain length was the dominant structural feature influencing solubility and sorption. Each CF2 moiety decreased the aqueous solubility by ∼0.78 log units (compared to 0.60 log units for each CH2 addition in hydrogenated primary alcohols), and increased OCnormalized sorption coefficients (Koc) by ∼0.87 log units. Good log-log linear correlations between Koc and both octanol-water partition coefficients and solubility were observed for the FTOHs.
Introduction Fluorotelomer alcohols (FTOHs) are a group of polyfluorinated alkyl chemicals that have been widely studied as possible precursors to perfluorocarboxylates (PFCAs), such as perfluorooctanoic acid (PFOA) (1-3). FTOHs are used to synthesize fluorotelomer surfactants and polymers, which serve as excellent stain-resistant and protective coatings for both domestic and industrial applications (4). They have sufficiently high vapor pressures to partition into air during manufacturing or application to textiles (5). Recent studies have shown that they can also be released as product impurities (6, 7) or from likely degradation of some fluorotelomer products. While fate of FTOHs in the atmosphere (8-10) is becoming well understood, knowledge is sparse on their fate in soils and landfills in which many commercial products that may be sources of FTOHs and related chemicals are disposed. * Corresponding author phone: 765-494-8612; fax: 765-496-2926; e-mail:
[email protected]. 10.1021/es070228n CCC: $37.00 Published on Web 06/30/2007
2007 American Chemical Society
In our previous work with 8:2 FTOH, the linear correlation between sorption and organic carbon (OC) content indicated that hydrophobic partitioning is the dominant process driving sorption of FTOHs (11). Higgins and Luthy (12) also found that organic matter was an important sorption domain in sediments for a series of anionic perfluorochemical surfactants. They further noted that log Koc values (OC-normalized sorption coefficients) were largely proportional to the perfluorocarbon chain length within the carboxylate or sulfonate homologues. Association of the 8:2 FTOH with organiccontaining aerosol particles has also been proposed as a potential route for its atmosphere transport (13). The OCwater partition coefficients for FTOHs have been estimated by Goss et al. (14) using the polyparameter-linear free energy relationship (pp-LFER) and several descriptors that describe various potential compound interactions; however, experimental data to validate these predictive tools are needed. The goal of the current study was to measure the solubility and sorption of smaller and longer chain FTOHs using methods we demonstrated previously for the 8:2 FTOH (11). Direct aqueous measurements were used for the 4:2 and 6:2 FTOHs while measurements in cosolvents and the log-linear cosolvency model were used for the longer chain FTOHs for which aqueous measurements were challenging. A summary of results, challenges, and characteristics as a function of perfluorinated chain length is presented.
Materials and Methods Chemicals. 4:2 FTOH (97%) and 10:2 FTOH (97%) were purchased from Apollo Scientific (Bredbury, UK), and 6:2 FTOH (g97%) was purchased from Fluka through SigmaAldrich (St. Louis, MO). 12:2 FTOH and the internal standard [1D, 1D, 2D, 2D-313C] 8:2 FTOH (96%) were provided by DuPont (Wilmington, DE). 4:2 and 6:2 FTOH are liquids at room temperature, while 10:2 FTOH (melting point, MP ) 92.7 °C) and 12:2 FTOH (MP unknown) are solids. LC/MS grade methanol and acetonitrile were obtained from SigmaAldrich, calcium chloride dihydrate (CaCl2‚2H2O) was purchased from Mallinckrodt Baker (Phillipsburg, NJ), and deionized water (g17.8 M cm) came from a Barnstead E-pure system. Soils. Three soils were selected from the five soils that we previously used for 8:2 FTOH sorption and solubility studies: soil 7CB2 is a silt loam with 8.18% organic carbon (OC) and pH of 5.30 (0.01 N CaCl2); Drummer-6 is a clay loam with 2.50% OC and pH of 5.80; and Oakville-24 is a sandy loam with 0.52% OC and pH of 4.40 (11). To sterilize soils and inhibit microbial activity in the aqueous sorption studies, 60Co(γ)-irradiation was used prior to addition of the test compounds. Air-dried ( 0.96 for 4:2 FTOH and R 2 > 0.99 for 6:2 FTOH (Figure 3 and Table 1). For 6:2 FTOH, the total mass recovery was close to 100% for only the Oakville-24 soil and the no-soil blanks. For the other two soils, for which complete replicate isotherms were measured, the average mass recoveries each time (n ) 12 for each isotherm) were 56.2 ( 2.4% and 86.1 ( 1.6% with 7CB2 and 51.8 ( 10.1% and 66.9 ( 4.3% with Drummer-6 (Table 1). Several possible causes of low recovery such as photolysis, volatility, irreversible sorption, and biotransformation were considered, but biotransformation was determined to be the predominant source of low mass recoveries (see supporting evidence and discussion in the 5360
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Supporting Information). Despite the low mass recoveries, the reversible sorption distribution coefficients of the 4:2 and 6:2 FTOHs (determined by direct analysis of the solution and sorbed concentrations) do not appear affected by the transformation process or the presence of anionic metabolites; isotherms appear identical from two independent measurements with different mass recoveries for two soils (Figure 3 and Table 1). Consistent with our previous 8:2 FTOH study (11), sorption of 4:2 and 6:2 FTOH were highly correlated with the fraction of soil OC. 4:2 FTOH showed a discernible amount of sorption only on the highest OC soil (7CB2, 8.18% OC) at a high soil to water ratio (1:2 g/mL) with a resulting log Koc value of 0.933. For 6:2 FTOH, the sorption coefficients on 7CB2, Drummer-6, and Oakville-12 are approximately proportional to their OC contents (Figure 3) with an average log Koc value of 2.43 (SE ) 0.12, n ) 3). Sorption of 10:2 FTOH from Acetone/Water Solutions. The sorption of 10:2 FTOH from aqueous solutions was not attempted given the uncertainty experienced with solubility measurements. Furthermore, the quantifiable concentration range was limited with an aqueous solubility of approximately 0.011 mg L-1 and the method quantitation limit of 0.001 mg L-1. The use of cosolvents widened the workable concentration range, yet several issues were still present at low acetone volume fractions (fc e 0.20). The no-soil controls reveal fcdependent mass recoveries (see Table 1 and Table S1 and related text in the Supporting Information) indicating the likelihood of volatile losses. The total mass recovery (including the mass in the bulk solution and rinsed from the glass) increased from 44.7% (SE ) 4.9, n ) 9) in fc ) 0.1 acetone to 94.4% (SE ) 4.2, n ) 9) in fc e 0.4 acetone (Table 1). No statistically significant trends in recovery as a function of 10:2 FTOH concentration was observed within any of the cosolvent/water systems (Table S2). At fc ) 0.10 and 0.20, a significant amount of mass was recovered from glass. Goss et al. (14) have shown that fluorotelomer compounds have a high tendency to adsorb to interfaces. Cosolvents help to minimize sorption to glassware, but apparently more than 20% acetone or a stronger solvent is needed for the longer chain FTOHs. Although it is likely that in the presence of soil, less mass was sorbed to the glassware than observed in the no-soil blanks, it does reflect the difficulty of generating reliable isotherms at low fc. In addition, S-shaped isotherms were observed at fc e 0.20 (Supporting Information, Figure S3); the equilibrium solution concentrations appeared to approach a plateau that was approximately 1 order of magnitude lower than the solubility limit for a given fc. The 0.01 N CaCl2 matrix would be expected to reduce FTOH solubility, but not to this magnitude. For these isotherms, only the three lowest concentrations, which appeared to represent the relatively linear region, were used in the regressions for estimating sorption coefficients. At higher cosolvent fractions (fc > 0.2), inclusion of isotherm data at all concentrations were well fit by the linear sorption model (e.g., 7CB in Figure 3C) as exemplified by good correlation coefficients (R 2) of 0.93, 0.97, and 0.98 for fc ) 0.24, 0.30, and 0.40, respectively. For the Drummer-6 and Oakville-24 soils, R 2 values of the linear isotherm fits were also best for data from fc ) 0.40 acetone (Table 1). The aqueous-phase sorption coefficient (Kd,w, L kg-1) for 10:2 FTOH was estimated by performing a log-linear extrapolation to fc ) 0 of experimentally determined sorption coefficients in mixed cosolvent/water solutions (Kd,mix) at multiple fc values (Figure 4), which has been illustrated in several studies including our previous study for 8:2 FTOH (11): log Kd,mix ) log Kd,w - Rσc fc where R is the solventsorbet interaction term. The resulting log Koc value for 10:2 FTOH is 6.20 (SE ) 0.18, n ) 3). For 8:2 FTOH, if we compare the directly measured log Koc values (3.84) and that estimated
TABLE 1. Summary of Total Mass Recovery and Sorption Coefficients for FTOHs in Water or Acetone/Water Solutions
OC%
4:2 FTOH
Experimental Sorption Coefficients Kd,w or Kd,mix (R 2)a 6:2 FTOH 10:2 FTOH
aqueous
aqueous
fc ) 0.1b
fc ) 0.17b
fc ) 0.2b
fc ) 0.24
fc ) 0.3
fc ) 0.4
(0.870)b
17.9 (0.898) 0.939 (0.977) Drummer-6 2.50 ND 6.09 (0.993) 2051(0.936) ND 199 (0.980) 57.4 Oakville-24 0.52 ND 1.94 (0.995) ND ND 88.9 (0.954) 23.8 (0.962)b 7.64 (0.933) 0.358 (0.970) 7CB2 8.18 0.701 (0.964) 17.0 (0.990) 3713 (0.983) 893 (0.952) 593 (0.931) 199 (0.927)c 33.9 (0.969)c 1.47 (0.983) % Total Mass Recovery (SE)e 4:2 FTOH isotherm 1
isotherm 2
6:2 FTOH isotherm 1
isotherm 2
10:2 FTOH
fc ) 0.1
fc ) 0.17
ND 51.8 (10.1) 66.9 (4.3) 87.1 (2.9) ND Drummer-6 ND f Oakville-24 ND ND 105.0 (2.1) ND ND ND 7CB2 88.4 (2.5) 95.9 (1.2) 56.2 (2.4) 86.1 (1.6) 86.3 (1.4) 100.4 (5.5) no-soil blank 89.8 (2.6) 101.4 (2.2) 44.7 (4.9)d ND
fc ) 0.2
fc ) 0.24
fc ) 0.3
fc ) 0.4
84.8 (1.9) 92.6 (2.4) 82.9 (1.8) 100.3 (2.0) 88.7 (2.4) 94.6 (1.6) 80.2 (3.7) 88.4 (1.2) 87.5 (2.5) 84.4 (3.1)c 90.1 (2.9)c 100.8 (1.4) 66.1 (5.0) ND 77.2 (3.9) 94.4 (4.2)
a Correlation coefficients for the linear isotherm model fits given in parentheses for each K b Regressions are based on 4-6 data d,w or Kd,mix. points at three concentrations. c Average is based on n ) 24. d Average is based on n ) 12. e Total mass recovery for soils ) (mass extracted from soil + mass in solution after equilibration)/applied mass × 100%. Average is based on n ) 12 unless otherwise specified. For no-soil controls, total mass recovery ) (mass in solution after equilibration + mass in tube rinsing)/applied mass × 100%. Average is based on n ) 9 unless otherwise noted. f Not determined.
by the log-linear extrapolation of measured Kd,mix values (4.13), the difference is only 0.36 log units, which offers validity to the cosolvency extrapolation approach when aqueous measurements are challenging. Using σc of 7.04 estimated from the 8:2 FTOH acetone/water solubility data, the resulting R for 8:2 FTOH on the five soils investigated is 1.02 (SE ) 0.01) (11). Using the σc of 9.46 estimated for 10:2 FTOH solubility from acetone/water solutions and 10:2 FTOH sorption data on three of the five soils used for 8:2 FTOH results in an average R of 1.20 (SE ) 0.02, n ) 3). An R > 1 indicates that addition of a cosolvent decreases sorption more than predicted from just changes in solution-phase activity (i.e., solubility). Plausible explanations include competition between cosolvent and solute for sorption domains, or preferential hydration of the sorbent by cosolvent, which results in a less hydrophobic soil organic matter domain. Values of R > 1 were previously noted by Rao et al. (21) for cosolvency of o-cresol (R ) 1.5) for anthracene in a 50/50 v/v dimethylsulfoxide/water solution, but discussion or confirmation of a mechanism was not included. FIGURE 4. Log-linear relationship (log Kd,mix ) log Kd,w - rσcfc) between 10:2 FTOH sorption coefficients (Kd,mix) in acetone/water solutions and volume fraction acetone (fc) in three soils. The lines are the linear regression fits through all Kd,mix values and extrapolated to fc ) 0 to estimate the aqueous sorption coefficient (Kd,mix).
Structural Features Affecting Sorption. The impact of the perfluorocarbon chain length on the log Koc values of homologous FTOHs is illustrated in Figure 5a along with log-log correlations between Koc and Kow (22) (Figure 5b) and Koc and Sw,l (Figure 5c). Log Koc correlates well with all
FIGURE 5. Relationships between the Log Koc and (a) total C-atoms, (b) Log Kow (22), and (c) Log Sw (mol L-1). Data points are the average log Koc value for each FTOH (Table 1, ref 11) estimated by either aqueous-based isotherms (b) or log-linear extrapolation from acetone/water solutions (O) and error bars represent the standard deviation. VOL. 41, NO. 15, 2007 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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three parameters. Based on the regression line with the number of aliphatic carbons (Figure 5a), addition of each CF2 moiety increased the FTOH sorption coefficients by 0.87 ( 0.12 log units (95% confidence interval of 0.61-1.13). Calculated water-humic acid partition coefficients for fluorotelomer olefins and alcohols (4 to 12 C-atoms) by Goss et al. (14) yields a contribution of ∼0.60 log units per CF2 moiety. For perfluorocarboxylates (8, 9, 10, and 11 C-atoms) and perfluorosulfonates (8 and 10 C-atoms) sorption by sediments, Higgins and Luthy (12) reported an increase of 0.50.6 log units per CF2 moiety. Although our data indicate a larger contribution per CF2 unit than either of the previously reported values, both of the previously reported values fall close to our lower 95% confidence interval. A further statistical analysis would be tenuous given the limited number of compounds for each data set combined with different sorbent classes used (soil, sediment, and humic acid), and/or the uneven weights for each log Koc value (sorbent-compound combinations varied from 1 to 5 within each data set). The measured solubility of the 4:2 to 10:2 FTOHs presented here confirms the conclusions drawn by Goss et al. (14) from their polyparameter linear free energy relationships that FTOHs and fluorotelomer olefins have a smaller tendency to be in water than expected, which for the most part, can be accounted for by simple increasing the molar volume attributed to the F atom. The large apparent contribution of the cavity term in dissolution is consistent with the fact that measured ∆Sm values are much lower than expected for a long chain compound. The latter indicates that the perfluorinated chains have an unusual propensity toward rigidity even in an aqueous solution, which is somewhat counterintuitive. In addition, the relatively similar log unit contribution per CF2 group estimated from measured solubility (0.78) and sorption by surface soils (0.87) for the FTOHs (Figure 5c) suggests that the escaping tendency for FTOHs from water is the primary force driving sorption from aqueous systems.
Acknowledgments We acknowledge funding from DuPont Center for Collaborative Research and Education (Wilmington, DE), National Science Foundation (BES-0606899), and a Purdue University Lynn Fellowship. We thank Stephen Sassman for general laboratory assistance and analytical methods development, and Sai Prasanth Chamarthy and Dr. Rodolfo Pinal (Industrial and Physical Pharmacy, Purdue University) for the entropy of melt determinations for 8:2 and 10:2 FTOHs.
Supporting Information Available Details of the LC/MS/MS methods and sample chromatograms, matrix effect examination, procedure and data table for mass recovery of 10:2 FTOH in blank controls, discussion on low mass recoveries for 4:2 and 6:2 FTOHs and soil sterilization, sample calculation of subcooled liquid aqueous solubility for 8:2 and 10:2 FTOHs, sorption isotherms of 10:2 FTOH at fc e 20% acetone, and tabulated sorption isotherm parameters of FTOHs. This material is available free of charge via the Internet at http://pubs.acs.org.
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Received for review January 29, 2007. Revised manuscript received May 8, 2007. Accepted May 21, 2007. ES070228N
