Environ. Sci. Technol. 2005, 39, 7535-7540
Solubility and Sorption by Soils of 8:2 Fluorotelomer Alcohol in Water and Cosolvent Systems JINXIA LIU AND LINDA S. LEE* Department of Agronomy, Purdue University, West Lafayette, Indiana 47907-2054
The solubility and sorption by five soils of 8:2 fluorotelomer alcohol (FTOH) were measured from water and cosolvent/ water solutions. Aqueous solubility and soil-water distribution coefficients (Kd,w, L kg-1) were extrapolated from cosolvent data using a log-linear cosolvency model and compared to direct aqueous measurements. Liquid chromatography tandem mass spectrometry with electrospray ionization was employed to analyze the 8:2 FTOH in solutions and soil extracts. The cosolvent-extrapolated water solubility is 0.224 mg L-1, in good agreement with the measured value of 0.194 mg L-1. All sorption isotherms were generally linear regardless of cosolvent composition or soil organic carbon (OC) content. Kd,w values extrapolated from cosolvent data were similar but consistently higher than those measured in aqueous solutions. The latter was hypothesized to be due to dissolved OC (DOC) in the aqueous slurries. An average log KDOC of 5.30 was estimated and supported by DOC and Kd,w measurements at two soil-water ratios. Sorption appeared to be driven by hydrophobic partitioning with a log Koc value of 4.13 ( 0.16. Irreversible sorption was also observed and appeared to be related to OC content, with the extraction efficiency reduced from 85% to 45% with increasing contact time from 3 to 72 h for the highest OC soil.
Introduction Synthetic alkyl polyfluorinated compounds have been detected in wildlife (1, 2), human blood (3), air (4, 5), river water (6), sediments, and sludge (7). Among the vast number of polyfluorinated compounds, perfluorooctanoic sulfonate (PFOS) and perfluorinated carboxylic acids (PFCAs), such as perfluorooctanoic acid (PFOA), are found to be most widespread and present at highest concentrations. The search for potential sources of these compounds has been extended to fluorotelomer alcohols (FTOHs), including 8:2 FTOH, whose biotransformation products have been confirmed to include low levels of PFOA (8, 9). FTOHs are polyfluorinated primary alcohols synthesized through a telomerization process. They are characterized by an even number of perfluorinated carbons and two hydrogenated carbons adjacent to a hydroxyl group and are named based on the ratio of fluorinated to hydrogenated carbons, such as 6:2, 8:2, and 10:2 FTOHs. The long fluorocarbon chain imparts unique physicochemical properties to the FTOHs, including low aqueous solubility and high vapor pressures, compared with those of their hydrocarbon or halogen analogues (10). The presence of these compounds in the atmosphere has been confirmed by recent air sampling campaigns, which found the total mean concentrations of FTOHs ranging from * Corresponding author phone: (765)494-8612; fax: (765)496-2926; e-mail:
[email protected]. 10.1021/es051125c CCC: $30.25 Published on Web 08/27/2005
2005 American Chemical Society
11 to 165 pg m-3, with 6:2 and 8:2 FTOHs being the dominant species (4, 5). One study on the atmospheric lifetime implied that FTOHs have the potential to undergo long-distance transport and, therefore, may contribute to the polyfluorinated compounds found in remote areas (11). FTOHs serve as raw industrial intermediates in the production of commercial fluorotelomer surfactants and polymers, which are widely used as excellent stain-resistant and protective coatings in carpets, upholstery, clothing, paper products, and various other industrial applications (12). These products are suspected to contain or degrade into FTOHs, and subsequently into perfluorinated acids, thus contributing to the global contamination by alky perfluorinated acids. This is of particular concern with regards to leachate from landfills where products such as polymer-coated carpets and paper utensils are disposed. Likewise, fluorotelomer-based coatings associated with clothes and carpets may slough off during use and cleaning and end up in wastewater discharge and biosolids. Therefore, characterizing the behavior of FTOHs in water and soil systems is imperative towards predicting their fate in surface and subsurface environments. We investigated the solubility and sorption behavior of the 8:2 FTOH (CF3-(CF2)7-(CH2)2-OH), one of the confirmed precursors of PFOA and the most abundant species among commercial FTOH mixtures in terms of weight percentage. The unique properties of the compound, including high vapor pressure, low aqueous solubility, and an affinity for glass surfaces and fluoropolymeric materials such as fluoropolymer tubing and septa (10, 13), make obtaining reliable data in aqueous systems difficult. The use of cosolvents minimizes volatilization losses, degradation, sorption to glassware, and mass-transfer limitations, which can bias direct aqueous measurements. Therefore, in addition to direct aqueous measurements, we employed the use of cosolvents and a log-linear cosolvency model to estimate aqueous solubility and soil-water distribution coefficients (Kd,w, L kg-1). Solubility was measured in water, methanol/water, and acetone/ water solutions. Sorption by five soils with a wide range in properties was measured in water and acetone/water solutions. Experimental variables such as temperature, soil-water ratio, and equilibration time were also assessed in relation to an interlaboratory comparison.
Materials and Methods Chemicals. 8:2 FTOH and deuterated 8:2 FTOH (1D,1D,2D,2D,313C-perfluorodecanol) at >99% purity were provided by DuPont Chemical Solutions Enterprise (Wilmington, DE). LC/MS grade methanol and Supelclean ENVI-Carb bulk packing were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO). Acetone, methanol, acetonitrile (Mallinckrodt, Chrom AR HPLC, Phillipsburg, NJ), calcium chloride dihydrate (CaCl2‚2H2O), and sodium hydroxide (NaOH) were of greater than 99% purity. Soils. Five soils previously used in other studies and representing a range in texture, pH, organic carbon content (OC), and cation exchange capacity (CEC) (Table 1) were selected. Drummer-6 is a typical agricultural surface soil from Indiana (14). Oakville-24 is sandy soil from northern Indiana. EPA-14 is a clayey soil from an eroded hillside in southeastern Ohio (15). Soil 7CB2 is a high organic matter agricultural soil from Costa Rica. SK961089 is a surface soil from England with a long-established history of deciduous coppice. SK961089 was one of the soils included in a previous study conducted to meet regulatory requirements in which the sorption of radio-labeled 8:2 FTOH was measured (16), thus serving as an interlaboratory comparison. Soils were passed VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 1. Selected Physical and Chemical Properties of Soils soil
texturea
EPA-14 clay Drummer-6 clay loam Oakville-24 sandy loam 7CB2 silt loam SK961089 clay loam
CEC pH (1:2 pH (0.01 sand silt clay OC (cmolc H2O) N CaCl2)b (%) (%) (%) (%)c kg-1)d 4.3 6.4 4.8
3.6 5.8 4.4
2 21 92
34 64 0.48e 18.9 43 36 2.50 23.3 4 4 0.52 3.8
5.9 8.0
5.3 7.5
32 38
50 18 8.18 28 34 4.60
10.9 38.2
a USDA texture class; particle size analysis by the hydrometer method (17). b pH at equilibrium with 0.01 N CaCl2 at the soil-to-solution volume ratio (g/mL) used for the isotherms. The ratios are 1:10 for EPA-14 and Oakville-24, 1:20 for Drummer-6, and 1:40 for 7CB2 and SK961089.c Loss on ignition method except where noted otherwise (18). d Ammonium acetate method at pH ) 7 (18). e Wet oxidation by the Walkley-Black method (18).
through a 2 mm sieve, homogenized, air-dried, and stored in closed containers at room temperature prior to use. Soils used in the aqueous sorption studies were wetted with 0.01 N CaCl2 solution, incubated for 48 h at 22.3 °C ( 0.4, and sterilized with cobalt-60 irradiation at 3.2 Mrad accumulative dosage to inhibit microbial activity. For sorption from acetone/water solutions, the presence of acetone at g10 vol % was assumed to adequately inhibit microbial activity; thus, cobalt-60 irradiation was not needed. Log-Linear Cosolvency Model. Yalkowsky et al. (19) described the effect of organic solvent on the solubility of a hydrophobic organic compound (HOC) with the following log-linear relationship:
log Smix ) log Sw + σfc
(1)
where Smix is the solubility in the cosolvent/water solution, fc is the volume fraction of the cosolvent, and the slope σ is the cosolvency power of the solvent for the solute. At fc ) 1, σ equals the logarithm of the ratio of solubility in pure cosolvent (Sc) and water (Sw)(log Sc/Sw). This method has been successfully used to estimate the aqueous solubility of organic compounds that are sparingly soluble in water and to enhance solubilization of pharmaceutical drugs (20). Based on the inverse relationship of solubility and sorption, Rao et al. derived a similar equation to describe the effect of a cosolvent on sorption of HOCs (21):
log Kd,mix ) log Kd,w - Rσfc
(2)
where Kd,mix is the sorption coefficient in binary cosolvent/ water solution and R is an empirical constant reflecting cosolvent-water-sorbent interactions. This relationship has been successfully applied to sorption of several HOCs (2225). Eqs 1 and 2 allow the estimation of aqueous solubility and Kd,w values, respectively, by extrapolating from data measured in cosolvent/water solutions to fc ) 0 assuming a log-linear behavior is exhibited. Solubility. Solubility of the 8:2 FTOH was measured at 22.3 °C ( 0.4 in water and in acetone/water and methanol/ water solutions of 10%, 20%, 30%, 40%, and 50% volume cosolvent. Excess 8:2 FTOH crystal was added into 8 mL glass centrifugation tubes, which were then filled completely (no headspace) with ultrapure water (conductivity g 17.8 × 106 Ω‚cm) or cosolvent/water solutions. Tubes were capped with aluminum-lined PTFE septa and shaken on an angular rotator at 35 rpm. Samples were removed periodically with a gastight syringe, placed in HPLC vials, and diluted with methanol (from 1:1 to 1:200 dilution). Equilibrium was achieved within a 48 h contact time. Sorption Isotherms. The sorption isotherms of 8:2 FTOH were measured at room temperature (22.3 °C ( 0.4) in water 7536
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(0.01 N CaCl2) and in acetone/water (0.01 N CaCl2) solutions of 10%, 17%, 24%, and 30% volume cosolvent for each of the five soils. Acetone was selected for the sorption studies, because the 8:2 FTOH exhibited a better log-linear cosolvency profile in acetone than in methanol cosolvent systems. Soil (g) to solution (mL) ratios (mass/vol) were varied with the solution matrix and soil type to achieve sufficient sorption while ensuring that solution concentrations remained above the analytical limits of detection (LOD); mass/vol ratios ranged from 1:2 to 1:8 g/mL for acetone/water solutions and from 1:10 to 1:40 g/mL for water (no cosolvent). 8:2 FTOH dissolved in methanol was injected with a microsyringe directly into the soils that had been preweighed into borosilicate glass centrifuge tubes followed by addition of the water or cosolvent solution to no headspace, which effectively reduced volatilization loss of 8:2 FTOH. A total of five initial concentrations (0.18-1.80 mg L-1 in the acetone/ water solutions, and 0.018-0.18 mg L-1 in the aqueous systems) in duplicate or triplicate prepared for each isotherm. The resulting methanol concentrations in soil slurries were less than 0.13 vol % in the aqueous system and less than 0.42 vol % in the acetone/water solutions. Tubes were shaken on an angular rotator at 35 rpm for 24 h, which was found to be sufficient to attain equilibrium in a preliminary kinetic study monitored over a period of 3-72 h, followed by centrifugation for 30 min at 700g. Supernatants were taken with a gastight syringe and diluted 1:1 with acetone in HPLC vials with about 0.1 mL of headspace left for addition of the deuterated 8:2 FTOH (10 ng) as an internal standard. Soil Extraction and Cleanup. Soils were extracted with acetone or with basic (20 mM NaOH) 90/10 v/v acetonitrile/ water solution (26) 2 to 3 times sequentially in the original tubes. Acetone extraction was sufficient to recover the sorbed 8:2 FTOH in sorption from cosolvent/water solutions, while the basic acetonitrile extraction was needed to improve mass recovery in the aqueous sorption studies. The soil extracts were separated by centrifugation for 30 min at 700g and spiked with a fixed amount of the internal standard of deuterated 8:2 FTOH. To reduce the potential matrix interferences in soil extracts, Supelclean ENVI-Carb bulk packing (26) was employed to remove the interfering substances. In 1.7 mL polypropylene microtubes, 25 mg of the bulk packing was added to a 1 mL soil extract aliquot spiked with the internal standard, and the material was mixed for 30 min. The soil extract was separated from the activated carbon by centrifugation (HERMLE Z233 M-2, Labnet International Inc., Edison, NJ) at 19 000g for 10 min, and the supernatants were analyzed with LC/MS/MS. LC/MS/MS Analysis. Analysis of the 8:2 FTOH in solutions and soil extracts was performed using liquid chromatography tandem mass spectrometry with electrospray ionization (LC/ MS/MS). No concentration step was needed, and only the simple sample cleanup step described above was required to reduce matrix effects and to achieve good sensitivity and rapid analysis. FTOHs in the gas phase have been successfully analyzed with gas chromatography (GC) using an electron capture detector or mass spectrometer (4, 8). However, for liquid phase analysis, the GC method requires extensive solvent extraction and cleanup, and column deterioration due to contamination from sample matrixes can be fast (27). To our knowledge, we are the first to employ LC/MS/MS to analyze 8:2 FTOH from soil samples, which could be further adapted for other environmental sample analyses. 8:2 FTOH was analyzed using a Shimadzu liquid chromatography system with a C-8 column coupled to an Applied Biosystem API3000 triple quadruple tandem mass spectrometer (MDS Sciex, Ontario, Canada) operating in the
negative electrospray ionization mode. A water/methanol mobile phase at a gradient similar to the one developed by Szostek et al. (28) was used. For the most acidic soil (EPA-14, pH < 4), 5 mmol L-1 of ethanolamine was added to the mobile phases to facilitate negative ionization. The detailed MS parameters and a sample chromatogram are provided in the Supporting Information. Values of 3 and 10 times the average noise level were considered the LOD and limit of quantification, respectively. The LOD in clean calibration solutions was about 250 ng L-1 with a 20 µL injection volume and about 100 ng L-1 with a 50 µL injection volume. Dissolved Organic Carbon (DOC) Analysis. The supernatants in direct aqueous sorption measurements were analyzed for nonpurgeable DOC levels after 24 h of equilibration. After 30 min of centrifugation at 700g, the supernatants were acidified, sparged, and then analyzed with a Shimadzu TOC-VCSH analyzer (Shimadzu America, Columbia, MD) using the high-sensitivity combustion catalytic oxidation/NDIR method. Quantification and Quality Control. The calibration solutions were made to match the solvent compositions in soil supernatant dilutions or soil extracts. Samples were stored in the dark at below -10 °C and analyzed within 2 weeks. The linear calibration curves of 6 to 8 data points were integrated with a 1/x weighting factor, which was found to be the best weighting scheme in this study. The deviation of standards from the nominal concentration was less than 15% at all concentration ranges, and the coefficient of determination was greater than 0.99 for each calibration curve. The calibration solutions were run at least twice at the beginning and the end of each sample batch. Analysis of Data. Aqueous solubility and soil-water distribution coefficients were determined by direct measurement and by extrapolation from data measured in cosolvent/water solutions. The solubilities determined at different cosolvent fractions were fitted to the log-linear solubility relationship (eq 1), and the aqueous solubility was estimated by extrapolation to fc ) 0 (y intercept ) log Sw). Sorption data were modeled with a linear sorption model (Cs ) Kd,wCw or Cs ) Kd,mixCmix, where Cs (µg g-1) is the equilibrium sorbed concentration, Cw (µg mL-1) and Cmix (µg mL-1) are the equilibrium concentrations in the aqueous or the mixed cosolvent/water solution, respectively, and Kd,w (L kg-1) and Kd,mix (L kg-1) are the sorption coefficients in water and cosolvent/water solution, respectively. For each soil, Kd,mix values determined at each fc were fitted into eq 2, and Kd,w was estimated by extrapolation to fc ) 0 (y intercept ) log Kd,w).
Results and Discussion Solubility. The water solubility of the 8:2 FTOH determined by direct aqueous phase measurements is 0.194 ( 0.032 mg L-1 (n ) 12). These values are similar but significantly higher (95% confidence level) than the value of 0.137 ( 0.053 mg L-1 (n ) 17) reported for 25 °C by Kaiser et al. (10). For a given volume of cosolvent, acetone was more effective at increasing solubility consistent with what has been observed for other HOCs. Acetone also exhibited a better log-linear cosolvency relationship for 8:2 FTOH (σ ) 7.04; R2 ) 0.98) than methanol (Figure 1) contrary for what has been observed for several nonpolar HOCs (29). The extrapolated water solubility from acetone/water solutions is 0.224 mg L-1, which is in good agreement with the value measured directly from water. Reduction of Matrix Effects in Soil Supernatants and Extracts. Analysis of 8:2 FTOHs with LC/MS/MS was found to be very susceptible to ion suppression induced by low pH conditions and coeluting species present in sample matrices. Therefore, addition of an appropriately selected internal standard such as the deuterated analogue was necessary. Sample cleanup was also employed to further reduce the
FIGURE 1. Measured solubility (mg L-1) of 8:2 FTOH in binary cosolvent mixtures and in water (O methanol/water, b acetone/ water, 9 water). The solid line is the linear regression of the measured solubility in acetone/water solutions extrapolated to zero cosolvent vol fraction. Each data point is the average of four values, and error bars represent the standard deviation (SD); in some cases, the SD is covered by the data point symbol. matrix effects. The soil supernatants were free of matrix effects except for the most acidic EPA-14 soil. Addition of 5 mmol L-1 ethanolamine to the HPLC mobile phase resulted in about a 10-fold increase in the signal from the EPA-14 supernatants (see Figure S2, parts a and b, in the Supporting Information) but had no significant effect on analysis of the other soil supernatants in which pH > 4 (Table 1). In the untreated soil extracts, matrix effects caused LOD to be ∼2 to ∼20 times poorer depending on soil type, with the greatest effect observed for the higher organic carbon soils. Cleanup of the extract using Supelclean ENVI-Carb bulk packing, a graphitized nonporous carbon, which presumably adsorbed the humic substances from the soil extract, alleviated the ion suppression (Figure S2, parts c and d, in the Supporting Information) and improved the signal by 2-3 times. Equilibration Time. The sorption coefficients of 8:2 FTOH were calculated based on measured solution concentration and extractable 8:2 FTOH mass from soil (“as-measured” in Table 2), rather than on disappearance of the compound from solution (“by difference” in Table 2). The latter is subject to large errors when there are other unquantifiable processes occurring, such as volatilization, sorption to glassware, or irreversible sorption. As shown in Table 2, the measured Kd,w (or Kd,mix) did not change after 24 h in aqueous solution or at 10% acetone. However, Kd,w (or Kd,mix) values estimated “by difference” continued to increase with time. It was speculated that the reversible sorption process was near equilibrium within 24 h. With increasing contact time, the amount of irreversible sorption increased and the solution concentration decreased. However, on the basis of the consistency of the distribution coefficients determined by extraction, it appears that as irreversible sorption occurred, equilibrium of the reversible process was reestablished. Extraction Efficiency and Mass Recovery. Average extraction efficiency and total mass recovery at different percent cosolvent values are summarized in Table 3 for each soil. Sequential extraction experiments indicated that in most cases only one extraction of the soil was necessary to recover all extractable 8:2 FTOH (data not shown), except for the higher soil-to-solution ratios of 1:2 and 1:4, in which case a VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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TABLE 2. Linear Sorption Coefficients for 7CB2 Soil at Three Equilibration Times in 10% Acetone (Kd,mix) and in Water (Kd,w)a Kd,mix (L kg-1) in 10% acetone (SD)a 3h 24 h 72 h
calculation method as-measuredb by differencec c
83.1 (4.7) 89.5 (3.0)
127 (13.3) 204.2 (16.7)
124 (15.5) 251.8 (28.1)
3h
Kd,w (L kg-1) (SD)a 24 h
72 h
230.8 (25.5) 271.1 (24.3)
452.2 (32.6) 601.4 (32.4)
453.1 (82.1) 806.6 (74.2)
a Average (n ) 3) from a single concentration; SD ) standard deviation. b Sorbed concentrations were determined by solvent extraction. Sorbed concentrations estimated by the difference between the initial mass applied and mass in solution phase after equilibration.
TABLE 3. Summary of 8:2 FTOH Soil Extraction Efficiency and Total Mass Recovery from Water and Acetone/Water Solutions % mean extraction efficiency (SD)a,b
EPA-14 Drummer-6 Oakville-24 7CB2 SK96108 9 no-soil blank
% mean mass recovery (SD)c,d
no acetone
10% acetone
17% acetone
24% acetone
30% acetone
no acetone
10% acetone
17% acetone
24% acetone
30% acetone
84.2 (9.3) 80.5 (3.8) 83.3 (5.6) 86.1 (7.9) 70.6 (7.8) NAe
87.3 (4.5) 90.6 (5.9) 90.1 (4.6) 84.1 (3.0) 94.7 (2.9) NA
84.8 (9.3) 90.6 (7.6) 77.1 (9.0) 91.5 (3.2) 94.9 (5.7) NA
96.0 (12.0) 90.7 (5.8) 79.4 (13.4) 95.4 (4.7) 99.5 (4.6) NA
81.4 (20.3) 85.0 (11.2) 80.4 (15.0) 90.4 (6.6) 94.7 (6.6) NA
88.5 (7.3) 82.3 (3.5) 87.2 (4.5) 87.1 (7.4) 73.2 (7.5) 78.6 (4.6)f
89.3 (3.9) 91.7 (5.3) 91.9 (3.8) 94.9 (3.0) 95.0 (2.7) 91.8 (8.1)
89.9 (6.5) 93.0 (5.9) 84.8 (6.3) 92.6 (3.1) 95.5 (5.2) 97.1 (5.3)
98.3 (5.1) 94.0 (3.8) 91.5 (4.8) 96.4 (3.6) 99.5 (3.8) 87.2 (7.3)
94.0 (4.3) 94.3 (4.5) 93.6 (5.8) 94.8 (4.0) 97.1 (3.5) 92.2 (4.2)
a Soil extraction efficiency ) mass extracted from soil/(mass applied initially - mass in solution after equilibration) × 100%. Average is based on n ) 10. b Standard deviation given in parentheses. c Total mass recovery (except for no-soil blanks) ) (mass extracted from soil + mass in solution after equilibration)/mass applied initially × 100%. Average is based on n ) 10. d Total mass recovery for the no-soil blank ) (mass in solution after equilibration + mass in tube rinsing)/mass applied initially × 100%. Average is based on n ) 10 unless otherwise noted. e NA: not applicable. f Average is based on n ) 12.
cosolvent extrapolation and direct aqueous measurement, respectively, which is comparable to that of a 3-ring aromatic hydrocarbon. Correlations between sorption and clay or surface area as reflected by soil textural class were not good.
FIGURE 2. Isotherms constructed from the solution phase concentrations and solid phase concentration by solvent extraction of 8:2 FTOH after a 24 h equilibration with 7CB2 soil in water (O) and in acetone/water solutions (9 10% acetone, [ 17% acetone, b 24% acetone, and 2 30% acetone). The solid lines are the linear sorption model fits. second extraction was needed. Overall, in cosolvent/water solutions slightly higher extraction efficiencies and mass recoveries were achieved, but there was no consistent correlation with fc. There was also no clear trend between recoveries and soil OC content, with the lowest extraction efficiency achieved for the sandy low OC Oakville-24 soil for a given fc. Sorption Studies. All sorption isotherms were well fitted by the linear sorption model as indicated by correlation coefficients of g0.94 and exemplified by representative isotherms in Figure 2. Sorption coefficients along with the results from application of the log-linear cosolvency sorption model (eq 2) are summarized in Table 4. Aqueous sorption coefficients are well correlated with soil OC, with average log Koc values of 4.13 ( 0.16 and 3.84 ( 0.16 estimated by 7538
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Sorption decreased exponentially with increasing fc, with a good log-linear relationship exhibited for all five soils in the cosolvent range employed (Figure 3). The extrapolated Kd,w values are 1.4-3.5 times higher than those estimated by direct measurement from aqueous solutions, with the difference being soil-dependent. The two lowest OC soils, EPA-14 and Oakville-24, have the biggest differences, with extrapolated Kd,w values being 2.6 and 3.5 times higher than direct aqueous measurements. Relative differences between the two methods are the smallest for the higher OC soils, with the Kd,w value determined from the highest OCcontaining soil (7CB2) falling within the 95% confidence bands of the extrapolated value. The other two higher OC soils (Drummer-6 and SK961089) exhibited reasonable agreement, with the aqueous-measured Kd,w value falling on the borders of the 95% confidence bands for the cosolventextrapolated value. Consistently higher aqueous Kd,w values estimated by extrapolation from cosolvent/water solutions may be due to cosolvent-enhanced mass-transfer, cosolvent modification of soil sorption characteristics, or reduced sorption in aqueous soil suspensions by dissolved organic carbon (DOC). On the basis of data collected in 10% acetone, equilibrium appeared to be similarly achieved in the presence and absence of acetone (Table 2). The solvent-sorbent interaction terms (R) estimated using σ (7.04) determined from the solubility data are near unity for all soils (Table 4) suggesting that the impact of acetone on sorption appears to be dominated by acetone-induced changes in solution phase activity of 8:2 FTOH. Therefore, the potential contribution of aqueous DOC to the differences between measured and cosolventextrapolated aqueous phase coefficients was further probed. Effects of DOC on Sorption. DOC concentrations of 8.55, 5.16, 3.59, 4.09, and 11.24 mg OC/L were estimated for the supernatants obtained from EPA-14, Drummer-6, Oakville24, 7CB2, and SK961089, respectively, at the mass/vol ratios used in the sorption isotherms. The presence of DOC can
TABLE 4. Sorption Coefficients at Different Cosolvent Fractions, Extrapolated from Log-Linear Cosolvency Relationship and Measured Directly in Aqueous Solutions Kd,mix (L kg-1) soil EPA-14 Drummer-6 Oakville-24 7CB2 SK96109 8
OC (%)
Kd,w (L kg-1)
10% 17% 24% 30% acetone acetone acetone acetone
0.48 16.34 2.50 53.41 0.52 17.82 8.18 115.09 4.60 109.56
6.03 19.90 6.09 38.79 39.99
2.19 6.16 1.98 14.54 14.28
0.63 1.88 0.64 3.94 3.79
slope (rσ)a
intercept (log Kd)
rb
6.964 7.251 7.185 7.174 7.182
1.944 2.491 1.990 2.805 2.800
0.99 1.03 1.02 1.02 1.02
average log Koc (SD)c
log Koc
from direct from direct extrapolation measurement extrapolation measurement 87.9 309.7 97.7 638.3 631.0
34.2 148.1 28.2 448.9 274.0
4.26 4.09 4.27 3.89 4.14
3.74 3.90 3.64 4.06 3.86
4.13 (0.16)
3.84 (0.16)
a Rσ from fitting of the log-linear sorption model (eq 2) to the sorption data (log K b R was calculated by dividing the slope in the d,mix vs fc). log-linear sorption model (eq 2) by σ ) 7.04, which was determined in the acetone/water solubility experiments (eq 1). c Standard deviation given in parentheses.
substituted in for Ktrue in eq 3 along with the DOC associated with that soil. The resulting log KDOC values are 5.26, 5.33, 5.84, 5.01, and 5.06 for EPA-14, Drummer-6, Oakville-24, 7CB2, and SK961089, respectively, with an overall average log KDOC of 5.30 ( 0.29. The general consistency in KDOC values across soils seems to support our speculation. It is noteworthy that this KDOC value is almost a log unit higher than the average Koc value; typically KDOC e Koc has been observed for other HOCs (30, 31).
FIGURE 3. Log-linear relationship between the linear sorption coefficients and the volume fraction of acetone for sorption of 8:2 FTOH by five soils. The solid lines are linear regression fits excluding the sorption coefficients measured at fc ) 0. The dashed lines are the 95% confidence bands. decrease the true sorption coefficient (Ktrue) according to the following relationship (30):
K*d )
Ktrue 1 + KDOC[DOC]
(3)
where K*d is the apparent sorption coefficient in the presence of DOC and [DOC] is the DOC concentration in mg L-1. Theoretically, the presence of cosolvents should minimize the effect of DOC on sorption, assuming that the [DOC] is small relative to the percent cosolvent present. With the application of this assumption and the additional assumption that the difference between direct and cosolvent-extrapolated sorption coefficients is only due to DOC, the Kd,w value estimated by cosolvent extrapolation for a given soil was
Interlaboratory Comparison. In a DuPont contract study to Covance Laboratories (16), sorption of radio-labeled 8:2 FTOH by the SK961089 soil was measured at 4 °C using a mass/vol ratio of 1:1 and a 3 h equilibration time. A linear sorption coefficient of 62.5 L kg-1 was reported, which is more than 4 times smaller than the value we measured (Table 4) with a longer equilibration period, higher temperature, and smaller mass/vol ratio. To explore which factors attributed to the difference between laboratories, we measured sorption at 4 and 22 °C with a 3 h equilibration time and the same 1:40 mass/vol we used previously, which was required to meet our analytical LOD. The resulting Kd,w values are 318 and 222 L kg-1 at 4 and 22 °C, respectively. The higher sorption at lower temperatures, indicative of an exothermic sorption process as has been shown for other HOCs (32), did not explain the differences in Kd,w values between the two laboratories. We speculated that at the 1:1 mass/vol ratio used by the Covance group, DOC concentrations would be much greater than present at our dilute 1:40 soil-to-water ratio. The nonpurgeable DOC concentrations obtained from the SK961089 soil equilibrated at 4 °C for 3 h at mass/vol ratios of 1:40 and 1:1 soil were 12.2 and 273.3 mg C L-1, respectively. Setting up eq 3 for each mass/vol ratio and solving simultaneously resulted in estimates of a “true” Kd value of 393.4 L kg-1 and a log KDOC of 4.29. This log KDOC value is smaller than the value of 5.30 estimated from cosolvent/water data for the same soil (24 h of equilibration at 22 °C) in which Ktrue was assumed to be the cosolventextrapolated Kd,w. However, the two log KDOC values are in reasonable agreement considering the difference in equilibration times and temperature between the two experiments. Although a 1:1 mass/vol ratio is more representative of a field scenario, DOC concentrations will change over time, with climate variations (wetting and drying events as well as temperature), and with soil type. To predict accurately the effect of DOC on sorption and subsequent transport, a measure of the ”true” unbiased sorption coefficient is needed. Environmental Implications. The structure of 8:2 FTOH suggests the possibility of hydrogen-bonding and electron donor-acceptor interactions; however, the small deviation about the average log Koc value (4.13 ( 0.16) and lack of correlation between sorption and other soil properties suggest that hydrophobic partitioning dominates the sorption to soils. VOL. 39, NO. 19, 2005 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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By analogy with other HOCs, it is not surprising that dissolved organic matter significantly affected the sorption of the 8:2 FTOH by soils; however, the estimated average log KDOC of 5.30 ( 0.29 is high relative to what would be expected given the estimated log Koc. Predicting behavior of 8:2 FTOH in soils is further complicated by the presence of an irreversible process, which was evident with the substantial reduction in extraction efficiency within a 3 day contact time. From these studies, the relatively high Koc value for the 8:2 FTOH coupled to irreversible binding processes suggests that this compound will have limited mobility in many environments. However, the high KDOC values estimated from the soil studies indicate a high potential for DOC-facilitated transport, which may be of particular concern where DOC levels are elevated as is common to many landfills.
Acknowledgments This work was funded in part by DuPont’s Center for Collaborative Research and Education in Wilmington, DE and through a Purdue University Lynn Fellowship. A special thanks to Stephen Sassman for general laboratory assistance and analytical methods development and Matt Ruark for dissolved organic carbon analysis.
Supporting Information Available Details of the liquid chromatography and mass spectrometry parameters, a sample chromatogram, and chromatograms demonstrating the reduction in matrix effects for low pH (
