Environ. Sci. Technol. 2008, 42, 6559–6565
Partitioning of Fluorotelomer Alcohols to Octanol and Different Sources of Dissolved Organic Carbon NADIA CARMOSINI AND LINDA S. LEE* Department of Agronomy, Purdue University, West Lafayette Indiana 47907-2054.
Received January 25, 2008. Revised manuscript received June 12, 2008. Accepted June 13, 2008.
Interest in the environmental fate of fluorotelomer alcohols (FTOHs) has spurred efforts to understand their equilibrium partitioning behavior. Experimentally determined partition coefficients for FTOHs between soil/water and air/water have been reported, but direct measurements of partition coefficients for dissolved organic carbon (DOC)/water (Kdoc) and octanol/ water (Kow) have been lacking. Here we measured the partitioning of 8:2 and 6:2 FTOH between one or more types of DOC and water using enhanced solubility or dialysis bag techniques, and also quantified Kow values for 4:2 to 8:2 FTOH using a batch equilibration method. The range in measured log Kdoc values for 8:2 FTOH using the enhanced solubility technique with DOC derived from two soils, two biosolids, and three reference humic acids is 2.00-3.97 with the lowest values obtained for the biosolids and an average across all other DOC sources (biosolid DOC excluded) of 3.54 ( 0.29. For 6:2 FTOH and Aldrich humic acid, a log Kdoc value of 1.96 ( 0.45 was measured using the dialysis technique. These average values are ∼1 to 2 log units lower than previously indirectly estimated Kdoc values. Overall, the affinity for DOC tends to be slightly lower than that for particulate soil organic carbon. Measured log Kow values for 4:2 (3.30 ( 0.04), 6:2 (4.54 ( 0.01), and 8:2 FTOH (5.58 ( 0.06) were in good agreement with previously reported estimates. Using relationships between experimentally measured partition coefficients and C-atom chain length, we estimated Kdoc and Kow values for shorter and longer chain FTOHs, respectively, that we were unable to measure experimentally.
Introduction Interest in the environmental fate and transport of persistent poly- and perfluorinated compounds (PFCs) has spurred efforts to determine their partitioning behavior. Fluorotelomer alcohols (FTOHs) are polyfluorinated compounds used to manufacture protective coatings and surfactants for a myriad of industrial and commercial applications. FTOHs are characterized by the formula F(CF2)nCH2CH2OH where n is an even number, and are named according to the ratio of fluorinated to hydrogenated carbons, such as 8:2 FTOH. Formulations known to be used in manufacturing contain primarily 4:2 (∼4%), 6:2 (∼16%), 8:2 (∼46%), and 10:2 or larger FTOHs (∼33%) (1). These compounds may be released to the environment through industrial emissions, as unre* Corresponding author phone: (765) 494-8612; fax: (765) 4962926; e-mail:
[email protected]. 10.1021/es800263t CCC: $40.75
Published on Web 07/25/2008
2008 American Chemical Society
acted and unbound residuals in widely used end-products, and by the degradation in soils of FTOH-derived fluoroacrylate polymers (2–5). Subsequently, as a result of their volatility and persistence under long-range atmospheric transport, FTOHs have been identified as an important source of persistent perfluorocarboxylates observed globally in environmental samples, including humans and wildlife (6–10). In the past few years, a handful of studies have reported equilibrium partition coefficients for FTOHs between various environmental compartments, such as air/water, octanol/ water, octanol/air, mineral surfaces/air, humic acid/water, soil/water, and dissolved organic carbon (DOC)/water (11–15). The majority of these values have originated from indirect experimental or modeling approaches due to substantial challenges encountered in experimental studies with FTOHs, such as sorption to labware, accumulation at surface interfaces, volatile losses, and large measurement variation (11–14, 16, 17). Within this data set, DOC/water partition coefficients (Kdoc, L/kg DOC) estimated from modeling techniques are substantially higher than soil OC/water partition coefficients (Koc, L/kg OC) determined experimentally. In batch equilibrium sorption experiments with several soils, Liu and Lee (11) obtained log Koc values for 8:2 FTOH of ∼4 and estimated log Kdoc values in the 5-6 range based on the apparent effect of solubilized OC on 8:2 FTOH sorption by soil. Goss et al. (14) arrived at a similar estimate of 5.48 using a polyparameter-linear free energy relationship (pp-LFER) describing 8:2 FTOH partitioning between humic acid (normalized to OC) and water. Although the Kdoc estimates from both studies appear in agreement, validation by direct measurements is needed. In soil environments rich with DOC, compounds with high Kdoc values have a high potential to undergo DOCfacilitated transport. This may be of concern in landfills and municipal wastewater treatment plants where FTOH contamination originating from product disposal or sloughing of PFC-coated products can combine with high DOC present in leachates or discharges. Contaminant partitioning to DOC has also been associated with reduced or modified degradation rates and pathways (18) and altered bioavailability (19, 20), thus, accurate information on Kdoc is essential for assessing the environmental fate of FTOHs. In this work we investigated the partitioning of FTOHs to DOC from soils, municipal biosolids, and commercially available humic acids using enhanced solubility and dialysis bag techniques for which only values for 6:2 and 8:2 FTOH were obtainable. The LFERs derived from previously reported Koc values were then used to estimate Kdoc values for the other FTOHs and compared to predicted OC-based partition coefficients. We also experimentally determined the octanol-water partition coefficients (Kow) for 4:2, 6:2, and 8:2 FTOHs, which until now have only been estimated.
Materials and Methods Chemicals. 8:2 FTOH (>99%) and isotopically labeled 8:2 FTOH (1D,1D,2D,2D,313C-perfluorodecanol; >99%) were provided by DuPont Chemical Solutions Enterprise (Wilmington, DE). 4:2 FTOH (97%) was purchased from Apollo Scientific (Bredbury, UK). 6:2 FTOH (g97%), Aldrich humic acid sodium salt, and LC/MS grade methanol were purchased from Sigma-Aldrich Chemical Co. (St. Louis, MO). Pahokee Peat and Leonardite reference humic acids were purchased from the International Humic Substances Society (St. Paul, MN). SK961089 soil was purchased from Land Research Associates (Lockington, Derby, UK). Potassium phosphate VOL. 42, NO. 17, 2008 / ENVIRONMENTAL SCIENCE & TECHNOLOGY
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monobasic (KH2PO4), potassium hydroxide (KOH), calcium chloride dihydrate (CaCl2 · 2H2O), and hydrochloric acid (HCl) were purchased from Mallinckrodt Baker (Paris, KY), and were analytical-reagent grade or higher. Reagent grade octanol was purchased from EM Scientific (Gibbstown, NJ). Distilled water was prepared with a Barnstead Mega-Pure System MP-3A (Dubuque, IA). Kow Measurements. Kow values were quantified using procedures modified from Karickhoff and Brown (21). For 4:2, 6:2, and 8:2 FTOH, solutions of approximately 1440, 1740, and 2500 mg/L, respectively, were prepared by dissolving measured amounts of the pure compounds in octanol purified by liquid extraction with 0.1 M NaOH and distilled water followed by distillation (21). The octanol solutions were equilibrated with varying volumes of high purity water in 9 mL glass centrifuge tubes for 24 h and 1 wk. Three to five aliquots were taken from both the aqueous and octanol phases of each tube and diluted with methanol for FTOH analysis. DOC Sources. DOC solutions were prepared from three commercial humic acid materials (Pahokee Peat, Leonardite, Aldrich), two soils (7CB2, SK961089), and two Class B municipal biosolids. Aldrich humic acid (AHA) was purified prior to use to reduce ash content as previously described (22), dialyzed against 0.01 M KCl for 48 h (1000 MWCO, Spectra/Por 6 Regenerated Cellulose Dialysis Membranes; Spectrum Laboratories, Rancho Dominguez, CA), freezedried, and stored at room temperature. Pahokee Peat (PP) and Leonardite (LEO) powders were used as received. Humic acid stock solutions were prepared by stirring each solid material in 10 mM KH2PO4 (pH 7) at a mass to volume ratio of 1 mg to 2 mL for 24 h, followed by filtering through a 0.2 µm nylon membrane filter (Pall Life Sciences, East Hills, NY). A DOC concentration solution series was prepared for each humic acid by diluting the stock solutions with 10 mM KH2PO4. All solutions were filter-sterilized through a 0.22 µm GP Express Plus Membrane Stericup (Millipore Co., Billerica, MA), stored in the dark at 4 °C, and used within 72 h. Soil DOC solutions were prepared from soils that have been used in previous FTOH sorption studies and described elsewhere (11, 12). Briefly, 7CB2 (8.18% OC) is an agricultural soil from Costa Rica, and SK961089 (4.60%) is a calcareous surface soil from England. DOC was extracted by agitating soil in 5 mM CaCl2 at a soil mass to solution ratio of 1 g to 2 mL for 24 h to parallel experimental conditions used in an earlier study on 8:2 FTOH sorption by these two soils. The suspensions were centrifuged for 1 h at 8000g, and the supernatants were filtered through a sequence of glass microfiber filters (GF/A, GF/C, and GF/F, Whatman, Florham Park, NJ) and nylon membrane filters (0.45 and 0.2 µm, Pall Life Sciences, East Hills, NY) previously rinsed with distilled water. The soil DOC extracts were analyzed for cation concentrations (described below) since ionic strength and dominant cations are known to impact sorption behavior by altering the macromolecular structure and charge of DOC, as well as the aqueous activity of the hydrophobic organic compounds (HOCs) (23–25). To maintain consistent pH and ionic strength, the DOC concentration series for the 7CB2 and calcareous SK961089 soil extracts were prepared by diluting with 5 mM CaCl2 at pH 6.3 and 13 mM CaCl2 at pH 8.0 solutions, respectively. The pH of the diluted solutions deviated less than 10% from the values measured for the stock solutions. All solutions were filter-sterilized, stored in the dark at 4 °C, and used within 72 h. DOC from biosolids was obtained by centrifuging Class B biosolids from the wastewater treatment facilities of the cities of West Lafayette (WL; anaerobic digestion) and Lafayette (LAF; aerobic digestion), Indiana, for 1 h at 10 000g. The supernatants were filter-sterilized and stored as described above. Due to the complexity of the supernatant electrolyte 6560
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matrix, only a single DOC concentration (that of the supernatant) was used in assessing the FTOH solubility enhancement by DOC from the biosolids. DOC Characterization. Nonpurgeable DOC concentrations were measured with a TOC-VCSH analyzer (Shimadzu, Columbia, MD) using the high-sensitivity combustion catalytic oxidation/nondispersive infrared method. Concentrations of dominant cations (Ca2+, Mg2+, Na+, K+) were measured with an Elan DRC-e inductively coupled plasmamass spectrometer (ICP-MS) (Perkin-Elmer Sciex, Waltham, MA). Specific UV absorbance at 254 nm (SUVA254), defined as absorbance divided by DOC concentration, was measured to estimate the aromatic content (% CAr) (26) (details provided in the Supporting Information). Solution pH was measured with an Accumet AR20 combined pH/EC meter (Fisher Scientific, Waltham, MA). Kdoc Measurement. Measurements of Kdoc (L/kg DOC), defined as the ratio of the DOC-bound (Cdoc, µg/kg DOC) and free aqueous phase concentrations (Cw, µg/L) at equilibrium, were attempted using solubility enhancement and dialysis techniques. For sparingly soluble HOCs, partitioning to DOC can result in a measurable increase in a compound’s apparent solubility (Sw*, µg/L). Apparent solubility includes the aqueous solubility of the free solute (Sw, µg/L) plus the DOC-bound solute concentration, and can be expressed in terms of Kdoc as follows: / Sw ) Sw(1 + [DOC] · Kdoc)
(1)
where [DOC] is the concentration of DOC in kg/L (27). A linear regression of Sw* versus [DOC] yields a slope equal to Sw · Kdoc and an intercept equal to Sw. Solubility enhancement was used only for 8:2 FTOH. The method is not appropriate for the smaller 6:2 and 4:2 FTOHs with moderate to high solubility (Sw ) 18.8 and 974 mg/L, respectively) (12, 27). It was also not attempted for the larger and more hydrophobic 10:2 and 12:2 FTOHs since previous research in our laboratory found that reliable and consistent values for their aqueous solubility were unattainable even with the use of organic cosolvents (12). For moderately soluble HOCs, dialysis is an appropriate technique to quantify Kdoc (24). Briefly, dialysis bags filled with a DOC solution are placed in vessels containing a solution with the HOC of interest. After an equilibration period, the concentration of the HOC inside and outside of the dialysis bags is measured. The freely dissolved solute concentration is assumed to be equal inside and outside. A higher concentration inside the bag relative to the outside is assumed to be the DOC-bound HOC. Therefore, Kdoc is determined from a ratio of HOC concentrations inside (Cin, µg/L) and outside (Cout, µg/L) of the bag according to the following relationship: Kdoc )
(Cin - Cout) Cout · [DOC]
(2)
Solubility Enhancement Technique. 8:2 FTOH dissolved in acetone was plated onto 9 mL glass centrifuge tubes that had been baked at 400 °C for 6 h and autoclaved. The solvent was gently evaporated with N2 passed through a sterile 0.2 µm filtration capsule (Polycap TF, Whatman, Florham Park, NJ). The tubes were filled (no headspace) with the DOC solutions, resulting in an 8:2 FTOH mass to volume ratio approximately 1000 times greater than the compound’s reported aqueous solubility of 137-224 µg/L (11, 16). The tubes were capped with sterile aluminum-lined PTFE septa, wrapped in aluminum foil to prevent photolytic transformation, and rotated at 30 rpm and 22.3 °C ( 0.4 (monitored laboratory temperature) for 6 days, which was found to be sufficient to attain near-equilibrium (Supporting Information Figure SI-1). After equilibration, the tubes were centrifuged
TABLE 1. Selected Properties of DOC Stock Solutions. cation concentration (mmol/L) DOC source soils: 7CB2 SK961089 humic acids: LEO PP AHA biosolids: LAF WL
K+
Na+
SUVA254a
% CArb
1.49 0.68
0.49 0.23
0.9 0.26
38 312
9 22
0.06 0.04 0.01
