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Sorption of Perfluoroalkyl Phosphonates and Perfluoroalkyl Phosphinates in Soils Holly Lee, and Scott Andrew Mabury Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b04395 • Publication Date (Web): 22 Feb 2017 Downloaded from http://pubs.acs.org on February 26, 2017
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Draft Manuscript
Sorption of Perfluoroalkyl Phosphonates and Perfluoroalkyl Phosphinates in Soils
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Holly Lee* and Scott A. Mabury¥ * Sciex, 71 Four Valley Drive, Concord, Ontario, Canada, L4K 4V8 (Corresponding author) Phone: (289) 982-2285 Email:
[email protected] ¥
Department of Chemistry, University of Toronto, 80 St. George St., Toronto, Ontario, Canada, M5S 3H6 Phone: (416) 978-1780 Email:
[email protected] ACS Paragon Plus Environment
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Abstract
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recently discovered perfluoroalkyl acids (PFAAs) that have been widely detected in house dust,
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aquatic biota, surface water, and wastewater environments. The sorption of C6, C8, and C10
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monoalkylated PFPAs and C6/C6, C6/C8, and C8/C8 dialkylated PFPiAs was investigated in
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seven soils of varying geochemical parameters. Mean distribution coefficients, logKd*, ranged
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from 0.2 to 2.1 for the PFPAs and PFPiAs and were generally observed to increase with
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perfluoroalkyl chain length. The logKd* of PFPiAs calculated here (1.6–2.1) were similar to
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those previously measured for the longer-chain perfluorodecane sulfonate (1.9, PFDS) and
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perfluoroundecanoate (1.7, PFUnA) in sediments, but overall when compared as a class, were
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greater than those for the perfluoroalkane sulfonates (-0.8–1.9, PFSAs), perfluoroalkyl
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carboxylates (-0.4–1.7, PFCAs), and PFPAs (0.2–1.5). No single soil-specific parameter, such as
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pH and organic carbon content, was observed to control the sorption of PFPAs and PFPiAs, the
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lack of which may be attributed to competing interferences in the naturally heterogeneous soils.
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The PFPAs were observed to desorb to a greater extent and likely circulate as aqueous
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contaminants in the environment, while the more sorptive PFPiAs would be preferentially
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retained by environmental solid phases.
Perfluoroalkyl phosphonates (PFPAs) and perfluoroalkyl phosphinates (PFPiAs) are
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Introduction
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documented in wastewater, surface water, and sediment samples collected downstream from
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wastewater treatment plants (WWTPs).1–7 Analysis of WWTP sludge has consistently reported
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ng/g concentrations of perfluoroalkyl carboxylates (PFCAs) and perfluoroalkane sulfonates
Urban discharge of perfluoroalkyl and polyfluoroalkyl substances (PFASs) has been well
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(PFSAs), with perfluorooctanoate (PFOA, C8 PFCA) and perfluorooctane sulfonate (PFOS, C8
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PFSA) typically observed as the dominant perfluoroalkyl acids (PFAAs), followed by longer
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chain PFCAs and PFSAs (>8 perfluorinated carbons, CF’s).7–15 This profile reflects legacy
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distribution in historical commercial production of PFASs, but is also consistent with the
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demonstrated preference of longer-chain PFAAs to sorb to sediments,16–19 soils, 20 and sludge,21
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all of which may act as potential sinks and reservoirs for the retention and remobilization of
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these chemicals to the aqueous environment, respectively.
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Laboratory experiments using freshwater sediments16 and topsoils 20 indicated the
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sorption of PFAAs exhibits a chain-length dependency in which their organic carbon-normalized
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distribution coefficients (KOC) increase with the number of CF’s present in the perfluorocarbon
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tail of the PFAAs studied. In the same studies, PFSAs were also observed to be more sorptive
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than PFCAs of equal perfluorocarbon chain length. In contrast, no information has been reported
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on the sorption capacity of the perfluoroalkyl phosphonates (PFPAs) and phosphinates (PFPiAs).
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PFPAs and PFPiAs are PFAAs with their perfluorocarbon tails attached through a
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carbon-phosphorus (C–P) bond to either a phosphonate (Rx-P(O)O2-; Cx PFPA) or phosphinate
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(Rx-P(Ry)(O)O-; Cx/Cy PFPiA) headgroup (Table 1). These chemicals are currently marketed as
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leveling and wetting agents in household cleaners22 and were historically used in United States
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(US) pesticide formulations23 until 2008.24 Despite widespread observations of the C6, C8, and
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C10 PFPAs in Canadian surface waters at pg/L concentrations,25 these chemicals were not
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detected in any lake trout fillet homogenate sampled from the Great Lakes, whereas the C6/C6
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and C6/C8 PFPiAs were observed at concentrations of up to 32 pg/g ww in the same samples.26
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A similar absence of PFPAs was also observed in benthic worms in which PFPiAs were present
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at 30–1900 pg/g ww.27 More recently, De Silva et al. reported ubiquitous detection of PFPiAs at
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112–15300 pg/g ww in the plasma of fish, dolphins, and birds sampled across North America.28
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This difference is consistent with faster depuration kinetics previously observed for the PFPAs in
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rainbow trout (4–5 days)29 and rats (1–3 days)30 than for the PFPiAs (6–53 days in rainbow trout;
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2–4 days in rats).29,30 The C6/C6 and C6/C8 PFPiAs have been observed at 2–3 ng/g in WWTP
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sludge,30 whereas analysis of various Dutch sludge and sediment samples did not reveal any
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detection of the monitored C6, C8, and C10 PFPAs.31
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The present study investigated the sorption of three PFPA (C6, C8, and C10) and three
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PFPiA (C6/C6, C6/C8, and C8/C8) congeners in seven soils of varying geochemical properties.
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The influence of structural features, such as the perfluorocarbon chain length and headgroup, on
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sorption was investigated by comparing distribution coefficients for the six congeners to those
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previously reported for the PFSAs and PFCAs. A number of laboratory and field studies have
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shown evidence of PFAAs leaching from soils, that have been spiked with PFAAs32 or exposed
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to contaminated street runoffs33 and WWTP sludge,32,34–36 to groundwater and surface water. As
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such, desorption experiments were also performed to determine which of the subject PFPAs and
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PFPiAs may be prone to remobilization into the aqueous phase and which to preferential
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residence in the soil phase.
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Materials and Methods Chemicals. Individual standards of C6, C8, and C10 PFPAs, and C6/C6, C6/C8, and
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C8/C8 PFPiAs were obtained from Wellington Laboratories (Guelph, ON). However, the
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amount of chemicals required and their costs to perform all sorption experiments precluded the
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use of these analytical standards. Instead, the technical product, Masurf®-780 (Mason Chemical
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Company, Arlington, IL), was used for the majority of these experiments. Using the individual
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analytical standards from Wellington Laboratories, the percent composition of this technical
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mixture was determined by standard addition to be 6.9±0.4% C6 PFPA, 5.8±0.4% C8 PFPA,
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3.2±0.5% C10 PFPA, 4.3±1.0% C6/C6 PFPiA, 4.7±0.9% C6/C8 PFPiA, and 0.6±0.1% C8/C8
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PFPiA. This percent composition was used to adjust the PFPA and PFPiA concentrations
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reported here. Details on how this composition compared to that previously reported by D’eon
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and Mabury30 and Lee and Mabury37 are provided in the Supporting Information (SI), as well as
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a list of all other chemicals used in this study.
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Soils. Four loam soils (A–D) were collected around Southern Ontario, Canada, while
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another loam soil (E) was collected in Athens, Georgia, US. The Elliott silt loam (F) and Florida
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Pahokee peat (G) were obtained, air-dried and pre-sieved, from the International Humic
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Substances Society (Golden, Colorado), and used as received. All other soils were sieved with a
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2 mm stainless steel mesh prior to oven drying at 105oC and homogenized finely using a mortar
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and pestle. Soil characterization data are provided in Table S1.
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Batch Sorption Experiments. All sorption experiments were performed in compliance
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with the Organization for Economic Cooperation and Development (OECD) guidelines for
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studying sorption.38 Prior to all experiments, the soils were pre-equilibrated with 0.10 mM
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mercuric chloride (HgCl2) and 0.01 M calcium chloride (CaCl2) in water by shaking overnight at
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200 rpm.
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A preliminary study was performed to determine the soil to solution ratio at which >50%
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by mass of the chemical has sorbed to the soil and the time to reach sorption equilibrium, the
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details of which are described in the SI. Except for C6 PFPA, 25) are likely necessary to retain these compounds in a larger aqueous
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compartment for measurements. However, using different soil to solution ratios optimized for
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each analyte’s sorptive capacity may not represent realistic field soil moisture conditions and
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precludes a robust comparison of sorption parameters. As a result, a ratio of 1:10 (5 g: 50 mL)
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and an equilibration time of 24 hours were chosen for the following experiments, as these
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parameters were consistent with those used previously for investigating sorption of PFAAs in
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sediments.16–19
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Sorption kinetics were determined by equilibrating 5 g of each soil type with 50 mL of
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0.01 M CaCl2, spiked with 10 μg/mL of Masurf, followed by sampling at 2, 4, 6, 8, and 24 hours.
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The percentage of analytes remaining in the aqueous phase and their corresponding distribution
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coefficients were determined based on separate analysis of the aqueous and soil phase upon
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reaching equilibrium. This experiment was repeated at four other concentrations (0.5, 1, 5, and
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50 μg/mL of Masurf), with the exception that the aqueous and soil phases were sampled only
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once at 24 hours, and together with the sorption data determined at 10 μg/mL, five-point sorption
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isotherms spanning two orders of magnitude were established. At the end of the experiment, the
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unused portions of the aqueous phases collected at 24 hours from the controls, soil blanks, and
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soils spiked with the target analytes were analyzed for dissolved organic carbon (DOC).
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Desorption kinetics were determined by agitating 10 μg/mL of Masurf with 5 g of Soil A
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and 50 mL 0.01 M CaCl2 until sorption equilibrium (i.e. 24 hours), after which the aqueous
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phase was entirely removed and replaced with an equal volume of 0.01 M CaCl2 without the test
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chemicals. The new mixtures were further agitated with periodic removal for analysis at 2, 4, 6,
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24, 30, and 52 hours.
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In all the experiments, blanks consisting of soil and 50 mL 0.01 M CaCl2 without the test
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chemicals were included to monitor for background contamination. A set of triplicate control
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samples (n = 3), consisting of only Masurf in 0.01 M CaCl2 in the absence of soil, were also
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included to test for the potential of abiotic degradation and adsorption to the container walls.
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Analysis of these controls revealed 75%) of the C8 and C10 PFPAs and all three
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PFPiAs to the container walls (Figure S2A), but similar declines in aqueous concentrations were 10 ACS Paragon Plus Environment
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also observed in Masurf-spiked water only. The fact that C6 PFPA remained mostly in the
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aqueous phase of both sets of controls and also in the aqueous phases equilibrated with the
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majority of the soils suggests complexation with Ca2+ and Hg2+ from the biocide may only
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minimally impact PFPA and PFPiA sorption and this behavior is expected to persist for the
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longer chain lengths as this interaction should be predominantly driven by the anionic headgroup
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of the PFPAs and PFPiAs. As expected, the presence of soil greatly reduced this surface
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adsorption to