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Sorption of Fluorotelomer Sulfonates, Fluorotelomer Sulfonamido Betaines, and a Fluorotelomer Sulfonamido Amine in National Foam Aqueous Film-Forming Foam to Soil Krista A. Barzen-Hanson, Shannon E. Davis, Markus Kleber, and Jennifer A. Field Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.7b03452 • Publication Date (Web): 02 Oct 2017 Downloaded from http://pubs.acs.org on October 3, 2017

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Environmental Science & Technology

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Sorption of Fluorotelomer Sulfonates, Fluorotelomer Sulfonamido Betaines, and a Fluorotelomer Sulfonamido Amine in National Foam Aqueous Film-Forming Foam to Soil

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Krista A. Barzen-Hanson,a Shannon E. Davis,a,b Markus Kleber,c and Jennifer A. Fieldd*

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a

Department of Chemistry, Oregon State University, 153 Gilbert Hall, Corvallis, Oregon 97331, United States b Currently at School for the Environment, University of Massachusetts, Boston, 100 William T Morrissey Boulevard, Boston, MA 02125, United States c Department of Crop and Soil Science, Oregon State University, 3017 ALS Building, 2750 SW Campus Way, Corvallis, OR 97331, United States d Department of Environmental and Molecular Toxicology, Oregon State University, 1007 ALS Building, 2750 SW Campus Way, Corvallis, Oregon 97331, United States *Corresponding author

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Address: Oregon State University, 1007 ALS Building, 2750 SW Campus Way, Corvallis, OR 97331. Phone: (541) 737-2265. Fax: (541) 737-0497. E-mail: [email protected]

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ABSTRACT

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During fire fighter training, equipment testing, and emergency responses with aqueous film-

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forming foams (AFFFs), mg/L concentrations of anionic, zwitterionic, and cationic per- and

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polyfluoroalkyl substances (PFASs) enter the environment. Since the behavior of zwitterionic

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and cationic PFASs in the subsurface is unknown, batch sorption experiments were conducted

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using National Foam AFFF, which contains anionic fluorotelomer sulfonates (FtSs), zwitterionic

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fluorotelomer sulfonamido betaines (FtSaBs), and cationic 6:2 fluorotelomer sulfonamido amine

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(FtSaAm). Sorption of the FtSs, FtSaBs, and 6:2 FtSaAm to six soils with varying organic

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carbon, effective cation exchange capacity, and anion exchange capacity was evaluated to

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determine sorption mechanisms. Due to poor recovery of the FtSaBs and 6:2 FtSaAm with

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published PFAS soil extraction methods, a new soil extraction method was developed to achieve

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good (90-100 %) recoveries. The 6:2 FtSaAm was depleted from the aqueous phase in all but

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one soil, which is attributed to electrostatic and hydrophobic interactions. Sorption of the FtSs

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was driven by hydrophobic interactions, while the FtSaBs behave more like cations that strongly

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associate with the solid phase relative to groundwater. Thus, the sorption mechanisms of the

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FtSs, FtSaBs, and 6:2 FtSaAm are more complex than expected and cannot be predicted by bulk

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soil properties.

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INTRODUCTION Per- and polyfluoroalkyl substances (PFASs) are ubiquitous contaminants in the

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environment1-3 and in wildlife,4,5 in part due to the release of aqueous film-forming foams

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(AFFFs) during fire-related emergencies, equipment testing, and training. AFFFs are complex

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mixtures that contain solvents, hydrocarbon surfactants, and anionic, zwitterionic, and cationic

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PFASs.6,7 Because AFFFs are proprietary mixtures, the composition8-11 and concentrations12 of

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PFASs in AFFFs were reverse-engineered to understand their environmental impact. At the time

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of application, AFFFs are diluted to either 3% or 6%,6,7 resulting in high (mg/L) concentrations

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of PFASs12 that subsequently enter the environment.

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Sorption of perfluoroalkyl carboxylates (PFCAs) and perfluoroalkyl sulfonates (PFSAs)

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increases with the organic carbon content in soils and sediments,13-16 as measured by solid-water

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partition coefficients (Kd) and organic carbon normalized solid-water partition coefficients. The

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contributions to sorption of PFSAs and PFCAs from electrostatic interactions with mineral

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phases increases when organic carbon is low (0-0.78%).14,17 Within the PFCA or PFSA

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homologous series, longer-chained homologs have higher partition coefficients than short-

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chained homologs.15,16,18-20

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In contrast, the anionic fluorotelomer sulfonates (FtSs) and zwitterionic and cationic

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PFASs present in groundwater,12 soil,21,22 and sediment23-25 have received little attention. Two

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studies report field-derived Kd values for the 6:2 FtS,23,26 while only one study reports field-

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derived Kd values for the zwitterionic 6:2 fluorotelomer sulfonamido betaine (FtSaB) and the

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cationic 6:2 fluorotelomer sulfonamido amine (FtSaAm).23 The ‘n:2’ fluorotelomer-based

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PFASs are characterized by carbon chains with ‘n’ perfluorinated carbons that are bonded to two

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non-fluorinated methylene carbons. For example, 6:2 FtS has six perfluorinated carbons that are

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bonded to two non-fluorinated methylene carbons that are then bonded to a sulfonate group.

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Typically, batch sorption experiments containing a mixture of PFCAs and PFSAs are

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performed under controlled conditions.15,16,20 However, a similar experimental set-up for

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sorption of zwitterionic and cationic PFASs is challenging due to the lack of authentic standards

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and mass-labeled internal standards for the zwitterionic and cationic PFASs, and few reference

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materials are available.12 Furthermore, mass balance of the PFCAs and PFSAs is typically 3 ACS Paragon Plus Environment


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achieved by extracting the soil or sediment.15,16 To date, all studies examining concentrations of

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zwitterionic and cationic PFASs on soil and sediment rely on extracting with methanol and either

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1.5-200 mM acid or base. However, this approach has only been validated by spiking dry soils

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with standards dissolved in methanol and allowing the methanol to evaporate for 2-48 h.22-24,27

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Others demonstrate that higher concentrations of acid (e.g. 0.5-1 M) in methanol are necessary to

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quantitatively recover cations, particularly if the cations have been allowed to equilibrate in

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water with the soil and/or sediment.28-31 If a higher concentration of acid in methanol for the

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extraction of zwitterionic and cationic PFASs from soil, then both concentrations24,25,27 and field-

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derived Kd estimates23 of zwitterionic and cationic PFASs may be significantly underestimated

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due to incomplete extraction of the (presumably) equilibrated soil and sediment.

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It is difficult to simulate the complexity of processes that occur at AFFF-impacted field

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sites during sorption experiments. To the best of our knowledge, only two studies report PFCA

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and PFSA sorption in the presence of ionized hydrocarbon surfactants,15,32 which occur at similar

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levels to PFASs in AFFFs along with solvents.6,7 Of the two studies, one reports an increase in

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the sorption of a single compound, perfluorooctane sulfonate (PFOS), in the presence of a single

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cationic hydrocarbon surfactant, and another single anionic hydrocarbon surfactant decreased

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PFOS sorption.32 In the second study, another anionic hydrocarbon surfactant caused either no

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change or a slight decrease in sorption of long-chained (n ≥ 6) perfluoroalkyl acids when present

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in a mixture of PFASs.15 However, neither of the two studies employed conditions that

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simulated the application of AFFF that contain mg/L levels of PFASs and other additives to

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pristine soil at a field site.

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Little is known about the sorption behavior of anionic, zwitterionic, and cationic PFASs

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present in AFFF, especially once concentrated AFFF is applied to soils at field sites. To the best

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of our knowledge, batch sorption experiments have not been conducted using AFFF containing a

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range of PFASs and concentrations in the same range as those found in groundwater (ng/L to

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mg/L) and AFFF applied to fight fires (mg/L to g/L). National Foam AFFF was selected because

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it was approved for use by the U.S. military since 1976,9 contains anionic FtSs, zwitterionic

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FtSaBs, and cationic 6:2 FtSaAm (Fig. 1),9,12 and is still available today on the Qualified Product

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List.33,34 The anionic FtSs are predicted to sorb via hydrophobic interactions as observed for

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PFAAs,14-16,35 while zwitterionic FtSaBs and cationic 6:2 FtSaAm are expected to sorb via 4 ACS Paragon Plus Environment

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electrostatic interactions in a manner analogous to zwitterionic and cationic pharmaceuticals.36-38

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Six uncontaminated soils with ranging levels of organic carbon, effective cation exchange

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capacity (ECEC), and anion exchange capacity (AEC) were used to investigate the sorption

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mechanisms of the FtSs, FtSaBs, and 6:2 FtSaAm contained in National Foam AFFF. While

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sorbate competition is possible, assessing sorbate competition was outside the scope of the study.

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Due to the potential for sorbates (FtSaBs, 6:2 FtSaAm), organic matter, and iron oxides to

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speciate over a range of pHs, the influence of pH on the sorption of the anionic FtSs, zwitterionic

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FtSaBs, and cationic 6:2 FtSaAm sorption was determined with a single soil titrated to pH 4 and

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pH 7 as well as the native pH (pH 5.1). Existing soil extraction methods for zwitterionic and

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cationic PFASs22-24,27 did not provide adequate mass balance; therefore, a new soil extraction

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method was developed to enable complete mass balance.

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EXPERIMENTAL

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Chemicals. A description of all materials, including solvents, chemicals, and analytical

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standards, is provided in the Supporting Information (SI).

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Soil Sample Collection and Characterization. Six uncontaminated soils (Soils 1-6) were

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selected from an archived soil collection at Oregon State University that constrained pH to 5.0-

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5.5 but covered a range of soil characteristics, including organic carbon, ECEC, and AEC (Table

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1, Table S1). Methods for soil characterization are provided in the SI. All soil characterization

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was performed prior to autoclaving, which is known to change organic matter structure and alter

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the clay fraction.39 Soils were sieved to < 2 mm and air dried prior to homogenization with a

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mortar and pestle before use in sorption experiments.

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Batch Sorption Experiments. Analyte Source Selection. Analyte sources and rationale are

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provided in the SI.

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Sorption Isotherm Experiment. All isotherms, with the exception of the preliminary experiment

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involving a commercial Capstone product (see SI), were created using National Foam AFFF.

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Material Safety Data Sheets for National Foam AFFF indicate 4-25 % solvent,7 but after dilution

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in surrogate groundwater, solutions were comprised of < 0.1 % organic solvent. A 21-point

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isotherm was constructed using Soil 1, with initial aqueous phase concentrations (Cwi) of 1.8 –

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240,000 nM for the 6:2 FtSaB. Eleven-point isotherms were constructed using Soils 2-6, and 5 ACS Paragon Plus Environment


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two additional isotherms were constructed with Soil 1 that was titrated to pH 4 and pH 7. For the

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11-point isotherms, Cwi ranged from 1.8 to 440 nM for the 6:2 FtSaB. The upper-end Cwi for the

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21-point and the 11-point isotherms were selected based on the 3% AFFF in water used in

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firefighting and a mid-range concentration for the 6:2 FtS in AFFF-impacted groundwater (21 –

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520 nM),12 respectively. Since all analytes were delivered as a mixture, only the concentration

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range for the most abundant analyte is reported here; the remaining concentration ranges and the

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relative ratios of each analyte are given in Table S2 and Table S3, respectively (see SI).

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Replicates, which consisted of homogenized soil spiked with the same volume of National Foam

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AFFF stock solution, were created for 6:2 FtSaB Cwi of 8.8 nM (n=4), 110 nM (n=3), and

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240,000 nM (n=3). All other data are derived from individual reactors.

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Homogenized soil was weighed (1.00 ± 0.02 g) into each 50 mL polypropylene

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centrifuge tube (VWR, Radnor, PA) and autoclaved for 40 min at 121 °C with a 20-min drying

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time (Consolidated Sterilizer Systems Model SSR-2A-ADVPB, Allston, MA). Vials contained

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the autoclaved soil and 10 mL of a groundwater surrogate (0.5 mM calcium chloride),16,35 to

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which only the diluted National Foam AFFF was spiked (Table S4; see SI). The soil-water ratio

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was optimized for the primary component in the AFFF (6:2 FtSaB). Because the ratios of all

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components were fixed in the AFFF and standards for the individual AFFF components are not

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available commercially, the solution composition could not be optimized for all components.

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Positive controls, consisting of groundwater surrogate and AFFF but no soil, were prepared at

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the Cwi of the replicates. One negative control, consisting of the autoclaved soil and groundwater

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surrogate but no AFFF, was prepared with each isotherm. Vials were allowed to equilibrate for

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24 h on a wrist action shaker at 10° rotation (Burrell Corporation, Model 75, Pittsburg, PA)

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based on previous literature (see SI).13,17,32,35

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Following equilibration, batch reactors were centrifuged at 2808 rcf for 20 min

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(Eppendorf, Model 5810R, Hauppauge, NY), and the supernatant was decanted into a separate

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15 mL centrifuge tube (VWR). The soil was transferred into a separate 50 mL centrifuge tube

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using deionized water. The empty vial from which the aqueous and soil phases had been

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removed (hereafter referred to as ‘vial phase’) and the aqueous phase were stored at -20 °C. The

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soil was freeze dried (VirTis Sentry 2.0, SP Scientific, Warminster, PA) for 48 h and stored in

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the dark at 22 °C until extraction. 6 ACS Paragon Plus Environment

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pH Adjustments to Pre-equilibrated Soil. Methods for pH adjusting the soils are provided

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in the SI. Briefly, soil was pre-equilibrated with groundwater surrogate for 24 h, over-titrated to

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either pH 3.3 or pH 8, and pre-equilibrated for another 24 h before AFFF was spiked.

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Sample Preparation. Aqueous Phase. The aqueous phase sample preparation was based on a

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slight modification of a micro liquid-liquid extraction, as established previously.12 A description

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is provided in the SI.

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Soil phase. Mass balance experiments consisted of quadruplicate vials of Soil 1 with 110 nM 6:2

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FtSaB Cwi. Following equilibration for 24 h, the three phases were separated and extracted

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individually. Initially, mass balance experiments were conducted using the aqueous phase

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extraction and a soil extraction typically used for cationic and zwitterionic PFASs.23,24,27 The

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vial and soil phases were extracted using 1.5 mM ammonium hydroxide in methanol as

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described elsewhere.27 Preliminary experiments indicated poor recovery of the zwitterionic 6:2

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FtSaB and the cationic 6:2 FtSaAm (Table S6) when soil and analytes were equilibrated in an

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aqueous phase (and not methanolic spikes) and extracted using relatively mild conditions (1.5-

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200 mM base in methanol22,24,27).

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Soil extraction methods for cationic, nonfluorinated surfactants typically use either 0.5

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M30 or 1 M HCl28,29,31 in methanol. The 1 M HCl in methanol extraction conditions proved to be

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problematic (unpublished data). Therefore, 0.5 M HCl in methanol was used in all subsequent

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extractions. The soil extraction procedure was modified slightly from Houtz et al.27 and is

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outlined in the SI.

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Mass balance experiments were repeated using the acidic methanol extraction for the vial

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and soil phases. Recoveries improved significantly for the 6:2 FtSaB and the 6:2 FtSaAm (90-

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100 % recovery; Table S6). Having achieved mass balance in separate experiments with an

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experimental set-up analogous to that used to generate isotherms, mass balance was assumed for

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all vials. Mass balances were not determined for each vial used to construct the isotherms due to

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the greater variability associated with subsampling the AFFF concentrate, and therefore, the

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variability in the mass of analyte added to each vial. Mass balance experiments also indicated

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that sorption to the vial walls was minimal for all analytes (unpublished data). Therefore, only



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the aqueous and soil phases were extracted for each vial (not the vial itself).

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LOD and LOQ parameters for soil extractions (Table S7) and descriptions are provided in the SI.

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Analytical Methods. A description of the analytical methods, including chromatography,

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MS/MS, calibration, analyte quality classification, and quality control measures, is based on

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previous literature10,12 and provided in the SI.

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Data Treatment. Measured soil phase concentrations for each analyte were adjusted to account

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for the mass of each analyte remaining in the pore water that was forced onto the soil during

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freeze drying (see SI for details).

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Whole method

Sorption isotherms were fitted using the following Freundlich isotherm model equations:

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= (Eqn 1) or = + (Eqn 2)

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where Kf [(nmol kg-1)(nmol L-1)-n] is the Freundlich sorption coefficient and n (unitless) is a

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measure of nonlinearity and represents the free energy associated with adding more sorbate to

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the sorbent.40 Kf values (Table S8) were converted to the concentration-specific Kd values using

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the following equation:

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= ∗ (Eqn 3)

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where Cw is the equilibrium concentration of the aqueous phase in the 8.8 or 110 nM Cwi vials, to

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enable comparisons between isotherms and across other sorption studies for other PFASs (see SI

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for details). The Kd values at Cwi = 110 nM are used for the FtSs and the 6:2 FtSaAm, while the

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Kd values at Cwi = 8.8 nM are used for the FtSaBs in the discussion below (see SI for rationale).

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Statistical Analysis. Correlation coefficients, 95% confidence intervals (CI), and p-values were

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calculated for all correlations using R (version 3.3.3).41 Correlations were computed using

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Pearson’s moment correlations and did not incorporate the 95% CI of each Kd value. Correlation

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coefficients and p-values are interpreted with caution due to the non-normal distribution of the

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soil property values.

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pKa. pKas for the functional groups on FtSs, FtSaBs, and FtSaAms were estimated by comparing

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the molecules to model compounds in the literature (see SI) and using the estimation software

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Chemicalize42 (Fig. 1). 8 ACS Paragon Plus Environment

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RESULTS AND DISCUSSION

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Commercial Product vs. AFFF. Preliminary experiments were performed to determine if the

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sorption of the 6:2 FtSaB from the Capstone product and National Foam AFFF at low

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concentrations (1.8 – 18 nM) was significantly different at the 95% CI (Fig. S3). This

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information was used to select the most appropriate product with which to conduct subsequent

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sorption experiments. No significant difference in the sorption of the 6:2 FtSaB between the

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Capstone product and National Foam AFFF was observed. The Capstone product is a simple

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mixture containing only 6:2 FtS, 6:2 FtSaB, and 6:2 FtSaAm with no hydrocarbon surfactants

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but percent levels of solvent,43 while National Foam AFFF contains the analytes plus percent

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levels of solvents and hydrocarbon surfactants.7 Given the AFFF’s greater complexity and the

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fact that the AFFF has been used since the 1970s and is still listed on the U.S. Military’s

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Qualified Products List,33,34 all subsequent sorption experiments were performed with National

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Foam AFFF.

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Sorption Isotherms. Sorption experiments with National Foam AFFF were conducted using six

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soils, with a subset of experiments conducted with a single soil (Soil 1) but at three different pHs

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(pH 4, 5.1, and 7). The objective was to test soils over a range of properties in order to

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investigate correlations between Kd and organic carbon, ECEC, AEC, and pH. Soil 6 is unique

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with regard to ECEC, since the clay fraction of Soil 6 is dominated by smectite with low

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(0.098%) organic carbon (Tables 1, S1). Soil 2, which also has low organic carbon and the

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highest ECEC (double that of Soil 6), contains some smectite as well as pedogenic iron oxides

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and other clay minerals. Although Soils 2 and 6 have similar organic carbon and ECEC, the

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additional minerals and pedogenic iron oxides present in Soil 2 may interact with the zwitterionic

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FtSaBs and cationic 6:2 FtSaAm through secondary interactions, such as sorbate-sorbate

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interactions. Sorption of some organic cations is influenced by the location of mineral and

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organic matter exchange sites.44 The greater complexity of the mineral phase in Soils 1-5

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indicates the possibility of sorbate-sorbent interactions through a variety of mechanisms not

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limited solely to electrostatic interactions, although the proportion of each sorption mechanism

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for a given analyte needs to be determined.

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Sorption was nonlinear for all soils (Table S8) for the 6:2 FtS (n = 0.8-1.2), 8:2 FtS (n = 0.74-1.1), 6:2 FtSaB (n = 0.8-1.5), 8:2 FtSaB (n = 0.41-1.8), and 6:2 FtSaAm (n = 0.17). Ranges 9 ACS Paragon Plus Environment


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for the Kd values (Table S9) are 3.1-12 L/kg (6:2 FtS, Cwi = 110 nM), 17-120 L/kg (8:2 FtS, Cwi =

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110 nM), 23-150 L/kg (6:2 FtSaB, Cwi = 8.8 nM), 34-400 L/kg (8:2 FtSaB, Cwi = 8.8 nM) , and

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650 L/kg (6:2 FtSaAm, Cwi = 110 nM). The ranges of n for the anionic FtSs are consistent with

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other batch sorption experiments for the anionic PFCAs and PFSAs.15,16,35 However, the n

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ranges for the zwitterionic FtSaBs and the 6:2 FtSaAm are wider than the anionic FtSs, which

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may indicate multiple sorption energies or mechanisms,40 as will be discussed below. Due to

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the experimental conditions used, complete depletion of several analytes (< 0.18 nM) limits the

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discussion to the behavior of 6:2 FtS, 8:2 FtS, 6:2 FtSaB, 8:2 FtSaB, and 6:2 FtSaAm (Soil 6

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isotherm only).

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Soil 1 Native pH (5.1) Isotherm. Log-transformed Freundlich isotherms (Eqn 2; Fig. 2) of the

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anionic 6:2 and 8:2 FtS and the zwitterionic 6:2 and 8:2 FtSaB for Soil 1 at pH 5.1 indicates the

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relative sorption strength of each sorbate. The values of the y-intercepts (i.e. log Kf) indicate that

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the strongest sorbed analyte is the 8:2 FtSaB, followed by the 8:2 FtS, 6:2 FtSaB, and 6:2 FtS,

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which is the weakest sorbed analyte. Within each class (FtS and FtSaB), sorption increases as

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the fluorinated chain length increases. The increase in sorption with fluorinated chain length for

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other PFASs was observed in previous sorption studies.15,16,19,20,45 The apparent increase of the

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last 3 points in the 6:2 FtS isotherm and the last 2 points of the 6:2 FtSaB isotherm may suggest

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an increase in secondary sorbate-sorbate interactions40,46,47 due to n > 1 for the 6:2 FtS and 6:2

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FtSaB, which may indicate sorbate-sorbate interactions48. The last 2-3 points of the isotherms

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were included in the analysis since the concentrations used to generate the points are

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environmentally relevant. Moreover, tentative exclusion of the data points did not significantly

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alter the observed trend or the quality of the correlation (Table S10). For the purposes of brevity,

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only the isotherm for Soil 1 at the native pH (5.1) are discussed here. Isotherms for the 6:2 and

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8:2 FtSs, 6:2 and 8:2 FtSaBs, and 6:2 FtSaAm (Soil 6 isotherm only) for Soils 2-6, Soil 1 pH 4,

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and Soil 1 pH 7 are provided in the SI (Fig. S4-S10) and are included in the discussion below.

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The sorption strength of each sorbate in a given soil (Soils 2-6, Soil 1 pH 4, and Soil 1 pH 7)

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may be different from the order discussed for the Soil 1 native pH isotherm.

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FtS, FtSaB, and 6:2 FtSaAm Sorption. Log Kd. Previous studies based on batch sorption

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experiments with PFASs report changes in log Kd as the number of CF2 groups increase but only

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for anionic PFASs. The log Kd increase per CF2 group between the 6:2 and 8:2 homologs for the 10 ACS Paragon Plus Environment

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FtSs and FtSaBs was 0.41 ± 0.059 and 0.20 ± 0.088, respectively (Table S11). The increase in

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log Kd per CF2 group for the anionic FtSs is consistent with the 0.5-1 log Kd increase for the

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anionic PFCAs and PFSAs.15,16,18,49 The log Kd increase per CF2 group for the zwitterionic

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FtSaBs is lower than that for FtSs, and the large 95 % CI may be partially attributed to greater

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analytical uncertainty about FtSaB concentrations (see Analytical Methods in SI). The increase

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in log Kd with an increase in the number of CF2 groups indicates that hydrophobic interactions

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may be an important sorption mechanism15 for the anionic FtSs and to a lesser extent for the

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zwitterionic FtSaBs. The lower log Kd per CF2 group for the FtSaBs indicates that a different

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primary sorption mechanism, such as electrostatic interactions, may influence sorption more

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heavily.

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For a given fluorinated chain length, the 6:2 FtSaB has a larger Kd (Cwi = 110 nM) than

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the 6:2 FtS for all soils and conditions, which is likely due to the greater size, molecular weight,

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and type of charges associated with 6:2 FtSaB. Since the 6:2 FtSaB head group has one positive

281

charge and a terminal negative charge at pH 5.1 (Fig. 1), the positive charge within the head

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group is contributing to increased interactions, presumably due to electrostatic interactions. The

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6:2 FtSaAm, which was completely sorbed to all soils except Soil 6, has one terminal positive

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charge (Fig. 1). The 6:2 FtSaAm has the highest observed Kd value (Cwi = 110 nM, Table S9), so

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the number of positive charges cannot explain the difference in sorption. Rather, the position of

286

the charges as well as the net charge appear to be relevant. In this case, the terminal positive

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charge on the 6:2 FtSaAm leads to greater sorption onto soil, whereas the 6:2 FtSaB, while

288

having a net neutral charge, also has a terminal negative charge, which may exert a repulsive

289

effect in close proximity to the soil surface, as was observed in the sorption of oxytetracycline

290

and ciprofloxacin50.

291

The single-point Kd (Cwi = 110 nM) values for the 6:2 FtS (3.1-12 L/kg; Table S9) are in

292

good agreement with the field-Kd values reported (5.8 L/kg23 and 3.2 L/kg26) for sediment-water

293

systems. The Kd (Cwi = 8.8 nM) values for the 6:2 FtSaB range from 23-240 L/kg (Table S9),

294

which are significantly greater than the single field-Kd of 4.5 L/kg23. The single Kd (Cwi = 110

295

nM) value obtained for the cationic 6:2 FtSaAm was 650 L/kg (Table S9), which is also

296

significantly greater than the single reported field-Kd of 34 L/kg23. The lower values reported by

297

Boiteux et al.23 may be due to incomplete extraction of zwitterionic and cationic PFASs from 11 ACS Paragon Plus Environment


Page 12 of 32

298

sediment using the milder extraction conditions (1% acetic acid in methanol) compared to the 0.5

299

M HCl in methanol in the present study. Alternatively, the organic matter and ECEC properties

300

of the sediment, which were not reported, may have contributed to the lower Kd values.23

301

Correlations. Organic Carbon. The experimental data indicate that anionic 6:2 FtS (0.83, p =

302

0.039) and 8:2 FtS (0.96, p = 0.0019) correlate with organic carbon (Fig. 3, Table S12). The Kd

303

(Cwi = 110 nM) values for the longer-chained 8:2 FtS are consistently greater than those of 6:2

304

FtS except for Soil 4 (Kd = 120 L/kg), in which the 8:2 FtS Kd is disproportionately high for an

305

unknown reason. Removing the Kd values for Soil 4 from the correlation analysis, no correlation

306

exists for the 6:2 FtS (0.21, p = 0.73) or the 8:2 FtS (0.42, p = 0.48), indicating that the Soil 4 Kd

307

(Cwi = 110 nM) values are likely outliers. Log transformations of both Kd and organic carbon did

308

not improve correlations (Table S13). At the lowest levels of organic carbon (0.098 – 0.12%),

309

the Kd (Cwi = 110 nM) values for the 6:2 and 8:2 FtS are similar in magnitude to those for the

310

soils with higher organic carbon (1.0 – 2.3%), which indicates that the mineral phase in the low

311

organic carbon soils (Soils 2, 6) influences sorption.14,17 The nonpolar fluorinated tail of the

312

anionic FtSs may interact with the hydrophobic (nonpolar) nanosites51-54 between the charged

313

sites on the mineral surface55 via weak hydrophobic interactions.14-16,56

314

The lack of a correlation in FtS sorption may be due to variations in the extent of organic

315

matter decomposition of the soils tested. The relationship between C/N in soil organic matter

316

and oxygen-containing functional groups is based on the following two premises: (a) the

317

majority of terrestrial organic matter is decomposed by oxidative depolymerization,57 which adds

318

oxygen-containing functional groups to oxidized compounds,58 and (b) oxidative

319

depolymerization of faunal debris (C/N > 25) is carried out by microbiota (C/N 6-10), in which

320

“fresh” organic debris with a large C/N (i.e. 30) are slowly converted into an amorphous

321

heterogeneous organic phase (humic substances) with a small C/N (i.e. 12).59,60 If the C/N ratio is

322

accepted as a rough proxy for the extent of organic matter decomposition61 with a large C/N

323

(C/N 25) indicating a relative prevalence of aliphatic C and nonpolar functionality (i.e., relatively

324

undecomposed organic matter), the C/N ratio can be treated as a rough proxy for the

325

decomposition stage ( = the "quality" of organic matter).62 Soils 1-3 and 5 have either low

326

organic carbon or a lower C/N ratio (greater number of carboxyl groups; Table 1, Table S1),

327

indicating a minimal influence of organic carbon on sorption (Soil 2) or less hydrophobic organic 12 ACS Paragon Plus Environment

Page 13 of 32


328

matter (Soils 1, 3, and 5). Therefore, the lack of correlation of FtS sorption with soil organic

329

carbon may be due to low hydrophobicity of the organic matter of the soils tested, with the

330

exception of Soil 4, which is more hydrophobic (higher C/N ratio; Tables 1 and S1) with fewer

331

carboxyl groups. Alternatively, the 6:2 and 8:2 FtS may require an organic carbon content

332

greater than 2.3% before hydrophobic interactions plays a significant role in their sorption. A

333

trend is difficult to discern without additional soils between 2.3% and 7.7% organic carbon.

334

To the best of our knowledge, studies examining PFAS sorption do not consider metrics

335

for the extent to which decaying organic matter is decorated with ionizable, oxygen containing

336

functional groups, which needs to be examined further. Many sorption studies of PFCAs and

337

PFSAs observe an increase in Kd with increasing organic carbon without any organic carbon

338

threshold,13-16 while a single study found little change in Kd with increasing organic carbon.63

339

No statistical evidence exists for correlations between the zwitterionic 6:2 FtSaB (-0.37, p

340

= 0.47) and the 8:2 FtSaB (-0.60, p = 0.40) and organic carbon (Fig. S11, Table S12), which is

341

consistent with reports that Kd values for zwitterionic pharmaceuticals do not correlate with

342

organic carbon content.36,37,64 The Kd (Cwi = 8.8 nM) values for the 8:2 FtSaB are consistently

343

and proportionally higher than the 6:2 FtSaB, due to the increased hydrophobicity of the

344

additional two CF2 groups. Relative to the anionic FtSs, the magnitude of the Kd values for the

345

zwitterionic FtSaBs are higher, which is likely due to the positive charge associated with the

346

polar head group, as will be discussed below. Cation exchange and electrostatic interactions

347

dominate,38 since electrostatic interactions are more thermodynamically favorable than

348

hydrophobic interactions65,66 or anionic ligand exchange,38 although hydrophobicity may help

349

exclude the sorbate from solution and aid in a higher extent of sorption.40 However, assessing

350

the thermodynamic favorability of the FtSaBs was outside the scope of the present study.

351

ECEC. No statistical evidence exists for correlations between the 6:2 FtS (-0.56, p = 0.25) and

352

the 8:2 FtS (-0.41, p = 0.42) and ECEC (Fig. 4a, Table S12), likely due to repulsion between

353

negatively charged organic matter and mineral surfaces and the negatively charged sulfonate

354

group. The high Kd (Cwi = 110 nM) of 120 L/kg for the 8:2 FtS in Soil 4 with an ECEC of 5.7 ±

355

0.80 meq/100 g is attributed to the more hydrophobic nature of the organic matter as discussed

356

above rather than cation exchange processes. The extent of cation bridging between Ca2+ and the

357

anionic FtSs was not assessed and is the focus of on-going research. 13 ACS Paragon Plus Environment


358

Page 14 of 32

Strong statistical evidence for positive correlations exist for the zwitterionic 6:2 FtSaB

359

(0.94, p = 0.0052) and 8:2 FtSaB (0.99, p = 0.0051) and ECEC (Fig. 4b, Table S12). The

360

proportionate increase in Kd (Cwi = 8.8 nM) from the 6:2 FtSaB to the 8:2 FtSaB may arise from

361

the greater affinity of the 8:2 FtSaB to the hydrophobic nanosites in between the localized

362

negative charges on the smectite surface.55 Alternatively, the hydrophobic perfluoroalkyl moiety

363

may allow easier exclusion from aqueous solution due to solvation effects.67 The cationic 6:2

364

FtSaAm was completely depleted from the aqueous phase in all but one soil (Soil 6; Fig. 4b),

365

which has the second highest ECEC (10 ± 3.4 meq/100 g). The terminal positive charge in the

366

cationic 6:2 FtSaAm may interact more readily with the cation exchange sites than the FtSaBs, in

367

which the terminal carboxyl group of the FtSaBs may repel the negatively charged CEC sites68.

368

Similarly, sorption studies with zwitterionic and cationic veterinary pharmaceuticals,36-38,64,69

369

organic cations,44,65 and cationic hydrocarbon surfactants70 indicate that electrostatic interactions

370

dominate, and contributions from hydrophobic partitioning are almost non-existent.38,44,65,70

371

Although the strong correlations between the zwitterionic FtSaBs and ECEC and the

372

single Kd (Cwi = 110 nM) value for the cationic 6:2 FtSaAm for Soil 6 indicate that electrostatic

373

interactions heavily influence sorption of zwitterionic and cationic PFASs, other mechanisms,

374

such as hydrophobic interactions, clearly have some impact since the 8:2 FtSaB has higher Kd

375

values than the 6:2 FtSaB. Similar to pharmaceuticals and other organic cations, traditional soil

376

properties may not be adequate to predict PFAS sorption, and new Kd models, such as those

377

developed by MacKay and Vasudevan,71 Droge et. al,44 and Jolin et. al,72 need to be incorporated

378

to assess the influence of electrostatic interactions, hydrophobic interactions, surface

379

complexation, and sorbate molecular volume.

380

AEC. No statistical evidence exists for a correlation between the anionic 6:2 FtS (0.58, p = 0.23)

381

and AEC, whereas some evidence exists for a correlation between the 8:2 FtS (0.83, p = 0.031)

382

and AEC (Fig. 5). However, the 8:2 FtS Kd value for Soil 4 is believed to be an outlier as

383

discussed in the Organic Carbon section; therefore, excluding Soil 4 from the correlation, no

384

evidence exists for a correlation between the 8:2 FtS (-0.50, p = 0.39) and AEC. Analysis of the

385

Kd values using AEC/ECEC73 and log-log transformations did not improve correlations (Table

386

S12, Table S13). The sorption of the 8:2 FtS is proportionally greater than the 6:2 FtS, except in

387

Soil 4. Since Soil 4 has both the highest organic carbon and the highest AEC, the influence of 14 ACS Paragon Plus Environment

Page 15 of 32


388

electrostatic interactions due to Soil 4’s positive charge and hydrophobic interactions cannot be

389

determined.

390

Anion exchange sites are fully protonated (pH ≤ 6; Fig. S13), singly coordinated

391

hydroxyl groups that arise from poorly crystalline forms of iron oxides (ferrihydrite) and

392

aluminosilicates.74 Soil 3 is a particularly oxide rich Ultisol, and Soil 4 has a poorly crystalline

393

mineral phase (“andic” soil properties) that contains singly coordinated hydroxyl groups (Table

394

1, Table S1). In the case of Soil 4 with the anionic 8:2 FtS, inner sphere complex formation via

395

anionic ligand exchange with the singly coordinated hydroxyl groups74 may contribute to the

396

observed increase in sorption. However, if contributions from inner sphere complex formation

397

strongly influenced the 8:2 FtS sorption in Soil 4, a proportional increase in the sorption of the

398

6:2 FtS would be expected. The lack of correlation between FtS sorption and AEC indicates that

399

the sulfonate group may not be as effective at surface complexation as carboxyl groups74,75 or

400

carboxyl groups with neighboring hydroxyl groups76.

401

No statistical evidence exists for correlations between the zwitterionic 6:2 FtSaB (-0.21, p

402

= 0.69) and 8:2 FtSaB (-0.58, p = 0.42) and AEC (Fig. S12). AEC trends with zwitterion

403

sorption are less frequently studied, but zwitterionic pesticide sorption experiments found either

404

an increase75 or no change77 in sorption with increasing AEC. The lack of correlation with the

405

zwitterionic FtSaBs and AEC is not unexpected because, in the case of zwitterionic

406

pharmaceutical sorption, the contribution of surface complexation and electrostatic interactions

407

with anion exchange sites has less influence than electrostatic interactions with cation exchange

408

sites.38,65,66

409

pH. As pH increases from 4 to 7 in Soil 1, Kd (Cwi = 110 nM) values remain unchanged for the

410

anionic 6:2 FtS (-0.55, p = 0.63) but decrease for the anionic 8:2 FtS (-0.99, p = 0.019; Fig. 6).

411

For the zwitterionic FtSaBs, however, Kd (Cwi = 8.8 nM) values decrease then remain constant as

412

pH increases (6:2 FtSaB: -0.81, p = 0.39; 8:2 FtSaB: -0.69, p = 0.51; Fig. 6). Due to the limited

413

statistical power with three data points, the correlations discussed should be interpreted

414

cautiously.

415 416

The decrease in Kd (Cwi = 110 nM) with increasing pH for the anionic 8:2 FtS is consistent with observed decreases in sorption for organic acids78 and other anionic PFASs16 15 ACS Paragon Plus Environment


Page 16 of 32

417

with an increase in pH. The decrease is approximately 0.17 log units per unit increase in pH,

418

assuming a linear fit, which is less steep relative to other anionic PFASs.16 Since the 6:2 and 8:2

419

FtS remain anions in the pH range studied (Fig. S13), the decrease in sorption is likely due to

420

deprotonation of the negatively charged organic matter (pKa ~ 4.5; Fig. S13)79,80 and thus anion

421

repulsion. Given that the zero-point-of-charge of iron and aluminum oxides is approximately

422

8.5, the oxide surfaces are increasingly protonated with decreasing pH and become increasingly

423

available to undergo anionic ligand exchange81 and, thus, result in a higher Kd at lower pH. The initial decrease in Kd (Cwi = 8.8 nM) that was then followed by no change in Kd (Cwi

424 425

= 8.8 nM) as pH increased for the zwitterionic FtSaBs is likely not due to the speciation of the

426

FtSaBs (Fig. S13) but rather the protonation of organic matter. Between pH 4 and pH 7, the

427

FtSaBs are zwitterionic with ~99 % deprotonated carboxyl group (negative charge) and a

428

positively charged quaternary amine, indicating that speciation does not significantly change in

429

the pH range studied. Soil 1 has an ECEC of 6.8 meq/100g at pH 5.1, and a batch reactor

430

containing Soil 1 and 440 nM 6:2 FtSaB Cwi has 6.8*10-2 mmol charge and 6.7*10-6 mmol

431

positively charged PFASs, indicating that the ECEC, even with changes in pH, would not be

432

exceeded in the current system.37 Instead, the speciation of the organic matter changes. At pH 4,

433

more than 50 % of carboxylic functional groups in soil organic matter will be protonated,

434

resulting in a joint contribution of hydrophobic interactions and electrostatic interactions through

435

cation exchange sites to FtSaB sorption. As pH increases and the carboxyl groups in the organic

436

matter become deprotonated, electrostatic interactions still influence sorption, but the negatively

437

charged organic matter likely repels the terminal carboxyl group of the FtSaB, causing sorption

438

to decrease. The finding contrasts those from sorption studies of veterinary pharmaceuticals in

439

which speciation changed as a function of pH and significantly influenced zwitterion sorption.36-

440

38,77

441

IMPLICATIONS

442

The low Kd values for the anionic 6:2 FtS suggest that the 6:2 FtS is highly mobile in

443

groundwater. Analysis of AFFF-impacted groundwater12,82 indicates that the 6:2 FtS is present

444

in high concentrations (up to mg/L). The higher Kd values indicate that the anionic 8:2 FtS,

445

zwitterionic FtSaBs, and cationic 6:2 FtSaAm are more likely to be associated with soil and

446

sediment. The anionic 8:2 FtS has been found in higher concentrations on AFFF-impacted soils 16 ACS Paragon Plus Environment

Page 17 of 32


447

and sediments,22,27,83 while the first reports of zwitterionic FtSaBs and cationic FtSaAms on

448

soil21 and sediment are currently being presented.22-25

449

By analogy, the anionic 8:2 FtS, zwitterionic FtSaBs, and cationic 6:2 FtSaAm are likely

450

to be found in PFAS source zones, especially where AFFFs were repeatedly applied at a field

451

site. However, current estimates of the mass of PFASs in the source zone are likely to be

452

significantly underestimated. Although the high Kd analytes may be immobilized in the source

453

zone, the complex interactions between anionic, zwitterionic, and cationic PFASs and soil may

454

impact their mobility in the subsurface. Furthermore, the high Kd values of the anionic 8:2 FtS,

455

zwitterionic FtSaBs, and the cationic 6:2 FtSaAm may not accurately predict the mobility of

456

these polyfluoroalkyl substances at all AFFF-impacted sites, as cationic PFASs have been found

457

in high abundance in AFFF-impacted groundwater.11 Mobile cationic PFASs have the potential

458

to impact drinking water sources, and non-fluorinated cations have been found to be more toxic

459

to aquatic species.84,85 Therefore, cationic PFASs might be more toxic by analogy. Removal of

460

mobile zwitterionic and cationic PFASs from drinking water sources is unknown, in part due to

461

the current analytical methods employed by the private sector, which only focus on well-studied

462

anionic PFASs (i.e. PFCAs and PFSAs).

463

Overall, the lack of correlations between the anionic FtSs, zwitterionic FtSaBs, and

464

cationic 6:2 FtSaAm and bulk soil parameters, such as organic carbon content, ECEC, and AEC,

465

indicates that the bulk parameters do not adequately predict sorption, at least for the compounds

466

studied. The influence of electrostatic interactions, hydrophobic interactions, cation bridging,

467

surface complexation, and structures of PFASs need to be understood in source zones before

468

robust remediation strategies can be proposed. For example, the multilayer sorption observed for

469

the anionic 6:2 FtS and the zwitterionic 6:2 FtSaB indicates that soil surfaces may become

470

coated in PFASs. Sorption of additional PFASs in AFFF to a soil already saturated with PFASs

471

may increase the sorption of the added PFASs, due to fluorophilic interactions between PFASs

472

sorbed to the surface and additional PFASs in solution.86 Furthermore, the impact of PFCAs and

473

PFSAs in the subsurface from AFFFs9,12 and biodegradation87-89 on the sorption of zwitterionic

474

and cationic PFASs remains unknown. Additionally, desorption and therefore the reversibility of

475

sorbed zwitterionic and cationic PFASs need to be determined, as the reversibly bound PFASs

476

may become mobile in the subsurface.20 17 ACS Paragon Plus Environment


477

Notes

478

The authors declare no competing financial interest.

Page 18 of 32

479 480

ACKNOWLEDGEMENTS

481

K. Barzen-Hanson was supported by the National Science Foundation’s Graduate Research

482

Fellowship Program Fellowship 1314109-DGE. The study was supported by the Strategic

483

Environmental Research and Defense Program Grant ER-2128 (J. Field). The authors would like

484

to thank Nicole Riddell and Alan McAlees of Wellington Laboratories, Inc. for providing the

485

pKa estimates and rationale, Steven Droge for suggesting Chemicalize for pKa values, and

486

Christopher Higgins of Colorado School of Mines for the insightful comments in the preparation

487

of this manuscript.

488 489

SUPPORTING INFORMATION AVAILABLE

490

Chemicals, details on isotherm set-up, absolute extraction efficiencies, soil extraction method

491

validation, analytical methods, pKa rationale, Tables S1-S13, Figures S1-S13. This information

492

is available free of charge via the Internet at http://pubs.acs.org.

493 494 495 496 497 498 499 500


Page 19 of 32

501


TOC Art

502



503

Page 20 of 32

Table 1. Soil properties. a

b

%N

C/N Ratio

CECc (meq/100g)

ECECd (meq/100g)

AECe (meq/100g)

% Clay

% Sand

% Silt

b

Soil #

pH

% OC

1

5.1

1.5

0.13

13

14 ± 0.59f

6.8 ± 0.68

0.20

22

6.3

71

2

5.5

0.12

0.02

6.8

22 ± 4.2

17 ± 2.2

0.58

26

10

64

3

5.1

2.3

0.20

13

17 ± 3.1

5.1 ± 0.56

1.2

48

14

39

4

5.0

7.7

0.43

21

36 ± 3.6

5.7 ± 0.80

2.2

18

37

45

5

5.0

1.0 ± 0.19

0.084 ± 0.0087

12 ± 1.1

16 ± 1.6

7.5 ± 0.44

0.28g

23 ± 3.2

4.8 ± 0.29

72 ± 3.0

6

5.2

0.098

0.004

30

16 ± 1.5

10 ± 3.4

0.33

24

3.6

73

504 505 506 507 508 509

a

pH at which the isotherm was conducted. bPercent organic carbon (% OC) and percent nitrogen, as determined by the method of Goni et al.90 cCation Exchange Capacity, measured by summation method, pH 7. dEffective cation exchange capacity measured at the native soil pH, as determined by the method of Gillman and Sumpter.91,92 eAnion exchange capacity at native soil pH (using water). fError computed as the 95% CI determined from a blind triplicate analysis. gOnly two values were valid, which has limited statistical power. No 95% CI was calculated.


Page 21 of 32


510 511

Figure 1. Structures of the dominant species of (A) the FtSs, (B) the FtSaBs, and (C) the 6:2

512

FtSaAm in National Foam AFFF at all native soil pHs.

513



Page 22 of 32

514 515

Figure 2. Log-transformed Freundlich isotherms for the anionic 6:2 and 8:2 FtSs and the

516

zwitterionic 6:2 and 8:2 FtSaBs for the Soil 1 native pH isotherm. The ovals highlight the sharp

517

increase in sorption.

518


Page 23 of 32


519 520

Figure 3. A lack of correlation for the anionic FtSs as a function of soil organic carbon. Error

521

bars indicate the 95% CI.

522



Page 24 of 32

523 524

Figure 4. A lack of correlation for (A) the anionic FtSs and (B) strong, positive correlations for

525

the zwitterionic 6:2 and 8:2 FtSaBs and the cationic 6:2 FtSaAm with increasing ECEC. Error

526

bars represent the 95% CI. The aqueous phase concentration was

Sorption of Fluorotelomer Sulfonates ... - ACS Publications

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