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Generation of perfluoroalkyl acids from aerobic biotransformation of quaternary ammonium polyfluoroalkyl surfactants Sandra Mejia Avendaño, Sung Vo Duy, Sébastien Sauvé, and Jinxia Liu Environ. Sci. Technol., Just Accepted Manuscript • DOI: 10.1021/acs.est.6b00140 • Publication Date (Web): 01 Aug 2016 Downloaded from http://pubs.acs.org on August 1, 2016
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Generation of perfluoroalkyl acids from aerobic
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biotransformation of quaternary ammonium polyfluoroalkyl
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surfactants
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Sandra Mejia Avendañoa, Sung Vo Duyb, Sébastien Sauvéb, Jinxia Liua,*
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a
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Department of Civil Engineering, McGill University, Montreal, Quebec, H3A 0C3, Canada
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Department of Chemistry, Université de Montréal, Montreal, Quebec, H3C 3J7, Canada
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*Corresponding author. Tel: +1 514 398 7938; fax: +1 514 398 7361
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E-mail address:
[email protected] 10 11 12
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Abstract
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The aerobic biotransformation over 180 d of two cationic quaternary ammonium
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compounds (QACs) with perfluoroalkyl chains was determined in soil microcosms and
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biotransformation pathways were proposed. This is the first time that polyfluoroalkyl cationic
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surfactants used in aqueous film-forming foam (AFFF) formulations were studied for their
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environmental fate. The biotransformation of perfluorooctaneamido quaternary ammonium
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salt (PFOAAmS) was characterized by a DT50 value (time necessary to consume half of the
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initial mass) of 142 d and significant generation of perfluoroalkyl carboxylic acid (PFOA) at
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a yield of 30 mol% by day 180. The biotransformation of perflurooctane sulfonamide
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quaternary ammonium salt (PFOSAmS) was very slow with unobservable change of the
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spiked mass; yet the generation of perflurooctane sulfonate (PFOS) at a yield 0.3 mol%
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confirmed the biotransformation of PFOSAmS. Three novel biotransformation intermediates
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were identified for PFOAAmS and three products including perfluorooctane sulfonamide
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(FOSA) for PFOSAmS through high-resolution mass spectrometry (MS) analysis and t-MS2
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fragmentation. The significantly slower PFOSAmS biotransformation is hypothesized to be
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due to its stronger sorption to soil owing to a longer perfluoroalkyl chain and a bulkier
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sulfonyl group, when compared to PFOAAmS. This study has demonstrated that despite
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overall high stability of QACs and their biocide nature, the ones with perfluoroalkyl chains
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can be substantially biotransformed into perfluoroalkyl acids in aerobic soil.
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Keywords: Polyfluoroalkyl and perfluoroalkyl substances (PFAS), aqueous film-forming
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foams (AFFF), quaternary ammonium compounds (QAC), biotransformation, soil
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Introduction
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Perfluoroalkyl and polyfluoroalkyl substances (PFAS) represent a major class of
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contaminants of emerging concern (CECs) found in groundwater, surface water, soil,
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sediment, and biota globally.1-5 Being designed to be chemically stable, PFAS species are
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highly persistent in the environment and some species are considered potential threats to
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human and environmental health.6, 7 In recent years, it has been recognized that practices
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of uncontrolled releases of PFAS-based aqueous film-forming foams (AFFFs) used for
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firefighting activities have resulted in many AFFF-impacted sites with elevated levels of
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PFASs.
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community, the research on AFFF contamination is still in its infancy. Consequently,
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regulatory frameworks for managing pollution associated with AFFFs mostly do not exist.
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Whereas the most prominent PFAS contaminant, perfluorooctane sulfonate (PFOS), is
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listed under the Stockholm Convention on Persistent Organic Pollutants for restricted
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production and use, numerous polyfluoroalkyl surfactants with various functionalities could
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be present in AFFF-impacted sites. Environmental fate, behaviors, and effect of these
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chemicals have not been sufficiently investigated.
8-10
Although there is extensive research on PFASs in the global scientific
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A major challenge in characterization and management of AFFF-impacted sites is the lack
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of information about identity and concentration of PFASs present in proprietary AFFF
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formulations. Thus, several recent studies have focused on identifying specific PFASs in
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major formulations using advanced mass spectrometry techniques.11-13 These studies have
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proposed structures of a large suite of cationic, anionic, and zwitterionic PFAS surfactants
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made by both Simon’s electrochemical fluorination (ECF) and fluorotelomerizaton
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chemistry. It is notable that all the newly identified PFASs are polyfluoroalkyl compounds, Page 3 of 33 ACS Paragon Plus Environment
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which have perfluoroalkyl carbon chain lengths varying from 4 to 12, and possess
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functionalities such as sulfonyl, thioether, tertiary amine, quaternary ammonium,
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carboxylate, sulfonate, amine oxide, betaine, etc. The extensive use of polyfluoroalkyl
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rather than perfluoroalkyl substances in AFFFs is probably due to a variety of functions that
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polyfluoroalkyl compounds can serve (e.g., as foaming, wetting, and dispersant agents), as
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well as high aqueous solubility, low volatility and low acid strength.14
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The identified cationic PFAS surfactants in these studies contain either tertiary amine or
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quaternary ammonium groups. Quaternary ammonium compounds (QACs) are functioning
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surfactants due to the presence of the permanent charge on the -NR4+ group, whereas
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tertiary amines are only precursor materials used for synthesizing QACs.14 Therefore, it is
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possible that the tertiary amines identified in these studies11-13 are only impurities carried
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from synthesis processes rather than effective ingredients, or breakdown products of
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corresponding QACs. The extensive use of PFAS-based QACs in AFFF formulations or as
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fire suppressants is documented in many patents.15-19 Thus, the question arises: what is the
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significance of cationic PFAS-based QACs, for which no data is available regarding
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environmental fate and effect, in AFFF-impacted sites?
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Another great challenge for characterizing and managing AFFF-impacted sites is
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inadequate understanding of the fate of PFAS surfactants once they are released into soil,
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surface and ground water environment. The primary environmental processes that can
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greatly complicate site investigation include microbially mediated degradation or abiotic
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transformation. It has been reported that despite the general persistence of PFASs, the
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polyfluoroalkyl compounds with non-fluorinated functionalities can undergo partial microbial Page 4 of 33 ACS Paragon Plus Environment
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transformation. Under aerobic conditions, many PFASs can be microbially degraded to a
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complex mixture of products including perfluoroalkyl acids (PFAAs) such as PFOS and
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perfluorooctane carboxylic acid (PFOA).20-23 Understanding biotransformation potential is
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thus essential for performing site investigation and conducting remediation if the latter is
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deemed necessary. Biotransformation products could have different physicochemical
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properties, mobility, and toxicity from their parent compounds. For instance, Phillips et al.
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demonstrated that the fluorotelomer carboxylic acids pose higher acute toxicity to aquatic
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organisms than their parent compounds of fluorotelomer alcohols.24 Knowing key
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biotransformation products or biomarkers could also reveal the sources of PFAAs, whether
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they are directly from AFFF products or from natural biotransformation of other PFAS
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surfactants.
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Several AFFF components, all of which are fluorotelomer-based, have recently been
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examined for their biotransformation potential: 6:2 fluorotelomer sulfonate (6:2 FTS),
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fluorotelomer thioether amido sulfonates (FTAoS), and 6:2 fluorotelomer sulfonamido
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betaine (6:2 FTAB).25,26 In contrast, no ECF-based AFFF components have been
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investigated as of date. Harding et al. found that under aerobic conditions, FTAoS can be
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readily biodegraded through a few intermediates to form FTSs.26 The later
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biotransformation of FTSs produced a range of characteristic fluorotelomer products that
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were previously identified by Wang et al., including x:2 fluorotelomer unsaturated carboxylic
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acids, x:3 fluorotelomer carboxylic acids, and perfluoroalkyl carboxylic acids (C4 – C8).27
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6:2 FTAB was also found to degrade aerobically in activated sludge following similar
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pathways as demonstrated by Harding et al.26, 28 As biotransformation pathways of these
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highly fluorinated compounds do not always follow known biotransformation pathways of Page 5 of 33 ACS Paragon Plus Environment
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non-fluorinated compounds, analysis by high-resolution mass spectrometry was
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indispensable in determining empirical molecular formula and structures of degradation
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products. Previous research on the biotransformation of PFAS has demonstrated
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differences between ECF-based compounds (non-AFFF components) and fluorotelomer
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derivatives. In both cases the non-fluorinated moiety undergoes degradation, but
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fluorotelomer derivatives undergo partial defluorination while ECF-based derivatives do not
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show any degradation of the perfluoroalkyl moiety.29 For this reason, it is expected that
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biotransformation of ECF-based AFFF surfactants will differ from previously reported
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fluorotelomer AFFF components.
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As there is no information on the transformation potential of ECF-based or cationic PFAS
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surfactants, the present study was conducted to fill such a knowledge gap. Two such ECF-
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based cationic surfactants were selected, which appear in several patents for their use in
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AFFFs: perflurooctane sulfonamide quaternary ammonium salt [PFOSAmS,
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F(CF2)8SO2NH(CH2)3N+(CH3)3] and perfluorooctaneamido quaternary ammonium salt
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[PFOAAmS, F(CF2)7CONH(CH2)3N+(CH3)3].15-19 Both compounds differ from the previously
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identified polyfluoroalkyl amines identified by D’Agostino and Mabury in the ammonium
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group: the identified compounds are tertiary amines, rather than quaternary ammonium
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salts.13 The corresponding perfluoroalkyl sulfonamide amine was also identified by Place
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and Field, and Backe et al. in AFFF formulations.11, 12 As previously stated, the QACs are
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the effective ingredients whereas the tertiary amine compounds are probably synthesis
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impurity or breakdown products of QACs; therefore QACs are studied instead of the tertiary
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amine compounds. The primary objective of the present study is to determine the aerobic
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biotransformation pathways and products. In addition, PFOSAmS and PFOAAmS mainly
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differ in the functional group that is immediately adjacent to the perfluoroalkyl chain, being
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sulfonyl and carbonyl groups, respectively; PFOAAmS has a C7 perfluoroalkyl chain
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whereas PFOS has C8. Therefore, it is of the interest of the authors to examine if the two
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cationic PFAS-based QACs have a similar potential to yield PFAAs as other negatively
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charged PFAS surfactants, despite their differences in structure, physicochemical
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properties and bioavailability. The knowledge generated in this study is expected to improve
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the understanding of the environmental fate of cationic PFAS surfactants, and provide more
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elements for a sound management in AFFF-impacted sites.
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Materials and methods
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2.1
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The model cationic surfactants of perfluorooctane sulfonamido ammonium iodide
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[PFOSAmS, CAS# 335-90-0, F(CF2)8SO2NH(CH2)3N+(CH3)3I, 98%] and
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perfluoroctaneamido ammonium iodide [PFOAAmS, CAS# 1652-63-7,
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F(CF2)7CONH(CH2)3N+(CH3)3I, 98%] were custom-synthesized at Beijing Surfactant
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Institute (Beijing, China). Three polyfluoroalkyl compounds that were used for identifying
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biotransformation products or as internal standards for quantitative chemical analysis were
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also synthesized: perfluoroctane sulfonamidoamido amine [PFOSAm, CAS# 13417-01-1,
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F(CF2)8SO2NH(CH2)3N(CH3)2, 96%], perfluoroctane sulfonamido amine oxide [PFOSNO,
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CAS# 30295-51-3, F(CF2)8SO2NH(CH2)3N(CH3)2O, 97%], and perfluoroctaneamido amine
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oxide [PFOANO, CAS# 30295-53-5, F(CF2)7CONH(CH2)3N(CH3)2O, 96%]. The synthesis
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processes for these compounds are briefly described in the Supporting Information (SI).
Standards and Reagents
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A standard mixture of PFAAs (2 µg mL-1, >98% purity), which included the PFOS (linear
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isomer) and PFOA (linear isomer) were obtained from Wellington Laboratories (Guelph,
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Canada). Perfluorooctane sulfonamide acetate (FOSAA, 50 µg mL-1, >98% purity) was also
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from Wellington Laboratories. Perfluorooctane sulfonamide (FOSA, 98% purity) and N-ethyl
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perfluorooctane sulfonamide (EtFOSA, 98% purity) were purchased from Advanced
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Technology & Industrial Co. (Hong Kong, China). The information on other chemicals and
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reagents can be found in SI.
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Test soil
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The soil used for setting up microcosms was collected in 2014 from a non-impacted area in
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proximity to a contaminated site (Quebec, Canada). As shown in Figure SI-1 in the SI, the
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soil was obtained from the zone next to Rivière Chaudière, upstream of the downtown area
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of Lac-Mégantic, where about 33,000 liters of AFFF concentrates were deployed for
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controlling crude oil fire in a major train derailment accident in July 2013.30 The top 20 cm
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layer was collected, sieved via a 2-mm sieve immediately after collection, and stored at 4°C
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until use. It is a sandy loam (59.2% sand, 32.2% silt, and 8.6% clay) with 3.1% organic
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matter (OM), pH of 5.0, CEC of 7.2 meg/100 g, 11 ppm phosphorus-P, and 64 ppm nitrate-
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N. As the levels of 18 selected PFAS species were found in the low levels of 0.02 – 0.77
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ng/g (listed in Table SI-1), comparable to other soils that had no known PFAS exposure
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history,31 the soil was deemed not impacted by the AFFF deployment. Before the
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commencement of the biotransformation experiment, part of the sieved soil was sterilized
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through three cycles of autoclaving (60 min per cycle at 121 °C and 15 psi) with intermittent
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incubation at ~22 °C for 24 h between autoclaving cycles. Three antibiotics
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approximate concentration of 100 mg/kg-soil to prevent the revival of microbial activity
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during long-term incubation.21
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Soil microcosms
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Closed test vessels were created by using 50 mL amber serum bottles fitted with crimp-
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sealed natural rubber stoppers, so as to minimize loss of potential volatile products. Similar
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closed systems have been previously used for evaluating biodegradability of fluorotelomer
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alcohols. 21, 32 21, 32 21, 32 21, 32 21, 32 21, 32 Approximately 5 g soil (dry weight equivalent at 103
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°C) was added to each vessel, adjusted to a gravimetric moisture content of 23 % (~80% of
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water holding capacity at 1/3 bar), and was pre-incubated at ~22°C for five days. Then the
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soil was dosed with 10 µL of a methanol stock solution of a single component (or methanol
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only for untreated soil) to result in a starting concentration of 1 μg/g (dw). The dosed soil
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was mixed manually using a sterile polypropylene spatula. Three treatments were prepared
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for each model compound: (a) untreated (matrix) live soil with 10 µL methanol; (b) treated
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live soil, and (c) treated sterile soil. A total of 30 vessels were prepared for each treatment
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and 90 vessels for each individual compound. At every sampling point, three vessels of
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each treatment were sacrificed for analysis.
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2.2
Sampling and sample preparation
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Sampling points were chosen to be 0, 3, 7, 14, 28, 60, 90, 120, and 180 days, and at each
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sampling point, the whole vessels of live, sterile and matrix samples were sacrificed. The
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concentration of headspace O2 was measured using a Quantek oxygen and carbon dioxide
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analyzer 902D (Grafton, MA) before sampling. When the level of O2 was below 10% of
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sterile air (filtered with 0.3 µm air filter) to re-aerate. The headspace was first sampled by
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flushing with air through a Maxi-Clean® C18 cartridge to capture potential volatile products.
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The cartridge was later eluted with acetonitrile, and the eluent was stored at -20°C before
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chemical analysis. Then the bottle’s crimp cap was opened to allow whole-bottle extraction
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and the original crimp cap was also extracted at the same time. The solvent extraction
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method was modified from Houtz et al.,8 and validated before the commencement of
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biotransformation experiments. Briefly, 10 mL of methanol with 0.1% NH4OH were added to
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each bottle and the cap was re-sealed. The whole bottle was sonicated for 30 min and
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shaken for 120 min on a horizontal shaker (240 rpm) at the ambient temperature. The
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extract was separated from the soil by centrifugation at 1600 g, and the supernatant was
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pipetted out. This extraction step was repeated two more times. The three extracts were
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combined, evaporated to dryness, and reconstituted with 10 mL methanol with 0.1% acetic
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acid and cleaned up with ENVI-Carb using the procedure developed by Powley, et al. 33
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Extracts were stored at -20°C before chemical analysis. Recoveries of the parent
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compounds and the quantifiable biotransformation products were found to be satisfactory in
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a spike recovery test (Table SI-2 in the SI). Methanol or acetonitrile alone could only
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recover no more than 40% of the dosed parent compounds (data not shown). Acetonitrile
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with sodium hydroxide, which provided complete recovery of perfluoroalkane sulfonamido
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derivatives from soil in our previous study,20 was deemed problematic because of possible
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amide hydrolysis and Stevens rearrangement and therefore not used.34
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2.3
Quantitative LC-MS/MS analyses
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Quantitative analysis was performed using a Shimadzu UHPLC system coupled to an AB
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(MRM) mode. Separation of PFOA, PFOS, EtFOSA, FOSA and FOSAA was achieved
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using an Ascentis Express F5 column (2.7 µm, 100 x 2.1 mm, Sigma-Aldrich, Canada) and
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the MS detection was under negative electrospray ionization. Separation of PFOAAmS,
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PFOSAmS, and PFOSAm was performed with a Kinetex C18 column (2.6 µm, 50 x 3.0
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mm, Phenomenex, USA) and ionization was achieved with both positive (for ammonium
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salts) and negative (for amine) electrospray ionization. In both cases, a Kinetex EVO C18
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column (5 µm, 50 x 3.1 mm, Phenomenex, USA) installed upstream of the UHPLC
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autosampler was used as a delay column to separate the PFASs leaching from
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polytetrafluoroethylene parts of the instrument, particularly the degasser. Immediately
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before the chemical analysis, the extracts were spiked with internal standards at 0.4 ng/mL.
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As no isotope-labeled standards were available for PFOAAmS, PFOSAmS, and PFOSAm,
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structurally similar compounds PFOANO and PFOSNO were used as the internal standards
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for PFOAAmS and PFOSAmS/PFOSAm, respectively. The details on chromatographic
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methods, mobile phases, monitored transitions, and calibration methods can be obtained
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from the SI.
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2.4
Identification of biotransformation products
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The soil extracts after concentration by nitrogen evaporation and the headspace extracts
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without concentration were analyzed under full scan to determine the exact mass of
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suspected biotransformation products. The analysis was performed on a Dionex UHPLC
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system coupled to a Q-Exactive Orbitrap mass spectrometer working in full scan mode (R:
249
70,000 at m/z = 200) with positive and negative heated electrospray ionization.
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Chromatographic separation was achieved with a Thermo C18 Hypersil aQ Gold column
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extracts were further analyzed in t-MS2 positive and negative ionization mode (normalized
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collision energy, NCE = 20-70%) using the same UHPLC-MS system. Qualitative analysis
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was based on a method previously developed by Munoz et al.35. The level of confidence in
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the structures identified is reported following the system established by Schymanski et al.36
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2.5
Determination of biotransformation kinetics
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Single first-order (SFO) kinetics model has been preferred in the past in describing
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biotransformation of PFAS-based compounds due to the simplicity of result interpretation.
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The assumption is that the number of molecules of a target compound is small relative to
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the number of degrading microorganisms. However, the SFO model cannot account for
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complex biotransformation processes (e.g., decreasing bioavailability over time due to slow
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sorption37; varying degradation kinetics spatially due to spatial variability of soil38) and is
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often inadequate in capturing concentration profile over a long-term incubation. Therefore,
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three additional kinetics models, which are used to evaluate soil degradation of pesticides
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in EU registration,39 are adapted herein to describe the disappearance of the PFAS
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compounds from soil. They are double first-order in parallel model (DFOP), hockey-stick
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model (HS), and first-order multiple compartment model (FOMC).40 Further information on
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these models is provided in the SI. It is noted that we intend to provide satisfactory
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mathematical description of the data, rather than suggest mechanisms. The experimental
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data were simulated using Kinetic Graphic User Interface (KinGUII) v2.1,40 which is an R-
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based software package designed specifically for determining kinetic parameters from
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environmental fate studies.39 Reweighted Least Squares (IRLS) method was used for data
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simulation. DT50 values, the time it takes for 50% mass to decline, were generated in the
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2.6
Prediction of biotransformation pathways
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The biotransformation pathways were predicted using CATALOGIC41 and EAWAG’s
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Biodegradation/Biocatalysis Database (EAWAG-BBD)42. EAWAG-BBD Pathway prediction
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system predicts biodegradation or biotransformation pathways based on reactions reported
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in literature and/or present in the database. CATALOGIC is a suite developed by Oasis
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Laboratory of Mathematical Chemistry, used for assessment of environmental fate of
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chemicals, including biotic and abiotic transformation, bioacccumullation and ecotoxicity
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endpoints.
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3
Results and Discussion
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3.1
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The gradual disappearance of PFOAAmS in live soil microcosms, whereas little change in
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concentrations occurred in sterile controls, suggested the disappearance was largely
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attributed to soil microbial activities (Figure 1). Approximately 43.0 % of the initially dosed
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PFOAAmS remained by the end of 180-day incubation. The concentration profile of live
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microcosms was fitted to the four kinetics models (SFO, DFOP, HS, and FOMC) that are
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commonly used in simulating biodegradation and biotransformation kinetics of pesticides in
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soil.39 All four models passed the test of goodness of fit with 95% confidence level whereas
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DFOP showed the smallest χ2 error, as detailed in the SI. DT50 value of PFOAAmS was
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estimated to be 142 d in the DFOP model, and 127 (or half-life) in the SFO model. For other
297
known PFOA precursors reported in the literature, which are largely fluorotelomer alcohol
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derivatives with C8 perfluoroethyl chain, their half-lives (as determined using SFO model) in
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aerobic soils ranged from
