Analytical methods for the determination of PFAS in the environment

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MSc Chemistry

Track Analytical Sciences

Literature Thesis

ANALYTICAL METHODS F OR DETERMINING

PER- AND POLYFLUOROALKYL SUBSTANCES

IN THE ENVIRONMENT

by

Laura Bucşă Bsc

UvA #: 12366404, VU #: 2654299

December 2019

12 ECTS

Period: July 1

st

_{to December 2}

nd

_{, 2019}

Supervisor:

Supervisor/Examiner:

Examiner:

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Abstract

Per- and polyfluoroalkyl substances (PFASs) are a group of man-made chemicals that have been used in industry and consumer products worldwide since they were first produced in the late 1940s. They are highly persistent with outstanding properties and they have been used in non-stick cookware, stain resistant fabrics and carpets, cosmetics, firefighting foams, in everyday products like protective clothing and products that resist grease, water, and oil.

Research on PFASs revealed that these compounds bind to proteins causing toxic effects to people and animals, and they are proven to bioaccumulate in the food chain and humans. Their presence has been detected worldwide in environmental matrices. Therefore, it is necessary to perform trace analysis for PFASs in various environmental matrices with a low limit of detection.

The aim of this thesis is to present and review the most recent analytical methods for determining PFASs in the environment. The topics presented are the definition and classification of these compounds, followed by properties, toxicity and methods of determination in water, air, biotic and abiotic solid matrices including sampling, pre-treatment and instrumental analysis; and some of the removal methods.

For the identification and quantification of PFASs in different environmental matrices, LC-MS/MS is the most used technique nowadays in literature, however different extraction methods are used such as liquid-liquid extraction (LLE), solid-liquid extraction (SLE) and solid-phase extraction (SPE).

Granular activated carbon, ion-exchange resins, chemical oxidation and chemical reduction are among the methods that are applied widely for the removal of PFAS from aqueous matrices. In order to remediate the PFAS-contaminated soil, several methods have been applied such as vapor energy generator (VEG) and stabilization and solidification (S/S) processes. However, a relative new method which is using microbes to defluorinated compounds is also of high interest.

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Abbreviations

µSIS micro-Selected Ion Storage mode ACN acetonitrile

ADONA 4,8-dioxa-3H-perfluorononanoic acid AFFF Aqueous-Film Forming Foams

AFFFF Aqueous-Film Forming Fire-Fighting Foam APCI Atmospheric Pressure Chemical Ionization APPI Atmospheric Pressure PhotoIonization ARP Advanced Reduction Processes

BC Branched Chain

Cl-PFESA chlorinated perfluoroether sulfonate Cl-PFOS chloro-perfluorooctane sulfonic acid CMC Critical Micelle Concentration DCM dichloromethane

DFHA 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl acrylate diPAP polyfluoroalkyl phosphatediester

diSAmPAP perfluorooctane sulfonamide-ethanol-based phosphate diester dMRM dynamic Multiple Reaction Monitoring

DVB/CAR/PDMS divinylbenzene/carboxen/polydimethylsiloxane

dw dry weight

DWTP Drinking Water Treatment Plants ECF ElectroChemical Fluorination EG-silicone ethylene glycol modified silicone Envi-Carb Graphitized Carbon

EPA Environmental Protection Agency ESI ElectroSpray Ionization

ETFE ethylene tetrafluoroethylene EtFOSA N-ethyl fluorooctane sulfonamide

EtFOSAA N-ethyl fluorooctane sulfonamidoacetic acid EtFOSE 2-(N-ethyl fluorooctane sulfonamido)-ethanol FASA perfluorinated sulfonamide

FASAA perfluoroalkane sulfonamido acetic acid FASE perfluorinated sulfonamido ethanol FDA Food and Drug Administration FHxSA perfluorohexane sulfonamide FIA Flow Injection Analysis

FISE Fluoride Ion Selective Electrodes FOSA perfluorooctane sulfonamide

FOSAA perfluorooctane sulfonamidoacetic acid FTA fluorotelomer sulfonamidoalkyl amine FTAB fluorotelomer sulfonamidoalkyl betaine FTAC fluorotelomer acrylate

FTB fluorotelomer betaine

FTCA fluorotelomer carboxylic acid FTMA fluorotelomer methacrylate

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FTOH fluorotelomer alcohol FTSA fluorotelomer sulfonic acid

FTSAS fluorotelomer thioether amido sulfonate FTSHA fluorotelomer thioether hydroxyammonium FTUCA fluorotelomer unsaturated carboxylic acid GAC Granular Activated Carbon

GC Gas Chromatography GCB Graphitized Carbon Black

GC-MS Gas Chromatography-Mass Spectrometry GenX 4,8-Dioxa-3H-perfluorononanoate GFF Glass-Fiber Filters

HBFA 4,4,4-trifluoro-1-phenyl-1,3-butanedione HDLs High-Density Lipoproteins

HFPO-DA hexafluoropropylene oxide-dimer acid HFPO-TA Hexafluoropropylene oxide-trimer acid H-PFOS 1H,1H,2H,2H-perfluorooctane sulfonate HPLC High-Performance Liquid Chromatography

HPLC-MS/MS High-Performance Liquid Chromatography-Tandem Mass Spectrometry HRMS High-Resolution Mass Spectrometry

IC Ion Chromatography

ICP Inducted Coupled Plasma Spectroscopy IDMS Isotope Dilution Mass Spectrometry IPE Ion-Pair Extraction

IUPAC International Union of Pure and Applied Chemistry

IX Ion-Exchange Resins

KAW air/water partition coefficient, dimensionless KOA octanol/air partitioning coefficient, dimensionless

KOC organic carbon water partition coefficient, dimensionless KOW octanol/water partition coefficient, dimensionless

LOD Limit of Detection LOQ Limit of Quantification LTQ Linear Trap Quadrupole

M4 6:2 Fluorotelomer sulfonamide propyl N/N-dimethylamine MA Monolith-based Adsorbent

MDL Method Detection Limit

MeFBSA N-methyl fluorobutane sulfonamide MeFOSA methyl perfluorooctane sulfonamide

MeFOSAA N-methyl fluorooctane sulfonamidoacetic acid MeFOSE methyl perfluorooctane sulfonamido ethanol

MeOH methanol

MMF-SPME Multiply Monolithic Fiber Solid-Phase Microextraction monoPAP polyfluoroalkyl phosphate monoester

MPFAC-MXA Mass-Labelled PFCAs and PFASs Solution/Mixture MQL Method of Quantitation Limit

MSPD Matrix Solid-Phase Dispersion MTBE methyl tert-butyl ether

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Na2CO3 sodium carbonate

NaHCO3 sodium hydrogen carbonate NaOH sodium hydroxide

n-Hex normal hexane

O-PFNS oxa-perfluorononane sulfonic acid O-PFOS oxa-perfluorooctane sulfonic acid

Pa Pascal

PAC Powdered Activated Carbon PAH polyaromatic hydrocarbons PCI Positive Chemical Ionization PDMS polydimethylsiloxane

PEEK polyether ether ketone PFA perfluorinated acid PFAA perfluoroalkyl acid

PFAAAm perfluroalkyl amidoalkyl amine PFAC perfluoroalkyl carboxylate PFAI perfluoroalkyl iodide

PFAS per- and polyfluoroalkyl substance

PFASAC perfluroalkyl sulfonamidoalkyl amino carboxylic acid PFASAm perfluoroalkyl sulfonamidoalkyl amine

PFBA perfluorobutanoic acid PFBS perfluorobutane sulfonate PFC perfluoroalkyl carboxylates PFCA perfluorinated carboxylic acid PFDA perfluorodecanoic acid PFDoDA perfluorododecanoic acid PFECA perfluoroether carboxylic acid

PFECHS perfluoroethylcyclohexane sulfonic acids PFESA perfluoroether sulfonic acid

PFHpA perfluoroheptanoic acid PFHxA perfluorohexanoic acid PFHxS perfluorohexane sulfonate PFNA perfluorononanoic acid PFOA perfluorooctanoic acid

PFOAB perfluorooctane amidoalkyl betaine PFOANO perfluorooctane amidoalkyl amine oxide PFOS perfluorooctane sulfonate

PFOSAmS perfluorooctane sulfonamidoalkyl ammonium PFOSB perfluorooctane sulfonamidoalkyl betaine PFOSK potassium salt of perfluorooctane sulfonate PFOSNO perfluorooctane sulfonamidoalkyl amine oxide PFPA perfluoroalkyl phosphonic acid

PFPeA perfluoropentanoic acid PFPiA perfluoroalkyl phosphinic acid PFSA perfluorinated sulfonic acid PFUnA perfluoroundecanoic acid

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PL subcooled liquid vapor pressure, Pa POP Persistent Organic Pollutant

PPARα peroxisome proliferator-activated receptor alpha PTFE polytetrafluoroethylene

PUF PolyUrethane Foam QFF Quartz-Fiber Filters

qNMR quantitative NMR

r2 _{squared correlation coefficient, dimensionless}

RP Reversed-Phase

RPF Risk Potency Factor

RRHD Rapid Resolution High Definition S/S Stabilization and Solidification

SAmPAP diester phosphate diester of N-ethylperfluorooctane sulfonamide ethanol SAmPAP triester phosphate triester of N-ethylperfluorooctane sulfonamide ethanol SBSE Stir Bar Sorptive Extraction

SL Subcooled Liquid solubility in water, mol L-1 SLE Solid-Liquid Extraction

SPME Solid-Phase Micro-Extraction

TBAS tetrabutyl ammonium hydrogen sulphate TES 2,2,2-trifluoroethane sulfonate

TFE tetrafluoroethylene TOF Time-Of-Flight

triPAP tri-substituted polyfluorinated phosphate ester UAcids Unsaturated Acids

UHPLC Ultra-High-Performance Liquid Chromatography UPC2 Ultra-Performance Convergence Chromatography VALLME Vortex Assisted Liquid-Liquid Microextraction VEG Vapor Energy Generator

VM Molar Volume, cm3 mol-1

VTAC (vinylbenzyl) trimethylammonium chloride WAX Weak Anion Exchange

WWTP WasteWater Treatment Plant XAD® hydrophobic polymeric resin ZVI Zero Valent Iron

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1. Introduction

Per- and polyfluoroalkyl substances (PFASs) have received much attention in recent years due to their persistence, bioaccumulation potential and adverse effects on biota and humans. A large percentage of PFASs, like PFOA (perfluorooctanoic acid) and PFOS (perfluorooctane sulfonate), can accumulate in the human body. In high doses intake, PFOS and PFOA may be carcinogenic with effects on the liver, gastrointestinal and thyroid hormone. (Lau et al., 2007)

PFASs are a group of synthetic aliphatic substances which contain multiple fluorine atoms. The strength and the stability of the C-F bond present in these compounds was proven to be high which give chemical and thermal stability. These properties together with the hydrophobic and lipophobic nature of a perfluoroalkyl moiety CnF2n+1- makes them and polymers in which this moiety is found highly useful as surfactants. (Buck et al., 2011)

Therefore, after 1950, PFASs, polymers and surfactants containing PFASs have been of great use in multiple commercial and industrial applications, such as surfactants in aqueous-film firefighting foams (AFFFs), processing aids for fluoropolymer manufacture, and constituents of side-chain-fluorinated polymers for grease- and waterproof textiles and food containers. (McCarthy et al., 2017) Among other applications should be mentioned the presence of PFASs in papers, carpets, leather, cosmetics, electronic productions and pesticides. (Z. Wang et al., 2017)

Among the PFASs, most attention has been given to the sulfonate and carboxylate forms and more specifically to PFOS and PFOA. PFOS and PFOS-related compounds have been listed under Annex B of the Stockholm Convention on persistent organic pollutants (POPs) since 2009. The PFOA, its salts and PFOA-related compounds have also been classified as POPs since May 2019. (Hamid et al., 2018; Kucharzyk et al., 2017; POPRC, 2019)

As a consequence of their widespread use and resulting emissions, an impressive range of these compounds have been universally detected in the environment, wildlife and humans, including remote locations. According to the latest update made by FDA (Food and Drug Administration) in October 2019, there are nearly 5000 types of PFASs currently in the environment. Taking this in consideration, research towards the physicochemical properties, fate and methods of detection in water and other matrices such as water, soil, air, and human samples is emerging. (S. F. Nakayama et al., 2019)

The classification of PFASs, their properties and toxicity will be presented in this review together with the recent advances in analytical method development for the determination of PFASs in different matrices and their removal. In addition, the advantages and disadvantages of the currently used analytical methods will be discussed.

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2. Theoretical background

Highly fluorinated substances (per- and polyfluorinated alkyl substances, PFASs) are used in many different chemical products and articles because of their desirable properties. There are various phases in the lifecycle of highly fluorinated substances when release can occur, with exposure to humans and the environment.

The first is the manufacture of the substance itself, after which there are various processing stages in which the substance may be used. An example of usage is in the production and formulation of chemical products such as fire-fighting foam or other numerous materials with domestic use which are shown in Figure 1. The final stage of a chemical is their use as complete products prior to entering the waste management stage.

Figure 1: PFAS-containing materials (Retrieved from: C. Lin, 2019)

As a consequence of its widespread production and use and its extreme properties of resistance to degradation mechanisms in the environment, PFASs have been universally detected and are even found in very remote areas such as Antarctica. Figure 2 shows an overview of sources from which the environment is being contaminated with PFASs.

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Figure 2: Overview of PFAS sources, contamination of water resources and human exposure pathways from tainted drinking water (Retrieved from: Dauchy, 2019)

2.1. Terminology and groups of PFASs

Definition

A clear and harmonized system for the classification of perfluoroalkyl and polyfluoroalkyl substances (PFASs) is yet to be defined which makes it difficult to recognize the major distinctions between PFASs. Generally, PFASs are defined as a class of fluorinated organic chemicals containing at least one perfluoroalkyl moiety CnF2n+1- which can be formed by linear, branched or cyclic carbon chains. In 2018, OECD defined PFASs as substances which contain a perfluoroalkyl moiety with three carbons (i.e. -CnF2n-, n ≥ 3) or a perfluoroalkylether moiety with two or more carbons (i.e. -CnF2nOCmF2m-, n and m ≥ 1). This definition was adapted in such a way to include compounds with having a functional group in the middle of two ends of perfluoroalkyl moiety

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(perfluoroalkyl dicarboxylic acids: HOOC-CnF2n-COOH) and the cyclic correspondents of linear PFASs (undeca fluorocyclohexane sulfonic acid potassium salt). (RIVM, 2019).

“Perfluoroalkyl substances are defined as aliphatic substances for which all of the H atoms

attached to C atoms in the nonfluorinated substance have been replaced by F atoms, except for those H atoms whose substitution would modify the nature of any functional groups present.” (Buck et al., 2011)

“Polyfluoroalkyl substances are defined as aliphatic substances for which all H atoms attached

to at least one (but not all) C atoms have been replaced by F atoms, resulting in containing at least one perfluoroalkyl moiety CnF2n+-.” (Buck et al., 2011)

Whereas the polyfluoroalkyl substances have the ability to be altered into perfluoroalkyl substances, under appropriate conditions, it cannot be stated that by starting from perfluoroalkyl substances, these can be degraded and obtain polyfluoroalkyl substances.

Due to their long alkyl chains which mainly consist of 4 to 18 fluorinated carbon atoms, PFASs can have long IUPAC names. For example, the IUPAC name for the substance C8F17SO2N(C2H5)CH2CH2OH is “N-ethyl-1,1,2,2,3,3,4,4,5,5,6,6,7,7,8,8,8-heptadecafluoro-N-(2-hydroxyethyl)octane-1-sulfonamide”. The easier way for this compound to be mentioned is to use a shorter terminology as “N-ethyl perfluorooctane sulfonamidoethanol” or the corresponding acronym EtFOSE.

This is why, in this review, the key terminology and definitions provided by Robert C Buck et al. in 2011 will be used. This is meant to be of help for the better understanding of the occurrence and connections between the diverse families of PFASs in the environment. (Buck et al., 2011)

2.1.1. Chain length terminology

The Organization for Economic Co-operation and Development (OECD, 2011) defined the “long-chain” substances as: perfluoroalkyl carboxylic acids (PFCAs) with eight carbons and higher, including perfluorooctanoic acid (PFOA); perfluoroalkane sulfonates (PFSAs) with six carbons and higher, including perfluorohexane sulfonic acid (PFHxS) and perfluorooctane sulfonic acid (PFOS).

“Short-chain” substances include: perfluoroalkyl carboxylic acids (PFCAs) with carbon chain lengths of seven and lower, including perfluoroheptanoic acid (PFHpA); perfluoroalkane sulfonates (PFSAs) with carbon chain lengths of five and lower, including perfluoropentane sulfonic acid (PFPeS).

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Both “short-chain” and “long-chain” PFASs may be produced and can be found in the environment. Examples of materials that can contain them are perfluoroalkyl sulfonyl fluoride-based raw materials and fluorotelomer-fluoride-based raw materials.

2.1.2. Chemical identity and example structures

Perfluoroalkyl substances and polyfluoroalkyl substances can be categorized into multiple families and this starts with dividing them into non-polymers and polymers. The first classification was made by Buck et al. in 2011 and it is presented in figure 3.

Figure 3: Classification of environmentally relevant perfluoroalkyl and polyfluoroalkyl substances (Reproduced from Buck et al., 2011) 2.1.2.1. Non-polymers

In the review made by Buck et al. in 2011 the non-polymeric PFAS were divided in nine groups which can be seen in Figure 2. Later on, in 2013 and 2015 OECD has concluded that another classification should be made which included only four main groups. (OECD, 2013, 2015)

Perfluoroalkyl acids (PFAAs);

Perfluoroalkane sulfonyl fluorides (PASFs) and compounds derived from PASFs, i.e. the FASAs and perfluoroalkane sulfonamido derivatives as mentioned by Buck;

Perfluroalkyl iodides (PFAIs), fluorotelomer iodides (FTIs) and fluorotelomer (FT)-based compounds (including the PFALs, SFAs and SFAenes);

Per- and polyfluoroalkyl ether-based compounds (PFAEs). (RIVM, 2019)

OECD rearranged the PFAS groups again in 2018 and has concluded that in the PFAA group per- and polyfluoroalkyl ether acids (PFAEs) will also be included. Some PFAEs and all PFAI and PASF are included in the group of the raw material from which PFAAs are produced. Some new groups which contain highly fluorinated substances were added, even some of them do not present surfactant properties. Additionally, linear PFCs (CnF2n+2) are included in the classification of PFAS groups, even if Buck et al. mentioned in the review from 2011 that their properties and functionalities of PFCs are not similar to those of PFAS. (OECD, 2018)

Non-polymers

Perfluoroalkyl substances:

• (Aliphatic) perfluorocarbons (PFCs) • Perfluoroalkyl acids (PFAAs) • Perfluoroalkane sulfonyl fluorides (PASFs) • Perfluoroalkane sulfonamides (FASAs) • Perfluroalkyl iodides (PFAIs) • Perfluoroalkyl aldehides (PFALs)

Polyfluoroalkyl substances:

• Perlfluoroalkane sulfonamido derivatives • Fluorotelomer-based compounds • Semifluorinated n-alkanes and alkenes

Polymers

Fluoropolymers:

• Carbon-only polymer backbone with fluorines directly attached. Perfluoropolyethers: • Carbon and oxygen polymer

backbone with fluorines directly attached to carbon Side-chain Fluorinated Polymers: • Fluorinated acrylate and methacrylate

polymers

• Flurorinated urethane polymers • Fluorinated oxetane polymers

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All the changes that have been imposed by OECD since the first classification of PFAS made by Buck et al. in 2011 are presented in the Figure 4. The following classification takes into consideration that in Buck et al. in 2011 and OECD (2013, 2015) the definition of a PFAS containing a CnF2n+1 moiety required that R1 and R3 to be a fluorine atom. In OECD (2018) it was decided that R1 and R3 can be any single atom or a larger moiety.

A short description of the main groups from the classification made in 2011 will be given in the following paragraphs. A detailed presentation of the subgroups of PFASs and some specific properties can be found in the Appendix 1.

Perfluoroalkyl acids (PFAAs) CnF2n+1R - have a charged functional moiety such as a sulfonate,

carboxylate or phosphonate. They are highly persistent substances and ideal surfactants; anionic and protonated forms have very different physical and chemical properties. Based on the functional group present in the structure of the compound, these substances can be classified in five subgroups: PFCAs (perfluoroalkyl carboxylic acids), PFSAs (perfluoroalkyl sulfonic acids), PFSIAs (perfluoroalkane sulfinic acids), PFPAs (perfluoroalkyl phosphonic acids), PFPIAs (perfluoroalkyl phosphinic acids). The most important compounds from this group are PFOS and PFOA which have been classified as toxic compounds in the Stockholm convention.

Perfluoroalkane sulfonyl fluorides (PASFs) CnF2n+1SO2F - are not only valuable as precursors

in the manufacture of the PFSA but also of a significant number of compounds which contain the perfluoroalkane sulfonamido group CnF2n+1SO2NH2. (Buck et al., 2011) Part of this category is perfluorooctane sulfonyl fluoride (POSF) has been defined as persistent organic pollutant in the Stockholm Convention in 2009 together with all PFOS-related compounds. (Martin et al., 2010)

Perfluoroalkane sulfonamides (FASAs) CnF2n+1SO2NH2 - are important for the production of

multiple fluorochemical products used as surfactants and also other applications such as protectors for paper packaging. (Renner et al., 2006)

Perfluoroalkanoyl fluorides (PAFs) CnF2n+1COF - are part of the major raw materials for the

production of PFOA made by using ECF process, and also for surfactants.

Perfluoroalkyl iodides (PFAIs) CnF2n+1I - are one of the first raw materials together with

fluorotelomer iodides used to obtain the polyfluoroalkyl products made by using telomerization.

Perfluoroalkyl aldehydes (PFALs) CnF2n+1CHO and aldehyde hydrates (PFAL·H2Os)

CnF2n+1CH(OH)2 - are the degradation products of fluorotelomer alcohols and their esters and are

used as intermediate environmental transformation product. (Ellis et al., 2004)

Perfluoroalkane sulfonamido substances are obtained by using as precursors the perfluoroalkane

sulfonyl fluorides PFASs discussed earlier and they are formed by four subgroups among which N-Alkyl perfluoroalkane sulfonamides (FASAs) and (N-alkyl)-perfluoroalkane sulfonamidoacetic

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acids (FASAAs) can be found. More details about these and the other subgroups can be found in the Appendix 1.

Fluorotelomer substances - are a source of building blocks for a significant amount of other

fluorinated compounds such as PFOA. PFCA can be produced by breaking down some fluorotelomer substances such as fluorotelomer acrylates and alcohols. Fluorotelomers with six fluorine atoms are the most used for the production of aqueous-film forming foams (AFFF). (Ding & Peijnenburg, 2013) The fluorotelomer substances can be classified into eleven subgroups. As examples from this family there are the n:2 fluorotelomer olefins (n:2 FTO), n:2 fluorotelomer iodides (n:2 FTI) and n:2 fluorotelomer alcohols (FTOHs). (Appendix 1)

Miscellaneous: Polyfluoroalkyl ether carboxylic acids CnF2n+1O(CmF2m)OCHF(CpF2p)COOH -

they are used as an alternative polymer processing aid. (Buck et al., 2011)

In this review, the first discovered perfluorinated compounds such as PFOA and PFOS and their related compounds will be referred to as being legacy or classic PFASs. The novel or emerging PFASs are the compounds which have been produced as a replacement for the classic PFASs after their toxicity has been proven. ADONA dioxa-3H-perfluorononanoic acid) and GenX (4,8-Dioxa-3H-perfluorononanoate) are such examples of the novel PFASs.

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Figure 4: Groups of non-polymer per- and polyfluoroalkyl substances based on OECD (2013; 2015; 2018) and (Buck et al., 2011) Green: groups are described by OECD and Buck et al. Orange and blue: group described only by Buck et al. (2011) or only by OECD (2013; 2015; 2018) respectively. Dashed blue boxes contain groups that are regarded by OECD (2018) as other high fluorinated substances matching the definition of PFASs. Dashed yellow and red boxes include groups which are designated by OECD (2018) as PFAA precursors and PFAAs, respectively. (Retrieved from RIVM, 2019)

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Some examples and structures of each group of nonpolymer PFASs are presented in table 1.

Table 1: Nonpolymer per- and polyfluoroalkyl substances, examples and structures

Group Example Structure of the

example

PFCAs Perfluorooctanoic acid (PFOA) -

C7F15COOH

PFSAs Perfluorooctane sulfonic acid (PFOS) -

C8F17SO3H

PFSIAs Perfluorooctane sulfinic acid (PFOSI)

C8F17SO2H

PFPAs Perfluorooctyl phosphonic acid

C8F17P(=O)(OH2)

PFPIAs Bis(heptadecafluorooctyl)phosphinic acid

C16HF34O2P

PAFSs Perfluorooctane sulfonyl fluoride (POSF),

C8F18SO2F

FASAs Perfluorooctane sulfonamide (FOSA)

C8F17SO2NH2

PAFs Perfluorooctanoyl fluoride (POF) C7F15COF

PFAIs Perfluorohexyl iodide (PFHxI) C6F13I

PFALs & PFAL·H2Os

Perfluoromethanal (PFMAL) C1F3CHO

N-alkyl FASAs N-Methyl perfluorooctane sulfonamide

(MeFOSA) C8F17SO2N(CH3)H

FASEs & N-alkyl FASEs

N-Ethyl perfluorooctane sulfonamidoethanol

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N-alkyl FASACs & N-alkyl FAS(M)ACs

N-Ethyl perfluorooctane sulfonamidoethyl acrylate (EtFOSAC),

C8F17SO2N(C2H5)CH2CH2OC(O)CH=CH2

FASAAs & N-alkyl FASAAs

N-Ethyl perfluorooctane sulfonamidoacetic acid (EtFOSAA),

C8F17SO2N(C2H5)CH2COOH

SFAs & SFAenes

1-(perfluoro-N-hexyl)decane C6F13(CH2)10H

n:2 FTIs 8:2 Fluorotelomer iodide (8:2 FTI),

C8F17CH2CH2I

n:2 FTOs 6:2 Fluorotelomer olefin (6:2 FTO),

C6F13CH=CH2

n:2 FTOHs 10:2 Fluorotelomer alcohol (10:2 FTOH),

C10F21CH2CH2OH

n:2 FTUOHs 8:2 Unsaturated fluorotelomer alcohol (8:2

FTUOH), C7F15CF=CHCH2OH

n:2 FTACs & n:2 FTMACs

6:2 Fluorotelomer methacrylate (6:2

FTMAC), C6F13CH2CH2OC(O)C(CH3)=CH2

n:2 PAPs 8:2 Fluorotelomer phosphate monoester (8:2

monoPAP), C8F17CH2CH2OP(=O)(OH)2

n:2 FTALs & n:2 FTUALs

8:2 Fluorotelomer aldehyde (8:2 FTAL),

C8F17CH2CHO

n:2 FTCAs & n:2 FTUCAs

8:2 Fluorotelomer carboxylic acid (8:2

FTCA), C8F17CH2COOH

n:3 Acids & n:3 UAcids

7:3 UAcid Vinyl Perflurooctanoate,

C7F15CH=CHCOOH

n:2 FTSAs 8:2 Fluorotelomer sulfonic acid (8:2 FTSA),

C8F17CH2CH2SO3H

Miscellaneous 4,8-Dioxa-3H-perfluorononanoate,

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2.1.2.2. Polymers

The polymeric PFAS are defined as being those: “1) whose synthesis involves the incorporation of one or more PFASs as monomers. In this case, there is some potential (theoretical or demonstrated) for the degradation of the polymer, during or after its useful lifetime, to lead to release of PFASs to the environment; or 2) whose manufacture requires the use of a PFAS as a processing aid.” (Buck et al., 2011)

According to Buck et al. 2011, KEMI 2015 and OECD 2013, 2015, 2018, the following groups of polymeric PFASs have been defined:

Fluoropolymers: fluorinated polymers which consist of carbon-only backbone with directly attached fluorines. Some of these fluoropolymers are obtained by using emulsion or dispersion polymerization. (Buck et al., 2011)

Side-chain fluorinated polymers: fluorinated polymers which consist of variable composition of non-fluorinated carbon backbones with polyfluoroalkyl (and possibly perfluoroalkyl) side chains which is in contrast to the other two groups of polymeric PFASs. They are separated into the following three sub-groups:

➢ Fluorinated (meth)acrylate polymers; ➢ Fluorinated urethane polymers; ➢ Fluorinated oxetane polymers.

Perfluoropolyethers (PFPEs): fluorinated polymers which consist of backbones containing carbon and oxygen with fluorines directly attached to carbon. This type of polymers cannot degrade into long-chain PFCAs due to their repeating units of only 2 or 3 Carbon atoms per Oxygen atom. (Buck et al., 2011)

Figure 5: General classification of polymer PFASs, according to (Buck et al., 2011) and OECD (2013, 2015, 2018) with the note that OECD (2018) does not mention the side-chain fluorinated polymers as a separate group. (Retrieved from RIVM, 2019)

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2.1.3. Manufacturing processes

In order to give a proper description of the many families of PFAS and their applications, it is appropriate to describe the most important manufacturing processes used to produce compounds containing the perfluoroalkyl moiety such as electrochemical fluorination (ECF) and telomerization.

2.1.3.1. Electrochemical fluorination

Electrochemical fluorination is an inexpensive process which involves the electrolysis of organic compounds in anhydrous HF which yields, in principal, perfluorinated sulfonyl and carbonyl fluorides. (Schultz et al., 2003) Due to the free-radical nature of the process, a mixture of isomers and homologues with even and odd number of carbon atoms is obtained. The majority of the chains have eight carbons, but their range is usually between 4 and 13 carbons. (John P. Giesy & Kannan, 2002) During this process, the ratio of branched to linear perfluorinated carbon chains obtained may differ due to the conditions used, but is usually between 20% to 30% branched and 70% to 80% linear. (Benskin et al., 2010)

The perfluoroalkyl sulfonyl and carbonyl fluorides obtained during the electrolysis process which is shown in table 2 are intermediates that will ultimately end up as perfluoroalkyl sulfonates and carboxylates or their corresponding acids. The last ones which are PFOA and PFOS will be the products that will be used for industrial or commercial purposes. (Schultz et al., 2003)

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Table 2: Principal PFASs’ production routes and products (Retrieved from Krafft & Riess, 2015)

2.1.3.2. Telomerisation

Telomerisation is the second important process for synthesizing perfluoroalkyl substances. During this process, tetrafluoroethylene, CF2=CF2 (TFE) is subjected to fluoroiodination with a pefluoroalkyl iodide, CmF2m+1I (PFAI). A mixture of perfluoroalkyl iodides is obtained with longer perfluorinated chains CmF2m+1(CF2CF2)nI. As a second step, the mixture is then reacted with ethylene to obtain fluorotelomer iodides CmF2m+1(CF2CF2)nCH2CH2I which are raw material intermediates that can be transformed into other intermediates such as olefins, sulfonyl chlorides, alcohols, thiols and thiocyanates. Depending on the desired product, these intermediates can result in perfluoroalkyl carboxylates (PFC) and fluorotelomer alcohols (FTOH). (Table 2)

In contrast to ECF, when using telomerisation as a manufacturing process for PFASs, if a linear PFEI is used, the resulting fluorinated substances are exclusively linear products of even numbers of carbon atoms. Similarly, if a branched pentafluoroiodide is used, the resulting product will be branched. (Buck et al., 2011; Schultz et al., 2003)

The type of manufacturing process used to synthesize PFASs is an influence in the terminology of these fluorinated compounds. The presence of the ethene in the intermediates which are obtained in the telomerisation process will be spotted in the telomeric terminology which will be different of its homologous obtained through the ECF process. Therefore, by reading the name of the

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products we can distinguish through which process this compound have been synthesized. For example, 1-octanesulfonic acid, 3,3,4,4,5,5,6,6,7,7,8,8,8-tridecafluoro-, ammonium salt, F(CF2CF2)3CH2CH2SO3-NH4+, can be named 6:2 fluorotelomer sulfonate (6:2 FTS) because of the presence of the six carbons with F and two hydrocarbons from the ethene. (Schultz et al., 2003)

2.2. Physicochemical properties of PFAS related to their performance

It is mandatory to understand how the chemical structure of PFASs influences the behavior and the impact on the environment in order to be able to design sustainable methods for removal and also help companies to synthesize greener products. Although a great importance has been given to studying the properties of these fluorinated compounds since they were first reported in the global environment, there is still much to learn about their unique toxicity and environmental chemistry. (J. P. Giesy et al., 2001)

By replacing the hydrogen atoms in the alkyl chains with fluorine, some of the compound’s properties are enhanced. Having a much higher electron affinity, ionization potential, the highest electronegativity (3.98 in Pauling scale), and a lower polarizability, makes the perfluoroalkyl chains very different than the alkyl chains. (Banks & Tatlow, 1986) The C-F bond is the strongest single bond than can be found in organic chemistry and has a strong dipole moment, due to the electrons being drawn towards the more electronegative fluorine. The F-alkyl chains require more space due to their higher surface area, and by combining this with low molecular forces gives higher hydrophobicity which is useful for being powerful fluorosurfactants. (Ki et al., 2015)

2.2.1. Physicochemical properties

Vapor pressure (PL) is defined as the partition behavior in an air/liquid, solid interface, volatilization from water, plants or soil, and plays an important role in knowing the long-range transport, distribution and fate of organic substances through the environment. Among the PFASs, the FTOHs are of great interest for the vapor pressure as they are the volatile precursors of PFCAs. (Ding & Peijnenburg, 2013)

Water solubility (SL) is an important parameter when aqueous environment is involved in the transport of the fluorinated compounds in the global environment. It is also useful in determining the air/water partition coefficient and also Henry’s law constant, which can be determined when combined with the vapor pressure. The intermolecular forces between solvent and solute are influencing the solubility, together with the enthalpy of the solute and the boiling/melting point. In defining subcooled liquid solubility it is important to take into consideration both molecular size and melting point, while when defining solid solubility, only molecular size is important. (Kim et al., 2015)

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The organic carbon water partition coefficient (KOC) is a dimensionless number defined as the ratio of a chemical's concentration absorbed per unit mass of soil, to its concentration in the aqueous phase, and it is valuable in estimating the mobility of organic soil contaminants. Taking into consideration that the KOC is depending on the functional group of the perfluorinated molecule, Gomis et al. in 2015 calculated this parameter by using COSMOtherm and the Seth equation from table 3. (Smith et al., 2016) This parameter for PFCAs and PFSAs are relatively low with values from 1.3 to 5. Long-chain PFASs, PFCAs were found to be bound to particles whereas the short-chain PFCAs were found only in the dissolved phase. (Muir et al., 2019) The log values of KOC for PFOS, PFOA and FOSA vary between laboratory studies, hence the range of values presented in table 4. The difference between the values could be caused by the variations of the composition, ionic strength and pH of the solutions used when conducting the studies. (Chen et al., 2012)

Octanol/water partition coefficient (KOW) is an important property which defines the proportions of the concentrations of organic compounds in n-octanol and water phases at equilibrium. The lipophilicity of compounds is characterized by this parameter and it expresses the inclination of the substances to divide between an organic phase and an aqueous phase. Because PFASs are both hydrophobic and lipophilic, it is difficult to obtain the KOW values by using usual methods. In order to quantify the octanol/water partition coefficient of PFAS, two kinds of this coefficient are used, one for the ionizable molecules which is Log P and one for the neutral solutes which is the distribution coefficient represented by Log D. At high pH values with pKa values reported to be between -0.5 and 3.8, the perfluorinated acids (PFAs) are considered to be dissociated, therefore the Log D values of the anionic species would be equal to Log P. (Hidalgo & Mora-Diez, 2016) By using this approach, Jing et al. in 2009 managed to determine a series of values by using ion-transfer cyclic voltammetry on perfluoroalkyl and alkyl carboxylates. (Ding & Peijnenburg, 2013; Krafft & Riess, 2015) It was concluded that the values for Log P for the perfluorinated substances are higher than its corresponding alkyl carboxylates. (Jing et al., 2009)

Air/water partition coefficient (KAW) is the concentration ratio of a compound in equilibrium between air and the corresponding aqueous phases. PFSA have a tendency to ionize and stay as anions in water, considering that they are strong acids. The data regarding this particular coefficient is very limited, therefore the values are estimated from Henry’s law constant (H’) in a study performed in 2015 by Kim et al. which are presented in table 4.

Octanol/air partition coefficient (KOA) is a parameter that defines the partitioning between the organic substances and atmosphere. For example, it measures the ratio of organic carbon in soil, organic films on aerosols or lipid portions of vegetation. (Ding & Peijnenburg, 2013). The KOA is important for those PFASs that are easily dispersed in the atmosphere and they are probable to remain there, such as FTOHs. (Ding & Peijnenburg, 2013; Krafft & Riess, 2015)

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Table 3: Quantitative structure property relationships between logarithm of properties (298 K and 760 mmHg) and molar volume (VM, cm3_mol-1_{) for PFAS (reproduced from Kim et al., 2015 except for the Koc which is reproduced from Gomis et al., 2015)}

Property Single molecular model r2 Vapor pressure (PL) log (PL/Pa) = -0.0081 x VM + 3.4361 0.8981

Water solubility (SL) Log (SL/mol L-1) = -0.0288 x VM + 3.0364 0.7229

Octanol/water partition coefficient (KOW) log KOW = 0.220 x VM - 3.4264 0.9768

Air/water partition coefficient (KAW) log KAW = 0.0207 x VM - 2.3439 0.8223

Octanol/air partition coefficient (KOA) log KOA = 0.0129 x VM + 2.4847 0.8989

Organic carbon water partition coefficient (KOC) 𝐾_𝑂𝐶𝐻𝐴=KOW x 0.35 -

By using the experimental data of various studies performed before and the formulas presented in table 3, Kim et al. in 2015 determined the physicochemical properties with predictive models for vapor pressure, water solubility, octanol/water partition coefficient, air/water coefficient and octanol/air coefficient using the QSPR model based on the molecular descriptor molar volume (VM) of PFASs. It was proven that the results for PFASs were accurate and the adjusted properties were linear with the calculated ones, therefore this model could be used to better understand the behavior and fate of fluorinated compounds in the environment. (Table 4)

Table 4: Physicochemical properties of selected PFASs calculated from equations in (Retrieved from Kim et al., 2015 except for the Koc which is reproduced from Smith et al., 2016)

Group Acronym log KOC (L/kg)

log (PL/Pa) log (SL/mol L-1)

log KOW log KAW Log KOA

PFSAs PFBS 1.00 2.12 -1.64 0.14 1.02 4.58 PFHxS 1.78 1.68 -3.23 1.35 2.15 5.29 PFOS 2.57-3.1 1.23 -4.81 2.56 3.29 5.99 PFCAs PFBA 2.82 2.40 -0.64 -0.62 0.30 4.13 PFPA 1.37 2.18 -1.43 -0.02 0.86 4.48 PFHxA 1.91 1.96 -2.22 0.59 1.43 4.84 PFHpA 2.19 1.74 -3.01 1.19 2.00 5.19 PFOA 1.31-2.35 1.51 -3.81 1.79 2.57 5.55 PFNA 2.39 1.29 -4.60 2.40 3.14 5.90 PFDA 2.76 1.07 -5.39 3.00 3.70 6.25 PFUnA 3.30 0.88 -6.05 3.51 4.18 6.55 PFDoDA - 0.62 -6.97 4.21 4.84 6.96 PFTA - 0.22 -8.40 5.30 5.87 7.60 FOSA 2.5-2.62 1.18 -4.99 2.70 3.42 6.07 FASAs MeFOSA 3.14 1.01 -5.61 3.17 3.86 6.35 EtFOSA 3.23 0.87 -6.09 3.53 4.21 6.57 EtFOSE - 0.64 -6.92 4.17 4.81 6.94 MeFOSE - 0.77 -6.45 3.81 4.46 6.73 MeFOSEA - 0.36 -7.91 4.92 5.51 7.38 FTOHs 4:2 FTOH 0.93 2.02 -1.99 0.41 1.26 4.73 6:2 FTOH 2.43 1.58 -3.57 1.62 2.40 5.44 8:2 FTOH 3.84 1.13 -5.16 2.82 3.54 6.15 10:2 FTOH 6.2 0.69 -6.14 4.03 4.76 6.86

Most PFASs are known to be strong acids and they ionize effectively in any environmental conditions. (Ellis et al., 2008) Under environmental conditions, less stable PFASs can suffer

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degradation and transform into products with high stability such as the perfluoroalkyl acids (PFAAs). PFCAs and PFSAs are a part of the PFAA family, they are fully fluorinated strong acids and have very low acid dissociation constants. (Muir et al., 2019)

The acidity constants of PFCAs were determined by Moroi et al in 2001 by electric conductivity, acid-base titration and solubility change with pH. Their results showed that the Ka values increase from C1 (ethanoic) to C3 (butanoic acid) and from C3 to C5 decreases. The authors noted that it was difficult to use the same methods to determine the Ka values to the intermediate PFCAs due to the almost impossible separation of the colloidal acid particles from the aqueous phase. However, they did manage to determine that in the case of PFCAs with longer chain length from C9 to C11 the Ka values increase with the increase of the chain length. These determinations were made on undefined oligomeric species of PFCAs.

Due to the presence of hydrophilic carboxylate and sulfonate groups on the molecules PFCAs and PFSAs have high values of water solubility. (Smith et al., 2016) Also, because of their low acid dissociation constant, PFCAs and PFSAs will usually be present in their anionic form in neutral pH under most environmental conditions. However, a limited number of PFAAs can be present in the environment in their protonated form with different physicochemical properties. As an example, there is the PFOA anion which has a high water solubility and a very low vapor pressure, whereas the protonated PFOA has a low solubility in water with high vapor pressure. (Cheng et al., 2009; Kaiser et al., 2005)

The vapor pressures of the PFAAs are known to have low values. Fluorotelomer alcohols, the precursor counterparts of PFAAs, have higher vapor pressure which classifies them as volatile. Vapor pressure is a parameter that decreases with the increase of the perfluorinated chain length. (Rayne & Forest, 2009)

Additionally, the pKa of PFOA has been the subject of study for several groups of scientists which brought much debate upon it. By using NaOH as the titrant, Brace in 1962 was among the first scientist to study the acidity of PFASs and he determined the pKa value of PFOA as being 2.80 in 50% aqueous ethanol for 0.005 M (2.07 mg L-1). Another value of pKa of 1.01 for 0.015 M (6.21 mg L-1) was obtained by Igarashi & Yotsuyanagi in 1992 by making several titrations with ethanol/water solutions with 0.1 M NaCl. Furthermore, in 2005, López-Fontán et al. obtained a pKa of 1.31 while investigating the aggregation of sodium perfluorooactanoate in water. Further investigations were done, however the conclusion reached was that more research efforts on the pKa of PFCAs need to be done in order to achieve a higher understanding of their environmental fate. (Goss et al., 2006)

Being one of the most studied compounds, it is known about PFOS that it is thermally stable, water soluble, strongly acid and that its potassium salt (PFOSK) has a mean solubility of 680 mg/L in

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pure water. Also, the PFOSK solubility in seawater is close to 12.4 mg/L, whereas the mean solubility in pure octanol was found to be 56 mg/L. (Jones et al., 2003)

2.2.2. Hydrophobic and lipophobic

The addition of the fluorine atom makes the PFASs to become both hydrophobic and lipophobic which is a unique characteristic for this type of organic compound. However, Liu & Lee in 2007 proved that with the increase of the F-alkyl chain, the water repellent properties will increase strongly. For n:2 fluorotelomer alcohols with n from 4 to 10, it was shown that the solubility in water decreases by 0.78 log units with every CF2 added to the chain. This ability of the fluorinated compounds to repel water, grease, dirt and fat, and microorganisms is being relied on by many applications. (Krafft & Riess, 2015)

2.2.3. Surface activity

As mentioned before, PFASs have the ability of being surfactant, and because of this, they have been extensively used for industrial uses as coating, detergents or emulsifiers. (B Bhhatarai & Gramatica, 2010) It is important to know the value of the critical micelle concentration (CMC) and for PFASs which represents the solubility of a surfactant at the Krafft point. The micelles are absent below the CMC value, therefore the transfer of water and the interaction with the perfluoroalkyl moiety are obstructed. (Ding & Peijnenburg, 2013)

Associating the F-chains with highly hydrophilic polar groups like acids or alcohols can lead to greater surface activity and stability against oxidative, reductive, acidic and alkaline reagents. This characteristic also gives stability against high temperatures. (Bhhatarai & Gramatica, 2011; Krafft & Riess, 2015) Compared to their non-fluorinated analogues, the lower values of CMC indicate higher efficiency at lowering the surface tension of water. Krafft et al. in 2015 showed that the efficiency expressed by the value of CMC is decreased exponentially with the increasing of the F-chain length expressed in figure 6. The structure of the hydrophilic group, the counter ion and temperature are other important parameters that affect the surface activity of the fluorosurfactants. (Shinoda et al., 1972)

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Figure 6: The CMC of fluorosurfactants: triangles: CnF2n+1COO-K+; open circles: CnF2n+1COOH; upside down triangle: H(CnF2n)COOH;

closed circles: H(CnF2n)-COO-NH4+; (Retrieved from Krafft & Riess, 2015)

2.2.4. Thermal stability and kinetic inertness

By adding the C-F bond and with the stability of the C-C bond in the F-chains, PFASs gain extremely stable properties against heat and they become non-flammable. Even by exposing them at 400 °C, F-alkylcarboxylic and F-alkane sulfonic acids will not suffer any significant decomposition. (Kissa, 2001) In addition to this resistance to heat, they can also resist UV radiation.

Additionally, these compounds have the characteristic to be extremely resistant to chemical reactions with strong acids and bases, even at high temperatures. Therefore, they can preserve their film-forming ability, elasticity, surface activity in extremely resilient conditions. This kinetic inertness is owed to the repellent electronic sheath that protects the large fluorine atoms which is covering carbon backbone of the molecule against attacks. Kissa in 2001 proved that PFOS can resist heating in concentrated nitric acid at 160 °C for 8 h. Also, the by-products of fluorotelomer alcohol were proven to maintain their surface activity in 25% sulfuric acid, 10% potassium hydroxide, 70% nitric acid or 37% hydrochloric acid. (Krafft & Riess, 2015)

2.2.5. Other valuable properties

PFASs can be used also to modulate wetting properties, lower friction, facilitate levelling, provide lubrication and readily adsorb on solids which makes them an invaluable asset for sky waxing and photographic industry. Even their optical properties are admirable as their refraction indices, is the lowest of all organic compounds, sometimes even lower than water. Other properties such as dielectric, piezoelectric and pyroelectric characteristics are also remarkable for this type of compounds. (Ding & Peijnenburg, 2013)

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3. Overview toxicity of PFAS

Multiple studies have found that the exposure to PFASs such as PFOA and PFOS could contribute to kidney and testicular cancer, thyroid disease, decreased sperm quality, developmental toxicity and reproductive toxicity. (Barry et al., 2013; Lopez-Espinosa et al., 2012; Piersma et al., 2011; Stein et al., 2009) The developmental effects observed in animals such as mice include skeletal effects, decreased survival, altered mammary gland development and altered puberty. (Dourson et al., 2019; Fair et al., 2011; Hines et al., 2009; Tucker et al., 2015)

In addition, a number of human epidemiology studies report associations between long-term exposure of PFOA and several disorders and diseases. As a matter of fact, a class action lawsuit was filed in West Virginia state court against the PFOA manufacturing company DuPont with the reason that the residents living in the proximity of the work facility had suffered from serious health effects caused by exposure to PFOA. (EBJ, 2019) The analysis study included around 70000 people and the reported health damages included but not limited to: decreased birth weight, ulcerative colitis, testicular and kidney cancer and liver disease. The association which conducted this study concluded that the connections between PFOA exposure and the diseases were not consistent, however they showed that the most stable connection was between PFOA and increased cholesterol and decreased birth weight. (RIVM, 2019)

In the animal studies made to analyze the effects of PFOA, change in the liver weight was the most common effect observed. This could be a cause of the activation of the cellular peroxisome proliferator-activated receptor alpha (PPARα) which is a major regulator of lipid metabolism in the liver. According to studies made by the Environmental Protection Agency (EPA) in 2016 and Health Canada in 2018, all the articles reviewed came to the common conclusion that PFOA induces cancerogenic effects in liver, pancreas and testicle in mice and rats.

Also described by EPA in 2016 together with Health Canada in 2018 are the toxic effects of PFOS which was proven in several studies made on rats, mice and monkeys. Among the effects of oral intake of PFOS, the most important are liver effects, neurological effects, immune effects and thyroid effects.

Studies on populations working at a PFOS manufacturing facility and residents that live in the proximity of another PFOS production facility were performed in the USA. The outcome of these studies was that a strong association could be outlined between PFOS exposure and increased total cholesterol and high-density lipoproteins (HDLs). Additionally, lower fertility in women and decreased birth weight were also the assigned effects to the exposure to PFOS. (Knutsen et al., 2018)

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The toxicity of other novel emerging PFAS such as HFPO-DA also known as GenX and ADONA were investigated. After careful revision, Beekman et al. in 2016 concluded that not enough studies on this matter have been conducted to decide if GenX is harmful to living organisms, even though it is very persistent as all the other perfluorinated compounds. Also, based on the studies performed so far, a definite statement cannot be offered regarding the toxicity of ADONA, therefore further research is required. (RIVM, 2019)

Zeilmaker et al. in 2018 developed a risk potency factor (RPF) approach were the PFASs risk assessment could be predicted by assuming that these compounds “act in a similar manner, with the same mechanism/mode of action, resulting in dose-responses with the same shape but with different potencies for each of the individual substances”.

In order to assess the toxic risk associated to the exposure of humans to PFASS, a relative potency factor approach was used. According to Zeilmaker et al. in 2018, “RPFs express the toxic potency of individual mixture components relative to the so-called Index Compound(IC), the latter being one of the mixture components with well-known occurrence and toxicity and”. Because the PFOA’s toxicity has already been proved, this compound was chosen to be the IC in this approach. In table 5, the relative potency factors (RPF) are predicted for 20 individual PFASs including GenX. In order to apply these factors to humans, according to EFSA’s CONTAM Panel, it is assumed that the RPF values of rats are equal to those of humans.

Table 5: Relative Potency Factors (RPFs) using PFOA as the Index Compound (Reproduced from: RIVM, 2019)

PFAS Group PFAS RPF

PFSA Perfluorobutanesulfonate (PFBS) 0.001

Perfluoropentane sulfonic acid (PFPeS) 0.001≤RPF≤0.6

Perfluorohexanesulfonate (PFHxS) 0.6

Perfluoroheptane sulfonic acid (PFHpS) 0.6≤RPF≤2

Perfluorooctanesulfonate (PFOS) 2

Perfluorodecane sulfonic acid (PFDS) 2

PFCA Perfluorobutyrate (PFBA) 0.05

Perfluoropentanoic acid (PFPeA) 0.1≤RPF≤0.05

Perfluorohexanoate (PFHxA) 0.01

Perfluoroheptanoic acid (PFHpA) 0.1≤RPF≤1

Perfluorooctanoic acid (PFOA) 1

Perfluorononanoic acid (PFNA) 10

Perfluorodecanoic acid (PFDA) 4≤RPF≤10

Perfluoroundecanoic acid (PFUnDA) 4

Perfluorododecanoic acid (PFDoDA) 3

Perfluorotridecanoic acid (PFTrDA) 0.3≤RPF≤3

Perfluorotertadecanoic acid (PFTeDA) 0.3

Perfluorohexadecanoic acid (PFHxDA) 0.02

Perfluorooctadecanoic acid (PFODA) 0.02

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4. Analysis of PFAS

The vast quantity of these compounds in the environment and their proven toxicity have brought upon a great attention which has been focused for the last few decades on new and efficient methods of identifying and quantifying them in different environmental matrices such as water and soils. Figure 7 shows the representative PFASs which are a matter of concern for the regulatory authorities and environmental monitoring agencies. (Feng et al., 2019) Therefore, comprehensive trace analytical methods which are able to determine not only the legacy PFASs, but also the emerging perfluorinated compounds are required and of great importance, because they will unavoidably be discovered into various environmental matrices in the following period. (Ruan & Jiang, 2017)

Figure 7: Occurrence of PFASs in the environment (Retrieved from: Feng, et al., 2019)

In the following paragraphs, the advances in the analytical methods used for PFASs in various matrices such as water and soil are presented and compared. The sampling and pretreatment techniques are also discussed, together with the extraction and clean-up steps, including the cartridges and solvents used in determining a wide range of PFASs.

4.1 PFAS analysis in aqueous matrices

In the early stages of PFAS research, various methods have been developed with a focus on the most known classes of PFAS such as PFOS and PFOA, which have been since phased out of production from the two of the biggest manufacturers in North America, 3M and DuPont. However, ever since these compounds have been banned from production, new structurally similar PFASs have been manufactured whose purpose is to replace the legacy PFASs but to have the same properties. Therefore, there is a need to develop analytical methods able to identify and quantify in aqueous matrices not only the known PFASs but also the many unregulated ones, which the scientists call the emerging novel PFASs.

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Currently, methods including liquid-liquid extraction (LLE), solid-phase extraction (SPE) cartridge clean up or ion-pair extraction (IPE) combined with HPLC-MS/MS or GC-MS were used by several authors to analyze some of the original PFASs and some novel PFASs. (Ayala-Cabrera et al., 2018; Coggan et al., 2019; Deng et al., 2018; Y. Huang et al., 2018; Janda et al., 2018; Tröger et al., 2018; X. Yao et al., 2018) More details about some of the methods used will be presented in the tables 6 and 7.

4.1.1. Sampling

The procedures used to collect the samples have a direct impact on the quality of the results of the analytical methods. The containers used for the samples should be previously cleaned and rinsed with methanol and water or acetone and hexane to avoid any trace of contamination. The samples of water should be collected in a stainless-steel bucket and transferred in polypropylene bottles (PP) (Habibullah-Al-Mamun et al., 2016) or high density polyethylene (HDPE) (Gallen et al., 2017). Samples should be stored safely by being refrigerated at 4°C (Ayala-Cabrera et al., 2018; Janda et al., 2018; Wei et al., 2018) If the samples are not treated within 24 hours, the should be frozen at - 20°C (Gallen et al., 2018). Aqueous samples with PFASs can bind with glass, therefore glass bottles should not be used under these circumstances. Also materials containing polytetrafluoroethylene (PTFE) which is known as Teflon should be avoided as it could contaminate the samples. (So et al., 2004) Additionally, any equipment used for sampling (buckets, submersible pumps etc) should be decontaminated between each sample collection using MilliQ water or low-phosphate laboratory cleaner. (Gallen et al., 2017)

Field personnel should avoid the usage of insect repellent or other personal care products which may contain PFASs during sampling. Also, water-resistant clothing and footwear should not be present in the proximity of the area where the samples are taken from. (Figure 8)

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4.1.2. Extraction, clean-up and concentration

The pre-treatment methods are summarized together with the analytical challenges for PFASs. In some studies, filtration is used after the sampling. However, this does not apply to all the methods presented in this review as it presents questions as it can add to the contamination of the samples or their adsorption to the glass filter prior to analysis.

Filters with cellulose acetate or syringe nylon membrane PP filters are among the materials used for the filtration of seawater. (Feng et al.,2019) For the SPE sorbents, there is a variety of them used such as Oasis HLB and WAX cartridges which are used for polar to semi-polar compounds. (Tröger et al., 2018) Recoveries of PFASs in HLB cartridges have usually been reported to be higher than 80%, except for when analyzing short-chained PFCAs whose recoveries have values under 30%. The recoveries reported for WAX cartridge have generally good values between 85% and 107%. (Taniyasu et al., 2005)

The most common extraction methods used are manual or automated SPE and LLE. However, other extraction procedures such as solid-phase microextraction (SPME) with direct immersion in water and dispersive liquid-liquid microextraction (DLLME) have been developed with the purpose of reducing the volume of the solvent. DLLME is performed using perfluoro-tert-butanol as an extraction solvent, which was chosen based on its fluorous affinity. This method caught a lot of interest among scientists for its low costs, simplicity and also for being environmental-friendly. SPME achieved better separation when performed in headspace (HS) acquisition. This method was developed in 2012 by Monteleone et al. and they used this type of extraction after a derivatization step of PFCAs in the aqueous sample by using propyl chloroformate/n-propanol. Also, a different technique used for extraction is vortex-assisted liquid-liquid microextraction (VALLME) which can achieve a detection limit as low as 1.6 ng/L for PFOS in tap water, river and well water only by using a vortex mixer. (Papadopoulou et al., 2011) Concha-Graña et al. in 2018 also proved that even if lower sample volumes were used for the VALLME, the results obtained were comparable with the ones obtained with SPE. Stir bar-sorptive extraction (SBSE) is another method that has been developed and applied to the analysis of PFAS by which sufficient recoveries can be achieved. (Y. Yao et al., 2018)

For the extraction of PFCAs several adsorbent materials have been tested such as silica gel, HLB, WAX Florisil, C18 and Envi-CarbTM. However, some of these materials suffer from low extraction capacity or bad recoveries as mentioned also above, unacceptable reusage or they require the use of complicated devices for performing the extraction. Using SPE cartridges filled with bamboo charcoal, a microporous biomaterial instead of a traditional Oasis WAX is also a new development. Due to its specific surface area and plentiful cavity construction which can be seen in figure 9, the bamboo charcoal is an effective sorbent and yields low limits of detection. (Deng et al., 2018)

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Figure 9: SEM micrograph of the bamboo charcoal at 1500x magnification (Retrieved from Deng et al., 2018)

Another shortage of the previously mentioned list of materials is that some have a high price which adds to the cost of the entire analysis. In order to overcome some of these limitations and embracing the principle “similarity dissolves similarity”, a new method was developed by synthesizing a monolith-based adsorbent (MA), which can be seen in figures 10 and 11, using 2,2,3,3,4,4,5,5,6,6,7,7-dodecafluoroheptyl acrylate (DFHA) and (vinylbenzyl) trimethyl ammonium chloride (VTAC) as mixed functional monomers. Therefore, the monolithic fibre solid-phase microextraction (MMF-SPME) was tested for the detection of ultra-low levels of PFCAs in water and the results reached the detection limits of 0.40-4.40 ng/L. (Huang et al., 2018)

The most used options for the elution of PFAS is pure MeOH or in combination with 0.1% M NH4 to facilitate the ionization and minimize retention of compounds in the sorbent. (Lorenzo et al., 2018)

4.1.3. Instrumental analysis

In regard to the instrumental analysis and quantification, a lot remained the same in the methods used in analyzing PFAS in aqueous matrices as in earlier years, HPLC-MS/MS being the most used technique. (Dalahmeh et al. 2018; Deng et al., 2018; Huang et al, 2018; X. Yao et al., 2018) The mobile phases mostly used are represented by ammonium acetate, methanol, acetonitrile or

Figure 11: Monolithic adsorbent and its mode of action (Retrieved from: Huang et al., 2018)

Figure 10: SEM of single monolithic fiber at 40× magnification (Retrieved from: Huang et al., 2018)

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their mixed solutions. The preferred choice in matter of analytical columns is the reversed phase bonded silica (RP-C18) column. (Y. Huang et al., 2018; Janda et al., 2018) A direct injection approach of the spiked water samples is a technique that is sometimes used when combining HPLC with tandem MS. This approach proved to be effective in avoiding any sample contamination, however compared with SPE, it has lower sensitivity and the matrix effects slightly increase. (Ciofi et al., 2018)

Regarding the mass spectrometers used coupled with LC, the QQQ and Q-TOF are among the most capable of determining PFASs. (Newton et al., 2017; Ruan et al., 2017) However, sometimes MS/MS does not show enough selectivity and some authors propose using a high-resolution mass spectrometer (HRMS) for the trace analysis of PFASs in complex environmental matrices. HRMS Orbitrap- or time-of-flight (TOF)-MS are some of the high-resolution mass analyzers that have been tested. Application of very narrow mass tolerance windows (<10 ppm) in TOF-MS resulted in low limits of detections and very good recoveries of PFASs. (Wille et al., 2010) Another study used a combination of VALLME with HPLC and LTQ (Linear Trap Quadrupole)-Orbitrap HRMS where samples of seawater were tested for PFAS. This type of MS used the exact mass while quantifying the target analyses which helped in avoiding interferences. (Concha-Graña et al., 2018) MS is usually operated in the ESI-negative mode given the fact that most PFAS targets are anionic. When analyzing neutral PFASs such as FTOHs, FASEs and FASAs other ionization techniques like atmospheric pressure photoionization (APPI) and atmospheric pressure chemical ionization (APCI) have been used. (Ayala-Cabrera et al., 2018)

In an article written by Coggan et al. in 2019 are presented some changes of the LC system that have been made in order to avoid background contamination once the samples are ready for analysis. A delay column was installed after the solvent mixer together with the peek tubing and steel solvent filters in the needle wash system. These changes have been made to avoid using ethylene tetrafluoroethylene (ETFE) line and the polytetrafluoroethylene (PTFE) or the glass filters. Additionally, the needle wash step included a 10-s wash using a solution of MQ:MeOH (50:50, v/v) and a 10-s needle seat backflush with MQ:MeOH (90:10, v/v) which is also an aspect introduced to avoid contamination caused by sorption after injection. By introducing at least one MeOH blank injection and one instrument blank at each extraction batch of 12 samples can help with the constant monitoring of the method for background contamination.

Even if it is not often used, another technique to analyze PFASs in aqueous matrices is GC-MS/MS. When analyzing PFSAs and PFCAs with GC-MS/MS, a derivatization step is needed to decrease the polarity and increase the volatility of these compounds. For the GC-MS analysis, helium was used as a carrier gas with a flow rate of 1 ml min-1_{and as a collision gas argon was} employed at a 2.3 mTorr pressure. A triple quantum MS (QQQ) was used in negative ion chemical