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\DOI{10.5802/crchim.391}
\datereceived{2024-09-30}
\daterevised{2024-11-14}
\dateaccepted{2025-03-05}
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\dateposted{2025-04-16}
\begin{document}

\begin{noXML}

\CDRsetmeta{articletype}{review}

\title{From fluorine's position in the periodic table to PFAS
environmental issues} 

\alttitle{De la position du fluor dans le tableau p\'{e}riodique aux
probl\`{e}mes environnementaux li\'{e}s aux PFAS}

\author{\firstname{Marie Pierre} \lastname{Krafft}\CDRorcid{0000-0002-3379-2783}}
\address{University of Strasbourg, Institut Charles Sadron (UPR CNRS 22), 23 rue du Loess, 67034 Strasbourg Cedex, France}
\email{krafft@unistra.fr}
                               
\keywords{\kwd{Fluorinated surfactants}
\kwd{Fluorocarbons}
\kwd{Fluoropolymers}
\kwd{Supramolecular self-assemblies}
\kwd{Adsorption at interfaces}
\kwd{Essential use}
\kwd{Regulation}
}

\altkeywords{\kwd{Tensioactifs fluor\'{e}s}
\kwd{Fluorocarbures}
\kwd{Fluoropolym\`{e}res}
\kwd{Auto-assemblages supramol\'{e}culaires}
\kwd{Adsorption aux interfaces}
\kwd{Utilisation essentielle}
\kwd{R\'{e}glementation}} 

\begin{abstract}
PFASs have become a global pollution issue. {These} {anthropogenic}
{and} {unusually} {stable chemicals have been} {in use} {since the
1940s.} {Thousands of them, whether molecular or polymeric, have been
developed and are found in innumerable industrial processes and
consumer products. Multiple air and water pathways ensure their
dissemination when released into the environment. They} {can adhere to
rock and sediment, concrete, tarmac, and asphalt.} {Humans, animals,
and plants have} {thus become} {exposed to PFASs through air, dust,
soil, diet, drinking water, and wastewater. Some PFASs have been
identified as very persistent in the environment, bioaccumulative, and
toxic by public health authorities.} {This review emphasizes that the
specific and outstanding properties of PFASs and of the C--F bond
derive directly from fluorine's position in the periodic table.}
{Fluorinated surfactants also have the outstanding capacity to form
large, exceptionally sturdy} {supramolecular} {self-assemblies in
aqueous solutions, on solid surfaces and at liquid interfaces. This
behavior may impact the fate of PFAS pollutants} {in the environment,
as well as their remediation procedures, a fact that has so far been
barely recognized. Finding alternatives to fluorinated surfactants and
fluoropolymers} {that deliver} {comparably high} {performances while
reducing} {their} environmental and biological impact can be extremely
challenging. The essential use concept distinguishes the uses for
which, in the current state, PFASs hardly find any replacement product
capable of matching the demanded properties from those for which
lesser performance is sufficient and attainable otherwise, not to speak
of those for which use of PFASs is downright futile. The need for
discerning, eco-responsible use of PFASs is accentuated if we want to
continue benefit from PFASs unmatchable performances.
\end{abstract}

\begin{altabstract}
Les PFAS sont devenus un probl\`{e}me de contamination plan\'{e}taire.
Ces produits chimiques anthropog\'{e}niques exceptionnellement stables
sont utilis\'{e}s depuis les ann\'{e}es 1940. Des milliers d'entre eux,
qu'ils soient mol\'{e}culaires ou polym\`{e}res, ont \'{e}t\'{e}
d\'{e}velopp\'{e}s et se retrouvent dans d'innombrables processus
industriels et produits de consommation. De multiples voies
a\'{e}riennes et aquatiques assurent leur diss\'{e}mination dans
l'environnement lors de leur rejet. Ils peuvent adh\'{e}rer aux roches
et aux s\'{e}diments, au b\'{e}ton et au tarmac. Les humains, les
animaux et les plantes ont ainsi \'{e}t\'{e} expos\'{e}s aux PFAS par
l'air, la poussi\`{e}re, le sol, l'alimentation, l'eau potable et
les eaux us\'{e}es. Certains PFAS ont \'{e}t\'{e} identifi\'{e}s comme
tr\`{e}s persistants dans l'environnement, bioaccumulables et
toxiques par les autorit\'{e}s de sant\'{e}. Cette revue souligne que
les propri\'{e}t\'{e}s sp\'{e}cifiques et exceptionnelles des PFAS et
de la liaison C--F d\'{e}coulent directement de la position du fluor
dans le tableau p\'{e}riodique. Les tensioactifs fluor\'{e}s ont une
capacit\'{e} accrue \`{a} former des auto-assemblages
supramol\'{e}culaires exceptionnellement robustes en solutions
aqueuses, sur des surfaces solides et aux interfaces liquides. Ce
comportement peut avoir un impact sur le devenir des polluants PFAS
dans l'environnement, ainsi que sur les proc\'{e}dures de
rem\'{e}diation, un fait jusqu'\`{a} pr\'{e}sent peu reconnu. Trouver
des alternatives aux tensioactifs fluor\'{e}s et aux
fluoropolym\`{e}res qui offrent des performances comparables tout en
r\'{e}duisant les impacts environnementaux et biologiques peut
s'av\'{e}rer difficile. La notion d'usage essentiel distingue les
usages pour lesquels, en l'\'{e}tat actuel, les PFAS ne trouvent
gu\`{e}re de produit de remplacement capable d'\'{e}galer les
propri\'{e}t\'{e}s requises, de ceux pour lesquels des performances
moindres sont suffisantes et/ou r\'{e}alisables autrement, sans parler
de ceux pour lesquels l'utilisation des PFAS est clairement futile. Une
utilisation raisonn\'{e}e et \'{e}coresponsable des PFAS s'impose si
on veut continuer \`{a} b\'{e}n\'{e}ficier de leurs performances
in\'{e}gal\'{e}es.
\end{altabstract}


\maketitle

{\vspace*{4pt}}

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\end{noXML}

\section{Introduction and scope}

The term ``per- and polyfluoroalkyl substances (PFASs)'' refers to a
wide range of organofluorine compounds that have diverse physical,
chemical, environmental, and biological properties. This term was
introduced in 2011 to provide a standardized language for the
scientific, industrial, and regulatory communities \cite{1}. PFASs have
gained attention since the identification in the environment of two
specific substances, perfluorooctanoic acid (PFOA) and
perfluorooctanesulfonic acid (PFOS)~\cite{2,3,4}. {Environmental} research
and regulatory efforts have since expanded to cover a considerably
broader range of compounds.

PFASs are synthetic compounds that have been in production and use
since the 1940s~\cite{5,6,7,8}. They include small molecules and
polymers and can have surface activity, hence be surfactants. The
chemical and thermal stability of perfluoroalkyl moieties, along with
their outstanding hydrophobic and lipophobic character, lead to
countless uses of PFASs in industrial processes and consumer
goods~\cite{5, 6,8,9,10,11,12,13,14}. The surfactant applications take
advantage of the unparalleled aqueous surface tension-lowering
properties of PFAS surfactants and include use as processing aids for
fluoropolymer manufacture, coatings, and aqueous film-forming foams
used to extinguish fires involving flammable liquids.

It is now recognized that some PFASs are globally distributed,
environmentally persistent, bioaccumulative, and potentially
harmful~\cite{1,2,3,15,16,17,18}. PFASs can travel long distances
through air, sea, and river routes, and can strongly adsorb to rocks
and sediments. A 2022 study presents an alarming picture of PFAS
pollution in the US, with approximately 50,000 factories either
producing or using PFAS, over 4000 contaminated water treatment
plants, 3500 polluted military sites, and more than 500 airports
affected~\cite{19}. The French newspaper Le Monde has published a map
showcasing the widespread PFAS contamination across Europe~\cite{20},
serving as a stark reminder that PFAS pollution is a pressing global
concern that demands action.

In 2023, the European Chemicals Agency (ECHA) published a proposal to
ban PFASs~\cite{21}. This is unprecedented considering the number of
substances covered by the USA Environmental Protection Agency (EPA),
which has listed over 14,000 PFAS structures, as well as a list of 1915
PFAS chemicals for which no explicit structure is documented (polymers
and unknown or variable composition, complex reaction products or
biological materials, collectively called UVCBs under REACH)~\cite{22}.
In 2023, the Organization for Economic Cooperation and
Development (OECD) defined PFASs as ``fluorinated substances that
contain at least one fully fluorinated methyl or methylene carbon atom
(without any H/Cl/Br/I atom attached to it), i.e.\ with a few noted
exceptions, any chemical with at least a perfluorinated methyl group
(--CF$_{3}$) or a perfluorinated methylene group
(--CF$_{2}$--) is a PFAS''~\cite{23}.

The present article begins by discussing the origins of the outstanding
properties of PFASs, starting with the exceptional stability of the
C--F bond. The second section clarifies why the widespread use of PFASs
in so many industrial branches and consumer goods directly results from
these characteristics, or a combination of them. The third section
presents the ``essential use'' concept, a useful tool that could help
to determine when PFASs can be phased-out. The fourth section is a
brief reminder of the recent regulation efforts. A last section focuses
on the outstanding tendency for amphiphilic PFASs to form
supramolecular aggregates in solutions and at interfaces, and the
implications of these strong self-assembling properties for both their
environmental fate and remediation techniques.

\section{Fluorine: \textit{nec pluribus impar} among elements}

Fluorine is the 13{th} most abundant element in the Earth's crust
(0.054\% by weight), ahead of carbon (0.02\%)~\cite{24}. It is
primarily extracted from fluorite (CaF$_{2}$, fluorspar) on a large
scale, and is also present in other mineral groups, namely bastnaesite
(Ca,La,Nd)(CO$_{3}$)F), cryolite (Na$_{3}$AlF$_{6}$), sellaite
(MgF$_{2}$), villiaumite (NaF), fluoroapatite
(Ca$_{5}$(PO$_{4}$)$_{3}$F), topaz (Al$_{2}$SiO$_{4}$(F,OH)$_{2}$) and
various phyllosilicate, such as biotite
(K(Mg,Fe)$_{3}$(AlSi$_{3}$O$_{10}$)\ubreak(OH,F)$_{2}$ \cite{25}. Use of
fluorite was first described by Georgius Agricola in 1529 as an
additive to metal ores to lower their melting points during smelting.
The Latin verb \textit{fluere}, meaning `to flow', gave the mineral its
name. Fluorine is scarcer in the oceans (1.3 ppm of fluoride compared
to 19,000 ppm for bromine)~\cite{25,26}. Concentrations can be higher
in waters that are in contact with rocks that contain high fluorine
concentrations~\cite{25}. By contrast, organically bound fluorine is
very rare in Nature, with only half a dozen naturally occurring
organofluorine compounds identified in plants and microorganisms; none
in mammals and insects~\cite{26,27,28}.\looseness=-1

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig01}
{\vspace*{-3pt}}
\caption{\label{fig1}Fluorine is in pole position in the periodic table
of the elements. All properties of PFASs and of the C--F bond directly
derive from this extreme location.}
{\vspace*{-3pt}}
\end{figure*}

Henri Moissan, Nobel laureate in chemistry in 1906~\cite{29}, isolated
elemental fluorine F$_{2}$ in 1886 using low-temperature electrolysis
of potassium hydrogen difluoride dissolved in anhydrous HF. F$_{2}$ is
extremely reactive and a strong oxidant (highest known standard
reduction potential: ${+}$2.87~V). Because of the difficulties in
handling such highly reactive and corrosive reagents, organofluorine
chemistry remained relatively undeveloped until the 1920s. The first
perfluoroalkanoic acid, trifluoroacetic acid, was synthesized in
1922~\cite{30}, and the first perfluorocarbon, CF$_{4}$, was isolated
in 1926~\cite{31}. Industrial production of F$_{2}$ began during World
War II as part of the Manhattan Project to produce liquid fluorocarbons
and fluoropolymers for use as coolants and materials exposed to the
highly reactive UF$_{6}$~\cite{32}. Commercial exploitation started
with little regulation in the early 1950s, following Plunkett's
serendipitous discovery of poly(tetrafluoroethylene) (PTFE, e.g.,
Teflon) in 1938~\cite{33}. After World War II, various methods were
developed to control direct fluorination by F$_{2}$. Vapor-phase
perfluorination was initially performed by carefully and progressively
adding F$_{2}$ diluted in an inert gas to the hydrocarbon substrate
(jet fluorination)~\cite{34}. The process was improved by starting the
reaction at low temperature (LaMar process)~\cite{35, 36}. Efficient
perfluorination was also obtained by adsorbing the hydrocarbon
substrate onto NaF particles (aerosol fluorination)~\cite{37}.
Liquid-phase fluorination provided improvements by adding the
hydrocarbon substrate into F$_{2}$ solubilized in an inert liquid, a
fluorocarbon or a chlorofluorocarbon, followed by UV
irradiation~\cite{38} or without UV irradiation (Exfluor-Lagow
method)~\cite{36}. The technique was further refined by transiently
grafting a long-chain fluorinated ester ligand on the hydrocarbon
substrate to enhance its solubility in a fluorinated solvent (PERFECT
method)~\cite{39}.

Two primary methods are used for industrial-scale production of
PFASs~\cite{1,5,7,9,40,41}. The first involves the electrochemical
fluorination of linear alkane sulfonic fluorides in anhydrous hydrogen
fluoride (HF), known as the Simons process~\cite{42,43}. This
electrochemical process is harsh and nonselective, resulting in complex
mixtures of linear, branched, and cyclic perfluorinated molecules with
various chain lengths. Notably, it produces a product signature that
can help trace the source of a contamination. Products labeled as PFOS
typically contain 70--80\% of the linear molecule. The second major
production method utilizes free radical telomerization of
tetrafluoroethylene ($\mathrm{CF}_{2}{=}\mathrm{CF}_{2}$) with a
fluorinated alkyl iodide, most commonly
$\mathrm{CF}_{3}\mathrm{CF}_{2}\mathrm{I}$. This process yields a
series of homologous, even carbon-numbered linear alkyl iodide
telomers, $\mathrm{CF}_{3}\mathrm{CF}_{2}(\mathrm{CF}_{2}
\mathrm{CF}_{2})_{{n}}\mathrm{I}$~\cite{44}. 
Subsequent distillation allows easy isolation of pure homologs within
the series, thanks to the significant molecular weight
difference---about 100---between consecutive telomers (\textit{n} and
$n+1$).

Due to its extreme top right position in the periodic table (noble
gases excepted), fluorine beholds unique characteristics
(Figure~\ref{fig1}). It is the smallest of the halogens. Compared to
hydrogen (1 proton, 1 electron), fluorine (9 protons, 9 electrons) has
a larger van der Waals radius (1.47~\AA{} vs.\ 1.20~\AA{}) and a
{significantly} larger single bond covalent radius (0.64~\AA{} vs.\ 
0.32~\AA{})~\cite{45}.\looseness=-1


Fluorine is the most electronegative element (3.98 on Pauling's scale
vs.\ 2.1 for H). It bears three lone pairs of electrons that are tightly
held by the nucleus. Fluorine has thus a much denser and less
polarizable electron cloud than hydrogen (polarizability $0.557 \times
10^{-24}~\mathrm{cm}^{3}$ vs.\ $0.667 \times 10^{-24}~\mathrm{cm}^{3}$).

The C--F bond is the strongest single bond known in organic chemistry
(${\sim}$485~kJ${\cdot}$mol$^{-1}$ compared to ${\sim}$413~kJ${\cdot}$mol$^{-1}$ for C--H
in hydrocarbon compounds)~\cite{46}. Due to fluorine's high
electronegativity, the C--F bond is highly polarized, with the fluorine
atom bearing the partial negative charge and the carbon atom the
positive charge~\cite{47, 48}. This endows the C--F bond with an
electrostatic character and generates a strong electric dipole with a
C--F orientation opposite to that found in C--H, which is a dramatic
change. An important consequence of the highly polarized nature of the
C--F bond is the availability of a low-energy ${\upsigma}^*$ antibonding
orbital that can accept electron density from nearby electron-donating
groups such as lone pairs or ${\upsigma}$ bonds~\cite{47,48}.
Fluorine's electron affinity is 328~kJ${\cdot}$mol$^{-1}$ (3.40~eV) vs.\  
73~kJ${\cdot}$mol$^{-1}$ for hydrogen. Fluorine also has a much higher ionization
energy than hydrogen (1677~kJ${\cdot}$mol$^{-1}$ vs.\ 1312~kJ${\cdot}$mol$^{-1}$). The
stability of the C--F bond increases with fluorine substitution (e.g.,
530~kJ${\cdot}$mol$^{-1}$ in CF$_{3}$CF$_{3}$ vs.\ 450~kJ${\cdot}$mol$^{-1}$ in
CH$_{3}$CH$_{2}$F). The backbone C--C bonds are significantly stronger
in fluorocarbons than in hydrocarbons. One can thus consider the C--F
bond as a highly polarized bond, short, strong, and unreactive.
Organofluorine compounds are subject to various stereoelectronic
effects, including dipole--dipole and charge--dipole interactions, and
hyperconjugation.

The unique characteristics of the fluorine atom and C--F bond endow
fluorinated chains $\mathrm{C}_{n}\mathrm{F}_{2n+1}$
({F}-chains)  with special properties~\cite{9,49,50}. Because
of the low polarizability of fluorine, {F}-chains display lower
intermolecular (van der Waals) attraction forces than hydrocarbon
chains ({H}-chains). {F}-chains are also bulkier than
{H}-chains with larger cross sections (27--30~\AA{}$^{2}$ vs.\ 
18--21~\AA{}$^{2}$). The volumes occupied by CF$_{2}$ and CF$_{3}$
groups in linear fluorocarbons are 38~\AA{}$^{3}$ and 92~\AA{}$^{3}$
vs.\ 27~\AA{}$^{3}$ and 54~\AA{}$^{3}$ for CH$_{2}$ and CH$_{3}$ groups,
respectively. The larger cross section of {F}-chains as compared
to {H}-chains explains their larger hydrophobicity. Molecular
simulations show that the extra work of cavity formation to accommodate
an {F}-chain in water, compared to an {H}-chain, is not
offset by enhanced energetic interactions with water~\cite{51}. The
enhanced hydrophobicity of fluorinated surfaces arises because
{F}-chains pack less densely on surfaces, leading to poorer van
der Waals interactions with water. {F}-chains longer than 6
carbons adopt a helical conformation rather than the all-trans planar
zig--zag conformation characteristic of alkyl chains~\cite{50,52,53}.
This particularity was recently re-assigned to hyperconjugation
stabilization through $\rmsigma_{\mathrm{cc}} \rightarrow
\rmsigma^{*}_{\mathrm{CF}}$ rather than to steric and electrostatic
effects~\cite{54}. The bonds between an {F}-chain and oxygen,
nitrogen, chlorine, or bromine atoms are also usually strengthened due
to the enhanced electronegative character of the fluorine-bearing
carbon atoms.\looseness=-1

\section{PFAS's exceptional properties produce a wide scope of applications}

Very different compounds are grouped under the PFAS denomination that
have distinctive properties and, consequently, different uses. Recent
reviews have provided eloquent images of the ``complex PFAS universe''
featuring the intermediates and final manufacturing
products~\cite{10,12,13}. A first distinction must be made between
polymers and substances of low molecular weight. In each class, it is
then important to differentiate the compounds that display substantial
surface activity from those that do not. For all of them, fluorine
contributes to creating or enhancing unique properties and has, for
this reason, been called ``the enabler''~\cite{55}.

\begin{table*}
\caption{\label{tab1}Chemical formula of some legacy PFASs and of compounds developed more
recently{\vspace*{-2pt}}}
\tabcolsep2pt
\begin{tabular}{cccc}
\tbody
\raisebox{30pt}{}
\parbox[t]{10pc}{\centering \inlinefig{fx01}\Lbreak PFOS} &amp; 
\parbox[t]{10pc}{\centering \inlinefig{fx02}\Lbreak PFHxS} &amp; 
\parbox[t]{5pc}{\centering \inlinefig{fx03}\Lbreak PFBS} &amp; 
\parbox[t]{10pc}{\centering \inlinefig{fx04}\Lbreak ADONA}{\vspace*{10pt}}\\
\parbox[t]{10pc}{\centering \inlinefig{fx05}\Lbreak PFOA} &amp; 
\parbox[t]{10pc}{\centering \inlinefig{fx06}\Lbreak PFHxA} &amp; 
\parbox[t]{5pc}{\centering \inlinefig{fx07}\Lbreak PFBA} &amp; 
\parbox[t]{10pc}{\centering \inlinefig{fx08}\Lbreak HFPO-DA}{\vspace*{10pt}}\\
\parbox[t]{10pc}{\centering \inlinefig{fx09}\Lbreak PFNA} &amp; 
\parbox[t]{10pc}{\centering \inlinefig{fx10}\Lbreak PFO3OA} &amp; 
\parbox[t]{5pc}{\centering \inlinefig{fx11}\Lbreak PFPrA} &amp; 
\parbox[t]{10pc}{\centering \inlinefig{fx12}\Lbreak GenX}{\vspace*{4pt}}
\botline
\end{tabular}
{\vspace*{-2pt}}
\end{table*}

\subsection{Fluorocarbons}

Fluorocarbons ($\mathrm{C}_{n}\mathrm{F}_{2n+2}$) are PFASs that are
devoid of polar head. They embody the essential properties of
{F}-chains~\cite{41,50,56,57,58,59}. The biological inertness
of liquid fluorocarbons and closely related, essentially
non-amphiphilic derivatives (e.g., perfluorooctylbromide,
perfluoroethers, or amines) is well documented. These compounds,
formulated as nanoemulsions, were investigated as injectable oxygen
carriers (temporary blood substitutes) in Phase III clinical trials in
Europe and the USA~\cite{41,56,60,61,62}. Fluorocarbons can
solubilize large amounts of gases (e.g., up to ${\sim}$50\% v/v of
O$_{2}$, while O$_{2 }$ solubility in water is only ${\sim}$2.5\% v/v),
which is a direct consequence of their low van der Waals interactions.
More generally, fluorocarbon nanoemulsions have the potential to
alleviate hypoxia in therapies that benefit from oxygenation, such as
photodynamic therapy, radiation therapy, sonodynamic therapy and
immunotherapy~\cite{63,64,65,66}. They are also actively investigated
as $^{19}$F MRI contrast agents (due to high $^{19}$F nuclei
concentration and absence of protons~\cite{60}), as ultrasound contrast
agents (microbubbles, as well as phase-change nanodroplets~\cite{60,67,68}), 
as $^{18}$F positron emission tomography (PET) contrast
agents and multimodal contrast agents~\cite{66,69}. Liquid
fluorocarbons have been used in biotechnology to supply oxygen and
other gases to cells in cultures~\cite{70}. Due to weak van der Waals
interactions, fluorocarbons have the lowest surface tensions,
refractive indices, and dielectric constants of all liquids. They have
higher density, fluidity, compressibility, gas solubilities, and
critical temperatures than \mbox{hydrocarbons} of similar length. As a
consequence of their outstanding hydrophobicity, their solubility in
water is extremely low and decreases exponentially with chain length.
Fluorocarbons are chemically inert, in particular nonflammable, and can
withstand very aggressive conditions (temperature and pH, think of
UF$_{6}$). They have found numerous applications as functional and heat
transfer fluids in the electronics industry and in energy storage.

\subsection{Fluorinated surfactants}

Fluorinated surfactants comprise an {F}-chain covalently bound
to a polar head. An alkyl spacer (e.g., --(CH$_{2}$CH$_{2}$)--) is
often inserted between the hydrophilic and hydrophobic moieties.
Fluorinated surfactants are generally more effective (they reduce the
surface tension of water to lower values) and more efficient (lower
concentrations are needed to reach lower surface tensions) than
hydrocarbon surfactants~\cite{61,71,72}. The incremental change in
free energy of adsorption at the water/air interface per CF$_{2}$ group
is ${-}$5.1~kJ${\cdot}$mol$^{-1}$ as compared to ${-}$2.6~kJ${\cdot}$mol$^{-1}$ for a
CH$_{2}$ group. They are superhydrophobic and lipophobic; in other
words, they repel oils as well as water. Their mechanical, optical, and
electrical properties are unrivaled.

Table~\ref{tab1} collects some PFASs among the better-known and most
used. First came PFOS and PFOA, and the higher homolog,
perfluorononanoic acid, PFNA, which are often called ``legacy PFASs''.
Shorter-chain derivatives, such as perfluorobutanoic acid (PFBA),
perfluorohexanoic acid (PFHxA) and their precursors, fluorotelomers
$\mathrm{C}_{4}\mathrm{F}_{9}(\mathrm{CH}_{2})_{2}\mathrm{OH}$ (4:2
FTOH) and
$\mathrm{C}_{6}\mathrm{F}_{13}(\mathrm{CH}_{2})_{2}\mathrm{OH}$ 6:2
FTOH, were subsequently used in an attempt to reduce toxicity and
environmental persistence~\cite{73}. Toxicological \mbox{evaluation} provided
surprises. Although the blood elimination half-lives of PFASs generally
decrease with chain length, PFHxS has a longer half-life in humans than
PFOA or PFOS. Likewise, although longer-chain PFASs tend to have longer
renal elimination half-lives in rats, PFBA has a slower renal clearance
than PFHxA. These short PFASs tend, in general, to be more soluble in
water and have a lower potential for sorption to particles than the
long-chain ones. They have, however, a higher potential for being
transported on long distances by waterways. Compounds in which the
fluorinated chain is \mbox{interrupted} by oxygen atoms have also been
introduced to facilitate their degradation. Unfortunately, some of
these compounds were eventually found to be more toxic than
PFOA~\cite{74}. These are examples of regrettable substitutions that
must be avoided in the future. More generally, the pharmacokinetics and
(eco)toxicological profiles of PFASs are characterized by still poorly
understood species, sex, and other variations.\looseness=-1

\begin{figure*}
{\vspace*{-4pt}}
\includegraphics{fig02}
{\vspace*{-7pt}}
\caption{\label{fig2}PFAS's specific features are exploited in hundreds
of uses ranging from essential to futile.}
{\vspace*{-6pt}}
\end{figure*}


Due to their unmatched properties, PFAS surfactants have found uses in
numerous industrial processes (Figure~\ref{fig2}). Selected examples
include uses as processing aids in the production of fluoropolymers
(PTFE and polyvinylidene fluoride, PVDF), in paints and varnishes,
medical devices, and in the aerospace industry (aircraft brakes and
hydraulic fluids). They are used to inhibit corrosion in the machinery
and equipment industry, including treatment and coatings of metals
(metal plating) and as mold release agents; in the high-technology
sector, as in electronics and semiconductors, for example, as additives
to photoresist formulations, energy, and electric vehicles. Surfactant
PFASs are used for \mbox{protection} and improved security, for example, to
protect from fire (fire-fighting foams, firefighter protective gears)
and contamination (medical workwear), in lubricants, coatings
(antireflective, antifogging or antistatic,\,{\ldots}) adhesives,
pesticides, etc. Finally, they are used in countless consumer and
comfort products, such as for waterproofing and as stain-repellent
treatment of fabrics, furniture, upholstery, cloth, carpet, leather,
paper, and cardboard (food packaging).


\subsection{Fluoropolymers}

Concerning polymers, there are two different situations to consider:
those that use fluorinated monomers and those that require a PFAS,
initially an ammonium salt of PFOA or PFNA, as a processing aid in
emulsion polymerization to achieve a homogenous, fine particle size
distribution. Traces of PFAS surfactants may then remain even after
curing of the material.

Notably, technologies currently exist to synthesize key fluoropolymers
such as PTFE, PVDF, and fluorinated elastomers (FKM) without using PFAS
surfactants as processing aids~\cite{75}. Fluoropolymers combine the
many advantages of fluorocarbons, such as hydrophobicity and
lipophobicity, excellent chemical stability and bio-inertness, low
surface \mbox{energy,} and aptitude for phase segregation, to those of
polymers that are well established in materials science~\cite{76}.
Fluoropolymers are resistant to acids, bases, oxidation, corrosion, and
heat. They have low refractive index, flammability, and 
\mbox{surface} \mbox{energy.}
Fluoropolymers have diverse structures ranging from semi-crystalline to
amorphous and can provide thermoplastics, thermosets, and elastomers.
Many fluoropolymers can be recycled~\cite{77}. Fluoropolymers are being
used for high-performance materials in a wide range of industries,
including the automotive, aerospace, chemical, nuclear industries,
electronics, and medical industries, and participate in green economy
initiatives~\cite{78}. Variously nanostructured fluoropolymers yield
superhydrophobic and chemically resistant surfaces when deposited on
solid surfaces that are minimally wetted by water and organic solvents,
and could be used for water collection, antifouling, and self-cleaning
applications. Some fluoropolymers (perfluorosulfonic acid membranes,
Nafion, Aquivion) are used in the important industrial chloralkali
process, the large-scale industrial process that produces common
chemical feedstocks, such as chlorine gas and sodium
hydroxide~\cite{79, 80}, for fuel cells and energy storage (lithium-ion
batteries). PVDF and fluorinated ethylene--propylene FEP, a copolymer
of hexafluoropropylene and tetrafluoroethylene, are used as binders for
the anode and cathode in nearly all commercial batteries~\cite{81}.
Besides their well-known applications in medical \mbox{coatings} and
implants~\cite{82}, fluoropolymers have found advanced uses in
biomedical applications during the past decade, including in gene
delivery, cytosolic protein delivery, drug delivery, magnetic resonance
imaging, photodynamic therapy, \mbox{anti-fouling} and anti-bacterial
applications, and tissue engineering~\cite{78,83}.

\begin{table*}
\caption{\label{tab2}The ``essential use'' concept can help phase-out
certain PFASs\vspace*{-2pt}}
\begin{tabular}{lcc}
\thead
Category &amp; Definition &amp; Examples of use\\
\endthead
1.~Non-essential &amp; 
\parbox[t]{6cm}{\raggedright Uses that are not essential for health or
safety and the functioning of the society.  Uses essentially driven by
market opportunity} &amp; 
\parbox[t]{6cm}{\raggedright Personal care products and cosmetics, ski
waxes, cooking ware, water- and stain-free textiles, leisure
clothing}{\vspace*{4pt}}\\
2.~Substitutable  &amp; 
\parbox[t]{6cm}{\raggedright Uses regarded as essential because they have
important functions, but for which alternatives have been developed
that have adequate performance} &amp; 
\parbox[t]{6cm}{\raggedright Class B fire-fighting foams,
perfluorosulfonic acid membranes for fuel cells}{\vspace*{4pt}}\\
3.~Essential &amp; 
\parbox[t]{6cm}{\raggedright Uses considered as essential for health or
safety and the functioning of the society and for which alternatives
are not yet established} &amp; 
\parbox[t]{6cm}{\raggedright Fluorinated drugs (regulated under other
regulations than REACH), perfluorosulfonic acid  membranes for the
chloralkali process, medical devices, occupational protective clothing}\vspace*{2pt}
\botline
\end{tabular}
\tabnote{Adapted from~\cite{84} under CC BY-NC 3.0.\vspace*{-4pt}}
\end{table*}

\section{The concept of essential uses}

It soon became obvious that determining when PFASs are not essential
and when alternative solutions existed was necessary. It is noteworthy
that in the revised OECD definition of PFASs, ``the term ``PFAS'' does
not inform whether a compound is harmful or not, but only communicates
that the compounds under this term share the same trait for having a
fully fluorinated methyl or methylene carbon moiety''~\cite{23}.
Discrimination between essential and non-essential uses seems to be the
key to effectively managing substances of concern~\cite{84,85}. To be
considered ``essential'', a use must be ``necessary for health, safety
or is critical for the functioning of society'' and ``there are no
available technically and economically feasible alternatives'' as
defined under the Montreal Protocol (Decision IV/25.19).

\begin{figure*}
{\vspace*{-2pt}}
\includegraphics{fig03}
{\vspace*{-1pt}}
\caption{\label{fig3}Some uses of PFASs that are considered essential
for safety (to protect from fire and contamination), health (drugs,
safe and functional implants, medical coatings), and chemical (key
processes), energy storage and aerospace industries.}
{\vspace*{-2pt}}
\end{figure*}

Most fluorinated pharmaceuticals fall in the latter category (category
3, Table~\ref{tab2}, Figure~\ref{fig3}). Fluorine-containing compounds
now constitute a quarter of all small-molecule drugs in the
pharmaceutical market and are found among the most prescribed and
top-selling drugs~\cite{86}. They are particularly efficient in the
treatment of cancers and neurodegenerative diseases (Parkinson's and
Alzheimer's diseases) Fluorination of pharmacological agents habitually
enhances their pharmacological effectiveness, increases their
biological half-life, and improves their biosorption~\cite{87}. Most
fluorine-containing pharmaceuticals only contain one or two fluorine
atoms. Some drugs contain one or two CF$_{3}$ groups or a short
perfluoroalkyl moiety. A semi-fluorinated alkane, perfluorohexyloctane
$\mathrm{C}_{6}\mathrm{F}_{13}\mathrm{C}_{8}\mathrm{H}_{18}$
(Miebo{\texttrademark}), was approved in 2023 by the Food and Drug
Administration (FDA) as a single-component eye drop to reduce tear
evaporation in the treatment of the dry eye disease~\cite{86}. It is
noteworthy that the active ingredients in pharmaceuticals and
pesticides are regulated \mbox{under} regulations other than REACH and benefit
from derogations in the EU restriction \mbox{proposal.} In view of the risk
associated with emission (particularly in the case of plant protection
products, the derogation comes with a recommendation to the EU to
address these concerns in the respective regulations to reduce the use
and emissions of PFAS as much as possible.

Another case of essential use is exemplified by perfluorosulfonic acid
(PFSA) polymer-based membranes (Nafion{{\textregistered}}, Chemours,
formerly DuPont) used in the chloralkali process
(Figure~\ref{fig3})~\cite{79,80}. These thermoplastics are used in a
wide range of chemical synthesis and separation procedures, and in
analytical instrumentation~\cite{88,89}. Nafion{\textregistered}\ provides 
the excellent conductivity of ions necessary for the
electrochemical process while operating under highly corrosive
conditions. It has replaced other processes that used more energy and
generated hazardous materials and byproducts such as asbestos or
mercury~\cite{90}. The use of PFSA membranes in the chloralkali process
is thus considered essential (category 3, Table~\ref{tab2}). On the
other hand, the use of PFSA membranes as proton exchange membranes for
high-efficiency fuel cells is considered substitutable. Lithium-ion
batteries cannot operate without PFAS today~\cite{91}. Moreover, PFAS
are indispensable in semiconductor manufacturing; a ban on these
substances would threaten the progression of innovative
technologies~\cite{91}.

Since their introduction in the 1950s, side-chain fluorinated polymers
have provided liquid repellency to many textile products. Their uses
range from nice-to-wear property in daily clothing to essential
protection in occupational protective attire (Figure~\ref{fig3})~\cite{92}.
Medical textiles are examples in which the technical standards for the
protection of human life (EU Medical Devices Directive 93/42/EEC) may
be challenging to meet without PFAS. Similarly, meeting the performance
standards set by the USA National Fire Prevention Association for
protective clothing for firefighters seems difficult without
fluorinated compounds.

\begin{figure*}
\includegraphics{fig04}
\caption{\label{fig4}Some extensive, high-tonnage everyday convenience
PFAS uses. Good to have, but indispensable?}
\vspace*{1pt}
\end{figure*}

Class B fire-fighting foams, designed to extinguish fires of flammable
liquids, such as hydrocarbons, contain either PFASs or proprietary
mixtures of hydrocarbon or silicone surfactants. Many companies have
developed fluorine-free class B foams that meet the standard
fire-fighting {performance} {certifications~\cite{93}.} There is some
debate about whether PFASs are still needed in certain situations, such
as fires occurring in refineries. Many airports, chemical industry
facilities, and national defense forces have already switched to
fluorine-free foams. Of note, the silicone-based foams may {contain}
{residual} amounts of cyclic siloxanes that are also listed as Substances
of Very High Concern (SVHC) under\break REACH.

\begin{table*}
\caption{\label{tab3}Recent regulations undertaken to restrict PFASs}
\fontsize{9.5}{12}\selectfont
\begin{tabular}{ccc}
\tbody
&amp; 2006: &amp;  \parbox[t]{28pc}{\raggedright -PFOS ban (restriction moved to EU POP Regulation)}\\
&amp; 2019: &amp; \parbox[t]{28pc}{\raggedright -TDFAs in solvent-based spray applications (Annex XVII entry 73)}\\
&amp; 2020:  &amp; \parbox[t]{28pc}{\raggedright -PFOA, salts and
related substances (restriction moved to EU POP Regulation)}\\
&amp; 2021:  &amp; \parbox[t]{28pc}{\raggedright -C9--C14 PFCAs
(Annex XII entry 68) and PFHxS and related substances (proposal
expected to be included in EU POP Regulation)}\\
&amp; 2022: &amp; \parbox[t]{28pc}{\raggedright  -Aqueous firefighting foams (proposal in preparation)}\\
&amp; 2023: &amp; \parbox[t]{28pc}{\raggedright  -PFHxA salts and
related substances (proposal waiting for decision
making)}{\vspace*{5pt}}\\

\morerows{6}{\parbox[t]{4.5pc}{\centering SVHC \mbox{identification}}} &amp; 2012:  &amp; \parbox[t]{28pc}{\raggedright
-C11--C14 PFCAs listed as very persistent and
very bioaccumulative (\textbf{vPvB})}\\
&amp; 2013:  &amp; \parbox[t]{28pc}{\raggedright -PFOA (${+}$salts)
listed as persistent, bioaccumulative and toxic (\textbf{PBT})}\\
&amp; 2015--16: &amp; \parbox[t]{28pc}{\raggedright  -PFNA and PFDA (${+}$salts) listed as \textbf{PBT}}\\
&amp; 2017: &amp; \parbox[t]{28pc}{\raggedright  -C6 PFCA (PFHx ${+}$ salts) listed as \textbf{vPvB})}\\
&amp; 2019: &amp; \parbox[t]{28pc}{\raggedright  -2,3,3,3-tetrafluoro-2-(heptafluoropropoxy)propionic acid (${+}$ salts
and acyl halides) listed as
Persistent Mobile and Toxic (Equivalent Level of Concern (\textbf{ELoC}))}\\
&amp; 2020: &amp; \parbox[t]{28pc}{\raggedright  -C4 PFSA (PFBS ${+}$ salts) listed as \textbf{ELoC}}\\
&amp; 2022: &amp; \parbox[t]{28pc}{\raggedright  -Perfluoroheptanoic acid (PFHpA) ${+}$ salts listed as
\textbf{PBT}, \textbf{vPvB} and \textbf{ELoC}}{\vspace*{5pt}}\\

\morerows{2}{On-going} &amp; 2023:  &amp; \parbox[t]{28pc}{\raggedright -ECHA PFAS ban proposal Annex XV}\\
&amp; 2024:  &amp; \parbox[t]{28pc}{\raggedright -USEPA
Significant New Use Rule (\textbf{SNUR}) list of PFAS listed on TSCA
(Toxic Substances Control Act) inventory}\vspace*{2pt}
\botline
\end{tabular}
\tabnote{TDFA: C$_{6}$F$_{13}$(CH$_{2}$)$_{2}$Si(OH)$_{3}$, PFCA:
perfluorocarboxylic acid, PFSA: perfluorosulfonic acid. Adapted
from~\cite{21}.}
\end{table*}

At the other end of the scale, the use of PFASs in personal care
products and cosmetics (waterproof makeup, hair products, skin creams,
etc.) is considered as non-essential (category 1, Table~\ref{tab2},
Figure~\ref{fig4}). A wide range of PFAS surfactants and PTFE are
concerned~\cite{94}. The use of PFAS in these products leads to direct
human exposure and potential health effects following dermal or oral
uptake. According to a recent OECD report, many companies have phased
out PFASs, suggesting that non-fluorinated alternatives can be or are
already found~\cite{14}.

\section{Recent regulations and voluntary actions}

Since the phase-out of PFOS by 3M in 2000, important regulatory actions
have been decided, and some of them have come into effect. In
particular, the European REACH legislation has considerably restricted
the use of long-chain PFAS and has listed certain short-chain compounds
as ``Persistent, Mobile and Toxic'' (Table~\ref{tab3}). 

\begin{figure*}
{\vspace*{2pt}}
\includegraphics{fig05}
\caption{\label{fig5}PFASs self-assemble in aqueous solutions to form
large and robust supramolecular assemblies that hydrocarbon surfactants
do not form. (a) Liposomes formed by a single-chain phosphatidylcholine
surfactant (adapted from~\cite{71}); (b--e) flexible and rigid fibers
formed by dimorpholinophosphates and glucophospholipids~\cite{71,98},
(f) polyaphron gels formed with an amine oxide
fluorosurfactant~\cite{99}. Structured nanodomains formed onto solid
surfaces by (g) semi-fluorinated alkanes (image size:  $1 \times
1~{\upmu}\mathrm{m}^{2}$)~\cite{53,100} and (h)  semi-fluorinated
carboxylic acids (image size: 500 ${\times}$ 500~nm$^{2}$ height: 
20~nm)~\cite{101}.}
\end{figure*}

The European Chemical Agency (ECHA) is presently examining a more
global restriction project. Derogations are proposed for certain PFASs
that are considered essential and bring decisive benefits to society,
in particular in medicine. Other countries also adopt some regulations,
like the United States Environmental Protection Agency, which now has
inventoried certain PFASs on a list of toxic substances.

\section{The propensity of PFAS to self-assemble and its likely impact
on their environmental fate and remediation methods}

Amphiphilic PFAS, like any surfactants, were reported to self-assemble
in aqueous solutions to form micelles above the critical micellar
concentrations (CMCs). CMC decreases, and micelle size increases with
increasing surfactant's {F}-chain length and in the presence of
an electrolyte~\cite{95,96,97}. The presence of organic compounds, such as
ethanol, decreases the CMC. However, fluorinated surfactants have an
exceptional aptitude for forming not only micelles but a wide variety
of more ordered supramolecular assemblies such as liposomes, fibers and
fibrils, and ribbons that can be extremely robust (Figure~\ref{fig5}). 


This specific behavior is supported by a large corpus of data collected
on fluorinated surfactants having a wide variety of structures with
different {F}-chain lengths and polar heads, such as sugars and
polyols, aminoacids, amine oxide and
morpholinophosphates~\cite{61,71,102}.   As a striking example, a
single-chain fluorinated phosphocholine derivative can form discrete
bilayered structures (liposomes, vesicles) that are extremely stable
(e.g., they can resist heat sterilization)~\cite{103,104}, while its
hydrocarbon analogs only form micelles. Even more organized,
quasi-crystalline tubular aggregates were obtained from a single-chain
derivative of \mbox{dimorpholinophosphate}~\cite{105,106} and from anionic
glucophospholipids~\cite{107}.

Semi-fluorinated alkanes were found to form nanometric domains at the
surface of solid surfaces spontaneously~\cite{53,100,108,109}.
Similar nanodomains were obtained with fluorinated carboxylic
acids~\cite{101}. These systems are extremely robust and often
quasi-crystalline, a behavior that contrasts sharply with that of
hydrocarbon surfactants which generally only afford labile micellar
solutions. Self-assembled nanodomains were also found to form at the
surface of water~\cite{110,111,112,53}, affording highly
viscoelastic 2D gels~\cite{113}.

The capacity of PFAS surfactants, especially long-chain ones, to adsorb
on solid surfaces and at fluid interfaces has become a hot topic
because PFAS pollutants adsorb on rocks and sediments. Natural and
artificial soils (e.g., concrete, tarmac, asphalt, and others) are
heterogeneous environments that can contain pockets of air and water
and, when contaminated, organic matter (nonaqueous phase liquids,
NAPL)~\cite{114}. These environments are very complex and still poorly
understood~\cite{115}.

Given the known physical chemistry of PFAS surfactants, it is
foreseeable that, even if their concentrations in the environment are
low, PFASs will concentrate at any gas/solid, gas/liquid, and
liquid/liquid interfaces present in the environment. At these
interfaces, they will likely form such supramolecular aggregates that
may act as PFAS reservoirs, which in turn may affect their fate in
their environment and also interfere with certain remediation
processes. Our understanding of these effects and possible
interferences is still essentially inexistent. Considering the
self-aggregation behavior of PFASs is therefore indispensable. One
prime example concerns the PFAS contamination by firefighting foams.
Many sites, particularly airports, are heavily contaminated because
they have served as training sites for many years by firefighters who
use PFAS-based extinguishing foams. Understanding what types of
supramolecular aggregates form under these conditions, their behavior,
and their stability is mandatory to help develop effective remediation
methods.

\section{Concluding remarks}

As a result of incautious, extensive usage, PFASs are now found all
over the planet. Longer-chain perfluoroalkylated surfactants are very
persistent, mobile, bioaccumulative, and biomagnifying. Exposure of the
biosphere, including human and animal populations, via food, beverages,
wastewater, air, dust and soil has become ubiquitous.

Some of these chemicals, such as PFOA and PFOS, have been associated
with untoward biological effects, including liver toxicity and reduced
response of children to vaccines. Other potential risks concern renal
and kidney cancers, reduced birth weight, increased cholesterol level
and the propensity of respiratory tract infections, thus raising
legitimate concern and calling for close regulation.

PFASs should obviously be phased out when not indispensable or when
safer and similarly effective functional options exist. Treating PFASs
as a single class and applying a global ban on them (and consequently
on their outstanding specific performances) seems unsatisfactory and
inappropriate, given the vast diversity of structures, properties,
\mbox{behaviors,} \mbox{potential} for bioaccumulation, and toxicity observed. Such a
drastic approach would also change the traditional way chemicals are
regulated. The proposed European ban on over 14,000 PFASs raises many
concerns and carries unintended potential negative consequences. Given
their much lower bioavailability and toxicity, there is no strong
scientific basis for the inclusion of fluoropolymers in the ban. Active
fluorinated pharmaceuticals (not excipients) for which the risk/benefit
ratio is significant and the tonnages are relatively small are
regulated separately. Regulation needs to consider both the technical
difficulties encountered in replacing PFASs and society's expectations
regarding product performance.

The search for less toxic, more environmentally friendly alternatives
to PFASs that display similarly high effectiveness for a given function
is imperative but challenging, if only because their {outstanding}
properties derive directly from fluorine's location in the periodic
table. It is also indispensable that the biological and environmental
toxicities of any alternative chemical be thoroughly investigated prior
to implementation so as to avoid regrettable substitutions.

Fortunately, many applications can do with lesser performances and
PFAS-free substitutes have been devised and are being successfully
implemented. For example, PFAS-free fire-extinguishing foams have been
developed to replace those that have {contaminated} many airports,
particularly in the USA.\looseness=-1

Substantial remediation efforts are definitely in order.
Decontamination of polluted sites requires the elaboration of dedicated
detection, monitoring, and identification techniques, and particularly
of inexpensive and portable instrumentation. The development of
effective remediation techniques also implies better consideration and
understanding of the propensity for PFASs to self-assemble in sturdy
aggregates, and of its dynamics. Likewise, for their probable impact on
PFAS destruction.


{Greater scrutiny is mandatory if we wish to preserve the unique
advantages and benefits of using PFASs when no alternatives yet exist.
Many high-efficacy niche products have negligible environmental impact.
The ``essential use'' concept should help sort out specific
high-performance-demanding uses for which no non-fluorinated
alternatives have yet been found, from uses for which sufficiently
effective substitutes have become available that fulfill the demanded
function, and from convenience uses that are definitely not essential,
and should thus help phase out non-essential uses. Like toxicity,
environmental dangerousness is a matter of dosage/amount released.
Altogether, the evaluation of PFAS essentiality is a formidable task in
view of the number of chemicals to be scrutinized and of the
often-insufficient public information available on the structure,
performances, tonnages, environmental behavior, and toxicity of certain
industrial products. It also supposes increased exchanges and
cooperation between industry, academia, regulatory agencies, research
institutions, and civil society.}

\section*{Declaration of interests}

The authors do not work for, advise, own shares in, or receive funds
from any organization that  could benefit from this article, and have
declared no affiliations other than their research organizations.

\section*{Note added in proofs}
PFAS no longer ``eternal'' pollutants? In a remarkable, newly published
paper, Gouverneur et al.~\cite{newgouv} describe a ground-breaking
mechanochemical approach for the complete destruction of PFAS that
involves the mineralization of the PFAS, once collected, through
reactions with potassium phosphate salts. Additionally, this technology
enables the recovery and reuse of fluorine to synthesize new high-value
fluorochemicals. PFAS could thus become partners in a virtuous circular
and sustainable fluorine chemistry.

\back{}

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\refinput{crchim20240798-reference.tex}

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