PFAS

The PFAS family includes twenty-six molecules in the IsoFind catalog, making it the second largest family after pesticides. It covers perfluoroalkyl carboxylic acids (PFCA C2 to C12), perfluoroalkyl sulfonic acids (PFSA C4 to C10), sulfonamides (FOSA), fluorinated telomers (FTS), Cl-PFAS, and next-generation substitutes such as HFPO-DA (GenX) and ADONA. This page presents the family structure, EU 2020/2184 regulatory regimes, the limited degradation pathways characterizing this class, and the few available CSIA data.

The Twenty-One Subfamilies

PFAS are grouped into twenty-one subfamilies reflecting both carbon chain length (C4, C5, C6... up to C12 for the longest) and the nature of the terminal functional group (carboxylate, sulfonate, sulfonamide, telomer). This fine granularity is necessary because toxicity, persistence, and mobility change significantly with chain length.

Class	Represented Subfamilies	Molecules
PFCA (carboxylates)	Short-chain PFCA, PFCA-C4 to C12, PFCA-alt	13 molecules: PFCA-C2, PFBA-C3, PFBA-C4, PFPeA, PFHxA, PFHpA, PFOA, PFNA, PFDA, PFUnDA, PFDoDA, HFPO-DA, ADONA
PFSA (sulfonates)	PFSA, PFSA-C4 to C10, PFSA-cyclic	7 molecules: PFBS, PFPeS, PFHxS, PFOS, linear-PFOS, PFDS, PFECHS
FOSA (sulfonamides)	FOSA	3 molecules: PFOSA, MeFOSA, EtFOSA
FTS (fluorinated telomers)	FTS	2 molecules: 6:2 FTS, 8:2 FTS
Cl-PFAS	Cl-PFAS	1 molecule: F-53B (Cl-PFOS)

Chain length is not just an inventory matter: C4 PFBS and C8 PFOS have very different regulatory thresholds, retention behaviors, and toxicities. IsoFind deliberately maintains subfamily granularity to allow for these distinctions rather than aggregating under a generic umbrella.

Two EU 2020/2184 Regulatory Regimes

The European Directive 2020/2184 on the quality of water intended for human consumption distinguishes two parameterizable sums, which translate into two distinct thresholds in the catalog.

Regime	Threshold	Summed Molecules
Sum of 4 PFAS (Art. 5)	0.10 µg/L	PFOA, PFNA, PFHxS, PFOS + direct precursors (PFOSA, MeFOSA, EtFOSA, linear-PFOS) and HFPO-DA
Sum of 20 PFAS	0.50 µg/L	Extended list of short and long PFCA, PFSA C4 to C10, FTS, ADONA, PFECHS

The first regime targets compounds considered of highest concern based on toxicological knowledge available in 2020. The second covers the class more broadly and reflects the philosophy of treating PFAS as a family where cumulative weight is meaningful even when no individual molecule exceeds specific thresholds.

The "Sum of 4 PFAS" concerns a minority of the twenty-six PFAS in the catalog. The remainder falls under either the "Sum of 20 PFAS" or does not yet have an EU threshold (e.g., F-53B, banned in China but not yet regulated in Europe). A measurement without individual exceedance can nonetheless contribute significantly to the aggregated sum, requiring both regimes to be checked in parallel.

Additional Regulatory Frameworks

Beyond the two EU 2020/2184 sums, several regulatory mechanisms affect the PFAS in the catalog depending on the specific molecule and its use. This information is recorded in the reglementation field of each datasheet.

Framework	Scope	Concerned PFAS (Examples)
REACH Annex XVII	Production and market placement restrictions	PFOS, PFOA, PFOSA precursors, and analogues
Stockholm Convention	Persistent Organic Pollutants (POPs), global elimination	PFOS (2009), PFOA (2019), PFHxS (2022)
ECHA SVHC	Substances of Very High Concern	PFNA (2013), PFDA, HFPO-DA (2022)
REACH 2023 Universal Restriction Proposal	Draft restriction for all PFAS	PFBS, PFBA, PFPeA, PFHxA, PFDS, PFHxS, and others

Tabulated Degradation Pathways

PFAS differ from other families in the catalog by the relative rarity of their known degradation pathways. The C-F bond is one of the strongest in organic chemistry, explaining both the extreme environmental persistence of these compounds and the scientific difficulty in documenting their degradation mechanisms. Seven pathways are currently tabulated for this family.

Molecule	Pathway	Category	Environment	Typical t½ (days)	Primary Metabolite
PFOS	Biotic reductive defluorination	Biological	Anaerobic (Eh −200 to +50 mV)	5,000	PFHxS
PFOS	Abiotic UV photolysis	Photolytic	Surface water	365	PFHxS
PFOA	Biotic defluorination (Acidimicrobium)	Biological	Anaerobic (Eh −100 to +100 mV)	7,500	PFHpA
PFOA	Advanced oxidation (Fenton)	Abiotic	Treated water (process)	7	PFHpA (mineralization possible)
PFHxS	Biotic defluorination	Biological	Anaerobic (Eh −150 to +50 mV)	4,000	PFBS
6:2 FTS	Biotic beta-oxidation	Biological	Aerobic (Eh +100 to +400 mV)	60	PFHxA
HFPO-DA (GenX)	Abiotic hydrolysis	Abiotic	Water	2,500	TFA

The range of half-lives—from 7 days for PFOA Fenton oxidation to 7,500 days for its biotic defluorination—illustrates how the environmental context controls kinetics. In natural environments, biotic pathways dominate, and half-lives are measured in decades. Short half-life abiotic pathways exist but are confined to industrial conditions (water treated by advanced oxidation processes) or surface UV exposure.

Only one of the seven tabulated pathways leads to complete mineralization (PFOA Fenton oxidation under treatment conditions). All others produce a shorter PFAS as a metabolite, which remains persistent. This property is key to the precursor concept: degrading a long PFAS does not make it disappear; it converts it into a shorter PFAS that still requires treatment.

Metabolite Cascades

The parent-metabolite relationships tabulated in the catalog form short cascades that feed into each other. The maximum molar yield and persistence of each metabolite are documented.

Parent	Metabolite	Max Molar Yield	Stability	Toxicity
PFOS	PFHxS (C6HF13O3S)	0.60	Persistent	Toxic
PFOS	PFBS (C4HF9O3S)	0.20	Persistent	Toxic
PFOA	PFHpA (C7HF13O2)	0.70	Persistent	Toxic
PFOA	PFHxA (C6HF11O2)	0.40	Persistent	Toxic
PFHxS	PFBS (C4HF9O3S)	0.70	Persistent	Toxic
6:2 FTS	PFHxA (C6HF11O2)	0.60	Persistent	Toxic
HFPO-DA (GenX)	TFA (C2HF3O2)	1.00	Persistent	Toxic

This cascade structure has a direct implication for reporting and simulation: the apparent disappearance of PFOS at a contaminated site should not be interpreted as remediation, but as a potential conversion into PFHxS and then PFBS. The IsoFind simulation engine automatically propagates parent-metabolite chains when the simulated molecule declares its metabolites in the catalog.

CSIA Data for PFAS

The catalog contains carbon isotope fractionation data for five PFAS. The rarity and low magnitude of observed fractionations are informative in themselves: they confirm the high stability of the C-F bonds, which carry the bulk of the masses.

Molecule	ε ¹³C (‰)	Range	Study Type	Reference
PFOS	-1.5	[-2.5 ; -0.8]	Contaminated site	Chiaia-Hernandez et al., 2020; Yamazaki et al., 2022
PFOA	-2.0	[-3.5 ; -1.0]	Pure microbial culture	Yamazaki et al., 2022
PFHxS	-1.8	[-3.0 ; -1.0]	Field	Chiaia-Hernandez et al., 2020
6:2 FTS	-3.5	[-5.0 ; -2.0]	Lab culture	Wang et al., 2011 (inference)
HFPO-DA (GenX)	-0.8	[-1.5 ; -0.3]	Lab	Pan et al., 2018

Fractionations are low (ε less than 5 ‰ in absolute value) because the breaking of C-F bonds releases little isotopic fractionation into the residual carbon. 6:2 FTS is the exception: its more pronounced fractionation (ε = −3.5 ‰) results from the beta-oxidation pathway attacking the hydrogenated chain rather than the perfluorinated chain, requiring a more fractionating C-C bond cleavage. For HFPO-DA, the cleavage occurs at the C-O-C ether bond (not a C-F bond), which also explains the very low fractionation.

CSIA on PFAS remains a developing tool. Despite low fractionations, it is useful for source apportionment: two industrial batches of PFOA may carry distinct δ¹³C signatures if their fluorination precursors have different origins. Since degradation-induced fractionation is limited, the source signature remains largely preserved at the contaminated site.

Precursors and Substitutes

The catalog includes several molecules in the precursors or substitutes category that warrant specific attention.

Molecule	Role	Point of Attention
PFOSA, MeFOSA, EtFOSA	Direct PFOS precursors	Included in the Sum of 4 PFAS; transform into PFOS
6:2 FTS, 8:2 FTS	Telomer substitutes for PFOS in AFFF foams	Degrade into PFHxA and PFDA respectively
HFPO-DA (GenX)	New generation Chemours PFOA substitute	Dordrecht industrial contamination 2018, ECHA SVHC 2022
ADONA	Dyneon/3M substitute	No individual threshold yet; counted in the Sum of 20
PFECHS	Cyclic PFSA	Present in aircraft hydraulic fluids; Sum of 20
F-53B (Cl-PFOS)	PFOS substitute used in China	Banned in China 2016; no EU/EPA threshold yet

Typical Case Study: AFFF Contamination

Contamination by AFFF (Aqueous Film-Forming Foam) firefighting foams is the most documented scenario in the environment. It typically leverages several features of the IsoFind PFAS catalog.

File Element	IsoFind Catalog Contribution
Identification of dominant PFAS	PFOS (legacy AFFF), 6:2 FTS (modern PFOS-free AFFF)
Compliance calculation	Simultaneous verification of Sum of 4 and Sum of 20 PFAS
Contamination age estimation	PFHxS/PFOS and PFBS/PFHxS ratios inform the degradation cascade
Legacy vs. modern source distinction	PFOS dominance = historical; 6:2 FTS dominance = post-2010
Isotopic attribution	PFOS δ¹³C helps refine origin if multiple potential sources exist

API Access

PFAS data are accessible via the molecule module endpoints by filtering for the family. Specific endpoints for this family are listed below.

Endpoint	Usage
GET /api/molecules/reference/catalogue?famille=PFAS	List of the 26 reference PFAS
POST /api/molecules/catalogue/seed/PFAS	Pre-populates the project with the PFAS family
GET /api/molecules/csia/PFOS/pathways	Lists tabulated degradation pathways for PFOS
POST /api/molecules/csia/resolve	CSIA resolution with local geochemical conditions
GET /api/molecules/{id}/conformite	Verification of Sum 4 and 20 PFAS thresholds for a sample