PFAS ‘Degradation’ Claims Often Hide Fluorine, New Evidence Framework Warns

Per- and polyfluoroalkyl substances, the notoriously persistent chemicals known as PFAS, are still being produced at a rate of more than one megatonne per year, and they continue to enter the environment through manufacture, use and disposal. A new opinion article published in Frontiers in Systems Biology argues that much of the language used to describe PFAS bioremediation has outrun the evidence behind it. Mohd Faheem Khan of University College Dublin contends that when researchers report that a microbe or treatment has removed PFAS from a solution, that observation alone says very little about what actually happened to the fluorine that makes these molecules so durable. The article proposes a fluorine-fate evidence framework that separates several outcomes which are often conflated: parent-compound disappearance, transformation-product formation, defluorination through carbon-fluorine bond cleavage, and true mineralisation of the carbon skeleton.

The chemical root of the problem is the carbon-fluorine bond itself. Reported bond dissociation energies depend on the molecular environment, but in highly fluorinated organic compounds a representative value is approximately 485 kilojoules per mole, accompanied by a short bond length, high electronegativity and limited biological accessibility. This stability underpins why PFAS are detected from nanograms to micrograms per litre across environmental matrices, why volatile PFAS and their precursors undergo long-range atmospheric transport before converting into persistent terminal compounds, and why contaminated agricultural and industrial soils can act as long-term sources to groundwater and food webs. Any remediation claim, the article argues, should be judged against this chemistry rather than against a simple disappearance measurement.

The concern is not academic. Parent-PFAS concentrations can fall because of adsorption, bioaccumulation, sequestration, precipitation, volatilisation or phase redistribution, none of which destroys anything. Even genuine molecular transformation may simply redistribute fluorine among shorter-chain PFAS, fluorotelomer products or other unmonitored organofluorines. Separation and stabilisation technologies, including adsorption, ion exchange, membrane separation and foam fractionation, can substantially lower aqueous concentrations, and soil stabilisation has reportedly reduced leaching by roughly 60 to 90 percent, but these approaches transfer or immobilise the fluorinated mass rather than destroying it. Destructive technologies, by contrast, are increasingly evaluated using defluorination and mineralisation rather than parent removal alone, and the article argues that biological studies should be held to the same standard.

Concrete examples illustrate how dramatically transformation and fluorine removal can diverge. The bacterium Labrys portucalensis F11 removed approximately 90 percent of PFOS and 58 percent of 5:3 FTCA after 100 days, with PFOS removal reaching 96 percent after 194 days, yet only 21 percent of 6:2 FTS was removed over 100 days. PFOS transformation generated PFHpS, PFHxS, PFHxA, PFPeA, PFBA, PFPrA and three defluorinated products, while 5:3 FTCA yielded shorter-chain and unsaturated fluorotelomer products and 6:2 FTS produced 4:2 FTS. The article characterises these results as demonstrating biological transformation and partial defluorination, but explicitly not complete destruction. A strikingly similar gap appears in non-biological systems: cold atmospheric plasma achieved 99.99 percent PFOS degradation but only 35 percent defluorination, with fluorine mass-balance recoveries of 31 to 106 percent, and cobalt(II) phthalocyanine with titanium(III) citrate defluorinated 10.9 percent of technical PFOS and 54.5 percent of branched PFOS isomers after 8 hours, whereas vitamin B12 managed only 2.4 percent.

Fungal and combined treatments tell the same story. The fungus Cunninghamella elegans completely consumed 100 milligrams per litre of 6:2 FTOH within 48 hours while accumulating 5:3 FTCA, a product identified using fluorine-19 NMR, GC-MS and LC-MS, with inhibitor studies implicating cytochrome P450 activity. Individual photocatalytic and fungal treatments of 100 parts per million PFOA achieved roughly 35 to 40 percent degradation but only 20 to 30 percent defluorination, whereas a sequential photocatalysis-fungal treatment raised degradation to 90 percent and defluorination to 60 percent. In that sequence, photocatalysis produced short-chain perfluorocarboxylic acids while the fungal step predominantly produced 5:3 FTCA. The lesson, according to the framework, is that parent-compound conversion and fluorine removal can diverge substantially even when transformation is extensive, and that each outcome must be measured independently.

Precursor chemistry adds a further warning. N-EtFOSAA can biologically transform into PFOS, meaning that the disappearance of a precursor can generate, rather than eliminate, a persistent terminal PFAS. Conversely, some structures can undergo transformation that initiates genuine defluorination: susceptible ether PFAS containing a CH2 group adjacent to an ether linkage or a carbon-carbon double bond can undergo oxidative or hydrolytic O-dealkylation to unstable fluoroalcohols that subsequently defluorinate. Transformation-product identification is therefore essential for resolving PFAS fate, but the article stresses that product profiles alone are mechanistically informative without establishing fluorine fate independently. Direct measurement of fluoride release provides stronger evidence of carbon-fluorine bond cleavage, as demonstrated when an organohalide-respiring community coupled product formation with direct fluoride release during reductive defluorination of two C6 per- and polyfluorinated compounds under lactate-fed conditions, even though Dehalococcoides present in the community was not responsible for the activity.

To make these distinctions quantitative, the framework centres on the fluorine mass balance, which should link the initial fluorine inventory to released fluoride and residual fluorine in parent PFAS, identified and unidentified transformation products, biomass, particulate phases and other compartments. Targeted LC-MS/MS should be complemented by total organic fluorine, extractable organic fluorine, adsorbable organic fluorine, high-resolution mass spectrometry and non-targeted analysis to capture organofluorine beyond the standard analyte list. The article cautions that these operationally defined fluorine pools are method-dependent and should not be treated as directly interchangeable or summed, because the pools may overlap and their recoveries may differ. Mass-balance closure must be interpreted in light of analytical recovery, measurement uncertainty and potentially unmeasured fluorine pools, and fluoride yields should be reported relative to the theoretical fluorine content of the substrate with corrections for background fluoride and stoichiometry.

The evidence base itself is uneven. A meta-analysis of 97 microbial PFAS biotransformation studies found that transformation was more likely under aerobic conditions, in defined or axenic cultures, at higher PFAS concentrations and for compounds containing fewer fluorine atoms, with activity also depending on chain length, branching and headgroup. Anaerobic systems with defined electron donors and acceptors, identified organisms or enzymes and comprehensive transformation-product data remain comparatively underrepresented. Meanwhile, microbial systems routinely combine multiple processes: in anaerobic wastewater cultures over 90 days, one study reported 10.9 to 38.3 percent adsorption without parent-PFAS transformation, while in aerobic cultures after 21 days adsorption ranged from 26.9 to 55.8 percent and biotic accumulation reached 13.35 to 17.55 percent, alongside transformation of PFOA at 47.87 percent, PFOS at 28.96 percent and 6:2 FTS at 43.1 percent with shorter-chain products and limited defluorination. Matrix effects in wastewater, biosolids, soil and sediment further complicate interpretation, since partitioning between water and solids can make apparent aqueous removal indistinguishable from transfer to sludge.

The framework also draws a hard line between defluorination and detoxification. Fluoride release demonstrates carbon-fluorine bond cleavage but does not show that the resulting products are less toxic or environmentally benign; detoxification requires separate assessment of transformation-product persistence, mobility and toxicity using appropriate chemical and biological endpoints. On the engineering side, the article distinguishes demonstrated performance from proposed capability. Recent work identified gut microbial defluorinases and used computational and protein-engineering approaches to predict defluorination activity with up to 95 percent accuracy for full-length sequences, and a multifunctional hydrolase from Serratia liquefaciens defluorinated monofluoroethylglycine and fluoroacetate, with an active-site mutation altering dehalogenase activity. These are proof-of-concept findings for defluorination and enzyme engineering, the article stresses, but they do not constitute evidence of PFAS defluorination unless activity against PFAS substrates is demonstrated. Metabolic engineering, directed evolution, synthetic biology and CRISPR-based pathway reconstruction remain future opportunities unless PFAS-specific activity is experimentally validated.

The practical prescription is a verification workflow applied from the outset of system development: quantify parent PFAS and transformation products, measure background-corrected fluoride release, assess fluorine mass balance, and, for mineralisation claims, demonstrate conversion of PFAS-derived carbon to inorganic carbon such as carbon dioxide or dissolved inorganic carbon, ideally with carbon-13-labelled PFAS to distinguish it from background carbon. Sterile, killed-cell, uninoculated, sorbent, matrix and substrate-free controls are required to separate biological activity from abiotic reactions and phase redistribution. The article argues that a statement such as 90 percent PFAS degradation may imply substantial environmental benefit even when most fluorine remains locked in transformation products, and that overstated biodegradation claims can misdirect discovery efforts toward organisms that merely sorb PFAS rather than cleave carbon-fluorine bonds. Screening directly for fluoride release, the author concludes, targets the biochemical function that destructive remediation actually requires, and reporting should shift from parent-compound disappearance toward endpoint-specific, quantitatively supported assessments of removal, biotransformation, biodefluorination and mineralisation.

Subject of Research: An evidence framework for distinguishing PFAS biotransformation, defluorination and mineralisation in remediation research

Article Title: Distinguishing PFAS biotransformation from defluorination and destruction: a fluorine-fate evidence framework

Article References: Khan, M. F. (2026). Distinguishing PFAS biotransformation from defluorination and destruction: a fluorine-fate evidence framework. Frontiers in Systems Biology, 6, Article 1991575. https://doi.org/10.3389/fsysb.2026.1991575

Image Credits: AI Generated

DOI: 10.3389/fsysb.2026.1991575

Keywords: PFAS, biodegradation, defluorination, fluorine mass balance, bioremediation, C-F bond cleavage, transformation products, mineralisation, microbiology, environmental chemistry, Distinguishing, biotransformation