The scale of the problem begins with the chemistry. Since the 1950s, PFAS have been manufactured on an industrial scale through two routes: electrochemical fluorination, which yields mixtures of branched and linear isomers, and telomerization, which produces shorter, linear chains with even numbers of carbon atoms. The OECD expanded the formal definition in 2021 to include any fluorinated substance containing at least one fully fluorinated methyl or methylene carbon atom, and a 2018 OECD/UNEP global inventory identified 4,730 PFAS-related CAS numbers that may have been on the world market. The structural secret of their staying power lies in the carbon-fluorine bond itself. With a bond dissociation energy of roughly 485 to 544 kilojoules per mole, a bond length of just 1.35 angstroms, and fluorine’s unmatched electronegativity of 4.0, the bond is among the strongest polar covalent bonds in organic chemistry. Although each individual bond is highly polarized, the symmetric tetrahedral arrangement of multiple carbon-fluorine bonds cancels the dipoles, leaving the perfluoroalkyl tail nonpolar, hydrophobic, lipophobic and nearly immune to attack by external electric fields.

That tail is only half of the story. Every PFAS molecule pairs its water-repellent fluorinated backbone with a polar, hydrophilic head group, typically a carboxylate or sulfonate, giving the compounds an amphiphilic, surfactant-like character that lowers the surface tension of water. This dual nature is precisely what makes PFAS so mobile and so troublesome: the head group drags the molecule into aqueous phases, while the tail refuses to let go of interfaces. In living systems, the consequences are severe. PFAS accumulate in serum proteins and hepatic tissue, disrupt the hypothalamic-pituitary-thyroid axis, interfere with androgen and estrogen signaling, activate the PPAR-alpha nuclear receptor and perturb lipid metabolism, and generate reactive oxygen species that oxidize lipids and damage DNA. Human half-lives stretch over years, and the compounds have been detected in blood, lung and kidney tissue. The United States Environmental Protection Agency responded on April 10, 2024 by finalizing drinking water standards for six PFAS compounds, while long-chain variants were phased out under REACH between 2011 and 2016.

The phase-out, however, created an unexpected paradox. The short-chain PFAS introduced as replacements bioaccumulate less in humans and animals, but they are dramatically more mobile in water, harder to remove in treatment plants, prone to uptake by plants and capable of climbing the food chain. They have already been flagged as substances of very high concern under REACH. The review emphasizes that this structural transition has been addressed only superficially in the literature, even though chain length and functional group chemistry fundamentally control how PFAS partition between air, water, soil and living tissue. Understanding those partitioning behaviors is now the central challenge of PFAS science.

At the air-water interface, the governing parameter is the dimensionless Henry’s law constant, the ratio of a compound’s gas-phase concentration to its aqueous concentration. Neutral, volatile precursors such as fluorotelomer alcohols exhibit constants between 0.31 and 2.82 and can travel thousands of kilometers through the atmosphere before degrading into persistent perfluorinated carboxylic acids. Ionic terminal compounds like PFBS show negligible volatility and remain locked in water. One of the most striking findings compiled in the review concerns sea spray aerosols. As waves burst bubbles at the ocean surface, perfluoroalkyl acids become enormously enriched in the ejected droplets relative to bulk seawater, and field studies show a significant positive correlation between PFAA concentrations and sodium ions, with PFOA correlating at r = 0.77. Aerosol particles a few micrometers in radius can carry PFAS more than 300 kilometers, with atmospheric residence times ranging from days to weeks, effectively allowing the ocean to re-emit these chemicals into the air alongside sea salt.

Beneath the surface, the vadose zone, the unsaturated soil layer between ground level and the water table, acts as a delayed-release reservoir. Two retention mechanisms operate simultaneously: sorption to soil solids and accumulation at air-water interfaces within soil pores. Because adsorption at these interfaces is nonlinear, lower concentrations paradoxically produce higher retardation factors; when PFOA concentrations dropped from 1.0 to 0.01 milligrams per liter, the retardation factor rose from 2.0 to 2.8, and PFOS retardation climbed from 1.35 to 3.69 under comparable dilution. In saturated soil, the soil-water distribution coefficient Kd governs mobility, and it is exquisitely sensitive to pH. At high pH, negatively charged organic matter and minerals electrostatically repel anionic PFAS head groups, weakening sorption; at low pH, protonated clay and organic carbon bind the compounds more tightly. Long-chain compounds such as PFOS show Kd values falling from 42 to 17 liters per kilogram across the pH 3 to 9 range, while short-chain PFBA barely responds, remaining stubbornly mobile throughout.

In living organisms, partitioning follows a different logic. Linear isomers accumulate more readily than branched ones, which are eliminated faster due to their lower octanol-water partition coefficients. Protein binding, quantified through the protein-water partition coefficient, dominates bioaccumulation: PFOS binds human serum albumin with a dissociation constant of just 0.62, stronger than PFOA, while short-chain PFBA interacts weakly. Fish sampled across global studies carry approximately 62 micrograms of PFAS per kilogram, exceeding regulatory advisory limits, and more than 90 percent of human exposure occurs through food and drinking water. Molecular dynamics simulations reveal why chain length matters at the membrane: short-chain PFAS are too stubby to pack against the hydrocarbon tails of lipid bilayers and interact only as a monolayer, whereas long-chain compounds form tightly cohesive van der Waals-packed layers that lodge deep in the membrane core and resist removal.

Degrading these molecules remains one of environmental engineering’s hardest problems. Microbial transformation offers little comfort: the carbon-fluorine bond’s dissociation energy of roughly 480 to 560 kilojoules per mole exceeds what most enzymes can attack, PFAS provide essentially no metabolic energy to bacteria, and degradation typically stalls at persistent shorter-chain intermediates rather than achieving mineralization. A notable 2020 study by Yu and colleagues did document reductive defluorination of two C6 compounds by an organohalide-respiring community, with fluoride release occurring only when the fluorinated compounds served as exclusive electron acceptors, and intriguingly, the activity was driven by minor phylogenetic groups rather than the dominant Dehalococcoides. On the abiotic side, photocatalysis begins with decarboxylation, peeling CF2 units from PFOA one at a time until only carbon dioxide, water and fluoride remain, and vacuum ultraviolet systems boosted PFOA degradation from 10 to 95 percent and defluorination from 17 to 94 percent. But most destructive technologies demand extreme conditions, with complete mineralization requiring temperatures near 850 degrees Celsius, making in situ application impractical.

Conventional wastewater treatment fares little better. Primary clarification merely shuttles PFAS between phases, activated sludge leaves most compounds untouched, coagulation-flocculation fails for short-chain variants, and adsorption and ion exchange simply concentrate the chemicals onto spent media requiring disposal. Reverse osmosis and nanofiltration achieve high removal but at substantial cost, energy demand and membrane fouling risk, and they too transfer rather than destroy the contamination. This is where computational modeling is transforming the field. Machine learning tools such as PFAsorptionML, trained on 1,274 measured soil-liquid distribution coefficients covering 51 PFAS, now predict sorption from molecular weight, hydrophobicity and soil organic carbon with robust validation performance, while quantitative structure-property relationships, symbolic regression and graph-based neural networks promise to extend predictions to the thousands of PFAS never experimentally characterized. The review’s authors caution that artificial intelligence should complement, not replace, mechanistic understanding, since extrapolation to poorly represented compounds and matrices remains unreliable.

The overall picture that emerges is sobering but actionable. Long-chain PFAS retard in soils and sediments, binding to iron oxides and organic matter and acting as slow secondary sources to groundwater; short-chain PFAS race through aquifers, dominate surface waters and slip past treatment barriers. Precursor transformation can even increase apparent concentrations of terminal acids over time, meaning that fate assessments must track chemistry, not just movement. The authors argue that future research must prioritize hybrid degradation systems combining chemical and biological processes, cost-effective capture technologies tailored to short-chain compounds, and AI-guided structure-property models capable of screening the more than 15,000 PFAS variants that no laboratory budget could ever test individually. Until then, the forever chemicals will keep traveling, on sea spray, through aquifers and into the food web, exactly as their molecular architecture dictates.

Subject of Research: Environmental fate, transport and degradation of per- and polyfluoroalkyl substances (PFAS)

Article Title: Fate and transport of per -and polyfluoroalkyl substances (PFAS) in environment

Article References: Seethalakshmi, B., Govindasamy, G., Ganesan, S., & Sivagami, K. (2026). Fate and transport of per-and polyfluoroalkyl substances (PFAS) in environment. Results in Chemistry, 31, Article 103846. https://doi.org/10.1016/j.rechem.2026.103846

Image Credits: AI Generated

DOI: 10.1016/j.rechem.2026.103846

Keywords: PFAS, forever chemicals, carbon-fluorine bond, environmental transport, short-chain PFAS, bioaccumulation, air-water partitioning, vadose zone, photocatalysis, machine learning, wastewater treatment, sea spray aerosols

The study, published in Nature Water by Jinyu Gao, Dandan Rao and Jinyong Liu, focuses on the UV/sulfite treatment system, one of the most promising advanced reduction processes for PFAS destruction. In this setup, sulfite ions dissolved in water absorb ultraviolet light and release hydrated electrons, highly reactive species that can attack carbon-fluorine bonds directly. For years, researchers assumed that these electrons would simply strip fluorine atoms from the carbon backbone one by one, replacing each with a hydrogen atom in a process called hydrodefluorination. The reality, it turns out, is far stranger.

Surprisingly, the analysis showed that most carbon-fluorine bonds are not reduced to carbon-hydrogen bonds at all. Instead, the degradation proceeds through a more elaborate sequence of reactions. The team found that a given fluorocarbon structure undergoes sequential reactions with three different chemical actors: first a hydrated electron, then a hydroxyl radical, and finally hydroxide ions. This alternating reductive and oxidative dance ultimately causes the carbon chain to cleave, breaking the molecule into progressively smaller fragments rather than simply peeling fluorine atoms off an intact skeleton.

To unravel this mechanism, the researchers examined transformation products across three legacy PFAS structural families, including perfluorooctane sulfonate (PFOS), perfluorooctanoic acid (PFOA) and trifluoroacetic acid, along with a series of fluorotelomer carboxylates. By tracking which products appeared, when they appeared, and how their concentrations changed over the course of irradiation, they could reconstruct the reaction network step by step. The picture that emerged explains puzzling products that earlier studies had observed but could not account for.

One of the most consequential corrections concerns what happens to carboxylic acid PFAS during treatment. A common misinterpretation in the literature held that chain-shortening from a structure of the form R–CF2–COO− to R–COO− represented straightforward reductive defluorination, as if the terminal CF2 unit had been cleanly replaced. The new analysis shows this interpretation is wrong. The apparent shortening actually reflects carbon-carbon bond cleavage driven by the combined action of hydrated electrons, hydroxyl radicals and hydroxide, not a simple substitution at the terminal carbon.

The carboxylate group itself emerged as a critical player in the degradation chemistry. In the fragmented products left behind after chain cleavage, the presence of a carboxylate moiety proved essential for enabling further defluorination to continue. This structural insight helps explain why some PFAS family members degrade readily under UV/sulfite conditions while others, notably sulfonates and fully fluorinated alkanes without such activating groups, resist attack. It also provides a design principle in reverse: if chemists understand which structural features make a fluorocarbon vulnerable, they can deliberately build those features into new fluorochemicals so that future products will be readily degradable at end of life.

The study also resolved the question of where the carbon ends up. The primary carbon-containing product of the degradation is formate, not carbon dioxide or carbon monoxide as might have been expected from complete mineralization pathways. This matters for practical treatment design, because formate is a benign, easily biodegradable one-carbon compound. Knowing that the process funnels fluorocarbon fragments toward formate rather than toward more toxic or persistent intermediates strengthens confidence that UV/sulfite treatment can genuinely destroy PFAS rather than merely converting one problematic chemical into another.

Perhaps the most practically important finding concerns pH. Beyond the well-known role of alkaline conditions in preserving hydrated electrons, which are otherwise quenched by bisulfite at lower pH, the researchers discovered that high pH actively promotes PFAS degradation through an additional mechanism. Elevated hydroxide concentrations enhance the fragmentation of fluorocarbon intermediates and promote the formation of carbon-carbon double bonds, creating unsaturated products that are more susceptible to subsequent attack. Comparisons of transformation products at pH 9.5 against those at pH 12 revealed how strongly the product distribution and degradation kinetics depend on this single operating parameter.

The implications for water utilities and environmental engineers are substantial. UV/sulfite systems have already been demonstrated at pilot scale, including a hybrid nanofiltration and UV-sulfite treatment train for contaminated groundwater, and integrated photo-electrochemical processes have achieved near-complete destruction of PFAS in aqueous film-forming foam. Understanding the true mechanism gives operators a rational basis for optimizing conditions. If fragmentation and double-bond formation accelerate at high pH, then running treatment systems alkaline is not merely a matter of electron economy but a direct lever on the destruction chemistry itself. The findings also inform how to handle complex matrices such as ion exchange resin regeneration brines, where PFAS concentrated from large volumes of water must be destroyed in a small volume of concentrated solution.

Beyond remediation, the work speaks to a growing movement in fluorochemical design. Recent years have seen remarkable advances in deliberately breaking carbon-fluorine bonds, from low-temperature mineralization of perfluorocarboxylic acids to photocatalytic defluorination of polymers like PTFE under visible light. The Riverside team’s mechanistic map adds a crucial piece: a detailed understanding of how hydrated-electron chemistry actually dismantles fluorocarbon chains in water. By identifying the carboxylate group as the structural key that unlocks deeper defluorination, the study offers chemists a concrete target for designing the next generation of readily degradable fluorochemicals, ones that perform their industrial functions but surrender their fluorine willingly when their useful life is over. For a class of pollutants that has contaminated drinking water supplies worldwide and defied destruction for decades, that combination of fundamental insight and practical guidance represents a meaningful step toward turning forever chemicals into finite ones.

Subject of Research: Mechanisms and pathways of PFAS degradation by hydrated electrons under ultraviolet irradiation

Article Title: Comprehensive mechanisms and pathways for per- and polyfluoroalkyl substances degradation under ultraviolet irradiation

Article References: Gao, J., Rao, D., & Liu, J. (2026). Comprehensive mechanisms and pathways for per- and polyfluoroalkyl substances degradation under ultraviolet irradiation. Nature Water, 4(9), 1177-1189. https://doi.org/10.1038/s44221-026-00696-9

Image Credits: AI Generated

DOI: 10.1038/s44221-026-00696-9

Keywords: PFAS, forever chemicals, hydrated electrons, UV/sulfite, defluorination, water treatment, perfluorooctanoic acid, PFOS, advanced reduction processes, carbon-fluorine bond, Nature Water, pH dependence