The research team set out to answer a deceptively simple question: under what conditions, and through what sequence of electron-transfer events, does PFOA surrender its fluorine atoms as harmless fluoride ions? To do so, they subjected PFOA to alkaline dimethyl sulfoxide/water (DMSO/H2O) solutions under a range of carefully controlled conditions, varying the amount of sodium hydroxide, the proportion of water in the solvent blend, and the reaction temperature. The fate of PFOA and its degradation by-products was tracked using ultra-high-pressure liquid chromatography coupled with mass spectrometry, a technique sensitive enough to detect and relatively quantify the short-chain fluorinated fragments that appear as the long molecule is dismantled. Destruction efficiency, in turn, was measured by potentiometric fluoride detection with a fluoride-selective electrode, complemented by scanning electron microscopy and energy-dispersive spectroscopy to examine solid residues.

The central conceptual contribution of the study lies in how the authors frame the degradation chemistry. Rather than treating hydroxyl and hydrogen radicals as freely diffusing species that randomly attack PFOA molecules, the researchers describe the entire process through redox couples involving only the carbon atoms within PFOA and its by-products. In this framework, the hydroxyl radical paired with hydroxide (OH·/OH⁻) and the hydrogen radical paired with water (H·/H2O) act as transient redox intermediates associated with discrete electron-transfer events. The electron donor in the system is hydroxide, while the electron acceptors are molecular oxygen, PFOA itself, and water. This carbon-focused redox description allows the team to explain why certain reactions proceed readily while others stall, simply by examining the oxidation state of each carbon atom in the molecule.

A key insight emerging from this analysis is that the nucleophilic or electrophilic character of each redox reaction depends on the oxidation state of the carbon atom involved. Carbons bearing multiple fluorine atoms sit at high oxidation states and behave as electrophilic targets, while carbons that have been partially reduced become susceptible to different modes of attack. The most favorable degradation pathway identified by the team involves the stepwise oxidation of carbon driven solely by the OH·/OH⁻ redox couple. Through this repeated sequence, the molecule is progressively shortened, with each cycle eliminating one –CF2– unit from the chain in the form of two fluoride ions and one carbonate ion. In effect, the perfluorinated backbone is unzipped two fluorine atoms at a time, converting the once-inert fluorocarbon chain into benign inorganic products.

The study also reveals that degradation is not purely an oxidative affair. The OH·/OH⁻ and H·/H2O redox couples together mediate secondary electron-transfer pathways in which two different carbon atoms within the same PFOA-derived molecule are simultaneously oxidized and reduced. This coupled oxidation-reduction within a single molecule helps explain the variety of by-products observed chromatographically, including shorter-chain perfluorinated species that retain some of their fluorine content. Understanding these parallel pathways matters for remediation engineering, because incomplete mineralization can leave behind shorter perfluorinated acids that are themselves persistent and, in some cases, more mobile in groundwater than the parent compound.

Perhaps the most practically important finding concerns the role of water. Although water is a participant in the redox chemistry, the researchers found that both the initial water content of the solvent mixture and the water continuously formed during the reactions actually limit the destruction process. The mechanism is subtle: water enhances the solvation of hydroxide ions, and a heavily solvated hydroxide is a weaker electron donor. Because the oxidizing capacity of the OH·/OH⁻ redox couple depends on the availability of unsolvated, reactive hydroxide, excess water effectively throttles the reaction. This finding provides a clear chemical rationale for why DMSO-rich mixtures outperform more aqueous media, and it suggests that managing water activity will be essential in any attempt to scale the chemistry beyond the laboratory bench.

The kinetic data translate into a straightforward recipe for maximizing defluorination. The higher the molar ratio of sodium hydroxide to PFOA, at 62:1 or greater, the lower the water content, with DMSO/H2O volume ratios of at least 3.87:1, and the higher the temperature, at 120 °C or above, the greater the extent of defluorination achieved. Each of these parameters pushes the chemistry in the same direction: abundant hydroxide supplies the electron donor that fuels the redox chain, a DMSO-rich environment keeps hydroxide minimally solvated and maximally reactive, and elevated temperature accelerates the electron-transfer steps that cleave the carbon-fluorine bonds. The conditions are demanding, but they remain far milder than the incineration temperatures, often exceeding 1000 °C, required to destroy PFAS thermally.

The significance of this work is best appreciated against the backdrop of the broader PFAS remediation challenge. Thousands of per- and polyfluoroalkyl substances are in commercial use, and their chemical inertness, which made them so valuable in industry, renders them essentially immune to conventional water treatment. Adsorption onto activated carbon or ion-exchange resins merely concentrates the problem rather than solving it, transferring the chemicals from water to a spent sorbent that still requires destruction. Advanced oxidation processes, which have proven effective against many organic pollutants, often fail against fully fluorinated compounds because hydroxyl radicals preferentially attack electron-rich moieties that perfluorinated chains simply do not possess. Reductive approaches using hydrated electrons have shown promise, but they too face structural and practical constraints. Chemical destruction in solvent systems, exemplified by the low-temperature mineralization of perfluorocarboxylic acids reported in Science in 2022, has emerged as one of the most exciting frontiers in the field.

The new study adds mechanistic depth to this frontier by explicitly mapping the electron-transfer choreography that underlies solvent-phase destruction. Prior work in alkaline DMSO systems had established that hydroxide can act as a one-electron reducing agent in aprotic solvents, generating superoxide and other reactive intermediates, and that DMSO itself participates in the radical chemistry of such mixtures. What the French team contributes is a unified, carbon-centered accounting of where electrons flow during PFOA degradation, showing that the apparent complexity of the product distribution can be rationalized by the oxidation states of individual carbon atoms and the redox couples that address them. This level of mechanistic resolution is precisely what engineers need to design reactors that push reactions down the most productive pathways while suppressing the side reactions that generate problematic intermediates.

Challenges remain before such chemistry can treat real-world contamination. The solvent volumes, hydroxide loadings, and temperatures required are substantial, and contaminated environmental matrices introduce water, dissolved oxygen, co-contaminants, and sorbed phases that the pristine laboratory solutions do not contain. The research was conducted under the European Union’s Horizon 2020 PROMISCES project, which targets the monitoring and elimination of emerging contaminants in soil and water, suggesting that the authors view the work as a step toward applied solutions rather than pure curiosity. Even so, the study’s message is ultimately an optimistic one: the carbon-fluorine bond, long considered an insurmountable barrier, yields predictably to a well-understood sequence of electron transfers when the solvent environment is engineered to keep hydroxide in its most reactive form. For a class of pollutants whose very name, forever chemicals, encodes despair, that predictability is a genuinely hopeful development, turning the destruction of PFOA from an empirical art into a science that can be rationally optimized.

Subject of Research: Chemical defluorination kinetics and redox mechanisms of perfluorooctanoic acid (PFOA) in alkaline dimethyl sulfoxide/water solutions

Article Title: Defluorination of perfluorooctanoic acid in alkaline dimethyl sulfoxide/water solutions: kinetics and carbon-focused redox mechanism insights

Article References: Tur, R., Betelu, S., Colombano, S., Davarzani, D., Bristeau, S., Grandclément, J., Perrault, A., Lions, J., van Hullebusch, E. D., & Ignatiadis, I. (2026). Defluorination of perfluorooctanoic acid in alkaline dimethyl sulfoxide/water solutions: kinetics and carbon-focused redox mechanism insights. Environmental Science and Pollution Research. https://doi.org/10.1007/s11356-026-38216-7

Image Credits: AI Generated

DOI: 10.1007/s11356-026-38216-7

Keywords: PFOA, PFAS, forever chemicals, defluorination, DMSO, redox chemistry, hydroxide, electron transfer, water treatment, environmental remediation, reaction kinetics, fluoride

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