UV Light Breaks Down Forever Chemicals in Ways Scientists Never Expected

Per- and polyfluoroalkyl substances, better known as PFAS, have earned their nickname as forever chemicals for good reason. The carbon-fluorine bond that defines them is among the strongest in organic chemistry, which is why these compounds have resisted nearly every conventional water treatment technology deployed against them. Now a team of researchers at the University of California, Riverside, has mapped out in unprecedented detail how ultraviolet irradiation actually dismantles these stubborn molecules, and their findings overturn several long-held assumptions about how the process works.

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