Understanding the Defluorination Mechanism of Per‐ and Polyfluoroalkyl Substances in Wastewater: From Microscopic Process to Practical Application

ABSTRACT

Per‐ and polyfluoroalkyl substances (PFAS) are called forever chemicals due to the extremely stable C─F bond (bond energy of 485 kJ·mol⁻¹), which leads the persistent existence in the natural environment thus poses a threat to human health. Compared with simple removal without destruction, defluorination is a more meaningful and thorough way for the harmless disposal of PFAS. In this review, the relationship between the structural properties of PFAS and defluorination mechanism are concluded. Thereafter, we discussed the advantages and limitations of defluorination techniques and specific defluorination pathways of active substances (free radicals and electron). This will provide a comprehensive theoretical foundation for the selection and improvement of defluorination technologies. The influence of key factors on the practical application of defluorination technologies in wastewater treatment plants are assessed in detail from the perspectives of reaction solvents, homogeneous and heterogeneous systems, and reactor design. Importantly, the pilot‐scale application of relevant PFAS defluorination technologies has also been discussed. Finally, we prospect for the future research direction of PFAS defluorination from the aspects of precise exploration of reaction pathways, fluorine balance, resource recovery, and life cycle assessment (LCA). This will provide a practical strategy for addressing the urgent issue of PFAS‐contaminated wastewater.

This review concluded the relationship between the structural properties of per‐ and polyfluoroalkyl substances (PFAS) and defluorination mechanism. The drawbacks and reaction pathways of different defluorination technologies have been also systematically analyzed. Importantly, the pilot‐scale application and future prospects of relevant PFAS wastewater defluorination technologies have also been summarized and discussed.

1. Introduction

Per‐ and polyfluoroalkyl substances (PFAS) are a class of artificial synthetic organofluorine compounds, usually consisting of a hydrophobic alkyl fluoride “tail” and a hydrophilic “head”. This affinity property makes PFAS an effective surfactant widely used in the manufacture of food packaging, cookware, clothing, furniture and other consumer products that are closely related to people's lives [1, 2]. Due to the extremely strong C─F bond (485 kJ·mol⁻¹), PFAS cannot be effectively defluorination by conventional wastewater treatment methods, and thus PFAS is also called as a “forever chemical compound” [3, 4, 5, 6]. The bioaccumulation, persistence, and mobility of PFAS result in a significant threat to the aquatic environment, and because of the neurotoxicity, immunotoxicity, and carcinogenicity of PAFS, they can pose a significant risk to human health [7, 8, 9, 10, 11]. The issue of PFAS has received more attention than ever before due to the massive use of PFAS, which has led to a significant increase in their presence in the environment [12, 13, 14, 15]. On April 10, 2024, the U.S. Environmental Protection Agency (U.S. EPA) published one of the most stringent drinking water standards in history [16]. Drinking water is required to contain no more than 4.0 ng·L⁻¹ of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS), and 10.0 ng · L⁻¹ of perfluorohexanesulfonate (PFHxS), perfluorononanoic acid, and hexafluoro‐propylene oxide‐dimer acid (GenX) [17]. The large accumulation of PFAS in the aqueous environment and the strict control of their concentration have prompted the need for innovations in wastewater treatment technology that can efficiently degrade stable PFAS. For example, GenX has been widely detected in various environmental media, including surface water (up to 631 ng·L⁻¹), drinking water, landfill leachate, air, and sediments [18, 19]. In addition, they have been linked to a variety of health issues, including cancer and immune disorders [20].

Due to the urgent need to degrade PFAS, the change in the number of published research papers on PFAS over the past 10 years shows a large increase in the field of environmental engineering about PFAS (Figure 1a). Many wastewater treatment technologies are used to treat PFAS in water, physical methods such as adsorption [21, 22, 23] and membrane separation [24, 25] can effectively removal PFAS from water are widely used, but they also face unavoidable problems. For example, the adsorption method cannot selectively adsorb PFAS efficiently, and the adsorption sites on the surface of the adsorbent are easily occupied by other substances, thus affecting the removal of PFAS. Membrane separation is prone to blocking and fouling when confronted with various impurities during wastewater treatment, and how to efficiently selectively separate PFAS is also a challenging issue. Microbiological methods can adsorb and degrade PFAS by microorganisms, mainly through the secretion of enzymes, or through aerobic/anaerobic activities [26, 27]. However, microbial methods for PFAS degradation also face some problems, first, how to accurately identify PFAS and perform effective degradation of PFAS in complex wastewater treatment environments. Second, fluoride (F⁻) released during microbial treatment of PFAS are cytotoxic. Unlike other monovalent anions, F⁻ inhibits central metabolism and interferes with microbial activity during wastewater treatment by complexing with important enzymes [28, 29, 30]. Therefore, it is difficult to achieve effective removal of PFAS through conventional wastewater treatment technologies, and wastewater containing PFAS may remain at concentrations that are seriously hazardous to humans after treatment in wastewater treatment plants. Due to the extreme stability of PFAS, when PFAS is enriched into the human body through PFAS, it can cause immeasurable harm to human health.

FIGURE 1.

(a) Summary of the number of papers on various PFASs in the past 10 years. (b) Number of papers published on different topics related to PFAS in the last 10 years. (c) Keyword relevance in papers liked to PFAS and C─F. (d) Schematic diagram of the themes of this study. Paper statistics from Web of Science as of July 1, 2025.

Effective defluorination and further F⁻ recovery, which is a strategy to turn pollutants into valuable substances, is the best way to solve the PFAS problem. This requires innovative treatment technologies and remediation strategies for effective destruction through deep defluorination. During deep defluorination, a significant number of C─F bonds of the PFAS molecule are broken, F⁻ is released and a less fluorinated/non‐fluorinated product is formed. Products with lower fluorine content are typically more mobile, less bioaccumulative and persistent in the environment [31, 32, 33]. Some technologies such as electrochemical [34, 35, 36, 37], photochemical [38, 39, 40, 41, 42], wet chemical [43, 44], piezoelectric [45, 46, 47] and metal catalysis [48, 49], which can efficiently activate and break C─F bonds of PFAS have become a hot research topic. Especially, electrocatalytic and photochemical techniques that can realize effective defluorination have been widely studied by researchers, and the number of articles on electrochemical and ultraviolet (UV) photochemical has increased dramatically in the last decade (Figure 1b). As can be seen from the relevance between the keywords of the papers with the topics PFAS and C‐F, most of these paper topics are related to different types of PFAS and degradation/defluorination (Figure 1c).

A review on how wastewater treatment technology can effectively activate and break the C─F bonds of PFAS is urgently needed to elucidate the microscopic mechanisms and practical applications of defluorination technologies (Figure 1d). This review summarizes the types, toxicity, and the structural properties of PFAS (e.g., electrostatic potential, bonding energy, and binding affinity of F, etc.), and a detailed understanding of the structural properties of the target pollutants is required to better design the material structure as well as the reaction system. Afterward, the commonly used wastewater treatment techniques which can achieve effective C─F bond breaking in PFAS are summarized, and the mechanism and defluorination pathways of different ways acting on C─F bonds breaking are systematically described. Meanwhile, key factors influences and pilot‐scale testing of PFAS defluorination technology in practical applications are explored. Finally, we have summarized and outlined the issues (in situ characterization of degradation pathways and processes) and future research directions (fluorine balance, recycling of defluorination products, life cycle assessment) that need to be focused on in the field of PFAS defluorination, which will be profoundly inspirational and helpful to related researchers.

2. Structure and Properties of PFAS

Some representative articles on defluorination in recent years are summarized in Figure 2, which is helpful for researchers to understand the latest research progress on PFAS defluorination. It can be observed that photochemical and electrochemical technologies are more prevalent due to proven superior defluorination ability. In order to achieve efficient defluorination for PFAS, the first step is to understand the structural properties of PFAS, including the type, toxicity and potential of PFAS, bond energy of C─F, and bind affinity of F.

FIGURE 2.

A brief history of the recent progress regarding the defluorination of PFAS. Reproduced with permission from refs [40, 91, 96, 103, 110, 128, 129, 130, 131]. Copyright 2020 American Chemical Society, 2021 American Chemical Society, 2022 Elsevier, 2022 Spring Nature, 2023 American Chemical Society, 2023 Springer Nature, 2024 American Chemical Society, 2025 Springer Nature, and 2025 Springer Nature, respectively.

2.1. Type and Toxicity of PFAS

PFAS are generally defined as fluorinated substances containing at least one perfluorinated methyl group (─CF₃) or methylene carbon atom (─CF₂‐), with no hydrogen (H), chlorine (Cl), bromine (Br), or iodine (I) atoms attached to these carbon atoms (Table S1). As of September 2023, the number of PFAS documented in the PubChem database—one of the largest open chemical repositories—has exceeded 7 million [50]. There are many types of PFAS, with the most common being perfluorocarboxylic acid (PFCA) and perfluorosulfonic acid (PFSA) as the two primary classes, the most representative compounds within these being PFOA and PFOS respectively. Then there are some emerging traditional PFAS alternatives, such as fluorotelomer unsaturated carboxylic acid (FTUCA), fluorotelomer carboxylic acid (FTCA), perfluoroethercarboxylic acid (PFECA), and chloro‐perfluoropolyether carboxylate (Cl─PFECA) are also receiving more and more attention [51, 52, 53]. Generally, hydrophilic contaminations can dissolve in water, making them more readily bioaccumulated through the food chain and thus entering the human body. These substances are more easily absorbed but have shorter half‐lives, allowing rapid excretion through urine, thus resulting in relatively lower long‐term hazards. Hydrophobic contaminations tend to accumulate in fatty tissues and organs. Once they enter the human body, they are extremely difficult to eliminate, leading to long‐term, chronic toxicity (carcinogenic, teratogenic, endocrine disrupting) [54, 55]. The carboxyl group (─COO) and sulfonic acid group (─SO₃) form the hydrophilic head groups of PFAS, while the abundance of C─F bonds confers hydrophobic properties, resulting in their dual hydrophilic‐hydrophobic nature [56]. Common alternative strategies also aim to enhance the hydrophilicity of PFAS, making them easier to eliminate from the human body. For instance, introducing an O atom between two C atoms in the traditional PFAS chemical structure converts the C─C bond into an ether oxygen bond, forming PFECA. Alternatively, replacing some C─F bonds in the chemical structure with C─H bonds reduces toxicity and increases hydrophilicity, leading to the preparation of compounds like FTCA [57, 58].

Besides the aforementioned substitution strategies, replacing long‐chain PFAS with short‐chain PFAS is also an option. However, this may result in reduced performance and require increased PFAS usage. Furthermore, research on the toxicity of short‐chain PFAS is growing, with studies indicating that short‐chain PFAS may exhibit greater toxicity. As Wang et al. discovered, the toxicity of PFAS is significantly correlated with carbon chain length. Long‐chain PFAS with six or more carbon atoms typically exhibit higher toxicity, while those with three to six carbon atoms show lower toxicity. Conversely, ultra‐short‐chain PFAS with three or fewer carbon atoms, such as pentafluoropropionic acid (PFPrA, C3) and trifluoroacetic acid (TFA, C3), demonstrate toxicity levels 1 to 3 orders of magnitude higher than conventional PFAS (Figure 3a) [59]. Many studies have reported that short‐chain PFAS exhibit greater mobility and stability than long‐chain PFAS, making them more difficult to realize defluorination and degradation [60, 61]. In recent years, short‐chain PFAS have garnered attention primarily because they are degradation products of long‐chain PFAS. Given the extreme toxicity of ultra‐short‐chain PFAS, this reminds us that we must also focus on removing the byproduct short‐chain PFAS when treating long‐chain PFAS. In the future, we need to replace PFAS with non‐fluorinated substances in order to fundamentally address the hazards posed by PFAS [62].

FIGURE 3.

(a) Relationship between PFAS chain length and toxicity. Reproduced with permission [59]. Copyright 2024 American Chemical Society. (b) Single‐electron reduction potential of PFAS. (c) Bond energy of C─F bonds in PFAS. (b,c) Reproduced with permission [65]. Copyright 2025 American Chemical Society. (d) Binding affinity of F with other substances. (e) The defluorination process of PFOA. (d,e) Reproduced with permission [66]. Copyright 2022 American Chemical Society. (f) Schematic diagram of PFAS adsorption mechanisms on different types of fluorinated surfaces. (g) The PFAS removal mechanism based on the synergistic effects of F···F interactions and other forces. (f,g) Reproduced with permission [17]. Copyright 2025 American Chemical Society.

2.2. Reduction Potential and Bond Energy of C─F Bonds

It is known that the C─F bond is one of the strongest single bonds in nature (with a bond energy of approximately 485 kJ·mol⁻¹), endowing it with extreme stability [63]. The defluorination process of PFAS involves the cleavage of C─F bonds, which can be selectively attacked by hydrated electrons (e_aq ⁻) and free radicals through photochemical and electrochemical reduction [38, 64]. The C─F bond of ─CF₃ group in PFOA and hexafluoropropylene oxide dimer acid (GenX) requires the lowest potential for single‐electron reduction, indicating that it demands greater energy input to achieve bond cleavage (Figure 3b). In contrast, the C─F bond of the ─CF₃ group is not the strongest, and is only slightly stronger than the C─F bonds adjacent to the head group. This indicates that the difficulty of C─F bond cleavage must be assessed based on multiple parameters, and the order of C─F bond cleavage should be verified through a combination of experimental and theoretical approaches (Figure 3c) [65]. The bond energies of C─F bonds in PFAS vary with the gain or loss of elements and electrons, providing a basis for designing oxidation/reduction experiments, while also strongly supporting the defluorination pathway inferred from our experimental results (Figure 3d). When the PFOA anion loses electrons through direct electron transfer, the resulting unstable intermediate is more prone to decarboxylation. Subsequently, hydroxyl radicals (OH·) generated at the electrochemical anode facilitate defluorination and chain‐shortening steps (Figure 3e) [66]. Theoretical calculations indicate that the C─C bond on the carboxylic acid group of the unstable intermediate can spontaneously cleave to decarboxylation (−25.7 kcal·mol⁻¹), explaining the phenomenon observed experimentally.

2.3. Binding Affinity of F

The F atom in PFAS is the most electronegative element in the periodic table (4.0 on the Pauling scale), can form diverse noncovalent interactions with various substances, such as C─H···F─C, C─F···F─C, and C─F···X (X = N, O, S, and halogens) [67, 68]. Among these interactions, the C─F···F─C contact between organic fluorine molecules (i.e., F···F interaction) has garnered significant attention in PFAS‐related research due to its unique physicochemical properties, including short contact distances, directional dependence, and specific recognition [17, 69]. Adsorption is the first step in the catalytic reaction, and the catalytic material must effectively adsorb PFAS to enable subsequent defluorination reactions. Typically, the adsorption capacity of catalysts toward PFAS is enhanced by synthesizing fluorinated sites on their surfaces. However, due to varying synthesis or modification strategies, the resulting fluorinated sites may appear on the surface as isolated, chain‐like, or continuous structures (Figure 3f). The appropriate type of fluorination site can be selected based on the structure of the catalytic material (e.g., spherical, tubular, etc.) and the configuration of the PFAS (e.g., chain length, chain type, etc) [70]. Fluorinated sites on material surfaces also interact with PFAS through electrostatic attraction, hydrogen bonds, π─π stacking, and van der Waals forces due to PFAS properties such as hydrophobicity/hydrophilicity and negative charge. F···F bonds can interact with these forces to enhance the catalyst's adsorption selectivity toward PFAS (Figure 3g). Different interactions significantly change the spatial orientation of PFAS, thereby facilitating radical/electron attack on the C─F bond [71].

3. Wastewater Treatment Technology for PFAS

Having known the structure and properties of PFAS, we will now briefly introduce wastewater treatment technologies capable of effectively defluorination of PFAS (Tables S2 and S3). These technologies can be broadly categorized into several major types: reduction methods, oxidation methods, and emerging defluorination methods (including hydrothermal alkali, supercritical water oxidation, mechanochemical methods, etc.).

3.1. Oxidation Technology

Electrochemistry is one of the earliest wastewater treatment technologies applied to PFAS treatment, typically employing anodes with high oxygen evolution potential (OEP) to generate OH· radicals for effective PFAS defluorination. Due to its exceptionally high OEP and outstanding stability, boron‐doped diamond (BDD) electrodes have become one of the simplest and most effective anodes in the field of electrochemical oxidation [72, 73, 74, 75]. However, the high cost of BDD limits its practical application at large scale. Researchers have begun preparing relatively cheap electrodes to replace BDD electrodes, such as Ti₄O₇, PbO₂ and CeO₂, but the disadvantages such as instability and toxicity remain equally unresolved (Figure 4a) [76]. Subsequently, researchers began preparing composite electrodes to enhance their stability and defluorination efficiency (Figure 4b,c) [36, 37]. For example, Yin et al. prepared a UiO‐66@BN composite electrode by doping hexagonal boron nitride with UiO‐66. Under pH 4.5 conditions, this electrode achieved a 99.5% removal efficiency and 70% defluorination efficiency for PFOA within 3 hours through electrochemical trap‐and‐zap [37].

FIGURE 4.

Electrochemical oxidation defluorination of PFAS by (a) CeO₂ anode. Reproduced with permission [76]. Copyright 2025 Elsevier. (b) [Ni_0.75Fe_0.25]–(OH)₂–hydrophilic carbon fiber paper anode. Reproduced with permission [36]. Copyright 2024 American Chemical Society. (c) UiO‐66 modified h‐BN anode. Reproduced with permission [37]. Copyright 2024 Elsevier. (d) Activated persulfate oxidation of nafion‐related ether sulfonates. Reproduced with permission [77]. Copyright 2024 American Chemical Society. (e) Activated persulfate by oligolayered Co@MXene to degrade PFAS. Reproduced with permission [78]. Copyright 2022 Elsevier. (f) Ultrasound‐enhanced electrochemistry with peroxydisulfate electrolyte. Reproduced with permission [35]. Copyright 2024 Elsevier. (g,h) Piezoelectric catalysis of BN‐SiO₂ for AFFF defluorination. Reproduced with permission [45]. Copyright 2025 American Chemical Society. (i) Piezoelectric catalysis of waste resin that containing PFAS. Reproduced with permission [47]. Copyright 2025 American Chemical Society. (j) Photo‐catalysis of gallium oxide for PFAS defluorination. Reproduced with permission [83]. Copyright 2024 Elsevier. (k) A (hydr)oxides/carbon sphere (FeO/CS) composite for the photo‐catalysis of PFAS. Reproduced with permission [84]. Copyright 2020 Elsevier. (l) Efficient photocatalytic PFOA degradation over boron nitride (BN). Reproduced with permission [85]. Copyright 2020 American Chemical Society.

Besides electrochemical oxidation technology, persulfate can be selectively activated by catalysts or energy (e.g., heat and electricity) to generate oxidizing radicals, thereby effectively degrading PFAS. Typically, persulfate can be activated directly by thermal energy without requiring a catalyst, generating different radicals (OH· and SO₄ ²⁻ · )− at varying pH levels to achieve PFAS defluorination (Figure 4d) [77]. Unlike thermal activation, Li et al. prepared a Co‐based oligolayered MXene (Co@MXene) catalyst that activates peroxydisulfate through a Co···SO₃ cation‐π bridge interaction, achieving 78.5% degradation of sodium p‐perfluoro‐n‐nonenoxybenzene sulfonate (OBS, a type of PFAS) within 2 min (Figure 4e) [78]. Subsequently, researchers employed ultrasound (US)‐assisted electrochemical oxidation (EO) to synergistically activate persulfate, achieving nearly 100% degradation efficiency and 80.01% defluorination within 3 hours (Figure 4f) [35].

Piezoelectric catalysis utilizes the piezoelectric field generated in piezoelectric materials under mechanical stress (e.g., ultrasonic vibration, stirring, or fluid shear forces) to drive the efficient separation and migration of charge carriers (electron‐hole pairs). Charge carriers migrating to the surface can directly react with organic pollutants or interact with H₂O/O₂ to generate highly reactive oxygen species (ROS) such as ·OH and ·O₂ ⁻, thereby non‐selectively oxidizing and degrading organic pollutants [79, 80]. Yang et al. achieved near‐complete defluorination (over 95% conversion of organic fluorine to F⁻) of undiluted aqueous film‐forming foam (AFFF) through a novel piezoelectric comilling with boron nitride [45]. Unlike the low concentrations of PFAS (mg·L⁻¹ or even ng·L⁻¹) found in water bodies, AFFF foam has a total organic fluorine concentration of 9,080 mg·L⁻¹ and a total organic carbon content of 234 g·L⁻¹ (Figure 4g,h). Later, Yang's team discovered that when treating PFOS using piezoelectric ball milling technology, adding waste ion exchange resin containing PFAS not only increased the defluorination rate of PFOS but also achieved nearly 100% defluorination of the waste resin containing PFAS (Figure 4i) [47]. Traditional landfill disposal of spent ion exchange resins still contaminates groundwater through soil permeation, whereas this method achieves their true harmless treatment.

Photocatalytic oxidation degradation of pollutants is essentially a photogenerated charge carrier‐driven advanced oxidation process. When light of a specific wavelength illuminates a semiconductor catalyst, it excites the formation of highly reactive electron‐hole pairs. The holes possess strong oxidizing power and can further react with H₂O or OH⁻ to generate OH· with even greater oxidizing capability. Simultaneously, electrons reduce O₂ to form reactive oxygen species such as ·O₂ ⁻. These free radicals can non‐selectively attack organic pollutants, ultimately mineralizing them into harmless small molecules like CO₂ and H₂O, thereby achieving highly efficient pollutant removal [81, 82]. Photocatalysis is equally applicable to the oxidative degradation of PFAS. Theoretical calculations simulated the most probable photocatalytic defluorination and chain‐shortening pathways for two ultrashort perfluoroalkyl acid model compounds (i.e., CF₃CF₂COO⁻ and CF₃CF₂SO₃ ⁻) on β‐Ga₂O₃ surfaces, consistent with experimental observations reported in the literature (Figure 4j) [83]. Xu et al. prepared an adsorptive photocatalyst consisting of iron (hydr)oxides and carbon spheres (FeO/CS) via a one‐step hydrothermal method, which adsorbed nearly 200 µg·L⁻¹ of PFOA within one hour. When FeO/CS enriched with PFOA was exposed to simulated sunlight under neutral pH conditions, the pre‐enriched PFOA achieved 95.2% photodegradation and 57.2% defluorination within 4 hours (Figure 4k) [84]. Interesting, Duan et al. discovered that in the presence of boron nitride, PFOA was degradation via a hole‐initiated reaction pathway similar to that of titanium dioxide (TiO₂), involving ·O₂ ⁻/H₂O₂ radicals and OH· radicals (Figure 4l) [85]. How photocatalysis effectively absorbs sunlight and its practical applications are key areas for future research.

Due to the unique structure of PFAS (consisting solely of head groups and C─F bonds), the first step in oxidation reactions reported in the literature always involves decarboxylation or desulfation, after which more efficient defluorination can be achieved [86, 87]. Once the head groups are removed, the PFAS as a whole becomes more hydrophobic. Therefore, the key to future oxidation technologies will lie in how to better design hydrophobic materials to interact with and activate the C─F bonds.

3.2. Reduction Technology

Unlike the oxidation method, which primarily relies on decarboxylation pathways, the reduction method achieves defluorination by selectively attacking the C─F bond of PFAS through radicals/electrons. Reduction reactions can be driven by electricity, light, or spontaneously by zero‐valent metals. Electrochemical reduction can be classified into heterogeneous systems and homogeneous systems based on the form of catalyst presence.

Current research often employs the preparation of plate electrodes capable of selectively adsorbing PFAS to achieve defluorination through direct electronic reduction (Figure 5a,b). Wang et al. modified the cathode with octadecyltrimethylammonium bromide (OTAB) to enhance selective adsorption of PFAS through hydrophobic and electrostatic interactions. The modified cathode achieved a 99.81% degradation efficiency and 76.67% defluorination efficiency for (E)‐perfluoro‐(4‐methylpent‐2‐enoic acid) (PFMeUPA) at a low potential of −1.6 V (vs Ag/AgCl) [88]. Although flat electrodes can effectively enable catalyst reuse and recovery, they cannot achieve the full contact between the catalyst and target pollutants. The appearance of molecular catalysts has effectively resolved the aforementioned issues, as they can fully interact with target substances in solution while utilizing electricity [89, 90]. Sinha et al. designed a molecular copper catalyst capable of rapidly cleaving the backbone of PFOA molecules, achieving a 93% degradation efficiency and 99% defluorination efficiency within 4 hours (Figure 5c) [91].

FIGURE 5.

Electrochemical direct electron defluorination of (a) OTAB and MWCNT‐COOH‐modified cathode. Reproduced with permission [88]. Copyright 2023 American Chemical Society. (b) Au cathode. Reproduced with permission [110]. Copyright 2024 American Chemical Society. (c) Molecular Cu catalyst. Reproduced with permission [91]. Copyright 2023 American Chemical Society. (d) Structure‐dependent defluorination of UV/SO₃ ²⁻ system. Reproduced with permission [40]. Copyright 2025 Springer Nature. (e) hydrated electron generation in the UV/phenol/dithionite system. Reproduced with permission [92]. Copyright 2025 Elsevier. (f) Hydrogen‐tuned 185 nm vacuum ultraviolet ((VUV/H₂) for the PFAS defluorination. Reproduced with permission [39]. Copyright 2024 American Chemical Society. (g) Zero‐valent zinc system for effective reductive defluorination. Reproduced with permission [48]. Copyright 2024 Elsevier; (h) Silicate‐confined hydrogen on nanoscale zerovalent iron for PFOA defluorination ref. Reproduced with permission [93]. Copyright 2025 American Chemical Society. (i) Cobalt complex catalyst for PFAS defluorination. Reproduced with permission [94]. Copyright 2018 American Chemical Society.

In addition to electrochemical reduction, photochemical reduction has emerged as one of the most promising technologies in the field of PFAS defluorination. The most common system involves generating e_aq ⁻ by UV excitation of SO₃ ²⁻, which has been proven effective for defluorination of PFAS. Tan et al. proposed an e_aq ⁻‐mediated, electron transfer (ET)‐limited degradation mechanism for PFAS through experimental and theoretical evaluations of 41 PFAS structures under UV/SO₃ ²⁻ conditions (Figure 5d). They found that defluorination proceeds in a stepwise way, where e_aq ⁻ transfer to PFAS represents the rate‐limiting step, rather than the subsequent C─F bond cleavage [40]. Since e_aq ⁻ are the key species for PFAS defluorination, researchers are dedicated to investigating how to enhance the yield and feasibility of e_aq ⁻ in photochemical systems. To enhance the feasibility of PFAS reduction processes, researchers employed phenol as the source chemical, generating e_aq ⁻ via stoichiometric methods. Meanwhile, they catalyzed the phenol recycling reaction using dithionite, achieving a sustained, and stable e_aq ⁻ yield (Figure 5e) [92]. Not only in the UV/ SO₃ ²⁻ system, but researchers have also discovered that brine anions, i.e., chloride and sulfate can promote the generation of e_aq ⁻ and the degradation of PFCA under vacuum ultraviolet/hydrogen photolysis (Figure 5f) [39]. Although photochemistry can effectively achieve defluorination, it faces significant challenges in practical applications. Both the harsh reaction environment requirements (strongly alkaline and anaerobic conditions) and interference from impurity ions are urgent issues that need to be resolved.

Besides photoelectrochemical reduction reactions, metal catalysis can achieve the reduction defluorination of PFAS without requiring external energy input. The most common approach involves reducing PFAS using zero‐valent metals, such as zero‐valent zinc (Zn⁰) [48] and nanoscale zerovalent iron (Fe⁰) [93] (Figure 5g,h). For example, Liao et al. modified nanoscale zero‐valent iron with silicates to enable it to effectively generate active hydrogen (H·) for efficient defluorination of PFOA. Apart from zero‐valent metals, studies have revealed that both bio‐derived cobalt‐porphyrin complexes (B12) and synthetic cobalt‐porphyrin complexes (Co‐PP) can selectively catalyze the defluorination of multiple C─F bonds in specific PFAS (Figure 5i) [94]. These technologies require no external energy input, significantly reducing operating costs. However, these reactions require strict anaerobic conditions, limiting their large‐scale application.

3.3. Emerging Defluorination Technology

In addition to traditional oxidation and reduction defluorination technologies, several emerging defluorination techniques have emerged in recent years. Hydrothermal alkali technology can achieve the defluorination of PFAS using alkaline reagents (typically NaOH) under relatively mild conditions (usually below 120 °C) without requiring catalysts (Figure 6a,b) [43, 44]. The hydrothermal alkali method requires organic solvents and typically processes PFAS concentrations at the g·L⁻¹ level. Therefore, this technology is suitable for treating concentrated PFAS samples, enabling their mineralization, defluorination, and F⁻ recovery.

FIGURE 6.

(a,b) Hydrothermal alkali technology for the PFAS defluorination. Reproduced with permissions [43, 44]. (a,b) Copyright 2022 The American Association for the Advancement of Science, 2024 American Chemical Society, respectively. (c) Mechanochemical method for PFAS defluorination. Reproduced with permission [97]. Copyright 2023 American Chemical Society. (d) The spontaneous degradation of PFAS in microdroplets. Reproduced with permission [98]. Copyright 2024 American Chemical Society. (e) Supercritical water oxidation for PFAS defluorination. Reproduced with permission [102]. Copyright 2023 Elsevier.

The mechanochemical method offers a solvent‐free approach capable of destroying PFAS under ambient temperature and pressure conditions [95, 96]. Gobindlal et al. found that mechanochemical method could achieve 99.95% to 100% destruction efficiencies for 5 different PFAS under the addition of quartz sand (SiO₂) (Figure 6c) [97]. This method could be further used to treat the solid substances to achieve harmless disposal, such as the adsorbent/membrane/resin that concentrated with PFAS [47]. Unlike the direct treatment for the PFAS‐contaminated wastewater, two step strategy containing concentration and destruction is an innovative way to efficiently treat PFAS wastewater at a relative low concentration.

With the development of defluorination technology, some researchers explored PFAS could be spontaneous degradation in droplets [98, 99]. This unexpectedly rapid degradation is achieved through a unique multicyclic redox reaction between PFOA and surfactant species at the droplet surface (Figure 6d) [98]. Subsequently, researchers demonstrated that wollastonite‐bearing microdroplets preferentially achieve defluorination of perfluoroalkyl chains rather than C─C bond cleavage through liquid‐solid‐gas three‐phase contact electrochemistry. This process enables complete mineralization of PFOA with virtually undetectable short‐chain PFAS byproducts [99]. The mechanism of spontaneous PFAS degradation on microdroplet surfaces offers a novel approach for future defluorination technology research, minimizing the need for energy and catalyst inputs to significantly reduce treatment costs.

Due to the large number of hydrophobic C─F bonds in PFAS, they may not dissolve well in aqueous solutions, thereby preventing effective contact with the catalyst/free radicals. Supercritical water oxidation technology is a highly efficient and clean waste treatment method that utilizes supercritical water as a reaction medium. Under high temperature and pressure conditions, PFAS compounds can fully interact with free radicals, enabling effective degradation and defluorination of PFAS [100, 101]. Li et al. designed a continuous‐flow supercritical water oxidation reactor that achieves its operation by introducing ignition fuel and compressed air. This system can reach sufficiently high temperatures to break down PFOS, one of the most persistent PFAS compounds (Figure 6e) [102]. Although this system can effectively achieve defluorination for PFAS, its reliance on high‐temperature and high‐pressure conditions. This will increase treatment costs and makes it difficult to apply in practical wastewater treatment.

Recently, several new technologies have been applied to the defluorination of PFAS. For example, the radiation method uses water as a source to generate redox radicals, with O·⁻/electron tandem reactions achieving complete defluorination of even the most stubborn TFA compounds [61]. In the future, more emerging technologies will be applied to PFAS defluorination, and achieving complete PFAS removal will require a collective effort.

4. Specific Defluorination Process of PFAS

4.1. Oxidation Pathway

The defluorination pathway is crucial for deepening our understanding of PFAS degradation mechanisms, which will guide the development of PFAS treatment technologies. EO technology is one of the most commonly used oxidation techniques, and its oxidation pathways are most representative for understanding the mechanisms of oxidative degradation [103, 104, 105, 106]. Currently, the degradation of PFAS in the EO field primarily relies on the synergistic effects of OH·, O₂, and H₂O with electrons (e⁻) (Figure 7). OH· and e⁻‐dominated cycles represent the most important pathways, primarily achieving defluorination and chain‐shortening cycles through four steps: decarboxylation, hydroxylation, α‐hydroxy hydrogen abstraction, and electron transfer [5, 107]. The first step of decarboxylation is crucial as it initiates PFAS degradation, removing the carboxyl group to produce CO₂ through direct electron transfer at the anode surface. The externally positioned C atom in the PFCA, after losing the carboxylate anion, exists in an unstable state and readily reacts with OH· through an addition reaction. Subsequently, the hydroxyl group at the outermost position will react with F via α‐hydroxy hydrogen abstraction, thereby producing a HF molecule and acyl fluoride (C_nF_2n+1COF). This structure is also extremely unstable, and the C atom that loses F will continue to have an addition reaction with OH·. Finally, the chain‐shortening cycle can be achieved through two methods: one involves removing an HF molecule via the same α‐hydroxy hydrogen abstraction reaction, while the other method removes F⁻ through direct electron transfer. The final products are both PFCA molecules that have lost a ‐CF₂ structure. Compared to the OH· cycle, the O₂ and H₂O cycles play a minor role in PFAS degradation. However, it is noteworthy that regardless of the pathway, direct electron decarboxylation initiates the entire reaction.

FIGURE 7.

The oxidation defluorination pathway of PFAS. Figure adapted from ref [107].

4.2. Reduction Pathway

Reductive defluorination is different from oxidation defluorination. The primary reducing species can selectively attack the C─F bond, thereby achieving deep defluorination. Currently, the literature reports two main reducing species that can effectively achieve defluorination: H· and e_aq ⁻. Multiple systems can effectively generate H·, with the most common being electrochemical reduction systems that produce H· through the Volmer reaction during water electrolysis [108, 109]. Some scholars have observed that PFOA occurred an electrochemical reduction reaction on the surface of Au electrodes. The nuclear magnetic resonance (NMR) spectra of H and F confirmed that one of the two F atoms in PFOA was replaced by H (Figure 8a–c) [110]. This reaction is not involved in the participation of H·, but only through the direct electron reduction at the Au electrode surface. Some researchers have also observed the generation of H· in photochemical systems and elucidated its role in the defluorination process of PFAS (Figure 8d,e) [65]. Interestingly, the C₇H₁₅CO·—the complete hydrogenation defluorination product of PFOA—was observed in the photochemical reaction involving H·. This represents a novel discovery enabling complete defluorination of PFAS without involving chain‐shortening processes. In addition to the light and electricity systems, the generation of H· has also been observed in the nanoscale zero‐valent iron system [93]. Researchers also observed the complete hydrogenation defluorination product (C₇H₁₅COOH) of PFOA (Figure 8f). If the production of C₇H₁₅COOH can be enhanced in the future, this will open up a new avenue for PFAS treatment—resource recovery.

FIGURE 8.

(a) F‐spectrum and (b) H‐spectrum of defluorination products of PFAS. (c) The electrochemical defluorination pathway of PFOA on the Au cathode. Reproduced with permission [110]. (a–c) Copyright 2024 American Chemical Society. The photochemical defluorination pathways of (d) PFOA and (e) Hexafluoropropylene oxide dimer acid (HFPO–DA, trade name GenX). Reproduced with permission [65]. (d,e) Copyright 2025 American Chemical Society. (f) H·‐dominated defluorination pathway of nanoscale zerovalent iron system. Reproduced with permission [93]. Copyright 2025 American Chemical Society. (g) Hydrated electron‐dominated defluorination pathway of UV/SO₃ ²⁻ system. Reproduced with permission [40]. Copyright 2025 Springer Nature.

It is noted that e_aq ⁻ is the most common active species in PFAS defluorination due to its exceptionally strong reducing ability. The most common system for generating e_aq ⁻ is UV/SO₃ ²⁻, which requires an alkaline and anaerobic environment. Relying solely on e_aq ⁻ cannot achieve deep defluorination, but requires synergistic role with electrons and SO₃ ⁻·₃ to accomplish chain shortening and defluorination. This is because e_aq ⁻ cannot achieve deep defluorination of PFAS without breaking C─C bonds, as mentioned in the literature above (Figure 8g). H₂O and OH⁻ both play crucial roles in chain shortening and defluorination, which explains why e_aq ⁻ defluorination tends to occur under alkaline conditions.

4.3. Redox and Emerging Technology Defluorination Pathways

After understanding the oxidation and reduction defluorination pathways of PFAS, we will introduce the several defluorination pathways associated with redox processes and emerging technologies. These pathways are more complex than the previous ones, but gaining a deep understanding of different defluorination pathways will help us solve PFAS contamination issues early.

When discussing reduction techniques, one challenge we face is that relying solely on reducing species (H· and e_aq ⁻) makes it difficult to achieve decarboxylation—a critical step in shortening PFAS chains. To address this issue, Wu et al. established a novel PFOA oxidation pathway coupled with advanced reduction processes. The e_aq ⁻ are responsible for reductive defluorination, while direct electron transfer can achieve oxidative decarboxylation and C─C bond cleavage, achieving a cyclic defluorination process through redox synergy (Figure 9a) [111]. Researchers have also achieved synergistic redox degradation of PFOA through a photoelectrocatalytic‐peroxymonosulfate (PEC‐PMS) system, with a MoSe₂/TiO₂ photoelectrode removing 93.6% of PFOA within 40 min (Figure 9b) [112]. Quenching experiments and electron paramagnetic resonance (EPR) spectroscopy confirm that electrons and superoxide radicals (·O₂) play a primary role. Unlike single defluorination technologies, synergistic redox strategies have more potential for future application in practical PFAS wastewater treatment.

FIGURE 9.

The synergic defluorination pathway of (a) the hydrated electron reduction and electron transfer oxidation. Reproduced with permission [111]. Copyright 2025 Elsevier. (b) Electron reduction and ·O₂ oxidation. Reproduced with permission [112]. Copyright 2025 Elsevier. (c) The defluorination pathway of PFAS by hydrothermal alkali treatment. Reproduced with permission [44]. Copyright 2022 The American Association for the Advancement of Science. (d) Spontaneous degradation pathway of PFAS in droplets. Reproduced with permission [99]. Copyright 2025 American Chemical Society.

Certain emerging technical pathways are equally important for understanding the defluorination mechanism of PFAS. Hydrothermal alkali technology enables spontaneous defluorination of PFAS under mild conditions without requiring energy input. This technology is suitable for treating highly concentrated PFAS after concentration. For instance, some researchers have employed hydrothermal alkali to treat spent granular activated carbon that has adsorbed PFAS [113]. The defluorination pathway of hydrothermal alkali technology is similar to that of conventional EO techniques, with the initial decarboxylation being the critical step (Figure 9c). Interestingly, the perfluoroalkene formed when the ─CF₂ unit loses one F atom during the hydrothermal alkaline defluorination pathway is crucial for the chain‐shortening cycle of PFCA [44]. It should be noted that the yield of trifluoroacetic acid (TFA) with an ultra‐short chain (which may exhibit higher toxicity and migration potential) is relatively high in the hydrothermal alkali treatment process, which is a key issue requiring attention in the future. Compared to hydrothermal alkali technology, which requires stringent conditions such as organic solvents and alkaline environments, some researchers have observed that PFAS can achieve defluorination through liquid‐solid‐gas three‐phase contact electrochemistry in microdroplets containing wollastonite (Figure 9d) [99]. Microdroplet‐mediated weathering of wollastonite triggers the formation of CaF₂–SiO₂ interface structures, thereby achieving fluoride immobilization with negligible leaching. In the future, we need to conduct more in‐depth research on the defluorination pathways of PFAS, which will help us resolve PFAS contamination issues as soon as possible.

5. The Influence of Key Factors on PFAS Defluorination

After understanding the pathways for PFAS defluorination, we will summarize and investigate the impact of key factors involved in the PFAS defluorination process.

The selection of solvents, homogeneous and heterogeneous systems, and reactor design all play a crucial role in the defluorination process of PFAS. Theoretical research on micro‐mechanisms forms the foundation for PFAS defluorination treatment, and discussions of these factors facilitate the advancement of defluorination technology from laboratory scale to practical application in wastewater treatment plants.

5.1. Reaction Solvent

Although the vast majority of PFAS defluorination technologies can be carried out in aqueous systems (such as EO technology, UV‐related photochemical technologies, piezoelectric catalysis and metal catalysis), certain defluorination techniques (electrochemical reduction and hydrothermal alkali technologies) requiring organic solvents have demonstrated outstanding performance (Figure 10a). Even though the electrochemical reduction technology has poor deep defluorination ability, some scholars have effectively improved the defluorination efficiency by designing molecular Cu catalysts in the organic solvents [91]. This method breaks through the barrier of traditional electrochemical reduction defluorination, and achieves 93% degradation and 99% defluorination of PFOA. In addition to the electrochemical reduction technology, the hydrothermal alkali technology also needs to be carried out in organic solvents. The defluorination mechanism and the pathway of this technology have been explored in detail by scholars, and great breakthroughs have been made in the laboratory scale [44, 113, 114, 115]. These defluorination technologies involving organic solvents are designed to treat exceptionally high concentrations of PFAS (at the g·L⁻¹ level), which makes them unsuitable for the direct treatment of PFAS‐contaminated wastewater. However, these technologies can be used to treat concentrated PFAS retained through adsorption/ion exchange resin/membrane technology. For example, Soker et al. treated PFAS‐adsorbed spent activated carbon using hydrothermal alkali technology, simultaneously achieving PFAS destruction and activated carbon regeneration [113].

FIGURE 10.

(a) The choice of reaction solvent for PFAS defluorination. (b) Heterogeneous and homogeneous system of PFAS defluorination. (c) Differential column batch reactor for electrochemical oxidation defluorination. Reproduced with permission [66]. Copyright 2022 American Chemical Society. (d) Redox‐electrodialysis reactor for desalination and defluorination. Reproduced with permission [34]. Copyright 2024 Springer Nature. (e) Column reactor of nanoscale zerovalent iron for PFAS defluorination. Reproduced with permission [93]. Copyright 2025 American Chemical Society. (f) Electrochemical dead‐end filtration membrane reactor for short‐chain PFAS. Reproduced with permission [103]. Copyright 2022 Elsevier.

5.2. Heterogeneous and Homogeneous Systems

The defluorination system is classified into homogeneous and heterogeneous systems based on the state of the catalyst and PFAS (especially, photochemical systems without catalyst are considered homogeneous systems). Homogeneous and heterogeneous systems each have their advantages and disadvantages. For instance, homogeneous systems enable thorough contact between the catalyst and PFAS, resulting in effective defluorination. However, they cannot achieve the reuse and recovery of catalysts like heterogeneous systems (Figure 10b). As previously mentioned, the application of a molecular catalyst in electrochemical reduction defluorination transforms the traditional heterogeneous electrochemical system into a homogeneous one, thereby significantly enhancing defluorination efficiency. Although molecular catalysts enhance defluorination efficiency, they cannot be effectively recovered and reused like flat plate and membrane electrodes. Membrane technology is an effective solution to this problem. For example, electrocatalytic membrane technology loads the catalyst completely into the pores of the membrane, enabling thorough contact between the catalyst and PFAS, and facilitating the recovery of the membrane material [104, 116]. In the future, Heterogeneous systems will undoubtedly hold greater potential for practical application in wastewater treatment. Our goal is to enhance the defluorination efficiency of heterogeneous systems while reducing operational costs.

5.3. Design of the Reactor

In the practical application of water treatment technology, reactor design is one of the most critical factors. This is also an issue that has received relatively little attention, with the most focus seemingly being placed on small‐scale material preparation and microscopic mechanisms. Next, we will introduce several types of reactors used in the PFAS defluorination field. The H‐type electrolytic cell is commonly used as a reactor in the EO system, but this reactor does not match the flow pattern of wastewater in actual wastewater treatment plants. To address this issue, Chen et al. designed a differential column batch reactor (DCBR) utilizing porous electrodes [66]. This reactor not only enables full contact between the catalyst and PFOA, but also shortens the distance between the anode and cathode, thereby promoting mass transfer. (Figure 10c). Some researchers have combined electrodialysis and electrosorption strategies to design a redox‐polymer electrodialysis system that can treat PFAS with chain lengths ranging from ultra‐short to long without causing membrane fouling (Figure 10d) [34]. Simultaneously, effective desalination of wastewater can be achieved while removing PFAS. This provides a feasible approach to treating PFAS in actual high‐concentration salt wastewater.

Nanoscale zerovalent iron (nZVI) have attracted a great deal of attention in the field of wastewater treatment because of their extremely powerful reducing ability [117, 118, 119, 120]. Research has also demonstrated that nZVI can realize effectively defluorination for PFAS. For instance, Chen et al. theoretically and experimentally proved that nZVI can induce the defluorination process of chlorinated polyfluoroalkyl substances (Cl‐PFAS) [121]. Liao et al. modified nZVI using surface‐modified silicates with significant proton affinity (Si‐nZVI), enabling it to efficiently generate and utilize H· for effective defluorination of PFOA [93]. Meanwhile, a column reactor capable of achieving continuous wastewater flow was designed to explore the application potential of Si‐nZVI technology (Figure 10e). The results indicate that Si‐nZVI can achieve over 90% defluorination of PFOA while effectively immobilizing F⁻. Finally, we will introduce the dead‐end filtration membrane reactor capable of conducting electrocatalysis, which effectively combines the advantages of membrane concentration and EO technology. PFAS can fully contact with the catalyst loaded within the membrane pores, thus enhancing the utilization efficiency of catalysts (Figure 10f) [103]. Future research should focus on membrane fouling issues, specifically how to achieve sufficient contact between PFAS and the catalyst while maintaining high membrane flux. The reactor design still requires more work in the future, particularly in achieving its implementation at the pilot scale—a critical step in advancing the practical application of defluorination technology.

5.4. Pilot‐Scale PFAS Defluorination

After discussing the reactor design, the pilot‐scale application of PFAS defluorination technology will be introduced. Significant progress has been made in laboratory studies on the PFAS defluorination, thus the focus of future research should now shift toward practical applications. EO technology has been effectively applied in pilot‐scale PFAS treatment due to its high defluorination efficiency and scalability of heterogeneous system. Smith et al. demonstrated a practical PFAS treatment process: first, foam separation technology was employed to concentrate PFAS from groundwater and landfill leachate, followed by EO technology to achieve PFAS degradation (Figure 11a) [122]. During the design treatment process, the average total degradation efficiency of PFAS reached 50%, with degradation rates for long‐chain and short‐chain PFAS reaching 86% and 31%, respectively. After 9 hours of processing, the energy consumption for 50 L experiments was approximately 270 kWh·m⁻³, while that for all 150 L experiments was approximately 93 kWh·m⁻³. Although the energy consumption has increased relative to laboratory scale, this is achieved while treating real PFAS wastewater at pilot‐scale conditions. To reduce energy consumption and enhance defluorination efficiency, researchers have explored a method that couples ion exchange resin (IXR) with EO technology [123]. The EO reactor consists of two main reaction chambers, each equipped with 12 pairs of magneli Ti₄O₇ plate anodes and stainless‐steel plate cathodes. It is additionally fitted with a circulation pump, water cooling system, and exhaust gas control system (Figure 11b). This system achieves a destruction efficiency of 80–98% for PFOA and PFOS, with energy consumption of 0.131–0.161 kWh·m⁻³ for PFOA and 0.071–0.094 kWh·m⁻³ for PFOS. This is several orders of magnitude lower in energy consumption than existing single EO technology.

FIGURE 11.

(a) Schematic diagram of the EO pilot system. Reproduced with permission [122]. Copyright 2023 American Chemical Society. (b) Schematic diagram of the IXR‐EO pilot system. Reproduced with permission [123]. Copyright 2022 Elsevier. (c) Schematic diagram of the NF/UV‐sulfite pilot system. Reproduced with permission [124]. Copyright 2021 Elsevier. (d) Schematic diagram of the supercritical water oxidation pilot system. Reproduced with permission [125]. Copyright 2022 Wiley.

Some progress has also been made in the pilot‐scale application of photochemical technology for the defluorination of PFAS, in addition to electrochemical technology. The UV/SO₃ ²⁻ system can generate e_aq ⁻ to achieve effective defluorination for PFAS, but its energy consumption is relatively high during practical application and it is susceptible to interference from impurities. To address this issue, Liu et al. achieved low‐energy defluorination of PFAS by combining nanofiltration (NF) with UV/SO₃ ²⁻ technology, while effectively eliminating interference from impurities (Figure 11c) [124]. NF technology can effectively retain and concentrate PFAS, while UV/SO₃ ²⁻ technology can achieve complete degradation of PFAS within 4 h. The specific energy consumption requirements for different pollutants vary significantly: PFCAs ≤ 13.1 kWh·m⁻³, PFOS at 14.1 kWh·m⁻³, while the more recalcitrant short‐chain PFSA analogues exceed 100 kWh·m⁻³. Emerging PFAS treatment technologies such as supercritical water oxidation (SCWO) technology have already been tested at pilot scale (Figure 11d). Following a series of experimental tests, the results demonstrate that it can achieve a degradation efficiency exceeding 99% and an average defluorination efficiency of 62.6% for wastewater containing 12 types of PFAS [125].

It is also worth noting that PFAS are widely present in industrial wastewater, primarily in four categories: fluorochemical, electronic, textile, and electroplating wastewater. PFAS concentrations in industrial wastewater span approximately 12 orders of magnitude 2.1 × 10⁻³ to 1.7 × 10⁹ ng·L⁻¹ [126]. Industrial wastewater contains a large number of interfering factors, such as various metal cations and anions, which can significantly interfere with PFAS defluorination systems. For example, in photochemical processes, hydrated electrons are severely quenched by NO₃ ⁻, thereby affecting treatment efficiency [93]. Therefore, the treatment of complex PFAS‐containing industrial wastewater requires a combination of technologies, such as membrane retention to concentrate and enrich PFAS, followed by electrooxidation to treat the resulting high‐concentration PFAS wastewater. This approach not only reduces the volume of wastewater requiring treatment but also significantly enhances mass transfer between the electrodes and the contaminants.

6. Summary and Prospect

In this review, the correlation between the structural properties of different types of PFAS and defluorination mechanisms was summarized, which provides a reference for the selection of catalysts and reaction systems in the future. Meanwhile, the reaction mechanism, process and limitation of different wastewater treatment technologies (including reduction, oxidation and emerging technologies) that can realize effective defluorination are discussed. The comprehensive summary about the defluorination process of different radicals and electrons will unveiling the mystery of complex defluorination mechanism. We not just focus on the microscopic defluorination process, our final goal is to promote the practical application of these technologies in the wastewater treatment plants. Thus, the influences of key factors on the PFAS defluorination, including the reaction solvent, heterogeneous and homogeneous systems, and reactor design were assessed by us. Importantly, the pilot‐scale application of relevant PFAS defluorination technologies has also been summarized and discussed. This review will supply a detailed and feasible strategy to treat the wastewater contaminated with PFAS.

Although many breakthroughs have been achieved, there are still some issues that need to be solved. First, precise characterization of the defluorination process requires focused attention, which is the key point of microscopic mechanism. Most of the related researchers pay more attention on the material characterization not the structure‐activity between the material and PFAS. The detailed defluorination process can be analyzed by the combination of experiments and theory calculation. NMR and Liquid chromatography‐mass spectrometry (LC‐MS) are powerful tools to explore the PFAS defluorination, which can precisely explore the reaction process (Figure 12a). It is worth noting that many current articles on PFAS degradation lack clarity regarding how LC‐MS is used to achieve precise quantification and qualitative analysis of PFAS, and there is currently no widely accepted standard for PFAS determination. Tan's research provides a detailed account of LC‐MS application, and it is recommended that researchers in this field use the analytical guidelines presented here as a benchmark for our own research projects [40]. Meanwhile, in‐situ characterization (in‐situ fourier transform infrared spectroscopy (FTIR) and Raman) can prove the specific C─F bond changes and the role of radicals, thus helping researchers understand deeper defluorination mechanism. Subsequently, the experimental results were further validated through theoretical calculations, which creates a closed‐loop system to better understand the defluorination process (Figure 12b).

FIGURE 12.

(a) Characterization of the PFAS degradation path by nuclear magnetic resonance (NMR) and liquid chromatography‐mass spectrometey (LC‐MS). (b) Analysis of the PFAS degradation process by in situ characterization technologies and theory calculation. (c) Assessment the F balance during the defluorination process.

Importantly, Fluorine balance should be considered in the aforementioned exploration of PFAS defluorination processes (Figure 12c). This helps us understand the product distribution of defluorination process, enabling us to better comprehend the defluorination mechanism. Changes in total organic carbon (TOC) before and after PFAS reaction can reveal the extent of mineralization, thereby determining the relative contribution of oxidation processes in this system. In addition to the organic reaction products, F⁻ is the component requiring our primary attention, which can be effectively detected using ion chromatography (IC) and fluoride ion‐selective electrode (ISE). TOC, LC‐MS, and fluorine concentration measurements can help us better evaluate defluorination performance.

During our research on PFAS defluorination, we noted that some scholars have observed the generation of octanoic acid (C₇F₁₅COOH) [65, 93]. Therefore, we are considering whether there may be opportunities in the future to achieve the resource recovery and utilization of PFAS. Achieving this in situ in actual wastewater is nearly impossible, but PFAS can be concentrated in pure aqueous solutions using adsorbents/IXR/membranes for resource recovery. Compared to oxidation technology, which achieves defluorination through chain‐shortening and thus cannot effectively preserve carbon chains, reduction technology possesses unique advantages in the resource recovery of PFAS. In the future, a multi‐technology synergistic approach can be employed to achieve deep defluorination of PFAS and enhance the selectivity for producing corresponding fatty acids. Meanwhile, the recovery of F⁻ is also a key focus in PFAS resource utilization. F⁻ can be used to synthesize synthetic fluorite (CaF₂), which serves as the starting point for the entire fluorochemical industry chain. Its end products are widely utilized in cutting‐edge fields such as modern industry, high‐end manufacturing, information technology, and new energy. Hydrothermal alkali technology can achieve over 90% F⁻ recovery from high‐concentration PFAS without requiring any catalysts or energy input (Figure 13a). The generated F⁻ ions can be recovered by adding calcium salts to form CaF₂ (Figure 13b). Meanwhile, it is worth noting that the recovery process of F⁻ can be accelerated by preparing activated seeds [127]. In addition to recovering fluorine in the form of CaF₂, recent literature reports that it can be upcycled into non‐PFAS products, opening up a new avenue for fluorine recovery (Figure 13c) [20].

FIGURE 13.

Defluorinaiton technologies for high concentration PFAS. (a) electrochemical reduction by molecular Cu catalysis. Reproduced with permission [91]. Copyright 2023 American Chemical Society. (b) F⁻ recycling for the heterogeneous and homogeneous system. (c) Schematic showing the workflow from mineralization of PFAS to separation of fluorides and valorization to a non‐PFAS product [20]. Copyright 2026 Springer Nature. (d) Life cycle asssessment during the entire process of PFAS defluorination.

The final key point is about the application of life cycle assessment (LCA) in the defluorination process of PFAS. Past research has largely focused on microscopic mechanisms, but solving PFAS issues requires the practical application of defluorination technologies in wastewater treatment plants. LCA is a powerful tool for assessing the practical application potential and optimizing operation parameters of different defluorination technologies. The focus is on three key areas: raw material usage, energy consumption, and disposal of defluorination byproducts (Figure 13d). The primary focus of LCA remains on assessing the environmental impacts of different processes, helping us in selecting and optimizing PFAS treatment process.

Conflicts of Interest

The authors declare no conflict of interest.

Supporting information

Supporting File: smll74007‐sup‐0001‐SuppMat.docx.

Data Availability Statement

Data sharing not applicable to this article as no datasets were generated or analysed during the current study.