Emerging Strategies for the Photoassisted Removal of PFAS from Water: From Fundamentals to Applications

Abstract

Per- and polyfluoroalkyl substances (PFAS) are a diverse group of synthetic fluorinated compounds widely used in industrial and consumer products due to their exceptional thermal stability and hydrophobicity. However, these same properties contribute to their environmental persistence, bioaccumulation, and potential adverse health effects, including hepatotoxicity, immunotoxicity, endocrine disruption, and increased cancer risk. Traditional water treatment technologies, such as coagulation, sedimentation, biological degradation, and even advanced membrane processes, have demonstrated limited efficacy in removing PFAS, as they primarily separate or concentrate these compounds rather than degrade them. In response to these limitations, photoassisted processes have emerged as promising alternatives capable of degrading PFAS into less harmful products. These strategies include direct photolysis using UV or VUV irradiation, heterogeneous photocatalysis with materials such as TiO₂ and novel semiconductors, light-activated persulfate oxidation generating sulfate radicals, and photo-Fenton reactions producing highly reactive hydroxyl radicals. Such approaches leverage the generation of reactive species under irradiation to cleave the strong carbon–fluorine bonds characteristic of PFAS. This review provides a comprehensive overview of emerging photoassisted technologies for PFAS removal from water, detailing their fundamental principles, degradation pathways, recent advancements in material development, and integration with hybrid treatment processes. Moreover, it discusses current challenges related to energy efficiency, catalyst deactivation, incomplete mineralization, and scalability, outlining future perspectives for their practical application in sustainable water treatment systems to mitigate PFAS pollution effectively.

1. Introduction

Per- and polyfluoroalkyl substances (PFAS) constitute a large family of synthetic fluorinated compounds (

Table 1. Overview of PFAS compounds.

PFAS Compound	Chemical Formula	Chemical Structure	Chain Length	Functional Group
PFOA	C7F15COOH		C8	Carboxyl
PFOS	C8F17SO3H		C8	Sulfonic acid
PFHxA	C6F13COOH		C6	Carboxyl
PFBA	C4F9COOH		C4	Carboxyl
PFBS	C4F9SO3H		C4	Sulfonic acid
PFHxS	C6F13SO3H		C6	Sulfonic acid
GenX	C6HF11O2C2HF4		C6	Carboxyl
PFOSA	C8F17SO2NH2		C8	Amide
PFDA	C10F21COOH		C10	Carboxyl

) widely used for over seven decades due to their remarkable thermal stability, hydrophobicity, and oleophobicity [1]. These properties have enabled their extensive application in industrial processes, firefighting foams, food packaging, textiles, and non-stick cookware, among others. However, the same chemical stability that makes PFAS valuable in manufacturing also confers environmental persistence and resistance to natural degradation pathways, earning them the designation of “forever chemicals” [2]. Their widespread occurrence in aquatic environments worldwide has raised significant concerns due to their recalcitrance, bioaccumulation potential, and diverse adverse health effects [3].

Toxicological studies have associated PFAS exposure with hepatotoxicity, immunotoxicity, endocrine disruption, developmental toxicity, and increased risk of certain cancers. For instance, perfluorooctanoic acid (PFOA) and perfluorooctane sulfonic acid (PFOS), two of the most extensively studied PFAS, have been linked to increased cholesterol levels, thyroid disease, and immunological effects in humans [4]. Furthermore, their high aqueous solubility and mobility facilitate their transport through groundwater and surface waters, leading to global distribution and contamination of drinking water sources [5]. These health and ecological risks have prompted regulatory agencies worldwide to implement stringent guidelines and, in some cases, to ban or phase out specific PFAS compounds [6]. Nonetheless, the challenges in mitigating PFAS pollution remain substantial due to their structural diversity and chemical inertness.

Traditional water treatment technologies, including coagulation-flocculation, sedimentation, and biological degradation, have demonstrated negligible removal efficiencies for PFAS [7]. Physical separation techniques such as granular activated carbon adsorption and ion exchange resins are commonly used in drinking water treatment plants, yet they primarily concentrate PFAS rather than destroy them, generating secondary waste streams that require further handling and disposal [8]. High-pressure membrane processes, such as reverse osmosis and nanofiltration, effectively reject PFAS but are energy-intensive and similarly produce concentrated brines [9]. Consequently, there is an urgent need to develop innovative treatment technologies capable of degrading PFAS into less harmful products rather than merely separating them from water matrices.

In recent years, photoassisted processes have emerged as promising alternatives for the degradation of PFAS due to their potential to generate highly reactive species capable of cleaving the robust carbon–fluorine bonds. These processes include direct photolysis, heterogeneous photocatalysis, photo-Fenton reactions, and light-activated persulfate oxidation. Direct photolysis utilizes ultraviolet (UV) or vacuum ultraviolet (VUV) light to induce the breakdown of PFAS molecules, while photocatalytic approaches employ semiconductors such as titanium dioxide (TiO₂) under irradiation to generate reactive oxygen species (ROS) that attack PFAS structures. Furthermore, hybrid systems integrating photoactivation with advanced oxidation processes (AOPs), such as persulfate or Fenton-based systems, enhance the generation of sulfate and hydroxyl radicals, thereby improving PFAS degradation efficiency [10].

The potential of photoassisted strategies lies not only in their ability to achieve mineralization or significant defluorination of PFAS but also in their compatibility with renewable energy sources, particularly solar irradiation, for sustainable water treatment [11,12]. Despite these advantages, challenges remain regarding their practical application, including energy requirements, incomplete mineralization, formation of toxic intermediates, catalyst deactivation, and scalability for real-world implementation [10]. Addressing these challenges necessitates a comprehensive understanding of the underlying degradation mechanisms (Figure 1), reactive species generation pathways, and factors influencing process efficiency, such as light wavelength, intensity, pH, and coexisting matrix constituents.

This review aims to provide an in-depth overview of emerging photoassisted strategies for PFAS removal from water, encompassing their fundamental principles, degradation pathways, and recent technological advancements. It systematically discusses direct photolysis, photocatalytic processes with traditional and novel materials, light-activated persulfate systems, and photo-Fenton reactions, highlighting their mechanistic insights, efficiencies, and limitations. Additionally, it outlines current challenges and future perspectives for upscaling these technologies and integrating them into treatment trains for efficient and sustainable PFAS remediation. By bridging fundamental knowledge with practical applications, this review seeks to guide researchers and practitioners in developing effective photoassisted treatment technologies to address the pressing global issue of PFAS contamination in water resources.

2. Systematic Review Methodology

This review offers an in-depth examination of how photoassisted technologies contribute to PFAS capture, degradation and mineralization, following a systematic framework inspired by the Preferred Reporting Items for Systematic Reviews and Meta-Analyses (PRISMA) guidelines. Applying PRISMA ensured that the process remained methodical, transparent, and reproducible, consistent with best practices for evidence synthesis. The first step consisted of an extensive literature search within the Web of Science Core Collection^®, where carefully constructed keyword combinations, such as “PFAS”, “per- and polyfluoroalkyl substances”, “photodegradation”, “adsorption”, “mineralization” and “removal”, were employed to capture all relevant studies. Only articles that explicitly investigated the application of photoassisted technologies for PFAS removal and/or degradation/mineralization were considered, while non-English works, conference proceedings, and unrelated studies were excluded.

The selection procedure involved a multi-stage screening, where at least two authors independently assessed titles and abstracts to verify compliance with the criteria. Eligible full-text articles underwent further evaluation, and disagreements between reviewers were addressed through discussion. For each included study, essential data were extracted regarding experimental conditions, degradation performance, and any identified limitations or gaps in the reported research results. Data collection was performed by one reviewer and cross-checked by another to ensure reliability. Additionally, the quality of the studies was judged using standardized benchmarks, emphasizing experimental rigor, reproducibility, and clarity of reported results, with consensus reached in cases of differing assessments.

3. PFAS: Chemistry, Properties, and Environmental Concerns

PFAS represent a vast and structurally diverse group of synthetic organofluorine compounds that have been produced and used since the mid-20th century [14]. Their widespread application in industrial and consumer products is largely due to their exceptional chemical and thermal stability, which arises from the presence of strong carbon–fluorine (C–F) bonds [15]. These bonds, among the strongest in organic chemistry, confer remarkable resistance to degradation processes such as hydrolysis, photolysis, microbial metabolism, and oxidation [16]. PFAS are most commonly defined by the presence of at least one fully fluorinated methyl or methylene group and are classified into two major categories: perfluoroalkyl substances, in which all hydrogen atoms on the carbon chain are replaced by fluorine, and polyfluoroalkyl substances, which include at least one perfluorinated moiety but retain other hydrogen or heteroatoms [17].

A fundamental structural characteristic of PFAS is the combination of a hydrophobic fluorinated carbon chain with a hydrophilic functional group, such as carboxylates, sulfonates, phosphates, or amides. This amphiphilic nature underpins their surfactant properties and has supported their use in numerous applications including firefighting foams, textile coatings, food packaging, lubricants, and cosmetics [4]. From an environmental and toxicological standpoint, PFAS are often further subdivided into long-chain and short-chain compounds [17]. Long-chain PFAS, such as PFOA and PFOS, typically contain eight or more fully fluorinated carbon atoms and have been associated with higher bioaccumulation and toxicity [18]. In contrast, short-chain PFAS, which have fewer carbon atoms, were introduced as replacements to mitigate these issues. While they may be less prone to bioaccumulation in organisms, they exhibit higher mobility in aquatic environments and, importantly, remain environmentally persistent [19].

Despite their utility, PFAS pose a significant environmental and public health challenge due to their chemical robustness and environmental behaviour. The physicochemical properties of PFAS, including high aqueous solubility, low volatility, and strong resistance to degradation, make them highly mobile in surface and groundwater. Their acid dissociation constants (pKa) are generally below 4, meaning that under environmental pH conditions, PFAS exist predominantly in their anionic form, which enhances solubility and environmental mobility [2]. Although they resist volatilization, certain precursors such as fluorotelomer alcohols (FTOHs) exhibit atmospheric transport potential and can degrade into terminal PFAS, further complicating their global distribution and environmental monitoring [20]. Their amphiphilic nature facilitates interaction with organic matter, solid surfaces, and proteins, contributing to their bioaccumulation in both terrestrial and aquatic organisms [21].

The environmental fate of PFAS is governed by their chemical structure [22]. Long-chain compounds tend to partition more strongly to sediments, organic matter, and biota, while short-chain variants exhibit higher mobility and are more challenging to remove from water matrices using conventional treatment technologies. In aquatic ecosystems, PFAS can be found in both dissolved and particulate-bound forms, with sorption behaviour influenced by pH, ionic strength, and organic content [7]. In soils, PFAS derived from land-applied biosolids, wastewater irrigation, or landfill leachates can persist for decades, gradually leaching into groundwater [23]. Atmospheric transport of volatile precursors also contributes to long-range contamination, with PFAS detected even in remote regions such as the Arctic and Antarctic [24,25]. This widespread occurrence underscores their resistance to natural attenuation and the urgent need for effective treatment and regulatory strategies.

In terms of toxicology, growing evidence has associated PFAS exposure with a range of adverse health outcomes (Figure 2). Epidemiological and toxicological studies, particularly those focused on legacy compounds like PFOS and PFOA, have linked chronic exposure to hepatotoxicity, endocrine disruption, immunotoxicity, reproductive and developmental toxicity, and carcinogenicity [26]. These effects have been observed at relatively low concentrations and over long-term exposure scenarios, raising concerns even at trace levels in drinking water [27]. In aquatic organisms, PFAS have been shown to cause oxidative stress, reproductive abnormalities, and behavioural changes, disrupting population dynamics and ecosystem functioning [28]. As a result, regulatory thresholds for PFAS in drinking water have become increasingly stringent, often in the low ng/L range. However, the vast diversity of PFAS structures, combined with the proprietary nature of many commercial formulations, poses significant challenges for risk assessment and regulation.

Toxicity studies of PFAS have consistently demonstrated a wide range of adverse outcomes in humans and ecological receptors, reflecting their persistence, bioaccumulative properties, and resistance to degradation. For instance, PFOA has been strongly associated with hepatotoxic effects, immune suppression, and developmental toxicity, while PFOS has been linked to endocrine disruption, altered thyroid hormone regulation, and impaired neurodevelopment [1]. Longitudinal epidemiological studies have revealed correlations between PFOA exposure and increased risks of kidney and testicular cancers, as well as associations of PFOS with dyslipidemia, immunosuppression, and reduced vaccine response in children [2]. Moreover, emerging PFAS alternatives, such as hexafluoropropylene oxide dimer acid (GenX), initially introduced as a safer substitute, have also shown evidence of hepatotoxicity and renal effects in animal models, highlighting the challenge of regrettable substitutions within this chemical class [3].

Mechanistically, PFAS toxicity is largely driven by their high affinity for serum albumin and other plasma proteins, enabling long half-lives in humans—estimated at 3.5 years for PFOS, 2.3 years for PFOA, and 5.3 years for perfluorohexane sulfonate (PFHxS) [4]. These compounds interact with nuclear receptors, particularly peroxisome proliferator-activated receptors (PPARs), thereby disrupting lipid metabolism and energy homeostasis [5]. Toxicological evidence also suggests cross-generational effects, as prenatal exposure to PFAS has been associated with low birth weight, developmental delays, and immune dysfunction in offspring [6]. Recent mixture studies further underscore the complexity of PFAS toxicity, as additive or synergistic effects among structurally diverse PFAS can exacerbate health risks beyond those predicted from single-compound assessments [7]. Collectively, these findings highlight the urgent need for comprehensive toxicological evaluation of both legacy and emerging PFAS to inform regulatory actions and safeguard public and environmental health.

The treatment of PFAS-contaminated water remains a pressing global concern due to the limited effectiveness of traditional water treatment technologies. Conventional methods such as coagulation, sedimentation, filtration, and biological degradation are largely ineffective because PFAS resist microbial breakdown and do not coagulate or flocculate under standard conditions [29]. Adsorption techniques, particularly using GAC and ion exchange resins, have been widely adopted and show relatively good performance for long-chain PFAS [30]. However, their effectiveness diminishes significantly for short-chain variants due to their lower sorption affinity, resulting in frequent media regeneration and higher operational costs [31]. Membrane processes such as reverse osmosis and nanofiltration provide excellent rejection rates for most PFAS but generate concentrated brines that require further treatment or safe disposal [32]. Moreover, these technologies primarily achieve physical separation rather than destruction, shifting the burden to waste management systems.

Advanced oxidation processes (AOPs), which are effective against a wide range of organic contaminants through the generation of reactive oxygen species like hydroxyl radicals, have shown limited success for PFAS [11]. This limitation stems from the extraordinary strength of the C–F bond and the low reactivity of PFAS toward nonspecific oxidants [33]. Thermal treatment, including high-temperature incineration, can destroy PFAS but demands rigorous control over temperature, residence time, and oxygen availability to avoid incomplete combustion and the formation of hazardous by-products. Furthermore, thermal technologies are energy-intensive and economically unfeasible for large-scale or decentralized applications [34].

Against this background, photoassisted processes have gained attention as innovative strategies for PFAS degradation. These methods include direct photolysis, photocatalysis, and photo-enhanced oxidation systems such as UV/persulfate and photo-Fenton reactions. Unlike traditional AOPs, photoassisted methods can exploit specific interactions between light and PFAS molecules or between light and photocatalysts to generate highly reactive species capable of attacking the C–F bond [10]. In some cases, reductive pathways involving hydrated electrons or superoxide radicals can be promoted under irradiation, leading to partial defluorination and breakdown of the PFAS structure. Such mechanisms are particularly attractive given the potential to use sunlight or low-energy light sources, reducing the environmental footprint of treatment operations [12].

Nevertheless, photoassisted degradation of PFAS is not without its own challenges. Efficiency is highly dependent on light source characteristics, reactor configuration, solution chemistry, and the nature of the PFAS target compounds [11]. Additionally, incomplete mineralization, formation of potentially toxic intermediates, photocatalyst fouling, and difficulties in scale-up represent ongoing limitations [35]. Even so, these approaches offer a promising pathway to move beyond PFAS removal toward actual destruction, particularly when coupled with energy-efficient systems and tailored catalyst design.

It is also important to recognize that while some legacy PFAS have been phased out through regulatory action, the industry has shifted toward short-chain and structurally modified alternatives. These compounds, often introduced as safer substitutes, are now raising new concerns due to their persistence, mobility, and unknown health effects [31]. This phenomenon of “regrettable substitution” highlights the need for a class-based regulatory and treatment approach rather than targeting individual compounds. Such an approach would acknowledge the shared chemical features that drive environmental persistence and treatment resistance across the PFAS family, and would better align with the development of broad-spectrum, degradation-based technologies such as photoassisted processes.

In conclusion, a detailed understanding of the chemistry, physicochemical properties, environmental behaviour, and health impacts of PFAS is essential to guide the development of effective treatment technologies. The limitations of conventional removal methods underscore the need for innovative degradation approaches. Photoassisted processes, by leveraging the interaction between light and chemical reactivity, offer one of the most promising avenues for addressing the persistent challenge of PFAS contamination in water resources.

4. Fundamentals of Photoassisted Processes for PFAS Removal

The persistent nature of PFAS, rooted in their strong C-F bonds and unique physicochemical properties, presents a significant challenge for conventional water treatment technologies. In this context, photoassisted processes have emerged as innovative and potentially transformative strategies capable of degrading PFAS rather than merely removing or concentrating them [10]. These processes harness the energy of light, often in the UV or visible spectrum, to activate either the PFAS molecules themselves or a catalytic system that facilitates their breakdown [36]. The core appeal of photoassisted strategies lies in their capacity to generate highly reactive species under irradiation, leading to direct bond cleavage or indirect oxidative or reductive degradation pathways [10]. This section outlines the fundamental principles underlying the main photoassisted mechanisms being explored for PFAS removal, including direct photolysis, photocatalysis, light-activated persulfate oxidation, and photo-Fenton reactions.

4.1. Direct Photolysis of PFAS

Direct photolysis refers to the process by which PFAS molecules absorb photons and undergo bond cleavage without the assistance of external photocatalysts or oxidants. For direct photolysis to be effective, the contaminant must exhibit sufficient absorbance in the spectral range of the irradiation source, typically ultraviolet-C (UV-C, ~254 nm) or vacuum ultraviolet (VUV, <200 nm). However, most PFAS, especially perfluoroalkyl acids (PFAAs), show limited absorption in this range due to the lack of chromophoric groups in their structure. As a result, conventional UV light often fails to directly excite PFAS molecules to a reactive state [37].

Despite this, VUV irradiation, particularly in aqueous media, offers a promising alternative. In water, VUV photons can photolyse water molecules, generating hydroxyl radicals and hydrated electrons capable of initiating PFAS degradation. This indirect pathway allows VUV-based systems to overcome the absorbance limitations of PFAS and achieve partial defluorination [38]. However, VUV systems are energy-intensive, require specialized reactors, and suffer from low penetration depth in turbid or matrix-rich waters. Moreover, direct photolysis is highly compound-specific and is generally more effective for PFAS precursors or fluorinated surfactants with light-absorbing functional groups than for terminal PFAAs. Therefore, while direct photolysis can contribute to PFAS degradation under specific conditions, its application is often limited in real-world treatment scenarios [39].

Previous investigations have examined the light absorption properties of various PFAS and revealed that their reactivity under photon exposure differs considerably. For example, PFOS absorbs weakly in the deep UV region up to about 220 nm, whereas PFOA shows strong absorption in this range but only limited absorption between 220 and 270 nm [40]. As a result, PFOS undergoes photolysis very slowly compared to PFOA, and even when exposed to UV or vacuum-UV (VUV, λ < 200 nm), its breakdown is practically negligible [38]. PFOA, on the other hand, does degrade under direct irradiation but requires both high photon intensities and extended exposure times; for instance, only around 30% removal was reported after 12 h using a powerful 200 W Xe–Hg lamp [41]. Interestingly, shorter-chain PFCAs (C4–C6) and fluorinated alternatives such as GenX and EEA appear even more resistant, degrading more slowly than PFOA itself [42,43,44].

When comparing light sources, higher efficiency was achieved with combined UV/VUV (254/185 nm) systems rather than with 254 nm UV alone, most likely due to the higher energy of 185 nm photons, stronger absorption of PFOA in the VUV range, and the generation of reactive intermediates [45]. However, in real aquatic systems, this advantage is largely offset because water strongly absorbs VUV light, preventing photons from interacting directly with PFAS [38]. Studies simulating solar irradiation have also demonstrated that natural sunlight is ineffective in breaking down PFAS; for instance, PFOA remained stable in seawater under simulated sunlight (290–800 nm), with estimated half-lives in surface ocean waters ranging from centuries to millennia [46]. Even under intense sunlight at high altitudes, degradation was minimal, with only about 5% PFOA loss over 106 days [47]. Moreover, photolysis of long-chain PFAS often produces shorter-chain analogues, which are even more persistent, suggesting that environmental levels of these smaller PFAS will continue to rise. Overall, direct photolysis alone contributes little to PFAS removal, given the need for strong radiation and long exposure times. This limitation has motivated research into the use of mediators that can produce highly reactive oxidative or reductive species to improve PFAS degradation efficiency.

The findings reported by Xin et al., 2023 [37] clearly indicate that perfluoroalkyl carboxylic acids (PFCAs) degrade much more efficiently under far-UVC light at 222 nm compared to the conventional 254 nm wavelength. This enhanced photolysis primarily occurs through decarboxylation and defluorination, producing shorter-chain PFCAs and releasing fluoride ions. Interestingly, GenX and fluorotelomer unsaturated carboxylic acids (FTUCAs) also undergo direct breakdown at 222 nm, although their degradation pathways appear more complex and remain less understood. In contrast, several other PFAS types (such as PFSAs, diPAPs, FTCA, FTS, FOSA, and FHxSA) showed little to no response under the same conditions, highlighting the selectivity of the process.

One of the strengths of this study is that it links photolysis rates not only to molar absorption coefficients but also to chain-length-related structural factors such as carboxylic acid pKa, chain conformation, and radical stability. However, the computational analysis also showed that simple bond dissociation energies are insufficient to explain the differences observed, and previously proposed mechanisms like HF elimination are unlikely due to high activation barriers. This reveals a knowledge gap: while we know photolysis works for some PFAS, the exact degradation routes (especially after decarboxylation) still need clarification. A positive aspect is that the efficiency of 222 nm irradiation was not significantly hindered by most water matrices, except in the presence of strong light absorbers like nitrate, suggesting broad applicability.

On the downside, the approach remains limited in scope since many PFAS resist degradation, which could restrict its standalone use in real-world treatment. Overall, I see great potential in far-UVC technologies, especially since 222 nm light is considered safer for human exposure and could reduce PFAS contamination without adding chemicals. Yet, future research should test excimer lamp performance under realistic water treatment conditions, expand to more PFAS types and co-contaminants, and explore whether combining far-UVC with oxidants or reductants could transform it into a more robust advanced treatment process.

Pioneering studies have revealed the critical role of irradiation wavelength in dictating photolysis performance. UV-C sources at 254 nm, though widely available and cost-effective, typically achieve only modest PFAS degradation rates due to insufficient photon energy to directly break C–F bonds in most PFAS [48]. In contrast, VUV radiation at 185 nm, either from low-pressure mercury lamps with dual emission lines or from specialized excimer lamps, has consistently demonstrated higher degradation efficiencies, often surpassing indirect photoassisted methods when applied under optimized conditions [38]. For example, systems employing Xe excimer lamps at 172 nm have achieved rapid defluorination of PFOA and related compounds without chemical additives, highlighting the intrinsic potential of photolysis for “chemical-free” PFAS removal [49]. Comparisons between pulsed laser photolysis and continuous lamp irradiation show that high-intensity, short-pulse irradiation can enhance instantaneous bond cleavage, potentially reducing overall treatment times [11]. These performance gains are particularly notable for shorter-chain PFAS, whose higher absorption cross-sections in the far-UV lead to faster degradation than long-chain analogs, a selectivity that can be exploited in treatment designs targeting mixed PFAS wastewaters [50].

Despite these advantages, photolysis is constrained by the optical properties of PFAS and the need for high-energy photons [12,51]. While VUV photons can efficiently initiate degradation, they also promote water photolysis, generating reactive species such as hydroxyl radicals and hydrated electrons that can either assist PFAS breakdown or divert energy toward competing reactions. This interplay can be beneficial when it accelerates mineralization but may also produce partially defluorinated by-products, such as shorter-chain perfluorinated acids, that retain environmental persistence [52]. These intermediates may require extended irradiation or integration with subsequent treatments, as discussed in Section 3 for hybrid systems [53]. From an engineering perspective, transmitting VUV light through water demands specialized materials such as quartz or synthetic silica, and the energy demands, while lower than some oxidation-based treatments on a per-mass basis, can still be significant for high-throughput applications [54]. Nevertheless, when compared to chemical-intensive advanced oxidation processes, photolysis remains attractive for its operational simplicity, absence of chemical storage needs, and low potential for introducing new contaminants into treated water.

Future advances in PFAS photolysis are likely to emerge from both light source innovation and reactor design optimization. The ongoing development of high-output, long-lifetime VUV light-emitting diodes (LEDs) promises to replace mercury-based lamps, eliminating toxic components while allowing for wavelength-tuning to match PFAS absorption maxima [11,12]. Photonic engineering, such as enhancing photon density through reflective reactor geometries or integrating light guides, can increase energy efficiency [55]. Hybrid approaches also hold promise: coupling direct photolysis with in situ radical generation (e.g., through persulfate activation or photocatalysis) could accelerate degradation while maintaining low chemical input, bridging the gap between reagent-free operation and high mineralization rates [56]. Because photolysis operates without the need for continuous dosing of chemicals, it is particularly well-suited for applications where PFAS concentrations are low but persistent, such as polishing steps after primary treatment, or in decentralized systems for remote communities. In this role, it can serve as a sustainable, maintenance-light option that complements the more aggressive and broad-spectrum processes discussed in previous sections, making it an important candidate in the next generation of PFAS remediation technologies.

4.2. Heterogeneous Photocatalysis

Heterogeneous photocatalysis involves the use of semiconducting materials, typically in solid form, that absorb light and generate electron-hole pairs. Upon irradiation with energy equal to or greater than their bandgap, these materials promote electrons from the valence band to the conduction band, leaving behind positively charged holes. These charge carriers can then participate in redox reactions with water and dissolved oxygen, producing ROS such as hydroxyl radicals, superoxide anions, and singlet oxygen [57]. It is important to note that reactive oxygen species (ROS) generally do not cleave the strong C–F bonds directly; rather, they preferentially attack the functional groups of PFAS (e.g., carboxylate or sulfonate moieties), leading to decarboxylation or desulfonation. The actual cleavage of C–F bonds is typically achieved through subsequent processes involving reductive species (such as hydrated electrons), high-energy UV/VUV irradiation, plasma, or catalytic pathways [58].

Metal oxides such as TiO₂, In₂O₃, and Ga₂O₃ have been widely tested for PFAS degradation, though their efficiencies vary significantly [59]. TiO₂, despite being the most traditional photocatalyst since its first use in water splitting in 1972, shows very limited activity toward PFAS due to its wide bandgap (~3.0 eV), low adsorption capacity, and fast electron–hole recombination [60]. To improve its performance, different modification strategies have been explored: metal doping (Fe, Cu, Pb, Pt, Pd, Ag) [61], non-metal doping [62], carbon loading (e.g., graphene, CNTs, activated carbon) [63,64], and heterojunction formation. Notably, BN/TiO₂ heterojunctions degraded PFOA 15 times faster than pristine TiO₂ [65], while Sb₂O₃–TiO₂ composites improved UV absorption and provided more active sites [66].

Although TiO₂ is the most widely studied photocatalyst due to its stability, availability, and strong oxidative potential, its efficiency in PFAS degradation is often modest due to its limited absorption in the UV-A range and the tendency of PFAS to adsorb poorly on its surface [67]. Moreover, recombination of photogenerated charge carriers limits the generation of ROS. To overcome these limitations, researchers have explored doped or hybrid photocatalysts, including modified TiO₂, bismuth-based materials, graphitic carbon nitride (g-C₃N₄), and layered double hydroxides. These materials can extend light absorption into the visible range, improve charge separation, or promote alternative degradation pathways such as reductive defluorination [60,68].

In₂O₃, with a narrower bandgap of 2.8 eV and visible-light sensitivity, has demonstrated much higher efficiency, achieving an 8.4-fold increase in the degradation rate constant of PFOA compared to TiO₂ under UV light [69]. Surface modifications such as introducing oxygen vacancies and designing nanostructures (nanospheres, nanosheets, nanocubes) have further improved its activity [70,71], while composites like MnOx–In₂O₃ enhance solar light utilization and generate abundant active sites [72].

On the other hand, Ga₂O₃, although having a wide bandgap of 4.9 eV, has shown outstanding results in PMS-assisted photocatalytic processes: in a Ga₂O₃/PMS/UV system, 100% PFOA degradation was achieved within 60 min, mainly via SO₄•⁻ and •O₂⁻ radicals [73]. Structural engineering (nanoparticles, rods, needles, sheets) and metal doping (e.g., Sn or In) have further boosted its reactivity by promoting charge separation and strong PFAS binding [74]. Overall, while pristine TiO₂ is limited, engineered TiO₂ composites, oxygen-deficient In₂O₃, and PMS-assisted Ga₂O₃ systems highlight promising directions for developing efficient photocatalysts for PFAS degradation.

Beyond metal oxides, several alternative photocatalysts have been tested for PFAS degradation in photocatalytic AOPs, including transition-metal photocatalysts, composite materials, and even non-metal-based semiconductors. For example, Fe-zeolite under UVA light (320–420 nm) achieved PFOA mineralization by cycling Fe²⁺/Fe³⁺ in the presence of O₂, offering broader light absorption and higher stability than homogeneous Fe³⁺ [75]. Other materials have shown remarkable efficiency: samarium-doped ferrite degraded 48.6% of PFOA in 1 h [76], while platinum-modified indium oxide nanorods (Pt/IONRs) achieved 98% removal within the same timeframe, thanks to Pt loading, the nanorod structure, and abundant oxygen vacancies that enhanced light capture and charge separation [77]. Among non-metal alternatives, boron nitride (BN) has emerged as a promising candidate: with a wide bandgap of 6.0 eV, ball-milled BN exhibited a fourfold higher degradation rate of PFOA compared to commercial TiO₂ under UV irradiation. Its photocatalytic activity relies on hole-driven oxidation (h⁺), complemented by •O₂⁻ and •OH radicals [78].

Overall, these results highlight that combining structural engineering (oxygen vacancies, nanostructures), noble-metal doping, and carbon loading can drastically accelerate PFAS degradation, while non-metallic photocatalysts like BN represent a novel, highly efficient pathway. In practice, the photocatalytic degradation of PFAS is a slow process, and complete mineralization is rarely achieved. The reaction rates are often constrained by mass transfer limitations, photocatalyst surface fouling, and the low reactivity of ROS with the fluorinated tail. Nonetheless, photocatalysis holds promise as a sustainable and low-footprint technology, especially when integrated with solar light or coupled with adsorption to enhance pre-concentration of PFAS on the photocatalyst surface. Advancements in photocatalyst engineering and reactor design are expected to further enhance the viability of this approach.

4.3. Light-Activated Persulfate Oxidation

Light-activated persulfate oxidation leverages the photolytic activation of persulfate (S₂O₈²⁻) or peroxymonosulfate (HSO₅⁻) to generate sulfate radicals (SO₄·⁻), which are highly oxidative and capable of degrading a wide range of organic contaminants. Upon UV or visible light irradiation, these oxidants undergo homolytic cleavage to yield SO₄·⁻ and other reactive species, including hydroxyl radicals and singlet oxygen, depending on the matrix and conditions [79].

In the context of PFAS degradation, sulfate radicals have shown superior reactivity compared to hydroxyl radicals due to their higher redox potential and longer half-life in aqueous systems [80]. The efficacy of UV/persulfate systems has been demonstrated for several PFAS, particularly when supported by alkaline pH, which promotes the generation of reactive intermediates and facilitates defluorination. However, the degradation mechanisms are complex (Figure 3) and often involve multi-step oxidation of the functional group, followed by gradual cleavage of the fluorinated carbon backbone [81].

An important consideration is the selectivity of sulfate radicals, which tend to react preferentially with electron-rich sites. Since PFAS molecules are typically electron-deficient and highly stable, the initial activation step is often the rate-limiting step. Moreover, persulfate consumption in the presence of natural organic matter or competing solutes can reduce the efficiency and increase the cost of treatment. Nevertheless, light-activated persulfate oxidation remains a promising tool due to its chemical tunability and relatively high efficiency in breaking down otherwise recalcitrant compounds [51].

Studies performed by Wang et al., 2021 [81] showed, for the first time, how PFBA behaves under different advanced oxidation processes, including UV alone, persulfate, UV/catalyst, UV/persulfate, and UV/persulfate/catalyst systems. Among these, the UV/persulfate process stood out, achieving 88% decomposition and nearly 40% mineralization of PFBA within 5 h at room temperature. Mechanistic analysis using scavenger tests and EPR confirmed that both SO₄•⁻ and •OH radicals play central roles, with degradation proceeding through a stepwise loss of CF₂ groups from PFBA and its intermediates. The reaction followed pseudo-first-order kinetics, with rate constants increasing under higher persulfate doses and stronger irradiation. Interestingly, solution chemistry had a notable influence: acidic conditions enhanced degradation by generating more SO₄•⁻, while alkaline conditions boosted •OH formation; in contrast, Cl⁻ strongly inhibited PFBA breakdown, whereas carbonate, sulfate, fluoride, and humic acids had only minor effects.

The main strength of this study lies in demonstrating that UV/PS can be a highly effective and relatively fast technology for short-chain PFAS removal under mild conditions. On the downside, while degradation efficiency is promising, the study raises an important concern: the toxicity and persistence of intermediate products remain poorly understood. Compared with other PFAS treatment methods, UV/persulfate shows clear advantages in reactivity and simplicity, but its long-term environmental safety still depends on fully clarifying the fate of the breakdown products.

4.4. Photo-Fenton and Modified Fenton Systems

The photo-Fenton process is a powerful variant of classical Fenton chemistry in which iron ions catalyze the decomposition of hydrogen peroxide to produce hydroxyl radicals, with the added contribution of light to accelerate iron cycling and increase radical yields. In the presence of UV or visible irradiation, Fe³⁺ is reduced back to Fe²⁺ much more rapidly, and additional reactive oxygen species, such as singlet oxygen or hydroperoxyl radicals, are also generated [82] (Figure 4). For PFAS degradation, this enhanced radical environment is particularly relevant because hydroxyl radicals alone are often insufficient to cleave the C–F bond efficiently, but the constant regeneration of Fe²⁺ and the contribution of other reactive pathways can lead to significant defluorination [10,11].

Unlike direct photolysis, which requires the pollutant itself to absorb photons at specific wavelengths, the photo-Fenton process relies on light–catalyst interactions, which broadens its applicability and makes solar radiation or moderate UV lamps viable energy sources. The fact that radicals are produced in the bulk solution, rather than only on a photocatalyst surface, ensures intimate contact between oxidants and PFAS molecules, giving this process an edge over heterogeneous photocatalysis in terms of reaction homogeneity and radical density [84]. Nevertheless, the classical limitation of the Fenton process (its optimal operation at acidic pH around 2.8) still constrains its direct application to natural waters, where neutral or slightly alkaline conditions prevail. This pH dependency remains the most significant hurdle for large-scale deployment, but within controlled laboratory or pilot systems, photo-Fenton has proven capable of producing faster and deeper degradation of PFAS than many other advanced oxidation techniques [85].

Some studies, such as Tang et al. 2012 [86], demonstrated that PFOA can be effectively degraded via UV-Fenton processes, where •OH was identified as the dominant oxidizing agent. Similarly, Barışçı and Suri, 2021 [87] showed that solar-driven •OH radicals could break down fluorotelomers, which are often used as PFAS substitutes. Olatunde et al., 2020 [88] argued that •OH alone is not efficient against PFAS, since these compounds lack the necessary reactive sites for attack. Supporting this, Mitchell et al., 2013 [89] compared several oxidants in catalyzed H₂O₂ propagation (CHP) reactions and found that while PFOA degradation reached 89% within 150 min when •OH, O₂•⁻, and HOO• worked together, •OH alone showed negligible reactivity. In contrast, O₂•⁻ and HOO• individually promoted 68% and 80% degradation, respectively, reinforcing the idea that •OH is not the sole driver in PFAS breakdown. From our perspective, these conflicting results highlight that hydroxyl radicals may play only a secondary role, with superoxide and hydroperoxide species likely being more effective in attacking the C–F bonds.

An interesting development in this field is anodic Fenton treatment (AFT), which combines classical Fenton chemistry (CFT) with electrochemical Fenton processes (EFT) [90]. Compared to CFT, AFT overcomes several limitations. For example, while CFT suffers from rapid conversion of reactive Fe²⁺ into less active Fe³⁺, AFT uses applied current to continuously regenerate Fe²⁺, maintaining high catalytic activity. Unlike CFT, which requires external H₂O₂ addition (a safety and logistical challenge), AFT produces H₂O₂ in situ through cathodic oxygen reduction, improving both safety and efficiency. Another advantage is reduced sludge generation: CFT often leads to large amounts of ferric sludge, but AFT minimizes this by keeping Fe²⁺ in circulation. In our opinion, these improvements make AFT a more sustainable and controllable oxidation process [91].

Still, while AFT has proven effective for pesticides and other organic pollutants, it has not yet been widely applied to PFAS degradation. A recent study from Yousefi et al., 2025 [92] explored a hybrid approach integrating cathodic electro-Fenton, anodic oxidation with a BDD anode, and membrane distillation (MD) to enhance PFOA degradation while reducing energy consumption. Results showed that increasing current density from 50 to 500 A/m² markedly boosted H₂O₂ production, from 0.25 mM to 2.3 mM, which in turn accelerated PFOA degradation and mineralization. At the lower current of 50 A/m², the EF/BDD system alone failed to mineralize PFOA, whereas the EF/BDD-MD configuration achieved 45% mineralization, attributed to higher PFOA concentration in the electrolytic cell. Under a higher current of 500 A/m², the hybrid EF/BDD-MD process reached 95% mineralization.

Analysis of the degradation pathway indicated that although EF-generated •OH radicals contributed to PFOA breakdown, the BDD anode was the dominant factor, responsible for approximately 80% of the reaction. Degradation began with direct electron transfer at the BDD surface, followed by further enhancement from both homogeneous and heterogeneous •OH radicals. Overall, this hybrid system not only improved degradation efficiency but also reduced energy requirements, demonstrating potential for scalable PFOA treatment.

To improve the effectiveness of the photo-Fenton approach for PFAS degradation, several modifications have been proposed. These include operating at circumneutral pH using chelating agents to stabilize iron, incorporating alternative radical initiators, or combining the Fenton reaction with persulfate activation [51]. Additionally, recent studies have explored the role of UV-activated ferric complexes in producing hydrated electrons under specific conditions, offering a potential reductive pathway for PFAS cleavage [93]. Such modifications may enable a shift from purely oxidative to mixed oxidative-reductive systems, enhancing the degradation potential for PFAS. Despite these innovations, the application of photo-Fenton processes to PFAS remains challenging due to competing side reactions, iron precipitation, and the need for careful control of pH and reagent dosing. However, the possibility of utilizing solar irradiation and recyclable iron catalysts provides an attractive avenue for sustainable treatment in decentralized settings or in regions with high solar availability.

4.5. Use of Persulfate Activation Under Light Irradiation (UV/PS, Visible Light/PS)

Persulfate activation under irradiation, often referred to as photo-activated persulfate systems, extends the principles of the photo-Fenton process by replacing hydrogen peroxide with persulfate salts such as peroxymonosulfate (PMS) or peroxydisulfate (PDS). Upon exposure to UV or visible light, these persulfates are activated to generate sulfate radicals (SO₄•⁻), which possess higher redox potential and greater selectivity than hydroxyl radicals [94], making them particularly effective against recalcitrant contaminants like PFAS [80]. One advantage of persulfate activation is its flexibility across the light spectrum: while UV irradiation efficiently cleaves the O–O bond in persulfates, visible-light activation becomes feasible when transition metals, photocatalysts, or dopants are introduced, enabling the use of lower-energy light sources or even sunlight [12]. Compared to photo-Fenton, photo-activated persulfate processes are less dependent on stringent pH conditions and can operate effectively in a wider range of aqueous environments, which makes them more adaptable to real-world water matrices [95]. Studies have shown that UV/PS systems can achieve substantial degradation of PFOA and PFOS [51,96], with fluoride ion release confirming partial mineralization, and that visible-light/PS systems, when coupled with tailored photocatalysts, can extend applicability to solar-driven treatment [97]. However, persulfate-based systems also face challenges, such as the potential generation of sulfate-rich by-products, the need for persulfate dosing, and sometimes slower reaction kinetics compared to photo-Fenton under optimal acidic conditions. Despite these drawbacks, persulfate activation offers a promising complementary pathway by diversifying the radical spectrum available for PFAS degradation, introducing sulfate radicals that may follow different degradation routes than hydroxyl radicals and thereby improving overall efficiency.

4.6. General Considerations and Mechanistic Insights

A common feature across photoassisted processes is the reliance on highly reactive species generated through light-driven reactions. The nature, lifetime, and reactivity of these species, whether oxidative (e.g., hydroxyl and sulfate radicals) or reductive (e.g., hydrated electrons, superoxide), ultimately determine the efficiency of PFAS degradation [98]. The reaction mechanisms vary depending on the type of PFAS, with sulfonates and carboxylates displaying different susceptibilities due to their differing electron distributions and molecular orbital configurations.

In most cases, degradation begins with the cleavage or transformation of the polar functional group, which may expose the fluorinated tail to further attack. Complete defluorination is rare and typically requires sustained treatment and high-energy inputs. Intermediates formed during the process may include shorter-chain perfluorinated acids or perfluoroalkyl alcohols, some of which retain environmental concern [99]. Therefore, comprehensive monitoring of transformation products and fluoride release is essential for evaluating the effectiveness and safety of photoassisted PFAS treatment.

Furthermore, the efficiency of these processes is significantly influenced by water matrix effects, including the presence of natural organic matter, bicarbonates, halides, and co-contaminants. These constituents can quench reactive species or compete with PFAS for reaction sites, reducing overall performance. Additionally, the choice of light source, its wavelength, intensity, and penetration plays a central role in determining reactor design and energy requirements [12].

In conclusion, photoassisted processes represent a diverse and promising toolkit for addressing the unique challenges posed by PFAS contamination. Each technique presents distinct advantages and limitations, shaped by fundamental photochemical principles, photocatalyst design, and environmental conditions. While no single approach has yet emerged as a universally applicable solution, the combination of light-based activation with tailored catalytic and chemical systems offers a versatile platform for achieving PFAS degradation under increasingly sustainable and practical conditions. The continued refinement of these processes, alongside mechanistic studies and pilot-scale validation, will be essential in translating laboratory successes into real-world water treatment applications.

4.7. Advantages, Limitations, and Recent Advances in Photocatalyst Design

The major strength of photo-Fenton and persulfate-based processes lies in their high radical generation rates, which translate into accelerated PFAS degradation compared to photolysis or conventional photocatalysis [100]. Their ability to exploit sunlight or moderate UV sources makes them intrinsically more energy-efficient and potentially scalable, especially in regions with abundant solar radiation. In terms of mineralization, both processes tend to produce higher defluorination yields than photocatalysis under comparable conditions, indicating that their radical-driven mechanisms are more effective at breaking down perfluoroalkyl chains [10]. Photocatalyst design has advanced considerably in recent years to overcome the operational hurdles that once limited these processes. For the photo-Fenton system, heterogeneous catalysts incorporating iron into clays, carbons, or biochars have been developed to reduce sludge formation and expand the applicable pH window closer to neutral [101]. Similarly, the use of iron chelating agents allows stable iron cycling without precipitation, maintaining high activity even outside acidic ranges. In persulfate systems, photocatalysts such as doped TiO₂, g-C₃N₄, or metal–organic frameworks have been employed to accelerate persulfate activation under visible light, thereby reducing energy requirements and enabling solar-based treatment schemes [102]. The immobilization of catalysts on membranes or fixed supports also represents a major advance, making continuous-flow operation more practical and reducing secondary contamination risks.

Nevertheless, limitations remain that temper the enthusiasm for immediate large-scale implementation. The acidic pH requirement of classical photo-Fenton remains a fundamental drawback unless alternative catalyst designs fully mitigate it. Both photo-Fenton and persulfate activation require chemical inputs (hydrogen peroxide or persulfate salts) that increase operating costs and introduce concerns about sustainability, particularly in large-scale water treatment. Moreover, while fluoride release indicates defluorination, complete mineralization is rarely achieved, and transformation products must be carefully monitored to ensure that partial breakdown does not result in more mobile or toxic species [103]. Despite these caveats, the trajectory of research in photocatalyst modification and hybrid system integration suggests that many of these limitations are surmountable. From a comparative perspective, photo-Fenton and persulfate-based systems currently stand out among photoassisted technologies for their radical productivity, scalability under solar light, and adaptability through photocatalyst design. If advances in heterogeneous catalysis and process integration continue at the current pace, these approaches could transition from promising laboratory studies to genuinely competitive candidates for real-world PFAS remediation.

5. Advanced Photocatalytic Processes

5.1. Fundamentals and Mechanistic Pathways

While direct photolysis relies exclusively on photon absorption by PFAS molecules, photocatalysis introduces a solid material that mediates the interaction between light and contaminant, broadening the spectrum of useful irradiation and amplifying degradation through radical-mediated pathways [104]. When irradiated with photons of sufficient energy, photocatalysts such as TiO₂, ZnO, Bi-based oxides, or carbon-modified semiconductors generate electron–hole pairs at their surface. These charge carriers can either react directly with PFAS molecules adsorbed at the interface or drive the production of secondary reactive species such as hydroxyl radicals, superoxide anions, or solvated electrons, all of which are known to attack C–F and C–C bonds [105,106].

Unlike direct photolysis, where the process is limited by the narrow absorption of PFAS in the far-UV, photocatalysis can utilize near-UV and even visible light depending on the bandgap and modifications of the photocatalyst [12,67]. This extension of usable light frequencies makes the method more versatile and potentially compatible with solar irradiation, reducing dependence on energy-intensive VUV lamps. Mechanistically, the debate remains whether PFAS degradation in photocatalysis occurs predominantly via radical attack or via electron transfer at the surface [107].

Evidence suggests that while hydroxyl radicals contribute to side reactions and partial defluorination, hydrated electrons and conduction-band electrons are more effective in inducing reductive cleavage of C–F bonds, especially in perfluorocarboxylates [13]. This duality in oxidative and reductive pathways adds complexity but also flexibility, as photocatalysts and reaction conditions can be tailored to favor one pathway over the other depending on the PFAS target. Compared to photolysis, photocatalysis introduces more variables (surface properties, charge separation efficiency, photocatalyst stability) but it also offers greater tunability, which is a powerful advantage for designing targeted treatment strategies.

5.2. Photocatalyst Systems and Performance Comparisons

Recent developments in PFAS photocatalysis have shifted from traditional TiO₂-based systems toward more complex, tunable materials capable of harvesting a broader light spectrum and enhancing charge separation [67]. Graphitic carbon nitride (g-C₃N₄) has emerged as a particularly promising material due to its visible-light activity, chemical stability, and ability to form heterojunctions with other semiconductors, which improves electron–hole separation and prolongs the lifetime of reactive species [108,109]. Doped metal oxides, such as N-, S-, or F-doped TiO₂ [61] or BiOCl variants [110], have similarly demonstrated enhanced absorption in the near-UV or visible range, boosting degradation kinetics while maintaining structural stability [111].

More recently, two-dimensional materials like Mxenes [112] and layered double hydroxides [113] have shown synergistic effects when combined with conventional semiconductors, offering high surface area, improved adsorption of PFAS, and rapid electron transfer at the interface. Beyond the material itself, photocatalyst immobilization and reactor configuration have become critical considerations for real-world implementation [114]. Powdered catalysts, while effective in laboratory batch studies, suffer from recovery and aggregation issues, prompting the exploration of immobilized systems, such as coatings on glass, ceramic, or carbon supports, or incorporation into membrane reactors [99].

These strategies maintain high photocatalytic activity while facilitating continuous operation and reducing operational complexity. Importantly, the combination of hybrid materials and intelligent reactor design allows for tunable residence times, optimal light penetration, and targeted PFAS adsorption, making these systems far more adaptable and potentially scalable than photolysis alone. While the development of novel photocatalysts adds complexity and cost, the potential for solar-driven operation, higher selectivity, and integration into continuous-flow reactors presents a clear advantage for future applications.

5.3. Advantages, Limitations, Perspectives and Future Directions in Advanced Photolysis of PFAS

The versatility of photocatalysis is further enhanced when combined with complementary water treatment processes, producing synergistic effects that can overcome the limitations of single-stage degradation. Adsorption-photocatalysis hybrid systems, for instance, utilize high-affinity adsorbents to concentrate PFAS molecules near reactive sites on the photocatalyst, increasing local pollutant concentration and facilitating faster degradation [115]. Similarly, coupling photocatalysis with electrochemical systems can supply additional electrons or generate reactive oxygen species in situ, enhancing the reductive or oxidative pathways depending on the PFAS species [13]. Ozonation, when combined with photocatalysis, also produces a richer mix of reactive radicals and increases mineralization efficiency, often leading to higher defluorination rates than either process alone [67,85].

These synergistic integrations, while operationally more complex, highlight one of the strongest advantages of photocatalysis: its adaptability. Unlike photolysis, which is limited by PFAS-specific absorption and light source intensity, photocatalysis can be tailored to various operational scenarios, combining material engineering, reactor optimization, and hybridization strategies to maximize both energy efficiency and pollutant removal. Critically, these hybrid approaches provide a pathway to tackle mixed PFAS contamination and real-world water matrices, addressing practical challenges that photolysis struggles to overcome. While further research is needed to quantify energy consumption, scale-up feasibility, and by-product formation in these systems, the evidence suggests that integrated photocatalytic platforms represent a highly promising direction for sustainable, high-efficiency PFAS remediation.

The future of PFAS photocatalysis lies not only in material development but also in system integration. Rational design of photocatalysts with engineered band structures, optimized surface chemistry, and improved resistance to deactivation will be central to making photocatalysis competitive with other advanced oxidation processes [12]. At the same time, coupling photocatalysis with adsorption stages, where PFAS are concentrated on photocatalyst surfaces prior to degradation, or with electrochemical assistance to boost electron availability, may provide synergistic effects that overcome the current bottlenecks of incomplete mineralization [116].

In comparison with photolysis, which may remain a niche technology for polishing low-concentration PFAS, photocatalysis has the potential to tackle a broader range of scenarios, from moderately contaminated groundwater to industrial wastewater. The emerging consensus is that no single technology will be universally applicable for PFAS removal, but photocatalysis distinguishes itself as one of the most adaptable and future-oriented approaches [106]. Its combination of tunable material properties, compatibility with renewable light sources, and potential for integration into hybrid systems gives it a distinct edge in the ongoing search for sustainable solutions. While direct photolysis offers a useful benchmark and an elegantly simple proof of concept, it is photocatalysis that embodies the translational potential necessary for real-world applications, especially if ongoing advances succeed in pushing degradation efficiencies closer to complete defluorination.

6. Recent Trends and Emerging Materials

6.1. Metal–Organic Frameworks (MOFs) and Covalent Organic Frameworks (COFs) in Photoassisted PFAS Removal

Metal–organic frameworks (MOFs) and covalent organic frameworks (COFs) have rapidly emerged as frontier materials in the field of photoassisted PFAS degradation due to their extraordinary tunability, high surface areas, and capacity for rational design [107,117]. Unlike conventional semiconductors, these frameworks allow precise engineering of pore size, chemical functionality, and bandgap, enabling selective adsorption of PFAS molecules in close proximity to photoactive sites [118,119].

MOFs, especially those incorporating Ti, Zr, Fe, Ru or Au centers (Figure 5), have demonstrated strong photocatalytic activity under UV and visible irradiation [107,120], while COFs, composed of entirely organic linkers, offer high stability, light-harvesting ability, and enhanced charge separation [121]. Their modular structures permit the incorporation of functional groups tailored for both hydrophobic PFAS backbones and polar head groups, resulting in dual functionality: enrichment of PFAS at the photocatalyst surface followed by accelerated degradation [122].

Compared to classical photocatalysts such as TiO₂, MOFs and COFs stand out for their flexibility and adaptability; however, challenges persist, including hydrolytic instability under aqueous conditions, possible leaching of metal ions, and high synthesis costs. Despite these drawbacks, the design freedom inherent to MOFs and COFs positions them as highly promising candidates for the next generation of PFAS treatment technologies, particularly if scalable synthesis and stability issues can be addressed.

Even though research on MOF-based photocatalysts for PFAS treatment is still emerging, recent work by Hou et al., 2021 [124] demonstrated the potential of lignin-based nanofiber composite membranes (lignin/PVA/bi-MOFs) incorporating bimetallic Co/Fe MOFs for PFOA degradation using peroxymonosulfate under sunlight. The degradation involved a combination of photocatalysis and advanced oxidation processes. The Co/Fe MOFs enhanced PFOA removal through adsorption, charge carrier generation, and formation of reactive intermediates upon photoactivation.

Under sunlight, about 90% of PFOA was degraded within 3 h, while complete degradation was achieved using 9 W UV light at 185 nm positioned 5 cm above the solution. Corresponding defluorination reached approximately 50%, indicating conversion to fluoride ions. The gap between overall PFOA degradation and fluoride release was attributed to adsorption of fluoride on the photocatalyst and the formation of shorter-chain perfluorocarboxylates, which may still pose environmental concerns. Overall, this study highlighted that combining the strong adsorption capabilities of MOFs with photocatalytic activity makes them promising candidates for PFAS remediation.

Another study performed by Wen et al., 2025 [107] evaluated the photocatalytic degradation and defluorination of PFOA using the titanium-based MOF MIL-177-HT. The material achieved approximately 83% PFOA removal and 32% defluorination over 24 h. The findings indicate that, beyond photocatalytic activity, other MOF characteristics significantly influence degradation efficiency. A comparison with various MOFs reported in the literature suggests that factors such as ligand hydrophobicity, structural dimensionality, and bandgap play key roles in controlling both adsorption and photocatalytic breakdown of PFOA.

Additionally, a Z-scheme Fe-BTC/BiOCl heterostructure was engineered by Yan et al., 2025 [111], taking advantage of its 3D nanostructure, strong electronic interactions, and interlaced energy band configuration, which promoted efficient charge-carrier separation and created abundant oxygen vacancies. When irradiated with an LED lamp, this heterostructure achieved 98.7% PFOA degradation within just 30 min. Mechanistic studies using electron paramagnetic resonance and scavenger experiments identified ROS and photogenerated electrons/hydrogen atoms as the main contributors to the high degradation efficiency. Further analysis of intermediates, supported by density functional theory calculations, indicated that PFOA degradation occurs via preferential cleavage of the carboxyl group, followed by a repeated unzipping process involving removal of −CF₂H groups and defluorination through C–C and C–F bond cleavage.

These studies collectively illustrate that MOF-based and MOF-derived photocatalysts are increasingly promising for PFAS remediation, yet their performance and mechanisms vary widely. Lignin/PVA/Co-Fe MOFs achieved rapid PFOA degradation, but defluorination was limited and the formation of short-chain perfluorocarboxylates raises lingering environmental concerns. MIL-177-HT, a titanium-based MOF, also showed respectable degradation but slower kinetics, highlighting how MOF structural features (ligand hydrophobicity, dimensionality, and bandgap) affect performance. The Z-scheme Fe-BTC/BiOCl heterostructure demonstrated the fastest and most efficient degradation by leveraging 3D nanostructures, oxygen vacancies, and strong charge separation, with reactive oxygen species and photogenerated electrons driving the process through stepwise C–C and C–F bond cleavage. Indirectly, these results suggest that while MOF-based systems hold high potential, gaps remain in understanding the factors controlling defluorination efficiency, the formation and fate of shorter-chain PFAS, and scaling these materials under sunlight or practical water conditions. The diversity of degradation rates and defluorination outcomes also indicates that photocatalyst design requires careful optimization of structural and electronic properties to balance rapid degradation with complete mineralization.

6.2. Carbon-Based Photocatalysts: Graphene Derivatives and Biochar-Based Materials

Carbon-based materials represent another significant trend in photoassisted PFAS remediation, combining low cost, environmental compatibility, and excellent electronic properties [125]. Graphene derivatives, such as graphene oxide (GO) and reduced graphene oxide (rGO), have proven effective as photocatalyst supports and electron conductors, facilitating rapid charge transport and minimizing electron–hole recombination. Their two-dimensional structure allows intimate interactions with PFAS molecules, while surface functional groups can be tuned to enhance adsorption and catalytic reactivity [126]. In parallel, biochar-derived photocatalysts have gained traction as sustainable, low-cost alternatives that harness agricultural or algal biomass to create porous, carbon-rich structures [127] capable of both adsorbing PFAS [128] and serving as photocatalyst platforms [129]. When doped with heteroatoms (N, S, P) or impregnated with metal nanoparticles, biochar-based materials exhibit enhanced light absorption and radical generation, positioning them as a bridge between green chemistry and advanced photocatalysis [130]. Compared with MOFs and COFs, carbon-based photocatalysts offer superior stability in aqueous environments and lower synthesis barriers, though they sometimes lack the precise structural control or light-harvesting efficiency of framework materials [131]. Their key advantage lies in their scalability and compatibility with circular economy concepts, which may become decisive factors for real-world applications.

Pervez et al., 2024 [126] evaluated different GO-based adsorbents, and between them, the material functionalized with CTAC displayed a markedly superior performance in capturing PFAS from aqueous systems. Structural and surface analyses confirmed that the modification was achieved without compromising the intrinsic characteristics of graphene oxide, while imparting a strong positive charge that appeared to drive the interaction with negatively charged PFAS molecules. The enhanced surface charge played a decisive role in explaining the high adsorption efficiency observed, particularly in controlled water matrices where near-complete removal could be achieved within a short contact time. Interestingly, this efficiency remained largely stable under variable environmental conditions, including fluctuations in pH, natural organic matter content, and ionic strength, suggesting that the adsorbent is resilient to the typical interferences present in natural waters.

Trials conducted for these authors with river water supported this robustness, showing nearly complete removal of a broad set of PFAS within a few hours, with the exception of shorter-chain compounds such as PFBA, which are generally more challenging to capture. Adsorption equilibrium analysis aligned well with a Sips model, with a relatively high maximum capacity, reinforcing the dual contribution of electrostatic attraction and hydrophobic interactions as the governing mechanisms. Taken together, these outcomes position the GO-CTAC system as a rapid and adaptable candidate material for PFAS removal, with potential to be scaled for practical applications in complex water environments.

The application of TiO₂/GO/PVA nanocomposite films has demonstrated remarkable promise for the degradation of persistent PFAS such as PFOS and PFOA under visible light. Among the different formulations, the composite incorporating an optimized fraction of TiO₂/GO and subjected to controlled thermal treatment achieved degradation efficiencies approaching complete removal, highlighting how synthesis parameters strongly influence photocatalytic performance, as reported by Boonchata et al., 2024 [114]. The interplay between graphene oxide content, heat-treatment conditions, and operational pH emerged as decisive factors, with the latter exerting a particularly strong impact on the rate of photodegradation and its variability across different PFAS species.

Such sensitivity to preparation and reaction conditions reinforces the need for fine-tuning in order to exploit the synergistic interaction between TiO₂ and GO, which facilitates charge separation and extends light absorption into the visible range. The results presented by these authors not only corroborate earlier evidence on the potential of hybrid photocatalysts but also underline the advantages of a relatively simple and low-cost fabrication route. In practical terms, this positions TiO₂/GO/PVA films as attractive candidates for wastewater treatment strategies, particularly in contexts where accessibility and affordability are critical, offering a sustainable pathway toward mitigating the environmental burden of PFAS.

6.3. Engineered Nanocomposites for Enhanced PFAS Degradation

A third major frontier involves the design of engineered nanocomposites that integrate multiple functional materials into hybrid systems with complementary properties. By coupling semiconductors with conductive carbons, plasmonic metals, or even MOFs/COFs, researchers have created photocatalysts capable of harvesting broader light spectra, accelerating charge separation, and providing active sites tailored for PFAS degradation [132]. For example, TiO₂–graphene composites achieve both strong adsorption of PFAS through the graphene component and efficient electron-driven degradation via TiO₂ under UV light. Similarly, plasmonic nanocomposites using Ag or Au nanoparticles extend photocatalytic activity into the visible range through localized surface plasmon resonance effects, increasing light utilization efficiency [133].

Designs that are more complex integrate multiple functions, adsorption, photocatalysis, and even persulfate activation, within a single material, effectively creating multifunctional treatment platforms [134]. The primary advantage of engineered nanocomposites lies in their ability to overcome the limitations of single-component systems, particularly in terms of electron–hole recombination and limited light absorption. However, these systems raise concerns about cost, synthesis complexity, and potential nanoparticle release into treated water. Despite these concerns, nanocomposites represent a practical step toward balancing laboratory efficiency with field applicability, especially when immobilized on membranes or integrated into continuous-flow reactors.

Alternative treatment strategies have explored coupling adsorption with advanced oxidation, such as the in situ formation of ferric nanoparticles followed by ozonation [135]. While this pathway enabled higher PFAS uptake than traditional sorbents, the overall removal efficiency remained moderate, highlighting the trade-off between capture capacity and destructive effectiveness. On the other hand, the use of titanate nanotubes has attracted growing interest because of their favourable photoelectronic behaviour. When combined with activated carbon and iron, these composites not only concentrated PFOA on their surface but also achieved substantial degradation under UV exposure (≥90% in 4 h, with ≥60% fully mineralised), with a significant fraction mineralized into fluoride [136]. A key advantage of this material lies in its ability to self-regenerate through photodegradation, maintaining efficiency across multiple treatment cycles without requiring additional reagents.

Other nanostructured catalysts, such as magnetite- and ferrite-based composites, provide further advantages due to their high surface area, structural robustness, and enhanced electron transfer, which are critical features for sustaining redox-driven PFAS degradation [137]. Building on this concept, modified titanate nanotube systems, including gallium-doped and carbon-functionalized variants, have demonstrated similar promise for degrading PFOS under photocatalytic conditions. Likewise, boron nitride has emerged as a highly active alternative to conventional TiO₂, showing superior degradation efficiency across different PFAS, including newer-generation compounds, and retaining performance over multiple cycles [78].

Beyond degradation, nanoscale technologies are also contributing to PFAS detection in complex matrices. For instance, the integration of COFs with lanthanide-doped nanocrystals has enabled ultra-sensitive PFOS sensing at picomolar levels in tap water [138], illustrating how nanoscale materials can extend applications beyond treatment toward monitoring and risk management. Taken together, these developments underscore the versatility of nanomaterials in PFAS remediation, where they serve not only as powerful adsorbents and photocatalysts but also as enabling components for next-generation sensing systems.

6.4. Perspectives on Emerging Materials

Taken together, MOFs/COFs, carbon-based materials, and engineered nanocomposites illustrate a clear trend in PFAS photocatalysis: the field is moving away from reliance on classical semiconductors toward highly engineered, multifunctional, and often sustainable materials. The development of nano-enabled approaches is opening new possibilities for PFAS treatment by combining high surface reactivity with tunable selectivity, features that are difficult to achieve using bulk materials. Their exceptionally large surface area facilitates strong sorption and rapid interactions, while surface functionalization can be tailored to promote either enhanced capture or catalytic degradation [74].

Yet, strategies based solely on physical removal inevitably generate contaminated residuals, which highlights the need for integrated concepts capable of not only concentrating PFAS but also destroying them in situ or through subsequent processes. Promising directions include the use of engineered nanostructures that simultaneously adsorb and drive degradation at or near the surface, or hybrid systems that enable capture followed by controlled release into destructive treatments. Mixed-metal catalysts incorporating iron, for example, can support photo-Fenton-like pathways that mineralize PFAS, although the requirement for strongly acidic conditions and significant energy inputs remains a challenge. Advances in catalyst surface modification and immobilization are helping mitigate these drawbacks, broadening operational windows to more neutral pH values and making the systems more compatible with realistic water matrices.

Equally important is the distinction between high removal efficiencies reported under idealized laboratory conditions and the performance observed in real waters. Many studies have relied on PFAS-spiked ultrapure water, whereas natural matrices with competing solutes, natural organic matter, and fluctuating ionic strength demand longer treatment times and shorten electrode lifetimes. Moreover, assessing mineralization rather than apparent removal is essential, as partial degradation may generate transformation products with unknown toxicity. Comprehensive analysis of intermediates and pathways is thus required to ensure that nano-enhanced technologies provide not only efficient PFAS elimination but also safe and environmentally sound outcomes. Overall, while the potential of these emerging approaches is clear, translating them into sustainable large-scale applications will require balancing energy efficiency, cost, and robustness with the assurance of complete and safe PFAS destruction.

MOFs and COFs provide unmatched design freedom, carbon-based materials deliver scalability and eco-friendliness, and nanocomposites offer synergistic performance that leverages the best features of each component. In comparing them, one sees that the greatest advantage of MOFs/COFs is their tunability, the strength of carbon-based catalysts lies in their practicality and green credentials, and nanocomposites excel in integrating multiple functionalities into robust platforms. The disadvantages (instability of MOFs/COFs, lower light utilization of carbon-based systems, and cost or environmental concerns for nanocomposites) should not overshadow the fact that these emerging materials are collectively advancing the field toward higher efficiency, greater selectivity, and improved feasibility for real-world deployment. In my view, the future of PFAS remediation will likely not rest on a single “ideal” material but on hybrid systems that combine the structural precision of frameworks, the sustainability of carbons, and the multifunctionality of nanocomposites into a single, scalable technology.

Recent advances have highlighted the role of single-atom catalysts (SACs) in enabling efficient PFAS destruction. For instance, platinum atoms dispersed on a silicon carbide support (Pt1/SiC) demonstrated exceptional activity in breaking C–F bonds during the hydrodefluorination of PFOA [139]. This performance was attributed to hydrogen “spillover” from isolated Pt sites, with the generated Si–H species facilitating redistribution toward C–F cleavage and subsequent covalent fluorine attachment to the substrate. Among current photocatalytic strategies, Pt1/SiC has been identified as one of the most energy-efficient technologies for PFOA degradation. Other SAC systems have also shown promise, such as facet-engineered TiO₂ modified with atomically dispersed Pt, which achieved degradation rates 15 times higher than comparable Pt nanoparticles and significantly outperformed unmodified TiO₂ [140]. Similarly, electrocatalytic oxidation of PFOA was enhanced using a Ti₄O₇ electrode fluorinated at the surface and decorated with amorphous palladium clusters [141]. This material enabled strong binding of PFOA and its intermediates while promoting electron transfer, leading to efficient mineralization without reliance on reactive oxygen species, thus avoiding inhibition by common anions in natural waters. Despite their potential, SAC-based technologies remain at an early development stage and are more applicable to concentrated waste streams at present.

Beyond SACs, nano-based photocatalysts continue to attract attention. Titanium dioxide, gallium oxide (Ga₂O₃), and indium oxide (In₂O₃) nanoparticles have each been investigated under UV light, with In₂O₃ nanostructures showing the greatest promise due to their large surface area and abundance of photogenerated surface holes [142]. Environmental conditions, however, were shown to influence degradation outcomes. Complementary studies [124,143] reported enhanced degradation of PFOA using combined systems such as TiO₂ with peroxymonosulfate (PMS), as well as Ga₂O₃/UV catalysis assisted by PMS, both of which improved photocatalytic efficiency in water and wastewater matrices. Novel membrane-based systems are also emerging, including lignin/polyvinyl alcohol/bimetallic MOF nanofiber composites, which activate PMS under solar light to generate sulfate and hydroxyl radicals. These membranes achieved 89% removal of PFOA within 3 h, could be regenerated by simple rinsing, and were reusable across multiple cycles [124].

6.5. Challenges and Limitations for Industrial Implementation

Although photoassisted technologies have shown promising results at laboratory scale, their transition to pilot and industrial levels remains limited. Some pilot-scale studies have demonstrated the feasibility of photocatalytic reactors for treating real wastewater streams (

Table 2. Pilot-scale and commercial efforts in PFAS removal technologies.

Project/Study	Scale/Field Type	Key Findings	Limitation Noted/ Relevance to Scale-Up
Field demonstration: hybrid NF + UV-sulfite for PFAS-impacted groundwater (U.S. DoD) [9]	Pilot-scale, treating NF reject + UV-sulfite in the field	NF achieved ≥95% rejection of many PFAS; UV-sulfite destroyed >75% of PFAS mass in NF reject after 4 h, >90% after 8 h.	Energy costs high for short-chain PFAS; long residence times required; scalability of UV-sulfite under varying water chemistries to be further validated.
Pilot-scale application of novel BiPO4 photocatalyst (BOHP/UV) (Strategic Environmental Research and Development Program (SERDP); Environmental Security Technology Certification Program (ESTCP). Pilot Scale Assessment of a Deployable Photocatalytic Treatment System Modified with BiPO4 Catalyst Particles for PFAS Destruction in Investigation-Derived Wastewaters. SERDP-ESTCP. https://serdp-estcp.mil/projects/details/ca952465-ff77-48a7-8038-6274812e38ff. Accessed on 17 September 2025)	Deployment to treat investigation-derived wastewater (IDW); continuous operation, batch & pilot systems	>95% reduction in GenX and perfluorocarboxylic acids under certain conditions; good photocatalyst robustness over use; evidence of catalyst stability over ~191 h usage.	Reactor design still relies significantly on photolysis/VUV; optimizing the photocatalytic contribution needed; full energy and cost-viability at large scale yet to be demonstrated.
Pilot plant adsorption of PFAS in RO concentrate/potable reuse systems (The Water Research Foundation. Pilot-Scale Adsorption of PFAS in RO Concentrate from Potable Reuse Systems. Project 5323. https://www.waterrf.org/research/projects/pilot-scale-adsorption-pfas-ro-concentrate-potable-reuse-systems. Accessed on 17 September 2025)	Adsorption (GAC, ion-exchange, etc.) pilot scale in water-reuse context	Evaluating multiple adsorbent media; useful data on removal efficiencies, media life, cost trajectories.	Adsorption removes PFAS but does not degrade; requires handling of PFAS waste adsorbents; cost of regeneration/disposal.
Photon Water/Jihostroj in situ containerised system (Photon Water. Photon Water and Jihostroj Jointly Fight PFAS with 99.5% Effectiveness. Photon Water. https://www.photonwater.com/article/photon-water-and-jihostroj-jointly-fight-pfas-with-99-5-effectiveness. Accessed on 17 September 2025)	Industrial pilot/commercial deployment for electroplating and rainwater wastewater	Over 99.5% PFAS removal; ~70% water recycling; system modular (containerised) and in situ operation.	Details on long-term performance, maintenance, energy consumption not yet fully published; how homogeneous the PFAS mix in the influent, or how short-chain PFAS perform, is less clear.
Commercial process “PerfluorAd®” by Fraunhofer UMSICHT & Cornelsen Umwelttechnologie (Fraunhofer UMSICHT. PerfluorAd®-Process: Energy and Resource Efficient Method for the Removal of Per- and Polyfluorinated Chemicals (PFAS) from Extinguishing Waters. Fraunhofer UMSICHT. https://www.umsicht.fraunhofer.de/en/projects/pfas-perfluorad.html. Accessed on 17 September 2025)	Mobile pilot plant/mobile removal process for firefighting water & groundwater	Process uses an additive to precipitate PFAS (“microflakes”) followed by filtration; demonstrated in pilot scale; process claimed economic and practical for some contaminated waters.	Does not directly degrade PFAS; requires handling of precipitated material; performance under high load, continuous operation, and varied PFAS profiles needs further validation.

); however, these systems often face challenges related to light penetration, photocatalyst recovery, and energy efficiency. The scalability of photoassisted processes is strongly influenced by reactor design, since uniform irradiation and sufficient contact between the photocatalyst and contaminants are difficult to achieve in large volumes. Moreover, the high operational costs associated with artificial light sources, as well as the variability of solar irradiation in solar-driven systems, represent significant barriers to commercialization [8].

Several laboratories and firms have initiated exploratory efforts to move toward commercialization, particularly in the development of immobilized photocatalysts and solar photoreactors designed for continuous operation. Nevertheless, these technologies are often constrained by limited throughput, photocatalyst stability under real conditions, and fouling phenomena that reduce efficiency over time. Integration with existing water treatment infrastructure also requires addressing regulatory standards, long-term performance validation, and life cycle assessments to ensure economic viability.

Overall, while photoassisted technologies hold considerable potential for sustainable water treatment, the path to large-scale industrialization is still at an early stage. Future research must focus on bridging the gap between lab-scale efficacy and real-world conditions through improved reactor engineering, cost-effective photocatalyst production, hybrid system integration, and robust pilot demonstrations. Addressing these challenges will be crucial to transform photoassisted processes from promising academic concepts into practical, commercially viable solutions.

The energy demand of photoassisted technologies represents one of the most critical bottlenecks for their industrial application. Comparative analyses show that deep-UV and VUV photolysis, while highly effective for PFAS degradation, are associated with high energy per order (EE/O) values, often exceeding 1 kWh·m⁻³·order⁻¹ for long-chain compounds such as PFOA and PFOS, and increasing significantly for short-chain PFAS [10]. Photocatalytic processes using TiO₂ and related materials can reduce energy consumption by enhancing quantum efficiency, but their performance remains highly dependent on photocatalyst stability, immobilization strategies, and water matrix effects.

Coupled systems, such as UV/sulfite or UV/persulfate, improve degradation kinetics and defluorination yields, yet the additional cost of chemical inputs must be considered when evaluating overall energy balances. Electrochemical processes (e.g., BDD anodes) generally exhibit higher energy intensities than photocatalysis, but they can achieve higher mineralization and fluoride release, making them attractive in cases where complete PFAS destruction is required. Plasma-based treatments, while capable of rapid degradation, are among the most energy-intensive technologies reported, with EE/O values an order of magnitude higher than UV-based methods, limiting their scalability [10].

Despite the variability among studies, one common limitation is the lack of standardized reporting of energy metrics, which hampers direct comparison across technologies. Metrics such as EE/O (kWh·m⁻³·order⁻¹), energy per mole of fluoride released, or kWh per gram of PFAS degraded are not consistently provided in the literature, particularly for pilot-scale demonstrations. To facilitate meaningful comparison, it is crucial that future studies systematically report reactor power, treatment time, treated volume, and mineralization data, enabling the calculation of comparable energy indicators. Overall, current evidence suggests that while UV/sulfite and photocatalytic hybrid systems may offer the most favorable balance between efficiency and energy consumption, further optimization and standardized reporting are essential to guide technology selection and industrial implementation.

To provide a clearer comparative perspective, the different AOPs reviewed in this work are summarized in

Table 3. Comparative overview of AOPs for PFAS treatment, including mechanisms, conditions, advantages, limitations, efficiencies, and scalability considerations.

AOP/Variant	Mechanism/Reactive Species	Typical Conditions	Advantages	Limitations	Reported Removal/Defluorination	Energy/Cost	Scale
Direct photolysis (UV254/VUV)	Direct photolysis of functional groups; VUV generates e−aq and high-energy photons	UV254 LP/MP lamps; VUV 185 nm; variable pH	Simple setup; no additives required	UV254: poor mineralization of C–F; VUV: high energy demand; by-product formation	UV254: functional group degradation but negligible defluorination; VUV: up to 50–90% defluorination in lab tests	VUV is very energy-intensive (high EE/O); UV254 lower energy but less effective	Lab/pilot scale; VUV applied in some pilot demonstrations
UV + Sulfite (e−aq generation)	Generation of hydrated electrons (e−aq) reductively attacking C–F bonds	UV254–VUV + sulfite (mM); alkaline pH	Highly effective for C–F cleavage; works on long- and short-chain PFAS	Requires chemical dosing; scavenger sensitivity	High degradation and defluorination (>70–95% under optimal conditions)	Moderate–high energy demand; additional cost for sulfite	Lab/pilot studies; promising for RO/NF concentrates
Photocatalysis (TiO2, BiOCl, BiPO4, doped)	ROS generation (•OH, h+) on catalyst surface	TiO2 UV-activated (<385 nm); immobilized/suspension	Abundant materials; potential solar use; immobilization feasible	Mainly attacks polar groups; limited direct C–F cleavage; catalyst fouling	Functional group degradation; partial defluorination, variable results	Energy depends on UV source; catalyst stability issues	Lab/pilot scale; better in hybrid systems
Photo-Fenton/Fenton-like	H2O2 + Fe2+ (UV-assisted) → •OH	Acidic pH (~3); H2O2/Fe dosing	High •OH production; effective for co-contaminants	Acidic conditions; sludge generation; limited C–F attack	Good removal of head groups; limited PFAS mineralization	Moderate cost (chemicals + pH adjustment)	Lab/pilot; often combined with other AOPs
Persulfate activation (UV, heat, Fe, photocatalysis)	SO4•− radicals generated from persulfate	Persulfate (mM); activation by UV/heat/Fe	Strong oxidant; more selective than •OH in complex waters	Requires activation; variable PFAS effectiveness; by-products possible	Functional group degradation; partial defluorination reported	Chemical cost + activation energy	Lab/pilot; often used as pretreatment
Electrooxidation (BDD, PbO2 anodes)	Direct anodic oxidation; adsorbed •OH; high mineralization potential	BDD anodes, high conductivity electrolyte	High mineralization; fluoride release possible	High energy demand; electrode cost; by-products	High defluorination/mineralization in lab and some pilots	Significant electricity demand; electrode replacement costly	Pilot/commercial in specific cases; strong candidate for complete destruction
Plasma (DBD, gliding arc, corona)	Plasma discharge generates UV, radicals, electrons	Non-thermal plasma reactors; direct contact with water/air	Fast PFAS degradation; C–F cleavage possible	Extremely energy-intensive; scalability issues	High degradation and defluorination in lab tests	Very high EE/O; engineering complexity	Lab/pilot only; scale-up limited
Sonolysis (ultrasound)	Cavitation → localized heat, radicals, pressure	High-frequency ultrasound; kHz–MHz range	Cavitation effects may attack PFAS at interfaces	Low volumetric rates; high energy; matrix effects	Partial PFAS degradation in some studies	High energy per volume treated	Lab scale; often combined with other AOPs
Hybrid systems (UV + plasma, UV + electro, etc.)	Synergistic ROS + reductive species	Integrated setups	Synergies improve degradation and defluorination	Complex operation; high cost	Higher performance than individual processes	High energy demand; complexity	Lab/pilot; emerging
Adsorption (GAC, ion-exchange resins)	Capture, not destruction	GAC, IEX resins; modular units	Effective PFAS removal from effluents; widely applied	Does not degrade PFAS; waste management required	>90% removal from effluents, depending on PFAS type	Cost of media and regeneration/disposal	Full-scale widely used; often combined with destructive AOPs
Membrane + AOP trains (RO/NF + destruction of concentrate)	Separation + destruction of retentate	NF/RO concentration + AOP treatment	Reduces treated volume; practical strategy	Concentrate management challenging	High removal in permeate; variable AOP performance on concentrate	Combined costs (membranes + AOP)	Pilot/full-scale; increasingly practical strategy

. The table compiles their main mechanisms, operational conditions, advantages, limitations, reported PFAS removal or defluorination efficiencies, energy considerations, and scalability aspects. This integrative overview highlights the diversity of approaches investigated and facilitates the identification of strengths and gaps across technologies.

As shown in Table 3, while several AOPs demonstrate promising removal efficiencies at laboratory scale, their high energy demands, variable performance in real waters, and scalability challenges remain key obstacles for industrial implementation.

7. Conclusions

The field of photoassisted PFAS degradation has advanced considerably over the past two decades, moving from proof-of-concept studies on direct photolysis to increasingly sophisticated catalytic, hybrid, and material-based systems. These advances demonstrate that light-driven processes can indeed break the formidable C–F bond and achieve significant levels of defluorination, an achievement that until recently seemed nearly unattainable. Nevertheless, translating laboratory successes into reliable large-scale applications remains a formidable challenge. Technical barriers include the inherent need for high-energy photons in many systems, the limited penetration depth of UV light in real water matrices, and the difficulty of designing reactors that ensure uniform irradiation and efficient contact between PFAS and photocatalysts. Immobilization strategies, flow-through reactors, and solar-assisted operation are promising steps, but their optimization at pilot and full scale is still in its infancy.

Environmental safety constitutes a second critical consideration. While many studies report high PFAS removal percentages, far fewer investigate the identity, persistence, and toxicity of degradation by-products. Intermediates such as shorter-chain perfluorocarboxylic acids, perfluorosulfonates, or mixed fluorinated organics may form during incomplete defluorination, and their environmental fate remains insufficiently characterized. Without rigorous analysis of by-products, there is a risk of replacing one persistent pollutant with another. Future research must therefore couple degradation experiments with high-resolution analytical monitoring to ensure that photoassisted treatments do not inadvertently generate harmful secondary pollutants.

From an economic and energetic perspective, photoassisted PFAS removal also faces challenges. Processes driven by deep-UV or VUV light sources are often energy intensive and costly, limiting their attractiveness for municipal or industrial treatment unless coupled with renewable power or integrated into broader treatment trains. Photocatalytic systems, particularly those that utilize visible light or solar irradiation, show greater promise in terms of cost–benefit balance, especially if paired with inexpensive, stable, and scalable catalysts such as doped carbons or biochar-based composites. However, precise energy consumption data, technoeconomic analyses, and life cycle assessments are still scarce, hindering meaningful comparison across technologies.

Looking ahead, several research gaps and opportunities are evident. First, the design of photocatalysts and catalytic frameworks responsive to visible light remains a priority, as it would allow direct exploitation of sunlight and reduce dependence on artificial UV sources. Second, most work remains at the laboratory scale under idealized conditions; robust pilot-scale studies using real contaminated waters are urgently needed to validate efficiency and durability. Third, photoassisted processes should not be considered in isolation but rather as components of treatment trains, complementing adsorption, electrochemical, or biological approaches to achieve complete remediation in a cost-effective manner. Finally, interdisciplinary research that combines material science, reactor engineering, and environmental toxicology will be essential to ensure that these technologies not only degrade PFAS efficiently but also safeguard ecosystem and human health.

In conclusion, photoassisted strategies for PFAS removal represent one of the most dynamic and promising areas of environmental remediation research. While current limitations in scalability, energy demand, and by-product control cannot be ignored, the continuous development of novel materials, synergistic processes, and reactor configurations suggests that practical, safe, and economically viable solutions are within reach. To realize this potential, future efforts must shift from proof-of-principle demonstrations toward integrative, field-relevant, and sustainability-driven approaches, paving the way for photoassisted PFAS treatment to evolve from experimental curiosity into a cornerstone of next-generation water purification technologies.