Strategies for the Removal of Per- and Polyfluoroalkyl Substances: A Review

Abstract

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are a class of synthetic fluorine-containing organic compounds that exhibit chemical and thermal stability due to the highly stable carbon–fluorine bonds present in their molecular structures. This characteristic makes them slow to degrade in the natural environment. With the widespread application of these compounds in the industrial and consumer goods sectors, environmental media such as water, air, soil, and food have been severely polluted, posing a range of significant threats to public health. Therefore, the development of efficient, economical, and environmentally friendly PFAS removal technologies has become a current research hotspot. This review systematically summarizes the current technologies for removing PFASs from four perspectives—physical, chemical, biological, and combined treatments—enabling a clear understanding of the existing treatment strategies to be discussed. In addition, suggestions for future research on PFAS removal are provided.

1. Introduction

Perfluoroalkyl and polyfluoroalkyl substances (PFASs) are a class of synthetic fluorine-containing organic compounds. The hydrogen atoms on the carbon chain are partially or completely replaced by fluorine atoms to form strong carbon–fluorine bonds, which require high amounts of energy to break, giving them both chemical and thermal stability [1,2]. In addition, the low polarizability of fluorine atoms also imparts PFASs with hydrophobic and oleophobic properties [3]. These characteristics mean that PFASs are widely used in food packaging, pesticide formulations, surfactants, waterproof fabrics, carpets, non-stick cookware, fire foam, and other products [3,4,5]. Unfortunately, it is precisely because of these characteristics and the large-scale application of PFASs that leads to their accumulation at soil–water and air–water interfaces, causing widespread pollution in aquatic environments, soil, and air [6,7,8]. Notably, researchers have also found that PFASs can be rapidly transferred from contaminated soil to plants, and that plants can retain high concentrations of PFAS compounds, thereby introducing them into the human food chain [9,10,11]. Exposure to PFASs can also cause a range of health problems, including cancer, an impaired immune system, thyroid hormone disorders, and reproductive dysfunction [1,12]. The prevalence of PFASs poses a threat to the environment and public health, and the development of efficient and feasible PFAS removal methods has become an urgent need for environmental governance.

At present, the methods of removing PFASs are mainly divided into four categories: physical, chemical, biological, and combined treatments. Physical treatment focuses on the separation and removal of PFASs, transferring them from one medium and then concentrating them into a new medium [13], rather than degrading them per se [13]. It usually includes adsorption and membrane filtration. Chemical treatment encompasses the use of thermal degradation, electrochemistry, photochemistry, sonochemistry, and plasma reaction techniques to destroy the carbon–fluorine bonds and other chemical bonds in PFAS molecules, converting them into small molecules or other harmless substances, thereby reducing or eliminating them [14]. Biological treatment mainly involves the use of the metabolism of microorganisms and plants to degrade PFASs [15,16,17]. Recent studies have found that some enzymes (such as oxidoreductase, laccase, soybean peroxidase, and chloroperoxidase) can be used for the biodegradation of new pollutants [18]. In recent years, combined treatment has attracted widespread attention. Combining physical treatment with chemical or biological treatment can not only reduce costs and repair efforts but also achieves removal rates that are difficult to achieve with a single technology [13,19].

Many studies have reviewed and discussed technologies for PFAS removal, but most of them have focused on the advantages and disadvantages of a single or a few specific technologies (such as hydrothermal treatment [20], photocatalytic degradation [21,22,23,24], electrochemical degradation [25,26,27,28,29], electron beam and plasma irradiation technology [30,31], and biodegradation [18,32,33,34,35]) and their removal mechanisms, or the application of a specific material (such as carbon-based materials [36,37], nanomaterials [38], framework materials [39,40,41], and membrane materials [1,42]) in PFAS removal. Few studies have classified and discussed technologies for PFAS removal. Therefore, this review systematically summarizes the recent research on PFAS removal technology from four perspectives—physical, chemical, biological, and combined treatment—and summarizes the removal efficiency and target PFASs of these technologies so as to provide a better understanding of existing treatment strategies.

2. Physical Treatment

Physical treatment is the most prominent and applicable method [43]. Its operation is relatively simple, and this method generally does not involve the addition of chemical agents, thus avoiding the secondary pollution problem that may be caused by chemical treatment, making it environmentally friendly. Recent studies have focused on the two methods of adsorption and membrane filtration, and some new PFAS removal technologies have also emerged. These methods will be introduced below.

2.1. Adsorption

The adsorption method is considered the most traditional method for PFAS treatment, offering the advantages of a strong versatility and simple operation [44,45,46]. The adsorption method involves the transfer of PFAS from water to a solid surface through the interaction between the adsorbent surface and PFAS molecules (e.g., electrostatic, hydrophobic, and coordination) (Figure 1). Granular activated carbon (GAC) and ion exchange resin are the most commonly used adsorbents for removing PFASs. GAC is a porous adsorption material made from carbon-containing raw materials (such as coal, coconut shells, fruit shells, wood, etc.) through the carbonization and activation processes. The high porosity of GAC makes it a good adsorbent for removing hydrophobic pollutants, but it exhibits a poor removal rate for short-chain PFASs and has problems such as a low adsorption capacity and slow mass transfer rate [9,47]. In contrast, anion exchange resins (AERs) usually show a higher adsorption capacity than GAC [9]. Fang et al. [48] found that the adsorption efficiency of AERs for anionic perfluoroalkyl acids (such as perfluorooctanesulfonic acid (PFOS) and perfluorooctanoic acid (PFOA)) was significantly higher than that of cation exchange resins (CERs), non-ionic resins (NIRs), and GAC, and the removal rate increased with the increase in carbon chain length and sulfonic acid groups. In recent years, significant progress has been made in the development of new adsorption materials, including covalent organic frameworks (COFs), metal–organic frameworks (MOFs), and carbon nanotubes (CNTs). MOFs are porous crystalline materials composed of inorganic nodes and organic linkers, which are assembled into multi-dimensional lattices by coordination bonds. MOFs have shown great potential in the adsorption and separation of small molecules due to their ultra-high surface area and pore volume, as well as their structural tunability [47]. Li et al. [47] compared the PFAS removal efficiency in groundwater samples contaminated by aqueous film-forming foam (AFFF) using three MOF materials (MOFs NU-1000, UiO-66, and ZIF-8). It was found that the Zr-based MOF NU-1000 has a high affinity for both anionic and non-ionic PFASs, which is superior to that of the other MOFs. The adsorption of anionic PFASs is based on the electrostatic interaction between the anionic PFAS and the cationic Zr₆ metal nodes of NU-1000. The key mechanism for the removal of non-ionic PFASs is the acid–base interaction between the amine group of non-ionic PFASs and the Zr₆ metal nodes of NU-1000. Loukopoulos et al. [44] further modified two Zr-based MOFs (MOFs DUT-67 and MOF-808) by introducing fluorine-containing monocarboxylic acids (2-fluorobenzoic acid (FBA), 2,6-difluorobenzoic acid (DFBA), or trifluoroacetic acid (TFA)) as non-structural ligands into Zr₆ clusters. In terms of long-chain PFASs, the adsorption capacity of the original MOF-808 for PFOA was 986 mg/g, and the modified TFA-MOF-808 was increased to 2496 mg/g. In terms of short-chain PFASs, the adsorption capacity of TFA-MOF-808 for heptafluorobutyric acid (PFBA, C₄) and perfluorohexanoic acid (PFHxA, C₆) was 311 mg/g and 436 mg/g, respectively, which was 33–66% higher than that of the original MOF. TFA-MOF-808 exhibits the efficient adsorption of C₄–C₆ PFASs, breaking through the limitation of traditional materials’ dependence on chain length and is suitable for complex polluted water bodies. In addition, Dixit et al. [49] found that the two-dimensional metal carbide Ti₃C₂ MXene has excellent performance in removing selective fluorotelomer zwitterionic PFASs in natural water. Compared with AER (A860), non-ion exchange resin (XAD 4 and XAD 7), and PFAS special resin (A694 and A592), Ti₃C₂ MXene showed a better removal efficiency, and its removal performance was not affected by the specific chain length of the compound, water pH, or water matrix.

2.2. Membrane Filtration

As a semi-permeable membrane-based material separation method, membrane treatment technology holds the potential for applications in the field of PFAS removal [45,50,51]. The membrane filtration method mainly relies on the use of physical screening, charge repulsion, hydrophobic interactions, and a hydration layer barrier to remove PFASs in water (Figure 1). According to its different retention capacities, membrane technology is mainly divided into ultrafiltration (UF), microfiltration (MF), nanofiltration (NF), and reverse osmosis (RO). Among these, low-pressure membranes (UF/MF) can effectively intercept particles and microorganisms, but their ability to remove soluble pollutants is limited, while high-pressure membranes (NF/RO) can effectively remove dissolved PFASs [6,52,53]. Compared with the cost advantages of adsorption methods such as GAC and ion exchange resin in the treatment of long-chain PFASs (such as PFOS and PFOA), high-pressure membrane technology shows a unique value in the removal of short-chain PFASs, the adsorption of which is difficult [54]. Liu et al. [54] found that spiral-wound NF and RO membranes have an excellent retention performance for PFASs in AFFF-contaminated water samples. Their experimental results showed that the retention rates of an NF membrane and RO membrane for PFASs in laboratory-simulated wastewater were stable at more than 98% and 99%, respectively, and the retention rate of PFASs by the NF membrane remained at >98% within 13 days of continuous operation. The β-lactoglobulin amyloid fiber membrane developed by Jin et al. [55] shows excellent environmental protection characteristics and economy, and the rejection rate of PFOA and PFOS is more than 98%. In order to further improve the PFAS removal efficiency, the researchers combined amyloid fiber with activated carbon to prepare a new hybrid membrane. The composite membrane exhibited a removal rate of PFAS (including PFBA, C₃) containing ≥4 perfluorocarbon atoms reaching more than 96%, and the removal rate of short-chain PFBA was significantly increased from 60% of the pure membrane to 96%, which fully reflected the synergistic effect of the composite material.

2.3. Emerging Technologies

In addition to the two traditional methods above, researchers have provided some new technologies in recent years. Chandler et al. [56] developed an aqueous electrostatic concentration (AEC) technology, which utilizes the polarity of PFAS molecules and drives them to migrate and deposit on special membranes using an electrostatic field, where they are then removed and destroyed [56]. In a small-scale test, the single removal efficiency of AEC for various PFASs can reach more than 99%. Compared with traditional technologies such as activated carbon adsorption, this technology only produces a small amount of harmless, PFAS-free clean solution and waste membrane materials, effectively avoiding the risk of secondary pollution [56]. Wang et al. [57] used foam fractionation to remove most PFASs in water effectively. The foam fractionation method uses the surface activity of PFASs to carry them through rising bubbles to form a foam, enrich pollutants from the water phase to the foam phase, and remove PFASs. In addition, they also found that the addition of hydroxypropyl methyl cellulose, a thickener, to water could further enhance the removal efficiency of PFASs.

Table 1. Types and sources of PFASs, removal efficiency (or adsorption capacity), and advantages of adsorption, membrane filtration, and the two emerging technologies. Abbreviations: PFCA, perfluoroalkyl carboxylic acid; PFSA, perfluorosulfonic acid; FTS, fluorotelomer sulfonic acid; FTCA, fluorotelomer carboxylic acid; PFBA, perfluorobutanoic acid; PFHxA, perfluorohexanoic acid; FTAB, fluorotelomer sulfonamide alkylbetaine; FTB, Fluoroalkyl Chain Betaine; PFAAs, perfluoroalkyl acids; and PFCAs, Perfluoroalkyl Carboxylic Acids.

Treatment Technology		Source of PFAS	Target PFAS	Removal Efficiency or Adsorption Capacity	Advantages	Disadvantages	References
Adsorption	Granular activated carbon	Contaminated groundwater	PFOA PFBS	0.86 μg/g 0.015 μg/g	The removal effect of long-chain PFAS is better.	The removal effect of short-chain PFAS is poor. Low adsorption capacity. The mass transfer rate is slow.	[58]
	Macroporous AERs	AFFF diluent	PFOA PFOS	0.26–0.88 μmol/mg 0.66–1.36 μmol/mg	Targeted selection. Renewable potential.	The removal effect of short-chain PFAS is poor. Special resins (e.g., PAER) are more expensive. Regeneration processes (e.g., salt solution elution) may lead to secondary pollution.	[48]
	MOF NU-1000	AFFF contaminated groundwater	Anionic PFAS (PFCA, PFSA, FTS, FTCA) Non-ionic PFAS (FASA)	58% 99%	High structural stability. Fast adsorption speed.	The ions (Cl−, NO3−, and CO32−) in the water matrix compete with PFAS for adsorption sites, resulting in a decrease in adsorption efficiency.	[47]
	TFA-MOF-808	Simulated water samples prepared in the laboratory	PFOA PFBA PFHxA	2496 mg/g 311 mg/g 436 mg/g	High adsorption capacity. Recycle regeneration.	The synthesis cost of functionalized MOF is high. Ligands such as trifluoroacetic acid released during the adsorption process may cause secondary pollution and require additional treatment.	[44]
	Ti3C2 MXenes	AFFF diluent	6:2 FTAB, 5:1:2 FTB	>80%	Low environmental pollution.	Ti3C2 MXenes need to be prepared by hydrofluoric acid (HF), and the synthesis process is dangerous and costly, which limits large-scale production.	[49]
Membrane Filtration	NF	Laboratory-simulated wastewater	PFAAs	>98%	Low energy consumption.	The removal effect of short-chain PFAS is poor.	[54]
		AFFF-contaminated groundwater	PFCAs Short-chain PFSAs Long-chain PFSAs	92–98% 92–95% >98%
	RO	Laboratory-simulated wastewater	PFAAs	>99%	Tolerance to high salt and organic matter.	High cost and energy consumption.
	Amyloid–carbon hybrid membrane	Water samples from Xiaoqing River Basin, China	PFAS (C ≥ 4), PFBA	>96%	Low pollution. Low energy consumption.	The preparation of the membrane requires a high temperature (90 °C) and acidic conditions (pH = 2), and industrial production may face energy consumption and cost challenges.	[55]
Aqueous electrostatic concentration		Actual polluted water samples	PFOA, PFOS	>99%	Low energy consumption. Less waste.	Long-term treatment of high suspended solids or organic wastewater may lead to membrane fouling.	[56]
Foam fractionation		Sewage treatment plant wastewater	PFOS PFOA	99% 94%	Adapting to complex matrices. Low energy consumption. Environmentally friendly.	The removal effect of short-chain PFAS is poor. A too high air flow rate can easily lead to liquid entrainment and reduce removal efficiency.	[57]

summarizes the types of PFASs treated with the different physical treatment technologies, their removal efficiencies, and their respective advantages.

3. Chemical Treatment

Currently, physical treatments such as adsorption and membrane filtration are effective for removing PFASs; however, these methods can only transfer pollutants from the liquid phase to the solid phase, producing concentrated waste streams (e.g., saturated adsorbents and concentrated liquid streams from membrane interception) [27,59,60]. These waste streams may contain harmful substances (such as PFOA and PFOS) under the CERCLA standard, and current treatment methods for these waste streams carry the risk of re-introducing PFASs into the environment [61]. Therefore, it is necessary to quantitatively defluorinate the C-F bonds in PFASs to create fluoride ions (F⁻) [61]. At present, chemical treatment methods such as thermal, electrochemical, photochemical, and sonochemical degradation, and plasma technology can achieve this purpose.

3.1. Thermal Degradation

PFASs have strong chemical stability and thermal stability, which means that a higher temperature is needed to remove them [62,63]. Heat treatment is an effective strategy for destroying PFASs because it can effectively decompose the strong halogen–carbon bonds in PFASs, such as C-F bonds [64,65]. Zhang et al. [66] found that hydrothermal treatment can significantly reduce PFAS occurrence in sewage sludge, but it is difficult to completely degrade PFASs using heat treatment alone. It is necessary to combine this with alkaline additives (e.g., Ca(OH)₂) to promote PFAS precursor conversion and pollutant release. Hao et al. [67] used hydrothermal alkali treatment (HALT) to treat AFFF-contaminated soil and groundwater at 350 °C and 5 M NaOH. Most PFASs were degraded below the detection limit within 90 min, and the refractory perfluoroalkyl sulfonic acid (PFSA) was also nearly completely removed after 180 min of reaction. Kim et al. [68] used an alkaline aprotic solvent system composed of dimethyl sulfoxide (DMSO) and NaOH to degrade non-fluorocarbon-containing fluorotelomers (e.g., 6:2 FTCA, 6:2 FTSA) and perfluorocarboxylic acids (PFCAs) within 4 h at mild temperatures (100–120 °C). Subcritical hydrothermal treatment is a thermochemical process using high-temperature (170–350 °C) and high-pressure (2–22 MPa) liquid water [60,69,70]. It has been used to treat refractory organic waste and has recently attracted attention as a promising PFAS degradation technology [20,71,72]. It was found that under subcritical hydrothermal conditions, strong alkaline reagents (e.g., NaOH) can significantly promote the degradation of short-chain perfluorobutanesulfonic acid (PFBS), mainly through the attack of hydroxyl groups on sulfonic acid groups to initiate S-C bond cleavage, followed by C-F bond cleavage and carbon chain shortening, which finally produces inorganic fluoride ions (F⁻), trifluoroacetic acid (TFA), CO₂, and other products [73]. Cai et al. [74] further enhanced the subcritical hydrothermal degradation of PFOS by adding Fe-based amorphous alloys. Under the optimized conditions of 325 °C, 1 M NaHCO₃, and 60 min of reaction, the degradation rate of PFOS was about 85%, which was higher than that of 56% without the alloy. Some researchers have found that PFAS can also achieve a high removal efficiency by supercritical water heat treatment technology [75,76]. Hydrothermal liquefaction technology provides another route for PFAS degradation, but PFCAs produce volatile hydrogen perfluoroalkanes during the degradation process, which may cause greenhouse gas emissions. In addition, Xiao et al. [77] proposed an innovative thermal degradation strategy to achieve efficient PFAS degradation (>90%), including refractory short-chain PFASs and perfluoroalkyl sulfonic acid, by utilizing induction heating to rapidly heat PFAS-containing solid materials (<40 s). This method is based on the Joule heating effect, which generates heat through electromagnetic induction in a metal reactor, and there is no direct contact between the induction heater and the reactor. The hysteresis loss of magnetic materials during heat treatment will generate additional heat. Figure 2 shows several thermal degradation technologies introduced in this review.

3.2. Electrochemical Degradation

At present, electrochemical methods are widely regarded as a feasible technology for repairing environmental media contaminated by PFASs because they can effectively degrade and remove PFASs through direct and indirect oxidation, electrochemical reduction, and other reaction mechanisms at the electrode–electrolyte interface [25,28,78,79]. Electrochemical oxidation technology has been developed in recent years, and it uses anodic oxidation instead of chemical oxidation [26,80]. It has the advantages of a high oxidation efficiency, fast reaction rate, simple operation, good environmental compatibility, and economical efficiency [81]. It can degrade toxic or refractory organic pollutants. It has also been proven that PFAS can be significantly degraded by reactive oxygen species (e.g., •OH, H₂O₂, and O₃). The reaction process is generally initiated by inactive anodes (e.g., BDD, PbO₂, SnO₂, and Magnéli phase Ti₄O₇) [64,82,83]. Boron-doped diamond (BDD) is one of the most commonly used anode materials for electrocatalytic PFAS degradation. It has a wide operating potential window, excellent chemical stability, and high oxidation potential [24]. Recent studies have found that BDD has great potential in the degradation of PFOA and long-chain PFASs (e.g., C₉ + sulfonic acids and C₁₂ + carboxylic acids) [84,85,86]. It is worth noting that important progress has also been made in new carbon-based anode materials. Duinslaeger et al. [87] found that a boron-doped graphene sponge anode (B-RGO) can effectively degrade C₄-C₈ PFASs through the synergistic effect of electrosorption and electrooxidation, and no chlorine-containing by-products are generated during the degradation process. They also used a borophene functionalized graphene sponge anode (Bph-RGO) to degrade C₄–C₈ PFASs, and found that borophene modification significantly improved the electrocatalytic activity of the electrode, achieving higher PFAS removal and defluorination efficiency under low-conductivity conditions, and no toxic chlorine-containing by-products were generated in a high-chlorine environment [88]. The wide application of traditional electrode materials such as platinum and iridium is limited due to their high cost. Non-noble metal oxide-based electrocatalysts (e.g., Ti₄O₇) have recently been studied as promising electrocatalysts [24]. The use of a porous Magnéli phase Ti₄O₇ film as an anode has been shown to improve the degradation efficiency of PFOS [24,89]. Gomri et al. [90] found that the coupling of electrochemical oxidation and the Electro-Fenton process based on the Magnéli-phase Ti₄O₇ anode could further improve the efficiency of PFOA and PFOS degradation.

3.3. Photochemical Degradation

The photocatalytic degradation of PFASs employs photocatalysts to produce active species (e.g., hydroxyl radicals, hydrated electrons) under light, thereby destroying PFASs’ stable chemical structure [21]. This technology is mainly divided into two categories: photooxidation technology and photoreduction technology. Photooxidation technology produces strong oxidizing substances (e.g., photogenerated holes (hvb⁺) and oxidative radicals) by a photocatalyst under light radiation [21]. These substances undergo an oxidation reaction with the PFAS and destroy its molecular structure. Studies have shown that nano-metal oxides (e.g., TiO₂, Ga₂O_3, and In₂O₃) can be used as photocatalysts to destroy and remove the molecular structure of PFAS through a photocatalytic oxidation process [91,92]. The photoreduction process uses electromagnetic radiation to produce strong reducing species (hydrated electrons (e⁻_aq), hydrogen radicals (H), or free radicals from the catalyst itself) to induce PFAS degradation [23,93]. UV radiation can usually interact with photocatalysts such as sulfites or iodides to produce hydrated electrons (e⁻_aq), thereby effectively degrading PFASs [64]. Hydrated electrons are a powerful reducing agent with a standard reduction potential of −2.9 V and a high tendency to react with PFASs through a single-electron transfer mechanism [64]. Fennell et al. [59] found that UV/persulfate preoxidation could effectively eliminate electron capture agents such as dissolved organic carbon and phosphate, so that the defluorination rate of subsequent UV/sulfite treatment could exceed 90% within 24 h. Recent studies have found that iodide can increase the yield and lifetime of e⁻_aq, thereby accelerating the defluorination and carbon chain rupture of PFASs [94]. The UV/chlorine system developed by Metz’s team significantly enhanced the degradation efficiency of perfluorooctanoic acid (PFOA) by generating chlorine radicals such as Cl• and Cl₂•⁻ [95]. In terms of new catalyst methods, metal–organic framework materials have great potential in photocatalytic PFAS degradation in addition to being used as adsorption materials. Wen et al. [96] used a titanium-based metal–organic framework material MIL-125-NH₂ to degrade PFOA by more than 98.9% within 24 h under ultraviolet light irradiation. In addition, Zhang et al. [97] designed an ultra-light reducing agent, KQGZ, with a highly twisted carbazole as its core to perform the photocatalytic reduction and defluorination degradation of various PFASs at 40–60 °C. After treatment, amorphous carbon and fluoride salts are primarily generated in PTFE, while oligomeric PFASs (e.g., PFCs, PFOS, and PFOA) produce defluorinated products, such as carbonates, formate, oxalate, and trifluoroacetate, thereby facilitating the recycling of fluorine in PFASs. Figure 3 shows several photochemical degradation techniques introduced in this review.

3.4. Sonochemical Degradation

Sonochemical degradation of PFASs is a technology that utilizes the cavitation effect generated by ultrasound (usually 20 kHz–1 MHz) to attack the C-F bond in PFASs through extreme high temperature and high pressure (local temperature > 5000 K, pressure > 100 MPa) and free radicals (e.g., •OH, •H) to achieve defluorination and molecular mineralization [3,98,99,100,101,102]. In recent years, researchers have focused on exploring the degradation effects of different PFASs through parameters such as the ultrasonic frequency, power intensity, power density, and PFAS structure and initial concentration [103,104,105,106,107]. Shende et al. [108] found that the degradation efficiency of PFOA and PFOS was significantly improved at a high power density (147–262 W/L). Fagan et al. [109] degraded n:2 fluorotelomer sulfonates (FTSAs, n = 4, 6, 8) with different chain lengths at 354 kHz ultrasonic frequency. It was found that short-chain FTSAs (e.g., 4:2 FTS) were more easily adsorbed into the cavitation bubble interface due to a higher diffusion coefficient, and the degradation rate was significantly faster than that of long-chain homologues (e.g., 8:2 FTS). Awoyemi et al. [110] further pointed out that the ultrasonic degradation efficiency was significantly affected by the PFAS structure and the matrix complexity. PFOA had the fastest degradation due to its simple molecular structure, followed by PFOS, and 6:2 FTS had the slowest degradation due to its hydrogenated carbon chain. In addition, Fuller et al. [111] found that a 1000 kHz ultrasound could effectively degrade PFASs in ion exchange regeneration waste liquid (e.g., distillation residue). A high chloride ion (Cl⁻) concentration promoted degradation, while a high total organic carbon (TOC) concentration and residual MeOH inhibited the reaction, but the addition of NaOH could partially alleviate TOC inhibition. In addition, volatile organic fluorine (VOF) formation was also detected during the ultrasonic process, indicating the presence of some organic fluorine intermediates.

3.5. Plasma-Based Technologies

Plasma-based treatment technology is also a promising PFAS degradation method, which uses electricity to convert water in the air or an environment with oxygen into a variety of active substances with strong oxidizing or strong reducing properties (e.g., •OH, •H, O₃, H₂O₂, and e⁻_aq) [3,13,112,113]. The stable carbon–fluorine bond in PFAS molecules will break under the attack of these active substances, and then PFASs will be decomposed into small molecular substances, and finally, degradation will be achieved [114]. Plasma can be divided into two categories: thermal plasma and non-thermal plasma (low-temperature plasma) [9,115]. Traditional plasma technology consumes a large amount of energy. Low-temperature plasma has been widely used in water treatment due to its low energy demand and selectivity [9,116,117,118,119]. It was found that non-thermal plasma–ozone tandem technology had a significant degradation ability for long-chain PFASs (PFOA and PFOS) [120]. Chen et al. [121] found that although the combination of non-thermal nitrogen (N₂) plasma and a denitrifying biofilm reactor effectively degrades PFOA and perfluorodecanoic acid (PFDA), it produces a large amount of inorganic nitrogen (NO₂⁻, NO₃⁻), which needs subsequent treatment. In terms of thermal plasma treatment, Lakkasandrum et al. [122] used a plasma rotating disk reactor (PSDR) to degrade PFASs with a chain length ≥ 4 (e.g., 6:2 FTS, PFHpA, PFHxA, PFPeA, and PFBA) to below the detection limit within 86 h, while PFASs with a chain length < 4 (e.g., PFPA, TFA) had a low degradation efficiency and took a longer time.

From the introduction of the above methods, we can see that electrochemical, photochemical, sonochemical, and plasma techniques can produce free radicals (e.g., •OH, SO₄•⁻). These free radicals destroy the C-F bond in the PFAS molecule through electron transfer or hydrogen atom capture, so that the C-F bond is broken, and the long-chain PFAS is gradually decomposed into short-chain compounds and finally mineralized into CO₂ and F⁻. Persulfate is a strong oxidant that can be dissociated in the aqueous phase to form persulfate ions (S₂O₈²⁻). After activation, it can also produce sulfate radicals (SO₄•⁻) and hydroxyl radicals (•OH). Using zero-valent iron-modified activated carbon (RAC) combined with persulfate, De et al. [123] achieved a PFOA removal rate of up to 83% in water at 60 °C through an “adsorption–decomposition” synergistic strategy. In this process, RAC first adsorbs and concentrates PFAS and then initiates oxidative decomposition by free radicals generated by persulfate activation, showing an efficient synergistic effect.

Figure 4 shows the basic mechanisms of PFAS degradation by the above chemical treatment technologies.

Table 2. The types and degradation efficiencies of PFASs treated by thermal, electrochemical, photochemical, and sonochemical degradation; plasma technology, and their respective treatment conditions. Abbreviations: HFPO-DA, Hexafluoropropylene Oxide Dimer Acid; PFPeA, Perfluoropentanoic acid.

Treatment Technology		Target PFAS	Treatment Conditions	Degradation Efficiency	References
Thermal Degradation	Hydrothermal treatment	PFCAs	300 °C, 0.5 h	100%	[66]
	Hydrothermal alkaline treatment	PFOS	350 °C, 0.5 h, 5 M NaOH	100%	[60]
	Subcritical hydrothermal treatment of Fe-based amorphous alloys	PFOS	325 °C, 1 h, 1 M NaHCO3	85%	[74]
	Induction heating	PFCAs, HFPO-DA	40 s (the temperature rose from 22 °C to 500 °C in the first 30 s and continued to rise to 845 °C in the last 30 s)	>99.5%	[77]
Electrochemical Degradation	Molecular copper electrocatalysts	PFOA	−5 mA, 4 h	93%	[65]
	Bph-RGO	PFOS PFOA	Landfill leachate, pH 5.6	95% 75%	[88]
	BDD	PFOA	0.5 Mm persulfate, 16.9 A/cm2, 2 h	>99%	[86]
Photochemical Degradation	UV/sulfite + iodide (UV/S + I) system	PFBS	24 h	>99.7%	[94]
	UV/chlorine system	PFOA	UV irradiation, 1.4 Mm NaOCl (106 mg/L), 0.5 h	12%	[95]
	Titanium-based MOF material MIL-125-NH2	PFOA	UV irradiation, 24 h	98.9%	[96]
Sonochemical Degradation		PFOA PFOS	30–262 W/L, 2 h	43–98% 34–97%	[108]
		4:2 FTS 6:2 FTS 8:2 FTS PFOS	354 kHz, 4 h	>99% >99% 86% 89%	[109]
Plasma-based Technologies	Falling film dielectric barrier discharge plasma technology	PFOA PFPeA	1 h 40 min	>95% 42.5%	[116]
	Non-thermal nitrogen (N2) plasma combined with a denitrifying biofilm reactor	PFOA PFOS	1 h	45% 60%	[121]

summarizes the PFASs and their degradation efficiency when treated using different chemical treatment techniques.

4. Biological Treatment

Although there are many physical and chemical methods for PFAS treatment, these methods have the disadvantages of high cost, high energy consumption, incomplete mineralization, toxic by-products, and the requirement of additional pretreatment steps [34,35,124]. Environmental pollutants can be naturally treated by biodegradation, which depends on the ability of organisms (mainly bacteria and fungi, plants, and earthworms) and their enzymes to degrade chemical bonds in pollutants, thereby converting them into harmless compounds [34,125,126,127,128]. Compared with the above physical and chemical methods, biological treatment has the advantages of cost-effectiveness, eco-friendliness, and sustainability, and is a more economical, green, and sustainable alternative [129]. However, biological treatment is a time-consuming process, which may be the reason why there are fewer studies on the removal of PFASs through biological methods than by physical and chemical methods [33].

In terms of microbial applications, Pseudomonas and white rot fungi seem to be good candidates for PFAS biodegradation [18,33,130,131,132,133,134]. In recent studies, Huang et al. [135] found that Acidimicrobium sp. Strain A6-enriched culture can degrade a variety of PFAAs (including C₄–C₁₀ PFCAs and PFSAs), which mainly provides electrons through the Feammox process, restores the C-F bond in the PFAS molecule, breaks it, and produces short-chain PFAAs and F⁻. In addition, they also found that linear and single-branched PFOA/PFOS can be effectively degraded, but double-branched isomers (e.g., 3,5-PFOA, 5,5-PFOS) are almost impossible to degrade.

Plants mainly absorb PFAS through their roots and accumulate it in above-ground parts (e.g., leaves) to remove PFAS from the environment. The absorption and transport of PFASs in plants usually involves three key processes: root absorption, intracellular movement, and long-distance transport [136]. The absorption of PFASs by plant roots occurs mainly through passive and active processes [136,137,138]. Passive transport involves the diffusion of PFASs (especially small-molecule short-chain PFASs) into plant cells [136]. Active transport depends on the selective uptake of PFASs by specific transporters (e.g., ion channels and aquaporins), thereby promoting their entry into plant cells [136]. Constructed wetland systems have shown good application prospects in PFAS pollution control because of their integration of physical adsorption, plant absorption, and biodegradation [136,139]. For example, the organic matter secreted by plant roots (e.g., organic acids and sugars) can promote the reproduction of rhizosphere microorganisms, enhance the ability of microorganisms to degrade PFAS, and form a “plant–microorganism” combined remediation system. Ferrario et al. [140] found that the PFAS removal efficiency by constructed wetland reeds can reach 35–68%, mainly relying on plant adsorption and physical filtration, which is suitable for the pretreatment or ecological restoration of low-intensity polluted water. Reed can also catalyze PFAS defluorination through the enzyme system in vivo. However, its ability to completely mineralize PFASs is limited, and it needs to be combined with other technologies (e.g., microbial degradation, advanced oxidation) to achieve efficient repair.

Enzymes are a class of biocatalysts, most of which are proteins that promote intracellular chemical reactions [124,141,142]. Laccase and peroxidase have garnered significant attention due to their ability to degrade a wide range of organic pollutants through free radical-mediated oxidation [18,34,143]. They can decompose macromolecular pollutants into smaller, less toxic, and biodegradable products [18]. Luo et al. [144] found that a composite system composed of laccase and soybean meal could degrade 40% of the PFOA after 140 days in a soil slurry environment, and the PFOA degradation rate reached 24% after 36 days in the aqueous phase system. In addition, recent studies have shown that laccase can also participate in the PFAS removal process through adsorption [145]. Horseradish peroxidase (HRP), lignin peroxidase (LiP), and manganese peroxidase (MnP) are the main peroxidases used to remove PFASs [34]. Among them, HRP is derived from horseradish plants, while LiP and MnP are found in wood-decaying fungi [34,146]. Colosi et al. [147] found that HRP could achieve 68% PFOA degradation within 6 h with hydrogen peroxide and 4-methoxyphenol as co-substrates. Existing studies have shown that the PFAS removal and defluorination rates with LiP and MnP are relatively low [34]. The enzymes used to degrade PFAS in most of the current studies are free enzymes. The activity of natural free enzymes is susceptible to environmental factors such as temperature and pH, and there are problems of poor stability and low reusability. Enzyme immobilization technology is expected to overcome these limitations and achieve the efficient degradation of PFAS.

Figure 5 shows the basic mechanisms of PFAS degradation by microorganisms, plants, and enzymes.

Table 3. Different biological treatments (bacteria, fungi, plants, and enzymes) for PFAS removal efficiency and their required treatment conditions. Abbreviations: FOSA, Perfluorooctane Sulfonamide.

Category		Target PFAS	Treatment Conditions	Removal Efficiency	References
Bacteria	Acidimicrobium sp. Strain A6	PFOA PFOS	120 days	59.1% 39.9%	[135]
	Pseudomonas aeruginosa	PFOA PFOS	96 h	27.9% 47.3%	[148]
	Pseudomonas putida	PFOA PFOS	96 h	19% 46.9%	[148]
Fungus	Trametopsis cervina	6:2 FTS	30 days	50%	[132]
Plants	Reed	PFOS PFBA PFBS PFOA	5 days	83.7% 71% 64.9% 61%	[140]
	Carrot	FOSA	98 days	100%	[149]
	Lettuce	FOSA	TOC 2.3%, 35 days TOC 53%, 35 days	50% 80%	[149]
Enzymes	Laccase	PFOA	Aqueous phase, 36 days Soil slurry, 140 days	24% 40%	[144]
		PFOA PFOS	Laccase and 1-hydroxybenzotriazole were added twice, 24 h	64% 67%	[145]
	Horseradish peroxidase	PFOA	Hydrogen peroxide and 4-methoxyphenol as co-substrates, 6 h	68%	[147]

summarizes the types and removal efficiencies of PFASs for the various biological treatment technologies mentioned above.

5. Combined Treatment

A new trend that arose in 2015 involves removing PFASs by combining multiple processing methods in one system solution [19,150]. The PFAS removal efficiency is affected by multiple factors, including treatment cost, initial pollutant concentration, the service life of treatment facilities, and the PFAS species’ composition in the system to be treated [19]. A single treatment method may be restricted by some of the above factors, and this makes it difficult to achieve the desired removal rate [151]. Traditional physical treatment technologies (e.g., adsorption and membrane filtration) have a low removal efficiency for short-chain PFASs, while chemical treatment technologies such as photocatalysis can effectively degrade short-chain PFAS molecules [19]. Therefore, the combined treatment can significantly improve the overall PFAS removal efficiency. The adsorption method has been widely used in the field of PFAS removal due to its simple operation and low cost [19]. In the combined treatment process, adsorption technology is usually used as a pretreatment to efficiently enrich most of the PFAS pollutants in water, and then the concentrated PFASs are degraded using a chemical treatment [19].

Recent studies have used a combination of different adsorbents (di-indole hexadecyl ammonium [152,153], magnetic modified clay [154]) and photocatalytic technologies, as well as the design and synthesis of materials with both adsorptive and photocatalytic properties (iron-doped and carbon-modified composite [155], silica-based granular media [156], and chitin/polyethylene imine/oxygen-doped graphitic carbon nitride sponges [157]) to achieve efficient PFAS removal from water. Smith et al. [158] preconcentrated PFASs through foam separation, and then effectively removed some PFASs from groundwater and landfill leachate using electrochemical oxidation technology. Kim et al. [159] used a combination strategy of electrodialysis and electrosorption to not only remove about 90% of the ultra-short-chain, short-chain, and long-chain PFASs in a single process but also desalted the water source to drinking water standards. De Souza et al. [123] found that granular activated carbon impregnated with zero-valent nano-iron combined with persulfate (RAC) could effectively remove perfluoroalkyl and polyfluoroalkyl substances in water and a water/soil slurry at 60 °C. They also found that carboxyl PFASs need to be removed by physical adsorption combined with chemical decomposition, while sulfonic PFASs can only be removed by physical adsorption. In addition, researchers have recently developed a physical–chemical–biological three-stage treatment system: first, PFASs are concentrated by reverse osmosis and foam fractionation, followed by laccase enzymatic hydrolysis, and finally, the residual PFASs are extensively degraded by electrochemical oxidation [160]. However, this process suffers from a low enzymatic hydrolysis efficiency, and PFASs need to be concentrated to more than 10 ppm to ensure the effectiveness of subsequent electrochemical oxidation [160].

Table 4. The PFAS types and removal efficiencies and processing times when treated with different combination modes. Abbreviations: FOSA, Perfluorooctane Sulfonamide; GenX, Hexafluoropropylene Oxide Dimer Acid.

Combination Mode	Target PFAS	Processing Times	Removal Efficiency	References
Physical treatment (di-indole hexadecyl ammonium) + chemical treatment (UV photocatalysis)	PFOA PFOS	10 s (physical treatment) 2 h (chemical treatment)	>99% (physical treatment) 95% (chemical treatment) >99% (physical treatment) 92% (chemical treatment)	[153]
Physical treatment (magnetic modified clay) + chemical treatment (UV photocatalysis)	PFBS GenX PFOA PFOS	48 h	27% (defluorination rate) 66% (defluorination rate) 42% (defluorination rate) 27% (defluorination rate) The removal efficiencies of the four substances with physical treatment were all 99%	[154]
Physical treatment (foam separation) + chemical treatment (electrochemical oxidation)	PFOA PFOS	9 h	92% (physical treatment) 68% (chemical treatment) 91% (physical treatment) 36% (chemical treatment)	[158]
Chitin/polyethylene imine/oxygen-doped graphitic carbon nitride sponges	PFOA PFOS	2 h (adsorption stage) 3 h (photocatalytic stage)	97.9% 99.7%	[157]
Physical treatment (reverse osmosis, foam fractionation) + biological treatment (laccase enzymatic hydrolysis) + chemical treatment (electrochemical oxidation)	PFOA PFOS	Biological treatment time was not mentioned 14 h (chemical treatment)	35% (biological treatment) >99% (chemical treatment) 35% (biological treatment) 97% (chemical treatment)	[160]

summarizes the PFAS types and removal efficiencies of treatment with the different combinations described above.

6. Conclusions

This paper systematically reviews the current removal technologies for PFASs, including physical, chemical, biological, and combined treatment methods. Physical treatment methods (e.g., adsorption and membrane filtration) can efficiently separate and concentrate PFASs but cannot completely degrade pollutants. Chemical treatment methods (e.g., thermal degradation, electrochemical degradation, and photochemical degradation) can achieve the mineralization of PFASs by destroying their carbon–fluorine bonds, but these methods suffer from high energy consumption and the possible generation of toxic by-products. Biological treatment methods (e.g., microbial degradation and plant uptake) have the advantages of environmental friendliness and sustainability, but their associated removal efficiency is low and time-consuming. Combined treatment technology significantly improves the PFAS removal efficiency by combining the advantages of various methods, while reducing the cost and secondary pollution risk. In general, different technologies have their advantages and disadvantages. In practical applications, appropriate methods should be selected according to pollution scenarios (e.g., PFAS types, concentrations, and environmental media). The current research shows that combined treatment technology (e.g., adsorption–photocatalysis, electrochemical–biodegradation) is the key direction of future development, as it can take into account both efficiency and economy.

At present, the removal of PFAS in industrial wastewater and drinking water is still dominated by physical treatment methods such as activated carbon adsorption and ion exchange [3]. Although some developed countries have eliminated C₈-based PFAS to alleviate pollution, the production of short-chain PFASs (C₂–C₇) has increased, and emerging PFASs such as GenX have also been detected in the environment [3]. The removal efficiency of short-chain PFAS by traditional treatment technology is limited. Although emerging technologies such as electrooxidation and plasma have shown application potential, they are faced with problems such as high energy consumption, high treatment cost, and secondary pollution. At present, most studies remain in the laboratory simulation stage, and the applicability of the technology has not been verified in actual complex water samples. Future research needs to focus on promoting pilot experiments, strengthening the adaptability of the technology to actual wastewater, and overcoming the economic costs, energy consumption, and stability problems in large-scale applications. At the same time, the combined application of multiple technologies should be strengthened to minimize the processing cost and energy consumption while improving the removal efficiency of PFAS.