Degradation of Per- and Polyfluoroalkyl Substances

Degradation of Per- and Polyfluoroalkyl Substances: Comparison

Please note this is a comparison between Version 1 by Xiaoyan Chen and Version 2 by Lindsay Dong.

Per- and polyfluoroalkyl substances (PFASs) are an emerging group of persistent organic pollutants in aquatic environments with high levels of toxicity and bioaccumulation. The risks posed by PFASs to the environment and health have attracted increasing attention. To remove them from water, advanced oxidation processes (AOPs), with the merits of high efficiency and low cost, are mainly used. Photo/electrocatalytic heterogeneous AOPs, with the assistance of nanostructured catalysts and external energy in the form of light/electricity, have emerged as one of the most powerful techniques, overcoming the difficulty associated with defluorination and achieving the effective and complete degradation of PFASs in water. The structures of photo/electrocatalysts play a critical role in the production of reactive oxygen species, the electron transfer process, and the degradation pathway and its efficiency.

per- and polyfluoroalkyl substances (PFASs)
heterogeneous advanced oxidation processes (AOPs)
photo/electrocatalytic degradation
photocatalysts
electrocatalysts

1. Introduction

Per- and polyfluoroalkyl substances (PFASs) are an emerging group of persistent organic pollutants in the environment which pose ecological and health risks [1]. PFASs are man-made chemicals that are widely used in various industrial and commercial products; their chemical structure includes a fully fluorinated carbon chain with a terminated functional group attached to it. The most common terminal groups are carboxylic acid (−COOH) and sulfonic acid (−SO₃H) groups, while the fluorinated carbon chain varies in length and number of branches, containing a number of carbon–fluorine bonds. Perfluorooctanoic acid (PFOA) and perfluorooctane sulfonate (PFOS), with structural formulas shown in Figure 1a, are the two most commonly detected PFASs. The carbon–fluorine bond in these molecules is one of the strongest single bonds due to its bond energy of 485–582 kJ/mol and its redox potential of F/F⁻ at 3.6 eV, making it difficult for PFASs to break down naturally [2]. Moreover, the fluorine atoms in PFASs provide a shielding effect that protects the carbon–fluorine bond from chemical and biological attack, further contributing to the persistence of these compounds. On the other hand, toxicological and epidemiological studies have linked PFAS exposure to adverse health effects.

Figure 1. (a) The structures of PFOA and PFOS, and the most commonly occurring PFASs in aquatic environments. (b) Volume of reported research on PFASs. (c) Heat map of works on PFASs, mainly addressing toxicity, occurrence, and control. Data collected from Web of Science (up to 2023).

Recent years have witnessed a boom in studies of PFASs, including strategies for their control in water, as displayed in Figure 1b,c. These strategies can be categorized into three principal types, i.e., physical separation ^[3][4][5,6], biological treatment ^[5][7], and chemical degradation (oxidation and reduction processes) ^[6][7][8][9][8,9,10,11]. Physical removal technologies, including adsorption, ion exchange, reverse osmosis, and nanofiltration, either have low PFAS elimination efficiency or high cost ^[4][6]. These non-destructive processes may generate waste such as spent adsorbents that can give rise to re-contamination with their re-entry into the environment. Biological degradation and chemical degradation are destructive technologies that permanently remove PFASs from water, but biodegradations are incomplete, have slow rates, and are highly dependent on environmental conditions ^[10][12]. Traditional wet chemical oxidation can barely break the very strong C–F bonds, whereas advanced oxidation processes (AOPs), originally introduced by Glaze in 1987 and distinguished by employing free radicals as highly reactive oxidant species, are considered to be highly efficient and have strong potential for the complete mineralization of PFASs ^[11][12][13,14]. To overcome the high overpotential of defluorination from C–F bonds and accelerate decomposition, more effective AOPs have been developed by introducing catalysts and external energy (light, electricity, heat, ultrasound, etc.) to lower the activation energy and overpotential of PFAS degradation reactions ^{[13][14][15][16]}[15,16,17,18]. Energy in the forms of light and electricity is more appropriate for in situ use, as such approaches are relatively low-cost and environmentally friendly. Therefore, photocatalytic and electrocatalytic AOPs in heterogeneous systems are promising for the efficient and complete removal of PFASs in practice.

2. Fundamentals of PFAS Degradation by AOPs

2.1. General Pathways and ROS for PFAS Degradation

The degradation by AOPs of PFASs occurring in water environments starts with their hydrolysis products (Equation (1)). Below, perfluorocarboxylic acid (PFCA, C_nF_2n+1COOH) represents the PFAS, while the effective ROS is represented by •OH. The degradation mechanisms of PFASs by AOPs are closely linked to ROS and their effective sites of action. However, these pathways can be summarized as a general reaction course ^[17][40]. Specifically, the oxidation of C_nF_2n+1COO⁻ to C_nF_2n+1COO• (Equation (2)) is initiated by radicals (e.g., •OH) generated from activated oxidizing agents ^[18][41]. Light and electrical energy can also trigger this reaction, utilizing, for example, photogenerated holes (h⁺) under ultraviolet radiation or the electron transfer process at the anode ^[19][42]. Subsequently, spontaneous decarboxylation of C_nF_2n+1COO• occurs due to its instability (Equation (3)), and the resulting perfluoralkyl radicals C_nF_2n+1• transform into C_nF_2n+1OH via hydroxylation (Equation (4)) ^[20][43]. After the spontaneous elimination of HF from C_nF_2n+1OH (Equation (5)), the resulting acyl halide, C_n−1F_2n−1COF, undergoes a hydrolysis process (Equation (6)), generating a short-chain PFCA (C_n−1F_2n−1COO⁻). Afterwards, this decarboxylation-hydroxylation-elimination-hydroxylation (DHEH) procedure is performed repeatedly, with the remove of a CF₂ unit and the release of CO₂ and HF in each cycle (Equations (2)–(6)) until complete mineralization is achieved ^[21][22][44,45]. The general degradation pathways of perfluorosulfonic acid (PFSA) are similar, but with the initial oxidation of C_nF_2n+1SO₃⁻ to C_nF_2n+1• by ROS attacking the C−S bond ^[23][46]. The resulting product then enters the defluorination cycle, similarly to PFCA.

C n F 2 n + 1 C O O H → C n F 2 n + 1 C O O − + H +

C n F 2 n + 1 C O O − + • O H ( / + h + / − e − ) → C n F 2 n + 1 C O O • + O H −

C n F 2 n + 1 C O O • → C n F 2 n + 1 • + C O 2

C n F 2 n + 1 • + • O H → C n F 2 n + 1 O H

C n F 2 n + 1 O H → C n − 1 F 2 n − 1 C O F + H F

C n − 1 F 2 n − 1 C O F + H 2 O → C n − 1 F 2 n − 1 C O O − + H F + H +

Effective ROS in AOPs of PFASs vary based on the oxidizing agents and their activation methods. Free radicals, such as •OH, sulfate radicals (SO₄⁻•), and superoxide radicals (•O₂⁻), as well as nonradicals like singlet oxygen (¹O₂) and holes (h⁺), have been identified as dominant ROS that contribute individually or synergistically to the defluorination and destruction of PFASs through advanced oxidation ^[24][47]. Fenton and Fenton-like processes, using hydrogen peroxide (H₂O₂) as the oxidant, generate •OH as the effective ROS when activated by Fe²⁺ or other transition metal-based chemicals or materials. Activated persulfate systems, utilizing persulfate (PDS, S₂O₈²⁻) or peroxymonosulfate (PMS, HSO₅⁻), primarily generate SO₄⁻• as the dominant ROS and exhibit reactivity due to the high redox potential (+ 2.5 V~3.1 V) and long lifetime (3.4 × 10⁻⁵ s) of SO₄⁻• ^[25][26][48,49]. Meanwhile, it should be noted that the activation methods of PMS make a big difference to the types of ROS, e.g., SO₄⁻• via activation with carbon materials, SO₄⁻• and •OH via thermal and radiation activation, SO₄⁻• and peroxymonosulfate anion radicals (SO₅•⁻) via transition metal activation, and •O₂⁻ and ¹O₂ via alkali activation ^[27][50]. In addition, photogenerated h⁺ acts as a powerful ROS that directly oxidizes PFASs or converts H₂O/O₂ to •OH/•O₂⁻, thereby generating more ROS [28]. Electrochemical processes induce the rapid generation of different ROS at the anode or facilitate electron transfer to the anode ^[29][51].

2.2. Principles of Photocatalytic AOPs for PFAS

Photocatalytic AOPs for FPAS are developed based on direct photo-degradation, achieved by breaking apart the C−F bonds using light of a specific wavelength. The direct photolysis process requires a match between the adsorption spectrum of the chemical bonds and the emission spectrum of the light, with the wavelength of the light playing a crucial role ^[30][52]. For example, PFOA has demonstrated strong UV adsorption and fast degradation at 185 nm ^[31][32][53,54], while light with a wavelength above 220 nm is barely absorbed by PFASs ^[33][34][29,55]. In photocatalytic AOPs of PFASs, the indirect photo-oxidation process is characterized by decarboxylation followed by defluorination, believed to be related to the photoinduced holes that exhibit a strong oxidizing capacity for organics. These holes work synergistically with other ROS to enhance PFAS degradation ^[35][36][56,57]. The system of photocatalytic AOPs consists of three components: the light, oxidant, and photocatalyst. There are two principles of PFAS degradation in photocatalytic AOPs: direct oxidation by photogenerated holes and co-oxidation with other ROS that are generated at the surface of catalysts with the assistance of the holes. The general process of PFAS degradation in photocatalytic AOPs can be described as follows: (1) Catalysts absorb light with energy (hv) equal to or greater than the band gap, which excites electrons from the valence band (VB) to the conduction band (CB), creating holes in the VB. (2) The generated electron−hole (e⁻−h⁺) pairs migrate to the surface of catalysts and react with the adsorbed PFAS. (3) The e⁻−h⁺ pairs react with precursors and generate ROS which assist in PFAS decomposition.

h + + H 2 O → • O H + H +

e − + O 2 → • O 2 − The general mechanism of photocatalytic AOPs for PFAS degradation is summarized in Figure 2, where the PFAS is represented by PFAC. Photocatalysts play a crucial role in this process, as they are responsible for generating effective ROS and binding PFAS molecules. Both of these factors determine the efficiency of degradation ^[23][46]. Therefore, the construction and structure engineering of photocatalysts have garnered significant research interest.

Figure 2. Mechanisms of photocatalytic AOPs for the degradation of PFASs.

2.3. Principles of Electrocatalytic AOPs for PFASs

There are two types of electrocatalytic AOPs for PFASs: direct electro-oxidation and indirect electrochemical oxidation. Direct electro-oxidation is a simple AOP that occurs on the surface of an electrode (anode) with a direct transfer of electrons. It relies on the in situ generation of ROS (e.g., •OH) or the direct transfer of electrons from the PFAS to the anode ^[37]. On the other hand, indirect electrochemical oxidation processes are the primary electrocatalytic AOP for organics treatment. Unlike direct electro-oxidation, electrons in this process act as mediators or assist in the generation of powerful ROS ^[29]. For example, the degradation of PFASs starts with the release of electrons, forming C

Mechanisms of photocatalytic AOPs for the degradation of PFASs.

2.3. Principles of Electrocatalytic AOPs for PFASs

There are two types of electrocatalytic AOPs for PFASs: direct electro-oxidation and indirect electrochemical oxidation. Direct electro-oxidation is a simple AOP that occurs on the surface of an electrode (anode) with a direct transfer of electrons. It relies on the in situ generation of ROS (e.g., •OH) or the direct transfer of electrons from the PFAS to the anode [60]. On the other hand, indirect electrochemical oxidation processes are the primary electrocatalytic AOP for organics treatment. Unlike direct electro-oxidation, electrons in this process act as mediators or assist in the generation of powerful ROS [51]. For example, the degradation of PFASs starts with the release of electrons, forming C

_2n+1COO• (Equation (2)); this occurs under an anode potential higher than the oxidation potential of the PFAS ^[38]. Additionally, electrocatalytic AOPs can produce radicals and oxidants during the electrode process. This includes •OH, which is strongly adsorbed onto the anode surface (M), as shown in Equation (9), H

COO• (Equation (2)); this occurs under an anode potential higher than the oxidation potential of the PFAS [61]. Additionally, electrocatalytic AOPs can produce radicals and oxidants during the electrode process. This includes •OH, which is strongly adsorbed onto the anode surface (M), as shown in Equation (9), H

₂

from the dimerization of •OH (Equation (10)), and ozone (O

₃) from the discharge of water molecules (Equation (11)) ^[39]. These products are highly reactive with certain intermediate products during the decarboxylation and defluorination processes of PFASs, contributing to the efficiency of degradation.

M + H 2 O → M ( • O H ) + H + + e −

2 M ( • O H ) → 2 M O + H 2 O 2

3 H 2 O → O 3 + 6 H + + 6 e −

The general mechanism of electrocatalytic AOPs for PFAS degradation is illustrated in

) from the discharge of water molecules (Equation (11)) [62]. These products are highly reactive with certain intermediate products during the decarboxylation and defluorination processes of PFASs, contributing to the efficiency of degradation.

The general mechanism of electrocatalytic AOPs for PFAS degradation is illustrated in

Figure 3

. According to previous studies, after the formation of C

_2n+1• ^[40], there several approaches for further mineralization. Process (a) and (b) initially undergo a reaction with •OH to form C

• [63], there several approaches for further mineralization. Process (a) and (b) initially undergo a reaction with •OH to form C

_2n+1OH (Equation (4)). In process (a), reactions from Equation (12) to Equation (13) occur ^[41], whereas in process (b), reactions from Equation (14) to Equation (16) take place ^[42][43]. Aside from •OH, other anodic ROS such as O

OH (Equation (4)). In process (a), reactions from Equation (12) to Equation (13) occur [64], whereas in process (b), reactions from Equation (14) to Equation (16) take place [65,66]. Aside from •OH, other anodic ROS such as O

₂

also react with C

_2n+1

• (Equation (17)), and the oxidation product C

_2n+1

OO• can react with other perfluoro-alkoxy radicals (R

COO•) (Equation (18)). The resulting C

_2n+1

O• then decomposes into C

_n−1

_2n−1• for further degradation in a new cycle (Equation (19)) ^{[40][41][42][44][45]}. The third pathway (c) follows the reactions from Equation (17) to Equation (19).

C n F 2 n + 1 O H + • O H → C n F 2 n + 1 O • + H 2 O

C n F 2 n + 1 O • → C n − 1 F 2 n − 1 • + C F 2 O

C n F 2 n + 1 O H → C n − 1 F 2 n − 1 C O F + H F

C n − 1 F 2 n − 1 C O F + • O H → C n F 2 n O 2 H •

C n F 2 n O 2 H • → C n − 1 F 2 n − 1 C O O • + H F

C n F 2 n + 1 • + O 2 → C n F 2 n + 1 O O •

C n F 2 n + 1 O O • + R F C O O • → C n F 2 n + 1 O • + R F C O • + O 2

C n F 2 n + 1 O • → C n − 1 F 2 n − 1 • + C F 2 O

• for further degradation in a new cycle (Equation (19)) [63,64,65,67,68]. The third pathway (c) follows the reactions from Equation (17) to Equation (19).

Figure 3. Mechanisms of electrocatalytic AOPs for the degradation of PFASs.

In electrocatalytic processes, oxidation primarily occurs on the anode. Therefore, the choice of anode materials (i.e., electrocatalysts) plays a crucial role in electrocatalytic AOPs for PFAS degradation. The behavior of PFAS degradation can vary depending on the type of anode material used. Anode materials are classified into two types based on the interactions between the adsorbed •OH on the anode surface and the degradation of organics: active anodes and nonactive anodes. Active anodes, such as Ti/SnO

₂

−Sb/MnO

₂ ^[40], have a low potential for O

[63], have a low potential for O

₂

evolution and are distinguished from non-active anodes, such as Ti/SnO

₂−Sb ^[46], by their ability to transform M(•OH) into strong oxidants. Generally, anode materials with higher O

−Sb [69], by their ability to transform M(•OH) into strong oxidants. Generally, anode materials with higher O

₂-evolution potential exhibit weaker interactions between M(•OH) and their surface, but they have higher reactivity towards PFASs ^[29].

-evolution potential exhibit weaker interactions between M(•OH) and their surface, but they have higher reactivity towards PFASs [51].

3. Photocatalysts in AOPs for PFAS Degradation

3.1. Metal Oxide-Based Materials

.1. Metal Oxide-Based Materials

Metal oxides, such as TiO

₂

, In

₂

₃

, and Ga

₂

₃, have long been used as traditional semiconductors in the photocatalytic degradation of organics in water. These metal oxides have been extensively studied for PFAS degradation ^{[23][47][48][49]}. TiO

, have long been used as traditional semiconductors in the photocatalytic degradation of organics in water. These metal oxides have been extensively studied for PFAS degradation [27,46,76,77]. TiO

₂

-based materials, in particular, have been widely used as photocatalysts since the discovery of water splitting on a TiO

₂ anode by Fujishima and Honda in 1972 ^[50]. Though TiO

anode by Fujishima and Honda in 1972 [78]. Though TiO

₂

has shown promise in heterogeneous photocatalysis due to its strong UV absorption, non-toxicity, and long-term photostability, it is not efficient for photocatalytic PFAS degradation. This is due to its narrow spectral range, wide bandgap (3.0 eV for the rutile phase and 3.0 eV for the anatase phase), low electron-hole separation efficiency, and poor adsorption performance. Therefore, modifications of TiO

₂ are necessary to enhance its photocatalytic activity. Strategies for modification include metal/nonmetal element doping, carbon material loading, and heterostructure construction. To date, doping with Fe ^[51], Cu ^[51], Pb ^[48][52], Pt ^[53], Pd ^[54], Ag ^[55] in TiO

are necessary to enhance its photocatalytic activity. Strategies for modification include metal/nonmetal element doping, carbon material loading, and heterostructure construction. To date, doping with Fe [79], Cu [79], Pb [24,76], Pt [80], Pd [81], Ag [82] in TiO

₂ for enhanced PFAS degradation has been studied, as well as the co-doping of metals, such as Fe/Nb ^[56]. Metal doping involves controlling the doping amount and regulating the pH of the solution to avoid the competitive adsorption of OH

for enhanced PFAS degradation has been studied, as well as the co-doping of metals, such as Fe/Nb [70]. Metal doping involves controlling the doping amount and regulating the pH of the solution to avoid the competitive adsorption of OH

⁻

on the catalyst surface under alkaline conditions.

₂

₃

is a PFAS affinity material with a narrow bandgap of 2.8 eV, exceptional photocatalytic activity, and sensitivity to visible light. When compared to TiO

₂

, In

₂

₃ has shown a remarkable 8.4-fold increase in the degradation rate coefficient of a PFAS (PFOA) under UV irradiation. These findings suggest it is a promising photocatalyst for PFAS decomposition ^[23]. Modifications are necessary for In

has shown a remarkable 8.4-fold increase in the degradation rate coefficient of a PFAS (PFOA) under UV irradiation. These findings suggest it is a promising photocatalyst for PFAS decomposition [46]. Modifications are necessary for In

₂

₃

due to its limitations, i.e., its low specific surface area and the rapid recombination of photogenerated electron-hole pairs. One effective approach is the generation of oxygen vacancies on the In

₂

₃ surface, which enhances its photocatalytic performance. Additionally, nanostructures like nanospheres ^[19], (porous) nanosheets, and nanocubes ^[57][58] have been developed to provide adsorption sites for PFOA and oxygen atom binding sites in the carboxyl groups. These modifications ultimately contribute to the improved photocatalytic decomposition of PFOA. Several composite materials have been reported, such as g-C

surface, which enhances its photocatalytic performance. Additionally, nanostructures like nanospheres [42], (porous) nanosheets, and nanocubes [91,92] have been developed to provide adsorption sites for PFOA and oxygen atom binding sites in the carboxyl groups. These modifications ultimately contribute to the improved photocatalytic decomposition of PFOA. Several composite materials have been reported, such as g-C

₃

₄

−In

₂

₃ ^[59], CeO

[93], CeO

₂

−In

₂

₃ ^[60], and MnOx−In

[94], and MnOx−In

₂

₃ ^[61].

[74].

₂

₃

has excellent conductivity and tunable optical properties, despite its wide band-gap (4.9 eV). Studies have demonstrated its remarkable UV photocatalytic activity against PFASs, specifically in the context of PMS-assisted photocatalytic AOPs. The Ga

₂

₃

/PMS/UV system, with SO

₄⁻

• and •O

₂⁻ as key ROS, achieves 100% degradation within 60 min ^[62].

as key ROS, achieves 100% degradation within 60 min [38].

3.2. Bi-Based Materials

Bismuth (Bi)-based photocatalysts have emerged as one of the most promising photocatalytic materials for catalysis, primarily due to their non-toxicity, high stability, and low cost. Commonly used Bi-based compounds for PFAS photocatalytic AOPs include bismuth oxyhalides (BiOX, where X is Cl, Br, I), bismuth ferrite (BiFeO

₃

, BFO), bismuth phosphate (BiPO

₄

), and bismuth hydroxyphosphate (Bi

₃

O(OH)(PO

₄

)

₂

, BiOHP). BiOX is a 2D layered compound with alternating double X ion layers and Bi

₂

layers along the

-axis. An internal electric field is formed between the halide planes and Bi

₂

₂ layers, promoting faster charge transfer, enhanced redox potential, and excellent photocatalytic performance for effective PFAS degradation ^[63]. BiOI has a narrow bandgap (E

layers, promoting faster charge transfer, enhanced redox potential, and excellent photocatalytic performance for effective PFAS degradation [39]. BiOI has a narrow bandgap (E

= 1.67~1.92 eV) and high visible light absorption, showing great potential for applications. However, the narrow bandgap leads to the easy recombination of photoinduced e

⁻/h+ pairs, which affects the photocatalytic activity ^[64]. Researchers have synthesized Br-doped BiOI (BiOI

/h+ pairs, which affects the photocatalytic activity [96]. Researchers have synthesized Br-doped BiOI (BiOI

_0.95

_0.05) for the photocatalytic degradation of PFOA ^[65]. Br doping not only increases PFOA adsorption but also expands the UV absorption range, leading to a significant enhancement in photocatalytic activity. However, the impact of non-metals on the structural properties of the semiconductor, in terms of stability, reproducibility, loading capacity, and practical applications of BiOI, requires further investigation. BiOCl, another noteworthy semiconductor with an indirect bandgap (E

) for the photocatalytic degradation of PFOA [97]. Br doping not only increases PFOA adsorption but also expands the UV absorption range, leading to a significant enhancement in photocatalytic activity. However, the impact of non-metals on the structural properties of the semiconductor, in terms of stability, reproducibility, loading capacity, and practical applications of BiOI, requires further investigation. BiOCl, another noteworthy semiconductor with an indirect bandgap (E

_g ranging from 2.62 to 3.46 eV) possesses excellent electronic and optical properties and has become a popular photocatalytic material ^[66].

3.3. Other Compounds and Composites

In addition to metal-oxides and Bi-based materials, various photocatalysts have been employed in photocatalytic AOPs for PFAS degradation. These include metal/transition metal-based materials, metal-free materials, and modified composite materials. Qian et al. proposed a heterogeneous photocatalytic degradation mechanism for PFOA using Fe-zeolite under UVA irradiation (wavelength range: 320–420 nm) with O

ranging from 2.62 to 3.46 eV) possesses excellent electronic and optical properties and has become a popular photocatalytic material [98].

3.3. Other Compounds and Composites

₂ as the terminal oxidant ^[67]. This Fe-zeolite catalyst, compared to homogeneous Fe

as the terminal oxidant [107]. This Fe-zeolite catalyst, compared to homogeneous Fe

³⁺

, exhibits a broader light absorption range and can oxidize Fe

²⁺

to Fe

³⁺

in the presence of O

₂, generating reactive species that contribute to PFAS mineralization. Other photocatalysts, such as samarium doped ferrite ^[68] and platinum-modified indium oxide nanorods (Pt/IONRs) ^[69], have also demonstrated significant PFOA degradation (48.6% and 98.0%) within a short period of time (1 h) in photocatalytic AOPs. The high degradation efficiency of Pt/IONRs can be attributed to Pt loading, the rod-like structure of the catalyst, and the presence of surface oxygen vacancies, which promote light harvesting, enhance the separation efficiency of the photogenerated charge carriers, and accelerate PFOA degradation.

, generating reactive species that contribute to PFAS mineralization. Other photocatalysts, such as samarium doped ferrite [108] and platinum-modified indium oxide nanorods (Pt/IONRs) [109], have also demonstrated significant PFOA degradation (48.6% and 98.0%) within a short period of time (1 h) in photocatalytic AOPs. The high degradation efficiency of Pt/IONRs can be attributed to Pt loading, the rod-like structure of the catalyst, and the presence of surface oxygen vacancies, which promote light harvesting, enhance the separation efficiency of the photogenerated charge carriers, and accelerate PFOA degradation.

4. Electrocatalysts in AOPs for PFAS Degradation

4.1. BDD-Based Materials

Boron-doped diamond (BDD) is a one of the most frequently used anode materials in the electrocatalytic degradation of PFASs, owing to its wide operational potential window, excellent chemical stability, and high oxidation potential (2.7 V vs. SHE) ^[42][70][71][65,126,127]. The BDD electrode plays a crucial role in the direct electrochemical oxidation of PFASs at low current densities, rather than relying on the •OH oxidation process at high current densities ^[9][72][11,128]. BDD films have been proven to be effective anodic electrocatalysts for PFOS degradation ^[71][127], with electron transfer from PFOS to the anode resulting in the formation of final products such as SO₄²⁻, F⁻, CO₂ and a small amount of trifluoroacetic acid. However, the widespread application of BDD as an anode material in electrocatalytic AOPs for PFASs is limited due to its high cost and a lack of suitable electrode substrates for BDD. Metals and nonmetals like Ta, Nb, W, and Si have been investigated as BDD substrates, and Si/BDD has been found to be a cost-effective option for PFOS degradation (>90%) ^[73][74][123,129].

4.2. Metal Oxide-Based Materials

The utilization of metal oxides as electrode materials has revealed the drawbacks associated with metal electrodes, such as lower oxygen evolution overpotential and susceptibility to oxidation. Additionally, it has made it possible to overcome the limitations of BDD electrodes, including low conductivity and low efficiency in terms of utilizing •OH radicals ^[75][131]. Currently, metal oxides commonly used as electrode materials include SnO₂, PbO₂, TiO₂, MnO₂, and others. Among these, SnO₂ and PbO₂ demonstrate outstanding electrocatalytic oxidation performance, with a higher overpotential of O₂ evolution, making them suitable for the degradation of organics. SnO₂, a semiconductor with a bandgap of 3.5eV, faces challenges in its direct use as an electrode material due to its high resistance. However, its conductivity can be improved through doping ^[76][77][78][22,122,132], particularly with antimony (Sb) ^[40][79][63,125]. In addition, SnO₂−Sb electrodes exhibit a high oxygen evolution potential, which contributes to their extensive application in PFAS degradation ^[40][42][80][63,65,110]. By doping Ti/SnO₂−Sb with multiple metals, a PFOA removal rate of 93.3% can be achieved ^[40][42][63,65]. It is important to note that the presence of SO₄²⁻ in the solution can cover the active sites on the electrode. This reduces the production of •OH radicals, subsequently decreasing the PFOA removal rate ^[80][110]. PbO₂ anodes offer several advantages for efficient PFAS degradation, such as low processing costs, simple preparation, high conductivity, and a high oxygen evolution potential ^[81][82][133,134]. However, the issue of PbO₂ detachment in Ti/PbO₂ electrodes hampers their stability. To address this, Ti/SnO₂−Sb/PbO₂, and TiO₂-Nanotubes (NTs)/Ag₂O/PbO₂ electrodes have been developed, effectively enhancing the stability and electrochemical degradation capability with the assistance of an interlayer ^[83][84][85][124,135,136]. A study was conducted ^[86][137], highlighting the contribution of interlayer metal/metal oxide anodes to the efficiency of PFOS degradation. However, PbO₂ electrodes are susceptible to Pb²⁺ leaching ^[81][82][133,134]; this can be mitigated through doping with elements to reduce the grain size, increase the electroactive surface area, improve the oxygen evolution potential, and enhance electron migration ability. Doping with cerium (Ce), ytterbium (Yb), and zirconium (Zr) has resulted in removal rates of over 88% for PFOA ^[44][45][87][67,68,138]. A Ti/SnO₂−Sb/PbO₂−Ce electrode exhibited removal rates of over 92% for PFDA and PFNA. It is worth noting that the electrochemical degradation rate of PFASs on Ti/SnO₂-based electrodes is influenced by the chain length, emphasizing the need to tailor the catalyst according to the PFAS chain structure ^[40][44][88][63,67,139]. Carbon-based materials also contribute to electrocatalytic PFAS degradation as substrates for PbO₂. For instance, a 3D graphene (3DG)−PbO₂ anode obtained through electro-deposition exhibited a degradation rate constant for PFOS that was 2.33 times higher than that of PbO₂ anodes ^[89][140]. This can be attributed to the porous nanostructures, resulting in a larger specific surface area and multiple electronic transfer channels in the anode.

4.3. Other Compounds and Composites

Composites and hybrids used as anode materials for the electrocatalytic degradation of PFASs can take various forms, employing different mechanisms to improve electrocatalysis performance. However, most of the composites or hybrids used as electrocatalysts are either BDD-based or metal oxide-based materials, as discussed above. Furthermore, there are only a few studies available on this topic, leaving ample room for further research ^[90][91][92][19,20,143]. In terms of electrode materials, one scarcely investigated type which has demonstrated high reactivity and chemical robustness is the multifunctional single-atom catalyst (SAC). SACs show promise in electrocatalytic PFAS degradation ^[93][144].

5. Conclusions

Material modification methods for enhancing PFAS degradation in photocatalytic and electrocatalytic AOPs are summarized as follows:

(1)

For catalysts in the photocatalytic oxidation of PFAS systems, current research primarily focuses on improving catalyst activity by addressing the rapid recombination of photogenerated e⁻−h⁺. Methods include introducing surface defects or oxygen vacancies, metal-doping, heterojunction construction, and crystal facet regulation ^{[28][61][63][64][94][95][96][97]}[21,26,28,39,71,74,96,101]. Furthermore, material composites and morphology regulation have been utilized to enhance reaction probabilities between PFASs and active groups, effectively enhancing the efficiency of PFAS degradation ^{[17][28][98][99]}[25,28,40,104].

(2)

For catalysts in the electrocatalytic oxidation of PFAS systems, current research mainly focuses on increasing the yield of active groups through various methods ^{[60][94][100]}[21,85,94]. Additionally, enhancing the efficiency of electron transfer and mass transfer processes is achieved through metal loading and constructing nano 3D structures ^{[20][83][89][100]}[43,85,124,140]. The stability of electrodes is also improved through the construction of intermediate layers ^[40][101][63,145].