Photodegradation and photocatalysis of per- and polyfluoroalkyl substances (PFAS): A review of recent progress

Per- and polyfluoroalkyl substances (PFAS) are oxidatively recalcitrant organic synthetic compounds. PFAS are an exceptional group of chemicals that have significant physical characteristics due to the presence of the most electronegative element (i.e., fluorine). PFAS persist in the environment, bioaccumulate, and have been linked to toxicological impacts. Epidemiological and toxicity studies have shown that PFAS pose environmental and health risks, requiring their complete elimination from the environment. Various separation technologies, including adsorption with activated carbon or ion exchange resin; nanofiltration; reverse osmosis; and destruction methods (e.g., sonolysis, thermally induced reduction, and photocatalytic dissociation) have been evaluated to remove PFAS from drinking water supplies. In this review, we will comprehensively summarize previous reports on the photodegradation of PFAS with a special focus on photocatalysis. Additionally, challenges associated with these approaches along with perspectives on the state-of-the-art approaches will be discussed. Finally, the photocatalytic defluorination mechanism of perfluorooctanoic acid (PFOA) and perfluorooctanesulfonic acid (PFOS) following complete mineralization will also be examined in detail.


Introduction
The extensive use of per-and polyfluoroalkyl substances (PFAS) in aqueous film-forming foams (AFFFs), non-stick technologies, and a variety of other coatings and industrial products has led to their widespread contamination in aqueous environments [1][2][3][4][5][6][7][8][9].These organic compounds are universally found and dispersed in the aquatic environment across their life cycle through manufacturing, across the supply chain, product use, end-use, and manufacturing materials [10][11][12][13][14][15].Consequently, PFAS have been introduced into aquatic environments from various non-point sources.The most significant volume of emitted PFAS (~95%) is directly released into the marine environment or ends in aquatic media as part of their fate and transport [16].Thus, PFAS pose a major threat to drinking water supplies due to their associated ecological and human health risks.PFAS, especially perfluorooctanesulfonic acid or perfluorooctane sulfonate (PFOS) and perfluorooctanoic acid (PFOA), have been widely detected in the blood of humans and wild animals as well as the environment [17][18][19][20].These compounds have been extensively used in commercial and industrial applications as coatings for textiles, paper products, and cookware surface treatment and to formulate some firefighting foams for almost 60 years.After careful evaluation of the toxicological effects of these compounds, the application and manufacture of longer-chain PFAS have been limited [16,21].
In 2016, the U.S. Environmental Protection Agency (US EPA) issued a lifetime health advisory for PFOA and PFOS, which was further revised to a near-zero standard in 2023 [22][23][24][25][26].However, several countries have continued manufacturing and using PFAS, contributing to additional environmental pollution with high concentrations of these compounds [27,28].At environmentally relevant pH values, PFAS are organic anions and tend to be mobile in groundwater [29].An extensive database of toxicity information is available for some other PFAS, but the toxicity of PFOA and PFOS remains the most studied [30][31][32].
Various methods have been used to separate PFAS from contaminated water and degrade these substances into harmless and environmentally friendly products.Treatment technologies span a wide range of in-situ processes, non-destructive methods, ex-situ separation, or destructive methods [33].Physical adsorption processes, including granular activated carbon (GAC), powdered activated carbon (PAC), anion exchange (AIX), molecularly imprinted polymers (MIP), and biomaterials, have been evaluated for the removal of PFAS from various complex water matrices.High-pressure-driven membrane processes (e.g., nanofiltration (NF) and reverse osmosis (RO)) have also been investigated (Fig. 1) [34].NF and RO processes have shown to be effective in removing a broad range of PFAS, but their wide implementation has been hampered by cost and membrane fouling.AIX has shown higher adsorption capacity than GAC and PAC [35,36].However, AIX is costly, making it economically infeasible for a large-scale application.Additionally, due to the slow dispersal of PFOA and PFOS in the pores of porous materials, AIX has shown a slow adsorption rate [37].Moreover, the presence of other constituents (e.g., natural organic matter (NOM)) in surface waters can significantly lower the adsorption capacity [38].
More importantly, activated carbon, ion-exchange resins, and reverse osmosis are separation processes that generate secondary waste (i.e., concentrated waste that requires secondary treatment).Adsorption processes require regenerating the spent adsorbents after the adsorption bed breakthrough using thermal desorption, or require fresh adsorbent [39][40][41][42].Recently, several promising PFAS degradation technologies, such as biological degradation [43] as well as advanced oxidation technologies (AOPs), including photochemical, sonolysis, electrochemical, thermolysis, chemical oxidation and reduction, plasma, subcritical and radiochemical treatment have been explored for the complete mineralization of PFAS [44].Among these degradative technologies, the photochemical process showed more PFAS dissociation and defluorination at ambient reaction conditions.Previous studies have suggested that photocatalysis is a promising technology for the degradation of PFAS [45].
This paper provides a critical review of recent investigations on various aspects of PFAS photodegradation with a particular emphasis on photocatalysis.Additionally, challenges with these approaches along with the mechanisms associated with the photocatalytic dissociation of PFAS will be critically evaluated.

Catalyst-free photodegradation of PFAS using ultraviolet (UV) light
Continued exposure of organic compounds such as PFAS to radiation with wavelengths less than 320 nm, known as actinic wavelengths, results in photodissociation via photolysis.In a challenge to eliminate PFAS accumulation in the environment, various studies have examined their possible UV degradation into harmless species under mild reaction conditions.In photolysis, the dissociation of organic pollutants is motivated by the adsorption of photons, thus providing a new reaction pathway via the formation of electronically excited reactive species [46,47].Two forms of photolysis are capable of degrading PFAS: (1) Direct photolysis and (2) Indirect photolysis [48,49].In direct photolysis of PFAS, photons are directly absorbed by the PFAS, causing it to undergo photodegradation.In fact, without using oxidants or photocatalysts, water is cleaved into hydrogen and hydroxyl radicals under UV radiation.For indirect photolysis (e.g., UV/H 2 O 2 , UV/Ozone, and photo-Fenton), separate compounds absorb photons (light energy/radiation) and then work as an intermediate to react with the contaminant (i.e., PFAS) [50,51].
In this regard, for a practicable photolysis process, the strong C-F bond in PFAS should be cleaved by UV irradiation into fluoride (F − ) ions.In water, F − ions can easily react with Ca 2+ ions, forming an environmentally friendly compound such as inorganic calcium fluoride (CaF 2 ) [52,53].Jing et al. [54] reported an efficient degradation of PFOA (initial concentration: 25 parts per million (ppm)) in water (61.7%)into fluoride ions, perfluoroheptanoic acid, perfluorohexanoic acid, perfluoropentanoic acid, and perfluorobutanoic acid within two hours of 185 nm vacuum ultraviolet (VUV) light irradiation.This study proposed a mechanism for the degradation of PFOA through the Higher temperatures boost PFOS photodegradation via the reductive process due to creating a chemically favored local environment and the enhanced interfacial mass transfer at the gas-liquid interface [57].The rate of PFOS dissociation increases with the increasing heating intensity under UV irradiation in the absence of photocatalyst.However, slow dissociation was reported under oxygenation and higher hydronium levels [57].
The effects of higher pH via a catalyst-free reductive route of PFOS showed a pseudo-firstorder decomposition rate constant of 0.91 h −1 in an aqueous solution [58], two orders of magnitude higher than earlier studies [59].Photolysis has limited effects on degrading PFAS under environmental conditions, but under higher VUV and UV energy, PFAS can be dissociated.Competitive UV irradiation absorption by NOM limits the rate of photolysis and photochemical degradation [60,61].NOM primarily hinders photodegradation by consuming UV photon energy, acting as a scavenger for reactive species, including hydroxyl and peroxyl radicals and generating NOM-derived oxidative intermediates [61].Table 1 reports a few non-catalytic photodegradation approaches for PFAS mineralization.
Hori et al. [81] showed that persulfate (S 2 O 8 2 − ) influenced the photochemical dissociation of PFOA and yielded 100% mineralization within 4 h of 254 nm UV light irradiation.In this process, PFOA reacted with SO 4 .− (from Eq. 6) and was then converted into C 7 F 15 COO • radicals (Eq.7), which was further degraded into short-chain perfluorinated carboxylic acids.In another study, under 185 nm irradiation, PFOA was equally dissociated via direct photolysis and by SO 4 .− (Scheme 4; Eq. 8) [82].The dissociation rate constant was reported to be 1.8 times greater than that at irradiation under 254 nm.This investigation demonstrated that the dissociation efficiency of PFOA was enhanced under 185 nm compared to that of 254 nm irradiation technology.
It was also observed that the presence of Fe 2+ accelerated the photodegradation of persulfate ions into sulfate radicals, SO 4 .− (Eq.9) [83].A combination of Fe 2+ /UV with persulfate ions enhanced the mineralization of PFOA.
In the presence of other organic compounds, the degradation of PFAS through persulfate/UV was inhibited [84].Therefore, pretreatment by removing NOM from the water matrix is needed prior to degrading PFAS.

Photocatalytic degradation of PFAS
Photocatalysts present in water under UV light irradiation produce reactive species such as hydroxyl radicals (•OH) as well as other photochemically generated reactive intermediates [87].These reactive species actively react with pollutants such as PFAS to degrade them into harmless intermediates.While a couple of review papers focused on the photocatalytic degradation of PFAS in solutions [88][89][90], this review covers the photodegradation pathways and catalytic performance as it relates to the photocatalyst (i.e., material).
Semiconductor photocatalysts absorb UV light to produce conduction band (CB) electrons (e − ) and valence band (VB) holes (h + ) to degrade PFOS through redox reaction.The reaction under UV light irradiation and at higher pH (pH=11.8)can also occur via chargetransfer-to-solvent to produce strongly oxidizing OH • .Then, e aq − acts as the main reductive species and can break C-C and C-F bonds in PFOS, leading to the complete mineralization of PFAS.These results support Qu et al. findings that the concentration of hydrated electron increased with an increase in the initial pH and enhanced the defluorination of PFOA in the UV-KI system [91].The reaction rate constant for PFOA degradation was reported as 0.0295 min −1 at pH=10, about 49 times higher than that at pH-5.Thus, initial pH conditions increased the concentration of hydrated electrons and supported the reductive mineralization of PFAS in a homogeneous catalytic system [92].This section discusses the role of nanocomposites and microparticle-based photocatalysis.

Nanocomposite-based photocatalysis for PFAS mineralization
The C-F bond in PFAS compound is very strong and requires an innovative method to facilitate C-F cleavage by directing energy transfer or contact with reactive species to the C-F locus [87].Semiconductor nano-photocatalysts, including titanium dioxide and nanostructured indium oxide, have been employed in water treatment applications [93].
The bandgap between the CB and VB of the material electrons plays a major role in the activation of a photocatalyst.The bandgap represents the minimum energy needed to stimulate an electron from the valance band to the conduction band and generates holes in the valance band.Further, these holes and electrons in combination generate reactive oxygen species such as superoxide (O 2 − ), and hydroxyl radical (OH • ) [94].The bandgap also depends on other factors, including size, structure, composition, and surface ligands of the nanocatalyst [95].This section, provides literature survey of nano-enabled photodegradation technologies for treating PFAS is provided.
A plausible reaction mechanism for the photodegradation of PFOA using TNTs under UV light is shown in Scheme 5.In aqueous media, PFOA exists in the anionic form, which can be adsorbed to the positively charged TNT to form TNTs-C 7 H 15 COO − complex (Scheme 5; Eq. 10 and 11).As a result, unused PFOA is converted into an excited state of PFOA and then photolyzed into C 7 F 15 and COOH − radicals under UV irradiation (Scheme 5; Eq. 12).Further, C 7 F 15 • radical adsorbed to the surface of excited TNTs to form TNTs-C 7 F 15 complex (Eq.13).In addition, this intermediate radical (C 7 F 15 • ) also reacted with water to form fluorinated alcohol (C 7 F 15 OH).In the next step, HF is eliminated from C 7 F 15 OH to form C 6 F 13 COF (Eq.14), followed by the formation of C 6 F 13 COOH (PFHpA) with one less CF 2 moiety (Eq. 15).

Alternative nano-enabled technologies for PFAS treatment
TiO 2 -based photocatalysts are effective in degrading most organic pollutants [104].However, TiO 2 has limited effectiveness in degrading PFOA [96][97][98][99][100][101][102]105,106].Other nanoenabled AOPs have been explored to treat PFAS (Table 3).Electron-hole pair segregation and bandgap variation have effectively enhanced photocatalyst reactivity [107].The introduction or immobilization of foreign metal on the surface of the photocatalyst/support can improve the electron-hole segregation.The usage of nanostructured metal oxides such as indium oxide (In 2 O 3 ) with narrower bandgap, as well as nanoplates and porous microspheres with high surface area, can also improve the photocatalytic performance of photocatalysts [108].
Li et al. [108] developed In 2 O 3 as a photocatalyst for the efficient degradation of PFOA under UV irradiation with rate constant ~8.4 times higher than TiO 2 .Their findings suggest that the COO − (carboxylate) functional group of PFOA strongly binds to the In 2 O 3 surface via co-ordination bond in a bridging or bidentate arrangement, which is useful in the direct PFOA photodegradation under UV irradiation (Fig. 2).Solid-state fluorine-19 nuclear magnetic resonance ( 19 F Mass NMR) analysis of TiO 2 confirmed the interaction of the inner CF 2 group of PFOA with OH group of TiO 2 surface via hydrogen bond (Fig. 2).PFOA molecules arrange onto the TiO 2 surface in a monodentate manner, then photogenerated holes favorably convert to OH  [113] for the efficient photocatalytic mineralization of PFOA into non-toxic by-products (CO 2 and fluoride ions) under UV light irradiation and mild reaction conditions (e.g., room temperature, weak acidic condition atmospheric pressure) (Fig. 4) (Table 4).Another needle-like nanostructured photocatalyst, gallium oxide (β-Ga 2 O 3 ), has been reported to decompose PFOA via photo-reductive decomposition in surface or wastewater following first-order rate constants of 3.51 and 4.03 h −1 , respectively [114].In wastewater treatment and under UV irradiation of 185 nm, β-Ga 2 O 3 exhibited higher efficacy for the elimination of trace amounts of PFOA.
Other studies have reported photocatalysts that have been deactivated due to their interaction with natural organic compounds typically present in wastewater during PFOA treatment.In another study, Huang et al. [115] reported using SiC-graphene as a catalyst to degrade PFOA via hydrodefluorination process (HDF) under UV light (Scheme 5).Using SiC-graphene as a catalyst, PFOA photodegradation occurred by photo-stimulating electrons on SiC that rapidly transferred to the PFAS group through graphene surface resulting in a reduction in the electron cloud density of C-F bond [115].
Studies have shown that the photodegradation of PFOA followed the hydrodefluorination mechanism through a two-step process that starts with reactive Si-H bonds forming on SiC-graphene surface under UV light irradiation.In the second step, F atoms at the α-position of the perfluoroalkyl group are substituted by hydrogen atoms (from Si-H) to form C n F 2n HCOOH via the Si-H/C-F redistribution because of the nucleophilic substitution reactions (Scheme 5).C n-1 F 2n-1 COOH is formed eliminating CH 2 carbene from C n F 2n HCOOH and the Photo-Kolbe decarboxylation reaction via carbon-carbon bond cleavage under UV light irradiation.C 7 F 15 radical is formed by the reaction of PFOA with a photogenerated electron from the SiC-graphene catalyst, which can then be mineralized through the HDF step (Scheme 6) [115].
Huang et al. [116] also studied the effectiveness of photodecomposition of both branched and linear PFOS using superior photocatalyst SiC/graphene quantum dots (SiC-GQDs), where GQDs acted as an electron donor under 254 nm UV light irradiation [116].It has been reported that PFOS was more challenging to degrade compared to PFOA.
GQDs were derived by oxidizing graphene using ultra-high frequency ultrasonication.This process is composed of SiC/GQDs nanocomposites, which involve the attachment of GQDs to SiC nanoparticles via a hydrothermal method.The synthesized material was tested for the photocatalytic degradation of PFOS.In this photoreaction, the photogenerated electrons (e cb − ) were generated from π−π * transition of C=C bond and the n−π * transition C=O bond.
Next, because of the heterojunction structure of SiC/GQDs, these electrons were transported from the LUMO of GQDs to the CB of silicon carbide (SiC) nanoparticles.The e cb − can be directionally transferred to the electron acceptor-PFOS, which is accumulated as a surfactant on the surface of the hydrophobic nanocomposite (SiC), resulting in the critical activation of -SO 3 − group.Next, PFOS was converted into C n F 2n + 1 • free radicals and further dissociated to short-chain perfluorinated carboxylic acids via hydrolysis and hydrodefluorination [116].
Recently, Xu et al. [117] reported promising platinum-modified indium oxide nanorods (Pt/IONRs) photocatalysts for PFOA mineralization under UV light irradiation.Loading of Pt and the rod-like morphology of indium oxide promote light-harvesting, which further increases the charge carrier separation rate, enhancing of the photocatalytic ability of this catalyst.Along with the oxygen vacancies on the Pt/INORs surface, it also stimulated the photooxidation of PFOA [117].
In summary, heterogeneous nanocomposites photocatalysis is a good alternative for eliminating of PFAS under UV light irradiation (Table 4).However, further work is needed to establish the toxicity of the reaction by-products.The stability and leaching of the catalyst should also be determined before scaling-up the technology.Thus, the nanomaterials should be measured cautiously to ensure the safety of this technology.In natural water, to access the practicability of these technologies, the effect of dissolved organic matter such as humic acid, fulvic acid, and other coexisting ions (e.g., CO 3 2 − , SO 4 2 − , NO 3 − ) should be evaluated.Sahu et al. [118] recently studied the rapid degradation and mineralization of PFOA using bismuth oxyhydroxphosphate (Bi 3 O(OH) (PO 4 ) 2 ) microparticle as a photocatalyst under UV light irradiation.His research group also synthesized other reference catalysts such as sheaf-like β-Ga 2 O 3 nanomaterial [119] and sub-micrometer particles BiPO 4 photocatalysts [120] and tested for PFOA photodegradation.Compared to these reference catalysts, BOHP microparticles showed intensely quicker PFOA degradation and mineralization.

Bismuth oxyhydroxphosphate (Bi 3 O(OH)(PO 4 ) 2 ) microparticle ultraviolet photocatalyst
The rate constant for photocatalytic dissociation of PFOA by BOHP was approximately 15 times higher than both β-Ga 2 O 3 and BiPO 4 in the presence of NOM [121].The BOHP photocatalyst was also examined at low PFOA concentrations and a rapid photocatalytic dissociation of PFOA was still achievable.

Photocatalytic degradation of PFAS via ozonation under UV light
Ozone (O 3 ) gas has been employed for the photodegradation of organic contaminants, PFOA, and PFOS [122][123][124][125][126]. Recently, Huang et al. [127] reported the efficient degradation of PFOA by photocatalytic ozonation using TiO 2 and UV light irradiation.Hence, the recombination of holes and photogenerated electrons decreased, leading to the enhanced PFOA photooxidative efficiency.In the presence of air, PFOA showed prolonged reductive degradation due to available reactive hydrated electrons (e aq − ) (generated from the photolysis of water) (Eq. 3) reacting with dissolved oxygen rapidly.Therefore, PFOA can be effectively photodegraded in the absence of O 2 [128].However, a recent study by Lashuk et al. [129] reported that photocatalytic ozonation inefficiently degraded PFAS with WO 3 / TiO 2 under UVA-visible radiation, where the degradation efficiency was comparable to photocatalysis.

Summary
PFAS are concerning compounds due to their bioaccumulative and persistence abilities.PFOA and PFOS are the main types of PFAS, which have been broadly detected in wildlife, humans, and the environment.Due to the strong C-F bond, these highly persistent perfluorinated chemicals are not easily degraded into unharmful compounds via biodegradation and hydrolysis in complex environmental matrices.Various degradative technologies such as electrochemical degradation, sonolysis, chemical redox, thermal degradation, and photolysis have been evaluated for PFAS elimination.In this review, photocatalytic degradation has shown promise in the effective mineralization of PFAS in aqueous media under UV light irradiation.PFAS treated with UV/photocatalyst undergoes photo-reduction or photooxidation to yield short-chain organic compounds, F-, CO 2 , etc.
Most photodegradation studies have been performed using synthetic PFOA solutions.Additional studies using actual complex wastewater containing NOM/other dissolved organic species are needed to thoroughly evaluate the process at a larger scale.In this regard, the present review also provides detailed information on the plausible PFOA and PFOS photodegradation mechanisms, which can assist researchers in establishing and designing the process scale-up.
State-of-the-art approaches have primarily employed heavy metals to develop photocatalysts with high electron conductivity, resistivity, and large surface area.The toxicity and scarceness of these metals call for environmentally friendly materials that can exhibit promising photocatalytic performance.To enhance the photoactivity of photocatalysts, future research should focus on the synthesis of diverse photocatalyst, which can also harvest energy under a broader visible light spectrum.The technology should be validated for varying water matrices to comprehensively assess the cost and environmental impacts associated with these technologies from energy consumption, kinetics, and life cycle perspectives.Further evolution of the enhanced photodissociation of PFAS technology may combine photo-and electrochemical systems for in-situ mineralization techniques.Moreover, the intermediate and free radical nature generated from the photodissociation methods still needs to be fully understood.. Investigation of reaction intermediates for fate, transport, ecotoxicity, and health risk should be conducted.This review is intended to provide researchers in this field a better understanding of recent advances and stimulate further discussions on the photodegradation of perfluorinated compounds.

Disclaimer
The U.S. Environmental Protection Agency funded and collaborated in the work described here.It has been subjected to the Agency's review and has been approved for publication.Note that approval does not signify that the contents necessarily reflect the views of the Agency.Overview of the state-of-the-art technologies employed for the degradation of PFOA and PFOS.Plausible reaction mechanisms for the PFOA photodissociation on TiO 2 and In 2 O 3 surface.

-induced oxidation for PFAS photochemical decomposition-Although persulfate
[101]pectively, in the presence of TiO 2 after 3 h of UV light irradiation.Following nitrogen purging of PFOA solution to remove dissolved oxygen, treatment with 3 mM oxalic acid (pH 2.47) resulted in 86.7% degradation.In contrast, the oxygenated solution of PFOA showed only 6.6% PFOA mineralization under the same reaction conditions.Several TiO 2 modifications have been established and explored to enhance the performance of PFAS dissociation.Tian et al. [99]used silver nanoparticles and molecularly imprinted polymers to modify TiO 2 nanotubes (MIP-Ag/TiO 2 NTs) for the photocatalytic degradation of PFOA into shorted chain fluorinated compounds.MIP-Ag/TiO 2 NTs decomposed 91% of the PFOA within 8 h of irradiation and showed higher reactivity than other photocatalysts such as TiO 2 .The improved photocatalytic properties of MIP-Ag/TiO 2 NTs can be attributed to footprint cavities created by molecularly imprinted polymer and electron traps of silver nanoparticles.Metal-doped titanium dioxide photocatalysts have also been investigated for adequate mineralization of PFAS.Sansotera et al. [100]synthesized iron and niobium co-doped titanium dioxide (Fe:Nb-TiO 2 ) via sol-gel method.Fe: Nb-TiO 2 catalyst did not completely degrade PFOA, where only 14.8% degradation was achieved after 3 h at pH= 4.3.However, this catalyst demonstrated higher activity compared to commercially available TiO 2 and un-doped TiO 2 .Later, Chen et al. reported the effective photocatalytic oxidative degradation of PFOA (initial concentration: 50 ppm) aqueous solution using transition metal (e.g., Cu and Fe) modified TiO 2 (Cu-TiO 2 and Fe-TiO 2 ) catalyst.Among the different modified TiO 2 , Cu-TiO 2 showed the highest reactivity.After 12 h of UV irradiation, Cu-TiO 2 decomposed PFOA into fluoride ions (F-) and shorter perfluorinated carboxylic acids (e.g., C 6 F 13 COOH, C 5 F 11 COOH, C 4 F 9 COOH, C 3 F 7 COOH, C 2 F 5 COOH, and CF 3 COOH); yielding 91% decomposition and 19% defluorination.Other modified TiO 2 photocatalysts such as noble metallic nanoparticles modified TiO 2 (M-TiO 2 , M = Pt, Pd, Ag)[101], Pbmodified TiO 2 (Pb-TiO 2 ) [102] and composites TiO 2 with multiple wall carbon nanotubes (MWCNTs)[103]have been investigated for the efficient and effective degradation of PFOA.These modified TiO 2 heterogeneous photocatalysts yield traps to capture photoinduced electrons or holes, showing higher PFAS degradation compared to pure TiO 2 or commercially available TiO 2P25 [102-104].
Gatto et al. reportedthe complete mineralization of 4.0 mM PFOA when treated with TiO 2 slurry (0.66 g/L) under UV irradiation of 95 W/m 2 for 6 h[97].The pseudo-first-order kinetic constant for PFOA photodegradation was reported as 0.1296 /h.Adding a hole-scavenger (e.g., oxalic acid) also enhances PFOA photodegradation using TiO 2 catalyst under a UV irradiation of 254 nm and nitrogen atmosphere.In contrast, adding an electron acceptor, such as potassium persulfate (K 2 S 2 O 8 ) prevents the decomposition of PFOA[98].In this case, oxalic acid, reactive carboxyl anion radical along with photogenerated electrons are formed, stimulating the photo-reductive decomposition of PFOA.Wang et al. [98]observed 10.5% and 12.4% of PFOA degradation in oxygen and nitrogen atmosphere • .These hydroxyl radicals are less reactive toward PFOA in which it leads to minimized photodegradation of PFOA.Electron spin resonance (ESR) investigations also support the above statement regarding the coordination of PFOA to TiO 2. Based on ESR studies, a plausible reaction mechanism of PFOA photodissociation on TiO 2 and In 2 O 3 surface has been proposed, as shown in Fig. 3.The proposed PFOA degradation mechanism consists of several stages.During the first step, perfluorinated alkyl radicals (C 7 F 15 COO • ) are generated via electron transfer from COO − to the photocatalyst (i.e., TiO 2 and In 2 O 3 ).In the next stage, perfluoroheptyl radical (C 7 F 15 • ) forms via Kolbe decarboxylation reaction and is followed by C 7 F 15 • reacting with water to form an unstable alcohol (C 7 F 15 OH).C 7 F 15 OH is then converted to C 6 F 13 COF, which in turn reacts with water forming perfluoroheptanoic acid (C 6 F 13 COOH).In the following steps, short chain perfluorinated compounds (e.g., perfluoropentanoic acid (C 4 F 9 COOH)) are produced stepwise until complete mineralization is achieved.A detailed description of the proposed mechanism has been previously described by our group [109].Zhang's research group further investigated other In 2 O 3 based heterogeneous nanophotocatalysts such as In 2 O 3 nanoporous, nanosphere (In 2 O 3 NPNSs) [110], nanostructured In 2 O 3 [111], In 2 O 3 -graphene nanocomposites [112] and CeO 2 -doped indium oxide (xCeO 2 /In 2 O 3 ) with various CeO 2 doping amounts Li's experiments showed that photocatalytic ozonation boosted the degradation efficiency of PFOA remarkably, which established the existence of the synergistic effect between ozonation and photocatalysis.The defluorination ratio of PFOA in UV/TiO 2 /O 3 was found to be 4.18, which was 3.01 times higher than UV/TiO 2 /O 2 and UV/O 3 within 4 h of UV light irradiation.O 3 is a stronger oxidizing agent than oxygen since it has high oxidizing ability (2.07 eV) and can easily react with a photogenerated electron to form reactive species Mention of trade names, products, or services does not convey official EPA approval, endorsement, or recommendation.S. Verma and B. Mezgebe were supported in part by appointment to the Postdoctoral Research Program at the Center for Environmental Solutions and Emergency Response (CESER) administered by the Oak Ridge Institute for Science and Education through Interagency Agreement No. DW-8992433001 between the U.S. Department of Energy and the U.S. Environmental Protection Agency.

Table 1
Summary of different non-catalytic photodegradative approaches for perfluorinated compounds (PFCs) mineralization.

Table 2
Photochemical mediators for the dissociation of PFCs under UV light irradiation.

Table 3
Role of TiO2 based nanocomposites as photocatalysts for PFCs mineralization.Next Mater.Author manuscript; available in PMC 2024 June 05.

Table 4
Role of alternative nano-empowered technologies in photocatalysis for PFCs mineralization.
Next Mater.Author manuscript; available in PMC 2024 June 05.