Perfluoro chemicals (PFCs) are man-made chemical compounds that have seen extensive use in various industrial products. They are commonly found in cleaning agents, firefighting foams, and the non-stick coatings on cooking pans [1]. Due to the high bonding energy of the C-F bond, which is approximately 485 kJ/mol in perfluoroalkyl moieties, perfluoro chemicals exhibit strong chemical and thermal stability. This characteristic renders them persistent compounds within the natural environment [2]. When an alkyl chain exceeds eight carbons, it is classified as a long-chain PFC, whereas if it contains fewer than eight carbons, it falls under the category of short-chain PFC. During wastewater treatment processes, long-chain PFCs degrade into shorter-chain products [3]. Other perfluoro compounds, such as fluorotelomer alcohol (FTOH), polyfluoroalkyl phosphates (PAPs), 8:2 fluorotelomer acrylate (8:2 FTAC), and fluorotelomer carboxylates (FTCAs), can also degrade into perfluorooctanoic acid (PFOA) [4]. As chain length decreases, so does toxicity. However, because short-chain PFCs are more hydrophilic than long-chain PFCs, they are more prevalent in natural water systems [5]. In natural water systems, approximately 88.8% of PFC contamination comprises short-chain PFCs [2].

A primary category of compounds in PFCs is referred to as per- and polyfluoroalkyl (PFAS) compounds, as indicated by USEPA. These compounds consist of a variable-length fluorinated alkyl chain followed by a functional group, as depicted in Figure 1. Figure 1 illustrates that there are primarily two types of PFAS: perfluorinated sulfonic acids (PFSAs) and perfluorinated carboxylic acids (PFCAs). PFSA consists of a variable-length fluorinated alkyl chain followed by a sulfonic acid functional group. Upon degradation and defluorination, the long alkyl chain breaks down into short-chain perfluorooctanesulfonic acid (PFOS), which contains eight carbon atoms in the alkyl chain. Another type of PFAS is PFCA, which is formed by various alkyl chains connected to a carboxylic functional group. When long-chain PFCA undergoes degradation, the length of the alkyl chain decreases, and one of the principal degradation products is PFOA [1].

Both PFOA and PFOS are common perfluoro compound contaminants that have been widely detected in the environment. This text will specifically focus on PFOA, which exists as a white waxy powder at room temperature. PFOA is utilized in the production of Teflon for non-stick cookware and is also employed in the manufacturing of surfactants such as shampoo and floor wax, taking the form of fluoropolymers and telomer alcohols. Following their use and disposal, these chemical compounds can eventually degrade into PFOA [6]. Due to its solubility in water (9.5 g/L) and its extended half-life in water systems (92 years), PFOA rarely degrades in the natural environment and is often referred to as a ‘forever chemical’ [7]. After PFOA is released from industrial and wastewater treatment plants, some of it is emitted into the atmosphere, while the rest travels through surface-water and groundwater systems [8]. It accumulates within the biota through the food chain and ultimately enters the human body through the consumption of food or drinking water [9]. The research indicates that the aquatic toxicity of PFOA for marine invertebrates is approximately 10–24 ppm, which is several orders of magnitude higher than the detected PFOA concentration in natural systems [10]. However, it has the potential to be transported through the food chain and ultimately bioaccumulate in aquatic life and humans. Recent studies have shown that trace concentrations of PFOA have been detected in human serum worldwide [9]. Typically, it targets sensitive organs such as the liver or kidney. When PFOA combines with albumin, it can reduce glomerular filtration in the kidney or lead to liver hypertrophy or necrosis [11]. The acute toxicity of PFOA in humans is still unknown, but the lethal dose (LD50) in male rats is 175–208 mg/kg [12]. The reference dose (RfD) for PFOA is 20 ng/kg/day, and an overdose can lead to accumulation in the human body, potentially increasing the risk of cancer [11]. To monitor and regulate this emerging contaminant, the US Environmental Protection Agency (USEPA) proposes healthy drinking water advisory levels for PFOA at 70 ng/L. In 2018, Health Canada published a drinking water standard of 200 ug/L for PFOA [13]. PFOA can cause aquatic toxicity, affecting the survival, growth, and reproduction of aquatic flora and fauna, and it can also be harmful to human health [10]. Due to PFOA’s toxicity, there is increasing attention on it as an emerging contaminant. Various methods have been developed to remove these contaminants and control pollutant concentrations to ensure they remain within safe levels.

Physical removal methods, such as granular activated carbon adsorption, ion exchange resin, membrane filtration, and coagulation, have been frequently documented ^{[1][14][15][16]}[1,14,15,16]. Recent research has demonstrated that ion exchange resins exhibit high adsorption capacities (525–1500 mg/g) in comparison to granular activated carbon adsorption (41–120 mg/g), and both methods can achieve over 90% removal [17]. These physical removal techniques have been developed at a full scale, making them applicable in drinking water treatment plants for PFOA removal. However, their primary drawback is that they solely extract contaminants from water resources and concentrate them without degradation [18]. Therefore, an additional degradation method or step is necessary to eliminate these compounds and prevent the introduction of secondary contaminants.

There are three primary methods for degrading these compounds. The first is a non-chemical degradation method, such as plasma technology, incineration, sonolysis, and photolysis. These methods require high-energy inputs, such as UV, ultrasound, or heat, to initiate degradation. During the reaction, the input energy can break down the C-F bond and convert PFOA into CO₂, free fluoride ions, and less toxic short-chain PFCA. This reaction typically occurs in a high-temperature, high-pressure environment without the involvement of any chemical compounds [19]. The second method is chemical degradation, which involves chemicals like ferric ions or persulfate as catalysts in the reaction. These chemicals lower the activation energy needed for the bond-breaking process, accelerating the degradation reaction [20]. The third degradation method is biological degradation, which represents a newer technology. During biodegradation, microorganisms consume PFOA as a carbon source [21]. Biodegradation boasts lower energy consumption and does not yield unpredictable by-products during the reaction [22].

3. Chemical Degradation

Chemical degradation is a well-established technology widely utilized in wastewater treatment systems. In most cases, it requires an energy source input, such as UV, electrical power, ultrasound, and microwave energy. With the presence of chemical catalysts, it generates free electrons or free radicals to break down the C-F bond in PFOA for degradation. The most employed chemical degradation methods include photochemical and electrochemical degradation ^[23][24][26,27]. Other forms of energy input are also used for degradation, including sonochemical degradation, microwave-enhanced degradation, and other chemically catalyzed degradation methods ^[25][26][27][28,29,30].

3.1. Photochemical Degradation

Most of the research on chemical degradation involves using UV as the input energy source to facilitate homogeneous or heterogeneous reactions ^[28][29][30][31,32,33]. Table 1 shows a selection of photocatalytic degradation studies that focused on low-pressure-to-medium-pressure mercury UV lamps with a wavelength of around 256 nm. This is because PFOA exhibits maximum absorbance at this wavelength, providing optimized energy for breaking down the chemical bonds in PFOA during the degradation process ^[30][31][33,34]. The UV lamp power varies from 9 W to 200 W, but generally, it utilizes low-power light, typically around 20 W, to maximize energy efficiency. ^[32][35] used a 15 W UV lamp with a wavelength of 240 nm to perform an anaerobic reduction reaction. Without the presence of chemicals as a reducing agent, the PFOA concentration only reduced by 13.3% in 6 h in an anaerobic environment. However, with the addition of KI as a catalyst, the PFOA concentration dropped to 93.9% in 6 h ^[32][35]. Table 1 demonstrates that photochemical degradation typically occurs at room temperature under a room atmosphere. The aerobic reaction takes place in a lower pH range, typically from 0.3 to 4.5, while the anaerobic reaction occurs in a higher pH range, usually between 8 and 12 ^[30][32][33,35]. There are two primary reasons for maintaining the aerobic reaction in an acidic environment. Firstly, the aerobic environment employs oxidation reagents to generate hydroxyl radicals for degradation. However, when the pH exceeds 4, it can render the oxidation agent unstable. For instance, at high pH levels, H₂O₂ degrades into H₂O and O₂, reducing the formation of free radicals. The second reason is that the precipitation of Fe(OH)₃ forms at high pH, which can obstruct ongoing reactions and significantly impede the degradation performance. Therefore, the reaction should occur in a low-pH environment ^[33][36]. An experiment conducted by Song et al. (2013) involved reductive defluorination of PFOA under anaerobic conditions. The results indicated that increasing the pH from 8.1 to 10.3 enhanced the defluorination from 58% to 85%. This is because free electrons drive a reduction reaction in the experiment, and the relative quasi-stationary concentration of eaq- (ROSC) is linked to pH. With an increase in pH, more free electrons are available in the solution for defluorination to occur ^[34][37]. The photocatalyst plays a crucial role in the photochemical degradation of PFOA. Wu et al. (2017) used ZnO as a catalyst that absorbs PFOA for degradation. However, an increase in pH reduces the adsorption of PFOA on the reaction surface and consequently decreases the reaction rate ^[35][38]. Furthermore, during application, elevated temperatures reduce the surface area of ZnO, leading to a reduction in reaction efficiency ^[35][38]. TiO₂ is another popular catalyst employed in photochemical degradation. It contributes electrons during the reduction reaction and provides a reaction surface ^[36][39]. The effectiveness of the TiO2 surface is influenced by pH, leading to changes in degradation efficiency ^[36][39]. Araniti et al. (2015) utilized Mg-aminoclay-coated nanoscale zero-valent iron as a catalyst for PFC degradation. After aging for 3 days, the degradation rate decreased by 15% due to the reduced amount of reactive iron in the coating ^[37][40]. The main degradation products of PFOA through photochemical degradation are fluoride ions, CO₂, and other short-chain PFOA compounds ^[38][41]. Perfluoroheptanoic acid (PFHpA) is the initial degradation by-product that emerges during the photochemical degradation of PFOA ^[39][42]. Throughout the degradation process, PFOA undergoes stepwise defluorination and shortens its alkyl chain, resulting in the production of C7 PFHpA ^[40][43]. Additionally, other short-chain PFCA compounds, such as C6 perfluorohexanoic acid (PFHxA), C5 per-fluoropentanoic acid (PFPeA), and C4 perfluorobutanoic acid (PFBA), have been detected in the final solution, each with a low concentration of approximately nanomoles per liter ^[34][37]. The degradation efficiency in photochemical degradation ranges from 70% to the complete removal of PFOA, with 30% to 90% defluorination occurring during a reaction time of 4–24 h ^[32][35]. It is worth noting that the defluorination rate is consistently lower than the PFOA removal rate; Song et al. (2013) achieved up to 100% removal of PFOA within 1 h but only managed a 40% defluorination rate ^[34][37]. This phenomenon is due to the higher energy of the C-F bond compared to the C-C bond, causing the degradation reaction to undergo chain shortening processes before complete defluorination is achieved. ^[25][53][28,56] The reaction temperature typically falls within the range of 10–20 °C, and the reaction occurs in an acidic solution. However, this method has several limitations. Firstly, it can only effectively degrade PFOA at low concentrations (10–100 ppb of PFOA). Secondly, when treating contaminated water sources such as landfill groundwater, it can be susceptible to the influence of volatile compounds like acetone, diisopropyl ether, and butanone, as these compounds can lower the interfacial temperature and hinder the formation of OH radicals ^[53][56]. Researchers have also observed that microwaves enhance degradation in sonochemical processes. In the study of Li et al. (2021), a 300 W microwave oven was used to generate a 2450 MHz microwave, heating specific compounds. This reaction, conducted under high temperatures in an acidic environment, produced free radicals, resulting in the removal of up to 80% of PFOA in just 4 h ^[27][30]. During sonochemical degradation, chemicals such as H₂O₂ are added to initiate Fenton-like processes or Na₂S₂O₈ to produce SO₄⁻ ions, which then engage in persulfate oxidation reactions ^[27][54][30,57]. However, similar to photochemical reactions, the degradation rate is significantly affected by pH and temperature and can also be hampered by the presence of Cl- ions ^[27][30]. Another notable drawback of this reaction is its high energy consumption, approximately 458 kWh/mol, owing to the dissipation of energy as heat during the reaction ^[26][29]. Some chemical degradation processes do not require additional power input to initiate degradation. For instance, ^[26][29] introduced a method that employs vitamin B13 as an electron transporter in combination with CuSO4 and NaBH4, resulting in the formation of zero-valence copper to expedite the reaction ^[26][29]. By adding 45 nM of titanium citrate as a reducing agent, they achieved a 65% removal of PFOA within 18 h ^[26][29]. In another approach, researchers utilized KMnO4 in combination with HCl for PFOA degradation. However, this method, although effective, required an extended period of up to 6 months to remove 90% of contaminants ^[55][58]. Comparing the experiment of Fang et al. (2016) with other chemical degradation methods reveals that even without the need for additional power input, degradation can occur. Nevertheless, due to the stubborn stability of PFOA, the reaction rate remains relatively low. Balancing energy consumption with degradation efficiency is a potential avenue for future research in chemical degradation.

4. Biological Degradation

Biological degradation represents an emerging technology when compared to chemical degradation, and research in this field remains rather limited. Only a handful of research articles have been published, with a specific focus on the biodegradation of PFOA. Previous research has demonstrated the thermodynamic favourability of dehalogenation reactions and the maturity of microbial dechlorination technology. This indicates that bacteria can indeed derive energy from breaking down perfluoro compounds [6]. However, the majority of biodegradation research centers on the degradation of fluorotelomer alcohols (FTOH), which ultimately yield PFOA as a final product without further breakdown ^[6][56][6,59]. Furthermore, it has been reported that biodegradation typically requires at least one hydrogen atom to be attached to the alkyl chain to initiate the reaction. In the case of PFOA, however, the alkyl chain only features fluorine atoms, forming high-energy C-F bonds (485 kJ/mol), which render them even more challenging to degrade compared to other perfluorinated compounds, such as fluorotelomer alcohols ^[57][60]. Schröder et al. (2004) employed sewage-treatment-plant sludge as a bacterial seed in a closed-loop system to treat PFOA for 28 days ^[58][59][61,62]. In recent years, scientists have managed to isolate and acclimate Pseudomonas Parafulv and Acidimicrobium sp A6 strains from PFOA-enriched environments, allowing them to perform biological degradation reactions using PFOA as a carbon source. Bacteria exhibit a certain level of tolerance to PFOA concentrations; when the concentration of PFOA reaches 500 ppm, the bacterial strain reaches a maximum population of 0.175 OD600 and achieves a degradation rate of 30% over 72 h [2]. However, if the concentration exceeds this threshold, both the OD600 and degradation rate begin to decline due to the inhibitory effects of PFOA on bacterial growth. In Table 3, biodegradation can occur either aerobically or anaerobically under culturing conditions of 30 °C and 150 rpm. However, the majority of experiments are conducted in anaerobic environments ^[60][63]. The primary reason for biological degradation occurring in an anaerobic environment rather than an aerobic one is the highly oxidized nature of PFOA. It struggles to donate electrons for further oxidation reactions in an aerobic environment [22]. In anaerobic conditions, PFOA can preferentially accept electrons to undergo reduction reactions, leading to no removal of PFOA under Liou’s aerobic degradation conditions but achieving 67% removal in Huang’s anaerobic degradation experiment ^[60][63]. Table 3 presents the anaerobic degradation of PFOA carried out by an Acidimicrobium sp A6 strain in an acidic environment with a pH range of 4.5–5.5. There has only one aerobic experiment, conducted by Yi et al. (2016), in a neutral environment using Pseudomonas parafulva for degradation. During their experiment, Yi et al. (2016) supplemented yeast extract and glucose to support metabolism. This can be used during the acclimation and isolation stages to boost bacterial populations during the degradation processes and increase the final removal rate of PFOA [2].