Electrochemical Advanced Oxidation Processes (EAOP) to degrade per- and polyfluoroalkyl substances (PFASs)

1.1 PFASs

Emerging contaminants (ECs) are chemicals, substances or materials showing emerging threat to human and / or environmental health [1, 2]. The contaminants could have existed in the environment for a long period of time but recent research has shown proof of their potential adverse effects. Examples of such ECs are per- and polyfluoroalkyl substances (PFASs) (or perfluorinated compounds, PFCs) [3]. Alternatively, the contaminants have just recently appeared in the environment (emerging compounds), such as nanomaterials [4, 5].

PFASs feature unique physical and chemical properties, such as hydrophobicity and oleophobicity, which have not being evidenced in other compounds. Therefore, they have been widely used in such applications as clothing, upholstery, carpeting, painting, food containers, cookware, aqueous film-forming foams (AFFFs) etc. [3, 6–8] On the other hands, PFASs also exhibit extreme stability with respect to thermal, chemical and biodegradation so that those compounds are also generally categorised as persistent organic pollutants (POPs) [9, 10]. That is, their inert fluoro-carbon skeletons are resistant to degradation under natural environmental conditions [11, 12]. Consequently, their usage and inertness have led to their global distribution and accumulation in the environment. This has in turn raised serious concerns about their impact on the environment and public health [13–15].

Once PFASs contamination is identified and monitored [16–19], the scientific community typically responds by developing remediation strategies. For example, electrochemical remediation has been well advanced towards the remediation of PFASs [20, 21]. The increasing attention can be evidenced in Figure 1whilst Table 1lists the typical PFASs discussed in this review.

Figure 1

Number of yearly published scientific articles. The data were collected via ScienceDirect Website (http://www.sciencedirect.com/) using the following typical words: “electrochemical advanced oxidation process”, “environment remediation”, “emerging contaminant” and “PFASs”, respectively.

1.2 PFASs’ degradation: Current approaches

Numerous remediation approaches currently exist, including adsorption, stabilisation, filtration, coagulation, separation, chemical oxidation / reduction, thermal decomposition, UV / photo-catalytic degradation, etc. [22] All these remediation approaches can be generally categorised into two types, removal (such as adsorption) and degradation (mineralisation). The latter demonstrates its advantage as a clean-up approach because the remediation products will be CO₂, water and other inorganic ions or molecules. However, for POPs’ degradation, the available approach is currently limited in its effectiveness. Taking PFOS / PFOA (Table 1) as an example, its degradation at industrial scale is currently only being achieved with high temperature combustion (>1100 °C) [12]. Other approaches have delivered only limited success, mostly in laboratories and the research is still ongoing.

1.3 AOP /EAOP

Advanced oxidation process (AOP) has received world-wide attention in last decades [23]. It uses radicals, typically hydroxyl radicals (●OH), to attack the POPs and degrade them into fragments or inorganic ions / molecules [24]. The AOP generally employs ozone (O₃), hydrogen peroxide (H₂O₂) or UV light with help of catalysis to produce the radical of ●OH, one of the most aggressive oxidants. The produced radicals usually behave short life-time, such as 2–4 µs in aqueous media for ●HO [25]. Therefore, the possible contamination from the residual reactants is thus effectively declined. Consequently, it has received increasing attention for the treatment of waste water [26].

However, the AOP requires chemicals or others (such as UV, catalysis) to generate radicals, which usually means harsh conditions and a low environmental compatibility. Electrochemical advanced oxidation process (EAOP) overcomes some of these limitations because the radicals can be generated in-situ via electrochemistry, which means the oxidation process can be driven by electricity rather than by chemicals to produce radicals. The less consumption on chemicals of EAOP promises a more environment-friendly approach [27].

Furthermore, the electrochemical process can directly oxidise the to-be-degraded targets or generate other radicals beyond ●OH, such as ●O₂⁻(superoxide radical), ●HO₂ (hydroperoxyl radical), etc., which can potentially break down some POPs that cannot be broken down by ●HO alone. This is true in the case of the remediation of PFASs [12]. The fluoro-carbon skeletons is so stable that it can resist the attack of ●HO [28]. However, the EAOP has been proven to be an effective method for breaking down PFASs.

Apart from its high efficiency and powerful degradation capacity, other advantages include environmental compatibility (using electricity rather than chemicals), versatility, and universal degradation capability, etc. [20, 24, 29] Consequently, EAOP has drawn increasing attention, which can be evidenced by the number of publications shown in Figure 1, and witnessed by recently published reviews [20, 23, 24, 26, 30–34].

Herein, we will discuss the principles of EAOP, then its application for the remediation of PFASs. In the conclusion section we will highlight the challenges and recommend possible directions for future research.

2.1 pH-potential diagram

In an EAOP cell, to generate the radical of ●HO, the electrode must be polarised, as shown in Figure 2(a). Note the exact potential of the radical’s redox depends on the measurement environment, which varies in the literatures [20, 35–37].

Figure 2

pH-potential diagram (a) and the schematic illustration of the anode reaction (b). In (a), the shadows mark the overpotential areas. The possible initial oxidation products and further products (bracketed) are suggested. In (b), water is electrolysed to generate radical of ●HO or adsorbed–OH group, which can be further oxidised as ●O●, or recombined as H₂O₂ (adsorbed), O₂, O₃ after further breaking the O-H bonds, from top to bottom. The process of indirect oxidation of target (R) to (O) with the involvement of radicals is shown on the top whilst the direct oxidation is shown on the bottom.

Most of the radicals are not as stable as the molecules of O₂ or O₃ (life-time of 2–4 µs in aqueous media for ●HO [25]). Their zone is thus marked with dashed line in Figure 2(a). In the meantime, overpotential is marked as shadow areas, which depends on the electrode materials [38].

In an effort to improve the degradation efficiency of EAOP, we hope to boost the productivity of radicals whilst suppressing the emission of O₂ and O₃. For this reason we must select the electrode materials so that their overpotential of O₂ and O₃ is high, such as boron-doped diamond, lead peroxide, which will be discussed in the following section.

2.2 Radicals and degradation reactions

The generation process of the radical is complicated [33]. In Figure 2(b) (top), we can see the simplified model of a water molecule that is adsorbed on the electrode surface [30, 36, 39]. In general, the oxidised products include ●HO, ●O⁻, ●HO₂, ●O₂⁻, H₂O₂, O₂, O₃. In principle, they have higher energy than that of H₂O, enabling them to act as oxidants to degrade organic molecules.

On the other hand, if there are other ions or moieties in the solution, the products might be more complicated. For example, the presence of Cl^- can generate Cl₂, HClO, HClO₂, HClO₃, HClO₄ [35] and the corresponding radicals [40], SO₄²⁻ can generate S₂O₈²⁻ and radicals as well [34]. In some cases, the EAOP can be enhanced, such as S₂O₈²⁻ as a strong oxidant and Fe^2+/3+ as a media that activates Fenton reaction [41]. In other cases, some by-products might bring about the secondary contamination, such as ClO₄⁻ (an EC itself). Hence, those parameters should be considered and optimised for subsequent improvement and development [34, 36].

Furthermore, the degradation reaction is predominantly a sequence of chain reactions, which involve initial and subsequent steps [24]. Taking PFOA as an example, the total reaction can be written as below,

According to Marcus [42], each subsequent step involves at most one electron exchange. Therefore, the initial step must be followed by a series of gentle subsequent steps, to completely mineralise PFOA to CO₂, HF and H⁺ with the involvement of 14 electrons. The HF appearance and the pH modification might affect the production of radicals and the mineralisation process, which is not well-known yet. This complicates the degradation mechanism and will be further discussed in the following.

2.3 Electrode materials

As stated previously, to boost the productivity of radicals and inhibit the emission of O₂, the electrode materials reported in literatures include platinum (Pt) [43], carbon or graphite (C) [44, 45], iridium dioxide (IrO₂) [46], ruthenium dioxide (RuO₂) [47], tin dioxide (SnO₂) [48], boron-doped diamond (BDD) [49], lead dioxide (PbO₂) [32], Ti₄O₇ (Ebonex) [39], TiO₂ nanotube array [34] as well [24, 36]. Among them, BDD has been reported to be the best for water treatment [49].

On the other hand, it is well-known that the initial oxidation of Pt surface (and other noble metals) can catalyse (direct) oxidation of some organics in a lower potential window than that of the emission of O₂ and the radical generation [50]. However, this process is slow and depends strongly on the electrocatalytic activity of the electrode, which hampers its application for remediation. Consequently, this aspect will not be discussed further here [31].

2.4 Cell

The EAOP is driven by the current flow. To maximise electrical efficiency, the main voltage loss should occur on the electrode surface, rather than on the solution part (or the external circuit). Therefore, a supporting electrolyte is added intentionally to increase the conductivity of the water sample. The common electrolyte is sodium sulphate (Na₂SO₄) due to its stability. Other supporting electrolytes such as sodium perchlorate (NaClO₄) have also been reported, but perchlorate itself is an EC. Some ions should be of concern, such as Cl^- which might poison the electrode surface and be oxidised to an EC of ClO₄⁻ [35].

The cell design usually requires two electrodes in parallel with an inter-distance of <1 cm, as presented in Figure 3(a) [48]. The cell can consist of an electrode array, as shown in Figure 3(b), which features an improved degradation efficiency [31]. On the other hand, the cathode can be replaced with cheap materials (c, d), such as a carbon electrode (graphite), even metal ones (cast iron, stainless steel, copper) or air-diffusion electrodes to boost the production of H₂O₂ [30, 41]. The configuration might also behave as a flow-through cell (cylinder or porous panels) (d, e) to increase the transportation efficiency [33, 36]. In Figure 3(d, e), in order to enhance the degradation efficiency, the space between the anode and cathode can be filled with nanomaterials (such as functionalised activated carbon) or an additional electrode couple [34, 51].

Figure 3

Configurations of the EAOP cells. (a) the same anode and cathode materials in parallel with a gap of <1 cm; (b) electrode array; (c) cathode is replaced with carbon, metal, or air-diffusion electrode; (d) and (e) flow through cells whilst the space between the anode and cathode can be filled with additional electrodes / gradual activated carbon. The stirring / flowing direction is indicated.

4.1 Efficiency improvement

EAOP is attracting increasing interest from the environmental remediation community, particularly for the degradation of some POPs, such as PFOS / PFOA. Unfortunately, EAOP will attack all compounds in the EAOP cell, although their individual oxidation kinetics might be different [33, 65]. In this scenario what it usually required is pre-treatment or extraction to: firstly, improve the efficiency by focusing on PFASs; secondly, protect some useful moieties in the sample, such as biomas, nutrients, etc.; and thirdly, avoid toxic products such as ClO₄⁻ and prolong the shelf life of electrode [66].

The additives in the electrolyte might enhance the degradation efficiency by boosting the yielding rate of radicals, such as via the Fenton reaction [41], by generating other kinds of radicals rather than ●OH (such as ●SO₄²⁻), or by producing other oxidants simultaneously (such as O₃, S₂O₈²⁻, BiO₃⁻, MnO₄⁻, FeO₄²⁻, etc.). Further research should also address the cost issue, such as by replacing the electrode materials (of BDD) with cheap ones, using solar panels to power the EAOP, etc. [20, 30]

4.2 Electrode materials and cell improvement

Currently electrode materials like BDD are expensive [36, 49]. In fact, PbO₂ has been reported as having a favourable performance, although its fabrication on wafer is a challenge too [48, 64]. Seeking a cheap replacement is thus urgent to significantly reduce the cost towards scale-up application. For example, the use of an iron electrode to generate FeO₄²⁻ might be another alternative [67]. In this case, however, EAOP must be differentiated from electrocoagulation [68].

To improve the electrode’s shelf life, the anode materials can be modified, such as by doping and nanofabricating [31, 32, 36, 45]. For example, PbO₂ doped with F^- was found to inhibit O₂ emission, accelerate oxidation and enhance degradation efficiency [32]. The explanation for is that the F^- is incorporated into the bulk structure and decreases the extent of adsorbed water, for both α-PbO₂ or β-PbO₂, although the latter is preferred for water treatment [33]. Similarly, SnO₂ doped with F^- is also reportedly promising for highly efficient treatment of PFOA in waste water [69].

Further attention should also be paid to the cathode and the reactions occurring there [31]. As a compulsory component of an electrochemical cell, the cathode reaction should be designed to maximise its benefit towards the degradation process. The emission of hydrogen on the cathode surface, as well as the production of H₂O₂ [41], the reduction of metal ions (such as from Fe³⁺ to Fe²⁺) and the dechlorination and debromination of organohalogens should also be noticed [51, 55, 70, 71].

4.3 Beyond water: Soil

Because the generation of radicals occurs in aqueous media, EAOP is generally utilised to remediate waste water, including surface water and ground water. However, even for the water sample, EAOP usually needs a pre-treatment or extraction module to address issues of efficiency and anode shelf life, as discussed above. This pre-treatment module might also be used for soil remediation [21]. In this process the PFASs will be extracted from the contaminated soils and transformed to the water media [72]. Subsequently the EAOP will take over the degradation process.

Note, there are several options in terms of electrochemical remediation [31], such as electrocoagulation [73], electrokinetics [21], electrodeposition [29] and geooxidation [74, 75], etc. Among these, the geooxidation process showed promise in breaking down some POPs from soil. Interestingly, it based on a similar mechanism to that used in EAOP [76]. Further research is thus highly desirable. For example, a similar approach has been described using a bipolar electrode to enhance EAOP’s performance [51].

4.4 Combination with other approaches

To increase the efficiency of remediation, a series of remediation stages is usually developed to target different contaminants [20, 65]. Currently the EAOP is employed as the last stage of remediation to break down persistent contaminants [30, 73, 77]. The possible reasons for this include: (i) its powerful degradation capacity for POPs; (ii) its high cost; and (iii) electrode shelf life through fouling of the surface by heavily polluted samples.

There are numerous other remediation approaches and a combined solution might work more effectively than an individual one. For example, sonochemistry is another newly developed technology for remediation [78, 79]. The basic idea is that the sonication-generated micro-bubble will experience a high temperature of >5000 °C where H₂O molecules can be split into the radicals of ●OH [26]. If EAOP is combined with this technology, the benefits will include: (i) improved radical generation efficiency [20]; (ii) accelerated transportation of POPs to and from the electrode surface [36]; and (iii) enhanced electrode self-cleaning function.

In general, with further investigation and development of the EAOP, wide applications for real sample remediation can be anticipated.