# Electrochemical advanced oxidation of PFOA by Ti4O7 reactive electrochemical membrane anode

Chaimaa Gomri, Diego Krzyzanowski, Matthieu Rivallin, François Zaviska, Eddy Petit, Mona Semsarilar, Marc Cretin

► To cite this version:

Chaimaa Gomri, Diego Krzyzanowski, Matthieu Rivallin, François Zaviska, Eddy Petit, et al., Electrochemical advanced oxidation of PFOA by Ti4O7 reactive electrochemical membrane anode. Journal of Environmental Chemical Engineering, 2024, 12 (5), pp.113495. 10.1016/j.jece.2024.113495. hal-05077040

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# Electrochemical advanced oxidation of PFOA by Ti₄O₇ reactive electrochemical membrane anode

Chaimaa Gomri, Diego Krzyzanowski, Matthieu Rivallin, François Zaviska, Eddy Petit, Mona Semsarilar, Marc Cretin*

Institut Européen des Membranes-IEM (UMR 5635), Univ Montpellier, CNRS, ENSCM, Montpellier 34095, France

## ARTICLE INFO

### Keywords:

Electrochemical advanced oxidation (EAOX)

Magnéli phase

Ti₄O₇

Reactive electrochemical membrane (REM)

Perfluorooctanoic acid (PFOA)

## ABSTRACT

In recent years, per and polyfluoroalkyl substances (PFAS) have garnered significant attention due to their persistent and harmful effects on the environment and human health. Traditional treatment methods often fall short of effectively removing PFAS from water, prompting the need for innovative techniques. One promising approach is electrochemical advanced oxidation, based on highly reactive chemical species generated through electrochemical reactions to degrade organic pollutants. In this study, a specific reactive electrochemical membrane (REM) anode was used to degrade perfluorooctanoic acid (PFOA). The anode is a tubular Ti₄O₇-based REM (mixture of Ti₄O₇ and Ti₅O₉ Magneli phases) with a water permeability of 3300 L m⁻² h⁻¹ bar⁻¹. The impact of various operating parameters, including current density, flux through REM, and feed concentration, on PFOA degradation efficiency was investigated. It was found that higher current densities, feed concentrations and lower flux through REM led to increased PFOA removal rates. With this type of REM, a total degradation of PFOA was observed at PFOA flux (passing throw the REM) under 0.2 g h⁻¹ m⁻² even at the lowest current density (71 A/m²). The degradation mechanism was investigated by evaluating the generated by-products after several degradation cycles, and the results confirmed that the degradation occurs via the decomposition cycle of CF₂. Energy consumption has been taken into account, as it is the major limitation of advanced oxidation processes, exhibiting value under 5 kWh/g for the lowest current density and the highest PFOA flux. The originality of this study relies on the use of a specific anode; a tubular shape Ti₄O₇-based REM (by Saint Gobain Company). Such concentric REM reactor is optimal for the treatment of recalcitrant effluent containing PFOA as it significantly improved mass transfer leading to a very interesting energy efficient system that can easily be scale up.

## 1. Introduction

Water pollution is an environmental issue resulting in the aquatic ecosystem's degradation and diseases that can lead to human death. According to the United Nations (UN) report, around 1.4 million children die each year because of diarrhea that can possibly be induced by polluted water [1]. Chronic diseases like organ cancer, high blood pressure, thyroid, obesity, etc., can develop due to contaminated water [2]. Knowing that water-related problems are increasing due to population growth and climate change, more research should be dedicated to the water treatment field.

One of the significant problems is refractory organic micro-contaminants that are resistant to conventional treatment techniques [3]. Their concentrations, are in the order of µg/L, and their recalcitrant behavior make their treatment challenging. Per and polyfluoroalkyl

substances (PFAS) are recalcitrant micropollutants that have garnered great interest lately. They have the highest stable bond (C-F, 536 kJ mol⁻¹), rendering their degradation challenging [4]. Exposure to PFAS can be through inhalation, ingestion, and skin contact, leading to diverse health issues such as infertility, newborn defects or deaths, liver damage, thyroid disease, kidney cancer, and testicular cancer [5-7].

PFAS are excessively used in many industries like aerospace, biotechnology, construction, chemical industry, electronic industry, food industry, and textile production [2,8]. Their properties, such as stability, non-reactivity, hydrophobicity, non-flammability, and resistance to harsh conditions and degradation, make them valuable for application [6], which led to vast contamination. PFAS has been detected in groundwater, drinking water, wastewater treatment plants (WWTPs), and landfill leachates [9]. Studies all over the world reported the dominant presence of perfluorooctanoic acid (PFOA) and

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: marc.cretin@umontpellier.fr (M. Cretin).</footnote>
perfluorooctane sulfonic acid (PFOS). These compounds share an eight-carbon atom chain but differ in their head groups: PFOA possesses a carboxylic group, while PFOS features a sulfonic group. Compared to other types of PFAS, their concentration in WWTPs varies between 0.28 and 1057 ng/L for PFOA and 0.44 and 462 ng/L for PFOS [10]. In in- dustrial contaminated sites, their concentration can reach hundreds of milligrams per liter [11] which explain why they are of significant concern for the environment and human health.

Regarding the danger, they present to human health and the envi- ronment, Lots of effort have been dedicated to PFAS treatment. The treatment techniques are divided into two categories: ex-situ and in-situ methods. The ex-situ techniques involve adsorption and filtration, while in-situ remedial imply degradation [12]. Electrochemical advanced oxidation processes are in-situ methods allowing the degradation of biorefractory pollutants like PFAS into less toxic and/or biodegradable subproducts. Whereas adsorption and filtration, both commonly used techniques for water remediation, need a further treatment of the concentrate or of the adsorbent solid phase.

AOPs represent promising technologies for degrading PFAS. These methods utilize highly reactive oxidant species to mineralize pollutants [13]. AOPs include various techniques such as photocatalysis, UV, electron beam, plasma, ozonation, sonolysis and electrochemical oxidation [13,14]. These approaches have demonstrated their effec- tiveness in breaking down and mineralizing a diverse range of recalcii- trant pollutants [15–18].

Among AOPs technologies, anodic electro-oxidation process has gained a growing interest due to its versatility, energy-efficiency and ability to operate under mild conditions [9,19]. For PFOA and PFOS it has been proved that energy consumption (e.g. kWh/m³ of treated wastewater) is up to an order of magnitude lower than other advanced processes [9]. Anodic electro-oxidation is based on direct electron transfer and oxidation mediated by hydroxyl radicals produced on the surface of the anode, so it does not require the addition of any chemical [20,21]. This technology showed outstanding results regarding the degradation of PFAS [22]. Different types of anodes have been employed for this purpose. The most common anode used is a boron-doped dia- mond (BDD), well-known for its good performance and high price [22, 23]. Another promising material that has been lately explored is Magnéli phase titanium sub-oxides; it comprises a series of compounds with the following formula Ti<sub>n</sub>O<sub>2n-1</sub>, (n from 4 to 8) with Ti<sub>4</sub>O<sub>7</sub> presenting the ideal form due to its greatest conductivity, stability, and low price [24, 25].

In an electrochemical cell used in electrolysis mode for remediation in a stirred-tank configuration, mass transfer of pollutants from the bulk of the solution to the electrode surface induces a strong limitation of process effectiveness. A breakthrough novel anode configuration is the reactive electrochemical membranes (REM). The porous anode operates as a membrane under a flow-through mode. With low pore size (i.e. a few micrometers), it is possible to decrease the limitation from diffusion and mass transfer can be enhanced by increasing convection through the porous anode ensuring a higher mass transfer and oxidation rate [17, 26]. With this configuration, more surface area is available, which in- creases the electrochemical reactions, and the mass transfer phenome- non is more privileged through convection-enhanced dispersion [27].

This work reports the degradation of PFOA using a Ti₄O₇ REM-based anode, a substoechiometric phase of TiO₂ (general formula TiₙO₂n-1 with n from 4 to 9 also called Magnéli phases [28]) allowing the generation of hydroxyl radicals from water oxidation useful for advanced oxidation of recalcitrant pollutants like PFAS. The effect of PFOA flow rate, current density, and feed concentration on the degradation efficiency and en- ergy cost was investigated. Furthermore, the degradation mechanism of PFOA has been explored by evaluating the generating by-products as a function of the degradation cycle number and feed concentration.

## 2. Materials and methods

### 2.1. Reagents

All chemicals ((PFOA, 95 %); Na₂SO₄; (Perfluoroheptanoic acid (C7); Perfluorohexanoic acid (C6); Perfluoropentanoic acid (C5); Per- fluorobutanoic acid (C4); Perfluoropropanoic acid (C3); analytical standard)) were purchased from Sigma Aldrich, France. Solutions were prepared by Ultrapure water obtained from a Milli-Q system (Millipore Co. Ltd, resistivity > 18.2 MΩ cm).

### 2.2. Experimental set-up and choice of membrane material

The experimental pilot reactor is described in a previous work real- ized by Ganzenko et al. [16] and a set-up schematic is proposed in Supplementary Material. Briefly, the pilot consists of a 3 L tank that feeds the electrochemical cell via a volumetric pump, a loop for recir- culating the retentate to increase the hydraulic retention time (HRT), and a DC power supply (ELC, model AL924A) working in galvanostatic mode. The electrochemical cell is inner encircled by the cathode, a 3 mm thick carbon felt, keeping a 3 cm distance from the anode. The REM was kindly provided by Saint Gobain CREE, France. Detailed synthesis and characterization of the membrane were reported previously [26]. The anode is a Ti₄O₇-based REM (mixture of Ti₄O₇ and Ti₅O₉ Magnéli pha- ses), have a hollow shape with an inner diameter of 6 mm, an outer diameter of 10 mm, and a length of 9 cm. The median pore size is 1.4 µm, ergo not allowing the retention of any dissolved substitute. The porosity volume is 41 % and the specific surface area 0.40 m. g⁻¹. The water permeability is 3300 L m⁻² h⁻¹ bar⁻¹. Amongst Magnéli phases based-membranes, this value is more than 50-folds higher than that of the commercial Ebonex electrodes (Vector Corrosion Technologies, Inc.), presenting a bimodal pore size distribution that dramatically in- creases the pressure drop [29].

Structural (X-ray diffraction data) and textural (data of Hg intrusion porosimetry analysis and scanning electron microscopy images of inner surface) characterization can be found in reference [26]

### 2.3. Operating conditions

In line with our previous work [16,30], the transmembrane pressure (TMP) was regulated using a micrometric valve from approximately 0.33 to 330 mbar, resulting in a variation of the flux through the REM ranging from 80 to 1000 L/h/m² to allow an adequate residence time of the pollutants within the membrane. Different current densities from 71 to 286 A m⁻², were applied, as well as different PFOA concentrations in the feed (from 0.2 to 20 ppm). High PFAS concentration can be found in industrial contaminated sites. According to the limit of detection of the UHPLC used (LOD = 15 ppb, see Section 2.5) and based on a degradation rate of 90 %, the lowest concentration of PFOA in the feed was fixed at 0.2 ppm (i.e. 200ppb). Moreover to highlight the effect of feed concen- tration, more concentrated synthetic effluents were tested (2 and 20 ppm). Current density values were chosen to get high degradation rate according to the residence time of the pollutants based on our previous work [15,16]. The system was operated under galvanostatic mode. The cell voltage between anode and cathode was from 7 to 14 V depending on the current Nitrogen was bubbled during the experiment to avoid oxygen reduction and H₂O₂ production at the carbon-felt cathode. For each variation, the permeate was collected after 10 min. of ru,n to reach a permanent regime and analyzed to evaluate the degradation efficiency. The operating conditions are summarized in Table 1.

Experiments were also conducted in cascade to evaluate the effect of number of cycles on the degradation mechanism. The permeate was reinjected into the electrochemical cell 3 times. In total, 4 degradation cycles were performed. After each cycle, the permeate was analyzed to evaluate the generated by-products and their concentration. The flux
<table><caption>**Table 1**
Operating conditions.</caption><tr><td>PFOA concentration (ppm)</td><td>0.2–20</td></tr><tr><td>Flux through the REM (L/h/m²)</td><td>80–1000</td></tr><tr><td>Transmembrane pressure (TMP mbar)</td><td>0.33–330</td></tr><tr><td>Cell voltage in galvanostatic mode (V)</td><td>7–14</td></tr><tr><td>Current density (A/m²)</td><td>71–286</td></tr></table>

was fixed at 150 L/h/m², and the applied current density was 286 A/m².

## 2.4. Energy consumption

The energy consumed per mg of PFOA eliminated (kWh mg⁻¹ of PFOA eliminated) was calculated following Eq. 1:

$$EC(kWhmg^{-1}) = \frac{E \times I}{Q \times (C_i - C_f)} \quad (1)$$

Where E is the average cell voltage (V), I is the current applied (A), Q is the permeate flux (m³ h⁻¹), Cᵢ is the concentration of PFOA in the feed (ppm), and C_f is the concentration of PFOA in the permeate (ppm).

## 2.5. PFOA and by-products analysis

The concentration of PFOA and the degradation products were determined by ultra-high performance liquid chromatography (UHPLC) coupled to a mass spectrometer (Shimadzu-LCMS-8050). Electrospray ionization (ESI), in negative mode, was used to identify and quantify molecules. Quantitative analysis was carried out using MRM mode (Multiple Reaction Monitoring). The UHPLC is equipped with a Phenomenex Kinetex® column (50 mm × 2.1 mm; 1.7 µm; C₁₈; 100 Å), and the temperature was maintained at 40°C. The injected volume was 1 µL. Mobile phase A consists of MilliQ water with formic acid (0.1 %), and mobile phase B consists of acetonitrile (UHPLC gradient) with formic acid (0.1 %). The flow rate of the elution gradient was 0.5 mL min⁻¹ and was applied as follows: 100 % of mobile phase A, from 0 to 1.5 min, 70 % of A and 30 % of B from 1.5 to 2 min, 100 % of B from 2 to 4 min and 100 % of A, from 4 to 5 min. The limit of detection (LOD) was 15 ppb (µg/L), and the limit of quantification (LOQ) was 50 ppb. Table 2 reports the m/z, retention time, formula, reference of PFOA and the by-products.

## 3. Results and discussion

Degradation of PFOA was carried out on the porous REM using three different current densities (71, 143, and 286 A m⁻²). A preliminary test was conducted without a current supply to evaluate if PFOA can be retained or adsorbed on the electrodes or any other part of the experimental set-up. Luckily, no variation in PFOA concentration was detected in the feed during the test. Fig. 1 presents the variation of PFOA flux through the REM as a function of PFOA elimination for an initial concentration of 2 ppm using varied current densities. Under 0.2 g h⁻¹ m⁻² passing through the REM (single pass), PFOA is eliminated. Upon this value, the degradation tends to decrease. At lower flux, the residence time is higher, enhancing the probability of electron transfer from the anode to PFOA to initiate the degradation.

Based on the work of Pierpaoli et al. [12], the degradation reaction of

<figure><img src="image_4.png" /><figcaption>**Fig. 1.** Degradation of an effluent doped with 2 ppm of PFOA as a function of current density and PFOA flux through the REM.</figcaption></figure>

PFOA can be described following Eqs. 2 to 7. Under anodic polarization physisorbed radicals are formed from water oxidation at the surface of the REM (Eq. 2).

$$Ti_4O_7 + H_2O \rightarrow Ti_4O_7(OH)^{\bullet-} + H^{+} + e^{-} \quad (2)$$

In a concomitant manner, the PFOA radical is formed via direct electron transfer from the carboxyl group at the electrode surface (Eq. 3).

$$C_nF_{2n+1}COO^{-} \rightarrow C_nF_{2n+1}COO^{\bullet-} + e^{-} \quad (3)$$

In a second step (Eq. 4) the low stability of the radical leads to Kolbe decarboxylation.

$$C_nF_{2n+1}COO^{\bullet-} \rightarrow C_nF^{\bullet-}_{2n+1} + CO_2 \quad (4)$$

Then (Eq. 5), perfluoro radicals can be oxidized by the physisorbed hydroxyl radicals formed at the REM surface according to Eq. 2.

$$C_nF^{\bullet-}_{2n+1} + OH^{\bullet-} \rightarrow C_nF_{2n+1}OH \quad (5)$$

And at the last step, defluorination may take place by intramolecular rearrangement (Eq. 6) as well as hydrolysis (Eq. 7).

$$C_nF_{2n+1}OH \rightarrow C_nF_{2n}O + HF \quad (6)$$

$$C_nF_{2n}O + H_2O \rightarrow C_{n-1}F_{2n-1}COO^{-} + HF + H^{+} \quad (7)$$

As shown in Fig. 1, the representation of PFOA elimination as a function of PFOA flux through the membrane presents a bell shape curve for which the maximum value of PFOA elimination depends on the flux through the membrane and the current density. It is clear that the higher the applied current density, the higher the limiting flux is. Compared to plate electrodes, reactive electrochemical membranes (i.e., porous electrodes) reduce the mass transport limitation by diffusion. Indeed, due to REM's low pore size, organics' diffusion time from the pore to the surface is lowered compared to diffusion time on a plate-dense electrode [31]. Then, by reducing the diffusion time, the mass transport will be controlled via convection and not diffusion. Increasing mass transport by convection (i.e., increasing the flux through the membrane), the electron transfer rate kinetic limit is reached (corresponding to the maxima of the bell curve). So, the system is ran under current limitation.

The effect of current density on PFOA degradation is noticeable at

<table><caption>**Table 2**
PFOA and by-products mass-to-charge ratio (m/z), retention time, formula, and their reference.</caption><thead><tr><th></th><th>413</th><th>363</th><th>313</th><th>263</th><th>213</th><th>163</th></tr></thead><tbody><tr><td>Parent m/z</td><td>413</td><td>363</td><td>313</td><td>263</td><td>213</td><td>163</td></tr><tr><td>Daughter fragment m/z</td><td>369</td><td>319</td><td>269</td><td>219</td><td>169</td><td>119</td></tr><tr><td>Retention time (s)</td><td>3.63</td><td>3.39</td><td>3.10</td><td>2.67</td><td>2.28</td><td>1.5</td></tr><tr><td>Formula</td><td>C₇F₁₅COO⁻</td><td>C₆F₁₃COO⁻</td><td>C₅F₁₁COO⁻</td><td>C₄F₉COO⁻</td><td>C₃F₇COO⁻</td><td>C₂F₅COO⁻</td></tr><tr><td>Reference</td><td>C8</td><td>C7</td><td>C6</td><td>C5</td><td>C4</td><td>C3</td></tr></tbody></table>
0.3 g h⁻¹ m⁻². 92 %, 85 %, and 77 % of PFOA were removed using, a current density of 286 A m⁻², 143 A m⁻², and 71 A m⁻² respectively, (Fig. 2). The electron transfer rate increase at high current density induces efficient removal compared to the lower current density.

Different concentrations of pollutants were tested, 0.2, 2, and 20 ppm, to evaluate the effect of concentration on the efficiency of PFOA removal. The results are presented in Fig. 3, using a current density of 286 A m⁻². At 20 ppm, the degradation is more efficient compared to 2 ppm and 0.2 ppm, even at higher flux. At lower flux, where the degradation is more efficient, PFOA was removed for an initial concentration of 2 and 20 ppm; in comparison, for 0.2 ppm, the higher removal achieved is 96 % (i.e., 4 % of PFOA remains in the permeate). The effect of concentration is accentuated at higher flux; more than 50 % of PFOA was removed for 20 ppm, and only 40 % was degraded for 2 ppm, while for 0.2 ppm, PFOA was not degraded. At higher concentrations, more reactive radical by-products (Eq. 4 & Eq. 5) are generated, enhancing the degradation efficiency. As a result the probability of the pollutants to react is increased.

Fig. 4 explores a broader range of PFOA flux through the membrane at 286 A m⁻². The bell curve remains considering the three concentrations, with breakpoints at approximately 2.4, 0.28, and 0.018 g h⁻¹ m⁻², for feed concentrations of 20, 2, and 0.2 ppm. These graphs prove the interest in adjusting the flux through the REM as a function of the pollutant concentration in the feed. It also confirms that at lower concentrations, the probability of the first direct oxidation of PFOA at the activated REM surface is low (Eq. 3). Consequently, less radical by-products are produced (Eqs. 4 to 7).

Fig. 5 presents the energy consumed per eliminated PFOA as a function of the PFOA flux. The energy consumed decreases with higher PFOA flux, which is observed when operating electro-oxidation under mass transport limitation [16]. The plateau corresponds to the process operating under current limitation, and EC values on the plateau depend on the used current density. It is to be noted that high current density values induce high cell voltages because of the ohmic characteristic of the electrolyzer. This results in higher energy consumption, despite the higher degradation rate of the pollutants for a given flux through the membrane. At lower PFOA flux, the energy consumed using a current density of 71 A.m⁻² is 2.5 times lower than when applying 143 A m⁻² and 5 times lower than when using 286 A m⁻².

In other words, because PFOA degradation occurs on the anode surface (or very near), EC depends strongly on both mass transfer (PFOA through the REM) and electron transfer (current density). The min EC is reached when the mass transfer is at its highest value and the lowest current density. It is important to consider both EC and degradation rates to optimize the process. For example, in Fig. 5, the best compromise to treat a solution at 2 ppm of PFOA will be to operate under a current density of 71 A.m⁻² and flux through the membrane of

<figure><img src="image_5.png" /><figcaption>**Fig. 2.** PFOA remaining in the permeate as a function of current density and PFOA flux through the REM, for a feed concentration of 2 ppm.</figcaption></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3.** PFOA remaining in the permeate as a function of feed concentration and flux through the REM at 286 A m⁻².</figcaption></figure>

0.18 g h⁻¹ m⁻², leading to a total degradation of the pollutant according to Fig. 1. From an industrial point of view, for a given treatment objective (Ci, Cf, and Q fixed) the compromise is between OPEX (Operational expenditure) and CAPEX (Capital expenditure). Increasing REM surface area will have a consequence to increase the CAPEX but decreasing EC strongly and thus OPEX.

Figs. 6 and 7 present the effect of concentration on energy consumption. For the same PFOA flux through the REM, the energy consumed for an initial concentration of 2 ppm is about 9 times lower than the energy consumed for 0.2 ppm. By comparing the energy consumed for 2 ppm and 20 ppm for the same PFOA flux, the energy consumed is 2 times lower for 20 ppm than 2 ppm, which confirms that the initial concentration of pollutant is a decisive parameter to optimize the process: the higher the concentration, the higher the probability of first oxidation of PFOA at REM surface (Eq. 3) and the higher the production of radical by-products (Eqs. 4 to 7).

Considering PFAS degradation, the energy consumption was recently estimated to be 5.1 and 6.7 kWh m⁻³ for PFOA (initial concentration 4.4 ppm) and PFOS (initial concentration 5 ppm) using a Ti₄O₇ based reactive electrochemical membrane showing that electrochemistry is a well-adapted tool to treat such type of pollutants [9].

The main advantage of REM configuration is that at lower concentrations, they still perform well compared to the plate electrode configuration. Enhancing feed concentration is recommended to reduce energy consumption, which is an important parameter to consider for implementing such a process.

To evaluate the effect of the concentration on the degradation mechanism of PFOA, cascade experiments were performed where the permeate was reinjected into the electrochemical cell. During each cycle, the permeate was analyzed to evaluate the generated sub-products. The flux through the membrane was fixed at 150 L/h/m², and the current density applied was 286 A m⁻². PFOA was nearly completely degraded from the first pass-through for all the concentrations tested: 0.2 ppm, 2 ppm, and 20 ppm at these hydrodynamic and electrochemical conditions. The degradation mechanism is presented by Eqs. 4 to 7. During the first cycle of degradation, for an initial concentration of 0.2 ppm, the main product generated is C7, followed by C6. At the 2nd and 3rd cycles, the concentration of C7 is reduced, while shorter PFAS chains (C4 and C5) are generated. At the 4th degradation cycle, 80 % of the initial PFOA is degraded to C4. When PFOA feed concentration is enhanced (2 and 20 ppm) PFOA is degraded primarily to shorter C3 chain. Then the concentration of C3 increased for the last cycle of 20 ppm. Additionally, the concentration of C3 starts to decrease and produce by-products of shorter chain. Fluorine ions were not detected, and the mass balance was less than 100 % for the sum of all degradation cycles. This could be explained by the production of volatile
<figure><img src="image_7.png" /><figcaption>Fig. 4. Degradation of PFOA effluent as a function of feed concentration and PFOA flux through the REM at a current density of 286 A m⁻².</figcaption></figure>

<figure><img src="image_8.png" /><figcaption>Fig. 5. Energy consumption per mg of PFOA eliminated as a function of PFOA flux through the REM and the current density applied, for a feed concentration of 2 ppm.</figcaption></figure>

by-products [9].

Based on the detected by-products, the degradation mechanism consists of a progressive cleavage of $CF_2$, as reported by prior studies

[32]. First, a perfluorinated radical, $C_7F_{15}COO^*$, is formed through direct electron transfer (Eq. 3). Then $C_7F_{15}^*$ is obtained via Kolbe decarboxylation of $C_7F_{15}COO^*$ (Eq. 4). Hydroxyl radicals will then interact with $C_7F_{15}^*$ to form $C_7F_{15}OH$ (Eq. 5), that afterward will be hydrolyzed to generate C7 (Eqs. 6 and 7). This degradation cycle will be reproduced until the total mineralization or production of shorter chain sub-products [13]. Le et al. reported that the rate-limiting step is the formation of $C_7F_{15}COO^*$ [9], which is consistent with the effect of concentration observed here. At higher PFOA concentrations, more $C_7F_{15}COO^*$ radical will be produced, inducing faster degradation of PFOA to shorter by-products.

## 4. Conclusion

Among the different AOPs, electro-oxidation using REM anode appear to be one of the most promising technologies to treat recalcitrant pollutants in water. Indeed, this process is based on direct electron transfer and oxidation mediated by hydroxyl radicals produced at the surface of the anode. It is an eco-friendly (when using green electricity), simple and versatile process. Using REM anode overcome mass transfer limitation due to diffusion, hence, significantly improving the performance and the energy efficiency of the conventional electro-oxidation process.

<figure><img src="image_9.png" /><figcaption>Fig. 6. Degradation efficiency and energy consumption per mg of PFOA eliminated as a function of feed concentration at a PFOA flux of A) 0.08 g h⁻¹ m⁻² and B) 0.62 g h⁻¹ m⁻².</figcaption></figure>
<figure><img src="image_10.png" /><figcaption>Fig. 7. Sub-products generated through different electro-oxidation cycles for different feed concentration.</figcaption></figure>

In this study, a specific concentric electro-oxidation reactor has been employed to treat PFOA using tubular Ti₄O₇ REM anode. The evaluation of the effect of parameters such as current density and the initial PFOA concentration conduct to the determination of the limiting PFOA flux to reach total degradation. Higher is the current density and the initial PFOA concentration and higher is the limiting flux. Furthermore, energy efficiency of the system has been evaluated for different PFOA concentrations and different current densities. The lowest energy consumption is obtained when the process is limited by the current density (high PFOA concentration with low current density) but significantly increase when the limitation is governed by mass transfer. There is thus a compromise between OPEX (i.e. current), CAPEX (i.e. REM surface) and performance (i.e. degradation rate). The determination of these boundaries (limiting PFOA flux and type of process limitation) coupled with such interesting concentric REM reactor configuration can help the scaling up and industrialization of the process.

### CRediT authorship contribution statement

Marc Cretin: Validation, Supervision, Project administration, Methodology, Investigation, Funding acquisition, Conceptualization.

### Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Data Availability

Data will be made available on request.

### Acknowledgements

This project was supported in part by Région Occitanie through the program ESR_PREMAT-00226/prematurat ion 2019 OXAMER. The authors would like to thank Région Occitanie for this financial support.

## Appendix A. Supporting information

Supplementary data associated with this article can be found in the online version at doi:10.1016/j.jece.2024.113495.

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