Reducing Energy Penalty in Wastewater Treatment: Fe-Cu-Modified MWCNT Electrodes for Low-Voltage Electrofiltration of OMC

Abstract

Pseudo-persistent organic pollutants, such as pharmaceuticals, personal care products (PPCPs), and organic dyes, are a major issue in current environmental engineering. Considering the limitations of traditional wastewater treatment plant methods and degradation technologies for organic pollutants, the search for new technologies more suitable for treating these new types of pollutants has become a research hotspot in recent years. Membrane filtration, adsorption, advanced oxidation, and electrochemical advanced oxidation technologies can effectively treat new organic pollutants. The electro-advanced oxidation process based on sulfate radicals is renowned for its non-selectivity, high efficiency, and environmental friendliness, and it can improve the dewatering performance of sludge after wastewater treatment. Therefore, in this study, octyl methoxycinnamate (OMC) was selected as the target pollutant. A new type of electrochemical filtration device based on the advanced oxidation process of sulfate radicals was designed, and a new type of modified carbon nanotube material electrode was synthesized to enhance its degradation effect. In a mixed system of water and acetonitrile, the efficiency of the electrochemical filtration device loaded with the modified electrode for degrading OMC is 1.54 times that at room temperature. The experimental results confirmed the superiority and application prospects of the self-designed treatment scheme for organic pollutants, providing experience and a reference for the future treatment of PPCP pollution.

1. Introduction

With the rapid development of the cosmetics industry, the environmental impact of harmful ingredients contained in cosmetics (e.g., heavy metals, hormones, antibiotics, sunscreens, etc.) is becoming increasingly significant. Many cosmetic ingredients can be harmful to plants and animals and even affect the health of the ecosystem when introduced into the environment. In recent years, there has been a growing awareness of the issue of ultraviolet radiation (UV)-filtering ingredients in sunscreens becoming an emerging contaminant in the aquatic environment, with these ingredients being released in large quantities [1]. The accumulation of these constituents in the aquatic environment has the potential to result in ecological contamination and bioaccumulation, particularly in the form of nanoparticles, which are considered to be particularly hazardous [2]. OMC is one of the most commonly used organic filters in the world and is classified as a new pollutant. A number of studies have reported that OMCs have toxic effects on the environment and act as endocrine disruptors, which can result in developmental toxicity [3]. Significantly, wastewater containing these organic pollutants is typically transported to local treatment plants together with domestic wastewater. However, conventional wastewater treatment processes are just able to deal with a limited number of simple and degradable organics in domestic wastewater. Entering the environment, these pollutants will undergo continuous migration and transformation, potentially resulting in irreversible ecological effects. Currently, a hot issue investigated by scholars in the field of water treatment is the development of methods for the specific targeting of novel organic pollutants [4].

At present, a range of advanced technologies, including membrane filtration, adsorption, advanced oxidation, electrochemical advanced oxidation, and others, have been demonstrated to be effective and efficient in the treatment of novel organic pollutants in different fields [5,6]. Traditional electrochemical oxidation is more inclined towards electron transfer or low-intensity oxidation, making it more suitable for primary treatment or conversion rather than complete degradation [7]. Advanced electrochemical oxidation represents an advanced form of electrochemical oxidation technology. Its core principle lies in the in situ generation of free radicals such as ·OH to achieve complete mineralization [8,9]. Compared with traditional electrochemical oxidation, electrochemical advanced oxidation has the characteristics of convenient, efficient, and non-selective treatment of persistent organic pollutants, and demonstrates excellent removal efficiency for hormones, antibiotics, and diverse persistent organic pollutants in wastewater [10,11]. Therefore, we chose electrochemical advanced oxidation technology for the treatment of OMC. Carbon nanotube (CNT) has proved to be excellent for the manufacture of nanocomposites in recent years because of its high hardness, high electrical conductivity, and low density, as well as its low toxicity, biocompatibility, bioactivity, and biodegradability [12]. Based on their structure, CNTs can be classified as either single-walled or multi-walled, and for electrochemical catalytic oxidation processes, the 3D architecture of multi-walled carbon nanotubes (MWCNTs) could hold greater promise than single-walled CNTs because of their greater specific surface areas, direct flow of water through the electrodes, and more active sites, which can increase the contact area between active free radicals and organic pollutants in the system, and also increased loaded sites of modified metals on CNT [13,14]. Studies have shown that CNTs have many applications in biomedicine and electrochemical advanced oxidation, etc., and grafting CNTs into bacterial cellulose can give them an advantage in polarization [15]; sensor performance for colchicine detection is markedly improved through the synergistic oxidation catalysis achieved by Au/Pt nanoparticle-decorated MWCNTs, boosting both sensitivity and selectivity [16]; graphite-functionalized MWCNTs exhibit remarkable catalytic performance in electro-Fenton processes, enabling superior Cr (VI) reduction and contaminated waste remediation [17]; the Fenton method with MWCNTs and carbon black co-modified graphite felt electrodes has been used for more efficient removal of amoxicillin from water [18]; MWCNTs modified with electrolytic manganese residue (EMR) exhibit outstanding catalytic performance when combined with electro-activated persulfate, eliminating 90.96% of roxarsone (ROX) contaminants [19]; Pt-Ru alloy-modified MWCNTs are able to degrade ibuprofen in a green and efficient manner [20,21]. Fe-Cu bimetallic functionalization of CNTs demonstrates enhanced catalytic activity compared to monometallic counterparts, revealing clear synergistic interactions [22]; and the carboxyl functionalization of MWCNTs creates additional active surface sites compared to their unmodified counterparts, which can accommodate a greater number of modified metal particles, allowing the system to be more fully exposed to organic contaminants, and are more compatible with other solvents [23].

This study aimed to utilize the electrochemical advanced oxidation process to simulate the treatment of wastewater containing organic sunscreens and to investigate the impact of various modified electrodes, electrolytes, persulfate concentration, and voltages on the degradation of organic pollutants from sunscreen products. By enhancing the CNT materials and developing electrodes with superior catalytic properties, this study aimed to identify optimal reaction conditions for the complete degradation of these pollutants. It is expected that this work, through the aspects of modifying the bimetallic system, using low-energy consumption new electrofiltration methods, and achieving efficient pollutant degradation, will fill the research gap in the efficient degradation of pharmaceutical wastewater for the Fe-Cu/MWCNT system and supply valuable suggestions and insights for developing more effective electrochemical advanced oxidation techniques for the industrial treatment of wastewater containing pharmaceuticals and PPCPs in the future.

2. Materials and Methods

2.1. Preparation of Electrodes with “Sandwich” Structure

Step 1: Disperse C-MCNT in TritonX-100, then subject it to ultrasonic treatment, centrifugation, purification, and drying. Remove the sample and enhance the hydrophilicity of the carboxylated multi-walled carbon nanotubes (CMWCNTs). Detailed steps can be found in Supplementary Material Text S4. Step 2: Perform metal modification on multi-walled carbon nanotubes using a low-temperature hydrothermal method. Impregnate C-MCNT with Fe³⁺/Cu²⁺ solution, then add polyethyleneimine for flocculation after ultrasonic treatment. React fully at 85 °C, followed by centrifugation, washing, and drying. Step 3: The obtained FeCu/CMWCNT was modified by m-phenylenediamine (MPD) coupling. During the reaction process, some MPD will be converted into poly-m-phenylenediamine (PMPD) [24]. Step 4: The modified material FeCu/CMWCNT (PMPD) was filtered on titanium mesh-polyvinylidene (PVDF) fluoride membrane to make the finished electrode. FeCu/CMWCNT (PMPD), Fe/CMWCNT (PMPD), Cu/CMWCNT (PMPD), CMWCNT, and other materials were prepared in this experiment to comparatively investigate their properties and effective reaction sites. The drugs used in the experiment are described in Supplementary Material Text S1.

2.2. Material Morphology Testing

Scanning electron microscopy (SEM) tests were carried out using a “Czech TESCAN MIRA LMS” instrument. The samples were examined by transmission electron microscope (TEM) using a TF20 model instrument and by X-ray diffraction (XRD) test with a RIKEN SmartLab-SE model. Fourier transform infrared spectroscopy (FTIR) was performed using a Thermo Scientific Nicolet iS20 model. Raman spectroscopy testing was conducted using a HORIBA Scientific LabRAM HR Evolution model.

2.3. Electrochemical Property Testing

The sample preparation process was as follows: Al203 polished glassy carbon electrode was washed with deionized water and ethyl alcohol in turn, and then wiped with mirror paper. Then, 10 mg of the awaiting testing sample was dissolved in a mixture of 925 μL ethyl alcohol and 75 μL of 5% Nafion solution, followed by ultrasonic treatment until the mixture was well dispersed. Subsequently, 10 μL of the prepared sample was measured and carefully pipetted onto the polished glassy carbon electrode. The sample was then left to naturally dry at normal atmospheric temperature.

The manufactured glassy carbon electrode was placed into the triquetrous electrode within the electrochemical workstation, the power of the workstation was turned on, and the computer software was started to repeat the cyclic voltammetry (CV) curves and electrochemical impedance spectroscopy (EIS) curves of each material sample. In addition, the contact angles of the two samples were videotaped with a JY-82B Kruss DSA modeler, and the images were captured every 50 ms, and the image when the droplet was just in contact with the sample was selected to measure the contact angle.

2.4. Degradation of Octyl Methoxycinnamate by Electrically Activated Persulfate at Sandwich Electrode

To create the standard stock solution of octyl methoxycinnamate, 2 mL of OMC sample was initially dispersed in 4 mL of acetonitrile. This mixture was then combined and poured into 400 mL of aqua destillata in a small beaker to achieve a uniform solution. Subsequently, the homogeneous mixture was transferred to a brown volumetric flask and stored on the test bench for standby use at any time.

The dissolution method was used to determine the concentration levels of sodium sulfate and sodium persulfate in electrolyte solution. Various masses of sodium sulfate electrolyte were added to 100 mL of distilled water, and the electrochemical filtration device was turned on to observe the currents of the electrolyte systems with different concentrations under 2.5 V voltage and current conditions. The current increases as the concentration of the added electrolyte rises, but the degradation effect does not grow any further after reaching a certain concentration [25]. The optimal reaction conditions are to add 20 g of sodium sulfate as an electrolyte and 200 mg of sodium persulfate as a source of sulfate radicals to the 400 mL degradation system.

Finally, the optimum voltage and removal of interference for electrochemical activation of sodium persulfate degradation reaction were explored. In this experiment, titanium mesh was used as the cathode of the electrochemical filtration device, and OMC standard stock solution with a concentration of 208.55 mg/L and 400 mL of sodium persulfate solution were mixed. The electrochemical filtration device (Figure S2) was connected, the peristaltic pump speed was 4, the voltage was adjusted to 2.5 V, and the amount of sodium persulfate mentioned earlier was added to the beaker. The time was counted from before the addition, and 15 mL of sample was collected at 0, 30, and 60 min intervals. Subsequently, the experiment was reiterated with the direct current (DC) power supply voltage set at 2.0 V, 1.5 V, 1.0 V, 0.5 V, and 0 V while keeping all other conditions constant. Ultimately, 6 groups of 18 samples each were obtained for total organic carbon (TOC) analysis.

3. Results

3.1. Analysis of Morphology Test Results

After the modification, active particles such as Fe, Cu, and PMPD were loaded onto the CMWCNT. By observing the SEM image (Figure 1a), it is evident that the modified material has more cluster particles aggregated or dispersed on the surface of CMWCNTs compared to the unmodified material (Figure 1b). This indicates that active particles, such as Fe, Cu, and PMPD, were sufficiently adsorbed by the CMWCNT, thereby increasing its specific surface area and confirming the success of the modification. From Figure 1c,d, it can be seen that the elements of Fe and Cu have obvious peaks, and the peaks of the elements of N and O are much higher than the peaks before modification, and the elements of Fe, Cu, N, and O accounted for 4.39%, 1.55%, 15.93%, and 22.94%, respectively, in the modified material. Therefore, energy dispersive X-ray spectroscopy (EDX) showed that the contents of various elements in the modified material were significantly higher than those in the pre-modified material, and the modified CMWCNT was successfully loaded with Fe, Cu, and PMPD. However, the coverage of PMPD, while improving the synergistic effect between Cu and CMWCNT, also masked many active sites inside the modified material, which hindered the electrochemical reaction of this part of the active sites [26,27]. The mapping plots of the elements before and after modification are shown in Figure S3, which can prove the presence of a large number of modified elements on the modified material. The TEM (Text S2) and XRD (Text S3) test results of the material can also prove the successful loading of the modified particulate matter. Metal materials possess excellent electrical conductivity and catalytic activity. Apart from iron and copper, there are many other metals that are commonly used as materials for modifying electrodes or catalysts. For example, metal-based nanocrystals (NCs) generally demonstrate superior catalytic performance [28]. Pd-based materials demonstrate efficient catalytic activity in a variety of electrochemical reactions, including hydrolysis, oxygen reduction, and the electrocatalytic oxidation of alcohols [29].

As depicted in Figure 2a, the XRD spectra of the materials exhibit substantial differences before and after modification. From the perspective of carbon peak enhancement, the incorporation of Fe, Cu, and N elements likely constitutes the primary reason for these changes. Furthermore, the particle sizes of Fe and Cu calculated using the Scherrer equation showed good agreement with TEM observations. Detailed calculation procedures are provided in Text S3. In Figure 2b, the FTIR spectrum of CMWCNT prior to modification (indicated by the gray line) shows characteristic peaks: the peak at 3044.29 cm⁻¹ conforms to the stretching vibration of the methyl group, the peak observed at 1714.45 cm⁻¹ is ascribed to the stretching vibration of the C=O, and the peaks at 1540.61 cm⁻¹ and 1188.88 cm⁻¹ are associated with the vibrational modes of the CNT backbone [30,31,32]. The peak at 936.09 cm⁻¹ is likely indicative of the peak of the C-H bond. These polar groups are predominantly located at the tail ends and defects on the outer surface of CMWCNT, thereby significantly enhancing its dispersion situation in polar solvents [33]. The disappearance of the absorption peak at 1714.45 cm⁻¹ in the modified CNT can be attributed to the substantial loading and embedding of Fe³⁺ and Cu²⁺ onto the ends, surface, and defect sites of CMWCNT. This process likely leads to the formation of new structural configurations, thereby replacing the original C=O bonding structures [34,35]. Two distinct absorption peaks are also observed at 838.32 cm⁻¹ and 606.82 cm⁻¹ for C-NO₂ and -NH₂ bonds, which prove that a portion of the -NH₂ functional groups in PMPD supported on CMWCNT remains intact after the modification process. Moreover, based on other studies, the absorption peaks for Fe and Cu metals are likely to be observed within the wavenumber range of 400–700 cm⁻¹ [36]. The red curve reveals a faint absorption peak in this region. However, considering the prior XRD characterization of the metal content in the modified material, the low concentration of Fe and Cu may cause their corresponding absorption peaks in the FTIR spectrum to be overshadowed by the peaks of CMWCNT and PMPD. Consequently, in the next work, we could investigate this summation ratio and augment the content of metal ions during the synthesis process to potentially enhance the visibility of the metal absorption peaks. Many studies have utilized MWCNTs for modification to enhance their electrochemical performance. For instance, the better interaction between functional MWCNTs and polyvinyl chloride significantly improved the thermal stability and electrical conductivity of the composite material [37]. A smartphone-powered, enzyme-free wearable photoelectrochemical (PEC) sensor utilizing 2D zinc porphyrin MOF nanosheets in combination with MWCNTs (2D-TCPP(Zn)/MCNT) has been developed to enable precise inspection of vitamin C in sweat [38].

3.2. Modified Material Performance Test Analysis

Figure 3 generally illustrates that the metal modification can significantly improve the electrochemical performance of the materials, but the improvement of the electrochemical performance of the materials with the addition of PMPD is rather insignificant. As shown in Figure 3a, the area of the CV curve for the Cu/C-CMWCNT(PMPD) is larger than that of the Cu/C-CMWCNT material, which suggests that the oxidative performance of the copper-modified monometallic material is significantly improved by PMPD and that PMPD and Cu may work together in a synergistic manner to promote the activation process of CMWCNT. This may be attributed to the activating effect of conductivity of Cu and CMWCNT by PMPD. However, the CV curve area of the Cu/C-CMWCNT(PMPD) was smaller than that of C-CMWCNT, while the areas of the curves for C-CMWCNT and CMWCNT were similar, which suggests that the material’s redox characteristics remain unaffected by graphite powder, whereas copper metal addition may lead to activity inhibition. This could be because metal Cu cannot form a stable physicochemical structure with CMWCNT when CMWCNT is loaded via the hydrothermal method, which affects the total electrochemical performance of the material, and the addition of PMPD may just make up for the shortcomings of this method. Unlike metallic Cu, the electrochemical activity of CMWCNT was significantly enhanced by Fe metal, but PMPD may have inhibited the synergistic effect of Fe with CMWCNT, as shown in Figure 3b. This may be due to the fact that PMPD was synthesized by completely encapsulating iron and CMWCNT in the process of combining the modifying materials so that they could not be exposed to the system.

The conductivity and electrochemical activity of CMWCNT can be greatly improved by simultaneous modification with bimetallic iron and copper. The synergistic effect of iron and copper enables the modified electrode to react with the organic pollutants in the system, thereby achieving its mineralization function and improving the removal efficiency. The two distinct peaks on the brown curve of Figure 3d may be the peaks produced by the load of Cu and Fe, respectively, indicating that Fe and Cu can both enhance the electrochemical activity of CMWCNT, but the incorporation of PMPD may degrade the electrochemical performance of CMWCNT decorated with Fe and Cu. Overall, the bimetallic-modified CMWCNT should possess a better effect on electrocatalytic oxidation ability and PMPD also exhibits an advantageous improvement effect on Cu-loaded CMWCNT. The comparison of the CV curve areas shows that the overall performance of the target modified material is better than that of the pre-modified CMWCNT.

By examining each EIS pattern (Figure 4) throughout the modification process, it is evident that the high-frequency region of all modified CMWCNTs features a straight line with a 45-degree inclination angle. This is the typical characteristic of the EIS presented by those porous films coated on the metal surface in the metal/film/electrolyte structure. Meanwhile, the half-round depression observed in the low-frequency region can be explained by the Faraday process of charge exchange at the carbon/electrolyte interface [39]. As can be inferred from Figure 4, the semidiameter of the semicircle enclosed by the FeCu/C-CMWCNT(PMPD) curve appears smaller than that of other materials that have not been modified with PMPD. Theoretically, a smaller radius typically indicates lower impedance. Consequently, it can be reasonably concluded that PMPD effectively enhances the overall electrical conductivity of the modified material [17]. In addition, the smallest radius in the semicircular curve is equivalent to FeCu/C-CMWCNT(PMPD) after treatment with N-methyl-2-pyrrolidone (NMP). This treatment is the last step of loading FeCu/C-CMWCNT(PMPD) onto the electrode. The observation further proves that the electrical conductivity of the modified composite is enhanced once more following NMP treatment. These findings validate the observed variations in the CV profiles, demonstrating that NMP serves not merely as a PVDF solvent but also enhances the modified material’s electrochemical performance. This effect likely stems from NMP’s ability to partially dissolve PMPD during processing, thereby exposing previously obscured active sites on both the metallic component and CMWCNT surfaces.

Finally, the contact angle of the modified material was tested, and the contact angle of the modified CMWCNT was less than 90° (Figure 5), which indicated that the modified composite material was still hydrophilic. This enhancement promotes improved adsorption of both aqueous solution and solutes onto the modified electrode surface, enabling thorough contact between organic pollutants and the material while providing sufficient reaction sites for increased pollutant accommodation.

3.3. Degradation of OMC by Electrically Activated Persulfate at Sandwich Electrodes

3.3.1. Principle of Electro-Activated Sodium Persulfate Oxidative Degradation of Organic Pollutants

It is documented that at different pH, persulfate produces different oxidation radicals in the system. Under alkaline conditions, persulfate dissolved in the system reacts with hydroxyl radicals to form hydroxide ions and persulfate radicals, while sulfate radicals are consumed by hydroxide ions to form sulfate ions and hydroxyl radicals, as in Equations (1) and (2) [40,41]. In contrast, under acidic conditions, persulfate accelerates the production of sulfate radicals catalyzed by hydrogen ions, as in Equation (3) [42].

$$S_2{O}_8^{2-}+·O{H}^{-}\to S_2{O}_8^{-}·+O{H}^{-}$$

(1)

$${OH}^{-}+{SO}_4^{-}·\to ·OH+{SO}_4^{2-}$$

(2)

$$S_2{O}_8^{2-}\stackrel{H^{+}}{\to }{2SO}_4^{2-}·$$

(3)

Under the condition of applied voltage, sodium persulfate dissolves in water so that it exists in the form of ions. In an electrochemical device, the energized anode tends to adsorb anions, while the cathode tends to adsorb cations, so theoretically the sodium ions produced by sodium persulfate dissolving in water should approach the cathode, and the persulfate anions should approach the anode, but the reaction of electrically activated sodium persulfate (PDS) is carried out on the cathode, and the mechanism of the reaction is shown in Equations (4) and (5), where PDS is reduced at the cathode to obtain electrons to produce, and then the organic pollutants are oxidized and degraded [43]. The reduced sulfate ions return to the anode and are oxidized to persulfate ions [44]. Therefore, based on the activation principle of PDS in the presence of electric field and the theoretical principle of attraction between charges, we decided to modify both the cathode and anode in an attempt to maximize the degradation of the target pollutant at neutral pH.

$$S_2{O}_8^{2-}+e^{-}\to {SO}_4^{2-}+{SO}_4^{-}·$$

(4)

$$2{SO}_4^{2-}\to S_2{O}_8^{2-}+2e^{-}$$

(5)

3.3.2. Degradation Effect of OMC by Electrically Activated Persulfate at Sandwich Electrodes

The effect of electroactivated degradation of OMC by sodium persulfate at different voltages on the titanium mesh electrode of an unmodified electrode is shown in Figure 5. The varying TOC concentrations observed in the samples at 0 min are likely a result of the samples being continuously exposed to ambient light for an extended period throughout the experiment. The longer the experiment lasts, the longer the OMC samples are exposed, and the higher the likelihood of photodegradation reactions occurring. The literature indicates that OMC exhibits enhanced photodegradation in concentrated non-aqueous solvent systems, such as in methanol [45]. Although the presence of minute quantities of refractory OMC in the environment remains a potential threat to human health, a preliminary conclusion can be drawn from the graphs showing TOC changes at different voltages. In the absence of modified materials, the degradation rate of TOC grows with voltage from 0 to 1.5 V. This is likely due to the increased current, which more fully activates the system with sodium persulfate and accelerates the direct mineralization reaction of OMC on the anode. The reduction in the TOC removal velocity at 2.0 V is likely attributable to the participation of specific electrons in the oxygen deposition reaction, as evidenced by the substantial amount of bubbles observed in the electrochemical filtration device at this voltage. Drawing upon the analysis in the academic literature, 1.5 V was ultimately determined to be the optimal reaction potential [46].

Furthermore, to eliminate the interference of acetonitrile in the OMC degradation process, a separate aqueous solution containing acetonitrile at an initial concentration of 4.04 mg/L was prepared under identical degradation conditions. Consequently, the TOC removal velocity was approximately 5.7% at this concentration during the 1 h electrochemical oxidation reaction. This indicates that the majority of the organic substances in the final reaction system remained OMC, thereby ensuring that the experimental results obtained were representative.

To further investigate the impact of modified composite incorporated into OMC degradation by various electrodes, experiments were conducted to compare the performance of several electrochemical filtration devices with different electrode configurations in degrading OMC. Additionally, this study examined the degradation effect of modified composite incorporated into copper mesh and carbon cloth for OMC. As depicted in Figure 6, when the cathode was modified, the degradation velocity of OMC reached 24.87% within 1 h, which is 53.9% higher than that under indoor lighting conditions. This demonstrates the excellent catalytic oxidation performance of the modified composites. When only the cathode or anode was loaded with the composite, the hourly degradation velocities were 16.53% and 16.97%, respectively. These rates represent increases of 13.8% and 16.9% compared with the cases where no composite loading was applied to these electrodes. This indicates that modifying single electrodes with composite nanomaterials can enhance the overall OMC mineralization effect, although to a much lesser extent than simultaneous modification of both electrodes. Other cases that utilize electrofiltration to treat organic substances have also shown excellent results. Nitrogen-doped graphene (N-GN) was used as the anode material for electrocatalytic degradation of acetaminophen (APAP), and the degradation results indicated that after 90 min of the reaction, the removal rate of APAP reached 93% on the N-GN (15.6 wt%) electrode. After 150 min of the reaction process, APAP was nearly fully degraded. The nitrogen doping increased the APAP degradation rate by 41.2% [47]. In the presence or absence of nitrate, using an electrocatalytic system consisting of Fe₃O₄ nanoparticles (NPs), humic acid, and Ni-Fe/Fe₃O₄ nanocomposite cathode to disinfect Escherichia coli, the nitrate reduction rate reached 72.8% within 120 min [48]. Moreover, when copper mesh and carbon cloth were employed as bases for double electrode modification, the degradation velocity of OMC was only 9.35% and 8.84%, respectively. This indicates that the performance of titanium mesh is significantly superior to that of carbon cloth and copper mesh. Additionally, copper mesh electrodes are deemed unsuitable for use as electrode bases due to their tendency to dissolve during the electrochemical reaction process.

4. Discussion

PMPD appears to be an effective material for enhancing electron transfer between Cu metal and CMWCNTs, and the development of a more rational loading method remains an important area for further investigation. The loading of PMPD can enhance the material’s specific surface area and promote contact between the target pollutants and the material’s surface. However, the aggregation of PMPD may potentially hinder the catalytic activity by covering some of the redox-active sites on the CNTs. The XRD data acquired were matched against the standard reference patterns for Fe and Cu, confirming the successful incorporation of the target metals into the material. The content of Fe, as determined by energy spectrum scanning of the material surface, is approximately three times that of Cu. This suggests that when a mixed solution of Fe³⁺ and Cu²⁺ ions at the same concentration is loaded onto CMWCNTs using the hydrothermal method, a higher proportion of Fe is incorporated compared to Cu. The previous study proves the conclusion that metal Fe³⁺ and Cu²⁺ have a synergistic effect when loading CNTs [49,50,51,52,53]. Meanwhile, this work also found that the use of chloride mixtures of the two ions with the same concentration produces a competitive effect in the hydrothermal synthesis of modified CNTs due to the varying charge sizes and atomic structures of Fe³⁺ and Cu²⁺, and the pores and defective sites of the CMWCNTs are not homogeneous, but the reason for this competitive mechanism is still to be further investigated. PMPD failed to produce a synergistic effect with bimetallic-modified CNTs, but it did produce a synergistic effect with monometallic Cu. This indicates that the interaction between Cu metal and m-phenylenediamine warrants further investigation. The relatively low content of Cu could also be a contributing factor to the activation performance of Cu-CMWWCNT-PMPD. Moreover, the nitrogen element in PMPD may also influence the electrocatalytic activity of the material, as suggested by the FTIR and Raman spectral evidence of the -NH₂ to C-NO₂ transition. Meanwhile, the increased defects in CMWCNTs create more active sites for electrochemical catalytic oxidation, instead of merely enhancing the material’s electrical conductivity. Given the electrode conditions composed of the newly modified materials with the aforementioned advantages, simultaneous loading of both the cathode and anode significantly facilitates the electrochemical mineralization of OMC compared to modification of individual electrodes. Thus, it can be hypothesized that the modified cathode and anode materials may exert a synergistic effect during the reaction process. The hydrothermal method is the key synthesis method in this study, and it is also the crucial step that determines the loading status of bimetal and PMPD. While this microstructure has the potential to enhance the specific surface area, it is important to note that the defect sites of the modified CNTs may become unrecognizable. It can thus be concluded that it is possible to increase the amount of NMP in order to achieve a more effective exposure of the defective active sites of the modified material without the necessity of changing the synthesis method. The hydrothermal method has been shown to be a feasible method for the CMWCNTs’ modification with metal ions, while the sticking of other polymers requires further improvement.

5. Conclusions

In this study, CNTs were modified using a bimetallic method. The morphology and electrochemical properties of the material were tested and analyzed before and after modification. The degradation efficiency of OMC was also compared. The following conclusions were drawn from this study: MWCNT composite materials modified with Fe and Cu exhibit catalytic activity and can effectively enhance the overall degradation efficiency of advanced oxidation reaction systems. Furthermore, the highest OMC degradation efficiency is achieved through the simultaneous modification of the cathode and anode. However, the cooperative and competitive effects of Fe³⁺ and Cu²⁺ during the modification process still require further study. This study provides a basis for the use of an efficient electrode-based electrofiltration system to treat new organic pollutants.