Novel MIL-53(Fe)@C Magnetic Composite Electrode for Efficient Dechlorination of Disinfection By-Product Trichloroacetic Acid in Water Treatment

Abstract

Electrochemical reduction is a promising strategy for the dechlorination of halogenated organic compounds, offering advantages such as enhanced electron transfer efficiency and increased hydrogen atom concentration. It has garnered significant attention for application in mitigating halogenated disinfection by-products (DBPs) in drinking water, owing to its high efficiency and simple operation. In this study, trichloroacetic acid (TCAA), a representative DBP, was selected as the target contaminant. A novel composite cathode comprising a metal–organic framework MIL-53(Fe)@C supported on an Nd magnet (MIL-53(Fe)@C-MAG) and its dechlorination performance for TCAA were systematically investigated. The innovative aspect of this study is the magnetic attachment of the MOF catalyst to the carbonized cathode surface treated through carbonization, which fundamentally differs from conventional solvent-based adhesion methods. Compared to the bare electrode, the MIL-53(Fe)@C-MAG achieved a TCAA removal efficiency exceeding 96.03% within 8 h of contact time. The structural characterization revealed that the α-Fe⁰ crystalline phase serves as the primary active center within the MIL-53(Fe)@C catalyst, facilitating efficient electron transfer and TCAA degradation. The scavenger experiments revealed that TCAA reduction involves a dual pathway: direct electron transfer and atomic hydrogen generation. The modified MIL-53(Fe)@C-MAG electrode exhibited robust electrolytic performance over a broad pH range of 3–7, with TCAA removal efficiency showing a positive correlation with current density within the range of 10–50 mA/cm². Furthermore, the electrode maintained exceptional stability, retaining more than 90% removal efficiency after five consecutive operational cycles. The versatility of the system was further validated by the rapid and efficient dechlorination of various chlorinated DBPs, demonstrating the broad applicability of the electrode. The innovative magnetic composite electrode demonstrates a significant advancement in electrochemical dechlorination technology, offering a reliable and efficient solution for the purification of drinking water contaminated with diverse halogenated DBPs. These results provide valuable insights into the development of electrolysis for dechlorination in water treatment applications.

1. Introduction

In engineering water treatment systems, disinfection is essential for the inactivation of pathogens and microorganisms [1,2]. Chlorine is perceived as an optimal disinfectant because of its cost-effectiveness in reducing waterborne diseases [3]. However, its reaction with natural organic matter (NOM) leads to the formation of halogenated disinfection by-products (DBPs), which pose significant adverse effects on human health [4,5,6]. Currently, over 700 DBPs have been identified in drinking water [7,8,9] and are categorized into carbonaceous and nitrogenous DBPs. Among these, trihalomethanes (THMs) and haloacetic acids (HAAs) are the most prevalent regulated species. Notably, HAAs exhibit lower volatility than THMs, limiting their elimination during common household activities, such as cooking or boiling. Consequently, HAAs pose a higher carcinogenic risk [10,11]. Therefore, effective and practical strategies are urgently required to mitigate HAAs contamination in drinking water.

Various methods have been reported for DBPs removal, including biologically active filtration, activated carbon adsorption, membrane separation, and advanced oxidation processes [12,13,14]. However, these methods are economically impractical or inconvenient to operate when applied after terminal chlorine disinfection. As an alternative, electrochemical reduction through electrolysis offers an efficient and environmentally friendly strategy for halogenated DBPs degradation, ensuring selective halogen atom removal without toxic by-product generation or chemical agent addition [15,16]. To elevate the electrolysis efficiency, catalytic materials, including MXenes, layered double hydroxides, and metal–organic frameworks (MOFs), have been employed to modify electrodes, facilitating electron transportation [17,18,19]. MOFs are self-assembled from metal cations/clusters and organic ligands, with their metal centers acting as electron transporters and catalysts in reduction reactions [18,19]. Yang et al. [20] developed an in situ-grown CuNi bimetallic MOF loaded on nickel foam as a composite cathode for efficient electrocatalytic reduction of nitrate. Among MOFs, the MIL-53(M^III) family is notable for its chemical versatility, high stability, and large specific surface area [21]. The MIL-53(Fe) has been widely utilized because of its multifunctional properties [22,23]. Recent studies have demonstrated that the biochar-supported MIL-53(Fe) cathodes exhibit excellent catalytic performance, significantly increasing the oxygen reduction reaction (ORR) activity and recovering energy while treating wastewater [24].

In our previous studies, vitamin B₁₂ (VB₁₂) was employed as a catalyst to promote dehalogenated reduction [25,26]. The cobalt-centered corrin macrocycle in VB₁₂ exhibits excellent electron transition activity in redox reactions [27,28]. However, when using Nafion as a binder for VB₁₂ coatings, we encountered significant challenges in simultaneously achieving low electrical resistance and strong interfacial adhesion, owing to the fundamental compromise between electrical conductivity and interfacial bonding strength [29]. Based on previous research, we propose an innovative composite cathode utilizing the MIL-53(Fe)@C as a coating catalyst applied using magnetic force. The purpose of this research includes (1) preparing the MIL-53(Fe)@C nanomaterials and fabricating a composite neodymium magnetic electrode; (2) elucidating the dechlorination efficiency and mechanism of TCAA by the composite electrode; and (3) characterizing the electric and chemical properties of the composite electrode.

2. Materials and Methods

2.1. Chemicals and Reagents

All solutions were prepared using Milli-Q water with a resistivity of 18.2 MΩ·cm. The N, N-dimethylformamide (DMF, 99.5%), iron (III) chloride hexahydrate (FeCl₃·6H₂O, 99.0%), ethanol (99.5%), and terephthalic acid (1, 4-H₂BDC, 99.0%) were purchased from Sinopharm Chemical Reagent Co., Ltd. (Shanghai, China). The trichloroacetic acid (TCAA), trichloroacetonitrile (TCAN), trichloroacetaldehyde (TCA), trichloroacetone (TCP), trichloromethane (TCM), sodium sulfate anhydrous (Na₂SO₄), 1,2-dibromopropane, tert-butanol (TBA) and sodium hypochlorite solution were obtained from Aladdin Reagent Corporation (Shanghai, China). The tert-butanol was purchased from Maclean Biochemical Technology Corporation (Shanghai, China). The methanol and methyl tert-butyl ether (MTBE) were purchased from CNW Technologies (Düsseldorf, Germany). The hydrochloric acid and concentrated sulfuric acid were supplied by Xilong Scientific Co., Ltd., China. The sodium hydroxide was purchased from Sinopharm Chemical Reagent Co., Ltd., China.

2.2. Preparation of MIL-53(Fe)@C-MAG Electrode

As shown in Figure 1, FeCl₃⋅6H₂O (3 mM) and 1, 4-H₂BDC (3 mM) were added to 65 mL of DMF solution. The mixture was stirred at a constant temperature to achieve uniformity and then heated at 150 °C for 15 h in a Teflon-lined autoclave. After automatic cooling to ambient temperature, the product was collected via centrifugation. The precipitate was washed three times with the DMF and ethanol, and finally dried in a vacuum oven at 60 °C for 12 h to obtain the precursor MIL-53(Fe).

The prepared precursors were ground and placed in a porcelain crucible in a tube furnace. Subsequently, thermal treatment was conducted at 800 °C with a heating rate of 5 °C/min for 2 h, resulting in the formation of the MIL-53(Fe)@C magnetic nanomaterials. The intermediate materials were then dispersed in a specific amount of ultrapure water and sonicated at 50 Hz for 30 min to obtain a MIL-53(Fe)@C nanomaterial dispersion.

For the pretreatment of the neodymium magnet, the surface was first polished with 20 wt% alumina, rinsed with ultrapure water, and dried at 60 °C. The MIL-53(Fe)@C nanomaterial dispersion was uniformly drop-coated onto the surface of the neodymium magnet electrode using a syringe and then dried in a vacuum oven at 60 °C to obtain the MIL-53(Fe)@C-MAG electrode.

2.3. TCAA Dechlorination Experiments

The experiments were conducted in a 2 L glass reactor wrapped in light-blocking aluminum foil to prevent potential photochemical reactions (Figure 2). The reactor was filled with a TCAA solution (100–300 µg/L) containing 0.1 M Na₂SO₄ as a supporting electrolyte. The initial pH of the reaction solution was adjusted using 0.5 M H₂SO₄ or 0.5 M NaOH, respectively. A ruthenium–iridium titanium electrode (40 mm × 70 mm × 2 mm) served as the anode, while the prepared electrode (50 mm × 50 mm × 2 mm) acted as the cathode. The electrodes were placed 3 cm apart, and a constant current was supplied using a direct current (DC) power source. A detailed diagram of the electrolysis unit is shown in Figure 2. The samples were collected at 0, 1, 2, 4, 6, and 8 h and analyzed immediately. The residual TCAA was measured after filtering 30 mL of the samples through a 0.22 µm membrane. In the cycling experiments, the used electrode was alternately rinsed three times with deionized water and ethanol. All the experiments were conducted in triplicate.

2.4. Analytical Methods

The TCAA was quantitatively extracted using MTBE-mediated liquid–liquid extraction (LLE). The aqueous sample was acidified with 4.0 g Na₂SO₄ and 2.0 mL of H₂SO₄. Subsequently, 3.0 mL MTBE containing 1,2-dibromopropane (300 μg/L, as an internal standard) was introduced for liquid–liquid extraction. The upper organic layer was transferred to a 20 mL derivatization vial containing 2.5 mL of freshly prepared sulfuric acid–methanol solution. The mixture was subjected to derivatization at 50 °C for 2 h in a thermostatic reactor and then cooled to room temperature. Following the derivatization, 7.0 mL of Na₂SO₄ (150 g/L) was added to dehydrate the organic phase of the sample. The aqueous phase was discarded, and the remaining organic phase was neutralized with 1.0 mL of saturated sodium bicarbonate solution. Finally, 2.0 mL of the treated organic extract was transferred to GC vials for analysis. For the TCM, TCAN, TCA, and TCP contaminants, direct extraction using MTBE without other treatment was performed before GC-ECD analysis. The pH was determined using a pH monitor (SX751; Shanghai San-Xin Instrumentation, Inc., Shanghai, China).

2.5. Characterization Methods

The crystallographic characteristics of the composite electrode surfaces were analyzed via X-ray diffraction (XRD) (PNAlytical X’Pert PRO, Malvern Panalytical, Almelo, The Netherlands). Field-emission scanning electron microscopy (FE-SEM; Zeiss Gemini 500, ZEISS GeminiSEM Family, Jena, Germany) was used to characterize the surface topography of the MIL-53(Fe) and its carbon-encapsulated counterpart MIL-53(Fe)@C during pyrolysis across thermal gradients (400–800 °C). The electrochemical analyses were performed with a 0.10 mol/L Na₂SO₄ electrolyte using an electrochemical workstation (Zhenhua CHI 660E, Shanghai Yidian Analytical Instrument Co., Ltd., Shanghai, China). The electrochemical impedance spectroscopy (EIS) was performed with an amplitude of 10 mV over a frequency range of 1 to 1,000,000 Hz. The linear sweep voltammetry (LSV) was performed with the test voltage set in the range of −1.45 to −0.20 V and a scan rate of 10 mV/s. The cyclic voltammetry (CV) curves were obtained by scanning at a rate of 0.1 V/s over a range of −1.5 to 2.0 V with Ag/AgCl intervals.

3. Results and Discussion

3.1. Preparation and Characterization of MIL-53(Fe) and MIL-53(Fe)@C Series

Figure 1 illustrates the fabrication process, morphological characteristics, and structure of the biconical nanorod-like MIL-53(Fe)@C architecture. The synthesis involves two key steps. First, the solvothermal preparation of uniform biconical nanorod-like MIL-53(Fe) precursors and second, the controlled carbonization to yield the final MIL-53(Fe)@C nanostructures. This unique nanorod morphology offers high accessibility for electrolytes and facilitates ion/electron transport pathways along the nanorod axis, as well as stable immobilization on the neodymium magnet electrode, owing to the material’s inherent ferromagnetic properties, thereby ensuring robust electrocatalytic performance.

The SEM images reveal the morphological evolution of the MIL-53(Fe)-derived materials at different carbonization temperatures. In Figure 3a,b, the precursor materials exhibit a uniform biconical composition of microporous three-dimensional framework structure with lengths of approximately 1000–1200 nm and widths of 800–1000 nm, displaying a typical bipyramidal hexagonal prism structure with sharp edges and a smooth surface. The initial carbonization at 400 °C (Figure 3c) yielded MIL-53(Fe)@C400, with nanorod structures accompanied by some morphologically incomplete particles. At the elevated temperatures, distinct structural changes were observed. When the calcination temperature reached 600 °C (Figure 3d), the MIL-53(Fe)@C600 appeared as fuzzy nanorod-like structures. As the temperature further increased to 800 °C, significant nanorods approximately 700–900 nm in length and 100–150 nm in width were obtained, as depicted in Figure 3e,f. This indicates that higher temperatures accelerate the formation of the MIL-53(Fe)@C nanorod structures while simultaneously reducing their characteristic dimensions.

The XRD patterns of the corresponding the MIL-53(Fe) derivatives carbonized at different temperatures under nitrogen conditions are presented in Figure 4. The evolution demonstrates significant temperature-dependent crystallographic transformations, with complete structural reorganization occurring at 800 °C to form homogeneous nanorod architectures. The X-ray diffractogram of MIL-53 (Fe) reveals four characteristic peaks at 9.2°, 9.7°, 18.5°, and 27.9°, corresponding to the (110), (200), (202), and (221) crystal planes, respectively (CCDC 734217) [24,30,31]. A characteristic peak at 35.5° at 400 °C was attributed to the crystalline Fe₃O₄ (JCPDS 19-0629) [30]. When the temperature was further increased above 600 °C, the characteristic peak at 44.7° began to appear as α-Fe⁰ (JCPDS 87-0722). At temperatures up to 800 °C, the characteristic peak at 43.9° appears to be Fe₃C (JCPDS 892867) [32], indicating the coexistence of Fe⁰ and graphitic carbon in the Fe-C composites.

3.2. Electrochemical Performance Analysis

The oxygen evolution reaction (OER) and oxygen reduction reaction (ORR) were conducted in aqueous Na₂SO₄ (0.1 mol/L) solution. The KCl-saturated Ag/AgCl and platinum electrodes were used as the counter and reference electrodes, respectively. The MIL-53(Fe)@C nanopowder was added to a 5 wt% Nafion ethanol solution, sonicated for 2 h, and then drop-coated onto a steel electrode (STE), which was subsequently dried to produce the working electrode. The MIL-53(Fe)@C-STE electrode was evaluated using linear sweep voltammetry (LSV), cyclic voltammetry (CV), and electrochemical impedance spectroscopy (EIS). The current response value of the MIL-53(Fe)@C-STE was as high as 5.70 mA/cm² compared to that of the bare STE (Figure 5a). This phenomenon suggests that the electron migration from the STE to the MIL-53(Fe)@C surface is the key electron transfer process throughout the reaction. Figure 5b shows the CV curves of the MIL-53(Fe)@C-STE over the 1.5 to 2.0 V range with a scan rate of 10 mV/s. A distinct oxidation peak appeared in the MIL-53(Fe)@C-STE system in the potential range of 1.50 to 1.75 V, corresponding to the oxidation of adsorbed H* [33]. The CV curves also indicate that the MIL-53(Fe)@C-STE exhibits a higher current density and stronger reduction capability than bare STE. In general, the arc radius of the electrochemical impedance spectrum (EIS) is related to the charge transfer impedance [34] for evaluating the electrical conductivity and redox capacity [35]. The semicircle diameter of the Nyquist curve of the MIL-53(Fe)@C-modified composite electrode was significantly smaller than that of the STE electrode (Figure 5c). The electrochemical impedance of the STE was approximately 20.49 Ω, which was significantly higher than the resistance of 11.96 Ω observed after loading the MIL-53(Fe)@C. This indicates that the MIL-53(Fe)@C reduced the electrochemical impedance and promoted electron transfer between the material surface and the contaminants.

3.3. Performance of TCAA Degradation Under Different Synthesis Conditions

The degradation of the TCAA using the MIL-53(Fe)@C-modified cathodes is illustrated in Figure 6a,b. The bare neodymium magnet exhibited a limited TCAA removal efficiency of 76.75% after 8 h, while the modified electrode with the MIL-53(Fe)@C/400 only slightly enhanced performance. However, the carbonization at 600 °C further improved the TCAA removal efficiency to 87.29%, and the carbonization at 800 °C improved it to 96.58%. This enhancement might be attributed to the formation of highly active α-Fe⁰ at elevated temperatures of 600 °C and 800 °C, which facilitated electron transfer. Moreover, at 800 °C, uniform carbon nanorods with a high specific surface area were formed, enriching the catalytically active sites on the electrode surface. Therefore, 800 °C was selected as the optimal carbonization temperature for subsequent experiments.

The dechlorination of TCAA by the electrolytic system employing steel (STE) and magnetic (MAG) electrodes was compared, as shown in Figure 6c,d. The TCAA removal efficiencies of the bare STE and MAG electrodes were only 75.77% and 76.69%, respectively. However, when the MIL-53(Fe)@C was directly dispersed into the STE-based reaction system (MIL-53(Fe)@C-STE), the removal efficiency increased significantly to 90.46%, owing to the enhanced electrocatalytic degradation driven by optimal current polarization and the generation of numerous charged microelectrodes with anode–cathode characteristics [36,37]. Further improvement was achieved by magnetically immobilizing the MIL-53(Fe)@C onto the MAG electrode to form the MIL-53(Fe)@C-MAG, which increased the TCAA removal efficiency to 96.03%. This superior performance might be attributed to the facilitation of electron acceptance by the MIL-53(Fe)@C loading on the electrodes. The comparison results indicated that the MIL-53(Fe)@C-MAG had the best configuration for catalytic electrolysis.

3.4. Effect of Influencing Factors on the Dechlorination of TCAA by MIL-53(Fe)@C-MAG

3.4.1. pH

Figure 7a,b shows the pH-dependent catalytic performance of the MIL-53(Fe)@C-MAG for TCAA removal. The system exhibited significantly reduced efficiency under alkaline conditions, with 86.44% and 79.10% TCAA removal at pH 9 and 11, within 8h, respectively. In contrast, at pH 3 and 5, almost complete efficiencies were obtained at 96.94% and 96.87%, respectively. Under alkaline conditions, Fe²⁺ and Fe³⁺ form flocs as hydroxide precipitates on the surface of the MIL-53(Fe)@C, leading to a loss of catalytic activity due to impaired iron complexation and redox cycling of Fe²⁺ and Fe³⁺ [38]. Acidic conditions facilitate the electronic cleavage of H₂O molecules to form H* atoms [25], which promotes the indirect electrolytic dechlorination of TCAA.

3.4.2. Current Density

Figure 7c,d illustrates the effect of current density on the TCAA removal efficiency. As the current density increased from 10 to 50 mA/cm², the TCAA removal efficiency increased from 52.57% to 96.03%. Notably, the enhancement was particularly significant in the range of 10–30 mA/cm² to 88.04%, with further improvement to 96.03% at 50 mA/cm². This trend can be explained by the dual role of the current density. The current density affects the polarization of the MIL-53(Fe)@C, as well as Fe²⁺ and Fe³⁺ cycling [39]. At lower current densities, insufficient polarization of the MIL-53(Fe)@C-MAG electrode resulted in limited electrocatalytic efficacy. In contrast, a higher current density substantially accelerates electron transfer, enhances the MIL-53(Fe)@C polarization, and expands the accessible reaction sites, thereby significantly facilitating multiphase catalytic action and redox processes [38]. However, when the current density exceeded 50 mA/cm², the TCAA removal remained almost constant during the initial 1 h. This was due to the excessive formation of hydrogen bubbles at the electrode surface, which hindered TCAA diffusion to the catalyst [40].

3.4.3. Initial TCAA Concentrations

Figure 8a,b shows the concentration-dependent performance of the TCAA removal system. As the initial TCAA concentration increased from 150, 200, 250 to 300 µg/L, the removal efficiencies decreased from 97.38%, 96.03%, 87.65% to 84.60%, accompanied by a corresponding decrease in pseudo-first order rate constants from 0.51915, 0.46262, 0.35955 to 0.31975 h⁻¹. This inverse relationship between the removal efficiency and TCAA initial concentration can be explained by the competitive adsorption dynamics at the electrode surface. The higher TCAA concentrations and their intermediate degradation products increasingly occupied the available active sites on the electrocatalytic electrode surface, thereby reducing the effective catalytic area for subsequent degradation [41]. The observed kinetic behavior further confirms that the degradation process is surface-dependent, where the accessibility of active sites becomes the limiting factor at elevated TCAA concentrations.

3.4.4. MIL-53(Fe)@C Loading

The TCAA removal efficiency gradually increased as the MIL-53(Fe)@C loading increased from 2 to 8 mg/cm². As shown in Figure 8c,d, the TCAA removal curves for 6–8 mg/cm² nearly overlapped, indicating a current density limitation at higher catalyst amounts. The increase in the MIL-53(Fe)@C loading enhanced the effective electrode surface area and active site density, which promoted electron transfer [42]. However, excessive loading and redundant active centers significantly exceed the requirements for H* generation, potentially inducing side reactions and short-circuit currents that reduce current efficiency [38,43]. The data indicate that the system reached optimal catalyst utilization under the given electrochemical conditions.

3.5. Reaction Mechanism

To verify the formation of atomic H* during the electrical reduction of TCAA, radical scavenger experiments were performed using tert-butanol (TBA) as a selective H* scavenger [44]. Figure 9a shows the concentration-dependent inhibitory effect of TBA on TCAA degradation. Even at low concentrations of 0.2–1.0 mg/L, TBA reduced TCAA efficiency by nearly 12.00% after 8 h of electrolysis. The inhibition effect increased with increasing TBA concentration, culminating in a significant reduction in TCAA removal to 55.70% at 20 mg/L. The reaction rate, constant with the addition of 20 mg/L TBA, was 0.09856 h⁻¹ (Figure 9b), revealing a reduction of 78.16% compared with the TBA-free system. The results confirmed the existence of H* and its crucial role in the MIL-53(Fe)@C-MAG electrocatalytic reduction system.

In our previous studies, electrolytic dechlorination of TCAA was verified by both direct and indirect electrochemical reduction [25,26]. The MIL-53(Fe)@C material plays an important catalytic role in facilitating electron transfer and atomic H* generation.

(1) Electrochemical direct reduction

The electrochemical direct dechlorination of TCAA proceeded through a combined mechanism involving direct chlorine atom cleavage and stepwise dechlorination [45,46], as described by the following reaction pathways (Equations (1)–(4)):

$${Cl}_{3}CCOOH+e^{-}\to {{Cl}_{2}CCOOH}^{·}+{Cl}^{-}$$

(1)

$${{Cl}_{2}CCOOH}^{·}+H^{+}+e^{-}\to {Cl}_{2}CHCOOH$$

(2)

$${Cl}_{2}CHCOOH+e^{-}\to {ClCHCOOH}^{·}+{Cl}^{-}$$

(3)

$${ClCHCOOH}^{·}+H^{+}+e^{-}\to ClC{H}_{2}COOH$$

(4)

(2) Electrochemical indirect reduction

The electrochemical indirect reduction of TCAA is primarily mediated by atomic H* [47,48]. The water adsorbed on the electrode surface was converted into H* through the Volmer reaction, which subsequently acted as a halide hydro-dehalogenation intermediate, leading to the degradation pathway described in Equations (5) and (6):

$$H_{2}O+e^{-}+M\to M+H^{*}+{OH}^{-} \left(Volmer\right)$$

(5)

$$RCl+H^{*}\to RH+{Cl}^{-}$$

(6)

where M is the metal catalyst, $RCl$ is TCAA, and $RH$ is AA.

3.6. Stability of MIL-53(Fe)@C-MAG Electrodes

The cycling employment of the MIL-53(Fe)@C-MAG electrode was performed to investigate the operational stability of the coating, and the results are shown in Figure 10a. The electrode maintained excellent catalytic performance after prolonged operation, achieving above 90.05% TCAA dechlorination efficiency during 40 h of continuous electrolysis. Although a gradual decline of approximately 6.00% was observed over multiple cycles, the electrocatalytic performance of the MIL-53(Fe)@C-MAG exhibited satisfactory stability. This durability can be attributed to the strong magnetic immobilization of the MIL-53(Fe)@C on the electrode substrate, which effectively prevents nanomaterial detachment while maintaining sufficient catalyst–electrode contact for efficient electrotransfer throughout the extended operation period.

The chemical stability of the electrode was evaluated by monitoring the Fe leaching. During the 8h electrolysis, a gradual release of Fe ions was observed, maintaining a consistently low concentration range of 1.72–4.81 μg/L (Figure 10b). This minimal leaching behavior suggests good structural stability of the electrode material. However, the observed Fe leaching may have partly contributed to the gradual decline in the electrode catalytic performance.

3.7. Dechlorination of Other DBPs by MIL-53(Fe)@C-MAG Electrodes

To evaluate the practical applicability of the MIL-53(Fe)@C-MAG electrode, its performance was assessed against the five representative DBPs including the trichloroacetaldehyde (TCA), trichloroacetone (TCP), trichloroacetic acid (TCAA), trichloroacetonitrile (TCAN), and trichloromethane (TCM) (Figure 10c). The MIL-53(Fe)@C-MAG electrode exhibited a superior removal efficiency of over 93.72% for the trichloroacetaldehyde (TCA) and trichloroacetone (TCP) within 2 h. The removal efficiencies of the trichloroacetic acid (TCAA), trichloroacetonitrile (TCAN), and trichloromethane (TCM) exceeded 87.16% within 6 h of electrolysis. The superior performance of the TCA and TCP can be explained by their carbonyl groups, which serve as electron transfer mediators [49], thereby accelerating the reduction reaction. However, the degradation of the TCM and TCAN was comparatively slower because of the strong electron-withdrawing effect of their chlorine atom and cyano group, respectively, which significantly enhanced the dissociation energy of the C-Cl bond [50]. Similarly, the carboxyl group of the TCAA inhibited its diffusion to the electrode surface [51]. It is now generally accepted that the reductive dehydrogenation of chlorides by metallic Fe is accomplished mainly through direct electron acquisition by chlorinated organics on the metal surface [52].

4. Conclusions

A novel magnetic MIL-53(Fe)@C-MAG electrode was synthesized. The MIL-53(Fe)@C catalyst was firmly coated on the surface of Nd through magnetic immobilization. Characterized by the α-Fe⁰ reactive species in the structure, a substantial specific surface area, and numerous active sites, the MIL-53(Fe)@C showed a significant catalytic effect on the dechlorination of the electrolysis system. The optimized MIL-53 (Fe)@C catalyst, obtained through pyrolysis at 800 °C, exhibited the highest TCAA removal of 96.58%. Quenching studies indicated that dual dechlorination mechanisms occurred, involving both direct dechlorination by electron transfer and indirect dechlorination mediated by H*. Consequently, the MIL-53(Fe)@C800 catalyst demonstrated excellent catalytic activity and long-term stability at pH 2–7, a current density of 30–50 mA/cm², and the MIL-53(Fe)@C loading of 4–8 mg/cm². In conclusion, the MIL-53(Fe)@C-MAG electrode is a promising technology for efficient DBPs control in drinking water treatment applications.