Preparation of Fe(3)O(4)@ACF Composite Catalytic Electrode and Study of Its Degradation of Antibiotics.

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Learn more: PMC Disclaimer | PMC Copyright Notice Nanomaterials (Basel) . 2026 Mar 31;16(7):431. doi: 10.3390/nano16070431 Search in PMC Search in PubMed View in NLM Catalog Add to search Preparation of Fe 3 O 4 @ACF Composite Catalytic Electrode and Study of Its Degradation of Antibiotics Xuan Liu Xuan Liu 1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing) at Karamay, Karamay 834000, China Conceptualization, Writing – original draft, Data curation, Methodology, Investigation Find articles by Xuan Liu 1 , Yanqiu Pang Yanqiu Pang 1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing) at Karamay, Karamay 834000, China Writing – original draft Find articles by Yanqiu Pang 1 , Hanyue Zhang Hanyue Zhang 1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing) at Karamay, Karamay 834000, China Writing – review & editing Find articles by Hanyue Zhang 1 , Yani Liu Yani Liu 1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing) at Karamay, Karamay 834000, China Writing – review & editing Find articles by Yani Liu 1 , Haiyi Yang Haiyi Yang 1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing) at Karamay, Karamay 834000, China Visualization Find articles by Haiyi Yang 1 , Junwei Hou Junwei Hou 1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing) at Karamay, Karamay 834000, China Funding acquisition, Project administration Find articles by Junwei Hou 1, * Editor: Antonio Guerrero-Ruiz 1 Author information Article notes Copyright and License information 1 State Key Laboratory of Heavy Oil Processing, China University of Petroleum (Beijing) at Karamay, Karamay 834000, China * Correspondence: [email protected] Roles Xuan Liu : Conceptualization, Writing – original draft, Data curation, Methodology, Investigation Yanqiu Pang : Writing – original draft Hanyue Zhang : Writing – review & editing Yani Liu : Writing – review & editing Haiyi Yang : Visualization Junwei Hou : Funding acquisition, Project administration Antonio Guerrero-Ruiz : Academic Editor Received 2026 Mar 11; Revised 2026 Mar 30; Accepted 2026 Mar 30; Collection date 2026 Apr. © 2026 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license . PMC Copyright notice PMCID: PMC13074807  PMID: 41972646 Abstract Antibiotics are extensively used in intensive livestock farming for disease prevention, resulting in the discharge of antibiotic-contaminated wastewater into aquatic environments. Addressing this issue, electrocatalytic oxidation has emerged as a promising alternative to conventional chemical oxidation due to its cost-effectiveness and minimal secondary pollution. Central to this technology is the development of catalytic electrodes with high specific surface area and superior electrocatalytic activity. In this work, an Fe 3 O 4 -modified activated carbon fiber electrode (Fe 3 O 4 @ACF) was fabricated via a co-precipitation method. The Fe 3 O 4 @ACF electrode exhibited a hierarchical porous structure with a specific surface area of 940.2 m 2 /g, and demonstrated significantly enhanced oxygen reduction reaction activity with a current density of 21.8 mA·cm −2 at –3.25 V vs. Ag/AgCl, which is 2.3 times higher than that of pristine ACF. EIS analysis revealed a low charge transfer resistance of 7.18 Ω, indicating improved electron transfer kinetics. In electro-Fenton degradation of tetracycline, the electrode achieved 82% removal within 120 min with a first-order rate constant of 0.01335 min −1 , and maintained over 94% of its initial activity after ten cycles. This study offers a viable and sustainable strategy for the efficient treatment of antibiotic-containing medical wastewater. Keywords: modified activated carbon fiber, heterogeneous electro-Fenton, iron-functionalized cathode 1. Introduction Antibiotics inhibit the growth and survival of microorganisms and are therefore widely used in healthcare, animal husbandry, and aquaculture [ 1 , 2 ]. Their increasing consumption in recent years has raised concerns about antibiotic residues in aquatic environments and the associated risks to ecosystem sustainability [ 3 ]. Tetracycline (TC), a widely used broad-spectrum antibiotic, is often regarded as an emerging contaminant due to its frequent detection and persistence in water bodies [ 4 ]. After entering aquatic and soil environments, TC can exert toxic effects on aquatic organisms and microbial communities, disrupt ecological balance, and pose potential risks to human health [ 5 ]. Moreover, continuous exposure to TC may promote the emergence and dissemination of antibiotic-resistant bacteria and resistance genes, further aggravating antibiotic resistance and threatening public health [ 6 ]. Therefore, developing efficient and sustainable water-treatment technologies to mitigate antibiotic contamination is urgently needed. Current treatments for emerging organic pollutants include adsorption [ 7 ], membrane separation [ 8 ], advanced oxidation [ 9 ], and biodegradation [ 10 ]. Among these, adsorption and advanced oxidation are extensively applied. Adsorption is attractive because of its low cost, simple operation, and rapid pollutant removal [ 11 , 12 , 13 ]. However, its practical application is limited by poor adsorbent regenerability, high regeneration energy consumption, finite adsorption capacity, and the inherently discontinuous nature of batch replacement/regeneration processes [ 14 , 15 ]. In contrast, advanced oxidation processes (AOPs) are a promising class of technologies capable of mineralizing non-biodegradable organic compounds or converting them into more biodegradable by-products [ 16 , 17 ]. While conventional advanced oxidation processes (AOPs) rely on the external addition of chemical oxidants such as ozone or hydrogen peroxide, electrochemical advanced oxidation processes (EAOPs) generate reactive species in situ via electrode reactions. This enables better process control, reduces chemical transport and storage risks, and enhances environmental compatibility [ 18 ]. Electrochemical advanced oxidation processes (EAOPs) have gained increasing attention due to their stable performance, facile controllability, low chemical demand, and environmental compatibility [ 19 , 20 ]. Among EAOPs, electro-Fenton (EF) technology produces H 2 O 2 in situ at the cathode via electrochemical reduction of dissolved O 2 , while simultaneously enabling continuous regeneration of the Fe 2+ /Fe 3+ redox catalyst to sustain Fenton reactions [ 21 ]. The resulting hydroxyl radicals (∙OH) are highly oxidative and can rapidly and non-selectively degrade refractory organic pollutants into intermediates, which can be further mineralized to CO 2 and H 2 O [ 22 , 23 ]. In situ H 2 O 2 generation eliminates the costs and safety concerns associated with transportation and storage [ 24 ]. In addition, cyclic Fe 2+ regeneration can reduce iron-sludge formation and enhance pollutant degradation efficiency [ 25 , 26 ]. For instance, studies have shown that heterogeneous electro-Fenton systems can reduce iron sludge production by over 90% compared to homogeneous Fenton processes, while maintaining degradation efficiencies above 95% [ 27 ]. Owing to its robustness against matrix interference, operational simplicity, and high degradation/mineralization capability [ 28 ], EF has been widely investigated for the treatment of various refractory organic wastewaters [ 29 , 30 , 31 ]. To further improve EF performance, extensive efforts have been devoted to developing heterogeneous EF cathodes/catalysts with enhanced electron transfer and H 2 O 2 utilization. As shown in the Table 1 , Cheng et al. [ 32 ] fabricated a Fe–Ni LDH@ZIF-67 catalyst-modified carbon-cloth (CC) cathode, achieving 95.6% TC removal within 60 min, compared with 75.7% for Fe–Ni LDH/CC. Under the synergistic action of heterogeneous EF and anodic oxidation, the maximum H 2 O 2 concentration reached 264 mg∙L −1 , indicating improved H 2 O 2 utilization. Cao et al. [ 33 ] prepared a Fe 3 O 4 /FeO/Fe 3 C-2-600 catalytic electrode by in situ growth of MIL-88B(Fe) on stainless-steel mesh followed by calcination; the resulting dual-cathode EF system achieved 97.7% TC degradation within 12 min. Cui et al. [ 34 ] synthesized Cu/CuFe 2 O 4 (CCFO) with different CuO ratios using a one-step solvothermal method and obtained 96.3% TC degradation at the optimal composition. Zhang et al. [ 35 ] reported CO 2 -activated CuFeC and CuMnC aerogel cathodes that enabled effective treatment of real printing and dyeing wastewater without external addition of Fe 2+ or H 2 O 2 , with removal efficiencies of 90.5% and 80.3%, respectively, within 60 min. In another study, Fe–Cu/kaolin electrodes were developed for rhodamine B (RhB) degradation in a three-dimensional EF (3D/EF) system, showing RhB removals of 91.6% and 81.1% at initial pH values of 6.71 and 9.0, respectively [ 36 ]. Luo et al. [ 37 ] investigated TC degradation using Cu-doped Fe@Fe 2 O 3 core–shell nanoparticles as catalysts and nickel foam as the cathode, achieving 98.1% TC degradation within 2 h and 89.8% mineralization after 6 h. Gao et al. [ 38 ] constructed a 3D/EF system using CuFe 2 O 4 as a heterogeneous catalyst and activated carbon as a granular electrode for semicoke wastewater treatment, achieving 80.9% COD removal under optimal conditions. The synthesis method and resulting morphology of electro-Fenton cathodes play critical roles in determining their catalytic performance. Different synthesis approaches yield distinct morphological features that directly impact key aspects of the electro-Fenton process. Active Site Exposure: Synthesis methods that promote uniform dispersion of catalytically active components maximize the exposure of active sites. Electron Transfer Efficiency: Morphologies that ensure intimate contact between the catalyst and conductive substrate facilitate efficient electron transfer. Mass Transport: Hierarchical pore structures facilitate rapid diffusion of reactants (O 2 , H 2 O 2 ) and pollutants to active sites. H 2 O 2 Generation and Utilization: Morphologies that provide abundant three-phase interfaces (solid catalyst–liquid electrolyte–dissolved O 2 ) promote efficient electrochemical reduction of oxygen to H 2 O 2 . Table 1. Previously reported iron-based cathode materials for Electro-Fenton degradation of organic pollutants in wastewater. Electrode Material Organic Dye Synthesis Method Morphology Concentration (mg/L) Removal Efficiency (Time) Current Density/ Voltage Ref. Fe-Ni LDH@ZIF-67 Tetracycline (TC) In situ growth Nanosheet array 10 95.6% (60 min) 100 mA∙cm −2 [ 32 ] Fe 3 O 4 /Fe0/Fe 3 C-2-600 Tetracycline (TC) MOF-derived calcination Nanoparticles 10 97.7% (12 min) 3.5 mA∙cm −2 [ 33 ] 3,6-Cu/CuFe 2 O 4 /CB@GF Tetracycline (TC) Solvothermal Nanoclusters 50 96.3 ± 1.8% (3120 min) 30 mA∙cm −2 [ 34 ] CuFeC 2,4,6-trichlorophenol (2,4,6-TCP) Pyrolysis Porous 40 90.5% (60 min) 7 mA∙cm −2 [ 35 ] Fe-Cu/kaolin RhodamineB (Rh B) Impregnation-calcination Particle electrodes 20 97.2% (60 min) 10 V [ 36 ] Cu-doped Fe@Fe 2 O 3 (CFF) Tetracycline (TC) Solvothermal Core–shell nanoparticles 20 98.1% (3120 min) 90.0% (5120 min) 72.2% (7120 min) 40 mA∙cm −2 [ 37 ] CuFe 2 O 4 200 mL semi-coking water Solvothermal Nanoparticles \ COD:80.9%, 120 min 4 V [ 38 ] FeCuC Methylene (MB) Sol–gel Porous 50 98% (30 min) 99% (60 min) 10 mA (4.1 V) [ 39 ] FeCuC Real dyeing waste water Sol–gel Porous \ TOC:64% (30 min) 83% (60 min) 10 mA (4.1 V) [ 39 ] Open in a new tab Carbon-based materials can act as a good electron acceptor and transfer channel to improve the conductivity and optimize the adsorption of H- or O-relevant intermediates of g-C 3 N 4 -based catalysts [ 40 , 41 , 42 ]. Emerging carbon architectures offer additional advantages. For instance, graphdiyne, with its unique sp-/sp 2 -hybridized carbon network and intrinsic bandgap, can modulate interfacial charge transfer more effectively than conventional carbons. Carbon nanocages provide continuous conductive pathways and geometric confinement effects that facilitate both electron transport and mass diffusion, which are critical for gas-involving reactions such as the oxygen evolution reaction (OER) and hydrogen evolution reaction (HER). Ordered mesoporous carbon frameworks, with their well-defined periodic porosity, enable uniform dispersion of g-C 3 N 4 and maximize accessible active sites while ensuring efficient charge transport [ 43 ]. The Fenton reaction is a contact reaction, and the poor electrical conductivity and small specific surface area of FeO or Fe 2 O 3 , which are used as Fenton materials, lead to low Fenton efficiency. Therefore, in this paper, high-performance electro-Fenton catalytic electrodes were prepared by loading Fe 3 O 4 nanoparticles onto the surface of activated carbon fibers (Fe 3 O 4 @ACF) by chemical precipitation method using carbon nanofibers as substrate. 2. Experimental Materials and Methods 2.1. Experimental Materials and Reagents All chemical reagents were analytically pure. Deionized (DI) water was used throughout the experiment. All the reagents used during the experiments, such as hydrochloric acid (HCl, 1 M), activated carbon fiber (ACF, 99.0%), iron sulfate heptahydrate (FeSO 4 ·7H 2 O, 99.0%), potassium nitrate (KNO 3 , 99.8%), sodium sulfate (Na 2 SO 4 , 99.0%), sodium hydroxide (NaOH, 98%), and other major analytical reagents, were purchased from the Shanghai Titan Scientific Co., Ltd. (Shanghai, China) 2.2. Subsubsection Pre-Processing of ACF ACF (2 cm × 2 cm, 99.8% purity) was placed in a beaker and ultrasonically cleaned 2–3 times with deionized water to remove residual impurities. Subsequently, a certain amount of 0.1 M HCl solution was added and soaked for 6–8 h to enhance its ability to adsorb ferric ions from solution [ 44 ], and finally, filtration was carried out for deionized water washing to remove residual acid and dried at 80 °C to obtain unmodified ACF. 2.3. Preparation of ACF-Loaded Fe 3 O 4 Catalyst Electrode 2.78 g of iron sulfate heptahydrate (FeSO 4 ·7H 2 O) and 1.01 g of potassium nitrate (KNO 3 ) were dissolved in 60 mL of distilled water, and the reaction was carried out for 20 min with a magnetic stirrer. Subsequently, after vigorous stirring at 60 °C for 20 min, activated carbon fibers were added to the solution, while 6mL of ammonium hydroxide (NH 3 ·H 2 O, 28%) was poured into the stirred solution, and the temperature was maintained at 60 °C (1 h). After the reaction, it was taken out and left to stand for 24 h, then washed with deionized water and dried at 80 °C in a preheated vacuum drying oven to obtain modified activated carbon fiber (Fe 3 O 4 @ACF). The pretreated pure ACF and optimized Fe 3 O 4 @ACF prepared by the co-precipitation method were stored in a sealed bag for subsequent experiments. 2.4. Analytical Instrument The surface morphology and elemental composition of the samples were characterized using a Hitachi SU4800 scanning electron microscope (SEM) (manufacturer: Hitachi High-Tech Corporation, Minato-ku, Tokyo, Japan) equipped with an energy dispersive X-ray spectrometer (HORIBA, 7593-H) (manufacturer: HORIBA, Ltd., Kyoto, Japan). Energy-dispersive X-ray spectroscopy (EDS) was used in conjunction with SEM to obtain characteristic spectra for qualitative and quantitative elemental analysis. The structure and crystal phase composition of the samples were investigated using an Ultimo IV X-ray diffractometer (CuKα radiation) (manufacturer: Thermo Fisher Scientific, Madison, Wisconsin, United States of America). The samples were analyzed by X-ray photoelectron spectroscopy (XPS) for surface composition and chemical valence states using a Kratos AXIS SUPRA XPS (manufacturer: Kratos Analytical Ltd., Manchester, United Kingdom). The samples were analyzed using a Micromeritics ASAP 2460 fully automated specific surface area and porosity analyzer (N 2 desorption) (manufacturer: Micromeritics Instrument Corporation, Norcross, Georgia, United States of America). The elemental distribution on the sample surface was characterized by transmission electron microscopy (TEM) mapping analysis using a JEOL JEM 2100F system (200 kV voltage) (manufacturer: JEOL Ltd., Akishima, Tokyo, Japan). 2.5. Electrochemical Performance Study Electrochemical tests, including linear scanning voltammetry (LSV) and electrochemical impedance spectroscopy (EIS) were carried out using Shanghai Chenhua CHI660E electrochemical workstation. These tests were carried out in 0.1 M Na 2 SO 4 aqueous solution using a three-electrode system, in which the cathode material prepared in this study was used as the working electrode, a platinum sheet was used as the counter electrode, and an Ag/AgCl electrode was used as the reference electrode. Linear Scanning Voltammetry (LSV): LSV has a test voltage range of –4.0 V to 2.0 V and a scan rate of 50 mV/S. Electrochemical Impedance Spectroscopy (EIS): EIS measurements are performed with an amplitude of 5 mV and a frequency range of 1 × 10 6 to 1 × 10 −2 Hz. 2.6. Study of Oxidative Degradation Properties of Electrofenton The experiments were carried out in a 250 mL vessel. A platinum sheet electrode was used as the anode for the reaction, while the Fe 3 O 4 @ACF material was used as the functionalized cathode (both measuring 2 cm × 2 cm). Subsequently, 100 mL of 20 mg/L TC solution was added to the vessel with 0.05 M Na 2 SO 4 as the supporting electrolyte. Throughout the Electro-Fenton reaction, the solution was stirred and homogenized using a magnetic stirrer, and the current was supplied by an adjustable constant voltage DC power supply. Then air was passed into the solution at a flow rate of 200 mL/min to establish a non-homogeneous Electro-Fenton oxidation system. Ten minutes before the start of the experiment, a mechanical stirrer was activated for rapid mixing and air was introduced to form an air-saturated Electro-Fenton system. The constant voltage DC power supply was then started by connecting the positive and negative terminals to the poles of the power supply. The tetracycline solution was scanned at full wavelength with a UV-visible spectrophotometer to determine its maximum absorption wavelength. Subsequently, the absorbance at the maximum wavelength was detected and monitored using a UV-visible spectrophotometer. The UV-visible degradation efficiency was calculated as: Removal rate (%) = (C 0 − C t )/C 0 × 100% (1) where C 0 is the initial concentration of tetracycline (mg/L); C t is the concentration of tetracycline at t min (mg/L); K is the quasi-primary rate constant for tetracycline degradation; and T is the reaction time (min). 2.7. Mechanistic Analysis of the Electro-Fenton System First, 3 mM TBHP, IPA and MeOH were used as the bursting agents, which were added to the Electro-Fenton oxidation system, and the concentration of tetracycline was monitored by UV-Vis spectrophotometer. Subsequently, free radicals that play an important role in tetracycline degradation were investigated using electron paramagnetic resonance spectroscopy (EPR) on a Bruker EMX PLUS instrument in Germany in the range of 3460–3560 G. The results of this study are summarized in the following table. 3. Results and Discussion 3.1. Microstructure and Composition Figure 1 shows SEM images and EDS energy spectra of the prepared Fe 3 O 4 @ACF, as well as quantitative data on the distribution of each element. In Figure 1 a–c, the samples show a large number of dense ACF filaments with a diameter of 11 μm. The ACF exhibits a unique, interlaced rod-like fibrous structure with many grooves on the surface. This unique morphology contributes to the large specific surface area and abundant active sites of the material. Figure 1 d–f shows increased surface roughness after Fe 3 O 4 loading. In addition, the aggregation of smaller Fe 3 O 4 @ACF particles to form larger-sized clusters of particles stacked in a disordered manner, as well as a layer of Fe 3 O 4 flocs loading, resulted in the roughness and inhomogeneity of the microstructure of the sample surface. The EDS spectrum ( Figure 1 h) shows uniform distribution of Fe, O, C, and K elements in the composite, with weight percentages of 69.40% (C), 27.46% (O), 3.13% (Fe), and 0.02% (K). From the above results, it is shown that Fe 3 O 4 particles have been successfully loaded onto the ACF surface. Figure 1. Open in a new tab SEM images of Fe 3 O 4 @ACF at different magnifications ( a – f ) and Particle size distribution histogram of Fe 3 O 4 @ACF ( g ); EDS of Fe 3 O 4 @ACF ( h ). The microscopic morphology of Fe 3 O 4 @ACF was observed by transmission electron microscopy, as shown in Figure 2 a,b. The composite particles are close to spherical and loaded on the ACF in a cluster shape, and it can be observed that a clear boundary appears between the Fe 3 O 4 crystals and the ACF material, indicating that Fe 3 O 4 is tightly adhered to the ACF surface. The high-resolution transmission electron microscopy (HRTEM) image ( Figure 2 c) shows that the lattice spacing of the as-prepared Fe 3 O 4 lattice is 0.153 nm and corresponds to the iron tetraoxide (511) crystal plane. In addition, the EDS elemental mapping in Figure 2 d confirms the presence of C, O, Fe and K elements in Fe 3 O 4 @ACF, where C, O and Fe are very uniformly distributed and K is sporadically dispersed. It further indicates that the Fe 3 O 4 @ACF non-homogeneous phase-like Fenton catalyst has been successfully constructed. Figure 2. Open in a new tab TEM ( a , b ) and HRTEM ( c ) images of Fe 3 O 4 @ACF, and elemental mapping image of Fe 3 O 4 @ACF ( d ). The XRD pattern of the Fe 3 O 4 @ACF composite ( Figure 3 ) exhibits diffraction peaks that can be indexed to the spinel structure of magnetite (Fe 3 O 4 , JCPDS No. 26-1136). Slight shifts to lower 2θ values are attributed to nanoscale particle size (~100 nm) and partial surface oxidation to maghemite (γ-Fe 2 O 3 ), as confirmed by XPS ( Figure 4 d). No detectable impurity phases such as α-Fe 2 O 3 or FeO are observed. The coexistence of Fe 3 O 4 and surface γ-Fe 2 O 3 is consistent with the literature on air-exposed magnetite nanoparticles and does not compromise the catalytic activity, as both phases are active in Fenton-like reactions. This result provides direct evidence for the successful crystallization of Fe 3 O 4 on the ACF substrate. Concurrently, the absence of any sharp diffraction features attributable to activated carbon indicates that the ACF matrix retains its amorphous structure within the composite. The HRTEM image in Figure 2 c reveals clear lattice fringes with an interplanar spacing of 0.153 nm, which corresponds to the (511) plane of Fe 3 O 4 . This assignment is consistent with the distinct XRD peak observed at 59.303° (2θ) indexed to the (511) reflection (JCPDS No. 26-1136). Furthermore, the XRD pattern in Figure 3 exhibits additional peaks corresponding to the (220), (311), (400), and (440) planes, confirming the phase purity of the Fe 3 O 4 nanoparticles. Figure 3. Open in a new tab X-ray powder diffraction pattern of Fe 3 O 4 @ACF. Figure 4. Open in a new tab Total XPS spectrum of Fe 3 O 4 @ACF ( a ); C 1s peak ( b ); O 1s peak ( c ); Fe 2p peak ( d ); Nitrogen adsorption–desorption isotherm of Fe 3 O 4 @ACF ( e ) and corresponding pore size distribution curve ( f ). The chemical states and surface composition of the Fe 3 O 4 @ACF composite were further elucidated by XPS. As presented in Figure 4 a,d, the high-resolution C 1s spectrum ( Figure 4 b) was deconvoluted into three peaks at 284.1 eV (C=C/C–C), 285.0 eV (C–O), and 290.9 eV (O=C–O). The presence of C–O species is corroborated by the O 1s spectrum ( Figure 4 c), which exhibits three components at 531.61 eV (C–O), 529.70 eV (metal oxides), and 532.83 eV (metal hydroxides). The metal hydroxide is Fe(OH) 3 and it is possible that some Fe(OH) 3 did not undergo dehydration to form Fe 3 O 4 due to incomplete drying at the sampling location. The metal oxides signal originates from a mixed phase of Fe 3 O 4 and γ-Fe 2 O 3 . This is confirmed by the Fe 2p spectrum ( Figure 4 d), where the Fe 2p 3 / 2 and Fe 2p 1 / 2 core-level peaks are centered at 711.2 eV and 724.9 eV, respectively, matching the standard values for Fe 3 O 4 [ 45 ]. The detailed fitting reveals contributions from both Fe 2+ (peaks at ~710.8 eV and ~724.3 eV) and Fe 3+ (peaks at ~712.9 eV and ~726.27 eV) oxidation states. Moreover, the distinct satellite peak near 718.7 eV is characteristic of Fe 3+ in γ-Fe 2 O 3 [ 46 ], indicating partial surface oxidation of the Fe 3 O 4 nanoparticles. Collectively, the XRD and XPS analyses provide consistent and conclusive evidence for the successful loading of Fe 3 O 4 onto the ACF surface. The specific surface area and pore architecture of the samples were characterized through N 2 adsorption–desorption measurements, with the corresponding isotherms and pore size distribution profiles presented in Figure 4 e–f [ 47 ]. The obtained isotherms exhibit a pronounced hysteresis loop within the relative pressure (P/P 0 ) range of 0.4 to 1.0. This feature is diagnostic of mesoporous materials, arising from the capillary condensation and evaporation of nitrogen within pores of specific geometry. According to the IUPAC classification, the observed pattern aligns with a Type II isotherm displaying an H3-type hysteresis loop. This classification is characteristic of aggregated plate-like particles, which generate slit-shaped or wedge-shaped pores. The complexity of the pore network, particularly the presence of such non-uniform slit pores, accounts for the noted divergence between the adsorption and desorption branches and the hysteresis loop morphology. Quantitative analysis of the desorption branch via the BJH method revealed a bimodal pore size distribution, confirming the coexistence of micropores (centered around 1.7–2.0 nm) and mesopores (in the range of 2.0–5.0 nm), indicative of a hierarchical pore structure within the Fe 3 O 4 @ACF composite. The morphological characteristics of the Fe 3 O 4 @ACF composite are intrinsically linked to its electrochemical degradation performance. The interlaced fibrous ACF network provides a high surface area that enables uniform dispersion of Fe 3 O 4 nanoparticles (~100 nm), preventing aggregation and maximizing exposure of active Fe sites. The tight adhesion between Fe 3 O 4 and the conductive ACF substrate facilitates efficient electron transfer, as evidenced by the reduced charge transfer resistance (7.18 Ω) and enhanced ORR activity. Furthermore, the hierarchical pore structure (micropores of 1.7–2.0 nm and mesopores of 2.0–5.0 nm) synergistically promotes pollutant adsorption and rapid mass transport of reactants, ensuring efficient utilization of electrogenerated reactive oxygen species. 3.2. Electrochemical Performance Analysis The electrocatalytic performance of the materials was first assessed through linear sweep voltammetry (LSV). As depicted in Figure 5 a, the Fe 3 O 4 @ACF composite exhibits significantly enhanced oxygen reduction reaction (ORR) activity compared to pristine ACF. At −3.25 V vs. Ag/AgCl, the current density reaches 21.8 mA·cm −2 for Fe 3 O 4 @ACF, which is 2.3 times higher than that of pure ACF (9.4 mA·cm −2 ). This improvement indicates that Fe 3 O 4 nanoparticles effectively promote the electrocatalytic process. Figure 5. Open in a new tab Linear sweep voltammetry curves ( a ) and electrochemical impedance spectra ( b ) of pure ACF and Fe 3 O 4 @ACF. To elucidate the origin of this enhanced activity, electrochemical impedance spectroscopy (EIS) was employed to probe the charge transfer kinetics [ 48 ] at the electrode-electrolyte interface ( Figure 5 b). Nyquist plots show distinct semicircles in the high-frequency region, corresponding to charge transfer resistance (Rct). Quantitative fitting shows that Fe 3 O 4 @ACF possesses a substantially lower Rct value (7.18 Ω) compared to pure ACF, indicating significantly facilitated electron transfer. In the low-frequency region, the linear Warburg component reflects mass transport limitations, with Fe 3 O 4 @ACF exhibiting a shallower slope, suggesting improved diffusion kinetics. The EIS fitting results demonstrate that the charge transfer resistance (Rct = 13.02 Ω) of Fe 3 O 4 @ACF is significantly lower than that of pure ACF (Rct = 27.35 Ω), indicating a marked reduction in electronic transfer impedance ( Table S1 ). This facilitates electron transfer during reactions and lowers electrochemical resistance, thereby enhancing power density and electron flux between acceptors and donors. These findings confirm that Fe 3 O 4 loading effectively improves the material’s electron transport efficiency. The similarity in CPE1 parameters suggests negligible differences in interfacial capacitance characteristics, indicating that the loading process did not compromise the material’s interfacial structural stability. The synergistic combination of LSV and EIS analyses provides compelling evidence for the superior electrocatalytic performance of Fe 3 O 4 @ACF. The substantially increased ORR current density directly demonstrates enhanced catalytic activity, while the reduced charge transfer resistance reveals more efficient interfacial electron transfer processes. These complementary findings collectively confirm that Fe 3 O 4 modification effectively optimizes both the catalytic sites and charge transport pathways in the ACF-based electrode. 3.3. Catalytic Degradation Properties of Tetracycline by Fe 3 O 4 @ACF To evaluate the versatility of the composite electrode, degradation experiments were conducted on tetracycline (TC) solutions. The effects of several parameters on degradation efficiency were systematically investigated, including the degradation method, electrode material, current density, pH value, electrode spacing, solution temperature, TC concentration, and electrode area. To elucidate the mechanism by which the Fe 3 O 4 @ACF composite electrode degrades tetracycline via the electro-Fenton process, a series of control experiments were performed ( Figure 6 b). The results demonstrate that the degradation efficiency was substantially suppressed under both adsorption-only and anaerobic conditions. In the adsorption-only test, the adsorption of tetracycline on ACF was below 2%, indicating a negligible contribution of physical adsorption to the overall removal. This minimal adsorption is likely attributable to the near-saturation of the ACF support during the material-preparation stage. Under oxygen-deficient operation, the interruption of continuous oxygen supply to the cathode severely hindered the oxygen-reduction reaction and the consequent electrogeneration of H 2 O 2 , leading to a pronounced decline in degradation. Nevertheless, a measurable residual degradation activity persisted, which can be explained by two concurrent pathways: oxygen produced at the anode via the oxygen-evolution reaction may diffuse to the cathode and be reduced to H 2 O 2 , while a small fraction of water molecules at the anode may be directly oxidized to yield ·OH radicals, both contributing to limited pollutant degradation in the absence of external aeration. Taken together, these findings confirm that the efficient degradation of tetracycline by Fe 3 O 4 @ACF is predominantly governed by the electro-Fenton catalytic oxidation when the composite acts as the cathode, rather than by physical adsorption or direct anodic oxidation pathways. Figure 6. Open in a new tab Comparison of Electro-Fenton oxidation on the degradation of pure ACF and Fe 3 O 4 @ACF ( a ); Comparison of different degradation methods (adsorption, anaerobic, cathodic) ( b ); Effect of current density on Electro-Fenton oxidation degradation ( c ); Effect of initial pH on Electro-Fenton oxidation degradation ( d ); Effect of electrode plate spacing on Electro-Fenton oxidation degradation ( e ); Effect of solution temperature on Electro-Fenton oxidation degradation ( f );Effect of TC concentration on Electro-Fenton oxidation and degradation ( g ); Effect of electrode area on Electro-Fenton oxidation and degradation ( h ). The comparison of Electro-Fenton oxidation performance of ACF and Fe 3 O 4 @ACF under the same conditions is shown in Figure 6 a. The removal rate of ACF at 120 min was only 64.21%, while Fe 3 O 4 @ACF exhibited a removal rate of 82% at 120 min. This phenomenon may be attributed to the fact that the iron particles in Fe 3 O 4 @ACF act as a bridge for electron transfer, which polarizes the cathode and improves electron transfer efficiency [ 49 ]. This observation is consistent with the electrochemical properties of the material. Therefore, in the Electro-Fenton reaction, the Fe 3 O 4 @ACF composite electrode, exhibiting superior oxygen reduction reaction (ORR) electrocatalytic activity, was able to produce more H 2 O 2 . This, in turn, reacted with the iron active sites on the catalyst surface to generate more reactive species, ultimately enhancing the TC removal rate. Figure 6 c shows the effect of different current densities (10, 15, 20, 25, 30, and 35 mA·cm −2 ) on TC degradation. It can be seen that the highest removal rate (82%) was achieved at a current density of 30 mA·cm −2 . This enhancement is attributed to iron particles acting as electron transfer bridges, polarizing the cathode and improving electron transfer efficiency, consistent with electrochemical properties. However, excessive current densities can promote side reactions [ 24 ], such as water electrolysis, which reduces the removal efficiency. The effect of the initial solution pH on TC removal was investigated. Initial pH values in the range of 3–7 resulted in over 80% removal at 120 min. TC removal slightly decreased with increasing initial pH, consistent with the optimal pH for ·OH generation in the Fenton system of approximately 4.0 [ 50 ]. At low pH, the scavenging effect of H + is enhanced, leading to a decrease in the oxidizing capacity of the Fenton process [ 51 ]. The formation of HO 2 − at higher pH may lead to a decrease in H 2 O 2 yield [ 52 ]. In fact, the removal of TC by Fe 3 O 4 @ACF at pH 9 still reached more than 75% at 120 min. Therefore, Fe 3 O 4 @ACF shows superior activity in Electro-Fenton systems under acidic, neutral, and even weakly alkaline conditions, expanding the pH applicability in practical applications. Figure 6 e shows the effect of electrode spacing on the oxidation performance of the Electro-Fenton process [ 53 ]. It can be seen that when the electrode spacing is 3 cm, the removal rate reaches 81.72% at 120 min. Deviations above or below this value result in a decrease in removal efficiency. Excessive spacing reduces the rates of electromigration and diffusive mass transfer during electrochemical reactions [ 54 ]. When the spacing is too small, concentration polarization occurs, resulting in fewer organic molecules being free to migrate, which decreases mass transfer efficiency and thus affects the treatment efficiency of the Electro-Fenton system [ 26 ]. Figure 6 f shows the effect of temperature on the performance of the Electro-Fenton process. It can be seen that the degradation efficiency decreases with increasing temperature, but the final degradation rate remains above 70%. At higher temperatures, the solubility of oxygen in water decreases, leading to reduced H 2 O 2 production via electrocatalysis. In addition, at elevated temperatures, some H 2 O 2 may decompose directly into H 2 O, significantly reducing the amount of ·OH available to degrade the contaminant and thus lowering the removal rate. Therefore, the Electro-Fenton system is not suitable for the direct treatment of high-temperature wastewater, and cooling the wastewater prior to the Electro-Fenton reaction is necessary to achieve better removal efficiency. To investigate the effect of initial TC concentration on the catalytic performance of Fe 3 O 4 @ACF, TC concentrations of 10, 20, 30, and 40 mg/L were tested, as shown in Figure 6 g. The catalytic degradation efficiency of the Fe 3 O 4 @ACF Electro-Fenton system decreased with increasing TC concentration. This inhibition can be attributed to the heterogeneous catalytic process at the interface of the liquid phase (TC solution) and solid phase (Fe 3 O 4 @ACF) [ 55 ]. The presence of a high TC concentration leads to competitive adsorption of TC and H 2 O 2 on the Fe 3 O 4 @ACF catalyst surface. As a result, it becomes difficult for H 2 O 2 to compete with TC molecules because the higher TC concentration limits the adsorption and subsequent degradation of H 2 O 2 . In addition, the decomposed TC intermediates occupy the active sites of the catalyst, which further inhibits the adsorption of H 2 O 2 and leads to a decrease in degradation efficiency. Figure 6 h shows the effect of different electrode areas on the electrocatalytic performance. It can be seen that the degradation efficiency gradually increases with increasing electrode area, providing preliminary evidence of the potential of the composite electrodes for future industrial-scale applications. 3.4. Reusability and Practicality of Fe 3 O 4 @ACF In addition to degradation efficiency, electrode stability is also important from a practical application perspective. Therefore, five consecutive degradation cycles were performed to evaluate the reusability of the Fe 3 O 4 @ACF electrode. As shown in Figure 7 a, the TC degradation rate was maintained at around 75% and did not decrease significantly over ten cycles. This indicates that the Fe 3 O 4 @ACF electrode is stable and reusable, and this nanomaterial shows great promise for sustainable use and recycling in Electro-Fenton systems. Figure 7. Open in a new tab Stability of Fe 3 O 4 @ACF electrodes ( a ) and degradation experiments of different organic compounds ( b ); Degradation experiments for interfering ions (K + , Ca 2+ , Mg 2+ , Cl − , NO 3− , and HCO 3− ) experiments ( c ); Time-dependent H 2 O 2 concentration curves generated by pristine ACF and Fe 3 O 4 @ACF electrodes in the electro-Fenton system ( d ). In addition, the performance of the Fe 3 O 4 @ACF composite electrode was evaluated by degrading other difficult-to-degrade pollutants, including Rhodamine B (RhB), arsenic-containing solutions (As), Methyl Blue, and Methyl Orange. As shown in Figure 7 b, the removal efficiencies of these pollutants reached 83.64%, 87.06%, 89.72%, and 88.95%, respectively, within 120 min. The results indicate that the Fe 3 O 4 @ACF composite electrode has broad applicability for the degradation of various pollutants, highlighting its potential for treating a wide range of organic contaminants. we have further supplemented the anti-interference experiments against common coexisting ions in actual industrial wastewater, and the results show that our system maintains stable degradation performance in the presence of 100 mg/L of common cations (K + , Ca 2+ , Mg 2+ ) and anions (Cl − , NO 3 − , HCO 3 − ), with only the bicarbonate ion showing a significant inhibitory effect, which provides solid data support for the practical application potential of the system in complex water matrices. The results of Figure 7 d reveal that the pristine ACF electrode achieves a peak H 2 O 2 concentration of 13.3 mg/L at 30 min, which gradually decreases to 12.0 mg/L by 120 min. In contrast, the Fe 3 O 4 @ACF electrode exhibits a lower peak H 2 O 2 concentration (6.5 mg/L at 30 min), followed by a slight recovery to 6.8 mg/L at 120 min. This observation aligns with the enhanced electro-Fenton catalytic activity of Fe 3 O 4 @ACF: the Fe 3 O 4 nanoparticles act as efficient Fenton catalysts, promoting the rapid decomposition of generated H 2 O 2 into highly reactive ·OH radicals, which accelerates pollutant degradation and results in a lower steady-state H 2 O 2 concentration compared to ACF. This provides direct experimental evidence linking Fe 3 O 4 modification to improved utilization of H 2 O 2 for pollutant mineralization, supporting our mechanistic hypothesis of enhanced ORR-derived electro-Fenton activity. 3.5. Analysis of the Main Controlling Factors of the Fenton Reaction The mechanism of TC degradation was investigated by quencher and EPR experiments. The reactive species ∙OH and ∙O 2 − produced during degradation were quenched by 3 mM MeOH (or TBHP) and IPA. In Figure 8 a, the removal efficiency of TC decreased from 82% to 61.44% and 62.3% when MeOH and TBHP were added, respectively. When IPA was added, the removal efficiency was 59.75% after 120 min. The contributions of ·OH and ·O 2 − to the removal process were calculated as 20.56% (or 19.7%) and 22.25%, respectively. To better study the kinetics of the TC degradation process, a first-order kinetic model was proposed as follows: ln(C 0 /C t ) = kt (2) where C 0 is the initial concentration of tetracycline (mg/L); C t is the concentration of tetracycline at t min (mg/L); k is the quasi-primary rate constant for tetracycline degradation; and t is the reaction time (min). Figure 8. Open in a new tab Effect of quenching agent on TC removal rate at 120 min ( a ); Kinetic analysis of the quenching group ( b ); pseudo-first-order ( c ) and pseudo-second-order ( d ) kinetic fitting plots for tetracycline hydrochloride degradation over Fe 3 O 4 @ACF and pristine ACF electrodes; Spin-capture EPR spectra of DMPO~∙OH in a methanol dispersion system for Fe 3 O 4 @ACF and ACF ( e ); Spin-capture EPR spectra of DMPO~∙O 2 − in a methanol dispersion system for Fe 3 O 4 @ACF and ACF ( f ). As shown in Figure 8 b, the first-order pseudo-kinetic constants k for each reaction after addition of quencher were calculated kinetically as 0.01335 (min −1 ), 0.00796 (min −1 ), 0.00767 (min −1 ), and 0.00747 (min −1 ), respectively. In summary, the addition of MeOH, TBHP and IPA significantly reduced the reaction rate, which corresponded to the decrease in the removal efficiency. This confirms that ∙OH and ∙O 2 − play a major role in the degradation of TC. This conclusion was further verified by EPR experiments. Samples were taken at 0 min, 15 min and 30 min of the energization reaction using pure-ACF and Fe 3 O 4 @ACF electrodes to detect the signal intensity of DMPO~∙OH and DMPO~∙O 2 − . As shown in Figure 8 c,d, at 0 min, no ∙OH and ∙O 2 − were produced by the catalytic reaction of both electrodes, and the quantitatively determined concentrations of ∙OH and ∙O 2 − signals were 0. After 15 min of the reaction, DMPO~∙OH and DMPO~∙O 2 − signals were detected for the Fe 3 O 4 @ACF electrode. Figure 8 c shows the characteristic peaks of DMPO~∙OH with intensity ratio of 1:2:2:1. Figure 8 d shows the signals with intensity ratio of 1:1:1:1, which is in agreement with the peaks of DMPO~∙O 2 − . This finding suggests that the presence of Fe 3 O 4 significantly enhances the catalytic ability of pure ACF in the Fenton reaction. Thus, the Fe 3 O 4 @ACF composite electrode produced ∙OH and ∙O 2 − in the catalytic reaction, confirming that the loading of Fe 3 O 4 promoted the production of ∙OH and ∙O 2 − . The results showed that the doping of Fe 3 O 4 effectively improved the efficiency of the Electro-Fenton reaction. The quenching and EPR experiments fully demonstrated that ∙OH and ∙O 2 − are the main radicals in the reaction process. Cathode: O 2 +2H + + 2e − → H 2 O 2 (3) Fe 2+ + H 2 O 2 → Fe 3+ + ·OH + OH − (4) Fe 3+ + H 2 O 2 → Fe 2+ + ·O 2 − + 2H + (5) ·OH/·O 2 − + organic pollutants → intermediate → CO 2 + H 2 O (6) In the Electro-Fenton reaction, the supplied oxygen is converted into H 2 O 2 , (Equation (3)) following the classical two-electron oxygen reduction reaction (ORR) pathway [ 56 ]. This process involves the redox cycling between Fe 2+ and Fe 3+ , which leads to the generation of ∙OH and ∙O 2 − (Equations (4) and (5)). Subsequently, the generated ∙OH or ∙O 2 − attack the organic pollutants, producing intermediates that are eventually mineralized into CO 2 and H 2 O (Equation (6)). Loaded Fe 3 O 4 particles act as electron transfer bridges, enhancing electron transport, reactive oxygen species generation, and degradation efficiency. 4. Conclusions In this study, a Fe 3 O 4 @ACF composite cathode was successfully fabricated via an optimized co-precipitation method. The Fe 3 O 4 nanoparticles (~100 nm) were uniformly loaded onto the ACF support, as confirmed by SEM, TEM, XRD, and XPS analyses. The Fe 3 O 4 @ACF electrode exhibited superior electrochemical performance, achieving a 2.3-fold higher ORR current density than pristine ACF and a low charge transfer resistance of 7.18 Ω. When employed as a cathode in an electro-Fenton system, it achieved 82% tetracycline degradation within 120 min under optimal conditions (30 mA·cm −2 , 3 cm electrode spacing, room temperature), following first-order kinetics with a rate constant of 0.01335 min −1 . The electrode demonstrated excellent stability, with less than 3% performance decay over five cycles, and broad applicability, achieving >80% removal for various organic pollutants. Mechanistic studies confirmed that ·OH and ·O 2 − are the dominant reactive species, with Fe 3 O 4 nanoparticles serving as electron-transfer mediators to enhance ROS generation. Overall, the Fe 3 O 4 @ACF composite offers a facile, stable, and efficient cathode material with broad pH applicability, presenting a promising solution for the treatment of antibiotic-contaminated wastewater. Acknowledgments The authors gratefully acknowledge the support of experimental facilities provided by China University of Petroleum (Beijing) at Karamay. Furthermore, we extend our thanks to the AI tools employed for language polishing and optimization throughout the manuscript preparation process. Supplementary Materials The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/nano16070431/s1 , Figure S1: Schematic diagram of electrode preparation; Figure S2: Pseudo-first-order (a) and pseudo-second-order (b) kinetic fitting plots for tetracycline hydrochloride degradation over Fe 3 O 4 @ACF and pristine ACF electrodes; Figure S3: TOC removal rates of tetracycline hydrochloride over pristine ACF and Fe 3 O 4 @ACF electrodes as a function of reaction time; Figure S4: Catalytic tetracycline degradation mechanism diagram using ACF-loaded Fe 3 O 4; Figure S5: Current-time curve of Fe3O4@ACF at 8 V constant voltage; Table S1: Parameters fitted to the EIS test; Table S2: Previously reported materials for Electro-Fenton degradation of TC [ 57 , 58 , 59 , 60 , 61 ]; Table S3: Specific surface area, pore volume and average pore diameter parameters of pristine ACF and Fe 3 O 4 @ACF samples. nanomaterials-16-00431-s001.zip (825.8KB, zip) Author Contributions Conceptualization, X.L.; formal analysis, resources, Y.P.; Data curation, investigation, and methodology, X.L.; writing—original draft preparation, X.L. and Y.P.; Writing—review and editing, H.Z. and Y.L.; Visualization, H.Z. and H.Y.; project administration, funding acquisition, J.H. All authors have read and agreed to the published version of the manuscript. Data Availability Statement The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors. Conflicts of 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. Funding Statement This work was financially supported by the National Natural Science Foundation of China (No. 52360003), and the Tianshan Talent Fund of Xinjiang Province (No. 2023TSYCJC0065). Footnotes Disclaimer/Publisher’s Note: The statements, opinions and data contained in all publications are solely those of the individual author(s) and contributor(s) and not of MDPI and/or the editor(s). 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[ DOI ] [ PMC free article ] [ PubMed ] [ Google Scholar ] Associated Data This section collects any data citations, data availability statements, or supplementary materials included in this article. Supplementary Materials nanomaterials-16-00431-s001.zip (825.8KB, zip) Data Availability Statement The original contributions presented in the study are included in the article, further inquiries can be directed to the corresponding authors. 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