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Antibiotics exhibit stubborn resistance to degradation in conventional wastewater treatment processes. To address this issue, advanced oxidation processes (AOPs) have attracted significant attention due to their strong oxidative capacity, environmental friendliness, and ability to completely mineralize antibiotics. Among them, the electro-Fenton advanced oxidation technology generates highly oxidative hydroxyl radicals through electrochemical reactions between iron-based catalysts and hydrogen peroxide, demonstrating remarkable degradation efficiency. To overcome reaction kinetic limitations, a flow-through cathodic electro-Fenton membrane can be employed to significantly shorten pollutant diffusion pathways, enhance mass transfer efficiency, and accelerate interfacial reaction kinetics. Furthermore, the introduction of nanoscale iron-based catalysts and carbon nanodefects can increase radical production, substantially improving antibiotic degradation efficiency.
Recently, Associate Professor Shuyan Yu's research team at the University of Science and Technology Beijing published their latest findings in the Journal of Materials Chemistry A entitled "The synergistic mechanism of an FeS₂/ACFs self-standing membrane in highly efficient electro-Fenton antibiotic degradation: carbon nanodefects and free radical analysis". The study developed a self-supporting FeS₂/ACFs electrocatalytic membrane by combining Fe-MOF-derived pyrite (FeS₂) with aligned carbon nanofiber membranes (ACFs), achieving 100% single-pass degradation of tetracycline (100 mg/L) in a flow-through electro-Fenton reactor.
By precisely regulating carbon defects and leveraging the synergistic effects of radicals (·OH/SO₄·⁻), the membrane significantly enhances the combined electrocatalytic and adsorption performance, overcoming the limitations of conventional electro-Fenton technology. This innovative membrane demonstrates efficient antibiotic degradation across a broad pH range (3-9), offering a novel strategy for antibiotic wastewater treatment.
Figure 1 Schematic of the synthesis process of the FeS₂/ACFs membrane.
The prepared ACFs and FeS₂/ACFs membranes exhibited a disc-like morphology with a diameter of 45 mm, demonstrating excellent flexibility and self-supporting properties. SEM characterization revealed that the ACFs membrane (Fig. 2e), fabricated at a rotational speed of 1500 rpm, displayed uniform fiber distribution without bead-like structures. The ACFs membrane showed remarkable alignment orientation, endowing it with superior mechanical properties and electrical conductivity compared to randomly arranged carbon nanofiber membranes.
The carbon nanofibers had an average diameter of approximately 200 nm, with FeS₂ growing as thin layers on their surfaces, thereby preserving the exposure of active sites on the carbon nanofibers. The outstanding electrical conductivity of the ACFs membrane effectively synergized with the catalytic performance of FeS₂ in the electro-Fenton system.
Figure 2 Photos of FeS₂/ACFs (a); SEM images of FeS₂/ACFs membrane (b–d) and ACFs membrane (e); representative EDS images (f) showing approximate content distributions of Fe (g), C (h), and S (i). XRD (j), Raman spectra (k), and XPS spectra of different elements: C 1s (l), N 1s (m), Fe 2p (n), and S 2p (o) in the FeS₂/ACFs membrane.
In the electro-Fenton process, both Fe²⁺ concentration and pH were identified as critical factors affecting TC removal. FeS₂/ACFs membranes with varying FeS₂ loadings were prepared through different hydrothermal concentrations. Remarkably, complete degradation of 100 mg·L⁻¹ TC was achieved in a single-pass operation.
Comparative analysis revealed that FeS₂/ACFs-2 exhibited superior catalytic activity over both FeS₂/ACFs-1 and FeS₂/ACFs-3. This demonstrates that while moderate FeS₂ loading enhances the reaction, excessive FeS₂ may aggregate on the membrane surface, potentially blocking carbon nanodefects and compromising both active site exposure and the H₂O₂-mediated ·OH generation process.
In the case of FeS₂/ACFs-3, the carbon nanofibers were substantially coated by FeS₂, which hindered their function as electron donors in the reaction system. Under these conditions, Fe³⁺ could only be reduced to Fe²⁺ through H₂O₂. Notably, the FeS₂/ACFs-2 membrane demonstrated significantly higher efficiency compared to the pristine ACFs membrane.
Figure 3 TC removal by different cathode membranes in Electro-Fenton treatment (100 mg/L TC (a), 150 mg/L TC (b)); CV curves of FeS₂/ACFs membrane with/without TC (c); TC removal curves by quenching (d); EPR spectra of FeS₂/ACFs membrane (e); comparison with other studies (f).
The electronic structure characteristics of FeS₂/ACFs and ACFs membranes were systematically analyzed through DFT calculations, with particular focus on their electrostatic potential (ESP) distributions. The ESP results revealed that the ACFs membrane exhibited relatively uniform electron distribution, featuring electron-rich central regions and positively charged nitrogen-doped edges.
Notably, the introduction of FeS₂ significantly altered this homogeneous ESP distribution in the ACFs membrane. The FeS₂/ACFs composite demonstrated markedly enhanced redox reactivity tendencies. This improved performance was further quantified by the reduced LUMO-HOMO energy gap (FeS₂/ACFs: 0.25 eV vs ACFs: 0.75 eV), indicating accelerated electron transfer kinetics.
These findings collectively demonstrate that FeS₂ incorporation effectively modulates the electronic properties of the ACFs membrane by lowering the energy barrier for electron transfer. Consequently, the FeS₂/ACFs composite facilitates more efficient electron transport compared to pristine ACFs, leading to substantially improved electrical conductivity and catalytic activity.
Figure 4 Carbon defect formation and nitrogen volatilization in ACFs membranes due to high temperature, and the impact of FeS₂/ACFs membrane loading on structural defects (a). DFT-based electrocatalytic activity of ACFs and FeS₂/ACFs. Molecular structure diagrams: ACFs membrane (b) and FeS₂/ACFs membrane (c); ESP distribution maps: ACFs membrane (d) and FeS₂/ACFs membrane (e); LUMO-HOMO energy gaps of ACFs and FeS₂/ACFs membranes (f).
The study employed LC-MS/MS technology to identify tetracycline (TC) degradation intermediates, coupled with density functional theory (DFT) calculations to elucidate the radical attack mechanisms. Fukui function analysis revealed that specific sites (C2, C6, C10, C11, C16, O23, O26, O27, and N29; f⁰=0.023-0.080) exhibited high susceptibility to ·OH/SO₄⁻· radical attacks. Notably, the heightened reactivity of N29 initiated Pathway I, where preferential attack on the amide group led to the formation of P1 (m/z=410).
Electronic structure analysis demonstrated that TC's LUMO (electron acceptor region) was primarily distributed across carbon ring No.1 and oxygen-containing functional groups, while the HOMO (electron donor region) concentrated on the amide group and carbon ring No.4. Based on these findings, three distinct degradation pathways were proposed:
Pathway I: Sequential demethylation/hydroxylation converted P1 into P5 (m/z=165)
Pathway II: Radical attack on carbon ring No.1 (C6 site ring-opening) generated P6 (m/z=449), ultimately degrading to P8 (m/z=114)
Pathway III: Amide group oxidation formed P9 (m/z=414), followed by successive dehydroxylation and demethylation to yield P13 (m/z=217)
All polycyclic intermediates (P14-P19) ultimately underwent cleavage into mono-/chain organic compounds before complete mineralization to CO₂ and H₂O. This combined experimental and theoretical approach comprehensively elucidated TC's ring-opening degradation and mineralization mechanisms.
Figure 5 Proposed TC degradation pathways.
Figure 6 illustrates the mechanism of Electro-Fenton TC degradation under neutral conditions by the FeS₂/ACFs membrane. Fe²⁺ and Fe³⁺ can interconvert on the membrane surface and in the bulk solution, generating ·OH during the process. S₂²⁻ in the FeS₂/ACFs membrane is converted to SO₄²⁻ in the solution, reacting with ·OH to produce SO₄·⁻. Both ·OH and SO₄·⁻ coexist. TC is adsorbed on the FeS₂/ACFs membrane surface and attacked by radicals in the solution, gradually degrading into smaller substances. In summary, the FeS₂/ACFs membrane effectively utilizes the synergistic effect of adsorption and catalysis, making it a promising candidate for Electro-Fenton treatment.
Figure 6 Schematic of the Electro-Fenton degradation mechanism of TC by FeS₂/ACFs.
This study demonstrates that the FeS₂/ACFs composite membrane significantly enhances the electro-Fenton degradation efficiency of tetracycline (TC). Quantum chemical calculations based on density functional theory (DFT) reveal that the FeS₂/ACFs membrane possesses exceptional electron transfer capability and redox activity, leading to markedly improved electrocatalytic performance. By incorporating FeS₂ dopants and optimizing carbonization temperature, we successfully increased carbon atom disorder and nanodefect concentration, substantially enhancing both adsorption capacity and catalytic activity. DFT-based electrostatic potential calculations further corroborated this catalytic mechanism.
In single-pass flow-through reactors, the FeS₂/ACFs membrane achieved complete degradation of 100 mg/L TC through synergistic adsorption-catalysis effects. Electron paramagnetic resonance (EPR) and quenching tests identified hydroxyl radicals (·OH) and sulfate radicals (SO₄⁻·) as the primary reactive species, with ·OH being the dominant attacking agent for TC degradation. DFT calculations of LUMO-HOMO energy gaps for TC and its degradation intermediates enabled detailed elucidation of the reaction pathways.
The FeS₂/ACFs composite membrane establishes a technical foundation for developing high-performance electrocatalytic membranes. Future research will focus on optimizing the self-supporting membrane structure to improve wastewater treatment stability and degradation efficiency
Paper link: https://doi.org/10.1039/d5ta00119f
