Carbon felt cathodes for electro-Fenton process to remove tetracycline via synergistic adsorption and degradation

Recent progress in electro-Fenton technology for the remediation of pharmaceutical compounds in aqueous environments

The global focus on wastewater treatment has intensified in the contemporary era due to its significant environmental and human health impacts. Pharmaceutical compounds (PCs) have become an emerging concern among various pollutants, as they resist conventional treatment methods and pose a severe environmental threat. Advanced oxidation processes (AOPs) emerge as a potent and environmentally benign approach for treating recalcitrant pharmaceuticals. To address the shortcomings of traditional treatment methods, a technology known as the electro-Fenton (EF) method has been developed more recently as an electrochemical advanced oxidation process (EAOP) that connects electrochemistry to the chemical Fenton process. It has shown effective in treating a variety of pharmaceutically active compounds and actual wastewaters. By producing H2O2 in situ through a two-electron reduction of dissolved O2 on an appropriate cathode, the EF process maximizes the benefits of electrochemistry. Herein, we have critically reviewed the application of the EF process, encompassing diverse reactor types and configurations, the underlying mechanisms involved in the degradation of pharmaceuticals and other emerging contaminants (ECs), and the impact of electrode materials on the process. The review also addresses the factors influencing the efficiency of the EF process, such as (i) pH, (ii) current density, (iii) H2O2 concentration, (iv) and others, while providing insight into the scalability potential of EF technology and its commercialization on a global scale. The review delves into future perspectives and implications concerning the ongoing challenges encountered in the operation of the electro-Fenton process for the treatment of PCs and other ECs.

Intensive investigation of the synergistic effects between electrocatalysis and peroxymonosulfate activation for efficient organic elimination

Hybrid systems combined eletrocatalysis and Fenton-like process attract a lot of attention due their outstanding performance and unique mechanism. Here, we proposed an efficient, cost-effective, and versatile electrochemical activation (ECA) system for efficient water purification, and intensively studied the synergistic effects between electrocatalysis and peroxymonosulfate (PMS)-based advanced oxidation. The ECA system achieved complete removal of 20 ppm tetracycline hydrochloride (TCH) in 15 min, with a rate constant of 0.338 min-1. Its performance was assessed across various operational parameters (PMS dosage, pH, applied voltage, electrode interval, temperature, co-existed ions, biomass, different oxidants), demonstrating its broad applicability and stability. Excellent degradation and mineralization for other 12 kinds of refractory organic pollutants were also achieved. The outstanding performance can be attributed to the synergistic effect in the system, in which electrocatalytic reduction of dissolved oxygen generated H2O2 and O2•-, boosting the number of reactive species, such as 1O2, by interacting with PMS. Furthermore, the presence of organic matter promotes electron transfer, amplifying the system's degradation capability. These findings not only highlight the ECA system's effectiveness in organic pollutant removal but also offer insights into the underlying degradation mechanisms, paving the way for future advancements in water purification technologies.

Coupling Electro-Fenton and Electrocoagulation of Aluminum-Air Batteries for Enhanced Tetracycline Degradation: Improving Hydrogen Peroxide and Power Generation

Electro-Fenton (EF) technology has shown great potential in environmental remediation. However, developing efficient heterogeneous EF catalysts and understanding the relevant reaction mechanisms for pollutant degradation remain challenging. We propose a new system that combines aluminum-air battery electrocoagulation (EC) with EF. The system utilizes dual electron reduction of O2 to generate H2O2 in situ on the air cathodes of aluminum-air batteries and the formation of primary cells to produce electricity. Tetracycline (TC) is degraded by ·OH produced by the Fenton reaction. Under optimal conditions, the system exhibits excellent TC degradation efficiency and higher H2O2 production. The TC removal rate by the reaction system using a graphite cathode reached nearly 100% within 4 h, whereas the H2O2 yield reached 127.07 mg/L within 24 h. The experimental results show that the novel EF and EC composite system of aluminum-air batteries, through the electroflocculation mechanism and ·OH and EF reactions, with EC as the main factor, generates multiple •OH radicals that interact to efficiently remove TC. This work provides novel and important insights into EF technology, as well as new strategies for TC removal.

Degradation of tetracycline under a wide pH range in a heterogeneous photo bio-electro-fenton system using FeMn-LDH/g-C3N4 cathode: Performance and mechanism

The widespread use of antibiotics and the inefficiency of traditional degradation treatments pose threats to the environment and human health. Previous studies have reported the potential of bio-electro-Fenton (BEF) processes for antibiotic removal. However, some drawbacks, such as a strict pH range of 2-3 and iron sludge generation, limit their large-scale application. Thus, to overcome the narrow pH range of traditional BEF processes, a photo-BEF (PBEF) system was established using a novel FeMn-layered double hydroxide (LDH)/graphitic carbon nitride (g-C3N4) (FM/CN) composite cathode. The performance of the PBEF system was investigated by degrading tetracycline (TC) under low-power LED lamp irradiation. The results indicated that the pH range of the PBEF system could be expanded to 3-11 using an FM/CN cathode, which exhibited a TC removal efficiency of 63.0%-75.9%. The highest TC removal efficiency was achieved at pH 7. The efficient mineralization of TC by the PBEF system can be high, up to 67.6%. In addition, the TC removal mechanism was discussed in terms of reactive oxygen species, TC degradation intermediate analyses, and density functional theory (DFT) calculations. Strong oxidative hydroxyl radicals (·OH) were the dominant reactive oxidizing species in the PBEF system, followed by ·O2- and h+. Three pathways of TC degradation were proposed based on the analysis of intermediates, and the reactive sites attacked by electrophilic reagents were explored using DFT modeling. In addition, the overall toxicity of TC degradation intermediates effectively decreased in the PBEF system. This work offers deep insights into the TC removal mechanisms and performance of the PBEF system over a wide pH range of 3-11.

Fabrication of magnetic MnFe2O4@HL composites with an in situ Fenton-like reaction for enhancing tetracycline degradation

Novel magnetic MnFe2O4@honey locust-derived carbon (MnFe2O4@HL) composites were synthesized via an in-situ hydrothermal precipitation method, and were characterized as an excellent Fenton-like catalyst for tetracycline (TC) degradation. Results showed that the vast majority of TC was mineralized in hydrogen peroxide (H2O2)/MnFe2O4@HL system after 120 min of reaction time with 92.3% of removal efficiency and the removal of 71.3% of total organic carbon (TOC). Systematic characterization approaches including scanning electron microscope (SEM), energy-dispersive X-ray spectrometry (EDS), X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectroscopy, X-ray photoelectron spectroscopy (XPS) and vibrating sample magnetometry (VSM) were introduced to reveal the microstructure and properties of magnetic MnFe2O4@HL composites. Hydroxyl radicals (•OH) were identified as the major reactive oxygen species (ROS) via the quenching experiments and electron spin resonance (ESR) analysis, while superoxide radicals (·O2-) played a negligible role. The dual cycles of both Fe3+/Fe2+ and Mn3+/Mn2+ were significant enhanced through the bimetallic redox effect and the electron transfer effect of the carbon-based functional group, accelerating the generation of •OH. The removal of TC was still up to 79.3% after five reuses of magnetic composites, demonstrating the MnFe2O4@HL with excellent stability and reuse performance. The influence of various experimental control conditions involving initial pH, catalyst and H2O2 dosage, temperature, as well as common anions (Cl-, NO3-, and HCO3-) on the degradation of TC were finally evaluated. This study provides an efficient in-situ generation method of emerging magnetic materials, and systematically reveals its mechanism of homogeneous Fenton-like catalysis, which shows promising applications for the degradation of environmental contaminants.

Facile and green synthesis of recyclable, environmentally friendly, chemically stable, and cost-effective magnetic nanohybrid adsorbent for tetracycline adsorption

Antibiotic contamination of water sources, particularly tetracycline (TC) contamination, has emerged as one of the global issues that needs action. In this research, ZnCoFe2O4@Chitosan (Ch) as a magnetic nanohybrid adsorbent was synthesized using the microwave-assisted co-precipitation method, and their efficiency for the TC adsorption process was investigated. FESEM (Field Emission Scanning Electron Microscope), EDX (Energy Dispersive X-ray), Mapping and line Scan, XRD (X-Ray Diffraction), FTIR (Fourier Transform Infrared Spectrometer), VSM (Vibrating Sample Magnetometer), Thermogravimetric analysis (TGA) and BET (Brunauer Emmett Teller) techniques were used to check and verify its physical and chemical properties. The removal of TC via the adsorption process from synthetic and real wastewater samples was investigated. The factors determining the TC adsorption process, comprising tetracycline concentration (5-30 mg/L), adsorbent dosage (0.7-2 g/L), contact time (2-45 min), and pH (3-11), were evaluated. The removal effectiveness for the synthetic sample and the real wastewater sample was 93 % and 80 %, respectively, under the ideal TC adsorption process parameters of pH 3, adsorbent dosage 1 g/L, TC initial concentration 5 mg/L, and contact time 30 min. According to kinetic and equilibrium studies, the adsorption of TC by ZnCoFe2O4@Ch follows pseudo-second-order kinetics and the Freundlich isotherm. Additionally, it was determined through the analysis of thermodynamic data that the process of exothermic adsorption is spontaneous and is followed by a decrease in disorder (ΔH = -15.16 kJ/mol, ΔS = -28.69 kJ/mol, and ΔG = -6.62 kJ/mol). After five cycles of recovery and regeneration, the ZnCoFe2O4@Ch magnetic nanocomposite was able to remove 65 % of the TC pollutant and had good chemical stability. The results showed that the magnetic nano-adsorbent ZnCoFe2O4@Ch is a novel magnetic nano-adsorbent with high adsorption capacity that can be utilized to eliminate pharmaceutical contaminants from aqueous solutions.

Facile synthesis of bimetallic ACF/CC@FeOCl-Cu composite cathode for efficient degradation of sulfamethoxazole at neutral pH by a flow-through heterogeneous electro-Fenton process

Flow-through heterogeneous electro-Fenton (FHEF) process shows a broad prospect for refractory organic pollutants removal. However, maintaining a long-term service life of higher catalytic cathode is crucial for the development of cathode materials, especially for iron-functionalized cathode operated under harsh conditions. In this study, a novel bimetallic CC@FeOCl-Cu composite was synthesized through one-step calcination, coupled with a series of microstructure characterization methodology, including XRD, SEM-EDS, XPS, and FTIR. The superior catalytic activity of CC@FeOCl-Cu could be ascribed to Fe-Cu synergy and better dispersion of FeOCl nanosheets. With the optimal Cu:Fe ratio of 1:60, the bifunctional ACF/CC@FeOCl-Cu cathode was employed in FHEF process, exhibiting an outstanding performance for sulfamethoxazole (SMX) removal over a wide pH range (3.0-9.0). Comparison of experimental results indicated that the ACF/CC@FeOCl-Cu-FHEF process showed higher performance than ACF/CC@FeOCl-FHEF and homogeneous EF processes. The average SMX removal efficiency was 98% and TOC removal efficiency was more than 57% even after 10 cycles. Radical quenching experiments and electron spin resonance test confirmed that •OH was the primary active species. More •OH was generated in the ACF/CC@FeOCl-Cu-FHEF process because the doping of Cu could enhance catalytic activity of cathode. In addition, the satisfactory performance could be observed in the ACF/CC@FeOCl-Cu-FHEF process for the treatment of real landfill leachate, indicating its potential for practical application in wastewater treatment.

Heterogenous electro-Fenton degradation of sulfamethoxazole on a polyethylene glycol-coated magnetite nanoparticles catalyst

We report the preparation and application of poly (ethylene) glycol (PEG) coated magnetite nanoparticles (MNPs) catalyst for the heterogeneous electro-Fenton (HEF) degradation of sulfamethoxazole in real wastewater PEG-coated MNPs of four MNP:PEG ratios were synthesised using the co-precipitation method. The synthesised MNP were characterised using FTIR, XRD, EDX, TEM, and CHN elemental analysis. It was observed that the coating of MNP with PEG influences the nanoparticle size, agglomeration tendencies and catalytic efficiency of MNPs properties in the HEF degradation process. A 1:1 optimal MNP:PEG catalyst yielded 91% sulfamethoxazole degradation and 48% total organic carbon removal in 60 min, which is an improvement of 11% over degradation with the uncoated MNP. The PEG-coated MNP showed higher stability in 10 consecutive reaction cycles, reduced leaching, and improved performance at a lower dosage and broader pH range than the uncoated MNPs. These results show that coating MNP with PEG enhances HEF catalytic performance in the degradation of sulfamethoxazole in wastewater.

Magnetic Adsorbent Fe3O4/ZnO/LC for the Removal of Tetracycline and Congo Red from Aqueous Solution

Zeolitic imidazolate frameworks (ZIFs) can be used as an adsorbent to efficiently adsorb organic pollutants. However, ZIF nanoparticles are easy to form aggregates, hampering the effective and practical application in practical adsorption. In this study, the ZIF-8 was successfully loaded onto lignocellulose (LC) to further produce ZnO/LC by in situ growth method and hydrothermal treatment, and then Fe3O4 nanoparticles (Fe3O4 NPs) were loaded onto ZnO/LC to prepare magnetic Fe3O4/ZnO/LC adsorbent for removing tetracycline (TC) and congo red (CR) pollutants from aqueous solution. The adsorption properties of the adsorbent were systematically analyzed for different conditions, such as adsorbent dosage, solution pH, contact time, temperature and initial concentration. The experimental data were fitted using adsorption kinetic and isotherm models. The results showed that the pseudo-second-order model and Sips model were well fitted to the adsorption kinetic and adsorption isotherm, respectively. The adsorption capacities of TC and CR reached the maximum value of 383.4 mg/g and 409.1 mg/g in experimental conditions. The mechanism of the removal mainly includes electrostatic interaction, hydrogen bonding and π-π stacking. This novel adsorbent could be rapidly separated from the aqueous solution, suggesting its high potential to remove pollutants in wastewater.

From Fenton and ORR 2e−-Type Catalysts to Bifunctional Electrodes for Environmental Remediation Using the Electro-Fenton Process

Currently, the presence of emerging contaminants in water sources has raised concerns worldwide due to low rates of mineralization, and in some cases, zero levels of degradation through conventional treatment methods. For these reasons, researchers in the field are focused on the use of advanced oxidation processes (AOPs) as a powerful tool for the degradation of persistent pollutants. These AOPs are based mainly on the in-situ production of hydroxyl radicals (OH•) generated from an oxidizing agent (H2O2 or O2) in the presence of a catalyst. Among the most studied AOPs, the Fenton reaction stands out due to its operational simplicity and good levels of degradation for a wide range of emerging contaminants. However, it has some limitations such as the storage and handling of H2O2. Therefore, the use of the electro-Fenton (EF) process has been proposed in which H2O2 is generated in situ by the action of the oxygen reduction reaction (ORR). However, it is important to mention that the ORR is given by two routes, by two or four electrons, which results in the products of H2O2 and H2O, respectively. For this reason, current efforts seek to increase the selectivity of ORR catalysts toward the 2e− route and thus improve the performance of the EF process. This work reviews catalysts for the Fenton reaction, ORR 2e− catalysts, and presents a short review of some proposed catalysts with bifunctional activity for ORR 2e− and Fenton processes. Finally, the most important factors for electro-Fenton dual catalysts to obtain high catalytic activity in both Fenton and ORR 2e− processes are summarized.

N-doped graphene aerogel cathode with internal aeration for enhanced degradation of p-nitrophenol by electro-Fenton process

A columnar N-doped graphene aerogel (NGA) was successfully fabricated by one-step hydrothermal synthesis using L-hydroxyproline as reductant, N-doping, and swelling agent, and it was used as the cathode with internal aeration mode for the electro-Fenton degradation of p-nitrophenol. Owing to the stable solid-liquid-gas three-phase interface, more active defects, and modulated nitrogen dopants, the NGA cathode exhibited enhanced electrocatalytic activity. H₂O₂ could be continuously electro-generated via a two-electron oxygen reduction, and the yield of H₂O₂ was 153.3 mg·L^-1·h^-1 with the low electric energy consumption of 15.3 kWh kg^-1. Simultaneously, the NGA cathode had better charge transfer capability with N-doping, which was conducive to the conversion of Fe³⁺/Fe²⁺. Under the optimal condition, nearly 100% removal of p-nitrophenol and 84% removal of TOC were obtained within 60 and 120 min, respectively. The NGA cathode also presented good stability and versatile applicability in different water matrices. Therefore, the NGA is a cost-effective cathode material in electro-Fenton system with adequate activity and reuse stabilization.

Laccase-coupled photoelectrocatalytic system for highly efficient degradation of bisphenol A

In the present work, a hybrid advanced oxidation process that combined laccase with photoelectrocatalysis (PEC) was explored for highly efficient degradation of bisphenol A (BPA). Visible light-responsive BiVO₄ film electrode with good optical and photoelectrochemical properties was prepared via an electrodeposition method and employed as photoanode for PEC degradation of BPA. After laccase was facilely introduced into the PEC system, the BPA removal efficiency was significantly promoted, attributed to the synergistic effect of enzymatic catalysis and PEC processes. To obtain the optimum operation conditions, the effects of initial pH and applied bias potential were investigated systematically. Radicals trapping experiments revealed that •O₂^- dominated the biophotoelectrocatalytic degradation process, and the possible degradation pathway for BPA was proposed by identifying intermediates using liquid chromatography-mass spectrometry.

How effective aerated continuous electrocoagulation can be for tetracycline removal from river water using aluminium electrodes?

The study assessed the effects of aeration on continuous electrocoagulation (EC) for tetracycline (TCL) removal from river water. Influence of hydraulic retention time (HRT) and initial drug concentration on treatment efficiency was tested. Best conditions for continuous EC operation were 12 min HRT, electrode spacing 2 cm, 9 V, and Al-Al electrode combination. Highest COD removal with non-aerated EC was 59.4% at 1 mg L^-1 initial TCL concentration and further increasing TCL concentration decreased COD removal efficiency. Maximum TCL removal was 66.6% at 10 mg L^-1 initial TCL concentration with non-aerated EC. Aerated EC enhanced COD and TCL removal to 61.4% and 71.5%, respectively. In XRD and FTIR spectra no new peaks were detected following EC treatment. XRD, FTIR and FESEM-EDS data supported that significant removal of TCL occurred by charge neutralization, entrapment, adsorption and precipitation driven by Al (OH)₃ flocs. Pseudo-second order reaction rate constants explained the kinetics of TCL removal from river water. Sludge volume produced with continuous mode EC non-aerated and aerated EC was 31 cm³ and 39 cm³, respectively. Operating cost was estimated to 0.018 US$/m³ for non-aerated EC and 0.025 US$/m³ for aerated EC. EC can be augmented by aeration for enhanced removal of TCL from river water.

An Ionic Supported Liquid Membrane for the Recovery of Bisphenol A from Aqueous Solution

In this work, a flat supported liquid membrane (FSLM) was applied for the extraction of bisphenol A (BPA) from aqueous solutions, using an ionic liquid as a carrier. The liquid membrane consists of tricaprylmethylammonium chloride (aliquat 336^®) diluted in 2-octanol. Furthermore, to obtain the best transport efficiency, the impacts of various experimental parameters were investigated. These parameters included aliquat 336^® concentration, the concentration of BPA in the feed phase, the pH of the feed phase, the concentration of NaOH in the receiving phase, the polymeric support nature, the percentage of extractant in the organic phase, and the solvent nature. The optimum conditions of the experiment were 50% (v/v) aliquat 336^®/2-octanol as the organic phase, a transport time of 8 h, and 1 × 10^-2 mol L^-1 NaOH as the receiving phase. The BPA was successfully recovered (the recovery percentage was about 89%). Supported liquid membrane-based aliquat 336^®/2-octanol displayed an acceptable stability with re-impregnation after 5 days of operation.

Current Progress in Natural Degradation and Enhanced Removal Techniques of Antibiotics in the Environment: A Review

Antibiotics are used extensively throughout the world and their presence in the environment has caused serious pollution. This review summarizes natural methods and enhanced technologies that have been developed for antibiotic degradation. In the natural environment, antibiotics can be degraded by photolysis, hydrolysis, and biodegradation, but the rate and extent of degradation are limited. Recently, developed enhanced techniques utilize biological, chemical, or physicochemical principles for antibiotic removal. These techniques include traditional biological methods, adsorption methods, membrane treatment, advanced oxidation processes (AOPs), constructed wetlands (CWs), microalgae treatment, and microbial electrochemical systems (such as microbial fuel cells, MFCs). These techniques have both advantages and disadvantages and, to overcome disadvantages associated with individual techniques, hybrid techniques have been developed and have shown significant potential for antibiotic removal. Hybrids include combinations of the electrochemical method with AOPs, CWs with MFCs, microalgal treatment with activated sludge, and AOPs with MFCs. Considering the complexity of antibiotic pollution and the characteristics of currently used removal technologies, it is apparent that hybrid methods are better choices for dealing with antibiotic contaminants.

Recent progress in Fenton/Fenton-like reactions for the removal of antibiotics in aqueous environments

The frequent use of antibiotics allows them to enter aqueous environments via wastewater, and many types of antibiotics accumulate in the environment due to difficult degradation, causing a threat to environmental health. It is crucial to adopt effective technical means to remove antibiotics in aqueous environments. The Fenton reaction, as an effective organic pollution treatment technology, is particularly suitable for the treatment of antibiotics, and at present, it is one of the most promising advanced oxidation technologies. Specifically, rapid Fenton oxidation, which features high removal efficiency, thorough reactions, negligible secondary pollution, etc., has led to many studies on using the Fenton reaction to degrade antibiotics. This paper summarizes recent progress on the removal of antibiotics in aqueous environments by Fenton and Fenton-like reactions. First, the applications of various Fenton and Fenton-like oxidation technologies to the removal of antibiotics are summarized; then, the advantages and disadvantages of these technologies are further summarized. Compared with Fenton oxidation, Fenton-like oxidations exhibit milder reaction conditions, wider application ranges, great reduction in economic costs, and great improved cycle times, in addition to simple and easy recycling of the catalyst. Finally, based on the above analysis, we discuss the potential for the removal of antibiotics under different application scenarios. This review will enable the selection of a suitable Fenton system to treat antibiotics according to practical conditions and will also aid the development of more advanced Fenton technologies for removing antibiotics and other organic pollutants.

Energy-efficient removal of carbamazepine in solution by electrocoagulation-electrofenton using a novel P-rGO cathode

In this study, carbamazepine (CBZ) decay in solution has been studied by coupling electrocoagulation with electro-Fenton (EC-EF) with a novel P-rGO/carbon felt (CF) cathode, aiming to accelerate the in-situ generation of •OH, instead of adding Fe²⁺ and H₂O₂. Firstly, the fabricated P-rGO and its derived cathode were characterized by XRD, SEM, AFM, XPS and electrochemical test (EIS, CV and LSV). Secondly, it was confirmed that the performance in removal efficiency and electric energy consumption (EEC) by EC-EF (k_obs=0.124 min^-1, EEC=43.98 kWh/kg CBZ) was better than EF (k_obs=0.069 min^-1, EEC=61.04 kWh/kg CBZ). Then, P-rGO/CF (k_obs=0.248 min^-1, EEC=29.47 kWh/kg CBZ, CE=61.04%) showed the best performance in EC-EF, among all studied heteroatom-doped graphene/CF. This superior performance may be associated with its largest layer spacing and richest C=C, which can promote the electron transfer rate and conductivity of the cathode. Thus, more H₂O₂ and •OH could be produced to degrade CBZ, and almost 100% CBZ was removed with k_obs being 0.337 min^-1 and the EEC was only 24.18 kWh/kg CBZ, under the optimal conditions (P-rGO loading was 6.0 mg/cm², the current density was 10.0 mA/cm², the gap between electrode was 2.0 cm). Additionally, no matter the influent is acidic, neutral or alkaline, no additional pH adjustment is required for the effluent of EC-EF. At last, an inconsecutive empirical kinetic model was firstly established to predict the effect of operating parameters on CBZ removal.

Progress of homogeneous and heterogeneous electro-Fenton treatments of antibiotics in synthetic and real wastewaters. A critical review on the period 2017-2021

Antibiotics are widely supplied over all the world to animals and humans to fight and heal bacteriological diseases. The uptake of antibiotics has largely increased the average-life expectancy of living beings. However, these recalcitrant products have been detected at low concentrations in natural waters, with potential health risks due to alterations in food chains and an increase in the resistance to bacterial infection, control of infectious diseases, and damage of the beneficial bacteria. The high stability of antibiotics at mild conditions prevents their effective removal in conventional wastewater treatment plants. A powerful advanced oxidation processes such as the electro-Fenton (EF) process is being developed as a guarantee for their destruction by ^•OH generated as strong oxidant. This review presents a critical, exhaustive, and detailed analysis on the application of EF to remediate synthetic and real wastewaters contaminated with common antibiotics, covering the period 2017-2021. Homogeneous EF and heterogeneous EF involving iron solid catalysts or iron functionalized cathodes, as well as their hybrid and sequential treatments, are exhaustively examined. Their fundamentals and characteristics are detailed, and the main results obtained for the removal of the most used antibiotic families are carefully described and discussed. The role of generated oxidizing agents is explained, and the by-products generated, and reaction sequences proposed are detailed.

A review on bio-electro-Fenton systems as environmentally friendly methods for degradation of environmental organic pollutants in wastewater

Bio-electro-Fenton (BEF) systems have been potentially studied as a promising technology to achieve environmental organic pollutants degradation and bioelectricity generation. The BEF systems are interesting and constantly expanding fields of science and technology. These emerging technologies, coupled with anodic microbial metabolisms and electrochemical Fenton's reactions, are considered suitable alternatives. Recently, great attention has been paid to BEFs due to special features such as hydrogen peroxide generation, energy saving, high efficiency and energy production, that these features make BEFs outstanding compared with the existing technologies. Despite the advantages of this technology, there are still problems to consider including low production of current density, chemical requirement for pH adjustment, iron sludge formation due to the addition of iron catalysts and costly materials used. This review has described the general features of BEF system, and introduced some operational parameters affecting the performance of BEF system. In addition, the results of published researches about the degradation of persistent organic pollutants and real wastewaters treatment in BEF system are presented. Some challenges and possible future prospects such as suitable methods for improving current generation, selection of electrode materials, and methods for reducing iron residues and application over a wide pH range are also given. Thus, the present review mainly revealed that BEF system is an environmental friendly technology for integrated wastewater treatment and clean energy production.

Direct 3D printing of zero valent iron@polylactic acid catalyst for tetracycline degradation with magnetically inducing active persulfate

Catalyst stability has become a challenging issue for advanced oxidation processes (AOPs). Herein, we report an alternative method based on 3D printing technology to obtain zero-valent iron polylactic acid prototypes (ZVI@PLA) in a single step and without post etching treatment. ZVI@PLA was used to activate persulfate (PS) for the removal of Tetracycline (TC) in recirculating mode under two different heating methodologies, thermal bath and contactless heating promoted by magnetic induction (MIH). The effect of both heating methodologies was systematically analysed by comparing the kinetic constant of the degradation processes. It was demonstrated that the non-contact heating of ZVI by MIH reactivates the surface of the catalyst, renewing the surface iron content exposed to the pollutant solution, which makes the ZVI@PLA catalyst reusable up to 10 cycles with no efficiency reduction. In contrast, by using a conventional thermal bath, the kinetic constant gradually decreases over the 10 cycles, because of the superficial iron consumption, being the kinetic constant 5 times lower in the 10th run compared to MIH experiment. X-ray diffraction and Mössbauer spectroscopy confirmed the presence of metallic iron embedded in the ZVI@PLA prototype, whose crystalline structure remained unchanged for 10th cycles of MIH. Moreover, it was proven that with no contact heating technology at low magnetic fields (12.2 mT), the solution temperature does not increase, but only the surface of the catalyst does. Under these superficial heated conditions, kinetic rate is increased up to 0.016 min^-1 compared to the value of 0.0086 min^-1 obtained for conventional heating at 20 °C. This increase is explained not only by PS activation by iron leaching but also by the contribution of ZVI in the heterogeneous activation of persulfate.

A comprehensive study on the heterogeneous electro-Fenton degradation of tartrazine in water using CoFe2O4/carbon felt cathode

In this study, cobalt ferrite coated carbon felt (CoFe₂O₄/CF) was synthesized by solvothermal method and applied as cathode for electro-Fenton (EF) treatment of tartrazine (TTZ) in water. The materials were characterized by SEM, XRD, FTIR, CV, and EIS to explore their physical, chemical, and electrical properties. The effects of solvothermal temperature and metal content on the TTZ removal were examined, showing that 220 °C with 2 mM of Co and 4 mM of Fe precursors were the best synthesis condition. Various influencing factors such as applied current density, pH, TTZ concentration, and electrolytes were investigated, and the optimal condition was found at 8.33 mA cm^-2, pH 3, 50 mgTTZ L^-1, and 50 mM of Na₂SO₄, respectively. By radical quenching test, , ¹O₂, and HO were recognized as the key reactive oxygen species and the reaction mechanism was proposed for the EF decolorization of TTZ using CoFe₂O₄/CF cathode. The reusability and stability test showed that the highly efficient CoFe₂O₄/CF cathode is very promising for practical application in wastewater treatment, especially for dyes and other recalcitrant organic compounds to improve its biodegradability.

A novel bio-electro-Fenton system with dual application for the catalytic degradation of tetracycline antibiotic in wastewater and bioelectricity generation

In this new insight, the potential application of the eco-friendly Bio-Electro-Fenton (BEF) system was surveyed with the aim of simultaneous degradation of tetracycline and in situ generation of renewable bioenergy without the need for an external electricity source. To shed light on this issue, catalytic degradation of tetracycline was directly accrued via in situ generated hydroxyl free radicals from Fenton's reaction in the cathode chamber. Simultaneously, the in situ electricity generation as renewable bioenergy was carried out through microbial activities. The effects of operating parameters, such as electrical circuit conditions (in the absence and presence of external resistor load), substrate concentration (1000, 2000, 5000, and 10 000 mg L⁻¹), catholyte pH (3, 5, and 7), and FeSO₄ concentration (2, 5, and 10 mg L⁻¹) were investigated in detail. The obtained results indicated that the tetracycline degradation was up to 99.04 ± 0.91% after 24 h under the optimal conditions (short-circuit, pH 3, FeSO₄ concentration of 5 mg L⁻¹, and substrate concentration of 2000 mg L⁻¹). Also, the maximum removal efficiency of anodic COD (85.71 ± 1.81%) was achieved by increasing the substrate concentration up to 2000 mg L⁻¹. However, the removal efficiencies decreased to 78.29 ± 2.68% with increasing substrate concentration up to 10 000 mg L⁻¹. Meanwhile, the obtained maximum voltage, current density, and power density were 322 mV, 1195 mA m⁻², and 141.60 mW m⁻², respectively, at the substrate concentration of 10 000 mg L⁻¹. Present results suggested that the BEF system could be employed as an energy-saving and promising technology for antibiotic-containing wastewater treatment and simultaneous sustainable bioelectricity generation.

In this new insight, the potential application of the Bio-Electro-Fenton system was surveyed with the aim of simultaneous degradation of tetracycline and in situ generation of renewable bioenergy without the need for an external electricity source.

Improved Degradation Efficiency of Levofloxacin by a Self-Powered Electrochemical System with Pulsed Direct-Current

With the excellent structural design, rotary triboelectric nanogenerator (R-TENG) is suitable for harvesting mechanical energy such as wind energy and water energy to build a self-powered electrochemical system for environmental science. The electrochemical performance has been greatly improved by using the pulsed direct-current (PDC) output of a TENG; however, a full-wave PDC (FW-PDC) is hardly realized in R-TENG devices due to existence of phase superposition. Here, a R-TENG with FW-PDC output is reported to perform a self-powered electro-Fenton system for enhancing the removal efficiency of levofloxacin (OFL). By adjusting the rotation center angle ratio between each rotator and stator unit, the phase superposition of R-TENG caused by multiple parallel electrodes can be effectively eliminated, thus achieving the desired FW-PDC output. Because of the reduced electrode passivation effect, the removal efficiency of OFL is improved by 30% under equal electric charges through using the designed R-TENG with FW-PDC output compared to traditional R-TENG. This study provides a promising methodology to improve the performance of self-powered electrochemical process for treating environment pollutions.

Anodic oxidation of ciprofloxacin using different graphite felt anodes: Kinetics and degradation pathways

Ciprofloxacin (CIP) is ubiquitous in the environment which poses a certain threat to human and ecology. In this investigation, the physical and electrochemical properties of graphite felt (GF) anodes which affected the anodic oxidation (AO) performance, and the CIP removal effect of GF were evaluated. The GFs were used as anodes for detection of ·OH with coumarin (COU) as molecule probe and removal of CIP in a 150 mL electrolytic cell with Pt cathode (AO-GF/Pt system). The results showed that hydrophilic GF (B-GF) owned higher sp³/sp² and more oxygen-containing and nitrogen-containing functional groups than the hydrophobic GF (A-GF). Moreover, B-GF possessed higher oxygen evolution potential (1.12 V), more active sites and stronger ·OH generation capacity. Above mentioned caused that B-GF exhibited more superior properties for CIP removal. The best efficiencies (96.95%, 99.83%) were obtained in the AO-B-GF/Pt system at 6.25 mAcm^-2 after 10 min (k₁, 0.356 min^-1) and 60 min (k₂, 0.224 min^-1), respectively. Furthermore, nine degradation pathways of CIP in AO-B-GF/Pt system were summarized as the cleavage of the piperazine ring, cyclopropyl group, quinolone ring and F atom by ·OH. It provides new insights into the removal and degradation pathways of CIP with GF in AO system.

Process optimization and mechanism study of acid red G degradation by electro-Fenton-Feox process as an in situ generation of H2O2

Dye-contaminated wastewaters are industrial wastewaters that are difficult to treat using traditional biochemical and physicochemical methods. In the present work, the acid red G was removed as a model pollutant by the electro-Fenton process for the first time. The anode and cathode used by the electro-Fenton process were iron plate and graphite felt, respectively. It was concluded that under the optimal conditions of current density = 20 mA cm-2, pH = 3 and initial Na2SO4 concentration = 0.2 M, the removal rate of acid red G (ARG) with an initial concentration of 300 mg L-1 could reach 94.05% after 80 min of electrolysis. This reveals that the electro-Fenton-Feox process used in this work has an excellent removal efficiency on acid red G. The required reagents (Fe2+ and H2O2) were generated by the electrode reaction, while the optimal generation conditions and mechanism of •OH, H2O2, and Fe2+ were investigated. By testing •OH, H2O2, and Fe2+ agents at different pH and current densities, it was revealed that the electro-Fenton reaction was most efficient when the current density was 20 mA cm-2, and the pH was 3. Moreover, the removal rate of ARG is consistent with first-order reaction kinetics.

Electrochemical removal of RRX-3B in residual dyeing liquid with typical engineered carbonaceous cathodes

Electro-catalytic activities of carbonaceous cathodes including graphite plate, graphite felt, carbon felt, activated carbon felt (ACF) and carbon fiber felt (CFF) for degradation of Reactive Red X-3B (RRX-3B) in residual dyeing liquid were compared. The best electrochemical performance was obtained using dimensional stable anode (DSA) and CFF cathode due to the higher capacity for electro-generation of H₂O₂ by selective two-electron oxygen reduction. The CFF/DSA electrolysis system realized 78.2% COD removal and complete decolorization over a wide pH range. The efficacy of RRX-3B degradation was found to be dependent on the nature of carbonaceous materials. Electrochemical measurements showed that CFF possessed higher electrochemical surface area and hydrogen evolution reaction over-potential. Furthermore, the intrinsic graphitic N in CFF was proved to be catalytic active site by DFT calculations. Reactive Red X-3B degradation intermediates with benzene structures and carboxylic acids via hydroxylation in RRX-3B oxidation were identified by GC-MS. It was found that S/Cl/N-containing groups in RRX-3B molecule were mineralized to SO₄^2-, NO₃^- and Cl^- ions in the electrolysis.

Tetracycline inhibition and transformation in microbial fuel cell systems: Performance, transformation intermediates, and microbial community structure

Tetracycline (TC) transformation in the anode of an air cathode microbial fuel cell (MFC) and in the cathode of an MFC-Fenton system was investigated. TC at 10 mg/L in the anolyte was removed by 43-74% in 14-d cycles, mainly attributed to adsorption. The electrochemical activity, COD and acetate consumption of the anodic biofilm were inhibited by TC; inhibition was reversed when TC addition was stopped. Over 84 d of MFC operation with TC, Geobacter and Mycobacterium in the anode biofilm decreased, while Janthinobacterium and Comamonas increased. Over 99% of TC at 10-40 mg/L was removed within 8 h in the MFC-Fenton cathode. O₂^-•/HO₂• and •OH were responsible for the cathodic TC degradation. The maximum current was 0.93 mA (at 250 Ω) and increased by 36.3% by the MFC-Fenton reaction. Cathodic MFC-Fenton is an efficient and energy-saving process for TC removal, compared to slow and problematic anodic TC bio-oxidation.

High-efficiency electro-catalytic performance of green dill biochar cathode and its application in electro-Fenton process for the degradation of pollutants

Biochar (BC) is a kind of carbon-rich, renewable and low-cost material, which can be prepared from various organic materials.

Fe3O4-Loaded g-C3N4/C-Layered Composite as a Ternary Photocatalyst for Tetracycline Degradation

A ternary photocatalyst, Fe₃O₄-loaded g-C₃N₄/C-layered composite (g-C₃N₄/C/Fe₃O₄) was fabricated by a facile sonication and in situ precipitation technique. Carbon nanosheets were prepared using the remaining non-metallic components of waste printed circuit boards as carbon sources. In this hybrid structure, g-C₃N₄ was immobilized on the surfaces of carbon nanosheets to form a layered composite, and 10-15 nm Fe₃O₄ nanoparticles are uniformly deposited on the surface of the composite material. The photocatalytic performance of the catalyst was studied by degrading tetracycline (TC) under simulated sunlight. The results showed that the photoactivity of the g-C₃N₄/C/Fe₃O₄ composite to TC was significantly enhanced, and the degradation rate was 10.07 times higher than that of pure g-C₃N₄, which was attributed to Fe₃O₄ nanoparticles and carbon nanosheets. Carbon sheets with good conductivity are an excellent electron transporter, which promotes the separation of photogenerated carriers and the Fe₃O₄ nanoparticles can utilize electrons effectively as a center of oxidation-reduction. Moreover, a possible photocatalytic mechanism for the excellent photocatalytic performance was proposed.

Degradation of sulfathiazole by electro-Fenton using a nitrogen-doped cathode and a BDD anode: Insight into the H2O2 generation and radical oxidation

In this work, nitrogen-doped cathodes for high H₂O₂ production and sulfathiazole (STZ) degradation in electro-Fenton (EF) systems were prepared by the carbonization of three carbon/nitrogen-enriched precursors. Among the cathodes elaborated from different precursors, the one using 1h-1,2,4-triazole-3,5-diamine as the precursor showed the best oxygen reduction reaction (ORR) ability with the normalized H₂O₂ accumulation of 9.49 ± 0.03 mg L^-1 h^-1 cm^-2 compared to the other two N-containing cathodes. The enhanced H₂O₂ accumulation was attributed to the high electroactive surface area and pyrrolic N (60.45%) content. Regarding reactive oxygen species in the absence of Fe²⁺, aside from the H₂O₂, O₂^-and ¹O₂ were identified using spectroscopic techniques and chemical probes. As a result, a degradation and mineralization efficiency of 98.25 ± 0.14% and 70.57 ± 0.27% of STZ were attained in the 180-min treatment, mainly coming from the homogeneous OH from classical Fenton, anodic OH on BDD anode and direct/indirect oxidation of O₂^-and ¹O₂. In addition, the plausible degradation pathway of STZ was proposed based on the density functional theory (DFT) combined with experimental data derived by ultra-performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS). The frontier orbital theory and Fukui function theoretically suggested the vulnerable sites of STZ for different active species including OH, O₂^- and ¹O₂. This study provides a new strategy for improving the ORR process and analyzing the generation and conversion of reactive oxygen species in the EF process.

Removal of tetracycline from an aqueous solution using manganese dioxide modified biochar derived from Chinese herbal medicine residues

Biochar (BC) derived from Chinese herbal medicine residues has been investigated for its performance as a potential adsorbent in tetracycline (TC) removal. In the present study, a chemical co-precipitation method was carried out to prepare manganese dioxide modified biochar (Mn-BC) to increase its sorption capacity. The properties of the modified biochar were characterized for further enhancing TC removal from an aqueous solution. Mn-BC was successfully synthesized and resulted in a much higher specific surface area, total pore volume and pore diameter. The sorption kinetics of TC on Mn-BC was described by the pseudo-second-order model. The sorption data of Mn-BC were fitted by Langmuir and Freundlich models. The study findings revealed a maximum adsorption capacity of Mn-BC (1:10) to TC was up to 131.49 mg/g. The adsorption process was endothermic and spontaneous. The degradation of TC was further enhanced by MnO₂ acting as an oxidizer on Mn-BC. Overall, the modified biochar derived from Chinese herbal medicine residues is a superior alternative for the removal of TC from an aqueous solution.

Kinetics and mechanisms of oxytetracycline degradation in an electro-Fenton system with a modified graphite felt cathode

The removal of trace antibiotics from the aquatic environment has received great interest. In this investigation, NaOH activated graphite felt (NaOH-GF) was characterized by multiple-methods, including scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), contact angle, linear sweep voltammetry (LSV) and electron paramagnetic resonance (EPR). The NaOH-GF was then used as the cathode in the electro-Fenton process for oxytetracycline (OTC) degradation, the experiment was carried out in an undivided and light-proof beaker with a Pt anode and a NaOH-GF cathode at pH 3. The results showed that the modification with NaOH enhanced the antibiotics degradation efficiency of graphite felt by increasing the oxygen reduction capacity and hydroxyl radicals yielding rate. Complete OTC removal was achieved at 5.17 mA cm^-2 after 40, 60 and 90 s with initial OTC concentration of 22, 44, and 66 μM, respectively. With an initial OTC concentration of 44 μM, after 30 min the removal rates of chemical oxygen demand (COD) by Raw-GF and NaOH-GF were 59.18% and 83.75%, respectively. The proposed degradation mechanism of OTC was an EF process, which consisted of hydroxylation, secondary alcohol oxidation, demethylation, decarbonylation, dehydration and deamination. This study demonstrates that NaOH activated GF cathode possesses high degradation capacity and good stability. It provides insight into the removal of non-biodegradable antibiotics and may shed light on future to its practical application.

Removal of Phenazopyridine from wastewater by merging biological and electrochemical methods via Azolla filiculoides and electro-Fenton process

In the present study, the potential of Azolla filiculoides (A. filiculoides) was first investigated for degradation of Phenazopyridine (PhP), an analgesic drug. The effects of main variables such as initial pharmaceutical concentration, amount of plant, and pH were studied on the efficiency of the biological process. It was observed that A. filiculoides was able to remove pharmaceuticals from contaminated water up to 85.90% during 48 h. Then, the electro-Fenton (EF) method was applied for further removal of PhP yielding a removal rate of about 98.72% under optimum conditions during 2 h. The effects of variables including the current, amount of catalyst, and pH were also studied in this phase. Also, the probability of adsorption was investigated during this step. Scanning electron microscopy (SEM) and X-ray diffraction (XRD) analysis were performed for the used magnetite nanoparticles, total organic carbon (TOC) were performed to investigate PhP removal efficiency during the reaction time and Gas chromatography-mass spectrometry (GC-MS) were performed to analyze degradation byproducts of PhP. Based on the results, it was found that a combination of these bioremediation and electrochemical removal steps were capable of PhP removal from contaminated water. Therefore, this approach may be effective for phytoremediation of pharmaceutical-contaminated aquatic ecosystems.

Microbial electro-Fenton: A promising system for antibiotics resistance genes degradation and energy generation

The widespread use of antibiotics has accelerated the development of antibiotic resistance genes (ARGs), which are now recognized as emerging environmental contaminants that pose a high risk to public health. In this study, simultaneous antibiotic and ARGs removal and bioelectricity generation was explored in a microbial electro-Fenton system using erythromycin (ERY) as a model antibiotic compound. The results showed that ERY could be degraded, with an average removal efficiency of 88.73% in 48 h, and the average removal efficiency of chemical oxygen demand in the microbial electro-Fenton with 50 μg L^-1 ERY reached 86.84% in 48 h, which was lower than that in the control group (89.11%). The produced ARGs were analyzed and degraded in a cathode chamber. The quantity of ermB was significantly reduced, with log removal reaching a value of 1.96. More importantly, all erm genes (ermB, ermC, ermG) showed a tendency to be degraded. Furthermore, the maximum power density obtained with respect to the electrode area was 0.193 W m^-2 when ERY was added, corresponding to a current density of 0.583 A m^-2 (external resistor = 1000 Ω), which was 14% larger than that of the control group (0.169 W m^-2). The results of this study demonstrate the potential of microbial electro-Fenton for ERY and ARGs removal.

Highly efficient electro-generation of H2O2by a nitrogen porous carbon modified carbonaceous cathode during the oxygen reduction reaction

In this work, active carbon fibers (ACFs) were modified with nitrogen-doped porous carbon (NPC) and carbon nanotubes (CNTs) to prepare a novel modified electrode as a cathode.

Strong adsorption of tetracycline hydrochloride on magnetic carbon-coated cobalt oxide nanoparticles

The overuse of antibiotics, including tetracycline hydrochloride (TC), seriously threatens human health and ecosystems. In this work, magnetic carbon-coated cobalt oxide nanoparticles (CoO@C) were prepared by one-step annealing method and used as an adsorbent for efficient removal of TC from aqueous solution. The characteristic of the materials was studied by SEM, TEM, and XRD, revealing CoO nanoparticles (≤10 nm) were coated by carbon layer. Several influencial parameters, such as annealing temperature and pH on adsorption of TC, were explored, and found that the maximum adsorption capacity of CoO@C on TC reached as high as 769.43 mg g^-1. Furthermore, CoO@C displayed excellent stability and reusability. After four repeated use of the adsorbent, the adsorption capacity still remained at 90% of the initial capacity. The pseudo-second order model and Temkin model proved that it was an exothermic chemical adsorption process. Furthermore, after analysis of FT-IR, Zeta-potential, XPS, the positive charge on the surface of CoO@C forms a strong electrostatic interaction with TC, and in addition, a surface bond is formed between the adsorbent and the TC molecule. This work provides a novel and efficient adsorbent for the purification of TC-containing wastewater.

Tris-Co(II)-H2O2 System-Mediated Durative Hydroxyl Radical Generation for Efficient Anionic Azo Dye Degradation by Integrating Electrostatic Attraction

The development of simple Fenton/Fenton-like systems with durative hydroxyl radical (^•OH) generation characteristics is significant to rapid organic pollutant degradation and cost-effective water treatment. In this study, a tris(hydroxymethyl)aminomethane (Tris)-incorporated Co(II)-H₂O₂ Fenton-like system has been successfully constructed for efficient Sunset Yellow (SY, a typical anionic azo dye) degradation under alkaline conditions. The mechanism of the enhanced degradation consists of two parts: first, the Tris-Co(II) complex triggers the durative generation of highly oxidized hydroxyl radicals; second, electrostatic attraction between SY and the Tris-Co(II) complex shortens the radical-SY interaction time and facilitates the degradation of SY. With the introduction of Tris to this proposed system, the decolorization rate of SY can be increased from 37.0 to 98.0% after 50 min and efficient SY degradation with a high total organic carbon removal efficiency (>59.0%) is achieved under a wide initial pH from 8.7 to 12.0. Moreover, the universality of the designed system for anionic azo dye degradation is verified with reactive red and congo red.

Modifying graphite felt cathode by HNO3 or KOH to improve the degradation efficiency of electro-Fenton for landfill leachate

Based on graphite felt (GF), the cathode of an electro-Fenton (EF) system was modified by HNO₃ and KOH respectively to improve the degradation efficiency for actual landfill leachate. The results of Fourier transform infrared spectroscopy (FTIR) spectra, Boehm titration experiments, contact angle, scanning electron microscopy (SEM) and adsorption experiments illustrated that the surface of the modified GFs had more oxygen-containing functional (OG) groups, and possessed better hydrophilicity and larger specific surface area. In 180 min H₂O₂ electrogeneration experiments, the cumulative amount of H₂O₂ produced by unmodified GF (GF-0), HNO₃ modified GF (GF-1) and KOH modified GF (GF-2) was 526 mg/L, 891 mg/L and 823 mg/L respectively. In 180 min EF reaction, the removal rate of chemical oxygen demand (COD) in GF-0, GF-1 and GF-2 EF systems was 31.88%, 60.65% and 52.08% respectively; the removal rate of NH₄ ⁺-N in GF-0, GF-1 and GF-2 EF systems was 43.37%, 98.10% and 94.81% respectively. In addition, both the performance of GF-1 and GF-2 for Fe²⁺ regeneration was greatly enhanced, and GF-1 was superior to GF-2. The degradation efficiency for landfill leachate was enhanced obviously by employing the modified EF system, suggesting that the two modified cathodes have great potential in practical production.