Advances in the Electro-Fenton Process for Remediation of Recalcitrant Organic Compounds

Bibliometric analysis and systematic review of electrochemical methods for environmental remediation

Electrochemical methods are increasingly favored for remediating polluted environments due to their environmental compatibility and reagent-saving features. However, a comprehensive understanding of recent progress, mechanisms, and trends in these methods is currently lacking. Web of Science (WoS) databases were utilized for searching the primary data to understand the knowledge structure and research trends of publications on electrochemical methods and to unveil certain hotspots and future trends of electrochemical methods research. The original data were sampled from 9080 publications in those databases with the search deadline of June 1st, 2022. CiteSpace and VOSviewer software facilitated data visualization and analysis of document quantities, source journals, institutions, authors, and keywords. We discussed principles, influencing factors, and progress related to seven major electrochemical methods. Notably, publications on this subject have experienced significant growth since 2007. The most frequently-investigated areas in electrochemical methods included novel materials development, heavy metal remediation, organic pollutant degradation, and removal mechanism identification. "Advanced oxidation process" and "Nanocomposite" are currently trending topics. The major remediation mechanisms are adsorption, oxidation, and reduction. The efficiency of electrochemical systems is influenced by material properties, system configuration, electron transfer efficiency, and power density. Electro-Fenton exhibits significant advantages in achieving synergistic effects of anodic oxidation and electro-adsorption among the seven techniques. Future research should prioritize the improvement of electron transfer efficiency, the optimization of electrode materials, the exploration of emerging technology coupling, and the reduction in system operation and maintenance costs.

Activated carbon fiber as an efficient co-catalyst toward accelerating Fe2+/Fe3+ cycling for improved removal of antibiotic cefaclor via electro-Fenton process using a gas diffusion electrode

The electro-Fenton (EF) based on gas-diffusion electrodes (GDEs) reveals promising application prospective towards recalcitrant organics degradation because such GDEs often yields superior H2O2 generation efficiency and selectivity. However, the low efficiency of Fe2+/Fe3+ cycle with GDEs is always considered to be the limiting step for the EF process. In this study, activated carbon fiber (ACF) was firstly employed as co-catalyst to facilitate the performance of antibiotic cefaclor (CEC) decomposition in EF process. It was found that the addition of ACF co-catalyst achieved a rapid Fe2+/Fe3+ cycling, which significantly enhanced Fenton's reaction and hydroxyl radicals (•OH) generation. X-ray photoelectron spectroscopy (XPS) results indicated that the functional groups on ACF surface are related to the conversion of Fe3+ into Fe2+. Moreover, DMSO probing experiment confirmed the enhanced •OH production in EF + ACF system compared to conventional EF system. When inactive BDD and Ti4O7/Ti anodes were paired to EF system, the addition of ACF could significantly improve mineralization degree. However, a large amount of toxic byproducts, including chlorate (ClO3-) and perchlorate (ClO4-), were generated in these EF processes, especially for BDD anode, due to their robust oxidation capacity. Higher mineralization efficiency and less toxic ClO4- generation were obtained in the EF + ACF process with Ti4O7/Ti anode. This presents a novel alternative for efficient chloride-containing organic removal during wastewater remediation.

Preparation of Ti3C2Tx modified rare earth doped PbO2 electrodes for efficient removal of sulfamethoxazole

In this study, we deposited Ti3C2Tx-modified, rare-earth-doped PbO2 on the surface of a carbon fabric via electrodeposition. The surface morphology and electronic structure of the electrode were characterized with SEM, XRD and XPS. The layered Ti3C2Tx did not change the structure of β-PbO2, and at the same time, it improved the crystallinity of the material and reduced the grains of PbO2. Electrochemical experiments showed that the addition of Ti3C2Tx increased the electrochemical activity of the electrode and produced more H2O2, which contributed to the degradation of pollutants. The efficiency of sulfamethoxazole (SMX) degradation reached 95% after 120 min at pH 3 with a current density of 50 mA/cm2. Moreover, the electrode has good cycling performance, and the degradation efficiency was still 80% after 120 min after 10 cycles of recycling. Based on the intermediates identified by HPLC‒MS, a mechanism for SMX degradation was proposed. Our results will provide a new idea for the development of efficient electrocatalytic degradation of antibiotics.

Study on diazinon toxicity reduction by electro-Fenton process: A bioassay using daphnia magna

The realm of diazinon reduction from polluted water has witnessed a surge in the significance of advanced oxidation processes (AOPs) in recent times. However, there is a dearth of research focusing on the mitigation of its toxicity through AOPs. Thus, the primary objective of this study was to evaluate the effectiveness of the Electro-Fenton process (EFP) in the eradication and detoxification of diazinon in aqueous solutions. Synthetic wastewater samples with concentrations of 2, 2.5 and 3 mg/L were prepared. A total of 27 samples were determined using Box Behnken Design. Reaction time, pH and iron to hydrogen peroxide ratio (Fe2+/H2O2) were examined as operational parameters under a constant current of 5.4 amps. The quantification of diazinon concentration was performed using High-Performance Liquid Chromatography (HPLC). To evaluate the detoxification of diazinon, the Daphnia magna bioassay was employed as a methodology in this study. According to the results, the EFP could reduce the diazinon to zero and the LC50 values are increased by applying the process. The LC50 values for diazinon were determined using the Daphnia magna bioassay, considering initial concentrations of 2, 2.5, and 3 mg/L at a pH of 5, a reaction time of 15 min, and an iron to hydrogen peroxide molar ratio of 2. The recorded LC50 values were 3.039, 3.076, and 3.106, respectively, indicating the lowest frequency of cumulative death in Daphnia magna. In this case, after 96 h, only 3 cases (30%) of Daphnia magna death were observed. However, for all the mentioned concentrations of diazinon, after 96 h of exposure to samples without applying the Daphnia Magna death process, it was observed between 60 and 100%. Reducing the diazinon concentration and increasing the 96-h LC50 showed that the EFP can reduce the toxicity of diazinon on Daphnia Magna at the same time. Therefore, EFP can be considered a superior method with low ecotoxicity.

Multivariate optimization of methylene blue dye degradation using electro-Fenton process with self-doped TiO2 nanotube anode

This paper reports the optimization of the electro-Fenton (EF) process using different anode materials for the degradation of Methylene Blue (MB) dye as a model compound. The cathode used was an air-diffusion PTFE, while three different anode materials (Pt, DSA, and self-doped TiO2 nanotubes - SD-TNT) were tested individually. A full factorial design (FFD) with a central point combined with response surface methodology (RSM) was employed to optimize the experimental variables, including solution pH, applied current, and anode material. The optimized EF conditions involved a pH of 4.0, a current of 100 mA, and an SD-TNT anode for 120 min of electrolysis. Under these conditions, the MB solution achieved complete decolorization and 45% of total organic carbon (TOC) removal after 120 min of EF treatment. The findings indicate that the hydroxyl radical (•OH) plays a crucial role as the primary oxidizing agent in the EF process. The decay of MB followed pseudo-first-order kinetics, reflecting a consistent formation of •OH radicals that effectively attacked the MB dye and its subproducts during mineralization. Moreover, the EF process exhibited superior performance in terms of energy consumption (EC) and mineralization current efficiency (ECM) in the initial treatment stages, while the presence of recalcitrant by-products and loss of anode self-doping impacted performance in the later stages. The optimized EF conditions and the understanding gained from this study contribute to the advancement of sustainable wastewater treatment strategies for the removal of organic dyes.

Carbon Gels-Green Graphene Composites as Metal-Free Bifunctional Electro-Fenton Catalysts

The Electro-Fenton (EF) process has emerged as a promising technology for pollutant removal. However, the EF process requires the use of two catalysts: one acting as an electrocatalyst for the reduction of oxygen to H2O2 and another Fenton-type catalyst for the generation of ·OH radicals from H2O2. Thus, the search for materials with bifunctionality for both processes is required for a practical and real application of the EF process. Thus, in this work, bifunctional electrocatalysts were obtained via doping carbon microspheres with Eco-graphene, a form of graphene produced using eco-friendly methods. The incorporation of Eco-graphene offers numerous advantages to the catalysts, including enhanced conductivity, leading to more efficient electron transfer during the Electro-Fenton process. Additionally, the synthesis induced structural defects that serve as active sites, promoting the direct production of hydroxyl radicals via a 3-electron pathway. Furthermore, the spherical morphology of carbon xerogels enhances the accessibility of the reagents to the active sites. This combination of factors results in the effective degradation of Tetracycline (TTC) using metal-free catalysts in the Electro-Fenton process, achieving up to an impressive 83% degradation without requiring any other external or additional catalyst.

Degradation of Rhodamine B in Wastewater by Iron-Loaded Attapulgite Particle Heterogeneous Fenton Catalyst

The water pollution caused by industry emissions makes effluent treatment a serious matter that needs to be settled. Heterogeneous Fenton oxidation has been recognized as an effective means to degrade pollutants in water. Attapulgite can be used as a catalyst carrier because of its distinctive spatial crystal structure and surface ion exchange. In this study, iron ions were transported on attapulgite particles to generate an iron-supporting attapulgite particles catalyst. BET, EDS, SEM and XRD characterized the catalysts. The particle was used as a heterogeneous catalyst to degrade rhodamine B (RhB) dye in wastewater. The effects of H2O2 concentration, initial pH value, catalyst dosage and temperature on the degradation of dyes were studied. The results showed that the decolorization efficiency was consistently maintained after consecutive use of a granular catalyst five times, and the removal rate was more than 98%. The degradation and mineralization effect of cationic dyes by granular catalyst was better than that of anionic dyes. Hydroxyl radicals play a dominant role in RhB catalytic degradation. The dynamic change and mechanism of granular catalysts in catalytic degradation of RhB were analyzed. In this study, the application range of attapulgite was widened. The prepared granular catalyst was cheap, stable and efficient, and could be used to treat refractory organic wastewater.

Two-Dimensional Nanostructures in the World of Advanced Oxidation Processes

Two-dimensional compounds with nanostructural features are attracting attention from researchers worldwide. Their multitude of applications in various fields and vast potential for future technology advancements are successively increasing the research progress. Wastewater treatment and preventing dangerous substances from entering the environment have become important aspects due to the increasing environmental awareness, and increasing consumer demands have resulted in the appearance of new, often nonbiodegradable compounds. In this review, we focus on using the most promising 2D materials, such as MXenes, Bi2WO6, and MOFs, as catalysts in the modification of the Fenton process to degrade nonbiodegradable compounds. We analyze the efficiency of the process, its toxicity, previous environmental applications, and the stability and reusability of the catalyst. We also discuss the catalyst’s mechanisms of action. Collectively, this work provides insight into the possibility of implementing 2D material-based catalysts for industrial and urban wastewater treatment.

Mechanism of bio-electrokinetic remediation of pyrene contaminated soil: Effects of an electric field on the degradation pathway and microbial metabolic processes

In this study, the mechanism of bio-electrokinetic (BIO-EK) remediation to improve the degradation of pyrene was evaluated based on an analysis of the intermediate products and the microbial community. The results show that BIO-EK remediation has a higher pyrene degradation efficiency on pyrene and its intermediate products than the bioremediation and electrokinetic (EK) remediation processes. A series of intermediate products were detected. According to the type of the intermediate products, two degradation pathways, biological metabolism and electrochemical oxidation, are proposed in the BIO-EK remediation of pyrene. Furthermore, the primary microbial taxa involved in the pollutant degradation changed, which led to variations in the functional gene components. The abundant and functional genes related to metabolism were specifically analyzed. The results indicate that the electric field promotes the expression of metabolisms associated with 14 carbohydrates, 13 lipids, 13 amino acids, five energies, and in particular, 11 xenobiotics. These results suggest that in addition to the promotion effect on the microbial metabolism caused by the electric field, BIO-EK remediation can promote the degradation of pollutants due to the coexistence of a microbial metabolic pathway and an electrochemical oxidation pathway.

Heterogeneous Electro-Fenton-like Designs for the Disposal of 2-Phenylphenol from Water

The hunt for efficient and environmentally friendly degradation processes has positioned the heterogeneous advanced oxidation processes as an alternative more interesting and economical rather than homogenous processes. Hence, the current study lies in investigating the efficiency of different heterogeneous catalysts using transition metals in order to prevent the generation of iron sludge and to extend the catalogue of possible catalysts to be used in advanced oxidation processes. In this study, nickel and zinc were tested and the ability for radical-generation degradation capacity of both ions as homogeneous was evaluated in the electro-Fenton-like degradation of 2-phenylphenol. In both cases, the degradation profiles followed a first-order kinetic model with the highest degradation rate for nickel (1 mM) with 2-phenylphenol removal level of 90.12% and a total organic reduction near 70% in 2 h. To synthesise the heterogeneous nickel catalyst, this transition metal was fixed on perlite by hydrothermal treatment and in a biochar or carbon nanofibers by adsorption. From the removal results using the three synthesized catalysts, it is concluded that the best catalysts were obtained by inclusion of nickel on biochar or nanofibers achieving in both with removal around 80% before 1 h. Thus, to synthetize a nickel electrocatalyst, nickel doped nanofibers were included on carbon felt. To do this, the amount of carbon black, nickel nanofibers and polytetrafluoroethylene to add on the carbon felt was optimized by Taguchi design. The obtained results revealed that under the optimised conditions, a near-complete removal was achieved after 2 h with high stability of the nickel electrocatalyst that open the applicability of this heterogeneous system to operate in flow systems.

Heterogeneous electro-Fenton catalysis with self-supporting CFP@MnO2-Fe3O4/C cathode for shale gas fracturing flowback wastewater

Self-supporting electrodes have triggered great interests in improving electro-Fenton (EF) system for degradation of refractory organic pollutants. In this work, a novel self-supporting carbon fiber paper (CFP) electrode modified by transition metals, e.g. Fe and Mn, was fabricated and employed as a heterogeneous EF cathode. The prepared electrode exhibited excellent degradation for a number of typical organic pollutants along with superior stability. Remarkably, a high removal efficiency was achieved in the EF treatment of shale gas fracturing flowback wastewater. Results indicated that 65.2% TOC and 74.8% COD were eliminated after 4 h degradation. The residual COD value of the real wastewater was 80 mg L^-1, meeting the emission requirement of the integrated wastewater discharge standard (COD＜100 mg L^-1) with a low specific energy consumption of 6.9kWhkg^-1COD^-1. This work demonstrates a competing alternative for efficient decontamination of real wastewater using an electro-Fenton strategy with a low-cost electrode.

A charcoal-shaped catalyst NiFe2O4/Fe2O3 in electro-Fenton: high activity, wide pH range and catalytic mechanism

A charcoal-shaped catalyst NiFe₂O₄/Fe₂O₃ in electro-Fenton (EF) was synthesized by a facile precipitation approach via sintering products of oxalate co-precipitation. This obtained NiFe₂O₄/Fe₂O₃ catalyst was easily separated via an external magnetic field and was used as a heterogeneous electro-Fenton catalyst for rhodamine B (RhB, a target pollutant) degradation. Characteristics of NiFe₂O₄/Fe₂O₃ catalyst were assessed using scanning electron microscope (SEM), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS) and Barrett-Emmett-Teller (BET), respectively. SEM results revealed that the proposed NiFe₂O₄/Fe₂O₃ was charcoal-shaped with the size in the range of 0.5-5 μm. Experiment results show that the EF process with the proposed catalyst could work in a wide pH range from 3 to 9. Under optimized conditions, estimated 90% RhB degradation was achieved in 60 min under the following conditions: 0.6 g/L NiFe₂O₄/Fe₂O₃, pH 3. Radical scavengers and electron spin resonance (ESR) spectra results demonstrated that the main oxidant species involved was ⋅OH, accounting for RhB degradation in EF. Moreover, according to our research on interfacial reaction, ⋅OH was mainly generated from the homogenous Fenton reaction rather than the surface Fenton reaction, stimulating by the dissolved Fe²⁺, Fe³⁺ and Ni²⁺ from catalyst. The reusability of NiFe₂O₄/Fe₂O₃ catalyst was evaluated for recycling the same catalyst for 5 runs. In conclusion, the facile fabrication NiFe₂O₄/Fe₂O₃ catalyst shows great potential in wastewater treatment with promising activity.

Carbonaceous cathode materials for electro-Fenton technology: Mechanism, kinetics, recent advances, opportunities and challenges

Electro-Fenton (EF) technique has gained significant attention in recent years owing to its high efficiency and environmental compatibility for the degradation of organic pollutants and contaminants of emerging concern (CECs). The efficiency of an EF reaction relies primarily on the formation of hydrogen peroxide (H₂O₂) via 2e^─ oxygen reduction reaction (ORR) and the generation of hydroxyl radicals (^●OH). This could be achieved through an efficient cathode material which operates over a wide pH range (pH 3-9). Herein, the current progresses on the advancements of carbonaceous cathode materials for EF reactions are comprehensively reviewed. The insights of various materials such as, activated carbon fibres (ACFs), carbon/graphite felt (CF/GF), carbon nanotubes (CNTs), graphene, carbon aerogels (CAs), ordered mesoporous carbon (OMCs), etc. are discussed inclusively. Transition metals and hetero atoms were used as dopants to enhance the efficiency of homogeneous and heterogeneous EF reactions. Iron-functionalized cathodes widened the working pH window (pH 1-9) and limited the energy consumption. The mechanism, reactor configuration, and kinetic models, are explained. Techno economic analysis of the EF reaction revealed that the anode and the raw materials contributed significantly to the overall cost. It is concluded that most reactions follow pseudo-first order kinetics and rotating cathodes provide the best H₂O₂ production efficiency in lab scale. The challenges, future prospects and commercialization of EF reaction for wastewater treatment are also discussed.

Reuse of PANI wastewater treated by anodic oxidation/electro-Fenton for the preparation of PANI

The present work investigated the treatment of the polyaniline (PANI) wastewater by anodic oxidation/electro-Fenton and reusing the treated PANI wastewater for the preparation of PANI. Organics were degraded by hydroxyl radical (OH) and sulfate radical (SO₄^-) formed simultaneously in the wastewater from electro-Fenton reaction and the anode surface. Under the conditions of 160 mL min^-1 oxygen flow rate, constant current density 14 mA cm^-2 and Fe²⁺ concentration 0.2 mM, 89% COD can be removed from the PANI wastewater after 360 min treatment. The energy consumption was 50 kWh (kg COD)^-1 and the current efficiency was 27.8%. After the PANI wastewater treatment, the aniline, aniline derivatives and aniline oligomers were removed from the wastewater. The PANI obtained using fresh solution, treated PANI wastewater and untreated PANI wastewater were recorded PANI-F, PANI-T and PANI-U, respectively. In the preparation of PANI-U, the reaction of p-benzoquinone with aniline or aniline oligomers could change the nucleation and growth, leading to the production of aggregated nanoparticles and low specific capacitance. However, the morphology and specific capacitance of PANI-T were similar to that of PANI-F. The PANI-T is three-dimensional sphere of nanofibers with high specific capacitance about 446.7 F g^-1. The yield of PANI-T using treated PANI wastewater could reach to 92.1%. These results demonstrate that the PANI wastewater treatment approach is efficient and environmentally friendly.

A highly efficient cathode based on modified graphite felt for aniline degradation by electro-Fenton

The graphite felt (GF) was modified through the concentrated sulfuric acid, potassium permanganate and ammonia activation with the aim of improving the specific surface area, hydrophilicity and the electrocatalytic activity for the electrogeneration of hydrogen peroxide and its application in the electro-Fenton (EF). The modified graphite felt (M-GF) was characterized by contact angle, scanning electron microscope, X-ray photoelectron spectroscopy, Raman spectrum, cyclic voltammetry and electrochemical impedance spectroscopy. The surface physic-chemical characteristics and morphology of the M-GF were considerably changed after the modification. The current efficiency for H₂O₂ generation was explored at an optimized current density of 10 mA cm^-2 and the value of H₂O₂ generation for the M-GF was 478.6 mg L^-1, while the GF was 276.5 mg L^-1 after 360 min of electrolysis. Finally, the M-GF was used for the mineralization of aniline (100 mg L^-1), reaching a mineralization percentage of 97% with EF process. The M-GF demonstrated good stability as the aniline mineralization percentage was above 90% after 10 cycles' degradation, indicating that the M-GF is a promising cathode material for improving the degradation and mineralization of others organic pollutants by the EF process.

Pharmaceutical wastewater treatment using UV-enhanced electro-Fenton process: Comparative study

In this research, treatment of a pharmaceutical wastewater (PhW) obtained from a factory by electro-Fenton (EF) and photoelectro-Fenton (PEF) processes was investigated. The effects of several parameters involving pH, current density, H₂ O₂ /Fe²⁺ molar ratio, volume ratio of H₂ O₂ /PhW, UVA light, and time were studied. The experiments were designed by Design Expert software, and response surface methodology (RSM) was applied to determine the optimum conditions for the highest COD removal. According to the analysis of variance (ANOVA), time was the most significant parameter on the process response (COD removal) followed by current density. The optimal conditions for 86.85% of COD removal through the EF process were at pH of 2.96, current density of 42.90 mA/cm² , H₂ O₂ /Fe²⁺ molar ratio of 3.78, volume ratio of H₂ O₂ /PhW of 1.37 ml/L, and reaction time of 58.49 min, while the optimal conditions for 93.00% of COD removal through PEF process were at pH of 2.91, current density of 43.71 mA/cm² , H₂ O₂ /Fe²⁺ molar ratio of 4.29, volume ratio of H₂ O₂ /PhW of 1.67 ml/L, UVA light of 6 W, and reaction time of 54.24 min. It was concluded that UVA light can increase the COD removal through PEF process around 7% more than that of the EF process at optimum conditions. PRACTITIONER POINTS: Treatment of a pharmaceutical wastewater by EF and PEF processes was investigated. Effects of several parameters were entirely studied on both the processes. RSM was applied to determine optimum conditions for the highest COD removal for both the processes. UVA light increased COD removal through PEF process (around 7%) at the optimum conditions.

Recent electrochemical methods in electrochemical degradation of halogenated organics: a review

Halogenated organics are widely used in modern industry, agriculture, and medicine, and their large-scale emissions have led to soil and water pollution. Electrochemical methods are attractive and promising techniques for wastewater treatment and have been developed for degradation of halogenated organic pollutants under mild conditions. Electrochemical techniques are classified according to main reaction pathways: (i) electrochemical reduction, in which cleavage of C-X (X = F, Cl, Br, I) bonds to release halide ions and produce non-halogenated and non-toxic organics and (ii) electrochemical oxidation, in which halogenated organics are degraded by electrogenerated oxidants. The electrode material is crucial to the degradation efficiency of an electrochemical process. Much research has therefore been devoted to developing appropriate electrode materials for practical applications. This paper reviews recent developments in electrode materials for electrochemical degradation of halogenated organics. And at the end of this paper, the characteristics of new combination methods, such as photocatalysis, nanofiltration, and the use of biochemical method, are discussed.

Fungi-Based Microbial Fuel Cells

Fungi are among the microorganisms able to generate electricity as a result of their metabolic processes. Throughout the last several years, a large number of papers on various microorganisms for current production in microbial fuel cells (MFCs) have been published; however, fungi still lack sufficient evaluation in this regard. In this review, we focus on fungi, paying special attention to their potential applicability to MFCs. Fungi used as anodic or cathodic catalysts, in different reactor configurations, with or without the addition of an exogenous mediator, are described. Contrary to bacteria, in which the mechanism of electron transfer is pretty well known, the mechanism of electron transfer in fungi-based MFCs has not been studied intensively. Thus, here we describe the main findings, which can be used as the starting point for future investigations. We show that fungi have the potential to act as electrogens or cathode catalysts, but MFCs based on bacteria–fungus interactions are especially interesting. The review presents the current state-of-the-art in the field of MFC systems exploiting fungi.

Electro-Fenton process for implementation of acid black liquor waste treatment

In this work, an eco-friendly solution for the remediation of wastewater generated in the lignin recovery process from eco-industrial paper mill has been proposed in their way towards a more circular economy strategy. Thus, the application of the electro-Fenton process for the degradation of the non-recovered lignin and other organic compounds form a scarcely studied acid black liquor waste (ABLW) was successfully performed. This treatment was able to operate in a range of COD loads (0.5-19.5mgO₂·L^-1) showing high degradation values of the ABLW determined by the abatement of color, total phenolic content and COD. Then, the optimization of the working conditions for the design of a sustainable treatment system with optimum efficiency was carried out using a response surface methodology. The experiment carried out in the calculated optimal conditions for the electro-Fenton degradation process (current intensity 132.5mA, catalyst dosage of 0.10mM, and temperature 40°C) showed a COD removal of 74.82% and current efficiency 77.79%, close to the theoretical value predicted by the model 73.12% and 77.06%, respectively. In addition, the identification of the final products permitted to confirm the mineralization efficiency.

Current advances and trends in electro-Fenton process using heterogeneous catalysts - A review

Over the last decades, advanced oxidation processes have often been used alone, or combined with other techniques, for remediation of ground and surface water pollutants. The application of heterogeneous catalysis to electrochemical advanced oxidation processes is especially useful due to its efficiency and environmental safety. Among those processes, electro-Fenton stands out as the one in which heterogeneous catalysis has been broadly applied. Thus, this review has introduced an up-to-date collation of the current knowledge of the heterogeneous electro-Fenton process, highlighting recent advances in the use of different catalysts such as iron minerals (pyrite, magnetite or goethite), prepared catalysts by the load of metals in inorganic and organic materials, nanoparticles, and the inclusion of catalysts on the cathode. The effects of physical-chemical parameters as well as the mechanisms involved are critically assessed. Finally, although the utilization of this process to remediation of wastewater overwhelmingly outnumber other utilities, several applications have been described in the context of regeneration of adsorbent or the remediation of soils as clear examples of the feasibility of the electro-Fenton process to solve different environmental problems.

Novel bio-electro-Fenton technology for azo dye wastewater treatment using microbial reverse-electrodialysis electrolysis cell

Development of sustainable technologies for treatment of azo dyes containing wastewaters has long been of great interest. In this study, we proposed an innovative concept of using microbial reverse-electrodialysis electrolysis cell (MREC) based Fenton process to treat azo dye wastewater. In such MREC-Fenton integrated process, the production of H₂O₂ which is the key reactant of fenton-reaction was driven by the electrons harvested from the exoelectrogens and salinity-gradient between sea water and fresh water in MREC. Complete decolorization and mineralization of 400mgL^-1 Orange G was achieved with apparent first order rate constants of 1.15±0.06 and 0.26±0.03h^-1, respectively. Furthermore, the initial concentration of orange G, initial solution pH, catholyte concentration, high and low concentration salt water flow rate and air flow rate were all found to significantly affect the dye degradation. This study provides an efficient and cost-effective system for the degradation of non-biodegradable pollutants.

Degradation of thiamethoxam by the synergetic effect between anodic oxidation and Fenton reactions

In this work, a comparative study using anodic oxidation, Fenton and electro-Fenton treatments was performed in order to determine the synergic effect for the removal of thiamethoxan. The results determined that electro-Fenton process showed high efficiency in comparison with Fenton or anodic oxidation. After that, this hybrid process was optimized and the influence of iron catalyst concentration and applied current intensity on the degradation and mineralization were evaluated. Degradation profiles were monitored by high performance liquid chromatography (HPLC) being satisfactorily described by pseudo-first order kinetic model. At the optimal experimental conditions (300mA and 0.2mM Fe(+2)), the complete degradation of thiamethoxam was achieved after 10min. On the other hand, mineralization of thiamethoxam was monitored by total organic carbon (TOC) decay reaching more than 92% of TOC removal after 8h. Furthermore, a plausible mineralization pathway for the thiamethoxam degradation was proposed based on the identification of by-products such as aromatic intermediates, carboxylic acids and inorganic ions released throughout electro-Fenton process.

In-situ generated H₂O₂ induced efficient visible light photo-electrochemical catalytic oxidation of PCP-Na with TiO₂

In this study, we developed a novel photo-electrochemical catalytic oxidation wastewater treatment system by interacting the cathodic in-situ generated H₂O₂ with TiO₂ suspension to form interfacial ≡ Ti(IV)OOH species, which endowed the PEC system with superior efficiency for degrading sodium pentachlorophenate (PCP-Na) under visible light irradiation at neutral pH. The apparent PCP-Na degradation rate constant of the PEC system was more than 10 times that of the electrochemical oxidation counterpart. In the PEC system, the interfacial ≡ Ti(IV)OOH species injected electrons to the conduction band of TiO₂ to initiate the activation of O₂ and the in-situ generated H₂O₂ adsorbed on the surface of TiO₂, lead to producing reactive oxygen species of superoxide anions and hydroxyl radicals, which were responsible for the dechlorination and mineralization of PCP-Na during the PEC process, respectively. The dosage of TiO₂ catalyst and the current intensity applied on PCP-Na degradation were optimized. This study develops a high efficient PEC oxidation system for wastewater treatment and provides new insight into the role of cathodic in-situ generated H₂O₂ on PEC oxidation of PCP-Na with TiO₂ under visible light irradiation.

Biodegradability and toxicity assessment of a real textile wastewater effluent treated by an optimized electrocoagulation process

In this work, the application of an iron electrode-based electrocoagulation (EC) process on the treatment of a real textile wastewater (RTW) was investigated. In order to perform an efficient integration of the EC process with a biological oxidation one, an enhancement in the biodegradability and low toxicity of final compounds was sought. Optimal values of EC reactor operation parameters (pH, current density and electrolysis time) were achieved by applying a full factorial 3(3) experimental design. Biodegradability and toxicity assays were performed on treated RTW samples obtained at the optimal values of: pH of the solution (7.0), current density (142.9 A m(-2)) and different electrolysis times. As response variables for the biodegradability and toxicity assessment, the Zahn-Wellens test (Dt), the ratio values of dissolved organic carbon (DOC) relative to low-molecular-weight carboxylates anions (LMCA) and lethal concentration 50 (LC50) were used. According to the Dt, the DOC/LMCA ratio and LC50, an electrolysis time of 15 min along with the optimal values of pH and current density were suggested as suitable for a next stage of treatment based on a biological oxidation process.

Critical review of electrochemical advanced oxidation processes for water treatment applications

Electrochemical advanced oxidation processes (EAOPs) have emerged as novel water treatment technologies for the elimination of a broad-range of organic contaminants. Considerable validation of this technology has been performed at both the bench-scale and pilot-scale, which has been facilitated by the development of stable electrode materials that efficiently generate high yields of hydroxyl radicals (OH˙) (e.g., boron-doped diamond (BDD), doped-SnO2, PbO2, and substoichiometic- and doped-TiO2). Although a promising new technology, the mechanisms involved in the oxidation of organic compounds during EAOPs and the corresponding environmental impacts of their use have not been fully addressed. In order to unify the state of knowledge, identify research gaps, and stimulate new research in these areas, this review critically analyses published research pertaining to EAOPs. Specific topics covered in this review include (1) EAOP electrode types, (2) oxidation pathways of select classes of contaminants, (3) rate limitations in applied settings, and (4) long-term sustainability. Key challenges facing EAOP technologies are related to toxic byproduct formation (e.g., ClO4(-) and halogenated organic compounds) and low electro-active surface areas. These challenges must be addressed in future research in order for EAOPs to realize their full potential for water treatment.

Electrochemical advanced oxidation and biological processes for wastewater treatment: a review of the combined approaches

As pollution becomes one of the biggest environmental challenges of the twenty-first century, pollution of water threatens the very existence of humanity, making immediate action a priority. The most persistent and hazardous pollutants come from industrial and agricultural activities; therefore, effective treatment of this wastewater prior to discharge into the natural environment is the solution. Advanced oxidation processes (AOPs) have caused increased interest due to their ability to degrade hazardous substances in contrast to other methods, which mainly only transfer pollution from wastewater to sludge, a membrane filter, or an adsorbent. Among a great variety of different AOPs, a group of electrochemical advanced oxidation processes (EAOPs), including electro-Fenton, is emerging as an environmental-friendly and effective treatment process for the destruction of persistent hazardous contaminants. The only concern that slows down a large-scale implementation is energy consumption and related investment and operational costs. A combination of EAOPs with biological treatment is an interesting solution. In such a synergetic way, removal efficiency is maximized, while minimizing operational costs. The goal of this review is to present cutting-edge research for treatment of three common and problematic pollutants and effluents: dyes and textile wastewater, olive processing wastewater, and pharmaceuticals and hospital wastewater. Each of these types is regarded in terms of recent scientific research on individual electrochemical, individual biological and a combined synergetic treatment.

Cr(VI) and azo dye removal using a hollow-fibre membrane system functionalized with a biogenic Pd-magnetite catalyst

This study investigates the application of a hybrid system combining hollow-fibre membrane technology with the reductive abilities of magnetic nanoparticles for the remediation of toxic Cr(VI) and the azo dye, Remazol Black B. Nano-scale biogenic magnetite (Fe3O4), formed by microbial reduction of the mineral ferrihydrite, has a high reductive capacity due to the presence of Fe(II) in the mineral structure. The magnetic nanoparticles (approximately 20 nm) can be arrayed with Pd0 nanoparticles (approximately 5 nm) making a catalytically active nanomaterial. Membrane units, with and without nanoparticles, were challenged with either Cr(VI) or azo dye and some were supplemented with sodium formate, as an electron donor for contaminant reduction promoted by the Pd. The combination of Pd-magnetite with formate resulted in the most effective remediation strategy for both contaminants and the lifetime of the membrane unit was also increased, with 55% (19 days) and 70% (23 days) removal of the azo dye and Cr(VI), respectively. Low flow rates of 0.1 ml/min resulted in improved efficiencies due to increased contact time with the membrane/nanoparticle unit, with 70-75% removal of each contaminant. Chemical analyses of the nanoparticles post-exposure to Cr(VI) in the membrane modules indicated Pd to be more oxidized when Cr removal was maximized, and that the Cr was partially reduced to Cr(III) at the surface of the magnetite. These results have demonstrated that hollow-fibre membrane units can be enhanced for the removal of soluble, redox sensitive contaminants by incorporation of a layer of palladized biogenic nanoparticulate magnetite.

Application of electro-Fenton technology to remediation of polluted effluents by self-sustaining process

The applicability of electro-Fenton technology to remediation of wastewater contaminated by several organic pollutants such as dyes and polycyclic aromatic hydrocarbons has been evaluated using iron-enriched zeolite as heterogeneous catalyst. The electro-Fenton technology is an advanced oxidation process that is efficient for the degradation of organic pollutants, but it suffers from the high operating costs due to the need for power investment. For this reason, in this study microbial fuel cells (MFCs) were designed in order to supply electricity to electro-Fenton processes and to achieve high treatment efficiency at low cost. Initially, the effect of key parameters on the MFC power generation was evaluated. Afterwards, the degradation of Reactive Black 5 dye and phenanthrene was evaluated in an electro-Fenton reactor, containing iron-enriched zeolite as catalyst, using the electricity supplied by the MFC. Near complete dye decolourization and 78% of phenanthrene degradation were reached after 90 min and 30 h, respectively. Furthermore, preliminary reusability tests of the developed catalyst showed high degradation levels for successive cycles. The results permit concluding that the integrated system is adequate to achieve high treatment efficiency with low electrical consumption.

Bacterial-fungal interactions enhance power generation in microbial fuel cells and drive dye decolourisation by an ex situ and in situ electro-Fenton process

In this work, the potential for sustainable energy production from wastes has been exploited using a combination fungus-bacterium in microbial fuel cell (MFC) and electro-Fenton technology. The fungus Trametes versicolor was grown with Shewanella oneidensis so that the bacterium would use the networks of the fungus to transport the electrons to the anode. This system generated stable electricity that was enhanced when the electro-Fenton reactions occurred in the cathode chamber. This configuration reached a stable voltage of approximately 1000 mV. Thus, the dual benefits of the in situ-designed MFC electro-Fenton, the simultaneous dye decolourisation and the electricity generation, were demonstrated. Moreover, the generated power was effectively used to drive an ex situ electro-Fenton process in batch and continuous mode. This newly developed MFC fungus-bacterium with an in situ electro-Fenton system can ensure a high power output and a continuous degradation of organic pollutants.

Using iron-loaded sepiolite obtained by adsorption as a catalyst in the electro-Fenton oxidation of Reactive Black 5

This study explores the possibility of using iron-loaded sepiolite, obtained by recovering iron from polluted water, as a catalyst in the electro-Fenton oxidation of organic pollutants in textile effluents. The removal of iron ions from aqueous solution by adsorption on sepiolite was studied in batch tests at iron concentrations between 100 and 1,000 ppm. Electro-Fenton experiments were carried out in an electrochemical cell with a working volume of 0.15 L, an air flow of 1 L/min, and 3 g of iron-loaded sepiolite. An electric field was applied using a boron-doped diamond anode and a graphite sheet cathode connected to a direct current power supply with a constant potential drop. Reactive Black 5 (100 mg/L) was selected as the model dye. The adsorption isotherms proved the ability of the used adsorbent. The removal of the iron ion by adsorption on sepiolite was in the range of 80-100 % for the studied concentration range. The Langmuir and Freundlich isotherms were found to be applicable in terms of the relatively high regression values. Iron-loaded sepiolite could be used as an effective heterogeneous catalyst for the degradation of organic dyes in the electro-Fenton process. Successive batch processes were performed at optimal working conditions (5 V and pH 2). The results indicate the suitability of the proposed combined process, adsorption to iron remediation followed by the application of the obtained iron-loaded sepiolite to the electro-Fenton technique, to oxidize polluted effluents.

Electro-Fenton decolourisation of dyes in an airlift continuous reactor using iron alginate beads

In this study, electro-Fenton dye degradation was performed in an airlift continuous reactor configuration by harnessing the catalytic activity of Fe alginate gel beads. Electro-Fenton experiments were carried out in an airlift reactor with a working volume of 1.5 L, air flow of 1.5 L/min and 115 g of Fe alginate gel beads. An electric field was applied by two graphite bars connected to a direct current power supply with a constant potential drop. In this study, Lissamine Green B and Reactive Black 5 were selected as model dyes. Fe alginate gel beads can be used as an effective heterogeneous catalyst for the degradation of organic dyes in the electro-Fenton process, as they are more efficient than the conventional electrochemical techniques. At optimal working conditions (3 V and pH 2), the continuous process was performed. For both dyes, the degree of decolourisation increases when the residence time augments. Taking into account hydrodynamic and kinetic behaviour, a model to describe the reactor profile was obtained, and the standard deviation between experimental and theoretical data was lower than 6%. The results indicate the suitability of the electro-Fenton technique to oxidise polluted effluents in the presence of Fe alginate gel beads. Moreover, the operation is possible in a continuous airlift reactor, due to the entrapment of iron in the alginate matrix.