Heterogeneous electro-Fenton Reaction Research Articles

Fe-Ce based metal-organic framework as a novel heterogeneous catalyst accelerating redox cycle for efficient degradation of sulfamethoxazole in wide pH ranges

The electro-Fenton (EF) technology holds immense potential for wastewater treatment. However, there persist certain factors that affect its degradation efficiency, such as the restricted pH range, production of iron sludge and sluggish conversion rate between Fe2+ and Fe3+. Herein, 2,3,6,7,10,11-hexahydroxytriphenyl (HHTP) was introduced as the organic ligand to synthesize a series of efficient iron-based MOF catalysts (FexCe1-HHTP (x = 1, 2, 3, 4)) with different molar ratios for sulfamethoxazole (SMX) degradation in heterogeneous EF system. HHTP with electron-rich functional groups is beneficial to enhance performance of oxygen evolution reaction. Among these catalysts, Fe3Ce1-HHTP catalyst achieved 97.3 % degradation efficiency in 30 min and mineralization rate was 89.7 % at 1 h. The introduction of Ce promoted the transition of Fe3+ and Fe2+ in heterogeneous EF reaction. The synergistic effects between Fe3+/Fe2+ and Ce4+/Ce3+ further catalyzed the generation of hydroxyl radicals and it was proved as the main active species. The heterogeneous catalyst showed good performance in wide pH range, good stability, and high degradation efficiency to other pollutants. Three possible SMX degradation routes are suggested on basis of intermediates detected by mass spectrum analysis and theoretical calculations. The toxicity of these intermediates was assessed using Toxicity Estimation Software Tool (T.E.S.T.). This work provides valuable guidance for the design of highly efficient and stable heterogeneous EF catalyst for antibiotic degradation and expand the application potential in wastewater treatment of organic pollutants.

The recent progress of cathode materials for heterogeneous electro-Fenton reactions

As one of environmentally friendly and efficient oxidation techniques, the electro-Fenton (EF) processes allow to remove and mineralize organic pollutants. However, the transitional EF processes require an acidic environment, adding ferrous ions, treatment of iron sludge, and costly solution pH adjustment. Heterogeneous EF (hetero-EF) processes enable EF to function over a broad working pH region and recycle across multiple reaction rounds which reduces overall cost and displays great potential for practical engineering applications. Here, this work reviewed the reported cathode materials for the hetero-EF processes. We firstly described the mechanisms of homogeneous and heterogeneous EF reactions. Then, we summarized different types of cathode materials for hetero-EF reactions, including metal-free carbon materials, transition metal single-atom catalysts (SACs)/carbon materials, alloys catalysts/carbon materials, metal compounds/carbon materials and other materials. Finally, we overviewed the challenges and proposal strategies of hetero-EF technique in order to expand its wide-scale applicability and practical significance for advanced wastewater treatment.

Electrochemically enhanced iron oxide-modified carbon cathode toward improved heterogeneous electro-Fenton reaction for the degradation of norfloxacin.

In this work, different iron-based cathode materials were prepared using two different approaches: a novel one-step approach, which involved the incorporation of iron oxide with Printex® L6 carbon/PTFE (PL6C/PTFE) on bare carbon felt (CF) and a two-step approach, where iron oxide is deposited onto CF previously modified with PL6C/PTFE. The results obtained from the physical characterization indicated that the presence of iron oxide homogeneously dispersed on the felt fibers with the CF 3-D network kept intact in the one-step approach; whereas the formation of iron oxide aggregates between the felt fibers for material obtained using the two-step approach. Among the iron oxide-based cathodes investigated, the iron-incorporated electrode exhibited the greatest efficiency in terms of the removal and mineralization of norfloxacin (NOR) under neutral pH (complete NOR removal in less than 30 min with around 50% mineralization after 90 min). The findings of this study show that the low cost and simple-to-prepare iron-modified carbon-based materials in HEF process led to the enhanced degradation of organic contaminants in aqueous solutions.

FeCo-ZIF derived carbon-encapsulated metal alloy as efficient cathode material for heterogeneous electro-Fenton reaction in 3-electron ORR pathway: Enhanced performance in alkaline condition

Oxygen reduction reaction via 3-electron pathway presents a promising approach for generating reactive oxygen species in heterogeneous electro-Fenton (H-EF) process. This approach overcomes the limitations posed by the slow accumulation of H2O2 via 2- electron ORR and the inefficient activation of H2O2 through Haber-Weiss cycle. In this study, bimetal alloy nanoparticles were encapsulated within carbon layers (FeCo/C) by a general MOF-derived strategy, resulting in an overall electron transfer number of 2.98 during the H-EF process. Under optimal modification conditions, the prepared FeCo/C exhibited exceptional performance in sulfamethoxazole removal. Furthermore, the performance was enhanced under alkaline conditions. The apparent rate constant increased from 0.02 to 0.03 min−1 as the pH value increased from 3 to 11. The mechanism underlying this alkalinity-enhanced activity was attributed to the involvement of a surface-independent outer sphere electron transfer component, which provides valuable insights into the potential application of the H-EF process for treating alkaline wastewater.

Ultrafast self-assembly Fe2O3 nanoparticles confined in carbon layers toward robust heterogeneous electro-Fenton reaction

Heterogeneous electro-Fenton (EF) is a promising advanced oxidation process for refractory wastewater treatment. However, typical calcination or hydrothermal method for heterogeneous EF cathode fabrication is time-consuming, energy-consuming, or harmful to the environment. High-temperature shock (HTS) technique is a novel and rapid material synthesis method with the advantages of low-cost, eco-friendly. In this study, the electrodes named Fe2O3@C/carbon cloth (Fe2O3@C/CC), were synthesized by HTS for the first time within an extremely short time (∼5 s). The Fe2O3 nanoparticles with an average diameter of 61.90 ± 15.09 nm are uniformly dispersed on carbon cloth (CC) with the confinement and protection of the carbon layers. The as-prepared Fe2O3@C/CC cathode exhibited excellent electrocatalytic properties in the EF system. The removal efficiency of methylisothiazolinone (MIT) could reach 90.60 ± 1.70% in 40 min at a low current density of 1.11 mA cm−2 under the oxidation of the hydroxyl radical. In addition, the degradation pathways of MIT were further studied. According to the results of theoretical calculation, the –S–N– bond is the primary active site attacked during MIT degradation process. Therefore, heterocyclic loop opening is the main pathway of MIT degradation. HTS is an ultrafast and convenient potential technique compared with other cathode synthesis methods.

3D Self-Supported Nitrogen-Doped Carbon Nanofiber Electrodes Incorporated Co/CoOx Nanoparticles: Application to Dyes Degradation by Electro-Fenton-Based Process.

We developed free-standing nitrogen-doped carbon nanofiber (CNF) electrodes incorporating Co/CoOx nanoparticles (NPs) as a new cathode material for removing Acid Orange 7 (AO7; a dye for wool) from wastewater by the heterogeneous electro-Fenton reaction. We produced the free-standing N-doped CNF electrodes by electrospinning a polyacrylonitrile (PAN) and cobalt acetate solution followed by thermal carbonation of the cobalt acetate/PAN nanofibers under a nitrogen atmosphere. We then investigated electro-Fenton-based removal of AO7 from wastewater with the free-standing N-doped-CNFs-Co/CoOx electrodes, in the presence or not of Fe2+ ions as a co-catalyst. The electrochemical analysis showed the high stability of the prepared N-doped-CNF-Co/CoOx electrodes in electrochemical oxidation experiments with excellent degradation of AO7 (20 mM) at acidic to near neutral pH values (3 and 6). Electro-Fenton oxidation at 10 mA/cm2 direct current for 40 min using the N-doped-CNF-Co/CoOx electrodes loaded with 25 wt% of Co/CoOx NPs led to complete AO7 solution decolorization with total organic carbon (TOC) removal values of 92.4% at pH 3 and 93.3% at pH 6. The newly developed N-doped-CNF-Co/CoOx electrodes are an effective alternative technique for wastewater pre-treatment before the biological treatment.

MgAl-LDH@Fe3O4 based heterogeneous electro-Fenton process for the concentrated liquor of gas field wastewater treatment: Optimization, mechanism, and stability

The concentrated liquor of gas field wastewater (CL-GFW) obtained by cryogenic multi-effect distillation is high-concentration organic wastewater. The MgAl-LDH@Fe3O4 particles were successfully synthesized as the catalysts of the heterogeneous electro-Fenton (EF) system to effectively degrade refractory organic pollutants in the CL-GFW. The particles broadened the pH range of the EF process under optimal conditions. The highest COD removal efficiency could achieve 88.18% after 4 h with pH of 5. The mesoporous particles were systematically studied via a series of characterization techniques. The heterogeneous degradation mechanisms were explored by radical quenching experiments, variation of H2O2 and Fe ion concentrations, and X-ray photoelectron spectroscopy (XPS) analysis. The results indicated that the synergistic effects of the active free radical oxidation, enhanced catalysis, and surface complexation contributed to the degradation of recalcitrant organics in the heterogeneous EF process. Electrocatalysis and homogeneous EF trials exhibited lower COD degradation rates of 45.68% and 62.86% comparing with the heterogeneous EF. The representative refractory organics before and after the heterogeneous EF reaction were identified by GC-MS and HPLC-MS methods. The excellent stability and reusability of the particles in the heterogeneous EF system were verified through five cyclic experiments with low sludge production and metal leaching rate.

Fe/Fe3C@CNTs anchored on carbonized wood as both self-standing anode and cathode for synergistic electro-Fenton oxidation and sequestration of As(III)

Considering the hypertoxicity and poor affinity of arsenite toward sorbent materials, development and construction of effective purification technologies are of vital importance for efficient conversion of As(III) to As(V) and concomitant capture of As(V). Herein, an electrochemical integrated system for As(III) removal is developed using 3D hierarchically porous hybrid monolith electrodes, constructed by Fe/Fe3C nanoparticles (NPs) and clusters encapsulated in N,O-codoped carbon nanotubes (CNTs) anchored on carbonized wood (CW) framework (denoted as Fe/Fe3C@CNTs/CW). Remarkably, the heterogeneous electro-Fenton reaction on the cathode promotes the oxidation of As(III) into As(V); while the electro-sorption on the anode is conducive to the subsequent arsenic immobilization. Under the optimized conditions, trace As(III) species (1 ppm) can be efficiently removed within 90 min, in which the residual arsenic concentration is below the threshold value (10 ppb) in the drinking water recommended by the World Health Organization (WHO). Significantly, the saturation electro-sorption capacity of the anode materials is approximately 10 times higher than that of physicochemical adsorption. The developed Fe/Fe3C@CNTs/CW electrode exhibits not only superior structural stability due to the protective action from the carbon chainmail but also satisfactory recycling capability with negligible decay in removal efficiency after five cycles, signifying its huge potential in the future sustainable remediation application.

OCNTs encapsulating Fe-Co PBA as efficient chainmail-like electrocatalyst for enhanced heterogeneous electro-Fenton reaction

Preparing heterogeneous electro-Fenton (HEF) catalysts without sacrificing activity towards the two-electron (2e−) oxygen reduction reaction (ORR) is a challenge, since Fenton-like active sites consist mainly of transition metals that prefer the 4e− ORR pathway. In this study, oxidized carbon nanotubes (OCNTs), which are promising ORR catalysts for the 2e− ORR, were uniformly attached to the surface of Prussian blue analogue (PBA) microcubes, forming a unique chainmail-like catalyst (OCNTs/PBA). 50 mg/L of sulfamethazine (SMZ) was completely removed within 30 min over OCNTs/PBA HEF process, with the first-order kinetic constant increased by 287.2 % compared with that PBA alone. To reveal the mechanism behind the enhanced catalytic activity, several possible interactions between the OCNTs and PBA were proposed and investigated further. This study not only offers a facile method to prepare a chainmail-like HEF catalyst, but also provides valuable insights into the rational design of HEF electrocatalysts with dual Fenton-like and ORR activity.

Efficient and stable heterogeneous electro-Fenton system using iron oxides embedded in Cu, N co-doped hollow porous carbon as functional electrocatalyst

Heterogeneous electro-Fenton reaction, constituted by Fenton chemistry and electrochemical synthesis of H2O2, exhibits the powerful ability for wastewater treatment by virtue of the continuous production of strong oxidizing ·OH radical. However, the slow reduction of Fe(III) to Fe(II) of Fenton phase hinders the continuous and effective ·OH generation. Strengthening the interfacial electron transfer from conductive carbon support to Fenton phase is a feasible strategy to promote Fe(II) regeneration, which can be achieved by chemical modifications of the carbon support with N and Cu doping. In this study, we constructed a functional electrocatalyst constituted by Cu, N co-doped hollow porous carbon with embedded iron oxide nanoparticles (FeOx/CuNxHPC) by carbonizing Cu doped iron metal-organic frameworks NH2-MIL-88B(Fe). The electro-Fenton system of FeOx/CuNxHPC exhibited almost 100% removal efficiency of phenol (50 ppm) within 90 min at the pH of 4, 6, 8 and 10 at a potential of −0.6 V vs SCE. Particularly, at pH 6, the phenol mineralization efficiency reached 81.6% within 180 min. Moreover, FeOx/CuNxHPC exhibited outstanding stability and reusability after consecutive ten tests of electron-Fenton reactions. Such enhanced electro-Fenton performance of FeOx/CuNxHPC could be ascribed to the facilitated surface Fe(II) regeneration, which greatly promoted in-situ decomposition of H2O2 into ·OH. By virtue of the excellent electro-Fenton performance and the facile preparation, this kind of iron-based metal-organic frameworks derived functional catalysts was proposed as the suitable cathodic material for the efficient and stable heterogeneous electro-Fenton catalysis.

A carbon nanotube-confined iron modified cathode with prominent stability and activity for heterogeneous electro-Fenton reactions

We demonstrate the importance of confinement catalysis in selectively controlling the position and valence of iron on CNTs, which could effectively increase heterogeneous electro-Fenton activity and decrease iron leaching to improve cathode stability.

Novel perovskite-based composites, La1-x Nd x FeO3@activated carbon, as efficient catalysts for the degradation of organic pollutants by heterogeneous electro-Fenton reactions.

Perovskites, which have excellent electrocatalytic properties, are promising for use in heterogeneous catalysis. However, the design and development of green and effective electrocatalysts for environmental water treatment remains an arduous challenge. To overcome such difficulties, we present a facile sol–gel method for the design and preparation of a series of perovskite-activated carbon (AC) composites (La1−xNdxFeO3@AC) for the degradation of methyl orange (MO) by heterogeneous electro-Fenton reactions. Furthermore, the as-made La0.6Nd0.4FeO3@AC composite anode had the strongest oxidation ability and stability, with MO wastewater and COD removal rates reaching 99.81% and 96.66% within 10 minutes, respectively. As far as we know, the La1−xNdxFeO3@AC composites can be regarded as a series of the most effective catalysts for the degradation of MO to date.

High efficient decolorization of Reactive Red 120 azo dye over reusable Fe-ZSM-5 nanocatalyst in electro-Fenton reaction

Abstract We report development of ZSM-5 nanocatalyst including Fe promoter for decolorization of Reactive Red 120 (RR120) azo dye in a heterogeneous electro-Fenton reaction. The nanocatalysts were characterized by XRD, FT-IR, FE-SEM, N 2 adsorption-desorption and NH 3 -TPD techniques. The influence of different parameters (Fe percentage, nanocatalyst loading and pH level) on the decolorization efficiency are studied. The results show that the optimum operational conditions are 1% Fe impregnation, 0.1 g nanocatalyst loading and pH 3.0 which result in the highest RR120 decolorization efficiency (ca. 98%). A pseudo-first order kinetic model fits the experimental data with the high correlation factor (R 2 > 0.98). The developed nanocatalyst is a reusable catalyst for the RR120 decolorization which significantly recovers the active sites through regeneration process and provides the high decolorization efficiency (>95%) in several batches. The results confirm the high capacity of developed ZSM-5 nanocatalyst for the dye decolorization in commercial scale.

Enhanced removal of Microcystis aeruginosa in BDD-CF electrochemical system by simple addition of Fe2+

This study presented a significant enhanced removal of Microcystis aeruginosa by simple addition of Fe2+ at circumneutral pH in the electrochemical system with boron-doped diamond (BDD) anode and carbon felt (CF) cathode. Results showed that over 99.9% of the M. aeruginosa cells were removed in 60min when 0.2mM Fe2+ was added to the system. The pseudo-first order rate constant for BDD-CF-Fe2+ system was almost 15 times higher than that (0.005min−1) for BDD-CF system. The excellent performance of the BDD-CF-Fe2+ system over the BDD-CF system was observed over a wide range of pH from 3 to 9. Based on the evolution of Fe2+/Fe3+, pH, H2O2 and OH, mechanism of enhanced removal of M. aeruginosa was revealed in the BDD-CF-Fe2+ system. H2O2 was electrochemically generated through reduction of dissolved oxygen on the surface of the CF cathode, and the added Fe2+ was rapidly adsorbed and precipitated also on the CF cathode. Comparisons of BDD-CF-Fe2+, BDD-CF-Fe3+ and BDD-CF-Fe2+/Fe3+ systems proved that the electrochemical reduction of Fe(III) to Fe(II) could guarantee the sustainable cycle between Fe(II) and Fe(III). Through heterogeneous electro-Fenton reactions, much higher amount of OH would be continuously generated compared with the BDD-CF system. Thereby, the lipid peroxidation on the cell membrane of M. aeruginosa was intensified through the addition of Fe2+. Moreover, the degradation of intracellular microcystin was remarkably accelerated, and the extracellular microcystin was controlled more safely due to Fe2+ addition. This study provided an alternative method to enhance the electrochemical mitigation of cyanobacteria-polluted waters.

Preparation and characterization of bimetallic Fe–Cu allophane nanoclays and their activity in the phenol oxidation by heterogeneous electro-Fenton reaction

Bimetallic (Fe–Cu) allophane nanoclays were synthesized using a two-step wet impregnation method with different Fe/Cu ratios. The catalytic activities of bimetallic (Fe–Cu) allophane were studied for phenol oxidation by heterogeneous electro-Fenton reaction (HEF) at different initial pHs (3.0 and 5.5), and were compared with Fe-allophane and Cu-allophane catalysts. A glassy carbon electrode modified with the bimetallic allophane nanoclays was used as working electrode. FTIR, SEM, X-ray diffraction, XPS, Mössbauer spectroscopy and N2 adsorption–desorption were used to characterize the catalysts, and indicated the formation of small copper oxide particles stabilized by iron oxide species. Phenol conversion by HEF process at initial pH 3.0 was near 100% for all bimetallic (Fe–Cu) allophane nanoclays in less than 2 h of reaction, following an exponential decay. The chemical oxygen demand (COD) removal was less than 47% for Cu-allophane and 65% for Fe-allophane, whereas for the bimetallic (Fe–Cu) allophane nanoclays the COD removal decreased with the amount of copper oxide in the catalyst, achieving an 80% COD removal with Fe5.4Cu0.6 catalyst. These results showed the synergetic effect between the Fe3+ and Cu2+ ions present in the bimetallic (Fe–Cu) allophane nanoclays. Similarly, when the reactions were performed at initial pH 5.5 the phenol conversion was near 100% after 4 h for Fe-allophane and bimetallic (Fe–Cu) allophane with lower copper content. In the bimetallic (Fe–Cu) allophane clays the leaching of iron and copper into the solution was less than 1.25 mg/L and 0.638 mg/L, respectively, indicative of the high stability of the bimetallic (Fe–Cu) allophane catalysts.

Heterogeneous electro-Fenton oxidation of catechol catalyzed by nano-Fe3O4: kinetics with the Fermi's equation

The degradation of catechol in a heterogeneous electro-Fenton system catalyzed by nano-Fe3O4 was studied. The effects of the main parameters such as temperature, pH, current density, catalyst load and initial catechol concentration on the degradation rate were investigated by carrying out batch experiments. Catechol was nearly completely degraded within 120 min when the current density was kept at 10 mA/cm2, with the addition of 1.0 g/L catalyst at an initial pH 3.0. Stability and reuse of the catalyst were also tested, which put evidence for its potential application. The catechol concentration evolutions were described with a simple semi-empirical kinetic model based on the Fermi's equation. A few adjustable parameters were involved to simultaneously describe the two-part degradation processes. The correlation between the model and the experimental data was remarkable. The apparent rate constant and transition time could be simultaneously obtained by model fitting. The heterogeneous electro-Fenton reaction was interface determined process according to the analysis of the apparent rate constant and surface rate constant.

Design of a neutral three-dimensional electro-Fenton system with various bentonite-based Fe particle electrodes: A comparative study

Three-dimensional electrochemical reactor coupling electro-Fenton degradation of phenol with three kinds of bentonite-based various iron species (Fe-Bent): Fe3O4-Bent, Fe2O3-Bent and Fe0-Bent, as particle electrodes at original pH (6·24) was investigated. At the optimal reaction conditions, a maximum of 92·48% phenol removal and 67·48% chemical oxygen demand removal were achieved in the presence of Fe0-Bent. By evaluating Fe leaching behaviours from the particle electrodes during the degradation of phenol, it was found that both homogeneous and heterogeneous electro-Fenton reactions were all involved in the phenol degradation. Moreover, the evolution of ·OH was evaluated and its relationship with Fe leaching and phenol removal was also discussed. The results show that Fe0-Bent can act as the most efficient catalyst in this system at natural conditions, and the feasibility of the proposed combined system of electro-Fenton and three-dimensional electrochemical reactor in the removal of pollutant from wastewater.