Electrogeneration of hydrogen peroxide for electro-Fenton system by oxygen reduction using chemically modified graphite felt cathode

Internal cycling of Fe3+/Fe2+ within Fe3O4 on cathode promotes green degradation of reactive brilliant red X-3B on anode

Reactive brilliant red X-3B (RBRX-3B) wastewater is difficult to decolourise, not readily biodegradable, and large in quantity. Therefore, the efficient removal of RBRX-3B is crucial. In this paper, a green and efficient electrochemical-electro-Fenton system with Fe3O4-modified carbon felt bag cathode (ECEF-Fe3O4) was set up to degrade RBRX-3B wastewater. Experiments confirmed that the removal of RBRX-3B by ·OH or H2O2 is quite low, and RBRX-3B can be completely oxidised and degraded directly on the anode. Long-cycle experimental data further shows that the degradation efficiency of RBRX-3B on the anode is 100% at 70 min at the reaction rate constants (k) of 0.071 min-1 in ECEF-Fe3O4 while that of RBRX-3B on the cathode is only 16.8 ± 0.9%. The generation of ·OH is mainly catalysed through the internal cycling of Fe3+/Fe2+ within Fe3O4 on the cathode, and the generation and annihilation of ·OH on the cathode enhance the oxidation efficiency of the anode, achieving the green and effective removal of RBRX-3B by the anode in ECEF-Fe3O4.

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.

Minerals as catalysts of heterogeneous Electro-Fenton and derived processes for wastewater treatment: a review

Advanced oxidation processes (AOPs) such as Fenton's reagent, which generates highly reactive oxygen species, are efficient in removing biorefractory organic pollutants from wastewater. However, Fenton's reagent has drawbacks such as the generation of iron sludge, high consumption of H₂O₂, and the need for pH control. To address these issues, Electro-Fenton (EF) and heterogeneous Electro-Fenton (HEF) have been developed. HEF, which uses solid catalysts, has gained increasing attention, and this review focuses on the use of mineral catalysts in HEF and derived processes. The reviewed studies highlight the advantages of using mineral catalysts, such as efficiency, stability, affordability, and environmental friendliness. However, obstacles to overcome include the agglomeration of unsupported nanoparticles and the complex preparation techniques and poor stability of some catalyst-containing cathodes. The review also discusses the optimal pH range and dosage of the heterogeneous catalysts and compares the performance of iron sulfides versus iron oxides. Although natural minerals appear to be the best choice for effluents at pH>4, no scale-up reports have been found. The need for further development in this field and the importance of considering the environmental impact of trace toxic metals or catalytic nanoparticles in the treated water on the receiving ecosystem is emphasized. Finally, the article acknowledges the high energy consumption of HEF processes at the lab scale and calls for their performance development to achieve environmentally friendly and cost-effective results using real wastewaters on a pilot scale.

Highly applicable dual-cathode electro-Fenton system with self-adjusting pH and ferrous species

Due to the high dependence on the pH of influent water and the level of ferrous species, the applicability of the electro-Fenton (EF) system is poor. A highly applicable dual-cathode (DC) EF system with self-adjusting pH and ferrous species is proposed: gas diffusion electrode (GDE) for generation H₂O₂ and Fe/S doped multi-walled carbon nanotubes (Fe/S-MWCNT) modification active cathode (AC) for adjusting pH and iron species. The strong synergistic enhancement effect between two cathodes (synergy factor up to 90.3%) improves the catalytic activity of this composite system about 12.4 times higher than that of cathode alone. Impressively, AC has the ability of self-regulate to shift towards the optimal Fenton pH (around 3.0) without adding reagents. Even pH can be adjusted from 9.0 to 3.4 within 60 min. This characteristic gives the system a wide range of pH applications, while avoiding the disadvantage of the high cost of traditional EF in pre-acidification. Furthermore, DC has a high and stable ferrous species supply, and the iron leaching amount is about twice less than that of heterogeneous EF system. Long-term stability of the DC system and its easy activity regeneration exhibit the potential of environmental remediation in industrial application.

Highly selective electrosynthesis of H2O2 by N, O co-doped graphite nanosheets for efficient electro-Fenton degradation of p-nitrophenol

The activity and selectivity of the cathode towards electrosynthesis of H₂O₂ are critical for electro-Fenton process. Herein, nickel-foam modified with N, O co-doped graphite nanosheets (NO-GNSs/Ni-F) was developed as a cathode for highly efficient and selective electrosynthesis of H₂O₂. Expectedly, the accumulation of H₂O₂ at pH= 3 reached 494.2 mg L^-1 h^-1, with the selectivity toward H₂O₂ generation reaching 93.0%. The synergistic effect of different oxygen-containing functional groups and N species on the performance and selectivity of H₂O₂ electrosynthesis was investigated by density functional theory calculations, and the combination of epoxy and graphitic N (EP + N) was identified as the most favorable configuration with the lowest theoretical overpotential for H₂O₂ generation. Moreover, NO-GNSs/Ni-F was applied in the electro-Fenton process for p-nitrophenol degradation, resulting in 100% removal within 15 min with the kinetic rate constant of 0.446 min^-1 and 97.6% mineralization within 6 h. The efficient removal was mainly attributed to the generation of bulk ·OH. Furthermore, NO-GNSs/Ni-F exhibited excellent stability. This work provides a workable option for the enhancement of H₂O₂ accumulation and the efficient degradation of pollutants in electro-Fenton system.

Application of electrochemical oxidation for the enhancement of antibiotic resistant bacteria removal in stormwater bioretention cells

Recently, increasing attention has been paid to the removal of antibiotic resistant bacteria (ARB) during electrochemical advanced oxidation processes. However, there is still no available literature about the application of electrochemical oxidation (EO) to enhance ARB removal in stormwater bioretention cells. Batch experiments were conducted to investigate target ARB (E. coli K-12 carrying blaTEM, tetR and aphA) removals in bioretention cells with different current densities and ratios of air to water (A/W). ARB removals for bioretention cells with 17.6 μA/m² of current density and 24:1 of A/W ratio was the largest with 5.28 log reduction, which was obviously higher than the one (3.68 log reduction) in the control (without EO). H₂O₂ production could be responsible for ARB removals in the used bioretention cells, where H₂O₂ levels increased at first and then decreased with the increase of current densities and A/W ratios. The evaluation for the application of EO implied that the highest antibiotic resistance (AR) conjugation frequency (3.8 × 10^-3) at 3.5 μA/m² of current density and 48:1 of A/W ratios was 124.5 % of the one in the control, while the largest AR transformation frequencies at 17.6 μA/m² of current density and 48:1 of A/W ratios was 366.9 % (tetR) and 216.2 % (aphA) of the corresponding in the control, and there were still stable for both dominant microflora and metabolic activities in bioretention cells with electricity and aeration, suggesting that EO could be promising for the enhancement of ARB removals in bioretention cells.

Antibiotic resistant bacteria inactivation through metal-free electrochemical disinfection with carbon catalysts and its potential risks

Recently, increasing attention has been paid to the inactivation of antibiotic resistant bacteria (ARB) during the electrochemical disinfection. However, no available information could be found on ARB inactivation in water during metal-free electrochemical disinfection. In this study, polyvinylidene fluoride (PVDF)-based carbon catalyst (PPC) was chosen as working electrode. Batch experiments were conducted to investigate key design for ARB inactivation, effects of water matrix and potential risks after the disinfection under the pre-determined conditions. The disinfection with current density at 2.25 mA/cm² and Air/Water ratio of 10:1 was optimal with the largest ARB inactivation (5.0 log reduction for 40 min), which was in line with the profile and yield of hydrogen peroxide (H₂O₂) during the disinfection. Effects of water matrix analysis implied that ARB inactivation efficiencies during the disinfection in acidic solutions were better than the one in alkaline solutions, which could be due to rich CC levels on surface of PPC cathode. After the optimal disinfection, ARB counts increased slightly at the first 2 h and then tended to disappear, and there were no conjugation transfer and little transformation for target antibiotic resistance genes, indicating that potential risks could be blocked after the disinfection for 40 min. Furthermore, intermittent flow was more effective in inactivating ARB compared with continuous flow. These suggested that the application of metal-free electrochemical disinfection with PPC to inactivate ARB in water was feasible and desirable in this study.

Elimination of recalcitrant micropollutants by medium pressure UV-catalyzed bioelectrochemical advanced oxidation process: Influencing factors, transformation pathway and toxicity assessment

Bio-electro-Fenton (BEF) processes have been widely studied in recent years to remove recalcitrant micropollutants from wastewater. Though promising, it still faces the critical challenge of residual iron and iron sludge in the treated effluent. Thus, an innovative medium-pressure ultraviolet-catalyzed bio-electrochemical system (MUBEC), in which medium-pressure ultraviolet was employed as an alternative to iron for in-situ H₂O₂ activation, was developed for the removal of recalcitrant micropollutants. The influence of operating parameters, including initial catholyte pH, cathodic aeration rate, and input voltage, on the system performance, was explored. Results indicated that complete reduction of 10 mg L^-1 of model micro-pollutants ibuprofen (IBU) and carbamazepine (CBZ) was achieved at pH 3, with an aeration rate of 1 mL min^-1 and a voltage of 0.3 V, following pseudo-first-order kinetics. Moreover, potential transformation pathways and the associated intermediates during the degradation were deduced and detected, respectively. Thus, the MUBEC system shows the potential for the efficient and cost-effective degradation of recalcitrant micropollutants from wastewater.

Electrosynthesis of H2O2 through a two-electron oxygen reduction reaction by carbon based catalysts: From mechanism, catalyst design to electrode fabrication

Hydrogen peroxide (H2O2) is an efficient oxidant with multiple uses ranging from chemical synthesis to wastewater treatment. The in-situ H2O2 production via a two-electron oxygen reduction reaction (ORR) will bring H2O2 beyond its current applications. The development of carbon materials offers the hope for obtaining inexpensive and high-performance alternatives to substitute noble-metal catalysts in order to provide a full and comprehensive picture of the current state of the art treatments and inspire new research in this area. Herein, the most up-to-date findings in theoretical predictions, synthetic methodologies, and experimental investigations of carbon-based catalysts are systematically summarized. Various electrode fabrication and modification methods were also introduced and compared, along with our original research on the air-breathing cathode and three-phase interface theory inside a porous electrode. In addition, our current understanding of the challenges, future directions, and suggestions on the carbon-based catalyst designs and electrode fabrication are highlighted.

Electrochemical Production of Hydrogen Peroxide in Perchloric Acid Supporting Electrolytes for the Synthesis of Chlorine Dioxide

This work focuses on the electrochemical production of hydrogen peroxide in supporting electrolytes containing perchlorate ions for being used as a reagent in the reduction of chlorates to produce chlorine dioxide, as a first step in the manufacture of portable ClO₂ production devices. This study evaluates the effect of the current density, pressure, and temperature on the production of hydrogen peroxide, and concentrations over 400 mg L^-1 are reached. The average rate for the formation of hydrogen peroxide is 9.85 mg h^-1, and the effect of increasing electrolyte concentration (3.0 and 30.0 g L^-1 perchloric acid), intensity, and pressure results in values of, respectively, -2.99, -4.49, and +7.73 mg h^-1. During the manufacturing process, hydrogen peroxide is decomposed through two mechanisms. The average destruction rate is 1.93 mg h^-1, and the effects of the three factors results in values of, respectively, +0.07, +0.11, and -0.12 mg h^-1. Solutions of this hydrogen peroxide produced electrochemically in a perchloric acid aqueous electrolyte were used to reduce chlorates in strongly acidic media and produce chlorine dioxide. Conversions of around 100% were obtained, demonstrating that this electrochemical product can be used efficiently to reduce chlorates to chlorine dioxide.

Shifts of surface-bound •OH to homogeneous •OH in BDD electrochemical system via UV irradiation for enhanced degradation of hydrophilic aromatic compounds

Indirect electrochemical oxidation by hydroxyl radicals is the predominant degradation mechanism in electrolysis with a boron-doped diamond (BDD) anode. However, this electrochemical method exhibits low reactivity in removal of hydrophilic aromatic pollutants owing to mass transfer limitation. In this study, the combination of ultraviolet light and BDD electrolysis could increase the degradation rate of hydrophilic aromatic pollutants by approximately 8-10 times relative to electrolysis alone. According to the results of the scavenging experiments and identification of benzoic acid oxidation products, surface-bound hydroxyl radical (•OH_(surface)) was the primary reactive species degrading aromatic pollutants in the BDD electrolysis process, whereas freely-diffusing homogeneous hydroxyl radical (•OH_(free)) was the major reactive species in the UV-assisted BDD electrolysis process. Cyclic voltammetry revealed that UV light decomposed H₂O₂ formed on the BDD anode surface, thus retarding O₂ evolution and facilitating •OH_(free) generation. This work also explored the potential application of UV-assisted BDD electrolysis in removing COD from bio-pretreated landfill leachate containing high concentrations of hydrophilic aromatic pollutants. This study shed light on the importance of the existing state of •OH on removal of pollutants during BDD electrolysis, and provided a facile and efficient UV-assisted strategy for promoting degradation of hydrophilic aromatic pollutants.

When microbial electrochemistry meets UV: The applicability to high-strength real pharmaceutical industry wastewater

Wastewater from pharmaceutical and related industries contains many residual pharmaceutical components rich in color and high COD contents, which cannot be removed through the traditional wastewater treatment processes. Recently, microbial electrolysis ultraviolet cell (MEUC) process has shown its promising potential to remove recalcitrant organics because of its merits of wide pH range, iron-free, and without complications of iron sludge production. However, its application to the real pharmaceutical-rich industrial wastewater is still unknown. In this study, the MEUC process was validated with real ciprofloxacin-rich (6863.79 ± 2.21 µg L^-1) industrial wastewater (6840 ± 110 mg L^-1 of COD). The MEUC process achieved 100% removal of ciprofloxacin, 100% decolorization, and 99.1% removal of COD within 12, 60 and 30 h, respectively, when it was operated at pH-controlled at 7.8, applied voltage of 0.6 V, UV intensity of 10 mW cm^-2, and cathodic aeration velocity of 0.005 mL min^-1 mL^-1. Moreover, fluorescence analysis showed that protein- and humic-like substances in such wastewater were effectively removed, providing further evidence of its high treatment efficiency. Furthermore, eco-toxicity testing with luminescent bacteria Vibro Feschri confirmed that the treated effluent was utterly non-toxic. The results demonstrated the broad application potential of MEUC technology for treating industrial wastewater.

Removal of Tricyclazole and Total Organic Carbon in Real Pesticide Wastewater by Electro-Fenton

In this study, electro-Fenton (EF) was employed for the removal of tricyclazole (TC) and total organic carbon (TOC) in real pesticide wastewater (RPW). The central composite design (CCD) with three parameters, that is, current density, Fe2+ concentration, and electrolysis time, has been conducted to predict the TOC and TC removal efficiency. The high correlation of the quadratic models of 0.9842 and 0.9781 for TC and TOC removal, respectively, indicates the significance of the models. The obtained results revealed that the high-efficiency removal of both TC and TOC in RPW. Approximately 99.6% of TC was decomposed under the optimum conditions of a current density of 2.2 mA/cm2 and an Fe2+ concentration of 0.2 mM during 188 min with an operating cost of 121.392 $/kg TC or ∼2.692 $/m3, while 84.2% TOC was eliminated at a current density of 2.2 mA/cm2 and an Fe2+ concentration of 0.2 mM during 217 min with an operating cost of 3.019 $/kg TOC or 3.916 $/m3. Acute toxicity tests at optimal condition revealed moderate exhibition toxicity of treated wastewater against Daphnia magna with LC50 values of 3.12%, 2.05%, 1.84%, and 1.36% at 24 h, 48 h, 72 h, and 96 h, respectively. The removal of TC and TOC followed pseudo-first-order kinetic with an R2 of ∼0.993 and 0.923, respectively.

Anodized graphite felt as an efficient cathode for in-situ hydrogen peroxide production and Electro-Fenton degradation of rhodamine B

This work investigated that the graphite felt anodized by NaOH, NH₄HCO₃, or H₂SO₄ aqueous, and then as the cathode materials for in-situ hydrogen peroxide (H₂O₂) production and its employed for rhodamine B (RhB) degradation via Electro-Fenton (EF) process. At -0.60 V (vs. SCE), after 120 min electrolysis, the H₂O₂ yield by graphite felt which anodized by 0.2 M H₂SO₄ achieved up 110.5 mg L^-1 in 0.05 M Na₂SO₄ electrolyte. Compared with the raw graphite felt used for cathode, the H₂O₂ yield increased by 15.85 times under the same conditions. The results of Raman spectroscopy demonstrated that graphite felt anodized by H₂SO₄ solution can be achieved the highest defect degree. For the degradation of RhB, the cathode which anodized by H₂SO₄ solution has the highest removal rate. For the degradation rate of RhB, the effect of applied current density, Fe²⁺ ions concentration, pH value were investigated. In addition, suggested that the efficient Fe³⁺ reduction reaction on the cathode surface was an important reason of the high efficiency of RhB degradation. 5-times continuous runs indicated that the modified cathode has remarkable stability and reusability during the EF process.

Graphite felt modified by nanoporous carbon as a novel cathode material for the EF process

Nanoporous carbon prepared by carbonizing ZIF-8@MWCNTs can greatly improve the performance of graphite felt as an electro-Fenton cathode.

Construction of a Microchannel Aeration Cathode for Producing H2O2 via Oxygen Reduction Reaction

Electrochemical oxygen reduction is a promising method for in situ H₂O₂ production. Its important precondition is that dissolved oxygen molecules have to diffuse to and arrive at the cathode surface for reacting with electrons. Obviously, shortening the diffusion distance is beneficial to improve the reaction efficiency. In this study, a novel microchannel aeration mode was proposed to confine the diffusion distance of O₂ to the micrometer level. For this mode, an aeration cathode was fabricated from a carbon block with microchannel arrays. The diameter of each channel was only 10-40 μm. Oxygen will be pumped into every microchannel from the top entry, while an aqueous solution will permeate into microchannels through the bottom entry and pores in the channel wall. This microchannel aeration cathode exhibited a H₂O₂ yield of up to 4.34 mg h^-1 cm^-2, about eight times higher than that of the common bubbling aeration mode. The corresponding energy consumption was only 7.35 kWh kg^-1, which was superior to most reported results. In addition to H₂O₂, this aeration cathode may also produce ^•OH via a one-electron reduction of H₂O₂. In combination with H₂O₂ and ^•OH, phenol, sulfamethoxazole, and atrazine were degraded effectively. We expect that this microchannel aeration cathode may inspire researchers focused on H₂O₂ production, water pollutant control, and other multiphase interfacial reactions.

Pesticide production wastewater treatment by Electro-Fenton using Taguchi experimental design

In this study, the electro-Fenton (EF) method was applied to remove total organic carbon (TOC) from the pesticide production wastewater containing tricyclazole (TC). Statistical Taguchi method was used to optimize the treatment performance. Analysis of variance (ANOVA) indicated that the polynomial regression model fitted experimental data with R² of 0.969. The optimal conditions for eliminating 75.4% TOC and 93.7% TC were 0.2 mM of Fe²⁺, 990 mg/L of Na₂SO₄, 180 min of reaction time at pH 3 with 2.22 mA/cm² of current density. The removal of TC present in the wastewater followed the first-order reaction kinetic model (R² = 0.993); while that was the second-order kinetic model in the case of the TOC removal (R² = 0.903). In addition, the experimental results and theory approaches (density functional theory and natural bond orbital calculations) also showed the C-N bond breaking and nitrate ions cleavage to ammonia. Acute toxicity of the pesticide wastewater after treatment (PWAT) on microcrustaceans showed that the treated wastewater still exhibited high toxicity against D. magna, with LC₅₀ values of 3.84%, 2.68%, 2.05%, and 1.78% at 24 h, 48 h, 72 h, and 96 h, respectively.

In-situ generation of reactive oxygen species using combination of electrochemical oxidation and metal sulfide

In-situ chemical oxidation (ISCO) is commonly practiced to degrade organic pollutants in various fields. However, ISCO is deteriorated the oxidation efficiency due to the non-selective and self-decomposition of reagents. Therefore, in-situ generation of oxidants is being proposed to compensate for the demerits of conventional ISCO. In this study, the aim is to suggest a novel in-situ generation system using the combination of electrochemical oxidation (EO) and pyrite oxidation. It is hypothesized that EO system can generate the oxygen species, which can activate the pyrite surface to produce more oxidants. We evaluated three systems (1) EO system (2) pyrite oxidation system (3) combined system using sulfanilamide as a common antibiotic. The EO system degraded completely sulfanilamide and generated 150 μM of H₂O₂ and 8 mg/L of DO even at 10 mA. In other words, EO system can directly oxidize the sulfanilamide and produce oxygen species. The pyrite system produced 204 and 24 μM of hydroxyl radicals at pH 3 under oxic and anoxic conditions, respectively, and 118 and 20 μM at pH 7. Pyrite oxidation can generate more reactive species in the presence of oxygen. The combined system enhanced the oxidation-rate constant to 1.5 times (from 0.2561 to 0.3502 h^-1). The additional supply of oxygen showed a higher oxidation rate to 1.5 and 1.3 times higher than single EO or pyrite oxidation, respectively. As a result, the co-presence of pyrite and oxygen shows a synergistic effect on the oxidation of the organic pollutant. Our results suggest that electrochemical generation of the oxygen species in the presence of pyrite is a promising technique to oxidize organic pollutants in groundwater.

A novel persulfate-photo-bioelectrochemical hybrid system promoting the degradation of refractory micropollutants at neutral pH

Bio-electro-Fenton is emerging as an alternative technology for the efficient and cost-effective removal of refractory micropollutants. Though promising, there are still several challenges that limit its wide application, including acidic operating conditions (pH at 2-3), the addition of supporting electrolytes (e.g., Na₂SO₄), and the issue of iron sludge generation. To address these challenges, a novel hybrid persulfate-photo-bioelectrochemical (PPBEC) system is proposed to remove model micropollutants (carbamazepine and clorfibric acid), from secondary effluent at low persulfate (PS) dosage and neutral pH. The effect of crucial operating parameters on the process was studied, including input voltage, cathodic aeration velocity, and PS dose. Under optimal conditions (0.6 V, 0.005 mL min^-1 mL^-1 and 1 mM), the PPBEC system achieved approx. 0.56-1.71 times greater micropollutant removal with 93% lower energy consumption when compared to the individual processes (UV/PS and PBEC). The improved performance was attributed to a faster production of sulfate radicals by UV irradiation, hydrogen peroxide activation and single-electron reduction, and hydroxyl radicals generated by UV irradiation. Furthermore, the transformation products of carbamazepine and clorfibric acid were identified and the probable pathways are proposed. Finally, the ecotoxicity of the PPBEC treated effluent was assessed by using Vibrio Fischeri, which exhibited a non-toxic effect.

Synergetic electrocatalytic degradation of isophorone by active oxygen species generated in the gas diffusion electrode and PbO2 anode

Significant efforts have been made to achieve efficient H₂O₂ generation via oxygen reduction reaction (ORR). Here, the acetylene black (AB)-based gas diffusion electrodes (GDEs) modified by PTFE were fabricated and applied in a closed autoclave for H₂O₂ generation for the first time. The surface morphology, BET, XPS, cyclic voltammetry curves and linear scanning voltammetry curves of GDEs were all characterized. Additionally, the H₂O₂ generation experiments of GDEs and the relationship between H₂O₂ yield and BET surface area (S_BET), double layer capacitance (C_dl) were all investigated. It could be found that the S_BET and C_dl of GDEs kept a good linear relationship with H₂O₂ content, and 2#GDE (AB:PTFE = 1:3) possessed the optimal H₂O₂ generation capacity. Also, the effect of oxygen pressure and current density on H₂O₂ production was evaluated detailedly, indicating higher oxygen pressure was beneficial to the H₂O₂ generation and the preferable current density was 20 mA cm^-2 due to the side reactions. And all the H₂O₂ electro-generation experiments exhibited a higher oxygen utilization efficiency (0.77%-2.93%). Finally, the efficient and synergetic electro-catalytic degradation of isophorone was achieved by self-made PbO₂ anode and GDE.

Degradation of norfloxacin wastewater using kaolin/steel slag particle electrodes: Performance, mechanism and pathway

In this work, kaolin/steel slag particle electrodes (KSPEs) were synthesized using a calcination method, and they were used to degrade norfloxacin (NOR) wastewater in three-dimensional (3D) reactor. Characterization methods used by KSPEs included SEM, XRF, XRD and BET. The effects of cell voltage, initial pH, KSPEs dosage and initial NOR concentration on NOR degradation were studied in the optimization experiment of operating parameters. The NOR degradation rate and COD removal rate can reach 96.02% and 93.45% under the optimal parameters within 30 min, and energy consumption is 0.99 kWh m^-3. As a result, KSPEs shows excellent catalytic performance and cycling, and still has high electrocatalytic activity after 10 cycles. Finally, the degradation mechanism and degradation pathways of KSPEs to treat NOR are proposed.

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.

Gas diffusion electrodes for H2O2 production and their applications for electrochemical degradation of organic pollutants in water: A review

Nowadays, it is a great challenge to minimize the negative impact of hazardous organic compounds in the environment. Highly efficient hydrogen peroxide (H₂O₂) production through electrochemical methods with gas diffusion electrodes (GDEs) is greatly demand for degradation of organic pollutants that present in drinking water and industrial wastewater. The GDEs as cathodic electrocatalyst manifest more cost-effective, lower energy consumption and higher oxygen utilization efficiency for H₂O₂ production as compared to other carbonaceous cathodes due to its worthy chemical and physical characteristics. In recent years, the crucial research and practical application of GDE for degradation of organic pollutants have been well developed. In this review, we focus on the novel design, fundamental aspects, influence factors, and electrochemical properties of GDEs. Furthermore, we investigate the generation of H₂O₂ through electrocatalytic processes and degradation mechanisms of refractory organic pollutants on GDEs. We describe the advanced methodologies towards electrochemical kinetics, which include the enhancement of GDEs electrochemical catalytic activity and mass transfer process. More importantly, we also highlight the other technologies assisted electrochemical process with GDEs to enlarge the practical application for water treatment. In addition, the developmental prospective and the existing research challenges of GDE-based electrocatalytic materials for real applications in H₂O₂ production and wastewater treatment are forecasted. According to our best knowledge, only few review articles have discussed GDEs in detail for H₂O₂ production and their applications for degradation of organic pollutants in water.

Fenton/Fenton-like processes with in-situ production of hydrogen peroxide/hydroxyl radical for degradation of emerging contaminants: Advances and prospects

Fenton processes based on the reaction between Fe²⁺ and H₂O₂ to produce hydroxyl radicals, have been widely studied and applied for the degradation of toxic organic contaminants in wastewater due to its high efficiency, mild condition and simple operation. However, H₂O₂ is usually added by bulk feeding, which suffers from the potential risks during the storage and transportation of H₂O₂ as well as its low utilization efficiency. Therefore, Fenton/Fenton-like processes with in-situ production of H₂O₂ have received increasing attention, in which H₂O₂ was in-situ produced through O₂ activation, then decomposed into hydroxyl radicals by Fenton catalysts. In this review, the in situ production of H₂O₂ for Fenton oxidation was introduced, the strategies for activation of O₂ to generate H₂O₂ were summarized, including chemical reduction, electro-catalysis and photo-catalysis, the influencing factors and the mechanisms of the in situ production and utilization of H₂O₂ in various Fenton/Fenton-like processes were analyzed and discussed, and the applications of these processes for the degradation of toxic organic contaminants were summarized. This review will deepen the understanding of the tacit cooperation between the in situ production and utilization of H₂O₂ in Fenton process, and provide the further insight into this promising process for degradation of emerging contaminants in industrial wastewater.

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.

Preparation of CeO2-doped carbon nanotubes cathode and its mechanism for advanced treatment of pig farm wastewater

The effluent from conventional treatment process (including anaerobic digestion and anoxic-oxic treatment) for pig farm wastewater was difficult to treat due to its low ratio of biochemical oxygen demand to chemical oxygen demand (BOD₅/COD_Cr) (<0.1). In the present study, electro-Fenton (EF) was used to improve the biodegradability of the mentioned effluent and the properties of self-prepared CeO₂-doped multi-wall carbon nanotubes (MWCNTs) electrodes were also studied. An excellent H₂O₂ production (165 mg L^-1) was recorded, after an 80-min electrolysis, when the mass ratio of MWCNTs, CeO₂ and pore-forming agent (NH₄HCO₃) was 6:1:1. Results of scanning electron microscopy (SEM), transmission electron microscope (TEM) and x-ray photoelectron spectroscopy (XPS) showed that addition of NH₄HCO₃ and the doping of CeO₂ could increase the superficial area of the electrode as well as the oxygen reduction reaction (ORR) electro-catalytic performance. The BOD₅/COD_Cr of the wastewater from the first stage AO process increased from 0.08 to 0.45 and COD_Cr reduced 71.5% after an 80-min electrolysis, with 0.3 mM Fe²⁺ solution. The non-biodegradable chemical pollutants from the first stage AO process were degraded by EF. The non-biodegradable pollutants identified by LC-MS/MS in the effluent from AO process including aminopyrine, oxadixyl and 3-methyl-2-quinoxalinecarboxylic acid could be degraded by EF process, with the removal rates of 81.86%, 34.39% and 7.13% in 80 min, and oxytetracycline with the removal rate of 100% in 20 min. Therefore, electro-Fenton with the new CeO₂-doped MWCNTs cathode electrode will be a promising supplement for advanced treatment of pig farm wastewater.

Efficient degradation of chloroquine drug by electro-Fenton oxidation: Effects of operating conditions and degradation mechanism

In this work, the degradation of chloroquine (CLQ), an antiviral and antimalarial drug, using electro-Fenton oxidation was investigated. Due to the importance of hydrogen peroxide (H2O2) generation during electro-Fenton oxidation, effects of pH, current density, molecular oxygen (O2) flow rate, and anode material on H2O2 generation were evaluated. H2O2 generation was enhanced by increasing the current density up to 60 mA/cm2 and the O2 flow rate up to 80 mL/min at pH 3.0 and using carbon felt cathode and boron-doped diamond (BDD) anode. Electro-Fenton-BDD oxidation achieved the total CLQ depletion and 92% total organic carbon (TOC) removal. Electro-Fenton-BDD oxidation was more effective than electro-Fenton-Pt and anodic oxidation using Pt and BDD anodes. The efficiency of CLQ depletion by electro-Fenton-BDD oxidation raises by increasing the current density and Fe2+ dose; however it drops with the increase of pH and CLQ concentration. CLQ depletion follows a pseudo-first order kinetics in all the experiments. The identification of CLQ degradation intermediates by chromatography methods confirms the formation of 7-chloro-4-quinolinamine, oxamic, and oxalic acids. Quantitative amounts of chlorides, nitrates, and ammonium ions are released during electro-Fenton oxidation of CLQ. The high efficiency of electro-Fenton oxidation derives from the generation of hydroxyl radicals from the catalytic decomposition of H2O2 by Fe2+ in solution, and the electrogeneration of hydroxyl and sulfates radicals and other strong oxidants (persulfates) from the oxidation of the electrolyte at the surface BDD anode. Electro-Fenton oxidation has the potential to be an alternative method for treating wastewaters contaminated with CLQ and its derivatives.

Enhancement of hydrogen peroxide production by electrochemical reduction of oxygen on carbon nanotubes modified with fluorine

It is vital to synthesis hydrogen peroxide via electrochemical reduction of oxygen since it is a green process to produce oxidant with wide applications including water/wastewater treatment. In this work, fluorine (F) was employed to modify carbon nanotube (CNT), and the obtained F doped CNT (F-CNT) catalyst was used to fabricate gas diffusion electrode (GDE). It was found that F doping could improve oxygen reduction activity and H₂O₂ selectivity, and then enhanced the H₂O₂ production. After modification, F-CNT prepared with 0.6 M HF (CNT-F-0.6) had much higher H₂O₂ production (47.6 mg L^-1) and current efficiency (89.5%) than that of CNT (29.6 mg L^-1, 70.1%) at bias voltage of -1.3 V (vs SCE) and pH 7. Moreover, the high catalytic activity of CNT-F-0.6 could maintain in 5 consecutive reaction cycles. The material characterization and electrochemical test indicated that F doping had no significant effects on the surface area of CNT, but improved the defect degree of CNT. The enhanced H₂O₂ production performance could be ascribed to the formation of CF₂ and CF₃ on the surface of F-doped CNT, which rendered the potential for practical application of novel carbon catalyst for GDE.

A novel Electro-Fenton process characterized by aeration from inside a graphite felt electrode with enhanced electrogeneration of H2O2 and cycle of Fe3+/Fe2

A novel Electro-Fenton process characterized by aeration from inside a graphite felt electrode with enhanced generation of H₂O₂ and cycle of Fe³⁺/Fe²⁺ was proposed. The new type of Electro-Fenton process was used to degrade organic pollutants via graphite felt electrode aeration (GF-EA). The H₂O₂ concentration by GF-EA could reach 152-169 mg/L in a wide pH range (3-10), which was much higher than that achieved by graphite felt using solution aeration (GF-SA, 37-113 mg/L). For the degradation of nitrobenzene (NB), benzoic acid (BA), bisphenol A (BPA), and sulfamethoxazole (SMX) at pH 5.5, the percentage degradation by GF-EA could reach 55%, 56%, 80%, and 60% higher than those obtained by GF-SA, respectively. The solution TOC removal by GF-EA were enhanced by 29-51% relative to GF-SA. Mechanism analysis showed both OH and ferryl species were involved in the reaction system, and the amounts of OH and dissolved iron species in GF-EA group were 7.7 times and 4-8 times higher than those in GF-SA group, respectively. Besides, the mass transfer rate of GF-EA system was 5.4 times higher than that of GF-SA system. High amounts of H₂O₂, dissolved iron species and OH were attributed to the enhanced mass transfer of O₂ and the solution.

Yellow 2G dye degradation by electro-Fenton process using steel electrode as catalysis and its phytotoxicity effect

The heterogeneous electro-Fenton process degradation of Yellow 2G from wastewater was studied using a batch reactor. The COD of the wastewater used in treatment experiments was 163 mg O₂·L^-1 and the BOD₅ was 17 mg O₂·L^-1 (hardly biodegradable). The treatment of the wastewater at different current densities (2.5 mA·cm^{- 2}-12.5 mA·cm^{- 2}), solution pH (3 and 6.6), reaction times (5-25 min), electrolyte nature (NaCl, Na₂SO₄) and electrolyte concentrations (0.15 g·L^{- 1}-1 g·L^{- 1}) was investigated. According to the results, the heterogeneous electro-Fenton process was suitable for the decolorization of wastewater containing Yellow 2G. The optimum conditions were current density of 12.5 mA·cm^-2, initial pH of the wastewater neutral, 25 min of electrolysis treatment using an additive steel electrode as a source of catalysis and in the presence of 1 g NaCl·L^-1. We obtained easily biodegradable water with a mineralization rate equal to 85% and non-toxicity confirmed by the pea grain germination test.

Modification of graphite felt doped with nitrogen and boron for enhanced removal of dimethyl phthalate in peroxi-coagulation system and mechanisms

To enhance the generation of hydrogen peroxide (H₂O₂), a modified graphite felt cathode doped with nitrogen and boron was developed and used in peroxi-coagulation system to degrade dimethyl phthalate (DMP). After a simple modification method, the yield of H₂O₂ on cathode increased from 9.39 to 152.8 mg/L, with current efficiency increased from 1.61 to 70.3%. Complete degradation of DMP and 80% removal of TOC were achieved within 2 h at the optimal condition with pH of 5, cathodic potential of - 0.69 V (vs. SCE), oxygen aeration, and electrode gap of 1 cm. Possible mechanism with synergistic effect of electro-Fenton and electrocoagulation process in the peroxi-coagulation system was revealed via quenching experiments. The prospect of this system in the effluent of landfill leachate and domestic sewage was studied, achieving 50% and 61% of DMP removal in 2 h. This efficient system with simple modified cathode had promising prospects in practical applications.

A three-dimensional electrode bioelectrochemical system for the advanced oxidation of p-nitrophenol in an aqueous solution

Three-dimensional electrodes serve as more efficient cathodes for the in situ generation of H₂O₂ in microbial fuel cells (MFCs) than two-dimensional electrodes and possess significant electric potentials in the advanced oxidation of organics. In this study, we investigated the performance of a three-dimensional MFC-Fenton system in degrading p-nitrophenol (PNP) in an aqueous solution with the objective of optimizing the operating parameters, including the initial pH, iron dosage, and loading resistance. A corresponding reaction pathway for PNP in the system was also proposed. The results showed that the three-dimensional electrode bioelectrochemical system efficiently oxidized PNP and removed total organic carbon over a short period (64 h). In addition, experiments showed that a lower initial pH enhanced the removal of PNP by the system. The highest removal efficiency of PNP was achieved with an initial iron concentration of 0.025 mol L⁻¹, and a lower or higher iron concentration resulted in decreased PNP degradation. Furthermore, the treatment capacity of the system was remarkably enhanced at a low loading resistance of 20 Ω. Under optimal conditions, the three-dimensional MFC-Fenton system achieved 95.7% PNP removal (within 8 h). Furthermore, the system showed a stable high treatment efficiency of approximately 90% for low PNP concentrations in wastewater over as long as 96 h.

PNP absorbed in and surrounded by GPEs has kinetic favorability in the degradation process by a three-dimensional electrode MFC-Fenton system.

Electro-generation of hydrogen peroxide using a graphite cathode from exhausted batteries: study of influential parameters on electro-Fenton process

In this work, the study of hydrogen peroxide (H₂O₂) electro-generation using graphite from exhausted batteries (Gr-Bat) was conducted. Linear sweep voltammetry and electrolysis experiments were carried out in a single compartment electrochemical cell. Study of the possibility to use this electrode revealed that it presents, as vitreous carbon (VC) electrode, a reduction of oxygen with two successive waves (bi-electronic reduction). The first wave corresponds to the reduction of O₂ to H₂O₂, while the second one corresponds to the reduction of H₂O₂ to H₂O. The cathodic potentials for electro-generation of H₂O₂ appeared at -600 and -700 mV vs. Ag/AgCl for Gr-Bat and VC electrodes, respectively. Subsequently, electrolysis experiments were conducted by imposing the potentials required for H₂O₂ formation. The effect of several operating parameters on H₂O₂ production, such as the nature and concentration of the electrolyte, the pH, the presence of ferrous ions and O₂ injection were studied using Gr-Bat and VC electrodes, respectively. For both electrodes, the acidic medium was more favorable for H₂O₂ electro-generation. The oxygen injection in solution promoted an increase of H₂O₂ concentration, but its effect was more pronounced in the case of VC electrode. Application for crystal violet degradation by electro-Fenton revealed that Gr-Bat had the best purification performance. A removal rate of 73.18% was obtained with Gr-Bat electrode against 62.27% with VC electrode for an electrolysis time of 120 min. This study has demonstrated the possibility of recycling Gr-Bat by using them as cathode materials in the electro-Fenton process.