Evaluating the synergistic effect of UVC/e-H2O2 applied for benzotriazole degradation in water matrices using catalyst-free printex L6 carbon-based gas diffusion electrode

The present work investigated the application of UVC combined with electrogenerated H2O2 (UVC/e-H2O2) for BTA degradation using a Printex L6 carbon-based (PL6C) gas diffusion electrode (GDE). The studies were carried out by analyzing the influence of the current density, pH and initial BTA concentration in the contaminant degradation process. Under optimal conditions using 0.05 mol L-1 K2SO4 as supporting electrolyte, BTA removal was evaluated in different water matrices. In all cases, 100% of BTA elimination was reached in the first 15 minutes of treatment under the UVC/e-H2O2 process, while mineralization rates ranging between 59.2 and 78.0% were obtained after 90 minutes of electrolysis. The active species O2•- and e- played an important role in the BTA removal. The toxicity test conducted on the river water sample post-treatment using the Lactuca sativa L seeds showed that the BTA by-products had low toxicity. The results obtained from the LC-ESI-MS/MS analyses showed the same BTA degradation by-products when the BTA-containing water matrices were treated using the UVC/e-H2O2 and photo-electro-Fenton processes. The PL6C-GDE developed in the study exhibited high efficiency when applied for H2O2 electrogeneration and BTA degradation. Additionally, the electrode demonstrated remarkable stability and durability, enabling the generation of reproducible data for up to 81 hours of continuous operation.

Principles, challenges and prospects for electro-oxidation treatment of oilfield produced water

The oil industry is facing substantial environmental challenges, especially in managing waste streams such as Oilfield Produced Water (OPW), which represents a significant component of the industrial ecological footprint. Conventional treatment methods often fail to effectively remove dissolved oils and grease compounds, leading to operational difficulties and incomplete remediation. Electrochemical oxidation (EO) has emerged as a promising alternative due to its operational simplicity and ability to degrade pollutants directly and indirectly, which has already been applied in treating several effluents containing organic compounds. The application of EO treatment for OPW is still in an initial stage, due to the intricate nature of this matrix and scattered information about it. This study provides a technological overview of EO technology for OPW treatment, from laboratory scale to the development of large-scale prototypes, identifying design and process parameters that can potentially permit high efficiency, applicability, and commercial deployment. Research in this domain has demonstrated notable rates of removal of recalcitrant pollutants (>90%), utilizing active and non-active electrodes. Electro-generated active species, primarily from chloride, play a pivotal role in the oxidation of organic compounds. However, the highly saline conditions in OPW hinder the complete mineralization of these organics, which can be improved by using non-active anodes and lower salinity levels. The performance of electrodes greatly influences the efficiency and effectiveness of OPW treatment. Various factors must be considered when selecting the electrode material, such as its conductivity, stability, surface area, corrosion resistance, and cost. Additionally, the specific contaminants present in the OPW, and their electrochemical reactivity must be considered to ensure optimal treatment outcomes. Balancing these considerations can be challenging, but it is crucial for achieving successful OPW treatment. Active electrode materials exhibit a high affinity for chloride molecules, generating more active species than non-active materials, which exhibit more significant degradation potential due to the production of hydroxyl radicals. Regarding scale-up, key challenges include low current efficiency, the formation of by-products, electrode deactivation, and limitations in mass transfer. To address these issues, enhanced mass transfer rates and appropriate residence times can be achieved using flow-through mesh anodes and moderate current densities, which have proven to be the optimal configuration for this process.

Influence of Water Hardness and Complexing Agents on Electrochemical Hydrogen Peroxide Generation

Recently, many studies have been published regarding electrochemical oxygen reduction reaction for generating hydrogen peroxide (H2O2) using gas diffusion electrodes (GDEs) for various applications. Sodium salts solved in deionized water were usually used as supporting electrolytes. In technical applications, however, tap water-based electrolytes with hardeners are particularly relevant and have only been considered in a few studies to date. In this work, we investigated the influence of hardeners on H2O2-generation at 150 mA cm-2 and were able to show that scaling occurs predominantly on the GDE-surface and not in its pore structure. With the novel method in electrochemical synthesis by using complexing agents to bind hardeners, we were able to significantly reduce the scaling. Even after 10 h of operation, the reactor still achieves a faradaic efficiency (FE) of above 70 % (>67 mg h-1 cm-2), comparable to the experiments without hardeners and complexing agents in the electrolyte. Furthermore, we demonstrate that the complexing agents are not electrochemically converted at the carbon-based GDE and behave inertly. If the cell is operated with complexing agents and rinsed with acidic liquid (anolyte) between batches, scaling can be completely avoided.

Advancing H2O2 electrosynthesis: enhancing electrochemical systems, unveiling emerging applications, and seizing opportunities

Hydrogen peroxide (H2O2) is a highly desired chemical with a wide range of applications. Recent advancements in H2O2 synthesis center on the electrochemical reduction of oxygen, an environmentally friendly approach that facilitates on-site production. To successfully implement practical-scale, highly efficient electrosynthesis of H2O2, it is critical to meticulously explore both the design of catalytic materials and the engineering of other components of the electrochemical system, as they hold equal importance in this process. Development of promising electrocatalysts with outstanding selectivity and activity is a prerequisite for efficient H2O2 electrosynthesis, while well-configured electrolyzers determine the practical implementation of large-scale H2O2 production. In this review, we systematically summarize fundamental mechanisms and recent achievements in H2O2 electrosynthesis, including electrocatalyst design, electrode optimization, electrolyte engineering, reactor exploration, potential applications, and integrated systems, with an emphasis on active site identification and microenvironment regulation. This review also proposes new insights into the existing challenges and opportunities within this rapidly evolving field, together with perspectives on future development of H2O2 electrosynthesis and its industrial-scale applications.

Efficient electrosynthesis of HO2- from air for sulfide control in sewers

Electrochemically in-situ generation of oxygen and caustic soda is promising for sulfide management while suffers from scaling, poor inactivating capacity, hydrogen release and ammonia escape. In this study, the four-compartment electrochemical cell efficiently captured oxygen molecules from the air chamber to produce HO2- without generating toxic by-products. Meanwhile, the catalyst layer surface of PTFE/CB-GDE maintained a relatively balanced gas-liquid micro-environment, enabling the formation of enduring solid-liquid-gas interfaces for efficient HO2- electrosynthesis. A dramatic increase in HO2- generation rate from 453.3 mg L-1 h-1 to 575.4 mg L-1 h-1 was attained by advancement in operation parameters design (flow channels, electrolyte types, flow rates and circulation types). Stability testing resulted in the HO2- generation rate over 15 g L-1 and the current efficiency (CE) exceeding 85%, indicating a robust stable operational capacity. Furthermore, after 120 mg L-1 HO2- treatment, an increase of 11.1% in necrotic and apoptotic cells in the sewer biofilm was observed, higher than that achieved with the addition of NaOH, H2O2 method. The in-situ electrosynthesis strategy for HO2- represents a significance toward the practical implementation of sulfide abatement in sewers, holding the potential to treat various sulfide-containing wastewater.

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.

Sustainable integrated process for cogeneration of oxidants for VOCs removal

This study upgrades the sustainability of environmental electrochemical technologies with a novel approach consisting of the in-situ cogeneration and use of two important oxidants, hydrogen peroxide (H2O2) and Caro's acid (H2SO5), manufactured with the same innovative cell. This reactor was equipped with a gas diffusion electrode (GDE) to generate cathodically H2O2, from oxygen reduction reaction, a boron doped diamond (BDD) electrode to obtain H2SO5, via anodic oxidation of dilute sulfuric acid, and a proton exchange membrane to separate the anodic and the cathodic compartment, preventing the scavenging effect of the interaction of oxidants. A special design of the inlet helps this cell to reach simultaneous efficiencies as high as 99% for H2O2 formation and 19.7% for Caro's acid formation, which means that the cogeneration reaches efficiencies over 100% in the uses of electric current to produce oxidants. The two oxidants' streams produced were used with different configurations for the degradation of three volatile organic compounds (benzene, toluene, and xylene) in a batch reactor equipped with a UVC-lamp. Among different alternatives studied, the combination H2SO5/H2O2 under UVC irradiation showed the best results in terms of degradation efficiency, demonstrating important synergisms as compared to the bare technologies.

Electrochemical transformations catalyzed by cytochrome P450s and peroxidases

Cytochrome P450s (Cyt P450s) and peroxidases are enzymes featuring iron heme cofactors that have wide applicability as biocatalysts in chemical syntheses. Cyt P450s are a family of monooxygenases that oxidize fatty acids, steroids, and xenobiotics, synthesize hormones, and convert drugs and other chemicals to metabolites. Peroxidases are involved in breaking down hydrogen peroxide and can oxidize organic compounds during this process. Both heme-containing enzymes utilize active Fe^IVO intermediates to oxidize reactants. By incorporating these enzymes in stable thin films on electrodes, Cyt P450s and peroxidases can accept electrons from an electrode, albeit by different mechanisms, and catalyze organic transformations in a feasible and cost-effective way. This is an advantageous approach, often called bioelectrocatalysis, compared to their biological pathways in solution that require expensive biochemical reductants such as NADPH or additional enzymes to recycle NADPH for Cyt P450s. Bioelectrocatalysis also serves as an ex situ platform to investigate metabolism of drugs and bio-relevant chemicals. In this paper we review biocatalytic electrochemical reactions using Cyt P450s including C-H activation, S-oxidation, epoxidation, N-hydroxylation, and oxidative N-, and O-dealkylation; as well as reactions catalyzed by peroxidases including synthetically important oxidations of organic compounds. Design aspects of these bioelectrocatalytic reactions are presented and discussed, including enzyme film formation on electrodes, temperature, pH, solvents, and activation of the enzymes. Finally, we discuss challenges and future perspective of these two important bioelectrocatalytic systems.

Highly Efficient Electrochemical Production of Hydrogen Peroxide Using the GDE Technology

This work examines the role of oxygen supply in the improvement of the hydrogen peroxide (H₂O₂) electrochemical production efficiency and the generation of high H₂O₂ concentrations in electrochemical processes operated in a discontinuous mode. To conduct this study, a highly efficient Printex L6 carbon-based gas diffusion electrode (GDE) as a cathode was employed for the electrogeneration of H₂O₂ in a flow-by reactor and evaluated the effects of lowering the operation temperature (to increase solubility) and increasing the air supply in the system on H₂O₂ electrogeneration. The results obtained in this study show that unlike what is expected in flow-through reactors, the efficiency in the H₂O₂ production is not affected by the solubility of oxygen when GDE is employed in the electrochemical process (using the flow-by reactor); i.e., the efficiency of H₂O₂ production is not significantly dependent on O₂ solubility, temperature, and pressure. The application of the proposed PL6C-based GDE led to the generation of accumulated H₂O₂ of over 3 g L^-1 at a high current density. It should be noted, however, that the application of the electrocatalyst at lower current densities resulted in higher energy efficiency in terms of H₂O₂ production. Precisely, a specific production of H₂O₂ as high as 131 g kWh^-1 was obtained at 25 mA cm^-2; the energy efficiency (in terms of H₂O₂ production) values obtained in this study based on the application of the proposed GDE in a flow-by reactor at low current densities were found to be within the range of values recorded for H₂O₂ production techniques that employ flow-through reactors.

Electrogeneration of H2O2 by graphite felt double coated with polytetrafluoroethylene and polydimethylsiloxane

Efficient and steady electrogeneration of H₂O₂ is a significant step in the Electro-Fenton water treatment process. This study fabricates a polytetrafluoroethylene (PTFE) coated graphite felt cathode with a polydimethylsiloxane (PDMS) damp-proof coating to generate H₂O₂ in a flow-through system without an external oxygen supply. We evaluated the effect of PDMS content, current, flow rate, and pH on H₂O₂ production. PDMS coating inhibits electrowetting to extend the longevity of the modified graphite felt for electrogeneration of H₂O₂. However, increasing PDMS content can decrease H₂O₂ production due to reduction of active sites on the graphite felt. Graphite felt electrodes (surface area = 14.5 cm²) coated with 500 mg of PDMS can generate 10 mg/L of H₂O₂ under a flow rate of 3 mL/min with only 2% production reduction after 24-hour use. This modified graphite felt has better performance in a neutral or alkaline environment than in an acidic condition. Up to 38.5 mg/L of H₂O₂ will be generated at optimum current (120 mA) at the flow rate of 3 mL/min. Increasing the flow rate decreases the concentration of H₂O₂ in the electrolyte but enhances total production after 145 mins.

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.

A novel UV-LED hydrogen peroxide electrochemical photoreactor for point-of-use organic contaminant degradation

The degradation of organic contaminants is typically achieved by exposure of hydrogen peroxide (H₂O₂) containing influent to ultraviolet (UV) lamps as the source of radiation that can convert H₂O₂ to hydroxyl radicals (·OH), which oxidize organic pollutants. However, two factors prevent this process from being scaled down: the need to introduce H₂O₂, which requires special handling, and the use of bulky UV lamps, which have a high electric power consumption. In this work, an electrochemical cell was developed for the efficient in situ generation of H₂O₂ from water and atmospheric oxygen in a process called a two-electron oxygen reduction reaction (2e-ORR), so that the external addition of H₂O₂ is no longer needed. Moreover, the electrochemical cell was equipped with ultraviolet light-emitting diodes (UV-LEDs) to convert H₂O₂ to ·OH. The reactor exhibited a current efficiency of ∼90% for the H₂O₂ production at a flow rate of 50 mL min^-1. The degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) was studied at 277 nm based on different operational parameters, such as UV fluence rate, initial concentration, and initial pH. A high degradation of >70% was obtained at a UV output of 900 mW. Our approach, the first of its kind, has novel features applied, including: optimal radiation distribution in the reactor by applying a new UV source, UV-LEDs that offer much more control for the radiation profile in the reaction system compared to traditional UV lamps, controlled hydrodynamics by implementing special flow channels to provide a more uniform residence time and offer enhanced mixing, and integrating UV reactor and electrochemical cell in a single unit which could lead to superior performance and space efficiency of the device. These features make the device very suitable for point-of-use (POU) water treatment applications to eliminate both microbial and chemical contaminants.

Recent Progress of Electrochemical Production of Hydrogen Peroxide by Two-Electron Oxygen Reduction Reaction

Shifting electrochemical oxygen reduction reaction (ORR) via two-electron pathway becomes increasingly crucial as an alternative/green method for hydrogen peroxide (H2 O2 ) generation. Here, the development of 2e- ORR catalysts in recent years is reviewed, in aspects of reaction mechanism exploration, types of high-performance catalysts, factors to influence catalytic performance, and potential applications of 2e- ORR. Based on the previous theoretical and experimental studies, the underlying 2e- ORR catalytic mechanism is firstly unveiled, in aspect of reaction pathway, thermodynamic free energy diagram, limiting potential, and volcano plots. Then, various types of efficient catalysts for producing H2 O2 via 2e- ORR pathway are summarized. Additionally, the catalytic active sites and factors to influence catalysts' performance, such as electronic structure, carbon defect, functional groups (O, N, B, S, F etc.), synergistic effect, and others (pH, pore structure, steric hindrance effect, etc.) are discussed. The H2 O2 electrogeneration via 2e- ORR also has various potential applications in wastewater treatment, disinfection, organics degradation, and energy storage. Finally, potential future directions and prospects in 2e- ORR catalysts for electrochemically producing H2 O2 are examined. These insights may help develop highly active/selective 2e- ORR catalysts and shape the potential application of this electrochemical H2 O2 producing method.

Electrogeneration of H2O2 utilizing anodic O2 on polytetrafluoroethylene-modified cathode in flow-through reactor

Efficient electrogeneration of hydrogen peroxide (H₂O₂) is critical for treatment of refractory pollutants by the electro-Fenton process. An effective strategy is developed by combining a flow-through reactor with a poly- tetrafluoroethylene (PTFE)-modified graphite felt cathode. In this design, anodic oxygen is directly used for efficient H₂O₂ generation at the modified cathode. Experimental results show that the modified cathode with the optimum PTFE content can produce 29.6 mg/L of H₂O₂, which is 16 times higher than the unmodified graphite felt cathode for a flow rate of 3 mL/min. Maximum H₂O₂ production, up to 30.7 mg/L, was obtained under the following conditions: 120 mA, 3 mL/min, initial pH 13, no external aeration.

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.

pH Transitions and electrochemical behavior during the synthesis of iron oxide nanoparticles with gas-diffusion electrodes

Gas diffusion electrocrystallization (GDEx) was explored for the synthesis of iron oxide nanoparticles (IONPs). A gas-diffusion cathode was employed to reduce oxygen, producing hydroxyl ions (OH^-) and oxidants (H₂O₂ and HO₂ ^-), which acted as reactive intermediates for the formation of stable IONPs. The IONPs were mainly composed of pure magnetite. However, their composition strongly depended on the presence of a weak acid, i.e., ammonium chloride (NH₄Cl), and on the applied electrode potential. Pure magnetite was obtained due to the simultaneous action of H₂O₂ and the buffer capacity of the added NH₄Cl. Magnetite and goethite were identified as products under different operating conditions. The presence of NH₄Cl facilitated an acid-base reaction and, in some cases, led to cathodic deprotonation, forming a surplus of hydrogen peroxide, while adding the weak acid promoted gradual changes in the pH by slightly enhancing H₂O₂ production when increasing the applied potential. This also resulted in smaller average crystallite sizes as follows: 20.3 ± 0.6 at -0.350 V, 14.7 ± 2.1 at -0.550 and 12.0 ± 2.0 at -0.750 V. GDEx is also demonstrated to be a green, effective, and efficient cathodic process to recover soluble iron to IONPs, being capable of removing >99% of the iron initially present in the solution.

Highly efficient electrosynthesis of hydrogen peroxide on a superhydrophobic three-phase interface by natural air diffusion

Hydrogen peroxide (H₂O₂) synthesis by electrochemical oxygen reduction reaction has attracted great attention as a green substitute for anthraquinone process. However, low oxygen utilization efficiency (<1%) and high energy consumption remain obstacles. Herein we propose a superhydrophobic natural air diffusion electrode (NADE) to greatly improve the oxygen diffusion coefficient at the cathode about 5.7 times as compared to the normal gas diffusion electrode (GDE) system. NADE allows the oxygen to be naturally diffused to the reaction interface, eliminating the need to pump oxygen/air to overcome the resistance of the gas diffusion layer, resulting in fast H₂O₂ production (101.67 mg h^-1 cm^-2) with a high oxygen utilization efficiency (44.5%–64.9%). Long-term operation stability of NADE and its high current efficiency under high current density indicate great potential to replace normal GDE for H₂O₂ electrosynthesis and environmental remediation on an industrial scale.

H₂O₂ electrosynthesis has garnered great attention as a green alternative to the anthraquinone process. Here the authors propose a cost-effective cathode to greatly improve the O₂ diffusion coefficient, resulting in a high H₂O₂ production without the need for aeration.

Removal of Gaseous Elemental Mercury in a Diffusion Electrochemical Reactor Based on a Three-Dimensional Electrode

A novel three dimensional electrochemical reactor with nickel foam and carbon paper used as the anode and stainless steel mesh used as the cathodewas studied in this research. Oxidation mercury removal is performed in a self-made diffusion reactor. The influence of the electrolysis voltage, pH, gas flow, and other factors on mercury removal is discussed, as well as the mechanism of anodization mercury removal is explored. The experimental results show that nickel foam has a significant effect on the removal of Hg0, and 80-85% removal can be achieved under optimal conditions. Meanwhile, nickel foam has stable performance at high temperatures (60 °C) and in strong alkaline electrolytes, which also play an effective role in anodized oxidation. Although carbon paper is more stable than nickel foam and less affected by experimental factors, it is sensitive to reaction temperature and can only work in the neutral electrolyte at low temperatures. In contrast, electrochemical catalytic oxidation technology using the nickel foam is more promising for Hg0 removal.

Hydrogen peroxide generation from O2 electroreduction for environmental remediation: A state-of-the-art review

The electrochemical production of hydrogen peroxide (H2O2) by 2-electron oxygen reduction reaction (ORR) is an attractive alternative to the present complex anthraquinone process. The objective of this paper is to provide a state-of-the-arts review of the most important aspects of this process. First, recent advances in H2O2 production are reviewed and the advantages of H2O2 electrogeneration via 2-electron ORR are highlighted. Second, the selectivity of the ORR pathway towards H2O2 formation as well as the development process of H2O2 production are presented. The cathode characteristics are the decisive factors of H2O2 production. Thus the focus is shifted to the introduction of commonly used carbon cathodes and their modification methods, including the introduction of other active carbon materials, hetero-atoms doping (i.e., O, N, F, B, and P) and decoration with metal oxides. Cathode stability is evaluated due to its significance for long-term application. Effects of various operational parameters, such as electrode potential/current density, supporting electrolyte, electrolyte pH, temperature, dissolved oxygen, and current mode on H2O2 production are then discussed. Additionally, the environmental application of electrogenerated H2O2 on aqueous and gaseous contaminants removal, including dyes, pesticides, herbicides, phenolic compounds, drugs, VOCs, SO2, NO, and Hg0, are described. Finally, a brief conclusion about the recent progress achieved in H2O2 electrogeneration via 2-electron ORR and an outlook on future research challenges are proposed.

Simultaneous electro-oxidation and in situ electro-peroxone process for the degradation of refractory organics in wastewater

During electro-oxidation (EO) wastewater treatment, the applied voltage must polarize both a dimensionally stable anode with a sufficiently high potential to effectively produce hydroxyl radicals (OH), as well as a cathode with a sufficiently low potential to catalyze the H₂ evolution reaction (HER). Nevertheless, H₂ does not contribute to pollutant degradation and yet increases energy consumption. Inspired by fuel cell technology, in which the O₂ reduction reaction (ORR) is catalyzed on the cathode, herein, a carbon nanotube-coated carbon-PTFE gas diffusion electrode was fabricated to catalyze ORR during EO for the treatment of leachate concentrates. In comparison to conventional HER-EO, ORR-EO was shown to save 17.7-23.2% energy consumption. Further, as the cathodic ORR byproduct, H₂O₂ can react with the ozone generated from the Ti/SnO₂-Sb₂O₅ anode to catalyze the peroxone process, which enhances OH generation for the degradation of organic products. This in situ electro-peroxone process was determined by salicylic acid OH trapping and liquid chromatography. The novel simultaneous EO and in situ electro-peroxone process described herein has great application potential and economic merit in the degradation of refractory organics in wastewater.

A Three-dimensional Floating Air Cathode with Dual Oxygen Supplies for Energy-efficient Production of Hydrogen Peroxide

The in situ and cleaner electrochemical production of hydrogen peroxide (H2O2) through two-electron oxygen reduction reaction has drawn increasing attentions in environmental applications as an alterantive to traditional anthraquinone process. Air cathodes avoid the need of aeration, but face the challenges of declined performance during scale-up due to non-uniform water infiltration or even water leakage, which is resulted from changing water pressures and immature cathode fabrication at a large scale. To address these challenges, a three-dimensional (3-D) floating air cathode (FAC) was built around the commercial sponge, by coating with carbon black/poly(tetrafluoroethylene) using a simple dipping-drying method. The FAC floated on the water-air interface without extensive water-proof measures, and could utilize oxygen both from passive diffusion and anodic oxygen evolution to produce H2O2. The FAC with six times of dipping treatment produced a maximum H2O2 concentration of 177.9 ± 26.1 mg L-1 at 90 min, with low energy consumption of 7.1 ± 0.003 Wh g-1 and stable performance during 10 cycles of operation. Our results showed that this 3-D FAC is a promising approach for in situ H2O2 production for both environmental remediation and industrial applications.

A bio-electro-Fenton system with a facile anti-biofouling air cathode for efficient degradation of landfill leachate

Bio-electro-Fenton (BEF) system holds great potential for sustainable degradation of refractory organics. Activated carbon (AC) air cathode was modified by co-pyrolyzing of AC with glucose and doping with nano-zero-valent iron (denoted as nZVI@MAC) in order to promote two-electron oxygen reduction reaction (2e^- ORR) for enhanced oxidizing performance. Single chamber microbial fuel cells (SCMFCs) with nZVI@MAC cathode was examined to degrade landfill leachate. It was revealed that nZVI@MAC cathode SCMFC showed higher degradation efficiency towards landfill leachate. Six landfill leachate treatment cycles indicated that nZVI@MAC cathode SCMFC exhibited higher COD removal efficiencies over AC and nZVI@AC and greatly enhanced columbic efficiency compared to AC and nZVI@AC cathode. Anti-biofouling effect was found on nZVI@MAC cathode because of the high Fenton oxidation effects at the vicinity of the cathode. Electrochemical characterizations indicated that MAC cathode had superior 2e^- ORR capability than AC and nZVI@AC cathode, which was further evidenced by higher H₂O₂ production from nZVI@MAC cathode in SCMFC. Graphitic structure of MAC was evidenced by High Resolution Transmission Electron Microscopy, and glucose pyrolysis also resulted in nano carbon spheres on the activated carbon skeletons. Raman spectra indicated more defects were generated on MAC during its co-pyrolyzation with glucose.

Tradeoff between groundwater arsenite removal efficiency and current production in the self-powered air cathode electrocoagulation with different oxygen reduction pathways

Naturally occurring arsenic enrichment in aquifers posts a huge threat to drinking water safety. To achieve energy-efficient arsenite [As(III)] removal, a self-powered iron electrocoagulation was developed that coupled iron corrosion anode with oxygen reduction air cathode for simultaneous As(III) oxidation and removal. Activated carbon (AC), which favored the four-electron oxygen reduction reaction (ORR, O₂+4e^-+4H⁺→2H₂O, E^0' = 0.816 V), and carbon black (CB), which favored two-electron ORR (O₂+2e^-+2H⁺→H₂O₂, E^0' = 0.283 V), were evaluated for As(III) removal efficiency and current production performance. The comparison showed a tradeoff between higher current (i.e., higher iron corrosion rate) attributed to the higher reduction potential with four-electron ORR, and higher H₂O₂ production for improved As(III) oxidation with two-electron ORR yet the lower reduction potential The CB cathode that favored H₂O₂ production had the best As(III) removal of 99.2 ± 0.4% and the lowest maximum power density of 60 ± 0.3 mW m^-2, while the AC cathode showed the opposite trend. These results suggested that cathode catalysts need to be carefully evaluated for the balance of As(III) removal and current production to provide a sustainable and effective solution for groundwater As(III) removal.

Synthesis, characterization and evaluations of TiO2 nanostructures prepared from different titania precursors for photocatalytic degradation of 4-chlorophenol in aqueous solution

BACKGROUND

The aim of present work, was to synthesize the titanium nanoparticles (TNPs) and titanium nanotubes (TNTs) through the hydrothermal method with different precursors including the Titanium(IV) isopropoxide (TTIP) and Titanium(IV) bis(ammonium lactato)dihydroxide (TALH).

METHODS

TiO₂ nanostructures from different titania precursors as heterogeneous photocatalysis via hydrothermal method were synthesized. The as-prepared photocatalysts were characterized by X-ray diffraction, UV-Vis diffuse reflectance spectra, surface area measurements, Fourier transform infrared spectroscopy and field emission scanning electron microscopy. The TiO₂ photocatalysts were tested on the degradation of 4-Chlorophenol (4-CP) aqueous solution under UVC irradiation in a fabricated photoreactor.

RESULTS

The effect of operating parameters including the; initial 4-CP concentration (50-150 mg/L), catalyst dosages (0-3 g/L) and solution pH (4-10) on the photocatalytic activity of the prepared catalysts were systematically investigated. The results show that amongst the TiO₂ nanostructures under best conditions (initial 4-CP concentration of 50 mg/L, catalyst dosage of 2 g/L, pH of 4.0, Time of 180 min) TNT-P2 exhibited much higher photocatalytic degradation efficiency (82%) as compared with TNT-P1 (77%), TNP-P2 (51%), and TNP-P1 (48%). Moreover, the mechanism and tentative pathways of 4-CP degradation were explored. Finally, the kinetic study was performed and the Langmuir-Hinshelwood kinetic model was aptly fitted with the experimental data.

CONCLUSION

The results of the photocatalytic activity measurement demonstrated that one-dimensional TNTs shows enhanced photocatalytic performance as compared to the TNPs, therefore, indicating the beneficial feature of TNTs as a photocatalyst for the degradation of pollutants. Besides, TiO₂ nanostructures prepared from TALH precursor (TNT-P2 82%, TNP-P2 51%) has higher photocatalytic degradation efficiency as compared with TTIP precursors (TNT-P1 77%, TNP-P1 48%).

Oxygen Reduction by Homogeneous Molecular Catalysts and Electrocatalysts

The oxygen reduction reaction (ORR) is a key component of biological processes and energy technologies. This Review provides a comprehensive report of soluble molecular catalysts and electrocatalysts for the ORR. The precise synthetic control and relative ease of mechanistic study for homogeneous molecular catalysts, as compared to heterogeneous materials or surface-adsorbed species, enables a detailed understanding of the individual steps of ORR catalysis. Thus, the Review places particular emphasis on ORR mechanism and thermodynamics. First, the thermochemistry of oxygen reduction and the factors influencing ORR efficiency are described to contextualize the discussion of catalytic studies that follows. Reports of ORR catalysis are presented in terms of their mechanism, with separate sections for catalysis proceeding via initial outer- and inner-sphere electron transfer to O₂. The rates and selectivities (for production of H₂O₂ vs H₂O) of these catalysts are provided, along with suggested methods for accurately comparing catalysts of different metals and ligand scaffolds that were examined under different experimental conditions.

Synthesis of new photosensitive H2BBQ2+[ZnCl4]2−/[(ZnCl)2(μ-BBH)] complexes, through selective oxidation of H2O to H2O2

A two-electron photosensitive H₂O to H₂O₂ oxidizer, H₂BBQ²⁺[ZnCl₄]²⁻/[(ZnCl)₂(μ-BBH)], has been synthesized. An aqueous {[(ZnCl)₂(μ-BBH)]||H₂O₂} solar rechargeable galvanic cell has been constructed.