Application and stability of cathodes with manganese dioxide nanoflowers supported on Vulcan by Fenton systems for the degradation of RB5 azo dye

Hydrogen peroxide electrogeneration from O2 electroreduction: A review focusing on carbon electrocatalysts and environmental applications

Hydrogen peroxide (H2O2) stands as one of the foremost utilized oxidizing agents in modern times. The established method for its production involves the intricate and costly anthraquinone process. However, a promising alternative pathway is the electrochemical hydrogen peroxide production, accomplished through the oxygen reduction reaction via a 2-electron pathway. This method not only simplifies the production process but also upholds environmental sustainability, especially when compared to the conventional anthraquinone method. In this review paper, recent works from the literature focusing on the 2-electron oxygen reduction reaction promoted by carbon electrocatalysts are summarized. The practical applications of these materials in the treatment of effluents contaminated with different pollutants (drugs, dyes, pesticides, and herbicides) are presented. Water treatment aiming to address these issues can be achieved through advanced oxidation electrochemical processes such as electro-Fenton, solar-electro-Fenton, and photo-electro-Fenton. These processes are discussed in detail in this work and the possible radicals that degrade the pollutants in each case are highlighted. The review broadens its scope to encompass contemporary computational simulations focused on the 2-electron oxygen reduction reaction, employing different models to describe carbon-based electrocatalysts. Finally, perspectives and future challenges in the area of carbon-based electrocatalysts for H2O2 electrogeneration are discussed. This review paper presents a forward-oriented viewpoint of present innovations and pragmatic implementations, delineating forthcoming challenges and prospects of this ever-evolving field.

Facile Gram-Scale Synthesis of NiO Nanoflowers for Highly Selective and Sensitive Electrocatalytic Detection of Hydrazine

The design and development of efficient and electrocatalytic sensitive nickel oxide nanomaterials have attracted attention as they are considered cost-effective, stable, and abundant electrocatalytic sensors. However, although innumerable electrocatalysts have been reported, their large-scale production with the same activity and sensitivity remains challenging. In this study, we report a simple protocol for the gram-scale synthesis of uniform NiO nanoflowers (approximately 1.75 g) via a hydrothermal method for highly selective and sensitive electrocatalytic detection of hydrazine. The resultant material was characterized by scanning electron microscopy, X-ray photoelectron spectroscopy, and X-ray diffraction. For the production of the modified electrode, NiO nanoflowers were dispersed in Nafion and drop-cast onto the surface of a glassy carbon electrode (NiO NF/GCE). By cyclic voltammetry, it was possible to observe the excellent performance of the modified electrode toward hydrazine oxidation in alkaline media, providing an oxidation overpotential of only +0.08 V vs Ag/AgCl. In these conditions, the peak current response increased linearly with hydrazine concentration ranging from 0.99 to 98.13 μmol L^-1. The electrocatalytic sensor showed a high sensitivity value of 0.10866 μA L μmol^-1. The limits of detection and quantification were 0.026 and 0.0898 μmol L^-1, respectively. Considering these results, NiO nanoflowers can be regarded as promising surfaces for the electrochemical determination of hydrazine, providing interesting features to explore in the electrocatalytic sensor field.

Integrated electro-Fenton system based on embedded U-tube GDE for efficient degradation of ibuprofen

Ibuprofen (IBP) is a carcinogenic non-steroidal anti-inflammatory drug (NSAID). It is of certain hazard to aquatic animals and may cause potential harm to human health. As traditional methods cannot effectively remove such a pollutant, many advanced oxidation processes (AOPs) have been developed for its degradation. The electro-Fenton process has the advantages of strong oxidative ability, a synergistic effect of various degradation processes, and a wide application range. This study developed a high-performance gas diffusion electrode (GDE) for electrochemical hydrogen peroxide (H2O2) production. The optimum system performance was found at the current density of 10 mA cm-2, pH of 7.0, and air flow rate at 0.6 L min-1, where the accumulation of H2O2 could reach as high as 769.82 mg L-1. The computational fluid dynamics (CFD) simulation results revealed a fast mass-transfer property in this electro-Fenton system with U-tube GDEs, which resulted in a deep-level degradation (∼100%) of the pollutant (IBP) and a low-concentration degradation of 10 mg L-1 within a 120-min reaction period. The high-performance liquid chromatography-mass spectrometry (LC-MS) studies demonstrated that the hydroxyl radicals were the primary active species in the electro-Fenton system and that the degradation intermediates of IBP were mainly 1-(4-isobutylphenyl) ethanol and 2-hydroxy-2-(4-isobutyl phenyl) propanoic acid through four probable electro-Fenton degradation pathways. This report provides a facile and efficient way to construct a high-performance electro-Fenton reactor, which could be effectively used in advanced oxidation processes (AOPs) to remove emerging contaminants in wastewater and natural water.

Decontamination of wastewater containing contaminants of emerging concern by electrooxidation and Fenton-based processes - A review on the relevance of materials and methods

In recent years, there has been an increasingly growing interest regarding the use of electrochemical advanced oxidation processes (EAOPs) which are considered highly promising alternative treatment techniques for addressing environmental issues related to pollutants of emerging concern. In EAOPs, electrogenerated oxidizing agents, such as hydroxyl radical (HO^•), can react non-selectively with a wide range of organic compounds, degrading and mineralizing their structures to unharmful molecules like CO₂, H₂O, and inorganic ions. To this date, a broad spectrum of advanced electrocatalysts have been developed and applied for the treatment of compounds of interest in different matrices, specifically aiming at enhancing the degradation performance. New combined methods have also been employed as alternative treatment techniques targeted at circumventing the major obstacles encountered in Fenton-based processes, such as high costs and energy consumption, which still contribute significantly toward inhibiting the large-scale application of these processes. First, some fundamental aspects of EAOPs will be presented. Further, we will provide an overview of electrode materials which have been recently developed and reported in the literature, highlighting different anode and cathode structures employed in EAOPs, their main advantages and disadvantages, as well as their contribution to the performance of the treatment processes. The influence of operating parameters, such as initial concentrations, pH effect, temperature, supporting electrolyte, and radiation source, on the treatment processes were also studied. Finally, hybrid techniques which have been reported in the literature and critically assess the most recent techniques used for evaluating the degradation efficiency of the treatment processes.

Photodegradation assessment of RB5 dye by utilizing WO3/TiO2 nanocomposite: a cytotoxicity study

Textile dyeing wastewater becomes one of the root causes of environmental pollution. Titanium dioxide (TiO₂) is one of the photocatalysts that shows prominent organic dye photodegradation ability. In this study, a porous tungsten oxide (WO₃)/TiO₂ composite was prepared through ultrasonic-assisted solvothermal technique with varying amounts of WO₃ ranging from 0.25 to 5 weight % (wt.%). The prepared 0.50 wt.% WO₃/TiO₂ (0.50WTi) composite exhibited the highest photodegradation activity (4.39 × 10^-2 min^-1) and complete mineralization in chemical oxygen demand (COD) reading towards 30 mg.L^-1 of Reactive Black 5 (RB5) dye under 60 min of light irradiation. Effects of large surface area, small crystallite size, high pore volume and size, and low electron-hole pair recombination rate attributed to the superiority of 0.50WTi. Besides, 0.50WTi could be reused, showing 86.50% of RB5 photodegradation at the fifth cycle. Scavenger study demonstrated that photogenerated hole (h⁺) was the main active species of 0.50WTi to initiate the RB5 photodegradation. Cytotoxicity assessment determined the readings of half-maximal inhibitory concentration (IC₅₀) were 1 mg.mL^-1 and 0.61 mg.mL^-1 (24 and 72 h of incubations) for the 0.50WTi composite.

A critical review on metal complexes removal from water using methods based on Fenton-like reactions: Analysis and comparison of methods and mechanisms

Metals mainly exist in the form of complexes in urban wastewater, fresh water and even drinking water, which are difficult to remove and further harm human health. Fenton-like reaction has been used for the removal of metal complexes. Effective removal of metal complexes using Fenton-like reaction requires the removal of both metals and organic ligands, meanwhile, the fate of metals and organic pollutions must be clearly understood. Thus, this review summarizes the relevant research on metal complex removal from using Fenton-like reactions in the past ten years, with the detailed removal approaches and mechanisms analyzed. Electro-, photo-, microwave/ultrasound-Fenton reactions or the synergistic Fenton reaction have been shown to exhibit excellent metal complex treatment capabilities. Furthermore, various catalysts, such as transition metals, bimetals and metal-free catalytic systems can expand the potential applications of Fenton-like reactions. Novel Fenton reaction methods without the addition of metals or H₂O₂, with construction of a dual active center catalyst, or with the introduction of other free radicals, are all worthy of further investigation. Due to increasing levels of environmental metal and organic pollutions remediation requirements, more research is required for the development of economical and efficient novel Fenton-like processes.

A review on the photoelectro-Fenton process as efficient electrochemical advanced oxidation for wastewater remediation. Treatment with UV light, sunlight, and coupling with conventional and other photo-assisted advanced technologies

Wastewaters containing recalcitrant and toxic organic pollutants are scarcely decontaminated in conventional wastewater facilities. Then, there is an urgent challenge the development of powerful oxidation processes to ensure their organic removal in order to preserve the water quality in the environment. This review presents the recent development of an electrochemical advanced oxidation process (EAOP) like the photoelectro-Fenton (PEF) process, covering the period 2010-2019, as an effective treatment for wastewater remediation. The high oxidation ability of this photo-assisted Fenton-based EAOP is due to the combination of in situ generated hydroxyl radicals and the photolytic action of UV or sunlight irradiation over the treated wastewater. Firstly, the fundamentals and characteristics of the PEF process are described to understand the role of oxidizing agents. Further, the properties of the homogeneous PEF process with iron catalyst and UV irradiation and the benefit of sunlight in the homogeneous solar PEF one (SPEF) are discussed, supported with examples over their application to the degradation and mineralization of synthetic solutions of industrial chemicals, herbicides, dyes and pharmaceuticals, as well as real wastewaters. Novel heterogeneous PEF processes involving solid iron catalysts or iron-modified cathodes are subsequently detailed. Finally, the oxidation power of hybrid processes including photocatalysis/PEF, solar photocatalysis/SPEF, photoelectrocatalysis/PEF and solar photoelectrocatalysis/SPEF, followed by that of sequential processes like electrocoagulation/PEF and biological oxidation coupled to SPEF, are analyzed.

Expanding the application of photoelectro-Fenton treatment to urban wastewater using the Fe(III)-EDDS complex

This work reports the first investigation on the use of EDDS as chelating agent in photoelectro-Fenton (PEF) treatment of water at near-neutral pH. As a case study, the removal of the antidepressant fluoxetine was optimized, using an electrochemical cell composed of an IrO₂-based anode an air-diffusion cathode for in-situ H₂O₂ production. Electrolytic trials at constant current were made in ultrapure water with different electrolytes, as well as in urban wastewater (secondary effluent) at pH 7.2. PEF with Fe(III)-EDDS (1:1) complex as catalyst outperformed electro-Fenton and PEF processes with uncomplexed Fe(II) or Fe(III). This can be explained by: (i) the larger solubilization of iron ions during the trials, favoring the production of ^•OH from Fenton-like reactions between H₂O₂ and Fe(II)-EDDS or Fe(III)-EDDS, and (ii) the occurrence of Fe(II) regeneration from Fe(III)-EDDS photoreduction, which was more efficient than conventional photo-Fenton reaction with uncomplexed Fe(III). The greatest drug concentration decays were achieved at low pH, using only 0.10 mM Fe(III)-EDDS, although complete removal in wastewater was feasible only with 0.20 mM Fe(III)-EDDS due to the greater formation of ^•OH. The effect of the applied current and anode nature was rather insignificant. A progressive destruction of the catalytic complex was unveiled, whereupon the mineralization mainly progressed thanks to the action of ^•OH adsorbed on the anode surface. Despite the incomplete mineralization using BDD as the anode, a remarkable toxicity decrease was determined. Fluoxetine degradation yielded F^- and NO₃^- ions, along with several aromatic intermediates. These included two chloro-organics, as a result of the anodic oxidation of Cl^- to active chlorine. A detailed mechanism for the Fe(III)-EDDS-catalyzed PEF treatment of fluoxetine in urban wastewater is finally proposed.

Removal of Orange II (OII) dye by simulated solar photoelectro-Fenton and stability of WO2.72/Vulcan XC72 gas diffusion electrode

The objectives of this study were to determine the viability of removing Orange II (OII) dye by simulated solar photoelectro-Fenton (SSPEF) and to evaluate the stability of a WO_2.72/Vulcan XC72 gas diffusion electrode (GDE) and thus determine its best operating parameters. The GDE cathode was combined with a BDD anode for decolorization and mineralization of 350 mL of 0.26 mM OII by anodic oxidation with electrogenerated H₂O₂ (AO-H₂O₂), electro-Fenton (EF) and photoelectro-Fenton (PEF) at 100, 150 and 200 mA cm^-2 and SSPEF at 150 mA cm^-2. The GDE showed successful operation for electrogeneration, good reproducibility and low leaching of W. Decolorization and OII decay were directly proportional to the current density (j). AO-H₂O₂ had a reduced performance that was only half of the SSPEF, PEF and EF treatments. The mineralization efficiency was in the following order: AO-H₂O₂ < EF < PEF ≈ SSPEF. This showed that the GDE, BDD anode and light radiation combination was advantageous and indicated that the SSPEF process is promising with both a lower cost than using UV lamps and simulating solar photoelectro-Fenton process. The PEF process with the lowest j (100 mA cm^-2) showed the best performance-mineralization current efficiency.

A nanoscale "yarn ball"-like heteropoly blue catalyst for extremely efficient elimination of antibiotics and dyes

Fenton system is one of the most popular methods to eliminate antibiotics and dyes in aquatic environment. However, the existed Fenton system is limited by various factors such as potential second pollution and narrow pH range. In this study, we report that the bottlenecks for high strength antibiotics and dyes wastewater treatment at a wide pH range can be well tackled by the nanoscale "yarn ball"-like Mo/W-containing heteropoly blue (HPB) catalyst Mg₂Ti₆Mo₂₃O₁₁₉SiW₁₂ (1). This novel catalyst displayed extremely efficient elimination for several typical organic contaminants such as malachite green (MG), tetracycline (TC) and methyl orange (MO). Compared with other materials reported in previous papers, the catalytic performance of 1 in degradation of the organic contaminants of high concentrations increased several times. More than 90% of antibiotics and dyes are degraded within 60 min. Electron spin resonance (ESR) experiments and UV-vis spectra confirmed that the catalytic mechanisms of 1 could mainly ascribe to the 1/H₂O₂ process and the possible photocatalytic oxidation of adsorbed H₂O by holes (h⁺) in the valence band (VB) of 1 surface generated ·OH for extremely efficient degradation of organic contaminants. This work widens the optimal pH values up to neutral condition and it's significant for the expansion of the heterogeneous Fenton-like catalyst family and its application in the field of water treatment.

Porous Organic Polymers Containing a Sulfur Skeleton for Visible Light Degradation of Organic Dyes

Three novel chemically stable porous organic polymers (POPs) were synthesized by the hydrothermal method; the POPs contain sulfone bonds (TpSD), no sulfur atoms (TpMD), or thioether bonds (TpTD). The catalytic mechanism of the POP with sulfone bonds was studied, and it was found that the wide visible light absorption range, high specific surface area, and the hydrophilicity of the material significantly promoted the catalytic efficiency of TpSD. The presence of O=S=O gives TpSD a higher degree of conjugation than TpMD and TpTD, so TpSD shows the strongest UV/Visible absorption and faster transmission of electrons. The photocatalytic degradation of Rhodamine B (RhB) molecules is approximately 100 % with TpSD and its pseudo-first-order rate constant is 0.0770 min^-1 , which is the highest among all reported non-metallic photocatalysts. Moreover, it is also the first time that sulfur-containing polymer have been used in photocatalytic degradation of dyes.

Efficient degradation of Orange G with persulfate activated by recyclable FeMoO4

In this study, FeMoO₄ was applied to activate persulfate (PS, S₂O₈^2-) for azo dye Orange G (OG) degradation. The catalyst was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM), transmission electron microscope (TEM), X-ray photoelectron spectroscopy (XPS) and nitrogen adsorption-desorption isotherms. FeMoO₄ showed excellent efficiency in activating PS for OG removal. More than 95% could be removed after 40 min under reaction conditions of 4 mM PS, 0.3 g L^-1 FeMoO₄ and 0.2 mM OG. The effect of different parameters (PS doses, FeMoO₄ doses and pH) were evaluated. The results showed that acid condition provided higher efficiency and overdosing FeMoO₄ and PS presented a scavenging effect. Major intermediates were identified and possible degradation pathway was proposed. Recycle tests presented that FeMoO₄ had excellent recyclable stability in activating PS for OG removal. Sulfate radicals and hydroxyl radicals all occurred in the oxidation reactions and the former came first. The oxidation reaction was involved in the translation of Fe²⁺/Fe³⁺ occurred on the surface layer. This study revealed that the FeMoO₄/PS system is a very promising method for degrading organic contaminants in the environment.