Electrochemical oxidation of methyl orange azo dye at pilot flow plant using BDD technology

Bimetallic Organic Gel for Effective Methyl Orange Dye Adsorption

A bimetallic organic gel (MOG-Fe/Al) was synthesized through the solvothermal method. The gel state of the product obtained under optimized gel formation conditions is sufficient to carry 2 g of weight for a long time. Scanning electron microscopy (SEM), X-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, Brunauer-Emmett-Teller (BET) technique, and X-ray photoelectron spectroscopy (XPS) analysis confirmed the structures and morphologies of the synthesized materials. MOG-Fe/Al, with good stability, excellent durability, and wide applicability, exhibited efficient MO adsorption capacity as high as 335.88 mg/g at 25 °C. Adsorption-influencing factors including solution pH, contact time, and temperature were investigated. The adsorption performance of the bimetallic organic gel was better than that of the monometallic organic gels (MOG-Fe and MOG-Al), and its adsorption processes were in accordance with the pseudo-second-order kinetic and Langmuir isothermal models. The excellent adsorption capacity of the MOG-Fe/Al is due to its surface structure, pore volume, π-π interactions, hydrogen bonds, and electrostatic interactions.

Electrochemical-based processes for produced water and oily wastewater treatment: A review

The greatest volume of by-products produced in oil and gas recovery operations is referred to as produced water and increasing environmental concerns and strict legislations on discharging it into the environment cause to more attention for focusing on degradation methods for treatment of produced water especially electrochemical technologies. This article provides an overview of electrochemical technologies for treating oily wastewater and produced water, including: electro-coagulation, electro-Fenton, electrochemical oxidation and electrochemical membrane reactor as a single stage and combination of these technologies as multi-stage treatment process. Many researchers have carried out experiments to examine the impact of various factors such as material (i.e, electrode material) and operational conditions (i.e., potential, current density, pH, electrode distance, and other factors) for organic elimination to obtain the high efficiency. Results of each method are reviewed and discussed according to these studies, comprehensively. Furthermore, several challenges need to be overcome and perspectives for future study are proposed for each method.

Multiple design and modelling approaches for the optimisation of carbon felt electro-Fenton treatment of dye laden wastewater

This study utilizes artificial intelligence and statistical modelling to optimize the operating parameters of a carbon-based electro-Fenton process for purifying model dye (RB19)-contaminated wastewater. Multilevel experimental Box-Behnken and uniform deisgns (BBD, UD) with four variables were analysed using polynomial regression analysis (PRA) and artificial neural networks (ANN), while the process optimisation was done using desirability function. For the given testing range but different design matrices and runs, both designs predicted a maximum RB19 removal (RB19-RR) of 90 ± 2.1% at lowest energy consumption (EC) of 0.44 ± 2.5 Wh, when voltage, Na2SO4, FeSO4, and time were maintained as follows: 4-5.3 V, 7-11 mM, 0.4-0.6 mM, and 35-40 min, respectively. All the design-model combinations portrayed the similar senitivity analyses, revealing that RB19 degradation and EC are primarily influenced by electrolysis time and voltage. The performance assessment demonstrated that all the design-model combinations also excellently predicted for unseen conditions as the maximum root mean squared error (RMSE) value for RB19-RR was 4.07, while it was 0.072 for EC, however, BBD-ANN performance proved to be slightly better than others. Having ∼57% less experimentation, UD based models managed to accurately predict the results for unseen conditions as the statistical errors were quite insignificant, even in some cases, RMSE found to be less for UD compared to BBD, elucidating the potential of uniform design as an alternative of conventional factorial designs. Nevertheless, the prediction accuracy is also dependent on modelling approach, as in some cases ANN failed to predict the response precisely specially when dealing with small data. Furthermore, techno-economic evaluation results spell out the efficacy of carbon felt based enhanced electro-Fenton process as promising environmental remediation technology and highlight its practical implication from view of operational cost.

Convex optimization for maximizing the degradation efficiency of chloroquine in a flow-by electrochemical reactor

The degradation efficiency of chloroquine phosphate (CQ), an anti-COVID-19 drug, was investigated in a flow-by electrochemical reactor (FBER) provided with two boron-doped diamond (BDD) electrodes (as cathode and anode) under batch recirculation mode. A central composite rotatable design (CCRD) was run down to model and assess the influence of initial pH in an interval of 3.71 to 11.28, the current density in an interval of 34.32 to 185.68 mA cm-2, and liquid volumetric flow rate in an interval of 0.58 to 1.42 L min-1, and conduct the convex optimization to obtain the maximum degradation efficiency. Experimental results were modeled through a second-order polynomial equation having a determination coefficient (R2) of 0.9705 with a variance coefficient of 1.1%. Optimal operating conditions found (initial pH of 5.38, current density (j) of 34.4 mA cm-2, and liquid flow rate (Q) of 1.42 L min-1) led to a global maximum degradation efficiency, COD removal efficiency, and mineralization efficiency of 89.3, 51.6 and 53.1%, respectively, with an energy consumption of 0.041 kWh L-1 within 9 h of treatment. Additionally, a pseudo-zero-order kinetic model was demonstrated to fit the experimental data and the calculated pseudo-zero-order kinetic constant (kapp) was 13.14 mg L-1 h-1 (2.54 × 10-5 mol dm-3 h-1). Furthermore, the total operating cost was of 0.47 US$ L-1. Finally, this research could be helpful for the treatment of wastewater containing an anti-COVID-19 drug such as CQ.

Supplementary Information

The online version contains supplementary material available at 10.1007/s10008-023-05452-7.

Electrochemical degradation and mineralisation of organic dyes in aqueous nitrate solutions

Electrochemical treatment of organic matter for environmental remediation necessitates the development of cheap and robust electrodes that are chemically and structurally stable. To address this challenging requirement, we demonstrate a new electrochemical approach using a simple copper electrode under cathodic conditions to electrochemically generate reactive nitrosonium ions for the degradation of different classes of synthetic organic dyes. This could be achieved in an aqueous HNO₃/KNO₃ electrolyte at a relatively low cathodic potential of -0.5 V RHE at room temperature. UV-visible absorption spectroscopy, Raman spectroscopy, liquid chromatography - mass spectrometry and total organic carbon measurements revealed the rapid decolorisation and mineralisation of several dye types such as triarylmethane dyes (crystal violet, cresol red), an azo dye (methyl orange) as well as a sulfur containing thiazine dye (toluidine blue). The total organic carbon content of a 50 mg L^-1 methyl orange solution was found to decrease by 83% after 1 h of electrolysis. Promisingly, locally sourced river and creek water samples spiked with 50 mg L^-1 methyl orange were also successfully treated for up to 6 cycles at a simple Cu electrode, demonstrating potential for the remediation of polluted waterways.

Electrochemical removal of synthetic methyl orange dyeing wastewater by reverse electrodialysis reactor: Experiment and mineralizing model

In this paper, the synthetic methyl orange (MO) dyeing wastewater treated by a reverse electrodialysis reactor (REDR) with 40 member pairs was investigated first. The boron-doped diamond (BDD) and carbon felt were adopted as an anode and a cathode in the REDR. The influences of operation parameters on the chemical oxygen demand (COD) removal efficiency were detected and explored. Then, a mathematical model of organic mineralizing was developed for the REDR to predict the variation of COD removal efficiency with treating time under the different operation conditions. Finally, the energy consumption of the wastewater treated by the REDR was analyzed. The results showed that raising the working fluid flowing velocity and electrode rinse solution flowrate improved the COD removal efficiency and instantaneous current efficiency (ICE), and reduced the total energy consumption (TEC) of the REDR. Raising the initial MO concentration could significantly reduce the TEC despite the COD removal efficiency being near. Since the main energy consumed by the REDR was salinity gradient energy (SGE) from waste heat conversion or the natural environment, the energy cost of REDR treating wastewater has been reduced significantly.

Electro-Chemical Degradation of Norfloxacin Using a PbO2-NF Anode Prepared by the Electrodeposition of PbO2 onto the Substrate of Nickel Foam

A novel three-dimensional network nickel foam/PbO2 combination electrode (PbO2-NF) with high electrochemical degradation efficiency to norfloxacin was successfully fabricated through the electrodeposition of PbO2 on the substrate of nickel foam. The characterization of an PbO2-NF electrode, including surface morphology, elemental components, electrochemical performance, and stability was performed. In electrochemical oxidation tests, the removal efficiency of norfloxacin (initial concentration for 50 mg/L) on PbO2-NF reached 88.64% within 60 min of electrolysis, whereas that of pure nickel foam was only 30%. In the presence of PbO2-NF, the optimum current density, solution pH, electrode spacing for norfloxacin degradation were 30 mA/cm2, 11, and 3 cm, respectively. The electric energy consumption for 80% norfloxacin was approximately 5 Wh/L. Therefore, these results provide a new anode to improve the removal of norfloxacin in the wastewater with high efficiency and low energy consumption.

Effect of additional Fe2+ salt on electrocoagulation process for the degradation of methyl orange dye: An optimization and kinetic study

The wastewater generated from textile industries is highly colored and contains dyes including azo dyes, which are toxic to human and water-living organisms. The treatment of these azo dyes using conventional treatment techniques is challenging due to their recalcitrant properties. In the current study, the effect of additional Fe²⁺ on electrocoagulation (EC) using Fe electrodes has been studied for the removal of methyl orange (MO) azo dye. pH between 4-5 was found to be optimum for EC and treatment efficiency decreased with increasing dye concentrations. With the addition of Fe²⁺ salt, dye removal for a certain concentration was increased with the increase of current density and Fe²⁺ up to a certain limit and after that, the removal efficiency decreased. The COD, color and dye removals were 88.5%, 93.1% and 100%, respectively, for EC of 200 mg.L⁻¹ dye solution using only 0.20 mmol.L⁻¹ Fe²⁺ for 0.40 mA cm⁻² current density, whereas for EC, the respective removal efficiencies were 76.7%, 63.4% and 82.4% for 32 min. The respective operating cost for EC was $768 kg⁻¹ removed dye ($0.342 m⁻³), whereas, for EC with additional Fe²⁺ salt, it was $350 kg⁻¹ removed dye ($0.189 m⁻³). The kinetic results revealed that the first-order kinetic model was fitted best for EC, whereas the second-order kinetic model was best fitted for Fe²⁺ added EC. For real textile wastewater, 57.6% COD removal was obtained for 0.15 mmol.L⁻¹ Fe²⁺ added EC compared to 27.8% COD removal for EC for 32 min. Based on the study we can conclude that Fe²⁺ assisted EC can be used for effective treatment of textile wastewater containing toxic compounds like azo dyes.

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EC represents limiting treatment performance for higher contaminant concentrations.

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0.20 mmol.L⁻¹ Fe²⁺ salt enhances the EC treatment performance of MO dye to 100%.

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EC followed first-order kinetic model, whereas Fe²⁺ added EC followed second-order kinetic model.

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Operating cost was reduced to $0.327 m⁻³ from $0.598 m⁻³ for EC with additional Fe²⁺.

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58% COD was removed for 0.15 mmol.L⁻¹ Fe²⁺ added EC for real textile wastewater.

Electro-oxidation of Ni (II)-citrate complexes at BDD electrode and simultaneous recovery of metallic nickel by electrodeposition

The simultaneous electro-oxidation of Ni (II)-citrate and electrodeposition recovery of nickel metal were attempted in a combined electro-oxidation-electrodeposition reactor with a boron-doped diamond (BDD) anode and a polished titanium cathode. Effects of initial nickel citrate concentration, current density, initial pH, electrode spacing, electrolyte type, and initial electrolyte dosage on electrochemical performance were examined. The efficiencies of Ni (II)-citrate removal and nickel metal recovery were determined to be 100% and over 72%, respectively, under the optimized conditions (10 mA/cm², pH 4.09, 80 mmol/L Na₂SO₄, initial Ni (II)-citrate concentration of 75 mg/L, electrode spacing of 1 cm, and 180 min of electrolysis). Energy consumption increased with increased current density, and the energy consumption was 0.032 kWh/L at a current density of 10 mA/cm² (pH 6.58). The deposits at the cathode were characterized by scanning electron microscopy (SEM), energy-dispersive spectrometry (EDS), X-ray diffraction (XRD), and X-ray photoelectron spectroscopy (XPS). These characterization results indicated that the purity of metallic nickel in cathodic deposition was over 95%. The electrochemical system exhibited a prospective approach to oxidize metal complexes and recover metallic nickel.

Electrochemical oxidation of sulfamethoxazole by nitrogen-doped carbon nanosheets composite PbO2 electrode: Kinetics and mechanism

In this study, nitrogen-doped carbon nanosheets (NCNSs) were prepared and successfully combined into the PbO₂ electrode by the composite electrodeposition technology, thereby NCNS-PbO₂ electrode was obtained. The electrochemical degradation of sulfamethoxazole (SMX) in aqueous solution by NCNS-PbO₂ electrode was studied. The main influence factors on the degradation of SMX, such as the initial concentration of SMX, current density, electrolyte concentration and initial pH value, were analyzed in detail. Under the optimal process conditions, after 120 min of treatment, the removal ratio of SMX and chemical oxygen demand (COD) reached 99.8 % and 60.7 %, respectively. The results showed that the electrochemical degradation of SMX fitted pseudo-first-order reaction kinetics. The electrochemical performance of NCNS-PbO₂ electrode was better than that of PbO₂ electrode by scanning electron microscopy, X-ray diffraction and X-ray photoelectron spectroscopy, as well as the use of cyclic voltammetry and electrochemical impedance spectroscopy for electrochemical performance testing. This was because the doping of nitrogen atoms improved the properties of carbon nanosheets. After the composite, the active sites on the surface of PbO₂ were improved, the particle size of PbO₂ was reduced, and the electrical conductivity and electrocatalytic activity of the electrode were improved. In addition, the intermediate products were determined by GC-MS method, and the possible degradation pathways of SMX were proposed.

Elucidation of a mechanism for the heterogeneous electro-fenton process and its application in the green treatment of azo dyes

Vast efforts are directed today toward the development of efficient, green methods for the degradation of toxic compounds, especially those that are water-soluble. Though Fenton reactions are commonly used in wastewater treatment, their mechanisms and the active species involved remain obscure due to their mechanistic complexity. In this work, the mechanism of an electro-Fenton reaction, in which a FeLaO₃ catalyst was entrapped in a sol-gel matrix, was studied in the presence of azo dyes as the model for toxic compounds. Increased knowledge about this important mechanism will confer greater control over related processes and enable a more efficient and green degradation method. DFT calculations showed that in the presence of Fe(IV), OH are formed under acidic conditions and that both the iron and hydroxyl species function as oxidation reagents in the degradation process. The structure of the formed Fe(IV) embedded in the solid matrix was not the typical tetravalent structure. Entrapment in the sol-gel matrix stabilized the catalyst, enhanced its efficiency and enabled it to be recycled. Sol-gel matrices constitute a simple method for the degradation of stable and toxic compounds under extreme pH conditions. The findings of this study are highly significant for the treatment of typically acidic wastewaters.

A comparative study of photo-Fenton process assisted by natural sunlight, UV-A, or visible LED light irradiation for degradation of real textile wastewater: factorial designs, kinetics, cost assessment, and phytotoxicity studies

The present work aims to evaluate the treatment of the effluent from the textile industry via advanced oxidative processes of photo-Fenton assisted by different sources (natural sunlight, UV-A or visible LED lamps). To identify the best operating conditions, a factorial design was carried out for each process. It was observed that after the optimization of the processes, chemical oxygen demand (COD) removals greater than 88% were achieved. In addition, it was observed that the use of the LED lamp required lower reagent concentrations compared to solar and UV-A sources. A kinetic study was carried out under the best conditions obtained and it was observed that the sources showed rapid evolution, reaching a COD removal equilibrium with 30 min of reaction. Reagent monitoring was also carried out, and it was observed that they were not limiting to the reaction. Phytotoxicity analysis was also satisfactory since the treated effluents allowed a higher relative growth and germination index of the cucumber roots compared to the raw effluent. Finally, the cost analysis indicated that the use of LED lamps resulted in a reduction in electrical consumption compared to the UV-A lamp, as well as a reduction in the cost of reagents due to the lower concentration of reagents required compared to processes assisted by natural sunlight and UV-A.

Landfill leachate treatment by a boron-doped diamond-based photo-electro-Fenton system integrated with biological oxidation: A toxicity, genotoxicity and by products assessment

A photo-electro-Fenton (PEF) reactor employing boron-doped diamond (BDD) and soft iron anodes was studied in landfill leachate (LL) treatment. The reactor operation parameters (ROP) H₂O₂ concentration, current intensity and flow rate were investigated in the removal of Abs 254 nm. The PEF process with BDD anode, operating at the best operational conditions, was used as a pre-treatment and enabled biological oxidation (BO). The treatment strategy of PEF followed by BO showed to be the most efficient, reaching reductions of 77.9% chemical oxygen demand (COD), 71.5% total carbon (TC) and 76.3% radiation absorbance in 254 nm (Abs 254 nm), as well as a significant reduction in the genotoxicity (Allium cepa), observed by an increase in the mitotic index (MI) (131.5%) and decrease in the abnormalities (47.8%). The reduction of the toxic potential of LL using the integration of processes was also observed in the gas chromatography-mass spectrometry (GC-MS) byproducts analysis, which indicated the removal of emerging contaminants, such as Bisphenol-A (BPA), N,N-Diethyl-3-methylbenzamide (DEET) and Diisooctyl phthalate (DIOP). Thus, the PEF process integrated with BO presented a considerable efficiency in the removal of contaminants present in LL, becoming an alternative for the minimization of the environmental impacts caused by the discharge of this effluent in the environment.

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

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

Parameter optimization and degradation mechanism for electrocatalytic degradation of 2,4-diclorophenoxyacetic acid (2,4-D) herbicide by lead dioxide electrodes

2,4-Dichlorophenoxyacetic acid (2,4-D) is one of the most commonly used herbicides in the world. In this work, the electro-catalytic degradation of 2,4-D herbicide from aqueous solutions was evaluated using three anode electrodes, i.e., lead dioxide coated on stainless steel 316 (SS316/β-PbO₂), lead dioxide coated on a lead bed (Pb/β-PbO₂), and lead dioxide coated on graphite (G/β-PbO₂). The structure and morphology of the prepared electrodes were studied by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). The process of herbicide degradation was monitored during constant current electrolysis using cyclic voltammetry (CV). In this study, the experiments were designed based on the central composite design (CCD) and were analyzed and modeled by response surface methodology (RSM) to demonstrate the operational variables and the interactive effect of three independent variables on 3 responses. The effects of parameters including pH (3–11), current density (j = 1–5 mA cm⁻²) and electrolysis time (20–80 min) were studied. The results showed that, at j = 5 mA cm⁻², by increasing the reaction time from 20 to 80 min and decreasing the pH from 11 to 3, the 2,4-D herbicide degradation efficiency using SS316/β-PbO₂, Pb/β-PbO₂ and G/β-PbO₂ anode electrodes was observed to be 60.4, 75.9 and 89.8%, respectively. Moreover, the results showed that the highest COD and TOC removal efficiencies using the G/β-PbO₂ electrode were 83.7 and 78.5%, under the conditions pH = 3, electrolysis time = 80 min and j = 5 mA cm⁻², respectively. It was also found that G/β-PbO₂ has lower energy consumption (EC) (5.67 kW h m⁻³) compared to the two other studied electrodes (SS316/β-PbO₂ and Pb/β-PbO₂). The results showed a good correlation between the experimental values and the predicted values of the quadratic model (P < 0.05). Results revealed that the electrochemical process using the G/β-PbO₂ anode electrode has an acceptable efficiency in the degradation of 2,4-D herbicide and can be used as a proper pretreatment technique to treat wastewater containing resistant pollutants, e.g., phenoxy group herbicides (2,4-D).

Optimization of process parameters by the CCD method and electrocatalytic degradation and the electrochemical degradation mechanism of 2,4-D using modified electrode anodes were investigated.

Study on the Performance of Nano-Titanium Nitride-Coated Stainless Steel Electrodes in Electro-Fenton Systems

The electro-Fenton (EF) process is a type of electrochemical oxidation process; ·OH radicals are generated on the cathode using electricity and decolorize dye wastewaters. Most studies on EF systems in the past have focused on the operating parameters of this process. In recent years, the influence of electrode performance on the EF process has begun to receive more attention. In this study, direct nitridation was used to prepare titanium nitride powders, which were thereafter coated on an SUS304 stainless steel substrate. The performance of this system in the treatment of rhodamine B dye wastewaters via the EF process was investigated. The experimental methods used in this work include: (1) scanning electron microscopy (SEM); (2) X-ray diffraction (XRD); (3) electrochemical Tafel curves; (4) linear sweep voltammetry (LSV); (5) and cyclic voltammetry (CV). It was shown that high-purity TiN can be formed at nitriding temperatures above 900 °C, and the strength of the (111) crystal plane increases with the increase in nitriding temperature; the TiN coating effectively activates the reactive surface of the electrode owing to its porous structure. In terms of corrosion resistance, the corrosion potential and corrosion current of the TiN 1000 °C/SUS304 electrode were 116.94 mV and 205 nA/cm², respectively, and the coating had a coating porosity of 0.89 × 10⁻⁷. As compared with SUS304 stainless steel, the TiN 1000 °C/SUS304 composite electrode had a significantly greater degree of corrosion resistance and exhibited higher redox activity in LSV tests. This composite electrode could achieve a decolorization rate of 49.86% after 30 min, and 94.46% after 120 min. In summary, the TiN 1000 °C/SUS304 composite electrode is very stable and has excellent decolorization efficacy in the EF process. Our findings will serve as a useful reference for future studies on EF electrodes.

An overview on the removal of synthetic dyes from water by electrochemical advanced oxidation processes

Wastewater containing dyes are one of the major threats to our environment. Conventional methods are insufficient for the removal of these persistent organic pollutants. Recently much attention has been received for the oxidative removal of various organic pollutants by electrochemically generated hydroxyl radical. This review article aims to provide the recent trends in the field of various Electrochemical Advanced Oxidation Processes (EAOPs) used for removing dyes from water medium. The characteristics, fundamentals and recent advances in each processes namely anodic oxidation, electro-Fenton, peroxicoagulation, fered Fenton, anodic Fenton, photoelectro-Fenton, sonoelectro-Fenton, bioelectro-Fenton etc. have been examined in detail. These processes have great potential to destroy persistent organic pollutants in aqueous medium and most of the studies reported complete removal of dyes from water. The great capacity of these processes indicates that EAOPs constitute a promising technology for the treatment of the dye contaminated effluents.

Nickel-foam-supported β-Ni(OH)2 as a green anodic catalyst for energy efficient electrooxidative degradation of azo-dye wastewater

Electrochemical oxidative degradation (EOD) is a particularly promising technique for removing organic pollutants from wastewater. However, due to the high overpotential of EOD in conventional anode materials, the energy cost of EOD is usually very high, which greatly promotes the search for highly active, stable, and energy-efficient anodic catalysts. Herein, we demonstrated that nickel-foam-supported (NF-supported) β-Ni(OH)₂ (NF/β-Ni(OH)₂) prepared via a facile hydrothermal method could be used as an energy efficient anode for EOD. The as-prepared 3D porous NF/β-Ni(OH)₂ exhibited high activity toward the electrochemical oxidation of methyl orange (MO) in the low potential region (<1.07 V vs. SCE). This property differs greatly from those of the conventional anode materials that require a high positive potential to keep them active for EOD, making NF/β-Ni(OH)₂ an energy-efficient and active anode material for EOD. With an oxidation current density of 0.25 mA cm⁻², the decolorization of MO was completed within 30 min, and the COD removal after 3h of reaction was 63.0%. The normalized energy consumption for the 3 h degradation of MO was 22.2 kW h (kg COD)⁻¹, which is only a fraction of (or even one tenth of) the values reported in the literature. Moreover, NF/β-Ni(OH)₂ had a good stability and recyclability for EOD. No activity decay was observed during 10 h of EOD and the COD removal remained almost unchanged after four consecutive reaction cycles. We demonstrated experimentally that the NF/β-Ni(OH)₂ anode could generate large amounts of hydroxyl radicals and that the oxidation of MO by hydroxyl radicals was the main mechanism during EOD. We believe that this work opens a new avenue for developing highly active and energy-efficient anode materials that can work in the low potential region for EOD.

A novel NF/β-Ni(OH)₂ catalyst for energy efficient electrochemical degradation of methyl orange was fabricated via a facile hydrothermal method.

Evidence for the electrochemical production of persulfate at TiO2 nanotubes decorated with PbO2

The aim of this work is to study the viability of using TiO₂ nanotube arrays decorated with PbO₂ to electrochemically produce persulfate

Functional group influences on the reactive azo dye decolorization performance by electrochemical oxidation and electro-Fenton technologies

Electrochemical water treatment technologies are highly promising to achieve complete decolorization of dyebath effluents, as demonstrated by several studies reported in the literature. However, these works are focused on the treatment of one model pollutant and generalize the performances of the processes which are not transposable since they depend on the pollutant treated. Thus, in the present study, we evaluate, for the first time, the influence of different functional groups that modify the dye structure on the electrochemical process decolorization performance. The textile azo dyes Reactive Orange 16, Reactive Violet 4, Reactive Red 228, and Reactive Black 5 have been selected because they present the same molecular basis structure with different functional groups. The results demonstrate that the functional groups that reduce the nucleophilicity of the pollutant hinder the electrophilic attack of electrogenerated hydroxyl radical. Thereby, the overall decolorization efficiency is consequently reduced as well as the decolorization rate. Moreover, the presence of an additional chromophore azo bond in the molecule enhances the recalcitrant character of the azo dyes as pollutants. The formation of a larger and more stable conjugated π system increases the activation energy required for the electrophyilic attack of ^•OH, affecting the performance of electrochemical technologies on effluent decolorization.

Efficient sonoelectrochemical decomposition of sulfamethoxazole adopting common Pt/graphite electrodes: The mechanism and favorable pathways

In this study, efficient degradation of sulfamethoxazole (SMX) with a high synergy factor of 14.7 was demonstrated in a sonoelectrochemical (US-EC) system adopting common Pt and graphite electrodes. It was found that the US-EC system could work effectively at broad pH range of 3-9, but would achieve good performances with appropriate electrochemical conditions at 20mA/cm² and 0.1M Na₂SO₄. Both OH attacking and the anode oxidation would be responsible for the SMX degradation in the US-EC system, while the multiple promotional roles of US would be played homogenously and heterogeneously. US could not only effectively accelerate the decomposition of cathode-generated H₂O₂ into OH, but also lead to the enhancement in the heterogeneous reactions on the two electrodes, i.e. the cathode generation of H₂O₂ as well as the anode oxidation of SMX and H₂O/OH^-. Besides, the US-EC system would decompose SMX molecule via similar and simple pathways, by using either Na₂SO₄ or NaCl electrolytes. It was interesting to note that the US-EC system could successfully avoid the formation of complex chlorinated byproducts that detected in the referring EC system with NaCl. This finding would make the sonoelectrochemical processes favorable in treating practical wastewaters by alleviating the environmental impact of disinfection byproducts.

Decolorization and degradation of reactive yellow HF aqueous solutions by electrochemical advanced oxidation processes

Textile manufacturing is the one responsible for water bodies' contamination through the discharge of colored wastes. This work presents the study of reactive yellow HF (RYHF) dye degradation under two different electrochemical advanced oxidation processes (EAOP), namely anodic oxidation (AO) and electro-Fenton (EF)/boron-doped diamond (BDD) process. For the AO, 100 and 300 mg/L solutions using Pt and BDD as anodes in a 100 mL stirred tank cell were used, with a supporting electrolyte of 0.05 mol/L of Na₂SO₄ at pH 3 under 30 and 50 mA/cm² current density. The EF/BDD process was carried out in a flow reactor at 4 and 7 L/min to degrade 100, 200, and 300 mg/L RYHF solutions under 50 and 80 mA/cm². UV-Vis determinations were used for decolorization evaluation, while high-performance liquid chromatography (HPLC) method provided information on dye degradation rate.

Preparation of transition metal composite graphite felt cathode for efficient heterogeneous electro-Fenton process

A composite graphite felt (GF) modified with transition metal was fabricated and used as cathode in heterogeneous electro-Fenton (EF) for methyl orange (MO) degradation. Characterized by scanning electron microscopy (SEM) and X-ray photoelectron spectroscopy (XPS), the morphology and surface physicochemical properties of the cathodes after modification were observed considerably changed. After loading metals, the current response became higher, the accumulation of H₂O₂ and the degradation efficiency of MO were improved. Under the same conditions, GF-Co had the highest catalytic activity for electro-reduction of O₂ to H₂O₂ and MO degradation. At pH 3, 99 % of MO degradation efficiency was obtained using GF-Co after 120 min treatment and even at initial pH 9, 82 % of that was obtained. TOC removal efficiency reached 93.8 % using GF-Co at pH 3 after 120 min treatment while that was 12.3 % using GF. After ten-time runs, the mineralization ratio of the GF-Co was still 89.5 %, suggesting that GF-Co was very promising for wastewater treatment. The addition of isopropanol proved that ^·OH played an important role in degradation of MO.

Diazo dye Congo Red degradation using a Boron-doped diamond anode: An experimental study on the effect of supporting electrolytes

Diazo dye Congo Red (CR) solutions at 100mg/L, were degraded using different supporting electrolytes in an electrochemical advanced oxidation process (EAOPs), like the anodic oxidation (AOx/BDD). All experiments were carried out in a 3L flow reactor with a Boron-doped diamond (BDD) anode and stainless steel cathode (AISI 304), at 7.5, 15, 30 and 50mA/cm(2) current densities (j). Furthermore, each experiment was carried out under a flow rate of 7L/min. Additionally, HClO4, NaCl, Na2SO4, and H2SO4 were tested as supporting electrolytes at a 50mM concentration. The degradation process was at all times considerably faster in NaCl medium. Solutions containing SO4(2-) or ClO4(-) ions were less prompted to degradation due to the low oxidation power of these species into the bulk. Dissolved organic carbon (DOC) analysis, was carried out to evaluate the mineralization of CR. The degradation of CR, was evaluated with the HPLC analysis of the treated solutions.

Mineralization of the textile dye acid yellow 42 by solar photoelectro-Fenton in a lab-pilot plant

A complete mineralization of a textile dye widely used in the Chilean textile industry, acid yellow 42 (AY42), was studied. Degradation was carried out in an aqueous solution containing 100mgL(-1) of total organic carbon (TOC) of dye using the advanced solar photoelectro-Fenton (SPEF) process in a lab-scale pilot plant consisting of a filter press cell, which contains a boron doped diamond electrode and an air diffusion cathode (BDD/air-diffusion cell), coupled with a solar photoreactor for treat 8L of wastewater during 270min of electrolysis. The main results obtained during the degradation of the textile dye were that a complete transformation to CO2 depends directly on the applied current density, the concentration of Fe(2+) used as catalyst, and the solar radiation intensity. The elimination of AY42 and its organic intermediates was due to hydroxyl radicals formed at the anode surface from water oxidation and in the bulk from Fenton's reaction between electrogenerated H2O2 and added Fe(2+). The application of solar radiation in the process (SPEF) yield higher current efficiencies and lower energy consumptions than electro-Fenton (EF) and electro-oxidation with electrogenerated H2O2 (E OH2O2) by the additional production of hydroxyl radicals from the photolysis of Fe(III) hydrated species and the photodecomposition of Fe(III) complexes with organic intermediates. Moreover, some products and intermediates formed during mineralization of dye, such as inorganic ions, carboxylic acids and aromatic compounds were determined by photometric and chromatographic methods. An oxidation pathway is proposed for the complete conversion to CO2.

Electrochemical Treatment of Water Contaminated with Methylorange

This study examines electrochemical degradation of water artificially contaminated by azo dye Methyl Orange (MO). Degradation is based on chemical electro-oxidation of MO molecules. Graphite was used as an electrode material for electrochemical oxidation of MO. In this work, the different operative parameters (electric current, NaCl content) and their effect on effectiveness as well as the treatment time/duration of MO degradation were tested. The highest dye removal (91.0 %) was obtained during the electrolysis at current density 3.032 mA/cm2, electrolyte with the content of NaCl 4 g/dm3 (NaCl) and the treatment time 35 min.

Photoassisted Electrochemical Treatment of Azo and Phtalocyanine Reactive Dyes in the Presence of Surfactants

An electrochemical treatment (EC) was applied at different intensities to degrade the chromophoric groups of dyes C.I. Reactive Black 5 (RB5) and C.I. Reactive Blue 7 (Rb7) until uncolored species were obtained. Decolorization rate constants of the azo dye RB5 were higher than the phtalocyanine Rb7 ones. In addition, the EC treatment was more efficient at higher intensities, but these conditions significantly increased the generation of undesirable by-products such as chloroform. The combination of EC with UV irradiation (UVEC) drastically minimized the generation of chloroform. The photo-assisted electrochemical treatment was also able to achieve decolorization values of 99%. Finally, mixtures of dyes and surfactants were treated by EC and UVEC. In the presence of surfactants, the decolorization kinetic of dyes was slowed due to the competitive reactions of surfactants degradation. Both methods achieved total decolorization and in both cases, the generation of haloforms was negligible.

Heterogeneous electro-Fenton oxidation of azo dye methyl orange catalyzed by magnetic Fe3O4 nanoparticles

Azo dye methyl orange (MO) degradation by heterogeneous electro-Fenton (EF) with a magnetic nano-Fe3O4 catalyst was investigated. In this study, Fe3O4 was synthesized by a coprecipitation method and characterized by X-ray diffraction and scanning electron microscopy. The influences of the main operating parameters such as current density, pH, catalyst dosage and aeration rate were studied. The results revealed that higher current density, catalyst dosage and aeration rate facilitated the degradation of MO, whereas the degradation efficiency of MO was decreased with an increase in the initial pH. After 90 min EF process, the volume of 500 mL, the initial concentration of 50 mg L(-1) MO solution could be degraded by 86.6% with the addition of 1 g L(-1) Fe3O4 under the current density of 10 mA cm(-2) and pH 3, compared with 69.5% for the electrolytic process alone. Meanwhile, a total organic carbon removal of 32% was obtained, up to 35.5 mg L(-1) accumulated H2O2 and less than 3.5 mg L(-1) Fe leaching were detected. Moreover, based on the results of natural bond orbital (at B3LYP/6-311G (d, p)) and liquid chromatograph-triple quadrupole mass spectrometer analysis, nine intermediates were identified and the proposed degradation pathways were investigated.

Single and Coupled Electrochemical Processes and Reactors for the Abatement of Organic Water Pollutants: A Critical Review

Traditional physicochemical and biological techniques, as well as advanced oxidation processes (AOPs), are often inadequate, ineffective, or expensive for industrial water reclamation. Within this context, the electrochemical technologies have found a niche where they can become dominant in the near future, especially for the abatement of biorefractory substances. In this critical review, some of the most promising electrochemical tools for the treatment of wastewater contaminated by organic pollutants are discussed in detail with the following goals: (1) to present the fundamental aspects of the selected processes; (2) to discuss the effect of both the main operating parameters and the reactor design on their performance; (3) to critically evaluate their advantages and disadvantages; and (4) to forecast the prospect of their utilization on an applicable scale by identifying the key points to be further investigated. The review is focused on the direct electrochemical oxidation, the indirect electrochemical oxidation mediated by electrogenerated active chlorine, and the coupling between anodic and cathodic processes. The last part of the review is devoted to the critical assessment of the reactors that can be used to put these technologies into practice.