Electrochemical advanced oxidation processes for wastewater treatment: Advances in formation and detection of reactive species and mechanisms

Over the past three decades, the knowledge of the mechanisms of electrochemical advanced oxidation processes (EAOPs) has progressively evolved with the advances in analytical and spectrometric techniques. A comprehensive understanding of the types and mechanisms of production of reactive species in EAOPs is a prerequisite to the understanding of their reactivities and elucidation of intermediate products generated during the oxidation process and degradation pathways. The type, nature, and quantity of reactive species generated in electrochemical treatment processes are controlled by many factors, including the type of the treatment technique, electrode/electrocatalyst materials, water/wastewater composition, water pH conditions, and operating parameters. Depending on the technique and operating parameters, single or multiple oxidants can be produced alone or combined electrochemical processes. However, the potency and reactivity of each oxidant are quite similar regardless of the technique, except in the case of heterogeneous and homogeneous hydroxyl radicals. This minireview presents the current state of mechanisms and models of reactive species generated in different EAOPs, highlighting different methods for their identification and reactivity.     

The electrochemical advanced oxidation processes coupling of oxidants for organic pollutants degradation: A mini-review

In this mini-review, the homogeneous and heterogeneous EAOPs-oxidant processes were summarized. The reaction mechanisms of different EAOPs combined with different oxidants are elucidated in detail, as well as the synergistic effect for improving the treatment efficiency.     

The electrochemical advanced oxidation processes coupling of oxidants for organic pollutants degradation: A mini-review

The electrochemical advanced oxidation processes(EAOPs) have been extensively applied in the treatment of organic pollutants degradation.Herein,the mini review provides the coupling systems about EAOPs and different oxidants(e.g.,persulfate(PS),peroxymonosulfate(PMS),and ozone(O_3)),including EAOPs-PS systems,EAOPs-PMS systems,EAOPs-peroxone systems,and photoelectro-oxidants systems,for the organic compounds degradation.The coupling system of EAOPs with oxidants is an effective way to improve the generated free radicals(e.g.,HO~·and SO_4~(·-)) concentration and to accelerate pollutant degradation.In this review,we make a summary of the homogeneous and heterogeneous EAOPs-oxidant processes.The reaction mechanisms of EAOPs combined with different oxidants are elucidated in detail,as well as the synergistic effect for improving the degradation and mineralization efficiency.     

Electrochemical Technologies to Decrease the Chemical Risk of Hospital Wastewater and Urine

The inefficiency of conventional biological processes to remove pharmaceutical compounds (PhCs) in wastewater is leading to their accumulation in aquatic environments. These compounds are characterized by high toxicity, high antibiotic activity and low biodegradability, and their presence is causing serious environmental risks. Because much of the PhCs consumed by humans are excreted in the urine, hospital effluents have been considered one of the main routes of entry of PhCs into the environment. In this work, a critical review of the technologies employed for the removal of PhCs in hospital wastewater was carried out. This review provides an overview of the current state of the developed technologies for decreasing the chemical risks associated with the presence of PhCs in hospital wastewater or urine in the last years, including conventional treatments (filtration, adsorption, or biological processes), advanced oxidation processes (AOPs) and electrochemical advanced oxidation processes (EAOPs).     

Recent advances and trends of heterogeneous electro-Fenton process for wastewater treatment-review

Electrochemical advanced oxidation processes （EAOPs） are effective and environmentally friendly for the treatment of refractory organic pollutants.Among EAOPs,heterogeneous electro-Fenton （EF） process with in-situ formation of hydrogen peroxide （H₂O₂） is an eco-friendly,cost-effective and easy-operable technology to generate hydroxyl radicals （^·OH） with high redox potential.The generation of^·OH is determined by the synergistic H₂O₂     

Applied photoelectrocatalysis on the degradation of organic pollutants in wastewaters

A large variety of electrochemical advanced oxidation processes (EAOPs) have been recently developed to remove organic pollutants from wastewaters to avoid their serious health-risk factors from their environmental accumulation and to reuse the treated water for human activities. The effectiveness of EAOPs is based on the in situ production of strong reactive oxygen species (ROS) like hydroxyl radical (OH). Photoelectrocatalysis (PEC) has emerged as a promising powerful EAOP by combining photocatalytic and electrolytic processes. It consists in the promotion of electrons from the valence band to the conduction band of a semiconductor photocatalyst upon light irradiation, with production of positive holes. The fast recombination of the electron/hole pairs formed is avoided in PEC by applying an external bias potential to the photocatalyst that extracts the photogenerated electrons up to the cathode of the electrolytic cell. Organics can be oxidized directly by the holes, ^•OH formed from water oxidation with holes and other ROS produced between the electrons and dissolved O₂. This paper presents a general and critical review on the application of PEC to the remediation of wastewaters with organic pollutants. Special attention is made over the different kinds of photocatalysts utilized and preparation methods of the most ubiquitous TiO₂ materials. Typical PEC systems and main operation variables that affect the effectiveness of the degradation process are also examined. An exhaustive analysis of the advances obtained on the treatment of dyes, chemicals and pharmaceuticals from synthetic solutions, as well as of real wastewaters, is performed. Finally, research prospects are proposed for the future development of PEC with perspectives to industrial application.     

The deposition of nanostructured β-PbO2 coatings from aqueous methanesulfonic acid for the electrochemical oxidation of organic pollutants

Highly crystalline, nanostructured, three-dimensional β-PbO₂ coatings were successfully obtained by galvanostatic deposition from baths containing aqueous lead(II) and methanesulfonic acid (CH₃SO₃H). This constitutes a much more environmentally friendly methodology compared to plating of β-PbO₂ in HNO₃. The deposits exhibited high quality and good adherence. The crystallite size was in the range 20–30 nm and AFM imaging revealed very uniform, rough deposits (i.e., 255–275 nm rms). The oxidative destruction of Methyl Orange azo dye was studied by electrochemical advanced oxidation processes (EAOPs). An electro-Fenton process with a high surface area carbon-felt cathode performed better than the single anodic oxidation. Rapid and complete decolorisation was achieved following pseudo first-order kinetics. The stability of the β-PbO₂ electrodes during the electrolyses was also demonstrated.     

Ozone-based electrochemical advanced oxidation processes

Novel processes have recently been developed that provide for the enhancement of ozonation through combination with electrochemical treatments. These are processes that can be included among those defined as advanced oxidation processes as they proceed via electrogeneration of highly oxidizing radical species.These processes are generally carried out by sparging ozone in both divided and undivided electrochemical cells in order to promote its decomposition through different mechanisms, depending on the electrode materials adopted, and in some cases still debated.This mini review presents the most recent advances in the field of electrochemically assisted ozonation.In particular, the first section is focused on the process known as electroperoxone (EP) where the ozone decomposition is enhanced by the adoption of carbon-based cathodes, due to the electrogeneration of hydrogen peroxide, while the second section is focused on the process that implies ozonation in a cell adopting metal-based cathodes.     

Fabrication of Ga<Subscript>2</Subscript>O<Subscript>3</Subscript>–PbO<Subscript>2</Subscript> electrode and its performance in electrochemical advanced oxidation processes

Owing to its high oxygen evolution potential, PbO₂ electrode is one of the candidates for electrochemical advanced oxidation processes (EAOPs). To further improve its performance as EAOP electrode, a Ga₂O₃-doped PbO₂ electrode (Ga₂O₃–PbO₂ electrode) was fabricated by the composite plating method. SEM and XRD results showed that the crystalline in the coating of Ga₂O₃–PbO₂ electrode is more uniform and in smaller size than that in the undoped PbO₂ electrode, which provided a higher specific surface area. The electrochemical studies showed that the Ga₂O₃–PbO₂ electrode had higher oxygen evolution potential (OEP) and smaller electrode surface impedance, which is a benefit for the formation of hydroxyl radicals (·OH). The electrochemically degradation test using bromocresol green sodium (BG) solution and glucose solution as the simulated wastewater showed that the kinetics of electrochemical catalytic degradation is a pseudo-first-order reaction, and the reaction rate constant on Ga₂O₃–PbO₂ electrode was 2 times accelerated.     

Relevance of gaseous flows in electrochemically assisted soil thermal remediation

Treatment of polluted soil is one of the priorities in the search of a more sustainable planet. Electrochemically assisted soil remediation has been considered a good option for removing organic contaminants contained in soil, including the removal of volatile organic compounds, associated with gaseous streams produced during the treatment. Also, recently, electrochemical gas treatment technologies have been appointed as promising for the treatment of volatile organic compounds. In this work, we review the current opinion about the most recent studies in both areas. The first section focuses on the production of gaseous compounds during soil remediation by conventional and electrochemical systems. The second section describes the recent progress in the integration of adsorption and absorption with electrochemical processes. Finally, we discuss the holistic application of assisted electrochemical technologies in soil remediation, considering also emerging processes recently published in the literature.     

Oxidation of positronium atoms on a surface of oxidic catalyst carrier containing acid centres

By Born approximation the cross section of positronium (Ps) oxidation on acid centres localized on the surface of oxide carriers is calculated. Analysis of the kinetics of elementary processes in porous carriers based on aluminum oxide including processes of annihilation of positron, formation of Ps and oxidation of Ps on acid centres is given.     

Decontamination of heavy metal complexes by advanced oxidation processes: A review

Heavy metal complexes with high mobility are widely distributed in wastewater from modern industries, which are more stable and refractory than free heavy metal ions. Their removals from wastewater draw increasing attentions and various technologies have been developed, among which advanced oxidation processes (AOPs) are more effectively and promising. Progresses on five representative types of AOPs, including Fenton (like) oxidation, electrochemical oxidation, photocatalytic oxidation, ozonation and discharge plasma oxidation for heavy metal complexes degradation are summarized in this review. Their rationales, advantages, applications, challenges and prospects are introduced independently. Combinations among these AOPs, such as electrochemical Fenton oxidation and photoelectrocatalytic oxidation, are also comprehensively highlighted. Future efforts should be made to reduce acid requirement and scale up for practical applications of AOPs for heavy metal complex degradation efficiently and cost-effectively.     

Modified Electrodes and Electrochemical Systems Switchable by Light Signals

This article is an overview of extensive research efforts in many laboratories in the last two decades in the area of light‐switchable electrochemical systems and modified electrodes. Electrochemical reactions, including electrocatalytic and bioelectrocatalytic processes, have been reversibly activated and inhibited upon irradiation with light at different wavelengths. In order to realize these light activated or inhibited processes, the electrodes or/and reacting molecules were functionalized with photoisomerizable molecules including various derivatives of diarylethene, phenoxynaphthacenequinone, azobenzene and spiropyran/merocyanine. Photochemical reactions of these species resulted in change of their redox activity, conformation and electrical charge. All these changes affected electrode surfaces or (bio)molecules resulting in switching ON‐OFF corresponding (bio)electrochemical processes. Various systems based on different light‐controlled reactions are reviewed and discussed with specific examples and with many illustrating figures. Possible extensions of the research area and future applications are briefly overviewed in the conclusion section. The present comprehensive review is addressed to a broad scientific community, including newcomers to the area.     

On-line electrochemistry--MS and related techniques

This review summarizes publications on the on-line coupling of electrochemistry with mass spectrometry. After a brief historic introduction it is divided into three parts, organized in order of increasing complexity of the experimental arrangement. The first section deals with the use of the electrospray ion source as an electrochemical reactor for oxidation or reduction reactions. It is followed by the second part which covers the hyphenation of different kinds of electrochemical flow cell with a variety of ionization interfaces. The last section focuses on the on-line coupling of chromatographic techniques with electrochemical flow cells and mass spectrometry.     

Fabrication of High‐aspect Ratio (HAR) Palladium Nanorod‐modified Electrodes for Raman Spectroelectrochemical Studies of Thiolate Desorption from HAR Nanomaterials

We describe a method to fabricate electrodes modified with high‐aspect ratio (HAR) palladium (Pd) nanorods derived from the electrochemical reduction of Pd salts in polycarbonate track‐etched membranes. The HAR Pd nanorod‐modified electrode platform enables direct spectroscopic observation of electrochemical processes particular to HAR forms of nano‐Pd using Raman spectroelectrochemistry. In the present study, we use this platform to observe an anomalous mechanism of oxidative electrochemical desorption of the probe molecule benzenethiol (BT) from the HAR Pd surface. At HAR Pd, the Pd?S bond between the Pd nanorods and BT severs upon oxidation of the Pd surface, whereas on the surface of spherical nano‐Pd, the C?S bond in BT is broken, as is more typically observed for sulfur‐containing organic molecules adsorbed to Pd surfaces. Using this probe reaction, we demonstrate the suitability of this HAR nanorod‐modified electrode platform for the investigation of anomalous electrochemical phenomena observed at HAR Pd for reactions that involve adsorbed intermediates in general – including not only adsorbed sulfur – but in principle also for the electrochemical oxidation of alternative fuels such as ethanol and methanol.     

A nanocarbon film electrode as a platform for exploring DNA methylation

We describe the quantitative nonlabel electrochemical detection of both cytosine (C) and methylcytosine (mC) in oligonucleotides using newly developed nanocarbon film electrodes. The film consists of nanocrystalline sp2 and sp3 mixed bonds formed by employing the electron cyclotron resonance (ECR) sputtering method. We successfully used this film to develop a simple electrochemical DNA methylation analysis technique based on the measurement of the differences between the oxidation currents of C and mC since our ECR nanocarbon film electrode can directly measure all DNA bases more quantitatively than conventional glassy carbon or boron-doped diamond electrodes. The excellent properties of ECR nanocarbon film electrodes result from the fact that they have a wide potential window while maintaining the high electrode activity needed to oxidize oligonucleotides electrochemically. Proof-of-concept experiments were performed with synthetic oligonucleotides including different numbers of C and mC. This film allowed us to perform both C- and mC-positive assays solely by using the electrochemical oxidation of oligonucleotides without bisulfite or labeling processes.     

Modeling of electrocatalysis at polyaniline-modified electrodes: autoacceleration of the process by the reaction product

We extend our previous model taking into account a possible autoacceleration mechanism of electrochemical processes. It is supposed that protons generated in an electrochemical oxidation of ascorbate are responsible for an increase of electric conductivity of polyaniline. Accordingly, the current–time profiles have been calculated taking into account a “threshold” value for proton concentration to increase the electric conductivity of polyaniline by one or more orders of magnitude. Different kinds of corresponding profiles, including ones with a sigmoid character and possessing sharp current flashes as well, were obtained and analyzed.     

Intense Electrochemical Oxidation on Graphitized Carbon Electrodes in the Presence of Ozone

A new intense oxidation process for water treatment in which oxidation with ozone is coupled to electrochemical processes is described, and the results from its application to water purification are presented along with the discussion of its practical implementation. The use of graphitized carbon materials for this process is explained and tested experimentally. The use of glassy carbon for the anode enables us to achieve very high (up to 25 vol %) concentrations of ozone in the generated ozone?oxygen mixture. The material used for the cathode—graphitized carbon cloth (GCC) reinforced with Ni allows different electrocatalytic processes to proceed on its developed surface, and combines the high sorption capacity of this cathode and potentialcontrolled selectivity of cathodic electrochemical processes.     

Direct electrochemical oxidation of a pesticide, 2,4-dichlorophenoxyacetic acid,at the surface of a graphite felt electrode: Biodegradability improvement

Pesticides’ biorecalcitrance can be related to the presence of a complex aromatic chains or of specific bonds, such as halogenated bonds, which are the most widespread. In order to treat this pollution at its source, namely in the case of highly concentrated solutions, selective processes, such as electrochemical processes, can appear especially relevant to avoid the possible generation of toxic degradation products and to improve biodegradability in view of a subsequent biological mineralization. 2,4-D was found to be electroactive in oxidation, but not in reduction, and the absence of hydroxyl radicals formation during the electrochemical step was demonstrated, showing that the pretreatment can be considered as a “direct” electrochemical process instead of an advanced electrochemical oxidation process. The presence of several degradation products in the oxidized effluent showed that the pretreatment was not as selective as expected. However, the relevance of the proposed combined process was confirmed since the overall mineralization yield was close to 93%.     

甲基磺酰氟Simons电化学氟化过程特性和机理研究

选择CH₃SO₂F电化氟化制备CF₃SO₂F过程为研究系统,研究了电化氟化过程操作电压和反应时间的关系、操作条件对氟化产物组成的影响规律以及Ni电极在电化氟化过程的变化情况. 实验结果表明,Simons电化学氟化过程主要由三个步骤组成：F-在阳极发生电化学氧化反应生成F,该步骤是Simons电化学氟化过程的控制步骤;在Ni电极上生成的F与Ni或NiF₂反应生成高价NiF_n (n≥3),NiFn为Simons电化学氟化过程的氟化剂;NiF_n可以在电极/电解液界面与有机物发生氟化反应生成氟化产物,也可以发生分解反应生成游离F²,NiF_n与有机物发生氟化反应的机理与用CoF₃等为氟化剂氟化有机物的机理相同. 但NiF_n的反应活性比CoF₃高,且在实验条件下极不稳定.