Electro-Fenton Process

This chapter is conceived as the gateway to more specific sections in the book. Its main aim is to introduce all the reactions of interest for fully understanding further development and applications of the EF process. The 50 reactions provided condense all the phenomena occurring in such a complex system and serve as the platform to justify the need of different devices and setups when treating water matrices of very different nature. In addition, all the key operation parameters for H₂O₂ electrogeneration and water decontamination are discussed. Subsections devoted to explaining the effect of the electrolyte composition, cell design, cathode and anode nature, catalyst source, hydrodynamic conditions, solution pH, and operation mode (potentiostatic or galvanostatic) are set out in summarized form, in order to present all the crucial information without intending to duplicate ideas that will be already given in subsequent chapters.

Biological treatments show insufficient removal efficiency in the case of recalcitrant organic compounds. Therefore, the necessity of upgrading wastewater treatment plants (WWTPs) with advanced treatment steps is unequivocal. Advanced oxidation processes (AOPs) are the most effective technologies for the removal of a large range of organic pollutants from water due to the generation of strong oxidizing species like hydroxyl radicals (^•OH). However, AOPs often involve high energy and/or reagent consumption and are considered as less cost-effective than biological processes. Hence, the combination of AOPs and biological treatments has been implemented aiming at maximizing efficient removal of recalcitrant organic pollutants while minimizing treatment costs. Among AOPs, electrochemical advanced oxidation processes (EAOPs) have been widely explored during coupled processes, since they possess remarkable advantages, such as high efficiencies, operability at mild conditions, economic feasibility, ease of automation, as well as eco-friendly character. The electro-Fenton process (EF) stands out as one of the most applied EAOPs and the present chapter is devoted to the advances and applications of EF process as a treatment step coupled with biological methods: the so-called bio-electro-Fenton (Bio-EF) process, which brings together the high oxidation power of EF and cost-effectiveness of biological methods.

The electro-peroxone (E-peroxone) process is a novel electrochemical advanced oxidation process (EAOP) that is enabled by in situ generation of hydrogen peroxide (H₂O₂) from cathodic oxygen (O₂) reduction during conventional ozonation. The electro-generated H₂O₂ can considerably enhance ozone (O₃) transformation to hydroxyl radicals (⋅OH), thus greatly enhancing pollutant degradation and total organic carbon (TOC) mineralization by the E-peroxone process than by conventional ozonation. Due to its higher kinetics of pollutant degradation, the E-peroxone process can also reduce reaction time and energy consumption required for water and wastewater treatment. In addition, the in situ generated H₂O₂ can effectively reduce bromate formation during the E-peroxone treatment of bromide-containing water compared to conventional ozonation. All oxidants (O₃, H₂O₂, and ⋅OH) are produced on site at controllable rates during the E-peroxone process using only clean oxygen and electricity. No chemicals or catalysts are added in the E-peroxone process nor does it produce secondary pollutants. By simply installing low-cost carbon-based cathodes in ozone contactors, conventional ozonation systems that are commonly used in water and wastewater utilities can be easily retrofitted to the E-peroxone process with minimal upgrade work and costs. Therefore, the E-peroxone process can provide a convenient and economical way to significantly improve the performance of existing ozonation systems in many aspects and has thus emerged as a promising EAOP for practical water and wastewater treatment.

Electro-Fenton (EF) process has received much attention among the various advanced oxidation process, due to its higher contaminant removal and mineralization efficiencies, simplicity in operation, in situ generation of hydrogen peroxide, etc. Heterogeneous EF process rectifies some of the drawbacks of conventional EF process by using solid catalyst for the generation of reactive hydroxyl radicals in water medium. The efficiency of various heterogeneous EF catalysts such as iron oxides, pyrite, iron supported on zeolite, carbon, alginate beads, etc. was tested by various researchers. All of these catalysts are insoluble in water; and most of them are stable and reusable in nature. Depending on the iron leaching characteristics, hydroxyl radicals are generated either in the solution or over the catalyst surface. Catalysts with higher leaching characteristics exhibit the first radical generation mechanism, while the stable catalyst with insignificant leaching exhibits the second radical generation mechanism. Adsorption of the pollutant over the surface of the catalyst also enhances the pollutant degradation. Overall, heterogeneous EF process is very potent, powerful, and useful for the pollutant decontamination from the water medium.

Electro-Fenton (EF) process is based on the continuous in situ production of hydrogen peroxide (H₂O₂) by a two-electron reduction of oxygen on cathode and the addition of ferrous ion to generate hydroxyl radical (^•OH) at the solution through Fenton’s reaction in acidic condition. Hence, cathode material has prominent effects on the H₂O₂ electro-generation efficiency and regeneration of ferrous ion. Carbonaceous materials are applied as suitable cathode in virtue of being highly conductive, stable, nontoxic, and commercially available. Besides, modification of cathode electrode with carbon-based nanomaterials (e.g., carbon nanotubes (CNTs), graphene, mesoporous carbon) can improve the electroactive surface area and the rate of oxygen mass transfer to the electrode, which increases the H₂O₂ electro-generation in the EF process. This chapter is to summarize the recent progress and advances in the modification of cathode electrode with carbon-based nanomaterials for EF process. The ability of different carbon-based nanomaterials to electro-generate H₂O₂ and degradation of pollutants is also discussed briefly.

In electro-Fenton process, carbon-based materials, particularly 3D carbon felt, are the best choices for the cathodic electrodes because of several advantages such as low cost, excellent electrolytic efficiency, high surface area, and porosity. In this chapter, various aspects of this material are discussed in detail. This chapter is divided into three main sections, including (1) characterization of carbon felt (CF), (2) modification of CF, and (3) application of CF in electro-Fenton (EF) process to remove biorefractory pollutants. First of all, the typical characteristics of CF such as morphology, porosity, and conductivity are discussed. Next, in the modification section, we introduce different methods to improve the performance of CF. We especially focus on the surface area and electrochemical activity toward electrodes applications. Finally, both modified and non-modified CF is used as cathode materials for EF systems like homogeneous, heterogeneous, hybrid, or pilot-scale types.

A cost-effective cathode is vital for electrochemical production of hydrogen peroxide and its application for organic pollutants degradation by electro-Fenton (EF). Graphite felt is one of the most extensively used cathodes for EF account for its good stability, conductivity, and commercial availability; however, its performance for hydrogen peroxide yield was not so satisfactory, and thus many cathode modification methods were investigated to improve the EF performance. This work systematically summarized our studies on the modification of graphite felt to improve EF performance, including chemical and electrochemical modification. Also, composite graphite felts with carbon black or graphene were reported. The preparation and characterizations of the cathode as well as their application for organic pollutants degradation by EF were described. Further, transition metal doping on the composite graphite felts to fulfill in situ heterogeneous EF was also attempted to overcome some drawbacks of homogeneous EF. Finally, an outlook for cathode modification was proposed. All these progresses would contribute to the application of EF using graphite felt cathode.

The cells used for electro-Fenton process look quite different, ranging from the simple open tanks, through the parallel-plate cells, to the sometimes complex designs with three-dimensional moving electrodes or microelectrodes. Recently, pressurized cells and microreactors have been used to improve the performance of the process. This chapter presents a general overview of the main cell configurations used in electro-Fenton process for the treatment of organic pollutants. A global perspective on the fundamentals and experimental setups is offered, and laboratory-scale and pilot-scale experiments are examined and discussed.

In order to increase the degradation efficiency and reduce the treatment cost of electro-Fenton (EF) process, many aspects have been attempted, among which the design of cost-effective reactors is very important. Flow-through EF reactor, i.e., the solution flow through the anode and cathode, is able to increase mass and electron transfer, which is favorable to improve electrochemical conversion, current efficiency and reduce energy consumption. Carbon-based materials, for example, graphite felt, are desirable cathodic electrodes for the flow-through EF system because of their stability, conductivity, high surface area and chemical resistance, as well as the filtration characteristics. The effects of some important parameters including current density, pH, and flow rate on organic pollutant removal efficiency were discussed. Moreover, some new attempts on coupled flow-through EF with other water/wastewater treatment technology (e.g., coagulation, adsorption, and ozonation) were extended to reach a higher treatment efficiency. The perspective of this process was also summarized. In conclusion, compared with conventional EF reactor, flow-through EF reactor was more energy-efficient and potential for degradation of organic pollutants.

Electrochemical reactor design for oxidation processes follows similar engineering principles used for typical electrosynthesis reactors and include considerations of the components materials, electrode and cell geometries, mass transport conditions, rate of reactions, space–time yield calculations, selectivity, modeling, and energy efficiencies. It is common practice to optimize these characteristics at laboratory scale level followed by more practical considerations to build a larger reactor able to accomplish a required performance that can be easily assembled and requires low maintenance and monitoring. The scaling-up process should involve testing a variety of electrode configurations and cell designs to maximize the degradation of a particular pollutant. In this chapter, we describe the general principles of reactor design and list the most typical reactor configurations and performance followed by some recent advances in modeling and further developments.

From the conventional Fenton process (H₂O₂ and Fe²⁺), the electro-Fenton process was derived to improve the hydroxylation method (partial organic oxidation). Thereafter, electro-Fenton was adapted to water remediation. Since then, this approach has received much attention for wastewater treatment because it is an eco-friendly process and its technological implementation is simple. Although electro-Fenton involves a few and very simple chemical species (H₂O₂, Fe²⁺, Fe³⁺, O₂), the interactions among them produce one of the most difficult set of chemical reactions. Therefore, the predictions of the main chemical reactions are a challenging task. The aim of this chapter is to propose a methodology for developing a general, practical, simple, semiempirical chemical model to predict organic pollutant abatement in a reliable electrochemical reactor by electro-Fenton process. The main outputs of this chemical model include the rate of H₂O₂ generation and its activation by Fe²⁺ to produce a strong oxidant. The organic pollutant degradation rate and the energy and time required to carry out the organic degradation are also included. Although under this approach it is not possible to follow a detailed evolution of concentration profiles of some by-products during the degradation time, this procedure is less complicated than others already available. Moreover, it can fulfil the main requirements of wastewater treatment: abatement of the organic pollutant.

Herein, an overview over the performance of emerging electrochemical advanced oxidation processes (EAOPs) such as solar photoelectro-Fenton (SPEF) and related solar-assisted methods to remove organic pollutants from acidic wastewaters is presented. These procedures generate ^•OH at the anode surface from water oxidation and in the bulk from Fenton’s reaction between added Fe²⁺ and H₂O₂ generated at a gas diffusion electrode (GDE) fed with pure O₂ or compressed air, similarly to the electro-Fenton (EF) process. SPEF involves the additional irradiation of the effluent with sunlight, which causes a synergistic effect on organic destruction due to the formation of more ^•OH from the photolysis of Fe(OH)²⁺ species and/or the photolysis of complexes of Fe(III) with generated carboxylic acids. Fundamentals of SPEF are explained to better clarify its characteristics on the removal of industrial chemicals, pesticides, dyes, pharmaceuticals, and real wastewaters. Examples with stirred tank reactors and pre-pilot flow plants equipped with electrochemical reactors containing a Pt or a boron-doped diamond anode and a GDE as cathode, coupled to a solar planar or CPC photoreactor, are given. The use of an autonomous flow plant powered by sunlight is examined. Coupled methods of SPEF with solar photocatalysis, photoelectrocatalysis, and biological treatment are described. The effect of experimental variables on the mineralization, current efficiency, and energy consumption is detailed. The decay kinetics of pollutants and the evolution of intermediates and released inorganic ions are discussed. SPEF is more efficient and less expensive than EAOPs like anodic oxidation and EF.

In this chapter critical discussion is provided on the recent innovations and the potential of the Electro-Fenton (EF) and EF-related processes as eco-engineered technologies in the field of water treatment. Emphasis is placed on the treatment of water and wastewater to eliminate a wide variety of synthetic organic pollutants, such as pesticides, pharmaceuticals, and dyes, the refractory nature of which requires the application of strong oxidants for their total elimination. In comparison to the general public acceptance of traditional and/or advanced water treatment technologies (e.g., activated carbon, membrane technologies, etc.), there is ambiguity or skepticism regarding EF adaptation. This is due to the lack of technology certification, the limited large-scale applications, or even the small number of demonstrations in realistic operational environments. In view of this state of technology, the parameters involved in designing and operating EF systems are discussed together with the appropriate engineering rules that can support optimal system design and operation so that these systems can be used at an efficient, effective, and profitable manner at industrial scale.

This chapter presents the degradation and mineralization of emerging trace contaminants artificial sweeteners (ASs) in aqueous solution by electro-Fenton process in which hydroxyl radicals were formed concomitantly by ^•OH formed from electrocatalytically generated Fenton’s reagent in the bulk solution and M(^•OH) from water oxidation at the anode surface. Experiments were performed in an undivided cylindrical glass cell with a carbon-felt cathode and a Pt or boron-doped diamond (BDD) anode. The effect of catalyst (Fe²⁺) concentration and applied current on the degradation and mineralization kinetics of ASs was evaluated. The absolute rate constants for the reaction between ASs and ^•OH were determined. The formation and evolution of short-chain carboxylic acids as well as released inorganic ions, and toxicity assessment during the electro-Fenton process have been reported and compared.

Soil remediation by electro-Fenton (EF) process has been recently proposed in literature. Being applied for solution treatment, EF is mainly combined with soil washing (SW)/soil flushing (SF) separation techniques to remove the organic pollutants. The main criteria influencing the combined process have been identified as (1) operating parameters (electrode materials, current density, and catalyst (Fe²⁺) concentration), (2) the matrix composition (nature and dose of extracting agent, pH, complexity of SW/SF solutions), and (3) the environmental impact (acute ecotoxicity and biodegradability of effluent as well as impact on soil microbial activity). The influence of these parameters on the SW/EF and SF/EF integrated processes has been reviewed. Energy consumption calculations have been finally considered as it constitutes the main source of operating cost in EF process.