Keywords

1 Introduction

The growing imbalance between water demand and water resources throughout the world, along with the scarcity of fresh water to sustain all human activities, has stimulated the development of a large variety of physical, chemical, photochemical, and electrochemical methods that show great efficiency in treating urban and industrial wastewater, aiming at its further reuse. The electro-Fenton (EF) process, initially called electrogenerated Fenton’s reagent, was first used for wastewater treatment in the mid ‘80s (Brillas et al. 2009). EF is considered an electrochemical advanced oxidation process (EAOP) since it consists in the generation of homogeneous hydroxyl radical (•OH). This is achieved upon the occurrence of Fenton’s reaction between Fe2+ and electrogenerated H2O2 (Brillas et al. 2009; Oturan and Aaron 2014; Sirés et al. 2014). •OH is the second strongest oxidizing species known after fluorine, showing a large ability to mineralize most organic pollutants in water. The most characteristic feature of EF is that H2O2 is continuously dosed to the reaction medium from the two-electron reduction of O2. This gas can be either directly fed as pure O2 or air into the wastewater or pumped through a gas-diffusion device, further accepting the electrons typically supplied to a carbonaceous cathode. Thanks to the production of H2O2 on site, expensive and dangerous steps like industrial synthesis, transportation, storage, and handling can be avoided. Conventional homogeneous EF involves the addition of Fe2+ to the wastewater, although Fe3+ can be used alternatively because it is reduced to the former ion at the cathode surface (Oturan and Aaron 2014; Martínez-Huitle et al. 2015). The continuous Fe2+ and H2O2 (re)generation represents a significant advantage over the classical chemical Fenton process. In addition, the use of an undivided cell can accelerate the decontamination because of the production of heterogeneous M(•OH) from water discharge at the surface of the anode M. On the other hand, one important drawback of EF when applied to organics removal is the production of Fe(III)-carboxylate complexes as final byproducts, which are quite refractory to •OH attack. This phenomenon also occurs in the dark Fenton. To overcome this problem, the photoelectro-Fenton (PEF) process was developed in the mid ‘90s (Brillas et al. 2009). It consists in the simultaneous irradiation of the solution with artificial UV light or, more recently, sunlight, thus promoting the photolysis of intermediates like the Fe(III)-carboxylate species. As a result, a larger mineralization can be usually attained. The application of EF and PEF, along with the related processes, to the treatment of synthetic and real wastewater has been discussed in several authoritative reviews (Brillas et al. 2009; Oturan and Aaron 2014; Sirés et al. 2014; Martínez-Huitle et al. 2015; Moreira et al. 2017). Recently, the fundamentals, reactions, and applications of these EAOPs have been summarized in a comprehensive book edited by Zhou et al. (2018).

Other drawbacks of homogeneous EF and PEF include the limited pH range, i.e., ~3, to avoid the catalyst loss by precipitation, and the increase of soluble iron content in the treated effluent (Martínez-Huitle et al. 2015). Over the last decade, many efforts have been made to overcome such limitations, thus re-enforcing the EF and PEF viability. Among the most valuable, nanoengineering has allowed the development of new nanomaterials as alternative cathodes, anodes, and catalysts. This chapter reviews the characteristics and applications of these materials, mainly prepared as: (i) cathodes to enhance the H2O2 electrogeneration and/or to promote heterogeneous Fenton’s reaction, (ii) anodes to favor the electrocatalytic and photoelectrocatalytic production of •OH, and (iii) Fe-based catalysts to extend the pH range. Nanomaterials for hybrid treatments involving EF or PEF in combination with adsorption are described as well.

2 Nanomaterials as Cathodes

Many nanomaterials have been utilized to enhance the H2O2 production at the cathode of an electrolytic cell from the two-electron reduction of O2 gas by reaction in Eq. 11.1. In some cases, they can simultaneously act as catalysts that favor the heterogeneous Fenton’s reaction, mimicking the conventional Fenton’s reaction (Eq. 11.2) with •OH generation (Brillas et al. 2009; Oturan and Aaron 2014; Sirés et al. 2014; Martínez-Huitle et al. 2015; Moreira et al. 2017). The use of these modified cathodes is described in this section.

$$ {\mathrm{O}}_{2\left(\mathrm{g}\right)}+2{\mathrm{H}}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2 $$
(11.1)
$$ {\mathrm{Fe}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{3+} +^{\bullet}\mathrm{OH}+{\mathrm{O}\mathrm{H}}^{-} $$
(11.2)

2.1 Carbon-Based Nanomaterials

Carbonaceous materials are appealing targets to be used as catalysts for H2O2 electrogeneration since they are abundant, cheap, durable, and show good faradaic efficiency in the reaction in Eq. 11.1 (Čolić et al. 2018). A recent theoretical study by Chai et al. (2017) has reported that the reduction of O2 molecules takes place by approaching the hydrogen sites on the carbon surface to form the hydroperoxide ion (HO2 −), whose subsequent protonation yields H2O2. The two-electron oxygen reduction reaction (ORR) is then very effective in acidic media, which agrees with the optimum pH of conventional Fenton’s reaction (Eq. 11.2), whereas in alkaline medium, the four-electron ORR to form H2O prevails. The use of carbon-based nanomaterials allows increasing the electroactive surface area of the cathode, and consequently higher O2 mass transport rates are achieved.

Carbon-based nanomaterials may exhibit outstanding electronic, photonic, electrocatalytic, chemical, and mechanical properties, depending on their nanoscale structure. Two main groups of materials can be distinguished: nanosized and nanostructured carbons (Khataee and Hasanzadeh 2017). The former is characterized by nanometric shell size and thickness; and among others includes carbon nanotubes (CNTs), graphene (Gr) and its derivatives (graphene oxide (GO) and reduced graphene oxide (rGO)), nanofibers, nanodiamonds, nanocoils, nanoribbon, and fullerene. The latter group is composed of ordered mesoporous carbons and carbon fibers, which are constructed through various methodologies. Some examples of nanocarbons are shown in Fig. 11.1.

Fig. 11.1
figure 1

Schematic illustration of some carbon-based nanomaterials. (Adapted from Khataee and Hasanzadeh (2017), Copyright 2017, with permission from Springer Nature)

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CNTs are ubiquitous carbon-based materials that allow the electrocatalysis of the reaction in Eq. 11.1. They have a hexagonal 1D structure with sp 2 hybridization (see Fig. 11.1), being formed upon the rolling up of a graphite layer that yields a nanoscale tube form (about 1 nm in diameter). The occurrence of two or more coaxial CNTs, with expanding diameters and tube separation of around 0.34 nm, gives rise to multi-walled CNTs (MWCNTs). CNTs sponge (Wang et al. 2014), classical CNTs (Khataee and Hasanzadeh 2017; Tian et al. 2016a), or MWCNTs (Roth et al. 2016) deposited onto gas-diffusion electrodes (GDEs), and graphite mixed with MWCNTs (Chu et al. 2013; Pajootan et al. 2014; Babaei-Sati and Parsa 2017) have been utilized to enhance the H2O2 generation, hence the removal of several organics by EF. For example, the Khataee’s group reported that the amount of H2O2 produced using CNTs-based GDE was up to three-fold higher than using conventional GDE with activated carbon (Khataee and Hasanzadeh 2017). Wang et al. (2014) degraded 120 mL of 50 mg/L dimethyl phthalate in 0.1 M Na2SO4 with 0.5 mM Fe2+ at pH 3.0 by EF using an undivided cell with a Pt anode and either a 4 cm2 CNTs sponge or graphite-based GDE as the cathode, at a cathodic potential (E cath) of −0.50 V/saturated calomel electrode (SCE). After 120 min of electrolysis, the pollutant removal was 96% with 75% of total organic carbon (TOC) abatement using the sponge. These values are much higher than the 48% and 35% obtained for graphite-based GDE, respectively. A superior degradation performance upon the incorporation of CNTs in the GDE has also been described during the EF treatment of the dye Rhodamine B (Tian et al. 2016a). Excellent removal has also been reported for Acid Red 14 with a MWCNTs-based GDE (Roth et al. 2016) and for m-cresol (Chu et al. 2013), Basic Blue 41 (Pajootan et al. 2014), and Acid Red 14 and Acid Blue 92 (Babaei-Sati and Parsa 2017) with graphite-MWCNTs.

Graphene is composed of a 2D planar sheet with monoatomic thickness, made of sp 2 carbon atoms densely organized into a honeycomb structure (see Fig. 11.1). As in the case with CNTs, multilayers are also feasible. Graphene has been applied to the EF process as a pristine nanomaterial (Mousset et al. 2016b; Chen et al. 2016a), coated onto carbon felt (Le et al. 2015, 2017; Yang et al. 2017), carbon cloth (Mousset et al. 2016a), carbon-cloth GDEs (Garcia-Rodriguez et al. 2018), or carbon fiber (Mousset et al. 2017a), and mixed with graphite and PTFE (Zhang et al. 2018). Table 11.1 collects the main results obtained for several target organic pollutants using these cathodes. All the trials were carried out at bench scale using undivided cells at the optimum pH for Fenton’s reaction (Eq. 11.2). Three-electrode cells with a constant E cath between the cathode and the reference electrode (SCE) or two-electrode cells with constant current (I ) or current density ( j ) were utilized.

Table 11.1 Selected results obtained for the EF treatment of several organic pollutants using undivided cells with cathodes based on graphene (Gr)
Full size table

In most cases, O2 was bubbled into the solutions to ensure their saturation, aiming at enhancing the largest H2O2 production by the reaction in Eq. 11.1. In setups equipped with carbon cloth GDE (Garcia-Rodriguez et al. 2018), O2 was pumped through the dry surface to produce H2O2 at the wet surface, which is in contact with the solution. Table 11.1 shows that very short electrolysis time was needed to degrade the model pollutants, whereas much longer time (up to 480 min) was necessary for achieving significant mineralization degrees because of the greater recalcitrance of byproducts to the attack of •OH. In general, worse results were found with raw cathodes without Gr. Figure 11.2 schematizes the preparation of a Gr/carbon-cloth cathode, involving the electrochemical exfoliation of a graphite rod to obtain graphene, which was mixed with Nafion® (PTFE suspension) and then ultrasonicated to obtain the graphene ink to coat the carbon cloth. Figure 11.2 also highlights the greater H2O2 electrogeneration with the Gr/carbon-cloth cathode compared with the uncoated cloth. This led to the greatest degradation (92%) and mineralization (57%) degrees for a 1.4 mM phenol solution using the coated cathode (see Table 11.1), only reaching 72% and 41% in the absence of Gr. Therefore, the larger H2O2 production favored a greater accumulation of •OH, thus resulting in the faster oxidation of organics.

Fig. 11.2
figure 2

Steps for the preparation of graphene-coated carbon cloth cathode. (Adapted from Khataee and Hasanzadeh (2017), Copyright 2017, with permission from Springer Nature). The graph represents the concentration of H2O2 accumulated in 80 mL of 0.05 M K2SO4 at pH 3.0 using an undivided cell with a Pt anode and uncoated or graphene-coated carbon cloth cathode, all with 15 cm2 area, at current density of 1.25 mA/cm2. (Adapted from Mousset et al. (2016a), Copyright 2016, with permission from Elsevier)

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2.2 Chemically Modified Carbonaceous Nanomaterials

The ability of carbonaceous cathodes to electrogenerate H2O2 from the reaction in Eq. 11.1 can be enhanced by chemically modifying their surface. Zhou et al. (2014) modified a graphite felt (GF) with pure ethanol, yielding a material (GF-A) with carbon nanoparticles of average diameter ca. 500 nm on the surface of the filaments. A second kind of modification employing 90:10 (v/v) ethanol/hydrazine hydrate mixtures yielded oxygen- and nitrogen-containing functional groups (GF-B), resulting in weaker hydrophobicity compared with GF and GF-A, thus favoring the H2O2 generation. Its accumulation increased in the sequence: GF (67 mg/L) < GF-A (110 mg/L) < GF-B (176 mg/L) upon electrolysis of 130 mL of an O2-saturated 0.05 M Na2SO4 solution at pH 3.0 using a three-electrode cell at E cath = −0.65 V/SCE for 120 min. The same trend was obtained when the performance of the EF process was assessed using 50 mg/L p-nitrophenol under the above conditions using 0.2 mM Fe3+ as catalyst, reaching 67%, 83%, and 99% substrate decay at 60 min and 22%, 32%, and 51% mineralization at 120 min with GF, GF-A, and GF-B, respectively. The superiority of GF-B containing N-doped carbon nanoparticles was related to the higher current recorded, which promoted the H2O2 production and the reduction of Fe3+ to Fe2+, thereby increasing the amount of •OH generated.

N-doping then favors the occurrence of the two-electron ORR to yield H2O2 since it increases the chemically active sites, the O2 chemisorption, and the hydrophilicity of the carbon surface (Khataee and Hasanzadeh 2017). This has been confirmed for the other modified nanocarbons like N-CNTs (Zhang et al. 2008) and an N-Gr@CNTs composite GDE (Liu et al. 2016), which were used to treat 250 mL of Methyl Orange and 100 mL of 50 mg/L dimethyl phthalate solutions in 0.05 M Na2SO4 with 0.2–0.5 mM Fe2+ at pH 3.0 by EF using three-electrode undivided cells with a Pt anode at E cath of −0.85 and −0.50 V/SCE, respectively. In the former system, 100% decolorization was obtained after 60 min using N-CNTs, a value higher than 82% found with an unmodified CNTs cathode. In the other case, the highest TOC removal of 52% at 180 min with the N-Gr@CNTs GDE was superior to 26%, 12%, 7.4%, and 38% reached with Gr@CNTs GDE, CNTs-based GDE, Gr-based GDE, and graphite-based GDE as the cathode, respectively. However, a contradictory interpretation of the behavior of the N-doped carbonaceous cathodes has been proposed in the recent work of Yang et al. (2018b). These authors prepared GF cathodes coated with Gr and N-Gr. Figure 11.3a and b evidence that the N-Gr particles deposited onto the graphite fibers were more uniform than the Gr ones. The degradation of 100 mL of 50 mg/L phenol in 0.05 M Na2SO4 at pH 3.0 with a DSA® anode at E cath = −0.90 V/SCE revealed the removal of 99% and 78% phenol using GF with N-Gr and Gr within 50 min, respectively. This was ascribed to the formation of •OH from the H2O2 reduction via the in situ metal-free processes illustrated in Fig. 11.3c, which was enhanced with the N-Gr-coated GF, as detected by ESR spectroscopy. Figure 11.3d shows phenol removal efficiency of 99% using the N-Gr-GF cathode at pH 3.0, quite similar to the 97% obtained using EF with Gr and 0.40 mM Fe2+. At pH 7.0, phenol disappearance slowed down under the latter conditions, attaining 41%. When using N-Gr-GF without an iron catalyst, removal efficiency of up to 92% was achieved. These surprising results should be confirmed in future works, because such a large formation of •OH from H2O2 reduction at GF coated with N-Gr is difficult to justify considering the state-of-the-art of EAOPs.

Fig. 11.3
figure 3

SEM images of raw graphite-felt cathode modified with (a) graphene (Gr) and (b) N-doped graphene (N-Gr). (c) Schematic illustration of the in situ metal-free reactions occurring in the above cathodes. (d) Percentage of phenol removal during the treatment of 100 mL of 50 mg/L phenol in 0.05 M Na2SO4 using a cell with a DSA® anode and each of the above cathodes at E cath = −0.90 V/SCE. The EF trials with Gr were made using 0.40 mM Fe2+. No Fe2+ was added using N-Gr. (Adapted from Yang et al. (2018b), Copyright 2018, with permission from Elsevier)

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On the other hand, Quan and coauthors have recently developed hierarchically porous carbon (HPC) as a novel structure with a high electrocatalytic activity for the ORR to form H2O2. These materials are derived from the carbonization of metal–organic frameworks (MOFs), giving rise to abundant micro-, meso-, or even macropores with transference of C-sp 2 to C-sp 3 that can act as active sites for O2 adsorption, eventually improving the kinetics of ORR (Liu et al. 2015b). F-doped HPC cathodes have been reported as good electrocatalysts for H2O2 generation (Zhao et al. 2018b). Liu et al. (2015a) prepared an HPC electrode from Zn2+ and 1,4-benzenedicarboxylic acid as precursors, carrying out the carbonization under H2 atmosphere. This electrode was used as the cathode of a three-electrode undivided cell with a Pt anode, yielding 97% of substrate decay and 91% TOC abatement after 180 and 240 min, respectively, upon EF treatment of 50 mg/L of perfluorooctanoate in 0.05 M Na2SO4 with 1.0 mM Fe2+ at pH 2.0 and E cath = −0.40 V/SCE.

2.3 Non-Ferrous Metal-Modified Carbon Nanomaterials

For a given cathode material, the H2O2 production can be maximized by optimizing main experimental parameters like the electrolyte composition, pH, temperature, cell configuration, and applied current or E cath (Yang et al. 2018a). Recently, the modification of carbonaceous cathodes with nonferrous metals has been addressed aiming at improving their electroactivity, i.e., trying to cause the ORR at less negative potential in order to reach higher current values at lower overvoltage. In this scenario, several authors have reported a large H2O2 enhancement by using Au-Pd (Pizzutilo et al. 2017) and Pt-Hg (Siahrostami et al. 2013) nanoparticles immobilized onto glassy carbon (GC). However, only few papers have described the use of bimetallic nanomaterials for the EF treatment of wastewater. For example, Félix-Navarro et al. (2013) deposited bimetallic Pt-Pd nanoparticles on MWCNTs, which were sprayed onto a reticulated vitreous carbon (RVC) GDE. This cathode allowed the accumulation of 71 mM H2O2 in 20 mL of 0.5 M H2SO4 after 20 min at E cath = −0.50 V vs. Ag/AgCl. This was much greater than 2.2 mM obtained with a similar cathode without nanoparticles, as a result of the great electroactive area of Pt-Pd nanoparticles. Under these conditions, nitrobenzene was rapidly and completely degraded by EF with 0.1 mM Fe2+.

Other authors have used metallic oxide nanoparticles, such as TaO2 deposited onto GDE (Carneiro et al. 2016), WO2.72 mixed with Vulcan carbon to prepare GDEs (Paz et al. 2018), and CexA1-xO2 (A = Zr, Cu, or Ni) immobilized onto carbon felt (Li et al. 2017), to enhance the electroactivity regarding the H2O2 production. The use of WO2.72@Vulcan GDE and Pt as the cathode and anode in a three-electrode cell to degrade 350 mL of a 0.260 mM Orange II solution in 0.1 M K2SO4 with 0.50 mM Fe2+ at pH 3.0 by EF yielded 100% decolorization after 120 min at E cath = −0.70 V vs Ag/AgCl. This potential was selected because it led to the maximum H2O2 accumulation (i.e., 480 mg/L) in the absence of a catalyst (Paz et al. 2018). The crucial role of •OH formed from Fenton’s reaction (Eq. 11.2) was confirmed from the poorer color loss, ca. 20%, achieved under analogous conditions but without Fe2+. In that so-called electrochemical oxidation with electrogenerated H2O2 (EO-H2O2) process, heterogeneous Pt(•OH) is formed from water discharge from the reaction in Eq. 11.3 (Brillas et al. 2009; Oturan and Aaron 2014; Sirés et al. 2014; Martínez-Huitle et al. 2015; Moreira et al. 2017). In EF, both •OH and Pt(•OH) contribute to the dye oxidation, with predominance of the former radical.

$$ \mathrm{Pt}+{\mathrm{H}}_2\mathrm{O}\to \mathrm{Pt}\left({}^{\bullet}\mathrm{OH}\right)+{\mathrm{H}}^{+}+{\mathrm{e}}^{-} $$
(11.3)

Co and Co-based materials also possess excellent characteristics for H2O2 electrogeneration in EF and PEF. Barros et al. prepared a GDE modified with 5% Co-phthalocyanine for the EF treatment of 400 mL of solutions containing 100 mg/L of the food dyes Amaranth (Barros et al. 2014b) and Tartrazine (Barros et al. 2014a), with 0.1 M K2SO4 and 0.15 mM Fe2+ at pH 2.5, using a three-electrode undivided cell with a Pt anode at E cath = −0.70 V vs. Ag/AgCl. After 90 min, 79% and 98% decolorization, with 67% and 75% TOC removal, related to 370 and 219 kWh per kg TOC removed, were obtained, respectively. Ridruejo et al. (2018) showed the viability of a CoS2/MWCNTs GDE cathode to degrade 150 mL of 0.112 mM of the anaesthetic tetracaine in 0.05 M Na2SO4 at pH 3.0 in a two-electrode cell with a BDD anode at j = 100 mA/cm2. The authors found the complete drug disappearance at 120 and 60 min of EO-H2O2 and PEF with a 6 W UVA lamp, respectively, with TOC reductions of 43% and 60% at 180 min. The superiority of PEF was explained by the generation of •OH from Fenton’s reaction (Eq. 11.2) and the photoreduction of photoactive Fe(OH)2+ via the reaction in Eq. 11.4, along with the photodecomposition of final Fe(III)-carboxylate species (Fe(OOCR)2+) via the reaction in Eq. 11.5:

$$ \mathrm{Fe}{\left(\mathrm{OH}\right)}^{2+}+\mathrm{h}\nu \to {\mathrm{Fe}}^{2+}{+}^{\bullet}\mathrm{OH} $$
(11.4)
$$ \mathrm{Fe}{\left(\mathrm{OOCR}\right)}^{2+}+\mathrm{h}\nu \to {\mathrm{Fe}}^{2+}+{\mathrm{CO}}_2+{\mathrm{R}}^{\bullet } $$
(11.5)

What is also worth highlighting, Liang et al. (2016, 2017) proposed a heterogeneous EF-like treatment to decolorize 100 mL of 50 mg/L of Methyl Orange in 0.05 M Na2SO4 at pH 3–9 in a two-electrode undivided cell, with a Ti/IrO2-RuO2 anode (DSA®) and a cathode composed of GDE or GF coated with Co, at j = 5 mA/cm2. A large H2O2 accumulation (about 595 mg/L) was obtained in the background electrolytes after 120 min of electrolysis, whereas 100% and 86% decolorization of the dye solution was attained after 180 min at pH 3.0 and pH 9.0, respectively, using coatings with 1.0–1.5 wt.% Co. The cathodes presented a great stability, with <4% of loss of color removal after ten cycles. The authors also showed the formation of •OH, which was ascribed to the Fenton-like reaction (Eq. 11.6) between Co2+ and H2O2 at the cathode surface. The Co3+ ion formed was further reduced to Co2+.

$$ {\mathrm{Co}}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Co}}^{3+}{+}^{\bullet}\mathrm{OH}+{\mathrm{O}\mathrm{H}}^{-} $$
(11.6)

2.4 Fe-Loaded Carbon Nanomaterials

During the twenty-first century, many research efforts have been made to develop heterogeneous electro-Fenton (hetero-EF) and photoelectro-Fenton (hetero-PEF) processes using carbonaceous materials loaded with nanoparticles of Fe (Li et al. 2011; Bañuelos et al. 2015), Fe-Fe2O3 (Li et al. 2009; Ai et al. 2007; Ding et al. 2017), Fe2O3 (Wang et al. 2013; Peng et al. 2015; Sklari et al. 2015), FeOOH (Zhang et al. 2012, 2015a), Fe3O4 (Chen et al. 2016b; Tian et al. 2016b; Zhang et al. 2017), γ-Fe2O3/Fe3O4 (Plakas et al. 2016), Fe-Cu (Garrido-Ramírez et al. 2016; Zhao et al. 2016, 2018a), CoFe hydroxide (Ganiyu et al. 2017), and Fe3S4/Fe7S8 (Choe et al. 2018). In these processes, no soluble iron catalyst is added to the solution since Fenton’s reaction (Eq. 11.2) occurs between Fe(II) adhered to the cathode surface and H2O2 produced at the same surface. Furthermore, leaching of iron ions from the catalyst can contribute to conventional homogeneous Fenton’s reaction. The main aim when using heterogeneous catalysts is to make the application of water treatments without pH regulation feasible, i.e., at natural pH, which is usually circumneutral. These materials minimize the formation of iron hydroxide, with the consequent reduction of costs associated to sludge management. A potential drawback of such catalysts is their gradual solubilization upon the use and the reuse, which limits their lifetime, especially under acidic conditions. Therefore, it is crucial to test their stability in consecutive degradation cycles. Selected degradation and mineralization results for some organic pollutants treated by hetero-EF and hetero-PEF are summarized in Table 11.2. A good performance can be observed for both processes within the pH range 2–7, although it is slightly better at pH 3.0, which is in agreement with the optimum conditions of Fenton’s reaction (Eq. 11.2). The data corroborate the viability of the heterogeneous treatments at neutral pH, showing acceptable effectiveness. A clear superiority of hetero-PEF over hetero-EF, leading to faster removal of the target pollutant and TOC, can also be observed under comparable conditions, which can be accounted for by the additional generation of •OH from photoreaction (Eq. 11.4) and the photolysis of intermediates such as final Fe(III)-carboxylate complexes via the reaction in Eq. 11.5.

Table 11.2 Selected results obtained for the hetero-EF and hetero-PEF treatment of several organic pollutants using undivided cells with cathodes based on Fe-loaded carbon nanomaterials
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As an example, Fig. 11.4a shows that magnetite (Fe3O4) presents a relatively uniform and spherical shape, with an average diameter of 40–50 nm, whereas Fig. 11.4b confirms its successful loading on the activated carbon acting as GDE (Zhang et al. 2017). Figure 11.4c illustrates the hetero-EF mechanism proposed to destroy tetracycline upon the attack of different reactive oxygen species (ROS) including: (i) the superoxide radical (O2 •-) produced from O2 via the reaction in Eq. 11.7, (ii) H2O2 formed from O2 via the reaction in Eq. 11.1, and (iii) •OH produced from heterogeneous Fenton’s reaction (Eq. 11.8), with the subsequent reduction of FeIII to FeII via the reaction in Eq. 11.9

$$ {\mathrm{O}}_{2\left(\mathrm{g}\right)}+{\mathrm{e}}^{-}\to {{\mathrm{O}}_2}^{\bullet -} $$
(11.7)
$$ \equiv {\mathrm{Fe}}^{\mathrm{II}}+{\mathrm{H}}_2{\mathrm{O}}_2\to \kern0.5em \equiv {\mathrm{Fe}}^{\mathrm{II}\mathrm{I}} +^{\bullet}\mathrm{OH}+{\mathrm{O}\mathrm{H}}^{-} $$
(11.8)
$$ \equiv {\mathrm{Fe}}^{\mathrm{II}\mathrm{I}}+{\mathrm{e}}^{-}\to \kern0.5em \equiv {\mathrm{Fe}}^{\mathrm{II}} $$
(11.9)

where ≡ means the Fe3O4 surface. On the other hand, Fig. 11.4d schematizes the generation of •OH to attack Acid Orange 7, using a CoFe hydroxide/carbon felt cathode (Ganiyu et al. 2017). •OH is formed homogeneously from the reactions in Eqs. 11.2 and 11.6 in acidic medium, owing to Fe2+ and Co2+ leaching from the cathode. In contrast, this radical originates from the heterogeneous reactions in Eqs. 11.10 and 11.11, with regeneration of the FeII/CoII-OH catalyst and no metal release, at neutral pH.

$$ \equiv {\mathrm{Fe}}^{\mathrm{II}\mathrm{I}}/{\mathrm{Co}}^{\mathrm{II}\mathrm{I}}\hbox{-} \mathrm{OH}+{\mathrm{H}}_2{\mathrm{O}}_2\to \kern0.5em \equiv {\mathrm{Fe}}^{\mathrm{II}}/{\mathrm{Co}}^{\mathrm{II}}\hbox{-} \mathrm{OH}+{{\mathrm{H}\mathrm{O}}_2}^{\bullet }+{\mathrm{H}}^{+}\kern0.5em $$
(11.10)
$$ \equiv {\mathrm{Fe}}^{\mathrm{II}}/{\mathrm{Co}}^{\mathrm{II}}\hbox{-} \mathrm{OH}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}^{\mathrm{II}\mathrm{I}}/{\mathrm{Co}}^{\mathrm{II}\mathrm{I}}\hbox{-} \mathrm{OH} +^{\bullet}\mathrm{OH}+{\mathrm{O}\mathrm{H}}^{-} $$
(11.11)
Fig. 11.4
figure 4

SEM images of (a) pure Fe3O4 and (b) fresh Fe3O4/GDE. (c) Schematic illustration of the hetero-EF mechanism with the latter cathode. (Adapted from Zhang et al. (2017), Copyright 2017, with permission from Elsevier). (d) Schematic illustration of Acid Orange 7 (AO7) degradation in the hetero-EF system with a Pt anode and a CoFe–LDH/CF cathode). (Adapted from Ganiyu et al. (2017), Copyright 2017, with permission from The Royal Society of Chemistry)

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3 Nanomaterials as Anodes

The EF and PEF treatments are usually carried out in cells equipped with conventional anodes like plates, rods/bars, or meshes. Alternatively, they can be performed with nanostructured anodes that can act as photoanodes or more efficient electrocatalysts. In the former case, hybrid processes with photoelectrocatalysis (PEC) have been devised. This section is focused on the preparation and application of such materials.

3.1 TiO2-Based Photoanodes

The PEC method involves the use of a photoanode, commonly a nanocrystalline TiO2-based material, in order to cause the light-induced oxidation of organic pollutants in aqueous media. This semiconductor has low cost and toxicity, and possesses a wide band gap of 3.2 eV (in the anatase form) (Garcia-Segura and Brillas 2017). Deposited as a thin film, it can absorb UV photons (λ < 380 nm) to promote an electron from the valence band to the conduction band (e− CB), with generation of a positively charged vacancy or hole (h+ VB) via the reaction in Eq. 11.12. Organics can then be oxidized by: (i) the hole, (ii) heterogeneous •OH formed from the reaction in Eq. 11.13 between h+ VB and adsorbed water, and (iii) different ROS initiated from the reduction of O2 by e− CB according to the reactions in Eqs. 11.14–11.17 (Sirés et al. 2014; Garcia-Segura and Brillas 2017).

$$ {\mathrm{TiO}}_2+\mathrm{h}\nu \to {\mathrm{e}}_{\mathrm{CB}}^{-}+{\mathrm{h}}_{\mathrm{VB}}^{+} $$
(11.12)
$$ {\mathrm{h}}_{\mathrm{VB}}^{+}+{\mathrm{H}}_2\mathrm{O}\to \bullet \mathrm{OH}+{\mathrm{H}}^{+} $$
(11.13)
$$ {\mathrm{e}}_{\mathrm{CB}}^{-}+{\mathrm{O}}_2\to {{\mathrm{O}}_2}^{\bullet -} $$
(11.14)
$$ {{\mathrm{O}}_2}^{\bullet -}+{\mathrm{H}}^{+}\to {{\mathrm{H}\mathrm{O}}_2}^{\bullet } $$
(11.15)
$$ 2{{\mathrm{H}\mathrm{O}}_2}^{\bullet}\to {\mathrm{H}}_2{\mathrm{O}}_2+{\mathrm{O}}_2 $$
(11.16)
$$ {\mathrm{H}}_2{\mathrm{O}}_2+{{\mathrm{O}}_2}^{\bullet -}\to {\kern0.5em }^{\bullet}\mathrm{OH}+{\mathrm{O}\mathrm{H}}^{-}+{\mathrm{O}}_2 $$
(11.17)

The recombination of the e− CB/h+ VB pair is the major drawback since it leads to significant efficiency loss. But this can be overcome by applying either an optimum constant I or a constant bias anodic potential (E anod) to the illuminated photoanode (Brillas et al. 2009; Sirés et al. 2014; Garcia-Segura and Brillas 2017). This ensures the continuous extraction of photoinduced electrons, which are conveyed to the cathode through the external electrical circuit.

When a photoanode is combined with an appropriate carbonaceous cathode that electrogenerates H2O2 in the presence of Fe2+ as catalyst, the hybrid PEC/EF process takes place (Peralta-Hernández et al. 2006; Xie and Li 2006; Li et al. 2007; Esquivel et al. 2009; Ramírez et al. 2010; Ding et al. 2012, 2014; Lin et al. 2013; Almeida et al. 2014; Mousset et al. 2017b). In this case, only the photoanode is exposed to UV radiation. In contrast, if also the solution is illuminated, the process is the so-called PEC/PEF (Mousset et al. 2017b; Almeida et al. 2015). For example, Fig. 11.5a schematizes the formation of the main oxidant •OH at a Bi2WO6 anode in PEC from the reaction in Eq. 11.13, which is combined with its formation via the reaction in Eq. 11.8 in hetero-PEF using an Fe@Fe2O3/activated carbon fiber (ACF) cathode. Both radicals are employed to destroy the dye Rhodamine B (Ding et al. 2012).

Fig. 11.5
figure 5

(a) Schematic view of the main reactions to destroy Rhodamine B (RhB) by PEC with a Bi2WO6 photoanode combined with hetero-EF with a Fe@Fe2O3/ACF cathode. (Adapted from Ding et al. (2012), Copyright 2012, with permission of Elsevier). (b) Total organic carbon decay vs. applied electric charge for the () PEC, () EF, and () PEC/PEF treatments of 500 mL of 85.4 mg/L Orange G solutions in 0.05 M Na2SO4 at pH 3.0. Anode in EF: Pt. Anode in PEC and PEC/PEF: Pt/TiO2 NTs. Cathode: GDE. Applied current: 50 mA. Fe2+ concentration: 0.50 mM. Irradiation: 80 W UVA lamp. (Adapted from Almeida et al. (2015), Copyright 2015, with permission of Elsevier))

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Good performance of PEC/EF and PEC/PEF processes for the removal of several organics can be observed in Table 11.3. Apart from TiO2, other effective photocatalysts like Bi2WO6 (Ding et al. 2012) and Pt/TiO2 NTs (Almeida et al. 2014; Mousset et al. 2017b) have been tested. Table 11.3 also evidences, as expected, greater efficiency of PEC/PEF when compared to PEC/EF for the treatment of a phenol solution under comparable conditions, which results from the acceleration of the oxidation process thanks to the contribution of the reactions in Eqs. 11.4 and 11.5 (Mousset et al. 2017b). In most of these works, classical anodes were comparatively tested to confirm the better performance of the hybrid treatments. In the case of hybrid PEC/EF, for example, Rhodamine B solutions were mineralized up to 94% in a cell equipped with a Bi2WO6 photoanode under 300 W tungsten halogen lamp irradiation and an Fe@ACF cathode (see Table 11.3). In contrast, only 78% and 14% mineralization were found by EF with Pt/Fe@Fe2O3 cell and PEC with Bi2WO6/Pt cell, respectively, under analogous conditions. The outperformance of PEC/EF was explained by: (i) better separation of e− CB/h+ VB pairs, with accumulation of a larger amount of oxidant holes at the photoanode surface, and (ii) the additional injection of photoinduced electrons to the Fe@Fe2O3/ACF cathode, enhancing the H2O2 production and yielding greater quantities of •OH. On the other hand, Fig. 11.5b illustrates the better performance of PEC/PEF with Pt/TiO2 NTs anode under 80 W UVA illumination and GDE as cathode, as compared to EF with a Pt/GDE cell and PEC with a Pt/TiO2 NTs photoanode and GDE as cathode (without added Fe2+) (Almeida et al. 2015). As can be seen, the TOC of an Orange G solution was reduced by 97%, 87%, and 80%, respectively, after 200 mA h/L of specific electrical consumption. The combined oxidation by •OH formed from the reactions in Eqs. 11.2, 11.4, and 11.13, along with the photolytic action of UVA photons, accounts for the faster removal of organics by PEC/PEF.

Table 11.3 Selected results obtained for the PEC/EF and PEC/PEF treatment of several organics using undivided cells with nanostructured photoanodes
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As closely related to the aforementioned processes, the Khataee’s group explored the characteristics of a PEF/photocatalysis treatment (Khataee et al. 2010, 2012, 2013, 2014; Zarei et al. 2010; Khataee and Zarei 2011), which can be considered as an alternative to the PEC/PEF system. Cubic undivided cells of 1.0–3.0 L capacity were equipped with the following elements: (i) a Pt anode, (ii) a CNTs-PTFE GDE as cathode fed with an O2 flow, (iii) a 6 W UVA, UVB, or UVC lamp within a quartz tube, and (iv) four glass or ceramic plates coated with TiO2 (Khataee et al. 2010, 2012, 2014; Zarei et al. 2010), N-doped TiO2 (Khataee et al. 2013), or ZnO (Khataee and Zarei 2011) nanoparticles, which were placed covering the four inner cell walls. Solutions of the dyes Acid Red 17 (Khataee et al. 2010), Basic Red 46 (Zarei et al. 2010), Acid Yellow 36 (Khataee et al. 2012), Direct Red 23 (Khataee et al. 2013), and Direct Yellow 12 (Khataee and Zarei 2011), as well as phenol (Khataee et al. 2014), in 0.05 M Na2SO4 with 0.1–0.2 mM Fe3+ at pH 3.0 were comparatively treated by the individual processes to show the benefits of the hybrid PEF/photocatalysis treatment. For example, for 2 L of 50 mg/L Direct Yellow 12 with 0.1 mM Fe3+ treated for 90 min at 100 mA under a 6 W UVC light irradiation (Khataee and Zarei 2011), the decolorization ability decreased as follows: PEF/photocatalysis (93%) > PEF (69%) > EF (56%), being much lower (39%) for photocatalysis alone (ZnO irradiated by UVC). The high mineralization power of the PEF/photocatalysis method was shown by large TOC reduction (97%) achieved at 360 min. This can be attributed to the destruction of organics by: (i) heterogeneous Pt(•OH) formed from the reaction in Eq. 11.3 and heterogeneous •OH from photogenerated holes on ZnO by the reaction in Eq. 11.13, (ii) homogeneous •OH formed from Fenton’s reaction (Eq. 11.2), from photolysis of Fe(OH)2+ species via the reaction in Eq. 11.4, and from UVC photolysis of electrogenerated H2O2, (iii) photoinduced holes produced on ZnO, similarly to the reaction in Eq. 11.12, and (iv) photodecomposition of intermediates under UVC radiation.

3.2 Electrocatalytic Anodes

Several authors have improved the EF method by synthesizing anodes with greater electrocatalytic ability as compared to conventional ones, thereby enhancing the production of heterogeneous •OH from water discharge, similarly to the reaction in Eq. 11.3. Examples of some stable, nanostructured anodes for such a purpose are β-PbO2 (Sirés et al. 2010), porous Ni/BDD/Ta (Li et al. 2018), and ZnO-TiO2/GF (El-Kacemi et al. 2017). The excellent performance of 3D β-PbO2 deposits obtained from carbon/polyvinyl-ester composite (Sirés et al. 2010) for the EF process has been well proven for the treatment of 275 mL of an O2-saturated solution with 0.25 mM Methyl Orange in 0.05 M Na2SO4 with 0.2 mM Fe2+ at pH 3.0, using an undivided glass cell with a carbon-felt cathode at I = 60 mA. Total decolorization was achieved after 60 min of electrolysis, a time shorter than 240 min needed for the analogous EO with Ni cathode in the absence of Fe2+, at 300 mA. This corroborates much greater oxidation power of •OH formed from Fenton’s reaction (Eq. 11.2) than PbO2(•OH) in EF. Further work of these authors showed the applicability of PbO2 deposits onto RVC to EO (Recio et al. 2011; Ramírez et al. 2016), being feasible to incorporate TiNTs (Ramírez et al. 2016), which could be extended in the future to EF and PEF. Similarly, for 60 mg/L Methylene Blue in 0.05 M Na2SO4 at pH 3.0, the use of a porous Ni/BDD/Ta anode allowed the overall loss of color after 80 min of the EF treatment with 0.5 mM Fe2+ at 120 mA. This was faster than the comparable EO process, which required 240 min (Li et al. 2018). On the other hand, a nanostructured ZnO-TiO2/GF anode was coupled to a GF cathode to treat 250 mL of an O2-saturated solution with 0.12 mM of the dye Amido Black 10, 0.06 M Na2SO4, and 0.1 mM Fe2+ at pH 3.0 by EF at I = 100 mA (El-Kacemi et al. 2017). Under these conditions, the dye solution was completely decolorized in 60 min, whereas 91% mineralization was reached after 360 min of the treatment, showing good effectiveness of this anode for dye destruction.

4 Suspended Nanocatalysts

The synthesis of nanocatalysts to be suspended in solution, with the ability to form heterogeneous •OH upon the reaction with electrogenerated H2O2, has widened the applicability of Fenton-based EAOPs for wastewater treatment through the development of hetero-EF and hetero-PEF. Similarly to what has been discussed in the case of Fe-loaded carbon nanomaterials used as cathodes, the main advantage of suspended nanocatalysts is that they allow operating within a larger pH range, and, therefore, neutralization of the final effluent may be avoided (Ganiyu et al. 2018; Poza-Nogueiras et al. 2018). This allows the minimization of sludge formation and prevents an excessive accumulation of iron ions in the treated effluent, which are common handicaps in the homogeneous processes. Moreover, the nanocatalyst is easy to handle, safe to store, it can be efficiently recovered and has the possibility of being reused (Ganiyu et al. 2018). As main drawbacks, suspended nanocatalysts may lose some of their active sites, thus reducing their catalytic ability, and they can undergo partial solubilization. This means that, on many occasions, there exists the conjunction of both homogeneous and heterogeneous processes during the treatment, even if this is disregarded by many authors.

A large number of nanomaterials has been synthesized and tested for hetero-EF, including: (i) Fe-based nanoparticles, like Fe3O4 (He et al. 2014; Es’haghzade et al. 2017), Pd/Fe3O4 (Luo et al. 2014; Huang et al. 2017), Fe molybdophosphate (Baiju et al. 2018), and zero-valent iron (Babuponnusami and Muthukumar 2012); (ii) Fe-carbon nanoparticles such as Fe-C/PTFE (Zhang et al. 2015b, c) and Fe3O4-CNTs (Shen et al. 2014); (iii) mineral-like nanocatalysts such as martite (Khataee et al. 2017); (iv) MOFs, only used in non-electrochemical systems so far, as described in detail in recent reviews (Dias and Petit 2015; Cheng et al. 2018); (v) supported Fe catalysts, such as Fe–zeolite (Rostamizadeh et al. 2018), Fe3O4–chitosan (Rezgui et al. 2018), Fe–silica (Jinisha et al. 2018), and Fe2O3–kaolin (Özcan et al. 2017); and (vi) non-Fe-based, like Cu/C (Xu et al. 2013). Table 11.4 collects selected results obtained in these works.

Table 11.4 Selected results obtained for the hetero-EF treatment of several organic pollutants using undivided cells with suspended nanostructured catalysts in solution
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Successful degradations can be observed within the pH range 2–7, which confirms the viability of nanomaterials to operate at neutral pH—the condition required for the treatment of wastewater under real conditions.

In the case of Fe-based nanocatalysts, the proposed heterogeneous mechanism involves the oxidation of organics with heterogeneous •OH produced via the reaction in Eq. 11.8, where H2O2 is generated at a suitable carbonaceous cathode (Ganiyu et al. 2018; Poza-Nogueiras et al. 2018). The regeneration of FeII at the catalyst surface can occur either by the reaction in Eq. 11.9, when the material comes into contact with the cathode upon stirring or recirculation, or by the heterogeneous Fenton-like reaction in Eq. 11.18.

$$ \equiv {\mathrm{Fe}}^{\mathrm{II}\mathrm{I}}+{\mathrm{H}}_2{\mathrm{O}}_2\to \equiv {\mathrm{Fe}}^{\mathrm{II}}+{{\mathrm{H}\mathrm{O}}_2}^{\bullet }+{\mathrm{H}}^{+}\kern0.5em $$
(11.18)

Some authors have suggested more complex reaction mechanisms for hetero-EF when H2O2 is not cathodically produced. Figure 11.6a schematizes the steps proposed for the removal of humic acids (HAs) and Cr(VI) in a BDD/Pt cell using Pd/Fe3O4 as catalyst (Huang et al. 2017). These include: (i) H2 and O2 gas generation at the cathode and anode, respectively, (ii) diffusion of O2 to the Pd/Fe3O4 nanoparticles to be reduced to the oxidant O2 •-, (iii) H2O2 formation from the reaction between H2 and O2 at the catalyst surface, (iv) generation of heterogeneous •OH from heterogeneous Fenton’s reaction (Eq. 11.8) between FeII at the Pd/Fe3O4 surface and produced H2O2, with subsequent FeII regeneration via the reaction in Eq. 11.9, and (v) •OH formation at the BDD surface from a reaction similar to the one in Eq. 11.3. HAs are then oxidized by O2 •- and •OH. According to step 6 shown in Fig. 11.6a, Cr(VI) is reduced to Cr(III) at the cathode, and by atomic H at the nanocatalyst surface, whereas as shown in step 7, Cr(III) can be removed upon the deposition of chromite (FeCr2O4). Similarly, for the treatment of phenol solutions with Cu/C nanoparticles (Xu et al. 2013), the H2O2 production from the heterogeneous reaction of H2 and O2 gases at the catalyst surface has been suggested. Hence, the oxidant •OH is formed homogeneously from Fenton’s reaction (Eq. 11.2) thanks to the addition of Fe2+ ion to the solution.

Fig. 11.6
figure 6

(a) Schematic mechanisms for the removal of humic acid (HAs) and Cr(VI) by hetero-EF process with a BDD/Pt cell using Pd/Fe3O4 as catalyst. (Adapted from Huang et al. (2017), Copyright 2017, with permission from Elsevier). (b) Normalized TOC removal with electrolysis time for the treatment of 175 mL of 0.25 mM of the antibiotic enoxacin in 0.05 M Na2SO4 at pH 3.0 by () homogeneous EF with 0.30 mM Fe2+ and () hetero-EF with 0.1 g of Fe2O3–kaolin using a Pt/carbon felt cell. Applied current: 60 mA. (Adapted from Özcan et al. (2017), Copyright 2017, with permission from Elsevier)

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Several authors have also compared the homogeneous EF and hetero-EF treatments to show the excellent performance of the latter method. Figure 11.6b illustrates the normalized TOC abatement from a 0.25 mM enoxacin solution at pH 3.0 by homogeneous EF with 0.3 mM Fe2+ and hetero-EF with 0.1 g of Fe2O3–kaolin using a Pt/carbon felt cell at 60 mA. After 360 min of electrolysis, 85% and 89% mineralization, respectively, was achieved, indicating the high effectiveness of the nanocatalyst, even superior to that of Fe2+ for •OH generation. This should further encourage the research into novel nanomaterials with a higher catalytic activity and great stability for practical wastewater treatment, aiming to implement hetero-EF at industrial scale.

5 Nanomaterials in Hybrid Processes

Some other hybrid processes have been described to upgrade the oxidation ability of the EF and PEF methods (Brillas and Sirés 2018). However, the use of nanomaterials is rather limited to date, mainly being reduced to different kinds of carbon materials as adsorbents. Recently, Zhao et al. (2017) proposed a hybrid method involving electrosorption/EF to treat 100 mL of an O2-saturated solution with 50 mg/L dimethyl phthalate in 0.05 M Na2SO4 at natural pH 6 using an undivided cell with a Pt anode and an Fe-doped carbon aerogel with Gr sheets as cathode at I = 135 mA. It was found that about 90% of the substrate was adsorbed on the cathode surface, whereas 98% of it was removed after 150 min of hetero-EF. The authors suggested a degradation mechanism involving the destruction of physisorbed dimethyl phthalate with heterogeneous •OH formed from the reaction in Eq. 11.8. Wang et al. (2018) synthesized nanomaterials like La1-xNdxFeO3@activated carbon as anodes with a high adsorption ability. In only 10 min of EF treatment of Methyl Orange at pH 2.0 using GDE as cathode at I = 60 min, 99% color and 97% chemical oxygen demand (COD) was found, attaining 17% and 36% removal of such by direct adsorption at the same time. Direct removal of the adsorbed dye by heterogeneous •OH formed from water oxidation at the anode and homogeneous •OH from Fenton’s reaction (Eq. 11.2) between the anodically leached Fe2+ and the produced H2O2 was proposed as the main degradation mechanism.

6 Conclusions

This chapter has reviewed the main current developments in the synthesis, modification, use, and reuse of key nanomaterials that enhance the performance of EF and PEF processes for the removal of organic pollutants from water. Since Fenton’s reaction relies on the decomposition of H2O2 upon the reaction with Fe(II) to produce large amounts of •OH in the bulk solution, efforts are particularly concentrated on new Fe-based nanocatalysts. We believe that research should especially focus on nanomaterials used as suspended catalysts since catalysts supported on the carbonaceous cathode surface tend to be detrimental for the two-electron ORR, and they may affect the material’s stability and diminish the exposed carbon surface area for H2O2 electrogeneration. Investigation into potential Fe-based catalysts should pay special attention to their robustness, reducing the iron leaching to ensure a minimum secondary contamination and a greater reusability with reproducible performance. The use of MOFs in heterogeneous electrochemical Fenton-based EAOPs is forecast as a relevant topic in the near future. Novel cathodes, particularly those based on non-metal and non-ferrous nanocatalysts, may become the material of choice to electrogenerate H2O2 on site, and new electrode configurations might contribute to a significant improvement of such in situ electrosynthesis. The preparation of all these mentioned catalysts should, hopefully, be based on green synthesis principles, as shown in the case of 3D PbO2 electrodeposited using biocompatible methanesulfonic acid. Regarding the use of nanomaterials in hybrid processes, adsorption with nanomaterials like Gr followed by EF or PEF post-treatments of the resulting effluents is an interesting way worth exploring, which can be combined with washing nanoadsorbents by means of Fenton-based EAOPs, as recently addressed by some authors. Finally, it is worth mentioning that the tests of these new materials and reactors should be carried out with real wastewater matrices in order to correctly assess their viability when compared to robust separation technologies like adsorption, (electro)coagulation, or membrane separation. Reusability tests in those complex matrices are thus needed to assess the impact of the catalyst degradation on the treatment costs as well as the reproducibility upon successive usage.