Catalyst for the Generation of OH Radicals in Advanced Electrochemical Oxidation Processes: Present and Future Perspectives

Abstract

Heterogeneous Advanced Oxidation Processes (H-AOPs) are considered a new process for removing emerging pollutants. In this case, the high reactivity of hydroxyl radicals is used to degrade persistent organic pollutants. This review explores the state-of-the-art catalyst for hydroxyl radical generation in AOPs. As a parasite reaction, chloride ions appear in alkaline conditions and compete with the active sites. The theoretical foundation of catalyst performance is explored, focusing on the fundamental principles that govern the efficiency and mechanism of hydroxyl or chloride radical production. The synthesis and electronic modification sections explore the modifications of catalysts. It discusses key methodologies for catalyst preparation, with a particular emphasis on electronic modification that enhances both activity and stability. Finally, laboratory and pilot applications highlight the effectiveness of novel or modified catalysts in different scenarios. These last findings provide insights into the future directions for research and application, aiming to draw attention to the gap between laboratory studies and real-world implementations.

1. Introduction

Water is considered a heritage and a human right. Industries, agriculture, and the general population consume water and discharge many compounds into wastewater. However, human practices play an important role in the issue of pollutants, which are called emerging pollutants. Many authors define emerging pollutants as new products or chemicals without regulatory status and whose effects on the environment and human health are unknown [1]. In 2014, the EU Water Framework Directive 2000/06/CE updated a list of key pollutants and 45 priority substances in Annexes VIII and X, including arsenic, metals, pesticides, endocrine disruptors, and so on. The presence of these compounds in waste, surface, and groundwater comes from anthropogenic activities and poses a serious threat to public health. In 2010, the General Assembly of the United Nations recognized that everyone has the right to water accessibility, including universal access to water and sanitation for domestic use. Therefore, wastewater remediation is a high priority, and consequently, efficient treatment of these pollutants is urgently needed. In Mexico, where the authors have better access to data, the administrative body responsible for water issues within the Ministry of Environment and Natural Resources is the National Water Commission (CONAGUA—Comisión Nacional del Agua). This organism concentrates data from wastewater treatment plants (WWTP) from all over the country in its water information platform [2]. Here, the conventional WWTPs have been steadily increasing. CONAGUA reported 2872 WWTPs working at 73.2% of the total installed flow capacity. However, to the best of our knowledge, the implementation of public policy in public health for emerging pollutants has not been considered yet in some products like naproxen, ibuprofen, diclofenac, and other compounds [3,4]. In India, a decentralized wastewater treatment system manages and treats wastewater from rural areas. Despite this, the Central Pollution Control Board reported an estimate of 29,129 ML Day⁻¹ of wastewater, based on the 2001 census. However, the installed wastewater treatment capacity is only 6190 ML Day⁻¹, making a difference of 22,939 ML Day⁻¹ [5]. Tripathi et al. [6] found various monomers, metals, and several carcinogenic compounds in wastewater. Other workers also found different pharmaceutical pollutants [7] or organochlorine pesticides [8]. China shows another similar scenario, where emerging organic pollutants persist in aquatic environments [9,10,11,12]. These compounds originate from anthropogenic activities such as domestic, industrial, and agricultural activities. Zhong et al. [9] recommend developing control strategies due to the toxicity of these compounds. In such cases, emerging pollutants are not considered in treated wastewater quality standards.

According to the degree of innovation, the available technologies for pollutant removal are classified as conventional, established recovery, and emerging removal methods. Treatment methods can be classified according to the process steps in which they are employed, as shown in Figure 1 [13]. In pre-treatment, primary, and secondary treatment, we can analyze that conventional treatment techniques only change the phase of these organic pollutants and do not specifically transform them into benign compounds [14]. In addition, the degree of treatment required for wastewater depends primarily on the effluent discharge limits, so a single treatment may not be sufficient to remove the pollutants present, and processes may consist of multiple stages.

In tertiary treatment, chemical wastewater treatments are an integral part of the wastewater treatment process and are used to remove pollutants, improving water quality prior to discharge or reuse. Common chemical wastewater treatments (chemical methods) include coagulation and flocculation, neutralization, precipitation, advanced oxidation, adsorption, disinfection, chemical dosing, and sludge treatment. These chemical methods (consolidated processes) are insufficient and not effective in removing emerging pollutants as a single step in wastewater plant treatments. From this problem, advanced oxidation processes (AOPs, as a new process) arise with the need to remove EPs [15]. Therefore, consolidated processes and new processes must be combined in advanced treatment to mitigate the concentration of these pollutants. As for new processes, we can classify the real effluent treatment methods into two different categories, namely homogeneous and heterogeneous methods, as shown in Figure 2. Based on the literature [16,17], this figure classifies advanced oxidation processes, where heterogeneous and homogeneous advanced oxidation processes (H-AOP and HOM-AOP) are applied to water effluents. In HOM-AOP, we can primarily find photolysis, radiation, Fenton-like processes, UV/H₂O_2, and UV/O₃ processes. A second option is to introduce advanced heterogeneous oxidation processes, where we can find heterogeneous photocatalysts such as photo-Fenton and electrochemical methods.

In H-AOP, many authors have reviewed developments in AOPs applied to different emerging pollutants such as endocrine disruptors [18], organic [19,20], pharmaceuticals [21,22], antibiotics [23], real effluent in wastewater [24,25,26,27,28], and industrial wastewater [29]. Here, TiO₂, ZnO, and g-C₃N₄-based material and reduced graphene oxide (rGO) have been used as photocatalysts. In the Fenton-type oxidation process, support metals and noble metals [30], metal-free carbon [31], and natural iron minerals [32] are used. In the field of heterogeneous catalytic ozonation, we found six cases of materials applied to AOP [33]. In electrochemical advanced oxidation processes (EAOP), different techniques are applied to WWTP, such as electrocoagulation (EC), electrochemical reduction (ER), electrochemical oxidation (EO), indirect electrochemical oxidation (IEO), and photo-assisted electrochemical methods (PhEM) [16,17]. In this connection, different types of materials, such as metals [34], metal oxide [35], supported metal [36,37], and boron-doped diamond [38], have been used. Depending on the type of electrocatalyst used, different highly reactive transient species can be formed, such as hydroxyl radicals $\left({\mathrm{OH}}^{•}\right)$, hydroperoxyl radicals (${\mathrm{HO}}_2^{•}$), sulfate radicals (${\mathrm{SO}}_4^{•-}, {\mathrm{S}}_2{\mathrm{O}}_8^{2-}$), superoxide anion radicals (${\mathrm{O}}_2^{•-}$), peroxydicarbonate (${\mathrm{C}}_2{\mathrm{O}}_6^{2-}$) and reactive chlorine species (${\mathrm{Cl}}_2, \mathrm{HOCl},{ \mathrm{ClO}}^{-}$, and ${\mathrm{ClO}}_2$) [39]. These radicals demonstrate remarkable performance in oxidizing organic substances [14]. For this reason, H-AOPs show remarkable efficiency in the production of reactive species, proving highly effective in removing recalcitrant substances present in water, even at low concentrations.

Taking into account the hydroxyl radicals, this radical was first identified in the Fenton reaction in 1934 [40]. Hydroxyl radicals are highly effective for wastewater treatment due to their high oxidation potential, followed by fluorine (cf.

Table 1. Comparison strengths of common oxidizing species.

Reagent Name	Reduction Potential (V/SHE)	Reference
Fluorine	3.03	[44]
Hydroxy radical	2.80
Sulfate radical	2.5	[45]
Atomic Oxygen	2.42	[44]
Ozone	2.07
Sodium persulfate	2.0	[45]
Hydrogen peroxide	1.78	[44]
Perhydroxyl radical	1.70
Permanganate	1.68
Chlorine dioxide	1.57
Hypochlorous acid	1.49
Chlorine	1.36
Oxygen	1.23	[45]
Superoxide ion	−2.4

). Detected in natural waters with lifetimes ranging from 10⁻⁶ to 10⁻³ s [41], they are the primary oxidants in the atmosphere [42]. This radical also exists in interstellar space and biological systems. Their reactions often involve single-electron abstraction, hydrogen abstraction, aromatic ring substitution, and addition to C=C double bonds [40]. This makes them highly reactive and non-selective, which is ideal for COD degradation. However, they are more susceptible to inhibition by natural organic matter (NOM), phosphates, and WWTP effluents compared to other reagents [43].

Finally, electrochemical technologies typically require little energy and can effectively break down specific refractory organic pollutants that would otherwise be difficult to remove by alternative methods. The selection of particular electrochemical processes depends on the desired physicochemical characteristics of the effluent and can influence the nature and quantity of reactive species. In this context, the present review focuses on Reactive Oxygen Species (ROS), principally the production of OH species and the reactive chlorine species, because of the lower production of OH radicals. Therefore, the synthesis method modifies the structural properties, surface area, stability, and selectivity of the materials, which are crucial for their scalability. In this sense, the task of moving from the laboratory scale to the pilot plant remains part of the engineering challenges.

2. Electrochemical Advanced Oxidation Processes

Electrochemical advanced oxidation processes (EAOPs) have been widely developed as an efficient and effective wastewater treatment technique over the past three decades. They are, therefore, characterized by their high environmental compatibility, as they use a very clean reagent, the electron. Other advantages are their high versatility and amenability, as well as their safety since they are performed under ambient conditions. Recently, several advances have been made to elucidate the mechanisms of reactive species generation and their reactivity during EAOPs. Spin-trap electro-paramagnetic spectrometry, chromatographic analysis, and scavenging techniques constitute efficient methods that have been used to identify reactive species generated in these processes [39]. The EAOPs system is based on an endergonic process (electrolytic cell), in which two bases are typically referred to as monopolar and bipolar configurations, as shown in Figure 3 [17].

Depending on the reducing or oxidizing agent, the electrode material may change. For example, the Fenton reaction is mediated by Fe species, while related reactions involving other metals, ligands, and/or peroxides are called Fenton-like processes. Some drawbacks of Fenton-like processes need to be improved. It is difficult to achieve complete mineralization of organic compounds in the Fenton process due to the formation of stable iron complexes such as oxalate and formate with partial oxidation products. However, oxalate complexes can be effectively destroyed by UV or visible light irradiation in the presence of a photocatalyst [46]. The general mechanism of the Fenton process can be represented by the following Equations (1)–(3):

$${\mathrm{Fe}}_{\left(\mathrm{aq}\right)}^{2+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}_{\left(\mathrm{aq}\right)}^{3+}+{\mathrm{OH}}^{•}+{\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}$$

(1)

$${\mathrm{Fe}}_{\left(\mathrm{aq}\right)}^{3+}+{\mathrm{H}}_2{\mathrm{O}}_2\to {\mathrm{Fe}}_{\left(\mathrm{aq}\right)}^{2+}+{\mathrm{HO}}_2^{•}+{\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}$$

(2)

$$\mathrm{Organic} \mathrm{matter}+{\mathrm{OH}}^{•}\to \mathrm{degradation} \mathrm{products}$$

(3)

Equation (3) suggested hydroxyl radical as a reactive intermediate, followed by ferryl ions as an intermediate. On the other hand, the electro-Fenton process is a modification of the original Fenton method, designed to eliminate the need for external reactants by generating them in situ. This is accomplished through the electro-reduction in molecular oxygen to peroxide at the cathode. The produced H₂O₂ reacts with dissolved Fe²⁺_(aq) to form hydroxyl radicals and Fe³⁺_(aq). The Fe³⁺_(aq) ion is then reduced back at the cathode, allowing for continuous regeneration of the reactants and sustaining the process, Equations (4) and (5) [47].

$${\mathrm{O}}_{2\left(\mathrm{g}\right)}+2{\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}+2{\mathrm{e}}^{-}\to {\mathrm{H}}_2{\mathrm{O}}_2$$

(4)

$${\mathrm{Fe}}_{\left(\mathrm{aq}\right)}^{3+}+{\mathrm{e}}^{-}\to {\mathrm{Fe}}_{\left(\mathrm{aq}\right)}^{2+}$$

(5)

The classical Fenton reaction has several drawbacks, such as requiring an acidic operating pH range (pH = 2–4) and producing sludges containing iron [48]. The precipitation of Fe³⁺_(aq) at pH above 3 imposes a limitation on the pH required to achieve a significant rate of pollutant degradation and to maintain the stability of the Fenton’s reagent at around pH 3.5. By using a low concentration of Fe²⁺_(aq) (<1 mM), it is possible to raise the pH to 6 without precipitation of iron hydroxides. Unfortunately, the requirement to maintain a low pH, together with the management and disposal of the generated and contaminated sludge, makes the Fenton process economically unfeasible [49]. Large-scale applications of heterogeneous electro-Fenton processes face limitations due to the need for precise pH control, high energy consumption, and catalyst deactivation caused by the leaching of metal ions [50].

In the ozonation process, there are two recently published reviews on this topic. Jin [51] and Chen et al. [33] share fundamental insights into the mechanism by which the breakdown of ozone molecules occurs on the surface of catalysts. Based on the same iron species, the oxidation of Fe_(s) produces FeO_(s), Fe₂O_3(s), Fe₃O_4(s), Fe³⁺_(aq), and Reactive Oxygen Species (ROS) such as O₃^•−, OH^•, O₂, OH⁻_(aq) in solution [33]. In the electro-ozonation process, ozone is electrochemically generated from the water molecule, Equations (6) and (7).

$$3{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to {\mathrm{O}}_3+6{\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}+6{\mathrm{e}}^{-}$$

(6)

$${\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}+{\mathrm{O}}_{2\left(\mathrm{g}\right)}\to {\mathrm{O}}_{3\left(\mathrm{g}\right)}+2{\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}+2{\mathrm{e}}^{-}$$

(7)

Additionally, the process can be enhanced with the cathodic electro-generation of H₂O₂, called the electro-peroxone process [52].

$$2{\mathrm{H}}_2{\mathrm{O}}_2+2{\mathrm{O}}_3\to {\mathrm{H}}_2\mathrm{O}+3{\mathrm{O}}_2+{\mathrm{HO}}_2^{•}+{\mathrm{OH}}^{•}$$

(8)

Among the tested ozone-generation electrodes, the boron-doped diamond (BDD) has significantly outperformed other leading candidates, such as platinum (Pt) and lead dioxide (PbO₂). It has demonstrated twice the current efficiency compared to Pt and does not suffer from the dissolution problems associated with Pb [53]. Electro-ozonation is not purely a hydroxyl radical formation process. It involves an electrochemical stage, where a different species is generated, followed by a chemical stage, where this species reacts to form OH^• through non-electrochemical pathways. Each step, such as the electro-generation of O₃, ozonation, and homogeneous and heterogeneous catalytic ozonation, can be individually studied and optimized. In the next section, we reviewed and focused on how monometallic oxide or composite materials can produce OH^• species and active chloride as subproducts to degrade organic matter.

2.1. Reactive Oxygen Species

2.1.1. Hydroxyl Radicals in Dark Conditions

As mentioned earlier, ${\mathrm{OH}}^{•}$ is formed in the first step of water oxidation in an acidic medium, Equation (9) [54].

$$\mathrm{M}\left(\mathrm{hkl}\right)+{\mathrm{H}}_2{\mathrm{O}}_{\left(\mathrm{l}\right)}\to \mathrm{M}{\left({\mathrm{OH}}^{•}\right)}_{\mathrm{ads}}+{\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}+{\mathrm{e}}^{-}$$

(9)

Here, M(hkl) denotes a metallic site. Alternatively, the first step of the ${\mathrm{OH}}^{•}$ formation in an alkaline medium is the reaction in Equation (10).

$$\mathrm{M}\left(\mathrm{hkl}\right)+{\mathrm{OH}}_{\left(\mathrm{aq}\right)}^{-}\to \mathrm{M}{\left({\mathrm{OH}}^{•}\right)}_{\mathrm{ads}}+{\mathrm{e}}^{-}$$

(10)

Because the subsequent electronic steps are kinetically fast, Equation (11) [55,56].

$$\mathrm{M}{\left({\mathrm{OH}}^{•}\right)}_{\mathrm{ads}}\to \mathrm{MO}+{\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}+{\mathrm{e}}^{-}$$

(11)

Therefore, ${\mathrm{OH}}^{•}$ has a shorter lifetime. Conversely, when the ${\mathrm{OH}}^{•}$ formed is weakly adsorbed on the surface, the electronic integrity of the radical is more clearly preserved, maintaining its high oxidative reactivity towards organic species [57]. Materials such as SnO₂, PbO₂, Ti_nO_2n−1, 4 ≤ n ≤ 10, and BDD fall into this category. The anodes most suitable for generating hydroxyl radicals active for the oxidation of organic molecules are also those that require a higher potential for water oxidation (between 1.8 and 2.8 V vs. SHE). At these potentials, organic molecules are also susceptible to direct oxidation at the electrode surface, according to Equation (12) [58].

$$\mathrm{R}\to {\mathrm{R}}^{•+}+{\mathrm{e}}^{-}$$

(12)

This is mainly an undesirable reaction, as it often results in partial oxidation, leading to a polymerization reaction that passivates the electrode surface. Moradi et al. [59] compiled the most common potentials provided by electrocatalysts, which are shown in

Table 2. Oxygen evolution reaction (OER) potentials for water oxidation in different anode materials [59].

Anode Material	OER (V/SHE)
RuO2	1.4–1.7
IrO2	1.5–1.8
Pt	1.6–1.9
Graphite	1.7
Ebonex® (Ti4O7)	1.7–1.8
PbO2	1.8–2.0
SnO2	1.9–2.2
BDD	2.2–2.6

.

Despite the extensive experience documented in the literature on these materials, the creation of an experimental scale for intrinsic electroactivity has proven challenging, as measurement of this characteristic is influenced by multiple factors. An additional complication is the lack of a standardized criterion for evaluating electrocatalysts [60,61], such as the decrease in initial concentration of pollutant species, chemical oxygen demand (COD) or total organic carbon (TOC) removal, current efficiency, energy consumption, and lifetime. With this caution, it is still possible to identify some general reaction trends: Costa et al. [62] compared current efficiency, COD removal, and TOC removal among Si/BDD, $\mathrm{Ti}/{\mathrm{SnO}}_2\text{-}\mathrm{Sb},$ and $\mathrm{Ti}/{\mathrm{SnO}}_2\text{-}\mathrm{Sb}\text{-}\mathrm{Ir},$ determining that BDD outperforms ${\mathrm{SnO}}_2\text{-}\mathrm{Sb}$, and that the latter is significantly better than the one containing Ir, Figure 4a. Comninellis [35] compared the reaction of N,N-dimethyl-p-nitrosoaniline (RNO) on ${\mathrm{SnO}}_2$, Pt, and ${\mathrm{IrO}}_2$, achieving an almost complete reduction in the UV–Vis absorption signal on ${\mathrm{SnO}}_2$ and decreasing significantly with Pt and IrO₂, Figure 4c. Xiao-yan Li et al. [63] used $\mathrm{Ti}/{\mathrm{SnO}}_2\text{-}\mathrm{Sb}$, Pt, and $\mathrm{Ti}/{\mathrm{RuO}}_2$ in phenol degradation, obtaining the best results with ${\mathrm{SnO}}_2$, followed by Pt, and then ${\mathrm{RuO}}_2$, Figure 4d. However, complete TOC removal only occurs with ${\mathrm{SnO}}_2$. Panizza and Cerisola [38] compared COD removed by Si/BDD and Ti/PbO₂, finding BDD to be more efficient. Polcaro et al. [36] evaluated the performance of ${\mathrm{PbO}}_2$ and ${\mathrm{SnO}}_2$ at lower 2-chlorophenol concentrations, with ${\mathrm{SnO}}_2$ showing superior performance. However, Vazquez-Gomez et al. [37] argued that ${\mathrm{PbO}}_2$ achieves better results for diethyl phthalate. Both groups agree that, in general, their behaviors are similar. Therefore, a possible trend in electrodegradation capacity could be followed by BDD > ${\mathrm{PbO}}_2$ ≈ ${\mathrm{SnO}}_2$ > Pt > ${\mathrm{IrO}}_2$ ≈ ${\mathrm{RuO}}_2$. These ideas are summarized in Figure 4, where these trends correlate with the adsorption energy of the ${\mathrm{OH}}^{•}$ radicals on the surface of the materials (Figure 4b). The great diversity of possible experimental conditions poses significant challenges in characterizing the best hydroxyl radical electrocatalysts. This has encouraged and made research using theoretical and computational methods essential.

2.1.2. Hydroxyl Radicals Produced by Photon

Photo-electrocatalysis is an advanced oxidation process that combines photocatalysis and electrochemistry to degrade organic pollutants in water. It employs a semiconductor like TiO₂, activated by light (UV or visible). The absorbed light generates electron–hole pairs, Equation (13), in the material [64].

$$\mathrm{SC}+h\nu \to {\mathrm{e}}_{\left(\mathrm{CB}\right)}^{-}+{\mathrm{h}}_{\left(\mathrm{VB}\right)}^{+}$$

(13)

Here, SC corresponds to semiconductor, hν for the energy of the photon, ${\mathrm{e}}_{\left(\mathrm{CB}\right)}^{-}$ for electrons in the conduction band (CB), and ${\mathrm{h}}_{\left(\mathrm{VB}\right)}^{+}$ for holes in the valence band (VB). This excitation promotes electrons from the valence band to the conduction band, leaving positively charged holes in the valence band. These positive holes can oxidize water molecules or hydroxide ions to produce hydroxyl radicals:

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{h}}_{\left(\mathrm{VB}\right)}^{+}\to {\mathrm{OH}}^{•}+{\mathrm{H}}^{+}$$

(14)

$${\mathrm{OH}}^{-}+{\mathrm{h}}_{\left(\mathrm{VB}\right)}^{+}\to {\mathrm{OH}}^{•}$$

(15)

To prevent the recombination of electron–hole pairs, electrons can be extracted through an electrode and an external circuit to the counter electrode; see Figure 5 [65,66].

Recently, several review articles have been published focusing on this topic, covering catalyst mechanisms and design [64], integrated systems [67], modified graphite [68], configuration and operational parameters [69], and reactors [66].

Key points include the relative energy positions of the bands in the solid compared to the oxidation potential of the species and the OH^•/H₂O pair, which largely determine whether the pollutant undergoes direct oxidation on the ${\mathrm{h}}_{\left(\mathrm{VB}\right)}^{+}$ of the material or indirect oxidation by OH^• generated in these systems (Equation (16)) [70,71].

$${\mathrm{H}}_2\mathrm{O}+{\mathrm{e}}_{\left(\mathrm{CB}\right)}^{-}\to {\mathrm{OH}}^{-}+{\mathrm{H}}^{•}$$

(16)

2.2. Active Chlorine (IEO, Indirect Electro-Oxidation)

While hydroxyl radicals exhibit the highest oxidation potential, surpassed only by fluorine [72], and offer high non-selectivity and efficiency for TOC degradation, their reaction rate depends on the mass transport of the contaminant species to the electrode-solution interface where OH^• resides. In practice, background impurities in the water impact OH concentration levels [39]. Here, active chlorine species have a lower oxidation potential than OH^•, resulting in a longer half-life but reduced mineralization capacity. In addition, the electrochemical generation of ROS takes advantage of the common presence of ${\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}$ in industrial effluents. The oxidation process starts from intermediates of the oxygen evolution reaction (OER) at the anode:

$$2{\mathrm{H}}_2\mathrm{O}\to {\mathrm{O}}_{2\left(\mathrm{g}\right)}+4{\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}+4{\mathrm{e}}^{-}$$

(17)

It specifically involves the initial formation of hydroxyl radicals, Equations (9) and (10) [73,74]. Chlorine evolution reaction (CER) thermodynamically proceeds at potentials above 1.36 V vs. SHE over these active sites.

$$2{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}\to {\mathrm{Cl}}_{2\left(\mathrm{g}\right)}+2{\mathrm{e}}^{-}$$

(18)

Subsequent homogeneous phase reactions occur [75]:

$${\mathrm{Cl}}_{2\left(\mathrm{g}\right)}+2{\mathrm{H}}_2\mathrm{O}\rightleftarrows \mathrm{HClO}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}+{\mathrm{H}}_3{\mathrm{O}}^{+} \left(\text{acidic} \text{medium}\right)$$

(19)

$${\mathrm{Cl}}_{2\left(\mathrm{g}\right)}+2{\mathrm{OH}}^{-}\rightleftarrows {\mathrm{ClO}}_{\left(\mathrm{aq}\right)}^{-}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}+{\mathrm{H}}_2\mathrm{O} \left(\text{alkaline} \text{medium}\right)$$

(20)

$$\mathrm{HClO}\rightleftarrows {\mathrm{ClO}}_{\left(\mathrm{aq}\right)}^{-}+{\mathrm{H}}_{\left(\mathrm{aq}\right)}^{+}, \text{pKa}=7.5$$

(21)

In Equation (21), free chlorine (Cl_2(g)) reacts with OH⁻ species and forms ClO⁻_(aq). Additionally, HClO and ${\mathrm{ClO}}_{\left(\mathrm{aq}\right)}^{-}$ can regenerate ROS in the presence of UV light, no longer confined to the interface [76], Equations (22)–(24).

$$\mathrm{HClO}+hv\to {\mathrm{OH}}^{•}+{\mathrm{Cl}}^{•}$$

(22)

$${\mathrm{ClO}}_{\left(\mathrm{aq}\right)}^{-}+hv\to {\mathrm{O}}^{•-}+{\mathrm{Cl}}^{•}$$

(23)

$${\mathrm{O}}^{•-}+{\mathrm{H}}_2\mathrm{O}\to {\mathrm{OH}}^{•}+{\mathrm{OH}}^{-}$$

(24)

Chloride ions are highly corrosive to electrode materials. While several metals achieve rapid organic contaminant degradation, Cu, Co, Ni, and Ag are prone to total corrosion, making noble metals like Ir, Pt, and Pd viable alternatives with excellent results [16]. However, their high cost limits industrial application. Metalloids like graphite and BDD offer greater flexibility and lower costs than noble metals, but BDD stands out for its exceptional mechanical, chemical, and electrochemical stability [77]. Despite this, BDD has one of the highest OER overpotentials, necessitating significant energy for CER [78]. Its superior ability to generate a large amount of ${\mathrm{OH}}^{•}$ radical facilitates the oxidation of active chlorine species into chlorates and perchlorates [79], which have a high health risk for living beings.

Noble metal oxides, such as Ti/IrO₂, Ti/RuO₂, and Ti/IrO₂-RuO₂, are effective and stable for chlorine evolution and more cost-effective than pure noble metals. Unlike BDD, RuO₂ and IrO₂ have the lowest OER overpotentials, resulting in significantly lower energy costs for CER; see Table 1 [16].

Given the crucial role of this reaction in the chloralkali industry, numerous investigations have been carried out using various catalysts [80], namely ${\mathrm{Co}}_3{\mathrm{O}}_4$, ${\mathrm{RuO}}_2$, ${\mathrm{NiCo}}_2{\mathrm{O}}_4$, ${\mathrm{IrO}}_2$, ${\mathrm{MnO}}_2$. However, the mechanisms that persist have been primarily elucidated by considering ${\mathrm{RuO}}_2$ as a catalyst, as it has been the state of the art of chlorine evolution in the field for several decades. In their recent review, Wang et al. [81], based on the computational-theoretical works of Hansen [82] and Exner [83], summarized the proposed mechanisms for the formation of active chlorine as three different mechanisms, reactions, as Equations (25)–(31).

Volmer–Tafel (V-T):

$$2{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}+2\mathrm{MO}{\left(\mathrm{hkl}\right)}^{*}\to 2{\mathrm{Cl}}^{*}+2{\mathrm{e}}^{-}$$

(25)

$${\mathrm{Cl}}^{*}+{\mathrm{Cl}}^{*}\to 2\mathrm{MO}{\left(\mathrm{hkl}\right)}^{*}+2{\mathrm{Cl}}_2$$

(26)

Volmer–Heyrovsky (V-H)

$$2{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}+\mathrm{MO}{\left(\mathrm{hkl}\right)}^{*}\to {\mathrm{Cl}}^{*}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}+{\mathrm{e}}^{-}$$

(27)

$${\mathrm{Cl}}^{*}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}\to \mathrm{MO}{\left(\mathrm{hkl}\right)}^{*}+{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}+{\mathrm{e}}^{-}$$

(28)

Krishatalik:

$$2{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}+\mathrm{MO}{\left(\mathrm{hkl}\right)}^{*}\to {\mathrm{Cl}}^{*}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}+{\mathrm{e}}^{-}$$

(29)

$${\mathrm{Cl}}^{*}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}\to {\mathrm{Cl}}^{•}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}+{\mathrm{e}}^{-}$$

(30)

$${\mathrm{Cl}}^{•}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}\to \mathrm{MO}{\left(\mathrm{hkl}\right)}^{*}+{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}$$

(31)

Here, MO(hkl)* denotes a metallic active site or surface oxygen. Among these possibilities, the V-H mechanism demonstrates a higher thermodynamic favorability. The use of the density functional theory (DFT) to model the adsorption of intermediates on Ruthenium (Ru) suggests that the active site for the initial adsorption of ${\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}$ in oxygen on-top position of the ${\mathrm{Ru}}_{\mathrm{cus}}$ (cus: coordinatively unsaturated sites), since this termination minimizes the surface free energy. The regions where the chlorine evolution reaction (CER) and oxygen evolution reaction (OER) are expected to be significant are delineated in Figure 6a. Only a few structures turn out to be thermodynamically stable in the considered potential-pH range, despite the fact that more than 100 adsorbate structures have been considered [84]. Consequently, the V-H mechanism can be expressed as follows in Equation (32):

$${\mathrm{O}}_{\mathrm{c}}+2{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}\to {\mathrm{ClO}}_{\mathrm{c}}+{\mathrm{Cl}}_{\left(\mathrm{aq}\right)}^{-}+{\mathrm{e}}^{-}\to {\mathrm{O}}_{\mathrm{c}}+{\mathrm{Cl}}_{2\left(\mathrm{g}\right)}+2{\mathrm{e}}^{-}$$

(32)

In Equation (32), the subscript “c” represents an absorbed species on the Ru_cus active site. The Volmer step, identified by Exner et al., was noted as the rate-determining step of the reaction. Figure 6a illustrates that according to the Volmer–Heyrovsky mechanism, chlorine adsorption on the catalyst surface is identified as an elementary step of the reaction. Based on these results, it follows that the Volmer–Heyrovsky mechanism mechanistically describes the CER on the surface of ${\mathrm{RuO}}_2$(110) completely covered with O. After introducing two water molecules into the surface unit cell of (2 × 1) due to solvent effects, Exner et al. [85] determined that the surface termination (1OH_br1O_br + 2O_ot) is the most stable under pH = 0 and 0 V < η < 0.19 V conditions. They reaffirmed that the Volmer–Heyrovsky mechanism prevails. In Figure 6b, the energy barriers of this mechanism are depicted with two configurations of active sites, where O_ot is situated adjacent to OH_br and O_br, respectively. In this figure, transition (i) to (ii) signifies the Volmer step, while transition (ii) to (iii) denotes the Heyrovsky step. The calculated barriers suggest that both mechanisms could act as rate-determining steps (RDS), contingent upon the chemical environment of the active site. On the other hand, Saha et al. [86] recalculated the Pourbaix diagrams, taking into account the remaining crystal faces not considered by the Exner group. Specifically, they considered the faces (100), (110), (111), (001), and (101). As depicted in Figure 6c, the lowest energy barrier corresponds to the (110) face for the CER process at pH = 0 and U = 1.36 V.

To mitigate the Gibbs free energy of this step, they opted to partially replace the Ru atoms in the surface models with alternative transition metal atoms, such as Zr, Rh, Pd, Pt, and Ir. Unfortunately, it turned out that Pt was the most efficient choice (Figure 6c). Foreign metal substitution alters the oxygen–chlorine bond strength in the structure of ${\mathrm{ClO}}_{\mathrm{ot}}$ precursor (ClO on top of the ${\mathrm{M}}_{\mathrm{cus}}$ site), consequently affecting the activity within the Volmer–Heyrovsky mechanism. Covering the fully O-coated ${\mathrm{RuO}}_2$(110) surface with a single monolayer of platinum oxide leads to an increase in activity since the Gibbs energy loss can be reduced by 50 meV. Consequently, the exploration of novel catalysts that are cost-effective and readily available remains a topic of debate in the literature.

3. Synthesis of Catalyst

As mentioned above, metal oxides are the best for producing OH and Cl species. However, these materials fall under the design strategies of MO oxides. Here, the synthesis of MO oxides is based on two general rules, namely, enhancing light utilization and improving photoinduced charge carriers’ separation [87]. Based on our literature review, to the best of our knowledge, there is no general classification of electrocatalysts’ synthesis methods. According to Castillo et al. [88], the synthesis method for mixed oxide photoelectrode preparation can be divided into (a) solution-assisted method, (b) electro-assisted method, and (c) advanced method. Spin-coating, dip-coating, hydrothermal, and SILAR are solution-assisted methods; electrospinning and electrodeposition are electro-assisted methods, whereas chemical vapor deposition, plasma spray, and pyrolysis are considered advanced methods. Although the use of advanced methods has increased in recent times [89,90,91], solution and electro-assisted methods continue to be the most used. Instead of the fact that solution and electro-assisted synthesis methods are still employed [92,93,94,95,96,97,98], in the last two years, the tendency is related to hybrid methods of fabrication of electrocatalysts [87,99,100,101,102,103,104,105,106], where solution and electro-assisted methods are one step of the process to obtain the desired material.

There is a growing interest in the development of electrocatalysts for the removal of pharmaceuticals from effluents, such as bisphenol A [103], tetrabromobisphenol A [107], ketorolac [108], levofloxacin [109], sulphonamide [102], propyphenazone [99], carbamazepine [110], tetracycline [111], endocrine disruptors [112], and anticancer drugs [113]. However, most reports continue focusing on dyes and phenolic compounds [87,92,93,96,97,98,101,104,105,106,114,115,116]. The inclusion of oxides such as WO₃ and BiVO₄, carbon nanotubes, or reduced graphene has been common in recent years. However, TiO₂ continues to be the most widely used material for the fabrication of electrodes for the degradation of water contaminants [108,117,118,119,120].

Two examples of hybrid methods are shown in Figure 7. In both, novel photo-electrocatalysts are obtained for application in water treatment. MoS₂-NHCS nanospheres were obtained by a multi-step synthesis method: a hard template was used to prepare NHCS with SiO₂ microspheres as a template [103]. Further calcination, hydrothermal reaction, and electrodeposition were performed to achieve the formation of MoS₂-NHCS; see Figure 7a. The morphology of MoS₂–NHCS was urchin-like with a 20 nm thick MoS₂ outer layer. The crystal phases of NHCS and MoS₂ were confirmed by XRD. Recently, synthesis of MoS₂-NHCS has been reported by Tian et al. [121], using hydrothermal and two annealing methods where the use of a hard template and HF etching is omitted, and the hydrothermal reaction is conducted in a shorter time (18 h vs. 24 h) under soft-temperature conditions (180 °C vs. 200 °C). This method results in the formation of nano-flowered structures. Both materials present high stability.

Another promising catalyst was synthesized to solve the problems related to the catalytic application of CaTiO₃. Synthesis of defective Ca_1−xAg_xTi_1−yCo_yO₃ by combining co-precipitation and microwave hydrothermal microwave-assisted was reported in 2022 for the first time [101]. The fabrication process is presented in Figure 7b. When these methods were used, the crystal structure of CaTiO₃ was not affected by doping with Ag and Co. The microstructure of doped material has more defects than CaTiO₃ and a lower optical bandgap.

The synthesis of B-doped TiO₂ conducted by Da Silva et al., using sol–gel and dip-coating methods, results in a film with irregular cracks and substrate detachment at the edges of the cracks, with a thickness of 597 nm. B-doped TiO₂ is usually synthesized by electrochemical anodization methods [95,122], leading to the formation of TiO₂ nanotubes. While Da Silva et al. synthesized TiO₂ using the sol–gel method, Bessegato and Kiziltas employed a titanium sheet. Both synthesis methods lead to the formation of the anatase phase of TiO₂ and hexagonal Ti, confirmed by XRD.

The novel chalcogenide Se-doped TiO₂/Ti electrode synthesis was first reported by Muzzakar et al. in 2019, using anodization, sol–gel, and dip-coating methods [123]. Under the synthesis conditions employed, a honeycomb structure was obtained on TiO₂/Ti; meanwhile, agglomerates around 5–10 μm were formed with the Se-doping. In more recent years, they adjusted some synthesis conditions [100], using a shorter immersion time for Se doping (1 min vs. 10 min) with an increase in calcination temperature (500 °C vs. 200 °C). By making these adjustments, the honeycomb structure of the TiO₂/Ti is maintained, while the Se-doped TiO₂/Ti surface is spread out in a layer of tiny particles that spread evenly on the surface of the electrode. Synthesis of Se-doped TiO₂/Ti nanotubes was achieved in 2023 [124] with a similar method. However, the variation in the sol–gel preparation of Se results in much larger particles with an average size of about 100 nm. This indicates that the morphology and properties of Se-doped TiO₂/Ti are highly influenced by the conditions of the sol–gel method.

The syntheses of Ti/SnO₂-Sb/PbO₂, Ti/SnO₂-Sb/PVP-PbO₂, Ti/SnO₂-Sb/Fe-PbO₂, and Ti/SnO₂-Sb/Fe-PVP-PbO₂ electrodes were achieved through a two-step process involving thermal decomposition followed by electrodeposition. This method resulted in a mixture of α and β-PbO₂, with β-PbO₂ as the main phase. The addition of 1 g L⁻¹ PVP promotes the growth of the β (211) plane. Conversely, with the addition of 6 mM Fe³⁺ ions to the CH₃SO₃H bath, the deposition of β-phase is significantly suppressed, and α-phase is preferentially deposited [115]. The pure PbO₂ film deposited from the CH₃SO₃H bath displayed a flat and dense surface with pebble-shaped grains. After PVP is added to the CH₃SO₃H bath, the surface morphology of the PbO₂ electrode is not significantly modified. However, the introduction of Fe³⁺ ions to the bath leads to a dense spindle-shaped morphology of the film surface.

The spray pyrolysis technique is considered the most simple, inexpensive, operated at a moderate temperature, easy to prepare on a large area, and good adherent of various metal oxide films [90]. The spray-deposited method of Bi₂WO₆ conducts the formation of polycrystalline samples [90,91]. Variation in spraying quantity influences the crystallinity and morphological surface of materials, directly impacting the photoelectrocatalytic activity. The spray pyrolysis method used to obtain yttrium-doped titanium dioxide thin films [89] results in the formation of the anatase phase of TiO₂ without a Y signal on XRD; however, Y incorporation on TiO₂, confirmed by XPS, showed a gradation trend in crystallite size (21–42 nm). Chemical vapor deposition (CVD) allows the formation of heterostructures with vertical, horizontal, or both growing orientations. Also, along with the HPHT method, CVD is one of the most used in the synthesis of B-doped diamond materials (BDD) [125], one of the few materials that have been proved at pilot scale for effluent treatment by the generation of OH and active chlorine [78,126]. Few reported studies have focused on the treatment of wastewater on a pilot or large scale using electrocatalytic treatments; however, there are multiple reports about the electrocatalytic treatment of synthetic and real wastewater at laboratory scale. In addition, this topic is beginning to be addressed in research reports. Interesting result to analyze the synthesis methods of electrocatalyst used in real wastewater solution and scalable configuration.

To the best of our knowledge and according to the literature review in the period between 2022 and 2024, the photoelectrocatalyst bifacial TiO₂–Bi/Sb-MOF synthesized by hydrothermal method followed by anodization was tested on 1 L of real municipal wastewater [106]. The use of these tandem synthesis methods resulted in the formation of uniform coating and spherical-shaped Bi-(mixed metal) MOF (diameter of 400 to 920 nm), randomly aligned over the TiO₂ surface. Sb-(mixed metal) MOF still shows a uniform but irregularly structured layer over the surface of TiO₂. According to the authors, there are no other reports about the use of these types of mixed metal MOF electrodes to treat municipal wastewater.

RuO₂-Ir-REM, Ti₄O₇-REM, and β-PbO₂-REM electrodes prepared by a two-step method (sol–gel and electrodeposition) allowed the production of electrodes to purify biologically treated landfills leachate using reactive electrochemical membranes (REMs). The tests were conducted at laboratory and pilot plant scales [127]. The three materials present porous structures with similar inner pore diameter distribution (30–60 μm), porosity (32–34%), and median pore size (42–49 μm). Ti/RuO₂ were prepared by thermal decomposition method and tested on the optimization of ammonia and COD removal in 1.3 L of real municipal wastewater by direct and indirect anodic electrooxidation. The catalyst exhibited a “mud” like structure [128]. Ti and RuO₂ were identified on the surface of the catalyst by XRD. The crystalline structure of the catalyst remains without changes after the catalytic performance.

Buoio et al. [117] synthesized Ti mesh coated with a photoactive TiO₂ film by plasma electrolytic oxidation. The obtained material was a mixture of anatase (45%) and rutile (55%) crystalline phases and sponge-like morphology, which was used as a photo and photo-electrocatalyst in a water remediation system for sustainable fish farming, mediated by the electro-chlorine advanced oxidation process. A similar anodization method was used by Thind et al. [120] for growing a TiO₂ nanoporous array on a Ti plate. However, they applied a further annealing process to obtain the anatase phase. This material was employed as a photocatalyst at an integrated electro and photocatalytic system for wastewater treatment from Lake Superior in Ontario, Canada. The RuO₂−IrO₂-based electrode was the electrocatalyst. It was prepared by brush painting on the Ti substrate surface, plus an annealing process to convert the metal chlorides to metal oxides. The cited work applied at the pilot scale, except for reactive electrode material (porous electrode material as a substrate) [127], involves the use of TiO₂ synthesized by the anodization method. Recently, the well-known sol–gel method allowed the synthesis of a great amount (146.5 kg) of Ti-Sn-Sb@γ-Al₂O_3. This catalyst was tested on pretreatment of hypersaline and high-organic wastewater with a three-dimensional electrocatalytic system. The high-organic and hypersaline wastewater in this study will induce the electrolytic generation of active chlorine (Cl₂, HClO) and ClO⁻_(aq), which serves as the main oxidant [129].

Solution-assisted methods offer versatility in catalyst synthesis. The sol–gel process is simple but requires controlled heating to avoid sintering. Dip- and spin-coating yield precise thin films but require clean substrates, limiting their use on large surfaces. Hydrothermal and solvothermal techniques produce highly crystalline materials; the latter use non-aqueous solvents for higher temperatures, while SILAR allows precise control of nanomaterial morphology. Electro-assisted methods facilitate the creation of well-defined films or nanostructures with high purity and adhesion by fine-tuning deposition parameters. Advanced techniques such as pyrolysis, CVD, plasma spray, and microwave-assisted synthesis produce high-quality films and nanostructures. Pyrolysis rapidly generates crystalline materials without post-treatment; CVD ensures precise deposition of thin films; plasma spray enables thick coatings on complex shapes; and microwave synthesis accelerates reactions to obtain high-crystallinity materials, often in combination with other methods to obtain optimal properties [88]. The aim of all these methods is to achieve good catalytic performance in reactions in the reaction of interest while avoiding catalyst poisoning, which is the main cause of the deactivation of electrocatalysts [130].

On the other hand, it is increasingly common to find works in which, in addition to the synthesis, characterization, and evaluation of the electrocatalytic activity, machine learning tools are incorporated to predict some property of interest of the catalysts, such as the stability in certain environments [131,132]. Machine learning improved processes for catalyst design, and a better understanding of electro/photocatalytic processes is essential for improving catalyst effectiveness. Recent advances in data science and artificial intelligence have great potential to accelerate electrocatalysis and photocatalysis research, particularly the rapid exploration of large materials chemistry spaces through machine learning [133,134].

4. Electronic Modification of Materials

As heterogeneous processes, catalysts play a major role in the generation of highly reactive species necessary for the efficient degradation of complex organic compounds [135]. The structural properties of these catalysts are decisive for their catalytic activity. Some key properties of interest in these catalysts include catalyst type, conductivity, mechanical and chemical stability, surface area, porosity, active sites, and interaction with reactive species, among others [136]. These properties collectively determine the effectiveness of catalysts in AOPs, making a thorough understanding of their structural characteristics essential for optimizing wastewater treatment and environmental remediation processes.

Table 3. Types of electronic modification applied to real and synthetic effluent.

Catalyst	Electrochemical Process	Type of Solution	Crystalline Phases	Electronic Modification	Reference
TiO2/ITO	PEC, Laboratory		Anatase	No modification	[98]
Ti/RuO2	CER, Laboratory	Synthetic and real municipal wastewater	RuO2, metallic Ti	No modification	[128]
Se doped-TiO2/Ti nanotubes	PEC, Laboratory	Synthetic diazinon	Anatase, rutile	Element doping	[124]
Bi2WO6-Bi2MoO6/TiO2 nanotubes	PEC, Laboratory	Synthetic Methyl Orange, MB, and RhB	Anatase, orthorhombic Bi2MoO6, orthorhombic Bi2WO6	Heterojunction	[97]
Au@TiO2	PEC, Laboratory	Synthetic Mo, phenol, and BPA	Anatase	strong-interfacial interaction	[87]
BHP@ZnO	PEC, Laboratory	Synthetic RhB	Hexagonal ZnO, orthorhombic BHP.	Heterojunction	[105]
MoS2/NHCS	PEC, Laboratory	Synthetic Bisphenol A	NHCS, MoS2	Heterojunction	[103]
Mo-doped BiVO4	PEC, Laboratory	Synthetic phenol	Monoclinic BiVO4	Defect engineering	[134]
TiO2–Bi (mixed metal)-MOF TiO2–Sb (mixed metal)-MOF	PEC, Laboratory	Real municipal wastewater	Bi2O3 tetragonal, anatase Valentinite, anatase	Heterojunction, element doping	[106]
TiO2/g-C3N4@AC	PEC, Laboratory	Synthetic levofloxacin solution	Anatase, g-C3N4	Heterojunction	[109]
TiO2-PVC	PEC, Laboratory	Synthetic orange 16	Anatase	Heterojunction	[118]
Y-doped TiO2	PEC, Laboratory	Synthetic PA and BA	Anatase	Element doping, defect engineering	[89]
TiO2 NTs	PEC, Laboratory	Synthetic ketorolac	Anatase, metallic Ti	No modifications	[108]
Ti/SnO2-Sb/Fe-PVP-PbO2	EO, Laboratory	Synthetic MO	α and β-PbO2	Element doping, heterojunction	[115]
WO3	PEC, Laboratory	Synthetic trimethoprim, diclofenac, sulfametho-xazole and carbamazepine	Monoclinic WO3	No modifications	[110]
WO3/W	PEC, Laboratory	Synthetic tetracycline	-	No modification	[111]
WO3/WO3-MoO3	PEC, Laboratory	Synthetic pesticide Imazalil	Monoclinic WO3, orthorhombic MoO3	Heterojunction	[116]
TiO2/TiO2-ZnO	PEC, Laboratory	Synthetic pesticide Imazalil	Anatase, wurtzite structure of ZnO	Heterojunction	[116]
TiO2/Ti	PEC, Laboratory	Synthetic Sulfamethazine	Anatase and rutile	No modifications	[119]
CdS/TiO2	PEC, Laboratory	Synthetic Ifosfamide, 5-fluorouracil and imatinib	Metal Ti, anatase	Element doping	[113]
Bi2WO6	PEC, Laboratory	Synthetic BA and MB	Polycrystalline orthorhombic Bi2WO6	Defect engineering	[90]
Bi2WO6	PEC, Laboratory	Synthetic RhB	Orthorhombic Bi2WO6	Defect engineering	[91]
Ti-W alloy oxide nanotubes	PEC, Laboratory	Synthetic endocrine disruptors and real deepwater reservoir	WO3−x (0 ≤ x ≤ 0.28)), Ti0.54W0.46O2, anatase, rutile	Element doping	[112]
Ca1−xAgxTi1−yCoyO3	PEC, Laboratory	Synthetic MB	Orthorhombic CaTiO3, CoTi2O5	Element doping	[101]
Se-doped TiO2/Ti	PEC, Laboratory	Synthetic reactive green 19	-	Element doping	[100]
B–Co/TiO2	PEC, Laboratory	Synthetic RhB	Hexagonal Ti, anatase	Element doping	[95]
Catalyst	Electrochemical Process	Type of Solution	XRD Phases	Electronic Modification	Reference
g-C3N4/Sn3O4/Ni	PEC, Laboratory	U(VI)	Triclinical Sn3O4, g-C3N4	Heterojunction	[92]
Au-RGO/TiO2 NTs	PEC, Laboratory	Synthetic MB	Anatase, RGO	Heterojunction	[93]
BiOI/TiO2 NTs	PEC, Laboratory	Synthetic RhB	Tetragonal BiOI, anatase	Heterojunction	[94]
B-TiO2	PEC, Laboratory	Synthetic propyphenazone	Anatase, hexagonal Ti,	Element doping	[99]
Co-doped ZnO	PEC, Laboratory	Synthetic sulphonamide, real tap water, and secondary sedimentation effluent	-	Element doping	[102]
PANI/TiO2 NTAs	PEC, Laboratory	Synthetic tetrabromobisphenol A	-	Heterojunction	[107]
Ag/Fe3O4/g-C3N4	PEC, Fenton, Laboratory	Synthetic 4-chlorophenol	Fe3O4 (magnetite), g-C3N4	Element doping	[114]
Ti-Sn-Sb@γ-Al2O3	CER, Pilot	Industrial wastewater	Anatase, rutile, cassiterite, valentinite	Element doping, heterojunction	[129]
RuO2−IrO2	PEC, Pilot	Real wastewater of Lake Superior	RuO2, IrO2	Heterojunction	[120]
TiO2/Ti	PEC, Pilot	Real water fish farming	Anatase (45%), rutile (55%)	Defect engineering	[117]

shows the electrochemical processes performed in the laboratory and pilot scale. In general, photo-electrochemical (PEC), Chlorine Evolution Reaction (CER), Electrochemical Oxidation (EO) are basically performed in real and synthetic effluent. Here, we focused on the electronic modification of the catalyst. Thus, we found that there are different types of electronic modifications, such as element doping, heterojunctions, defect engineering, and strong-interfacial interaction.

According to Table 3, most of the materials and their electronic modifications have only been implemented at the laboratory scale, with minimal applications at the pilot scale. This could be due to challenges in scaling up the materials as well as the limitations of the synthesis methods used. Therefore, Figure 8 classifies these examples that we found in the literature with respect to AOPs in general [137].

Here, we observe several key strategies for material design:

• Heterojunction formation: This involves creating a junction between two different materials, resulting in unique properties at the interface [92,93,94,116,138].
• Defect engineering: This focuses on defect handling, such as oxygen vacancies, to tailor the material’s behavior [90,91,117,138].
• Element doping: This strategy involves substituting atoms within the main crystal structure with different elements to achieve desired properties [95,99,100,101,102,112,113,114].
• Strong-interfacial interaction: This refers to a strong interaction between the material and a metal oxide support, named Strong-Metal Support Interaction (SMSI) [87]. It is important to distinguish from Strong-Metal Oxide Carbon Interaction (SMOCI) [139], where this refers to a strong bond formed between the material and support through oxygen–carbon bonds at the interface.

These electronic modifications are part of the strategies in defect engineering. The goal is to increase the active site and stability of the catalyst [136]. In the following section, we discussed the case studies on laboratory and pilot scale, where these aspects of the materials are crucial.

5. Case Studies

5.1. Electrochemical Advanced Oxidation Processes (EAOP)

In this section, we present different methods by which OH^• in Heterogeneous Advanced Oxidation Processes (H-AOP) are applied to real effluents. Here, we consider studies on a laboratory scale and their comparison with a reactor volume larger than one liter, classified as pilot plants or larger-scale plants [140].

Evaluating the intrinsic electroactivity of catalysts on the laboratory scale presents several challenges. Researchers often neglect the critical role of electronic and microstructural properties in controlling the macroscopic response of the catalyst. Electronic modifications are fundamental, directly affecting the active sites themselves (catalytic centers). Additionally, the microstructure of the material, including features like dislocations, terraces, vacancies, and impurities, plays a crucial role. These engineering defects influence various properties that impact catalytic effects, such as topography, roughness, porosity, surface wettability, and grain size [137]. In addition, controllable factors from experimental design, even if not standardized, include:

• Electrochemical reactor design: electrode area/electrolyte volume ratio, cell and electrode geometry, hydrodynamic conditions.
• Electrochemical reactor parameters: Electrical current and potential intervals, electroactive species and supporting electrolyte concentrations, test durations, working cycles, collected and reported data such as Chemical Oxygen Demand (COD), Total Organic Carbon (TOC), pH, current efficiency (ICE), and electrical consumption (kW-h m⁻³ or kW-h kg⁻¹).

Surface area has been a central topic in heterogeneous catalysis research for decades [141,142,143,144]. It depends on porosity, roughness, and irregularities ranging from macroscopic to atomic scales, making it a fractal property with indices between 2 and 3 [145]. In catalysis, the most relevant metric is the active surface area, which is not necessarily correlated with the total surface area. The active or electroactive area in electrocatalysis depends on the density and distribution of the active sites and the wettability of the surface in aqueous systems. Unfortunately, active-site detection techniques are not standardized due to the inherent dependence on specific reaction mechanisms [60]. This limitation is due to the fact that the “active site” is often linked to the adsorption energy of key intermediates, resulting in different meanings depending on the evaluation method used [146]. The BET (Brunauer–Emmett–Teller) area is still the most widely used method for characterizing catalysts. In recent years, it has been used to measure the surface area of TiO₂ modified in photoelectrocatalysis and active chlorine generation [92,98,99,103,106,129], with values ranging from 4.39 to 2792 m²/g. When no direct correlation between surface area and activity is found, it is presumed that the density of active sites varies between materials.

Considering these complexities, this section reviews some of the most promising catalysts reported at the laboratory scale over the past two years. We subsequently evaluate their feasibility for larger-scale applications. Finally, we report pilot-scale experiences with these electrocatalysts, emphasizing key characteristics that influenced their selection by researchers.

One way to minimize the impact of various factors is by evaluating different electrocatalysts within the same experimental setup. This approach was used by Li et al. [147], who compared the current efficiency (ICE) and COD degradation of BPA between BDD, PbO₂, and IrO₂, finding the order BDD >> PbO₂ >> IrO₂. They used Na₂SO₄ electrolyte, identifying ^•OH and SO₄^•− as oxidizing agents. While this study does not introduce novel insights into electrocatalysis, it reaffirms the electro-degradation properties of these materials previously discussed. The authors do not report energy consumption but do mention an optimal current density of 60 mA cm⁻². Combined with the high overpotential for oxygen evolution on BDD, this leads to inherent scalability issues.

Lin et al. [127] confirm that the β-PbO₂-REM gave superior results in generating OH^•. It produces 23 times more OH^• than RuO₂-Ir-REM and five times more than Ti₄O₇-REM. The authors also present reactivity tests of materials with Fe(CN)₆⁴⁻ to compare the active sites of REMs. However, the use of this technique to evaluate electrocatalysts stems from a misinterpretation of electrocatalysis. Using a universal test molecule is not suitable for comparison with inner-sphere reactions such as those occurring during the oxidation of water intermediates. Furthermore, in these analyses, it is crucial to consider the active area [60]. The Fe(CN)₆⁴⁻ shows itself inner-sphere behavior, making it an unsuitable candidate as a universal test molecule. Therefore, this test did not yield consistent results with OH^• generating ability, classifying it as RuO₂-Ir-REM > β-PbO₂-REM > Ti₄O₇-REM.

Among the most notable examples of novel materials reported in the last two years are MoS₂ nanosheets grown on nitrogen-doped hollow carbon spheres (MoS₂–NHCS) linked to a foamed nickel matrix. Their separation efficiency and carrier lifespan improved, enhancing BPA degradation by OH^•, ${\mathrm{h}}_{\left(\mathrm{VB}\right)}^{+}$, and O₂^•− with less than 10% activity loss over four cycles when used as a photoanode [103]. Additionally, their stability improved for Pb²⁺ removal, showing less than 3% activity loss over 20 cycles when used as a photocathode [121]. Bi₂WO₆ has gained popularity in recent years and has been tested for the removal of benzoic acid and methyl blue, achieving 72% and 98% degradation, respectively, in a short time [90]. One of the most promising catalysts for the degradation of organic pollutants is CaTiO₃, which was improved by doping it with Co and Ag to obtain Ca_1−xAg_xTi_1−yCo_yO₃ [101]. This material exhibits a band gap of 2.78 eV and features Ag–Ca and Co–Ti donor–acceptor defect pairs, which provide exceptional durability and reusability. In contrast, leveraging the success of TiO₂ in the field has led to doping the oxide with various elements, such as Bi-Sb [106], B-Co [95], Se [100], and B [95,122]. These modifications have demonstrated improved performance in terms of photocurrent and degradation efficiency compared to undoped TiO₂.

5.2. Pilot Plant Scale AOPs

The HOM-AOP and H-AOP processes are considered new processes, where implementation and full-scale operation are still a challenge. Advanced treatment is considered tertiary treatment, where the best technology depends on local conditions. Here, established processes (i.e., active carbon, ozonation, and membrane filtration processes) and new processes are used as combined methods in WWTP [148]. In the past two decades, advanced oxidation processes involving OH^• have been implemented on both pilot and large scales. Most of these treatments are homogeneous phase treatments, including UV, H₂O₂, O₃, Fe²⁺, Fe³⁺, HSO₅⁻, and combinations thereof [149,150,151,152,153,154,155,156,157,158,159,160,161,162,163,164,165]. Although research on H-AOP new materials and processes has been prolific in heterogeneous catalysis over the past 20 years, these have not progressed beyond the laboratory scale. Regarding applications at the pilot scale, the continuous operation time of the catalysts ranges from a few hours to two months, treating everything from mixtures of contaminants in synthetic effluents to wells and wastewater from hospitals [166], agriculture [167,168], personal care [169], chemical [170,171,172], municipal sources [173,174,175], and industrial [176]. Excessive fossil energy consumption has caused a global energy crisis and environmental issues. Hence, photocatalysis and photo-electrocatalysis are regarded as the most promising technologies for simultaneously solving the above two issues. The application of these materials can take advantage of abundant and clean energy. In addition, photo-electrocatalysis cannot only convert solar energy or electric energy into green and clean hydrogen energy, as proved in water splitting. These technologies also convert carbon dioxide (CO₂) into fuels and high-value-added chemicals [177].

TiO₂ and ZnO commercial catalysts are the most used catalysts, either alone or modified. Here, TiO₂ is widely used independently or in conjunction with other techniques [178]. Even in wastewater for which treatment by conventional methods is not feasible, such as that from hospitals [166]. Sun et al. [179], in their review on photocatalysts, mentioned nearly 30 pilot plant experiments, of which 11 used commercial TiO₂ directly, 14 used modified TiO₂, 3 used ZnO, and one used Ag/BiVO₄. They suggest that catalyst modifications often perform worse than commercial TiO₂ or ZnO, and more pilot-scale experimental trials are needed. We also identified pilot plant photocatalysis as the most reliable H-AOP option for researchers prior to 2022. This is evidenced by the large number of studies conducted with unmodified TiO₂ [180,181,182,183,184,185,186,187,188,189,190,191,192,193,194,195,196,197,198,199,200,201,202,203]. Only a few modifications to TiO₂ have reached the pilot scale [204,205,206,207,208]. As for other photocatalysts, experiences at this scale are fewer [209,210,211].

In the photo-electrocatalytic process, every year, many new catalysts are reported, but they sometimes include non-environmentally friendly elements in their composition. For instance, some studies use cadmium or lead-based catalysts, which risk leaching toxic heavy metals during water treatment. Most researchers particularly agree on the high competitiveness of TiO₂-based photoelectrodes for water treatment. Pristine TiO₂ and TiO₂-based nanocomposites have been widely used as photoanodes in this field, leading to many scientific publications. However, despite the satisfactory performance found for the removal of organic pollutants from wastewater by PEC pre-eminently with TiO₂ nanocomposites [19], there are few reports about its application at pilot and large-scale levels [117,118].

In the area of electrodegradation, we can mention BDD, Ti/RuO₂–IrO₂, and Ti/IrO₂/Ta₂O₅ anodes. Only one case has been identified with a different catalyst at this scale, CuFeMnO spinel oxide, by Huang et al. [212], which features superior PMS catalytic capacity. In the current state of treatment technology applications, research on catalyst selection tends to reinforce widely accepted explanations rather than introducing novel insights. Only a handful of authors provide justification for their choices [213,214,215]. Nevertheless, few researchers provide information regarding the post-treatment characterization of their catalysts [212,214,215,216]. Concerning other advanced oxidation processes at the pilot scale, we have the following: For the removal of per- and polyfluoroalkyl substances, Franke et al. [217] used iron oxides and heterogeneous catalytic ozonation. Monteil et al. [218] used a BDD anode and a carbon felt cathode for electro-oxidation treatment to eliminate hydrochlorothiazide as a model pollutant. Roccamante et al. [219] used Nb-BDD and a carbon-PTFE GDE cathode in a combination of electro-oxidation and ozonation to remove pentachlorophenol, terbutryn, chlorfenvinphos, and diclofenac. Shestakova et al. [220] employed Ti/Ta₂O₅–SnO₂ electrodes to eliminate methylene blue. Voglar et al. [221] used graphite and BDD for the remediation of soils contaminated with metals.

During the application of these procedures under near real-world conditions, the importance of catalytic activity must be considered along with other factors, such as catalyst service life. However, these tests are not feasible under real working conditions, as they would require several months or years of experimentation with modifications of key variables (current density, pH, concentration and type of supporting electrolyte, impurities, and undesirable reactions). Numerous factors have been identified as determinants of electrode durability under operational conditions [59]. The uniformity and adhesion of catalytic films to the substrate are crucial [222], as electrolytes can permeate them, leading to reactions accompanied by changes in the lattice parameter. This, in turn, induces irregular stresses in the structure, leading to breakage and crack formation. These cracks allow further electrolyte leakage, which accelerates the electrode failure process. In contrast, surface irregularities, defined as fine roughness, facilitate the formation and release of gas bubbles, preventing them from exerting significant pressure on the catalytic film structure [223], thus prolonging its lifetime. Doping with other elements [224] can contribute not only to charge transfer at the interface but also to corrosion resistance and conductivity. Surface deposition of hydrated compounds increases the resistance of the catalytic film [225], leading to high anodic potential in constant current processes and energetic conditions that enhance undesirable reactions. Careful handling of electrodes is crucial, as accidental impacts are a common source of cracking and detachment of electrocatalyst films. Table 4 presents some representative cases of pilot plant studies.

According to Table 4, most of these experiments have been performed with real effluents that have lasted up to one year of continuous operation [76,221,226]. Figure 9a,b and Table 4 show the pilot plant case used by Otter et al. [76] and Min et al. [226], respectively. Otter et al. used Ru-Ir mixed oxide electrodes and UV light to treat water from the Elbe River for 10 months without loss of electroactivity, while Min et al. discussed a treatment train for pesticide wastewater using Ti/PbO₂ anodes. Long work cycles make the use of synthetic effluents uneconomical. Only two cases explicitly report satisfactory performance and no catalyst degradation [76,127]. Notably, these processes use well-studied catalysts like Ir, Ru, Sn, Ti, Pb oxides, and BDD, which are known for generating OH^•, or active chlorine species. BDD is not economical [78,126,227], with β-PbO₂ and reactive electrochemical membranes being more cost-effective [127]. BDD is not recommended in the presence of Cl⁻, as Anglada et al. [126] have shown. This is because BDD exhibits a high overpotential for oxygen evolution, which is a prerequisite for Cl₂ evolution and the formation of active chlorine. At high current densities, the formation of dangerous chlorates occurs due to their reaction with OH^•. The production of OH^• is favored at high overpotentials, which consequently increases energy consumption. The flow diagram of the pilot plant used by Anglada et al. is shown in Figure 9c, with details in Table 4, as a representative example of the use of BDD in that scale. Efficient stirring to enhance mass transport is essential, as demonstrated by Acosta-Santoyo et al. [227], who showed decreasing TOC removal efficiency with increased current density.

With the aim of making the results reported by different researchers comparable, “accelerated life” tests have been suggested [228]. In these tests, a constant current density ten times higher than that applied under normal working conditions is used while recording the anode potential with a reference electrode. The premise is that, as long as the electrode promotes the same electrochemical reactions, the anodic potential will remain constant and at low values. Once the nature of the electrode is altered, the reactions will change to maintain the current density, and the anodic potential will increase sharply. A decrease in the (opposite) potential is unusual since the failure of an electrode is indicated by a reduction in its electronic conductivity. This issue is crucial and requires conscious intervention from the scientific community to standardize results and establish universal evaluation parameters.

Table 4. Pilot plant case studies involve processes utilizing both hydroxyl radicals and active chlorine species.

Pilot Scale System	Type of Treated Water	Catalyst	Plant Volume	Catalyst Performance	Energy Consumption	Ref.
Electro-degradation processes: Reactive electrochemical membranes	Biologically treated landfills leachate from a municipal landfill in Dongguan, Guangdong, China	RuO2-Ir-REM, Ti4O7-REM, and β-PbO2-REM	300 and 370 L in a single filtration step	The β-PbO2-REM gave superior results due to its remarkable generation of OH•. Five tests demonstrated reproducibility without deterioration of the electrocatalytic capabilities	3.6 kW-h m−3	[127]
Electro-degradation processes: DiaCells treatment cells	Leachate from landfills was previously biologically processed. Located in the municipality of Meruelo, Cantabria, Spain.	BDD	750 L feed tank and three treatment lines, incorporating a total of 150 cells	BDD achieves total COD oxidation and nearly complete mineralization. Partial oxidation of ammonium to nitrates requires an additional operational step. The authors do not address the stability of the anodic yield	Not calculated. Given the high j (300–1200 A m−2), considerable energy consumption could be expected	[126]
Electro-degradation processes: ultrafiltration plant	Commercial oxyfluorfen solution	BDD	Multi-tubular ceramic membrane with 19 channels (Dh = 3.5 mm), 38 cm long, 285.1 L h−1 m−2.	Surfactants are not retained by the membrane in ultrafiltration, affecting the formation of persulfates that favor the oxidation of oxyfluorfen. ICE for TOC removal decreases with increasing current density, even as oxyfluorfen degradation increases	2900 kW-h kg−1, and 590.2 kW-h kg−1 with and without the concentration stage	[227]
Electro-degradation processes: electrochemically assisted hydrolysis stage	Pesticide treatment of a wastewater effluent from a company in Dalian, China	Ti coated with a nanocrystalline PbO2 film (MAGNETO, Suzhou, China)	8.2 m3 volume with a flow rate of 2.5 to 5.5 tons day−1	The BOD5/COD ratio was doubled, allowing the effluent to meet the quality standards required for urban wastewater treatment plants. No specific evaluation of the catalysts’ performance, but they do not mention any problems related to its operation for a period longer than one year	Reduces operating costs by 14%, with a payback period of 10 years	[226]
In situ active chlorine generation: UV/ECl2 pilot system	Elbe River water, filtered by a UF system (Pall, Port Washington, NY, USA)	RuO2/IrO2 mixed oxide	1023 m3, 3.4 m3d−1	Gabapentin and oxipurinol were nearly completely degraded by the Cl2. Benzotriazole, 4 and 5-methyl-benzotriazole, and iomeprol were degraded by 5–11%. However, combining UV radiation increased degradation to 49–89%. The field test lasted 10 months. The system operated without technical interruptions, requiring only monthly manual cleaning of quartz glass sleeves. The electrolytic cells showed no wear by the end of the trial, and the water’s hardness did not cause calcareous deposits on the cell surface with polarity inversion intervals of three hours	1.35 kWh m−3 including pumping	[76]
In-situ active chlorine generation: Electrochemical pilot plant	Five different real saline industrial effluents	BDD	750 L tank and three pumps feeding the electrolyte into three parallel fluid lines	Chloride oxidation was the primary reaction. Complete ammonia removal was achieved for all wastewaters, while TOC removal reached up to 90%	68 kW-h m−3	[78]
In-situ active chlorine generation: Three-dimensional electrocatalytic system	Industrial hypersaline and high-organic wastewater from Jingzhou City, Hubei Province, China	Ti coated with Ru-Ir-Ti-Sn. And Ti-Sn-Sb@γ-Al2O3 particles	Two reactors with 75 and 71 kg of catalyst. A retention time of 1.5 h, and a flow rate of 3 m3 day−1.	Total biological oxygen demand (BOD) increased from 40 mgL−1 to 1050 mgL−1 with oxide particles, implying a 690% increase in biodegradability. Active chlorine formation is the primary COD removal mechanism.	102.80 kW-h kg−1 COD and 88.09 kW-h m−3	[129]
Photo-electro-catalysis processes. Stainless-steel continuous tubular reactor	Rainbow trout (Oncorhynchus mykiss) culture.	Ti mesh coated with a photoactive TiO2 film	Three 50 L tanks equipped with the PEC purification system: Continuous tubular reactor with 1 L free volume.	The water of the PEC group showed lower ammonia and nitrite concentrations and higher nitrate concentrations, also leading to gaseous N2, compared with conventional biological filters. Histological analysis did not reveal any pathological alteration in the gills and liver of both groups.	Not reported	[117]

Table 4. Pilot plant case studies involve processes utilizing both hydroxyl radicals and active chlorine species.

Pilot Scale System	Type of Treated Water	Catalyst	Plant Volume	Catalyst Performance	Energy Consumption	Ref.
Electro-degradation processes: Reactive electrochemical membranes	Biologically treated landfills leachate from a municipal landfill in Dongguan, Guangdong, China	RuO2-Ir-REM, Ti4O7-REM, and β-PbO2-REM	300 and 370 L in a single filtration step	The β-PbO2-REM gave superior results due to its remarkable generation of OH•. Five tests demonstrated reproducibility without deterioration of the electrocatalytic capabilities	3.6 kW-h m−3	[127]
Electro-degradation processes: DiaCells treatment cells	Leachate from landfills was previously biologically processed. Located in the municipality of Meruelo, Cantabria, Spain.	BDD	750 L feed tank and three treatment lines, incorporating a total of 150 cells	BDD achieves total COD oxidation and nearly complete mineralization. Partial oxidation of ammonium to nitrates requires an additional operational step. The authors do not address the stability of the anodic yield	Not calculated. Given the high j (300–1200 A m−2), considerable energy consumption could be expected	[126]
Electro-degradation processes: ultrafiltration plant	Commercial oxyfluorfen solution	BDD	Multi-tubular ceramic membrane with 19 channels (Dh = 3.5 mm), 38 cm long, 285.1 L h−1 m−2.	Surfactants are not retained by the membrane in ultrafiltration, affecting the formation of persulfates that favor the oxidation of oxyfluorfen. ICE for TOC removal decreases with increasing current density, even as oxyfluorfen degradation increases	2900 kW-h kg−1, and 590.2 kW-h kg−1 with and without the concentration stage	[227]
Electro-degradation processes: electrochemically assisted hydrolysis stage	Pesticide treatment of a wastewater effluent from a company in Dalian, China	Ti coated with a nanocrystalline PbO2 film (MAGNETO, Suzhou, China)	8.2 m3 volume with a flow rate of 2.5 to 5.5 tons day−1	The BOD5/COD ratio was doubled, allowing the effluent to meet the quality standards required for urban wastewater treatment plants. No specific evaluation of the catalysts’ performance, but they do not mention any problems related to its operation for a period longer than one year	Reduces operating costs by 14%, with a payback period of 10 years	[226]
In situ active chlorine generation: UV/ECl2 pilot system	Elbe River water, filtered by a UF system (Pall, Port Washington, NY, USA)	RuO2/IrO2 mixed oxide	1023 m3, 3.4 m3d−1	Gabapentin and oxipurinol were nearly completely degraded by the Cl2. Benzotriazole, 4 and 5-methyl-benzotriazole, and iomeprol were degraded by 5–11%. However, combining UV radiation increased degradation to 49–89%. The field test lasted 10 months. The system operated without technical interruptions, requiring only monthly manual cleaning of quartz glass sleeves. The electrolytic cells showed no wear by the end of the trial, and the water’s hardness did not cause calcareous deposits on the cell surface with polarity inversion intervals of three hours	1.35 kWh m−3 including pumping	[76]
In-situ active chlorine generation: Electrochemical pilot plant	Five different real saline industrial effluents	BDD	750 L tank and three pumps feeding the electrolyte into three parallel fluid lines	Chloride oxidation was the primary reaction. Complete ammonia removal was achieved for all wastewaters, while TOC removal reached up to 90%	68 kW-h m−3	[78]
In-situ active chlorine generation: Three-dimensional electrocatalytic system	Industrial hypersaline and high-organic wastewater from Jingzhou City, Hubei Province, China	Ti coated with Ru-Ir-Ti-Sn. And Ti-Sn-Sb@γ-Al2O3 particles	Two reactors with 75 and 71 kg of catalyst. A retention time of 1.5 h, and a flow rate of 3 m3 day−1.	Total biological oxygen demand (BOD) increased from 40 mgL−1 to 1050 mgL−1 with oxide particles, implying a 690% increase in biodegradability. Active chlorine formation is the primary COD removal mechanism.	102.80 kW-h kg−1 COD and 88.09 kW-h m−3	[129]
Photo-electro-catalysis processes. Stainless-steel continuous tubular reactor	Rainbow trout (Oncorhynchus mykiss) culture.	Ti mesh coated with a photoactive TiO2 film	Three 50 L tanks equipped with the PEC purification system: Continuous tubular reactor with 1 L free volume.	The water of the PEC group showed lower ammonia and nitrite concentrations and higher nitrate concentrations, also leading to gaseous N2, compared with conventional biological filters. Histological analysis did not reveal any pathological alteration in the gills and liver of both groups.	Not reported	[117]

6. Conclusions and Perspectives

With advances in wastewater treatment plants, emerging pollutants are not taken into account in tertiary treatment. The application of conventional methods is not suitable enough in practice. In this regard, AOPs are considered a promising process in real effluent treatments, namely H-AOPs. However, despite this advantage, several challenges persist that hinder the widespread technological application of these processes in many countries.

First, the synthesis methods discussed have multiple advantages on their own; however, more research studies employ multi-step synthesis methods, which allow for greater control of the physicochemical properties of the catalyst. However, the incorporation of a larger number of steps results in longer synthesis times, potentially increasing production costs and affecting the scalability of the catalysts. On the other hand, catalyst scale-up faces the same challenges intrinsic to any chemical process. Failures at the industrial scale are often the result of insufficient testing at smaller scales. Problems can arise from underestimating synthesis steps, difficulties in acquiring secondary raw materials, insufficient knowledge of the relationship between raw material properties and catalyst quality, or poor understanding of desirable catalyst characteristics. In addition, an incomplete understanding of the synthesis chemistry and non-compliance with safety and environmental regulations in the early stages contribute to setbacks. Thorough testing and knowledge of these factors are essential for successful industrial application.

At the laboratory scale, different materials are synthesized and applied to synthetic samples with different electronic modifications, such as heterojunction, element doping, and defect engineering. However, boron-doped diamond (BDD) continues to gain popularity for hydroxy radical (OH) generation, showing high stability during continuous operation and efficiency in contaminant degradation and mineralization. Surprisingly, lead dioxide, another OH generator, has shown remarkable performance at the pilot plant level, meeting water quality standards with greater economic efficiency than BDD and demonstrating high stability even after more than a year of operation.

A few studies describe the use of element doping and heterojunction modification in pilot plants. Here, energy consumption is crucial in the scalability of the electrochemical systems, based on the OH radical or Cl species. In a municipal landfill, a value of 3.6 kW-h m⁻³ is used for the OH radical. The reduction in energy consumption is due to the β-PbO₂-REM materials, where the reactive electrochemical membrane is more efficient. For active Cl species, 88.09 kW-h m⁻³ are applied to hypersaline industrial wastewater with high organic content, using Ti-Sn-Sb@γ-Al₂O₃ as an electrode. The advantage is that no sludge is produced during the process.

Instead, to bridge the gap between promising laboratory catalysts and their commercial use, several general recommendations can be made: (i) Promising catalysts should be tested in a variety of scenarios to generate experience and data for decision-making; (ii) The academic community should prioritize reproducibility and stability of the synthesis over purely novel ideas. (iii) Improving the understanding of catalytic mechanisms can help to focus industrial production on key properties. (iv) Introducing artificial intelligence techniques could be an option once sufficient data are available, and (v) incorporating basic chemical engineering principles into laboratory research would also help to scale up successfully, as industrialization requires significant capital investment.

From an engineering standpoint, projects with the highest risk of failure often involve poor design processes, overly complex flow diagrams, or multi-step synthesis with custom-built equipment. Insufficient investment to ensure optimal equipment can also lead to failure. Successful industrial scale-ups often turn optimized procedures into trade secrets. Therefore, laboratory synthesis must prioritize unit operations using equipment aligned with established technologies. To test a new material, the evaluation should consider a laboratory scale and a pilot scale as a post-test. Despite some promising results, a universal catalyst adapted to the unique characteristics of various industrial or municipal wastewaters remains elusive. Depending on the emerging pollutant, wastewater treatment plants must be adapted to the actual conditions based on the background impurities in the water streams. Finally, before scaling any potentially risky step, extensive testing should be conducted to ensure safety and feasibility in the earlier stages of development.