# A Comprehensive Review on Modification of Titanium Dioxide-Based Catalysts in Advanced Oxidation Processes for Water Treatment

Fida Tanos, Antonio Razzouk, Geoffroy Lesage, Marc Cretin, Mikhael Bechelany

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Fida Tanos, Antonio Razzouk, Geoffroy Lesage, Marc Cretin, Mikhael Bechelany. A Comprehensive Review on Modification of Titanium Dioxide-Based Catalysts in Advanced Oxidation Processes for Water Treatment. ChemSusChem, 2023, 17 (6), pp.e202301139. 10.1002/cssc.202301139. hal-04816356

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# A Comprehensive Review on Modification of Titanium Dioxide-Based Catalysts in Advanced Oxidation Processes for Water Treatment

Fida Tanos,[a] Antonio Razzouk,[b] Geoffroy Lesage,[a] Marc Cretin,[a] and Mikhael Bechelany*[a, c]

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It has become necessary to develop effective strategies to prevent and reduce water pollution as a result of the increase in dangerous pollutants in water reservoirs. Consequently, there is a need to design new catalyst materials to promote the efficiency of advanced oxidation processes (AOPs) in the field of wastewater treatment plant to ensure the mineralization of trace organic contaminants. A notable approach gaining attention involves the coupling of sulfate radicals-based AOPs to photocatalysis or electrocatalysis processes, aiming to achieve the complete removal of refractory contaminants into water and carbon dioxide. Titanium dioxide as metal oxide has received great attention for its catalytic application in water purification. TiO₂ catalysts offer a multitude of advantages in AOPs. They are characterized by their high photocatalytic

activity under both ultraviolet and visible light, making them environmentally friendly due to the absence of toxic byproducts during oxidation. Their versatility is remarkable, finding utility in various AOPs, from photocatalysis to photo-Fenton processes. TiO₂'s durability ensures long-lasting catalytic activity, which is crucial for continuous treatment processes, and their cost-effectiveness is particularly advantageous. Furthermore, their chemical stability allows it to withstand varying pH conditions. However, the large band gap energy and low electrical conductivity hinder the catalytic reaction effectiveness. This review aims to examine various approaches to enhance the catalytic performance of titanium dioxide, with the objective of enabling more efficient water purification methods.

# 1. Introduction

Water resources as well as their quality are continually decreasing. At the same time, human activities are increasing exponentially with the consumption of several chemicals due to population growth, industrialization and world development. Pollutants have been severally introduced into aquatic sources. Emerging contaminants, or contaminants of emerging concern, represent a category of environmental pollutants with potentially harmful effects on ecosystems and human health. These contaminants often exhibit properties like low concentrations, persistence, bioactivity, and diverse chemical structures, making their detection and management challenging. They can be categorized into various types, such as pharmaceuticals and personal care products, endocrine disruptors, per- and polyfluoroalkyl substances, pesticides, nanomaterials, and plastics. Their pollution characteristics encompass widespread distribution, bioaccumulation, toxicity, and ecological disruption, underscoring the need for increased efforts in monitoring, regulation, and research to address this issue.[1-4] Uncontrolled discharge of untreated water containing these micropollutants from various plants ends up in the environment and results in water pollution. The detection of emerging contaminants, even at low concentrations can have adverse effects on human

health and a drastic impact on ecosystem.[3,5-7] Therefore, it has become urgent to prevent the world from water scarcity problem by development of appropriate purification systems to remove target pollutants. Since, conventional wastewater treatments are insufficient to achieve overall treated water quality, biorefractory organic traces are still detected in aquatic sources. Consequently, these emerging pollutants require the implementation of innovative processes that have been intensively studied over the past decades.[3,4,8]

Various processes have demonstrated their ability to effectively remove traces of recalcitrant compounds from water, such as reverse osmose, nanofiltration, granular activated carbon adsorption and advanced oxidation processes (AOPs). AOPs are a relevant answer to tackle water crisis.[9-11] These methods have been applied in pre-treatment processes and/or post-treatment to the conventional wastewater treatment. Photocatalysis, electrocatalysis and persulfate radicals-based AOPs allow the removal of micropollutants from water using light sources, electrical sources and chemicals respectively. Owing to their efficiency, cost-effectiveness and environmental friendliness, these techniques have attracted a lot of interest for the removal of drugs.[12-15]

The fundamental principle of photocatalysis is based on the excitation of electron in a semiconductor by the absorption of irradiation to photogenerated carries that will destroy recalcitrant pollutant.[16-18] In electrocatalysis process, electrical energy is converted to chemical energy to initiate the redox reaction at the surface of anode and oxidize organic compound with different radicals.[19,20] Sulfate radicals-based AOPs consists on the in-situ generation of *SO₄²⁻* radicals through active component such as peroxymonosulfate. Thus, the combination between these remediation methods could be effective in achieving the complete removal of persistent pollutants.[21-24]

Among possible semiconductors, titanium dioxide represents an important class of catalysts for the removal of organic compounds due to its advantages. Nevertheless, it is wide bang gap energy, rapid recombination of electron hole-pair and low conductivity limit its performance in the above processes.[25-28] There drawbacks could be overcome by forming a heterostructure composite through the combination of TiO₂ with different metal oxide-graphene-metal materials. Furthermore,

[a] Dr. F. Tanos, Dr. G. Lesage, Prof. M. Cretin, Dr. M. Bechelany
Institut Européen des Membranes, IEM, UMR 5635
Univ Montpellier, ENSCM, Centre national de la recherche scientifique
(CNRS)
Place Eugène Bataillon, 34095 Montpellier (France)
E-mail: mikhael.bechelany@umontpellier.fr

[b] Prof. A. Razzouk
Laboratoire d'Analyses Chimiques, Faculty of Sciences
LAC-Lebanese University
Jdeidet 90656 (Lebanon)

[c] Dr. M. Bechelany
Gulf University for Science and Technology, GUST
32093 Hawally
(Kuwait)

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© 2023 The Authors. ChemSusChem published by Wiley-VCH GmbH. This is an open access article under the terms of the Creative Commons Attribution License, which permits use, distribution and reproduction in any medium, provided the original work is properly cited.
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reducing TiO₂ to sub-stochiometric TiₙO₂n⁻¹ can be an effective solution to promote the properties of titanium dioxide. Mechanochemical methods are considered as promising approach for the synthesis of semiconductor-semiconductor composite catalysts to obtain nano-scale powder with homogeneous mixture.[29–33]

An overview on these various subjects and a state of the art in relation to these topics are provided in this review as well. In the first section, an overview of the different types of environmental pollution is presented with the occurrence of pharmaceuticals in water resources. In the second section, the different classes of water treatment plants are described. Photocatalysis and electrocatalysis systems are presented in the third and fourth part. The fifth section concerns sulfate radials-based advanced oxidation processes. A state of the art concerning the modification of titanium dioxide-based catalysts with various methodologies for the effective removal of micropollutants in the context of environmental remediation is also provided. Finally, the methods for the synthesis and modification of titanium dioxide material are discussed.

## 2. Environmental pollution

The main types of pollution that have an impact on the environment and human health will be discussed in detail in

this section. The biggest problem that is facing the planet currently is pollution. Environmental pollution is the effect caused by introducing harmful contaminants that produces damage to the ecosystem and undesirable effects on human health. With increasing in human activities such as globalization, industrialization, agriculture and population, pollution has become the biggest environmental challenge facing the world today.[34–36] When the pollutants are released in the nature, different categories are detected like emerging pollutants,[37] persistent organic pollutants (POPs)[38] and heavy metals.[39] Their amounts are higher than their natural abundance and the environment can no longer counterbalance these quantities without outright damage of its ecosystem.[40]

### 2.1. Major types of pollution

#### 2.1.1. Air pollution

The emission of large amounts of hazardous chemicals and various forms of waste into the air has resulted in the phenomenon known as air pollution, leading to the deterioration of air quality. Figure 1 shows the predominate sources of natural process and human activities that change the atmospheric composition.[41–43] Different types of pollutants can escape to the environment from these sources including the

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Fida Tanos, born in June 1996 in Lebanon, completed her BS in Chemistry at the Department of Chemistry Faculty of Sciences, Lebanese University in 2017. Subsequently, she earned her Master's degree in Petrochemical Analysis from the Faculty of Science, Lebanese University. Her academic journey continued with a Ph.D. at the University of Montpellier, European Institute of Membranes, under the supervision of Prof. Marc Cretin and Dr. Mikhael Bechelany. This Ph.D. program involved collaborative research with the Lebanese University under the supervision of Prof Antonio Razzouk. Fida's research focuses on the synthesis of heterostructure composites and their application for pollutant removal.

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Marc Cretin, a Professor at the University of Montpellier since 2012, previously served as a lecturer at the Chemistry Graduated School of Montpellier from 1998. Specializing in Electrochemistry, Chemistry in Solution, and Thermodynamics, he earned a PhD in Electrochemistry in 1996. With a research background at the University of Geneva, Switzerland, he currently focuses on electrochemistry for water treatment and energy, particularly the development of electrochemical interfaces for anodic oxidation and the electro-Fenton process. Co-author of approximately 130 scientific publications as of September 2023.

ResearcherID: C-3119-2015

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Antonio Razzouk, with an M.Sc. (2003) and Ph.D. (2006) in Analytical Chemistry from the University of Lyon I, specializes in predicting complex mixtures separations. In postdoctoral research at the Laboratory of Thermodynamics and On-Line Analysis, he developed chromatographic and electroanalytical methods for physicochemical property determination. Joining the Faculty of Sciences at the Lebanese University in 2009, his research centers on groundwater contamination by petroleum products, plasticizer migration from packaging to food, and assessing novel nanomaterials for water pollutant degradation. With expertise in energy, environment, and industrial processes, Antonio has a comprehensive background in analytical chemistry.

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Mikhael Bechelany, born in March 1979, holds a Ph.D. in materials chemistry (2006) from the University of Lyon. Following postdoctoral research at EMPA in Switzerland, he joined the European Institute of Membranes in 2010 as a CNRS Research Scientist. Specializing in one-dimensional nanostructures and employing innovative synthesis methods like atomic layer deposition and 3D printing, his research, reflected in 325 publications and 13 patents (h-index=62), focuses on inorganic membranes for energy, health, and environmental applications. By October 2023, Bechelany has demonstrated impactful academic contributions, extensive industrial collaborations, and co-founded 5 startups.
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<figure><img src="image_13.png" /><figcaption>**Figure 1.** Sources of air pollutants with the chemical compounds that change the atmosphere composition.</figcaption></figure>

primary pollutants. When these contaminants undergo further modification like photochemical reactions by UV light, toxic gases can be generated leading to the formation of secondary pollutants. In the other part, the contamination of air by emerging contaminants is a concerning issue. Emerging contaminants in the context of air pollution encompass a diverse range of substances that were not traditionally considered primary air pollutants. These contaminants can become airborne through various pathways, including industrial processes, emissions from vehicles, and the volatilization of chemicals from products and materials. They may include volatile organic compounds from household products and industrial processes, pharmaceuticals, personal care products, microplastics, and perfluorinated compounds, among others.[42,44] All the chemical compounds that pose health risk are reported in numerous health effects can damage the cardiovascular, respiratory, and neurological systems, like shortness of breath, pulmonary insufficiency and heart attacks leading to the diseases.[44,45] Furthermore, air pollution also contributes to the degradation of soil and groundwater quality. After the dispersion of pollutants in air they can be moved through terrestrial and aquatic environment with various process.[46]

### 2.1.2. Soil pollution

Soil has the ability of absorbing various emerging contaminants due to anthropogenic origins of contaminants, some of them are mentioned in Figure 2. Emerging contaminants, when introduced into the soil, can disrupt the intricate web of life that depends on healthy soil. Soil microorganisms, essential for nutrient cycling and maintaining soil structure, can be adversely affected by these substances. Furthermore, the potential bioaccumulation of these contaminants in plants and animals further disrupts the balance of ecosystems, which rely on soil health for their vitality.[47] A deviation in the regular composition of soil occurs when these constitutions present at higher concentration than normal leading to the pollution.[47,48] The majority of compounds involved in affecting soil quality are represented in Figure 2. Although, these pollutants can be transferred to human food chain, which can threaten public health. They contribute also in water pollution after their accumulation in fresh water sources.[49,50]
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<figure><img src="image_15.png" /><figcaption>**Figure 2.** Overview of soil pollution and their contaminants.</figcaption></figure>

### 2.1.3. Water pollution

The discharge of untreated wastewater, chemicals and hazardous waste from various industries into the environment causes the major source of water pollution. The contaminants are released easily in surface water like rivers, lakes and reservoirs in comparison with groundwater due to their higher accessibility.[51-53]

Water bodies can be affected by conventional pollutant such as nitrate iron, fluoride iron, pathogens and heavy metals. In this review, our focus has been directed towards another aspect of pollutants referred to as emerging contaminants (ECs). The various sources from where ECs have originated have been mentioned in Figure 3.

The contamination of water by emerging contaminants, including plasticizers, pesticides, personal care products, drugs, fertilizers, and fire retardants, poses a critical environmental challenge with diverse implications. Plasticizers like phthalates and or Per- and Polyfluoroalkyl (PFAS) Substances persistently affect aquatic ecosystems and human health. Pesticides from agriculture runoff, personal care products with compounds like parabens, and nutrient-rich fertilizers contribute to water pollution and ecosystem disruption. In addition, fire retardants and the emerging pollutants in the form of drugs further compound the issue, demanding comprehensive regulatory, awareness, and research-driven solutions to safeguard our environment and public health.[51,52,54-56] In this review the main focus is on pharmaceutical pollutants as emerging contaminants

due to their extensive global consumption and their presence in water sources. Furthermore, the persistence of these pharmaceuticals makes them exceptionally difficult to eliminate from water, even when present at low concentrations.

#### 2.1.3.1. Environmental concern of pharmaceutical pollutants

Pharmaceutical compounds are used in medical field for different applications in order to treat, diagnose, prevent diseases or modify organic functions. However, their inadequate treatment before discharge has led now to identify these substances as the chemical of emerging environment concern.[57,58] In the last decade, there is a continuously growing on the demand of drugs in Europe market by effect of development of new technologies and population growth.[59] The most common classes of pharmaceuticals include analgesics, anti-inflammatory, antibiotics, psycho-stimulants and hormones compounds.[60,61] These drugs are released in wastewater treatment plant, surface water, groundwater and drinking water.[61] Hence special attention on these types of contaminants is presented in this paper as they can release in multiple mode into the water resources and cause serious effects on all the human beings.[57] For instance, these products are primarily released in the environment via wastewater discharged from pharmaceuticals manufacturing plants, hospital effluent and industrial effluent. Another important sources of pollution are agricultural runoff, domestic effluent containing pharmaceuticals
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<figure><img src="image_17.png" /><figcaption>**Figure 3.** Contaminants of emerging concern</figcaption></figure>

cals excreted in native form or in metabolites form of humans, municipal landfills and veterinary waste.[58,60,62,63]

### 2.1.3.2. Occurrence of pharmaceuticals in aquatic environment

The presence of drugs in aquatic environment is recognized as hazardous emerging contaminants due to their persistence, bioaccumulation in life form and toxicological profile.[61] Their quantification show that they can be found in nature in very low concentration (ng.L⁻¹and µg.L⁻¹). Nevertheless, even at trace concentrations, exposure to pharmaceuticals in drinking water can potentially have adverse effects on human health and wildlife.[62] Here in, we will mention occurrence data in some research studies concerning the presence of pharmaceuticals in water resources.

In November 2020, the French Government reported the environmental contamination by antibiotics over the French territory.[64] They found that the main sources of antibiotics in aquatic environment in France come from the effluents of wastewater treatment plants. The data showed a reduction in the concentration of the contaminants in the dissolved phase of water surface when the distance between those environments and Waste Water Treatment Plants (WWTPs) increases. In fact, this decrease can be related to different factors such as dilution, degradation and sorption to particle matter. Figure 4 shows an example on the decrease of the concentration of two antibiotics and their frequency along the contamination

continuum. Taking Ciprofloxacin and Sulfamethoxazole as examples, they detected that the quantity of these two pollutants was very high in the raw hospital wastewater. However, WWTPs reduce their amounts and that concentrations in dissolved phase of surface water and groundwater surface water are lower than treated wastewater.[64]

A monitoring study along Lisbon's drinking water supply system showed that sixteen compounds were quantified in the analyzed samples. The results showed that the concentration of pharmaceuticals was detected to be in the range of 0.005 to 46 ng.L⁻¹ in raw water and from 0.09 to 46 ng.L⁻¹ in drinking water. Figure 5 resumes the maximum concentration of the sixteen pharmaceuticals detected in different water resources. In addition, the River Tagus presented the higher quantity for most of the pollutants and this can be explained by the fact of the discharged in this river several effluents from wastewater treatment plants with high pollution load.[62]

During the spring and the fall of 2011, a screening of 411 emerging pollutants was done at 494 groundwater locations around France.[65] Among the emerging contaminants, pharmaceuticals products were quantified in sixty-eight percent of the sampled places.

Figure 6 presents the frequency of quantification for thirteen pharmaceuticals compounds in more than 1% of total samples. It is clearly seen that the occurrence of paracetamol quantified in different groundwater was high (27%) compared to the other pharmaceuticals. The analysis of data shows that paracetamol (as non-prescription analgesic) is most often detected in urban-industrial sites.[65]
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<figure><img src="image_19.png" /><figcaption>Figure 4. Concentration levels of Ciprofloxacin and Sulfamethoxazole along the contamination continuum.[64]</figcaption></figure>

<figure><img src="image_20.png" /><figcaption>Figure 5. Concentration of pharmaceuticals in different water resources. Reproduced from ref. [62]. Copyright (2014), with permission from Elsevier.</figcaption></figure>

Paracetamol (PCM) known as acetaminophen is ranked as the
one of the most extensively used and prescribed drugs for

minor pain and reducing fever. Due to its low cost, efficacy and
relative absence of undesirable side effects, this product was
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<figure><img src="image_22.png" /><figcaption>**Figure 6.** Frequency of quantified pharmaceuticals in the groundwater samples in France. Reproduced from ref. 65. Copyright (2015), with permission from Elsevier.</figcaption></figure>

considered as a first-line therapy in the world. The above reports show that paracetamol can be frequently accumulated in the water system i.e., sewage treatment plant effluents, surface water, groundwater and drinking water. Although, the amount of PCM in aqueous environment is in the range of ng.L⁻¹ to µg.L⁻¹, the continued consumption of these traces could impose potential health chronic consequences and harmful effects on aquatic organisms.[66-68] For these reasons, paracetamol was chosen as referent pollutant in our study. Different European countries have detected PCM in numerous water resources. After treatment by conventional methods, paracetamol was degraded from wastewaters and the concentration was reduced from 65 µg L⁻¹ to less than 10 µg L⁻¹ which indicate that WWTPs are inadequate to eliminate completely this type of pollutant.[67] The occurrence of paracetamol in various water streams of multiple countries is summarize in Table 1.

The high frequency of paracetamol in the aqueous environment can be related to the relationship between pharmaceutical consumption and performance of WWTP. Herein, it is important to describe the different water recycling techniques that ensure the suitable filtration and decontamination of water.

### 3. Water Treatment

In this section, we will outline the processes that water must go throw in wastewater treatment plants before it can be considered clean for consumption in order to better understand the difference in the efficiency of pollutant removal in treatment facilities. In the past few years, many countries have agreed to use recycled water from wastewater treatment plant as drinking water but in France it is not the case. The French water law allows the consumption of treated wastewater to irrigate the land provided that its characteristics are compatible with the requirements of the protection of public health and the environment. However, this law prohibits the use of treated wastewater as potable one. Therefore, we can distinguish two types of purification plants: drinking water and sewage treatment. In the first type, surface water such as rivers and lakes were used for drinking water while domestic wastewater and industrial discharge were used in the second one.

#### 3.1. Drinking water treatment process

Potable water is naturally accessible in the environment, but it cannot be directly used due to the presence of high content of a number of impurities. The presence of floating objects, hardness, pathogens, suspended solids, algae, dissolved gasses, taste, odor, or color induce what is known by water contamination. Herein, the importance of drinking water treatment
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<table><caption>**Table 1.** The concentration of paracetamol in different aquatic environment of some countries.</caption><thead><tr><th>Chemical structure</th><th>Concentration (µg.L<sup>-1</sup>)</th><th>Water resources</th><th>Country</th><th>Ref.</th></tr></thead><tbody><tr><td rowspan="10">Paracetamol<br><img src="chemical_structure.png" alt="Chemical structure of paracetamol"></td><td>0.002</td><td>Drinking water</td><td>USA</td><td>69</td></tr><tr><td>0.298–0.017</td><td>Drinking water</td><td>Canada</td><td>70</td></tr><tr><td>0.045</td><td>Drinking water</td><td>France</td><td>69</td></tr><tr><td>0.034</td><td>Groundwater</td><td>Spain</td><td>71</td></tr><tr><td>0.16</td><td>Groundwater</td><td>USA</td><td>72</td></tr><tr><td>292</td><td>WWTP influent</td><td>France</td><td>69</td></tr><tr><td>11.308</td><td>WWTP effluent</td><td>France</td><td>73</td></tr><tr><td>1000</td><td>WWTP influent</td><td>USA</td><td>74</td></tr><tr><td>150.079</td><td>WWTP effluent</td><td>USA</td><td>75</td></tr><tr><td>211.38</td><td>WWTP influent</td><td>UK</td><td>76</td></tr><tr><td>11.733</td><td>WWTP effluent</td><td>UK</td><td>76</td></tr><tr><td>148</td><td>Sewage water</td><td>Australia</td><td>69</td></tr><tr><td>6</td><td>Sewage water</td><td>German</td><td>77</td></tr><tr><td>1.9</td><td>Sewage water</td><td>Canada</td><td>78</td></tr></tbody></table>

plants is to produce potable water by improving the quality of water that meets public health regulations prior to consumption. The water treatment plant's input can be dirty water of rivers, lakes or reservoirs. In general, the treatment processes for water pollution management encompass a series of steps such as coagulation, sedimentation, filtration, and disinfection, as depicted in Figure 7.

During the coagulation step, chemicals known as coagulants are added to the influent in order to turn the small contaminants into larger flocks through a rapid mixing. After that, the coagulated water is transferred to the flocculation basins and the application of gently mixing will allow the formation of flocs particles. In the next step, the water flows to a large sedimentation tank. The concept of this process is to settle the flocs to the bottom of the tank in the form of sludge where they are eliminated periodically via backwashing. The collected water then sent to the filtration; sand filters are used with different pore sizes to clear water from suspended solids and bacteria remains from the previous stages. The last step of the drinking water treatment plants is disinfection. The objective of this stage is to kill or inactivate the harmful microorganisms present in the water. There are different types of disinfection that can be applied using chemical method of physical technique such as chlorination, reverse osmosis and UV radiation. Finally, the treated water it is ready to be pumped to the storage tank which connected to the distribution system for the consumers. Different parameters should be taken into consideration before distribution such as pH, alkalinity, temperature, turbidity, hardness, color, suspended solids, etc. The monitoring of these parameters after purification is an important point to improve the quality of the produced water.[79–83]

## 3.2. Wastewater treatment plants

Different activities such as agricultural or domestic but also industrial discharge effluents with undesirable pollutants pose negative impact on biodiversity and aquatic environment. The hazardous waste contaminants contain various toxigenic microorganisms in addition to solids, metals and inorganic salts. In this context, wastewater treatment plant is an important process to expel these constituents from water by improving the quality of the treated effluent. In addition, due to the population growth and overwhelming urbanization, a large volume of sewage water has been discharge for purification.[84–86] In Europe, the instructions are progressively severe and require that all wastewaters from different municipalities should be treated before being discharged in the nature. Hence, different approach should be taken into consideration to reach the best purification and the decontamination goals.[87,88]

### 3.2.1. Conventional waste water treatment

Conventional wastewater treatment facilities are designed to remove suspended solids, easily biodegradable organic matter, metals and nutrients from sewage water. The purification processes include several stages based on the combination of physical, chemical and biological techniques such as coagulation, flocculation, ion exchange, membrane separation and electrochemical methods.[87,89–92] However, the use of conventional processes in wastewater treatment plants do not completely remove recalcitrant pollutants and they end up consequently in the environment. As a result, pharmaceuticals as an example of these pollutants could be found in receiving
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<figure><img src="image_25.png" /><figcaption>**Figure 7.** Schematic diagram of the drinking water treatment plants.</figcaption></figure>

surface water. Various studies have confirmed the detection of drugs in the effluent of the conventional methods.[92–95] Bijlsma et al. present a study concerning the occurrence of pharmaceuticals in a conventional WWTP that collects sewage water from an urban region and a near-by hospital. The monitoring data, showed that approximatively 50% of the pharmaceutical compounds were completely or partially cleared while using conventional biological method. Although still 50% of these pollutants were not eliminated.[93] In fact, drugs are characterized by their low hydrophobicity, high solubility, and negative charge at neutral pH that induce poor sorption affinity on biological sludge.[92] Due to the limitations of conventional techniques, efforts have been reinforced to improve the development of alternative and effective treatments capable to ensure the complete removal of these emerging contaminants.[94] To resume, wastewater treatment plants are an essential component of our urban infrastructure, serving as the initial line of defense in the treatment of wastewater. The conventional pollutants are generally larger in size and are more readily biodegradable, making them prime targets for removal through the conventional treatment processes used in WWTPs. However emerging contaminants are often present in wastewater at lower concentrations, and they can be quite resistant to the traditional treatment methods employed by WWTPs. This is where the need for coupling advanced oxidation

processes with WWTPs becomes apparent in order to ensure the total removal of all pollutants.

### 3.2.2. Advanced oxidation processes

The persistence of toxic and refractory contaminants even at trace amounts render conventional treatments inefficient for this type of pollutants. Over the last decades, a variety of alternative technologies have been investigated to solve the problem of pollution by recalcitrant compounds. Fortunately, advanced oxidation processes are considered to be the most effective green technologies for the degradation of a wide range of organic contents from the sewage water. They have been widely used to destroy specific contaminates structure, decrease organic matter content and improve biodegradability. Advanced oxidation processes (AOPs) involve the generation of reactive oxygen species that oxide pollutants and degrade them into carbon dioxide (CO₂) and water. Hydroxyl radical (*OH*) is the common free radical generated by AOPs which destroy rapidly and non-selectively the pollutants.[96–100] Other free radicals such as sulfate (*SO₄²⁻*) radical has attracted highly attention as a potential substitute in AOP due to its wide pH range (2.0–8.0), high oxidation capacity (2.5–3.1 V), and long half-life life (30–40µs).[99,101]
<figure><img src="image_26.png" /></figure>

Figure 8 presents the oxidation potential of different oxidizing species.[99,102,103] Until now, conventional system cannot be replaced by AOPs, but they are classified as complement existing systems for better quality effluent. They could be applied in pre-treatment process or/and in post treatment to the biological process. The various advanced oxidation processes that produce radicals by utilizing various factors will be discussed in more detail in the following section.

These free radicals could be produced by oxidants through a variety of activation condition as shown in Figure 9.

### 3.2.3. Electrochemical AOPs

Electrochemical advanced oxidation processes (EAOPs) were recently encouraged for the removal of pharmaceuticals through the generation of reactive species upon the application

of electrical energy without the addition of chemicals. Multiple efforts have been made to illustrate the concept of radical's formation and their reactivity in EAOPs. The most common EAOPs are anodic oxidation, Electro-Fenton (EF) and photo-EF and the different reactions that occur during these three mechanisms are shown in Table 2. Anodic oxidation is the simplest EAOP technique, based on the removal of organic pollutants through direct electron transfer or/and indirect oxidation that electro-generated radicals at the anode surface leading to the decontamination process.[98,100,104-107]

EAOPs are characterized by the operation over a wide range of pH solution, energy efficiency, cost effective, versatility, ecofriendly and require shorter reaction times. Hence, it is important to improve the type of cathodes, the anodic material and the operating parameters to totally mineralize highly refractory contaminants persistent in wastewater.[104,105,107]

<figure><img src="image_27.png" /><figcaption>Figure 8. Oxidation potential of different radicals and reactive oxygen species.</figcaption></figure>

<table><caption>Table 2. Various electrochemical reaction occurring at anode and cathode surface for the EAOPS processes.</caption><thead><tr><th>EAOP</th><th>Anodic reaction</th><th>Cathodic reaction</th></tr></thead><tbody><tr><td>Anodic oxidation</td><td>M + H<sub>2</sub>O → M(OH) + H<sup>+</sup> + e<sup>-</sup></td><td>O<sub>2</sub> + 2H<sup>+</sup> + 2e<sup>-</sup> → H<sub>2</sub>O<br>Or<br>O<sub>2</sub> + 4H<sup>+</sup> + 4e<sup>-</sup> → 2H<sub>2</sub>O</td></tr><tr><td>Electro-Fenton</td><td>M + H<sub>2</sub>O → M(OH) + H<sup>+</sup> + e<sup>-</sup></td><td>O<sub>2</sub> + 2H<sup>+</sup> + 2e<sup>-</sup> → H<sub>2</sub>O<br>Fe<sup>3+</sup> + e<sup>-</sup> → Fe<sup>2+</sup><br>Fe<sup>2+</sup> + H<sub>2</sub>O + H<sup>+</sup> → Fe<sup>3+</sup> + H<sub>2</sub>O + OH<sup>-</sup></td></tr><tr><td>Photo-EF</td><td>M + H<sub>2</sub>O → M(OH) + H<sup>+</sup> + e<sup>-</sup></td><td>O<sub>2</sub> + 2H<sup>+</sup> + 2e<sup>-</sup> → H<sub>2</sub>O<br>Fe<sup>3+</sup> + e<sup>-</sup> → Fe<sup>2+</sup><br>Fe<sup>2+</sup> + H<sub>2</sub>O + H<sup>+</sup> → Fe<sup>3+</sup> + H<sub>2</sub>O + OH<sup>-</sup><br>Fe(OH)<sup>2+</sup> + hν → Fe<sup>2+</sup> + OH<sup>-</sup></td></tr></tbody></table>
<figure><img src="image_28.png" /><figcaption>Figure 9. Different advanced oxidation processes employ various mechanisms for the degradation of pollutants during wastewater treatment.</figcaption></figure>

### 3.2.4. Physical AOPs

Physical process such ultrasound (US) and microwave (MW) assisted have been reported as activation method in AOPs. During the sonochemical process, aqueous medium sonolysis requires the formation of sound waves with a range of 20–1000 KHz leading to create the cavitation bubbles with the generation of •OH radicals as shown in Eq. (1).[108–110]

$$H_2O \rightarrow \infty H + \cdot OH \quad (1)$$

Microwave assisted system is based on magnetic and electric fields perpendicular to each other and their interaction with materials, which create dielectric and magnetic losses that facilitate heat generation.[101] However, these processes are still in their initial stages and necessitates the optimization of different operating parameters in order to reduce the cost associated with these methods' efficiency. Therefore, the combination of physical process with others AOPs can enhance the removal and the mineralization of the studied pollutants. For example, the addition of H₂O₂ as oxidant exerts synergistic enhancement on the reaction rate of ROS through the

decomposition of H₂O₂ via microwave irradiation. The possible reaction is presented in Eq. (2).[101,108]

$$H_2O_2 \rightarrow^{MW} 2(OH) \quad (2)$$

### 3.2.5. UV based AOPs

UV based AOPs consist on the combination of UV radiation with powerful oxidants and catalysis (Figure 1. 10). These processes can destroy contaminants and disinfect water through different possible reactions like photodecomposition, excitation and degradation of refractory compounds and oxidation by direct action of oxidants. All these reactions enhance the generation of different oxygen species specially •OH radicals as shown in Eqs. (3–8).[96,97,103,111–113]

$$\text{UV/H}_2\text{O}_2: \text{H}_2\text{O}_2 + h\nu \rightarrow 2(\text{OH}) \quad (3)$$

$$\text{UV/}^{\cdot}\text{SO}_4: \text{HSO}_5^{-} + h\nu \rightarrow \cdot\text{SO}_4^{-} + \cdot\text{OH} \quad (4)$$
$$ \mathrm{UV/O_3} : O_3 + H_2O + h\nu \rightarrow O_3 + H_2O_2 \quad (5) $$

$$ H_2O_2 + h\nu \rightarrow 2(OH) \quad (6) $$

$$ \mathrm{UV/Fe^{2+}} : Fe^{3+} + H_2 + h\nu \rightarrow Fe^{2+} + OH^{-} + H^{+} \quad (7) $$

$$ \mathrm{UV/Cl_2} : HOCl + h\nu \rightarrow OH^{-} + Cl^{-} \quad (8) $$

The different types of UV based advanced oxidation process depending on the radical promoter added to the solution are listed in Figure 10.

UV combination with these oxidants offers better mineralization efficiencies for the abatement of pharmaceuticals. However, these processes are governed by various factors such as UV light intensity, type of pollutants also water quality (pH, temperature...). In addition, replacing UV irradiation with visible light/sunlight for initiating reaction has attracted considerable attention in term of reducing UV energy cost.[99,100]

### 3.2.6. Ozone based AOPs

Ozonation is also considered as an efficient process used in AOP for the treatment of recalcitrant compound. There are two types of oxidizing reactions through which ozone ($O_3$) reacts with contaminated water. One mechanism consists on direct oxidation to dissociate dissolved organic compounds while the second involves the generation of *OH radicals that act as secondary oxidant through the decomposition of $O_3$ (indirect mechanism). However, the high energy cost of ozonation in direct reaction and the low solubility of $O_3$ in water have limited the application of this process. To overcome this drawback, numerous studies have proved that the conjunction of several catalysts with ozone are much more effective. Hence, the combinations $O_3/H_2O_2$, $O_3/UV$ and $O_3/catalysts$ could enhance

the generation of sufficient *OH radicals that increase the rate of oxidations reactions.[114–118]

### 3.2.7. Catalytic AOPs

In order to enhance the generation of *OH radicals, different catalysts have been added during the treatment of pollutants. Homogeneous photo-Fenton and heterogeneous photocatalysis processes are the most extensively investigated in catalytic AOPs.

#### 3.2.7.1. Homogeneous photo-Fenton process

Fenton based technologies consist on the use of $Fe^{2+}$ and $H_2O_2$ to generate *OH radicals under low pH conditions. In photo-Fenton process, the introduction of UV or visible light could promote the direct formation of *OH species and the recycling of $Fe^{2+}$ ions through the reduction of $Fe^{3+}$ [Eqs. (9–12)]. As a result, additional *OH radicals will be produced and the concentration of $Fe^{2+}$ could be increased leading to enhance the oxidation reaction. Therefore, complete mineralization of drugs could be achieved with the application of these methods.[100,104,107]

$$ Fe^{2+} + H_2O_2 \rightarrow Fe^{3+} + OH^{-} + OH^{-} \quad (9) $$

$$ Fe^{3+} + H_2O_2 \rightarrow Fe^{2+} + HO_2^{-} + H^{+} \quad (10) $$

$$ Fe(OH)^{2+} + h\nu \rightarrow Fe^{2+} + OH^{-} \quad (11) $$

$$ H_2O_2 + h\nu \rightarrow 2(OH) \quad (12) $$

<figure><img src="image_29.png" /><figcaption>Figure 10. Combination of UV radiation with different radical promoters.</figcaption></figure>
### 3.2.7.2. Heterogeneous photocatalysis process

This process is established with the presence of UV irradiation or visible light but also with solar radiation. After illumination, different redox reactions will occur on the surface of the photocatalyst for the photodegradation of refractory contaminants. To date, heterogeneous photocatalysis, is the most promising process applied in wastewater treatment as green technology.[98,107,112] This method will be discussed with more details in part 5.

Numerous studies have reported the application of AOPs in wastewater treatment for the removal of various type of emerging contaminants. Table 3 presents different examples on the elimination of pharmaceuticals from water using AOPs techniques and $TiO_2$ as catalyst. The inclusion of a catalyst like $TiO_2$ is vital in AOPs. For example, in photocatalysis, $TiO_2$ plays a key role in boosting reaction efficiency by catalyzing the production of highly reactive species like hydroxyl radicals under UV light, which break down contaminants. $TiO_2$ not only speeds up the removal of emerging contaminants but also offers significant environmental and economic benefits by reducing energy consumption and costs. In the photo-Fenton reaction, $TiO_2$ is crucial, serving as a photocatalyst in this modified version of the Fenton reaction that utilizes iron ions and hydrogen peroxide alongside UV or visible light exposure. The presence of $TiO_2$ as a catalyst is essential for the reasons mentioned above.

### 3.2.8. AOPs limitations

The use of AOPs have been improved to be an emerging and promising wastewater treatment technology, offering high removal efficiency for a broad spectrum of organic pollutants without secondary pollution. Figure 11 presents the different activation methods that can be potentially used, in which the AOPs are classified into two major categories: homogeneous and heterogeneous techniques.[96,124] After addressing all the

advanced oxidation processes for trace pollutant elimination, it becomes imperative to evaluate the cost and energy efficiency of these methods. Therefore, the integration of eco-friendly and sustainable energy resources becomes a crucial aspect of this assessment. When considering costs, it's essential to scrutinize the economic viability of AOPs since various AOPs may entail distinct expenses related to catalysts, reactants, and equipment, which play a role in determining their overall affordability, particularly in large-scale applications. Assessing the energy efficiency of each AOP is key to identifying the most sustainable and cost-effective approach. Moreover, the transition to green and renewable energy sources is highly advantageous for mitigating energy expenses and environmental impacts. For instance, the utilization of solar, wind, hydropower, and other renewable energy alternatives holds the potential to substantially diminish the carbon footprint linked to AOPs, furnishing a sustainable and environmentally conscious energy foundation for these methods. This integration of green and renewable energy sources can further bolster the overall sustainability of AOPs, with solar energy, for instance, facilitating the power supply for photocatalytic reactions, thereby diminishing reliance on conventional electricity sources. Similarly, the harnessing of wind and hydropower resources can provide sustainable energy inputs for various AOPs.

Overall, different parameters could affect the performance of these processes such as harsh reaction and high treatment cost (i.e., energy consumption, chemical input). To overcome these drawbacks, the combination of different AOPs was introduced and reported by various researchers. Exploration and improvement of AOPs is more worthy of scientific's attention. Therefore, elaboration of green and simple preparation of catalysts, in addition to reducing the cost of efficient catalysts and process treatment play a key factors in enhancing the application of AOPs.[97,98,100] In the next parts (5, 6) we will present two promising methods of AOPs: photocatalysis and electrocatalysis with their advantage on the removal of pharmaceuticals.

<table><caption>**Table 3.** AOPs reported recently for the removal of drugs.</caption><thead><tr><th>Catalyst</th><th>Pharmaceutical product</th><th>AOP process</th><th>Optimal experimental conditions</th><th>Removal efficiency (%)</th><th>Ref.</th></tr></thead><tbody><tr><td>Pristine TiO<sub>2</sub></td><td>Ciprofloxacin</td><td>Photocatalysis</td><td>Initial concentration 5 mg.L<sup>-1</sup>, PH:7, Time:30 min under UV Light</td><td>77</td><td>119</td></tr><tr><td>TiO<sub>2</sub></td><td>endocrine disrupting steroid hormones</td><td>Catalytic ozonation</td><td>pH: 6.8, Ozone dose: 10 mg.L<sup>-1</sup>, Initial concentration 100 µg.L<sup>-1</sup>, Catalyst dosage: 0.05 g.L<sup>-1</sup>, Time: 10 min</td><td>88</td><td>120</td></tr><tr><td>TiO<sub>2</sub></td><td>endocrine disrupting steroid hormones</td><td>Photo Catalytic ozonation</td><td>pH: 6.8, Ozone dose: 10 mg.L<sup>-1</sup>, Initial concentration 100 µg.L<sup>-1</sup>, Catalyst dosage: 0.05 g.L<sup>-1</sup>, Time: 10 min, UV light intensity = 50</td><td>100</td><td>120</td></tr><tr><td>TiO<sub>2</sub></td><td>cloxacillin</td><td>Photocatalysis</td><td>Initial concentration: 203 µmol.L<sup>-1</sup> Light power: 150 W, Catalyst dosage:2 g.L<sup>-1</sup>, pH: 6, Time :60 min dosage: 2 mM, Time: 6 h</td><td>96</td><td>121</td></tr><tr><td>TiO<sub>2</sub></td><td>Acetaminophen</td><td>Photoelectrocatalysis</td><td>Initial concentration 40 mg.L<sup>-1</sup>, UV-light PMS = 1 mM Time: 240 min, pH:7 potential of 0.5 V Ag/AgCl</td><td>47</td><td>122</td></tr><tr><td>TiO<sub>2</sub></td><td>sulfaclozine</td><td>Light/Persulfate(PS)</td><td>Initial concentration:25mgL<sup>-1</sup>, PDS = 2.5 g.L<sup>-1</sup> Time: 60 min, Catalyst dosage: 2.5 g.L<sup>-1</sup>, Time:30 min, pH:7</td><td>100</td><td>123</td></tr></tbody></table>
<figure><img src="image_30.png" /><figcaption>Figure 11. Schematic classification of the most frequently treatments based on AOP.</figcaption></figure>

## 4. Photocatalytic Oxidation Process

In recent years, heterogeneous photocatalysis has emerged as a promising technology against water pollution. Applying this process in water treatment plant, offers the ability of destroying recalcitrant organic pollutants by producing more biologically degradable and less harmful compounds. In this technique, contaminants are destroyed through the combination between light energy and semiconductor photocatalysts. Moreover, photocatalysis is considered efficient, economic and green technology via the application of visible or solar light. Based on these benefits, this process has attracted great attention in AOPs for the degradation of pharmaceuticals, dyes, pesticides....[125-128]

### 4.1. Principle

The fundamental principle of semiconductor-based heterogeneous photocatalysis is illustrated in Figure 12. Illuminating the surface of a semiconductor with a wavelength equal to or greater than its band gap leads to photocatalytic reactions. The absorbed photon causes the excitation of an electron (e⁻) from the valence band to the conduction band of the photocatalyst. During the photoexcitation, positive holes (h⁺) are generated in the valence band. The holes oxidize water molecules and produce *OH radicals, while superoxide anions (·O₂⁻), are formed by the reaction between oxygen and electrons. The

generated radicals on the photocatalyst surface are highly reactive and can destroy recalcitrant pollutants into innocuous compounds such as CO₂ and H₂O. The detailed mechanism of degradation can be presented by the following Equations (13-18):[125,129-131]

$$ \text{Photocatalyst} + h\nu \rightarrow e^{-} + h^{+} \quad (13) $$

$$ e^{-} + O_{2} \rightarrow -O_{2}^{-} \quad (14) $$

$$ h^{+} + H_{2}O \rightarrow -OH + H^{+} \quad (15) $$

$$ -O_{2}^{-} + e^{-} + 2H^{+} \rightarrow -H_{2}O_{2} \quad (16) $$

$$ H_{2}O_{2} + e^{-} \rightarrow -OH + OH^{-} \quad (17) $$

$$ -OH + \text{organic} + \text{intermediates} \rightarrow CO_{2} + H_{2}O \quad (18) $$

### 4.2. Parameters affecting the mechanism of photocatalysis

The reaction conditions to ensure high removal performance were significantly different and depend on various parameters such as type of semiconductor, pH, light source and catalyst/pollutant ratio.[126]
<figure><img src="image_31.png" /></figure>

<figure><img src="image_32.png" /><figcaption>**Figure 12.** Basic mechanism of photocatalytic degradation of pollutants.</figcaption></figure>

### 4.2.1. Semiconductor material

In the photocatalytic process, the type of catalyst material is crucial. It is normal to expect that any semiconductor employed as a photocatalyst will have a proper band gap and have photochemical stability in aqueous conditions. Such a material ought to have a significant capacity for organic pollutant adsorption as well as a high quantum yield. In order to address the issue of sustainability and the process' overall financial ramifications, it is also preferable that such photocatalyst material be able to gather a significant amount of the visible spectrum.[132]

#### 4.2.1.1. Titanium dioxide semiconductor

Titanium dioxide (TiO₂) is one of the most suitable semiconductors for the heterogeneous photocatalysis application. TiO₂ photocatalyst exhibits chemical stability, low toxicity and high oxidation efficiency for the mineralization of a wide range of organic contaminants such as drugs. However, the fast electron-hole pairs recombination and its wide band gap energy (3.2 eV) limited its application under visible light. In fact, TiO₂ can only absorb irradiation with a wavelength of light in the UV region that corresponds to less than 5% of the total sunlight spectrum. Several studies have been reported on the modification of TiO₂ in order to reduce the recombination of photogenerated electron-hole pairs and to extend the absorption edge of this catalyst to the visible region of sunlight.[133-136] The different strategies that have been applied to improve the properties of TiO₂ semiconductor will be discussed further in section 6.

#### 4.2.2. pH

The photochemical reactions are highly affected by the pH of solution due to its controls to the surface charge and oxidation potential of the photocatalyst. In addition, the pH has an impact on the generation of •OH radicals due to the interaction between the positive holes and hydroxide ions on the titanium dioxide surface. At low pH, the photogenerated holes are considered as the dominant oxidizing species while •OH radicals are the major reactive oxygen species at neutral or high pH value. The effect of pH control on the photocatalysis efficiency using TiO₂ as catalyst has been recently reported in literature.[127,137,138] The influence of initial pH solution on the photocatalytic degradation of paracetamol was studied in pH range of 2.5-10 using TiO₂ as photocatalyst. Figure 13 shows that the removal efficiency of paracetamol is significantly affected by the value of pH solution. At a pH value of 8, the higher photodegradation is observed due to the excess of hydroxyl radicals on the TiO₂ surface. However, the photodegradation rate decrease when the solution become more alkaline (pH 10). This decrease can be explained by the electrostatic repulsion between the existence of paracetamol as anion form (pka = 9.5) and the surface of negatively charged TiO₂ which reduces the adsorption of paracetamol leading to a low efficiency.[136]

#### 4.2.3. Light intensity

The light source is one of the key parameters influencing the formation of photogenerated electron-hole pairs thought the absorption of irradiation by the semiconductor at a given wavelength. Therefore, light intensity enhances the amount of photogenerated charge carriers and thus the number of reactive oxygen species which in turn improves the degradation performance.[127,138]
<figure><img src="image_33.png" /></figure>

<figure><img src="image_34.png" /><figcaption>**Figure 13.** Effect of pH on the degradation efficiency of paracetamol [Paracetamol]o = 20 mg. L⁻¹, [catalyst] = 0.4 g.L⁻¹, light intensity = 50.0 mW.cm⁻²].
Reproduced from ref. [136]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

#### 4.2.4. Catalyst/pollutant ratio

The initial concentration of contaminants affects notably the kinetics of removal efficiency in addition to the optimal catalyst loading. Generally, at optimum catalyst dosing and low pollutants concentration, a sufficient number of radicals ensures the mineralization of organic compounds. However, above the optimal concentration, the insufficient amount of *OH radicals induces incomplete removal. In fact, adsorption of more reactant molecules on the surface of the catalyst reduces its ability to absorb light and the generation of *OH radicals due to the presence of fewer active sites for adsorption of hydroxyl anions. Therefore, it is necessary to control the catalyst/ pollutant ratio for optimal degradation efficiency.[125,127,137,139]

#### 4.2.5. Other parameters

The efficacy of photocatalysis employing TiO₂ as a catalyst is intricately tied to an additional range of parameters. Notably, solutions with elevated ionic strength tend to curtail photocatalytic efficiency, as the heightened presence of ions engenders intensified competition for active sites on the TiO₂ catalyst's surface. Furthermore, natural organic matter present in water bodies can diminish efficiency by vying for adsorption sites and, in some cases, serving as scavengers for the reactive species generated during the photocatalytic process. In contrast, an increase in temperature typically augments photo-

catalytic efficiency by bolstering the kinetic energy of molecules, thereby expediting the chemical reactions involved. However, the influence of temperature is somewhat context-dependent, as its precise impact can vary based on the specific reaction and the type of catalyst employed. It is also important to note that extremely high or low temperatures could pose a risk to the stability of the catalyst itself, potentially undermining the overall effectiveness of the photocatalytic process.

### 5. Electrocatalysis Oxydation Process

Recent studies have proved the great potential of electrochemical advanced oxidation processes as cleaner technologies for degrading biorefractory organic contaminants, as a result of their environmental compatibility, cost effectiveness and high efficiency. Electrocatalysis, known as anodic oxidation, is one of the widespread electrochemical processes. Due to its advantages such as the absence of secondary waste stream, its versatility and the non-necessity of oxidants, anodic oxidation has been widely applied to mineralize pollutants discharged from various industrial wastewaters containing pharmaceuticals, dyes and pesticides.[140–143]
## 5.1. Fundamental principle of electrocatalysis

Instead of light irradiation, electrical energy can be applied to initiate the chemical reaction. This technique can be also considered as green oxidation process by producing energy from renewable sources. During the electrocatalysis process, electrical energy is converted into chemical energy upon the application of an external potential between two electrodes, resulting in a redox reaction at the surface of the anode and the cathode. The principal of anodic oxidation can be divided

into two types of mechanisms namely direct oxidation and indirect oxidation.[144,145]

### 5.1.1. Direct oxidation

During direct oxidation, organic contaminants are first adsorbed onto the anode surface and then oxidized on the electrode by direct electron transfer. Consequently, these pollutants can be completely mineralized into CO₂ and H₂O as presented in Figure 14.[146–148]

### 5.1.2. Indirect oxidation

As shown in Figure 15, indirect oxidation is based on the generation of oxidizing species mainly *OH radicals produced at the anode surface from the water discharge, so that organic compounds are oxidized through the mediation of these radicals. The reaction processes are given by the following Eqs. (19–21) where M is the metal surface and R is organic species.[146,147,149,150]

$$M + H_2O \rightarrow M(-OH) + H^+ + e^- \quad (19)$$

$$M(-OH) + R \rightarrow M + \text{oxidation intermediates} \quad (20)$$

$$\text{Intermediates} + M(-OH) \rightarrow M + CO_2 + H_2O \quad (21)$$

<figure><img src="image_35.png" /><figcaption>Figure 14. Schematic illustration of direct oxidation of electrocatalysis process.</figcaption></figure>

## 5.2. Anodic materials

The electrocatalytic system consists of an anode, a cathode, a power supply and an electrolyte cell as the main elements. Among them, anodic materials are a key point in the electrochemical reaction. Therefore, the selection of electrode materials plays a crucial role in the performance of electrocatalytic degradation. Up to now, researchers have investigated various types of electrodes, such as boron-doped diamond (BDD), graphite, tin oxide (SnO₂), ruthenium and titanium oxides (RuO₂ and TiO₂) characterized by their strong oxygen transient progression and the generation of sufficient *OH radicals.[143,151,152] Titanium dioxide is a kind of semiconductor material with different advantages. However, the poor conductivity of this catalyst imposes a strong limit on its electrocatalytic activity. To overcome this problem, the TiO₂ electrode was modified with different metal oxides or carbon materials, or doped with metals, thus improving its properties.[144,152–154] In addition, the reduction of TiO₂ under hydrogen atmosphere to sub-stoichiometric titanium dioxide (Ti₃O₂n⁻¹) can be another approach to improve the catalytic activity of TiO₂.[140,147] These types of modification will be discussed in details in section 8.

<figure><img src="image_36.png" /><figcaption>Figure 15. Schematic illustration of indirect oxidation of electrocatalysis process.</figcaption></figure>
<figure><img src="image_37.png" /></figure>

### 5.3. Factors influencing electrocatalytic degradation

The anodic oxidation reaction does not only depend on the choice of the anode itself, but is also influenced by many factors, such as current density, pH and initial pollutant concentration.[144–146] Wu *et al.* have studied the effect of the applied potential on the electrocatalytic degradation of crystal violet using TiO₂/CF-1000 as anode. Figure 16 shows that without the application of the potential, a slight decrease in the concentration of crystal violet was observed after seven hours due to weak adsorption. Increasing the applied voltage leads to an increase in removal efficiency with 90.3% removal at 1.5 V. However, when the applied potential exceeds this value, a decrease in electrocatalytic activity was observed. This decrease can be explained by the onset of an oxygen evolution reaction resulting in competition between water splitting and organic oxidation which affects the degradation rate of crystal violet.[154]

In addition, initial pH can have a remarkable effect on the efficiency of the decomposition of organic contaminants. Therefore, Hu *et al.* present the effect of different pH solutions during the anodic oxidation of acid brown 14 (AB-14) as dye pollutant. From Figure 17, the results show an improvement in AB-14 removal under acidic solution conditions. This can be explained by the generation of active chlorine as the dominant oxidizing species that ensure the mineralization of dye at acid pH.[155]

Furthermore, it is crucial to study the effect of initial concentration of contaminants on the degradation efficiency. Ansari *et al.* have tested three experiments for the removal of p-dinitrobenzene(p-DNB) with three different initial concentration (85, 170 and 250 mg.L⁻¹) in an electrocatalytic system using Ti/SnO₂-Sb/β-PbO₂ as working electrode. As shown in Figure 18, the removal of p-DNB decreases with increasing initial concentration. At high amount of p-DNB, a large number of p-DNB

molecules were transferred to the anode surface, while the generation of reactive oxygen species such as *OH radicals is insufficient for the oxidation of p-DNB pollutants.*[156]

## 6. Sulfate Radical-based AOPs

In addition to the two remediation methods mentioned above, sulfate radical-based AOPs (SR-AOPs) have shown a higher performance in the degradation of pollutants in wastewater treatment. This superiority is related to the various advantages (cited in Section 4.2.2) presented with the generation of •SO₄²⁻ radicals over conventional •OH radicals.[157–159] The principle of this method is based on the in-situ generation of •SO₄²⁻ radicals through two precursor salt, peroxymonosulfate (PMS, HSO₃⁻) and peroxydisulfate (PDS, S₂O₈²⁻). Oxone (2KHSO₃·KHSO₄·K₂SO₄) is used to form PMS, while sodium persulfate (Na₂S₂O₈) and potassium persulfate (K₂S₂O₈) are employed to obtain PDS. The oxidants PMS and PDS can be easily dissolved in water and provide a series of oxidizing processes by breaking O–O bonds due to their long bond distance 1.460 and 1.497 Å respectively.[159–161] The next section lists some of the different ways that allow the PDS/PMS activation to produce radicals.

### 6.1. Types of activators used in persulfate oxidation process

Different ways can be used to generate •SO₄²⁻ radicals through the activation of PDS/PMS including UV, metal ions, electrochemical, metal oxides, carbon material.[157,159,162,163] Acknowledging both UV-induced and transition metal-induced activation of PMS/PS is essential when discussing hybrid activation processes in advanced oxidation for water treatment, providing

<figure><img src="image_38.png" /><figcaption>**Figure 16.** Effect of applied potential on the electrocatalytic degradation performance of crystal violet [Crystal violet] o = 6 mg.L⁻¹ using TiO₂/CF-1000 as anode. Reproduced from ref. [154]. Copyright (2022), with permission from Elsevier.</figcaption></figure>
<figure><img src="image_39.png" /><figcaption>Figure 17. Effect of pH on the removal of AB 14[AB 14]o = 100 mg/L by electrocatalysis. Reproduced from ref. [155]. Copyright (2017), with permission from Elsevier.</figcaption></figure>

<figure><img src="image_40.png" /><figcaption>Figure 18. Influence of the initial p-DNB concentration on the apparent rate constant during electrocatalysis. Reproduced from ref. [156]. Copyright (2019), with permission from Elsevier.</figcaption></figure>

a comprehensive view of their applicability in environmental
remediation. Additionally, the cooperative use of TiO₂-based

catalysts alongside metal oxides can synergistically enhance
PMS/PS activation, further emphasizing their combined effec-
tiveness in these processes.

6.1.1. UV irradiation

The combination of UV irradiation with the PS/PMS process has been widely applied in AOPs for degrading emerging compounds. UV irradiation as an ecological method consists on the breaking of O–O bonds from the energy transfer of UV light leading to the decomposition of PDS/PMS and the generation of •SO₄⁻ radicals as shown in the following Eqs. (22 and 23).[160,164,165]

$$
S_2O_8^{2-} + h\nu \rightarrow 2(-SO_4^{2-}) \qquad (22)
$$

$$
HSO_3^- + h\nu \rightarrow -SO_4^{2-} + \OH^- \qquad (23)
$$

In addition, with the presence of water molecules, electrons
can be produced with light irradiation, which activates PDS and
PMS by electron conduction as presented in Eqs. (24 to 26).[161]

$$
H_2O + h\nu \rightarrow -H + \OH^- \qquad (24)
$$

$$
S_2O_8^{2-} + -H \rightarrow -SO_4^{2-} + SO_4^{2-} + H^+ \quad (25)
$$

$$
HSO_3^- + -H \rightarrow -SO_4^{2-} + H_2O \qquad (26)
$$

Đurkić et al. have evaluated the behavior of 1,2,3-trichlor-
obenzene pollutant during its degradation using UV/SR-AOPs.
The results indicate that about 97–99% of 1,2,3-trichlorobenzene was removed in both systems UV/S₂O₈²⁻ at pH 5 and UV/HSO₅⁻ at pH 9.5. This confirms that the application of SR-AOPs with the assistance of a light source could promote the mineralization of micropollutants.[166]

### 6.1.2. Transition metal ions and metal oxides

The activation of PDS and PMS with transition metal ions and their oxides are frequently used as activating species in SR-AOPS. The interaction between transition metal such as Cu²⁺, Co²⁺, Fe²⁺ etc. and their oxidants produces *SO₄²⁻* radicals as they can easily gain and lose electrons. The proposed mechanism of decomposition is illustrated by the following Eqs. (27 and 28).[160,164]

$$S_2O_8^{2-} + M^n \rightarrow M^{n+1}SO_4^{-} + SO_4^{2-} \quad (27)$$

$$HSO_5^- + M^n \rightarrow M^{n+1}SO_4^{-} + OH^- \quad (28)$$

This type of activation can be divided into two categories homogeneous and heterogeneous phase. Figure 19 presents the application of different homogenous activation system (PMS/metal) for the removal of 2,4-dichlorophenol. The results show that cobalt ion is very effective in activation PMS to generate sulfate radicals and ensure complete removal of the pollutant 2,4-dichlorophenol. For other metals, the low efficiency may be related to the reaction between dissolved metal ions and PMS and thus reduce the mass transfer effect on PMS

activation. Therefore, heterogeneous activation can overcome the limitation of metal ion leaching and thus increase the generation of sufficient sulfate radicals without the need for additional treatment to eliminate metals from the water.[160,161]

### 6.1.3. Carbon based catalysts

Carbon-based materials have also been used as catalysts for PMS and PDS activation due to their many advantages such as high specific surface area, cost effectiveness, thermal stability and particularly low leaching of toxic metal. Therefore, graphene oxide, carbon nanotube and activated carbon are the most used structures for wastewater decontamination by generating sulfate radicals through PDS/PMS activation. This activation capacity can be explained by the structural defect, the presence of delocalized π electrons and large surface area to enhance mass transfer and the greater exposure of active sites.[158,161,167]

### 6.1.4. Electrochemical activation

Electrochemical oxidation (EO) can be used as an activator in PS-AOPs while using anodic oxidation at the surface of anode with a synergistic effect on the degradation of emerging compounds while using both systems. Another advantage of combining EO with PS-AOPs is to reduce the energy consumption in electrocatalysis system. The generation of sulfate radicals increase mass transfer by increasing the half time of the

<figure><img src="image_41.png" /><figcaption>Figure 19. Removal efficiency 2,4-dichlorophenol through the activation of PMS with different metals. Reproduced from ref. [161]. Copyright (2017), with permission from Elsevier.</figcaption></figure>
<figure><img src="image_42.png" /></figure>

reactive species (\*SO₄⁻, \*OH). As a result, the activated radicals can diffuse rapidly in the electrolyte solution and destroy the pollutants more completely, thus reducing the reaction time and energy consumption. The mechanism of electro-activated \*SO₄⁻ radicals is similar to the anodic oxidation of \*OH radicals. After adsorption of the contaminants, they will be removed by direct or indirect oxidation and mineralized to CO₂ and H₂O.[160,168,169]

### 6.1.5. Hybrid activation

The use of each activation process has its limitations for activating \*SO₄⁻ radicals alone. Scientists have proposed to combine several activation methods in order to take full advantage of their benefits and reduce their drawbacks.[168] Kohantorabi et al. have evaluated the PMS-enhanced-photocatalytic activity of synthetized ZnO@NiFe₂O₄ nanorods in facilitating the removal of paracetamol. For this purpose, they studied the effect varying concentration of Zn²⁺ solution during the preparation of ZnO on NiFe₂O₄. The obtained results were reported in Figure 20. The concentration of the zinc solution in the ZnO preparation process has negligible impact on the photodegradation of paracetamol under UV irradiation. After adding PMS oxidant to the catalyst, an increase in the removal efficiency was observed, indicating the great potential of PMS activation with the ZnO@NiFe₂O₄ catalyst. Furthermore, 93.6% of paracetamol was eliminated after combining the UV/PMS process with the catalyst synthetized with 0.4 M of zinc solution,

indicating the importance of coupled ZnO as metal oxide for the photocatalytic PMS activation.[170]

Yu et al. have investigated a new electrochemical anodic (EA) activated PMS process by combining Ti/La₂O₃–PbO₂ anode with PMS. As shown in Figure 21, only 19.97% of cefadroxil (CFR) was removed in 1 h using the single system PMS. Better performance was observed in the electrocatalytic degradation using Ti/La₂O₃–PbO₂ as anode with a removal rate of 80.22% during 1 h. After the addition of PMS to the electrocatalytic process, the removal rate increased further and reached 98.07% in 1 h. The improvement in CFR removal rate is associated to the anodic oxidation on electrode surface to degrade pollutant in addition to the electroactivation of PMS across the anode to generate strongly oxidizing \*SO₄⁻ radicals and improve the mineralization of cefadroxil. The incorporation of the Ti/La₂O₃–PbO₂ electrocatalyst provides additional beneficial contribution to the removal of pollutant in EA-PMS process.[169]

## 7. Titanium Dioxide Based Catalysts

To date, titanium dioxide is considered an increasingly attractive catalyst for the elimination of micropollutants in the context of environmental remediation, due to its excellent physical and chemical characteristics.[171,172] The adsorption properties of TiO₂-based catalysts are pivotal in material-enhanced AOPs. These catalysts are characterized by their high surface area and porous structure, facilitating efficient contact between the catalyst and contaminants in the aqueous solution.

<figure><img src="image_43.png" /><figcaption>**Figure 20.** Influence of ZnO concentration on catalytic activity of ZnO@NiFe₂O₄ through the UV/PMS degradation of paracetamol. Reproduced from ref. [170]. Copyright (2021), with permission from Elsevier.</figcaption></figure>
<figure><img src="image_44.png" /></figure>

<figure><img src="image_45.png" /><figcaption>**Figure 21.** Removal of CFR under different systems. Reproduced from ref. [169]. Copyright (2022), with permission from Elsevier.</figcaption></figure>

TiO₂ exhibits both chemisorption, involving strong chemical bonding, and physisorption, entailing weaker physical interactions, making it versatile for a wide range of organic and inorganic species' adsorption. Additionally, TiO₂'s rapid adsorption kinetics are advantageous for the swift removal of contaminants, promoting efficient degradation reactions. Moreover, these catalysts are often reusable and exhibit pH sensitivity, necessitating pH optimization for maximized adsorption efficacy. Altogether, the careful consideration and optimization of TiO₂'s adsorption behavior are instrumental in the success of water treatment and environmental remediation efforts within AOPs.

As discussed above, some drawbacks limit its application in photocatalysis and electrocatalysis processes. Therefore, the improvement of its catalytic activity in wastewater treatment is under continuous investigation with various modifications, including metal doping, non-metal doping and coupling with other catalysts to form a heterojunction.[173-175] Numerous methods have proven their ability to improve TiO₂'s characteristics for effective removal of contaminants. These methods are mentioned in the following part.

## 7.1. Metals doped TiO₂ catalyst

Doping TiO₂ with metal ions could improve its photocatalytic activity by extending the absorption of this catalyst to the visible range and enhancing the efficiency of charge carrier separation.[176-178] Furthermore, it has been proven by scientists that the introduction of metal ions into TiO₂ is a common strategy to improve the conductivity of the anode in the electrocatalysis process.[179] Different transition metals such as

Fe, Cu, Ni, Co, Zn and Mn are used as dopants to enhance the photocatalytic degradation of pharmaceuticals under visible light. The incorporation of metal ions into the TiO₂ structure creates an impurity band near the conduction band (CB) that helps delay charge carrier recombination by acting as a carrier trap. In addition, due to this incorporation, the band gap has been shifted to a higher wavelength.[180-182] Kaur et al. have synthesized Cu-doped TiO₂ as photocatalyst for the removal of ofloxacin (OFX) as an antibiotic under visible light. The optical properties show that pure TiO₂ has a band gap of 3.14 while Cu-doped TiO₂ has a low energy of 2.91 eV. This decrease in band gap energy is related to the incorporation of Cu²⁺ which generates an intermediate new energy level in TiO₂ that are positioned near the CB and thus shift the optical response towards the visible region. As a result, Cu-doped TiO₂ improves the high removal efficiency for the decomposition of (OFX) compared to pure TiO₂ photocatalyst after 3 h of visible illumination. Figure 22 illustrates the photocatalytic degradation mechanism using Cu-doped TiO₂ under visible light.[180]

## 7.2. Metal oxides modified TiO₂

The coupling of TiO₂ with semiconducting metal oxides has been considered as an interesting approach to improve the physico-chemical, electrical and photocatalytic properties of TiO₂ catalyst.[171,172,176] This combination induces additional electric fields that improve the performance of TiO₂ by accelerating charge carrier separation. Using this strategy, metal oxides like ZnO, CuO, Fe₂O₃, SnO₂ etc. have been widely studied for the synthesis of mixed oxide composite and have shown increased catalytic activity compared to pure TiO₂.[182,183] Behara
<figure><img src="image_46.png" /></figure>

<figure><img src="image_47.png" /><figcaption>Figure 22. Schematic mechanism of Cu-doped TiO₂ for OFX degradation.</figcaption></figure>

et al. have combined ZnO₂ semiconductor to TiO₂ for electro-catalytic degradation of Crystal violet dye. After 12 h, 80% of the dye was removed using TiO₂ as anode. The heterostructure of TiO₂/ZnO composite used as electrode shows high performance with 85% decolorization after 6.5 h. This increase in electrocatalytic activity is linked to the synergistic interaction between these two metal oxides with a synchronized charge transport (Figure 23) leading to a rapid elimination of pollutant with low energy consumption.[184]

In addition, Mirmasoomi et al. synthetized TiO₂/Fe₂O₃ nano-composite by ultrasonic-assisted impregnation method. The prepared material was used as photocatalyst for degradation of diazinon. The authors found that the absorption edge of TiO₂/Fe₂O₃ nanocomposite was shifted to the visible region with low

band gap. This photocatalyst exhibits high degradation efficiency (95.07%) compared to TiO₂, which implies that the heterojunction between TiO₂ and Fe₂O₃ could significantly improve the inhibition of electron-hole recombination.[185]

### 7.3. Carbonaceous materials modified TiO₂

Although, carbon materials such as activated carbon, carbon nanotube (CNT), graphene and graphene oxide (GO) can be associated with TiO₂ due to their various advantages such as high surface area, excellent electron transfer characteristics and great number of oxygenated functional groups. Graphene component have been used in AOPs on different process such

<figure><img src="image_48.png" /><figcaption>Figure 23. Schema representing the charge carrier in the electrocatalytic removal of Crystal violet using TiO₂/ZnO anode.[184]</figcaption></figure>
<figure><img src="image_49.png" /></figure>

electrocatalysis and photocatalysis. Graphene is known to be a semiconductor with zero band gap energy and high electron-hole pair mobility. Doping graphene with TiO₂ could promote more active sites for the reaction with better electrical conductivity at high dopant concentration. In addition, this association could enhance the photocatalytic properties and allow activation by visible light.[135,176,186,187] Dong *et al.* successfully prepared a combined material of carbon nanotube and immobilized titanium dioxide for the photocatalytic removal of methylene orange. High photocatalytic activity was achieved with the CNT-TiO₂ heterostructure due to the low recombination of electron-hole pairs and the excellent photo-absorption induced by the synergistic effects of the hybrid material.[188] Sharma *et al.* reported the elaboration of TiO₂-GO nanocomposite as photocatalyst. After 150 min, 63% of crystal violet removal was detected using this composite while 43% of crystal violet was eliminated in the presence of TiO₂ nanoparticles. These results show that the photocatalytic activity of TiO₂-GO heterostructure is two time higher than pure TiO₂.[189]

7.4. Heterogeneous TiO₂-based composite

The elaboration of composites by combining different semi-
conductors with different energy levels has received a lot of
attention in the last decades. Numerous composite materials
can be coupled to TiO₂ semiconductor, creating a heterojunc-
tion with two or more oxides. These composite heterostructures
are interesting for extending the absorption edge of TiO₂ into

the visible light region. They also increase the lifetime of
electron-hole pairs, accelerate the separation of charge carriers
and improve the conductivity of the resulting composite.
Therefore, various metal oxide-graphene-metal composites
based on TiO₂ are an important research topic.[172,190–193]

In the case of incorporating oxides with suitable properties,
perovskite-type oxide is one of the most common minerals for
application in AOPs to remove emerging pollutants. Perovskite
is a kind type of material with a stable structure, a narrow band
gap and crystal structure similar to that of calcium titanate
(CaTiO₃).[194-196] As shown in Figure 24, comprehensive studies
on the use of this type of material with different AOPs have
been published in recent years.[98,197-200]

Chen et al. prepared a composite heterojunction photo-
catalyst by coupling TiO₂ to LaNiO₃ perovskite by *in situ* sol-gel
technique with different calcination temperatures. Figure 25a
presents the synergistic interaction mechanism between the
synthesized composites. The results show that LaNiO₃/TiO₂
significantly improves the removal of Methyl Orange (MO) by
photocatalysis (Figure 25b). After 75 min of irradiation with
visible light, the MO degradation rate was 57.6% which
confirms that LaNiO₃/TiO₂ improves the separation of the
photogenerated charges and increases the number of

catalytically active sites with high utility efficiency compared
to pure TiO₂ and LaNiO₃.[¹⁹⁵]

Among the perovskite material, calcium copper titanate (CaCu₃Ti₄O₁₂) is widely reported over the last decades. This material is a mixed oxide belonging to the semiconductor family known as Ti-based perovskite. It has several interesting

<figure><img src="image_50.png" /><figcaption>**Figure 24.** Representative study on perovskite application in AOPs. Reproduced from ref. [194]. Copyright (2021), with permission from Elsevier.</figcaption></figure>
<figure><img src="image_51.png" /></figure>

<figure><img src="image_52.png" /><figcaption>Figure 25. (a) Schematic diagram of LaNiO$_{3}$/TiO$_{2}$ involved in photocatalytic process</figcaption></figure>

properties such as a very high dielectric constant of up to 10⁵ over a wide temperature range of 100–600 K and wide frequency (10⁻² – 10⁻⁶ Hz). CaCu$_{3}$Ti$_{4}$O$_{12}$ (CCTO) has a ABO$_{3}$ perovskite structure characterized by the combination of several band gaps of TiO$_{2}$ and CuO mainly. The combination of these properties in the CCTO complex, offers an improvement of its behavior under visible light exposure, leading to the photogeneration of a free movement of electron holes in its crystallographic network. Therefore, this complex is recently used as a photo-electrocatalyst active under visible light in photo-electrocatalytic processes, in the degradation of dyes and pharmaceutical pollutants.[201–203]

### 7.5. Sub-stochiometric titanium dioxide

In view of the increasing demand of conductive materials, chemically reduced TiO$_{2}$, Ti$_{n}$O$_{2n-1}$ (4 ≤ n ≤ 10) called Magnéli phases have attracted a lot of attention and have been used in environmental remediation as electrocatalysts and photocatalysts. They can be produced by heat treatment of TiO$_{2}$ with specific reducing agents. In particular, Magnéli titanium suboxides have been considered promising candidates for anodic oxidation due to their high electrical conductivity, chemical inertness and corrosion resistance.[204–206] Ti$_{4}$O$_{7}$, as sub-stochiometric Magnéli phase of TiO$_{2}$, has proven to be the most

popular and cost-effective electrode material, as it has the highest conductivity with exceptional performance in degrading refractory contaminants.[204,207–209] Wang et al. prepared high quality Magnéli phase Ti$_{4}$O$_{7}$ using the spark plasma sintering method. This material was used as anode for the electrochemical oxidation of MO azo dye. The results show that Ti$_{4}$O$_{7}$ electrode exhibited the fastest electrochemical oxidation rate compared to commercial graphite, dimension stable anode and stainless-steel electrodes under the same experimental conditions. The superior electrochemical oxidation performance of Ti$_{4}$O$_{7}$ anode can be explained by the excellent electrical conductivity (~961.5 S cm⁻¹).[210] The possible removal mechanism for electrocatalysis of MO using Ti$_{4}$O$_{7}$ was illustrated in Figure 26.

Table 4 presents a concise summary of various instances of TiO$_{2}$ catalyst modification, accompanied by their respective applications and the resultant impact of these modifications on catalyst properties

### 8. Synthesis and modification of TiO$_{2}$

The design of highly active TiO$_{2}$-based catalysts has attracted great interest from researchers in order to ensure the best properties such as specific surface area, crystal structure, phase composition and uniform morphology. There are several

<table><caption>Table 4. Different types of TiO$_{2}$ modification and their effects.</caption><thead><tr><td>Catalyst</td><td>Type of modification</td><td>Application</td><td>Effect in the properties</td><td>Ref</td></tr></thead><tbody><tr><td>TiO<sub>2</sub>-Bn-Pd</td><td>heterojunctions with semiconductor/ doping with metal ions</td><td>Photocatalysis</td><td>extension of the spectral response to the visible range</td><td>211</td></tr><tr><td>N-doped TiO<sub>2</sub></td><td>doping with non-metals</td><td>Photocatalysis</td><td>reduce the recombination of photo-generated electron-hole pairs in TiO<sub>2</sub></td><td>212</td></tr><tr><td>Cu-TiO<sub>2</sub>-GO</td><td>metal doping/modification with carbon material</td><td>photocatalysis</td><td>Increase in the adsorption capacity of TiO<sub>2</sub></td><td>213</td></tr><tr><td>TiO<sub>2</sub>- Carbon quantum dots</td><td>modification with carbon material</td><td>Photo-electrocatalysis</td><td>decrease the energy band gap into the visible region/ decrease the recombination of electron -holes</td><td>214</td></tr><tr><td>SnO<sub>2</sub>-TiO<sub>2</sub></td><td>heterojunctions with metal oxide</td><td>Photo-electrocatalysis</td><td>increase the electron transport rate</td><td>215</td></tr></tbody></table>
<figure><img src="image_53.png" /></figure>

<figure><img src="image_54.png" /><figcaption>**Figure 26.** Schematic representation of electrocatalytic oxidation of MO using Ti₄O₇ anode. Reproduced from ref. 210. Copyright (2019), with permission from Elsevier.</figcaption></figure>

methods for the synthesize of TiO₂ including sol-gel, hydro-
thermal technique, chemical vapor deposition and
electrospinning.[216–218] This section describes the design of a
TiO₂-based active catalyst using several approaches.

**8.1. Sol-gel method**

The sol-gel is the most common method used in wet chemical
technique. It is characterized by two steps: hydrolysis and
condensation of a molecular precursor. The precursor used for
the preparation of TiO₂ can be titanium isopropoxide or
inorganic salt such as titanium tetrachloride.[219,220] As discussed
by Macwan *et al*. the morphology and size of particles can be
optimized by controlling synthetize parameters like calcination
temperature.[221] This method offers several advantages includ-
ing low temperature operation, high purity, better homogeneity

of the material and the simplicity of the procedure and the
good specific surface of the materials.[222,223]

8.2. Hydrothermal

Hydrothermal synthetize is another type of wet chemical
technique, frequently used for the fabrication of TiO₂
nanomaterials.[216,217] Several examples of hydrothermal syn-
thesis, including the work of Guo *et al*. are cited in the literature
for the production of different forms of TiO₂ such as nanotubes
and nanowires. The results show the important of temperature
during the hydrothermal synthesis on the morphology of TiO₂
in which they obtained nanotubes at 150°C while nanowires
are formed at 180°C. Figure 27 shows the morphology of TiO₂
prepared at different temperatures obtained by high-resolution
transmission electron microscopy (HRTEM).[224] This process

<figure><img src="image_55.png" /><figcaption>**Figure 27.** HRTEM images of (a) TiO₂ nanotubes and (b) TiO₂ nanowires. Reproduced from ref. [224]. Copyright (2010), with permission from Elsevier.</figcaption></figure>
<figure><img src="image_56.png" /></figure>

allows better control of the size, shape and crystallinity of
TiO₂.[²¹⁷]

## 8.3. Electrospinning

One dimensional TiO₂ nanomaterials such as nanofibers (NFs) have been extensively used in the application of catalytic activity due to their extraordinary flexibility and their physical and chemical properties.[²²⁵,²²⁶] Electrospinning is a simple and efficient method used to fabricate nanofibers for a wide variety of suitable materials. This method is reproducible and can be scaled up with low cost consumption. The preparation of TiO₂ NFs by electrospinning is based on the use of TiO₂ precursor dispersed in a polymer solution like polyvinyl alcohol (PVA) with the application of a high voltage from a power supply. Figure 28 illustrate this process.[²²⁷⁻²²⁹]

In a typical electrospinning experiment, the polymer solution is pumped from the syringe at a constant and controllable rate. The application of voltage makes the pendent drop of the solution highly electrified. When the voltage is sufficiently high, electrostatic forces will overcome the surface tension of the polymer solution and the ejection of liquid jet from the nozzle is possible. During the stretching and whipping process, the solvent evaporates and solid fibers are formed on the rotating drum. Subsequently, the precipitated fibers are collected and processed in a muffle furnace to remove the polymer fraction and obtain pure TiO₂ NFs. Different parameters can affect the properties of the NFs such as applied voltage, viscosity of the solution and the distance between the nozzle and the collector.[²³⁰]

Recently, TiO₂- NFs have been used for the elaboration of photocatalysts for water remediation. Sayegh *et al.* synthetized TiO₂/BN/Pd nanofibers by electrospinning. The hybrid composites were used as photocatalysts for degradation of paracetamol. The authors found that TiO₂ NFs with a deposition of 100 cycles of boron nitride and 100 cycled of palladium

(TiO₂-BN100-Pd100) by atomic layer deposition was the optimum, which was confirmed by UV-Vis absorbance spectroscopy. The absorption edge of TiO₂-BN100-Pd100 composite nanofiber is red-shifted with the lowest band gap. This photocatalyst exhibits higher degradation of paracetamol above all the synthetized samples. Electrospinning is a promising method for the synthetize of TiO₂ catalyst with well dispersed nanofibers and easy integration of other materials to form composites.[²¹¹]

## 8.4. Atomic layer deposition

As an advanced gas- phase deposition method, atomic layer deposition (ALD) is a thin film growth technique known as a self-limiting deposition process characterized by unique capabilities. ALD is based on chemisorption between substrate-surface and on surface-surface reaction to deposit materials at an atomic scale layer-by layer as presented in Figure 29.[²³¹⁻²³³] The ALD process consists mainly of four steps. In the first step, precursor A is pulsed into the reaction chamber and reacts with the active sites of the substrate during the exposure time. Then, an inert gas is purged into the chamber to eliminate excess precursor and by-products (step 2). Step 3 consists on the pulse of precursor B (co-reactants) followed by exposure in which it will react with be adsorbed precursor A and create a layer of the target composite. Finally, a second purge phase is initiated in step 4 to remove the reaction products from the reactor for the next deposition cycle. All these steps are involved in the ALD automatically by controlling the valves at high speed. Many cycles can be repeated in order to achieve the desired film thickness.[²³⁴]

Recently, ALD has attracted great importance in the design of high efficiency catalysts. In particular, the synthesis of TiO₂ as a catalyst with different structures using this method have been widely reported due to the homogenous and conformal coatings with controlled thickness.[²³²,²³⁵⁻²³⁷] Sayegh *et al.* used TiCl₄ and water as precursors to synthesize TiO₂ nanotubes by ALD

<figure><img src="image_57.png" /><figcaption>**Figure 28.** Schematic representation of electrospinning method.</figcaption></figure>
<figure><img src="image_58.png" /></figure>

<figure><img src="image_59.png" /><figcaption>**Figure 29.** Schematic representation of one cycle of atomic layer deposition.</figcaption></figure>

for photocatalysis. The ALD cycle consisted of a 0.2 s TiCl₄ pulse, 10 s exposure and 60 s Ar purge followed by a 2 s H₂O pulse, 10 s exposure and 60 s Ar purge.[212] In order to adjust the thickness of the TiO₂ shell, the number of ALD cycles was varied between 500 and 1000. The results showed that N-doped TiO₂ nanotubes promote the generation of radicals with high photocatalytic efficiency for the removal of paracetamol. Furthermore, they found that after 5 consecutive cycles of photocatalysis using the same N-doped TiO₂ nanotubes, the degradation rate of paracetamol was still high with a slight decrease in efficiency. This recyclability of the photocatalyst is related to the use of ALD method as an interesting technique for the synthesis of high stable photocatalysts. Figure 30 shows

the steps involved in obtaining TiO₂ nanotubes with the removal efficiency of paracetamol using different samples.

## 8.5. Mechanosynthesis method

Mechanochemical techniques have been reported as another approach for the synthesis of semiconductor-semiconductor composite catalysts. Among these techniques, ball milling (BM) is a promising synthesis route for the elaboration of nano-structured oxide at room temperature for efficient water treatment.[238–240] This method has various advantages such as low cost, simple operation, high potential for large-scale

<figure><img src="image_60.png" /><figcaption>**Figure 30.** Schematic representation of the elaboration of N-doped TiO₂ nanotubes by ALD for paracetamol degradation. Reproduced from ref. [212]. Copyright (2022), with permission from Elsevier.</figcaption></figure>
<figure><img src="image_61.png" /></figure>

application, environmentally friendly technique. Besides, a solid-state reaction is involved during the mechanical force generated in the grinding. The ball milling process is based on the generation of mechanical force on the precursor material through the grinding balls when the milling jar rotate with a certain regularity. This mechanical impact could reduce the crystallite size of the precursors to the nano scale with a homogeneous powder mixture. Different parameters can affect the mechanical reaction and thus the properties of the resulting composite such as grinding time, grinding speed and ball/ powder ratio. These reactions are considered as chemical reaction that are induced with the help of mechanical energy.[241-244] Kucio et al. prepared Ni-TiO₂ photocatalyst using mechanochemical synthesis in a planetary ball mill. TiO₂ and Nickel acetylacetonate were chosen as initial precursors. The powders were placed in Si₃N₄ jar containing 25 balls of the same material. The conditions were fixed at a milling time of 1 h with a mill speed of 250 rpm. The resulting mixtures were then calcinated at 800 °C for 1 h and different samples were obtained with different percentages of nickel. The UV/Vis spectra show that mechanochemical treatment of pure TiO₂ under the same condition as mentioned above, improves its optical properties with a change of the band gap Eg = 3.05 eV compared to the initial precursor of TiO₂ Eg = 3.11 eV. Furthermore, photocatalytic results confirm that ball milling is a simple and efficient way to prepare photocatalysts without by-products.[245]

In addition, mechanochemical methods are widely used to develop high quality perovskite ceramic oxides.[246] Kawrani et al. mixed a stoichiometric amount of CaCO₃, TiO₂ and CuO using a ball mill in an alumina balls and jar for 5 h to obtain CaCu₃Ti₄O₁₂ as a Ti-based perovskite material. The results show that after calcination at 900 °C for 3 h under air, a pure and homogeneous power of CCTO was obtained without any impurities. Therefore, pellets of pure phase of CCTO were used as photoelectrocatalyst in water splitting.[203,247]

## 9. Future perspectives

The advancement of titanium dioxide based catalysts is expected to continue in response to the growing demand for efficient and sustainable catalytic processes. Future research will likely focus on improving the catalytic activity, selectivity, and stability of TiO₂ catalysts by modifying their surface structures, such as controlling the crystalline phase, engineering the bandgap, and enhancing porosity. Additionally, the incorporation of other materials into TiO₂ to enhance its catalytic properties is an essential aspect of catalyst development. There are several hybrid materials that have been investigated for the improvement of TiO₂ catalysts in AOPs. For example, carbon-based materials like graphene, carbon nanotubes, or activated carbon can be combined with TiO₂ to form hybrid structures. These hybrids provide high surface area, improve light absorption, and enhance electron transfer, leading to improved catalytic activity. In addition, polymer-TiO₂ composites have been developed to improve the stability and reusability of TiO₂

catalysts. Polymers like polyvinyl alcohol, polyethylene glycol, or polyaniline can be incorporated with TiO₂ to form hybrid materials with enhanced catalytic performance. Incorporating other metal oxides, such as ZnO, Fe₂O₃, or SnO₂, with TiO₂ can enhance its photocatalytic activity. These metal oxides can enhance the generation of reactive oxygen species for pollutant degradation. The incorporation of co-catalysts can further enhance their performance.[248]

Exploring new synthetic methods and techniques will contribute to the development of TiO₂-based catalysts with customized functionalities for specific reactions. For instance, surface modification techniques like chemical grafting, atomic layer deposition, or electrodeposition can provide precise control over the catalyst's surface chemistry. Nanostructuring methods such as sol-gel synthesis, hydrothermal methods, or template-assisted synthesis can be employed to create TiO₂ nanostructures with larger surface areas, increased active sites, and improved mass transport properties. These nanostructured TiO₂ catalysts exhibit superior catalytic performance compared to bulk TiO₂ materials in specific reactions. By combining various synthetic approaches, researchers can create TiO₂-based catalysts optimized for a wide range of reactions. The ability to tailor the functionalities of TiO₂ catalysts will play a crucial role in advancing catalysis and addressing challenges in fields like water remediation and other applications.

A significant focus lies in designing catalysts that can remove multiple emerging contaminants simultaneously, such as pharmaceuticals and microplastics. For example, coupling TiO₂ with activated carbon or metal oxides can create hybrid catalysts with enhanced adsorption and catalytic capabilities towards both contaminant types.

Furthermore, future research in the development of TiO₂ catalysts may focus on creating advanced catalysts with improved visible light response. This would enable efficient pollutant degradation even in the absence of UV light. While TiO₂ is an excellent photocatalyst under UV light, its performance is limited in the visible light region of the electromagnetic spectrum. Enhancing the visible light response of TiO₂ catalysts is crucial for expanding their practical applications and maximizing their environmental remediation potential.

In future research, there is a focus on developing integrated photoelectrocatalytic systems that combine photocatalysis and electrocatalysis. These systems utilize solar energy to generate charge carriers through photocatalysis, which are then used in electrochemical reactions for efficient pollutant degradation. This approach takes advantage of the benefits offered by both techniques, resulting in effective pollutant removal. Another area of interest involves integrating electro-Fenton with photocatalysis. Electro-Fenton involves the electrochemical reduction of hydrogen peroxide (H₂O₂) to produce hydroxyl radicals (*OH) for pollutant degradation. When combined with photocatalysis, this hybrid process exhibits synergistic effects, enhancing pollutant removal capabilities. The electrochemical reduction of H₂O₂ in electro-Fenton regenerates oxidizing species consumed in photocatalysis, ensuring a continuous supply of reactive oxygen species for improved pollutant removal efficiency and reaction kinetics.
<figure><img src="image_62.png" /></figure>

One of the main challenges faced by TiO₂-based catalysts is the gradual decline in their catalytic performance after multiple reaction cycles, a phenomenon known as catalyst deactivation. To address this issue, future research endeavors may concentrate on developing strategies aimed at minimizing catalyst deactivation and prolonging the catalytic performance of TiO₂-based catalysts. One promising approach for enhancing the stability of these catalysts involves the utilization of atomic layer deposition (ALD) to deposit a layer of aluminum oxide (Al₂O₃) onto the TiO₂ surface. ALD is a highly precise thin film deposition technique capable of creating ultra-thin layers with atomic-level accuracy. The deposition of Al₂O₃ through ALD can serve as a surface passivation layer for TiO₂ catalysts, creating a protective barrier that hinders undesired reactions with the surrounding environment. This passivation layer plays a crucial role in inhibiting surface reactions like photocorrosion and unwanted side reactions, which can lead to catalyst degradation and a loss of catalytic activity.

In the other side, transitioning from a batch system to a scaled-up system is a crucial step in realizing the practical application of optimized TiO₂-based catalysts in water treatment. Several key factors need to be considered to ensure a successful transition. By addressing reactor design, catalyst loading, process control, safety, and economic factors, the transition from a batch system to a scaled-up system can be accomplished successfully. This contributes to the development of more sustainable and efficient water treatment solutions, ultimately improving water quality and environmental preservation.

In conclusion, the future of titanium dioxide-based catalysts is bright, with immense potential for advancements in performance, expanded applications, and significant contributions to a greener and more sustainable future. Continued research efforts in this field will drive innovation and unlock new possibilities, paving the way for the widespread adoption of TiO₂-based catalysts in diverse industries and catalytic processes. Recognizing the limitations of TiO₂-based catalysts in AOPs provides opportunities for innovation and research. Ongoing efforts focus on developing new materials, catalyst designs, and process modifications to enhance AOP efficiency and versatility. Researchers explore hybrid systems that combine TiO₂ with other catalysts or treatment methods, nanomaterial advancements that improve TiO₂'s catalytic properties, and pH control strategies to mitigate pH dependencies. Catalyst recovery and regeneration methods are pursued to reduce operational costs, and light source optimization, including energy-efficient options and solar-based systems, seeks to reduce energy consumption. Real-world challenges drive a deeper understanding of the interplay between catalysts and water quality, while growing environmental and regulatory concerns encourage the development of sustainable AOPs with reduced environmental footprints, including catalyst production and disposal. These perspectives underscore the dynamic evolution of TiO₂-based AOPs in water treatment and environmental remediation applications.

## 10. Conclusion and challenges

Water is a vital element for humans, and it is therefore necessary to preserve it. Today, water is increasingly used by different activities which produce wastewater in high quantities, thus containing multiple sources of pollution. Among the chemical pollutants, emerging biorefractory organic pollutants deserve special attention. In particular, pharmaceutical products that are released from hospital effluents or industries, despite their presence at low concentrations of a few ng/L-µg/L, prove to be very harmful for humans and for the ecosystem. Indeed, the presence of these substances in water causes disturbances to aquatic fauna and according to several ecotoxicological studies, they would be the cause of carcinogenic, mutagenic or endocrine disrupting effects. To deal with this problem, conventional wastewater treatment methods such as physical-chemical and biological treatments are used, but these do not completely eliminate these types of contaminants and generate other secondary problems, such as the additional use of chemical reagents, the formation of by-products or the production of sludge that is difficult to treat. That is why, efficient and ecological strategies have been developed. Among them we can mention the advanced oxidation processes which are nowadays considered as one of the best methods to remove efficiently many contaminants by oxidizing organic pollutants into harmless products. Advanced oxidation processes include various technologies such as photocatalysis, electrocatalysis and sulfate radical-based AOPs.

Heterogeneous photocatalysis is as an environmentally friendly technique for water treatment based on the absorption of irradiation by a semiconductor with energy equal to or greater than its activation energy. This activation will generate electrons and holes, they can combine or react with chemical species present on the surface of the semiconductor to lead to the formation of oxidizing species.

Electrochemical processes have proven to be more efficient methods for the removal of refractory products from wastewater. Electrocatalysis as an electrochemical advanced oxidation process, has attracted considerable interest in the field of water remediation due to its many advantages, such simple operation. It is also environmentally friendly and it has the ability to mineralize contaminants without secondary pollution. This process can be divided into two parts, the direct oxidation which is characterized by electron transfer between the contaminant and the anode surface and the indirect oxidation which involves the generation of reactive oxygen species. In the case of indirect oxidation, the electrical energy by transforming into chemical energy helps to produce reactive oxygen species and oxidize pollutants to form CO₂ and H₂O, or small biodegradable organic compounds.

In addition, these two processes can be combined with sulfate radical-based AOPs to enhance the generation of *SO₄²⁻* radicals through the activation of peroxymonosulfate by the photo-generated electrons or the application of an external potential. The scientists confirmed that this combination is a promising and innovative approach to ensure complete removal of contaminants.
<figure><img src="image_63.png" /></figure>

Titanium dioxide, commonly known as titania or TiO₂, has been used in a variety of applications in recent decades, such as water treatment, due to its excellent physicochemical properties, good stability, relatively low cost and its compatibility with the environment. It is a well-known multi-functional ceramic material, the advantages of TiO₂ as based material (photo-catalyst or anode) are numerous. However, it has major drawbacks such as its low electrical conductivity, wide band gap energy, fast recombination of electrons and holes, low activity under visible light irradiation and self-aggregation of pure nanostructured TiO₂ which considerably limit its activity in AOPs. Hence, several ways were applied to solve those problems such as metal doping, metals oxide modified TiO₂, carbonaceous material modified TiO₂, sub-stochiometric titanium dioxide and coupling with other catalysts to form a heterojunction. Plenty of researchers have confirmed that these modifications are able to enhance the separation efficiency of photo-generated charge carries, to reduce the recombination of electron-hole pairs, increase the conductivity and promote the catalytic activity of TiO₂. The preparation and modification of TiO₂ can be achieved with different strategies. Several parameters can affect the properties of the resulting composite in each method. While TiO₂-based catalysts offer significant advantages in AOPs for water treatment and environmental remediation, such as their photocatalytic properties and environmental compatibility, they do come with certain limitations. Understanding these limitations is important when designing AOP processes and choosing the most suitable treatment methods for specific applications. It's often necessary to consider these constraints and potentially combine TiO₂-based AOPs with other treatment technologies to address complex and diverse water treatment challenges effectively.

Finally, considering the constraints of using TiO₂, it will be interesting to develop a new catalyst with high performance by using the appropriate and simple method for excellent removal of pollutants. Once the designed catalyst is optimized for the application in a laboratory scale, the question of larger-scale use should be addressed for the integration of the new catalyst into continuous system to treat real wastewater.

## Conflict of Interests

The authors declare no conflict of interest.

## Data Availability Statement

The data that support the findings of this study are available from the corresponding author upon reasonable request.

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Manuscript received: August 2, 2023

Revised manuscript received: November 19, 2023

Accepted manuscript online: November 21, 2023

Version of record online: December 19, 2023