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Masood, Z.;  Ikhlaq, A.;  Akram, A.;  Qazi, U.Y.;  Rizvi, O.S.;  Javaid, R.;  Alazmi, A.;  Madkour, M.;  Qi, F. Nanocatalysts Used in AOPs for Wastewater Treatment. Encyclopedia. Available online: (accessed on 19 June 2024).
Masood Z,  Ikhlaq A,  Akram A,  Qazi UY,  Rizvi OS,  Javaid R, et al. Nanocatalysts Used in AOPs for Wastewater Treatment. Encyclopedia. Available at: Accessed June 19, 2024.
Masood, Zafar, Amir Ikhlaq, Asia Akram, Umair Yaqub Qazi, Osama Shaheen Rizvi, Rahat Javaid, Amira Alazmi, Metwally Madkour, Fei Qi. "Nanocatalysts Used in AOPs for Wastewater Treatment" Encyclopedia, (accessed June 19, 2024).
Masood, Z.,  Ikhlaq, A.,  Akram, A.,  Qazi, U.Y.,  Rizvi, O.S.,  Javaid, R.,  Alazmi, A.,  Madkour, M., & Qi, F. (2022, July 21). Nanocatalysts Used in AOPs for Wastewater Treatment. In Encyclopedia.
Masood, Zafar, et al. "Nanocatalysts Used in AOPs for Wastewater Treatment." Encyclopedia. Web. 21 July, 2022.
Nanocatalysts Used in AOPs for Wastewater Treatment

The increase in population demands for industrialization and urbanization which led to the introduction of novel hazardous chemicals in our environment. The most significant parts of these harmful substances found in water bodies remain in the background, causing a health risk to humans and animals. It is critical to remove these toxic chemicals from the wastewater to keep a cleaner and greener environment. Hence, wastewater treatment is a challenging area these days to manage liquid wastes effectively. Therefore, scientists are in search of novel technologies to treat and recycle wastewater, and nanotechnology is one of them, thanks to the potential of nanoparticles to effectively clean wastewater while also being ecologically benign. Advanced oxidation processes (AOPs) have been extensively studied. Advanced oxidation processes can be defined as the processes and technologies which involve the generation of active species such as hydroxyl radicals (•OH) which act as efficient oxidants to decompose pollutants in wastewater treatment. 

advanced oxidation processes challenges nanocatalysts

1. Nanocatalysts Used in AOPs for Wastewater Treatment

The nanoparticles possess a high surface area and high density of active site mainly due to their unique size ranging between 1–100 nm [1]. These unique characteristics enable nanomaterials for a variety of applications in wastewater treatment. Various nanomaterials as catalysts were used in the past for such applications; these include metals and their oxides, metal-organic frameworks (MOFs), carbon nanotubes (CNTs), and zeolites, etc. [1]. Nanocatalysts in advanced oxidation processes (AOPs) and their challenges for practical application are shown in Figure 1.
Figure 1. Nanocatalysts in AOPs and challenges for practical applications.

2. Graphene-Based Materials

Graphene is an allotropic form of graphite having a systematic honeycomb network. The reduced graphene and its modified forms were used as advanced oxidation catalysts in water and wastewater treatment since reduced graphene oxide (RGO) is less conductive than the parent form [1], and it has been observed that surface hydroxyl groups, Lewis acid sites, and π-electrons play an important role in generating hydroxyl radicals in various AOPs. Moreover, it has a reasonably high surface area and different functional groups (epoxy, carbonyl, hydroxyl) that enable graphene-based catalysts to adsorb pollutants on their surface [1] and efficiently degrade via adsorbed reactive oxygen species (ROS). The multiple pollutants may be a challenge for graphene-based catalysts, as per previous findings, the nature of the pollutant may affect the catalytic ability in AOPs. For example, pollutants may adsorb on the surface of the catalyst and block active sites. Each catalyst type may behave differently for a particular pollutant [2][3][4][5]. RGO-based nanocatalysts were studied in various AOPs to degrade a variety of pollutants. During the photocatalysis reaction, the charge transfer mechanism in the RGO/PEI/Ag nanocatalyst occurred, and the dye molecules moved from the aqueous solution to the composite surface and adsorbed with offset direct orientation via π–π coupling between MB (and RhB) and graphene aromatic sections. When UV was applied on the surface of RGO/PEI/Ag nanocatalyst, the electrons which were photoexcited had a tendency of being rapidly injected into graphene sheets and then reacted with adsorbed oxygen molecules on the surface of graphene to produce O2 or/and O2−2 radicals. In such a way, more electrons and holes could be generated by the prepared composite, and more superoxide anions and/or peroxide species produced, which disintegrated the dyes into the water, carbon dioxide, and other mineralization. As the result of the electron transfer process, recombination of charge was repressed in RGO/PEI/Ag nanocatalyst and consequently, it enhanced the efficacy of the photocatalytic properties [6].
It has been observed that graphene-based catalysts were mostly tested for the treatment of drinking water or synthetic wastewaters by AOPs. However, for large-scale applications, it is crucial to test these catalysts using real wastewater since constituents of real wastewater may affect the overall performance of these catalysts.
Another challenge to applying graphene-based nanocatalysts for water treatment is their organic nature [7]. Since AOPs involve ROS generation that may react with organic-based catalysts to denature them. It is pertinent to mention here that in most of the studies which involve the application of RGO or its modified forms, the loss of catalyst and its reactions with ROS were ignored. On the other hand, some findings indicate that the presence of hydroxyl radical scavengers (such as chlorides) may enhance the activity of RGO [7], which suggested that the catalyst reactivity may reduce in the presence of radical scavengers, and hence its performance may increase. Therefore, it is essential to apply these nanocatalysts using a real wastewater matrix. Table 1 shows examples of research conducted on applying graphene and its modified forms for wastewater treatment by AOPs. In this Table 1, the type of nanocatalysts, wastewater type, and removal efficiencies are addressed. For example, for the removal of methylene blue (MB) dye from the aqueous solution, ZnFe2O4-reduced graphene oxide was applied as a nanocatalyst in the photocatalytic process using H2O2 resulting in 70% MB removal at optimum conditions [8].
Table 1. Graphene and its modified forms as a catalyst used in AOPs for wastewater treatment.

3. Metals and Metal Oxides

Metals and their oxides were extensively implied as catalysts in both homogeneous and heterogeneous AOPs. It was reported that their surface hydroxyl groups and Lewis acid sites were the main active sites in AOPs [13][14]. Recently various modified forms of metal oxides were tested successfully as nanocatalysts for wastewater treatment. Metal oxide nanoparticles such as ZnO, TiO2, and CeO2 have been widely studied to degrade contaminants in aqueous solutions [15][16][17].
Ye et al. [18] carried out photocatalytic degradation of pharmaceuticals using TiO2 nanotube arrays (TNAs) for the removal of β-blocker metoprolol (MTP) from aqueous solution through free hydroxyl radicals. In order to elaborate on the degradation mechanism, experiments with the addition of specific scavengers were performed. In their study, the maximum contribution of reactive species to MTP degradation was estimated at 88% by free hydroxyl radicals (·OH) in bulk solution, and around 9% by hydroxyl radicals (·OH) and photo-generated holes (h+). Tert-butanol and formic acid were added as a scavenger for ·OH and h+, respectively. A major part of MTP degradation happened due to free hydroxyl radicals whereas minor degradation occurred on the catalysis surface through the reaction of h+ and ·OH adsorbed on the surface of catalysts. Other reactive species such as superoxide radical anions and photo-generated electrons participated in minor degradation of MTP over TNAs of about 3%. Due to their better photocatalytic performance and high surface area, metal oxide nanoparticles are considered better photocatalysts for water purification. Among the metal oxides, iron-based catalysts were extensively studied and were highly effective catalysts for the degradation of various environmental contaminants [15][19]. Iron oxides have advantages of recycling, reusability, and relatively lower usage cost and environmental risks. For example, higher efficiencies were obtained for the degradation of salicylic acid (20 g) using α-Fe2O3 in photocatalyst advanced oxidation process using batch mode [19].
The metals, metal oxides, and their various forms may not exist independently in aqueous environments. In the presence of water molecules, they may hydrate and form different complexes; this process may be pH-dependent [5]. Moreover, the addition of metal oxides and the contaminants present on them may alter the pH of water. Since the AOPs are pH-dependent processes, their mechanism and effectiveness (of various AOPs) depend on water pH [5]. Catalytic ozonation, Fenton-like processes, and UV-based processes are all pH-sensitive processes. For example, the ozonation process requires alkaline pH. Whereas the catalytic hydrogen peroxide decomposition using Fe-based catalysts (Fenton-like process) requires acidic pH to efficiently generate hydroxyl radicals which are necessary for the decomposition of pollutants. For this process, a pH 3 is considered as the optimum and most suitable pH regardless of the target pollutant [20]. At higher pH, Fe3+ forms Fe(OH)3 which decreases the efficiency of the Fenton process, as less Fe3+ is present to react with hydrogen peroxide to generate hydroxyl radicals [20]. On the other hand, a lower pH than 3 causes the formation of Fe complex ([Fe(H2O)6·]2+), which reacts with hydrogen peroxide in the solution, hence lesser hydrogen peroxide is available as an oxidant. In addition, at very low pH, hydrogen peroxide forms stable oxonium ions [H3O2]+, which are stable and less reactive compared to hydroxyl radicals, reducing its efficiency in oxidizing the pollutants [20]. The pH of the water may also affect the nature of active sites and the effectiveness of the catalysts. For example, the point of zero charge is an important property of a material that may determine the surface charges on a material at a particular pH and the nature of active sites (involvement of Lewis and Bronsted acid sites). Therefore, various materials have a characteristic point of zero charge [21][22][23]. Hence, it is indeed important to study the effect of pH on various materials in order to understand their ability to act as a catalyst for wastewater treatment. However, in many published works, the pH changes during the process, and due to contaminants on catalysts, were ignored. Therefore, the mentioned factor should be considered for further application of metals and metal oxides as nanocatalysts in wastewater treatment. Table 2 summarizes various studies applying metal oxides for wastewater treatment by AOPs. For example, Soltani et al. [24] applied sonocatalysis for the removal of chemical oxygen demand (COD) in textile wastewater by using the ZnO nanoparticles (catalyst dosage of 6 mg/L) at 9 pH for 150 min of reaction time resulting in 44% COD removal.
Table 2. Metal oxides in AOPs for wastewater treatment.

4. Zeolites and Modified Zeolites

Zeolites are referred to as a family of aluminosilicate materials that consist of microporous structures [27]. Zeolites were extensively investigated for the removal of contaminants in water and wastewater. Their excellent stability, adsorption, and ion exchange capabilities make them unique from other nanomaterials [28]. Most of the zeolites-based AOPs were used to remove pollutants from aqueous synthetic solutions. However, in many recent investigations, real wastewater samples were used to study the effectiveness of these materials. Ikhlaq et al. [29] used iron-loaded zeolites-A to treat municipal wastewater in catalytic ozonation-based AOP. The results revealed that about 90% reduction in COD values was achieved in 1 h ozonation (O3 = 0.9 mg/min) [30]. Another recent study showed a successful application of zeolite A to treat veterinary pharmaceutical wastewater in a synergic electro-flocculation and catalytic ozonation process [31]. In their study, the COD and turbidity removal efficiencies were compared. Moreover, the removal efficiency of identified pharmaceuticals was also investigated [31].
In most studies, zeolites were employed as support, and the metal nanoparticles were deposited on their surfaces. Most of the published work lacks the investigation of the reuse performance of zeolite-based nanocatalysts [32]. Deposited, doped, or impregnated nanoparticles may leach out in wastewater. Therefore, it is essential to consider their reuse performance and leach out the tendency of metals or metal oxides deposited on various types of zeolites. Table 3 summarizes multiple research applications utilizing zeolites for wastewater treatment. For the removal of pollutant COD from veterinary pharmaceutical wastewater Fe-zeolite A utilized as a catalyst in the synergic electro-flocculation–catalytic ozonation process. When an ozone dose of 0.4 mg/min was provided in a reactor at neutral pH and the Fe-zeolite A dosage was 1.5 g/L, maximum COD removal of 85.12% was achieved [31].
Table 3. Zeolites used as nanocatalysts in AOPs for wastewater treatment.

5. Carbon Nanotubes

Carbon is a unique and valuable element due to its many allotropes and catenation characteristics. Carbon nanotubes (CNTs) have a large surface area, which allows them to have strong chemical activity and good adsorption properties. CNTs have brought a revolution in the field of water and wastewater treatment. Therefore, these materials should be extensively applied to investigate their effectiveness and utility. CNTs have been studied under various categories such as single-walled or one-dimensional CNTs, multi-walled, and composite CNTs [1]. CNTs have been studied for their ability to remove a variety of contaminants. Many other modified CNTs used to treat various pollutants have been presented in Table 4. CNTs are highly recommended materials in AOPs due to their high removal efficiencies to treat highly resistant pollutants.
Table 4. Carbon nanomaterials and nanotubes as nanocatalysts in AOPs for wastewater treatment.
However, most of the studies found in the literature revealed that CNTs were tested in an aqueous environment by using single or multiple pollutants. Only a few studies were conducted using real wastewater to scale up the process using CNTs. Since actual conditions may be more challenging, having multiple contaminants, pollutants, interfering chemicals, and scavengers may affect a catalyst’s performance. Therefore, it is highly required to apply CNTs in real wastewater treatment. Since the real wastewater matrix may affect the ability and effectiveness of a catalyst, the real wastewater is a complex matrix that contained a variety of chemicals that can compete with the reactions of pollutants and oxidants (hydroxyl radicals). For example, carbonates, bicarbonates, phosphates, sulfates, etc., are hydroxyl scavengers. Moreover, the heavy metals and organic acids present in wastewater, if any, may adsorb on the catalyst and may block their active sites. Table 4 summarizes the practical applications of carbon nanomaterials and nanotubes as nanocatalysts in AOPs for wastewater treatment. A study conducted for the removal of TOC from real wastewater contaminated with dyes using Fe-CNTs in the Fenton-like and photo-Fenton process showed a maximum of 40% TOC removal [38].

6. Metal-Organic Frameworks

Metal-organic frameworks (MOFs) are widely utilized as fillers in water purification membranes. These materials are referred to as adsorbents to remove contaminants from the environment [41]. Many researchers focus on their applications as a catalyst in AOPs. Due to their unique properties, such as three-dimensional structures, surface areas, and metal-containing active sites, MOFs have been recently studied in various types of AOP processes (Table 5). Table 5 presents the details of MOFs that have been recently studied in various types of AOP processes. Sun et al. [42] applied catalyzed Fenton process utilizing the Fe(BDC) (DMF,F) as MOFs for the removal of an aromatic compound such as phenol from the solution. It was found that higher removal efficiency of more than 99% was achieved by this treatment process. Although this process is efficient in terms of pollutant removal on a lab scale, there is a need to evaluate the application of this process on real textile wastewater.
Table 5. Metal organic frameworks in AOPs for wastewater treatment.
However, the studies on the practical applications of MOFs are limited, and there is a need to test these materials using real wastewater samples. Moreover, it is also essential to investigate the self-degradation of MOFs in the presence of reactive oxygen species (ROS) as MOFs are organic-based materials that might react with radicals produced in AOP systems.

7. Clay-Based Materials

Various researchers investigated the clays due to their availability and economic considerations to promote the production of reactive oxygen species (ROS) to remove multiple pollutants. Among the various AOPs, wet catalytic oxidation and ozone-based catalytic processes were successfully studied to remove pollutants such as phenols and dyes [47][48]. Clays modified with various metals were widely applied to treat wastewater (Table 3). Boudissa et al. [48] suggested that protonated silanol groups (Bronsted acid sites) on clays may play an important role in the production of reactive oxygen species (ROS) while interacting with the dye molecules. Moreover, it was suggested that the charge on the dyes and surface charge on the catalyst might play an important role in the adsorption of various pollutants on the surface of the catalyst that might affect the overall efficiency [48].
Despite several successful published applications of clay-based catalysts in AOPs as summarized in Table 3, they have not been implied on a larger scale for commercial applications. This might be due to the materials’ limitations (recovery of clay waste catalyst and addition of turbidity to wastewater) and a lack of investigations with real wastewaters. The materials’ limitations include a lack of maintaining high porosity and stability, resulting in turbidity to water [49]. Moreover, the leaching of metal nanoparticles deposited to these clay-based supports is very frequently observed during AOPs which also limits their applications in wastewater treatment. These above-stated limitations affect the catalysts’ life and cause the deactivation of the catalysts. Such catalysts are not suitable for long duration processes and cannot be reused [50]Table 6 summarizes the application of clays as nanocatalysts in AOPs for wastewater treatment. Kalmakhanova et al. [36] applied catalytic ozonation to the degradation of methylene blue, methyl green, methyl orange, and methyl-thymol blue in their aqueous solutions by using an acid-treated clay catalyst. The results showed removal of 49–96% dyes achieved in 20 min of reaction time.
Table 6. Application of clays as nanocatalysts in AOPs for wastewater treatment.


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