Photocatalytic Dye Degradation from Textile Wastewater: A Review

Abstract

The elimination of dyes discharged from industrial wastewater into water bodies is crucial due to its detrimental effects on aquatic organisms and potential carcinogenic impact on human health. Various methods are employed for dye removal, but they often fall short in completely degrading the dyes and generating large amounts of suspended solids. Hence, there is a critical need for an efficient process that can achieve complete dye degradation with minimal waste emission. Among traditional water treatment approaches, photocatalysis stands out as a promising method for degrading diverse toxic and organic pollutants present in wastewater. In this review, the heterogeneous photocatalysis process is well explained for dye removal. This comprehensive review not only provides insightful illumination on the classification of dyes but also thoroughly explains various dye removal methods and the underlying mechanisms of photocatalysis. Furthermore, factors which effect the activity of the photocatalysis process are also explained in detail. Likewise, we categorized the heterogeneous photocatalyst in three generations and observed their activity for dye removal. This review also addresses the challenges and effectiveness of this promising field. Its primary aim is to offer a comprehensive overview of the photocatalytic degradation of pollution and to explore its potential for further future applications.

1. Introduction

The textile industry is one of the most prominent and mature sectors in many countries, such as Pakistan, Srilanka, India, and China, but it faces the challenge of unplanned production of wastewater effluent. During the wet fabric manufacturing process, in which different stages are involved, such as printing, dyeing, and finishing, an excessive amount of water (120–280 L) and a variety of dyes are used to accomplish the process. These synthetic pigments (dyes) are produced by coal and petroleum intermediates with an estimated annual production of 7 × 10⁵ tons. In the course of a dying process, all dyes are not firmly attached to the fabric, and almost 11–15% are discharged into the water as industrial effluent, with a rise of chemical oxygen demand (COD) and biological oxygen demand (BOD).^1,2 Therefore, to protect consumers by preventing and minimizing environmental damage, the Ecological and Toxicological Association of the Dyestuff Manufacturing Industry (ETAD) was established in 1974 by completely collaborating with the government on issues relating to the ecological effect.³ According to ETDA, mostly reactive dyes were present in the industrial effluents. Quansah et al. reported that crystal violet and methylene blue are protein-based pigments which are nondegradable in nature.⁴ These reactive dyes are harmful to human and aquatic life when flushed into the water and produce a carcinogenic effect as well as disturb the reproductive system.⁵ Furthermore, wastewater also affects crops when used for irrigation systems; therefore, proper treatment of wastewater is essential before discharging it into the environment.

Conventional dye removal methods (physical and biological processes) are insufficient for removing all dyes, especially reactive dyes. These dyes have more than one functional group, forming rigid bonding with the substrate and creating hindrances for degradation. Therefore, an advanced oxidation process (AOP) is used to remove the soluble dyes which cannot be removed through traditional methods. In AOP, photocatalysis is a promising technique to remove organic pollutants by oxidation. During the photocatalysis, the source of light and catalysts are used to accelerate the process of the degradation of organic pollutants. According to the source data, photocatalysis can remove 70–80% of pigments from industrial effluent. Several researchers are working on the heterogeneous photocatalysts such as ZnO, TiO₂, Cds, and WO₃ for dye degradation.⁶

The mechanism of photocatalysis is divided into classes: (1) Homogeneous Photocatalysis, in which reactant and catalyst are both in the same phase, usually in the solution phase. In homogeneous photocatalysis, the catalyst, often a transition metal complex or a metal-containing compound such as ruthenium or iridium complexes, absorbs light and undergoes a photochemical transformation, leading to the generation of reactive species. These reactive species, such as radicals or excited states, then initiate chemical reactions with the organic molecules present in the solution. The advantage of homogeneous photocatalysis lies in the direct and rapid interaction between the catalyst and the reactants in the same phase.⁷ (2) Heterogeneous Photocatalysis, a process where the photocatalyst and the reactants are in different phases. Typically, the photocatalyst is a solid material, such as titanium dioxide (TiO₂), zinc oxide (ZnO), or other metal oxides, while the reactants are in the liquid or gaseous phase. This type of photocatalysis involves the absorption of light by the solid photocatalyst, leading to the generation of electron–hole pairs and subsequent redox reactions with adsorbed species on the catalyst.⁸

Among classes of photocatalysis, heterogeneous photocatalysis is superior as compared to homogeneous photocatalysis for dye removal from wastewater due to several reasons. First, the ability to recover and reuse photocatalysts makes heterogeneous systems more cost-effective and environmentally sustainable for large-scale applications. Additionally, heterogeneous photocatalysis exhibits enhanced efficiency and selectivity due to the direct interaction between the solid catalyst and dye molecules, leading to higher yields of degradation products and reduced side reactions. The stability and durability of solid photocatalysts further contribute to their superiority, as they can withstand harsh reaction conditions without significant degradation. This results in longer catalyst lifetimes and improved process sustainability. Moreover, the reduced environmental impact of heterogeneous photocatalysis with minimal contamination of the effluent by catalyst residues ensures compliance with regulatory standards. Lastly, the scalability and industrial applicability of heterogeneous systems make them suitable for commercial dye removal applications in wastewater treatment facilities.⁹⁻¹²

2. Classification of Synthetic Dyes

Synthetic dyes are artificially produced colorants used in various industries, including textiles, cosmetics, and food. They are created through chemical processes and are often more vibrant and stable than natural dyes. Synthetic dyes are classified into ionic and nonionic compounds based on their chemical properties and application method.¹³

In between the two categories are ionic and nonionic dyes (as shown in Figure 1). Ionic compounds contain cationic or anionic charge ions, and cationic dyes carry a positive charge and are commonly used to color materials with negatively charged surfaces, such as acrylic fibers and certain types of paper. They establish electrostatic attractions with negatively charged groups on the substrate, resulting in robust bonding. These dyes interact with the substrate through ionic interactions, forming strong bonds with materials that possess opposite charges. Anionic dyes carry a negative charge and are often employed to color materials with positively charged surfaces such as natural fibers like cotton and wool. They form bonds with positively charged groups on the substrate through electrostatic interactions.^14,15 In contrast, nonionic dyes do not possess a net charge on their molecules. Instead, they rely on weaker forces such as van der Waals forces, hydrogen bonding, and hydrophobic interactions to adhere to the substrate. Nonionic dyes are frequently utilized for dyeing materials with neutral surfaces or surfaces that do not readily interact with charged molecules. They offer versatility in terms of substrate compatibility and can be used across a wide range of materials. Examples of nonionic dyes include dispersed dyes, which are commonly used to color synthetic fibers like polyester and nylon, and vat dyes, which are employed for dyeing natural fibers such as cotton and linen. Ionic dyes are commonly used in dyeing processes and can have significant impacts on water quality when discharged as wastewater.^16,17 The discharge of dye-containing wastewater from industrial processes poses significant threats to aquatic ecosystems. Synthetic dyes can lead to water contamination, alter their physical, chemical, and biological properties, and render them unsuitable for various uses, including drinking water sources and aquatic habitats. Additionally, highly pigmented dyes can reduce light penetration into water bodies, disrupting photosynthesis and oxygen levels and thereby affecting overall ecosystem health. Certain classes of synthetic dyes, such as azo dyes (−N=N−), exhibit toxicity to aquatic organisms, leading to adverse effects on fish, invertebrates, and other life forms. Moreover, synthetic dyes can alter water chemistry parameters, disrupt nutrient cycling processes, and persist in the environment for extended periods, posing ongoing risks to aquatic ecosystems. Effective wastewater treatment processes, regulatory measures, public awareness campaigns, and eco-friendly dyeing techniques are essential for mitigating the environmental impact of synthetic dyes on water resources.¹⁸⁻²¹

Figure 1.

Classification of synthetic dyes. Reproduced with permission from ref (14). Copyright 2023, Springer Nature.

3. Fundamental Techniques for Dye Degradation

Multiple techniques, i.e., biological, physical, and advanced oxidation methods, are utilized for the degradation of dyes from the textile wastewater; their strength and limitation concerning dye degradation are mentioned in Table 1.

Table 1. Strengths and Limitations of Different Techniques for the Dye Removal.

Techniques	Strength	Limitation	Ref
Physical Method	Good for removal of insoluble dyes	Not applicable to all dyes	(22−26)
Adsorption	Low operational cost	The high cost of some activated carbon
Membrane Filtration	Effective at lab scale	Inadequate quality for reusing the permeate
Nanofiltration	No adsorbent loss (regeneration)	Disposal of suspended effluent is difficult
Microfiltration
Ion Exchange
Coagulation
Reverse Osmosis
Biological Method	Enzymes are used for the degradation of dyes	Low removal rate	(27, 28)
Micro-organism	Eco-friendly process	Cannot degrade all types of dyes
Plant	Economically attractive method, no additional operation cost is required
Enzymes	Less sludge is produced as compared to other methods	Large space is required
	Effective for azo dyes
Chemical Method/Advanced Oxidation Process	Quick and efficient process	Sludge is produced during the process	(29−32)
Ozonation	Good elimination of pigments	High cost of electricity required
Fenton	Efficient for soluble and nonsoluble dyes	Producing aromatic chemicals, which can be harmful to human health and the environment
UV Hydrogen Peroxide	Electrons are used for the degradation of pigments
Photocatalysis

The aforementioned information makes it clear that biological techniques are quite famous because of their economic nature, but this technique is kinetically slow and cannot remove all of the dyes from wastewater. Similarly, physical techniques like adsorption, flocculation, precipitation, reverse osmosis, and coagulation are also not very effective to remove all of the dyes from the wastewater.³³ However, the advanced oxidation process (AOP) holds promise which can degrade the ionic and nonionic nature of dyes from wastewater. AOP is characterized by its nonselective nature, making it effective in degrading a wide range of organic pollutants, through the generation of highly reactive hydroxyl radicals (OH^•–). These hydroxyl radicals react rapidly with organic compounds, breaking down complex dye molecules into smaller, less harmful byproducts. AOP offers advantages such as (1) high efficiency, (2) versatility, and (3) applicability to various wastewater matrices, making it a promising approach for the treatment of dye-containing wastewater. The AOP constitutes a diverse array of methodologies designed for the degradation of organic pollutants, particularly synthetic dyes, within wastewater treatment. AOP incorporates both homogeneous and heterogeneous chemical reactions, where the catalyst and reactants may be in the same or different phases, respectively. Fenton processes, a part of AOP, leverage iron salts and hydrogen peroxide to generate highly reactive hydroxyl radicals (OH^•–), which effectively break down organic compounds. Additionally, electrochemical oxidations involve the application of an electrical current to drive oxidation reactions and produce reactive species such as hydroxyl radicals. Photolysis, using both gamma and ultraviolet (UV) irradiation, plays a significant role in AOP.³⁴⁻³⁷

Photocatalysis has emerged as a particularly superior technique for the degradation of organic pollutants, including synthetic dyes, in wastewater treatment. Photocatalysis harnesses the power of catalysts, such as titanium dioxide (TiO₂), under UV irradiation to generate highly reactive hydroxyl radicals (OH^•–). This process exhibits several advantages over conventional wastewater treatment techniques. First, photocatalysis offers exceptional efficiency in degrading organic pollutants due to the rapid generation of hydroxyl radicals upon exposure of light. Second, it operates under relatively mild conditions of low temperature and pressure, minimizing energy consumption and operational costs. Third, photocatalysis produces no harmful side products, ensuring environmentally friendly treatment of wastewater. Additionally, photocatalytic processes are readily scalable and adaptable to various wastewater matrices, making them applicable in diverse industrial and environmental settings. These inherent advantages position photocatalysis as a leading technology for the removal of synthetic dyes and other organic pollutants from wastewater, offering a sustainable and efficient solution to water contamination issues.³⁸⁻⁴⁰

4. General Mechanism of Photocatalysis for Dye Degradation

Photocatalysis is a process where a photocatalyst (TiO₂) absorbs photons (typically ultraviolet light) to generate electron–hole pairs, which then participate in redox reactions with adsorbed species on the catalyst surface, leading to the degradation of synthetic dyes in wastewater. The general mechanism of photocatalysis involves three steps:⁴¹⁻⁴³

• (1)Generation of electron–hole pairs: When the photocatalyst absorbs photons with energy greater than its bandgap, electrons are excited from the valence band to the conduction band, leaving behind positively charged holes in the valence band.
• (2)Redox reactions: The electrons in the conduction band and the holes in the valence band are highly reactive and can participate in redox reactions with adsorbed species on the catalyst surface, such as water and oxygen, or organic pollutants.
• (3)Degradation of pollutants: The reactive species, such as hydroxyl radicals (OH^•–) formed from water molecules, can react with organic pollutants adsorbed on the catalyst surface, leading to their degradation into smaller, less harmful byproducts.

The surface of the catalyst also plays a major role in the removal of dye. As the surface of the catalyst is large, the maximum amount of dye degraded during the photolysis. The decomposition of dyes on the surface of the catalyst can be done by two pathways: one is the direct photocatalytic mechanism and the second is the indirect photocatalytic mechanism.

4.1. Direct Photocatalytic Mechanism

Two methods have been suggested to explain the direct mechanism, the Langmuir–Hinshelwood method and the Eley–Rideal method.

In 1921 Langmuir was the first one who described the adsorption theory, and then in 1926 Hinshelwood refined this theory, and the adsorption theory also known as the Langmuir–Hinshelwood theory⁴⁴ is presented in Figure 2. It explains that both the electron and hole are produced by photoexcitation of the catalyst, and they react with the stuck dye molecule on the catalyst surface and form active radical species followed by dye decomposition after the combination with the electron and catalyst is regenerated.^44,45

Figure 2.

Representation of the adsorption phenomena by the Langmuir–Hinshelwood mechanism. Replicated with permission from ref (44). Copyright 2018, Elsevier.

The Langmuir–Hinshelwood equation to find the rate of degradation is:⁴⁶

where K_a is the adsorption constant; K_c is the specific rate constant; and C₀ is the initial concentration.

In 1938, Eley–Rideal described the second method, as shown in Figure 3. In this method, first the hole was trapped on the surface of the defect, and then photoexcited species react with dye which becomes an intermediate species. The intermediate compound further spoils the products or recombines with electrons.⁴⁴

Figure 3.

Representation of the adsorption phenomena by the Eley–Rideal method mechanism. Replicated with permission from ref (44). Copyright 2018, Elsevier.

The reaction scheme for dye removal is given below^45,47,48

In this reaction, h_v is the intensity of light; “S” is the surface of the catalyst; and e^•– and h^• are the electron and hole respectively, where the dot ^• represents the radical.^49,50

4.2. Indirect Photocatalytic Mechanism

In the second method, the electron and hole are both photogenerated on the surface of the catalyst as shown in Figure 4. When the beam of light (UV or solar) strikes on the surface of the catalyst, it absorbs the photon energy greater than its band gap which generates electron (e^•–) and hole (h⁺) bands, as represented below⁵¹

This mechanism is comprised of the following steps:

• (1)Photoexcitation: The semiconductor consists of the valence band (VB) and conductance band (CB). The difference is called the energy gap (E_g). When the beam of light (photon) strikes with a semiconductor, the photoelectron is transferred from the valence band to the conduction band as a result of absorption of ultraviolet radiations. Due to this photoexcitation, a hole (h⁺) is generated on the valence band and a charge carrier (e^•–) on the conduction band.
• (2)Dissociation of water: On the valence band, OH^•– is generated as a result of the reaction of photogenerated holes with water molecules. The irradiated semiconductor surface acts as a powerful oxidizing agent, and organic pollutants present on the dyed surface chemically react with OH^•– radicals and formulate a chain of radicals by consuming oxygen and after decomposition of organic matter into carbon dioxide and water.
• (3)Oxygen ion sorption: During the formation of hydroxyl radical molecules by the reaction of surface bound water and photogenerated electrons, molecular oxygen fills up the conduction band and produces an anionic superoxide radical. This superoxide not only takes part in the oxidation process but also prevents the recombination of the electron–hole.
• (4)Protonation process of the superoxide: The reduction process involves the easy reduction of oxygen and the production of hydrogen. The electrons present on the conduction band react with dissolved O₂^•– and form superoxide anions, which then attached to the intermediate product produced from oxidative reaction and resulted in peroxide or converted them into an ultimate product of hydrogen peroxide and H₂O.⁵²

Figure 4.

Mechanism of dye degradation occurring on an ideal photocatalyst imbued with dye reduction and oxidation reaction. Reproduced with permission from ref (53). Copyright 2020, Elsevier.

The detailed mechanism of photolysis in the form of an equation is represented as⁵⁴⁻⁵⁹

Finally, the dye reacts with an OH radical and for intermediate (dye degradation) and end products.

The mathematical form to find out the % age degradation of dyes is⁶⁰

where C_i is the initial concentration of dye before degradation, and C_f is the final concentration of dye after degradation.

5. Parameters Affecting the Photocatalytic Dye Degradation

There are different parameters affecting the process of degradation of dyes such as pH, temperature, intensity of light, dose of photocatalyst, initial concentration of dye, shape of the catalysts, etc.

5.1. Effect of pH

In the process of the degradation of dye by heterogeneous photocatalysis, pH is a critical operating parameter that affects (a) the charge on the surface of a photocatalyst, (b) the charge on a dye molecule, (c) photocatalyst aggregate size, and (d) the positions of the valence band and conductance band. The pH of polluted water contaminated by dye regulates the electrostatic interaction present between the substrate, catalyst surfaces, and radicals generated during the degradation process.

Chen et al. in 2007 exposed that the adsorption of malachite green (MG) dye on the photocatalyst surface was reduced when photogenerated charge carriers recombine and accelerate the charge transfer process at interfaces.⁶³ The MG is a cationic organic dye with low pH, and more H⁺ was available for adsorption and in turn masked the surface of the catalyst, preventing the photoexcitation of the semiconductor particle and reducing the generation of free radicals. So, the strong force of repulsion between the positive charge semiconductor metal oxide surface and cationic MG molecules makes it difficult for MG molecules to get adsorbed on the surface of semiconductor oxide under acidic conditions. Therefore, this repulsion decelerates the degradation efficacy of the photocatalyst at low pH. However, under basic conditions, strong adsorption of MG molecules occurs at high pH, but excessive adsorption blocks the light harvesting on the photocatalyst surface and hinders the process of photoexcitation. Gaya et al. studied the effect of change in pH on the photocatalytic oxidation proficiency of TiO₂. They witnessed the strong oxidation performance of TiO₂ at pH less than 2. However, the reaction rate tends to decrease when pH < 2.⁶¹⁻⁶³

5.2. Effect of Temperature

As the photocatalytic process is light dependent, the lower effect of temperature is observed in the range of 20–80 °C. At temperatures below 0 °C or temperatures above 80 °C, a declining trend is observed in photocatalytic activity. Thus, photocatalytic activity does not need particular temperature control. A slight benefit effect is witnessed at a high temperature. It might be due to rapid diffusion of OH^– ions from the surface of the catalyst toward the pollutant. However, the dissolved oxygen concentration of the dissolved oxygen ratio in solution tends to decrease at high temperature, and this low oxygen concentration after a certain point allows electron–hole recombination on the TiO₂ surface. Dominant electron–hole recombination takes place, except the electron acceptor like oxygen is available to absorb the photoexcited electron. Hassan et al. verified the role of temperature (25–40 °C) on the speed of anthracene degradation by using green NPs of ZnO. The degradation efficiency revealed that an incline in temperature leads to a small increase in the rate of reaction. This might be due to the amplified collision frequency of the targeted molecules. However, at higher temperatures, desorption of the contaminant from the catalyst surface can result in a low reaction rate.^64,65

5.3. Efficacy under Visible Light Irradiation

Photocatalytic degradation of the dye relies on the intensity of energy delivered by the light quanta. Absorption of a photon of energy equal to/greater than the band gap generates the electron–hole pairs in the valence and conduction band. The dye removal efficiency significantly increases with the increase of irradiation power from 16 to 32 W and then slightly increases with subsequent incline from 32 to 48 W followed by the dye removal process to be independent of the light radiation intensity. Some experiments also indicate that the speed of the dye removal process becomes almost double with an increase of light intensity from 10 to 70 mW/cm². In another study, a negative relation of light intensity with the degradation process was observed on the Congo Red dye in the range of 50–90 J/cm². Initially the dye removal rate was high until a light intensity of up to 80 J/cm², but at 90 J/cm², the degradation rate tends to drop due to thermal effects linked with an increase of temperature of the dye-tainted solution.⁶⁶⁻⁶⁸

In 2010, Yoon et al. elaborated on the potential of light for eco-friendly chemical reactions due to its nontoxic nature, absence of waste generation, and renewable sourcing. However, they noted that traditional organic photochemical processes rely on high-energy ultraviolet radiation, limiting their practicality and environmental benefits on a large scale. The researchers explored the use of metal polypyridyl photocatalysts in organic transformations to overcome this limitation. Catalysts like Ru(bpy)₃²⁺ were highlighted for their ability to harness visible light from sources such as fluorescent bulbs or sunlight to enhance practicality and eco-friendliness for industrial applications.⁶⁹

In 2023, Mugumo et al. degraded Rhodamine B (RhB) dye in the presence of visible light irradiation and heterojunction nanocomposite CuS/ZnS. This results in 97% of 5 ppm RhB dye being removed after 270 min of visible light irradiation.⁷⁰

In 2023, Alwared et al. conducted a study of the solar-induced photocatalytic degradation of reactive red and reactive turquoise dyes using TiO₂ immobilized in xanthan gum. The research found that the catalyst exhibited a high degradation efficiency, achieving 92.5% degradation of reactive red dye and 90.8% degradation of reactive turquoise dye within 120 min under solar light.⁷¹

In 2024, Karuppaiya et al. conducted a study focusing on the removal of cationic (Rhodamine B) and anionic (Eosin yellow) dyes utilizing nanocomposite SnO₂–Zr–F under solar light exposure. The research observed a significant degradation of both dyes, with a maximum degradation efficiency of 92% for cationic dye (Rhodamine B) and 98% for anionic dye (Eosin yellow) achieved over a 150 min period under sunlight.⁷²

5.4. Effect of Photocatalyst Loaded Amount

The concentration of the photocatalyst in a wastewater has a noteworthy effect on the reaction rate as well as dye degradation capability. Increasing the loaded amount of photocatalyst generates additional electron–hole pairs and leads to a subsequently higher and faster dye degradation process. However, at higher concentrations, high turbidity of wastewater increases the light scattering tendency and decreases its penetration power in the reaction mixture. Hosseini and colleagues in 2018 reported the increased photocatalytic activity with increase of concentration, but the photocatalytic activity and decolorization efficiency started to decrease after a further increase in the amount of photocatalyst. In recent times, the negative effect of composite concentration on the degradation process was reported. The study of degradation of para-nitrophenol was scrutinized against different concentrations of composite dosage (0.05, 0.15, 0.25, 0.35, and 0.45 g/L), and the best removal tendency (97%) was attained at the optimized composite concentration of 0.25 g/L. The decrease in removal activity at higher concentration was due to lessened penetration and amplified light scattering. In 2015, Tahir et al. reported that with an increase of Ag NP concentration from 2–8 mg/mL the MB decomposition raised from 40 to 96%. At lower dosage, the particles are normally very small and extremely dispersed, which delivers a huge surface area with more active sites and results in increased dye degradation efficiency. A further upsurge in the catalyst amount, from 8–12 mg/mL, does not further support the degradation process due to agglomeration of large size Ag NP catalysts and consequent low surface area and reduced number of active sites.⁷³⁻⁷⁵

5.5. Effect of Dye Concentration

The adsorption of pollutants (dyes) on the surface of a photocatalyst depends on the binding affinity and electrostatic interactions between the catalyst and dye molecule. Adsorption of a moderate amount of dye molecules on the surface of the photocatalyst is beneficial for the process of degradation based on the synergies between photocatalysis and the adsorption process. Composite materials possessing upright adsorption properties exploit a synergistic effect for concurrent adsorption and dye degradation. However, a high concentration of adsorbed dye molecules on the surface of the catalyst overpowers the quantity of photons approaching the surface-active sites. Moreover, dye molecules can also work as sensitizers which absorb the electrons and then scatter them in an unwanted direction. The synergistic effect between these two mechanisms for methylene blue degradation was reported by Anwer and colleagues. A composite of graphene oxide with TiO₂ caused 50% augmentation in the photodegradation of dye compared to TiO₂ alone. The concentration of dye after the optimized amount seems to be disadvantageous for reaction.^74,76

5.6. Effect of Electron Acceptors

One practical issue with the utilization of the TiO₂ photocatalyst is the loss of energy during electron–hole recombination that results in squat degradation efficiency. Henceforth, the inhibition of electron–hole recombination was found to be very important. Molecular oxygen is normally employed as an operative e^– acceptor in photocatalytic applications. In heterogeneous conditions, molecular oxygen from air was selected as an electron acceptor to restrict the electron–hole recombination. Another approach to mitigate electron–hole recombination in photocatalysis is by incorporating hydrogen peroxide (H₂O₂) as an electron acceptor. H₂O₂ serves as an electron scavenger, effectively trapping electrons from the conduction band of the photocatalyst, thereby preventing their recombination with holes and enhancing the efficiency of photodegradation. This process has several beneficial effects. First, by accommodating the electrons from the conduction band, H₂O₂ effectively reduces the likelihood of recombination, allowing more electrons to participate in the degradation reactions. Second, H₂O₂ increases the concentration of hydroxyl radicals (OH^•–) through reactions with holes or other intermediates. Hydroxyl radicals are highly reactive species and play a crucial role in the degradation of organic pollutants. In addition to H₂O₂, other suitable options for electron scavenging and recombination suppression include persulfate ions (S₂O₈^–2) and bromate ions (BrO^–). These species act similarly to H₂O₂ by capturing electrons and preventing their recombination with holes, thereby enhancing the overall photodegradation efficacy.^63,77,78

5.7. Effect of Intermediate Species

The degradation of dyes through photocatalysis involves the generation of reactive oxygen species (ROS) and other intermediates, each playing a vital role in the overall process. Reactive oxygen species like hydroxyl radicals (OH^•–), superoxide radicals (O₂^•–), and hydrogen peroxide (H₂O₂) are instrumental in breaking down dye molecules into less toxic or nontoxic compounds. However, the presence of inorganic anions, such as carbonates (CO₃^2–), chlorides (Cl^–), nitrates (NO^3–), and sulfates (SO₄^2–), can impact the photocatalytic degradation process by scavenging hydroxyl radicals (OH^•–) and other ROS, thereby reducing their concentration and efficiency. Conversely, the addition of oxidizing agents such as hydrogen peroxide (H₂O₂), ammonium persulfate ((NH₄)₂S₂O₈), and potassium bromate (KBrO₃) can enhance the photocatalytic degradation of dyes by providing additional oxidizing power. It is crucial, however, to carefully control the concentration of these agents to avoid inhibiting the photocatalytic process. In summary, the influence of intermediate species on photocatalytic dye degradation is intricate and contingent on specific conditions and reactants. Proper management of these factors is essential to optimize the efficiency of dye degradation and minimize the formation of undesirable byproducts. While the ideal outcome of a photocatalytic reaction is the production of water and carbon dioxide, the breakdown of organic dyes can yield intermediate molecules that may have more detrimental effects than the original dye compound. Monitoring and understanding these transitional products are crucial, as they can pose potential risks.

Under ideal situations, the photocatalytic reaction irradiated by UV light yields water and carbon dioxide as the final products. However, the organic dyes due to their large structures can be broken down into smaller molecules during the process of photocatalytic degradation, and these intermediate molecules can have a more detrimental effect than the original dye compound. Generally, the reaction intermediates are not monitored, and no work is done to explore the transitional products formed during dye degradation. Chen et al. studied the degradation of N,N,N,N-tetraethylsulforhodamine-B dye and recognized the reaction intermediates possessing a dissimilar number of N-ethyl groups. These reaction intermediates were radically identical with parent dye molecules. However, it would be erroneous to assume that the same situation will happen in all reactions. These transitional products can be more dangerous than the parental dye. For example, one of the side products of phenol degradation is catechol, and convulsions and hypertension developed in animals from catechol are greater than from phenol.^79,80

5.8. Effect of Inorganic Ions on Photodegradation

The efficiency of the photodegradation processes is influenced by the type and concentration of inorganic ions in the solution. Inorganic ions have the potential to either enhance or inhibit the photocatalytic activity of TiO₂, depending on their specific characteristics and concentrations. For instance, chloride (Cl^•–) and sulfate (SO₄^•2–) ions have been observed to enhance the degradation efficiency of TiO₂. Conversely, ions, such as carbonate (CO₃^•2–) and phosphate (PO₄^•3–), can impede the photocatalytic activity of TiO₂. Furthermore, the presence of inorganic ions can also impact the adsorption of dyes onto the TiO₂ surface, thus influencing the overall degradation process.⁸¹ Hu et al. performed the experiment to degrade the cationic blue (CBX) and red MX-5B which are the subclass of azo dyes using the inorganic ions SO₄^•2–, H₂PO₄^•–, ClO₄^•–, and F^•–. They observed that at pH 2.4 the decolorization rates of MX-5B and CBX are enhanced on the surface of TiO₂. At pH 10.8, most of the selected anions inhibited the photocatalytic oxidation to decolorize and degrade CBX and MX-5B. These results demonstrated that inorganic anions affect the photodegradation of dyes by their adsorption onto the surface of TiO₂ and trapping positive hole (h⁺) and OH. Inorganic cationic ions, such as Cu^•2+ and Ni^•2+, had strong inhibition on the decolorization of MX-5B at pH 10.8. In strong basic conditions (pH 10.8), the main anions in solution were HPO₄^•2– and PO₄^•3–. These anions reacted with OH to form HPO₄^•– and PO₄^•2–, which are somewhat less reactive. The presence of these anions had a stronger inhibition effect on the photodegradation of dyes at high pH conditions.⁸²

5.9. Impact of Photocatalyst Morphology on Degradation

The morphology of photocatalysts significantly influences the degradation of dyes during the photocatalytic processes. Various morphological characteristics, such as surface area, porosity, crystallinity, and particle size, can impact the efficiency of dye degradation. Catalysts with high surface area-to-volume ratios generally exhibit enhanced photocatalytic activity due to the increased availability of active sites for adsorption and reaction of dye molecules. Additionally, well-defined crystalline structures with specific facets can facilitate efficient charge separation and migration, leading to improved degradation kinetics.^83,84

In 2023, Cigdem et al. conducted a study where they designed different morphologies of zinc oxide (ZnO) nanostructures, including nanoflower (NF), nanosponge (NS), and nanourchin (NU), to investigate their photocatalytic capabilities for dye degradation. Among these morphologies, ZnO NSs exhibited notably superior performance in photocatalytic dye degradation compared to that of the others. The photocatalytic activity of ZnO nanocatalysts was influenced by factors such as defect structure, pore diameter, and crystallinity. Their research study emphasized the importance of developing ZnO catalysts with fewer core defects, increased oxygen vacancies, near band emission, larger crystallite size, and larger pore diameter to achieve optimal photocatalytic activity in dye degradation.⁸⁵

In 2023, Das et al. conducted a study on Ga₂O₃ photocatalysts for dye (RhB) degradation. They designed different morphologies of Ga₂O₃ photocatalysts such as spherical nanoparticles and fractured nanobricks using various Gallia precursors. It was observed that the nanobricks originating from nitrate salt showed superior efficiency in degrading rhodamine B (RhB) compared to the nanospheres produced from the chloride salt. The nanobricks exhibited a degradation rate constant of 0.0394 min^–1, while the nanospheres had a lower degradation rate constant of 0.0057 min^–1. This disparity in degradation performance was attributed to differences in electronic band positions and morphological features of the photocatalysts. The nanobricks possessed higher specific surface area, porosity, and aspect ratio, facilitating faster degradation of RhB.⁸⁶

5.10. Photocatalyst’s Resistance to Photocorrosion

The stability and durability of a photocatalyst material are crucial factors for ensuring its long-term performance and cost efficiency, particularly in applications such as water and wastewater remediation. Photocatalyst corrosion resistance denotes the ability of a photocatalyst material to withstand degradation or deterioration upon exposure to corrosive environments or during photocatalytic reactions. Corrosion in photocatalysts can arise from chemical and electrochemical reactions induced by photoactivation, resulting in the dissolution of the material. Photocorrosion is a prevalent concern for semiconducting materials like zinc oxide (ZnO), which can limit their practical utility in photocatalysis.⁸⁷

In 2023, Warren et al. conducted a study where they doped zinc oxide (ZnO) with 1% and 2% Co, Ni, or Cu salts, resulting in a notable enhancement in stability with a significant reduction in zinc ion leaching following exposure to light. However, this modification also caused a decline in the material’s photocatalytic activity. They explained that the doping process caused a decrease in the band gap of the material and increased its resistance to photocorrosion. Nonetheless, the decrease in photocatalytic activity was attributed to alterations in the material’s energy level positions. Further research is warranted to maintain the photocatalytic efficiency of ZnO while simultaneously enhancing its stability in water.⁸⁸

5.11. Strategies for Photocatalyst Regeneration

The primary aim of photocatalyst regeneration is to restore the catalytic functionality of the photocatalyst after it has been deactivated or saturated with contaminants. This regenerative process facilitates the reuse of the photocatalyst, reducing the need for frequent replacement and minimizing the generation of waste. The restoration of the photocatalyst enhances the efficiency and effectiveness of photocatalytic processes, ensuring continuous and sustainable operation. In the specific context of water purification, where the photocatalyst interacts with pollutants, regeneration becomes particularly vital to revive the catalyst to an active state for the ongoing degradation of pollutants.^89,90

In 2014, Miranda et al. investigated the photocatalytic degradation of emerging contaminants using TiO₂ photocatalysts immobilized on glass spheres. Their study involved evaluating the performance of these photocatalysts over four cycles and exploring methods for recovering the photoactivity. The results revealed the efficient removal of fluorine-containing contaminants, although compounds with amine or amide groups exhibited lower degradation rates. Regeneration techniques utilizing H₂O₂/UV or calcination were found to be effective in restoring photocatalytic activity. The study also underscored the preferential adsorption of certain compounds on the active sites of the photocatalyst.⁹¹

In 2022, Kim et al. explored the use of TiO₂-coated zeolite (TiO₂/zeolite) as a photoregenerative adsorbent for volatile organic compound (VOC) filters, aiming to prolong their service life. Through their experiments, they found that the TiO₂/zeolite filter demonstrated a photoregeneration efficiency exceeding 90% during the initial two regeneration cycles under ultraviolet (UV) illumination. The presence of TiO₂ on zeolite significantly contributed to enhancing the regeneration efficiency of VOC filters based on zeolite. Compared to filters solely composed of bare zeolite, the TiO₂/zeolite filters exhibited notably improved regeneration efficiency, suggesting their potential suitability as adsorbents for UV-regenerative air filtration systems capable of multiple uses. Additionally, the TiO₂/zeolite filter maintained a photoregeneration efficiency of over 60% for up to five cycles, further supporting its candidacy as a promising adsorbent for photoregenerative VOC filters.⁹²

In their 2022 study, Yong et al.⁹³ introduced a formic-acid-mediated regeneration approach for spent V₂O₅–WO₃/TiO₂ catalysts, enabling the removal of toxic arsenic (As) while preserving the catalytic activity of the catalysts. The regenerated catalysts exhibited a specific activity of 98.3% compared to fresh catalysts, achieving 99.1% As removal and less than 1.8% vanadium (V) loss within a 15 min time frame.

5.12. Applicability to Real Industrial Wastewater

Photocatalytic technologies have emerged as effective solutions for industrial wastewater treatment, employing advanced materials and processes to eliminate contaminants from water. The primary emphasis is on enhancing photocatalytic efficiency and devising economically viable approaches that are tailored for industrial use. Investigating metal-oxide-based semiconductors, including TiO₂, SnO₂, CeO₂, ZrO₂, WO₃, and ZnO, researchers aim to leverage these photocatalysts in degrading organic pollutants within wastewater.^93,94

In their 2023 study, Mei et al. investigated the challenges still preventing the widespread use of photocatalytic wastewater treatment technology in industries. They found that even though scientists have been working on photocatalysis for over three decades it is still not widely used because of various challenges. They pointed out that the efficiency of photocatalysis is not good enough, and making the treatment reactors costs a lot. To fix these problems, the researchers suggested some practical ideas, like making better photocatalysts and designing reactors that work well and can be used on a large scale.⁹³

6. Composition of Photocatalysts

A large variety of photocatalysts (metals, metal oxides, carbon-based materials, semiconductors, quantum dots, magnetic cored dendrimer, metal–organic frameworks (MOFs), and other materials) are extensively studied for the dye degradation process in wastewater. Among the innumerable materials, we mainly focus on three generations of metal oxide for this process.

6.1. First-Generation Photocatalysts

Among the three generations, the first-generation photocatalysts (Figure 5), also known as single-component metal oxide photocatalysts, comprise TiO₂, ZnO, ZrO₂, ZnS, SnO₂, CuO₂, and NiO, etc. The operational mechanism of these photocatalysts generally depends on electron–hole pair generation under the source of UV–visible light. Among the entire single-component metal oxide photocatalysts, TiO₂ and ZnO and particularly TiO₂ are extensively used.

Figure 5.

Representative schematics of the mechanism of single component metal oxide photocatalyst. Reprinted with permission from ref (102). Exclusive common copyright 2020, Elsevier.

TiO₂ mostly exists in three phases, anatase, brookite, and rutile. However, anatase is crystalline and preferably used for dye removal due to its better adsorptive affinity and higher photocatalytic affinity.^95,96 This semiconductor acts as a good photocatalyst due to its inherent qualities like noncorrosive, nature, economical, and complete degradation of almost all the dyes in the presence of light.^97,98

Liu et al. in 2006 synthesized TiO₂ (anatase) by a chemical vapor deposition method for the removal of C.I. Acid Yellow17 dye. The highest dye degradation was achieved at a lower pH value and the highest intensity of light. At pH = 3, almost 70.6% dye was removed within 375 min in the presence of UV light.⁵⁸

Gnanasekaran et al. in 2017 performed an experiment by using different metal oxides, namely, CeO₂, CuO, NiO, Mn₃O₄, SnO₂, and ZnO for the degradation of textile dyes such as methyl orange and methyl blue by using the UV–vis light. The dose of catalyst (nanoparticles) was 100 mg/100 mL solution for the experiment carried out for 2 h, and the sample was collected during the process of photocatalysis after 20 min. The results indicate that ZnO is a highly effective catalyst for the removal of methyl orange and methyl blue as compared to other metal oxides.⁹⁹

Al Qarni et al. in 2019 worked on eco-friendly green catalyst TiO₂ synthesized from the coffee husk extract. The structure of green nanocomposites observed by scanning electron microscopy confirms that the green TiO₂ has macro- and micropore (8–10 nm) channels and possesses a band gap of 3.21 eV, close to the band gap of TiO₂ obtained from ethanol. The absorbance of methylene blue by using the TiO₂ (coffee husk extract) was rapid in the presence of irradiation (λ_max = 668 nm) as compared to that of TiO₂ of ethanol. Moreover, the low cost green catalyst could be regenerated and recyclable during the reaction, and it was found that the recycled catalyst can cause 98% dye degradation from the wastewater.¹⁰⁰

In 2020, Mirgane et al. described the ecofriendly catalyst ZnO synthesized from leaves of Abelmoschus esculentus Linn (lady finger leaves) at room temperature and pH of 9–11. The SEM and TEM analyses confirm the polycrystalline structure of ZnO. The ZnO NPs were tested for removal of methylene blue and methyl orange. The solution turns to colorless after dye degradation, and the rate of degradation increases with the passage of time (96% at 540 min). The results concluded that that this catalyst is not only eco-friendly but also very cheap and recyclable 3–4 time with good efficiency.¹⁰¹

Barakat et al. in 2011 removed Procion yellow H-EXL (commercial dye) from the wastewater by using the TiO₂ suspension as a photocatalyst in the presence of UV light (100 W). They quantified that 100% degradation of dye was achieved by using only 1 g/L dosage of photocatalyst (TiO₂), at pH ∼ 5 with the 10 mg/L concentration of dye.¹⁰³

Chakarabati et al. in 2004 reported a ZnO photocatalyst for the removal of methylene blue and Eosin Y.A. from the aqueous solution in the presence of UV light (16 W). They illustrated that the dose and pH of the catalyst are directly proportional to the removal of the dyes. As the dose of catalysts is increased from 0.2 to 1 g, the % age dye degradation increased from 47 to 74% for Eosin Y.A. degraded and 58 to 76% for methylene blue during the 2 h reaction time. Meanwhile, as the pH increased from 5.5 to 9.7, the rate of degradation increased from 49 to 62%. However, in contrast as the concentration of dyes increased in the water the degradation of dyes decreased from 87% to 40% in the case of methylene blue, while Eosin Y.A. degraded from 93 to 63%.¹⁰⁴

Mrunal et al. in 2019 synthesized Cu₂O via the green synthesis approach and found that 97% methylene blue destruction occurred at pH 5.2 of the aqueous solution under the 250 W UV lamp in 120 min.¹⁰⁵

Raheem et al. in 2016 utilized ZnO as a photocatalyst for the desolation of reactive green dye. They proved that 94.14% dye was destructed in the presence of 0.12 g of ZnO, while the concentration was 40 ppm.¹⁰⁶

However, the first generation has certain limitations such as: (a) the catalysts of this generation, TiO₂, ZnO, and ZrO₂, have a large energy gap (mostly ≥3.0 eV) between CB and VB that requires only UV light for photoexcitation; (b) photocatalysts have a single VB and CB, so as the photostability increases, these electrons go back, and the process of degradation of dyes stops and in turn the efficiency of the photocatalysts decreases; and (c) these photocatalysts are used for limited dye removal, not for all dyes.

6.2. Second-Generation Photocatalysts

To address limitations of first-generation photocatalysts, second-generation photocatalysts were developed, featuring multicomponent combinations that significantly alter charge carrier dynamics. These photocatalysts are divided into two categories: homojunction and heterojunction photocatalysts. In homojunction photocatalysts, the semiconductor layers are composed of the same material but may have different doping levels or compositions in various regions, leading to the formation of a junction within the material itself. When these photocatalysts are exposed to light, electrons are excited from the valence band (VB) to the conduction band (CB), generating electron–hole pairs. Due to the built-in potential gradient within the semiconductor material, electrons are directed toward the region with higher CB energy, while holes migrate toward the region with lower VB energy. At the junction, electrons accumulate in the CB of the higher energy region, while holes accumulate in the VB of the lower energy region. This spatial separation of charge carriers at the junction effectively reduces recombination, thereby promoting efficient photocatalytic reactions. In contrast, heterojunction photocatalysts are designed to form junctions between different semiconductor materials, offering several advantages such as improved charge separation, enhanced catalytic activity, and broader light absorption due to varied bandgaps. In these photocatalysts, two distinct semiconductor materials are brought together, each with its own valence band (VB) and conduction band (CB) energy levels. Typically, the energy levels of one semiconductor differ from those of the other, resulting in a band offset at their interface. When illuminated, photons with energy exceeding the bandgap of the semiconductors generate electron–hole pairs within both materials. Due to the band offset, electrons migrate from the CB of one semiconductor to the CB of the other, while holes move in the opposite direction. This separation of charge carriers prevents recombination, ensuring that more electrons and holes are available for participation in photocatalytic reactions. Homojunction photocatalysts find applications in areas such as water purification, air pollution control, hydrogen production, and solar energy conversion. Heterogeneous photocatalysts are utilized in similar fields but are particularly advantageous for wastewater treatment, pollutant degradation, and environmental remediation due to their enhanced charge separation and catalytic activity at junction interfaces.^107−109

In Figure 6, a heterogeneous photocatalytic phenomenon is illustrated wherein electrons are confined in the conduction band (CB) of one semiconductor, while holes are confined in the valence band (VB) of another semiconductor. The proposed mechanism depicts electron transfer from the CB of one semiconductor (with a high energy level) to the VB of another semiconductor (with a low energy level), or vice versa, wherein holes move from the VB of one semiconductor (with a low energy level) to the other (with a high energy level). The distinct electronic configurations lead to the generation of active sites, facilitating the degradation of dyes, achieving nearly 99% removal in a short duration. Graphitic carbon nitride (g-C₃N₄) and Ag₃VO₄ serve as exemplary instances of second-generation photocatalysts.⁹⁸

Figure 6.

Representative schematics of the two-component heterojunction photocatalyst. Reprinted with permission from ref (102). Exclusive common copyright 2020, Elsevier.

Rodríguez León et al. in 2016 synthesized the gold nanoparticles by using natural zeolite and ascorbic acid as a reducing agent. The synthesized green nanocomposite of Au–zeolite possesses a size of 2 nm and is used for the degradation of methylene blue (MB) under LED and sunlight. The dose of catalyst was 0.15 g. The achieved degradation of methylene blue was 87% under LED light within 45 min, while the degradation of methylene blue was 100% under sunlight within 30 min. The reason for this response was that the catalyst (Au–zeolite) absorbed the oxygen into the pores of zeolite and excited the electrons on the surface of Au, which caused the formation O₂^– species. As a result of the electron transfer, a partially positive charge is produced at the 5d level. The promoted electron transfer that can be neutralized by the electrons of organic molecules causes the degradation of the dyes. The results indicate that this low cost catalyst can be reused during the process and can remove the dyes 100%.¹¹⁰

Magdalene et al. in 2017 described another photocatalyst (Ce), which shows the best activity due to its shape, size, phases, and crystallographic properties. It is an environmentally friendly material, and the only limitation is that it does not produce satisfactory results alone for the degradation of dyes. So, the composite of cerium oxide with yttrium oxide (CeO₂/Y₂O) was synthesized by the hydrothermal process and structurally characterized by X-ray diffraction (XRD). Analysis confirms that it is a crystalline cubic face structure and has a larger amount of reactive oxygen species (ROS). For the removal of rhodamine-B (RHB), the CeO₂/Y₂O composite in 2:1 weight ratio was used under UV-light and as well as under visible light at different pH (3, 7, and 9). The results indicate that at different pH levels (9, 7, and 3) the degradation of dye was 98%, 95.7%, and 92.0%, respectively. When the pH was 9, the solution became colorless within 120 min; at pH 7, the solution became colorless within 180 min; and at pH 3 the degradation of dye was achieved within 180 min. The same experiment was repeated with CeO₂/Y₂O (2:1) at pH 9 under visible light. For the decline in absorption spectra after 553 nm with an increase of intensity of visible light, it was found that degradation of dye (RHB) was 98%, which was due to the increase in pH and surface area of the catalyst with high vicinity of oxygen which increases the efficiency of catalyst for the removal of dyes.¹¹¹

In 2019, Zuorro et al. reported that reactive violet 5 and azo dyes are widely used in textile industries, and their presence in wastewater also increases the harmful human health effect; so, the removal of these dyes is an important issue. The nanocomposite particle Fe-doped TiO₂ with a bandgap of 2.63 eV was utilized by them under visible light (LED) with the wavelength range of 460–470 nm, and hydrogen peroxide was added as a strong oxidizing agent. The experiment was performed by addition of 10 to 100 mM H₂O₂ to a dye solution at pH 10, while the concentration of dye was 30 ppm and catalyst load amount 3 g/L. The experiment reveals that due to hydrogen peroxide the reaction becomes fast and makes the solution colorless. It is due to reaction of H₂O₂ with TiO₂ and production of peroxy compounds that degrade the dye more efficiently.¹¹²

In 2020, Hua et al. reported that C-doped TiO₂ produces better results than TiO₂. The basic reason is that in the case of single TiO₂ mostly UV light has been used in various experiments, but C-doped TiO₂ not only makes possible the transfer of light from UV to visible but also gives a better result for the removal of dyes. Hydrothermally synthesized C-doped TiO₂ shows 90% degradation of organic pollutant under visible light in a short time due to its conductive nature.¹¹³

In 2021, Cruz Arias et al. reported the gold-doped TiO₂ particle (Au-TiO₂) for the removal of dyes. The nanocomposite of Au-TiO₂ synthesized by using a titanium oxide precursor was tested at pH 3 for 6 h. The doping of gold was carried out by the deposition of HAuCl₄·3H₂O on the surface of a semiconductor. Methylene blue (MB) in different concentrations of 5, 10, 15, 20, 25, 30, 35, 40, 45, and 50 ppm was used as a model sample to check the performance of the catalyst (Au-TiO₂) under the UV-light irradiation, and 97% dye removal was observed with an optimized dose of 30 ppm.¹¹⁴

Although the second-generation metal oxide photocatalysts are very effective and increase the % age degradation of dyes, there are some limitations such as C-doped TiO₂ which produces improved results as compared to single TiO₂, but the perpetration of C-doped TiO₂ is more difficult due to the crystal lattice structure of carbon. Therefore, future research work and development is needed to make new strategies to construct the more reliable C-doped TiO₂ with high doped amount and controlled distribution of carbon. Besides this, an extra amount of energy and cost is required for the effluent treatment to recover the second-generation photocatalyst.¹¹³

Saroyan et al. in 2019 illustrated that graphene oxide tapping on manganese oxide (Go-MnO) was considered a superior catalyst for the removal of organic contents due to the presence of graphene which is a strong carbonaceous material. They performed the experimental work for the removal of Reactive Black5 (RB5) in the absence of light irradiation and found that 98% of dye degraded by using only 20% manganese oxide.¹¹⁵

Ajmal et al. in 2016 designed a novel catalyst, Cu₂O-CuO/TiO₂, for the degradation of textile dye. They observed that under the optimal conditions almost all dyes were degraded. They reported that by using 125 mg/L of catalyst dose and at 5 mg/L dye concentration almost all the commercial dyes and specifically reactive blue 49 (RB49) were 100% degraded from the wastewater.⁵³

Ma et al. in 2015 synthesized a nanocomposite photocatalyst Cu₂O/ZnO by the coprecipitation method for the removal of orange II dye. They reported that by using the 200 mg/L dose of catalyst for the 50 mg/L initial concentration of dyes almost 80% of dye was removed from the aqueous solution.¹¹⁶

In 2020, Taddesse et al. designed a ternary metal oxide photocatalyst (Cu₂O/ZnO/Ag₃PO₄) by a coprecipitation method for the removal of dye from the textile effluent. They illustrated that ternary heterogeneous metal oxide destructed 81.1% of methyl orange from the textile wastewater.¹¹⁷ Magdalane et al. in 2017 synthesized a photocatalyst CeO₂/Y₂O₃ by the hydrothermal method for the degradation of dyes from urban wastewater. They observed that within 150 min under visible light at pH 9, by using 40 mg of catalyst, 94% of rhodium blue was removed from the wastewater.¹¹⁸

6.3. Third-Generation Photocatalysts

As in the second-generation system, there is some difficulty to remove the photocatalyst from the residue, so extra energy and cost are required for the separation; however, still all amounts of photocatalyst cannot be separated. So to avoid the separation process, the researchers reported that if a metallic photocatalyst can be attached to an organic functional group of the membrane then the separation and regeneration of photocatalysis becomes easy. So the discovery of third-generation photocatalysis gives a better solution. Third-generation photocatalysis (Figure 7) is presently attractive due to its excellent certain properties such as (i) huge surface area, (ii) high absorption tendency, (iii) complete degradation of dye, and (iv) high structure stability.¹¹⁹

Figure 7.

Representative schematics of the properties of three components of metal oxide (3rd generation). Reprinted in part with permission from ref (102). Exclusive common copyright 2020, Elsevier.

Floating photocatalysts are specialized materials designed for wastewater treatment, engineered to remain suspended on the water’s surface. These photocatalysts are typically affixed to buoyant substrates such as foam or lightweight materials, allowing them to float instead of sinking. This unique design maximizes their interaction with water pollutants and exposure to sunlight, which serves as an energy source for the photocatalytic reactions. By enhancing the contact between the photocatalyst and contaminants, floating photocatalysts efficiently degrade organic pollutants and other harmful substances in wastewater.¹²⁰ Nasir et al. note that these innovative photocatalysts utilize full solar irradiation in nonoxygenated and nonstirred reservoirs. Various fabrication techniques, such as sol–gel, surface coating, and hydrothermal methods, have been employed to synthesize these photocatalysts. Immobilizing potent photocatalysts on floatable substrates enhances their effectiveness by increasing the active surface area, optimizing light utilization, and facilitating maximum contact with pollutants. This approach holds great promise for efficient and sustainable wastewater treatment applications.¹²¹

In 2019, Dalponte and colleagues introduced a novel floating catalyst, TiO₂/CaAlg, demonstrating outstanding performance under UV light with an impressive 89% decolorization rate. The researchers emphasize that this easily recoverable floating photocatalyst holds promise for enhancing the photocatalytic treatment of industrial wastewater. The design allows for efficient photoactivation without the need for mechanical stirring, offering a practical and effective solution for wastewater remediation.¹²²

Khan et al. in 2012 revealed the single-step photocatalytic activity of CdS/ZnO and CdS/Al₂O₃ prepared using the hydrothermal process in the presence of graphene oxide (GO). During the experiment methyl orange was used as a reference dye and visible light as the source of light, and their photocatalytic activity after 10 and 60 min was observed. It was noticed that CdS/ZnO/GO and CdS/Al₂O₃/GO both show the highest efficiency for the degradation of organic dyes (methyl orange). However, after 10 min, the degradation efficiency of the CdS/ZnO/GO and CdS/Al₂O₃/GO was 85% and 51%, respectively, while after a 60 min interval, the CdS/ZnO/GO and CdS/Al₂O₃/GO dye degradation efficiency was 99% and 90%, respectively. The results showed that photocatalytic properties increased due to greater surface area and effective separation of photoinduced charge carriers by using the sheet like nature of GO.¹²³

In 2019, Ahmed et al. synthesized a Sn-WO₃/g-C₃N₄ composite in different compositions by using the calcination method and used for the removal of dyes. In all composites different amount of tin was incorporated and structure was thoroughly studied through SEM to confirm the uniform distribution of Sn-WO3 on g-sheet material. Among all composites, the 8% Sn-WO₃/g-C₃N(8-SnWg) structure was well ordered and more suitable for dye removal. Furthermore, the binding energy of molecular oxygen was checked by X-ray photoelectron spectroscopy (XPS) and it was found that the higher molecular oxygen adsorption on the surface of the as-prepared composite material can easily remove the organic components. The experiments performed by using 8-SnWg as a catalyst for the removal of cationic dye (Rhodamine B) and anionic dye (Methyl orange) under the visible light showed that cationic dye (Rhodamine B) removed was almost 87% in 120 min and anionic dye (Methyl orange) removal was 99% within 50 min and it proved that molecular oxygen plays an important role in the degradation of dye.¹²⁴

Jangam et al. in 2021 synthesized a novel compound of Zn_1–xCo_xFeMnO₄ with different contents of Zn with Co (x = 0.0, 0.25, 0.50, 0.75, 1.0) by using the sol–gel method with the addition of glycerin. The prepared samples labeled as SF-1 (X = 0.0), SF-2 (X = 0.25), SF-3 (X = 0.50), SF-4 (X = 0.75), and SF-5 (X = 1.0) possess bandgaps of 0.74, 2.66, 2.58, 2.24, and 2.41 eV, respectively, and these compounds were used to investigate the dye degradation efficiency under sunlight. For methylene blue (concentration 10 ppm), the observed the kinetic rate of SF-1, SF-2, SF-3, SF-4, and SF-5 was 0.063, 0.068, 0.074, 0.106, and 0.091 min^–1, respectively. The final data prove that the rate of degradation of methylene blue was a maximum with SF-4 (0.106 min^–1) that was attributed to the low band gap of 2.24. Further experiments carried out different metal-doped Zn ferrite nanocomposites under different sources of light to confirm that the efficiency of dye removal by the Zn_1–xCo_xFeMnO₄ nanocomposite was best, as it can degrade maximum dye within 60 min under sunlight.¹²⁵

In the same year, Anandkumar et al. introduced a novel single-phase multicomponent equiatomic oxide for the reduction of dyes under the exposure of sunlight. The nanoparticles of Gd_0.2La_0.2Ce_0.2Hf_0.2Zr_0.2O₂ and Gd_0.2La_0.2Y_0.2Hf_0.2Zr_0.2O₂ synthesized by a simple coprecipitation method at 500 °C were named as photocatalysts GLCHZ-500 and GLYHZ-500. The analysis of these compounds by XRD, UV–vis spectroscopy, and HRTEM reflects that only a single-phase cationic component was present, and their structure is in crystal lattice form. The band gap of GLCHZ-500 is 2.52 eV and of GLYHZ-500 is 3.09 eV. The experiment was performed by using these photocatalysts (GLCHZ-500, GLYHZ-500) and claimed that these semiconductors are more effective for the degradation of Cr(VI) to Cr(III) dyes, and these dyes are too toxic and cause a carcinogenic effect when dissolved in water. By using the single-phase multicomponent catalyst these carcinogenic dyes were 100% degraded, while 90% of methylene blue was degraded during the reaction.^126,127

Bhattacharya et al. in 2019 designed a novel ternary metal oxide catalyst CuCo_0.5Ti_0.5O for the removal of brilliant green (BG). They demonstrated that as the catalyst loading and pH of the solution increased the dye degradation efficiency also increased. At 75g/L catalyst dosage and pH ∼ 8, above almost 90% of dye was removed from the aqueous water.¹²⁸

Li et al. in 2008 reported Ag₂ZnGeO₄ with a (multiple-metal oxide) crystallite-related crystal structure with a band gap of 2.29 eV as a Vis light-sensitive photocatalyst for the process of dye degradation. The physical characterization was carried out by XRD, SEM, and UV–vis diffuse reflectance spectroscopy. The photocatalytic activity of Ag₂ZnGeO₄ was confirmed by using rhodamine B (RhB) and Orange II for photodegradation in an aqueous phase. After 6 h of visible light (l > 420 nm) exposure, the conversions of Orange II and RhB reached 69.2 and 100%, respectively. The band structure and DFT calculations reveal that the hybridized O 2p⁶ and Ag 4d¹⁰ orbitals develop a valence band top of Ag₂ZnGeO₄ and in turn narrowed the band gap as compared to the Na₂ZnGeO₄ parent sample. They proposed that the definite cristobalite structure of the photocatalyst favors the movement of photogenerated charge carriers and contributes positively to the experiential photocatalytic response for the dye degradation process.¹²⁹

In Table 2, photocatalysts with a source of light which are utilized for dye degradation are summarized.

Table 2. Comparison of Photocatalytic Activity of Different Metal Oxides for Different Types of Dyes under Reported Conditions.

Photocatalyst	Dye	Source of light	Dye concentration (mg/L)	pH	% of Dye degradation	Ref
TiO2	C.I. Acid yellow 17	UV	50	3	70.6	(31)
ZnO	Methyl orange	UV	16	7–8	93	(99)
TiO2 (CHE)	Methylene blue	Sunlight	20	-	98	(100)
ZnO	Methyl orange	UV	-	9–11	96	(101)
TiO2	Procion yellow H-EXL	UV	10	5	85.2	(57)
ZnO	Methylene blue	UV	50	7	76	(104)
ZnO	Eosin Y	UV	50	7	74	(104)
Cu2O NPs	Methylene blue	UV	0.1	5.2	97	(105)
ZnO	Reactive green	UV	40	-	94.14	(106)
Au-zeolite	Methyl orange	Sunlight	15	-	100	(110)
CeO2/Y2O	Rhodamine B	Visible		9	98	(111)
Fe-doped TiO2	Reactive violet 5	Visible	3	9	87.2	(112)
C-doped titania	Methylene blue	UV	-	-	90	(113)
Au-TiO2	Methylene blue	UV	30	3	97.4	(114)
GO-MnO2	Reactive black 5	UV	60	3	85	(115)
4 wt % Cu2O-CuO/TiO2	Reactive Blue 49	UV	5	6.5	100	(53)
Cu2O/ZnO	Orange II	Visible	50	6.4	80	(116)
Cu2O/ZnO/Ag3PO4	Methyl orange	Visible	10	6	81	(117)
CeO2/Y2O3	Rhodamine B	UV–vis	40	9	98	(118)
CdS/ZnO/GO	Methylene blue	UV	-	-	99	(123)
Sn-WO3/g-C3N4	Rhodamine B	UV–vis	10		99	(124)
Zn0.25Co0.75 MnFeO4	Methylene blue	UV–vis	-	2.5	99	(125)
GLCHZ-500	Methylene blue	Sunlight	10	-	75	(127)
CuCo0.5Ti0.5O2	Brilliant green	Visible	5	8	95	(128)
Ag2ZnGeO4	Orange II	Visible	12		69.2	(129)
Ag2ZnGeO4	Rhodium blue	Visible	12	-	100	(129)

7. Conclusion and Future Prospective

Dye removal from wastewater is imperative to preventing detrimental impacts on ecosystems and human health. Various techniques are employed for wastewater treatment, including biological treatment, physiochemical treatment, membrane filtering, advanced oxidation processes (AOPs), and photocatalytic treatment. However, conventional methods have limitations such as an inability to remove a wide range of impurities, reliance on external chemicals, production of slurry and sludge, and the need for routine maintenance. In contrast, the photocatalytic treatment of wastewater offers several advantages. It results in the complete breakdown of pollutants into simpler compounds without the use of external chemicals. Additionally, photocatalysts can be regenerated and reused for further processing. Photocatalysis has shown significant potential for decomposing dyes, hydrocarbons, insecticides, germs, and microorganisms and reducing dangerous metal ions in wastewater. During photocatalysis, up to 80–90% of pigments can be removed from wastewater without the use of chemicals. However, the performance of photocatalysts depends on various factors such as catalytic morphology, electron–hole pair generation and recombination rate, pH, temperature, and light intensity, among others. Optimization of these parameters is crucial to achieving better results.

In this review, three generations of metal oxides are thoroughly studied. The first generation has been widely used, while its limitations in degrading all dyes and reliance on UV light have prompted the exploration of second- and third-generation metal oxides. These newer generations exhibit better charge separation, faster reaction kinetics, utilization of visible light, and improved dye degradation tendencies. Additionally, immobilization technology in third-generation photocatalysts offers opportunities for catalyst regeneration. The shape and surface area of the catalyst also influence dye removal, with larger surface areas enhancing the removal efficiency.

Despite that photocatalysis is an advanced technology for wastewater treatment and especially for dye degradation, work is still required to enhance the efficiency of the process. So there are several future recommendations.

• Further research is required to focus on maximizing the utilization of natural sunlight in photocatalysis processes to enhance the economic feasibility of the treatment.

• There is a need for in-depth investigation into the mechanism of photocatalysis at a molecular level to gain a comprehensive understanding of the process. Understanding the formation and transformation of intermediates will enable the optimization of photocatalytic degradation pathways and improve overall process efficiency.

• Additional research is needed to optimize the design of photocatalysis reactors by enhancing catalyst efficiency, integrating mathematical modeling for process optimization, and implementing automated control systems to enhance operational efficiency and overall effectiveness.

The authors declare no competing financial interest.