From conventional treatment to light-driven remediation: a review of strategies for treatment of dye-contaminated wastewater

1 Introduction

The textile industry is one of the key contributors to water pollution due to the discharge of substantial quantities of dye-containing wastewater. These dyes possess complicated aromatic structures that grant high chemical stability, making them resistant to natural degradation processes (Armaković et al., 2022). Dyes are extensively used in textile, leather, paper, plastic, and cosmetic industries, and their complex aromatic structures impart high chemical stability and resistance to biodegradation (Wawrzkiewicz and Hubicki, 2015).

When dye-laden effluents are released into water systems, they block light, disrupt photosynthesis, and harm aquatic life and human health as these are toxic, mutagenic, and cancer-causing (Jabbar et al., 2024). Dyes provide water a strong color even at low levels, so removal of them is vital for the environment (Rauf and Ashraf, 2009). Biological procedure, adsorption, coagulation-flocculation, membrane filtration, and chemical oxidation are some of the usual ways to get dyes eliminated (Anwer et al., 2019). But these methods have problems like not breaking down completely, creating sludge, causing secondary pollution, and being expensive to run (Mirgane et al., 2020).

Photocatalysis has appeared as a transformative and sustainable technology for wastewater remediation due to its capability to achieve complete mineralization of organic pollutants under mild conditions. In contrast to conventional treatment methods, photocatalytic systems utilize semiconductor materials to harness solar or artificial light energy for the formation of highly reactive species such as hydroxyl radicals (•OH), enabling degradation of constant pollutants at the molecular level. Recent developments have demonstrated that photocatalysis can achieve degradation efficiencies exceeding 90% under optimized conditions, with slight sludge production and reduced secondary pollution. Moreover, the integration of visible-light-responsive and heterojunction photocatalysts has significantly increased solar utilization efficiency, making photocatalysis a promising candidate for large-scale and energy-efficient wastewater treatment applications (Shishir et al., 2025a).

Advanced oxidation processes have garnered considerable attention in recent times due to their ability to produce highly reactive radical species for the reduction of persistent organic pollutants (Zhang Q. et al., 2019). Among these processes, heterogeneous photocatalysis has emerged as an efficient and environmentally friendly approach for dye degradation in wastewater (Qasem et al., 2021). Photocatalytic degradation is based on the excitation of a semiconductor photocatalyst under light irradiation, resulting in the generation of electron–hole pairs. These photogenerated charge carriers initiate redox reactions leading to the formation of reactive oxygen species such as hydroxyl radicals and superoxide radicals (Park et al., 2015).

The generated reactive species are capable of oxidizing dye molecules and mineralizing them into simpler, non-toxic end products such as carbon dioxide and water. Semiconductor photocatalysts such as titanium dioxide (TiO₂) and zinc oxide (ZnO) have been extensively investigated for dye degradation due to their high oxidative power, chemical stability, non-toxicity, and low cost (Pavel et al., 2023). TiO₂ exists mainly in anatase, rutile, and brookite phases, among which anatase exhibits superior photocatalytic activity for dye degradation applications. ZnO has also demonstrated comparable photocatalytic efficiency and, in some cases, higher quantum efficiency than TiO₂. Despite their advantages, the practical application of these photocatalysts is restricted due to rapid electron–hole recombination and limited utilization of visible light. Current studies have evidenced significant advancements in photocatalytic materials and reactor designs for dye degradation. For example, advanced heterojunction systems and nanocomposites have shown redefined visible-light activity and improved degradation efficiencies under real wastewater conditions (Kumari et al., 2024a; Kumari et al., 2025a; Kumari et al., 2024b; Kumari et al., 2025b; Kumari et al., 2023). These developments showcase the rising interest in sustainable and scalable photocatalytic technologies for environmental remediation. Compared to earlier reviews, this work provides a detailed analysis of conventional and advanced treatment techniques, with a particular focus on the adaptation of photocatalytic systems from single-component materials to advanced composite and immobilized structures, along with a consideration of operational parameters and scalability challenges.

2 Classifications of dyes

Based on their source of origin, dyes are typically classified as Natural and Synthetic dyes. Natural dyes are generally deemed less toxic; however, their safety depends on constituents and concentration, and some may still reveal environmental impacts. These are mainly derived from plants, animals, fungi or coloured ore-based sources. Indigo, betacyanin, curcumin, crocin, etc., are a few examples of natural dyes (Tan et al., 2018). Synthetic dyes are predominantly organic compounds; even so, certain dyeing processes may involve metal-based mordants or additives such as Pb, Hg, Cr, etc., that contribute to toxicity. (Alegbe and Uthman, 2024). Based on the type of fabric material on which the synthetic dyes are to be used, these are primarily classified into three distinct categories as seen in Figure 1 i.e.,. Cellulose fiber dyes, Protein fiber dyes and Synthetic fiber dyes (Singh, 2024b).

These are much more affordable than natural dyes and hence easily accepted by various industries. As some of these dyes contain heavy metal ions, these dyes cause a threat to the environment upon slipping into the food chain (Jarrar et al., 2024). For example, Blue 1, Red 40, Yellow 5, and Yellow six can potentially cause cancer in humans (Jarrar et al., 2024; Al-Hamdi et al., 2014; Hosseini et al., 2022; Fu et al., 2011). As a result, the removal of these materials is more worked upon when compared to their decomposition.

3 Methods for dye treatment

A variety of technologies have been employed for the efficient decolorization of dye-laden wastewater. Each treatment method possesses a unique operational approach; however, generally, the material interacts with the dye particles in the contaminated water, leading to a gradual reduction in dye concentration and the subsequent creation of clean water. At first, the decolorization process speeds up, then slows down, and finally stabilizes after a few minutes or hours. There are different ways through which dye-containing water can be treated, which are diagrammatically represented in Figure 2.

3.1 Adsorption and filtration

Adsorption is a widely used physical method for removing dye that relies on the binding of its molecules to a solid surface. Because they have a lot of surface area and can adsorb, carbon dioxide, zeolites, and other porous substances are often used as adsorbents (Khan et al., 2022). There are a number of things that impact the process, such as the adsorbent’s properties, the pH, the temperature, and the initial dye concentration. Adsorption may successfully decrease dye concentrations, making it ideal for both industrial and laboratory settings. Adjusted operational parameters help with problems like adsorbent regrowth and saturation (Al-Jabri et al., 2020). Whereas, in the process of filtering dye-containing water through an agent, the pigment particles are separated from the fluid phase. There are numerous methods to filter dye particles, depending on their size and type. Sand filtration captures dye particles within a sand bed (Althafiri, 2016); however, membrane filtration selectively isolates dye molecules from water through semi-permeable membranes with designated pore dimensions. The filtration process is based on the size of the dye molecules, the level of purification, and the parameters of the wastewater (Bhat et al., 2024). Such as activated carbon and biochar-based adsorbents have illustrated dye removal efficiencies exceeding 90% for methylene blue and congo red, although renewal and disposal of spent adsorbents remain significant challenges.

3.2 Sedimentation and photocatalysis

Sedimentation is the process that separates dye particles from water. It works by using gravity to settle the dye particles in a liquid (Carmen and Daniel, 2012). Larger, denser particles that have a tendency to sink to the bottom of a container are particularly well-suited for this technique because of their effectiveness. Coagulants or flocculants may be added to the mixture in order to improve particle aggregation and makes the sedimentation process more quickly. Sedimentation is an easy physical procedure; nevertheless, the effectiveness of this method is contingent on a number of parameters, including the size of the particles, the velocity at which they settle, and the presence of compounds that interfere with the process (Nguyen et al., 2018).

Photocatalysis is a complex physical method that speeds up the breakdown of dye molecules by using light-driven chemical reactions as seen in Figure 3. In this process, TiO₂, also known as titanium dioxide, is a common photocatalyst (Bhat et al., 2024). When TiO₂ is exposed to ultraviolet (UV) light, it makes reactive oxygen species (ROS) that speed up the breakdown of dye molecules into simpler, less harmful parts. Photocatalysis is very effective for organic dyes that do not respond well to other methods of treatment. Development in this field involves enhancing catalyst properties, light sources, and reactor designs to optimize the overall effectiveness of the photocatalytic process (Karimi-Maleh et al., 2020). Recent studies have reported that TiO₂-based and g-C₃N₄-based photocatalysts can obtain degradation efficiencies above 85%–95% under optimized conditions, with nanocomposite systems showing improved performance due to strengthened charge separation and visible-light activity.

3.3 Electrochemical methods

Electrochemical techniques use an electric field or current to help dye ions move and be removed from water. For example, electrocoagulation makes use of electric current to destabilize charged particles, leading them to form aggregates that can be easily separated from the water (Tejero-Martin et al., 2019). Electrochemical technologies offer advantages such as ease of control, diminished chemical consumption, and the potential for metal recovery through electrodeposition. The effectiveness of the electric treatment is influenced by the choice of electrodes, voltage, and duration of the reaction (Zong et al., 2010).

3.4 Coagulation and flocculation

Flocculation and coagulation are chemical methods that use chemicals to group and destabilize dye particles, which makes it easier to get them out of water. Coagulants, such as aluminum sulfate (alum) or ferric chloride, cancel out the negative charges on dye particles. Polymers are the most common type of flocculant. They help bigger flocs stick together so that they can be easily separated from water. These technologies are widely used in industries that need to get rid of small, suspended particles, like dyes, quickly and effectively (Iwuozor, 2019). Coagulation and flocculation are used in municipal wastewater treatment plants and industries like textiles, paper, and tanneries. The effectiveness of coagulation and flocculation depends on factors like the type of dye, the pH, the temperature, and the amount of chemicals used. The best conditions are often different for each dye, so different strategies are needed. Residual coagulants and flocculants in treated water may pose challenges, underscoring the importance of effective post-treatment (Al-Hamdi et al., 2014).

3.5 Advanced oxidation processes (AOPs)

Advanced Oxidation Processes (AOPs) generate very reactive hydroxyl radicals (•OH) to break down and oxidize color molecules. Ozonation, Fenton’s reaction (Fe²⁺/H₂O₂), and photocatalysis using semiconductor materials like titanium dioxide (TiO₂) activated by ultraviolet light are all common advanced oxidation processes (AOPs). AOPs are effective in treating stubborn and complicated dye compounds that do not respond to standard treatment methods (Hosseini et al., 2022). They are used to clean up industrial wastewater that has adamant dyes that will not go away easily. Advanced Oxidation Processes (AOPs) can use a lot of energy and make by-products, so they need to be carefully optimized. The efficiency depends on things like the type of dye, the setup of the reactor, and whether or not there are other organic or inorganic substances in the effluent (Fu et al., 2011). Advanced oxidation processes including Fenton and ozonation exhibit rapid dye degradation; however, they are linked with high chemical consumption, sludge generation, and operational costs, limiting their broad-scale application.

3.6 Biological treatment

Microorganisms are used in biological treatment methods to break down and degrade color compounds. Aerobic and anaerobic biodegradation are common methods. Microorganisms use oxygen to break down dyes in aerobic conditions, but they do not need oxygen to do so in anaerobic conditions. Biological treatment is particularly effective for the remediation of textile wastewater, which often contains complex dye chemicals (Chahal et al., 2022). Biological treatment methods involve constructed wetlands, activated sludge processes, and microbial fuel cells. Biological treatment may not work so well if the pH, temperature, or chemicals that stop it from working are present. The issues of slow degradation rates and the chance of biomass buildup are lessened by optimizing and making systems (Repeta et al., 2016).

Table 1 provides summary of biological catalytic methods for breaking down dyes using microorganisms and enzymes that are good for the environment. These methods let dyes change through metabolic and enzymatic pathways, but they get impacted by when they are used and usually take longer to work. Biological treatment methods are environmentally friendly; such as, their efficiency is frequently restricted to biodegradable dyes, and degradation rates are significantly lower rate compared to advanced oxidation processes.

TABLE 1. Catalytic degradation of dyes by biological methods.

As soon as microorganisms break down dye compounds in an area with a lot of oxygen, rapid decay happens. Whenever bacteria are in the presence of oxygen, they use it to oxidize organic pollutants, which makes them into simpler and less harmful by-products (Thevarajah et al., 2005). Microorganisms that break out organic matter in the presence of oxygen include bacteria, fungi, and algae. Effluent from the textile and dyeing industries, which often contains azo dyes and other organic compounds, can be treated with aerobic methods. Dye properties, microbial activity, pH, and temperature are all things that can affect how well aerobic biodegradation works. Some dyes are very hard to break down, so they need more treatment (Xiao et al., 2018).

Anaerobic biodegradation is a type of process which uses bacteria that do well in low oxygen environments. It turns color molecules into simpler chemicals and methane. Anaerobic biodegradation can break down wastewater that has dyes that are hard to get rid of. It works well on azo dyes, which are hard to break down. Process optimization and system design deal with how fast anaerobic degradation happens and how fast intermediate metabolites build up, adding co-substrates and controlling ambient parameters are two important things (Huang et al., 2021c).

3.7 Chemical precipitation

Chemical precipitation involves using reagents that react with dye ions to make precipitates that do not dissolve. Common precipitants are sodium carbonate, sodium hydroxide, and lime (calcium hydroxide) (Hao et al., 2021). Chemical precipitation is a method to remove heavy metal ions and certain dyes out of wastewater. It is easy to separate the precipitates that are made by sedimentation or filtration. The effectiveness of chemical precipitation depends on factors such as the solubility of the precipitate, the pH, and the reaction’s stoichiometry. System optimization takes into consideration the need for careful pH control and the amount of sludge produced (Fatima et al., 2020).

3.8 Constructed wetlands and microbial fuel cells (MFCs)

It uses natural processes in man-made systems to clean up wastewater that contains dyes. Plants and microorganisms that live in wetlands are very important for breaking down dye compounds. The wetland matrix is a good place for microbial communities to live. Constructed wetlands can be used to treat many different kinds of dye pollutants, making them a flexible and long-lasting option for treatment. They are especially good for distributed treatment in small to medium-sized facilities. The effectiveness of built wetlands is influenced by variables including plant selection, hydraulic retention time, and the properties of dye pollutants (Abdel et al., 2008). MFCs not only clean wastewater, but they also use the metabolic activity of microorganisms to make electricity. Microbial oxidation of organic molecules, releases electrons that help make power. Microbial fuel cells can make electricity and clean up wastewater at the same time. They can be used in decentralized applications because they can handle wastewater with low to moderate amounts of organic pollutants. MFCs have issues with reactor design, power output, and for making their technology work on a larger scale (Sharma and Bhattacharya, 2016).

3.9 Sequential batch reactors (SBRs)

In this method, a single reactor goes through the fill, react, settle, and decant phases over and over again. Microorganisms get used to certain dye chemicals during the react phase, which helps them break down the dye chemicals in a natural way. SBRs can clean up a wide range of dye pollutants because they can be used in many different ways. They are often used in treatment plants that are small to medium-sized, and they can be changed to work in different conditions. Some of the problems with SBRs are that they need to be monitored from time to time, they may not always work as well as they should, and they need to be controlled. To solve these problems, people use monitoring and advanced control methods (Kushwaha et al., 2022).

Conventional treatment methods such as adsorption, coagulation, and biological degradation exhibit dye removal optimization levels ranging between 60% and 90%; however, these ways often fail to achieve complete mineralization and create secondary waste such as sludge. For example, adsorption using activated carbon can achieve removal efficiencies above 90% but requires regular regeneration and disposal of spent adsorbents. Likewise, coagulation–flocculation is cost-effective but generates significant sludge, increasing post-treatment costs. On the other hand, photocatalytic processes demonstrate mineralization efficiencies exceeding 90%–95% under optimized conditions, transforming dyes into CO₂ and H₂O without secondary pollution. This clearly shows the superiority of light-driven processes over conventional treatment techniques.

Table 2 shows that among different dye treatment technologies photocatalytic degradation has superior mineralization efficiency with minimal secondary pollution.

TABLE 2. Comparative analysis of dye treatment technologies.

4 Parameters affecting photocatalytic dye degradation

Photocatalytic dye degradation is firmly dependent on several operational parameters, each exhibiting an optimum range for maximum degradation efficiency. The optimum pH typically lies between five and 8, depending on the type of dye and catalyst, ensuring favorable electrostatic interactions and enhanced adsorption. Photocatalyst incorporation is generally optimized in the range of 0.5–1.5 g/L, where adequate active sites are available without causing light scattering or particle aggregation. The beginning dye concentration is ideally maintained between 10 and 50 mg/L, as higher concentrations restrict light penetration and reduce degradation efficiency. Temperature shows a medium influence, with optimal performance usually observed around 25 °C–40 °C, while excessive temperatures may encourage recombination of charge carriers. Light intensity must also be carefully handled, as an increase enhances electron–hole generation up to an optimum level, after which recombination effects dominate and reduce efficiency.

4.1 Effect of pH and temperature

The pH significantly impacts photocatalytic activity by changing the surface charge of the catalyst, ionization of the dye, way the dye molecules stick together, and the placement of the band edges. Electrostatic interactions between dye molecules, catalyst surfaces, and produced radicals control how well adsorption and degradation work (Priya et al., 2021). In acidic environments, cationic dyes are electrostatically pushed away from semiconductor surfaces that have a positive charge. This makes them less likely to stick and break down. Higher pH levels may lead to better adsorption, but having too much surface area can make photoexcitation less effective and block light absorption. So, very high or very low pH levels usually make breakdown less effective (Zhao et al., 2022). Temperature has a very small effect on it in the middle range (20 °C–80 °C). A minor rise in the rate of decomposition could happen because molecules are moving around more and colliding with each other more often. At high temperatures, less dissolved oxygen and contaminants coming off the catalyst surface make it easier for electrons and holes to recombine, which lowers activity (Niu et al., 2019).

4.2 Effect of light intensity and visible light irradiation

The charge of photons and the strength of the radiation affect how quickly dyes break down. Increasing the intensity of light speeds up degradation at first because it makes more electron-hole pairs. Further increase after optimal level show little or no effect, or even a negative effect, because of recombination processes and heat effects (Etacheri et al., 2015). Light-sensitive catalysts and heterojunction devices that respond to visible light make it possible for dyes to break down in sunlight or low-energy light sources.

4.3 Effect of photocatalyst dosage and initial dye concentration

Higher the concentration of photographic catalyst improves dye degradation by giving the dye more active sites and charge carriers. On the other hand, too much catalyst loading causes light to scatter, particles to clump together, and the solution to become cloudy, all of which make it harder for photons to get through and lower the active surface area, which lowers degradation efficiency (Yusoff et al., 2021). Moderate dye concentrations facilitate photocatalytic degradation by allowing for effective adsorption and surface interactions. Too much dye covering at higher concentrations slows down degradation rates by blocking active sites, making it harder for light to get through, and causing unwanted sensitization effects (Krishnan et al., 2016).

4.4 Effect of electron acceptors and intermediate species

Electron-hole recombination makes photocatalytic effectiveness very low. Electron acceptors like dissolved oxygen, hydrogen peroxide, persulfate, or bromate improve degradation efficiency by suppressing recombination. It is done by scavenging photogenerated electrons and encouraging the production of reactive oxygen species (Somsesta et al., 2019). Inorganic anions can pick up reactive radicals and stop degradation, while oxidants speed it up. It is important to keep an eye on reaction pathways and by-products because some degradation intermediates may be more harmful than the original dyes (Sasaki et al., 2016).

4.5 Effect of inorganic ions and photocatalyst morphology

Inorganic ions can either help or stop photocatalytic destruction, depending on what kind they are and how many there are. Some anions slow down degradation by getting rid of radicals or blocking active sites, while others speed it up by encouraging charge transfer (Qasem et al., 2021). These effects are additionally affected by pH-dependent ion speciation. The catalyst’s shape is essential for breaking down dyes. A high surface area, the right amount of porosity, beneficial crystallinity, and certain exposed facets all help with mass transfer, charge separation, and adsorption, which all improve photocatalytic performance (Kumar et al., 2019).

4.6 Photocatalyst resistance to photo-corrosion

Photocorrosion affects the long-term efficiency and lifespan of catalysts, especially in semiconductors like ZnO. Material modification and doping can make things more stable by stopping metal ions from leaking out, but they can also change the band structure and lower photocatalytic performance (Enaime et al., 2020). One of the most important problems is still finding the right balance between stability and efficiency.

Generally, the optimization of these parameters is critical for achieving maximum photocatalytic efficiency, as even marginal deviations from optimal conditions can significantly affect degradation kinetics and mineralization efficiency.

5 Composition of photocatalysts

A number of photocatalytic materials have been looked into to see if they can break down dyes in wastewater. These materials include metals, metal oxides, carbon-based materials, electronic quantum dots, magnetic composites, and metal-organic frameworks (Wu et al., 2015). Metal oxide-based photocatalysts remain the most extensively studied due to their effectiveness, stability, and availability. There are three main types of photocatalysts based on how complex their structure is and how well they work (Serrà et al., 2019). Figure 4, shows evaluation of photocatalysis from first generation to third generation system.

5.1 First-generation photocatalysts

First-generation photocatalysts are made up of single-component metal oxides like TiO₂, ZnO, ZrO₂, SnO₂, ZnS, CuO, and NiO. When exposed to UV or UV-visible light, their photocatalytic activity depends on making electron-hole pairs (Bantawal et al., 2020). TiO₂ and ZnO are the most commonly used of these compounds because they are chemically stable, not very toxic, cheap, and can mineralize a wide range of dyes. The three main types of TiO₂ are anatase, rutile, and brookite. Anatase is the best choice because it can hold more and is more effective at photocatalysis. ZnO is also very easy to recycle and works very well at breaking down dyes. First-generation photocatalysts work, but they have some big problems. For example, they have wide band gaps that need to be activated by UV light, electrons and holes recombine quickly, and they can only be used with certain types of dyes (Ani et al., 2018).

5.2 Second-generation photocatalysts

Second-generation photocatalysts were made to fix the problems with single-component systems. Most of the time, these materials are made up of many parts that make up homojunction or heterojunction structures. Junctions effectively separate charges by directing photogenerated electrons and holes into separate areas and stopping them from recombining (Abarca et al., 2023).

Heterojunction photocatalysts, which are made up of two or more semiconductors with different band structures, work better as catalysts, transfer charge better, and absorb light when exposed to visible light. Adding metals or nonmetals and combining them with carbon-based materials makes degradation more effective and makes the material more responsive to light (Gong et al., 2021). Still, there are problems with recovering treated water, making the process more complicated, and raising operating costs. Composite photocatalysts illustrate advanced performance compared to single-component systems due to enhanced charge separation, extended light absorption range, and reduced electron–hole recombination, which substantively improve photocatalytic efficiency. Moreover, immobilized photocatalyst systems provide practical advantages over conventional slurry systems, including easy recovery, improved reusability, and reduced post-treatment separation costs. However, slurry systems may provide better degradation efficiency due to greater surface area exposure, showing a trade-off between operational efficiency and practical applicability.

5.3 Third-generation photocatalysts

The aim of third-generation photocatalysts is to address the challenges of catalyst recovery and reusability in suspended systems. Most of the time, these photocatalysts are stuck to solid membranes or supports, like composite frameworks, floating substrates, or polymer matrices. Immobilization makes it easy to separate, regenerate, and keep things stable over time without significantly reducing photocatalytic activity. The use of floating photocatalysts is a big step forward because they improve the interaction between surfaces and contaminants and solar energy use without needing to be moved around. These systems show that they are good for treating large amounts of wastewater because they are very good at breaking down waste, are stable in structure, and are good for large-scale use. Third-generation photocatalysts are very promising for cleaning up the environment because they have a large surface area, collect light well, and are easy to recover (Anwer et al., 2019; Liu et al., 2016; Sellet et al., 2023; Chandrapal et al., 2022; Medina-RamÃRez and Hernández-RamÃRez, 2014).

Table 3 briefs the major application areas of photocatalytic dye degradation systems as well as their advantages.

TABLE 3. Commonly used catalyst and their advantages.

Table 4 analyzes commonly reported photocatalysts based on their degradation efficiency, highlighting the high-quality performance of composite and supported systems under visible and solar light conditions.

TABLE 4. Comparative efficiency of photocatalysts for dye degradation.

6 Characterization techniques for photocatalysts

Photocatalysts are characterized using various techniques to recognize their structural, morphological, and optical properties. X-ray diffraction (XRD) is used to identify crystal structure, while scanning electron microscopy (SEM) and transmission electron microscopy (TEM) give morphological details. Brunauer–Emmett–Teller (BET) analysis evaluates surface area, and X-ray photoelectron spectroscopy (XPS) recognizes elemental composition and oxidation states. UV–Vis spectroscopy is used to evaluate optical absorption properties, while Fourier transform infrared spectroscopy (FTIR) identifies functional groups. These characterization techniques are important for correlating material properties with photocatalytic performance (Shishir et al., 2025b; Bayzid et al., 2025).

7 Challenges and future perspectives

In spite of significant advancements, photocatalytic dye degradation faces several challenges that restrict its large-scale application. One of the main limitations is the rapid recombination of photogenerated electron–hole pairs, which lowers efficiency. In addition, many conventional photocatalysts are only UV-active, restricting solar energy utilization. Catalyst recovery and stability, especially in slurry systems, also remain critical challenges. Moreover, the formation of intermediate by-products during degradation escalates concerns regarding toxicity and requires further investigation. Future research should target the development of visible-light-active and heterojunction photocatalysts with enhanced charge separation efficiency. The framework of immobilized and magnetic photocatalysts can address recovery and reusability issues. Integration of photocatalysis with other advanced oxidation processes and the operation of artificial intelligence for process optimization are emerging trends. Furthermore, pilot-scale studies and real wastewater treatment investigations are essential to bridge the gap between laboratory research and industrial application.

8 Conclusion

Chemically stable and toxic dyes are still very hard for the environment to get rid of from wastewater. Even though traditional methods are widely used, their flaws show that we need better and more effective methods. Photocatalytic degradation is a good option because it lets dye mineralization happen in a way that is safe for the environment. The performance of photocatalytic systems relies on their operational mechanisms and the characteristics of the catalyst, particularly metal oxide photocatalysts, which are crucial. New ways to change and fix catalysts have made them more stable, reusable, and better at using light. Photocatalytic systems need to keep getting better at being efficient and long-lasting if they are going to be useful for treating large amounts of wastewater. From an economic and sustainability perspective, photocatalysis provides benefits such as low chemical consumption, potential solar energy utilization, and reduced sludge generation, making it a favourable alternative for long-term wastewater treatment solutions.

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Footnotes

Edited by: Jianhua Zhang, Victoria University, Australia

Reviewed by: Sheetal Kumari, Harbin Institute of Technology, China

Md. Khalid Hossain Shishir, Chittagong University of Engineering & Technology, Bangladesh