Harita Kumari,Sonia,Suman,Rohit Ranga,Surjeet Chahal,Seema Devi,Sourabh Sharma,Sandeep Kumar,Parmod Kumar,Suresh Kumar,Ashok Kumar,Rajesh Parmar
A Review on Photocatalysis Used For Wastewater Treatment: Dye Degradation
Metal oxides,Photocatalytic degradation,Rare earth dopants,Nanocomposites,Core–shell,Band gap tuning
Water pollution is a global issue as a consequence of rapid industrialization and urbanization. Organic compounds which are generated from various industries produce problematic pollutants in water. Recently, metal oxide (TiO2, SnO2, CeO2, ZrO2, WO3, and ZnO)-based semiconductors have been explored as excellent photocatalysts in order to degrade organic pollutants in wastewater. However, their photocatalytic performance is limited due to their high band gap (UV range) and recombination time of photogenerated electron–hole pairs. Strategies for improving the performance of these metal oxides in the fields of photocatalysis are discussed. To improve their photocatalytic activity, researchers have investigated the concept of doping, formation of nanocomposites and core–shell nanostructures of metal oxides. Rare-earth doped metal oxides have the advantage of interacting with functional groups quickly because of the 4f empty orbitals. More precisely, in this review, in-depth procedures for synthesizing rare earth doped metal oxides and nonocomposites, their efficiency towards organic pollutants degradation and sources have been discussed. The major goal of this review article is to propose high-performing, cost-effective combined tactics with prospective benefits for future industrial applications solutions.
A Review on Photocatalysis Used For Wastewater Treatment: Dye Degradation

Water pollution is a global issue as a consequence of rapid industrialization and urbanization. Organic compounds which are generated from various industries produce problematic pollutants in water. Recently, metal oxide (TiO2, SnO2, CeO2, ZrO2, WO3, and ZnO)-based semiconductors have been explored as excellent photocatalysts in order to degrade organic pollutants in wastewater. However, their photocatalytic performance is limited due to their high band gap (UV range) and recombination time of photogenerated electron–hole pairs. Strategies for improving the performance of these metal oxides in the fields of photocatalysis are discussed. To improve their photocatalytic activity, researchers have investigated the concept of doping, formation of nanocomposites and core–shell nanostructures of metal oxides. Rare-earth doped metal oxides have the advantage of interacting with functional groups quickly because of the 4f empty orbitals. More precisely, in this review, in-depth procedures for synthesizing rare earth doped metal oxides and nonocomposites, their efficiency towards organic pollutants degradation and sources have been discussed. The major goal of this review article is to propose high-performing, cost-effective combined tactics with prospective benefits for future industrial applications solutions.

With the current economic development and population growth, protecting water resources is a pressing concern. Every day, about 2 million tons of waste pollutants are carelessly released into the public water supply, gravely affecting human health and resulting in numerous deaths (Yan et al., 2021).The most representative pollutants are residual toxic dyes, monoaromatic hydrocarbons, and antibiotics originating from industrial wastes, such as rhodamineB (RhB), 4-nitrophenol (4-NP), and naproxen (NPX), which exhibit thermal and photo-resistant stability. According to reports, the maximum amount of 4-NP in water should be kept to less than 10 ppb (Devi et al., 2023; Yang et al., 2020a). Every year, more than 175,000 tons of dyes and 54,000 tons of antibiotics are arbitrary released into worldwide water resources (He et al., 2019; Zhang et al., 2015). Concern has grown greatly and deeply as water pollution has become an intractable challenge. In fact, numerous methods of waste water purification, including ion-exchange techniques, adsorption, membrane separation techniques, and electrochemical purification, have been proposed and put into action to address the water resources (Li et al., 2021; Suman et al., 2020a, b). Although much efforts have been put towards solving this problem, there are still several obstacles, including limited adsorption capacity, ready inactivity, secondary pollutants, and high energy consumption. To address this challenging issue, photo-induced catalytic degradation technology has recently attracted a lot of attention (Feng et al., 2023). Solar energy is most environmentally friendly, cost-effective, and even endless source of energy. On the other hand, innovative semiconductor photocatalysts that use active radicals to degrade organic pollutants can be rationally designed and produced as a result of advancements in nanoscience and nanotechnology (He et al., 2019; Hu et al., 2019). Cost-effectiveness, environmental friendly nature, and minimal energy usage are further benefits of photocatalysis process (Huang et al., 2018; Wei et al., 2019; Wu et al., 2020). Despite semiconductor photocatalysis technology being a talent self-driving potential technique by harvesting inexhaustible solar energy, the efficiency of photocatalysts in converting solar energy is considerably lower than predicted. This is because it is challenging to discover a single photocatalyst that can absorb all of the solar energy while maintaining low recombination rates of photo-induced carriers and strong photo stability (Bie et al., 2019; Wei et al., 2018; Xu et al., 2019). Since the components of composite have some synergistic effects, it is believed that composite of co-photocatalysts will partially satisfy the difficult acquisition. Many materials can be used for photocatalytic process, but metal oxides specifically titanium oxide (TiO2), cerium oxide (CeO2), zinc oxide (ZnO), zirconium oxide (ZrO2), manganese oxide (MnO2), and tin oxide (SnO2) are considered as excellent photocatalysts in this class due to their high stability in wide pH range, higher efficiency, low-cost, easy availability, less toxicity, being eco-friendly, and highly oxidizing photo-generated holes (Alam et al., 2018; Keerthana et al., 2021; Shaheen et al., 2020; Singh et al., 2018). However, there are several drawbacks of utilizing these metal oxides for photocatalytic degradation, such as incomplete mineralization and a wide band gap (Vaiano et al., 2018; Wang et al., 2019), which significantly limit their photocatalytic activity. Furthermore, a significant factor in dye degradation is the quick transport of electrons and holes (Rani & Shanker, 2021; Xing, 2017). However, rapid recombination of electrons (e−) and holes (h+) greatly reduces their potential for photocatalytic degradation (Liu et al., 2019; Rani et al., 2019). Hence, photocatalytic effectiveness of pure metal oxides was not up to mark. So as to improve their efficiency, rare earth doped metal oxides, metal oxide nanocomposites, core–shell nanostructures of metal oxides, etc. have been investigated (Joshi et al., 2022; Shanmuganathan et al., 2020; Wahba et al., 2020). However, a wider variety of catalysts are described here to give a more comprehensive understanding of the materials utilized. These are classified into three categories: single metal oxide, doped, and composite photocatalysts.

Water consisting of toxic heavy metals, microorganisms, organic materials, and soluble inorganic compounds are treated as sources of wastewater. These pollutants interfere with freshwater’s chemical, biological, and physical properties (Abou El-Nour et al., 2010). Sources of wastewater can be classified mainly into three categories as shown in Fig. 1 (Adams et al., 2006). Further, different types of pollutants which are found in water are shown in Fig. 2. World Health Organization (WHO) estimates that more than 13.7 million deaths worldwide occurred in 2016 as a result of people working and living in unhealthy environment. Particularly, air pollution resulted in almost 6 million deaths, while water contamination resulted in over 1.8 million deaths, demanding extensive remediation techniques (Dharwal et al., 2020; Kumar et al., 2021). Nowadays, treatment of wastewater has become crucial due to the harmful impact of pollutants and risk of polluted wastewater on animals, human beings, as well as on agriculture. In order to protect the environment from contamination, effective actions must be taken at personal and government level for wastewater treatment. Physical, biological, and chemical procedures can be involved for the treatment of wastewater. Dye and other substances can be physical pollutants in wastewater (fixed, volatile, dissolved, and suspended). Wastewater may contain organic, inorganic, or gaseous substances as chemical pollutants. Figure 3 shows an overview of chemicals used by textile industries, which is the major source of water pollution. Along with physical and chemical characteristics biological characteristics are additionally prevalent in the contaminated water. Biological pollutants consist of pathogenic microorganisms like bacteria, protozoa, and viruses causing intense and chronic health impacts (Shon et al., 2007).

Contaminated water is a serious problem that has far-reaching consequences for the environment. Numerous insecticides, dyes, metal ions, chemical and inorganic substances, and other toxins pollute our environment. The non-biodegradable and potentially dangerous nature of dyes is largely attributable to the presence of stable aromatic rings in majority of dyes. Even a trace amount of dye can contaminate a system (Kapoor et al., 2021). Therefore, dye usage is one of the detrimental types of pollutants that need to be reduced in environment. Most of the dyes are stable in presence of light, heat, and oxidizers; hence, they are not biodegradable. Dyes have an effect on aquatic ecology because they alter the habitat’s aesthetic value. Dyes may be classified as cationic, anionic, and non-ionic. Dyes can also be categorized on the basis of functional groups: azo, indigo, phthalocyanine, anthraquinone, sulfur, etc. (Benkhaya et al., 2020). Dyes can be grouped together as basic, dispersed, vat, acidic, reactive, etc. For each color, textile dyes have interesting chemical structure (Solayman et al., 2023). Figure 4 presents many industries that are to blamed for the existence of dye effluents in atmosphere. Every year, world creation of textile dyes is assessed in excess of 10,000 tons and roughly 100 tons of dyes are delivered to wastewater consistently (Semeraro et al., 2015). Not only that, but textile industry is currently most polluting in the entire globe. Therefore, textile production is biggest cause of adverse environmental effects (Lellis et al., 2019). Dyes have capability of absorbing light radiation in the range of visible region (380–700 nm).When white light reflects off a surface or is diffused or transmitted, it changes from white to colored light as a result of selective energy absorption by particular atoms (chromophoric groups). To put it another way, a dye is a substance with the capacity to absorb some light radiations and subsequently reflect complimentary colors. Before the middle of nineteenth century, natural materials like beet root were used as sources for dyes. The natural dyes might be acquired from insects, plants, minerals, and animals. Biodegradation can be utilized to treat wastewater produced by natural dyes as they are usually less hazardous and allergic than synthetic dyes (Sivakumar et al., 2009). Table 1 provides a general overview of the different dyes.

In the upcoming future for upholding a better environment, fostering a better and sustainable ecosystem is an obligatory requirement. Because of industrial outlets, the environment gets polluted, resulting harmful health effects to human being, aquatic creatures, terrestrial animals, and plants. Because effluents from textile industry play a significant role in environmental pollution, we were inspired and motivated to take on task of authoring this review study. In order to establish a sustainable environment, this review article focuses on combining modern and classical approaches to remove pollutants from textile wastewater. To get rid of textile effluents, industrial wastewater is treated utilizing a number of chemical, biological, physical, and electrochemical techniques. The main focus of this article is on to explore the behavior of various dyes resulting from textile and pharmaceutical industries followed by their abatement using metal oxide–based photocatalysts.

Many more dyes in addition to those listed in Table 1 also pose serious risks to human health and the ecosystem. The most dangerous ones are RB and phenol dyes. Over previous many years, various physical, chemical, and biological techniques like membrane filtration, precipitation, ion exchange, distillation, electrolysis, electrodialysis, flotation, conventional adsorption, ion exchange, chemical oxidation, biological treatment, ultrasonic-assisted adsorption, and chemical coagulation have been done to treat dye effluents and to eliminate dyes from aqueous solutions (Karimi-Maleh et al., 2020; Katheresan et al., 2018). Techniques for removing dye via physical, chemical, and biological means are illustrated in Fig. 5. Tables 2, 3, and 4 provide several physical, chemical, and biological dye removal strategies along with their benefits and drawbacks, accordingly. Since we have a lot of methods for wastewater treatment, but highly efficient and cheaper methods need to be explored for providing safe water access to every human being. Among all these methods, photocatalysis has gained much attention since it is most effective and eco-friendly method till date. Photocatalytic frameworks with required band gap, high stability, large surface area, and suitable morphology are of extreme significance. The photocatalytic process can be carried out with a variety of different materials; however, TiO2, SnO2, CeO2, ZrO2, and ZnO are usually considered as most effective photocatalysts in this category.

Perovskite catalysts and advanced oxidation processes (AOPs) have been used in recent years to remove organic pollutants from wastewater. AOPs have been fully implemented in wastewater treatment, pharmaceutical industry, textiles, and general circular economy scale globally. AOPs can be divided into multiple categories based on various processes that generate free radicals. AOPs have been developed recently that differ in the ways to generate reactive oxygen species (ROS) and kinds of these species. Categorization of processes and applications mentioned in research can be used to evaluate the effect of each AOP separately, as well as in combination. Single AOPs may be used to purify and treat waste and wastewater at very efficient rates; however, combinations of AOPs have been implemented in many cases, with prominent results. Basic classification of AOPs comprises of processes of UV/H2O2, Fenton and photo-Fenton, ozone-based (O3) processes, photocatalysis, and sonolysis. In real water matrices, energy requirements are strongly associated with the nature of contaminant. Regarding removal of a pollutant, a comparison table (Table 5) is presented depicting possible concentrations of water purification for above mentioned AOPs and total degradation percentages of heavy wastewater, such as leachates, and low-concentration pollutants such as antibiotics. Tetracycline (TC), an antibiotic, is frequently provided to cattle feed and used to treat infections in both humans and animals. These applications are attributed to its inexpensive cost and broad spectrum of activity. Even in recent literature, TC has been referred as a potential COVID-19 treatment. The unfortunate fact is that only 25% of TC is metabolized by humans; remaining 75% is released into the environment. Residual TC has potential to adversely affect human health through biological food chain, impact the growth and development of aquatic flora and fauna, and cause endocrine disorders, mutagenicity, and antibiotic resistance. Therefore, it is crucial to figure out how to remove TC from water (Gopal et al., 2020; Lundström et al., 2016; Sodhi & Etminan, 2020). Additionally, photo-Fenton is an important direction in photocatalytic technology. In particular, the photo-Fenton reaction can degrade dye wastewater efficiently and quickly. C. Xiao et al. successfully synthesized novel Ni-doped FeOx catalyst through a simple co-precipitation method for simultaneous removal of Cr(VI) and various organic pollutants in heterogeneous photo-Fenton system. The removal efficiency of Cr (VI) and RhB still reached 90% and 91% in 10-NiFeOx/H2O2/visible light system. C. Xiao et al. successfully synthesized the burger-like α-Fe2O3 using FeCl3 as iron source through a facile hydrothermal process. The α-Fe2O3 samples with spherical, elliptic, olive-like, and burger-like morphologies were obtained by adjusting hydrothermal reaction time, respectively. Under optimum operating conditions, degradation ratio of acid red G (ARG) by burger-like α-Fe2O3 reached 98% under visible light irradiation within 90 min (Xiao et al., 2018). The kaolin–FeOOH catalyst was successfully prepared by C. Xiao et al. by using a facile method and exhibited excellent photo-Fenton catalytic performance with the assistance of oxalic acid. Benefiting from the high photosensitivity of iron–oxalate complexes of oxalic acid, 98.8% degradation percentage and 49.1% removal rate of TOC due to RhB degradation were achieved under optimum operating conditions (Chen et al., 2020). Q. He et al. prepared a kaolin-Fe2O3 (K-Fe2O3) photocatalyst by a facile method. In a K-Fe2O3/PS/visible light system, a 99.8% degradation ratio of RhB was reached under optimal experimental conditions (He et al., 2020). FeWO4 nanosheets as an example, the activation of oxalic acid (OA) based 13 on facet engineering for the enhanced generation of active radical species is reported, revealing 14 unprecedented surface Fenton activity for pollutant degradation. Density functional theory calculations confirmed more efficient generation of reactive oxygen species over FeWO4 nanosheets with {001} facet exposed (FWO-001) under visible light irradiation compared to efficiency of FeWO4 nanosheets with {010} facet exposed (FWO-010), which could be attributed to higher density of iron and efficient activation of OA on {001} facet (Li et al., 2019). Advanced oxidation processes can also be divided into Fenton systems, represented by H2O2 (•OH), and systems, represented by peroxymonosulfate (PMS) and persulfate (PDS) (SO4•−), based on various free radicals. The oxidative power of SO4•− was higher (2.5–3.1 V) and its half-life was longer (30–40 s) than those of •OH (1.8–2.7 V and 20 ns), making it more efficient at degrading aqueous pollutants (Huang et al., 2021a; Huo et al., 2020). Researchers have paid increased attention to persulfate-based advanced oxidation processes (PS-AOPs) in recent years, largely as a result of increased prevalence of carbon materials and discovery of a non-radical pathway. Graphene (G), carbon nanotubes (CNTs), and biochar (BC) have all seen extensive application in persulfate-based advanced oxidation processes (PS-AOPs). H. Luo et al. demonstrated active locations of G, CNTs, BC, and other carbon materials, and generalized the methods for promoting carbon material activity and switching reaction pathways in PS-AOPs. Reactive oxygen species (ROS) and structures were discussed in context of functions of carbon materials in PS-AOPs. However, predominant ROS formation is linked to active sites on carbon materials, despite the fact that ROS are often complex in AOPs (Luo et al., 2022). To further activate persulfate (PS) for tetracycline (TC) degradation, nitrogen and copper co-doped biochar (N-Cu/biochar) material was also studied. With a 200 mg/L catalyst concentration, 0.5 mM PS concentration, and pH of 7.0, all TC was removed in 120 min. Bioluminescence inhibition assay was employed to figure out the toxicity of TC and its metabolites. The N-Cu/biochar/PS system, with its high catalytic efficiency and low consumables, could be a novel approach to wastewater remediation (Zhong et al., 2020). The work by Y. Yuan, H. Luo, et al. provided new insight on the process of removing hexavalent chromium (Cr(VI)), which helped researchers to better define the scope of applications for sulfidated nanoscale zero-valent iron (S-nZVI) in Cr(VI) remediation (Yuan et al., 2022). The removal of Cd(II) and Pb(II) from water was also studied by R. Huang and H. Luo et al. adopting three inexpensive adsorbents: pure raw attapulgite (A-ATP), high-temperature-calcined attapulgite (T-ATP), and hydrothermal loading of MgO (MgO-ATP). The goal of this research was to shed light on MgO-ATP and its potential as an economic and effective adsorbent for cleaning up polluted environments that have been contaminated with harmful metals like Cd(II) and Pb(II) (Huang et al., 2020). Porous carbon nanomaterials have received a lot of attention recently as PMS activators (Hu et al., 2021). The oxidation of Fe0 can be slowed down by metal particles enclosed in hierarchical carbon layer of these materials. Additional favorable factors for PMS activation include carbon layer’s tunable surface chemistry and their outstanding electron transport capabilities (Huang et al., 2021b, c). Having metal ions or clusters in the center and organic legends acting as a binding agent, metal organic frameworks (MOFs) are highly arranged porous materials (Ye et al., 2020). MOFs are extensively used in adsorption catalytic oxidation and other similar processes (Zhang et al., 2021, 2022a). Because of their large surface area and porosity, adjustable pore size, high conductivity, and stability, carbon compounds generated from MOF have recently received exhaustive study for the activation of PDS and PMS (Hao et al., 2021). Through straightforward pyrolysis of [Fe, Cu]-BDC precursor, Tang et al. effectively created a three-dimensional flower-like catalyst (i.e., FeCu@C) that contains iron-copper bimetallic NPs within a mesoporous carbon shell and was used to degrade sulfamethazine. Cu species facilitated quick Fe3+/Fe2+ redox cycles, which increased •OH production (Tang & Wang, 2019). Since addition of FeNx sites regulates the electronic structure of catalysts, He et al. effectively synthesized carbon nanocubes containing a lot of FeNx active sites by calcining xFe@ZIF-8. These electron-deficient Fe centers serve as electron acceptors to take in electrons that adsorbed PMS transmits, producing highly reactive 1O2 for quick phenol oxidation. (He et al., 2021). In order to achieve degradation of phenol by activating PDS, Li et al. synthesized iron/carbon composites by pyrolyzing MIL-88A (Fe). During this process, optimized catalyst FexC-600 performed superbly (Li et al., 2020). The advantage of carbon-coated metal nanoparticles is uniform distribution of metal particles throughout carbon layer, which is achieved by calcining MOFs at high temperatures in an Ar environment. Additionally, carbon encapsulation of metal particles can prevent them from aggregating (Liu et al., 2018).

Materials of Institute Lavoisier (MILs) are one of the typical representatives of MOF materials, which are synthesized by different transition metal elements (such as Fe2+) and dicarboxylic acid ligands such as succinic acid and glutaric acid. Compared with metal oxides, MILs have unique advantages like porosity and unsaturated metal sites, which make them widely applicable in the field of catalysis. In sulfate radical advanced oxidation processes (SR-AOPs), coordinated unsaturated metal ions (CUS) mainly supplied by MILs can activate PMS/PS to form SO4•− and •OH, which have strong oxidizing properties and thus degrade organic pollutants in water. Also, MILs have a high specific surface area and pore volume, which not only provides high-density active centers and large reaction space, but also effectively adsorbs organic pollutants in water, thus facilitate the formation of SO4•−, and •OH to react with organic substances(Huang et al., 2021c). Iron-containing MILs have been widely used in catalysis field. The richness, environmental friendliness, excellent magnetic separability and cost-effectiveness of Fe-based heterogeneous catalysts make them powerful candidates for SR-AOPs. Therefore, they have received particular attention in SR-AOPs. For example, (Li et al., 2016) synthesized four kinds of pure MILs including MIL-101(Fe), MIL-100(Fe), MIL 53(Fe), and MIL-88B(Fe) by simple solvothermal method. The azo dye acid orange 7 (AO7) degradation experiment showed that these MILs could accelerate PS activation to produce SO4•−. The MIL-101(Fe)/PS system presented the best degradation efficiency, which indicated that catalytic capacity was closely related to the active sites of catalyst and different cage sizes.

Graphene-based materials, which usually have good chemical durability and adsorption capacity, have been demonstrated as eco-friendly candidates for environmental catalysis. In previous studies, nanostructured graphene and graphene derivatives such as graphene oxide (GO) and reduced graphene oxide (rGO) was employed to activate PMS/PDS for degradation of target pollutant and graphene showed much poorer catalytic performance than GO and rGO. Although graphene has been employed as an adsorbent for various pollutants, its efficiency for the activation of persulfates is limited, which is ascribed to poor electron supply capacity by its stable π-conjugated system (Bekris et al., 2017). Pristine graphene oxide is also highly stable with poor reactivity due to the excess of oxygen content. In contrast, GO with relatively lower oxygen content has higher catalytic activity (Hu et al., 2017). GO requires surface modification, such as heteroatom doping and chemical/thermal reduction to rGO, to improve the catalytic activity. The reduction of GO will change the amounts and distributions of oxygen-containing functional groups on its surface, which influences catalytic activity and adsorption ability. It was found that adsorption ability was generally enhanced with increase of reduction degree of GO by introducing electron-donating functional groups, which benefited oxidation process occurred on rGO (Oh & Lim, 2018). Duan et al. found that rGO-900/PMS achieved complete removal of phenol in 150 min, and degradation efficiency was promoted when rGO synthesis temperature was elevated, due to removal of excessive oxygen groups (Duan et al., 2016). For degradation of various organic pollutants persulfates activation by carbon materials supported with metal has depicted in (Table 6). In every AOP, production of free and strong OH radicals is achieved through different sources of power and materials. Firstly, in UV peroxide AOPs, power from UV radiation is capable of producing necessary OH radicals. In contrast to Fenton processes, addition of Fe3+/Fe2+ ions with H2O2 (Fenton reagent) produces highly active Fe2+/Fe3+ ions, respectively, with OH radicals providing the necessary catalytic system to properly treat specific wastewater. In electro-Fenton reactions, the process utilizes electrical energy to split the water molecule into OH free radicals with simultaneous presence of Fe ions. Ozone-based processes use photons (hv) to initiate the production of hydrogen peroxide which, in turn, produces OH radicals. In photocatalysis, a metaloxide catalyst is used, which utilizes the UV/Vis radiation to create electron hole pairs for the generation of OH free radicals. Ultimately, energy from ultrasound (sonolysis) is efficient for rapid production of H and OH radicals with high yield. In Table 7, the main advantages and disadvantages of each AOP are presented.

The majority of fluorine-containing wastes are currently dumped in landfills or used for low-value purposes; thus, it is necessary to carry out high value-added resource treatment for slag utilization. Industrial effluents, such as colorless antibiotics and colored dyes, have recently risen to forefront of concern due to their widespread distribution in water bodies, which threatens the natural system. The decomposition of organic pollutants using photocatalytic technology is proven to be successful and promising. Under the influence of ultraviolet (UV) or visible light, a catalyst is used to speed up chemical reactions in a photocatalyst. The words “photo” and “catalysis” come from ancient Greek. Light is the “photo” in “photocatalysis,” which increases the rate of a reaction using an exogenous material (the “catalyst”) that is not itself consumed in the process. A catalyst minimizes activation energy required for a chemical reaction, boosting the rate of reaction. To sum up, photocatalysis is the technique of using light and catalysts to speed up a chemical reaction. Photocatalysis can be either homogeneous or heterogeneous, depending on the photocatalysts used (Esplugas et al., 2007). To degrade the pollutant dyes, heterogeneous photocatalysis includes a number of reactions. However, homogeneous photocatalysis includes metal complexes as a catalyst. In recent years to degrade harmful dyes, excellent heterogeneous photocatalysts are wide band gap semiconductors such as ZrO2, TiO2, SnO2, CeO2, ZrO, ZnO, etc. Band gap of certain metal oxides is given in Fig. 6. Production of electron–hole pairs on the illumination of light is materials properties as a photocatalyst. The recombination time of electron–hole pair is higher for a decent photocatalyst. Further, these e−/h+ pairs produce hydroxyl radicals (OH) and superoxide radicals (O2−) which have been demonstrated as excellent scavengers for photocatalytic degradation of harmful dyes.

Band gap energy (Eg) is difference of energy between the top of valence band (VB) and the bottom of conduction band (CB). If energy of light (which is illuminating on the solution of catalyst and organic dye) is higher than band gap of catalyst, excitation of e− will takes from VB due to which generation of h+ occurs in VB. These electrons and holes will migrate to catalyst's surface. The e− form superoxide radicals (O2•−) upon reacting with O2 available from surroundings and holes h+ form hydroxyl radicals (OH•) on reacting with water in the solution. During photocatalytic oxidation, these radicals react with organic dyes in order to get H2O and CO2 as final products (Rehman et al., 2009). Basic photocatalytic mechanism is shown by Fig. 7 and can also be understood with the help of given reactions (Suman et al., 2021a): The following benefits make photocatalysis best technology for treating wastewater: Photocatalysis produces non-hazardous byproducts, which is a major advantage. Complete mineralization of organic dyes. The band gap of photocatalytic material determines whether or not ultraviolet (UV) light can be used instead of sunlight. Dye degradation can make use of the environment's plentiful oxygen supply. For photocatalysis, scientists turn to semiconductor metal oxide photocatalysts since they're cost effective, stable chemically, recyclable, and non-toxic. Photocatalysis is a cost-effective and eco-friendly strategy for dye degradation.

Metal oxides are crystalline solids consisting of an oxide anion and a metal cation. Normally, they interact with acids to produce salts and with water to produce bases. These nanoparticles possess exclusive optical properties like UV absorption, dichoism, specific color absorption in visible range of spectrum, photoluminescence, etc. Also, these nanoparticles have unique physical and chemical properties which are ascribed to large density and small size of edges as well as corners on their surface. Since particle dimension/size is small at nanoscale, a significant portion of atoms would be on the surface of particles. The reactivity of nanoparticles increases as the effective surface area increases because they are capable to react to a larger extent in comparison to that of normal crystals. Consequently, there are many metal oxide nanoparticles on the surface in order to react with incoming molecules. In metal oxide nanoparticles specifically like ZnO, CeO2,SnO2, TiO2, CuO, ZrO2, etc., stress or strain and adjoining structural perturbation are generated by increasing number of interface and surface atoms as size decreases in case of nanoparticles (Hosseini et al., 2022). In comparison to reactivity of bulk, metal oxide nanocrystals exhibit intrinsically greater chemical reactivity due to their unique surface chemistry (Naseem & Durrani, 2021). Due to size-related structural alterations, changes in cell parameters have been observed in nanoparticles of SnO2, TiO2, CeO2, ZnO, ZrO, AgO, MgO, CuO, etc. Due to their small size and significant surface area, metal oxide nanoparticles exhibit exceptional catalytic properties. For ecological remediation, particularly in the decomposition of organic pollutants present in wastewater, photocatalytic active materials dependent on nanomaterials of metal oxides are frontliners. These nanoparticles are well-suited for photocatalytic oxidation and removal of organic pollutants ion because to their superior surface, structural, and crystalline features (Ahmed & Mohamed, 2022). Hence, metal oxide nanomaterials like TiO2, CeO2, SnO2, ZrO2, MnO2, WO3, Cu2O, CuO, and ZnO focusing on photocatalytic applications have outstanding performances (Park et al., 2022; Rajendiran et al., 2022). The structural design, size, and shape of nanostructures have a significant impact on aforementioned properties of metal oxide nanoparticles. Consequently, structural characteristics and fundamental properties of functional devices are affected by number of factors which in turn have influence on their performance. For the manufacture of metal oxide nanoparticles with tunable structural features and properties, a sustainable and acceptable technique should include optimization of all process parameters that could result in unfavorable adjustment of obtained characteristics (Theerthagiri et al., 2018).

Initially photocatalysis mechanism requires retainment of light irradiation higher in comparison to threshold band gap energy, which is a property of semiconducting materials. Absorption of light quantum results in photogeneration of e− and h+ in CB and valence band VB, respectively. These electron–hole pairs degrade organic pollutants upon reacting with them by diffusing on surface of metal oxides during photocatalytic degradation reaction. Metal oxides have emerged as most important photocatalysts for environmental rehabilitation of polluted drinking water and industrial wastewater over past few decades. Many semiconducting photocatalysts, including WO3, ZrO2, SnO2, CeO2, Cu2O, and others, have been used because of their superior optical, electronic, and anisotropic properties, such as high surface area, environmental friendliness, efficiency, cost, toxicity, availability, stability, and resistance to different irradiation wavelengths. Although, most of metal oxides show good photocatalytic properties but TiO2 has gained much importance towards photocatalytic activity because of its outstanding electrical, optical properties and chemical stability. TiO2 nanoparticles have three crystalline phases: (1) rutile, (2) anatase, and (3) brookite. Out of all three phases, anatase is highly efficient and stable in photocatalytic degradation of organic pollutants due to its open structure in comparison to that of rutile phase. Among rutile and brookite phases, photocatalytic activity is also shown by rutile phase but to a smaller extent; however, no appreciable photocatalytic activity is displayed by brookite phase. Charge transfer properties, favorable combination of electronic structure and lifetime of exciton (MONPs) has made it feasible for their utilization in the field of photocatalysis. Although MONPs have various useful properties but there are also some disadvantages for certain photocatalysts which prohibit their large-scale applicability in the field of photocatalysis. Broad band gap (3.2 to 3.4 eV) of metal oxides (ZnO, TiO2 etc.) limits their photocatalytic activities to ultraviolet (UV) region of light. Besides of it, narrow band gap (2.6–3.0 eV) of metal oxides like CuO, V2O5, WO3, Fe2O3 etc. have the capability to retain maximum solar light which is responsible for hasty recombination of e−/h+ pairs which in turn decreases charge kinetics process and eventually overcome photocatalytic activity of semiconducting metal oxides. Consequently, there is a solid need to plan and use effective metal oxide photocatalytic materials with better properties to overcome these drawbacks. Doping of rare earth elements like La, Ce, Er, Gd, Yb, Sm etc. is vastly employed strategy for enhancement of photocatalytic performance of metal oxide nanoparticles (MONPs). Modification of metal oxides can be done by creating point defects like oxygen vacancies, doping impurities and metal interstitials (Buelens et al., 2019; Raizada et al., 2021; Yuan et al., 2016). It has been investigated by many researchers that rare earth doping improves the photocatalytic, surface and optical properties of metal oxides. Rare earth doping has also been shown to shift the working wavelength, which in turn shrinks band gap to visible region of the spectrum. Thus, narrowing of band gap improves MONPs' photocatalytic performance. Rare-earth doping also leads to formation of oxygen vacancies as defects, which are responsible for increasing degradation efficiency because their presence and character lower the band gap and shift the oxide’s response towards visible spectrum (Zhang et al., 2022b). As a result of the doping with rare earth elements, the following occurs: Enhancement in photocatalytic activity Visible light absorption increases Active surface area increases Enhancement in thermal stability Band gap decreases Effectual separation of photogenerated e−/h+ pairs To observe the photocatalytic effectiveness of La doped CeO2 for degradation of Rose Bengal (RB) dye, photocatalytic test was performed. The absorption curves at various time durations under exposure of UV light for RB dye solution are shown in Fig. 8. Degradation of dye is shown by regular decrement in intensity of peak. It has been found that degradation percentage enhances from 76.7 to 86.7% for 2% La-doping to 8% La-doping. Figure 9a depicts decrement in relative concentration of dye with exposure time. Figure 9b depicts percentage degradation efficiency versus. Linear fitting of –ln (C/Co) with respect to time is shown in Fig. 9c and variation in electricity cost with dopant (La) content is calculated in Fig. 9d. Comparative study of cerium (Ce), lanthanum (La), and neodymium (Nd) doped SnO2 was observed for photodegradation of phenol in aqueous solution. Figure 10a shows the relative concentration of dye versus time for La-doped SnO2. Degradation efficiency continuously increases with increment in Ce-doping in host lattice as displayed by Fig. 10b. Figure 10c depicts relative dye concentration versus time for Nd-doped SnO2. Upon utilizing 0.4 wt% Ce: SnO2 catalyst, 86% phenol was found to be degraded after 120 min. of irradiation time. It has also been observed that the photocatalytic performance increases from 57% (0.2 wt% Nd: SnO2) to 95% using 0.6 wt% Nd: SnO2 as a catalyst. However, only 24% of phenol has been found to be eliminated upon using undoped SnO2 catalyst under same irradiation time. 100% degradation of phenol was observed for 0.6% La: SnO2 catalyst. Figure 15 presents comparison of decrement in the concentration of phenol dye with the help of different rare-earth (La, Ce, Nd) doping SnO2 nanoparticles. Hence, it can be concluded that amongst three dopants, La-doped-SnO2 samples are highly photoactive as 100% phenol was degraded under equivalent exposure time. The photocatalytic degradation efficiency of pristine ZnO, gadolinium (Gd)-doped ZnO, La-doped ZnO, and Gd/La co-doped ZnO nanoparticles was observed for degradation RhB dye and colorless pollutant tetracyanonickelate (TCN) under irradiation of sunlight as depicted in Fig. 11. In comparison to pristine ZnO photocatalysts, La/Gd co-doped ZnO showed greater degradation percentage. After 100 min of light irradiation, greatest RhB degradation efficiency of Gd/La@ZnO nanoflower was 91%, reflected in Fig. 11a. The dye degradation percentage for ZnO, Gd@ZnO, and La@ZnO was calculated to be 20%, 42%, and 26%, respectively. Using pseudo-1st-order kinetics, rate constant (k) for RhB degradation reaction was estimated, as shown in Fig. 11b Gd/La@ZnO photocatalyst has a k value of 0.023 min−1, which is 4.6 and 7.7 times greater than Gd@ZnO and La@ZnO photocatalysts, respectively. In addition, photocatalytic effectiveness and kinetic plot of a colorless pollutant (TCN) was examined, as depicted in Fig. 11c, d. Gd/La@ZNO had a maximum degrading efficiency of 74% and a “k” of 0.013 min−1. The degradation of RhB dye under visible light illumination has been employed to test photocatalytic performance of pristine and Ce@WO3nanorod samples (see Fig. 12a). Under illumination of visible light, blank experiment exhibit negligible degradation. Within 30 min, 11.5%, 23.3%, and 24% of dye were absorbed by tungsten oxide (WO3), 3% Ce@WO3 and 5% Ce@WO3 before light on. The degrading efficiency of catalyst 3% Ce@WO3 was observed to be 96%, which is 1.28 times higher than pure WO3. The rate of photocatalytic degradation of RhB dye is shown in Fig. 12b. Notably, 3% ce@WO3 catalyst has a higher K value (0.0318447 min−1) than pure WO3, which is 2.6 times greater. Figure 12c depicts the 3% Ce@WO3 catalyst’s cyclic RhB dye degradation performance. The efficiency decreases from 90 to 89% when third cycle is completed. It demonstrates that 3% Ce@WO3 nanorods is a fantastic photocatalyst. After the recycling test, XRD profile was collected, as shown in Fig. 12d. The catalyst’s XRD pattern has not changed significantly. Thus, it examined the catalyst's enhanced photostability and reusability. The photocatalytic activities of pure and rare earth (RE) ions (Ce, Eu) doped TiO2 nanowires were investigated using degradation (C0/C) of organic TBO dye in an aqueous solution shown in Fig. 13. Where, Co and C are the starting concentration and concentration of organic dye in reaction solution at time (t), respectively. Ce3+:TiO2 and Eu3+:TiO2 nanowire arrays, when exposed to UV light, degrade almost 90% of toluidine blue O (TBO) within 80 and 120 min, respectively. High surface area with large aspect ratio is strongly related to improvement in photocatalytic performance for RE-doped TiO2 thin films (Bandi et al., 2013). The impact of dopant concentration on photocatalytic performance of Gd-doped TiO2 is shown in Fig. 14. The efficiency of MB decomposition rises initially and then falls when Gd doping content rises. When Gd content is 5 wt%, degradation percentage of MB reaches 90% in visible light with a wavelength of 405 nm, whereas pristine TiO2 only reaches 76% after 10 min. This finding suggests that a small quantity of Gd doping can boost TiO2’s photocatalytic effectiveness (Tang et al., 2022). The decrease in Congo Red (CR) concentration vs time for ZS (ZnO:Sm) sample is displayed in Fig. 15a, starting from varied values of initial dye concentration. Furthermore, Fig. 15b shows relationship between initial reaction rate (r0) and apparent initial dye concentration (C0) for ZS (ZnO:Sm) material. Finally, process optimization was done empirically using gradient approach. The optimal parameters for CR dye degradation in presence of ZS sample were found to be an initial dye concentration of 10.7 mg/L and a catalyst dose of 0.236 g/L. In these settings, a maximum color removal performance of 95.8% was measured experimentally. Thermal activation also allowed for successful recovery of wasted catalyst (Pascariu et al., 2019). How much rare earth doped metal oxides are efficient for water remediation are illustrated in Table 8.

To improve photocatalytic performance of these wide band gap semiconducting metal oxide nanoparticles, use of transition metals as dopants, such as vanadium (V), tungsten (W), iron (Fe), cupper (Cu), silver (Ag), niobium (Nb), manganese (Mn), and so on, appears as a promising strategy (Chahal et al., 2022). A detailed literature survey of photocatalytic performance of transition metal ion doped MONPs are given in Table 9.

High photocatalytic efficiency is achieved by improving optoelectronic properties of photocatalysts, which in turn improves their light absorption, photo-induced charge separation, and charge transfer. As it has been already discussed that semiconducting metal oxides have shown great promise in the field of photocatalysis. Unfortunately, high recombination rates of these photogenerated e− and h+ pairs make these materials inefficient for photocatalytic activities (Suman et al., 2021b). In addition, wide band gaps of these materials also hinders their practical application in visible-light region of solar energy, which is the greatest source of energy on earth and sufficiently available. Moreover, when the surfaces of these nanomaterials are covered with products of catalysis reactions, interactions between incoming target molecules and surface are restricted by some extent. As a result, photoexcited e− and h+ pairs cannot be transferred for further photocatalytic reactions. Therefore, it has been realized that designing multi-component systems by assembling functional nanomaterials with metal oxide photocatalysts would be a better way to solve these issues (Rani et al., 2020; Suman et al., 2021c). Many metal oxide-based photocatalysts have been functionalized to boost their photocatalytic activity and electrical/optical features in response to these difficult problems. The fictionalization of metal oxide photocatalyst nanomaterials by coupling with transition metal oxides, ferrites or graphene nanostructures has attracted considerable interest in the development of high-performance photocatalytic systems in recent years. These multi-component nanocomposite systems composed of metal oxides, graphene (or its derivatives) and other metal oxides are highly significant as alternatives to extensively studied bi-component systems for the design of high-performance photocatalytic systems. The outstanding electrical and surface plasmon resonance capabilities of these nanomaterials make coupling of metal oxides with graphene and metals advantageous for the enhancement of optoelectronic features (Chahal et al., 2023). Under influence of visible light, vanadium oxide (V2O5) nanorods operate effectively as photocatalysts. However, within 210 min of exposure time, 66.85% photodegradation was noted due to recombination of photogenerated electron–hole. In comparison to UV and Visible light, photocatalytic performance of as-prepared grapheme oxide-vanadium oxide (GO-V2O5) nanocomposite was better under direct sunlight. The GO-V2O5 nanocomposite demonstrated nearly complete degradation of 97.95% of VB dye in 90 min under direct sunlight; however, degradation of VB dye under visible (96.93%) and UV light (78.35%) sources takes 180 and 210 min, respectively. Due to its effective charge transfer capabilities, which inhibit recombination of electron–hole pairs, as-synthesized GO-V2O5 nanocomposite exhibited greater photocatalytic activity compared to pure V2O5 (Beaula Ruby Kamalam et al., 2021). Malachite green (MG) and methylene blue (MB) photocatalytic degradation with TiO2 and rGO-Fe3O4/TiO2 were observed under illumination of visible light by glancing at major absorption peak at 617 nm and 658 nm, respectively, as shown in Fig. 16a and b. The findings showed that rGO-Fe3O4/TiO2 has a photocatalytic efficiency of 99% for MG and 97% for MB as compared to TiO2, which has a photocatalytic efficiency of MG (67%) and MB (62%), respectively. Increased photocatalytic performance of rGO-Fe3O4/TiO2 is correlated with the presence of reduced graphene oxide (rGO), which is significant for photocatalytic decomposition of MG (Bibi et al., 2021). Figure 17 illustrates photo-Fenton performance of various samples (ZnO, Fe3O4, Fe3O4/GO, ZnO-Fe3O4/rGO) toward MB dye degradation. ZnO-Fe3O4/rGO nanocomposite sample needed 150 min to remove 97% of MB dye from an aqueous solution when exposed to visible light, whereas Fe3O4/GO could only decompose approximately 45% of total MB concentration. The electrons in VB of ZnO can thus be stimulated into CB of ZnO when exposed to visible light in ZnO-Fe3O4/rGO system, and they can then be successfully transferred to Fe3+/Fe2+redox pair in Fe3O4 across the graphene interlayer (Ojha et al., 2017). Table 10 lists various nanocomposites of metal oxides that serve as photocatalysts.

By using a schematic illustration such as shown in Fig. 18, it is possible to understand how electrons and holes move between shell and the core and how it affects photocatalytic performance. The •OH and O2 radicals play a crucial part in decomposition of dye, contributing in transformation of hazardous dye molecules into non-toxic product. Excitation of VB and CB of core and shell during photodegradation process can result in generation of electrons and holes. A shift of energetic electrons from shell’s CB to core’s CB and a transfer of holes from shell's VB to core’s VB can both occur during the core–shell interface. The hole that moved onto the surface of the shell improved the core's photostability. This prevents the recombination of photogenerated electron and hole. Superoxide (O2·−) and hydroxyl (OH•) radicals can be produced when excited electrons interact with adsorbed oxide species on core@shell. Furthermore, adsorbed H2O molecules can react with holes in VB of core and shell to produce OH• radicals. Therefore, photocatalytic activity of core@shell for degradation of dye is considerably enhanced by these O2·−, OH·, and h+ species. These strong oxidizing agents, OH• and O2·−, are responsible for causing degradation of polluting dyes in non-toxic products. This demonstrates how core–shell nanoparticles (NPs) can increase production of OH• radicals by lowering the rate at which photo-induced charge carriers recombine. It supports the development of core–shell NPs’ improved photocatalytic performance (Sonia et al., 2023). Table 11 depicts various semiconducting metal oxide–based core–shell nanostructures as an efficient photocatalysts. RB dye was photodegraded under ultraviolet light to test photocatalytic performance of hematite and ceria-based core–shell nanostructures. For degradation of RB, CeO2 and α-Fe2O3 were found to have very low photocatalytic activity. Because recombination and charge generation processes of pure CeO2 and α-Fe2O3 are suppressed, heterojunction between two types of nanoparticles can be produced by connecting CeO2 to α-Fe2O3 or vice versa. The effectiveness of photocatalyst could be enhanced by this heterojunction. When exposed to UV light for 75 min, α-Fe2O3@CeO2 core–shell nanostructures showed 93% degradation of Rose Bengal dye, compared to bare CeO2 and α-Fe2O3, which were 78% and 75%, respectively, as seen in Fig. 19 (Suman et al., 2021d). Under conditions similar to sunlight, photocatalytic performance of samples of α-Fe2O3@SnO2/Mxene-3.5 with various RhB dye reaction periods is investigated. The photodegradation curves of α-Fe2O3@SnO2/Mxene-3.5 with various reaction times for RhB dye are shown in Fig. 20a. As reaction time increases, degradation impact also does. The photocatalytic efficiency of α-Fe2O3@SnO2/Mxene-3.5 sample in 140 min (E140) can increase up to 72.43%, particularly when reaction duration was 24 h. The fitting outcomes in accordance with first-order kinetics are shown in Fig. 20b. The 24-h reaction rate constant of α-Fe2O3@SnO2/Mxene-3.5 would show maximum (Zhang et al., 2022c).

Metal oxides have been considered as potential materials for wastewater remediation due to their chemical stability, nontoxic nature, and excellent optical and electrical properties. The drawbacks of metal oxide nanoparticles/nanostructures such as wide band gap, electron–hole pair recombination, and UV irradiation working range can be significantly resolved by nanocomposites of metal oxides, their core–shell nanostructures and doping/co-doping of rare-earth as well transition metal ions. Rare-earth doping improves the optical, photocatalytic, and surface properties of nanoparticles of metal oxides. Surface defects like paramagnetic centers as well as oxygen vacancies are created due to rare-earth doping which reduces the recombination rate of electron–hole pairs. Because of these surface defects, appearance of additional visible light absorption edges takes place, which are answerable for the increment in photocatalytic performance of metal oxide photocatalysts in visible range of spectrum. Thus, this review showed the current aspects of photocatalytic process which is best economical and environment friendly way for treatment of wastewater.

The global photocatalysts market should reach $5.1 billion by 2026 from $3.1 billion in 2021 at a compound annual growth rate (CAGR) of 10.5% for the forecast period of 2021 to 2026. The construction segment of global photocatalysts market is expected to grow from $2.7 billion in 2021 to $4.5 billion in 2026 at a CAGR of 10.3% for the forecast period of 2021 to 2026. The consumer products segment of global photocatalysts market is expected to grow from $238.7 million in 2021 to $421.9 million in 2026 at a CAGR of 12.1% for the forecast period of 2021 to 2026. In terms of material type, photocatalyst market is segmented into • Titanium dioxide • Tungsten trioxide-based • Others In terms of end use, photocatalyst market is segmented into • Construction • Consumer products • Others In terms of geography, photocatalyst market is segmented into • North America • Europe • Asia-Pacific • Rest of the World The global market for photocatalyst-based products is expected to increase from REDACTED in 2020 to REDACTED in 2026 growing at a CAGR of REDACTED during the forecast period. Increased use in construction and consumer product sectors and in general improvement of global economic conditions is the main drivers for this expansion, which corresponds to a compound annual growth rate (CAGR) of REDACTED during the period 2020–2026. Since 2018, a marked increase in product demand has been partially offset by a significant reduction in unit prices of raw materials, resulting in lower revenues. With COVID-19 outbreak, subdued demand from construction sector has reduced demand for photocatalysts in 2021, which is expected to recover by the end of 2022. Photocatalysts are a category of catalysts that use photons to activate chemical reactions. Superior oxidizing properties and super-hydrophilicity characterize these products. Since late 1960s, these unique properties have been used in various commercial products. Although anatase titanium dioxide is most common material used to produce photocatalysts, other compounds are available (or are under development) to achieve higher photocatalytic efficiency. Photocatalysts have current and potential use across seven main industry sectors: automotive, construction, consumer products, energy, environmental, mechanical/chemical, and medical/dental. From literature survey, electricity cost for many materials has been calculated. Table 12 shows a brief view of electricity cost in the field of photocatalysis for various photocatalysts. The power consumption can be estimated using following relation:where t90 signifies time taken by any dye to be degraded 90% of its initial concentration, EC is electricity cost, P is power consumed (in Watt) of UV light source. Power consumers consuming a maximum 500 units of electricity per month pay INR 4.68 per unit in our locality (INR-Indian rupees).

Seeing the immense literature on photocatalytic development, it seems that nothing is left; however, still multiple problems regarding the sensible operational performances has to be enhanced. Overall to derive photocatalysis for wastewater treatment, following aspects should be under scrutiny: Simplified, cost-efficient, and environment-friendly photocatalysts are required. The chemical process driven by photocatalysts is not simply understood. Once large-scale operations are final word factor, exploration focus ought to be directed towards the employment of extensive and noble-essence free co-catalysts. The employment of these catalysts within the natural system is restricted because of low recycling capability, small size, and lower mechanical strength of these catalysts.