Recent Advances in Efficient Photocatalytic Degradation Approaches for Azo Dyes

Chowdhury, Arpita Paul; Anantharaju, K. S.; Keshavamurthy, K.; Rokhum, Samuel Lalthazuala

doi:https://doi.org/10.1155/2023/9780955

Journal of Chemistry

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Abstract Introduction Conclusion Data Availability Conflicts of Interest Authors’ Contributions References Copyright Related Articles

Review Article | Open Access

Volume 2023 | Article ID 9780955 | https://doi.org/10.1155/2023/9780955

Recent Advances in Efficient Photocatalytic Degradation Approaches for Azo Dyes

Arpita Paul Chowdhury,¹K. S. Anantharaju,¹K. Keshavamurthy,²and Samuel Lalthazuala Rokhum³

Academic Editor: Kamala Kanta Nanda

Received04 Sept 2023

Revised15 Nov 2023

Accepted07 Dec 2023

Published26 Dec 2023

Abstract

In recent decades, the textile industry has contributed to continuous pollution in the environment. Synthetic dyes which are commonly found in waste water are azo, sulfur, anthraquinone, triphenylmethyl, indigoid, and phthalocyanine derivatives. These pollutants block the light penetration in water bodies and prevent photosynthesis activity, thereby affecting aquatic life. As an environmental crisis, several technologies have been explored to control pollution. Among all the techniques, the photocatalysis process is considered as a green, simple, and economical process. To improve the photocatalytic activity, researchers worldwide have investigated various photocatalysts such as metal oxides, metal ferrites, and heterostructured nanocomposites. The major goal of this review article is to propose a high-performing, cost-effective hybrid photocatalyst reported to date for prospective azo dye pollutant remediation. This review article also aimed to highlight the challenges and uncertainties associated with dye degradation in the photocatalytic process.

1. Introduction

Industrialization growth worldwide drastically leads to environmental problems due to release of the pollutants. According to the reports, about 20% of the total production of dyes is being discharged into the water surface [1]. These toxic chemicals cause great concern for both the environment and public health. The extreme discharge of the effluent adversely affects aquatic life and soil fertility and disturbs the ecosystem integrity by increasing the chemical oxygen demand and biochemical oxygen demand, altering the pH, and affecting water quality [2]. The wastewater containing organic dye pollutants blocks the ecosystem and causes threats to environmental sustainability. The intense color of the wastewater discharges due to the textile dyeing industry cannot be hidden, which is persistent due to the poor biodegradability of these dyes [3]. Dyes absorb the visible spectrum light from 380 to 750 nm. The dye molecule consists of three essential groups: the chromophore, the auxochrome, and the matrix. The chromophore is the active site of the dye in which upon excitation, the electrons of a molecule absorb light energy. Chromophore-containing molecules feature extensively in surgical practice, with synthetic dyes gaining popularity over endogenous optical adjuncts [4]. The common chromophore are ─NO₂, ─N=N─, ─C=S, ─C=O, ─C=C─, and ─N=O. The organic molecules containing these groups are known as chromogenic. The chromogenic molecule with the accumulation of auxochrome groups shows dyeing possibilities. These auxochrome groups allow the fixation of the dyes and can modify the color of the dye. The auxochrome groups are acidic (COOH, SO₃, and OH) or basic (NH₂, NHR, and NR₂), while the third part of the molecule is the rest part of the atom called as matrix [5]. The azo dyes with the (-N=N-) chromophore group account for 70% market share and are highly soluble in water [6].

The nitro group presence makes the dye mutagenic and highly toxic. The breakdown products, such as 1,4-phenylenediamine and o-tolidine, are highly poisonous [7]. However, the main limitation of these dyes is their potential carcinogenicity which limits its application [8]. Recent documentation has found an interplay of azo dyes with human serum albumin and hemoglobin [9]. Consequently, the elimination and prior treatment of this colored pollutant are essential and challenging. Many technologies have emerged for the treatment of polluted water. The treatment methods such as the membrane process, biological process, reverse osmosis, chemical and electrochemical technology, ion exchange, electrodialysis, electrolysis, adsorption techniques, and advanced oxidation processes are commonly used in conventional treatment technologies [10]. These methods are not efficient to completely destruct the chromophoric structure of the dye. Advanced oxidation processes (AOPs), such as the photocatalytic degradation process recently considered as an eco-friendly, cost-effective, and complete mineralization of the dyes are observed compared to traditional methods. The active radicals such as hydroxyl radicals (•OH), superoxide anion (•O₂⁻), and hydrogen peroxide (H₂O₂) are generated in situ to degrade organic dye compounds [11]. Thus, photocatalysis is a greener way to debase the pollutants of water with semiconductors and is intensely investigated worldwide [12].

The objective of this present review article is to explore applications of different nanomaterials as a robust photocatalyst for the treatment of wastewater containing azo dyes generated from textile industries. In particular, the authors focussed on the recent developments and included studies only published in the years, 2015–2023. Additionally, the effect of different parameters which includes the dye concentration effect of light irradiation, pH, and catalyst loading is discussed for the optimization of the degradation efficiency of textile wastewater.

2. Chemical Structure and Classification of Azo Dyes

The chemical structure of an azo dye, in general, consists of a solubilizing group, auxochrome groups, and chromophoric groups [13]. The structure of an azo dye is depicted in Figure 1. The nature of the azo bonds and the associated chromophore and auxochrome groups decide the color of the dyes. The number of azo linkages present in an azo dye molecule classifies the molecule as monoazo, diazo, triazo, and polyazo. Furthermore, azo dyes can be subdivided into acidic, basic (cationic), direct (substantive), disperse (nonionic), reactive, vat, and sulfur dyes [14]. Azo dyes in the color index (CI) system are categorized with numbers that range from 11,000 to 39,999 in correspondence with the chemical structure [15–17]. Figures 2 and 3 illustrate the classification of the dyes.

3. Dye Removal Technologies

The dyes containing waste from the textile industries need to be treated to remove the pollutants for the safety of the marine environment and ecosystem. Many technologies have emerged for the remediation of these dyestuffs. The treatment technologies are broadly classified as (a) biological treatment, (b) physicochemical treatment, (c) advanced approaches, and (d) combinatorial treatment [18]. Figure 4 represents the overview of different dye removal technologies.

3.1. Biological Treatment

Biological treatment is a low-cost and effective method for azo dye removal. The azo bond cleavage is difficult under the aerobic condition, but in the anaerobic condition, it can be easily accomplished. At present day, the technology comprising the anaerobic and aerobic conditions is used to degrade azo dye wastewater [19].

3.1.1. Fungal-Assisted Degradation

Fungi are present everywhere in the natural ecosystem, and it has a strong ability to adopt metabolism to various carbon and nitrogen sources. Recent studies reported that complex organic pollutants can be degraded in the presence of fungi by secreting various intracellular and extracellular enzymes [20]. The decolorization of azo dyes by fungi is triggered by adsorption onto hyphae, followed by the breakage of chemical bonds through extracellular enzymes [21]. Various reports have been published that showed fungal biomass possesses the potential to degrade and decolorize a wide variety of azo dyes. Gomaa et al. [22] explored Aspergillus arcoverdensis SSSIHL-01 fungi for the removal of CR dye and reported 98.6% efficiency. Ali et al. [23] used Meyerozyma caribbica SSA1654 and showed biodegradation of Acid orange 7 dye. Pulp wash was used to synthesize Pleurotus sajor-caju-fungal lignin-modifying enzymes for an inducing substrate, and for cell immobilization, L. cylindrica (L.C) was used. The fungus showed higher Lac activity (126.5 IU·mL⁻¹) and was used for RB 5 decolorizing of the dye with 89.4% decolorization as demonstrated by Fernandes et al. [24].

3.1.2. Yeast Assisted

Yeasts have great capability for bioremediation of xenobiotics deposited in wastewater. However, only a few reports are present so far that deal with the biodegradation of aromatic dye pollutants [25, 26]. Pichia occidentalis A2, a halotolerant yeast, was recently reported by Wang et al. [27] that can degrade acid red B azo dye. The CR dye with 100% efficiency was successfully removed by the adsorption process step with brewery yeast reported by Soh et al. [28]. Yeast consortium Meyerozyma guilliermondii, Yarrowia sp., and Sterigmatomyces halophilus showed degradation of RR 120 as demonstrated by Ali et al. [29].

3.1.3. Algal- and Bacterial-Assisted Degradation

The potential cyanobacteria, Rotifera, and Cercozoa can show AO-2 biodegradation with 95% efficiency demonstrated by Zhang et al. [30]. The study was conducted by Hamouda et al. [31] to explore the green microalga Scenedesmus obliquus growth using a disperse orange 2RL azo dye. After 7 days of algal growth with dye, the highest decolorization of dispersed orange 2RL azo dye (98.14%) was observed. El-Sheekh [32] reported Chlorococcum sp. for the elimination of reactive orange 2RL and reactive red M-2BF azo dye. At 40 ppm for reactive orange 2RL and 20 ppm for reactive red M-2BF, maximum decolorization was obtained. Agarwal et al. [33] developed a biofilm reactor for the decolorization of dyes. The reactor was continuously operated for 264 days, achieving a ≥96% reduction in COD and ADMI values. The study highlighted by Patel and Gupta [34] for the dye decolorization of acid maroon V by a potential bacterial consortium EDPA containing Enterobacter dissolvens AGYP1 and Pseudomonas aeruginosa AGYP2 in the presence of redox mediators.

3.2. Phytoremediation

The scientific community focussed on the alternative eco-friendly technique for the remediation of toxic dye wastage accumulated in soil and water bodies. It is considered as a sustainable alternative method for carrying out needed degradation [35]. Plant species such as Phaseolus mungo, Sorghum vulgare, and Brassica juncea have been evaluated for decolorization. Wetlands decolorization obtained by cocoyam plantations is 72%–77% efficient. Tagetes patula L. root system can efficiently degrade the reactive orange M2RL, reactive red M5B, methyl red, MO, and navy blue HE2R dyes [36, 37]. Though biological treatment is efficient, it is considered less flexible in design and operation. Moreover, satisfactory color removal and mineralization are not achieved. The complex chemical structure due to xenobiotic nature the biodegradability of azo dyes are limited [38]. However, in the anaerobic condition, it usually generates aromatic amines, which possess high biotoxicity and cannot be completely degraded. Hence, efficient and effective treatment processes need to be developed for the complete removal of azo dyes from wastewater [39].

3.3. Physicochemical Treatment

Physicochemical treatment is a separation technique that involves chemical reaction and includes processes such as the addition of coagulant, adsorption studies, chemical oxidation, flocculation, and electrocoagulation. Some of the processes are discussed as follows.

3.3.1. Adsorption

Adsorption is a surface phenomenon, in which only the adsorbent surface is concerned, and adsorbate should not penetrate inside the structure of the adsorbent [40]. Adsorption can be classified as physical adsorption and chemical adsorption. In physical adsorption, weak electrostatic forces bind the adsorbate to the adsorbent surface, and in chemisorption, strong covalent bonds exist between the adsorbate and adsorbent surfaces [41, 42]. The advantages of the adsorption process include the reusability of adsorbents, high efficiency, and the short time required for dye removal from wastewater [43]. Recently, He et al. [44] synthesized activated carbon based on edamame shell for the effective adsorption and removal of CR dye. Graphene oxides-TiO₂ is synthesized by Cano et al. [45] through the ball milling process. Efficient adsorption for methylene blue dye is observed within 9 min. Wang et al. [46] designed CoFe₂O₄/chitosan magnetic adsorbent supported on Mxene for the removal of CR, RhB, and MG dyes. The adsorption capacity for CR dye is up to 2095.9 mg·g⁻¹. In another work, Ti-MOFs were reported by Zhao and his coworkers [47] for the adsorption of methylene blue, methyl orange, and indigo dyes. Rana and Sharma [48] fabricated Fe₃O₄ embedded oxidized Sterculia gum/Gelatin hybrid and evaluated for the enrichment of MO and MB with maximum adsorption of 90% and 88%, respectively. Qasem et al. [49] reported Co-MOF@CNT nanocomposite for adsorption of cationic dye with 98% removal of MB dye at neutral pH. MnO₂-NP-CPC efficient adsorbent synthesized by Belcaid et al. [50] was analyzed for the removal of three different toxic dyes. In the same line, Sudan et al. [51] synthesized biochar from rice husk and investigated the adsorption of EBT dye and found 94% removal within 2 h contact time at 1.5 g catalyst dosage.

3.3.2. Ion Exchange

The ion exchange process significantly removes dyestuffs. The ion exchange resin has capability to disperse in concentrate contaminants [52]. Ion exchange resins have been classified based on the charge of the exchangeable counter ion and the ionic strength of the bound ion. Ion exchange resins are classified as (a) strong cation-exchange resins (—SO₃-H⁺), (b) weak cation-exchange resins (—COO-H⁺), (c) strong anion exchange resins of quaternary ammonium groups (type I resins contain —CH₂N(CH₃)₃⁺Cl⁻ groups and type II resins contain —CH₂N(CH₃)₂(CH₂CH₂OH)⁺Cl⁻ groups), and (d) weak anion exchange resins of primary (-NH₂), secondary (=NH), or tertiary-amine (≡N) functional groups in the chloride or hydroxide form [53]. Lu et al. [54] reported the magnetic cation-exchange microbeads for the removal of CV and acid green 9 dye. Cellulose films were grafted with p(GMA) via the ATRP method, and the epoxy groups were converted into phosphate (RCF-g-p(GMA)-PO₃H₂) and sulfate (RCF-g-p(GMA)-SO₃H) groups and used for the removal of chlorazole Black E by Bayramoglu and Arica [55]. In the meantime, Marin et al. [56] used the resin amberlite IRA 400, a basic anionic exchange resin for the treatment of acid orange 7. The sorption capacity at 25°C and 65 min was 317.6 mg·g⁻¹. The ion exchange behavior of microsized ammonium phosphomolybdate (APM) was investigated by Joseph and his coworkers [57] for the removal of MB, EY, MG, and MR dyes.

3.3.3. Coagulation and Flocculation

Coagulation-flocculation is a low capital cost, simple operation and is widely used for dye removal without the formation of any secondary pollutants. In this process, the dye solution system is destabilized to form agglomerates or flocs which settle down under gravity [58]. It is a well-known process, and here, with the addition of a coagulant, destabilization of the colloidal particles is carried out. The various operational factors are colloidal characteristics, turbidity, alkalinity, pH, and colloidal [59]. Hadadi et al. [60] in their research demonstrated Pinus halepensis Mill. seed extract’ biocoagulant efficacy for treating Congo red-containing wastewater. Around 80% dye removal with coagulant dosages from 3 to 12 mL·L⁻¹ is achieved. In the current investigation, Shende and his coworkers [61] have found that synthetic dyes MO, CR, MB, and safranin were removed from wastewater by cocoyam bioflocculant. The electrostatic force of attraction and hydrogen bonding mechanisms were observed for the dye removal process. The work by Ihaddaden et al. [62] reported the use of bentonite and Opuntia ficus-indica as coagulant-flocculant for the MB dye removal. The dye removal efficiency obtained is 98.29% within 19 min. Duan et al. [63] studied the removal of reactive orange using magnesium hydroxide and polyacrylamide and described the coagulation performance and floc characteristics. Obiora-Okafo et al. [64] explored natural organic polymers, Brachystegia eurycoma coagulant, and Vigna subterranean coagulant for the decolorization of AR44 found in wastewater. The kinetics of the coagulation-flocculation process was confirmed to be a pseudo-second-order model with > 0.997.

3.3.4. Membrane Filtration

Membranes have been used in several studies to treat colored wastewater and are considered one of the best solutions to clean wastewater due to their high separation efficiency, reliability, cost-effectiveness, and simplicity. Thus, ultrafiltration, nanofiltration, and microfiltration are an attractive technology. Polymeric materials, for example, PES, PAN, PVDF, and BPPO, are efficiently used [65, 66]. Li et al. [67] in their research reported MXene-PANI/PES membrane for CR and MB dyes removal [67]. Yusaf et al. [68] employed micellar-enhanced ultrafiltration for the removal of Procion blue MxR. Subrahmanya and his coworkers [69] reported a syringe filter membrane composed of Whatman 42 filter, GO₁₀/(SBA-15/PDA)₂ membrane to effectively screen MB and EBT dyes from wastewater. PVDF/IL-Na⁺MMT membrane is fabricated by Zhao et al. [70] for removal efficiency for both anionic and cationic dyes at a high level of over 90% efficiency. Li et al. [71], in their study, prepared a novel antifouling loose nanofiltration (LNF) membrane by cross-linking and grafting of hydrolyzed polyacrylonitrile ultrafiltration substrate with a PEI-based zwitterionic polymer PEI-CA through the covalent amide bond for the treatment of dye brilliant blue R, CR, MB, and chrome black T. Dmitrenko et al. [72] reported PAN membranes by various TiO₂ modifications [72]. Nanofiltration is a separation technology, where pore size ranges about 0.5–2.0 nm with molecular weight cutoff ranging from 200 to 2000 Da. The smaller pore size, higher permeability, and lower transmembrane pressure are advantages over ultrafiltration and microfiltration. Recently, Ahmadipouya et al. [73] reported polysulfone mixed-matrix nanofiltration membranes containing UiO-66 metal-organic framework and incorporated for MG, MB, MO, and MR dye adsorption. Li et al. [74] in their work prepared the antibacterial NF membrane using a hydrophilic antibiotic streptomycin with trimesoyl chloride for desirable performance for excellent dyes rejections such as DR 23, CR, AR66, Victoria blue B, and chrome black T dyes.

Nevertheless, the physicochemical process is not considered suitable for wastewater treatment due to its high costs and low efficiency. Moreover, disposal problems of the secondary waste generated which gets converted into toxic intermediates.

3.4. Advanced Approaches

3.4.1. Ozonization

Ozonation technology is a chemical oxidation treatment affected with ozone which possesses a strong ability to remove color and smell and is widely used for the treatment of wastewater. Li et al. [75] investigated the ozonization process on reactive dyes such as reactive blue FL-RN, reactive red FL-2BL, and reactive yellow FL-2RN. The synthetic azo dye, reactive black 5, and ozonization were probed by Chu et al. [76] with a microbubble generator. The catalytic ozonation reaction by Ru impregnated on SBA-15 material as a catalyst for reactive orange 4 was reported by Ghuge and Saroha [77]. Le et al. [78] in his work studied the decolorization of reactive red 239 and DB 71 using an ozonation membrane contactor containing PVDF. Fe-RGO oxidation catalyst is prepared by Qazi et al. [79] to investigate the ozonization process for the removal of MB dye by catalytic ozonation at pH 7. The catalyst dose was 0.02 g, and the ozone dose was 0.5 mg/min. Around 96% removal efficiency was achieved after 10 min. Dabuth and his coworkers [80] reported a catalyst consisting of coconut and coal-based activated carbon and eucalyptus biochar coated by titanium dioxide for catalytic ozonation. It is reported that the ozonation technique in wastewater dye pollutant removal is effective, but the complete mineralization of organic substances is still a challenge.

3.4.2. Electrocoagulation

The electrocoagulation technique is a simple, reliable, and low-cost method for the treatment of dyes contained in wastewater. The process has good removal efficiency for COD, color, turbidity, and dissolved solids. The advantage of electrocoagulation over coagulation is the use of fewer reagents, lower conductivity, and neutral pH value, which suits the treated water for some industrial applications [81]. In this technique, a direct current source is applied between metal electrodes, which produces metal hydroxides immersed in polluted water. The metal hydroxides formed lead to the removal of the contaminants by acting as coagulants [82]. A study by Márquez et al. [83] deals with electrocoagulation techniques for the removal of brilliant green dye. The electrocoagulation process investigated by Lach et al. [84] revealed a high azo dye removal efficiency (96.5%) with TOC and COD of 93.5% and 85.0%, respectively. Liu et al. [85] presented a reverse electrocoagulation treatment of methyl orange and alizarin yellow dyes using Fe-Fe electrodes. The efficiency for methyl orange and alizarin yellow achieved was 98.7% and 98.6%, respectively, using NaCl as the electrolyte. Yao et al. [86] designed an electrocoagulation reactor and studied methyl orange as the target pollutant and found 92.16% removal efficiency with 16.8% COD removal efficiency. Brilliant yellow azo dye removal by the electrocoagulation method was reported by Ahmed et al. [87] using an Al helical coil and obtained 92% efficiency in dye removal within 90 min. Handaoui et al. [88] reported indigo dye removal with 98.4% removal at a pH of 7.5 and in 60 mg/L solution concentration. The electrocoagulation is an eco-efficient electrolysis-based technology, but the electrodes need to be replaced which is a disadvantage and may also lead to electrode passivation [89].

3.4.3. Fenton Oxidation

The Fenton oxidation is an advanced alternative process for the treatment of textile azo dyes. The Fenton reaction process involves the reaction between hydrogen peroxide and iron which further produces reactive oxygen and hydroxyl species that enhance the degradation of pollutants in the absence or presence of light (photo-Fenton) [90]. Eskikaya et al. [91] studied the degradation of BR18 and RR180 using the hydrochars obtained from laurel leaves and watermelon peels and observed removal efficiency of 98.8% and 99.8% within 60 min and with 100 mg·L⁻¹ initial dye concentration. Magnetic Fe₃O₄-NIO calcium alginate beads were prepared by Ayed et al. [92] for the degradation of Novacron blue dye. The researcher observed 80% efficiency in the photo-Fenton process compared to only the Fenton process (50%) after 120 min reaction time. Jain et al. [93] fabricated zinc oxide-graphene oxide nanohybrid and employed it for brilliant green degradation. Tang et al. [94] prepared Cu-FeOOH/TCN composites and acquired 97.8% degradation efficiency with an 85.5% mineralization rate. The SrWO₄@MIL-88A(Fe) composites were reported by Wei et al. [95] for the degradation of MB within 4 min and 93.75% degradation rate was achieved. Dai et al. [96] synthesized ferrous ion-modified Laponite@diatomite composite for photo-Fenton degradation of MO and obtained a degradation rate of 99% in 240 min.

Though Fenton oxidation is an effective technology, it is limited to large-scale applications due to the excessive iron sludge production and formation of toxic secondary by-products.

3.4.4. Nanophotocatalysis

The researchers to date focussed on various conventional approaches for the remediation of harmful chemical contaminants. These approaches are time-consuming and very costly, and their usage has been limited. Recently, nanotechnology emerged as an efficient, clean, and environmentally benign alternative technique originating from the unique properties of nanomaterials. The nanophotocatalysis process has an advantage, and it is a powerful way to clean up the extremely noxious and stable molecules from the environment and has received much attention due to its low cost, high efficiency, and low toxicity [97]. In 1938, TiO₂ was discovered by Goodeve and Kitchener [98] for bleaching of dyes in the presence of O₂. Since then, scientists have been keen on investigating the fundamental principles in photocatalysis reactions and continuous search for new efficient photocatalysts. In the photocatalysis phenomenon, a semiconductor is capable of conducting electricity in the presence of light at room temperature. The photocatalyst provides a site for both oxidation and reduction reactions simultaneously for the mineralization of pollutants [99].

3.4.5. Principle of the Photocatalytic Oxidation Process

The mechanism of photocatalysis can be described by the following four important steps: (i) generation of electron-hole pairs, (ii) excited charges separation, (iii) migration of photogenerated electrons and holes to the surface of photocatalysts, and (iv) redox reactions. The holes oxidize the pollutant-containing water molecules and produce hydroxyl radicals, and electrons reduce the dissolved oxygen in water to produce superoxide anion [100]. The main processes and key reactions involved in nanophotocatalysis are presented in Figure 5.

The photocatalysis reaction can be categorized as homogeneous photocatalysis and heterogeneous photocatalysis [102].

(1) Homogeneous Photocatalysis. Here, both the reactants and catalyst are uniformly distributed in the same phase. The molecular catalysts, catalytic sites for oxidation and reduction processes, are all in a solution form. The ozone and photo-Fenton systems are the most used homogeneous photocatalysis systems. The reactive species is OH^•, which is used for various purposes.

(2) Heterogeneous Photocatalysis. In heterogeneous photocatalysis, the photocatalyst is in contact with aqueous organic compounds or pollutants. Here, the reactant species are present in different physical states. The semiconductor-mediated heterogeneous photocatalysis is generally preferred because of its easy separation of the catalysts, energy-efficiency, and mild reaction conditions. Considerable efforts have been placed on the design and fabrication of heterojunctions for improving photocatalytic activity. Based on the interface of the different materials, the heterojunction photocatalysts are categorized into four steps: (i) the semiconductor-semiconductor (abbreviated as S-S) heterojunction; (ii) the semiconductor-metal (abbreviated as S-M) heterojunction; (iii) the semiconductor-carbon group (abbreviated as S-C) heterojunction (carbon group: activated carbon, carbon nanotubes (CNTs), and graphene); and (iv) multicomponent heterojunction [103].

In semiconductor-based composites, the charge may migrate from one component, typically the semiconductor absorbing irradiation to the other component, with proper band-edge positions. Reasonably, a composite photocatalyst system, composed of one semiconductor as the light absorber and other components to enhance the photocatalytic processes, is supposed to be an effective prototype to overcome the above shortcomings. Moreover, the second or third component in the composites may also provide active centers for the activation of reactants for the subsequent reduction/oxidation reactions, by working as a cocatalyst.

3.4.6. Basics of Heterojunction

The heterojunction is defined as the interface between two different semiconductors with unequal band structure. The internal structure of semiconductors comprises some band alignments which act as the basis for the formation of heterostructures. Based on the alignment of energy levels, heterostructures/heterojunction can be categorized into three types: (a) straddling type, (b) staggered type, and (c) those with a broken gap, as shown in Figure 6 [104].

(a)

(b)

(c)

In straddling type heterojunctions (type I), semiconductor A has a wider energy band gap than B. This results in the accumulation of the charge carriers on the semiconductor with a smaller band gap, and so, recombination of charge carriers is still possible since both electrons and holes are accumulated on the same semiconductor. In staggered-type heterojunction (type II), the VB and CB of semiconductor A are higher and lower than those of B. This results in the spatial separation of charge carriers which prevents the recombination of electrons and holes. The broken gap (type-III heterojunction) is the same as the staggered type except that the staggering gap becomes so wide that electron-hole migration is not possible, and therefore, the separation cannot occur [105–107]. Another, new Z-scheme photocatalyst is identified from the natural photosynthesis process. In this mechanism, two semiconductor photocatalysts are connected, an oxidation photocatalyst and another is reduction photocatalyst (Figure 7) [108].

The low VB position of the oxidation photocatalyst and high CB position of reduction photocatalysts are usually found in the Z-scheme photocatalyst. The strong reduction ability of photogenerated electrons in the CB of photocatalyst 1 and oxidation abilities in the VB of PC II are observed. Thus, a Z-scheme photocatalyst simultaneously has a strong redox ability.

4. Photocatalytic Degradation of Azo Dyes

The advanced oxidation processes (AOPs) are able to deal with the problem of dye destruction in aqueous systems. In the AOPs, reactive species such as hydroxyl radicals (•OH) are generated that oxidize a broad range of pollutants. The combined process of solar technology and catalysis proved to be a useful process for the reduction of water pollution. Table 1 summarizes the overview of different catalysts used in the photocatalysis process. Different catalysts are used for the degradation of the dyes.

4.1. Transition Metal Oxide Catalysts

Transition metals possess multiple valency, which results in a variety of oxides and crystal structures. They are a potential catalyst due to their low toxicity and ease of preparation. Jiang et al. [109] synthesized iron oxides, namely, 2-line ferrihydrite, α-Fe₂O₃, γ-Fe₂O₃, and Fe₃O₄ by the sol-gel method. The researcher reported the orange II degradation in the presence of visible light and H₂O₂ at neutral pH. Spherical γ-Fe₂O₃ was synthesized by Liang et al. [110] for the photocatalytic removal of orange I of 20 mg/L concentration. Around 48.89% degradation and 36.5% mineralization were achieved within 6 h of visible-light irradiation. Iron oxide nanoparticles of size less than 100 nm were biosynthesized for the decolorization of orange S. Under optimal conditions, 87.2% of the dye was removed at pH 9 by Karthikeyeni and his coworkers [111]. Sharma et al. [112] reported (α-Fe₂O₃) nanocubes synthesized by a low-temperature aqueous solution process for efficient degradation of MO with 80% efficiency under UV light. Ali et al. [113], in this study, prepared α-Fe₂O₃ from the mandarin peels and showed the photodegradation of basic maxilon blue GRL 300% and acidic neolan blue 2G. Burger shape morphology of α-Fe2O3 catalyst reported by Xiao et al. [114] synthesized by the facile hydrothermal method. The catalyst exhibited 98% degradation efficiency and 62% COD removal rate for acid redG in aqueous solution. Weldegebrieal et al. [115] synthesized Fe₂O₃ using V. amygdalina leaves extract. The degradation of MB and MO dyes was investigated under sunlight irradiation. Gupta et al. [116] reported coral-shaped Fe₂O₃. The photocatalyst showed a fast degradation efficiency of around 94% within 5 min in UV/H₂O₂ in a 32 W system for azo dye MO. Recently, α-Fe₂O₃ nanoparticles were synthesized by the combustion process of organic fuels. The degradation efficiency under UV radiation obtained for MO degradation is 95.31% which was synthesized using sucrose [117].

Sheikh et al. [118] synthesized TiO₂ at low temperature via a greener sol-gel approach. The researcher reported the photocatalytic potentiality for the photodegradation of azo dyes, MO, and CR. The effect of thermal treatment on photocatalytic activity and morphology of TiO₂ was reported by Allen et al. [119]. Raju and Kale [120] designed flower-like rutile TiO₂ using the solvothermal method. The photocatalyst under mercury and sunlight showed 96 and 94% degradation within 50 and 40 min, respectively. Dave and Chauhan [121] synthesized TiO₂ for the dye degradation of DB 14 under visible light. Around 99% of the dye degraded after 10 min. TiO₂ nanofibres were reported by Secundino-Sánchez et al. [122] and they evaluated the photocatalytic activity by decoloration of Remazol Black B azo dye. Saensook and Sirisuk [123] synthesized black TiO₂ by a chemical reduction for MO dye degradation under both UV and visible-light irradiations. The researcher found 82.17% efficiency under UV light. Vacca et al. [124] performed the optimization of MO dye removal with TiO₂ nanotubes. Redha et al. [125] prepared TiO₂ nanofilms by the sol-gel method, and a reactor was fabricated. The degradation of novacron red c-2BL (98%) was achieved within 100 min under UV light.

Copper oxide nanoparticles were synthesized by Lingaraju et al. [126] using the solution-combustion method using green fuel Rauvolfia serpentina. The photodegradation performance was evaluated by degrading trypan blue dye. Scuderi et al. [127] reported the catalytic efficiency of synthesized CuO and Cu₂O nanowires by evaluating MO degradation by a UV lamp. Novikova et al. [128] prepared CuO nanostructures by the precipitation method for the investigation of the photocatalytic activity of MO. CuO nanoparticles were synthesized using Spirulina platensis. The CR dye degradation was evaluated by Alsamhary et al. [129]. Aroob et al. [130] reported CuO nanoparticles by a mechanochemical approach using a green reducing agent Seriphidium oliverianum. The degradation rate obtained for MO is 65.078%. Cu₂O catalyst was evaluated for decolorization of RR180 and BR18 dye by Polat et al. [131]. The removal efficiency for RR180 and BR18 was observed to be 100% and 94.34%.

Bhat et al. [132] synthesized FCC-structured NiO by the coprecipitation method. The concentration of the initial dye on photocatalytic effect was evaluated by degrading CR dye. Sabouri et al. [133] synthesized NiO nanosheets by the coprecipitation method, and degradation of MO is reported. Around 82% of MO degradation was achieved within 210 min. Hydrothermal route was adopted for the preparation of NiO nanocubes by Sun et al. [134]. The degradation efficiency for CR was achieved within 30 min of irradiation. Roopan et al. [135] prepared NiO particles using the capping agent Calotropis gigantea latex. Under sunlight irradiation, 95% of CR degradation was observed.

Chen at al. [136] reported a sol-gel method for ZnO photocatalyst, and 99.70% removal of MO was observed under UV radiation. Sakthivel et al. [137] compared and investigated acid brown 14 degradation under solar light with commercially available TiO₂. The decolorization was achieved within 120 min of irradiation. Al-Zahrani et al. [138] created ZnO-NPs using leaf extract of P. granatum for photocatalytic degradation of orange 16 dye. Lanjwani et al. [139] synthesized ZnO by complexation reaction between zinc acetate and sodium diethyldithiocarbamate trihydrate. The photocatalytic efficiency was evaluated for EBT dye under UV, visible, and mercury light irradiation. Meanwhile, the ZnO nanosphere catalyst is synthesized by hydrothermal route for MO dye under UV radiation [140]. Sansenya et al. [141] prepared ZnO by a hydrothermal method for the complete photodegradation of RR141 and CR dye. ZnO nanoswirlings fabricated by Umar et al. [142] observed 79% AR183 dye degradation under UV light within 80 min.

4.1.1. Perovskite-Type Oxides

The general formula of perovskite is ABO₃ where cation A is of larger size and belongs to perovskite-type oxides. These materials have drawn outstanding interest towards energy conversion and environmental remediation due to their unique optical, physiochemical, electrical, structural, and thermal properties. Lin et al. [143] synthesized perovskite cobalt titanate and employed it for the degradation of AR 27, AB 120, and AY 17 azoic dyes. Perovskite SrSnO₃ photocatalyst is synthesized by Teixeira and his coworkers [144]. The degradation of Remazol golden yellow was observed under UV radiation with 98% efficiency. CeNiO₃ was synthesized by hydrothermal approach by Maridevaru et al. [145] and assessed the photocatalyst under UV irradiation for the removal of orange G. Merrad et al. [146] reported the synthesis of deficient perovskite SrTiO_3-δ and photooxidation of CR under both UV light and solar irradiation, respectively. Maridevaru et al. [147] synthesized nanocubes of LaNiO₃ and examined visible degradation with 96% rate for orange G azo dye. The catalytic wet air oxidation process was investigated by Palas et al. [148] for the removal of RB 5 of 100 mg/L concentration.

4.2. Spinel Metal Ferrites

A spinel ferrite crystal possesses a unit formula of AFe₂O₄. These are formed by the combination of a trivalent cation and another divalent metallic cation, and they have a face-centered cubic core. The cations are arranged according to the different crystallographic sites, i.e., tetrahedral sites and octahedral crystallographic sites surrounding oxygen atoms.

NiFe₂O₄ NPs were synthesized by Taj et al. [149], and photocatalytic degradation of CR was achieved with 85% of removal. Manganese ferrite nanoparticle (MFN) was prepared by Mahoodi [150] for the degradation of RR198 and RR 120 azo dye as model compounds. Ćirković et al. [151] reported a single-phase BiFeO₃ultrasound-assisted sol-gel route for Mordant blue 9 degradation. The synthesis of Mg_0.5Ni_xZn_0.5-xFe₂O₄ was reported by Dhiman et al. [152], and within 90 min, visible-light degradation can be achieved with 99.5% efficiency. ZnFe₂O₄ photocatalyst was fabricated by Cai et al. [153] for catalytic degradation of orange II decolorization under 150 W visible light. Ramadoss et al. [154] synthesized MnFe₂O₄ and revealed 98.3% CR dye in a photocatalytic reactor within 30 min. El-Salamony et al. [155] reported photodegradation of Amido black b10 dye using ZnFe₂O₄ with 92% degradation efficiency.

4.3. Bismuth Oxyhalide

An emerging class of materials, bismuth oxyhalide (BiOX), exhibits excellent photocatalytic properties due to its exceptional layered and attractive band structure. BiOX materials were earlier used for storage and as ferroelectric materials. However, recently, its wide applications have been explored in various fields, especially in the purification of wastewater.

Sun et al. synthesized BiOCl via a solvothermal process using methanol as solvent, and degradation of MO is reported under sunlight [156]. Flower-like BiOI was prepared by Wang et al. [157] for MO degradation under the visible-light irradiation. BiOBr was reported by Mera et al. [158] for MO dye, with 97% degradation within 60 min under simulated solar radiation. Nourzad et al. [159] synthesized BiOI by solvothermal technique and evaluated degradation of MO and EBT with a removal efficiency of 47.4% and 86.7%, respectively, using an LED light. Yu and Han [160] prepared BiOX (X = Cl, Br), and MO degradation is investigated. BiOCl microspheres were synthesized by Mendez-Alvarado and his coworkers [161] by the solvothermal method for catalytic degradation of RO84 under sunlight.

4.4. Hybrid Nanomaterial

Among these, semiconductor/semiconductor-based photocatalysts instead of single semiconductors are more efficient and practical approaches to enhance photocatalytic performance. Due to the coupling and synergistic effects resulting from heterogeneous interfaces, the system not only possesses the corresponding physicochemical properties of its individual component but also exhibits superior or completely new characteristics. Christoforidis et al. [162] prepared hybrid β-Fe₂O₃/g-C₃N₄ nanomaterial via a facile in situ growth strategy embedded in a solid state process. Their photocatalytic activity was evaluated under both simulated solar light and pure visible-light irradiation against the photodegradation of MO dye. The β-Fe₂O₃ content in the final material was tuned to optimize the photocatalytic performance, with particular attention to the activity under visible light. Kumar et al. [163] synthesized a hybrid Fe₃O₄@SiO₂@Ru composite for the removal of MR and MO under irradiation of visible light. The composite showing better stability and repeating cycles greater than 90% was obtained. Palanisamy et al. [164] fabricated ZnS/ZnCr₂O₄ by the simple precipitation method. The amino acid (methionine) is used as a capping agent and showed enhanced photodegradation MO with four repeating cycles. A heterogeneous catalyst, Au-WO₃/SBA-15 catalyst, fabricated by Malik et al. [165] efficiently degraded high concentration of MO dye at a pH 2 value pollutants (C = 0.3 g·l⁻⁼) within 30 min and can retain a good degradation efficiency at a pH value of 2. Aiello et al. [166] synthesized MNC-PMAO-PN nanohybrids and performed the photodegradation experiments for acid black 10B azo dye. Around 76% degradation efficiency is observed without the addition of an external oxidising agent. Tin-doped bismuth ferrite nanoparticles-graphene nanoplatelets (BFSO/GNP) were prepared by Fatima and Rizwan [167]. The efficient dye removal is observed with 10 cycles of catalytic activities. Pradeeba et al. [168] degraded azo dye, cationic azo dye, and Bismarck brown by poly (azomethine)/TiO₂ photocatalyst. Ravikumar et al. [169] reported a CdS/Ag photocatalyst for efficient degradation of RR120, AB 1, and DB 15 azo dyes under UV and solar light irradiation. The studies demonstrated 93% of degradation within 75 min and a high repeatability cycle. A doped carbon nitride photocatalyst was prepared by Yang et al. [170]. The doping of F and S elements with (g-C₃N₄) enhanced the degradation rate of MO dye. Ranjithkumar et al. [171] by modified hydrothermal route fabricated CS-BiOCl/ZnO nanocomposite for the photodegradation of Congo red. The catalyst degraded 93% dye under UV light within 40 min. Zhang et al. [172] demonstrated rGO-poly[ViEtIm]Br as a platform to degrade MO dye. The degradation efficiency is 98.7% in 3 hours. Razavi et al. [173] synthesized Pt–Au/TiO₂/BaFe₁₂O₁₉ nanofibres and investigated for AR 14 dye with 95.2%, with the reaction rate coefficient of 0.0247 min^−c. The Ag₂O/CuWO₄ composite demonstrated effective degradation of acid orange 7 [174]. Codoped ZnO-NPs were synthesized by using polyvinyl alcohol hydrogel for MO dye degradation. The CoZnO/PVA composite showed photocatalytic efficiency and completely mineralized MO in 48 min [175]. Kang et al. [176] synthesized Fe₃O₄ @Al-MOF for the removal of CR with 83.7% upto five cycles. Cubic mesoporous ZnAl_xB_2−xO₄ were synthesized and calcined at 700°C for 3 h by the solution-combustion method by Mukherjee and his coworkers [177] and they evaluated for the degradation of reactive black 5 dye. NiO/TiO₂ composite was prepared by Gul et al. [178] for the degradation of orange II dye with 96% degradation within 30 min. Mukonza et al. [179] adopted the sol-gel technique and reported Fluorine-Sm³⁺ codoped TiO₂ (0.6% Sm³⁺) catalyst for the degradation of brilliant black BN bisazo dye within 3 hours of simulated solar irradiation. The total organic carbon removal was 82%. The researcher demonstrated that F and Sm³ incorporation improved the optical properties, surface chemistry, and quantum efficiency properties. Further reaction kinetics increased, and reaction time was improved by the incorporation of MWCNTs. Meanwhile, Al-Musawi et al. [180] completely degraded AB113 dye at pH = 3 using MWCNTs/CoFe₂O₄ by the sonophotocatalytic process within 40 min. The optimum parameters for 100% AB113 removal with catalytic dose is 0.4 g/L, at pH 3. The UV light of 36 W was used as a light source. Highly porous three-dimensional TiO₂@Cd-MOF was fabricated by Ramasubbu et al. [181] by sol-gel technique and they further prepared TiO₂@Cd-MOF)@ZnPp and evaluated for MO degradation. Around 94.1% degradation efficiency was achieved with good recycle tests. The researcher revealed that optical absorption and efficient electron injection further improved the photocatalytic activity. A multifunctional TiO₂@Fe₃O₄@C–NF hybrid material is reported by Yilmaz et al. [182] for azo dye degradation. The degradation efficiencies reached 80–100% within 8–125 min under UV irradiation. The prepared sample showed high repeatability, efficiency, and sustainability. Kardeş et al. [183] fabricated flexible photocatalyst 3D polyurethane (PU) foam struts aligned with ZnO nanorods. The hybrid photocatalyst structural integrities analysis showed that ZnO layers remained intact and adherent on the surfaces even after 1 h of sonication. Azo dye, AR 88, was tested in a laboratory-scale photoreactor for color removal efficiency and total organic carbon content. The efficiency obtained is 97% within 180 min under UVA light. The hydrothermal method is adopted by Ishfaq et al. [184] for synthesizing doped CeO₂ (Ce_0.92Cu_0.04Bi_0.04O₂). The researcher demonstrated that doping copper and bismuth improved the current conductivity and light harvesting synergistically. Under solar irradiation, the catalyst degraded 95.79% MO dye within 50 min at the rate of 0.0314 min^−t. The dye mineralization is also five times faster than a CeO₂ photocatalyst with robust reusability tests. Orange G photodegradation was evaluated by chitosan-modified CeO₂/ZrO₂ under solar light by Nisha et al. [185]. The synthesized catalyst obtained band gap energy of 2.9 eV with an average crystalline size of 30 nm. Zhang and Shih [186] reported FeO_xrGO/Ti photoanodes and photoelectrocatalytic degradation of azo dye. The incorporated rGO enhanced the electrochemical surface area of the anode which substantially improved the catalytic property. FeNi@GR nanoparticles prepared by Zou et al. [187] demonstrate that the synergistic effect between bimetals and the electron transfer ability of graphene within 150 min efficiently removed CR dye. Thao et al. [188] evaluated the orange G degradation by MnFe₂O₄/α-MnO₂ in peroxymonosulfate activation. Around 96.8% of orange G degradation was achieved within 30 min regardless of solution pH changes. Chowdhury et al. [189] synthesized ternary BiOCl-Cu₂CoSnS₄-TiO₂ nanocomposite and evaluated the degradation of direct blue 71 azo dye. Gogoi et al. [190] fabricated quaternary Ag/Ag₃PO₄-BiOBr-C₃N₄ composite for anionic pollutant RR 120 dye.

5. Photocatalyst Surface Charge Interaction with Dyes

The surface charge of a photocatalyst (zeta potential) and dye interaction is considered as a vital factor for enhancing the photocatalytic activity. The redox reaction and charge carrier movements occur at the photocatalyst surface. Thus, improvement in surface modification can pave an effective photocatalytic activity. The surface charge (zeta potential) of the catalyst is influenced by the pH of the medium. Borthakur et al. [191] reported the Ag/rGO nanocomposite for the CR anionic diazo dye degradation. The zeta potential of Ag/rGO nanocomposite materials was analyzed by the following equation:where ζ denotes the zeta potential, μ signifies the electrophoretic mobility, η is the viscosity, and Dt is the dielectric constant of the medium. The experiments were conducted in the pH range from 2 to 11. It was found that the Ag/rGO nanocomposite surface is negatively charged. As the pH medium increases, the surface charge of the nanocomposite becomes more negative. This is due to the functional group present on the graphene sheets. Conversely, with a decrease in the acidic medium, the surface becomes positive. The CR dye is an anionic molecule. Therefore, in an acidic medium, the interaction of the dye is high, and a maximum degradation of 95.55% is observed at pH 3. Meanwhile, another study by Mittal and Khanuja [192] for the MoSe₂-PANI composite found that the highest photocatalytic degradation efficiency is obtained in MP21 due to its optimized surface charge. MoSe₂ zeta potential is −23.01 mV, whereas PANI is positively charged with 9.51 mV. The zeta potential of the composite MP21 sample is −5.7 mV. Thus, RhB molecules being a cationic dye would preferably adsorb on the MoSe₂ surface and CR molecules being an anionic dye adsorb on the PANI surface. Another work investigated by Borthakur and Das [193] reported the surface potential values of NiS₂-rGO and CoS-rGO nanocomposite and evaluated the photodegradation efficiency of CR dye. Verma et al. [194], in their experimental results, obtained that 3% Au–ZnO nanorods composite found that with an increase in alkaline medium 10, the zeta potential was found to be about 5.5 mV, while at pH 3, it is 33.1 mV. Thus, in an acidic medium, there was an increase in zeta potential. At the basic condition, hydroxylated surface deprotonation occurred, which results into decreased zeta potential, or at lower pH medium, hydroxylated surface protonation attributed to positive surface charge.

6. Factors Affecting the Degradation of Dyes

The dye degradation by a photocatalyst depends on certain parameters such as pH, photocatalyst particle concentration, initial dye concentration, and light intensity. These parameters are discussed influencing the photodegradation of dyes.

6.1. Catalyst Dosage

Generally, with the increase in catalyst amount, photodegradation increases. This is due to the availability of more active sites which can absorb more photons under irradiation and produce more OH• radicals and positive holes. However, beyond a certain limit, if catalyst dosage is increased, the photodegradation rate decreases which might be due to the formation of a turbid solution so that UV/Vis radiation is not percolated in the water. The nanoparticles agglomerate which reduces the active surface sites available for exposure results in a coagulation of catalyst particles [195, 196].

6.2. Effect of pH

The pH is an effective factor in the photocatalyst degradation of azo dyes. The pH can affect the ionization level of the photocatalyst. The pH variation impacts the photocatalyst oxidation process. The hydroxyl radicals are formed by the reaction between hydroxide ions and positive holes. Also, in the lower pH, the positive holes are vital oxidising species, whereas at neutral or higher pH, hydroxyl radicals are the dominant species [197].

6.3. Effect of Dye Concentration

The initial dye concentration in photocatalytic degradation plays an important role. On increasing dye concentration and with a fixed amount of photocatalyst, the degradation rate decreases. This might be due to the presence of more organic pollutants which are adsorbed on the surface of the photocatalyst, and so, photons are unable to get on the catalyst surface. Hence, there is less active species formed and so a decrease in decolorization percentage [198, 199].

6.4. Effect of Irradiation Time and Intensity of Light

The light intensity, source, and irradiation time greatly influence the photodegradation of pollutants. In general, a photocatalyst absorbs light energy equal to or more than the band gap energy. This further causes the electrons to transfer from VB to CB leaving behind holes in the VB. Thus, when the intensity of incident light is high, the excitation of the photocatalyst is also high, and thus, the photodegradation rate can be enhanced [200, 201]. Natural sunlight is a cost-effective light source. However, nowadays, artificial UV and solar simulators are more reproducible. Ollis et al. [202] investigated that the rate of degradation occurs linearly (first order) on increasing the light intensity. At intermediate, the light intensity rate would depend on the square root of the light intensity (half order). Hence, by increasing the number of photons, the probability of activation of photons on the surface of the catalyst enhances.

7. Role of Active Species in Photodegradation of Dye

Photocatalytic degradation is the light-driven process for pollutant degradation. A redox reaction occurs by the photocatalyst upon interaction with the dye molecule in the presence of light. The photon absorption in the first step leads to the formation of photogenerated electron and hole pairs [203]. These electron/hole pairs generated further react with oxygen, water, and hydroxyl groups and form reactive oxygen species such as hydroxyl (•OH) and superoxide (•O₂⁻) radicals and hydrogen peroxide (H₂O₂). To determine the active species responsible for the photocatalytic experiment, different scavengers are added to trap and remove active species [204]. The hydrogen peroxide is a stable molecule compared to the superoxide radical. The hydroxyl radicals (•OH) are usually trapped by adding isopropanol (IPr), ethanol (EtOH), and methanol (MeOH). [205, 206]. For superoxide anion radical (•O₂⁻), the scavengers used are p-benzoquinone (p-BQ) and ascorbic acid (AA) [207–209]. The scavengers such as ethylenediaminetetraacetic acid (EDTA), triethanolamine (TEA), sodium oxalate (Na₂C₂O₄), and sodium iodide (NaI) are usually used to scavenge holes [210–212]. For the electrons, the scavengers used are potassium bromate (KBrO₃), potassium persulfate (K₂S₂O₈), and ammonium persulfate (NH₄)₂S₂O₈) [212–214]. The singlet oxygen (¹O₂) is determined by adding sodium azide (NaN₃) [215]. Christoforidis et al. [162] investigated the active species role by control experiments on the FCN-3 sample. For the reaction, tert-butyl alcohol (TBA) was used as hydroxyl radical (OH∙) and triethanolamine (TEOA) for hole (h⁺) scavenging. The superoxide radicals (O₂−∙) were analyzed by control experiments under a saturated argon atmosphere. The photogenerated h⁺ is the dominant species responsible for the degradation of MO, as the degradation process was greatly suppressed in the case of the FCN-3 sample. Palanisamy [164] in their study confirmed the reactive species role by using isopropanol for OH∙ radical, benzoquinone for (O₂−∙), and triethanolamine for h⁺ radical. The researcher observed that with the addition of BQ and IPA scavenger, degradation efficiency decreased to 60.1 and 24.2% from 98.66%, respectively. This indicates that the holes did not take part in the photodegradation process and OH∙ and O₂−∙ species initiated the breaking of MO dye molecules. Kardeş and his group evaluated the scavenger experiments for AR88 dye solutions by conducting the experiments under UVA light for 240 min. Here, propanol, formic acid, and l-ascorbic acid were used as trapping agents for •OH, h⁺, and ·O₂, radicals, respectively. Figure 8 illustrates the degradation efficiencies for the dye ZnO/PU samples in the presence of scavengers [183].

(a)

(b)

It was observed that the degradation efficiency (83.5%) is slightly suppressed in the presence of formic acid and 2-propanol (63.2%) indicating that OH and h+ radicals have little effect on degradation process. The efficiency intensely decreased from 95.3 to 58.1% after the addition of l-ascorbic acid. The results indicated that O₂⁻ radicals are markedly inhibited by l-ascorbic acid.

Another researcher group investigated reactive species formed in the degradation process by trapping experiments for AB113 dye in the presence of isopropanol (IPA), ethylenediaminetetraacetic acid (EDTA), and benzoquinone (BQ) for •OH, h+, and •O₂⁻, radicals, respectively. The results showed that 100% efficiency is obtained in the absence of any scavenger. In addition to IPA and EDTA, removal efficiencies decrease by 42% and 35% indicating that •OH and h⁺ are solely responsible for the degradation process [180]. Thao et al. [188] showed that after the addition of TBA, FFA, p-BQ, and EtOH in the orange G dye solution with MnFe₂O₄/α-MnO₂/PMS catalyst, degradation efficiency decreased to a great extent compared to without adding scavenger 96.8%. The order of active radicals formed in the system is SO₄ •− > O₂ •− > 1O₂ > HO•. Furthermore, DMPO and TEMP analyses were used to detect radicals. Also, the EPR results confirmed the active role of SO₄ •−, O₂ •−, and 1O₂, in catalytic reaction.

8. Conclusion and Future Prospects

Dye effluent’s presence causes water pollution and is a major concern for the environment. Till date, efficient dye removal techniques have been explored. In this review, we concluded various dye removal techniques such as biological, chemical, and physical methods. The idea was to make readers informed of the existence along with modern methods available for removing organic pollutants from wastewater. The aim of this paper is to highlight an advanced nanophotocatalysis process that is recognized as an effective method for removing dye completely from waste water. In this review, the basic theory of photocatalysis and its mechanism is discussed. The photocatalysis technique is simple, powerful, and a promising technology for dye remediation, and the photocatalysts used have several advantages such as nontoxic, stable, and reusable and have a high effective activity. Further various parameters affecting degradation rate and active radical species generated in the photocatalytic process are highlighted.

Forthcoming studies can be focussed and addressed on the following:(1)Despite modern strategies adopted, still photocatalyst suffer from problems such as extended visible-light absorption, efficiency, and prolonged lifespan(2)The photocatalyst reusability and large scale remain a significant challenge(3)The researcher should perform experiments with real industrial dye effluents(4)There is a lack of sufficient information on the spent catalyst disposal(5)The structure-activity, reactions pathway, and photocatalytic mechanism still needs depth examination with advanced characteristic technology and supported theoretical calculations

Notations

VB:	Valence band
CB:	Conduction band
UV:	Ultraviolet
Ppm:	Parts per million
COD:	Chemical oxygen demand
TOC:	Total organic carbon
ADMI:	Dye Manufacturers’ Institute
APM:	Ammonium phosphomolybdate
PANI:	Polyaniline
F-SCN:	Sulfur fluoride-doped carbon nitride
RB 5:	Reactive black 5
MO:	Methyl orange
MB:	Methylene blue
EBT:	Eriochrome Black T
EY:	Eosin yellow
MG:	Malachite green
MR:	Methyl red
AR:	Acid red
DR:	Direct red
DB:	Direct blue
BR:	Brilliant red
RR:	Reactive red
AR:	Acid red
AB:	Acid blue
RO:	Reactive orange
OG:	Orange G
RNL:	Remazol golden yellow
AB10B:	Amido black 10b.

Data Availability

The data supporting this review are available from previously reported studies and datasets, which have been cited.

Conflicts of Interest

The authors declare that they have no conflicts of interest.

Authors’ Contributions

Arpita Paul Chowdhury wrote the original manuscript and reviewed the draft, K. S. Anantharaju and K. Keshavamurthy reviewed the manuscript, and Samuel Lalthazuala Rokhum supervised the manuscript.

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Copyright © 2023 Arpita Paul Chowdhury et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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