Recent Advances in Efficient Photocatalytic Degradation Approaches for Azo Dyes

. 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. Tese pollutants block the light penetration in water bodies and prevent photosynthesis activity, thereby afecting 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. Te major goal of this review article is to propose a high-performing, cost-efective hybrid photocatalyst reported to date for prospective azo dye pollutant remediation. Tis review article also aimed to highlight the challenges and uncertainties associated with dye degradation in the photocatalytic process.


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].Tese toxic chemicals cause great concern for both the environment and public health.Te extreme discharge of the effuent adversely afects aquatic life and soil fertility and disturbs the ecosystem integrity by increasing the chemical oxygen demand and biochemical oxygen demand, altering the pH, and afecting water quality [2].Te wastewater containing organic dye pollutants blocks the ecosystem and causes threats to environmental sustainability.Te 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.Te dye molecule consists of three essential groups: the chromophore, the auxochrome, and the matrix.Te 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].Te common chromophore are ─NO 2 , ─N=N─, ─C=S, ─C=O, ─C=C─, and ─N=O.Te organic molecules containing these groups are known as chromogenic.Te chromogenic molecule with the accumulation of auxochrome groups shows dyeing possibilities.Tese auxochrome groups allow the fxation of the dyes and can modify the color of the dye.Te auxochrome groups are acidic (COOH, SO 3 , and OH) or basic (NH 2 , NHR, and NR 2 ), while the third part of the molecule is the rest part of the atom called as matrix [5].Te azo dyes with the (-N=N-) chromophore group account for 70% market share and are highly soluble in water [6].
Te nitro group presence makes the dye mutagenic and highly toxic.Te 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.Te 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].Tese methods are not efcient 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-efective, and complete mineralization of the dyes are observed compared to traditional methods.Te active radicals such as hydroxyl radicals (•OH), superoxide anion (•O 2 − ), and hydrogen peroxide (H 2 O 2 ) are generated in situ to degrade organic dye compounds [11].Tus, photocatalysis is a greener way to debase the pollutants of water with semiconductors and is intensely investigated worldwide [12].
Te objective of this present review article is to explore applications of diferent 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 efect of diferent parameters which includes the dye concentration efect of light irradiation, pH, and catalyst loading is discussed for the optimization of the degradation efciency of textile wastewater.

Chemical Structure and Classification of Azo Dyes
Te chemical structure of an azo dye, in general, consists of a solubilizing group, auxochrome groups, and chromophoric groups [13].Te structure of an azo dye is depicted in Figure 1.
Te nature of the azo bonds and the associated chromophore and auxochrome groups decide the color of the dyes.Te number of azo linkages present in an azo dye molecule classifes 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][16][17].Figures 2  and 3 illustrate the classifcation of the dyes.

Dye Removal Technologies
Te 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 dyestufs.Te treatment technologies are broadly classifed as (a) biological treatment, (b) physicochemical treatment, (c) advanced approaches, and (d) combinatorial treatment [18].Figure 4 represents the overview of diferent dye removal technologies.
3.1.Biological Treatment.Biological treatment is a lowcost and efective method for azo dye removal.Te azo bond cleavage is difcult 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].Te 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% efciency.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 ligninmodifying enzymes for an inducing substrate, and for cell immobilization, L. cylindrica (L.C) was used.Te 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].[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.

Phytoremediation.
Te scientifc 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% efcient.Tagetes patula L. root system can efciently degrade the reactive orange M2RL, reactive red M5B, methyl red, MO, and navy blue HE2R dyes [36,37].Tough biological treatment is efcient, it is considered less fexible in design and operation.Moreover, satisfactory color removal and mineralization are Journal of Chemistry not achieved.Te 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, efcient and efective treatment processes need to be developed for the complete removal of azo dyes from wastewater [39].Cl − groups and type II resins contain -CH 2 N(CH 3 ) 2 (CH 2 CH 2 OH) + Cl − groups), and (d) weak anion exchange resins of primary (-NH 2 ), 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 flms were grafted with p(GMA) via the ATRP method, and the epoxy groups were converted into phosphate (RCF-g-p(GMA)-PO 3 H 2 ) and sulfate (RCF-g-p(GMA)-SO 3 H) 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. Te sorption capacity at 25 °C and 65 min was 317.6 mg•g −1 .Te 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-focculation 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 focs 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.Te 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 efcacy for treating Congo redcontaining 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 biofocculant.Te electrostatic force of attraction and hydrogen bonding mechanisms were observed for the dye removal process.Te work by Ihaddaden et al. [62] reported the use of bentonite and Opuntia fcus-indica as coagulant-focculant for the MB dye removal.Te dye removal efciency 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 foc 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.Te kinetics of the coagulationfocculation process was confrmed to be a pseudo-secondorder model with R 2 > 0.997.

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 efciency, reliability, cost-efectiveness, and simplicity.Tus, ultrafltration, nanofltration, and microfltration are an attractive technology.Polymeric materials, for example, PES, PAN, PVDF, and BPPO, are efciently 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 ultrafltration for the removal of Procion blue MxR.Subrahmanya and his coworkers [69] reported a syringe flter membrane composed of Whatman 42 flter, GO 10 /(SBA-15/ PDA) 2 membrane to efectively screen MB and EBT dyes from wastewater.PVDF/IL-Na + MMT membrane is fabricated by Zhao et al. [70] for removal efciency for both anionic and cationic dyes at a high level of over 90% efciency.Li et al. [71], in their study, prepared a novel antifouling loose nanofltration (LNF) membrane by crosslinking and grafting of hydrolyzed polyacrylonitrile ultrafltration 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 TiO 2 modifcations [72].Nanofltration is a separation technology, where pore size ranges about 0.5-2.0nm with molecular weight cutof ranging from 200 to 2000 Da.Te smaller pore size, higher permeability, and lower transmembrane pressure are advantages over ultrafltration and microfltration.Recently, Ahmadipouya et al. [73] reported polysulfone mixed-matrix nanofltration 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 efciency.Moreover, disposal problems of the secondary waste generated which gets converted into toxic intermediates.

Advanced Approaches
3.4.1.Ozonization.Ozonation technology is a chemical oxidation treatment afected 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.Te synthetic azo dye, reactive black 5, and ozonization were probed by Chu et al. [76] with a microbubble generator.Te 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. Te catalyst dose was 0.02 g, and the ozone dose was 0.5 mg/min.Around 96% removal efciency 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 efective, but the complete mineralization of organic substances is still a challenge.

Electrocoagulation. Te electrocoagulation technique
is a simple, reliable, and low-cost method for the treatment of dyes contained in wastewater.Te process has good removal efciency for COD, color, turbidity, and dissolved solids.Te 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.Te 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.Te electrocoagulation process investigated by Lach et al. [84] revealed a high azo dye removal efciency (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.Te efciency 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 efciency with 16.8% COD removal efciency.Brilliant yellow azo dye removal by the electrocoagulation method was reported by Ahmed et al. [87] using an Al helical coil and obtained 92% efciency 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.Te electrocoagulation is an eco-efcient electrolysis-based technology, but the electrodes need to be replaced which is a disadvantage and may also lead to electrode passivation [89].

Fenton Oxidation.
Te Fenton oxidation is an advanced alternative process for the treatment of textile azo dyes.Te 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 efciency of 98.8% and 99.8% within 60 min and with 100 mg•L −1 initial dye concentration.Magnetic Fe 3 O 4 -NIO calcium alginate beads were prepared by Ayed et al. [92] for the degradation of Novacron blue dye.Te researcher observed 80% efciency 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 efciency with an 85.5% mineralization rate.Te SrWO 4 @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-modifed Laponite@diatomite composite for photo-Fenton degradation of MO and obtained a degradation rate of 99% in 240 min.
Tough Fenton oxidation is an efective technology, it is limited to large-scale applications due to the excessive iron 6 Journal of Chemistry sludge production and formation of toxic secondary byproducts.

Nanophotocatalysis.
Te researchers to date focussed on various conventional approaches for the remediation of harmful chemical contaminants.Tese approaches are timeconsuming and very costly, and their usage has been limited.Recently, nanotechnology emerged as an efcient, clean, and environmentally benign alternative technique originating from the unique properties of nanomaterials.Te 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 efciency, and low toxicity [97].In 1938, TiO 2 was discovered by Goodeve and Kitchener [98] for bleaching of dyes in the presence of O 2 .Since then, scientists have been keen on investigating the fundamental principles in photocatalysis reactions and continuous search for new efcient photocatalysts.In the photocatalysis phenomenon, a semiconductor is capable of conducting electricity in the presence of light at room temperature.Te photocatalyst provides a site for both oxidation and reduction reactions simultaneously for the mineralization of pollutants [99].

Principle of the Photocatalytic Oxidation Process.
Te mechanism of photocatalysis can be described by the following four important steps: (i) generation of electronhole pairs, (ii) excited charges separation, (iii) migration of photogenerated electrons and holes to the surface of photocatalysts, and (iv) redox reactions.Te holes oxidize the pollutant-containing water molecules and produce hydroxyl radicals, and electrons reduce the dissolved oxygen in water to produce superoxide anion [100].Te main processes and key reactions involved in nanophotocatalysis are presented in Figure 5.
(1) Homogeneous Photocatalysis.Here, both the reactants and catalyst are uniformly distributed in the same phase.Te molecular catalysts, catalytic sites for oxidation and reduction processes, are all in a solution form.Te ozone and photo-Fenton systems are the most used homogeneous photocatalysis systems.Te 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 diferent physical states.Te semiconductormediated heterogeneous photocatalysis is generally preferred because of its easy separation of the catalysts, energyefciency, 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 diferent 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 efective 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.

Basics of Heterojunction.
Te heterojunction is defned as the interface between two diferent semiconductors with unequal band structure.Te 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].
In straddling type heterojunctions (type I), semiconductor A has a wider energy band gap than B. Tis 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 staggeredtype heterojunction (type II), the VB and CB of semiconductor A are higher and lower than those of B. Tis results in the spatial separation of charge carriers which prevents the recombination of electrons and holes.Te 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][106][107].Another, new Z-scheme photocatalyst is identifed from the natural photosynthesis process.In this mechanism, two semiconductor photocatalysts are connected, an oxidation photocatalyst and another is reduction photocatalyst (Figure 7) [108].
Te low VB position of the oxidation photocatalyst and high CB position of reduction photocatalysts are usually found in the Z-scheme photocatalyst.Te strong reduction ability of photogenerated electrons in the CB of photocatalyst 1 and oxidation abilities in the VB of PC II are observed.Tus, a Z-scheme photocatalyst simultaneously has a strong redox ability.

Photocatalytic Degradation of Azo Dyes
Te 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.Te 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 diferent catalysts used in the photocatalysis process.Diferent catalysts are used for the degradation of the dyes.

Transition Metal Oxide Catalysts.
Transition metals possess multiple valency, which results in a variety of oxides and crystal structures.Tey 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-Fe 2 O 3 , c-Fe 2 O 3 , and Fe 3 O 4 by the sol-gel method.Te researcher reported the orange II degradation in the presence of visible light and H 2 O 2 at neutral pH.Spherical c-Fe 2 O 3 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  8 Journal of Chemistry      Journal of Chemistry conditions, 87.2% of the dye was removed at pH 9 by Karthikeyeni and his coworkers [111].Sharma et al. [112] reported (α-Fe 2 O 3 ) nanocubes synthesized by a lowtemperature aqueous solution process for efcient degradation of MO with 80% efciency under UV light.Ali et al. [113], in this study, prepared α-Fe 2 O 3 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.Te catalyst exhibited 98% degradation efciency and 62% COD removal rate for acid redG in aqueous solution.Weldegebrieal  Te degradation efciency under UV radiation obtained for MO degradation is 95.31% which was synthesized using sucrose [117].Sheikh et al. [118] synthesized TiO 2 at low temperature via a greener sol-gel approach.Te researcher reported the photocatalytic potentiality for the photodegradation of azo dyes, MO, and CR.Te efect of thermal treatment on photocatalytic activity and morphology of TiO 2 was reported by Allen et al. [119].Raju and Kale [120] designed fower-like rutile TiO 2 using the solvothermal method.Te photocatalyst under mercury and sunlight showed 96 and 94% degradation within 50 and 40 min, respectively.Dave and Chauhan [121] synthesized TiO 2 for the dye degradation of DB 14 under visible light.Around 99% of the dye degraded after 10 min.TiO 2 nanofbres 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 TiO 2 by a chemical reduction for MO dye degradation under both UV and visible-light irradiations.Te researcher found 82.17% efciency under UV light.Vacca et al. [124] performed the optimization of MO dye removal with TiO 2 nanotubes.Redha et al. [125] prepared TiO 2 nanoflms by the sol-gel method, and a reactor was fabricated.Te 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 Rauvolfa serpentina.Te photodegradation performance was evaluated by degrading trypan blue dye.Scuderi et al. [127] reported the catalytic efciency of synthesized CuO and Cu 2 O 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.Te 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.Te degradation rate obtained for MO is 65.078%.Cu 2 O catalyst was evaluated for decolorization of RR180 and BR18 dye by Polat et al. [131].Te removal efciency for RR180 and BR18 was observed to be 100% and 94.34%.
Bhat et al. [132] synthesized FCC-structured NiO by the coprecipitation method.Te concentration of the initial dye on photocatalytic efect 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].Te degradation efciency 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 TiO 2 .Te 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.Te photocatalytic efciency 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.

Perovskite-Type Oxides.
Te general formula of perovskite is ABO 3 where cation A is of larger size and belongs to perovskite-type oxides.Tese 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 SrSnO 3 photocatalyst is synthesized by Teixeira and his coworkers [144].Te degradation of Remazol golden yellow was observed under UV radiation with 98% efciency.CeNiO 3 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 defcient perovskite SrTiO 3-δ and photooxidation of CR under both UV light and solar irradiation, respectively.Maridevaru et al. [147] synthesized nanocubes of LaNiO 3 and examined visible degradation with 96% rate for orange G azo dye.Te catalytic wet air oxidation process was investigated by Palas et al. [148] for the removal of RB 5 of 100 mg/L concentration.

Spinel Metal Ferrites.
A spinel ferrite crystal possesses a unit formula of AFe 2 O 4 .Tese are formed by the combination of a trivalent cation and another divalent metallic cation, and they have a face-centered cubic core.Te cations are arranged according to the diferent crystallographic sites, i.e., tetrahedral sites and octahedral crystallographic sites surrounding oxygen atoms.
NiFe 2 O 4 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 BiFeO 3 ultrasound-assisted sol-gel route for Mordant blue 9 degradation.Te synthesis of Mg 0.5 Ni x Zn 0.5-x Fe 2 O 4 was reported by Dhiman et al. [152], and within 90 min, visible-light degradation can be achieved with 99.5% efciency.ZnFe 2 O 4 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 MnFe 2 O 4 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 ZnFe 2 O 4 with 92% degradation efciency.

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 felds, especially in the purifcation 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 visiblelight 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 efciency 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 efcient and practical approaches to enhance photocatalytic performance.Due to the coupling and synergistic efects 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-Fe 2 O 3 /g-C 3 N 4 nanomaterial via a facile in situ growth strategy embedded in a solid state process.Teir photocatalytic activity was evaluated under both simulated solar light and pure visible-light irradiation against the photodegradation of MO dye.Te β-Fe 2 O 3 content in the fnal material was tuned to optimize the photocatalytic performance, with particular attention to the activity under visible light.Kumar et al. [163] synthesized a hybrid Fe 3 O 4 @SiO 2 @Ru composite for the removal of MR and MO under irradiation of visible light.Te composite showing better stability and repeating cycles greater than 90% was obtained.Palanisamy et al. [164] fabricated ZnS/ZnCr 2 O 4 by the simple precipitation method.Te amino acid (methionine) is used as a capping agent and showed enhanced photodegradation MO with four repeating cycles.A heterogeneous catalyst, Au-WO 3 / SBA-15 catalyst, fabricated by Malik et al. [165] efciently 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 efciency 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 efciency is observed without the addition of an external oxidising agent.Tin-doped bismuth ferrite nanoparticlesgraphene nanoplatelets (BFSO/GNP) were prepared by Fatima and Rizwan [167].Te efcient 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)/TiO 2 photocatalyst.Ravikumar et al. [169] reported a CdS/Ag photocatalyst for efcient degradation of RR120, AB 1, and DB 15 azo dyes under UV and solar light irradiation.Te 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].Te doping of F and S elements with (g-C 3 N 4 ) enhanced the degradation rate of MO dye.Ranjithkumar et al. [171] by modifed hydrothermal route fabricated CS-BiOCl/ZnO nanocomposite for the photodegradation of Congo red.Te 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.Te degradation efciency is 98.7% in 3 hours.Razavi et al. [173] synthesized Pt-Au/ TiO 2 /BaFe 12 O 19 nanofbres and investigated for AR 14 dye with 95.2%, with the reaction rate coefcient of 0.0247 min −c .Te Ag 2 O/CuWO 4 composite demonstrated efective degradation of acid orange 7 [174].Codoped ZnO-NPs were synthesized by using polyvinyl alcohol hydrogel for MO dye degradation.Te CoZnO/PVA composite showed photocatalytic efciency and completely mineralized MO in 48 min [175].Kang et al. [176] synthesized Fe 3 O 4 @Al-MOF for the removal of CR with 83.7% upto fve cycles.Cubic mesoporous ZnAl x B 2−x O 4 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/TiO 2 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-Sm 3+ codoped TiO 2 (0.6% Sm 3+ ) catalyst for the degradation of Journal of Chemistry brilliant black BN bisazo dye within 3 hours of simulated solar irradiation.Te total organic carbon removal was 82%.Te researcher demonstrated that F and Sm 3 incorporation improved the optical properties, surface chemistry, and quantum efciency 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/CoFe 2 O 4 by the sonophotocatalytic process within 40 min.Te optimum parameters for 100% AB113 removal with catalytic dose is 0.4 g/L, at pH 3. Te UV light of 36 W was used as a light source.Highly porous three-dimensional TiO 2 @Cd-MOF was fabricated by Ramasubbu et al. [181] by sol-gel technique and they further prepared TiO 2 @Cd-MOF)@ZnPp and evaluated for MO degradation.Around 94.1% degradation efciency was achieved with good recycle tests.Te researcher revealed that optical absorption and efcient electron injection further improved the photocatalytic activity.A multifunctional TiO 2 @Fe 3 O 4 @C-NF hybrid material is reported by Yilmaz et al. [182] for azo dye degradation.Te degradation efciencies reached 80-100% within 8-125 min under UV irradiation.Te prepared sample showed high repeatability, efciency, and sustainability.Kardes ¸et al. [183] fabricated fexible photocatalyst 3D polyurethane (PU) foam struts aligned with ZnO nanorods.Te 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 efciency and total organic carbon content.Te efciency obtained is 97% within 180 min under UVA light.Te hydrothermal method is adopted by Ishfaq et al. [184] for synthesizing doped CeO 2 (Ce 0.92 Cu 0.04 Bi 0.04 O 2 ).Te 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 .Te dye mineralization is also fve times faster than a CeO 2 photocatalyst with robust reusability tests.Orange G photodegradation was evaluated by chitosan-modifed CeO 2 / ZrO 2 under solar light by Nisha et al. [185].Te synthesized catalyst obtained band gap energy of 2.9 eV with an average crystalline size of 30 nm.Zhang and Shih [186] reported FeO x rGO/Ti photoanodes and photoelectrocatalytic degradation of azo dye.Te 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 efect between bimetals and the electron transfer ability of graphene within 150 min efciently removed CR dye.Tao et al. [188] evaluated the orange G degradation by MnFe 2 O 4 / α-MnO 2 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-Cu 2 CoSnS 4 -TiO 2 nanocomposite and evaluated the degradation of direct blue 71 azo dye.Gogoi et al. [190] fabricated quaternary Ag/Ag 3 PO 4 -BiOBr-C 3 N 4 composite for anionic pollutant RR 120 dye.

Photocatalyst Surface Charge Interaction with Dyes
Te surface charge of a photocatalyst (zeta potential) and dye interaction is considered as a vital factor for enhancing the photocatalytic activity.Te redox reaction and charge carrier movements occur at the photocatalyst surface.Tus, improvement in surface modifcation can pave an efective photocatalytic activity.Te surface charge (zeta potential) of the catalyst is infuenced by the pH of the medium.Borthakur et al. [191] reported the Ag/rGO nanocomposite for the CR anionic diazo dye degradation.Te zeta potential of Ag/rGO nanocomposite materials was analyzed by the following equation: where ζ denotes the zeta potential, μ signifes the electrophoretic mobility, η is the viscosity, and Dt is the dielectric constant of the medium.Te 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.Tis is due to the functional group present on the graphene sheets.Conversely, with a decrease in the acidic medium, the surface becomes positive.Te CR dye is an anionic molecule.Terefore, 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 MoSe 2 -PANI composite found that the highest photocatalytic degradation efciency is obtained in MP21 due to its optimized surface charge.MoSe 2 zeta potential is −23.01 mV, whereas PANI is positively charged with 9.51 mV.Te zeta potential of the composite MP21 sample is −5.7 mV.Tus, RhB molecules being a cationic dye would preferably adsorb on the MoSe 2 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 NiS 2 -rGO and CoS-rGO nanocomposite and evaluated the photodegradation efciency 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.Tus, 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.

Factors Affecting the Degradation of Dyes
Te dye degradation by a photocatalyst depends on certain parameters such as pH, photocatalyst particle concentration, initial dye concentration, and light intensity.Tese parameters are discussed infuencing the photodegradation of dyes.Journal of Chemistry 6.1.Catalyst Dosage.Generally, with the increase in catalyst amount, photodegradation increases.Tis 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.Te nanoparticles agglomerate which reduces the active surface sites available for exposure results in a coagulation of catalyst particles [195,196].

Efect of pH.
Te pH is an efective factor in the photocatalyst degradation of azo dyes.Te pH can afect the ionization level of the photocatalyst.Te pH variation impacts the photocatalyst oxidation process.Te 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].

Efect of Dye Concentration.
Te initial dye concentration in photocatalytic degradation plays an important role.
On increasing dye concentration and with a fxed amount of photocatalyst, the degradation rate decreases.Tis 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].

Efect of Irradiation Time and Intensity of Light.
Te light intensity, source, and irradiation time greatly infuence the photodegradation of pollutants.In general, a photocatalyst absorbs light energy equal to or more than the band gap energy.Tis further causes the electrons to transfer from VB to CB leaving behind holes in the VB.Tus, 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 costefective light source.However, nowadays, artifcial UV and solar simulators are more reproducible.Ollis et al. [202] investigated that the rate of degradation occurs linearly (frst 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.

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.Te photon absorption in the frst step leads to the formation of photogenerated electron and hole pairs [203].Tese electron/hole pairs generated further react with oxygen, water, and hydroxyl groups and form reactive oxygen species such as hydroxyl (•OH) and superoxide (•O 2 − ) radicals and hydrogen peroxide (H 2 O 2 ).To determine the active species responsible for the photocatalytic experiment, diferent scavengers are added to trap and remove active species [204].Te hydrogen peroxide is a stable molecule compared to the superoxide radical.Te hydroxyl radicals (•OH) are usually trapped by adding isopropanol (IPr), ethanol (EtOH), and methanol (MeOH).[205,206].For superoxide anion radical (•O 2 − ), the scavengers used are p-benzoquinone (p-BQ) and ascorbic acid (AA) [207][208][209].Te scavengers such as ethylenediaminetetraacetic acid (EDTA), triethanolamine (TEA), sodium oxalate (Na 2 C 2 O 4 ), and sodium iodide (NaI) are usually used to scavenge holes [210][211][212].For the electrons, the scavengers used are potassium bromate (KBrO 3 ), potassium persulfate (K 2 S 2 O 8 ), and ammonium persulfate (NH 4 ) 2 S 2 O 8 ) [212][213][214].Te singlet oxygen ( 1 O 2 ) is determined by adding sodium azide (NaN 3 ) [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.Te superoxide radicals (O 2 −•) were analyzed by control experiments under a saturated argon atmosphere.Te 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 confrmed the reactive species role by using isopropanol for OH• radical, benzoquinone for (O 2 −•), and triethanolamine for h + radical.Te researcher observed that with the addition of BQ and IPA scavenger, degradation efciency decreased to 60.1 and 24.2% from 98.66%, respectively.Tis indicates that the holes did not take part in the photodegradation process and OH• and O 2 −• species initiated the breaking of MO dye molecules.Kardes ¸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 •O 2 , radicals, respectively.Figure 8 illustrates the degradation efciencies for the dye ZnO/PU samples in the presence of scavengers [183].
It was observed that the degradation efciency (83.5%) is slightly suppressed in the presence of formic acid and 2propanol (63.2%) indicating that OH and h+ radicals have little efect on degradation process.Te efciency intensely decreased from 95.3 to 58.1% after the addition of l-ascorbic acid.Te results indicated that O 2 − 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 •O 2 − , radicals, respectively.Te results showed that 100% efciency is obtained in the absence of any scavenger.In addition to IPA and EDTA, removal efciencies decrease by 42% and 35% indicating that Journal of Chemistry •OH and h + are solely responsible for the degradation process [180].Tao et al. [188] showed that after the addition of TBA, FFA, p-BQ, and EtOH in the orange G dye solution with MnFe 2 O 4 /α-MnO 2 /PMS catalyst, degradation efciency decreased to a great extent compared to without adding scavenger 96.8%.Te order of active radicals formed in the system is SO 4 •− > O 2 •− > 1O 2 > HO•.Furthermore, DMPO and TEMP analyses were used to detect radicals.Also, the EPR results confrmed the active role of SO 4 •−, O 2 •−, and 1O 2 , in catalytic reaction.

Conclusion and Future Prospects
Dye efuent's presence causes water pollution and is a major concern for the environment.Till date, efcient dye removal techniques have been explored.In this review, we concluded various dye removal techniques such as biological, chemical, and physical methods.Te idea was to make readers informed of the existence along with modern methods available for removing organic pollutants from wastewater.Te aim of this paper is to highlight an advanced nanophotocatalysis process that is recognized as an efective method for removing dye completely from waste water.In this review, the basic theory of photocatalysis and its mechanism is discussed.Te 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 efective activity.Further various parameters afecting 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 sufer from problems such as extended

Figure 8 :
Figure 8: (a) Percentage color removal efciencies of the AR88 dye solutions for ZnO/PU nanohybrid samples and (b) percentage color removal as a function of scavengers, reproduced from reference [178] (copyright 2023, with permission from ACS).

Table 1 :
An overview of photocatalytic degradation of azo dyes using diferent photocatalysts.
[116].[115]synthesizedFe 2 O 3 using V. amygdalina leaves extract.Te degradation of MB and MO dyes was investigated under sunlight irradiation.Gupta et al.[116]reported coral-shaped Fe 2 O 3 .Te photocatalyst showed a fast degradation efciency of around 94% within 5 min in UV/H 2 O 2 in a 32 W system for azo dye MO.Recently, α-Fe 2 O 3 nanoparticles were synthesized by the combustion process of organic fuels.