Recent Advances in Congo Red Degradation by TiO₂-Based Photocatalysts Under Visible Light

Abstract

Congo Red is a complex aromatic azo dye whose metabolites can be toxic due to their carcinogenicity, mutagenicity, and various associated toxic effects on flora, fauna, and humans. Different technologies have been employed to degrade this dye, including biodegradation, radiation-based degradation, and chemical degradation with catalysts and photocatalysis. Among these, the use of TiO₂-based materials combined with photocatalysis has proven to be an effective technology for its degradation. However, the wide bandgap of TiO₂ limits its efficiency under visible light, prompting the need for modifications such as doping with metals, metalloids, and organic compounds. These modifications enhance its photocatalytic performance under visible light, achieving degradation efficiencies of up to 100% under optimal conditions. This article explores recent advances (from 2020 to the present) in the degradation of Congo Red using TiO₂-based photocatalysts under visible light, focusing on their characteristics, synthesis methods, and degradation efficiencies. Additionally, it compares the TiO₂-based photocatalysis with visible light to other available technologies, emphasizing its potential as a sustainable and efficient approach while addressing the importance of monitoring degradation byproducts to prevent the generation of equally or more toxic compounds.

1. Introduction

Congo Red is a synthetic azo dye widely recognized for its distinctive color change depending on pH levels; it appears blue-violet at a pH of 3.0 and red at pH 5.0. Chemically, its structure corresponds to 3,3′-(biphenyl-4,4′-diyldidiazene-2,1-diyl) bis (4-aminonaphthalene-1-sulfonate) and is functionally related to a 3,3′-(biphenyl-4,4′-diyldidiazene-2,1-diyl) bis (4-aminonaphthalene-1-sulfonic acid) as described by the NIH (2024). The chemical structure is shown in Figure 1 [1]. This compound, derived from benzidine, is a complex aromatic molecule whose metabolites can be highly toxic [2]. Although Congo Red is not directly toxic [2], it poses significant environmental concerns due to its carcinogenicity, mutagenicity, and harmful effects on flora and fauna [3]. Moreover, the dye is known to cause infertility, increase chemical oxygen demand in water bodies, and trigger allergic reactions through its toxic metabolites. It also negatively impacts the aesthetic and ecological quality of aquatic ecosystems [2].

Figure 1. Chemical structure of Congo Red [1].

This dye is extensively used as a colorant in the textile, printing, dyeing, and rubber industries, which often leads to its discharge into aquatic systems [3]. Once released into the environment, Congo Red persists due to its stable aromatic structure and high resistance to natural degradation. With a maximum absorption wavelength of 497 nm at a pH of 5, its detection in environmental monitoring efforts is relatively straightforward [4].

Regarding its toxicity, evaluations have been conducted using animal models. For instance, a study on New Zealand albino rabbits observed 100% mortality at doses of 6.0 and 8.0 g kg⁻¹ of body weight, highlighting its high toxicity potential at elevated concentrations [5]. This underscores the need for strict monitoring and mitigation strategies.

To address the environmental challenges associated with Congo Red, various degradation technologies have been developed. These include photocatalysis, chemical, physical, and biological treatments, and irradiation techniques. Relevant information regarding these degradation methods is presented in Table 1.

Table 1. Common methods and technologies for the degradation of Congo Red.

Method or Technology	Working Conditions	% of Degradation, Elimination, or Rate Constant/Toxicity Bioassays Performed	Reference
Biological/Aspergillus niger, producer of lignin peroxidase and manganese peroxidase	2 g mycelia incubated at pH 5 in the presence of 200 mg L−1 of dye for 6 days at 28 °C and under 120 to 150 rpm.	97% decolorization/Phytotoxicity bioassays with Zea mais and Solanum lycopersicum, and microtoxicity bioassays with Bacillus cereus ATCC 11,778 and Escherichia coli ATCC 10,536 strains found metabolites less toxic	[6]
Biological/native laccase enzyme from Ganoderma lucidum	6 h of treatment at pH 4.0 and 40 °C, total volume of 50 mL containing 50 mg L−1 of Congo red and native laccase (5 U)	Decolorization of 80%/Toxicity study with lettuce seeds has shown Congo red inhibited the hypocotyl growth, with a significant reduction of toxicity (p ≤ 0.05)	[7]
Biological/laccase from Oudemansiella canarii	5 U of the enzyme, 50 mg L−1 Congo red within 24 h at 30 °C and pH 5.5	Decolorization of 80%/Microtox test determined a diminution of toxicity of 92.5%	[8]
Biological-Photocatalytic/Extract of Carissa edulis	Extract of Carissa edulis obtained by microwave-assisted extraction at 70 °C and 400 W, capped with ZnO nanoparticles (NPs) (1 mg) and mixed with 1 mM Congo red dye in water, for 130 min, at 365 nm in a photoreactor	97% degradation/No toxicity bioassays were performed	[9]
Photocatalytic/Cobalt ferrite (CoFe2O4) nanostructures	Aqueous solution of dye (10 mg L−1), 10 mg of CoFe2O4 nanostructures were thoroughly mixed and kept at room temperature for 30 min in the dark and constant stirring was maintained; UV irradiation for 90 min, pH 9; centrifugation at 4500 rpm for 15 min	91% degradation/No toxicity bioassays were performed	[10]
Physical-Chemical/Dalspinin Mediated gold nanoparticles (DLP-AuNPs)	Congo Red solution (1 × 10−5 M) in the presence of NaBH4 (1 × 10−3 M); 100 μL of DLP-AuNPs was added into a quartz tube containing dye solution (3.0 mL) and NaBH4, with a reaction time of 10 min	Degradation with a rate constant of 4.5 × 10−3 s−1/Toxic metabolites monitored by Fourier transform infrared (FTIR) spectroscopy/Toxicity not specified	[11]
Photocatalytic/ZnO	pH 8, 60 min of irradiation time, ZnO concentration of 0.5 g L−1, dye concentration of 20 mg L−1, temperature of 298 K, I = 90 j/cm2, λ = 365 nm	95.02% degradation/No toxicity bioassays were performed	[12]
Photocatalytic/ZnO and ZnO/CuO composites	A 350 W Xe lamp was used, specific quartz reactors (100 mL), and 50 mg of each composite was added into 50 mL of dye aqueous solution (50 mg L−1); initial pH value 5.6, sonicated for 30 min in the dark	25% and 95% degradation for ZnO and ZnO/CuO/No toxicity bioassays were performed	[13]
Physical-Chemical/ZnMn2O4 nanoparticles (NPs) and visible light	Room temperature, visible light (fluorescent lamp (λ > 400 nm, 90 W)), 15 min, dye concentration of 20 mg L−1, 30 mg of catalyst, natural pH	96% degradation/No toxicity bioassays were performed	[14]
Physical–chemical/Ultraviolet photolysis of nitrate (UV/NO3−)	The photon flux of (I 253.7 nm) was calculated to be 1.23 × 10−7 Einstein⋅L−1⋅s−1; dye concentration of 20 μM and NO3− of 50 mM; sodium nitrate (50 mM) and Congo Red (20 μM) were added to deionized water (100 mL), 25 ± 1 °C, pH 7 ± 0.4	81.9% degradation/No toxicity bioassays were performed	[15]
Physical and chemical/Pd-doped ZnO catalyst and UV irradiation	UV irradiation (100 W, strongest emission at 365 nm) for 1 h, 50 mg of photocatalyst in 100 mL of dye solution (16 mg L−1), stirred for 30 min before irradiation	98% degradation/No toxicity bioassays were performed	[16]
Physical-Chemical/Photoelectrochemical Fenton’s based	0.260 mM of dye with 0.50 mM Fe2+ at 100 mAcm−2, current efficiency and 0.45 kWh (g DOC)−1 energy consumption, 240 min, pH 3, boron-doped diamond (BDD) air-stirred reactor	Mineralization current efficiency (MCE) of 49%/Liquid-Chromatography-Mass Spectrometry (LC-MS) used for monitoring metabolites/No toxicity bioassays were performed	[17]
Physical–Chemical/Co-60 gamma radiation	5 kGy, 0.5 mL of H2O2 (37%) was added to the dye solution (50 mg L−1)	100% decolorization/Radiolytic end products were monitored by FTIR and Gas-Chromatography-Mass Spectrometry (GC-MS)/No toxicity bioassays were performed	[18]
Physical–Chemical/gamma rays	A series of 1 × 10−4 M Congo Red aqueous solutions were bubbled with high-purity (99.99%) N2, N2O and O2, respectively, for 15 min in Pyrex glass tubes before gamma-ray irradiation at 11.9 kGy	Total Organic Carbon (TOC) decreased by 76% and 86% for solutions saturated with O2 and N2O, respectively/No toxicity bioassays were performed	[19]
Physical–Chemical/Activated carbon powder under microwave irradiation	A volume of reaction of 25 mL with a dye concentration of 50 mg L−1, 3.6 g L−1 of activated carbon powder, and 2.5 min of microwave irradiation at a microwave frequency of 2450 MHz and output power of 800 W	High-performance liquid chromatography (HPLC) monitored 96.49% degradation and the intermediate degradation products/No toxicity bioassays were performed	[20]

As shown in Table 1, photocatalysis is the most employed method for degrading Congo Red, with decolorization ranging from 81.9% to 98% [10,12,13,14,15,16]. This technique uses advanced materials such as titanium dioxide (TiO₂) and zinc oxide (ZnO), often enhanced with dopants or co-catalysts, to decompose the dye under ultraviolet (UV) or visible light. However, while there are high degradation rates, these processes often lack direct toxicity assessments. Conversely, the combination of physical–chemical degradation with irradiation using visible light, UV light, gamma radiation, or microwaves necessitates shorter treatment times, often less than an hour, making it faster than other technologies (see Table 1).

Irradiation techniques, such as gamma radiation from sources like Co-60 [18,19], or microwave-assisted degradation [20], have also shown high efficiency, with degradation rates ranging from 86% to 100%. These technologies often monitor intermediate products through advanced analytical methods such as liquid chromatography-mass spectrometry (LC-MS), Fourier-transform infrared spectroscopy (FTIR), or gas chromatography-mass spectrometry (GC-MS). For example, Muneer et al. identified radiolytic products of Congo Red using FTIR and GC-MS, while Sales Solano et al. employed LC-MS to analyze metabolites formed during the Fenton’s process [18].

Biological treatments focus on using microorganisms, such as bacteria and fungi, or plant systems to degrade Congo Red. Previous studies have demonstrated that these methods achieve degradation rates between 80% and 97% [6,7,8,9], while also incorporating toxicity bioassays to ensure the environmental safety of the resulting products. For example, Fowsiya et al. demonstrated the ability of certain microorganisms to degrade the dye while reducing its toxicity [9]. Unlike physical–chemical methods that use irradiation, biological enzymes typically require longer treatment times, ranging from several hours to 6 days. This can be a significant drawback (see Table 1). Additionally, some enzymes, particularly purified ones, can be quite expensive [21] and may denature under extreme conditions such as high pressure, temperature [22], pH [23], and salt concentrations [24]. In contrast, materials like TiO₂ are more affordable [25] and stable [26].

Assessing the toxicity of degradation byproducts is crucial to ensuring the environmental safety of treatments. Depending on the degradation method used, toxic intermediates can form. For example, during the ozonation of diclofenac, antiestrogenic compounds with dichloroaniline structures were detected, which were not present during photodegradation using UVA and UVC light [27]. Findings like these highlight the importance of thorough monitoring to ensure that degradation processes do not produce byproducts more hazardous than the original pollutant.

2. Degradation of Congo Red with TiO₂-Based Photocatalysts: Advantages and Limitations

When selecting a material for degradation processes, several crucial factors must be taken into account to ensure efficiency and feasibility. These include the material’s availability, affordability, and suitability for large-scale applications. Additionally, a high specific surface area and the capability for surface functionalization are highly desirable characteristics. In photocatalysis applications, the material’s ability to enhance the absorption of UV radiation is particularly advantageous. TiO₂ exemplifies many of these characteristics [28]. It is chemically inert, non-toxic, and possesses functional groups that can facilitate further surface modifications, making it well-suited for environmentally friendly processes [29].

Despite its advantages, TiO₂ has a notable limitation: its wide band gap of approximately 3.2 eV (in the anatase crystalline form) restricts its photocatalytic activity to UV light. This dependence on UV radiation is problematic because UV constitutes only about 5% of the solar spectrum, whereas visible light represents a significantly larger portion at approximately 45%. Consequently, TiO₂ demonstrates low energy efficiency when exposed to natural sunlight. Addressing this limitation by shifting its optical response from the UV to the visible spectrum could greatly enhance its practical applications, particularly in solar-driven photocatalysis [30].

Numerous studies have demonstrated the effectiveness of TiO₂ in degrading Congo Red when combined with UV light. For instance, Nurdin et al. explored using TiO₂ and UV radiation in degrading a 5 mg L⁻¹ solution of Congo Red. Over 60 min, they observed a degradation rate (k’) of 0.018 ppm min⁻¹ in a 4 mL solution [29]. Similarly, Harun et al. achieved a 66.99% degradation of 250 mL of 4 mg L⁻¹ of Congo Red within 30 min using TiO₂ and a UV lamp [31]. Another significant study by Turcu et al. used TiO₂ and UV light, a pH of 5.5, and 0.05 g of powdered TiO₂ in a 50 mL solution with 50 mg L⁻¹ of Congo Red, achieving a remarkable 98.28% degradation in 90 min [32].

These studies underscore the potential of TiO₂ as an effective photocatalyst for the degradation of Congo Red under UV light. However, they also highlight the limitations imposed by its dependence on UV radiation, emphasizing the importance of further research into strategies to improve its optical response. Possible approaches include doping with other elements, coupling TiO₂ with other semiconductors, or developing composite materials to extend its absorption range into the visible spectrum. Enhancing the solar utilization efficiency of TiO₂ could significantly advance its application in sustainable, large-scale environmental remediation processes.

While other materials such as Fe₂O₃, WO₃, ZnSe, CdSe, CdS, and MoS₂ have lower band gaps [33], TiO₂ stands out due to its numerous advantages. These include low cost, easy accessibility, the ability to modify its surface with different molecules, physical and chemical stability, non-toxicity, photosensitivity, and biocompatibility. Additionally, TiO₂ nanoparticles can be surface-modified to create stable, non-aggregating formulations [34]. These characteristics make TiO₂ particularly suitable for the photocatalytic degradation of organic compounds, such as Congo Red, which will be explained further.

3. Degradation of Congo Red Using TiO₂-Based Photocatalysts Under Visible Light

TiO₂ has proven to be an efficient photocatalyst with a wide range of applications. However, its effectiveness and suitability for large-scale use can be significantly enhanced by modifying its properties to function under visible light. This is primarily achieved by reducing its band gap, allowing for a broader absorption spectrum. Over the years, extensive research has been conducted to optimize TiO₂ for visible light photocatalysis, yielding promising results, as summarized in Table 2.

Table 2. TiO₂-based materials used in the degradation of Congo Red with visible light.

Photocatalyst	Catalyst Synthesis/Degradation Conditions	Band Gap and Absorption Wavelength/% of Degradation or Elimination/Toxicity Bioassays Perfomed	Reference
TiO2/reduced graphene oxide sheets (rGO/TiO2)	Synthesis: hydrothermal method, 10 mL of Titanium tetra isopropoxide (TTIP) stock was diluted with 40 mL of distilled water and irradiated using manual irradiation at 15 W in ice bath. Dried at 80 °C for 5 h and calcined at 500 °C for 2 h. The GO was synthesized by the Hemmer modified method. The rGO/TiO2 nanocomposite was carried out using the hydrothermal method. Degradation: 0.01 g of catalyst 5% in 100 mL of 10 mg L−1 dye solution, stirred in the dark for 30 min. Direct sunlight with an intensity of 1200 W·cm2, 100 min, pH = 7, and 50 mg L−1 of dye concentration.	Band gap of 2.7 eV, 330–500 nm/Up to 99% of degradation and mineralization/Monitoring of products by LC-MS/No toxicity bioassays were performed	[35]
TiO2-doped cobalt ferrite (CoFe2O4) nanoparticles	Synthesis: Microwave-assisted method, using a 2:1 molar ratio of ferric nitrate and cobalt nitrate solutions, in a domestic microwave oven for 15 min at 2:54 GHz frequency at 850 W power, ground in a mortar. TTIP (0.1 M) was added and irradiated at 2:54 GHz (850 W power) in a microwave oven for 15 min and washed with ethanol. CoFe2O4 (95%) and titania (5%) were mixed with 150 mL of tetrahydrofuran (THF) and stirred for 1 h. 1 g of ascorbic acid was added, filtered at 25 ◦C, and dried for 48 h. Finally, it was calcinated at 500 °C for 3 h. Degradation: 80 mg of TiO2-doped CoFe2O4 nanostructures were gagged for the degradation of 100 mL dye (10 mg L−1) under visible light irradiation (150 W metal Halide lamp; λ > 400 nm), and 120 min. Also, an assay was carried out with 5 mL of 10% of H2O2 to increase -OH radicals.	Band gap of 2.88 eV, 330–500 nm/85% degradation without H2O2 and 97% with H2O2/No toxicity bioassays were performed	[36]
Fe-doped TiO2 activated carbon (AC) nanocomposite	Synthesis: the ultrasonic–hydrothermal method was used, 0.15 g of TiO2 nanoparticles was added to 1 g of 100 mL ferric nitrate solution while stirred for 30 min. Then, 0.3 g activated carbon was poured into the mixture and ultra-sonicated for 1 h. Later, the mixture was shifted into a hydrothermal autoclave and kept at 150 °C inside an oven for 24 h. After the hydrothermal treatment, the solution was washed with double-refined water in a centrifuge machine to neutralize the solution pH. Finally, the solution was dried at 65 °C for 12 h. Degradation: 20 mg L−1 of dye solution for 60 min, pH 1, 0.06 g of catalyst, and irradiated at a maximum wavelength of 520 nm with a high-pressure mercury lamp, 350 W, located at the top of the reactor, 10 cm away from the reaction solution.	Band gap 2.3 eV, 468 nm/100% degradation/No toxicity bioassays were performed	[37]
TiO2 co-doped with Fe, Co, and S	Synthesis: TiO2 nanoparticles co-doped with Fe, Co, and S were prepared using the sol-gel method. TTIP solution was poured continuously and stirred for 1 h. The product was isolated using centrifugation. It was washed several times with C2H5OH and deionized water. After drying at 100 °C, this product was calcinated in a box furnace at 500 °C for 3 h. Degradation: under optimized conditions, 30 mg L−1 of dye was degraded at a slightly acidic pH (~6.5), with 0.14 g of photocatalyst within 70 min of irradiation time.	Band gap from 1.46–2.85 eV, maximum absorbance at 500 nm/99.3% degradation/No toxicity bioassays were performed	[38]
Cu-ZnO/TiO2 Nanocomposite	Synthesis: ultrasonic-assisted and solvothermal method, 100 mL TiO2 NPs suspension in distilled water was prepared using an ultrasound probe sonicator (20 kHz, 20 mm tip diameter, 220 W) operated with pulse mode (5 sec ON and 5 sec OFF) for 10 min. An appropriate quantity of CuSO4.5H2O, C4H6O4Zn·2H2O, and polyvinylpyrrolidone (PVP) was added as a stabilizer. The solution was dried at 80 °C for 6 h to provide the ternary Cu-ZnO/TiO2 nanocomposite photocatalyst. Degradation: direct sunlight, during 20 min, 0.025 g of catalyst, dye solution of 75 mg L−1.	Band gap of 2.68 eV, 500–650 nm/98% degradation/No toxicity bioassays were performed	[39]
C,N Co-Doped TiO2	Synthesis: hydrothermal method, 1 g of TiO2 suspended in water was mixed with chicken egg white (2 g) accompanied by constant stirring at 500 rpm for 2 h, and the mixture was transferred into an autoclave to be heated in the oven at 150 °C for 4 h. Afterward, the photocatalyst was collected, dried at 100 °C for 2 h, and calcined at 500 °C for 2 h. Degradation: 10 mg L−1 Congo Red dye in 100 mL of the solution under visible irradiation could be degraded by applying TiO2-C,N prepared from 2 g of the egg white within 45 min, at pH 7, and 50 mg of the photocatalyst mass.	Band gap from 2.69 to 3.04, absorbance from 415 to 430 nm/98% degradation/No toxicity bioassays were performed	[40]
Modified P25-TiO2 with Cobalt-Carbon Supported on SiO2 Matrix	Synthesis: solvothermal method, cobalt–carbon silica nanocomposite CoC@SiO2-bipy (s1) and CoC@SiO2-phen (s2) were prepared by ultrasonication of TiO2 (75%) and CoC@SiO2-bipy (s1) or CoC@SiO2-phen (s2) (25%) (weight ratio) for 30 min. The mixture was then filtered, dried at 60 °C, and milled Degradation: initial concentration of dye of 10µM; irradiation time of 60 min; solar power (UV index 5.0), volume of 2 mL; pH of 4.0, P25-TiO2 concentration of 56.25 mg L−1.	92% degradation/No toxicity bioassays were performed	[41]
Green synthesized TiO2 (CG-TiO2)	Synthesis: CG-TiO2 was prepared from TTIP precursor using aqueous leaf extract of Calotropis gigantea (30 g leaves in 100 mL distilled water boiled for 2 h at 90 °C); TTIP:CG—1:3; mixing time and temperature—2 h and 27 °C, respectively; calcination temperature—300 °C.	97% degradation/No toxicity bioassays were performed	[42]
Bentonite/TiO2 quantum dots (Bent/TiO2 QD)	Synthesis: co-precipitation method, using TTIP and calcination at 280 °C for 60 min in a muffle furnace. TiO2 was prepared via the sol-gel method using TTIP as the precursor. The resulting gel was aged at room temperature for 24 h, and after that was dried in an oven at 105 °C for 7 h and calcined at 280 °C for 4 h in a furnace. Degradation: 240 min of irradiation by xenon photoreactor, irradiance of 70 W/cm2, 500 mL of dye solution (4 × 10−5 M), pH 6.9, 0.5 g of catalyst, maintained in darkness for 20 min previous to irradiation. Then, it was centrifugated at 12,000 rpm/30 min to separate the catalyst from the dye solution.	Band gap of 3.15 eV/Removal efficiency of 84.5% and a Total Organic Carbon (TOC) reduction of 76.2%/No toxicity bioassays were performed	[43]
Cr doped into TiO2	Synthesis: Sol–gel method. The doping was conducted by reacting TTIP as TiO2 precursor and Cr-containing tannery wastewater through the sol–gel method. Degradation: 10 mg L−1 of Congo red in 50 mL of reaction solution, 20 mg of TiO2-Cr (1:0.5) photocatalyst weight, solution pH at 5 in 60 min.	Band gaps from 2.21 to 2.59 eV/98% degradation/No toxicity bioassays were performed	[44]
Cu2O/TiO2-Quantum Dots (QD)	Synthesis: precipitation method. 10 mg copper (I) oxide nanoparticles in 10 mL ethanol were stirred for 30 min on a magnetic stirrer, and TiO2-QD sol was added. Then, the mixture was centrifuged at 8000 rpm, and the precipitate was filtered and washed several times using ethanol/water (1:1) solvent. To get Cu2O/TiO2-QD composite, the obtained precipitate was dried at room temperature. Degradation: 9 mg L−1 of dye, 150 mg L−1 of catalyst, 110 min, pH 6, visible light.	Band gap of 2.48 eV, 300–650 nm/89% degradation/No toxicity bioassays were performed	[45]
Doped TiO2 with Fe from rusty iron waste	Synthesis: sol–gel method, by interacting the TTIP solution with a solution containing Fe3+ dissolved from the rusty iron waste. The Fe3+ solution was prepared by dissolving 0.80 g of the rusty iron waste in 7.5 mL of HCl and HNO3 mix (3:1) then diluted up to 100 mL. The resulting gels were calcined at 500 °C for 3 h. Degradation: pH 5, in a closed box equipped with visible lamps, 60 mg of catalyst, irradiated with visible light accompanied by stirring magnetically for 60 min.	Band gap from 2.38 to 3.15 eV, 424, 554 nm/99% degradation/No toxicity bioassays were performed	[46]
Ag and Fe co-doped TiO2 Nanoparticles	Synthesis: The nanoparticles were synthesized by sol–gel method, using TTIP as precursor, AgNO3 and FeCl3. The obtained sol was kept for 24 h at room temperature to convert into gel, filtered, washed with methanol and distilled water several times, and then dried at 100 °C for 24 h in a hot oven. The dried material was ground with a mortar and pestle and annealed at 400 °C for 4 h. Similarly, AgxFe0.05Ti0.95-xO2; x = 0.08, 0.12 mol % were prepared by using 0.7 and 1.0 gm of AgNO3 in TTIP–methanol solution. Degradation: concentration of Ag (x = 0.12), custom-built reactor, 0.05 g of catalyst, 120 min, 100 mL of 10−5 M of dye, stirred 1 h in the dark, then illuminated with a phosphorous coated Hg vapor lamp with cutoff filters from 420–520 nm.	Band gap from 2.30 to 2.92, max absorbance with 0.12 mole% from 25′ to ~430 nm/82% degradation/No toxicity bioassays were performed	[47]
Bimetallic Au-Pd/TiO2	Synthesis: synthesis of sol. Aqueous solutions of PdCl2 and HAuCl4 3H2O were prepared. PVA (1% weight solution) was supplied to the solution previously prepared. A 0.1 M NaBH4 solution was mixed to synthesize a dark brown sol. Titania and carbon and the H2SO4 were stirred and the catalyst produced was filtered before being thoroughly washed with 2 L of distilled water. Then, it was oven-dried at 120 °C for 16 h. Degradation: the best conditions were 28 mg L−1 of dye, 0.09 g. 75 mL−1 of dye solution of catalyst, 7 µL. 75 mL−1 of dye solution of H2O2, pH 5, 55 °C, 4 h.	96% degradation/No toxicity bioassays were performed	[48]
Sol–Gel Immobilized TiO2 Thin Layers	Synthesis: TTIP was used as the precursor for TiO2. TiO2 thin films were synthesized using the sol–gel technique, involving hydrolysis and condensation of TTIP in a controlled solution. TiO2 thin films were synthesized using the sol–gel technique through hydrolysis and condensation in a controlled solution environment. The coated glass tube column underwent calcination at 400 °C to transform the deposited gel into crystalline TiO2 thin films. Degradation: 50 mg L−1 of dye, 240 min, batch reactor, 415 nm, a glass tube column with immobilized TiO2 thin film used as a catalyst.	Band gap of 3.2 eV, absorption started at 390 nm/99% degradation/No toxicity bioassays were performed	[49]
Cuprous oxide/titanium dioxide and cuprous oxide/zinc oxide p-n heterojunction photocatalyst (Cu2O/TiO2-Cu2O/ZnO	Synthesis: The nanoparticles were produced using a solvothermal one-pot process in a Teflon-coated stainless-steel autoclave under autogenous pressure. Cu(NO3)2⋅3H2O (1.5 M) was dissolved with absolute ethanol and autoclaved for 8 h at 170 °C. The final product (i.e., Cu2O) was obtained by filtration followed by washing thoroughly after the autoclave had cooled down. The product was then dried for 12 h at 60 °C. After that, 1.5 M Cu(NO3)2⋅3H2O and 1.5, 1.0, and 0.5 M Ti(OBu)4 were separately dissolved with 30 mL of absolute ethanol under stirring. The solutions were then combined, stirred, and transferred to the autoclave. The autoclave was operated at 170 °C for 8 h, cooled to room temperature, filtered, then washed and dried for 12 h at 60 °C. Degradation: 30 mg of photocatalysts into 100 mL dye solution (30 mg L−1), 10 min. Previously, the dye solution with the catalyst was stirred for 30 min in the dark. Next, a 300 W xenon lamp with a cut-off filter of 420 nm was switched on to begin photocatalytic degradation under visible light illumination.	Band gap of 2.61 eV, 500–570 nm/100% degradation/No toxicity bioassays were performed	[50]
In3+ and Sb5+ doped and co-doped TiO2 semiconductors	Synthesis: Chemical precipitation. The desired TiO2 compositions were synthesized by using titanium butoxide (Ti(OBu)4, 98%), indium chloride (InCl3, 99.9%), antimony acetate (Sb(CH3CO2)3, 99%). Pure TiO2 powder was synthesized by the addition of 30 mL n-butanol (C4H10O) to 10 titanium butoxide (Ti(C4H9O)4) solutions under constant stirring for 30 min. After that, the precipitate was produced by adding NH4OH solution at pH 8.5 under continuous stirring. The acquired precipitate was washed and dried under normal air using deionized water, followed by calcination at 700 °C for 4 h to form TiO2 powder. Degradation: 15 mg L−1 of dye, 100 mL of solution, 0.05 g of catalyst, stirred for 30 min before irradiation, then irradiated under natural sunlight for 80 min.	Band gap from 2.91 to 3.21, 500–1250 nm/81 and 86% for Ti0·97Sb0·03O2 and Ti0·94In0·03Sb0·03O2, respectively/No toxicity bioassays were performed	[51]
Ternary TiO2/Y2O3@ g-C3N4 nanohybrid	Synthesis: thermal and sonochemical method. The nanosheets of g-C3N4 were fabricated by thermally degrading urea. Ternary TiO2/Y2O3@g-C3N4 nanostructures were fabricated using a straightforward ultrasonication technique, including agitating 0.90 g of pure g-C3N4 in 0.150 L ethanol solvent. The ethanolic solution was combined with 0.2 g of TiO2 and Y2O3 (15% each) nanopowders to generate a suspension solution. The mixture was sonicated at 500 MHz for 80 min before being dried at 85 °C for 24 h, pulverized with a ceramic mortar, pestled, and heated at 150 °C for 60 min in a muffle furnace. The same protocol was adopted to fabricate TiO2@g-C3N4 and Y2O3@g-C3N4. Degradation: 0.05 g of catalyst, 100 mL of dye solution (30 mg L−1), stirred (400 rpm) in the darkness. Then, it was exposed for 60 min to visible light radiation (OSRAM lamp 58 IM/W).	Band gap from 2.47 to 2.55 eV, 440 nm/100% degradation/No toxicity assays.	[52]
Z-Scheme CuO@ TiO2@ halloysite heterostructure	Synthesis: two-step microwave and ultrasonic-assisted. Degradation: 50 mL of dye solution (10 mg L−1), 30 mg of catalyst, left in the dark for 30 min before irradiation, irradiated for 150 min with a Xenon lamp (300 W) with visible light (450 nm).	Band gap of 3.1 eV, max absorption at ~450 nm/82% degradation/No toxicity bioassays were performed	[53]
Carbon nanotubes - silver modified -titanium dioxide (CNTs-Ag-TiO2)	Synthesis: thermal and ultrasound-assisted method. 0.5 g of carbon nanotubes (CNTs), 10 mL concentrated nitric acid, and 30 mL concentrated sulfuric acid were stirred at 300 rpm for 30 min and heated to 80 °C; then, CNTs were washed with deionized water, taken out of the solid and dried at 70 °C, and stored in containers for subsequent use. The modified CNTs and nano silver powder (AgNPs) were added into 30 mL absolute ethanol at the same time, and the CNTs-Ag composite material was obtained by standing after ultrasonic treatment for 30 min, taking out the solid after centrifugal treatment for 5 min, and grinding. Degradation: 100 mL of dye (100 mg L−1), stirred in the dark for 60 min, then was irradiated with a 1000 W xenon lamp for 140 min.	Band gap and wavelength not specified/100% degradation/No toxicity bioassays were performed	[3]
TiO2 quantum dots (TDS)	Synthesis: synthesized via a low-temperature precipitation method using a syringe, 6.0 mL of TTIP was dispensed into a 250 mL beaker containing 180 mL of isopropyl alcohol, and the mixture was stirred continually at 0 °C for over 60 min to create solution A (pH 7). After 24 h at room temperature (25 °C), a white powder was produced by adding 0.03 mol of CTAB to solution (A). The dry white powder was removed from the crucible, placed in the mortar, coarsely crushed and deposited in an aluminum oxide crucible. Subsequently, the samples were subjected to the calcination process for 45 min at 280 °C. Degradation: Industrial textile effluent from a dying plant with a pH of 6.9 was treated immediately under direct sunlight. Throughout the experiment, the daily dosage of UV radiation was 4.7 mW/cm2, and the dose of visible light received during the middle of the day was 1635 mW/cm2, 100 min.	Band gap from 2.97–3.09 eV/Photodegradation rate of 22.49 × 10−3 S−1/No toxicity bioassays were performed	[4]
(TiO2)/Cellulose biochar	Synthesis: Sol–gel method. 5 mL of TTIP was dissolved in 90 mL of isopropanol and stirred for 5 min. 1 g of cellulose biochar was added, and continued stirring. After that, 9 mL of distilled water was added and stirred for 2 h to form a homogeneous gel. Then, the gel was dried at 100 °C for 5 h and ground using a mortar and pestle. Next, the solid materials were calcined at 500 °C for 2 h, washed with distilled water several times until the supernatants looked clear, and dried in an oven at 100 °C overnight. Degradation: The optimum conditions for the photocatalytic degradation of Congo Red were 0.5 g L−1 of cellulose biochar/TiO2−700 with 0.3 mM of potassium peroxymonosulfate (PMS) used to treat an initial dye concentration of 10 mg L−1 under 30 °C and solution pH 5, in 30 min.	Band gap and wavelength not specified/100% removal/No toxicity bioassays were performed	[33]
Cu-doped mesoporous TiO2 (mTiO2)	Synthesis: Sol–gel, ultrasonic, hydrothermal treatment, and calcination methods. 0.6 g of PVA was added in boiling water and stirred for 30 min to prepare solution A, and then 3 mL of TTIP were hydrolyzed in 1.5 mL of acetic acid to prepare solution B. Solution B was then mixed with solution A. An Ultrasonic Disruptor UD-21 was used to sonicate (sonication power: 70 W, frequency: 59 KHz, and speed: 24,000 rpm) the mixture for 60 min at room temperature. The intermediate products were placed inside a Teflon-sealed vessel for 17 h of hydrothermal treatment at 105 °C. The prepared material was then calcined for 5 h at 500 °C. The same technique was used to synthesize mesoporous TiO2 with 1, 2, and 3 w% Cu-doping. Degradation: 100 mL of dye (40 mg L−1), 45 mg of Cu-doped TiO2 (3 w%), pH 5, kept in the dark for 30 min, then irradiated with a Hg lamp with UV light output of 5.8 × 102 W/cm C/Co.	Band gap of 2.6 eV, wavelength not specified/99% degradation/No toxicity bioassays were performed	[54]
Magnetically Recyclable Wool/Fe3O4@ TiO2/UiO-66 Core-Shell Structured Composite	Synthesis: wool/Fe3O4@TiO2/UiO-66 composite was prepared by solvothermal and hydrothermal methods, using 0.2332 g of ZrCl4 and 0.1661 g of H2BDC, dissolved in 50 mL of N,N-dimethylformamide (DMF) solution, and 6 mL of glacial acetic acid was subsequently added to control the morphology of UiO-66. Then, 0.6 g of TBT was dissolved in 18 mL of absolute ethanol, and 12 mL of deionized water and 2 mL of PEG400 were added successively under vigorous stirring to obtain the TiO2 precursor solution B. After that, according to the mass ratios of TiO2 to UiO-66 1:2, 1:1, and 2:1 individually, a certain volume of B solution was added into a specific volume of A solution under continuous stirring and treated at 120 °C for 4 h in a 100 mL Teflon-lined stainless steel autoclave. In addition, the UiO-66 and TiO2 particles were prepared using the precursor solution A and B under solvothermal and hydrothermal conditions, respectively. Degradation: 0.01 g of catalyst, 30 mL of dye solution (40 mg L−1), and left in the darkness. Then, exposure to a white light diode lamp (LED, 25 W) with an optical power density of 155 Mw/cm2 or 1.3 W/cm2 under stirring.	Band gap from 2.49 to 3.03 eV, 450 nm/77.1% degradation/No toxicity bioassays were performed	[55]

The efficiency of TiO₂-based photocatalysis in degrading pollutants like Congo Red depends on several factors, including the concentration of the catalyst, pH levels, the initial concentration of the dye, and reaction time. Studies have extensively characterized these variables, presenting key findings in the Supplementary Data. For example, Asaad Mahdi et al. proposed a detailed mechanism for the degradation of Congo Red using TiO₂ under visible light. According to their findings, the degradation process involves the cleavage of amine groups and oxygenation of the dye, leading to the formation of new compounds. The results from LC-MS revealed two intensity peaks corresponding to hydroxy-napthalene-1-sulfonic acid, which resulted from the cleavage of the C₆H₆ ring, along with the bond cleavages of N‚N, and C-N. Additionally, the sustained activity of hydroxy free radicals contributed to the formation of 4-carboxy-butanoate and naphthalene, with respective molecular weights of 138 and 122. Reactive oxygen species (ROS) can cause the cleavage or reduction of sulfonic, amine, nitro, and hydroxyl groups bonded to the C₆H₆ ring, leading to the production of 3-carboxy propanoate, malonic acid, and malonate [35]. The authors also proposed a mechanism of action for the decolorization of Congo Red (see Figure 2).

Figure 2. Mechanism of decolorization of Congo Red via reactive oxygen species (ROS), according to Asaad Mahdi et al. [35].

3.1. Modification of TiO₂ Photocatalyst Using Metals

A critical development in this field has been the doping of TiO₂ with various materials to decrease its band gap and enhance its performance under visible light. Numerous studies have demonstrated that doping TiO₂ with metals can significantly improve its photocatalytic efficiency, including Fe [36,37,38,46,47,55], Co [38,41], Cu [39,45,50,54], Zn [39,50,51], Cr [44], Ag [47], Au [48], Pd [48,51], and Sn [56], as well as metalloids like Sb [51]. Degradation rates as high as 99% [35,38,46,54] and even 100% [37,50] have been reported in studies using these doped catalysts (see Figure 3).

Figure 3. Percentage of Congo Red degradation using TiO₂ along with various dopants and co-dopants.

According to Zaleska [57], co-doping with metallic species can create a new energy level within the band gap of TiO₂. In a typical photocatalytic reaction, the process begins with the absorption of a photon (hv1) that has energy equal to or greater than the band gap of TiO₂ (approximately 3.3 eV for the anatase phase). This absorption generates an electron-hole pair on the surface of the TiO₂ nanoparticle. The absorbed energy promotes an electron to the conduction band, resulting in the formation of a positive hole in the valence band. The excited electrons and holes can recombine, dissipating the input energy as heat, or they may become trapped in metastable surface states. Alternatively, they can react with electron donors and acceptors that are adsorbed on the semiconductor surface or within the surrounding electrical double layer of the charged particles. When these holes react with water, they can produce hydroxyl radicals, which possess a high redox oxidizing potential. Depending on the reaction conditions, the holes, hydroxyl radicals, superoxide anions (O₂⁻), H₂O₂, and O₂ can play vital roles in the photocatalytic reaction mechanism. Furthermore, when TiO₂ is metal-doped, a new energy level is generated in the band gap due to the dispersion of metal nanoparticles within the TiO₂ matrix. In this scenario, an electron can be excited from the defect state to the TiO₂ conduction band by a photon with energy equal to hv2. An added benefit of transition metal doping is the improved trapping of electrons, which helps to inhibit electron–hole recombination during irradiation. Additionally, the reduction in charge carrier recombination leads to enhanced photoactivity [57].

3.2. Modification of TiO₂ Photocatalyst Using Non-Metals

Additionally, the efficiency of TiO₂ under visible light can be enhanced using other materials. For instance, graphene oxide has achieved a degradation rate of up to 99% [35], while bentonite combined with quantum dots has also shown high effectiveness [43]. Organic compounds like cellulose with biochar have reached a complete degradation of 100% [33]. In contrast, recyclable wool achieved only a 77.1% degradation rate [55] (see Figure 3). Furthermore, activated carbon as a dopant has resulted in a 100% degradation rate [37] and 98% when used as a co-dopant [40]. Notably, 100% degradation has also been attained with nano Y₂O₃@g-C₃N₄ hybrids [52]. The enhancement of efficiency in TiO₂ co-doped with non-metal can be explained by three distinct mechanisms [57]: (a) band gap narrowing, (b) impurity energy level, and (c) oxygen vacancies. In the band gap narrowing mechanism, the nitrogen 2p states hybridize with oxygen 2p states in anatase TiO₂ doped with nitrogen, as their energy levels are very close. This interaction narrows the band gap of N-TiO₂, enabling it to absorb visible light. The impurity energy level mechanism involves the substitution of oxygen sites in TiO₂ with nitrogen atoms, which create isolated impurity energy levels above the valence band. Under UV light irradiation, electrons are excited from both the valence band and the impurity energy levels; however, when illuminated with visible light, only electrons in the impurity energy level are excited. Lastly, in the oxygen vacancies mechanism, oxygen-deficient sites form at the grain boundaries, which are critical for enhancing visible light activity. The presence of nitrogen dopants in these oxygen-deficient sites helps to prevent reoxidation, further improving performance [57].

3.3. Impact of the Synthesis Process on Degradation Ability

The synthesis process also plays a crucial role in determining the efficiency of these photocatalysts. Many catalysts prepared via the hydrothermal method [3,35,37,40,52] achieved the highest degradation percentages [35,37,40,54], often comparable to or exceeding other methodologies and technologies described in Table 2. In contrast, sol–gel methods were the most frequently employed [33,38,44,46,47,49,50,54,55], yielding degradation percentages above 90%. Only one study utilized a solvothermal method and reported a high degradation percentage (98%) [39]. Most photocatalysts with lower degradation percentages were produced using alternate methods, except for a study by Tian et al., which achieved a 77.1% degradation using both solvothermal and hydrothermal techniques [55]. Rani also reported an 82% degradation rate with the same synthesis method. Additionally, Magdalane et al. achieved an 85% degradation using a microwave-assisted method [36], Sayqal et al. [43] reported an 84.5% degradation from a co-precipitation method, Musmade et al. [45] obtained a rate of 89% with a precipitation method, and Moulahi et al. achieved 86% using a precipitation method [51]. Pouthika et al. reported 81% degradation with a microwave ultrasonic-assisted method [53], while Ali et al. obtained 80% degradation with a co-precipitation method for synthesizing the photocatalyst [56]. In some instances, the photocatalytic process using TiO₂ was enhanced with chemical reagents like H₂O₂. For example, Magdalane et al. increased the degradation percentage from 85% (using only TiO₂-doped cobalt ferrite nanoparticles) to 97% when utilizing TiO₂-doped cobalt ferrite nanoparticles combined with H₂O₂ [36].

The synthesis process significantly influences the characteristics of the materials. Factors such as catalyst concentration, pH, initial concentration of the colorant, and reaction time were primarily considered by different authors (see Table 2). The degradation methods using TiO₂-based photocatalysts and visible light that achieved 100% degradation typically operated in an acidic pH range (1–5), with catalyst concentrations from 0.03–0.06% w/v, dye concentrations from 10 to 100 mg L⁻¹, and reaction times spanning 10 to 140 min [3,33,37,50,52]. Moreover, in TiO₂-doped catalysts, the anatase phase predominated, and smaller particle sizes were associated with increased degradation efficiency. The doping materials were vital in decreasing the band gap, making the photocatalysts suitable for use under visible light (see Supplementary Data, Table S1).

4. Perspectives

As summarized in Table 2, TiO₂-based photocatalysts are often co-doped with other materials to enhance various features such as surface area, band gap, absorptivity, particle size, reusability, affordability, and the ability to degrade different pollutants. The sustainability and development of green processes are becoming increasingly important, leading to the production of more composites from eco-friendly and recyclable sources. In terms of synthesis methods, hydrothermal, solvothermal, and sol–gel techniques are expected to remain the predominant approaches, as they yield photocatalysts with greater degradation capabilities and are simpler to implement compared to other methods. However, while some TiO₂-based photocatalysts can achieve 100% degradation or mineralization of Congo Red, further studies are needed to assess the toxicity of intermediate and final products in cases where complete degradation is not attained. Analytic techniques such as HPLC, MS, TOC, FTIR, LC, and bioassays can be effective in identifying degradation products, as indicated in Table 1.

5. Conclusions

Congo Red is an organic dye known to have detrimental effects on both the environment and human health. Various methods have been proposed for its degradation, with photocatalysis utilizing different materials being a promising approach. Titanium dioxide (TiO₂) stands out as a potential photocatalyst for the degradation of this dye. Despite its many advantages—such as availability, affordability, suitability for large-scale use, high specific surface area, enhanced absorption of ultraviolet (UV) radiation, and the ability for further surface functionalization—TiO₂ has some limitations. Notably, it is inert, non-toxic, and possesses a wide bandgap, which restricts its effectiveness under visible light. However, this limitation can be addressed by employing various dopants and synthesis methods to enhance its performance in visible light. Several factors, including the preparation method, irradiation time, initial concentration of both the photocatalyst and the dye, pH, and temperature can influence the degradation efficiency of TiO₂. Among the synthesis methods, sol–gel is frequently used, though the hydrothermal method often achieves the highest degradation rates, sometimes approaching 100%. Doping TiO₂ with different elements or compounds can provide advantages such as predominance of the anatase phase, reduced particle size (which increases efficiency), and a decrease in the band gap. It is also crucial to conduct monitoring tests for degradation products and bioassays to ensure that the degradation products do not possess equal or greater toxicity than the original compound.