In-Depth Photocatalytic Degradation Mechanism of the Extensively Used Dyes Malachite Green, Methylene Blue, Congo Red, and Rhodamine B via Covalent Organic Framework-Based Photocatalysts

Abstract

Photocatalytic degradation technology has received much attention from researchers in the last few decades, due to its easy and cost-effective nature. A lot of review articles have been published on dyes via photocatalytic degradation, but most of the review articles lack a detailed and in-depth photocatalytic degradation mechanism of dyes. Numerous review articles are available on photocatalysis. Here, in this review article, we are mainly focused on the complete and in-depth photocatalytic degradation mechanism of four commonly used dyes such as Malachite Green, Methylene Blue, Congo Red and Rhodamine B, which will be highly useful for the new researchers that work on dyes’ photocatalytic degradation. Initially, various aspects of dyes have been included in this review article, comprehensively. The main focus was on the covalent organic framework-based photocatalysts for dyes’ photocatalytic degradation, due to their porous nature and various unique properties. Various synthesis routes and the photocatalytic performance of covalent organic frameworks and composite of covalent organic frameworks have been highlighted in this review article. In the last section of this review article, the main stimulus was the four mentioned dyes’ properties, uses, and toxicity, and the photocatalytic degradation mechanism through various paths into environmentally friendly and less-harmful compounds in the presence of photocatalysts. Factors effecting the photocatalytic degradation, economic cost, challenges and future aspects of photocatalytic technology were also included in this review article. This review article will be highly useful for those researchers that work on the photocatalytic degradation of various dyes and search for the complete degradation of complex dye molecules.

1. Introduction

Water is a “universal solvent” and has the ability to dissolve an extensive range of compounds, which is the main reason that it can polluted easily by numerous chemical ingredients such as fertilizers, heavy metals, herbicides, pesticides, insecticides, pharmaceutical wastes, oils, and dyes [1]. According to statistical calculations released by UNESCO in 2023, approximately two billion people, or 26% of the global population, lack access to clean and fresh drinking water. Another report indicates that around four billion individuals worldwide do not have access to clean and sterile water. These figures are expected to worsen due to the massive discharge of pollutants into water sources [2]. Water safety is one of the prime indicators of society’s progress. However, 12 years ago the human right to water and hygiene was explicitly acknowledged by the United Nations General Assembly, when they predicted that by 2025, half of the ecosphere’s populations might be living in water-stress areas and by 2040, one children out of every four on the Earth will be living in a severe water-scarcity area. Therefore, the well-organized recycling of wastewater is becoming an essential focal theme due to the fact that numerous pollutants are evolving and intimidating the water integrity, of which dye wastes have been demonstrated to be one of the main and most common culprits. Other culprits comprise heavy metals, algal toxins, pesticides, microorganisms, etc. [3].

Dyes are highly hazardous in nature, but their complete stoppage is impossible because their implementation is vital in multiple fields and applications. They have more complex structures, with multiple functional groups. The chromophore is mainly responsible for importing color to dye, which varies in different dyes. Dyes have multiple applications and are implemented in numerous field such as pharmaceutical industries, clothes industries, food industries, cosmetics, polishing of leather shoes and purses, making paints, coloring of hair, etc., which leads to the direct production of dyes comprising colored wastewater [4]. According to an estimation, other industrial processes release 2% of water-soluble dyes, while cloth and associated industries discharge 10% of dyes in sewage [5]. The toxic dyes containing wastewater are not acceptable under the particular environmental regulations [6]. Mostly dyes are organic in nature, because they contain mainly carbon in their structure. Dyes are different from pigments in the sense that pigments are bright in color and may be insoluble in the employed medium. Throughout history, dyes have been major objects of commerce worldwide. Naturally-occurring dyes were the foremost source for coloring objects till the 19th century. Plants and lichens were the key origins of color in nature, and mollusks and insects came next [7]. The production of almost all commercial products passes through coloring at some stage, and nowadays over nine thousand (9000) colorant materials with more than fifty thousand (50,000) trade names are used. Before the 1850s, nearly all types of dyes were acquired from natural resources (vegetable plants, trees, lichens, and insects). Firm evidence indicates that coloring approaches are ancient, dating back more than 4000 years, as demonstrated by colored fabrics found in Egyptian tombs. The blue dye indigo is possibly the oldest dye, first obtained from the leaves of the dyer’s woad (Isatis tinctoria) in Europe, and from the indigo plant (Indigofera tinctoria) in Asia [8].

2. Classification of Dyes and Their Hazardous Effect

As one of the key pollutants, dyes have a complex chemical structure. Due to their carcinogenic nature, their treatment requires prompt action before entering into the water bodies, because of their high effect on aquatic life. The color dye deposition could greatly obstruct the sunlight penetrating water bodies, thus interfering with the water ecosystems. Additionally, the water utilization rate would decrease greatly. Such a type of wastewater is also not useful for land irrigation because it greatly affects crops production. Around 70% of the contribution of the dye stuff comes from textile industries [4]. These industries mainly use vat dyes, azo dyes and reactive dyes for the dyeing and printing of the cotton fibers. A total of 21% of the market is contributed to by the usage of disperse dyes, while direct dyes are only responsible for 16% [9].

Dyes are mainly divided into classes (natural dyes and synthetic dyes). The natural dyes resources are animals, plants, insects and minerals without any chemical treatment [10]. The dyes synthesized chemically in labs or factories are called synthetic dyes, and these are exceedingly stable and noxious for living bodies. The synthetic dyes can be classified on the basis of color, structure and methods of application [11]. Dyes can also be categorized, usually based on charges in the aqueous medium, i.e., cationic, anionic and non-ionic dyes. Cationic dyes possess cationic functionality, which is the ability to dissociate into ions in an aqueous medium [12,13]. Examples are Malachite Green (MG), Methylene Blue (MB), and Crystal Violet (CV). Anionic dyes comprise anionic functionality (carboxylic or sulfonic groups), which means they will efficiently interrelate with those materials having hydrophilic surfaces. Examples are Methyl Orange (MO), Congo Red (CR), Sunset Yellow (SY), and Rose Bengal (RB) [12]. The classification on the basis of chemical structure is depicted in Figure 1. Dyes can also be divided into two categories: water-soluble dyes (hydrophilic) and water-insoluble dyes (hydrophobic). The presence of chromophore gives color to dyes, and auxochrome allows their fixation [14]. The Common chromophore groups comprise the azo group (–N=N–), carbonyl group (=C=O), nitroso groups (=C=C=C=, –CH=N–, C=NH, N–OH, NO), nitro groups (NO–OH or NO₂) and the sulfur group (C=S), and auxochromes comprise –COOH, –NH₃, –OH and HSO₃, which contribute to enhancing both the absorption intensity and dye color [15,16]. The electron receivers are chromophores which impart color to the dyes, while electron donors (auxochromes), intensify the color on the substrate by enhancing the power of solubility and adhesion of color to the respective substrate [16]. Most of the dyes are water soluble, imparting a minute amount of color, whereas certain dyes, even at minor concentrations, impart darker colors. Hydrophobic dyes are those dyes which are insoluble in water, for example, disperse dyes, Vat dyes, sulfur dyes, solvent dyes, etc.

Researchers need to use dyes in numerous field, but the use of all these dyes has a hazardous effect on all living organisms on the planet [17,18,19,20]. Generally, since ancient periods, man has mainly relied on different dyes for numerous purposes, like their use in the pharmaceuticals field, textiles industries, paintings, food materials and countless other industries. Statistically, the global production of dyes per year is about 700 million kg, from which 10 to 20% across the world enter the aquatic environment during the industrial processes, particularly from the textile industry [21]. It is also considered that roughly 1.6 million tons of damaging dyes are produced each year, and about 10 to 15% of this capacity is discharged in wastewater form [22].

Unluckily, dyes with lower concentrations (1 mg/L) from industrial waste are considered as leading hazardous contaminants, and their discharge into the predominantly marine environment has caused adverse effects such as mutagenicity, photosynthesis inhibition, the reduction of oxygen content in water bodies, carcinogenicity, and an upsurge in oxygen demand, chemically as well as bio-chemically, etc. [23,24]. These above-mentioned antagonistic effects have compelled the research community to constantly pursue environmentally friendly methods, since time immemorial [25]. The effluents of dye have augmented soluble solids, pH, metals, salts, COD (chemical oxygen demand) and BOD (biological oxygen demand), in various water bodies. The existence of augmented dye quantities poses a risk for biota, as they comprise noxious ingredients like heavy metals and aromatics. Moreover, the existence of the –N=N– bond makes commercial dyes carcinogenic and recalcitrant. Furthermore, dyes’ complex aromatic structure makes them barely biodegradable [16,26]. Untreated dyes’ effluent mainly discharged into the waterbodies and farmland has consequences in the occurrence of a huge amount of harmful dyes in crops. Additionally, these toxic dyes are promptly transmitted to human beings and animals via the food chain, jeopardizing the health of humans, marine creatures, and animals, and leading to the causing of cancer and additional diseases (carcinogenicity, mutagenicity, genotoxicity, endocrine disruption, neurotoxicity, and skin irritation) [27]. According to the literature survey, over 100,000 commercially available dyes are already known, and the annual production of dyes exceeds over 700,000 tons, of which roughly 15% is released into the environment after implementation, and their breakdown is very tough. Approximately 70% of dyes contained in clothing effluents comprise azo chromophores (–N=N–), which are considered to be accountable for changes in color, chemical oxygen demand, salinity, and pH of aquatic systems. Furthermore, only 47% of all dyes are biodegradable in nature, and as a consequence, diverse physical and biological method regarding approaches for remediation were reported during the last two decades [28]. The developed approaches for dye and other organic-pollutant processing are physical methods (adsorption, ion-exchange, chemical precipitation, chemical degradation, flocculation, coagulation, and ultra-filtration), chemical methods [29,30,31,32] (electro-catalytic degradation, the photochemical process, the Fenton process, oxidation, irradiation, ozonation and photocatalytic degradation) [33], and biological methods (enzymatic activity, microbial activity and phytoremediation) [18,20,34,35,36,37,38,39,40,41,42]. Coagulation and sedimentation approaches are frequently implemented; however, these methods lead to the accumulation of pollutants, which are challenging to manage. Moreover, chemical oxidation often produces secondary pollutants, further complicating their environmental impact [2]. Another commonly used disinfection method involves the use of chlorine and other oxidizing agents, but many of their by-products are hazardous and mutagenic [14]. Researchers are increasingly interested in photocatalytic materials, which utilize reactive species generated via UV–visible light, particularly for the degradation of organic pollutants. Photocatalytic degradation is considered a green technique that converts photonic energy into usable chemical energy. While adsorption methods, especially those using natural sorbents, offer a simple and eco-friendly approach for dye removal, photocatalytic degradation is often more effective for completely eradicating organic pollutants before they enter water bodies.

Here, in this review article, the main focus is on four important hydrophilic dyes (Malachite Green, Methylene Blue, Rhodamine B, and Congo Red), which have been used for multiple applications and have a complex chemical structures. These dyes are used as model dyes during photocatalysis by numerous researchers in the laboratory to observe the effectiveness of their designed and developed photocatalysts. Researchers have also focused their attention on the possible degradation mechanism of dye molecules in the presence of a photocatalyst. We will focus on all aspects of the above-mentioned dyes (properties, uses, toxicity and the complete photocatalytic degradation mechanism using advanced photocatalysts, specifically COF-based photocatalysts. We will highlight photocatalysis, photocatalysts (specifically COFs), properties, uses, toxicity and the comprehensive photocatalytic degradation mechanism of dye molecules (Malachite Green (MG), Methylene Blue (MB), Congo Red (CR), and Rhodamine B (RhB)), along with their economic cost and future prospects of photocatalytic technology.

3. Photocatalysis with a General Mechanism

The term Photocatalysis is the combination of two Greek words: “photo” from “phos” means light, and “catalysis” from “katalyo” means decompose or degrade. Photocatalysis is quite an environmentally friendly and useful process for the degradation of pollutants in comparison to chemical degradation methods. It is quite an old technique, and titanium oxide (TiO₂) was first implemented as a photocatalyst during this process. During this process, catalytic oxidation reaction takes place by absorption of light in the presence of photocatalysts (generally semiconductors). The process of photocatalysis includes photochemistry as well as the catalytic material and can be used as a foundation step for the removal of recalcitrant, hazardous, and non-biodegradable organic pollutants from wastewater by improving their biodegradability [17,43,44]. It is already known and documented that semiconductors possess a small band gap between conduction bands and valence bands, and electrons can be easily excited from the valence band to the conduction band, providing visible or ultraviolet radiations. The general mechanism of photocatalysis is shown in Figure 2. Photocatalysis is divided into three types [45].

(a)

Direct degradation method.

(b)

Indirect degradation method.

(c)

Charge injection dye sensitization method.

(a). The direct degradation method, which is quite slow and steady, is also called photolysis, and is fundamentally independent on photocatalytic materials, (b). The second most famous photocatalysis mechanism is the indirect one, in which absorption of energy causes the promotion of the electron to the higher LUMO orbital from the lower HOMO orbital, responsible for the generation of electron pair as well as the positive hole (h⁺). As an outcome, molecular oxygen (O₂) is reduced to superoxide radicals (^⦁O₂⁻) and hydroxyl (OH^⦁) radicals, produced by the reaction between h⁺ and H₂O molecules. Formation of the electron-and-hole pair under the UV–visible light was chiefly accredited to photocatalyst presence. Hence, photocatalysts exhibited marvelous photocatalytic degradation efficiency for the degradation of organic pollutants, and for the method (c). In this route, striking photons absorb equivalent energy or a little higher energy than the respective band gap of the photocatalytic materials, triggering excitation of electron from the valence band to conduction band; as a consequence, the creation of pairs of electrons and h⁺ occurs [46]. The produced h⁺ and electrons predominantly interact with the targeted dye molecule, leading to the generation of excited dye molecules, symbolized by dye*. In the next step, excited dye* (unstable dye) is transformed to free radicals (anionic dyes (dye⁻) or cationic dyes (dye⁺)). In the final step, the produced dye radicals instinctively degrade/reduce because the present free radicals in the system are exceedingly unstable and a prerequisite is stability, which is the fundamental reason for the degradation of dye molecules [47]. The general mechanism of photocatalysis is displayed in Figure 2 [48]. When striking solar energy surpasses the bandgap of a photocatalyst such as TiO₂ (i.e., the energy of a photon equals or surpasses their bandgap energy), the photocatalyst surface becomes excited, and transit of electrons occurs from the valence band (VB) to the conduction band (CB).

Figure 2. General mechanism of photocatalysis. Reprinted from Ref. [48] with permission.

Figure 2. General mechanism of photocatalysis. Reprinted from Ref. [48] with permission.

These whole process is responsible for generating electron–hole pairs (i.e., generating electron (e⁻) and hole (h⁺) pairs). VB holes have the ability for robust oxidation reaction activity, since they have lost electrons and act as reducing agents, and CB electrons have good reducibility when they endure reduction reaction. The produced holes can either generate OH^⦁ or react directly with dyes/organic molecules, which successively oxidizes the dyes/organic molecules [49]. The electrons can also have the ability to react with organic compounds to yield reduction products. The role of oxygen is significant, as it reacts with the photo-generated electrons. Organic compounds can then undergo oxidative degradation via their reactions with OH^⦁ and ^⦁O₂⁻ radicals and VB holes, as well as reductive cleavage via reactions with electrons yielding various byproducts, and finally mineral end-products.

4. Photocatalysts

Photocatalytic materials can effortlessly change solar energy for usage in oxidation and reduction activities. In the last few decades, photocatalytic materials have gained great attention and interest due to their potential for eradicating noxious and hazardous compounds from the surrounding environment [50]. The main purpose of a catalyst is to speed up the chemical reaction. It cannot effect the rate of the chemical reaction, but only save time, because the world is becoming computerized day-by-day. Some of the chemical reactions proceed in the presence of chemicals such as a reducing agent (sodium borohydride, lithium aluminum hydride, ascorbic acid, etc.) along with a catalyst, which is named chemical degradation, while some of the reactions are processed in the presence of sunlight along with catalyst, which is typically called photocatalytic degradation and the catalyst named as photocatalyst. Photocatalysts are of two types. (1) Homogeneous photocatalysts: those photocatalysts that have same nature as that of the solution. For example, liquid- and powder-like photo-catalysts that can dissolve in the solution [43,51]. Such types of photocatalysts cannot regenerate easily, and need centrifugation for recovery. (2) Heterogeneous photocatalysts: those photocatalysts that are different in nature from the solution. For example, 3D-constructed photocatalysts that are solid in nature and those powder photocatalyst that cannot dissolve in the solution. Such types of photocatalysts can implement and regenerate easily after implementation [52]. Numerous photocatalysts have been used for dye degradation such as carbonaceous photocatalysts, metal organic frameworks (MOFs) photocatalysts, layered double hydroxides (LDHs) photocatalysts, hybrid polymeric photocatalysts, metal oxides (MOs), MXenes photocatalysts and covalent organic framework (COF) photocatalysts. To cover all the aspects of all the above-mentioned photocatalysts is quite difficult in one review article. Therefore, in this review article, our main focus is only on advanced COF-based photocatalysts for dye photodegradation, which we will discuss below, in detail. COF is an emerging porous material with a controllable crystal structure. Two-dimensional COF-based photocatalysts exhibit tremendous photocatalytic capabilities due to their extended sp² hybridized orbital construction, which facilitates efficient charge-carrier transport [53]. Importantly, the presence of both electron donors and acceptors within the same COF structure is crucial for stabilizing photoexcited electrons and holes, which are readily available for redox reactions [54]. The band gap energy is an important parameter of any designed photocatalyst. The capability of the developed photocatalyst to generate e⁻ and h⁺ pairs could be preferably evaluated by the bandgap energy of the respective photocatalyst. Hence, bandgap study is indispensable for probing the photocatalyst response towards degradation. The bandgap energy depends upon the material’s shape, size, and dimensions. The synthesized photocatalysts’ optical bandgap was resolute via the Tauc plot method, as shown in Equation (1) [55].

$$C\left(hv-E_g\right)=({ahv)}^n$$

(1)

In Equation (1), C stands for the constant, v stands for light energy, h stands for Planck’s constant, E_g stands for bandgap energy, α stands for absorption coefficient, and n represents the continuous (2 or 1/2), which demonstrates the direct or indirect electronic transitions. The E_g values of the fabricated photocatalyst can assessed by drawing a graph of (ahv)² vs. hv.

4.1. Mechanism of Dye Degradation over COF Photocatalysts

In the initial stage, when a COF absorbs photons of light, it triggers the generation of electron–hole pairs. Subsequently, organic pollutants, such as dyes, adhere to the surface of the COF. The excited electron transfers energy to the dye molecule, initiating oxidation and reduction reactions. This process leads to the formation of reactive species such as hydroxyl radicals (OH^⦁), superoxide radicals (^⦁O₂⁻) and hole (h⁺), as the excited electron reacts with dissolved species. These radicals then attack the dye molecules, facilitating their transformation into less harmful products, as explained below in detail, in the photocatalytic degradation mechanism schemes of dyes. Wu et al. reported a porphyrin-based COF photocatalyst for the degradation of Methylene Blue and Rhodamine B, as shown in Figure 3 [56]. During photocatalytic degradation, OH^⦁ radicals, (^⦁O₂⁻), and (h⁺) are produced respectively, and are responsible for the photocatalytic degradation of dye molecules. The mentioned mechanism is the typical mechanism for all kind of photocatalytic degradation, but it is still debatable and under observation as to which radical (OH^⦁/(^⦁O₂⁻) is more responsible for degradation of dye molecules. This observation will explain this below in more detail, via different scavenging experiments. The comprehensive and detailed photocatalytic degradation mechanisms of all four dyes are shown in Section 4 in Scheme 1, Scheme 2, Scheme 3 and Scheme 4. But first, we will focus on the COF properties and synthetic route here.

4.2. COF and Its Synthesis Routes

COFs are also called conjugated micro/mesoporous polymers (CMPs). Normal COFs are more stable than metal organic frameworks (MOFs). COFs are highly porous organic materials with exceedingly periodic and well-organized structural network. COFs were first reported by Yaghi et al. in 2005 [57]. COFs are newly emerging highly porous materials with a crystalline nature, large surface area, low density, low thermal conductivity and tunable bandgaps. More significantly, the COF backbone structure is designable at the molecular level [58]. Since then, a lot of attention has been diverted towards COF materials in the science community, due to their distinctive structural features and properties for various applications. To acquire COFs with the requisite crystalline structures and targeted functionality, numerous covalent-bonding approaches were employed to direct 2D/3D alignment and determine the associated topology structure [59]. The building blocks of COF materials are based on light elements such as H, C, N, O, and B, which are interconnected via covalent bonding to form extended 2D/3D architectures [60]. The strategies of covalent-bond construction could be classified into the succeeding categories, such as boronate ester-linked, hydrazone-linked, boroxine-linked, triazine-linked, azine-linked, carbon–carbon linked, imine-linked, b-ketoenamine-linked, imide-linked, and so on [61,62]. COFs are usually produced from building blocks by the reversible condensation method, leading to better crystalline structures with high chemical and thermal stabilities. COFs’ improved chemical stability has been accredited to their metal-free configurations and pure covalently bonded nature [63]. Till the present date, numerous synthetic routes have been reported for the synthesis of COFs, including the ionothermal method, the solvothermal method, the sonochemical method, the microwave-assisted method, the interfacial method, and the mechanochemical method, etc., which will be discussed below in detail. Based on the structural characteristics of COFs, organic units and proper linker selection during fabrication mainly regulate the skeleton, pore size and symmetry of resulting 3D frameworks [64].

Covalent organic frameworks (COFs) represent an intriguing category of porous organic polymeric materials meticulously crafted from organic building blocks through covalent bonding [65,66,67,68]. Their appeal lies in a crystalline structure that can be customized during synthesis to showcase various advantageous features, enabling their application across a broad spectrum of uses [69]. COFs are defined by low skeleton density, expansive specific-surface area, high porosity, commendable stability, and remarkable selectivity. These inherent qualities make COFs exceptionally well-suited for sample pre-treatment in critical fields such as food safety detection and environmental pollutant analysis [70]. Their distinctive structure facilitates the extraction and concentration of diverse pollutants from various sample matrices, highlighting their versatility and adaptability [71]. COFs play a pivotal role in composite materials when combined with other functional counterparts, resulting in hybrids exhibiting superior performance characteristics [72]. The synergistic effects of such combinations broaden COFs’ utility across multiple domains, amplifying their efficacy in various applications.

COFs are extraordinary materials distinguished by their adaptable crystalline structures and versatile functionalities. Their impressive properties render them invaluable across various applications, offering innovative solutions in critical areas such as environmental analysis, energy storage, healthcare, and materials science. The ongoing investigation and incorporation of COFs into diverse functional systems persistently reveal their potential for transformative advancements in various technological domains. Covalent organic frameworks (COFs) constitute a category of crystalline porous polymers that enable the atomically precise integration of organic units to form predesigned skeletons and nanopores [73]. They have emerged as a novel molecular platform for the design of organic materials with applications in gas storage, catalysis, and optoelectronics [74,75]. The design and synthesis of COFs involve reversible dynamic covalent reactions, various building blocks, and geometry retention [66]. The initial members of COFs, COF-1, and COF-5, have been successfully synthesized, showcasing high thermal stability, permanent porosity, and substantial surface areas. COFs provide confined molecular spaces that enable the interaction of various particles and molecules, resulting in distinctive properties and functionalities. Recent advancements in COFs encompass the development of novel design principles, synthetic strategies, and functional designs. Future research on COFs will concentrate on addressing fundamental issues and exploring new directions in chemistry, physics, and materials science.

The synthesis of the periodic arrangement of organic polymers involves ketoenamine linkages with imines through azine and hydrazone bonds [76]. Due to their crystallinity, porous nature, and numerous attractive properties, COFs have become a focal point for researchers [77,78,79]. The formation of 3D COFs occurs when atomic layers are stacked into overlapping layers through π–π interactions [80]. The precise structure of COFs aids in understanding the link between structure and properties [81]. COFs play a crucial role in forming highly ordered organic structures, making them suitable for catalysis [82], ion conduction [83], and gas storage [84]. In comparison to traditional materials like Silica [85], Metal-Organic Frameworks (MOFs) [86], or zeolites [87,88] COFs exhibit more efficient characteristics. Terephthalaldehyde-based boronic ester COFs behave as semiconductors, and ppy-COFs in self-condensation photocurrent measurements were reported by Jiang et al. in 2008 [89]. The advantages of using COFs in photocatalysis include (i) a high surface area and high porosity, providing more active sites; (ii) the presence of covalent linkages making them more stable; (iii) strong π–π bonds in layers for the transport of charge carriers; and (iv) tunable morphology and responsive characteristics in visible light.

4.3. Synthesis of COFs

Achieving enhanced porosity and improved crystallinity in the synthesis of COFs poses a challenging task. Balancing crystallization and COF formation requires a suitable combination of solvents, specific temperature, pressure, and time [76]. COFs can be fabricated via different synthetic methods [90,91]. Below is the overview of all synthetic routes for COFs given in

Table 1. Synthetic routes for COFs.

Scheme.	Solvent	Temperature	Time	Examples	Advantages	Drawbacks	Ref.
Room-temperature solution synthesis	Organic solvent	293 K	1–30 min	COF-39	Fast reaction rate, mild reaction conditions	Consumption of solvent	[92,93]
Solvothermal synthesis	Organic solvent	353–473 K	2–9 days	COF-1	Excellent crystallinity	Longer time for reaction and consumption of solvent	[76,94]
Microwave synthesis	Organic solvent	373.16 K	20–60 min	COF-5	Fast reaction rate, high yield	Consumption of polar solvent	[95,96]
Mechanochemical synthesis	No solvent or organic solvent	293 K	30 min–3 days	COF-66	Mild reaction conditions, high yield	Lack of crystallinity	[97]
Ionothermal synthesis	Molten salts or ionic liquids	673.15 K or 293 K	12–40 h	COF-6	No solvent consumption	Intensive reaction conditions	[98,99]

.

4.3.1. Room-Temperature Solution Synthesis

The synthesis of COFs at room temperature in ambient air has garnered significant attention, especially for sensitive polymers, where room temperature plays a crucial role. Zamora et al. successfully synthesized imine-based COFs at room temperature. For instance, RT-COF-1 was synthesized in m-cresol at room temperature by reacting 1,3,5-benzenetricarbaldehyde (BTCA) and 1,3,5-tris(4-aminiphenyl) benzene (TAPB) for 1 min in the air. This COF exhibits porosity for both N₂ and CO₂ and demonstrates stability at 723 K. Yan et al. reported the fabrication of spherical COFs through the reaction between DABP or 1,3,5-triformylphloroglucinol (Tp) for 30 min at room temperature in the presence of ethanol [93]. COFs can be synthesized at room temperature in a solution without requiring specialized equipment. This method is simple and convenient, enabling the synthesis of COFs with various functional groups [100].

4.3.2. Solvothermal Synthesis

The solvothermal method is commonly employed to synthesize COFs, utilizing COF precursors and specific solvents in a sealed Pyrex tube. In this process, the mixture is initially heated at a high temperature for several hours and then cooled at room temperature. The resulting COFs are obtained through filtration and purification using the Soxhlet extraction method [101]. Crucial factors in this method include time and temperature, typically ranging from 353 to 473 K, with a required reaction time of 2–9 days [102]. In 2005, Yaghi et al. utilized the solvothermal process to produce COF-1 and COF-2. In this process, 1,4-benzenediboronic acid (BDBA) dissolved in a mesitylene solution at 77 K is flash-frozen in a Pyrex tube. COF-1 is synthesized with the tube’s internal pressure maintained at 150 torr, and evacuated. COF-1 and COF-5 exhibit high thermal stability, even up to 873 K [76]. The success of COF synthesis through the solvothermal method hinges on selecting a suitable solvent, a crucial factor influencing nucleation and growth [73]. This method has been employed to generate thin films, as demonstrated by Dichtel et al., who grew a COF film on a single-layer graphene substrate. In this process, a cylindrical pressure vessel was used to mix HHTP, BDBA, 1,4-dioxane, and mesitylene, followed by 30 min of sonication. Substrates containing graphene were added, and the mixture was heated for 30 min at 363 K [103]. Despite the time requirement and the need for specific equipment, the solvothermal synthesis method is the most versatile and reliable approach for COFs. This method can achieve high temperatures and pressures, facilitating faster reaction rates and increased product formation [104]. Additionally, solvothermal methods offer precise control over various reaction parameters such as pressure, temperature, and solvent conditions. Materials synthesized through solvothermal methods often possess unique characteristics not replicated by other methods and are amenable to recycling [105].

4.3.3. Microwave-Assisted Method

Microwave-assisted synthesis has emerged as a promising strategy for crafting COFs [106,107,108]. This method provides notable advantages, particularly in expediting processes and ensuring improved reproducibility [109]. Various types of COFs, encompassing imine, triazine, and diverse 2D and 3D structures, have been successfully synthesized using microwave-assisted techniques [110]. The application of microwaves as an energy source has proven effective in accelerating reaction rates, enabling swift and scalable COF synthesis. Moreover, microwave-assisted methods have been pivotal in creating COFs, employing various linkers and synthesis strategies, including nanoparticles, thin films, and powder synthesis routes.

For the synthesis of COFs, microwave heat is applied during the reaction, offering several advantages, such as obtaining a clean product with higher yield, energy efficiency, and easy controllability. Polar solvents are commonly preferred in microwave-assisted methods, as they absorb microwaves, providing uniform heat conducive to the synthesis process [109]. Polar solvents enhance contact between reactants, increasing product formation [111]. Additionally, polar solvents promote condensation reactions, especially when the intermediate form is polar [112]. Cooper discovered COF-5 in just 20 min, which was 210 times faster than the solvothermal process. COF-5 comprises repeating units of boronic acid with boronate ester linkages. It is a crystalline two-dimensional (2D) material with a large surface area, high-temperature stability, and tunable chemical properties [113]. The surface area of the solvothermal process is 1590 m²/g, which is less than that of the microwave-assisted method [95]. Enamine-linked COFs are synthesized by reacting 1,3,5-triformylphloroglucinol with phenylenediamine in a mixture of 3 M acetic acid, 1,4-dioxane, and mesitylene for 60 min with stirring at 373 K. Microwave synthesis yields an 83% yield, while the conventional method produces an 8% yield, indicating its efficacy. TpPa-COF is used for CO₂ storage and synthesized using a microwave [96]. Microwave synthesis proves to be a quick and effective technique for COF synthesis. Shorter reaction times are often achievable compared to traditional heating methods. However, microwave-assisted approaches in COF synthesis are somewhat limited, but their potential for future research and industrial-scale production appears promising, necessitating further exploration and development in this field.

4.3.4. Mechanochemical Synthesis

Mechanochemical synthesis has emerged as a viable technique for creating COFs [56]. This innovative method utilizes mechanical force to initiate chemical reactions, facilitating the formation of COFs. In a noteworthy study, cellulose nanocrystals were ingeniously employed for COF composite preparation using a mechanochemical approach [114]. Another significant investigation demonstrated a three-component synthesis methodology capable of simultaneously producing extended aromatics and COFs, allowing the seamless fabrication of building blocks and COFs together [115,116]. This groundbreaking approach resulted in COFs with complete precursor conversion and an impressive surface area [114].

In this form of COF synthesis, the reaction occurs at room temperature, and a mortar and pestle are employed. Banerjee’s group synthesized three isoreticular COFs in 2013, using this method. Various materials like 1,3,5-triformylphloroglucinol (Tp) and Pa are utilized in the process, and grinding them, mostly at room temperature, results in different colors [117]. Additionally, a small amount of solvent can be introduced to enhance the crystalline structure of COF. Zhao et al. synthesized two sulfonated COFs with high yields [97]. Banerjee et al. reported the synthesis of COFs through the Schiff base reaction involving 2,2-bipyridine-5,5-diamine (Bpy) and 1,3,5-triformylphloroglucinol (Tp) [118]. For certain COFs, mechanochemical synthesis is a valuable solvent-free and environmentally friendly process. In its entirety, mechanochemical synthesis presents a promising avenue for COF fabrication, providing a scalable and efficient method for their production. The method’s versatility and capacity to streamline COF synthesis processes underscore its potential as a viable and resourceful approach for future advancements in COF material development.

4.3.5. Ionothermal Synthesis

Ionothermal conditions, often employing ionic liquids and molten salts as solvents, are utilized for COF synthesis. Metal salts are commonly preferred as solvents to synthesize highly thermally stable COFs, especially at high temperatures. When molten salt is employed, it leads to the synthesis of covalent triazine frameworks (CTFs). For instance, a mixture of ZnCl₂ or 1,4-dicyanobenzene heated at 673 K for 40 h forms stable and porous CTFs, where ZnCl₂ acts both as a catalyst and a solvent [98]. Another study by the Thomas group involves the transformation of a non-porous and amorphous polymer into microporous CTFs through an ionothermal process in two steps. The trimerization of the DCB monomer in the presence of Bronsted acid produces an amorphous polymer, which is further converted into crystalline CTFs by heating for 40 h at 673 K with ZnCl₂ [99]. While ionothermal synthesis can prepare specific COFs under specialized conditions, its applicability may not be as widespread as other methods.

5. Detailed Mechanism of Dye Degradation over COF Photocatalysts

Ensuring human health requires the removal of organic pollutants from water. Various methods, including biological processes, advanced oxidation processes (AOPs), and adsorption, are employed for this purpose. Within AOPs, heterogeneous photocatalysis breaks organic pollutants into less harmful byproducts (water and carbon dioxide). COFs have emerged as promising photocatalysts for this task, showcasing significant potential. Their inherent high stability, modifiable porosity, and versatile functionalities make them well-suited for various catalytic and sorption applications. COFs play dual roles as adsorbents and catalysts, demonstrating effectiveness in sorbing and reducing diverse pollutants, including organic chemicals, heavy metal ions, and radionuclides [119,120]. COFs engage with pollutants through intricate mechanisms, including electrostatic attraction, surface complexation, and reduction reactions, highlighting their versatile capabilities in pollutant removal [121]. Furthermore, COFs demonstrate proficiency as photocatalysts, utilizing irradiation-induced reduction reactions to degrade environmental pollutants such as dyes, carbon dioxide and other pollutants [122,123]. The optimization of COFs with precise structures and active sites is crucial for enhancing their photocatalytic performance. Recently, COF-based materials featuring functional groups with inherent photosensitivity have garnered significant attention within the realm of photocatalytic degradation [124]. Their porous morphology, broad light-absorbing capabilities, and π-π stacking interactions among adjacent COF layers are pivotal to their remarkable photocatalytic performance [125]. Moreover, COF-based materials serve as excellent alternatives to organic semiconductors, due to their fully aromatic conjugated structure, which offers enhanced pathways for electron (e-) and hole (h+) transport [126]. This structural attribute is immensely consequential for advancing photocatalytic processes and effectively facilitating photocatalytic reduction reactions [127]. However, challenges remain in effectively implementing COFs for catalytic purposes, especially when transitioning from laboratory successes to real-world applications. This presents a crucial area for future research and development. COF proves to be a more efficient method for removing organic pollutants, including various dyes [128,129,130,131,132,133,134,135]. The combination of electron-deficient monomers results in a photoactive COF with a triazine unit, serving as a photoactive component capable of degrading organic pollutants [130]. This process generates reactive hydroxyl and oxygen species, which subsequently oxidize the organic pollutants. In the degradation of Rhodamine B in water, a copper porphyrin and g-C₃N₄-based heterogeneous photocatalyst COF proves effective [129]. The ultra-small pores of COF prevent the aggregation of nanoparticles. By introducing thiol groups, Lu and colleagues prepared COF pores through photosynthetic modification [130]. Hong-Yi Yu and colleagues researched the degradation of methyl orange using a xenon lamp. Before irradiation, the reaction was kept in the dark for 60 min to facilitate the adsorption and desorption processes [136]. Wu et al. presented COFs with dual functionality prepared through condensation, serving as a removal agent and as a photocatalyst for Methylene Blue (MB) dye [56]). The developed COFs exhibited a narrow band gap of 1.02 eV, demonstrating promising efficiency (99%, 180 min) as a photocatalyst for MB degradation. Moreover, they maintained stability over four cycles without a decline in photocatalytic performance. The improved performance of the material can be attributed to its porous nature, facilitating easy diffusion of pollutants and enhancing degradation performance. The detail photocatalytic degradation mechanism of Malachite Green, Methylene Blue, Congo Red and Rhodamine B is shown in Scheme 1, Scheme 2, Scheme 3 and Scheme 4, with a possible degradation route to form friendly and harmless products. Yao et al. reported a titanium dioxide/covalent organic framework (TiO₂/COF) photocatalyst for the photocatalytic degradation of Malachite Green (MG). The result shown that the fabricated photocatalyst has a better photocatalytic degradation efficiency of 93.64 % in 120 min. The degradation mechanism was studied in the presence of scavengers (IPA, BQ, AgNO₃, and EDTA-2Na) to evaluate which radicals are responsible for MG degradation. The result for the experiments confirm that ^•O₂⁻ and h⁺ are the radicals mainly responsible for MG degradation [137]. Xue et al. reported the photocatalytic degradation of azo dye (MB) using an Imine-linked covalent organic framework with 100% degradation efficiency in 60 min. During this process, four active species were generated including the hydroxyl radical (OH^⦁), the superoxide radical (^⦁O₂⁻), the singlet oxygen (¹O₂) and the hole (h⁺). 5,5-dimethyl-1-pyrroline N-oxide (DMPO) in the COF network work to spin-trap capture ^⦁O₂⁻, h⁺ and OH^⦁, while 2,2,6,6-tetramethylpiperidinooxy (TEMPO) in the COF network was cast-off to capture ¹O₂ [138]. The ^⦁O₂⁻ radical is the key contributor to the photocatalytic degradation of dye molecules under visible light. Furthermore, the OH^⦁ radicals are outcomes from the produced anion radical (^⦁O₂⁻) during this whole process, which triggers the degradation of dye molecules. Xiao et al. reported Fe-porphyrin-based COFs with exceedingly effective and selective photocatalytic degradation of Congo Red (CR). The Fe-porphyrin-based photocatalyst shown better efficiency for the degradation of Congo Red, with 88.3% in 6 min [139]. During CR degradation, the possible active species were elevated via the scavenging experiment and it was confirmed that the ^•O₂⁻ radical, among other active species such as ^•OH, h⁺, is mainly responsible for CR photocatalytic degradation. Khaing et al. reported a COF@MoS₂ photocatalyst for the photocatalytic degradation of Rhodamine B (RhB) with a better efficiency, of 98%. For this experiment, he used the trapping agent to confirm which radical is mainly responsible for the RhB degradation mechanism. To investigate the role of the reactive species (^•OH, h⁺, and ^•O₂⁻), isopropylalcohol (IPA), triethanolamine (TEOA), and ρ-benzoquinone (BQ) were used as the scavenging agent. In all three scavengers, it was demonstrated that ^•OH, h⁺, and ^•O₂⁻ are all highly responsible for the photocatalytic degradation process. It has been confirmed that the intimate interfacial interaction achieved within the 2D–2D layered structure significantly increases the contact area and markedly enhances charge migration and transport to the interface [138]. The inclusion of cyano groups (–C≡N) as active sites in COF-based photocatalysts [21] endows them with commendable electron-absorption capacity, thereby facilitating charge transfer and separation, ultimately enhancing photocatalytic performance and augmenting light-absorption capacity. Hence, these cyano groups play a crucial role in catalyzing photocatalytic reactions [140]. For instance, Wang et al. introduced a cyano-conjugation approach to activate the carbonyl oxygen sites within β-ketoenamine-linked COFs, resulting in enhanced photocatalytic hydrogen generation rates. This innovative method efficiently boosts photoinduced charge separation and lowers the energy barrier for H₂ production in functionalized COFs, thereby significantly improving the performance of the developed materials [141].

The conjugated π-electrons in porphyrins possess unique photophysical properties, making them excellent candidates for implementation as photocatalysts. When integrated into COFs, porphyrin units exhibit distinctive photocatalytic activity. The two-dimensional, well-ordered structure of COFs enhances the π-conjugated system of porphyrins, thereby significantly improving their light-absorption capability [142]. Moreover, the incorporation of porphyrin units into COF-based materials enhances their dispersibility, preventing self-aggregation in aqueous media and ensuring efficient utilization in photocatalytic processes. [143]. Incorporating donor–acceptor moieties into COF-based materials has been validated as an effective strategy for promoting charge separation. In this context, facilitating charge transfer between bridging units and porphyrins efficiently mitigates electron–hole recombination, thereby enhancing photocatalytic activity [53,54,55,56,57,58]. As a result, porphyrin-based COFs emerge as promising photocatalysts, capable of effectively degrading organic dyes and pollutants [138].

The photocatalytic degradation concept of dyes is still debatable, and it is under investigation which radical is more highly responsible for dye molecule degradation. Most researchers claim that the OH^⦁ radical is commonly responsible for dye degradation, although one cannot omit the responsibility of the ^•O₂⁻ radical during this process, because it is also responsible for the generation of OH^⦁ radicals.

Table 2. COFs as a photocatalytic material for dyes.

COF Name	Dye Name	Efficiency (%)	Ref.
TiO2/COF	MG	93.64	[137]
Imine-linked covalent organic framework	MB	100	[138]
Porphyrin-based COFs	MB	99	[56]
Organic framework with carbon nanotubes	Mordant Black 17	71	[144]
UPC-CMP-1	Congo Red	88.3	[139]
MoS2/COF	RhB	98	[145]
COP-NT	MO	67	[146]
COP-NT	RhB	78	[146]
CTF-A	MB	100	[132]
TpMA(1 mL)	Phenol	83.5	[147]
TpMA(3 mL)	Phenol	100	[147]
DCN@COF/PMS	Orange 2	93	[148]
UCN@COF/P	Orange 2	100	[148]
MCN@COF/PMSMS	Orange 2	64.1	[148]
COF of 2DZnPc@ZnO	Methylene Violet Eosin Y	98 92	[149]
Metal-free TpTt-COF	RhB	83.2	[150]
C6-TRZ-TPA COF	Rose Bengal	99	[151]
NH2-MIL-68@TPA-COF	RhB	-	[134]
Fe3O4@MOFUiO-66@Tz-Dz-COF	MG/CR	99/97	[152]
CuPor-Ph-COF@g-C3N4	RhB	86	[129]
TTO-COF	MO/MB	99/99	[153]

provides an overview of various reported COFs for the degradation of different dyes. The literature survey above serves to confirm and validate the fact that both radicals play pivotal roles in pollutant degradation. Their interlinking during the photocatalytic process underscores their high degree of responsibility for pollutant degradation.

The demonstrative equations for the photocatalytic degradation of dye molecules are summarized below, in Equations (2)–(5):

$$COF+hv\to COF[e^{+}+h^{+}]$$

(2)

$$O_2+e^{-}\to {}^{⦁}{͘O}_2^{-}$$

(3)

$$H_2{O}_2+e^{-}\to {}^{⦁}OH+{OH}^{-}$$

(4)

$${}^{⦁}{O}_2^{-}+h^{+}{}^{⦁}OH+Dye\to DegradationProducts\left(C{O}_2+H_{2}O+N{O}_3^{-}+S{O}_4^{2-}\right)$$

(5)

Photocatalytic degradation of any dye, in percent (%), can be calculated by Equations (6) and (7).

$$PhotocatalyticDegradationRate\left(\mathrm{\%}\right)=\frac{(C_0-C)}{C_0}\times 100$$

(6)

$$PhotocatalyticDegradationRate\left(\%\right)=\frac{(A_0-A)}{A_0}\times 100$$

(7)

In the above two equations, C₀ stands for the initial dye concentration, C stands for the concentration of dye after reaction, A₀ stands for the initial absorbance of dye, and A stands for the dye absorbance after the reaction.

6. Detailed Description and Photocatalytic Degradation of MG, MB, CR and RhB

Here, in this section, we highlight all the properties, uses, toxicity and complete photocatalytic degradation mechanism of each dye, one-by-one. After complete degradation, the color of each respective dye changes to a colorless product, as depicted in Figure 4.

6.1. Malachite Green

Properties: Malachite Green (MG) is highly water-soluble, cationic, and is commonly named N-methylated diaminotriphenylmethane; it is primarily used in aquaculture as a therapeutic agent [154]. In solution form, MG exists as a mixture of the cation (chromatic MG) and its carbinol base with a different ratio depending on the solution pH. The MG can also undergo metabolic and chemical reduction to a leuco derivative. MG contains a chromophore of triphenylmethane of a vivid, bluish green, and is inspired by the malachite gemstone with its banded striations of green. MG usage was forbidden internationally by the Food and Drug Administration (FDA) in 2002, though it is still used in various situations, due to its low price and the deficiency of appropriate substitutes.

Uses: Its extensive uses include the textile industries, acrylic industries, aquaculture, food, and medical disinfectants. MG is broadly used as a parasiticide in aquaculture, as a food coloring agent, for dyeing wool and silk, for biological staining, in the textile industries, and in other industries for multiple purposes. It also controls fungal attack, protozoan infection and some additional diseases triggered by helminths in a varied diversity of fish as well as other aquatic organisms [155], although the major practice of MG in fish culture has been to stop the overgrowth in incubating fish eggs of oomycete fungi. There is also an extensive use of MG for the treatment of adult fish. MG therapy can be used to control proliferative kidney disease in rainbow trout. This parasite disease is mainly responsible for effecting trout farming in various countries. During treatment of the above-mentioned fish diseases, MG is normally added to an aqueous solution in ponds and tanks, generally intermittently. Consequently, MG is released straightaway into the aquatic atmosphere, which also affects the life of all aquatic organisms [156]. MG is also implemented as a biological stain for probing tissue samples and cell biology. MG is used as a blue-green counterstain during the Gimenez staining process.

Toxicity: Its long-term presence causes chromosomal fractures, teratogenicity, carcinogenesis, mutagenesis, and respiratory toxicity in addition to other numerous side effects. Multi-organ tissue damage is one of the most histopathological effects of MG. In mammalian cells, it displays noticeable cytotoxicity and has the capability to cause the transformation of the cell, as well as lipid peroxidation. It also acts as a tumor promoter, maybe because of its capability to cause the formation of reactive oxygen species [157]. MG-treated fishes have always faced serious changes in biochemical parameters in the blood. Muscles, kidney, liver, serum, tissues, eggs and fry have also contained MG, as well as leucomalachite green. Organ damage, carcinogenic, mutagenic, and developmental aberrations are all indications of toxicity in some mammals, due to MG.

Photocatalytic degradation mechanism of Malachite Green: Primarily, Malachite Green dissolves in water and becomes ionized. After ionization, in the presence of light and a photocatalyst, hydroxyl radicals (OH^⦁) and superoxide radicals (^⦁O₂⁻) are generated, in which (OH^⦁) radicals alternatively attack the dye’s molecules to degrade it completely. Furthermore, the OH^⦁ radicals are outcomes of the produced anion radical (^⦁O₂⁻) during this whole process, which triggers the degradation of dye molecules. The process looks very simple through visualization, as the color changes after photocatalytic degradation during the experiment. However, this process is very complex nature-wise, as is shown in Scheme 1. Initially, MG dye molecules were attacked by OH^⦁ and transformed to (4-(dimethylamino) cyclohexa-2,5-dien-1-yl) (4-(dimethylamino) phenyl) (phenyl) methanol. After that, the process was probably completed in two paths such as Path A and Path B. In both probable processes, a lot of intermediate products were generated, which at last were converted into simple products of CO₂ and H₂O, along with sulphate and nitrate ions [158]. In the initial step, MG was converted to the compound “leucomalachite green” with an m/z value of 330. In the next steps, other products formed, including 4,4-Bis(dimethylamino) benzophenone (m/z: 268), 4-aminobenzophenone (m/z: 225), p-nitrobenzoic acid (m/z: 227), 4-hydroxybenzoic acid (m/z: 198), 4-(dimethylamino) phenol (m/z: 165), p-nitrophenol (m/z: 138), p-benzosemiquinone (m/z: 110), and oxalic acid (m/z: 90). In the last step, CO₂ and H₂O, along with sulphate and nitrate ions, formed, and the MG dye completely degraded in a colorless solution [158]. The photocatalytic degradation efficiency depends on the nature of the photocatalysts. Some have better efficiency in comparison to others, and COF is one of the advanced photocatalysts that have shown better efficiency due to their highly porous nature and unique properties.

6.2. Methylene Blue

Properties: Methylene Blue (MB) is a fluorescent in nature. MB was discovered by the German chemist Heinrich Caro in 1876 and has since been used in numerous scientific fields [159]. MB is highly water-soluble (40 g/L at 20 °C) with a melting point of 190 °C, odorless, green in color in the oxidized state, and colorless in the reduced form, with specific gravity and density of 0.98 and 1.0 g/mL at 20 °C, respectively. The color is mainly imported by chromophore (N–S group) and auxochrome (N-containing groups with lone-pair electrons on the benzene ring). MB is powdery in nature at room temperature.

Uses: MB is used as a medication during management and treatment of methemoglobinemia (hemoglobin decreases its capability to carry enough oxygen). So, due to this activity, MB is used as a valuable agent in the treatment of methemoglobinemia, Plasmodium falciparum, vasoplegic syndrome, and ifosfamide-induced encephalopathy, as well as for diagnostic purposes such as sentinel lymph-node mapping during breast procedures and parathyroid gland mapping during parathyroidectomies. Lately, it has been used for the detection of enterovesical, intestinal, and bronchopleural fistulas. It can also be used as an antidote to cyanide poisoning [160]. MB is also identified as a thiazine dye that can be used as a medication, and can be administered intravenously and orally. It should be administered intravenously gradually, and usually over 3–10 min [161]. MB, together with a special near-infrared (NIR) system used for the detection of fluorescence, permits the imagining of previously unseen assemblies during surgery. It is also implemented as a potential candidate in dye-sensitized solar-cell sensors, capacitors, microbial fuel cells, etc.

Toxicity: Methylene Blue is carcinogenic in nature, causing skin allergies, and is also responsible for respiratory toxicity. It also causes diarrhea, nausea, vomiting, cyanosis, shock, jaundice, gastritis, tissue necrosis, and increased heart rate, and leads to the death of premature cells in tissues; it causes itching and eye/skin irritations. One of the most important limitations while implementing MB to see the ureters is the potential damage of renal function, due its excretion by the kidneys [162]. Another drawback is the constraint of MB use only for those patients who are proficient in adapting MB into the non-fluorescing leucomethylene blue, which is triggered by the reduction in, or the acidity of, the surrounding environment. MB in a high dose (5 mg/kg) always causes allergic reactions. Therefore, MB should be used in the smallest efficacious dose [163].

Photocatalytic degradation mechanism of Methylene Blue: The intermediate products of Methylene Blue (MB) photocatalytic degradation were recognized by liquid chromatography–mass spectrometry, as displayed in Scheme 2. The mass spectra attained at various time intervals show the foundation of various products which are resultant from the hydroxylation process. The proposed mechanism confirmed that MB degrades in two ways. In both degradation pathways, MB leads to the formation of less harmless compounds such as CO₂ and to the conversion of S and N heteroatoms into inorganic ions, such as ammonium, nitrate, and sulfate ions, respectively. In the initial phase, the complex structure was converted into a double-ring structure, then to a simple single-ring structure, and in the last stage it was converted into simple harmless compounds [164]. This process is completed in different times, depending on the nature of the catalyst band gap. Some catalysts have the ability to complete the photodegradation in a very quick time, which is the demand of the present era. Various researchers are working to design better photocatalysts for the swift photodegradation of dyes, and COF is one of them that have been considered to be better for this application, due to its porous nature and low band gap [138].

6.3. Congo Red

Properties: Congo Red (CR) dye was first discovered by Paul Bottinger in 1884. It is an anionic dye with two azo groups (–N=N–). The azo (–N=N–) group belongs to the chromophores and the sulfonic (–SO₃H) group belongs to the acidic auxochromes. CR is also named as an acidic diazo dye [165]. CR belongs to the direct dyes, which are soluble in water with the presence of an azo group [20]. CR is water-soluble, resulting in a red colloidal solution. Its solubility is better in organic solvents. CR is an indicator dye, and changes its color with a pH change (blue-violet at pH 3.0 and red at pH 5.0). Typically, CR has a most stable aromatic structure, is non-biodegradable, and is harmful to human health and aquatic animals.

Uses: CR is mainly used in the textile industry for coloring cellulosic fibers. It is extensively use in the rubber, plastic, textile, paper, printing and dyeing industries. It is also widely used as a histological dye to reveal the manifestation of deposits of amyloid in tissue, as well as for coloring the cell walls of fungi, plant and Gram-negative bacteria [166]. It is used in testing for hydrochloric acid in gastric contents. CR is used in optoelectronic materials. Kocyigit et al. reported the light-detection presentation of CR in a Schottky-type photodiode [167].

Toxicity: CR has multiple applications, but CR is still carcinogenic in nature, and associated with allergenic effects, asthma, dermatitis, and skin irritation. It is also responsible for decrease in light penetration, and also modifies the photosynthetic process. It also causes phytotoxicity and unaesthetic surface water, and increases COD, infertility, and mutagenicity [165]. As reported previously, CR has been identified as causing an inhibitory influence on the activities of alanine aminotransferase and aspartate aminotransferase, which are mostly found in the liver. During tissue suffering like cell growth and cellular necrosis, both of the enzymes are spread intracellularly and there is seepage into the main bloodstream. Occasionally, when tissue injury happens, the activities of the enzymes are raised and highly effected [168]. The biological poisonousness of CR was seen in numerous pathogenic species such as Vibrio fischeri, V. fischeri, Selenastrum capricornutum, S. capricornutum, Tetrahymena pyriformis, T. pyriformis, Daphnia magna, D. magna, Ceriodaphnia rigaudi, C. rigaudi, Danio rerio. Pseudokirchneriella subcapitata, and D. rerio. P. subcapitata [169]. The severity of CR is also noticeable in marine plant life. CR, on mixing with water, decreases the diffusion of light, which changes the process of photosynthesis, and, consequently, the ecosystem is adversely affected. CR is non-mutagenic in nature, but its metabolite benzidine is a well-known mutagen and already associated with human urinary-bladder cancer [165].

Photocatalytic degradation mechanism of Congo Red: The proposed photocatalytic degradation mechanism for Congo Red is shown in Scheme 3. Numerous possibilities for the degradation of CR have been identified, including the following: (a) direct breakage of the benzene ring, (b) breakage of the –C–S– linkage present between the sulfonate groups and aromatic ring, and (c) breakage of both chromophores (–C–N– and –C–C–) and, lastly, through the breakage of the azo bonds (–N=N–) existing in the CR molecular structure. The CR photocatalytic degradation is also completed in two pathways, and at the end of both pathways, the generated intermediates are exposed to oxidative breakdown, comprising the aromatic-ring opening, which produces aliphatic carboxylic acids such as maleic acid, oxamic acid, formic acid, oxalic acid, and acetic acid, which ultimately produce CO₂ and H₂O products, leading to the complete mineralization of CR [170]. A comprehensive mechanism for CR degradation was proposed by Moeinzadeh et al. [170], as shown in Scheme 3. Initially, the degradation of CR starts by attacking the OH^⦁ radical on the azo bond (–N=N–) of the dye molecule and converting it to sodium 3-([1,1′-biphenyl]-4-yldiazenyl)-4-aminonaphthalene-1-sulfonate with an m/z value of 425. In the next steps, in both pathways, other degradation compounds form with lower m/z values, such as sodium 4-amino-3-((4′-amino-[1,1′-biphenyl]-4-yl)diazenyl)naphthalene-1-sulfonate (m/z: 440), 2-((4′-amino-[1,1′-biphenyl]-4-yl)diazenyl)naphthalen-1-amine (m/z: 338), 4′-((1-aminonaphthalen-2-yl)diazenyl)-[1,1′-biphenyl]-4-ol (m/z: 339), sodium 4-amino-3-(hydroxydiazenyl)naphthalene-1-sulfonate (m/z: 289), sodium 3,4-diaminonaphthalene-1-sulfonate (m/z: 260), sodium 3-aminonaphthalene-1-sulfonate (m/z: 245), 3,4-diaminonaphthalene-1-sulfonic acid (m/z: 238), sodium 4-aminonaphthalene-1-sulfonate (m/z: 245), 3-aminonaphthalene-1-sulfonic acid (m/z: 223), 3-aminonaphthalen-1-ol (m/z: 159), naphthalene-1,2-diamine (m/z: 158), 3,4-diaminonaphthalen-1-ol (m/z: 174), 4-aminonaphthalen-1-ol (m/z: 159), naphthalene-1,4-diol (m/z: 160), naphthalene-1,4-dione (m/z: 158), phthalic acid (m/z: 166), benzidine (m/z: 184), 4′-amino-[1,1′-biphenyl]-4-ol (m/z: 185), [1,1′-biphenyl]-4,4′-diol (m/z: 186), [1,1′-biphenyl]-4-amine (m/z: 169), hydroquinone (m/z: 110), benzoquinone (m/z: 108), phenol (m/z: 93), maleic acid (m/z: 116), adipic acid (m/z: 147), carbamic acid (m/z: 61), succinic acid (m/z: 118), and oxalic acid (m/z: 90). In the last step, simple and harmless products formed such as H₂O and CO₂, produced along with NO₃⁻ and NH₄⁺.

6.4. Rhodamine B

Properties: Rhodamine B (Rh B) is an inexpensive basic red dye included in the xanthene class. It has a reddish violet color in powder form and comes under the trade name of D & C Red No. 19 [171]. RhB always exist in two forms: the fluorescent/open form and the nonfluorescent/closed spironolactone form. These two form are always in equilibrium. The fluorescent form always dominates in an acidic environment, while the nonfluorescent form is always colorless in a basic environment [172]. RhB fluorescence intensity decrease with an increase in temperature.

Uses: It is used extensively in the textile and food industries as a colorant, and is also used as a renowned water-tracing fluorescent. It is also employed in the engineering of various products such as paints, ball pens, leather, carbon sheets, dye lasers, crackers, stamp-pad inks, and explosives [173]. Generally, RhB is used as a laser dye, due to its high photostability as well as its photophysical properties, such as polarization and quantum yield, and as a fluorescent probe to analyze the surface of polymeric nanoparticles, examine the structure and dynamics of micelles, and in single-molecule imaging and imaging in living cells [174]. RhB is regularly used in biological operations, due to its low cost and chemically stability, and has a very high extinction coefficient. RhB is often used as a systemic marker in numerous animals, due to its better persistence level. RhB is also used mostly in various applications of biotechnology, including flow cytometry, fluorescence microscopy, and fluorescence correlation spectroscopy. RhB is used as a dye, particularly for paper, as well as a reagent for bismuth, antimony, niobium, cobalt, manganese, gold, molybdenum, mercury, thallium, tantalum, and tungsten, and as a biological stain. It is registered conditionally for usage in the cosmetic and drug industries.

Toxicity: RhB is a weak basic nitrogenous molecule and always dissociates into extremely stable and non-biodegradable colorful cations. The colorful cations produced from RhB enter into water reservoirs, causing noteworthy changes in the aquatic ecosystem which are considered hazardous for both aquatic and terrestrial living things [173]. It is the main source of oncogenic and mutagenic variations in living things. It is highly dangerous if swallowed by living organisms, and is always considered as the foundation of skin irritation, eye irritation and respiratory tract irritation. There have been worries that RhB can cause mutations or cancer.

Photocatalytic degradation mechanism of Rhodamine B: In comparison to other dyes, Rhodamine B (RhB) degrades easily because of its simple structure. The proposed photocatalytic degradation mechanism for RhB is shown in Scheme 4. Based on the LC-MS investigation of m/z values, the probable photocatalytic degradation pathway of RhB was determined. During degradation, due to the N-de-ethylation process, RhB lost 28 units of mass to generate a product with a value of m/z 415. The other generated products are the consequences of the conjugate-structure destruction. The conjugate structure of the Rh B molecule is constructed of a single bond, which is much easier to break. The generated product, with value of m/z 152, is possibly due to the azo group cleavage. The values of m/z 90 and 60 are allocated to intermediates of thering-opening reactions. At the end of the reaction, the generated intermediates break down into the non-toxic products of CO₂ and H₂O, and the color become colorless [175].

7. Techniques for Photocatalytic Degradation Confirmation

The following are the few techniques that are used to confirm the process of photocatalysis and the degradation products of the dye molecules.

UV is the most common technique for studying the photocatalytic degradation of dyes. Each dye gives an absorbance peak at a specific wavelength. After using the process of photocatalytic degradation, the intensity of the absorbance peak decrease with time, and ultimately completely finishes after a certain time, depending on the nature of the photocatalyst.

The photocatalytic degradation process of dye can also be visualized by the naked eye, with which one can observe the change in color with time. This visualization confirms the fact that the photocatalytic degradation process take place but depends on the color of the dye. If the dye has no color, then this observation is useless. The photocatalytic degradation intermediate and the by-products of dye molecules can usually be verified via the gas chromatography/mass spectrometry (GC/MS) technique, which is the combination of two analytical tools. This is a useful technique, which confirms all the possible intermediates during the degradation process.

8. Factors Effecting Photocatalytic Degradation

Numerous factors are responsible for the photocatalytic degradation of dyes, such as dye concentration, catalyst dosage, temperature, pH, porosity, surface area, nature of the dyes, light intensity, irradiation time, effect of dopants and composite, and oxidants and scavengers.

The concentration of dyes greatly affected the mechanism of photocatalytic degradation. It is a common observation that photocatalytic degradation decreases with an increase in dye concentration, because at a higher dye concentration more molecules of dye will go through the degradation mechanism for which the amount of photocatalyst will not be enough. In the photocatalytic degradation mechanism, dye molecules are adsorbed on the photocatalyst surface during the photocatalytic degradation mechanism, and the photocatalyst surface will be not enough for the dye molecules, and vice versa. This phenomenon can be explained by the principle that as the concentration of dye upsurges, additional dye molecules are adsorbed on the surface of photocatalyst, whereas fewer generated photons are available to reach the surface of the photocatalyst and so fewer ^•OH radicals are generated, consequently leading to a lower percentage of dye degradation.

The dosage of photocatalysts is also responsible for photocatalytic dye degradation. Generally, photocatalytic degradation enhances with increases in the amount of photocatalyst. This concept mainly explains the fact that more active sites will be available with enhances in the photocatalyst amount, leading to an upsurge in the generation of ^•OH radicals which can mainly take part in the actual photocatalytic degradation of the dye solution, and vice versa. Beyond a certain limit of photocatalyst quantity, the dye solution becomes turbid, leading to a blocking of the UV-irradiation needed for the respective reaction to ensue, and hence the percentage of dye degradation starts declining.

Porosity is also the key factor that affects the photocatalytic degradation of dye. Highly porous photocatalysts are always considered better for photocatalytic degradation in comparison to dense materials. The reason behind this concept is the easy diffusion of dye molecules towards the active site, due to large pore size in comparison with dense materials, in which the diffusion of the dye molecules is comparatively difficult due to the pack structure. Due to this reason, porous photocatalysts are always better candidates for photocatalytic dye degradation. Hybrid polymeric materials, MOFs and COFs, are examples of the few porous photocatalysts used extensively for the photocatalytic degradation of dyes [24].

The surface area of the photocatalysts is a fundamental aspect in the photocatalytic degradation of dyes. Surface area is directly related to the size of the photocatalysts [176]. Photocatalysts with a smaller size will have a larger surface area and are always considered better photocatalysts for photocatalytic dye degradation, and vice versa. Surface morphology is a vital factor of any photocatalyst, because all the chemical proceedings take place at the surface. The nanostructured type of photocatalysts with a small size (below 20 nm) are always considered of great interest in comparison to bulk materials [177]. This is due to the fact that a higher surface area always comprises more active sites in comparison to low surface-area materials.

Intensity of light is a vital factor during the photodegradation process. It greatly affects the rate of photocatalytic degradation up to certain limit, due to the existence of inadequate active sites on the photocatalyst surface. With higher light intensity, the photocatalytic rate will be higher, until all the active sites are occupied during this process. Further enhancement of the light will have no effect on the photocatalytic degradation process because there will be no available active site on the surface of the photocatalyst [178].

pH is another factor that is highly responsible for the photocatalytic degradation of dye molecules. Countless studies have revealed that acidic pH is better for the degradation of dyes, but depending on the nature of the dyes. More positive charges generated on the surface of the photocatalyst in an acidic pH makes the surface of the photocatalyst positively charged, which attracts dye molecules which are negatively charged, more efficiently. On the contrary, the dyes which are positively charged are degraded in a better way in alkaline pH, because basic pH makes the surface of the photocatalyst more negatively charged. Shubha et al. reported the effect of pH on MB photocatalytic degradation. The results evidently revealed that at higher pH (pH = 10) the degradation was up to 98%, while at low pH (pH = 4) the degradation efficiency was only 52% [179].

It is a common observation that rate of reaction increases with rise in temperature, and vice versa. According to kinetic theory a rise in reaction temperature leads to enhanced photocatalytic activity. However, a reaction temperature has a certain range, and above that temperature the recombination of charge carriers promotes and disfavors the organic compound adsorption on the photocatalyst surface. In certain cases, a temperature lower than 80 °C favors more adsorption of dye molecules but a further decrease in reaction temperature to below 20 °C leads to enhancement in the apparent activation energy. Consequently, a temperature in the range of 20 °C to 80 °C has been regarded as the preferred temperature for better photocatalytic degradation of organic contents.

Oxidant nature has also affected the photocatalytic degradation of dye molecules. A strong oxidant is always useful for fast and efficient photocatalytic degradation. Among the oxidants, hydrogen peroxide (H₂O₂) is a strong oxidant which enhances the formation rate of hydroxyl radicals (OH^⦁), leading to boosts in the photocatalytic degradation of dye molecules at lower concentrations, too [180]. The reason is the generation of efficient ^•OH radicals and the inhibition of the electron–hole pair recombination, as H₂O₂ act as an electron acceptor in this whole process. H₂O₂ is always considered one of the most effective potential oxidizing catalysts, and has the capability to generate 2 mol of the ^•OH radicals, followed by the interaction with targeted dye molecules. KMnO₄, Citrate ions and air are also used as potential oxidizing agents for the photocatalytic degradation of dye molecules [181,182].

In the photocatalytic degradation process, various species ((^•OH, h⁺, & ^•O₂⁻)) are the key reactive species contributing to the degradation of organic pollutants. Numerous radicals’ scavengers have been reported in order to comprehend the complete mechanism and to observe the primary active species’ responsibility during the process of photocatalytic degradation. These radicals comprise t-butanol (^•OH), ammonium oxalate (h⁺), and 1,4-benzoquinone (^•O₂⁻) [183]. So, scavengers are also highly responsible in decreasing the photocatalytic degradation activity of dye molecules.

Dopants and composite photocatalyst have a pronounced effect on the photocatalytic degradation of dye molecules. Dopants and composites are mainly responsible for reducing the recombination rate of e⁻/h⁺ pairs [184]. It is reported that, with the increase in dopant concentration, the adsorption of solar light enhances, which lead to acceleration of the process of photocatalysis [185]. It is also reported that composite-photocatalyst efficiency is better than that of the pristine photocatalyst [186].

9. Advantages and Disadvantages of Photocatalytic Degradation

Photocatalytic degradation is an advanced and emerging approach for the degradation of numerous organic pollutants in wastewater [187]. Photocatalytic degradation progression is a feasible alternative for the effective degradation of all kinds of petrochemical contaminants in water as well as in air [188]. The usages of substitute light sources like LEDs are an auspicious way for saving power consumption. Photocatalytic degradation of organic pollutants is always considered as a friendly, easy, and less-energy-consuming route over other route for dye processing because of the presence of abundant solar energy on the Earth’s crust. But it is still essential to improve and find more effective techniques in which solar energy is used for the photocatalytic process [189].

This process is always considered a slow degradation process in comparison to the chemical degradation method. It is typically not so effective, particularly because of its nonvisible-light-driven capacity. Numerous studies proposed that the intermediates formed during this process also produce noxiousness for various creatures in the surroundings. Investigators have studied the adverse effects of nano photocatalysts on floras, comprising the inhibition of root development in a few plant species [190].

10. Economic Cost of Photocatalytic Technology

Economic cost is a fundamental factor that determines the implementation of any design of photocatalytic technology on an industrial scale. However, a few solitary reported works are available on the cost assessment of photocatalytic materials, as most of the investigation is restricted to the laboratory scale. Hence, cost estimation of the photocatalytic materials is an inclusive process with plentiful features that should be considered, such as synthetic route, availability, transportation, and lifetime issues [191,192]. The worldwide photocatalysts market is estimated to reach from USD 5.1 billion (2021) to USD 3.1 billion (2026), at an annual growth rate (CAGR) of 10.5%. Moreover, the manufacturing sector of the worldwide photocatalyst market is predicted to grow from USD 2.7 billion (2012) to USD 4.5 billion (2026), at a CAGR of 10.3%. Additionally, the consumer products section of the worldwide photocatalysts market is predicted to grow from USD 238.7 million (2021) to USD 421.9 million (2026), at a CAGR of 12.1%. The power consumption during photocatalytic technology can be assessed by using the following relation (Equations (8) and (9)) [193]:

$$t_{90}=\frac{\mathrm{l}\mathrm{n}\left(10\right)}{K}$$

(8)

$$E_C=\frac{P\times t_{90}\times Unitprice}{1000\times 60}$$

(9)

where “t₉₀” stands for the time taken by the dye to be degraded to up to 90% of its initial concentration, “E_C” stands for electricity cost, and “P” stands for power consumed (in Watts) of the UV light source. The “unit price” of electricity will be different for different countries. To the best of our knowledge, few investigators respect the cost evaluation of photocatalytic materials. Therefore, a detailed study is desirable to explore this inclusive area further [194].

11. Summary, Challenges and Future Prospects

The occurrence of dyes in natural water is dangerous for fauna and flora, due to their venomous nature. Photocatalytic degradation is an effective and cost-effective method for the complete degradation of various dyes into nontoxic and less harmful compounds. COF-based photocatalysts was the main theme of this review article, with their unique properties and different synthetic routes. The comprehensive degradation mechanism and different reaction-pathway analysis revealed that MG, MB, CR and RhB dyes were initially transformed into different intermediate products and were then entirely mineralized into less harmless products (CO₂, H₂O, NO₃⁻, SO₄²⁻). The photocatalytic degradation process depends on the structure of the dye molecules. In the case of complex structures such as MG and CR, the degradation path is highly complex in comparison to MB and RhB. Photocatalytic degradation processes are greatly affected by numerous factors including dye concentration, catalyst dosage, temperature, pH, porosity, surface area, nature of the dyes, light intensity, irradiation time, and the effect of dopants, oxidants and scavengers, which confirms that we should be careful about all these factors during photocatalytic technology. Economic cost also confirms that photocatalytic technology is the most environmentally friendly and cost-effective in comparison to other technologies.

There are a few aspects that still necessitate thorough examination, not only to efficiently remove the dye molecules, but also to upsurge their real-world usage on an industrial scale.

To accomplish the full effectiveness of photocatalytic technology, the key complications obstructing large-scale photocatalytic applications for environmental cleaning and solar hydrogen generation require resolving. The primary significant hurdle to achieving this objective is the construction of exceedingly active photocatalytic materials on a large-scale, via low-energy, economical and environmentally friendly fabrication approaches. The subsequent obstacle is choosing the ideal structure of the substrate, the immobilization method, and the respective material. In addition, complications arise in choosing the essential design and geometry of photocatalytic devices, depending on the mode of operation and conditions.

The construction of the most efficient photocatalytic materials goes with the prospect of multiple recycles with no loss, or minimal loss, of photocatalytic activity. Additional determinations are prerequisites for focusing on the effective usage of highly available free sunlight for the industrial-scale photocatalytic method via the introduction of additional systems proficient in concentrating and focusing direct and diffuse beams. Photocatalytic reactor modifications with optimal designs enhance the mass transfer of reaction products and the maximum usage of incident photons to upsurge the overall performance. Thoughts are of coalescing photocatalytic development with additional methods for resolving environmental as well as energy difficulties, so helping to balance the disadvantages and improve the advantages of each. To conduct an inclusive cost approximation of a photocatalytic technology for commercialization and to compare it with additional technologies, it is essential to provide data on the prices of the materials used and the prices of operation. In conclusion, photocatalytic technology is a “green” technology, and experts are the prerequisite for more determination in supplying its benefits in order to generate discourse on energy and environmental difficulties by overwhelming obstacles to commercialization and improving economic efficacy.