Advancing Sustainable Approaches for the Removal and Recycling of Toxic Dyes from the Aquatic Environment

Abstract

The widespread usage of synthetic dyes and chemicals across industries leads to the production of a considerable amount of wastewater. Textile industries, in particular, frequently release harmful dyes directly into the environment, presenting significant threats to human health. Discharging untreated sewage from numerous textile industries contributes to severe environmental consequences. To address these concerns, there is growing emphasis on developing efficient and cost-effective nano-adsorbents, leveraging their distinctive properties to mitigate the ecological impact of dyes. In light of these challenges, significant efforts have been focused on developing efficient adsorbents to remove unwanted substances from wastewater. Effectively and selectively eliminating dyes is increasingly acknowledged as essential for addressing environmental concerns. A comprehensive approach involves comparing the degradation efficiency of different catalysts, considering the search for a suitable adsorbent. This chapter comprehensively examines numerous advanced adsorbents in the literature for dye degradation. In addition to outlining the physicochemical characteristics of various adsorbents, the chapter delves into the mechanisms and effectiveness of the adsorption process. It is crucial to tackle the challenges inherent in dye degradation to alleviate the adverse environmental effects of dyes. A critical evaluation of next-generation adsorbents can advance the adoption of clean and cost-effective water purification practices.

1. Introduction

Water is Earth’s most precious natural resource, but several kinds of pollution are constantly tainting it. The demand for clean water is steadily rising because of the population explosion, severely threatening our ecosystem [1]. In today’s life, lack of access to sanitary facilities and clean drinking water is one of the most inescapable problems affecting people. Water pollution is a critical global issue, exacerbated by widespread industrialization, human activities, unsustainable water resource exploitation, unbridled growth of dye-utilizing industries, and inadequate effluent treatment methods [2, 3, 4]. The wastewater liberated from factories and laboratories involved in the textile industry have led to serious pollution issues because unused dyes are released into the environment, even in minimal amounts (less than 1 ppm for some specific dyes), which are hazardous due to their high toxicity and color [1, 5, 6]. The United Nations (UN) has reported that significant polluting elements are not removed from over 80% of wastewater produced by anthropogenic activities before it is released into waterways. By 2030, it is predicted that 47% of people on Earth will reside in areas with limited access to clean water [6]. In recent years, the dye industry has expanded rapidly. According to the US “Color Index,” the quantity of commercial dyes has surged to tens of thousands. Annually, approximately 60,000 tons of dyes, with around 80% being azo dyes, are disposed of into the environment globally as waste [5, 7]. The paper and textile industries utilize a lot of synthetic dyes. Over 10,000 dyes and pigments are used in the textile industry, along with an ineffective dyeing water treatment procedure that poses a severe risk to human health [6]. In rivers where treated wastewater is discharged, dyes can undergo partial degradation or transformation in the water and sediment. Certain pigments and compounds they convert into are toxic carcinogens [5, 8].

Globally, there is increasing concern about eliminating microorganisms, hazardous dyes, and toxic contaminants to provide potable water. Most countries now use costly water purification systems, making them unaffordable for many low- and middle-income nations [6]. This chapter aims to present a thorough analysis of several cutting-edge sustainable approaches in wastewater treatment techniques for eliminating toxic dyes from aqueous solutions. Furthermore, a detailed method compares the effectiveness of various catalysts in degrading materials while considering the need to find an appropriate adsorbent. Moreover, this chapter comprehensively reviews various state-of-the-art adsorbents documented in the literature for dye degradation. In addition to emphasizing the various adsorbent’s physicochemical characteristics, the chapter also covers the mechanisms and efficiencies associated with the adsorption process. To lessen the harmful effects of dyes on the environment, addressing the difficulties posed by the dye degradation process is essential. The features, benefits, limitations, and underlying mechanisms of various sorbents have all been thoroughly examined. The difficulties in creating a dye degradation process that is both efficient and environmentally benign are emphasized. This chapter will improve knowledge of the state-of-the-art dye degradation process and be helpful in the actual use of various sorbents for particular dye adsorption.

2. Textile dyes

The textile industry has become a primary contributor to pollution in surface and groundwater reservoirs. The main uses of dyes are to color textiles and materials such as plastics, leather, fur, and hair. In several nations, including China and the estuaries of South Africa, the textile industries significantly contribute to environmental degradation and the growth of the global economy [6]. The textile industry utilizes over 10,000 tons of synthetic dyes annually, accounting for a fraction of the approximately 7 × 10⁷ produced globally each year [9]. Dyes can absorb or emit light within the visible range (400–700 nm), making color. Typically, a dye requires a complex aromatic conjugated system for resonance, essential for imparting color. Within a dye’s aromatic rings exist color-bearing groups known as chromophores and other chemical groups such as bridging groups (auxochromes). These auxochromes intensify or modify chromophores of color and facilitate the fixation of dyes [10]. Dye is commonly classified into multiple categories based on its source, composition, and usage. Among synthetic dyes, the textile industries predominantly employ azo, direct, reactive, mordant, acid, basic, dispersion, and sulfide dyes. The textile business uses natural fibers, such as acrylic, polyester, wool, cotton, and silk [9]. Two main categories of dyes exist: synthetic and natural. Synthetic dyes are extensively utilized across various industries, offering a broader spectrum of colors than natural dyes. Synthetic dyes can be divided into two primary groups: ionic and non-ionic. There are cationic and anionic dyes within the ionic group. Even in small amounts, all of these dyes have the potential to be harmful when dissolved in water. Based on their specific applications, they can be further divided into subcategories, such as reactive, direct, essential, and dispersed dyes depicted in Figure 1 [11]. Most synthetic dyes are sourced from two primary origins: petroleum-based intermediates and coal tar. These dyes are typically available in various forms, including liquid dispersions, pastes, granules, or powders [12]. A recent investigation found that 100,000 commercially available dyes were used globally, with an annual consumption record of 10,000 tons [13]. During the washing of colored or printed textile goods, a substantial volume of liquid waste containing dyes and pigments is generated. This waste constitutes approximately 60% of the total volume [14, 15, 16]. Most dyes are complex organic compounds that can adhere to various surfaces, including leather, textiles, and so on. The complexity of effluent and the massive dissipation of textile colors disturb the ecological system [13].

Figure 1.

Classification of dyes.

Inefficient textile dyeing techniques, where dyes fail to bind to fibers and fabrics properly, lead to the release of 15–50% of azo dyes into the wastes of textile industries [14]. Industrial effluents contaminated with dyes pose significant environmental hazards due to elevated levels of biological oxygen demand (BOD), chemical oxygen demand (COD), total dissolved solids (TDS), as well as various toxic heavy metals and harmful chemical compounds [10].

For instance, textile effluents have a high concentration of dyes and numerous other compounds added during the coloring process. The high intensity of color and wide compositional variability of textile effluent make treatment extremely challenging [4].

3. Environmental and health impacts of toxic dyes

Wastewater containing dyes presents substantial risks, carrying the potential for adverse effects like cancer and mutagenesis in humans if released without treatment. This risk is predominantly attributed to these dyes’ complex chemical structure and non-biodegradable characteristics. Globally, an estimated annual volume of at least 200 billion liters of wastewater from dyeing is generated [17]. Given the global health risks associated with textile wastewater discharge, new initiatives for environmental restoration are being launched, driven by both ecological preservation and commercial interests [18]. The extensive usage of textile dyes and their potential to create hazardous aromatic amines make them environmentally important. Additionally, they pose a risk to ecosystems because they block sunlight, which lowers photosynthetic activity and dissolved oxygen concentration [10]. Azo dyes pose a hazard due to toxic amines in the effluents. Additionally, anthraquinone dyes maintain their color for extended periods in textile rejects owing to their strong resistance to deterioration. Numerous adverse environmental effects can be traced back to the textile industry [13]. Particulate matter, dust, sulfur and nitrogen oxides, and volatile organic compounds are a few examples of the created air pollution. The primary sources of solid waste in the textile industry include leftover yarns, fabric remnants, and empty packaging. In contrast, textile sludge presents challenges related to excessive volumes and undesirable composition, often containing high concentrations of organic matter, micronutrients, heavy metal cations, and pathogenic microorganisms [16, 17].

The consumption of azo dye through food or water can result in acute vomiting, cyanosis, allergic reactions, and genetic mutations [8]. One of the most critical consequences of textile waste is the depletion of dissolved oxygen in water. Dissolved oxygen is essential for marine life, and its depletion endangers aquatic ecosystems and hinders the self-purification ability of water bodies [18]. Growing amounts of dye in water bodies impact several algae parameters, including growth protein, pigment, and other nutritional content. The possible effects of various dyes on algae vary. Algae are 50% more sensitive to pollutants when measuring contamination in aquatic habitats than species typically utilized in toxicological testing [12]. Numerous dyes harm flora and fauna, hinder the growth of microorganisms, and are hazardous to fish and mammals. In addition, several dyes and the byproducts of their breakdown are dangerous to aquatic life [18]. Even the treated water contains pigments that may be primarily destroyed or transformed in the water and sediments of streams. Both textile dyes and their byproducts have the potential to cause cancer, mutagenesis, and teratogenicity. Moreover, almost 40% of colors and dyes used worldwide contain chemically bound chlorine [19]. Along with the wastewater, these hazardous substances are transported over great distances. After that, they stay in the water and soil for extended periods, endangering living things’ lives and decreasing soil fertility and aquatic plant photosynthetic activity, which causes anoxic conditions to develop for marine fauna and flora [9]. The pictorial representation of the environmental and health impacts of dye contamination is depicted in Figure 2.

Figure 2.

Schematic representation of environmental and health impacts of dye pollution.

4. Conventional treatment techniques for toxic dyes

The presence of dyes and other chemicals, in the effluent produced by the cotton dyeing and textile industries, is contaminated with high levels of color, BOD, COD, and total solids. Drinking water should be colorless and devoid of harmful substances to prevent problems with the river’s treatment costs. Thus, before being released into any freshwater body, textile wastewater must undergo various treatment procedures, such as physical, chemical, and biological [20]. To address the issue of dye removal from wastewater, numerous techniques based on several methods, such as combination treatment processes and physical, chemical, and biological approaches, have been developed to treat textile dyeing wastewater economically and effectively. These processes are typically very effective in wastewater from textile dyeing [11, 12]. The adsorption, sedimentation, ultrafiltration, flotation, coagulation, flocculation, filtration, photoionization, and incineration techniques are subdivided under the physical approaches. The methods, including neutralization, oxidation, reduction, electrochemical oxidation, wet-air oxidation, and photochemical degradation, are the subgroups of chemical techniques. Activated sludge, trickling filters, aerated lagoons, stabilization ponds, anaerobic digestion, and various microbial strains are all used in biological treatment techniques [21, 22, 23]. Compared with other conventional techniques, the adsorption technique has several benefits, such as high efficiency, rapid reaction times for wastewater color removal, and the capacity to reuse adsorbent; also, it is one of the promising techniques in sequestering toxic dyes [9]. The schematic representation of conventional dye wastewater treatment technologies is depicted in Figure 3.

Figure 3.

Schematic representation of conventional dye wastewater treatment technologies.

5. Adsorption of dyes

Adsorption is a surface phenomenon where molecules or ions that have been adsorbed are drawn toward the surface of a solid adsorbent. It is divided into two types based on how the dye molecule is adsorbed onto the adsorbent surface: physisorption and chemisorption [9]. Adsorption is an eco-friendly, effective, and reasonably priced technique for sequestering contaminants from wastewater. The most popular adsorbents utilized to remove dyes and heavy metals are composites, metal oxides, polymeric polymers, and activated carbon [8, 24, 25]. The adsorption method is a desirable treatment for dye-contaminated wastewater. Because of its effectiveness and affordability, adsorption has become one of the widely used wastewater treatment techniques in industries [5]. The adsorption process is supported by various forces such as hydrogen bonding, van der Wall, hydrophobic attraction, and electrostatic forces. The basic principle of adsorption is that the porous structure present on the surface of the adsorbent enhances the adsorption in the total exposed surface area necessary for the quick and effective adsorption of dye molecules from wastewater [9]. The adsorbent may be recycled and regenerated simultaneously without producing further pollution. As a result, adsorption is a separation technology with several uses. The most crucial phase in adsorption is preparing superior adsorbents that can efficiently remove the dye [26]. Adsorption is one of those methods that can help remove organic chemicals that are difficult for the body to break down effectively. Because of their broad surface area and porous nature, typical activated carbons (ACs) effectively remove the organic molecules [27]. A biosorbent significantly aids the elimination of dye. Utilizing household and agricultural wastes as adsorbents has become a practical substitute. Many adsorbents from biomass waste have been developed and are used as very efficient agents to remove contaminants from wastewater and water. These waste products were used either precisely as they were intended to be used or with the appropriate changes. Several agricultural and food waste materials have shown promise when used as adsorbents to remove contaminants. These materials include Azolla, banana peel, cabbage waste, chitosan, citrus peel, Citrus limonum leaves, corn cob, orange peel, peanut hull, rice husk, sawdust, and sugar cane bagasse [28]. Adsorbents of all kinds, including hydrogels, metal-organic frameworks, organic polymers, nanomaterials, and others, have been effectively developed for the remediation of wastewater containing dyes [29, 30]. The morphological characteristics such as size, shape, and structure of the adsorbent strongly influence the adsorption mechanism, and the capability of the adsorbent’s chemical structure is incredibly crucial. As a result, adsorbents for removing pollutants have been prepared using a large amount of natural and mineral resources [31].

5.1 Carbon-based adsorbents

Biochar is a resultant product of pyrolysis of forestry and agricultural waste with many surfaces, chemical functional groups, and a porous structure. Adsorbing organic dyes from wastewater using biochar is a promising sustainable adsorbent that aligns with sustainable development and cleaner production. Regarding removing dye, biochar’s adsorption efficiency is inferior to typical adsorbents and must be significantly enhanced throughout pyrolysis [32, 33]. Carbonaceous materials have become the most widely utilized adsorbents because of their many benefits, including their well-developed pore structure, exceptional physical and chemical stability, and ease of functionalization. However, conventional porous carbon materials often have a low adsorption capacity. They are challenging to recover from, severely restricting their practical use and encouraging the development of effective porous carbon adsorbents [34]. In many diverse treatment applications, such as industrial wastewater treatment from the textile, sugar, leather, pharmaceutical, and petroleum industries, activated carbons are frequently employed. Various surface functional groups, such as quinones, carbonyl, and carboxyl, can absorb pollutants from wastewater contamination. According to published research, activated carbon has a very effective capacity for adsorbing various contaminants, depending on the adsorbate and functional groups of the adsorbent. Altering the activated carbon’s surface is an additional method that successfully displays alterations in the configuration of different surface functional groups [35]. Many studies have been conducted on the adsorption of dyes using carbon-based materials, such as activated carbon (AC), carbon quantum dots (CQDs), carbon nitride (C₃N₄, CN), and graphene oxide (GO) due to their high physical and chemical stability, thermal stability, and high adsorption capacity [36]. Due to these modifications, the adsorbent’s surface area increased, and a new functional group and texture were created by adding atoms. Adsorption of contaminants from wastewater is substantially improved by altering physical and chemical activation processes, mainly when removing heavy metal ions and industrial colors [35]. In the work conducted by Fan et al., it is reported that through free radical polymerization of a new class of polyacrylic acid grafted lignosulfonate (SLS)/carboxymethyl cellulose (CMC) and biochar (BC) composites (SLS/CMC/BC), which can efficiently adsorb Pb²⁺ and methylene blue (MB). The highest adsorption capacity of MB and Pb²⁺ in a monopollutant system is 113 mg g⁻¹ and 204 mg g⁻¹, respectively. Furthermore, adding biomass carbon significantly enhanced the hydrogel’s stability and adsorption capacity [26]. Through pyrolysis, activated carbon (AC) from watermelon shells was converted into the unique magnetic nanocomposite activated carbon (MWMSC). The experimental data were tested using adsorption isotherms, and the results are well-fitted to the Langmuir model, with MB and MO having maximal adsorption capacities of 303.30 and 345.70 mg g⁻¹, respectively [37]. The present understanding of techniques such as pyrolysis, physical activation, and chemical activation processes helps develop activated carbon (AC) and biochar from organically sourced materials, previously living materials considered waste materials and would otherwise be conventionally disposed of Medeiros et al. [27]. Numerous studies have been conducted to increase the adsorption capacity of activated carbon adsorbents; these studies have primarily focused on altering process parameters, such as raw material or adsorbent, carbonization, and activation procedure, under the right conditions [35]. A carbon-based adsorbent was created by carbonizing the pitaya peel (Hylocereus undatus) in the presence of ZnCl₂ to remove the Metanil Yellow dye (MY) from colored fluids. At 298 K, the Langmuir model reached its maximum adsorption capacity of 144.07 mg g⁻¹, demonstrating the best adjustment of the system’s equilibrium isotherms [38].

5.2 Nanotube and graphene-based adsorbents

Carbon nanotubes (CNTs) are composed of graphene or graphite sheets coiled into tubular shapes. Their lengths are in the micrometer range, and their interior diameters are in the nanometer range. Single and multiwalled carbon nanotubes are distinguished from one another by their layered structure [39]. Zhang et al. have investigated the adsorption activity of mesoporous carbon nanospheres for the sequestration of toxic dyes such as Congo red (CR), malachite green (MG), and methylene blue (MB). The research findings concluded that the distinct porosity structure of mesoporous carbon nanospheres gives them excellent adsorption powers for dyes, such as CR (1726.60 mg g⁻¹), MG (980.55 mg g⁻¹), and MB (708.82 mg g⁻¹), indicating that they can adsorb both anionic and cationic dyes [34]. Since CNTs are considered a superior adsorption material, several studies have been conducted on their ability to remove contaminants from aqueous systems. Their high specific area, high porosity, large ratio of accessible micropores, hollow internal structure, layered structure, appropriately established mesopores, hydrophobicity, distinct sidewall curvature, Pi-conjugative structure, ease of chemical activation, lightweight, tunable surface properties, high corrosion resistance, excellent support for coating or depositing other materials, and high water permeability make them exceptional adsorbents [39]. Researchers are drawn to carbon-based materials among various inorganic substances, such as graphene oxide (GO) and carbon nanotubes (MWCNTs), due to their rich specific surface area and abundance of chemical modification sites [29]. A straightforward ultrasound irradiation technique was used to create a variety of carbon-based MgAl₂O₄ adsorbents using activated carbon (AC), carbon quantum dots (CQDs), C₃N₄ (CN), and graphene oxide (GO) as carbon sources. The Congo red (CR) dye was adsorbed using carbon-based MAO adsorbents, which produced an 89.7% dye removal rate [36].

5.3 Metal-based adsorbents

Metal-organic frameworks (MOFs) and materials generated from MOFs are becoming more and more attractive as adsorption candidates because of their essential characteristics, which include a large surface area, adjustable porosity, and a variety of functions [40]. Metal oxide nanoparticles such as ZnO, TiO₂, BiVO_4, and WO₃ have drawn increased interest because of their potential uses in solar fuel, energy conversion and storage, and photocatalysis. Moreover, metal oxides are employed in various chemical redox processes, including eliminating dye contamination from wastewater [41]. A recently discovered class of two-dimensional (2D) transition metal carbides, nitrides, or carbonitrides known as MXenes has shown promise in several domains. Because of their graphene-like morphology and distinctive qualities such as their large specific surface area, highly active sites, hydrophilic nature, environmental friendliness, thermal and electrical conductivity, anisotropy, anti-microbial properties, large inter-layer spacing, high chemical stability, superior sorption/reduction capacity, and high negative zeta-potential—these materials can be used in a variety of applications. Because of their layered structure, high hydrophilicity, wide specific surface area, and negatively charged surface, MXenes are the perfect adsorbent materials to adsorb dyes from the aqueous environment [42]. One notable adsorbent that is affordable, simple to synthesize, and non-toxic to human health is magnesium oxide (MgO). This work used a straightforward sol-gel process to manufacture magnesium oxide, which is inexpensive and environmentally benign. For the first time, MgO microparticles were used to remove Reactive Red 21 azo dye, and their effectiveness in actual textile effluent was examined. The adsorption studies inferred that the MgO adsorbent effectively sequestered about 98% of the azo dye in real-time textile effluent [43]. The ZnO/MgO nanocomposite was made using the Pechini sol-gel method, with tartaric acid as a chelating agent and ethylene glycol as a crosslinker. A ZnO/MgO nanocomposite was employed to remove Congo red dye from aqueous solutions rapidly. The ZnO/MgO nanocomposite has a maximal Congo red dye adsorption capability of 317.46 mg g⁻¹ [44]. On the other hand, the inability of MOFs to withstand severe environmental conditions led to the development of more stable MOF-derived materials for adsorption purposes. A growingly effective technique for creating stable metal oxide-carbon hybrids as adsorbents for removing pollutants is the carbonization of MOFs [40]. Metal-organic porous frameworks with large surface area, adjustable porosity, and usefulness have garnered much interest in adsorption. Because of their distinctive structural characteristics, MOFs have the potential to be used as materials for a variety of other purposes, such as medication delivery, gas storage, sensors, heterogeneous catalysis, and separation. MIL-100(Fe)-derived mesoporous LaFeO₃@C nanocomposites for the efficient adsorption removal of sulforhodamine B (SRB) and methyl orange (MO) were studied by Lalan et al. Regarding SRB and MO, the material formed at 700°C (LFO@C-700) showed greater removal effectiveness of 99% and 97.1%, respectively [40].

5.4 Polymer and gel-based adsorbent

The wastewater treatment industry extensively uses polymer materials as functional materials because of their distinctive structure, ease of modification, and processing. Polymeric materials offer a wide range of pore diameters, chemical stability under a variety of environmental circumstances, and affinity for target dyes based on the functional groups of the polymer as well as the dyes; they are a viable candidate for the adsorption of dyes from aqueous fluids [26]. Recently, several natural polymers, including gelatin, cellulose, polyvinyl alcohol, chitosan, and sodium alginate, as well as mineral minerals like clay, nano-hydroxyapatite, calcium oxide, and zinc, have been used to synthesize adsorbents because of their cost-effectiveness and efficacy [31]. In the study conducted by Zhou et al., an efficient and unique porous alginate/gelatin/n-hydroxyapatite/magnetic nanoparticle was fabricated and utilized to sequester methylene blue [31]. In the work conducted by Brahmi et al., porous hydroxyapatite-metakaolin geopolymer (HAP-MK-GP) granules are used as a unique adsorbent for capturing bright green dye. Metakaolin, solid sodium metasilicate, and finely ground hydroxyapatite were combined in a high-shear granulator for the preparation. According to the findings, it is inferred that the HAP-MK-GP granules are a workable adsorbent for the removal of color from industrial effluents [45]. Researchers have focused on hydrogels, among other adsorbent materials, because of their abundance of pore architectures, wealthy functional groups, ease of synthesis and separation, and other benefits. As a result, using hydrogel adsorbent to remove organic dyes from wastewater is extremely promising. When hydrogels are formed, the insertion of inorganic compounds is frequently preferred due to the unique functionality and structural stability of hydrogels [29]. Preetha and Visalakshi fabricated an effective gel adsorbent material by grafting Karaya gum with N, N′-dimethyl acrylamide, which showed strong adsorption properties for cationic dyes. The gel is economical and environmentally friendly because it utilizes natural polysaccharides. The synthetic material exhibited swelling behavior that was pH-responsive [46] in this study conducted by Li and colleagues; graphene oxide (GO), carbon nanotubes (MWCNTs), diethylenetriamine pentaacetic acid (DTPA), acrylic acid (AA), N-vinyl imidazole (NVI), acryloyl chloride modified β-cyclodextrin (A-β-CD) as raw materials were first used to synthesize a novel and environmentally friendly pH-responsive carbon-based hydrogel adsorbent [29]. Joshiba et al. investigated the noxious Rhodamine B dye sequestration from an aqueous system using an ecologically friendly Fe₃O₄@SiO₂ nanocomposite that was immobilized with Pseudomonas fluorescens biomass in calcium alginate beads (MSAB). The research findings concluded that Rhodamine B was sequestered with a Langmuir monolayer adsorption capacity of about 229.6 mg g⁻¹ [47]. Song et al. investigated methyl orange sequestration using an amine-rich gel (ARG) adsorbent derived from peach gum polysaccharide. The results showed that the ARG showed superior adsorption capacity and selectivity for anionic dyes. ARG can uptake methyl orange up to 1949.5 mg g⁻¹ [48]. Li et al. created an effective sodium alginate gel adsorbent with a highly porous nature and adsorption capability. In the presence of cations, specifically Ca²⁺, sodium alginate underwent fast cross-linking, forming a very stable and porous gel bead adsorbent (CPS-Bead). During the gel bead manufacturing process, a 3D composite carbon nanomaterial consisting of graphene oxide and carbon nanotubes modified with poly-ethylenimine (PEI) was used as a modifier of sodium alginate to enhance the adsorbent’s adsorption ability further. The number of adsorption sites in adsorbents was increased by including 3D carbon materials [49]. Okara biochar was produced by pyrolysis and then further processed into the shape of beads to prevent biochar from leaking into the surrounding environment. The findings demonstrate that OBB has an adsorption percentage of up to 80% and may be readily isolated from water [30].

5.5 Photocatalyst-based adsorbents

Photocatalytic degradation employs a plentiful resource that can directly harness solar radiation. Adsorption with photocatalysis degradation techniques will be a successful strategy for managing azo-dye contaminants. The contaminants that have been adsorbed are continually photodegraded and can be eliminated because of the synergistic impact of adsorption and photodegradation. The binding sites on the materials will be liberated to allow the pollutants’ continuous adsorption as the pollutants degrade concurrently. Therefore, the limitations of conventional adsorbents can be addressed, and the treatment impact of contaminants can be improved by developing materials that have the synergistic effects of adsorption and photocatalytic degradation [50]. Zero-dimensional carbon nanomaterials have demonstrated great promise as photocatalysts in the treatment of water purification as adsorbents and ion detectors and in the remediation of hazardous contaminants due to their ultra-small size, exceptional properties, and antibacterial action [51]. Heterojunction hydrogen and sodium titanate nanosheets are fabricated using microwave-assisted hydrothermal, which is easy and economical to synthesize. The synthesized material was checked for the ability of the materials to decolorize Rhodamine B, and methylene blue dyes were investigated for photocatalytic effectiveness. With a half-life period of only 5 minutes for RhB discoloration and a high adsorption capacity of around 90% in 10 minutes in the MB medium, the photocatalyst exhibiting the highest efficiency was found [52]. Rhodamine B (RB) and malachite green (MG) removal from wastewater is achieved with a one-pot approach employing microwave heating to create a biochar-based magnetic photocatalyst (ZnFe-BC). According to the Langmuir model, the adsorption capacities of MG and RB are 576.73 mg g⁻¹ and 334.89 mg g⁻¹, respectively [32]. In the study conducted by Yang et al., ZnS/CuFe₂O₄/MXene (ZSCFOM) composite with ternary heterostructures was utilized in photodegrading the azo dyes. With the support of electrostatic interactions and hydrogen bonding, the azo dyes, such as direct brown M and direct black RN, have adsorption capacities of about 377 mg g⁻¹ and 390 mg g⁻¹, respectively [50]. Cheng et al. reported that the ZnFe-BC synthesized in a single pot method utilizing microwave heating is used in effectively sequestering to remove malachite green and Rhodamine from wastewater. The research findings demonstrated the potential of ZnFe-BC as a magnetic adsorbent-photocatalyst for extracting malachite green and Rhodamine from sewage. According to the Langmuir model, the maximum adsorption quantities of malachite green and Rhodamine are 576.73 mg g⁻¹ and 334.89 mg g⁻¹, respectively [32].

6. Conclusion and future perspectives

Water is the most fundamental element for all living things. In recent years, several countries have faced widespread contamination of natural water resources by organic and inorganic pollutants. It is estimated that over 800 million people worldwide still lack access to safe drinking water suitable for residential use. Textile effluent contains a broad spectrum of hazardous xenobiotics that are dangerous to the environment and public health. The daily release of millions of gallons of highly polluted wastewater by textile mills worldwide presents a significant issue with toxic dye-containing wastewater. Textile dyes are stable and can linger in the environment long if not adequately treated. However, because there is yet to be a single, practical method for effectively handling this kind of issue, treating wastewater that contains dyes is a significant challenge. Numerous established and novel methods of treating wastewater containing dyes have been documented. Adsorption is a cost-effective, efficient, simple, and eco-friendly technique compared to conventional treatment technologies. The selection of absorbent material is challenging due to various factors, including cost-effectiveness, operational simplicity, reusability, safety, and environmental impact. While low-cost carbon materials have shown efficiency in wastewater decolorization, their separation from water post-reaction poses difficulties and incurs high costs. Activated carbon is the most commonly used adsorbent for reducing wastewater pollution, but it faces significant constraints due to high regeneration and synthesis costs. Additionally, the pH sensitivity of these materials limits their effectiveness in reducing a wide variety of contaminants in water. Carbon nanotubes (CNTs) are another prominent nanomaterial for water purification, capable of removing both organic and inorganic pollutants as catalysts. However, their long-term stability is reduced due to oxidation during reactions, affecting reproducibility. Graphene-based catalysts show promise in meeting commercial demands for wastewater treatment due to their high surface area and efficient reduction of various contaminants, but they suffer from high synthesis costs. Coordination polymers have also shown significant pollutant removal from water by anchoring organic or inorganic ligands onto heavy metal complexes, though mass production remains a challenge. Harvesting atmospheric pure water presents a novel strategy for water purification. Photocatalytic dye degradation relies on factors such as light intensity and sensitization of dyes, with a need for adjustments in energy band gaps and cost reduction of photocatalysts for commercially applicable processes. Despite the laboratory-scale success, challenges in engineering, economics, and environmental factors must be addressed for widespread application. Understanding the photochemical stability under natural conditions is crucial for evaluating the effect of surface functions on dye degradation. It appears unrealistic to rely on a single globally applicable adsorbent as an end-of-pipe solution. Instead, a combination of suitable adsorbent techniques is imperative to develop technically and economically feasible options.