A comprehensive review on adsorptive removal of azo dyes using functional materials

Dyes are most frequently used in our daily life. These dyes make their way into the environment via industrial processes. All these industries pump their untreated effluents into the water, which causes water pollution. Between 2000 and 2050, the demand for fresh water is predicted to rise by 55% due to the growth of industrialization. However, the contamination of water by azo dyes further worsens the situations (Khan et al. 2020a, 2022a, 2022b). Azo dyes constitute the largest group of synthetic dyes and are widely used due to ease of use and a wide range of colors from yellow to black. They are highly light-resistant and used in multiple industries such as printing, textile, paint, and varnish to dye various products. Azo dyes can color most artificial, natural, and synthetic materials including rubber products, leather, and plastic. The majority of azo dyes are soluble in water, and their physical adsorption, absorption, or mechanical fixing is what gives different materials their color. For example, methyl orange and Congo red are widespread water-soluble azo dyes commonly used in various industries. The high desirability of these toward fibers makes them useful in textile industry. These need to be handled both before and after entering the water system since azo dyes released into water body reduce light penetration, impairing the performance of algae and growing aquatic plants. They also result in serious health issues to humans pertaining to the kidneys, brain, liver, respiratory system, excretory, reproductive system, and central nervous system (Khan et al. 2022a; Mahmoudi et al. 2023).

Numerous methods like reverse osmosis, biological degradation, flocculation, coagulation, ultrasonic techniques, electrochemical processes, flotation, photo-degradation, and adsorption have been used to remove azo dyes from water. Adsorption is a promising technique because it is simple, effective in pipeline systems, cost-effective, environmentally friendly, involves sludge-free cleaning operations, non-destructive, biocompatible, and straightforward (Ozeken et al. 2023). So far, significant research has been devoted to establishing various functional materials, including nanomaterials, porous materials, polymer materials, and biomaterials for the treatment of azo dyes with high selectivity, high sensitivity, and large removal capacity. Every functional material has distinct benefits and has the ability to significantly reduce dye pollution in real-world applications (Khan et al. 2020b; Kalia et al. 2023).

First, nanomaterials such as metal nanoprobes and carbon dots have attracted attention in functional systems development due to their unique magnetic, electronic, and optical properties. Nanomaterials provide new strategies for constructing functional materials that can rapidly, accurately, and conveniently remove azo dyes from water. Secondly, polymer materials possess favorable structures and abundant reactive groups, which make them the preferred option for constructing high-performance chemical adsorbers (Pandey et al. 2022a, 2022b). In addition, metal-organic frameworks, mesoporous silica, and covalent organic frameworks, among other porous materials, have emerged as popular systems for the removal of azo dyes due to their significant advantages, including large surface areas, extended p-conjugated frameworks, tunable functionality, and inherent porous structures. Apart from the above-mentioned three types of functional materials, biocompatible biomaterials have gained significant attention due to their multifunctionality and compatibility with living systems. Functional materials are currently the subject of significant advancements to meet the increasing demands of water pollution control. This review attempts to compile the most recent developments in the removal of azo dyes using various functional materials, such as nanomaterials, polymers, porous materials, and biomaterials, taking into account the benefits of these materials (Khan et al. 2020a; Isaev et al. 2023).

The main purpose and novelty of this review article are to explore the application of new, green, and low-cost adsorbents and biosorbents for the adsorption of azo dyes from drinking and industrial wastewater by batch and fixed-bed column systems. Furthermore, chemical and physical adsorption processes and parameters influencing adsorption capacity are given. Adsorption kinetics, thermodynamics, adsorption capacity, and isotherm models are collected under various conditions. Therefore, the current review paper offers a summary of the most thorough updated data on the adsorption of various azo dyes from aqueous solutions by a variety of adsorbents. Over the past two decades, attempts have also been made to utilize a variety of materials to analyze the adsorption information azo dyes.

Azo dyes are mixtures of phenol and diazotized amines that have numerous azo groups (–N = N–). These dyes are synthetic since they make up around half of all dyes that are created each year and display the majority of the color spectrum. The majority of dyes used in textiles are azo dyes, which are also widely used in the food, cosmetic, paper, and printing industries (Mossavi et al. 2022b; Isaev et al. 2023). In the 1980s, it was estimated that approximately 280,000 tons of fabric dyes are disposed of annually into industrial waste worldwide. Azo dyes are used in the textile industries and make up about 70% of the dyestuffs by weight. They are the most used materials of synthetic dyes discarded into the surrounding. Compared to other dyestuff, azo dyes have good fiber-fixation properties, showing up to 85% fixation; however, this explains why so much dye is released into the environment. A few azo dyes are Congo red, methyl orange, and acid red. They are easily found in industrial wastewater because of their bright colors, broad application, good solubility, and large-scale preparation. Regrettably, azo dyes are highly problematic synthetic dyes that are difficult to remove using conventional dye effluent treatment techniques (Isaev et al. 2023).

Workers who handle azo dyes may come into contact with them at work and absorb the dyes into their skin, adding to the issues caused by contaminated food and water. Similarly, all people are exposed to these dyes through oral and food chains if they enter the water system and contaminate it. The final point may be very concerning in places with inadequate waste treatment techniques or with lax sanitary regulations pertaining to the treatment of industrial waste (Khan et al. 2022b).

Numerous azo dyes are being studied to determine why they have toxic effects on a range of organisms, including humans and bacteria. Chung & Cerniglia (1992) investigated the cytotoxic, mutagenic, and genotoxic effects of disperse blue. They found that azo dye exhibits dosage-dependent effects, producing micronuclei, breaking down DNA, and increasing the apoptotic index in human hepatoma cells. Numerous azo dyes have induced mutagenic responses in Salmonella and mammalian species, indicating that their effectiveness depends on the position and nature of the aromatic group and the amino group. For example, 2-methoxy-4-aminazobenzene is a very poor mutagen, though in the same circumstances, 3-methoxy-4-aminoazobenzene is a powerful hepato-carcinogen in rats and a powerful mutagen in Salmonella typhimurium and Escherichia coli (Chung & Cerniglia 1992). Chung & Cerniglia (1992) published a review paper on numerous azo dyes investigated using the microsomal/Salmonella assay. According to reports, all of the nitro groups found in the azo dyes under investigation explained the mutagenic action. Without metabolic activation, acid Alizarin and acid yellow showed this effect. Both with and without metabolic activation, the nitro group–containing dyes orasol navy and basic red were exposed to mutagenic conditions. The results verified that Chrysodin was mutagenic when synthesized in rat liver, as demonstrated by the microsomal/Salmonella test of azo dyes with benzene amines (Brown et al. 1978; Fujita & Peisach 1978; Venturini & Tamaro 1979).

These dyes have mutagenic and carcinogenic properties due to the compound itself, aryl amine and free radical derivatives created during the reductive biotransformation of the azo connection, or even metabolites generated following oxidation via cytochrome P450 (Brown et al. 1978). Walker (1970) revealed that dog urine has sulfanilic acid, which showed for the first time that the reductive breaking of the azo groups could be the mechanism by which azo dyes are metabolized. An azo dye may be reduced to aromatic rings by anaerobic intestinal microflora and most likely by a mammalian azo reductase in the intestinal wall or liver after it has been taken orally. These kinds of bio transformations can occur in humans and in a wide variety of mammalian species. A small number of these aromatic amines are carcinogenic and can build up in food chains. For example, biphenylamines found in the environment, such as 4-biphenylamine and benzidine, pose a risk to ecosystems and human health in general (Van der Zee & Villaverde 2005).

Numerous industries are accountable for the release of dyes into the environment in inappropriate and uncontrolled conditions, which lead to serious environmental problems. The primary factor determining humanity's future is unquestionably the importance of treating and controlling environmental contamination. Wastewater from the textile industry that is improperly handled and released into the surrounding environment can seriously harm residential areas' natural water systems and land. Wastewater from textile dyeing has high concentrations of organic, inorganic, and brightly colored compounds, which can have a growing effect and increase the likelihood of entering the food chain. Reactive azo dyes cause wastewater that is highly contaminated and dark in color, which increases water turbidity. This is because of the dyes' higher chemical, oxygen, and biological demands as well as their coloration and salt content (Khan et al. 2022a, 2020b). Because colored water deprives marine organisms of light, which is necessary for their growth, the ecosystem is destroyed. However, colored river water should not be used for drinking, since this will increase the cost of treatment. Strict regulations have been implemented by the governments of several nations to prevent wastewater from entering water bodies. Industries and government representatives are looking for ways to reduce pollution in an appropriate manner. Therefore, research on wastewater treatment is crucial (Khan et al. 2022a, 2020b).

Depending on the material to be dyed, the range of azo dye concentrations in dyeing wastewater is 5–1,500 mg/L. Because azo dyes are resistant to biodegradation, they must first undergo a variety of physicochemical pretreatments. The main purpose of physicochemical and biological techniques for treating azo dye–containing wastewater is to eliminate the color of the solutions. Every color removal technique has benefits and drawbacks of its own, depending on the type of dye used, the wastewater's composition, concentration, toxicity, cost of chemicals used, equipment, and processing costs per unit of wastewater. The viability of each technique for treating azo dye effluents in wastewater is determined by these factors. It should be mentioned that eliminating azo dye with just one technique might not be adequate. As a result, researching the processes involved in azo dye removal has become crucial, and efforts are currently being made to find environmentally friendly ways to remove dye to lessen the anthropogenic load that wastewater containing azo dyes places on ecosystems (Khan et al. 2020a; Mossavi et al. 2022b). Previously, a variety of techniques, including chemical, physicochemical, and biological combined treatment methods, were tested in an effort to find an affordable and appropriate way to treat the wastewater from textile dyeing. Since every technique has benefits and drawbacks, there is not one that is accepted by everyone. Many of these conventional methods for treating wastewater containing dyes are not widely applied on an industrial scale in paper and textile industries because of their high cost and problems with sludge disposal. Because of the complex chemical structure of the dyes, there is no single approach that can provide adequate treatment (Khan et al. 2020a; Mossavi et al. 2022b).

Yasin et al. (2007) employed both activated carbon and activated carbon treated with KOH for the adsorptive removal of methyl orange. To maximize the amount of methyl orange removed from water, the experimental setup was optimized by examining a number of variables, including pH, adsorbent quantity, and time. It was determined that both materials exhibited a similar uptake of methyl orange, with an adsorption capacity of between 80 and 90%. The outcomes verified that 120 min was the required time to reach equilibrium for removal using treated activated carbon. The pH value was adjusted from 1.5 to 12, and the amount of methylene blue removed was 50 and 100 ppm by activated carbon and 30% by activated carbon treated with KOH. These results demonstrated that the removal process increased as the pH value increased. It was also observed that the removal of dyes increases as the dose of the adsorbent increases. The Langmuir adsorption isotherm was used to describe the data. The study showed that activated carbons can effectively remove methyl orange, especially when treated with KOH, which resulted in a significant reduction in both color and contact time.

Kir et al. (2023) described the biosynthesis and characterization of a novel ZnO/BaMg₂ nanocomposite with high adsorption for azo dye. Visible and ultraviolet light spectroscopies were used to characterize the synthesized nanoparticles (NPs), revealing absorption peaks at about 277 nm. FT-IR spectroscopy analyses were used to confirm the ZnO/BaMg₂ chemical bond configurations. ZnO NPs and synthesized ZnO/BaMg₂ were compared for their catalytic activity in the degradation of methyl orange and rose bengal dyes using visible and UV-Vis spectroscopy. In the ZnO/BaMg₂ composite, the decolorization ratios were 90.2 and 98.71% of rose bengal and methyl orange, respectively, whereas in the ZnO NPs, the ratios were 75.57 and 88.69% of methyl orange and rose bengal after 120 min, respectively.

Mossavi et al. (2022a) documented the adsorption of azo dyes from wastewater media by a renewable nanocomposite based on graphene sheets and ZnO/hydroxyapatite NPs. The maximum removal rates of 94.83% and 96.5% were possible under ideal adsorption conditions. The processes were chemisorption, endothermic, and homogeneous confirmed by the Langmuir isotherm model (R² = 0.982) and thermodynamic parameters. An as-prepared nanocomposite has a higher adsorption capacity (700 mg/g) than previously reported in a short period of time (4 min). It can also be applied for up to six cycles without experiencing a significant decline in performance, which makes it a promising option for treating dye-containing aqueous environments.

Chen et al. (2021) reported the use of a novel nanomaterial to remove azo dyes. The highest removal efficiencies of 74.22, 45.72, and 37.75% for Congo red, acid orange, and amino black, respectively, were obtained when the ideal adsorption rates were reached. For three dyes, the ideal adsorption temperature was 30 °C, and the adsorption equilibrium was attained in 150 min. For the three azo dyes, the adsorption kinetic model of Fe₃O₄–N–GO@SA matched the quasi-second-order reaction model, and the adsorption isotherm more closely resembled the Freundlich adsorption. Chemisorption was the primary mechanism for controlling the speed in the multilayer heterogeneous adsorption process, which was mediated by both physical and chemical adsorption.

Bensalah et al. (2020) reported the adsorption of azo dye on hydroxyapatite adsorbent transformed from industrial waste. The anionic azo dye Congo red was adsorbed onto both samples, considering the effects of the initial dye concentration (100–500 mg/L), the dosage of the adsorbent (0.5–30 g/L), the contact time (5–180 min), and the pH of the solution (2–12). According to kinetic studies, chemisorption is the rate-limiting step in the pseudo-second-order model of Congo red adsorption. It was discovered that the Freundlich isotherm was the best model to describe the adsorption of Congo red, meaning that adsorption is a multilayer process. At pH 5.5 and a dosage of 2 g/L, the maximum adsorption capacity of the synthesized B93-HAp adsorbent using Brij-93 surfactant was determined to be 139 mg/g. Studies using X-ray photoelectron spectroscopy and FT-IR spectroscopy identified two main mechanisms for Congo red adsorption, electrostatic attraction, and hydrogen bonding. According to the results of the multi-cycle sorption/desorption tests, waste-transformed adsorbent could be recycled and used up to six times.

Azha et al. (2019) reported azo dye adsorption on iron-modified composite adsorbent. The adsorption of acid red on an iron-modified composite adsorbent coating was successful, as evidenced by the photo-Fenton process, which allowed for up to 10 adsorption–regeneration cycles. A statistical physics model was utilized to comprehend the mechanism of dye adsorption. The adsorption capacities at various temperatures and the adsorption geometry of this dye were estimated using this model. The interaction between the dye molecule and the iron-modified composite adsorbent coating surface was described by this statistical physics model by taking an energetic consideration into account.

Wang et al. (2018) discussed the adsorption of anionic azo dyes from aqueous solution on flax shives modified with cationic gemini surfactants. The adsorption of acid orange, acid red, and acid black on gemini surfactant (MFS) was analyzed with kinetical and equilibrium analyses. The Langmuir model provided a good fit for the adsorption equilibrium data. The pseudo-second-order and Elovich models were followed during the adsorption process. Three anionic azo dyes were found to adsorb spontaneously and exothermically, according to thermodynamic investigations. The temperature, initial pH, adsorbent dosage, and ionic strength all had an impact on the adsorption of acidic azo dyes on adsorbent.

Konicki et al. (2017) discussed aqueous solution–based anionic azo dyes' adsorption onto graphene oxide. The effects of pH, temperature, and dye initial concentration on acid orange and direct red adsorption onto graphene oxide were examined. Model equations such as Langmuir, Freundlich, Temkin, Dubinin–Radushkevich, and Redlich–Peterson isotherms were used to analyze equilibrium data; Langmuir and Redlich–Peterson isotherm models were the most representative. The intraparticle diffusion model, the pseudo-second-order kinetic model, and the pseudo-first-order kinetic model were all used to analyze the kinetic adsorption data. A pseudo-second-order kinetic model provided a good fit for the adsorption kinetics. Thermodynamic parameters suggested that acid orange and direct red adsorbed spontaneously onto graphene oxide, whereas direct red adsorption onto graphene oxide was endothermic in nature and acid orange adsorption onto graphene oxide was exothermic.

Xue et al. (2023) documented anionic azo dye adsorption on porous hetero-structured MXene/biomass-activated carbon composites. The Langmuir model, pseudo-second-order kinetic model, and intraparticle diffusion model all agreed with the dyes' adsorption. The possible adsorption directions of the dye molecules on the adsorbent surface under various operating conditions were given by the simulation results. The adsorption energy computations demonstrated that azo dye adsorption was exothermic and adsorption of acid red and Congo red was heat-absorbing. Physical adsorption, hydrogen bonds, and electrostatic interactions work in concert to produce the adsorption mechanism. The results of kinetic models for the dyes’ adsorption at 298 K and actual pH are given in Table 2.

Sharma & Kaur (2011) reported using sugarcane bagasse to filter aqueous waste of erythrosin B and methylene blue. They carried out batch experiments to look into the effects of various parameters, such as temperature, time, adsorbent weight, pH, and primary dye concentration, on bagasse's ability to adsorb azo dyes. It was observed that as the pH and amount of adsorbent increase, the elimination efficiency improves. The rate of uptake first increased with time and subsequently remained almost constant. It was observed that as the amount of adsorbate increased, the amount adsorbed at equilibrium increased while the amount adsorbed decreased as the dye concentration increased. Freundlich and Langmuir isotherms provided a better fit for the experimental data.

dos Santos et al. (2013) revealed how methyl orange could be eliminated using organic clay made from sodium bentonite. To improve the methyl orange's adsorption onto the substrate, a number of factors were looked at, including temperature, the amount of adsorbent, primary concentrations, time, and pH. The procedure was carried out at room temperature. A very slight pH influence was observed during this study. Contact time was another variable that was examined. It was found that 60 min was required for the primary dye concentration (50–70 mg L⁻¹), whereas 120 and 300 min were required for the primary dye concentrations of 200 and 100 mg L⁻¹, respectively. When the initial dye amount was increased from 50 to 200 mg L⁻¹, the amount adsorbed ranged from 46 to 190 mg g⁻¹.

Mahesh et al. (2010) revealed how both chemically activated and raw bagasse effectively removed violet dyes through adsorption. The Freundlich adsorption isotherm was followed by this set of dyes and sorbent. A rise in dye concentration resulted in a decrease in dye adsorption. Bagasse that had not been chemically activated was more beneficial. The equilibrium was reached in 30–60 min. It was observed that elevated temperatures have an adverse effect on the rate of dye removal.

Consolin Filho et al. (2007) carried out the methylene blue adsorption study using formic lignin. Temperature, pH, and ionic strength were the variables that were measured to determine how methylene blue adsorption was affected on sugar bagasse. It was discovered that the ideal temperature range for the elimination of methylene blue was between 40 and 50 °C and it was observed that the ionic strength of the solutions and the adsorption data that fit well into Langmuir adsorption isotherms affected the uptake rate.

Raghuvanshi et al. (2004) published an equilibrium and adsorption study using raw and chemically activated bagasse as an adsorbent. Compared to raw bagasse, it was found that chemically treated bagasse exhibited superior and more efficient adsorption. Under various experimental settings, it was discovered that there was an 18% uptake difference between the two adsorbents. The Freundlich adsorption isotherm provided the best fit for the removal data.

Abechi et al. (2011) reported use of activated carbon to remove methylene blue. They looked at a variety of factors, such as temperature, time, and dye concentration. The pseudo-second-order expression is followed by the kinetic data. The Lagergren kinetic model provided an excellent explanation of elimination. At 25 °C, it was discovered that the optimum initial methylene blue concentration was 20 ppm, which was 99% removed.

Santhi & Manonmani (2009) documented methylene blue on Ricinus communis carbon. By adjusting the adsorbent dose, pH, shaking duration, dye initial concentration, and shaking time, the impact on the removal of methylene blue was tracked. It was observed that the adsorption capacity at pH 7 was 62.5 mg g⁻¹ at 25 °C. The Langmuir isotherm was followed by the adsorption data.

Ahmad et al. (2013) artificially produced activated carbon for the adsorptive removal of reactive black from water by chemically activating the waste bamboo. Many variables were assessed, including the pH of the solution, time, and concentration of the primary dye. The Freundlich adsorption isotherm demonstrates the equilibrium data. The pseudo-second-order kinetic equation was found to be followed by the process.

Santhi et al. (2009) examined the removal of dyes on Cucumis sativa, such as methyl orange, malachite green, and methylene blue. Many variables, such as the impact of pH, dye concentration, and adsorbent quantity were investigated during adsorption process. To guarantee the maximum adsorption of dyes, the impact of adsorbent quantity was investigated. The ideal pH for the adsorption of the previously mentioned dyes was discovered to be 6. In nearly every instance, equilibrium was reached in a single hour. They were observed to adhere to the Freundlich and Langmuir expressions. Furthermore, the increased amount of adsorbent greatly accelerated the rate of dye adsorption.

Oidde et al. (2009) examined how different factors, such as time and pH, affected the rice husk ability to adsorptively remove methylene blue. The results showed that pH 7 was the ideal pH and that a 40-min contact time was best for methylene blue adsorption. Rice husk ash had a favorable adsorbent dose of 2.5 and 20 mg L⁻¹, respectively. On experimental data, the Langmuir equation was found to fit the data quite well.

Ashiq (2010) investigated the dye uptake onto alumina under various circumstances, including pH, temperature, shaking periods, and adsorbent dosage. It was found that the ideal shaking duration was 40 min. For dyes such as methylene orange, bromophenol, methyl blue, methyl violet, and Congo red, the appropriate amount of adsorbent to use was found to be 2 g, while eriochrome black and phenol red required 3 g. This investigation found a good fit with the Langmuir adsorption isotherm. The greatest elimination happened at low pH levels.

Jaikumar et al. (2010) examined how acid dyes were adsorbed on adsorbent spent brewery grains made from waste products from the brewery industry. Temperature, pH, adsorbent quantity, and dye concentration are among the adsorption parameters that were investigated. It was discovered that while dye adsorption decreased at low temperatures and dye amounts, it increased with time and biosorbent dose.

Hong et al. (2009) investigated the impact of temperature on bentonite ability to remove methylene blue. It was observed that at higher temperatures, the removal of dyes increased. Numerous variables related to thermodynamics were computed. The adsorption process was endothermic, as indicated by the positive ΔH value, and it was spontaneous in nature. Entropy had a positive value, which was associated with the growing disorderliness that occurred when the dye was removed.

Abdelkader et al. (2016) used the sol–gel method to create SnO₂ nanomaterials, materials that have been prepared using different methods. A study on batch adsorption was conducted, and the effects of different parameters were examined. The outcomes verified SnO₂’s high crystallinity. It was thought that the primary adsorption mechanism was the hydrogen bond. The most suitable adsorption model was Langmuir's.

Dadvar et al. (2017) published a review article on removal of azo dyes on semiconductive TiO₂ with graphene oxide on the base of different polymeric membrane. There are numerous attached groups, such as carbonyl, carboxyl, and epoxy, to graphene oxide NPs. These groups that contain oxygen form a membrane with a high degree of hydrophilicity. The photocatalytic TiO₂ in this membrane is another excellent feature for the degradation of azo dyes in aqueous solutions. The authors discussed in-depth invitations on the TiO₂ and graphene oxide photocatalysts. In this review article, the photocatalytic degradation mechanism was fully explored. TiO₂ was found to be the best semiconductor because of its nontoxicity and chemical stability, and it has an excellent photocatalytic activity in the presence of UV irradiation

Zafar et al. (2019) made zinc oxide NPs by co-precipitation, which were then effectively used for adsorption of amaranth and methyl orange from aqueous systems. The adsorbent was characterized by XRD, SEM, FT-IR, and Brunauer–Emmett–Teller (BET) analysis. The 0.3 g ZnO NPs show maximum adsorption capacity with an initial dye concentration of 40 ppm at a pH of 6. The kinetic data follow pseudo-second-order kinetic models (R² > 0.99), while the adsorption of both dyes follows the Langmuir adsorption isotherm. The electrostatic force of attraction between the active sites of the adsorbent and dyes was found to be the actual mechanism. The maximum adsorption of trisodium was observed at a shaking speed of 125 rpm and an adsorbent dose of 0.3 g at pH 6, and the maximum uptake was recorded at 175 rpm and 0.3 g and a pH of 6. The trisodium and methyl orange adsorption processes onto ZnO NPs were confirmed by adsorption studies to fit the Langmuir model properly. The effect of pH was investigated at room temperature from 2 to 11, while keeping the other parameters constant. The results showed that the removal of amaranth increases up to 94% at pH 5 and remains constant up to pH 7, and the removal of methyl orange also increased up to pH 6. From intraparticle diffusion models, it was concluded that the first linear section is the external surface adsorption during which a large amount of methyl orange and amaranth were rapidly adsorbed by the exterior surface of the adsorbent. The second linear portion was a slow adsorption phase related to intraparticle diffusion of dye molecules within the pores of the adsorbent.

Large-scale applications are not possible for the data obtained from batch adsorption studies because they are typically restricted to small-scale investigations. Investigating fixed-bed columns is crucial to gathering the information required for dye removal with continuous flow. Column experiments are, therefore, necessary to provide data for large-scale direct applications (Khan et al. 2022b).

A review article on the adsorption of phenol from aqueous medium in fluidized- and packed-bed columns was studied by Girish & Ramachandra (2013). An in-depth analysis was derived from their exploration of the packed- and fluidized-bed columns used to treat phenol-containing wastewater. They noticed that more effort was needed to be put into the column design to improve the device efficacy for phenol removal.

Gayatri & Ahmaruzzaman (2010) described the use of inexpensive materials in the adsorption of azo dyes from aqueous media. Despite being an effective material, activated carbon's widespread use is restricted due to its high cost and significant loss during regeneration. The experimental data from this study closely matches the available adsorption models to correlate the break through time and break through curve for the adsorption process.

Ekpete et al. (2011) used fluted pumpkin and commercial activated carbon to remove chlorophenol from fixed beds. Data were contrasted with activated carbon that was sold commercially. The purpose of the fixed-bed work was to investigate the primary concentration (100–200 mg/L), flow rate (2–4 mL/min), and bed height (3–9 cm). Bed length increases the column bed capacity and exhaustion time. In addition, the bed ability decreases as the flow rate increases. They observed that at the lowest flow rate of 2 mL min⁻¹, the column functioned well.

El-Ashtoukhy et al. (2013) investigated the removal of dyes from petroleum waste by electrocoagulation using a fixed-bed electrochemical reactor. Dye adsorption under various conditions was investigated. As bed length increases, the saturation time rises from 700 to 3,500 minutes. As the starting dye concentration rises, the saturation time drops from 2,000 to 400 min. In a similar vein, the breakthrough curve steepens and the saturation time and dye adsorption decrease with increasing flow rate. The Thomas model was utilized to determine the maximum adsorption at different flow rates based on the adsorption data, and the Yoon–Nelson model was employed to pinpoint the precise saturation point. This confirmed that the rate of reaction increases as the flow rate increases.

Sorour et al. (2006) investigated phenol removal by adsorption using a packed-bed reactor model. They conducted the procedure to determine the dye concentration vs. different adsorption support depths and at different times, as well as the Langmuir coefficients (Xm and α). The model expressions, which combine particle and transport kinetics, were used to determine the relationships between the concentration and flow rate of sorbate as variables with column depth at any given time. During testing, phenol was used as the adsorbate and anthracite and granular active carbon as the adsorbents, over a range of dye concentrations (100–300 mg L⁻¹).

The study on elimination of phenol in fixed-bed adsorbents was conducted by Sulaymon et al. (2012). For the adsorptive removal of lead and phenol by activated carbon in a double system, they used fixed-bed adsorbents. To find the fixed-bed breakthrough curves for the double component system, a common model that takes into account both internal and external mass transfer resistances as well as axial dispersion with nonlinear multi-component models was utilized. The results verified that the adsorption could be explained by a common rate model.

The work on fixed-bed studies for the adsorptive elimination of nitrophenol from water by starch-based polymer was carried out by Garg et al. (2012). It was mentioned that the polymer column adsorption experiment showed that removal increased as the concentration increased. The useful materials for the adsorption of para-nitrophenol from water were summarized as the adsorbent. The maximum para-nitrophenol adsorption capacity of 42 at 100 mg L⁻¹ of 7.5 cm bed height and an effluent flow rate of 4 mL min ⁻¹ was observed. The equilibrium adsorption capability was found to increase with bed height and influent concentration and to decrease with flow rate.

Coated sand was applied to eliminate phenol from water in the batch study by Al-Obaidy (2013). They reported the effect of different factors for example pH, time, primary concentration, and adsorbent quantity on the adsorption capability of phenol.

The adsorptive elimination of phenol from water by zeolite was conducted by Ebrahim (2013). The purpose of the experiments was to determine how the column efficiency was affected by the amount of influent, the flow rate, the temperature, and the bed depth. It was observed that a satisfactory explanation of the adsorption is given by the uniform surface diffusion model, which incorporates surface diffusion resistance and film mass transfer. The breakthrough curve steepened as the amount of dye increased because the driving force was enhanced.

The review article published by Xu et al. (2013) examined that the common rate models were usually well-fitted in most cases to the experimental data; however, they are comparatively time consuming. Clark's equation was fit to explain column adsorption following the Freundlich adsorption isotherm.

Prigione et al. (2008) reported the removal of dyes from textile effluents by fungal biosorption. The fungal biomasses show strong sorption capacities, resulting in up to 94% decolorization and a 58% reduction in chemical oxygen demand. Following biosorption treatments, the Lemna minor toxicity test revealed a considerable reduction in toxicity, suggesting that the removal of dyes actually corresponds to a detoxification of the treated wastewater. Three simulated wastewaters, designed to mimic effluents released during cotton textile dyeing processes, were prepared using mixed industrial dyes at high concentrations. The industrial dyes utilized in these studies were chosen because they are common in the textile industry and provide a representative sample of many structurally significant commercial dye types. These results show that biosorption treatment by Caenorhabditis elegans, Rhizomucor pusillus, and Rhizopus stolonifer biomasses determine a substantial and fast day removal of the three tested effluents. Good yield of removal (65%) was also achieved with the more concentrated (5,000 ppm). CS was generally less effective compared to fungal biomasses. The three selected biomasses can be regarded as powerful candidates for biotechnology of dye biosorption due to their high ability to remove industrial mixed dyes.

Jahan et al. (2023) investigated the adsorption of azo dyes from textile effluents using activated carbon. This study focuses on the removal of methylene blue and reactive blue, which are cationic and anionic in nature, respectively, using two different types of activated carbon adsorbents prepared from fish scale and sawdust. Dye removal capacity was tested as a function of temperature, initial concentration of dye, contact time, adsorbent dosage, and pH during the process. The applicability of Freundlich and Langmuir isotherms was investigated. The morphology and surface configuration was identified by SEM images, and they show that adsorbents with a large surface area have high dye removal potential, whereas the changes in adsorption occur due to changes in surface area. In the current investigation, maximum removal of 95 and 87% was found for methylene blue and reactive blue-250, respectively, by sawdust, and 90% removal of methylene blue by using fish scale.

Kulasooriya et al. (2020) reported the removal of textile dyes from industrial effluents by burnt brick pieces. Equilibrium, kinetic, and isotherm studies were conducted. Brick clay was found to exhibit the highest adsorption efficiency among several adsorbents for the removal of dyes from aqueous solutions. The optimum values of experimental parameters, such as adsorbent dosage, temperature, settling time, shaking time, and initial solution pH were determined to be 4 g, 200 °C, 15 min, 15 min, and <10, respectively. The Langmuir model was best-fitted to the experimental data of the dyes leading to the highest removal ability under optimized conditions as 1,668, 3,334, and 2,500 mg kg⁻¹ for Dye 1, Dye 2, and Dye 3, respectively. All three dyes follow the pseudo-second-order model for their adsorption and confirm the initial rate of adsorption in the following order: Dye 1 ≈ Dye 2 (10,000 mg kg⁻¹ min⁻¹) > Dye 3 (3,333 mg kg⁻¹ min⁻¹); further, real dye effluent containing each of the above dyes having calculated initial concentrations of 42 mg L⁻¹ (Dye 1), 106 mg L⁻¹ (Dye 2), and 39 mg L⁻¹ (Dye 3) shows 88, 87, and 46% removal efficiencies at their λ_max values with the treatment of burnt brick clay particles under optimized circumstances. Desorption of the dyes from the surface indicates that desorption prefers basic conditions.

Fito et al. (2023) reported the adsorption of methylene blue from textile industrial effluents by activated carbon produced from Rumex abyssinicus plant. The adsorbent was activated by thermal and chemical methods, and then it was characterized by FT-IR, SEM, XRD, BET, and pH zero-point charge. The data were interpreted by kinetic and isotherm models. The experimental system was composed of four factors such as initial MB concentration (100, 150, and 200 mg L⁻¹), pH (3, 6, and 9), contact time (20, 40, and 60 min), and adsorbent dosage (20, 40, and 60 mg/100 mL). The absorbent and adsorbate interaction were assessed using response surface methodology. The characterization of activated carbon was found to have various functional groups (FT-IR), cracks with up and down morphologies (SEM), an amorphous structure (XRD), a point of zero charge (PZC) of 5, and a high surface area of 2,523 m² g⁻¹. The optimization of methylene blue dye removal was carried out using the response surface methodology coupled with the Box–Behnken approach. The maximum removal efficiency of 99.9% was recorded at optimum conditions of methylene blue concentration of 100 mg L⁻¹, pH of 9, contact time of 60 min, and the adsorbent dosage of 60 mg/100 ml. The Freundlich isotherm model was found to be the best fit with an experimental value at an R² of 0.99 and confirms that the adsorption process was multilayer and heterogeneous whereas the kinetics study confirmed that pseudo-second-order at an R² of 0.89.

Tamjidi et al. (2024) investigated a review study on industrial by-products as potentially economic and promising adsorbents for removing azo dyes from industrial effluents. This review paper highlights that using economic adsorbents in the adsorption process was highly cost-effective compared to traditional adsorbents. A review of kinetic, isothermal, and thermodynamic studies revealed that industrial wastes commonly followed Freundlich and Langmuir isotherm models and the pseudo-second-order kinetic model. Furthermore, the values obtained from thermodynamic factors demonstrated that the adsorption method was generally exothermic and spontaneous and is accompanied by increased irregularities. This study emphasizes how treating industrial dye with by-products from the manufacturing process can be a creative way to promote a pollution-free world.

Water pollution from colored effluent is one of the biggest environmental problems in the world. Numerous treatment technologies, including enzymatic decolorization, photocatalytic degradation, sonochemical degradation, electrochemical processes, sonophotocatalytic degradation, oxidation processes, ozonation, biological degradation, and adsorption have been developed to remove color-producing dyes from wastewater in response to the increasingly stringent environmental regulations. The adsorption process has been widely used to remove color from wastewater. To remove dyes from an aqueous environment, a variety of adsorbents are presented in this review article, including activated carbon, non-conventional low-cost materials, nanomaterials, composites, and nanocomposites. The mechanism and adsorption kinetics are thought to be dependent on the chemical makeup of the materials as well as a number of physicochemical experimental parameters, including solution pH, the initial concentration of the adsorbate, the dosage of the adsorbent, and system temperature. The literature review acknowledges that improving the adsorbent modification increases the removal efficiency when the adsorbent is inexpensive. However, very little work has been done, particularly to understand the mechanism of adsorption. The Langmuir and Freundlich adsorption isotherm models are frequently used to evaluate the adsorption capacity of various adsorbents; the kinetic data of dye adsorption typically follows the pseudo-second-order kinetic model; and the adsorption process was found to be endothermic and spontaneous in the majority of cases. The equilibrium adsorption isotherm, kinetic, and thermodynamic data of different adsorbents were also reviewed.

Even though much research is constantly published on azo dye adsorption using different adsorbents, a lot of the publications covered here focus on azo dye adsorption in a single system. There are a few articles on ternary and binary systems accessible. Therefore, rather than concentrating just on color removal, more study on azo dye adsorption from binary or ternary systems under varied working circumstances is needed. Also, many articles reviewed herein describe the azo dyes adsorption in a batch adsorption system. Only a few papers are available on industrial-scale water treatment studies have been investigated. Therefore, studies on industrial wastewater adsorption in the real world are required. Adsorption on a suitable virgin and modified type of adsorbents and biosorbents may be included at the end of dye treatment processes as a fail-safe mechanism. This will prevent the release of untreated dyes or toxic degradation intermediates into the environment. Desorption of contaminants from the adsorbent and treatment of resulting concentrated eluate will be another area for intensive research in the future. In the future, these integrated technologies will probably be validated more broadly and scaled up to create a process that is both profitable and efficient.

All relevant data are included in the paper or its Supplementary Information.

The authors declare there is no conflict.