This wastewater treatment method involves chemicals such as hydroxides, carbonates, and sulfides to react with impurities present in the solution to form precipitates that can settle easily. Chemicals introduced in the coagulation tank interact with molecules of dye to form easily removable precipitates (Mohan et al., 2004). The common procedures comprise: 1) the addition of appropriate chemicals into targeted pollutant-containing solutions, 2) precipitate will be formed after interaction of molecule of dye, 3) allow the suspension to settle (i.e. insoluble particles), and 4) separation of the sludge. The most common chemical precipitation method for dye removal is hydroxide precipitation (Ahmad et al., 2015). Vimonses et al. (2010) studied the mechanisms involved in the decolorization of Congo red dye. The removal was governed by combined physiochemical reactions of adsorption, ion-exchange, and precipitation. The precipitation contributed over 70% of total dye removal, followed by adsorption and ion-exchange (Vimonses et al., 2010). Sludge generation, the extra operational cost for sludge disposal, and high maintenance costs are major challenges in applying this process.

Complex formation in the treatment process has been considered as a highly effective method for utilization to improve economical adsorbents for wastewater treatment. The hydrophobic dye can be removed from the aqueous solution through inclusion complex formation (Saifi et al., 2021). The potential of β-cyclodextrin (β-CD) was studied to remove highly toxic oil orange SS (OOSS) dye by complexion. This process needs the synthesis of complexes that can interact with the particular contaminant. The complexion mechanism may occur simultaneously with the ion-exchange mechanism (Kubra et al., 2021). Hisada et al. (2019) reported the occurrence of simultaneous mechanisms in the removal of Basic Orange 2 (BO2) by poly-γ-glutamic acid (PGA) in different pH ranges. Adsorption and complexation/precipitation have occurred in the pH range of 4-5 and hydrogen-bonded organic ammonium carboxylate salt complexes were formed (Hisada et al., 2019). When the pH was greater than 6, complexation/precipitation was significantly suppressed due to the neutral nature of the dye and van der Waals adsorption and hydrophobic interaction’s domination. Moreover, 99.5% removal was reported at pH = 5.

The ion-exchange technique of wastewater treatment is categorized under widely used techniques. The process is based on the presence of ion-exchange resins that interact strongly with functional groups and charged dyes so that various dyes can be effectively removed from the aqueous solutions. The ion-exchange resins can be anion exchangers or cation exchangers (Ahmad et al., 2015). In the ion-exchange process, strong bonds are formed between solutes (dye molecules) and resins through an exchange of positive and negative ions. Conceptually, anionic (e.g., acid, mordant, reactive, direct, metal complexes) as well as cationic (basic) dyes, if treated with ion-exchange resins, should form complexes in the form of large flocs, able to be separated by filtration. Among different resins, quarternized cellulose and quarternized sugarcane bagasse were reported as viable ion-exchange resins capable of binding hydrolyzed reactive dyes (Singh and Arora, 2011). Several studies are reported in this area, for example, Acid orange 10 was removed from the environment by an Amberlite IRA 400 anion-exchange resin with up to 96.8% removal efficiency (Marin et al., 2019). Ion exchange dominates the mechanism of Congo red adsorption on the magnetic ion-exchange (MIEX) resin (Jia et al., 2020). In ion-exchange processes, there is no loss of adsorbent on regeneration, reclamation of solvent after use is possible and can remove soluble dyes effectively, but technology and organic solvent are expensive (Gnanadoss, 2013).

Membrane filtration is an alternative physical treatment that can be used to remove the color from wastewater. It has some advantages such as appreciable resistance to temperature, good resistance to adverse chemical environment, and excellent performance of color removal (Roy et al., 2018). Also worthy of note are disadvantages, among which the one linked to the high cost of disposal of the remaining concentrated residue following the separation, the high management costs and the inability to deal with high flow rates, and frequent clogging of membrane pores by the dye molecules (Collivignarelli et al., 2019). Reverse osmosis and nanofiltration membrane systems effectively removed dyes such as acid red, reactive black, and reactive blue dyes as an alternative treatment method of wastewater discharged from Iraqi textile mills (Abid et al., 2012). However, Zhou et al. (2019) pointed out the limitation of using membrane technologies for treating dye-containing wastewater due to the short service life of the membrane and the ease to be pollution (Zhou et al., 2019).

Flotation is a solid-liquid separation process, applicable to particles having a lower density than the solution, then the impurities floated on the top are removed by collectors. It involves the introduction of the transport medium (gas bubbles). The flotation technique was originally applicable in the mineral processing practices to extract minerals and particulate solids from water. The application of these techniques mainly comprises the treatment of heavy metals and dyes containing water and wastewater (Deliyanni et al., 2017; Gross et al., 2017). The efficiency of floatation techniques are dependent on the surface properties of different particles, pH value, temperature, current density, initial dye concentration, ionic strength, and stirring speed (El-Hosiny et al., 2017). This process can achieve effective treatment of dyes when combining with conventional coagulation, flotation, and electrochemistry in wastewater treatment (El-Hosiny et al., 2018; Hu et al., 2020; Tara et al., 2019). The flotation techniques can be divided into three processes: 1) dissolved air flotation (DAF) process, 2) ionic flotation, and 3) precipitate flotation. The generation of a bubble could be applied for techniques (Kyzas and Matis, 2018). The dissolved air flotation method has been commonly used in wastewater treatment (Al-Zoubi et al., 2015). Ion flotation removes surface-inactive ions from the solution by adding oppositely charged surfactants or collectors followed by introducing gas bubbles through the solutions (Deliyanni et al., 2017).

Recently, advanced oxidation processes (AOPs) have been effectively investigated in the removal of a variety of contaminants from water and wastewater. Advanced oxidation processes (AOPs) involve the generation of oxidizing species such as hydroxyl radicals and sulfate radicals in sufficient quantity to interact with the organic compounds of the medium (Wang and Wang, 2020; Duan et al., 2020). AOPs comprise all the catalytic and noncatalytic processes that take advantage of the high oxidizing capacity of the hydroxyl radical (OH), and they differ from each other in the way in which this radical is generated (Cuerda-Correa et al., 2020). These processes are mainly based on the in-situ generation of the hydroxyl radical that reacts rapidly with most organic compounds. Thus, such a radical is generated in sufficient quantity to interact with organic compounds (Babu et al., 2019; Mokif, 2019).

Photolysis, O₃-based processes (ozonation and catalytic ozonation), H₂O₂-based processes (Fenton, photo-Fenton), sonochemical oxidation, electrochemical oxidation, persulfate-based oxidation, wet air oxidation, and heterogeneous photocatalysis are the major advanced oxidation processes (Cuerda-Correa et al., 2020; Sharma and Feng, 2019; Miklos et al., 2018).

Several studies used advanced oxidation processes (AOPs) to treat different dyes in the wastewater stream. Zero-valent aluminum (ZVAl)-based AOPs were utilized to treat textile wastewater treatment. A color removal efficiency of 94.4% was achieved after the oxidation process (Khatri et al., 2018). Some of the other studies include: Fenton and photo-Fenton processes for the degradation of methylene blue with an efficiency of 65 and 83% respectively at pH of 3 and 30 min contact time (Gowtham and Pauline, 2021), combined sono-photo-electro-Fenton (SPEF) process for the removal of Acid Black 172 and Disperse Blue 56 with a removal efficiency of 97.4 and 95.5% respectively (Mahmoudi et al., 2021a), dark-Fenton process conditions for the removal of methylene blue and acid blue 29 with the removal efficiency of 82 and 95% respectively (Nasuha et al., 2021), electro-Fenton (EF) process and Ozone oxidation for the removal of Basic Blue 9 (BB9) dye with more than 97% removal efficiency (Bustos-Terrones et al., 2021), and electro-Fenton processes for the complete degradation of reactive red 195 (Elbatea et al., 2021) and more than 94% of neutral red (NR) dye (Ebratkhahan et al., 2021). Temperature, pH, the concentration of the oxidant, the initial concentration of dyes, current density, irradiation time, and reaction time affect the catalytic decomposition of dyes (Javaid and Qazi, 2019; Titchou et al., 2021; Mahmoudi et al., 2021b). Sludge production in the case of Fenton reagent, formation of toxic byproduct in the case of photocatalyst, and high cost due to the use of expensive reagents (for example, H₂O₂) and energy consumption (generation of O₃ or UV radiation) are the major drawback of AOPs for dye removal (Cuerda-Correa et al., 2020; Moosavi et al., 2020).

This treatment process is a comparatively novel technique, and usually does not require the consumption of chemicals (Roy and Saha, 2021). The anode continuously disintegrates as a result of electrical current flowing through it and produces in-situ cations, which acts as electro coagulants that attract the contaminants in the solution. The dyes attached to the electro coagulant either precipitate or flocculate, and thereby are separated. The principle of EC is based on three parameters: 1) the ionic charge, 2) particulate size, and 3) droplet (or vapor) spatial density. The generation of the charged agglomeration involves three stages: 1) coagulant formation by electrolytic disintegration of the anode, 2) destabilization of the suspension, and 3) aggregation to form the floating material (flocs). The EC process is successful in the removal of organic dyes, fine suspended particles, heavy metals, and oil and grease (or other heavier hydrocarbons) from diverse industrial effluents (Mondal et al., 2018). A report revealed that up to 94% of indigo dye could be decolorized at a pH of 7.5 using a continuous electrocoagulation process (Hendaoui et al., 2021). The limitation of this process includes the necessity of electrolytic apparatus, high-cost electricity, and less color removal for some dyes due to the higher flow rate.

Coagulation-flocculation is among the most widespread physio-chemical processes for the removal and distraction of coloring substances in wastewater. The removal of color by coagulation is strongly dependent on the structure of dye molecules (chromophores and auxochromes), so structural considerations of dyes are very important when choosing the most appropriate coagulant for dye removal (Mcyotto et al., 2021). The method can be separated into coagulation (destabilization of the colloidal suspensions by charge neutralization), flocculation, and sedimentation (physical processes that allow the smaller particles to be grouped so that they can be easily separated by sedimentation). The removal efficiency of color up to 99% has been reported for treating simulated water containing Brilliant green and Congo red (Collivignarelli et al., 2019). Moreover, many reports are accessible on dye removal by coagulation and flocculation from simulated and real wastewater. Magnesium chloride, lime, alum have been widely used coagulants in the process (Roy et al., 2018).

Adsorption is found to be a very effective, cheap, and commonly used method among all available dye removal techniques (Kandisa and Saibaba KV, 2016). Adsorption technique in wastewater treatment comprises removals of contaminants using commercial activated carbon and various available potentially low-cost adsorbents including biomass-based adsorbents. Several potential low-cost adsorbents have been studied in the application of wastewater treatment containing dyes (Figure 3) and widespread studies on dyes, their classification, and harmfulness in conjunction with various dye-containing wastewater treatment approaches, and adsorption characteristics of various nonconventional and cost-effective sustainable adsorbents were reported (Dawood and Sen, 2014; Mondal et al., 2018). The application of waste organic materials (e.g., compounds extracted from peels, leaves, barks) and microbial biomasses (fungus bio-sorbent, bacterial, and green algal biomasses) is gaining an increasing interest being effective, low-cost, and ecologically friendly sorbents. Moreover, newly discovered carbon nanomaterials (carbon nanotubes, graphene, and their derivatives) have been used for dye removal processes (Piaskowski et al., 2018).

Activated carbon (AC) is a commonly used adsorbent that refers to a group of carbon materials with high oscillation and high internal surface which are unique because of their remarkable interior area, pore structure, high adsorption capacity, surface reactivity, and low cost compared to inorganic adsorbents such as zeolite (Hasanzadeh et al., 2020). Special processes for the production of activated carbons in powdered, granular, and spherical forms have so far been developed. Activated carbon can be commercially produced from pyrolysis of carbon or carbon-containing plant materials, such as coal, bamboo wood, charcoal, kernels, or fruit shells, such as coconut shells, and is subsequently activated (Heidarinejad et al., 2020; Yeow et al., 2021; Hassan and Carr, 2021).

There are three main processes for the activation of carbon which are steam activation, activation with carbon dioxide, chemical activation. Among the above three methods, steam activation is the best environmentally and economically viable option, while chemical activation results in the highest surface area and porosity (Rahimian and Zarinabadi, 2020). In the chemical activation process, the chemically active agents (such as phosphoric acid, KOH, H₂SO₄) are utilized initially to prepare the precursor followed by heat treatment up to 450–700°C. The charcoal is then washed with water to remove the acid from the carbon and dry.

The adsorption method is attractive and preferable, and carbonaceous adsorbents fit that purpose due to their high dye-binding capacity (Hassan and Carr, 2021). The performance of some commercial activated carbons (CAC) was evaluated for the removal of the Reactive Black dye from synthetic wastewaters (Table 2) (Giannakoudakis et al., 2016). The activated carbon samples were Norit Darco 12 × 20 (DARCO), Norit R008 (R008), and Norit PK 1–3 (PK13) with the maximum theoretical adsorption capacities 348, 527, 394 mg/g respectively at 25°C for all carbon samples. Moreover, Shu et al. (2017) modified the commercial activated carbon by loading copper metal and used it in the removal of dyes containing Rhodamine B, MB, Amaranth, Congo red, and Eosin-Y from the wastewater; they found that the adsorption capacity is better in MB than in Rhodamine B (Shu et al., 2017). Although the removal of dye using CAC is efficient, its cost is high for industrial applications. And some studies reported that the adsorption capacities of some biomass-based activated carbons prepared in the laboratory scale showed superior adsorption capacity than CAC; however, limited information is available on the application of biomass-based activated carbon at the industrial level. Coal-derived activated carbon was supposed to be the most efficient type of activated carbon in adsorption. The efficiency of treatment is 99.8%, and it can treat various types of dyes (Yeow et al., 2021). The capacity of removal of Reactive Red 120 was compared using Spirulina platensis (microalgae) and CAC. The batch adsorption system revealed that the maximum adsorption capacities of microalgae and CAC to dye were 482.2 and 267.2 mg/g respectively at pH of 2 and 25°C (Cardoso et al., 2012). The percentage removal of the dye using AC was 93.6–97.7%, while it was 94.4–99.0% using microalgae. In another study, the adsorption capacity for Remazol Red on CAC has higher than activated carbon prepared from olive stone (Uǧurlu et al., 2008). The substitution of CAC by emerging alternatives needs a detailed study like a method of activation and other desirable properties of adsorbents.

Several factors that affect dye adsorption capacity include initial dye concentration, pH, temperature, the dosage of adsorbents and their type, contact time, etc. For certain dye adsorption, the optimum parameters of each sorbent vary significantly (Yadav et al., 2021), the optimization of each factor is helpful to large-scale industrial applications and the understanding of the adsorption mechanism. For an efficient treatment process, adsorbent should have enough mechanical strength and can tolerate various conditions of wastewater, high adsorption amount, and rapid adsorption rate, effective to a variety of dyes or have selectivity for a certain pollutant, and easy to be regenerated and reused (Zhou et al., 2019).

Dyes from the industrial wastewater effluents are effectively removed by using activated carbon; however, it has some constraints such as capital cost, energy consumption, and loss of its ability after sorption-desorption cycles. To have effective removal of dyes, several economically available nonconventional adsorbents are required (Kandisa and Saibaba KV, 2016). Therefore, bioadsorbents obtained from fungal, bacterial, algal, and agricultural, or forest biomass are promising environmentally friendly adsorbents utilized for dye and heavy metal removal.

Fungal biomasses are composed of sugars, proteins, and lipids, and different functional groups (alcohols, carboxyl, and alkanes) which provide it certain properties and applications as a biosorption in treatments of wastewater (Isaza-Pérez et al., 2020; Ahmed and Ebrahim, 2020). Biotreatment of dye-containing wastewater effluent by fungal cell was provided a cost-effective, easily applicable, eco-friendly (Salem et al., 2019; (Argumedo-Delira et al., 2021), and absence of nutrient needs (Ghany et al., 2019). Different species of fungus have been used as an effective candidate for the removal of a variety of dyes from effluents such as Trichoderma sp. (Argumedo-Delira et al., 2021), Sarocladium sp. (Nouri et al., 2021), growing Rhizopus arrhizus (Gül, 2013), different varieties of white-rot fungi (Gnanadoss, 2013; Dai et al., 2018; Forgacs et al., 2004), Aspergillus niger (Salem et al., 2019; Asses et al., 2018), Aspergillus flavus (Mahmooda Takey, 2014), Lentinus concinnus (Bayramoglu and Yilmaz, 2018), Penicillium simplicissimum (Chen et al., 2020), Pycnoporus cinnabarinus, Pleurotus ostreatus and Trametes hirsute (Pecková et al., 2020), macro fungi spent mushroom waste (SMW) (Agaricus bisporus) (Ahmed and Ebrahim, 2020), Trichoderma harzianum (Karthik et al., 2020), and many others.

Bayramoglu and Yilmaz (2018) studied the removal of azo dye (Reactive Yellow 86 dye (RY-86)) using free (Lentinus concinnus) and immobilized fungal biomasses with polyvinyl alcohol/polyethylene oxide hydrogels (PVA/PEO) along with isotherms, kinetics, and thermodynamic studies (Bayramoglu and Yilmaz, 2018). They reported, maximum RY-86 dye adsorption for the free fungal and composite fungal biomasses was 190.2 and 87.6, respectively, using 200 mg/L initial dye concentration at pH 5.0 with 2.0 h contact time (Table 3). Moreover, the equilibrium data were well described with the Freundlich and Temkin isotherm models. The adsorption of RY-86 dye was fitted best by the pseudo-second-order kinetic model. The adsorption performance of fungal biomass is dependent on the chemical structure and surface charge of several dyes associated with the electrostatic interaction. From FTIR studies, it was observed that the carboxylate, hydroxyl, and amine groups were involved in the adsorption of the RY-86 dye. Similarly, Bayramoglu and Arica (2018) reported that the amine, carboxyl, and hydroxyl groups are involved in the adsorption of the Congo Red by iminodiacetic acid and triethylenetetramine modified fungal biomasses of Funalia trogii (Bayramoglu and Arica, 2018).

Fungal biomass (Trichoderma harzianum) was used to remove Reactive Red-3 (RR3) from an aqueous solution (Table 3). The biosorption mechanism revealed that the fitted kinetics (pseudo-second-order) suggested the adsorption mode of chemisorption in which the adsorbate (RR3 dye) is monopolized over the external layer of the biomass by electrostatic interaction (Karthik et al., 2020). After all the vacant spots are monopolized, the RR3 dye starts propagating into the adsorbent opening for further interactions.

Table 3 summarizes the application of fungal immobilization to dye removal. Most species of fungi potentially achieved more than 90% of a specific type of dye except Pleurotus ostreatus, Aspergillus flavus, and Diaporthe schini which had a removal efficiency of 53%, 53.62, and 87%, respectively. This variation might be due to the poor monitoring of experimental conditions, the processing of bioadsorbents, and the availability of functional groups that have a limited binding capacity with the dye molecule.

It has been observed that (Table 3) in low pH solutions, the removal rate of anionic dyes is increased while the removal rate of cationic dyes is lower. Conversely, a high pH solution increases the removal of the cationic dyes and causes a low removal percentage for anionic dyes (Ahmed and Ebrahim, 2020). The point of zero charges (pH_pzc) is an essential parameter for understanding the mechanism and the favorability of the adsorption process. The pH_pzc value indicates the type of active sites and the adsorption ability of adsorbents. When pH is greater than pH_pzc, it is favorable for cationic dye adsorption due to the existence of functional groups (OH ⁻ , COO ⁻ ), whereas anionic dye adsorption is favorable at pH less than pH_pzc because the surfaces of adsorbents become positively charged (Zhou et al., 2019). In general, the use of fungal biomass is a promising substitute to the current technologies for dye decoloration and adsorption. In addition to optimizing the environmental parameters, considering the genotype and the preparation of the biomass are important for effective dye adsorption performance.

Algae are considered one of the sources of the most favorable types of bio-sorbents because they have high biosorption capacity and are easily available in large amounts (Singh and Singh, 2017; Azam et al., 2020). The biosorption depends on the composition and structure of the algal cell wall, which is made up of several polysaccharides: xylan, mannan, alginic acid, chitin. These components, along with the proteins present, can provide acid-binding sites such as amino, amine, hydroxyl, imidazole, phosphate, and sulfate groups (Singh et al., 2018). Encapsulation and surface modification can be used as pretreatment methods to improve the adsorption capacity of algae. Citric acid-functionalized brown algae were reported to improve the adsorption capacity for textile dye (crystal violet) removal in aqueous solutions (Table 4). An optimum monolayer uptake capacity of 279.14 mg/g was reported for the modified algal adsorbent (Essekri et al., 2021). Moreover, electrostatic attractions and π-π interactions were responsible for the dye adsorption toward the algal biomass surface. Magnetically responsive brown algae (Sargassum horneri) was utilized for the adsorption of five water-soluble dyes. The magnetic modification leads to rapid and selective separation through an external magnetic field by using microwave-synthesized iron oxide nano and microparticles (Angelova et al., 2016). The sorbent had maximum sorption capacity for acridine orange (193.8 mg/g) but lower for malachite green (110.4 mg/g) at 2 h contact time.

Table 4 revealed that Spirulina and Chlorella species are common types of microalgae studied and achieved up to 99% removal efficiency. Similarly, Sargassum species of macro algae are commonly studied and can remove more than 90% of dyes. MB is among the common dyes removed by various dye species. The type of dye to be removed significantly depends on the pH of the solution. Anionic dyes are removed in acidic conditions and cationic dyes (e.g. MB) in alkaline conditions. This is due to the involvement of H ⁺ ions in the biomass-pollutant interaction process. If the surface of the adsorbent possesses a positive charge, the hydrogen ions (H⁺) may compete productively with the cations present in the dye solution, resulting in a decrease in the amount of adsorbed dye. The carboxyl groups have a negative charge at a higher pH, which results in electrostatic binding of the cationic dyes. Moreover, the biosorption performance is strongly influenced by other parameters including the processing of biomass into adsorbent, initial contaminant concentration, biomass dosage, temperature, and contact time.

The role of bacteria in bioremediation applications can be based on the adsorption of contaminants from aqueous media (Sarvajith et al., 2018) through different mechanisms using dead biomasses (Roy et al., 2018). Their small size, ubiquity, and capability to grow under varying environmental conditions make them good adsorbents (Singh et al., 2018). Bacterial species have been proved to be highly effective to treat wastewater containing reactive dyes by adsorption under optimized environmental conditions (San Keskin et al., 2015; Mishra and Maiti, 2018; Zabłocka-Godlewska et al., 2018). The bacterial dye decolorization rates vary with the type of bacteria, the reactivity of dye, and operational parameters such as temperature, pH, co-substrate, electron donor, and dissolved oxygen concentration. As shown in Table 5 the literature revealed that Pseudomonas spp. are relatively more successful to treat reactive dyes-containing wastewater (Mishra and Maiti, 2018; Eslami et al., 2016). Treatment of textile dyes using extremophiles can effectively be achieved. For example, 87% of Reactive Black and 85% of Reactive Red dyes were removed by haloalkaliphilic bacteria grown from textile wastewater (Seyedi et al., 2020).

Kim et al. (2015) isolated the bacterial strains (Bacillus catenulatus JB-022) from polluted ponds and soils for the biosorption of cationic dye. The isolate achieved 58% of cationic basic blue dye removal at the initial concentration of 2000 mg/L. The maximum sorption capacity for basic blue dye was found to be 139.74 mg/g from the Langmuir adsorption isotherm. The presence of carboxyl and phosphonate groups at the adsorbent surfaces can act as a potential surface functional groups capable of binding to cationic pollutants (Kim et al., 2015). Several functional groups on the Penaeus indicus biomass surface were likely involved in Acid Blue 25 dye binding, but the amino groups, alpha-chitin were by far the most important ones (Kousha et al., 2015). Bacillus subtilis had been immobilized in calcium alginate bead and utilized in batch and continuous reactor for the removal of MB. The kinetic study in the batch and continuous contactor gave greater than 90% removal (Upendar et al., 2016).

Different biocomposites have been reported for improved performance of dye removal (Chen et al., 2019; Hu et al., 2019). A bacterial strain (Clavibacter michiganensis) was studied for the removal of a reactive dye by joining with electrospun polycaprolactone and polylactic acid nanofibrous polymeric webs (Sarioglu et al., 2017b). The biocomposite was a promising material for the treatment of textile dyes with reusable and improvable properties. Moreover, electrospun nanofibrous-encapsulated bacterial cells were utilized for MB dye treatment (Sarioglu et al., 2017a). In another study, a design of a paper-like multifunctional purifier that has a biomass-based structure by embedding polyethyleneimine derived from ammonium compounds onto the substrate of bacterial cellulose was reported with higher adsorption capacities toward anionic dyes (including methyl orange, congo red, and methyl red) (Hu et al., 2019). Kang et al. (2020) used polysulfone and Escherichia coil biomass to prepare polysulfone Escherichia coil biomass composite fiber (PSBF) and they combined poly(acrylic acid) (PAA) at the surface of the composite. The adsorption performance of the prepared material on MB was studied by considering several parameters; they found an adsorption capacity of 225.2 mg/g at a pH of 7 fitted by Langmuir isotherm (Kang et al., 2020). Moreover, the regeneration ability of the composite was tested by desorption using an HCl-acidified solution at a pH of 2 as an eluent and the material can be reused at least 3 times.

Table 4 revealed that Langmuir isotherm was fitted by most biosorption studies, indicating that the monolayer coverage adsorption was the dominant process. Similarly, the pseudo-2^nd -order kinetic model showed the best model fit. The adsorption involves valency forces through the sharing or exchange of electrons between the adsorbent and dye molecules by covalent forces and ion exchange (Mohd Nasir et al., 2021). At acidic pH maximum biosorption has been occurred in the case of metallic cations due to the involvement of the carboxyl functional group present in the bacterial cell walls, which is responsible for binding metal ions via different mechanisms. The fidelity of biosorption depends not only on the kind of ions but also on the type of bacterial species due to variation in their cell wall compositions (Roy et al., 2018). The presence of different anionic functional groups, gram-negative bacteria containing phospholipids, peptidoglycan, and lipopolysaccharides, and in the gram-positive bacteria including peptidoglycan, teichoic acids were responsible for the metal-binding capacity of the bacterial cell wall. Extracellular polysaccharides have also the ability to bind ions.

Yeast is a single-celled organism, a member of the kingdom of fungi that have many favorable applications in the treatment of dye-containing solutions due to their high capacity to adsorb and accumulate pollutants, fast growth, faster decolorization than filamentous fungi, and the ability to survive unfavorable environments (Sen et al., 2016). The yeast surface consists of several functional groups including carboxyl hydroxide, polymer, amino, and phosphate, etc. that affect the desired pH of the solution to be analyzed (Singh et al., 2020).

Several studies revealed the bio-adsorption of different dyes using yeast biomass. Different species of yeast that could potentially remove different types of dyes are summarized in Table 6. Several factors including pH of the solution, the concentration of pollutants, and mass of yeast, temperature contact time influence the bio-sorption method (Al-Najar et al., 2021). For example, in the optimal condition of pH = 2, biomass dose = 2.0 g, and temperature = 30°C, complete decolorization of indigo dye was achieved by yeast (Diutina rugosa) biomass (Bankole et al., 2017). NADH-DCIP reductase and lignin peroxidase played a major role in the asymmetric cleavage, initial reduction, and deamination of indigo dye. Ramazole blue was also decolorized effectively by Baker’s yeast cells, but pH of the solution, bio-sorbent dosage, initial dye concentration, and contact times significantly affect the removal process (Mahmoud, 2016; Dil et al., 2017).

Ruscasso et al. (2021) investigated the biosorption of two reactive dyes (Reactive Blue 19 (RB 19) and Reactive Red 141 (RR 141)) using the Antarctic yeast (Debaryomyces hansenii F39A) with considering several factors. They found that 90% of RR 141 and 50% of RB 19 was adsorbed at pH 6.0, initial dye concentration of 100 mg/L, and 2 g/L biomass dose, but when the dosage of biomass was increased to 6 g/L the adsorption of RB 19 increased to 90%. Moreover, the adsorption process followed pseudo-second-order kinetics for each dye system and the Langmuir isotherm was the best fitted model (Ruscasso et al., 2021).

Semião et al. (2020) studied the adsorption performance of residual yeast and diatomaceous earth for the removal of Reactive Blue 160 dye (RB 160). They found that the two biosorbents had a dye removal capacity of 8.66 mg/g and 7.96 mg/g respectively at a pH of 2 (Semião et al., 2020). The positive charge in the biomass functional group is responsible for the chemical interaction with the anionic dye. Biosorption of yeast isotherm data was better fitted by the Freundlich isotherm model, but for diatomaceous earth, Langmuir isotherm was fitted indicating that the active sites of residual yeasts are more heterogeneous. In another research, the brewer’s yeast biomass was assessed as a biosorbent for removal of basic dyes (safranin O (SO), MB, and malachite green (MG)) from aqueous solutions within 1 h (Table 6). The adsorption kinetics of MB and MG followed pseudo-second-order, while SO followed pseudo-first-order. The maximum monolayer adsorption capacities of BYB for MB, MG, and SO were 212.05, 212.05, and 76.77 mg/g, respectively. Hydroxyl group, cyano group, and other functional groups are the major sources of adsorption by yeast (Lin et al., 2019). Table 6 revealed that most studies showed the best fit for Langmuir isotherm and pseudo-second-order kinetic models, suggesting the dominance of monolayer chemo-adsorption except some yeast species in which the Freundlich and Temkin isotherm models showed the best fit model.

The utilization of agricultural wastes and plants for the adsorptions of organic pollutants and inorganic pollutants is treated as an alternative to conventional wastewater treatment methods (Mo et al., 2018). Several studies have been conducted in the use of nonconventional, naturally occurring, low-cost biomass as adsorbents such as seeds, straw, fruit peels, leaves, bark, sawdust, ash, sludge, and others that are easily available. Different studies show that the dye adsorption abilities of those biomasses mainly depend on the types of dyes as well as the techniques of processing (Deniz and Kepekci, 2017); (Kharat, 2015).

Rice husk, sawdust, and bark are commonly used abundant, cheap, and low-value by-products for the preparation of adsorbent materials that are used in the removal of dye (Chuah et al., 2005); (Amirza et al., 2017). Sewu et al. (2017) prepared biochar from Korean cabbage (KC), rice straw, and wood chip and can be used as an alternative to activated carbon to remove congo red and CV (Sewu et al., 2017), but KC had better absorption performance than others. Potato peel biochar was considered a promising adsorbent for removing Cibacron Blue dye. The sorption ability of this waste follows the order: calcined > activated > native materials (Bouhadjra et al., 2021).

Activated carbon provided from Persea Americana was suggested for basic yellow dye removal from wastewater with 98% removal. This activated carbon can substitute the commercially available adsorbents (Regti et al., 2017). Pathania et al. (2013) synthesized Ficus carica bast (FCBAC) AC that showed an encouraging adsorption capacity for MB with a low amount of operating parameters such as an initial dye concentration of 0.5 g/L, sorbent dosage (5 g/L), contact time 1.3 h, and temperature (30°C) at a pH of 7.8 (Pathania et al., 2013). The isotherm models confirmed that the sorption is heterogeneous and occurred through physicochemical interactions.

Watermelon peels were used in the preparation of modified biochar with ozone initially and further modification with ammonium hydroxide and triethylenetetramine for the removal of Acid Yellow 11 (AY11) dye (El Nemr et al., 2020). The sorption study revealed that the capacity of prepared watermelon biochars was ranged between 76.94 and 462.18 mg/g at pH of 1.0, contact time of 3 h, and room temperature, and more than 96% removal of AY11 was achieved. Moreover, the isotherm model fitted Freundlich and Langmuir isotherm and the kinetics followed the pseudo-second-order model. Electrostatic interaction was responsible for the primary mechanism controlling the adsorption of AY11 dye onto the biochars. In another study, pea (Pisum sativum) peels were used to prepare biochar for the removal of Acid Orange 7 (AO7) dye with a similar modification process stated in the utilization of watermelon peels for AY11 removal. The adsorption capacity of the modified biochar was significantly enhanced from 78.18 to 523.12 mg/g at equilibrium. Up to 98% of AO7 was removed by triethylenetetramine-modified biochar, while 96% removal was achieved using unmodified biochar (El-Nemr et al., 2020).

Mandarin (Citrus reticulata) peel (MP) was utilized in the removal of SO from aqueous solution with the maximum adsorption capacity of 464 mg/g, 0.4 g/L adsorbent dose, 2 h contact time, and 84.75% removal. Hydrogen bonds, π-interactions, and electrostatic interactions were the possible adsorptive mechanisms of SO removal (Januário et al., 2021). Moreover, the desorption study revealed that the adsorption capacity was maintained more than 50% after four cycles.

Agricultural and forestry wastes are generally rich in cellulose, hemicellulose, and lignin. Its surface comprises several active groups including hydroxyl, carboxyl, amino, carboxyl, methyl, and so on (Zhou et al., 2019). These functional groups can adsorb dyes through mechanisms of complexation, hydrogen bonding, ion exchange, etc (Dai et al., 2018). Various agricultural and forest waste materials are summarized in Table 7 along with the biosorption capacity and the required reagents to activate them. Different acids have been used to activate the bio-sorbents to enhance the binding sites, the chemistry of the aqueous solution, to improve porosity and specific surface area. For example, phosphoric acid stimulates bond cleavage in agricultural waste biomass to increase the carbon yield (Bello et al., 2019). Phosphoric acid promoted dye biosorption by grafting phosphate functions onto the biomass and enhancing the acid functions involved in dye fixation (Benabbas et al., 2021). Moreover, H₂SO_4, NaOH, and KOH are the common activating agents during the preparation of agricultural waste-based bioadsorbents (Table 7).

Several environmental conditions including adsorbent dosage, temperature, contact time, pH of the solution, the particle size of the plant-based adsorbent, agitation, and initial dye concentration significantly affect the biosorption process. The pH of the solution affects both the chemistry of the aqueous solution and the binding sites present on the surface of adsorbents (Kumar et al., 2021).

Several authors reported that the cost of bioadsorbents prepared from microbes and forest and agricultural waste is low as compared with conventional treatment technologies, although they did not incorporate cost-benefit analysis in their study. The advantages of biosorption are related to the nature of adsorbent material used in the adsorption processes of dyes. To be economical, the availability of a large quantity of adsorbent material and the ease of preparation or processing, and the requirements of activation are significantly important along with the concepts of green chemistry (Bulgariu et al., 2019). The cost of forest wastes is only associated with the transport cost from the storage place to the site where they will be utilized (Adegoke and Bello, 2015). Some authors argue that the naming of “low-cost adsorbents” actually only indicates their original prices so that their local availability, transportation, treatment process, and both recycle and lifetime issues, and the processes of regeneration and treatment should be critically investigated (Zhou et al., 2019). Unfortunately, most studies on biomass-based adsorption have been conducted on the laboratory scale using simulated wastewater, so that the cost analysis of the biomass-based adsorbents is limited, particularly for microbial biomass.

A comprehensive cost analysis was carried out for the biochars made from agro-waste (Coconut shell, Groundnut Shell, and Rice husk) for their adsorption performance of the Basic Red 09 dye removal from wastewater. It was estimated that the cost of 1 g of adsorbent is ₹4.54, ₹0.91, and ₹0.97 for Coconut shell, Groundnut shell, and Rice husk, respectively, by considering operational costs including manufacturing costs, maintenance costs, feedstock costs, transportation costs, labor costs, and the distribution costs (Praveen et al., 2021). Groundnut shell-based biochar had maximum adsorption capacity (46.3 mg/g) as well as the least cost (₹0.91) per unit gram of Basic Red 09 dye removal. Bello et al. (2019) prepared phosphoric acid-functionalized locust bean pod AC for the removal of RhB dye and studied the preliminary cost analysis of this adsorbent. They concluded that the AC prepared from this plant material was approximately six times cheaper than conventional AC. Phosphoric acid and deionized water contribute most of the cost (Bello et al., 2019).

After the adsorption process is accomplished, the desorption process is of great importance to regenerate and reuse those used adsorbents and to reduce the generation of waste as well as to keep the process price down (Zhou et al., 2019). The common desorption methods include thermal treatment, acid (H₂SO₄, HNO₃, HCl, H₃PO₄, NaOH) treatment, organic solvents (methanol and ethanol), biological methods, and vacuum methods (Hassan and Carr, 2021). In the case of the solvent desorption process, the eluent (dye-saturated substrate) is mixed with the appropriate solvent to extract the dye and then separate the adsorbent by filtration. The dye-solvent mixture is dried to evaporate the solvent (Zhou et al., 2019).

The regeneration process should be cost-effective and should have potentially low energy consumption. Some authors stated that the recovery step of powder-activated carbons is very costly, time-consuming, and not very efficient so that they are not suitable for industrial applications (Moosavi et al., 2020), and more particularly reusability is almost impossible for low-cost products (Saha et al., 2020). However, some studies report the regeneration of biomass-based adsorbents (Essekri et al., 2021; Elgarahy et al., 2019; Kang et al., 2020; Pathania et al., 2013; Yang et al., 2021). Saha et al. (2020) studied Congo red loaded java citronella (Cymbopogon winterianus) based bio-adsorbent and commercial AC regeneration using ethanol. They found 90.3 and 83.7% desorption for biomass and commercial AC, respectively, from dye-loaded adsorbents (Saha et al., 2020). Moreover, the adsorbents were effective up to two adsorption-desorption cycles. In another study, the adsorption capacity of biosorbent prepared from mandarin (Citrus reticulata) peels was maintained more than 50% after four cycles (from 77.90 to 41.55 mg/g) during the removal of Safranin orange (SO) dye. HCl was the best chamical that had a good performance during desorption (Januário et al., 2021). Dahiru et al. (2018) reported the efficiency of the banana peel-based adsorbent for the removal of Malachite green (MG) and Methylene blue (MB) decreased from 100 to 64% and 76% after 5 consecutive usages respectively (Dahiru et al., 2018).

Various adsorbents can be prepared, modified, and applied to different contaminants in the environment. The preparation process for the activation and modification of adsorbents determines their characteristics. In general, the most critical characteristics of a good adsorbent are surface area, void active sites, and reproducibility in the activation processes (Leng et al., 2021). Among all the adsorbents, the biomass-derived adsorbent is preferred comparatively having the highest surface area, regardless of the activation techniques, depending on source types resulted in increasing the adsorption of the contaminants.

Among the adsorbents, bioadsorbents are the most popular for the adsorption of dyes, contaminants in general, from wastewater due to their versatility, cost-effectiveness. Also, their byproducts are not hazardous, and in addition to easy operation. Moreover, the biomass-derived adsorbents are commonly cellulose-based structures and can produce a good porous carbon that can adsorb various types of contaminants in the natural environment. One of the important features for the use of biomass-derived adsorbent for removal of dyes is that it can be used on a large scale because of its huge availability. Also, biomass-derived adsorbents have high affinity/binding capacity for various types of contaminants, and also have good regeneration capacities. In addition, their good surface chemistry such as pore distribution, specific surface area, and functional groups helps for the removal of dyes in the wastewater (Sherugar et al., 2022).

However, environmental factors such as temperature, pH, ionic strength, nutrient availability, photodegradation through natural light, which can arise in some seasonal and temporal variations, may affect structural characteristics and chemical constituents of the biomass. Thus, this can strongly affect the real adsorption capacity of biomass-based adsorbents. As a result, the biomass-derived adsorbents should be well characterized to understand the adsorption mechanisms for certain contaminants. In addition, studies should be conducted in a multi-pollutant system to meet the requirements of real wastewater treatment containing diversified contaminants that can assure stability resulted in the suitability of biomass for the practical biosorption process (Zhou et al., 2019).

The other most significant constraint for utilizing biomass-derived adsorbents for dye removal is the scaling up and cost analysis. Although there are several available low-cost adsorbents, their processing and improvements (activation, cross-linking, drying, autoclaving, etc.), modification, and transportation cost should be considered to improve the selectivity of a certain pollutant with the corresponding sorption capacity. In addition, the recovery or regeneration procedure may also need a significant amount of energy and solvents depending on the activation processes used. After the dye-removal from wastewater, the effective utilization, and handling of the adsorbent and the eluent containing the contaminants more concentrate is important. The major solution for this management is the reuse and regeneration of the adsorbent several times using efficient regenerating agents (Patel, 2021). After complete exhaustion of the adsorbents, dumping, storing, or disposing of are options but safe disposal is necessary (Vakili et al., 2019). The potential solutions for safely disposing of the final effluent include stabilization and immobilization, for example, using a cement-based system as a binding agent (Dey et al., 2017). Consequently, difficulties are faced in the comparison among them. This indicates the necessity for more researches that can compare the superiority of an effective dye removal technique over the other.