2.1. Natural Dye

The dyes obtained from natural sources are called natural dyes. They are mostly applied to textiles using mordants. These mordants are metallic salts that have a high affinity for fiber as well as any coloring matter. These metallic mordants after mixing with dye in fiber form an insoluble precipitate, leading to both dye and mordant getting fixed [23]. On the basis of the chemical constitution, a natural dye can further be classified namely, indigoid dyes, alpha-hydroxynaphthoquinones, flavones, dihydropyrans, anthocyanidins, and carotenoids. Indigoid dye’s most common examples are indigo and Tyrian purple [24]. This dye type is obtained from the leaves of a wood plant as it possesses indigo as the key component in dyeing. Oppositely, alpha-hydroxy-naphthoquinones is a lawsone also known as Heena. It is mainly cultivated in Egypt and India. This dye type is also found in the unripe walnuts shell which gives orange color shades [25]. Flavones are colorless organic compounds. In general, natural yellows are derivatives of methoxy and hydroxyl substituted isoflavones and flavones. Weld (containing luteolin pigment) is a common example, which gives bright and fast colors to silk and wool [26]. Dihydropyrans are similar in chemical structure to flavones. Some examples are hematin and its leuco form, hematoxylin. These categories of natural dyes give dark shades to wool, silk, and cotton. Some of the common sources of these dyes are brazilwood, sappan wood, and logwood. Another dye, anthocyanidins, naturally possesses orange dye for cotton, and these are extracted from Bignonia chica leaves. Carotene is derivative of orange pigment found in carrots. Carotenoids owe their color to the conjugated double bonds which are present in their structure. Annatto and saffron are examples of carotenoids [27].

2.2. Synthetic Dyes

Synthetic dyes are man-made petrochemicals that may consist of lead, mercury, copper, sodium chloride, toluene, chromium, and benzene. Synthetic dyes have a brighter and wider range of colors, longer color permanence, and are easier, faster, and cheaper to produce in comparison with natural dyes. There are over 10,000 different types of synthetic dyes. Synthetic dyes are categorized on the basis of their chemical composition and the method of their application in the dyeing process. Although natural dyes are important from the chemical and historical point of view, these dyes are very expensive, need purification, and do not bind well because they lack the chemical grouping required to react with the binding sites of a fabric. Therefore, for dyeing applications, synthetic colors are utilized in place of natural dyes. This is because synthetic dyes are generally purer, less expensive, and their colors bind faster to the fabric [28]. Synthetic dyes are classified into three categories including anionic, non-ionic acid, and azo dyes. Anionic synthetic dyes include acid and whereas non-ionic dyes are basic, dyes are also known as cationic. On the other hand, dyes can be classified based on their chemical structure into anthraquinone dyes and azo dyes [29].

Azo dyes are characterized by being strong, having good all-around properties, and being less expensive. This dye has at least one azo bond (-N=N-), along with one or more aromatic structures. Azo dyes are intended to transport resistance and high photolytic stability toward major oxidizing agents. They have wide variety of applications in textile, food making, and cosmetic industries [3]. After azo dyes, anthraquinone dyes are widely used textile dyes [30]. Anthraquinone dyes have a wide color range and it almost covers the whole visible light spectrum. Dyes based on anthraquinone are the most resistant to degradation because of their fused aromatic structures, which retain the color for a duration [31]. Acid dyes are soluble in water and is used for silk, wool, nylon, modified acrylics, leather, paper, ink-jet printing, cosmetics, and food. The principal chemical classes of acid dyes are azo (including premetallized), anthraquinone, azine, triphenylmethane, nitro, nitroso, and xanthene [32]. Metal complex dyes consist of dyestuff and metal (usually chrome such as chromium, cobalt, nickel, and copper) and are being used since the 1940s. On the other hand, direct dyes are compounds of poly azo having some phthalocyanines, oxazines, and stilbenes. These dyes types are the anionic dye having water-soluble properties. They are utilized in dying rayon, cotton, nylon, leather, and paper [33]. Basic dyes are utilized on polyacrylonitrile, paper, cation dyeable polyethylene terephthalate modified polyesters, modified nylons, and in medicines. Dyes yield colored cations in a solution and they are known as cationic dyes. The chemical structure of basic dyes encompasses triarylmethane, aromatic methane, xanthene, thiazine, hemicyanine, oxazine, cyanine, and acridine [32,33].

Another dye, reactive dye is mainly used for dyeing cotton, silk, wool, nylon, and leather. This class of dyes makes a covalent bond with the fiber. It is so tightly fixed that they cannot be removed even in hash conditions [34]. Sulfur dyes have a complex structure and it contains sulfur. This class of dyes is economically friendly as they are low priced and has good wash fastness properties. These dyes are not brightly colored and are used for dyeing cotton, rayon, polyamide fibers, paper, leather, silk, and wood [33]. Vat dyes are fast dyes and are used for coloring cellulosic fiber, mainly cotton fiber. These dyes belong to the chemical class comprising indigoids and anthraquinone derivatives (including polycyclic quinones) [32]. Disperse dyes are primarily used on polyester and to a lesser amount on nylon, acrylic fibers, cellulose, and cellulose acetate. They are insoluble in water and non-ionic in nature. Disperse dyes molecule is based on azobenzene or anthraquinone molecules having nitro, hydroxyl, and amine groups attached [32,33].

4.1. Physical Method

There are several physical methods through which dye is biodegraded such as adsorption, membrane filters, coagulation/flocculation, ion exchange, and radiation. Adsorption is an effective and attractive method for dye elimination from wastewater, particularly if the adsorbent is not expensive and commonly available [47]. Activated carbon is mainly utilized for dye removal with great success due to its higher adsorption capacity [48]. The cellular structure of peat makes it a perfect choice as an adsorbent. It can adsorb polar organic compounds and transition metals from colored effluents. Unlike activated carbon, it requires no activation and is less expensive [49,50]. Wood chips have high-quality adsorption capabilities for acid dyes, although because of their hardness and longer contact times are required [51]. Many low-priced adsorbents have been studied on fly ash for dye adsorption [52]. Its adsorption ability depends on various properties of the adsorbent: Porous chemical structure, porous structure, and surface area [53]. Silica gel could be considered effective for removing basic dyes. However, side reactions such as air binding and air fouling with particulate matter prevent it from becoming effective for commercial use [18].

Membrane filter methods have several applications by improving the finished product quality, increasing the amount of yield generation, saving raw materials, or recovery of products from waste [54]. This method has some special features which are not found in other methods, namely, resistance to temperature, an adverse microbial attack, and chemical environment [18]. Concentrated sludge production is the main disadvantage. Ion exchange resins were used to decolorize the textile wastewater and to reduce the COD [55]. Most of the dyes are either anionic or cationic in nature; therefore, it was possible to remove them through ion exchange resins. These are not utilized extensively for the treatment of colored effluents, mainly because they couldn’t accommodate a wide spectrum of dyes. The whole process is possible only in the presence of organic solvents during regeneration time [18].

Coagulation/flocculation is the only cost-effective method used for the removal of color from the effluent. The primary wastewater treatment method proceeds with coagulation and flocculation by removing colloidal particles of turbidity, color, and bacteria. Coagulants such as Fe (III) or Al (III) salts are used in high concentrations for initiating precipitation and settling along with it [56]. Generation of the huge toxic sludge and disposal amount has been the major drawback of this method. Radiation, in general, is the emission of any rays or particles from a source. Radiation is classified into two main categories: Ionizing radiation and non-ionizing [57]. The rate of reaction was monitored by radiation dose and the amount of oxygen present in the solution. It is a proven effective method for the removal of the dye at the same time toxic organic compounds. Almost 90% of the color has been removed from the effluent. The whole process of radiation is very expensive which makes it less feasible for use. Finally, the drawbacks of all such processes have been mainly due to low efficiency, high cost, problems in the disposal, and limited versatility. Thus, most of the chemical and physical methods for the treatment of colored wastewater are not widely used in textile industry sites or plants [37].

4.2. Chemical Method

In the chemical method, various types of chemical reagents and treatments are applied to degrade the dyes such as ozone treatment, Fenton’s reagent, photochemical oxidation, sodium hypochlorite (NaOCl), and electrochemical. Fenton’s reagent is an effective decolorization chemical method of the textile wastewater which is unaffected to a biological method or is toxic to living biomass but not useful in reducing its COD if not combined with one more process such as coagulation [58]. In the acidic mixture, iron (II) acts as a catalyst which when acts on hydrogen peroxide, immediately leads to the formation of hydroxide radicals. These hydroxide radicals help in decolorizing dye wastes. This method can act on both insoluble and soluble dyes. Sludge generation is the major disadvantage of this process [59]. Ozone treatment is widely applied in the treatment of water; ozone either alone or in combinations (O₃-UV or O₃-H₂O₂) is used for treating industrial effluents [60]. Chromophore groups are accountable for color which can be fragmented by ozone either directly or indirectly through the formation of smaller fragments [61]. The only disadvantage is the short shelf life (20 min). Photochemical oxidation is the traditional technique used for industrial textile effluent treatment [62]. UV light activates the degradation of H₂O₂ into hydroxyl radicals which may attack and release hydrogen atoms from organic molecules capable of the oxidizing organic compound. The formation of byproducts is a major drawback [39,63].

NaOCl acts on the amino group of the dye molecule by the Cl⁻ which starts and speeds up azo bond cleavage [64]. NaOCl, when added to effluent may remove residual colors effectively. Moreover, this method is not favored much as it has been observed to lead to the generation of toxic chlorinated compounds which are dangerous for the environment as well as human health [65]. An electrochemical method in current years has been a growing interest in wastewater treatment. This type of method has been effectively functional in the detoxification of textile wastewater [66]. Electrochemical reactions are mainly used in sulfur and vat dyeing. Electrochemical oxidation leads to the generation of hypochlorite or hydroxyl radical. These electrogenerated species are used to degrade dyes present in wastewater. However, the electricity price used is high compared to the chemicals cost [18].

4.3. Biological Method

In the last few years, awareness among the scientific community about biological techniques has increased tremendously. These techniques have several benefits over conventional techniques such as low cost, environment-friendly, safe operation, and less sludge production. Bioremediation is now considered an upcoming treatment option for dye removal in diverse conditions. The bioremediation method can use natural and recombinant microorganisms to degrade toxic materials because of flexibility in operating conditions and design. Flexibility in this technique is understood by the fact that they could be employed ex situ (off-site) or in situ (on-site) and even plants can be used (phytoremediation). Biodegradation could be defined as the biologically facilitated dye molecule breakdown into several by-products by the action of a variety of enzymes. It is a process that is energy dependent [67]. Dye biodegradation results in decolorization and the breakdown of the dye molecules into smaller fragments. Diverse microbes such as bacteria, fungi, and algae are employed for the decolorization and degradation of synthetic dyes. Microbes have various capabilities for decolorizing a variety of dyes. Some groups of microorganisms have specific advantages over others in synthetic dyes biodegradation. The bioremediation of dyes effectiveness is subject to the activity and adaptability of the microbes [38].

4.3.1. Fungi

Fungi are most effective in breaking down or sometimes completing the mineralization of synthetic dyes [68]. These degradation properties are attributed to the presence of a powerful extracellular and intracellular enzyme system comprising laccase, manganese peroxidase, and lignin peroxidase, robust morphology, and various metabolic activity [69]. The mycelia of fungal species have an advantage over unicellular organisms that they solubilize the insoluble substance by enzymes. They have a high cell-to-surface ratio which helps them to have greater enzymatic and physical contact with pollutants. Extra-cellular fungal enzymes are also advantageous in tolerating high toxicants concentration [67]. Fungi initiate the process by adsorption of dye onto hyphae, followed by the breakdown of chemical bonds by enzymes [70]. However, fungi application for the removal of the dye from textile wastewater has some inherent drawbacks such as a long growth cycle and the need for nitrogen limiting conditions [71]. Gill et al. [72] reported Congo red decolorization by Pha. chrysosporium and Dichomitus squalens. Svobodová et al. [73] have reported maximum decolorization of Reactive Orange 16 up to 80% by fungal strain Irpex lacteus. Jayasinghe et al. [74] have reported Ganoderma lucidum, Pycnoporus cinnabarinus, Pleurotus pulmonarius, Stereum ostrea, and Trametes suaveolens, for their ability to decolorize Congo red (100 mg/L).

In a report, fungal species Fusarium oxysporum, Penicillium lanosum, and Ganoderma resinaceum was reported for Blue 21 dye decolorization [75]. Synthetic dye amaranth was reported to be degraded by Bjerkandera adusta [76]. White rot fungus Armillaria spp. was confirmed for degrading azo, anthraquinone, and triphenylmethane dyes. [77]. Noval species of Alternaria alternata was able to decolorize Congo red (600 mg/L) within 48 h of incubation [78]. Ganoderma spp. has also been reported to decolorize Reactive Orange 16 [79]. Chen et al. [80] reported Coriolopsis spp. for decolorizing triphenylmethane dyes. In a report, the novel fungal strain Absidia spinosa was reported for biotransformation of Cresol Red up to 65% [81]. Barapatre et al. [82] reported Aspergillus flavus for biodegradation of Malachite Green in which intermediate N-demethylated and N-oxidized metabolites were identified [Figure 1]. Fungal strain Aspergillus bombycis was reported for having the ability to degrade Reactive Red 31 [83]. Asses et al. [84] reported Aspergillus niger having the ability to decolorize Congo red (200 mg/L) within 6 days of incubation [Figure 2]. Oudemansiella canarii, the white rot fungi, was reported for degrading Congo Red dye [85]. In another report, Aspergillus terreus was reported for degrading Direct Blue-1 [86]. Krishnan et al. [87] reported Fusarium equiseti for the degradation of methylene blue dye.

Figure 1: Biodegradation pathway of malachite green using Aspergillus flavus. Source: Adapted with permission from Barapatre et al. [82]. [Click here to view]

Figure 2: Decolorization of Congo red through Aspergillus niger. Source: Adapted with permission from Asses et al. [84]. [Click here to view]

4.3.2. Bacteria

Dyes decolorization through bacteria is faster than fungal decolorization as bacteria take less time to grow [88]. Bacterial cells represent a promising and inexpensive tool for the several azo dyes removal from the textile dye effluents. Bacteria have many advantages as compared to filamentous fungi such as higher hydraulic retention time, faster growth rate, and could be efficient in treating high strength organic wastewaters [89]. In general, the azo dyes decolorization occurs under conventional anaerobic, facultatively anaerobic, and aerobic conditions by different bacterial groups [90]. A bacteria like Pseudomonas spp. could decolorize Reactive Orange 16 (100 mg/L) to 98% within 24 h of incubation [91]. Park et al. [92] reported decolorization of Congo red (1 g/L) by Staphylococcus spp. up to 96%. In a report, Pseudomonas spp. SUK1 could decolorize methyl orange dye (300 mg/L) within 6 h of incubation [93]. On the other hand, strain Kocuria rosea exhibited complete decolorization [94]. In another study, Nocardiopsis alba could decolorize Reactive Orange 16–95% (1000 mg/L) dye within 24 h of incubation [95]. Ghanem et al. [96] reported decolorization of methyl orange (38 μg/ml 10 μg/ml, 36 μg/ml, 32 μg/ml, and 40 μg/ml) by Acinetobacter baumannii, Cytophaga columnaris, Corynebacterium spp., Escherichia coli, and Pseudomonas fluorescence up to 76, 20, 72, 64, and 80%, respectively.

A study by Shah et al. [97] showed complete decolorization of methyl orange (2.9 mg/L) by Bacillus spp. complete methyl orange decolorization has also been reported for a much higher concentration of dye (50–200 mg/L) by several researchers. In a finding by Cui et al. [98] reported 100% decolorization of Methyl orange (100 mg/L) dye by Klebsiella spp. strain Y3. Ng et al. [99] reported decolorization up to 96% for 200 mg/L of Methyl orange by Shewanella xiamenensis. Marine bacteria A. baumannii could decolorize Congo red (100 mg/L) to 99.1% within 30 h and 5 days of incubation under optimized conditions [100]. Mycobacterium could decolorize Reactive Orange 16 (250 mg/L and 100 mg/L) to 96% within 24 h [101]. Bacillus cereus, Ochrobactrum spp., and Achromobacter xylosoxidans could decolorize Congo red (25 mg/L) 93%, 62%, and 94%, respectively [102]. Bacterium Alishewanella spp. was reported for azo dyes degradation [103]. Mani et al. [104] reported Shewanella oneidensis for decolorization of acid orange 7 [Figure 3]. Lysinibacillus fusiformis was reported for degrading methyl red [105]. Saha and Rao [106] studied the degradation of Reactive Orange 16 up to 89% by Bacillus flexus. In a report, halo-alkaliphilic bacterium Nesterenkonia lacusekhoensis was reported for decolorization of Reactive Red-35 dye [107]. Akansha et al. [108] reported Bacillus stratosphericus for biodegradation of reactive orange 16 [Figure 4].

Figure 3: Acid Orange 7 biodegradation pathways by Shewanella oneidensis. Source: Adapted with permission from Mani et al. [104]. [Click here to view]

Figure 4: Biodegradation pathway of Reactive Orange 16 through Bacillus stratosphericus. Source: Adapted with permission from Akansha et al. [108]. [Click here to view]

6.1. pH

In general, effective decolorization or degradation of dyes using bacteria takes place at basic or neutral pH, while yeast and fungi at neutral or acidic pH [67]. At pH below 4, H⁺ ions compete effectively with cations of dye, causing a reduction in efficiency of color removal, while at pH higher above this point charge, the biomass surface gets negatively charged, which attracts the dye positively charged cations through electrostatic force of attraction [193]. The study on 27 different dyes by 21 various basidiomycetes reveals that optimum pH was found to be in the 3–5 range for dye decolorization [194]. A study demonstrated Enterobacter spp. decolorizes Reactive Black 5 at pH 7.0 [195]. The study revealed that decolorization and degradations of Scarlet R by Proteus vulgarius and Micrococcus glutamicus occurred in the range of pH 7–8 with optimum pH 7 [90]. In a study, bacterial mixtures consisting of total of six bacterial species, namely, Bacillus spp. (four strain), Lysinibacillus spp., and Ochrobacterium spp. were reported for decolorization and removal of azo dye-Reactive Violet 5R at neutral pH i.e, 7.0 [151]. In another investigation, the bacterium A. flavus sorted out from the effluent disposal area soil was reported for degrading Reactive Red 198 effectively at low pH [196].

Another report has reported bacterium Lysinibacillus spp. isolated from the textile industrial area which was degrading and decolorizing the toxic sulfonated azo dye C.I. Remazol Red at pH 7.0 [197]. In a study by Khan et al. [198], Reactive Red 195 dye was degraded by the bacterial consortium consisting of, Bacillus spp., Pseudomonas spp., and Ochrobactrum spp. at pH 8.0. In an investigation, the bacterium Lactobacillus paracase isolated form the deep sea sediments were reported for removal of Acid Black azo dye between the pH range from 5.0 to 7.0 [199]. In another study, the most effective pH for the Crystal Violet decolorization by the fungi A. niger was reported 5.5 [200]. Bacillus spp., isolated from an alkaline lake, was reported for degradation of Reactive Red 239 dye. The most efficient pH for dye degradation by the bacterium was 10 [201]. In another investigation, Enterobacter spp. was degrading Crystal Violet at pH 6.5 [202]. In a report, bacteria obtained from textile industry released effluent were identified as Alcaligenes faecalis, Bacillus spp., and B. cereus which were reported for biodegradation of Novacron Super Black G dye at pH 8.0 under static conditions [203]. Javadzadeh and Asoodeh [204] isolated Bacillus spp. from the gut of termite which was having indigo dye biodegrading properties. The bacterium was reported for biodegrading the dye at pH 8.0. In an investigation, the microbial species Micrococcus yunnanensis was reported for degrading Methyl Orange at pH 7 [205].

6.2. Temperature

Temperature is a vital factor and plays an imperative role in the environment during the process of biodegradation. The metabolic activity of microorganisms is greatly affected by temperature. It is commonly experiential that the decomposition process is faster in summer as compared to winter as a warmer climate favors the growth and multiplication of various microorganisms [206]. However, the process is not the same after a certain temperature. Beyond this optimum temperature, there is a reduction in growth, metabolic activity, and deactivation of enzymes which ultimately lead to a decline in the decolorization process [193]. Thus, various studies conclude that biodegradation of dye by the microorganism is possible at ambient temperature which is responsible for their metabolic activities and reproduction [207]. Various microorganisms require different ambient temperatures for growth, with most growing at a temperature range of 25–35°C [208]. In a study by Shedbalkar et al. [209], the ambient temperature required to decolorize Cotton Blue by Penicillium ochrochloron was found to be 25°C. Enterobacter spp. could decolorize Reactive Black 5 at an optimum temperature of 37°C and on further increasing the temperature to 42°C there was a drastic decrease in decolorization activity [195]. In a study, Pseudomonas spp. was reported for degrading Congo red dye along with textile industry released effluent at 40°C temperature [210]. An isolate Sphingomonas paucimobilis was reported for bioremoval of Methyl Red dye between a broad range of temperatures i.e. 4–40°C [211]. In another report, B. vallismortis isolated from the disposal site of the textile industry was reported for degrading triphenylmethane dyes, including Aniline blue, Malachite Green, and Brilliant Green at high temperature range of 70°C [212].

A study has reported, Reactive Black-5 dye was reported to be degraded by the halotolerant bacterial strain Pseudomonas spp. at a temperature 25°C [213]. Similarly, the bacterium M. osloensis was reportedly degrading Mordant Black 17 at 35°C temperature [214]. In a report, Direct Orange 16 was degraded at 37°C temperature by bacterial isolate Micrococcus luteus [215]. Reactive Black 5 dye was reported to be biodegraded by the Aeromonas hydrophila ate 35°C temperature [216]. In a study, the bacterial strain Bacillus algicola was reported for decolorizing red, yellow, and blue dye at temperatures 25°C, 35°C, and 45°C. The highest degradation was observed at 25°C temperature [217]. In a similar report, two bacterial strains, namely, Acinetobacter and Klebsiella were decolorizing the azo dyes, that is, diazo dye Reactive Green 19 and monoazo dye Reactive Orange 16 at a temperature range of 20–40°C [218]. Ajaz et al. [219] reported Alcaligenes aquatilis for decolorization of Synazol Red 6HBN dye at 37°C temperature in 4 days [Figure 6]. In a report, B. subtilis was reported for decolorizing Reactive Blue 160 dye at 35°C temperatures [220]. Enterobacter hormaechei isolated from textile effluent was reported to degrade Reactive Yellow 145 and Reactive Red 180 at a temperature 37°C [221].

Figure 6: Biodegradation of Synazol Red HF 6BN dye Alcaligenes aquatilis. Synazol Red HF 6BN enters the cell of A. aquatilis through unknown process. After the entrance dye is processed enzymatically into several end products. First, the azo group of the dye is reduced which is followed by cleavage reaction to form various end products. Second, the desulfonation and oxidative deamination results in synthesis of pyrrolo[1,2-a] pyrazine-1,4-dione derivative which can be used as substrates in amino acid metabolism. The amino acid catabolism can synthesize pyruvate (3C compound) which can be converted into acetyl-CoA. The acetyl-CoA undergoes Krebs cycle to produce NADH2 and FADH2 (substrates of electron transport chain). Moreover, dye desulfonation, oxidative deamination, and carboxylation lead to produce phthalate derivatives, which can be transformed into different fatty acids and aldehydes. The phthalate, fatty acids, and aldehydes can directly/indirectly enter into fatty acid oxidation reactions (β-oxidation) to produce acetyl-CoA, NADH2 and FADH2. Source: Adapted with permission from Ajaz et al. [219]. [Click here to view]

6.3. Initial Dye Concentration

The impact on microbial insisted decolorization of the dye initial concentration was investigated. Studies show that there is a lowering in the efficiency of dye decolorization by microbes with an initial concentration increase of dye. This is because the dye toxicity increases at a higher concentration which inversely affects the growth of the microbial cells [193]. A similar report was demonstrated by Parshetti et al. [222] which indicated that higher concentration Malachite Green (100 mg/L) was toxic to K. rosea. Jirasripongpun et al. [223] found that Reactive Red 195 at concentrations 50 and 100 mg/L had a lethal effect on Enterobacter spp. and was not able to grow. A report on Congo red decolorization by Bacillus spp. reported that the decolorization rate declined with the dye concentration increase [224]. In a report, P. aeruginosa obtained from the dyestuff contaminated sediments was reported for 93.06% decolorization of Direct Orange 39 with 50 mg/L concentration within 45 ± 5 min and the maximum concentration degraded by the strain was 1.5 g L⁻¹ with 60% decolorization [225]. The consortium of Providencia spp. and Pseudomonas aeuroginosa was reported for 100% bioremediation of dye Red HE3B at 50 mg/L initial concentration and as the dye concentration rises to 200 mg/L and 250 mg/L the decolorization decreases to 85% and 70%, respectively [147].

A study reported Pseudomonas putida was decolorizing dye Orange II at maximum up to 1000 mg/L initial concentration and maximum decolorization was achieved at 100 mg/L concentration [242]. Shah et al. [53] reported that as the concentration of methyl orange was increased (3, 4, and 5 g/L), the incubation time required for decolorization was varied from 66 to 90 h. In another report, Pichia kudriavzevii was reported for degrading Acid Red B dye with maximum initial concentration 400 mg/L within 40 h and maximum decolorization was achieved with 100 mg/L initial concentration within 40 h [241]. Saroj et al. [245] concluded, the fungal consortium containing Penicillium oxalicum, A. niger, and A. flavus strains was able to degrade three different azo dyes, namely, Direct Blue 15, Direct Red 75, and Acid Red 183, with the initial concentration range 200–400 mg/L. All these three dyes were degraded at lower initial concentrations by the fungal consortium. Bacterium Aeromonas veronii sorted out from acclimated textile effluent was reported to decolorize the azo dye up to 1000 mg/L initial concentration [227].

Reactive Blue 4, the anthraquinone dye, was degraded by the bacterial granules up to 1000 mg/L initial concentration [246]. In a report, thermophilic microflora was reported for detoxification of azo-dye Direct Black G with an initial concentration 600 mgL⁻¹ [247]. In another report, Reactive Black-5 dye decolorization through bacterial (Gordonia, Pseudoarthrobacter, Sphingomonas, and Stenotrophomonas) consortium was tested with three different initial concentrations, that is, 25, 50, and 100 mg/L. The best dye decolorization by the consortium of bacterial strains was found at 50 mg/L initial concentration [156]. Amin et al. [248] reported Bacillus spp. to detoxify the diazo dye at an initial concentration of 100 mg/L. In a study, Ochrobactrum anthropic from textile wastewater was reported for biodegradation of Reactive Black 5 at the initial concentration of 400 mg/L [240].

6.4. Sodium Chloride Concentration

Effluents from the textile industry contain various salts or metal ions, acids, and alkalis as impurity in addition to dyes [67]. The salt concentration varies from 15 to 20% and has been calculated in dye industry wastewater. Thus, a microbial strain that can tolerate high concentrations of salt facilitates the degradation of dye wastewater. The biological treatment of the dye effluent containing various components, and identification of salt tolerant microorganisms is important. Rudakiya, Pawar [249] have shown the decolorization efficacy of a bacterial strain using salt concentrations up to 10%. In their study, the result come out to be that 6% salinity was effective in promoting both cell biomass and degradation of Reactive Orange 16. A similar study was done on Shewanella marisflavi and S. algae algae which were able to degrade single or mixed azo dyes at lower concentrations of NaCl (2–3%). It has been concluded that a lower level of salinity induces the activities of azoreductase, laccase, and NADH-DCIP reductase enzymes [250]. Cui et al. [98] reported that Klebsiella spp. was able to decolorize Methyl red, Orange I, Congo red, and Methyl orange efficiently over the salinity range (1–4%). In a study, M. luteus bacterium was reported for the detoxification of the Direct Orange 16 dye with 3% NaCl concentration in 6 h of incubation [215]. In another report, halotolerant S. marisflavi reported for decolorization of Xylidine Ponceau 2R under the 20% concentration of NaCl [251]. Song et al. [252] reported a yeast Pichia occidentalis for the Acid Red B dye biodegradation in 16 h with 30 g L⁻¹ of NaCl. In another report, Bacillus spp. was biodegrading 96% of sulfonated dye, that is, methyl orange with 5–20 g/L of sodium chloride concentration [253]. Salt tolerant bacterium Halomonas was reported for the biodegradation of Toluidine Red dye where the concentration of NaCl was 5% [254]. Zhuang et al. [255] reported Methyl Orange and Reactive Yellow 84 decolorization by the Shewanella indica and Oceanimonas smirnovii. These strains were decolorizing the dye in the presence of NaCl (0–70 g L⁻¹). Similarly, A. baumannii was reported for the 87% and 90% degradation of Reactive Black 5 and Reactive Blue 221, respectively, with 5% NaCl concentration [256].

6.5. Agitation/Static Condition

There are conflicting reports on azo dye decolorization through microbes under shaking/static conditions. According to studies, the decolorization rate increases by shaking culture conditions while other reports suggest static conditions. A higher rate of decolorization was observed during shaking due to the easy transport of oxygen and nutrients over static conditions [257,258]. On the other hand, decolorization by Pseudomonas spp. under agitation showed no decolorization while static culture conditions showed 96% Reactive Red 2 decolorization [88]. Similarly, Direct Red 81 and Reactive Red 120 decolorization by A. niger were more efficient under static conditions [259]. The rate of enzymatic activity was higher in the still condition [67]. Complete Navy Blue dye decolorization by Trichosporon beigelii was observed under static conditions whereas it declined to 30% under shaking conditions [22]. Another study concluded K. rosea for 100% Methyl Orange decolorization under the static conditions [94]. In a report, halophilic and halotolerant Halomonas variabilis and Halomonas glaciei were reported for degrading reactive textile dye in batch mode static condition [235]. Shah et al. [260] reported A. faecalis for degradation of Reactive Orange 13 under static anoxic condition. In a study, white rot fungi, that is, B. adusta, Coriolopsis gallica, Trametes trogii, and T. versicolor were reported for decolorizing Lanaset Gray G dye under the static conditions [231]. In agitation condition, Issatchenkia orientalis was reported for the five azo dyes decolorization, that is, Reactive Red 198, Reactive Orange 16, Direct Yellow 12, Direct Black 22, and Direct Blue 71 [236]. Nor et al. [244] reported Trichoderma harzianum for degrading Cresol Red dye under agitation conditions. In a study, Chaetomium globosum was tested for dye detoxification under stirred and static conditions and the report concluded that fungi were efficiently degraded the dye under agitation conditions in comparison with static conditions [261]. Bhattacharya et al. [238] reported under static conditions, halotolerant and alkalophilic N. lacusekhoensis was efficiently detoxifying Methyl Red. Similarly, A. baumannii from the sea sediments was reported for Reactive Red dye degradation at both agitation and static conditions [226]. In a comparative study, L. fusiformis was experimented for azoreductase degradation under both static and agitation conditions and results showed that under agitation conditions, dye degradation was better [105]. In a study, under static conditions, P. aeruginosa and Thiosphaera pantotropha were reported for decolorization and Reactive Yellow 145 dye detoxification [243].

6.6. Aerobic/Anaerobic Culture Conditions

Azo dyes are usually resistant to the attack of bacterial species under aerobic conditions [4]. Anaerobic degradation of dye has been more effective than aerobic, but the intermediates formed are carcinogenic and toxic in nature and must be degraded before being discharged to the main water stream [262]. Apart from few, all aromatic amines produced after azo dyes decolorization has recalcitrant properties under oxygen limiting condition [152]. However, the process when combined shows much more efficient output as reported by many authors [262]. They reported that the dye degradation by bacteria under anaerobic conditions is usually accompanied by the colorless aromatic amine production which is easily metabolized under aerobic conditions. The breakdown products are non-toxic in nature [139]. Bacterium P. aeruginosa was able to degrade, Navitan Fast Blue SSR, under an aerobic condition in the presence of glucose. The organism was also able to discolorate various other textile dyes [263]. Aerobic bacteria consisting of B. cereus, B. megaterium, B. pumilus, B. subtilis, and B. vallismortis were found to be efficient in decolorizing microbial strains individually as well as the mixture of dyes. Almost 80–90% decolorization was noticed in four out of six azo dyes (Congo red, Bordeaux, Blue BCC, and Ranocid fast blue) when present in mixture [134]. In a study, thermophilic microbial strains Anoxybacillus pushchinoensis, A. kamchatkensis, and A. flavithermus blend were reported for detoxifying 80% of Reactive Black 5 in aerobic conditions [163]. A comparative study has reported, microbial strain A. hydrophila was efficiently degrading the synthetic dye basic fuchsin, Crystal violet, Solophenyl red 3BL, Polar brilliant red B, Safranin, and Nigrosine under microaerophilic conditions in comparison to aerobic conditions [264]. Aerobic detoxification of Acid Scarlet GR was reported by a halotolerant yeast strain G. geotrichum [118]. A study has reported efficient degradation of dye Acid Red 14 by microbial strain Oerskovia paurometabola under anaerobic conditions [265]. In another report, Pseudomonas spp. and Clostridium spp. were reported to degrade the dye under aerobic conditions [266].

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