Keywords

5.1 An Introduction on History and Discovery of Dyes

The history of dye begins in 2600 BC, according to the earliest written record, with the use of dye stuffs in China. During that time these dyes were originally obtained from animal and vegetable sources. Also, Egyptian mummies were found to be wrapped with red-dyed clothes made of madder plants. The Egyptians commonly dye clothes using plant dyes and natural earth dyes. They also had very good knowledge of attaching the dye to the fabrics. Anthraquinone dye requires a metallic salt to impart color to the fabrics, and it is believed that to accomplish this, Egyptians used the salt alum (Nicholson and Shaw 2000). The majority of dyes that are used in the present world are chemically synthesized, and origin of these dyes can be traced back to organic chemistry. W.H. Perkin (1856) who is known as the “father of dye industry” accidentally discovered mauveine dye while trying to synthesize quinine, an antimalarial drug (Tyagi and Yadav 2001). Synthesis of dye is a very complex process. Distillate aromatic molecules should undergo reduction, oxidation, condensation, and nitration. Bismarck brown dye was the first commercial azo dye obtained by diazotization, and most of the dye used today is obtained through the same procedure. Based on the different chemical structures, dyes are divided into different classes. Azo dyes are the largest class of dye compounds since among the 100,000 existing dyes, more than 2000 dyes belong to azo dye group (Stolz 2001; Vijaykumar et al. 2007). Azo dyes are the most commercially important and extensively studied ones; few of those are shown in Figs. 5.1 and 5.2. This is because of the superior properties that are found in these dye classes when compared to other dyes. The chemical structure that is found in azo dyes and the bond that is responsible for the nondegradable property of these dyes are R-N=N-R. Azo dyes can be synthesized easily and can attach well to the fabrics and will not fade easily (Jeong 2008). Based on the number of N=N, azo dyes are classified as mono azo dyes, diazo dyes, triazo dyes, and polyazo dyes. Degradation of azo dyes is a very difficult process due to the presence of N=N. A total number of azo bonds, functional groups, and their arrangements greatly influence its degradation capacity (Rani et al. 2009; Grekova et al. 2012).

Fig. 5.1
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Four different azo dyes which are used in almost all textile industries of India: (a) Reactive Red 195A, (b) Reactive Blue 198, (c) Reactive Brown F3B, (d) Reactive Yellow 145

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Fig. 5.2
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Structure of toxic azo dyes Methyl Red, Disperse Yellow, Reactive Orange II, Congo Red, and Methyl Orange. (Sudha et al. 2014)

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5.1.1 Impact of Azo Dyes

Increasing urbanization, globalization, and industrialization have caused different types of environmental pollution. Among various industries, the textile industries discharge large volume of wastewater after dyeing process. As azo dyes have poor exhaustion properties, the remaining unbounded dye particles to the fiber get released into the environment and lead to bioaccumulation (Zolinger 1987). Among all available synthetic dyes , azo dyes are the largest class of dyes with the wide range of colors and structures, and it represents a major portion of the total dyes used in textile industries (Lang et al. 2013). In every textile industry, to dye 1 kg of fabric, 40–60 g of dyestuff is required, and after the dyeing process, approximately 15–20% of dye remains in the effluent (Baban et al. 2003; Babu et al. 2007). This effluent then becomes a highly toxic solution with toxic chemicals like reactive dyes and azo dyes (N=N). Toxic effluents after discharge into the environment cause adverse effects on the fertility of soil, plants, animals, aquatic organisms, and human beings (Mester and Tien 2000; Puvaneswari et al. 2006; Solıs et al. 2012; Saratale et al. 2013). Phytoplanktons present in the environment show abnormal coloration and reduction in the photosynthesis ability due to the absorbance of light by these dyes that enters the water ecosystem (Duran and Esposito 2000; Mester and Tien 2000). This also affects the pH, biochemical oxygen demand, and chemical oxygen demands and provided intense coloration in water and hence decreases the quality of water. The presence of these toxic and unnatural colors in water is aesthetically unpleasant and shows the presence of contamination in water. These dyes and their contamination will remain in the environment for longer period of time if not treated adequately (Olukanni et al. 2006). So far many physicochemical and biological methods were adapted for the removal of these toxic dyes. Some of those methods are found to be effective but also showed many negative side effects, and few are very expensive. Chemical methods used for the degradation of azo dye are found to increase the toxicity in the environment since most of the organic compounds are normally toxic. The advantages and disadvantages of the methods used for the removal of dyes from the textile effluents are summarized in Table 5.1 (Andrea et al. 2005). Azo dyes are mainly used in textile industries, but its applications are also seen in food, pharmaceutical, paper, cosmetics, and leather industries (Saratale et al. 2011). The more we use these toxic dyes, the more our environment will get polluted. Industrial effluent samples consisting of these azo dyes lead to bioaccumulation that causes severe toxic effects on the environment since these N=N make azo dyes highly toxic. Most of the dyes are soluble in water and can be absorbed by skin contact and also inhalation which can lead to allergy, risk of cancer, and skin and eye irritation and cause high toxicity if inhaled or consumed (Nikulina et al. 1995). Para-phenylenediamine is an aromatic amine which is present in almost all dyes and causes skin irritation, chemosis, permanent blindness, and lacrimation. Entry of para-phenylenediamine inside the body causes edema on the neck, tongue, and face and also respiratory distress. These dyes also cause disease like acute tubular necrosis, vomiting gastritis, hypertension, vertigo, urinary bladder cancer, splenic sarcomas, nuclear anomalies, and chromosomal aberrations (Table 5.2). Degradation of azo dyes is a bioremediation process which will remove toxicity from the environment . Therefore there is an urgent need for their removal and to reduce its toxicity before discharge of the waste effluent into the environment (Ayed et al. 2011). Research has been initiated in the field of biodegradation of azo dyes, i.e., azo dye degradation using microorganisms . Microbial degradation of azo dyes will also depend on the microbes such as bacteria, fungi, actinobacteria, bacterial consortium, and yeast and also on the culture condition provided. Biodegradation of azo dye is an easy, effective, and eco-friendly approach for the degradation and removal of toxic azo compounds from the environment. This review summarizes the recent achievements and methods that are used for the degradation of toxic azo dyes and also discusses the toxicity of degraded compounds and future perspective on the degradation of textile azo dyes.

Table 5.1 Advantages and disadvantages of the current physical and chemical methods that are used for the removal of toxic azo dye from industrial effluents (Andrea et al. 2005)
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Table 5.2 Different azo dyes and their severe effects reported on humans and animals
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5.2 Biodegradation of Azo Dye

Physical and chemical methods available for the removal of azo dyes include coagulation, precipitation, adsorption, flotation, flocculation, mineralization, and electrochemical destruction (Gogate and Pandit 2004). Mentioned techniques have many disadvantages such as high cost, release of the residue, time, and also inability to reduce the toxicity of degraded compounds (Copper 1993; Maier et al. 2004). Moreover these techniques will only minimize the toxicity level and not be able to completely remove the toxicity of the dyes (Copper 1993; Maier et al. 2004). To replace these techniques, microbial degradation methods can be used which show complete degradation of azo dyes and also detoxify the toxic compounds (Pandey et al. 2007). Biological treatment of textile effluents is an eco-friendly approach, and it is also gaining much importance in today’s scenario. Microorganisms are very active in reducing azo dyes by secreting different enzymes like azoreductase, laccases , peroxidase , and hydrogenase. These reduced compounds are then broken down into smaller compounds which are then utilized as their energy source (Stozl 2001). The location of these reactions may be either intracellular or extracellular sites (Fig. 5.3). According to the available literature, microbes are more active under combined effect of aerobic and anaerobic conditions (Waleed and Muhammad 2014). Almost all microorganisms are capable of degrading azo dyes including bacteria, yeast, actinomycetes, fungi, algae, and consortium of these microbes (Table 5.3). All these microorganisms have developed special enzyme systems for the discoloration and degradation of azo dyes under certain environmental conditions (Anjali et al. 2007).

Fig. 5.3
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Biological method mechanism behind degradation of reactive azo dyes using bacteria (RM redox mediator, ED electron donor, B bacteria)

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Table 5.3 Studies reported on degradation of azo dyes using different microbes such as bacteria, fungus, yeast, algae, and actinobacteria
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5.2.1 Degradation Using Bacteria

Degradation of azo dyes using bacteria is normally nonspecific and faster (Sudha et al. 2014). Many aerobic and anaerobic bacteria such as Staphylococcus sp., Enterococcus sp., Bacillus subtilis, Rhabdobacter sp., Xenophilus sp., Clostridium sp., Klebsiella sp., Acinetobacter sp., and Pseudomonas sp. have been reported in many studies for the degradation of toxic azo dyes (Olukanni et al. 2006; Vijaykumar et al. 2007; Lin and Leu 2008). Most of the bacteria produces azoreductase enzyme for the degradation of N=N, and few bacteria show their activity in the presence of specific carbon and nitrogen sources (Caughlin et al. 2002). Bacterial degradation, when done individually under the aerobic or anaerobic condition, will show only the degradation, but there will not be any mineralization. Many researchers have experimentally proved that the combined effect or aerobic and anaerobic treatment methods can be an effective method (Feigel and Knackmuss 1993; Chen et al. 2003). Many reports are available on degradation of mixture of azo dyes using bacteria (Table 5.4). Kolekar and Kisan (2013) reported degradation of mixture of textile dyes using Shewanella sp. strain KMK6 isolated from soil sample contaminated with dyes. This study using bacteria showed a decrease in the color of the mixture of dye and chemical oxygen demand. Also the toxic mixture got converted into nontoxic degraded product. In another study, 87% of degradation in the mixture of dyes was observed using novel bacterial strain Lysinibacillus sp. RGS with a reduction within 48 h (Saratale et al. 2013). In a recent report, two bacterial isolates Bacillus sp. and Aeromonas hydrophila isolated from textile mill effluent showed more than 90% of Reactive Green and provisional pink dye within 5 days with a dye concentration of 50 mg/L (Parimala and Suruthi 2016). Few other strains of bacteria like Pseudomonas fluorescens and Shewanella have also been reported for degradation of azo dyes (Liu et al. 2013; Godlewska et al. 2014). In another study plant and bacterial synergistic systems were used for treatment of textile effluents, and their consortium was used for the degradation of mixture of dyes. This treatment method showed 100% degradation for the mixture of dyes (Kabra et al. 2013). Anoxic culture of Aeromonas hydrophila was isolated and selected as dye degrading bacteria at a pH of 5.5–10 and at an optimum temperature of 20–30 °C (Naik and Singh 2012). Degradation parameter of textile effluent showed color and chemical oxygen demand removal when treated with culture of Bacillus subtilis (Jayan et al. 2011). Mixture of seven different dyes with different chemical structures showed 87% of degradation using B. laterosporus within 24 h when provided an optimum temperature of 40 °C. This study also came up with a very less toxic end product (Kurade et al. 2011). Biodegradation of selected dyes Reactive Black 5, Reactive Orange 16, Disperse Red 78, and Direct Red 81 was reported using bacterial isolates Providencia rettgeri and Pseudomonas sp. In this study, both isolates showed 97–99% degradation of all dyes within 30 h at a concentration of 100 mg/L (Harshad et al. 2014). It has been reported that mixed culture of bacteria can give better results when compared with the results shown by individual isolates.

Table 5.4 Percentage decolorization of different toxic azo dyes using bacterial strains isolated from industrial effluent
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5.2.1.1 Degradation Using Bacterial Consortium

Many bacterial isolates showed good azo dye degradation when applied together as a consortium rather than individually. Many reports are available on the dye degrading assays using bacterial consortium. Nigam et al. (1996) reported for the first time the combined ability of M. luteus, Micrococcus sp., and P. polymyxa in the degradation of azo dyes but individually showed no degradation. Similar work was carried by Moosi et al. (2007) using the same three isolates isolated from contaminated sites. A consortium of four bacterial isolates such as P. putida, P. fluorescens, B. cereus, and S. acidaminiphila showed degradation of Acid Red 88 within 24 h, but when inoculated individually, each isolate took more than 72 h for degradation (Khehra et al. 2005). In an another study, the effect of isolates Klebsiella sp., Bacillus sp., and Clostridium sp. showed good degradation ability under aerobic condition, while the same consortium showed no change under anaerobic condition (Cui et al. 2012). The fungal and bacterial consortium also plays a very good role and shows the high effect in azo dye degradation. Aspergillus sp. and Pseudomonas sp. together detoxified Rubine dye within 30 h (Lade et al. 2012). The same consortium showed very promising results by degrading 98% of textile effluents which consist of reactive dyes, disperse azo dyes, and sulfate within 35 h (Lade et al. 2012).

5.2.2 Degradation Using Fungi

A wide variety of fungal organisms are capable of decolorizing a wide range of textile azo dyes. Many of these fungi are employed either in living or inactive forms. Degradation of azo dyes using fungi has an advantage as it is cost-effective and production of sludge is very less and environmentally friendly. Fungi possess a strong ability to degrade complex organic molecules by producing extracellular enzymes such as laccase and lignin peroxidase ; hence researchers are paying more attention toward fungi-mediated dye degradation (Sudip et al. 2016). The mechanism of fungal degradation involves adsorption and enzymatic degradation or combination of both. Recently Wang et al. (2017) reported decolorization and degradation of Congo Red using Ceriporia lacerata a newly isolated white rot fungus isolated from decayed mulberry branches. This study showed 90% degradation of Congo Red dye with 48 h when 3 g of mycelia was inoculated in 20 mL of 0.1 mg/mL concentration of Congo Red solution. In another study two endophytic fungi, Phlebia sp. and Paecilomyces formosus, showed decolorization of Reactive Blue 19 and Reactive Black 5. Both isolates showed degradation activity with 0.1 g/ml of dye solution after 30 days (Ligia et al. 2017). Anand et al. (2017) reported biodegradation of Malachite Green using Aspergillus flavus. Aspergillus flavus showed complete degradation of 150 mg/L of dye solution within 8 days in Kirk’s medium under static condition in the presence of sucrose and sodium nitrate as effective carbon and nitrogen sources, respectively. Table 5.5 depicts some of the different dye mixtures decolorized by fungal degradation.

Table 5.5 Percentage decolorization of different toxic azo dyes using fungal strains isolated from industrial effluent
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5.2.3 Degradation Using Yeast

The growth rate of filamentous fungi is normally slow when compared with yeast; hence yeasts have an advantage over fungi from a biotechnological view for degradation of azo dyes. Yeast is a resilient microbe and is able to resist different environmental conditions like pH, organic wastewater, and high salt concentration. According to our literature survey, the first study presenting degradation of azo dyes by breaking N=N was published by Mecke and Schmahl (1957). However, this subject was actually brought into action after several years (Olteanu et al. 2008). Many reports available on yeast-mediated degradation are using Candida curvata and Geotrichum candidum with 90% and above degradation effect. Kluyveromyces marxianus showed the removal of diazo dye Remazol Black with 89% of degradation (Ertugrul et al. 2009). Similarly Candida catenulata and Candida kefyr degraded 90% of amaranth dye using biosorption techniques (Zeroual et al. 2007). S. cerevisiae and C. tropicalis are very active yeast isolates with the capacity of degrading more than one azo dye including Remazol Blue, Reactive Black, and Reactive Red. The action of these strains changes according to the dye concentration and exposure time (Aksu 2013; Donmez 2012). In a recent study, 12 out of the 44 isolated yeast colonies showed degradation; Reactive brilliant red K2 and those isolates were identified as S. cerevisiae, Torulopsis candida, and Saccharomycopsis lipolytica. Hence this feasible and metabolically versatile yeast should be considered for bioremediation process since a majority of yeast species have never been studied for azo dye degradation process.

5.3 Enzyme Involved in the Degradation of Azo Dyes

A number of microorganisms have been reported for the degradation of reactive azo dyes which include bacteria, yeast, fungi, and consortium of microorganisms and plants (Wesenberg et al. 2003; Olukanni et al. 2006). All these microbes have developed special enzyme systems for the degradation and discoloration of toxic azo dyes under suitable environmental conditions (Anjali et al. 2006). Although azo dyes have highly complex structural variations, they are degraded by a selected number of enzymes. Dye degrading enzymes are redox-active molecules which require a specific substrate for their action (Duran and Esposito 2000; Mester and Tien 2000). Microbes can either excrete the active enzymes into the used medium or the dye molecules move inside the microbial cell. Active enzymes are also potential in reducing or removing the toxicity from the dyes and effluents. The degrading capacity of microbes gets decreased by an increase in the concentration of dyes due to the microbial growth inhibition caused by the target molecules. To overcome this problem, we can extract the dye degrading enzymes from active microbes in bulk, and those enzymes can be used directly (Joshin and Chacko 2011). There are many reports on biologically synthesized dye degrading enzymes. Peroxidase , azoreductase, and laccases are the major and most promising enzymes involved in azo dye degradation (Abadulla et al. 2000). Azoreductase is a major and the most important group of enzyme synthesized from bacteria and fungi. The mechanism of these enzymes is reductive cleavage of azo bonds and converting them into colorless aromatic amines (Pandey et al. 2007). Figure 5.4 shows the proposed mechanism for the degradation of azo dyes using azoreductase under anaerobic condition. In intracellular and extracellular sites of the bacterial cell wall, the reducing molecules such as NADH, NADPH, and FADH2 help in the breaking of N=N (Zimmermann et al. 1982 and Zimmermann et al. 1984), while azoreductase plays a major role in the degradation process of bacteria, viz., Escherichia coli, Staphylococcus aureus, Rhodobacter sphaeroides, Enterococcus faecalis, and Bacillus sp. (Blumel and Stolz 2003, Yan et al. 2004 and Chen et al. 2005). Azoreductase enzyme extracted from E. faecalis YZ66 was able to degrade sulfonated azo dye Direct Red 18 and also detoxified their toxic effect (Sahasrabudhe et al. 2014). On the other hand, degradation of azo dyes using fungi generates two types of enzymes that are peroxidases and phenol oxidase (Ramya et al. 2010). Peroxidase enzymes are catalyzed in the presence of hydrogen peroxide (Fig. 5.5). These heme peroxidases are divided into different groups based on the organisms produced, substrate, and primary structure (Gumiero et al. 2010). The oxidative process of H2O2 which is catalyzed by chloroperoxidase was used for the degradation of azo dyes such as Orange G and S Yellow (Zhang et al. 2012). Lignin peroxidase enzyme isolated from Tagetes patula for the degradation of Reactive Blue 160 was reported by Patil and Jadha (2013). Another major enzyme that helps in the degradation of azo dyes is laccase enzyme (Fig. 5.6). These enzymes are also known as multicopper oxidase enzymes (MCO) as it belongs to the family of copper-containing polyphenol oxidases (Birhanli and Yesilada 2006; Arora and Sharma 2010; Giardina et al. 2010). Bertrand (1985) discovered laccase from the sap of a tree, Rhus vernicifera. Husain (2006) reported for the first time the importance of laccase enzyme in the degradation of textile color effluent. The major property of this enzyme that makes it a best azo dye degrading agent is its nonspecific oxidation capacity, a non-requirement of cofactors, and they do not require oxygen as an electron acceptor (Kalyani et al. 2012). In a recent study, laccase enzyme synthesized from white rot fungus, Ganoderma lucidum BCR 36123, showed 90% and above degradation azo dye Acid Orange AO7 (Chin et al. 2017). Purified laccases from mushroom Hypsizygus ulmarius showed degradation of azo dye Methyl Orange without using any redox mediator (Ravikumar et al. 2013). Enzymatic degradation of azo dyes also has significant potential to solve this problem due to their eco-friendly, inexpensive nature and also due to less production of sludges. Enzymatic processes are very promising for the degradation of toxic azo dyes; hence these enzymes can be considered as a molecular weapon for bioremediation of these dyes.

Fig. 5.5
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Proposed mechanism for the degradation of azo dyes by azoreductase by converting toxic chromophore group N=N into nontoxic NH2. (Courtesy: Keck et al. 1997)

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Fig. 5.4
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Proposed mechanism for alternative asymmetrical and symmetrical cleavage of sulfonated azo dye by peroxidase enzymes generated from fungi. (Courtesy: McMullan et al. 2001)

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Fig. 5.6
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Proposed mechanism of degradation using laccase enzyme, another major enzyme that helps in the degradation of azo dyes. (Courtesy: Andrea et al. 2005)

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5.4 Nanoparticle-Mediated Photocatalytic Degradation

Photocatalytic degradation involves acceleration of a photoreaction in the presence of a catalyst. Light energy is absorbed by a provided semiconducting material which helps in the degradation of dyes. The concept of photocatalytic degradation using TiO2 as a substrate for water decomposition was brought by Akira and Honda (1972), which is known as Honda-Fujishima effect. One of the advanced methods that are used for degradation of azo dyes is photocatalytic degradation (Zhao and Zhang 2008). The concept of nanoparticle-mediated degradation is very simple: the semiconducting materials absorb light of equal or more energy that will lead to the generation of electrons. These electrons can then further generate free radicals for the oxidation of substrates (organic matters). In many previous studies, this method has been broadly and highly explored (Yizhong 2000; Loannis and Triantafyllos 2004). Andrea et al. (2014) reported UV-induced degradation of Methyl Red and Methyl Orange azo dyes in the presence of TiO2 nanoparticles which was immobilized at the bottom of the effluent passing channel. The concept of azo dye degradation using nanoparticles and photocatalysis is related to each other. Thermally active zinc oxide was used in a study for photocatalytic degradation of Congo Red, where the system showed 96% of degradation (Tapas and Naba 2014). In a recent report, ferric oxide Gallic nanostructures were used for the degradation of azo dyes. The nanostructure synthesized by two different ways was subjected to photocatalysis, and degradation percentage was compared (Minoo and Ali 2016). All these studies used chemically synthesized nanoparticles for azo dye degradation. Although these methods reduce the toxicity by a certain extent by breaking the azo bond, they can sustain moderate toxicity which may be due to the presence of nanoparticles. However, the toxic effect is less when compared with the chemical methods for effluent treatment . To overcome the abovementioned problem, researchers started green synthesis of nanoparticles and used them in azo dye degradation. Priyaragini et al. (2014) showed 84% and 85% degradation of Acid Red 79 and Acid Red 80, respectively, using marine actinobacterial-mediated TiO2 nanoparticles. Photocatalytic degradation of Rhodamine Blue in the presence of actinobacterial-mediated TiO2 nanoparticles showed 95% and above degradation effect (Veena et al. 2016). Similarly in another study, UV and solar photocatalytic degradation of azo dyes and dye effluents in different time intervals was tested using the crude extract of coconut-mediated silver nanoparticles (Mariselvam et al. 2016). Photocatalytic degradation of azo dye can be considered as an easy approach, however it is time consuming and effective method when compared to all other techniques. Photocatalytic activity was used first for self-cleaning property by Akira Fujishima (1972), and now it is used in the process of cleaning our environment and protects it from becoming toxic for the coming generations. When we talk about nanoparticle-mediated azo dye degradation, we should focus on biologically synthesized nanoparticles rather than going with chemically synthesized nanoparticles, since our purpose is to remove toxicity completely without leaving a single trace of toxic compounds. Also, the green synthesis of nanoparticles will make azo dye degrading product cheaper when compared with chemically synthesized nanoparticles. Hence, it can be concluded that nanoparticle-mediated photocatalytic degradation of azo dye is an easy, economical, fast, and eco-friendly technique.

5.5 Degraded Compounds and Their Toxicity

Degradation and decolorization of azo dyes only are not enough; emphasis should also be given to verify the detoxification of azo dyes as well. The degraded dye should break down into nontoxic compounds. After degradation, analysis researchers should focus whether the highly toxic azo dyes got converted to nontoxic compounds or not, and also the reduction in the toxicity levels can be checked. In this field the first attempt was done by removing the toxicity and mutagenicity of direct red in the presence of B. velezensis strain (Bafana et al. 2008). Toxic Remazol Black B was converted into nontoxic derivatives by using Zinnia angustifolia and Exiguobacterium aestuarii, a plant and bacterial remediation, respectively (Khandare et al. 2012). In another study, toxicity evaluation was done using Daphnia magna under microaerophilic process (Harshad et al. 2015). Here complete detoxification of all the selected textile azo dyes was observed. Few reports are available on toxicity removal by a combined effect of ozonation and biofilm reactor. Toxicity of azo dye solution decreased within 2 min when subjected to ozonation, but toxicity increased when kept for longer time. Along with the removal of toxicity, evaluation of toxicity is also a necessary process. Biotoxicity and phytotoxicity assays are two majorly used assays to evaluate the toxicity of degraded compounds. In many reported works, biotoxicity assay was done using brine shrimp eggs, and the test is known as brine shrimp hatchability test. This test is used widely since it uses convenient organisms for the evaluation of toxicity and is a simple and inexpensive method. In a study less toxic nature of degraded dye was evaluated by observing the survival of 50% of brine shrimp eggs at a much higher concentration than that of the azo dyes (Arun and Bhaskara Rao 2012). In a recent report, toxicity evaluation of degraded azo dye Direct Yellow 4 was reported using phytotoxicity assay. Here phytotoxicity assay showed a considerable decrease in the toxicity of degraded dyes when compared with the pure dye (Shazia et al. 2017). Mutagenicity, cytotoxicity, and phytotoxicity of biodegraded textile effluent using fungal ligninolytic enzyme have been evaluated in a recent report (Muhammad et al. 2016). The cytotoxicity (Allium cepa, Daphnia magna, and brine shrimp), phytotoxicity (Triticum aestivum), and mutagenicity study using Ames test revealed that biodegradation of textile effluent using fungal-mediated enzymes detoxifies the toxic compounds present.

5.6 Future Perspective

Wastewater discharge by textile industries has become a great environmental concern for scientists because of the prevailing hazards in our ecosystem. Accumulation of industrial dyestuffs and dye wastewater not only creates environmental pollution, but it can also lead to medical problems and problems in the exquisiteness of our environment. There should be technically possible and cost-effective treatment methods for the removal of these toxic dyestuffs from the environment since in the present world environment regulations are becoming even stricter. Dye degradation using microbes bears a significant potential in solving these problems since microbes and their products are eco-friendly, inexpensive, and easily available. This review clearly stated the importance of microbial dye degradation, nanoparticle -based degradation, photocatalytic degradation, enzymatic degradation, and toxicity of degraded compounds. As an emerging technique, using microbes and their mediated nanoparticles is an eco-friendly, less expensive, and easy way to degrade toxic dyes and to remove toxicity from our environment . Lab-scale work will be entirely different when it reaches to the industrial level. Microbial degradation of azo dyes should be focused using small-scale effluent treatment fermenter designing which later can be applied to different textile industries to treat these toxic dye-filled effluents. Similarly, all techniques should be studied with a design so that they can be applied at the industrial level. Industries should get involved with universities or research institutes to carry out these lab works to the next level. The enzyme responsible for degradation of these toxic dyes should be produced in a large amount with the help of industries and should be brought in action as soon as possible at least for small-scale textile industries or dyeing units. Also, there’s a need to formulate the effective product that can be delivered to remote places. These dyes get concentrated at the end of the food chain and lead to severe medical problems such as tumor, cancer, asthma, nervous disorder, and even death. So to avoid these entire problems and to protect our environment, textile effluents have to be free from toxic azo dyes and its toxicity before it reaches the environment.

5.7 Conclusion

Azo dyes constitute the largest and most versatile class of synthetic dyes used in a variety of industries including textile, pharmaceutical, food and cosmetics industries and represent major components in wastewater from these industrial dyeing processes. The presence of dyes imparts an intense color to effluents which leads to environmental as well as aesthetic problems. Many researchers are working on degradation of azo dyes; however, there is still a need to generate relative performance data on industrial effluents. Hence this review concludes that azo dye degradation is an extremely serious topic to be focused on, and it can be done using microbes, nanoparticles, and photocatalytic methods. As we are aware of the effect of water scarcity in our country, wastewater treatment is an issue that should be taken into consideration. Also, it’s our duty to keep our environment clean and to protect our natural resources from all toxic compounds. Azo dye also comes in the list of toxic compounds or environmental pollution-causing products; hence removal of these toxic dyes using all motioned techniques will help in the process of keeping our environment clean and healthy.