Bio-removal of Azo Dyes: A Review

Synthetic dyes are widely used in textile, paper, food, cosmetics and pharmaceutical industries with the textile industry as the largest consumer. Among all the available synthetic dyes, azo dyes are the largest group of dyes used in textile industry. Textile dyeing and finishing processes generate a large amount of dye containing wastewater which is one of the main sources of water pollution problems worldwide. Several physico-chemical methods have been applied to the treatment of textile wastewater but these methods have many limitations due to high cost, low efficiency and secondary pollution problems. As an alternative to physico-chemical methods, biological methods comprise bacteria, fungi, yeast, algae and plants and their enzymes which received increasing interest due to their cost effectiveness and eco-friendly nature. Decolorization of azo dyes by biological processes may take place either by biosorption or biodegradation. A variety of reductive and oxidative enzymes may also be involved in the degradation of dyes. This review provides an overview of decolorization and degradation of azo dyes by biological processes and establishes the fact that these microbial and plant cells are significantly effective biological weapon against the toxic azo dyes.


Introduction
Dyes are the important industrial coloring chemical compounds. Organic chemicals that own color are known as dyes. Dyes are classified by (i) chromophore groups in their chemical structures as azo dyes, anthraquinone dyes and phthalocyanine dyes etc. and (ii) their usage or application method as disperse dyes for polyester and reactive dyes for cotton (Singh et al., 2012). Azo (monoazo, diazo, triazo and polyazo), anthraquinone, triarylmethane and phthalocynine dyes are main groups of dyes. Azo dyes absorb light in the visible region because of their chemical structure, which is characterized by one or more azo bonds (-N=N-) (Chang et al., 2001). Globally, 2.8×10 5 tons of textile dyes are poured into water ecosystem every year (Jin et al., 2007). Azo dyes are the most common (more than 3000 different varieties) of all textile dyes produced because of their easier biosynthesis, chemical stability and the diversity of colors available as compared to natural dyes (Chang et al., 2004). About 80% of azo dyes are used in the dyeing process of textile industries. They are widely used in the textile, leather, food, paper, cosmetics and pharmaceutical industries. It has been estimated that about 10-15% of the dyes used in dyeing process goes unbound with the textile fibers and are discharged into the environment (Asad et al., 2007). Release of dye containing effluent derived from various industrial practices into water bodies and surrounding industrial areas is of major concern (Mugdha and Usha, 2012) which have several adverse effects on life including decreased aquatic photosynthesis, ability to exhaust dissolved oxygen and toxic effect on flora, fauna and humans. Presence of dyes in the textile effluent causes an unpleasant appearance by imparting the color and also their breakdown products (colorless amines) are toxic, carcinogenic and mutagenic (Xu et al., 2005). P.K.   Int. J. Appl. Sci. Biotechnol. Vol 5(2): [108][109][110][111][112][113][114][115][116][117][118][119][120][121][122][123][124][125][126] This paper can be downloaded online at http://ijasbt.org&http://nepjol.info/index.php/IJASBT potent mutagen in bacteria (Ferraz et al., 2011). A commercial textile azo dye Acid Violet 7 has ability to induce lipid peroxidation, chromosomal aberrations and inhibition of enzyme acetyl cholinesterase. The toxicity of Acid Violet 7 dye increases during the biodegradation process by Pseudomonas putida due to its corresponding metabolites 4-aminoacetanilide and 5-acetamido-2-amino-1-hydroxy-3,6-naphtalene disulfonic acid (Mansour et al., 2010). Similarly, Methyl Red is also a mutagenic dye and its microbial degradation product N, N-dimethylphenylenediamine (DMPD) is a toxic and mutagenic aromatic amine (Wong and Yu, 1999) that remains unchanged in the culture (Ayed et al., 2011). Tsuboy et al. (2007) tested azo dye Disperse Blue 291 and found that it has genotoxic, mutagenic, cytotoxic effect and also leads to the formation of micronuclei and DNA fragmentation in human hepatoma cells. Sunset Yellow, Carmoisine, Quinoline Yellow, Allura Red, Tartrazine and Ponceau 4R are azo compounds which are proved to be harmful to children when used as additives in food and drinks (Parliament, Council, The, & Union, 2008). Some dyes are also mentioned which induces bladder cancer in humans and splenic sarcomas, hepatocarcinomas and nuclear abnormality in experimental animals (Rafii et al., 1997;Puvaneswari et al., 2006). Metanil Yellow, an azo dye, has been proved to be hepatotoxic in albino rats (Singh et al., 1987(Singh et al., , 1988. The metabolic disposition of Metanil Yellow and Orange II has also been studied using rat and guinea pig as model systems (Singh, 1989;Singh et al., 1991a,b).
Due to these harmful effects of azo dyes on human health and on the environment, there is a crying need to restrict their entry in the environment. But, practically it is not feasible. So a solution to this problem is adopting such treatments methods that reduce or remove the dyes from the wastewaters.
This review compiles all the dimensions of biological methods used for biodecolorization and biodegradation of azo dyes from different sources.

Biological Methods of Decolorization and Degradation of Azo Dyes
Decolorization and degradation of azo dyes may take place by two methods either adsorption on the microbial biomass (biosorption) or biodegradation of the dyes by the living cells. Biological treatment of textile azo dyes has been proved to be the best method due to its ability to degrade almost all dye stuff and also overcome many disadvantages posed by the physico-chemical processes. Numerous studies, available with emphasis on the use of microorganisms to degrade dyes, suggest that biodegradation is an eco-friendly and cost effective method for dye containing wastewater treatment (Vitor and Corso, 2008;Pajot et al., 2011).
Microorganisms (bacteria, fungi, algae and yeast), plants and their enzymes can be successfully utilized in removal of color of a wide range of azo dyes through anaerobic, aerobic and sequential anaerobic-aerobic treatment processes.

Biosorption
The uptake or accumulation of chemicals by microbial mass has been termed as biosorption (Kumar et al., 1998). Biomass of bacteria, yeast, filamentous fungi and algae has been used successfully to remove toxic dyes by biosorption (Bhatnagar and Sillanpaa, 2010). This property of microorganism is due to the cell wall components such as heteropolysaccharides and lipids, which consists of different functional groups including, amino, hydroxyl, carboxyl, phosphate and other charged groups, creating strong attractive forces between the azo dye and cell wall (Srinivasan and Viraraghavan, 2010;Das and Charumathi, 2012). Won et al. (2005) found bacterium Corynebacterium glutamicum as a latent biosorbent of azo dye Reactive Red 4. Mnif et al. (2015) isolated bacterial strain Bacillus weihenstephanensis RI12 which removes Congo Red by biosorption with the use of SPB1 biosurfactant. Similar to bacteria mediated biosorption; fungi have also been used as latent sorbent for decolorization of azo dyes from industrial effluents and have achieved considerable attention. Effective biosorption depends on several conditions such as pH, temperature, ionic strength, time of contact, adsorbent, dye concentration, dye structure and type of used microorganism (Renganathan et al., 2006;Vijayaraghavan and Yun, 2007;Erden et al., 2011;Ambrosio et al., 2012).
In addition to application of bacterial and fungal biomass in biosorption of azo dyes, algae are also considered as potent biosorbents due to their availability in both fresh and salt water (Wu and Jean, 2012). Donmez and Aksu (2002) revealed that potential of algae for biosorption is attributed to their relative high surface area and high binding affinity. Electrostatic attraction and complexation in the cell wall of algae are the physical processes that mediate algal biosorption (Satiroglu et al., 2002).
Dead algal cells proved to be advantageous as biosorbents over living cells as they do not have nutrients demand, can be stored and used for long periods and can be regenerated using organic solvents or surfactants (Fu and Viraraghavan, 2001). The key advantages of biosorption process is its high selectivity, efficiency, economic, works at low concentration (Hammaini et al., 2002) and have good removal performance as it is proved to be an efficient method over presently available physico-chemical technique such as ion exchange (Aksu and Donmez, 2003). An important disadvantage associated with biosorption is early saturation problem and almost no biological control over characteristic biosorbent. P.K. Singh and R.L. Singh (2017) Int. J. Appl. Sci. Biotechnol. Vol 5(2): 108-126 This paper can be downloaded online at http://ijasbt.org&http://nepjol.info/index.php/IJASBT

Enzymatic Decolorization and Degradation
There are many reports on the use of physico-chemical methods for color removal from dye containing effluents (Golab et al., 2005;Lopez-Grimau and Gutierrez, 2006). These methods include adsorption, chemical treatment, coagulation and ion pair extractions etc. but they are linked to problems such as high cost and produce large amounts of sludge after treatment which requires safe disposal.
The enzymatic approach offers alternative strategy for decolorization/degradation of azo dyes from wastewater over conventional physico-chemical treatments as it ends with less sludge production and is also cost effective. There are several enzymes involved in removal of azo dyes which proved an effective molecular weapon for decolorization of azo dyes . Enzymes that mediate azo dye decolorization are grouped into two broad classes; Reductive and Oxidative.

Reductive enzymes
Azoreductases are the catalytic proteins, encoded by microorganisms such as bacteria (Misal et al., 2011), algae (El-Sheekh et al., 2009) and yeast (Vitor and Corso, 2008). Several bacterial species have been identified that have the potential of degrading azo dyes under reduced (anaerobic) conditions (Oturkar et al., 2011). During the process of dye decolorization azo bond is initially cleaved by bacterial azoreductase enzyme which leads to the formation of toxic colorless aromatic amines under reduced conditions (Pandey et al., 2007). Azoreductases completely rely upon reducing equivalents (e.g., NADPH, NADH and FADH) for decolorization of azo dyes (Van der Zee and Cervantes, 2009). Most of the azo dyes are substituted with sulfonate groups and have high molecular weight due to which dyes can't pass through bacterial membranes. This fact reveals that dye reducing activity is not dependent on the intracellular uptake of the dye (Pearce et al., 2003). On the basis of reducing equivalent used to reduce azo linkages, enzymes have been identified as FMN-dependent reductases (Burger and Stolz, 2010), FMN-independent reductases (Burger and Stolz, 2010), NADH-dependent reductases (Misal et al., 2011), NADPH-dependent reductases (Mendes et al., 2011) and NADH-DCIP reductases (Phugare et al., 2010). NADH-DCIP reductases are believed to be marker enzymes of bacterial and fungal mixed function oxidase systems, and mediate the detoxification of xenobiotic compounds (Bhosale et al., 2006).

Oxidative enzymes
Microorganisms also encode number of oxidative enzymes for degradation of azo dyes such as polyphenol oxidases (PPO), manganese peroxidase (MnP), lignin peroxidase (LiP), laccase (Lac), tyrosinase (Tyr), N-demethylase (Oturkar et al., 2011;Martorell et al., 2012), dye decolorizing peroxidases (Liers et al., 2010) and cellobiose dehydrogenase (Tilli et al., 2011). These oxidases have been reported in bacteria, filamentous fungi, yeast and plants. These enzymes catalyze conversion of broad range of substrates into less toxic insoluble compounds. The removal of toxic compound from waste takes place by a mechanism involving the formation of free radical followed by insoluble product (Torres et al., 2003). Peroxidase is a heme containing enzyme and widely distributed in plants, microorganisms and animals (Duarte-Vazquez et al., 2003). In the last few years massive research, to develop methods, relied on peroxidases from plants and fungi for the treatment of wastewater containing colored pollutants. Ferreira-Leitao et al. (2007) reported the role of plant Horseradish peroxidase and LiP from Penicillium chrysosporium in the oxidation of Methylene Blue (Basic Blue 9) and Azure B dyes. Recently Versatile Peroxidase (VP) has been purified and described as a new family of ligninolytic peroxidases, along with LiP and MnP obtained from P. chrysosporium (Martinez, 2002). Interestingly, these enzymes have shown the activity of both LiP and MnP and they have potential to oxidize Mn 2+ to Mn 3+ at around pH 5.0 while aromatic compounds at around pH 3.0, despite the presence of Mn 2+ (Heinfling et al., 1998;Ruiz-Duenas et al., 2001).
Laccases are multi-copper oxidases and most studied enzyme which facilitates removal of dyes due to their distinct features such as non specific oxidation capacity, no requirement for co-factors and also they do not use readily available oxygen as an electron acceptor (Telke et al., 2011). Laccases catalyze decolorization of textile dyes either by direct oxidation or via indirect oxidation by using redox mediators (e.g., ABTS) to accelerate the reaction (Khlifi et al., 2010).
The catalytic cycle of MnP involves the oxidation of manganous ions (Mn 2+ ) to Mn 3+ (Hofrichter, 2002) which is further chelated with organic acids (e.g., oxalic acid). The chelated Mn 3+ readily diffuses from the active site of the enzyme which in turn oxidizes the secondary substrates (Mester and Field, 1998).
Polyphenol oxidase is a tetramer enzyme that contains four atoms of copper per molecule, and binding sites for two aromatic compounds and oxygen involves in the removal of aromatic pollutants from various contaminated sites. PPO catalyzes o-hydroxylation of monophenols to odiphenol. They can further catalyze the oxidation of odiphenols to o-quinones. Waghmode et al. (2011a) proposed degradation pathway by G. geotrichum (Fig. 1a), B. laterosporus (Fig. 1b) and consortium GG-BL ( Fig. 1c) which utilizes different microbial enzymes for the breakdown of azo dye Golden Yellow HER (GYHER). P.K. Singh and R.L. Singh (2017) Int. J. Appl. Sci. Biotechnol. Vol 5(2): 108-126 This paper can be downloaded online at http://ijasbt.org&http://nepjol.info/index.php/IJASBT

Bacteria
Extensive work has been done to screen out the role of diverse groups of bacteria in the decolorization of azo dyes. These bacteria can be isolated from several ecological niches such as soil, water, human and animal excreta and even from contaminated food materials. Bacteria are suited fine for decolorization and complete mineralization of azo dyes as they are easy to cultivate and grow rapidly. The process of decolorization by a bacterial system may be anaerobic or aerobic or involve a combination of both (Fig. 2).
The bacterial decolorization and degradation of these dyes has been taken into account as it can ensure a higher degree of biodegradation and mineralization, is applicable to a wide range of azo dyes, low cost, eco-friendly and produce less sludge (Verma and Madamwar, 2003;Khehra et al., 2006).

Pure bacterial culture
Bacteria mediated decolorization and degradation of azo dyes involve azoreductase assisted breakdown of azo bond (-N=N-) under anaerobic condition which results in the formation of colorless hazardous aromatic amines (Van der Zee and Villaverde, 2005) which are further removed aerobically or anaerobically (Joshi et al., 2008). Horitsu et al. (1977) first reported Bacillus subtilis culture capable of degrading azo dyes and after that Aeromonas hydrophilia (Idaka et al., 1978) and Bacillus cereus (Wuhrmann et al., 1980) were reported to degrade azo dye.
It is reported that azo dyes are generally resistant to bacteria mediated degradation under aerobic conditions (Ola et al., 2010) because oxygen rich environment usually hampers breakdown of the azo bond. However, some aerobic bacterial strains are reported to have ability to reduce the azo bond by oxygen insensitive or aerobic azoreductases (Lin and Leu, 2008) proving that decolorization and degradation demands oxygen rich environments (Fig. 3).
Extremophiles, an interesting bacterial community that are used in bioremediation of dye containing effluents, where normal bacterial strains could not sustain due to the high temperature and high NaCl concentration (Amoozegar et al., 2011). Staphylococcus, Exiguobacterium (Chen et al., 2011 and Aeromonas hydrophila (Ogugbue et al., 2011)    Bacterial Consortium Consortium based treatment system achieves higher extent of biodegradation and mineralization due to the synergistic metabolic properties of the microbial community and has significant advantages over the use of pure cultures in the degradation of azo dyes (Saratale et al., 2010). Consortia can be constructed with bacteria, fungi or a combination of both (Yang et al., 2009). Generally during reduction of azo bonds production of toxic, carcinogenic aromatic amines takes place but by using microbial consortium these P.K. Singh and R.L. Singh (2017) Int. J. Appl. Sci. Biotechnol. Vol 5(2): 108-126 This paper can be downloaded online at http://ijasbt.org&http://nepjol.info/index.php/IJASBT aromatic amines get degraded by complementary organisms, making the process more effective and efficient (Moosvi et al., 2007). It is found that in a microbial consortium the individual microbial strains may attack the dye molecule at different positions or may act on metabolites produced by the co-existing strains for further decomposition (Patil et al., 2008;Saratale et al., 2009b (Moosvi et al., 2007). Similarly, Phugare et al. (2011) reported that raw effluent was decolorize more effectively with a consortium (Fig. 4) containing Providencia sp. SDS and Pseudomonas aeuroginosa strain BCH than the individual cultures alone. Several studies reported for biodegradation of azo dyes using bacterial consortium are given in Table 2.

Algae (Phycoremediation)
Living and nonviable algae have been used in bioremediation of textile wastewater (Lim et al., 2010). Several research groups concluded that microalgae offer a solution to global environmental problems such as the greenhouse effect and wastewater treatments (Craggs et al., 1997;Parikh and Madamwar, 2005). Algae utilize three different intrinsic mechanisms for decolorization of azo dyes including use of chromophore for production of algal biomass via assimilation, production of CO2 and H2O during conversion of color to non-color molecule and chromophore adsorption by algal biomass. However, release of azo dyes into natural water ecosystem causes toxic effect on aquatic life but it does not significantly reduce algal growth (Acuner and Dilek, 2004) and algae have been reported to grow in industrial effluents (Dubey et al., 2011). Mechanisms of algal decolorization can involve enzymatic degradation, adsorption or both. Most studied algae that are involved in decolorization and degradation of azo dyes are blue green algae, green algae and diatoms. Several species of Chlorella (Acuner and Dilek, 2004) and Oscillatoria (Jinqi and Houtian, 1992) were found to be potent decolorizer of azo dyes and generate their corresponding aromatic amines which further metabolize these toxic aromatic amines into simpler organic compounds or CO2 and H2O. Similar to bacteria, algal mediated degradation of azo dyes depends upon induced form of enzyme azoreductase for breakdown P.K.

Filamentous Fungi (Mycoremediation)
Several studies are available about the potential of fungi to oxidize phenolic, nonphenolic, soluble and nonsoluble dyes (Tekere et al., 2001;Libra et al., 2003). The mycoremediation of dyes is a cost effective possible option to the present treatment technologies. The most widely used fungi in decolorization and degradation of dye are the lignolytic fungi of class basidiomycetes. The fungal potential of bioremediation of different complex organic pollutants is devoted to a metabolic product of a large set of intra and extracellular enzymes (Humnabadkar et al., 2008). Fungi mediate a variety of complex conversion reactions such as hydroxylation of complex polyaromatic hydrocarbons, dye effluents, organic waste and steroid compounds (McMullan et al., 2001). It was found that fungal mediated degradation of aromatic compound is a secondary metabolic event that arises when nutrients (C, N and S) become limiting, then these aromatic compounds are used as source of energy and nutrients essential for the propagation of the cultures (Christian et al., 2005). White rot fungi produces lignin peroxidase (LiP), manganese peroxidase (MnP) and laccase which play major role in degradation of many aromatic compounds due to its nonspecific enzyme activity ( Although P. chrysosporium is the most widely studied white rot fungi in the decolorization and degradation of dyes, others fungal species have also contributed in bioremediation of azo dyes (Table 3).
Now it is a well established fact that fungal mediated removal of dyes is either by adsorption or enzymatic process. The growth, enzyme production and subsequent dye degradation by filamentous fungi are dependent and affected by many factors such as culture conditions, nutrient conditions, especially regarding nitrogen limitation, carbon source, time, pH, agitation, temperature, oxygen supply, additives and salts (Mielgo et al., 2001;Zouari-Mechichi et al., 2006;Evangelista-Barreto et al., 2009;Parshetti et al., 2010;Pilatin and Kunduhoglu, 2011;Grinhut et al., 2011).
Although different fungal species are successfully employed in the decolorization and degradation of azo dyes but at the same time they encounter many problems for the removal of dyes from textile wastewaters such as large volume produces, control of biomass and the nature of synthetic dyes (Palma et al., 1999;Stolz, 2001). Despite these decolorization related problems, fungal mediated decolorization is a promising alternative to replace or supplement present treatment processes.

Yeast
Similar to microalgae, the mechanisms adopted by yeast for dye decolorization are adsorption (Yu and Wen, 2005), enzymatic degradation, or a combination of both. In recent years, there has been rigorous work on dye removal by different yeast species because it shows attractive features over bacteria and filamentous fungi, as it offers lots of advantages. They not only grow rapidly like bacteria, but they also have the capacity to sustain in adverse environmental condition, such as at low pH (Yu and Wen, 2005;Martorell et al., 2012). Several simple azo dyes were degraded in liquid aerated batch cultures by yeast strain Candida zeylanoides, the tune of color removal ranged from 44 to 90%, after 7 days of treatment (Martins et al., 1999). It was observed that enzyme mediated biodegradation followed decolorization of several azo dyes which is done by some ascomycetes yeast species including Candida tropicalis, Debaryomyces polymorphus (Yang et al., 2003) and Issatchenkia occidentalis (Ramalho et al., 2004). Lucas et al. (2006) used Candida oleophila for efficient decolorization of commercial textile diazo dye Reactive Black 5. The most preferred physical condition for decolorization by yeast biomass is a low pH for example Candida albicans adsorb maximum Direct Violet 51 dye at pH 2.5 (Vitor and Corso, 2008) and C. tropicalis perform similar role at pH 4.0 for Violet 3 (Charumathi and Das, 2010). Yang et al. (2003) have shown that two yeasts (D. polymorphus, C. tropicalis) and a filamentous fungi (Umbelopsis isabellina) could completely decolorize 100 mg of Reactive Black 5 within 16-48 hours. Earlier S. cerevisiae MTCC463 strain was studied for decolorization of two azo dyes Malachite Green and Methyl Red (Jadhav and Govindwar, 2006;Jadhav et al., 2007). Waghmode et al. (2011b) reported that the decolorization, COD and TOC removal of a mixture of Golden Yellow HER, Remazol Red, Rubine GFL, Scarlet RR, Methyl Red, Brilliant Blue and Brown 3 REL is higher under aerobic than under anoxic or anaerobic conditions by using G. geotrichum MTCC 1360.

Decolorization and Degradation of Azo Dyes by Plants (Phytoremediation)
Phytoremediation is emerging as the most promising technology for the remediation of soils and groundwater contaminated with heavy metals and organic pollutants which provide an innovative, cost effective alternative approach for biotreatment of wastewater (Patil et al., 2009). Recently, some studies describe the use of plants for bioremoval of dye from wastewaters. Researchers utilized the phytoremediation potential of Petunia grandiflora Juss., for remediation of a mixture of dyes and dye containing wastewater (Watharkar et al., 2013). Three plant species Brassica juncea, Phaseolus mungo and Sorghum vulgare have been studied for its decolorization potential against azo dyes of textile effluents. These plants (B. juncea, P. mungo and S. vulgare) showed color removal of textile effluent up to 79, 53 and 57%, respectively (Ghodake et al., 2009). Similarly, various other plants are also reported for dye decolorization for instance, Blumea malcolmii and Typhonium flagelliforme P.K.   The main advantages of dye removal by plants is that it is an autotrophic system with a large biomass which requires little nutrient cost, is easier to handle, and is generally accepted by the public due to both its aesthetic demand and environmental sustainability (Ghodake et al., 2009;Kagalkar et al., 2009).
Although extensive research have been carried out to establish effective and efficient phytoremediation techniques for decolorization, and degradation of azo dyes, large scale application of phytoremediation is not feasible and presently it faces a number of problems including the extent of pollutants tolerated by the plant, the bioavailable fraction of the contaminants and evapotranspiration of volatile organic pollutants as well as requirement of large areas to establish the treatment plants (Williams, 2002).

Role of Nanotechnology in Decolorization and Degradation of Azo Dyes
Biological treatment for the decolorization and degradation of azo dyes has advantages and disadvantages, which have been earlier described in this review. Bioremediation is an eco-friendly approach for the treatment of textile wastewater, but on the other hand physico-chemical characteristics of the effluents, including pH, temperature, the content of NaCl and other salts and the presence of organic compounds, can result in the deactivation of enzymes and microbial cells. Therefore, it is necessary to search for more active and versatile enzymes, microorganisms and techniques with high stability and low cost that is suitable to meet the requirements of textile industry wastewater treatment.
In the last few years various nanoparticles are widely used for the degradation of azo dyes. Nanotechnology is a very promising technique, having an important role in improvement of manufacturing technologies, telecommunications, electronics, health and even in environmental remediation (Gross, 2001;Kim et al., 2005;Moore, 2006). Recently, nanomaterials (NMs) have been used as efficient, cost effective and eco-friendly alternative to existing treatment materials, from the standpoints of both resource conservation and environmental remediation (Dimitrov, 2006;Dastjerdi and Montazer, 2010). With unique physical and chemical properties nanomaterials have a remarkable potential for toxic contaminant removal. Nanotechnology offers production and utilization of a different range of NMs, with size ranging from 1 to 100 nm and exhibit unique properties which are not found in bulk-sized materials (Stone et al., 2010;Wang et al., 2010). under anaerobic and aerobic conditions. The complete decolorization of Methyl Red at a concentration of 100 mg l -1 by A. jandaei strain SCS5 was recorded within 6 h for both anaerobic and aerobic suspended cultures, but the decolorization rate was faster in acidic than basic P.K. Singh and R.L. Singh (2017) Int. J. Appl. Sci. Biotechnol. Vol 5(2): 108-126 This paper can be downloaded online at http://ijasbt.org&http://nepjol.info/index.php/IJASBT conditions. Decolorization of azo dye MR involved the use of mediators anthraquinone-2,6-disulfonate and Fe3O4 nanoparticles.

Conclusions
Azo dyes account for the majority of all dyestuffs produced and employed in the textile industries. They are considered as electron-deficient xenobiotic compounds containing the aromatic functionality with one or more azo (-N=N-) groups. Textile industries are largest generator of dye containing wastewater. The release of dye containing effluent into the environment is of great concern due to its aesthetic value, toxicity, mutagenicity and carcinogenicity. Therefore, effluents from textile industries have serious environmental concern and the removal of dyes from effluent is necessary prior to their disposal. Biological processes including microorganisms such as bacteria, fungi, yeast, algae and plants overcome the limitations of physico-chemical techniques and provide an alternative approach to existing technologies for removal of azo dyes. These methods are inexpensive, environment friendly and applicable to different structural varieties of dyes. This review emphasizes the potential of biological systems and their possible mechanisms for decolorization and degradation of azo dyes. Bacteria is the most frequently applied microorganisms for the removal of dyes from textile effluents because they are easy to cultivate, adapted to survive in extreme environmental conditions and decolorize the azo dyes at a faster rate as compared to other available microorganisms. The effectiveness of microbial processes for bioremoval of azo dyes depends on the adaptability and the activity of selected microorganisms. For mineralization of azo dyes, treatment systems having mixed microbial populations or consortium are more effective due to concerted metabolic activities of the microbial community. The role of oxidative and reductive enzymes and their possible mechanisms to understand the biochemical basis of decolorization and degradation of azo dyes are also addressed in this review. Moreover, the development of innovative nanotechnologies is the future solution to the problem of colored wastewater of textile dyeing industries. Nanotechnology coupled with conventional biological processes will play a critical role in increasing environmental protection.