"LITERATURE REVIEW ON AZO DYESClassification of dyesDyes are compounds that impart specific colour to the substance being coloured(Pohorecki and Bridgwater 2011). Dyes are a common industrial pollutant belonging to the classof synthetic organic compounds. A large amount of dyes synthesized are not biodegradable oreco-friendly (Pereira and Alves 2012). A student at the Royal College of Chemistry, WilliamHenry Perkin accidently discovered a synthetic purple fast dye that would stain silk. The dye washighly resistant to washing and did not fade in the presence of light (D. Bhimani 2011). Fig 1: Structure of azo dyes Aromatic compounds are known to absorb electromagnetic energy; however thecompounds absorbing in the visible range (~400-800 nm) are colored. Dyes show the presence ofeither a chromophore (-C=C-, -C=O, -NO2, - N=N-, -C=N-and quinoid rings) or an auxochrome(–OH, -COOH, -NH3 and -SO3H). According to the chemical structure of chromophores, dyesare classified into 20-30 groups. Amongst these groups the important and widely used groups areazo (monoazo, diazo, triazo, polyazo), phthalocyanine, anthraquinone and triarylmethane dyes.Azo dyes can be further divided into following classes: Basic dyes, Acid dyes, Direct dyes,Mordant dyes, Disperse dyes, Solvent dyes and Reactive dyes. They can even be divided on thebasis of the chromophore group contained in the dye (D. Bhimani 2011). Although there arevarious azo dyes, we are interested in Direct blue 53, Direct Blue 2B and Direct red 28IUPAC NAME Molecular Formula SynonymsDyeDirect tetrasodium;(6E)-4- C H N Na O S 1. Azovan Blue34 24 6 4 14 4 blue 53 amino-6-[[4-[4-[(2E)-2- 2. Blue, Azovan(8-amino-1-oxo-5,7- 3. Blue, Evan'sdisulfonatonaphthalen- 4. Blue, Evans2-ylidene)hydrazinyl]-3- methylphenyl]-2- 5. C.I. 23860methylphenyl]hydraziny 6. C.I. Direct Blue 53lidene]-5- 7. Evan Blueoxonaphthalene-1,3- disulfonate8. Evan's Blue9. Evans BlueDirect disodium;4-amino-3-[[4- C H N Na O S 1. Congo Red32 22 6 2 6 2 Red 28 [4-[(1-amino-4- 2. Red, Congosulfonatonaphthalen-2- 3. Direct Red 28yl)diazenyl]phenyl]phen yl]diazenyl]naphthalene-1-sulfonateDirect 4,4' - biphenylylene) bis C H N Na O S directly Blue 2B; indigo32 20 6 4 14 4 Blue (The pre)) bis (5-amino- directly 2B; direct Cyan 2B;2B 7-naphthalenedisulfonic indigo directlyacidairedale blue 2d; aizen hasdirect blue; amanil blue2BX ; ATLANTIC blue 2b;Atul direct blue 2b; C. I.Direct Blue 6 Table 1: Information of Azo dyesDye IndustriesThe major reason for increase in dye industries is the increasing demand for textiles,leather and colored paper. More dye is used to obtain dark or bright colors in contrast to lightcolors. The world market for dyes and pigments accounts for around $ 23 billion.Fig 2: Pie chart representing World consumption of synthetic dyes in 2013 The overall growth of dye industries have slowed down recently, still approximately 2%per annum increase is observed. From the world consumption of synthetic dyes-2013 (Fig 2), we can see the major contribution of India in dye consumption. Indian dye industry has flourished tosuch an extent that today, India exports dye and dye intermediates to countries, from where itused to import dyes a decade back.Method for dye degradation-Physical chemical and biologicalVarious methods Physical chemical and biological methods are available for dyedegradation.Physical methodsChemical methods Biological methodsPrecipitation/Sedimentation Catalytic degradation Fungal degradationCoagulation Chemical precipitation Algal treatmentFiltration Reduction Anaerobic digestionAdsorption Oxidation StabilizationFlotation Neutralization Trickling filterFlocculation Ion exchange Activated SludgeElectrolysis Enzymatic processReverse osmosisMembrane treatment Advanced oxidation processes Combinatorial systemsDistillation Ozonation Surface immobilizationElectrochemical oxidation Enzyme immobilizationSolvent extractionPhoto catalytic degradation Fenton reactions Aerate lagoons Table 2: Physical, Chemical and Biological methods for dye degradationChemical methodThe primary goal of any oxidation reaction is generation and use of free radical that arestrong oxidantswhich react with contaminating organic compounds to destroy the compoundthat cannot be transformed by conventional method (Adal et al. 2005).Fenton reagent plays an important role in decolorizing textile effluents which are resistantto biological methods, however it is toxic to living biomass. Ozone works by destroying thechromophores which are responsible for coloration of compound by either directly or indirectlyto form smaller molecule. It is alone or in combination (O3-UV or O3-H2O2) now widely usedto treat industrial effluents. Catalytic degradation , chemical precipitation , reduction, oxidation, Neutralization, Ionexchange, Electrolysis, Advanced Oxidation process Ozination Electrochemical oxidation ,Fenton reaction are all chemical methods of dye removal(Swaminathan et al., 2006). Physical methodAdsorption and ion exchange are two important methods for physical removal of dyefrom effluents (Slokar and Le Marechal, 1997). These methods are influenced by manyphysicochemical properties such as particle size, pH, temperature, dye-adsorbent interaction,sorbent surface area and contact time.(Anjaneyulu, Sreedhara Chary, and Samuel Suman Raj2005)Cheap and wide availability of adsorbents makes them an effective and attractive choicefor dye removal. Other methods which use silica gel, membrane filtration, wood chips are alsowidely used to treat effluent sample.Radiolysis by radiation of samples like environment wastewater, contaminating soil,textile effluent and sediment is also a promising treatment technology. Effect of radiation can beintensified in aqueous solutions since primary product from radiolysis of water can degrade dyemolecules effectively. These methods however face drawbacks of high cost, limited versatility,and low efficiency and disposal problems. Precipitation/sedimentation, Coagulation, Filtration,Adsorption , Flotation, Flocculation, Reverse osmosis , Distillation, Solvent extraction, Photo catalytic degradation are method involved in physical method of dye removal (Getoff 1996).Biological method Physico-chemical methods to decolorize dye effluents are efficient but often costly and face theproblem of disposal (Dias et al. 2003). Biological methods are cheap and their simplicity makesthem widely acceptable for treatment of effluents worldwide. Microbial decolourization involvesusing suitable bacteria, algae and fungi (McMullan et al. 2001) Several actinomycetes strainshave also been reported to have the capacity to decolorize reactive dyes, including azo dyes,anthraquinone and pthalocyanine. A number of azo dyes have been degraded in liquid batchculture using stain of Candida zeylanoides (Zhou and Zimmermann 1993) Fungal mycelia have advantage over single cell organisms as they solubilize insolublesubstrates by producing extracellular enzymes and due to increased cell-to cell surface ratiofungi have greater physical and enzymatic contact with environmental samples. Fungaldegradation, algal treatment , anaerobic digestion, stabilization, trickling filter, activated sludge,enzymatic process, combinatorial system , enzyme immobilization, aerate lagoons etc. aremethods for biological degradation of dye (Kaushik and Malik 2009)Need for dye degradationOne of the major dyes that are manufactured in India is azo dyes. Textile effluents havingazo dyes and their secondary products when discharged improperly in the water bodies lowersthe penetration of sunlight, photosynthesis, dissolved oxygen concentration, quality of water andis toxic to aquatic plants and animals, leading to serious threat to environment throughout theworld (Vandevivere et al. 1998). Azo dyes cannot be naturally degraded. Even if we observesome degradation the compounds produced on degradation are toxic. Some azo-dyes have alsobeen shown to be associated to various types of cancers (human bladder cancer,hepatocarcinoma, splenic carcinomas etc) in experimental laboratory animals and mammaliancells (N Puvaneshwari, J Muthukrishnan, and P Gunasekaran 2006).Due to azo dyes in aquatic systems, the biological oxygen demand (BOD), total organiccarbon (TOC), chemical oxygen demands (COD) are also affected severely (Saratale et al. 2009). Most of the azo dyes can lead to cancer, mutations in genes and also causes toxicity (Myslaket.al 1998). Plants play many important roles in natural ecosystem like preventing soil erosion,providing habitat to wildlife and providing organic matter that is important for fertility of soil.Many reports have also shown that azo dyes in textile effluents also affect severely the plantgermination and biomass rate (Chandra Ram 2016).Therefore, before the final discharge oftextile effluents containing the azo dyes and their secondary products, it should be treatedproperly.One of the greatest sources of liquid effluent pollutants is textile industry since largeamount of water is used in process of dyeing. Waste water obtained from the dyeing processaffects the aqueous ecosystem in terms of salinity, suspended solids, recalcitrance, colour, widerange of pH (Akan et al 2009). Anaerobic breaking of azo dyes produces aromatic amines whichare carcinogenic and toxic.Due to development of more than ten thousand synthetic dyes and their use inmanufacturing has led to large amount of increase in pollution due to textile effluentscontaminated with dyes (D. Bhimani 2011). The textile effluents from the industries containmany different types of dyes and other components like sodium chloride, bases, acids,detergents, salts, humectants, dispersants, oxidants, hardness, etc. (Tchobanoglous and Burton1995)There are many physicochemical methods that are used to treat textile effluentscontaining dyes like coagulation and adsorption. These methods don’t destroy the dyescompletely rather change them from one form to another; therefore, problem remains unsolved(Sokolowska-Gajda, Freeman, and Reife 1996). Reports have been made of azo dye degradationusing different microorganisms (McMullan et al. 2001), bacteria (Rajaguru P et.al 2000), yeast(Martins et al., 1999) and fungi (Pointing 2001).There are many disadvantages of using physicochemical methods like cost (since use oflarge amount of chemicals and energy), pollution due to secondary metabolites (since nocomplete degradation), complex procedures (Forgacs, Cserháti, and Oros 2004). On the otherhand, degradation and decolourization of textile effluents containing azo dyes usingmicroorganisms and enzymes is eco-friendly, cost effective alternative to physicochemicalmethods (Ogugbue, Akubuenyi, and Ibiene 2012). Problems have been created due to reactive dye processes of waste waters since fixationof dyes with fibres is very low. Dyes are highly water soluble and cannot be removed byconventional methods. Municipal sewerage systems cannot decolourize the textile effluent dyes(Wilmot et al, 1998).In 1974, the Ecological and Toxicological Association of the Dye stuffManufacturing Industry (ETAD) was form aiming to decrease the damage of environment,consumer protection in cooperation with government (Aniker 1979). Over 90% of some 4000dyes tested in an ETAD survey had LD50 values greater than 2x103 mg /kg. The highest rates oftoxicity were found amongst basic and diazo direct dyes (Shore 1996).The gastro-intestinal tract of humans harbors a wide variety of microbial species. Watersoluble as well as water insoluble azo dyes can be reduced by theses intestinal microbes. Themetabolites produced by the intestinal microbes have shown carcinogenic effects on human,although the parent compound may not be toxic or carcinogenic. Several intestinal microbeshave shown the presence of azo-reductase activity (Feng, Cerniglia, and Chen 2012)Microbial degradation of textile dye:Aeromonas hydrophila shows decolorization of 70-80 % of reactive red 141 of 3.8 g/L in24hrs.. Psedominas demolyticum shows decolorization of direct blue 6 and red HE7B with 70and 80 percent reduction within 5 hours with the help of LiP, Laccase tyrosinase enzyme.Rhizobium radiobacter gave 90 % decolorization of reactive red 141, methyl violet (10-50 mg/L)with time interval of 8-49 hours using enzyme LiP, MG reductase, DCIP reductase. Kocuriarosea MTCC 1532 shows dye decolorization of Malachide green, methyl orange (50 mg/L each)with time interval of 5 hours showing 80-100 % decolorization using DCIP reductase, MGreductase, LiP . laccase and tyrosinase. Sphingomonas sp BN6 gave decolorization of acid azodye, direct azo dye and amaranthwith the help of enzyme Falvin reductase (Saratale et al.2011).Factor affecting bacterial bioremediation of textile dyes The nature of the textile dye varies depending upon the highly fluctuating levels ofinorganic ions, heavy metals, dye concentration, toxic compounds, nutrients and salts. These "