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A dye can generally be described as a colored substance that has an affinity to the substrate to which it is being applied. It is generally applied in an aqueous solution and may require a mordant to improve the fastness of the dye on the fiber (http://en.wikipedia.org/wiki/Dye). India is the second largest exporter of dyestuffs and intermediates developing countries, after China. The textile industry accounts for the largest consumption of dyestuffs, nearly 80%.
The textile industries are to satisfy the ever-growing demands in terms of quality, variety, fastness and other technical requirements. However, a recent study conducted under the National Biodiversity Strategy and Action Plan (BSAP) has revealed that chemical colors have all but wiped out India’s wonderful vegetable dyes. A large variety of dyes and chemicals were used in an attempt to make more attractive popular shades of fabrics for a competitive market, rendering them very complex (Rajagopalan, 1990). Synthetic dyes are frequently found in textile and paper industry effluents and these are generally toxic.
One example of these synthetic dyes is the azo dye (e.g., monoazo, diazo, triazo and polyazo). Azo dyes represent the largest class of organic colorants listed in the color index (i.e., 60-70% of the total). They make up the vast majority of the dyes discharged (Van der Zee, 2002). At recent times, synthetic dyes are the major cause of concern as they are the major pollutants in water.
Azo dye is a synthetic dye that has the azo group of two nitrogen atoms (N=N) connecting aromatic ring compounds. Most of the azo dyes are water soluble, and they color different substrates by becoming physically attached. The attachment may be due to adsorption, absorption or mechanical adherence.
Congo red is the sodium salt of benzidinediazo-bis-1-naphtylamine-4-sulfonic acid (formula: C32H22N6Na2O6S2; molecular weight: 696.66 g/mol). Congo red yields a red colloidal solution. Its solubility is better in organic solvents such as ethanol.
Various physical, chemical and biological pretreatment as well as main treatment techniques were reported to remove color from dye-containing wastewater (Cooper, 1993 as cited by Van der Zee, 2002). Biological techniques include bacterial and fungal biosorption and biodegradation in aerobic or anaerobic/aerobic treatment processes. Over the last two decades, considerable work has been done with the goal of using microorganisms as bioremediation agents in the treatment of wastewater-containing textile dyes (Ramalho et al., 2004). Various bacterial strains reduce azo dyes under anaerobic condition. The most generally accepted hypothesis for this phenomenon is that many bacterial strains possess rather unspecific cytoplasmic enzymes which act as azoreductases (Walker, 1970). Rafii et al. (1990) isolated anaerobic bacteria that produced azoreductases constitutively and released them extracellularly. An azoreductase was purified from
E. faecalis by hydrophobic, anion exchange and affinity chromatography (Punj and John, 2008). Maier et al. (2004) showed that azoreductase is responsible for azo dye reduction by Bacillus strain SF. In an earlier study, two types of bacteria in monoculture and two consortia were isolated and were found to decolorize congo red under fermentative, nitrate reducing and denitrifying condition (Decenaala and Barraquio, 2004). Their study showed that in the presence of oxygen as electron acceptor, no decolorization of the dye occurred. In a succeeding study, Decenaala (2006) isolated congo red-decolorizing bacteria and consortia from polluted and non-polluted samples.
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