Effect of Dyes on Water Chemistry, Soil Quality, and Biological Properties of Water

Effect Of Dyes On Water Chemistry

Textile wastewater has a major problem due to the presence of colored effluent. Although we can say that all dyes are not as much toxic but they have an adverse aesthetic effect because they are visible pollutants. The colored water reduces the entrance of sunlight into the water bodies affecting the aquatic life. In many cases, if the concentration of dye is present below 1 ppm, that produces an obvious water coloration. The source of major pollution of textile wastewater comes from the dyeing and finishing processes. The major pollutants include highly suspended solids (SS), biochemical oxygen demand (BOD), chemical oxygen demand (COD), heat, color, acidity, basicity and other inorganic contaminants. The water insoluble dyes or unreactive dyes like disperse and vat dyes, normally exhibit good exhaustion properties, like these types of dyes have fibre binding properties and can be removed by physical processes such as flocculation. On the other hand, if we have water soluble dyes or reactive dyes as effluents, the removal by conventional methods is not that much useful for color removal. The reactive dyes are colored molecules which are used to dye cellulose fibres generally characterized by N=N azo bonds. The azo bond gives color to azo dyes associated with chromophores. The process includes absorption of azo dyes onto the cellulose and further reaction with fibre. The reaction includes covalent bond formation between the dye molecule and the fibre which is much more resistant to unusual conditions. The reactive parts of the dye react with ionised hydroxyl groups on the cellulose substrate. But in alkaline dyeing condition, the hydroxyl groups are present in the dye bath compete with the hydroxyl groups present on the cellulose fibre that results in the formation of hydrolysed dyes which no longer react with the fibre giving rise to a highly colored effluent. Therefore, it becomes crucial to find an appropriate and effective method of the treatment of wastewater to remove color from textile effluents. Currently, various methods have been used including chemical and physical processes, such as chemical precipitation and separation of pollutants, electrocoagulation, elimination by adsorption on activated carbon etc. But all these methods have a common drawback that they are not destructive instead they transfer the contamination from one phase to another causing a new type of pollution. It becomes necessary to find a new method for further treatment of these effluents (Slokar & Marechal 1998, Galindo et al. 2001, Tunay et al., 1996). Recently, an alternative to conventional methods, is “Advanced Oxidation Processes” (AOPs), which includes the generation of very reactive species such as hydroxyl radicals that has proved very useful for degradation of wide variety of organic pollutants (Kuo & Ho 2001, Legrini et al. 1993). The possible reaction pathways involving the attack of hydroxyl radicals onto organic compounds are: the electrophilic addition of hydroxyl radical to organic compounds (unsaturated or aromatic) that contain a π bond leading to the formation of organic radicals (Equation 1), the hydrogen abstraction by reaction of the hydroxyl radical with a saturated aliphatic compound (Equation 2) and electron transfer with reduction of the hydroxyl radical into a hydroxyl anion by an organic substrate (Bossmann et al., 1998, Tang 2004) (Equation 3).