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

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

Kiran Meghwal, Srishti Kumawat, Chetna Ameta, Nirmala Kumari Jangid
Copyright: © 2020 |Pages: 25
DOI: 10.4018/978-1-7998-0311-9.ch005
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As the textile industries use aqueous method for dyeing processes, the color that is released in the environment is associated with the incomplete absorption of dyes on fibres. So, there is a strong need to reduce the amount of residual dye in textile effluent. Large amounts of liquid wastes are produced from textile industries that contain both organic and inorganic compounds. The degradation of azo dyes is difficult using the conventional processes. These complex azo dyes containing N=N bond have been found to show carcinogenic evidences on reductive cleavage. Azo dyes have capability to alter physical and chemical properties of soil, causing harm to the water bodies. Dyes are toxic in nature, which is lethal for microorganisms present in soil affecting agricultural productivity. The presence of azo dyes in water decreases its water transparency and water gas solubility. This reduces light penetration through water, decreases its photosynthesis activity, causing oxygen deficiency and de-regulating the biological cycles of aquatic system.
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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).

Key Terms in this Chapter

Chemical Oxygen Demand (COD): Chemical oxygen demand (COD) is a measure of the capacity of water to consume oxygen during the decomposition of organic matter and the oxidation of inorganic chemicals.

Advanced Oxidation Process (AOPs): Advanced oxidation processes (AOPs) are a set of chemical treatment procedures designed to remove organic or sometimes inorganic materials in water and wastewater by oxidation through reactions with hydroxyl radicals (OH).

Biological Oxygen Demand (BOD): Biological Oxygen Demand (BOD) is the amount of dissolved oxygen needed by aerobic biological organisms to break down organic material present in a given water sample at certain temperature over a specific time period.

Azo Dyes: Azo dyes are organic compounds, which contain the colouring azo function (N=N) which is often bound to an aromatic ring.

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