**1. Introduction**

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176 Eco-Friendly Textile Dyeing and Finishing

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> The exact amount of dyes produced in the world is not known. Exact data on the quantity of dyes discharged in the environment are also not available. It is assumed that a loss of 1-2% in production and 1-10% loss in use (after being garment) are a fair estimate [1]. For reactive dyes, this figure can be about 10-20% due to low fixation. Due to large-scale production and extensive application, synthetic dyes can cause considerable environmental pollution and are serious health-risk factors. Although, the growing impact of environmental protection on industrial development promotes the development of eco-friendly technologies, reduced consumption of freshwater and lower output of wastewater [1], the release of important amounts of synthetic dyes to the environment causes public concern, legislation problems and is a serious challenge to environmental scientists.

> Globally, accessing to freshwater is becoming more acute every day. In the dyeing of textile materials, water is used firstly in the form of steam to heat the treatment baths, and secondly to enable the transfer of dyes to the fibers. Cotton, which is the world's most widely used fiber, is also the substrate that requires the most water in its processing [2]. The dyeing and rinsing of 1 kg of cotton with reactive dyes demands from 70 to 150 L water, 0.6 to 0.8 kg NaCl and anywhere from 30 to 60 g dyestuff. More than 80,000 tn of reactive dyes are pro‐ duced and consumed each year, making it possible to estimate the total pollution caused by their use. After the dyeing is completed, the various treatment baths are drained out, includ‐ ing the first dye bath, which has a very high salt concentration, is heavily coloured and con‐ tains a substantial load of organic substances [2]. One solution to this problem consists in mixing together all the different aqueous effluents, then concentrating the pollution and re‐ using the water either as rinsing water or as processing water, depending on the treatment

© 2013 Kyzas et al.; licensee InTech. This is an open access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. © 2013 Kyzas et al.; licensee InTech. This is a paper distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

selected (either nanofiltration or reverse osmosis for the membrane processes). These treat‐ ments apply only to very dilute dye baths [2]. This is generally not the case of the first dye baths recovered which are the most heavily polluted ones. The wastewater produced by a reactive dyeing contains [2,3]: (i) hydrolyzed reactive dyes not fixed on the substrate, repre‐ senting 20-30% of the reactive dyes applied (on average 2 g/L) (this residual amount is re‐ sponsible for the coloration of the effluents and cannot be recycled); (ii) dyeing auxiliaries or organic substances, which are non-recyclable and responsible for the high BOD/COD of the effluents; (iii) textile fibres, and (iv) 60-100 g/L electrolyte, essentially NaCl and Na2CO3, which is responsible for the very high saline content of the wastewater.

**Chromophore group Name Auxogroup Name**

−N=N− Azo –NH<sup>2</sup> Amino

−N=N<sup>+</sup>−O- Azoxy –NHCH<sup>3</sup> Methyl amino −N=N−NH Azoamino –N(CH3)2 Dimethyl amino −N=O, N−OH Nitroso –SO3H Sulphonic acid >C=O Carbonyl –OH Hydroxy

>C=C< Ethenyl –COOH Carboxylic acid

To further examine the interactions between dyes and substrates, the classification of dyes is required. It is very important to know the chemistry of the dyes in dyeing effluents, in order to synthesize a suitable adsorbent with the appropriate functional group. Hunger et al [7] mentioned that dyes are classified in two methods. The main classification is related to the chemical structure of dyes and particularly considering the chromophoric structure present‐ ed in dye molecules. Another type of classification is based on their usage or applying. The classification of dyes by usage or application is the most important system adopted by the

*Reactive dyes*. These dyes form a covalent chemical bond with fiber is ether or ester linkage under suitable conditions. Majority of reactive dyes contains azo that includes metallized azo, triphendioxazine, phthalocyanine, formazan, and anthraquinone. The molecular struc‐ tures of these dyes are much simpler than direct dyes. They also produce brighter shades than direct dyes. Reactive dyes are primarily used for dyeing and printing of cotton fibers.

*Direct dyes*. In the presence of electrolytes, these anionic dyes are water-soluble in aqueous solution. They have high affinity to cellulose fibers. Most of the dyes in this class are polya‐ zo compounds, along with some stilbenes, phthalocyanines, and oxazines. To improve wash fastness, frequently chelations with metal (such as copper and chromium) salts are applied to the dyestuff. Also, their treatment with formaldehyde or a cationic dye-complexing resin.

*Disperse dyes*. These are substantially water insoluble nonionic dyes applied to hydrophobic fibers from microfine aqueous dispersion. They are used predominantly on polyester, polya‐ mide, polyacrylonitrile, polypropylene fibers to a lesser, it is used to dye nylon, cellulose acetate, and acrylic fibers. Chemical structures of dyes are mainly consisted of azo and an‐

–CN Cyano –COCH<sup>3</sup> Acetyl –CONH<sup>2</sup> Amido

Decolorization of Dyeing Wastewater Using Polymeric Absorbents - An Overview

http://dx.doi.org/10.5772/52817

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>C=S Thio –Cl Chloro −NO<sup>2</sup> Nitro –CH<sup>3</sup> Methyl >C=NH, >C=N− Azomethine –OCH<sup>3</sup> Methoxy

**Table 1.** Names of chromophore and auxochrome groups of dyes

Colour Index (CI). Briefly [8]:

In addition, these effluents exhibit a pH of 10-11 and a high temperature (50-70 o C). The le‐ gal regulations respecting the limit values for the release of wastewater are changing and are becoming increasingly severe, including the limits with respect to salinity.

A typical effluent treatment is broadly classified into preliminary, primary, secondary, and tertiary stages [4,5]. The preliminary stage includes equalization and neutralization. The pri‐ mary stage involves screening, sedimentation, flotation, and flocculation. The secondary stage reduces the organic load and facilitates the physical/chemical separation (biological oxidation). The tertiary stage is focused on decolorization. In the latter, adsorption onto vari‐ ous materials can be broadly used to limit the concentration of colour in effluents [3].
