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Dyes may be defined as compounds that, when applied to a substrate provide colour by a process that alters, at least temporarily, any crystal structure of the coloured substances. Such substances with considerable colouring capacity are widely employed in the textile, pharmaceutical, food, cosmetics, plastics, photographic and paper industries. The dyes can adhere to compatible surfaces by solution, by forming covalent bond or complexes with salts or metals, by physical adsorption or by mechanical retention.
2.1.1 Classification of dyes
Hunger et al mentioned that dyes can be classified in two ways. Dyes are classified according to their application and chemical structure, and are composed of a group of atoms known as chromophores, responsible for the dye color. These chromophore-containing centers are based on diverse functional groups, such as azo, anthraquinone, methine, nitro, arilmethane, carbonyl and others. In addition, electrons withdrawing or donating substituents so as to generate or intensify the color of the chromophores are denominated as auxochromes. Another type of classification is based on their usage or to the type of material they are applied to. The classification of dyes by usage or application is the most important system adopted by the Colour Index International (CI).
2.1.2 Reactive dyes.
Reactive dyes are mainly used for printing and dyeing of cotton fibres. These are dyes that will chemically bond with the fibrous molecules. Most of reactive dyes have an azo group. Examples of such dyes include metalized-azo, triphendioxazine, phthalocyanine, formazan, and others. These dyes are much simpler molecular structure than direct dyes. Compared to direct dye, reactive dye produces brighter shades.
2.1.3 Direct dyes.
These anionic dyes are soluble in water to form aqueous solution. Direct dyes have high affinity to cellulosic material and fibres. The majority of the dyes of this category are compounds containing polyazo groups and oxazine groups. To improve wash fastness, chelations with metal salts such as chromium and copper salts are commonly applied to the dyestuff
2.1.4 Vat dyes.
These dyes are water insoluble and can apply mainly to cellulose fibres by converting them to their leuco compounds. The latter was carried out by reduction and solubilization with sodium hydro-sulphite and sodium hydroxide solution, which is called ââ‚¬Å“vatting processââ‚¬Â. The main chemical structural and functional groups of vat dyes are anthraquinone and indigoid.
2.1.5 Basic dyes (cationic dyes)
These dyes are cationic and water soluble. They are applied on paper, polyacrylonitrile, modified nylons, and modified polyesters. In addition, they are used to apply with silk, wool, and tanninââ‚¬"mordant cotton when brightness shade was more necessary than fastness to light and washing.
They are water soluble anionic dyes and are applied on nylon, wool, silk, and modified acrylics. Moreover, they are used to dye paper, leather, printing industry, food products, and in the cosmetic industry.
2.1.7 Mordant dyes.
These type dyes have mordant dyeing characteristics with good quality in the presence of certain groups in the dye molecule structure. These groups are capable to hold metal residuals by formation of covalent and coordinate bonds involving a chelate compound.The salts of aluminium, chromium, copper, cobalt, nickel, iron, and tin are used as mordant that their metallic salts
They are water insoluble and are applied to cotton in the form of sodium salts by the reduction process using sodium sulphide as the reducing agent under alkaline conditions. The low cost and good wash fastness properties of dyeing makes these dyes economic attractive.
2.2 Dyeing process
Modern dyeing technology consists of several steps selected according to the nature of the fiber and properties of the dyes and pigments for use in fabrics, such as chemical structure, classification, commercial availability, fixing properties compatible with the target material to be dyed, economic considerations and many others .Dyeing methods have not changed much with time. Basically water is used to clean, dye and apply auxiliary chemicals to the fabrics, and also to rinse the treated fibers or fabrics. Dyeing process involves three steps namely preparation, dyeing and finishing.
This is the step in which unwanted impurities are removed from the fabrics before dyeing. This can be carried out by cleaning with aqueous alkaline substances and detergents or by applying enzymes. Many fabrics are bleached with hydrogen peroxide or chlorine containing compounds in order to remove their natural color, and if the fabric is to be sold white and not dyed, optical brightening agents are added.
Dyeing is the aqueous application of color to the textile substrates, mainly using synthetic organic dyes and frequently at elevated temperatures and pressures in some of the steps. It is important to point out that there is no dye which dyes all existing fibers and no fiber which can be dyed by all known dyes. During this step, the dyes and chemical aids such as surfactants, acids, alkali/bases, electrolytes, carriers, leveling agents, promoting agents, chelating agents, emulsifying oils, softening agents etc are applied to the textile to get a uniform depth of color with the color fastness properties suitable for the end use of the fabric. This process includes diffusion of the dye into the liquid phase followed by adsorption onto the outer surface of the fibers, and finally diffusion and adsorption on the inner surface of the fibers.
Finishing involves treatments with chemical compounds aimed at improving the quality of the fabric. Permanent press treatments, water proofing, softening, antistatic protection, soil resistance, stain release and microbial/fungal protection are all examples of fabric treatments applied in the finishing process.
2.3 Textile wastewater, Characteristics and Environmental impacts
The contamination of natural waters has become one of the biggest problems in modern society, and the economical use of this natural resource in production processes has gained special attention, since in predictions for the coming years, the amount of water required per capita is of concern. This environmental problem is related not only to its waste through misuse, but also to the release of industrial and domestic effluents.
2.3.1 Textile effluents
Of the industries with high-polluting power, the textile dyeing industry, responsible for dyeing various types of fiber, stands out. Independent of the characteristics of the dyes chosen, the final operation of all dyeing process involves washing in baths to remove excesses of the original or hydrolyzed dyes not fixed to the fiber in the previous steps . In these baths, it is estimated that approximately 10-50% of the dyes used in the dyeing process are lost, and end up in the effluent, contaminating the environment with about one million tons of these compounds. The dyes end up in the water bodies due mainly to the use of the activated sludge treatment in the effluent treatment plants, which have been shown to be ineffective in removing the toxicity and coloring of some types of dye. The release of improperly treated textile effluents into the environment can become an important source of problems for human and environmental health. The major source of dye loss corresponds to the incomplete fixation of the dyes during the textile fiber dyeing step.
2.3.2 Environmental impacts
In addition to the problem caused by the loss of dye during the dyeing process, within the context of environmental pollution, the textile industry is also focused due to the large volumes of water used by its industrial park, consequently generating large volumes of effluent. It has been calculated that approximately 200 liters of water are needed for each kilogram of cotton produced. These effluents are complex mixtures of many pollutants, ranging from original colors lost during the dyeing process, to associated pesticides and heavy metal, and when not properly treated, can cause serious contamination of the water sources. So the materials that end up in the water bodies are effluents containing a high organic load and biochemical oxygen demand, low dissolved oxygen concentrations, strong color and low biodegradability. In addition to visual pollution, the pollution of water bodies with these compounds causes changes in the biological cycles, particularly affecting the photosynthesis and oxygenation processes of the water body by hindering the passage of sunlight through the water.
Moreover, studies have shown that some classes of dye, especially azo dyes and their byproducts, may be carcinogenic and / or mutagenic, endangering human health, since the wastewater treatment systems and water treatment plants (WTP) are ineffective in removing the color and the mutagenic properties of some dyes .The difficulty in removing them from the environment can be attributed to the high stability of these compounds, since they are designed to resist biodegradation to meet the demands of the consumer market with respect to durability of the colors in the fibers, consequently implying that they also remain in the environment for a long time .Based on all the problems cited above regarding the discharge of effluents into the environment, it is obvious there is a need to find alternative treatments that are effective in removing dyes from effluents.
2.4 Treatment of Textile effluents
Generally, treatment of textile effluent is rather difficult as those textile plants generate complex type of wastewater which can contain multiple components. The concentration of colorants present in that type effluent undergoes constant variations daily or even hourly. The strongly coloured textile effluent contains enormous amount of suspended solids and is characterized with a high COD content and irregular variation in pH which can make it even more difficult to treat. As treatment of textile effluents as it is not usually the most economical, some small scale dyeing industries are facing closure since they cannot do so (Rao and Rao, 2006). Various methods for treatment of the textile effluent exist, but the best combination of treatment methods and system will differ from plant to plant which will based upon the size of the plant, the nature of the pollutants and extent of treatment needed for compliance to norms. Generally the treatment options can be divided into three main categories namely biological, chemical and physical methods.
2.4.1 Biological method
Various types of biological treatment methods exist for the remediation of textile effluents. These include trickling filters, activated sludge process, anaerobic process, oxidation pond etc. Up to now, the most common options for used for the remediation of dye effluent are aerobic biological processes, consisting primarily of conventional and extended activated sludge system. The principle of biological methods for removing pollutants from wastewater involves the action of bacteria and other microorganism on the degradation of the organic waste. Biological methods are normally cheap and simple to apply and are presently used to eliminate organic matter and colour from textile factory effluents (Kim et al., 2004). Both the combination of anaerobic and aerobic type of treatments have been utilized for the treatment of textile effluents but additional physical or chemical treatment are usually required to achieve compliance. Removal of colour from textile effluents contaminated with azo dyes by aerobic treatment system was found to be quite ineffective in (Oââ‚¬â„¢Neill et al., 2000). As several textile dyes are harmful to micro-organism present in biological treatment system problems such as sludge bulking, rising sludge and pin flock may arise which makes biological process not quite an effective way for treatment of dyestuff wastewater (Ahn et al., 1999). Also due to their low biodegradability, many textiles chemicals and dyes may not be removed hence making biological treatment is not really an effective way for treating textile industry wastewater (Pala and Tokat, 2002).
Chemical method includes coagulation/flocculation and Oxidation. Conventional coagulation and flocculation system is removes dye molecules from the dye effluent rather than partially decomposing them into simpler aromatic molecules which evenly more be harmful and hazardous (Golob et al., 2005). Chemical treatment techniques are more effective than biological processes in terms colour removal, but those processes are however more energy intensive and may involve the use of large quantity of chemicals. The major drawback of coagulation/flocculation is that sludge is produced (Golob et al., 2005). Large quantity of sludge is generated during the process and may ultimately become another pollutant itself hence increasing the cost of treating the water (Ahn et al., 1999).
In contrast chemical oxidation makes use strong oxidizing agents to force the degradation of resistant organic pollutant. Chemicals agents can degrade dye molecules in textile wastewater and can also bleach the waste stream. Presently, Fenton oxidation and ozone oxidation are regularly used in treatment of wastewater.
Fenton method is mostly used as a pre-treatment for wastewater which resistant to microbial degradation or which contain compound which are toxic to biomass. In large scale treatment facilities, the process is commonly carried out at ambient temperature in the presence excess of ions as well as hydrogen peroxide. The major disadvantages of the technique are the addition of considerable amount of acid and alkali to achieve the desired pH, the need to decrease the residual ion concentration which is too elevated for release in final effluent, and the related production of high amount of sludge (Sheng.H et al.,1997) .
Ozonation is also a very effective and fast way of decolorizing effluents as double bonds present in most of the dyes are easily broken. Ozonation also inhibit the formation of foam due to residual surfactants .It can also degrade an important fraction of COD. Furthermore, the biodegradability of those effluents can increase, as a high fraction of non-biodegradable and toxic components through will be converted (by limited oxidation) into more easily biodegradable intermediates. Moreover, the treatment does increase neither the volume of wastewater nor the sludge mass. Full scale applications are growing in number, mainly as final polishing treatment. Generally up-stream treatments are required to decrease the amount of suspended solids and hence increase the efficiency of colour removal. However, the negative aspect is the release of potential carcinogenic aromatic amines and other toxic by-products (Sheng.H et al, 1997).
2.4.3 Physical method