Dyes may be defined as compounds that, when applied to a substrate provide coloring material by a procedure that alters, at least temporarily, any crystal construction of the colored substances. Such substances with considerable coloring capacity are widely employed in the fabric, pharmaceutical, nutrient, cosmetics, plastics, photographic and paper industries. The dyes can adhere to compatible surfaces by solution, by organizing covalent bond or composites with salts or metals, by physical surface assimilation or by mechanical keeping.
2.1.1 Categorization of dyes
Hunger et Al mentioned that dyes can be classified in two ways. Dyes are classified harmonizing to their application and chemical construction, and are composed of a group of atoms known as chromophores, responsible for the dye colour. These chromophore-containing centres are based on diverse functional groups, such as azo, anthraquinone, methine, nitro, arilmethane, carbonyl and others. In add-on, negatrons retreating or donating substituents so as to bring forth or escalate the colour of the chromophores are denominated as auxochromes. Another type of categorization is based on their use or to the type of stuff they are applied to. The categorization of dyes by use or application is the most of import system adopted by the Colour Index International ( CI ) .
2.1.2 Reactive dyes.
Reactive dyes are chiefly used for printing and dyeing of cotton fibers. These are dyes that will chemically bond with the hempen 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 construction than direct dyes. Compared to direct dye, reactive dye produces brighter sunglassess.
2.1.3 Direct dyes.
These anionic dyes are soluble in H2O to organize aqueous solution. Direct dyes have high affinity to cellulosic stuff and fibers. The bulk of the dyes of this class are compounds incorporating polyazo groups and oxazine groups. To better wash speed, chelations with metal salts such as Cr and Cu salts are normally applied to the dyestuff
2.1.4 Vat dyes.
These dyes are H2O indissoluble and can use chiefly to cellulose fibers by change overing them to their leuco compounds. The latter was carried out by decrease and solubilization with sodium hydro-sulphite and sodium hydrated oxide solution, which is called aa‚¬A“vatting processaa‚¬A? . The chief chemical structural and functional groups of vat dyes are anthraquinone and indigoid.
2.1.5 Basic dyes ( cationic dyes )
These dyes are cationic and H2O soluble. They are applied on paper, polyacrylonitrile, modified rayonss, and modified polyesters. In add-on, they are used to use with silk, wool, and tanninaa‚¬ ” black cotton when brightness shadiness was more necessary than speed to visible radiation and lavation.
They are H2O soluble anionic dyes and are applied on nylon, wool, silk, and modified acrylics. Furthermore, they are used to dye paper, leather, publishing industry, nutrient merchandises, and in the decorative industry.
2.1.7 Black dyes.
These type dyes have black dyeing features with good quality in the presence of certain groups in the dye molecule construction. These groups are capable to keep metal remainders by formation of covalent and organize bonds affecting a chelate compound.The salts of aluminum, Cr, Cu, Co, nickel, Fe, and Sn are used as mordant that their metallic salts
They are H2O indissoluble and are applied to cotton in the signifier of Na salts by the decrease procedure utilizing sodium sulfide as the cut downing agent under alkalic conditions. The low cost and good wash speed belongingss of dyeing makes these dyes economic attractive.
2.2 Dyeing procedure
Modern dyeing engineering consists of several stairss selected harmonizing to the nature of the fibre and belongingss of the dyes and pigments for usage in cloths, such as chemical construction, categorization, commercial handiness, repairing belongingss compatible with the mark stuff to be dyed, economic considerations and many others.Dyeing methods have non changed much with clip. Basically H2O is used to clean, dye and use subsidiary chemicals to the cloths, and besides to rinse the treated fibres or cloths. Dying procedure involves three stairss viz. readying, dyeing and coating.
This is the measure in which unwanted drosss are removed from the cloths before dyeing. This can be carried out by cleaning with aqueous alkaline substances and detergents or by using enzymes. Many cloths are bleached with H peroxide or Cl incorporating compounds in order to take their natural colour, and if the cloth is to be sold white and non dyed, optical brightening agents are added.
Dyeing is the aqueous application of colour to the fabric substrates, chiefly utilizing man-made organic dyes and often at elevated temperatures and force per unit areas in some of the stairss. It is of import to indicate out that there is no dye which dyes all bing fibres and no fibre which can be dyed by all known dyes. During this measure, the dyes and chemical AIDSs such as wetting agents, acids, alkali/bases, electrolytes, bearers, leveling agents, advancing agents, chelating agents, emulsifying oils, softening agents etc are applied to the fabric to acquire a unvarying deepness of colour with the colour speed belongingss suited for the terminal usage of the cloth. This procedure includes diffusion of the dye into the liquid stage followed by surface assimilation onto the outer surface of the fibres, and eventually diffusion and surface assimilation on the interior surface of the fibres.
Completing involves interventions with chemical compounds aimed at bettering the quality of the cloth. Permanent imperativeness interventions, H2O proofing, softening, antistatic protection, dirt opposition, stain release and microbial/fungal protection are all illustrations of fabric interventions applied in the finishing procedure.
2.3 Textile effluent, Characteristics and Environmental impacts
The taint of natural Waterss has become one of the biggest jobs in modern society, and the economical usage of this natural resource in production procedures has gained particular attending, since in anticipations for the coming old ages, the sum of H2O required per capita is of concern. This environmental job is related non merely to its waste through abuse, but besides to the release of industrial and domestic wastewaters.
2.3.1 Fabric wastewaters
Of the industries with high-polluting power, the fabric dyeing industry, responsible for dyeing assorted types of fibre, stands out. Mugwump of the features of the dyes chosen, the concluding operation of all dyeing procedure involves rinsing in baths to take surpluss of the original or hydrolyzed dyes non fixed to the fibre in the old stairss. In these baths, it is estimated that about 10-50 % of the dyes used in the dyeing procedure are lost, and stop up in the wastewater, polluting the environment with about one million dozenss of these compounds. The dyes end up in the H2O organic structures due chiefly to the usage of the activated sludge intervention in the outflowing intervention workss, which have been shown to be uneffective in taking the toxicity and colouring of some types of dye. The release of improperly treated fabric wastewaters into the environment can go an of import beginning of jobs for homo and environmental wellness. The major beginning of dye loss corresponds to the uncomplete arrested development of the dyes during the fabric fibre dyeing measure.
2.3.2 Environmental impacts
In add-on to the job caused by the loss of dye during the dyeing procedure, within the context of environmental pollution, the fabric industry is besides focused due to the big volumes of H2O used by its industrial park, accordingly bring forthing big volumes of wastewater. It has been calculated that about 200 litres of H2O are needed for each kg of cotton produced. These wastewaters are complex mixtures of many pollutants, runing from original colourss lost during the dyeing procedure, to associated pesticides and heavy metal, and when non decently treated, can do serious taint of the H2O beginnings. So the stuffs that end up in the H2O organic structures are wastewaters incorporating a high organic burden and biochemical O demand, low dissolved O concentrations, strong colour and low biodegradability. In add-on to ocular pollution, the pollution of H2O organic structures with these compounds causes alterations in the biological rhythms, peculiarly impacting the photosynthesis and oxygenation procedures of the H2O organic structure by impeding the transition of sunshine through the H2O.
Furthermore, surveies have shown that some categories of dye, particularly azo dyes and their by-products, may be carcinogenic and / or mutagenic, jeopardizing human wellness, since the effluent intervention systems and H2O intervention workss ( WTP ) are uneffective in taking the colour and the mutagenic belongingss of some dyes.The trouble in taking them from the environment can be attributed to the high stableness of these compounds, since they are designed to defy biodegradation to run into the demands of the consumer market with regard to lastingness of the colourss in the fibres, accordingly connoting that they besides remain in the environment for a long clip.Based on all the jobs cited above sing the discharge of wastewaters into the environment, it is obvious there is a demand to happen alternate interventions that are effectual in taking dyes from wastewaters.
2.4 Treatment of Textile wastewaters
By and large, intervention of fabric wastewater is instead hard as those fabric workss generate complex type of effluent which can incorporate multiple constituents. The concentration of colorants present in that type wastewater undergoes changeless fluctuations daily or even hourly. The strongly coloured fabric wastewater contains tremendous sum of suspended solids and is characterized with a high COD content and irregular fluctuation in pH which can do it even more hard to handle. As intervention of fabric wastewaters as it is non normally the most economical, some little graduated table dyeing industries are confronting closing since they can non make so ( Rao and Rao, 2006 ) . Assorted methods for intervention of the fabric wastewater exist, but the best combination of intervention methods and system will differ from works to works which will based upon the size of the works, the nature of the pollutants and extent of intervention needed for conformity to norms. By and large the intervention options can be divided into three chief classs viz. biological, chemical and physical methods.
2.4.1 Biological method
Assorted types of biological intervention methods exist for the redress of fabric wastewaters. These include dribbling filters, activated sludge procedure, anaerobiotic procedure, oxidization pool etc. Up to now, the most common options for used for the redress of dye wastewater are aerophilic biological procedures, dwelling chiefly of conventional and extended activated sludge system. The rule of biological methods for taking pollutants from effluent involves the action of bacteriums and other micro-organism on the debasement of the organic waste. Biological methods are usually inexpensive and simple to use and are soon used to extinguish organic affair and coloring material from fabric mill wastewaters ( Kim et al. , 2004 ) . Both the combination of anaerobiotic and aerophilic type of interventions have been utilized for the intervention of fabric wastewaters but extra physical or chemical intervention are normally required to accomplish conformity. Removal of coloring material from fabric wastewaters contaminated with azo dyes by aerophilic intervention system was found to be rather uneffective in ( Oaa‚¬a„?Neill et al. , 2000 ) . As several textile dyes are harmful to microorganism nowadays in biological intervention system jobs such as sludge bulking, lifting sludge and pin flock may originate which makes biological procedure non rather an effectual manner for intervention of dyestuff effluent ( Ahn et al. , 1999 ) . Besides due to their low biodegradability, many fabrics chemicals and dyes may non be removed therefore doing biological intervention is non truly an effectual manner for handling fabric industry effluent ( Pala and Tokat, 2002 ) .
Chemical method includes coagulation/flocculation and Oxidation. Conventional curdling and flocculation system is removes dye molecules from the dye wastewater instead than partly break uping them into simpler aromatic molecules which equally more be harmful and risky ( Golob et al. , 2005 ) . Chemical intervention techniques are more effectual than biological procedures in footings colour remotion, but those procedures are nevertheless more energy intensive and may affect the usage of big measure of chemicals. The major drawback of coagulation/flocculation is that sludge is produced ( Golob et al. , 2005 ) . Large measure of sludge is generated during the procedure and may finally go another pollutant itself therefore increasing the cost of handling the H2O ( Ahn et al. , 1999 ) .
In contrast chemical oxidization makes usage strong oxidising agents to coerce the debasement of immune organic pollutant. Chemicals agents can degrade dye molecules in fabric effluent and can besides decolor the waste watercourse. Soon, Fenton oxidization and ozone oxidization are on a regular basis used in intervention of effluent.
Fenton method is largely used as a pre-treatment for effluent which resistant to microbic debasement or which contain compound which are toxic to biomass. In big graduated table intervention installations, the procedure is normally carried out at ambient temperature in the presence surplus of ions every bit good as H peroxide. The major disadvantages of the technique are the add-on of considerable sum of acid and base to accomplish the coveted pH, the demand to diminish the residuary ion concentration which is excessively elevated for release in concluding wastewater, and the related production of high sum of sludge ( Sheng.H et al.,1997 ) .
Ozonation is besides a really effectual and fast manner of bleaching wastewaters as dual bonds present in most of the dyes are easy broken. Ozonation besides inhibit the formation of froth due to residuary wetting agents.It can besides degrade an of import fraction of COD. Furthermore, the biodegradability of those wastewaters can increase, as a high fraction of non-biodegradable and toxic constituents through will be converted ( by limited oxidization ) into more easy biodegradable intermediates. Furthermore, the intervention does increase neither the volume of effluent nor the sludge mass. Full graduated table applications are turning in figure, chiefly as concluding shining intervention. By and large up-stream interventions are required to diminish the sum of suspended solids and hence addition the efficiency of coloring material remotion. However, the negative facet is the release of possible carcinogenic aromatic aminoalkanes and other toxic byproducts ( Sheng.H et al, 1997 ) .
2.4.3 Physical method