The dyes comprise of two functional groups known as chromophores and auxochromes. Chromophores In Greek, chroma means colour and phore means bearer have a radical configuration consisting of conjugated double bonds containing delocalised electrons, which is responsible for colour. Auxochromes ("Auxo" means augment) are atoms with non−bonded electrons which render the dye molecule soluble in water, intensify colour and enhance the dye affinity to substrate. Common examples of chromophores and auxochromes are summarised in the Table 2.1.
Dyes exhibit considerable structural diversity and are classified in several ways. Hunger (2003) classifies dyes by their chemistry (e.g. Azo, Anthraquinone, Thrioryl methane) , by their end use (e.g. Food, Textile, Paper) and by their application class. In this review, dyes will be grouped according to their application class rather than their chemical structure due to the complexities of the dye nomenclature. It is also the principal system adopted by the Colour Index (C.I.). This system includes the name of the dye class, its hue, and a number (Gupta and Suhas, 2009).
(Adapted: Hubbe et al., 2012)
Figure 2.1: Dye classification by application
Focus on Reactive Dye
Commercially launched in 1956, reactive dyes are mainly used for the dyeing of cellulosic fibres such as cotton, linen, ramie, rayon and lyocel. These cellulosic fibres represent half of the world's fibre consumption (Klemola et al., 2007). Direct and acid dyes can also be used for the same purpose, but reactive dyes have more advantages. Their chemical structures are much simpler, their absorption spectra show narrower absorption bands, and the colour ability is brighter. The reactive groups present in the dye are capable of forming a covalent bond with the hydroxyl group of the fibre. The principal classes of reactive groups are azo, triphendioxazine, phthalocyanine, formazan, and anthraquinone. However, the dye is highly soluble. A high fraction is therefore hydrolysed and remains in aqueous form due to its low affinity to the fibre. This represents up to 0.6−0.8 g of dye per m3 of waste water effluent discharged (Errais et al., 2011).
C.I. Reactive Blue 221 (C.I. RB 221) Dye
C.I. Reactive Blue 221 is one of the fundamental reactive dyes used when mixing different colour conbinations (Klemola, 2008). It is commonly known as Reactive Blue B−RN or Synazol Blue KR and is classified as a copper formazan complex dye. The main functional groups present are monochlorotriazin and vinylsulphone (Schramm et al., 2002). The simple molecular structure of the dye is illustrated in Figure 2.2.
Source: (KaraoÄŸlu et al., 2010)
Figure 2.2: Simple structure of C.I. RB 221
For better understanding of the dye chemistry, a more detailed structure is provided in Figure 2.3 where the copper formazan and the different chromophores and auxochromes can be easily identified.
Functional Group: vinylsulphone
Functional Group: monochlorotriazin
Auxochrome
Chromophores:
Copper Formazan
Chromophores:
Copper Formazan
Auxochrome
Chromophores:
Copper Formazan
Auxochrome
Chromophores:
Copper Formazan
Functional Group: monochlorotriazinhttp://www.worlddyevariety.com/wp-content/uploads/2012/05/Reactive-Blue-221.gif
Source: ( Xiel et al., 2011)
Figure 2.3: Detailed structure of C.I. RB 221
Like all reactive dyes, part of C.I. RB 221, notably the vinylsulphone group is hydrolysed and remains in an aqueous solution during its application. Depending on the reaction conditions, about 20−40% of the dye derivatives invariably remain in aqueous phase (Murugananthan and Raju., 2010). Table 2.2 provides information about the properties of C.I. RB 221 dye.
Table 2.2: Properties of C.I. RB 221 dye
Appearance
Blue powder
Odour
Odourless
Biodegradability
25−50%
Molecular Formula
C33H24ClCuN9Na3O15S4
Molecular weight (g/mol)
1082.83
Optimum Wave-length(nm)
601 (KaraoÄŸlu et al., 2010)
602 (Alkan et al., 2007)
614 ( Nyanhongo et al.,2002)
Dye Consumption and Wastewater Generation
The textile industry is the largest consumer of synthetic dyes and water and consequently the major generator of coloured wastewater. Out of the 1 million tonnes of synthetic dyes produced annually, 60−70 % is meant for the textile industries (Molen, 2008). The coloured wastewater can arise not only during its application in industries but also during the manufacturing process of dyes. Easton (1995) gave an estimate that 2 % of dyes produced are discharged from the dye manufacturing operation and 10% are discharged from the operation of textile industries.
Waste water during manufacturing of dyes
The dye manufacturing process involves the following steps (Molen, 2008): Chemical synthesis and precipitation of the dye, filtration of the slurry, drying, grinding (Pulverising) and blending of residue before packing. The filtration step generates the highest amount of coloured wastewater (Molen, 2008).
Waste water from textile industry
The wet processing stages in a textile industry are considered to be the most polluting operations ( US EPA, 1997). Coloured wastewater results mainly from dyeing and printing processes, with the dyeing stage being the largest contributor to colour pollution. The flow diagram in Figure 2.4 gives an overview of effluents being discharged from these wet processes.
Large contributor of TSS and BOD: Enzyme and starch
Desizing: Enzyme or acid wash of fabric together with hot water to impart tensile strength.
High BOD loading: sizing material, NaOH & fabric fragments
Scouring: Use of alkaline solution to break down natural oil and emulsify/suspend impurities present in the fabric.
Alkaline effluent with high solid content and low/moderate BOD
Bleaching: Use of hydrogen peroxide, sodium hypochlorite or sodium chlorite to eliminate unwanted coloured matter, rendering the fabric white.
Highly alkaline effluent with low BOD and TSS level
Mercering: Fabric passes through 15−20% Na OH solution, followed by stretching and spraying of hot water to remove most NaOH. Increase fabric's dyeability, luster, and appearance.
Dyeing: Addition of colour to fabric followed by washing and rinsing to remove excess dye.
Printing: Imparting color pattern and design onto the fabric
Largest volume of coloured effluent with high COD, low BOD &toxicity
Effluent containing BOD, COD, SS, solvents and toxic.
Effluent containing colour, high COD, foam and solid
Finishing: Chemical or mechanical treatment (Brushing/Ironing) to improve appearance, texture or performance of fabric.
Adapted: (US EPA, 1997: Tubtimhin, 2002 : Dos Santos et al.,2003)
Figure 2.4: Wet processes in a textile industry
Why is dye wastewater treatment necessary?
Scenario analyses were mentioned the values of dye concentration in some receiving rivers of 5−10 mg/L (average value, 50 days each year) or 1300−1555 mg/L (the worst case, 2 days each year) for batch wise dyeing of cotton with reactive dyes (Zaharra and Suteu, 2012). According to Mauritius EPA 2003, colour is one parameter that shall be controlled in effluent from dye houses and washing units in the Textile sector. Unlike others countries, like Taiwan, where the true color discharge limit is 400 American Dye Manufacturer Institute (ADMI) units ( Kao et al., 2001), Mauritius has not yet established any colour discharge limit. It is only stipulated in the EPA (2003) that the presence of colour should be non−objectionable. However, it should be pointed out that less than 1 ppm of dye content causes obvious water coloration, alerting the authorities (Banat et al., 1996: Rafatullaha et al., 2010). There is therefore increasing pressure on the textile industry to adopt the most versatile treatment method, in order to regulate its coloured effluent discharge.
In addition, the discharge of such highly coloured dye effluents is a real treat to the environment and its biodiversity. Reactive dyes consist of toxic compounds such as metal chlorides and aromatics which interfere with light penetration: inhibiting light penetration and leading to the direct destruction of aquatic life. For C.I. RB 221 dye, a LC50 > 100mg/l is deemed to be detrimental to aquatic animal (As per MSDS provided by Space International in India). Dyes may also reach human beings through the food chain. Various studies have also shown that reactive dyes exhibit carcinogenic properties (Klemola et al.,2007) and can cause diseases and disorders ranging from nausea and allergic dermatomes to damage of the reproductive system, liver, brain and nervous system (Daneshwar et al., 2003; Solpan et al., 2003).
Traditionally, most dye houses tend to be hostile towards water reduction policies based on the belief that high quality product will be produced only by using high water volumes. U.S EPA (1997) reported a waste water generation of around 15−20 gallons/lb of reactive dyes from dyeing processes. This represents a considerable amount of water which can be recuperated and reused if appropriate treatment methods are established.
Review on Existing Dye Removal Technology
The split of C=C, N=N and heterocyclic and aromatic ring present in reactive dyes can be accomplished by physical, chemical and biological methods (Kobal, 2007). Different techniques under these methods have been established to reduce the colour from polluted effluent. The tables below give a brief description of these techniques and their associated limitations.
Physical Treatment Method
Description
Limitation
Established Recovery Process
Membrane Filtration: Microfiltration, ultrafiltration, nano−filtration, and reverse−osmosis.
High working pressure, expensive, incapable of treating large volume (Grini, 2006).
Significant energy consumption (Gupta and Suhas, 2009).
Relatively short membrane life (Gupta and Suhas, 2009).
Concentrate sludge production (Jain et al., 2011).
High membrane area and frequent membrane replacement due to clogging, particularly with ultra−filtration and nano−filtration (Gupta and Suhas, 2009).
Ion Exchange: A reversible chemical process wherein an ion from dye solution is exchanged for a similarly charged ion attached to an immobile solid particle.
Economic constraints and not effective to all dyes (Grini, 2006: Jain et al., 2011).
Table 2.3: Physical Decolourisation Techniques
Chemical Treatment Method
Description
Limitation
Conventional Treatment
Chemical Coagulation / Flocculation: Use of alum, ferrous sulphate and ferric chloride to remove suspended and colloidal particles.
High cost of coagulants and flocculants and excessive production of sludge, requiring appropriate disposal and treatment method.
Ineffective on water soluble dyes such as reactive dye (Gupta and Suhas, 2009).
Electrochemical: Electro oxidation with non−soluble anodes or Electro−coagulation using consumable materials.
Sludge production and also pollution from chlorinated organic, heavy metals due to indirect oxidation (Gupta and Suhas, 2009).
High cost of electricity (Jain et al., 2011).
Established Recovery Process
Oxidation: Chemical Oxidation and/ or U.V assisted oxidation using chloride, hydrogen peroxide, Fenton's reagent , ozone and potassium manganate
Required long retention time for decolourisation of reactive dyes with chloride (Gupta and Suhas, 2009).
Release of amines as by-products with H2O2/UV oxidation (Mahmoud et al., 2007).
Concentrated sludge production with Fenton reagent (Jain et al., 2011).
Oxidation of H2O2 with O3 involves handling of toxic and hazardous substances (Soklar and Le Marechal, 1997).
Emerging Technology:AOP
Photocatalysis: Advanced oxidation using catalyst such as TiO2, ZnO, ZrO2, CeO2 in the presence of U.V light.
Limitation of light penetration, fouling of catalyst and difficulty to separate the fine catalyst from treated liquid (Gupta and Suhas, 2009).
Sonolysis: Use of ultrasonic waves.
High capital investment, noise pollution and extreme conditions are required (Aline et a.l, 2006).
Table 2.4: Chemical Decolourisation Technology
Table 2.5: Biological Decolourisation Technology
Biological Treatment Method
Conventional Treatment
Description
Limitation
Aerobic Treatment: Degradation in the presence of O2 by aerobes such as bacteria and fungi.
Incapable to obtain satisfactory colour elimination (Robinson et al., 2001).
Reactive dyes are recalcitrant to biological breakdown (Rai et al., 2005).
Large land area requirement and less flexible in design and operation (Bhattacharya and Sharma, 2003).
Anaerobic Treatment: Degradation in the absence of O2 by anaerobes or facultative microorganisms.
Waste water from vinylsulphone reactive dye (Such as RB 221) bath is potentially toxic to anaerobes (Murugananthan and Raju., 2010).
Sulphates present in dye are degraded to toxic sulphide (Gupta and Suhas, 2009).
Previous Studies on C.I. RB 221 Dye
Natural clay materials such as sepiolite (Alkan et al., 2007) and kaolinite (KaraoÄŸlu et al., 2010) were tested on Reactive Blue Dye 221. However, low adsorption capacity was achieved (as indicated in Table 2.6.) since these materials tend to exhibit a negatively charged structure, favouring cationic dye adsorption rather than anionic dye adsorption (Errais et al.,2012).
Table 2.6: Adsorption study on RB 221 using clay materials
Adsorbent
Absorbent capacity, mol/g
