Synthetic Dyes

A dye is a coloured compound that can be applied on a substrate by one of the various processes of dyeing, printing, surface coating, and so on. Most of the dyes have been made from natural sources such as various parts of plants or certain animals until synthetic dyes were developed in the late 1800s and early 1900s. Most of the synthetic dyes are aromatic organic compounds and generally, the substrate that is the material on which dye is being applied includes textile fibers, polymers, foodstuffs, oils, leather, and many other similar materials. Dyestuffs are produced over 700,000 tons annually estimated from more than 100,000 commercially available dyes and applied in many different industries. A dye molecule consists of two major elements known as chromophores and auxochromes whose structures. Chromophores ( ' 'chroma'' means colour and ''phore'' means bearer) is an unsaturated group basic that is responsible for colour while auxochromes ( ' ' Auxo'' means augment) are the characteristic groups which intensify color or improve the dye affinity to substrate. Dyes are classified firstly, according to their chemical structure particularly considering the chromo-phoric structure presents in the dye molecule and secondly based on their usage or application (Fig. 1).

RR158 also known as Levafix brilliant red E4B4 (C29H19OnN7S3ClNa3) utilises a chromophore containing substituent that is capable of directly reacting

Fig. 1 Structure of Reactive Red-158 dye

Fig. 1 Structure of Reactive Red-158 dye

Parameter

Value

Table 1 Characteristics of RR-158 dye

Parameter

Value pH

Wavelength (nm)

Molar absorptivity (mol/L cm)

Limit quantification (blank, mg/L)

Slope

Intercept

6.3 513

with the fiber substrate. The covalent bonds that attach reactive dye to natural fibers make it among the most permanent of dyes. Cold reactive dyes, such as Procion MX, Cibacron F, Levafix Brilliant and Drimarene K, are very easy to use because the dye can be applied at room temperature. Due to their strong interaction with many surfaces of synthetic and natural fabrics, reactive dyes are highly used for dyeing of wool, cotton, nylon, silk, and modified acrylics. Some classes of dyes are harmful to aquatic life even at lower concentrations. It is pointed out that less than 1.0 mg/L of dye content causes obvious water coloration. Dye concentrations of 10 mg/L up to 25 mg/L have been cited as being present in dyehouse effluents. After mixing with other water streams, the concentration of dyes is further diluted. The limit of concentration of some toxic dyes in water is 1.0 ng/L. Table 1 provides information about the characteristics of RR158 dye.

2.1.1 Dye as a Water Pollutant

The discharge of highly coloured synthetic dye effluents is an indicator of water pollution and it is aesthetically displeasing. The World Bank estimates that 17-20% of industrial water pollution comes from textile dyeing and treatment. They have also identified 72 toxic chemicals in our waters solely from textile dyeing, 30 of which cannot be removed (Khehra et al. 2005). Dye wastewaters contain many toxic organic residues. Their disposal is always a matter of great concern since they are considered as a quite dangerous source of environmental pollution that may cause direct destruction to aquatic life due to the presence of aromatic and metal chlorides. Moreover, colorant may interfere with light penetration and oxygen in water bodies decreasing the biological oxygen demand (BOD) level in the water streams. The dyes comprise likewise an aesthetic problem and colour restricts which lead to conflict between upstream discharger and downstream user water. Thus, effective and economical treatment of effluents with diversify textile dyes has become a necessity for clean production technology for all industries.

Water pollution by dye contamination may arise from different manufacturing industries such as textiles, paper, cosmetic, leather, food, paint, electroplating, galvanized and powder batteries processing units and pharmaceutical industries. The textile industry is considered to be the largest group using synthetic dyes constituting of 60-70% of all dyestuffs [10]. Textile dyes form a large group of textile chemicals and comprise over 8,000 different compounds with almost 40,000 commercial names. The textile industry is the largest consumer of dyes with a consumption of around 60% of that produced using them in conjunction with a wide range of auxiliary reagents for various dyeing, printing and finishing processes. About 20-50% of the reactive dye can be released into waterways depending upon dyestuff type, the application route and depth of shade required due to the chemical reactions involved in the fixation of reactive dyes to fabrics [6]. This waste water derived from the textile industries needs suitable treatment(s) before its release in order to avoid dangerous increase of colour, pH, BOD and chemical oxygen demand (COD) in rivers or in drainage areas.

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