Abstract
Colour is the most important part to make any fabric attractive, but its use for dying has become a major environmental hazard. Colloidal particles present with color increases the turbidity, gives bad appearance, foul odour, and obstructs the penetration of sunlight into water bodies required for the photosynthesis process, which interferes with the oxygen flow mechanism and hence marine life. So, it is essential to remove such pollutants from the waste water before its final disposal into water bodies. Photocatalysis is one of the advance oxidation processes, mainly carried out in the presence of light and suitable photocatalytic materials for the degradation of dyes. This chapter outlines the background of dye chemistry, the application areas, and the impact of dyeing effluents in the environment and on living beings.
TopIntroduction
Dyes have been very popularly used as coloring agents, additives in textile, food, papermaking leather, printing, ink, drugs, cosmetic and pharmaceutical industries. Since many years, synthetic dye especially azo dyes is interestingly used in large amount as compared to natural dye due to its high color fastness and sustainable life, and cost effectiveness. It was proclaimed that nearly more than 103 tons of dyes are produced every year (Forgacs et al. 2004). Therefore people are very much concerned about the treatment of such waste water. The considerable concern which arises is due to the release of wastewater containing large concentration of dyes that is highly toxic and colored and poses a threat to the environment. The released waste water is colored and it affects the visibility of water in the littoral zone and prevents the penetration of sunlight in the stream (Namasivayam et al. 1997). Different kinds of chemicals present in synthetic dyes can cause carcinogenic and mutagenic effects to human beings and fauna and ultimately affects the environment . In the textile industries large amounts of dye effluents and aqueous wastes are discharged from the dyeing process, which last long with a high biological oxygen demand (BOD), both of which are environmentally and aesthetically undesirable (Wang et al. 2007).
Nearly 2×105 tons of these synthetic dyes are released into the water bodies every year during the dyeing and finishing processes, because of the inefficiency of the dyeing processes (Ogugbueand Sawidis 2011). As a result most of these azo dyes are unable to degrade by classical conventional wastewater treatment processes and remain in the environment for long time, due to its great stability to light, water, temperature, soap, detergents, chemicals, bleaching etc (Couto 2009). Furthermore, anti-microbial agents are antagonistic to the biological degradation are very popularly used in the textile manufacturing, specifically for natural fibers such as cotton (Couto 2009 and O’Neill C 1999). The complex origin and aromatic structures of these azo dyes make them more fractious for biodegradation (Forgacs et al 2004 and Przystaś 2012). However, environmental legislation constrain the different industries to remove the color from their dye-containing effluents, before disposal into water bodies so that the environment of water body doesnot get contaminated (Ogugbue 2011 and O’Neill 1999).
Textile wastewaters are characterized by inconstancy in various parameters such as biochemical oxygen demand (BOD), chemical oxygen demand (COD), salinity, pH and color. The fractional composition of the wastewater will mainly depends on the different chemicals, organic-based compounds, and dyes used in the wet-dry processing steps (Dos et al. 2007 and Talarposhti et al. 2001). The main pollutants in the textile effluents are color, toxicant, surfactant, chlorinated compounds, recalcitrant’s organics and salts (Ben et al. 2012).
Key Terms in this Chapter
Basic Dyes: Basic dyes are water-soluble cationic dyes that are mainly applied to acrylic fibers, but find some use for wool and silk. Usually acetic acid is added to the dye bath to help the uptake of the dye onto the fiber. Basic dyes are also used in the coloration of paper.
Synthetic Dyes: Synthetic dyes are found in a wide range of products such as clothes, leather accessories, and furniture. These dyes are commonly used every day. However, a side effect of their widespread use is that up to 12% of these dyes are wasted during the dying process and about 20% of this wastage enters the environment (mainly into water supply).
Azoic Dyeing: Azoic dyeing is a technique in which an insoluble Azo dye is produced directly onto or within the fiber. This is achieved by treating a fiber with both diazoic and coupling components. With suitable adjustment of dyebath conditions the two components react to produce the required insoluble azo dye. This technique of dyeing is unique, in that the final color is controlled by the choice of the diazoic and coupling components.
Photocatalysis: Photocatalysis is the acceleration of a photoreaction in the presence of a catalyst. In catalysed photolysis, light is absorbed by an adsorbed substrate. In photogenerated catalysis, the photocatalytic activity (PCA) depends on the ability of the catalyst to create electron-hole pairs, which generate free radicals (e.g., hydroxyl radicals: OH) able to undergo secondary reactions.
Acid Dyes: Acid dyes are water-soluble anionic dyes that are applied to fibers such as silk, wool, nylon and modified acrylic fibers using neutral to acid dye baths. Attachment to the fiber is attributed, at least partly, to salt formation between anionic groups in the dyes and cationic groups in the fiber. Acid dyes are not substantive to cellulosic fibers. Most synthetic food colors fall in this category.