ABSTRACT
An urban population demands high quantities of raw materials, water, and energy, some of which turns into environmental pollution. Water is a vital substance for all creatures and its quality is crucial for the future of humankind. One of the main problems confronted by the human race is to provide clean water to the vast majority of the population around the world (Mollah et al. 2001). When consumers make use of water, it is disposed of as wastewater, most often containing a variety of substances and pollutants. Indeed, the “tragedy of the commons” arises when significant amounts of pollutants are constantly and sometimes ignorantly released into the environment, which should be a safe and sound place for all creatures, imposing extremely devastating effects and problems (Feeny et al. 1990). The use of water by humans in several industrial activities increases the amount of dissolved chemicals in water, consequently reducing water quality. Water polluted with chemicals threatens public health and the environment and increases the costs of water remediation. Among chemicals, dyes have long been utilized in dyeing, paper and pulp, textiles, plastics, leather, cosmetics, and food industries. Dyes are compounds that absorb light within the visible range, between 380 and 700 nm. They are very detectable contaminants, because of their intense color in water bodies (dos Santos et al. 2007). Dyes have complex aromatic molecular structures, making them more stable and resistant to microbial degradation (Yavuz et al. 2012). Dyes are composed of a group of atoms responsible for their color, which is called the chromophore, as well as an electron-withdrawing or donating substituent that influences the color of the chromophore, called the auxochrome (dos Santos et al. 2007). Dyes are classified based on their chemical structures, physical properties, and applications in various processes. The most important chromophores are azo (–N=N-), carbonyl (–C=O), nitro (–NO2), and quinoid groups. Moreover, the major classification of synthetic dyes normally used includes anthraquinone, indigoide, triphenylmethyl, xanthene, azo derivatives, and so on (Panakoulias et al. 2010; Yavuz et al. 2012). Table 3.1 shows the structures of various dyes with some of their properties. Dyes can also be classified according to their usage, such as reactive, disperse, direct, vat, sulfur, cationic, acid, mordant, ingrain, and solvent dyes that are listed in the color index (C.I.) by the Society of Dyers and Colourists and the American Association of Textile Chemists and Colorists. The C.I. name of a dye indicates how it is used with materials and its hue, and its number specifies the chronological order of its commercial introduction (Salleh et al. 2011; Hao et al. 2000; O’Neill et al. 1999).
