Removal of Organic Dyes from Industrial Effluents: An Overview of Physical and Biotechnological Applications - Green Chemistry for Dyes Removal from Wastewater - page 6

azo dyes (strong, good all-round properties, low-cost) and anthraquinone
dyes (weak, expensive). Second, there are a number of manageable chemi-
cal groups. Most importantly, it is the classification used most widely by
both the synthetic dye chemist and technologist. Thus, both chemists and
technologists can readily identify with phrases such as an azo yellow, an
anthraquinone red, and a phthalocyanine blue [27].
The application classification of dyes arranged according to the C. I.
(Color Index) is given in Table 1.2, which includes the principal substrates,
the methods of application, and the representative chemical types for each
application class [27]. Although not shown in Table 1.2, dyes are also used
in high-tech applications, such as in medical, electronics, and especially
the nonimpact printing technologies [27,38].
Acid Dyes, which are water-soluble anionic dyes, are applied to nylon,
wool, silk and modified acrylics. They are also used to some extent for
paper, leather, inkjet printing, food, and cosmetics.
Direct Dyes are water-soluble anionic dyes. When dyed from aqueous
solution in the presence of electrolytes, they are substantive to, i.e., have
high affinities for cellulosic fibers. Their principal use is in the dyeing of
cotton and regenerated cellulose, paper, leather, and, to a lesser extent,
nylon. Most of the dyes in this class are polyazo compounds, along with
some stilbenes, phthalocyanines, and oxazines. Treatments applied to the
dyed material to improve wash fastness properties include chelation with
salts of metals, which are usually copper or chromium, and treatment with
formaldehyde or a cationic dye-complexing resin.
Azoic Dyes are applied via combining two soluble components impreg-
nated in the fiber to form an insoluble color molecule. These dye compo-
nents, which are sold as paste-type dispersions and powders, are chiefly
used for cellulosic fibers, especially cotton. Dye bath temperatures of
16-27 C (60-80 F) are generally used to make the shade [39].
Disperse Dyes, which are substantially water-insoluble nonionic dyes for
application to hydrophobic fibers from aqueous dispersion, are used pre-
dominantly on polyester and to a lesser extent on nylon, cellulose, cellu-
lose acetate, and acrylic fibers. Thermal transfer printing and dye diffusion
thermal transfer (D2T2) 2 T 2 ) processes for electronic photography represent
rich markets for selected members of this class.
Sulfur Dyes are used primarily for cotton and rayon. The application of sulfur
dyes requires carefully planned transformations between the water-soluble
reduced state of the dye and the insoluble oxidized form. Sulfur dyes, which
generally have a poor resistance to chlorine, and are not applicable to wool or