Page 114 - Materials Chemistry, Second Edition

Textile Wastewater Treatment by Advanced Oxidation Processes 95

colorant receives a Generic Name CI, which incorporates an application class, a hue,
and a number that reflects the chronological order of dyes since they became com-
mercially available.

6.1.2 cHeMical classificaTion of coloranTs
According to the chemical classification method, dyes are grouped depending on
similar chemical structural features. The families of dyes and pigments of out-
standing relevance are the azo dyes (-N = N-) followed by the carbonyl (C = O),
phthalocyanine, arylcarbonium ion, sulfide, polymethine, and, finally, nitro types
(Christie, 2001).

6.1.2.1 Azo Dyes and Pigments
Azo dyes and pigments are by far the most important chemical family among com-
mercial organic dyes, and they account for about 60%–70% of the dyes used in tradi-
tional textile applications (Chequer et al., 2013). They are produced mainly in China,
India, Korea, Chinese Taipei, and Argentina (OECD, 2005).
Azo dyes, as their name implies, contain the azo functional group (-N = N-),
2
attached on either side to carbon atoms with sp hybridization, and it is usually
bonded to two aromatic rings. Most azo dyes of commercial importance have one
single azo group and are therefore referred to as monoazo dyes, but many of them
have two, three, or more groups. The versatility of the synthesis sequence of azo dyes
creates an immense number of different dyes. Moreover, the synthesis process uses
inexpensive and readily available organic starting materials such as aromatic amines
and phenols as well as low energy.
Regarding their color properties, azo dyes can provide a broad range of tones
with high color intensity. They can present satisfactory technical properties, such as
fastness to light, heat, water, and other solvents, but they perform slightly worse than
other chemical groups such as carbonyl dyes and phthalocyanines (Christie, 2001).
However, they can be improved thanks to the mordant process, in which certain
metal ions are used; nowadays, this is primarily restricted to the coupling of certain
azo dyes with chromium (III) in wool.

6.1.2.2 Carbonyl Dyes and Pigments
This chemical family of colorants arises as the second in importance, and it is char-
acterized by the presence of the carbonyl group (C = O), acting as its main chromo-
phore. Carbonyl-type colorants are found in a far more extensive variety of structural
arrangements than in the case of azo dyes and pigments. The main reason for the
importance of carbonyl dyes is that they are capable of presenting absorption bands
at long wavelengths with relatively short conjugated systems.
The chemistry developed in the manufacture of the carbonyl dyes is usually more
elaborate than in the case of azo dyes and often involves multiple steps and the use
of specific intermediates. Consequently, the number of commercial products is more
limited, and these tend to be more expensive. The most relevant group of carbonyl
dyes is the family of compounds known as anthraquinones (Gürses et al., 2016).