Removal of Organic Dyes from Industrial Effluents: An Overview of Physical and Biotechnological Applications - Green Chemistry for Dyes Removal from Wastewater - page 4

Table 1.1 Wavelengths of ligh t absorption versus the color of organic dyes.

Wavelength range
absorbed (nm)

Color absorbed Color observed

400-435 Violet Yellow-Green

435-480 Blue Yellow

480-490 Green-Blue Orange

490-500 Blue-Green Red

500-560 Green Purple

560-580 Yellow-Green Violet

580-595 Yellow Blue

595-605 Orange Green-Blue

605-700 Red Blue-Green

With regard to their solubility, organic colorants fall into two classes,
dyes and pigments. The key distinction is that dyes are soluble in water
and/or an organic solvent, while pigments are insoluble in both types of
liquid media. Dyes are used to color substrates to which they have a spe-
cific affinity, whereas pigments can be used to color any polymeric sub-
strate by a mechanism quite different than that of dyes [22,21].

1.1.2 Historical Development of Dyes

Humans discovered that certain roots, leaves, or bark could be manipu-
lated, usually into a liquid form, and then used to dye textiles. They used
these techniques to decorate clothing, utensils, and even the body, as a reli-
gious and functional practice. Records and cloth fragments dating back
over 5000 years ago indicate intricate dyeing practices. Certain hues have
historical importance and denote social standing [23]. The dye made from
the secretions of shellfish, which is a clear fluid that oxidizes when exposed
to the air, was used to produce a red to bluish purple. This dye was difficult
to create and used only on the finest garments; hence it became associated
with aristocrats and royalty [23]. Until the middle of the last century most
of the dyes were derived from plants or animal sources by long and elabo-
rate processes. Ancient Egyptian hieroglyphs contain a thorough descrip-
tion of the extraction of natural dyes and their application in dyeing [18].
In the past, only organic matter was available for use in making dyes.
Today, there are numerous options and methods for the colorization of
textiles. While today's methods capitalize on efficiency, there is question as
to whether the use of chemicals is harmful to the environment.