Advanced Oxidation Processes for Removal of Dyes from Aqueous Media - Green Chemistry for Dyes Removal from Wastewater - page 84

3.1 Introduction
Dyes have a synthetic origin and complex aromatic molecular structures.
Based on the chemical structure or chromophore, approximately 30 dif-
ferent groups of dyes are present. Anthraquinone, phthalocyanine, tri-
arylmethane and azo dyes are quantitatively the most important groups
of dyes [1].
Azo dyes are the most widely used commercial reactive dyes and char-
acterized by nitrogen to nitrogen double bond. The color of dyes is due to
azo bond and associated chromophores [2-4]. Within the overall category
of dyestuffs, azo dyes represent the largest class of dyes [3,5,6] and there-
fore constitute a significant portion of the dye pollutants.
Dyes are widely used in various industrial applications such as textile,
plastics, pharmaceutical, paper, paint and food. Wastewater from these dye
processing or dye using industries is one of the major environmental prob-
lems due to the characteristics of wastewater such as strong color, high
chemical oxygen demand (COD) derived from additives and surfactants
and a low biodegradability. These dyes are the most problematic pollutants
of industrial wastewaters. For example, in the literature it is reported that
more than 15% of the textile dyes is lost in wastewater stream during dye-
ing operation [1,7-9].
Chemical, physical and biological methods have been used to treat dye-
containing effluents, however these conventional treatment technologies
were found to be unsatisfactory [1] due to the complex structure and syn-
thetic nature of the dyes. The dye containing effluents are chemically and
photolytically stable, toxic and mostly nonbiodegradable and also contain
many organic compounds, which are not easy to degrade; in other words,
they are resistant to destruction by conventional treatment methods. The
inappropriate disposal of dyes is a big challenge as it can disturb the eco-
system and constitutes potential environmental and health problems such
as reducing light penetration and photosynthesis [10,11]. Consequently,
it is necessary to find an effective treatment technology that leads to com-
plete degradation and decolorization. Hence removal of dyes from aque-
ous media has been the cause of much concern to societies, researchers
and regulation authorities around the world.
Recently, alternative to conventional treatment methods, the Advanced
Oxidation Processes (AOPs), through which highly oxidizing species like
hydroxyl radicals are produced, have been of major concern. These radicals
can be produced by means of oxidizing agents such as O 3 , H 2 O 2 , ultraviolet
irradiation, ultrasound, and catalysts (homogeneous or heterogeneous).