Photocatalytic Processes for the Removal of Dye - Green Chemistry for Dyes Removal from Wastewater - page 120

various sources of water pollution, some main sources are oil spills, sew-
age, industrial waste, nuclear waste, and agricultural waste [1]. According
to a World Health Organization (WHO) report, in 2008 about 884 million
people still relied on inferior water sources, out of which 84% live in rural
areas. Many of them are suffering from severe waterborne diseases due to
their daily usage of microbiologically unsafe water [2]. Dye is one of the
most important classes of pollutants which results in colored wastewater
that is sometimes hard to degrade because of its complex structure [3].
Dye-containing wastewater generates from textile, tannery, dying, pulp
and paper, and paint industries. The majority of dyes arise from dyeing
and finishing processes in textile industries. The colored wastewater is not
only unpleasant on aesthetic grounds, but also is characterized by high bio-
chemical and chemical oxygen demands (BOD 5 : 80-6000 mg. L -1 ; COD:
150-12000 mg. L -1 ) [4].
Estimated dye production from different sources is reported in the
range of 7 10 5 to 1 10 6 tons/annum. A significant amount of dye
(10-15% of the produced quantity) enters into the environment. A dye
molecule has two main components: i) chromophores, and ii) auxo-
chromes. Chromophore is responsible for color, whereas auxochromes
facilitate attachment towards fibers [3]. There are fourteen different types
of textile dyes among which six classes (acid dye, direct dye, azonic dye,
dispersed dye, sulfur dye, and fiber reactive dye) are classified by the US
Environmental Protection Agency (EPA) as major toxic elements [5].
Dye pollutants result in several health hazards such as skin and eye-
related diseases. Most dyes are toxic in nature and pose a threat to aquatic
living organisms. To comply with strict environmental regulations, several
conventional methods such as adsorption, membrane processes, biologi-
cal processes, electrocoagulation, etc., have been utilized for the removal of
dyes from water and wastewater. Table 4.1 shows a comparative review of
conventional dye removal techniques with their advantages/disadvantages.
Conventional oxidation processes are inferior in oxidizing dyestuff with
complex structures, and thus advanced oxidation processes (AOPs) (UV/O 3 ,
UV/H 2 O 2 , photo-Fenton, etc.) are introduced for dye degradation [4].
Among the AOPs, TiO 2 /UV-based photocatalytic oxidation processes have
received significant attention in recent years as an alternative method for
water detoxification [6]. To make the TiO 2 -based photo-oxidation process
economical, solar light can be used as a potential replacement for commer-
cial UV lamps. This again requires modification of TiO 2 and TiO 2 -based
photocatalysts, for utilization of solar visible spectra [1].
This chapter summarizes photocatalytic oxidation process for dye degra-
dation under both UV and visible light, application of solar light and solar