Photocatalytic Processes for the Removal of Dye - Green Chemistry for Dyes Removal from Wastewater - page 140

5.1 Introduction
Over the last few decades, the awareness of society towards the protec-
tion of the environment has vastly increased. In this regard, the chemi-
cal industry has become one of the foremost targets of environmentalists,
since its potential adverse impacts can have irreparable damage to the
ecosystem. Closely related to the environment is the presence of chemi-
cals, which may seriously jeopardize human health if not treated properly
[1,2]. Dyes and pigments are one of the most criticized such chemicals that
pollute the environment, although they are not as toxic as other pollut-
ants such as heavy metals. This can be related to the visible nature of these
contaminants, which not only causes toxicity in aqueous media, but also is
at the forefront of aesthetic pollution even at very low concentrations, thus
receiving significant public concern. These substances can also hinder the
penetration of light into the water streams, endangering aquatic life. Since
the degradation of dye molecules by aquatic microorganisms is almost
negligible, they potentially accumulate in the medium [3,4].
This is why governments have ratified stringent legislations regard-
ing the discharge of dye-containing wastewater into the environment and
more specifically into the water bodies. This has obliged industries to look
for technologies to remove such contaminants from aqueous streams.
Some of the physicochemical and biological technologies employed for
this purpose are ozonation, coagulation-flocculation, photodecompo-
sition, flotation, membranes, electrochemical destruction, adsorption
and ion exchange. However, these technologies have been impeded due
to several drawbacks such as low efficiency, generation of secondary
byproducts, short half-life of oxidizing agents, high cost, low removal
rate, high sludge formation, technical constraints and high energy con-
sumption [5-12].
Adsorption, among others, has been proved to be one of the most prom-
ising and efficient dye removal techniques in industry [13,14]. Nevertheless,
its high cost has been known to be a major disadvantage [15-17], which
adds financial burden to the treatment of wastewater and might appear in
the form of higher final product price. Therefore, in practice although con-
sumers are the ultimate beneficiaries of pollution remediation, the onus
of the financial responsibility is imposed on them [18]. This, in turn will
probably influence the overall demand of consumers. All these interactions
have obliged industries to endeavor to reduce the cost of the adsorption
process while maintaining the high efficiency. In this regard, the applica-
tion of low cost carbonaceous or siliceous waste materials is found to be
highly attractive with its significant cost reduction despite the challenges