Advanced Oxidation Processes for Removal of Dyes from Aqueous Media - Green Chemistry for Dyes Removal from Wastewater - page 104

or by direct reaction with holes
R + h + R + degradation products (3.19)
The benefits of this method are mineralization of organic compounds,
no additional wastewater problem and operating in mild pressure and
temperature [149].
In literature, there are many studies on degradation and decolorization
of azo dyes using advanced oxidation methods comprising the photoca-
talysis in the presence of various types of photocatalysts. Some of these
studies are summarized in Table 3.6.
3.2.2.1.1 Factors Influencing Photocatalytic Degradation
In photocatalytic degradation of dyes in wastewaters, the main operating
parameters which affect the process efficiency are UV intensity, photocata-
lyst type and loading, initial dye concentration and pH. These parameters
will be considered one after the other as they influence the photocatalytic
processes of the degradation of dyes in wastewaters.
UV Intensity
Generally it is expected that the treatment efficiency steadily increases with
increased UV intensity since UV intensity determines the amount of pho-
ton absorbed by the catalyst [143].
Under the higher intensity of light irradiation, the removal is expected
to be higher because the electron-hole formation is predominant and,
hence, electron-hole recombination is negligible. In contrast to this, at
lower light intensity, electron-hole pair separation competes with recom-
bination resulting in a decrease in the formation of free radicals, causing
less of an effect on the percentage degradation of the dyes [150,151].
Photocatalyst Type and Loading
The use of semiconductors such as TiO 2 , ZnO, Fe 2 O 3 , and CdS as pho-
tocatalysts is inevitable for the degradation of organic pollution. Due to
its optical and electrical properties, low cost, remarkable photocatalytic
activity, chemical stability, nontoxicity and photocorrosion resistance,
nano-titanium dioxide is preferred as a common photocatalyst [152,153].
The catalytic activity of TiO 2 catalyst has been raised by using transi-
tion metals. Recently, photocatalytic degradation of organic pollutants
has been improved by immobilization of the transition metals onto solid
matrices such as activated carbon, zeolite, clay, sand, glass, stainless steel
and various biomaterials [154,155]. These supporting materials provide a
large surface area for adsorption, prevent leaching and allow the catalyst
recovery [141,153].