Photocatalytic Processes for the Removal of Dye - Green Chemistry for Dyes Removal from Wastewater - page 126

4.3 Photo-Oxidation Mechanism

The mechanisms of photocatalytic processes have been well discussed by
several authors, but the most interesting explanation of photocatalysis was
given by Chen et al. [6], who elucidated the existence of light source (elec-
tronic phase) as the fourth phase along with solid (photocatalyst), liquid
(aqueous or organic solvent) and gaseous phases (O 2 ).
Photo-oxidation mechanism for dye molecule can be different under dif-
ferent light sources depending on the energy of the incident photons. In the
presence of UV light, the conventional photo-oxidation mechanism will be
applied with different photocatalysts such as TiO 2 , ZnO, SrTiO 3 , ZnS, WO 3 ,
etc. The reaction mechanism for UV photo-oxidation is shown below [16]:

h

TiO TiO h e

2 2

.

Oe O

2 2

HO . HO h

. .

HO RH R H O CO H O mineral acid

2 2 2

The reaction mechanism would be similar to this in the presence of vis-
ible light if metal or nonmetal doped semiconductor materials (TiO 2 , ZnO,
etc.) are used as photocatalysts. However, dye-sensitization mechanism
would play a major role under visible light in the presence of sensitizing
materials (dye, polymer, commercial pigment, etc.) [17,18,19].
Chatterjee et al. [20] discussed the degradation of a non-sensitizing dye
(Acid Blue 1 (AB1)) with the help of TiO 2 and a sensitizing dye (eosin
Y(EY)) via dye-sensitization mechanism. The degradation process gener-
ated O 2 - / HO 2 as the reactive species which were primarily responsible for
the reactions. At first, EY absorbed visible photon and formed an excited
state (EY * ). hen EY * injected electron into the conduction band of TiO 2
and transformed to oxidized species (EY + ). Finally, EY + was reduced to
ground state (EY) by AB1, which acted as electron donor producing oxi-
dized dye AB1 + (Figure 4.1).

4.4 Solar Photocatalysis/Photoreactors

The earth's surface receives 1.5 10 18 kWh of sunlight per year. Two major
issues regarding the use of solar light for photocatalysis are: i) incident
solar light is relatively diluted before reaching the earth surface and needs