Page 188 - Industrial Wastewater Treatment, Recycling and Reuse

162 Industrial Wastewater Treatment, Recycling, and Reuse

producing a positively charged hole in the valence band and an electron in
the conduction band. Because of the generation of positive holes and elec-
trons, oxidation-reduction reactions take place at the surface of semiconduc-
tors. The photo-generated electrons could reduce the organic molecule or
react with electron acceptors such as O 2 adsorbed on the catalyst surface or
•
dissolved in water, reducing it to super oxide radical anion O 2 . The
+
photo-generated holes can oxidize the organic molecule to form R ,or
•
react with OH or H 2 O, oxidizing them into OH radicals, which subse-

quently act as oxidizing species.
The scheme of various reactions due to the photocatalytic effect can be
given as follows:
+
UV + MO ! MO h +eð Þ (3.18)
Here MO stands for metal oxide
•
+
+
h +H 2 O ! H + OH (3.19)
+ +
2h +2H 2 O ! 2H +H 2 O 2 (3.20)
• •
H 2 O 2 ! HO + OH (3.21)
Reductive reaction due to the photocatalytic effect have been given as
follows:
•

e +O 2 ! O (3.22)
2
• + •
O +H ! HO (3.23)
2 2
• • +
O +HO +H ! H 2 O 2 +O 2 (3.24)
2 2
•
•
HOOH ! HO + OH (3.25)
Hydroxyl radicals are thus generated, which have the highest oxidation
potential, and they react with organic pollutants to oxidize/mineralize them.
3.4.1 Reactor Used for Photocatalytic Oxidation

The photocatalytic process can be carried out by simply using slurry of the
fine particles of the solid semiconductor material dispersed in the liquid
phase in a reactor irradiated with UV light either directly or indirectly.
The proper dispersion of the catalyst in the liquid phase can be achieved
using either mechanical or magnetic stirrers. Aeration is usually maintained
•
for scavenging the electrons (HO 2 radicals are formed in the valence band)
and prevent electron/hole charge recombination also helps in achieving the