Environmental Engineering Reference
In-Depth Information
Coinciding with the advancements in nanotechnology applications, photocatalysis has
been successfully used by using immobilized TiO
2
nanoparticles for the degradation
and mineralization of water containing organic pollutants. For photocatalytic treatment
of water containing organic pollutants, immobilized nano-TiO
2
catalyst has been found
highly effective in reactors using UV radiation [14].
Lately, optical iber photoreactors have been designed for uniform distribution of light
and decreased internal relectivity. Nano-TiO
2
coatings were applied on optical ibers using
titanium butoxide. The size of TiO
2
ilms was 15-25 nm, and the thickness of ilm applied
ranged from 60 to 600 nm. Photodecomposition of ammonia showed the superior conduc-
tivity of TiO
2
-coated optical ibers. It has been stated that OH
−
radicals produced by the
photocatalytic effect are 1-10,000× more effective than chlorine and ozone. The production
of (OH
∙
) radicals and
O
2
is a well-known phenomenon. It can be summarized as follows:
TiO
2
has a moderate band gap (1.33 eV) between their valence and conduction bands. Upon
illumination by a semiconductor by photons of energy equal to or greater than the band
gap, e.g., photons are absorbed, which excite the valence band (“vb”) electrons, to the con-
duction band (“cb”) creating electron
e
c
( )
charge carriers. On migration to the
solid surface without recombining, the electrons and holes undergo transfer processes with
the adsorbates. For instance, adsorbed water reacts with holes to produce OH
∙
radicals,
which can oxidize organic compounds. In certain cases, the inal product is CO
2
and H
2
O.
The following reactions exemplify the steps described above:
()
hole
h
vb
(
)
h
ν
−
+
TiO iO
→
e
cb
+
h
2
2
vb
(
)
→
Recombination
2
−
+
TiOe
→+
h
TiO eat
+
cb
vb
2
−
+
HOadsorbed
(
)→+
OH
H
2
ads
ads
+
+ →
hOH
H
vb
ads
eO O
cb
−
+→
2
−
2
In addition to various forms of active ions such as
O
2
−
, OH
−
, the free radical/active spe-
cies OH
∙
, and
O
2
−
are produced.
−
−
+→
2
eO Oads
cb
2
(
)
2
−
+→
O ds
(
)
H
Oads
(
)
2
2
+
(
+
hHOOHads
+ → +
2
)
H
vb
+
−
+ →
2
hOads
(
)
2
Oads
(
)
vb
The photocatalytic technique is a versatile and eficient disinfection process. The inal
decomposition products of organic pollutants are water and CO
2
.
