Environmental Engineering Reference
In-Depth Information
range. TiO
2
can only be excited under irradiation of UV light at wavelengths <380 nm, which
covers only ~5% of the solar spectrum.
47
The antibacterial activity of TiO
2
is related to ROS
production, especially hydroxyl free radicals and peroxide formed under UV-A irradiation
via oxidative and reductive pathways, respectively.
48
The development of photocatalysts capa-
ble of absorbing light in the visible region of the spectrum is therefore of great interest. Metal
doping has been shown to be a successful approach to enhance the photocatalytic activity
of TiO
2
under visible light. Ag/TiO
2
composite materials have attracted particular attention
wherein the tiny-sized Ag nanoparticles with a plasmon effect are exploited to enhance the
visible light photocatalytic activity of the composites.
49-52
Recently, Ag/AgBr/TiO
2
that con-
tains small particles has been prepared, and these systems are indeed more eficient than
the Ag/TiO
2
composite materials with large TiO
2
particles in the photodegradation of
E. coli
under visible light.
53,54
(For more information, refer to reviews in references 55 and 56.)
A high-performance inorganic iltration membrane has been fabricated by directly
growing titanate nanotube separation layers on a porous titanium membrane substrate.
The resultant titanate nanotube membrane retains membrane structure, and successful
separation of
E. coli
demonstrates the applicability of the titanate nanotube membrane for
waterborne pathogen removal, which would be of great interest to the water puriication
applications.
57
Fluorescence and scanning electron microscopy (SEM) studies proved the
eficiency of the membrane in pathogen removal (Figure 12.9).
Combining TiO
2
with carbonaceous nanomaterials is being increasingly investigated as a
means to increase photocatalytic activity, and demonstrations of enhancement are plentiful.
TiO
2
-carbonaceous materials combining the photocatalytic antibacterial activity of TiO
2
and
adsorbent properties of carbonaceous materials have a great potential for water disinfection.
Graphene oxide/TiO
2
nanocomposites as photocatalysts for degradation of
E. coli
bacteria
in an aqueous solution under solar light irradiation have been reported.
58
Photocatalytic
disinfection of
E. coli
by carbon-modiied TiO
2
photocatalysts was tested under UV and vis-
ible light irradiation. For modiication purposes, ive alcohols were used (methanol, ethanol,
n-
butanol, 2-butanol, and
tert
-butanol) as a source of carbon. It was found that photocatalysts
with a low amount of carbon have better antibacterial ability under visible light irradiation;
photocatalysts modiied with methanol reduced 100% of
E. coli
after 45 min of irradiation
with visible light. The photocatalysts with a higher amount of carbon had better antibacterial
ability under UVA light irradiation; photocatalysts modiied with 2-butanol and
n-
butanol
reduced 100% of bacteria after 20 min of irradiation with UV light.
59
TiO
2
/MWNT hetero-
junction arrays were synthesized and immobilized on an Si substrate as photocatalysts for
inactivation of
E. coli
bacteria (Figure 12.10a). The visible-light-induced photoinactivation of
(a)
(b)
(c)
(d)
E. coli
E. coli
200 nm
5 µm
500 nm
FIGURE 12.9
(See color insert.)
Fluorescent microscopic images of
E. coli
feed (a) and permeate (b). (c) SEM image of retained
E. coli
on the Titanate Nanotube Membrane (TNM) after iltration (low magniication) and a high-magniication SEM
image (inset). (d) Cross-section SEM image of TNM after iltration of
E. coli.
(From Zhang, H. et al.
J. Membrane Sci.,
343, 212, 2009. With permission.)
