Nanotechnology for domestic water purification - Nanotechnology in Eco-efficient Construction - page 372

mechanism involves the metal ion induced fl occulation of negatively
charged alumina nanoparticles (alkaline pH conditions). Alumina is used
as support for heterogeneous catalysis (Nair and Pradeep, 2007). The major
reasons behind the use of oxides for water purifi cation are: high surface
area for adsorption, mesoporous structure, presence of surface charge, sta-
bility and low solubility in water.
Titanium dioxide (TiO 2 ) is one of the most important materials used in
water treatment applications because of its photocatalytic properties. Two
different types of photocatalytic applications can be distinguished in water
treatment: solar photocatalysis and photocatalytic systems equipped with
artifi cial ultraviolet (UV) light. Both systems can be applied at ambient
temperature to degrade various chemical and microbiological pollutants in
water and air. As it makes use of sunlight, solar photocatalysis technology
is inexpensive, environmentally friendly and universally applicable. The
equipment needed is minimal and also appropriate for developing countries
or remote sites with no access to electricity. Nanoparticles that are activated
by light, such as the large band-gap semiconductors titanium dioxide (TiO 2 )
and zinc oxide (ZnO), are frequently studied for their ability to remove
organic contaminants from various wastewater. These nanoparticles have
the advantages of ready availability, being inexpensive, and having low
toxicity. The rapid recombination of photo-generated electron hole pairs
and the non-selectivity of the system are the main problems that limit the
application of photocatalysis processes. The specifi c chelating agents such
as arginine, lauryl sulfate and salicylic acid can modify the surface proper-
ties of nanocrystal TiO 2 and inhibit rapid recombination of photo-generated
electron hole pairs. A comprehensive review of photocatalytic nano-TiO 2
for environmental applications is found in Kwon et al. , (2008).
Conventional TiO 2 photocatalysts are utilized only under UV light due
to its wide bandgap of 3.2 eV and, therefore, cannot be used indoors or
inside vehicles. In order to obtain the photocatalytic activity under visible
light, various types of TiO 2 -based photocatalysts have been created (Sato,
1986; Kisch et al. , 1998; Zang et al. , 1998; Asahi et al. , 2001; Umebayashi
et al. , 2002; Irie et al. , 2003; Sakthivel and Kisch, 2003; Miyauchi et al. , 2004);
and nitrogen-doped TiO 2 (TiO 2− x N x ) is regarded as one of the most effec-
tive and practical catalysts (Irie et al. , 2003; Miyauchi et al. , 2004; Irokawa
et al. , 2006). Nitrogen doping has extended the photoactive wavelengths up
to 520 nm as a result of bandgap narrowing, realizing the equivalent
photoactivity in conventional TiO 2 under UV light since the number of
carrier-recombination centres was minimized. The change in band gap and
generation of free radicals are shown in Fig. 16.3.
Synthesized titanium dioxide nanoparticles of both anatase and rutile
forms were used for wet oxidation of phenols by hydrothermal treatment
(Andersson et al. , 2002). In another study, a novel composite reactor with

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