Nanotechnostructured Catalysts TiO Nanoparticles for Water Purification - Nanotechnologies for Water Environment Applications - page 72

photocatalytic reaction in water purification (Fox and Dulay, 1993; Mills and LeHunte,
1997; Herrmann, 1999; Blake, 2001; Augugliaro et al., 2002; Bhatkhande et al., 2002;
Gogate and Pandit, 2004; Konstantinou and Albanis, 2004; Kwon et al., 2008). In this
section, three most widely investigated pollutants are discussed.

3.5.1 Halogenated Compounds Degradation and Detoxification

Photocatalytic degradation of halogenated compounds has received considerable
attention due to the high toxicity nature and wide occurrence in the environment.
Photocatalytic treatment has been demonstrated to be capable of destructing or even
completely mineralizing a variety of aliphatic and aromatic halogenated compounds.
Although mineralization requires prolonged treatment in most cases, partial
detoxification of highly concentrated halogenated compounds can be achieved in a short
time scale. This makes it possible for further treating the photocatalytically treated
streams by biological processes, which are not able to degrade the parent compounds at
high concentrations due to their toxic nature. Table 3.3 summarizes some halogenated
compounds treated with photocatalytic reaction by various research groups.

If complete mineralization can be achieved, a general stoichiometry for the
photocatalytic oxidation of a generic halogenated hydrocarbon can be written as
(Hoffmann et al., 1995):

yz − yz −

⎛ ⎞ ⎛ ⎞ + −

CHX ++ x O → xCO + + + zH zX HO (Eq. 3.31)

⎜ ⎟ ⎜ ⎟ xyz 2 2 2

4 2

⎝ ⎠ ⎝ ⎠

Detailed reaction pathway for individual compounds, especially simply
structured ones, has also been studied based on intermediates and final products detected.
Huang and co-workers have investigated the degradation of 4-CP in TiO 2 suspension
(Figure 3.16) and proposed that the oxidation of OH• radicals is the major reaction route
in addition to several reaction pathways (Dong and Huang, 1995). It should be noted
that, even for the same compounds, the reaction pathway might differ due to different
photocatalyst and reaction conditions.

3.5.2 Dyes

Textile dyes and other industrial dyestuffs constitute one of the largest
waterborne pollutants causing a variety of environmental issues. The release of dye
containing wastewater is considered not only unaesthetic; it also causes eutrophication,
and posing latent eco-risks when toxic by-products are generated through unintended
natural oxidation or bio-reactions. Photocatalytic oxidation of dyes appears to be very
promising as a cost effective method to solve this notorious environmental problem. On