Environmental Engineering Reference
In-Depth Information
Irradiation of an aqueous TiO 2 suspension offers an oxidation capable of mineraliza-
tion of many of the recalcitrant organic substances into CO 2 , H 2 O, and other associated
inorganic components. Numerous studies have reported that the photocatalytic activity
would be affected by the crystalline structure of the TiO 2 photocatalyst. TiO 2 in the anatase
form appears to be photocatalytically more active compared with the rutile form and con-
venient for water puriication, water treatment, hazardous waste control, air puriication,
and water disinfection. The lower photocatalytic activity of rutile compared with that of
anatase is attributed to the lower capacity of the rutile surface to adsorb molecular oxygen,
leading to higher rates of e -h + pair recombination. Furthermore, the difference between
rutile and anatase particles is that the main product from molecular oxygen on rutile par-
ticles is superoxide ion and it is hydrogen peroxide on anatase particles. Moreover, control
of the morphology, particle size, particle size distribution, phase composition, and poros-
ity of TiO 2 is vital to achieve optimum photocatalytic activity.
A variety of methods have been proposed for the preparation of n -TiO 2 such as lame
synthesis, sol-gel routes, thermal hydrolysis, and reverse microemulsion method. Among
these techniques, sol-gel thin ilm deposition offers several advantages over other tech-
niques. The main disadvantage of the sol-gel process is agglomeration of hydrolyzed TiO 2
particles (Mohammadi et al., 2006). Many methods have been developed to disperse the
aggregated TiO 2 particles such as washing with organic solvent, freeze-drying, followed
by heating and adding surfactants. The most economical and eficient way to prevent the
aggregation of TiO 2 is the addition of surfactants. Surfactants are well known as stabiliz-
ers, template agents, and shape directors. The highly photocatalytically active mesopo-
rous TiO 2 with particle sizes ranging from submicron to micron were synthesized using
nonionic surfactants. The preparation of surfactant-assisted n -TiO 2 microspheres showed
better catalytic eficiency than Degussa P-25 TiO 2 .
25.6 Factors Affecting the UV/TiO 2 Process
The effects of operating parameters on the photocatalytic eficiency of UV/TiO 2 are
described below.
25.6.1 Substrate Concentration
The substrate concentration can inluence the extent of adsorption and rate of reaction at
the surface of the photocatalyst. It is an important parameter for optimization between
high degradation rate and eficiency. For a steady-state balance in a photocatalytic system,
the rate of mass transfer must be equal to the rate of reaction.
25.6.2 Light Intensity
Light irradiance plays a signiicant role in all photocatalytic reactions and determines the
number of e and h + pairs created. Accordingly, an increase in the incident photon rate
would result in an increase in the photocatalytic reaction rate. For a simple set of reac-
tions including only charge-carrier generation, recombination, reduction, and oxidation,
the rate of oxidation of a particular compound is proportional to the light intensity. This
phenomenon indicates that high photon lux increases the probability of collision between
photons and activated sites on the catalyst surface and enhances the rate of photocatalytic
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