Page 191 - Industrial Wastewater Treatment, Recycling and Reuse

Advanced Oxidation Technologies for Wastewater Treatment: An Overview 165

3.4.2.4 Radiant Flux
The reaction rate is directly proportional to the intensity of radiation; usu-
ally, linear variation is observed at low intensities and beyond a certain mag-
nitude of optimum intensity (depending on the reactor conditions as well as
the nature of the effluent), the rate of the reaction shows a square root
dependence on the intensity (Ollis et al., 1991). Lower dependence on
the intensity of irradiation is usually attributed to the increased contribution
of the recombination reaction between the generated holes and electrons
when their density is high. Usually, this is handled by choosing different
polymorphs of TiO 2 (anatase, which is photocatalytically active, and rutile,
which is photocatalytically inactive) in appropriate proportions. Another
important factor regarding the incident light is the wavelength of irradiation.
Shorter wavelengths are recommended for better results. Moreover, the

angle of incidence of the UV light should always be 90 because maximum
rates are observed at this angle of incidence (Ray and Beenackers, 1997).

3.4.2.5 Medium pH
The pH may affect the surface charge on the photocatalyst and also the state of
ionization of the substrate and hence its adsorption. Medium pH has a com-
plex effect on the rates of photocatalytic oxidation, and the observed effect is
generally dependent on the type of the pollutant as well as the zero point
charge (zPc) of the semiconductor used in the oxidation process, and, more
specifically, on the electrostatic interaction between the catalyst surface and
the pollutant. The adsorption of the pollutant, and hence the rates of degra-
dation, will be maximum near the zPc of the catalyst (Subramanian et al.,
2000). For some of the pollutants, which are weakly acidic, the rate of photo-
catalyticoxidationincreasesatlowerpHbecauseofanincreaseintheextentof
adsorption of the pollutant on the catalyst surface under acidic conditions
(Bhatkhande et al., 2002). Some of the pollutants, which undergo hydrolysis
under alkaline conditions or undergo decompositionovera certain pHrange,
may show an increase in the rate of photocatalytic oxidation with an increase
in the pH (Choi and Hoffmann, 1997). Fox and Duley (1993) and Davis and
Huang (1991) reported that pH had a marginal effect on the extent of degra-
dation over the range of pH used in their work. Since the effect of pH cannot
be generalized, it is recommended that laboratory-scale studies should be
performed for establishing the optimum conditions for the operating pH
unless data are available in the literature with closely matched operating con-
ditions, i.e., type of equipment as well as the range of operating parameters,
including the composition of the effluent stream.