It was also reported that the photocatalytic degradation rate of azo dyes in the
presence of UV/TiO 2 is strongly dependent on the chemical structure of the dye.
Indeed, monoazo dyes are more easily decomposed than triazo dyes since the
stability of both triazine nucleus and cyanuric acid intermediates makes triazo dyes
difficult to fully oxidize (Reutergadh and Iangphasuk 1997; Minero et al. 1997).
Fortunately, such intermediates are not toxic.
Competitive adsorption between water molecules and the target molecules can
occur due to photogenerated oxidizing species, which may not migrate far away
from their formation centers, explaining that there is no or very slow rate of
degradation at few nanometer layers around the catalyst particles surface
(Hufschmidt et al. 2002).
According to the configuration of the reactor, light intensity, nature of the dye,
and morphology of TiO 2 catalyst, the optimum quantity of the catalyst required for
an efficient removal of the dye can change. Several studies have been realized in
order to determine this optimum quantity by studying the reaction rate of the
photooxidation process. Indeed, when all dye molecules are adsorbed on TiO 2
surface, it clearly appears that the addition of higher quantities of photocatalyst
would have no further enhancing effect on the degradation efficiency. Moreover,
Zhu et al. reported in their study that the excess of photocatalyst particles may
increase opacity of the suspension which may retard the degradation rate (Zhu et al.
2012). Furthermore increasing the quantity of catalyst in the medium will increase
particle–particle interaction. In that case, activated molecules can be deactivated by
collision with ground-state titanium dioxide particles.
On the other hand, the quantity of the initial dye can also influence the mechanism
of photodegradation. Indeed, when increasing dye concentration, dye ions will cover
more active sites preventing the formation of OH radicals on the surface of TiO 2
catalyst as described in paragraph 3.2. Several authors described this phenomenon
by increasing (Zhu et al. 2012) (Ilinoiu et al. 2013) or decreasing (Kansal et al. 2008)
the quantity of initial dye. In order to increase photooxidation rate, adding oxidative
species such as hydrogen peroxide (H 2 O 2 ), ammonium persulfate ((NH 4 ) 2 S 2 O 8 ),
potassium bromate (KBRO 3 ), or potassium peroxydisulfate (K 2 S 2 O 8 ) (Qamar et al.
2005; Saquib et al. 2008a, b) is often used in the photocatalytic process. Indeed, the
electron/hole recombination causes waste of energy, and to avoid this, it is essential
to prevent this latter phenomenon. In heterogeneous catalysis, molecular oxygen
plays the role of an electron acceptor, so the addition of external oxidant (electron
acceptor) in the medium will increase the photocatalytic degradation of contaminants. This phenomenon is well described in literature and based in three steps
(Ahmed et al. 2010):
– Removal of the electron-hole recombination
– Increase of the hydroxyl radical concentration and so of the intermediate rate
compounds
– Generation of more radicals and other oxidizing species in order to increase the
degradation photocatalytic rate of the compounds
50
B. Lebeau et al.
presence of UV/TiO 2 is strongly dependent on the chemical structure of the dye.
Indeed, monoazo dyes are more easily decomposed than triazo dyes since the
stability of both triazine nucleus and cyanuric acid intermediates makes triazo dyes
difficult to fully oxidize (Reutergadh and Iangphasuk 1997; Minero et al. 1997).
Fortunately, such intermediates are not toxic.
Competitive adsorption between water molecules and the target molecules can
occur due to photogenerated oxidizing species, which may not migrate far away
from their formation centers, explaining that there is no or very slow rate of
degradation at few nanometer layers around the catalyst particles surface
(Hufschmidt et al. 2002).
According to the configuration of the reactor, light intensity, nature of the dye,
and morphology of TiO 2 catalyst, the optimum quantity of the catalyst required for
an efficient removal of the dye can change. Several studies have been realized in
order to determine this optimum quantity by studying the reaction rate of the
photooxidation process. Indeed, when all dye molecules are adsorbed on TiO 2
surface, it clearly appears that the addition of higher quantities of photocatalyst
would have no further enhancing effect on the degradation efficiency. Moreover,
Zhu et al. reported in their study that the excess of photocatalyst particles may
increase opacity of the suspension which may retard the degradation rate (Zhu et al.
2012). Furthermore increasing the quantity of catalyst in the medium will increase
particle–particle interaction. In that case, activated molecules can be deactivated by
collision with ground-state titanium dioxide particles.
On the other hand, the quantity of the initial dye can also influence the mechanism
of photodegradation. Indeed, when increasing dye concentration, dye ions will cover
more active sites preventing the formation of OH radicals on the surface of TiO 2
catalyst as described in paragraph 3.2. Several authors described this phenomenon
by increasing (Zhu et al. 2012) (Ilinoiu et al. 2013) or decreasing (Kansal et al. 2008)
the quantity of initial dye. In order to increase photooxidation rate, adding oxidative
species such as hydrogen peroxide (H 2 O 2 ), ammonium persulfate ((NH 4 ) 2 S 2 O 8 ),
potassium bromate (KBRO 3 ), or potassium peroxydisulfate (K 2 S 2 O 8 ) (Qamar et al.
2005; Saquib et al. 2008a, b) is often used in the photocatalytic process. Indeed, the
electron/hole recombination causes waste of energy, and to avoid this, it is essential
to prevent this latter phenomenon. In heterogeneous catalysis, molecular oxygen
plays the role of an electron acceptor, so the addition of external oxidant (electron
acceptor) in the medium will increase the photocatalytic degradation of contaminants. This phenomenon is well described in literature and based in three steps
(Ahmed et al. 2010):
– Removal of the electron-hole recombination
– Increase of the hydroxyl radical concentration and so of the intermediate rate
compounds
– Generation of more radicals and other oxidizing species in order to increase the
degradation photocatalytic rate of the compounds
50
B. Lebeau et al.