ws0">,
about 7% against 12 wt%, respectively [5].
AC
2
N
AC
2
CO
AC
AC
AC
AC AC
Figure 7 shows the kinetic of hydroquinone (HQ) and
benzoquinone (BQ) appearance and disappearance dur-
ing phenol photodegradation under UV-irradiated some
selected solids. These two molecules were the main in-
termediates products observed in all samples studied and
under both types of irradiation. The maximum time of
appearance and time require for the total disappearance
of intermediates are lower for the binary materials
TiO2-AC with respect to TiO2 alone, only in presence of
mixed system that showed higher photocatalytic activity.
This fact is an indicative that intermediates products are
also photodegradated in shorter irradiation time than that
on TiO2 reported by our group for the case of 4-chloro-
phenol [7,8]. As we appointed above, an explanation for
the apparent synergy effect can be based on the conven-
tional Langmuir-Hinshelnwood mechanism with the rate
being proportional to the surface coverage θ varying as:

ads eqads eq
rkkK C1K CKiCi
 
. (1)
being: Kads and Ki correspond to the adsorption constants
of phenol and the intermediate i, Ceq and Ci is the phenol
and intermediate concentration in solution after achieve
the equilibrium adsorption in the dark. Owing to the
similarity of the reactants and of the main initial aromatic
intermediates formed, the term ΣKi·Ci can be estimated
as constant, thus explaining the apparent first order:

ads eqads eq
rkK C1KiCiK C



(2)
The nature of the intermediate main products (HQ and
BQ) is the same for TiO2-AC as for neat TiO2. This con-
firms that reaction mechanism has not been altered nor
changed by the addition of AC, or at least, for these car-
bons [15]. UV photons create electron hole pairs in Tita-
nia
2
TiO hvep

 (3)
which separate because of electron transfer reactions:

22
OOae
 ds
H
(4)
2
OOp
. (5)
As we have already appointed in previous works [15]
radicals created by Equation (5) react with phenolic
compounds to produce hydroxylated aromatic compounds,
mainly hydroquinone in equilibrium with benzoquinone
(Figure 7), and then aliphatic fragments resulting from
the opening before producing CO2 such as picric acid,
oxalic acids, and humic acids, are difficult to well quan-
tified by HPLC. Thus, synergy effect can also be pointed
out in the kinetics of intermediate products appearance
and disappearance. For hydroquinone, its kinetics can be
summarized as:
OH
65 2
123
CH-OHHQBQCO
kkkkn
 (6)
4. Conclusions
For the case of MB photodegradation, the binary materi-
als TiO2-AC showed a clear increase in the photocata-
lytic activity with respect to TiO2 alone, under the two
lamps studied. Under the MH lamp which has a higher
proportion of visible light, TiO2 in presence of H-type
AC showed higher photocatalytic activity with respect to
TiO2 in presence of L-type AC. This beneficial effect has
been attributed to the specific properties of H-type AC
with a high surface area and basic pHPZC. By contrast, for
he case of phenol photodegradation, only TiO2- t
2
CO -800
AC
Copyright © 2012 SciRes. MRC
J. MATOS ET AL.
8
0
1
2
3
4
5
6
7
8
9
10
0100 200 300 400 500 600
HQ (μmo l)
Time (min)
TiO2 P25 Lamp Hg
TiO2-AC CO2 800 Lam
p
H
g
TiO2-AC N2 1000 Lamp Hg
TiO
2
P25 Lamp Hg
TiO
2–
AC CO
2
800 Lamp Hg
TiO
2–
AC N
2
1000 Lamp Hg
0
1
2
3
4
5
6
7
8
9
10
0100 200 300 400 500 600
BQ (μmol)
Time (min)
TiO2 P25 Lamp H
g
TiO2-AC CO2 800 Lamp H
g
TiO2-AC N2 1000 Lamp Hg
TiO
2
P25 Lamp H
g
TiO
2–
AC CO
2
800 Lamp Hg
TiO
2–
AC N
2
1000 Lamp Hg
Figure 7. Hydroquinone (HQ) and benzoquinone (BQ) appearance and disappearance during phenol photodegradation with
some selected solids under Hg Lamp.
presented higher photocatalytic activity than TiO2.
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