Degradation of phenol in water by titanium dioxide photocatalysis

Article abstract of DOI:10.13005/ojc/290328

The photocatalytic degradation of phenol in a static-batch photoreactor, using a UV lamp as a light source and TiO₂ as a photocatalyst, was investigated. The effects of various parameters, such as photocatalyst dosage, initial substrate concentration, reaction pH and addition of oxidants, on the phenol degradation rate of TiO₂ photocatalysis were examined. The degradation rates proved to be strongly influenced by these parameters. Adsorption of phenol is a prerequisite for the TiO₂-assisted photodegradation. In the presence of both UV light illumination and TiO ₂ catalyst, phenol was more effectively degraded than with UV alone. Phenol showed higher photocatalytic oxidation efficiency at natural pH. The reaction rate was found to obey pseudo first-order kinetics represented by the Langmuir-Hinshelwood model. A complete photomineralization of phenol can be easily achieved.

Full text of DOI:10.13005/ojc/290328

ISSN: 0970-020 X
CODEN: OJCHEG
2013, Vol. 29, No. (3):
Pg. 1055-1060
ORIENTAL JOURNAL OF CHEMISTRY
An International Open Free Access, Peer Reviewed Research Journal
www.orientjchem.org
Est. 1984
Degradation of Phenol in Water by
Titanium Dioxide Photocatalysis
NOUREDDINE BARKA1*, IDRISS BAKAS2, SAMIR QOURZAL2,
ALI ASSABBANE2 ANDYHYA AIT-ICHOU2
1Université Hassan 1, Laboratoire CAE, Equipe de Recherche Gestion de l’Eau et Développement
Durable (GEDD), Faculté Polydisciplinaire de Khouribga, BP. 145 Khouribga, Morocco.
2Equipe de Matériaux, Photocatalyse et Environnement, Département de Chimie, Faculté des
Sciences, Université Ibn Zohr, B. P. 8106 Cité Dakhla, Agadir, Morocco.
*Corresponding author E-mail: barkanoureddine@yahoo.fr
DOI: http://dx.doi.org/10.13005/ojc/290328
(Received: June 15, 2013; Accepted: July 24, 2013)
ABSTRACT
The photocatalytic degradation of phenol in a static-batch photoreactor, using a UV lamp
as a light source and TiO2 as a photocatalyst, was investigated. The effects of various parameters,
such as photocatalyst dosage, initial substrate concentration, reaction pH and addition of oxidants,
on the phenol degradation rate of TiO2 photocatalysis were examined. The degradation rates
proved to be strongly influenced by these parameters. Adsorption of phenol is a prerequisite for
the TiO2-assisted photodegradation. In the presence of both UV light illumination and TiO2 catalyst,
phenol was more effectively degraded than with UV alone. Phenol showed higher photocatalytic
oxidation efficiency at natural pH. The reaction rate was found to obey pseudo first-order kinetics
represented by the Langmuir-Hinshelwood model. A complete photomineralization of phenol can
be easily achieved.
Key words: Advanced Oxidation Processes; Photocatalysis; TiO2;
Photomineralization; Phenol; Water treatment.
INTRODUCTION
Phenol is a kind of refractory organic
compound, having the toxicity of carcinogenesis,
teratogenesis and mutagenicity2. The treatment of
phenol in micro-polluted water has always been
paid attention by many researchers.
A wide variety of organic pollutants are
introduced into the water system from various
sources, such as industrial effluents, agricultural
run-off and chemical spills1. Their toxicity, stability
to natural decomposition and persistence in the
environment has been the cause of much concern
to the societies and regulation authorities around
the world.
The traditional purification technique has
been not suitable for the existing water source and
water quality standards. Therefore, it is necessary
to develop low cost and high efficient treatment
technique.

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Several specific new technologies, called Photoreactor and light source
Advanced Oxidation Processes (AOP), have been
The static-batch photoreactor was a
developed to eliminate dangerous chemicals such cylindrical flask of ca. 2 l, opened to air and fitted
as organics from polluted waters.The photocatalytic with a bottom optical Pyrex window ca. 4 cm in
process, based on UV irradiated semiconductor, diameter (Figure 1). It was mounted at a distance of
represents one of AOP that provide an interesting 3 cm from the top of the lamp assembly, which
route to the destruction of many organic substances consisted of an HPK 125 W mercury lamp supplied
to CO2, H2O and corresponding mineral acids3-5.
by Phillips and a circulating-water cuvette (ca. 2.2
cm thick). A filter (Corning 0-52) was used to cut off
Semiconductor such as titanium dioxide the wavelengths shorter than 340 nm which can
TiO2-based photocatalytic oxidation has received cause a direct photochemical transformation of
much attention in environmental remediation field, phenol.
especially for mineralization of many toxic and non-
biodegradable organic pollutants in wastewater6,7.
Among the photocatalysts, TiO2 (in anatase phase)
has been most widely used because it is easily
available, inexpensive, non-toxic, and shows a
relative high chemical stability.
The process of the photocatalytic
degradation of organic pollutants by the
semiconductor powders has been considered to
be very promising. Under UV light illumination,
absorption of photons creates an electron-hole pair
if the energy is higher than the bandgap (3.2 eV) of
Fig. 1: Schematic diagram
of photoreactor system
TiO2. The pairs migrate at the surface, are trapped
by the titanium and OH surface groups, and finally
from OH° and HO2° radicals. These free radicals
cause the oxidation of organic compounds.
Procedure
In the experiments, 1000 ml of phenol at
The aim of the present work is: (i) to
natural pH (6) was magnetically stirred in presence
investigate the influence of various parameters on
of TiO2 in the dark during 30 min to reach the
photocatalytic degradation of phenol in the
adsorption equilibrium before UV irradiation. The
presence of TiO2 particles and irradiated by the UV
suspensions were continuously purged with
light and (ii) to find out the optimum conditions for
oxygen throughout in each experiment. The
efficient degradation.
temperature during the experiment was maintained
at 20°C. Samples of 0.5 ml were collected at regular
intervals, filtered trough 0.45 nylon filters (Millipore)
EXPERIMENTAL
and analyzed by HPLC.
Materials
All chemicals were reagent grade and
Analyses
were used without further treatment. Phenol was
The quantitative and qualitative analysis
purchased from Fluka (> 99% purity). The
of the organic compounds in the samples was
photocatalyst was TiO2 “Degussa P-25” mainly
performed by High Performance Liquid
anatase (80% anatase and 20% rutile) with a
Chromatography HPLC (Jasco-type). The
surface area of 50 m2/g, a density of 3.85 g/cm3 and
wavelength of detector was 270 nm. A reverse-
an average particle size of 30 nm. The other
phase column (length, 25 cm; internal diameter, 4.6
chemicals used in this study such as K2S2O8 and
mm) ODS-2 Spherisorb (Chrompack) was used.
H2O2 were obtained from Merck.
The mobile phase was composed of acetonitrile

BARKA et al., Orient. J. Chem., Vol. 29(3), 1055-1060 (2013)
1057
(20%) and doubly distilled water (80%). The flow
rate was 0.4 ml/min.
Effect of photocatalyst dosages
To optimize the TiO2 suspension
concentration, the effect of photocatalyst dosages
on the degradation of phenol in aqueous solution
was studied. The results were illustrated in Figure
3. The degradation efficiency increased with
increasing the amounts up to 1g/l. Then, the
efficiency decreased slightly, and above 1.25 g/l of
TiO2 became nearly flat. The increase in the
efficiency seems to be due to the increase in the
total surface area, namely number of active sites,
available for the
Complete mineralization of organic
samples to CO2 is very often obtained by using the
photocatalytic method. The kinetic of the evolution
of CO2 formed was followed by using the method of
Chemseddine and Boehm8, which consists of
flushing the CO2 produced by oxygen into a flask
containing 500 ml of barium hydroxide (1.2 x 10-2
mol/l) and to follow the conductivity of the solution
with a conductimeter Orion model 150. CO2
precipitates as BaCO3, thus decreasing the ionic
conductivity in water.
Photocatalytic reaction as the dosage of
photocatalyst increased. However, when TiO2 was
overdosed, the number of active sites on the TiO2
surface may become almost constant because of
the decreased light penetration, the increased light
scattering and the loss in surface area occasioned
by agglomeration (particle-particles interactions) at
high solid concentration9. Therefore, 1 g/l of TiO2
was selected as the optimal amounts of photocatlyst
for the sequential experiment.
RESULTS AND DISCUSSION
Kinetics of phenol disappearance
The elimination of phenol was studied at
two different experimental conditions: (i) under UV
illumination in absence of TiO2 (photolysis), and (ii)
under UV illumination in presence of TiO2
(photocatalysis). In Figure 2, the normalized
concentration of phenol is plotted as a function of
reaction time. There was no observable loss of
phenol when the irradiation was carried out in the
absence of TiO2. However, in the presence of TiO2,
a rapid degradation of phenol occurred by
irradiation. The phenol concentration was
completely removed after 120 min.Therefore, it can
be concluded that phenol degradation in
photocatalysis proceed by the oxidation of phenol
by hydroxyl radical.
Fig. 3: Influence of catalyst concentration on
the degradation rate for the decomposition of
phenol (Co = 10-4 mol/l). Experimental
conditions: PhotocatalystTiO2 (0.15 g/l – 2 g/l),
and irradiation time 60 min
Effect of initial phenol concentration
Figure 4 presents the degradation of
phenol, in solutions containing different
concentrations of the starting compound and 1 g/l
of TiO2 at its natural pH (6), as a function of the
illumination time. The elimination of phenol
proceeds in a shorter time period at lower
concentrations of the pollutant, as expected.
Fig. 2: Photocatalytic disappearance of phenol
(Co = 10-4 mol/l) degradation under the control
conditions: photolysis, and photocatalysis.

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BARKA et al., Orient. J. Chem., Vol. 29(3), 1055-1060 (2013)
it dictates the surface charge properties of the
photocatalyst and size of aggregates it forms.
Therefore, the degradation of the pollutant was
studied at different pH values (in the range 2-10).
The pH of the reaction mixture was adjusted by
adding a dilute aqueous solution of HNO3 or NaOH.
The degradation rate for the decomposition of
phenol as a function of reaction pH is shown in
Figure7.
Fig. 4: Influence of the initial
concentration of phenol
run, when the concentration of phenol is
increased until it attains a plateau, at around 1 .10-
4
mol/l. This behavior indicates saturation-type
Langmuir kinetics. This is confirmed by the linear
plots of 1/ro versus 1/Co (Figure 6) with an intercept
on the ordinate, in agreement with Eqs. (1) and (2):
Fig. 5:Variation of the initial degradation rate of
phenol as a function of its initial concentration
kKCo
ro =
...(1)
1+ KCo
hence
1
1
1
=
+
kKCo
ro
k
...(2)
where Co is the initial phenol concentration.
The factor k is often interpreted in the literature10 as
a pseudo-first-order Langmuir-Hinshelwood (L-H)-
type rate coefficient relating toTiO2-catalysed primary
oxidation events on a surface monolayer (0.15 .10-
4 mol/l.min) and K is a pseudo-equilibrium constant
related to the monolayer adsorption (1.03 .104 l/
mol). Such a Langmuir-type relationship between
the initial degradation rate and the concentration,
has been also reported by other authors11,12. This
type of reaction kinetics suggests that the adsorption
plays a key role in the photocatalytic degradation
mechanism.
Fig. 6:The linear transformation of 1/ro vs. 1/Co
The degradation efficiency increased with
increase in pH. The zero point charge (zpc) pHzpc of
TiO2 particles is around 6 13.TiO2 surface is positively
charged in acidic media (pH < 6) whereas it is
negatively charged under alkaline condition (pH >
6). Generally, the pH changes can have a non-
insignificant result not only on the mode of
adsorption of the phenol substrate on TiO2 surface,
but also on the selectivity of the photodegradative
reaction occurring on the particle surface redox
reactions are very sensitive to changes in the
surface potential. On the other hand, at high initial
pH, more hydroxide ions (OH-) in the solution
Effect of pH
An important parameter in the
photocatalytic reactions taking place on the
particulate surfaces is the pH of the solution, since

BARKA et al., Orient. J. Chem., Vol. 29(3), 1055-1060 (2013)
1059
induced the generation of hydroxyl free radicals
(OH°), which came from the photooxidation of OH-
by holes forming on the titanium dioxide surface.
Since hydroxyl free radical is the dominant oxidizing
species in the photocatalytic process, the
photocatalytic decay of phenol may be accelerated
in an alkaline medium. Similar reaction has been
acceptors such as H2O2 and K2S2O8 on the
photocatalytic degradation of the model compound
under investigation. These acceptors are known to
generate reactive species according to the following
Eqs. (3-5):
H2O2 + e- → OH° + OH-
...(3)
...(4)
...(5)
suggested by a number of researches14,15
.
Consequently, pH 6 was selected for the optimal
experimental conditions, because of the
unnecessary of chemical treatment including
neutralization process.
S2O82- + e-→SO42- + SO4°-
SO42- + H2O→SO42- + OH° + H+
As expected, all the additives markedly
influence the degradation rate for the
decomposition of the compound. These results
indicate that all the additives are more effective
electron acceptors than molecular potential oxygen
as indicated by the one electron reduction potential
of different species formed from these additives:
°-
E(O2/O2 ) = -155 mV, E(H2O2/OH°) = 800 mV,
E(S2O82-/SO4
)
=
1100 mV16. From the
°-
thermodynamic point of view all employed additives
should therefore be more efficient electron
acceptors than molecular oxygen.
Fig. 7: Photodegradation of phenol
at different initial pH values
Effect of electron acceptors
Since hydroxyl radical appears to play an
important role in photocatalysis, electron acceptors
such as hydrogen peroxide and potassium
persulphate were added into the solution in order
to enhance the formation of hydroxyl radicals and
also to inhibit (e-/h+) pair recombination. The
degradation rate for the decomposition of phenol
in the presence of various electron acceptors is
shown in Figure 8. All the additives showed a
beneficial effect on the degradation of the model
compound. One practical problem in using TiO2 as
a photocatalyst is the undesired electron/hole
recombination, which, in the absence of proper
electron acceptor or donor, is extremely efficient
and thus represents the major energy-wasting step
thereby limiting the achievable quantum yield. One
strategy to inhibit electron-hole pair recombination
is to add other (irreversible) electron acceptors to
the reaction. In highly toxic wastewater where the
degradation of organic pollutants is the major
concern, the addition of additives to enhance the
degradation rate may often be justified. In this
connection, we have studied the effect of electron
Fig. 8: Comparison of degradation rate for the
decomposition of phenol (Co = 10-4 mol/l) in the
presence of different electron acceptors.
Experimental conditions: PhotocatalystTiO2 3
Degussa P-253 , electron acceptors: H2O2 (5
.10-4 mol/l) and K2S2O8 (5 .10-4 mol/l),
and irradiation time 60 min
Photomineralization
The evolution of CO2 as a function of
irradiation time in the photocatalytic degradation of
phenol is shown in Figure 9. The same reactivity
order was found for the total mineralization followed

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BARKA et al., Orient. J. Chem., Vol. 29(3), 1055-1060 (2013)
by CO2 evolution. Taking into account the fact that
complete disappearance of phenol in the irradiated
reactor occurs after 120 min, whereas the
CONCLUSION
The results of this study show that the TiO2
stoichiometric (equation 12) formation of CO2 is can efficiently catalyze the decomposition of phenol
observed after 240 min under the same working in the presence of light and oxygen. The
conditions. This implies that total degradation photooxidation of phenol followed first-order kinetics.
requires a longer time for degrading all the possible The apparent rate constant depends on the initial
reaction intermediate produced involved.
phenol concentration. The parameters such as
concentration of the catalyst and pH play an
...(6) important role affecting the reaction rate. The
addition of electron acceptor markedly enhances
the degradation rate of the pollutant. Complete
photomineralization of phenol was achieved. The
final degradation product was carbon dioxide, and
all carbon atoms of phenol were transformed to CO2.
C6H5OH + 7O2 → 6CO2 +3H2O
The observations of these investigations
clearly demonstrate the importance of choosing the
optimum degradation parameters to obtain high
degradation rate, which is essential for any practical
application of photocatalytic oxidation processes.
Fig. 9: Kinetic of CO2 appearance during
the photocatalytic degradation of phenol
in the presence ofTiO2
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