Photocatalytic degradation of dibenzothiophene using La/PEG-modified TiO2 under visible light irradiation

Article abstract of DOI:10.1007/s11164-013-1519-z

While the photocatalytic degradation of various organic compounds under UV light irradiation has been widely investigated, visible-light-induced photocatalytic degradation of low levels of pollutants such as dibenzothiophene (DBT) is occasionally reported. In the present work, lanthanide/polyethylene glycol-modified TiO₂ (La/PEG/TiO₂) has been successfully synthesized by a sol-gel method. The photocatalyst was characterized by X-ray diffraction, scanning electron microscopy, transmission electron microscopy, UV-Vis diffusive reflectance spectroscopy, and energy dispersive X-ray analysis. Moreover, the photocatalytic degradation of DBT under visible light irradiation is investigated for the first time using the newly synthesized photocatalyst. The effects of important operational parameters such as initial DBT concentration, catalyst loading, pH, and amount of H₂O₂ on the degradation efficiency were studied. Kinetics parameters of the photocatalytic oxidation of DBT were also calculated. The results show that DBT decomposition occurs according to the Langmuir-Hinshelwood mechanism. The degradation products were analyzed by the GC-MS technique.

Full text of DOI:10.1007/s11164-013-1519-z

Res Chem Intermed
DOI 10.1007/s11164-013-1519-z
Photocatalytic degradation of dibenzothiophene using
La/PEG-modified TiO2 under visible light irradiation
•
Sina Moradi Manouchehr Vossoughi
•
•
Mehrzad Feilizadeh S. Mohammad Esmaeil Zakeri
•
•
Mohammad Moein Mohammadi
•
Davood Rashtchian Amin Yoosefi Booshehri
Received: 5 September 2013 / Accepted: 17 December 2013
ꢀ Springer Science+Business Media Dordrecht 2014
Abstract While the photocatalytic degradation of various organic compounds
under UV light irradiation has been widely investigated, visible-light-induced
photocatalytic degradation of low levels of pollutants such as dibenzothiophene
(DBT) is occasionally reported. In the present work, lanthanide/polyethylene gly-
col-modified TiO2 (La/PEG/TiO2) has been successfully synthesized by a sol–gel
method. The photocatalyst was characterized by X-ray diffraction, scanning elec-
tron microscopy, transmission electron microscopy, UV–Vis diffusive reflectance
spectroscopy, and energy dispersive X-ray analysis. Moreover, the photocatalytic
degradation of DBT under visible light irradiation is investigated for the first time
using the newly synthesized photocatalyst. The effects of important operational
parameters such as initial DBT concentration, catalyst loading, pH, and amount of
H2O2 on the degradation efficiency were studied. Kinetics parameters of the pho-
tocatalytic oxidation of DBT were also calculated. The results show that DBT
decomposition occurs according to the Langmuir–Hinshelwood mechanism. The
degradation products were analyzed by the GC–MS technique.
S. Moradi ꢀ M. Vossoughi ꢀ M. Feilizadeh ꢀ M. M. Mohammadi ꢀ D. Rashtchian
Department of Chemical and Petroleum Engineering, Sharif University of Technology, Tehran, Iran
M. Vossoughi (&)
Institute for Nano-science and Nano-technology, Sharif University of Technology, Tehran, Iran
e-mail: vosoughi@sharif.edu
S. M. E. Zakeri
Department of Engineering, University of Kashan, Kashan, Iran
A. Yoosefi Booshehri
School of Chemical & Biomedical Engineering, Nanyang Technological University, 62 Nanyang
Drive, Singapore, Singapore
A. Yoosefi Booshehri
Singapore Membrane Technology Centre, Nanyang Environmental and Water Research Institute,
1 Cleantech Loop, Singapore, Singapore
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S. Moradi et al.
Keywords Photocatalytic degradation ꢀ Dibenzothiophene ꢀ La/PEG/TiO2 ꢀ
Visible light illumination ꢀ Langmuir–Hinshelwood
Introduction
Dibenzothiophene (DBT) is a heterocyclic molecule containing sulfur present in
petroleum, and its degradation has been studied as the model organic sulfur
compound in crude oil [1]. The presence of sulfur in petroleum products is
considered as one of the most important sources of environmental contamination,
since it leads to the release of sulfur dioxide (SO2), as a major air pollutant
responsible for acid rain, upon combustion [2]. Hence, the level of sulfur in such
compounds is continuously being reduced by regulations to the lowest levels ever.
For instance, the Environmental Protection Agency has issued a series of tenacious
regulations in order to decrease sulfur content in diesel fuel to\15 ppm in 2013 [3].
The widely used technology to remove sulfur content from liquid petroleum
streams is the conventional hydrodesulfurization (HDS) method [4, 5]. Although
HDS is highly efficient in removing thiols, sulfides, and disulfides, it has several
inherent disadvantages such as heavy cost in operating and capital, energy-
intensiveness, and its ineffectiveness in removing sulfur-containing compounds
such as DBT [6, 7]. Therefore, new approaches have attracted attention for being
more efficient alternatives.
Oxidative desulfurization (ODS) is one of the promising new methods being
investigated as an alternative technology for the deep desulfurization of fuel oils. In
comparison with conventional HDS, ODS has attractive features such as high
efficiency, lower cost of operation, and mild temperature and pressure conditions [8–
10]. In particular, photocatalytic ODS is of great interest. In this process, free radicals
are formed via irradiation by some kind of energy source in the presence of a
photocatalyst in order to photo-oxidize sulfur compounds to CO2, SO42-, sulfoxides,
and sulfones [11–13]. TiO2 has been widely used as a photocatalyst because of its
merits which include optical and electronic properties, high level of photocatalytic
activity, low cost, chemical stability, and non-toxicity [14–18]. For instance, to
improve sulfur removal, TiO2 has been investigated for the oxidation of DBT [19–21].
Matsuzawa et al. [19] studied the effect of adding H2O2 and ultrasound irradiation to
the TiO2-containing system for the photooxidation of DBT. They compared four
kinds of TiO2 powder as photocatalysts and found that P25 showed the highest photo-
oxidation rate. However, the conversion of DBT was\40 % after irradiation for 10 h.
Robertson and Bandosz [20] evaluated a new titanium(IV) oxide–hectorite nanofilm
photocatalyst in the photo-oxidation of DBT in tetradecane, as a model for diesel fuel.
They made a comparison with a commercially available TiO2 catalyst, Degussa P25.
Their results indicated that the outlined process was effective in reducing sulfur levels
to below 10 ppm. Zhang and coauthors [21] investigated photocatalytic oxidation of
DBT using titanium silicalite zeolite as photocatalyst with hydrogen peroxide at
ultraviolet lamp irradiation. They found that DBT was photooxidized successfully. In
all the mentioned works, the degradation of DBT was performed under UV light,
123

Photocatalytic degradation of dibenzothiophene
while, to our knowledge, there is no publication on the photocatalytic degradation of
DBT under visible light irradiation.
Although TiO2 is now the most widely researched photocatalyst for various
applications, it is restricted to UV light due to its wide band gap energy of *3.2 eV
[14, 22]. Therefore, the development of a TiO2 photocatalyst that shows a high level
of activity under visible light with a view towards industrial applications is needed.
Notably, in order to extend its light absorption into the visible region, several
attempts have been made to lower the band gap energy by doping TiO2 with
transition metals/non-metal elements or other semiconductors, dyes, and metal and
carbon nanoparticles [23–27]. For example, N, S, or C anion-doped TiO2
photocatalysts have been reported, having an anatase form that shows a relatively
high level of activity when irradiated by visible light [28–30]. In addition, Adams
et al. [31] reported that the use of lanthanide doping in a titania sol–gel enhances the
electron hole separation and also widens the absorption potential into the visible
region. Furthermore, as a strategy to achieve a more efficient photocatalyst,
polyethylene glycol (PEG) was added to the photocatalyst during the synthesis
process. It increases not only the specific surface areas but also the visible light
adsorption of the synthesized photocatalyst [32–34].
In the present work, lanthanide/PEG-modified TiO2 (La/PEG/TiO2) has been
synthesized for the first time using a sol–gel method. Its application has been
evaluated through the photocatalytic degradation of DBT in acetonitrile under
visible light. Moreover, the effects of important operational parameters such as
initial DBT concentration, catalyst loading, pH, and the amount of H2O2 on the
degradation efficiency were studied. Kinetics parameters of the photocatalytic
oxidation of DBT were also calculated.
Experimental
Chemicals
All reagents and solvents used in experiments are available commercially and were
used without further purification. DBT ([98 %) was purchased from Fluka (Buch,
Switzerland). Acetic acid, acetonitrile, titanium(IV) isopropoxide (TTIP), lantha-
num(III) nitrate hexahydrate and PEG (MW = 4,000) were obtained from Merck.
The following equipment was used: a 400 W xenon lamp to produce visible light at
420 nm, an ultraviolet cutoff filter (Kenko,) to allow the passage of light with
wavelengths longer than 400 nm in order to ensure that the irradiation occurred
under visible light only, a glass vessel, X-ray diffraction (XRD) patterns (Philips
X’pert Pro MPD model) with Cu Ka radiation (40 kV, 40 mA) at 2h angles from
10ꢁ to 80ꢁ and with a scan speed of 4 min-1, field emission scanning electron
microscope Hitachi S4160, TEM Hitachi Model H-9000NAR instrument with
accelerating voltage of 200 kV, UV–Vis spectrometer (Ava Spec-2048TEC
spectrometer from 350 to 750 nm wavelengths), a zeta meter (Zeta Meter, USA)
and an Agilent 7890A GC coupled with an Agilent 5975C MSD (Agilent
Technologies, Santa Clara, CA, USA).
123

S. Moradi et al.
Catalyst preparation
The catalyst was prepared by a sol–gel process as follows: TTIP was dissolved and
hydrolyzed in glacial acetic acid at a mole ratio of 1:10 at 0 ꢁC. The solution was
stirred magnetically for 30 min (mixture A). La(NO3)3ꢀ6H2O (La:Ti mol
ratio = 0.3 %) and PEG (PEG: Ti = 0.5 molar ratio) were dissolved in appropriate
amount of deionized water (mixture B). Mixture A was then added dropwise into
mixture B for 70 min under vigorous stirring. The resulting solution was ultrasoni-
cated for 20 min at 0 ꢁC and stirred continuously for 260 min. Subsequently, the
solution was again ultrasonicated and held for a day at room temperature in the dark.
The solution was then gelated at 76 ꢁC and dried at 120 ꢁC. The dried photocatalyst
was ground with a mortar and pestle into fine powders, and finally calcined at 550 ꢁC
for 3 h to prepare the required nanoparticle photocatalysts. Bare TiO2 was
synthesized similar to Philips X’pert Pro MPD model) with Cu K radiation above
method, without use of La(NO3)3ꢀ6H2O and PEG in mixture B.
Photocatalytic test
Photo-oxidation trials were performed in a quartz tube reactor using the 400 W xenon
lamp which produces light spectra similar to that of solar radiation. Photocatalytic
tests were carried out by stirring at room temperature an acetonitrile solution of the
sulphur compound (25–500 ppm) in the presence of photocatalyst (from 10 to
150 mg). Some experiments were carried out by adding the required amount of H2O2
to obtain an initial 10-3 M concentration of H2O2 before starting the irradiations.
Photocatalytic experiments comprised two phases: an equilibration phase and a
reaction phase. In the first phase (equilibration), the reaction suspension was stirred
continuously in the dark for 20 min to ensure adsorption/desorption equilibrium. For
the second phase, the solution was irradiated for 5 h and samples (5 ml) were
collected from the glass vessel and analyzed to determine substrate concentration and
reaction products. Prior to analysis, samples were centrifuged to remove fine
photocatalyst particles. A gas chromatograph (GC-7890A; Agilent) equipped with the
mass spectrometer (MS-5975C; Agilent) was mainly used for the analysis.
Results and discussion
Characterization of synthesized photocatalyst
XRD patterns analysis
Figure 1 shows the XRD patterns of TiO2 and La/PEG/TiO2. The diffraction patterns
of TiO2 and the sol–gel-prepared catalyst only show anatase as the crystalline phase.
Therefore, the broader peak width and slight shift to a higher angle was observed for
the La/PEG/TiO2 sample. Evidently, the dominant peaks of anatase at 25.2ꢁ and
25.48ꢁ for TiO2 and La/PEG/TiO2, respectively, may indicate slight lattice shrinkage
of some of the TiO2 lattices caused by substitution of the dopants. Furthermore, the
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Photocatalytic degradation of dibenzothiophene
Fig. 1 XRD patterns of the La/PEG/TiO2 and pure TiO2
broader XRD peak width possibly implied that the crystallites of the doping sample
were smaller relative to pure TiO2. Therefore, based on these results, the average
crystalline particle size of TiO2 was calculated by the Scherrer equation from the
broadening of the anatase peaks. The La/PEG/TiO2 sample revealed unique small
crystallite sizes (23 nm) compared to TiO2 (36 nm) suggesting that the presence of La
and PEG retarded the growth of the TiO2 crystalline phase.
Scanning electron microscope (SEM) analysis
The typical SEM image of La/PEG/TiO2 is shown in Fig. 2. It can be clearly seen
that the catalyst is composed of nanoparticles.
As can be seen, the surface of La/PEG/TiO2 cannot be described as a
homogeneous layer of material (Fig. 2). Rather, it is made up of agglomerations of
nanoscale particles of hectorite (outer layer), stuck together into features, which in
some cases are nearly a micrometer across. The individual crystals appear to span a
range of about 15–25 nm.
Transmission electron microscopy (TEM) analysis
Figure 3 is the TEM image of La/PEG/TiO2. As shown in Fig. 3, the synthesized
photocatalyst had a mesoporous structure and the average particle size of the La/
PEG/TiO2 was about 20 nm.
UV–Vis diffuse reflectance spectra
The optical absorption properties that are relevant to the electronic structure feature
of the photocatalytic materials are recognized as the key factors in determining their
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S. Moradi et al.
Fig. 2 SEM image of La/PEG/TiO2 catalyst
Fig. 3 TEM image of La/PEG/TiO2 catalyst
photocatalytic activity. Here, the optical absorption properties of the as-prepared La/
PEG/TiO2 were investigated by UV–Vis/DRS; for comparison, the La-doped TiO2
as well as pure TiO2 were also tested (Fig. 4).
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Photocatalytic degradation of dibenzothiophene
The band gap energies (Eg) of La/PEG/TiO2 and La-doped TiO2 can be estimated
from the wavelength values corresponding to the intersection point of the vertical
and horizontal parts of the spectra using the following equation [35, 36]:
hc
Eg ¼
;
k
where Eg is the band gap energy (eV), h is the Planck’s constant, c is the light
velocity (m/s) and k is the wavelength (nm). The band gap energies of La/PEG/TiO2
and La-doped TiO2 were calculated, and are 2.45 and 2.55 eV, respectively.
The red shift of the absorption edge and the lower band gap of La/PEG/TiO2 is
probably due to the lower band gap (Eg) of La/PEG/TiO2 in comparison to that of
La-doped TiO2. It is known that a red shift of the absorption edge would mean a
reduction in band gap energy and an enhancement of photocatalytic efficiency. In
other words, with lower band gap energy in comparison to that of La-doped TiO2,
La/PEG/TiO2 can be excited to produce more electron–hole pairs under visible
light, which could lead to higher photocatalytic activity. Furthermore, the addition
of PEG introduces some extra trap states (impurity levels) within the valence and
conduction bands of TiO2 [37]. The results indicate that the whole La/PEG/TiO2
sample had a better optical absorption in the region of 300–700 nm owing to the
presence of both La and PEG compared with the La-doped TiO2 and the pure TiO2.
The enhancement of absorbance in the UV–Vis region increases the number of
photogenerated electrons and holes to participate in the photocatalytic reaction,
which can enhance the photocatalytic activity of TiO2 [38].
Fig. 4 The UV–Vis diffuse reflectance spectra of pure TiO2, La-doped TiO2, and La/PEG/TiO2
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S. Moradi et al.
Energy dispersive X-ray (EDX) analysis
Energy dispersive X-ray spectroscopy (EDX) is an analytical technique for element
analysis, and the chemical characterization of the La/PEG/TiO2 has been obtained.
EDX analysis of the La/PEG/TiO2 is illustrated in Fig. 5. This figure shows the
presence of Ti, La, and O elements on the as-prepared photocatalyst. The Ti and O
peaks result from the TiO2 particles; the La element indicates the presence of
lanthanide in the La/PEG/TiO2 photocatalyst.
Effect of initial DBT concentration
The effect of the presence of La/PEG/TiO2 on the photooxidation of DBT in
ꢀ
ꢁ
Ct
acetonitrile was assessed from plots of ꢁln
=
versus t obtained with and
C0
without the photocatalyst (Fig. 6). It is important to analyze the effect of different
DBT concentrations because DBT concentration varies in different crude oil
streams. In our study, the different DBT concentrations ranged from 25 to
500 mg L-1. The results showed that the degradation efficiency decreased with the
increased concentration of DBT (Fig. 6). When the concentration of DBT increases,
the photons can be interrupted before they can reach the photocatalyst surface;
hence, the absorption of photons by the photocatalyst decreases, and, consequently,
the degradation efficiency decreases. It is suggested that the optimal initial DBT
concentration for the photocatalytic degradation in the experiment is 250 mg L-1
and the photocatalytic degradation reached 67.24 % in this condition.
,
Fig. 5 EDX spectrum of La/PEG/TiO2 catalyst
123

Photocatalytic degradation of dibenzothiophene
Fig. 6 Effect of initial DBT concentration on photocatalytic oxidation, [La/PEG/TiO2]: 0.5 g L-1, pH
7.00
Effect of photocatalyst concentration
The influence of the photocatalyst concentration on the degradation of DBT has
been investigated using different concentrations of La/PEG/TiO2 varying from 0.1
to 1.5 g L-1 for DBT (Fig. 7). The initial concentration of DBT and irradiation
conditions are identical on all the runs. In the absence of the catalyst, almost no
oxidization of DBT was observed. However, it can be seen that the addition of the
photocatalyst promotes the oxidation of DBT, since the rate of OH• formation and
charge transfer can be accelerated by the addition of the photocatalyst.
A photocatalyst concentration of 0.5 g L-1 appears to be the best condition
considering the degradation of DBT, whereas, in the presence of lower photocat-
alyst loading smaller and in the case of higher loading, almost identical photonic
efficiencies were observed. The reason for this is that, at a certain photon flux
impinging on the suspension, the number of photons absorbed increases with the
amount of photocatalyst up to a point when the presence of additional amounts of
photocatalyst do not lead to further photon absorption.
pH effect
The pH of the aqueous solution is one of the important environmental parameters,
significantly influencing the physicochemical properties of semiconductors, includ-
ing the charge on the photocatalyst particle, the aggregation numbers of particles,
and the positions of the conductance and valence bands [39].
The effect of initial pH on the photocatalytic degradation of DBT was studied. In
the alkaline range, the pH was varied using aqueous NaOH, whereas in the acidic
123

S. Moradi et al.
Fig. 7 Effect of photocatalyst concentration on photocatalytic oxidation of DBT in acetonitrile, [DBT]0:
250 mg L-1, pH 7.00
range, the pH was varied using HClO4. An initial DBT concentration of
250 mg L-1 and catalyst loading of 0.5 g L-1 were used in all these experiments.
The experimental results are shown in Fig. 8.
As observed in Fig. 8, no significant differences were found in the DBT removal
within the tested reaction period of 5 h with different pH of the initial solution.
However, the acidic pH showed a better performance than that of neutral or alkaline
pH. To investigate the surface charge of La-doped TiO2, and La/PEG/TiO2, the zeta
potentials (f) of the photocatalysts were measured on a zeta meter. A f value of
-40.3 4.12 mV was measured for pure undoped TiO2. Upon doping with La, the
f value of La-doped TiO2 decreased to about -27.3 2.95 mV, and this value was
further decreased to -22.8 4.06 mV upon addition of PEG to the photocatalyst
structure. Possible explanations for the above results are: at acidic pH, the HO2•
radical is also able to form, and this can compensate for the decrease in the
generated OH• radicals. At theeutral and alkaline pH, the results indicated that the
as-prepared photocatalyst possessed surface negative charges, resulting in negative
f values. Moreover, the DBT anions have been studied [40]. Therefore, there is a
Coulombic repulsion between the surface of La/PEG/TiO2 and DBT anion at the
neutral and alkaline pH. The Coulombic repulsion can limit the adsorption of the
DBT anion on the surface of La/PEG/TiO2 and the diffusion of OH• radicals toward
the DBT anions for the subsequent reaction. Therefore, the photocatalytic
degradation rate would be unavoidably decreased in neutral or alkali solutions.
Effect of addition of hydrogen peroxide (H2O2)
The effect of the oxidant amount on the desulfurization rate were investigated
experimentally. The experimental results are shown in Fig. 9. According to Fig. 9,
123

Photocatalytic degradation of dibenzothiophene
Fig. 8 Effect of pH on photocatalytic oxidation of DBT in acetonitrile, [La/PEG/TiO2]: 0.5 g L-1
,
[DBT]0: 250 mg L-1
when the oxidant amount rose from 1 to 3 mL, a steady increase in the
desulfurization rate of DBT was observed, and the value of the desulfurization
rate reached the maximum at 3 mL. When the oxidant amount was further
increased, the desulfurization rate decreased.
It is clear that the addition of hydrogen peroxide in this process has two
competing effects: (1) increasing the concentration of hydroxyl radicals in the
reaction media; and (2) decreasing the average light intensity within the solution
due to the absorption of visible light by H2O2. The proper H2O2 concentration has
been found to enhance the degradation rate of DBT due to the more efficient
generation of hydroxyl radicals (OH•) and the inhibition of the recombination of
electron/hole (e-/h?) pairs according to the following equation [41]:
eꢁ þ H2O2 ! OHꢁ þ OHꢂ:
ð1Þ
When excessive H2O2 is present, it is possible that H2O2 scavenges reactive
oxidative species like OH• radicals (Eq. 2), and inhibits the subsequent degradation
reactions [42].
H2O2 þ OHꢂ ! H2O þ HOꢂ2:
ð2Þ
Although confirmative evidence is needed, the competition for OH• and the
formation of less reactive HO•2 may explain the reduced reaction rate at a higher
concentration of H2O2 shown in Fig. 9.
In addition, in order to exhibit the role of La and PEG in the improvement of
photocatalytic activity of La/PEG/TiO2 under visible light irradiation, at and
undoped TiO2 loading of 0.5 g L-1, and with the oxidant amount of 3 mL on the
condition of visible light irradiations, the effects of the presence of only H2O2, H2O2
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S. Moradi et al.
Fig. 9 Effect of addition of hydrogen peroxide on photooxidation of DBT in acetonitrile, [La/PEG/
TiO2]: 0.5 g L-1, pH 7.00, [DBT]0: 250 mg L-1
and undoped TiO2 on the desulfurization efficiency were investigated experimen-
tally. The experimental results are shown in Fig. 10.
As can be seen from Figs. 9 and 10, the DBT desulfurization efficiency in the
presence of only H2O2 and undoped TiO2 are slower than the photo-oxidation of
DBT when La/PEG/TiO2 was used. Therefore, the increase in the photooxidation of
DBT for the La/PEG/TiO2–H2O2 system may be attributed to direct oxidation by
H2O2 as well as the role of La and PEG in the improvement of photocatalytic
activity of La/PEG/TiO2 under visible light irradiation.
Kinetics of photocatalytic oxidation of DBT
Kinetics studies on the photocatalytic rate of the contaminants are useful for process
scale-up. In general, the reaction kinetics follows the Langmuir–Hinshelwood
mechanism given in the following equation [43]:
dC
dt
kKC
r0 ¼ ꢁ
¼
:
ð3Þ
1þKC
where r0 is the reaction rate (mg L-1 min -1), C is the concentration of reactant in
solvent (mg L-1), k is the limiting degradation rate constant at maximum coverage
of the adsorbed pollutant (mg L-1 min -1), K is the Langmuir adsorption constant
of pollutants that adhere to the photocatalyst surface (L mg-1), and t is the illu-
mination time (min). This model assumes adsorption of reactants, surface reaction,
and desorption of products; where the reaction is the rate limiting step. The
expression can be rewritten and integrated to:
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Photocatalytic degradation of dibenzothiophene
Fig. 10 The effects of the presence of only H2O2, H2O2 and undoped TiO2 on the desulfurization
efficiency of DBT in acetonitrile, pH 7.00, [DBT]0: 250 mg L-1
À Á
C0
C
ln
k ꢀ K
¼
t ꢁ K;
ð4Þ
ðC0 ꢁ CÞ ðC0 ꢁ CÞ
where C0 is the initial concentration. It has been suggested that, at very dilute
concentrations (C0 \10-3 mol L-1), KC becomes ꢃ1, and the reaction is of
apparent first-order [44] and the equation is simplified to:
ꢂ
ꢃ
C0
C
ln
¼ k ꢀ Kt ¼ k0t;
ð5Þ
where k0 represents the pseudo-first-order rate constant. A plot of the logarithm of
C0
À À ÁÁ
the relative concentration of the pollutant ln
as a function of illumination time
C
yields a straight line with the slope corresponding to the first-order constant (Fig. 6).
From the data obtained during this study, the values of k0 for different initial con-
centrations of DBT are given in Table 1 and the k, K, and kꢀK for various CDBT are
listed in Table 2.
According to Table 1, it is clear that the reported rate constants are not pseudo-
first-order rate constants at all since they change with the initial concentration of
C
0
ln
ð Þ
C
t
DBT. In Table 2, the k, K, and kꢀK obtained from plots of
versus
are
ðC0ꢁCÞ
ðC0ꢁCÞ
shown. As can be seen from the table, the kꢀK values are of the same order of
magnitude, but not equal, to the k0 values reported in Table 1. It could also be
Table 1 Apparent pseudo-first-
order rate constants for DBT
degradation in acetonitrile
C0 (mg L-1
)
k0 9 10 (min-1
)
R2
25
3.25
1.64
1.26
0.78
0.988
0.981
0.973
0.982
100
250
500
123

S. Moradi et al.
Table 2 DBT Langmuir–
Hinshelwood parameters at the
studied initial concentration
values
C0
(mg L-1
k
K 9 10
(L mg-1
kꢀK 9 10
R2
-1
)
(mg L-1 min
)
)
(min-1
)
25
2.36
2.41
2.65
2.47
1.35
0.98
0.68
0.61
3.18
2.36
1.80
1.51
0.983
0.987
0.954
0.961
100
250
500
Table 3 Comparison between
the rate constants obtained for
mineralization and degradation
of DBT at pH 3, 7, and 10
pH
k0 9 10 (min-1
mineralization
)
k0 9 10 (min-1
)
degradation
3.00
7.00
10.00
1.12
0.94
0.51
4.43
3.18
1.58
proved that the decrease in the k0 values with increasing C0 is due to the light as
limiting reagent. From these tables it can be concluded that, in fact, for
photocatalytic degradation of DBT in acetonitrile the Langmuir–Hinshelwood
mechanism is operating since the experimental points can be fit to Eq. (3).
Products formed by photocatalytic oxidation of DBT
An attempt was made to identify the photo-oxidation products formed in the
photocatalytic degradation of DBT in acetonitrile through GC–MS analysis. The
main products identified by GC–MS were DBT 5-oxide (DBT sulfoxide) and DBT
5,5-dioxide (DBT sulfone). The products were identified based on their molecular
ion and mass spectrometric fragmentation peaks. The similarities of the main
products compared with GC–MS NIST library ranged from 88 to 95 %. The
formation of these products is consistent with the results obtained by Matsuzawa
et al. [19].
DBT mineralization
DBT mineralization during the photocatalytic oxidation was followed by measuring
the COD (chemical oxygen demand) at pH 3, 7, and 10. An initial DBT concentration
of 25 mg L-1 and catalyst loading of 0.5 g L-1 were used in all these experiments. In
a typical sample, reaction aliquots (3 mL) were taken at different reaction times and
the COD in each aliquot was determined using a colorimetric method [45], in which
the total organic sample content was oxidized in a closed system using sulfuric acid
and potassium dichromate for 2 h at 150 ꢁC. The solution absorbance at 600 nm was
measured [12]. From this absorbance, the solution COD was obtained using a
calibration curve made with potassium hydrogen phthalate. The apparent pseudo-
first-order rate constants for DBT mineralization are obtained from the slope of plots
of ln COD versus time. These rate constants are shown in Table 3 where they are
compared with the corresponding rate constants for DBT degradation.
123

Photocatalytic degradation of dibenzothiophene
As shown in Table 3, the rate constants for mineralization (following COD) are
75, 70, and 67 % slower than the DBT degradation at pH 3, 7, and 10, respectively.
The mineralization rate far below the degradation rate indicates the formation of
some stable organic intermediates during the photocatalytic process.
Conclusion
La/PEG/TiO2 was successfully synthesized using a sol–gel method. The addition of
lanthanide and PEG to the photocatalyst structure improved the photocatalysis of
TiO2 under visible light irradiation. This new photocatalysis was used for the
degradation of DBT under visible light. The results of this study clearly demonstrate
the importance of choosing the optimum degradation parameters (DBT concentra-
tion, pH, catalyst concentration, etc.) to obtain a high degradation rate of the model
pollutant. The optimal values of the operation parameters under the related
constraint conditions were found at initial DBT concentration of 250 mg L-1, H2O2
amount 3 mL, initial pH of 3, and catalyst loading 0.5 g L-1. Moreover, DBT
decomposition occurs according to the Langmuir–Hinshelwood mechanism.
Acknowledgment The authors acknowledge the Biochemical and Bioenvironmental Engineering
Research Center (BBRC) at Sharif University of Technology for financial support.
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