ADSORPTION BEHAVIOR OF CHLOROBENZENE ON TiO2 SURFACE IN
RELATION TO THE PHOTOCATALYTIC DEGRADATION IN UV/TiO2
PROCESS
Hsin-Hsu Huang*, Dyi-Hwa Tseng* and Lain-Chuen Juang**
*
Graduate Institute of Environmental Engineering, National Central University,
Chungli, Taiwan 320, Republic of China (dhtseng@ncuen.ncu.edu.tw)
**
Department of Environmental Engineering, Van Nung Institute of Technology,
Chungli, Taiwan 320, Republic of China
ABSTRACT
The effect of adsorption on photocatalytic degradation of MCB in UV/TiO2 process
was investigated in this study with two commercial TiO2. Higher affinity of TiO2 for
water vapor gave rise to higher adsorption capacity than MCB in gaseous system.
Because of the hydroxyl groups on the hydroxylated TiO2 could combine with MCB
by hydrogen bonding or covalent bond formation in aqueous system, the adsorption
behavior of MCB would be affected by the status of TiO2 surface and competition of
water molecules. The theoretical maximum adsorption capacities of MCB on Degussa
P-25 and Janssen in aqueous system were 0.08 and 0.32 moles MCB/m2 TiO2,
respectively. Moreover, higher affinity of water vapor to Degussa P-25 resulted in the
reduction of MCB adsorption in aqueous system. When the solution pH was in the
range of 3 to 9, the rate of MCB photodegradation in UV/TiO2 system had the same
trend as compare to the rate of MCB adsorption on TiO2 particles, and the adsorption
of MCB on TiO2 was contributed to the photocatalytic degradation of MCB in
UV/TiO2 process. In addition, alkaline surrounding was unfavorable to the MCB
adsorption, but beneficial for hydroxyl radical production, which proceeds to MCB
photocatalytic degradation.
KEYWORDS
Adsorption behavior, Chlorobenzene, Titanium dioxide, Photocatalytic degradation
INTRODUCTION
Heterogeneous photocatalysis is one of Advanced Oxidation Processes (AOPs)
technologies lead to more complete destruction of numerous organics than that of
traditional oxidation methods (Davis et al., 1994; Hager et al., 2000; Muneer et al.,
2001; Al-Qaradawi and Salman, 2002; Garcia and Takashima, 2003). The ability of
photoativated TiO2 to oxidize organic molecules major based on hydroxyl radical
which is the primary oxidant in the photocatalytic system. In recent years, applications
of UV/TiO2 system to environmental cleanup have been one of the most active areas
in heterogeneous photocatalytic degradation.
The photocatalytic degradation strategies employ reactive TiO2 surface to capture and
degrade the contaminants to begin aqueous-phase and surface-bound products (Turchi
and Ollis, 1990). Many researches indicated that photoinduced reactions took place at
the surface of TiO2 in UV/TiO2 system, and the adsorption of target compound onto
TiO2 might play very important role to the photocatalytic reaction (Fox and Dulay,
1993; Ilisz et al., 2002; Wiszniowski et al., 2002). Since surface adsorption is
required for interaction between chemical compounds and TiO2 surface, differences in
the affinity for the adsorption of these species should give rise to a broad range of
observed reaction rates. However, the adsorption behavior of various compounds on
TiO2 surface was seldom studied.
Monochlorobenzene (MCB) is one of hydrophobic and volatile organic pollutants that
widely appear in the effluent of plastics, pesticides and chemical industry. Our
previously study has demonstrated that MCB in aqueous phase could be degraded
with UV/TiO2 process (Juang and Tseng, 1997). However, based on the properties of
MCB and characteristics of TiO2, further investigation is necessary to evaluate the
adsorption phenomena of MCB on TiO2 and to realize the importance of adsorption
behavior in relation to the efficiency of photocatalytic degradation of MCB with
UV/TiO2 process.
MATERIALS AND METHODS
Titanium dioxide (TiO2) used in this study was obtained from Degussa (P-25) and
Janssen without further treatment. The surface characteristics of TiO2 are described in
Table 1, which indicated that the crystalline of Degussa P-25 was in the form of
anatase and contained a certain amount of rutile and Janssen was in the form of
anatase. Furthermore, the specific surface area of Degussa P-25 was 5 times greater
than that of Janssen. Reagent grade MCB was purchased from Merck and used
directly as received. Milli-Q water was used through this study for the preparation of
solutions. Acetonitrile was HPLC grade and used as HPLC eluents. All other
chemicals used in analysis were reagent grade or better and used as received.
In order to evaluate the MCB adsorption behavior in TiO2 suspension with single
component system, the adsorption experiments were carried out in both gaseous and
aqueous conditions. Experiments of gaseous phase adsorption took place in a vacuum
system at 25 0C, which putting TiO2 particles in one side of gravimetric adsorption
apparatus Cahn D-200 recording balance and weight standard in the other side. Then,
pure gaseous MCB or water vapor passed through the vacuum system and adsorbed on
TiO2 surface. When equilibrium was achieved, measured weight change of TiO2 to
estimate adsorption capacity of MCB and H2O vapor, respectively. For the
experiments of aqueous phase adsorption, 0.25 g TiO2 was suspended in various
concentration of MCB solutions that were filled in 25 mL centrifugal tubes. Also, the
solution pHs were adjusted by NaOH and HClO4 to expected values. Furthermore, the
centrifugal tubes were shaked in dark with thermostatic shaker at 120 rpm and 25 0C
until anticipative adsorption time or equilibrium reached. Finally, the tubes were
centrifuged at 6000 rpm to separate the TiO2 particles and then the supernatants were
analyzed by HPLC for the change of MCB concentrations.
Photocatalytic degradation of MCB was carried out in a 2.5 L hollow cylindrical
photoreactor equipped with a water jacket. The inter wall of the water jacket is made
Table 1. Characteristics of TiO2 used
Commercial
name
Specific surface area Particle size Crystalline
(m2/g)
(nm)
Degussa P-25
47.05 ±0.17
Janssen
9.31 ±0.03
A：anatase, R：rutile
30
150
A/R
A
Anatase content
(%)
70
100
of quartz and the outer wall is Pyrex. A UVP blacklight lamp (F15T8 BLB, 15 W)
with a maximum UV emission at 365 nm was positioned within the inner part of the
photoreactor. Batch experiments were conducted at 30 0C. A standard operation of
experiment involved prepares 2.5 L 10-4 M MCB in preaerated distilled water with
saturated oxygen. After 2.5 g TiO2 was added into the solution, the suspension was
magnetically stirred and equilibrated in dark for 30 mins prior to Illumination. All test
solutions were run at various pHs after adjustment with NaOH or HClO4. The
concentrations of MCB were analyzed at different time intervals by high performance
liquid chromatography (HPLC) equipped with a variable UV detector.
RESULTS AND DISCUSSION
Adsorption behavior of MCB on TiO2
The relationship of adsorption capacity and relative pressure in gaseous system is
exhibited in Figure 1. The results in Figure 1 depicted that the adsorption capacity of
MCB and water vapor for Degussa P-25 was similar to that of Janssen based on the
surface area of TiO2 particles. Also, the data shown in Figure 1 revealed that the
relationship of adsorption capacity and relative pressure was fitted to the following
BET equation over the relative pressure P/P0 in the range of 0.05 to 0.30, especially.
(C  1)P
1

V(Po  P) Vm C Vm CPo
V = volume of gas adsorbed at pressure P
Vm = volume of gas adsorbed in monolayer
Po = saturation pressure of adsorbate gas at experimental temperature
C = a constant related exponentially to the heats of adsorption and liquefaction of
the gas
P

Using the experimental data for P/P0 in the range of 0.05 to 0.30, a plot of the linear
form of BET equation could obtain the monolayer coverage, Vm, and then the
monolayer saturated adsorption capacity, Q, could be further calculated. The result of
Q for Degussa P-25 was 3.073 mole MCB/m2 TiO2, which was close to 3.100 mole
MCB/m2 TiO2 for Janssen. Consequently, the adsorption capacity of gaseous phase
MCB on TiO2 was independent of the type of crystalline. Additionally, Figure 1 also
pointed out that the shape of adsorption curves for water vapor were analogous to
those of MCB. The Q for Degussa P-25 was around 11.406 mole H2O/m2 TiO2 that
slightly higher than 10.443 mole H2O/m2 TiO2 for Janssen.
Adsorption Capacity,
μmole/m2
50
40
30
Water vapor
20
MCB
10
0
0.0
0.2
0.4
0.6
0.8
Relative pressure, P/Po
1.0
Figure 1. The relationship of adsorption capacity and relative pressure in gaseous
adsorption system ()Degussa P-25, ()Janssen. The solid symbols are for
MCB adsorption and the hollow symbols for water vapor adsorption.
Comparing the values of monolayer saturated adsorption capacity mentioned above, it
found that TiO2 particles had higher ability to adsorb water vapor than MCB. The
benefit for water vapor adsorption indicated that the surface of TiO2 usually adsorbed
H2O molecules dissociatively to form surface hydroxyls, on which further H2O
molecules were physisorbed through hydrogen bonding (Krischok et al., 2002). On
the other side, electrostatic van-der Waals interactions played a major role in the
adsorption of aromatic molecules i.e. MCB, which was a weak adsorption (Diebold,
2003).
For isotherm adsorption in aqueous system, the adsorption capacity versus
equilibrium MCB concentration is illustrated in Figure 2. Three adsorption models
(Langmuir, BET, and Freundlich) were used to estimate the adsorption capacity of
MCB on TiO2 and the modeling curves are also plotted in Figure 2. According to the
results shown in Table 2 indicated that the experimental data fit quite well to these
adsorption models, however, BET model had the best correlation coefficient.
Therefore, it could assume that the adsorption of MCB on hydroxylated TiO2 surface
was to be a multi-layer adsorption.
A calculation based on the BET model indicated that the theoretical maximum
adsorption capacity of MCB in aqueous phase was 0.08 and 0.32 moles MCB/m2
TiO2 for Degussa P-25 and Janssen, respectively. Compare these results with the
outcome in gaseous system, it revealed that the adsorption of MCB was restrained in
aqueous system obviously. Once more, when the TiO2 particles come in contact with
aqueous solution, the TiO2 surface is readily hydroxylated first. Both dissociated and
molecular water are bound to the surface, which consistent with a particle almost
completely covered by bound OH- and H2O (Krischok et al., 2002). Therefore, the
adsorption sites for MCB are decreased due to H2O molecule occupy the main surface
area of TiO2. Furthermore, the adsorption capacity of MCB for Degussa P-25 was
similar to that for Janssen in gaseous system, but lower than Janssen in aqueous
system. The fact of this phenomenon was primarily due to Degussa P-25 had higher
affinity for H2O molecule than Janssen, thus resulted in lower capacity for MCB
adsorption.
(a)
(b)
0.04
0.02
0.00
2
2
0.06
0.40
μmole /m
Adsorption capacity,
0.50
0.08
μmole /m
Adsorption capacity,
0.10
0.30
0.20
0.10
0.00
0
30
60 90 120 150 180
[MCB]eq, mg/L
0
30
60 90 120 150 180
[MCB]eq, mg/L
Figure 2. Comparison of adsorption isotherm model and experiment data (a)Degussa
P-25, (b)Janssen, and () experiment data, () Langmuir, (– –) BET, ()
Freundlich.
Table 2. The fitting results of MCB adsorption on TiO2 in aqueous system with three
isotherm adsorption models
Isotherm
Degussa P-25
Janssen
parameters
R2
parameters
R2
Langmuir
*Q0 = 0.13
*Q0 = 0.50
0.80
0.97
Q 0 bc
qe 
b = 0.02
b = 0.04
1  bc
BET
qe 
BCQ 0
(C s  C )[1  ( B  1)(C / C s )]
Freundlich
*Q0 = 0.08
B = 15.95
n = 1.74
qe  kF C
kF = 0.03
0
*: the unit of Q is adsorbate mole/m2 TiO2
1/ n
0.92
*Q0 = 0.32
B = 36.8
0.99
0.93
n = 1.86
kF = 0.04
0.93
The adsorption kinetic of MCB on TiO2 was measured by the change of MCB
concentration in aqueous solution as a function of adsorption time. The normalized
MCB concentration versus adsorption time at various initial solution pHs are shown
in Figure 3. The results indicated that the adsorption of MCB on the surface of TiO2
was rapid at the first 4 hrs and then became slower thereafter, which means the
adsorption reached its equilibrium at very short time. The results in Figure 3 also
showed that the adsorption rate of MCB on TiO2 was enhanced at neutral and
near-to-neutral pH values. This was probably due to the maximum adsorption was
occurred at the pH close to the pHzpc of TiO2. The literature indicated that pHzpc of
TiO2 for pure anatase is 6.39 and for pure rutile is 5.80 (Szczepankiewicz, 2001).
Therefore, when the solution pH was less or greater than pHzpc, the surface hydroxyl
groups of TiO2 would undergo a proton association or dissociation reaction and lead
to positive or negative charge on TiO2 surface.
Considering a MCB molecule, the electronegativity difference between the carbon
and chlorine atom should be not ignored and the C–Cl bond in MCB is polar. The
chlorine atom, with its higher electronegativity, pulls the bonding electrons closer to it.
This makes the carbon atom somewhat electron deficient and gives it a partial
(b)
1.00
1.00
0.95
0.95
C/Co
C/Co
(a)
0.90
0.90
0.85
0.85
0.80
0.80
0
5
10
15
20
Adsorption time, hr
25
0
5
10
15
20
Adsorption time, hr
25
Figure 3. The normalized MCB concentration as a function of adsorption time at
various initial solution pH levels (a)Degussa P-25, (b)Janssen, and ()pH 3, () pH 5,
() pH 7, () pH 9, () pH 11 ([TiO2] = 10 g/L, [MCB] = 5  10-4 M).
positive (+). Alternatively, the chlorine atom becomes somewhat electron rich and
bears a partial negative charge (–). At acidity condition, the positive TiO2 surface
can easier connect with partial negative charge chlorine of MCB. Consequently,
experimental results showed that the MCB was more easily adsorbed by the
electrostatic attraction at lower pH. In contrast, a lack of adsorption of MCB in
alkaline solutions seems consistent with the theory of electrostatic nature of
adsorption. The adsorption kinetics as illustrated in Figure 3 for both of two types of
TiO2 were in the same order of pH 7 > pH 3 > pH 5 > pH 9 > pH 11, but the effect of
pH on the adsorption rate constant was insignificant.
In gaseous adsorption system, the adsorption of MCB on TiO2 is in compliance with
the electrostatic van-der Waals force and independent of catalyst crystalline. In
aqueous solution, however, the adsorption behavior of MCB affected by the status of
TiO2 surface and competition of water molecules. The outcome of aqueous adsorption
reveals that the hydroxyl groups on the hydroxylated TiO2 can combine with MCB by
hydrogen bonding or covalent bond formation. Simultaneously, it can also form
hydrogen bonding with water molecules, creating a competition for adsorption
between MCB and water.
Photocatalytic degradation of MCB with TiO2 slurry
Heterogeneous photocatalytic reactions are caused by photon irradiation, when a TiO2
suspension is illuminated with UV light the concentration of MCB markedly
decreases with reaction time is shown in Figure 4. In this study, complete
decomposition of MCB was observed within 3 hrs of illumination. According to the
results presented in Figure 4, it can be estimated that Degussa P-25 has higher
photodegradation rate constant at initial pH 3, 7, 11; otherwise Janssen has higher
value at initial pH 3 and 11. Compared the results of photocatalytic degradation of
MCB by UV/TiO2 and adsorption of MCB by TiO2 particles, it found that the
tendency for photocatalytic degradation of MCB affected by pH was consistent with
that for adsorption of MCB at solution pH values were less than 9. However, in
contrast to lower MCB adsorption rate constant at alkaline condition (pH=11), the
(b)
1.000
1.000
0.800
0.800
0.600
0.600
C/Co
C/Co
(a)
0.400
0.400
0.200
0.200
0.000
0.000
0
50
100
150
200
Illumination time, min
250
0
50
100
150
200
Illumination time, min
250
Figure 4. The normalized MCB concentration as a function of illumination time at
various initial solution pH levels (a)Degussa P-25, (b)Janssen, and ()pH 3, () pH 5,
() pH 7, () pH 9, () pH 11 ([TiO2] = 1 g/L, [MCB] = 10-4 M, light intensity =
5.6 mW/cm2).
photocatalytic degradation rate constant was higher.
In aqueous photocatalytic system, the TiO2 particles were surrounded by water and
almost completely covered by bound OH- and H2O. When TiO2 was photoactivated by
UV light and then resulted in formation of electrons and holes at the surface. Since
OH- and H2O groups were the most abundant adsorbates, it seemed likely that holes
would react with these species to form OH radicals, which were presented as mobile
radicals. The generated radicals could then oxidize MCB at the solid-liquid interface;
therefore, adsorption of MCB to the TiO2 surface was a limited step in photocatalytic
degradation. However, the adsorption of MCB on surface hydroxyl group by hydrogen
bonding or covalent bond has demonstrated in this study. Although the acidity
situation was unfavorable for hydroxyl radical generation, the adsorption of MCB on
TiO2 assisted in the photodegradation of MCB in UV/TiO2 process. Moreover,
hydroxyl radical generation was constructive at alkaline solution and the larger
quantity of hydroxyl radical produced would overcome the restraint of MCB
adsorption at pH 11.
CONCLUSIONS
In gaseous system, two types of TiO2 in this study had similar MCB adsorption
capacity resulted from the electrostatic van-der Waals force. Compared the adsorption
capacity of MCB with that of water vapor demonstrated that TiO2 had higher ability to
adsorb water vapor than MCB. In aqueous solution, however, the hydroxyl group on
hydroxylated TiO2 could react with MCB through hydrogen bonding or covalent bond
formation and the adsorption behavior of MCB on TiO2 was affected by the status of
TiO2 surface and competition of water molecules. Isotherm adsorption tests indicated
that experimental data fit to BET model. The theoretical maximum adsorption
capacities of MCB on Degussa P-25 and Janssen in aqueous system were 0.08 and
0.32 moles MCB/m2 TiO2, respectively. Moreover, higher affinity of H2O to
Degussa P-25 resulted in less MCB adsorption in aqueous system than that for Janssen.
The tendency that affected by pH in the range of 3 to 9 for MCB photodegradation in
UV/TiO2 system was same as that for MCB adsorption on TiO2. Therefore, the
adsorption of MCB on TiO2 would be contributed to the photodegradation of MCB in
UV/TiO2 process. Interestingly, the adsorption of MCB on TiO2 was unfavorable in
alkaline solution, but hydroxyl radical production was beneficial at high pH, thus
photocatalytic degradation of MCB was enhanced due to the radical oxidation.
ACKONWLEDGEMENT
The authors would like to thank the National Science Council, R.O.C. for financial
support of this study under Contract No. NSC 90-2211-E-238-003.
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