PHOTODEGRADATION OF METHYL TERT-BUTYL ETHER (MTBE) VAPOR BY
USING PHOTOCATALYST IMMOBILIZED NONWOVEN FIBER TEXTILES
Ting-Nien Wu*, Chao-Ming Huang
Department of Environmental Engineering
Kun Shan University
Tainan 71003, Taiwan
Key Words : MTBE, TiO2, ZnO, metal-doping, nonwoven fiber texile, photocatalysis
ABSTRACT
The commercial TiO2 (ST-01), ZnO, and sol-gel prepared metal-doped TiO2 were
individually immobilized onto the nonwoven fiber textiles, and three levels of
photocatalyst loadings were immobilized by hot pressing. In this study, the photocatalyst
immobilized nonwoven fiber textiles were utilized to perform photocatalytic degradation of
methyl tert-butyl ether (MTBE) vapor in a 1520 cm3 photoreactor. The photocatalytic
experiments were conducted 240 min under the irradiation of a fluorescent lamp. The use
of commercial TiO2 (ST-01) provided the remarkable performance of 75% MTBE
photodegradation with 0.75 mg cm-2 TiO2 loading, and the complete removal of MTBE was
reached as increasing TiO2 loading to 3.0 mg cm-2. Comparatively, the ZnO immobilized
nonwoven fiber textiles only performed 17% to 29% MTBE photodegradation. The
prepared W-doped, Fe-doped, and Ag-doped TiO2 were individually immobilized 1.5 mg
cm-2 onto the nonwoven fiber textiles for the photocatalytic experiments. The use of
W-doped, Fe-doped, and Ag-doped TiO2 provided the performance of 61%, 47%, and 37%
MTBE photodegradation, respectively. However, the metal-doped TiO2 did not provide the
superior performance to the commercial TiO2 (ST-01) on MTBE degradation. According to
intermediate identification by GC/MS, MTBE photodegradation can proceed in the route of
pathway A to form TBF, TBA, and acetone or pathway B to form methyl acetate. The
detected CO2 concentrations showed an increasing trend during all photocatalytic
experiments, and it implicated that MTBE vapor can be successfully mineralized through
fluorescence-irradiated photocatalysis. The experimental results supported the possible
application of the photocatalyst immobilized nonwoven fiber textiles to lower the MTBE
level in ambient air around the gasoline stations.
*
To whom all correspondence should be addressed.
E-mail address: wutn@mail.ksu.edu.tw
INTRODUCTION
Due to the air pollution problem regarding
lead particles caused by the use of alkyl lead
additives, methyl tertiary butyl ether (MTBE,
C5H12O) was introduced to replace alkyl lead
additives in the late 1970s. MTBE did maintain
the adequate octane rating, prevent engine
knocking, improve gasoline combustion, and
reduce auto emission of volatile organic
compounds and suspended lead particles.
Typically, reformulated gasoline blends with 10
to 15% MTBE before sale in order to reduce
carbon monoxide and hydrocarbon emissions
from vehicle exhaust. It has been noticed that
the continual use of MTBE has imposed
significant adverse impacts on groundwater
supply [1]. Besides, the presence of MTBE in
the troposphere was about 10 mg m-3 that is
expected to rise with increasing use [2-3].
MTBE has been classified as a possible human
carcinogen [4], and thus the risk regarding the
exposure of MTBE vapor in ambient air around
the gasoline stations is a focal environmental
issue.
Numerous treatment processes have been
proposed for removing MTBE from water
include aerobic biodegradation [5], air stripping
[6], adsorption [6-7], and advanced oxidation.
However, these conventional remedy schemes
have generally indicated either low efficiency or
high costs for MTBE removal. Advanced
oxidation technologies (AOTs) can employ the
generation of hydroxyl radicals to degrade or
even mineralize MTBE with a high reaction rate
such as ozone/hydrogen peroxide [8], Fenton’s
reagent [9], UV/peroxide [10], potassium
permanganate [11], persulfate [12], gamma
radiolysis [13], electrochemical oxidation [1416], and photocatalysis [17].
The studies of applying photocatalysis to
degrade MTBE in the gas phase are rather few.
Titanium dioxide (TiO2) is a widely-used
photocatalyst, which has successfully applied on
the degradation of volatile organic compounds
[18]. This study is aimed at applying TiO2 to the
degradation of MTBE vapor in ambient air
around the gasoline stations. For the practical
application, the photocatalysts including TiO2,
ZnO, metal-doped TiO2, and metal-doped ZnO
were immobilized onto the unwoven fiber
textiles. The fluorescence lamp was selected as
light source to create the surroundings close to
the reality. In this paper, experimental data of a
bench-scale photocatalytic system was provided
for further improving the performance of the
TiO2 immobilized unwoven fiber textiles on
solving the potential air pollution of MTBE
vapor around the gasoline stations.
MATERIALS AND METHODS
1. Materials
Analytical-grade MTBE (99.5%) was
purchased from Merck Ltd. Acetone (99.7%)
obtained from Uni-Ward Co. (Taiwan) and
tert-butyl alcohol (TBA) from TEDIA (U.S.)
were used for the quantification of degradation
intermediates. TiO2 powder with particle size 7
nm and surface area 300 m2g-1 was ST-01
commercial product from Ishihara Co. ZnO
powder with particle size 30 nm and surface
area 40 to 60 m2g-1 was obtained from CBT Co.
2. Preparation of the Photocatalyst
Immobilized Nonwoven Fiber Textiles
The purchased commercial ST-01 TiO2
and ZnO powder was used to prepare the
photocatalyst immobilized nonwoven fiber
textiles by the sol-gel method. Nonwoven fiber
textiles were cut in a size of 5 cm × 24 cm and
dried in an oven before immersion processes.
The sol-gel solution was prepared by blending
the solutions of 3 M titanium tetraisopropoxide
(TTIP) with 4 M ethanol, and the TTIP mixture
was heated up to 90 ℃ and recirculated 30min
for completely mixing. The nonwoven fiber
textile was stepwise dipped in the prepared
sol-gel solution and subsequently dried out in a
110℃ oven for 1 h. The dip-coating process was
repeated one more time until the desired amount
of photocatalyst coating is reaching. Finally, hot
pressing process was applied at 110 ℃ for 10 s
to fix photocatalyst coating onto the nonwoven
fiber textile.
Besides, the metal-doped TiO2 powder was
also utilized to prepare the photocatalyst
immobilized nonwoven fiber textiles. The
metal-doping solution was prepared by blending
AgNO3, FeSO4, or H26N6O41W12 with HNO3 for
preparing Ag-doped, Fe-doped, or W-doped
photocatalyst. Next, the sol-gel solution
containing TTIP and ethanol was decanted into
the metal-doping solution, and the mixture was
magnetically stirred 24 h for completely mixing.
The mixture was oven-dried at 110 ℃ and
subsequently annealed at 400 ℃ for 4 h. The
residual crystalline fracture after annealing was
ground to obtain metal-doped TiO2 powder that
was further used to prepare the photocatalyst
immobilized nonwoven fiber textiles by the
sol-gel method.
3. Photocatalytic Experiments
The experimental setup of the bench-scale
photoreactor was shown in Fig 1. The volume of
the tubular Pyrex photoreactor is 1520 cm3. The
photoreactor was double jacketed with the
installation of a fluorescent lamp. The measured
illumination intensity of the fluorescent lamp is
0.32 to 0.35 MW cm-2. In the photoreactor, the
1
photocatalyst immobilized nonwoven fiber
textile encircled the fluorescent lamp to attain
the best illumination.
3.
3.
4
1
1. MTBE injection
.
2. sampling port
2
3. fluorescence lamp
4. photocatalyst immobilized
nonwoven fiber textile
3
5. Pyrex photoreactor
4
4
.
2
.
5
.
Fig.1. Schematic diagram of the experimental
setup
Before each experiment, 1μL of MTBE
was injected into the photoreactor at room
temperature. The stabilization of a simulated
MTBE contaminated ambient environment was
reached in the photoreactor after waiting 120
min for MTBE vaporization and fiber textile’s
adsorption. Then, the fluorescent lamp was
turned on to conduct the photocatalytic
experiment. Each run was lasted for 240 min,
and 1 ml gas sample was taken with a gas
syringe every 30 min. The sampled gas was
instantly subjected to the analysis of gas
chromatography/ mass spectrometry (GC/MS)
for the determination of residual MTBE as well
as the degradation intermediates.
4. Analytical Methods
MTBE concentration was analyzed using
Perkin Elmer model Clarus 500 GC/MS system
equipped with a Equity-5 column (30 m L ×
0.25 mm ID × 0.5 m thick). In this study,
operating condition was setup as: N2 carrier gas
at 1 ml min-1, injector temperature at 40℃, and
detector temperature at 200℃. The column was
isothermal at 40℃ for 4 min, ramped at 10℃
min-1 to 100℃, held for 2 min, continually
ramped at 30℃ min-1 to 180℃, and held for 2
min.
1
RESULTS AND DISCUSSION
1. Effect of Photocatalyst Coating on
MTBE Removal
In this study, a fluorescent lamp ( > 387.4
nm) was utilized as light source for
photocatalysis of MTBE. The photocatalytic
mechanisms by visible light are given in
reaction 1 to 6 [19]. The chemisorbed MTBE
molecules were excited under visible light
irradiation, and then the electrons were
transferred from excited MTBE molecules to
semiconductor particles. After electron injection
onto the conduction band of the semiconductor,
the conduction band electrons are scavenged by
2
adsorbed oxygen molecules to yield superoxide
radical anions. The formed superoxide radical
anions can combine with hydrogen cations to
produce OOH radicals, and continually react to
generate H2O2 and OH radicals. Accordingly,
photocatalytic degradation of MTBE can occur
through the attack of OH and OOH radicals
under visible light irradiation.
MTBEads + h → MTBEads*
(1)
TiO2(e-) + O2 → O2－
(2)
O2－ + H+ → OOH
(3)
OOH + O2－ + H+ → O2 + H2O2
(4)
H2O2 + O2－ → OH- + OH + O2
(5)
(CH3)3COCH3 + OH (or OOH)
→ intermediates → mineralized products (6)
100
C/Co
80
60
40
TiO2 0.75 mg/cm2
20
TiO2 1.5 mg/cm2
TiO2 3.0 mg/cm2
0
0
60
120
Time (min)
180
240
(a)
100
C/Co
80
60
40
after photocatalyzing 210 min with 3.0 mg cm-2
TiO2 loading. However, the rate of photocatalytic reactions may not increase linearly
with increasing catalyst loading or surface area
because only excited catalyst can create
electron/hole charge pairs [20]. In this study, the
upper bond of photocatalyst loading to hinder
light penetration for photocatalysis is not
reached. The first-order rate constant of MTBE
photodegradation was 0.0045, 0.0056, and
0.0099 min-1 with the TiO2 loading of 0.75, 1.5,
and 3.0 mg cm-2, respectively.
In Fig. 2b, the growing trend of MTBE
removal efficiency with increasing ZnO loading
holds valid. The efficiencies of MTBE removal
are between 17% and 29% after photocatalyzing
240 min when using the ZnO immobilized
unwoven fiber textiles. Comparing with TiO2,
ZnO provided the much poor performance on
photodegradation efficiency and reaction rate
due to less surface area and photocatalytic
characteristics. The photocatalytic degradation
of MTBE follows the first-order kinetics with
the linear R2 value of 0.967 and 0.987. The rate
constants of MTBE photodegradation were
0.0009 and 0.0014 min-1 with the ZnO loading
of 1.5 and 3.0 mg cm-2.
ZnO 0.75 mg/cm2
20
2. Effect of metal-doped Photocatalysts
on MTBE Removal
ZnO 1.5 mg/cm2
ZnO 3.0 mg/cm2
0
0
60
120
Time (min)
180
240
(b)
Fig.2. MTBE removals through photocatalysis
by using (a) TiO2 immobilized nonwoven fiber
textiles, and (b) ZnO immobilized nonwoven
fiber textiles.
For photocatalytic reactions, light can
penetrate to a certain depth due to the absorption
coefficient, the applied wavelength, and the
amount of the scattered light. The amount of
light adsorbed by the photocatalyst is regularly
limited by the amount of immobilized
photocatalyst on nonwoven fiber textiles. Fig. 1
provided the comparison of MTBE removal
efficiencies through photocatalysis at three
levels of photocatalyst loading. As shown in Fig.
1a, the efficiency of MTBE removal increased
with the amount of TiO2 loading as expected.
The complete removal of MTBE was noticed
It is well known that TiO2 can only be
excited by high energy UV irradiation due to its
high energy band gap. The use of sunlight might
result in a low rate of electron transfer and a
high rate of recombination of excited
electron-hole pairs, and thus the low quantum
yield limited the efficiency of photocatalysis
(Woo, 2007). An alternative to extend the
absorption threshold of TiO2 to visible light is to
dope some transition metal onto TiO2 to modify
its photocatalytic properties. In this study, Fe,
Ag, or Wu doped TiO2 nanostructured powder
were synthesized by the sol-gel method, and
subsequently hot-pressed to prepare the
photocatalyst immobilized nonwoven fiber
textiles. The effect of some transition metal ion
dopants on TiO2 photocatalytic activity was
illustrated in Fig. 3. The best efficiency of
MTBE removal is 75% after photocatalyzing
210 min with 1.5 mg cm-2 TiO2 loading, and the
metal-doped TiO2 did not enhance the photo-
catalytic efficiency. By using the metal-doped
TiO2, the photocatalytic degradation of MTBE
seems to follow the first-order kinetics with the
linear R2 value between 0.7 and 0.951. The rate
constants of MTBE photodegradation were
0.004, 0.003, and 0.0021 min-1 with 1.5 mg cm-2
of W-doped, Fe-doped, and Ag-doped TiO2.
Some transition metals such as Fe, Ag, and
Wu were also doped onto the ZnO nanoparticles to investigate the modification of
photocatalytic activity. The efficiencies of
MTBE removal is between 18% and 38% after
photocatalyzing 240 min with 1.5 mg cm-2
metal-doped ZnO. Comparing with all the tested
metal-doped TiO2, ZnO, and metal-doped ZnO,
the commercial ST-01 TiO2 offered the best
photocatalytic efficiency on degrading MTBE.
TiO2 1.5 mg/cm2
W/TiO2 1.5 mg/cm2
Fe/TiO2 1.5 mg/cm2
Ag/TiO2 1.5 mg/cm2
y=0.0056x
y=0.004x
y=0.003x
y=0.0021x
1.8
1.6
ln (C0/C)
1.4
1.2
1
0.8
0.6
0.4
0.2
0
0
60
120
Time (min)
180
240
Fig.3. MTBE removals through photocatalysis
by using the metal-doped TiO2 immobilized
nonwoven fiber textiles
3. Degradation of MTBE through
Photocatalysis
Two possible routes of MTBE degradation
in aqueous phase were identified: one pathway
to breakdown MTBE to tert-butyl formate
(TBF), tert-butyl alcohol (TBA), and acetone
before complete mineralization, and the other to
form methyl acetate as degradation intermediate
[14]. In this study, all the sampled gases were
subjected to the analysis of GC/MS to identify
the degradation intermediates and measure the
disappearance of MTBE. Based on the
identification of MS ion fragments with
mass-to-charge ratio, the intermediate of MTBE
100%
carbon based distribution
2
degradation through photocatalysis are methyl
acetate, TBF, TBA, and acetone. For each
experiment, the level of carbon dioxide showed
a growing trend during photocatalytic
degradation of MTBE vapor.
The carbon based distribution in the
residual MTBE and the formed intermediates
during photocatalysis were illustrated in Fig. 4.
Because the measured carbon based distribution
in methyl acetate is only 2% to 4%, it is clear
that the major photocatalytic degradation of
MTBE is to form TBF, TBA, and acetone before
final mineralization. In Fig. 4a, the measured
carbon based distribution in TBF, TBA, or
acetone is mostly oscillating between 2% and
7% during photocatalytic degradation of MTBE
vapor. After photocatalyzing 180 min with 3.0
mg cm-2 TiO2 loading, the distributions of
carbon atoms in methyl acetate, TBF, TBA,
acetone, CO2, and MTBE were 3.0 %, 5.1 %,
6.3%, 1.5%, 73.9% and 10.2 %. In Fig. 4b, the
measured carbon based distribution in TBF,
TBA, or acetone is negligible (< 0.5%) due to
the poor photocatalytic degradation of MTBE
vapor with ZnO. The growing trend of CO2
level implicated that MTBE can be mineralized
in a slow rate through photocatalysis with ZnO.
Accordingly, the distributions of carbon atoms
in CO2 and MTBE were 22% and 78% after
photocatalyzing 180 min with 3.0 mg cm-2 ZnO
loading.
80%
60%
40%
20%
0%
30
methyl acetate
acetone
60
90
120
Time (min)
TBF
CO2
(a)
150
TBA
MTBE
180
carbon based distribution
100%
80%
60%
40%
20%
0%
30
methyl acetate
acetone
60
90
120
Time (min)
TBF
CO2
150
180
TBA
MTBE
(b)
Fig.4. The carbon based distribution in the
residual MTBE and the formed intermediates
during photocatalysis by using (a) a 3.0 mg cm-2
TiO2 immobilized nonwoven fiber textile, and (b)
a 3.0 mg cm-2 ZnO immobilized nonwoven fiber
textile.
carbon based distribution
100%
80%
60%
40%
20%
0%
TiO2
methyl acetate
acetone
W/TiO2 Fe/TiO2 Ag/TiO2
TBF
CO2
TBA
MTBE
Fig.5. The carbon based distribution in the
residual MTBE and the formed intermediates
after photocatalysis 240 min by using the
metal-doped TiO2 immobilized nonwoven fiber
textiles.
Fig. 5 showed the comparison of the
carbon based distribution in the residual MTBE
and the formed intermediates after photo-
catalysis 240 min by using the metal-doped TiO2
immobilized
nonwoven
fiber
textiles.
Metal-doped TiO2 did not enhance the
photocatalytic efficiency nor affect the
photocatalytic degradation pathway. The carbon
based distributions in the formed intermediates
are not significant, and the extent of
mineralization is 73%, 55%, 47%, and 36% for
the use of TiO2, W-TiO2, Fe-TiO2, and Ag- TiO2.
The decrease of photocatalytic efficiency by
using the metal-doped TiO2 can be explained by
the possible cutback of surface area after metal
doping.
CONCLUSIONS
The band gap of TiO2 is 3.2 eV, and high
energy UV irradiation is generally required to
excite the catalyst. This study demonstrated the
photocatalytic degradation of MTBE vapor
under the irradiation of visible light. The
commercial ST-01 TiO2 immobilized nonwoven
fiber textile gave the best performance on
photocatalysis that MTBE can be completely
degraded through photocatalysis within 210 min
with 3.0 mg cm-2 TiO2 loading. The photocatalytic degradation of MTBE vapor seems to
follow the first-order kinetic, and the rate
constants of MTBE photodegradation were
0.0099 and 0.0014 min-1 with 3.0 mg cm-2 TiO2
and ZnO loading. The prepared W-doped,
Fe-doped, and Ag-doped TiO2 did not improve
photocatalytic degradation of MTBE vapor. The
major pathway of MTBE degradation through
photocatalysis is to form TBF, TBA, and
acetone before final mineralization. Based on
the measured carbon based distribution, the
degradation intermediates including TBF, TBA,
and acetone are typically oscillating between
2% and 7% during photocatalysis. By using
TiO2 immobilized nonwoven fiber textile,
MTBE vapor can be mostly mineralized to CO2
through photocatalysis with visible light. This
study provided an economical and simple
treatment alternative of lowering the MTBE
level in ambient air around the gasoline stations
by using the photocatalyst immobilized
nonwoven fiber textiles.
ACKNOWLEDGEMENTS
This work was partially supported by the
National Science Council (NSC), Taiwan,
R.O.C. under Grant No. NSC-93-2211-E-168001. The authors would like to thank Mr.
Zong-Chih Lin and Kuo-Hua Ling for his
support on analytical work and Department of
Environmental
Engineering,
Kun
Shan
University for the financial support on the
establishment of analytical instruments.
9.
10.
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