Enhanced photocatalytic degradation of methylene blue under vacuum ultraviolet irradiation

Article abstract of DOI:10.1016/j.cattod.2012.06.022

A vacuum ultraviolet (VUV) lamp was used as the irradiation of photocatalysis to enhance the degradation of methylene blue (MB), a model dye compound. In contrast to photocatalysis under 254 nm UV irradiation (UV/TiO ₂), photocatalysis under VUV irradiation (VUV/TiO₂) exhibited much higher efficiency in MB degradation and mineralization and needed less TiO₂ dosage. The rate constant of MB degradation in VUV/TiO₂ (0.0793 min^-1) is about 4 times of that of UV/TiO₂ (0.0205 min^-1). The MB mineralization rate is also greatly increased from 12.5% in UV/TiO₂ to 47.7% in VUV/TiO ₂ after 60 min of irradiation. Multiple advanced oxidation processes including photocatalytic oxidation and VUV photo-oxidation coexist in VUV/TiO₂ to substantially produce highly reactive species (such as OH and energetic photons), which is responsible for efficient degradation and mineralization of MB.

Full text of DOI:10.1016/j.cattod.2012.06.022

Catalysis Today 201 (2013) 189–194
Contents lists available at SciVerse ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
Enhanced photocatalytic degradation of methylene blue under vacuum
ultraviolet irradiation
Haibao Huanga,b, Dennis Y.C. Leunga,∗, Philip C.W. Kwongc, Jing Xionga, Lu Zhangb
a Department of Mechanical Engineering, The University of Hong Kong, Pokfulam Road, Hong Kong
b School of Environmental Science and Engineering, Sun Yat-Sen University, China
c School of Chemical Engineering, The University of Adelaide, Australia
a r t i c l e i n f o
a b s t r a c t
Article history:
A vacuum ultraviolet (VUV) lamp was used as the irradiation of photocatalysis to enhance the degra-
dation of methylene blue (MB), a model dye compound. In contrast to photocatalysis under 254 nm UV
irradiation (UV/TiO2), photocatalysis under VUV irradiation (VUV/TiO2) exhibited much higher efficiency
in MB degradation and mineralization and needed less TiO2 dosage. The rate constant of MB degradation
in VUV/TiO2 (0.0793 min−1) is about 4 times of that of UV/TiO2 (0.0205 min−1). The MB mineralization
rate is also greatly increased from 12.5% in UV/TiO2 to 47.7% in VUV/TiO2 after 60 min of irradiation. Mul-
tiple advanced oxidation processes including photocatalytic oxidation and VUV photo-oxidation coexist
in VUV/TiO2 to substantially produce highly reactive species (such as •OH and energetic photons), which
is responsible for efficient degradation and mineralization of MB.
Received 15 January 2012
Received in revised form 21 June 2012
Accepted 25 June 2012
Available online 11 August 2012
Keywords:
Photocatalytic degradation
Photolysis
Vacuum ultraviolet irradiation
Methylene blue
© 2012 Elsevier B.V. All rights reserved.
Mineralization
1. Introduction
application is greatly limited by inefficient oxidant generation and
low degradation capacity [8,9]. In order to improve the efficiency
A large amount of wastewater containing dyestuffs with inten-
sive color and toxicity can be introduced into aquatic systems from
textile industry. It causes a great deal of environmental problems
around the world [1–3] and has become one of the key concerns of
sewage treatment [4]. Many technological approaches, including
physical methods (such as coagulation and adsorption) [4], bio-
logical methods [5] and chemical methods (such as NaClO and
ozonation) [6] have been developed for reducing dye pollution.
However, conventional treatment methods are difficult to meet the
stringent environmental quality standards required these days. For
example, biological methods are not ineffective in decolorizing tex-
tile effluents since many dye compounds are recalcitrant organic
molecules or non-biodegradable [7]. Coagulation flocculation is not
efficient for the removal of soluble reactive dyestuff. In addition,
substantial sludge will be generated in these two processes and the
sludge require further treatment [1], which may greatly increase
the cost of wastewater treatment.
of PCO, many modification methods have been adopted, includ-
ing assistance of hydrogen peroxide [10], microwave [11,12] and
ozone [13]. Nevertheless, all these processes need additional oxi-
dants or equipment to generate •OH, making it cost prohibitive for
full-scale operation [14]. More recently, vacuum ultraviolet (VUV)
photo-oxidation of wastewater has attracted much attention since
it exhibits an advantage of being an additive-free process, and
thereby may make commercially viable [15]. It has been shown
to be effective for the treatment of certain types of wastewater
such as perfluorodecanoic acid, SDBS surfactants, geosmin and 2-
methylisoborneol [15–19]. Nevertheless, unfavorable by-products
and intermediates radicals can be formed depending on the com-
plexity of the organics degraded in this process [16–18]. In addition,
VUV photo-oxidation has a limited capacity of •OH formation,
resulting in restricted oxidation and mineralization rate [17].
Methylene blue (MB) is an intensely colored compound which
is used in dyeing and printing textiles and is a common water pol-
lutant [20]. In this study, a VUV lamp was used as the irradiation of
photocatalysis to enhance the degradation of MB. Its performance
was compared with photocatalysis under 254 nm UV irradiation
(UV/TiO2). The mechanism leading to excellent performance of
VUV/TiO2 was discussed. This is the first attempt to remove dye pol-
lutants using VUV/TiO2. This efficient process presents a promising
technology for dye treatment.
Photocatalytic oxidation (PCO) is regarded as a promising tech-
nology for dye wastewater treatment due to its strong capacity
of oxidization and environmentally friendliness [8,9], whereas, its
∗
Corresponding author. Fax: +852 2858 5415.
E-mail addresses: ycleung@hku.hk, seabao@126.com (D.Y.C. Leung).
0920-5861/$ – see front matter © 2012 Elsevier B.V. All rights reserved.
http://dx.doi.org/10.1016/j.cattod.2012.06.022

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H. Huang et al. / Catalysis Today 201 (2013) 189–194
experiment, 400 mL of 20 mg/L dye solution was poured into the
Stainless steel
encloser
beaker. TiO2 powder was added into the solution and well spread
by a magnetic stirrer with a mixing rate of 800 r/min. Two types of
low-pressure mercury lamps (7 W) were used to provide UV illumi-
nation: a UV lamp emitting at 254 nm (ZW7D12W-H115, Cnlight)
and a VUV lamp emitting about 10% 185 nm UV radiation and
about 90% 254 nm UV radiation (ZW7D12Y-H115, Cnlight). Both
UV lamps are H-type with a length of 11.5 cm. The effective length
of lamp in MB solution is 10 cm. Prior to irradiation, the solution
and TiO2 powder were allowed to be fully mixed under stirring
and equilibrate in the dark for 30 min. Samples were collected at
regular intervals, and centrifuged to remove the catalyst particles
prior to analysis by UV–Vis spectrometer and TOC analyzer. For oxy-
gen rich conditions, oxygen from cylinder was bubbled at a flow of
0.2 L/min. This ensured the reaction solution was kept saturated by
the continuous supply of oxygen at a constant rate when required.
Degradation efficiency and mineralization rate were calculated
using the following equations:
VUV lamp
Beaker
MB solution
TiO2 powder
Magnetic
stirrer
Fig. 1. The schematic diagram of the PCO reactor.
2. Experimental and methods
2.1. Reagents and materials
MB (Tianjing Chemical Co., Ltd., China) was analytical grade and
used as received. Commercial TiO2 powder, Degussa P25 with BET
surface area of 50.8 m2/g and average particle size of 22.1 nm, was
utilized as the photocatalyst.
C0 − Ct
Degradation efficiency (%) =
× 100
(1)
(2)
C0
TOC0 − TOCt
Mineralization rate (%) =
TOC0
where C0 and Ct represents the initial concentration of MB and
those at time t, and TOC0 and TOCt represents the initial concen-
tration of TOC and those at time t, respectively. The degradation
efficiency and mineralization rate may not be identical since
organic intermediates can be generated during the degradation of
pollutants. Compared with degradation efficiency, mineralization
rate provide a better understanding of the capability of transferring
pollutants into harmless inorganic products.
2.2. Measurement scheme
The MB solution was analyzed by a UV–Vis spectrometer
(Jenway 6105, Keison) at the wavelength of 664 nm. The MB
concentration in water was calculated using calibration curves.
The total organic carbon (TOC) in the solution was measured
with a Shimadzu VCPH TOC analyzer. The inorganic ion products
from MB degradation were determined using ionic chromatogra-
phy (IC, Metrohm 882) with a Metrosep A Supp 5 guard column
(250 mm/4 mm). A gradient elution of a 3.2 mM Na2CO3 and1 mM
NaHCO3 solution was used. The intermediates of MB degradation
were identified by a liquid chromatography–mass spectrometry
(LC–MS) system (LCQ DECA XP, Thermo Finnigan) using an Agilent
Zorbax SB-C18 column (150 mm × 4.6 mm) packed with 5 ␮m par-
ticle size. The mobile phase consisted of two solutions namely A and
B. Solution A was made from 0.1 M ammonium acetate and acetic
acid (pH 5.3), whereas solution B was acetonitrile [21]. The gradient
elution was from 5% to 95% in 30 min, the flow rate was 0.8 mL/min
and the injection volume was 100 ␮L. The mass spectrometer was
equipped with an electrospray ionization source and operated in
positive polarity. The mass range was from 50 to 300 m/z.
3. Results and discussion
3.1. MB degradation
Fig. 2a compares the MB concentration as a function of irra-
diation time in different processes. The MB concentration slightly
dropped from 20 mg/L to 19.2 mg/L in the presence of TiO2 after
reaching adsorption equilibrium, revealing that the adsorption
from TiO2 is not significant. Almost no reaction was observed
for 254 nm UV irradiation alone, indicating that the direct photo-
degradation by 254 nm UV irradiation can be neglected. The
concentration of MB tended to decrease exponentially with irra-
diation time in the other processes. VUV/TiO2 obtained a much
faster degradation of MB than UV/TiO2. The MB concentration in
UV/TiO2 dropped 6.2 mg/L after 20 min of irradiation; however, it
The schematic diagram of the PCO reactor is presented in Fig. 1.
One 500 mL glass beaker with a diameter of 10 cm was used as
the reaction vessel and placed on a magnetic stirrer. In a typical
8
20
(a)
(b)
k
k
k
k
VUV/TiO2=0.0793 min-1
UV/TiO2=0.0205 min-1
VUV=0.0468 min-1
UV=0
6
4
2
0
15
UV
VUV
UV/TiO
UV
2
10
VUV
VUV/TiO
2
UV/TiO
2
VUV/TiO
2
5
0
0
20
40
60
80
0
20
40
60
80
Irradiation time (min)
Irradiation time (min)
Fig. 2. Time course of MB concentration (a) and rate constant of MB degradation (b) (TiO2 dosage = 0.5 g/L, C0 = 20 mg/L).

H. Huang et al. / Catalysis Today 201 (2013) 189–194
191
was decreased to only 1.1 mg/L in VUV/TiO2. In contrast to UV/TiO2,
VUV/TiO2 can provide more processes such as photo-oxidation for
MB degradation. It can be seen from that the MB concentration in
VUV dropped to 4.2 mg/L, indicating that VUV photo-oxidation can
directly degrade MB in the absence of TiO2.
To analyze degradation kinetics, simplified apparent pseudo-
first-order of kinetics rates were obtained for MB degradation using
Eq. (3):
declined sharply along with an increased distance from the UV lamp
due to serious light scattering and agglomeration resulting from a
high number of catalyst particles in aqueous suspension [14,22].
Considering both the favorable and the unfavorable effect of the
increased TiO2 dosage, the overall degradation efficiency of MB did
not change too much as it is over 1 g/L. Similarly, an optimum TiO2
dosage of 1 g/L was also observed for the VUV/TiO2 process (Fig. 3b).
However, the removal efficiency of MB was kept at very high level of
98.2% even with 0.5 g/L TiO2 dosage. The TiO2 dosage had less influ-
ence in the VUV/TiO2 process than the UV/TiO2 process. As shown
in Fig. 2, VUV photo-oxidation, which happened in the absence of
TiO2, was also much involved in MB degradation besides PCO in
the VUV/TiO2 process. One of the disadvantages of conventional
photocatalysis is the requirement of large amounts of TiO2 photo-
catalyst. Compared with UV/TiO2 process, the use of photocatalyst
can be substantially reduced due to the presence of multiple oxida-
tion processes in the VUV/TiO2 system, which can greatly cut down
the cost and makes it more capable for industrial application.
C0
ln
= ktt
(3)
Ct
where kt was the apparent pseudo-first-order rate constant of ini-
tial degradation (expressed in units of min−1), Ct and C0 represents
the transient and initial concentrations of MB, respectively. The
rate constant kt was calculated from the slope of the ln (C0/Ct)
versus t, as shown in Fig. 2b. It can be found that the degrada-
tion of MB shows apparent pseudo-first-order of kinetics. The rate
constant of MB degradation was 0.0205 and 0.0468 min−1 in the
UV/TiO2 and VUV process, respectively. However, it was greatly
increased to 0.0793 min−1 for the VUV/TiO2 process, which was
about 4 and 2 times of that in the UV/TiO2 and VUV process. The
rate constant of VUV/TiO2 process was even higher than the sum of
that of UV/TiO2 and VUV process, indicating that synergetic effect
occurred during the MB degradation in the VUV/TiO2 process. The
VUV photo-oxidation and UV/TiO2 process coexisted and synergis-
tically degraded pollutants in the VUV/TiO2 system.
3.3. Mineralization rate
Both UV/TiO2 and VUV/TiO2 obtained high MB removal effi-
ciency with TiO2 dosage of 1 g/L, as previously described. Their
degradation and mineralization rate of MB under this condition
is compared and the results are presented in Fig. 4. With TiO2
dosage of 1 g/L, UV/TiO2 achieved higher degradation efficiency
than VUV, which is different from the result with TiO2 dosage of
0.5 g/L (Fig. 2a). Both the degradation and the mineralization rate,
especially the latter, were significantly raised in VUV/TiO2 when
compared with the UV/TiO2. The mineralization rates were only
24.2% and 3.5% after 30 min of irradiation in the UV/TiO2 and VUV
process, respectively. However, it is greatly increased to 35.5% in
VUV/TiO2. Comparing Fig. 4a and b, it can be observed that the
MB mineralization rate is much smaller than the degradation effi-
ciency. For example, nearly 100% degradation efficiency of MB can
be obtained on all the processes after 60 min of irradiation (Fig. 4a)
while the corresponding mineralization rate is only 47.7%, 35.8%
and 12.5% for the VUV/TiO2, UV/TiO2 and VUV process (Fig. 4b),
respectively. The results suggest that MB is quickly converted to
intermediate products while it takes a relatively long time to min-
eralize intermediate products. This observation agrees with SDBS
oxidation in the previous studies, in which intermediates such as
ketones, alcohols, and carboxylic acids have been observed [17,23].
Compared with VUV alone, photocatalysis under irradiation of
either UV or VUV light both obtained much higher mineraliza-
tion rate, indicating that the presence of TiO2 can promote the
3.2. Effect of TiO2 dosage
The dosage of photocatalyst is an important factor of PCO for the
sake of the operating cost and its effect on the removal efficiency.
Various dosage of TiO2 was used in the PCO processes and its effect
on MB degradation is presented in Fig. 3. The removal efficiency of
MB after 60 min of irradiation is only 79.6% with 0.5 g/L TiO2 dosage
while it greatly increased to 98.5% with 1 g/L TiO2 dosage in the
UV/TiO2 process (Fig. 3a). The increase of TiO2 dosage can improve
the molar ratio of TiO2/MB and provide more active centers for pho-
tocatalytic oxidation, which favors MB degradation. The maximum
removal efficiency was obtained with 1.5 g/L TiO2 dosage while fur-
ther increase of TiO2 dosage contrarily only caused a slight decrease
of removal efficiency. In aqueous photoreaction systems, the exis-
tence of gradient light intensity throughout the reaction solution
is one of important restrictions to the efficiency of overall photore-
action. It highly relies on the optical properties of the catalyst such
as light absorption and scattering effects, the particle size of the
catalyst and the degree of agglomeration in its slurry [22]. Beyond
such an optimum dosage, the light intensity inside the photoreactor
100
90
100
90
80
70
80
60
50
70
0 g/L
40
0 g/L
0.5 g/L
1 g/L
1.5 g/L
2 g/L
0.5 g/L
60
50
40
30
20
10
0
1 g/L
1.5 g/L
2 g/L
0
20
40
60
80
100
0
20
40
60
80
100
Irradiation time (min)
Irradiation time (min)
(a) UV/TiO2
(b) VUV/TiO2
Fig. 3. Effect of TiO2 dosage on degradation efficiency of MB (C0 = 20 mg/L).

192
H. Huang et al. / Catalysis Today 201 (2013) 189–194
80
70
60
50
40
30
20
100
90
80
70
60
50
40
30
20
10
0
VUV/TiO
2
VUV/TiO
2
UV/TiO
2
UV/TiO
10
0
2
VUV
140
VUV
20
40
60
80
100
120
160
0
20
40
60
80
100
120
Irradiation time (min)
Irradiation time (min)
(a) Degradation efficiency
( b) Mineralization rate
Fig. 4. Time course of degradation and mineralization rate (TiO2 dosage = 1 g/L, C0 = 20 mg/L).
mineralization of pollutants. Similar phenomena were also
observed in the degradation of p-chlorobenzoic acid in these pro-
cesses [24]. This is probably due to intermediate degradation on the
surface of photocatalyst [24] and the strong oxidation capability of
•OH in the PCO process.
easier it is for old bonds to break and new bonds to form [31]. Com-
pared with water molecular, MB molecular can more easily to be
destructed by energetic photons.
Fig. 4 gives another evidence of the contribution of direct pho-
tolysis in degradation of MB. UV/TiO2 and VUV obtained similar MB
degradation efficiency after 60 min of irradiation. Nevertheless, the
mineralization rate in the former is 36%, which is about 3 times of
the mineralization rate of 12.5% in VUV. The results indicate that
the reactive species responsible for MB degradation are possibly
different in the two processes. There should be additional reactive
species involved in MB degradation besides •OH in the VUV pro-
cess. In the VUV process, the degradation efficiency (95.8%) is much
larger than the mineralization rate (12.5%) after 60 min of irradi-
ation, indicating that many organic intermediates are generated
from MB dissociation by energetic photons. This was confirmed
by LC–MS results. Azure B and thionin were identified in the MB
solution after degradation using LC–MS. However, •OH is the main
oxidant in the UV/TiO2 process [26,28,29]. It is well known that •OH
is highly reactive, non-selective, and capable of complete oxidation
of organic pollutants [16]. The different potential of complete oxi-
dation of pollutants between energetic photons and •OH may be
responsible for their discrepancy in mineralization rate.
3.4. Degradation mechanism of VUV/TiO2 process
The detailed mechanism of the photocatalysis has been inten-
sively discussed in the literature [25–29]. Electron–hole pairs and
•OH are considered as the major oxidative species in PCO [26,28,29].
As for VUV, it is usually regarded that •OH, generated from the
photo-oxidation of water, is mainly responsible for the degrada-
tion of pollutants [16,18]. The •OH is produced mainly via a series of
reactions ((R1)–(R4)) [16,18,24], among which (R1) is the principal
reaction.
H2O + hv(185 nm) → •OH + •H
(R1)
(R2)
(R3)
(R4)
O2 + •H → HO2
•
•
HO2
+
•H → H2O2
H2O2 + hv(254 nm) → 2•OH
The anion products were fur−ther identified using IC method. It
2−
were formed during
However, the contribution of direct VUV photolysis, i.e. ener-
getic photons, is generally ignored during degradation of pollutants
in previous studies [16,18]. It should not be excluded during the
degradation of pollutants. It is known that 185 nm wavelength cor-
responds to photon energy of 6.7 eV, which is higher than the bond
dissociation energy (BDE) of water molecular [30] and can dissoci-
ate water to form •OH [14,17]. In addition, it can directly dissociate
organic pollutants with BDE lower than 6.7 eV. The BDE data of MB
molecules and water are shown in Table 1. It can be seen that the
BDE of MB molecular is much smaller than photon energy of 185 nm
radiation as well as water molecular. According to the BDE theory,
the lower the BDE is, the more active the chemical bond is and the
can be observed that Cl−, NO3 and SO4
MB degradation. Fig. 5 is a representative IC spectrum of MB solu-
tion after reaction of 120 min in the VUV/TiO2 process. The anion
concentrations at different reaction times were measured and the
results are shown in Fig. 6. It is interesting to find that Cl−, NO3− and
2−
SO4
show different generation rates. Cl− concentration quickly
reached the calculated Cl− concentration (2.2 mg/L) of complete
MB degradation after a reaction of 10 min, indicating that Cl− is
easily detached from MB molecules. Although NO3− concentration
can reach a stable value after−a reaction of 10 min, it is only about
of 1/3 of the calculated NO3 concentration (11.6 mg/L) of com-
plete MB degradation, indicating that only partial N C of MB was
oxidized into NO3−. Unlike Cl− and NO3−, the SO42− concentration
gradually increased with reaction time and reached the maximum
value, which is close to the calculated SO42− concentration (5 mg/L)
of complete MB degradation, after a reaction of 120 min. This result
shows that the S C bond of MB is more difficult to be broken down
than its N C bond.
Table 1
BDE in a MB and water molecule [30,31].
Molecule bonds
Bond energy (kcal/mol)
Bond energy (eV)
CH3 N(CH3)C6H5
N(CH3)2 C6H5
C6H5 NH C6H5
70.8
93.2
87.4
76.0
102.6
120.2
3.07
4.04
3.79
3.30
4.45
5.21
Based on BDE analysis of MB, and the inorganic ions and inter-
mediates from MB degradation indentified in this and previous
work [21,31], the possible pathways of MB degradation by the
VUV/TiO2 process are proposed in Fig. 7. Different from the path-
ways of MB PCO in which •OH is mainly involved in the opening of
C6H5
C6H5 NH2
OH
S
C6H5
H
The chemical bonds corresponding to the given BDE are shown as “ ”.

H. Huang et al. / Catalysis Today 201 (2013) 189–194
193
4.0
3.5
3.0
2.5
2.0
1.5
1.0
0.5
0.0
SO42-
-
NO3
-
Cl
0
5
10
15
20
25
30
Time (min)
Fig. 5. Determination of Cl−, NO3− and SO42− anions of MB degradation by VUV/TiO2
after reaction of 120 min by ion chromatography (TiO2 dosage = 1 g/L, C0 = 20 mg/L).
aromatic ring and oxidation [21,31], energetic photons also play an
important role in VUV. During the dissolution of MB, Cl− is first ion-
ized and exists in the detached state. The methyl (CH3) connected
to 7C and 12C has the lowest BDE value in the MB molecular struc-
ture and is first broken down by radicals and oxidized into HCHO,
Azure B and thionin. Azure B and thionin were identified by LC–MS,
as previously described. N C and S C are the most active bonds of
thionin molecule and are −further broken down[31]. N in the N
C
bond is oxidized into NO3 and S in the S C bond is oxidized into
HSO3, which is still connected to 9C. The intermediates of thionin
oxidation were further oxidized into a single ring structure, which
finally oxidized into harmless substances (H2O and CO2) or ions
2−
(such as SO4 and NH4+).
VUV/TiO2 is a more complex system than VUV alone. Numerous
sub-processes and reactions are involved in formation of oxidants
and the degradation of pollutants in VUV/TiO2. Multiple AOPs (such
as VUV photolysis and PCO) coexist to substantially produce highly
reactive species (such as •OH and energetic photons). In addition,
TiO2 can absorb both MB and VUV irradiation and provide active
sites for MB degradation by VUV photolysis. The VUV/TiO2 pro-
cess can overcome the limitations of individual conventional PCO
or VUV photo-oxidation.
Fig. 7. Proposed pathway of MB degradation by VUV/TiO2 process.
No air bubbling
0.08
10 min
Air bubbling
5
4
3
2
1
0
30 min
60 min
120 min
0.06
0.04
0.02
0.00
VUV/TiO
UV/TiO
VUV
SO42-
2
-
2
-
Cl
NO3
Fig. 8. Effect of oxygen bubbling on reaction constant rate of MB degradation (TiO2
dosage = 0.5 g/L, C0 = 20 mg/L).
−
2−
Fig. 6. Time course of concentration Cl−, NO3 and SO4 anions during MB degra-
dation by VUV/TiO2 (TiO2 dosage = 0.5 g/L, C0 = 20 mg/L).

194
H. Huang et al. / Catalysis Today 201 (2013) 189–194
3.5. Effect of oxygen bubbling
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ozone in air [32]. It is well known that ozone is a very strong oxidant
and may affect MB degradation. Fig. 8 shows the effect of oxygen
bubbling on reaction constant rate of MB degradation. It can be
found that oxygen did not significantly increase reaction constant
rate in all processes, which well agrees with the observation in the
previous studies [17,24]. This is attributed to weak absorption coef-
ficient of oxygen at 185 nm and low oxygen concentration in water
[24]. Oxygen itself hardly directly absorbed 185 nm light to form
•OH or ozone in water [24]. Nevertheless, highly reactive •OH can
be abundantly produced by different ways either in the presence
or in the absence of dissolved oxygen in aqueous solution [17].
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4. Conclusions
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VUV/TiO2 obtained much higher degradation efficiency and
mineralization rate than conventional UV/TiO2. The MB concen-
tration was dropped from 6.2 mg/L in UV/TiO2 to only 1.1 mg/L in
VUV/TiO2 after 20 min of irradiation. The rate constant of MB degra-
dation in VUV/TiO2 (0.0793 min−1) is about 4 times of that of the
former (0.0205 min−1). The MB mineralization rate is also greatly
increased from 12.5% in UV/TiO2 to 47.7% in VUV/TiO2 after 60 min
of irradiation. Multiple advanced oxidation processes including
PCO and VUV photo-oxidation coexist in VUV/TiO2 to substan-
tially produce highly reactive species (such as •OH and energetic
photons), which is responsible for higher MB degradation and min-
eralization efficiency. In addition, VUV/TiO2 has advantages such as
lower cost and less TiO2 dosage over conventional UV/TiO2, which
makes VUV/TiO2 more capable for industrial application.
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Acknowledgment
[26] I.K. Konstantinou, T.A. Albanis, Applied Catalysis B: Environmental 49 (2004)
1.
[27] U.G. Akpan, B.H. Hameed, Journal of Hazardous Materials 170 (2009) 520.
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T. Sano, H. Ichinose, N. Negishi, Y. Seto, K. Takeuchi, Journal of Physical Chem-
istry C 114 (2011) 2305.
The authors gratefully acknowledge the financial support from
the Committee on Research and Conference Grants (CRCG) of
the University of Hong Kong (Grant Nos. 200907176159 and
201109176115).
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