Gas-phase photocatalytic degradation and detoxification of o-toluidine: Degradation mechanism and Salmonella mutagenicity assessment of mixed gaseous intermediates

Article abstract of DOI:10.1016/j.molcata.2010.10.009

The photocatalytic degradation of toluidine over titanium oxide (TiO ₂) thin films under UV irradiation was investigated. The degradation efficiency of 98.7% was obtained for a toluidine concentration of about 4500 μg L^-1 and illumination of 240 min. The degradation intermediates produced during photocatalytic oxidation were identified using Fourier transform-infrared spectrometry (FTIR) and gas chromatography-mass spectrometry (GC-MS). Only a small amount of intermediates, including phenol and toluene, were found in the gas phase. Many other trace amount intermediates, such as 2-hydroxybenzaldehyde, 2-nitrobenzaldehyde, 2-hydroxybenzenemethanol, 2-hydroxybenzoic acid, phenol etc., were detected on the TiO₂ surface. An Ames assay of the Salmonella typhimurium strains TA98 and TA100 was employed to evaluate the mutagenicity of toluidine and its gaseous photocatalytic degradation intermediates. With or without rat liver microsomal fraction (S9 mix) activation, neither toluidine nor its gaseous intermediates presented mutagenic activity against strains TA98 (±S9) and TA100 (-S9) at all tested doses. Toluidine, however, can induce a weak positive response to the TA100 strain with an S9 mix at doses as high as 4000 μg plate ^-1. An increase of revertants per plate was obtained after 30 min photocatalysis in the TA100 strain with S9 mix. As reaction time further increased, photocatalytic technology exhibited the ability to completely and efficiently detoxify toluidine. Both our chemical analysis and toxic evaluation indicate that all mutagenic intermediates in the gas can be completely eliminated within 240 min, which further suggests that photocatalytic technology is an effective approach for degrading aromatic amines.

Full text of DOI:10.1016/j.molcata.2010.10.009

Journal of Molecular Catalysis A: Chemical 333 (2010) 128–135
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
Gas-phase photocatalytic degradation and detoxification of o-toluidine:
Degradation mechanism and Salmonella mutagenicity assessment
of mixed gaseous intermediates
Taicheng Ana,∗, Lei Suna,b, Guiying Lia,∗, Shungang Wana,b
a State Key Laboratory of Organic Geochemistry and Guangdong Key Laboratory of Environmental Resources Utilization and Protection,
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
b Graduate School of Chinese Academy of Sciences, Beijing 100049, China
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 23 March 2010
Received in revised form 4 October 2010
Accepted 7 October 2010
Available online 15 October 2010
The photocatalytic degradation of toluidine over titanium oxide (TiO2) thin films under UV irradiation
was investigated. The degradation efficiency of 98.7% was obtained for a toluidine concentration
of about 4500 ␮g L−1 and illumination of 240 min. The degradation intermediates produced during
photocatalytic oxidation were identified using Fourier transform-infrared spectrometry (FTIR) and
gas chromatography–mass spectrometry (GC–MS). Only a small amount of intermediates, including
phenol and toluene, were found in the gas phase. Many other trace amount intermediates, such as
2-hydroxybenzaldehyde, 2-nitrobenzaldehyde, 2-hydroxybenzenemethanol, 2-hydroxybenzoic acid,
phenol etc., were detected on the TiO2 surface. An Ames assay of the Salmonella typhimurium strains
TA98 and TA100 was employed to evaluate the mutagenicity of toluidine and its gaseous photocatalytic
degradation intermediates. With or without rat liver microsomal fraction (S9 mix) activation, neither
toluidine nor its gaseous intermediates presented mutagenic activity against strains TA98 ( S9) and
TA100 (−S9) at all tested doses. Toluidine, however, can induce a weak positive response to the TA100
strain with an S9 mix at doses as high as 4000 ␮g plate−1. An increase of revertants per plate was
obtained after 30 min photocatalysis in the TA100 strain with S9 mix. As reaction time further increased,
photocatalytic technology exhibited the ability to completely and efficiently detoxify toluidine. Both
our chemical analysis and toxic evaluation indicate that all mutagenic intermediates in the gas can
be completely eliminated within 240 min, which further suggests that photocatalytic technology is an
effective approach for degrading aromatic amines.
Keywords:
o-Toluidine
Photocatalytic oxidation
Degradation mechanism
Mutagenicity
Ames assay
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
industry workers as “aniline cancer” [3]. o-Toluidine can also be
metabolized in vivo into a number of compounds, some of which
o-Toluidine, a monosubstituted aniline, has been commercially
available since its first synthesis in 1844. It is often used as an
intermediate in the dye industry, as well as in a number of other
applications in the fields of rubber processing, pharmaceutical
production, etc. [1]. o-Toluidine also can be released during the
manufacturing and processing of coal oil gasoline, etc. According to
the International Agency for Research on Cancer (IARC, 1982), the
maximum permitted exposure level to o-toluidine in the workplace
ranges from 3 to 22 mg m−2 or 5 mg L−1 in those countries which
have set limits [2], especially since o-toluidine has been demon-
strated to be a carcinogen in mice and rats. In fact, aromatic amines
had been recognized to be carcinogenic as far back as 1895, when
Ludwig Rehn described the high incidence of bladder cancer in dye
are active genotoxins [2]. Due to the various environmental con-
cerns it raised and its adverse effects on human health, o-toluidine
has received increasing attention in recent decades [4].
Research on organic genotoxicity has become increasingly
understood with the application of bacteria [5] and mammalian
cells [6] in animals, as well as through epidemiological investi-
gations among large groups of people [7,8]. Among many assay
methods, the Salmonella mutagenesis assay (Ames assay) has been
conveniently and reliably used [9]. Most of the research available
has focused on the genotoxicity of o-toluidine and its co-mutagenic
action with norharman [10,11], rather than the degradation inter-
mediates produced.
Photocatalytic technology is a highly effective method for the
complete degradation of a wide spectrum of organic pollutants
into less toxic or harmless compounds without selection [12–16].
Most of the previous works in this field were carried out in aque-
ous systems [17–19], but only limited investigations focused on
∗
Corresponding authors. Tel.: +86 20 85291501; fax: +86 20 85290706.
E-mail addresses: antc99@gig.ac.cn (T. An), ligy1999@gig.ac.cn (G. Li).
1381-1169/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.molcata.2010.10.009

T. An et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 128–135
129
the evaluation the detoxification of gaseous organic pollutants. Mo
et al. [20] theoretically evaluated the toxicity of the photocatalytic
by-products of gas-phase toluene at indoor levels and concluded
that the by-products had no negative effects to human health via
the health-related index (HRI). However, HRI values do not rep-
resent a standard method. Rather, they are the user-defined sum
of all HRIi parameters (i.e., the ratio of the concentration versus
its recommended exposure limit of compounds i). To date, few
studies have been reported that experimentally assess the muta-
genic change trends of volatile organic compounds (VOCs) and
their gaseous mixed degradation intermediates during the photo-
catalytic degradation.
rium. The photocatalytic degradation of o-toluidine was carried out
via a circulation reaction process with a gas circulation pump join-
ing with short circulation Teflon tubing. The mixed reaction gas was
circulated from bottom to top. The o-toluidine equilibrium concen-
tration was about 4500 ␮g L−1 and the relative humidity was ca.
45%. Once the concentration of o-toluidine has stabilized, the o-
toluidine gas was then irradiated with UV light, signaling the start
of photocatalysis. Subsequently, at photocatalytic intervals of 30,
60, 120, 180, and 240 min, a portion of the gaseous samples was
collected and sealed in a 1-L volumetric Teflon bag for mechanism
studies. Other portions of the gaseous samples were trapped with
DMSO solution for mutagenicity assessment.
In this work, a typical aromatic amine, o-toluidine, is used
as a model VOC to determine photocatalytic detoxification fea-
sibility by chemically identifying its degradation intermediates.
We aim to experimentally examine the potential mutagenicity of
o-toluidine before and during photocatalytic degradation and eval-
uate the safety discharge of photocatalytic technology as compared
to conventional methods for the treatment of aromatic amines. A
tentative degradation pathway is also proposed to explain why
photocatalytic technology is an effective method for detoxifying
gaseous o-toluidine.
2.3. Analysis
A gas chromatography (GC) (HP 5890, Series II, equipped with a
split/splitless injector and a flame ionization detector) was used
to analyze the concentration of gaseous o-toluidine before and
after photocatalytic degradation. The injector temperature and the
detector temperature were set at 250 and 280 ◦C, respectively.
Gaseous samples amounting to 500 ␮L were taken using a 500 ␮L
gas-tight locking syringe (Agilent, Australia) at given intervals and
injected into the GC for o-toluidine determination in splitless mode.
The typically programmed temperature of the column was main-
tained at 40 ◦C for 2 min, then increased to 100 ◦C at a rate of
2. Materials and methods
6 ◦C min−1, and finally increased to 280 ◦C at a rate of 10 ◦C min−1
.
2.1. Reagents
The concentrations of the target compounds were quantified by
external standard calibration. The degraded gaseous intermediates
were analyzed by a GC (Agilent 7890A)–mass spectrometer detec-
tor (Agilent 5975C with Triple-Axis Detector) (GC–MSD) with a
DB-5 capillary column (30 m × 0.32 mm × 0.25 ␮m, Agilent Tech-
nology). The carrier gas was ultra-high purity helium fed at a
constant flow rate of 1.3 mL min−1. The analysis method and pro-
grammed procedure were the same as for GC quantification. The
MSD was operated in full scan mode with m/z 40–300 amu. The
gaseous target species were identified via their mass spectrum
using the Wiley database (Wiley Mass Spectral Library).
To identify the intermediates adsorbed onto the surface of TiO2
thin films, two sets of experiments were performed. First, solid
intermediates deposited onto the thin films were directly analyzed
by Nicolet 330 Fourier transform-infrared spectroscopy (FTIR).
Next, these intermediates were extracted ultrasonically using a
small amount of methanol for 30 min. The extract was filtered
through a 0.45 ␮m filter membrane. The filtrate was collected, con-
centrated with a gentle stream of high-purity nitrogen, averaged
into two similar 1.5 mL vials, and then completely dried with a
gentle stream of high-purity nitrogen. One portion of the sample
was re-dissolved in 1.0 mL ethyl acetate and then injected into the
GC–MSD for the direct determination of intermediates. The remain-
ing portion was derived at room temperature using 50 ␮L BSTFA
as the derivatization reagent and 50 ␮L pyridine as the catalyst.
The resulting solution was injected into the GC–MSD to identify
polar intermediates. Samples amounting to 1.0 ␮L were injected in
the splitless mode. The analysis method was the same as for the
detection of gaseous target species. These intermediates were also
identified via their mass spectrum according to the library spectra
as described previously.
Dimethyl sulfoxide (DMSO) (purity: >99.9%), glucose-6-
phosphate (purity: >98%) and dexon (purity: 99%) were
purchased from Sigma Chemical Corp. (Saint Louis, MO, USA).
2-Aminofluorene (purity: >97.0%) was purchased from Fluka
Chemical Corp. (Ronkonkoma, NY, USA). Nicotinamide ade-
nine dinucleotide phosphate (NADP), d-biotin, l-histidine, agar,
nutrient broth, and other chemicals of analytical grade were
purchased from Huankai (Guangzhou, China). Rat liver enzymes
(S9) and S. typhimurium strains, TA98 and TA100, were obtained
from the Guangzhou Sanitation Prevention Station, China. o-
Toluidine (purity: 99.5%) was purchased from Acros Organics
(Geel, NV, Belgium). The methanol used in the procedures was
of chromatography grade (Germany). N,O-bis (trimethylsilyl)-
trifluoroacetamide (BSTFA) (purity: 98%) was obtained from Acros
Organics (NJ, USA). All other reagents were used as received
without further purification.
2.2. Photocatalytic characterization
Nanocrystalline titanium dioxide(TiO2) thin films were pre-
pared via our reported method [21] with the experimental flow
chart shown in Fig. 1. All photocatalytic experiments were per-
formed in a sealed Pyrex gas–solid reactor with a total volume of
5 L [22]. Briefly, a 125-W high-pressure mercury lamp that emits
maximum radiation of 365 nm (GGZ125, Shanghai Yaming Light-
ing Co., Ltd.) was used as the light source. Film photocatalysts were
installed facing the light source, which was sleeved in the cen-
ter of a double-walled quartz cylinder. The reaction temperature
was maintained at room temperature by a continuous circulation
of cooling water in the quartz glass jacket around the light source
(light intensity: 5.3 mW cm−2). A mini-type fan stirrer at the bot-
tom was used as the air blender for the reaction gas during the
operation. For a typical experiment procedure, four identically pre-
pared TiO2/ITO films were installed into the reactor, which was
then filled with dry air. Liquid o-toluidine and a small fraction of
distilled water were injected into the vaporizing tube from the
injection port. The vaporizing tube was heated and o-toluidine was
allowed to vaporize, mix, and reach gas–solid adsorption equilib-
2.4. Mutagencity assessment
Ames assays were performed according to the standard plate
incorporation method with the classic S. typhimurium strains, TA98
and TA100, which are capable of detecting base frame shift types
and base pair substitution-type mutagenicity, respectively [23]. o-
Toluidine and its photocatalytic degradation intermediates at 30,
60, 120, 180, and 240 min were trapped and dissolved completely

130
T. An et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 128–135
TiO2 thin film
O toluidine
Fan blend
Fan blend
Vapor
Gas-solid reactor
Gas-solid reactor
Dry air
Distilled water
Photocatalytic
degradation
Detected
concentration
Adsorption
equilibrium
Chemical analysis
Discuss the security
in the process of
photocatalysis
N2 blow
Degradation pathways
Dissolved in
DMSO
Genotoxicity
assessment
Fig. 1. Experimental flowchart.
in 5 mL of DMSO. All the experimental conditions employed were
identical for both strains with and without S9 metabolic activa-
tion, except for the positive control regents used. Four different
dose groups (4000, 2000, 1000, and 500 ␮g plate−1) for each sam-
ple were assayed in triplicates. Two known mutagens, dexon and
2-aminofluorene, were used as the positive control regents without
S9 and with S9, respectively. The strains were tested without the
addition of any foreign compounds as negative controls. Solvent
controls were also assessed for each determination. The revertant
colonies on each plate were counted and scored after 48 h incuba-
tion at 37 ◦C, and the number of his+ revertants in each sample was
recorded as the mean value from three plates. Mutagenic index (MI)
was also calculated for each dose as the ratio of the average num-
bers of revertants per plate in a sample and in the negative control.
A sample was considered mutagenic when the MI value was equal
to or greater than 2 for at least one of the tested doses and if it had
a reproducible dose–response curve [24].
3. Results and discussion
3.1. Photocatalytic degradation kinetics
The photocatalytic degradation kinetics of o-toluidine was
investigated, and the results are shown in Fig. 2. As seen in Fig. 2,
gaseous o-toluidine can reach adsorption–desorption equilibrium
after 120 min in dark conditions. This equilibrium concentration
is about 4500 ␮g L−1. o-Toluidine can be very slowly photolyzed
under UV light irradiation in the absence of film photocatalysts
with increasing reaction time. The photocatalytic efficiency of o-
toluidine was found to be 21.2% within 240 min. In the presence
of TiO2 films, however, o-toluidine can be degraded with a degra-
dation efficiency of 98.7% within the same time course. The linear
plot of ln(C0/Ct) versus time for the photocatalytic degradation is
shown in the inset of Fig. 2. In a heterogeneous solid–gas reac-
tion, the photocatalytic degradation of o-toluidine on the TiO2 thin
1.0
0.8
5
y = 0.0179x- 0.02
R2= 0.9936
4
3
2
1
0
0.6
0.4
0.2
0.0
3
1
(f)
(e)
(d)
(c)
(b)
(a)
2
0
60
120
Time (min)
180
240
Adsorption
Photolysis
PCO
0
30
60
90
120 150 180 210 240
4
8
12
16
20
Time ( min)
Time (min)
Fig. 2. Photocatalytic degradation kinetics of o-toluidine. Inset: linear plots of
ln(C0/Ct) versus reaction time.
Fig. 3. TIC profile for gaseous compounds in the photocatalytic degradation of o-
toluidine: (a) 0 min; (b) 30 min; (c) 60 min; (d) 120 min; (e) 180 min; and (f) 240 min.

T. An et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 128–135
131
film surface follows a pseudo-first-order reaction, which is con-
5000
4000
3000
2000
1000
0
sistent with the Langmuir–Hinshelwood (L–H) model. Its kinetic
equation may be simplified as follows: ln(C0/Ct) = kt [25], where k
is the apparent rate constant of the photocatalytic degradation, C0
and Ct are the initial and the final concentrations of o-toluidine,
respectively, and t is the degradation time in minutes. Rate con-
stant, which is the slope of the linear fit of ln(C0/Ct) versus time,
was calculated as 0.018 min−1. We also calculated the half-life of
our system and found it to be 38.5 min. The lower rate constant and
longer half-life obtained under the same conditions indicate that o-
toluidine was more difficult to be degraded compared to toluene,
as reported in our previous work [22]. The difference in the com-
pound structures of o-toluidine and toluene may have affected the
extent of photocatalytic reaction observed.
-toluidine
phenol
toluene
3.2. Identification of the photocatalytic degradation
intermediates
0
60
120
180
240
Time (min)
Fig. 4. The concentration conversion curve of o-toluidine, toluene, and phenol in
the gas phase system.
The resultant gaseous mixtures after gas–solid phase photo-
catalytic degradation were subject to GC–MSD analysis. Total ion
current (TIC) profiles of gaseous compounds recorded at different
reaction intervals are shown in Fig. 3. For samples without degra-
dation (Fig. 3a), a peak (3) with retention time (tR) of 10.9 min
was observed, signifying the presence of the original o-toluidine
molecule. For 240-min photocatalytic degradation samples, the
degradation intermediates 1 (tR = 3.7 min) and 2 (tR = 8.8 min) were
detected in the gas phase. Two most probable intermediates 1
and 2 were identified with GC–MS analysis as toluene and phe-
nol in the air from degradation of o-toluidine. In the mass spectra,
a couple m/z values of 92/91 corresponding to M+/M+ − 1 fragmen-
tation of intermediates 1 was obtained with a molecule weight of
toluene. Also, an m/z value of 94 corresponding to M+ − 1 frag-
mentation of intermediates 2 also was obtained with a molecule
weight of phenol. Of course, both the MS cleavage patterns of
intermediates 1 and 2 also conform to that of toluene and phe-
nol very well. During first stage of the photocatalytic degradation,
the concentration of toluene in the degradation mixture decreased
slightly, while the concentration of phenol increased to its max-
imum. When the degradation time was further increased, both
toluene and phenol decreased gradually, ultimately reaching levels
below detection limits after 240 min photocatalytic degradation.
It must be noted that o-toluidine (3) remained the dominant
compound, although its concentration in the reaction mixture
decreased rapidly with the increase in reaction time. This indicates
that o-toluidine can be degraded effectively, while the gaseous
intermediates of toluene and phenol were produced and then
degraded again during the degradation course. The plots of the
concentrations of o-toluidine, toluene, and phenol against the reac-
tion time are shown in Fig. 4. The equilibrium concentration of
o-toluidine was about 4500 ␮g L−1 before the photocatalytic degra-
dation. In the gaseous mixture, the concentrations of o-toluidine,
phenol, and toluene were 3000, 510, and 180 ␮g L−1 within 30 min
photocatalytic degradation, and 1520, 560, and 160 ␮g L−1 after
60 min photocatalytic degradation, respectively. For 120-min pho-
tocatalytic degradation samples, the concentrations of o-toluidine,
phenol, and toluene were 440, 250, and 86 ␮g L−1, respectively. The
concentration of o-toluidine decreased to 60 ␮g L−1 and approxi-
mately 98.7% of o-toluidine was degraded, while both the phenol
and toluene intermediates could not be detected after 240 min pho-
tocatalytic degradation. Thus, we can conclude that o-toluidine and
both its gaseous intermediates, phenol and toluene, may be com-
pletely mineralized into CO2 and H2O given enough degradation
time.
FTIR spectroscopy and GC–MSD with and without BSTFA deriva-
tion. The FTIR spectra shown in Fig. 5(a)–(f) reveals the information
from TiO2 thin films before (0 min) and after photocatalytic degra-
dation at different intervals of 30, 60, 120, 180, and 240 min. Fig. 5a
shows that the spectrum of a fresh TiO2 thin film catalyst was
relatively flat. However, the trend of an initially increasing and
then decreasing band was observed near 1000–1300 cm−1 (C–O
stretching vibration) with increasing of the degradation time. The
intensity of this band in the FTIR spectrum after 180-min degrada-
tion (Fig. 5e) was significantly higher than that in the other samples.
Thus, we can conclude that the C–O bond may have accumulated
onto the catalysts and reached maximum amounts at 180 min pho-
tocatalytic degradation. This indicated that the intermediates with
C–O stretching vibration band were formed first and then subse-
quently mineralized with further the increases in reaction time. The
intensity of the wide band at 3200–3700 cm−1 (OH group charac-
teristics) also initially increased, peaked at 120 min degradation,
and then decreased subsequently, indicating that the appearance
and disappearance of OH group of alcohol or phenol species [26,27].
The weak band at 1680–1720 cm−1 was also assigned to be the C
stretching vibration of aldehydes or carboxylic acids [28]. The weak
bimodal band at 1580–1640 cm−1 can be recognized as the C
O
C
stretching vibration of the aromatic ring [26]. Thus, it was deduced
(f)
(e)
(d)
(c)
(b)
(a)
500
1000
1500
2000
2500
3000
3500
4000
Wavenumber ( cm-1)
In order to obtain full insights into the produced intermedi-
ates involved in the reaction process, the trace solid by-products
adsorbed onto the TiO2 films were also studied by both in situ
Fig. 5. FTIR spectra of TiO2 thin films during the photocatalytic degradation: (a)
0 min; (b) 30 min; (c) 60 min; (d) 120 min; (e) 180 min; and (f) 240 min.

132
T. An et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 128–135
121
106
OH
100
80
60
40
20
0
100
80
60
40
20
OH
CH2OH
CHO
65
78
104
53
50
44
51
76
79
44
91
65
55
40
60
80
100
120
121
40
60
80
66
100
120
94
100
80
60
40
20
0
100
80
60
40
20
0
OH
NO2
65
CHO
50
44
41
93
76
55
63
104
68
50
62
55
75
42
40
79
80
40
60
80
100
120
50
60
70
90
100
m/z
Fig. 6. Mass spectra of adsorbed intermediates on TiO2 thin film during the photocatalytic degradation identified by GC–MS without silylation.
that the skeleton structure of the aromatic ring was still presented
onto TiO2 surfaces. It is also noted that the weak band appeared near
1350 and 1560 cm−1, were assigned to be the stretching vibrations
of –NO2 [29,30]. These observations can be doubly confirmed by
the gradual changes in color of the TiO2 thin films: from white to
pale yellow and then to yellow brown with increases in reaction
time. From the analysis above, the FTIR spectra indicate that some
alcohols, phenols, carboxylic acids, and nitro compounds as well
as aromatic ring compounds may be produced and absorbed onto
TiO2 surfaces during the photocatalytic degradation of gaseous o-
toluidine.
tation coupling with the cleave fragmentation of 165, 135 and
91 was identified as 2-methylphenyl trimethylsilyl ether. As for
benzoic acid trimethylsilyl ester, a very weak molecule ion peak
of 194 was also found coupling with cleavage fragmentation of
179, 147, 135 and 105. Also, as for third intermediates, salicylic
acid, a m/z value of 267 was also identified as the molecule ion
peak of bis(trimethylsilyl) derivative of salicylic acid after BSTFA
derivatization. Thus, from above-mentioned discussion, all three
intermediates were identified as 2-methylphenol, benzoic acid, and
salicylic acid, respectively, in the adsorbed solid sample.
Thus, the adsorbed intermediates on the TiO2 thin films
were further confirmed with GC–MSD after extraction with
methanol. The analysis results of the mass spectra of sam-
ples with and without BSTFA derivatization are shown in
Figs. 6 and 7, respectively. Seen from the mass spectra of Fig. 6,
besides above identified phenol, 2-hydroxybenzenemethanol,
2-nitrobenzaldehyde, and 2-hydroxybenzaldehyde were also iden-
tified in the adsorbed solid samples without BSTFA derivatization.
In the mass spectra of 2-hydroxybenzaldehyde, a couple m/z val-
ues of 122/121 corresponding to M+/M+ − 1 fragmentation can
be obtained. However, we cannot get the molecule ion peaks of
2-hydroxybenzenemethanol and 2-nitrobenzaldehyde of 124 and
151 from both the mass spectra, respectively. It is fortunate that
we can easily find the cleave fragmentation of 106 and 78 from
the mass spectra of 2-hydroxybenzenemethanol. Similarly, the
cleave fragmentation of 121, 104, 76 and 65 also can be found
from the mass spectra of 2-nitrobenzaldehyde. Thus, combined
above FTIR spectra with mass spectra patters, all four intermediates
were identified reasonably. It was also noted that all these identi-
fied intermediates were doubly confirmed with authentic standard
reagents.
91
100
75
50
25
0
S
165
O
i
180
179
135
135
CH
3
45
73
77
149
105
105
59
40
80
77
120
120
160
200
100
75
50
25
0
S
i
O
O
51
147
45
73
189
194
59
89
40
80
160
200
267
100
75
50
25
0
S
O
i
73
O
S
i
O
135
209
45
40
149
91
193
200
249
240
80
120
160
m/z
280
As the similar way, three intermediates were also found in the
adsorbed solid sample with the derivative form of trimethylsiloxy
ester after BSTFA derivatization (as shown in Fig. 7). In the mass
spectra, a m/z value of 180 corresponding to M+/M+ − 1 fragmen-
Fig. 7. Mass spectra of adsorbed intermediates on TiO2 thin film during the photo-
catalytic degradation identified with BSTFA silylation.

T. An et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 128–135
133
Fig. 8. Proposed photocatalytic degradation pathways for o-toluidine in the gas phase.
Based on the identified intermediates from both the gas and
adsorbed solid phases in our experimental system, a possible pho-
tocatalytic degradation mechanism of o-toluidine is tentatively
proposed in Fig. 8. It is suggested that three distinct pathways
might occur simultaneously during the photocatalytic degradation.
Following pathway 1, o-toluidine is oxidized to 2-methylphenol
by the attack of •OH radical. Direct hydrogen abstraction by pho-
tohole from the methyl group, which leads to a benzyl radical,
is followed by the formation of 2-hydroxybenzenemethanol. This
compound is then further oxidized to 2-hydroxybenzaldehyde and
2-hydroxybenzoic acid. Phenols would then be produced by the
photo-decarboxylation of 2-hydroxybenzoic acid [31]. In pathways
2 and 3, o-toluidine is first oxidized to o-nitrotoluene by photo-
hole and oxygen. The hydrogen abstraction from the methyl group
then further leads to a benzyl radical, followed by the formation
of 2-nitrobenzaldehyde in pathway 2. Then, 2-nitrobenzaldehyde
may be readily oxidized to 2-hydroxybenzaldehyde with the simul-
taneous elimination of –NO2 groups because the –NO2 group
can be easily removed [32]. The further oxidation intermedi-
ates of 2-hydroxybenzaldehyde, 2-hydroxybenzoic acid, can be
further converted into phenols followed with the decarboxyla-
tion reaction from the aromatic ring by partial mineralization.
In pathway 3, o-nitrotoluene is first formed by photohole and
oxygen as in the first step as the pathway 2. This intermedi-
ate can be further attacked by photoholes removing the –NO2
group from aromatic ring to form the daughter intermediate,
toluene. And the photocatalytic degradation mechanism of toluene
in this system can be referenced in our recent publication
[22].
From the viewpoint of chemical analysis, the identified degra-
dation intermediates from both gaseous and adsorbed phase can
be finally mineralized into CO2 and H2O, although some carcino-
genic intermediates may be produced during the photocatalytic
degradation of o-toluidine. Therefore, a mutagenicity assessment
of the mixed intermediates is necessary during the course of the
photocatalytic degradation.

134
T. An et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 128–135
Table 1
Results of the Ames assay during photocatalytic degradation of o-toluidine.
Sample
Dose (␮g plate−1
)
Number of revertants plate−1 in S. typhimuriuma
TA98
TA100
−S9
35 5.6
+S9
39 3.6
−S9
+S9
136 3.8
0b
132 6.0
500
32 4.0 (0.9)
37 2.1 (1.1)
35 4.2 (1.0)
22 2.1 (0.6)
41 2.3 (1.1)
40 6.7 (1.0)
42 2.9 (1.1)
40 5.9 (1.0)
134 4.7 (1.0)
133 7.2 (1.0)
140 8.6 (1.1)
132 5.7 (1.0)
149 8.0 (1.1)
234 4.6 (1.7)
329 17.2 (2.4)
421 7.0 (3.1)
1000
2000
4000
o-Toluidine
PC 30 min
PC 60 min
PC 120 min
PC 180 min
PC 240 min
0
500
1000
2000
4000
32 3.8
41 6.6
132 6.6
138 3.5
36 3.1 (1.1)
31 4.2 (1.0)
32 7.1 (1.0)
30 2.5 (0.9)
39 3.1 (1.0)
41 3.5 (1.0)
37 6.2 (0.9)
34 2.9 (0.8)
131 8.1 (1.0)
134 6.0 (1.0)
140 10.9 (1.1)
138 3.0 (1.0)
202 8.5 (1.5)
294 14.0 (2.1)
399 11.7 (2.9)
501 18.0 (3.6)
0
500
1000
2000
4000
31 1.7
34 4.0
137 5.0
138 7.8
32 4.0 (1.0)
28 3.6 (0.9)
30 6.6 (1.0)
31 5.8 (1.0)
39 2.1 (1.1)
34 5.5 (1.0)
33 3.8 (1.0)
37 1.2 (1.1)
139 4.2 (1.0)
142 14.0 (1.0)
143 8.5 (1.0)
139 7.5 (1.0)
144 6.9 (1.0)
148 10.5 (1.1)
150 9.8 (1.1)
248 12.3 (1.8)
0
500
1000
2000
4000
33 3.1
40 6.1
130 8.0
144 4.0
37 1.2 (1.1)
35 3.1 (1.1)
31 5.5 (0.9)
34 1.7(1.0)
38 2.9 (0.9)
41 2.6 (1.0)
42 2.6 (1.1)
42 5.5 (1.1)
137 4.6 (1.1)
141 12.7 (1.1)
132 7.0 (1.0)
131 4.6 (1.0)
142 8.6 (1.0)
148 5.0 (1.0)
142 8.5 (1.0)
134 12.1 (0.9)
0
500
1000
2000
4000
27 3.2
33 3.6
136 6.0
147 6.4
30 2.6 (1.1)
31 3.0 (1.1)
30 2.1 (1.1)
27 5.0 (1.0)
35 5.0 (1.1)
31 3.1 (0.9)
38 5.1 (1.2)
32 1.5 (1.0)
134 4.0 (1.0)
145 13.1 (1.1)
141 8.0 (1.0)
143 12.5 (1.1)
152 3.8 (1.0)
148 10.5 (1.0)
145 5.5 (1.0)
147 4.2 (1.0)
0
500
1000
2000
4000
29 4.6
36 6.1
133 9.4
136 6.1
30 3.2 (1.0)
34 4.7 (1.2)
28 4.5 (1.0)
30 1.5 (1.0)
41 6.0 (1.1)
30 3.5 (0.8)
34 4.9 (0.9)
35 2.5 (1.0)
130 10.6 (1.0)
135 5.6 (1.0)
136 8.9 (1.0)
135 5.1 (1.0)
143 5.7 (1.1)
141 4.4 (1.0)
134 9.3 (1.0)
145 5.6 (1.1)
Solvent controlc
Positive controld
100 (␮L plate−1)
33 6.5 (0.9)
41 4.0 (1.0)
139 10.6 (1.1)
604 19.9 (4.6)
134 6.5 (1.0)
50
20
1315 33.2 (37.6)
2937 128 (75.3)
1181 67.2 (8.7)
a
Mean standard deviation.
Negative control.
b
c
DMSO only.
d
50 ␮g plate−1 dexon without S9 and 20 ␮g plate−1 2-aminofluorene with S9.
3.3. Mutagencity assay of gaseous intermediates during the
degradation of o-toluidine
in the presence of S9 metabolic activation because the MI value
is greater than 2 at the 4000 ␮g plate−1 and it had a reproducible
dose–response curve.
From our previous discussion, it is known that some carcino-
genic intermediates may be produced during the photocatalytic
degradation of o-toluidine. Considering that gaseous organic pol-
lutants can directly endanger human health via inhalation and
exposure, the Ames assay was employed to evaluate the detoxi-
fication ability of gaseous intermediates at different photocatalytic
degradation intervals (Table 1). The range of spontaneous rever-
sions (negative control) for the TA98 strain without S9 mixture
metabolic activation (−S9) was at 27–35 revertants per plate,
while the TA100 strain reverted spontaneously (−S9) with 130–137
revertants per plate in this study. The spontaneous reversions
slightly increased with S9 mixture metabolic activation (+S9) for
the TA98 strain at 33–41 revertants per plate and for the TA100
strain at 136–147 revertants per plate. For o-toluidine, at the high-
est tested dose of 4000 ␮g plate−1, the number of revertants per
plate obtained were 22 2.1 (−S9) and 40 5.9 (+S9) for the TA98
strain. This means that no obvious increase in the number of rever-
tants was found for TA98 compared with the negative control. The
numbers of revertants per plate obtained were 122 8.5 (−S9) and
421 7.0 (+S9) (MI = 3.1) for the TA100 strain. This indicates that
o-toluidine has weak mutagenic activity against the TA100 strain
At different photocatalytic degradation intervals of 30, 60, 120,
180, and 240 min for the TA98 strain, the numbers of revertants per
plate at the highest tested dose (i.e., 4000 ␮g plate−1) were 30 2.5,
31 5.8, 34 1.7, 27 5.0, and 30 1.5 without S9, and 34 2.9,
37 1.2, 42 5.5, 32 1.5, and 35 2.5 with S9, respectively. All
MI values were less than 2 and all the results were negative against
the TA98 strain with and without S9 in all the dose groups of 4000,
2000, 1000, and 500 ␮g plate−1, which indicate that the interme-
diates in the gaseous phase will not cause base frameshift-type
mutagenicity to organisms. In addition, compared with the neg-
ative control in all the dose groups, no increase in the number
of revertants per plate was detected for the TA100 strain with-
out S9. However, different results were obtained for the TA100
strain with S9. For a 30 min photocatalytic degradation, the num-
bers of revertants per plate obtained were 501 18.0 (MI = 3.6) at
the 4000 ␮g plate−1, 399 11.7 (MI = 2.9) at the 2000 ␮g plate−1
,
and 294 14.0 (MI = 2.1) at the 1000 ␮g plate−1. The MI values were
larger than 2 for this group and a reproducible dose–response curve
was obtained, indicating that the gaseous intermediates at 30 min
photocatalytic degradation may cause base pair substitution-type
mutagenicity to organisms in the presence of S9 metabolic acti-

T. An et al. / Journal of Molecular Catalysis A: Chemical 333 (2010) 128–135
135
500
400
300
200
100
0
Thus, by combining our chemical analysis with the mutagenicity
assessment of intermediates, we can conclude that photocatalytic
technology is an effective detoxification method for o-toluidine in
the gaseous phase. This work is of great importance in the applica-
tion of photocatalytic technology for detoxifying aromatic amines
in the environmental waste management field.
4000 μg plant-1
2000 μg plant-1
1000 μg plant-1
500 μg plant-1
negative control
Acknowledgements
This is contribution No. IS-1255 from GIGCAS. This work was
financially supported by the National Natural Science Foundation
of China (40572173) and the Science and Technology Project of
Guangdong Province, China (2007A032301002, 2009B030400001
and 2009A030902003).
0 min
30 min
60 min
180 min
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Gaseous o-toluidine was successfully detoxified using photocat-
alytic technology within a 240 min reaction, and the degradation
kinetics of o-toluidine followed a pseudo-first-order reaction. Two
intermediates of toluene and phenol were found in the gas phase.
Small amounts of 2-hydroxybenzaldehyde, 2-nitrobenzaldehyde,
2-hydroxybenzenemethanol, 2-methylphenol, 2-hydroxybenzoic
acid, and phenol were also identified on the surface of the thin
films photocatalyst during the photocatalytic process. Further-
more, Ames assay results showed that o-toluidine presented weak
mutagenic activity against the TA100 strain in the presence of S9
metabolic activation at tested doses. The mutagenic toxicity of its
gaseous intermediates also increased slightly after a 30-min photo-
catalytic degradation. Finally, the samples in gas phase did not show
any mutagenic activity after 240 min photocatalytic degradation.