TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine taste and odor compound in aqueous phase: Kinetics, degradation pathways and toxicity evaluation

Article abstract of DOI:10.1016/j.cattod.2014.03.027

In recognition of the growing demand regarding the control of undesired taste and odor (T&O) problems in natural water resources, the photocatalytic degradation of 2-isopropyl-3 methoxy pyrazine (IPMP), a common metabolite of soil actinomycetes which contributes a rotten vegetable odor to water, was investigated under simulated solar irradiation. Under the studied conditions (C = 10 mg L^-1, C_TiO2 = 100 mg L^-1and I = 600 W m^-2), 95% of IPMP was removed within 20 min of irradiation. The reaction intermediates were completely mineralized to CO₂ and the nitrogen was predominantly released as NH₄⁺ ions after 240 min irradiation. The major transformation products of TiO₂ photocatalysis of IPMP have been determined by the use of high resolution accurate liquid chromatography-orbitrap mass spectrometry as well as gas chromatography-mass spectrometry (GC-MS) techniques. Hydroxylation of the isopropyl and methoxy groups has been identified as the main reaction pathway. Scavenging experiments indicated the important role of HO^?, h⁺ and O₂^?- in the photocatalytic process. Toxicity assessment revealed the efficiency of the photocatalytic treatment to achieve almost complete detoxification of the irradiated solution.

Full text of DOI:10.1016/j.cattod.2014.03.027

G Model
CATTOD-8974; No. of Pages8
ARTICLE IN PRESS
Catalysis Today xxx (2014) xxx–xxx
Contents lists available at ScienceDirect
Catalysis Today
journal homepage: www.elsevier.com/locate/cattod
TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine taste and odor
compound in aqueous phase: Kinetics, degradation pathways and
toxicity evaluation
M. Antonopoulou, I. Konstantinou∗
Department of Environmental and Natural Resources Management, University of Patras, 30100 Agrinio, Greece
a r t i c l e i n f o
a b s t r a c t
Article history:
In recognition of the growing demand regarding the control of undesired taste and odor (T&O) problems
in natural water resources, the photocatalytic degradation of 2-isopropyl-3 methoxy pyrazine (IPMP), a
common metabolite of soil actinomycetes which contributes a rotten vegetable odor to water, was inves-
Received 14 January 2014
Received in revised form 10 March 2014
Accepted 13 March 2014
Available online xxx
tigated under simulated solar irradiation. Under the studied conditions (C = 10 mg L−1, CTiO = 100 mg L−1
2
and I = 600 W m−2), 95% of IPMP was removed within 20 min of irradiation. The reaction intermediates
+
were completely mineralized to CO2 and the nitrogen was predominantly released as NH4 ions after
Keywords:
Taste and odor
TiO2
Photocatalysis
Transformation products
Scavengers
240 min irradiation. The major transformation products of TiO2 photocatalysis of IPMP have been deter-
mined by the use of high resolution accurate liquid chromatography–orbitrap mass spectrometry as well
as gas chromatography–mass spectrometry (GC–MS) techniques. Hydroxylation of the isopropyl and
methoxy groups has been identified as the main reaction pathway. Scavenging experiments indicated
the important role of HO•, h+ and O2 in the photocatalytic process. Toxicity assessment revealed the
•−
efficiency of the photocatalytic treatment to achieve almost complete detoxification of the irradiated
solution.
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
clean water has led to the development of alternative more
efficient water treatment processes [4,5]. In last decades, hetero-
The increasing occurrence of taste and odor (T&O) compounds in
drinking water supplies has been recognized as an important water
quality problem receiving substantial public concern throughout
the world [1]. Although, changes in esthetic values are typically
not problematic from toxicological points of view, they impose
serious considerations for the quality of drinking water [2]. A
great number of organic compounds either naturally occurring in
aquatic compartments or from industrial sources, or even produced
during water treatment, can cause undesirable tastes and odors
in drinking water in extremely low concentrations. Of particular
importance is the group of naturally occurring compounds that
produce earthy–musty odors in drinking water [3].
In general, the removal of the majority of T&O compounds
from drinking water is a difficult task since their odor threshold is
extremely low, usually in the range of ng L−1 [4]. Their incomplete
removal in potable water by conventional treatment processes
in combination with the growing demand for good quality
geneous photocatalytic process employing catalysts such as TiO2,
has demonstrated promising results for the degradation of differ-
ent taste and odor compounds. However, only the photocatalytic
removal 2-methylisoborneol and geosmin is extensively docu-
mented in the literature [6–9]. Among T&O compounds, we have
focused on IPMP, a representative compound of the group of nat-
urally occurring compounds that produce earthy–musty odors in
drinking water. IPMP is a metabolite of soil actinomycetes which
contributes a rotten vegetable odor to water and has been fre-
quently found in natural water serving as drinking water resources.
Although this compound does not have a direct negative effect on
the public health, its presence, even at trace levels, gives rise to
esthetic issues due to its low threshold of 0.2 ng L−1 [1]. Accord-
ing to our knowledge this is the first research article of the current
state of knowledge regarding the application of the heterogeneous
photocatalysis for the removal of IPMP from aqueous phase. Thus,
the aim of this work is to comprehensively study and evaluate the
application of TiO2 photocatalysis for the treatment of IPMP. In this
context, the main aspects investigated have been: the efficiency
of TiO2 photocatalytic process for IPMP degradation, the identifi-
cation of transformation products (TPs) of IPMP, the elucidation of
∗
Corresponding author. Tel.: +30 2641074186.
E-mail addresses: iokonst@upatras.gr, iokonst@cc.uoi.gr (I. Konstantinou).
http://dx.doi.org/10.1016/j.cattod.2014.03.027
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: M. Antonopoulou, I. Konstantinou, TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine
taste and odor compound in aqueous phase: Kinetics, degradation pathways and toxicity evaluation, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.027

G Model
CATTOD-8974; No. of Pages8
ARTICLE IN PRESS
M. Antonopoulou, I. Konstantinou / Catalysis Today xxx (2014) xxx–xxx
2
mechanistic details of the photocatalytic reaction using scavenging
experiments, the evaluation of mineralization as well as the toxicity
evolution during the process.
2.4. By products evaluation
2.4.1. LC–MS and GC–MS analysis
The intermediates generated during IPMP photocatalysis was
characterized by an UPLC–ESI–MS system in positive ionization
mode. The LC system was equipped with an Accela Autosampler,
an Accela LC pump and a LIT Orbitrap mass spectrometer (Thermo
Fisher Scientific, Germany). The chromatographic separations were
run on a C18 Hypersil Gold, 100 mm × 2.1 mm i.d., 1.9 ␮m particle
size (Thermo Fisher Scientific, San Jose, USA), thermostated at 40 ◦C
and the injection volume was 10 ␮L. Mobile phases A and B were
water/5 mM ammonium formate and methanol/5 mM ammonium
formate, respectively, at a flow rate of 300 ␮L min−1. A linear gra-
dient progressed from 90% A (initial conditions) to 0% A in 14 min,
followed by a linear gradient to 90% A in 20 min. The ESI-source
parameters were as follows: sheath and auxiliary gas flow rate 30
and 8 (nitrogen, arbitrary units), respectively; source voltage at
3.70 kV; capillary temperature was maintained at 320 ◦C. For frag-
mentation study, the voltage of the HCD collision cell was set at
35 eV. Prior to analysis, the orbitrap mass analyzer was externally
calibrated, in the scan range m/z 70–650, to obtain mass accuracy
with 5 ppm. A resolving power of 60,000 was applied. Chemi-
cal compositions and accurate masses of the protonated molecules
and their fragments were determined by means of chemical for-
mula calculator, included in Xcalibur software. For the GC–MS
analysis, the SPE method reported in an earlier study [11] was
applied to the samples previous to the injection. GC–MS analy-
sis was performed using a gas chromatograph–mass spectrometer
(GC–MS), QP-2010 Shimadzu (Shimadzu, Kyoto, Japan). The GC–MS
was equipped with a split/splitless auto-injector model AOC-20i
and an auto sampler model AOC-20s. The GC was fitted with a
fused silica capillary column SLB-5ms column (30m × 0.25 mm and
0.25 ␮m film thickness) from Supelco (Bellefonte, PA, USA). The
operating chromatographic conditions were as follows: injector
temperature 220 ◦C, column program of temperatures 50 ◦C (4 m−in1)
2. Experimental
2.1. Reagents and materials
IPMP, analytical grade >98%, was purchased from TCI (TOKYO
Chemical Industry CO.). Titanium dioxide P25 from Degussa
(Germany) was used as photocatalyst. HPLC grade solvents (ace-
tonitrile, isopropanol and methanol) were supplied by Merck
(Darmstadt, Germany). Sodium azide (NaN3), potassium iodide
(KI) and p-benzoquinone (BQ) were obtained from Sigma-Aldrich.
Ultrapure water was obtained from a Millipore Waters Milli-Q
water purification system.
2.2. Photocatalytic degradation experiments
Photocatalytic experiments were carried out in a solar sim-
ulator Atlas Suntest XLS+ (Germany). Illumination was provided
with a xenon lamp (2.2 kW) which was jacketed with special fil-
ters restricting the transmission of wavelengths below 290 nm.
Irradiation experiments were performed using a Pyrex glass UV-
reactor containing 250 mL of aqueous solutions and the appropriate
amount of TiO2 (100 mg L−1) at natural pH. The suspension was kept
in the dark for 30 min, prior to illumination to reach adsorption
equilibrium onto semiconductor surface. A relatively low amount
of TiO2 (100 mg L−1) and a higher initial concentration of IPMP
(10 mg L−1) than the typical values found in drinking waters have
been selected in the experiments in order to obtain slower kinetics
and provide favorable conditions for the identification and struc-
tural elucidation of transformation products and the mineralization
study.
to 280 ◦C (2 min) at 6 ◦C min−1 to 300 ◦C (2 min) at 10 ◦C min
Helium was used as the carrier gas at a flow rate of 1.7 mL min−1
.
.
2.3. Analytical procedures
The ion source and transfer line were kept at 200 and 250 ◦C, respec-
tively. The quadrupole mass spectrometer was operated in electron
impact (EI) ionization mode at 70 eV and the spectra were obtained
at a scan range from m/z 50 to 450. The splitless mode was used for
injection of 1 ␮L volume.
2.3.1. Kinetic study
IPMP concentrations were determined by a Dionex P680 HPLC
chromatography equipped with a Dionex PDA-100 Photodiode
Array Detector using a Discovery C18, (250 mm length × 4.6 mm ID,
5 ␮m particle size) analytical column from Supelco (Bellefonte, PA,
USA). The HPLC mobile phase was a mixture of LC-grade water pH 3
(30%) and acetonitrile (70%) with a flow rate of 1 mL min−1. Column
temperature was set at 40 ◦C. The detection of IPMP was performed
at 212 nm.
2.5. Toxicity measurements
Acute toxicity of the treated samples was evaluated by mon-
itoring changes in the natural emission of marine luminescence
bacteria Vibrio Fischeri. The analysis was conducted by Microtox
Model 500 Toxicity Analyzer (Azur Enviromental). A briefly descrip-
tion of the procedure has been reported in our previous study [14].
2.3.2. Mineralization studies
Total organic carbon (TOC) was measured on filtered suspen-
sions (0.22 ␮m), using a Shimadzu TOC V-csh Analyzer equipped
with a non-dispersive infrared detector. NO3− and NO2− ions, were
determined by a Dionex ICS-1500 equipped with ASRS Ultra II
self-regenerating suppressor. NH4+ ions were analyzed by the colo-
metric method based on indophenol blue formation [10] using an
UV–vis spectrophotometer Hitachi, U-2000.
3. Results and discussion
3.1. Kinetics of disappearance and mineralization by
TiO2-photocatalytic process
Preliminary adsorption and photolysis experiments were car-
ried out at the initial IPMP concentration of 10 mg L−1 to assess
the extent of adsorption and photolysis of IPMP. Negligible adsorp-
tion of the target compound on the catalyst’s surface was observed
under dark conditions (data not shown). The direct photolysis (sim-
ulated solar radiation alone) showed that only 20% of IPMP was
removed after 30 min of simulated solar illumination. However,
after 4 h of irradiation almost 85% of the initial concentration of
IPMP was degraded (Fig. 1). In contrast, no mineralization was
2.3.3. Scavenging experiments for reactive species
The role of HO•, O2•−, h+ and 1O2 in the reaction was determined
with the addition of isopropanol (1000 mg L−1), p-benzoquinone
(20 mg L−1), iodine anions (100 mg L−1) and azide (10 mg L−1) [11]
to the solutions containing 10 mg L−1 IPMP, respectively. The
employed concentrations were sufficient to inhibit the reactive
species HO•, O2•−, h+ and 1O2 as reported in previous works [11–13]
using similar experimental conditions.
Please cite this article in press as: M. Antonopoulou, I. Konstantinou, TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine
taste and odor compound in aqueous phase: Kinetics, degradation pathways and toxicity evaluation, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.027

G Model
CATTOD-8974; No. of Pages8
ARTICLE IN PRESS
M. Antonopoulou, I. Konstantinou / Catalysis Today xxx (2014) xxx–xxx
3
−
+
Fig. 1. Disappearance of IPMP, TOC removal and evolution of NO3 and NH4 ions under photolytic and photocatalytic conditions ([IPMP] = 10 mg L−1, [TiO2] = 100 mg L−1
,
I = 600 W m−2).
observed after the same irradiation time. Complete degradation of
IPMP is rather a quick process and is achieved within 20 min in the
presence of TiO2 with a half time of 3.3 min (Fig. 1) following appar-
ent first-order kinetic model. Consequently, the decomposition that
is observed in the presence of TiO2 is ascribed to the catalyst’s activ-
ity. TOC removal followed a much slower rate compared to IPMP,
showing a half time of 100.4 min following also first-order kinetics.
High TOC removal percentages (approximately 80%) have been suc-
ceeded after 240 min irradiation. At the end of the process, nitrogen
was mainly transformed into NH4+ ions (45%) and in a lesser extent
into NO3− (almost 10%). NO3− and NH4+ ions reach about 55% of the
expected stoichiometric nitrogen amount, indicating either that N-
containing species remained adsorbed in the photocatalyst surface
or that NH3 was produced and transferred to the gas phase. The
lack of nitrogen stoichiometric recovery can be correlated with the
remaining 20% of TOC indicating the presence of derivatives which
still containing the organic nitrogen such as aliphatic amines (i.e.
methylamine, ethylamine, dimethylamine, n-propylamine etc.) or
of iodine ions in the photocatalytic system provokes 91.2% inhibi-
tion at 30 min reaction with a calculated value of k = 0.0188 min−1
(Table 1). Slightly lower inhibition of the degradation was observed
compared to the inhibition in the presence of isopropanol (HO• rad-
ical scavenger) suggesting that reactions occur mainly at the surface
of the photocatalyst via surface-bound HO• radicals and/or photo-
generated holes. The addition of azide provokes partial inhibition of
IPMP photocatalytic degradation as shown in Table 1 (around 62%
inhibition). Moreover, when p-BQ was added to the IPMP solution,
89.6% inhibition of IPMP photocatalytic degradation was observed,
•−
proving the significant contribution of O2 to the photocatalytic
degradation of IPMP.
3.3. Identification of degradation products by mass spectrometric
techniques
In photocatalytic processes, numerous intermediates and TPs
can be formed as a consequence of the non-selectivity of HO• and
the participation of the other photogenerated reactive species in
various oxido-reductive reactions [17,18]. In some cases the trans-
formation products can be more toxic and persistent than the
parent compound [18] as well as can contribute to the organoleptic
quality of the water [19]. This emphasize that the identification of
transformation products is essential task in assessing the reliability
of the degradation treatment for the pollutant and in optimizing
the overall process efficiency. In the present study, identifica-
tion and structural elucidation of intermediates is based on the
use of accurate mass measurements, MS2, MS3 fragmentation and
electron-impact mass spectra. The mass spectra characteristics of
the detected TPs are presented in Tables 2 and 3 for HR-LC–MS and
GC–MS analysis, respectively. TPs evolution as a function of irradi-
ation time was also followed and the evolution profiles of the TPs
are depicted in Fig. 2. Up to ten intermediate products were eluci-
dated by high resolution accurate LC–MS analysis. Errors between
the measured and the calculated accurate mass lower than 5 ppm
were obtained in all cases providing a high degree of confidence in
assigning the empirical formula and possible structures of the inter-
mediates generated. Two TPs were identified by GC–MS through
interpretation of the mass spectra and investigation of their char-
acteristic ions as well as using the identification program of the
NIST library with a fit value higher than 75%.
intermediates bearing the C
N C fragment arising from opening of
pyrazine ring, as identified elsewhere [15,16]. In the present study,
we focused on the characterization and identification of the pri-
mary TPs resulting from the photocatalytic degradation of IPMP.
These primary TPs undergo ring opening, oxidation and demeth-
ylation mechanisms and finally transformed to lower molecular
weight products before total mineralization. The higher amount of
NH4+ ions compared to NO3− is consistent with previous study that
reported that the photocatalytic degradation of heterocyclic two-
nitrogen containing compounds with a C
N C fragment leads to
+
the generation of a large amount of NH4 and a minor amount of
−
NO3 [15].
3.2. Contribution of reactive species to photodegradation of IPMP
By using isopropanol, p-benzoquinone, iodine and azide ions as
hydroxyl, superoxide anion, holes and singlet oxygen scavengers,
respectively, the contribution of the reactive species on IPMP degra-
dation was determined [11]. The pseudo-first order rate constants
with or without the presence of the scavenging reagents are sum-
marized in Table 1. The inhibitory effect was evaluated by the
percent reduction of kinetic constants, denoted as % ꢀk in Table 1.
The addition of isopropanol leads to almost completely inhibi-
tion on TiO2 photocatalytic degradation of IPMP (93.9% inhibition).
Extensive inhibition of the process by isopropanol indicates the
essential participation of HO• in the oxidation of IPMP. The addition
Three major monohydroxylated compounds with m/z 169.0971
(A–C) were identified at the first stages of the photoctalytic
reaction as a consequence of the non-selectivity of the HO•
Please cite this article in press as: M. Antonopoulou, I. Konstantinou, TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine
taste and odor compound in aqueous phase: Kinetics, degradation pathways and toxicity evaluation, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.027

G Model
CATTOD-8974; No. of Pages8
ARTICLE IN PRESS
M. Antonopoulou, I. Konstantinou / Catalysis Today xxx (2014) xxx–xxx
4
Table 1
Kinetic parameters (rate constants and % RSD, correlation coefficients (R2), half-lives (t1/2)) of IPMP (10 mg L−1) photocatalytic degradation in the presence of TiO2 suspension
and with the addition of scavengers.
Treatment system
k (min−1) (% RSD)
t1/2 (min)
R2
% ꢀk
Control (no scavenger)
TOC
Isopropanol
0.2127 (0.9)
0.0069 (12.2)
0.0130 (11.9)
0.0226 (7.7)
0.0188 (8.1)
0.0806 (4.4)
3.3
100.4
53.3
30.7
36.9
8.6
0.9779
0.9736
0.9886
0.9890
0.9916
0.9756
93.9
89.4
91.2
62.1
p-Benzoquinone
Iodine ions
−
N3
Fig. 2. Time evolution profiles of identified TPs based on HR-LC–MS data.
Please cite this article in press as: M. Antonopoulou, I. Konstantinou, TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine
taste and odor compound in aqueous phase: Kinetics, degradation pathways and toxicity evaluation, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.027

G Model
CATTOD-8974; No. of Pages8
ARTICLE IN PRESS
M. Antonopoulou, I. Konstantinou / Catalysis Today xxx (2014) xxx–xxx
5
Table 2
High resolution accurate mass data ([M + H+], MS2, MS3 product ions, molecular formulas and relative error ꢀ(ppm) for the identified transformation products.
Rt
Molecular
formula
[M + H]+ m/z
ꢀ (ppm)
−0.590
MS2 m/z
ꢀ (ppm)
−2.712
MS3
ꢀ (ppm)
Molecular structure
10.09
C8H13N2O
153.1022
(IPMP)
138.0784
C7H10N2O
(–CH3) (100)
123.0550
C6H7N2O
(–CH3) (100)
−2.856
C7H13N2 (–CO)
(30)
83.0597
−3.6226
−4.6800
C4H7N2
(–C4H5O) (20)
6.26
C8H13N2O2
169.0971
(169-A)
−4.697
151.0865
C8H11N2O
(–H2O) (100)
−0.791
136.0621
C7H8N2O
(–CH3) (5)
−4.787
C6H7N2O
(–C2H4) (100)
119.0600
−0.990
−4.156
C7H7N2
(–CH4O) (10)
6.55
C8H13N2O2
169.0971
(169-B)
−4.970
151.0868
C8H11N2O
(–H2O) (100)
−0.003
136.0621
C7H8N2O
(–CH3) (5)
−4.980
C6H7N2O
(–C2H4) (100)
119.0600
−2.352
−2.980
C7H7N2
(–CH4O) (10)
7.44
C8H13N2O2
169.0971
(169-C)
−4.460
151.0864
C8H11N2O
(–H2O) (100)
−0.394
123.0550
C6H7N2O
(–C2H4) (100)
−2.514
C7H9N2
0.703
1.468
(–CH2O) (20)
119.0600
C7H7N2
(–CH4O) (10)
6.76
C8H13N2O3
185.0921
(185-A)
−4.586
167.0813
C8H11N2O2
(–H2O) (40)
−1.521
−1.809
124.0628
C6H8N2O
(–C2H5O2)
(100)
7.05
C8H13N2O3
185.0921
(185-B)
−4.211
151.0865
C8H11N2O
(–H2O2) (100)
−0.725
−0.684
123.0549
C6H7ON2
(–C2H4) (100)
−3.246
138.0787
C7H10N2O
(–CH3O2) (40)
4.13
C8H13N2O4
201.0869
(201-A)
−5.238
183.0755
C8H11N2O3
(–H2O) (40)
−5.000
97.0757
C5H9N2
(–CH3OH)
(100)
−3.244
129.1018
−5.000
−1.172
C6H13N2O
(–C2O3) (100)
157.0970
C7H13N2O2
(–CO2) (25)
Please cite this article in press as: M. Antonopoulou, I. Konstantinou, TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine
taste and odor compound in aqueous phase: Kinetics, degradation pathways and toxicity evaluation, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.027

G Model
CATTOD-8974; No. of Pages8
ARTICLE IN PRESS
M. Antonopoulou, I. Konstantinou / Catalysis Today xxx (2014) xxx–xxx
6
Table 2 (Continued)
Rt
Molecular
formula
[M + H]+ m/z
ꢀ (ppm)
MS2 m/z
ꢀ (ppm)
MS3
ꢀ (ppm)
−4.185
Molecular structure
4.45
C8H13N2O4
201.0869
(201-B)
−5.218
183.0755
C8H11N2O3
(–H2O) (40)
−4.704
155.0819
C7H11N2O2
(–CH2O2) (100)
129.1017
C6H13N2O
−4.180
(–C2O3) (100)
155.0819
2.811
C7H11N2O2
(–CH2O2) (35)
5.52
5.01
C7H9N2O2
153.0658
139.0866
−4.600
−3.448
C7H11N2O
121.0756
C7H9N2 (–H2O)
(100)
−3.427
−4.665
5.44
C8H11N2O2
167.0807
−0.774
153.0664
C7H9O2N2
(–CH2) (100)
radical attack [11]. The formation of an intense product ion at m/z
151.0865–151.0868 for all the isomers was observed, originating
from H2O loss, well matched with OH group addition on the IPMP
molecule. Through the analysis of their MS/MS spectra it could be
proposed the position for hydroxyl group substitution. The absence
of fragments which indicates modified pyrazine ring suggests the
hydroxylation of either the isopropyl or methoxy chain. This assign-
ment was further supported by the presence of a common fragment
at m/z 119.0600 which can be attributed to the loss of methanol and
enabled the chemical modification of the pyrazine ring moiety to
be excluded. A major fragment at m/z 123.0550 occurs in the MS3
spectra of the three hydroxylated derivatives of pyrazine. This frag-
ment corresponding to C6H7N2O could be attributed to the loss of
the ethene. This can occur by hydrogen migration from the methoxy
group to the tertiary carbon followed by loss of ethene [20]. As
both isomers A and B show identical fragments corresponding to
the loss of methyl group and methanol molecule, it is not possible
to attribute the position of the hydroxyl group from MS2 and MS3
spectra. The absence of other product ions does not allow to dis-
tinguish these two hydroxyl TPs which can bear the hydroxyl on
the methyl carbon of the methoxy group or on the tertiary carbon
linked to the isopropyl group.
corresponding loss of formaldehyde ion is a significant fragment
only when a methoxy-group is situated in ortho position in rela-
tion to the alkyl side chain [20]. Similarly to our results, previous
works on photocatalytic transformation of aromatic organic com-
pounds bearing an isopropyl group have reported that, the attack
of HO• radicals is favored on the isopropyl groups instead of the
aromatic ring [21,22]. According to literature data single and multi-
ple hydroxylation of the isopropyl group appeared to be kinetically
favored in photocatalytic processes [21].
Two derivatives with [M + H]+ ions at m/z 185.0921 (185-A,B)
were detected in 6.76 and 7.05 min (Table 2). By analyzing isomer
185-A, two main product ions at m/z 167.0813 (loss of a water
molecule) and m/z 124.0628 (loss of C2H5O2) derived from MS2
spectrum, which allow to propose the formation of hydroperoxy
derivative. Similarly, in the case of 185-B isomer, a favored loss of
a H2O2 molecule with the consequent formation of a species at m/z
151.0865 (C8H11ON2) was observed and could be interpreted as a
hydroperoxide derivative.
The structural assignment of hydroperoxy derivatives was based
on the key losses of H2O2 (34 Da) and H2O (18 Da), which are
indicative of the formation of a peroxy bond [23,24]. The proposed
mechanism for the formation of hydroperoxy derivative involves
hydrogen abstraction (low step of the reaction) by the produced
HO• followed by the very fast reaction with O2 [25].
As regards isomer 169-C, the OH group is located on a terminal
carbon of isopropyl chain, as assessed by the structural-diagnostic
MS3 ion at m/z 121.0762 which fit well with the loss of formalde-
hyde and by the absence of fragment at m/z 136.0621, indicating
the presence of a modified CH3 group. The m/z 121.0776 and the
The TPs at [M + H]+ at m/z 201.0869 (two isomers 201-A,B) were
detected, all derived by the hydroxylation of hydroperoxy TPs. This
assignment is supported by the mass difference of 15.9948 from the
Table 3
Retention times and mass spectra characteristics of IPMP and its oxidation products by GC–MS.
Rt (min)
Product
m/z fragments (% relative intensity)
Similarity (%)
11.45
16.28
11.63
IPMP
152(47), 137(100), 124(35)
138(100), 123(35) 137(37), 105(21)
152(12), 137(41), 43(100)
95
89
75
2-ethyl-3-methoxy pyrazine (P1)
1-(2-methoxy-3pyrazinyl-)-
ethanone (P2)
Please cite this article in press as: M. Antonopoulou, I. Konstantinou, TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine
taste and odor compound in aqueous phase: Kinetics, degradation pathways and toxicity evaluation, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.027

G Model
CATTOD-8974; No. of Pages8
ARTICLE IN PRESS
M. Antonopoulou, I. Konstantinou / Catalysis Today xxx (2014) xxx–xxx
7
Fig. 3. Proposed degradation pathways of IPMP photocatalytic degradation in the presence of TiO2 and simulated solar light.
parent compounds and the formation of MS2 product ions at m/z
183.0755, which is consistent with the loss of a water molecule.
The MS2 and MS3 spectrums reveal also key fragment ions pre-
sented in Table 2 which come from a loss of methanol molecule, CO
and formic acid and evidence the presence of hydroperoxy bond
in the studied species [23,26]. The formation of a species at m/z
139.0866 and an empirical formula C7H11ON2 invokes the cleavage
of the methoxy group and the formation of 2-isopropyl-pyrazinol.
Two TPs with m/z 153.0658 and 167.0807 sharing empirical for-
mulas consistent with the oxidation of the isopropyl-group were
also detected but it was not possible to accomplish a fragmentation
study on these two TPs. However, the high resolution and accu-
rate measurements, allowed to propose TPs structures which are
depicted in Table 2.
The TP at m/z 153.0658 1-(2-methoxy-)3-pyrazynil ethanone
was detected using GC–MS, as well. One more intermediate arising
from dealkylation of isopropyl chain, 2-ethyl-3-methoxy pyrazine
was also eluted in 16.28 min by GC–MS system. The time evolu-
tion profiles of TPs in Fig. 2 show that all products attained their
maximum concentrations within the same time scale (5–10 min)
showing that the formation of hydroxy and hydroperoxy deriva-
tives can take place simultaneously. Reduction and/or hydrolysis of
the hydroperoxy derivatives could be also followed for the gener-
ation of the corresponding hydroxy derivatives [27]. On the other
hand, the demethylated and oxidized TPs showed slower forma-
tion within 10 min of irradiation that could be formed after the
HO• radical attack at the alkyl groups (Fig. 2). On the basis of the
identification of intermediates, their evolution profile as well as
the mineralization and scavenging studies, a tentative photocat-
alytic degradation pathway of IPMP was proposed in Fig. 3. The
pathways include single hydroxylation that occurred mainly in
the isopropyl chain (pathway A), HO• radical electrophilic attack
with the subsequent O2 addition (pathway B) and the cleavage of
methoxy group (pathway C) bonds via positive holes. Although the
high reactivity of hydroxyl radicals toward aromatic rings, dealky-
lation of the isopropyl side chain is one of the dominant processes.
The proposed mechanism for photocatalytic degradation of iso-
propyl group involves reaction of a hydroxyl radical with either
the secondary or a primary carbon, which via the corresponding
hydroperoxide leads to formation of aldehyde and keto structures
[28]. The dealkylated intermediates were suggested to proceed via
further oxidation of the keto TPs and subsequent decarboxylation
reactions [22,28]. After successive dealkylation, all the above inter-
mediates were further transformed by an oxidative opening of the
aromatic ring via hydroxyl radicals and/or positive holes continu-
ous attack, giving rise to the formation of lower molecular weight
and more oxidized molecules as reported elsewhere [15,16] and
depicted in Fig. 3.
As regards the organoleptic quality of TPs, no information is
available on their taste and odor properties because all of them are
new chemical entities. According to literature data, the majority
of the identified TPs arising from MBT photocatalytic degrada-
tion holding the benzothiazole-moiety have been reported to pose
threats to the esthetic value of the water [19]. Following the current
state of knowledge for the degradation of mercaptobenzothiazole
[19] and taking into account the fact that the identified TPs bearing
the heterocyclic pyrazine ring, it is assumed that the TPs are poten-
tially odor compounds contributing to the organoleptic quality of
the water samples. However, all the identified TPs completely dis-
appear very quickly after 30 min. This clearly proves the efficiency
of heterogeneous photocatalysis for the degradation of the IPMP,
rendering this process a sustainable treatment technology for taste
and odor control.
3.4. Toxicity evaluation
Acute toxicity was evaluated by monitoring changes in the natu-
ral emission of the luminescent bacteria V. fischeri when challenged
with toxic compounds and is expressed as percentage of inhibi-
tion of the bacteria luminescence. Toxicity (inhibition %) evolution
during the photocatalytic treatment is depicted in Fig. 4. The ini-
tial toxicity of IPMP solution showed an initial inhibition of 19%
that is slightly increases at 30 min of irradiation and reaching the
value of 25%. It is worth mentioning that the highest toxicity is
observed at irradiation times where the majority of the identified
TPs attain almost their minimum concentration. Thus, the slight
Please cite this article in press as: M. Antonopoulou, I. Konstantinou, TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine
taste and odor compound in aqueous phase: Kinetics, degradation pathways and toxicity evaluation, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.027

G Model
CATTOD-8974; No. of Pages8
ARTICLE IN PRESS
M. Antonopoulou, I. Konstantinou / Catalysis Today xxx (2014) xxx–xxx
Acknowledgments
8
This research has been co-financed by the European Union
(European Social Fund—ESF) and Greek National Funds through
the Operational Program “Education and Lifelong Learning” of the
National Strategic Reference Framework (NSRF)—Research Fund-
ing Program: Heracleitus II. Investing in knowledge society through
the European Social Fund.
The authors would like to thank also the Unit of Environmental,
Organic and Biochemical high resolution analysis-orbitrap-LC–MS
analysis of the University of Ioannina for providing access to the
facilities.
References
[1] N. An, H. Xie, N. Gao, Y. Deng, W. Chu, J. Jiang, Clean–Soil, Air, Water (12) (2012)
1349–1356.
[2] A. Peter, U. von Gunten, Environ. Sci. Technol. 41 (2007) 626–631.
[3] C. Liang, D. Wang, M. Yang, W. Sun, S. Zhang, J. Environ. Sci. Health, A 40 (2005)
767–778.
[4] A. Peter, O. Köster, A. Schildknecht, U. von Gunten, Water Res. 43 (2009)
2191–2200.
[5] K. Zoschke, N. Dietrich, H. Börnick, E. Worch, Water Res. 46 (2012) 5365–5373.
[6] L.A. Lawton, P.K.J. Robertson, R.F. Robertson, F.G. Bruce, Appl. Catal., B: Environ.
44 (2003) 9–13.
Fig. 4. % Inhibition of the marine luminescence bacteria Vibrio Fischeri as a function
of photocatalytic treatment ([IPMP] = 10 mg L−1, [TiO2] = 100 mg L−1, I = 600 W m−2).
[7] H. Tran, G.M. Evans, Y. Yan, A.V. Nguyen, Water. Sci. Technol. 9 (2009) 477–483.
[8] E.E. Bamuza-Pemu, E.M.N. Chirwa, Chem. Eng. Trans. 24 (2011) 91–96.
[9] M. Antonopoulou, E. Evgenidou, D. Lambropoulou, I. Konstantinou, Water Res.
53 (2014) 215–234.
[10] L. Solorzano, Limnol. Oceanogr. 14 (5) (1969) 799–801.
[11] M. Antonopoulou, A. Giannakas, Y. Deligiannakis, I. Konstantinou, Chem. Eng.
J. 231 (2013) 314–325.
[12] R. Palominos, J. Freer, M.A. Mondaca, H.D. Mansilla, J. Photochem. Photobiol.,
A: Chem. 193 (2008) 139–145.
[13] S. Zheng, Y. Cai, K.E. O’Shea, J. Photochem. Photobiol., A: Chem. 210 (2010)
61–68.
toxicity increase can be related to the generation of more toxic
ring opened and/or aliphatic amine species during the photocat-
alytic treatment, while synergistic effects between them can also
be considered. Quantifiable toxicity response of aliphatic amines
to Microtox tests was reported elsewhere [29]. Obviously, bio-
luminescence inhibition values ranged always below 25%, which
indicates that IPMP and its degradation products have low acute
toxicity effects on the tested bacteria. Thereafter, inhibition %
decreases leading to almost total detoxification after 300 min irra-
diation.
[14] M. Antonopoulou, V. Papadopoulos, I. Konstantinou, J. Chem. Technol. Bio-
technol. 87 (2012) 1385–1395.
[15] S. Horikoshi, H. Hidaka, J. Photochem. Photobiol., A: Chem. 141 (2001) 201–207.
[16] F. Qi, B. Xu, Z. Chen, L. Feng, L. Zhang, D. Sun, Chem. Eng. J. 219 (2013) 527–536.
[17] D. Fatta-Kassinos, M.I. Vasquez, K. Kümmerer, Chemosphere 85 (2011)
693–709.
4. Conclusions
[18] D.A. Lambropoulou, I.K. Konstantinou, T.A. Albanis, A.R. Fernández-Alba,
Chemosphere 83 (2011) 367–378.
The photocatalytic degradation of IPMP in the presence of TiO2
was investigated in detail focusing on the kinetic and mechanistic
study. Results of the present study clearly point out that heteroge-
neous TiO2 photocatalytic treatment is suitable for the elimination
of IPMP from the aqueous phase. Degradation for both IPMP and
TOC followed pseudo-first order kinetic model. Scavenging stud-
ies indicated that HO• are responsible for the major degradation of
IPMP whereas O2•− and h+ play also significant roles during the pro-
cess. The photoinduced transformation of IPMP proceeded through
the formation of numerous intermediate products and involved:
hydroxylation, oxidation and demethylation pathways. Investiga-
tions on the intermediates suggest that the hydroxyl radical attack
occur preferentially on the isopropyl chain of the IPMP before the
ring opening and total mineralization. A significant abatement of
the overall toxicity was accomplished as revealed by Microtox
bioassay.
[19] F.B. Li, X.Z. Li, K.H. Ng, Ind. Eng. Chem. Res. 45 (2006) 1–7.
[20] H.M. Maher, T. Awad, J. DeRuiter, C.R. Clark, J. Chromatog. Sci. 50 (2012) 1–9.
[21] I. Losito, A. Amorisco, F. Palmisano, Appl. Catal., B: Environ. 79 (2008) 224–236.
[22] M.V.P. Sharma, V.D. Kumari, M. Subrahmanyam, Chemosphere 72 (2008)
644–651.
[23] M.-C. Reinnig, J. Warnke, T. Hoffmann, Rapid Commun. Mass Spec. 23 (2009)
1735–1741.
[24] F. Giuffrida, F. Destaillats, L.H. Skibsted, F. Dionisi, Chem. Phys. Lipids 131 (2004)
41–49.
[25] I.K. Konstantinou, T.M. Sakellarides, V.A. Sakkas, T. Albanis, Environ. Sci. Tech-
nol. 35 (2001) 398–405.
[26] Y. Mochida, Y. Yokoyama, T. Kawai, S. Nakamura, M. Tsuchiya, J. Mass Spectrom.
Soc. Jpn. 42 (1994) 207–215.
[27] M.N. Möller, D.M. Hatch, H-Y.H. Kim, N.A. Porter, J. Am. Chem. Soc. 134 (2012)
16773–16780.
[28] E. Pelizzetti, C. Minero, V. Carlin, M. Vincenti, E. Pramauro, M. Dolci, Chemo-
sphere 24 (1992) 891–910.
[29] W-L. Gong, K.J. Sears, J.E. Alleman, E.R. Blatchley, Environ. Toxicol. Chem. 23
(2004) 239–244.
Please cite this article in press as: M. Antonopoulou, I. Konstantinou, TiO2 photocatalysis of 2-isopropyl-3-methoxy pyrazine
taste and odor compound in aqueous phase: Kinetics, degradation pathways and toxicity evaluation, Catal. Today (2014),
http://dx.doi.org/10.1016/j.cattod.2014.03.027