Photocatalytic oxidation of pirimicarb in aqueous slurries containing binary and ternary oxides of zinc and titanium

Article abstract of DOI:10.1016/j.jphotochem.2014.10.014

The photocatalytic degradation of pirimicarb in pure water has been studied using zinc oxide (ZnO), titanium dioxide (TiO₂) and zinc titanates (Zn₂TiO₄ and ZnTiO₃) under artificial light (300-460 nm) irradiation. Comparison of catalysts showed that TiO₂ is the most efficient for the removal of pirimicarb and their transformation products. To obtain a better understanding of the mechanistic details of these ZnO-TiO₂-assisted photodegradation of pirimicarb, the transformation products of the processes were identified by liquid chromatography tandem mass spectrometry (LC-MS²) and liquid chromatography time of flight mass spectrometry (LC-TOF/MS) techniques. The probable photodegradation pathways are proposed and discussed. The main steps involved: N-dealkylation of the 2-dimethylamino group to form the carbamate and decarbamoylation of the carbamate moiety with further N-dealkylation of the 2-dimethylamino group to have the hydroxypyrimidines.

Full text of DOI:10.1016/j.jphotochem.2014.10.014

Journal of Photochemistry and Photobiology A: Chemistry 298 (2014) 24–32
Contents lists available at ScienceDirect
Journal of Photochemistry and Photobiology A:
Chemistry
journal homepage: www.elsevier.com/locate/jphotochem
Photocatalytic oxidation of pirimicarb in aqueous slurries containing
binary and ternary oxides of zinc and titanium
a
a
a
b
c
José Fenoll a, , Isabel Garrido , Pilar Hellín , Pilar Flores , Nuria Vela , Simón Navarro
*
a
Departamento de Calidad y Garantía Alimentaria, Instituto Murciano de Investigación y Desarrollo Agrario y Alimentario (IMIDA), C/Mayor s/n. La Alberca,
30150 Murcia, Spain
Facultad de Enfermería, Universidad Católica San Antonio de Murcia, Campus de Los Jerónimos, s/n. Guadalupe, 30107, Murcia, Spain
b
c
Departamento de Química Agrícola, Geología y Edafología, Facultad de Química, Universidad de Murcia, Campus Universitario de Espinardo, 30100, Murcia,
Spain
A R T I C L E I N F O
A B S T R A C T
Article history:
Received 27 June 2014
Received in revised form 10 October 2014
Accepted 17 October 2014
Available online 22 October 2014
The photocatalytic degradation of pirimicarb in pure water has been studied using zinc oxide (ZnO),
titanium dioxide (TiO2) and zinc titanates (Zn2TiO4 and ZnTiO3) under artificial light (300–460 nm)
irradiation. Comparison of catalysts showed that TiO2 is the most efficient for the removal of pirimicarb
and their transformation products. To obtain a better understanding of the mechanistic details of these
ZnO–TiO2-assisted photodegradation of pirimicarb, the transformation products of the processes were
identified by liquid chromatography tandem mass spectrometry (LC-MS2) and liquid chromatography
time of flight mass spectrometry (LC-TOF/MS) techniques. The probable photodegradation pathways
are proposed and discussed. The main steps involved: N-dealkylation of the 2-dimethylamino group
to form the carbamate and decarbamoylation of the carbamate moiety with further N-dealkylation of the
2-dimethylamino group to have the hydroxypyrimidines.
Keywords:
Pirimicarb
Transformation products
Photocatalytic Oxidation
Semiconductor materials
ã 2014 Elsevier B.V. All rights reserved.
1. Introduction
photocatalytic process is based on the generation of electron–hole
pairs by irradiation of semiconductor particles with energy h
n
ꢀ Eg
Carbamate pesticides have been extensively used in pest control
owing to their high effectiveness and low persistence. Pirimicarb (2-
dimethylamino-5,6-dimethylpyrimidin-4-yldimethylcarbamate) is
an N,N-dimethylcarbamate insecticide that is widely employed
against aphids in fruit and vegetable. Pirimicarb exerts its toxic
effects by inhibiting the enzyme acetylcholinesterase [1]. This
systemic insecticide is suspected carcinogen and mutagen [2]. In
addition, theirhigh solubility, pirimicarb mayleach throughthe soil
profile [3,4]. Consequently, it is of primary importance to apply
remediation strategies to polluted waters in order to protect hydric
resources.
In this context, the degradation of organic pollutants in water
by photocatalysis, using semiconductors such as TiO2 and ZnO,
has been widely used by numerous authors in the past two decades
[5–9]. Solar radiation, a renewable source of energy, inexhaustible
and pollution-free, can be widely utilized in the photocatalytic
degradation of the dyes, pesticides and detergents [10–13]. The
(band gap energy). These electron–hole pairs are capable of
initiating a series of chemical reactions that eventually mineralize
the pollutants [14].
These semiconductors have many advantages such as a low
price, high photocatalytic activity, simple preparation, etc.
However, ZnO has a low stability at high pH, while TiO2 can only
absorb a small portion of solar spectra in the UV region [15]. Thus,
the photocatalytic activity of ZnO–TiO2 systems (Zn2TiO4 with
cubic spinel crystal structure, Zn2Ti3O8 with cubic defect spinel
structure and ZnTiO3 with cubic or hexagonal structure) is being
investigated with the purpose of looking for more efficient
materials for photocatalytic applications [16–18]. Although the
photocatalytic degradation of pirimicarb using ZnO and TiO2 has
been studied from previous researchers [5,19], the effect of
ZnO–TiO2 system for the removal of pirimicarb from water has
not been studied. The main goal of this work was to assess the
effectiveness of the photocatalytic process for the decontamina-
tion of water polluted by pirimicarb, to evaluate the kinetics
aspects of the process and the reaction intermediates for
understanding the mechanistic details of the photodegradation
in the ZnO–TiO2 systems (ZnO, TiO2, Zn2TiO4 and ZnTiO3).
* Corresponding author: Tel.: +34 968366798; fax: +34 968366792.
E-mail address: jose.fenoll@carm.es (J. Fenoll).
http://dx.doi.org/10.1016/j.jphotochem.2014.10.014
1010-6030/ã 2014 Elsevier B.V. All rights reserved.

J. Fenoll et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2014) 24–32
25
2. Experimental
Initially, 2000 mL of type II analytical-grade water (pH 7.1, ORP
212 mV, resistivity >5 M
cm (25 ꢁC); conductivity <1 S cmꢂ1
TOC <30
g Lꢂ1; microorganisms < 10 cfu mLꢂ1) was mixed with
V
m
,
2.1. Pirimicarb, transformation products and reagents
m
commercial product (APHOX, Syngenta) to reach a spiking level of
about 0.1 mg Lꢂ1 of pirimicarb, homogenizing the mixture for
20 min to constant concentration in the dark. After this time the
appropriate amount of catalyst (200 mg Lꢂ1) was added to the
reaction solution. The respective mixtures were maintained for
30 min in the dark, prior to illumination, in order to achieve the
maximum adsorption of pirimicarb onto semiconductor surface.
Analytical reagents were purchased from Dr. Ehrenstorfer
(Augsburg, Germany) with purity >99%. Pirimicarb and their main
organic transformation products are listed in Table 1. Zinc oxide
(ZnO) and zinc titanium oxide (Zn2TiO4) were purchased from Alfa
Aesar (Karlsruhe, Germany). zinc titanate (ZnTiO3) was purchased
from Sigma–Aldrich Química S.A. (Madrid, Spain). Titanium
dioxide P25 Degussa (TiO2) was supplied from Nippon Aerosil
Co., Ltd. (Osaka, Japan). Acetonitrile was supplied by Scharlau
(Barcelone, Spain).
Several samples from
0 to 60 min were taken during the
illumination period. In all cases, assays were replicated three
times.
2.2. Photocatalyst characterization
2.4. Analytical determinations
The photocatalysts were characterized by
a
PerkinElmer
The extraction procedure involves a pre-concentration of
pirimicarb from water samples (50 mL) step based on solid-phase
extraction with a silica-based bonded C18 cartridge (Sep-Pak Plus
500 mg, 6 mL, Waters). The SPE cartridge was first conditioned
with 5 mL of acetonitrile, followed by 5 mL of deionised water.
Extraction of water samples was carried out at a 8 mL minꢂ1 flow
rate. The samples were loaded onto the cartridges, and washed
with deionised water and dried for 15 min under a vacuum. The
analytes retained were eluted with 5 mL of acetonitrile. The
Lambda 750S UV/VIS spectrophotometer equipped with 60 mm
integrating sphere accessory for diffuse reflectance spectra.
Software: UV WinLab DPV 1.0 (Shelton, USA). A BaSO4 standard
was used as the reference spectrum. The crystalline structure of
the photacatalysts was characterized by means of powder X-ray
diffractometry (XRD), on a Philips PW 1700. The samples were
measured at 40 kV and 24 mA using Cu-K
a radiation at a scanning
speed of 1ꢁ (2
u
). The morphology of the solids was examined by
field emission scanning electron microscopy (FE-SEM) using a Carl
Zeiss MERLIN VP Compact microscope (Oberkochem, Germany)
with high resolution imaging up to 0.8 nm @15 KV, acceleration
voltage of 0.02–30 kV and energy dispersive X-ray (EDX) attached
to SEM. The surface area (SBET) of the semiconductors was
measured according to the BET method by nitrogen adsorption–
desorption isotherms at 77 K on a Micromeritics Tristar 3000 in-
strument (Micromeritics Instruments Co., USA).
solution was filtered through a 0.45 mm filter and analyzed by LC-
MS2 according to the procedure described by Fenoll et al. [21]
Analysis of pirimicarb and their transformed products: 2-
[(methylformyl) amino]-5,6-dimethylpyrimidin-4-yl dimethylcar-
bamate (Metabolite I), 2-(dimethylamino)-5,6-dimethyl-4-hydrox-
ypyrimidine (Metabolite II), 2-(methylamino)-5,6-dimethyl-4-
hydroxypirimidine (Metabolite III) and 2-(methylamino)-5,6 dime-
thylpyrimidin-4-yl dimethylcarbamate (MetaboliteV) was achieved
bycomparisonwithcommerciallyavailablestandardsusinganHPLC
system (Agilent Series 1100, Agilent Technologies, Santa Clara, CA,
USA) equipped with a G6410A triple quadrupole mass spectrometer.
A preliminarystudyof the optimal single reaction monitoring (SRM)
transitions for each compound was carried out by injecting
2.3. Photoreactor setup
The photocatalytic and photolytic experiments were performed
in a photochemical reactor [20]. The reactant solution containing
different amounts of the semiconductor powder (100–400 mg Lꢂ1
)
individual analytes at a concentration level of 10
fragmentor voltages (FV) and collision energies (CE) were applied to
m
g mLꢂ1. Various
was circulated at a flow rate of 600 mL minꢂ1
.
Table 1
Analytical conditions of pirimicarb and their transformation products by HPLC-MS2 and HPLC-TOF/MS.
Compounds
MFa
trb(min) HPLC-MS2
HPLC-TOF/MS
m/z
Error DBE
(ppm)
c
c
SRM1
F1d(V) CE1e(V) SRM2
F2d(V) CE2e(V) Theorical Experimental
Pirimicarb(2-(dimethylamino)-5,6-
dimethylpyrimidin-4-yl
dimethylcarbamate)
Metabolite I(2-[(methylformyl) amino]-
5,6-dimethylpyrimidin-4-yl
dimethylcarbamate)
C
C
11H18N4O2 13.7
239 !72
90
20
239 !182 90
15
239.1503 239.1504
ꢂ0.77
5
6
11H16N4O3 18.9
253 !72 110
10
253 ! 225 110
5
253.1295 253.1294
0.31
Metabolite II(2-(dimethylamino)-5,6-
dimethyl-4-hydroxypyrimidine)
Metabolite III(2-(methylamino)-5,6-
dimethyl-4-hydroxypirimidine)
Metabolite IV(2-(formylamino)-5,6-
dimethylpyrimidin-4-yl
dimethylcarbamate)
Metabolite V(2-(methylamino)-5,6
dimethylpyrimidin-4-yl
C8H13N3O
C7H11N3O
3.25
3.1
168 ! 98 110
154 ! 98 110
239 !166 90
20
20
10
168 ! 71
110
30
10
20
168.1131 168.1132
154.0975 154.0967
239.1139 239.1136
ꢂ0.41
4.81
4
4
6
154 !137 110
C
C
10H14N4O3 16.0
239 !72
225 !72
90
0.93
10H16N4O2 11.2
225 !168 110
10
110
20
225.1346 225.1348
ꢂ1.06
5
dimethylcarbamate)
a
Molecular formula.
Retention time.
Transitions (m/z).
Fragmentor.
b
c
d
e
Collision energy.

26
J. Fenoll et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2014) 24–32
the compounds under study. Table 1 lists the pesticides along with
their retention times and optimized SRM transitions with a dwell
time of 60 ms.
were calculated from the signal-to-noise (S/N) ratio 3 and 10,
respectively. The calibration samples were analyzed by spiking
pesticides at 0.5–200 m
g Lꢂ1 levels into water samples in five
Blank samples were used to establish the limit of detection
(LOD) and the limit of quantification (LOQ). Both, LODs and LOQs
replicates. The correlation coefficient was found to be >0.99 and
the limits of quantification were in the range 5–50 ng Lꢂ1 for
Fig. 1. XRD patterns of TiO2 P25 Degussa (A), ZnO (B), ZnTiO3(C) and Zn2TiO4(D).

J. Fenoll et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2014) 24–32
27
Table 2
contents of anatase (A) and rutile (R) phases in TiO2 P25 Degussa
powder was 70A:30R. ZnO was hexagonal wurtzite structure,
while Zn2TiO4 and ZnTiO3 powders were cubic structure. Table 2
presents the percentages of TiO2 and ZnO, particle size and BET
surface area of the studied photocatalysts. In comparison, SEM
showed that the ZnO nanoparticles were predominantly smaller
than 200 nm in diameter with hexagonal structure (Fig. 2B)
whereas, some larger particles (Zn2TiO4) or agglomerates of TiO2
and ZnTiO3 nanoparticles were observed (Fig 2A, C and D)
Physico-chemical properties and band gaps of studied photocatalysts.
Photocatalysts
TiO2 P25 Degussa ZnO
Zn2TiO4
ZnTiO3
Composition compounds
TiO2(%)
ZnO(%)
100
37.32
62.68
51.68
48.32
100
Particle size (nm)
32 ꢃ 5
55 ꢃ 6
3.0
194 ꢃ 16 4125 ꢃ 650 56 ꢃ 7
BET surface (m2 gꢂ1
)
7 ꢃ 1
0.6 ꢃ 0.2
45 ꢃ 8
Band-gap energy (eV)
3.1
3.3
2.8
The optical properties were studied by UV–vis diffuse
reflectance (UV-DR) spectroscopy for TiO2 P25 Degussa, ZnO,
Zn2TiO4 and ZnTiO3 samples. The diffuse reflectance spectra of
different ZnO–TiO2 systems are illustrated in Fig. 3. The direct
band gaps energy was calculated by plotting [F(R) hv]1/2 vs.
[hv ꢂ Eg], where F(R) = (1 ꢂ R)2/(2R), h is Plank’s constant, Eg is the
pirimicarb and 2-(methylamino)-5,6-dimethyl-4-hydroxypirimi-
dine (Metabolite III), respectively.
For other products, identification was carried out using an HPLC
system connected to a TOF/MS (Agilent Technologies). Liquid
chromatography was performed under the same conditions used
in LC-MS2.
A Shimadzu TOC-L Analyzer (Kyoto, Japan) equipped with an
NDIR detector (680 ꢁC combustion catalytic oxidation technique)
was used. The DOC (dissolved organic carbon) procedure requires
band gap energy,
v is the frequency of light and R is diffuse
reflectance based on the Kubelka–Monk theory of diffuse
reflectance. The indirect band gap estimated from the intercept
of the tangents to the plots were 3.0, 3.1, 3.3 and 2.8 eV for the
samples of TiO2 P25 Degussa, ZnO, Zn2TiO4 and ZnTiO3,
respectively, which are consistent with the experimental band
gaps determined by other authors [22–26].
the sample be passed through a 0.45-mm filter prior to analysis to
remove particulate OC from the sample.
2.5. Statistical analysis
3.2. Effect of catalyst concentration and initial pH during the
photocatalytic degradation of pirimicarb
The curve fitting and statistical data were obtained using
SigmaPlot version 12.0 statistical software (Systat, Software Inc.,
San Jose, CA).
The catalyst concentration and the initial pH of the solution are
two important parameters in the evaluation of photocatalytic
reactions [26]. The effect of these two parameters on the
photocatalytic degradation of pirimicarb has been investigated
using a multivariate method. Minimum differences were observed
in pH values during the irradiation time. Fig. 4 shows the response
surface diagram obtained. In general, the rate constant initially
increases by increasing the catalyst concentration. However, above
a certain catalyst concentration a negative effect is observed. This
unfavourable effect could be due to light scattering and the
reduction of light penetration at a relatively high catalyst loading
[27,28]. The optimum value for catalyst concentration was
3. Results and discussion
3.1. XRD, SEM, UV–VIS diffuse reflectance spectra, percentages of TiO2
and ZnO, BET surface area, and band gap
The degree of crystallinity, the particle size and the surface area
of the catalysts can significantly influence the efficiency of
photocatalytic oxidation of water contaminants. The X-ray
diffractograms of ZnO–TiO2 systems are given in Fig. 1. The
Fig. 2. SEM images of TiO2 P25 Degussa (A), ZnO (B), ZnTiO3(C) and Zn2TiO4(D).

28
J. Fenoll et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2014) 24–32
80
60
40
20
TiO2
ZnO
ZnTiO3
Zn2TiO4
0
300
350
400
450
500
Wavelength(nm)
Fig. 3. Diffuse reflectance spectra of different catalysts (TiO2 P25 Degussa, ZnO, ZnTiO3 and Zn2TiO4).
200 mg Lꢂ1 for all the ZnO–TiO2 systems. As consequence, further
assays were carried out at this catalyst loading.
efficiency of the catalysts in the photooxidation of pirimicarb in
optimal conditions was evaluated in view of the work practical
nature.
The influence of the initial pH value on the rate constant of the
process using different ZnO–TiO2 systems was also investigated. In
general, at relatively low pH values, pirimicarb (pKa = 4.4) might be
protonated, which could provoke repulsion between the substrate
and catalyst according to point of zero charge (PZC) of catalyst. As
reported, the pH of the suspension in the case of TiO2 P25 Degussa,
Zn2TiO4 and ZnTiO3 appeared to have an insignificant effect on the
rate of disappearance of pirimicarb between pH 5.0 and pH 9.0. The
PZC of TiO2 depends on the production method the most frequent
value for TiO2 P25 and ZnO are 6.3 and 9.0, respectively [29], below
or above which the catalyst surface is positively or negatively
charged, respectively. Therefore, adsorption is an important
parameter in the apparent kinetic order of degradation. Apparent
half-order kinetics for TiO2 suggests dissociative adsorption [30].
Moreover, the pH of the solution affects the formation of OH by the
reaction between OH- and photo-induced holes on the TiO2
surface. The h+ are considered to be mainly responsible for
oxidation at low pH, whereas OH are considered as the
predominant species in neutral or alkaline medium [31]. In case
of ZnO, the optimum value for initial pH was 7.0, it can be due to
acid–base properties of this catalyst [32]. Bearing in mind the
results obtained for ZnO and the insignificant effect on the rate of
disappearance of pirimicarb between pH 5.0 and pH 9.0 for TiO2
P25 Degussa, Zn2TiO4 and ZnTiO3, the photocatalytical treatments
were carried out at an initial pH of around 7.0.
The photocatalytic degradation of pirimicarb using the ZnO–
TiO2 systems was conducted under optimal conditions ([photo-
catalyst] = 200 mg Lꢂ1, pH0 7.1). The degradation profile of pirimi-
carb is shown in Fig. 5. The combined action of radiation and the
catalyst (UV/TiO2–ZnO system) leads to the elimination of the
insecticide. In the presence of ZnO and TiO2 P25 Degussa complete
disappearance was achieved after 60 min and 30 min of illumina-
tion, respectively. The residual levels of pirimicarb after 60 min in
the presence of ZnTiO3 and Zn2TiO4 were 28 and 57 m ,
g Lꢂ1
respectively. In the absence of a catalyst, the photolytic decompo-
sition of pirimicarb occurred at a lower rate than that observed in
the photocatalyisis process, and only a 23% reduction of its initial
concentration was achieved after 60 min of light exposure.
The kinetic parameters of pirimicarb in optimal conditions
are shown in Table 3 where the apparent rate constants and half-
lives are listed. The degradation fitted well the exponential
decay curve, following first-order behaviour consistent to the
Langmuir–Hinshelwood model with R2 ranging from 0.958 to
1.000. The half-lives for this compound in the presence of TiO2,
ZnO, ZnTiO3 and Zn2TiO4 were about 0.6, 3.8, 37.7 and 68.0 min,
respectively.
Comparison of TiO2–ZnO systems showed that TiO2 is the most
efficient for catalyzing the removal of this insecticide. ZnTiO3 and
Zn2TiO4 appear to be less effective than ZnO and TiO2, although it
can be used as photocatalyst for the pirimicarb oxidation. The
absorption threshold of photocatalysts might be responsible for
the observed higher rates in ZnO and TiO2. The corresponding
absorption threshold of ZnO, TiO2, and Zn2TiO4 are 413, 400 and
376 nm, respectively, suggesting that ZnO and TiO2 absorbs a large
fraction of UV, and probably absorbed more photons than Zn2TiO4
from the light source used (Fig. 3). In addition, Zn2TiO4 exhibits a
much minor specific surface area than the other photocatalysts
(Table 2). ZnTiO3 has smaller band gap (2.8 eV) which permits rapid
recombination of hole and electron and so conduction band
electron in this semiconductor cannot move into the electron
acceptors in the solution rapidly. The photocatalytic activity of
semiconductors is also dependent on the crystallinity, particle size,
3.3. Photocatalytic activity and kinetics
Evaluating catalytic activity at equal catalyst concentration can
lead to inconclusive results since each material may display
different optical properties. To address the above limitations,
avoiding the precise measurement of the optical properties of the
catalysts, the photoactivity of each material can be evaluated at
constant volumetric rate of photon absorption in the photoreactor
[33]. However, in this work, a comparison of photocatalytic
degradation of pirimicarb with ZnO, TiO2, ZnTiO3 and Zn2TiO4
cannot be evaluated at equal level of photon absorption due to
large surface particle diameter of Zn2TiO4 (4 mm). Therefore, the

J. Fenoll et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2014) 24–32
29
Fig. 4. Optimization of semiconductor (TiO2 P25 Degussa, ZnO, ZnTiO3 and Zn2TiO4) load and initial pH for photocatalytic degradation of pirimicarb (0.1 mg Lꢂ1) using a
response surface diagram.
surface area and concentration of the impurities in the catalysts
[34]. The found photocatalytic activity order (TiO2 > ZnO) agreed
with the previously reported [35–38]. Besides that aqueous
solutions of pirimicarb are unstable to UV light, the main reason
for the greater activity of TiO2 can be due to its smaller size.
Therefore it exhibits a much larger specific surface area than ZnO
for applications using artificial light (Table 2). However numerous
works have reported that ZnO presents a better efficiency for
catalyzing the removal of organic pollutants than that exhibited by
TiO2 for solar applications [20,39,40]. It is due to large fraction
absorption of the solar spectrum [26]. In addition, electronic
mobility in ZnO is much higher, indicating lower charge
recombination [41]. In addition, ZnO has the disadvantage of
undergoing photodissolution (induced by self-oxidation). Howev-
er, this problem can be solved by adjusting the pH of solution to
about 9.5 [32].
Although decay in the DOC was observed during the irradiation
time, 11% and 8% of the organic carbon was still present after
60 min in ZnO and TiO2, respectively. These percentages should be
due to the formation of non-degradable organic transformation
products produced during the experiments and/or other organic
compounds present in the commercial insecticide sample.

30
J. Fenoll et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2014) 24–32
Fig. 5. Degradation kinetics of the pirimicarb and quantitative analysis of derivatives detected in water by photolysis and heterogeneous photocatalysis (TiO2 P25 Degussa,
ZnO, ZnTiO3 and Zn2TiO4) during the photoperiod.
3.4. Identification and evolution of the organic transformation
products
identification of the most relevant is a very important task to
predict the environmental impact of the original compound or
to evaluate the efficiency of the photocatalytic treatment. In order
to investigate and elucidate the yielded transformation products
during pirimicarb degradation, concentrated SPE extracts of the
samples, collected at different illumination times, were analyzed
by HPLC-MS2 and HPLC-TOF/MS. Four transformation products
(Metabolites I, II, III, V) were isolated and identified by
comparison with authentic analytical standards using quantita-
tive HPLC-MS2. These transformation products were also identi-
fied in published papers [42–44].
In addition, after detailed examination of all the peaks present
in the chromatograms obtained by HPLC-TOF/MS, five compounds
(Metabolites I, II, III, IV, V) were identified as possible transforma-
tion products based on their accurate mass and the information
provided by the system software, which calculates the elemental
The transformation of pirimicarb under photolysis or photo-
catalysis processes leads to the formation of several products. The
Table 3
Kinetic parameters for photocatalysis (TiO2, ZnO, Zn2TiO4 and ZnTiO3) of pirimicarb
in water under artificial light.
0 t
Semiconductor
R2
Sy/x
t1/2 (min)
a
Ct ¼ C0 ek
t
t
TiO2
ZnO
Zn2TiO4
ZnTiO3
Ct = 100.7 eꢂ1.155
1.000
0.978
0.966
0.958
0.11
5.86
3.31
6.03
0.6
3.8
67.9
37.7
Ct = 93.6 eꢂ0.183
Ct = 98.9 eꢂ0.010
t
t
Ct = 105.7 eꢂ0.018
a
Standard deviation of the fitting (standard error of estimate).

J. Fenoll et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2014) 24–32
31
Fig. 6. Photocatalytic degradation pathways of pirimicarb. See Table 1 for identification of derivative compounds (dotted arrows connect transformation products not
identified).
composition and empirical formula of protonated molecules, with
an accuracy threshold of 5 ppm (standard for unknown identifica-
tion) and the number of rings and double bonds present in the
of principal transformation products was also followed during the
irradiation experiment (Fig. 5). The rapid transformation of
pirimicarb was accompanied by the appearance of five transfor-
mation products. On the one hand, TiO2 and ZnO treatments led to
similar evolution profiles for most of the obtained transformation
products but different from those observed for ZnTiO3 and Zn2TiO4
catalysts whose effect on the metabolite profile were similar to
that showed with the photolysis treatment. The most abundant
transformation product was Metabolite I followed by Metabolite V
and Metabolite IV. Metabolite I was detected in concentrations of
molecule (DBE, double-bond equivalency). Therefore,
a new
compound (Metabolite IV) was identified as possible transforma-
tion product in addition to those identified by HPLC-MS2. Table 1
shows summary data related to the experimental and calculated
masses of the protonated ion, the error between them and the DBE.
Experimental and calculated masses of the protonated ion
corresponding to the transformation product identified (Metabo-
lite IV) were 239.1136 and 239.1139, respectively. Accurate mass of
transformation product was found with an error of less than
0.93 ppm, thus providing a high degree of certainty in the
assignment of formula (C10H14N4O3). In addition, MS/MS experi-
ment was conducted through the selection of the ions m/z 72 and
166. We suggest the tentative assignment of this formula with the
compound 2-(formylamino)-5,6-dimethylpyrimidin-4-yl dime-
thylcarbamate. This compound was also identified by Pirisi et al.
[42] in aqueous solutions or solid phase of pirimicarb exposed to
sunlight or lamps in the laboratory. Other transformation products
as 2-amino-5,6-dimethylpyrimidine-4-yl dimethylcarbamate
(Metabolite VI), 2-amino-5,6-dimethyl-4-hydroxypyrimidine (Me-
tabolite VII), 2-(formylamino)-5,6-dimethyl-4-hydroxypyrimidine
(Metabolite VIII) and 2-[(methylformyl) amino]-5,6-dimethyl-4-
hydroxypyrimidine (Metabolite IX) were not detected by HPLC-
TOF/MS.
15
15
m
m
g Lꢂ1 for ZnO (after 5 min), 39
m
g Lꢂ1 for TiO2 (after 2 min),
g Lꢂ1 for Zn2TiO4 (after
g Lꢂ1 for ZnTiO3 (after 60 min) and 11
m
60 min). In the case of TiO2, only Metabolite I was found in the
medium at the end of the experiment (60 min). Observed
differences in the concentration of the transformation products
found for ZnO–TiO2 systems may be due to the different reaction
rates obtained. Based on the results, a possible photocatalytic
degradation pathway of pirimicarb consisting of several steps was
proposed in Fig. 6. As can be seen, all the transformation products
are formed in mainly two different ways: In the first case, through
successive N-dealkylation of the 2-dimethylamino group to form
the carbamate (step A). In the second case, through decarbamoy-
lation of the carbamate moiety with further N-dealkylation of the
2-dimethylamino group to have the hydroxypyrimidines (step B).
4. Conclusions
Little information is available comparing the photodegradation
route of pirimicarb in aqueous suspension of ZnO–TiO2 systems. In
order to better understand the reaction mechanisms involved in
the photocatalytic degradation of pirimicarb, the kinetic evolution
The photodegradation of pirimicarb in aqueous suspensions of
binary (ZnO and TiO2) and ternary (Zn2TiO4 and ZnTiO3) oxides of
zinc and titanium under artificial light (300–460 nm) irradiation

32
J. Fenoll et al. / Journal of Photochemistry and Photobiology A: Chemistry 298 (2014) 24–32
was investigated. Pirimicarb was totally degraded in aqueous
suspensions of ZnO and TiO2 under artificial light irradiation
(300–460 nm) after 60 and 30 min, respectively. In general, the
efficiency of the catalysts in the photooxidation of pirimicarb was
in the order: TiO2 > ZnO > ZnTiO3 > Zn2TiO4. However, the process
using ZnO would present drawbacks in comparison with TiO2 since
the pH must be adjusted to precipitate the Zn2+ and then be
neutralized to be released in the environment, as it is the case with
Fe in photo-Fenton process. Five transformation products were
identified by HPLC-MS2 and HPLC-TOF/MS during the experiments.
Based on this derivative identification, a simple degradation
pathway, mainly trough dealkylation and decarbamoylation, is
proposed for the parent compounds.
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Acknowledgements
We acknowledge financial support from the Ministerio de
España de Ciencia e Innovación (Project AGL2010-20458-C02-01),
Ramón and Cajal Subprogram and the Instituto Nacional de
Investigación y Tecnología Agraria y Alimentaria (Project RTA2011-
00022-00-00). We are also grateful to Inmaculada Garrido, Juana
Cava, María V. Molina and María J. Yáñez for technical assistance
and to FEDER and European Social Funds.
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