Photocatalytic mitigation of triazinone herbicide residues using titanium dioxide in slurry photoreactor

Article abstract of DOI:10.1016/j.cattod.2014.12.011

Heterogeneous photocatalysis offers an alternative for the treatment of wastewater containing refractory pollutants. The photocatalytic degradation of metamitron and metribuzin, asymmetrical triazine compounds used worldwide as herbicides, was investigate

Full text of DOI:10.1016/j.cattod.2014.12.011

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Photocatalytic mitigation of triazinone herbicide residues using
titanium dioxide in slurry photoreactor
N. Velaa, J. Fenollb, I. Garridoc, G. Navarroc, M. Gambínc, S. Navarroc,∗
a Universidad Católica de Murcia, Campus de Los Jerónimos, s/n. Guadalupe, 30107 Murcia, Spain
b Instituto Murciano de Investigación y Desarrollo Agrario y Alimentario (IMIDA), C/ Mayor s/n. La Alberca, 30150 Murcia, Spain
c 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 11 July 2014
Received in revised form 17 October 2014
Accepted 10 December 2014
Available online xxx
Heterogeneous photocatalysis offers an alternative for the treatment of wastewater containing refrac-
tory pollutants. The photocatalytic degradation of metamitron and metribuzin, asymmetrical triazine
compounds used worldwide as herbicides, was investigated in aqueous suspension of different commer-
cial TiO2 nanopowders with different composition of anatase/rutile phases, using artificial UV-A light. In
addition, we have studied the role of the most important operating parameters (catalyst loading, effect of
electron acceptor, pH, light intensity, initial concentration of pollutants, and interfering substances) on
the photooxidation of both herbicides. TiO2 P25 (70% anatase/30% rutile) was found to be highly active
for herbicide decomposition. The progress of photocatalytic degradation of both herbicides has been
observed by monitoring the change in substrate concentration using HPLC/MS2. Results showed that the
addition of TiO2 in tandem with an electron acceptor like Na2S2O8 strongly enhances the degradation
rate of metamitron and metribuzin in comparison with photolytic test. The degradation of these herbi-
cides followed a pseudo-first order kinetics. The time required for 50% degradation was lower than 8 min
for both. Thus, complete disappearance was achieved after 30 min of illumination in the TiO2/Na2S2O8
system. Both herbicides were rapidly deaminated.
Keywords:
Assymetrical triazines
Heterogeneous photocatalysis
Metamitron
Metribuzin
Titania
Water detoxification
© 2014 Elsevier B.V. All rights reserved.
1. Introduction
be seriously contaminated by some of them. Triazine parent com-
pounds and their metabolites have been detected in surface and
Modern agriculture depends to a large degree on the use of pes-
ticides. All pesticides are subject to an approval procedure under
EU legislation, and often under national legislation. Plant Protection
Products Regulation 1107/2009 and the Sustainable Use Directive
2009/128/EC are essential elements in this context. However, some
risks are inevitable because the use of pesticides has an impact on
the natural environment through spray drift, leaching and run-off
into water, or even affect non-target organisms.
The pollution of water bodies with agrochemical products can
pose a significant threat to aquatic ecosystems and drinking water
resources. More specially, triazine compounds have been among
the most widely used pesticides in recent decades [1]. As a conse-
quence, their residues are the most commonly recovered in water.
The decrease in triazine usage in Europe is chiefly due to legal pro-
visions limiting the use of these products. The banning of triazines
in the EU was precipitated mainly by the fact that water sources can
groundwater in several European countries and USA [2–4]. How-
ever, other authorized compounds of this class, such as metribuzin
and metamitron (asymmetrical triazine class), which are used
worldwide as herbicides, are chemically stable and can penetrate
through the soil and cause long-term pollution of ground water [5].
The triazinones forms a small group of herbicides developed
in the 1970s for the pre- and postemergence control of grasses
and broad-leaved weeds. Both metribuzin and metamitron inhibit
photosystem II and are similar to s-triazines in this respect. The as-
triazines are not acutely toxic but metribuzin causes liver toxicity at
high doses in mice. Both compounds are deaminated photochem-
ically, chemically, and biochemically [6]. The S-methyl group of
metribuzin compared with the methyl group of metamitron is the
most important distinguishing feature between the two herbicides.
Both are highly water soluble and adsorption in low-organic sandy
soils is rather weak.
Numerous studies have demonstrated that pollutants can be
removed from wastewater by advanced oxidation processes (AOPs)
such as O3/UV, H2O2/UV, H2O2/O3/UV, homogeneous photo-
Fenton, and heterogeneous photocatalysis [7,8]. AOPs have been
∗
Corresponding author. Tel.: +34 868 887477; fax: +34 868 884148.
E-mail address: snavarro@um.es (S. Navarro).
http://dx.doi.org/10.1016/j.cattod.2014.12.011
0920-5861/© 2014 Elsevier B.V. All rights reserved.
Please cite this article in press as: N. Vela, et al., Photocatalytic mitigation of triazinone herbicide residues using titanium dioxide in
slurry photoreactor, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2014.12.011

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Power supply
broadly defined as near ambient temperature treatment processes
based on highly reactive radicals, especially the hydroxyl radical
(•OH), as the primary oxidant while the other radical and active
oxygen species are superoxide radical anions (O2•−), hydroper-
oxyl radicals (HO2•−), triplet oxygen (3O2), and organic peroxyl
radicals (ROO−). Clearly, the •OH radical (E0 = 2.8 V) is among the
strongest oxidizing species used in water and wastewater treat-
ment and offers the potential to greatly accelerate the rates of
contaminant oxidation. Among AOPs, heterogeneous photocata-
lysis is a process of great potential for pollutant abatement in
water. The basic principles of this process have been extensively
reviewed in the recent literature [9–13]. This treatment allows
the degradation and in most cases, the complete mineralization
of some triazine herbicides [14,15]. The photocatalytic degradation
of these compounds is largely dependent on the mass, composition
and type of photocatalyst, pollutant type and initial concentration,
interfering substances, solution pH, wavelength and light intensity
and oxidizing agents/electron acceptors. An ideal photocatalyst is
characterized by photo-stability, biologically and chemically inert
nature, low cost and availability, and capability to adsorb reac-
tants under efficient photonic activation (hv ≥ Eg). TiO2 (mainly
anatase/rutile mixture) is the most widely photocatalyst studied
[16].
Thermometer
pH-meter
Air
Reagents
Glass water-jacketed reactor
Cooling
water
oulet
Water inlet
Circulaꢀon bath
Lamp
Jacket
Cooling
water
inlet
Water outlet
In this view, the aim of this paper was to assess the photoactivity
of TiO2 on the degradation of two triazinone (as-triazine) herbi-
cides, metamitron and metribuzin, under artificial light. For this
purpose, we have studied the role of the most important operating
parameters (proportion of anatase/rutile, catalyst loading, effect of
electron acceptor, pH, light intensity, initial concentration of pol-
lutants, and interfering substances) on the photooxidation of both
herbicides.
Magneꢀc sꢀrring bar
Sampling valve
Pump
Fig. 1. Schematic drawing of the photochemical reactor.
2. Materials and methods
2.3. Photocatalysis experiment using artificial light
2.1. Herbicides and reagents
The photocatalytic and photolytic experiments were performed
in a 2000 mL cylindrical glass (250 mm long, 100 mm diameter)
photochemical reactor (SBS, Barcelona, Spain) equipped with a
magnetic stirring bar, and a 8 W low pressure mercury lamp.
The diagram of the experimental system used is shown in Fig. 1.
The irradiated volume of suspension (Vreactor) was 1660 mL. The
intensity of the light was approximately 10 mW cm−2 with major
emission output at 366 nm (based on manufacturer’s data). The
photon flux from the lamp was controlled on the wall of the reactor
using a portable photoradiometer Delta Ohm HD 2102.2 (Caselle di
Selvazzano, Italy) fitted with a UVA sensor (range 315–400 nm).
The study was carried out in a batch recirculation mode
(600 mL min−1). The reaction system was bubbled with air
every 10 min, because dissolved oxygen acts as electron sink
in the photodegradation process, forming the superoxide rad-
ical (O2 + e− → O2•−) and avoiding the recombination of h+/e−.
The system was continuously stirred to achieve a homogeneous
suspension and thermostated by circulating water to keep the tem-
perature at 23 1 ◦C during the time of irradiation.
Metamitron (MM) and metribuzin (MB) with purity >98% were
purchased from Dr Ehrenstorfer (Augsburg, Germany). Titanium
dioxide anatase (99.9%), titanium dioxide mixture of rutile and
anatase (99.5%) and titanium dioxide P25 Degussa (99.5%) were
purchased from Alfa Aesar (Karlsruhe, Germany), Sigma–Aldrich
Química S.A. (Madrid, Spain) and Nippon Aerosil Co. Ltd. (Osaka,
Japan), respectively. Sodium peroxydisulfate (98%) was purchased
from Panreac Química (Barcelona, Spain). Acetonitrile was supplied
by Scharlau (Barcelona, Spain).
2.2. Photocatalyst characterization
The photocatalysts were characterized by
Lambda 750S UV/VIS spectrophotometer equipped with
a PerkinElmer
a
PerkinElmer 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 crys-
talline structure of the photocatalysts 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␣ radia-
tion at a scanning speed of 1◦ (2ꢀ). 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 semi-
conductors was measured according to the BET method by nitrogen
adsorption–desorption isotherms at 77 K on a Micromeritics Tristar
3000 instrument (Micromeritics Instruments Co., USA).
Initially, 2000 mL of type II analytical-grade water (pH 7.1, ORP
225 mV, resistivity >5 Mꢁ cm (25 ◦C); conductivity <1.09 ␮S cm−1
,
TOC <30 ␮g L−1; microorganisms <10 cfu mL−1) were mixed with
both active ingredients (at 0.2 mg L−1 of each herbicide), homog-
enizing the mixture for 20 min in the dark. After this time the
appropriate amount of catalyst was added to the reaction solu-
tion. The mixture was maintained for 30 min in the dark prior to
illumination in order to achieve the maximum adsorption of the
herbicides onto the semiconductor surface. After this time Na2S2O8,
used as electron acceptor, was added. Several samples were taken
during the illumination period (60 min). Two parallel blank assays,
without semiconductor and oxidant (photolytic test) and only
Please cite this article in press as: N. Vela, et al., Photocatalytic mitigation of triazinone herbicide residues using titanium dioxide in
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Table 1
3.2. Effect of crystalline phase and catalyst loading on the
herbicide decomposition
Physico-chemical properties of TiO2 photocatalysts.
Photocatalysts
The photooxidation of both herbicides in the absence/presence
of the studied photocatalysts is presented in Fig. 2(A) and (B). It
is clear that the photocatalytic degradation of the herbicides in
the presence of the studied oxides (200 mg L−1) leads to the dis-
appearance of these substances although at different rates. Under
irradiation (366 nm), the herbicides degraded very slowly com-
pared with the reaction rates obtained using oxides. By the end
of the photolytic test (120 min) less than 10% of the initial amount
(200 ␮g L−1) was removed. As clearly seen in Fig. 2, TiO2 Degussa
P25 and TiO2 Alfa Aesar were highly active in the photodecompo-
sition of the studied herbicides, yielding more than 80% conversion
after 120 min of reaction. On the contrary, when TiO2 Sigma Aldrich
was used the degradation of both herbicides was lower than 20%
after the same reaction time. This behaviour can be due to the
smaller band gap of TiO2 Sigma Aldrich (3.0 eV) and surface area
(24 m2 g−1) than the other TiO2 samples. Regarding the two more
active catalysts, Degussa P25 and Alfa Aesar, the former owes its
activity to a slow electron/hole recombination rate, while the high
photoreactivity of the latter is due to a fast interfacial electron
transfer rate which is typical of pure anatase [18]. Both mecha-
nisms appear to play a critical role for herbicide degradation and
this would explain the nearly activities of both shown in Fig. 2.
Nonetheless, TiO2 P25 has larger specific surface area and lower
particle size than TiO2 Alfa Aesar (Table 1) and this may explain
its increased activity. This is so since the rates of electron transfer
amongst electrons, holes and the reactants increase with increasing
specific surface area particularly at conditions of slow electron/hole
recombination [19].
P25 Degussa
Sigma–Aldrich
Alfa-Aesar
Composition
Anatase (%)
Rutile (%)
70
30
<21
55
25
75
<100
24
90
10
<32
45
Particle size (nm)
BET surface (m2 g−1
)
Band-gap energy (eV)
3.1
3.0
3.2
oxidant were carried out. In all cases, assays were replicated three
times.
2.4. Analytical determinations
Water samples were extracted and analyzed according to the
procedure described by Fenoll et al. [17]. Briefly, the procedure
involves initial single phase extraction of samples with acetonitrile
by sonication, followed by liquid-liquid partition aided by “salt-
ing out” process using NaCl. Finally, the residue was redissolved in
acetonitrile, and analyzed by HPLC-MS2.
The identification of MM and MB and their transformation
products was carried out using an HPLC system (Agilent Series
1100, Agilent Technologies, Santa Clara, CA, USA) equipped with
a reversed phase C8 analytical column of 150 mm × 4.6 mm and
5 ␮m particle size (Zorbax Eclipse XDB-C8). The HPLC system is
connected to a TOF/MS (Agilent Technologies) with an electro-
spray interface. The mobile phases A and B were acetonitrile and
0.1% formic acid, respectively. The gradient program started with
10% A, constant for 5 min, followed by a linear gradient to 100% A
after 35 min. After this 35 min run time, 10 min of post-run time
followed using initial 10% of A. The flow rate was held constant
(0.6 mL min−1) during the whole process and 20 ␮L of samples were
injected in every case.
At the end of the irradiation time the remaining percentages
using TiO2 P25 were lower than 10% for both herbicides (Fig. 2).
The higher photoactivity of TiO2 P25 Degussa is well known and it
can be attributed to its crystalline composition of rutile and anatase,
surface area and particle size. Anatase is generally regarded as the
more photochemically active phase of titania, presumably due to
the combined effect of lower rates of recombination, higher sur-
face adsorptive capacity and higher possibility to adsorb oxygen by
higher density of superficial hydroxyl groups [20–22]. In view of
these results, all subsequent assays were done with TiO2 Degussa
P25.
LC-MS2 analyses were performed under the same conditions
used in TOF/MS. Table S1 (Supplementary material) lists the ana-
lytical conditions for both system.
2.5. Statistical analysis
On the other hand, the influence of the photocatalyst concen-
tration on the disappearance kinetics of both herbicides has been
investigated using different concentrations of TiO2 P25. As can be
seen in Fig. 2(C) and (D), significant differences were observed
in the reaction rate when the concentration of the catalyst was
increased from 50 to 300 mg L−1. The best results were obtained
with 200 mg L−1 because higher levels may cause light scattering
and agglomeration of the catalyst, resulting in the loss of the surface
area available for light absorption.
In slurry photocatalytic processes, the amount of photocatalyst
is an important parameter that can affect the degradation rates of
the organic compounds. The reaction rate is directly proportional to
the mass of catalyst. However, according to Herrmann [9] above a
certain value the reaction rate levels off and becomes independent
of the concentration of the catalyst depending on the geometry
and the working conditions of the photoreactor. This plateau fits
with the maximum amount of catalyst in which all the particles are
totally illuminated, after this limit a screening effect excess particle
occurs.
The curve fitting and statistical data were obtained using
SigmaPlot version 12.0 statistical software (Systat, Software Inc.,
San Jose, CA).
3. Results and discussion
3.1. Catalysts properties
Table 1 presents the composition, particle size, BET surface area
and band gap energy (Eg) of the studied photocatalysts. The Eg of
the catalyst was estimated by plotting [F(R)hv]1/2 vs [hv–Eg], where
F(R) = (1 − R)2/(2R), h is Plank’s constant, Eg is the 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 was in the
range of 3.0–3.2 eV for TiO2 samples.
Figure S1 (Supplementary material) shows the XRD patterns
of TiO2 samples. The content of anatase (A) and rutile (R)
phases in TiO2 nanopowders was 70A:30R (P25 Degussa), 30A:70R
(Sigma–Aldrich) and 90A:10R (Alfa-Aesar). In comparison, the
recovered SEM images (Figure S2, Supplementary material) shows
that particle size of TiO2 30A:70R is about 3–5 times greater and
BET surface 2 times smaller than the other TiO2 samples.
3.3. Influence of electron acceptor and light intensity
In order to examine the role of peroxydisulfate, experiments of
the photocatalytic degradation of both herbicides using different
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Fig. 2. Evolution of metamitron and metribuzin residues in aqueous slurry of TiO2 (200 mg L−1) with different proportions of anatase and rutile (A and B) and effect of the
catalyst (TiO2 P25 Degussa) loading on their degradation (C and D).
initial concentrations of the oxidant were conducted. The best
results were obtained with 100 mg L−1 in both cases as depicted
in Fig. 3(A) and (B) where the effect of adding Na2S2O8 on the
disappearance kinetics of both herbicides was studied using
several concentrations of Na2S2O8 (50–200 mg L−1).
3.4. Effect of pH and herbicide concentration
The surface charge of the photocatalyst and the ionization (pKa)
of the herbicides may be strongly affected by the solution pH. MM is
not dissociated while MB has a pKa = 1. Therefore, at pH above this
value the herbicide attains a negative charge. Electrostatic interac-
tion between a semiconductor surface, substrate, solvent molecules
and charged radicals formed during photocatalytic oxidation are
profoundly influenced by the solution pH [12,26]. Fig. 4(A) and (B)
shows the influence of the initial pH value on the photodegradation
rate of the studied herbicides for the TiO2 P25 suspensions. In the
illuminated TiO2 system, the greatest degradation rate occurred at
pH 6–7, which is near its point of zero charge (PZC). The PZC (the pH
at which the surface exhibits a neutral net electrical charge) of tita-
nia is quite insensitive to the crystallographic structure (anatase
vs rutile) and the experimental method used. The recommended
value is pH 5.9 for both polymorphs [27]. Although the PZC of TiO2
depends on the production method the most frequent value for TiO2
P25 is 6.3 [28], below or above which the catalyst surface is pos-
itively or negatively charged, respectively. The positive holes are
considered as the major oxidation step at low pH, whereas hydroxyl
radicals are considered as the predominant species at neutral or
high pH levels [29].
The electron–hole recombination is one of the main drawbacks
in the application of semiconductor photocatalysis as it causes
waste of energy. In the absence of suitable electron acceptor,
recombination step is predominant, and thus, it limits the quan-
tum yield [12]. Na2S2O8 traps the photogenerated electron and
reduces the probability of recombination with the positive hole,
•−
generating SO4 radicals, which are also a very strong oxidizing
species (reduction potential of SO4•− E0 = 2.6 V), and more hydroxyl
radicals.
On the other hand, the dependency of pollutant degradation rate
on the light intensity has been studied in numerous investigations
of many pesticides [12]. The electron–hole formation in the photo-
chemical reaction is strongly dependent on the light intensity [23].
Fig. 3(C) and (D) shows the effect of light intensity on the pho-
todegradation of MM and MB. As can be seen, a small increase in
the reaction rate was observed for both herbicides when using two
lamps of 10 mW cm−2 for each one. As reported by some authors
[24], at high intensity, the reaction rate is independent of light
intensity. It has been also reported that the rate is proportional
to the radiant flux for ˚ < 25 mW cm−2. Above this value, the rate
is shown to vary as ˚1/2, indicating a too high value of the flux and
an increase of the electron–hole recombination rate [25].
On the other hand, Fig. 4(C) and (D) shows the effect of initial
concentration of MM and MB in the TiO2 P25/S2O8 system. Reac-
tion rate of both herbicides significantly decreases with increasing
initial concentration from 0.2 to 1.0 mg L−1
.
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1.2
1.2
METRIBUZIN
=
METAMITRON
B
A
=
50 S O
2 8
50 S
O
2
8
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
=
=
=
=
=
=
100 S
150 S
200 S
O
100 S
150 S
200 S
O
8
2
2
2
8
2
2
2
O
O
8
8
O
O
8
8
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Time (min)
Time (min)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
METAMITRON
METRIBUZIN
C
D
1 lamp
2 lamps
1 lamp
2 lamps
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Time (min)
Time (min)
Fig. 3. Effect of electron acceptor (A and B) and light intensity (C and D) on the degradation of herbicides using TiO2 P25 Degussa (200 mg L−1) as photocatalyst.
3.5. Effect of interfering substances
inhibitive effect of some anions in the order SO4 < NO3− < Cl−
during the photolytic degradation of diazinone and imidacloprid
attributing the detrimental effect to their competition for the active
sites on the TiO2 surface and catalyst deactivation which subse-
quently, decreased the reaction rate. Moreover, ions such as Ca2−+
and Mg2+ [31], Fe2+ [32], Mn2+, Zn2+, and Cu2+ [12,33] or NO2
As reviewed by Ahmed et al. [12] a number of studies have
demonstrated that water components like calcium, magnesium,
iron, zinc, copper, bicarbonate, phosphate, nitrate, sulphate, chlo-
ride, and dissolved organic matters can affect the photodegradation
rate of organic pollutants since they can be adsorbed onto the sur-
face of TiO2. To evaluate the effect of water matrix, 0.2 mg L−1 of
each herbicide were spiked in tap water whose composition is
shown in Table 3. The results plotted in Fig. 5 show that reac-
tion rates significantly increase in tap water in comparison with
deionised water especially for metribuzin. No significant differ-
ences were observed in the runs carried out with tap water
regarding the content of cations and anions except for bicarbon-
ate and sulphate. In the case of bicarbonate, the final concentration
falls to 113.8 mg L−1 while sulphate increases its concentration t−o
298.5 mg L−1 after 60 min of irradiation. The decrease of HCO3
is due to its transformation in CO2 while the increase of SO4 is
due to effect of S2O8 added according to the following reactions:
S2O8 + e− → 2SO4•− → SO4•− + H2O → SO4 + •OH + H+. As a conse-
quence, a little increase in electrical conductivity was observed at
the end of irradiation time.
(0.04 mg L−1) and NO3 (2.15 mg L−1) [34–37] can also enhance.
−
Nitrite and nitrate absorb light and undergo homolysis to pro-
duce •OH radicals and nitrogen reactive species (NO•, NO2•, N2O3,
and N2O4) leading to the degradation of pesticides•, although •OH
−
may further be scavenged by NO2 to form NO2 [34–36]. The
NO3−/NO2 photo-induced degradation (mainly NO3−) of three
−
chlorophenylurea herbicides (chlorotoluron, diuron and linuron)
in water has been demonstrated to occur quite efficiently under
solar irradiation [37]. Similar findings were obtained in the case
of fluometuron since a greater increase in the reaction rate was
reported after the addition of nitrate ions to distilled water [38].
On the other hand, the photocatalytic effect of dissolved organic
matter (DOM) is contradictory in the literature [39,40]. The sunlight
absorbance by DOM could provide a rich variety of photochemi-
cals reactions resulting in reactive transients produced mainly by
humic substances (HS) which could participate in energy and elec-
tron transfer and free radical generation. On the contrary, HS can
reduce the pesticide photodegradation because the sensitization
effect is hidden by the strong filter effect (quenching). They could
act as one of the important sunlight-absorbing (especially on UV
range), they undergo photolysis under incident UV–vis light and
The presence of ions either enhances or decreases the reaction
rate depending on the mechanism. Most of the anions and cations in
tap water such as chloride (51.9 mg L−1), sulphate (213.8 mg L−1),
sodium (87 mg L−1) or potassium (4.6 mg L−1) are transparent to
solar irradiation. However, Mahmoodi et al. [30] observed an
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1.2
1.2
METRIBUZIN
METAMITRON
pH 5
pH 6
pH 7
pH 8
B
A
pH 5
pH 6
pH 7
pH 8
1.0
0.8
0.6
0.4
0.2
0.0
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Time (min)
Time (min)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
1.2
1.0
0.8
0.6
0.4
0.2
0.0
METAMITRON
METRIBUZIN
C
D
0.2
0.5
1
0.2
0.5
1
0
10
20
30
40
50
60
0
10
20
30
40
50
60
Time (min)
Time (min)
Fig. 4. Effect of pH (A and B) and herbicide concentration (C and D) on the degradation of herbicides using TiO2 P25 Degussa (200 mg L−1) as photocatalyst.
they can also quickly react with •OH radicals. According to Kon-
Table 2
Kinetic parameters calculated for the TiO2 P25 (200 mg L−1)/S2O8 (100 mg L−1
system.
)
stantinou et al. [39], the presence of HS in the lake, river, and marine
water samples (high content of HS) reduces photodegradation rates
of triazine compounds in comparison with the distilled and ground
water (low content of HS). In our case, the drinking water used had
a small DOM value (1.5 mg L−1). As a consequence, the quenching
can be considered negligible.
app t
Parameter (C = C0e−k
)
Metamitron
Metribuzin
C/C0
1.050***
0.062**
0.956***
0.095
1.016***
0.107***
0.977***
0.063
kapp (min−1
)
R2
Sy/x
TD50/TD90 (min)
8/27
7/22
3.6. Photocatalytic activity of P25 Degussa and kinetics
**
P < 0.01.
P < 0.001.
***
Although an exhaustive kinetic analysis was outside of this
work, an attempt was made to describe degradation kinetics. At
low substrate concentrations, the photocatalytic degradation rate
of organic compounds, including pesticides, is commonly described
by a pseudo-first order kinetic expression, which is rationalized in
terms of the Langmuir–Hinshelwood (L-H) model:
The decomposition of MM and MB fitted reasonably well with
the exponential decay curve, following first-order behaviour. The
kinetic parameters of both compounds obtained in the selected
conditions (200 mg L−1 of TiO2 and 100 mg L−1 of Na2S2O8) are
shown in Table 2 where the apparent rate constants and half-
lives are listed. As shown, the R2 values were ≥0.96 for both with
standard error of the estimate (Sy/x) lower than 0.1. The DT90 (the
time needed for disappearance of 90% of the herbicide under lab-
oratory conditions) in the TiO2 P25/Na2S2O8 system were 27 min
and 22 min for MM and MB, respectively.
dC
−
= kapp[C]n
dt
For n = 1, integration gives:
t
C = C0e−k
or ln C0/Ct = kapp
t
app
where kapp (in units of time−1) is the apparent reaction rate con-
stant, t the reaction time, C0 the initial concentration of the target
compound in aqueous solution and Ct is the residual concentration
of pesticide at time t. First-order degradation rate constants were
determined by regression analysis.
3.7. Identification and evolution of the organic intermediates
In order to better understand the reaction mechanisms involved
in the photocatalytic degradation of MM and MB, the kinetic
evolution of principal intermediates was also followed during
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7
1.2
METAMITRON
Deionised water
Tap water
1.0
0.8
0.6
0.4
0.2
0.0
0
10
20
30
40
50
60
Fig. 6. Photodegradation kinetics of metamitron (MM) and metribuzin (MB) and
quantitative analysis of derivatives, deamino-metamitron (DA MM) and deamino-
metribuzin (DA MB) detected in water by heterogeneous photocatalysis using TiO2
P25 Degussa/Na2S2O8 during the photoperiod.
Time (min)
1.2
1.0
0.8
0.6
0.4
0.2
0.0
METRIBUZIN
Deionised water
Tap water
the irradiation experiment (Fig. 6). Several samples from 0 to
240 min were taken during the illumination period. Both herbi-
cides were rapidly deaminated. After detailed examination of all
the peaks present in the chromatograms obtained by HPLC-TOF/MS,
two compounds were identified as possible photo-transformation
products (PTPs). In addition, MS/MS experiment was conducted
through the selection of two transitions for each PTP.
A peak, holding [M+H]+ 188 has been detected at retention
time of 13.85 min. The theoretical accurate mass of this specie
+
(187.0752) gave a unique elemental composition of C10H9N3O2
(error 3.29 ppm). The difference of 15 amu with respect to the par-
ent molecule numerically corresponds to a •NH loss and it can
be assigned to deamino-metamitron (DA MM). Specie having pro-
tonated molecular ions of m/z 200 was eluted at retention time
18.20 min suggesting a deamination of MB. The calculated mass
of this ion and the error on this assignment were 199.0779 and
−0.38 ppm, respectively, indicating that the calculated elemental
composition (C8H13N3OS+) can be considered reliable. In addi-
tion, MS/MS experiment was conducted through the selection of
the ions m/z 172 and 116. We suggest the tentative assignment
of this formula with deamino-metribuzin (DA MB). Other deriva-
tives such as diketo-metribuzin (C7H12N4O2), deaminodiketo-
metribuzin (C7H11N3O2), hydroxy-metamitron (C10H9N4O2), and
deaminohydroxy-metamitron (C10H9N3O2) were not found during
the irradiation time.
0
10
20
30
40
50
60
Time (min)
Fig. 5. Influence of water matrix (deionised vs tap water) on the photodegradation of
metamitron and metribuzin using TiO2 P25 Degussa (200 mg L−1) as photocatalyst.
Table 3
Composition of tap water used in this study (n = 3).
Parameter
pH
8.38
0.76
1.48
EC (dS m−1
)
DOC (mg L−1
)
mg L−1
51.9
0.2
BDL
0.04
2.1
163.7
213.8
BDL
3.8. Conclusions
Cl−
Br−
F−
As conclusion we can affirm that the use of TiO2 P25 Degussa in
tandem with an electron acceptor like Na2S2O8 in the conditions
used in this study is an effective and rapid method to remove MM
and MB residues, triazinone compounds used worldwide as herbi-
cides, and their main phototransformation products (DA MM and
DA MB) in water.
−
NO2−
NO3
−
HCO3
SO4
PO4
2−
3−
Na+
K+
87.0
4.6
61.8
25.8
Ca2+
Mg2+
Acknowledgements
The authors are grateful to FEDER and European Social Funds
and the Ministerio de Espan˜a de Ciencia e Innovación through the
Ramón and Cajal Subprogram and the Instituto Nacional de Inves-
tigación y Tecnología Agraria y Alimentaria through the project
RTA2011-00022-00-00 for financial support. Thanks are also given
to Dr. J. Yán˜ez for recording of SEM images.
Fe
0.007
0.005
0.010
0.950
Cu
Mn
Zn
BDL, below detection limit.
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Please cite this article in press as: N. Vela, et al., Photocatalytic mitigation of triazinone herbicide residues using titanium dioxide in
slurry photoreactor, Catal. Today (2015), http://dx.doi.org/10.1016/j.cattod.2014.12.011