American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com
36
|El Mouraille Nadia
1
| Belmouden Moustapha *
1
| and | Ait Ichou Yahia
1
|
1.
Ibn Zohr University | Department of Chemistry | Agadir | Morocco |
| Received | 01 July 2018 | | Published 26 July 2018 | | ID Article | Nadia-ManuscriptRef.2-ajira040718 |
ABSTRACT
Background: The use of pesticides massively in agriculture is considered among the causes of pollution most probable of surface and
ground water. These organic compounds are very hazardous and harmful to health, as most are toxic and carcinogenic, even at low
concentrations. For these reasons, in recent years, environmental regulations in many countries have become strict for the production
of drinking water and wastewater treatment, in particular concerning pollution by pesticides. Objective: The removal of pesticides
from contaminated water is an extremely complex problem due to the wide range of pesticides of multiple chemical structures and
properties. Among the unconventional methods of treatment of polluted water, heterogeneous photocatalysis has been used for the
treatment of several pollutants such as pesticides. The objective of this work is the evaluation of this method for the elimination of a
pesticide under different operating conditions Methods: Photocatalytic degradation of aqueous commercial pesticide "Mythos" solution
has been tested by using TiO
2
as a photocatalyst and a new batch reactor using low voltage UV lamps as an irradiation source. The
influence of some parameters such as the presence of UV irradiation, aeration, pH, pesticide concentration and time of reaction was
examined. The effect of some inorganic anions, such as Cl-, SO
4
2-
and NO
3
-
, commonly present in real effluents, on the photocatalytic
degradation, was also studied. Results: The degradation rates were found to be strongly influenced by all the above parameters,
except pH which has a moderate effect. It is found that the photocatalytic degradation process follows first-order reaction kinetics
represented by the Langmuir-Hinshelwood mechanism. The presence of Cl- and SO
4
2-
anions led to a slight decrease of the
effectiveness of the photocatalytic degradation. However, the observed inhibitive effect on the degradation of the tested fungicide is
shown to follow the order: Cl
-
< SO
4
2-
< CO
3
2-
< PO
4
2-
< NO
3
-
. The aeration of the medium bulk enhanced the photodegradation rate of
the pollutant. Conclusion: based on the results, the photocatalytic degradation using a low-voltage UV light could be a useful method
for the removal, of wastewaters containing Mythos.
Keywords: Photocatalysis, TiO
2
, low-voltage UV, Mythos, kinetic model.
1. INTRODUCTION
A diversity of organic pollutants especially pesticides are introduced into the water system from different sources such as
industrial water discharges, agricultural runoff, and chemical spills [1,2]. Their toxicity, stability to natural decomposition,
and persistence in the environment have been the cause of much concern to the societies and regulatory authorities
around the world [3,4].
The control of organic pollutants in water is an important measure in environmental protection. Among many processes
proposed and/or being developed for the destruction of the organic contaminants, biodegradation has received the
greatest attention. However, many organic chemicals, especially those that are toxic or refractory, are not amendable to
microbial degradation [5].
On recent years, novel methods for water and air purification have been developed including chemical, electrochemical
and photochemical processes [6,7]. Indeed; photocatalytic degradation has been shown to be a promising technology for
the treatment of water contaminated with organic and inorganic pollutants [8,9].
Heterogeneous photocatalysis involves the use of a semiconductor material which is excited by light with energy equal to
or greater than the band gap. This leads to the formation of electron/hole pairs which react at the particle-water
interface resulting in the degradation of chemical species by both oxidative and reductive pathways. Titanium dioxide
(TiO
2
) is the photocatalyst of choice for water decontamination treatment because it is not soluble under normal pH
ranges found in natural water. In the other hand, it is photoactive, photostable, and relatively inexpensive. However, TiO
2
is a wide band gap semiconductor (E
bg
= 3.2 eV for anatase) and can only be excited by UV light, meaning that only 5%
of the solar spectrum can be utilised.
ORIGINAL ARTICLE
UV/TiO
2
PHOTOCATALYTIC OXIDATION OF COMMERCIAL
PESTICIDE IN AQUEOUS SOLUTION
*Corresponding Author: | Belmouden Moustapha | Author Copyright © 2018: | El Mouraille Nadia |. All Rights Reserved. All articles published in American Journal of
Innovative Research and Applied Sciences are the property of Atlantic Center Research Sciences, and is protected by copyright laws CC-BY. See: http://creativecommons.org/licenses/by-nc/4.0/.
American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com
37
In our previous paper [10], we mentioned that during the adsorption phase, consider as the vital stage in photocatalytic
degradation of organic pollutants on TiO
2
surface, the amount of pyrimethanil fixed by the particles of TiO
2
, and depend
on several parameters. The time needed to reach adsorption equilibrium increase with concentration, but not exceed 30
minutes. The uptake of pyrimethanil was found to increase with the increase of salt’s concentration especially in the
presence of NaCl and NaHPO
4
.
In the present work, photocatalysis based on TiO
2
and a batch reactor using low voltage UV lamps as an irradiation
source were applied to degrade the commercial solutions of pyrimethanil, named Mythos. The effects of the major
system parameters (pH, initial concentrations of Mythos, concentration of ions and presence of O
2
) on the degradation of
this fungicide in presence of TiO
2
were studied.
2. MATERIALS AND METHODS
2.1. Chemicals
Pyrimethanil is an anilinopyrimidine, 4,6-dimethyl-N-phenyl-2-pyrimidinamine. Imazalil sulfate: 1-[2-(2,4-dichlorophenyl)-
2-(2-propenyloxy)ethyl]-1H- imidazole sulfate. The commercial pyrimethanil used is named MYTHOS SC300 purchased
from Bayer Corporation. The molecular structure of this pesticide is shown in Figure 1. All chemicals used in the
experiments were of analytical pure grade and used without further purification. A stock pyrimethanil standard solution
(100 mg in water) was prepared in doubly distilled water.
The photocatalysts used in this experiment were technical grade: Titanium (IV) oxide, anatase (purity > ≥99%), was
purchased from Sigma-Aldrich and used as received, and TiO
2
Degussa P-25 (anatase/rutile, surface area 56 m
2
g
-1
,
nonporous).
Figure 1: The figure presents the molecular structure of Pyrimethanil.
2.2. Photocatalytic Reactor: This photo-reactor, of a capacity of 1 L, has a cylindrical form made with Pyrex vessel.
The cover of the photoreactor has several ports for sampling and for introducing oxygen into the reaction mixture. It's
placed in the center of a stainless steel cylinder with 6 low-pressure UV lamps (8 watts, TUV G8T5, Philips), with
maximum emission wavelength mainly around 365 nm, on axial position allowing that the intensity received by the
effluent in the Pyrex reaction vessel is uniform. The photoreactor has placed in a closed box (1m
3
). The ventilation of this
enclosure is ensured by two fans.
2.3. Analysis and procedure: Absorbance was measured using UV-Vis spectrophotometer (Jasco V630). The
photocatalytic experiments were performed using a volume of the solution of 1L containing the concentration 15 mg/L of
Mythos in the presence suspension of 1 g/L TiO
2
powder. Before irradiation, the suspension was magnetically stirred in
the dark for 60 min to reach adsorption–desorption equilibrium. During irradiation, a test of samples was withdrawn with
the help of a syringe and filtered through syringe (45 µm) filters to remove TiO
2
particles before analysis. Total volume of
the samples withdrawn from each experiment was less than 10% (by volume) of the reaction solution. The pH of the
reaction solution was adjusted by adding H
2
SO
4
or NaOH. Materials and Methods are written in this area. Describe in
detail the technic used, the Name and the references of laboratory materials used should be cited.
3. RESULTS
3.1. Photolysis
As shown in Figure 2, the degradation of Mythos in aqueous solution by photolysis at 365 nm (without TiO
2
) is very slow
in comparison with photocatalytic degradation, as only 10% of the initial amount of the compound is degraded after 5 h
of irradiation without TiO
2
.
American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com
38
Figure 2: The figure presents the degradation
kinetics of Mythos in the presence and the absence of
TiO
2
P25 and TiO
2
Sigma, using UV/O
2
system
([Mythos] = 15 mg/L; TiO
2
= 1 g/L).
This fact could be attributed to a relatively low substrate adsorption at this wavelength and the quantum yield of
photolysis. Conversely, in the presence of TiO
2
91% and 82% of initial concentration the Mythos was degraded,
respectively for TiO
2
P-25 and TiO
2
anatase. Consequently, the decrease of the Mythos concentration in the presence of
TiO
2
is mainly due to the photocatalytic degradation. Similar results were observed for photocatalysis of diuron,
imidacloprid, formetanate and methomyl [10]. As the TiO
2
P-25 gives a slightly better result than the other form, it will be
used for the rest of this work.
3.2. Effect of substrate initial’s concentration
Figure 3 presents the evolution of illuminated suspensions containing different concentrations of Mythos in the presence
1 g/L of TiO
2
, at free pH, as a function of time. The increase in the initial concentration in the range of 15-60 mg/L is
traducing by a decreased of a reaction rate. Thus, the higher the initial concentration of the pesticide, the longer the time
it takes to remove it. Similar trends have been observed for the photocatalytic degradation of triclopyr and daminozid
[12], carbofuran [13], propham and tebuthioron [14].
Figure 3: The figure presents the influence of the
initial concentration on the photodegradation of Mythos
in the presence of 1 g/L of TiO
2
P-25.
As indicated in several investigations, this trend maybe explains considering that as the concentration of the target
pollutant increase, more molecules of the compound are adsorbed on the surface of the photocatalyst. Therefore, the
reactive species (OH
•
and O
2
•
) required for the degradation of the pollutant must increases. However, the formation of
OH
•
and O
2
•
on the catalyst surface remains constant for a given light intensity, catalyst amount, and duration of
irradiation. Hence, the available OH
•
radicals are inadequate for pollutant degradation at higher concentrations.
Consequently, the pollutant degradation rate decreases as the concentration increases [15]. In addition, an increase in
substrate concentration can lead to the generation of higher concentration of intermediates, which may adsorb on the
surface of the catalyst. Slow diffusion of the generated intermediates from the catalyst surface can result in the
0
0.2
0.4
0.6
0.8
1
1.2
-60 0 60 120 180 240 300
C/Co
temps (min)
Photocatalysis (P25)
Photocatalysis (T-Sigma)
Photolysis
0
0.2
0.4
0.6
0.8
1
1.2
-60 0 60 120 180 240 300
Ce/Co
Time (min.)
5 mg/L
10 mg/L
15mg/l
30mg/l
45 mg/L
60 mg/L
American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com
39
deactivation of active sites on the photocatalyst and result in a reduction in the degradation rate. However, at low
concentration, the number of catalytic sites will not be limiting factor and the rate of degradation will be proportional to
the substrate concentration. Similar trends have been reported for the photocatalytic degradation of propachlor [14],
acephate [16], 2,4-DNP [17], isoproturon [18], and diphenamid [19].
3.3. Kinetic study
A Linear relationship was found by plotting ln(C
e
/C
o
) versus irradiation time (t) (Figure 4) displayed that Mythos
photocatalytic degradation followed the pseudo-first-order kinetic for the different concentrations studied. The pseudo-
first-order rate constants (k
app
) were determined from the slope value of the line upon linear regression according to "Eq.
(1) ":
(1)
Figure 4: The figure presents the variation of ln(C
e
/C
o
) versus irradiation
time for Mythos ([TiO
2
] = 1g/L).
3.3.1. Analysis of degradation using the Langmuir-Hinshelwood (L-H) Kinetic Model
The Langmuir-Hinshelwood model has been used to describe the kinetics of the reaction [21, 22]. This model has been
used extensively to describe experimental results in heterogeneous photocatalysis reactions [23, 24]. The rate of the
photocatalytic reaction depends on the fraction of the sites covered by the Pyrimethanil and the k
LH
a rate constant which
depends on the concentration of HO
•
produced.
Although some authors consider the Langmuir-Hinshelwood model not to be a sufficient process (Yu et al., 2003; Zhu et
al., 2005), this model is widely used because it correlates the rate of degradation to the instantaneous concentration. The
equation used is:
(2)
The linear form of "Eq. (2)" is presented in "Eq. (3)"
=
+
(3)
Where,
C
o
: Mythos initial concentration (mg/L),
k
L-H :
Adsorption equilibrium constant under irradiation (L/mg),
V
o
: Substrate degradation reaction rate (mg/L.min),
-6
-5
-4
-3
-2
-1
0
0 60 120 180 240 300
Ln(Ce/Co)
Time (min.)
5 mg/L 10 mg/L
15 mg/l 30 mg/l
45 mg/L 60 mg/L
American Journal of Innovative Research and Applied Sciences. ISSN 2429-5396 I www.american-jiras.com
40
T: irradiation time (min),
K
r
: Specific rate constant for the oxidation of the organic compound (mg /L min).
The slop of
as a function of
is linear, and the calculated constants k
r
and K
L-H
were k
r
= 0.0373 mg/L.min and K
L-H
=
0.572 L/mg, respectively. Hence, the calculated values of K
L-H
(0.572 L/mg) and K
ads
(0.0422 L/mg L) were different. The
difference obtained in this study can be attributed to the inexactitude of Langmuir and Langmuir–Hinshelwood
alignments. Thus, the adsorption affinity of Mythos on the surface of TiO
2
can be reflected by the parameter K
L-H
.
3.3.2. Half-life of the reaction
One of the most useful indications for evaluating the reaction rate of first order kinetics is the calculation of the half-life
of the reaction. The integration of the Langmuir-Hinshelwood equation (L-H) over time gives "Eq. (4)":
(4)
Where C
0
: Initial pollutant concentration (mg/L),
C: Concentration of organic compound (mg/L),
t: Reaction time during substrate degradation (min).
The half-life of the reaction corresponds to the disappearance of half of the initial amount of the substrate. The
theoretical reaction time t
1/2
is estimated by "Eq. (5)":
(5)
On the other hand, for the apparent reaction constant reaction, the half-life (t'
1/2
) is calculated according to "Eq. (6)":
(6)
The different values of t
1/2
(estimated) and t'
1/2
(calculated) obtained for Mythos for different initial (15 to 60 mg/L) in the
presence of TiO
2
(1 g/L) are given in the Table below.
Table 1: Estimated and calculated half-life values for the various
initial concentration of Mythos.
[Mythos] mg/L
t'
1/2
cal.
(min.)
t
1/2
est.
(min.)
∆t
1/2
(min.)
5
46.2
33.8
12.4
10
70.7
56.5
14.2
15
99.0
79.3
19.7
30
173.3
147.5
25.8
45
261.6
215.8
45.1
60
346.6
284.0
62.6
The graphical exploitation of the half-life time as a function of the initial concentration gives straight lines (Figure 5).