Photocatalytic degradation of imidacloprid in soil: Application of response surface methodology for the optimization of parameters

Article abstract of DOI:10.1039/c5ra02224j

The photocatalytic mineralization of imidacloprid (IMI) in soil to inorganic ions and the formation of various intermediates using TiO₂ as the photocatalyst have been investigated under UV light. Various parameters, viz., catalyst concentration, soil depth and pH, intensity of light and initial concentration of IMI were optimized theoretically by using a central composite design based on a response surface methodology and were correlated with experimental results. The statistical analysis from the modelling results indicates that the degradation efficiency of IMI is affected by the depth of soil and the intensity of light, but the effects of the pH and the initial concentration of imidacloprid are more dominant. The optimum conditions obtained for the maximum degradation of imidacloprid were at pH = 3, intensity of UV light = 30 W m^-2, soil depth = 0.2 cm and initial concentration of imidacloprid = 10 mg kg^-1 of soil. Under these optimum conditions, the highest degradation efficiency of 83% was achieved after 18 h of UV light irradiation. The identification of various photoproduced intermediates of IMI by LC-MS analysis revealed its degradation, whereas the increase in the formation of inorganic ions with time of UV light irradiation confirms its mineralization. This journal is

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Graphical Abstract

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Title: Photocatalytic degradation of imidacloprid in soil: Application of
response surface methodology for optimization of parameters
Authors name: Teena Sharma† a, Amrit Pal Toor b*, Anita Rajor a
†Designation: First author is PhD student, second author is Professor and third author is
Associate professor
Address:
School of Energy & Environment,
Thapar University,
Patiala 147004
Punjab, India
Corresponding author:
Amrit Pal Toor
Dr. S. S. Bhatnagar Institute of Chemical Engg. & Tech.
Panjab University, Chandigarh, 160014 India
Eꢀmail: aptoor@yahoo.com
Tel: +91ꢀ172ꢀ253ꢀ4904
1

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Abstract
The photocatalytic mineralization of imidacloprid to inorganic ions and various intermediates
formation using TiO2 as photocatalyst in soil was investigated under UVꢀlight. Various
parameters viz., catalyst concentration, soil depth & pH, intensity of light and initial
concentration of IMI were optimised theoretically by using central composite design based on
response surface methodology and were correlated with the experimental results. The statistical
analysis from the modelling results indicated that degradation efficiency of IMI was affected by
depth of soil, intensity of light also, but effect of pH and initial concentration of imidacloprid
were more dominant. The optimum conditions obtained for maximum degradation of
imidacloprid were at pH = 3; intensity of UV light =30Wmꢀ2; soil depth= 0.2cm and initial
concentration of imidacloprid =10 mg kgꢀ1 of soil. Under these optimum conditions, the highest
degradation of 82% was achieved after 18 h of UV light irradiation. The identification of various
photoproduced intermediates of IMI by LCꢀMS analysis revealed its degradation, whereas the
increase in formation of inorganic ions with time of UV light irradiation confirms its
mineralization.
Keywords: Soil, TiO2, Photocatalytic degradation, Response Surface Methodology,
Mineralization, Mechanism
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1 Introduction
Soil, an ultimate sink for accumulation of applied pesticide have attracted the current research
interest inꢀterms of analysing leaching behaviour, formation of intermediate products and role of
parameters affecting degradation of pesticides. Upon contact of pesticide with soil, binding
between them takes place, with the extent being dependent on the nature, composition, texture,
organic matter, pH, cation exchange capacity, electrical conductance and moisture of the soil (1).
Adsorption of pesticide with soil is a crucial factor, as it is well known to influence (2) their
persistency, degradation and fate in the soil. For instance, adsorption and desorption process
were reported (3) to determine the mobility of pesticides and were found to be influenced by
physicoꢀchemical properties of soil. Some studies have shown that organic matter affects binding
of pesticide molecules with soil (4) and is reported as a major controller component in the
sorption, transformation, and transport of many organic pollutants in soil (5).
Imidacloprid (IMI), belonging to neonicotinoids family of insecticides, effective on insects
that are resistant to carbamates, organophosphates and pyrethroids, has been admitted from past
two decades over 120 crops worldwide. This insecticide because of its high water solubility,
leaching behaviour and halfꢀlife has contaminated the soil and water reservoirs nearby the
agricultural fields, and is of concern for the environment (6). Its natural degradation is slow in
soil under natural conditions, decomposes sometime more toxic and persistent intermediates than
IMI itself. Therefore, efforts have been made to degrade and study the formation of metabolites
of IMI biologically (7ꢀ11) in soil and broth. The degradation of IMI has also been studied
photocatalytically in water (12, 13) using TiO2 as photocatalyst. The available reports reveal that
approx 95% degradation of IMI could be done by photocatalysis in water (14). No studies have
been reported on photocatalytic degradation of IMI in soil however for other molecules such
diuron (15), PNP and polycyclic aromatic hydrocarbons (16, 17), the same process is well
reported. Therefore, using TiO2 as a photocatalyst in soil could be useful for mineralization of
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IMI, and seems to be unexplored. Hence, degradation of IMI using commercially available
Degussa P25ꢀTiO2 as photocatalyst, under UVꢀlight irradiation in soil has been studied.
Generally, Singleꢀvariableꢀatꢀaꢀtime (SVAT) method is used to study photocatalytic
process (18,19), however it suffers from some disadvantages viz., time consumption, incapability
to account for interactions between different variables and hence in predicting the optimum
conditions (20). On the contrary, central composite design (CCD) based on response surface
methodology (RSM) is found to be more convenient, as it provide the good correlation between
the various parameters in much less time (21,22). Although, this approach has been employed for
the degradation of various pollutants in air and water (23ꢀ28), yet much less work has been
carried out using the same in soil.
Hence in this study, various parameters such as intensity of light, pH, initial concentration
of IMI and depth of soil affecting the degradation of IMI were optimized using CCD based on
RSM. The predicted response values from RSM were compared with experimental photoꢀ
catalytic degradation efficiency of IMI. Under these optimized parameters degradation of IMI
was experimentally studied using HPLC along with identification of metabolites formed by LCꢀ
MS technique, and the fate of heteroatom (N,O, Cl) present were studied by ion chromatography.
2 Experimental Methods
2.1 Materials
Imidacloprid (99%) and TiO2 (Degussa P25, average particle size of 30ꢀ50 nm, anatase: rutile ::
80:20) were obtained as gift samples from Bayer crop science India Ltd., Mumbai and Evonik
Industries, respectively. Acetonitrile (HPLC grade), deꢀionized water, H2SO4 and NaOH were
obtained from Loba Chemi, India.
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2.2 Experimental procedures
The soil samples from surface (0ꢀ10 cm) collected from Thapar university campus, Patiala
(Punjab), India, were firstly air dried and passed through 1 mm sieve. Sieved soil samples were
autoclaved (121°C, 3 × 30 min) and stored in the dark. Adsorption and photodegradation studies
(with and without TiO2) were performed in UV reactor equipped with six UV lamps (Phillips,
20W) with such an arrangement that height of the soil samples with respect to UV light can be
varied (Electronic Supplementary Information (ESI) fig.1) as reported previously (29) by our
group. The internal temperature of the chamber was maintained by using exhaust fan. The pH (3,
7 and 11) of the soil was maintained by using 0.1 N HCl/NaOH and temperature was maintained
by circulating the water below soil samples petriplates. The soil samples were spiked to 50 mg
kgꢀ1 of the soil using acetonitrile solution of IMI and known amount of soil (5 g) was evenly
spread on glass petriplates (90 mm of diameter), forming a layer of 0.2 cm estimated from soil
bulk density and petriplate area. Moreover, the soil samples spiked with IMI have been irradiated
without TiO2 and hereafter termed as photolysis.
Further soil samples (5g) from irradiated and nonꢀirradiated experiments were extracted by
stirring with acetonitrile for in 50 ml beaker followed by sonication in an ultrasonic water bath
(EN 60 US, tank size 12’’×6”×6” , 100W, 33±3 KHz). After extraction solution was separated
from the soil by centrifugation and washed with acetonitrile. The extract solution thus obtained
was filtered (0.22 µm Millipore syringe filter) and analysed by using UV–VIS spectrophotometer
(λ max= 270 nm, Hitachi Vꢀ500 UV/VIS, Japan).
The photoproducts formed during degradation of IMI were quantitatively analysed by LCꢀ
MS technique using Waters Alliance 2795 LCꢀMS (Waters, UK) linked with a Micromass QꢀTof
system equipped with a Shimadzu column C18 (250 × 4.6 mm, 5 ꢁm), at 20°C. The acidified
(0.1% acetic acid) mobile phase (acetonitrile: water:: 50:50) was isocratically flow at a rate of
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0.3ml/min. Mass analysis was performed with a Zꢀspray source for positive electrospray
ionization at using multiplereaction monitoring (MRM) scan mode in a range of 50ꢀ300 m/z. The
o
source and probe were isothermally kept at 140 and 250 C, respectively and N2 (1 ml/min) was
used as source of nebulisation. The sample injection volume for LCꢀMS was 10 µl.
Quantification of inorganic anions (nitrate, nitrite and chloride) produced has been
estimated by injecting 100 µl of the sample into Ion chromatograph equipped with a Waters
501pump, a Waters 431 conductivity detector, and ion pack (50 mm × 4.6 mm, Metrhom)
column using methanol: water :: 60:40 as mobile phase @ 0.6 ml min−1.
2.3 Experimental design and data analysis
Parameters such as UV light intensity, initial conc. of IMI, pH, TiO2 catalyst dose, reactor
configuration etc., are some of the factors responsible for photocatalytic oxidation (30).
However, it is very difficult to carry out an experimental design including abovementioned
factors because of large number of experiments (31). Therefore, four variables pH, initial
concentration of IMI, intensity of UV light and depth of soil were investigated which includes all
the possible combinations for each factor. This multivariable experimental design was done
according to CCD based on RSM and the photocatalytic degradation efficiency was selected as
response. Analysis of experimental data was supported by DesignꢀExpert Software (trial version
9.0.3.1, StatꢀEase, Inc., MN, USA) (21, 32, 33). These variables were firstly converted to
dimensionless ones (A,B,C,D) with coded values at levels:ꢀ ꢀα,1,0,+1,+α as shown in ESI- table1.
These five levels depicting controlling factors are 3ꢀ11 for pH values (A); 10ꢀ30 Wmꢀ2 for
intensity of light (B); 5ꢀ 25 g amount of soil (C) in petriplates of fixed diameter of 90 mm
(correspond to soil depth ranges from 0.2ꢀ1cm respectively); 10ꢀ90 mg of IMI kgꢀ1 of soil for
initial concentration of IMI (D) in soil. The 30 combinations obtained by software were
experimentally performed in the present study and shown in table1.
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3 Result and Discussions
3.1 Preliminary experiments
Preliminary experiments were carried out in order to evaluate the photolysis and adsorption of
IMI in soil. It was observed that change in the concentration of IMI was not significant when
kept under dark for 18 h ascribed to its adsorption in soil. As pH has significant effect on
adsorption of pollutants, therefore adsorption studies for IMI on soil at different pH (3, 7 and 11)
were performed (Fig.1a). The adsorption of IMI in soil was ~5% at pH = 11 and it increases up to
~9% at pH = 3. This indicates protoꢀnation of imdazole ring and is found be in good agreement
with the work of Jodeh et al. (34) where protoꢀnation of imidacloprid reported to be a cause for
decrease in its sorption in alkaline silty clay soil. Moreover, photolysis of IMI in soil at optimum
adsorption pH was carried out, showing no notable change in its amount even after 18 h of UVꢀ
light irradiation. This clearly depicts the stability of IMI molecule and is accordance of reports
(12) where its half life is reported to be 40ꢀ60 days. However, when soil having IMI was
irradiated with UVꢀlight in presence of TiO2 (0.1ꢀ 0.5g Kgꢀ1) the degradation was perceived and
found to be dependent upon its amount (Fig.1b). It was observed that degradation increases with
increase in photocatalyst doses upto an amount of 0.3 g kgꢀ1 and thereafter it decreases.
Generally, increase in the amount of catalyst actually increases the number of active sites on the
photocatalyst surface, thus causing an increase in the number of •OH radicals which actually
participate in IMI degradation. But with further increase in amount of TiO2, the light penetration
declines causing decrease in formation of excited charge carriers and hence oxidative hydroxyl
and superoxide radicals (35, 36) therefore there is decrease in degradation rate.
Thus 0.3g TiO2 per kg of the soil was found to be optimum which was further used for
optimizing other four experimental parameters (A,B,C,D).
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3.2 Response surface modelling and data analysis
The experimental design concludes 30 experiments and the results came from experimental data
are presented in table 1. A theoretical coꢀrelation between these parameters could be expressed as
follows:
R = +64.69562 ꢀ1.20421 pH + 1.41174 Intensityꢀ 0.36298 soil + 0.010786 conc.ꢀ0.098242 pH Intensityꢀ
0.11836 pH soil ꢀ0.042773 pH conc.ꢀ 0.024414 soil Intensityꢀ0.014629 Intensity conc.+ 2.30469Eꢀ
003 soil conc. +6.67969Eꢀ003 pH Intensity soil+ 2.48047 Eꢀ003 pH Intensity conc. + 1.52344Eꢀ
003 pH soil conc. ꢀ8.98438Eꢀ 005 Intensity soil conc. ꢀ7.42188E ꢀ005 pH Intensity soil conc.
In the above expression, R is the response variable for degradation efficiency of
imidacloprid. Adequacy and significance of the model (response surface reduced quadratic
model) was checked by applying ANNOVA which is one of the most important test for the
evaluation of data. Thus from table 2, the probability value for A and D are very low (<0.0001)
as compared to B and C. This shows that variables A (pH) and D (concentration of IMI) have
much significant effect than B (intensity of light) and C (depth of soil) in the degradation of IMI.
The ANNOVA results showed that the factors with very low value of probability have more
significant effect on the degradation as compared to the factors having high value of probability.
Additionally, adequacy of selected model with real system was confirmed by analyzing the
correlation between observed and predicted values. The parity between experimental and
predicted values are given in Figure 2a where the experimental data has been reasonably fitted
well within ±10 %.The response factor of calculated residual values were plotted and shown in
fig 2b which depicts that all data points lie close to straight line and within 95% confidence
intervals line with mean values near zero. The plot of internally studentized residuals vs
predicted values (ESI-fig.2) depicted that points are randomly scattered and values lie within the
same range (ꢀ3 to +3).
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Moreover, the 3ꢀdimensional response surface plots (Fig. 3, 4 and 5) and two dimensional
contours (ESI-fig.3, 4 & 5) further confirms that highest degradation (82%) could be achieved
under the optimized parameters. Figure 3 reveals the significant impact of soil pH and initial IMI
concentration on degradation of IMI. It can be seen that in acidic soil and lower concentration of
IMI, the degradation is maximum. This could be accredited to the fact that with decrease in pH,
the TiO2 surface become cationic (37), causing increase in attractive interaction between the O
atom (partially negative) of resonance stabilized (ESI- fig.6) ꢀNO2 group in IMI. Consequently,
adsorption of IMI increases favouring more number of IMI molecules to be present at the interface
of TiO2 and hence the degradation efficiency. However, at pH= 9ꢀ11, reduction in degradation
efficiency (50% to 15%) could be ascribed to the protonation of imidazole ring as revealed by the
dark adsorption studies. At low concentration of IMI, the degradation is faster than at high
concentration of IMI. With the increase in concentration of IMI the screening effect dominates and
prevents the penetration of the light and hence reduction in the generation of OHꢂ radicals on the
catalyst surface (38). The degradation of IMI is further found to be affected by soil depth (Fig. 4 &
5) and intensity of light. Increase in the soil depth (0.2ꢀ1cm) causes decrease in degradation
efficiency. This is because the sunlight cannot reach deep inside the soil and the necessary
conditions for the photocatalytic degradation are absent in this part of soil hence degradation
decreases as the soil layer becomes thicker (39,40). The degradation efficiency was found to
increase with increase in light intensity. This could be credited to more availability of photons for
excitation of valance band electrons leading to predominate formation of electronꢀhole pair and
thereby diminishing the charge recombination process (41ꢀ43). However, at lower light intensity,
electron–hole pair separation competes with recombination which in turn decreases the formation
of free radicals, thereby, causing less effect on the degradation of the IMI on soil surfaces.
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Experiments which were carried out, yielded maximum degradation of 83%, in agreement
with the predicted response of 86% verified the validity of optimal point, indicating that CCD
could be effectively used to optimize photocatalytic degradation of pollutants.
3.3 Degradation kinetics under optimum conditions
The photocatalytic oxidation kinetics of many organic compounds is fitted to Langmuir–
Hinshelwood (L–H) model as :
ꢀ
Where, k is the reaction rate constant, K is the equilibrium adsorption constant, C the substrate
concentration at any time t (44). In case of low concentration of reacting substrate this equation
simplifies to apparent first order kinetics:
ꢀln(C/Co) = krK t= kt
The plot for –ln(C/Co) vs. time of irradiation, found to be a straight line with slope (k) 0.0957
minꢀ1 as shown in Fig. 6.
3.4 Degradation reaction mechanism under optimized conditions
A clear contrast has been found for the LC chromatograms obtained for the photolysis and
photodegradation of IMI (ESI-fig. 7). It can be clearly seen that peak height of the IMI after 18 h
of its photolysis is comparatively higher than that found for the sample after photodegradation.
Moreover, some new peaks at Rt = 1.6, 1.9, 2.3 and 2.5 min have appeared for the sample after
photodegradation while only peak at Rt = 1.6 min was found after photolysis sample. The various
intermediate products were identified by LCꢀMS analysis (ESI-fig.8) as compound B (Rt =
1.95min), C (Rt =1.63 min), D (Rt =2.09 min) and E (Rt =3.32 min). The formation of these
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identified intermediates could be explained on the basis of proposed pathway (Fig. 7) suggesting
various possible attack positions by the hydroxyl radical at IMI.
The preliminary attack of hydroxyl radicals at NꢀN in IMI expected to yield compound D
which further degrade to compound E. The hydrolysis of IMI could result in the formation of
intermediate compound B, which degraded to compound C via attack of hydroxyl radicals and is
reported to be an intermediate product of IMI photooxidation. The formation of these identified
intermediates during photolysis and photodegradation is in accordance to the previous reports
(45) concerning the same in aqueous media.
Investigation for the fate of heteroatoms showed that the amount of inorganic ions formed
after 18h of photodegradation is notably higher than that found after photolysis of IMI (Fig. 8),
indication less mineralization in case of photolysis. The increases in formation of inorganic ions,
with time of UVꢀlight irradiation further confirm the mineralization of IMI, during its
photodegradation. The formulated (eq.1) balanced stoichiometric chemical equation for nitrate,
nitrite and chloride ions during the photodegradation of IMI in present study found to deviate
from the expected balance equation (eq.2) after complete its degradation. The existence of
indentified intermediates composing the heteroatoms accounts for the incomplete mineralization.
The increase in the amount of nitrate ion formation with irradiation time is probably due to
mineralization of the imidazole or pyridineꢀsubstituted ring(s). These results are in correlation
with a previous study (46, 47) for the dissipation of cyrpoconazole and fenhexamid, where the
nitrogen atoms present in the triazole moiety were found to decompose to nitrate and ammonium
ions.
_
_
_
TiO
2
C9H10N5O2Cl + (3/2) O2
0.10 NO2 + 0.63 NO3 + 0.57 Cl + Intermediates
hv (18 h)
.....eq. 1
_
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_
_
C9H10N5O2Cl + O2
9CO2 + 5NO3 + Cl + 6H+ + 2H2O
.....eq. 2
4 Conclusions
Photocatalytic degradation of IMI could be effectively done using TiO2 as photocatalyst in soil
surfaces. Present study showed the dependence of degradation of IMI on the pH, intensity of
light, depth of soil and initial concentration of IMI. The abovementioned parameters were firstly
optimized using CCD in RSM supported by DesignꢀExpert Software and the number of
experiments came from software were selected and experimentally performed to find out the
photocatalytic degradation efficiency. Therefore, under the optimized conditions, mineralization
of IMI to inorganic ions and various metabolites could be achieved to overcome the problem of
persistence of IMI in soil.
Acknowledgements
The financial and instrumentation support from Department of Science and Technology under
DST WOSꢀA project (SR/WOSꢀA/LSꢀ353/2012) and Central Instrumentation Laboratory (CIL),
Punjab University, Chandigarh is highly acknowledged.
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Tables and Figure Captions:
Table1. Experimental and calculated values for degradation of imidacloprid during photocatalytic
process.
Table 2. Response surface model regression coefficients and Pꢀvalue for responses.
Fig. 1 (a) Adsorption studies of IMI at different pH on soil surfaces and (b) Effect of TiO2
concentration on degradation of imidacloprid (50 mg Kgꢀ1 of soil) after 18 h of UV light
irradiation (20 Wmꢀ2) in soil (pH = 7, depth 0.2 cm).
Fig. 2 (a) Experimental and calculated values for degradation of imidacloprid and (b) internally
studentized residuals plot during photocatalytic process.
Fig. 3 Response surface graph for the interaction between pH and initial imidacloprid
concentration at fixed depth of soil (0. 2cm) and light intensity 30 Wmꢀ2.
Fig. 4 Response surface graph for the interaction between light intensity and initial imidacloprid
concentration at fixed depth of soil (0.2 cm) and pH= 3.
Fig. 5 Response surface graph for the interaction between depth of soil and initial imidacloprid
concentration at fixed intensity of light 30 Wmꢀ2 and pH = 3.
Fig. 6 Kinetics and mineralization profiles of imidacloprid at optimum conditions in soil (initial
IMI conc. 50mg kgꢀ1, pH=3, intensity of light 30 Wmꢀ2). Inset: Time course UVꢀVisible spectrum
for degradation of imidacloprid.
Fig. 7 Proposed pathway for the degradation of imidacloprid.
Fig. 8 Evolution of inorganic ions during photocatalytic degradation of imidacloprid.
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Figures
Figure 1a
Figure 1b

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Figure 2a & 2b

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Figure3.

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Figure 4

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Figure 5

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2.5
2.0
1.5
1.0
0.5
0.0
1.0
0.8
0.6
0.4
0.2
0.0
Degradation
Time (h)
0
6
12
15
18
24
200
250
300
350
Wavelength (nm)
0
4
8
12
16
20
24
Time (h)
Figure 6
Figure 7

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Figure 8