Kinetics of photocatalytic removal of imidacloprid from water by advanced oxidation processes with respect to nanotechnology

In this study, the kinetics of photocatalytic removal of imidacloprid, a systemic chloronicotinoid insecticide, from water using two advanced oxidation systems (ZnO(normal)/H2O2/artificial sunlight and ZnO(nano)/H2O2/artificial sunlight) were investigated. Moreover, the effects of pH, insecticide concentration, catalyst concentration, catalyst particle size, and water type on the photocatalytic removal of imidacloprid were evaluated. Furthermore, total mineralization of imidacloprid under these advanced oxidation systems was evaluated by monitoring the decreases in dissolved organic carbon (DOC) concentrations and formation rate of inorganic ions (Cl and NO2 ) with irradiation time using total organic carbon (TOC) analysis and ion chromatography to confirm the complete detoxification of imidacloprid in water. The degradation rate of imidacloprid was faster under the ZnO(nano)/H2O2/artificial sunlight system than the ZnO(normal)/artificial sunlight system in both pure and river water. The photocatalytic degradation of imidacloprid under both advanced oxidation systems was affected by pH, catalyst concentration, imidacloprid concentration, and water type. Almost complete mineralization of imidacloprid was only achieved in the ZnO(nano)/H2O2/artificial sunlight oxidation system. The photogeneration rate of hydroxyl radicals was higher under the ZnO(nano)/H2O2/artificial sunlight system than the ZnO(normal)/H2O2/artificial sunlight system. Advanced oxidation processes, particularly those using nanosized zinc oxide, can be regarded as an effective photocatalytic method for imidacloprid removal from water. This is an Open Access article distributed under the terms of the Creative Commons Attribution Licence (CC BY 4.0), which permits copying, adaptation and redistribution, provided the original work is properly cited (http://creativecommons.org/licenses/by/4.0/). doi: 10.2166/wh.2019.259 om http://iwaponline.com/jwh/article-pdf/17/2/254/845982/jwh0170254.pdf 021 A. Derbalah M. Sunday R. Chidya W. Jadoon H. Sakugawa (corresponding author) Graduate School of Biosphere Science, Hiroshima University, 1-7-1, Kagamiyama, Higashi-Hiroshima, Japan E-mail: hsakuga@hiroshima-u.ac.jp A. Derbalah Pesticides Chemistry and Toxicology Department, Faculty of Agriculture, Kafrelsheikh University, 33516, Egypt M. Sunday Department of Chemistry, Federal University of Technology Akure, P.M.B 704, Ondo State, Nigeria R. Chidya Department of Water Resources Management and Development, Faculty of Environmental Sciences, Mzuzu University, P/Bag 201, Luwinga, Mzuzu, Malawi W. Jadoon Department of Environmental Sciences, Hazara University, Mansehra, Pakistan This article has been made Open Access thanks to the generous support of a global network of libraries as part of the Knowledge Unlatched Select initiative.


INTRODUCTION
Pesticides are widely used in agriculture to reduce the loss of production caused by pests; however, they also contribute to pollution of the environment (Derbalah et al. , , ). The presence of pesticide residues in water poses a risk to aquatic organisms as well as to human health (Sathiyanarayanan et al. ). The United Nations has reported that less than 1% of applied pesticides reach their target pests, while the remainder become distributed in different environmental components such as soil, air, and water (Readman et al. ; Kopling et al. ; Meyer & Thurman ). Because organic pesticides are generally toxic, persistent and difficult to destroy biologically, their presence in aquatic environments leads to unexpected hazards (Hayo ; Hilz & Vermeer ). Therefore, the process of treating contaminated water with pesticides is a major concern because pesticides are toxic and sometimes carcinogenic, resulting in significant health risks (Derbalah et al. , ).
Imidacloprid is a neonicotinoid insecticide registered in more than 140 countries to control many of the sucking insects that affect field crops such as rice, wheat, and cotton.
Imidacloprid is also used to control pests in crops grown in greenhouses. This pesticide can be applied as a systemic insecticide to soil and seeds, or by spraying on crops ( In order to minimize the residues of pesticides in water to levels established by the World Health Organization and the US Environmental Protection Organization, which do not have harmful health effects on humans and the environment, as well as to meet the standards of water quality of pesticides to protect human health, methods of removal of pesticide residues from water must be more effective and highly sensitive. Therefore, the ideal procedures for removing pesticide residues from water are non-selective methods that achieve the rapid and complete destruction of organic pesticides to non-toxic inorganic products that are effective in small-scale remediation units (Blanco & Malato ; Derbalah et al. , , , ).
Multiple techniques such as advanced oxidation processes, adsorption and biological methods are used for the removal of pesticide residues from water. The elimination of many toxic agrochemicals, including pesticides in water, using advanced oxidation processes such as ( The photocatalysts created by nanotechnology are unique and differ in their morphology and characteristics when compared with the same materials prepared without nanotechnology; thus, they represent an important step in improving the efficiency of the photocatalytic removal of pesticides from water. Metal oxides that are synthesized using nanotechnology like zinc oxide and titanium dioxide can take different forms, such as nanoparticles, nanorods, nanowires, nanotubes, nanosheets, and nanoflowers. The reduction of metal oxides particle size to the nanoscale increases their surface area, and subsequently their active sites on the surface for oxidation and degradation of pesticides (Colon et al. ; Derbalah et al. , ).
Zinc oxide is a semi-conductive material that can be fabricated with various morphological forms in normal and nanoscale sizes (Kumari et al. ). Zinc oxide is environmentally friendly and does not have harmful effects on living organisms (Ismail et al. ). Fabrication of ZnO in nanostructure can change its properties. Light absorption of ZnO as a photocatalyst in the visible region is limited because of its wide band gap. Therefore, its optical properties must be modified using nanotechnology to enable the economic use of zinc oxide in advanced oxidation systems for removal of pesticide residues from water in the presence of sunlight. Additionally, zinc oxide created by nanotechnology can be used very efficiently to eliminate pesticide residues from water depending on the used fabrication methods (Baruah et al. ).
In this study, the photocatalytic removal of imidacloprid from water using two advanced oxidation systems (ZnO(normal)/H 2 O 2 and ZnO(nano)/H 2 O 2 ) was investigated.
Artificial sunlight (solar simulator) was used in this study to examine the photocatalytic removal of imidacloprid since the sunlight advanced oxidation system is not much studied, compared with the UV light advanced oxidation system. Moreover, the effects of pH, catalyst concentration, insecticide concentration, and water type on the photocatalytic removal of imidacloprid were evaluated. Furthermore, the complete mineralization of imidacloprid in water to CO 2 and H 2 O, as well as inorganic ions, was evaluated using a total organic carbon (TOC) analyzer and ion chromatography to confirm the complete detoxification of imidacloprid in water. To interpret the differences in the degradation rate of imidacloprid under both oxidation systems, the photoformation rates of hydroxyl radicals were determined.

Chemicals
Imidacloprid technical standard (99% purity) was obtained from Sigma-Aldrich, USA, as were zinc oxide nanopowder (<100 nm with a specific surface area of 15-25 m 3 /g) and zinc oxide powder with a purity of 99.99%. Benzene, acetonitrile, and phenol (99.5% purity) were obtained from Nacalai Tesque, Japan, and methanol was obtained from Wako Chemical Company, Japan. For TOC analysis, imidacloprid was dissolved directly in Milli-Q water to avoid the influence of methanol on dissolved organic carbon (DOC) levels. Acetonitrile (HPLC grade) was obtained from Kanto Chemical Company, Japan.

Zinc oxide nanoparticles
Zinc oxide nanopowder (with a particle size 80 nm and specific surface area of 15-25 m 3 /g) and zinc oxide powder with a purity of 99.99% (particle size 130 nm and surface area of 5-10 m 3 /g) were obtained from Sigma Aldrich Company, USA. Fabrication of ZnO oxide nanopowder as reported by the company was conducted by the physical vapor synthesis (PVS) method to produce ZnO nanoparticles with unique characteristics.

Photocatalytic degradation of imidacloprid in water
This experiment was designed to assess the efficiency of the advanced oxidation process using zinc oxide nanopowder (synthesized) and normal-sized zinc oxide (commercial) to remove imidacloprid ( Figure 1) from water. River water samples of the chemical composition given in Table 1  For photocatalytic removal of imidacloprid, a water solution containing imidacloprid (1 mg/L) and zinc oxide (1,000 mg/L) in normal or nanosize was stirred for 15 min.
This was done in the dark to attain equilibrium while taking the loss of imidacloprid caused by adsorption into account. Hydrogen peroxide was then added at a concentration of 20 mM and the final volume of the solution in the beaker was diluted to 100 mL with Milli-Q water. The pH of the solution was subsequently adjusted to 7, which was the optimum pH for the zinc oxide catalyst, using a Jenway pH/mV/temperature meter, Model 3510 (Derbalah ).
During irradiation, the solution in the quartz glass cell (60 mL) was mixed well with a stir bar while the temperature was kept constant at 20 C. After irradiation started, the  To evaluate the effects of catalyst concentration on the degradation rate of imidacloprid, different initial concentrations of ZnO were used (100, 500, and 1,000 mg/L). To investigate the effects of pH on the degradation process, different pH values were employed (5, 7, and 9). The effects of imidacloprid concentration on its degradation rate were investigated using different initial concentrations (100, 500, and 1,000 μg/L). Finally, the effects of water type on the degradation rate of imidacloprid were evaluated in river and Milli-Q water.
The efficacy of H 2 O 2 alone without catalyst in the pres- Direct photolysis of imidacloprid under solar simulator without ZnO or hydrogen peroxide was also examined.

HPLC analysis
The treated water samples were analyzed directly using an HPLC system with a mixture of acetonitrile and Milli-Q water (30:70) as the mobile phase. The flow rate was maintained at 1.0 ml min À1 and the UV detector wavelength was 270 nm (Dewangan et al. ). The detection and quantification limit of imidacloprid in water were 1.2 and 3 μg/L, respectively. The degradation rate constant (k) and half-life (t 1/2 ) was calculated as described by Bondarenko et al. () and Evgenidou et al. (). The photodegradation rate constants were normalized as described by Arakaki et al. ().

Mineralization experiments
To evaluate the mineralization rate of imidacloprid in Milli-Q water by the two tested oxidation systems, losses in DOC and formation rate of inorganic ions (Cl À and NO 2 À ) under irradiation using the solar simulator described above in this study were measured using a TOC analyzer and ion chromatography, respectively. For the loss of DOC with different times of solar light exposure (0, 2, 4, 6, and 8 h) a concentration of 3 mg C/L of imidacloprid was used. After exposure to solar light, the water samples were acidified using HCl and injected directly into a TOC analyzer that had been calibrated using standard solutions of potassium hydrogen phthalate. To monitor the process of imidacloprid (7 mg/L) mineralization to its inorganic ions, the yield of the selected major ions of chloride (Cl À ) and nitrite (NO 2 À ) were where R phenol is the photoformation rate of phenol in each irradiated sample; Y phenol is the yield of phenol formed in the reaction between benzene and . OH, and F benzene-OH is ZnO concentration, solution pH, catalyst particle size (nano and normal), and water type (Milli-Q and river waters) were evaluated to identify the optimum conditions for photocatalytic removal of imidacloprid in water.

Effect of imidacloprid concentration
The effects of imidacloprid initial concentration (100, 500, and 1,000 μg/L) on its degradation rate were evaluated. As shown in

Effect of catalyst concentration
The effects of zinc oxide (normal and nanosized) initial concentration (100, 500, and 1,000 mg/L) on the degradation  (Table 3).   The different letters represent significant differences using Fisher's LSD test at P 0.05.

Effect of pH
Data presented in mean value (three replicates) ± standard deviation. were observed in Milli-Q and river water, respectively.

Effect of catalyst particle size
The effects of zinc oxide particle size (normal and nanosized) on the photocatalytic degradation of imidacloprid in Milli-Q and river water were investigated. The results showed that the particle size of zinc oxide catalyst significantly influenced the photocatalytic degradation rate of imidacloprid, with nanosized zinc oxide resulting in faster degradation than normal sized zinc oxide (Table 5). As shown in Table 5, the half-lives for imidacloprid were 85.89 and 72.68 min in Milli-Q water when normal and nano sized zinc oxide were used, respectively. However, for river water, the half-lives of imidacloprid were 123.23 and 88.57 min using normal and nanosized zinc oxide, respectively.

Total mineralization of imidacloprid
The total mineralization of imidacloprid in water was con- H 2 O 2 ¼ 20 mM, ZnO ¼ 1 g/ L, and pH ¼ 7.

DISCUSSION
In this study, significant degradation of imidacloprid with a  (2)). These electron (e À cb )hole (h þ vb ) pairs can either recombine again (Equation (3)) or react in a separate way with other chemical species.
It is possible that adsorbed H 2 O or OH À can be oxidized by the holes in the ZnO valence band and generate • OH radicals (Equation (4)). Moreover, as described in Equation (5) can produce • OH radicals as described in Equations (8)   Statistical comparisons were made among treatments within a single column.
The different letters represent significant differences using Fisher's LSD test at P 0.05.
Data presented in mean value (three replicates) ± standard deviation.

) and other inorganic ions.
ZnO  As a result, this reduction in the rate of hydroxyl radical generation in river water slows the degradation rate of imidacloprid. In addition, DOC, which is present in river water, absorbs most of the emitted photons, thereby slowing the degradation of imidacloprid (Konstantinou et al. ).
As shown in

CONCLUSIONS
The degradation rate of imidacloprid using the ZnO(nano)/ H 2 O 2 /artificial sunlight oxidation system was faster than that of the ZnO(normal)/H 2 O 2 /artificial sunlight oxidation system in Milli-Q or river water. The photoformation rate of • OH using ZnO (nano)/H 2 O 2 /artificial sunlight was higher than that of the ZnO (normal)/H 2 O 2 /artificial sunlight system. Some parameters such as water type, pH, catalyst, and pesticide concentration as well as the catalyst particle size significantly affect the efficacy of advanced oxidation processes during imidacloprid removal from water. Total mineralization of imidacloprid was only achieved by using the ZnO(nano)/H 2 O 2 /artificial sunlight oxidation system, which confirms the complete detoxification of imidacloprid in treated water. Advanced oxidation processes, particularly with zinc oxide nanocatalyst, can be regarded as an effective photocatalytic method for imidacloprid removal from water.