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Facile synthesis and characterization of magnetic nanocomposite ZnO/CoFe2O4 hetero-structure for rapid photocatalytic degradation of imidacloprid

This work has attempted to investigate the potential of ZnO/CoFe2O4 magnetic nanocomposite to mineralize imidacloprid completely to have sustainable pollutant free and safe water supply. The co-precipitation method was performed to prepare the composites; was performed to characterize composites, scanning electron microscope (SEM), transmission electron microscopy (TEM), energy dispersive x-ray crystallography (EDX), x-ray diffraction (XRD), Fourier transform infrared (FT-IR) spectroscopy, and vibrating sample magnetometer (VSM). It was attempted to explore and enhance parameters influencing the process and the percentage of imidacloprid degradation, including photocatalyst amount, pesticide concentration, pH, radiation time, and temperature. UV-Vis spectrophotometer was used for the degradation percent of organochlorine pesticides. Parameters affecting the process, including photocatalyst amount, pesticide concentration, pH, radiation time, and temperature effect on the percentage of imidacloprid degradation were Investigated and optimized. 0.05 g of photocatalyst, with a concentration of 5 ppm for 45 min under light exposure was obtained at pH 10 at room temperature.


Introduction
A big problem in the world is the pollution of water, hence it is necessary to evaluate and review water resource policies. It is believed that food demands will be doubled globally within the next 50 years [1]. The food production has been boosted by applying in agriculture through managing many pests at various stages of crop production and storage; however, it polluted the soil and groundwater severely [2,3,4]. Some studies revealed that around 2.5 million tons of pesticides are used globally every year. In Iran, 60 thousand tons of pesticides are used annually, which is increasing at an annual rate of about 6-15%. Surprisingly, only 0.1 % of pesticides target the crops and, 99.9 % is wasted through the air, soil, and water, which leads to pollution inevitably [5]. Therefore, pesticides rank among the most common pollutants in surface and ground waters of agricultural areas. Human health is significantly influenced by the presence of pesticides in drinking water, and some of these pesticides are known to be carcinogens. They not only influence the people involved in agriculture but also are carried to long distances because of their longer half-lives and entering the water bodies in the end [6,7,8,9,10]. This scenario is worsened in countries like Iran due to limited water resources and existing resources being polluted by the reckless and unnecessary use of the agrochemicals [11]. Therefore, developing a well-organized and cost-effective technology seems essential to remove these dangerous pollutants from waters and wastewaters completely. Some methods have been proposed for removal of the pollutants from the waters and wastewaters such as adsorption, filtration, chemical oxidation, and biological treatments [12,13]. Recently, the semiconductor photocatalysis has been favored because it considers the solution of environmental problems. These semiconductor photocatalysts generate electron and hole pairs (e À -h þ ) by irradiation of light energy that could be used to start oxidation and decrease reactions of the pesticide, respectively [14]. The number of photons striking the photocatalyst controls the reaction rate. One of the frequently studied photo-catalyst is ZnO due to its chemical stability, availability, and is low-cost. Besides, it is similar to TiO 2 in terms of the energy band gap [15]. Cobalt, iron, and zinc metals were successfully used to decrease the half-lives of pesticides such as chlorpyrifos, imidacloprid, etc. These metals are capable of boosting the degradation rate and thus could be a catalyst for the degradation. Light energy can be collected by semiconductors like TiO 2 /ZnO for photo-degradation. Both of them are capable of boosting the degradation rate if combined in composite form through the reduction of the energy consumption for degradation and gathering additional energy. Suitable photocatalysts are selective, active, and highly stable; it is possible to re-recover and separate them effortlessly from the reaction medium. The industrial application of nanotechnology has expanded the production of nanocatalysts. The miniature size of the nanocomposites is not beneficial due to isolation issue; thus, when magnetic nanoparticles are used as nano photocatalyst, isolating and reusing the nanocomposites would be feasible at the end of the reaction using a suitable magnetic field. These magnetic nanophotocatalysts can substitute heterogeneous and homogenous catalysts because of separation capability and high levels and activity. In this paper, the co-precipitation method was applied to prepare ZnO/CoFe 2 O 4 magnetic nanocomposite. This method has numerous advantages including: (1) the acidification treatment was used to increase the specific surface area of ZnO, and to ease post-functionalization, oxygen-containing functional groups were proposed as new adsorption and active sites; (2) the combination of CoFe 2 O 4 separated photogenerated electrons and to photo-less, thus, the photocatalytic activity of the composite material was enhanced; and (3) CoFe 2 O 4 supported on ZnO also provided new adsorption sites to boost the materials in terms of the catalytic performance. Besides, the activation of ZnO/CoFe 2 O 4 magnetic nanocomposite under the irradiation containing visible light would result in its synergistic catalytic role with the CoFe 2 O 4 leading to enhanced photocatalytic activity of the composite material. Imidacloprid as neonicotinoid insecticide has been used frequently in agricultural production; they are identified in the environment because they are soluble in water. Photolysis and hydrolysis are the main natural degradation modes of imidacloprid [16]. Therefore, this compound was selected to act the organic contaminants to exam the photocatalytic activity of the as-prepared ZnO/CoFe 2 O 4 photocatalysts under visible light irradiation [17,18].

Materials and apparatus
In the present study, chemicals including zinc acetate, ammonium oxalate, absolute ethanol, ammonia solution, FeCl 3 .6H 2 O (98 %), and CoCl 2 .6H 2 O (98%) were analytical grades and purchased from Merck Company. Imidacloprid was purchased from Sigma. Fig. 1 shows the structure of the imidacloprid.
Distilled water was used for all solutions. An electronic balance Mettler AE-160 (Greifensee, www.mt.com) was used to weigh the samples. Varian Cary 50 spectrophotometer made in Australia. A Metrohm 827 pH-meter equipped with a consolidated glass-calomel electrode was used for pH calibration. To make the samples homogenized, magnetic stirrer hot plate (2000 rpm) and oven model 100 (Memmert, www.me mmert.com) were used. A Sonorex digitec model DT 225H with 35 kHz ultrasonicator (Bandelin (www.bandelin.com), was applied for separating the nanoparticles in solutions. A Tensor 27 spectrometer (Bruker, www.bruker.com) was used to have the Fourier transform infrared (FT-IR) spectra. EM10C transmission electron microscope (TEM), (Zeiss, www.zeiss.com), was used with an accelerating voltage of 100 kV. Field emission-scanning electron microscopy (FE-SEM) images were obtained on a model Sigma (Zeiss, www.zeiss.com). The powder X-ray diffraction (XRD) patterns were examined on a model X'PertPro diffractometer (Panalytical, www.panalytical.com) using Cu Kα radiation in the 2ɵ range 10-80 . A vibrating sample magnetometer (VSM) model MDKFD (www.lakeshore.com) was used for magnetic measurements.

Synthesis of ZnO nanoparticles
A new precipitation method was used to prepare ZnO NPs. 100 mL of distilled water was used to dissolve 0.01 M Zinc acetate (2.19 g). After precise weighing, 0.01 M of solid ammonium oxalate (1.42 g) powder was added to the zinc acetate solution. To dissolve the ammonium oxalate completely, the suspension was stirred. To increase the pH of the solution to 10, the ammonia solution was added in drops. The Afterward, 12 mL of 25% NH 3 was quickly added to the solution. Then, the bulk solution turned from orange to brown straight away. After washing several times first with deionized water and second with ethanol, a magnet was used to collect the magnetite precipitates. Ultimately, they were put to dry in an oven at 80 C for 6 h followed by calcination at 350 C for 3 h to form nanocrystalline ZnO/CoFe 2 O 4 .

Photocatalytic degradation
To explore the photocatalytic degradation of the synthesized nanocomposite, the experiments were performed as follows: 0.05 g of the weighed composition was poured into 5 mL of a 10-ppm solution of imidacloprid pesticide, and the solution was put under direct exposure to visible light and was stirred for 30 min. Then, UV-Vis spectroscopy was applied to explore the reaction mixture in terms of studying the degradation of imidacloprid and determining the amount of imidacloprid pesticide remaining in the solution. This process was done for all stages of final catalyst synthesis, and Table 1 shows its degradation percentage. Scheme 1 depicts the proposed mechanism of imidacloprid degradation [20,21,22]. As shown, the products of degradation have been turned into inorganic compounds; besides, UV peak was not observed in a region of 254 nm; thus, no organic intermediates were logged as reported in the literature [23]. The products of decomposition were Cl À , NO 3 -, and NO 2 -. The tests for halide ions using silver nitrate solution confirmed the presence of Cl À and the method in our research group [24] was used to verify the presence of nitrate and nitrite.

Results and discussion
3.1. Characteristics of synthesized nanoparticles 3.1.1. Fourier transforms infrared spectroscopy (FT-IR) analysis As shown in Fig. 2A FTIR spectrum provides data on the chemical bonding between Zn and O. The spectrum indicated a broad peak of 432 cm À1 and shoulder 541 cm À1 corresponding to ZnO nanoparticle. The remaining spectrum was smooth with a few peaks of CO 2 . Fig. 2B shows IR spectra of the co-precipitated ZnO/CoFe 2 O 4 powders. The absorption band at 579 cm À1 is associated with the stretching vibrations of the M-O bonds in the tetrahedral site of spinel ferrite. The broadband between 3600 and 3300 cm À1 centered at 3420 cm À1 is the result of the O-H stretching vibration of physically adsorbed water onto the nanoparticle surface [25,26].

FESEM images
FE-SEM images were used to characterize the surface morphological images of the synthesized ZnO nanoparticles and ZnO/CoFe 2 O 4 magnetic nanocomposite that were shown in different magnification. As shown in Fig. 3, the uniform morphology and hexagonal rods shape of the ZnO nanoparticles and spherical shape of CoFe 2 O 4 nanoparticles dispersed on the ZnO nanoparticles are confirmed. Fig. 4 shows the analysis of EDX to explain the elemental composition of the pure ZnO nanoparticles and ZnO/CoFe 2 O 4 magnetic nanocomposite. As expected, zinc and oxygen are observed in the spectrum in Fig. 4A; iron, oxygen, cobalt, and zinc elements are observed in spectrum in Fig. 4B.

TEM images
As shown in Fig. 5, the fine dispersion of black particles (CoFe 2 O 4 ) on the grey surface (ZnO) is verified by the TEM analysis of ZnO/CoFe 2 O 4 magnetic nanocomposite. It is also assumed that the grain boundaries disperse the Co-Fe phase on the surface of ZnO. TEM Figure that the black dispersion confirmed the successful occurrences of CoFe 2 O 4 in the ZnO/CoFe 2 O 4 magnetic nanocomposite. Also, the diameter of the magnetic core is less than 10 nm. can be indexed to hexagonal structure of the ZnO nanoparticles with space group hexagonal P 63 mc. 3.1.6. Magnetic measurements As shown in Fig. 7, the saturation magnetization value was 15 emu/g. Moreover, a strong magnetic effect is made by ZnO/CoFe 2 O 4 magnetic nanocomposite, and this characteristic could be applied for photocatalyst separation. Fig. 8 shows the UV-visible absorption spectra of ZnO nanoparticles. The light source restricted our spectrometer, but a blue shift is observed in the absorption band of the ZnO nanoparticles because of the quantum confinement of the excitations present in the sample as compared with the bulk ZnO particles. This optical phenomenon implies that these nanoparticles have a quantum size effect. Fig. 8 shows UV-visible spectra absorbance peaks at 368 and 382 nm for the ZnO and CoFe 2 O 4 nanoparticles dispersed in water [27]. In this section, it was attempted to explore and enhance parameters influencing the process and the percentage of imidacloprid degradation, including photocatalyst amount, pesticide concentration, pH, radiation time, and temperature.

Effect of photocatalyst amount
It was also tried to show how the synthesized photocatalyst influences the degradation percentage of imidacloprid pesticide from 0.02 to 0.1 g. As shown in Fig. 9, an increased amount of ZnO/CoFe 2 O 4 magnetic nanocomposite, leads to an increased percentage of imidacloprid pesticide degradation. Some adsorbed molecules cause the increased destruction rate on the photocatalyst surface; besides, more photons were observed [28,29,30]. Above the specified amount, the solution is opaque, the less light enters the solution, and the photocatalytic process is reduced [31]. Fig. 10 shows how the initial concentration of imidacloprid influences the degradation rate by changing the concentration from 5-50 ppm, keeping the time at 60 min with a photocatalyst amount of 0.05 g, and the degradation percentage. The increased concentration of imidacloprid pesticides decreased the degradation percentage. The formation of Hydroxyl radicals is the result of holes reacting with OH À and H 2 O (through Radical trapping experiments). Separately, isopropyl alcohol, benzoquinone, silver nitrate, and potassium iodide were added to the pesticide solution in the presence of a catalyst. The results showed that after the addition of potassium iodide, the degradation process is decreased showing that the holes (h þ ) led to the water oxidation or the capture of an electron from the hydroxyl ion, which resulted in the pesticide degradation. Organochlorine ions replaced OH À ions adsorbed on the photocatalyst surface. OH, radicals influence the rate of imidacloprid degradation. Hydroxyl radicals are decreased in concentrations higher than the optimal concentration of organochlorine compounds due to reduced active sites accessible for the production of OH radicals. Moreover, the increased concentration of the solution had a preventive role in light penetration through the screening effect [32,33].   magnetic nanocomposite surface forms the electron-hole pairs. After the start of the reaction, they bind to the nanostructured sites very fast because of the high amount of imidacloprid; thus, the degradation is increased. 45 min was the optimal interval, according to the obtained data. The photocatalytic reactions are greatly influenced by pH. Fig. 12 shows the solutions reached under the optimal conditions with the pH range 2-10 and the photocatalytic degradation in the present study. As shown, at pH ¼ 10, the best performance was observed due to the positive reaction of the hydroxide radicals formed on the catalyst surface positively with holes, and positive holes act as dominant species and oxidation sites at low pH; whereas hydroxide radicals in pH higher than the environment (pH > 7). Consequently, in the pH > 7, the formation of the hydroxide radicals took place on the surface of ZnO/  CoFe 2 O 4 magnetic nanocomposite leading to the degradation of imidacloprid pesticides.

Effect of temperature
The relationship between the photocatalytic efficacy of imidacloprid and temperature under. optimal conditions was explored. As shown in Fig. 13, increasing temperature from 20-80 C led to increased photocatalytic efficacy of imidacloprid. Besides, increasing the temperature, increased the percentage of degradation and the highest degradation rate was logged at 80 C.

Reuse of synthesized photocatalysts
As a photocatalyst in the degradation of imidacloprid pesticide, ZnO/CoFe 2 O 4 magnetic nanocomposite was examined in terms of sustainability and reuse by applying the optimal experimental conditions. The magnet was isolated from the solution after the photocatalyst test ended; then, the solution was washed thoroughly with distilled water and allowed to dry. According to Fig. 14, the percentage of degradation of the imidacloprid with the photocatalyst synthesized after about five times remains about to 80%. In other words, that is, it is possible to use the magnetic nanoscale photocatalyst again for at least five times without losing its effectiveness.

Comparison with other methods
Our nanocatalyst was compared to other photocatalysts for the degradation of imidacloprid pesticide and results are shown in Table 2.

Conclusion
Many photocatalysts have been used for the degradation of pesticides. Among them, a composite nanostructure was synthesized possessing photocatalytic properties, separation capability, high absorption capacity, etc., which could be used to mitigate environmental issues. FT-IR, FESEM, EDX, TEM, XRD, VSM, and UV-Visible were used to characterize the photocatalyst. According to the results of this work, the ZnO/ CoFe 2 O 4 magnetic nanocomposite could be an effective nano photocatalyst in the degradation of pesticides, including imidacloprid. For the degradation of pesticides, the photocatalytic activity of the synthesized nanostructure was boosted, and the results revealed that the optimum photocatalyst value could degrade 0.05 g of at least 5 ppm of pesticide at pH ¼ 10. These results suggest that the ZnO/CoFe 2 O 4 magnetic nanocomposite is very suitable for potential applications in degradation of pesticides.

Author contribution statement
Matin Naghizadeh: Performed the experiments; Analyzed and interpreted the data; Wrote the paper.
Mohammad Ali Taher: Analyzed and interpreted the data; Wrote the paper.
Ali-Mohammad Tamaddon: Analyzed and interpreted the data.

Funding statement
This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.