Removal of Neonicotinoid Insecticides in a Flat-plate Photoreactor

The aim of this study was to investigate the photolytic and photocatalytic degradation of neonicotinoids in an aqueous solution. Acetamiprid (ACE) and thiacloprid (TIA), two widely used insecticides, were used as model components. Experiments were performed in a flat-plate photoreactor under conditions of recirculation of the reaction mix - ture over an immobilised photocatalyst layer (TiO 2 modified by urea) using two artificial lamps for simulation of solar irradiation (2.4 % UVB and 12 % UVA; 300–700 nm). The catalyst used was characterised by XRD, UV/Vis-DRS, BET, SEM/EDX, and CHNS analysis. All experiments were performed at room temperature and atmospheric pressure, at a recirculation flow rate of 200 cm 3 min −1 , and at an initial concentration of ACE and TIA of 10 mg dm −3 . For most measurements, the reaction mixture was sonicated for 15 min immediately before charging the reactor. The study focused on the influence of the pH of the initial solution on the efficiency of photocatalytic and photolytic degradation. It was found that photocatalytic deg - radation of the two model components was most effective under acidic operating conditions, i.e. , at pH 4.5, while photolysis resulted in their minimum degradation. It was also observed that pretreatment of the reaction mixture with ultrasound promoted photocatalytic degradation, while in the case of photolytic degradation, the application of ultrasound did not contribute to better degradation. Finally, photocatalytic degradation of TIA proved to be more successful than photodegradation of ACE (66.4 % vs. 25.8 %) under identical process conditions.


Introduction
Neonicotinoids are a relatively new and very popular class of insecticides that are currently registered in more than 120 countries around the world.Since their introduction in the late 1980s, the use of neonicotinoids has increased due to their unique mode of action and relatively low toxicity to non-target organisms and the environment. 1lthough a new generation of neonicotinoids has been developed in recent years, imidacloprid and thiacloprid with five-membered rings in their structure, thiamethoxam with a six-membered ring, and the four noncyclic compounds acetamiprid, clothianidin, dinotefuran, and nitenpyram have been widely used and studied.However, their high solubility in water and very slow degradation in the environment result in residues of neonicotinoids entering soil, sediments, groundwater, and surface waters.Despite their effectiveness in minimising crop damage, an increasing number of studies report adverse effects of neonicotinoids on humans, non-target insects, and aquatic invertebrates.In general, the excessive and uncontrolled use of neonicotinoids poses a risk to the entire ecosystem.[4][5] Due to increasing concentrations of neonicotinoid insecticides in surface and groundwater, advanced techniques and methods need to be developed to prevent bioaccumulation of insecticides and other undesirable persistent compounds in the environment, to enable their complete degradation, and ensure favourable conditions for environmental remediation.In the last decade, heterogeneous photocatalysis has attracted great attention as one of the advanced oxidation processes.Photocatalysis is a "green" and energy-saving technology with great potential.It can be applied to difficult-to-biodegrade, complex, and highly concentrated pollutants found in wastewater that cannot be degraded by classical water treatment methods.7][8] Although TiO 2 has many desirable properties, its practical application in photocatalysis is severely limited due to a high carrier recombination rate and a relatively wide band gap (≈ 3.0 eV for rutile and ≈ 3.2 eV for anatase), which only allows absorption of the ultraviolet component of sunlight (3-5 %).Considerable efforts have been made to improve the use of visible light by TiO 2 and to reduce the light-induced electron-hole recombination rate at the surface of TiO 2 .As shown in Table 1, the most common strategies to affect the absorption range, include: i) photosensitising the semiconductor surface with organic dyes, ii) doping TiO 2 with metals, especially oxidation-and corrosion-resistant noble metals such as Au, Ag, and Pt, and iii) doping TiO 2 with non-metals (nitrogen, fluorine, sulphur, carbon, oxygen).0][11][12][13][14] Various physical and chemical methods have been used to introduce nitrogen into the TiO 2 crystal lattice, including ball milling, sputtering, plasma or ion implementation, sol-gel method, solvothermal method, hydrothermal method, direct hydrolysis of organic/inorganic salts, and oxidation of titanium nitride. 14Ammonium chloride, guanidine hydrochloride, hydrazine, trimethylamine, urea, and many other organic compounds can be used as nitrogen sources.Urea is a hydrocarbon with high nitrogen content according to its molecular formula (CH 4 N 2 O), and is a potential additive for the preparation of nitrogen-doped titanium dioxide. 15e aim of this study was to modify the original TiO 2 -P25 photocatalyst in a suitable way to reduce the band gap or energy gap between valence and conduction band (E g ), so that the photocatalyst can work efficiently under simulated solar irradiation.The studies were carried out in a flat-plate reactor with recirculation of the reaction mixture using an immobilised photocatalyst layer (nitrogen-doped TiO 2 , N-TiO 2 ).The insecticides acetamiprid (C 10 H 11 ClN 4 ) and thiacloprid (C 10 H 9 ClN 4 S) were used as model components, and the influence of the initial pH of the reaction mixture on the efficiency of photodegradation of acetamiprid and thiacloprid under the conditions of simulated solar radiation was investigated.Based on the obtained experimental data, the kinetics of heterogeneous photocatalytic degradation of the model components was studied and the model parameters estimated.

Materials
All reagents were of analytical grade and used without further purification.The analytical standards acetamiprid and thiacloprid (PESTANAL TM ) (purity ≥ 98.0 %, ≤ 100 %) used for HPLC analysis were provided by Sigma Aldrich Company Ltd.Laboratory-grade acetamiprid, Boxer Mospilan 200 SP (w = 20 %), was provided by Genera Inc., Kalinovica, Croatia.Ultrapure water (18.2MΩ cm −1 ) from a Nirosta Ultrapure Water System, Nirosta, Osijek, Croatia, was  used in this study to prepare solutions for the irradiation experiments.The initial pH values of the reaction mixture were adjusted with sulphuric acid (H 2 SO 4 ) and sodium hydroxide (NaOH) supplied by VWR International S.A.S., Fontenay-sous-Bois, France.The photocatalyst, titanium dioxide (TiO 2 -P25) nanopowder containing 80 % anatase and 20 % rutile with a primary particle size of 30-50 nm and a BET surface area of 50 m 2 g −1 was purchased from Evonik, Essen, Germany, and modified with urea (Kemika d. d., Zagreb, Croatia).The mineral binder Procol (Lasselsberger-Knauf d. o. o., Đurđevac, Croatia) was used to immobilise the photocatalyst.Formic acid 98 %, p.a. and HPLC-grade acetonitrile were purchased from VWR International, Radnor, Pennsylvania, USA.

Synthesis and characterisation of photocatalyst
Nitrogen-doped TiO 2 (N-TiO 2 ) was prepared by mechanically mixing urea as a nitrogen precursor with the TiO 2 -P25 powder in a 4 : 1 ratio.Mechanical mixing was followed by heat treatment at atmospheric pressure and a temperature of 400 °C for 1 h.After cooling the mixture to room temperature, the resulting product was ground to a fine yellowish powder, which as such was ready for further immobilisation on an abrasive material used as a carrier.For immobilisation of the TiO 2 catalyst, a dense paste was prepared from 1.5 g of modified TiO 2 , 0.5 g of commercial mineral binder (Procol), and 10 cm 3 of distilled water.After mixing, the paste was applied to the substrate in a thin layer using a brush.It was then dried at room temperature for 24 h.The immobilised layer thus prepared was adhered to the metal plate with double-sided adhesive tape so that it could be inserted into the photoreactor.
The TiO 2 -P25 and N-TiO 2 catalysts used were characterised by several characterisation methods, including XRD, UV/Vis-DRS, BET, SEM /EDX, and CHNS analysis.X-ray powder diffraction (XRD) measurements were performed using a Shimadzu XRD 6000 diffractometer in the range 2θ ≈ 20-65° with Cu-Kα.UV-Vis diffuse reflectance spectra (UV-Vis DRS) were recorded using a Perkin-Elmer Lambda 35 UV/Vis spectrophotometer at room temperature in the wavelength range of 200-800 nm.The textural properties (specific surface area, total pore volume, and average pore diameter) were determined according to the Brunauer-Emmet-Teller model (BET) using a nitrogen adsorption device at 77 K (Micromeritics ASAP 2000).Scanning electron microscopy (SEM/EDX) images of the photocatalysts were acquired using an Oxford Instruments energy dispersive X-ray analyser EDS/INCA 350.The content of C, H, N, and S was determined using a Perkin-Elmer 2400 Series II CHNS analyser.

Experimental setup
Photodegradation experiments were performed in a 240 cm 3 plate photoreactor under recirculation conditions over an immobilised layer of nitrogen-doped TiO 2 .Irradiation was performed with two commercial lamps (Arcadia, 8W, T5, 300 × 16 mm) simulating solar radiation (300-700 nm; 0.30-0.68mW cm −2 ), placed above the photoreactor.Light intensity was measured with a UVX radiometer (Labormed, Zagreb, Croatia) and with the appropriate sensors for UVA, UVB, and UVC radiation before and after the end of each experiment.Photocatalytic degradation of acetamiprid (ACE) and thiacloprid (TIA) at an initial concentration of 10 mg dm  The optical absorption spectra of the measured samples are shown in Fig. 3.For N-TiO 2 , a slight shift of the absorption edge to a lower energy in the visible light region was observed (Fig. 3a).The band gap energies were calculated using the Kubelka-Munk function by plotting [F(R)E] 1/2 versus the light energy, and correspond to 3.38 and 2.92 eV for TiO 2 -P25 and N-TiO 2 , respectively (Fig. 3b) .
The BET surface area, pore size, and pore volume were estimated to be 56.0 m 2 g −1 , 15.2 nm, and 0.22 cm 3 g −1 for N-TiO 2 , respectively.These properties increased slightly in regard to TiO 2 -P25, which had 53.6 m 2 g −1 , 10.4 nm, and 0.15 cm 3 g −1 as BET surface area, pore size, and pore volume, respectively, indicating that N-TiO 2 could be useful for photodegradation.To investigate the chemical composition and chemical state of the catalysts, SEM/EDX analysis was performed.As listed in Table 2, TiO 2 -P25 contained Ti, C, and O with atomic compositions of 33, 3, and 64 at.%, respectively.Results of SEM/EDX did not confirm the presence of nitrogen in the N-TiO 2 , and the atomic compositions of Ti, C, and O were 29, 5, and 66 at.%, respectively.
Since EDX only measures the presence of nitrogen on the surface, CHNS elemental analysis was performed to provide secondary information on nitrogen content, which yielded a nitrogen content of 8.3 wt%.

Photodegradation measurements
Photolytic degradation of ACE and TIA was carried out at pH 4.5 in two ways: i) with ultrasound pretreatment of the reaction mixture, and ii) without ultrasound pretreatment.The results obtained are shown in Fig. 4. It was found that ultrasonic pretreatment of the reaction mixture slightly affected the obtained results.Photolytic degradation led to insignificant degradation of the stable model components, i.e., the most important requirement for photodegradation of ACE and TIA is the presence of a suitable photocatalyst.To investigate the photocatalytic activity of nitrogen-doped TiO 2 , the degradation of ACE and TIA was performed using two commercial lamps simulating solar radiation (Fig. 5).It could be observed that the presence of the photocata-lyst greatly improved the degradation of the two pesticides compared to that without the photocatalyst.These results confirm the positive role of the use of N-TiO 2 for the degradation of pesticides.It was also found that pretreatment of the reaction mixture with ultrasound for a period of 15 min had a positive effect on the efficiency of photocatalytic degradation.A possible explanation for these results is the additional activation of the centres on the photocatalyst by the effect of ultrasound and the formation of additional reactive species (e.g., hydroxyl radicals) by the ultrasound pretreatment of the reaction mixture.The maximum ACE and TIA removal efficiencies at an initial solution pH of 6.5 were 20.2 and 50.3 %, respectively.In conclusion, a nitrogen-doped TiO 2 photocatalyst was prepared by a relatively practical and environmentally friendly method using urea as a nitrogen source, which resulted in a reduction of E g , and consequently, higher degradation rates of pesticides present in wastewater.
In heterogeneous photocatalysis, pH has a significant effect on the photodegradation of organic pollutants.The surface charge of the photocatalyst depends on the pH of the reaction solution. 14The interaction of the reactant molecule with the photocatalyst, the size of the TiO 2 particles, and the nature of the radicals and intermediates formed during photodegradation are strongly dependent on the pH, which also affects the adsorption of the reactant on the photocatalyst surface, and ultimately the efficiency of the process.In order to determine the optimal pH for performing photocatalytic decomposition, it is necessary to know the so-called point of zero charge (PZC).The PZC refers to the pH at which the surface of the photocatalyst has a net electrical charge of zero.Although the PZC for TiO 2 depends on the method of its preparation, a value of 6.25 is reported in the literature mainly for commercial TiO 2 -P25. 15From the results shown in Fig. 6, it could be concluded that, after 240 min of irradiation with Arcadia lamps used to simulate solar irradiation, the highest conversion of both ACE and TIA was obtained at a pH of 4.5-5.5, while no clear trend was observed at other pH values.
The better degradation rate at a lower pH can be attributed to the fact that, at a pH of < 6.25, the TiO 2 surface is positively charged, while ACE is negatively charged at the same time due to the characteristic value of its dissociation constant of pK a = 4.16.This leads to a better adsorption of ACE on the surface of the photocatalyst, and thus to a more successful degradation. 16No characteristic pK a values were found in the literature for TIA, which is reported not to dissociate or whose pK a is very low (0.01 +/− 0.1).
It follows that TIA does not behave like an acid or alkali, i.e., the efficiency of photocatalytic degradation cannot be related to the electrostatic interactions between TIA and TiO 2 .Therefore, it is very difficult to predict theoretically the effects of pH on the photodegradation of TIA on a TiO 2 photocatalyst.However, the experimental results obtained in this work are in agreement with those published by Černigoj et al. 17 These authors reported the synergistic effect of ozone (O 3 ) and TiO 2 in the photodegradation of TIA in the acidic and neutral pH range, but the synergy disappeared in the alkaline range, which they attributed to the faster ozone degradation.The values of the estimated rate constants of photocatalytic degradation of ACE and TIA and the values of the root mean square deviation, RMSD, are shown in Table 3.The characteristic values for the rate constants k determined for TIA are higher than the corresponding values for ACE under identical operating conditions, indicating that TIA is degraded more successfully, i.e., that its degradation results in higher conversions.

Conclusion
The aim of this study was to investigate the efficiency of nitrogen-doped TiO 2 in the form of an immobilised layer as a potential photocatalyst for the photocatalytic degradation of acetamiprid and thiacloprid in aqueous solutions under simulated solar irradiation.The nitrogen-doped TiO 2 was synthesised using low-cost urea as a nitrogen precursor.
The N-doping shifted the energy band gap of TiO 2 to lower energy, i.e., the absorption edge was shifted to the visible light region.The research focused on the effect of pH during photodegradation and was studied in a pH range of 4.5 to 8.5.In addition, the effect of pretreating the reaction mixture with ultrasound for 15 min on the photolytic and photocatalytic degradation results was investigated.The results of this study show that N-TiO 2 can efficiently catalyse the photodegradation of the insecticides acetamiprid and thiacloprid in the presence of simulated solar irradiation.The best degradation rate was obtained at pH 4.5, while pretreatment of the reaction mixture with ultrasound promoted photocatalytic degradation.Photocatalytic degradation of thiacloprid proved to be more successful than photodegradation of acetamiprid under the same process conditions.

List of abbreviations and symbols
Popis kratica i simbola