Preparation of mesoporous ammonium phosphotungstate/silica catalyst and study on the degradation performance of chlorothalonil

Pesticide is an essential means of production for modern agricultural production. The use of a large number of pesticides not only guarantees agricultural production and income, but also causes a large number of environmental problems (Zinatloo-Ajabshir et al. 2020), such as the Lipton tea pollution incident in 2012, the pesticide pollution accident in Guangzhou on the Pearl river (de Souza et al. 2017; Prabavathi et al. 2020), the pesticide fish-poisoning incident in Shien, Hubei, etc. Water pollution caused by pesticides has attracted widespread attention. Reducing or eliminating the hazards of pesticides and secondary pollution is also an important direction of environmental chemistry research at this stage. Chlorothalonil is a widely used substitutive benzene fungicide (Huang et al. 2019; Lv et al. 2020), not only used in agriculture to prevent and control diseases of vegetables, melons and fruits, peanuts, rice, wheat and other crops, but also used in industry for mildew-proof coatings and additives (Lu et al. 2005). Chlorothalonil is a low-toxicity fungicide. Its annual production in China exceeds 8,000 t (He et al. 2018), the solubility in water is 0.9 mg/L, and its half-life in water is 8–9 days. Under normal conditions, chlorothalonil powder acute transoral LD50 in both rats and rabbits was greater than 10,000 mg/kg, and rats with acute inhalation LD50 > 4.7 mg/L. However, some studies have shown that the harm of chlorothalonil to the environment should not be underestimated. For example, Tang Mingde and others found that chlorothalonil can induce cell mutation and sister chromatid exchange in mouse bone marrow lymphocytes (Tang & Yi 1985). Córdova Juárez et al. (2010) found that chlorothalonil can cause genetic mutations in aquatic organisms, amphibians and plant cells. Wang et al. (1986) found that chlorothalonil has sensitive effects on the human body, and skin contact can cause contact dermatitis. Mozzachio et al. (2008) explored the relationship between chlorothalonil and colon, lung, and prostate cancers (Meng et al. 2019). The United States Environmental Protection Agency (USEPA) has listed chlorothalonil as one of the possible inducements of human cancer, and it is a potential threat to environmental and food safety (Lawruk et al. 1995; He et al. 2018). It is imperative to eliminate the harm of residual chlorothalonil in the water environment to the environment and human body.

In the water environment, the degradation pathway of chlorothalonil is mainly biodegradation, hydrolysis and photochemical degradation (Zhang et al. 2017). Caux et al. (1996) studied the degradation of chlorothalonil in a simulated marine environment, and the results showed that the half-life of chlorothalonil degradation in seawater was 8–9 days; Li et al. (2004) studied the influence of several factors on the hydrolysis of chlorothalonil and the results showed pH has a significant effect on the hydrolysis of chlorothalonil. The hydrolysis half-life of chlorothalonil in distilled water is 117.46d, while the hydrolysis half-life of chlorothalonil in groundwater and Chaohu Lake water is 58.24d and 56.8d, respectively. Although biodegradation is environmentally protective, it has a long degradation period. Hydrolysis is greatly affected by pH and the temperature of the water body. Photochemical degradation has the best effect, but its efficiency is greatly affected by photosensitizers (Zhang & Wang 2018). Therefore, choosing a suitable photocatalyst is the key to degrading chlorothalonil in a water body (Wang et al. 2015; Li et al. 2019), the compound being a kind of polynuclear inorganic polymer compound containing an oxygen bridge. It is composed of heteropoly anions, cations (including metal cations, protons and other organic cations) and crystal water or organic molecules (Zhao 2008). Its unique structure and spectral properties make it suitable for becoming an efficient photocatalyst (Chen et al. 2011; Bayal et al. 2017). With regard to the synthesis and application of heteropoly acid photocatalysts (Chen et al. 2014; Shaban et al. 2017), scholars at home and abroad have carried out relevant research in this field, such as Friesen et al. (1999), who showed that CsCl and H₃PW₁₂O₄₀ react to generate Cs₃PW₁₂O_40, and Cs₃PW₁₂O₄₀ can rapidly degrade N-methylpyrrolidone under 254 nm ultraviolet light exposure. However, traditional heteropoly acid photocatalysts have a strong water solubility, are easily soluble in water and oxygen-containing organic solvents, have low reuse efficiency and small specific surface area and are difficult to recycle and reuse. Therefore, the preparation of water-insoluble photodegradation catalysts for organic pesticides has great theoretical and practical significance (Li et al. 2004, 2014; Chang et al. 2017).

D/max-38 X-ray Diffractometer (incident light source is CuKα target, incident wavelength λ = 0.154 nm, voltage 40 kV, current 40 mA). Nicoleis10 Fourier Infrared Spectrometer (resolution 4 cm⁻¹, wavenumber range 4,000–400 cm⁻¹, KBr tabletting method), QUANTA 200 Electron Microscope, UV-2550 Spectrophotometer (with diffuse reflection measuring device – integrating sphere, measuring wavelength 200–900 nm, calcium sulfate as reference), XPA Photochemical Reaction Instrument (light source is 100 W mercury lamp, Trace 1300 Gas Chromatograph (ECD detector, TR-5 column). Phosphotungstic acid, ammonium chloride and ethyl acetate are analytically pure. Florisil Solid Phase Extraction Column (500 mg/6 mL), chlorothalonil purity 98.2%, is provided by the National Pesticide Quality Supervision and Inspection Center.

Firstly, a certain amount of CTAB was dissolved in distilled water and stirred to be clarified in a constant temperature water bath at 40 °C. Then, anhydrous ethanol was added and stirred for 10 min. Secondly, the ammonia water was added drop by drop and stirred for 10 min. TEOS was added until the solution turned into opalescent turbidities (n_ethanol: n_TEOS:n_CTAB = 60:2.5:1). The stirring continued for 2 h. Finally, ammonium phosphotungstate prepared by reflux adsorption was added to the solution and stirred for 2 h. After washing the white sediment with distilled water until Cl-free, it was first dried at 105 °C, then dried overnight at 70 °C in vacuum. Finally, the NH₄PW/SiO₂ composite material was obtained by forging and burning at 500 °C in a Muffle furnace for 3 h.

The NH₄PW/SiO₂ was characterized by D/max-38 X-ray diffractometer (the incident light source was CuKα target, the incident wavelength was λ = 0.154 nm, the voltage was 40 kV, the current was 40 mA, the scanning rate was 0.095°/min, the measuring range was 2θ = 0.5°–8°), NOVA 2000 specific surface tester (under 7 K condition, N₂ was used as adsorbent; degassing conditions: degassing stage, heating rate 10 °C/min, final temperature 90 °C, sample degassed at 363 K for 3 h before analysis), NICOLET IS10 Fourier infrared spectrometer (KBr tabletting test, resolution 4 cm⁻¹, scanning range 4,000 cm⁻¹–400 cm⁻¹).

The photocatalytic activity of NH₄PW/SiO₂ was carried out on an XPA-7 photochemical reactor, using a 100W mercury lamp as light source. Chlorothalonil was detected according to the literature method (Wang et al. 2014). The retention time of chlorothalonil was 2.28 min and the detection limit was 0.0035 mg/L.

The N₂ adsorption–desorption experiments were carried out on mesoporous SiO₂ and NH₄PW/SiO₂. The adsorption–desorption isotherms of SiO₂ and NH₄PW/SiO₂ were obtained as shown in Figure 1. As can be seen from the figure, there are obvious hysteresis loops caused by capillary condensation in the range of relative pressure of 0.25–1.0, belonging to the class IV Langmuir adsorption–desorption isotherm, and belonging to the isotherm peculiar to the adsorption of porous substances by a multi-molecular layer. This indicates that the prepared sample is a typical mesoporous material. The hysteresis loop shape on the desorption branch is between H₂ and H₃, which also indicates the existence of mesopore structure in the catalyst (Chen et al. 2006; Cao et al. 2008). Table 1 shows the Brunauer–Emmett–Teller (BET) analysis of SiO₂ and NH₄PW/SiO₂. As shown in Table 1, the mesoporous NH₄PW/SiO₂ has a pore diameter of 2.93 nm, a specific surface area of 1,368.15 m²/g, and a pore volume of 1 cm³/g. Compared with mesoporous SiO₂, there is no law that the specific surface area decreases and the pore diameter becomes lower after loading. It indicates that the mesoporous structure of silica is not destroyed after the ammonium phosphotungstate loading.

As shown in Figure 2, it can be seen that both have strong diffraction peaks in the range of 2θ = 2°–3°, and there is still one diffraction peak in the range of 4°–6°, corresponding to the characteristic crystal face peaks (200) and (210). This shows that both mesoporous SiO₂ and mesoporous NH₄PW/SiO₂ have mesoporous structures, and also shows that the introduction of ammonium phosphotungstate does not destroy the mesoporous structure, but weakens the crystallinity of SiO₂.

IR characterization of NH₄PW/SiO₂ is shown in Figure 3. As can be seen from the figure, there is a strong O-H contraction vibration peak at 3,191 cm⁻¹, which may be related to the presence of bound water inside the material. After SiO₂ is loaded with ammonium phosphotungstate, phosphotungstic acid is at 1,079 cm⁻¹ (P-O); 989 cm⁻¹ (W = O); 889 cm⁻¹(W-Oc-W). The characteristic absorption peak of 782 cm⁻¹(W-Oc-W) is still obvious. The characteristic peaks of SiO₂ are as follows: 466 cm⁻¹ is the stretching vibration absorption peak of the Si-O-Si bond. The bending vibration absorption peak of the Si-O-Si bond is 803 cm⁻¹ and 953 cm⁻¹ is the stretching vibration absorption peak of Si-O in silanol. The asymmetric vibration absorption peak of the Si-O-Si bond in siloxane is 1,062 cm⁻¹ (Yang 2010). At 1,076 cm⁻¹, there is also a broad peak formed by the overlapping 1,079 cm⁻¹ peak of phosphotungstic acid and 1,062 cm⁻¹ peak of SiO₂, which also shows that the heteropoly acid interacts with the carrier, but does not destroy the Keggin structure of phosphotungstate and the mesoporous structure of SiO₂.

The effect of different dosages of NH₄PW/SiO₂ on the degradation of chlorothalonil is shown in Figure 4. As can be seen from the figure, when the dosage of NH₄PW/SiO₂ is 0.02 g, the degradation effect is the best, the degradation rate reaches 82.55%, and the degradation rate of NH₄PW (64.76%) is 17.79%. When the amount of NH₄PW/SiO₂ exceeds the optimum amount, the degradation rate decreases. This may be because the turbidity of the solution will affect the photocatalytic effect. When the amount of the catalyst is large, the turbidity of the suspension increases, the light transmittance decreases, and eventually the degradation effect is lowered.

In Figure 5, the degradation effect becomes better with the increase of the degradation time within 0–120 min. When the degradation time is 120 min, NH₄PW/SiO₂ has the best degradation effect on chlorothalonil, with the degradation rate reaching 68.53%. It may be due to the recombination of some electrons and holes as time increases, the electron transfer rate decreases, and the degradation rate of chlorothalonil decreases.

Figure 6 is a graph showing the effect of different chlorothalonil concentrations on the degradation rate of NH₄PW/SiO₂. From the graph, it can be seen that with the increase of chlorothalonil concentration, the degradation rate of NH₄PW/SiO₂ decreases. This is because the larger chlorothalonil concentration affects the transmittance of light, so that the absorption of light by the photocatalyst is lowered, and thus the degradation rate is lowered. In the following experiments, the concentration of chlorothalonil was selected at 5 mg/L.

The effect of different pH on chlorothalonil degradation by NH₄PW/SiO₂ is shown in Figure 7. From Figure 7, NH₄PW/SiO₂ has the best degradation effect on chlorothalonil when the pH is about 5, with the degradation rate reaching 71.21% respectively. The degradation effect is not good in chlorothalonil solution with low or high pH, which may be due to different pH values affecting the structure of mesoporous silicon dioxide, the stability of NH₄PW/SiO₂ and the dispersibility in chlorothalonil solution, thus affecting their degradation effect.

The degradation effects of NH₄PW and NH₄PW/SiO₂ are shown in Figure 8. As can be seen from Figure 8, the removal rate of chlorothalonil by SiO₂ loaded with NH₄PW is obviously increased compared with that before loading, with the removal rate increasing from 64.16% to 83.23%.

Table 2 shows the comparison of the degradation effect of chlorothalonil in different methods. It can be seen that the mesoporous material NH₄PW/SiO₂ has the best degradation effect on chlorothalonil, and the removal rate can reach 83.23%. By preparing the mesoporous material NH₄PW/SiO₂, the degradation activity of the photocatalyst is effectively improved. Therefore, the photocatalytic degradation of chlorothalonil by the mesoporous material NH₄PW/SiO₂ is of great significance.

The changes of CODcr before and after chlorothalonil degradation are shown in Table 3. From the data in Table 3, it can be seen that SiO₂ loaded with NH₄PW can more effectively remove chlorothalonil from aqueous solution, and the COD change value is 22.74, which is 10.76 higher than for NH₄PW (11.98).

According to the photocatalytic degradation of chlorothalonil by NH₄PW/SiO₂, the degradation process is fitted by pseudo-first-order and second-order kinetic models and intraparticle diffusion models. The fitting parameters are shown in Table 3. It can be seen from the data in Table 4 that the degradation of chlorothalonil by NH₄PW/SiO₂ is more in line with the first-order kinetic model. The correlation coefficient of the linear equation fitted by the first-order kinetic model was 0.9882, and the calculated Q_e value of NH₄PW/SiO₂ to chlorothalonil was 9.213 mg/g, which was close to the actual value (9.431 mg/g).

The degraded water samples were extracted three times with 5 mL of chloroform, ethyl acetate, toluene and n-hexane, respectively, and the organic phases were combined. After evaporation by a rotary evaporator, the samples were fixed in volume with 1 mL of toluene and analyzed by gas chromatography–mass spectrometry (GC-MS). The results are shown in Figure 9. The peak of the intermediate product appeared at t = 7.68 min and was identified as 1,2,3-trimethylbenzene by mass spectrometry.

The solution before and after degradation was centrifuged, and the supernatant was taken for IC analysis. As a result, the ion chromatogram before and after degradation was obtained as shown in Figure 10. It can be seen from the spectrum that the concentration of Cl⁻ and NO³⁻ changed before and after NH₄PW loading SiO₂, Cl⁻ increased from 3.80 mg/L before loading to 17.16 mg/L, and NO³⁻ from 0.5237 mg/L before loading. Increased to 0.6147 mg/L, with the formation of NO²⁻ after the load, the concentration is 4.1221 mg/L. It can be seen that the photocatalytic degradation activity of NH₄PW is increased by the load, which is consistent with the conclusions drawn from the performance. The degradation activity of chlorothalonil after SiO₂ loading NH₄PW is improved, mainly due to the large specific area of SiO₂ and porous channels, which is beneficial to the adsorption of chlorothalonil to its surface, and finally improves the photocatalytic degradation performance.

According to the analysis of chlorothalonil degradation products, it is preliminarily concluded that NH₄PW/SiO₂ photocatalytic degradation of chlorothalonil is a surface photocatalytic reaction caused by ·OH attack. When ultraviolet light is irradiated onto the surface of the photocatalyst, an electron–hole pair will be generated on the surface thereof, and OH⁻ and H₂O adsorbed on the surface of the catalyst by oxidation of the hole will form ·OH, thereby promoting oxidation of chlorothalonil. However, the increase of the chlorothalonil degradation activity of SiO₂ loaded with NH₄PW is due to the fact that mesoporous SiO₂ can adsorb chlorothalonil to its surface by using its large specific area and pore channel characteristics, thus increasing the contact surface between ·OH and chlorothalonil and finally improving the photocatalytic degradation performance.

Non-water-soluble ammonium phosphotungstate was added to an alkaline alcohol water system by using cetyltrimethylammonium bromide and ethyl orthosilicate as raw materials, and mesoporous phosphorus was synthesized by one-step synthesis by the sol-gel method. Ammonium tungstate/silica photocatalyst (NH₄PW/SiO₂): BET and XRD analysis show that the prepared NH₄PW/SiO₂ is a mesoporous material. IR characterization indicated that the SiO₂ supported NH₄PW retained the Keggin structure of phosphotungstic acid and preserved the mesoporous structure of SiO₂. When the amount of NH₄PW/SiO₂ was 0.02 g, the light was 180 min, the concentration of chlorothalonil was 5 mg/L, and the pH = 5, the degradation rate of NH₄PW/SiO₂ to chlorothalonil reached 83.23%, and the degradation kinetics conformed to the first-order kinetic equation. Mesoporous SiO₂ effectively enhanced the photocatalytic activity of NH₄PW (64.16%). Therefore, NH₄PW/SiO₂ is an excellent photocatalyst, which not only makes up for the shortcomings of difficult recovery and low efficiency of phosphotungstate, but also can be used in water environment remediation and the chemical industry.

This work was funded by the Yunnan Provincial Department of Education YMU-DEAKIN International Functional Materials Related Laboratory (2017ZZX087), China University Students Innovation and Entrepreneurship Training Program (2018JWC-DC-30).

All relevant data are included in the paper or its Supplementary Information.