A titanium dioxide-bismuth tungstate heterojunction photocatalyst and its application in degrading patulin in fruit juice

Abstract

The present invention discloses a titanium dioxide-bismuth tungstate heterojunction photocatalyst and its application in degrading patulin in fruit juice. The method first uses bismuth nitrate and sodium tungstate to react to form a bismuth tungstate precursor, and then titanium dioxide is added for compounding, enabling bismuth tungstate to grow in-situ on the surface of titanium dioxide to form a titanium dioxide-bismuth tungstate heterojunction photocatalyst. The present invention uses a one-pot hydrothermal method, the process method is simple, and when the constructed heterojunction catalyst is used to degrade patulin in fruit juice, compared with bismuth tungstate and titanium dioxide monomers, the photocatalytic activity of the composite catalyst is significantly improved.

Description

Technical Field

[0001] The present invention belongs to the technical field of photocatalytic materials. Specifically, it relates to the construction of a titanium dioxide-bismuth tungstate heterojunction photocatalyst and its application in the degradation of patulin in fruit juice. Background Art

[0002] Patulin is a natural toxic substance widely present in fruits and vegetables. It has acute toxicity, carcinogenicity, genotoxicity, immunotoxicity, skin toxicity, intestinal toxicity, hepatotoxicity, nephrotoxicity, etc., and has extensive harm to the human body system.

[0003] Photocatalytic degradation is a chemical method for degrading patulin in fruit juice. Its safe and efficient characteristics have attracted wide attention in the field of food safety control. After the photocatalyst absorbs light, electrons and holes are generated and transferred to the surface of the catalyst to react with the adsorbed electron acceptors and donors, thereby generating hydroxyl radicals (·OH) and superoxide anions (·O2 - -), and then these strong oxidizing substances react with the toxins adsorbed on the surface of the catalyst, thereby achieving the degradation effect.

[0004] The selection of photocatalysts is particularly important in photocatalytic degradation. At present, due to the safety, stability of titanium dioxide and its excellent energy band structure, it has an absolute advantage in the field of photocatalytic degradation. Therefore, the application of photocatalysts in food is mainly carried out around titanium dioxide. The titanium dioxide photocatalyst is limited by its low charge transfer efficiency and high electron-hole recombination rate, resulting in low photocatalytic efficiency. The methods for improving the photocatalytic performance of titanium dioxide mainly include element doping, sensitization, crystal plane control, noble metal deposition, and heterojunction construction, etc. The principle is mainly to reduce the band gap of the forbidden band and inhibit the ineffective recombination of photo-generated holes and electrons.

[0005] Bismuth tungstate is a new type of bismuth-based photocatalyst with a lamellar structure, an orthorhombic crystal structure, and is composed of perovskite-like (WO4) 2- octahedral layers sandwiched between (Bi2O2) 2+ layers, with a band gap of about 2.8 eV, and the absorption spectrum range can reach around 450 nm. Due to its good biocompatibility and stable optoelectronic properties, its application scope has been extended to treating polluted wastewater, sterilization, food fresh-keeping packaging, drug carriers, etc.

[0006] CN 115501869 A discloses a preparation method of a heterojunction photocatalyst, comprising the following steps: (1) dissolving bismuth salt and tungsten salt in an organic solvent respectively to obtain two metal salt solutions; (2) dispersing titanium dioxide in one of the metal salt solutions obtained in step (1), mixing evenly to obtain a suspension; (3) adding the other salt solution obtained in step (1) into the suspension obtained in step (2), mixing evenly to obtain a reaction gel; (4) transferring the reaction gel to a reaction vessel, reacting at a certain temperature for a certain time, and fully washing and drying the obtained product to obtain the heterojunction photocatalyst. However, this method uses a large amount of organic solvents, with high costs and easy to cause organic matter residues.

[0007] CN 115025769 A discloses a preparation method of a photo-generated electron-thermoelectron enhanced plasmonic photocatalyst, comprising the following steps: S1. preparing titanium dioxide nanosheets; S2. mixing bismuth nitrate, sodium tungstate, ethanol and water, adjusting the pH to 0.1 - 1 with nitric acid to form a bismuth tungstate precursor; S3. dispersing the titanium dioxide nanosheets obtained in step S1 in a mixed solution of ethanol and water to form a titanium dioxide dispersion; adding the bismuth tungstate precursor obtained in step S2 into the titanium dioxide dispersion, carrying out a hydrothermal reaction, and then washing and drying to obtain the product. However, when using ethanol to control the morphology of the catalyst, it is not conducive to the crystallization of the catalyst, and higher temperature and longer time are required during the reaction.

[0008] CN 104607178 A discloses a preparation method of a bismuth tungstate - titanium dioxide heterojunction composite photocatalytic material. This method first prepares a TiO2 sol and a Bi2WO6 precursor solution, and then mixes the two and places them in a hydrothermal reaction kettle, and uses the hydrothermal method to synthesize the Bi2WO6 and TiO2 heterojunction composite photocatalytic material in one step. However, this method has a high reaction temperature and requires the preparation of titanium dioxide colloid. Summary of the Invention

[0009] In view of the deficiencies of the prior art, the purpose of the present invention is to provide the construction of a titanium dioxide - bismuth tungstate heterojunction photocatalyst and its application in the degradation of patulin in apple juice.

[0010] In order to achieve the above technical purpose, the inventor of the present invention through a large number of experimental studies and unremitting exploration, finally adopts the following method to construct a titanium dioxide - bismuth tungstate heterojunction photocatalyst: First, react bismuth nitrate and sodium tungstate to form a bismuth tungstate colloid, and then add titanium dioxide for compounding, so that bismuth tungstate grows in-situ on the surface of titanium dioxide to form a titanium dioxide - bismuth tungstate heterojunction photocatalyst.

[0011] Specifically, the object of the present invention is achieved by the following technical solutions: A preparation method of a titanium dioxide-bismuth tungstate heterojunction photocatalyst, the method comprising the following steps:

[0012] (1) Dissolve bismuth nitrate in acetic acid or an acetic acid aqueous solution to obtain solution A;

[0013] (2) Dissolve sodium tungstate in pure water to obtain solution B;

[0014] (3) Slowly pour solution B into solution A and stir at room temperature to obtain solution C;

[0015] (4) According to the molar ratio of bismuth tungstate to titanium dioxide being 1:(4 - 7), add titanium dioxide to solution C, mix evenly to form dispersion D;

[0016] (5) Transfer dispersion D into a hydrothermal reaction kettle, react at 160 - 180 °C for 12 - 16 h, naturally cool after the reaction ends, filter the reaction product by suction, and cross - wash the precipitate with ethanol and pure water 3 - 8 times, then dry to obtain the titanium dioxide - bismuth tungstate heterojunction photocatalyst.

[0017] Further preferably, in the preparation method of the titanium dioxide - bismuth tungstate heterojunction photocatalyst as described above, the molar ratio of bismuth nitrate to acetic acid in step (1) is 1:(2.19 - 15.30).

[0018] Further preferably, in the preparation method of the titanium dioxide - bismuth tungstate heterojunction photocatalyst as described above, the bismuth nitrate in step (1) is bismuth nitrate pentahydrate.

[0019] Further preferably, in the preparation method of the titanium dioxide - bismuth tungstate heterojunction photocatalyst as described above, the sodium tungstate in step (2) is sodium tungstate dihydrate.

[0020] Further preferably, in the preparation method of the titanium dioxide - bismuth tungstate heterojunction photocatalyst as described above, in step (3), solution B is slowly poured into solution A according to the molar ratio of W:Bi = 1:2.

[0021] Further preferably, in the preparation method of the titanium dioxide - bismuth tungstate heterojunction photocatalyst as described above, the stirring speed in step (3) is 500 - 700 rpm and the stirring time is 10 - 40 min.

[0022] Further preferably, in the preparation method of the titanium dioxide - bismuth tungstate heterojunction photocatalyst as described above, the mixing method in step (4) is: first stir for 8 - 12 min, then ultrasonicate for 15 - 25 min under ultrasonic waves, and stir the solution after ultrasonication for 20 - 40 min, with the stirring speed being 500 - 700 rpm.

[0023] Further preferably, in the preparation method of the titanium dioxide-bismuth tungstate heterojunction photocatalyst as described above, the drying method in step (5) is: drying in an oven at 60-70°C for 8-12 h.

[0024] In addition, the present invention also provides the application of the titanium dioxide-bismuth tungstate heterojunction photocatalyst prepared by the above method in degrading patulin in fruit juice. Further preferably, the fruit juice is apple juice.

[0025] Compared with the prior art, the preparation method and the obtained titanium dioxide-bismuth tungstate heterojunction photocatalyst involved in the present invention have the following advantages and remarkable progress: This method uses a one-pot hydrothermal method, and the process method is simple. When the constructed heterojunction is used to degrade patulin in fruit juice, the photocatalytic activities compared with bismuth tungstate and titanium dioxide monomers are significantly improved. Under 254 nm ultraviolet light, the titanium dioxide-bismuth tungstate heterojunction photocatalyst prepared by the present invention can efficiently degrade patulin in fruit juice. When the original concentration of patulin is 500 μg / kg, the concentration of patulin can reach the national standard requirements after 30 min, and patulin can be basically completely degraded (98.46%) after 60 min. Description of the Drawings

[0026] Figure 1 : SEM images of Bi2WO6 nanoflowers (a, b); SEM image of TiO2 (P25) (c); SEM image of BT-5 (d); elemental distribution maps of BT-5, Bi, W, Ti, and O (e-i).

[0027] Figure 2 : XRD patterns of Bi2WO6, TiO2, and BT-5.

[0028] Figure 3 : XPS spectra of BT-5: full spectrum (a); Bi 4f (b); W 4f (c); Ti 2p (d); O 1s (e).

[0029] Figure 4 : Nitrogen adsorption-desorption isotherms (a) and pore size distribution maps (b) of Bi2WO6, TiO2, and BT-5.

[0030] Figure 5 : Infrared spectra of Bi2WO6, TiO2, and BT-5.

[0031] Figure 6 : UV-Vis diffuse reflectance spectra (a) and band gap diagrams (b) of Bi2WO6, TiO2, and BT-5.

[0032] Figure 7 : Photoluminescence spectra of Bi2WO6, TiO2, and BT-5.

[0033] Figure 8 : Standard curve of patulin in the HPLC detection standard sample.

[0034] Figure 9 : Photocatalytic degradation of patulin in the control group.

[0035] Figure 10 : Degradation activities of photocatalysts with different molar ratios.

[0036] Figure 11 : Recycling experiment of patulin degradation by BT-5 in acetic acid water.

[0037] Figure 12 : Influence of catalyst concentration on the degradation rate of patulin.

[0038] Figure 13 : Influence of lamp power on the degradation rate of patulin.

[0039] Figure 14 : Influence of reaction temperature on the degradation rate of patulin.

[0040] Figure 15 : Influence of initial toxin concentration on the degradation rate of patulin.

[0041] Figure 16 : Influence of Bi2WO6, TiO2, and BT-5 on the degradation rate of patulin under the optimal process conditions. Specific implementation manners

[0042] The following uses examples to further describe the implementation process and beneficial effects of the method of the present invention. The test examples are only for illustrative purposes and do not limit the protection scope of the present invention. At the same time, the obvious changes made by those of ordinary skill in the art according to the examples are also included within the scope of the present invention.

[0043] Example 1: Construction of titanium dioxide-bismuth tungstate heterojunction catalyst

[0044] Weigh 4 mmol of bismuth nitrate pentahydrate (Bi(NO3)3·5H2O), dissolve it in 2.5 ml of acetic acid by ultrasonic treatment for 10 min. Then dissolve 2 mmol of sodium tungstate dihydrate (NaWO4·2H2O) in 60 ml of ultrapure water. After that, slowly pour the sodium tungstate aqueous solution into the acetic acid solution of bismuth nitrate to obtain a milky white solution. Stir it at 600 rpm for 30 min at room temperature. Then pour 10 mmol of TiO2 (P25) into the milky white solution respectively to make the molar ratio of Bi2WO6 to TiO2 1:5. Then stir for 10 min, and then ultrasonicate for 20 min under ultrasonic treatment. And stir the ultrasonicated solution for 30 min. After that, pour the milky white suspension into a 100 mL polytetrafluoroethylene inner liner and react at 180 °C for 12 h. After the reaction is completed and the hydrothermal reaction kettle is naturally cooled to room temperature, filter it by suction and wash the precipitate alternately with ethanol and ultrapure water 6 times. Then put it into an oven at 65 °C and dry it for 8 h. Then grind the precipitate into powder to obtain light yellow nanoparticles.

[0045] Example 2: Characterization and Activity Verification of Titanium Dioxide-Bismuth Tungstate Heterojunction Catalyst

[0046] 1. Preparation of Bismuth Tungstate (Bi2WO6) Nanoparticles

[0047] Weigh 4 mmol of bismuth nitrate pentahydrate (Bi(NO3)3·5H2O) and place it in 2.5 mL of acetic acid, ultrasonicate for 10 min to completely dissolve it. Then dissolve 2 mmol of sodium tungstate dihydrate (Na2WO4·2H2O) in 60 mL of ultrapure water. After that, slowly pour the sodium tungstate aqueous solution into the acetic acid solution of bismuth nitrate to obtain a milky white colloidal solution. Stir it at a speed of 600 rpm / min for 30 min at room temperature. Then pour the white solution into a 100 mL polytetrafluoroethylene inner liner and react at 180 °C for 12 h. After the reaction is completed and the hydrothermal reaction kettle is naturally cooled to room temperature, filter it by suction and wash the precipitate alternately with ethanol and ultrapure water 6 times. Then put it into an oven at 65 °C and bake it for 12 h. Finally, grind the precipitate into powder to obtain light yellow Bi2WO6 nanoparticles.

[0048] 2. Preparation of Titanium Dioxide-Bismuth Tungstate (TiO2 / Bi2WO6) Heterojunction Catalyst

[0049] The Bi2WO6 was in-situ grown on TiO2 by using a simple hydrothermal method to construct a heterojunction structure. The preparation process of the composite catalyst was similar to that of Bi2WO6. That is, after the sodium tungstate aqueous solution was poured into the acetic acid solution of bismuth nitrate, the milky white solution was first stirred at room temperature for 10 min, and then 4 mmol, 6 mmol, 8 mmol, 10 mmol, 12 mmol, 14 mmol of TiO2 (P25) were respectively poured into the milky white solution to make the molar ratio of Bi2WO6 to TiO2 be 1:2, 1:3, 1:4, 1:5, 1:6, 1:7. Then it was stirred for 10 min, ultrasonicated for 20 min, and the ultrasonicated solution was stirred for 30 min to fully mix TiO2 into the Bi2WO6 precursor solution. The subsequent operations were the same as those in the preparation process of Bi2WO6 nanoparticles. The obtained catalysts were denoted as BT-2, BT-3, BT-4, BT-5, BT-6, and BT-7.

[0050] 3. Characterization of titanium dioxide-bismuth tungstate (TiO2 / Bi2WO6) heterojunction catalysts

[0051] Scanning electron microscope (SEM) was used to observe the surface structure and micro-morphology of the catalysts. The test conditions were: scanning voltage: 20 KV; scanning magnification: 5000 times - 10000 times;

[0052] X-ray diffractometer (XRD) could be used to characterize the crystallization of the catalysts and analyze the crystal structure of the catalysts. The test conditions were: Cu target Working current 40 mA, working voltage 40 kV, scanning range 2θ = 20° - 80°, scanning rate 10° / min;

[0053] X-ray photoelectron spectroscopy (XPS) obtained the electron binding energy by irradiating X-rays on the surface of the object, so as to infer the elemental types and valence states on the surface of the object. The measurement range scanning range was 0 - 1500 eV, and the spectrometer for collecting spectra was a 180° hemispherical energy analyzer.

[0054] Specific surface area and porosity analyzer (BET), through nitrogen adsorption test, could obtain the specific surface area, pore size and its distribution of the catalysts. The sample needed to be degassed at 150 °C under vacuum for 3 h before the test;

[0055] Fourier transform infrared spectrometer (FT-IR) analyzed the bonding situation of the catalysts through the intensity and position of the infrared absorption peaks, so as to obtain the molecular structure and chemical composition of the catalysts. The test conditions were: mixing spectroscopic grade potassium bromide and the catalyst and pressing them into tablets, using air as the background, and the number of scanning times was 64 times;

[0056] Ultraviolet-visible spectrophotometer (UV-Vis): The catalyst has different absorption intensities for light of different wavelengths. The absorption curve of the catalyst for light with wavelengths of 200 nm - 800 nm is obtained, and the band gap width of the catalyst can be calculated. The test conditions are as follows: Using barium sulfate as the blank background, the scanning wavelength is 200 nm - 800 nm, the scanning speed is in the medium-speed mode, and the slit width is 0.5 nm;

[0057] Photoluminescence spectrometer (PL): Analyze the separation degree of photo-generated electrons and holes on the surface of the catalyst. The higher the peak intensity, the lower the separation degree. The test conditions are as follows: Excitation wavelength: 290 nm, excitation slit: 5.0 nm, emission slit: 20.0 nm, scanning voltage: 400 V.

[0058] (1) Scanning electron microscope (SEM)

[0059] Figure 1 (a, b) show the morphology of Bi2WO6 nanoflowers. From Figure 1 (b), it can be seen that Bi2WO6 nanoflowers are composed of many Bi2WO6 nanosheets, and most of the diameter sizes are between 2 μm - 4 μm, with the majority being in the range of 3 μm - 4 μm; Figure 1 (c) is the morphology of TiO2 (P25). It can be seen that this TiO2 is spherical, with a diameter size between 25 nm - 50 nm, and there is a serious agglomeration phenomenon; Figure 1 (d) is the heterojunction structure composed of Bi2WO6 and TiO2. The catalyst forming BT-5 has two morphologies, which are composed of flaky Bi2WO6 and granular TiO2 mixed. Due to the in-situ growth method used, the flower-like three-dimensional morphology of Bi2WO6 is damaged and becomes irregular flakes, and in some places, TiO2 is embedded into the Bi2WO6 flakes; compared with pure TiO2, the introduction of Bi2WO6 significantly improves the agglomeration phenomenon of TiO2, which is also confirmed by the elemental mapping images. Figure 1 (e - i) are the elemental mapping images of BT-5. Figure 1 (e) is the total elemental mapping image. Figure 1 (f - i) are the mapping images of Bi, W, Ti, and O elements respectively. They have the same position, indicating that the two catalysts are evenly distributed, proving the successful synthesis of the composite material.

[0060] (2) X-ray diffraction (XRD)

[0061] From the XRD pattern ( Figure 2)It can be seen that the diffraction peaks of all catalysts are sharp and intense, without other miscellaneous peaks, indicating good crystallinity and high purity. The diffraction peaks at 28.41°, 32.91°, 47.23°, 56.05°, 58.60°, 69.06°, 76.16°, 78.61°, 87.73° in Bi2WO6 correspond to (103), (200), (220), (303), (107), (400), (109), (307), (318) in the Bi2WO6 standard card NO.39-0256 (Maksoud et al 2021) respectively; the diffraction peaks at 25.38°, 27.56°, 37.86°, 48.02°, 53.90°, 55.09°, 62.81°, 69.02°, 70.34°, 75.16°, 82.68° in TiO2 correspond to (101), (004), (200), (105), (211), (204), (116), (220), (215), (224) in the TiO2 standard card NO.21-1272 respectively, where (101), (004), (200), (105) correspond to the crystal planes of anatase TiO2 (Chandrika et al 2022). From the spectrum of the composite material BT-5, it can be seen that this spectrum has the main crystal planes of both, further proving that this catalyst is obtained by the combination of Bi2WO6 and TiO2; at the same time, it can also be seen that there is no obvious shift in the diffraction peaks of TiO2 and Bi2WO6, indicating that Bi2WO6 mainly grows on the surface of TiO2 during growth, rather than being embedded in the TiO2 lattice.

[0062] (3)X-ray photoelectron spectroscopy (XPS)

[0063] Figure 3 is the XPS spectrum of BT-5. The binding energy is corrected through the data of C1s, and the binding energy of contaminated carbon is selected as 284.8 eV. From the full spectrum (a), it can be seen that the main constituent elements of BT-6 are Bi, W, Ti, and O, which is consistent with the previous EDS scan. Figures (b-e) are the fine spectra of Bi4f, W4f, Ti2p, and O1s respectively. The 161.16 eV and 158.86 eV in Figure (b) correspond to Bi4f 5 / 2 and Bi 4f 7 / 2 , and the absence of other peaks indicates that Bi exists in the form of Bi 3+ , without other valence states; the 37.28 eV and 35.17 eV in Figure (c) correspond to W4f 5 / 2 and W4f 7 / 2 , indicating that W 6+The existence (Zhang et al 2018). The orbits corresponding to 458.69 eV and 464.84 eV in Figure (d) are Ti2p 3 / 2 and Ti2p 1 / 2 , indicating that titanium mainly exists as Ti 4+ . The binding energy difference (near 6.2 eV) and positions of the two orbits exactly meet the requirements of anatase TiO2 (Zhang et al 2021). Meanwhile, the O orbit in Figure (e) is O1s, indicating that the oxidation state of oxygen in this catalyst is all O 2- . The 529.32 eV and 530.00 eV at points A and B correspond to the Ti-O bond and the -OH group on the catalyst surface respectively (Zhang et al 2022). Moreover, compared with pure Bi2WO6 and TiO2, the orbits of Bi, W, Ti, and O in BT-5 all tend to shift towards lower binding energies. This may be due to the formation of new chemical bonds caused by the presence of TiO2, which increases the electron concentration, leading to an enhanced electron shielding effect and thus a decrease in the binding energy, which also verifies the formation of the heterojunction (Yang et al 2017).

[0064] (4) Specific surface area and pore size distribution tester (BET)

[0065] As can be seen from Figure 4 (a), all three catalysts produced obvious hysteresis loops between (P / P0) = 0.8 - 1.0, indicating the presence of many mesopores. At the same time, it can also be seen that the hysteresis loops of the three extended to the relative pressure P / P0 ≈ 1, indicating that the catalysts have macropores (>50 nm). From Figure 4 (b), the pore size distribution of the three catalysts can be seen. It can be seen that BT-5 indeed has micropores and mesopores, which is consistent with Table 1 and Figure 4 (a). However, the main pore sizes of the three still distribute between 0 - 5 nm. At the same time, combining Table 1, the specific surface areas of the three can be known. A high specific surface area means having more active sites, which is beneficial to improving the photocatalytic efficiency. Among them, the specific surface area of Bi2WO6 is the smallest, 24.418 m 2 / g. The compounding of TiO2 is beneficial to the increase of the catalyst's specific surface area, making the specific surface area of BT-5 in the middle state, which is 1.62 times that of Bi2WO6. This is consistent with the trends in Figure 4 (a) and Figure 4 (b).

[0066] Table 1: BET specific surface area related parameters

[0067]

[0068]

[0069] (5) Fourier transform infrared spectrometer (FT-IR)

[0070] Figure 5 are the infrared spectra of the three catalysts. In all spectra, the broad absorption band at 3433 cm -1 is the stretching vibration of -OH on the catalyst surface, and the peak at 1630 cm -1 is the bending vibration of the H-O-H bond in the water molecules adsorbed on the catalyst surface; the two peaks at 2920 cm -1 and 2856 cm -1 belong to the C-H stretching vibration, and the peak at 2365 cm -1 is the stretching vibration of C=O, which comes from CO2 in the air (Chandrika et al 2022, Wang et al 2019). In the fingerprint region, for Bi2WO6, the absorption peaks at 578 cm -1 , 734 cm -1 , 825 cm -1 and 1385 cm -1 are caused by the asymmetric stretching of Bi-O-Bi, W-O, Bi-O and the stretching vibration of the W-O-W bond bridge (Chen et al 2017, J. Yu et al 2005); for TiO2, the broad peak between 820 cm -1 and 450 cm -1 is caused by the stretching vibrations of Ti-O-Ti and Ti-O (Coromelci et al 2022). The infrared spectra verify again that the catalyst is composed of Bi2WO6 and TiO2. (6) Ultraviolet-visible diffuse reflectance spectroscopy (UV-Vis DRS)

[0071] Figure 6 (a) are the ultraviolet-visible diffuse reflectance spectra of TiO2, BT-5, and Bi2WO6, Figure 6(b) was obtained by calculating the band gaps of the three catalysts using αhv = A(hν - Eg) (Mane et al 2021). It can be visually seen from Figure (a) that compared with TiO2, BT-5 has increased its absorption intensity in the visible light range, and compared with Bi2WO6, it has increased the absorption intensity in the ultraviolet light range. In short, the light absorption range of BT-5 has undergone a partial red shift, and the absorption intensity of light with different wavelengths has also increased. As can be seen from Figure (b), the band gaps of TiO2, BT-5, and Bi2WO6 are 3.13 eV, 2.92 eV, and 2.73 eV respectively. The light absorption of BT-5 is very similar to that of Bi2WO6. The introduction of Bi2WO6 is beneficial to reducing the band gap of BT-5, which means that BT-5 can be excited by some visible light and can generate more electron-hole pairs compared with TiO2.

[0072] (7) Photoluminescence spectrum (PL)

[0073] Figure 7 The photoluminescence spectra of TiO2, BT-5, and Bi2WO6 are shown. It can be visually seen the recombination rates of photo-generated electrons and holes in the three catalysts. The higher the fluorescence intensity, the higher the recombination rate, which is more unfavorable for the photocatalytic reaction (Li et al 2017). It can be seen from the figure that the order of fluorescence intensity is TiO2 > Bi2WO6 > BT-5, which indicates that after the combination of the two catalysts, the recombination rate of photo-generated electrons and holes in BT-5 is the lowest, and more electrons and holes transfer to the surface of the catalyst to participate in the reaction. Therefore, the final result is that the photocatalytic activity of BT-5 is the best.

[0074] Example 3: Activity verification of titanium dioxide-bismuth tungstate heterojunction catalyst

[0075] 1. Preparation of experimental simulated juice

[0076] Patrezin obtained by dissolving in ethyl acetate was pipetted into a rotary evaporation flask and evaporated to dryness, and then patulin was redissolved with acetic acid water with pH = 4.0 to prepare a patulin acetic acid aqueous solution with a patulin concentration of 500 μg / kg.

[0077] 2. Photocatalytic degradation experiment

[0078] First, add 250 mg of the catalyst to the reaction vessel, and then add 250 g of acetic acid aqueous solution with pH = 4.0. Sonicate for 2 min while stirring. The operation equipment of the reaction equipment refers to the equipment in Li Weiwei (Master's Thesis, Research on the Removal of Patulin in Fruit Juice by TiO2 Photocatalysis, Huazhong Agricultural University, 2019). Then, carry out a dark reaction for 30 min under stirring in the dark. Next, turn on the ultraviolet lamp for the photocatalytic reaction. Take samples every 5 min, 4 ml each time. The termination time of the photocatalytic reaction is 35 min. During the reaction process, control the reaction temperature at 25.0 ± 1.0 °C.

[0079] 3. Measurement of patulin in simulated apple juice by HPLC

[0080] Refer to the detection method of GB 5009.185-2016 and improve it.

[0081] (1) Standard stock solution of patulin

[0082] Dissolve 5 mg of patulin standard in ethyl acetate and make up to 100 mL with ethyl acetate to prepare an ethyl acetate solution of 50 mg / mL, and store it in an environment at 4 °C for use as needed.

[0083] (2) Preparation of a series of standard solutions

[0084] Take the above 50 mg / mL patulin ethyl acetate solution. After evaporating a certain amount of patulin ethyl acetate solution to dryness using a rotary evaporator, dissolve it with acetic acid aqueous solution with pH = 4.0 to prepare a patulin acetic acid aqueous solution, and then dilute it with acetic acid water to standard solutions of 10 μg / kg, 25 μg / kg, 50 μg / kg, 100 μg / kg, 250 μg / kg, 500 μg / kg, 1000 μg / kg. The standard curve is as Figure 8 shown.

[0085] (3) Pretreatment of sample preparation

[0086] Filter the reaction solution using a 0.45 μm organic nylon 66 filter head, and the obtained filtrate can be directly detected by the instrument.

[0087] (4) HPLC chromatographic conditions

[0088] Liquid chromatography conditions: C18 chromatographic column (5 μm, 4.6 mm × 250 mm); isocratic elution with acetonitrile - water (V:V = 10:90), flow rate 1.0 mL / min; ultraviolet detector, 276 nm; injection volume, 50 μL; sample elution time, 12 min.

[0089] 4. Recycling experiment of titanium dioxide - bismuth tungstate (TiO2 / Bi2WO6) catalyst

[0090] The reaction process was the same as that of the above photocatalytic degradation experiment. After the reaction, the reaction solution was filtered by suction, and the precipitate was washed 6 times alternately with ethanol and water. After drying in an oven at 65 °C for 12 h, the photocatalyst after the reaction was obtained. Then, the photocatalytic degradation reaction was carried out, and the experiment was repeated four times. The final degradation rate was used as the evaluation criterion.

[0091] 5. Experimental Results and Analysis

[0092] (1) Patulin Degradation Curve of the Control Group

[0093] Figure 9 They were the degradation curves under light illumination, dark adsorption, and monomer photocatalyst. Patulin was relatively sensitive to ultraviolet light, and the degradation rate reached 58.91% after 35 min under ultraviolet light; while under the condition of no light illumination, the adsorption amount of TiO2 / Bi2WO6 to patulin was very small, about 2%. In the experiment of monomer photocatalysis, the effect of TiO2 (94.0%) was much better than that of Bi2WO6 (42.71%). For Bi2WO6, its degradation effect was not as good as that under pure ultraviolet conditions. It may be because the generation of ·OH was restricted under acidic solution, affecting the photocatalytic performance of lBi2WO6, resulting in that although Bi2WO6 absorbed part of the ultraviolet light, it did not show catalytic activity (Carina and Chu 2013); in addition, the molar mass of Bi2WO6 was relatively large, 697.8 g / mol. At the same mass, its molar number was much lower than that of TiO2, resulting in relatively fewer active sites.

[0094] (2) Influence of the Ratio of Bi2WO6 to TiO2 on the Degradation Rate of Patulin in Simulated Apple Juice

[0095] Figure 10 It shows the influence of different molar ratios of Bi2WO6 and TiO2 on the photocatalytic performance of the composite catalyst during the preparation of the heterojunction catalyst. The molar number of TiO2 was 2, 3, 4, 5, 6, and 7 times that of Bi2WO6. As can be seen from the figure, the photocatalytic activity of all catalysts was improved. After 35 min, the degradation rates of basically all catalysts with heterojunction structures reached more than 98%, exceeding the degradation rate of the TiO2 experimental group (94.0%). Moreover, the improvement of the degradation efficiency had a certain regularity. With the increase of the relative content of TiO2, the degradation rate showed a trend of first increasing and then decreasing. Among them, when the molar ratio of the two was 1:5, the catalyst activity was the strongest, and 53.1% of patulin could be degraded at 5 min.

[0096] (3) Reusability of Titanium Dioxide - Bismuth Tungstate (TiO2 / Bi2WO6) Catalyst

[0097] The stability and reusability of a catalyst are intuitive indicators for evaluating the quality of the catalyst. From Figure 11 It can be seen that although the degradation efficiency of the catalyst decreased before 20 min in the early stage, patulin could still be completely degraded. After four cycles, the degradation efficiency of the photocatalyst began to decline at 30 min, reaching 91.5%, without an obvious decline, indicating that the properties of the catalyst are relatively stable and it has good recyclability. Example 4: Practical application of titanium dioxide-bismuth tungstate heterojunction catalyst for the degradation of patulin in apple juice 1. Preparation of positive apple juice for the experiment

[0098] In this experimental design, patulin was directly added to the Longshang Huaniu apple juice purchased from the supermarket as the reaction system. First, 5 mg of patulin was dissolved in 100 mL of ethyl acetate to prepare a high-concentration patulin ethyl acetate solution with a concentration of 50 μg / mL. Then, a quantitative amount of the patulin ethyl acetate solution was pipetted into a 50 mL rotary evaporation flask each time. After rotary evaporation to dryness, a part of the apple juice was poured into the rotary evaporation flask. After the patulin was completely dissolved, it was mixed evenly with the remaining apple juice. To ensure the uniform dispersion of patulin in the apple juice, the apple juice was stirred with a magnetic stirrer for more than 2 h before use and kept stirring during use until the apple juice was completely used up.

[0099] 2. HPLC measurement of patulin in apple juice

[0100] Compared with the simple simulated apple juice system, the apple juice has a complex chemical system. Therefore, liquid-liquid extraction and purification of the apple juice are required. Since the aqueous solutions of Na2CO3 and NaHCO3 are both alkaline and likely to destroy patulin during the sample purification process, referring to the experimental operation of Algarra (2009), NaH2PO4 was used as the purification reagent, and only the instrument used for extraction was changed. The specific steps are as follows: First, the apple juice was centrifuged at 4000 rpm for 4 min to obtain the supernatant. Then, 2.00 g of apple juice was accurately weighed into a 10 mL centrifuge tube, 2 mL of ethyl acetate-n-hexane mixed liquid (V:V = 96:4) and 200 mg of NaH2PO4 were added, vortexed for 4 min, and then centrifuged at 3000 rpm for 3 min. 1 mL of the upper organic phase was transferred to a 50 mL rotary evaporation flask, rotary evaporated to dryness at 45 °C, 2 mL of acetic acid water was added, and the rotary evaporation flask was fixed on an oscillator and shaken at a speed of 250 rpm for 20 min. Then, it was filtered through a 0.45 μm organic phase filter head nylon 66, and the obtained filtrate could be used for on-machine detection.

[0101] 3. Single-factor experiment on photocatalytic degradation of patulin

[0102] Weigh 250 g of the positive apple juice prepared in 3.3.1 into a reaction vessel, then add a certain amount of catalyst, and use stirring and ultrasonic intermittently to ensure that the photocatalyst can be evenly dispersed in the apple juice. The ultrasonic time is 2 min. After that, set up the experimental equipment and react at a certain temperature and a certain lamp power for 60 min. Take 5 ml of samples every 5 min in the first 30 min and 5 ml of samples every 10 min in the last 30 min. Among them, the catalyst concentration gradients are set as 0.1 g / kg, 0.5 g / kg, 1.0 g / kg, 1.5 g / kg (other conditions: temperature 4, initial toxin concentration 500 μg / kg, lamp power 4 w); the temperature gradients are set as 4 °C, 15 °C, 25 °C, 35 °C (other conditions: catalyst concentration 1.0 g / kg, initial toxin concentration 500 μg / kg, lamp power 4 w); the lamp power gradients are set as 1 w, 2 w, 4 w (other conditions: catalyst concentration 1.0 g / kg, initial toxin concentration 500 μg / kg, temperature 25 °C); the initial toxin concentration gradients are set as 250 μg / kg, 500 μg / kg, 1000 μg / kg, 2000 μg / kg (other conditions: catalyst concentration 1.0 g / kg, temperature 25 °C, lamp power 4 w).

[0103] 4. Orthogonal Experiment on Photocatalytic Degradation of Patulin

[0104] Since the deviation of the degradation rate of patulin is mainly caused by environmental factors during catalytic degradation, and during the processing, requirements such as energy saving and low-temperature processing need to be met. Therefore, based on the results of the single-factor experiment, the catalyst dosage, lamp power, and reaction temperature are selected as the influencing factors, and the orthogonal experiment with three factors and three levels is designed using SPSS software. The specific experimental arrangements are as follows:

[0105] Table 2 Orthogonal Experiment Factor Design Table

[0106]

[0107]

[0108] Table 3: Orthogonal Experiment Arrangement Table

[0109]

[0110] 5. Experimental Results and Analysis

[0111] 5.1 Single-Factor Experimental Results of Photocatalytic Degradation of Patulin by BT-5

[0112] 5.1.1 Influence of Catalyst Concentration on Patulin Degradation Rate

[0113] As Figure 12As shown in the figure, the influence of catalyst concentration on the degradation efficiency is relatively obvious. When the catalyst concentration increases from 0.1 g / kg to 1.5 g / kg, the degradation efficiency of the catalyst first increases and then decreases. When the catalyst concentration is 1.0 g / kg, the degradation effect reaches the best state. When the catalyst concentration is 1.5 g / kg, in the first 20 minutes of the reaction, its catalytic efficiency is not much different from that of the experimental group with a catalyst concentration of 1.0 g / kg, but after 20 minutes, its catalytic efficiency decreases significantly. The phenomenon of high catalytic efficiency in the early stage may be due to the adsorption of toxins by the catalyst. With a high catalyst concentration, more toxins are adsorbed during the dark reaction period. In the light reaction stage, the photoinduced adsorption effect of the high-concentration catalyst is also better. Generally speaking, it is because the high-concentration photocatalyst adsorbs more toxins in the early stage. However, in the later stage, due to the high light scattering of the high-concentration catalyst and the easy agglomeration of the high-concentration catalyst, these lead to a low light utilization rate of the catalyst. When the adsorption effect of the photocatalyst on toxins is not significantly stronger than that of other experimental groups with low photocatalyst concentrations, its degradation rate begins to gradually decrease, and the gap with other groups becomes more and more obvious (Zhu et al 2022).

[0114] 5.1.2 Effect of lamp power on the degradation rate of patulin

[0115] As can be seen Figure 13 from the figure, with the increase of lamp power, the photocatalytic efficiency of patulin has been greatly improved. There are mainly two reasons for this: First, patulin itself is very sensitive to ultraviolet light. When the ultraviolet intensity increases, patulin has the opportunity to absorb more ultraviolet light, so it is beneficial to the degradation of patulin. Second, when the photocatalyst contacts more ultraviolet light, more photogenerated electrons and holes will be generated, and more active substances will be produced. The final result is that the opportunity for patulin to combine with active substances is increased.

[0116] 5.1.3 Effect of reaction temperature on the degradation rate of patulin

[0117] As can be seen Figure 14 from the figure, when the temperature gradually increases, the rate of photocatalytic degradation also gradually increases. However, in terms of the final result, after 30 minutes, the difference in its degradation rate becomes less and less obvious with the increase of degradation time. When the reaction temperature is 4℃, the degradation rate at 60 minutes reaches 96.6%. In apple juice with an initial concentration of 500 μg / kg, the residual amount of patulin has reached the national standard requirements, indicating that the catalyst still has high activity in a low-temperature environment and has the potential to be applied in the low-temperature juice processing technology.

[0118] 5.1.4 Effect of initial concentration of patulin on the degradation rate

[0119] As can be seen Figure 15It can be seen that when the initial concentration is 250 μg / kg, the degradation rate of patulin is the lowest, and the final degradation rate is also the lowest. As the concentration of patulin gradually increases, the degradation rate of patulin first increases and then decreases. This may be because the concentration of patulin is relatively high at the beginning, and the generated active substances have more opportunities to contact patulin. However, when patulin gradually decreases, this contact opportunity also decreases. Therefore, after the degradation time reaches 30 min (93.06%), the degradation degree of the 250 μg / kg experimental group can no longer be significantly improved. Finally, the degradation rate of patulin is 98.05%, but the residual rate of patulin is still within the national standard range, which is 4.9 μg / kg. For the 2000 μg / kg experimental group, although its degradation rate and degradation degree (99.66%) are relatively high, the residual amount of patulin is the largest, which is 6.8 μg / kg. This shows that when the toxin concentration is very low, the degradation efficiency of BT-5 is relatively low, and it also shows that the range of toxin concentrations suitable for BT-5 is very large, which can meet the detoxification requirements of apple juice containing different concentrations of patulin during the production process.

[0120] 5.2 Orthogonal test results of photocatalytic degradation of patulin by BT-5

[0121] Table 4 Orthogonal test results and analysis

[0122]

[0123]

[0124] In practical applications, since the initial concentration of the toxin is a raw material characteristic during product processing and has great uncertainty, it is not included in the orthogonal test factor table. Table 4 shows the results and analysis of the orthogonal test. It can be seen that the order of the influence of catalyst concentration, lamp power, and reaction temperature on the degradation rate of patulin is: lamp power > reaction temperature > catalyst concentration. It can also be seen that the best degradation process of patulin under this set of equipment is: lamp power is 4 W, reaction temperature is 25 °C, and catalyst concentration is 1.0 g / kg.

[0125] Figure 16 It is the degradation rate curves of three photocatalysts under the optimal process conditions. It can be seen from the figure that after the two catalysts are loaded, both the degradation rate and the degradation degree are further improved. It is worth pondering that when Bi2WO6 is specifically applied in apple juice, its catalytic activity is stimulated, but it has no effect in acetic acid water. Since the light exposure opportunities before and after are not very different, according to the principle of photocatalytic reaction, it is speculated that the photoinduced adsorption phenomenon generated by Bi2WO6 is different in the two solutions. Substances such as pectin and sugars in the juice may have a certain positive effect on the adsorption of toxins, and the specific reasons need to be further explored.

Claims

1. Application of a titanium dioxide-bismuth tungstate heterojunction photocatalyst in degrading patulin in fruit juice under low-temperature conditions of 4 to 15 °C. The preparation method of the titanium dioxide-bismuth tungstate heterojunction photocatalyst comprises the following steps: (1) Dissolve bismuth nitrate in acetic acid or an acetic acid aqueous solution to obtain solution A; (2) Dissolve sodium tungstate in pure water to obtain solution B; (3) Slowly pour solution B into solution A and stir at room temperature to obtain solution C; (4) According to the molar ratio of bismuth tungstate to titanium dioxide of 1:(4 - 7), add titanium dioxide into solution C, mix evenly to form dispersion D; (5) Transfer dispersion D into a hydrothermal reaction kettle, react at 160 - 180 °C for 12 - 16 h, naturally cool after the reaction, filter the reaction product by suction, cross-wash the precipitate with ethanol and pure water for 3 - 8 times, and dry to obtain the titanium dioxide-bismuth tungstate heterojunction photocatalyst.

2. The application according to claim 1, wherein In step (1), the molar ratio of bismuth nitrate to acetic acid is 1:(2.19 - 15.30).

3. The application according to claim 1, characterized in that, In step (1), the bismuth nitrate is bismuth nitrate pentahydrate.

4. The application according to claim 1, characterized in that In step (2), the sodium tungstate is sodium tungstate dihydrate.

5. The application according to claim 1, wherein In step (3), pour solution B slowly into solution A according to the molar ratio of W:Bi = 1:

2.

6. The application according to claim 1, characterized in that, In step (3), the stirring speed is 500 - 700 rpm and the stirring time is 10 - 40 min.

7. The application according to claim 1, wherein In step (4), the mixing method is: first stir for 8 - 12 min, then ultrasonicate for 15 - 25 min under ultrasound, and stir the solution after ultrasonication for 20 - 40 min, with the stirring speed being 500 - 700 rpm.

8. The application according to claim 1, wherein In step (5), the drying method is: dry in an oven at 60 - 70 °C for 8 - 12 h.

9. The application according to claim 1, wherein The fruit juice is apple juice.

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