Biochar / titanium dioxide composite photocatalytic material, preparation method therefor, and use thereof

Abstract

The present invention relates to the field of photocatalysis, and relates specifically to a biochar / titanium dioxide composite photocatalytic material, a preparation method therefor, and a use thereof. The preparation method comprises the following steps: pretreating wheat straw, then transferring same into a crucible, and carrying out a carbonization treatment to obtain straw biochar; mixing the straw biochar with hydrochloric acid, followed by washing and then drying to obtain biochar; mixing butyl titanate with absolute ethanol to obtain solution A, and thoroughly and uniformly mixing absolute ethanol with distilled water and acetic acid to obtain solution B; adding solution B dropwise into solution A, performing magnetic stirring and then adding the biochar, continuing to stir, followed by aging and then drying, grinding, and then calcining to obtain a biochar / titanium dioxide composite photocatalytic material. The present invention provides an efficient, sustainable, and economical solution for the treatment of hexavalent chromium wastewater, promoting the application of biochar and nano titanium dioxide composite materials in the environment field.

Description

A biochar / titanium dioxide composite photocatalytic material and its preparation method and application Technical Field

[0001] The present invention belongs to the technical field of photocatalysis, and in particular relates to a biochar / titanium dioxide composite photocatalytic material and a preparation method and application thereof. Background Art

[0002] Hexavalent chromium (Cr(VI)) in water is a common toxic pollutant, posing a serious threat to human health and the environment. Hexavalent chromium is widely used in industrial processes such as electroplating, leather making, metallurgy, dyeing, and wastewater treatment, resulting in the generation of large amounts of hexavalent chromium wastewater. Hexavalent chromium is highly toxic and persistent, readily soluble in water, and accumulates in plants, animals, and crops through surface water and agricultural irrigation, posing a potential threat to human health. Commonly used technologies for treating hexavalent chromium include chemical precipitation, ion exchange, and membrane separation. However, these technologies are limited and challenged by their high cost, high energy consumption, and significant byproduct pollution. Therefore, the development of efficient, economical, and environmentally friendly treatment technologies for the removal of hexavalent chromium wastewater is of great significance.

[0003] In recent years, photocatalysis has attracted considerable research attention as a potential environmental remediation method. Photocatalysis utilizes light to excite catalysts to generate highly active free radicals and free electrons, which in turn degrade and redox organic and inorganic pollutants. Among numerous photocatalytic materials, titanium dioxide (TiO2) has garnered widespread attention due to its excellent photocatalytic performance. However, pure TiO2 exhibits several limitations in photocatalysis, such as a large band gap, easy recombination of photogenerated electron-hole pairs, and difficulty separating from water. To overcome these limitations, the preparation of composite materials has become an effective approach. Biochar (BC), a porous, high-surface-area carbon material, has shown potential for applications in adsorption and catalysis. BC is produced by pyrolysis or charring of renewable biomass, such as straw and wood chips. Its unique structure and chemical properties provide excellent adsorption properties for the removal of organic and inorganic pollutants. Its surface is easily modified and can be composited with catalytic materials, making it possible to load TiO2 onto it. BC as a carrier has a high specific surface area and rich functional groups, which can provide the ability to adsorb hexavalent chromium in wastewater. Nano-titanium dioxide as a catalyst can convert the adsorbed hexavalent chromium into harmless trivalent chromium through photocatalytic reaction. At the same time, BC as an e - reservoir, by placing e -The photocatalytic reduction of hexavalent chromium by biochar-supported titanium dioxide is a promising approach to improve the efficiency of the adsorption-photocatalytic reaction. However, there are relatively few reports on the adsorption-photocatalytic reduction of hexavalent chromium by biochar-supported titanium dioxide in recent years. Therefore, there is an urgent need to provide an efficient, sustainable, and economical solution for the treatment of hexavalent chromium wastewater and to promote the application of biochar-nano-titanium dioxide composites in the environmental field. Summary of the Invention

[0004] The purpose of the present invention is to provide a biochar / titanium dioxide composite photocatalytic material and its preparation method and application, to provide an efficient, sustainable and economical solution for the treatment of hexavalent chromium wastewater, and to promote the application of biochar and nano-titanium dioxide composite materials in the environmental field.

[0005] The object of the present invention is achieved through the following technical solutions:

[0006] The present invention provides a method for preparing a biochar / titanium dioxide composite photocatalytic material, comprising the following steps:

[0007] (1) Pre-treating wheat straw;

[0008] (2) The pretreated wheat straw is transferred into a crucible for carbonization to obtain straw biochar;

[0009] (3) mixing the straw biochar obtained in step (2) with hydrochloric acid, washing it, and then drying it to obtain biochar;

[0010] (4) mixing butyl titanate and anhydrous ethanol to obtain solution A, and thoroughly mixing anhydrous ethanol with distilled water and acetic acid to obtain solution B;

[0011] (5) adding solution B in step (4) dropwise to solution A, adding the biochar prepared in step (3) after magnetic stirring, and continuing stirring to obtain a black sol;

[0012] (6) The black sol obtained in step (5) is aged, dried, ground, and then calcined to obtain a biochar / titanium dioxide composite photocatalytic material.

[0013] Furthermore, in step (1), the specific operations of the pretreatment are ultrasonicating the wheat straw, washing with deionized water, drying at 65° C. for 24 h, crushing with a wall-breaking machine, and sieving with 100 mesh.

[0014] Furthermore, in step (2), the specific operation of the carbonization treatment is to perform slow pyrolysis in a tubular furnace under N2 atmosphere at a gas flow rate of 100 mL / min, 500°C, a heating rate of 5°C / min, for 2 hours, and then cool to room temperature.

[0015] Furthermore, in step (3), the mixing is to mix the straw biochar with 1M hydrochloric acid for 2 hours; and the parameters in the drying process are drying at 65°C for 24 hours.

[0016] Furthermore, in step (4), the content of butyl titanate in the solution A is 10 mL, and the content of anhydrous ethanol is 30 mL; the content of anhydrous ethanol in the solution B is 10 mL, the content of distilled water is 4 mL, and the content of acetic acid is 1 mL.

[0017] Furthermore, in step (5), the magnetic stirring time is 30 minutes; the continued stirring time is 2 hours; and the amount of biochar added is 0.025 g.

[0018] Furthermore, in step (6), the aging time is 24 hours; the parameters of the drying process are drying at 65°C for 12 hours; and the specific operation of calcining is placing it in a muffle furnace and calcining it at 300°C for 2 hours, with a heating rate of 5°C / min.

[0019] The present invention also provides a biochar / titanium dioxide composite photocatalytic material.

[0020] The present invention also provides an application of a biochar / titanium dioxide composite photocatalytic material in the removal of hexavalent chromium wastewater.

[0021] Furthermore, the removal of the hexavalent chromium wastewater is specifically carried out by reducing the hexavalent chromium using a biochar / titanium dioxide composite photocatalytic material.

[0022] The beneficial effects of the present invention are:

[0023] This invention synthesizes a straw biochar / titanium dioxide nanocomposite photocatalytic material that exhibits higher adsorption-photocatalytic reduction activity than pure TiO2 under illumination, effectively promoting the synergistic adsorption-photocatalytic reduction of Cr(VI). Combining the different structural and photochemical characterization results of the prepared composite photocatalysts with the results of photocatalytic reduction of Cr(VI), it is confirmed that TiO2 effectively adheres to the biochar (BC) material. The high specific surface area and low average pore size of the BC / TiO2 composite explain its enhanced photocatalytic reduction activity due to the induced adsorption and further reduction of Cr(VI) on its mesoporous surface. Furthermore, the low band gap and low impedance of the BC / TiO2 composite also explain its excellent photocatalytic effect. The enhanced photocatalytic activity efficiency is primarily attributed to the synergistic effect of photogenerated carriers on the composite surface under illumination and their easy transfer to the biochar, resulting in suppressed photogenerated hole-electron recombination and enhanced electron transfer. Furthermore, it possesses efficient and recyclable regeneration capabilities. In summary, this invention provides an efficient, sustainable, and economical solution for the treatment of hexavalent chromium wastewater, promoting the application of biochar and nano-titanium dioxide composites in the environmental field. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 shows the XRD patterns of different materials;

[0025] Figure 2 is the XPS spectrum, where a is the spectrum of the composite photocatalyst components, b, c, and d are the XPS spectra of C1s, O1s, and Ti2P, respectively;

[0026] Figure 3 shows the diffuse reflectance spectra and DRS of different components;

[0027] Figure 4 is a SEM-EDX analysis diagram, where the first three figures are SEM images of BC at different scales, the middle three figures are SEM images of TBC-3 at different scales, and the last three figures are SEM images of TiO2 at different scales;

[0028] Figure 5 shows the morphology of the TBC-3 sample. The first three images are EDS elemental mapping images, the middle three images are typical TEM images of the prepared samples, and the last three images are EDX spectra and SEM images of TBC-3.

[0029] Figure 6 shows the N2 adsorption-desorption isotherms of BC, TiO2, and TBC-3 photocatalysts;

[0030] FIG7 is a Barrett-Joyner-Halenda (BJH) curve of BC, TiO2 and the prepared composite material;

[0031] Figure 8 shows the TGA curves of BC and TBC-3;

[0032] FIG9 is an electrochemical impedance spectroscopy measurement diagram;

[0033] Figure 10 shows the results of the photocatalytic reduction experiment of the prepared composite materials and TiO2 and the BC adsorption experiment, where a is the photocatalytic reduction of Cr (VI) by BC, TiO2, TBC-1, TBC-2, TBC-3, TBC-4 and TBC-5 (a), b is the photocatalytic reduction of Cr (VI) by TBC-3 composite in the presence of K2S2O8, EDTA, BQ and MT scavengers, and c is the cyclic experiment of photocatalytic reduction of Cr (VI) using TBC-3 composite material. DETAILED DESCRIPTION

[0034] The following experimental examples and embodiments are intended to further illustrate the present invention, but are not intended to limit the present invention.

[0035] Example 1

[0036] 1. Preparation of biochar

[0037] Wheat straw was collected from the experimental fields of Anhui University of Science and Technology in Fengyang County, Chuzhou City. The straw was ultrasonically treated, washed with deionized water, dried at 65°C for 24 hours, crushed with a wall breaker, sieved through a 100-mesh sieve, and transferred to a crucible. In a tube furnace, under a nitrogen atmosphere, with an airflow rate of 100 mL / min, at 500°C and a heating rate of 5°C / min, the straw biochar was slowly pyrolyzed for 2 hours. The resulting biochar was then cooled to room temperature. The resulting biochar was mixed with 1 M hydrochloric acid for 2 hours, washed with deionized water until neutral, and dried at 65°C for 24 hours to obtain the biochar for later use.

[0038] 2. Preparation of BC / TiO2 composite materials

[0039] Solution A was prepared by mixing 10 mL of tetrabutyl titanate and 30 mL of anhydrous ethanol. Solution B was prepared by thoroughly mixing 10 mL of anhydrous ethanol, 4 mL of distilled water, and 1 mL of acetic acid. Solution B was added dropwise to solution A and magnetically stirred for 30 minutes. Then, 0.100 g, 0.050 g, 0.025 g, 0.010 g, and 0.001 g of biochar were added, respectively, and stirring continued for 2 hours to obtain a black sol, which was then aged for 24 hours. The solid was then dried at 65°C for 12 hours to obtain a black solid mass, which was then ground. The solid was then calcined in a muffle furnace at 300°C for 2 hours at a heating rate of 5°C / min to obtain BC / TiO2 composite samples, labeled TBC-x, where x is 1, 2, 3, 4, and 5, corresponding to the different BC masses. Pure titanium dioxide was prepared using the same method as above, without the addition of BC, and designated TiO2. An appropriate amount of commercial titanium dioxide (Degussa) was prepared and designated P25.

[0040] 3. Characterization

[0041] The structure was analyzed using X-ray diffraction (XRD - Cu Kα1 - λ = 1.54060 Å) at an applied voltage (45 kV), current (100 mA), and scattering angle (2θ / 20-80), and X-ray photoelectron spectroscopy (XPS - Al Kα - 1486.6 eV) at an applied voltage (12 kV) and current (23 mA). Optical properties were measured using UV-vis diffuse reflectance spectroscopy (Shimadzu UV-3600). The elemental content and distribution were analyzed using high-resolution transmission electron microscopy (HRTEM-JEM-2100F) at an accelerating voltage (200 kV), scanning electron microscopy (SEM, Hitachi, SU-8100) at an applied voltage (5.0 kV), and energy dispersive spectroscopy (EDS, Oxford, X-MaxN 80T). The specific surface area and pore size were determined by Brunauer-Emmeter-Teller (BET) analysis with nitrogen adsorption-desorption isotherms (Micromeritics ASAP 2460) at 77 K. Thermal stability tests were performed on a thermogravimetric analyzer (TG-DTG, STA-449F5, Netzsch, Germany) at a heating rate of 10°C / min from 40°C to 800°C. Electrochemical impedance spectroscopy (EIS) was recorded using a three-electrode system (Ag / AgCl reference electrode, carbon rod, and nickel foam covered with the prepared sample as the working electrode) on an electrochemical workstation (CHI-660E, Chenhua Instruments Co., Shanghai, China). A working electrode sample was prepared by dispersing 10 mg of the photocatalyst in 0.5 mL of ethanol and 0.5 mL of ultrapure water. After ultrasonic treatment for 2 h, 0.1 mL of the slurry was repeatedly dripped onto a nickel foam substrate (1 × 2 cm). The sample was dried and then added repeatedly until the addition was complete. Electrochemical impedance spectroscopy (EIS) measurements were performed in a 1 M Na₂SO₄ electrolyte solution at room temperature (25 ± 0.2°C). Measurements were performed at 1 V over a frequency range of 0.01 Hz to 100 kHz, with an AC current amplitude of 5 mV.

[0042] As shown in Figure 1, only anatase TiO2 is formed in the prepared photocatalyst, with almost no impurities found. Samples with different BC doping ratios show significant peak broadening. Diffraction peaks corresponding to the (101), (004), (200), (105), (211), (204), (116), (220), (215), and (224) crystal planes are generated. It can be observed that when the amount of biochar added is less than 0.01g, the anatase diffraction peak of TiO2 is more obvious, while when the amount of biochar added is higher than 0.01g, its diffraction peak becomes significantly wider and smaller. The crystallinity is calculated using the formula, and the calculated crystallinity percentages of TiO2, TBC-1, TBC-2, TBC-3, TBC-4 and TBC-5 are 31.7, 22.3, 29.5, 39.6, 45.1 and 44.9%, respectively. Obviously, the crystallinity of the TBC-3 composite material is not the highest, which indicates that excessive BC has a negative impact on the growth of TiO2 crystals, which may be due to the fact that the indeterminate structure of the excessive biochar masks the diffraction of the anatase crystal plane. The crystallite sizes of TiO2, TBC-1, TBC-2, TBC-3, TBC-4, and TBC-5 calculated using the Scherrer formula were 24.2, 15.8, 17, 17.4, 19.8, and 22.3 nm, respectively. Bragg's law equation was used to calculate the d-spacings of TiO2, TBC-1, TBC-2, TBC-3, TBC-4, and TBC-5 to be 3.7, 3.5, 3.6, 3.6, 3.7, and 3.7 Å, respectively. This is consistent with the SEM observations (d = 0.357). Due to their natural composition, no distinct characteristic peaks were observed in the BC material. However, the characteristic peaks of the composite material indicate that they were successfully and effectively constructed on the BC, providing a possible mechanism for the adsorption-photocatalytic reduction reaction.

[0043] XPS analysis is to determine the chemical state of elements and obtain information about chemical bonds. In order to study the existence of elemental components and the successful synthesis of specified materials, XPS measurements were performed, and the results are shown in Figure 2. The XPS spectrum shown in Figure 2a shows that the composite photocatalyst mainly contains three elements: Ti, O, and C. Figures 2bcd are high-resolution XPS spectra of C1s, O1s, and Ti2p of the prepared composite photocatalyst. It can be seen that the spectrum of Ti2p is deconvoluted into two Ti 4+ The spin-orbit peaks are respectively attributed to Ti(2p 1 / 2 ) and Ti(2p 3 / 2 ), whose energy bonds are 459.4 and 465.1eV. In this composite photocatalytic material, Ti(2p 1 / 2 ) and Ti(2p 3 / 2) peak levels is 5.7 eV, indicating that anatase TiO2 is formed in the composite photocatalytic material. 3 / 2 (458.6 eV) and Ti 2p 1 / 2 (464.1 eV) compared to the standard binding energy of Ti 2p in the composite photocatalytic material. 3 / 2 and Ti 2p 1 / 2 The binding energy position of TiO2 nanoparticles is mainly located in the Ti 4+ oxidation state. In addition, Ti2p 3 / 2 and Ti2p 1 / 2 The change in the peak value may also be based on the effect of biochar material on the electronic valence state of Ti element in the TiO2 lattice.

[0044] UV-visible diffuse reflectance spectroscopy was performed on the prepared TiO2, P25, and composite photocatalyst in the 200-800 nm range. The resulting spectra are shown in Figure 3. As shown in Figure 3a, all samples exhibit a typical anatase absorption edge around 390 nm. As shown in Figure 3b, the band gaps of P25 and TiO2 are 3.53 eV and 3.65 eV, respectively, while those of TBC-1, TBC-2, TBC-3, TBC-4, and TBC-5 are 3.43 eV, 3.41 eV, 3.64 eV, and 3.62 eV, respectively. Notably, the band gap of TBC-3 is significantly smaller than that of P25, reaching 3.39 eV. This material also outperforms other composite photocatalysts in the adsorption and photocatalytic reduction of hexavalent chromium. Since biochar has strong light absorption properties, the composite material still has a high absorbance in the visible light band. As can be seen from the figure, the higher the amount of biochar doping, the higher its absorbance in the visible light band. This effect is more obvious in TBC-2 and TBC-3 because these samples have a low TiO2 content, but TBC-1 is the exception. This may be because some C atoms in the biochar combine with Ti atoms to form a Ti-OC structure, resulting in a zero background value similar to TiO2 in the visible light band. As mentioned above, the band gap value is calculated according to the Tauc relationship (αhν=A(E g - hν) n ), where A is a constant, α is the absorption coefficient, Eg is the average band gap of the semiconductor, n depends on the transition type, and h is Planck's constant (6.626×10 -34 J·s), ν is the frequency of photons. The band gap energy is given by (αhν) 2The tangent of the linear portion with hν was estimated. These values ​​are very similar to those of TiO2. Although it has been reported that the binding of TiO2 to carbonaceous supports can produce a significant red-shift in the absorption edge, this has not been observed in our material structure, indicating the absence of an intermediate band gap level. This may be due to the relatively limited interaction between TiO2 and BC compared to other carbon supports such as graphene or carbon nanotubes.

[0045] The morphology and elemental composition of the composite materials were analyzed using SEM-EDX, as shown in Figure 4. Figure 4 shows SEM images of BC, TBC-x, and TiO2, respectively. The SEM image of BC reveals the long-range order and void-rich structure of the straw biochar, which provides a good carrier for the successful loading of TiO2 on the biochar. The SEM image of TBC-3 shows that the surface of BC is successfully loaded with TiO2 nanoparticles, forming a relatively uniform and complete layer. Furthermore, TiO2 particles are deposited not only on the surface of the BC material but also within the micropores and mesopores of the material. The SEM image of TiO2 shows that pure TiO2 is mainly composed of large agglomerated particles with a large particle size, as can be seen from the BET data.

[0046] The morphology of the prepared TBC-3 sample was characterized by TEM (Figure 5). Disordered TiO2 crystals with an average size of 17.4 nm were clearly observed, and the d spacing between their crystal planes (0.357 nm) was confirmed by high-resolution imaging and diffraction patterns to correspond to the (101) planes of the tetragonal anatase crystal structure, which is consistent with the XRD results. In the TBC-3 sample, the image shows smaller TiO2 nanoparticles and surrounding BC particles, as already observed in SEM. The EDS spectrum of the composite photocatalyst shows that the composite photocatalyst contains only C, Ti, and O elements. The synthesis of TBC-3 matches the data obtained by HRTEM, XRD, and XPS.

[0047] The N2 adsorption-desorption isotherms of the prepared BC, TiO2, and TBC-3 photocatalysts are shown in Figure 6, and the specific surface area, pore size, pore volume, and most probable pore size are shown in Table 1. The Barrett-Joyner-Halenda (BJH) curves of BC, TiO2, and the prepared composites are shown in Figure 7.

[0048] Table 1 Results of specific surface area, pore size, pore volume and most probable pore size

[0049] Sample specific surface area (m 2 / g) pore volume (cm 3 / g)Average pore size (nm)Most probable pore size (nm)BC24.5550.0118.03526.737TiO2122.0700.0652.1291.746TBC-3306.6090.4505.8678.854

[0050] Figure 8 shows the TGA curves of BC and TBC-3. From 40°C to 800°C, the BC curve exhibits two phases. Between 40°C and 150°C, BC loses approximately 1.6% of its water molecules. Between 250°C and 550°C, the organic components of BC decompose, resulting in a weight loss of up to 63%. Above 550°C, the organic components of BC decompose almost completely, leaving only a small amount of inorganic ash and other impurities. On the other hand, a continuous weight loss of the gel can be observed between 40°C and 550°C. For the TBC-3 gel, its weight gradually decreases by approximately 15% between 40°C and 250°C due to water loss and the decomposition of titanium hydroxide to form TiO2. Based on the added BC amount (0.025 g) and the dry weight of the TiO2 sol (0.27 g / 10 mL), a steep downward trend appears between 250°C and 550°C, likely due to the decomposition of organic components in BC and the continued decomposition of the gel, resulting in a weight loss of up to 18%. The TGA results also show that when the calcination temperature is lower than 220 °C, the organic components of the gel do not change, but the lower calcination temperature may lead to incomplete decomposition of titanium hydroxide, and when the calcination temperature is higher than 550 °C, it may lead to complete decomposition of the organic components in BC.

[0051] Electrochemical impedance spectroscopy (EIS) measurements were used to investigate the electrical properties of the BC / TiO2 composite and compared them with those of BC, TiO2, and P25, as shown in Figure 9. The semicircular Nyquist plot represents the total impedance, or charge transfer resistance, at the interface between the working electrode covering the sample and the electrolyte solution. It is generally believed that the electronic conductivity of a sample is determined by the arc radius at high frequencies on the EIS plot; the smaller the arc radius, the greater the electronic conductivity. It can be seen that the arc radius of the synthesized samples first increases and then decreases with the addition of BC. This indicates that the charge transfer capacity of the BC / TiO2 composite is enhanced when loaded with BC. Furthermore, the arc radius of the TBC-3 sample is the smallest, indicating that the TBC-3 sample has the highest conductivity, indicating that the TBC-3 sample has the highest interface separation efficiency and the fastest transfer rate of photogenerated carriers.

[0052] 4. Photocatalytic activity test

[0053] The synergistic adsorption-photocatalytic reduction of Cr(VI) by the synthesized BC / TiO2 composites and TiO2 and BC under light irradiation was evaluated. A PLS-SSX 300 xenon lamp (Perfect Light) was used as the light source, providing a stable light intensity (50 W / m 2 ). The reaction system was maintained at room temperature using a constant temperature cooling bath. To conduct the photocatalytic test, 100 mg of photocatalyst was mixed with 100 ml of Cr(VI) (10 ppm) and stirred in the dark for 30 minutes to determine the maximum adsorption capacity of Cr(VI) on the photocatalyst surface. The mixture was then irradiated with light, with the xenon lamp kept at a distance of approximately 10 cm from the reaction liquid surface. The photocatalytic degradation process lasted for 80 minutes, with 2 ml of the aqueous solution extracted every 5 minutes and filtered using a 0.22 um filter membrane. The Cr(VI) concentration was determined using the DPC method, i.e., an appropriate amount of solution was taken and the absorbance of the pink complex formed by Cr(VI)-diphenylcarbohydrazide was measured at 540 nm using a UV / visible spectrophotometer. The photocatalytic activity was represented by a (C / Co) versus irradiation time (t) curve, where (C) is the concentration at a certain moment and (Co) is the initial concentration.

[0054] As mentioned above, in all experiments, the photocatalyst (1 g / L) and hexavalent chromium solution (10 mg / L) were stirred in the dark for 30 minutes to reach adsorption-desorption equilibrium. Then, the xenon lamp was turned on for illumination. In all experiments, the solution pH was adjusted to 3 because it is known that acidic pH media favors the photocatalytic adsorption-reduction of Cr(VI) in thermodynamics. Since Cr(VI) in solution changes with pH, ​​it may exist in different ionic forms, such as CrO4 2− 、Cr2O7 2− 、H2CrO4、HCrO4 − , and HCr2O7 − , and HCrO4 −It is the primary form of Cr(VI) at pH 3. The results of the prepared composite material and TiO2 photocatalytic reduction experiments and BC adsorption experiments are shown in Figure 10. The experiments show that under light irradiation, Cr(VI) cannot be reduced and removed under natural conditions without a photocatalyst. After the addition of the photocatalyst, pure TiO2 exhibited virtually no adsorption and reduction of Cr(VI) within -30 minutes (dark adsorption), while BC and the BC / TiO2 composite materials exhibited higher Cr(VI) adsorption efficiencies than pure TiO2 (only 6% after 30 minutes). TBC-1, TBC-2, and TBC-3 achieved adsorption efficiencies of approximately 40%, while TBC-4 and TBC-5 achieved adsorption efficiencies of approximately 30% and 25%, respectively. However, once light irradiation began, the results in the figure demonstrate that the prepared composite photocatalyst exhibited higher adsorption-photocatalytic efficiencies than pure TiO2. TBC-1, TBC-2, and TBC-3 completely adsorbed and reduced Cr(VI) after 25, 40, and 70 minutes of light irradiation, respectively. TBC-4 and TBC-5, however, only reached 70% and 60% at 80 minutes. In contrast, pure TiO2 only reached approximately 40% at 80 minutes. Clearly, TBC-3 exhibits the best synergistic adsorption-photocatalytic performance, with a BC doping level of 0.025g being the optimal doping level for this composite photocatalytic material, consistent with the characterization results above. Furthermore, we can conclude that when TiO2 is effectively loaded onto the BC surface, its adsorption-photocatalytic efficiency is significantly enhanced.

[0055] In the study of sacrificial agents for TBC-3 materials, potassium persulfate (K2S2O8), methanol (MT), ethylenediaminetetraacetic acid (EDTA) and benzoquinone (BQ) (1 M each) were used as e − 、h + OH· - and O2 - The scavenger was immersed in the photocatalytic reaction and the main reaction species were identified. In the presence of K2S2O8, the cooperative adsorption-photocatalytic efficiency of Cr(VI) decreased significantly to 32%, indicating that the e − It plays a major role in the photocatalytic mechanism. In the presence of MT, FDTA and BQ, the reduction rate is significantly higher than that of K2S2O8, which are 99.97%, 78.3% and 88.9% respectively. This behavior may be due to the fact that OH· - and H + The combination of two substances indicates that h + OH· - and O2 - Unlike e in photocatalytic reaction −That plays a major role. It's worth noting that the composite material, composed of an adsorbent material and a photocatalyst, has a significant impact on the photocatalytic removal of Cr(VI). This influences the "adsorption and shuttling" (A&S) process, which utilizes the adsorbent material's structure to increase the number and variety of pollutants near the TiO2 photocatalytic sites, significantly improving photocatalytic efficiency compared to TiO2.

[0056] The composite photocatalytic material prepared by the present invention exhibits a higher synergistic adsorption-photocatalytic reduction effect of Cr(VI) than pure TiO2 under light:

[0057] 1. Structural morphology and photochemical characterization confirmed that TiO2 was effectively attached to the BC material. The band gap energy of the composite material was relatively smaller than that of TiO2. The optimal doping amount of the composite photocatalytic material was found to be 0.025g of BC.

[0058] 2. The high specific surface area and low average pore size of the composite material explain that the enhanced photocatalytic reduction activity is due to the induction of Cr(VI) adsorption on its mesoporous surface and further reduction. By adding potassium persulfate as an electron sacrificial agent, the photogenerated electrons were explored. - Plays the main reducing role;

[0059] 3. Under light irradiation, the photogenerated carriers on the surface of the composite material can migrate from the conduction band (CB) of TiO2 to the surface of BC (BC acts as an electron trap), resulting in the synergistic effect of suppressing photogenerated hole-electron recombination and enhancing electron transfer;

[0060] 4. The self-photosensitization function of BC, which is similar to the organic structure of dye photosensitizers and its conductivity, is also a potential reason for enhancing the photocatalytic activity efficiency.

[0061] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and are not limiting. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that the technical solutions of the present invention may be modified or replaced by equivalents without departing from the purpose and scope of the technical solutions of the present invention, which should all be covered by the scope of the claims of the present invention.

Claims

1. A method for preparing a biochar / titanium dioxide composite photocatalytic material, characterized in that: The steps include: (1) Pre-treating wheat straw; (2) moving the pretreated wheat straw into a crucible for carbonization to obtain straw biochar; (3) mixing the straw biochar obtained in step (2) with hydrochloric acid, washing and drying to obtain biochar; (4) mixing butyl titanate and anhydrous ethanol to obtain solution A, and thoroughly mixing anhydrous ethanol with distilled water and acetic acid to obtain solution B; (5) adding solution B in step (4) dropwise into solution A, adding the biochar prepared in step (3) after magnetic stirring, and continuing stirring to obtain a black sol; (6) The black sol obtained in step (5) is aged, dried, ground, and then calcined to obtain a biochar / titanium dioxide composite photocatalytic material.

2. The method for preparing the biochar / titanium dioxide composite photocatalytic material according to claim 1, characterized in that: In step (1), the specific operations of the pretreatment are ultrasonication of the wheat straw, washing with deionized water, drying at 65° C. for 24 h, crushing with a wall-breaking machine, and sieving with 100 mesh.

3. The method for preparing the biochar / titanium dioxide composite photocatalytic material according to claim 1, characterized in that: In step (2), the specific operation of the carbonization treatment is to perform slow pyrolysis in a tubular furnace under a N2 atmosphere at a gas flow rate of 100 mL / min, 500°C, a heating rate of 5°C / min, perform slow pyrolysis in the furnace for 2 hours, and cool at room temperature.

4. The method for preparing the biochar / titanium dioxide composite photocatalytic material according to claim 1, characterized in that: In step (3), the mixing is to mix the straw biochar with 1M hydrochloric acid for 2 hours; the parameters in the drying process are drying at 65° C. for 24 hours.

5. The method for preparing the biochar / titanium dioxide composite photocatalytic material according to claim 1, characterized in that: In step (4), the content of butyl titanate in solution A is 10 mL, and the content of anhydrous ethanol is 30 mL; the content of anhydrous ethanol in solution B is 10 mL, the content of distilled water is 4 mL, and the content of acetic acid is 1 mL.

6. The method for preparing the biochar / titanium dioxide composite photocatalytic material according to claim 1, characterized in that: In step (5), the magnetic stirring time is 30 minutes; the continued stirring time is 2 hours; and the amount of biochar added is 0.025 g.

7. The method for preparing the biochar / titanium dioxide composite photocatalytic material according to claim 1, characterized in that: In step (6), the aging time is 24 hours; the parameters of the drying process are drying at 65° C. for 12 hours; the specific operation of calcination is placing in a muffle furnace and calcining at 300° C. for 2 hours, with a heating rate of 5° C. / min.

8. A biochar / titanium dioxide composite photocatalytic material prepared by the preparation method according to any one of claims 1 to 7.

9. Use of the biochar / titanium dioxide composite photocatalytic material as claimed in claim 8 in the removal of hexavalent chromium wastewater.

10. The use according to claim 9, characterized in that: The removal of the hexavalent chromium wastewater is specifically carried out by reducing the hexavalent chromium using a biochar / titanium dioxide composite photocatalytic material.

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