Nitrogen-doped supported titanium dioxide modified biochar composite material as well as preparation method and application thereof

Abstract

The invention relates to the technical field of soil chromium pollution remediation, in particular to a nitrogen-doped titanium dioxide-loaded modified biochar composite material as well as a preparation method and application thereof. The preparation method comprises the following steps: dipping biochar in a nitrogen source solution to obtain nitrogen-doped biochar; carrying out carbonization treatment on the nitrogen-doped biochar to obtain carbonized nitrogen-doped biochar; taking butyl titanate as a titanium source, mixing the carbonized nitrogen-doped biochar with the butyl titanate to form gel, and performing aging and drying treatment to obtain the nitrogen-doped loaded titanium dioxide modified biochar composite material with efficient adsorption performance, reduction performance and good stability. The defects that existing modified biochar is low in removal efficiency, insufficient in reducing capacity and limited in adsorption site under the weakly alkaline condition are overcome, the adsorption-reduction synergistic performance, soil passivation durability and heavy metal stabilization efficiency of Cr (VI) are remarkably improved, and an effective solution is provided for treatment of chromium-contaminated soil.

Description

Technical Field

[0001] This invention relates to the field of soil chromium pollution remediation technology, specifically to a nitrogen-doped titanium dioxide-modified biochar composite material, its preparation method, and its application. Background Technology

[0002] Chromium and its compounds are widely used in industries such as electroplating, leather making, and metallurgy, resulting in a large amount of chromium-containing waste entering the environment and causing serious soil chromium pollution. Among them, hexavalent chromium (Cr(VI)) poses a serious threat to the ecological environment and human health due to its high toxicity, high mobility, and strong carcinogenicity. Therefore, conducting research on the remediation of chromium-contaminated soil is of great practical significance.

[0003] Currently, remediation technologies for chromium-contaminated soil mainly include physical, chemical, and biological methods. Biochar, due to its large specific surface area, rich pore structure, and diverse surface functional groups, is widely used for the remediation of heavy metal-contaminated soil. However, the adsorption capacity of raw biochar is limited, and its adsorption efficiency is low under weakly alkaline conditions, with insufficient reduction capacity, which restricts its application in practical remediation. In recent years, researchers have modified biochar through heteroatom doping and metal loading to improve its performance. Introducing non-metallic elements such as nitrogen, sulfur, and phosphorus into biochar can significantly improve its adsorption capacity, catalytic performance, and electrochemical characteristics, while providing more adsorption sites. Impregnation, co-precipitation, or pyrolysis of biochar to introduce metals such as iron, magnesium, and aluminum, or their oxides, can fix Cr(VI) through physical adsorption and chemical precipitation, while simultaneously promoting the stabilization of heavy metals.

[0004] However, the practical application of existing modified biochar technology in soil chromium remediation still faces many severe challenges. Modified biochar is heavily influenced by soil pH; under neutral and slightly alkaline conditions, its removal efficiency and reduction capacity for hexavalent chromium often decrease sharply, severely limiting its applicability in various typical farmland soils. Furthermore, the long-term stability of biochar in soil and the persistence of heavy metal immobilization still lack systematic and long-term comprehensive evaluation. Summary of the Invention

[0005] To address the shortcomings of existing technologies, this invention provides a nitrogen-doped titanium dioxide-modified biochar composite material, its preparation method, and its application. This invention uses waste biomass as raw material, first preparing biochar through pyrolysis; then impregnating the biochar in a nitrogen source solution and carbonizing it to form carbonized nitrogen-doped biochar; finally, using tetrabutyl titanate as a titanium source, it is mixed with the carbonized nitrogen-doped biochar to obtain the nitrogen-doped titanium dioxide-modified biochar composite material. The nitrogen-doped titanium dioxide-modified biochar composite material of this invention possesses high adsorption performance, good reduction performance, and stability. When used as an adsorbent to treat Cr(VI) contaminated soil, it not only overcomes the shortcomings of existing modified biochar, such as low removal efficiency, insufficient reduction capacity, and limited adsorption sites under weakly alkaline conditions, but also significantly improves the synergistic adsorption-reduction performance of Cr(VI), soil passivation persistence, and heavy metal stabilization efficiency, providing an effective solution for the remediation of chromium-contaminated soil.

[0006] To achieve the above objectives, the technical solution adopted by the present invention is as follows: The first objective of this invention is to provide a method for preparing nitrogen-doped titanium dioxide-modified biochar composite materials, comprising the following steps: S1. Biochar is impregnated in a nitrogen source solution to obtain nitrogen-doped biochar; the nitrogen-doped biochar is then carbonized. During the carbonization process, the nitrogen source decomposes to produce gas, which etches the carbon skeleton of the nitrogen-doped biochar, forming micropores and mesopores, thus obtaining carbonized nitrogen-doped biochar.

[0007] S2. Using tetrabutyl titanate as the titanium source, carbonized nitrogen-doped biochar is mixed with tetrabutyl titanate to form a gel. This gel is then subjected to aging and drying treatments. During aging and drying, tetrabutyl titanate undergoes a hydrolysis-condensation reaction to generate TiO2 nanoparticles, which are then loaded onto the surface of the carbonized nitrogen-doped biochar, resulting in a nitrogen-doped titanium dioxide-modified biochar composite material. The advantages of using tetrabutyl titanate as the titanium source are that the process is mild and controllable, and easy to operate precisely in both laboratory and industrial production. Compared to inorganic titanium salts (such as titanium tetrachloride and titanium sulfate), the hydrolysis process of tetrabutyl titanate does not produce strongly acidic or highly corrosive byproducts, avoiding severe corrosion and damage to the carbon skeleton structure of the carbonized nitrogen-doped biochar during loading, thus preserving the inherent porous structure of the carbonized nitrogen-doped biochar. This method effectively prevents the aggregation of TiO2 nanoparticles, ensuring their high dispersion on the surface and in the pores of the carbonized nitrogen-doped biochar.

[0008] The nitrogen source is selected from ammonium sulfate or ammonium chloride. Ammonium sulfate decomposes during carbonization to produce gas, which can etch the carbon framework of nitrogen-doped biochar, forming micropores and mesopores. The increased porosity facilitates the loading of TiO2 nanoparticles. Ammonium chloride, during carbonization, produces HCl gas, which also has an etching effect on the carbon framework of nitrogen-doped biochar. - At high temperatures, it is easy to corrode equipment or affect the stability of biochar, and even produce harmful gases. Therefore, ammonium sulfate is a better nitrogen source. Urea was also used as a nitrogen source and it was found that urea mainly produces NH3 and nitrogen-containing small molecules during pyrolysis. Its main function is to dope nitrogen elements, and it has almost no gas pore-forming effect.

[0009] Preferably, the mass fraction of the ammonium sulfate solution is 5% to 15%.

[0010] Preferably, the mass ratio of biochar to nitrogen source is 1:1~3.

[0011] Preferably, the mass ratio of carbonized nitrogen-doped biochar to tetrabutyl titanate is 1:1~4.

[0012] Preferably, the carbonization conditions are: pyrolysis at 400℃~600℃ for 2h~2.5h under an inert atmosphere.

[0013] Preferably, the aging treatment conditions are: placed at room temperature in the dark for 24h~28h.

[0014] Preferably, biochar is prepared according to the following steps: In an inert atmosphere, waste biomass is heated to 400℃~500℃ at 10℃ / min and then pyrolyzed for 2h~2.5h. After grinding, it is passed through a 100-mesh sieve to obtain biochar.

[0015] Preferably, the waste biomass is rice husk. Rice husk, after pyrolysis, has the characteristics of low ash content, high aroma and rich functional groups. Biochar made from rice husk has the characteristics of strong adsorption capacity.

[0016] A second objective of this invention is to provide a nitrogen-doped titanium dioxide-modified biochar composite material prepared by the above-described method.

[0017] Preferably, the nitrogen-doped titanium dioxide-modified biochar composite material has a honeycomb porous structure, with micropores distributed in the macropores, and micropores being the main component.

[0018] A third objective of this invention is to provide the application of the above-mentioned nitrogen-doped titanium dioxide-modified biochar composite material in the preparation of Cr(VI) heavy metal ion adsorbent materials.

[0019] The preferred application method is as follows: Nitrogen-doped titanium dioxide-modified biochar composite material was mixed with Cr(VI) contaminated soil; wherein the mass ratio of nitrogen-doped titanium dioxide-modified biochar composite material to Cr(VI) contaminated soil was 1~4:100.

[0020] Preferably, the pH value of Cr(VI) contaminated soil is 2 to 8.

[0021] Compared with the prior art, the beneficial effects of the present invention are as follows: 1. This invention provides a method for preparing a nitrogen-doped titanium dioxide-modified biochar composite material. The biochar is impregnated in a nitrogen source solution and carbonized. During the carbonization process, the nitrogen source decomposes to generate gas, which etches the carbon skeleton of the nitrogen-doped biochar, forming micropores and mesopores, thus obtaining carbonized nitrogen-doped biochar. Using tetrabutyl titanate as a titanium source, the carbonized nitrogen-doped biochar is mixed with tetrabutyl titanate to form a gel. After aging and drying, the nitrogen-doped titanium dioxide-modified biochar composite material is obtained. This invention uses waste biomass as raw material and employs a stepwise process of nitrogen doping and titanium dioxide loading to successfully prepare a nitrogen-doped titanium dioxide-modified biochar composite material with high adsorption and reduction performance and good stability. When used to treat Cr(VI) contaminated soil, it not only effectively overcomes the shortcomings of existing modified biochars in neutral and weakly alkaline conditions, such as low removal efficiency, weak reduction capacity, and limited adsorption sites, but also significantly improves its synergistic adsorption-reduction performance for Cr(VI), enhances its passivation persistence and heavy metal stabilization efficiency in soil, and provides a low-cost, high-efficiency, environmentally friendly, and scalable remediation material for chromium-contaminated soil.

[0022] Among them, under neutral or weakly alkaline conditions, although H + Despite extremely low concentrations, the NH functional groups on the surface of nitrogen-doped titanium dioxide-modified biochar composites, due to their low oxidation potential, can directly oxidize to nearby Cr(VI) anions (such as CrO4). 2- TiO2 provides electrons to reduce Cr(VI) to Cr(III). Simultaneously, TiO2 and carbonized nitrogen-doped biochar form a heterojunction interface through Ti-OC bonds. This structure facilitates the efficient transfer of electrons to Cr(VI) through the TiO2 interface, completing the reduction of Cr(VI) to Cr(III), and further achieving stable fixation of chromium through the interfacial reaction (TiO2 + Cr2O3 → 2CrTiO3).

[0023] Structurally, some TiO2 nanoparticles optimize the pore structure by occupying or dividing larger mesopores, transforming them into smaller micropores. Furthermore, TiO2 forms Ti-OC bonds with the carbon framework of carbonized nitrogen-doped biochar, which not only facilitates charge transfer but also effectively prevents the migration and aggregation of TiO2 nanoparticles, thus ensuring that the composite material maintains a high specific surface area and high activity.

[0024] 2. The nitrogen-doped titanium dioxide-modified biochar composite material of the present invention exhibits excellent adsorption performance and good stability for the remediation of chromium-contaminated soil. This is mainly attributed to the unique microstructure of the nitrogen-doped titanium dioxide-modified biochar composite material: TiO2 is uniformly loaded in the form of nanoparticles on the surface of carbonized nitrogen-doped biochar, filling or covering some macropores, forming a multi-level porous structure dominated by micropores. This structure not only increases the specific surface area of ​​the nitrogen-doped titanium dioxide-modified biochar composite material, but also provides abundant surface functional groups and a large number of adsorption sites, thereby further enhancing its catalytic degradation ability for pollutants.

[0025] 3. This invention uses waste biomass as raw material and successfully prepares nitrogen-doped titanium dioxide-modified biochar composite material through a simple doping and co-impregnation process. The raw materials used in this invention are widely available and inexpensive, and the preparation process is simple and efficient. This not only realizes the resource utilization of agricultural waste but also effectively solves the problem of rice husk waste disposal.

[0026] 4. The nitrogen-doped titanium dioxide-modified biochar composite material of the present invention has broad application potential. It can not only maintain high efficiency in removal performance in a wide pH range (2~8), but also has excellent passivation and remediation effect on chromium-contaminated soil, showing excellent stability and durability. Attached Figure Description

[0027] Figure 1 The images shown are SEM images of NT-BC and BC from Example 1, where a represents NT-BC and b represents BC.

[0028] Figure 2 The figures are nitrogen adsorption-desorption isotherms for NT-BC and BC in Example 1, where a represents BC, and the inset in a represents the pore size distribution curve of BC; b represents NT-BC, and the inset in b represents the pore size distribution curve of NT-BC.

[0029] Figure 3 The image shows the XRD patterns of NT-BC and BC in Example 1.

[0030] Figure 4 The image shows the FTIR plots of NT-BC and BC in Example 1.

[0031] Figure 5The graphs show the removal rate and adsorption capacity of NT-BC and BC for Cr(VI) under different pH conditions in Example 1. a is the removal rate graph and b is the adsorption capacity graph.

[0032] Figure 6 The graph shows the effect of NT-BC on the concentration of Cr in different valence states in the polluted liquid in Example 1.

[0033] Figure 7 The graph shows the soil cation exchange capacity of chromium-containing soil after adding different proportions of NT-BC and BC from Example 1 for 7 days, 15 days, and 45 days. In the graph, a represents NT-BC and b represents BC.

[0034] Figure 8 The graph shows the content ratio of heavy metal occurrence forms after adding different proportions of NT-BC and BC from Example 1 to chromium-containing soil, where a represents 7 days, b represents 15 days, and c represents 45 days.

[0035] Figure 9 The percentage of available Cr(VI) in soil after adding different proportions of NT-BC and BC from Example 1 to chromium-containing soil is shown in the figure.

[0036] Figure 10 A comparison of Cr(VI) removal rates of nitrogen-doped biochar prepared for different nitrogen sources. Detailed Implementation

[0037] The technical solution of the present invention will be clearly and completely described below with reference to the data in the embodiments of the present invention. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0038] It should be noted that the technical terms used in this invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of this invention. Unless otherwise specified, all raw materials, reagents, instruments and equipment used in the following embodiments of this invention can be purchased on the market or prepared by existing methods.

[0039] In the current technology, the application of biochar in the field of soil remediation has the following shortcomings: First, the adsorption capacity of raw biochar is limited, and its adsorption efficiency is low and its reduction capacity is insufficient under weakly alkaline conditions, which limits its application in actual remediation; Second, the existing modified biochar technology is limited by soil pH value, and its removal efficiency and reduction capacity for hexavalent chromium drop sharply under neutral and weakly alkaline conditions, which cannot meet the remediation needs of various soil environments; Third, there is a lack of systematic evaluation of the long-term stability of biochar in soil and the persistence of heavy metal fixation.

[0040] To address the problems of the existing technologies, this invention uses waste biomass as raw material and produces biochar through pyrolysis. By modifying the biochar with nitrogen doping and titanium dioxide loading, the invention overcomes the shortcomings of existing biochar, such as limited adsorption capacity and low efficiency under weakly alkaline conditions. Furthermore, by optimizing the preparation process, the invention solves the problem of performance degradation of modified biochar under neutral and weakly alkaline conditions in the existing technologies, thereby improving the stability and durability of the material.

[0041] To enable those skilled in the art to more clearly understand the technical solution of the present invention, the following will provide a detailed description in conjunction with specific embodiments: Example 1 A method for preparing a nitrogen-doped titanium dioxide-modified biochar composite material includes the following steps: S1. After washing the rice husks with deionized water, dry them at 65℃ for 24 hours, crush them with a wall-breaking machine and pass them through a 100-mesh sieve to obtain rice husk powder. Put the rice husk powder into a tube furnace, introduce nitrogen gas to ensure that the tube furnace is in a vacuum state, and then heat it to 400℃ at 10℃ / min and pyrolyze it for 2 hours. After pyrolysis is completed, let it cool naturally to room temperature and pass it through a 100-mesh sieve to obtain rice husk biochar, i.e., original biochar, denoted as BC.

[0042] S2. Take 10g of BC and 5% ammonium sulfate solution and place them together in a beaker and mix thoroughly. After sonication for 30min, stir at room temperature for 3h. After stirring, filter and wash the solid until it is neutral. Then dry and grind it in sequence, pass it through a 100-mesh sieve, and place it in a tube furnace. Purge with nitrogen gas and heat to 400℃ at 10℃ / min for 2h to obtain carbonized nitrogen-doped biochar, denoted as N-BC. The mass ratio of BC to nitrogen source is 1:1.

[0043] S3. N-BC and tetrabutyl titanate were mixed at a mass ratio of 1:4. After stirring vigorously for 30 minutes, a small amount of deionized water was added and stirring was continued until a gel was formed. The mixture was then placed at room temperature in the dark for 24 hours and dried at 60°C for 24 hours. After drying, the mixture was ground and passed through a 100-mesh sieve to obtain a nitrogen-doped titanium dioxide-modified biochar composite material, denoted as NT-BC.

[0044] Example 2 A method for preparing a nitrogen-doped titanium dioxide-modified biochar composite material includes the following steps: S1. After washing the rice husks with deionized water, dry them at 65℃ for 24 hours, crush them with a wall-breaking machine and pass them through a 100-mesh sieve to obtain rice husk powder. Put the rice husk powder into a tube furnace, introduce nitrogen gas to ensure that the tube furnace is in a vacuum state, and then heat it to 400℃ at 10℃ / min and pyrolyze it for 2 hours. After pyrolysis, let it cool naturally to room temperature and pass it through a 100-mesh sieve to obtain rice husk biochar, denoted as BC.

[0045] S2. Take 10g of BC and 10% ammonium sulfate solution and place them together in a beaker and mix thoroughly. After sonication for 30min, stir at room temperature for 3h. After stirring, filter and wash the solid until it is neutral. Then dry and grind it in sequence, pass it through a 100-mesh sieve, and place it in a tube furnace. Purge with nitrogen gas and heat to 400℃ at 10℃ / min for 2h to obtain carbonized nitrogen-doped biochar, denoted as N-BC. The mass ratio of BC to nitrogen source is 1:2.

[0046] S3. N-BC and tetrabutyl titanate were mixed at a mass ratio of 1:1 and stirred vigorously for 30 min. A small amount of deionized water was added and the mixture was stirred until it reached a gel state. The mixture was then placed at room temperature in the dark for 24 h and dried at 60 °C for 24 h. After drying, the mixture was ground and passed through a 100-mesh sieve to obtain nitrogen-doped titanium dioxide-modified biochar composite material, denoted as NT-BC.

[0047] Example 3 A method for preparing a nitrogen-doped titanium dioxide-modified biochar composite material includes the following steps: S1. After washing the rice husks with deionized water, dry them at 65℃ for 24 hours, crush them with a wall-breaking machine and pass them through a 100-mesh sieve to obtain rice husk powder. Put the rice husk powder into a tube furnace, introduce nitrogen gas to ensure that the tube furnace is in a vacuum state, and finally heat it to 400℃ at 10℃ / min and pyrolyze it for 2 hours. After pyrolysis, let it cool naturally to room temperature and pass it through a 100-mesh sieve to obtain rice husk biochar, denoted as BC. S2. Take 10g of BC and 15% ammonium sulfate solution and place them together in a beaker and mix thoroughly. After sonication for 30min, stir at room temperature for 3h. After stirring, filter and wash the solid until it is neutral. Then dry and grind it in sequence, pass it through a 100-mesh sieve, and place it in a tube furnace. Purge with nitrogen gas and heat to 400℃ at 10℃ / min for 2h to obtain carbonized nitrogen-doped biochar, denoted as N-BC. The mass ratio of BC to nitrogen source is 1:3.

[0048] S3. N-BC and tetrabutyl titanate were mixed at a mass ratio of 1:2. After stirring vigorously for 30 minutes, a small amount of deionized water was added and stirring was continued until a gel was formed. The mixture was then placed at room temperature in the dark for 24 hours and dried at 60°C for 24 hours. After drying, the mixture was ground and passed through a 100-mesh sieve to obtain nitrogen-doped titanium dioxide-modified biochar composite material, denoted as NT-BC.

[0049] Comparative Example 1 A method for preparing a nitrogen-doped titanium dioxide-modified biochar composite material is the same as that in Example 1, except that ammonium sulfate solution is replaced with ammonium chloride to obtain the nitrogen-doped titanium dioxide-modified biochar composite material.

[0050] Comparative Example 2 A method for preparing a nitrogen-doped titanium dioxide-modified biochar composite material is the same as that in Example 1, except that ammonium sulfate solution is replaced with urea to obtain the nitrogen-doped titanium dioxide-modified biochar composite material.

[0051] Characterization: The BC and NT-BC were characterized, and the results are as follows: observe Figure 1 It was found that although BC has preliminary pores, the distribution is uneven, there are many smooth and non-porous areas, and it is mainly mesoporous with limited pore structure. The surface of NT-BC is significantly roughened due to the uniform adhesion of TiO2 nanoparticles, forming a multi-level pore structure dominated by micropores. The specific surface area and the number of active sites are greatly increased, providing sufficient space and reaction sites for the efficient adsorption of Cr(VI).

[0052] observe Figure 2 The results showed that the isotherm of BC was type IV, with a predominantly mesoporous structure, and capillary condensation occurred in the medium-pressure region; the isotherm of NT-BC was type I, with its microstructure dominated by a micropore filling mechanism. The slight increase in adsorption in the high-pressure region indicated that a small number of mesopores and macropores still existed in NT-BC. This suggests that while forming a structure dominated by micropores, NT-BC still retained some mesopores / macropores as transport channels, which ensured the rapid diffusion of Cr(VI) and provided sufficient adsorption sites through micropores, thereby synergistically improving the removal performance.

[0053] Depend on Figure 3The diffraction pattern of NT-BC showed characteristic crystal plane diffraction peaks of nitrogen-containing functional groups and TiO2, indicating that N and TiO2 were successfully loaded onto the surface of biochar. In addition, the diffraction peaks were weak and broadened, indicating that N and TiO2 were mainly distributed inside the pores of biochar, forming a smaller microporous structure. The diffraction pattern of NT-BC showed an overall amorphous state with significantly reduced crystallinity, which is more conducive to the diffusion of heavy metal ions into the pores and enhances its adsorption capacity for pollutants.

[0054] Depend on Figure 4 It was found that N and TiO2 were successfully loaded onto BC and grown at 1500 cm⁻¹. -1 (CN), 3347.11cm -1 (NH) and 590cm -1 (Ti-O / Ti-O-Ti) exhibits a significant vibrational characteristic peak; the main functional groups on NT-BC have a peak at 1603.33 cm⁻¹. -1 (C=C), 1109cm -1 (CCOC changes significantly), 2924.62cm -1 and 808cm -1 (CH stretching vibration), 1425cm -1 (-COOH stretching vibration), 1500cm -1 (CN stretching vibration), 3347.11cm -1 (-OH / NH) and 590cm -1 (Ti-O / Ti-O-Ti deformation vibration) indicates that the strength and types of functional groups on the NT-BC surface are significantly increased, providing more adsorption sites, resulting in higher adsorption efficiency and better stability.

[0055] In Examples 1-3 of this invention, nitrogen-doped titanium dioxide-modified biochar composite materials were prepared with parallel effects. The adsorption performance of NT-BC and BC from Example 1 is compared below. The research methods and results are as follows: application: The study on Cr(VI) removal, including the methods and results, is as follows: (1) BC and NT-BC were used to treat simulated soil Cr(VI) contaminants at different pH levels: A 100 mg / L Cr(VI) contaminated solution was diluted to 10 mg / L. The pH of the contaminated solution was adjusted to 2, 3, 4, 5, 6, 7, and 8 using 0.1 mol / L NaOH and HNO3 solutions. 50 mL of each pH solution was taken, and 0.05 g of BC and NT-BC were added respectively, with three control groups for each pH value. The solution was shaken on a constant-temperature shaker at 120 r / min for 24 h at 25 °C. After filtration through a filter membrane, the contamination concentration of the filtrate was determined.

[0056] like Figure 5 As shown, NT-BC is more effective at removing Cr(VI) under lower pH conditions, but overall, different pH values ​​have a relatively small impact on the removal of Cr(VI) by NT-BC. NT-BC still maintains a removal rate of 79.8% at pH 8. In contrast, the removal rate of BC is significantly affected by pH. At pH 2.0, the removal rate is approximately 58%, which decreases rapidly with increasing pH, dropping to near zero at pH 6.0, and remaining at a low level within the neutral to slightly alkaline range. Comparing BC and NT-BC, it was found that NT-BC exhibits excellent and stable Cr(VI) removal performance over a wider pH range.

[0057] (2) Effects of NT-BC on the concentrations of Cr(VI), Cr(III) and total chromium in simulated soil Cr(VI) contaminated liquid: A Cr(VI) contaminated solution with a mass concentration of 100 mg / L was diluted to 10 mg / L, and the pH of the contaminated solution was adjusted to 5 ± 0.1 using 0.1 mol / L NaOH and HNO3 solutions. 0.05 g of NT-BC was weighed into a 100 mL Erlenmeyer flask, and 50 mL of each of the diluted contaminated solution was added to the flask, resulting in three parallel samples. The solution was shaken on a constant-temperature shaker at 120 r / min for 24 h at 25 °C. After filtration through a filter membrane, the total chromium and Cr(VI) content in the supernatant was determined.

[0058] The results are as follows Figure 6 As shown, by comparing the valence state distribution of chromium in the polluted solution before and after NT-BC treatment, it was found that the total chromium concentration in the solution was significantly reduced after the reaction, with a removal rate of 90.5%. Among them, the removal rate of Cr(VI) was 91.4%, with only trace amounts of Cr(III) (0.012 mg / L) remaining. The results indicate that NT-BC can effectively adsorb and fix chromium pollutants and can also reduce highly toxic Cr(VI) to less toxic Cr(III), exhibiting a good reduction-adsorption synergistic removal effect.

[0059] (3) Different proportions of BC and NT-BC were added to chromium-containing soil and cultured. The results of the treatment of the contaminated soil were as follows: The obtained chromium-containing soil was pretreated, then ground and sieved, and stored in a dry environment. The soil pH was measured to be 8. Three experimental groups were set up: a control group with 0% biochar; a BC group and an NT-BC group, with BC and NT-BC added at mass ratios of 1%, 2%, and 4%, respectively; after addition, the soil was incubated for 45 days while maintaining moisture. Samples were taken at 7, 15, and 45 days to detect and analyze changes in cation exchange capacity (CEC) and the speciation of heavy metal Cr, as well as the content of available chromium in the soil under different treatment conditions. Each group had three parallel samples. The experimental groups and biochar dosage are shown in Table 1.

[0060] Table 1 shows the different biochar dosages. ① In chromium-containing soil with a pH of 8, 1%, 2%, and 4% NT-BC and BC were added respectively. Samples were taken at 7, 15, and 45 days after incubation to analyze the cation exchange in the soil. The results are as follows: Figure 7 As shown.

[0061] ② In chromium-containing soil with a pH of 8, 1%, 2%, and 4% NT-BC and BC were added respectively. Samples were taken at 7, 15, and 45 days after incubation to analyze various occurrence forms of the heavy metal chromium in the soil. The results are as follows: Figure 8 As shown.

[0062] ③ In chromium-containing soil with a pH of 8, 1%, 2%, and 4% NT-BC and BC were added respectively. The content of available Cr(VI) in the soil was analyzed 7 days after incubation. The results are as follows: Figure 9 As shown.

[0063] like Figure 7 As shown, both BC and NT-BC biochars can increase soil cation exchange capacity (CEC), and CEC increases significantly with increasing biochar dosage. Compared with BC, NT-BC has a more pronounced effect on increasing CEC, approximately twice that of the control group (CK). The effect of NT-BC on CEC initially increases with incubation time, followed by a slight decrease, but the decreased value is still much higher than that of the CK group. At 15 days of incubation, NT-BC-4% showed the best effect, reaching 16.36 cmol. + / kg.

[0064] like Figure 8As shown, BC initially influenced the distribution of chromium speciation, but gradually reached equilibrium later. NT-BC, on the other hand, continuously played a transformative role throughout the entire incubation period, with the proportions of various speciations constantly changing, indicating its more durable and stable solidification ability and remediation potential, making it more suitable for long-term application in actual chromium-contaminated soils. The NT-BC-4% treatment group showed the most significant effect. After 45 days of incubation, the proportion of weakly acid-extractable chromium decreased from the initial 54.473% to 7.309%, indicating a significant reduction in its environmental mobility and bioavailability; the proportion of residual chromium increased by 41.475%, demonstrating excellent chemical stability and solidification ability. This is mainly due to the abundant oxygen-containing functional groups on the surface of NT-BC and the loaded titanium dioxide, which jointly promoted the transformation of reducible chromium into a more stable form.

[0065] like Figure 9 As shown, the content of available chromium in the soil changed significantly under different treatment conditions. Compared with the blank control group (CK), the content of available Cr(VI) in all treatment groups with added biochar was significantly reduced, and the decrease increased with the increase of the addition ratio. In the BC treatment group, the content of available Cr(VI) decreased by 12.31 mg / kg, 17.28 mg / kg, and 25.62 mg / kg at the addition ratios of 1%, 2%, and 4%, respectively; while the reduction effect of the NT-BC treatment group was more significant, with decreases of 41.03 mg / kg, 62.11 mg / kg, and 81.72 mg / kg at the corresponding addition ratios. The results indicate that NT-BC has a better immobilization effect on available Cr(VI) in soil than BC, and has a significant dose-effect. Modified biochar NT-BC mainly converts available Cr(VI) into stable chromium through the reduction effect mediated by surface functional groups, significantly reducing its mobility and bioavailability, thereby more effectively inhibiting the ecological risks of chromium in the soil environment.

[0066] like Figure 10 As shown, the original biochar removed only 6% of Cr(VI), while the nitrogen-doped titanium dioxide-modified biochar composites prepared using ammonium sulfate, ammonium chloride, and urea as nitrogen sources achieved 18%, 10%, and 4% removal rates of Cr(VI), respectively. The comparison revealed that the nitrogen-doped titanium dioxide-modified biochar composites prepared using ammonium sulfate as the nitrogen source exhibited the best Cr(VI) adsorption effect. Therefore, this invention selected ammonium sulfate as the nitrogen-doped raw material for subsequent titanium dioxide loading modification to synergistically enhance the Cr(VI) remediation performance of the nitrogen-doped titanium dioxide-modified biochar composites.

[0067] It should be noted that when numerical ranges are involved in this invention, it should be understood that both endpoints of each numerical range, as well as any value between the two endpoints, can be selected. Since the steps and methods used are the same as in the embodiments, preferred embodiments are described here to avoid redundancy. Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make other changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments as well as all changes and modifications falling within the scope of this invention.

Claims

1. A method for preparing a nitrogen-doped, titanium dioxide-modified biochar composite material, characterized in that, Includes the following steps: Nitrogen-doped biochar was obtained by impregnating biochar with a nitrogen source solution. Nitrogen-doped biochar is carbonized. During the carbonization process, the nitrogen source decomposes to produce gas, which etches the carbon skeleton of the nitrogen-doped biochar, forming micropores and mesopores, thus obtaining carbonized nitrogen-doped biochar. Using tetrabutyl titanate as the titanium source, carbonized nitrogen-doped biochar was mixed with tetrabutyl titanate to form a gel. Subsequently, the gel was subjected to aging and drying treatments. During the aging and drying treatments, tetrabutyl titanate underwent a hydrolysis and polycondensation reaction to generate TiO2 nanoparticles, which were then loaded onto the surface of the carbonized nitrogen-doped biochar to obtain a nitrogen-doped titanium dioxide-modified biochar composite material. The nitrogen source is selected from ammonium sulfate or ammonium chloride.

2. The method for preparing nitrogen-doped supported titanium dioxide modified biochar composite material according to claim 1, characterized in that, The mass ratio of biochar to nitrogen source is 1:1~3.

3. The method for preparing nitrogen-doped supported titanium dioxide modified biochar composite material according to claim 1, characterized in that, The mass ratio of carbonized nitrogen-doped biochar to tetrabutyl titanate is 1:1~4.

4. The method for preparing nitrogen-doped supported titanium dioxide modified biochar composite material according to claim 1, characterized in that, The carbonization conditions are: pyrolysis at 400℃~600℃ for 2h~2.5h under an inert atmosphere.

5. The method for preparing nitrogen-doped supported titanium dioxide modified biochar composite material according to claim 1, characterized in that, The aging treatment conditions are: place at room temperature in the dark for 24-28 hours.

6. A nitrogen-doped, titanium dioxide-modified biochar composite material prepared by the preparation method according to any one of claims 1 to 5, characterized in that, The nitrogen-doped titanium dioxide-modified biochar composite material exhibits a honeycomb porous structure with micropores distributed within macropores, and micropores are the dominant type.

7. The application of the nitrogen-doped supported titanium dioxide modified biochar composite material according to claim 6 in the preparation of Cr(VI) heavy metal ion adsorbent materials.

8. The application according to claim 7, characterized in that, The application method is as follows: Nitrogen-doped titanium dioxide-modified biochar composite material was mixed with Cr(VI) contaminated soil, and the nitrogen-doped titanium dioxide-modified biochar composite material was used to adsorb Cr(VI) heavy metal ions in the Cr(VI) contaminated soil. The mass ratio of nitrogen-doped titanium dioxide-modified biochar composite material to Cr(VI)-contaminated soil is 1-4:

100.

9. The application according to claim 8, characterized in that, The pH value of Cr(VI) contaminated soil is 2-8.

10. The application according to claim 9, characterized in that, The pH value of Cr(VI) contaminated soil is 7-8.

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