A nitrogen-containing magnetic hydrothermal carbon material and its preparation method and its application in Cr(VI) removal.

Abstract

This invention relates to a nitrogen-containing magnetic hydrothermal carbon material and its preparation method, as well as its application in Cr(VI) removal. The method includes the following steps: mixing biomass powder, iron salt solution, and alcohol; then adding sodium acetate and a nitrogen source to obtain a precursor solution; subjecting the precursor solution to a hydrothermal reaction at 170–190°C; and finally, post-processing the resulting product to obtain the magnetic hydrothermal carbon material. This invention primarily utilizes a simple and easy hydrothermal method to prepare the nitrogen-containing magnetic hydrothermal carbon material. The introduction of nitrogen element achieves efficient photoelectron transfer while simultaneously promoting the removal of Cr(VI) by Fe. 2+ and Fe 3+ The conversion of electrons into an electron transfer bridge improves the interfacial reaction rate. In Cr(VI) removal experiments, it exhibits strong reducing properties, adsorption capacity, and excellent cycling stability.

Description

Technical Field

[0001] This invention belongs to the field of biochar materials and photochemical technology, specifically relating to a nitrogen-containing magnetic hydrothermal carbon material and its preparation method, as well as its application in Cr(VI) removal. Background Technology

[0002] Biomass waste is a widely applicable, low-cost, and renewable natural polymer material with broad application prospects. For biomass waste generated in agriculture, combustion is one of the direct treatment methods. However, burning biomass, such as straw, releases fixed carbon into the atmosphere, which exacerbates the emission of the greenhouse gas carbon dioxide. Excessive carbon dioxide emissions can easily lead to a series of environmental and ecological problems. Every year, a large amount of biomass waste is incinerated. In addition to carbon dioxide, many pollutants such as PM2.5 particles, particulate matter, nitrogen oxides, aldehydes, toluene, and metal or metal oxide particles are generated through this method, causing environmental pollution. Currently, the main treatment methods for industrial and domestic biological waste are landfill, bioconversion, and thermal treatment. However, landfills occupy a large amount of land resources and pollute soil and groundwater; bioconversion processes are time-consuming; high-temperature pyrolysis converts biomass waste into gaseous fuels, requiring high energy input and is only suitable for biomass with a moisture content of <5%. Existing methods for treating biomass typically convert large biomass molecules into efficiently usable small molecules. In addition, biomass can be converted into porous carbon, graphite, or graphene with metals, carbon / metal, or carbon / metal oxide composites through hydrothermal or pyrolysis methods. The products of biomass waste treatment can be used in a variety of applications, such as adsorption, bioimaging, catalysis, capacitor energy storage, electrodes and lithium-ion batteries, CO2 reduction, or hydrogen production.

[0003] Hydrothermal processing is a common method for treating biomass waste. In this process, biomass waste is typically heated in a sealed container at 150–300°C with water or an organic solvent, using the steam pressure of the water or solvent for treatment. Hydrothermal char (HTC) treats biomass instead of burning it, avoiding carbon dioxide emissions and contributing to the reduction of greenhouse gas emissions. The HTC process is relatively low-cost with near-zero net emissions. Biomass-derived HTC is used for the following applications: I) Templates for synthesizing other hollow porous oxide materials; the surface of HTC contains numerous oxygen-containing functional groups, such as hydroxyl and carboxyl groups, and can adsorb metal ions; the HTC template can be removed to form hollow metal oxide structures. II) Metal catalyst supports; metals can be well dispersed on HTC, and iron nanoparticles supported on mesoporous hydrogen-carbon reduce the aggregation of metal nanoparticles, which helps improve catalytic activity and stability. III) HTC is a promising photocatalyst that can be obtained through the hydrothermal treatment of biomass. After hydrothermal treatment, HTC can be formed, which is mainly composed of sp...2 Hybridized chain composition. 2 Hybridized chains are photoactive, capable of generating photoexcited electrons and holes under visible light irradiation. These photoexcited charge carriers can contribute to the generation of reactive oxygen species, such as ·OH and ·O2. - Free radicals can be used to degrade or disinfect organic pollutants. Therefore, HTC can be widely used as a visible light-responsive material in photocatalytic applications.

[0004] Studies on the photoactivity of hydrothermal carbon (HTC) have revealed that while charge transfer within individual polymer chains is efficient, the efficiency of charge transfer between polymer chains and interfacial reactions remains a bottleneck for its wider application and higher activity. Effectively increasing the rates of charge transfer and interfacial reactions between molecular chains are two key steps to enhancing photocatalytic performance. Summary of the Invention

[0005] The purpose of this invention is to overcome the above-mentioned technical deficiencies and provide a nitrogen-containing magnetic hydrothermal carbon material and its preparation method, as well as its application in Cr(VI) removal, thereby solving the technical problem that the low photocatalytic performance of hydrothermal carbon in the prior art is caused by the efficiency bottleneck of charge transfer and interfacial reaction between polymer chains.

[0006] To achieve the above-mentioned technical objectives, the technical solution provided by this invention is as follows:

[0007] In a first aspect, the present invention provides a method for preparing a nitrogen-containing magnetic hydrothermal carbon material, comprising the following steps: mixing biomass powder, iron salt solution and alcohol, then adding sodium acetate and nitrogen source to obtain a precursor solution; subjecting the precursor solution to a hydrothermal reaction at 170-190°C, and after the reaction is completed, the product is post-treated to obtain the magnetic hydrothermal carbon material.

[0008] Preferably, the biomass powder is obtained by washing, drying, crushing, and sieving biomass.

[0009] Preferably, the iron salt solution is a ferric chloride solution with a concentration of 4-6 mmol / L; the ratio of biomass powder to ferric chloride solution is 1 g: (3-5) mL.

[0010] Preferably, the alcohol used is ethylene glycol; the ratio of biomass powder to ethylene glycol is 1g:(40-60)mL.

[0011] Preferably, the mass ratio of biomass powder to sodium acetate is 1:(3-8).

[0012] Preferably, the mass ratio of biomass powder to nitrogen source is 1:(0.1-3).

[0013] Preferably, the nitrogen source is urea.

[0014] Preferably, the hydrothermal reaction time is 22–26 h; the vacuum drying temperature is 60–80 °C.

[0015] Secondly, the present invention provides a nitrogen-containing magnetic hydrothermal carbon material.

[0016] Thirdly, the present invention provides an application of a nitrogen-containing magnetic hydrothermal carbon material as a photo-induced removal material for Cr(VI) in water.

[0017] Compared with the prior art, the beneficial effects of the present invention include:

[0018] This invention mainly prepares nitrogen-containing magnetic hydrothermal carbon materials through a simple and easy-to-implement hydrothermal method for photoinduced removal of Cr(VI) from wastewater. The introduction of N elements achieves efficient transfer of photoelectrons while simultaneously creating Fe on the surface of the magnetic hydrothermal carbon. 2+ and Fe 3+ The conversion of the electron transfer bridge improves the interfacial reaction rate. In the Cr(VI) removal experiment, it exhibits strong reducing power, adsorption capacity, and excellent cycle stability. Secondly, the process of this invention is simple; the target product can be obtained directly through hydrothermal reaction of biomass, without the need for subsequent heat treatment. This method is highly feasible, easily scaled up, conforms to the characteristics of green chemistry, and is conducive to market promotion. Attached Figure Description

[0019] Figure 1 This is the XRD pattern of the nitrogen-containing magnetic hydrothermal carbon material of Example 3 of the present invention;

[0020] Figure 2 This is a SEM image of the nitrogen-containing magnetic hydrothermal carbon material of Example 3 of the present invention;

[0021] Figure 3 This is a graph showing the Cr(VI) removal performance of the nitrogen-containing magnetic hydrothermal carbon material in Example 3 of the present invention;

[0022] Figure 4 This is a diagram showing the optimal Cr(VI) removal performance of the nitrogen-containing magnetic hydrothermal carbon material in Example 3 of this invention;

[0023] Figure 5 This is a graph showing the Cr(VI) removal cycle performance of the nitrogen-containing magnetic hydrothermal carbon material in Example 3 of the present invention;

[0024] Figure 6 This is a photocurrent performance diagram of the nitrogen-containing magnetic hydrothermal carbon material of Embodiment 3 of the present invention. Detailed Implementation

[0025] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative and not intended to limit the invention.

[0026] Definition of Abbreviations

[0027] HTC: Hydrocarbon

[0028] CFO: Magnetic hydrothermal carbon without introducing N

[0029] HTCN: Nitrogen-containing hydrothermal carbon

[0030] This invention provides a nitrogen-containing magnetic hydrothermal carbon material and its preparation method. The preparation method is controllable, the process is simple, meets the requirements of green chemistry, and is easy to scale up. On this basis, the nitrogen-containing magnetic hydrothermal material also has excellent photoinduced Cr(VI) removal performance and can be used as a raw material for the adsorption and reduction of heavy metal Cr(VI).

[0031] In the nitrogen-containing magnetic hydrothermal carbon material of this invention, the magnetic material is cubic Fe3O4, and the phase is completely consistent with the Fe3O4 standard sample with card number 79-0418, and the space group is Fd-3m. Simultaneously, the introduction of nitrogen increases the carrier separation efficiency under biochar illumination.

[0032] The present invention discloses a method for preparing nitrogen-containing magnetic hydrothermal carbon materials, comprising the following steps:

[0033] 1) Cleaning, drying, crushing, and sieving of the biomass from the leaves of the Chinese parasol tree;

[0034] 2) Add 4-6 mmol / L ferric chloride aqueous solution (FeCl3·6H2O) and sycamore biomass to ethylene glycol and stir magnetically to mix thoroughly;

[0035] 3) Add sodium acetate and stir well;

[0036] 4) Add urea and stir well to obtain the precursor solution;

[0037] 5) Transfer the precursor solution obtained in step 4) to the reactor for hydrothermal reaction;

[0038] 6) The obtained active material was washed several times with pure water and then dried in a vacuum drying oven to finally obtain nitrogen-containing magnetic hydrothermal carbon material.

[0039] According to the above scheme, the sieve mesh number in step 1) is 200 mesh.

[0040] According to the above scheme, the amount of ferric chloride solution added in step 2) is 3-5 mL per 1 g of biomass.

[0041] According to the above scheme, the amount of sodium acetate added in step 3) is 3-8g per 1g of biomass.

[0042] According to the above scheme, the amount of urea added in step 4) is 0.1-3g relative to 1g of biomass; preferably 0.3-1.5g, and more preferably 0.5-1g.

[0043] According to the above scheme, the hydrothermal temperature in step 5) is a relatively low 170-190℃ to achieve the energy-saving goal, and the hydrothermal time is 22-26 hours to ensure the hydrolysis process of biomass. This invention controls the hydrothermal temperature and other conditions to control the dehydration energy of the obtained hydrothermal carbon, enabling it to condense and form sp... 2 The hybrid chain gives it the potential to be used as a photocatalytic material.

[0044] According to the above scheme, the vacuum drying temperature in step 6) is 60-80℃.

[0045] The application of the nitrogen-containing magnetic hydrothermal carbon material in photoinduced removal of Cr(VI).

[0046] Main mechanism of action of this invention:

[0047] In the nitrogen-containing magnetic hydrothermal carbon (Fe3O4 / HTCN) prepared by this invention, HTC is mainly composed of sp 2 The hybrid chain composition allows for the generation of photoexcited electrons and holes under visible light irradiation; the nitrogen element introduced by urea increases charge transfer between molecular chains, which is beneficial for improving the carrier diffusion rate, ultimately enhancing the material's photoinduced Cr(VI) removal performance, and the Fe on the surface... 2+ and Fe 3+ The conversion of electrons into an electron transfer bridge enhances the interfacial reaction rate of photogenerated carriers; and the nitrogen-containing magnetic hydrothermal carbon prepared has a blocky porous structure that can efficiently adsorb Cr(VI).

[0048] The present invention will be further described in detail below through specific embodiments.

[0049] Example 1

[0050] A method for preparing nitrogen-containing magnetic hydrothermal carbon materials includes the following steps:

[0051] 1) Add 1g of *Firmiana simplex* biomass and 4mL of ferric chloride (FeCl3·6H2O) with a concentration of 5mmol / L to 50mL of ethylene glycol and stir thoroughly to make the mixture uniform;

[0052] 2) Add 5g of sodium acetate and stir well;

[0053] 3) Add 0.3g of urea and stir well to obtain the precursor solution;

[0054] 4) Transfer the precursor solution obtained in step 3) to the reactor, and the hydrothermal temperature is 180℃ for 24 hours;

[0055] 5) The obtained active material was washed several times with pure water and then dried in a vacuum drying oven at 60℃ to finally obtain nitrogen-containing magnetic hydrothermal carbon material.

[0056] The application of the nitrogen-containing magnetic hydrothermal carbon material obtained in this embodiment as a photoinduced Cr(VI) removal material is as follows: 15 mg of the prepared nitrogen-containing magnetic hydrothermal carbon material is used as both an adsorbent and a catalyst. 50 mL of a Cr(VI) solution with a concentration of 20 ppm is used as the target pollutant (pH = 2). The removal rate of Cr(VI) over time is tested under irradiation with a xenon lamp with a light intensity of 3 W. The removal rate is 90.2% after 60 min of irradiation.

[0057] Example 2

[0058] A method for preparing nitrogen-containing magnetic hydrothermal carbon materials includes the following steps:

[0059] 1) Add 1g of *Firmiana simplex* biomass and 4mL of ferric chloride (FeCl3·6H2O) with a concentration of 5mmol / L to 50mL of ethylene glycol and stir thoroughly to make the mixture uniform;

[0060] 2) Add 5g of sodium acetate and stir well;

[0061] 3) Add 0.5g of urea and stir well to obtain the precursor solution;

[0062] 4) Transfer the precursor solution obtained in step 3) to the reactor, and the hydrothermal temperature is 180℃ for 24 hours;

[0063] 5) The obtained active material was washed several times with pure water and then dried in a vacuum drying oven at 60℃ to finally obtain Fe3O4 / hydrothermal carbon material.

[0064] Taking the nitrogen-containing magnetic hydrothermal carbon material obtained in this embodiment as an example, the test conditions were the same as in Example 1. Under irradiation with a 3W xenon lamp for 60 minutes at pH=2, the removal rate of Cr(VI) was 92.6%.

[0065] Example 3

[0066] A method for preparing nitrogen-containing magnetic hydrothermal carbon materials includes the following steps:

[0067] 1) Add 1g of *Firmiana simplex* biomass and 4mL of ferric chloride (FeCl3·6H2O) with a concentration of 5mmol / L to 50mL of ethylene glycol and stir thoroughly to make the mixture uniform;

[0068] 2) Add 5g of sodium acetate and stir well;

[0069] 3) Add 1.0g of urea and stir well to obtain the precursor solution;

[0070] 4) Transfer the precursor solution obtained in step 3) to the reactor, and the hydrothermal temperature is 180℃ for 24 hours;

[0071] 5) The obtained active material was washed several times with pure water and then dried in a vacuum drying oven at 60℃ to finally obtain nitrogen-containing magnetic hydrothermal carbon material.

[0072] Taking the nitrogen-containing magnetic hydrothermal carbon material obtained in this embodiment as an example, the test conditions were the same as in Example 1. Under irradiation with a 3W xenon lamp for 60 minutes at pH=2, the removal rate of Cr(VI) was 100%.

[0073] Depend on Figure 1 XRD analysis revealed that the magnetic material was Fe3O4, which has a cubic crystal system. The phase composition perfectly matched that of the Fe3O4 standard sample with card number 79-0418. Observation... Figure 2 The SEM images show that it is evenly distributed on the hydrothermal carbon material.

[0074] Figure 2 The image shows a SEM image of the nitrogen-containing magnetic hydrothermal carbon material obtained in Example 3. As can be seen from the image, the nitrogen-containing magnetic hydrothermal carbon material has a blocky porous structure. Its main structure is HTC obtained from *Firmiana simplex* biomass after hydrothermal treatment, and its interior is composed of sp... 2 The hybrid chain composition allows for the generation of photoexcited electrons and holes under visible light irradiation. The nitrogen element introduced by urea can increase charge transfer between molecular chains, which is beneficial for improving the carrier diffusion rate. In the aforementioned magnetic hydrothermal carbon material, the hydrothermal carbon (HTC) has a nanosheet structure with a width of approximately 100-300 nm.

[0075] Comparative Example 1 (Examining the effect of nitrogen content)

[0076] The only difference from Example 3 is that the amount of urea is adjusted to 0. The other steps and conditions are the same as in Example 3, and finally, nitrogen-free Fe3O4 / hydrothermal carbon material (nitrogen-free magnetic hydrothermal carbon, referred to as CFO material) is obtained.

[0077] Under the same test conditions as in Example 3, the CFO material obtained in this comparative example had a Cr(VI) removal rate of 82.4% at pH=2 after 60 min of irradiation with a xenon lamp of 3W light intensity.

[0078] As can be seen from Comparative Example 1 and Examples 1-3, whether the target product contains nitrogen and the amount of nitrogen both affect the removal rate of Cr(VI). When there is no nitrogen or the nitrogen content is very low, the removal rate of Cr(VI) is low. When the amount of urea is 50% of the biomass mass, the removal rate of Cr(VI) can be significantly improved. After the amount is increased to a certain extent, the removal rate of Cr(VI) reaches 100%. Further increases will lead to increased costs or even side effects. Therefore, the preferred mass ratio of urea to biomass in this invention is (0.5~1):1.

[0079] Comparative Example 2

[0080] The difference from Example 3 is that the sycamore biomass was directly subjected to a hydrothermal reaction to produce HTC, and the hydrothermal conditions were the same as in Example 3.

[0081] Comparative Example 3 (Examining the effect on the reaction system)

[0082] The only difference from Example 3 is that ethylene glycol is not added; the other steps and conditions are the same as in Example 3.

[0083] The results showed that without the addition of ethylene glycol, magnetic substances could not be formed in the final product.

[0084] Similarly, the absence of sodium acetate will also prevent the formation of magnetic substances in the final product. Only in the presence of ethylene glycol and sodium acetate can the hydrothermal reaction generate the desired magnetic hydrothermal carbon.

[0085] The following reference groups were set up to compare the Cr(VI) removal performance of the nitrogen-containing magnetic hydrothermal carbon material obtained in Example 3. The test conditions were the same as those in Example 1.

[0086] Reference group A: Pure Fe3O4 (the amount added is the same as that of nitrogen-containing magnetic hydrothermal carbon material, and other test conditions are the same as in Example 1);

[0087] Reference group B: HTC obtained from Comparative Example 2.

[0088] Figure 3 The graph shows the Cr(VI) removal performance of the nitrogen-containing magnetic hydrothermal carbon material obtained in Example 3 under the same test conditions as in Example 1. As can be seen from the graph, compared to pure Fe3O4 in reference group A and hydrothermal carbon (HTC) in reference group B, the nitrogen-containing magnetic hydrothermal carbon material obtained in Example 3 can completely remove Cr(VI) within 40 minutes. This indicates that in the nitrogen-containing magnetic hydrothermal carbon material obtained in this invention, the synergistic effect of HTC composite nitrogen and Fe3O4 is achieved, and the combined effect of N doping and Fe3O4 improves the photoinduced Cr(VI) removal performance of the magnetic hydrothermal carbon.

[0089] To determine optimal performance, with all other test conditions unchanged, the Cr(VI) concentration was increased to 50 ppm for testing. Figure 4 and Figure 5 As shown.

[0090] Figure 4 This indicates that the nitrogen-containing magnetic hydrothermal char material obtained in Example 3 can remove 157.7 mg / g within 90 minutes. Comparing light and dark environments, it was found that ambient light significantly enhances the Cr(VI) removal capacity of the magnetic biochar.

[0091] exist Figure 5 The nitrogen-containing magnetic hydrothermal carbon material obtained in Example 3 showed that the Cr(VI) removal rates were maintained at 100%, 90.3%, 90.1%, and 89.8% respectively in four cycles. The experimental test conditions for each cycle were consistent with those in Example 1. Since about 3 mg of nitrogen-containing magnetic hydrothermal carbon material was lost at the end of each cycle, 3 mg of nitrogen-containing magnetic hydrothermal carbon material was added to the recovered material after each test before reuse.

[0092] Figure 6 To test the photocurrent performance of the nitrogen-containing magnetic hydrothermal carbon material using an electrochemical workstation, the same mass of magnetic biochar was coated onto FTO conductive glass. Using a xenon lamp as the light source and maintaining a constant light intensity, the photocurrent intensity was measured in a three-electrode electrolytic cell. Compared to the HTC obtained in Comparative Example 2 and the magnetic hydrothermal carbon (CFO) material without N incorporation in Comparative Example 1, the nitrogen-containing magnetic hydrothermal carbon material of this invention exhibits a significantly increased photocurrent density within the same timeframe, further demonstrating that the nitrogen-containing magnetic hydrothermal carbon material of this invention can more effectively utilize ambient light to remove Cr(VI).

[0093] Compared with existing technologies, this invention provides a nitrogen-containing magnetic hydrothermal carbon material and its preparation method, as well as its application in treating Cr(VI). The preparation steps are as follows: first, clean and crush the *Platycladus orientalis* leaf biomass and sieve it; add FeCl3 and biomass powder to ethylene glycol and stir thoroughly to ensure uniform mixing; add sodium acetate (CH3COONa) and urea (CH4N2O) and stir evenly; transfer the precursor to a reaction vessel for hydrothermal reaction; wash the prepared material several times with pure water and ethanol, then dry it to finally obtain the nitrogen-containing magnetic hydrothermal carbon material. This invention directly obtains nitrogen-containing magnetic hydrothermal carbon material through a single hydrothermal method, and the prepared composite material has a blocky porous structure. Due to its special preparation conditions, hydrothermal carbon forms sp... 2 Hybrid chains possess the potential to serve as photocatalytic materials. Furthermore, the abundant functional groups on the surface of hydrothermal biochar provide an excellent medium for adsorbing Cr(VI). Fe3O4 can bind well with hydrothermal biochar and provides good magnetic properties, facilitating collection and reuse. Simultaneously, the introduction of N can react with both hydrothermal biochar and Fe to enhance the photocatalytic activity of the biochar.

[0094] Therefore, using nitrogen (N) to connect the molecular chains of hydrothermal carbon can serve as an electron transport channel, enabling efficient transport of photogenerated electrons. Furthermore, carbon materials used as adsorbents in wastewater treatment often face difficulties in recycling; combining magnetic materials with catalytic materials can leverage magnetism to achieve efficient material recovery. Simultaneously, during magnetization, N can further combine with biochar to transfer electrons, enhancing the efficiency of Cr(VI) removal by photoluminescence on magnetic hydrothermal carbon, reaching 90.2%–100%.

[0095] This invention features a simple process, mild reaction conditions, and excellent photo-induced Cr(VI) removal performance of the material.

[0096] The specific embodiments of the present invention described above do not constitute a limitation on the scope of protection of the present invention. Any other corresponding changes and modifications made in accordance with the technical concept of the present invention should be included within the scope of protection of the claims of the present invention.

Claims

1. The application of a nitrogen-containing magnetic hydrothermal carbon material as a photo-induced removal material for Cr(VI) in water, characterized in that, The preparation method of the nitrogen-containing magnetic hydrothermal carbon material includes the following steps: 1) Add 1 g of *Firmiana simplex* biomass and 4 mL of 5 mmol / L FeCl3·6H2O to 50 mL of ethylene glycol and stir thoroughly to mix evenly; 2) Add 5 g of sodium acetate and stir well; 3) Add 1.0 g of urea and stir well to obtain the precursor solution; 4) Transfer the precursor solution obtained in step 3) to the reactor, and the hydrothermal temperature is 180 °C for 24 h. 5) The obtained active material was washed several times with pure water and then dried in a vacuum drying oven at 60 ℃ to finally obtain nitrogen-containing magnetic hydrothermal carbon material; The nitrogen-containing magnetic hydrothermal carbon material includes hydrothermal carbon (HTC), and the hydrothermal carbon HTC has a nanosheet structure with a width of 100-300 nm.

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