Preparation method and application of temperature fractionation reinforced nitrogen-doped biochar

Abstract

The application relates to a preparation method of a temperature grading reinforced nitrogen-doped biochar, and belongs to the field of water body / atmosphere pollutant treatment. The specific scheme comprises the following steps: step one, pretreatment: crushing and drying biomass to obtain biomass powder; step two, carbonization: carrying out high-temperature pyrolysis treatment on the biomass powder under a nitrogen atmosphere to obtain biochar precursor I; step three, mixing: mixing the biochar precursor I, a nitrogen source compound and an activating agent in anhydrous ethanol, carrying out water bath heating and stirring treatment until the ethanol is naturally volatilized to obtain a nitrogen-containing mixture; step four, doping and activation: sufficiently grinding the nitrogen-containing mixture, carrying out gradient temperature rising in a nitrogen atmosphere and carrying out heat preservation treatment in different temperature intervals to obtain biochar precursor II; and step five, post-treatment: carrying out acid washing on the biochar precursor II, carrying out water washing until neutral, and carrying out drying to obtain nitrogen-doped biochar. The prepared biochar has the characteristics of high specific surface area, porous structure, high adsorption capacity and rapid adsorption.

Description

Technical Field

[0001] This invention belongs to the field of water / gas pollutant treatment, specifically relating to a method for preparing and applying temperature-graded enhanced nitrogen-doped biochar. Background Technology

[0002] Water and air pollution are characterized by strong diffusion, susceptibility to secondary pollution, and wide-ranging harmful effects. Furthermore, current waste biomass presents challenges due to its large volume and difficulty in treatment; improper utilization can exacerbate environmental pollution. Therefore, there is an urgent need to find new treatment methods.

[0003] Studies have shown that biochar has many advantages, including a wide variety of raw materials, low cost, recyclability, and rich functional groups. It can effectively remove organic dyes and other organic matter from water bodies. Currently, conventional activated carbon has a small adsorption capacity and slow adsorption rate for pollutants. The raw materials for preparing high specific surface area activated carbon in China are generally coal, petroleum coke, and coconut shells. However, due to the high cost of these raw materials, especially coconut shell carbonization which still needs to be imported, production costs are high, the finished product price is expensive, and market promotion is difficult. Furthermore, the activation reagents are not environmentally friendly.

[0004] Chinese patent application publication number CN 111533125 A discloses a method for preparing nitrogen-doped hierarchical porous carbon materials. The method uses a mixture of lavender residue, potassium hydroxide, and urea to prepare porous carbon, which is then applied to the treatment of environmental water pollutants. Hexavalent chromium is selected as the adsorbent, with a maximum adsorption capacity of 276.2 mg / g and a removal rate of up to 92.9%. However, the KOH used in this method's embodiments is highly corrosive and can damage equipment. Furthermore, the direct linear heating method reduces the doping and activation effects.

[0005] Chinese patent application publication number CN 1111534902 A discloses a method for preparing multi-purpose oxygen-nitrogen co-doped waste biomass-derived carbon materials. The method involves mixing palm flower spikes and KOH for activation and then directly linearly heating to prepare porous carbon. At a current density of 1 A / g, the highest specific capacitance of a single electrode can reach 562 F / g, and the adsorption capacity of CO2 at room temperature and pressure can reach 5.73 mmol / g. However, it also faces the problem of easy corrosion of KOH reagent.

[0006] Therefore, there is still a need in this field to study novel activated carbon materials with high adsorption performance and low raw material costs, as well as to optimize their preparation techniques. Summary of the Invention

[0007] To address the problems existing in the prior art, this invention proposes a green and economical method for preparing high-performance biochar by using common agricultural waste biomass as a carbonization precursor and through temperature-graded enhanced nitrogen doping and activation.

[0008] To achieve the above objectives, the present invention adopts the following technical solution: a method for preparing temperature-graded enhanced nitrogen-doped biochar, comprising the following steps:

[0009] Step 1, Pretreatment: The biomass is crushed and dried to obtain biomass powder;

[0010] Step 2, Carbonization: The biomass powder is subjected to high-temperature pyrolysis under a nitrogen atmosphere to obtain biochar precursor 1;

[0011] Step 3, Mixing: Mix biochar precursor 1, nitrogen source compound and activator in anhydrous ethanol, heat in a water bath, and stir until the ethanol evaporates naturally to obtain a nitrogen-containing mixture;

[0012] Step 4, Doping and Activation: After thoroughly grinding the nitrogen-containing mixture, heat it in a nitrogen atmosphere at a rate of 10℃ / min from room temperature, and hold it at 300℃, 600℃ and 800℃ for 0.5h respectively. After completion, allow it to cool naturally to room temperature to obtain biochar precursor II.

[0013] Step 5, Post-processing: After washing the biochar precursor II with sulfuric acid or hydrochloric acid, wash with water until neutral, and dry to obtain nitrogen-doped biochar.

[0014] Furthermore, in step one, the biomass includes one or more combinations of sawdust, rice husks, and corn stalks.

[0015] Furthermore, in step two, the high-temperature pyrolysis process is as follows: the temperature is increased from room temperature to 500℃ at a rate of 5℃ / min, maintained at a constant temperature for 1 hour, and then naturally cooled.

[0016] Furthermore, in step three, the mass ratio of biochar precursor one, nitrogen source compound, and activator is 1:1:3.

[0017] Furthermore, in step three, the nitrogen source compound is melamine, and the activator is potassium carbonate.

[0018] Furthermore, in step three, the water bath heating temperature is 80℃, and the stirring method is magnetic stirring at a speed of 500 rpm.

[0019] Furthermore, in step five, the sulfuric acid concentration is 0.2M, the hydrochloric acid concentration is 0.5mol / L, and the pickling time is 2 hours.

[0020] An application of the nitrogen-doped biochar prepared by the aforementioned preparation method, wherein the nitrogen-doped biochar is used for wastewater or gas purification.

[0021] Compared with the prior art, the beneficial effects of the present invention are:

[0022] The functional design of biochar is essentially a process in which biochar reacts with an activating reagent to achieve the migration and conversion between reacting elements. Temperature is the determining factor for the degree and rate of reaction. Most current activation technologies are based on direct programmed heating to the target temperature and holding the reaction. Direct linear heating may have the following drawbacks: (1) The activation reaction has a short residence time in the suitable temperature range, resulting in incomplete reaction. (2) Excessive temperature reduces the reaction rate and may even lead to the decomposition of reactants. For example, melamine begins to decompose at around 330°C, and other nitrogen sources also begin to decompose successively within 500°C. Therefore, this invention proposes a temperature-graded enhanced doping strategy, using melamine as the nitrogen source, with a low holding temperature to further reduce energy consumption. Three stepped temperatures are set at 300°C, 600°C, and 800°C, each held for half an hour. This allows carbon materials to undergo low-temperature nitrogen doping, medium-temperature carbonization to retain oxygen, and high-temperature activation to increase pore size, thereby maximizing atom economy.

[0023] This invention utilizes biochar prepared by the above method to conduct adsorption tests on methylene blue, a representative aquatic pollutant. A certain concentration of methylene blue solution was added to the biochar prepared by the above method, and the mixture was magnetically stirred at room temperature for 12 hours. The saturated adsorption capacity of methylene blue was then determined. Small amounts of solution were aspirated at set time points, and the concentration of methylene blue solution was measured using a UV-Vis spectrophotometer to determine its kinetic adsorption process. The biochar prepared in this invention exhibited a maximum adsorption capacity of 1874.72 mg / g for methylene blue in aquatic pollutants, while recent studies have shown adsorption levels of only 310-1075.29 mg / g for methylene blue. For a 50 mL methylene blue solution with a concentration of 1 g / L, 40 mg of the sample prepared according to this invention can achieve a purification rate of 99% within 30 minutes; for a 50 mL methylene blue solution with a concentration of 40 mg / L, after removing the influence of carbon particles suspended in the water, 10 mg of the sample prepared according to this invention can achieve a 100% purification rate in just 3 minutes. The above data show that the sample prepared according to this invention can achieve rapid and efficient adsorption of methylene blue solutions of any concentration.

[0024] The biochar prepared by the above method was used in this invention to conduct adsorption tests on toluene, a representative air pollutant. The toluene concentration was 500 ppm, the gas mixture was argon, and the temperature was room temperature. The adsorption capacity was: RH-3 > CS-3 > SD-3. RH-3 and CS-3 had similar adsorption capacities, with saturation capacities close to 780 mg / g. Recent studies have shown adsorption levels of 132.9-606.6 mg / g for toluene. This demonstrates the superiority of the temperature-graded enhanced nitrogen-doped activated biochar technology proposed in this invention. To prove the scientific advancement of this technology, while keeping other conditions constant, a control group of biochar was prepared using the same biomass through direct heating. The yield comparison is shown below. Figure 1As shown. The temperature-stage activation strategy proposed in this invention, based on chemical reaction kinetics, effectively solves the problems of incomplete reaction and reactant decomposition caused by direct heating, resulting in higher yields. The biochar yield prepared by temperature-stage heating using straw as a carbon precursor is nearly 5% higher than that prepared by direct heating. Simultaneously, the adsorption performance of biochar prepared by the two different heating methods was compared, as shown... Figure 2 and Figure 4 As shown, the sample prepared by this invention exhibits not only a higher saturated adsorption capacity but also a faster adsorption rate, confirming that the temperature-grading method can effectively improve the adsorption capacity of biochar. The biochar prepared by this invention possesses characteristics such as high specific surface area, porous structure, high adsorption capacity, and rapid adsorption. The biochar material of this invention is derived from agricultural waste biomass, which is readily available and inexpensive. The use of the environmentally friendly activator K₂CO₃ instead of the expensive KOH activator effectively saves costs, protects the environment, and achieves added value from waste. Attached Figure Description

[0025] Figure 1 This is a comparison chart of the biochar yields prepared by the preparation methods described in Examples 1-3 and the direct heating method described in Comparative Example 1;

[0026] Figure 2 This is a comparison chart of the adsorption performance of biochar prepared by the preparation method described in Example 3 and the direct heating method described in Comparative Example 1 on methylene blue.

[0027] Figure 3 These are the kinetic adsorption curves of methylene blue solutions of different concentrations prepared by the preparation method described in Examples 1-3;

[0028] Figure 4 The adsorption curves of toluene on the biochar prepared by the preparation method described in Examples 1-3 are shown.

[0029] Figure 5 This is a comparison chart of the toluene adsorption performance of biochar prepared by the preparation method described in Example 3 and the direct heating method described in Comparative Example 1. Detailed Implementation

[0030] The technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings and embodiments. Obviously, the described embodiments are only some embodiments of the invention, not all embodiments. All other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are within the scope of protection of the present invention.

[0031] Example 1:

[0032] Preparation of rice husk biochar:

[0033] (1) Material pretreatment: The rice husks are crushed and screened to 60-120 mesh using a crusher and then placed in a forced-air drying oven to dry overnight at 105℃.

[0034] (2) Rice husk carbonization: The pretreated rice husks were placed in a tube furnace, connected to the pipeline, and inert gas N2 was introduced. After the air in the tube was exhausted, the carrier gas flow rate was maintained at 1000 mL / min. The tube furnace was heated from room temperature to 500℃ at the set heating rate (5℃ / min), and kept at a constant temperature for 1 hour before being naturally cooled to obtain biochar precursor one;

[0035] (3) Mixing carbon source, nitrogen source and K2CO3: Mix biochar precursor 1, melamine and K2CO3 in a mass ratio of 1:1:3, add an appropriate amount of anhydrous ethanol as solvent, and magnetically stir in an 80℃ constant temperature water bath at a speed of 500rpm until the mixture is uniform and the ethanol is completely evaporated, and then take it out to obtain a nitrogen-containing mixture.

[0036] (4) N doping and K2CO3 activation: After the nitrogen-containing mixture is thoroughly ground, it is heated from room temperature at a rate of 10℃ / min in N2 atmosphere (flow rate 500mL / min), and kept at 300℃, 600℃ and 800℃ for 0.5h respectively. After completion, it is naturally cooled to room temperature to obtain biochar precursor II.

[0037] (5) Post-treatment: Biochar precursor 2 was acid-washed with 0.2M H2SO4 for 2h, washed with water until neutral, and then dried to obtain nitrogen-doped biochar, denoted as RH-3.

[0038] Example 2

[0039] Preparation of wood chip biochar:

[0040] (1) Material pretreatment: The wood chips are crushed and screened to 60-120 mesh using a crusher and then placed in a forced-air drying oven to dry overnight at 105℃.

[0041] (2) Wood chip carbonization: The pretreated wood chips were placed in a tube furnace, connected to the pipeline, and inert gas N2 was introduced. After the air in the tube was exhausted, the carrier gas flow rate was maintained at 1000 mL / min. The tube furnace was heated from room temperature to 500℃ at the set heating rate (5℃ / min), and kept at a constant temperature for 1 hour before being naturally cooled to obtain biochar precursor one;

[0042] (3) Mixing carbon source, nitrogen source and K2CO3: The biochar precursor 1, melamine and K2CO3 are mixed uniformly in a mass ratio of 1:1:3. An appropriate amount of anhydrous ethanol is added as a solvent. The mixture is magnetically stirred in a constant temperature water bath at 80℃ and a speed of 500rpm until it is uniform and the ethanol is completely evaporated, and then the mixture containing nitrogen is obtained.

[0043] (4) N doping and K2CO3 activation: After the nitrogen-containing mixture is thoroughly ground, it is heated from room temperature at a rate of 10℃ / min in N2 atmosphere (flow rate 500mL / min), and kept at 300℃, 600℃ and 800℃ for 0.5h respectively. After completion, it is naturally cooled to room temperature to obtain biochar precursor II.

[0044] (5) Post-treatment: Biochar precursor 2 was acid-washed with 0.2M H2SO4 for 2h, washed with water until neutral, and then dried to obtain nitrogen-doped biochar, denoted as SD-3.

[0045] Example 3

[0046] Preparation of straw biochar:

[0047] (1) Material pretreatment: After crushing the straw to 60-120 mesh using a crusher, put it into a blower drying oven to dry overnight at 105℃.

[0048] (2) Straw carbonization: The pretreated straw was placed in a tubular furnace, connected to the pipeline, and inert gas N2 was introduced. After the air in the tube was exhausted, the carrier gas flow rate was maintained at 1000 mL / min. The tubular furnace was heated from room temperature to 500℃ at the set heating rate (5℃ / min), and kept at a constant temperature for 1 hour before being naturally cooled to obtain biochar precursor one;

[0049] (3) Mixing carbon source, nitrogen source and K2CO3: The biochar precursor 1, melamine and K2CO3 are mixed uniformly in a mass ratio of 1:1:3. An appropriate amount of anhydrous ethanol is added as a solvent. The mixture is magnetically stirred in a constant temperature water bath at 80℃ and a speed of 500rpm until it is uniform and the ethanol is completely evaporated, and then the mixture containing nitrogen is obtained.

[0050] (4) N doping and K2CO3 activation: After the nitrogen-containing mixture is thoroughly ground, it is heated from room temperature at a rate of 10℃ / min in N2 atmosphere (flow rate 500mL / min), and kept at 300℃, 600℃ and 800℃ for 0.5h respectively. After completion, it is naturally cooled to room temperature to obtain biochar precursor II.

[0051] (5) Post-treatment: The biochar precursor 2 was acid-washed with 0.2M H2SO4 for 2h, washed with water until neutral, and then dried to obtain nitrogen-doped biochar, denoted as CS-3.

[0052] Comparative Example 1

[0053] The preparation method of this comparative example is the same as that of Example 3, except that the heating method in step (4) is changed to: directly heating from room temperature to 800℃ at a rate of 10℃ / min and holding at that temperature for 0.5h. The final product of Comparative Example 1 is denoted as TCS-3.

[0054] Test Example 1

[0055] Biochar adsorption of methylene blue test:

[0056] The adsorption capacity of different biochar samples for methylene blue was tested by liquid-phase adsorption experiments to characterize the adsorption capacity of the biochar prepared in this invention for typical liquid-phase organic pollutants.

[0057] (1) Prepare a methylene blue solution of a certain concentration, measure its absorption spectrum using a UV-Vis spectrophotometer, and select the wavelength of 665.9 nm as its characteristic peak to determine the concentration of the methylene blue solution to be tested.

[0058] (2) Use methylene blue solutions of known concentrations of 0.001 / 0.0025 / 0.005 / 0.01 / 0.02 / 0.03 / 0.04 / 0.05 / 0.1 / 0.5 / 0.6 / 0.8 / 1 / 1.2 / 1.5 g / L to calibrate the concentration curve, and fit the results in segments.

[0059] (3) Take three 50ml portions of methylene blue solution with a concentration of 1g / L and add 40mg of SD-3, RH-3 and CS-3 respectively. Stir magnetically for 12h at room temperature and determine the saturated adsorption capacity of methylene blue.

[0060] (4) Further, 50 ml of methylene blue solution with a concentration of 1.5 g / L was added to 40 mg of CS-3, and the mixture was magnetically stirred at room temperature for 12 h. The actual saturated adsorption capacity of CS-3 was then determined.

[0061] (5) The methylene blue solution from steps (3) and (4) above is subjected to a set time.

[0062] A small amount of solution was taken at 5 / 10 / 15 / 30 / 45 / 60 / 75 / 90 / 120 / 180 / 240 / 300 min, and the concentration of methylene blue solution was measured using a UV-UV spectrophotometer to determine its kinetic adsorption process.

[0063] (6) To test the adsorption performance of biochar in low concentration methylene blue solution, 50 ml of methylene blue solution with a concentration of 40 mg / L was added to 10 mg of RH-3 for kinetic adsorption test.

[0064] Depend on Figure 3 The kinetic adsorption curves and saturated adsorption capacities after 12 hours of absorption show that CS-3 exhibits the highest saturated adsorption capacity among SD-3, RH-3, and CS-3, reaching 1249.20 mg / g, indicating that it adsorbs all the methylene blue in the solution. Meanwhile, based on... Figure 2A comparison between CS-3 and TCS-3 shows that CS-3 prepared using the temperature-graded enhancement method not only has a higher saturated adsorption capacity but also exhibits a faster adsorption rate.

[0065] In a 1.5 g / L methylene blue solution, CS-3 reached an absorption peak at 10 min and then desorbed slightly. From 15 min onwards, the adsorption capacity continued to increase rapidly, and the saturated adsorption capacity measured after 12 h was as high as 1874.72 mg / g.

[0066] For a 40 mg / L methylene blue solution, after about 3 minutes, the solution became basically clear and transparent after removing the influence of carbon particles suspended in the water. This indicates that the sample can achieve rapid and efficient adsorption of low concentrations of methylene blue.

[0067] Test Example 2

[0068] Biochar adsorption of toluene test:

[0069] The adsorption capacity of different biochar samples for toluene was tested by gas-phase toluene adsorption experiments, which characterizes the adsorption capacity of the biochar prepared in this invention for typical gas-phase organic pollutants.

[0070] (1) The toluene adsorption experiment was conducted on a fixed bed experimental platform combined with online mass spectrometry. Toluene was generated by a VOCs generator at a concentration of 500 ppm, and Ar was used as the gas to be mixed. The mass of the adsorption raw material was 100 mg.

[0071] (2) Take a blank tube without biochar loading, pass toluene vapor through it, and record the change of toluene concentration over time.

[0072] (3) Take 100 mg of SD-3, RH-3 and CS-3 respectively and place them in the reactor. Then, introduce toluene vapor at room temperature.

[0073] Record the change in toluene concentration over time.

[0074] Depend on Figure 4 The adsorption capacity curves show that the adsorption capacity is in the order RH-3 > CS-3 > SD-3, with RH-3 and CS-3 having similar adsorption capacities and saturation capacities close to 780 mg / g. According to... Figure 5 A comparison of the adsorption results of CS-3 and TCS-3 shows that the temperature-stage enhanced activation strategy can effectively improve the adsorption capacity of biochar for toluene, and has significant advantages over the direct linear heating method.

[0075] Furthermore, it should be understood that although this specification describes embodiments, not every embodiment contains only one independent technical solution. This narrative style is merely for clarity. Those skilled in the art should consider the specification as a whole, and the technical solutions in each embodiment can also be appropriately combined to form other embodiments that can be understood by those skilled in the art.

Claims

1. The application of temperature-graded enhanced nitrogen-doped biochar in gas purification, wherein the gas is toluene, characterized in that, The method for preparing the nitrogen-doped biochar includes the following steps: Step 1, Pre-treatment: The biomass is crushed and dried to obtain biomass powder. The biomass is selected from one or two types of rice husks and corn stalks. Step 2, Carbonization: The biomass powder is subjected to high-temperature pyrolysis under a nitrogen atmosphere to obtain biochar precursor 1; Step 3, Mixing: Mix biochar precursor 1, nitrogen source compound and activator in anhydrous ethanol, heat in a water bath, and stir until the ethanol evaporates naturally to obtain a nitrogen-containing mixture. The nitrogen source compound is melamine and the activator is potassium carbonate. Step 4, Doping and Activation: After thoroughly grinding the nitrogen-containing mixture, heat it in a nitrogen atmosphere at a rate of 10℃ / min from room temperature, and hold it at 300℃, 600℃ and 800℃ for 0.5h respectively. After completion, allow it to cool naturally to room temperature to obtain biochar precursor II. Step 5, Post-processing: After washing the biochar precursor II with sulfuric acid or hydrochloric acid, wash with water until neutral, and dry to obtain nitrogen-doped biochar.

2. The application according to claim 1, characterized in that: In step two, the high-temperature pyrolysis process is as follows: the temperature is increased from room temperature to 500℃ at a rate of 5℃ / min, kept constant for 1 hour, and then naturally cooled.

3. The application according to claim 1, characterized in that: In step three, the mass ratio of biochar precursor I, nitrogen source compound, and activator is 1:1:

3.

4. The application according to claim 1, characterized in that: In step three, the water bath heating temperature is 80℃, and the stirring method is magnetic stirring at a speed of 500 rpm.

5. The application according to claim 1, characterized in that: In step five, the concentration of sulfuric acid is 0.2 M, the concentration of hydrochloric acid is 0.5 mol / L, and the pickling time is 2 hours.

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