Nitrogen-doped graded porous biochar as well as preparation method and application thereof

Abstract

The invention belongs to the technical field of biochar, and provides nitrogen-doped graded porous biochar as well as a preparation method and application thereof. The preparation method comprises the following steps: performing hydrothermal reaction on coconut shell biomass powder and urea to obtain nitrogen-doped biomass powder, and mixing the nitrogen-doped biomass powder with an activating agent for carbonization; the activating agent comprises potassium hydroxide and potassium nitrate. The agricultural waste coconut shells are prepared into the porous biochar, the raw material sources are wide, the production cost is low, the porous biochar is environmentally friendly, and the preparation method has the advantage of being high in yield which is 2.85 times that of a traditional alkali activation method; the nitrogen-doped graded porous biochar has the characteristics of excellent graded porous structure, strong chemical stability, high nitrogen content and high specific surface area, the adsorption capacity of gaseous iodine is effectively improved, the adsorption capacity of 6000 mg / g or above can still be kept after five times of adsorption, and the nitrogen-doped graded porous biochar has relatively good iodine retention capacity.

Description

Technical Field

[0001] The present invention relates to the technical field of biochar, and in particular to nitrogen-doped graded porous biochar and a preparation method and application thereof. Background Art

[0002] The environmental hazards and resource shortages of fossil fuels have forced the rapid development of alternative energy sources. Compared with fossil fuels, nuclear energy produces almost no carbon dioxide and plays an important role in research on climate change. However, the application of nuclear energy is hindered by nuclear leakage, spent fuel disposal, etc., and radioactive nuclear elements pose a serious threat to the environment and human health. Radioactive gaseous iodine ( 129 I. 131 I) is the main volatile pollutant produced when processing spent fuel in nuclear power plants. It needs to be effectively treated before it spreads to curb its harm.

[0003] Dry adsorption is widely used for treating radioactive gaseous iodine due to its simplicity and high adsorption efficiency. However, materials such as silver-loaded zeolites are expensive, and emerging adsorption materials such as MOFs and COFs are only in the laboratory synthesis stage and have not yet been mass-produced. Porous carbon, with its large surface area, abundant pores, and strong chemical stability, is suitable for widespread application. However, the precursors currently used to prepare porous carbon are generally expensive and have low iodine adsorption capacity. Improvements to porous carbon could make it a promising candidate for adsorbing gaseous iodine.

[0004] The porous biochar prepared from agricultural waste has the characteristics of large specific surface area, rich pore structure, strong chemical stability, rich organic functional groups, etc., which can provide good adsorption performance. Ma et al. [Separation and Purification Technolog 347 (2024): 127613] used rapeseed pollen with micron-scale ellipsoidal shape, uniform size, porous structure with sparse internal connection network, excellent mechanical strength and good thermal stability as a porous carbon precursor for adsorbing gaseous iodine. Through ethanol pretreatment, 350℃ pre-carbonization and 600℃ KOH activation carbonization, porous carbon was successfully prepared. The specific surface area after 350℃ pre-carbonization was 24.7m 2 / g, the specific surface area of ​​KOH activated carbonization reaches 602m 2 / g, and the pore volume reaches 0.47cm 3 / g, and the adsorption capacity of gaseous iodine is as high as 3842mg / g. Li et al. [Chinese Environmental Science, 2024, 44(10): 5406-5414] used walnut shell as a precursor to prepare microporous biochar by KOH activation, with a specific surface area of ​​1589m 2 / g, and the gaseous iodine adsorption capacity reached 2027mg / g. However, the adsorption capacity of porous biochar for gaseous iodine is still lower than that of MOF, COP and other materials.

[0005] Therefore, providing an iodine adsorption material with high yield and high adsorption capacity has good application prospects. Summary of the Invention

[0006] The purpose of the present invention is to provide a nitrogen-doped hierarchical porous biochar and its preparation method and application in response to the deficiencies in the prior art.

[0007] In order to achieve the above-mentioned object of the invention, the present invention provides the following technical solutions:

[0008] The present invention provides a method for preparing nitrogen-doped hierarchical porous biochar, comprising the following steps:

[0009] 1) hydrothermally reacting coconut shell biomass powder and urea to obtain nitrogen-doped biomass powder;

[0010] 2) mixing the nitrogen-doped biomass powder and the activator and carbonizing the mixture to obtain nitrogen-doped hierarchical porous biochar;

[0011] The activator comprises potassium hydroxide and potassium nitrate.

[0012] Preferably, in step 1), the mass ratio of the coconut shell biomass powder to urea is 1:1-3.

[0013] Preferably, the temperature of the hydrothermal reaction in step 1) is 180-220° C., and the time of the hydrothermal reaction is 1.5-3 h.

[0014] Preferably, in step 2), the mass molar ratio of the nitrogen-doped biomass powder to the activator is 15 g: 0.2-0.5 mol.

[0015] Preferably, the molar ratio of potassium hydroxide to potassium nitrate is 1:0.05-0.25.

[0016] Preferably, the carbonization in step 2) is carried out in a nitrogen atmosphere, the carbonization temperature is 600-1000° C., the carbonization time is 1.5-3 h, and the heating rate to the carbonization temperature is 3-8° C. / min.

[0017] Preferably, after the carbonization in step 2) is completed, washing is performed to obtain nitrogen-doped graded porous biochar;

[0018] The washing process is stopped when the pH value of the washing solution reaches 6.5 to 7.5.

[0019] The present invention also provides nitrogen-doped hierarchical porous biochar prepared by the preparation method, wherein the specific surface area of ​​the nitrogen-doped hierarchical porous biochar is 2300-3300 m 2 / g, and the total pore volume of nitrogen-doped hierarchical porous biochar was 1.6-3.2 cm 3 / g, and the micropore volume of nitrogen-doped hierarchical porous biochar is 1-1.4 cm 3 / g.

[0020] The present invention also provides the use of the nitrogen-doped graded porous biochar in adsorbing gaseous iodine.

[0021] The beneficial effects of the present invention include the following:

[0022] 1) The present invention prepares porous biochar from coconut shells, an agricultural waste product. The raw material source is wide, the production cost is low, and it is green and environmentally friendly, which is conducive to industrial promotion. The preparation method has the characteristics of high yield, which is more than 2.85 times that of the traditional alkaline activation method.

[0023] 2) The nitrogen-doped hierarchical porous biochar prepared by the present invention has an excellent hierarchical porous structure and strong chemical stability. It also has the characteristics of high nitrogen content and high specific surface area. The nitrogen content is 6.9 times that of the traditional alkaline activation method, and the specific surface area is 2300-3300m 2 / g, with a total pore volume of 1.6 to 3.2 cm 3 / g, micropore volume is 1~1.4cm 3 / g, effectively improving the adsorption capacity of gaseous iodine, the adsorption capacity is as high as 7221mg / g, and it can still maintain an adsorption capacity of more than 6000mg / g after 5 adsorptions, and has good iodine retention ability. After adsorption, only 6.31% is desorbed one day and 20.99% is desorbed after 5 days. BRIEF DESCRIPTION OF THE DRAWINGS

[0024] Figure 1 This is a scanning electron microscope image of the biochar prepared in Comparative Example 2;

[0025] Figure 2 This is a scanning electron microscope image of the biochar prepared in Comparative Example 1;

[0026] Figure 3 This is a scanning electron microscope image of the nitrogen-doped hierarchical porous biochar prepared in Example 1;

[0027] Figure 4 N2 isothermal adsorption-desorption curve of the biochar prepared in Comparative Example 2 at 77K;

[0028] Figure 5 N2 isothermal adsorption-desorption curve of the biochar prepared in Comparative Example 1 at 77K;

[0029] Figure 6 N2 isothermal adsorption-desorption curve of the nitrogen-doped hierarchical porous biochar prepared in Example 1 at 77K;

[0030] Figure 7 This is the pore size distribution diagram of the biochar prepared in Comparative Example 2;

[0031] Figure 8 This is the pore size distribution diagram of the biochar prepared in Comparative Example 1;

[0032] Figure 9 This is the pore size distribution diagram of the nitrogen-doped hierarchical porous biochar prepared in Example 1;

[0033] Figure 10 This is the adsorption kinetic curve of iodine by the biochar prepared in Comparative Example 2;

[0034] Figure 11 This is the adsorption kinetic curve of iodine by the biochar prepared in Comparative Example 1;

[0035] Figure 12 This is the adsorption kinetic curve of iodine on the nitrogen-doped hierarchical porous biochar prepared in Example 1. DETAILED DESCRIPTION

[0036] The present invention provides a method for preparing nitrogen-doped hierarchical porous biochar, comprising the following steps:

[0037] 1) hydrothermally reacting coconut shell biomass powder and urea to obtain nitrogen-doped biomass powder;

[0038] 2) mixing the nitrogen-doped biomass powder and the activator and carbonizing the mixture to obtain nitrogen-doped hierarchical porous biochar;

[0039] The activator comprises potassium hydroxide and potassium nitrate.

[0040] In the present invention, the coconut shell biomass powder in step 1) is obtained by sequentially washing, drying, and crushing coconut shell biomass; the drying temperature is preferably 70-80° C., more preferably 75° C.; the particle size of the coconut shell biomass powder is preferably ≤0.15 mm, more preferably ≤0.1 mm.

[0041] In the present invention, the mass ratio of the coconut shell biomass powder to urea in step 1) is preferably 1:1-3, more preferably 1:1.5-2.5, and more preferably 1:2.

[0042] In the present invention, in the hydrothermal reaction in step 1), the mass volume ratio of coconut shell biomass powder to water is preferably 10 g:40-80 mL, more preferably 10 g:50-70 mL, and even more preferably 10 g:60 mL.

[0043] In the present invention, the temperature of the hydrothermal reaction in step 1) is preferably 180-220° C., more preferably 190-210° C., and more preferably 200° C.; the time of the hydrothermal reaction is preferably 1.5-3 h, more preferably 2-2.5 h.

[0044] In the present invention, after the hydrothermal reaction in step 1) is completed, it is preferably dried to obtain nitrogen-doped biomass powder; the drying temperature after the hydrothermal reaction is preferably 70 to 90° C., more preferably 80° C.; the drying time after the hydrothermal reaction is preferably 20 to 30 hours, more preferably 24 to 26 hours.

[0045] In the present invention, the mass molar ratio of the nitrogen-doped biomass powder and the activator in step 2) is preferably 15 g: 0.2-0.5 mol, and more preferably 15 g: 0.3-0.4 mol.

[0046] In the present invention, the molar ratio of potassium hydroxide to potassium nitrate is preferably 1:0.05-0.25, more preferably 1:0.1-0.2, and even more preferably 1:0.13-0.18.

[0047] In the present invention, the mixing in step 2) is carried out by ball milling, and the ball milling speed is preferably 200-580 r / min, more preferably 300-500 r / min; the ball milling time is preferably 5-15 min, more preferably 10 min; the ball-to-material ratio is preferably 3-1:1, more preferably 2.5:1.

[0048] In the present invention, the carbonization in step 2) is preferably carried out in a nitrogen atmosphere, and the carbonization temperature is preferably 600-1000°C, more preferably 700-900°C, and more preferably 800°C; the carbonization time is preferably 1.5-3h, more preferably 2-2.5h; the heating rate to the carbonization temperature is preferably 3-8°C / min, more preferably 4-7°C / min, and more preferably 5-6°C / min.

[0049] In the present invention, after the carbonization in step 2) is completed, washing is preferably performed to obtain nitrogen-doped graded porous biochar;

[0050] The reagent used for the washing is preferably water, and the washing is preferably stopped when the pH value of the washing liquid reaches 6.5 to 7.5, more preferably 7.

[0051] The present invention also provides nitrogen-doped hierarchical porous biochar prepared by the preparation method, wherein the specific surface area of ​​the nitrogen-doped hierarchical porous biochar is 2300-3300 m 2 / g, and the total pore volume of nitrogen-doped hierarchical porous biochar was 1.6-3.2 cm 3 / g, and the micropore volume of nitrogen-doped hierarchical porous biochar is 1-1.4 cm 3 / g.

[0052] The present invention also provides the use of the nitrogen-doped graded porous biochar in adsorbing gaseous iodine.

[0053] The technical solutions provided by the present invention are described in detail below with reference to the embodiments, but they should not be construed as limiting the scope of protection of the present invention.

[0054] Example 1

[0055] The coconut shell biomass was washed and dried at 75°C, then crushed in a crusher and sieved through a standard 0.15 mm sieve to obtain coconut shell biomass powder with a particle size of ≤0.15 mm. Coconut shell biomass powder and urea were mixed in a 1:1 mass ratio of coconut shell biomass powder to water (mass to volume ratio of 10 g coconut shell biomass powder to 70 mL). The mixture was then placed in a hydrothermal synthesis reactor and subjected to a hydrothermal reaction at 210°C for 2 hours. After the hydrothermal reaction, the mixture was naturally cooled to room temperature, and the hydrothermal reaction product was dried at 80°C for 24 hours to obtain nitrogen-doped biomass powder.

[0056] 15g of nitrogen-doped biomass powder, 0.34mol of potassium hydroxide, and 0.06mol of potassium nitrate were placed in a corundum ball mill. The mill was then placed in a planetary ball mill using corundum balls as the milling medium. The milling process was carried out for 10 minutes at a speed of 580r / min and a ball-to-batch ratio of 2.5:1. The milled product was placed in a corundum crucible and transferred to a tube furnace. Under a nitrogen atmosphere, the temperature was increased to 900°C at a heating rate of 5°C / min and carbonized at 900°C for 2 hours. After carbonization, the product was cooled to room temperature and transferred to a filtration unit. The product was then washed with deionized water until the pH of the washing solution reached 7. The washed product was dried at 80°C for 20 hours to obtain nitrogen-doped hierarchical porous biochar weighing 1.68g.

[0057] Example 2

[0058] The amount of potassium hydroxide and potassium nitrate in Example 1 were modified to 0.32 mol and 0.08 mol, respectively. Other conditions were the same as in Example 1.

[0059] Example 3

[0060] The amount of potassium hydroxide and potassium nitrate in Example 1 were modified to 0.38 mol and 0.02 mol, respectively. Other conditions were the same as in Example 1.

[0061] Example 4

[0062] The coconut shell biomass was washed and dried at 70°C, then crushed in a crusher and sieved through a standard 0.15 mm sieve to obtain coconut shell biomass powder with a particle size of ≤0.15 mm. Coconut shell biomass powder and urea were mixed in a 1:2 mass ratio of coconut shell biomass powder to water (mass to volume ratio of 10 g coconut shell biomass powder to water: 60 mL). The mixture was then placed in a hydrothermal synthesis reactor and subjected to a hydrothermal reaction at 180°C for 3 hours. After the hydrothermal reaction, the mixture was naturally cooled to room temperature, and the hydrothermal reaction product was dried at 70°C for 30 hours to obtain nitrogen-doped biomass powder.

[0063] 15 g of nitrogen-doped biomass powder, 0.4 mol of potassium hydroxide, and 0.1 mol of potassium nitrate were placed in a corundum ball mill. The mill was then placed in a planetary ball mill using corundum balls as the milling medium. The milling process was carried out for 10 minutes at a speed of 580 rpm and a ball-to-batch ratio of 2.5:1. The milled product was placed in a corundum crucible and transferred to a tube furnace. The furnace was heated to 600°C at a heating rate of 8°C / min under a nitrogen atmosphere and carbonized at 600°C for 3 hours. After carbonization, the product was cooled to room temperature and transferred to a filtration unit. The product was then washed with deionized water until the pH of the washing solution reached 7. The washed product was dried at 70°C for 30 hours to obtain nitrogen-doped hierarchical porous biochar.

[0064] Example 5

[0065] The coconut shell biomass was washed and dried at 80°C, then crushed in a crusher and sieved through a standard 0.15 mm sieve to obtain coconut shell biomass powder with a particle size of ≤0.15 mm. Coconut shell biomass powder and urea were mixed in a 1:3 mass ratio of coconut shell biomass powder to water (mass to volume ratio of 10 g coconut shell biomass powder to 50 mL). The mixture was then placed in a hydrothermal synthesis reactor and subjected to a hydrothermal reaction at 220°C for 1.5 hours. After the hydrothermal reaction, the mixture was naturally cooled to room temperature, and the hydrothermal reaction product was dried at 90°C for 20 hours to obtain nitrogen-doped biomass powder.

[0066] 15 g of nitrogen-doped biomass powder, 0.17 mol of potassium hydroxide, and 0.03 mol of potassium nitrate were placed in a corundum ball mill. The mill was then placed in a planetary ball mill using corundum balls as the milling medium. The milling process was carried out at a speed of 580 rpm and a ball-to-batch ratio of 2.5:1 for 10 minutes. The milled product was placed in a corundum crucible and transferred to a tube furnace. The furnace was heated to 1000°C at a heating rate of 3°C / min under a nitrogen atmosphere and carbonized at 1000°C for 3 hours. After carbonization, the product was cooled to room temperature and transferred to a filtration unit. The product was then washed with deionized water until the pH of the washing solution reached 7. The washed product was dried at 90°C for 24 hours to obtain nitrogen-doped hierarchical porous biochar.

[0067] Comparative Example 1

[0068] The potassium nitrate in Example 1 was omitted, and the amount of potassium hydroxide was modified to 0.4 mol. Other steps were the same as in Example 1.

[0069] The weight of the biochar prepared in this comparative example was 0.59 g.

[0070] Comparative Example 2

[0071] The potassium hydroxide and potassium nitrate in Example 1 are omitted, and the rest are the same as in Example 1.

[0072] Figure 1 This is a scanning electron microscope image of the biochar prepared in Comparative Example 2. Figure 2 This is a scanning electron microscope image of the biochar prepared in Comparative Example 1. Figure 3 This is a scanning electron microscope image of the nitrogen-doped hierarchical porous biochar prepared in Example 1. Figure 1 It can be seen that the biochar prepared only by hydrothermal reaction with urea exhibits irregular morphology and a rough surface. Figure 2 It can be seen that the biochar prepared by hydrothermal reaction with urea and activation with potassium hydroxide can be observed to have obvious pore structure, and the surface around the pores is relatively smooth. Figure 3 It can be seen that more porous structures can be observed in the porous biochar prepared by hydrothermal reaction with urea and co-activation with potassium hydroxide and potassium nitrate. This result shows that co-activation with potassium hydroxide and potassium nitrate is more conducive to the generation of rich pores than single potassium hydroxide activation.

[0073] Figure 4 This is the N2 isothermal adsorption-desorption curve of the biochar prepared in Comparative Example 2 at 77K. Figure 5 This is the N2 isothermal adsorption-desorption curve of the biochar prepared in Comparative Example 1 at 77K. Figure 6 This is the N2 isothermal adsorption-desorption curve of the nitrogen-doped hierarchical porous biochar prepared in Example 1 at 77K. Figure 4It can be seen that the biochar prepared in Comparative Example 2 has poor adsorption and desorption capacity for N2, showing an I-type isotherm model classified by IUPAC. This result shows that single nitrogen doping cannot produce biochar with rich pore structure. Figure 5 、 Figure 6 As can be seen, the biochars prepared in Comparative Example 1 and Example 1 both exhibit an IUPAC-classified IV isotherm model. N₂ adsorption increases linearly in the low-pressure region. In the medium-pressure region, N₂ adsorption slowly increases with increasing relative pressure, forming a hysteresis loop with N₂ desorption and N₂ adsorption. These results demonstrate that the biochars prepared in Comparative Example 1 and Example 1 possess a rich pore structure, including abundant mesopores.

[0074] Figure 7 This is the pore size distribution diagram of the biochar prepared in Comparative Example 2. Figure 8 This is the pore size distribution diagram of the biochar prepared in Comparative Example 1. Figure 9 This is the pore size distribution diagram of the nitrogen-doped hierarchical porous biochar prepared in Example 1. Figures 7-9 It can be seen that the pore richness of Comparative Example 2 is much lower than that of Comparative Example 1 and Example 1, and the specific surface area (S BET ) and total pore volume (V Total ) is low, and presents a pore structure dominated by micropores (pore diameter ≤ 2nm); the biochar prepared in Comparative Example 1 and Example 1 both present a pore structure dominated by micropores (pore diameter ≤ 2nm) and mesopores (pore diameter 2-10nm) in terms of pore distribution, but the nitrogen-doped hierarchical porous biochar prepared by potassium hydroxide and potassium nitrate co-activation in Example 1 shows a better pore structure than that of Comparative Example 1 (single potassium hydroxide activation). This result shows that with the addition of potassium hydroxide, the specific surface area (S BET ) and total pore volume (V Total ) were greatly improved (the specific surface area and total pore volume of Comparative Example 1 were 9.35 times and 13.27 times of those of Comparative Example 2, respectively); after a portion of potassium hydroxide was replaced by potassium nitrate, the specific surface area and total pore volume of the prepared biochar were further improved (the specific surface area and total pore volume of Example 1 were 1.4 times and 1.39 times of those of Comparative Example 1, respectively), and the mesopore ratio was also improved (the micropore volume (V Micro ): mesopore volume (V Total -V Micro ) is 1.44); although the overall amount of potassium element does not change, the pore structure of biochar can be effectively changed by replacing part of potassium hydroxide with potassium nitrate.

[0075] From the weight comparison of the biochar prepared in Example 1 and Comparative Example 1, it can be seen that the yield of nitrogen-doped graded porous biochar prepared by co-activation of potassium hydroxide and potassium nitrate in Example 1 is significantly higher than that of Comparative Example 1 which is activated only by potassium hydroxide. The yield of the porous biochar in Example 1 is 2.85 times that of Comparative Example 1.

[0076] The contents of C, H, N, and O elements in the biochars prepared in Example 1 and Comparative Examples 1-2 were analyzed using an elemental analyzer. The results are shown in Table 1.

[0077] Table 1 Elemental composition (atomic percentage) of biochar prepared in Example 1 and Comparative Examples 1-2

[0078] biochar C / ％ N / ％ H / ％ O / ％ Example 1 86.49 2.56 0.31 10.64 Comparative Example 1 80.78 0.37 0.17 18.68 Comparative Example 2 86.39 3.56 0.5 9.55

[0079] As shown in Table 1, the biochars prepared in Example 1 and Comparative Examples 1-2 all had high carbon contents. The biochar prepared in Comparative Example 2, without potassium activation, had the highest nitrogen content and the lowest oxygen content compared to Comparative Example 1 and Example 1. The biochar prepared in Comparative Example 1, activated only with potassium hydroxide, had the highest oxygen content and the lowest nitrogen content. These results indicate that potassium hydroxide activation introduces more oxygen and consumes more nitrogen. However, combined activation with potassium hydroxide and potassium nitrate did not significantly reduce nitrogen content, favoring nitrogen retention.

[0080] The adsorption performance of gaseous iodine by the biochar prepared in Example 1 and Comparative Examples 1-2 was tested respectively. The test conditions were as follows: elemental iodine was used as the gaseous iodine source, adsorbed at 75°C for 6 hours, and the mass of the biochar before and after adsorption was recorded. The mass of the biochar was calculated using the formula q e =(m2-m1 / m1)×1000 to calculate the adsorption capacity, where q e is the adsorption capacity (mg / g), m2 is the mass of biochar after adsorption (g), and m1 is the mass of biochar before adsorption (g). The adsorption capacity test results are shown in Table 2.

[0081] Table 2 Adsorption capacity of biochar prepared in Example 1 and Comparative Examples 1-2 for gaseous iodine

[0082] biochar Example 1 Comparative Example 1 Comparative Example 2 Adsorption capacity / mg / g 7221 4898 80

[0083] As shown in Table 2, the adsorption capacity of gaseous iodine in Example 1 is significantly higher than that in Comparative Examples 1 and 2. This result shows that the adsorption performance of the nitrogen-doped hierarchical porous biochar prepared by the present invention using co-activation of potassium hydroxide and potassium nitrate is far superior to that of the traditional alkaline activation method.

[0084] After the adsorption performance test of gaseous iodine was completed, the biochar adsorbed in Example 1 was left to stand at room temperature, and the mass of the biochar was recorded every 24 hours. The mass of the biochar was calculated using the formula A = (m2 - m 2+x) / (m2-m1)×100% to calculate the iodine desorption rate, where A is the iodine desorption rate (%), m2 is the mass of biochar after adsorption (g), m1 is the mass of biochar before adsorption (g), and m 2+x is the mass of biochar after standing for x days (g). The test results are shown in Table 3.

[0085] Table 3 Iodine desorption rate of nitrogen-doped hierarchical porous biochar prepared in Example 1

[0086] Standing time 1 day 2 days 3 days 4 days 5 days Iodine desorption rate / % 6.31 11.20 15.18 18.33 20.99

[0087] As shown in Table 3, the iodine desorption rate of the nitrogen-doped hierarchical porous biochar prepared in Example 1 was only 6.31% after standing at room temperature for one day, and 20.99% after standing for five days. This result demonstrates that the nitrogen-doped hierarchical porous biochar prepared in the present invention has good iodine retention capacity.

[0088] The porous biochar prepared in Example 1 was subjected to a cyclic adsorption stability test. Following the gaseous iodine adsorption test, the iodine-adsorbed porous biochar was dispersed in anhydrous ethanol and repeatedly washed until the solution became transparent. The solution was then dried in a forced air drying oven at 120°C for 12 hours. The adsorption test was then repeated for gaseous iodine. This constituted one adsorption cycle. This cycle was repeated four times, and the adsorption capacity for each cycle was recorded. The test results are shown in Table 4.

[0089] Table 4 Cyclic adsorption stability of nitrogen-doped hierarchical porous biochar prepared in Example 1

[0090] Adsorption times 1 time 2 times 3 times 4 times 5 times Adsorption capacity / mg / g 7221 6829 6573 6407 6384

[0091] As shown in Table 4, the nitrogen-doped hierarchical porous biochar prepared in Example 1 still had an iodine adsorption capacity greater than 6000 mg / g after four regeneration cycles. This result demonstrates that the nitrogen-doped hierarchical porous biochar prepared in the present invention has high cyclic adsorption stability.

[0092] The adsorption type of gaseous iodine by the biochar prepared in Example 1 and Comparative Examples 1-2 was analyzed, and the specific steps were as follows:

[0093] Place 0.05g of biochar and 1g of elemental iodine in two glass culture dishes respectively, and place the two glass culture dishes together in a 250mL sealed container. This is one set of experimental devices. Repeat to make 12 sets of experimental devices. Place the 12 sets of experimental devices in a forced air drying oven at a temperature of 75°C, start timing, and take out one set of experimental devices at 10min, 20min, 30min, 60min, 90min, 120min, 150min, 180min, 240min, 300min, 600min and 1200min respectively, take out the biochar in it, weigh it and calculate the adsorption amount. Through the pseudo-first-order kinetic model ln(q e -q t )=ln(q t )-k1t and pseudo-second-order kinetic model t / q=1 / k2q e 2 +t / q e , fitting the functional relationship between adsorption amount and time, where q e is the equilibrium adsorption capacity (mg / g), q t is the adsorption amount at different time t, k1 and k2 are the rate constants of pseudo-first-order kinetic model and pseudo-second-order kinetic model respectively. Table 5 shows the kinetic model of biochar prepared in Example 1 and Comparative Examples 1-2.

[0094] Table 5 Kinetic model of biochar prepared in Example 1 and Comparative Examples 1-2

[0095]

[0096] Figure 10 This is the adsorption kinetic curve of iodine by the biochar prepared in Comparative Example 2. Figure 11 This is the adsorption kinetic curve of iodine by the biochar prepared in Comparative Example 1. Figure 12 This is the adsorption kinetic curve of iodine by the nitrogen-doped hierarchical porous biochar prepared in Example 1. Figure 10 、 Figure 11 、 Figure 12 As can be seen from Table 5, the adsorption of iodine by the biochar prepared in Example 1 and Comparative Examples 1-2 is closer to the pseudo-first-order kinetic model.

[0097] The present invention uses a strategy of nitrogen doping assisted by potassium hydroxide and potassium nitrate co-activation to successfully prepare high-yield nitrogen-doped graded porous biochar. The biochar has an adsorption capacity of up to 7221 mg / g for gaseous iodine, reaching adsorption equilibrium within 4 hours. After adsorption, the biochar has a strong iodine retention capacity (the iodine desorption rate is only 6.31% after one day of standing and 20.99% after five days of standing) and good cyclic adsorption stability (the adsorption capacity remains above 6000 mg / g after five adsorption cycles). The method of the present invention overcomes the problems of low yield and insufficient nitrogen content of traditional alkaline activation methods, not only improving the biochar's iodine adsorption performance, but also addressing the high raw material costs and potential environmental pollution issues.

[0098] The above is only a preferred embodiment of the present invention. It should be pointed out that for ordinary technicians in this technical field, several improvements and modifications can be made without departing from the principles of the present invention. These improvements and modifications should also be regarded as within the scope of protection of the present invention.

Claims

1. A method for preparing nitrogen-doped hierarchical porous biochar, characterized in that: The following steps are included: 1) hydrothermally reacting coconut shell biomass powder and urea to obtain nitrogen-doped biomass powder; 2) mixing the nitrogen-doped biomass powder and the activator and carbonizing the mixture to obtain nitrogen-doped hierarchical porous biochar; The activator comprises potassium hydroxide and potassium nitrate.

2. The preparation method according to claim 1, characterized in that In step 1), the mass ratio of the coconut shell biomass powder to urea is 1:1-3.

3. The preparation method according to claim 1 or 2, characterized in that The temperature of the hydrothermal reaction in step 1) is 180-220° C., and the time of the hydrothermal reaction is 1.5-3 hours.

4. The preparation method according to claim 3, characterized in that Step 2) The mass molar ratio of the nitrogen-doped biomass powder to the activator is 15g:0.2-0.5mol.

5. The preparation method according to claim 4, characterized in that The molar ratio of potassium hydroxide to potassium nitrate is 1:0.05-0.

25.

6. The preparation method according to claim 4 or 5, characterized in that Step 2) The carbonization is carried out in a nitrogen atmosphere, the carbonization temperature is 600-1000° C., the carbonization time is 1.5-3 h, and the heating rate to the carbonization temperature is 3-8° C. / min.

7. The preparation method according to claim 6, characterized in that Step 2) washing after the carbonization is completed to obtain nitrogen-doped graded porous biochar; The washing process is stopped when the pH value of the washing solution reaches 6.5 to 7.

5.

8. The nitrogen-doped hierarchical porous biochar prepared by the preparation method according to any one of claims 1 to 7, characterized in that: The specific surface area of ​​the nitrogen-doped hierarchical porous biochar is 2300-3300 m 2 / g, and the total pore volume of nitrogen-doped hierarchical porous biochar was 1.6-3.2 cm 3 / g, and the micropore volume of nitrogen-doped hierarchical porous biochar is 1-1.4 cm 3 / g.

9. Use of the nitrogen-doped hierarchical porous biochar according to claim 8 in adsorbing gaseous iodine.

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