Method for preparing biochar by regulating biomass by utilizing freeze-thaw cycles, and use

Abstract

A method for preparing biochar by regulating biomass by utilizing freeze-thaw cycles, and a use. The method comprises the following steps: S1. crushing the biomass, then sieving and drying the crushed biomass, adding water, uniformly stirring, then soaking the mixture, performing freeze-thaw cycle treatment at -80°C to -10°C, and drying to obtain a precursor; and S2. putting the precursor into a container for pyrolysis at 300-700°C, and after the pyrolysis is complete, cooling the pyrolysis product to room temperature, and performing grinding and sieving to obtain the biochar. The simple freeze-thaw cycles are used to pretreat the biomass to regulate the structure of biochar pores. The freeze-thaw cycle pretreatment only requires water as a medium, has advantages of a simple process, a low cost, no pollution, easy mass production, etc., and is a potential method for precisely regulating micropores in activated carbon.

Description

A method and application of preparing biochar by regulating biomass through freeze-thaw cycles Technical Field

[0001] The present invention belongs to the technical field of biochar preparation, and specifically relates to a method and application of preparing biochar by regulating biomass through freeze-thaw cycles. Background Art

[0002] Biochar, with its well-developed pores and rich functional groups, has been widely used in soil remediation, carbon sequestration and emission reduction, and wastewater and exhaust gas treatment. Currently, biomass-based carbon materials, favored by scientists due to their broad raw material availability and stable physical and chemical properties, are highly valued. The pore structure of biochar determines its application, with micropores primarily responsible for adsorption, while mesopores and macropores play a role in transport and conduction. Therefore, regulating the pore structure of biochar has both practical and theoretical implications for the further utilization of porous biochar.

[0003] In the prior art, for example, application number 202010620030.8, a device and method for rapidly expanding the pores of biochar based on low-temperature freezing, uses a pressure device to control a pore-expanding agent to squeeze and expand the pores of the biochar, thereby increasing the pores of the biochar; application number 202010590461.4, a preparation method for precisely adjusting the microporous structure of biomass-based activated carbon and the resulting biomass-based activated carbon, controls the cellulose content and crystallinity of the biomass through cellulose-degrading microorganisms, and achieves a microporous structure of 0- Precise adjustment of 2nm; Su Deli, in the effect of freeze-thaw cycles on the physical and chemical properties and adsorption performance of biochar, pine sawdust was carbonized to prepare biochar, then ground, sieved, and labeled, and then the biochar was subjected to freeze-thaw cycle tests. It was found that freeze-thaw cycles can dissolve soluble minerals inside the biochar. The object of the freeze-thaw cycle is biochar, and the main effect of the freeze-thaw cycle should be the volume change of "water-ice" conversion, which in turn brings about physical effects; the ways to adjust the pore size also include enzyme reagents, microwave devices or chemical reagents, as well as the interaction between various factors.

[0004] However, the above-mentioned microbial agents, chemical reagents, or pressure devices, microwave devices, etc. are required to regulate the pore structure of biochar, which has complex processes and high preparation costs, and the use of chemical reagents is prone to pollution or by-products.

[0005] Summary of the Invention

[0006] The purpose of the present invention is to provide a method and application of preparing biochar by regulating biomass, in response to the problems of complex process, high preparation cost and easy pollution or by-products in the existing regulation of the pore structure of biochar. The present invention adopts a simple freeze-thaw cycle to pretreat biomass to regulate the pore structure of biochar. The freeze-thaw cycle pretreatment only requires water as a medium, has the advantages of simple process, low cost, no pollution, easy large-scale production, etc., and is a potential method for precise regulation of activated carbon micropores.

[0007] In order to achieve the above objectives, the technical solutions adopted in this application are:

[0008] The first object of the present invention is to provide a method for preparing biochar by regulating biomass through freeze-thaw cycles, comprising the following steps:

[0009] S1. The biomass is crushed, sieved and dried, added with water, stirred evenly and then soaked, and then subjected to freeze-thaw cycle treatment at -80 to -10°C and dried to obtain a precursor;

[0010] S2. The precursor is placed in a container and pyrolyzed at 300-700°C. After the pyrolysis is completed, the precursor is cooled to room temperature and ground and sieved to obtain biochar.

[0011] Preferably, in S1, the biomass is crop straw, which is crushed and then passed through an 18-mesh sieve.

[0012] Preferably, in S1, the mass ratio of the biomass to water is 1:8-15.

[0013] Preferably, in S1, the soaking time is 24 to 36 hours.

[0014] Preferably, in S1, the freeze-thaw cycle treatment is first frozen at -80 to -10°C for 15 to 24 hours, and then thawed at 20°C for 9 to 24 hours, and the number of freeze-thaw cycles is 1 to 15 times.

[0015] Preferably, the pyrolysis time is 2 to 4 hours, and the heating rate is 10 to 20° C. / min.

[0016] Preferably, in S2, the precursor is placed in a container, air is exhausted and the container is sealed, and the container is ground and then passed through an 80-mesh sieve.

[0017] The second object of the present invention is to provide the use of biochar prepared by the above method in removing ferrous ions in water.

[0018] Compared with the prior art, the present invention has the following beneficial effects:

[0019] (1) The present invention uses a simple freeze-thaw cycle to pretreat biomass to regulate the pore structure of biochar. The freeze-thaw cycle pretreatment only requires water as a medium to change the biomass structure and then regulate the pore structure of biochar. No additional equipment is required. It has the advantages of simple process, low cost, no pollution, and easy large-scale production.

[0020] (2) The present invention uses freeze-thaw cycle pretreatment to regulate the pore structure of biomass, mainly relying on the "water-ice" conversion process during the freeze-thaw process. Unlike general properties, "heat shrinkage and cold expansion" is a characteristic of water. After freezing, the cells and cell walls of the biomass are affected by the freezing effect and produce a "cold expansion" effect. After multiple freeze-thaw cycles, the "cold expansion" effect is strengthened, which has a pore expansion effect on the biomass to a certain extent. At the same time, it may also cause some water-soluble sugar products to dissolve and consume. Therefore, the freeze-thaw cycle mainly has the effect of pore expansion. BRIEF DESCRIPTION OF THE DRAWINGS

[0021] FIG1 shows the effect of freeze-thaw times on the pH of biomass according to an embodiment of the present invention;

[0022] FIG2 shows the pH change of biochar prepared in an embodiment of the present invention;

[0023] FIG3 shows the yield change of biochar prepared in an embodiment of the present invention;

[0024] FIG4 is a scanning electron microscope image of the biochar of corn straw of the present invention, comparative example 1, soaked corn straw, comparative example 2, example 1 and example 2 at 2000 times magnification;

[0025] FIG5 is a Fourier transform infrared spectra of corn straw and biochar before and after pretreatment in Example 1, Example 2, and Comparative Example 1 of the present invention;

[0026] FIG6 is an X-ray diffraction pattern of biochar prepared in Example 1-2 of the present invention and Comparative Example 1-2;

[0027] FIG7 shows the crystallinity of biochar prepared in Examples 1-2 of the present invention and Comparative Examples 1-2;

[0028] FIG8 shows the adsorption amount of ferrous ions by biochar according to an embodiment of the present invention. DETAILED DESCRIPTION

[0029] The following will clearly and completely describe the technical solutions in the embodiments of the present invention in conjunction with the data and drawings in the embodiments of the present invention. Obviously, the embodiments described are only part of the embodiments of the present invention, not all of the embodiments. Based on the embodiments of the present invention, all other embodiments obtained by ordinary technicians in this field without making creative efforts are within the scope of protection of the present invention.

[0030] It should be noted that the professional terms used in the present invention are only for the purpose of describing specific embodiments and are not intended to limit the scope of protection of the present invention. Unless otherwise specified, the various raw materials, reagents, instruments and equipment used in the following embodiments of the present invention can be purchased from the market or prepared by existing methods.

[0031] Example 1

[0032] A method for preparing biochar by regulating biomass through freeze-thaw cycles comprises the following steps:

[0033] S1. The biomass corn stalks were crushed and screened through an 18-mesh screen using a sieve. The sieved material was the obtained biomass powder. The biomass powder was mixed with deionized water at a mass ratio of 1:8, stirred evenly, and soaked for 24 hours. The mixture was placed in a freezer at a temperature of -10°C for 15 hours, and then thawed at 20°C for 9 hours. This freeze-thaw cycle was repeated 15 times. After the freeze-thaw cycle treatment, the precursor was dried to obtain the precursor.

[0034] S2. Put the precursor into a crucible, fill the crucible to exclude air, wrap it with tin foil and seal it, put it into a muffle furnace and pyrolyze it at 500℃ for 2h. The heating rate of the muffle furnace is 10℃ / min. After the pyrolysis is completed, cool it to room temperature, take out the pyrolytic carbon, put it in a mortar, and grind it through an 80-mesh sieve to obtain biochar.

[0035] Example 2

[0036] A method for preparing biochar by regulating biomass through freeze-thaw cycles comprises the following steps:

[0037] S1. The biomass corn stalks were crushed and screened through an 18-mesh screen using a sieve. The sieved material was the obtained biomass powder. The biomass powder was mixed with deionized water at a mass ratio of 1:8, stirred evenly, and soaked for 24 hours. The mixture was placed in a freezer at a temperature of -80°C for 15 hours, and then thawed at 20°C for 9 hours. This freeze-thaw cycle was repeated 15 times. After the freeze-thaw cycle treatment, the precursor was dried to obtain the precursor.

[0038] S2. Put the precursor into a crucible, fill the crucible to exclude air, wrap it with tin foil and seal it, put it into a muffle furnace and pyrolyze it at 500℃ for 2h. The heating rate of the muffle furnace is 10℃ / min. After the pyrolysis is completed, cool it to room temperature, take out the pyrolytic carbon, put it in a mortar, and grind it through an 80-mesh sieve to obtain biochar.

[0039] Example 3

[0040] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 1.

[0041] Example 4

[0042] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 2.

[0043] Example 5

[0044] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 3.

[0045] Example 6

[0046] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 4.

[0047] Example 7

[0048] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 5.

[0049] Example 8

[0050] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 6.

[0051] Example 9

[0052] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 7.

[0053] Example 10

[0054] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 8.

[0055] Example 11

[0056] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 9.

[0057] Example 12

[0058] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 10.

[0059] Example 13

[0060] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 11.

[0061] Example 14

[0062] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 12.

[0063] Example 15

[0064] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 13.

[0065] Example 16

[0066] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 1, except that the number of freeze-thaw cycles in S1 is 14.

[0067] Example 17

[0068] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 1.

[0069] Example 18

[0070] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 2.

[0071] Example 19

[0072] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 3.

[0073] Example 20

[0074] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 4.

[0075] Example 21

[0076] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 5.

[0077] Example 22

[0078] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 6.

[0079] Example 23

[0080] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 7.

[0081] Example 24

[0082] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 8.

[0083] Example 25

[0084] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 9.

[0085] Example 26

[0086] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 10.

[0087] Example 27

[0088] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 11.

[0089] Example 28

[0090] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 12.

[0091] Example 29

[0092] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 13.

[0093] Example 30

[0094] A method for preparing biochar by regulating biomass through freeze-thaw cycles is basically the same as Example 2, except that the number of freeze-thaw cycles in S1 is 14.

[0095] Comparative Example 1

[0096] A biochar method comprising the following steps:

[0097] The biomass corn stalks were crushed and passed through an 18-mesh sieve using a screening machine. The sieved material was the obtained biomass powder. The biomass powder was placed in a crucible, filled to exclude air, and sealed with tin foil. The crucible was placed in a muffle furnace for pyrolysis at 500°C for 2 hours with a muffle furnace heating rate of 10°C / min. After the pyrolysis was completed, it was cooled to room temperature, the pyrolytic charcoal was taken out and placed in a mortar, and ground through an 80-mesh sieve to obtain biochar.

[0098] Comparative Example 2

[0099] A biochar method comprising the following steps:

[0100] S1. Grind the biomass corn stalks and pass them through an 18-mesh sieve using a sieving machine. The sieved material is the obtained biomass powder. The biomass powder is mixed with deionized water at a mass ratio of 1:8, stirred evenly, and soaked for 24 hours, and then dried to obtain a precursor;

[0101] S2. Put the precursor into a crucible, fill the crucible to exclude air, wrap it with tin foil and seal it, put it into a muffle furnace and pyrolyze it at 500℃ for 2h. The heating rate of the muffle furnace is 10℃ / min. After the pyrolysis is completed, cool it to room temperature, take out the pyrolytic carbon, put it in a mortar, and grind it through an 80-mesh sieve to obtain biochar.

[0102] The pH of the biomass after freeze-thaw cycles prepared in the above example was measured: 1 g of the biomass sample after freeze-thaw cycles in the above example was weighed using an electronic scale and placed in a 100 ml conical flask. 20 ml of deionized water was added at a mass-to-volume ratio of 1:20, and the flask was placed in a constant temperature shaking box. The flask was shaken for 2 hours at 25±1°C and a speed of 150 r / min. After the flask was taken out and allowed to stand for 30 minutes, the pH of the solution was measured using a pH meter after filtration. The test was repeated 3 times and the average value was taken.

[0103] Figure 1 shows the effect of freeze-thaw cycles on the pH of biomass in an example of the present invention. As shown in Figure 1, the pH of the corn straw solution gradually decreases with increasing freeze-thaw cycles. Furthermore, the pH of the corn straw pretreated with repeated freeze-thaw cycles at -80°C was even lower, indicating that the freeze-thaw cycle disrupts the internal fiber structure of the corn straw, leading to acidification.

[0104] The pH value of the biochar prepared in the above embodiment was measured as follows: 1 g of the biochar sample prepared in the above embodiment was weighed into a 100 ml conical flask using an electronic scale, 20 ml of deionized water was added at a mass-to-volume ratio of 1:20, and the flask was placed in a constant temperature shaking box. The flask was shaken for 2 h at 25±1°C and a rotation speed of 150 r / min, and then taken out and allowed to stand for 30 min. After filtering, the pH value of the solution was measured using a pH meter. The test was repeated 3 times and the average value was taken.

[0105] Figure 2 shows the pH change of the biochar prepared in the embodiment of the present invention. As shown in Figure 2, the biochar prepared in the embodiment is alkaline, and the pH value is stable at about 10, indicating that the temperature and number of repeated freeze-thaw cycles have no significant effect on the pH value of the pyrolyzed biochar. It can be seen that the pH value of biochar is related to the raw materials and the pyrolysis temperature conditions during preparation. The pH value of biochar is easily affected by acidic or alkaline dissolved substances, and its pH value represents the ability of biochar to provide or accept protons in solution. The reason why the biochar prepared by the present invention is alkaline is mainly due to the presence of alkaline functional groups on its surface and the concentration of mineral elements in the biochar.

[0106] Figure 3 shows the yield of biochar produced in an example of the present invention. As shown in Figure 3, corn straw was pyrolyzed at 500°C for 2 hours. During the pyrolysis process, the lignocellulose in the corn straw undergoes hydrolysis and other reactions to produce aromatic compounds. The biochar yields before and after pretreatment were relatively stable. The yield of biochar from pretreated corn straw was slightly lower than that from untreated corn straw. This is because the cellulose, hemicellulose, and lignin structures in the corn straw are destroyed by repeated freeze-thaw pretreatment, fully exposing the various types of lignocellulose, which is more conducive to carbonization during pyrolysis. This should increase the biochar yield, but in reality, the biochar yield actually decreases. This is because the destruction of the lignocellulose structure also destroys some lignin and separates with the liquid. Since biochar yield is primarily related to lignin, the pyrolysis biochar yield decreases. This also confirms that repeated freeze-thaw cycles damage the lignocellulose structure, indicating that repeated freeze-thaw pretreatment of pyrolysis biochar has performance potential.

[0107] SEM analysis was performed using a ZEISS MERLIN Compact instrument: biochar samples were affixed to the sample stage with conductive adhesive and observed at magnifications between 1000 and 5000. Because biochar has poor electrical conductivity, gold spraying was used to enhance its conductivity and improve imaging. Scanning electron micrographs of straw biochar prepared under different pretreatment conditions were used to observe the formation and changes in the biochar's pore structure and explore its physical properties.

[0108] Figure 4 is a scanning electron microscope image of the biochar of corn straw, comparative example 1, soaked corn straw, comparative example 2, embodiment 1 and embodiment 2 of the present invention at a magnification of 2000 times, wherein a is corn straw, b is comparative example 1, c is soaked corn straw, d is comparative example 2, e is embodiment 1, and f is embodiment 2. As can be seen from Figure 4 a and c, corn straw itself has a pore structure, but the porosity is very low. After 24 hours of soaking, the impurities on the surface of the corn straw are significantly reduced, and the pore structure is more obvious. This is because the impurities on the surface and in the pores of the corn straw are washed away with deionized water, which increases the pore capacity of the corn straw and makes the structure clearer. It can be concluded from the scanning electron microscope images of the biochar in Figure 4b and f that compared with corn straw, the biochar has a more obvious pore structure, a tubular structure appears, and the pore wall surface is relatively smooth. The biochar prepared under different treatment conditions The pore structure, number and size of the biochar are all different. The untreated corn straw biochar (Comparative Example 1) has the thickest pore wall, fewer pores, and contains impurities, which is not conducive to the adsorption effect of biochar. The treated corn straw biochar (Comparative Example 2) has a clearer structure and is not interfered with by impurity particles. As can be seen from Figure 4 e and f, the pore structure formed by the freeze-thaw pretreated corn straw biochar is more dense and regular, with a honeycomb structure and clear pore boundaries. These image features confirm that repeated freeze-thaw pretreatment has an impact on the structure of biochar. High-temperature pyrolysis increases the ash content in biochar, reduces volatile substances, opens the pore channels inside the biomass, and forms a porous structure with a high specific surface area. High-temperature pyrolysis transforms the aliphatic carbon phase of cellulose and hemicellulose in biomass into aromatic carbon monomers with higher thermal stability, increases the content of fixed carbon, and makes biochar have a more stable aromatic structure. It also provides active sites for the adsorption performance of biochar, which is better applied in the field of pollutant adsorption. However, the internal structure of corn straw biochar that was repeatedly frozen and thawed at -80℃ for 15 times collapsed significantly. Based on the destructive effect of repeated freezing and thawing on plant cell walls, it was found that when the freezing and thawing temperature is too low and the number of repeated freezing and thawing is too many, the damage to the lignocellulose is too great when a certain number of times is reached. Not only does it break up the intertwined structure of the lignocellulose, but it also damages structures such as lignin, which reduces its charcoal yield and causes the biochar structure to break and collapse. In comparison, corn straw biochar that was repeatedly frozen and thawed at -10℃ for 15 times also showed structural collapse, but it was not as obvious as the straw biochar treated at -80℃. This shows that in the case of 15 repeated freezing and thawing, the freezing and thawing temperature of -10℃ is more advantageous in terms of the structure of biochar.

[0109] Fourier transform infrared spectroscopy was performed on the corn stover and biochar prepared in Example 1, Example 2, and Comparative Example 1 before and after pretreatment to analyze their surface functional groups. Figure 5 shows Fourier transform infrared spectra of the corn stover and biochar prepared in Example 1, Example 2, and Comparative Example 1 before and after pretreatment. In Figure 5 , a shows the corn stover prepared in Example 1, Example 2, and Comparative Example 1 before and after pretreatment, and b shows the biochar prepared in Example 1, Example 2, and Comparative Example 1.

[0110] As shown in Figure 5a, the spectra of corn straw before and after pretreatment are not much different, and the spectral range is also the same as that of straw biochar, both in the range of 500-4000 cm -1 . At about 3860-3837cm -1 A large peak appears in the spectral band around 2950-3308 cm, which corresponds to the vibration peak of -OH. It is caused by the stretching displacement vibration of the hydroxyl group (-OH) unique to water, phenol and alcohol. The characteristic peak of the -CH structure unique to alkanes appears at 2950-3308 cm -1 The -CH structure in this band is not obvious in the infrared spectra of corn straw and straw biochar, and exists in a small fluctuation form; -1 The obvious absorption peaks appearing in the spectral bands around are caused by the stretching vibration of C=C and C=O; at 1543cm -1 The characteristic peaks formed at the spectral band are caused by the stretching vibrations of C=C and C=N in the aromatic structure;

[0111] As shown in Figure 5b, the infrared spectra of corn straw before and after pretreatment changed significantly after being pyrolyzed into biochar. -1 The -OH vibration peak on the left and right spectral bands has a significant shift and is transferred to 3632-3650cm -1 In the band around 2950-3308cm -1 The CH structure with no obvious fluctuation in the spectral band at 3308 cm -1 Unsaturated CH bond stretching vibration absorption occurs in the band around 1650cm -1 There is a sharp characteristic peak at the wavelength of 1543cm, which can clearly confirm that the peak is caused by the stretching of the C=O bond. At the same time, there is also some -COO in this band. -1 The characteristic peak of C=C in the spectral band of -1 The band, and this band also has saturated CH; at 1149cm -1 The spectral bands of CO and -CH3 structural bond stretching vibrations exist. Before and after pyrolysis, the wavelengths of 861-852 cm-1 The spectral band of SiO2 is the characteristic peak. The samples before and after pyrolysis contain some mineral elements. -1 A vibration peak of -CH2 is produced in the spectral band, but the fluctuation is not obvious.

[0112] After the corn straw was pyrolyzed to generate biochar, the stretching vibrations of CO, -COO, -CH3 and -CH2 structural bonds were newly added, proving that the pyrolysis process produced the polymerization and deformation of the chemical structure, the increase of oxygen-containing functional groups in the biochar, and the formation of aromatic structures. The contraction of the C=O structure, the bias of the unsaturated CH bond and the weakening of the vibration peak of -OH all indicate the changes of lignocellulose, the reduction of cellulose and hemicellulose content, the increase of aromatic chemical structure in the biochar, and the better stability of the biochar. -1 The C=C characteristic peak of the wavelength band increased compared to the untreated straw biochar, demonstrating that the pretreated biochar has a higher fixed carbon content and a more stable carbon structure. As the temperature rises, the chemical structure of the various functional groups in the biomass undergoes changes such as bending vibration, deformation, and polymerization, causing the lignocellulose components of the biomass to decompose and reform, ultimately producing biochar with a more stable chemical structure. The structure of biochar that has undergone repeated freeze-thaw pretreatment is even more stable.

[0113] The biochar was subjected to X-ray diffraction analysis using a Rigaku Ultima IV instrument. The sample was irradiated with X-rays, and the intensity and diffraction angle of the reflected rays were collected and recorded to obtain a diffraction pattern. The crystal structure and lattice parameters of the biochar were determined by analyzing the pattern. The test parameters selected the Cu target ray, with a wavelength of 1.5418, an operating voltage of 40kV, an operating current of 40mA, a diffraction scanning range of 5°-90°, and a scanning speed of 5° / min. Figure 6 is the X-ray diffraction pattern of the biochar prepared in Example 1-2 of the present invention and Comparative Example 1-2. As shown in Figure 6, the XRD pattern analysis of the straw biochar material found that the broad and slow diffuse diffraction peak appearing at about 7° was caused by the amorphous phase or microcrystalline phase structure in the material. The diffraction peaks at about 7° correspond to the manifestation of the amorphous phase and the manifestation of some amorphous or microcrystalline ordered regions in the straw biochar material, respectively, which mainly correspond to the incompletely decomposed wood cellulose in the biochar. Among them, the peak appearing at around 7° mainly comes from the scattering caused by the ordered structure of the amorphous state, while the peak appearing at around 23° comes from some ordered areas of the amorphous or microcrystalline state, which may contain some crystalline particles. The peak at around 23° is related to its ordered crystal structure with a graphite structure, which is usually due to the fact that the disordered carbon structure in the original biomass is transformed into an ordered graphite crystal structure through a high-temperature carbonization reaction during the carbonization process. A peak of graphite structure was also found at around 43°. In addition, the preparation conditions and carbonization temperature of different samples may also lead to differences in their structures, thereby affecting the peak height intensity, lattice size, order degree and diffraction angle of the diffraction peak. Among the four materials, after the raw materials were soaked and repeatedly frozen and thawed at -10°C for 15 times (Example 1), the amorphous phase peak and the amorphous or crystalline phase peak became stronger, which may indicate that the amorphous part in the material increased. Since the amorphous phase contains more disordered structures, when the amorphous part increases, the number of lattices in the entire sample decreases, and the distance between each crystal will also become smaller, thereby increasing the peak intensity. After 15 cycles of freezing and thawing at -80°C (Example 2), the peak intensities of the amorphous phase and the microcrystalline phase were relatively weakened compared to those at -10°C, indicating that the ordered structures of the amorphous and crystalline phases of the biochar material were severely damaged by the 15 cycles of freezing and thawing at -80°C.

[0114] In addition, there is a small amount of silicon in the straw biochar material. In Figure 6, 28.4°, 47.3° and 56.1° correspond to the (111), (220) and (311) planes of silicon, respectively. During the repeated ultra-low temperature freeze-thaw process at -80°C (Example 2), the weak characteristic peak of silicon almost disappeared, which may indicate that the silicon content in the material or its crystallinity has changed. This change may be due to the repeated freeze-thaw action that destroys the crystal structure in the material. In addition, the infiltration of water during the freeze-thaw process may also cause changes in the chemical properties of silicon. As can be seen from Figure 6, the crystallinity of biochar (Comparative Example 1) is not high. This is because the porous structure and multi-fold characteristics of biochar reduce the graphitization and crystallization of biochar, and reduce the crystallinity of biochar.

[0115] Figure 7 shows the crystallinity of the biochar prepared in Examples 1-2 and Comparative Examples 1-2 of the present invention. Calculation of the biochar crystallinity indicates that soaking and freeze-thaw cycles significantly increase the crystallinity of the biochar, indicating that the graphitic and crystalline structures of the pretreated biochar increase. However, the crystallinity of the straw biochar subjected to 15 freeze-thaw cycles at -80°C is the lowest among the pretreated biochars (compared to Example 1 and Comparative Example 1). This is because the multiple freeze-thaw cycles at ultra-low temperatures severely damage the corn straw structure, causing structural collapse and significantly damaging the biochar structure, which in turn reduces the crystallinity of the biochar.

[0116] The specific surface area and pore size of the biochar prepared in Examples 1-2 and Comparative Example 1 were measured, as shown in Table 1.

[0117] Table 1 BET data of biochar prepared in Examples 1-2 and Comparative Example 1

[0118] As shown in Table 1, the specific surface area of ​​the straw biochar (Example 1) pretreated with 15 freeze-thaw cycles at -10°C is 16.1733 m 2 ·g -1 , which is 2.6731m2 higher than the specific surface area of ​​untreated corn straw biochar (Comparative Example 1). 2 ·g -1 The specific surface area of ​​the straw biochar (Example 2) that was pre-treated at -80°C and subjected to 15 freeze-thaw cycles was only 3.8395 m 2 ·g -1, which is about 1.5 times that of untreated corn straw biochar; by comparison, it was found that the pore diameter of the -80°C freeze-thaw cycle 15 times (Example 2) was 10.9697nm, while the pore diameter of the -10°C freeze-thaw cycle 15 times (Example 1) was 6.8162nm, and the average pore diameter of the straw biochar that was not pretreated by freeze-thaw cycle (Comparative Example 1) was 1.843nm, indicating that the freeze-thaw cycle has achieved the pore expansion of the biochar, and the lower the freezing temperature, the larger the average pore diameter.

[0119] The effect of the biochar prepared in the comparative example on the biochar adsorption of ferrous ions was evaluated as follows: 100 ml of a 50 mg / L ferrous sulfate solution was added to a 250 ml conical flask, and 0.1 g of the biochar sample prepared in the example was added at a ratio of 1 g / L. The mixture was shaken at a temperature of 25 ± 1 ° C and a rotation speed of 150 r / min for 24 hours. The sample was removed, allowed to stand for 5 minutes, and filtered through a 0.45 μm filter membrane. The treated sample was injected into a graphite tube via an automatic sampler, electrothermally atomized, and the characteristic absorption spectrum at a wavelength of 248.3 nm was measured for absorbance. The element content was then calculated based on the measured standard curve, and the effect on the ferrous ion adsorption capacity was finally compared.

[0120] Figure 8 shows the adsorption capacity of ferrous ions by biochar according to an embodiment of the present invention. As shown in Figure 8, under constant adsorption conditions, the removal rate of ferrous ions also shows an increasing trend with the increase in the number of freeze-thaw cycles. Under -10°C freeze-thaw conditions, there is a more obvious growth phenomenon after 1-5 repetitions, followed by a more obvious small fluctuation after 10-12 repetitions, but the overall trend is an upward trend, indicating that repeated freeze-thaw pretreatment has a certain effect on the adsorption performance of biochar. Under -10°C freeze-thaw conditions, 1-5 freeze-thaw treatments have the greatest impact on the biochar structure, and 6-15 freeze-thaw treatments have a relatively stable effect on the biochar properties, indicating that after 5 repeated freeze-thaw cycles at -10°C, the pretreatment method will not have a significant effect on the biochar structure, but will increase the risk of biochar structural instability. Under -80°C freeze-thaw conditions, the adsorption capacity of ferrous ions begins to decrease after 4 repeated freeze-thaw cycles, and a fault-like decline occurs. After 6 repeated freeze-thaw cycles, the biochar performance is relatively stable. The reason for the decrease in the adsorption capacity of biochar for ferrous ions is speculated to be that multiple ultra-low temperature freeze-thaw treatments have severely damaged the internal structure of the biochar, resulting in structural collapse, which has led to a reduction in the binding sites for biochar to adsorb metal ions, affecting its adsorption effect.

[0121] Further iron ion adsorption experiments showed that the maximum adsorption capacity of biochar obtained by pyrolyzing biomass at a freezing temperature of -10°C and 15 freeze-thaw cycles was 191.28 mg / g, while the maximum adsorption capacity of biochar obtained by pyrolyzing biomass at a freezing temperature of -80°C and 3 freeze-thaw cycles was 179.07 mg / g.

[0122] In summary, the present invention performs freeze-thaw cycle pretreatment on biomass, and biomass has a cellular structure, is hydrophilic, water-soluble, and has a non-rigid structure. Water molecules can enter the interior of the cells, destroy the structure of the cell wall, and may also break the interwoven structure of macromolecules such as cellulose, hemicellulose, and lignin. The present invention investigates the effect of freeze-thaw cycle pretreatment on biomass (freezing temperature is -10 and -80°C, thawing temperature is 20°C, and freeze-thaw cycles are 15 times), and then uses biomass with different freeze-thaw cycles as raw material to carbonize and prepare biochar, change the biomass structure, and then regulate the pore structure of the biochar. No additional equipment is required, and the process has the advantages of simple process, low cost, pollution-free, and easy large-scale production. The prepared biochar has a denser and regular pore structure, a honeycomb structure, and clear pore boundaries. High-temperature pyrolysis increases the ash content in the biochar and reduces volatile substances, opens the pore channels inside the biomass, forms a porous structure with a high specific surface area, increases the content of fixed carbon, and makes the biochar have a more stable aromatic structure. It also provides active sites for the adsorption performance of the biochar, and is better applied in the field of pollutant adsorption.

[0123] It should be noted that when numerical ranges are mentioned in the present invention, it should be understood that both endpoints of each numerical range and any value between the two endpoints may be selected. Since the steps and methods used are the same as those in the embodiments, in order to avoid redundancy, the present invention describes preferred embodiments. Although preferred embodiments of the present invention have been described, those skilled in the art may make additional changes and modifications to these embodiments once they understand the basic inventive concepts. Therefore, the appended claims are intended to be interpreted as including the preferred embodiments and all changes and modifications that fall within the scope of the present invention.

[0124] Obviously, those skilled in the art may make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if such changes and modifications fall within the scope of the claims and their equivalents, the present invention is intended to include such changes and modifications.

Claims

1. A method for preparing biochar by regulating biomass through freeze-thaw cycles, characterized in that: The following steps are involved: S1, crushing the biomass, sieving and drying, adding water, stirring evenly and soaking, and then subjecting the mixture to a freeze-thaw cycle at -80 to -10°C, and drying to obtain a precursor; S2. Place the precursor in a container and perform pyrolysis at 300-700°C. After the pyrolysis is completed, cool it to room temperature and grind and sieve it to obtain biochar.

2. The method for preparing biochar by regulating biomass through freeze-thaw cycles according to claim 1, characterized in that: In S1, the biomass is crop straw, which is crushed and passed through an 18-mesh sieve.

3. The method for preparing biochar by regulating biomass through freeze-thaw cycles according to claim 1, characterized in that: In S1, the mass ratio of the biomass to water is 1:8-15.

4. The method for preparing biochar by regulating biomass through freeze-thaw cycles according to claim 1, characterized in that: In S1, the soaking time is 24 to 36 hours.

5. The method for preparing biochar by regulating biomass through freeze-thaw cycles according to claim 1, characterized in that: In S1, the freeze-thaw cycle treatment is firstly frozen at -80 to -10°C for 15 to 24 hours, and then thawed at 20°C for 9 to 24 hours, and the number of freeze-thaw cycles is 1 to 15 times.

6. The method for preparing biochar by regulating biomass through freeze-thaw cycles according to claim 1, characterized in that: The pyrolysis time is 2 to 4 hours, and the heating rate is 10 to 20° C. / min.

7. The method for preparing biochar by regulating biomass through freeze-thaw cycles according to claim 1, characterized in that: In S2, the precursor is placed in a container, air is exhausted and the container is sealed; and the precursor is ground and passed through an 80-mesh sieve.

8. Biochar prepared by the method according to any one of claims 1 to 7.

9. Use of the biochar according to claim 8 in removing ferrous ions in water.

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