WO2025091190A1 - Method for preparing biochar by regulating biomass by utilizing freeze-thaw cycles, and use - Google Patents

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

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

Biochar has well-developed pores and rich functional groups, and has been widely used in soil remediation, carbon fixation and emission reduction, wastewater and waste gas treatment, etc. At present, biomass-based carbon materials are favored by scientists because of their wide source of raw materials and stable physical and chemical properties. The pore structure determines the use of biochar, among which the microporous structure mainly plays an adsorption role, while the mesopores and macropores play a role in transmission and conduction. Therefore, regulating the pore structure of biochar has practical and theoretical significance for the further utilization of biomass-based porous carbon.

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 expander to extrude and expand the pores of biochar, thereby increasing the pores of biochar; application number 202010590461.4, a preparation method for accurately adjusting the microporous structure of biomass-based activated carbon and the prepared biomass-based activated carbon, controls the cellulose content and crystallinity of biomass through cellulose-degrading microorganisms, and realizes a 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.

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.

Summary of the invention

The purpose of the present invention is to provide a method and application of preparing biochar by regulating biomass, in view of 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 pretreatment of biomass to regulate the pore structure of biochar. The freeze-thaw cycle pretreatment only requires water as a medium. It 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.

In order to achieve the above purpose, the technical solution adopted in this application is:

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:

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.

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

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

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

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.

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

Preferably, 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.

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.

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

(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, and the process is simple, low cost, pollution-free, and easy to mass produce.

(2) The present invention uses freeze-thaw cycle pretreatment to regulate the pore structure of biomass, which mainly relies on the "water-ice" conversion process during the freeze-thaw process. Different from the general characteristics, "thermal contraction and cold expansion" is a characteristic of water. After freezing, the cells and cell walls of the biomass are subjected to 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

FIG1 is a graph showing the effect of freeze-thaw times on the pH of biomass according to an embodiment of the present invention;

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

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

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 magnified 2000 times;

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

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

FIG7 shows the crystallinity of biochar prepared in Example 1-2 of the present invention and Comparative Example 1-2;

FIG8 shows the adsorption amount of ferrous ions by biochar according to an embodiment of the present invention.

DETAILED DESCRIPTION

The following will be combined with the data and drawings in the embodiments of the present invention to clearly and completely describe the technical solutions in the embodiments of the present invention. Obviously, the described embodiments 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 creative work are within the scope of protection of the present invention.

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 specifically stated, 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.

Example 1

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

S1. The biomass corn stalks are crushed and sieved through an 18-mesh 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; frozen at a freezing temperature of -10°C for 15 hours, and then thawed at 20°C for 9 hours, and freeze-thawed for 15 times. After freeze-thaw cycle treatment, the precursor is dried;

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

Example 2

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

S1. The biomass corn stalks are crushed and sieved through an 18-mesh 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; frozen at a freezing temperature of -80°C for 15 hours, and then thawed at 20°C for 9 hours, and freeze-thawed for 15 times. After freeze-thaw cycle treatment, the precursor is dried;

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

Example 3

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.

Example 4

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.

Example 5

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.

Example 6

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.

Example 7

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.

Example 8

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.

Example 9

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.

Example 10

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.

Embodiment 11

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.

Example 12

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.

Example 13

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.

Embodiment 14

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.

Embodiment 15

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.

Example 16

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.

Embodiment 17

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.

Embodiment 18

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.

Embodiment 19

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.

Embodiment 20

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.

Embodiment 21

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.

Embodiment 22

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.

Embodiment 23

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.

Embodiment 24

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.

Embodiment 25

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.

Embodiment 26

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.

Embodiment 27

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.

Embodiment 28

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.

Embodiment 29

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.

Embodiment 30

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.

Comparative Example 1

A biochar method comprising the following steps:

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 put into a crucible, which was filled to exclude air. The crucible was sealed with tin foil and placed in a muffle furnace for pyrolysis at 500°C for 2h. The heating rate of the muffle furnace was 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.

Comparative Example 2

A biochar method comprising the following steps:

S1. The biomass corn stalks are crushed and passed 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;

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

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

FIG1 shows the pH change of biomass due to the number of freeze-thaw cycles in an embodiment of the present invention. As shown in FIG1 , as the number of freeze-thaw cycles increases, the pH value of the corn stalk solution gradually decreases. Moreover, the pH value of the stalk pretreated with repeated freeze-thaw cycles at -80°C is even lower, indicating that the freeze-thaw treatment breaks the internal fiber structure of the corn stalk and produces acidification.

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

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 pyrolysis 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 alkaline functional groups contained on its surface and the concentration of mineral elements in the biochar.

FIG3 shows the yield change of biochar prepared by an embodiment of the present invention. As shown in FIG3 , corn stalks are pyrolyzed at a temperature of 500° C. for 2 h. During the pyrolysis process, the lignocellulose in the corn stalks undergoes hydrolysis and other reactions to generate aromatic compounds. The biochar yields before and after pretreatment are relatively stable. The yield of pretreated corn stalk biochar is slightly lower than that of untreated stalk biochar, because the cellulose, hemicellulose and lignin structures in the corn stalks pretreated by repeated freeze-thaw cycles are destroyed, and various types of lignocellulose are fully exposed, which is more conducive to carbonization reaction during pyrolysis. The biochar yield should be increased, but in reality, the biochar yield is reduced. This is because after the lignocellulose structure is destroyed, part of the lignin is also destroyed and separated with the liquid, and the charcoal yield of biochar is mainly related to lignin, so the yield of pyrolysis biochar is reduced. However, this also confirms that repeated freeze-thaw cycles have a destructive effect on the lignocellulose structure, so that the performance of pyrolysis biochar pretreated by repeated freeze-thaw cycles has potential.

SEM analysis was performed using a ZEISS MERLIN Compact instrument: the biochar sample was glued to the sample stage with conductive glue and observed at a magnification of 1000-5000. Since biochar has poor conductivity, gold spraying technology was used to enhance its conductivity and improve the shooting effect. Through the SEM images of straw biochar under different pretreatment conditions, the formation and changes of the pore structure of biochar were observed, and the physical properties of biochar were explored.

Figure 4 is a scanning electron microscope image of the biochar of corn stalks of the present invention, comparative example 1, soaked corn stalks, comparative example 2, example 1 and example 2 magnified 2000 times, where a is corn stalks, b is comparative example 1, c are soaked corn stalks, d is comparative example 2, e is example 1, and f is example 2. From a and c in Figure 4, it can be seen that corn stalks have a porous structure, but the porosity is very low. After 24 hours of soaking, the impurities on the surface of the corn stalks 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 stalks are washed away by deionized water, making the pore capacity of the corn stalks larger and the structure clearer. From the scanning electron microscope images of biochar in Figure 4b and f, it can be concluded that compared with corn stalks, biochar has a more obvious pore structure, a tubular structure, and a smoother pore wall surface. The biochar prepared under different treatment conditions has a more obvious pore structure. The pore structure, quantity and size of biochar are all different. The untreated corn straw biochar (Comparative Example 1) has the thickest pore wall, fewer holes, 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 by impurity particles. From Figure 4 e and f, it can be seen that the pore structure formed by the corn straw biochar after freeze-thaw pretreatment is more dense and regular, with a honeycomb structure and clear pore boundaries. These image features confirm that repeated freeze-thaw pretreatment has an effect 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. According to 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 lignocellulose is too great when a certain number of times is reached. Not only does it open up the intertwined structure of lignocellulose, but the damage to structures such as lignin also 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 freeze-thaw cycles, the freeze-thaw temperature of -10℃ is more advantageous in the structure of biochar.

Fourier transform infrared spectroscopy was performed on the corn stalks and biochar before and after pretreatment in Example 1, Example 2 and Comparative Example 1 to analyze their surface functional groups. FIG5 is a Fourier transform infrared spectra of corn stalks and biochar before and after pretreatment in Example 1, Example 2 and Comparative Example 1 of the present invention, FIG5 In the figure, a is corn stover before and after pretreatment in Example 1, Example 2 and Comparative Example 1, and b is biochar prepared in Example 1, Example 2 and Comparative Example 1.

As shown in Figure 5a, the spectra of corn straw before and after pretreatment are not much different, and the spectrum range is the same as that of straw biochar, both of which are 500-4000cm ^-1 . A large peak appears in the spectral band of about 3860-3837cm- ¹ , which corresponds to the vibration peak of -OH, which is caused by the stretching displacement vibration of hydroxyl (-OH) unique to water, phenol and alcohol; the characteristic peak of the -CH structure unique to alkanes appears in the spectral band of about 2950-3308cm ^-1 , but 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; the relatively obvious absorption peaks appearing in the spectral band of about 1925-1887cm ^-1 are caused by the stretching vibration of C＝C and C＝O; the characteristic peak formed at the spectral band of 1543cm ^-1 is caused by the stretching vibration of C＝C and C＝N in the aromatic structure;

As shown in Figure 5b, the infrared spectra of corn stalks before and after pretreatment after high-temperature pyrolysis into biochar have obvious changes. The -OH vibration peak in the spectral band around 3860-3837cm ^-1 has a significant shift and is transferred to the band around 3632-3650cm ^-1 ; the CH structure with no obvious fluctuation in the spectral band at 2950-3308cm- ¹ produces unsaturated CH bond stretching vibration absorption in the band around 3308cm ^-1 after pyrolysis; there is a sharp characteristic peak in the band of 1650cm ^-1 after pyrolysis, which can be clearly confirmed that the peak is produced by the stretching of the C=O bond, and there is also part of -COO in this band; and the characteristic peak of C=C in the spectral band of 1543cm- ¹ , the stretching vibration shifts to the band of 1438-1448cm ^-1 , and there is also saturated CH in this band; CO and _-CH3 structural bond stretching vibration exist in the spectral band of 1149cm ^-1 . The characteristic peak of _-SiO2 in the spectral band of 861-852cm ^-1 before and after pyrolysis, the samples before and after pyrolysis contain some mineral elements. The vibration peak of _-CH2 is generated in the spectral band of 660-676cm ^-1 of biochar after pyrolysis, but the fluctuation is not obvious.

After corn straw was pyrolyzed to produce biochar, new stretching vibrations of CO, -COO, _-CH3 and _-CH2 structural bonds were added, proving that the pyrolysis process produced polymerization and deformation of the chemical structure, increased oxygen-containing functional groups in the biochar, and formed aromatic structures. The shrinkage of the C=O structure, the bias of unsaturated CH bonds, and the weakening of the vibration peak of -OH all indicate changes in lignocellulose, a decrease in the content of cellulose and hemicellulose, and an increase in the aromatic chemical structure in the biochar, which also proves that the better the stability of the biochar. The characteristic peak of C＝C in the 1438-1448cm ^-1 band of biochar pretreated by repeated freeze-thaw increases compared with that of untreated straw biochar, proving that the pretreated biochar has a higher fixed carbon content and a more stable carbon structure. As the temperature rises, the chemical structure of various functional groups of biomass undergoes changes such as bending vibration, deformation and polymerization, which decomposes and reforms the lignocellulose components of biomass, and finally generates biochar with a more stable chemical structure. The structure of biochar pretreated by repeated freeze-thaw is more stable.

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 spectrum. The crystal structure and lattice parameters of the biochar were determined by analyzing the spectrum. The test parameters selected the rays of the Cu target, 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 an X-ray diffraction pattern of the biochar prepared by Examples 1-2 and Comparative Examples 1-2 of the present invention. As shown in Figure 6, the XRD spectrum analysis of the straw biomass charcoal material found that the broad and slow diffuse diffraction peaks appearing at about 7° were caused by the amorphous or microcrystalline structure in the material. The diffraction peaks at about 7° correspond to the manifestation of the amorphous phase in the straw biomass charcoal material and the manifestation of some amorphous or microcrystalline ordered regions, respectively, which mainly correspond to the incompletely decomposed wood cellulose in the biochar. Among them, the peak appearing at about 7° mainly comes from the scattering caused by the ordered structure of the amorphous state, while the peak appearing at about 23° comes from some ordered regions of the amorphous or microcrystalline state, which may contain some crystalline particles. The peak at about 23° is related to its ordered crystal structure with a graphite structure, which is usually due to the disordered carbon structure in the original biomass during the carbonization process being transformed into an ordered graphite crystal structure through a high-temperature carbonization reaction. A peak of a graphite structure was also found at about 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 freeze-thaw cycles at -80°C (Example 2), the peak intensities of the amorphous phase and microcrystalline phase were relatively weakened compared with those at -10°C, indicating that 15 freeze-thaw cycles at -80°C and the treatment method had an impact on the amorphous phase of the biochar material. The ordered structure of the crystalline phase has been severely damaged.

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 freezing and thawing 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 freezing and thawing that destroys the crystal structure in the material. In addition, the infiltration of water during the freezing and thawing process may also cause changes in the chemical properties of silicon. It can be seen from Figure 6 that the crystallinity of the biochar (Comparative Example 1) is not high. This is because the porous structure and multi-fold characteristics of the biochar reduce the graphitization and crystallization of the biochar, and reduce the crystallinity of the biochar.

Figure 7 shows the crystallinity of biochar prepared in Examples 1-2 and Comparative Examples 1-2 of the present invention. The calculation of the crystallinity of biochar shows that the soaking and repeated freeze-thaw pretreatment methods significantly increase the crystallinity of biochar, indicating that the graphite structure and crystal structure of the pretreated biochar increase. However, the crystallinity of the straw biochar that was repeatedly frozen and thawed at -80°C for 15 times is the lowest among the pretreated biochars (compared to Example 1 and Comparative Example 1). This is because the multiple freeze-thaws under ultra-low temperature conditions severely damage the structure of corn straw, resulting in structural collapse, which seriously damages the structure of the biochar and reduces the crystallinity of the biochar.

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.

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

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 significantly higher than that of the untreated corn straw biochar (Comparative Example 1). The specific surface area is 2.6731m ² ·g ^-1, which is nearly eight times higher. However, the specific surface area of the straw biochar (Example 2) pretreated with 15 freeze-thaw cycles at -80℃ is only 3.8395m ² ·g ^-1 , which is about 1.5 times that of the untreated corn straw biochar; by comparison, it is found that the pore size of the -80℃ freeze-thaw cycle 15 times (Example 2) is 10.9697nm, while the pore size of the -10℃ freeze-thaw cycle 15 times (Example 1) is 6.8162nm, and the average pore size of the straw biochar (Comparative Example 1) that has not been pretreated with freeze-thaw cycles is 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 size.

The effect of the biochar prepared in the comparative example on the adsorption of ferrous ions by biochar is as follows: 100 ml of 50 mg/L ferrous sulfate solution is taken into a 250 ml conical flask, and 0.1 g of the biochar sample prepared in the example is added at a ratio of 1 g/L. Under the conditions of a temperature of 25 ± 1 ° C and a rotation speed of 150 r/min, the sample is shaken for 24 hours and taken out. After taking it out, it is allowed to stand for 5 minutes and filtered with a 0.45 μm filter membrane. The treated sample is injected into a graphite tube through an automatic sampler, and after electrothermal atomization, the characteristic spectrum is absorbed at a wavelength of 248.3 nm and the absorbance value is measured. Then, according to the measured standard curve, the element content is calculated, and finally the effect on the adsorption capacity of ferrous ions is compared.

FIG8 is the adsorption amount of ferrous ions by biochar of an embodiment of the present invention. As shown in FIG8 , under constant adsorption conditions, the removal rate of ferrous ions also shows an increasing trend with the increase of freeze-thaw times. 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 effect on the biochar structure, and 6-15 freeze-thaw treatments have a relatively stable effect on the biochar characteristics, indicating that the pretreatment method will not have a significant effect on the biochar structure after 5 freeze-thaw cycles at -10°C, but will increase the risk of biochar structure instability. Under -80°C freeze-thaw conditions, the adsorption amount of ferrous ions began to decrease after 4 repeated freeze-thaw cycles, and a fault-like decrease occurred. After 6 repeated freeze-thaw cycles, the biochar performance was relatively stable. The reason for the decrease in the adsorption of ferrous ions by biochar is speculated to be that multiple ultra-low temperature freeze-thaw treatments have severely damaged the internal structure of the biochar, causing structural collapse, which has led to a reduction in the binding sites for biochar to adsorb metal ions, affecting its adsorption effect.

Further experiments on the adsorption of iron ions in the solution showed that the freezing temperature was -10℃ and the number of freeze-thaw cycles was The maximum adsorption capacity of biochar obtained by pyrolysis of biomass 15 times was 191.28 mg/g. The maximum adsorption capacity of biochar for iron ions obtained by pyrolysis of biomass at a freezing temperature of -80°C and 3 freeze-thaw cycles was 179.07 mg/g.

In summary, the present invention performs freeze-thaw cycle pretreatment on biomass, and biomass has a cell 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 influence of freeze-thaw cycle pretreatment on biomass (freezing temperature is -10 and -80°C, thawing temperature is 20°C, and freeze-thaw cycle is 15 times), and then uses biomass with different freeze-thaw cycle times as raw material to prepare biochar by carbonization, changes the biomass structure, and then regulates the pore structure of the biochar, and has no additional device requirements, simple process, low cost, pollution-free, easy large-scale production, etc. 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, 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. Active sites are provided for the adsorption performance of the biochar, and the biochar is better applied in the field of pollutant adsorption.

It should be noted that when the present invention involves a numerical range, it should be understood that the two endpoints of each numerical range and any value between the two endpoints can 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 a preferred embodiment. Although the 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 know the basic creative concept. Therefore, the attached 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.

Obviously, those skilled in the art can make various changes and modifications to the present invention without departing from the spirit and scope of the present invention. Thus, if these modifications and variations of the present invention fall within the scope of the claims of the present invention and their equivalents, the present invention is also intended to include these modifications and variations.

Claims (9)

A method for preparing biochar by regulating biomass through freeze-thaw cycles, characterized in that it comprises the following steps: 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. 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. 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 to 15. 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. The method for preparing biochar by regulating biomass using freeze-thaw cycles according to claim 1 is characterized in that, 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. The method for preparing biochar by regulating biomass through freeze-thaw cycles according to claim 1 is characterized in that the pyrolysis time is 2 to 4 hours and the heating rate is 10 to 20°C/min. The method for preparing biochar by regulating biomass through freeze-thaw cycles according to claim 1 is 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. A biochar prepared by the method according to any one of claims 1 to 7. Use of the biochar according to claim 8 in removing ferrous ions from water.