A magnesium modified biochar and a method of making the same

Abstract

The application discloses a kind of magnesium modified biochar and preparation method thereof, belong to the field of agricultural and forestry waste resource utilization and printing and dyeing wastewater treatment adsorption material.The preparation method includes: corn straw is pretreated and mixed with magnesium chloride hexahydrate aqueous solution and impregnated, by solid-liquid separation, drying obtains magnesium salt loaded straw precursor;The precursor is heated to 500~800 DEG C under the condition of limiting oxygen and is pyrolyzed carbonization, after natural cooling, grinding is crushed, and magnesium modified biochar is obtained.The application is impregnated by the synergistic effect of load and limited oxygen pyrolysis, and magnesium element is uniformly loaded in the pore structure of biochar in the form of crystalline magnesium oxide, and has excellent adsorption performance on cationic organic dyes such as rhodamine B, and the adsorption rate can reach more than 98%, and after anhydrous ethanol ultrasonic regeneration, it still maintains more than 90% of the adsorption rate for many cycles, the raw material is cheap and easy to obtain, the process is simple and controllable, the adsorption performance is superior, and it can be recycled and regenerated, and is suitable for the field of printing and dyeing wastewater treatment.

Description

Technical Field

[0001] This invention relates to the field of adsorption materials technology for the resource utilization of agricultural and forestry waste and the treatment of dyeing and printing wastewater, specifically to a magnesium-modified biochar and its preparation method. Background Technology

[0002] The current state of pollution from organic dye wastewater in textile and printing industries: The textile, printing and dyeing, and papermaking industries use large quantities of organic dyes, resulting in wastewater with high color and poor biodegradability. Direct discharge of this wastewater severely damages aquatic ecosystems. Rhodamine B, a widely used basic synthetic dye, is highly toxic and carcinogenic, chemically stable, and difficult to degrade naturally. Once in water bodies, it inhibits aquatic organism reproduction, disrupts the food chain, pollutes soil, reduces soil fertility, and can accumulate through the food chain, damaging the liver, kidneys, and nervous system in humans. Conventional flocculation and biochemical treatment processes have low removal efficiency for Rhodamine B, failing to meet wastewater discharge standards. Therefore, low-cost, high-efficiency adsorption materials are urgently needed.

[0003] The disposal of corn stalk solid waste generates a massive amount of corn stalk waste. Open burning of stalks easily causes air pollution, while returning them to the field has problems such as long decomposition cycles and the breeding of pests and diseases. Traditional resource utilization methods have low added value. Corn stalks are rich in cellulose and lignin, making them a high-quality biomass raw material for biochar production. If efficient adsorbent carbon can be produced from corn stalk solid waste, the dual challenges of solid waste pollution and wastewater treatment can be solved simultaneously.

[0004] Ordinary biochar has obvious shortcomings in adsorption performance. Virgin corn straw biochar prepared by direct pyrolysis has obvious shortcomings: low specific surface area, few pores, limited surface active functional groups, insufficient adsorption sites for cationic dyes, and limited adsorption capacity and removal rate. Simple acid-base and physical modification can only optimize the pore structure, but it is difficult to increase the surface chemical adsorption sites in large quantities, resulting in poor recycling performance.

[0005] Existing metal-modified biochar technologies are inadequate. Existing iron, zinc, and manganese-modified biochars have problems such as the risk of metal ion leaching, high raw material costs, and severe degradation of adsorption performance after regeneration. Existing magnesium-modified biochars are mostly used for the treatment of nitrogen, phosphorus, and heavy metals in water bodies, lacking a targeted preparation process for Rhodamine B organic dyes. The optimal pyrolysis temperature, magnesium salt impregnation ratio, and optimal wastewater adsorption conditions are not clearly defined, and a complete regeneration and recycling scheme is lacking, making industrial application difficult.

[0006] In summary, existing technologies lack a magnesium-modified corn stalk biochar material and its supporting preparation process that is readily available, simple to prepare, has high adsorption efficiency, can be regenerated and recycled multiple times, and is suitable for treating Rhodamine B dye wastewater. This invention addresses these deficiencies by achieving technological innovation. Summary of the Invention

[0007] This invention aims to solve the following technical problems existing in the prior art: First, it provides a method for preparing magnesium-modified biochar using agricultural waste corn stalks as raw material, realizing the resource utilization of solid waste and reducing the production cost of adsorption materials. Second, it provides a magnesium-doped modified biochar composite material, significantly improving the specific surface area and pore structure of primary biochar, increasing Mg-O active sites, and enhancing the adsorption capacity for cationic organic dyes. Third, it provides a complete and controllable magnesium-modified biochar preparation process, clearly defining the optimal process parameters. Fourth, it provides an application method and regeneration and recycling process for this magnesium-modified biochar in the treatment of wastewater containing cationic organic dyes, reducing the operation and maintenance costs of wastewater treatment.

[0008] To achieve the above objectives, the present invention provides the following technical solution: a magnesium-modified biochar, comprising the following components: The magnesium-modified biochar has a phase composition of amorphous carbon and crystalline magnesium oxide, wherein the crystalline magnesium oxide is uniformly loaded inside the pores and on the surface of the biochar; the BET specific surface area of ​​the magnesium-modified biochar is 210 m². 2 / g~230m 2 / g, total pore volume is 0.20cm³ 3 / g~0.30cm 3 / g.

[0009] Preferably, the magnesium-modified biochar surface contains Mg-O bonds, CO bonds, and aromatic ring CH functional groups; the magnesium-modified biochar has a rich mesoporous structure, and its Fourier transform infrared spectrum is within 400 cm⁻¹. -1 ~420cm -1 Characteristic absorption peaks of Mg-O appear at [location].

[0010] Preferably, a method for preparing magnesium-modified biochar includes the following steps: S1. First, corn stalks need to be pre-treated: wash the corn stalks to remove impurities, dry them, crush them and sieve them to obtain stalk powder.

[0011] S2. Then, magnesium salt impregnation modification is required: the straw powder is mixed with an aqueous solution of magnesium chloride hexahydrate, and impregnation loading is carried out under stirring conditions so that magnesium ions are fully adsorbed on the surface and internal pores of the straw powder. Then, solid-liquid separation is carried out, and the resulting solid is dried to obtain magnesium salt loaded straw precursor.

[0012] S3. Finally, oxygen-limited high-temperature pyrolysis carbonization is required: The magnesium salt-loaded straw precursor is placed in an oxygen-limited atmosphere and subjected to high-temperature pyrolysis carbonization in a programmed heating manner. The pyrolysis temperature is 500℃~800℃, the holding time is 1h~3h, and after natural cooling, it is crushed to obtain magnesium-modified biochar.

[0013] Preferably, the specific conditions for the corn stalk pretreatment are as follows: the corn stalks are repeatedly rinsed with deionized water to remove mud and sand impurities, dried in a forced-air drying oven at 100℃~110℃ until constant weight, and then crushed and passed through a standard sieve of 1mm~3mm to obtain stalk powder.

[0014] Preferably, the concentration of the magnesium chloride hexahydrate aqueous solution is 0.2 mol / L; the solid-liquid ratio of the straw powder to the magnesium chloride hexahydrate aqueous solution is 1 g: 10 mL; the impregnation loading temperature is 25°C, the time is 24 h, and the stirring speed is 100 r / min to 300 r / min.

[0015] Preferably, the oxygen-limited atmosphere is achieved by loading the magnesium salt-loaded straw precursor into a crucible and sealing it; the temperature rise rate of the programmed heating is 3℃ / min to 8℃ / min; the temperature of the high-temperature pyrolysis carbonization is 800℃; and the holding time is 2h.

[0016] Preferably, the application of magnesium-modified biochar in the adsorption treatment of wastewater containing cationic organic dyes includes the following steps: adding the magnesium-modified biochar to the wastewater containing cationic organic dyes, adjusting the pH of the wastewater to 2~5, and performing adsorption by shaking at 35℃~55℃ for 60min~180min, followed by solid-liquid separation to complete the adsorption treatment.

[0017] Preferably, the cationic organic dye is Rhodamine B; the initial concentration of Rhodamine B in the wastewater is 10 mg / L to 100 mg / L; the dosage of the magnesium-modified biochar is 1 g / L to 5 g / L; the adsorption treatment temperature is 45°C, the wastewater pH is 3, and the shaking adsorption time is 120 min.

[0018] Preferably, the regeneration method of magnesium-modified biochar involves placing the adsorbed saturated magnesium-modified biochar in anhydrous ethanol and ultrasonically desorbing it at 20°C to 30°C for 20 to 40 minutes. After solid-liquid separation, the biochar is washed with deionized water and dried to complete the regeneration. After five regeneration cycles, the adsorption rate of the magnesium-modified biochar for cationic organic dyes remains above 90%.

[0019] Explanation of Key Technology Principles The core technical principle of this invention lies in achieving magnesium doping modification of biochar through the synergistic effect of magnesium salt impregnation and oxygen-limited pyrolysis. During the impregnation stage, the Mg in the magnesium chloride hexahydrate aqueous solution... 2+ Magnesium chloride binds to oxygen-containing functional groups in straw cellulose and lignin through electrostatic adsorption and coordination, and is uniformly loaded onto the surface and internal pore structure of straw. During the subsequent high-temperature oxygen-limited pyrolysis process, magnesium chloride undergoes hydrolysis and decomposition at high temperature to generate magnesium oxide, while the straw organic matter is carbonized to form a biochar skeleton.

[0020] The optimal pyrolysis temperature is crucial. When the pyrolysis temperature is between 500℃ and 600℃, magnesium chloride decomposes incompletely, magnesium oxide has extremely low crystallinity, and there are insufficient adsorption sites. When the pyrolysis temperature is 700℃, the crystallinity of magnesium oxide increases somewhat, but pore development is still insufficient. When the pyrolysis temperature is 800℃, magnesium chloride is completely hydrolyzed and decomposed to generate highly crystalline magnesium oxide particles, which are uniformly loaded inside the biochar channels, and the specific surface area and pore volume reach their peak values, resulting in the optimal adsorption performance.

[0021] Optimizing the magnesium salt impregnation concentration is equally crucial. When the magnesium chloride concentration is too low, the magnesium loading is insufficient, resulting in fewer active sites; when the concentration is too high, magnesium salt crystals will clog the straw pores, reducing the specific surface area of ​​the material. The 0.2 mol / L concentration determined in this invention is the optimal concentration for balancing the loading and pore structure.

[0022] The adsorption of cationic organic dyes by magnesium-modified biochar is the result of multiple synergistic mechanisms: First, physical adsorption—the well-developed mesoporous structure provides a huge specific surface area, relying on van der Waals forces to capture dye molecules; Second, chemical adsorption—Mg-O and CO functional groups on the material surface coordinate with dye molecules and transfer electrons, and the adsorption process conforms to a pseudo-second-order kinetic model; Third, monolayer homogeneous adsorption—the isothermal fitting conforms to the Langmuir model, and dye molecules are uniformly immobilized in a monolayer on the active sites of the material; Fourth, thermodynamic endothermic reaction—heating is beneficial for the spontaneous adsorption process, and the removal effect is better under high-temperature conditions.

[0023] In an acidic adsorption environment (pH=3), Rhodamine B is a cationic dye. The MgO on the surface of magnesium-modified biochar is protonated, which reduces the positive charge density on the material surface and weakens the repulsion effect of like charges. At the same time, the synergistic effects of electrostatic attraction, ion exchange, and hydrogen bonding are enhanced, which greatly improves the adsorption capacity of the dye.

[0024] Compared with the prior art, the beneficial effects of the present invention are: This invention utilizes inexpensive and readily available raw materials, achieving the resource utilization of solid waste. Using discarded corn stalks from farmland as raw material, it eliminates the need to purchase specialized carbon-based raw materials, not only solving the environmental pollution problem caused by stalk burning but also significantly reducing the production cost of adsorbent materials. The modification process is simple and controllable, with mild preparation conditions. This invention employs only a combination of room-temperature impregnation and a one-step high-temperature carbonization process, eliminating the need for complex ball milling, gas activation, or multi-step oxidation treatments. Oxygen-limited pyrolysis is achieved in a muffle furnace with a sealed lid, eliminating the need for continuous supply of protective gas. Equipment investment is low, and the process is easily scalable for large-scale production.

[0025] This invention significantly improves adsorption performance and treatment efficiency. Magnesium doping introduces crystalline magnesium oxide, which multiplies the specific surface area and pore volume, and adds a large number of Mg-O chemical active sites. Under optimal conditions, the adsorption rate of Rhodamine B by the magnesium-modified biochar of this invention can reach 98.16%, which is much higher than the 67.3% of the unmodified biochar. It is suitable for acidic dyeing and printing wastewater and adapts to actual working conditions. Dyeing and printing wastewater is mostly acidic. The material of this invention has the best adsorption effect in an acidic system with pH=3, eliminating the need for a large amount of alkali solution to adjust the pH value of the wastewater and reducing the cost of water treatment agents.

[0026] This invention features reversible adsorption and can be regenerated and recycled multiple times. It employs anhydrous ethanol ultrasonic regeneration technology, which is simple to operate and eliminates the risk of secondary heavy metal leaching pollution. After five regeneration cycles, the adsorption rate of Rhodamine B remains above 90%, significantly reducing the frequency of adsorption material replacement, lowering engineering operation and maintenance costs, and ensuring high environmental safety with no secondary pollution. Magnesium is an environmentally friendly mineral element that is stably fixed in the carbon skeleton as magnesium oxide after pyrolysis. The amount of magnesium ions leached during wastewater treatment is extremely low. Waste saturated biochar can be directly returned to the field to improve saline-alkali soil, thus possessing the added value of carbon sequestration and soil remediation. Attached Figure Description

[0027] Figure 1 This is a flow chart of the magnesium-modified biochar preparation process of the present invention; Figure 2 The XRD pattern of the magnesium-modified biochar of this invention; Figure 3 The images show the SEM microstructures of the primary biochar and magnesium-modified biochar of this invention. Figure 4 The M-BC Fourier transform infrared (FT-IR) curve of this invention; Figure 5 The BET isotherm of primary carbon and M-BC nitrogen adsorption-desorption in this invention; Figure 6 This invention illustrates the effect of different pyrolysis temperatures on the adsorption rate. Figure 7 This invention illustrates the effect of initial pH on adsorption performance. Detailed Implementation

[0028] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0029] Example 1: Large-scale preparation of magnesium-modified biochar S1. Straw pretreatment: Large-scale crushing equipment crushes corn stalks, washes them with water in a drum to remove impurities, and dries them with continuous hot air until the moisture content is <5%, and then screens them to obtain straw fragments of less than 2 mm.

[0030] S2. Continuous impregnation section: 0.2 mol / L magnesium chloride solution is circulated and sprayed onto the straw pile, impregnated at room temperature in a sealed environment for 24 hours, and then separated into solid and liquid by a vacuum belt filter.

[0031] S3. Drying and carbonization: The filter cake enters a continuous drying kiln and is dehydrated at 105 ℃; a closed rotary muffle furnace is used to heat the filter cake to 800 ℃ at a rate of 5 ℃ / min, and the temperature is maintained for 2 hours for oxygen-limited carbonization. The filter cake is then discharged, cooled, crushed, and packaged. The finished product, M-BC, is sealed and stored in a moisture-proof container.

[0032] Example 2: Complete Application Process for Dyeing and Printing Wastewater Treatment S1. Pretreatment: Dilute hydrochloric acid is added to the dyeing and printing wastewater equalization tank to control the pH of the wastewater to be stable at around 3.

[0033] S2. Adsorption reaction: After adjustment, the wastewater enters the adsorption reactor, and the magnesium-modified biochar of this invention is added at 3 g / L. The mixture is stirred at 45 ℃ for 120 min for adsorption.

[0034] S3. Solid-liquid separation: Plate and frame filter press separates adsorbed saturated carbon from purified effluent, and the dye concentration in the effluent meets the discharge standards.

[0035] S4. Regeneration section: The filter cake is sent to the ultrasonic desorption tank, where it is desorbed by circulating anhydrous ethanol for 30 minutes. After washing and drying, it is returned to the adsorption section for reuse. The desorbed ethanol solution is recovered by distillation, and the ethanol is recycled. The concentrated dye residue is disposed of in a harmless manner.

[0036] Example 3: Resource Utilization of Waste Saturated Biochar M-BC, whose adsorption performance has declined after multiple cycles, can be crushed and directly applied to saline-alkali farmland. The MgO inside the material can improve the soil pH, and the biochar skeleton can enhance the soil organic matter, water retention and fertilizer retention capacity, thus realizing the dual resource utilization of "wastewater treatment and soil remediation".

[0037] Key technical points and protection points extracted Protection Point 1: A complete preparation process for preparing magnesium-modified biochar using corn stalks as a substrate, 0.2 mol / L magnesium chloride hexahydrate impregnated at room temperature for 24 h, and then subjected to oxygen-limited pyrolysis at 800 ℃ for 2 h; Protection Point 2: Magnesium-modified biochar composite material, characterized by an amorphous carbon + crystalline MgO composite structure, with a specific surface area of ​​228.93 m². 2 / g; Protection Point 3: Optimal adsorption conditions for magnesium-modified biochar in the treatment of Rhodamine B wastewater (pH=3, dosage 0.3 g / 100 mL, temperature 45 ℃); Protection point 4: The recycling process of magnesium-modified biochar regenerated by anhydrous ethanol ultrasonic regeneration. Protection Point 5: The combined application of magnesium-modified biochar in the treatment of cationic organic dye wastewater and the improvement of saline-alkali soil.

[0038] Example 4: Preparation of Magnesium-Modified Biochar S1. Pretreatment of corn stalks: Collect waste corn stalks from farmland in Yingkou area, rinse them repeatedly with deionized water 3 times to remove mud and sand impurities, dry them in a 105℃ forced-air drying oven for 12 hours until constant weight, crush them and pass them through a 2mm standard sieve to obtain straw powder, which is then sealed, dried and stored for later use.

[0039] S2. Magnesium Salt Impregnation Modification: Accurately weigh 40.66 g of magnesium chloride hexahydrate (MgCl2·6H2O), dissolve it in 1 L of deionized water to prepare a 0.2 mol / L magnesium chloride hexahydrate aqueous solution. Place 20 g of sieved corn stalk powder in a 250 mL Erlenmeyer flask, add 200 mL of the above magnesium chloride solution to completely submerge the stalk powder, and stir with a magnetic stirrer at 180 r / min for 24 h at a constant temperature of 25 °C to achieve sufficient adsorption and loading of magnesium ions. After impregnation, separate the solid-liquid mixture by vacuum filtration, and dry the resulting filter cake in a 105 °C oven for 10 h to constant weight to obtain the magnesium salt-loaded stalk precursor.

[0040] S3. Oxygen-limited high-temperature pyrolysis carbonization: The dried magnesium salt-loaded straw precursor was loaded into a ceramic crucible, sealed to achieve an oxygen-limited atmosphere, and placed in a muffle furnace. A programmed temperature rise was set at a rate of 5℃ / min, and the temperature was maintained at 800℃ for 2 hours. After pyrolysis, the mixture was allowed to cool naturally to room temperature. The solid product was then removed, ground, pulverized, sealed, dried, and stored to obtain magnesium-modified biochar, labeled M-BC.

[0041] Example 5: Effect of different pyrolysis temperatures on adsorption performance Following the preparation method of Example 4, only the pyrolysis temperature in step S3 was changed, and the pyrolysis temperatures were set to 500℃, 600℃, 700℃, and 800℃ respectively to prepare magnesium-modified biochar samples at different pyrolysis temperatures.

[0042] Adsorption experiments were conducted on all samples under the same conditions (initial Rhodamine B concentration 50 mg / L, wastewater pH=3, dosage 0.3 g / 100 mL, temperature 45℃, and adsorption by shaking for 120 min). The results showed that the adsorption rate of Rhodamine B by magnesium-modified biochar gradually increased with the pyrolysis temperature from 500℃ to 800℃, reaching its maximum at 800℃. This indicates that 800℃ is the optimal pyrolysis temperature, at which magnesium chloride completely decomposes to form highly crystalline magnesium oxide, and the biochar pore structure is most fully developed.

[0043] Example 6: Effect of different impregnation concentrations on adsorption performance Following the preparation method of Example 4, only the concentration of the magnesium chloride hexahydrate aqueous solution in step S2 was changed, and magnesium chloride solutions of 0.05 mol / L, 0.1 mol / L, 0.2 mol / L, 0.3 mol / L and 0.5 mol / L were prepared for impregnation modification to prepare magnesium-modified biochar samples with different magnesium loadings.

[0044] Rhodamine B adsorption experiments were conducted on all samples under the same conditions. The results showed that the adsorption rate of magnesium-modified biochar gradually increased as the magnesium chloride concentration increased from 0.05 mol / L to 0.2 mol / L; the adsorption performance was optimal at a concentration of 0.2 mol / L; however, the adsorption rate decreased when the concentration was further increased to above 0.3 mol / L. Analysis suggests that too low a concentration results in insufficient magnesium loading and fewer active sites; while too high a concentration leads to magnesium salt crystallization, which blocks the pores of the straw and reduces the specific surface area of ​​the material. Therefore, 0.2 mol / L is the optimal impregnation concentration.

[0045] Example 7: Effect of different pH values ​​on adsorption performance The magnesium-modified biochar M-BC prepared in Example 4 was used to investigate the effect of different pH values ​​on adsorption efficiency by adjusting the pH values ​​of wastewater to 3, 5, 7, 9, and 11 under the conditions of an initial Rhodamine B concentration of 50 mg / L, a dosage of 0.3 g / 100 mL, and a temperature of 45 °C.

[0046] Experimental results show that under strongly acidic conditions (pH=3), M-BC exhibits the highest adsorption efficiency for Rhodamine B, reaching its maximum at pH=3. The adsorption efficiency gradually decreases with increasing pH. Analysis suggests that Rhodamine B is a cationic dye. Under acidic conditions, the MgO on the surface of the magnesium-modified biochar undergoes protonation, reducing the positive charge density on the material surface and weakening the repulsion effect of like charges. Simultaneously, the synergistic effects of electrostatic attraction, ion exchange, and hydrogen bonding are enhanced, thereby significantly increasing the dye adsorption capacity. This characteristic makes the magnesium-modified biochar of this invention particularly suitable for the treatment of acidic dyeing and printing wastewater.

[0047] In summary, the magnesium-modified biochar preparation method provided by this invention utilizes widely available and inexpensive raw materials, has a simple process route, low equipment requirements, and is easy to operate, making it readily applicable for large-scale industrial production. The resulting magnesium-modified biochar exhibits excellent adsorption performance for cationic organic dyes and can be repeatedly regenerated and recycled, demonstrating broad application prospects and significant economic and social benefits in the field of dyeing and printing wastewater treatment.

[0048] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A magnesium-modified biochar, characterized in that: Includes the following components: The magnesium-modified biochar has a phase composition of amorphous carbon and crystalline magnesium oxide, wherein the crystalline magnesium oxide is uniformly loaded within the pores and on the surface of the biochar; the BET specific surface area of ​​the magnesium-modified biochar is 200 m². 2 / g~260m 2 / g, total pore volume is 0.20cm³ 3 / g~0.30cm 3 / g.

2. The magnesium-modified biochar according to claim 1, characterized in that: The magnesium-modified biochar contains Mg-O bonds, CO bonds, and aromatic ring CH functional groups on its surface; the magnesium-modified biochar has a rich mesoporous structure, and its Fourier transform infrared spectrum is within 400 cm⁻¹. -1 ~420cm -1 Characteristic absorption peaks of Mg-O appear at [location].

3. A method for preparing magnesium-modified biochar according to claims 1-2, characterized in that: Its preparation method includes the following steps: S1. First, corn stalks need to be pre-treated: wash the corn stalks to remove impurities, dry them, crush them and sieve them to obtain stalk powder. S2. Then, magnesium salt impregnation modification is required: the straw powder is mixed with an aqueous solution of magnesium chloride hexahydrate, and impregnation loading is carried out under stirring conditions so that magnesium ions are fully adsorbed on the surface and internal pores of the straw powder. Then, solid-liquid separation is carried out, and the obtained solid is dried to obtain magnesium salt loaded straw precursor. S3. Finally, oxygen-limited high-temperature pyrolysis carbonization is required: The magnesium salt-loaded straw precursor is placed in an oxygen-limited atmosphere and subjected to high-temperature pyrolysis carbonization in a programmed heating manner. The pyrolysis temperature is 500℃~800℃, the holding time is 1h~3h, and after natural cooling, it is crushed to obtain magnesium-modified biochar.

4. The method for preparing magnesium-modified biochar according to claim 3, characterized in that: The specific conditions for the pretreatment of corn stalks are as follows: the corn stalks are repeatedly rinsed with deionized water to remove mud and sand impurities, dried in a forced-air drying oven at 100℃~110℃ until constant weight, and then crushed and passed through a standard sieve of 1mm~3mm to obtain stalk powder.

5. The method for preparing magnesium-modified biochar according to claim 3, characterized in that: The concentration of the magnesium chloride hexahydrate aqueous solution is 0.2 mol / L; the solid-liquid ratio of the straw powder to the magnesium chloride hexahydrate aqueous solution is 1 g: 10 mL; the impregnation loading temperature is 25℃, the time is 24 h, and the stirring speed is 100 r / min to 300 r / min.

6. The method for preparing magnesium-modified biochar according to claim 3, characterized in that: The oxygen-limited atmosphere is achieved by loading the magnesium salt-loaded straw precursor into a crucible and sealing it; the temperature rise rate of the programmed heating is 3℃ / min to 8℃ / min; the temperature of the high-temperature pyrolysis carbonization is 800℃; and the holding time is 2h.

7. The application of magnesium-modified biochar according to claims 1-6 in the adsorption treatment of wastewater containing cationic organic dyes, characterized in that, The process includes the following steps: adding the magnesium-modified biochar to wastewater containing cationic organic dyes, adjusting the pH of the wastewater to 2-5, and performing adsorption by shaking at 35℃-55℃ for 60-180 minutes, followed by solid-liquid separation to complete the adsorption treatment.

8. The application according to claim 7, characterized in that: The cationic organic dye is Rhodamine B; the initial concentration of Rhodamine B in the wastewater is 10 mg / L to 100 mg / L; the dosage of magnesium-modified biochar is 1 g / L to 5 g / L; the adsorption treatment temperature is 45℃, the wastewater pH is 3, and the shaking adsorption time is 120 min.

9. A method for regenerating magnesium-modified biochar according to claims 1-6, characterized in that: The magnesium-modified biochar, which is saturated with adsorption, is placed in anhydrous ethanol and ultrasonically desorbed at 20℃~30℃ for 20min~40min. After solid-liquid separation, it is washed with deionized water and dried to complete the regeneration. After 5 regeneration cycles, the adsorption rate of the magnesium-modified biochar for cationic organic dyes is still maintained at over 90%.

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