Preparation method of natural-synthetic copolymer saline-alkali soil modifier

Abstract

The invention discloses a preparation method of a natural-synthetic copolymer saline-alkali soil modifier. The preparation method comprises the following steps: 1, preparing cotton straw biomass charcoal; 2, preparing a carboxymethyl cellulose free radical initiation system; 3, preparing a copolymer precursor mixed dispersion liquid; 4, preparing the natural-synthetic copolymer saline-alkali soil modifier; according to the invention, natural macromolecular carboxymethyl cellulose is adopted to modify an acrylic polymer, so that the acrylic polymer has good water absorption and salt tolerance; the biomass charcoal is added into a copolymer network, so that pores of the soil can be effectively improved, formation of soil aggregates can be promoted, and meanwhile, the content of organic matters in the soil can be increased; the method has the advantages of mild reaction conditions and green and environment-friendly materials, and can improve the physicochemical properties of the saline-alkali soil.

Description

Technical Field

[0001] The invention relates to the field of saline-alkali land improvers, and in particular to a method for preparing a natural-synthetic copolymer saline-alkali land improver. Background Art

[0003] At present, the most widely used soil conditioners are synthetic ones. Synthetic conditioners are mainly artificially synthesized high molecular organic polymers, such as polyacrylamide and polyvinyl alcohol. The improvement effect is significant, but most of them are difficult to degrade and are not environmentally friendly.

[0004] Natural macromolecules usually have good water absorption and sustainability, and are usually widely distributed in nature and inexpensive. Using natural macromolecules to modify synthetic improvers can effectively alleviate these problems. Its main function is to improve the structure and physical and chemical properties of the soil, improve soil fertility, alleviate soil erosion and improve the salinity of arid soils. Summary of the invention

[0005] In view of this, the present invention provides a method for preparing a natural-synthetic copolymer saline-alkali land improver.

[0006] In order to solve the above technical problems, the present invention adopts the following technical solutions:

[0007] A method for preparing a natural-synthetic copolymer saline-alkali land improver, wherein the amount "parts" are parts by weight, comprises the following steps:

[0008] Step 1: Preparation of cotton straw biochar

[0009] Cotton straw powder and potassium bicarbonate were added into a quartz boat, placed into a tube furnace, pyrolyzed under the protection of argon atmosphere, naturally cooled, washed with deionized water to remove ash, and dried to obtain cotton straw biomass charcoal;

[0010] Step 2: Preparation of carboxymethyl cellulose free radical initiation system

[0011] Take carboxymethyl cellulose, add deionized water, and stir under heating to dissolve it; introduce nitrogen protection, add ammonium persulfate solution while stirring, and continue the reaction to obtain a carboxymethyl cellulose free radical initiation system solution;

[0012] Step 3: Preparation of copolymer precursor mixed dispersion

[0013] In a sealed kettle filled with nitrogen, sodium hydroxide solution and acrylic acid are added, and stirred at zero degrees until the solution is clear and transparent; then acrylamide is added, and stirred at room temperature to completely dissolve the acrylamide; then the cotton straw biochar and N,N′-methylenebisacrylamide solution are added, and stirred at room temperature to uniformly disperse the biochar in the solution, so as to obtain a precursor mixed dispersion of the copolymer;

[0014] Step 4: Preparation of natural-synthetic copolymer saline-alkali land improver

[0015] Under nitrogen protection and heating conditions, the copolymer precursor mixed dispersion prepared in the third step is added to the carboxymethyl cellulose free radical initiation system solution prepared in the second step while stirring, and stirring is continued to ensure that the mixture is fully and evenly mixed; then the system temperature is increased, and the reaction is continued with stirring, and the product after the reaction is dried to a constant weight and crushed to obtain a natural-synthetic copolymer saline-alkali land improver.

[0016] Preferably, in the first step, the cotton straw powder is 3 to 5 parts of dry cotton straw powder passed through an 80-mesh sieve, and the amount of potassium bicarbonate added is 6 to 10 parts.

[0017] Preferably, in the first step, the pyrolysis method is to increase the temperature to 400-600° C. at a heating rate of 5° C. / min and maintain it for 2-4 hours.

[0018] Preferably, in the first step, the drying condition is 60°C.

[0019] Preferably, in the second step, the amount of carboxymethyl cellulose added is 3 to 6 parts, the amount of deionized water added is 100 to 200 parts, the heating conditions are 60 to 80° C., and the stirring time is 2 to 4 hours.

[0020] Preferably, in the second step, the amount of ammonium persulfate solution added is 2 to 3 parts, the mass fraction is 12 to 15%, and the reaction time is 30 to 60 minutes.

[0021] Preferably, in the third step, the amount of sodium hydroxide solution added is 5 to 12 parts, with a mass fraction of 10 to 25%; the amount of acrylic acid added is 25 to 40 parts, and the amount of acrylamide added is 60 to 75 parts.

[0022] Preferably, in the third step, the amount of cotton straw biochar prepared in the first step is 1 to 5 parts, the amount of N,N′-methylenebisacrylamide solution is 3 to 5 parts, the mass fraction is 1 to 3%, and the stirring time at room temperature is 10 to 30 minutes.

[0023] Preferably, in the fourth step, the heating condition is 50° C. and the continuous stirring time is 60 to 90 minutes.

[0024] Preferably, in the fourth step, the system temperature is raised to 65-80° C., the stirring reaction time is continued for 2-3 h, the drying temperature is 60° C., and the product is crushed through an 80-mesh sieve.

[0025] Compared with the prior art, the present invention has achieved the following technical effects:

[0026] (1) The present invention uses natural macromolecular carboxymethyl cellulose to modify acrylic polymers to make them have good water absorption and salt resistance;

[0027] (2) Adding biochar to the copolymer network can effectively improve the porosity of the soil and promote the formation of soil aggregates, while also increasing the content of soil organic matter;

[0028] (3) The method of the present invention has mild reaction conditions, green and environmentally friendly materials, and can improve the physical and chemical properties of saline-alkali soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0029] Figure 1 The SEM image of the natural-synthetic copolymer saline-alkali land improver of the preparation method of the natural-synthetic copolymer saline-alkali land improver of the present invention;

[0030] Figure 2 A synchronous thermal analysis diagram of a natural-synthetic copolymer saline-alkali land improver according to a method for preparing the natural-synthetic copolymer saline-alkali land improver of the present invention;

[0031] Figure 3 The improved effect diagram of the natural-synthetic copolymer saline-alkali land improver is a preparation method of the natural-synthetic copolymer saline-alkali land improver of the present invention. DETAILED DESCRIPTION

[0032] The following will be combined with the 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.

[0033] like Figure 1-3 As shown, the present invention discloses a method for preparing a natural-synthetic copolymer saline-alkali land improver, comprising the following steps:

[0034] Step 1: Preparation of cotton straw biochar

[0035] 3-5 parts of dry cotton straw powder passed through an 80-mesh sieve and 6-10 parts of potassium bicarbonate are added to a quartz porcelain boat, placed in a tube furnace, and heated to 400-600° C. at a heating rate of 5° C. / min under argon atmosphere protection, and maintained for 2-4 hours. After natural cooling, the powder is washed with deionized water to remove ash, and dried at 60° C. to obtain cotton straw biomass charcoal for later use;

[0036] Step 2: Preparation of carboxymethyl cellulose free radical initiation system

[0037] Take 3-6 parts of carboxymethyl cellulose, add 100-200 parts of deionized water, stir at 60-80°C for 2-4 hours to dissolve it. Pass nitrogen protection, add 2-3 parts of 12-15% ammonium persulfate solution while stirring, continue the reaction for 30-60 minutes, and obtain a carboxymethyl cellulose free radical initiation system solution for standby use;

[0038] Step 3: Preparation of copolymer precursor mixed dispersion

[0039] In a sealed kettle filled with nitrogen, add 5 to 12 parts of a sodium hydroxide solution with a mass fraction of 10 to 25%, add 25 to 40 parts of acrylic acid, and stir at 0°C until the solution is clear and transparent; then add 60 to 75 parts of acrylamide, and stir at room temperature to completely dissolve the acrylamide; then add 1 to 5 parts of the cotton straw biochar prepared in the first step and 3 to 5 parts of a 1 to 3% N,N′-methylenebisacrylamide solution, and stir at room temperature for 10 to 30 minutes to uniformly disperse the biochar in the solution, so as to obtain a precursor mixed dispersion of the copolymer, which is set aside;

[0040] Step 4: Preparation of natural-synthetic copolymer saline-alkali land improver

[0041] Under nitrogen protection and at 50° C., add the copolymer precursor mixed dispersion prepared in the third step to the carboxymethyl cellulose free radical initiation system solution prepared in the second step while stirring, and continue stirring for 60 to 90 minutes to fully mix; the system temperature is raised to 65 to 80° C., and the reaction is continued with stirring for 2 to 3 hours. The product after the reaction is dried at 60° C. to constant weight, and crushed through an 80-mesh sieve to obtain a natural-synthetic copolymer saline-alkali land improver.

[0042] Example 1

[0043] Step 1: Preparation of cotton straw biochar

[0044] Take 3 parts of dry cotton straw powder passed through an 80-mesh sieve and 6 parts of potassium bicarbonate, add them into a quartz porcelain boat, put them into a tube furnace, and heat them to 400°C at a heating rate of 5°C / min under the protection of argon atmosphere, and keep them for 2 hours. After natural cooling, wash them with deionized water to remove ash, and dry them at 60°C to obtain cotton straw biochar for later use;

[0045] Step 2: Preparation of carboxymethyl cellulose free radical initiation system

[0046] Take 3 parts of carboxymethyl cellulose, add 100 parts of deionized water, stir at 60°C for 2 hours to dissolve it; pass nitrogen protection, add 2 parts of 12% ammonium persulfate solution while stirring, continue to react for 30 minutes, and obtain a carboxymethyl cellulose free radical initiation system solution for standby use;

[0047] Step 3: Preparation of copolymer precursor mixed dispersion

[0048] In a sealed kettle filled with nitrogen, add 5 parts of 10% sodium hydroxide solution by mass, add 25 parts of acrylic acid, and stir at 0°C until the solution is clear and transparent; then add 75 parts of acrylamide, and stir at room temperature to completely dissolve the acrylamide; then add 1 part of the cotton straw biochar prepared in the first step and 3 parts of 1% N,N′-methylenebisacrylamide solution by mass, and stir at room temperature for 10 minutes to evenly disperse the biochar in the solution, so as to obtain a precursor mixed dispersion of the copolymer, which is set aside;

[0049] Step 4: Preparation of natural-synthetic copolymer saline-alkali land improver

[0050] Under nitrogen protection and at 50°C, add the copolymer precursor mixed dispersion prepared in the third step to the carboxymethyl cellulose free radical initiation system solution prepared in the second step while stirring, and continue stirring for 60 minutes to mix thoroughly; the system temperature is raised to 65°C, and the reaction is continued with stirring for 2 hours. The reaction product is dried at 60°C to constant weight, crushed and passed through an 80-mesh sieve to obtain a natural-synthetic copolymer saline-alkali land improver.

[0051] Take 5 parts of natural-synthetic copolymer saline-alkali land conditioner and 1000 parts of air-dried saline-alkali soil, mix them evenly, and put them into a soil column with a filling height of 30 cm. Add deionized water. When the wetting front depth reaches 30 cm, stop adding water. After leaving the soil column for one week, measure the pH, conductivity and total amount of water-soluble salt of the blank treated and improved soil samples respectively.

[0052] Example 2

[0053] Step 1: Preparation of cotton straw biochar

[0054] Take 3 parts of dry cotton straw powder passed through an 80-mesh sieve and 7 parts of potassium bicarbonate, add them into a quartz boat, put them into a tube furnace, and heat them to 400°C at a heating rate of 5°C / min under the protection of argon atmosphere, and keep them for 2.5 hours. After natural cooling, wash them with deionized water to remove ash, and dry them at 60°C to obtain cotton straw biochar for later use;

[0055] Step 2: Preparation of carboxymethyl cellulose free radical initiation system

[0056] Take 4 parts of carboxymethyl cellulose, add 100 parts of deionized water, stir at 65°C for 3 hours to dissolve it; pass nitrogen protection, add 2 parts of 12% ammonium persulfate solution while stirring, continue to react for 45 minutes, and obtain a carboxymethyl cellulose free radical initiation system solution for standby use;

[0057] Step 3: Preparation of copolymer precursor mixed dispersion

[0058] In a sealed kettle filled with nitrogen, add 6 parts of 12% sodium hydroxide solution by mass, add 30 parts of acrylic acid, and stir at 0°C until the solution is clear and transparent; then add 70 parts of acrylamide, and stir at room temperature to completely dissolve the acrylamide; then add 2 parts of the cotton straw biochar prepared in the first step and 3 parts of 2% N,N′-methylenebisacrylamide solution by mass, and stir at room temperature for 15 minutes to evenly disperse the biochar in the solution, so as to obtain a precursor mixed dispersion of the copolymer, which is set aside;

[0059] Step 4: Preparation of natural-synthetic copolymer saline-alkali land improver

[0060] Under nitrogen protection and at 50°C, add the copolymer precursor mixed dispersion prepared in the third step to the carboxymethyl cellulose free radical initiation system solution prepared in the second step while stirring, and continue stirring for 70 minutes to mix thoroughly. The system temperature is raised to 70°C, and the reaction is continued with stirring for 2.5 hours. The product after the reaction is dried at 60°C to constant weight, and crushed through an 80-mesh sieve to obtain a natural-synthetic copolymer saline-alkali land improver.

[0061] Take 7 parts of natural-synthetic copolymer saline-alkali land conditioner and 1000 parts of air-dried saline-alkali soil, mix them evenly, and put them into a soil column with a filling height of 30 cm. Add deionized water. When the wetting front depth reaches 30 cm, stop adding water. After leaving the soil column for one week, measure the pH, conductivity and total amount of water-soluble salt of the blank treated and improved soil samples respectively.

[0062] Example 3

[0063] Step 1: Preparation of cotton straw biochar

[0064] Take 4 parts of dry cotton straw powder passed through an 80-mesh sieve and 8 parts of potassium bicarbonate, add them into a quartz porcelain boat, put them into a tube furnace, and heat them to 500°C at a heating rate of 5°C / min under the protection of argon atmosphere, and keep them for 3 hours. After natural cooling, wash them with deionized water to remove ash, and dry them at 60°C to obtain cotton straw biochar for later use;

[0065] Step 2: Preparation of carboxymethyl cellulose free radical initiation system

[0066] Take 5 parts of carboxymethyl cellulose, add 200 parts of deionized water, stir at 70°C for 3 hours to dissolve it, pass nitrogen protection, add 3 parts of 14% ammonium persulfate solution while stirring, continue to react for 45 minutes, and obtain a carboxymethyl cellulose free radical initiation system solution for standby use;

[0067] Step 3: Preparation of copolymer precursor mixed dispersion

[0068] In a sealed kettle filled with nitrogen, add 7 parts of 13% sodium hydroxide solution by mass, add 35 parts of acrylic acid, and stir at 0°C until the solution is clear and transparent; then add 65 parts of acrylamide, and stir at room temperature to completely dissolve the acrylamide; then add 3 parts of the cotton straw biochar prepared in the first step and 3.5 parts of 2% N,N′-methylenebisacrylamide solution by mass, and stir at room temperature for 20 minutes to evenly disperse the biochar in the solution, so as to obtain a precursor mixed dispersion of the copolymer, which is set aside;

[0069] Step 4: Preparation of natural-synthetic copolymer saline-alkali land improver

[0070] Under nitrogen protection and at 50°C, add the copolymer precursor mixed dispersion prepared in the third step to the carboxymethyl cellulose free radical initiation system solution prepared in the second step while stirring, and continue stirring for 80 minutes to mix thoroughly. The system temperature is raised to 70°C, and the reaction is continued with stirring for 2.5 hours. The product after the reaction is dried at 60°C to constant weight, and crushed through an 80-mesh sieve to obtain a natural-synthetic copolymer saline-alkali land improver.

[0071] Take 12 parts of natural-synthetic copolymer saline-alkali land conditioner and 1000 parts of air-dried saline-alkali soil, mix them evenly, and put them into a soil column with a filling height of 30 cm. Add deionized water. When the wetting front depth reaches 30 cm, stop adding water. After leaving the soil column for one week, measure the pH, conductivity and total amount of water-soluble salt of the blank treated and improved soil samples respectively.

[0072] Example 4

[0073] Step 1: Preparation of cotton straw biochar

[0074] Take 4 parts of dry cotton straw powder passed through an 80-mesh sieve and 8 parts of potassium bicarbonate, add them into a quartz porcelain boat, put them into a tube furnace, and heat them to 500°C at a heating rate of 5°C / min under the protection of argon atmosphere, and keep them for 3 hours. After natural cooling, wash them with deionized water to remove ash, and dry them at 60°C to obtain cotton straw biochar for later use;

[0075] Step 2: Preparation of carboxymethyl cellulose free radical initiation system

[0076] Take 5 parts of carboxymethyl cellulose, add 200 parts of deionized water, stir at 80°C for 3 hours to dissolve it, pass nitrogen protection, add 3 parts of 14% ammonium persulfate solution while stirring, continue to react for 50 minutes, and obtain a carboxymethyl cellulose free radical initiation system solution for standby use;

[0077] Step 3: Preparation of copolymer precursor mixed dispersion

[0078] In a sealed kettle filled with nitrogen, add 10 parts of 20% sodium hydroxide solution by mass, add 40 parts of acrylic acid, and stir at 0°C until the solution is clear and transparent; then add 60 parts of acrylamide, and stir at room temperature to completely dissolve the acrylamide; then add 4 parts of the cotton straw biochar prepared in the first step and 4 parts of 3% N,N′-methylenebisacrylamide solution by mass, and stir at room temperature for 30 minutes to evenly disperse the biochar in the solution, so as to obtain a precursor mixed dispersion of the copolymer, which is set aside;

[0079] Step 4: Preparation of natural-synthetic copolymer saline-alkali land improver

[0080] Under nitrogen protection and at 50°C, add the copolymer precursor mixed dispersion prepared in the third step to the carboxymethyl cellulose free radical initiation system solution prepared in the second step while stirring, and continue stirring for 90 minutes to mix thoroughly. The system temperature is raised to 75°C, and the reaction is continued with stirring for 2.5 hours. The product after the reaction is dried at 60°C to constant weight, and crushed through an 80-mesh sieve to obtain a natural-synthetic copolymer saline-alkali land improver.

[0081] Take 10 parts of natural-synthetic copolymer saline-alkali land conditioner and 1000 parts of air-dried saline-alkali soil, mix them evenly, and put them into a soil column with a filling height of 30 cm. Add deionized water. When the wetting front depth reaches 30 cm, stop adding water. After leaving the soil column for one week, measure the pH, conductivity and total amount of water-soluble salt of the blank treated and improved soil samples respectively.

[0082] Example 5

[0083] Step 1: Preparation of cotton straw biochar

[0084] Take 5 parts of dry cotton straw powder passed through an 80-mesh sieve and 10 parts of potassium bicarbonate, add them into a quartz porcelain boat, put them into a tube furnace, and heat them to 600°C at a heating rate of 5°C / min under the protection of argon atmosphere, and keep them for 4 hours. After natural cooling, wash them with deionized water to remove ash, and dry them at 60°C to obtain cotton straw biochar for later use;

[0085] Step 2: Preparation of carboxymethyl cellulose free radical initiation system

[0086] Take 6 parts of carboxymethyl cellulose, add 200 parts of deionized water, stir at 80°C for 4 hours to dissolve it, pass nitrogen protection, add 3 parts of 15% ammonium persulfate solution while stirring, continue to react for 60 minutes, and obtain a carboxymethyl cellulose free radical initiation system solution for standby use;

[0087] Step 3: Preparation of copolymer precursor mixed dispersion

[0088] In a sealed kettle filled with nitrogen, add 12 parts of 25% sodium hydroxide solution and 40 parts of acrylic acid, and stir at 0°C until the solution is clear and transparent; then add 60 parts of acrylamide, and stir at room temperature to completely dissolve the acrylamide; then add 5 parts of cotton straw biochar prepared in the first step and 5 parts of 3% N,N′-methylenebisacrylamide solution, and stir at room temperature for 30 minutes to evenly disperse the biochar in the solution, so as to obtain a precursor mixed dispersion of the copolymer, which is set aside;

[0089] Step 4: Preparation of natural-synthetic copolymer saline-alkali land improver

[0090] Under nitrogen protection and at 50°C, add the copolymer precursor mixed dispersion prepared in the third step to the carboxymethyl cellulose free radical initiation system solution prepared in the second step while stirring, and continue stirring for 90 minutes to mix thoroughly. The system temperature is raised to 80°C, and the reaction is continued with stirring for 3 hours. The product after the reaction is dried at 60°C to constant weight, and crushed through an 80-mesh sieve to obtain a natural-synthetic copolymer saline-alkali land improver.

[0091] Take 15 parts of natural-synthetic copolymer saline-alkali land conditioner and 1000 parts of air-dried saline-alkali soil, mix them evenly, and put them into a soil column with a filling height of 30 cm. Add deionized water. When the wetting front depth reaches 30 cm, stop adding water. After leaving the soil column for one week, measure the pH, conductivity and total amount of water-soluble salt of the blank treated and improved soil samples respectively.

[0092] The above description is only a preferred embodiment of the present invention and does not limit the technical scope of the present invention. Therefore, any slight modifications, equivalent changes and modifications made to the above embodiments based on the technical essence of the present invention are still within the scope of the technical solution of the present invention.

Claims

1. A method for preparing a natural-synthetic copolymer saline-alkali land improver, characterized in that: The amount of material taken "parts" is weight parts, including the following steps: Step 1: Preparation of cotton straw biochar Cotton straw powder and potassium bicarbonate were added into a quartz boat, placed into a tube furnace, pyrolyzed under the protection of argon atmosphere, naturally cooled, washed with deionized water to remove ash, and dried to obtain cotton straw biomass charcoal; Step 2: Preparation of carboxymethyl cellulose free radical initiation system Take carboxymethyl cellulose, add deionized water, and stir under heating to dissolve it; introduce nitrogen protection, add ammonium persulfate solution while stirring, and continue the reaction to obtain a carboxymethyl cellulose free radical initiation system solution; Step 3: Preparation of copolymer precursor mixed dispersion In a sealed kettle filled with nitrogen, sodium hydroxide solution and acrylic acid are added, and stirred at zero degrees until the solution is clear and transparent; then acrylamide is added, and stirred at room temperature to completely dissolve the acrylamide; then the cotton straw biochar and N,N′-methylenebisacrylamide solution are added, and stirred at room temperature to uniformly disperse the biochar in the solution, so as to obtain a precursor mixed dispersion of the copolymer; Step 4: Preparation of natural-synthetic copolymer saline-alkali land improver Under nitrogen protection and heating conditions, the copolymer precursor mixed dispersion prepared in the third step is added to the carboxymethyl cellulose free radical initiation system solution prepared in the second step while stirring, and stirring is continued to ensure that the mixture is fully and evenly mixed; then the system temperature is increased, and the reaction is continued with stirring, and the product after the reaction is dried to a constant weight and crushed to obtain a natural-synthetic copolymer saline-alkali land improver.

2. The method for preparing a natural-synthetic copolymer saline-alkali land improver according to claim 1, characterized in that: In the first step, the cotton straw powder is 3 to 5 parts of dry cotton straw powder passed through an 80-mesh sieve, and the amount of potassium bicarbonate added is 6 to 10 parts.

3. The method for preparing a natural-synthetic copolymer saline-alkali land improver according to claim 1, characterized in that: In the first step, the pyrolysis method is to increase the temperature to 400-600°C at a heating rate of 5°C / min and maintain it for 2-4h.

4. The method for preparing a natural-synthetic copolymer saline-alkali land improver according to claim 1, characterized in that: In the first step, the drying condition is 60°C.

5. The method for preparing a natural-synthetic copolymer saline-alkali land improver according to claim 1, characterized in that: In the second step, the amount of carboxymethyl cellulose added is 3 to 6 parts, the amount of deionized water added is 100 to 200 parts, the heating condition is 60 to 80° C., and the stirring time is 2 to 4 hours.

6. The method for preparing a natural-synthetic copolymer saline-alkali land improver according to claim 1, characterized in that: In the second step, the amount of ammonium persulfate solution added is 2 to 3 parts, the mass fraction is 12 to 15%, and the reaction time is 30 to 60 minutes.

7. The method for preparing a natural-synthetic copolymer saline-alkali land improver according to claim 1, characterized in that: In the third step, the amount of sodium hydroxide solution added is 5 to 12 parts, with a mass fraction of 10 to 25%; the amount of acrylic acid added is 25 to 40 parts, and the amount of acrylamide added is 60 to 75 parts.

8. The method for preparing a natural-synthetic copolymer saline-alkali land improver according to claim 1, characterized in that: In the third step, the amount of the cotton straw biochar prepared in the first step is 1-5 parts, the amount of the N,N′-methylenebisacrylamide solution is 3-5 parts, the mass fraction is 1-3%, and the stirring time at room temperature is 10-30 minutes.

9. The method for preparing a natural-synthetic copolymer saline-alkali land improver according to claim 1, characterized in that: In the fourth step, the heating condition is 50° C. and the continuous stirring time is 60 to 90 minutes.

10. The method for preparing a natural-synthetic copolymer saline-alkali land improver according to claim 1, characterized in that: In the fourth step, the system temperature is raised to 65-80° C., the stirring reaction time is continued for 2-3 hours, the drying temperature is 60° C., and the product is crushed through an 80-mesh sieve.

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