A soil conditioner for saline-alkali land and its preparation method

Abstract

This invention discloses a soil conditioner for saline-alkali land and its preparation method, belonging to the field of saline-alkali land improvement technology. Its core carriers are chitosan, β-cyclodextrin, and debranched short-chain starch, all of which are natural polysaccharide biomass materials. These can be gradually degraded by bacteria in the soil and possess good and long-lasting salt control effects, alleviating soil compaction and improving the crop growth environment. Debranched starch-modified chitosan composite microspheres are used as the main active ingredient. Through a porous structure, zero-valent iron is fixed inside, achieving stable dispersion and loading of zero-valent iron, allowing heavy metal ions to fully contact the adsorption sites. Furthermore, the hydroxyl groups of debranched starch and the amino groups of chitosan themselves have coordination adsorption capabilities, which, combined with the cavity inclusion effect of cyclodextrin, form multiple coordination adsorption effects, further enhancing the heavy metal adsorption capacity. Moreover, the branches of debranched starch form steric hindrance on the surface of zero-valent iron, which, together with the electrostatic repulsion of chitosan and the cavity isolation of cyclodextrin, prevents aggregation.

Description

Technical Field

[0001] This invention belongs to the field of saline-alkali land improvement technology, specifically a soil conditioner for saline-alkali land and its preparation method. Background Technology

[0002] Saline-alkali land is characterized by high salinity, easy compaction, poor structure, low organic matter content, and weak water and fertilizer retention capacity. This makes agricultural utilization extremely difficult during the initial reclamation phase and the process of desalination to normal soil conditions. Furthermore, improper agricultural practices lead to recurring soil salinization. Saline-alkali land is a widespread form of land degradation globally, particularly prevalent in arid and semi-arid regions. Its formation is primarily due to the accumulation of salt in the soil, which severely impacts soil structure, reduces fertility, restricts crop growth, and ultimately leads to decreased agricultural yields. The remediation of saline-alkali land is a complex and extremely challenging task, involving the improvement of soil physical properties, reduction of salinity, and enhancement of soil biological activity.

[0003] Currently, biological methods are considered the most effective and safest methods for improving saline-alkali land. By introducing salt-tolerant plants or microorganisms, the accumulation of soil organic matter and microbial diversity are promoted. This method is environmentally friendly and sustainable. However, existing biological conditioners have a slow effect, are not obvious, and are greatly limited by environmental conditions.

[0004] Chinese patent CN105001876B discloses a saline-alkali soil conditioner and its dedicated microcapsule-type organic acid sustained-release formulation. The microcapsule-type sustained-release formulation is prepared by using an acrylic polymer as the capsule wall and an organic acid as the core. However, the acrylic polymer in this scheme is usually a synthetic polymer material, which degrades relatively slowly in the natural environment. Due to the stability of its molecular structure and the lack of microbial enzymes that can effectively degrade it in the natural environment, it is easy to remain in the soil for a long time, leading to changes in soil structure and potentially having a negative impact on the soil ecosystem. Summary of the Invention

[0005] The purpose of this invention is to provide a soil conditioner for saline-alkali land and its preparation method. The core carriers are chitosan, β-cyclodextrin, and debranched short-chain starch. All three are natural polysaccharide biomass materials that can be gradually degraded by bacteria in the soil and have good and long-lasting salt control effects, alleviate soil compaction, and improve the crop growth environment.

[0006] The objective of this invention can be achieved through the following technical solutions: A method for preparing a soil conditioner for saline-alkali land includes the following steps: Step 1: After forming a water / oil (W / O) emulsion with chitosan and cyclodextrin, chitosan / cyclodextrin composite microspheres are obtained after mechanical shearing and demulsification with sodium hydroxide.

[0007] Step 2: Carboxylated chitosan composite microspheres are prepared by nucleophilic acylation of the amino groups on the surface of chitosan / cyclodextrin composite microspheres with succinic anhydride.

[0008] Step 3: After debranching the glutinous corn starch with pullulanase, a large amount of short linear glucan can be obtained. It is more likely to spontaneously form a left-handed helical structure than natural amylose, and hydrophobic cavities are generated inside, thus obtaining debranched short amylose.

[0009] Step 4: Debranched starch-modified chitosan composite microspheres are obtained by condensation esterification reaction between the hydroxyl groups of debranched short-chain starch and the carboxyl groups of carboxylated chitosan composite microspheres.

[0010] Step 5: Utilizing the unique structure of the debranched starch-modified chitosan composite microspheres, ferrous sulfate heptahydrate is introduced into the channels of the microspheres. Under the reduction of potassium borohydride, a soil conditioner for saline-alkali land is obtained. Further, the specific preparation steps for the chitosan / cyclodextrin composite microspheres are as follows: Chitosan, β-cyclodextrin, and acetic acid were mixed at a ratio of 20-22g:22-24g:4000-4200mL to obtain a mixed solution. Span-20 and Tween-20 were added to a reaction vessel and stirred for 10-12 minutes at 20-25℃ and 500-600r / min. Liquid paraffin was added and stirring was continued for 15-20 minutes. Then the mixed solution was added and stirring was continued for 30-40 minutes to form a homogeneous emulsion. A 3mol / L sodium hydroxide ethanol solution was added and stirring was continued for 15-20 minutes. The mixture was centrifuged at 6000-7000r / min for 10-12 minutes, and the supernatant was discarded. The precipitate was washed 2-4 times with a 50-60% ethanol solution and deionized water, respectively, to obtain chitosan / cyclodextrin composite microspheres.

[0011] Furthermore, the ratio of Span-20, Tween-20, liquid paraffin, mixed solution, and sodium hydroxide ethanol solution is 19-20g: 1-2g: 2-3L: 400-500mL: 60-70mL.

[0012] Furthermore, the specific preparation steps of carboxylated chitosan composite microspheres are as follows: Chitosan composite microspheres, deionized water, and calcium chloride were added to a reaction vessel and stirred for 40-50 minutes at 20-25℃ and 500-600 r / min. The mixture was then filtered, vacuum dried at 60-70℃ for 1-2 hours, and ground to obtain carboxylated chitosan composite microspheres.

[0013] Furthermore, the ratio of chitosan composite microspheres, deionized water, and calcium chloride is 30-35g: 600-700mL: 30-32g.

[0014] Furthermore, the specific preparation steps of chitosan composite microspheres are as follows: Chitosan / cyclodextrin composite microspheres and dimethyl sulfoxide were added to a reaction vessel and stirred for 10-12 min at 20-25℃ and 500-600 r / min. Then, the mixture was heated to 65-70℃, succinic anhydride was added, and stirring was continued for 12-14 h. The mixture was then filtered, and the product was immersed in ethanol for 1-2 h. The pH value was then adjusted to 9-10 with 2 mol / L NaOH solution, filtered, and vacuum dried at 60-70℃ for 1-2 h to obtain chitosan composite microspheres.

[0015] Furthermore, the ratio of chitosan / cyclodextrin composite microspheres, dimethyl sulfoxide, and succinic anhydride is 40-45g: 800-900mL: 10-12g.

[0016] Furthermore, the specific preparation steps for debranched short-chain starch are as follows: Add glutinous corn starch and deionized water to a reaction vessel and stir for 10-12 minutes at 20-25℃ and 500-600 rpm. Then heat to 80-85℃ and continue stirring for 30-32 minutes to completely gelatinize the glutinous corn starch. After natural cooling, adjust the pH to 5-6 with phosphate buffer, add 16-18 NPUN / g pullulanase, heat to 58-60℃, and enzymatically hydrolyze for 24-26 hours. Heat the hydrolyzed glutinous corn starch to 90-95℃ and keep it at that temperature for 15-17 minutes. Centrifuge at 8000-9000 rpm for 3-5 minutes, retain the supernatant, precipitate with anhydrous ethanol, and freeze-dry at -20℃ for 1-2 hours to obtain debranched short amylose with a debranching degree of 90-95%.

[0017] Furthermore, the ratio of glutinous corn starch to deionized water is 13-14g: 400-410mL.

[0018] Furthermore, the specific preparation steps of the debranched starch-modified chitosan composite microspheres are as follows: Carboxylated chitosan composite microspheres, debranched short-chain starch, and toluene were added to a reaction vessel and stirred for 10-12 min at 20-25℃ and 500-600 r / min. Then, 4-dimethylaminopyridine and triethylamine were added, and the mixture was heated to 90-95℃ under nitrogen protection and stirred for 12-14 h. After naturally cooling to room temperature, anhydrous diethyl ether was added to precipitate the polymer. The precipitate was washed 2-4 times with anhydrous diethyl ether and deionized water, respectively, and then freeze-dried to obtain debranched starch-modified chitosan composite microspheres.

[0019] Furthermore, the ratio of carboxylated chitosan composite microspheres, debranched short-chain starch, toluene, 4-dimethylaminopyridine and triethylamine is 15-17g: 16-18g: 300-400mL: 0.3-0.4g: 0.2-0.4mL.

[0020] Furthermore, the specific preparation steps for soil conditioners used in saline-alkali land are as follows: Debranched starch-modified chitosan composite microspheres, N,N-dimethylformamide, deionized water, and anhydrous ethanol were added to a reaction vessel and stirred for 20-30 minutes at 20-25℃ and 500-600 r / min. Then, ferrous sulfate heptahydrate was added, and stirring was continued for another 20-30 minutes. Next, a 50-60% potassium borohydride solution was added, and the reaction was continued for 4-5 hours. The mixture was then filtered, and the filter cake was washed 2-4 times with deionized water and anhydrous ethanol, respectively. The cake was then vacuum dried at 60-70℃ for 2-5 hours to obtain a soil conditioner for saline-alkali land.

[0021] Furthermore, the ratio of debranched starch-modified chitosan composite microspheres, N,N-dimethylformamide, deionized water, anhydrous ethanol, ferrous sulfate heptahydrate, and potassium borohydride solution is 10-14g: 200-300mL: 200-300mL: 100-120mL: 0.9-1.0g: 1-2g.

[0022] The beneficial effects of this invention are: 1. The core carrier of the soil conditioner for saline-alkali land prepared in this invention is chitosan, β-cyclodextrin, and debranched short-chain starch. All three are natural polysaccharide biomass materials that can be gradually degraded by bacteria in the soil and have good and long-lasting salt control effects, alleviate soil compaction, and improve the crop growth environment.

[0023] 2. The soil conditioner for saline-alkali land of the present invention uses debranched starch-modified chitosan composite microspheres as the main active ingredient. Through its special porous structure, zero-valent iron is fixed inside, achieving stable dispersion and loading of zero-valent iron. This allows heavy metal ions to fully contact the adsorption sites. Furthermore, the hydroxyl groups of debranched starch and the amino groups of chitosan themselves have coordination adsorption capabilities. Combined with the cavity inclusion effect of cyclodextrin, multiple coordination adsorption effects are formed, further enhancing the heavy metal adsorption capacity. Moreover, the branches of debranched starch form steric hindrance on the surface of zero-valent iron. Combined with the electrostatic repulsion of chitosan and the cavity isolation of cyclodextrin, multiple fixation effects are formed, resulting in better dispersion of zero-valent iron within the microspheres and preventing aggregation. The hydrophilic hydroxyl groups of debranched starch form a thicker hydration film on the surface of the microspheres, which can more effectively isolate oxidizing substances in saline-alkali land and delay the oxidative deactivation of zero-valent iron.

[0024] 3. The debranched starch-modified chitosan composite microspheres of the present invention have a hybrid framework that is more stable than the hydrogen-bonded framework of pure chitosan. The branches of debranched starch can form an intermolecular cross-linking network, so that the debranched starch-modified chitosan composite microspheres will not disintegrate or be lost under the high salinity leaching of saline-alkali land, and can be fixed on the surface of soil particles for a long time, thereby ensuring that the action sites of zero-valent iron do not shift.

[0025] 4. The debranched starch-modified chitosan composite microspheres of the present invention are pH-responsive. In locally high pH soil, the hydroxyl groups of debranched starch form hydrogen bonds with the amino groups of chitosan, resulting in a denser skeleton and a low swelling rate of the microspheres, which firmly seals in zero-valent iron. When the local pH decreases, the chitosan threshold is reached, and the chitosan protonates, breaking the hydrogen bonds. The hydroxyl groups of debranched starch absorb water and swell, driving the microspheres to swell in a gradient. The pores open slowly, achieving a gradient slow release of zero-valent iron and avoiding ineffective consumption caused by a one-time release of zero-valent iron. Attached Figure Description

[0026] Figure 1 This is a SEM image of the soil conditioner used in Example 3 for saline-alkali land. Detailed Implementation

[0027] The technical solutions of the present invention will be clearly and completely described below with reference to the embodiments of the present invention. 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 of ordinary skill in the art without creative effort are within the scope of protection of the present invention.

[0028] Example 1: A method for preparing a soil conditioner for saline-alkali land, comprising the following steps: S1: Mix 20g chitosan, 22g β-cyclodextrin and 4000mL acetic acid to obtain a mixed solution; add 19g Span-20 and 1g Tween-20 to the reactor and stir for 10min at 20℃ and 500r / min. Add 2L liquid paraffin and continue stirring for 15min. Then add 400mL of the mixed solution and continue stirring for 30min to form a homogeneous emulsion. Add 60mL of 3mol / L sodium hydroxide ethanol solution and continue stirring for 15min. Centrifuge at 6000r / min for 10min, discard the supernatant, and wash the precipitate twice with 50% ethanol solution and deionized water, respectively, to obtain chitosan / cyclodextrin composite microspheres.

[0029] S2: Add 40g of chitosan / cyclodextrin composite microspheres and 800mL of dimethyl sulfoxide to a reaction vessel, stir for 10min at 20℃ and 500r / min, then heat to 65℃, add 10g of succinic anhydride, continue stirring for 12h, filter, immerse the product in ethanol for 1h, then adjust the pH to 9 with 2mol / L NaOH solution, filter, and vacuum dry at 60℃ for 1h to obtain chitosan composite microspheres; Add 30g of chitosan composite microspheres, 600mL of deionized water and 30g of calcium chloride to a reaction vessel, stir for 40min at 20℃ and 500r / min, filter, vacuum dry at 60℃ for 1h, grind to obtain carboxylated chitosan composite microspheres.

[0030] S3: Add 13g of glutinous corn starch and 400mL of deionized water to the reactor, stir for 10min at 20℃ and 500r / min, then heat to 80℃ and continue stirring for 30min to completely gelatinize the glutinous corn starch. After natural cooling, adjust the pH to 5 with phosphate buffer, then add 16NPUN / g pullulanase, heat to 58℃ and enzymatically hydrolyze for 24h. Heat the enzymatically hydrolyzed glutinous corn starch to 90℃, keep warm for 15min, centrifuge at 8000r / min for 3min, retain the supernatant, precipitate with anhydrous ethanol, and freeze-dry at -20℃ for 1h to obtain debranched short amylose with a debranching degree of 90%.

[0031] S4: 15g of carboxylated chitosan composite microspheres, 16g of debranched short-chain starch and 300mL of toluene were added to a reaction vessel and stirred for 10min at 20℃ and 500r / min. Then, 0.3g of 4-dimethylaminopyridine and 0.2mL of triethylamine were added. Under nitrogen protection, the mixture was heated to 90℃ and stirred for 12h. After naturally cooling to room temperature, anhydrous diethyl ether was added to precipitate the polymer. The precipitate was washed twice with anhydrous diethyl ether and deionized water, respectively, and then freeze-dried to obtain debranched starch-modified chitosan composite microspheres.

[0032] S5: Add 10g of debranched starch-modified chitosan composite microspheres, 200mL of N,N-dimethylformamide, 200mL of deionized water and 100mL of anhydrous ethanol to a reaction vessel, stir for 20min at 20℃ and 500r / min, then add 0.9g of ferrous sulfate heptahydrate, continue stirring for 20min, then add 1g of 50% potassium borohydride solution, continue the reaction for 4h, filter, wash the filter cake twice with deionized water and anhydrous ethanol respectively, and vacuum dry at 60℃ for 2h to obtain a soil conditioner for saline-alkali land.

[0033] Example 2: A method for preparing a soil conditioner for saline-alkali land, comprising the following steps: S1: 21g chitosan, 23g β-cyclodextrin and 4100mL acetic acid were stirred and mixed to obtain a mixed solution; 19.5g Span-20 and 1.5g Tween-20 were added to the reaction vessel and stirred for 11min at 22.5℃ and 550r / min. 2.5L liquid paraffin was added and stirred for another 17.5min. Then 450mL of the mixed solution was added and stirred for another 35min to form a homogeneous emulsion. 65mL of 3mol / L sodium hydroxide ethanol solution was added and stirred for another 17.5min. The mixture was centrifuged at 6500r / min for 11min, the supernatant was discarded, and the precipitate was washed three times with 55% ethanol solution and deionized water, respectively, to obtain chitosan / cyclodextrin composite microspheres.

[0034] S2: 42.5 g of chitosan / cyclodextrin composite microspheres and 850 mL of dimethyl sulfoxide were added to a reaction vessel and stirred for 11 min at 22.5 °C and 550 r / min. Then, the mixture was heated to 67.5 °C, 11 g of succinic anhydride was added, and stirring was continued for 13 h. The mixture was then filtered, and the product was immersed in ethanol for 1.5 h. The pH value was then adjusted to 9.5 with 2 mol / L NaOH solution, filtered, and vacuum dried at 65 °C for 1.5 h to obtain chitosan composite microspheres. 32.5 g of chitosan composite microspheres, 650 mL of deionized water, and 31 g of calcium chloride were added to a reaction vessel and stirred for 45 min at 22.5 °C and 550 r / min. The mixture was then filtered, vacuum dried at 65 °C for 1.5 h, and ground to obtain carboxylated chitosan composite microspheres.

[0035] S3: Add 13.5g of glutinous corn starch and 405mL of deionized water to the reactor. Stir for 11min at 22.5℃ and 550r / min. Then heat to 82.5℃ and continue stirring for 31min to completely gelatinize the glutinous corn starch. After natural cooling, adjust the pH to 5.5 with phosphate buffer. Add 17NPUN / g pullulanase and heat to 59℃ for 25h of enzymatic hydrolysis. Heat the hydrolyzed glutinous corn starch to 92.5℃ and incubate for 16min. Centrifuge at 8500r / min for 4min. Retain the supernatant and precipitate with anhydrous ethanol. Freeze-dry at -20℃ for 1.5h to obtain debranched short amylose with a debranching degree of 92.5%.

[0036] S4: 16g of carboxylated chitosan composite microspheres, 17g of debranched short-chain starch and 350mL of toluene were added to a reaction vessel and stirred for 11min at 22.5℃ and 550r / min. Then, 0.35g of 4-dimethylaminopyridine and 0.3mL of triethylamine were added. Under nitrogen protection, the mixture was heated to 92.5℃ and stirred for 13h. After naturally cooling to room temperature, anhydrous diethyl ether was added to precipitate the polymer. The precipitate was washed three times with anhydrous diethyl ether and deionized water, respectively, and then freeze-dried to obtain debranched starch-modified chitosan composite microspheres.

[0037] S5: 12g of debranched starch-modified chitosan composite microspheres, 250mL of N,N-dimethylformamide, 250mL of deionized water and 110mL of anhydrous ethanol were added to a reaction vessel and stirred for 25min at 22.5℃ and 550r / min. Then, 0.95g of ferrous sulfate heptahydrate was added and stirring was continued for 25min. Then, 1.5g of 55% potassium borohydride solution was added and the reaction was continued for 4.5h. The mixture was filtered and the filter cake was washed three times with deionized water and three times with anhydrous ethanol. The mixture was then vacuum dried at 65℃ for 3.5h to obtain a soil conditioner for saline-alkali land.

[0038] Example 3: A method for preparing a soil conditioner for saline-alkali land, comprising the following steps: S1: Mix 22g chitosan, 24g β-cyclodextrin and 4200mL acetic acid to obtain a mixed solution; add 20g Span-20 and 2g Tween-20 to the reactor and stir for 12min at 25℃ and 600r / min. Add 3L liquid paraffin and continue stirring for 20min. Then add 500mL of the mixed solution and continue stirring for 40min to form a homogeneous emulsion. Add 70mL of 3mol / L sodium hydroxide ethanol solution and continue stirring for 20min. Centrifuge at 7000r / min for 12min, discard the supernatant, and wash the precipitate four times with 60% ethanol solution and deionized water, respectively, to obtain chitosan / cyclodextrin composite microspheres.

[0039] S2: 45g of chitosan / cyclodextrin composite microspheres and 900mL of dimethyl sulfoxide were added to a reaction vessel and stirred for 12min at 25℃ and 600r / min. Then, the mixture was heated to 70℃, 12g of succinic anhydride was added, and stirring was continued for 14h. The mixture was filtered, and the product was immersed in ethanol for 2h. The pH value was then adjusted to 10 with 2mol / L NaOH solution. The mixture was filtered again and vacuum dried at 70℃ for 2h to obtain chitosan composite microspheres. 35g of chitosan composite microspheres, 700mL of deionized water and 32g of calcium chloride were added to a reaction vessel and stirred for 50min at 25℃ and 600r / min. The mixture was filtered again, vacuum dried at 70℃ for 2h, and ground to obtain carboxylated chitosan composite microspheres.

[0040] S3: Add 14g of glutinous corn starch and 410mL of deionized water to the reactor, stir for 12min at 25℃ and 600r / min, then heat to 85℃ and continue stirring for 32min to completely gelatinize the glutinous corn starch. After natural cooling, adjust the pH to 6 with phosphate buffer, then add 18NPUN / g pullulanase, heat to 60℃ and enzymatically hydrolyze for 26h. Heat the hydrolyzed glutinous corn starch to 95℃, keep warm for 17min, centrifuge at 9000r / min for 5min, retain the supernatant, precipitate with anhydrous ethanol, and freeze-dry at -20℃ for 2h to obtain debranched short amylose with a debranching degree of 95%.

[0041] S4: 17g of carboxylated chitosan composite microspheres, 18g of debranched short-chain starch and 400mL of toluene were added to a reaction vessel and stirred for 12min at 25℃ and 600r / min. Then, 0.4g of 4-dimethylaminopyridine and 0.4mL of triethylamine were added. Under nitrogen protection, the mixture was heated to 95℃ and stirred for 14h. After naturally cooling to room temperature, anhydrous diethyl ether was added to precipitate the polymer. The precipitate was washed four times with anhydrous diethyl ether and deionized water, respectively, and then freeze-dried to obtain debranched starch-modified chitosan composite microspheres.

[0042] S5: 14g of debranched starch-modified chitosan composite microspheres, 300mL of N,N-dimethylformamide, 300mL of deionized water and 120mL of anhydrous ethanol were added to a reaction vessel and stirred for 30min at 25℃ and 600r / min. Then, 1.0g of ferrous sulfate heptahydrate was added and stirring was continued for 30min. Then, 2g of 60% potassium borohydride solution was added and the reaction was continued for 5h. The mixture was filtered and the filter cake was washed 4 times with deionized water and anhydrous ethanol, respectively. The mixture was then vacuum dried at 70℃ for 5h to obtain a soil conditioner for saline-alkali land.

[0043] Comparative Example 1: Based on Example 3, chitosan in step S1 was omitted.

[0044] Comparative Example 2: Based on Example 3, the debranched short-chain starch in step S4 was replaced with the raw material glutinous corn starch in step S3.

[0045] Comparative Example 3: Based on Example 3, the debranched starch-modified chitosan composite microspheres prepared in step S4 were directly used as a soil conditioner for saline-alkali land.

[0046] The performance of a soil conditioner for improving saline-alkali soil prepared in Examples 1-3 and Comparative Examples 1-3 was tested. The test site was a saline-alkali area in northeastern Suzhou City, Anhui Province. Two weeks before corn planting, the topsoil of the planting area was deep-tilled using a deep-tilling machine to a depth of 23 cm, repeated 3 times. The soil conditioner was applied at a rate of 2 kg / mu (approximately 0.067 hectares), followed by one reclamation at a depth of 30 cm. The physicochemical properties of the soil were tested before corn planting and at corn harvest. Twenty samples were taken from each test field, and the average values ​​of each physicochemical property were calculated. The content of heavy metals (cadmium) in the soil was also measured. The results are shown in Table 1. Table 1

[0047] As shown in Table 1, in Comparative Example 1, the microspheres without chitosan-formed stable hybrid frameworks easily disintegrate and are lost under high-salt leaching in saline-alkali soil, failing to adhere to soil particles for long periods, resulting in rapid loss of zero-valent iron and functional components. The lack of chitosan amino group coordination adsorption prevents the formation of multiple adsorptions with debranched starch and cyclodextrin, significantly weakening the removal capacity of heavy metal cadmium. Without the electrostatic repulsion of chitosan, zero-valent iron easily aggregates, and without the hydration film protection provided by chitosan, it is rapidly oxidized and deactivated. Without the protonation swelling regulation of chitosan, the gradual release of zero-valent iron cannot be achieved, resulting in extremely poor effects in controlling salt, reducing alkali, and improving soil compaction.

[0048] In Comparative Example 2, the skeleton strength was insufficient, the glutinous corn starch was not debranched, and it could not form an intermolecular cross-linking network. The hydrogen bond skeleton of the microspheres was fragile and easily swelled and broke under high salt conditions. The unbranched starch could not form effective steric hindrance on the surface of zero-valent iron, and zero-valent iron was easy to agglomerate. Moreover, the hydrophilic hydroxyl groups were not exposed, the hydration film was thin, and it could not isolate oxidizing substances in saline-alkali soil. Zero-valent iron was easily oxidized and deactivated. The synergistic effect of coordination adsorption and cyclodextrin inclusion was weakened, and the effect of controlling salt, reducing heavy metals, and improving soil porosity and organic matter was far lower than that of the Example.

[0049] In Comparative Example 3, the lack of zero-valent iron (ZFI) for the reduction and fixation of heavy metal ions, relying solely on the physical adsorption of microspheres, resulted in a significant reduction in cadmium removal efficiency. Without ZFI, soil structure regulation could not effectively alleviate soil compaction, improve porosity and water holding capacity, and the improvement in soil pH and salinity was minimal. Without ZFI loading, the core function of pH-responsive gradient release was lost, making it impossible to achieve long-term and stable soil improvement in saline-alkali land.

[0050] 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 variations can be made to these embodiments without departing from the principles and spirit of the invention.

Claims

1. A method for preparing a soil conditioner for saline-alkali land, characterized in that, Includes the following steps: Step 1: After forming a water / oil (W / O) emulsion with chitosan and cyclodextrin, chitosan / cyclodextrin composite microspheres are obtained after mechanical shearing and demulsification with sodium hydroxide. Step 2: Carboxylated chitosan composite microspheres were prepared by nucleophilic acylation of the amino groups on the surface of chitosan / cyclodextrin composite microspheres with succinic anhydride. Step 3: After debranching the glutinous corn starch with pullulanase, a large amount of short linear dextran can be obtained. It is more likely to spontaneously form a left-handed helical structure than natural amylose, and hydrophobic cavities are generated inside, thus obtaining debranched short amylose. Step 4: Debranched starch-modified chitosan composite microspheres are obtained by condensation esterification reaction between the hydroxyl groups of debranched short-chain starch and the carboxyl groups of carboxylated chitosan composite microspheres. Step 5: Through the special structure of debranched starch-modified chitosan composite microspheres, ferrous sulfate heptahydrate is introduced into the channels of the debranched starch-modified chitosan composite microspheres. Under the reduction of potassium borohydride, a soil conditioner for saline-alkali land is obtained.

2. The method for preparing a soil conditioner for saline-alkali land according to claim 1, characterized in that, The specific preparation steps for the chitosan / cyclodextrin composite microspheres are as follows: Chitosan, β-cyclodextrin, and acetic acid were mixed at a ratio of 20-22g:22-24g:4000-4200mL to obtain a mixed solution. Span-20 and Tween-20 were added to a reaction vessel and stirred for 10-12 minutes at 20-25℃ and 500-600r / min. Liquid paraffin was added and stirring was continued for 15-20 minutes. Then the mixed solution was added and stirring was continued for 30-40 minutes to form a homogeneous emulsion. A 3mol / L sodium hydroxide ethanol solution was added and stirring was continued for 15-20 minutes. The mixture was centrifuged at 6000-7000r / min for 10-12 minutes, and the supernatant was discarded. The precipitate was washed 2-4 times with a 50-60% ethanol solution and deionized water, respectively, to obtain chitosan / cyclodextrin composite microspheres.

3. The method for preparing a soil conditioner for saline-alkali land according to claim 2, characterized in that, The ratio of Span-20, Tween-20, liquid paraffin, mixed solution, and sodium hydroxide ethanol solution is 19-20g: 1-2g: 2-3L: 400-500mL: 60-70mL.

4. The method for preparing a soil conditioner for saline-alkali land according to claim 1, characterized in that, The specific preparation steps for the carboxylated chitosan composite microspheres are as follows: Chitosan composite microspheres, deionized water and calcium chloride were added to a reaction vessel and stirred for 40-50 min at 20-25℃ and 500-600 r / min. The mixture was then filtered, dried under vacuum at 60-70℃ for 1-2 h, and ground to obtain carboxylated chitosan composite microspheres. The ratio of chitosan composite microspheres, deionized water, and calcium chloride is 30-35g: 600-700mL: 30-32g.

5. A method for preparing a soil conditioner for saline-alkali land according to claim 1, characterized in that, The specific preparation steps of the chitosan composite microspheres are as follows: Chitosan / cyclodextrin composite microspheres and dimethyl sulfoxide were added to a reaction vessel and stirred for 10-12 min at 20-25℃ and 500-600 r / min. Then the mixture was heated to 65-70℃, succinic anhydride was added, and stirring was continued for 12-14 h. The mixture was then filtered, and the product was immersed in ethanol for 1-2 h. The pH value was then adjusted to 9-10 with 2 mol / L NaOH solution. The mixture was then filtered and vacuum dried at 60-70℃ for 1-2 h to obtain chitosan composite microspheres. The ratio of chitosan / cyclodextrin composite microspheres, dimethyl sulfoxide, and succinic anhydride is 40-45g: 800-900mL: 10-12g.

6. A method for preparing a soil conditioner for saline-alkali land according to claim 1, characterized in that, The specific preparation steps for the debranched short amylose are as follows: Add glutinous corn starch and deionized water to a reaction vessel and stir for 10-12 minutes at 20-25℃ and 500-600 rpm. Then heat to 80-85℃ and continue stirring for 30-32 minutes to completely gelatinize the glutinous corn starch. After natural cooling, adjust the pH to 5-6 with phosphate buffer, add 16-18 NPUN / g pullulanase, heat to 58-60℃, and enzymatically hydrolyze for 24-26 hours. Heat the hydrolyzed glutinous corn starch to 90-95℃ and keep it at that temperature for 15-17 minutes. Centrifuge at 8000-9000 rpm for 3-5 minutes, retain the supernatant, precipitate with anhydrous ethanol, and freeze-dry at -20℃ for 1-2 hours to obtain debranched short amylose with a debranching degree of 90-95%. The ratio of glutinous corn starch to deionized water is 13-14g: 400-410mL.

7. A method for preparing a soil conditioner for saline-alkali land according to claim 1, characterized in that, The specific preparation steps of the debranched starch-modified chitosan composite microspheres are as follows: Carboxylated chitosan composite microspheres, debranched short-chain starch, and toluene were added to a reaction vessel and stirred for 10-12 min at 20-25℃ and 500-600 r / min. Then, 4-dimethylaminopyridine and triethylamine were added, and the mixture was heated to 90-95℃ under nitrogen protection and stirred for 12-14 h. After naturally cooling to room temperature, anhydrous diethyl ether was added to precipitate the polymer. The precipitate was washed 2-4 times with anhydrous diethyl ether and deionized water, respectively, and then freeze-dried to obtain debranched starch-modified chitosan composite microspheres. The ratio of carboxylated chitosan composite microspheres, debranched short-chain starch, toluene, 4-dimethylaminopyridine and triethylamine is 15-17g: 16-18g: 300-400mL: 0.3-0.4g: 0.2-0.4mL.

8. A method for preparing a soil conditioner for saline-alkali land according to claim 1, characterized in that, The specific preparation steps of the soil conditioner for saline-alkali land are as follows: Debranched starch-modified chitosan composite microspheres, N,N-dimethylformamide, deionized water, and anhydrous ethanol were added to a reaction vessel and stirred for 20-30 minutes at 20-25℃ and 500-600 r / min. Then, ferrous sulfate heptahydrate was added, and stirring was continued for another 20-30 minutes. Next, a 50-60% potassium borohydride solution was added, and the reaction was continued for 4-5 hours. The mixture was then filtered, and the filter cake was washed 2-4 times with deionized water and anhydrous ethanol, respectively. The cake was then vacuum dried at 60-70℃ for 2-5 hours to obtain a soil conditioner for saline-alkali land.

9. A method for preparing a soil conditioner for saline-alkali land according to claim 8, characterized in that, The ratio of the debranched starch-modified chitosan composite microspheres, N,N-dimethylformamide, deionized water, anhydrous ethanol, ferrous sulfate heptahydrate, and potassium borohydride solution is 10-14g: 200-300mL: 200-300mL: 100-120mL: 0.9-1.0g: 1-2g.

10. A soil conditioner for saline-alkali land, characterized in that, It is prepared by the preparation method described in any one of claims 1-9.

Images