A passivation culture medium for reducing cadmium in rice and its preparation method

Abstract

This invention relates to the field of agricultural environmental remediation and plant nutrition regulation technology, specifically to a passivation culture medium for reducing cadmium in rice and its preparation method. The culture medium consists of ferric citrate complex solution, sodium polyaspartate graft, potassium dihydrogen phosphate, choline-stabilized orthosilicate mother liquor, and zinc disodium ethylenediaminetetraacetate hydrate. During preparation, a stable system is synergistically constructed through multi-coordination complexation, electrostatic regulation, and organic framework stabilization mechanisms. This culture medium maintains good dispersibility and stability under acidic and hard water conditions, forming a weakly acidic to neutral buffer zone in the rice rhizosphere, effectively reducing the migration and absorption of cadmium ions while maintaining the availability of nutrients such as iron and zinc. This significantly reduces the cadmium content in rice grains, improves crop nutritional quality and environmental adaptability, and is suitable for the efficient and safe remediation of cadmium-contaminated paddy fields.

Description

Technical Field

[0001] This invention relates to the field of agricultural environmental remediation and plant nutrition regulation technology, and in particular to a passivation culture medium for reducing cadmium in rice and its preparation method. Background Technology

[0002] Cadmium (Cd) pollution in rice is particularly prominent in acidic rice-growing areas of southern my country. High soil acidity causes cadmium to primarily exist in exchangeable or carbonate-bound forms, making it easily absorbed by plants. To reduce cadmium accumulation in rice, current research primarily employs passivating culture media, soil conditioners, or foliar sprays to achieve chemical fixation or inhibition of cadmium bioavailability. Phosphate and silicate passivating agents are widely used, utilizing co-precipitation reactions to generate insoluble salts, thereby reducing migratable cadmium. However, phosphate and single-silicon source systems have several limitations in practical applications.

[0003] First, under hard water and acidic conditions, phosphate ions react with Ca... 2+ Mg 2+ Precipitates such as Ca3(PO4)2 and MgHPO4 are easily formed, leading to turbidity, severe sedimentation, and reduced nutrient availability. In silicate systems, orthosilicic acid rapidly polymerizes at pH > 7 or under high electrolyte strength, forming SiO2·nH2O gel, which reduces silicon activity and makes it difficult for crops to absorb. This is particularly evident in hard water areas (hardness > 250 mg / L based on calcium carbonate), where silicates react with calcium and magnesium ions and precipitate out polysilicic acid within a short time, causing nozzle clogging and sedimentation problems after field spraying, significantly reducing field operability and efficacy stability.

[0004] Secondly, existing phosphate or single-silicon-source passivation solutions rely solely on simple co-precipitation or physical adsorption to immobilize cadmium, exhibiting poor selectivity for essential elements such as Fe and Zn. This often reduces the availability of cadmium while also decreasing the bioavailability of trace elements like iron and zinc. Particularly in iron-deficient paddy fields, insufficient Fe supply induces plants to upregulate metal transmembrane transporters, leading to enhanced competitive cadmium absorption and a negative feedback effect. Therefore, simple chemical passivation cannot simultaneously achieve the dual goals of cadmium suppression and iron / zinc retention.

[0005] Third, traditional passivation solution systems lack interfacial compatibility design. The pH windows of phosphates and silicon sources differ significantly, making them difficult to integrate within the same system. While organic ligands (such as citrates and amino acid salts) can improve solubility, they readily react with Ca under hard water conditions. 2+ Competitive complexation leads to system instability. Most existing products are single-component or two-component mixtures, failing to consider constructing multi-coordination networks at the molecular level that involve both electrical matching, complexation, and precipitation. Therefore, the system is prone to precipitation or inactivation during storage and application, resulting in unstable field effects.

[0006] Fourth, from the perspective of rhizosphere processes, phosphate-based passivation only temporarily increases soil pH and forms precipitates; with changes in the redox environment, Cd is easily re-dissolved. Silicate-based passivation mainly relies on root surface silicification or downregulation of translocation, but single silicon sources lack stability in acidic paddy fields, making it difficult to form a sustained silicification barrier. Silicate-based passivators lacking organic framework support cannot maintain a long-term presence in the rhizosphere and do not possess the ability to effectively control Cd. 2+ High affinity complexing ability.

[0007] Furthermore, traditional systems neglect the crucial factor of rhizosphere microenvironment dynamics. Root respiration and microbial metabolism cause local pH fluctuations and redox cycles, which a single inorganic passivation system struggles to cope with, easily leading to short-term effectiveness followed by failure. Due to the lack of organic components with electrochemical regulation and coordination buffering capabilities, the system has poor tolerance to environmental disturbances, and Cd may still be reactivated and migrate towards the grains in the later stages of growth.

[0008] Finally, existing technologies also suffer from poor compatibility with fertilizers and pesticides. Most phosphate or silicate passivating agents immediately produce white precipitates and increased turbidity when mixed with fertilizers under hard water conditions, failing to meet the dispersion requirements for field spraying or drip irrigation, thus greatly limiting their widespread application. The lack of intermediate components with electrical compatibility and colloidal stability makes it difficult for the system to maintain uniform dispersion and sustained effectiveness in acidic, hard water, and multi-component environments.

[0009] Therefore, there is an urgent need for a composite passivation culture medium that can remain stable under acidic and hard water conditions, and has the ability to achieve multi-coordination synergy and interfacial electrical regulation, so as to achieve highly selective cadmium fixation while taking into account crop nutrition and environmental adaptability. Summary of the Invention

[0010] In view of this, the purpose of this invention is to propose a passivation culture medium for reducing cadmium in rice and its preparation method, so as to solve the problems that existing phosphate or single silicon source passivation solutions are prone to precipitation under hard water / acidic conditions, and that simple co-precipitation alone leads to poor Cd selectivity and negative effects on Fe / Zn nutrients.

[0011] To achieve the above objectives, the present invention provides a method for preparing a passivation culture medium for reducing cadmium in rice, comprising the following steps:

[0012] Step 1: Preparation of sodium polyaspartate grafts: Dissolve sodium poly-(α,β)-DL-aspartate in deionized water, adjust the pH to 5.8, add N-hydroxysuccinimide and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and react for 10-20 min, then add cysteine ​​hydrochloride and 3-dimethylaminopropylamine sequentially and stir for 3-5 h, add iodomethane and stir for 1.5-2.5 h, and obtain sodium polyaspartate grafts by dialysis and vacuum drying;

[0013] Step 2: Prepare choline-stable orthosilicic acid mother liquor. Mix glycerol and deionized water, add choline chloride to dissolve, add hydrochloric acid dropwise, and add sodium silicate aqueous solution dropwise while stirring continuously. During the dropwise addition, use hydrochloric acid to maintain the pH in the range of 2.0-2.5. Stir for 20-40 minutes to obtain choline-stable orthosilicic acid mother liquor.

[0014] Step 3: Prepare ferric citrate complex solution. Dissolve citric acid monohydrate in deionized water, add ferric chloride hexahydrate, adjust the pH to 4.0 with sodium hydroxide solution, and stir for 20-40 minutes to obtain ferric citrate complex solution.

[0015] Step 4: Preparation of passivation culture medium. Add ferric citrate complex solution, sodium polyaspartate graft solution, potassium dihydrogen phosphate, choline-stabilized orthosilicate mother liquor, and zinc disodium ethylenediaminetetraacetate hydrate to deionized water in sequence, while controlling the pH in the range of 5.6-6.2 to obtain the passivation culture medium.

[0016] Preferably, the weight-average molecular weight of the poly-(α,β)-DL-aspartic acid sodium salt in step 1 is 5000.

[0017] Preferably, in step 1, the amount of N-hydroxysuccinimide is 900-1260 mg, and the amount of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride is 1500-2100 mg.

[0018] Preferably, the amount of cysteine ​​hydrochloride used in step 1 is 350-520 mg, the amount of 3-dimethylaminopropylamine is 400-580 mg, and the amount of iodomethane is 700-1100 mg.

[0019] Preferably, in step 1, a dialysis bag with a molecular weight cutoff of 3.5 kDa is used for dialysis, and the dialysis water is changed 3 times during the 24-hour dialysis period, with an interval of 8 hours between each change.

[0020] Preferably, in step 2, the amount of choline chloride used is 8-12g, the concentration of hydrochloric acid is 37wt% and the amount used is 2.5-3.5g, and the solid content of the sodium silicate aqueous solution is 40%, the modulus is 3.4 and the amount used is 8-12g.

[0021] Preferably, the amount of citric acid monohydrate used in step 3 is 850-1250 mg, and the amount of ferric chloride hexahydrate used is 1100-1600 mg.

[0022] Preferably, the amounts of each component in step 4 are as follows: 3-10g of ferric citrate complex solution, 0.5-2.0g of sodium polyaspartate graft aqueous solution, 80-200mg of potassium dihydrogen phosphate, 1-4g of choline-stabilized orthosilicate mother liquor, and 20-60mg of zinc disodium ethylenediaminetetraacetate hydrate.

[0023] Preferably, the concentration of the sodium polyaspartate graft aqueous solution in step 4 is 10 wt%.

[0024] The present invention also provides a passivation culture medium for reducing cadmium in rice, which is obtained by the above-described method for preparing the passivation culture medium for reducing cadmium in rice.

[0025] The beneficial effects of this invention are:

[0026] The passivation culture medium provided by this invention employs an integrated technical solution of silicon-containing components, iron-containing complexes, and multi-coordinated organic frameworks, enabling the construction of a stable and functionally coordinated microenvironment regulation system in the plant rhizosphere. The orthosilicic acid in this culture medium effectively maintains the long-term stability of its active silicon form through a dual stabilization mechanism of choline and polyols. Upon entering the root surface region, it significantly promotes the silicification process of plant cell walls and the densification modification of the extracellular polymer matrix, thereby effectively reducing the transmembrane migration flux of heavy metal cadmium ions under the dual mechanisms of physical barrier and chemical passivation. The iron citrate complex in the culture medium provides a highly bioavailable iron nutrient source for plant roots. This complex exhibits excellent solubility and migration performance in weakly acidic to neutral soil environments, significantly mitigating the competitive interference effect of divalent heavy metal ions at root surface absorption sites, thus avoiding the non-specific excessive absorption of heavy metal ions induced by iron deficiency.

[0027] The polyaspartic acid graft copolymer backbone provides a high density of multifunctional coordinating groups. Carboxyl groups are responsible for long-range electrostatic capture and primary complexation fixation of heavy metal ions, while thiol groups introduced through thiolation modification provide a highly selective soft acid-soft base strong coordination binding ability for cadmium ions, ensuring that cadmium ions are preferentially fixed at binding sites in the plant extracellular matrix and cell wall, significantly reducing their migration and translocation into the plant. The permanent positively charged side chains introduced through quaternization modification effectively improve the electrocompatibility between the culture medium and soil containing silicate anions, citrate complexes, and various trace element chelates. Even in hard water environments containing high concentrations of calcium and magnesium ions, the system maintains good dispersibility and spray uniformity, effectively avoiding the adverse effects of calcium and magnesium ion-induced secondary aggregation on field application.

[0028] The comprehensive effects of this technical solution are as follows: It achieves highly selective passivation of cadmium in the soil without altering existing crop cultivation and management practices, while effectively maintaining or even enhancing the bioavailability of essential nutrients such as iron and zinc and their normal deposition and distribution in crop grains. This ensures the safety of agricultural products while comprehensively improving their nutritional quality. The culture medium exhibits good environmental adaptability and stable treatment effects in acidic to slightly acidic paddy soils, making it suitable for use in conjunction with conventional fertilization, irrigation, and drainage systems. It also possesses good scalability for transitioning from experimental plots to large-scale production conditions. Attached Figure Description

[0029] To more clearly illustrate the technical solutions in this invention or the prior art, the accompanying drawings used in the description of the embodiments or the prior art will be briefly introduced below.

[0030] Figure 1 This is a flowchart illustrating the preparation process of the passivation culture medium used for cadmium reduction in rice in this invention.

[0031] Figure 2 The infrared spectrum of the sodium polyaspartate graft in Example 2 of this invention is shown. Detailed Implementation

[0032] To make the objectives, technical solutions, and advantages of this invention clearer, the invention will be further described in detail below with reference to specific embodiments.

[0033] Example 1

[0034] Step 1: Preparation of sodium polyaspartate graft

[0035] Add 100g deionized water and 5g sodium poly-(α,β)-DL-aspartate (weight average molecular weight 5000) to a 500ml glass beaker. Stir with a glass rod until completely dissolved. Adjust the pH to 5.8 with 0.1mol / L hydrochloric acid. Then add 900mg N-hydroxysuccinimide and 1500mg 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride. Stir the mixture at room temperature for 10min. Then add 350mg cysteine ​​hydrochloride and 400mg... 3-Dimethylaminopropylamine was added and stirred for 3 hours, during which the pH was maintained in the range of 5.8-6.2 with 0.1 mol / L sodium hydroxide solution. Then 700 mg of iodomethane was added and the mixture was stirred for 1.5 hours. The mixture was then transferred to a dialysis bag with a molecular weight cutoff of 3.5 kDa and dialyzed in 2000 g of deionized water for 24 hours. The dialysate was changed 3 times during the dialysis process, with an interval of 8 hours between each change. After the dialysis was completed, the dialysis bag was removed and vacuum dried to obtain sodium polyaspartate grafts.

[0036] Step 2: Preparation of choline-stabilized orthosilicic acid mother liquor

[0037] Add 50g of glycerol and 50g of deionized water to a 250ml glass beaker and stir with a glass rod until well mixed. Then add 8g of choline chloride and continue stirring until completely dissolved. Slowly add 2.5g of hydrochloric acid (37wt%). While stirring continuously, slowly add 8g of sodium silicate aqueous solution (40% solid content, modulus 3.4). During the addition, maintain the pH in the range of 2.0-2.5 with hydrochloric acid. After the addition is complete, continue stirring for 20 minutes to obtain a choline-stable orthosilicic acid mother liquor.

[0038] Step 3: Prepare ferric citrate complex solution

[0039] Add 100g of deionized water and 850mg of citric acid monohydrate to a 250ml glass beaker and stir until completely dissolved. Add 1100mg of ferric chloride hexahydrate and adjust the pH to 4.0 with 0.1mol / L sodium hydroxide solution. Continue stirring for 20min to obtain ferric citrate complex solution.

[0040] Step 4: Preparation of passivation culture medium

[0041] Add 1000g of deionized water and 3g of ferric citrate complex solution to a 1000ml polypropylene graduated cylinder, stir for 2 minutes, then add 0.5g of sodium polyaspartate graft aqueous solution (concentration 10wt%), continue stirring for 5 minutes, then add 80mg of potassium dihydrogen phosphate, stir for 10 minutes, during which the pH is controlled within the range of 5.6-6.2 with 0.1mol / L sodium hydroxide solution, then add 1g of choline-stabilized orthosilicic acid mother liquor, stir for 10 minutes, then add 20mg of zinc disodium ethylenediaminetetraacetate hydrate, stir for 5 minutes to obtain the passivation culture medium for cadmium reduction in rice.

[0042] Example 2:

[0043] Step 1: Preparation of sodium polyaspartate graft

[0044] Add 100g of deionized water and 5g of sodium poly-(α,β)-DL-aspartate (weight average molecular weight 5000) to a 500ml glass beaker. Stir with a glass rod until completely dissolved. Adjust the pH to 5.8 with 0.1mol / L hydrochloric acid. Then add 1080mg of N-hydroxysuccinimide and 1800mg of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride. Stir the reaction at room temperature for 15min. Then add 432mg of cysteine ​​hydrochloride and 494mg of... 3-Dimethylaminopropylamine was added and stirred for 4 hours, during which the pH was maintained in the range of 5.8-6.2 with 0.1 mol / L sodium hydroxide solution. Then 880 mg of iodomethane was added and stirred for another 2 hours. The mixture was then transferred to a dialysis bag with a molecular weight cutoff of 3.5 kDa and dialyzed in 2000 g of deionized water for 24 hours, with the dialysate changed 3 times at 8-hour intervals. After dialysis, the dialysis bag was removed and vacuum dried to obtain sodium polyaspartate grafts.

[0045] Step 2: Preparation of choline-stabilized orthosilicic acid mother liquor

[0046] Add 50g of glycerol and 50g of deionized water to a 250ml glass beaker and stir with a glass rod until well mixed. Then add 10g of choline chloride and continue stirring until completely dissolved. Slowly add 3g of hydrochloric acid (37wt%). While stirring continuously, slowly add 10g of sodium silicate aqueous solution (40% solid content, modulus 3.4). During the addition, maintain the pH in the range of 2.0-2.5 with hydrochloric acid. After the addition is complete, continue stirring for 30 minutes to obtain a choline-stable orthosilicic acid mother liquor.

[0047] Step 3: Prepare ferric citrate complex solution

[0048] Add 100g of deionized water and 1050mg of citric acid monohydrate to a 250ml glass beaker, stir until completely dissolved, add 1350mg of ferric chloride hexahydrate, adjust the pH to 4.0 with 0.1mol / L sodium hydroxide solution, and continue stirring for 30min to obtain ferric citrate complex solution.

[0049] Step 4: Preparation of passivation culture medium

[0050] Add 1000g of deionized water and 5g of ferric citrate complex solution to a 1000ml polypropylene graduated cylinder, stir for 2 minutes, then add 1g of sodium polyaspartate graft aqueous solution (concentration 10wt%), continue stirring for 5 minutes, then add 130mg of potassium dihydrogen phosphate, stir for 10 minutes, during which the pH is controlled within the range of 5.6-6.2 with 0.1mol / L sodium hydroxide solution, then add 2g of choline-stabilized orthosilicate mother liquor, stir for 10 minutes, then add 40mg of zinc disodium ethylenediaminetetraacetate hydrate, stir for 5 minutes to obtain the passivation culture medium for cadmium reduction in rice.

[0051] Example 3

[0052] Step 1: Preparation of sodium polyaspartate graft

[0053] Add 100g of deionized water and 5g of sodium poly-(α,β)-DL-aspartate (weight average molecular weight 5000) to a 500ml glass beaker. Stir with a glass rod until completely dissolved. Adjust the pH to 5.8 with 0.1mol / L hydrochloric acid. Then add 1260mg of N-hydroxysuccinimide and 2100mg of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride. Stir the reaction at room temperature for 20min. Then add 520mg of cysteine ​​hydrochloride and 580mg of... 3-Dimethylaminopropylamine was added and stirred for 5 hours, during which the pH was maintained in the range of 5.8-6.2 with 0.1 mol / L sodium hydroxide solution. Then 1100 mg of iodomethane was added and the mixture was stirred for 2.5 hours. The mixture was then transferred to a dialysis bag with a molecular weight cutoff of 3.5 kDa and dialyzed in 2000 g of deionized water for 24 hours, with the dialysate changed 3 times at 8-hour intervals. After dialysis, the dialysis bag was removed and vacuum dried to obtain sodium polyaspartate grafts.

[0054] Step 2: Preparation of choline-stabilized orthosilicic acid mother liquor

[0055] Add 50g of glycerol and 50g of deionized water to a 250ml glass beaker and stir with a glass rod until well mixed. Then add 12g of choline chloride and continue stirring until completely dissolved. Slowly add 3.5g of hydrochloric acid (37wt%). While stirring continuously, slowly add 12g of sodium silicate aqueous solution (40% solid content, modulus 3.4). During the addition, maintain the pH in the range of 2.0-2.5 with hydrochloric acid. After the addition is complete, continue stirring for 40 minutes to obtain a choline-stable orthosilicic acid mother liquor.

[0056] Step 3: Prepare ferric citrate complex solution

[0057] Add 100g of deionized water and 1250mg of citric acid monohydrate to a 250ml glass beaker, stir until completely dissolved, add 1600mg of ferric chloride hexahydrate, adjust the pH to 4.0 with 0.1mol / L sodium hydroxide solution, and continue stirring for 40min to obtain ferric citrate complex solution.

[0058] Step 4: Preparation of passivation culture medium

[0059] Add 1000g of deionized water and 10g of ferric citrate complex solution to a 1000ml polypropylene graduated cylinder, stir for 2 minutes, then add 2.0g of sodium polyaspartate graft aqueous solution (concentration 10wt%), continue stirring for 5 minutes, then add 200mg of potassium dihydrogen phosphate, stir for 10 minutes, during which the pH is controlled within the range of 5.6-6.2 with 0.1mol / L sodium hydroxide solution, then add 4g of choline-stabilized orthosilicate mother liquor, stir for 10 minutes, then add 60mg of zinc disodium ethylenediaminetetraacetate hydrate, stir for 5 minutes to obtain the passivation culture medium for cadmium reduction in rice.

[0060] Comparative Example 1:

[0061] The difference between Comparative Example 1 and Example 2 is that no sodium polyaspartate graft was added, while the other conditions were the same as in Example 2.

[0062] Comparative Example 2:

[0063] The difference between Comparative Example 2 and Example 2 is that no choline-stabilized orthosilicic acid mother liquor was added, while the other conditions were the same as in Example 2.

[0064] Comparative Example 3:

[0065] The difference between Comparative Example 3 and Example 2 is that ferric citrate complex solution was not added, while the other conditions were the same as in Example 2.

[0066] Comparative Example 4:

[0067] The difference between Comparative Example 4 and Example 2 is that cysteine ​​hydrochloride is not added in the preparation step of the polyaspartic acid sodium graft, while the other conditions are the same as in Example 2.

[0068] Comparative Example 5:

[0069] The difference between Comparative Example 5 and Example 2 is that iodomethane was not added in the preparation step of the sodium aspartate graft, while the other conditions were the same as in Example 2.

[0070] Comparative Example 6:

[0071] The difference between Comparative Example 6 and Example 2 is that choline chloride was not added during the preparation of the choline-stabilized orthosilicic acid mother liquor, while the other conditions were the same as in Example 2.

[0072] Performance testing:

[0073] Pot Experiment: A randomized block design was used in the pot experiment, with a total of 8 treatments: CK (no application), Examples 1-3, and Comparative Examples 1-6, with 5 pots replicated per treatment. The pots were 28cm in diameter and 30cm in height, each containing 8.0kg of cadmium-contaminated paddy soil (preliminary pH ≈ 5.2). Two clumps of conventional rice from the same batch were transplanted into each pot, maintaining a shallow water layer of 2-3cm throughout. Basal fertilizer was applied at a rate of 0.80g pure N, 0.40g P2O5, and 0.80g K2O per pot (basal application of P, K, and 50% N, with the remaining 50% N applied as topdressing at tillering). The passivation culture solution was slowly injected into the rhizosphere at 3 days, 20 days, and 15 days before heading, at a rate of 200mL each time, totaling 600mL per pot. The CK was injected with an equal volume of deionized water. Rhizosphere soil samples were collected at 30 days and harvest for Cd and pH extraction using aqua regia. Polished rice was prepared after harvest for Cd determination (GB). 5009.15; Limit determination shall be made in accordance with GB 2762).

[0074] Infrared spectroscopy: Characterized by Fourier transform infrared spectroscopy, KBr pellet.

[0075] Hard water compatibility stability (turbidity): According to HJ 1075-2019, the hard water hardness is 300 mg / L as calcium carbonate. The turbidity was measured at 1 min and 10 min after the passivation culture medium and hard water were mixed at a volume ratio of 1:9. The results are shown in Table 1.

[0076] Table 1 Performance Test Results

[0077]

[0078] Data Analysis:

[0079] Analysis of Examples 1-3 in Table 1 reveals that this system effectively inhibits cadmium transport to grains while maintaining the availability of essential elements by creating a weakly acidic to neutral buffer environment in the rhizosphere and utilizing a multi-coordination synergistic mechanism. The carboxyl and reducible thiol sites in the system selectively complex cadmium, and the choline-stabilized orthosilicic acid promotes silicon deposition in the cell wall and extracellular polymers, reducing the expression activity of transmembrane transporters. Furthermore, ferric citrate provides a matching and stable iron source for the root surface, mitigating the antagonistic effect of cadmium and iron at absorption sites; zinc chelates help increase zinc accumulation in grains without increasing the risk of cadmium absorption. Overall, this system significantly improves Cd safety while ensuring that nutrients such as iron and zinc remain within reasonable ranges, maintaining good dispersion stability even in hard water environments. These results indicate that this system is suitable for paddy fields with moderate to mild cadmium pollution, and can simultaneously ensure rice quality and food safety without altering conventional irrigation and fertilization practices.

[0080] As shown in Table 1, the data from Example 2 and Comparative Example 1 reveal that the absence of the polyaspartic acid graft copolymer significantly reduces the system's ability to fix and translocate cadmium, leading to an increased risk of cadmium in the grains. The mechanism lies in the absence of multidentate carboxyl / thiol sites in the polymer, which hinders the formation of a dense complex barrier, making it easier for cadmium to enter the vascular system under transpiration. Simultaneously, the absence of quaternized segments weakens the electrostatic neutralization capacity, resulting in enhanced calcium and magnesium-induced microflocculation and dispersion instability in hard water, ultimately increasing the volatility of field performance.

[0081] Comparing the data from Example 2 and Comparative Example 2 in Table 1, the removal of choline-stabilized orthosilicic acid resulted in a dual weakening of the rhizosphere pH increase and silicon deposition effect, leading to unsatisfactory performance in both grain cadmium and solution turbidity. The fundamental reason is that orthosilicic acid readily polymerizes and silicifies in the absence of choline and polyol stabilization, resulting in the loss of available silicon forms and making it difficult to inhibit cadmium transport through cell wall silicification and transporter regulation. Simultaneously, hard water environments easily cause silicon polymer coagulation, increasing the system turbidity.

[0082] As can be seen from the data in Table 1 of Example 2 and Comparative Example 3, the iron content in the grains was significantly reduced and cadmium inhibition was insufficient when ferric citrate was lacking. Ferric citrate complexes have high coordination stability under weakly acidic to neutral conditions, which can maintain the function of rhizosphere iron channels and reduce cadmium-iron competition; without this component, iron deficiency induces non-selective absorption of divalent metals, indirectly increasing cadmium flux.

[0083] Comparing Example 2 and Comparative Example 4 in Table 1, the cadmium inhibition effect was weakened without thiol modification. This is mainly because thiol groups, as soft-soft coordination sites, can capture cadmium with high affinity. After deletion, the density of high-affinity complexing sites on the root surface decreases, making it more difficult for cadmium to be fixed in the cell wall and extracellular matrix, and the proportion entering the xylem relatively increases.

[0084] Comparing Example 2 and Comparative Example 5 in Table 1, the static inhibition of Cd by the system was acceptable without quaternization modification, but the compatibility was severely deteriorated under hard water conditions. Since the polymer does not contain a permanent positive charge, it is difficult to neutralize and stabilize the complex containing silicon anions and organic acid salts, which leads to enhanced secondary aggregation induced by calcium and magnesium, significantly increased turbidity of the system, and a significant reduction in spray uniformity and field consistency.

[0085] Comparing Example 2 and Comparative Example 6 in Table 1, the absence of choline chloride in the silicon source mother liquor significantly increased the turbidity of hard water, and worsened both grain cadmium inhibition and rhizosphere pH improvement. The main reason is that orthosilicic acid rapidly polysilicifies under acidic and high ionic strength conditions, reducing the supply of active silicon and weakening the effects of cell wall silicification and transporter downregulation. Simultaneously, the lack of quaternary ammonium-type compatibility stabilizers leads to secondary flocculation of silicon polymers and calcium-magnesium salts, resulting in poor application uniformity and insufficient effective dose in the rhizosphere. Therefore, choline stabilization plays a crucial synergistic role in maintaining active silicon and compatibility stability, demonstrating an interfacial stability gain greater than the sum of its parts (1+1>2).

[0086] from Figure 2 It can be seen that the sample is at 1650cm -1 Strong amide I appeared and 1602 / 1405cm -1 Carboxylate symmetric / asymmetric stretching coexistence, 1542cm -1 Amide II is clear, 970 and 1170 cm. -1 Quaternary ammonium characteristic peak is significant and at 2550 cm⁻¹ -1 The presence of a weak SH band indicates that the carboxyl group has been partially amidated and that quaternization and the introduction of thiol groups have been successful.

[0087] Those skilled in the art should understand that the discussion of any of the above embodiments is merely exemplary and is not intended to imply that the scope of the invention is limited to these examples; within the framework of the invention, the technical features of the above embodiments or different embodiments can also be combined, the steps can be implemented in any order, and there are many other variations of the different aspects of the invention as described above, which are not provided in detail for the sake of brevity.

Claims

1. A method for preparing a passivation culture solution for cadmium reduction in rice, characterized by, Includes the following steps: Step 1: Preparation of sodium polyaspartate grafts: Dissolve sodium poly-(α,β)-DL-aspartate in deionized water, adjust the pH to 5.8, add N-hydroxysuccinimide and 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride and react for 10-20 min, then add cysteine ​​hydrochloride and 3-dimethylaminopropylamine sequentially and stir for 3-5 h, add iodomethane and stir for 1.5-2.5 h, and obtain sodium polyaspartate grafts by dialysis and vacuum drying; Step 2: Prepare choline-stable orthosilicic acid mother liquor. Mix glycerol and deionized water, add choline chloride to dissolve, add hydrochloric acid dropwise, and add sodium silicate aqueous solution dropwise while stirring continuously. During the dropwise addition, use hydrochloric acid to maintain the pH in the range of 2.0-2.

5. Stir for 20-40 minutes to obtain choline-stable orthosilicic acid mother liquor. Step 3: Prepare ferric citrate complex solution. Dissolve citric acid monohydrate in deionized water, add ferric chloride hexahydrate, adjust the pH to 4.0 with sodium hydroxide solution, and stir for 20-40 minutes to obtain ferric citrate complex solution. Step 4: Preparation of passivation culture medium. Add ferric citrate complex solution, sodium polyaspartate graft solution, potassium dihydrogen phosphate, choline-stabilized orthosilicate mother liquor and zinc ethylenediaminetetraacetate disodium salt hydrate to deionized water in sequence, and control the pH in the range of 5.6-6.2 during the process to obtain the passivation culture medium. The weight-average molecular weight of the poly-(α,β)-DL-aspartic acid sodium salt described in step 1 is 5000.

2. The method for preparing the passivation culture medium for reducing cadmium in rice according to claim 1, characterized in that, In step 1, the dosage of N-hydroxysuccinimide is 900-1260 mg, the dosage of 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide hydrochloride is 1500-2100 mg, the dosage of cysteine ​​hydrochloride is 350-520 mg, the dosage of 3-dimethylaminopropylamine is 400-580 mg, and the dosage of iodomethane is 700-1100 mg.

3. The method for preparing the passivation culture medium for reducing cadmium in rice according to claim 1, characterized in that, In step 1, a dialysis bag with a molecular weight cutoff of 3.5 kDa was used for dialysis. The dialysis water was changed 3 times during the 24-hour dialysis period, with an interval of 8 hours between each change.

4. The method for preparing the passivation culture medium for reducing cadmium in rice according to claim 1, characterized in that, In step 2, the amount of choline chloride used is 8-12g, and the concentration of hydrochloric acid is 37wt% with an amount of 2.5-3.5g.

5. The method for preparing the passivation culture medium for reducing cadmium in rice according to claim 1, characterized in that, In step 2, the sodium silicate aqueous solution has a solid content of 40%, a modulus of 3.4, and a dosage of 8-12g.

6. The method for preparing the passivation culture medium for reducing cadmium in rice according to claim 1, characterized in that, In step 3, the dosage of citric acid monohydrate is 850-1250 mg, and the dosage of ferric chloride hexahydrate is 1100-1600 mg.

7. The method for preparing the passivation culture medium for reducing cadmium in rice according to claim 1, characterized in that, The amounts of each component in step 4 are as follows: 3-10g of ferric citrate complex solution, 0.5-2.0g of sodium polyaspartate graft aqueous solution, 80-200mg of potassium dihydrogen phosphate, 1-4g of choline-stabilized orthosilicic acid mother liquor, and 20-60mg of zinc disodium ethylenediaminetetraacetate hydrate.

8. The method for preparing the passivation culture medium for reducing cadmium in rice according to claim 1, characterized in that, The concentration of the sodium polyaspartate graft aqueous solution in step 4 is 10 wt%.

9. A passivation culture medium for reducing cadmium in rice, characterized in that, It is obtained by the method for preparing the passivation culture medium for reducing cadmium in rice according to any one of claims 1-8.

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