A method for combined reduction and remediation of cadmium contaminated paddy soil

Abstract

The application discloses a kind of composite reduction repair methods of cadmium contaminated paddy field soil, comprising the following steps: S1, 10-14 days before transplanting, add composite slow-release agent to paddy field, plough into soil;S2, plant high cadmium accumulation rice varieties;S3, gradient dry-wet alternate water management is carried out in three stages during the growth period of rice;S4, all aboveground parts are harvested at the end of grain filling and transported out of the field for centralized disposal;S5, after harvesting, deep plough the field surface, and flood irrigation to make the root system float, and collect all the roots for removal from the field.The composite reduction repair method of cadmium contaminated paddy field soil integrates soil chemical activation, plant absorption enhancement and root removal from the field, and builds a complete cadmium reduction path of activation-enrichment-removal.The technical means are connected on the time axis, and the overall repair efficiency is significantly higher than that of the conventional scheme of applying soil modifier alone or planting superaccumulator alone.

Description

Technical Field

[0001] This invention belongs to the field of soil remediation technology, specifically relating to a composite reduction remediation method for cadmium-contaminated paddy field soil. Background Technology

[0002] Cadmium (Cd) is one of the most toxic heavy metal pollutants in the soil environment. Its main sources include phosphate fertilizer application, sewage irrigation, atmospheric deposition, and mining and smelting activities.

[0003] Currently, the remediation strategies for cadmium-contaminated paddy fields are mainly divided into two categories: one is engineering physical remediation, represented by topsoil, deep plowing, and electrodynamic remediation. Although these methods have direct effects, they involve large-scale engineering and are costly, making them difficult to promote on a large scale in agricultural production. The other is in-situ reduction technology that combines chemical passivation with agronomic regulation. This type of technology has received widespread attention because it is relatively simple to operate and does not affect normal agricultural production.

[0004] In terms of chemical passivation, the application of passivating agents such as lime, calcium silicate materials, iron-based materials, and phosphates can reduce the content of available cadmium in the soil through mechanisms such as increasing soil pH, precipitation, or co-precipitation, thereby reducing the absorption of cadmium by rice. However, single passivating agents often have a simple mechanism of action, short duration of effect, and are difficult to maintain a stable passivation effect throughout the entire growth period of rice; the large-scale application of some materials also poses a risk of causing secondary soil pollution or affecting the structure of soil microbial communities.

[0005] Regarding water management, studies have shown that continuous flooding reduces soil redox potential, which is conducive to the formation of insoluble cadmium compounds such as cadmium sulfide, significantly inhibiting cadmium migration to rice roots. However, continuous flooding increases methane emissions, exacerbating the greenhouse effect, and may also lead to decreased soil aeration and weakened root activity, which are detrimental to high and stable rice yields. Therefore, optimizing water management systems while balancing yield and environmental benefits remains a key challenge in current research.

[0006] In phytoremediation, using rice varieties with high cadmium accumulation genotypes to concentrate cadmium in the aboveground parts of the soil and then harvesting it is a promising bioremediation approach. However, existing research shows that phytoremediation alone has limited efficiency, and the amount remediated in a single season is insufficient to meet the remediation needs of moderately to severely polluted paddy fields. Furthermore, the cadmium absorption capacity of high cadmium accumulation varieties is constrained by multiple factors, including the available cadmium levels in the soil, moisture conditions, and the rhizosphere microenvironment, resulting in unstable actual remediation efficiency.

[0007] Furthermore, the transport and distribution of cadmium in rice is regulated by multiple physiological processes, with the key link being the transport of cadmium from the roots to the aboveground parts via xylem loading after absorption. Currently, agronomic intervention research targeting the cadmium transport process in rice is still immature, and practical technologies to effectively block cadmium migration to grains are still relatively lacking.

[0008] In summary, existing single remediation technologies all have significant limitations, making it difficult to simultaneously achieve efficient cadmium fixation in soil, complete inhibition of cadmium absorption by rice, and substantial removal of cadmium from soil. Therefore, it is necessary to develop a comprehensive cadmium reduction and remediation method that combines multiple technologies, covers the entire rice growth cycle, and balances remediation efficiency with agricultural sustainability. Summary of the Invention

[0009] In view of the shortcomings of the existing technology, the purpose of this invention is to provide a composite reduction and remediation method for cadmium-contaminated paddy field soil.

[0010] To achieve the above objectives, the present invention provides the following technical solution: A composite method for reducing and remediating cadmium-contaminated paddy field soil includes the following steps: S1. 10 to 14 days before transplanting, add compound slow-release agent to the paddy field and plow it into the soil; S2. Planting rice varieties with high cadmium accumulation; S3. Implement gradient wet-dry alternation water management in three stages during the rice growth period: mild wet-dry alternation during the tillering stage, intensive wet-dry alternation during the jointing-booting stage, and dry-drying the field during the grain-filling stage; apply ammonium thiosulfate solution with irrigation water during each re-irrigation during the tillering and jointing-booting stages; apply N-acetyl-L-cysteine ​​solution with each dry-drying stage during the jointing-booting stage; and apply zinc sulfate solution with the last batch of irrigation water when switching to dry-drying during the grain-filling stage. S4. Harvest all above-ground parts close to the ground at the end of the grouting stage and transport them out of the field for centralized disposal. S5. After harvesting, deeply plow the field and flood it with water to make the roots float to the surface. Then, collect all the roots and remove them from the field.

[0011] Preferably, the preparation method of the composite sustained-release agent in step S1 includes the following steps: (a) Mix phytic acid solution with FeCl3 solution, then adjust the pH of the mixture to 6.5-7.0 with NaOH solution, and stir to obtain phytic acid iron complex gel; (b) Mix elemental sulfur with bentonite to obtain a mixed powder; add the mixed powder to the sulfur-oxidizing thiobacillus suspension, then add the phytic acid iron complex gel from step (a), stir evenly to obtain a core mixture; (c) Add the kernel mixture from step (b) to the sodium alginate solution and disperse it evenly. Then add a mixed solution of MnCl2 and CaCl2 dropwise to carry out the cross-linking reaction. After the reaction is completed, wash and air dry to obtain the composite sustained-release agent.

[0012] Preferably, in step (a), the phytic acid solution has a mass concentration of 8-12%, and the FeCl3 solution has a mass concentration of 5-8%. The phytic acid and FeCl3 solution contain phytic acid and Fe... 3+ The molar ratio is 1:2 to 3, and the stirring treatment temperature is 20 to 30°C, and the time is 30 to 40 minutes.

[0013] In this invention, phytic acid contains six phosphate groups, exhibiting strong metal chelating ability, and combines with FeCl3 to provide Fe... 3+ Highly efficient coordination complexation occurs under pH conditions of 6.5–7.0, forming a dense phytic acid-iron gel; by controlling the reaction between phytic acid and Fe... 3+ The molar ratio is 1:2 to 3, which makes Fe 3+ A relative excess ensures that the phytic acid phosphate groups are fully bridged to form a stable gel rather than a loose complex; the entire mixture is adjusted to a neutral range to ensure that the phosphate groups are fully deprotonated to improve complexation efficiency, while avoiding excessive alkalinity that would lead to Fe(OH)3 precipitation, and is also compatible with the growth pH of Thiobacillus thiooxidans in subsequent steps; this gel serves a dual function in the system: as a physical binding matrix to bind the solid functional components in step (b) into a core, and as an active functional component, after being applied to the soil, it reacts with the periodic fluctuations in redox potential caused by alternating wet and dry conditions, resulting in Fe2+ oxidation. 2+ / Fe 3+ The dynamic transformation of valence states leads to the formation and dissolution of an iron oxide film on the root surface, which is effective against Cd. 2+ It has surface complexing buffering capacity, which can suppress the peak concentration of cadmium in the rhizosphere soil solution, thus reducing Cd concentration. 2+ The cadmium enters the root system and is transported upwards at a relatively stable flux through the symptom pathway, thus avoiding the large amount of cadmium remaining in the root system and unable to migrate to the aboveground parts when the cadmium concentration in the soil solution rises sharply, thereby improving the overall transport efficiency of cadmium from the root system to the aboveground parts.

[0014] Preferably, in step (b), the mass ratio of elemental sulfur to bentonite is 3-4:1, and the viable count of *Thiobacillus thiooxidans* in the suspension is 1×10⁻⁶. 8 ~5×10 8 The cfu / mL concentration of the *Thiobacillus thiooxidans* suspension and the mass ratio of the mixed powder are 1:9 to 11, the mass ratio of the phytic acid iron complex gel and the mixed powder is 1:7 to 9, and the stirring time is 20 to 30 min.

[0015] In this invention, the core is composed of four components: elemental sulfur, bentonite, Thiobacillus thiooxidans bacterial suspension, and ferric phytate gel. Each component has complementary functions and works synergistically. Elemental sulfur, as the core functional substrate, is continuously oxidized by Thiobacillus thiooxidans to produce H2SO4, maintaining a low local pH in the microdomain surrounding the slow-release agent. This inhibits the oxidation of Cd by soil colloids during the drying and oxidation phase.2+ Re-adsorption (the negative charge of soil colloids decreases at low pH, affecting Cd) 2+ (This reduces the adsorption capacity of Cd), causing Cd to... 2+ Maintaining a high concentration of iron oxide in the soil solution is beneficial for continuous and efficient root absorption. On the other hand, a locally acidified environment can promote the dynamic dissolution and reformation of the iron oxide film, strengthening the regulatory effect of the iron cycle on the root's cadmium uptake flux. Bentonite, with its high specific surface area, serves as a carrier framework to disperse sulfur particles, increase the inoculum-substrate contact area, and provide the core with sufficient mechanical strength to prevent breakage during processing and application. Thiobacillus thiooxidans, at a concentration of 1×10⁻⁶... 8 ~5×10 8 A cfu / mL viable bacteria count is incorporated to ensure sufficient initial bacterial count while avoiding excessive early acid production that could damage the roots. Phytic acid-iron gel is used as a binder at a mass ratio of 1:7–9 to bind the solid components together. This core is the biochemical functional core of the slow-release agent, promoting continuous biological acid production and reducing rhizosphere Cd levels. 2+ The synergistic mechanism of solution-state maintenance and dynamic regulation of absorption flux by iron film provides a continuous and stable cadmium source supply to the roots throughout the entire rice growth period, ensuring the efficient extraction of cadmium from the soil by high cadmium-accumulating varieties.

[0016] Preferably, the mass concentration of the sodium alginate solution in step (c) is 2-3%; the concentration of MnCl2 in the mixed solution is 7-9 mmol / L, and the concentration of CaCl2 is 40-60 mmol / L; the mass ratio of the core mixture, sodium alginate solution, and mixed solution is 1:3-5:2-4; the temperature of the crosslinking reaction is 20-30°C, and the time is 15-25 min; the air-drying temperature is ≤35°C, and the air-drying is carried out until the moisture content is ≤15%.

[0017] In this invention, the kernel is dispersed in a sodium alginate solution, and then a solution containing Ca is added dropwise. 2+ and Mn 2+ The mixed crosslinking liquid, Ca 2+ Specific ionic crosslinking occurs with the guluronic acid fragments of sodium alginate, forming a gel shell that imparts good mechanical strength to the particles; Mn 2+ Simultaneously participating in cross-linking and accumulating in the shell, it can be slowly released in the soil and then synergistically regulate rhizosphere Cd through the Mn redox cycle. 2+ Morphological transformation; the final gel shell acts as a diffusion barrier, matching the release rate of the core active components with the duration of the entire rice growth period, while isolating the bacteria from the original antagonistic microorganisms in the soil and drastic environmental changes, ensuring the stable survival and continuous acid production of the functional flora in the soil, and achieving slow-release regulation of cadmium-contaminated rice fields throughout the entire growth period.

[0018] Preferably, the amount of the compound slow-release agent added in step S1 is 20-33 kg / mu, and the depth of tilling into the soil is 10-15 cm.

[0019] Preferably, the high cadmium-accumulating rice variety mentioned in step S2 is one or more of R900, Longliangyou 1212, and 9311.

[0020] Preferably, the specific operation of implementing mild wet-dry alternation during the tillering stage in step S3 is as follows: irrigate to a water depth of 3-5 cm on the field surface, keep the water for 5-7 days and then let it dry naturally until there is no standing water on the field surface, repeat 3-4 times; each time the field is re-irrigated, apply an 8%-12% ammonium thiosulfate solution with the irrigation water at a rate of 13-27 L / mu.

[0021] Preferably, the specific operation of strengthening the alternation of dry and wet during the jointing-booting stage in step S3 is as follows: irrigate to a water depth of 5-8 cm on the field surface, retain water for 5-7 days, and then drain until small cracks appear on the field surface, repeating this cycle 3-4 times; each time the field is re-irrigated, apply an 8%-12% ammonium thiosulfate solution with the irrigation water at a rate of 27-40 L / mu; each time the field is drained until the soil moisture content drops to 25%-35%, spray the leaves with an N-acetyl-L-cysteine ​​solution at a concentration of 0.5-1.0 mmol / L at a rate of 47-53 L / mu. The specific operation of drying the field during the grouting period is as follows: completely dry the field and do not re-irrigate; when switching to drying, apply a zinc sulfate solution with a concentration of 4-6 mmol / L along with the last batch of irrigation water, at a rate of 37-43 L / mu.

[0022] In this invention, when switching to drying during the grouting period, a zinc sulfate solution is applied as a single top dressing along with the last batch of irrigation water. 2+ with cd 2+ The plant shares transport proteins during root and xylem loading, and the two have a competitive absorption relationship; a large amount of Zn is supplemented during the grain-filling period. 2+ It can competitively inhibit Cd at the transporter protein level. 2+ The transport to the ear causes cadmium to be preferentially retained in the vegetative parts of the stems and leaves rather than distributed to the grains. This measure is applied only once during the grain-filling period, with a clear timing. It does not affect the active accumulation of cadmium during the tillering and jointing-heading stages, nor does it cause zinc toxicity due to long-term high zinc treatment.

[0023] Preferably, the depth of deep plowing in step S5 is 15-20cm, and the water depth of flood irrigation to the field surface is 15-20cm.

[0024] Compared with the prior art, the present invention has the following beneficial effects: (1) The composite reduction and remediation method for cadmium-contaminated paddy field soil provided by this invention integrates three links: soil chemical activation, plant absorption enhancement, and complete removal of roots and stems from the field, thus constructing a complete cadmium reduction pathway of activation-enrichment-removal; the composite slow-release agent is plowed into the soil before transplanting and continuously releases acidic substances and Fe produced by sulfur-oxidizing bacteria metabolism during the rice growth period. 2+ / Fe 3+ Maintaining the concentration of available cadmium in the rhizosphere soil; gradient wet and dry water management combined with the phased application of ammonium thiosulfate and N-acetyl-L-cysteine ​​to promote the continuous absorption and enrichment of cadmium-accumulating varieties from the tillering stage to the grain-filling stage; harvesting all aboveground parts and removing the roots from the field at the end of the grain-filling stage to achieve the physical output of cadmium from the soil; the various technical means are interconnected on the time axis, effectively avoiding the bottleneck of remediation efficiency caused by insufficient soil cadmium activation or limited cadmium accumulation in plants due to single measures, and the overall remediation efficiency is significantly higher than the conventional scheme of applying soil conditioners alone or planting hyperaccumulating plants alone.

[0025] (2) The composite reduction remediation method for cadmium-contaminated paddy field soil provided by the present invention prepares a composite slow-release agent with sodium alginate / Mn 2+ -Ca 2+ A dual-ion cross-linked gel forms the outer shell, encapsulating *Thiobacillus thiooxidans*, elemental sulfur, and a bentonite core. The physical barrier effect of the sodium alginate gel allows for the slow release of the bacteria and the substrate elemental sulfur into the soil, preventing rapid inactivation of the bacteria due to direct exposure to the soil environment; Mn 2+ With Ca 2+ Double cross-linking endows the gel shell with good structural stability under acidic soil conditions, prolongs the release cycle, and the released sulfur-oxidizing thiobacilli continuously produce sulfuric acid using elemental sulfur as a substrate, locally lowering the rhizosphere pH and promoting the conversion of insoluble cadmium in the soil to available form; phytic acid-iron complexed gel contains Fe 3+ Under microbial reduction conditions, cadmium is released, forming a rhizosphere iron oxide-organic acid complex, which further enhances the dissolution of cadmium and its adsorption and uptake by plant roots. Bentonite, as a carrier, also has a certain buffering effect, preventing excessive local acidification from affecting the normal growth of rice.

[0026] (3) The composite reduction and remediation method for cadmium-contaminated paddy soil provided by the present invention adopts a three-stage gradient wet-dry alternation from the tillering stage to the grain filling stage, instead of maintaining a uniform moisture state throughout the entire growth period. The mild wet-dry alternation during the tillering stage causes periodic fluctuations in the soil redox potential (Eh), promoting the migration of cadmium from the solid phase to the liquid phase in the soil, while preventing the roots from being subjected to strong oxidative stress. The enhanced wet-dry alternation during the jointing-booting stage further expands the amplitude of Eh fluctuations, making the activation level of cadmium in the rhizosphere synchronize with the rapid accumulation stage of rice biomass in time, which is conducive to the large-scale transport of cadmium to the aboveground parts by transpiration. The complete drying and sun-drying of the field during the grain filling stage inhibits the further transfer of cadmium from the roots to the grains through strong soil oxidation, locking the enrichment site of cadmium in the stems and leaves, rather than entering the edible parts, while providing the drying and hardening operation conditions for subsequent root harvesting. The three-stage gradient design makes the activation-absorption-locking of cadmium each have their own emphasis in time, without interfering with each other.

[0027] (4) The composite reduction and remediation method for cadmium-contaminated paddy soil provided by this invention, in the process of gradient wet-dry alternating water management, ammonium thiosulfate is applied with irrigation water each time it is re-irrigated. The thiosulfate ions produced by its hydrolysis act as electron donors in the micro-aerobic-anaerobic alternating environment, promoting the reducing metabolism of rhizosphere microorganisms, reducing rhizosphere Eh, and releasing cadmium bound to iron and manganese oxides in the soil into pore water, thereby increasing the concentration of cadmium available to plants; at the same time, the continuous supply of thiosulfate ions maintains a high concentration of reducing sulfur in the rhizosphere microdomain, providing substrate supplementation for the activity maintenance of thiobacillus thiooxidans; N- When acetyl-L-cysteine ​​is applied foliarly during the jointing-booting stage and the subsequent drying phase, the thiol group in its molecule can form a stable metal-thiol complex with cadmium, increasing the mobility of cadmium within the plant and promoting long-distance transport of cadmium from the roots to the stems and leaves. At the same time, N-acetyl-L-cysteine, as a precursor to glutathione, can alleviate the damage of high cadmium accumulation to oxidative stress in rice to a certain extent, maintain the normal physiological state of the plant under high cadmium load, and ensure the continuous accumulation of subsequent biomass. The staggered application times of the two substances allow them to act on the soil-root interface and the root-stem transport link, respectively, forming a relay reinforcement from source to sink. Attached Figure Description

[0028] Figure 1 The amount of cadmium accumulated in the soil by rice in the remediation methods of each embodiment and comparative example. Detailed Implementation

[0029] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0030] Unless otherwise specified, all chemical reagents and materials in this invention are purchased from the market or synthesized from raw materials purchased from the market.

[0031] Experimental location: A cadmium-contaminated paddy field in Hunan Province. The soil pH was 5.3, the total cadmium content in the topsoil was 2.5 mg / kg, the available cadmium content was 1.35 mg / kg, and the soil texture was clay loam.

[0032] Example 1 A composite method for reducing and remediating cadmium-contaminated paddy field soil includes the following steps: S1. Ten days before transplanting, add compound slow-release agent to the paddy field at a rate of 27 kg / mu, and plow it into the soil to a depth of 10-15 cm. S2. Planting the high-cadmium-accumulating rice variety R900; S3. Implement gradient wet-dry alternation water management in three stages during the rice growth period: Tillering stage: Irrigate to a water depth of 3-5cm on the field surface, keep the water for 5-7 days and let it dry naturally until there is no standing water on the field surface, repeat 4 times; each time you re-irrigate, apply a 10% ammonium thiosulfate solution with the irrigation water at a rate of 20L / mu. During the jointing-booting stage: irrigate to a water depth of 5-8 cm, retain water for 5-7 days, then drain until small cracks appear on the field surface, repeating this cycle 4 times; each time the field is re-irrigated, apply a 10% ammonium thiosulfate solution at a rate of 34 L / mu with the irrigation water; each time the field is drained until the soil moisture content drops to 30%, spray the leaves with a 0.8 mmol / L N-acetyl-L-cysteine ​​solution at a rate of 50 L / mu. Grouting period: Completely dry out and do not re-irrigate; when switching to dry out, apply a 5 mmol / L zinc sulfate solution once with the last batch of irrigation water, at a rate of 40L / mu; S4. Harvest all above-ground parts close to the ground at the end of the grouting stage and transport them out of the field for centralized disposal. S5. After harvesting, plow the field to a depth of 15-20cm and flood the field with water to a depth of 15-20cm to make the roots float to the surface. Then, collect all the roots and remove them from the field.

[0033] The preparation method of the composite sustained-release agent in step S1 includes the following steps: (a) Mix 660g of 10% phytic acid solution with 580g of 7% FeCl3 solution, then adjust the pH of the mixture to 6.8 with 1mol / L NaOH solution, and stir at 25℃ for 35min to obtain phytic acid iron complex gel. (b) Mix 1050g of elemental sulfur with 300g of bentonite to obtain a mixed powder; add 1000g of the mixed powder to 100g of Thiobacillus thiooxidans suspension (viable count 3×10⁻⁶).8 (cfu / mL), then add 125g of phytic acid iron complex gel from step (a), stir for 25min to obtain the core mixture; (c) Add 1000g of the kernel mixture from step (b) to 4000g of sodium alginate solution with a mass concentration of 2.5% and disperse evenly. Then add 3000g of a mixed solution of MnCl2 and CaCl2 (MnCl2 concentration of 8mmol / L and CaCl2 concentration of 50mmol / L) dropwise. Perform a crosslinking reaction at 25℃ for 20min. After the reaction is complete, wash three times with deionized water and air dry at 30℃ until the moisture content is 12% to obtain the composite sustained-release agent.

[0034] Example 2 A composite method for reducing and remediating cadmium-contaminated paddy field soil includes the following steps: S1. 12 days before transplanting, add compound slow-release agent to the paddy field at a rate of 20 kg / mu, and plow it into the soil to a depth of 10-15 cm. S2. Plant the high-cadmium-accumulating rice variety Longliangyou 1212; S3. Implement gradient wet-dry alternation water management in three stages during the rice growth period: Tillering stage: Irrigate to a water depth of 3-5cm on the field surface, keep the water for 5-7 days and let it dry naturally until there is no standing water on the field surface, repeat 3 times; each time you re-irrigate, apply an 8% ammonium thiosulfate solution with the irrigation water at a rate of 27L / mu. During the jointing-booting stage: irrigate to a water depth of 5-8 cm, retain water for 5-7 days, then drain until small cracks appear on the field surface, repeating this cycle 3 times; each time the field is re-irrigated, apply an 8% ammonium thiosulfate solution at a rate of 40 L / mu with the irrigation water; after each draining until the soil moisture content drops to 25%, spray the leaves with a 0.5 mmol / L N-acetyl-L-cysteine ​​solution at a rate of 53 L / mu. Grouting period: Completely dry out and do not re-irrigate; when switching to dry out, apply a 4 mmol / L zinc sulfate solution once with the last batch of irrigation water, at a rate of 43 L / mu; S4. Harvest all above-ground parts close to the ground at the end of the grouting stage and transport them out of the field for centralized disposal. S5. After harvesting, plow the field to a depth of 15-20cm and flood the field with water to a depth of 15-20cm to make the roots float to the surface. Then, collect all the roots and remove them from the field.

[0035] The preparation method of the composite sustained-release agent in step S1 includes the following steps: (a) Mix 825g of phytic acid solution with a mass concentration of 8% with 650g of FeCl3 solution with a mass concentration of 5%, then adjust the pH of the mixture to 6.5 with 1mol / L NaOH solution, and stir at 20℃ for 40min to obtain phytic acid iron complex gel. (b) Mix 900g of elemental sulfur with 300g of bentonite to obtain a mixed powder; add 900g of the mixed powder to 100g of Thiobacillus thiooxidans suspension (live count 1×10⁻⁶). 8 (cfu / mL), then add 100g of phytic acid iron complex gel from step (a), stir for 20min to obtain the core mixture; (c) Add 1000g of the kernel mixture from step (b) to 3000g of sodium alginate solution with a mass concentration of 2% and disperse evenly. Then add 2000g of a mixed solution of MnCl2 and CaCl2 (MnCl2 concentration of 7mmol / L and CaCl2 concentration of 4mmol / L) dropwise. Crosslink the mixture at 20℃ for 25min. After the reaction is complete, wash the mixture three times with deionized water and air dry it at 30℃ until the water content is 12% to obtain the composite sustained-release agent.

[0036] Example 3 A composite method for reducing and remediating cadmium-contaminated paddy field soil includes the following steps: S1. 14 days before transplanting, add compound slow-release agent to the paddy field at a rate of 33 kg / mu, and plow it into the soil to a depth of 10-15 cm. S2. Planting the high-cadmium-accumulating rice variety 9311; S3. Implement gradient wet-dry alternation water management in three stages during the rice growth period: Tillering stage: Irrigate to a water depth of 3-5cm on the field surface, keep the water for 5-7 days and let it dry naturally until there is no standing water on the field surface, repeat 4 times; each time you re-irrigate, apply a 12% ammonium thiosulfate solution with the irrigation water at a rate of 13L / mu. During the jointing-booting stage: irrigate to a water depth of 5-8 cm, retain water for 5-7 days, then drain until small cracks appear on the field surface, repeating this cycle 4 times; each time the field is re-irrigated, apply a 12% ammonium thiosulfate solution at a rate of 27 L / mu with the irrigation water; each time the field is drained until the soil moisture content drops to 35%, spray the leaves with a 1.0 mmol / L N-acetyl-L-cysteine ​​solution at a rate of 47 L / mu. Grouting period: Completely dry out and do not re-irrigate; when switching to dry out, apply a 6 mmol / L zinc sulfate solution once with the last batch of irrigation water, at a rate of 37 L / mu; S4. Harvest all above-ground parts close to the ground at the end of the grouting stage and transport them out of the field for centralized disposal. S5. After harvesting, plow the field to a depth of 15-20cm and flood the field with water to a depth of 15-20cm to make the roots float to the surface. Then, collect all the roots and remove them from the field.

[0037] The preparation method of the composite sustained-release agent in step S1 includes the following steps: (a) Mix 550g of phytic acid solution with a mass concentration of 12% with 600g of FeCl3 solution with a mass concentration of 8%, then adjust the pH of the mixture to 7.0 with 1mol / L NaOH solution, and stir at 30℃ for 30min to obtain phytic acid iron complex gel. (b) Mix 1200g of elemental sulfur with 300g of bentonite to obtain a mixed powder; add 1100g of the mixed powder to 100g of Thiobacillus thiooxidans suspension (viable count 5×10⁻⁶). 8 (cfu / mL), then add 150g of phytic acid iron complex gel from step (a), stir for 30 min, and obtain the core mixture; (c) Add 1000g of the kernel mixture from step (b) to 5000g of sodium alginate solution with a mass concentration of 3% and disperse evenly. Then add 4000g of a mixed solution of MnCl2 and CaCl2 (MnCl2 concentration of 9mmol / L and CaCl2 concentration of 60mmol / L) dropwise. Perform a crosslinking reaction at 30℃ for 15min. After the reaction is complete, wash three times with deionized water and air dry at 30℃ until the moisture content is 12% to obtain the composite sustained-release agent.

[0038] Comparative Example 1 A composite method for reducing and remediating cadmium-contaminated paddy field soil includes the following steps: S1. Ten days before transplanting, add compound slow-release agent to the paddy field at a rate of 27 kg / mu, and plow it into the soil to a depth of 10-15 cm. S2. Planting the high-cadmium-accumulating rice variety R900; S3. Harvest all above-ground parts close to the ground at the end of the grouting stage and transport them out of the field for centralized disposal. S4. After harvesting, plow the field to a depth of 15-20cm and flood it with water to a depth of 15-20cm to make the roots float to the surface. Then, collect all the roots and remove them from the field.

[0039] The preparation method of the composite sustained-release agent in step S1 includes the following steps: (a) Mix 660g of 10% phytic acid solution with 580g of 7% FeCl3 solution, then adjust the pH of the mixture to 6.8 with 1mol / L NaOH solution, and stir at 25℃ for 35min to obtain phytic acid iron complex gel. (b) Mix 1050g of elemental sulfur with 300g of bentonite to obtain a mixed powder; add 1000g of the mixed powder to 100g of Thiobacillus thiooxidans suspension (viable count 3×10⁻⁶). 8 (cfu / mL), then add 125g of phytic acid iron complex gel from step (a), stir for 25min to obtain the core mixture; (c) Add 1000g of the kernel mixture from step (b) to 4000g of sodium alginate solution with a mass concentration of 2.5% and disperse evenly. Then add 3000g of a mixed solution of MnCl2 and CaCl2 (MnCl2 concentration of 8mmol / L and CaCl2 concentration of 50mmol / L) dropwise. Perform a crosslinking reaction at 25℃ for 20min. After the reaction is complete, wash three times with deionized water and air dry at 30℃ until the moisture content is 12% to obtain the composite sustained-release agent.

[0040] Compared with Example 1, this comparative example only applied a compound sustained-release agent and did not perform dry-wet alternation moisture management.

[0041] Comparative Example 2 A composite method for reducing and remediating cadmium-contaminated paddy field soil includes the following steps: S1. Ten days before transplanting, add compound slow-release agent to the paddy field at a rate of 27 kg / mu, and plow it into the soil to a depth of 10-15 cm. S2. Planting the high-cadmium-accumulating rice variety R900; S3. Implement gradient wet-dry alternation water management in three stages during the rice growth period: Tillering stage: Irrigate to a water depth of 3-5cm on the field surface, keep the water for 5-7 days and let it dry naturally until there is no standing water on the field surface, repeat 4 times; each time you re-irrigate, apply a 10% ammonium thiosulfate solution with the irrigation water at a rate of 20L / mu. During the jointing-booting stage: irrigate to a water depth of 5-8 cm, retain water for 5-7 days, then drain until small cracks appear on the field surface, repeating this cycle 4 times; each time the field is re-irrigated, apply a 10% ammonium thiosulfate solution at a rate of 34 L / mu with the irrigation water; each time the field is drained until the soil moisture content drops to 30%, spray the leaves with a 0.8 mmol / L N-acetyl-L-cysteine ​​solution at a rate of 50 L / mu. Grouting period: Completely dry out and do not re-irrigate; when switching to dry out, apply a 5 mmol / L zinc sulfate solution once with the last batch of irrigation water, at a rate of 40L / mu; S4. Harvest all above-ground parts close to the ground at the end of the grouting stage and transport them out of the field for centralized disposal. S5. After harvesting, plow the field to a depth of 15-20cm and flood the field with water to a depth of 15-20cm to make the roots float to the surface. Then, collect all the roots and remove them from the field.

[0042] The preparation method of the composite sustained-release agent in step S1 includes the following steps: (a) Mix 660g of 10% phytic acid solution with 580g of 7% FeCl3 solution, then adjust the pH of the mixture to 6.8 with 1mol / L NaOH solution, and stir at 25℃ for 35min to obtain phytic acid iron complex gel. (b) Mix 1050g of elemental sulfur with 300g of bentonite to obtain a mixed powder; add 1000g of the mixed powder to 125g of phytic acid iron complex gel in step (a), stir for 25min to obtain a core mixture; (c) Add 1000g of the kernel mixture from step (b) to 4000g of sodium alginate solution with a mass concentration of 2.5% and disperse evenly. Then add 3000g of a mixed solution of MnCl2 and CaCl2 (MnCl2 concentration of 8mmol / L and CaCl2 concentration of 50mmol / L) dropwise. Perform a crosslinking reaction at 25℃ for 20min. After the reaction is complete, wash three times with deionized water and air dry at 30℃ until the moisture content is 12% to obtain the composite sustained-release agent.

[0043] Compared with Example 1, no Thiobacillus thiooxidans was added to the compound sustained-release agent in this comparative example.

[0044] Comparative Example 3 A composite method for reducing and remediating cadmium-contaminated paddy field soil includes the following steps: S1. Ten days before transplanting, add compound slow-release agent to the paddy field at a rate of 27 kg / mu, and plow it into the soil to a depth of 10-15 cm. S2. Planting the high-cadmium-accumulating rice variety R900; S3. Implement gradient wet-dry alternation water management in three stages during the rice growth period: Tillering stage: Irrigate to a water depth of 3-5cm on the field surface, keep the water for 5-7 days and let it dry naturally until there is no standing water on the field surface, repeat 4 times; each time you re-irrigate, apply a 10% ammonium thiosulfate solution with the irrigation water at a rate of 20L / mu. During the jointing-booting stage: irrigate to a water depth of 5-8 cm, retain water for 5-7 days, then drain until small cracks appear on the field surface, repeating this cycle 4 times; each time the field is re-irrigated, apply a 10% ammonium thiosulfate solution at a rate of 34 L / mu with the irrigation water; each time the field is drained until the soil moisture content drops to 30%, spray the leaves with a 0.8 mmol / L N-acetyl-L-cysteine ​​solution at a rate of 50 L / mu. Grouting period: Completely dry out and do not re-irrigate; when switching to dry out, apply a 5 mmol / L zinc sulfate solution once with the last batch of irrigation water, at a rate of 40 L / mu; S4. Harvest all above-ground parts close to the ground at the end of the grouting stage and transport them out of the field for centralized disposal. S5. After harvesting, plow the field to a depth of 15-20cm and flood the field with water to a depth of 15-20cm to make the roots float to the surface. Then, collect all the roots and remove them from the field.

[0045] The preparation method of the composite sustained-release agent in step S1 includes the following steps: (a) Mix 1050g of elemental sulfur with 300g of bentonite to obtain a mixed powder; add 1000g of the mixed powder to 100g of Thiobacillus thiooxidans suspension (with a viable count of 3×10⁻⁶). 8 (cfu / mL), stir for 25 min to obtain the kernel mixture; (b) Add 1000g of the kernel mixture from step (a) to 4000g of sodium alginate solution with a mass concentration of 2.5% and disperse evenly. Then add 3000g of a mixed solution of MnCl2 and CaCl2 (MnCl2 concentration of 8mmol / L and CaCl2 concentration of 50mmol / L) dropwise. Perform a crosslinking reaction at 25℃ for 20min. After the reaction is complete, wash three times with deionized water and air dry at 30℃ until the moisture content is 12% to obtain the composite sustained-release agent.

[0046] Compared with Example 1, the composite sustained-release agent of this comparative example does not introduce phytic acid iron gel, that is, step (a) is omitted.

[0047] Comparative Example 4 A composite method for reducing and remediating cadmium-contaminated paddy field soil includes the following steps: S1. Ten days before transplanting, add compound slow-release agent to the paddy field at a rate of 27 kg / mu, and plow it into the soil to a depth of 10-15 cm. S2. Planting the high-cadmium-accumulating rice variety R900; S3. Implement gradient wet-dry alternation water management in three stages during the rice growth period: Tillering stage: Irrigate to a depth of 3-5cm on the field surface, keep the water for 5-7 days and then let it dry naturally until there is no visible water on the field surface. Repeat this cycle 4 times. Jointing-heading stage: Irrigate to a water depth of 5-8cm on the field surface, keep the water for 5-7 days, then drain until small cracks appear on the field surface, repeat 4 times; Grouting period: Completely dry out and do not re-irrigate; when switching to dry out, apply a 5 mmol / L zinc sulfate solution once with the last batch of irrigation water, at a rate of 40L / mu; S4. Harvest all above-ground parts close to the ground at the end of the grouting stage and transport them out of the field for centralized disposal. S5. After harvesting, plow the field to a depth of 15-20cm and flood the field with water to a depth of 15-20cm to make the roots float to the surface. Then, collect all the roots and remove them from the field.

[0048] The preparation method of the composite sustained-release agent in step S1 includes the following steps: (a) Mix 660g of 10% phytic acid solution with 580g of 7% FeCl3 solution, then adjust the pH of the mixture to 6.8 with 1mol / L NaOH solution, and stir at 25℃ for 35min to obtain phytic acid iron complex gel. (b) Mix 1050g of elemental sulfur with 300g of bentonite to obtain a mixed powder; add 1000g of the mixed powder to 100g of Thiobacillus thiooxidans suspension (with a viable count of 3×10⁻⁶). 8 (cfu / mL), then add 125g of phytic acid iron complex gel from step (a), stir for 25min to obtain the core mixture; (c) Add 1000g of the kernel mixture from step (b) to 4000g of sodium alginate solution with a mass concentration of 2.5% and disperse evenly. Then add 3000g of a mixed solution of MnCl2 and CaCl2 (MnCl2 concentration of 8mmol / L and CaCl2 concentration of 50mmol / L) dropwise. Perform a crosslinking reaction at 25℃ for 20min. After the reaction is complete, wash three times with deionized water and air dry at 30℃ until the moisture content is 12% to obtain the composite sustained-release agent.

[0049] Compared with Example 1, this comparative example did not apply ammonium thiosulfate solution during the tillering stage and did not apply ammonium thiosulfate solution or NAC solution during the jointing-heading stage when implementing gradient wet-dry alternating water management.

[0050] After remediation, the cadmium content in rice and soil was tested in Examples 1-3 and Comparative Examples 1-4, respectively. Based on an average rice planting density of 12,000 plants / acre, the remediation efficiency for soil cadmium reduction and removal was calculated using the following formula: (Cumulative cadmium content in rice (g) = cadmium content in rice (mg / kg) × dry biomass of rice (kg) / 1000; cadmium content in topsoil (g) = (cadmium content in topsoil - cadmium pollution risk screening value in soil) (mg / kg) × topsoil weight (kg) / 1000), where, when the soil pH is less than or equal to 5.5, the cadmium pollution risk screening value in soil is 0.3 mg / kg, and the topsoil weight is calculated at 150 tons per mu. The test results are shown in Table 1 below: Table 1. Remediation efficiency of rice for cadmium reduction in soil in each group As can be seen from Table 1 above, the composite reduction remediation method for cadmium-contaminated paddy field soil provided by the present invention can significantly improve the remediation efficiency of cadmium pollution in soil and shorten the remediation period, showing good application prospects.

[0051] The above description is a further detailed explanation of the present invention in conjunction with specific implementation examples. It should not be considered that the specific implementation of the present invention is limited to these descriptions. For those skilled in the art, several simple deductions or substitutions can be made without departing from the concept of the present invention, and all such deductions or substitutions should be considered to fall within the protection scope of the present invention.

[0052] Those skilled in the art will readily understand that the above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, and improvements made within the spirit and principles of the present invention should be included within the scope of protection of the present invention.

Claims

1. A composite method for reducing and remediating cadmium-contaminated paddy field soil, characterized in that, Includes the following steps: S1. 10 to 14 days before transplanting, add compound slow-release agent to the paddy field and plow it into the soil; S2. Planting rice varieties with high cadmium accumulation; S3. Implement gradient wet-dry alternation water management in three stages during the rice growth period: mild wet-dry alternation during the tillering stage, intensive wet-dry alternation during the jointing-booting stage, and dry-drying the field during the grain-filling stage; apply ammonium thiosulfate solution with irrigation water during each re-irrigation during the tillering and jointing-booting stages; apply N-acetyl-L-cysteine ​​solution with each dry-drying stage during the jointing-booting stage; and apply zinc sulfate solution with the last batch of irrigation water when switching to dry-drying during the grain-filling stage. S4. Harvest all above-ground parts close to the ground at the end of the grouting stage and transport them out of the field for centralized disposal. S5. After harvesting, plow the field surface deeply and flood it with water to make the roots float to the surface. Then, collect all the roots and remove them from the field. The preparation method of the composite sustained-release agent in step S1 includes the following steps: (a) Mix phytic acid solution with FeCl3 solution, then adjust the pH of the mixture to 6.5-7.0 with NaOH solution, and stir to obtain phytic acid iron complex gel; (b) Mix elemental sulfur with bentonite to obtain a mixed powder; add the mixed powder to the sulfur-oxidizing Bacillus thiocyanate suspension, then add phytic acid iron complex gel, stir evenly to obtain a core mixture; (c) The kernel mixture was added to a sodium alginate solution, followed by the addition of a mixed solution of MnCl2 and CaCl2 to carry out a cross-linking reaction. After the reaction was completed, the mixture was washed and air-dried to obtain a composite sustained-release agent.

2. The composite reduction and repair method according to claim 1, characterized in that, The phytic acid solution in step (a) has a mass concentration of 8-12%, and the FeCl3 solution has a mass concentration of 5-8%. The phytic acid and FeCl3 solutions contain phytic acid and Fe... 3+ The molar ratio is 1:2 to 3, and the stirring treatment temperature is 20 to 30°C, and the time is 30 to 40 minutes.

3. The composite reduction and repair method according to claim 1, characterized in that, In step (b), the mass ratio of elemental sulfur to bentonite is 3–4:1, and the viable count of *Thiobacillus thiooxidans* in the suspension is 1 × 10⁻⁶. 8 ~5×10 8 The cfu / mL concentration of the *Thiobacillus thiooxidans* suspension and the mass ratio of the mixed powder are 1:9 to 11, the mass ratio of the phytic acid iron complex gel and the mixed powder is 1:7 to 9, and the stirring time is 20 to 30 min.

4. The composite reduction and repair method according to claim 1, characterized in that, The mass concentration of the sodium alginate solution in step (c) is 2-3%; the concentration of MnCl2 in the mixed solution is 7-9 mmol / L, and the concentration of CaCl2 is 40-60 mmol / L; the mass ratio of the core mixture, sodium alginate solution, and mixed solution is 1:3-5:2-4; the temperature of the crosslinking reaction is 20-30℃, and the time is 15-25 min; the air-drying temperature is ≤35℃, and the air-drying is carried out until the moisture content is ≤15%.

5. The composite reduction and repair method according to claim 1, characterized in that, The amount of compound slow-release agent added in step S1 is 20-33 kg / mu, and the depth of tillage into the soil is 10-15 cm.

6. The composite reduction and repair method according to claim 1, characterized in that, The high cadmium-accumulating rice varieties mentioned in step S2 are one or more of R900, Longliangyou 1212, and 9311.

7. The composite reduction and repair method according to claim 1, characterized in that, The specific operation of implementing mild wet-dry alternation during the tillering stage in step S3 is as follows: irrigate to a water depth of 3-5 cm on the field surface, keep the water for 5-7 days and then let it dry naturally until there is no standing water on the field surface, repeat 3-4 times; each time the field is re-irrigated, apply an 8%-12% ammonium thiosulfate solution with the irrigation water, at a rate of 13-27 L / mu.

8. The composite reduction and repair method according to claim 1, characterized in that, The specific operation of strengthening the alternation of dry and wet during the jointing-booting stage described in step S3 is as follows: irrigate to a water depth of 5-8 cm, retain water for 5-7 days, and then drain until small cracks appear on the field surface. Repeat this cycle 3-4 times. Each time the field is re-irrigated, apply an 8%-12% ammonium thiosulfate solution with the irrigation water at a rate of 27-40 L / mu. Each time the field is drained until the soil moisture content drops to 25%-35%, spray the leaves with a 0.5-1.0 mmol / L N-acetyl-L-cysteine ​​solution at a rate of 47-53 L / mu. The specific operation of drying the field during the grouting period is as follows: completely dry the field and do not re-irrigate; when switching to drying, apply a zinc sulfate solution with a concentration of 4-6 mmol / L along with the last batch of irrigation water, at a rate of 37-43 L / mu.

9. The composite reduction and repair method according to claim 1, characterized in that, The deep plowing in step S5 is 15-20cm deep, and the flood irrigation reaches a water depth of 15-20cm on the field surface.

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