Slow-release activating agent for strengthening phytoremediation of heavy metal contaminated soil and preparation method of slow-release activating agent

Abstract

The invention discloses a slow-release activating agent for strengthening phytoremediation of heavy metal contaminated soil and a preparation method of the slow-release activating agent, and belongs to the technical field of heavy metal contaminated soil remediation. The method comprises the following steps: by taking sodium alginate as a carrier, embedding a biodegradable chelating agent glutamic acid tetrasodium diacetate (GLDA), and carrying out Fe < 3 + > gradient cross-linking and ethanol phase separation treatment, so as to construct the microsphere slow-release agent with a hierarchical porous structure with a compact outer layer and a loose inner layer. The microsphere slow-release agent prepared by the invention can realize intelligent slow release of GLDA, avoids initial burst release, reduces the risk of heavy metal leaching, improves the absorption efficiency of plants to heavy metals, has a remarkable repair effect, and has good environmental adaptability and popularization and application prospects.

Description

Technical Field

[0001] This invention relates to the field of heavy metal contaminated soil remediation technology, specifically to a slow-release activator for enhancing phytoremediation of heavy metal contaminated soil and its preparation method. Background Technology

[0002] Human activities such as mining, metal smelting, wastewater irrigation, and the irrational use of chemicals containing heavy metals have led to increasingly serious soil heavy metal pollution problems, with high rates of contamination at arable land sampling points. Cadmium (Cd), lead (Pb), and zinc (Zn) have become major pollutants. These pollutants are characterized by high toxicity, poor degradation, and easy accumulation, and can be continuously transferred and accumulated through the food chain, ultimately posing a serious threat to human health. Therefore, the remediation and restoration of contaminated soil has become a major and unavoidable challenge. Faced with large areas of low- to medium-level contaminated sites, traditional physicochemical remediation technologies (such as topsoil replacement, leaching, and solidification stabilization) often have limitations such as high cost, complex operation, and damage to soil structure and ecological functions. Phytoremediation technology, which utilizes hyperaccumulating plants to absorb, fix, and transfer heavy metals from the soil, is considered a promising green remediation approach. However, the widespread application and remediation efficiency of this technology are constrained by a key bottleneck: most heavy metals in the soil have low bioavailability, resulting in poor plant absorption efficiency and a long remediation cycle.

[0003] By adding chelating agents to the soil, soluble complexes are formed with heavy metal ions, thereby "activating" them from the soil solid phase, significantly improving their bioavailability, and making them available for plant absorption. This ultimately enhances remediation efficiency and shortens the remediation cycle. While traditional chelating agents (such as EDTA) have significant activation effects, they are difficult to degrade in the environment, have long residual times, and easily cause intense activation and downward leaching of heavy metals, posing a serious risk of secondary pollution. The new generation of biodegradable green chelating agent, tetrasodium glutamate diacetate (GLDA), has solved the problems of biotoxicity and residue to some extent. However, its direct application faces the challenge of initial explosive release. Large-scale release of GLDA in a short period not only leads to rapid loss of effective components, but the instantaneous high-concentration complexes formed with heavy metals may also be toxic to plant roots, far exceeding the instantaneous absorption capacity of plants, greatly increasing the risk of heavy metal leaching into deeper soil or groundwater. Therefore, there is an urgent need to develop a material that can efficiently activate heavy metals in contaminated soil and improve plant absorption efficiency. Summary of the Invention

[0004] To address the issues of low bioavailability of most heavy metals in soil, poor plant uptake efficiency, and the initial explosive release of tetrasodium glutamate diacetate (GLDA), the present invention aims to provide a slow-release activator for enhancing phytoremediation of heavy metal-contaminated soil. This slow-release activator is a novel functional material capable of precisely regulating GLDA release behavior, which can slowly release GLDA, effectively prolonging the duration of action and continuously enhancing the effect of phytoremediation of heavy metal-contaminated soil.

[0005] Another objective of this invention is to provide a method for preparing a slow-release activator for enhancing phytoremediation of heavy metal-contaminated soil, specifically comprising the following steps: S1. Encapsulation: Mix the tetrasodium glutamate diacetate aqueous solution with the sodium alginate aqueous solution and stir to obtain a mixture.

[0006] S2, Droplet molding and gradient crosslinking: The mixture in S1 is dropped into FeCl3 aqueous solution, and gradient crosslinking is performed in FeCl3 aqueous solutions of different concentrations. After washing, the microsphere sustained-release agent is preliminarily formed.

[0007] S3, Dehydration and Stabilization Structure: The preliminary microspheres in S2 are immersed in an aqueous ethanol solution, allowed to stand, filtered, and washed to obtain a stable microsphere sustained-release agent.

[0008] S4. Post-processing: The stable microsphere slow-release agent in S3 is pre-frozen and vacuum freeze-dried to obtain the slow-release activator.

[0009] Preferably, in step S1 of the present invention, the mass percentage concentration of the tetrasodium diacetate aqueous solution is 47%, and the mass percentage concentration of the sodium alginate aqueous solution is 3%.

[0010] Preferably, in step S1 of the present invention, the volume ratio of tetrasodium glutamate diacetate aqueous solution and sodium alginate aqueous solution is 1:100.

[0011] Preferably, the stirring conditions in step S1 of the present invention are: magnetic stirring by water bath heating for 10-20 minutes at 45-65°C.

[0012] Preferably, the conditions for adding the mixture in step S2 of the present invention are: the mixture is added at a rate of 2-3 mL / min at a distance of 8-12 cm from the liquid surface.

[0013] Preferably, the static crosslinking in step S2 of the present invention specifically involves: first dripping the mixture into a 1% FeCl3 aqueous solution and letting it stand for 6-15 minutes, then removing the microspheres, and then placing them in a 1.5% FeCl3 aqueous solution and letting them stand for 6-15 minutes, then removing the microspheres again, and then placing them in a 2% FeCl3 aqueous solution and letting them stand for 6-15 minutes, and then removing the microspheres again.

[0014] Preferably, the washing conditions in step S2 of the present invention are: washing with water 3-5 times.

[0015] Preferably, the volume percentage concentration of the ethanol aqueous solution in step S3 of the present invention is 20%.

[0016] Preferably, the settling time in step S3 of the present invention is 15-25 minutes.

[0017] Preferably, the washing conditions in step S3 of the present invention are: washing with water 3-5 times.

[0018] Preferably, the pre-freezing conditions in step S4 of the present invention are: freezing at -18°C for 12-24 hours.

[0019] Preferably, the conditions for vacuum freeze-drying in step S4 of the present invention are: freeze-drying at -80°C for 12-20 hours, with a vacuum degree ≤10 Pa.

[0020] Mechanism of the invention: The invention utilizes a triple synergistic strategy of "embedding-gradient crosslinking-ethanol phase separation" to jointly construct intelligent sustained-release microspheres with a multi-level porous structure of dense outer layer and porous inner layer, thereby achieving controllable and continuous release of GLDA. First, sodium alginate (SA) physically encapsulates GLDA uniformly within a three-dimensional network, forming hydrogen bonds and electrostatic interactions that initially delay GLDA dissolution, laying the foundation for sustained release. Then, a gradually increasing FeCl3 concentration gradient (1%→1.5%→2%) is used for cross-linking, creating a cross-linking density gradient in the microspheres: the highly cross-linked outer layer forms a dense barrier, effectively inhibiting the initial burst release of GLDA; the low-cross-linked inner layer maintains a loose structure to store GLDA, supporting slow release in the later stages. This allows for a spatiotemporally programmed match with the absorption rhythm of heavy metals by plant roots, avoiding toxicity and leaching risks. Ethanol treatment induces phase separation in the SA network, forming interconnected multi-level channels, significantly enhancing porosity and connectivity, reducing GLDA diffusion resistance, preventing structural density, and ensuring efficient release of internal GLDA. The synergistic effect of encapsulation, ethanol phase separation, and gradient cross-linking significantly improves GLDA release efficiency and utilization while maintaining the overall stability of the microspheres.

[0021] Compared with existing technologies, this invention provides a slow-release activator for enhancing phytoremediation of heavy metal contaminated soil and its preparation method, which has the following beneficial effects: (1) The present invention is simple to operate, the materials are readily available, and it can be used on a large scale.

[0022] (2) This invention breaks through the limitations of traditional uniform cross-linking and innovatively adopts Fe from the surface to the inside. 3+The gradient cross-linking strategy aims to construct a gradient structure with a dense outer layer and a loose inner layer inside the microsphere, thereby spatially controlling the diffusion rate of GLDA and achieving a slow and continuous release mode.

[0023] (3) Based on gradient crosslinking, this invention introduces an ethanol phase separation post-processing process. This step aims to induce phase separation in the microsphere network and construct a multi-level pore system, fundamentally solving the problem of high mass transfer resistance inside hydrogel materials and ensuring that the internal GLDA can be efficiently released and utilized.

[0024] (4) This invention prepares a smart slow-release material that matches the plant’s absorption rhythm by using the dual synergistic effect of gradient crosslinking to control release kinetics and ethanol phase separation to optimize release channels. This material can avoid the initial burst release of GLDA, reduce the risk of leaching of heavy metals into deep soil or groundwater, and reduce plant toxicity. It can also maintain the effective activation concentration in the rhizosphere for a long time and continuously improve the bioavailability of heavy metals. This achieves the unity of maximizing activation effect and minimizing environmental risk, providing a new material basis and technical solution for the green, efficient and safe remediation of heavy metal contaminated soil.

[0025] (5) The slow-release activator prepared by the present invention can promote plant growth and promote the absorption of heavy metals in heavy metal polluted soil. The removal rate of Cd can reach 86.33%, the removal rate of Pb can reach 70.11%, and the removal rate of Zn can reach 85.91%. Detailed Implementation

[0026] 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.

[0027] The tetrasodium diacetate aqueous solution used in the embodiments and comparative examples of this invention has a mass percentage concentration of 47%, the sodium alginate aqueous solution has a mass percentage concentration of 3%, and the ethanol aqueous solution has a volume percentage concentration of 20%. The soil used in the embodiments and comparative examples is soil from the vicinity of a mining area in Yunnan Province, and the plants are all purchased from the same seller, with plants of similar growth selected for planting.

[0028] Example 1 The preparation of a slow-release activator for enhancing phytoremediation of heavy metal contaminated soil includes the following steps: S1. Encapsulation: Mix tetrasodium glutamate diacetate aqueous solution and sodium alginate aqueous solution at a volume ratio of 1:100, and stir magnetically for 15 min by heating in a water bath at 55°C to obtain a mixture encapsulated with GLDA.

[0029] S2. Droplet molding and gradient crosslinking: The mixture in S1 was dripped into FeCl3 aqueous solution at a rate of 2 mL / min from a distance of 10 cm above the liquid surface using a 1 ml syringe (needle inner diameter of 0.5 mm and length of 20 mm). Gradient crosslinking was carried out in FeCl3 aqueous solutions of different concentrations. Specifically, the mixture was first dripped into FeCl3 aqueous solution with a mass percentage concentration of 1% and allowed to stand for 10 min. The microspheres were then removed and placed into FeCl3 aqueous solution with a mass percentage concentration of 1.5% and allowed to stand for 10 min. The microspheres were then removed and placed into FeCl3 aqueous solution with a mass percentage concentration of 2% and allowed to stand for 10 min. The microspheres were then removed and washed three times with ultrapure water to obtain the pre-formed microsphere sustained-release agent.

[0030] S3, Dehydration and Stabilization Structure: The preliminary microspheres in S2 are immersed in an ethanol aqueous solution and allowed to stand for 20 minutes, then washed three times with ultrapure water to obtain the stabilized microsphere sustained-release agent.

[0031] S4. Post-processing: The stable microsphere slow-release agent in S3 is pre-frozen in a refrigerator at -18℃ for 12 hours, and then transferred to a freeze dryer and completely dried for 20 hours at a temperature of -80℃ and a vacuum degree of ≤10p to obtain the final slow-release activator.

[0032] The slow-release activator prepared in this embodiment was mixed with soil and planted with Sedum aizoon. After 90 days of planting, the plant height was 18.01 cm. The contents of Cd, Pb and Zn in the soil before and after planting were measured. It was calculated that the slow-release activator prepared in this embodiment had a removal rate of 86.33% for Cd, a removal rate of 70.11% for Pb and a removal rate of 85.91% for Zn.

[0033] Example 2 The preparation of a slow-release activator for enhancing phytoremediation of heavy metal contaminated soil includes the following steps: S1. Encapsulation: Mix tetrasodium glutamate diacetate aqueous solution and sodium alginate aqueous solution at a volume ratio of 1:100, and stir magnetically for 20 min by water bath heating at 45℃ to obtain a mixture encapsulated with GLDA.

[0034] S2. Droplet molding and gradient crosslinking: The mixture from S1 was dripped into FeCl3 aqueous solution at a rate of 2.5 mL / min from a distance of 8 cm above the liquid surface using a 1 ml syringe (needle inner diameter of 0.5 mm and length of 20 mm). Gradient crosslinking was performed in FeCl3 aqueous solutions of different concentrations. Specifically, the mixture was first dripped into FeCl3 aqueous solution with a mass percentage concentration of 1% and allowed to stand for 6 min. The microspheres were then removed and placed into FeCl3 aqueous solution with a mass percentage concentration of 1.5% and allowed to stand for 6 min. The microspheres were then removed and placed into FeCl3 aqueous solution with a mass percentage concentration of 2% and allowed to stand for 6 min. The microspheres were then removed and washed 4 times with ultrapure water to obtain the pre-formed microsphere sustained-release agent.

[0035] S3, Dehydration and Stabilization Structure: The preliminary microspheres in S2 were immersed in an ethanol aqueous solution for 15 min for dehydration treatment, and then washed 4 times with ultrapure water to obtain the stabilized microsphere sustained-release agent.

[0036] S4. Post-processing: The stable microsphere slow-release agent in S3 is pre-frozen in a refrigerator at -18℃ for 18 hours, and then transferred to a freeze dryer and completely dried for 16 hours at a temperature of -80℃ and a vacuum degree of ≤10p to obtain the final slow-release activator.

[0037] The slow-release activator prepared in this embodiment was mixed with soil and planted with Sedum aizoon. After 90 days of planting, the plant height was 17.89 cm. The contents of Cd, Pb and Zn in the soil before and after planting were measured. It was calculated that the slow-release activator prepared in this embodiment had a removal rate of 85.47% for Cd, a removal rate of 68.55% for Pb and a removal rate of 82.31% for Zn.

[0038] Example 3 The preparation of a slow-release activator for enhancing phytoremediation of heavy metal contaminated soil includes the following steps: S1. Encapsulation: Mix tetrasodium glutamate diacetate aqueous solution and sodium alginate aqueous solution at a volume ratio of 1:100, and stir magnetically for 10 min by water bath heating at 65℃ to obtain a mixture encapsulated with GLDA.

[0039] S2. Droplet molding and gradient crosslinking: The mixture in S1 was dripped into FeCl3 aqueous solution at a rate of 3 mL / min from a distance of 12 cm above the liquid surface using a 1 ml syringe (needle inner diameter of 0.5 mm and length of 20 mm). Gradient crosslinking was carried out in FeCl3 aqueous solutions of different concentrations. Specifically, the mixture was first dripped into FeCl3 aqueous solution with a mass percentage concentration of 1% and allowed to stand for 15 min. The microspheres were then removed and placed into FeCl3 aqueous solution with a mass percentage concentration of 1.5% and allowed to stand for 15 min. The microspheres were then removed and placed into FeCl3 aqueous solution with a mass percentage concentration of 2% and allowed to stand for 15 min. The microspheres were then removed and washed 5 times with ultrapure water to obtain the pre-formed microsphere sustained-release agent.

[0040] S3, Dehydration and Stabilization Structure: The preliminary microspheres in S2 were immersed in an ethanol aqueous solution for 25 min for dehydration treatment, and then washed 5 times with ultrapure water to obtain a stable microsphere sustained-release agent.

[0041] S4. Post-processing: The stable microsphere slow-release agent in S3 is pre-frozen in a refrigerator at -18℃ for 24 hours, and then transferred to a freeze dryer and completely dried for 12 hours at a temperature of -80℃ and a vacuum degree of ≤10p to obtain the final slow-release activator.

[0042] The slow-release activator prepared in this embodiment was mixed with soil and planted with Sedum aizoon. After 90 days of planting, the plant height was 17.88 cm. The contents of Cd, Pb and Zn in the soil before and after planting were measured. It was calculated that the slow-release activator prepared in this embodiment had a removal rate of 85.02% for Cd, a removal rate of 67.98% for Pb and a removal rate of 81.69% for Zn.

[0043] The slow-release agents prepared in Examples 1-3 of this invention exhibit significant growth-promoting effects on plants and enhance the activation and absorption of Cd, Pb, and Zn by plants.

[0044] Comparative Example 1 The difference between this comparative example and Example 1 is that in S1, the volume ratio of tetrasodium glutamate diacetate aqueous solution to sodium alginate aqueous solution is 0.7:100, that is, the concentration of GLDA in the mixture is changed, while the other steps are the same.

[0045] The slow-release activator prepared in this comparative example was mixed with soil and planted with Sedum aizoon. After 90 days of planting, the plant height was 12.82 cm. The contents of Cd, Pb and Zn in the soil before and after planting were measured. It was calculated that the slow-release activator prepared in this example had a removal rate of 43.57% for Cd, 40.98% for Pb and 60.32% for Zn. The slow-release activator prepared in this comparative example had insufficient encapsulation strength due to the reduced SA concentration, resulting in excessively rapid release of GLDA, poor plant growth and low heavy metal removal capacity.

[0046] Comparative Example 2 The difference between this comparative example and Example 1 is that the mass percentage concentration of the gradient crosslinked FeCl3 aqueous solution in S2 is 0.5%, 1%, and 1.5% respectively. That is, the mechanical strength of the sustained-release microspheres is changed by changing the gradient of the crosslinking liquid. All other steps are the same.

[0047] The slow-release activator prepared in this comparative example was mixed with soil and planted with Sedum aizoon. After 90 days of planting, the plant height was 14.06 cm. The contents of Cd, Pb and Zn in the soil before and after planting were measured. It was calculated that the slow-release activator prepared in this example had a Cd removal rate of 60.43%, a Pb removal rate of 44.12% and a Zn removal rate of 65.56%. The slow-release activator prepared in this comparative example had a poor slow-release effect due to the change in crosslinking concentration, resulting in poor plant growth and low heavy metal removal capacity.

[0048] Comparative Example 3 The difference between this comparative example and Example 1 is that no ethanol phase separation treatment is performed; all other steps are the same.

[0049] The slow-release activator prepared in this comparative example was mixed with soil and planted with Sedum aizoon. After 90 days of planting, the plant height was 15.81 cm. The contents of Cd, Pb and Zn in the soil before and after planting were measured. It was calculated that the slow-release activator prepared in this example had a Cd removal rate of 73.08%, a Pb removal rate of 56.46% and a Zn removal rate of 71.27%. The slow-release activator prepared in this comparative example had poor pore structure due to insufficient phase separation caused by not being treated with ethanol, which hindered GLDA release, resulting in poor plant growth and low heavy metal removal capacity.

[0050] This invention selects *Sedum aizoon*, which has a strong ability to accumulate heavy metals, as a model plant. The slow-release agents prepared in Examples 1-3 and Comparative Examples 1-3 were added to the soil and mixed at a mass ratio of soil to slow-release agent of 200:1. One *Sedum aizoon* seedling with uniform growth was transplanted into each pot and managed uniformly in a controlled greenhouse. The soil was watered regularly and quantitatively to maintain soil moisture. The growth cycle was 90 days. After 90 days, the *Sedum aizoon* was harvested, and the biomass of the plants was measured. The contents of Cd, Pb, and Zn in the soil before and after planting were measured, and the removal rate of each heavy metal was calculated. The specific results are shown in Table 1.

[0051] Table 1 In summary, this invention uses sodium alginate as a carrier to encapsulate the biodegradable chelating agent tetrasodium glutamate diacetate (GLDA), via Fe... 3+Gradient crosslinking and ethanol phase separation treatment were used to construct a microsphere slow-release agent with a multi-level porous structure of dense outer layer and loose inner layer. This achieved the best match between the slow-release microsphere structure and release behavior, significantly improving the efficiency of phytoremediation and providing reliable technical support for the green remediation of heavy metal contaminated soil.

[0052] Although embodiments of the invention have been shown and described, it will be understood by those skilled in the art that various changes, modifications, substitutions and alterations can be made to these embodiments without departing from the principles and spirit of the invention, the scope of which is defined by the appended claims and their equivalents.

Claims

1. A method for preparing a slow-release activator for enhancing phytoremediation of heavy metal contaminated soil, characterized in that, Specifically, the following steps are included: S1. Encapsulation: Mix the tetrasodium glutamate diacetate aqueous solution with the sodium alginate aqueous solution and stir to obtain a mixture; S2, Drop casting and gradient crosslinking: The mixture in S1 is dropped into FeCl3 aqueous solution, and gradient crosslinking is performed in FeCl3 aqueous solutions of different concentrations. After washing, a preliminary microsphere sustained-release agent is obtained. S3, Dehydration and Stabilization Structure: The preliminary microspheres in S2 are immersed in an ethanol aqueous solution, allowed to stand, filtered, and washed to obtain a stable microsphere sustained-release agent; S4. Post-processing: The stable microsphere slow-release agent in S3 is pre-frozen and vacuum freeze-dried to obtain the slow-release activator.

2. The method for preparing the slow-release activator for enhancing phytoremediation of heavy metal contaminated soil according to claim 1, characterized in that, The mass percentage concentration of the tetrasodium diacetate aqueous solution in step S1 is 47%; the mass percentage concentration of the sodium alginate aqueous solution is 3%.

3. The method for preparing the slow-release activator for enhancing phytoremediation of heavy metal contaminated soil according to claim 1, characterized in that, In step S1, the volume ratio of tetrasodium glutamate diacetate aqueous solution to sodium alginate aqueous solution is 1:

100.

4. The method for preparing the slow-release activator for enhancing phytoremediation of heavy metal contaminated soil according to claim 1, characterized in that, The stirring conditions in step S1 are: magnetic stirring in a water bath at 45-65℃ for 10-20 minutes.

5. The slow-release activator for enhancing phytoremediation of heavy metal contaminated soil according to claim 1, and its preparation method, characterized in that, The conditions for adding the mixture in step S2 are as follows: the mixture is added at a rate of 2-3 mL / min from a distance of 8-12 cm from the liquid surface.

6. The method for preparing the slow-release activator for enhancing phytoremediation of heavy metal contaminated soil according to claim 1, characterized in that, The static crosslinking described in step S2 is as follows: the mixture is first dropped into a 1% FeCl3 aqueous solution and left to stand for 6-15 minutes. The microspheres are then removed and placed in a 1.5% FeCl3 aqueous solution and left to stand for 6-15 minutes. The microspheres are then removed and placed in a 2% FeCl3 aqueous solution and left to stand for 6-15 minutes. The microspheres are then removed again.

7. The method for preparing the slow-release activator for enhancing phytoremediation of heavy metal contaminated soil according to claim 1, characterized in that, The washing conditions in step S2 are: wash with water 3-5 times.

8. The method for preparing the slow-release activator for enhancing phytoremediation of heavy metal contaminated soil according to claim 1, characterized in that, The volume percentage concentration of the ethanol-water solution in step S3 is 20%; the standing time is 15-25 min; and the washing conditions are: washing with ultrapure water 3-5 times.

9. The method for preparing the slow-release activator for enhancing phytoremediation of heavy metal contaminated soil according to claim 1, characterized in that, The pre-freezing conditions in step S4 are: freezing at -18℃ for 12-24 hours; the vacuum freeze-drying conditions are: freeze-drying at -80℃ for 12-20 hours, with a vacuum degree ≤10 Pa.

10. A slow-release activator for enhancing phytoremediation of heavy metal contaminated soil prepared by the method according to any one of claims 1 to 9.

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