Soil remediation method combining plants and heavy metal composite treatment agent

Abstract

The invention discloses a soil remediation method combining plants and a heavy metal composite treatment agent, and relates to the technical field of environmental remediation, according to the method, core-shell structure microcapsules with rhizosphere response release characteristics are adopted as the composite treatment agent, a shell layer is sulfydryl functionalized nano-hydroxyapatite, and rapid fixation of heavy metal is achieved; the inner shell layer is a PHBV controlled release layer and responds to degradation of plant root exudates; and the core is loaded with an EDDS chelating agent and a pH buffering agent which are selectively released at the rhizosphere to activate heavy metals. The technical problems that antagonism exists between existing chemical immobilization and phytoextraction technologies, and synergistic interaction is difficult are solved, the contradiction between chemical remediation and phytoremediation is overcome, safe, efficient and sustainable soil heavy metal synergistic remediation and space-time coupling of non-rhizosphere region heavy metal stabilization and rhizosphere micro-region efficient activation are achieved, and the soil heavy metal remediation effect is improved. The heavy metal absorption efficiency of the hyperaccumulator is obviously improved.

Description

Technical Field

[0001] This invention relates to the field of environmental remediation technology, and in particular to a soil remediation method combining plant and heavy metal composite treatment agents. Background Technology

[0002] With the acceleration of global industrialization and the in-depth exploitation of mineral resources, soil pollution caused by heavy metals has become increasingly serious, posing a long-term and far-reaching threat to ecological environment security, agricultural product quality, and human health. Against this backdrop, developing efficient, economical, and environmentally friendly soil remediation technologies has become a core issue in the field of environmental science and engineering. Among numerous remediation strategies, the method combining phytoremediation and chemical stabilization is considered a highly promising technological development direction because it can balance the low disturbance of in-situ remediation, the ecological sustainability of bioremediation, and the high efficiency of chemical treatment.

[0003] The basic idea behind this type of technology is to utilize specific plant systems to absorb, enrich, or degrade pollutants in the soil, while simultaneously using chemical treatment agents to passivate heavy metal activity and improve soil physicochemical properties, aiming to achieve synergistic remediation. However, with in-depth research into the mechanisms of material migration and transformation within the remediation system, researchers have gradually realized that the strategy of simply combining phytoremediation with chemical treatments has a profound and subtle technical contradiction at the level of its underlying mechanism. This contradiction constitutes the core bottleneck restricting the efficiency and stability of the combined remediation technology. The reason for this is that chemical stabilization remediation and phytoremediation pursue diametrically opposed goals in the transformation of the target heavy metals into their forms. Therefore, when a composite treatment agent with the primary goal of reducing the bioavailability of heavy metals is applied to the soil, while achieving the immobilization of heavy metals, it inevitably forms a "competitive relationship" with the roots of the remediation plants for the target heavy metals. That is, the chemical treatment agent "locks" heavy metal ions that could be absorbed by plants into the soil mineral lattice or organic macromolecules, thus significantly inhibiting the extraction efficiency of plants. This inherent antagonistic effect means that the final remediation effect is not a simple arithmetic sum of the two technologies. In fact, the excessive passivation of chemical agents may lead to the near failure of the plant remediation pathway, which prolongs the entire remediation cycle and makes the subsequent treatment of plant biomass lose its fundamental significance as a "heavy metal carrier".

[0004] The current technological challenge is no longer simply about combining plants with chemical agents, nor merely about how to mix agents more evenly into the soil, but rather about fundamentally resolving the inherent conflict between chemical immobilization and plant uptake. This requires that the designed compound treatment agents and their application methods can achieve differentiated and precise control over heavy metal speciation. That is, while effectively reducing the environmental risks of some highly toxic and highly mobile heavy metals, they must selectively maintain or even enhance the bioavailability of target heavy metals required by specific hyperaccumulating plants. This dual function of both "immobilization" and "activation" places far more stringent demands on the formulation design, timing of action, and interaction with the plant root microenvironment than existing technologies. Summary of the Invention

[0005] The purpose of this invention is to provide a soil remediation method that combines plant and heavy metal composite treatment agents to solve the technical problem that existing chemical immobilization and plant extraction technologies have antagonistic effects and are difficult to synergistically enhance.

[0006] To address the aforementioned technical problems, this invention provides a soil remediation method combining plant and heavy metal composite treatment agents, the method comprising the following steps:

[0007] Step S1: Matching Analysis of Contaminated Site Investigation and Remediation System. This step aims to quantitatively characterize the soil physicochemical properties, types, concentrations, and speciation of heavy metal pollutants at the target remediation site, and select suitable hyperaccumulating plant species based on the characterization results. Specifically, soil samples were collected at a depth of 0-40 cm using a grid sampling method, and the total concentrations of cadmium, lead, arsenic, copper, and zinc in the soil were determined by inductively coupled plasma mass spectrometry. The European Community Standard Material Bureau (ECLS) continuous extraction method was used to analyze the speciation of the target heavy metal pollutants, quantifying the distribution ratios of their acid-extractable, reducible, oxidizable, and residual forms. Simultaneously, the soil pH, cation exchange capacity, organic matter content, and texture were measured. Based on the above analysis results, especially the types and concentrations of major pollutants, one or more hyperaccumulating plants with high bioaccumulation coefficients (BCF > 1000) and high translocation coefficients (TF > 1) for the target heavy metals were selected. Furthermore, high-performance liquid chromatography-mass spectrometry was used to qualitatively and quantitatively analyze the components of root exudates secreted by the selected hyperaccumulating plants under sterile hydroponic conditions, clarifying the types and secretion rates of characteristic low molecular weight organic acids, amino acids, and phenolic substances. The results of this analysis served as the basis for designing the response layer of the composite treatment agent in subsequent steps.

[0008] Step S2: Preparation of a rhizosphere-responsive core-shell composite treatment agent. This step aims to synthesize a core-shell microcapsule with dual functions, the functional switching of which is controlled by plant root exudates. The microcapsule consists of a core, an inner shell, and an outer shell.

[0009] The core component is a heavy metal activation unit, whose components, by mass percentage, include: 30-50% biodegradable chelating agent, 5-15% rhizosphere microzone pH regulator, and the remainder a porous carrier. The biodegradable chelating agent is S,S-ethylenediamine-N,N'-disuccinic acid (EDDS), which functions to form stable and water-soluble complexes with heavy metal ions for absorption by plant roots. The rhizosphere microzone pH regulator is a citrate-disodium hydrogen phosphate buffer system, designed to adjust the soil microenvironment pH in the release zone after microcapsule rupture to 5.0-5.8, a pH range optimized for heavy metal absorption by most hyperaccumulating plants. The porous carrier has an average pore size of 20 nm and a specific surface area of ​​400 nm. Mesoporous silica microspheres are used to load and stabilize chelating agents and pH adjusters.

[0010] The inner shell layer is a rhizosphere-responsive controlled-release layer made of poly(β-hydroxybutyrate-valerate) copolymer (PHBV), in which the molar fraction of valerate units is 10-15%. This PHBV layer maintains its structural integrity in the main soil environment, but degrades due to ester bond hydrolysis under the synergistic action of specific esterases secreted by the roots of hyperaccumulating plants and low-molecular-weight organic acids, thereby achieving the targeted release of the core substance. The thickness of the inner shell layer is controlled at 5-10 micrometers.

[0011] The outer shell layer is a rapid heavy metal immobilization layer, made of nano-hydroxyapatite (n-HAP) functionalized with thiol (-SH). The nano-hydroxyapatite has a particle size of 20-50 nm, and its surface is loaded with thiol functional groups through a grafting reaction with 3-mercaptopropionic acid, achieving a loading density of 0.5-1.5 mmol / g. This outer shell layer utilizes the ion exchange and precipitation effects of hydroxyapatite on divalent metal ions such as lead (Pb), and the strong complexing effect of thiol groups on soft metal ions such as cadmium (Cd). In the initial stage of microcapsule application to the soil, it rapidly adsorbs and immobilizes highly reactive heavy metal ions in the soil solution, reducing their migration and biotoxicity.

[0012] The preparation process of the composite treatment agent includes the following sub-steps:

[0013] Sub-step S2-1: Preparation of core materials. EDDS and citrate-disodium hydrogen phosphate buffer are dissolved in deionized water to form a homogeneous solution with a mass concentration of 20%. This solution is mixed with mesoporous silica microspheres that have been activated at 120°C for 4 hours under vacuum impregnation conditions. After impregnation for 12 hours, the mixture is vacuum dried at 60°C for 24 hours to obtain core particles loaded with the activated components.

[0014] Sub-step S2-2: Coating of the inner shell layer. Fluidized bed bottom spray coating technology is used, dissolving PHBV in dichloromethane to form a 5% (w / w) coating solution. Using the core particles obtained in sub-step S2-1 as the fluidized bed substrate, spray coating is performed under conditions of 70℃ inlet air temperature, 45℃ material temperature, and a spray rate of 15 g / min, until the coating weight gain reaches 20-30% of the core particle mass, forming an intermediate coated with a PHBV inner shell layer.

[0015] Sub-step S2-3: Coating of the outer shell layer. Thiol-functionalized nano-hydroxyapatite is dispersed in an aqueous solution containing 0.1% polyvinyl alcohol (PVA) to form a stable suspension with a mass fraction of 10%. The intermediate obtained in sub-step S2-2 is added to this suspension. Under mechanical stirring, the pH is controlled at 7.5. The nano-hydroxyapatite is guided to deposit on the PHBV surface by utilizing the difference in surface charge of the particles. Moisture is removed by spray drying, ultimately forming complete core-shell structured composite microcapsules with an average particle size of 300-600 micrometers.

[0016] Step S3: Application of the compound treatment agent and land preparation. Based on the total heavy metal analysis results of the soil in Step S1, determine the application rate of the compound treatment agent, calculated according to the standard of 2-5 grams of compound treatment agent per kilogram of contaminated soil (dry weight). Use a rotary tiller to evenly mix the compound treatment agent into the top 0-20 cm of soil. After application, irrigate once to bring the soil moisture content to 70-80% of field capacity, and then allow it to mature for 7-10 days. This maturation process aims to fully utilize the rapid immobilization effect of the compound treatment agent's outer shell.

[0017] Step S4: Transplanting and Field Management of Hyperaccumulating Plants. After the maturation period, the hyperaccumulating plants selected in Step S1 and in the seedling stage were transplanted to the remediation site at a density of 20 cm between plants and 20 cm between rows. Throughout the plant's growth cycle, routine field management was carried out, including regular irrigation to maintain soil moisture and application of a compound nitrogen-potassium fertilizer free of phosphorus and target heavy metals to avoid competitive adsorption of exogenous phosphorus with the hydroxyapatite outer shell of the treatment agent, while meeting the basic nutritional needs of plant growth.

[0018] Step S5: Monitoring and endpoint determination of the remediation process. During key plant growth stages—tillering, vigorous growth, and maturity—soil and plant samples are collected periodically for dynamic monitoring. Soil sampling is conducted in two areas: rhizosphere soil and non-rhizosphere soil. Rhizosphere soil is defined as soil within 2 mm of the root surface, and non-rhizosphere soil is defined as soil beyond 20 cm from the root surface. The total and available heavy metal content in the soil of both areas is determined using the DTPA extraction method. Plant samples are divided into root and aboveground parts. After drying and digestion, the heavy metal content is determined. The synergistic effect of the remediation process is assessed by comparing the differences in available heavy metal content between rhizosphere and non-rhizosphere soils, and the differences in heavy metal accumulation in plants between the treatment group and the control group (plants were planted but no treatment agents were applied). When the total concentration of the target heavy metal in the soil is lower than the risk screening value in the national "Soil Environmental Quality Agricultural Land Soil Pollution Risk Control Standard" after two consecutive growing seasons of harvesting, and the heavy metal content in the aboveground parts of the plant tends to stabilize and no longer increases significantly after three consecutive sampling results, the remediation is deemed to have reached its endpoint.

[0019] Step S6: Plant Harvesting and Subsequent Disposal. At the end of the remediation period or each growing season, the above-ground parts of the hyperaccumulating plants are harvested. The harvested plant biomass is dried to constant weight at 80°C and then transported to a specialized incineration facility for incineration under controlled oxygen conditions at 850°C. The fly ash rich in heavy metals produced during incineration is collected by a bag filter and then solidified / stabilized to meet the requirements of the Hazardous Waste Identification Standard before being safely disposed of in a hazardous waste landfill. Alternatively, the high-concentration heavy metal ash can be extracted using hydrometallurgical methods to recover valuable metals.

[0020] In a preferred embodiment of the present invention, in step S2, the thiol-functionalized modified nano-hydroxyapatite of the outer shell layer can be further compounded with 10-20% by mass of montmorillonite. The introduction of montmorillonite can improve the dispersibility of the composite treatment agent in the soil and utilize its layered structure to provide additional heavy metal ion adsorption sites, thereby enhancing the initial immobilization effect.

[0021] Furthermore, in the field management of step S4, specific rhizosphere microbial agents can be applied synergistically. These agents include phosphate-solubilizing bacteria and siderophore-producing bacteria, such as *Bacillus megaterium* and *Pseudomonas aeruginosa*. Phosphate-solubilizing bacteria can activate insoluble phosphates in the soil, providing phosphorus nutrition to plants and reducing dependence on phosphate fertilizers; siderophores secreted by siderophores can form stable complexes with iron ions, reducing competition between iron ions and EDDS, thereby improving the chelation efficiency of EDDS for target heavy metals.

[0022] Specifically, in step S5, the monitoring system of the remediation method further includes deploying an in-situ microelectrode sensor array to monitor the pH and redox potential (Eh) of the rhizosphere microzone soil in real time. This real-time data can provide a basis for fine-tuning the remediation process. For example, when the rhizosphere pH is detected to deviate from the optimal absorption window, a small amount of acid or alkali can be added through the drip irrigation system for fine-tuning.

[0023] Compared with existing methods, the beneficial technical effects of the present invention are as follows:

[0024] The composite treatment agent of this invention ingeniously solves the inherent contradiction between chemical immobilization and plant absorption: initially, the outer shell plays the role of "guardian", quickly reducing the activity of toxic heavy metals in the soil and controlling the risk of pollution spread; subsequently, when the roots of the repair plant extend to the vicinity of the treatment agent, its secretions become a "key", precisely opening the inner shell and releasing the core "activator".

[0025] This invention, through activation, strictly confines heavy metals within a few millimeters of space around the root system, forming a local hotspot area with "high bioavailability," which greatly facilitates absorption by plant roots, while in the vast non-rhizosphere soil, heavy metals remain in a stable state fixed by the outer shell.

[0026] This invention enables a paradigm shift in the entire remediation process from "mutual competition" to "orderly relay" through precise decoupling and coupling in time and space. This significantly shortens the remediation cycle, increases the amount of heavy metals removed per unit of biomass, and ultimately achieves the goal of efficient, safe, and economical soil remediation. Attached Figure Description

[0027] Figure 1 This is a schematic flowchart of a soil remediation method combining plant and heavy metal composite treatment agents according to the present invention. Detailed Implementation

[0028] To make the objectives, technical solutions, and advantages of this invention clearer, a soil remediation method combining plant and heavy metal composite treatment agents disclosed in this invention will be described in detail below with reference to the accompanying drawings and specific embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and not intended to limit it.

[0029] Reference Figure 1The flowchart shown illustrates a soil remediation method combining phytoremediation and heavy metal composite treatment agents provided by this invention. The core of this method lies in the use of a specially designed heavy metal composite treatment agent with rhizosphere microenvironment-responsive release characteristics. Through a series of precise and synergistic steps, this method achieves the orderly coupling of the chemical immobilization of heavy metal pollutants in the soil with the phytoremediation process in both time and space, thereby fundamentally solving the antagonistic effect between the two processes in traditional combined remediation technologies.

[0030] In a specific implementation process, this remediation method begins with step S1, namely, a detailed investigation of the contaminated site and a compatibility analysis of the remediation system. This step is fundamental to the success of the entire remediation project. Specifically, for the target site to be remediated, a checkerboard or quincunx sampling method is used, with a grid density of 10 meters × 10 meters, to collect soil samples from a depth of 0 to 40 centimeters below the surface using a soil sampling drill. After removing stones, plant debris, and other impurities, the soil sample collected from each sampling point is reduced to approximately 1 kilogram using a quartering method, placed in clean polyethylene self-sealing bags, numbered, and stored in a portable refrigerator at 4°C for analysis. After air-drying, grinding, and passing through a 100-mesh nylon sieve, the collected soil samples are digested using inductively coupled plasma mass spectrometry (ICP-MS) to determine the total concentration of target heavy metal pollutants, such as cadmium, lead, arsenic, copper, and zinc. Meanwhile, to gain a deeper understanding of the environmental behavior and bioavailability of pollutants, the sequential extraction method recommended by the European Community Standard Material Bureau was used to analyze the speciation of key heavy metal pollutants, thereby quantifying the relative distribution ratios of their acid-extractable, reducible, oxidizable, and residual forms in the soil. The acid-extractable form represents the relative distribution ratio with the highest activity and bioavailability; the reducible form is the relative distribution ratio bound to iron and manganese oxides; the oxidizable form is the relative distribution ratio bound to organic matter and sulfides; and the residual form is the relative distribution ratio stably existing in the mineral lattice. Furthermore, a comprehensive characterization of the basic physicochemical properties of the soil was required, including measuring soil pH (water-to-soil ratio 2.5:1) using a pH meter, determining cation exchange capacity using the ammonium acetate exchange method, determining organic matter content using the potassium dichromate volumetric method, and determining soil texture using a laser particle size analyzer.

[0031] Based on the detailed site characterization data mentioned above, especially the types and concentration levels of dominant heavy metal pollutants in the soil, hyperaccumulating plant species were screened and matched. The selection criteria were extremely stringent, requiring the selected plants to have extremely high bioaccumulation coefficients for one or more target heavy metals, defined as the ratio of the heavy metal concentration in the aboveground parts of the plant to the concentration of that heavy metal in the soil, typically requiring a value greater than 1000; at the same time, their translocation coefficients, defined as the ratio of the heavy metal concentration in the aboveground parts of the plant to the heavy metal concentration in the roots, needed to be greater than 1, to ensure that pollutants can be efficiently transferred from the roots to the easily harvested aboveground parts.

[0032] For soils primarily contaminated with cadmium and zinc, the Cd / Zn hyperaccumulating ecotype *Sedum aizoon* is a suitable choice; for arsenic-contaminated sites, *Pteris vittata* is an ideal option. After selecting the plant species, the next step is not immediately followed by a detailed analysis of the chemical composition of its root exudates. This process involves cultivating the selected plants in a sterile hydroponic environment, periodically collecting the culture medium, and analyzing it using high-performance liquid chromatography-mass spectrometry (HPLC-MS). The analysis aims to qualitatively and quantitatively identify the main low-molecular-weight organic acids, amino acids, phenols, and sugars exuded by the plant roots, and to determine their exudation rates at different growth stages. These data are crucial for designing the inner shell response characteristics of the composite treatment agent in subsequent step S2, ensuring that the release behavior of the treatment agent precisely matches the physiological activities of the specific plant.

[0033] Step S2, namely the preparation of the rhizosphere-responsive core-shell composite treatment agent, is the core material of this invention, with a sophisticated structure and distinct functions. The composite treatment agent microcapsule comprises, from the inside out, a core, an inner shell, and an outer shell. The core is a heavy metal activation unit, whose function is to enhance the solubility and mobility of heavy metals in the rhizosphere microzone after release triggered by plant roots. Its components, by mass percentage, include 30% to 50% of a biodegradable chelating agent, 5% to 15% of a rhizosphere microzone pH regulator, and the remainder a porous carrier. In a specific formulation, the biodegradable chelating agent is S,S-ethylenediamine-N,N'-disuccinic acid (EDDS), which has a strong chelating ability for various divalent heavy metal ions and can be degraded by microorganisms in the environment, avoiding secondary pollution. The preferred rhizosphere microenvironment pH regulator is a citrate-disodium hydrogen phosphate buffer system, precisely calculated to stabilize the local soil microenvironment's pH within a slightly acidic range of 5.0 to 5.8 after the release of the core substances. This range represents the physiologically optimal range for most hyperaccumulating plants to absorb heavy metal ions. The porous carrier is selected from mesoporous silica microspheres with high specific surface area and regular pore structure. Its function is to efficiently load and protect the chelating agent and pH regulator, preventing premature inactivation during encapsulation and storage.

[0034] The inner shell layer is crucial for achieving the core function of this invention: the rhizosphere-responsive controlled-release layer. It is made of poly(β-hydroxybutyrate-valerate) copolymer (PHBV), a biodegradable polyester. By controlling the molar fraction of valerate units in the copolymer between 10% and 15%, its crystallinity and degradation rate can be precisely adjusted. In the soil environment far from plant roots, this PHBV layer maintains its structural integrity due to relatively mild microbial activity, providing effective physical barrier properties. However, when the roots of hyperaccumulating plants grow nearby, the esterases specific to root exudates and the high concentration of low-molecular-weight organic acids (the types and concentrations of which have been determined in step S1) synergistically catalyze the hydrolysis of the ester bonds in PHBV, leading to the gradual disintegration of the inner shell layer structure and precisely triggering the release of the core substance in the rhizosphere region. The thickness of the inner shell layer is strictly controlled between 5 and 10 micrometers to ensure sufficient mechanical strength and barrier performance, while also enabling degradation within a reasonable timeframe induced by root exudates.

[0035] The outer shell layer plays a crucial role in the rapid immobilization of heavy metals. It is made of surface-functionalized nano-hydroxyapatite (n-HAP). Specifically, nano-hydroxyapatite with a particle size ranging from 20 to 50 nanometers is selected and grafted with 3-mercaptopropionic acid, resulting in the covalent bonding of numerous thiol (-SH) functional groups to its surface. The loading density can be controlled between 0.5 and 1.5 mmol / g. This outer shell layer design embodies the synergistic effect of multiple immobilization mechanisms. On one hand, hydroxyapatite itself can efficiently immobilize heavy metal ions such as lead through ion exchange and surface precipitation. On the other hand, the introduced thiol groups, as soft affinity groups, have a strong complexing ability for soft metal ions such as cadmium, forming highly stable thiolates. Therefore, when the composite treatment agent microcapsules are applied to the soil, their outer shell layer can rapidly react with highly reactive heavy metal ions in the soil solution during the initial remediation phase, immobilizing them on the microcapsule surface. This effectively reduces their migration and early toxicity to plants, creating a safer environmental window for plant establishment and growth.

[0036] The preparation process of the composite treatment agent is itself a precise engineering operation. First, sub-step S2-1, namely the preparation of the core material, is performed. Analytical grade EDDS is dissolved in deionized water with a calculated ratio of citric acid and disodium hydrogen phosphate buffer to prepare a homogeneous solution with a mass concentration of 20%. Simultaneously, mesoporous silica microspheres are activated in a muffle furnace at 120°C for 4 hours to remove adsorbed moisture and impurities. Subsequently, in a vacuum impregnation device, the activated mesoporous silica microspheres are mixed with the above solution, and a vacuum of -0.09 MPa is applied and maintained for 12 hours to ensure that the solution fully penetrates the mesoporous structure of the silica. After impregnation, the mixture is transferred to a vacuum drying oven and vacuum-dried at 60°C for 24 hours until all moisture is removed, thereby obtaining core particles loaded with the activated components.

[0037] Next, sub-step S2-2, namely the coating of the inner shell, is performed. This process employs fluidized bed bottom spray coating technology, an advanced technology capable of achieving uniform and dense film coating. PHBV with a specific molar fraction of valerate is dissolved in dichloromethane to form a coating solution with a mass fraction of 5%. The core particles obtained in sub-step S2-1 are used as the fluidized substrate and placed in a fluidized bed coating machine. Spraying is carried out under strictly controlled process parameters, such as setting the inlet air temperature to 70°C, stabilizing the material temperature at 45°C by adjusting the spray rate and atomizing pressure, and controlling the spray rate at 15 g / min. By monitoring the weight gain of the particles online, when the coating weight gain reaches 20% to 30% of the original mass of the core particles, spraying is stopped, and fluidized drying continues for several minutes to obtain intermediate particles completely coated with a PHBV inner shell.

[0038] Finally, sub-step S2-3, namely the coating of the outer shell layer, is completed. First, a stable suspension of the outer shell layer material is prepared. Thiol-functionalized nano-hydroxyapatite powder is dispersed in an aqueous solution containing 0.1% polyvinyl alcohol, and dispersed using a high-shear homogenizer or ultrasonic dispersion to form a uniform and stable suspension with a mass fraction of 10%. Then, the intermediate particles coated with PHBV obtained in sub-step S2-2 are added to this suspension. Under gentle mechanical stirring, the pH of the suspension is precisely adjusted to 7.5 by adding dilute acid or dilute alkali. At this pH, the surface of PHBV is typically negatively charged, while the surface of nano-hydroxyapatite exhibits a different charge. Utilizing this difference in surface charge between particles and the bridging effect of PVA, heterogeneous deposition of nano-hydroxyapatite particles on the PHBV surface is guided. After deposition for a period of time, the entire system is rapidly dried using a spray-drying method to remove moisture, ultimately obtaining a composite microcapsule product with uniform particle size distribution, intact structure, and a three-layer core-shell structure.

[0039] In step S3, the compound treatment agent is applied and the land is prepared. Based on the total heavy metal concentration and contamination depth of the soil measured in step S1, the theoretical application rate of the compound treatment agent is calculated. Typically, the application standard is determined by applying 2 to 5 grams of compound treatment agent per kilogram of contaminated soil, which translates to approximately 3 to 7.5 tons per hectare. Using a rotary tiller paired with a tractor, the calculated compound treatment agent is evenly spread on the surface, followed immediately by rotary tillage to ensure thorough and uniform mixing of the treatment agent with the top 0 to 20 cm of soil. After mixing, a thorough irrigation is immediately carried out to bring the soil moisture content to 70% to 80% of field capacity. This humidity level is conducive to the contact and reaction between the treatment agent's outer shell and heavy metal ions in the soil. Subsequently, the treated land is left to mature for 7 to 10 days. This maturation stage is crucial, as it allows sufficient time for the rapid immobilization function of the compound treatment agent's outer shell to take effect, minimizing the initial concentration of heavy metals in the soil solution.

[0040] After the maturation period, step S4, namely the transplanting and field management of hyperaccumulating plants, is initiated. The hyperaccumulating plant seedlings selected in step S1, nurtured to a robust state in the nursery, are carefully transplanted to the treated remediation site at an optimized planting density. Throughout the plant's growth cycle, meticulous field management is implemented. This includes regular irrigation based on weather conditions and soil moisture monitoring results, consistently maintaining soil moisture content at levels suitable for plant growth. Regarding fertilization, special attention must be paid to applying compound nitrogen and potassium fertilizers free of phosphorus and target heavy metals. For example, urea can be used to provide nitrogen, and potassium sulfate to provide potassium. Phosphate fertilizers are avoided to prevent exogenous phosphate ions from competitively adsorbing with the hydroxyapatite coating of the treatment agent, or forming new precipitates with target heavy metals in the soil, thereby interfering with the pre-defined remediation mechanism. Simultaneously, sufficient nitrogen and potassium nutrition is fundamental to ensuring rapid plant growth and high biomass.

[0041] Step S5, namely dynamic monitoring and endpoint determination, is crucial throughout the entire remediation process. During key phenological stages of plant growth, such as tillering, vigorous growth, and maturity, soil and plant samples need to be collected regularly to track remediation progress. Soil sample collection is meticulously designed, specifically distinguishing between rhizosphere and non-rhizosphere soil sampling areas. Rhizosphere soil is defined as soil tightly attached to the root surface within 2 mm of the root surface, collected by carefully shaking off the root ball and brushing it off with a soft brush; non-rhizosphere soil is collected from an area more than 20 cm away from the base of the plant. For both types of soil samples, the total concentration and bioavailable content of heavy metals are determined. Bioavailable content is typically determined using the DTPA extraction method, which effectively characterizes the bioavailability of heavy metals to plants. Plant samples are divided into root and aboveground parts, dried to constant weight in an 80℃ oven, pulverized, digested with acid, and then their heavy metal content is determined using atomic absorption spectrometry or ICP-MS. The synergistic effect of the technical solution of this invention is demonstrated by quantitatively comparing the significant differences in the content of available heavy metals in rhizosphere and non-rhizosphere soils, predicting that the content of available heavy metals in rhizosphere soil is much higher than that in non-rhizosphere soil, and by the huge difference in the total amount of heavy metal accumulation in plants in the treated group and the control group without treatment. The criteria for judging the remediation endpoint are twofold: on the one hand, the total concentration of the target heavy metal in the soil is consistently lower than the risk screening value specified in the national standard "Soil Environmental Quality Standard for Agricultural Land Soil Pollution Risk Control" after two consecutive growing seasons; on the other hand, the heavy metal content in the aboveground parts of the plant tends to a plateau value in three consecutive sampling analyses at the end of the growing season, with no further significant increase, indicating that the plant's extraction capacity has reached the limit under current soil conditions.

[0042] When remediation reaches its predetermined endpoint or at the end of each growing season, step S6, namely the harvesting and subsequent safe disposal of the plants, is performed. The above-ground parts of the hyperaccumulating plants are harvested at ground level using a harvester. The harvested plant biomass is first dried to constant weight at 80°C in a large drying facility to reduce volume and weight for easier transport and subsequent processing. The dried plant biomass is then transported to a qualified hazardous waste treatment center or a specially designed biomass incineration facility for processing under controlled oxygen incineration conditions at approximately 850°C. The incineration process completely decomposes the organic matter within the plants, while heavy metals are concentrated in the fly ash produced. This heavy metal-rich fly ash is collected using a high-efficiency bag filter. The collected fly ash is considered hazardous waste and must undergo solidification / stabilization treatment. The solidified material must undergo a toxicity leaching procedure test to ensure that the heavy metal concentration in the leachate is below the limits specified in the "Hazardous Waste Identification Standard" before it can be disposed of in a safe hazardous waste landfill. As a way to utilize resources, hydrometallurgical technology can also be considered for ash slag rich in high-value metals. Through processes such as acid leaching, extraction, and electrolysis, valuable metals can be recovered, turning waste into treasure. Among them, high-value metals include copper, zinc, and cadmium.

[0043] In a preferred embodiment of the present invention, during the preparation of the composite treatment agent in step S2, the thiol-functionalized modified nano-hydroxyapatite of the outer shell layer can be further compounded with 10% to 20% by mass of montmorillonite. Montmorillonite is a natural layered silicate mineral with a large specific surface area and excellent cation exchange capacity. Introducing it into the outer shell layer can, on the one hand, improve the dispersibility and suspension stability of the composite treatment agent in the soil medium, facilitating more uniform distribution during application; on the other hand, its layered structure can provide additional adsorption sites for heavy metal ions, especially for some hydroxyapatite and thiol-based cations with relatively weak adsorption capacity, thereby further enhancing the overall immobilization effect of the treatment agent in the early stage of remediation.

[0044] Furthermore, to maximize the efficiency of the entire remediation system, specific rhizosphere microbial agents can be applied synergistically during the field management phase of step S4. These agents can be designed as a composite formulation containing highly efficient phosphate-solubilizing bacteria (Bacillus megaterium) and siderophore-producing bacteria (Pseudomonas aeruginosa). Phosphate-solubilizing bacteria can secrete organic acids and phosphatases to convert fixed, insoluble phosphates (such as calcium phosphate and iron phosphate) in the soil into soluble phosphorus that plants can absorb, thus meeting the plant's phosphorus requirements without the application of exogenous phosphate fertilizers. Siderophore-producing bacteria secrete siderophores, small molecules with a high affinity for ferric ions, which can capture iron ions in the soil. This not only provides iron to plants but, more importantly, reduces the competitive complexation between iron ions and the core activator EDDS, allowing EDDS to focus more intently on chelating target heavy metal pollutants, thereby significantly improving the specificity and efficiency of the activation process.

[0045] In a more refined implementation, the monitoring system of the remediation method in step S5 can also integrate an in-situ microelectrode sensor array. This array consists of multiple miniaturized pH electrodes and redox potential (Eh) electrodes, systematically buried at different depths and locations within the rhizosphere region of the plants. These sensors are connected to the central control system via a data acquisition unit, enabling continuous, real-time monitoring of key parameters of the soil chemical environment in the rhizosphere microzone. This high-resolution real-time data provides unprecedented possibilities for the refined control of the remediation process. For example, when the system detects that the rhizosphere pH value in a certain area deviates from the optimal absorption window of 5.0-5.8 due to changes in plant physiological activity or soil buffering capacity, the central control system can automatically trigger the linked drip irrigation system to precisely supplement the area with trace amounts of dilute acid or alkali for immediate correction, ensuring that the plants are always in the most efficient state for heavy metal absorption.

[0046] To illustrate the significant technical effects of the present invention more specifically, a specific embodiment and its comparison with several comparative examples will be used below.

[0047] Example 1

[0048] This embodiment relates to a method for remediating farmland soil contaminated with Cd-Pb composite pollution.

[0049] The contaminated site soil type is alluvial soil, with a pH of 6.8 and an organic matter content of 1.8%. ICP-MS analysis showed that the average Cd concentration in the top 0-20 cm soil was 3.5 mg / kg and the average Pb concentration was 280 mg / kg, both exceeding the national screening values ​​for agricultural land soil pollution risk. Analysis using the BCR continuous extraction method showed that 45% of Cd and 35% of Pb were acid-extractable, indicating high bioavailability. The selected hyperaccumulating plant was the Cd / Zn hyperaccumulating ecotype of *Sedum aizoon*.

[0050] The composite treatment agent is prepared according to step S2, wherein the core comprises 40% EDDS, 10% citrate-disodium hydrogen phosphate buffer (target pH 5.5) and 50% mesoporous silica carrier; the inner shell is PHBV with a valerate molar fraction of 12%; and the outer shell is nano-hydroxyapatite with a mercapto loading density of 1.0 mmol / g.

[0051] Following step S3, the prepared compound treatment agent was evenly applied to the experimental field at a rate of 4 tons / hectare, and then mixed thoroughly with the top 20 cm of soil using a rotary tiller. Irrigation was then carried out to bring the soil moisture content to 75% of field capacity, and the soil was allowed to mature for 10 days. Afterwards, following step S4, the robust seedlings of *Sedum aizoon* were transplanted into the field at a spacing of 20 cm × 20 cm. During the 120-day growth cycle, conventional irrigation and phosphorus-free nitrogen-potassium fertilizer management were implemented.

[0052] The results showed that after remediation, the total Cd concentration in the soil decreased to 0.5 mg / kg, and the total Pb concentration decreased to 155 mg / kg. The aboveground biomass dry weight of *Sedum aizoon* reached 5.8 t / ha, with an aboveground Cd dry weight concentration of 150 mg / kg and a Pb dry weight concentration of 1250 mg / kg. Compared with control group 1, which only planted *Sedum aizoon*, the removal rates of Cd and Pb in the soil increased by 150% and 110%, respectively, and the enrichment of Cd and Pb by plants increased by 2.2 times and 1.8 times, respectively.

[0053] Comparative Example 1

[0054] The same contaminated soil and Sedum sarmentosum plants as in Example 1 were used, but no compound treatment agents were applied, and only conventional planting and management were carried out.

[0055] Harvest after 120 days of growth.

[0056] The results showed that the total Cd concentration in the soil after remediation was 2.9 mg / kg, and the total Pb concentration was 255 mg / kg, with no significant decrease in concentration. The aboveground biomass dry weight of *Sedum aizoon* was 4.2 t / ha, indicating weak growth. The accumulated Cd concentration in its aboveground parts was 68 mg / kg, and the Pb concentration was 700 mg / kg. These results indicate that phytoremediation alone is inefficient and time-consuming in heavily polluted soils.

[0057] Comparative Example 2

[0058] The same contaminated soil and Sedum aizoon plants as in Example 1 were used, but only a chemical fixative was applied. The fixative used was the outer shell material of the composite treatment agent in Example 1, namely thiol-functionalized nano-hydroxyapatite, and the amount applied was equivalent to the mass of the outer shell material contained in the treatment agent in Example 1.

[0059] After applying a fixative and allowing it to mature for 10 days, transplant the Sedum aizoon and harvest it after 120 days of growth.

[0060] The results showed that the concentrations of available Cd and Pb in the soil via DTPA were significantly reduced, with a decrease exceeding 80%, indicating a good immobilization effect. However, the growth of *Sedum aizoon* was severely inhibited, with a dry biomass weight of only 3.1 t / ha. The accumulated Cd concentration in the aboveground parts was 15 mg / kg, and the Pb concentration was 150 mg / kg, far lower than that in Comparative Example 1. This demonstrates that chemical immobilization severely inhibits the absorption of heavy metals by plants, reflecting the antagonistic effect described in the background section.

[0061] Comparative Example 3

[0062] The same contaminated soil and Sedum sarmentosum plants as in Example 1 were used. The applied treatment agent was a simple physical mixture of the components in Example 1, that is, EDDS, buffer, mesoporous silica, PHBV powder, and thiol-functionalized nano-hydroxyapatite were directly mixed in the same proportions and then applied to the soil, rather than being prepared into core-shell microcapsules.

[0063] Transplant the Sedum aizoon immediately after applying the mixture, and harvest after 120 days of growth.

[0064] The results showed that because EDDS was directly released into the soil, a large amount of Cd and Pb entered the soil solution, resulting in excessively high heavy metal concentrations in the initial soil solution, which had a toxic effect on plants and prolonged the seedling recovery period. Simultaneously, EDDS directly competed with hydroxyapatite for heavy metal ions, leading to poor immobilization. Ultimately, the aboveground biomass dry weight of the plant was 3.8 t / ha, the aboveground Cd concentration was 95 mg / kg, and the Pb concentration was 850 mg / kg. The remediation effect was inferior to Example 1 of this invention, demonstrating the necessity of a core-shell structure for achieving spatiotemporal ordered regulation.

[0065] Comparative Example 4

[0066] The same contaminated soil and Sedum sarmentosum plants as in Example 1 were used. A commercially available general-purpose resin-coated slow-release fertilizer microcapsules were applied, with the core components replaced by those in Example 1, namely EDDS, buffer, and mesoporous silica. The outer shell of these slow-release microcapsules is a common polymer membrane, and its release mechanism mainly relies on water permeation and membrane pore diffusion, lacking rhizosphere responsiveness.

[0067] After applying the slow-release microcapsules, the Sedum aizoon plants were transplanted and harvested after 120 days of growth.

[0068] After a 120-day growth cycle, samples were taken from all treatment groups for analysis, including changes in total heavy metal content in the soil, aboveground plant biomass, and the concentration of heavy metals accumulated in the aboveground parts. The comparison results of various key performance indicators are summarized in the table below:

[0069]

[0070] The superiority of the technical solution of this invention is clearly evident from the data comparison in the table above. The treatment in Example 1 significantly reduced the total amount of Cd and Pb in the soil, with removal rates as high as 85.7% and 44.6%, respectively, far exceeding all comparative examples. Comparative Example 1 (phytoremediation only) shows that without exogenous intervention, phytoremediation efficiency is low, and the reduction in soil pollutant concentration is limited. Comparative Example 2 (chemical immobilization only), while effectively reducing the bioavailability of heavy metals, severely inhibited plant absorption of heavy metals, leading to a significant decrease in plant biomass and extremely low heavy metal accumulation concentrations, almost eliminating the role of phytoextraction. This directly demonstrates the antagonistic effect between chemical immobilization and plant absorption. Comparative Example 3 (physical mixing of components) showed poor remediation results due to the disordered and premature release of the chelating agent EDDS, which caused toxicity to early plant growth and interfered with the immobilizer. This highlights the absolute necessity of the core-shell structure of this invention to achieve spatiotemporal ordered regulation. While Comparative Example 4 (a non-responsive slow-release system) outperformed the first three comparative examples, its chelating agent release was indiscriminate and slow, resulting in a significant waste of activator in non-rhizosphere regions, leading to a utilization efficiency far lower than that of this invention. In contrast, Example 1 of this invention not only achieved the highest plant biomass but also the highest concentrations of Cd and Pb accumulated in the aboveground parts of the plant, reaching 150 mg / kg and 1250 mg / kg, respectively. This resulted in an astonishing total extraction of Cd and Pb by a single-season plant at 870 g / ha and 7250 g / ha, respectively, representing an increase in extraction efficiency of 205% and 147% compared to Comparative Example 1, which relied solely on phytoremediation. These data strongly demonstrate that this invention, through a rhizosphere-responsive core-shell composite treatment agent, successfully transforms the mutually restrictive relationship between chemical aids and phytoremediation processes into a highly efficient and synergistic relay, achieving a breakthrough improvement in soil remediation efficiency.

Claims

1. A soil remediation method combining plant and heavy metal composite treatment agents, characterized in that, Includes the following steps: Step S1: Compatibility analysis of contaminated site investigation and remediation systems; Step S2: Preparation of rhizosphere-responsive core-shell composite treatment agent; Step S3: Application of compound treatment agent and land preparation; Step S4: Planting and field management of hyperaccumulating plants; Step S5: Monitoring the repair process and determining the endpoint; Step S6: Plant harvesting and subsequent treatment.

2. The soil remediation method combining plant and heavy metal composite treatment agents according to claim 1, characterized in that, The preparation of the rhizosphere-responsive core-shell composite treatment agent aims to synthesize a core-shell microcapsule with dual functions, the functional conversion of which is controlled by plant root exudates. The core-shell structured microcapsule comprises, from the inside out: The core is a heavy metal activation unit, which is used to enhance the bioavailability of heavy metals in the rhizosphere microzone of the hyperaccumulating plant after release. An inner shell, which is a root-responsive controlled-release layer, is wrapped around the core. The material of the inner shell is designed to degrade under the specific action of the root secretions of the hyperaccumulated plant, thereby triggering the localized release of substances within the core. The outer shell layer, which is a rapid immobilization layer for heavy metals, is coated outside the inner shell layer and is used to rapidly adsorb and immobilize highly active heavy metal ions in the soil in the early stage of the application of the composite treatment agent to the soil.

3. The soil remediation method combining plant and heavy metal composite treatment agents according to claim 2, characterized in that: The outer shell is made of thiol-functionalized nano-hydroxyapatite; the nano-hydroxyapatite has a particle size of 20-50 nanometers, and its surface is loaded with thiol functional groups through a grafting reaction with 3-mercaptopropionic acid. The loading density of the thiol functional groups is 0.5-1.5 mmol / g. The ion exchange and precipitation effects of the hydroxyapatite and the complexation effect of the thiol groups are used to fix heavy metal ions in the soil. The outer shell layer is also composited with 10-20% montmorillonite by mass. The montmorillonite is used to improve the dispersibility of the composite treatment agent in the soil and to provide additional heavy metal ion adsorption sites through its layered structure.

4. The soil remediation method combining plant and heavy metal composite treatment agents according to claim 2, characterized in that: The inner shell is made of poly-β-hydroxybutyrate-valerate copolymer, and the molar fraction of valerate units in the copolymer is 10-15%. The thickness of the inner shell is controlled at 5-10 micrometers. The poly-β-hydroxybutyrate-valerate copolymer is degraded by ester bond hydrolysis under the synergistic effect of esterase secreted by the roots of the hyperaccumulating plant and low molecular weight organic acids.

5. The soil remediation method combining plant and heavy metal composite treatment agents according to claim 2, characterized in that: The core components, by mass percentage, include: 30-50% biodegradable chelating agent; 5-15% rhizosphere microzone pH regulator; and the remainder being a porous carrier.

6. The soil remediation method combining plant and heavy metal composite treatment agents according to claim 5, characterized in that: The biodegradable chelating agent is S,S-ethylenediamine-N,N'-disuccinic acid (EDDS); the rhizosphere microzone pH regulator is a citrate-disodium hydrogen phosphate buffer system, designed to adjust the pH of the local soil microenvironment after the core is released to the range of 5.0-5.8; the porous carrier is mesoporous silica microspheres with an average pore size of 20 nanometers and a specific surface area of ​​400 square meters / gram.

7. The soil remediation method combining plant and heavy metal composite treatment agents according to claim 2, characterized in that, The preparation process of the core-shell composite treatment agent includes the following sub-steps: Preparation of core materials: The biodegradable chelating agent and the rhizosphere microzone pH regulator are dissolved to form a homogeneous solution, and the solution is loaded onto the activated porous carrier under vacuum impregnation conditions, followed by vacuum drying to obtain core particles loaded with activated components; Coating of the inner shell layer: Fluidized bed bottom spray coating technology is adopted, using the core particle as the fluidized substrate, and coating liquid containing the inner shell layer material is sprayed and coated until the coating weight gain reaches 20-30% of the mass of the core particle, forming an intermediate body coated by the inner shell layer. Coating of the outer shell layer: The material of the outer shell layer is dispersed in an aqueous solution to form a stable suspension. The intermediate is added to the suspension. The pH value is adjusted to utilize the difference in surface charge of the particles to guide the material of the outer shell layer to be deposited on the surface of the inner shell layer (2). The moisture is removed by spray drying to finally form the core-shell structure composite treatment agent.

8. The soil remediation method combining plant and heavy metal composite treatment agents according to claim 1, characterized in that: The method further includes a step of maturing the soil treated with the compound treatment agent before planting the hyperaccumulating plant; the application amount of the compound treatment agent is calculated according to the standard of applying 2-5 grams of the compound treatment agent per kilogram of contaminated soil, and the compound treatment agent is evenly mixed into the top 0-20 cm of soil using a rotary tiller; the maturation treatment includes irrigating the soil once to bring the soil moisture content to 70-80% of field capacity, and letting it stand for 7-10 days; during the growth cycle of the hyperaccumulating plant, a compound nitrogen and potassium fertilizer free of phosphorus and target heavy metals is applied.

9. The soil remediation method combining plant and heavy metal composite treatment agents according to claim 2, characterized in that: When planting the hyperaccumulating plant, a rhizosphere microbial agent is synergistically applied. The agent contains phosphate-solubilizing bacteria and siderophore-producing bacteria. The phosphate-solubilizing bacteria is Bacillus megaterium, which is used to activate insoluble phosphate in the soil to supply phosphorus nutrition to the plant. The siderophore-producing bacteria is Pseudomonas aeruginosa, which is used to secrete siderophores to reduce the competitive complexation of iron ions with the biodegradable chelating agent in the core.

10. The soil remediation method combining plant and heavy metal composite treatment agents according to claim 2, characterized in that, The method further includes a step of monitoring the remediation process during the growth cycle of the hyperaccumulating plant; the monitoring step includes: deploying an in-situ microelectrode sensor array in the rhizosphere microzone of the plant to monitor the pH value and redox potential of the rhizosphere soil in real time; and, when the rhizosphere pH value is detected to deviate from the preset heavy metal absorption optimization range, supplementing a trace amount of acid or alkali through a drip irrigation system for fine adjustment; and collecting rhizosphere soil and non-rhizosphere soil samples respectively, measuring and comparing the difference in the content of available heavy metals in the two, and evaluating the local release effect of the activation unit in the core.

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