A method for screening rhododendrons and a soil remediation method for remediating low-concentration cadmium-contaminated soil in phosphate mine waste sites.

Abstract

This invention discloses a method for selecting Rhododendron and a soil remediation method for remediating low-concentration cadmium-contaminated soil in phosphate mine waste sites, belonging to the field of contaminated soil remediation technology. It aims to solve the problems of low biomass and low cadmium accumulation rates of herbaceous plants in the high-phosphorus and alkaline environment of phosphate mining areas. This invention selects Rhododendron as the core remediation plant, and seedlings are planted after soil evaluation; soil moisture is maintained through quantitative irrigation, and nitrogen and potassium fertilizers are supplemented to optimize nutrients; the above-ground parts are mechanized and the underground roots are regularly cleaned 8-10 months after planting, followed by harmless disposal. By adopting the above technical solution, this invention utilizes the high biomass and strong stress resistance of Rhododendron to achieve efficient extraction and stripping of low-concentration cadmium, reducing cadmium pollution in the soil while increasing vegetation coverage in the mining area, demonstrating excellent ecological benefits.

Description

Technical Field

[0001] This invention relates to the field of contaminated soil remediation technology, specifically to a method for screening azaleas and a soil remediation method for remediating low-concentration cadmium-contaminated soil in phosphate mine waste sites. Background Technology

[0002] Heavy metal pollution in soil is a serious challenge facing global environmental governance, and its effectiveness directly affects the safety of ecosystems and the sustainable development of human health. Among numerous heavy metal pollutants, cadmium (Cd) is recognized as a priority target for soil environmental remediation due to its extremely high biotoxicity, strong environmental migration capacity, and significant bioaccumulation effect. Particularly in areas with concentrated phosphate mining and processing, such as Xingshan County in the Three Gorges Reservoir area, large areas of abandoned phosphate mines have been formed due to historical mining activities and the long-term evolution of tailings. The soils in these areas generally exhibit low concentrations of cadmium pollution, typically ranging from 0.21 to 1.0 mg / kg. Although the absolute concentration does not yet reach the level of severe pollution, the vast geographical area, complex topography, and the highly concealed and stable nature of pollutants in the soil matrix make traditional physical engineering methods (such as topsoil replacement and deep tillage) or chemical leaching techniques face insurmountable bottlenecks, including extremely high remediation costs, easy damage to the original soil physical structure, and the risk of secondary environmental consequences.

[0003] Against this backdrop, phytoremediation technology, with its significant advantages such as environmental friendliness, in-situ treatment, low cost, and aesthetic appeal, is considered the most promising technological approach for resolving soil contaminated with low concentrations of heavy metals. The core logic of phytoremediation lies in utilizing specific hyperaccumulating or highly accumulating plants. Through their root systems, these plants absorb heavy metals from the soil, transport them across membranes, and then facilitate their synergistic migration and accumulation in the aboveground organs and tissues. Ultimately, the removal of heavy metals and soil purification are achieved through the regular harvesting and harmless disposal of the aboveground parts. However, a closer examination of existing technologies reveals a series of profound and irreconcilable technical contradictions when dealing with specific conditions such as phosphate mine waste sites.

[0004] First, most of the cadmium-accumulating plants screened in existing technologies are herbaceous plants, such as *Centipeda minima*, *Solanum nigrum*, or *Sedum aizoon*. While these plants exhibit extremely high bioaccumulation factor (BCF) and transfer factor (TF) in controlled laboratory environments or high-concentration contaminated soils, in practical applications, their biomass is relatively small, their root distribution is limited, and their growth cycle is generally short. When facing vast areas with low-concentration cadmium pollution, the insufficient effective absorption surface area provided by plants per unit area leads to a significant, non-linear decrease in remediation efficiency as soil cadmium concentration decreases. A deeper problem is that these herbaceous plants are mostly annuals or short-lifecycle species, with weak tolerance to environmental stress, making it difficult for them to form stable successor populations in harsh outdoor habitats like phosphate mine wastelands. To maintain long-term remediation effects, a large amount of human and material resources must be invested in repeated seedling propagation, transplanting, and harvesting, which significantly increases the lifecycle cost of remediation, contradicting the original intention of low-cost phytoremediation.

[0005] Secondly, the soil physicochemical properties of phosphate mine waste sites are highly unique, often exhibiting alkalinity (pH values ​​frequently above 7.48) and abnormally high phosphorus content (total phosphorus content can reach over 3 g / kg). This unique geochemical environment creates a complex intervention mechanism for cadmium bioavailability. Traditionally, high concentrations of phosphate ions are believed to readily form insoluble cadmium phosphate precipitates with cadmium ions, significantly inhibiting cadmium uptake by plant roots. Many accumulating plants screened using existing technologies rely on high abundances of soluble cadmium in the soil for their absorption mechanisms. However, when placed in the antagonistic environment of high phosphorus and high pH in phosphate mine areas, their original accumulation capacity often degrades drastically, even leading to stunted growth due to their inability to adapt to the complex soil nutrient balance. This means that simply transplanting known highly accumulating herbaceous plants to phosphate mine waste sites often fails to achieve the expected remediation targets, and existing screening criteria lack in-depth exploration of "non-obvious" accumulating resistance varieties with low concentrations and specific physicochemical characteristics.

[0006] Furthermore, considering the sustainability of remediation schemes and the synergy of ecological functions, existing screening logic often overemphasizes the absolute content of heavy metals in plants, neglecting the core parameter of total cadmium extraction per plant determined by "biomass × cadmium content." While woody plants are considered slow-growing in some studies, they possess vast root systems, highly resilient stress-resistant structures, and biomass far exceeding that of herbaceous plants. However, in current technologies, due to limitations such as long screening cycles and the lack of preliminary evaluation models, woody plants with high remediation potential are often excluded from mainstream screening. In multi-objective remediation scenarios like phosphate mine waste sites, which require consideration of soil and water conservation, windbreak and sand fixation, and heavy metal removal, there is an urgent technological need to develop a woody plant remediation scheme that can adapt to high-phosphorus alkaline habitats, exhibits stable growth, and possesses high cadmium extraction capacity per plant.

[0007] In summary, how to overcome the inhibition of heavy metal availability by soil physicochemical properties in complex phosphate mine wasteland habitats, and how to find and establish a woody plant remediation technology system that can achieve efficient enrichment and absorption of cadmium while taking into account long-term ecological stability and low maintenance costs, has become a key scientific problem and technical challenge that urgently needs to be addressed in the fields of environmental engineering and soil remediation. Summary of the Invention

[0008] This invention provides a method for screening rhododendrons and a soil remediation method for remediating low-concentration cadmium-contaminated soil in phosphate mine waste sites, in order to solve the problems of existing technologies being limited to herbaceous plants, having single indicators, and not being combined with the specific characteristics of soil in mining areas.

[0009] The technical solution adopted in this invention is as follows:

[0010] One objective of this invention is to provide a method for remediating low-concentration cadmium-contaminated soil in phosphate mine waste sites. This method uses rhododendron as the core remediation plant, leveraging the biomass advantage of the rhododendron in the phosphate mine habitat to achieve cadmium removal from the soil. Specific steps include:

[0011] (1) Habitat assessment and regional delineation: Grid sampling was carried out on the target phosphate mine wasteland, soil physicochemical indicators were measured, and plots with total phosphorus content of 3.0~3.5 g / kg, pH value of 7.0~8.0 and total cadmium content of 0.1~5 mg / kg were selected as remediation target areas;

[0012] (2) Seedling selection and adaptation hardening: Select rhododendron seedlings with a growth period of 2 years, clear genetic background and stable phenotypes. The seedlings should have uniform basal diameter and plant height, complete root ball and no diseases or pests. The seedlings should be subjected to an adaptation hardening treatment for two weeks in a facility that simulates the environment of a phosphate mine wasteland.

[0013] (3) Engineered planting: When the spring temperature rises to above 10℃, the surface of the target area is cleared and planting holes are dug in a matrix pattern with a row spacing of 60cm and a plant spacing of 50cm. The depth and diameter of the planting holes are reserved with 20% of the redundant space compared with the size of the seedling root ball. When planting, ensure that the roots are spread out and in full contact with the soil, and then irrigate with rooting water.

[0014] (4) Water and fertilizer coupling regulation and habitat maintenance: Soil moisture content is regulated in real time through an automated monitoring system, and precise fertilization is carried out in response to the nutrient imbalance characteristics of phosphate mine wasteland to ensure that plants maintain biomass accumulation under cadmium stress;

[0015] (5) Monitoring of physiological growth dynamics: During the repair period, the net growth rate of plant height and net growth rate of basal diameter of azalea were monitored monthly, and the activity of leaf antioxidant enzyme system and cell membrane damage indicators were calibrated to ensure that the plant's physiological metabolism was in a stable state.

[0016] (6) Harvesting of plant tissues and migration of heavy metals: In the 8th to 10th month after planting, after the biomass of rhododendrons reaches its peak, the above-ground stem and leaf tissues and underground roots are harvested and removed by mechanization or manual means to achieve the directional transfer of cadmium from the soil matrix to the organisms.

[0017] (7) Harmless disposal of the remains: The harvested rhododendron tissues are subjected to pyrolysis to reduce the volume and lock the cadmium element in the biochar, so as to prevent the spread of secondary pollution.

[0018] Preferably, the engineered planting in step (3) further includes: the planting time is preferably in March each year, when the natural coupling of rainfall and temperature promotes the establishment of root dominance; the planting process adopts the technical procedure of layered backfilling and layered compaction, which aims to eliminate the gaps between the roots and the phosphate rock soil particles; the planting density is designed to ensure the maximum utilization of photosynthetically effective radiation per unit area; for phosphate rock wasteland with a large slope, a stepped horizontal strip is constructed before planting to increase the on-site interception rate of rainwater and prevent the loss of remediation matrix caused by soil erosion.

[0019] Preferably, the water-fertilizer coupling regulation and habitat maintenance in step (4) specifically involve:

[0020] a) Precise water management: By deploying an automated water sensor grid in the remediation area, using 80% of field capacity as the irrigation trigger threshold, the intermittent sprinkler system is driven to form a dynamic water cycle in the rhizosphere microenvironment. The water gradient induces dissolved cadmium ions in the soil solution to converge to the root surface, while maintaining the dissolved oxygen balance in the rhizosphere microenvironment.

[0021] b) Nutrient balance intervention: In response to the geochemical barrier of phosphorus-rich but nitrogen- and potassium-deficient phosphate mine waste sites, soil nutrient dynamics are monitored regularly, and water-soluble nitrogen and potassium fertilizers are supplemented through sprinkler irrigation systems to optimize the nutrient ratio in plants. The competitive effect of potassium ions and cadmium ions is used to enhance plant stress resistance and extend the restoration service life.

[0022] Preferably, the harmless disposal process of the residue in step (7) is as follows: after the harvested rhododendron stem and leaf residues are dried and crushed, they are put into a closed pyrolysis reactor and subjected to high-temperature pyrolysis in an oxygen-free environment at 450°C to realize the energy conversion of organic components and enrich cadmium in chemically stable biochar. Then, through subsequent chemical sealing technology, it is locked in the solid building matrix.

[0023] In summary, compared with the prior art, the present invention has the following advantages and benefits:

[0024] 1. This invention, through in-depth exploration of the physiological potential of rhododendrons, reveals their exceptionally strong cadmium tolerance even in the specific high-phosphorus, alkaline soil environment of phosphate mining areas. By immobilizing and adsorbing cadmium through their roots, the bioavailability of cadmium in the soil is reduced. Simultaneously, through a certain degree of absorption by the aboveground parts, and with periodic harvesting of both the aboveground and underground roots, partial removal of pollutants is achieved. Combined with the massive biomass of woody plants, this results in a breakthrough increase in the extraction yield per plant. This discovery overturns the conventional wisdom that "herbaceous plants are the only option for phytoremediation," providing a completely new technological paradigm for soil remediation in complex industrial and mining areas.

[0025] 2. The restoration method described in this invention also has significant value in landscape construction. Phosphate mine wastelands often exhibit desertification; large-scale planting of rhododendrons can quickly create green coverage, and their flowering period is highly ornamental, transforming previously barren mining areas into suburban parks with ecotourism potential. This "synergistic approach of restoration and landscaping" adds more social value to environmental engineering and is a vivid practice of the concept of ecological civilization construction in the field of soil remediation. The implementation of this invention can significantly enhance the land value of phosphate mine wastelands and inject lasting momentum into the comprehensive recovery of damaged ecosystems. Detailed Implementation

[0026] The present invention will be described in detail below with reference to specific embodiments and examples, thereby making the advantages and various effects of the present invention more clearly apparent. Those skilled in the art should understand that these specific embodiments and examples are for illustrative purposes only and are not intended to limit the present invention.

[0027] Throughout this specification, unless otherwise specified, the terminology used herein should be understood as having the meaning commonly used in the art. Therefore, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention pertains. In the event of any conflict, this specification shall prevail.

[0028] Unless otherwise specified, all raw materials, reagents, instruments and equipment used in this invention can be purchased from the market or prepared by existing methods.

[0029] The present application will now be described in detail with reference to embodiments and experimental data.

[0030] Example 1

[0031] (1) Test plants and test soil

[0032] Select 2-year-old azalea seedlings that are uniform in thickness and growth.

[0033] The test soil was taken from the topsoil (0–30 cm) of an uncontaminated area, air-dried naturally, impurities removed, and sieved through a 2 mm sieve. The basic physicochemical properties of the soil were: pH 7.2, organic matter 25.6 g·kg⁻¹, total nitrogen 1.8 g·kg⁻¹, total phosphorus 0.8 g·kg⁻¹, total potassium 18.5 g·kg⁻¹, and Cd background value 0.12 mg·kg⁻¹.

[0034] (2) Cd stress treatment settings

[0035] The Cd stress treatment concentrations were set based on the background Cd values ​​in the study area, the national agricultural land soil environmental quality risk screening values, and previous survey results. Four Cd addition levels were set: 0, 5, 25, and 50 mg / kg (25 mg / kg and 50 mg / kg were mainly used to verify the plant's tolerance and accumulation potential under higher Cd stress conditions), corresponding to no pollution, low pollution, moderate pollution, and high-intensity stress scenarios. Exogenous Cd was added to the soil in the form of a CdCl2·2.5H2O aqueous solution. Table 1 shows the soil physicochemical properties before planting for the four Cd stress treatment groups.

[0036] Table 1. Soil physicochemical properties before planting

[0037]

[0038] After Cd is added, the treated soil is aged at room temperature for more than 15 days and stirred regularly to ensure that Cd is distributed relatively evenly in the soil.

[0039] (3) Experimental design and cultivation management

[0040] The pot experiment was conducted in a greenhouse from March to November 2025, using round plastic pots with accompanying trays. Each pot contained 13 kg of soil and planted one seedling. The experiment employed a completely randomized design, with five replicates for each Cd treatment. Water management was implemented using a quantitative irrigation method during the experiment to maintain the relative soil moisture content at approximately 80% of field capacity. Plant height and basal diameter were measured monthly during the growing season.

[0041] (4) Sample collection and processing

[0042] Destructive sampling was conducted after the experiment. Soil adhering to the surface of plant roots and not shaken off was collected as rhizosphere soil samples, while non-rhizosphere soil samples were collected from different locations within the pot and mixed together. The soil samples were air-dried naturally and then used for subsequent determination of physicochemical properties and heavy metal content.

[0043] After plant samples were collected, roots, stems, and leaves were separated. Root samples were treated with EDTA solution to remove adsorbed Cd from the surface, followed by thorough rinsing with deionized water. All organ samples were blanched at 105℃ for 30 min, then dried at 80℃ to constant weight. The dry weight was measured and recorded for subsequent analysis.

[0044] (5) Measurement indicators and analytical methods

[0045] All indicators in this experiment were measured in triplicate.

[0046] The measurement indicators and methods are as follows:

[0047] 1) Plant nutrient content

[0048] After separating the above-ground and underground parts of the plant, the plants were blanched at 105℃ for 30 minutes and then dried at 70℃ to constant weight. The samples were then pulverized and passed through a 1mm sieve. The nitrogen and phosphorus contents were determined according to the NY / T 2017-2011 standard. Nitrogen was determined using a KDN-19K fully automated Kjeldahl nitrogen analyzer, while phosphorus was analyzed using a UV-1750 ultraviolet absorption spectrophotometer.

[0049] 2) Basic physical and chemical properties of soil

[0050] Soil pH was determined using the potentiometry method; organic matter was determined using the potassium dichromate-concentrated sulfuric acid method; total nitrogen was determined using the Kjeldahl method; total phosphorus was determined using the alkali fusion-molybdenum antimony colorimetric method; and total potassium was determined using the flame photometry method. Available phosphorus was determined using 0.5 mol·L⁻¹. -1 Sodium bicarbonate extraction-molybdenum antimony colorimetric method for determination; available potassium was determined using 1 mol·L⁻¹. -1 Determination by ammonium acetate extraction-flame photometry.

[0051] 3) Cd content in plants and soil

[0052] Plant samples (above-ground and below-ground parts) were thoroughly washed with deionized water after collection. Roots were soaked in 0.1 MEDTA solution for 10 minutes to remove adsorbed Cd²⁺ from the root surface, then dried at 105℃ for 30 minutes, transferred to 70℃ to constant weight, and ground through a 100-mesh sieve for later use. Soil samples were air-dried, impurities removed, ground through a 2 mm nylon sieve, and further passed through a 100-mesh sieve. Plant samples were pre-digested with concentrated nitric acid, followed by gradient temperature increases (120℃ to 160℃) combined with hydrogen peroxide for complete decomposition. Soil samples were digested with a 9:1 (v / v) HNO₃:HClO₄ mixture. After volume adjustment and filtration, the Cd content in plants and soil was determined by inductively coupled plasma mass spectrometry (ICP-MS).

[0053] The calculations for the above-mentioned indicators are as follows:

[0054] The formula for calculating the relative growth rate in height / basal dimeter (RGR_H / D) is as follows:

[0055] RGR=

[0056] In the formula: H1 and H2 are the plant height (or basal diameter) values ​​at times t1 and t2, respectively; The time interval between the two measurements.

[0057] The formula for calculating the Bioconcentration Factor (BCF) is as follows:

[0058] BCF=

[0059] In the formula: Cd content (mg·kg) in plant tissues (above-ground parts or roots) -1 ); The initial total Cd content of the soil (mg·kg) -1 ).

[0060] The formula for calculating the transfer factor (TF) is as follows:

[0061] TF=

[0062] In the formula: Cd content in the aboveground parts of the plant (mg·kg) -1 ); (mg·kg) -1() represents the Cd content in plant roots.

[0063] The formula for calculating the Cd extraction yield per plant is as follows:

[0064] =

[0065] In the formula: and The concentrations of Cd in the aboveground parts and roots of the plant (mg·kg) are respectively. -1 ); and The dry matter (g) of the aboveground and underground parts of a single plant are respectively.

[0066] (6) Measurement results

[0067] 1) Growth response characteristics of rhododendron under Cd stress

[0068] As shown in Table 2, under different Cd treatments, the overall changes in various growth indicators of Rhododendron were relatively small, demonstrating strong Cd tolerance. Although the net growth rate of plant height decreased with increasing Cd concentration (from 0.08 in the control group to 0.02), there were no significant differences among the treatments, indicating that Cd stress did not significantly inhibit plant height growth at this concentration. The net growth rate of basal diameter responded significantly to Cd treatment, reaching 0.18 at the 5 mg / kg treatment (p < 0.05), while the 25 mg / kg and 50 mg / kg treatments were 0.112 and 0.15, respectively, at intermediate levels, indicating that low concentrations of Cd had a certain stimulating effect on stem thickening. Root length showed no significant differences among the treatments, remaining between 14.33 and 16.67 cm, indicating that the Rhododendron root system has a strong adaptability to Cd stress. The aboveground biomass was significantly lower than the control at the 25 mg / kg treatment (p < 0.05), while the 5 mg / kg and 50 mg / kg treatments showed no significant difference from the control; the underground biomass showed a similar trend to the aboveground biomass, with the lowest value at the 25 mg / kg treatment.

[0069] Overall, low concentrations of Cd (5 mg / kg) had a certain promoting effect on basal diameter growth, while medium concentrations of Cd (25 mg / kg) had the most significant inhibitory effect on biomass accumulation. In contrast, most growth indicators were better under the 50 mg / kg treatment than under the 25 mg / kg treatment, indicating that rhododendrons may have a certain tolerance to higher concentrations of Cd stress or maintain growth through physiological regulation.

[0070] Table 2. Rhododendron growth parameters under different Cd concentrations.

[0071]

[0072] Note: Different lowercase letters indicate significant differences (p<0.05) between different Cd treatments for the same indicator. The same applies below.

[0073] 2) Physiological regulatory response of rhododendrons under Cd stress

[0074] As shown in Table 3, under different Cd treatments, the overall changes in antioxidant enzyme activity and MDA content in rhododendron leaves were relatively small, but the responses of different indicators to Cd stress varied to some extent. SOD activity showed no significant difference among treatments, with the value decreasing from 885.67 U·g in the CK treatment. -1 The concentration decreased slightly with increasing Cd concentration, reaching 807.33 U·g for the 50 mg / kg treatment. -1 This indicates that the SOD system remains relatively stable under different Cd levels. POD activity also did not show significant differences among treatments, generally remaining within the range of 0.47–0.65 U·g. -1 Within the specified range, this indicates that the enzyme system did not respond significantly to Cd stress. CAT activity, however, showed some treatment-specific differences, being significantly higher than the control (CK) at 5 mg / kg and 25 mg / kg treatments (p<0.05), reaching 0.87 and 0.80 mmol·h, respectively. -1 ·g -1 The 50 mg / kg treatment resulted in a decrease to 0.60, which was at the same significance level as the control (CK), indicating that low to medium concentrations of Cd stress can induce increased CAT activity, thereby enhancing hydrogen peroxide scavenging capacity. MDA content showed no significant differences among treatments, generally remaining between 28.60 and 33.23 nmol·g⁻¹. -1 The range indicates that the degree of cell membrane lipid peroxidation did not change significantly, and no obvious membrane structure damage was observed in rhododendrons under Cd stress.

[0075] Overall, the antioxidant system of Rhododendron remained relatively stable under Cd stress. The activity of CAT was enhanced under low to medium concentrations of Cd, indicating that it can alleviate Cd-induced oxidative stress by regulating the activity of antioxidant enzymes.

[0076] Table 3. Antioxidant enzyme activity and malondialdehyde (MDA) content in rhododendron leaves under different Cd treatments.

[0077]

[0078] 3) Characteristics of Cd enrichment and translocation in Rhododendron under Cd stress

[0079] The accumulation characteristics of Cd in Rhododendron are shown in Table 4. Under different Cd stress conditions, the Cd content in Rhododendron significantly increased with increasing soil Cd concentration. The Cd content in the aboveground parts gradually increased from 1.92 mg / kg in the CK treatment to 28.09 mg / kg in the 50 mg / kg treatment; the Cd content in the underground parts increased significantly from 2.13 mg / kg to 90.63 mg / kg, with a more pronounced increase. The differences among the treatments were all statistically significant.

[0080] Table 4. Cd content in rhododendrons under different Cd stress conditions.

[0081]

[0082] Table 5 shows the effects of Cd stress on the available Cd content in soil after Rhododendron planting. Under different Cd treatments, the available Cd content in the soil generally increased with increasing Cd concentration. Compared with the soil before the experiment, the available Cd content in the soil increased after planting Rhododendron under the CK and 5 mg / kg treatments, rising from 0.06 mg / kg to 0.30 mg / kg and from 2.26 mg / kg to 2.74 mg / kg, respectively. However, under the 25 mg / kg and 50 mg / kg treatments, the changes in available Cd content before and after planting were relatively small, remaining at similar levels. Significant differences in available Cd content were observed among the different treatments, showing an overall trend of increasing with increasing Cd concentration. The results indicate that Rhododendron may have a certain activating effect on soil Cd under low Cd concentrations, while its effect on available Cd content in the soil is relatively stable under medium and high concentrations.

[0083] Table 5. Soil bioavailable Cd content after Rhododendron treatment under Cd stress

[0084]

[0085] Table 6 shows the enrichment and translocation characteristics of Rhododendron and the Cd extraction amount. Under different Cd treatments, the enrichment and translocation characteristics of Rhododendron showed a certain regularity with Cd concentration. With increasing soil Cd concentration, the aboveground BCF, belowground BCF, and total BCF gradually decreased, with the total BCF decreasing from 33.69 in the CK treatment to 2.48 in the 50 mg / kg treatment, showing significant differences among treatments. In contrast, the transfer coefficient (TF) did not differ significantly among treatments, but the values ​​generally showed a decreasing trend, indicating that the translocation capacity of Cd to the aboveground parts weakened with increasing Cd concentration. Conversely, the total Cd extraction amount from the plant increased significantly with increasing soil Cd concentration, from 30.15 mg / plant in the CK treatment. -1 The concentration of the strain increased to 700.59 mg / kg in the treatment. -1Overall, rhododendrons exhibit strong enrichment capacity at low Cd concentrations, while at higher Cd concentrations, although the enrichment coefficient decreases, a large extraction volume can still be achieved through the high in vivo Cd content, demonstrating certain remedial potential.

[0086] Table 6. Enrichment, translocation coefficients, and Cd extraction yield of Rhododendron simsii.

[0087]

[0088] In summary, within the Cd stress range of 0–50 mg / kg, *Rhododendron* exhibited stable overall growth, with no significant impact on plant height and root length. Only the biomass decreased in the 25 mg / kg treatment, indicating strong tolerance to Cd stress. Physiological results showed that the antioxidant enzyme system remained stable overall, while CAT activity increased under low to medium Cd concentrations, suggesting that *Rhododendron* can mitigate oxidative damage caused by Cd stress through physiological regulation. Regarding Cd accumulation and translocation, the Cd content in both the aboveground and belowground parts of *Rhododendron* increased significantly with increasing soil Cd concentration, but Cd was mainly accumulated in the belowground parts, with a generally low translocation coefficient, indicating relatively limited translocation capacity to the aboveground parts. Nevertheless, *Rhododendron* showed high accumulation capacity under low Cd concentrations, and under medium to high concentrations, it achieved a large Cd extraction yield per plant due to its high in vivo Cd content and stable biomass, reaching a total extraction yield of 700.59 mg / plant at the 50 mg / kg treatment. -1 Therefore, although rhododendrons, as woody plants, do not possess a strong ability to translocate Cd to the aboveground parts, they still have good potential for Cd remediation due to their high root accumulation capacity and Cd extraction yield per plant. They can be used for phytoremediation of low-concentration Cd-contaminated soils such as those found in phosphate mine wastelands. The aforementioned pot experiment not only verified the tolerance and accumulation potential of rhododendrons to Cd stress but also provided experimental evidence for mine remediation.

[0089] Finally, it should be noted that the terms "comprising," "including," or any other variations are intended to cover a non-exclusive inclusion, such that a process, method, article, or apparatus that comprises a list of elements includes not only those elements but also other elements not expressly listed, or elements inherent to such a process, method, article, or apparatus. Although preferred embodiments of the invention have been described, those skilled in the art, upon learning the basic inventive concept, can make further changes and modifications to these embodiments. Therefore, the appended claims are intended to be interpreted as including both the preferred embodiments and all changes and modifications falling within the scope of the invention.

[0090] The embodiments described above merely illustrate specific implementation methods of this application, and while the descriptions are detailed and specific, they should not be construed as limiting the scope of protection of this application. It should be noted that those skilled in the art can make various modifications and improvements without departing from the concept of the technical solution of this application, and these modifications and improvements all fall within the scope of protection of this application.

Claims

1. A method for remediating low-concentration cadmium-contaminated soil in phosphate mine waste sites, characterized in that, This method uses rhododendron as the core remediation plant, leveraging the biomass advantage of rhododendron in phosphate rock habitats to achieve cadmium removal from the soil. Specific steps include: (1) Habitat assessment and regional delineation: Grid sampling was carried out on the target phosphate mine wasteland, soil physicochemical indicators were measured, and plots with total phosphorus content of 3.0~3.5 g / kg, pH value of 7.0~8.0 and total cadmium content of 0.1~5 mg / kg were selected as remediation target areas; (2) Seedling selection and adaptation hardening: Select rhododendron seedlings with a growth period of 2 years, clear genetic background and stable phenotypes. The seedlings should have uniform basal diameter and plant height, complete root ball and no diseases or pests. The seedlings should be subjected to an adaptation hardening treatment for two weeks in a facility that simulates the environment of a phosphate mine wasteland. (3) Engineered planting: When the spring temperature rises to above 10℃, the surface of the target area is cleared and planting holes are dug in a matrix pattern with a row spacing of 60cm and a plant spacing of 50cm. The depth and diameter of the planting holes are reserved with 20% of the redundant space compared with the size of the seedling root ball. When planting, ensure that the roots are spread out and in full contact with the soil, and then irrigate with rooting water. (4) Water and fertilizer coupling regulation and habitat maintenance: Soil moisture content is regulated in real time through an automated monitoring system, and precise fertilization is carried out in response to the nutrient imbalance characteristics of phosphate mine wasteland to ensure that plants maintain biomass accumulation under cadmium stress; (5) Monitoring of physiological growth dynamics: During the repair period, the net growth rate of plant height and net growth rate of basal diameter of azalea were monitored monthly, and the activity of leaf antioxidant enzyme system and cell membrane damage indicators were calibrated to ensure that the plant's physiological metabolism was in a stable state. (6) Harvesting of plant tissues and migration of heavy metals: In the 8th to 10th month after planting, after the biomass of rhododendrons reaches its peak, the above-ground stem and leaf tissues and underground roots are harvested and removed by mechanization or manual means to achieve the directional transfer of cadmium from the soil matrix to the organisms. (7) Harmless disposal of the remains: The harvested rhododendron tissues are subjected to pyrolysis to reduce the volume and lock the cadmium element in the biochar, so as to prevent the spread of secondary pollution.

2. The repair method as described in claim 1, characterized in that, The engineered planting in step (3) also includes: the planting time is preferably in March each year, when the natural coupling of rainfall and temperature promotes the establishment of root dominance; the planting process adopts the technical procedure of layered backfilling and layered compaction, which aims to eliminate the gaps between the roots and the phosphate rock soil particles; the planting density is designed to ensure the maximum utilization of photosynthetically effective radiation per unit area; for phosphate rock wasteland with a large slope, a stepped horizontal strip is constructed before planting to increase the on-site interception rate of rainwater and prevent the loss of remediation matrix caused by soil erosion.

3. The repair method as described in claim 1, characterized in that, The water-fertilizer coupling regulation and habitat maintenance in step (4) specifically involve: a) Precise water management: By deploying an automated water sensor grid in the remediation area, using 80% of field capacity as the irrigation trigger threshold, the intermittent sprinkler system is driven to form a dynamic water cycle in the rhizosphere microenvironment. The water gradient induces dissolved cadmium ions in the soil solution to converge to the root surface, while maintaining the dissolved oxygen balance in the rhizosphere microenvironment. b) Nutrient balance intervention: In response to the geochemical barrier of phosphorus-rich but nitrogen- and potassium-deficient phosphate mine waste sites, soil nutrient dynamics are monitored regularly, and water-soluble nitrogen and potassium fertilizers are supplemented through sprinkler irrigation systems to optimize the nutrient ratio in plants. The competitive effect of potassium ions and cadmium ions is used to enhance plant stress resistance and extend the restoration service life.

4. The repair method as described in claim 1, characterized in that, The specific process of harmless disposal of the residue in step (7) is as follows: after the harvested rhododendron stem and leaf residues are dried and crushed, they are put into a closed pyrolysis reactor and subjected to high-temperature pyrolysis in an oxygen-free environment at 450°C to realize the energy conversion of organic components and enrich cadmium in chemically stable biochar. Then, through subsequent chemical sealing technology, it is locked in the solid building matrix.

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