A method for repairing cadmium contaminated soil by intercropping ipomoea aquatica with tagetes erecta and applying a passivation agent

Abstract

The application discloses a method for repairing cadmium contaminated soil by using Ipomoea aquatica intercropping with Tagetes erecta and adding a passivator, and relates to the technical field of soil repair. The method comprises the following steps: preparing HAP modified biochar by reacting hydroxyapatite and biochar, and adding the passivator into the cadmium contaminated soil; and intercropping Ipomoea aquatica and Tagetes erecta on the cadmium contaminated soil to which the passivator is added, so as to reduce the cadmium content in the soil. The method uses the passivator to chemically repair the cadmium in the soil, reduces the availability of heavy metals in the soil, and intercropping with plants that can enrich heavy metals, so as to reduce the content of heavy metals in the soil, and is a heavy metal pollution repair method that can ensure the safety of crop production.

Description

Technical Field

[0001] This invention relates to the field of soil remediation technology, and in particular to a method for remediating cadmium-contaminated soil by intercropping water spinach with marigolds and applying a passivating agent. Background Technology

[0002] Soil is a crucial natural resource for the sustainable development of agriculture in my country, serving as the material foundation and fundamental guarantee for human survival and development. With the rapid advancement of industrialization and urbanization, problems such as soil pollution and declining soil quality have gradually become prominent. Heavy metal pollution in soil, in particular, has a long latency period, poor mobility, and is difficult to remediate. It not only damages the environment but also affects crop growth and development and can enter the human body through the food chain, endangering human health. Therefore, soil protection and the remediation of polluted soil are among the key focuses of our country's soil environmental protection work. Cadmium is a well-known heavy metal, characterized by a long decomposition cycle, high mobility, high toxicity, and difficulty in degradation. Therefore, how to effectively remediate and control cadmium pollution in soil, reduce the absorption and accumulation of cadmium by crops, and achieve the safety of the environment and agricultural products is a crucial issue in the fields of environmental ecology and agriculture.

[0003] Currently, methods for remediating cadmium pollution in soil include physical remediation, chemical remediation, bioremediation, and agroecological measures. Among these, physical remediation is costly and difficult to implement on a large scale. Phytoremediation, a form of bioremediation, utilizes plants with a high capacity for cadmium accumulation to extract and centrally process cadmium from the soil. Passivating agent treatment, a type of chemical remediation, involves adding agents to cadmium-contaminated soil to convert cadmium into a stable state, thereby reducing its migration and bioavailability. This is achieved by altering soil pH, generating precipitates, and through adsorption, ion exchange, and organic coordination complexation, effectively reducing the concentration and bioavailability of available cadmium ions in the soil. Agroecological measures primarily address cadmium pollution in soil by changing cropping systems. Intercropping, a traditional agricultural practice, can effectively control the absorption and accumulation of heavy metals such as cadmium by crops through a reasonable intercropping system, while also promoting efficient utilization of soil nutrients, increasing crop yield and land utilization.

[0004] In the prior art, CN113843272A discloses a method for remediating cadmium- and lead-contaminated farmland using marigold as an alternative crop. Specifically, marigolds are planted after applying biochar, a passivating agent, to the lead- and cadmium-contaminated soil. The high biomass and short growth cycle of marigolds significantly reduce cadmium and lead levels. CN108817048A discloses a method for remediating cadmium-contaminated soil by intercropping herbaceous plants with water spinach. Specifically, water spinach and herbaceous plants, including Napier grass, are intercropped on the cadmium-contaminated soil to be remediated. These intercropped herbaceous plants have high biomass and rapid growth, enabling them to accumulate heavy metals such as cadmium and effectively reduce the cadmium content in the contaminated soil. However, different intercropped plants, through their different root characteristics, drive the diversity of farmland soil biodiversity and ecological functions, thereby affecting the absorption and translocation of heavy metals by crops.

[0005] Therefore, selecting suitable intercropping plants in conjunction with appropriate passivating agents to remediate cadmium pollution in soil is of great significance for protecting the ecological environment and ensuring crop safety. Summary of the Invention

[0006] To address the aforementioned limitations of existing technologies, the present invention aims to provide a method for remediating cadmium-contaminated soil by intercropping water spinach with marigolds and applying a passivating agent. This invention uses HAP-modified biochar, prepared by reacting hydroxyapatite and biochar, as a passivating agent. The passivating agent is first added to the cadmium-contaminated soil, and then water spinach and marigolds are intercropped on this soil to reduce the cadmium content. This invention utilizes a passivating agent for chemical remediation of cadmium in the soil, reducing the availability of heavy metals in the soil, and intercropping with plants that accumulate heavy metals, thereby reducing the heavy metal content in the soil. This method is a reliable way to remediate heavy metal pollution and ensure the safety of crop production.

[0007] To achieve the above objectives, the present invention adopts the following technical solution:

[0008] This invention provides a method for remediating cadmium-contaminated soil by intercropping water spinach with marigolds and applying a passivating agent, comprising the following steps:

[0009] (1) Disperse hydroxyapatite in water to obtain hydroxyapatite solution, then add biochar to hydroxyapatite solution and stir to obtain mixture. Filter the mixture, collect the precipitate and wash and dry it. Then activate the dried precipitate to obtain HAP modified biochar.

[0010] (2) HAP modified biochar was first added to the cadmium-contaminated soil to be remediated, and then water spinach and marigold were intercropped and cultivated and managed to achieve the purpose of remediating cadmium-contaminated soil.

[0011] Preferably, in step (1), the ratio of hydroxyapatite to water in the hydroxyapatite solution is (0.5-2) g: 100 mL.

[0012] Preferably, the hydroxyapatite has a pH of 7.02 and an EC value of 37.36 μg·cm⁻¹. -1 Available phosphorus content: 96.02 mg / kg -1 The particle diameter is less than 200 nm.

[0013] Preferably, in step (1), the biochar is prepared by carbonizing cow manure biogas residue under anaerobic conditions and then sieving it to obtain biochar.

[0014] Further preferred, the carbonization temperature is 400-500℃ and the carbonization time is 0.8-1.2h.

[0015] Preferably, in step (1), the biochar has a pH of 9.43 and an EC value of 286.13 μg·cm⁻¹. -1 The total nitrogen content is 11.56 g·kg. -1 The available phosphorus content is 1.03 mg·kg. -1 The readily available potassium content is 24.32 mg / kg. -1 Organic matter content: 536.23 g·kg -1 .

[0016] Preferably, in step (1), the mass ratio of biochar to hydroxyapatite in the hydroxyapatite solution is (18-19.5):(0.5-2).

[0017] Preferably, in step (1), the stirring time is 2.5-3.5h.

[0018] Preferably, in step (1), the activation temperature is 450-550℃ and the activation time is 60-70min.

[0019] Preferably, in step (2), the water content of the cadmium-contaminated soil after adding HAP-modified biochar is 55-65% of field capacity.

[0020] Preferably, in step (2), the amount of HAP-modified biochar applied is 0.6-0.7 g / kg.

[0021] Preferably, in step (2), the ratio of water spinach and marigold intercropping is 1:(1-2), that is, 1-2 rows of marigolds are intercropped in every row of water spinach.

[0022] Preferably, in step (2), the planting density of water spinach in the planting strip is 11-13 plants / m². 2 The planting density of marigolds in the planting area is 4-6 plants / m². 2 The spacing between the water spinach planting strip and the marigold planting strip is 8-12cm.

[0023] Preferably, in step (2), water spinach and marigold need to be treated with seedlings before intercropping, and then the treated water spinach and marigold are sown together.

[0024] Further preferred methods include: soaking water spinach seeds in water at 25-30℃ for 12-28 hours, wrapping them in damp gauze, and germinating them at 28-30℃ until they sprout; and sowing marigold seeds in water at 35-40℃ for 2.5-3.5 hours, followed by air drying.

[0025] Preferably, in step (2), the cultivation management is as follows: 7 days before sowing water spinach and marigold, apply base fertilizer to the soil, and apply urea one week before harvesting water spinach.

[0026] More preferably, the base fertilizer is a mixture of urea, diammonium phosphate and potassium sulfate in a mass ratio of 1:(0.2-0.3):(0.4-0.6).

[0027] Further preferred, the application rate of base fertilizer is 0.083-0.33 g / kg, and the application rate of urea is 0.30-0.35 g / kg.

[0028] The beneficial effects of this invention are:

[0029] This invention uses HAP-modified biochar prepared by reacting hydroxyapatite and biochar as a passivating agent. The passivating agent is first added to cadmium-contaminated soil, and water spinach and marigold are intercropped on the cadmium-contaminated soil with the passivating agent added to achieve the effect of reducing the cadmium content in the soil.

[0030] This invention utilizes passivating agents to chemically remediate cadmium in soil through complexation, adsorption, precipitation, oxidation, and reduction, thereby reducing the availability of heavy metals in the soil. Simultaneously, intercropping with plants that accumulate heavy metals further reduces the heavy metal content in the soil, making it a remediation method for heavy metal pollution that effectively ensures crop production safety. On one hand, the passivating agent used in this invention combines the properties of hydroxyapatite and biochar, exhibiting excellent passivation effects on cadmium in the soil. Furthermore, biochar-loaded hydroxyapatite avoids the disadvantage of hydroxyapatite's poor dispersion in the soil, achieving a better passivation effect. On the other hand, this invention leverages the high biomass, well-developed root system, and certain pollution tolerance of water spinach, and the large biomass and excellent cadmium accumulation effect of marigolds. Moreover, the growth cycles of marigolds and water spinach are essentially the same. Therefore, selecting water spinach in conjunction with intercropping the cadmium hyperaccumulator marigold achieves a good remediation effect on cadmium-contaminated soil. Attached Figure Description

[0031] Figure 1(A) Scanning electron microscope image of biochar prepared in Example 1; (B) Scanning electron microscope image of 10% HAP modified biochar prepared in Example 1.

[0032] Figure 2 The effect of HAP-modified biochar on the content of weakly acidic cadmium in soil;

[0033] Figure 3 The effect of HAP-modified biochar on the content of reduced cadmium in soil;

[0034] Figure 4 The effect of HAP-modified biochar on soil oxidized cadmium content;

[0035] Figure 5 The effect of HAP-modified biochar on the residual cadmium content in soil;

[0036] Figure 6 The effects of different treatments on the available Cd content in soil;

[0037] Figure 7 The effects of different treatments on total Cd in soil;

[0038] Figure 8 : Speciation of Cd in soils under different treatments;

[0039] Figure 9 The effect of different treatments on the transfer coefficient of water spinach. Detailed Implementation

[0040] It should be noted that the following detailed descriptions are illustrative and intended to provide further explanation of this application. Unless otherwise specified, 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 application pertains.

[0041] To enable those skilled in the art to better understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below with reference to specific embodiments.

[0042] The experimental materials used in the embodiments of this invention are all conventional experimental materials in the art and can be purchased through commercial channels.

[0043] The cadmium-contaminated soil used in this invention was taken from the periphery of the mine (0-20cm), with a pH of 6.31 and EC of 286.13 μg·cm³. -1 Total nitrogen 1.28 g·kg -1 Available phosphorus 10.67 mg·kg -1 Available potassium 109.84 mg·kg -1 Organic matter content: 15.06 g·kg -1 Cd 3.21 mg·kg -1;

[0044] The hydroxyapatite (HAP) is nano-hydroxyapatite produced by Shaanxi Zhenhe Biotechnology Co., Ltd., with a particle diameter of less than 200 nm. The cow dung biogas residue was purchased from CGN Yangxin Bioenergy Technology Co., Ltd. The Cd content in both the hydroxyapatite and biochar is less than 0.01 mg / kg.

[0045] Example 1: Preparation of HAP-modified biochar

[0046] (1) Weigh 0.5g, 1g and 2g of hydroxyapatite respectively and dissolve them in 100mL of deionized water. Sonicate for 30min to disperse them evenly, and obtain the first hydroxyapatite solution, the second hydroxyapatite solution and the third hydroxyapatite solution respectively.

[0047] (2) Carbonize the cow dung swamp at 450℃ for 1 hour under anaerobic conditions, and pass it through a 100-mesh sieve to obtain biochar;

[0048] (3) Weigh 19.5g, 19g, and 18g of biochar respectively and add them to the first hydroxyapatite solution, the second hydroxyapatite solution, and the third hydroxyapatite solution. Stir for 3 hours to mix them evenly to obtain a mixture. After filtering the mixture, collect the precipitate, wash it with ultrapure water and dry it. Then activate the dried precipitate at 500℃ for 2 hours. After cooling to room temperature, pass it through a 300-mesh sieve to obtain 2.5% HAP modified biochar (HB), 5% HAP modified biochar (DH), and 10% HAP modified biochar (QH).

[0049] The biochar and 10% HAP-modified biochar (QH) prepared in this example were characterized using scanning electron microscopy (SEM, JSM-6360LV, JEOL, Japan). The results are as follows: Figure 1 As shown.

[0050] in, Figure 1 (A) is a scanning electron microscope image of biochar. Figure 1 (B) is a scanning electron microscope image of 10% HAP-modified biochar (QH). Figure 1 (A) It can be seen that the surface of biochar is rough and there is no obvious polymer attached, which is due to... Figure 1 (B) It can be seen that the surface of 10% HAP modified biochar (QH) is smooth and a large number of white agglomerates appear. The morphology and structure of the agglomerates are basically the same as those of HAP. This shows that HAP was successfully loaded onto the surface of biochar.

[0051] Example 2: A method for remediating cadmium-contaminated soil by intercropping water spinach with marigolds and applying a passivating agent.

[0052] The 2.5% HAP-modified biochar prepared in Example 1 was first added to the cadmium-contaminated soil to be remediated. The application rate was 0.67 g / kg of soil. The moisture content of the cadmium-contaminated soil after adding 2.5% HAP-modified biochar was adjusted to 60% of the field capacity. Water spinach and marigold were intercropped and cultivated and managed to achieve the purpose of remediating the cadmium-contaminated soil.

[0053] Before intercropping, water spinach and marigolds are treated with seedlings, and then sown uniformly. The specific treatment is as follows: For water spinach, the seeds are soaked in water at 28℃ for 24 hours, wrapped in damp gauze, and placed at 29℃ to germinate. Once the seeds show white sprouts, they can be sown. For marigolds, the seeds are soaked in water at 38℃ for 3 hours and then air-dried before sowing.

[0054] During intercropping, the ratio of water spinach to marigolds is 1:2, meaning that 1-2 rows of marigolds are planted per row of water spinach, and the planting density of the water spinach strip is 12 plants / m². 2 The planting density of marigolds in the planting area is 5 plants / m². 2 The planting strips for water spinach and marigolds are spaced 10cm apart.

[0055] Cultivation management is as follows: Seven days before sowing water spinach and marigolds, apply base fertilizer to the soil, and one week before harvesting water spinach, apply urea as a top dressing. The base fertilizer is a mixture of urea, diammonium phosphate, and potassium sulfate in a mass ratio of 1:0.25:0.5, with an application rate of 0.20 g / kg soil and 0.33 g / kg soil for urea.

[0056] Experiment 1: Pot Experiment

[0057] The pot experiment was conducted at the experimental station of Panhe Campus of Shandong Agricultural University, and the experiment started on July 18, 2023 and ended on October 18, 2023.

[0058] This experiment included 10 treatments, as detailed below:

[0059] Treatment group 1: CK group, this treatment group was not treated with any passivating agent and was not intercropped with marigolds;

[0060] Treatment Group 2: Group BC. In this treatment group, biochar prepared in Example 1 was applied as a passivating agent and marigolds were not intercropped. The preparation method of the biochar was the same as in Example 1.

[0061] Treatment Group 3: HB Group, this treatment group was treated with 2.5% HAP modified biochar (HB) prepared in Example 1 as a passivating agent and without intercropping marigolds;

[0062] Treatment Group 4: DH Group. This treatment group was treated with 5% HAP-modified biochar (DH) prepared in Example 1 as a passivating agent and without intercropping marigolds.

[0063] Treatment Group 5: QH Group. This treatment group was treated with 10% HAP-modified biochar (QH) prepared in Example 1 as a passivating agent and without intercropping marigolds.

[0064] Treatment group 6: OJ group, this treatment group was not treated with passivating agent and was intercropped with marigolds;

[0065] Treatment Group 7: Group BJ. In this treatment group, biochar prepared in Example 1 was applied as a passivating agent and marigolds were intercropped. The preparation method of the biochar was the same as in Example 1.

[0066] Treatment Group 8: HJ Group, this treatment group was treated with 2.5% HAP modified biochar (HB) prepared in Example 1 as a passivating agent and intercropped with marigolds;

[0067] Treatment Group 9: DJ Group. This treatment group was treated with 5% HAP-modified biochar (DH) prepared in Example 1 as a passivating agent and intercropped with marigolds.

[0068] Treatment Group 10: QJ Group. This treatment group was treated with 10% HAP-modified biochar (QH) prepared in Example 1 as a passivating agent and intercropped with marigolds.

[0069] Ensure that the amount of passivating agent applied and the management measures are consistent across all treatment groups, and repeat each treatment group 4 times.

[0070] 1. Specific experimental procedures

[0071] (1) Cadmium-contaminated soil was used as pot test soil. 3 kg of the soil was passed through a 10-mesh sieve and placed in a plastic pot (33 cm long, 23 cm wide, 15 cm high). The soil moisture was adjusted to 60% of field capacity. The test crop was large-leaf water spinach, and the intercropping crop was marigold. Seven days before sowing, 1.75 g of basal fertilizer was applied to the soil. The basal fertilizer consisted of a mixture of urea, diammonium phosphate, and potassium sulfate in a mass ratio of 1:0.25:0.5. Before sowing, both water spinach and marigold were treated with seedlings. The water spinach seedling treatment involved soaking the seeds in water at 28°C for 24 hours, followed by applying a wet compress... Wrap the seeds in gauze and place them at 29℃ to germinate. Once the seeds show white sprouts, they can be sown. The seedling treatment for marigolds is as follows: soak the marigold seeds in water at 38℃ for 3 hours, and then sow them after air drying. Intercrop the treated water spinach seeds and marigold seeds, sowing 10 water spinach seeds and 15 marigold seeds evenly in each pot. After emergence, retain three plants with better growth from each variety. All other management measures are the same. Apply 1g of urea one week before each harvest. Harvest three crops of water spinach on August 18, September 18, and October 18, respectively, and collect soil and marigold samples on October 18.

[0072] (2) Sample collection

[0073] Plant sampling: The above-ground parts of the first two crops of water spinach were taken, and the whole plants of the third crop and marigolds were taken back to the laboratory for biomass determination. A portion was stored in a -20℃ refrigerator, and the other portion was blanched, dried, and pulverized through an 80-mesh sieve to determine the cadmium content in the plants.

[0074] Soil sampling: Potted experimental soil was bagged and brought back to the laboratory, where it was air-dried, ground, and passed through 20-mesh and 100-mesh sieves respectively to determine soil nutrients, cadmium content, and other indicators. The rhizosphere soil sampling method adopted was the "root shaking method," in which large soil particles were shaken off from the roots, and the soil adhering to the roots was brushed off with a sterile brush along the root surface, which was regarded as the rhizosphere soil.

[0075] 2.1 Determination of Soil Indicators

[0076] 2.1.1 Total Cd, available Cd, and Cd speciation in soil

[0077] Total Cd in the soil was extracted with aqua regia; the content of available Cd was extracted with diethylenetriaminepentaacetic acid (DTPA); Cd speciation was determined according to the improved BCR continuous extraction method (1985), and the preparation method of the BCR continuous extraction method is shown in Table 1. The Cd speciation was determined by inductively coupled plasma optical emission spectrometry (ICP-OES) (Thermo ScientifiiCAP 7200).

[0078] Table 1 BCR Continuous Extraction Method

[0079]

[0080] 2.2 Plant index measurement

[0081] 2.2.1 Determination of plant biomass

[0082] After retrieving the plant samples, the surface soil was washed off with water. All plant tissues were blanched at 105℃ for 30 minutes and then dried at 80℃ to constant weight. The water spinach was weighed separately for the above-ground and underground parts, and the marigold was weighed as a whole to obtain the biomass of each part.

[0083] 2.2.2 Cd content in plants

[0084] Weigh 0.3g (accurate to 0.0001g) of seeds, stems, leaves, or roots into a conical flask, add a few glass beads, and add 10mL of a nitric acid-perchloric acid mixture (9:1). Cover and soak overnight. Place a small funnel on a hot plate for digestion. If the solution turns brownish-black, add more nitric acid until white fumes are emitted. The digestion solution should be colorless and transparent or slightly yellow. After cooling, wash the digestion solution into a 25mL volumetric flask. Wash the conical flask three times with a small amount of nitric acid solution (1%) and dilute to the mark with nitric acid solution (1%). Mix well and set aside. At the same time, perform a reagent blank test and determine the Cd concentration in the digestion solution using an inductively coupled plasma optical emission spectrometer (ICP-OES) (ThermoScientifiiCAP 7200).

[0085] Among them, the bioconcentration factor (BCF) represents the ability of a plant to accumulate Cd, and the calculation formula is shown in Equation (I). The translocation factor (TF) is the ratio of Cd in the aboveground part to the underground part of the plant, and the calculation formula is shown in Equation (II).

[0086] BCF=C plant / C soil Formula (I);

[0087] TF=C i / C j Equation (II);

[0088] In equation (I), C plant It is the Cd concentration (mg / kg) in plant tissues; C soil This represents the Cd concentration (mg / kg) in the soil. In formula (II), C i It is the above-ground Cd concentration (mg / kg); C j This indicates the underground Cd concentration (mg / kg) (An et al., 2022).

[0089] The cadmium accumulation in plants is the product of the cadmium content in the plant and the plant biomass.

[0090] 3. Results and Analysis

[0091] 3.1 Effects of applying different proportions of HAP-modified biochar on soil cadmium speciation

[0092] 3.1.1 Effects of applying different proportions of HAP-modified biochar on the weakly acidic soluble state of soil

[0093] Depend on Figure 2 It was found that on day 5 of incubation, the soil cadmium content in both biochar and HAP-modified biochar treatments significantly decreased (P<0.05), with reductions of 30.75%, 29.82%, 21.96%, and 25.54% compared to the control (CK), respectively. On day 10, the reduction in soil cadmium content in each treatment was significantly different from the control (CK) (P<0.05), with significant reductions of 26.17%, 33.40%, 37.11%, and 28.94%, respectively. There were no significant differences between treatments applying different proportions of HAP-modified biochar on days 5 and 10 (P>0.05). On day 20, the treatments with added biochar and modified biochar significantly reduced cadmium content by 32.77%, 49.04%, 43.55%, and 47.69% compared to the control (CK). Furthermore, there were significant differences in soil cadmium content between the BC and HB treatments.

[0094] analyze Figure 2 It was found that the HB treatment reduced the weakly acidic cadmium content by 24.20% compared to the BC treatment (P<0.05). In the data collected on day 40, the QH treatment significantly reduced the weakly acidic cadmium content in the soil by 51.05% and 22.27% compared to the CK and HB treatments (P<0.05), respectively. The BC, HB, and DH treatments significantly reduced the weakly acidic cadmium content in the soil by 45.14%, 37.03%, and 46.80% compared to the CK treatment (P<0.05), respectively. There were no significant differences among these three treatments. On day 60 of the incubation experiment, the weakly acidic cadmium content in the soil was significantly reduced in the BC, HB, DH, and QH treatments compared to the CK treatment (P<0.05). There were no significant differences between the treatments with biochar application and those with modified biochar (P>0.05).

[0095] In the CK treatment, there was no significant difference in soil weakly acidic cadmium content among different time points during the incubation experiment (P>0.05). The soil weakly acidic cadmium content in the BC, HB, DH, and QH treatments all reached its minimum on day 60. Under the BC treatment, there were no significant differences in soil weakly acidic cadmium content on days 5, 10, and 20 (P>0.05), but on day 40, it decreased significantly by 19.82% and 20.39% compared to days 5 and 10, respectively (P<0.05). On day 60, it decreased significantly by 42.86% compared to the initial incubation period (5 days) (P<0.05). Under the HB treatment, the soil weakly acidic cadmium content showed significant differences on days 5, 10, and 20. The levels of cadmium in the soil decreased during the first and second 20 days, but showed an upward trend from 20 to 40 days, before decreasing again to a minimum on day 60, with a significant decrease compared to the previous four periods (P<0.05). The DH treatment showed a significant difference in weakly acidic cadmium content compared to the CK treatment, but there was no significant difference in soil weakly acidic cadmium content at any of the 5-60 day periods (P>0.05). Under the QH treatment, soil weakly acidic cadmium levels decreased significantly in both the 5-20 day and 40-60 day phases (P<0.05), with the levels at 20, 40, and 60 days significantly decreasing by 29.98%, 33.46%, and 52.07% respectively compared to day 5. Figure 2 ).

[0096] 3.1.2 Effects of applying different proportions of HAP-modified biochar on soil reduced state

[0097] from Figure 3 It can be seen that on days 5 and 10 of the incubation experiment, the reduction effect of each treatment on the soil reduced cadmium content was not significant compared with the CK treatment (P>0.05). On day 20, the soil reduced cadmium content of the treatments with biochar and HAP-modified biochar was significantly lower than that of CK, with reductions of 18.70%, 23.43%, 19.02%, and 23.49%, respectively (P<0.05). The reduced cadmium content of the CK treatment was also significantly higher than that of other treatments on day 40 (P<0.05). By day 60 of the incubation experiment, the soil reduced cadmium in the QH treatment was significantly lower than that in CK and DH, decreasing by 61.01% and 21.79%, respectively (P<0.05).

[0098] analyze Figure 3Similar to the weakly acidic state, the reduced cadmium content in the soil under the CK treatment showed no significant difference at any stage of the incubation experiment (P>0.05); under the BC treatment, the reduced cadmium content in the soil did not change significantly on days 5, 10, and 20 (P>0.05), but decreased significantly on days 40 and 60 compared to day 5, decreasing by 35.53% and 47.97% respectively (P<0.05); under the HB treatment, the reduced cadmium content in the soil decreased significantly from day 20 compared to the early stage of the incubation experiment (5 days) (P<0.05), and then reached its minimum on day 60, significantly decreasing by 49.41% compared to CK (P<0.05); compared to the early stage of the incubation experiment (5 days),

[0099] The DH treatment significantly reduced the soil reduced cadmium content after 10 days of cultivation (P<0.05), reaching its lowest level on day 60, a decrease of 45.44% compared to day 5; the QH treatment significantly reduced the soil reduced cadmium content by 56.75% on day 60 compared to day 5 (P<0.05).

[0100] 3.1.3 Effects of applying different proportions of HAP-modified biochar on soil oxidative state

[0101] analyze Figure 4 It was found that on days 5, 10, 20 and 40 of the incubation experiment, there was no significant difference in soil oxidizable content between the treatments with added biochar and HAP-modified biochar and the control (P>0.05). On day 60 of the incubation experiment, the soil oxidizable cadmium content in the BC and DH treatments was significantly higher than that in the control (P<0.05), by 16.88% and 14.35%, respectively. The HB and QH treatments had no significant effect on the soil oxidizable cadmium content (P>0.05).

[0102] Depend on Figure 4 It can be seen that under different proportions of HAP-modified biochar treatment, the content of oxidized cadmium in the soil gradually increased with the increase of cultivation time. The content of oxidized cadmium in the soil at the end of cultivation (60d) of the biochar treatment and the modified biochar treatment increased significantly by 17.81%, 8.62%, 16.64%, and 12.29% respectively compared with the early cultivation period (5d) (P<0.05).

[0103] 3.1.4 Effects of applying different proportions of HAP-modified biochar on soil residual state

[0104] like Figure 5As shown, in the initial stage (5 days) of the incubation experiment, the residual cadmium content in the soil of the HB and DH treatments was significantly higher than that of the CK treatment (P<0.05), increasing by 17.31% and 16.62%, respectively. On day 10 of the incubation experiment, only the QH and CK treatments showed significant differences, with QH showing a 14.03% increase in residual cadmium content compared to CK (P<0.05). When the incubation experiment reached days 20, 40, and 60... Compared with the control (CK), the residual cadmium content in soil treated with BC, HB, DH, and QH was significantly increased. At 20 days, the cadmium content in each treatment increased by 21.24%, 28.15%, 30.77%, and 34.09%, respectively (P<0.05); at 40 days, the cadmium content in each treatment increased by 29.96%, 29.18%, 29.08%, and 36.59%, respectively (P<0.05); and at 60 days, the cadmium content in each treatment increased by 37.21%, 42.11%, 36.78%, and 45.13%, respectively (P<0.05).

[0105] Depend on Figure 5 It was found that under different ratios of HAP-modified biochar treatments, the residual cadmium content in the soil showed a significant upward trend with increasing incubation time. Specifically, the residual cadmium content in the soil began to increase significantly on day 20 for BC, HB, and DH treatments, while the QH treatment showed a significant difference from day 5 on day 10. Under the BC treatment, the residual cadmium content in the soil increased significantly by 10.25%, 24.46%, and 30.24% on days 20, 40, and 60, respectively, compared to the initial incubation period (5 days) (P<0.05). Under the HB and DH treatments, the residual cadmium content on days 20–60 increased significantly by 13.32%–31.16% and 16.33%–26.99%, respectively, compared to the initial incubation period (5 days) (P<0.05). From day 10 to 60, the residual cadmium content in the soil under the QH treatment increased significantly by 11.39%–33.27% (P<0.05).

[0106] 3.2 Effects of water spinach intercropping with marigolds and application of passivating agents on available Cd, total Cd in soil, and Cd speciation.

[0107] 3.2.1 Effects of water spinach intercropping with marigolds and application of passivating agents on the content of available cadmium in the soil

[0108] from Figure 6As can be seen, the available cadmium content in the soil was significantly reduced under all other treatments compared to the control group (CK). Treatments with only biochar (BC group) and HAP-modified biochar showed a 34.52%–40.13% decrease in available cadmium content compared to the control group (P<0.05). Intercropping with marigolds was more effective than treatments with only biochar and HAP-modified biochar in reducing available cadmium content. Specifically, intercropping with only marigolds (OJ group) reduced cadmium content by 15.99% compared to the control group (P<0.05). Intercropping with both biochar and HAP-modified biochar reduced cadmium content by 41.03%–48.78% compared to the control group and by 29.80%–39.04% compared to OJ. Among the treatments with biochar and HAP-modified biochar and their corresponding intercropping treatments, DJ significantly reduced available cadmium content by 17.96% compared to DH, and QJ significantly reduced it by 16.10% compared to QH (P<0.05). Among all treatments, the intercropping of marigolds and the application of 2.5% HAP-modified biochar (HJ group) showed the most significant effect in reducing soil cadmium.

[0109] 3.2.2 Effects of water spinach intercropping with marigolds and application of passivating agents on total soil cadmium levels

[0110] Figure 7 This study investigated the changes in total soil cadmium under different treatments. The application of biochar alone and HAP-modified biochar had no significant effect on reducing total soil cadmium content (P>0.05), but decreased by 0.79%–7.5% compared to the control (CK). Intercropping marigolds alone also had no significant effect on total soil cadmium content. The application of biochar and HAP-modified biochar combined with intercropping significantly reduced total soil cadmium content, decreasing by 13.12%–22.12% compared to the CK treatment (P<0.05). The BJ, DJ, and QJ treatments significantly reduced cadmium content by 19.23%, 14.89%, and 14.89% respectively compared to the OJ treatment. The percentages were 12.69% and 12.69% (P<0.05); among the treatments of applying biochar and HAP-modified biochar and their corresponding intercropping, the total soil cadmium in BJ was significantly reduced by 15.75% compared to the BC treatment, DJ by 12.17% compared to DH, and QJ by 15.15% compared to QH (P<0.05); the treatments of applying biochar and HAP-modified biochar and intercropping with marigolds showed a greater reduction in total soil cadmium than the treatments of applying only biochar and HAP-modified biochar, with the effects of significantly reducing total soil cadmium in the order of BJ>HJ>QJ>DJ.

[0111] 3.2.3 Effects of water spinach intercropping with marigolds and application of passivating agents on cadmium speciation in soil

[0112] Figure 8The distribution of cadmium speciation under different treatments shows that most cadmium in the soil exists in residual form, accounting for 44.54%–55.04% of the total cadmium in the soil; followed by weakly acidic form, accounting for 20.98%–34.88% of the total cadmium in the soil; the contents of reduced and oxidized cadmium in the soil are relatively low, accounting for 8.97%–16.23% and 7.97%–17.52% of the total cadmium in the soil, respectively. Compared to the control (CK), all treatments significantly reduced the proportion of weakly acidic cadmium in the soil. The HJ and DJ treatments showed the best reductions, decreasing by 39.85% and 37.41%, respectively (P<0.05). Only the marigold-planted treatment (OJ) significantly reduced the proportion of weakly acidic cadmium by 12.87% compared to CK (P<0.05). There were no significant differences in the proportion of weakly acidic cadmium among treatments using biochar, HAP-modified biochar, and their corresponding marigold intercropping treatments (P>0.05). Under the HB and QH treatments, the proportion of reduced cadmium in the soil significantly increased by 41.98% and 53.60% compared to CK, respectively (P<0.05). The QJ treatment reduced the proportion of reduced cadmium by 15.16% compared to CK (P<0.05). Significant differences in the proportion of reduced cadmium were observed between HB and HJ, and between QH and QJ among treatments using biochar, HAP-modified biochar, and their corresponding marigold intercropping treatments. The differences were as follows: HB increased by 38.14% compared to HJ (P<0.05), and QH increased by 80.98% compared to QJ (P<0.05). Except for OJ, which showed no significant difference in the proportion of oxidized cadmium in the soil compared to the control (CK) (P>0.05), all other treatments showed a significant increase in the proportion of oxidized cadmium in the soil compared to the control (CK), ranging from 58.86% to 119.92% (P<0.05). Among the treatments with biochar application, HAP-modified biochar application, and corresponding intercropping with marigolds, BJ showed a significantly higher proportion of oxidized cadmium in the soil compared to the control (BC) (P<0.05). There was no significant difference in the proportion of residual cadmium in the soil compared to the control (CK) in any treatment (P>0.05). Among the treatments with intercropping with marigolds, HJ, DJ, and QJ showed a significantly higher proportion of residual cadmium in the soil compared to BJ (P<0.05), by 21.04%, 22.36%, and 23.58%, respectively.

[0113] 3.3 Effects of water spinach intercropping with marigold and application of passivating agents on soil cadmium absorption and accumulation by water spinach and marigold.

[0114] 3.3.1 Effects of HAP-modified biochar on marigold biomass and soil cadmium uptake and accumulation

[0115] Table 2 shows that the addition of biochar and HAP-modified biochar can increase the biomass of marigolds. The biomass of marigolds under the BJ and HJ treatments is significantly higher than that under the CK treatment, increasing by 52.90% and 52.68%, respectively. The cadmium content of marigolds decreased after the addition of HAP-modified biochar, but there was no significant change compared with the CK treatment. Compared with the CK, the enrichment coefficient of cadmium in soil by marigolds treated with HAP-modified biochar did not change significantly. After the application of HAP-modified biochar, the cadmium accumulation of marigolds in the HJ and DJ treatments was significantly higher than that in the CK treatment, increasing by 32.83% and 26.15%, respectively.

[0116] Table 2. Effects of marigold on soil cadmium absorption and accumulation under different treatments.

[0117]

[0118] 3.3.2 Effects of intercropping water spinach with marigold and applying a deactivating agent on the biomass of different parts of water spinach

[0119] Table 3 shows that after applying biochar and HAP-modified biochar, the biomass of the first crop of water spinach was significantly higher than that of the control (CK) treatment, increasing by 69.28%, 71.89%, 31.69%, and 79.41%, respectively. After intercropping with marigolds, the biomass of water spinach was still significantly higher than that of the CK, increasing by 55.56%, 71.89%, 41.83%, and 51.63%, respectively, with no difference compared to the corresponding unintercropped treatments. The change in water spinach biomass after intercropping with marigolds alone was not significant compared to the CK. Compared with C... Compared with intercropping and non-intercropping, the application of biochar and HAP-modified biochar in treatment K had no significant effect on the aboveground biomass of the second crop of water spinach. The aboveground biomass of the third crop of water spinach in treatments QH and QJ was significantly higher than that in control (CK), increasing by 217% and 255% respectively. Other treatments had no significant effect on the aboveground biomass of the third crop of water spinach. The biomass of the lower part of water spinach in all treatments with biochar and HAP-modified biochar was significantly higher than that in control (CK), increasing by 143% to 253%.

[0120] Table 3. Effects of different treatments on biomass of different parts of water spinach.

[0121]

[0122] 3.3.3 Effects of intercropping water spinach with marigold and applying a passivating agent on cadmium content in different parts of water spinach

[0123] As shown in Table 4, the OJ treatment, in which only marigolds were intercropped in the first crop of water spinach, resulted in significantly higher cadmium content in the aboveground parts of the water spinach compared to other treatments, increasing by 31.33% compared to the control (CK) treatment (P<0.05). The BC, HB, DH, and QH treatments, which applied only biochar and HAP-modified biochar, all showed significantly lower cadmium content in the aboveground parts of water spinach compared to the control (CK) treatment, decreasing by 25.90%, 33.73%, 28.01%, and 33.13%, respectively (P<0.05). The BJ, HJ, DJ, and QJ treatments, which intercropped with marigolds, all showed significantly lower cadmium content in the water spinach compared to the control (CK) treatment. Among them, the QJ treatment had the best effect on reducing cadmium content in the aboveground parts of water spinach, decreasing by 25.01% compared to the control (CK) treatment and by 42.59% compared to the OJ treatment (P<0.05). In the first crop of water spinach, there were no significant differences between the treatments of intercropping marigolds with biochar and HAP-modified biochar and the corresponding treatments of not intercropping marigolds (BC and BJ, DH and DJ, QH and QJ) (P>0.05). The cadmium content of water spinach was significantly reduced by 21.15% compared with HJ.

[0124] In the second crop of water spinach, the cadmium content in the aboveground parts of the CK and OJ treatments was significantly higher than that in other treatments, while there was no significant difference in cadmium content in the aboveground parts of water spinach between the two treatments. Treatments with only biochar and HAP-modified biochar (BC, HB, DH, and QH) showed a reduction in cadmium content in the aboveground parts of water spinach of 18.69%, 20.39%, 26.91%, and 39.66%, respectively, compared to the CK treatment (P<0.05). Treatments with intercropping marigolds and application of both biochar and HAP-modified biochar also showed significant reductions. Compared with the control (CK), the cadmium content in the aboveground parts of water spinach in the BJ, HJ, DJ, and QJ treatments was significantly reduced by 32.29%, 30.88%, 23.79%, and 19.83%, respectively, and significantly reduced by 31.51%, 30.09%, 22.92%, and 18.91%, respectively, compared with the OJ treatment. Among the treatments intercropped with marigolds and their corresponding non-intercropping treatments, the cadmium content in the aboveground parts of water spinach in the QH treatment was significantly reduced by 24.73% compared with the QJ treatment. There were no significant differences among the other treatments (Table 4).

[0125] In the third crop of water spinach, the cadmium content in the aboveground parts of water spinach treated with biochar and HAP-modified biochar alone was significantly lower than that in the control (CK) treatment (BC, HB, DH, and QH), decreasing by 22.73%, 24.33%, 33.96%, and 45.99%, respectively (P<0.05). Compared with the CK treatment, the OJ treatment also significantly reduced the cadmium content in the aboveground parts of water spinach in the third crop, decreasing it by 30.21%. Compared with the CK treatment, the treatment of intercropping marigolds with biochar and HAP-modified biochar resulted in a significant decrease in the cadmium content in the aboveground parts of water spinach. Compared with the CK treatment, the BJ, HJ, DJ, and QJ treatments significantly reduced the cadmium content in the aboveground parts of water spinach by 29.43%, 33.69%, 27.46%, and 30.21%, respectively (Table 4).

[0126] Analysis of Table 4 shows that the cadmium content in the underground parts of water spinach was significantly higher in the CK and OJ treatments than in other treatments. Specifically, the treatments with biochar alone and HAP-modified biochar significantly reduced the cadmium content in the underground parts of water spinach compared to the CK treatment, decreasing by 41.14%, 38.30%, 46.71%, and 36.49%, respectively (P<0.05), with no significant differences among these treatments. In the treatments intercropped with marigolds, the BJ, HJ, DJ, and QJ treatments reduced the cadmium content by 32.63%, 27.82%, 34.69%, and 24.17%, respectively, compared to the CK treatment (P<0.05). There were no significant differences in cadmium content in the underground parts of water spinach between the treatments with biochar alone and HAP-modified biochar and their corresponding intercropped marigolds (P>0.05).

[0127] After applying HAP-modified biochar, the overall cadmium content of water spinach was significantly lower than that of the control (CK) treatment, decreasing by 28.31%–40.32%. After intercropping marigolds with biochar and HAP-modified biochar, the overall cadmium content of water spinach decreased by 22.71%–25.21%, which was significantly lower than that of the marigold-only intercropping treatment by 25.91%–28.41% (Table 4).

[0128] Table 4. Effects of different treatments on cadmium content in different parts of water spinach.

[0129]

[0130]

[0131] 3.3.4 Effects of intercropping water spinach with marigold and applying a passivating agent on the cadmium enrichment coefficient of different parts of water spinach

[0132] As shown in Table 5, the BCF of the first crop of water spinach in the OJ treatment with only marigold intercropping was significantly higher than that in other treatments, increasing by 36.06% compared to the CK treatment. Compared to the CK, the BCF of the aboveground parts of water spinach in the BC, HB, DH, and QH treatments decreased significantly, by 20.12%, 27.51%, 27.51%, and 32.67%, respectively (P<0.05). Among the treatments with marigold intercropping, the BJ, HJ, DJ, and QJ treatments significantly reduced the BCF of the aboveground parts of water spinach compared to the OJ treatment, with the QJ treatment showing the most significant effect, decreasing by 34.47%. The BCF of the aboveground parts of water spinach changed to some extent between the single application of biochar and HAP-modified biochar and their corresponding marigold intercropping treatments, with significant differences between the BC and BJ treatments, the HB and HJ treatments, and the QH and QJ treatments (P<0.05).

[0133] In the second crop of water spinach, the biochar content (BCF) of the aboveground parts of water spinach was higher in the CK, OJ, and QJ treatments than in other treatments. Among the treatments with biochar alone and HAP-modified biochar, the DH and QH treatments significantly reduced the BCF of the aboveground parts of water spinach compared to the CK treatment, by 26.06% and 39.66%, respectively (P<0.05). The application of biochar and HAP-modified biochar in combination with marigold intercropping had no significant effect on the BCF of the aboveground parts of water spinach in the second crop. Among the treatments with biochar alone and HAP-modified biochar and their corresponding intercropping marigold, there was a significant difference in BCF between the QH and QJ treatments, with QJ increasing the BCF by 57.14% compared to QH (Table 5).

[0134] In the third harvest of water spinach, the enrichment coefficients of the aboveground parts of the water spinach in the treatments of applying biochar alone and HAP-modified biochar were significantly lower than those in the control (CK) treatment. BC, HB, DH, and QH were significantly reduced by 16.69%, 18.88%, 33.49%, and 45.61% compared to CK, respectively. However, the treatment with marigolds alone was significantly reduced by 27.82% compared to CK. In the treatment with marigolds intercropped with biochar and HAP-modified biochar, HJ, DJ, and QJ were significantly reduced by 19.41%, 16.95%, and 17.29% compared to CK (Table 5).

[0135] As shown in Table 5, the OJ treatment had the most significant effect on increasing the BCF of the underground parts of water spinach, which was 51.33% higher than that of the DJ treatment; the DH and QH treatments significantly reduced the BCF by 46.33% and 35.85% respectively compared with the CK treatment; there were no significant differences between the single application of biochar and the HAP-modified biochar treatment and their corresponding intercropping with marigold (P>0.05).

[0136] The enrichment coefficient of water spinach treated with HAP-modified biochar alone was significantly lower than that of the control (CK) treatment, decreasing by 22.58%–39.89%. After intercropping with marigolds and applying modified biochar, the enrichment coefficient of only the DJ treatment was significantly lower than that of the CK treatment, decreasing by 13.98%. Compared with the treatment without intercropping with marigolds, the enrichment coefficient of water spinach in the BJ treatment was significantly higher than that in the BC treatment, and the QJ treatment was higher than that in the HJ treatment, by 24.21% and 52.62% (Table 5).

[0137] Table 5. Effects of different treatments on the enrichment coefficients of different parts of water spinach.

[0138]

[0139] 3.3.5 Effects of intercropping water spinach with marigold and applying a passivating agent on cadmium accumulation in water spinach

[0140] Table 6 shows the cadmium accumulation in the soil by water spinach under different treatments. The cadmium content in the aboveground parts of the first crop of water spinach under the BJ and DJ treatments was significantly higher than that under the control (CK) treatment, increasing by 46.89% and 40.98%, respectively. Among the treatments where biochar and HAP-modified biochar were applied and marigolds were intercropped, the cadmium accumulation in the aboveground parts of water spinach under the DJ treatment was significantly higher than that under the DH treatment (48.60% higher than the corresponding unintercropped treatment). In the second crop of water spinach, the cadmium accumulation in the aboveground parts of water spinach under the QJ treatment was the highest, significantly higher than that under the OJ treatment (intercropped only with marigolds) and the DH and QH treatments (unintercropped with marigolds), increasing by 23.14%, 29.16%, and 27.01%, respectively. There were no significant differences in cadmium accumulation among the other treatments. The cadmium accumulation in the aboveground parts of water spinach in the third crop under the QH and HJ treatments was significantly lower than that under the CK treatment, decreasing by 18.16% and 17.06%, respectively. In the treatment without intercropping with marigolds, the cadmium accumulation in the aboveground parts of water spinach under the HB treatment was significantly higher than that under the DH and QH treatments, increasing by 19.07% and 27.93%, respectively. There were significant differences in the cadmium content in the aboveground parts of water spinach between the marigold intercropping treatment and its corresponding unintercropping treatments, HB and HJ, with HJ reducing it by 20.78% compared to HB.

[0141] In the treatments involving the application of biochar and HAP-modified biochar, the cadmium content in the underground parts of water spinach in the BC, HB, DH, and QH treatments was significantly higher than that in the control (CK) treatment, increasing by 92.39%, 72.52%, 49.19%, and 98.97%, respectively. Intercropping marigold alone significantly increased the cadmium content in the underground parts of water spinach by 57.81% compared to the CK treatment. Intercropping marigold with biochar and HAP-modified biochar significantly increased the cadmium content in the underground parts of water spinach by 83.97%–154.66%. Under the marigold intercropping treatment, the cadmium content in the underground parts of water spinach was higher than that in the unintercropped treatment, with HJ showing a significant increase of 32.26% compared to HB (Table 6).

[0142] Analysis of Table 6 shows that the application of biochar and HAP-modified biochar alone, as well as the application of biochar and HAP-modified biochar during intercropping with marigolds, can increase the accumulation of cadmium in the soil by water spinach. In the non-intercropping treatments, the cadmium accumulation in water spinach under the BC, HB, and QH treatments was significantly higher than that under the CK treatment by 31.62%, 23.97%, and 16.87%, respectively. Under the treatment of intercropping with marigolds and applying biochar and HAP-modified biochar, the cadmium content in water spinach was significantly higher than that under the CK treatment by 27.28% to 40.52%, with the highest cadmium accumulation in water spinach under the BJ treatment. Furthermore, the total cadmium accumulation level in the soil from water spinach and marigolds under the BJ treatment reached 4.73 mg / kg.

[0143] Table 6. Effects of different treatments on cadmium accumulation in different parts of water spinach.

[0144]

[0145] 3.3.6 Effect of water spinach intercropping with marigold and application of a passivating agent on the transfer coefficient of water spinach

[0146] Depend on Figure 9 It can be seen that among the treatments with biochar and HAP-modified biochar alone, the BC, HB, and DH treatments significantly increased the cadmium transfer coefficient of water spinach by 34.09%, 20.27%, and 32.23% respectively compared with the control (P<0.05). QH was the treatment with the lowest cadmium transfer coefficient among all treatments with biochar and HAP-modified biochar alone, decreasing by 14.88%, 25.01%, and 16.70% respectively compared with BC, HB, and DH. The treatment of intercropping marigold with biochar and HAP-modified biochar had no significant effect on the cadmium transfer coefficient of water spinach. Among the intercropping treatments and their corresponding non-intercropping treatments, the cadmium transfer coefficient of water spinach was significantly lower in BJ than in BC, in HJ than in HB, and in DJ than in DH.

[0147] 4. Conclusion

[0148] Applying biochar or modified biochar alone significantly reduced the available cadmium (Cd) content in the soil. Intercropping marigolds with biochar or modified biochar resulted in significantly lower available and total Cd contents compared to the control. Compared to the control, application of biochar alone or HAP-modified biochar reduced available Cd content by 34.52%–40.13%, while intercropping marigolds with biochar or HAP-modified biochar reduced available Cd content by 41.03%–48.78% and total Cd content by 13.12%–22.12%. Intercropping marigolds with 2.5% HAP-modified biochar showed the most significant effect on reducing available Cd content, while intercropping marigolds with unmodified biochar showed the best effect on reducing total Cd content.

[0149] Applying biochar or HAP-modified biochar alone can reduce the cadmium content and cadmium accumulation capacity of water spinach in soil, while increasing the biomass and cadmium accumulation. Compared with the control, the treatments with biochar and modified biochar significantly reduced the cadmium content and accumulation coefficient of water spinach, decreasing by 28.31%–40.32% and 2.58%–39.89%, respectively. The biomass of water spinach significantly increased by 61.80%–95.89% after applying biochar and modified biochar. Due to the increased biomass, the cadmium accumulation in water spinach was also higher than the control; the cadmium accumulation in water spinach treated with unmodified biochar was significantly higher than the control by 31.62%.

[0150] When marigolds were intercropped with modified biochar, the cadmium accumulation in water spinach was significantly higher than that in the control treatment and the treatment with marigolds only. When biochar and 2.5% HAP-modified biochar were applied to marigolds, the cadmium accumulation was significantly higher than that in the treatment without passivating agent. After intercropping marigolds with biochar and HAP-modified biochar, the cadmium content of water spinach was significantly reduced by 22.71%–25.21% compared with the control, while the biomass of water spinach was significantly increased by 64.74%–87.37% and the cadmium accumulation was significantly increased by 27.28%–40.52%. Among them, the treatment with marigolds intercropped with unmodified biochar had the highest cadmium accumulation in water spinach, reaching 2.81 mg / kg. Under this treatment, the total cadmium accumulation of water spinach and marigolds reached 4.73 mg / kg. After applying biochar and HAP-modified biochar, the biomass of marigolds was significantly increased by 52.90% and 52.68% compared with the treatments without biochar and modified biochar, respectively, and the cadmium accumulation was significantly increased by 32.83% and 26.15%, respectively.

[0151] The above description is merely a preferred embodiment of this application and is not intended to limit this application. Various modifications and variations can be made to this application by those skilled in the art. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of this application should be included within the protection scope of this application.

Claims

1. A method for remediating cadmium-contaminated soil by intercropping water spinach with marigolds and applying a passivating agent, characterized in that, Includes the following steps: (1) Mix hydroxyapatite and water at a ratio of (0.5-2) g: 100 mL to obtain a hydroxyapatite solution; carbonize cow manure biogas residue at 400-500℃ for 0.8-1.2 h under anaerobic conditions and then sieve to obtain biochar; add the biochar to the hydroxyapatite solution and stir to obtain a mixture; filter the mixture, collect the precipitate and wash and dry it; then place the dried precipitate at 450-550℃ for 60-70 min to obtain HAP-modified biochar. The mass ratio of hydroxyapatite in the biochar and hydroxyapatite solution is (18-19.5):(0.5-2). (2) HAP modified biochar was first added to the cadmium-contaminated soil to be remediated at an application rate of 0.6-0.7 g / kg. Then, water spinach and marigolds were intercropped and cultivated and managed to achieve the purpose of remediating cadmium-contaminated soil. The intercropping ratio of water spinach and marigold is 1:(1-2); the planting density of water spinach in the planting strip is 11-13 plants / m². 2 The planting density of marigolds in the planting area is 4-6 plants / m². 2 The spacing between the water spinach planting strip and the marigold planting strip is 8-12cm.

2. The method for remediating cadmium-contaminated soil by intercropping water spinach with marigolds and applying a passivating agent as described in claim 1, characterized in that, In step (2), the water content of cadmium-contaminated soil after adding HAP-modified biochar is 55-65% of field capacity.

3. The method for remediating cadmium-contaminated soil by intercropping water spinach with marigolds and applying a passivating agent as described in claim 1, characterized in that... In step (2), before intercropping water spinach and marigold, seedling treatment is required, and then the water spinach and marigold seedlings are sown together. The seedling treatment for water spinach is as follows: soak water spinach seeds in water at 25-30℃ for 12-28 hours, wrap them in damp gauze, and place them at 28-30℃ to germinate. Sow them after they sprout white. The seedling treatment for marigold is as follows: soak marigold seeds in water at 35-40℃ for 2.5-3.5 hours, and sow them after they are air-dried.

4. The method for remediating cadmium-contaminated soil by intercropping water spinach with marigolds and applying a passivating agent as described in claim 1, characterized in that, In step (2), the cultivation management is as follows: seven days before sowing water spinach and marigold, apply base fertilizer to the soil, and one week before harvesting water spinach, apply urea; the base fertilizer is a mixture of urea, diammonium phosphate and potassium sulfate in a mass ratio of 1:(0.2-0.3):(0.4-0.6).

5. The method for remediating cadmium-contaminated soil by intercropping water spinach with marigolds and applying a passivating agent as described in claim 4, characterized in that... The application rate of base fertilizer is 0.083-0.33 g / kg, and the application rate of urea is 0.30-0.35 g / kg.

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