Method for repairing cadmium-polluted soil by intercropping water spinach and marigold and applying passivating agent

Abstract

The invention discloses a method for repairing cadmium-polluted soil by intercropping water spinach and marigold and applying a passivating agent, and relates to the technical field of soil repairing. According to the method, HAP modified biochar is prepared through a reaction of hydroxyapatite and biochar and serves as a passivator, the passivator is added into the cadmium-contaminated soil, and water spinach and marigold are intercropped on the cadmium-contaminated soil with the passivator, so that the effect of reducing the cadmium content in the soil is achieved. According to the method, the passivating agent is used for conducting chemical remediation on soil cadmium, the effectiveness of heavy metal in the soil is reduced, plants rich in heavy metal are intercropped, and therefore the content of the heavy metal in the soil is reduced, and the method is a heavy metal pollution remediation mode capable of practically guaranteeing crop production safety.

Description

Technical Field

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

[0002] Soil is a vital natural resource for the sustainable development of agriculture in my country, and is the material basis and fundamental guarantee for human survival and development. With the rapid advancement of industrialization and urbanization, problems such as soil pollution and soil quality decline have gradually become prominent. Among them, soil heavy metal pollution has a long incubation period, poor mobility, and difficulty in remediation. It will not only damage the environment, but also affect the growth and development of crops, and enter the human body through the food chain, endangering human health. Therefore, the protection of soil and the treatment of contaminated soil are one of the focuses of our country's soil environmental protection work. Cadmium is one of the well-known "five poisonous" heavy metal elements, with the characteristics of long decomposition cycle, high mobility, high toxicity, and difficulty in degradation. Therefore, how to effectively repair and control soil cadmium pollution, reduce the absorption and enrichment of cadmium by crops, and achieve the safety of the environment and crop products is one of the important issues in the field of environmental ecology and agriculture.

[0003] At present, the methods for treating soil cadmium pollution include: physical remediation, chemical remediation, biological remediation and agricultural ecological measures. Among them, physical remediation is costly and difficult to promote on a large scale. Phytoremediation, as a form of biological remediation, uses plants with high cadmium enrichment ability to extract cadmium from the soil for centralized treatment. Passivation agent treatment, as a kind of chemical remediation, is added to cadmium-contaminated soil to convert cadmium elements into a stable state, thereby reducing its migration and bioavailability. It effectively reduces the concentration and bioavailability of effective cadmium ions in the soil by changing the pH value of the soil, generating precipitation, adsorption and ion exchange, organic coordination and complexation, etc. Agricultural ecological measures mainly treat soil cadmium pollution by changing the farming system and other methods. Among them, intercropping is a traditional farming model in agricultural production. A reasonable intercropping system can effectively control the absorption and accumulation of heavy metals such as cadmium by crops, and promote the efficient use of soil nutrients by crops, thereby increasing crop yields and land utilization.

[0004] In the prior art, CN113843272A discloses a method for repairing cadmium-lead contaminated cultivated land by replacing marigolds with other plants. Specifically, marigolds are planted after applying biochar, a passivating agent, in the lead-cadmium contaminated soil. The marigolds have large biomass and short growth cycle, and the effect of removing cadmium and lead is remarkable. CN108817048A discloses a method for repairing cadmium-contaminated soil by intercropping herbaceous plants and water spinach. Specifically, water spinach and herbaceous plants are intercropped on the cadmium-contaminated soil to be repaired. The herbaceous plants include: Pennisetum, etc. The intercropped herbaceous plants have high biomass and grow fast, can enrich heavy metals such as cadmium, and can effectively reduce the cadmium content in the contaminated soil. However, different intercropped plants drive the diversity of farmland soil organisms and the diversity of ecological functions through different root characteristics, thereby affecting the absorption and transport of heavy metals by crops.

[0005] Therefore, selecting suitable intercropped plants in combination with suitable passivators to repair soil cadmium pollution is of great significance to protecting the ecological environment and crop safety. Summary of the invention

[0006] In view of the above-mentioned prior art, the purpose of the present invention is to provide a method for repairing cadmium-contaminated soil by intercropping water spinach with marigold and applying a passivating agent. The present invention adopts hydroxyapatite and biochar to react to prepare HAP-modified biochar as a passivating agent, first adds the passivating agent to the cadmium-contaminated soil, and intercrops water spinach and marigold on the cadmium-contaminated soil with the passivating agent to achieve the effect of reducing the cadmium content in the soil. The present invention uses a passivating agent to chemically repair soil cadmium, reduce the effectiveness of heavy metals in the soil, and intercrop plants that are rich in heavy metals, thereby reducing the heavy metal content in the soil, which is a heavy metal pollution repair method that can effectively ensure the safety of crop production.

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

[0008] The present invention provides a method for repairing cadmium-contaminated soil by intercropping water spinach with marigold and applying a passivating agent, comprising the following steps:

[0009] (1) dispersing hydroxyapatite in water to obtain a hydroxyapatite solution, adding biochar to the hydroxyapatite solution and stirring to obtain a mixture, filtering the mixture, collecting the precipitate, washing and drying it, and then activating the dried precipitate to obtain HAP-modified biochar;

[0010] (2) First, add HAP-modified biochar to the cadmium-contaminated soil to be repaired, then intercrop water spinach and marigold and carry out cultivation management to achieve the purpose of repairing the cadmium-contaminated soil.

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

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

[0013] Preferably, in step (1), the method for preparing the biochar is as follows: the cow dung sludge is carbonized under anaerobic conditions and then sieved to obtain the biochar.

[0014] More preferably, the carbonization temperature is 400-500° C., and the carbonization time is 0.8-1.2 h.

[0015] Preferably, in step (1), the pH of the biochar is 9.43 and the EC value is 286.13 μg·cm -1 , total nitrogen content is 11.56g·kg -1 , the available phosphorus content is 1.03 mg kg -1 , the content of fast-acting potassium is 24.32 mg kg -1 , organic matter content 536.23g·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 2.5-3.5 h.

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

[0019] Preferably, in step (2), the moisture content of the cadmium-contaminated soil after adding HAP-modified biochar is 55-65% of the field water holding 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 the intercropping of water spinach and marigold is 1:(1-2), that is, 1-2 rows of marigold are intercropped for every 1 row of water spinach.

[0022] Preferably, in step (2), the planting density of the water spinach planting belt is 11-13 plants / m 2 The planting density of marigold planting belt is 4-6 trees / m 2 The distance between the planting belt of water spinach and the planting belt of marigold is 8-12cm.

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

[0024] Further preferably, the water spinach seedling treatment operation is: soak the water spinach seeds in 25-30°C water for 12-28 hours, wrap them with wet gauze, and place them at 28-30°C for germination, and sow them after they turn white; the seedling treatment operation of marigold is: soak the marigold seeds in 35-40°C water for 2.5-3.5 hours, and sow them after they are air-dried.

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

[0026] Further 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 preferably, the application amount of basal fertilizer is 0.083-0.33 g / kg, and the application amount of urea is 0.30-0.35 g / kg.

[0028] Beneficial effects of the present invention:

[0029] The invention adopts hydroxyapatite and biochar to react to prepare HAP modified biochar as a passivator, firstly adds the passivator into cadmium contaminated soil, and intercrops water spinach and marigold on the cadmium contaminated soil with the passivator, so as to achieve the effect of reducing the cadmium content in the soil.

[0030] The present invention utilizes a passivating agent to carry out chemical repairs such as complexation, adsorption, precipitation, oxidation, reduction, etc. to soil cadmium, reduce the effectiveness of heavy metals in the soil, and utilize intercropping of plants that enrich heavy metals, thereby reducing the content of heavy metals in the soil, and is a heavy metal pollution repair method that can effectively ensure the safety of crop production. On the one hand, the passivating agent used in the present invention has the characteristics of hydroxyapatite and biochar, and has a good passivation effect on the cadmium in the soil, and biochar-loaded hydroxyapatite can avoid the disadvantage that hydroxyapatite is not easily dispersed in the soil, and achieves a better passivation effect. On the other hand, the present invention utilizes water spinach biomass high, well-developed root system, and has certain pollution resistance, the biomass of marigold is large, and has a good enrichment effect on cadmium, and the growth cycle of marigold and water spinach is basically the same, therefore, water spinach is selected to cooperate with intercropping cadmium hyper-enriched plant marigold, thereby achieving a good repair effect on cadmium contaminated soil. BRIEF DESCRIPTION OF THE DRAWINGS

[0031] Figure 1: (A) Scanning electron micrograph of the biochar prepared in Example 1, (B) Scanning electron micrograph of the 10% HAP-modified biochar prepared in Example 1;

[0032] Figure 2 :Effect of HAP modified biochar on weakly acidic cadmium content in soil;

[0033] Figure 3 :Effect of HAP modified biochar on soil reduced cadmium content;

[0034] Figure 4 :Effect of HAP modified biochar on soil oxidized cadmium content;

[0035] Figure 5 :Effect of HAP modified biochar on the residual cadmium content in soil;

[0036] Figure 6 :Effects of different treatments on soil available Cd content;

[0037] Figure 7 : Effects of different treatments on the total amount of soil Cd;

[0038] Figure 8 : Distribution of soil Cd forms in different treatments;

[0039] Fig. 9 : Effects of different treatments on the transfer coefficient of water spinach. DETAILED DESCRIPTION

[0040] It should be noted that the following detailed descriptions are illustrative and are intended to provide further explanation of the present application. Unless otherwise specified, all technical and scientific terms used herein have the same meanings as those commonly understood by those skilled in the art to which the present application belongs.

[0041] In order to enable those skilled in the art to more clearly understand the technical solution of the present invention, the technical solution of the present invention will be described in detail below in conjunction with specific embodiments.

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

[0043] The cadmium-contaminated soil used in the present invention was taken from the periphery of the mine (0-20 cm), pH 6.31, EC 286.13 μg·cm -1 , total nitrogen 1.28g·kg -1 , available phosphorus 10.67 mg kg -1 , fast-acting potassium 109.84 mg kg -1 , organic matter content 15.06g·kg -1 , Cd3.21mg·kg -1;

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

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

[0046] (1) Weighing 0.5 g, 1 g, and 2 g of hydroxyapatite, respectively, and dissolving them in 100 mL of deionized water, and ultrasonically vibrating for 30 min to make them evenly dispersed, to obtain a first hydroxyapatite solution, a second hydroxyapatite solution, and a third hydroxyapatite solution, respectively;

[0047] (2) carbonizing the cow dung swamp at 450°C for 1 h under anaerobic conditions and passing through a 100-mesh sieve to obtain biochar;

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

[0049] The biochar prepared in this example and the 10% HAP modified biochar (QH) were characterized by scanning electron microscopy (SEM, JSM-6360LV, JEOL, Japan). Figure 1 shown.

[0050] in, Figure 1 (A) is the scanning electron microscope image of biochar. Figure 1 (B) is the scanning electron micrograph of 10% HAP modified biochar (QH). Figure 1 (A) It can be seen that the surface of biochar is rough and no obvious polymer is attached. 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 consistent with that of HAP. This shows that HAP is successfully loaded on the surface of biochar.

[0051] Example 2: Method for repairing cadmium-contaminated soil by intercropping water spinach with marigold and applying a passivating agent

[0052] 2.5% HAP modified biochar prepared in Example 1 was first added to the cadmium contaminated soil to be repaired, and the application amount was 0.67 g / kg soil. The moisture content of the cadmium contaminated soil after adding 2.5% HAP modified biochar was adjusted to 60% of the field water holding capacity. Water spinach and marigold were intercropped, and cultivation management was carried out to achieve the purpose of repairing the cadmium contaminated soil.

[0053] Among them, before intercropping, water spinach and marigold are treated with seedlings, and the water spinach and marigold after seedling treatment are sown uniformly, as follows: the seedling treatment of water spinach is as follows: after soaking the water spinach seeds in 28°C water for 24 hours, wrap them with wet gauze, and place them at 29°C for germination, and they can be sown after they turn white; the seedling treatment of marigold is as follows: after soaking the marigold seeds in 38°C water for 3 hours, they can be sown after they are air-dried.

[0054] During the intercropping process, the ratio of water spinach and marigold is 1:2, that is, 1-2 rows of marigold are planted for every 1 row of water spinach, and the planting density of the water spinach planting belt is 12 seeds / m 2 The planting density of marigold planting belt is 5 seeds / m 2 The distance between the planting belt of water spinach and the planting belt of marigold is 10cm.

[0055] The cultivation management is as follows: 7 days before sowing water spinach and marigold, basal fertilizer is applied to the soil, and one week before harvesting water spinach, urea is applied. The basal fertilizer is a mixture of urea, diammonium phosphate and potassium sulfate in a mass ratio of 1:0.25:0.5, the application amount of the basal fertilizer is 0.20g / kg soil, and the application amount of urea is 0.33g / kg soil.

[0056] Test example 1: Potted plant experiment

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

[0058] There are 10 treatments in this experiment, as follows:

[0059] Treatment group 1: CK group, this treatment group did not apply any passivation agent and did not intercrop marigold;

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

[0061] Treatment group 3: HB group, in this treatment group, 2.5% HAP modified biochar (HB) prepared in Example 1 was applied as a passivating agent and marigold was not intercropped;

[0062] Treatment group 4: DH group, in this treatment group, 5% HAP modified biochar (DH) prepared in Example 1 was applied as a passivating agent and marigold was not intercropped;

[0063] Treatment group 5: QH group, in this treatment group, 10% HAP modified biochar (QH) prepared in Example 1 was applied as a passivating agent and marigold was not intercropped;

[0064] Treatment group 6: OJ group, this treatment group did not apply passivation agent and intercropped marigold;

[0065] Treatment group 7: BJ group, in this treatment group, the biochar prepared in Example 1 was applied as a passivating agent and marigold was intercropped, and the preparation method of the biochar was the same as that in Example 1;

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

[0067] Treatment group 9: DJ group, this treatment group applied 5% HAP modified biochar (DH) prepared in Example 1 as a passivating agent and intercropped marigold;

[0068] Treatment group 10: QJ group. In this treatment group, 10% HAP-modified biochar (QH) prepared in Example 1 was applied as a passivating agent and marigold was intercropped.

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

[0070] 1. Specific test steps

[0071] (1) Cadmium-contaminated soil was used as potted test soil, passed through a 10-mesh sieve, and 3 kg was weighed in a plastic pot (33 cm long, 23 cm wide, and 15 cm high). The soil moisture was adjusted to 60% of the field water holding capacity. Large-leafed water spinach was selected as the test crop, and marigold was selected as the intercrop crop. 7 days before sowing, 1.75 g of basal fertilizer was applied to the soil. The basal fertilizer was a mixture of urea, diammonium phosphate, and potassium sulfate in a mass ratio of 1:0.25:0.5. Before sowing, water spinach and marigold were treated with seedlings. The water spinach seedling treatment operation was as follows: after soaking water spinach seeds in 28°C water for 24 hours, the seeds were wetted with water. Wrap them with gauze and place them at 29℃ for germination. They can be sown after they turn white. The seedling treatment operation of marigold is as follows: soak the marigold seeds in 38℃ water for 3h, and sow them after they are air-dried. Intercrop the water spinach seeds and marigold seeds after seedling treatment. Sow 10 water spinach seeds and 15 marigold seeds evenly in each pot. After emergence, retain three plants with good growth. Other management measures are the same. Apply 1g of urea one week before each harvest. Take three crops of water spinach on August 18, September 18 and October 18, and collect soil and marigold samples on October 18.

[0072] (2) Sample collection

[0073] Plant sampling: The aboveground parts of the first two crops of water spinach were taken, and the third crop and the whole plant of marigold were taken back to the laboratory for biomass determination. One part was stored in a -20℃ refrigerator, and the other part was withered, dried, crushed and passed through an 80-mesh sieve to determine the cadmium content in the plants.

[0074] Soil sampling: The potted test soil was bagged and brought back to the laboratory for air drying, grinding and passing through 20-mesh and 100-mesh sieves respectively to determine soil nutrients, cadmium content and other indicators. The rhizosphere soil sampling method adopted the "root shaking method", which was to shake off the large particles of soil in the root system, and use a sterile brush to brush off the attached soil along the root surface, which was regarded as the rhizosphere soil.

[0075] 2.1 Determination of soil indicators

[0076] 2.1.1 Total soil Cd, available Cd and Cd forms

[0077] The total Cd in soil was extracted with aqua regia; the available Cd content was extracted with diethylenetriaminepentaacetic acid (DTPA); the Cd form fractionation was based on the improved BCR continuous extraction method (1985), the preparation method of the BCR continuous extraction method is shown in Table 1, and the Cd content was determined by inductively coupled plasma optical emission spectrometer (ICP-OES) (Thermo Scientific Cap 7200).

[0078] Table 1 BCR continuous extraction method

[0079]

[0080] 2.2 Plant index determination

[0081] 2.2.1 Determination of plant biomass

[0082] After retrieving the plant samples, the surface soil was washed with water, all plant tissues were sterilized at 105°C for 30 minutes, and then dried at 80°C to constant weight. The water spinach was weighed separately according to the aboveground part and the underground part, and the whole marigold was weighed to obtain the biomass of the corresponding parts.

[0083] 2.2.2 Plant Cd content

[0084] Weigh 0.3 g (accurate to 0.0001 g) of seeds, stems, leaves or roots respectively into a conical flask, put a few glass beads, add 10 mL of nitric acid-perchloric acid mixed solution (9:1), cover and soak overnight, add a small funnel and digest on a hot plate. If it turns brown-black, add nitric acid until white smoke comes out and the digestion solution is colorless and transparent or slightly yellowish. After cooling, wash the digestion solution into a 25 mL volumetric flask, wash the conical flask with a small amount of nitric acid solution (1%) 3 times, and make up to the scale with nitric acid solution (1%), mix well and set aside; at the same time, do a reagent blank test, and determine the Cd concentration in the digestion solution by inductively coupled plasma optical emission spectrometer (ICP-OES) (ThermoScientifiiCAP 7200).

[0085] Among them, the bioconcentration factor (BCF) represents the ability of plants to accumulate Cd, and the calculation formula is shown in formula (I). The transfer 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 formula (II).

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

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

[0088] Wherein, in formula (I), C plant is the Cd concentration in plant tissue (mg / kg); C soil represents the Cd concentration in the soil (mg / kg). i is the aboveground Cd concentration (mg / kg); C j represents underground Cd concentration (mg / kg) (An et al., 2022).

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

[0090] 3. Results and Analysis

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

[0092] 3.1.1 Effects of applying different proportions of HAP-modified biochar on soil weak acid soluble state

[0093] Depend on Figure 2 It can be seen that on the 5th day after the start of cultivation, the weak acid cadmium content in the soil treated with biochar and HAP modified biochar decreased significantly (P<0.05), and each treatment decreased by 30.75%, 29.82%, 21.96% and 25.54% compared with CK, respectively; on the 10th day after cultivation, the effect of reducing the weak acid cadmium content in the soil of each treatment compared with CK was also significantly different (P<0.05), significantly reducing 26.17%, 33.40%, 37.11% and 28.94%; there was no significant difference between the treatments of applying different proportions of HAP modified biochar on the 5th and 10th days (P>0.05); on the 20th day after cultivation, the treatments with biochar and modified biochar significantly reduced the weak acid cadmium content in the soil by 32.77%, 49.04%, 43.55% and 47.69% compared with CK. In addition, there was a significant difference in the weak acid cadmium content in the soil between the BC treatment and the HB treatment.

[0094] analyze Figure 2 It can be seen that the weakly acidic cadmium content in the HB treatment was reduced by 24.20% compared with the BC treatment (P<0.05); in the data sampled on the 40th day, the weakly acidic cadmium content in the soil of the QH treatment was significantly reduced by 51.05% and 22.27% compared with the CK and HB treatments, respectively (P<0.05), and the weakly acidic cadmium content in the soil of the BC, HB and DH treatments was significantly reduced by 45.14%, 37.03% and 46.80% compared with the CK, respectively (P<0.05), and there was no significant difference among the three treatments; on the 60th day of the incubation experiment, the weakly acidic content in the soil of the BC, HB, DH and QH treatments was significantly lower than that of the CK treatment (P<0.05), and there was no significant difference between the treatments with biochar and modified biochar (P>0.05).

[0095] There was no significant difference in the weak acid cadmium content in the soil of CK treatment at different periods in the incubation experiment (P>0.05); the weak acid cadmium content in the soil of BC, HB, DH and QH treatments reached the minimum on the 60th day; there was no significant difference in the weak acid cadmium content in the soil of BC treatment at the 5th, 10th and 20th days (P>0.05), and on the 40th day, it decreased significantly by 19.82% and 20.39% compared with the 5th and 10th days, respectively (P<0.05), and on the 60th day, it decreased significantly by 42.86% compared with the initial incubation period (5d) (P<0.05); the weak acid cadmium content in the soil of HB treatment increased significantly at the 5th, 10th and 20th days. The soil weak acid cadmium content decreased during the first four periods, but increased during the first four periods, and decreased to the minimum value during the 60th day. The decrease was significant compared with the previous four periods (P<0.05). The weak acid cadmium content in the DH treatment was significantly different from that in the CK treatment, but there was no significant difference in the weak acid cadmium content in the soil during the 5-60 days period (P>0.05). Under the QH treatment, the weak acid cadmium content in the soil decreased significantly during the 5-20 days and 40-60 days period (P<0.05), and the weak acid cadmium content in the soil on the 20th, 40th and 60th days decreased by 29.98%, 33.46% and 52.07% compared with that on the 5th day (P<0.05). Figure 2 ).

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

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

[0098] analyze Figure 3It can be seen that, similar to the weakly acidic state, the reduced cadmium content in the soil under the CK treatment had no significant difference in the various periods of the incubation experiment (P>0.05); the reduced cadmium content in the soil under the BC treatment had no significant change on the 5th, 10th and 20th days (P>0.05), and decreased significantly on the 40th and 60th days compared with the 5th day, by 35.53% and 47.97% respectively (P<0.05); the reduced cadmium in the soil under the HB treatment began to decrease significantly on the 20th day compared with the early stage of the incubation experiment (5d) (P<0.05), and then reached the minimum on the 60th day, which was significantly decreased by 49.41% compared with CK (P<0.05); compared with the early stage of the incubation experiment (5d),

[0099] DH treatment could significantly reduce the reduced cadmium content in soil after 10 days of cultivation (P<0.05), reaching the lowest level on the 60th day, a decrease of 45.44% compared with the 5th day; under QH treatment, the reduced cadmium content in soil on the 60th day decreased significantly by 56.75% compared with the 5th day (P<0.05).

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

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

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

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

[0104] like Figure 5As shown in the figure, in the treatments of adding biochar and HAP modified biochar, the residual cadmium content in the soil of HB and DH treatments was significantly higher than that of CK treatment at the beginning of the incubation experiment (5d) (P<0.05), increasing by 17.31% and 16.62%, respectively; on the 10th day of the incubation experiment, only QH and CK showed significant differences in the treatments of adding biochar and modified biochar, and the residual cadmium in the soil of QH increased by 14.03% compared with CK (P<0.05); when the incubation experiment was carried out to the 20th, 40th and 60th days, the residual cadmium in the soil of QH increased by 14.03% compared with CK. The residual cadmium content in soil of BC, HB, DH and QH treatments was significantly higher than that of CK. At 20 days, the content of cadmium in each treatment increased by 21.24%, 28.15%, 30.77% and 34.09% (P<0.05), at 40 days, the content of cadmium in soil of each treatment increased by 29.96%, 29.18%, 29.08% and 36.59% (P<0.05), at 60 days, the content of cadmium in soil of each treatment increased by 37.21%, 42.11%, 36.78% and 45.13% (P<0.05).

[0105] Depend on Figure 5 It can be seen that under different ratios of HAP modified biochar treatments, the residual cadmium content in the soil showed a significant upward trend with the increase of incubation time. The residual cadmium content in the soil in the BC, HB, and DH treatments began to increase significantly on the 20th day, and the QH treatment began to show significant differences on the 10th day compared with the 5th day. Under the BC treatment, the residual cadmium content in the soil on the 20th, 40th, and 60th days increased significantly by 10.25%, 24.46%, and 30.24% (P<0.05) compared with the initial stage of the incubation experiment (5d); the residual cadmium in the HB and DH treatments from 20 to 60 days increased significantly by 13.32% to 31.16% and 16.33% to 26.99% (P<0.05) compared with the initial stage of the experiment (5d); the residual cadmium content in the soil under the QH treatment increased significantly by 11.39% to 33.27% (P<0.05) from 10 days to 60 days.

[0106] 3.2 Effects of water spinach intercropping with marigold and application of passivators on available Cd, total Cd and Cd forms in soil

[0107] 3.2.1 Effects of water spinach intercropping with marigold and application of passivation agent on available cadmium content in soil

[0108] from Figure 6It can be seen that compared with the CK group, the available cadmium content in the soil under other treatments was significantly reduced. The treatments of only applying biochar (BC group) and HAP modified biochar decreased the available cadmium content in the soil by 34.52% to 40.13% (P<0.05) compared with the CK treatment. The effect of intercropping marigold on reducing the available cadmium content in the soil was better than that of the treatments of only applying biochar and HAP modified biochar. Among them, the treatment of only intercropping marigold (OJ group) decreased by 15.99% (P<0.05) compared with the CK group. The intercropping treatments of applying biochar and HAP modified biochar decreased by 41.03% to 48.78% compared with CK, and decreased by 29.80% to 39.04% compared with OJ. In the treatments of applying biochar and HAP modified biochar and their corresponding intercropping treatments, the available cadmium in the soil of DJ was significantly reduced by 17.96% compared with DH, and that of QJ was significantly reduced by 16.10% compared with QH (P<0.05). Among all the treatments, planting marigolds in the middle and applying 2.5% HAP-modified biochar (HJ group) had the most significant effect on reducing soil cadmium.

[0109] 3.2.2 Effects of water spinach intercropping with marigold and application of passivation agent on the total amount of soil cadmium

[0110] Figure 7 The changes of total soil cadmium content under different treatments, among which only applying biochar and HAP modified biochar had no significant effect on reducing the total soil cadmium content (P>0.05), which was reduced by 0.79% to 7.5% compared with CK; intercropping marigold alone had no significant effect on the changes of total soil cadmium content; applying biochar and HAP modified biochar and intercropping could significantly reduce the total soil cadmium content, which was reduced by 13.12% to 22.12% compared with CK treatment (P<0.05), and BJ, DJ and QJ treatments significantly reduced the total soil cadmium content by 19.23%, 14.89% and 16.89% compared with OJ. % and 12.69% (P<0.05); in the treatments of applying biochar and HAP-modified biochar and their corresponding intercropping treatments, the total soil cadmium content in BJ was significantly reduced by 15.75% compared with BC, DJ was reduced by 12.17% compared with DH, and QJ was reduced by 15.15% compared with QH (P<0.05); the degree of decrease in the total soil cadmium content in the treatments of applying biochar and HAP-modified biochar and intercropping marigold was higher than that in the treatments of only applying biochar and HAP-modified biochar, among which the effects of significantly reducing the total soil cadmium content were BJ>HJ>QJ>DJ.

[0111] 3.2.3 Effects of water spinach intercropping with marigold and application of passivators on cadmium forms in soil

[0112] Figure 8The distribution of cadmium forms under different treatments showed that most of the cadmium in the soil existed in the residual form, accounting for 44.54% to 55.04% of the total cadmium in the soil; the next highest form was the weak acid form, accounting for 20.98% to 34.88% of the total cadmium in the soil; the contents of reduced and oxidized cadmium in the soil were low, accounting for 8.97% to 16.23% and 7.97% to 17.52% of the total cadmium in the soil, respectively. Compared with CK, the proportion of weakly acidic cadmium in soil of all treatments decreased significantly, among which HJ and DJ treatments had the best effect, decreasing by 39.85% and 37.41%, respectively (P<0.05). Only the proportion of weakly acidic cadmium in soil of marigold treatment (OJ) decreased significantly by 12.87% compared with CK (P<0.05). There was no significant difference in the proportion of weakly acidic cadmium in soil between the treatments of biochar and HAP-modified biochar and their corresponding intercropped marigold (P>0.05). The proportion of reduced cadmium in soil under HB and QH treatments increased significantly by 41.98% and 53.60% compared with CK (P<0.05), and the proportion of reduced cadmium in QJ treatment decreased by 15.16% compared with CK (P<0.05). In the treatments of biochar and HAP-modified biochar and their corresponding intercropped marigold, the proportion of reduced cadmium in soil showed significant differences between HB and HJ, and between QH and QJ. Compared with HJ, the percentage of oxidized cadmium in HB was 38.14% higher (P<0.05), and that in QH was 80.98% higher (P<0.05) higher than that in QJ. Except for OJ and CK, which showed no significant difference in the percentage of oxidized cadmium in soil (P>0.05), the percentage of oxidized cadmium in soil in other treatments was significantly higher than that in CK, ranging from 58.86% to 119.92% (P<0.05). Compared with BC, the percentage of oxidized cadmium in soil in BJ treated with biochar and HAP-modified biochar and their corresponding intercropped marigold was significantly higher by 38.44% (P<0.05). Compared with CK, the percentage of residual cadmium in soil in all treatments showed no significant difference (P>0.05). Compared with BJ, the percentage of residual cadmium in soil in HJ, DJ and QJ treatments treated with intercropped marigold was significantly higher by 21.04%, 22.36% and 23.58% (P<0.05).

[0113] 3.3 Effects of intercropping water spinach with marigold and applying passivators 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] As can be seen from Table 2, the addition of biochar and HAP-modified biochar can increase the biomass of marigold. The biomass of marigold in BJ and HJ treatments was significantly higher than that in CK treatment, increasing by 52.90% and 52.68%. The cadmium content of marigold decreased after adding HAP-modified biochar, but there was no significant change compared with the CK treatment. Compared with CK, the enrichment coefficient of soil cadmium of marigold treated with HAP-modified biochar did not change significantly; after applying HAP-modified biochar, the accumulation of marigold in HJ and DJ treatments was significantly higher than that in CK, increasing by 32.83% and 26.15%, respectively.

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

[0117]

[0118] 3.3.2 Effects of intercropping marigold with passivation agent on the biomass of different parts of water spinach

[0119] As can be seen from Table 3, after applying biochar and HAP modified biochar, the biomass of the first crop of water spinach was significantly higher than that of CK treatment, increasing by 69.28%, 71.89%, 31.69% and 79.41% respectively; after intercropping with marigold, the biomass of water spinach was still significantly higher than that of CK, increasing by 55.56%, 71.89%, 41.83% and 51.63% respectively, and there was no difference between the corresponding non-intercropping treatments. The biomass change of water spinach after intercropping with marigold alone was not significant compared with CK; compared with C Compared with intercropping and non-intercropping treatments, the application of biochar and HAP-modified biochar under K treatment 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 under QH and QJ treatments was significantly higher than that in CK, which was significantly increased by 217% and 255% compared with CK, respectively, and 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 increased compared with CK, which increased by 143% to 253%.

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

[0121]

[0122] 3.3.3 Effects of intercropping marigold with passivation agent on cadmium content in different parts of water spinach

[0123] As can be seen from Table 4, in the first crop of water spinach collected, only the OJ treatment with marigold intercropped had significantly higher cadmium content in the aboveground part of water spinach than other treatments, which was 31.33% higher than that in the CK treatment (P<0.05); the cadmium content in the aboveground part of water spinach in the BC, HB, DH and QH treatments with only biochar and HAP-modified biochar decreased significantly compared with the CK treatment, by 25.90%, 33.73%, 28.01% and 33.13%, respectively (P<0.05); the cadmium content in water spinach in BJ, HJ, DJ and QJ intercropped with marigold was significantly lower than that in the CK treatment, among which the QJ treatment had the best effect on reducing the cadmium content in the aboveground part of water spinach, which was 25.01% lower than that in the CK treatment and 42.59% lower than that in the OJ treatment (P<0.05). In the first crop of water spinach, there was no significant difference (P>0.05) between BC and BJ, DH and DJ, QH and QJ in the intercropping marigold combined with biochar and HAP modified biochar treatments and their corresponding non-intercropping marigold treatments. The cadmium content of water spinach in HB was significantly reduced by 21.15% compared with that in HJ.

[0124] In the aboveground part of the second crop of water spinach, the cadmium content of water spinach in CK and OJ treatments was significantly higher than that in other treatments, and there was no significant difference in the cadmium content of water spinach between the two treatments; the cadmium content of water spinach aboveground in the treatments of BC, HB, DH and QH, which only applied biochar and HAP-modified biochar, was reduced by 18.69%, 20.39%, 26.91% and 39.66% respectively compared with that in the CK treatment (P<0.05); The cadmium content in the aboveground part of the second crop of water spinach in BJ, HJ, DJ and QJ treatments was significantly reduced by 32.29%, 30.88%, 23.79% and 19.83% compared with CK, and was significantly reduced by 31.51%, 30.09%, 22.92% and 18.91% compared with OJ treatment. In the intercropping treatment of marigold and its corresponding non-intercropping treatment, the cadmium content in the aboveground part of water spinach in QH was significantly reduced by 24.73% compared with QJ. There was no significant difference among other treatments (Table 4).

[0125] In the third crop of water spinach, the cadmium contents in the aboveground parts of water spinach in the treatments of only applying biochar and HAP-modified biochar were significantly lower than those in the CK treatment, decreasing by 22.73%, 24.33%, 33.96% and 45.99%, respectively (P<0.05); compared with the CK treatment, the OJ treatment also had a significant effect on reducing the cadmium content in the aboveground parts of the third crop of water spinach, which could be reduced by 30.21%; the cadmium content in the aboveground parts of water spinach in the treatments of intercropping marigold on the basis of applying biochar and HAP-modified biochar was significantly reduced compared with the CK treatment; the cadmium content in the aboveground parts of water spinach in the BJ, HJ, DJ and QJ treatments was significantly reduced by 29.43%, 33.69%, 27.46% and 30.21% compared with CK (Table 4).

[0126] Analysis of Table 4 shows that the cadmium content in the underground part of water spinach in the CK and OJ treatments was significantly higher than that in other treatments. The cadmium content in the underground part of water spinach in the treatments of single application of biochar and HAP modified biochar was significantly reduced compared with the CK treatment, which was reduced by 41.14%, 38.30%, 46.71% and 36.49% respectively (P<0.05), and there was no significant difference between these treatments; in the treatment of intercropping marigold, the cadmium content in the BJ, HJ, DJ and QJ treatments was reduced by 32.63%, 27.82%, 34.69% and 24.17% respectively compared with the CK treatment (P<0.05). There was no significant difference in the cadmium content in the underground part of water spinach between the treatments of single application of biochar and HAP modified biochar and their corresponding intercropping marigold treatments (P>0.05).

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

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

[0129]

[0130]

[0131] 3.3.4 Effects of intercropping marigold with passivation agent on cadmium enrichment coefficients in different parts of water spinach

[0132] As can be seen from Table 5, in the OJ treatment with only intercropping of marigold, the BCF of the aboveground part of water spinach in the first crop was significantly higher than that in other treatments, and increased by 36.06% compared with the CK treatment; compared with CK, the BCF of the aboveground part 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 intercropped with marigold, the BCF of the aboveground part of water spinach in the BJ, HJ, DJ, QJ treatments could all be significantly reduced compared with the OJ treatment, among which the effect of the QJ treatment was the most significant, decreasing by 34.47%; there were certain changes in the BCF of the aboveground part of water spinach between the single application of biochar and HAP modified biochar and their corresponding intercropped marigold treatments, among which there were 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 BCF of the aboveground part of water spinach in CK, OJ and QJ treatments was higher than that in other treatments. Among the treatments with biochar and HAP-modified biochar alone, the BCF of the aboveground part of water spinach in DH and QH treatments was significantly reduced compared with CK, by 26.06% and 39.66%, respectively (P<0.05). Intercropping marigold with biochar and HAP-modified biochar had no significant effect on the BCF of the aboveground part of the second crop of water spinach. Among the treatments with biochar alone and HAP-modified biochar and their corresponding intercropping marigold, there was a significant difference in BCF between QH and QJ treatments, and QJ increased by 57.14% compared with QH (Table 5).

[0134] In the third harvest of water spinach, the enrichment coefficient of the third aboveground part of water spinach treated with biochar and HAP-modified biochar alone was significantly lower than that of CK. BC, HB, DH and QH were significantly reduced by 16.69%, 18.88%, 33.49% and 45.61% respectively compared with CK. However, the treatment with marigold alone was significantly reduced by 27.82% compared with CK. In the treatment with marigold intercropped with biochar and HAP-modified biochar, HJ, DJ and QJ were significantly reduced by 19.41%, 16.95% and 17.29% compared with CK (Table 5).

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

[0136] The enrichment coefficient of water spinach in the treatment with HAP modified biochar alone was significantly lower than that in the CK treatment, decreasing by 22.58% to 39.89%. After intercropping marigold with modified biochar, only the enrichment coefficient of DJ treatment was significantly lower than that of CK, decreasing by 13.98%. Compared with the treatment without intercropping marigold, the enrichment coefficient of water spinach in BJ was significantly increased by 24.21% compared with BC, and in QJ compared with HJ (Table 5).

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

[0138]

[0139] 3.3.5 Effects of intercropping marigold with passivation agent on cadmium accumulation in water spinach

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

[0141] The cadmium content in the underground part of water spinach in the treatments of BC, HB, DH and QH with biochar and HAP modified biochar was significantly higher than that in the CK treatment, increasing by 92.39%, 72.52%, 49.19% and 98.97% respectively. The cadmium content in the underground part of water spinach intercropped with marigold alone increased significantly by 57.81% compared with the CK treatment, and the cadmium content in the underground part of water spinach after intercropping marigold with biochar and HAP modified biochar increased significantly by 83.97% to 154.66%. The cadmium content in the underground part of water spinach under the treatment of intercropping marigold was higher than that of the treatment without intercropping, among which HJ increased significantly by 32.26% compared with HB (Table 6).

[0142] From the analysis of Table 6, it can be seen that the use of biochar and HAP-modified biochar alone or intercropping marigold with biochar and HAP-modified biochar can increase the accumulation of soil cadmium in water spinach. In the non-intercropping treatments, the cadmium accumulation of water spinach under BC, HB and QH treatments was significantly increased by 31.62%, 23.97% and 16.87% compared with the CK treatment. The cadmium content of water spinach under intercropping marigold with biochar and HAP-modified biochar was significantly increased by 27.28% to 40.52% compared with CK. Among them, the cadmium accumulation of water spinach under BJ treatment was the highest, and the total accumulation level of soil cadmium of water spinach and marigold under BJ treatment reached 4.73 mg / kg.

[0143] Table 6 Effect of different treatments on cadmium accumulation in various parts of water spinach

[0144]

[0145] 3.3.6 Effect of intercropping marigold with water spinach and applying passivation agent on the transfer coefficient of water spinach

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

[0147] 4. Conclusion

[0148] Applying biochar and modified biochar alone can significantly reduce the available cadmium content in soil. The available cadmium and total cadmium content in soil after intercropping marigold with biochar and modified biochar were significantly lower than those in the control treatment. Compared with the control, the available cadmium content in soil decreased by 34.52% to 40.13% after applying biochar and HAP modified biochar alone, and the available cadmium content in soil decreased by 41.03% to 48.78% after intercropping marigold with biochar and HAP modified biochar, and the total cadmium content in soil decreased by 13.12% to 22.12%. Among them, intercropping marigold with 2.5% HAP modified biochar had the most significant effect on reducing the available cadmium content in soil, and intercropping marigold with unmodified biochar had the best effect on reducing the total cadmium content in soil.

[0149] Only applying biochar and HAP modified biochar can reduce the cadmium content of water spinach and the enrichment capacity of water spinach for soil cadmium, and increase the biomass and cadmium accumulation of water spinach. Compared with the control, the treatments of applying biochar and modified biochar significantly reduced the cadmium content and enrichment coefficient of water spinach by 28.31% to 40.32% and 2.58% to 39.89%, respectively; the biomass of water spinach increased significantly by 61.80% to 95.89% after applying biochar and modified biochar. Due to the increase in water spinach biomass, the cadmium accumulation of water spinach was also higher than that of the control. The cadmium accumulation of water spinach with unmodified biochar was significantly increased by 31.62% compared with the control.

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

[0151] The above description is only the preferred embodiment of the present application and is not intended to limit the present application. For those skilled in the art, the present application may have various modifications and variations. Any modification, equivalent replacement, improvement, etc. made within the spirit and principle of the present application shall be included in the protection scope of the present application.

Claims

1. A method for repairing cadmium-contaminated soil by intercropping water spinach with marigold and applying a passivating agent, characterized in that: The following steps are involved: (1) dispersing hydroxyapatite in water to obtain a hydroxyapatite solution, adding biochar to the hydroxyapatite solution and stirring to obtain a mixture, filtering the mixture, collecting the precipitate, washing and drying it, and then activating the dried precipitate to obtain HAP-modified biochar; (2) First add HAP-modified biochar to the cadmium-contaminated soil to be repaired, then intercrop water spinach and marigold and carry out cultivation management to achieve the purpose of repairing the cadmium-contaminated soil.

2. The method according to claim 1, characterized in that In step (1), in the hydroxyapatite solution, the material-liquid ratio of hydroxyapatite to water is (0.5-2) g:100 mL.

3. The method according to claim 1, characterized in that In step (1), the preparation method of the biochar is as follows: the cow dung residue is carbonized under anaerobic conditions and then sieved to obtain the biochar; the carbonization temperature is 400-500° C., and the carbonization time is 0.8-1.2 h.

4. The method according to claim 1, characterized in that In step (1), the mass ratio of biochar to hydroxyapatite in the hydroxyapatite solution is (18-19.5): (0.5-2); the activation treatment temperature is 450-550° C., and the activation treatment time is 60-70 min.

5. The method according to claim 1, characterized in that In step (2), the moisture content of the cadmium-contaminated soil after adding the HAP-modified biochar is 55-65% of the field water holding capacity.

6. The method according to claim 1, characterized in that In step (2), the amount of HAP-modified biochar applied is 0.6-0.7 g / kg.

7. The method according to claim 1, characterized in that In step (2), the ratio of intercropping of water spinach and marigold is 1:(1-2); the planting density of water spinach planting belt is 11-13 plants / m 2 The planting density of marigold planting belt is 4-6 trees / m 2 The distance between the planting belt of water spinach and the planting belt of marigold is 8-12cm.

8. The method according to claim 1, characterized in that In step (2), water spinach and marigold need to be treated with seedlings before intercropping, and then the water spinach and marigold after the seedling treatment are sown uniformly; the water spinach seedling treatment is as follows: soak the water spinach seeds in water at 25-30°C for 12-28 hours, wrap them with wet gauze, and place them at 28-30°C for germination, and sow them after they turn white; the marigold seedling treatment is as follows: soak the marigold seeds in water at 35-40°C for 2.5-3.5 hours, and sow them after they are air-dried.

9. The method according to claim 1, characterized in that In step (2), the cultivation management is as follows: 7 days before sowing water spinach and marigold, basal fertilizer is applied to the soil, and one week before harvesting water spinach, urea is applied as topdressing; the basal 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).

10. The method according to claim 9, characterized in that The application amount of base fertilizer is 0.083-0.33g / kg, and the application amount of urea is 0.30-0.35g / kg.

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