Method for degrading cadmium-polluted soil based on heavy metal passivator

Abstract

The invention relates to the technical field of soil regeneration, in particular to a method for degrading cadmium-polluted soil based on a heavy metal passivator, which comprises the following steps: S1, preparing PB-MgAl-LDH; s2, carrying out magnetic modification on the PB-MgAl-LDH; s3, preparing a passivating agent; and S4, soil remediation. Solid powder with a good specific surface area and adsorption activity is prepared through steady-state coprecipitation of pine barks and magnesium-aluminum hydrotalcite at a preferable specific growth position, a unique interlayer structure after magnetic modification has magnetism, negative charges are easily generated, adsorption of cadmium is accelerated for balancing the charges, and the adsorption efficiency of cadmium is improved. Meanwhile, lattice diffusion, isomorphous replacement and the like occur, so that the cadmium ion adsorbent shows relatively high ion exchange capacity and adsorption capacity to cadmium ions.

Description

Technical Field

[0001] The invention relates to the technical field of soil regeneration, and in particular to a method for degrading cadmium-contaminated soil based on a heavy metal passivator. Background Art

[0002] Cadmium pollution in soil mainly comes from industrial production, mining and irrational use of pesticides and fertilizers. Cadmium is a toxic heavy metal element. Once it enters the soil, it will not only poison crops and reduce crop yields and quality, but also enter the human body through the food chain, posing a potential threat to human health, such as causing bone diseases and kidney diseases.

[0003] When repairing and treating cadmium-contaminated soil, physical (soil repair, electrical repair), chemical (leaching) or biological methods (microorganisms) are generally used to remove cadmium from the soil. The principles of soil passivation remediation technology mainly include adjusting the soil pH, adding organic matter, and applying microbial preparations. Through these measures, the soil environment can be changed to restore it to a state suitable for plant growth. Especially for soil contaminated by heavy metals, passivation technology can reduce the absorption of heavy metals by plants by changing the form of heavy metals, reducing their biological effectiveness and mobility, thereby ensuring the safe production of crops. Passivation remediation technology not only works quickly, but also has the advantages of low cost, simple method and environmental friendliness. It has been widely used in the treatment of cadmium-contaminated land and achieved good results.

[0004] However, a single type of passivator has limited effect, poor stability, is prone to other environmental problems, and uses a large amount of raw materials. The invention patent with patent publication number CN113481014A discloses a cadmium-contaminated soil solid waste-based passivator, including modified blast furnace slag, modified fly ash and modified biochar. The modified blast furnace slag is prepared by modifying blast furnace slag with hydrochloric acid, the modified fly ash is prepared by modifying fly ash with NaOH, and the modified biochar is prepared by modifying biochar with chitosan. It also involves a method for preparing a cadmium-contaminated soil solid waste-based passivator, wherein the modified blast furnace slag, the modified fly ash and the modified biochar are uniformly mixed, stirred with water, and dried to obtain a cadmium-contaminated soil solid waste-based passivator. The beneficial effect is that the cost of the solid waste-based passivation agent for cadmium-contaminated soil is low, and the specific surface area of ​​the modified blast furnace slag, modified fly ash and modified biochar formed after modification is increased and the adsorption sites are increased, so as to increase the contact area with the cadmium ions in the cadmium-contaminated soil, thereby improving its adsorption effect and reducing the effective cadmium content in the cadmium-contaminated soil. However, this method still cannot avoid the secondary pollution problem caused by fly ash and blast furnace slag. Summary of the invention

[0005] In order to solve the above problems, the present invention provides a method for degrading cadmium-contaminated soil based on a heavy metal passivator, comprising the following steps: S1. Preparation of PB-MgAl-LDH: Take pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder, add them into deionized water as solid phase, dissolve and stir to obtain base liquid, add the base liquid and the first alkaline solution dropwise into deionized water, dissolve and stir to obtain a first mixed liquid, filter the first mixed liquid to obtain a first precipitate, wash and dry the first precipitate to obtain PB-MgAl-LDH solid powder; The mass ratio of pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder is 2:6~8:3~4, the solid content in the base liquid is 15~20wt%, the volume ratio of the base liquid to the first alkaline solution is 1~2:1~2, and the pH value of the first mixed solution is controlled to be 10±0.5 by adjusting the amount of deionized water in the first mixed solution; S2, PB-MgAl-LDH magnetic modification: adding the PB-MgAl-LDH solid powder to deionized water, adding a mixed solution of FeSO4·7H2O and FeCl3·6H2O under the condition of water bath heating and continuous stirring, then dropping a second alkaline solution to control the pH value at 10±0.5, continuing stirring for 1-2 hours and then cooling to obtain a second mixed solution, filtering the second mixed solution and magnetically separating to obtain a second precipitate, washing the second precipitate and drying to obtain a magnetically modified PB-MgAl-LDH solid powder; Wherein, Fe in the mixed solution of FeSO4·7H2O and FeCl3·6H2O 2+ and Fe 3+ The molar ratio of FeSO4·7H2O and FeCl3·6H2O is 3-4:5, the mass fraction of the PB-MgAl-LDH solid powder in deionized water is 25-30%, and the volume ratio of the mixed solution of FeSO4·7H2O and FeCl3·6H2O to deionized water is 1-2:1-2; S3, passivator preparation: the magnetic modified PB-MgAl-LDH solid powder is mixed with MnO2 powder and organic fertilizer to obtain a passivator; Wherein, the mass ratio of the magnetic modified PB-MgAl-LDH solid powder to the MnO2 powder and the organic fertilizer is 10-15:3-6:2-10; S4. Soil remediation: The passivating agent is evenly stirred and applied to the cadmium-contaminated soil to be treated, with an application amount of 100-300 g / m 3 The stirring depth is 5~20cm.

[0006] Furthermore, the preparation method of the pine bark powder in S1 is: grinding the bark on the shady side of the pine tree on the trunk with a grinding roller, the grinding depth is 0.2~0.6cm, to obtain pine bark coarse powder, and then grinding the pine bark coarse powder through a 100 mesh sieve to obtain pine bark powder.

[0007] Description: The pine bark on the shaded side is selected, which has a slower growth rate, rough bark, many surface cracks, a larger specific surface area and a well-developed pore structure, and is a good adsorption carrier.

[0008] Furthermore, the method for preparing the pine bark powder in S1 is as follows: peeling the bark on the shady side of the felled pine tree to a peeling depth of 0.5 to 1 cm, and grinding the peeled bark through a 100-mesh sieve to obtain the pine bark powder.

[0009] Furthermore, the preparation method of the first alkaline solution in S1 is: placing NaOH solid powder and Na2CO3 solid powder in deionized water, stirring and mixing to obtain a first alkaline solution with a mass concentration of 5-10%; Among them, the mass ratio of NaOH solid powder to Na2CO3 solid powder is 5~6:1.

[0010] Note: Adding NaOH solid powder and Na2CO3 solid powder to the first alkaline solution simultaneously makes the first alkaline solution more soluble and thermally stable than a single NaOH solution.

[0011] Furthermore, the second alkaline solution in S2 is prepared by placing NaOH solid powder in deionized water, stirring and mixing to obtain a second alkaline solution with a mass concentration of 15-20%.

[0012] Note: The addition of the second alkaline solution facilitates rapid adjustment of the pH value of the second mixed solution.

[0013] Furthermore, the Mg(NO3)2·6H2O solid powder, the Al(NO3)3·9H2O solid powder and the MnO2 powder are all sieved through a 200-mesh sieve.

[0014] Note: By optimizing and adjusting the powder particle size of each raw material, it is ensured that the prepared passivator has good surface morphology and passivation effect.

[0015] Furthermore, the water bath heating temperature in S2 is 60-65° C., and the stirring speed is 300-400 rpm.

[0016] Note: The reaction efficiency of water bath heating can be improved by adjusting the stirring speed.

[0017] Furthermore, the organic fertilizer in S3 is cow dung or sheep dung.

[0018] Description: Adding organic fertilizer to the passivator system can accelerate the recovery of soil to arable state.

[0019] Furthermore, in S4, when the passivating agent is uniformly stirred and applied to the cadmium-contaminated soil to be treated, the soil moisture content is maintained at 20-30% by controlling the amount of water added.

[0020] Note: By controlling the moisture content of the soil within a reasonable range, the adsorption effect of the passivator on heavy metal cadmium is ensured.

[0021] Compared with the prior art, the present invention has the following beneficial effects: (1) The present invention discloses a method for degrading cadmium-contaminated soil based on a heavy metal passivator. The method comprises preparing a PB-MgAl-LDH solid powder having good specific surface area and adsorption activity by steady-state coprecipitation of pine bark from a preferred specific growth position and magnesium aluminum hydrotalcite MgAl-LDH. The PB-MgAl-LDH solid powder is then magnetically modified so that its unique interlayer structure has magnetism and is prone to generate negative charge, thereby accelerating the adsorption of cadmium to balance the charge. At the same time, the adsorbed ions in the interlayer lattice undergo lattice diffusion and isomorphous substitution with cadmium ions, thereby exhibiting a strong ion exchange capacity and adsorption capacity for cadmium ions.

[0022] (2) The method of the present invention for degrading cadmium-contaminated soil based on a heavy metal passivator also limits the method for preparing pine bark, by preferably selecting pine bark on the shady side or peeling off pine bark at a specified position, thereby improving the access to raw materials, and allowing magnesium and aluminum to undergo an organic complex reaction with hydroxyl and carboxylic acid groups to form chemical bonds, thereby allowing the LDH-like substance to be constructed on the PB. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] Figure 1 This is a graph showing the change in the effective cadmium content in the soil of each experimental group at 15 days in the experimental example of the present invention; Figure 2 This is a graph showing the change in the effective cadmium content in the soil of each experimental group at 3 months in the experimental example of the present invention. DETAILED DESCRIPTION

[0024] In order to further illustrate the method adopted by the present invention and the effect achieved, the technical solution of the present invention will be clearly and completely described in combination with experiments below.

[0025] Example 1: This example describes a method for degrading cadmium-contaminated soil based on a heavy metal passivator, comprising the following steps: S1. Preparation of PB-MgAl-LDH: Take pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder, add them into deionized water as solid phase, dissolve and stir to obtain base liquid, add the base liquid and the first alkaline solution dropwise into deionized water, dissolve and stir to obtain a first mixed liquid, filter the first mixed liquid to obtain a first precipitate, wash and dry the first precipitate to obtain PB-MgAl-LDH solid powder; the filtration, washing and drying in this step are all routine operations, and are not specifically limited here. Those skilled in the art can perform the operations according to common sense and corresponding needs; The mass ratio of pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder is 2:6:3, the solid content in the base liquid is 16wt%, the volume ratio of the base liquid to the first alkaline solution is 1:1, and the pH value of the first mixed solution is controlled to be 10 by adjusting the amount of deionized water in the first mixed solution; The preparation method of pine bark powder is as follows: the bark on the shady side of the pine tree is ground on the trunk with a grinding roller to a grinding depth of 0.4 cm to obtain pine bark coarse powder, and then the pine bark coarse powder is ground through a 100-mesh sieve to obtain pine bark powder; The first alkaline solution is prepared by placing NaOH solid powder and Na2CO3 solid powder in deionized water, stirring and mixing to obtain a first alkaline solution with a mass concentration of 5%, wherein the mass ratio of the NaOH solid powder to the Na2CO3 solid powder is 5:1; S2, PB-MgAl-LDH magnetic modification: PB-MgAl-LDH solid powder is added to deionized water, and a mixed solution of FeSO4·7H2O and FeCl3·6H2O is added under conditions of water bath heating and continuous stirring, the water bath heating temperature is 62°C, the stirring speed is 350rpm, and then a second alkaline solution is added dropwise to control the pH value at 10, and the stirring is continued for 1.5 hours and then cooled to obtain a second mixed solution, the second mixed solution is filtered and magnetically separated to obtain a second precipitate, and the second precipitate is washed and dried to obtain a magnetically modified PB-MgAl-LDH solid powder; the filtration, washing and drying in this step are all conventional operations, and are not specifically limited here. Those skilled in the art can operate according to common sense and corresponding needs; Among them, Fe in the mixed solution of FeSO4·7H2O and FeCl3·6H2O 2+ and Fe 3+ The molar ratio of FeSO4·7H2O and FeCl3·6H2O is 3.5:5, the mass fraction of PB-MgAl-LDH solid powder in deionized water is 26%, and the volume ratio of the mixed solution of FeSO4·7H2O and FeCl3·6H2O to deionized water is 1:1; The second alkaline solution is prepared by placing NaOH solid powder in deionized water, stirring and mixing to obtain a second alkaline solution with a mass concentration of 17%; Mg(NO3)2·6H2O solid powder, Al(NO3)3·9H2O solid powder and MnO2 powder were all sieved through a 200-mesh sieve; S3, passivator preparation: magnetic modified PB-MgAl-LDH solid powder is mixed with MnO2 powder and organic fertilizer, wherein the organic fertilizer is sheep manure, to obtain a passivator; Among them, the mass ratio of magnetic modified PB-MgAl-LDH solid powder to MnO2 powder and organic fertilizer is 12:4:5; S4. Soil remediation: The passivating agent is evenly mixed and applied to the cadmium-contaminated soil to be treated, with an application amount of 200g / m 3 The mixing depth is 10 cm. When the passivating agent is evenly mixed and applied to the cadmium-contaminated soil to be treated, the soil moisture content is maintained at 25% by controlling the amount of water added.

[0026] Example 2: The difference between this example and Example 1 is that in S1, the mass ratio of pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder is 2:8:4, the solid phase content in the base liquid is 17wt%, the volume ratio of the base liquid to the first alkaline solution is 1.5:2, and the pH value of the first mixed liquid is 10.5.

[0027] Example 3: The difference between this example and Example 1 is that in S1, the mass ratio of pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder is 2:7:3, the solid phase content in the base liquid is 15wt%, the volume ratio of the base liquid to the first alkaline solution is 2:1, and the pH value of the first mixed liquid is 10.

[0028] Example 4: The difference between this example and Example 1 is that in S1, the mass ratio of pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder is 2:8:3, the solid phase content in the base liquid is 20wt%, the volume ratio of the base liquid to the first alkaline solution is 1:2, and the pH value of the first mixed liquid is 9.5.

[0029] Note: In Examples 1 to 4, the parameters that play a key role in influencing the final passivator effect are the ratio of pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder. When the content of pine bark powder is increased, the structure between MgAl-LDH and PB may become loose, and when the content of Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder is increased, a portion of Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder may not be utilized. Therefore, the ratio between the three should be controlled within a reasonable range.

[0030] Example 5: This example is different from Example 1 in that, in S1, the grinding depth is 0.2 cm.

[0031] Example 6: This example is different from Example 1 in that, in S1, the grinding depth is 0.6 cm.

[0032] Example 7: This example is different from Example 1 in that, in S1, the method for preparing pine bark powder is as follows: the bark on the shady side of the felled pine tree is peeled off to a depth of 0.5 cm, and the peeled bark is ground through a 100-mesh sieve to obtain the pine bark powder.

[0033] Example 8: This example is different from Example 7 in that, in S1, the peeling depth is 0.8 cm.

[0034] Example 9: This example is different from Example 7 in that, in S1, the peeling depth is 1 cm.

[0035] Note: In Examples 5 to 9, we mainly limit the preparation of pine bark powder and specific parameters. Among them, we have mentioned in the beneficial effects that the shady side and the rougher surface of the pine bark are preferred as the acquisition positions to obtain the pine bark powder that is most suitable for the method of the present invention. However, in the actual operation process, it is difficult to control strictly. Therefore, the parameters selected in Examples 1 and 5 to 9 can achieve similar technical effects.

[0036] Example 10: The difference between this example and Example 1 is that in S1, the method for preparing the first alkaline solution is: placing NaOH solid powder and Na2CO3 solid powder in deionized water, stirring and mixing to obtain a first alkaline solution with a mass concentration of 10%, wherein the mass ratio of NaOH solid powder to Na2CO3 solid powder is 6:1.

[0037] Example 11: The difference between this example and Example 1 is that in S1, the method for preparing the first alkaline solution is: placing NaOH solid powder and Na2CO3 solid powder in deionized water, stirring and mixing to obtain a first alkaline solution with a mass concentration of 8%, wherein the mass ratio of NaOH solid powder to Na2CO3 solid powder is 5.5:1.

[0038] Example 12: This example is different from Example 1 in that, in S2, the specific operating parameters of the magnetic modification of PB-MgAl-LDH are different.

[0039] S2. Magnetic modification of PB-MgAl-LDH: Add PB-MgAl-LDH solid powder to deionized water, add a mixed solution of FeSO4·7H2O and FeCl3·6H2O under conditions of water bath heating and continuous stirring, the water bath heating temperature is 60°C, the stirring speed is 300 rpm, then add a second alkaline solution dropwise to control the pH value at 9.5, continue stirring for 1 hour and then cool to obtain a second mixed solution, filter the second mixed solution, and magnetically separate to obtain a second precipitate, wash and dry the second precipitate to obtain magnetically modified PB-MgAl-LDH solid powder.

[0040] Example 13: This example is different from Example 1 in that, in S2, the specific operating parameters of the magnetic modification of PB-MgAl-LDH are different.

[0041] S2. Magnetic modification of PB-MgAl-LDH: Add PB-MgAl-LDH solid powder to deionized water, add a mixed solution of FeSO4·7H2O and FeCl3·6H2O under conditions of water bath heating and continuous stirring, the water bath heating temperature is 65°C, the stirring speed is 400 rpm, then add a second alkaline solution dropwise to control the pH value at 10.5, continue stirring for 2 hours and then cool to obtain a second mixed solution, filter the second mixed solution, and magnetically separate to obtain a second precipitate, wash and dry the second precipitate to obtain magnetically modified PB-MgAl-LDH solid powder.

[0042] Example 14: This example is different from Example 1 in that, in S2, the raw material parameter ratios of the magnetic modification of PB-MgAl-LDH are different.

[0043] Among them, Fe in the mixed solution of FeSO4·7H2O and FeCl3·6H2O 2+ and Fe 3+ The molar ratio of FeSO4·7H2O and FeCl3·6H2O is 3:5, the mass fraction of PB-MgAl-LDH solid powder in deionized water is 25%, and the volume ratio of the mixed solution of FeSO4·7H2O and FeCl3·6H2O to deionized water is 2:1.5; The second alkaline solution is prepared by placing NaOH solid powder in deionized water, stirring and mixing to obtain a second alkaline solution with a mass concentration of 15%.

[0044] Example 15: This example is different from Example 1 in that, in S2, the raw material parameter ratios of the magnetic modification of PB-MgAl-LDH are different.

[0045] Among them, Fe in the mixed solution of FeSO4·7H2O and FeCl3·6H2O 2+ and Fe 3+ The molar ratio of FeSO4·7H2O and FeCl3·6H2O is 4:5, the mass fraction of PB-MgAl-LDH solid powder in deionized water is 30%, and the volume ratio of the mixed solution of FeSO4·7H2O and FeCl3·6H2O to deionized water is 1.5:2; The second alkaline solution is prepared by placing NaOH solid powder in deionized water, stirring and mixing to obtain a second alkaline solution with a mass concentration of 20%.

[0046] Note: In Example 14 and Example 15, the volume ratio of the mixed solution of FeSO4·7H2O and FeCl3·6H2O to deionized water is maintained at 1:1 as much as possible, and the remaining parameters are selected within the range given in the present invention.

[0047] Example 16: This example is different from Example 1 in that, in S3, the mass ratio of magnetically modified PB-MgAl-LDH solid powder to MnO2 powder and organic fertilizer is 10:3:2.

[0048] Example 17: This example is different from Example 1 in that, in S3, the mass ratio of magnetically modified PB-MgAl-LDH solid powder to MnO2 powder and organic fertilizer is 15:6:10, and the organic fertilizer is cow dung.

[0049] Note: In Example 16 and Example 17, the application amount of organic fertilizer is reasonably adjusted according to the application scenario of the cadmium-contaminated soil to be treated. If the cadmium-contaminated soil to be treated will be used as arable land later, the use of organic fertilizer can be appropriately increased.

[0050] Example 18: This example is different from Example 1 in that the specific parameters of soil remediation in S4 are different.

[0051] S4. Soil remediation: The passivating agent is evenly mixed and applied to the cadmium-contaminated soil to be treated, with an application amount of 100g / m 3 The mixing depth is 5 cm. When the passivating agent is evenly mixed and applied to the cadmium-contaminated soil to be treated, the soil moisture content is maintained at 20% by controlling the amount of water added.

[0052] Example 19: This example differs from Example 1 in that the specific parameters of soil remediation in S4 are different.

[0053] S4. Soil remediation: The passivating agent is evenly mixed and applied to the cadmium-contaminated soil to be treated, with an application amount of 300g / m 3 The mixing depth is 20 cm. When the passivating agent is evenly mixed and applied to the cadmium-contaminated soil to be treated, the soil moisture content is maintained at 30% by controlling the amount of water added.

[0054] Note: In Example 18 and Example 19, different application amounts are reasonably selected according to the degree of cadmium contamination in the soil. At the same time, when the application amount is higher, the soil moisture content and the depth of stirring are appropriately increased to ensure the activity of the passivator.

[0055] Experimental Example: The description of this experimental example is based on the scheme described in Example 1, and is intended to illustrate the practical application effect of the present invention.

[0056] 1. Experimental design: In order to illustrate the adsorption and degradation performance of the passivator prepared by the present invention on cadmium pollution in soil, the following experimental groups were designed: Blank group: no passivator was added; Control group 1: Compared with Example 1, the pine bark powder was replaced with poplar bark powder, and the remaining steps were the same as those in Example 1; Control group 2: Compared with Example 1, the sampling position of the pine bark powder is not limited, and any pine bark is mixed and crushed, wherein there are pine barks on the shady side and the sunny side, and the remaining steps are the same as in Example 1; Control group 3: Compared with Example 1, step S2 and PB-MgAl-LDH magnetic modification were not performed, and the remaining steps were the same as those in Example 1; Control group 4: Compared with Example 1, in step S3, the magnetically modified PB-MgAl-LDH solid powder was replaced with pine bark biochar, and the remaining steps were the same as those in Example 1. The pine bark biochar was obtained by carbonizing pine bark at 700°C; Control group 5: Compared with Example 1, the magnetically modified PB-MgAl-LDH solid powder was replaced with a conventional fly ash passivator in step S3, and the remaining steps were the same as those in Example 1.

[0057] 2. Related performance experiments: First, the microstructure and specific surface area of ​​the passivator raw materials in each experimental group were observed. The results are shown in Table 1.

[0058] Table 1 Specific surface area of ​​passivation agent raw materials in each experimental group

[0059] It can be seen from the data in Table 1 that the specific surface area of ​​PB powder can be significantly increased by stable co-precipitation of PB powder and MgAl-LDH.

[0060] Then we applied each experimental group to actual cadmium-contaminated soil, where the cadmium content in the original soil was 2.2 mg / kg. After the application and moisture content control were completed according to the application method in Example 1, the changes in the effective cadmium content in the soil were measured at 15 days and 3 months respectively. The results are as follows: Figure 1 and Figure 2 shown.

[0061] It can be seen that at 15 days, the effective cadmium content in the soil of each experimental group decreased, among which the decrease in control group 4 and control group 5 was the most obvious. This is because the pine bark biochar and fly ash have a large specific surface area and have a good adsorption effect in the initial stage; secondly, there is no obvious difference between Example 1 and control groups 1 and 2, Example 1 is slightly higher, and control group 3 has the worst effect, because the PB-MgAl-LDH solid powder has not been magnetically modified, and its specific surface area is small, so the adsorption effect is not good in the initial stage; After 3 months, the passivator in Example 1 has the best adsorption and degradation rate of effective cadmium in the soil, and is significantly better than other control groups, while the difference in the effect between control group 4 and control group 5 from 15 days to 3 months is not much. This is because the control group 4 and control group 5 lack the modification of the internal structure of the passivator. Although they have good pore effect and specific surface area, the adsorption effect of the raw material itself is limited, and the adsorption efficiency will drop quickly. In other control groups, different degrees of modification were performed. The bark and magnesium aluminum hydrotalcite MgAl-LDH were coprecipitated to obtain X-MgAl-LDH solid powder with different good specific surface area and adsorption activity. It has a unique layered molecular structure and is easy to generate negative charge. Therefore, it will act on Cd in the soil for a long time. 2+ Continuous adsorption; in particular, in Example 1, pine bark Pine Bark and magnesium aluminum hydrotalcite MgAl-LDH in a preferred specific growth position are prepared by steady-state coprecipitation to obtain a PB-MgAl-LDH solid powder with good specific surface area and adsorption activity, and then the PB-MgAl-LDH solid powder is magnetically modified to make its unique interlayer structure magnetic, easy to generate negative charge, accelerate the adsorption of cadmium to balance the charge, and at the same time, the adsorbed ions in the interlayer lattice undergo lattice diffusion and isomorphous substitution with the cadmium ions, showing a strong ion exchange capacity and adsorption capacity for cadmium ions. Therefore, compared with the control group 3 that did not perform step S2 and PB-MgAl-LDH magnetic modification, the adsorption effect of effective cadmium in the soil was significantly improved both in the early stage of treatment and in the middle and late stages of treatment.

[0062] Compared with control group 1 and control group 2, the preparation method of pine bark is optimized, and the pine bark on the shady side is preferred. This is because magnesium and aluminum undergo organic complexation reactions with hydroxyl and carboxylic acid groups to form chemical bonds, thereby allowing LDH-like structures to be constructed on PB. The acquisition position of pine bark is optimized to obtain pine bark powder that is most suitable for the method of the present invention. This is because the pine bark on the shady side grows slower, the bark is rough, and there are many surface cracks. It has a larger surface and a well-developed pore structure, and is a good adsorption carrier. Poplar bark is relatively smooth, and the particles after grinding are finer and more uniform, with a smaller specific surface area, and smaller pore size and pore volume. The adsorption effect on cadmium in actual soil is not as good as that in Example 1. Therefore, the passivator in Example 1 is preferred as the best.

Claims

1. A method for degrading cadmium-contaminated soil based on a heavy metal passivator, characterized in that: The following steps are involved: S1. Preparation of PB-MgAl-LDH: Take pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder, add them into deionized water as solid phase, dissolve and stir to obtain base liquid, add the base liquid and the first alkaline solution dropwise into deionized water, dissolve and stir to obtain a first mixed liquid, filter the first mixed liquid to obtain a first precipitate, wash and dry the first precipitate to obtain PB-MgAl-LDH solid powder; The mass ratio of pine bark powder, Mg(NO3)2·6H2O solid powder and Al(NO3)3·9H2O solid powder is 2:6~8:3~4, the solid content in the base liquid is 15~20wt%, the volume ratio of the base liquid to the first alkaline solution is 1~2:1~2, and the pH value of the first mixed solution is controlled to be 10±0.5 by adjusting the amount of deionized water in the first mixed solution; S2, PB-MgAl-LDH magnetic modification: adding the PB-MgAl-LDH solid powder to deionized water, adding a mixed solution of FeSO4·7H2O and FeCl3·6H2O under the condition of water bath heating and continuous stirring, then dropping a second alkaline solution to control the pH value at 10±0.5, continuing stirring for 1-2 hours and then cooling to obtain a second mixed solution, filtering the second mixed solution and magnetically separating to obtain a second precipitate, washing the second precipitate and drying to obtain a magnetically modified PB-MgAl-LDH solid powder; Wherein, Fe in the mixed solution of FeSO4·7H2O and FeCl3·6H2O 2+ and Fe 3+ The molar ratio of FeSO4·7H2O and FeCl3·6H2O is 3-4:5, the mass fraction of the PB-MgAl-LDH solid powder in deionized water is 25-30%, and the volume ratio of the mixed solution of FeSO4·7H2O and FeCl3·6H2O to deionized water is 1-2:1-2; S3, passivator preparation: the magnetic modified PB-MgAl-LDH solid powder is mixed with MnO2 powder and organic fertilizer to obtain a passivator; Wherein, the mass ratio of the magnetic modified PB-MgAl-LDH solid powder to the MnO2 powder and the organic fertilizer is 10-15:3-6:2-10; S4. Soil remediation: The passivating agent is evenly stirred and applied to the cadmium-contaminated soil to be treated, with an application amount of 100-300 g / m 3 The stirring depth is 5~20cm.

2. A method for degrading cadmium-contaminated soil based on a heavy metal passivator as claimed in claim 1, characterized in that: The preparation method of the pine bark powder in S1 is as follows: the bark on the shady side of the pine tree is ground on the trunk by a grinding roller to a grinding depth of 0.2-0.6 cm to obtain coarse pine bark powder, and then the coarse pine bark powder is ground through a 100-mesh sieve to obtain pine bark powder.

3. A method for degrading cadmium-contaminated soil based on a heavy metal passivator as claimed in claim 1, characterized in that: The method for preparing the pine bark powder in S1 is as follows: peeling the bark on the shady side of the felled pine tree to a peeling depth of 0.5-1 cm, and grinding the peeled bark through a 100-mesh sieve to obtain the pine bark powder.

4. A method for degrading cadmium-contaminated soil based on a heavy metal passivator as claimed in claim 1, characterized in that: The preparation method of the first alkaline solution in S1 is: placing NaOH solid powder and Na2CO3 solid powder in deionized water, stirring and mixing to obtain a first alkaline solution with a mass concentration of 5-10%; Among them, the mass ratio of NaOH solid powder to Na2CO3 solid powder is 5~6:

1.

5. A method for degrading cadmium-contaminated soil based on a heavy metal passivator as claimed in claim 1, characterized in that: The second alkaline solution in S2 is prepared by placing NaOH solid powder in deionized water, stirring and mixing to obtain a second alkaline solution with a mass concentration of 15-20%.

6. A method for degrading cadmium-contaminated soil based on a heavy metal passivator as claimed in claim 1, characterized in that: The Mg(NO3)2·6H2O solid powder, the Al(NO3)3·9H2O solid powder and the MnO2 powder are all sieved through a 200-mesh sieve.

7. A method for degrading cadmium-contaminated soil based on a heavy metal passivator as claimed in claim 1, characterized in that: The water bath heating temperature in S2 is 60-65° C., and the stirring speed is 300-400 rpm.

8. The method for degrading cadmium-contaminated soil based on a heavy metal passivator according to claim 1, characterized in that: The organic fertilizer in S3 is cow dung or sheep dung.

9. The method for degrading cadmium-contaminated soil based on a heavy metal passivator according to claim 1, characterized in that: In S4, when the passivating agent is uniformly stirred and applied to the cadmium-contaminated soil to be treated, the soil moisture content is maintained at 20-30% by controlling the amount of water added.

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