A composite soil conditioner and method for immobilizing cadmium in soil

Abstract

The application belongs to the field of environmental protection and discloses a composite soil modifier for improving the cadmium fixation capacity of soil aggregates and a soil cadmium fixation method. The composite soil modifier comprises straw, montmorillonite and goethite, and the weight ratio of the straw, the montmorillonite and the goethite is 1-3:1-3:1-3. After the modifier is dispersed into soil, it can increase the residual Cd content of soil aggregates with a particle size of less than 0.053 mm and can optimize the cadmium fixation effect of the straw as a cadmium fixation material in a traditional scheme.

Description

TECHNICAL FIELD

[0001] The present application relates to the field of environmental protection, and in particular to a composite soil conditioner and a method for fixing cadmium in soil. BACKGROUND

[0002] The main heavy metal pollutants in soil are mercury, cadmium, lead, copper, chromium, arsenic, nickel, iron, manganese and zinc. These heavy metal elements are generally not easily leached with water in soil and cannot be decomposed by soil microorganisms. On the contrary, organisms can enrich heavy metals, which often causes heavy metals to gradually accumulate in the soil environment. Even some heavy metal elements in the soil can be transformed into more toxic methyl compounds. Some heavy metals accumulate in the human body in harmful concentrations through the food chain, which seriously endangers human health. Compared with water pollution, the treatment of heavy metal pollution in soil is more difficult and the harm is greater.

[0003] The treatment methods for cadmium pollution can be generally divided into:

[0004] The guest soil method or soil replacement method, adding lime substances to the soil to increase the pH of the soil environment, so that Cd(OH)3 or CdCO3 precipitates that are not easily absorbed by plants are formed from cadmium, using organic matter that promotes reduction in the soil to make cadmium in the soil form CdS precipitates with sulfur in the soil, planting cadmium-enriched plants such as amaranth plants to absorb cadmium in contaminated soil;

[0005] As a method that is now more valued in the art, the adsorption method can be seen in the following references:

[0006] Patent application No. CN109762574A, entitled Passivation agent for passivating soil heavy metal cadmium, preparation method thereof and method for passivating soil heavy metal cadmium using the same, discloses a passivation agent prepared from bentonite, quicklime, rice husk powder or rice straw powder, calcium phosphate and calcium hydrogen phosphate, wherein the rice husk powder or rice straw powder is mainly used to increase soil organic matter.

[0007] In other documents, it is also disclosed that straw powder can fix cadmium by complexation after modification and composting.

[0008] In addition, in some documents, it is disclosed that plant powders such as straw can fix cadmium by the principle of porous adsorption.

[0009] In summary, the mechanisms of straw cadmium fixation obtained by different documents are not the same, but it is not unique that straw can fix cadmium.

[0010] The research angle of the present application is to demonstrate a new cadmium fixation mechanism of the related soil conditioner through the relationship between soil aggregates and straw. SUMMARY

[0011] The application aims to provide a composite soil conditioner for improving the cadmium fixation capacity of soil aggregates, which can increase the residual Cd content of soil aggregates with a particle size of <0.053 mm and optimize the effect of cadmium fixation in the traditional scheme using straw as a cadmium fixation material after being dispersed into the soil.

[0012] Meanwhile, the application also provides a soil cadmium fixation method.

[0013] To achieve the above-mentioned purpose, the application discloses a composite soil conditioner for improving the cadmium fixation capacity of soil aggregates, which comprises straw, montmorillonite and goethite, and the weight ratio of the straw, the montmorillonite and the goethite is 1-3:1-3:1-3.

[0014] The application compares the effects of the straw, the combination of the straw and the montmorillonite, the combination of the straw and the goethite and the combination of the three on the cadmium content in the aggregates, and it can be found through verification that the cadmium content in the soil aggregates with different particle sizes is not greatly affected in any case; through analyzing the content of Cd in different forms in the soil aggregates with a particle size of <0.053 mm, it can be found that at least in red soil, the straw and the combination of the straw and any compound cannot significantly increase the content of residual Cd, and the content of residual Cd is significantly increased when the three are combined.

[0015] The possible reason is that the existing technology about cadmium fixation of straw is mostly used for improvement in paddy soil or other soil rich in organic matter, and the organic matter and other substances can improve the cadmium fixation effect of the straw. The related performance is difficult to exhibit in red soil. The combination of the straw, the montmorillonite and the goethite can effectively improve the situation.

[0016] In the composite soil conditioner, the weight ratio of the straw, the montmorillonite and the goethite is 2:1:1.

[0017] In the composite soil conditioner, the particle size of the straw is 1-3 mm.

[0018] In the composite soil conditioner, the straw is one or a combination of corn straw, rice straw and wheat straw.

[0019] Meanwhile, the application also discloses a soil cadmium fixation method, which comprises adding the composite soil conditioner as described above into the soil, and the weight ratio of the composite soil conditioner and the soil is 3-9:100.

[0020] In the soil cadmium fixation method, the soil is red soil.

[0021] In the soil cadmium fixation method, the temperature of the mixture of the soil conditioner and the soil is 15-45 DEG C.

[0022] In the above-mentioned method for fixing cadmium in soil, the temperature of the mixture of the soil conditioner and the soil is 25-35℃.

[0023] The present application has the following advantages and effects over the prior art:

[0024] The present application compares the effects of straw, straw and montmorillonite, straw and goethite, and the combination of the three on the cadmium content in the aggregates. Through verification, we can find that in any case, the cadmium content in the soil aggregates of different particle sizes is not greatly affected. By analyzing the content of different forms of Cd in the soil aggregates of <0.053mm particle size, it can be found that at least in red soil, straw and the combination of straw and any compound cannot significantly increase the content of residual Cd, and when the combination of the three is used, the content of residual Cd is significantly increased.

[0025] Through the present application, a solution is provided to further improve the cadmium fixation effect of red soil. DETAILED DESCRIPTION

[0026] The present application will be described clearly and completely below with reference to the embodiments of the present application. In the description of the present application, it should be noted that if the specific conditions are not specified in the embodiments, the conventional conditions or the conditions recommended by the manufacturer are used. If the reagents or instruments used are not specified by the manufacturer, they are all conventional products that can be purchased on the market.

[0027] Soil source: red soil in Changsha, Hunan, taken from Changsha, Hunan (North Latitude 28° East Longitude 113°), subtropical monsoon humid climate, and the soil parent material is Quaternary red clay. The main minerals present in the red soil are kaolinite, vermiculite and illite; the general chemical composition is as follows: sand particles 41.42wt%, silt particles 39.75wt%, clay particles 18.83wt%; soil organic matter (g / kg): 6.7; cation exchange capacity (cmol / kg): 6.8; pH: 5.97; free iron (g / kg): 21.86; amorphous iron (g / kg): 2.35; Cd (mg / kg): 0.21.

[0028] Goethite: CAS 1310-14-1;

[0029] Montmorillonite: CAS No. 1318-93-0;

[0030] Straw: naturally dried rice straw, particle size 2mm.

[0031] Grouping of soil:

[0032] The soil is divided into blank group (Con), straw group (RS), composite group 1 (RS+Fe), composite group 2 (RS+Mt) and composite group 3 (Com);

[0033] The weight ratio of straw and goethite in composite group 1 is 1:1;

[0034] The weight ratio of straw and montmorillonite in composite group 2 is 1:1;

[0035] The weight ratio of straw, goethite and montmorillonite in composite group 3 is 1:0.5:0.5;

[0036] The amount of straw added in each experimental group is 2wt%; the amount of goethite in composite group 1 is 2wt%, the amount of montmorillonite in composite group 2 is 2wt%, and the amount of goethite and montmorillonite in composite group 3 is 1wt% respectively. At the same time, 4mg / kg Cd is added to the soil in the above soil cadmium immobilization method, and the temperature of the mixture of the soil conditioner and the soil is 25~35℃, to obtain each soil sample, and each group of soil samples is 2, for cultivation experiment at different temperatures;

[0037] Each soil sample is placed in an environment of 25℃ and 35℃ for cultivation, and after 2 months of cultivation, the soil is sampled, and the sampled soil aggregates are classified into three particle size aggregates of >0.25mm, >0.053mm and <0.053mm.

[0038] Detection content:

[0039] 1. The Cd content in each particle size soil aggregate is determined, and the determination method is: after the soil is classified according to different aggregates, nitration is carried out, and then the Cd content in the nitration solution is determined by ICP-MS;

[0040] 2. Cd chemical form classification in <0.053mm particle size, determination method: refer to the method in Table 1 of the literature "Tessier sequential extraction method for soil chromium analysis Cr(Ⅵ)-Cr(Ⅲ) transformation and applicability", Vol. 15, No. 7, July 2021, theme, which records the detection of metal binding state, and the detection of Cd binding state is applicable to this method;

[0041] The results of Cd content in each particle size soil aggregate are shown in Tables 1 and 2;

[0042] Table 1 Cd content in each particle size soil aggregate under 25℃ cultivation conditions (unit: mg / kg)

[0043] > 0.25 mm > 0.053 mm <0.053 mm Con 1.6 1.7 3.0 RS 2.2 3.4 3.1 RS+Fe 2.2 3.5 3.1 RS+Mt 2.3 3.0 3.0 Com 2.6 3.3 3.1

[0044] Table 2 Cd content in different size soil aggregates under 35℃ incubation condition

[0045] > 0.25 mm > 0.053 mm <0.053 mm Con 1.8 2.3 3.2 RS 2.7 3.6 3.3 RS+Fe 2.3 3.5 3.1 RS+Mt 2.7 3.5 3.2 Com 2.6 3.7 3.2

[0046] Note: Aggregates separation requires constant flushing with water, which will cause a part of Cd with weak combination to soil to be lost, so the total content of Cd in any group of samples is not higher than 4mg / kg.

[0047] Cd chemical fractionation results in <0.053 particle size under 25℃ incubation condition can be referred to Table 3;

[0048] Table 3 Cd chemical fractionation results in <0.053 particle size (unit: mg / kg)

[0049] Exchangeable Cd Specifically adsorbed Cd Cd bound to Fe and Mn oxides Cd bound to organic matter Residual Cd Con 1.35 0.53 0.82 0.19 0.11 RS 1.72 0.36 0.63 0.25 0.12 RS+Fe 1.58 0.39 0.74 0.26 0.14 RS+Mt 1.56 0.36 0.63 0.25 0.11 Com 1.65 0.34 0.68 0.23 0.19

[0050] Cd chemical fractionation results in <0.053 particle size under 35℃ incubation condition can be referred to Table 4;

[0051] Table 4 Cd chemical fractionation results in <0.053 particle size (unit: mg / kg)

[0052] Exchangeable Cd Specifically adsorbed Cd Cd bound to Fe and Mn oxides Cd bound to organic matter Residual Cd Con 1.65 0.42 0.73 0.21 0.16 RS 1.79 0.43 0.65 0.28 0.17 RS+Fe 1.72 0.32 0.61 0.27 0.15 RS+Mt 1.7 0.41 0.59 0.32 0.18 Com Exchangeable Cd Specifically adsorbed Cd Cd bound to Fe and Mn oxides Cd bound to organic matter Residual Cd Con RS RS+Fe RS+Mt Com 1.75 0.39 0.54 0.28 0.25

[0053] Based on the Cd content in Table 1 and Table 2 and the Cd fractionation results in Table 3 and Table 4, we can get the following conclusions:

[0054] 1. From Table 1 and Table 2, we can see that the Cd content in <0.053mm soil aggregates in different composite groups changes little; the Cd content in each size soil aggregates in Table 2 is higher than that in Table 1, which indicates that the composition of soil aggregates changes significantly under different temperature conditions; generally, the content of <0.053mm soil aggregates will fluctuate within 75wt%~85wt%.

[0055] 2. From Table 3 and Table 4, we can see that under 25℃ incubation condition, the RS+Fe experimental group has better Cd immobilization effect and can improve the content of residual Cd; under 35℃ incubation condition, each single component has no obvious contribution to the improvement of Cd immobilization effect.

[0056] 3. From the comparison between Com group and other groups, we can see that no matter under 25℃ or 35℃ condition, the Com group has more obvious synergy.

Claims

1. A composite soil conditioner for improving the cadmium fixation capacity of soil aggregates, characterized in that, It is composed of straw, montmorillonite, and goethite; the weight ratio of straw, montmorillonite, and goethite is 1~3:1~3:1~3; the soil is red soil.

2. The composite soil conditioner according to claim 1, characterized in that, The weight ratio of straw, montmorillonite, and goethite is 2:1:

1.

3. The composite soil conditioner according to claim 1, characterized in that, The straw has a particle size of 1~3mm.

4. The composite soil conditioner according to claim 1, characterized in that, The straw is one or more of the following: corn straw, rice straw, and wheat straw.

5. A method for cadmium fixation in soil, characterized in that, The composite soil conditioner as described in any one of claims 1 to 4 is added to the soil, wherein the weight ratio of the composite soil conditioner to the soil is 3 to 9:

100.

6. The method for cadmium fixation in soil according to claim 5, characterized in that, The temperature of the soil conditioner and soil mixture is 15~45℃.

7. The method for cadmium fixation in soil according to claim 6, characterized in that, The temperature of the soil conditioner and soil mixture is 25~35℃.