A dithiocarbamate modified magnesium modified biochar and a preparation method and application thereof

Abstract

This invention provides a magnesium-modified biochar modified with dithiocarbamate, its preparation method, and its application. The preparation method includes: (1) mixing magnesium-modified biochar, a solvent, and an amine, then adding a solution containing an aldehyde to carry out a cross-linking reaction to obtain magnesium-modified biochar grafted with an amine; (2) mixing the magnesium-modified biochar grafted with an amine, an alkaline solution, and carbon disulfide, and reacting to obtain the magnesium-modified biochar modified with dithiocarbamate. The magnesium-modified biochar modified with dithiocarbamate prepared by the method of this invention has a strong adsorption effect on heavy metals.

Description

Technical Field

[0001] This invention belongs to the field of water treatment technology, and relates to a magnesium-modified biochar modified with dithiocarbamate, its preparation method and application. Background Technology

[0002] Heavy metals in water can seriously harm human health, damaging multiple systems such as the nervous, renal, cardiovascular, and endocrine systems. Common methods for treating heavy metal wastewater include chemical precipitation, membrane separation, and adsorption. Among these, adsorption is widely used due to its simplicity, speed, and low cost.

[0003] Biochar materials, due to their porous structure, large specific surface area, and abundant surface functional groups, are excellent adsorbent materials. Currently, in biochar adsorption research, various products and methods for adsorbing heavy metals in water have been developed. For example, CN109529783A discloses a method for preparing amino-modified biochar, which uses biomass as raw material to prepare biochar, and then modifies it with triethylenediamine to obtain amino-modified biochar, achieving an adsorption capacity of 398.42 mg / g for lead ions. While the modified biochar material prepared above shows improved heavy metal adsorption compared to original biochar, the improvement is limited. Therefore, developing a biochar material with superior heavy metal adsorption performance is of great significance. Summary of the Invention

[0004] To address the shortcomings of existing technologies, the present invention aims to provide a magnesium-modified biochar modified with dithiocarbamate, its preparation method, and its applications. This invention prepares magnesium-modified biochar grafted with amines through a cross-linking reaction, and then prepares magnesium-modified biochar modified with dithiocarbamate through a reaction with carbon disulfide. On the one hand, the magnesium oxide introduced onto the biochar surface can treat heavy metal ions in water through ion exchange and mineralization; on the other hand, the modified dithiocarbamate (DTC) group is a highly efficient functional group for capturing heavy metal ions and treating heavy metal pollution in wastewater. It contains two sulfur atoms with specific binding capabilities for most heavy metals, and these two donor sulfur atoms can interact with heavy metal ions to form stable chelate complexes. Therefore, under the synergistic effect of magnesium oxide and dithiocarbamate, the biochar material exhibits strong adsorption of various heavy metals in water, and the heavy metal adsorption effect is excellent.

[0005] To achieve this objective, the present invention adopts the following technical solution:

[0006] In a first aspect, the present invention provides a method for preparing magnesium-modified biochar modified with dithiocarbamate, the method comprising:

[0007] (1) Mix magnesium-modified biochar, solvent and amine, then add a solution containing aldehyde to carry out a cross-linking reaction to obtain magnesium-modified biochar grafted with amine.

[0008] (2) The magnesium-modified biochar grafted with amines, an alkaline solution, and carbon disulfide are mixed and reacted to obtain the magnesium-modified biochar modified with dithiocarbamate.

[0009] It should be noted that the "magnesium" in the magnesium-modified biochar exists in the form of magnesium oxide.

[0010] This invention provides a method for preparing magnesium-modified biochar modified with dithiocarbamate. The method utilizes the hydroxyl groups on the surface of magnesium-modified biochar to obtain magnesium-modified biochar grafted with amines through a cross-linking reaction. Then, it reacts with carbon disulfide to obtain magnesium-modified biochar modified with dithiocarbamate. On one hand, the magnesium oxide introduced onto the biochar surface can treat heavy metal ions in water through ion exchange and mineralization. On the other hand, the modified dithiocarbamate group is a highly efficient functional group for capturing heavy metal ions and treating heavy metal pollution in wastewater. It contains two sulfur atoms with specific binding capabilities for most heavy metals. These two donor sulfur atoms can interact with heavy metal ions to form stable chelate complexes. Therefore, under the synergistic effect of magnesium oxide and dithiocarbamate, the biochar material exhibits strong adsorption of various heavy metals in water, and the heavy metal adsorption effect is excellent.

[0011] Preferably, the method for preparing the magnesium-modified biochar includes:

[0012] The magnesium-modified biochar is obtained by mixing biomass raw materials, magnesium source and solvent, impregnating the mixture, and then freeze-drying and pyrolyzing it under vacuum.

[0013] In this invention, the magnesium-modified biochar synthesized by impregnation-pyrolysis method can significantly increase the specific surface area and porosity of biochar, and magnesium oxide is uniformly distributed on the surface of biochar.

[0014] Preferably, the biomass raw material includes at least one of water hyacinth, orange peel and straw, with water hyacinth being the most preferred.

[0015] In this invention, water hyacinth, rich in cellulose and lignin, produces biochar with more surface functional groups through pyrolysis, making it an ideal raw material for biochar preparation. Furthermore, water hyacinth is an invasive alien species, and processing it into biochar can improve the efficient utilization of waste resources. However, its adsorption and removal of pollutants in water is limited by its surface functional groups and electrochemical properties. Therefore, it is necessary to modify the surface of the prepared biochar to functionalize it and improve its affinity for metal ions, thereby providing a highly efficient and economical biochar composite adsorbent for the water treatment field.

[0016] Preferably, the magnesium source includes at least one of magnesium bicarbonate, magnesium carbonate, and magnesium acetate.

[0017] Preferably, the mass ratio of the biomass raw material to the magnesium source is 1:(0.05-5), for example, it can be 1:0.05, 1:0.1, 1:0.2, 1:0.5, 1:1, 1:2, 1:3, 1:4 or 1:5, etc.

[0018] In this invention, if the mass ratio of biomass raw material to magnesium source is too large, the magnesium loading will be small, affecting the adsorption effect; if the mass ratio of biomass raw material to magnesium source is too small, the cost will be greatly increased.

[0019] Preferably, the soaking time is 8-24 hours, for example, 8 hours, 10 hours, 15 hours, 20 hours, 22 hours, or 24 hours.

[0020] Preferably, the pyrolysis temperature is 500-800℃, for example, it can be 500℃, 550℃, 600℃, 650℃, 700℃ or 800℃.

[0021] Preferably, the pyrolysis time is 2-4 hours, for example, 2 hours, 3 hours, 3.5 hours or 4 hours.

[0022] Preferably, the heating rate of the pyrolysis is 5-20℃ / min, for example, it can be 5℃ / min, 10℃ / min, 15℃ / min or 20℃ / min, etc.

[0023] Preferably, the atmosphere for pyrolysis is an oxygen-free atmosphere.

[0024] Optionally, the gas in the oxygen-free atmosphere includes at least one of argon, nitrogen, carbon dioxide, and helium.

[0025] Preferably, the solvent in step (1) includes a methanol solution.

[0026] Preferably, the amine substance includes at least one of polyethyleneimine (PEI), polyacrylamide, tetraethylenepentamine, and ethylenediamine.

[0027] Preferably, the polyethyleneimine comprises branched polyethyleneimine and / or linear polyethyleneimine.

[0028] In this invention, branched polyethyleneimine is used more effectively because its branched structure allows the generated DTC groups to be densely distributed on the surface of the biochar.

[0029] Preferably, the molecular weight of the polyethyleneimine is 600-70000, for example, it can be 600, 800, 1000, 1500, 2000, 5000, 10000, 20000, 50000 or 70000.

[0030] Preferably, the aldehydes include at least one of glutaraldehyde, formaldehyde, and glyoxal.

[0031] Preferably, the mass concentration of the aldehyde-containing solution is 0.5-10%, for example, it can be 0.5%, 1%, 2%, 5%, 8% or 10%.

[0032] Preferably, the mass ratio of the magnesium-modified biochar to the amine is (1-2):(0.5-10), wherein the range of magnesium-modified biochar (1-2) can be, for example, 1, 1.2, 1.5, 1.8 or 2, and the range of amine can be, for example, 0.5, 1, 2, 3, 4, 5, 6, 7, 8, 9 or 10.

[0033] In this invention, if the mass ratio of magnesium-modified biochar to the amine is too large, it will result in insufficient introduction of amine groups, affecting the formation of DTC groups; if the mass ratio of magnesium-modified biochar to the amine is too small, it will result in pore blockage of the biochar, affecting the diffusion and adsorption of heavy metals.

[0034] Preferably, the ratio of the mass of the amine substance to the volume of the aldehyde-containing solution is (0.5-10) g:25 mL, for example, it can be 0.5 g:25 mL, 1 g:25 mL, 2 g:25 mL, 3 g:25 mL, 4 g:25 mL, 5 g:25 mL, 6 g:25 mL, 7 g:25 mL, 8 g:25 mL, 9 g:25 mL, or 10 g:25 mL, etc.

[0035] Preferably, the crosslinking reaction takes 4-8 hours, for example, 4 hours, 5 hours, 6 hours, 7 hours, or 8 hours.

[0036] Preferably, the alkaline solution in step (2) includes sodium carbonate solution and / or sodium hydroxide solution.

[0037] Preferably, the pH value of the alkaline solution is 11-13, for example, it can be 11, 12 or 13.

[0038] Preferably, the mass ratio of the magnesium-modified biochar grafted with amines to the volume of the carbon disulfide is 1g:(5-10)mL, for example, it can be 1g:5mL, 1g:6mL, 1g:7mL, 1g:8mL, 1g:9mL or 1g:10mL, etc.

[0039] Preferably, the reaction time in step (2) is 12-24 hours, for example, 12 hours, 15 hours, 18 hours, 20 hours, 22 hours or 24 hours.

[0040] As a preferred technical solution of the present invention, the preparation method specifically includes:

[0041] (I) Mix biomass raw materials and magnesium source, then add water and stir. After impregnation, freeze dry under vacuum, then pyrolyze at 500-800℃ for 2-4 hours in an oxygen-free atmosphere. After cooling, wash and dry to obtain magnesium-modified biochar.

[0042] The biomass raw materials include at least one of water hyacinth, orange peel and straw; the magnesium source includes at least one of magnesium bicarbonate, magnesium carbonate and magnesium acetate.

[0043] (II) Stir and mix the magnesium-modified biochar and methanol solution for 1-3 hours, then add amine substances and continue stirring and mixing, then add a solution containing aldehyde substances and carry out a cross-linking reaction for 4-8 hours, then wash, separate solid and liquid and dry to obtain magnesium-modified biochar grafted with amine substances.

[0044] The amines include at least one of polyethyleneimine, polyacrylamide, tetraethylenepentamine, and ethylenediamine; the aldehydes include at least one of glutaraldehyde, formaldehyde, and glyoxal; the mass ratio of the magnesium-modified biochar to the amines is (1-2):(0.5-10); the mass ratio of the amines to the volume of the aldehyde-containing solution is (0.5-10) g:25 mL.

[0045] (III) The magnesium-modified biochar grafted with amines is added to an alkaline solution, and then carbon disulfide is added to react. After washing, solid-liquid separation and drying, the magnesium-modified biochar modified with dithiocarbamate is obtained.

[0046] In a second aspect, the present invention provides a magnesium-modified biochar modified with dithiocarbamate, wherein the magnesium-modified biochar modified with dithiocarbamate is prepared by the preparation method described in the first aspect.

[0047] Thirdly, the present invention provides an application of magnesium-modified biochar modified with dithiocarbamate as described in the second aspect, wherein the magnesium-modified biochar modified with dithiocarbamate is used to adsorb heavy metals in water.

[0048] This invention provides magnesium-modified biochar modified with dithiocarbamate, suitable for adsorbing all heavy metals. Exemplarily, the heavy metals may be lead ions, nickel ions, cadmium ions, and copper ions, etc.

[0049] The numerical range described in this invention includes not only the point values ​​listed above, but also any point values ​​within the numerical ranges not listed above. Due to space limitations and for the sake of brevity, this invention will not exhaustively list all the specific point values ​​included in the range.

[0050] Compared with the prior art, the beneficial effects of the present invention are as follows:

[0051] This invention provides a method for preparing magnesium-modified biochar modified with dithiocarbamate. The method utilizes the hydroxyl groups on the surface of magnesium-modified biochar to obtain magnesium-modified biochar grafted with amines through a cross-linking reaction. Then, it reacts with carbon disulfide to obtain magnesium-modified biochar modified with dithiocarbamate. On one hand, the magnesium oxide introduced onto the biochar surface can treat heavy metal ions in water through ion exchange and mineralization. On the other hand, the modified dithiocarbamate group is a highly efficient functional group for capturing heavy metal ions and treating heavy metal pollution in wastewater. It contains two sulfur atoms with specific binding capabilities for most heavy metals. These two donor sulfur atoms can interact with heavy metal ions to form stable chelate complexes. Therefore, under the synergistic effect of magnesium oxide and dithiocarbamate, the biochar material exhibits strong adsorption of various heavy metals in water, and the heavy metal adsorption effect is excellent.

[0052] The magnesium-modified biochar material modified with dithiocarbamate prepared by the method of the present invention can adsorb lead ions up to 1028.3 mg / g and nickel ions up to 287.4 mg / g at a dosage of 1 g / L. Attached Figure Description

[0053] Figure 1 The infrared spectra of Mg-BC, Mg-PEIBC and Mg-DTBC provided in Example 1 of the present invention are shown.

[0054] Figure 2 The images show the adsorption-desorption isotherms of WHBC, Mg-BC, and Mg-DTBC in Example 1 and Comparative Example 1 of this invention. Detailed Implementation

[0055] The technical solution of the present invention will be further illustrated below through specific embodiments.

[0056] Example 1

[0057] This embodiment provides a method for preparing magnesium-modified biochar modified with dithiocarbamate, the preparation method comprising:

[0058] (1) Clean the water hyacinth and remove the roots, dry and crush to obtain water hyacinth powder; mix 10g water hyacinth powder and 10g magnesium bicarbonate thoroughly, add 100mL ultrapure water for impregnation treatment, then perform vacuum freeze drying, heat to 600℃ in a tube furnace under argon atmosphere at a heating rate of 5℃ / min and hold for 2.5h, cool, wash and dry to obtain magnesium modified biochar (Mg-BC).

[0059] (2) Mix 1g of magnesium-modified biochar with 25mL of methanol solution, add 5g of branched polyethyleneimine (PEI) with a molecular weight of 600 and continue stirring at a constant temperature, then add 25mL of glutaraldehyde aqueous solution with a mass concentration of 5% to carry out cross-linking reaction. After the reaction is completed, wash and dry with ethanol and ultrapure water to obtain magnesium-modified biochar grafted with PEI (Mg-PEIBC).

[0060] (3) 1g of PEI-grafted magnesium-modified biochar was added to 50mL of sodium carbonate aqueous solution with pH 11.5, and then 5mL of carbon disulfide was added. After reacting for 12h, the biochar was washed with ethanol and ultrapure water, filtered and dried to obtain magnesium-modified biochar modified with dithiocarbamate (Mg-DTBC).

[0061] Infrared analysis of Mg-BC, Mg-PEIBC, and Mg-DTBC was performed using Fourier transform infrared absorption spectroscopy (FTIR). The results are as follows: Figure 1 As shown, the left and right figures respectively illustrate Mg-BC, Mg-PEIBC, and Mg-DTBC at 2000-4000 cm⁻¹. -1 and 500-3500cm -1 The infrared spectrum shows that 1650 cm⁻¹ -1 The presence of a C=O peak in the amide group at 1139 cm⁻¹ indicates that polyethyleneimine molecules were introduced onto the biochar surface through the formation of amide groups. -1 and 1421cm -1 The successful introduction of DTC groups on the surface of the infrared peaks at C=S and NC=S bonds respectively.

[0062] Elemental analysis was performed on Mg-BC, Mg-PEIBC and Mg-DTBC, and the results are shown in Table 1.

[0063] Table 1

[0064] element / % C N O S H Mg-BC 18.651 0.047 8.59 0.537 2.719 Mg-PEIBC 20.706 1.527 9.45 1.761 2.965 Mg-DTBC 31.431 0.582 14.36 3.931 2.826

[0065] As shown in Table 1, the N content and S content increased significantly after grafting PEI, which is due to the introduction of the DTC group.

[0066] Example 2

[0067] The difference between this embodiment and embodiment 1 is that the amount of branched polyethyleneimine added in step (2) is 0.5g.

[0068] The remaining preparation methods are the same as in Example 1.

[0069] Example 3

[0070] The difference between this embodiment and embodiment 1 is that the amount of branched polyethyleneimine added in step (2) is 1g.

[0071] The remaining preparation methods are the same as in Example 1.

[0072] Example 4

[0073] The difference between this embodiment and Embodiment 1 is that the amount of branched polyethyleneimine added in step (2) is 2g.

[0074] The remaining preparation methods are the same as in Example 1.

[0075] Example 5

[0076] The difference between this embodiment and embodiment 1 is that the amount of branched polyethyleneimine added in step (2) is 10g.

[0077] The remaining preparation methods are the same as in Example 1.

[0078] Example 6

[0079] The difference between this embodiment and Embodiment 1 is that the amount of branched polyethyleneimine added in step (2) is 0.1g.

[0080] The remaining preparation methods are the same as in Example 1.

[0081] Example 7

[0082] The difference between this embodiment and embodiment 1 is that the amount of branched polyethyleneimine added in step (2) is 10.5g.

[0083] The remaining preparation methods are the same as in Example 1.

[0084] Example 8

[0085] This embodiment provides a method for preparing magnesium-modified biochar modified with dithiocarbamate, the preparation method comprising:

[0086] (1) Clean the water hyacinth and remove the roots, dry and crush to obtain water hyacinth powder; mix 10g water hyacinth powder and 30g magnesium bicarbonate thoroughly, add 200mL ultrapure water for impregnation, then perform vacuum freeze drying, heat to 700℃ in a tube furnace under argon atmosphere at a heating rate of 5℃ / min and hold for 3h, cool, wash and dry to obtain magnesium modified biochar.

[0087] (2) Mix 1g of magnesium-modified biochar with 25mL of methanol solution, add 5g of branched polyethyleneimine (PEI) with a molecular weight of 600 and continue stirring at a constant temperature, then add 25mL of glutaraldehyde aqueous solution with a mass concentration of 8% to carry out cross-linking reaction. After the reaction is completed, wash and dry with ethanol and ultrapure water to obtain magnesium-modified biochar grafted with PEI.

[0088] (3) Add 5g of PEI-grafted magnesium-modified biochar to 50mL of sodium carbonate aqueous solution with pH 11.5, then add 8mL of carbon disulfide. After reacting for 12h, wash, filter and dry with ethanol and ultrapure water to obtain magnesium-modified biochar modified with dithiocarbamate.

[0089] Example 9

[0090] The difference between this embodiment and embodiment 1 is that in step (2), branched polyethyleneimine is replaced by linear polyethyleneimine.

[0091] The remaining preparation methods are the same as in Example 1.

[0092] Example 10

[0093] The difference between this embodiment and Embodiment 1 is that the amount of magnesium bicarbonate added in step (1) is adjusted to 0.3g.

[0094] The remaining preparation methods are the same as in Example 1.

[0095] Example 11

[0096] The difference between this embodiment and Embodiment 1 is that the amount of magnesium bicarbonate added in step (1) is adjusted to 52g.

[0097] The remaining preparation methods are the same as in Example 1.

[0098] Comparative Example 1

[0099] This comparative example provides a method for preparing raw water hyacinth biochar, the method comprising:

[0100] The water hyacinth was cleaned and the roots were removed. It was dried and crushed to obtain water hyacinth powder. 10g of water hyacinth powder was heated to 600℃ in a tube furnace under argon atmosphere at a heating rate of 5℃ / min and held at that temperature for 2.5h. After cooling, washing and drying, the raw water hyacinth biochar (WHBC) was obtained.

[0101] Figure 2The adsorption-desorption isotherms of WHBC, Mg-BC, and Mg-DTBC in Example 1 and Comparative Example 1 are shown. The adsorption-desorption isotherms of the three biochar materials conform to the typical H3 type hysteresis curve of the IV adsorption isotherm, indicating that the pores of these materials are mesoporous. The specific surface area of ​​the biochar modified with magnesium bicarbonate is significantly increased, while that modified with dithiocarbamate is somewhat reduced, but still much larger than that of the original water hyacinth biochar. The larger specific surface area is beneficial to the diffusion and adsorption of pollutants.

[0102] Performance testing

[0103] The biochar materials prepared in the above examples and comparative examples were added at a dosage of 1 g / L to shakers containing 2000 mg / L lead nitrate solution and 500 mg / L nickel chloride solution, respectively. Adsorption experiments were conducted on both shakers at a rotation speed of 200 rpm / min. After the adsorption experiments, the solutions were filtered using a needle filter / finger-press filter (0.45 μm aqueous system), and the concentration of remaining heavy metal ions was measured and the adsorption capacity was calculated using inductively coupled plasma optical emission spectrometry (ICP-OES, PQ9000, Germany).

[0104] Table 2

[0105] Lead ion adsorption capacity (mg / g) Nickel ion adsorption capacity (mg / g) Example 1 1028.3 287.4 Example 2 945.2 234.5 Example 3 956.7 246.7 Example 4 989.6 257.8 Example 5 1024.6 279.6 Example 6 936.4 212.4 Example 7 945.2 247.8 Example 8 1045.3 282.8 Example 9 1014.6 279.5 Example 10 689.5 147.6 Example 11 1434.5 312.7 Comparative Example 1 251 20

[0106] analyze:

[0107] As demonstrated in Examples 1-5 and Example 8, the magnesium-modified biochar modified with dithiocarbamate prepared by the method of this invention exhibits strong adsorption of lead and nickel ions under the synergistic effect of magnesium oxide and dithiocarbamate, resulting in excellent adsorption performance. At a dosage of 1 g / L, the adsorption capacity for lead ions reaches 1028.3 mg / g, and the adsorption capacity for nickel ions reaches 287.4 mg / g.

[0108] As can be seen from Examples 1 and 6-7, Example 6 added less branched polyethyleneimine, resulting in an excessively high mass ratio of magnesium-modified biochar to branched polyethyleneimine, which would lead to insufficient introduction of amine groups and affect the formation of DTC groups. Example 7 added more branched polyethyleneimine, resulting in an excessively low mass ratio of magnesium-modified biochar to branched polyethyleneimine, which would lead to pore blockage of the biochar and affect the diffusion and adsorption of heavy metals.

[0109] As can be seen from Examples 1 and 9, compared with branched polyethyleneimine, the adsorption effect of grafting with straight-chain polyethyleneimine is slightly less than that of biochar grafted with branched polyethyleneimine. This is because the branched structure of branched polyethyleneimine enables the generated DTC groups to be densely distributed on the surface of biochar.

[0110] As can be seen from Examples 1 and 10-11, Example 10 added less magnesium bicarbonate, resulting in an excessively high mass ratio of water hyacinth powder to magnesium bicarbonate, which led to a low magnesium loading and affected the adsorption effect. Example 11 added more magnesium bicarbonate, resulting in an excessively low mass ratio of water hyacinth powder to magnesium bicarbonate, which led to a small improvement in the adsorption effect, but greatly increased the cost.

[0111] The above description is only a specific embodiment of the present invention, but the protection scope of the present invention is not limited thereto. Those skilled in the art should understand that any changes or substitutions that can be easily conceived by those skilled in the art within the technical scope disclosed in the present invention fall within the protection and disclosure scope of the present invention.

Claims

1. Use of a dithiocarbamate salt-modified magnesium modified biochar, characterized in that, The magnesium-modified biochar modified with dithiocarbamate is used to adsorb heavy metals lead and nickel in water. The dithiocarbamate-modified magnesium-modified biochar is prepared by the following method, which includes: (1) Magnesium-modified biochar, solvent and amine are mixed, and then a solution containing aldehyde is added to carry out a cross-linking reaction to obtain magnesium-modified biochar grafted with amine; the mass ratio of the magnesium-modified biochar to the amine is (1-2):(1-10); the amine is polyethyleneimine; the molecular weight of the polyethyleneimine is 600-70000; the aldehyde includes at least one of glutaraldehyde, formaldehyde and glyoxal; the mass ratio of the amine to the volume of the solution containing aldehyde is (0.5-10) g:25 mL; (2) The magnesium-modified biochar grafted with amines, an alkaline solution and carbon disulfide are mixed and reacted to obtain magnesium-modified biochar modified with dithiocarbamate; the mass ratio of the magnesium-modified biochar grafted with amines to the volume ratio of the carbon disulfide is 1g:(5-10)mL. The method for preparing the magnesium-modified biochar includes: The magnesium-modified biochar is obtained by mixing biomass raw material, magnesium source and solvent, impregnating, and then freeze-drying and pyrolyzing under vacuum; the mass ratio of biomass raw material to magnesium source is 1:(0.05-5); the biomass raw material is water hyacinth; the magnesium source is magnesium bicarbonate; the pyrolysis temperature is 500-800℃; and the pyrolysis time is 2-4h.

2. Use according to claim 1, characterized in that, The soaking time is 8-24 hours.

3. Use according to claim 1, characterized in that, The heating rate of the pyrolysis is 5-20℃ / min.

4. Use according to claim 1, characterized in that, The atmosphere for pyrolysis is an oxygen-free atmosphere.

5. The use according to claim 1, characterized in that, The solvent in step (1) includes a methanol solution.

6. The application according to claim 1, characterized in that, The mass concentration of the aldehyde-containing solution is 0.5-10%.

7. The application according to claim 1, characterized in that, The cross-linking reaction takes 4-8 hours.

8. The application according to claim 1, characterized in that, The alkaline solution in step (2) includes sodium carbonate solution and / or sodium hydroxide solution.

9. The application according to claim 1, characterized in that, The pH value of the alkaline solution is 11-13.

10. The application according to claim 1, characterized in that, The reaction time in step (2) is 12-24 hours.

11. The application according to claim 1, characterized in that, The magnesium-modified biochar modified with dithiocarbamate is used to adsorb heavy metals lead and nickel in water. The dithiocarbamate-modified magnesium biochar was prepared by the following method, which specifically includes: (I) Mix biomass raw materials and magnesium source, then add water and stir. After impregnation, freeze dry under vacuum, then pyrolyze at 500-800℃ for 2-4 hours in an oxygen-free atmosphere. After cooling, wash and dry to obtain magnesium-modified biochar. The biomass raw material is water hyacinth; the magnesium source is magnesium bicarbonate. (II) The magnesium-modified biochar and methanol solution are stirred and mixed for 1-3 hours, then amine substances are added and stirred and mixed again, followed by the addition of a solution containing aldehyde substances, and a cross-linking reaction is carried out for 4-8 hours. After washing, solid-liquid separation and drying, magnesium-modified biochar grafted with amine substances is obtained. Wherein, the amine is polyethyleneimine; the aldehyde includes at least one of glutaraldehyde, formaldehyde and glyoxal; the mass ratio of the magnesium-modified biochar to the amine is (1-2):(1-10); the mass ratio of the amine to the volume of the aldehyde-containing solution is (0.5-10) g:25 mL; (III) The magnesium-modified biochar grafted with amines is added to an alkaline solution, and then carbon disulfide is added to react. After washing, solid-liquid separation and drying, the magnesium-modified biochar modified with dithiocarbamate is obtained.

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