Preparation method and application of magnesium alkali modified biochar

Abstract

The invention discloses a preparation method and application of magnesium alkali modified biochar, and relates to the technical field of sewage treatment. Specifically, magnesium and alkali are selected to modify the biochar, the specific surface area, cation exchange capacity and surface precipitation capacity of the biochar can be remarkably increased, magnesium oxide is successfully loaded on the surface and in pores of the biochar, the modified biochar has very strong adsorption capacity on cadmium and lead in sewage, and the adsorption capacity of the modified biochar is greatly improved. The performance is stable under a wide pH range, the modification method is simple, and the price is low.

Description

Technical Field

[0001] The invention belongs to the field of adsorbent preparation, and specifically relates to a preparation method and application of magnesium-alkali modified biochar. Background Art

[0002] With the rapid development of industrialization, heavy metal pollution has become a global environmental problem. How to remove heavy metal ions from wastewater in an effective and economical way has become an urgent problem to be solved. Lead and cadmium are heavy metal elements that are toxic to the environment and human body. They are difficult to degrade in the natural environment and are easily enriched in the human body through the food chain. They can accumulate in the human body and cause neurological and kidney diseases. Therefore, it is very important to remove toxic heavy metals cadmium and lead from wastewater.

[0003] There are many ways to remove heavy metals from wastewater, including ion exchange, microbial methods, membrane treatment technology, and adsorption. Among them, adsorption is widely favored because of its simple preparation method and excellent adsorption performance. At present, various types of adsorbents have been used to treat polluted water, such as activated carbon, graphene oxide, carbon nanofibers, metal organic frameworks, bentonite and biochar. Among them, biochar has been widely studied because of its wide source and low price.

[0004] Biochar is a pyrolysis carbonized product produced by thermal degradation of carbon-rich biomass in an oxygen-restricted environment, and its pore structure is well developed. The ability of biochar to remove inorganic and organic pollutants is closely related to the characteristics of biochar, such as surface area, functional groups, negative charge and cation exchange capacity. The poor adsorption capacity of pure biochar for heavy metals in water is mainly attributed to its few functional groups on the surface and poor cation exchange capacity. However, biochar has low production costs and is environmentally friendly, and can be used as a good carrier. Therefore, researchers at home and abroad have made many modifications to biochar, including surface oxidation and functional modification. Although most of the modification methods have obvious changes in the physical and chemical properties of biochar, the removal effect of heavy metals in aqueous solution is still poor. Summary of the invention

[0005] In view of this, the present invention provides a magnesium-alkali modified biochar and a preparation method and use thereof. The preparation method can realize the resource utilization and reduced utilization of lavender straw. It not only uses lavender straw in the field of heavy metal removal for the first time, but also has a significant adsorption effect on cadmium and lead in sewage.

[0006] To achieve the above object, the present invention provides the following technical solutions:

[0007] A method for preparing magnesium-alkali modified biochar, the preparation method comprising the following steps:

[0008] S1. Wash, dry and chop the crop straw.

[0009] S2. Add the crop straw obtained in S1 to the magnesium salt solution to obtain a mixed solution, and stir at room temperature.

[0010] S3. Add sodium hydroxide solution to the mixed solution obtained in S2, adjust the pH value of the solution, stir at room temperature, filter and dry.

[0011] S4. The product obtained in S3 is loaded into a porcelain boat and placed in a tube furnace. After N2 is introduced, it is pyrolyzed. After cooling, it is washed and dried.

[0012] Furthermore, the straw in S1 is lavender straw, and the length of the chopped lavender straw is 2-3 cm.

[0013] Furthermore, the lavender straw in S2 is 10 g.

[0014] Furthermore, the magnesium salt solution in S2 is a magnesium chloride solution with a specification of 200 mL and a concentration of 0.8 mol / L.

[0015] Furthermore, the stirring condition in S2 is magnetic stirring at 400 r / min for 1 h.

[0016] Furthermore, the sodium hydroxide solution in S3 is 5 mol / L.

[0017] Furthermore, the pH of the solution is adjusted to 10-11 in S3.

[0018] Furthermore, the stirring condition in S3 is 500 r / min magnetic stirring for 2 h, and the drying condition is 60° C. for 12 h.

[0019] Furthermore, during the pyrolysis treatment in S4, the heating rate was 5 °C / min and the nitrogen flow rate was 0.2 L / min.

[0020] Furthermore, the pyrolysis treatment temperature in S4 is 500-800°C, preferably 700°C, and the constant temperature time is 2 hours. The drying conditions after washing are 60°C and 12 hours.

[0021] The second aspect of the present application provides a magnesium-alkali modified biochar, which is prepared according to the above method.

[0022] The third aspect of the present application provides the application of the above-mentioned magnesium-alkali modified biochar in sewage treatment.

[0023] It should be understood that the above general description and the following detailed description are only exemplary and are not intended to limit the application of the present invention.

[0024] The beneficial technical effects of the present invention are as follows.

[0025] (1) The present invention uses magnesium chloride and sodium hydroxide for modification. The use of sodium hydroxide can increase the specific surface area and porosity of biochar on the one hand, and on the other hand, the magnesium hydroxide generated by the reaction of sodium hydroxide and magnesium chloride in an aqueous solution can be evenly wrapped on the lavender straw. At high temperature, the magnesium hydroxide is thermally decomposed to produce magnesium oxide, which makes the probability of magnesium oxide being loaded on the lavender straw biochar higher; the chemical reagents selected in the preparation process are low in price, the preparation process will not cause pollution or harm to the environment, is green and environmentally friendly, and the preparation process is simple.

[0026] (2) The raw materials of the material of the present invention are derived from agricultural and forestry wastes, which are sufficient and easy to obtain. The use of lavender straw to prepare biochar not only expands the application scope of lavender straw, but also reduces the secondary pollution that the agricultural and forestry wastes may cause to the environment, reduces their processing costs, and can improve the efficient utilization of agricultural and forestry waste resources.

[0027] (3) The modified biochar prepared by the present invention through resource recycling and reuse of lavender straw, an agricultural and forestry waste, not only has the excellent properties of biochar itself: well-developed pores, good adsorption selectivity, and fast adsorption rate, but also the specific surface area of ​​the modified biochar is 50 times that of the original biochar. The magnesium oxide metal oxide loaded thereon can promote the adsorption of cadmium and lead ions in sewage by promoting the chemical precipitation of heavy metal ions on the surface of biochar, cation exchange, etc. The modified lavender straw biochar loaded with magnesium oxide can achieve a good adsorption effect on cadmium and lead in sewage within a wide pH range, has strong anti-interference ability, and the adsorption effect is 8-10 times that of the original biochar. BRIEF DESCRIPTION OF THE DRAWINGS

[0028] Figure 1 XRD diffraction analysis diagrams of the magnesium-alkali modified biochar prepared in Example 1 and the biochar prepared in Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4. The five biochar materials prepared in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 are respectively recorded as MBC-7, BC, MBC-5, MBC-6 and MBC-8.

[0029] Figure 2 Scanning electron microscope images of the magnesium-alkali modified biochar prepared in Example 1 and the original biochar prepared in Comparative Example 1; (a) is the original biochar; (b) is the magnesium-alkali modified biochar.

[0030] Figure 3 This is the element distribution diagram of the magnesium-alkali modified biochar prepared in Example 1.

[0031] Figure 4This is a comparison chart of the adsorption of cadmium and lead in aqueous solution by the magnesium-alkali modified biochar prepared in Example 1 and the biochar prepared in Comparative Examples 1, 2, 3 and 4 under the same experimental conditions.

[0032] Figure 5 The adsorption kinetics of cadmium (a) and lead (b) by the magnesium-alkali modified biochar prepared in Example 1 and the adsorption kinetics of cadmium (c) and lead (d) by the original biochar prepared in Comparative Example 1 are shown.

[0033] Figure 6 The adsorption isotherm fitting diagrams of cadmium (a) and lead (b) by the magnesium-alkali modified biochar prepared in Example 1 and the adsorption isotherm fitting diagrams of cadmium (c) and lead (d) by the original biochar prepared in Comparative Example 1.

[0034] Figure 7 The adsorption amount of the magnesium-modified biochar prepared in Example 1 varies with pH.

[0035] Figure 8 The adsorption amount of the magnesium-alkali modified biochar prepared in Example 1 varies with ionic strength. DETAILED DESCRIPTION

[0036] The exemplary embodiments of the present invention are described in detail below. This description shows the optional embodiments of the present invention, however, it is more clear that the present invention can be implemented in various forms and is not intended to limit the present invention. The addition of these specific embodiments is to increase the completeness and thoroughness of the content of the present invention, and can clearly convey the scope of the application of the present invention to researchers and technicians in the field.

[0037] The terms used in this application are not intended to limit the present application, but are only used to describe the embodiments. The singular forms "a", "the", etc. used in this application and the claims do not only represent one form, but are intended to include multiple forms. The open-ended terms used herein, such as "having", "including", etc., mean including but not limited to, unless it is specifically stated that a certain form is specifically referred to.

[0038] For the numerical ranges herein, it is not specifically intended or represented that a certain intermediate value is included, but each intermediate value in the specific range is included. Each smaller range within any stated value or stated range is also included in the present application.

[0039] Unless otherwise specified herein, all technical and scientific terms used herein in the relevant field are the same meanings understood by conventional technicians. Although the present invention only describes preferred materials and embodiments, any methods and materials similar or equivalent to those described herein can also be used in the implementation or testing of the present invention.

[0040] The following is a principle explanation of the implementation content:

[0041] The present invention discloses a method for preparing magnesium-alkali modified biochar, comprising:

[0042] S1. Remove the remaining flower buds on the discarded lavender straw, leaving only the straw, wash, dry and cut the straw into pieces. This step is mainly to remove impurities.

[0043] S2. Add the lavender straw obtained in S1 to the magnesium chloride solution and stir at room temperature to allow the magnesium chloride and lavender straw to fully react; then add the sodium hydroxide solution to the mixed solution stirred in the previous step, and adjust the pH value of the solution to 10-11. In a strongly alkaline pH environment, on the one hand, it can ensure that the magnesium chloride is completely converted into magnesium hydroxide, and at the same time, excess sodium hydroxide can react with the lavender straw to modify it; after the overall mixture is stirred and reacted at room temperature, it is filtered and dried; in short, the adsorption capacity of biochar depends on its specific surface area and porosity to a certain extent. The purpose of adding sodium hydroxide is, on the one hand, to convert magnesium chloride into magnesium hydroxide, and on the other hand, the sodium hydroxide modification can make the lavender straw after pyrolysis have a larger specific surface area and a more developed porosity.

[0044] S3, put the product obtained in S2 into a magnetic boat, put it into a high-temperature tube furnace, pass 0.2 L / min of nitrogen, and heat it to 500-800℃ at a heating rate of 5℃ / min, preferably 700℃, and keep it constant for 2 hours. When the tube furnace cools to room temperature, take it out, wash the biochar with deionized water, and dry it. In an environment of 500-800℃, the magnesium hydroxide wrapped in the lavender straw will be thermally decomposed to produce water and magnesium oxide. The water evaporates at high temperature, leaving magnesium oxide loaded on the biochar. The surface of the biochar after pyrolysis is rough and has a short rod-like structure, which shows that the high temperature does not destroy its macroscopic skeleton structure.

[0045] This will be described in detail through the following examples.

[0046] Example 1

[0047] Collect lavender straw, remove the flower buds remaining on the discarded lavender straw, leaving only the straw, wash the lavender straw, dry it, and cut it into short strips of 2-3 cm. Weigh 32.5285 g of MgCl2·6H2O solid and dissolve it in 200 mL of deionized water, stir it magnetically at 400 r / min for 0.5 h at room temperature, then weigh 10 g of the cut lavender straw and add it to the 0.8 mol / L magnesium chloride solution stirred in the previous step, stir the mixed solution magnetically at room temperature and 400 r / min for 1 h, then adjust the pH value of the mixed solution to 10-11 with 5 mol / L NaOH solution, and then stir the solution magnetically at room temperature for 2 h to make the solution evenly mixed, and then stir the solution magnetically at room temperature for 2 h, and the speed of the magnetic stirrer is set to 500 r / min. After stirring, filter it, transfer the solid sample to a drying oven, set the temperature to 60℃, and dry it for 12 h. After the sample is dried, it is loaded into a porcelain boat and placed in a tubular furnace. After passing 0.2 L / min of N2, the temperature is raised to 700°C at a heating rate of 5°C / min and kept constant for 2 h. When the tubular furnace is cooled to room temperature, the modified biochar is taken out, washed with deionized water, and transferred to a drying oven for drying at 60°C for 12 h.

[0048] Comparative Example 1

[0049] Collect lavender straw, remove the remaining flower buds on the discarded lavender straw, leaving only the straw, wash, dry, and cut the lavender straw into short strips of 2-3 cm, put it into a porcelain boat and put it into a tubular furnace. After passing 0.2 L / min of N2, increase the temperature to 700℃ at a rate of 5℃ / min and keep it constant for 2 h. When the tubular furnace is cooled to room temperature, take it out, wash the obtained biochar with deionized water, transfer it to a drying oven, and dry it at 60℃ for 12 h.

[0050] Comparative Example 2

[0051] Same as Example 1, except that the pyrolysis treatment temperature is 500°C.

[0052] Comparative Example 3

[0053] Same as Example 1, except that the pyrolysis treatment temperature is 600°C.

[0054] Comparative Example 4

[0055] Same as Example 1, except that the pyrolysis treatment temperature is 800°C.

[0056] The following is a detailed description of the application of magnesium-alkali modified biochar in the present invention in conjunction with the accompanying drawings, as follows:

[0057] The magnesium-alkali modified biochar prepared in Example 1 was compared with the original biochar prepared in Comparative Example 1. The results showed that the specific surface area of ​​the magnesium-alkali modified biochar prepared in Example 1 was 158.277 m 2 / g, the specific surface area of ​​the original biochar is 2.908m 2 / g, the former is 54.43 times that of the latter. It can be seen that the specific surface area of ​​biochar modified by sodium hydroxide and magnesium chloride is significantly increased. This may be due to the gas escape produced during the reaction of sodium hydroxide and carbon, and secondly, the magnesium oxide metal oxide produced by the thermal decomposition of magnesium hydroxide covers the surface of biochar and increases its specific surface area.

[0058] Figure 1 The X-ray diffraction patterns (XRD) of Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3 and Comparative Example 4 are shown in the XRD pattern. As shown in the XRD pattern, five new diffraction peaks appear at 2θ equal to 36.9°, 42.9°, 62.3°, 74.7° and 78.7°, corresponding to the MgO (1 1 1), (2 0 0), (2 2 0), (3 1 1) and (2 2 2) crystal planes, which indicates that magnesium oxide is successfully loaded on the modified biochar, and with the increase of temperature, the more obvious the peak of magnesium oxide, the better the crystallinity of magnesium oxide.

[0059] Figure 2 The scanning electron micrographs of the magnesium-alkali modified biochar (b) prepared in Example 1 and the original biochar (a) prepared in Comparative Example 1 show that after modification with sodium hydroxide and magnesium chloride, a large amount of loading material appears on the surface and in the pores of the biochar. Figure 3 The element distribution map of magnesium-alkali modified biochar shows that a large amount of Mg element is distributed on the surface and pores of biochar, which indicates that the load observed on the surface and pores of the modified biochar is magnesium oxide.

[0060] The adsorption performance of the magnesium-alkali modified biochar prepared by the present invention will be described in detail below through experimental examples.

[0061] Experimental Example 1

[0062] The five biochar materials prepared in Example 1, Comparative Example 1, Comparative Example 2, Comparative Example 3, and Comparative Example 4 were recorded as MBC-7, BC, MBC-5, MBC-6, and MBC-8, respectively. 600 mg / L cadmium chloride and lead nitrate solutions were prepared respectively, and the initial pH of the solution was adjusted to 5.0±0.1. The five biochar materials were ground and sieved and added to the prepared centrifuge tubes containing 20 mL and 600 mg / L cadmium chloride or lead nitrate solutions, respectively, with a dosage ratio of 0.5 g / L (m / V), and placed in a constant temperature oscillating box at 25°C and 160 r / min for 24 h. The supernatant of the solution was aspirated with a rubber-tipped dropper, filtered with a 0.45μm filter, and placed in a 5 mL centrifuge tube. The concentration of heavy metals after adsorption in the solution was determined by inductively coupled plasma emission spectrometry. In addition, for all adsorption experiments, the content of heavy metal cadmium and lead was determined by inductively coupled plasma emission spectrometry.

[0063] The adsorption amount of cadmium and lead in sewage by biochar was calculated by the following equation when the adsorption time was 24 h:

[0064] (1)

[0065] Where C0 represents the concentration of heavy metal ions in the solution before adsorption, C e represents the concentration of heavy metal ions in the solution after adsorption, V represents the volume of the solution, and m represents the amount of adsorbent used.

[0066] The adsorption amount of cadmium and lead in aqueous solution in the embodiment and the comparative example at the same adsorption time (24 h) changes as follows: Figure 4 As shown in the figure, the magnesium-alkali modified biochar prepared at different pyrolysis temperatures has better effects than the unmodified pure biochar in the adsorption experiments of cadmium and lead. The order of the adsorption of cadmium in aqueous solution by the five biochars is: MBC-7>MBC-6>MBC-8>MBC-5>BC; the order of the adsorption of lead in aqueous solution by the five biochars is: MBC-7>MBC-8>MBC-6>MBC-5>BC. Among them, the adsorption of cadmium by MBC-7 reached 527 mg / g, and the adsorption of lead reached 842 mg / g, which is about ten times that of the unmodified original biochar BC.

[0067] Experimental Example 2

[0068] The adsorption kinetics and adsorption isotherms of magnesium alkali modified lavender straw biochar and unmodified original lavender straw biochar were investigated.

[0069] (1) Adsorption kinetics: 250 mL of 600 mg / L cadmium chloride and lead nitrate solutions were added to four 500 mL conical flasks, two of which were added in a group. The initial pH of the solution was 5.0±0.1. Then, 0.125 g of MBC-7 and BC were added as adsorbents. At 25°C and 160 r / min, samples were taken within 5 min to 36 h and filtered with a 0.45 μm filter. The heavy metal concentrations in the solution were detected in each time period, and the pseudo-first-order kinetic model and pseudo-second-order kinetic model were fitted. The pseudo-first-order kinetic equation (Equation 2) and pseudo-second-order kinetic equation (Equation 3) are as follows:

[0070] (2)

[0071] (3)

[0072] Where Q t and Q e (mg / g) represents the adsorption time (min) and the adsorption capacity at equilibrium.

[0073] like Figure 5 As shown, (a) and (b) are the adsorption kinetics fitting diagrams of MBC-7 for cadmium and lead, (c) and (d) are the adsorption kinetics fitting diagrams of BC for cadmium and lead. As shown in the figure, the adsorption amount increases continuously with the change of time. The adsorption of cadmium and lead by modified biochar with a pyrolysis temperature of 700°C reaches equilibrium at about 25 h, and the adsorption of cadmium and lead by pure biochar reaches equilibrium at about 1-2 h. The adsorption of cadmium and lead by MBC-7 at a concentration of 600 mg / L is approximately 527 mg / g and 842 mg / g, and the adsorption kinetics fitting is more consistent with the quasi-second-order kinetic model, indicating that magnesium-alkali modified biochar has excellent adsorption effect on cadmium and lead, and the adsorption process is mainly chemical adsorption.

[0074] (2) Adsorption isotherm.

[0075] 20 mL of cadmium chloride solution with concentrations of 600 mg / L, 500 mg / L, 350 mg / L, 175 mg / L, 80 mg / L, and 20 mg / L were added to multiple 50 mL centrifuge tubes, and 20 mL of lead nitrate solution with concentrations of 600 mg / L, 400 mg / L, 200 mg / L, 100 mg / L, 50 mg / L, and 20 mg / L were added to multiple 50 mL centrifuge tubes, respectively. Then, 0.01 g of adsorbent, i.e., magnesium-alkali modified biochar MBC-7 in Example 1, was added to each centrifuge tube. After 24 h of oscillation adsorption at 25°C and 160 r / min, the supernatant of the solution was aspirated and filtered with a 0.45 μm filter, the heavy metal concentration in the solution was detected, and the adsorption data was fitted. At the same time, the original biochar BC in Comparative Example 1 was used as an adsorbent for a control experiment.

[0076] like Figure 6 As shown in the figure, in low concentration (0-100 mg / L) cadmium or lead solution, the lead ion concentration in the solution after adsorption of MBC-7 is 0.38-0.48 mg / L, and for cadmium, the cadmium ion concentration in the solution after adsorption is 0.008-0.109 mg / L. It can be seen that at low concentrations, the removal rate of cadmium and lead by magnesium-alkali modified biochar can reach 99%. When the concentration of cadmium or lead in the initial solution increases, the adsorption amount of cadmium and lead by magnesium-alkali modified biochar also increases within a certain range. The adsorption amount of cadmium or lead by unmodified biochar BC is lower than that of MBC-7 at various concentrations.

[0077] Experimental Example 3

[0078] Investigate the influence of different pH and coexisting ions:

[0079] (1) 20 mL of cadmium chloride or lead nitrate solution with a concentration of 600 mg / L and initial pH values ​​of 2, 3, 4, 5, and 6 were added to multiple 50 mL centrifuge tubes, and then 0.02 g of adsorbent, i.e., magnesium-alkali modified biochar MBC-7 in Example 1, was added to each centrifuge tube. After oscillation adsorption for 24 h at 25 °C and 160 r / min, the supernatant was aspirated and filtered with a 0.45 μm filter, and the heavy metal concentration in the solution was detected.

[0080] Figure 7 The left middle graph shows the variation of the adsorption of cadmium by MBC-7 with pH, ​​and the right middle graph shows the variation of the adsorption of lead by MBC-7 with pH. Figure 7It can be seen that when the initial pH value of the solution is 2, the adsorption effect of the modified biochar on cadmium and lead in water is poor. This may be because at a low pH value, it is not conducive for cadmium and lead ions to precipitate on the surface of the biochar; after the pH value is greater than 2, the adsorption effect of the modified biochar on cadmium and lead increases dramatically, and increases with the increase of the pH value. This shows that the magnesium-alkali modified biochar prepared by the present invention can achieve a good adsorption and removal effect on cadmium and lead ions in polluted water within the pH range of 3-6.

[0081] (2) 100 mg / L cadmium chloride solution was used to prepare 50 mmol / L, 100 mmol / L, and 200 mmol / L sodium chloride solutions, respectively. The pH value of the solution was adjusted to 5. 20 mL of each solution was added to 50 mL centrifuge tubes. Then, 0.02 g of adsorbent, i.e., magnesium-alkali modified biochar MBC-7 in Example 1, was added to each centrifuge tube. After oscillation adsorption for 24 h at 25 °C and 160 r / min, the supernatant was aspirated and filtered with a 0.45 μm filter to detect the heavy metal concentration in the solution. At the same time, for the lead coexistence ion experiment, only the cadmium chloride solution was replaced with 100 mg / L lead nitrate solution, and the other conditions remained unchanged.

[0082] from Figure 8 It can be seen that the ionic strength in the solution has little effect on the adsorption performance of the modified biochar, which indicates that the concentration of NaCl in the solution will not affect the adsorption process. This may be because the Zeta potential of the modified biochar is negative, the ionic valence of the heavy metal ions cadmium and lead in the aqueous solution is +2, and the cation Na in NaCl is + The valence is +1, which has little effect on the electrostatic attraction of heavy metal ions by modified biochar.

Claims

1. A method for preparing magnesium-alkali modified biochar, characterized in that: The following steps are involved: The crop straw is washed, dried and chopped; the chopped straw is added to a magnesium salt solution to obtain a mixture, and the mixture is stirred at room temperature; an alkaline solution is added to the mixture obtained in the previous step, the pH value of the solution is adjusted, and the mixture is stirred and reacted at room temperature, filtered and dried, and the obtained product is loaded into a porcelain boat and placed in a tubular furnace, and N2 is introduced to perform pyrolysis treatment, and the mixture is washed and dried after cooling.

2. The preparation method according to claim 1, characterized in that: The straw is lavender straw, and the shredded length is 2-3 cm.

3. The preparation method according to claim 1, characterized in that: The mass of the straw added to the magnesium salt solution is 10 g, and the concentration of the magnesium salt solution is (0.1-1) mol / L.

4. The preparation method according to claim 1, characterized in that: The magnesium salt solution is one or more of magnesium chloride, magnesium nitrate and magnesium sulfate solutions.

5. The preparation method according to claim 1, characterized in that: The alkaline solution is a sodium hydroxide solution with a concentration of (2-5) mol / L, and the pH of the solution is adjusted to (10-11).

6. The preparation method according to claim 1, characterized in that: During the pyrolysis treatment, the heating rate was 5 °C / min and the nitrogen flow rate was 0.2 L / min.

7. The preparation method according to claim 1, characterized in that The pyrolysis treatment temperature is 500-800°C and the constant temperature time is 2 h.

8. The preparation method according to claim 1, characterized in that: The drying condition after washing is 60°C.

9. A magnesium-alkali modified biochar, characterized in that: Prepared according to the preparation method according to any one of claims 1 to 8.

10. Application of the magnesium-alkali modified biochar as claimed in claim 9 in sewage treatment technology.

11. The use according to claim 10, characterized in that: The sewage is sewage containing cadmium or lead.

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