Silicon-calcium modified biochar composite material as well as preparation method and application thereof

Abstract

The invention discloses a silicon-calcium modified biochar composite material and a preparation method and application thereof.The preparation method of the silicon-calcium modified biochar composite material comprises the following steps that straw powder is dispersed in a sodium silicate solution, ultrasonic treatment is conducted for at least 10 min, stirring is conducted for at least 8 h, filtering is conducted to obtain a solid, the solid is washed to be neutral, drying and sieving are conducted, and the silicon-calcium modified biochar composite material is obtained; the preparation method comprises the following steps: preparing a silicon-modified material, uniformly mixing eggshell powder and the silicon-modified material, cracking at 650-850 DEG C for 1.5-2.5 h in the atmosphere of nitrogen or inert gas, cooling to room temperature, and sieving to obtain the silicon-calcium modified biochar composite material, the silicon-calcium modified biochar composite material has a good adsorption effect on phosphorus-containing wastewater, the maximum adsorption capacity on phosphate can reach 207.0156 mg / g, and the silicon-calcium modified biochar composite material can be used for adsorbing phosphorus-containing wastewater. According to the silicon-calcium modified charcoal composite material disclosed by the invention, the silicon-calcium modified charcoal phosphorus adsorption composite material can be obtained while phosphorus-containing wastewater is subjected to adsorption treatment, and the silicon-calcium modified charcoal phosphorus adsorption composite material can be applied to soil as a fertilizer, so that the recycling of waste phosphorus resources is promoted.

Description

Technical Field

[0001] The present invention belongs to the field of biochar composite material technology for treating phosphorus-containing wastewater and soil improvement technology, and specifically relates to a silicon-calcium modified biochar composite material and a preparation method and application thereof. Background Art

[0002] Nowadays, due to the huge amount of phosphorus-containing wastewater discharged and its wide range of sources, eutrophication of water bodies has become one of the environmental challenges. Therefore, adsorption and recovery of phosphorus from eutrophic water bodies has become an important measure for sewage treatment and phosphorus resource utilization. Common traditional fertilizers such as phosphate fertilizers mainly provide phosphorus, which also reflects the importance of phosphorus resources from the side. Traditional fertilizers have the characteristics of high application amount and rapid nutrient release, and are prone to nutrient loss through leaching and soil runoff. Not only are they inefficient, they may also pollute groundwater resources. Therefore, in order to achieve the goals of efficient, safe and environmentally friendly agricultural production, environmentally friendly fertilizer solutions are bound to become the mainstream direction of future agricultural development.

[0003] Biochar is widely used in environmental adsorption materials and soil improvement fields due to its unique characteristics such as high carbon content, larger specific surface area, cation exchange capacity, strong adsorption and stability. For different purposes, the raw material selection, modification method and preparation process of biochar vary greatly. At the same time, silicon-calcium biochar has good compatibility with soil. After saturation with phosphorus adsorption, the phosphorus-saturated biochar can be applied to the soil. The biochar after phosphorus adsorption can carry a large amount of phosphate and slowly release nutrients into the soil. It can be used as a natural phosphorus fertilizer to provide the nutrients needed by plants. Biochar modified with calcium metal, thanks to its strong chemical combination with phosphate, can slow down the release rate of phosphorus in the soil and improve soil fertility and carbon fixation capacity.

[0004] Patent CN105536704A discloses a calcium-modified biochar and its preparation method and use. Calcium is introduced to the surface of biochar by an appropriate amount of CaCl2 solution. Although the biochar prepared in this way can increase the sites of biochar to adsorb and fix phosphorus by loading calcium on the surface of biochar, the modification process is relatively cumbersome and difficult, and there is also a certain risk. Patent CN115504825A discloses a soil-improving biofertilizer and its preparation method. The preparation method mixes peat, organic fertilizer, inorganic fertilizer and microbial agent and reacts them in a reactor, and then mixes them with algae powder, urea formaldehyde, sodium fluorosilicate salt, potassium salt and high molecular polymer to obtain soil-improving biofertilizer. Too many components are added during the modification process, and the operation is complicated and difficult. Summary of the invention

[0005] In view of the deficiencies in the prior art, an object of the present invention is to provide a method for preparing a silicon-calcium modified biochar composite material.

[0006] Another object of the present invention is to provide a silicon-calcium modified biochar phosphorus adsorption composite material obtained by the above preparation method.

[0007] Another object of the present invention is to provide a silicon-calcium modified biochar composite material for use in treating phosphorus-containing wastewater.

[0008] The purpose of the present invention is achieved through the following technical solutions.

[0009] A method for preparing a silicon-calcium modified biochar composite material comprises the following steps:

[0010] Step 1, dispersing straw (CS) powder in a sodium silicate solution, ultrasonicating for at least 10 minutes, stirring for at least 8 hours, filtering to obtain a solid, washing the solid to neutrality, drying, and sieving to obtain a silicon-modified material (Si-CS), wherein the mass percentage of sodium silicate in the sodium silicate solution is 0.1-0.5%.

[0011] In the step 1, the particle size of the straw (CS) powder is 40-60 meshes.

[0012] In the step 1, the method for preparing the straw (CS) powder comprises: crushing and grinding the straw, and sieving to obtain the straw (CS) powder.

[0013] In the step 1, the straw (CS) may be a mixture of one or more of rice straw, wheat straw and corn straw.

[0014] In the step 1, the ratio of the mass fraction of the straw (CS) powder to the volume fraction of the sodium silicate solution is (5-10):100, the unit of the mass fraction is g, and the unit of the volume fraction is mL.

[0015] In step 1, the ultrasonic time is 10 to 15 minutes, and the ultrasonic frequency is 80 to 120 kHz.

[0016] In step 1, the stirring temperature is room temperature, and the stirring time is 8 to 12 hours.

[0017] In step 1, the drying temperature is 80-100° C., and the drying time is 8-12 hours.

[0018] In the step 1, the solid is washed with deionized water to neutrality.

[0019] In step 1, the sodium silicate solution is a mixture of sodium silicate and water.

[0020] Step 2, the eggshell powder and the silicon modified material (Si-CS) are mixed evenly, cracked at 650-850°C for 1.5-2.5h in a nitrogen or inert gas atmosphere, cooled to room temperature, sieved, and a silicon-calcium modified biochar composite material (Ca / Si-CS) is obtained. The ratio of the eggshell powder to the silicon modified material is (1-4):1 by mass.

[0021] In step 2, the eggshell comprises: a mixture of one or more of chicken eggshell, duck eggshell and goose eggshell.

[0022] In step 2, the method for preparing the eggshell powder includes: crushing and grinding the eggshells, and sieving to obtain the eggshell powder.

[0023] In step 2, the flow rate of the gas in the nitrogen or inert gas atmosphere is 80 to 90 mL / min.

[0024] In the step 2, the rate of heating to the temperature of 650-850° C. is 5-20° C. / min.

[0025] In step 1 and step 2, the sieve has a mesh size of 40 to 60.

[0026] The silicon-calcium modified biochar composite material obtained by the above preparation method.

[0027] Application of the above-mentioned silicon-calcium modified biochar composite material in treating phosphorus-containing wastewater.

[0028] In the above technical solution, the silicon-calcium modified biochar composite material is dispersed in phosphorus-containing wastewater and oscillated.

[0029] A method for preparing a silicon-calcium modified biochar phosphorus adsorption composite material comprises the following steps: dispersing the silicon-calcium modified biochar composite material in a phosphorus solution, oscillating, filtering, and drying to obtain the silicon-calcium modified biochar phosphorus adsorption composite material, wherein the phosphorus solution contains soluble phosphate.

[0030] In the above technical solution, the concentration of soluble phosphate in the phosphorus solution is 20 to 150 mg / L.

[0031] In the above technical solution, the phosphorus solution is a mixture of soluble phosphates and water, the mass fraction ratio of the silicon-calcium modified biochar composite material to the volume fraction ratio of the phosphorus solution is 0.05:(10-50), the unit of mass fraction is g, and the unit of volume fraction is mL.

[0032] In the above technical solution, the soluble phosphate includes: potassium dihydrogen phosphate.

[0033] In the above technical solution, the oscillation speed is 180-200 r / min, and the oscillation time is 22-24 h.

[0034] In the above technical solution, the drying temperature is 80-100° C., and the drying time is 8-12 hours.

[0035] The above-mentioned silicon-calcium modified biochar composite material is used in the soil. The silicon-calcium modified biochar composite material first absorbs phosphorus and then serves as a fertilizer in the soil.

[0036] Compared with the prior art, the present invention has the following beneficial effects:

[0037] (1) The silicon-calcium modified biochar composite material of the present invention has a good adsorption effect on phosphorus-containing wastewater, and the maximum adsorption amount of soluble phosphate can reach 207.0156 mg / g. The silicon-calcium modified biochar composite material of the present invention can obtain a silicon-calcium modified biochar adsorbed phosphorus composite material while adsorbing and treating phosphorus-containing wastewater. The biochar adsorbed phosphorus composite material can be applied to the soil as a fertilizer, thereby promoting the recycling of waste phosphorus resources and complying with the concept of green circular development.

[0038] (2) The silicon-calcium modified biochar phosphorus adsorption composite material of the present invention is a natural fertilizer and soil conditioner that slowly releases nutrients. It can not only improve soil conductivity and help increase soil salt content, thereby enriching soil properties, but also significantly increase the total phosphorus, effective phosphorus and available potassium content in the soil, thereby improving soil fertility and carbon fixation capacity, improving soil and improving soil fertility, and has very good prospects and economic benefits.

[0039] (3) The raw materials used in the present invention are of extremely low cost and widely available, non-toxic and harmless, and are environmentally friendly products. The eggshell powder is recycled through waste to achieve resource utilization and is in line with the concept of green and sustainable development. BRIEF DESCRIPTION OF THE DRAWINGS

[0040] Figure 1 The figures are scanning electron microscopy images (SEM) and energy dispersive X-ray (EDS) spectra, wherein (a) is straw powder, (b) is the silicon-calcium modified biochar composite material prepared in Example 1, and (c) is the silicon-calcium modified biochar phosphorus adsorption composite material prepared in Example 4;

[0041] Figure 2 X-ray diffraction patterns (XRD) of the silicon-calcium modified biochar composite material prepared in Example 1 and the silicon-calcium modified biochar phosphorus adsorption composite material prepared in Example 4;

[0042] Figure 3 This is the adsorption isotherm curve of soluble phosphate on the silicon-calcium modified biochar composite material prepared in Example 1. DETAILED DESCRIPTION

[0043] The technical solution of the present invention is further described in detail below in conjunction with the accompanying drawings and embodiments.

[0044] Examples 1 to 3

[0045] A method for preparing a silicon-calcium modified biochar composite material comprises the following steps:

[0046] Step 1, suspend and disperse straw (CS) powder (particle size is 60 mesh) in sodium silicate solution, ultrasonically mix at a frequency of 80kHz for t min to make the mixture uniform, stir at room temperature of 20-25°C for 12h to fully load sodium silicate on the straw powder, filter to obtain solid, wash the solid with deionized water to neutrality, dry in a vacuum drying oven at 80°C for 12h, grind, pass through a 60-mesh sieve, and obtain silicon-modified material (Si-CS), wherein the method for preparing straw powder includes: crushing and grinding straw (the straw is corn straw), passing through a 60-mesh sieve, obtaining straw powder, the ratio of the mass fraction of straw powder to the volume fraction of sodium silicate solution is 10:100, the unit of mass fraction is g, and the unit of volume fraction is mL. The sodium silicate solution is a mixture of sodium silicate and water, and the mass percentage of sodium silicate in the sodium silicate solution is C.

[0047] Step 2, adding eggshell powder (particle size of 60 mesh) into the silicon-modified material (Si-CS) and mixing them evenly, heating to T℃ at a heating rate of 10℃ / min in a vacuum tube high-temperature sintering furnace under a nitrogen atmosphere with a flow rate of 90mL / min, and cracking mh at T℃, cooling to room temperature, grinding, and passing through a 60-mesh sieve to obtain a silicon-calcium modified biochar composite material (Ca / Si-CS), wherein the method for preparing eggshell powder comprises: crushing and grinding the eggshell (the eggshell is a chicken egg shell), passing through a 60-mesh sieve, and obtaining eggshell powder, and the ratio of the mass fraction of the eggshell powder to the mass fraction of the silicon-modified material is W.

[0048] The parameters (W, C, t, m and T) of the silicon-calcium modified biochar composite materials prepared in Examples 1 to 3 are shown in Table 1.

[0049] Table 1 Parameters of silicon-calcium modified biochar composites

[0050]

[0051]

[0052] Example 4

[0053] A method for preparing a calcium-silicon modified biochar phosphorus adsorption composite material (Ca / Si-CS-P) comprises the following steps:

[0054] The silicon-calcium modified biochar composite material prepared in Example 1 was dispersed in a phosphorus solution, oscillated in a constant temperature oscillating box at a speed of 180 r / min for 24 h, filtered, and dried in a vacuum drying oven at 80°C for 12 h to obtain a silicon-calcium modified biochar adsorption phosphorus composite material (Ca / Si-CS-P), wherein the phosphorus solution is a mixture of phosphate and water, the phosphate is potassium dihydrogen phosphate, the concentration of potassium dihydrogen phosphate in the phosphorus solution is P mg / L, P=150, the mass fraction of the silicon-calcium modified biochar composite material and the volume fraction ratio of the phosphorus solution is 0.05:50, the unit of mass fraction is g, and the unit of volume fraction is mL.

[0055] The straw powder (corn straw) used in Example 1, the silicon-calcium modified biochar composite material prepared in Example 1, and the silicon-calcium modified biochar adsorbed phosphorus composite material prepared in Example 4 were subjected to scanning electron microscopy (SEM) and energy dispersive X-ray (EDS) tests. Figure 1 As shown by Figure 1 It can be seen that compared with corn stalks, the surface of the silicon-calcium modified biochar composite material prepared in Example 1 is rough, with a large number of granular structures, and many fine particles are dispersed in its pores. This may be CaO and Ca(OH)2 generated by the thermal decomposition of CaCO3 in the eggshell. The EDS energy spectrum shows that the surface of the silicon-calcium modified biochar composite material prepared in Example 1 is composed of C, Si, O, and Ca, indicating that Si and Ca are successfully loaded on the surface of corn stalks. After adsorbing potassium dihydrogen phosphate, flocculent precipitation appears on the surface of the silicon-calcium modified biochar phosphorus adsorption composite material prepared in Example 4. Figure 2 It can be seen that the flocculent precipitate is hydroxyapatite (Ca5(PO4)3(OH)).

[0056] Figure 2 The X-ray diffraction patterns (XRD) of the silicon-calcium modified biochar composite material prepared in Example 1 and the silicon-calcium modified biochar adsorbed phosphorus composite material prepared in Example 4 are shown in FIG. Figure 2 It can be seen that in the XRD of the silicon-calcium modified biochar composite material prepared in Example 1, an obvious CaCO3 crystal diffraction peak was observed, indicating that CaCO3 is the main form of calcium in the silicon-calcium modified biochar material. In the XRD of the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4, hydroxyapatite (Ca5(PO4)3(OH)) is the main calcium-phosphorus conjugate of the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4. The peak map position obtained by jade software shows that these peaks also conform to the crystal corresponding peaks of Ca3(PO4)2·2H2O.

[0057] Referring to the preparation method of the silicon-calcium modified biochar phosphorus adsorption composite material prepared in Example 4, the concentration of potassium dihydrogen phosphate after oscillating the silicon-calcium modified biochar composite material prepared in Example 1 in phosphorus solutions with different initial P values ​​(the initial P values ​​were 0 mg / L, 10 mg / L, 15 mg / L, 30 mg / L, 60 mg / L, 100 mg / L, 120 mg / L and 150 mg / L) for 24 hours was determined, and the adsorption capacity of potassium dihydrogen phosphate in the phosphorus solution of the silicon-calcium modified biochar composite material prepared in Example 1 was calculated according to the adsorption capacity formula. The adsorption capacity formula is as follows:

[0058]

[0059] Among them, Q e (mg / g) is the adsorption capacity of potassium dihydrogen phosphate by the silicon-calcium modified biochar composite material prepared in Example 1; C0 (mg / L) is the initial concentration of potassium dihydrogen phosphate in the phosphorus solution (i.e., the initial P value); C e (mg / L) is the concentration of potassium dihydrogen phosphate in the equilibrium state of the phosphorus solution (i.e., the concentration of potassium dihydrogen phosphate in the phosphorus solution after 24 hours of oscillation); V(L) is the volume of the phosphorus solution; m(g) is the mass of the silicon-calcium modified biochar composite material prepared in Example 1.

[0060] Further, the adsorption capacity of potassium dihydrogen phosphate in the phosphorus solution by the silicon-calcium modified biochar composite material prepared in Example 1 and the concentration C of potassium dihydrogen phosphate in the phosphorus solution in the equilibrium state are e The Langmuir and Freundlich isotherm models were used to fit the adsorption behavior of potassium dihydrogen phosphate on the silicon-calcium modified biochar composite material prepared in Example 1, and the adsorption isotherm curve was obtained. e The adsorption isotherm fitting curve of the silicon-calcium modified biochar composite material prepared in Example 1 is as follows: Figure 3 The adsorption isotherm fitting parameters are shown in Table 2.

[0061] Table 2 Adsorption isotherm fitting parameters

[0062]

[0063] Depend on Figure 3 It can be seen that with the gradual increase of the initial concentration of potassium dihydrogen phosphate in the phosphorus solution, the amount of phosphorus adsorbed by the silicon-calcium modified biochar composite material prepared in Example 1 shows a rapid growth trend. The Langmuir isotherm model mainly describes the single-layer adsorption process, involving forces such as precipitation and hydrogen bonding, while the Freundlich isotherm model considers more multi-layer adsorption phenomena, including factors such as van der Waals forces and electrostatic attraction. The adsorption data of the silicon-calcium modified biochar composite material of the present invention is more consistent with the Langmuir isotherm model, but both isotherm adsorption models have higher regression parameters (R2 ), indicating that electrostatic adsorption also plays a certain role in the adsorption of phosphate by silicon-calcium modified biochar composites.

[0064] Depend on Figure 3 As shown in Table 2, compared with the Freundlich isotherm model of the silicon-calcium modified biochar composite material prepared in Example 1, in the Langmuir isotherm model, the maximum adsorption capacity (Q max ) reaches 207.0156 mg / g, has a high removal rate for potassium dihydrogen phosphate, and has high adsorption capacity.

[0065] In order to determine the improvement effect of biochar composite materials on soil and their application potential as soil improvement fertilizers, a soil cultivation experiment was conducted. The specific method was as follows: 3 g of biochar composite materials were added to 300 g of soil, and normal soil without the addition of biochar composite materials was used as a comparison. The biochar composite materials were the silicon-calcium modified biochar phosphorus adsorption composite materials prepared in Example 4 or the silicon-calcium modified biochar composite materials prepared in Example 1. A 30-day soil cultivation test was conducted, and soil samples were collected after 30 days for statistical analysis. The nutrients and their effective heavy metal contents in the soil with the addition of biochar composite materials and the normal soil (without the addition of biochar composite materials) were tested. The test indicators of the soil cultivation experiment are shown in Table 3.

[0066] Table 3 Soil culture experiment test indicators

[0067]

[0068] It can be seen from Table 3 that compared with normal soil (without adding biochar composite material), the pH of the soil to which the silicon-calcium modified biochar phosphorus adsorption composite material prepared in Example 4 and the silicon-calcium modified biochar composite material prepared in Example 1 were added did not show significant changes.

[0069] It can be seen from Table 3 that compared with normal soil (without adding biochar composite material), the electrical conductivity of the soil to which the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 and the silicon-calcium modified biochar composite material prepared in Example 1 were added respectively was improved, and the electrical conductivity of the soil reflects the salt content in the soil, indicating that the addition of the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 and the silicon-calcium modified biochar composite material prepared in Example 1 helps to increase the salt content of the soil, thereby enriching the properties of the soil. Among them, the addition of the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 has a more obvious effect on improving the electrical conductivity of the soil, reaching 979μs / cm.

[0070] As can be seen from Table 3, adding the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 and the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 1 to the soil respectively has a significant promoting effect on increasing the soil organic matter (SOM) content. In addition to providing nutrients through adsorbed phosphates, the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 is also rich in organic carbon. Therefore, when it is applied to the soil, it can significantly increase the organic carbon in the soil, thereby increasing the organic matter content in the soil. Obviously, the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 has a more obvious effect on improving soil organic matter. Compared with normal soil, SOM increased by 21.4%.

[0071] Further exploring the impact on soil nutrients, it can be seen from Table 3 that compared with normal soil (without adding biochar composite material), the addition of the silicon-calcium modified biochar composite material prepared in Example 1 and the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 can increase the content of total phosphorus, available phosphorus, available silicon and available potassium in the soil, but the increase in the total nitrogen content in the soil is small. Among them, the addition of the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 significantly promoted the content of total phosphorus, available phosphorus and available potassium in the soil, reaching a significant difference, indicating that the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 provides rich phosphorus elements for the soil. Although the silicon-calcium modified biochar composite material prepared in Example 1 does not load phosphate, it can also increase the total phosphorus, available phosphorus and available potassium in the soil because it itself contains rich organic matter.

[0072] At the same time, the silicon-calcium modified biochar composite material prepared in Example 1 and the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 are both loaded with a certain amount of silicon, which can provide effective silicon for the soil and promote the formation of silicon skeletons and plant growth in plants. Among them, the effective silicon content of the soil added with the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 is 266.70 mg / kg, and its effective silicon enhancement effect is better than the effective silicon content of the soil added with the silicon-calcium modified biochar composite material prepared in Example 1 (236.41 mg / kg). In summary, the addition of the silicon-calcium modified biochar composite material prepared in Example 1 and the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 can significantly increase the content of nutrients in the soil, which is beneficial to improving soil fertility, enriching the soil nutrient environment, and promoting the growth of vegetation in the soil, but the addition effect of the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 is better.

[0073] As can be seen from Table 3, for most heavy metals, the addition of the silicon-calcium modified biochar composite material prepared in Example 1 and the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 can effectively reduce the content of effective heavy metals in the soil. This is because it is rich in pores and various functional groups, and can adsorb and passivate the effectiveness of heavy metals in the soil, thereby reducing the migration of heavy metals in the soil. The silicon-calcium modified biochar composite material prepared in Example 1 has a better effect on reducing the effective states of heavy metals Cu, Cd and Pb than the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4. This may be because the pore adsorption sites on the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 are occupied by solid substances (i.e., Ca-P precipitates) precipitated from calcium-bound phosphorus in the phosphorus solution, and its functional groups participate in the phosphorus adsorption reaction, thereby weakening the adsorption of heavy metals, but it is still better than normal soil (no biochar composite material added) to a certain extent, and the silicon-calcium modified biochar adsorption phosphorus composite material prepared in Example 4 has a better effect on reducing the effective states of heavy metals Zn and Cr.

[0074] The present invention is described above by way of example. It should be noted that, without departing from the core of the present invention, any simple deformation, modification or other equivalent replacement that can be made by those skilled in the art without inventive effort falls within the protection scope of the present invention.

Claims

1. A method for preparing a silicon-calcium modified biochar composite material, It is characterized in that The following steps are involved: Step 1, dispersing straw powder in a sodium silicate solution, ultrasonicating for at least 10 minutes, stirring for at least 8 hours, filtering to obtain a solid, washing the solid to neutrality, drying, and sieving to obtain a silicon-modified material, wherein the mass percentage of sodium silicate in the sodium silicate solution is 0.1-0.5%; Step 2, mix the eggshell powder and the silicon-modified material evenly, crack them at 650-850°C for 1.5-2.5h in an atmosphere of nitrogen or inert gas, cool to room temperature, and sieve to obtain a silicon-calcium modified biochar composite material, wherein the ratio of the eggshell powder to the silicon-modified material is (1-4):1 by mass.

2. The preparation method according to claim 1, It is characterized in that In step 1, the particle size of the straw powder is 40-60 mesh; In step 1, the ultrasonic time is 10 to 15 minutes, and the ultrasonic frequency is 80 to 120 kHz; In step 1, the stirring temperature is room temperature and the stirring time is 8 to 12 hours; In step 1, the drying temperature is 80-100° C. and the drying time is 8-12 hours; In the step 1, the solid is washed with deionized water until it becomes neutral; In step 1, the sodium silicate solution is a mixture of sodium silicate and water; In step 2, the eggshell comprises: a mixture of one or more of chicken eggshell, duck eggshell and goose eggshell; In step 2, the method for preparing the eggshell powder comprises: crushing and grinding the eggshells, and sieving to obtain the eggshell powder; In step 2, the flow rate of the gas in the nitrogen or inert gas atmosphere is 80 to 90 mL / min; In the step 2, the rate of heating to the temperature of 650-850°C is 5-20°C / min; In step 1 and step 2, the sieve has a mesh size of 40 to 60.

3. The silicon-calcium modified biochar composite material obtained by the preparation method as described in claim 1 or 2.

4. Application of the silicon-calcium modified biochar composite material as described in claim 3 in treating phosphorus-containing wastewater.

5. The use according to claim 4, It is characterized in that The silicon-calcium modified biochar composite material is dispersed in phosphorus-containing wastewater and shaken.

6. A method for preparing a calcium-silicon modified biochar phosphorus adsorption composite material, It is characterized in that The method comprises the following steps: dispersing the silicon-calcium modified biochar composite material described in claim 3 in a phosphorus solution, shaking, filtering, and drying to obtain a silicon-calcium modified biochar adsorbed phosphorus composite material, wherein the phosphorus solution contains soluble phosphate.

7. The method according to claim 6, It is characterized in that The concentration of soluble phosphate in the phosphorus solution is 20-150 mg / L.

8. The method according to claim 6, It is characterized in that The phosphorus solution is a mixture of soluble phosphate and water, and the mass fraction ratio of the silicon-calcium modified biochar composite material to the volume fraction of the phosphorus solution is 0.05:(10-50), the unit of the mass fraction is g, and the unit of the volume fraction is mL.

9. The method according to claim 6, It is characterized in that The oscillation speed is 180-200 r / min, and the oscillation time is 22-24 h; The drying temperature is 80-100° C., and the drying time is 8-12 hours.

10. Use of the calcium-silicon modified biochar composite material in soil as claimed in claim 1, It is characterized in that The calcium-silicon modified biochar composite material first adsorbs phosphorus and then acts as a fertilizer in the soil.

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