Composite biochar material with gradient pore structure and preparation method thereof

Abstract

The invention provides a composite biochar material with a gradient pore structure and a preparation method thereof.The composite biochar material is obtained through high-temperature carbonization of composite gel with the gradient pore structure, and the composite gel with the gradient pore structure is obtained through centrifugal separation and directional light curing; the composite gel is composed of pre-activated dolomite and bean dregs subjected to alkali pretreatment. According to the invention, the density difference between the bean dregs and the dolomite is utilized, a layered structure in gradient distribution is formed during centrifugation, and the layered structure is cured, so that the gradient pore structure of the material in the subsequent carbonization process is maintained.

Description

Technical Field

[0001] The present invention relates to the field of biochar materials, and particularly relates to a composite biochar material with a gradient pore structure and a preparation method thereof. Background Art

[0002] With the rapid development of industrialization and economy, the aquatic ecosystem has been increasingly disturbed, and water environment problems have attracted more and more attention. Among them, water eutrophication is one of the common pollution phenomena; usually, there are excessive nitrogen, phosphorus and other compounds in water, resulting in excessive growth of organisms in water, especially algae and phytoplankton. Among them, the excess content of phosphate is one of the main reasons for water eutrophication. High concentrations of phosphate will exacerbate the growth of algae in water, thus deteriorating the quality of the aquatic ecosystem. The severity of phosphorus pollution and the progress of prevention and control have attracted extensive attention from researchers. At present, the adsorption method has become a commonly used and effective method for phosphorus removal in water treatment because of its advantages such as simple operation, environmental friendliness, economic efficiency, etc., and it can be used for water quality under different conditions, has strong adaptability to water quality changes, and has no obvious interference to microorganisms in the biochemical treatment process. There have been many studies on phosphate adsorbents currently, including synthetic metal oxides / hydroxides, carbonate minerals, clay minerals, activated carbon and biochar, polymers, as well as bio-derived materials and industrial wastes, etc. The selection and modification optimization of adsorption materials have always been one of the research hotspots of phosphate adsorption. Issues such as the economy of adsorbent raw materials, the complexity of preparation processes, the adsorption performance for phosphate, and the recycling and reuse after phosphorus adsorption will all affect their selection. Biochar is generally a solid material obtained by thermochemical conversion of biomass by heating to 200-900 °C under oxygen-limited conditions. The sources of biochar materials are simple and inexpensive, and can be agricultural wastes, forestry wastes, animal feces, sludge, etc. A large amount of waste biomass is generated in China every year that needs to be disposed of, and the quantity of waste biomass in China is huge, which is a natural and green low-cost raw material. In recent years, research reports on modified biochar have been continuously updated, and there have been improvements to varying degrees in enhancing its adsorption performance, reducing production costs and increasing its reuse rate. At the same time, the application of a large number of chemical reagents has directly increased production costs and caused indirect pollution to the environment. Therefore, to solve the above problems, this study uses dolomite instead of chemical reagents to prepare a mineral-soybean residue biochar composite material for removing phosphate in water. Natural dolomite is a carbonate mineral containing abundant elements such as Ca, Mg, Fe, and Mn. Its main chemical composition is CaMg(CO3)2, which has the characteristics of low cost, wide distribution, and easy availability. Secondly, the cultivation of legumes and related processing industries have a long history in China. Soybean residue is a by-product of soybean product production, with an annual output of about 20 million tons in China. Due to its high moisture content and easy mildew and decay, most of the soybean residue can only be used as feed, or even directly discarded, with low recovery and utilization rates. It not only occupies land resources and causes environmental pollution, but also wastes resources due to its high content of nutrients such as protein, fat, and minerals. Therefore, preparing a dolomite-soybean residue biochar composite adsorbent with high efficiency for phosphorus removal from the non-toxic and non-side-effect and high-yield soybean residue biomass is expected to reduce greenhouse gases, solve the problem of wastewater phosphorus pollution, and the difficult disposal of waste biomass, etc. Summary of the Invention

[0003] Technical problems to be solved: The present invention provides a composite biochar material with a gradient pore structure and a preparation method thereof, and uses the gradient porous structure of the composite biochar material and the metal ions contained in dolomite to achieve the adsorption of phosphorus elements in water.

[0004] Technical solution: A composite biochar material with a gradient pore structure, wherein the biochar composite material is obtained by high-temperature carbonization of a composite gel with a gradient pore structure, the composite gel with a gradient pore structure is obtained by centrifugal separation and directional photocuring, and the composite gel is composed of pre-activated dolomite and soybean residue pretreated with alkali. Preferably, the preparation method comprises the following steps: S1. Pre-cool the pre-activated dolomite / sodium alginate methacrylate solution and inject it into a centrifuge tube, and then add the soybean residue pretreated with alkali / sodium alginate methacrylate solution, and obtain a mixed solution with upper and lower layers after centrifugation; S2. Heat the product obtained in S1 and perform directional photocuring, and then obtain a composite gel with a gradient porous structure; S3. Perform high-temperature carbonization on the composite gel obtained in S2, take out the product and wash it with water until neutral to obtain a composite biochar material with a gradient pore structure. Preferably, the mass ratio of the pre-activated dolomite / sodium alginate methacrylate solution to the soybean residue pretreated with alkali / sodium alginate methacrylate solution in S1 is 1-9:1-9. Preferably, the preparation method of the sodium alginate methacrylate solution in S1 comprises the following steps: S11. React sodium alginate with methacrylic anhydride in a molar ratio of 1 - 4:0.3 - 0.8 to obtain methacrylated sodium alginate; S12. Add the methacrylated sodium alginate obtained in S11 to water, and add lithium phenyl - 2,4,6 - trimethylbenzoylphosphinate to obtain a methacrylated sodium alginate solution with a mass fraction of 2 - 6 wt%. Preferably, the preparation of the pre - activated dolomite / methacrylated sodium alginate solution in S1 includes the following steps: S21. Mix dolomite and deionized water in a mass ratio of 1 - 3:2 - 7, and microwave - treat for 3 - 6 min to obtain pre - activated dolomite; S22. Add the pre - activated dolomite obtained in S21 to the methacrylated sodium alginate solution, and add 1 - 4 wt% of Pluronic F127 thermosensitive gel to obtain a pre - activated dolomite / methacrylated sodium alginate solution with a mass fraction of 60 - 80 wt%. Preferably, the preparation of the alkali - pretreated soybean dregs / methacrylated sodium alginate solution in S1 includes the following steps: S31. Immerse soybean dregs in sodium bicarbonate with a pH value of 8 - 9 for 1 - 2 h, wash and dry to obtain alkali - pretreated soybean dregs; S32. Add the alkali - pretreated soybean dregs to the methacrylated sodium alginate solution to obtain an alkali - pretreated soybean dregs / methacrylated sodium alginate solution with a mass fraction of 60 - 80 wt%. Preferably, for the upper - layer photocuring in S2, use 330 - 370 nm UV to irradiate from the top for 50 - 70 s, and for the lower - layer photocuring, use 390 - 410 nm laser to irradiate through the transparent tube bottom for 25 - 35 s. Preferably, the temperature of the heat - up treatment in S2 is 25 - 40 °C, and the time is 4 - 8 min. Preferably, the temperature of carbonization in S3 is 500 - 900 °C, the heating rate is 8 - 12 °C / min, and the heat - preservation time is 1.5 - 2.5 h. Beneficial effects: The present invention has the following advantages: 1. The calcium and magnesium ions rich in dolomite provide active sites for phosphate adsorption. As agricultural waste, a large amount of organic components in soybean dregs form a porous structure and rich functional groups after pyrolysis. The synergistic effect of the two not only solves the problem of waste treatment but also improves the overall adsorption performance of the material; 2. In the present invention, by utilizing the density difference between soybean dregs and dolomite, a layered structure with a gradient distribution is formed during centrifugation. Subsequently, through the synergistic effect of thermosensitive gel and photocuring, that is, the dual synergy of physical and chemical processes, a physical gel is formed after centrifugation by heating treatment to temporarily fix the layered structure. Then, chemical crosslinking is achieved through directional photocuring. UV curing is used for the upper layer to gently crosslink the soybean dregs - sodium alginate layer to form an open microporous network, and laser curing is used for the lower layer to penetrate the dolomite layer. While achieving high-density crosslinking, the positions of mineral particles are locked to prevent the collapse of pore channels caused by particle migration during carbonization, thus maintaining the gradient pore structure of the material during the subsequent carbonization process. 3. The triple synergistic mechanism of the gradient pore structure - charge - chemical site of the composite biochar obtained in the present invention realizes the efficient adsorption of phosphorus elements in water (the adsorption efficiency reaches 99.3%); and the adsorbed phosphorus can be continuously released, prolonging the effective release period of phosphorus in the soil, improving the utilization rate of phosphate fertilizers, and breaking through the application dilemma of traditional phosphate fertilizers. Description of the Drawings Figure 1 Shows the adsorption capacity and removal rate of the biochar composite material obtained in Example 8 for phosphorus in water with different initial pH solutions. Figure 2 Shows the adsorption capacity and removal rate of dolomite - soybean dregs biochar composite materials prepared with different composite ratios for phosphorus in water. Detailed Embodiments The present invention will be further described below in conjunction with embodiments. The following embodiments are explanations of the present invention and the present invention is not limited to the following embodiments: Example 1 A composite biochar material with a gradient pore structure and its preparation method, the preparation method comprising the following steps: S1. Pre - cool the pre - activated dolomite / methacrylated sodium alginate solution to 0 °C and then inject it into a centrifuge tube. Subsequently, add the soybean dregs / methacrylated sodium alginate solution pretreated with alkali. The mass ratio of the pre - activated dolomite / methacrylated sodium alginate solution to the soybean dregs / methacrylated sodium alginate solution pretreated with alkali is 7:3. After centrifugation, a mixed solution with upper and lower layers is obtained. S2. Heat the product obtained in S1 to 25 °C, treat it for 4 min and perform directional photocuring. For the upper - layer photocuring, irradiate it from the top with 330 nm UV for 50 s, and for the lower - layer photocuring, irradiate it through the transparent tube bottom with 390 nm laser for 25 s to obtain a composite gel with a gradient porous structure. S3. Perform high - temperature carbonization on the composite gel obtained in S2. The carbonization temperature is 500 °C, the heating rate is 8 °C / min, and the holding time is 1.5 h. After taking out the product, wash it with water until neutral to obtain a composite biochar material with a gradient porous structure. Among them, the preparation method of the methacrylated sodium alginate solution in S1 includes the following steps: S11. React sodium alginate with methacrylic anhydride in a molar ratio of 1:0.3 to obtain methacrylated sodium alginate; S12. Add the methacrylated sodium alginate obtained in S11 to water, and add lithium phenyl-2,4,6-trimethylbenzoylphosphinate to obtain a 2% methacrylated sodium alginate solution by mass fraction. Among them, the preparation of the pre-activated dolomite / methacrylated sodium alginate solution in S1 includes the following steps: S21. Mix dolomite and deionized water in a mass ratio of 2:5, and perform microwave treatment for 3 min to obtain pre-activated dolomite; S22. Add the pre-activated dolomite obtained in S21 to the methacrylated sodium alginate solution, and add 1 wt% of Pluronic F127 thermosensitive gel to obtain a 60 wt% pre-activated dolomite / methacrylated sodium alginate solution; Among them, the preparation of the alkali-pretreated soybean dregs / methacrylated sodium alginate solution in S1 includes the following steps: S31. Immerse soybean dregs in sodium bicarbonate with a pH value of 8 for 1 h, and obtain alkali-pretreated soybean dregs after washing and drying; S32. Add the alkali-pretreated soybean dregs to the methacrylated sodium alginate solution to obtain a 60 wt% alkali-pretreated soybean dregs / methacrylated sodium alginate solution. Example 2 A composite biochar material with a gradient pore structure and a preparation method thereof, the preparation method includes the following steps: S1. Pre-cool the pre-activated dolomite / methacrylated sodium alginate solution to 1 °C and inject it into a centrifuge tube, and then add the alkali-pretreated soybean dregs / methacrylated sodium alginate solution. The mass ratio of the pre-activated dolomite / methacrylated sodium alginate solution to the alkali-pretreated soybean dregs / methacrylated sodium alginate solution is 5:5. After centrifugation, a layered mixed solution is obtained; S2. Heat the product obtained in S1 to 30 °C and treat it for 4 min and perform directional photocuring. The upper layer is photocured by irradiating with 340 nm UV from the top for 55 s, and the lower layer is photocured by irradiating with 390 nm laser through the transparent tube bottom for 30 s to obtain a composite gel with a gradient porous structure; S3. Perform high-temperature carbonization on the composite gel obtained in S2. The carbonization temperature is 600 °C, the heating rate is 10 °C / min, the holding time is 1.5 h. After taking out the product, wash it with water until neutral to obtain a composite biochar material with a gradient porous structure; Among them, the preparation method of the methacrylated sodium alginate solution in S1 includes the following steps: S11. React sodium alginate with methacrylic anhydride at a molar ratio of 2:0.8 to obtain methacrylated sodium alginate; S12. Add the methacrylated sodium alginate obtained in S11 to water, and add lithium phenyl-2,4,6-trimethylbenzoylphosphinate to obtain a methacrylated sodium alginate solution with a mass fraction of 3 wt%. Among them, the preparation of the pre-activated dolomite / methacrylated sodium alginate solution in S1 includes the following steps: S21. Mix dolomite and deionized water according to a mass ratio of 3:2, and perform microwave treatment for 4 min to obtain pre-activated dolomite; S22. Add the pre-activated dolomite obtained in S21 to the methacrylated sodium alginate solution, and add 2 wt% of Pluronic F127 thermosensitive gel to obtain a pre-activated dolomite / methacrylated sodium alginate solution with a mass fraction of 70 wt%; Among them, the preparation of the alkali-pretreated soybean dregs / methacrylated sodium alginate solution in S1 includes the following steps: S31. Immerse soybean dregs in sodium bicarbonate with a pH value of 8 for 1 h, and obtain alkali-pretreated soybean dregs after washing and drying; S32. Add the alkali-pretreated soybean dregs to the methacrylated sodium alginate solution to obtain an alkali-pretreated soybean dregs / methacrylated sodium alginate solution with a mass fraction of 70 wt%. Example 3 A composite biochar material with a gradient pore structure and a preparation method thereof, the preparation method includes the following steps: S1. Pre-cool the pre-activated dolomite / methacrylated sodium alginate solution to 2 °C and inject it into a centrifuge tube, and then add the alkali-pretreated soybean dregs / methacrylated sodium alginate solution. The mass ratio of the pre-activated dolomite / methacrylated sodium alginate solution to the alkali-pretreated soybean dregs / methacrylated sodium alginate solution is 3:7. After centrifugation, a layered mixed solution is obtained; S2. Heat the product obtained in S1 to 30 °C and treat it for 5 min and perform directional photocuring. The upper layer is photocured by irradiating with 340 nm UV light from the top for 60 s, and the lower layer is photocured by irradiating with 400 nm laser light through the transparent tube bottom for 30 s to obtain a composite gel with a gradient porous structure; S3. The composite gel obtained in S2 is subjected to high-temperature carbonization at a carbonization temperature of 700 °C, a heating rate of 12 °C / min, and a heat preservation time of 2 h. After taking out the product, it is washed with water until neutral to obtain a composite biochar material with a gradient porous structure; Among them, the preparation method of the methacrylated sodium alginate solution in S1 includes the following steps: S11. React sodium alginate with methacrylic anhydride in a molar ratio of 3:0.3 to obtain methacrylated sodium alginate; S12. Add the methacrylated sodium alginate obtained in S11 to water, and add lithium phenyl-2,4,6-trimethylbenzoylphosphinate to obtain a methacrylated sodium alginate solution with a mass fraction of 4 wt%. Among them, the preparation of the pre-activated dolomite / methacrylated sodium alginate solution in S1 includes the following steps: S21. Mix dolomite and deionized water in a mass ratio of 2:7, and perform microwave treatment for 4 min to obtain pre-activated dolomite; S22. Add the pre-activated dolomite obtained in S21 to the methacrylated sodium alginate solution, and add 3 wt% of Pluronic F127 thermosensitive gel to obtain a pre-activated dolomite / methacrylated sodium alginate solution with a mass fraction of 65 wt%; Among them, the preparation of the alkali-pretreated soybean dregs / methacrylated sodium alginate solution in S1 includes the following steps: S31. Immerse soybean dregs in sodium bicarbonate with a pH value of 9 for 1.5 h, and obtain alkali-pretreated soybean dregs after washing and drying; S32. Add the alkali-pretreated soybean dregs to the methacrylated sodium alginate solution to obtain an alkali-pretreated soybean dregs / methacrylated sodium alginate solution with a mass fraction of 70 wt%. Example 4 A composite biochar material with a gradient pore structure and a preparation method thereof, the preparation method includes the following steps: S1. Pre-cool the pre-activated dolomite / methacrylated sodium alginate solution to 2 °C and then inject it into a centrifuge tube, and then add the alkali-pretreated soybean dregs / methacrylated sodium alginate solution. The mass ratio of the pre-activated dolomite / methacrylated sodium alginate solution to the alkali-pretreated soybean dregs / methacrylated sodium alginate solution is 2:8, and a layered mixed solution is obtained after centrifugation; S2. Heat the product obtained in S1 to 30 °C and treat it for 5 min and perform directional photocuring. The upper layer is photocured by irradiating with 350 nm UV light from the top for 55 s, and the lower layer is photocured by irradiating with 400 nm laser light through the transparent tube bottom for 30 s to obtain a composite gel with a gradient porous structure; S3. The composite gel obtained in S2 is subjected to high-temperature carbonization at a carbonization temperature of 600 °C, a heating rate of 8 °C / min, and a heat preservation time of 1.5 h. After taking out the product, it is washed with water until neutral to obtain a composite biochar material with a gradient porous structure; Among them, the preparation method of the methacrylated sodium alginate solution in S1 includes the following steps: S11. React sodium alginate with methacrylic anhydride at a molar ratio of 2:0.5 to obtain methacrylated sodium alginate; S12. Add the methacrylated sodium alginate obtained in S11 to water, and add lithium phenyl-2,4,6-trimethylbenzoylphosphinate to obtain a methacrylated sodium alginate solution with a mass fraction of 3 wt%. Among them, the preparation of the pre-activated dolomite / methacrylated sodium alginate solution in S1 includes the following steps: S21. Mix dolomite and deionized water at a mass ratio of 2:5, and perform microwave treatment for 4 min to obtain pre-activated dolomite; S22. Add the pre-activated dolomite obtained in S21 to the methacrylated sodium alginate solution, and add 2 wt% of Pluronic F127 thermosensitive gel to obtain a pre-activated dolomite / methacrylated sodium alginate solution with a mass fraction of 70 wt%; Among them, the preparation of the alkali-pretreated soybean dregs / methacrylated sodium alginate solution in S1 includes the following steps: S31. Soak soybean dregs in sodium bicarbonate with a pH value of 9 for 1.5 h, and obtain alkali-pretreated soybean dregs after washing and drying; S32. Add the alkali-pretreated soybean dregs to the methacrylated sodium alginate solution to obtain an alkali-pretreated soybean dregs / methacrylated sodium alginate solution with a mass fraction of 75 wt%. Example 5 A composite biochar material with a gradient pore structure and a preparation method thereof, the preparation method includes the following steps: S1. Pre-cool the pre-activated dolomite / methacrylated sodium alginate solution to 2 °C and inject it into a centrifuge tube, and then add the alkali-pretreated soybean dregs / methacrylated sodium alginate solution. The mass ratio of the pre-activated dolomite / methacrylated sodium alginate solution to the alkali-pretreated soybean dregs / methacrylated sodium alginate solution is 2:8. After centrifugation, a mixed solution with upper and lower layers is obtained; S2. Heat the product obtained in S1 to 30 °C and treat it for 6 min, then perform directional photocuring. For the upper layer photocuring, irradiate it from the top with 360 nm UV for 60 s, and for the lower layer photocuring, irradiate it through the transparent tube bottom with 400 nm laser for 35 s to obtain a composite gel with a gradient porous structure; S3. Perform high-temperature carbonization on the composite gel obtained in S2. The carbonization temperature is 700 °C, the heating rate is 10 °C / min, and the heat preservation time is 2 h. After taking out the product, wash it with water until neutral to obtain a composite biochar material with a gradient porous structure; Among them, the preparation method of the methacrylated sodium alginate solution in S1 includes the following steps: S11. React sodium alginate with methacrylic anhydride in a molar ratio of 3:0.6 to obtain methacrylated sodium alginate; S12. Add the methacrylated sodium alginate obtained in S11 to water, and add lithium phenyl-2,4,6-trimethylbenzoylphosphinate to obtain a 5 wt% methacrylated sodium alginate solution. Among them, the preparation of the pre-activated dolomite / methacrylated sodium alginate solution in S1 includes the following steps: S21. Mix dolomite and deionized water in a mass ratio of 2:5, and perform microwave treatment for 5 min to obtain pre-activated dolomite; S22. Add the pre-activated dolomite obtained in S21 to the methacrylated sodium alginate solution, and add 2 wt% Pluronic F127 thermosensitive gel to obtain a 65 wt% pre-activated dolomite / methacrylated sodium alginate solution; Among them, the preparation of the alkali-pretreated soybean dregs / methacrylated sodium alginate solution in S1 includes the following steps: S31. Soak soybean dregs in sodium bicarbonate with a pH value of 9 for 1.5 h, and after washing and drying, obtain alkali-pretreated soybean dregs; S32. Add the alkali-pretreated soybean dregs to the methacrylated sodium alginate solution to obtain a 65 wt% alkali-pretreated soybean dregs / methacrylated sodium alginate solution. Example 6 A composite biochar material with a gradient pore structure and its preparation method, the preparation method includes the following steps: S1. Pre-cool the pre-activated dolomite / methacrylated sodium alginate solution to 2 °C and inject it into a centrifuge tube, and then add the alkali-pretreated soybean dregs / methacrylated sodium alginate solution. The mass ratio of the pre-activated dolomite / methacrylated sodium alginate solution to the alkali-pretreated soybean dregs / methacrylated sodium alginate solution is 7:5. After centrifugation, obtain a layered mixed solution; S2. Heat the product obtained in S1 to 25 °C, treat it for 5 min, and perform directional photocuring. For the upper layer photocuring, irradiate it from the top with 340 nm UV for 60 s, and for the lower layer photocuring, irradiate it through the transparent tube bottom with 400 nm laser for 25 s to obtain a composite gel with a gradient porous structure; S3. Perform high-temperature carbonization on the composite gel obtained in S2. The carbonization temperature is 900 °C, the heating rate is 8 °C / min, and the heat preservation time is 1.5 h. After taking out the product, wash it with water until neutral to obtain a composite biochar material with a gradient porous structure; Among them, the preparation method of the sodium alginate methacrylate solution in S1 includes the following steps: S11. React sodium alginate with methacrylic anhydride at a molar ratio of 2:0.8 to obtain sodium alginate methacrylate; S12. Add the sodium alginate methacrylate obtained in S11 to water, and add lithium phenyl-2,4,6-trimethylbenzoylphosphinate to obtain a 2 wt% sodium alginate methacrylate solution. Among them, the preparation of the pre-activated dolomite / sodium alginate methacrylate solution in S1 includes the following steps: S21. Mix dolomite and deionized water according to a mass ratio of 3:7, and perform microwave treatment for 5 min to obtain pre-activated dolomite; S22. Add the pre-activated dolomite obtained in S21 to the sodium alginate methacrylate solution, and add 2 wt% Pluronic F127 thermosensitive gel to obtain a 65 wt% pre-activated dolomite / sodium alginate methacrylate solution; Among them, the preparation of the alkali-pretreated soybean dregs / sodium alginate methacrylate solution in S1 includes the following steps: S31. Soak the soybean dregs in sodium bicarbonate with a pH value of 8 for 1.5 h, and after washing and drying, obtain alkali-pretreated soybean dregs; S32. Add the alkali-pretreated soybean dregs to the sodium alginate methacrylate solution to obtain a 70 wt% alkali-pretreated soybean dregs / sodium alginate methacrylate solution. Example 7 A composite biochar material with a gradient pore structure and a preparation method thereof. The preparation method includes the following steps: S1. Pre-cool the pre-activated dolomite / sodium alginate methacrylate solution to 2 °C and then inject it into a centrifuge tube. Subsequently, add the alkali-pretreated soybean residue / sodium alginate methacrylate solution. The mass ratio of the pre-activated dolomite / sodium alginate methacrylate solution to the alkali-pretreated soybean residue / sodium alginate methacrylate solution is 7:2. After centrifugation, a layered mixed solution is obtained. S2. Heat the product obtained in S1 to 30 °C and treat it for 5 min, then perform directional photocuring. For the upper layer, photocure it with 345 nm UV light irradiated from the top for 60 s, and for the lower layer, photocure it with 405 nm laser light irradiated through the transparent tube bottom for 35 s to obtain a composite gel with a gradient porous structure. S3. Perform high-temperature carbonization on the composite gel obtained in S2. The carbonization temperature is 650 °C, the heating rate is 10 °C / min, and the holding time is 2 h. After taking out the product, wash it with water until it is neutral to obtain a composite biochar material with a gradient porous structure. Among them, the preparation method of the sodium alginate methacrylate solution in S1 includes the following steps: S11. React sodium alginate with methacrylic anhydride in a molar ratio of 3:0.7 to obtain sodium alginate methacrylate. S12. Add the sodium alginate methacrylate obtained in S11 to water, and add lithium phenyl-2,4,6-trimethylbenzoylphosphinate to obtain a 5 wt% sodium alginate methacrylate solution. Among them, the preparation of the pre-activated dolomite / sodium alginate methacrylate solution in S1 includes the following steps: S21. Mix dolomite and deionized water in a mass ratio of 3:5 and microwave-treat for 4 min to obtain pre-activated dolomite. S22. Add the pre-activated dolomite obtained in S21 to the sodium alginate methacrylate solution, and add 3 wt% Pluronic F127 thermosensitive gel to obtain a 70 wt% pre-activated dolomite / sodium alginate methacrylate solution. Among them, the preparation of the alkali-pretreated soybean residue / sodium alginate methacrylate solution in S1 includes the following steps: S31. Soak the soybean residue in sodium bicarbonate with a pH value of 9 for 1.5 h, and after washing and drying, obtain the alkali-pretreated soybean residue. S32. Add the alkali-pretreated soybean residue to the sodium alginate methacrylate solution to obtain a 70 wt% alkali-pretreated soybean residue / sodium alginate methacrylate solution. Example 8 A composite biochar material with a gradient pore structure and a preparation method thereof. The preparation method includes the following steps: S1. Pre-cool the pre-activated dolomite / sodium alginate methacrylate solution to 0 °C and then inject it into a centrifuge tube. Subsequently, add the alkali-pretreated soybean dregs / sodium alginate methacrylate solution. The mass ratio of the pre-activated dolomite / sodium alginate methacrylate solution to the alkali-pretreated soybean dregs / sodium alginate methacrylate solution is 9:1. After centrifugation, a layered mixed solution is obtained. S2. Heat the product obtained in S1 to 25 °C and treat it for 5 min for directional photocuring. For the upper layer, photocure it with 340 nm UV light irradiated from the top for 65 s. For the lower layer, photocure it with 405 nm laser light irradiated through the transparent bottom of the tube for 25 s to obtain a composite gel with a gradient porous structure. S3. Subject the composite gel obtained in S2 to high-temperature carbonization. The carbonization temperature is 900 °C, the heating rate is 10 °C / min, and the holding time is 2 h. After taking out the product, wash it with water until neutral to obtain a composite biochar material with a gradient porous structure. Among them, the preparation method of the sodium alginate methacrylate solution in S1 includes the following steps: S11. React sodium alginate with methacrylic anhydride in a molar ratio of 3:0.7 to obtain sodium alginate methacrylate. S12. Add the sodium alginate methacrylate obtained in S11 to water and add lithium phenyl-2,4,6-trimethylbenzoylphosphinate to obtain a 4 wt% sodium alginate methacrylate solution. Among them, the preparation of the pre-activated dolomite / sodium alginate methacrylate solution in S1 includes the following steps: S21. Mix dolomite and deionized water in a mass ratio of 3:5 and microwave-treat for 5 min to obtain pre-activated dolomite. S22. Add the pre-activated dolomite obtained in S21 to the sodium alginate methacrylate solution and add 3 wt% Pluronic F127 thermosensitive gel to obtain a 70 wt% pre-activated dolomite / sodium alginate methacrylate solution. Among them, the preparation of the alkali-pretreated soybean dregs / sodium alginate methacrylate solution in S1 includes the following steps: S31. Soak the soybean dregs in sodium bicarbonate with a pH value of 9 for 1.5 h, and after washing and drying, obtain the alkali-pretreated soybean dregs. S32. Add the alkali-pretreated soybean dregs to the sodium alginate methacrylate solution to obtain a 75 wt% alkali-pretreated soybean dregs / sodium alginate methacrylate solution. Comparative Example 1 The difference between Comparative Example 1 and Example 8 is that in S1, dolomite and soybean dregs are placed in a quartz boat in a mass ratio of 9:1 and then sent into a tubular furnace for carbonization. Comparative Example 2 The difference between Comparative Example 2 and Example 8 is that the soybean dregs in S1 are not pretreated with alkali. Comparative Example 3 The difference between Comparative Example 3 and Example 8 is that in S2, both the upper layer and the lower layer are cured using 340 nm UV. Comparative Example 4 The difference between Comparative Example 4 and Example 8 is that in S2, both the upper layer and the lower layer are cured using 405 nm laser. Comparative Example 5 The difference between Comparative Example 5 and Example 8 is that lithium phenyl-2,4,6-trimethylbenzoylphosphinate is not added in S12. Comparative Example 6 The difference between Comparative Example 6 and Example 8 is that Pluronic F127 thermosensitive gel is not added in S22. Comparative Example 7 The difference between Comparative Example 7 and Example 8 is that the alkali-pretreated soybean dregs are directly placed in a quartz boat for carbonization without adding dolomite. Comparative Example 8 The difference between Comparative Example 8 and Example 8 is that dolomite is directly placed in a quartz boat for carbonization without adding soybean dregs. Comparative Example 9 The difference between Comparative Example 9 and Example 8 is that in S1, the pre-activated dolomite / sodium alginate methacrylate solution is pre-cooled to 0 °C and then injected into a centrifuge tube without adding the alkali-pretreated soybean dregs / sodium alginate methacrylate solution; in S2, only 405 nm laser is used to irradiate through the transparent bottom of the tube for 25 s. Comparative Example 10 The difference between Comparative Example 10 and Example 8 is that in S1, the alkali-pretreated soybean dregs / sodium alginate methacrylate solution is added without adding the pre-activated dolomite / sodium alginate methacrylate solution; in S2, only 340 nm UV is used to irradiate from the top for 65 s. Performance Test The phosphorus adsorption amounts of the materials obtained in Examples 1 to 8 and Comparative Examples 1 to 10 in water bodies were tested, and the results are shown in the following table. After the materials obtained in Examples 1 to 8 and Comparative Examples 1 to 10 adsorbed phosphorus elements, the release rate of phosphorus elements from the materials and the cumulative release rate in 28 days were tested, and the results are shown in the following table. Obviously, the above examples are only illustrations for clear explanation and not limitations on the implementation manners. For those of ordinary skill in the art, other different forms of changes or modifications can be made based on the above description. It is not necessary and impossible to list all implementation manners here. And the obvious changes or modifications derived therefrom still fall within the protection scope of the present invention.

Claims

1. A composite biochar material with a gradient pore structure, characterized in that: The composite biochar material is obtained by high-temperature carbonization of a composite gel with a gradient pore structure. The composite gel with a gradient pore structure is obtained by centrifugal separation and directional photocuring. The composite gel is composed of pre-activated dolomite and soybean dregs pretreated with alkali.

2. The preparation method of the composite biochar material with a gradient pore structure according to claim 1, characterized in that: The preparation method includes the following steps: S1. Pre-cool the pre-activated dolomite / sodium alginate methacrylate solution and inject it into a centrifuge tube. Subsequently, add the soybean dregs pretreated with alkali / sodium alginate methacrylate solution. After centrifugation, a mixed solution with upper and lower layers is obtained. S2. Heat-treat the product obtained in S1 and perform directional photocuring. Subsequently, a composite gel with a gradient porous structure is obtained. S3. Perform high-temperature carbonization on the composite gel obtained in S2. After taking out the product, wash it with water until it is neutral to obtain a composite biochar material with a gradient porous structure.

3. The preparation method of the composite biochar material with a gradient pore structure according to claim 2, wherein: In S1, the mass ratio of the pre-activated dolomite / sodium alginate methacrylate solution to the soybean dregs pretreated with alkali / sodium alginate methacrylate solution is 1-9:1-9, and the pre-cooling temperature is 0-4°C.

4. The preparation method of the composite biochar material with a gradient pore structure according to claim 2, characterized in that: The preparation method of the sodium alginate methacrylate solution in S1 includes the following steps: S11. React sodium alginate with methacrylic anhydride in a molar ratio of 1-4:0.3-0.8 to obtain sodium alginate methacrylate. S12. Add the sodium alginate methacrylate obtained in S11 to water, and add lithium phenyl-2,4,6-trimethylbenzoylphosphinate to obtain a sodium alginate methacrylate solution with a mass fraction of 2-6 wt%.

5. The preparation method of the composite biochar material with a gradient pore structure according to claim 2, characterized in that: The preparation of the pre-activated dolomite / sodium alginate methacrylate solution in S1 includes the following steps: S21. Mix dolomite and deionized water in a mass ratio of 1-3:2-7, and perform microwave treatment for 3-6 min to obtain pre-activated dolomite. S22. Add the pre-activated dolomite obtained in S21 to the sodium alginate methacrylate solution, and add 1-4 wt% of Pluronic F127 thermosensitive gel to obtain a pre-activated dolomite / sodium alginate methacrylate solution with a mass fraction of 60-80 wt%.

6. The preparation method of the composite biochar material with a gradient pore structure according to claim 2, characterized in that: The preparation of the soybean dregs pretreated with alkali / sodium alginate methacrylate solution in S1 includes the following steps: S31. Soak the soybean dregs in sodium bicarbonate with a pH value of 8-9 for 1-2 h, and obtain the soybean dregs pretreated with alkali after washing and drying. S32. Add the soybean dregs pretreated with alkali to the sodium alginate methacrylate solution to obtain a soybean dregs pretreated with alkali / sodium alginate methacrylate solution with a mass fraction of 60-80 wt%.

7. The preparation method of the composite biochar material with a gradient pore structure according to claim 2, characterized in that: In S2, for the upper-layer photocuring, irradiate with 330-370 nm UV at the top for 50-70 s, and for the lower-layer photocuring, irradiate with 390-410 nm laser through the transparent tube bottom for 25-35 s.

8. The preparation method of the composite biochar material with a gradient pore structure according to claim 2, characterized in that: In S2, the temperature of the heat treatment is 25-40°C, and the time is 4-8 min.

9. The preparation method of the composite biochar material with a gradient pore structure according to claim 2, characterized in that: In S3, the temperature of carbonization is 500-900°C, the heating rate is 8-12°C / min, and the heat preservation time is 1.5-2.5 h.

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