Layered double hydroxide-biochar composite material as well as preparation method and application thereof

Abstract

The invention discloses a layered double hydroxide-charcoal composite material as well as a preparation method and application thereof, and belongs to the technical field of composite materials. The double hydroxide-biochar composite material comprises a biochar matrix, wherein layered double hydroxide is loaded on the biochar matrix; phosphate is loaded between layers of the layered double hydroxide. The phosphate is strong acid extracted phosphate or strong base extracted phosphate. The invention also discloses a preparation method of the composite material and application of the composite material as a slow release fertilizer. The layered double hydroxide-biochar composite material has good phosphorus holding capacity, MgAl-LDH not only increases the pore structure and the specific surface area of biochar, but also enables phosphate to exist in the composite material in a stable form through the interlayer adsorption capacity to phosphate, and the slow release performance of phosphorus is improved.

Description

Technical Field

[0001] The present application relates to a layered double hydroxide-biochar composite material, a preparation method and an application thereof, and belongs to the technical field of composite materials. Background Art

[0002] In the severe situation of the increasingly scarce global phosphorus resources and the average utilization rate of phosphate fertilizers being only 12.6%, the research and development of environmentally friendly and efficient slow-release phosphate fertilizers has become a key topic in the research of sustainable agricultural development. Biochar slow-release phosphate fertilizers, with their unique advantages, can not only significantly improve the utilization rate of phosphorus, effectively reduce phosphorus loss, but also convert agricultural waste into fertilizers with high added value, realizing the recycling of resources and opening up a new path for the green development of agriculture. The porous structure and strong adsorption capacity of biochar make it a natural carrier for nutrients, capable of precisely regulating the nutrient release rate, ensuring that crops obtain stable and continuous nutrient supply throughout the growth cycle, thus significantly improving the fertilizer utilization rate and reducing nutrient loss and environmental pollution. At the same time, biochar can also promote the proliferation and activity of microorganisms in the soil, thereby enhancing soil fertility and biodiversity, providing strong support for the stable development of the agricultural ecosystem.

[0003] However, biochar surfaces usually carry negative charges, which may affect their ability to adsorb and retain anionic nutrients (such as phosphates). Due to the presence of electrostatic repulsion, anionic nutrients are difficult to firmly attach to the biochar surface, resulting in too rapid release of nutrients in the soil and failure to achieve the desired slow-release effect. In this regard, the research by Yao Y et al. found that the negative charge on the biochar surface affects its phosphate adsorption capacity, and iron modification can reduce this effect (YAO Y, GAOB, INYANG M, et al. Biochar derived from anaerobically digested sugar beet tailings: characterization and phosphate removal potential[J]. Bioresource Technology, 2011.); Li Yc et al. also pointed out that unmodified biochar has poor adsorption effects on anionic and cationic heavy metals (LIY, WANG S, OUYANG XF, et al. Acetate anions intercalated Fe / Mg-layered double hydroxides modified biochar for efficient adsorption of anionic and cationic heavy metal ions from polluted water[J]. Chemosphere, 2024.); Secondly, the surface functional group density of biochar is relatively limited, which restricts its binding ability with nutrients. These functional groups are the key sites for chemical reactions and physical adsorption between biochar and nutrients, but the quantity and type of surface functional groups are significantly affected by precursors and pyrolysis conditions (JIAHOU H, XI Z, SHAOJIE Z, et al. Biochar as a highly efficient adsorption carrier for sewage sludge-derived nutrients and biostimulants: component fixation and mechanism[J]. Biochar, 2024, 6(1)). If the surface functional group density is insufficient, the adsorption and fixation ability of biochar to nutrients will be restricted, thus affecting the slow-release effect and stability of fertilizers. These limiting factors have to some extent reduced the potential for the widespread application of biochar in the environmental field. Therefore, in order to improve the stability and reliability of raw biochar, some studies have investigated the preparation of engineered / modified biochar composites.

[0004] Layered double hydroxides (LDH) composites are composed of positively charged brucite-type mixed metal (including divalent and trivalent metals) hydroxide host layers. LDH has a large specific surface area, can effectively carry out anion exchange, and at the same time has the characteristic of being non-toxic, so it is used to improve biochar slow-release fertilizers. The general chemical structure of LDH can be expressed as [M 2+ 1-x M 3+ x (OH)2] x+ (A n- ) x / n ·yH2O, where M 2+ and M 3+ represent divalent ions (Mg 2+ , Fe 2+ , Ca 2+ , Mn 2+ , Ni 2+ , etc.) and trivalent ions (Al 3+ , Fe 3+ , Co 3+ , etc.), and A n- is the charge-balancing interlayer anion (CO3 2- , NO3 - , Cl - , etc.); the value of x represents the molar ratio of [M 3+ / ([M 2+ +[M 3+ ). Shin J et al. used MgAl layered double hydroxide-functionalized coffee ground waste biochar to adsorb phosphate and nitrate ions and used them as slow-release fertilizers to stimulate plant growth (SHIN J, KWAK J, KIM S, et al. Enhanced selectivity and recovery of phosphate and nitrate ions onto coffee ground waste biochars via co-precipitation of Mg / Al layered double hydroxides: A potential slow-release fertilizer[J]. Environmental Research, 2023.).

[0005] Based on the above research, this application aims to develop a new type of biochar slow-release fertilizer that can not only improve the slow-release performance of biochar slow-release fertilizers but also improve soil quality. Summary of the Invention

[0006] According to one aspect of the present application, a layered double hydroxide-biochar composite material is provided. By combining layered double hydroxide and biochar and loading stable-state phosphate, the slow-release effect of phosphorus in the composite material is improved.

[0007] The layered double hydroxide-biochar composite material includes a biochar matrix, and layered double hydroxide is loaded on the biochar matrix;

[0008] Phosphate is loaded between the layers of the layered double hydroxide;

[0009] The phosphate is strong-acid extractable phosphate or strong-base extractable phosphate.

[0010] Optionally, the divalent metal cation in the layered double hydroxide is Mg 2+ , and the trivalent metal cation is Al 3+ ;

[0011] In the layered double hydroxide, the molar ratio of Mg to Al is 2-5:1.

[0012] Specifically, the molar ratio of Mg to Al can be independently selected from 2:1, 2.5:1, 3:1, 3.5:1, 4:1, 4.5:1, 5:1, or any ratio between the above values. Preferably, the molar ratio of Mg to Al is 4:1.

[0013] Optionally, the content of the layered double hydroxide is 4-14 wt%.

[0014] Optionally, the content of the phosphate is 14-16 wt%.

[0015] According to another aspect of the present application, a preparation method of the layered double hydroxide-biochar composite material is provided. The method at least includes the following steps:

[0016] Obtain layered double hydroxide;

[0017] Co-pyrolyze the biochar raw material, calcium source, phosphate source root, and the layered double hydroxide under a protective atmosphere to obtain the layered double hydroxide-biochar composite material.

[0018] Optionally, the biochar raw material uses crop straw;

[0019] The phosphate source root is selected from at least one of Ca3(PO3)2, Ca(H2PO4)2, KH2PO4, K3PO4, (NH4)3PO4, NH4H2PO4.

[0020] Preferably, the phosphate source root uses Ca3(PO3)2.

[0021] In this application, the biochar raw material can be selected from common crop straws such as corn straw, sorghum straw, and rice straw. In actual use, the powder of crop straw is adopted.

[0022] Optionally, the addition amount of the layered double hydroxide is 5-20% of the mass of the biochar raw material;

[0023] The addition amount of the phosphate source is 15-25% of the mass of the biochar raw material.

[0024] Specifically, the lower limit of the addition amount of the layered double hydroxide can be independently selected from 5%, 6%, 8%, 10%, 12% of the mass of the biochar raw material, and the upper limit of the addition amount of the layered double hydroxide can be independently selected from 13%, 14%, 15%, 18%, 20% of the mass of the biochar raw material. Preferably, the addition amount of the layered double hydroxide is 10% of the mass of the biochar raw material.

[0025] Specifically, the lower limit of the addition amount of the phosphate source can be independently selected from 15%, 16%, 17%, 18%, 19% of the mass of the biochar raw material, and the upper limit of the addition amount of the phosphate source can be independently selected from 20%, 21%, 22%, 23%, 25% of the mass of the biochar raw material.

[0026] Preferably, the addition amount of the phosphate source is 20% of the mass of the biochar raw material.

[0027] Optionally, the conditions of the co-pyrolysis reaction are as follows:

[0028] The heating temperature is 400-600 °C, the heating time is 1.5-4 h, and the heating rate is 8-20 °C / min.

[0029] Optionally, the feeding rate of the protective atmosphere is 80-120 mL / min.

[0030] Specifically, the lower limit of the heating temperature can be independently selected from 400 °C, 425 °C, 450 °C, 475 °C, 500 °C, and the upper limit of the heating temperature can be independently selected from 525 °C, 540 °C, 550 °C, 575 °C, 600 °C. Preferably, the heating temperature is 400 °C.

[0031] Specifically, the heating time can be independently selected from 1.5 h, 2 h, 2.5 h, 3 h, 3.5 h, 4 h, or any ratio between the above values. Preferably, the heating time is 2 h.

[0032] Specifically, the lower limit of the heating rate can be independently selected from 8 °C / min, 10 °C / min, 12 °C / min, 14 °C / min, 15 °C / min; the upper limit of the heating rate can be independently selected from 16 °C / min, 17 °C / min, 18 °C / min, 19 °C / min, 20 °C / min. Preferably, the heating rate is 10 °C / min.

[0033] Optionally, the flow rate of the protective atmosphere is 80-200 mL / min.

[0034] Specifically, the lower limit of the introduction rate of the protective atmosphere can be independently selected from 80 mL / min, 90 mL / min, 100 mL / min, 110 mL / min, 120 mL / min; the upper limit of the introduction rate of the protective atmosphere can be independently selected from 130 mL / min, 140 mL / min, 150 mL / min, 180 mL / min, 200 mL / min. Preferably, the introduction rate of the protective atmosphere is 150 mL / min.

[0035] Specifically, the protective atmosphere can be an inert atmosphere such as nitrogen or argon, and those skilled in the art can select the protective atmosphere according to the actual production situation. The present application preferably uses nitrogen as the protective atmosphere.

[0036] According to another aspect of the present application, there is provided an application of a layered double hydroxide-biochar composite material as a slow-release fertilizer.

[0037] The beneficial effects that the present application can produce include:

[0038] 1) The layered double hydroxide-biochar composite material has good phosphorus retention ability. MgAl-LDH not only increases the pore structure and specific surface area of biochar, but also makes phosphate exist in a stable form in the composite material through the interlayer adsorption ability of MgAl-LDH to phosphate, increasing the slow-release performance of phosphorus.

[0039] 2) Using the layered double hydroxide-biochar composite material of the present application as a biochar slow-release phosphate fertilizer can increase the phosphorus that plants can utilize in the medium and long term, improve the nutrient utilization efficiency of plants, and can better promote plant growth compared with commercial phosphate fertilizers; and can improve soil conductivity, improve soil aggregate structure, and improve the water retention ability of the soil.

[0040] 3) In this application, phosphate is added to the biochar and layered double hydroxide composite material through co-pyrolysis. During the co-pyrolysis process, MgAl-LDH can adsorb reactive phosphorus (such as water-soluble phosphorus) and labile phosphorus (such as sodium bicarbonate extractable phosphorus) in the sediment, thereby reducing their contents. At the same time, the metal cations in MgAl-LDH can react with phosphate to form insoluble phosphate precipitates. And MgAl-LDH may transfer phosphate from the reactive or labile form to a more stable sodium hydroxide extractable or hydrochloric acid extractable form through its interlayer anion exchange effect, thereby increasing the slow-release rate of phosphorus and increasing the plant-available phosphorus in the composite material. Description of the Drawings

[0041] Figure 1 SEM images and EDS maps of the samples obtained in Example 1 of this application. Among them, Figures a - e are the SEM images of samples BCF, 2:1LDH@BCF, 3:1LDH@BCF, 4:1LDH@BCF, and 5:1LDH@BCF respectively, and Figure f is the EDS map of 4:1LDH@BCF;

[0042] Figure 2 N2 adsorption - desorption isotherms of the samples obtained in Example 1 of this application. Among them, Figures a - e are the N2 adsorption - desorption isotherms and pore size distribution curves of samples BCF, 2:1LDH@BCF, 3:1LDH@BCF, 4:1LDH@BCF, and 5:1LDH@BCF respectively;

[0043] Figure 3 Pore size distribution curves of the samples obtained in Example 1 of this application. Among them, Figures a - e are the pore size distribution curves of samples BCF, 2:1LDH@BCF, 3:1LDH@BCF, 4:1LDH@BCF, and 5:1LDH@BCF respectively;

[0044] Figure 4 FTIR spectra of the samples obtained in Example 1 of this application;

[0045] Figure 5 XRD patterns of the samples obtained in Example 1 of this application;

[0046] Figure 6 XPS spectra of the samples obtained in Example 1 of this application. Among them, Figures a - e are the XPS spectra of samples BCF, 2:1LDH@BCF, 3:1LDH@BCF, 4:1LDH@BCF, and 5:1LDH@BCF respectively;

[0047] Figure 7 Slow-release experiment results of different samples in water in the examples of this application;

[0048] Figure 8 The experimental results of the slow-release kinetics in water of different samples in the embodiments of the present application;

[0049] Figure 9 The experimental results of the sequential extraction of phosphorus from the samples in the embodiments of the present application;

[0050] Figure 10 The photos of the effects of phosphate fertilizer and the samples obtained in Example 1 of the present application on the growth of peppers, where Figure a shows the growth status of the potted plants, and Figure b shows the growth status of the seedlings in the potted plants;

[0051] Figure 11 The changes in the indexes of the potted plants after different fertilizer treatments, where Figure a shows the influence results of the germination rate of pepper seeds, Figure b shows the changes in the height of the pepper plants, and Figure c shows the average fresh weight and dry weight results of the pepper plants after 30 days of cultivation;

[0052] Figure 12 The changes in the physical and chemical indexes of the soil after different fertilizer treatments, where Figure a shows the changes in the soil pH and electrical conductivity, Figure b shows the changes in the dry matter content and moisture of the soil, and Figure c shows the changes in the available phosphorus content of the soil. Detailed implementation manners

[0053] The present application will be described in detail below in conjunction with the embodiments, but the present application is not limited to these embodiments.

[0054] Unless otherwise specified, the raw materials in the embodiments of the present application are all purchased through commercial channels. Among them, analytical pure magnesium chloride hexahydrate (MgCl2·6H2O), anhydrous aluminum chloride (AlCl3), calcium phosphate (Ca3(PO3)2), and other sodium reagents, including sodium hydroxide (NaOH) and sodium bicarbonate (NaHCO3), are purchased from Shanghai Titan Technology Co., Ltd. All reagents are used as received without further purification. All experiments use deionized water (Shanghai Heta Instrument Co., Ltd., Master-S15). The corn straw is purchased from a farm in Xi'an, Shaanxi. Before use, the corn straw is cut into small pieces less than 5 cm, washed several times with deionized water, dried overnight in an 80 °C oven, then crushed with a high-speed rotary cutter, passed through an 80-mesh sieve, and stored in a sealed container for standby.

[0055] The preparation method of the layered double hydroxide-biochar composite material of the present application includes the following steps:

[0056] Step 1: Obtain layered double hydroxides

[0057] The metal elements in the layered double hydroxides selected in the present application are Mg and Al respectively. The preparation method of the layered double hydroxide MgAl-LDH is a conventional method in the art. Specifically, the present application prepares MgAl-LDH by the coprecipitation method:

[0058] According to Mg 2+ and Al 3+ with a molar ratio of 2-5:1, a certain amount of MgCl2·6H2O and AlCl3 were weighed and dissolved in pure water, where the concentration of Mg 2+ in the system was 0.2-0.5 M. At room temperature, it was stirred at a speed of 800 rpm / min. The pH of the system was adjusted to 10±0.1 using 1 M NaOH and 0.5 M NaHCO3, stirred for 4 h, aged at room temperature for 18 h, then the sample was collected. The washed sample was placed in an oven at 80°C and dried for 12 hours to obtain dry MgAl-LDH powder.

[0059] Step 2: According to the addition amount of layered double hydroxide being 5-20% of the mass of the biochar raw material and the addition amount of the phosphate source being 15-25% of the mass of the biochar raw material, the layered double hydroxide MgAl-LDH, the phosphate source, and the biochar raw material were weighed respectively, and co-pyrolyzed at 400-600°C for 1.5-4 h in a protective atmosphere to obtain a layered double hydroxide-biochar composite material. During the co-pyrolysis process, the heating rate was controlled at 10-20°C / min, and the flow rate of the protective atmosphere was 80-200 mL / min.

[0060] Example 1

[0061] MgAl-LDH, Ca3(PO3)2, and corn stover powder were weighed and added to a tube furnace (Anhui Kemi Instrument Co., Ltd., TFH-1200-100-440) for co-pyrolysis. Among them, the addition amount of MgAl-LDH was 15% of the mass of the corn stover powder, and the addition amount of Ca3(PO3)2 was 20% of the mass of the corn stover powder. At 400°C, the N2 flow rate was controlled at 150 mL / min, the heating rate was 10°C / min, and heated for 2 h to obtain a layered double hydroxide-biochar composite material sample, denoted as LDH@BCF. According to the molar ratio of Mg 2+ and Al 3+ in the added MgAl-LDH, samples with molar ratios of Mg 2+ and Al 3+ of 2:1, 3:1, 4:1, and 5:1 were obtained, and were respectively denoted as 2:1LDH@BCF, 3:1LDH@BCF, 4:1LDH@BCF, and 5:1LDH@BCF.

[0062] In addition, the sample obtained by heating the corn stover powder alone under the above pyrolysis conditions was denoted as BCF.

[0063] Example 2

[0064] Prepare 4:1 LDH@BCF according to the method of Example 1, with the difference that the heating temperatures are controlled at 500 °C, 550 °C, and 600 °C respectively.

[0065] Example 3

[0066] Prepare 4:1 LDH@BCF according to the method of Example 1, with the difference that the addition amounts of MgAl-LDH are 5%, 10%, and 20% of the mass of corn straw powder respectively.

[0067] Example 4

[0068] Prepare 4:1 LDH@BCF according to the method of Example 1, with the difference that Ca3(PO3)2 is replaced by Ca(H2PO4)2 and K3PO4.

[0069] Example 5

[0070] Weigh MgAl-LDH (the molar ratio of Mg 2+ to Al 3+ is 4:1), Ca3(PO3)2 and corn straw powder and add them to a tube furnace (Anhui Kemi Instrument Co., Ltd., TFH-1200-100-440) for co-pyrolysis. The addition amount of MgAl-LDH is 15% of the mass of corn straw powder, and the addition amount of Ca3(PO3)2 is 15% of the mass of corn straw powder. Control the N2 flow rate at 200 mL / min at 400 °C, the heating rate at 20 °C / min, and heat for 1.5 h to obtain a layered double hydroxide-biochar composite material.

[0071] Example 6

[0072] Weigh MgAl-LDH (the molar ratio of Mg 2+ to Al 3+ is 4:1), Ca3(PO3)2 and corn straw powder and add them to a tube furnace (Anhui Kemi Instrument Co., Ltd., TFH-1200-100-440) for co-pyrolysis. The addition amount of MgAl-LDH is 25% of the mass of corn straw powder, and the addition amount of Ca3(PO3)2 is 15% of the mass of corn straw powder. Control the N2 flow rate at 80 mL / min at 400 °C, the heating rate at 8 °C / min, and heat for 4 h to obtain a layered double hydroxide-biochar composite material.

[0073] Comparative Example 1

[0074] Weigh MgAl-LDH (the molar ratio of Mg 2+ to Al 3+The molar ratio of (4:1) and corn straw powder were added to a tubular furnace, where the addition amount of MgAl-LDH was 15% of the mass of the corn straw powder. At 400 °C, the N2 flow rate was controlled at 150 mL / min, the heating rate was 10 °C / min, and heating was carried out for 2 h to obtain a phosphate-free co-heated composite material; the phosphate-free co-heated composite material and Ca3(PO3)2 with an additional amount of 15% of the mass of the corn straw powder were blended to obtain P-LDH@BCF. Through static water release experiments, the phosphorus release rate of P-LDH@BCF reached 60% within 24 h, and the cumulative phosphorus release rate at 10 d was nearly 80%, with a poor slow-release effect.

[0075] Characterization and performance experimental verification were carried out on the layered double hydroxide-biochar composite material.

[0076] 1. The layered double hydroxide-biochar composite materials prepared in Examples 1-6 were characterized, and typically, the sample obtained in Example 1 was used for specific description.

[0077] The characterization methods used in this application include: using Cu Kα radiation on a Rigaku Miniflex 600 diffractometer (Rigaku, Japan). The XRD pattern was recorded at a scanning rate of 10.0 ° / min in the 2θ range of 7-70°.

[0078] The SEM micrograph was obtained using a Hitachi Limited FlexSEM1000 scanning electron microscope (Hitachi, Japan) with a working voltage of 10.0 kV. The surface microstructure of LDH@BC was observed using the SEM.

[0079] The FTIR spectrum was measured using a NICOLET 5700 spectrometer in the range of 4000-400 cm -1 (NICOLET, USA).

[0080] XPS (Thermo escalab 250Xi, USA) was used to test the three characteristic peaks of C1s, Mg1s, and P2p on the surface of the sample.

[0081] The BET specific surface area, total pore volume, average pore diameter, and N2 adsorption-desorption curve of the biochar slow-release phosphate fertilizer were measured using a TriStar IIPlus high-throughput surface area and porosity analyzer.

[0082] 1) The scanning electron micrographs of BCF and LDH@BCF are as Figure 1 shown. The sample prepared without adding MgAl-LDH has a relatively regular pore structure, indicating that the addition of calcium phosphate source promotes the formation of the biochar structure during co-pyrolysis, but there are collapsed structures. Figure 1a). There are flaky substances on the sample prepared after adding MgAl-LDH, indicating that MgAl-LDH was successfully loaded onto the biochar during the co-pyrolysis process. Figure 1 b, 1c). At the same time, after adding MgAl-LDH, it can be observed that the surface roughness decreases and the pore structure becomes denser and more uniform. Figure 1 d, 1e). This indicates that the Mg-Al interlayer structure of LDH helps to form a more uniform pore structure during the pyrolysis of biomass. The EDS map shows that Mg, Al, and P elements were successfully loaded onto the biochar material, further confirming that LDH was successfully loaded onto the sample material. Figure 1 f).

[0083] At the same time, the physical properties of different LDH@BCF were detected, and it was found that the pH of the sample increased after adding MgAl-LDH, which was caused by the alkaline environment during the preparation of MgAl-LDH; the conductivity of BCF was 198, while the conductivities of the samples with different Mg / Al ratios (as the molar ratio of Mg increased) were 221 S / m, 327 S / m, 482 S / m, and 484 S / m respectively. It can be seen that with the addition of MgAl-LDH, the conductivity of the composite material increased, which means that as a fertilizer, the soluble nutrients and micronutrient elements in the composite material increased, thus improving the fertilizer efficiency. 2+

[0084] 2) The N2 adsorption-desorption method was used to analyze the structural properties of BCF and different LDH@BCF samples. The N2 adsorption-desorption isotherm and pore size distribution curve are as Figure 2 、 Figure 3 shown. The corresponding pore structure parameters, including BET specific surface area, total pore volume, and average pore diameter, are shown in Table 1. It can be seen that after adding MgAl-LDH, the BET specific surface area and total pore volume of the material increased, and the BET specific surface area and total pore volume of LDH@BCF with a Mg / Al ratio of 4:1 were the largest, indicating that it had the optimal pore structure. This is because the layered double metal hydroxide (LDH) creates pores through gas release during pyrolysis, and the metal oxide template effect in the interlayer structure expands the specific surface area and pore volume. At the same time, the layered structure of LDH hinders the densification of the biochar precursor during pyrolysis, preventing excessive shrinkage of the carbon structure, thus retaining more pores. The abundant hydroxyl groups (-OH) on the LDH surface react with the oxygen-containing functional groups (such as carboxyl groups and phenolic hydroxyl groups) in the biochar to form a stable three-dimensional network structure, promoting pore generation and significantly optimizing the pore structure of the biochar. Among them, the interlayer structure composed of MgAl-LDH with a Mg / Al ratio of 4:1 achieves the optimal pore distribution by balancing the pore-forming ability of MgO and the structural stability of Al2O3, thereby improving the nutrient controlled-release performance of the slow-release fertilizer.

[0085] ​Table 1 Physical and Chemical Properties of Various Slow-Release Fertilizers

[0086]

[0087] 3) The FTIR spectra of different LDH@BCF are as Figure 4 shown. There is a stretching vibration absorption peak of hydroxyl groups at a wavelength of 3440 cm -1 . The strong hydroxyl peak indicates that there may be more associated hydroxyl groups, which is due to the incomplete removal of moisture and the formation of hydrogen bonds between the hydroxyl groups in the layered double metal hydroxide and surrounding molecules. The absorption peak of carbonated apatite is near a wavelength of 563 cm -1 , which indicates that the addition of calcium phosphate forms carbonated apatite, promoting the enhancement of the slow-release performance of the material. In the sample added with MgAL-LDH, the absorption peak at a wavelength of 875 cm -1 may be related to the vibration of the P-O-Mg bond, which indicates the binding mode of magnesium and phosphate in the layered double metal oxide (LDH). At the same time, the absorption peak at 765 cm-1 may be related to the vibration of the P-O bond, and the addition of LDH increases its intensity, indicating that the addition of LDH during the co-pyrolysis process increases the number of phosphorus-oxygen bonds. An absorption peak is observed at 1050 cm-1, which is related to the vibration of phosphate ions, indicating the presence of phosphate groups in the sample. As the magnesium-aluminum ratio increases, the intensity of the absorption peak at 1050 cm-1 weakens, indicating a decrease in the number of phosphate ions in the sample, indicating that some phosphate ions are adsorbed between the layers of LDH. However, too high a Mg / Al ratio may lead to a decrease in the layer spacing and changes in the interlayer environment, thus reducing the effective adsorption sites for phosphate ions and resulting in a decrease in the number of phosphate ions adsorbed between the LDH layers. The infrared spectroscopy results show that the interlayer structure of LDH can adsorb phosphate, which is beneficial to the slow release of nutrients in biochar slow-release phosphate fertilizers.

[0088] 4) The X-ray diffraction experimental results of different LDH@BCF are as Figure 5As shown, where ☆ represents Ca3(PO4)2F, ★ represents KCl, ! represents SiO2, and ▽ represents CaCO3. It can be seen from the XRD pattern that there are no other characteristic diffraction peaks in BCF except for a small amount of inherent elements Ca, K, Na, and Si formed during the high-temperature pyrolysis of biomass and Ca3(PO4)2. The main diffraction peaks of 2:1 LDH@BCF, 3:1 LDH@BCF, 4:1 LDH@BCF, and 5:1 LDH@BCF are CaCO3 (2θ = 28.9°, 39.3°), Ca3(PO4)2F (2θ = 25.8°, 31.8°, 46.6°), and SiO2 (2θ = 25.8°). In addition, KCl diffraction peaks (2θ = 28.2°, 40.6°) appear, which is due to the replacement of LDH interlayer anions by phosphate ions, indicating that LDH adsorbs phosphate ions through the interlayer structure, converting unstable and easily released phosphorus into stable phosphorus with better slow-release effect.

[0089] 5) To further clarify the phosphorus retention mechanism of the composite material as a slow-release fertilizer by the interlayer structure of LDH, XPS analysis was performed on different samples prepared in Example 1, and three characteristic peaks of C1s, Mg1s, and P2p were tested. The results are as Figure 6 shown. BCF and LDH@BCF are mainly composed of C, O, Ca, and P. Compared with Figure (6a), an obvious Mg1s signal appears after adding LDH. The spectrum of Mg1s can be divided into two peaks with binding energies of 1305.3 eV and 1304.5 eV, respectively, which belong to Mg-O and Mg-P. As the Mg / Al ratio increases, the area of the Mg-P peak increases. When Mg:Al is 4:1, the appropriate presence of Al 3+ ensures the positive charge distribution and structural integrity of the layer board, and sufficient Mg 2+ is exposed on the surface to combine with the phosphorus source to form Mg-P bonds, resulting in the largest area of the Mg-P peak. When Mg:Al is further increased to 5:1, Al 3+Too low content leads to charge imbalance in the lamellar, which may cause structural collapse or the formation of an amorphous phase. This reduces the effectively exposed Mg sites, and some Mg may be encapsulated in the disordered structure and unable to bind with P, thus resulting in a decrease in the Mg-P peak area. The C1s peak can be divided into C-C / C-Si (~284.8 eV), C=O (~285.5 eV), and O-C=O (288.2 eV), corresponding to the graphitized carbon skeleton, carboxyl group, and ester group, respectively. The intensity of the C=O peak in LDH@BCF is lower than that in the original BCF, indicating that a condensation reaction occurred between the interlayer hydroxyl group and the carboxyl group of LDH during the co-pyrolysis process[, reducing the competitive adsorption sites and enhancing the selective adsorption of phosphate. Analyzing the XPS spectra of P2p of BCF and different LDH@BCF shows two phosphorus compounds with binding energies of 133.3 eV and 134.4 eV, respectively, which are attributed to HPO4 2- and PO4 3- , after adding MgAl-LDH, the content of HPO4 2- decreases, and the content of PO4 3- increases. This is because some unstable P is adsorbed between the LDH layers to form more stable PO4 3- , which is consistent with the results of XRD. As the Mg / Al ratio increases, the interlayer spacing and electrical properties of LDH will change. The interlayer spacing and the electrical properties of LDH compounds are key features for anion adsorption. As the Mg / Al ratio increases, the LDH interlayer spacing expands, and the lamellar charge density decreases, further enhancing the interlayer fixation ability of PO4 3- . When Mg / Al = 4:1, the proportion of PO4 3- reaches the peak ( Figure 6 d), indicating that a high Mg ratio significantly improves the slow-release performance of phosphorus by optimizing the interlayer chemical environment. However, when the Mg / Al increases to 5:1, too low Al 3+ content may lead to charge imbalance in the lamellar: insufficient positive charge on the lamellar weakens the electrostatic adsorption ability for PO4 3- . At the same time, the "crosslinking" effect of Al 3+ weakens, and the lamellar may collapse or form an amorphous phase, resulting in a reduction in the interlayer space and hindering the embedding of PO4 3- ; meanwhile, the surface Mg 2+ is encapsulated in the disordered structure, reducing the effective binding sites, which leads to a decrease in the content of PO4 3- . The results of the XPS spectra show that the LDH layered structure provides more coordination anchoring sites for phosphates, reduces the competitive adsorption sites, enhances the selective adsorption of phosphates, and some unstable P is adsorbed between the LDH layers to form more stable PO4 3- .

[0090] 2. Performance experiments of layered double hydroxide-biochar composites

[0091] 1) Slow-release behavior of layered double hydroxide-biochar composite in water

[0092] The extraction of nutrients from slow-release fertilizers was carried out according to the method described in the "Standard for Slow-release Fertilizers (GB / T 23348-2009) of the People's Republic of China". Specifically, at 25 °C, 0.1 g of MgAl-LDH@BCF (accurate to 0.001 g) was weighed and placed in a small bag made of a nylon mesh with a pore size of 150 μm (100 mesh). The small bag was placed in a conical flask containing 50 mL of deionized water, covered and left standing. The sampling times were 24 h, 3 d, 5 d, 7 d, 10 d, 14 d, 21 d, 28 d, 42 d, and 56 d. After sampling, the content of phosphorus in the sample was determined by the ammonium molybdate spectrophotometric method.

[0093] To study the phosphorus release effect of different prepared composites at different times, the prepared samples were placed in static water for slow-release experiments. The cumulative phosphorus release amount of all samples increased with time. The phosphorus release amount was the largest on the first day and then gradually decreased. The maximum phosphorus amount released on the first day was mainly related to the water-soluble phosphorus in the slow-release phosphate fertilizer. Figure 7 a shows the slow-release performance of the composites prepared after adding LDH with different magnesium-aluminum molar ratios. Compared with BCF prepared without adding MgAl-LDH, adding MgAl-LDH could reduce the cumulative release rate and improve its slow-release performance. Among them, the 28-day cumulative release rate of BCF was 56.7%, while the 28-day cumulative release rates of LDH@BCF added with 2:1 LDH, 3:1 LDH, 4:1 LDH, and 5:1 LDH were 55.3%, 44.8%, 32.8%, and 48.3% respectively. At the same time, the prepared composites had a tendency of continuous phosphorus release. In addition, the slow-release performance of the composites prepared by adding different ratios (5-20 wt%) of MgAl-LDH during the co-pyrolysis process was further studied (as Figure 7 shown in b). It was also found that adding different ratios of MgAl-LDH could improve its slow-release performance. The 28-day cumulative release rates of LDH@BCF obtained with addition amounts of 5 wt%, 10 wt%, 15 wt%, and 20 wt% were 45.2%, 51.4%, 32.8%, and 50.5% respectively.

[0094] The effect of pyrolysis temperature on the slow-release performance of LDH-modified biochar slow-release phosphate fertilizer was further studied (as Figure 7As shown in c), when the sustained-release time was 28 d, the cumulative release rates of LDH@BCF prepared at pyrolysis temperatures of 500 °C, 550 °C, and 600 °C were 48.2%, 41.6%, and 54.66% respectively. Compared with LDH@BCF prepared at 400 °C, the cumulative release rate was 32.8%. Some studies have shown that at higher temperatures (above 500 °C), magnesium-aluminum layered double hydroxides will decompose to form corresponding metal oxides, mainly MgO and Al2O3. The destruction of the LDH layered structure affects the slow-release performance of biochar slow-release phosphate fertilizers. The static water slow-release results of this application show that the layered structure of LDH can improve the phosphorus-holding ability of biochar slow-release phosphate fertilizers and enhance their slow-release performance.

[0095] 2) Slow-release kinetics of layered double hydroxide-biochar composites in water

[0096] The slow-release kinetics of the prepared Mg / Al-LDH@BCF was analyzed using kinetic models, and the equations are as follows:

[0097] Zero-order kinetic model M t = a + bt (1)

[0098] First-order kinetic model

[0099] Ritger-pepass model

[0100] Higuchi model

[0101] Elovich model M t = a - b ln(t) (5)

[0102] Parabolic diffusion model

[0103] In the formula, M t is the phosphorus cumulatively released at different time intervals, M ∞ is the maximum released phosphorus. M t / M ∞ is the cumulative release rate of phosphorus at different time intervals, k1, k2, and k3 are the release rate constants calculated by the model, n is the release exponent; a is the initial release amount of phosphorus; b is the phosphorus release constant.

[0104] To explore the slow-release mechanisms of the prepared BCF and LDH@BCF, the release kinetics of phosphorus in BCF and LDH@BCF were studied. Six commonly used mathematical models (zero-order kinetic model, first-order kinetic model, Ritger-Peppas model, Higuchi model, Elovich model, and Parabolic diffusion model) were used to describe the slow-release kinetics of phosphorus in BCF and different LDH@BCF ( Figure 8 ). Through the fitting study of the above models (as shown in Tables 2, 3, and 4), the first-order kinetic model was more in line with the release behavior of phosphorus in water, and the correlation coefficient R 2 was greater than 0.95. It indicates that the release mechanism of phosphorus in the composite material is mainly controlled by diffusion. During the slow-release process, as time goes by, the phosphorus concentration near the surface of the composite material gradually decreases, resulting in a gradual slowdown in the diffusion rate of phosphorus ions to the external environment. When the phosphorus concentration around the composite material particles reaches equilibrium, further release will depend on the migration of phosphorus in the internal structure of the composite material, and this migration rate is usually constant, so it conforms to the kinetic characteristics of the first-order reaction.

[0105] Table 2 Slow-release kinetic fitting models of LDH@BCF prepared from BCF and LDH with different Mg / Al ratios

[0106]

[0107] Table 3 Slow-release kinetic fitting models of LDH@BCF prepared from BCF and LDH with different added LDH ratios

[0108]

[0109]

[0110] Table 4 Slow-release kinetic fitting models of LDH@BCF prepared from BCF and different pyrolysis temperatures

[0111]

[0112] 3) Influence of the LDH layered structure on the slow-release performance of the composite material

[0113] Sequential extraction of phosphorus is a method used to study the distribution of different forms of phosphorus in soil or sediment and its transformation under different environmental conditions. The morphological distribution of phosphorus affects the slow-release performance.

[0114] Sequential fractionation of MgAl-LDH@BCF was carried out according to the soil phosphorus classification method. This method can effectively distinguish inorganic phosphorus and organic phosphorus in MgAl-LDH. Among them, P is divided into five categories: H2O-P (water-soluble P), NaHCO3-P (P extracted by NaHCO3), NaOH-P (P extracted by NaOH), HCl-P (P extracted by HCl), and Res-P (residual P). H2O-P (water-soluble phosphorus): represents the free phosphorus that can be rapidly released, directly reflecting the initial release potential of fertilizers; NaHCO3-P (weakly bound phosphorus): represents the phosphorus adsorbed on the surface or interlayer domain of the material through electrostatic interaction, which is significantly affected by environmental pH and ionic strength; NaOH-P (Fe / Al-bound phosphorus): phosphorus fixed through coordination bonds or coprecipitation, with medium stability; HCl-P (Ca-bound phosphorus): stable apatite phases formed by binding with Ca 2+ to form (such as Ca3(PO4)2F), and its release is controlled by mineral dissolution kinetics; Res-P (residual phosphorus): refers to the phosphorus embedded in the carbon matrix lattice or forming insoluble compounds, which requires strong oxidation conditions to be released.

[0115] Water-soluble phosphorus and phosphorus extracted by sodium bicarbonate are short-term available to plants and are easily directly absorbed by plants. Phosphorus bound to iron and aluminum oxides and insoluble calcium and magnesium phosphate compounds can be released under specific environmental conditions, providing a long-term phosphorus source for plants. NaHCO3-P is the phosphorus extracted by sodium bicarbonate. Although it is relatively unstable, its effectiveness is high and it can be rapidly utilized by plants. The contents of active phosphorus and unstable phosphorus in BCF account for 35.7% and 27.8% of the total phosphorus content respectively. Phosphorus that can be utilized by plants in the medium and long term accounts for 27.0%, and residual phosphorus accounts for 9.4%. Residual phosphorus includes some phosphorus-containing minerals and stable organic phosphorus. Phosphorus in these forms cannot be directly absorbed by plants and usually needs to be converted into an available form through microbial action or other chemical processes. Such as Figure 9As shown in a, adding MgAl-LDH with different Mg / Al ratios during pyrolysis can reduce the contents of available phosphorus and labile phosphorus, and increase the phosphorus that can be utilized by plants in the medium and long term. The contents of available phosphorus and labile phosphorus in 4:1 LDH@BCF account for 29.6% and 17.4% of the total phosphorus content, respectively. The phosphorus that can be utilized by plants in the medium and long term accounts for 44.1%, and the residual phosphorus accounts for 8.7%. This may be because LDH has excellent anion exchange ability and can adsorb phosphates through mechanisms such as electrostatic attraction, ligand exchange, and inner-sphere complex formation. During co-pyrolysis, MgAl-LDH can adsorb available phosphorus (such as water-soluble phosphorus) and labile phosphorus (such as sodium bicarbonate-extractable phosphorus) in sediments, thereby reducing their contents. At the same time, the metal cations in MgAl-LDH can react with phosphates to form insoluble phosphate precipitates, and MgAl-LDH may transfer phosphates from the active or labile form to the more stable sodium hydroxide-extractable or hydrochloric acid-extractable form through its interlayer anion exchange effect.

[0116] As Figure 9 shown in b, with the increase of the addition amount of MgAl-LDH, the contents of available phosphorus and labile phosphorus decrease, and the contents of phosphorus in the more stable sodium hydroxide-extractable or hydrochloric acid-extractable form increase. At the same time, the transformation of phosphorus forms at different co-pyrolysis temperatures was studied. As Figure 9 shown in c, with the increase of pyrolysis temperature, the contents of available phosphorus and labile phosphorus decrease. When the pyrolysis temperature is 400 °C, the contents of available phosphorus and labile phosphorus are 37.6% and 21.4% respectively. When the pyrolysis temperature is 600 °C, the contents of available phosphorus and labile phosphorus are 12.1% and 9.9% respectively. However, the content of residual phosphorus also increases, from 10.8% at 400 °C to 37.3% at 600 °C, which is not very conducive to the utilization of phosphorus by plants.

[0117] 4) Pot experiment

[0118] To verify whether the prepared LDH@BCF can effectively promote plant growth and improve nutrient use efficiency, a pot experiment was carried out using pepper plants as experimental plants. Specifically: 1 g of different slow-release fertilizers was added to 100 g of soil (from Lintong, Xi'an, Shaanxi). Subsequently, 6 pepper seeds of similar size were sown in each flower pot (40% humidity) and germinated at room temperature. After the plants grew for 30 days, the roots and seedlings were carefully rinsed with distilled water, dried with absorbent paper, and then their biological characteristics were measured.

[0119] The experimental results are as Figure 10As shown, the growth of pepper plants treated with LDH@BCF is better than that of pepper plants treated with BCF and commercial phosphate fertilizers. The average plant heights of pepper plants treated with commercial phosphate fertilizers (produced by Lvpan Environmental Protection Technology Co., Ltd., batch number 100022459224), BCF, 2:1 LDH@BCF, 3:1 LDH@BCF, 4:1 LDH@BCF, and 5:1 LDH@BCF reached 6.12 cm, 6.34 cm, 7.71 cm, 7.58 cm, 7.73 cm, and 7.53 cm, respectively. According to Figure 6 the nutrient slow-release results shown, it can be reasonably inferred that the prepared LDH@BCF has better nutrient slow-release performance and can provide more sufficient nutrients for the growth of pepper plants.

[0120] Figure 11 a shows the germination rates of pepper seeds in pots after different fertilizer treatments. Among them, the germination rates after 4:1 LDH@BCF treatment were increased by 16.7% and 11.1% compared with those after commercial phosphate fertilizer and BCF treatments, respectively. Figure 11 c shows the average fresh weight and dry weight of pepper plants after 30 days of cultivation with different fertilizers. The results show that the average fresh weight and dry weight of pepper plants after LDH@BCF treatment are higher than those of pepper plants after commercial phosphate fertilizer and BCF treatments. Among them, the dry weight and fresh weight of pepper plants after 4:1 LDH@BCF treatment were increased by 44.4%, 31.1% and 60.9%, 37.5% compared with commercial phosphate fertilizer and BCF, respectively, which also confirmed that the promotion effect of applying LDH@BCF on the growth of pepper plants is better than that of commercial phosphate fertilizer and BCF.

[0121] Finally, the physical and chemical properties of the soil after different fertilizer treatments were measured to further study the effects of LDH@BCF application on plants and soil. First, the pH and electrical conductivity of the soil in different treatment groups were measured. As Figure 12 shown in a, there is little difference in the pH of the soil among different fertilizer treatments, indicating that the addition of MgAl-LDH has less impact on the pH of the soil than BCF. The electrical conductivities of the soils treated with BCF and LDH@BCF are both higher than that of the soil treated with commercial phosphate fertilizer. This may be because when biochar is added to the soil, it can release a certain amount of electrolyte ions, thereby increasing the electrical conductivity of the soil. The dry matter content and moisture of the soil after different fertilizer treatments were measured. As Figure 12 shown in b, the results show that adding LDH@BCF to the soil can increase the soil moisture content compared with adding BCF. This may be because the addition of layered MgAl-LDH increases the pores of the LDH@BCF biochar slow-release phosphate fertilizer, improves the soil aggregate structure, and enhances the water retention capacity of the soil. At the same time, the available phosphorus content in the soil of different treatment groups was measured. As Figure 12As shown in c, the results indicate that the available phosphorus content in the soil treated with BCF and LDH@BCF is higher than that of commercial phosphate fertilizers. A significant amount of commercial phosphate fertilizers is lost in the soil. BCF and LDH@BCF reduce phosphorus loss due to their high phosphorus retention capacity.

[0122] In summary, in this study, a novel biochar slow-release phosphate fertilizer with good slow-release performance was developed by adding magnesium-aluminum layered double hydroxide (MgAl-LDH) during the co-pyrolysis of biomass and phosphate fertilizer. The results show that the presence of MgAl-LDH effectively improves the pore structure of the biochar slow-release phosphate fertilizer, increasing its BET specific surface area by 2.48 to 10.94 times. During the 56-day static slow-release experiment, the cumulative release rate of phosphorus in LDH@BCF is lower than that in BCF. In particular, the leaching rate of phosphorus in 4:1 LDH@BCF is 36.8%, lower than 66.5% of BCF. The prepared LDH@BCF has excellent slow-release performance, mainly attributed to the adsorption of phosphorus by the porous structure and adsorption capacity of the biochar itself and the interlayer adsorption of phosphates by MgAl-LDH, enabling the MgAl-LDH-modified biochar slow-release phosphate fertilizer to provide stable and continuous nutrient supply throughout the growth cycle of crops, thus significantly improving the fertilizer utilization rate and reducing nutrient loss and environmental pollution.

[0123] The above are only several embodiments of this application and do not impose any form of limitation on this application. Although this application is disclosed with preferred embodiments as above, it is not intended to limit this application. Any person skilled in the art, without departing from the scope of the technical solution of this application, makes some changes or modifications using the disclosed technical content, which are equivalent to equivalent implementation cases and all fall within the scope of the technical solution.

Claims

1. Layered double hydroxide-biochar composite material, characterized in that, It includes a biochar matrix, on which layered double hydroxides are loaded; Phosphates are loaded between the layers of the layered double hydroxides; The phosphates are strong acid-extractable phosphates or strong base-extractable phosphates.

2. The composite material according to claim 1, wherein The divalent metal cation in the layered double hydroxide is Mg 2+ , and the trivalent metal cation is Al 3+ ; In the layered double hydroxides, the molar ratio of Mg to Al is 2-5:

1.

3. The composite material according to claim 1, characterized in that, In the composite material, the content of the layered double hydroxides is 4-14 wt%.

4. The composite material according to claim 1, characterized in that, In the composite material, the content of the phosphates is 14-16 wt%.

5. The preparation method of the layered double hydroxide-biochar composite material according to any one of claims 1-4, characterized in that, The method at least includes the following steps: Obtain layered double hydroxides; Co-pyrolyze a biochar raw material, a calcium source, a phosphate source, and the layered double hydroxides under a protective atmosphere to obtain the layered double hydroxide-biochar composite material.

6. The preparation method according to claim 5, characterized in that, The biochar raw material uses crop straws; The phosphate source is selected from at least one of Ca3(PO3)2, Ca(H2PO4)2, KH2PO4, K3PO4, (NH4)3PO4, and NH4H2PO4.

7. The preparation method according to claim 5, characterized in that, The addition amount of the layered double hydroxides is 5-20% of the mass of the biochar raw material; The addition amount of the phosphate source is 15-25% of the mass of the biochar raw material.

8. The preparation method according to claim 5, characterized in that, The conditions for the co-pyrolysis reaction are as follows: The heating temperature is 400-600 °C, the heating time is 1.5-4 h, and the heating rate is 8-20 °C / min.

9. The preparation method according to claim 5, wherein The flow rate of the protective atmosphere introduced is 80-200 mL / min.

10. Use of the layered double hydroxide-biochar composite material according to any one of claims 1-4 or the layered double hydroxide-biochar composite material prepared by the method according to any one of claims 5-9 as a slow-release fertilizer.

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