Layered double hydroxide-biochar composite material, method of preparation and use thereof

Abstract

The application discloses layered double hydroxide-biochar composite material and 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, and a layered double hydroxide is loaded on the biochar matrix; phosphate is loaded between layers of the layered double hydroxide; and the phosphate is strong-acid-extractable phosphate or strong-alkali-extractable phosphate. The application also discloses a preparation method of the composite material and application of the composite material as slow-release fertilizer. The layered double hydroxide-biochar composite material has good phosphorus retention capacity, 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 interlayer adsorption capacity of the phosphate, thereby improving the slow-release performance of the phosphate.

Description

TECHNICAL FIELD

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

[0002] Under the grim situation of global phosphorus resources becoming increasingly scarce and the average utilization rate of phosphorus fertilizer being only 12.6%, developing environment-friendly and efficient slow-release phosphorus fertilizer has become a key research topic for sustainable agricultural development. Biochar slow-release phosphorus fertilizer has unique advantages, which can not only significantly improve the utilization rate of phosphorus and effectively reduce the loss of phosphorus, but also convert agricultural waste into high-value fertilizer, realize the recycling of resources, and open up a new path for the green development of agriculture. The porous structure and strong adsorption capacity of biochar make it a natural carrier of nutrients, which can precisely control the release rate of nutrients and ensure that crops obtain stable and continuous nutrient supply during the whole growth period, thereby significantly improving the utilization rate of fertilizer 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 improving soil fertility and biodiversity, and providing strong support for the stable development of agricultural ecosystems.

[0003] However, the surface of biochar is usually negatively charged, which can affect its adsorption and retention capacity for anionic nutrients such as phosphates. Due to the presence of electrostatic repulsion, anionic nutrients are difficult to firmly adhere to the surface of biochar, resulting in too fast release of nutrients in the soil and failing to achieve the desired slow-release effect. In this regard, Yao Y et al. found that the negative charge on the surface of biochar affects its adsorption capacity for phosphates, and iron modification can reduce this effect (YAO Y, GAO B, IN YANG 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 effect on anionic and cationic heavy metals (LI Y, WANG S, OUYANG X F, 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 relative limited density of surface functional groups of biochar limits its binding capacity with nutrients, these functional groups are the key sites for chemical reaction and physical adsorption between biochar and nutrients, but the number and types of surface functional groups are significantly affected by the precursor 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 density of surface functional groups is insufficient, the adsorption and fixation capacity of biochar for nutrients will be limited, which will affect the slow-release effect and stability of fertilizer. These limiting factors to some extent reduce the potential of biochar in the field of environmental application. Therefore, in order to improve the stability and reliability of the original biochar, some studies have carried out research on the preparation of engineered / modification biochar composite materials.

[0004] Layered double hydroxides (LDH) composite materials are composed of positively charged brucite-type mixed metal (including divalent and trivalent metal) hydroxide host layers. LDH has a large specific surface area, can effectively exchange anions, and at the same time has the characteristics of non-toxicity, and is used to improve biochar slow-release fertilizer. The general chemical structure of LDH can be represented as [M 2+ 1-x M 3+ x (OH)2] x+ (A n- ) x / n ·yH2O, wherein 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.), A n- is a 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. functionalized coffee grounds waste biochar with MgAl layered double hydroxides 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, the present application aims to develop a new type of biochar slow-release fertilizer, which can not only improve the slow-release performance of biochar slow-release fertilizer, but also improve soil quality. SUMMARY

[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 phosphate, the sustained release effect of phosphorus of the composite material is improved.

[0007] The layered double hydroxide-biochar composite material comprises a biochar matrix, on which a layered double hydroxide is loaded;

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

[0009] The phosphate is a strong acid extractable phosphate or a strong alkali extractable phosphate.

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

[0011] In the layered double hydroxide, the molar ratio of Mg to Al is 2 to 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 therebetween. Preferably, the molar ratio of Mg to Al is 4:1.

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

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

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

[0016] obtaining a layered double hydroxide;

[0017] The biochar raw material, a calcium source, a phosphate source and the layered double hydroxide are subjected to a co-pyrolysis reaction under a protective atmosphere to obtain the layered double hydroxide-biochar composite material.

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

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

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

[0021] In the present application, the biochar raw material can be selected from corn stalks, sorghum stalks, rice straw and other common crop stalks, and in specific use, the powder of crop stalks is used.

[0022] Optionally, the layered double hydroxide is added in an amount of 5-20% of the mass of the biochar raw material.

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

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

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

[0026] Preferably, the phosphate source is added in an amount of 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℃, the heating time is 1.5-4h, and the heating rate is 8-20℃ / min.

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

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

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

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

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

[0034] Specifically, the lower limit of the flow 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; and the upper limit of the flow 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 flow rate of the protective atmosphere is 150 mL / min.

[0035] Specifically, the protective atmosphere can be nitrogen, argon or other inert atmosphere, and those skilled in the art can select the protective atmosphere according to the actual production situation. Nitrogen is preferably used as the protective atmosphere in the present application.

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

[0037] The beneficial effects that can be produced by the present application include:

[0038] 1) The layered double hydroxide-biochar composite material has good phosphorus retention capacity. MgAl-LDH not only increases the pore structure and specific surface area of biochar, but also makes the phosphate exist in a stable form in the composite material through the interlayer adsorption capacity of MgAl-LDH for phosphate, thereby 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 phosphorus fertilizer can increase the long-term available phosphorus for plants, improve the nutrient use efficiency of plants, and better promote plant growth compared to commercial phosphorus fertilizers; and can improve the soil conductivity, improve the soil aggregate structure, and improve the water retention capacity of the soil.

[0040] 3) The application adds phosphate into the biochar and layered double hydroxide composite material by means of co-pyrolysis. In the co-pyrolysis process, MgAl-LDH can adsorb active phosphorus (such as water-soluble phosphorus) and unstable phosphorus (such as sodium bicarbonate extractable phosphorus) in the sediment, thereby reducing their content. At the same time, the metal cations in MgAl-LDH can react with the phosphate to generate insoluble phosphate precipitates, and MgAl-LDH can transfer the phosphate from the active or unstable form to the more stable sodium hydroxide extractable form or hydrochloric acid extractable form through its interlayer anion exchange, thereby improving the slow-release rate of phosphorus and increasing the plant available phosphorus in the composite material. BRIEF DESCRIPTION OF DRAWINGS

[0041] Figure 1 SEM and EDS images of the samples obtained in Example 1 of the application, wherein Figs. a-e are SEM images of the sample BCF, the sample 2:1 LDH@BCF, the sample 3:1 LDH@BCF, the sample 4:1 LDH@BCF, and the sample 5:1 LDH@BCF, respectively, and Fig. f is an EDS image of 4:1 LDH@BCF;

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

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

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

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

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

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

[0048] Figure 8 Results of slow release kinetics experiment in water for different samples in the embodiments of the present application;

[0049] Figure 9 Results of phosphorus sequential extraction experiment for samples in the embodiments of the present application;

[0050] Figure 10 Photos of the effects of the phosphorus fertilizer and the sample obtained in Embodiment 1 of the present application on the growth of peppers, wherein Fig. a is the potting growth condition, and Fig. b is the potting seedling growth condition;

[0051] Figure 11 Photos of the changes in the indicators of the potting plants after different fertilizer treatments, wherein Fig. a is the effect result of the pepper seed germination rate, Fig. b is the change condition of the pepper plant height, and Fig. c is the average fresh weight and dry weight results of the pepper plants after 30 days of culture;

[0052] Figure 12 Photos of the changes in the physical and chemical indicators of the soil after different fertilizer treatments, wherein Fig. a is the change condition of the soil pH and conductivity, Fig. b is the change condition of the dry matter content and moisture of the soil, and Fig. c is the change condition of the available phosphorus content of the soil. DETAILED DESCRIPTION

[0053] The present application will be described in detail below with reference to 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 purchased through commercial channels, wherein the 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 Science and Technology Co., Ltd., and all reagents are used as received without further purification. Deionized water (Shanghai Haitai Instrument Co., Ltd., Master-S15) is used in all experiments, and corn stalks are purchased from a farm in Xi'an, Shaanxi Province. Before use, the corn stalks are cut into small pieces of <5 cm, washed several times with deionized water, dried in an 80°C oven overnight, then pulverized with a high-speed rotary cutter, sieved through an 80-mesh sieve, and stored in a sealed container.

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

[0056] Step 1, obtaining a layered double hydroxide

[0057] The metal elements in the layered double hydroxide used 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 MgAl-LDH is prepared by a coprecipitation method in the present application:

[0058] According to Mg 2+ The molar ratio of Mg 3+ and AlCl3 was 2-5:1. A certain amount of MgCl2·6H2O and AlCl3 was weighed and dissolved in pure water, wherein the concentration of Mg 2+ in the system was 0.2-0.5 M, stirring at a speed of 800 rpm / min at room temperature, adjusting the pH of the system to 10±0.1 using 1 M NaOH and 0.5 M NaHCO3, stirring for 4 h, after aging at room temperature for 18 h, collecting the sample, and drying the washed sample in an oven at 80°C for 12 hours to obtain dry MgAl-LDH powder.

[0059] Step 2, according to the amount of layered double hydroxide added to the mass of the biochar raw material is 5-20%, the amount of phosphate source added to the mass of the biochar raw material is 15-25%, respectively, layered double hydroxide MgAl-LDH, phosphate source and biochar raw material are weighed, co-pyrolysis in a protective atmosphere at 400-600°C for 1.5-4 h, to obtain layered double hydroxide-biochar composite material. The heating rate is controlled to be 10-20°C / min during co-pyrolysis, and the flow rate of the protective atmosphere is 80-200 mL / min.

[0060] Example 1

[0061] MgAl-LDH, Ca3(PO3)2 and corn straw powder were weighed and co-pyrolyzed in a tubular furnace (Anhui Kepu Instrument Co., Ltd., TFH-1200-100-440), wherein the amount of MgAl-LDH added was 15% of the mass of the corn straw powder, the amount of Ca3(PO3)2 added was 20% of the mass of the corn straw powder, the N2 flow rate was controlled to be 150 mL / min at 400°C, the heating rate was 10°C / min, and heating was performed 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, respectively, denoted as 2:1 LDH@BCF, 3:1 LDH@BCF, 4:1 LDH@BCF and 5:1 LDH@BCF.

[0062] In addition, the sample obtained by heating corn straw powder alone according to the above pyrolysis conditions is denoted as BCF.

[0063] Example 2

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

[0065] Example 3

[0066] 4:1 LDH@BCF were prepared according to the method of Example 1, except that the added amounts of MgAl-LDH were 5%, 10%, and 20% of the mass of the corn straw powder, respectively.

[0067] Example 4

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

[0069] Example 5

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

[0071] Example 6

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

[0073] Comparative Example 1

[0074] Weigh MgAl-LDH (Mg 2+ With Al 3+MgAl-LDH (molar ratio of Mg:Al = 4:1) and corn stalk powder were added into a tube furnace, the amount of MgAl-LDH added was 15% of the mass of corn stalk powder, the N2 flow rate was controlled at 150 mL / min at 400°C, the heating rate was 10°C / min, and heating was performed for 2 h to obtain a phosphate-free co-thermal composite material; the phosphate-free co-thermal composite material and Ca3(PO3)2 added in an amount of 15% of the mass of corn stalk powder were mixed to obtain P-LDH@BCF. Through a static water release experiment, the release rate of phosphorus of P-LDH@BCF within 24 h was 60%, and the cumulative release rate of phosphorus within 10 d was nearly 80%, and the slow-release effect was poor.

[0075] Characterization and performance experiment verification of the layered double hydroxide-biochar composite material

[0076] 1. The layered double hydroxide-biochar composite material prepared in Examples 1-6 was characterized, and the sample obtained in Example 1 was typically described in detail.

[0077] The characterization methods used in the present application include: Cu Kα radiation on a Rigaku Miniflex 600 diffractometer (Rigaku, Japan) at a scanning rate of 10.0° / min in the 2θ range of 7-70° to record the XRD pattern. 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) at a working voltage of 10.0 kV. The surface microstructure of LDH@BC was observed using a SEM electron microscope.

[0079] The FTIR spectrum was obtained 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 size, and N2 adsorption-desorption curve of the biochar slow-release phosphorus fertilizer were determined using a TriStar IIPlus high-throughput surface area and porosity analyzer.

[0082] 1) The scanning electron micrographs of BCF and LDH@BCF are shown in Figure 1 The sample prepared without adding MgAl-LDH has a relatively regular pore structure, indicating that the addition of a calcium and phosphorus source to the co-thermal pyrolysis promotes the formation of the biochar structure, but there is a collapsed structure Figure 1a) The presence of sheet-like structures on the sample prepared after the addition of MgAl-LDH indicates that MgAl-LDH was successfully loaded onto the biochar during the co-pyrolysis process Figure 1 b, 1c) At the same time, after the addition of MgAl-LDH, it can be observed that the roughness is reduced and the pore structure becomes more compact and uniform Figure 1 d, 1e), which indicates that the magnesium-aluminum interlayer structure of LDH helps the biomass to form a more uniform pore structure during pyrolysis. The EDS map shows that Mg, Al and P elements are successfully loaded onto the biochar material, further confirming that the LDH is 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 the addition of MgAl-LDH, which was caused by the alkaline environment during the preparation of MgAl-LDH; the conductivity of BCF was 198, while the conductivity of samples added with LDH with different magnesium-aluminum ratios (Mg 2+ The molar ratio increased) was 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 increases, which means that the soluble nutrients and trace elements in the composite material as a fertilizer increase, thereby improving the fertilizer efficiency of the fertilizer.

[0084] 2) The structural properties of BCF and different LDH@BCF samples were analyzed by N2 adsorption-desorption method. The N2 adsorption-desorption isotherms and pore size distribution curves are shown in Figure 2 , Figure 3 The corresponding pore structure parameters, including BET specific surface area, total pore volume and average pore size, are shown in Table 1. It can be seen that after the addition of MgAl-LDH, the BET specific surface area and total pore volume of the material increase, and the BET specific surface area and total pore volume of the LDH@BCF with a magnesium-aluminum ratio of 4:1 are the largest, indicating that it has the optimal pore structure, because the layered double hydroxide (LDH) forms pores through gas release during the pyrolysis process, and the metal oxides in the interlayer structure template to expand the specific surface area and pore volume. At the same time, the layered structure of LDH hinders the densification of biochar precursors during pyrolysis, preventing excessive shrinkage of the carbon structure, thereby retaining more pores, cross-linking reactions between the abundant hydroxyl groups (-OH) on the surface of LDH and the oxygen-containing functional groups (such as carboxyl and phenolic hydroxyl) in the biochar, forming a stable three-dimensional network structure, promoting pore formation, and significantly optimizing the pore structure of the biochar. Among them, the interlayer structure composed of MgAL-LDH with a magnesium-aluminum ratio of 4:1 balances the pore-forming ability of MgO and the structural stability of Al2O3, achieving the optimal pore distribution, 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) FTIR spectra of different LDH@BCF Figure 4 As shown, at a wavelength of 3440 cm -1 There is a stretching vibration absorption peak of hydroxyl group at the wavelength of 563cm. The strong hydroxyl peak indicates that there may be more associated hydroxyl groups. This is due to the incomplete removal of water and the formation of hydrogen bonds between the hydroxyl groups in the layered double hydroxide and the surrounding molecules. The absorption peak of carbonate apatite is at a wavelength of 563cm -1 This indicates that the addition of calcium phosphate forms carbonate apatite, which promotes the enhancement of the sustained release performance of the material. In the sample after adding MgAL-LDH, the wavelength is 875cm -1 The absorption peak at 765 cm-1 is likely related to the vibration of the PO-Mg bond, indicating the binding mechanism between magnesium and phosphate in the layered bimetallic oxide (LDH). The absorption peak at 765 cm-1, likely related to the vibration of the PO bond, increases with the addition of LDH, suggesting that the addition of LDH during the co-pyrolysis process increases the number of phosphorus-oxygen bonds. An absorption peak at 1050 cm-1 is associated with the vibration of phosphate ions, indicating the presence of phosphate groups in the sample. With increasing magnesium-aluminum ratios, the intensity of the absorption peak at 1050 cm-1 decreases, indicating a decrease in the number of phosphate ions in the sample. This suggests that some phosphate ions are adsorbed between the LDH layers. However, an excessively high Mg / Al ratio may reduce the interlayer spacing and alter the interlayer environment, thereby reducing the number of available adsorption sites for phosphate ions and reducing the number of phosphate ions adsorbed between the LDH layers. Infrared spectroscopy results indicate that the interlayer structure of the LDH can adsorb phosphate, which is beneficial for the slow release of nutrients from biochar-based slow-release phosphate fertilizers.

[0088] 4) X-ray diffraction experimental results of different LDH@BCF are as follows Figure 5indicated, where ☆ represents Ca3(PO4)2F, ★ represents KCl,! represents SiO2, and ▽ represents CaCO3. As can be seen from the XRD pattern, in addition to the small amount of inherent elements Ca, K, Na, and Si formed in the high-temperature pyrolysis process of biomass and Ca3(PO4)2, there are no other characteristic diffraction peaks in the BCF. 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 to which, KCl diffraction peaks (2θ = 28.2°, 40.6°) also appear, which is due to the replacement of LDH interlayer anions and phosphate ions, which also indicates that LDH adsorbs phosphate ions through the interlayer structure, and converts unstable and easily released phosphorus into stable and slow-release phosphorus.

[0089] 5) In order to further clarify the mechanism of the interlayer structure of LDH in the slow-release of the composite material as a slow-release fertilizer, 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 shown in Figure 6 As shown, BCF and LDH@BCF are mainly composed of C, O, Ca, and P. Compared with Figure (6a), a relatively obvious Mg1s signal appears after the addition of LDH, and the Mg1s spectrum 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, respectively. With the increase of the magnesium aluminum ratio, the Mg-P peak area increases, and when the Mg:Al is 4:1, Al 3+ The appropriate existence of Mg 2+ is exposed on the surface and combines with the phosphorus source to form Mg-P bonds, resulting in the largest Mg-P peak area. When the Mg:Al is further increased to 5:1, Al 3+The low content of Mg can cause the imbalance of the layer charge, which may lead to the collapse of the structure or the formation of amorphous phase. This reduces the effective exposure of Mg sites, and part of Mg may be wrapped in disordered structures, which cannot be combined with P, 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), which correspond to the graphitized carbon skeleton, carboxyl, and ester group, respectively. The C=O peak intensity in LDH@BCF is lower than that in the original BCF, indicating that the condensation reaction between the interlayer hydroxyl group of LDH and the carboxyl group occurs during the co-pyrolysis process, which reduces the competitive adsorption sites and enhances the selective adsorption of phosphates. Analysis of the P2p XPS spectrum 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- , respectively. After the addition of MgAl-LDH, the content of HPO4 2- decreases, and the content of PO4 3- increases, because part of the unstable P is adsorbed by the interlayer of LDH to form more stable PO4 3- , which is consistent with the XRD results. With the increase of the magnesium aluminum ratio, the interlayer distance and electrical properties of LDH change, and the interlayer distance and electrical properties of LDH compounds are key features of anion adsorption. With the increase of the magnesium aluminum ratio, the interlayer distance of LDH expands, and the layer charge density decreases, further enhancing the interlayer fixation ability of PO4 3- . When Mg / Al = 4:1, the PO4 3- occupies a peak value ( Figure 6 d), indicating that a high magnesium ratio significantly improves the phosphorus slow-release performance by optimizing the interlayer chemical environment. However, when Mg / Al increases to 5:1, the Al 3+ content is too low, which may cause the imbalance of the layer charge: the positive charge of the layer is insufficient, which weakens the electrostatic adsorption ability of PO4 3- . At the same time, the "cross-linking" effect of Al 3+ is weakened, and the layer may collapse or form an amorphous phase, resulting in a decrease in the interlayer space and hindering the embedding of PO4 3- . At the same time, the surface Mg 2+ is wrapped in a disordered structure, and the effective binding site is reduced, which leads to a decrease in the PO4 3- content. The XPS spectrum results show that the layered structure of LDH provides more coordination anchoring sites for phosphates, reduces the competitive adsorption sites, and enhances the selective adsorption of phosphates. Part of the unstable P is adsorbed by the interlayer of LDH to form more stable PO4 3- .

[0090] 2. Performance experiment of layered double hydroxide-biochar composite material

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

[0092] The slow release fertilizer nutrient was extracted according to the method shown in the "Standard of Slow Release Fertilizer of the People's Republic of China (GB / T 23348-2009)". Specifically, 0.1 g of MgAl-LDH@BCF (weighed to 0.001 g) was placed in a small bag made of 150 μm (100 mesh) nylon gauze at 25°C. The small bag was placed in a conical flask containing 50 mL of deionized water, covered and placed, and the sampling time was 24 h, 3 d, 5 d, 7 d, 10 d, 14 d, 21 d, 28 d, 42 d, 56 d. After sampling, the content of phosphorus in the sample was determined by ammonium molybdate spectrophotometry.

[0093] In order to study the effect of different composites prepared at different times on the release of phosphorus, the prepared samples were placed in static water for slow release experiments. The cumulative release amount of phosphorus of all samples increased with time. The amount of phosphorus released was the largest on the first day, and then gradually decreased. The maximum amount of phosphorus released on the first day was mainly related to the water-soluble phosphorus in the slow-release phosphorus fertilizer. Figure 7 a shows the slow release performance of the composites prepared after adding LDH with different molar ratios of magnesium and aluminum. Compared with BCF prepared without adding MgAl-LDH, the addition of MgAl-LDH can reduce the cumulative release rate and improve the slow release performance. Among them, the cumulative release rate of BCF 28d is 56.7%, while the cumulative release rates of LDH@BCF added with 2:1 LDH, 3:1 LDH, 4:1 LDH and 5:1 LDH are 55.3%, 44.8%, 32.8% and 48.3%, respectively. At the same time, the prepared composites have a sustained release trend for phosphorus. In addition, the slow release performance of the composites prepared by adding different proportions (5-20wt%) of MgAl-LDH in the co-pyrolysis process was further studied (as shown in Figure 7 b), and it was found that the addition of different proportions of MgAl-LDH can improve the slow release performance. The cumulative release rates of LDH@BCF added with 5wt%, 10wt%, 15wt% and 20wt% MgAl-LDH are 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 phosphorus fertilizer was further studied (as shown in Figure 7The cumulative release rates of LDH@BCF prepared at 500℃, 550℃, 600℃ were 48.2%, 41.6%, 54.66% respectively, compared with 32.8% of LDH@BCF prepared at 400℃, when the release time was 28d. Studies have shown that at higher temperatures (above 500℃), magnesium aluminum layered double hydroxide will decompose to generate the corresponding metal oxide, mainly MgO and Al2O3. The destruction of the LDH layered structure affects the slow-release performance of the biochar slow-release phosphorus fertilizer. The static water release results of the present application show that the layered structure of LDH can improve the phosphorus retention capacity of the biochar slow-release phosphorus fertilizer and improve its slow-release performance.

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

[0096] The slow-release kinetics of the prepared Mg / Al-LDH@BCF was analyzed by using a kinetic model, and the equation is 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 cumulative release of phosphorus at different time intervals, M ∞ is the maximum release of phosphorus. M t / M ∞ is the cumulative release rate of phosphorus at different time intervals, k1, k2 and k3 are release rate constants calculated by the model, n is the release index; a is the initial release amount of phosphorus; b is the release constant of phosphorus.

[0104] To investigate the release mechanism of the prepared BCF and LDH@BCF, the release kinetics of phosphorus in BCF and LDH@BCF was studied. Six commonly used mathematical models (zero-order kinetics model, first-order kinetics model, Ritger-Peppas model, Higuchi model, Elovich model and Parabolic diffusion model) were used to describe the 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 kinetics model was more consistent with the release behavior of phosphorus in water, and the correlation coefficient R 2 was greater than 0.95. It showed that the release mechanism of phosphorus in the composite material was mainly controlled by diffusion. During the slow-release process, the phosphorus concentration near the surface of the composite material gradually decreased, resulting in a gradual slowdown of the diffusion speed of phosphorus ions to the external environment. When the phosphorus concentration around the composite material particles reached equilibrium, further release would depend on the migration of phosphorus in the internal structure of the composite material, and the migration speed was usually constant, so it was consistent with the kinetic characteristics of the first-order reaction.

[0105] Table 2 Release kinetics fitting model of BCF and LDH@BCF prepared by different MgAl ratio LDH

[0106]

[0107] Table 3 Release kinetics fitting model of BCF and LDH@BCF prepared by different LDH addition ratio

[0108]

[0109]

[0110] Table 4 Release kinetics fitting model of BCF and LDH@BCF prepared by different pyrolysis temperature

[0111]

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

[0113] Sequential extraction of phosphorus is a method for studying the distribution of different forms of phosphorus in soil or sediment and their transformation under different environmental conditions. The distribution of phosphorus forms affects the slow-release performance.

[0114] MgAl-LDH@BCF was sequentially fractionated 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 H2O-P (water-soluble P), NaHCO3-P (NaHCO3 extracted P), NaOH-P (NaOH extracted P), HCl-P (HCl extracted P) and Res-P (residual P) five categories. H2O-P (water-soluble phosphorus): represents the free state of phosphorus that can be quickly released, directly reflects the initial release potential of the fertilizer; NaHCO3-P (weakly bound state phosphorus): represents the phosphorus adsorbed on the material surface or interlayer domain through electrostatic interaction, which is significantly affected by environmental pH and ionic strength; NaOH-P (iron and aluminum combined state phosphorus): phosphorus fixed by coordination bond or coprecipitation, with moderate stability; HCl-P (calcium combined state phosphorus): combined with Ca 2+ Combined with the formation of stable apatite phase (such as Ca3(PO4)2F), its release is controlled by mineral dissolution kinetics; Res-P (residual state phosphorus): refers to the phosphorus embedded in the carbon matrix lattice or forming a difficult compound, which can be released under strong oxidation conditions.

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

[0116] As shown in Figure Figure 9 b, with the increase of the amount of MgAl-LDH added, the content of active phosphorus and unstable phosphorus decreases, and the content of more stable sodium hydroxide extractable or hydrochloric acid extractable phosphorus increases. The transformation of phosphorus forms at different co-pyrolysis temperatures was also studied, as shown in Figure Figure 9 c, with the increase of pyrolysis temperature, the content of active phosphorus and unstable phosphorus decreases. When the pyrolysis temperature is 400°C, the content of active phosphorus and unstable phosphorus is 37.6% and 21.4%, respectively. When the pyrolysis temperature is 600°C, the content of active phosphorus and unstable phosphorus is 12.1% and 9.9%, respectively. However, the content of residual phosphorus is also increasing, from 10.8% at 400°C to 37.3% at 600°C, which is not 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 conducted 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), then 6 pepper seeds of similar size were sown in each pot (40% humidity), and germination was carried out at room temperature. After 30 days of plant growth, the roots and seedlings were carefully rinsed with distilled water, dried with absorbent paper, and then the biological characteristics were measured.

[0119] The experimental results are shown in Figure Figure 10As shown, the growth of the LDH@BCF treated pepper plants was better than the BCF and commercial phosphate fertilizer treated pepper plants. The average plant height of the commercial phosphate fertilizer (produced by green 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 treated pepper plants 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 As shown in the nutrient release results, it can be reasonably inferred that the prepared LDH@BCF has better nutrient release performance and can provide more sufficient nutrients for the growth of pepper plants.

[0120] Figure 11 a shows the germination rate of pepper seeds in pots after different fertilizer treatments, wherein the germination rate after 4:1 LDH@BCF treatment is increased by 16.7% and 11.1% compared with the germination rate after commercial phosphate fertilizer and BCF treatment. Figure 11 c shows the average fresh weight and dry weight of the pepper plants treated with different fertilizers after 30 days of culture, and the results show that the average fresh weight and dry weight of the pepper plants treated with LDH@BCF are higher than those of the pepper plants treated with commercial phosphate fertilizer and BCF, wherein the dry weight and fresh weight of the pepper plants treated with 4:1 LDH@BCF are increased by 44.4%, 31.1% and 60.9%, 37.5% compared with those of the commercial phosphate fertilizer and BCF, which also confirms that the promotion effect of 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 determined to further study the effect of LDH@BCF application on plants and soil. First, the pH and conductivity of the soil in different treatment groups were determined, as shown in Figure 12 a shows that there is little difference in the pH of the soil between different fertilizer treatments, indicating that the addition of MgAl-LDH has less effect on the pH of the soil than BCF. The conductivity of the soil treated with BCF and LDH@BCF is higher than that of the soil treated with commercial phosphate fertilizer, which may be because when biochar is added to the soil, it can release a certain amount of electrolyte ions, thereby increasing the conductivity of the soil. The dry matter content and water content of the soil after different fertilizer treatments were determined, as shown in Figure 12 b shows that the addition of LDH@BCF to the soil can increase the water content of the soil compared with the addition of BCF, which may be because the addition of layered MgAl-LDH increases the porosity of LDH@BCF biochar slow-release phosphate fertilizer, improves the soil aggregate structure and improves the water retention capacity of the soil. The effective phosphorus content in the soil in different treatment groups was also determined, as shown in Figure 12As shown in Fig. c, the results show that the available phosphorus content of the soil treated by BCF and LDH@BCF is higher than that of the commercial phosphorus fertilizer, and the loss of the commercial phosphorus fertilizer in the soil is more. BCF and LDH@BCF reduce the loss of phosphorus due to their high phosphorus retention capacity.

[0122] In summary, in this study, by adding MgAl-layered double hydroxide (MgAl-LDH) in the process of co-pyrolysis of biomass and phosphorus fertilizer, a new type of biochar slow-release phosphorus fertilizer with good slow-release performance was developed. The results show that the presence of MgAl-LDH effectively improves the pore structure of the biochar slow-release phosphorus fertilizer, and the BET specific surface area is increased by 2.48 to 10.94 times. In the static water release experiment for 56 days, the cumulative release rate of phosphorus in LDH@BCF is lower than that in BCF. Especially, the leaching rate of phosphorus in 4:1 LDH@BCF is 36.8%, which is lower than 66.5% of BCF. The prepared LDH@BCF has excellent slow-release performance, which is mainly due to the adsorption of phosphorus elements by the porous structure and adsorption capacity of biochar itself and the interlayer adsorption of MgAl-LDH to phosphate. The biochar slow-release phosphorus fertilizer modified by MgAl-LDH can provide stable and sustained nutrient supply during the whole growth period of crops, thereby significantly improving the utilization rate of fertilizer, reducing nutrient loss and environmental pollution.

[0123] The above is only a few embodiments of the present application, and does not limit the present application in any form. Although the preferred embodiments are disclosed as above, the present application is not limited thereto. Any person skilled in the art can make some changes or modifications to the above disclosed technical contents without departing from the scope of the technical solutions of the present application, which are equivalent to equivalent embodiments, and all belong to the scope of the technical solutions.

Claims

1. A method for the preparation of layered double hydroxide-biochar composites, characterized by, The method comprises at least the following steps: obtaining a layered double hydroxide; co-pyrolyzing a biochar raw material, a calcium source, a phosphate source and the layered double hydroxide under a protective atmosphere to obtain the layered double hydroxide-biochar composite material; the biochar substrate is loaded with the layered double hydroxide; the layered double hydroxide is loaded with phosphate between layers; the phosphate is strong acid extractable phosphate or strong alkali extractable phosphate; The divalent metal cation in the layered double hydroxide is Mg 2+ The trivalent metal cation is Al 3+ ; the molar ratio of Mg to Al in the layered double hydroxide is 2-5:1; the content of the layered double hydroxide in the composite material is 4-14 wt%; the content of the phosphate in the composite material is 14-16 wt%.

2. The method of claim 1, wherein, the biochar raw material is crop straw; the phosphate source is at least one selected from Ca3(PO3)2, Ca(H2PO4)2, KH2PO4, K3PO4, (NH4)3PO4 and NH4H2PO4.

3. The preparation method according to claim 1, characterized in that the amount of the layered double hydroxide added is 5-20% of the mass of the biochar raw material; the amount of the phosphate source added is 15-25% of the mass of the biochar raw material.

4. The production method according to claim 1, characterized by, the co-pyrolysis reaction conditions are as follows: the heating temperature is 400-600℃, the heating time is 1.5-4h, and the heating rate is 8-20℃ / min.

5. The preparation method according to claim 1, characterized in that the flow rate of the protective atmosphere is 80-200mL / min.

6. The layered double hydroxide-biochar composite material prepared by the method of any one of claims 1-5 is used as a slow-release fertilizer.

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