A metal organic framework type salt-alkali soil conditioner and a preparation method thereof

Abstract

The application discloses a metal organic framework salt-alkali soil modifier and a preparation method thereof. The modifier is formed by bridging coordination of a metal cluster, inorganic ligand phosphoric acid, organic ligand and structure directing agent urea under hydrothermal conditions; the metal cluster is ferric chloride hexahydrate and / or zinc sulfate heptahydrate; the organic ligand is one or a combination of two of oxalic acid dihydrate, citric acid monohydrate and tartaric acid; and the molar ratio of the metal cluster, phosphoric acid, organic ligand, urea and deionized water in the hydrothermal reaction is 1-2:6:1-2:2.5-3:100. The modifier has the functions of improving salt-alkali soil and slow-release fertilizer, can provide sufficient nutrients for crops and significantly reduce the salt effect of the nutrients.

Description

Technical Field

[0001] This invention belongs to the field of soil conditioners, specifically relating to a metal-organic framework-based saline-alkali soil conditioner and its preparation method. Background Technology

[0002] Saline-alkali soil is formed when soluble salts accumulate in the topsoil due to the alternating influences of climate, topography, soil texture, groundwater level, and river / seawater, damaging the soil's physical, chemical, and biological properties and affecting crop growth. Maximizing the development and utilization of saline-alkali land resources is crucial for achieving sustainable agricultural development; therefore, improving saline-alkali land is of paramount importance.

[0003] The improvement of saline-alkali soil is a systematic project with a long cycle and significant challenges. Currently, the improvement and management of saline-alkali soil involves multiple approaches, including physical, chemical, hydraulic, and biological methods. Whether using irrigation facilities, applying organic fertilizers, or planting salt-tolerant crops, all methods suffer from slow results and high costs, hindering effective promotion. Chemical soil conditioners are an economical and practical method; they can retain water and fertilizer, improve soil physicochemical properties, and reduce salt ion aggregation, thus achieving a certain improvement effect. However, the effective improvement time of current conditioners is relatively short, and they may even pose potential hazards to soil health. Therefore, there is an urgent need to develop a new type of green soil conditioner.

[0004] Metal-organic frameworks (MOFs) are porous crystalline materials formed by the bridging coordination of inorganic metal ions or metal ion clusters with organic ligands. Due to their high specific area, diverse and tunable pore structures, abundant active sites, and ease of modification and functionalization, they have been widely used in gas storage and separation, catalysis, sensing, adsorption, and drug delivery. Previous reports have covered the application of MOFs in controlled-release fertilizer research. For example, Chinese patents ZL 202010424615.2 and ZL 202110166554.9 disclose novel MOF-based fertilizers and their preparation methods, as well as a method for solid-phase synthesis of MOF-based fertilizers. These synthesized fertilizers all exhibit good slow-release performance, but research reports on saline-alkali land improvement are lacking. In addition, the applicant previously prepared a novel Fe-MOF iron fertilizer using a hydrothermal synthesis method, but it had problems such as long reaction time (24h) and low nutrient content (containing about 5.1% N, about 14.7% P and about 19.7% Fe). Furthermore, the application did not involve research on the adsorption and improvement of saline-alkali soil. Summary of the Invention

[0005] The first objective of this invention is to provide a metal-organic framework-based soil conditioner for saline-alkali soils.

[0006] To achieve the above objectives, the present invention adopts the following technical solution:

[0007] A metal-organic framework-based soil conditioner for saline-alkali soil, wherein the conditioner is formed by bridging coordination of a metal cluster, an inorganic ligand phosphate, an organic ligand, and a structure-directing agent urea under hydrothermal conditions;

[0008] The metal cluster is ferric chloride hexahydrate and / or zinc sulfate heptahydrate; the organic ligand is one or a combination of two of oxalic acid dihydrate, citric acid monohydrate, and tartaric acid.

[0009] During the hydrothermal reaction, the molar ratio of metal cluster, phosphoric acid, organic ligand, urea and deionized water is 1-2:6:1-2:2.5-3:100.

[0010] In a preferred embodiment, the hydrothermal conditions are 110-120℃ for 18-20 h.

[0011] Furthermore, the hydrothermal conditions are a reaction at 120°C for 18 hours.

[0012] In a preferred embodiment, the metal cluster is ferric chloride hexahydrate or a combination of ferric chloride hexahydrate and zinc sulfate heptahydrate.

[0013] Furthermore, when the metal cluster is a combination of ferric chloride hexahydrate and zinc sulfate heptahydrate, the molar ratio of ferric chloride hexahydrate to zinc sulfate heptahydrate is 1:1; and the molar amounts of ferric chloride hexahydrate and zinc sulfate heptahydrate are equal to the molar amounts when ferric chloride hexahydrate is the only metal cluster.

[0014] In a preferred embodiment, the organic ligand is oxalic acid dihydrate, or a combination of oxalic acid dihydrate and citric acid monohydrate, or a combination of oxalic acid dihydrate and tartaric acid.

[0015] Furthermore, when the organic ligand is oxalic acid dihydrate and citric acid monohydrate, or a combination of oxalic acid dihydrate and tartaric acid, the molar ratio of the two organic acids is 1:1.

[0016] Furthermore, the molar amounts of each of the two organic acids are equal to the molar amounts when oxalic acid is used as the sole organic ligand.

[0017] In a preferred embodiment, the metal cluster is ferric chloride hexahydrate, and the organic ligand is oxalic acid dihydrate and tartaric acid;

[0018] During the hydrothermal reaction, the molar ratio of ferric chloride hexahydrate, phosphoric acid, oxalic acid dihydrate, tartaric acid, urea, and deionized water is 1:6:1:1:2.5:100.

[0019] A second objective of this invention is to provide a method for preparing the aforementioned modifier, comprising:

[0020] Weigh the metal cluster, phosphoric acid, organic ligand and urea in molar ratio and dissolve them in deionized water to obtain a mixed solution;

[0021] The mixed solution was poured into a reaction vessel and sealed, and then transferred to an oven for reaction.

[0022] After the reactor has cooled to room temperature, the reactants are removed, filtered through filter paper, washed with deionized water, and dried to obtain the product.

[0023] In a preferred embodiment, the filtered reactants are dried at 60 °C to obtain the product.

[0024] The technical solution of this invention uses inorganic metal ion clusters, phosphoric acid, natural organic acid organic ligands, and urea to prepare a metal-organic framework modifier. This modifier is then applied to saline-alkali soil. Because the framework material itself contains a microporous structure, it has a certain adsorption capacity for salt ions, while NH4... + -N, as a guest molecule, resides in the interstitial spaces within the framework, maintaining charge balance. During the slow release of nitrogen, the framework material also adsorbs salt ions such as K / Na / Ca / Mg under electrostatic forces. On the other hand, the metal framework structure can slowly release nutrients such as nitrogen, phosphorus, and iron, enhancing soil fertility. Furthermore, it can lower soil pH and reduce soil alkalization. By adsorbing soil salt ions and lowering pH, this amendment can alleviate soil salinity and alkalinity. In a preferred embodiment, this invention introduces the natural organic ligand tartaric acid, a functional ligand possessing both hydroxyl and carboxyl coordination active groups, exhibiting multiple bridging coordination modes and satisfying coordination modes with different metals. Moreover, this invention relates to two different organic / inorganic ligands competing for coordination modes, thereby influencing MOFs and modifying them to a certain extent.

[0025] This invention synthesizes a series of metal-organic framework (MOF) modifiers under hydrothermal conditions, using ferric chloride hexahydrate / zinc sulfate heptahydrate as inorganic metal clusters, natural organic acids such as oxalic acid dihydrate / citric acid monohydrate / tartaric acid as organic ligands, urea as a structure-directing agent, and phosphoric acid. The frameworks are rich in nitrogen, phosphorus, and iron nutrients. Soil cultivation experiments have shown that the frameworks possess excellent slow-release nutrient properties. Furthermore, simulated saline-alkali soil tests have revealed that the modifiers can effectively reduce soil pH and salinity. Therefore, the metal-organic framework modifiers of this invention simultaneously possess controlled-release nutrient properties and improve saline-alkali soil characteristics. This study explores the improvement effect of MOFs on saline-alkali soils, broadens the application scope of MOFs, and effectively supports the efficient utilization of saline-alkali soil resources. Attached Figure Description

[0026] Figure 1 The cumulative release rate (%) of mineral nitrogen in saline-alkali soil.

[0027] Figure 2 The cumulative release rate of available phosphorus in saline-alkali soil (%).

[0028] Figure 3Salt content and pH for different treatment groups.

[0029] Figure 4 The potassium / sodium / calcium / magnesium content of different treatment groups. Detailed Implementation

[0030] The present invention will be further described below with reference to specific embodiments, but the present invention is not limited to the following embodiments. Unless otherwise specified, the methods described are conventional methods and can be referred to in previously published literature (Wu Ke, Zhou Jianmin, Du Changwen. Development of novel iron fertilizers based on metal-organic frameworks [J]. Journal of Huazhong Agricultural University, 2023, 42(6):73-79).

[0031] Example 1

[0032] This embodiment provides a method for preparing a metal-organic framework-based saline-alkali soil conditioner, which specifically includes the following steps:

[0033] Weigh the substrate raw materials in the following molar ratio: ferric chloride hexahydrate (FeCl3·6H2O): phosphoric acid (H3PO4): oxalic acid dihydrate (H2C2O4·2H2O): urea (CO(NH2)2): deionized water = 1:6:1:2.5:100. Completely dissolve the ferric chloride hexahydrate, phosphoric acid, oxalic acid dihydrate, and urea in deionized water, and stir thoroughly with a glass rod. Pour the mixture into a reaction vessel (Xi'an Changyi Instrument Equipment Co., Ltd.), tighten and seal the lid, and then transfer it to an oven at 120 ℃ for 18 h. After the vessel cools to room temperature, open the reaction vessel, filter the reactants through filter paper, wash three times with deionized water, and then dry at 60 ℃ to obtain the product. Compared with the previous Fe-MOF novel iron fertilizer (Comparative Example 1), the preparation method of this embodiment adjusted the substrate molar ratio and hydrothermal synthesis conditions, increased the hydrothermal synthesis temperature, and shortened the reaction time. Compared with Comparative Example 1, the metal-organic framework modifier prepared in this embodiment has significantly improved the nutrient phosphorus content, iron content, and pH reduction rate, as shown in Table 1.

[0034] Example 2

[0035] The only difference between this embodiment and Example 1 is that the organic ligand oxalic acid is replaced with oxalic acid and tartaric acid (C4H6O6) (1:1), and the molar ratio of the reaction substrate is ferric chloride hexahydrate: phosphoric acid: oxalic acid dihydrate: tartaric acid: urea: deionized water = 1:6:1:1:2.5:100.

[0036] Example 3

[0037] The only difference between this embodiment and Example 1 is that the inorganic metal cluster ferric chloride hexahydrate is replaced with ferric chloride hexahydrate and zinc sulfate heptahydrate (ZnSO4·7H2O) (1:1), and the molar ratio of the reaction substrate is ferric chloride hexahydrate: zinc sulfate heptahydrate: phosphoric acid: oxalic acid dihydrate: urea: deionized water = 1:1:6:1:2.5:100.

[0038] Example 4

[0039] The only difference between this embodiment and Example 1 is that the organic ligand oxalic acid is replaced with oxalic acid and citric acid monohydrate (H8C6O7·H2O) (1:1), and the molar ratio of the reaction substrate is ferric chloride hexahydrate: phosphoric acid: oxalic acid dihydrate: citric acid monohydrate: urea: deionized water = 1:6:1:1:2.5:100.

[0040] Example 5

[0041] The only difference between this embodiment and Example 1 is the urea substrate ratio. The molar ratio of the reaction substrate is ferric chloride hexahydrate: phosphoric acid: oxalic acid dihydrate: urea: deionized water = 1:6:1:3:100.

[0042] Comparative Example 1

[0043] This embodiment is prepared according to the novel Fe-MOF iron fertilizer synthesized in previously published literature. The difference from Example 1 is that the hydrothermal reaction conditions are 100 °C for 24 h, and the molar ratio of the substrate is ferric chloride hexahydrate: phosphoric acid: oxalic acid dihydrate: urea: deionized water = 1:5:3:3:80.

[0044] Comparative Example 2

[0045] The only difference between this embodiment and Example 1 is that the organic ligand oxalic acid is replaced with tartaric acid, and the molar ratio of the reaction substrate is ferric chloride hexahydrate: phosphoric acid: tartaric acid: urea: deionized water = 1:6:1:2.5:100.

[0046] Comparative Example 3

[0047] The only difference between this embodiment and Example 1 is that the organic ligand oxalic acid is replaced with citric acid. The molar ratio of the reaction substrate is ferric chloride hexahydrate: phosphoric acid: citric acid monohydrate: urea: deionized water = 1:6:1:2.5:100.

[0048] Comparative Example 4

[0049] The only difference between this embodiment and Example 1 is the substrate ratio of phosphoric acid and urea. The molar ratio of the reaction substrate is ferric chloride hexahydrate: phosphoric acid: oxalic acid dihydrate: urea: deionized water = 1:5:1:2.5:100.

[0050] Example 6

[0051] This embodiment utilizes soil experiments to test the nutrient and salinity improvement effects of the metal-organic framework amendments prepared in Examples 1-5 and Comparative Examples 1-4. The testing environment is as follows:

[0052] Soil preparation: The soil used was saline-alkali soil with the following physicochemical properties: organic matter 12.45 g·kg⁻¹ -1 Total nitrogen 0.62 g·kg -1 Ammonium nitrogen 1.22 mg·kg -1 Nitrate nitrogen 12.47 mg·kg -1 Available phosphorus 6.72 mg·kg -1 Available iron 8.73 mg·kg -1 The soil sample had a pH of 9.02, a salt content of 0.44%, and an electrical conductivity of 6328.82 μS / cm. A soil sample (100 g) was placed in an 8 cm diameter petri dish, and the soil moisture content was uniformly adjusted to 38% (w / w).

[0053] Two treatments were administered: a control and a MOF treatment. The control received no MOF, while the MOF treatment received MOF at a rate of 75 kg N / ha. Each treatment was replicated three times. All petri dishes were incubated at 25°C in a PPX-450B incubator (Saifu Corporation, China) and loosely covered with plastic wrap to reduce soil moisture evaporation. Soil samples were collected from 36 dishes on days 1, 3, 5, 10, 20, and 30 of incubation. All soil samples were air-dried and sieved.

[0054] Mineral nitrogen (ammonium nitrogen and nitrate nitrogen) content in soil samples was determined using a Smart Chem 200 automated analyzer (AMS Alliance, Frepillon, France); available phosphorus and available iron content in the soil were measured using an ICP-OES spectrometer (iCAP 7000, Thermo Fisher Scientific, USA); and soil pH was determined using a pH meter (Orion Star A211, Thermo Fisher Scientific, USA).

[0055] Meanwhile, another 5 g of the above-mentioned saline-alkali soil was weighed and placed in a 50 mL centrifuge tube. 25 mL of deionized water was added, and two treatments were set up: a treatment with 0.2 g of MOF (in each example and comparative example) and a control (blank group) without MOF. Each treatment was repeated three times. The samples were then placed on a shaker and shaken at 200 r / min for 12 h at 25 °C. After the shake, the liquid in the tubes was analyzed. K / Na / Ca / Mg content in the liquid was determined using an ICP-OES spectrometer, soil conductivity was measured using a conductivity meter (SD30, Mettler Toledo, Zurich, Switzerland), total water-soluble salts in the soil were determined using the residue drying-mass method, and soil pH was measured using a pH meter.

[0056] The detection results of Examples 1-5 and Comparative Examples 1-4 are shown in Table 1:

[0057] Table 1. Nutrient content and salinity improvement effect of metal-organic framework modifiers

[0058]

[0059] As can be seen, this invention prepared a series of metal-organic framework amendments under hydrothermal reaction. The average nutrient content of these amendments is approximately 5.32% C, 4.61% N, 17.01% P, 20.79% Fe, and 10.44% Zn. After application, the amendments reduced soil pH by an average of approximately 16.05% and desalinated soil by approximately 7.85%. When applied to saline-alkali soil, the cumulative nitrogen release rate after 30 days was approximately 11.03%, demonstrating a good controlled nutrient release effect. While providing sufficient nutrients to crops, it significantly reduced the salinity effect of the nutrients themselves. Compared to the comparative example, the nutrient content and salinity improvement effect of the examples were improved to a certain extent. Therefore, the synthesized metal-organic framework amendments can alleviate soil salinity stress and improve saline-alkali soils.

Claims

1. A metal-organic framework-based soil conditioner for saline-alkali soil, characterized in that, The modifier is formed by bridging coordination of a metal cluster, an inorganic ligand phosphoric acid, an organic ligand, and a structure-directing agent urea under hydrothermal conditions. The metal cluster is ferric chloride hexahydrate; the organic ligand is oxalic acid dihydrate and citric acid monohydrate, or a combination of oxalic acid dihydrate and tartaric acid, wherein the molar ratio of the two organic acids in the combination is 1:

1. During the hydrothermal reaction, the molar ratio of metal cluster, phosphoric acid, organic ligand, urea and deionized water is 1-2:6:1-2:2.5-3:

100.

2. The modifier according to claim 1, characterized in that, The hydrothermal conditions are 110-120℃ for 18-20 hours.

3. The modifier according to claim 2, characterized in that, The hydrothermal conditions were 120℃ for 18 h.

4. The modifier according to claim 1, characterized in that, The organic ligands are oxalic acid dihydrate and tartaric acid; During the hydrothermal reaction, the molar ratio of ferric chloride hexahydrate, phosphoric acid, oxalic acid dihydrate, tartaric acid, urea, and deionized water is 1:6:1:1:2.5:

100.

5. A method for preparing the modifier according to any one of claims 1-4, characterized in that, include: Weigh the metal cluster, phosphoric acid, organic ligand and urea in molar ratio and dissolve them in deionized water to obtain a mixed solution; The mixed solution was poured into a reaction vessel and sealed, and then transferred to an oven for reaction. After the reactor has cooled to room temperature, the reactants are removed, filtered through filter paper, washed with deionized water, and dried to obtain the product.

6. The preparation method according to claim 5, characterized in that, The filtered reactants were dried at 60 °C to obtain the product.

Images