CN112608190B

CN112608190B - Preparation method of pH/salt-sensitive porous 3D structure slow-release nitrogen fertilizer based on MOF (Cu) @ biochar

Info

Publication number: CN112608190B
Application number: CN202011520269.4A
Authority: CN
Inventors: 吴称意; 汪争先; 但悠梦; 段正超; 田大听; 向东山
Original assignee: Hubei University for Nationalities
Current assignee: Hubei University for Nationalities
Priority date: 2020-12-21
Filing date: 2020-12-21
Publication date: 2022-06-17
Anticipated expiration: 2040-12-21
Also published as: CN112608190A

Abstract

The inventionRelates to the technical field of nitrogen fertilizers, and particularly discloses a preparation method of a pH/salt sensitive porous 3D structure slow-release nitrogen fertilizer based on MOF (Cu) @ biochar. The slow-release nitrogen fertilizer is prepared by crosslinking carboxymethylcellulose (CMC), Gelatin and MOF (Cu) @ biochar in the presence of ammonium chloride to prepare a novel NH coated hybrid aerogel (CMC/Gelatin/MOF (Cu) @ biochar)₄And Cl to obtain the Slow Release Fertilizer (SRF). The preparation method of the slow-release nitrogen fertilizer has simple process and low cost; the prepared slow-release nitrogen fertilizer has good slow-release performance and excellent fertilizer and water retention capacity, and can be used for agriculture and horticulture to improve soil and vegetable productivity.

Description

Preparation method of pH/salt-sensitive porous 3D structure slow-release nitrogen fertilizer based on MOF (Cu) @ biochar

Technical Field

The invention relates to the technical field of nitrogen fertilizers, in particular to a preparation method of a pH/salt sensitive porous 3D structure slow-release nitrogen fertilizer based on MOF (Cu) @ biochar.

Background

Increasing crop yields is a fundamental goal to meet the continuing growth of the world population. Fertilizer and water are two important input factors for increasing crop yield. In China, the contribution of fertilization to crop yield is 56.81%. However, about 40-70% of nitrogen, 80-90% of phosphorus and 50-70% of normal fertilizer potassium cannot be effectively absorbed, so that the nitrogen, the phosphorus and the potassium are lost to the surrounding environment, and serious economic loss and damage to the agricultural ecological environment are caused. Therefore, there is a need to improve their application efficiency. Slow Release Fertilizers (SRFs) provide a strategy to increase nutrient utilization, extend fertilizer cycle, and reduce environmental problems caused by nutrient loss. In recent years, encapsulation has been a conventional and simple method to produce polymer-related slow release N-fertilizers. The support material is prepared using various polymers: from non-environmentally degradable polymers (polyolefins, polyurethanes, and polysulfonates) to degradable polymers (polysaccharides and aliphatic polyesters). In addition, water plays an indispensable role in crop growth and fully exerts the function of a fertilizer. However, the use of SRF is limited due to the lack of water resources in some arid and semi-arid regions. Therefore, SRFs with good water retention properties have been developed in recent decades.

Aerogels are constructed from three-dimensional (3D) porous networks and have many excellent properties, including extremely low density (typically 0.004-0.005 g-cm)^-3) Large specific surface area and high porosity. In recent years, natural hydrophilic polymers and their derivatives, including xanthan gum, lignin, starch, sodium alginate, chitosan and cellulose, have been selected from their low cost, non-toxic, renewable and biodegradable synthetic polymers. In addition, these natural hydrophilic polymers can also form aerogels with good water retention. Carboxymethyl cellulose (CMC) is an anionic linear polymer containing a large number of hydroxyl and carboxyl groups. It has excellent biocompatibility and biodegradability, and can be widely applied to the fields of drug carriers, slow release fertilizers, adhesives, mineral removal, medicines, foods and the like.

The gelatin is composed of a unique amino acid sequence, is rich in glycine, proline and hydroxyproline, is a natural hydrophilic polymer, and has good non-toxicity and biodegradability. Is widely applied to the industrial, pharmaceutical and medical fields. However, the application of the slow release fertilizer has not been reported.

However, the main disadvantages of aerogels are the high production costs and the poor mechanical properties, especially after absorption of high water. The hybrid aerogel is composed of an organic polymer and an inorganic material, wherein the organic component has toughness, low density, good elasticity and formability, and the inorganic component has rigidity, hardness and thermal stability. Therefore, hybrid aerogels are widely used based on their organic and inorganic properties. Metal-organic framework Materials (MOFs) are prepared by self-assembly of metal ions and organic ligands, which are mainly carboxylates and sulfonates. Due to their three-dimensional network, high porosity, specific surface area, tunable functionality and biodegradability, they are used as adsorbents in many fields. However, up to now, MOFs cannot provide strong dispersion force to bind small molecules due to their low atomic density in the structure. In addition, most MOFs are less stable in aqueous solutions. In recent years, MOF-based composite materials have attracted much attention in combination with other materials such as carbon nanotubes, graphite oxide, metals, and activated carbon. The biochar has the unique characteristics of high carbon content, large specific surface area and stable structure. In addition, biochar is produced from various organic waste materials, such as agricultural waste and municipal sewage sludge, which are more easily collected.

Disclosure of Invention

Aiming at the defects of the prior art, the invention aims to provide a preparation method of a pH/salt sensitive porous 3D structure slow-release nitrogen fertilizer based on MOF (Cu) @ biochar, which comprises the following steps:

(1) preparation of mof (cu) @ biochar: dispersing biochar in a DMF-water mixture (preferably DMF and water are mixed according to the volume ratio of 1: 1), and carrying out ultrasonic treatment for 1h in an ice bath; dissolving copper nitrate in water to prepare a solution (the concentration of the copper nitrate is preferably 1 mol/L); dissolving terephthalic acid (TPA) in DMF to obtain a solution (preferably, the concentration of the terephthalic acid is 1 mol/L);

mixing the three solutions, transferring the mixture into a glass reactor, and reacting for 24-30 h under the condition of 120 ℃ oil bath; after cooling to room temperature, sequentially washing with DMF and chloroform for at least 3 times, and then placing the collected solid in an oven for vacuum drying at 40-60 ℃ for 12-24 h to obtain black powder, namely MOF (Cu) @ biochar;

further, in the step (1): the dosage relation of the biochar, namely copper nitrate and terephthalic acid is 0.1g, 0.01mol and 0.01 mol; the dosage relationship of the biochar and the DMF-water mixture is 0.1g: 100-200 mL;

further, in the step (1), the preparation method of the biochar comprises the following steps: pulverizing dried corn cob, sieving with 40 mesh sieve, and placing in inert atmosphere (such as N)₂) Under protection, putting the biomass into a tube furnace, heating to 600 ℃ at the heating rate of 5-10 ℃/min, and pyrolyzing for 2h to obtain biochar, which is marked as biochar;

(2) preparing a slow-release nitrogen fertilizer: dissolving carboxymethyl cellulose (CMC) and gelatin in distilled water at 50-65 ℃, and adding NH into the solution₄After Cl is completely dissolved, adding the MOF (Cu) @ biochar obtained in the step (1) into the solution, and carrying out ultrasonic treatment for 15-30 min to obtain a uniform system; the homogeneous system was transferred to an ice bath and pouredDropwise adding 1% (w/v, g/mL) glutaraldehyde aqueous solution into the uniform system by using an injector, crosslinking the obtained mixture at 50 ℃ for 20-30 min, and freeze-drying at the temperature below-20 ℃ to obtain NH coated with CMC/Gelatin/MOF (Cu) @ biochar hybrid aerogel₄Cl, obtaining a slow release fertilizer, and recording as SRF;

further, in the step (2): carboxymethyl cellulose: the mass ratio of the gelatin is 1: 2; the mass ratio of MOF (Cu) @ biochar to (carboxymethyl cellulose + gelatin) is 0-36.67%, preferably 10% -36.67%, more preferably 10%; NH added₄The mass ratio of Cl to (carboxymethylcellulose + gelatin) was 1: 6.

Further, in the step (2): the dosage ratio of the added cross-linking agent 1% (w/v) glutaraldehyde aqueous solution to (carboxymethyl cellulose + gelatin) is (0.5-1) mL:1 g.

The slow-release nitrogen fertilizer prepared by the preparation method is used as a fertilizer.

Further, the slow-release nitrogen fertilizer is used as a special fertilizer for vegetable planting, and 1-2 g of the SRF fertilizer is applied to 100g of sandy soil.

Compared with the prior art, the invention has the advantages and beneficial effects that:

the invention prepares a novel CMC/Gelatin/MOF (Cu) @ biochar hybrid aerogel and applies the novel CMC/Gelatin/MOF (Cu) @ biochar hybrid aerogel to a slow-release N-fertilizer. The preparation method of the slow-release nitrogen fertilizer based on the pH/salt sensitive porous 3D structure of MOF (Cu) @ biochar has the advantages of simple process and low cost; the slow-release nitrogen fertilizer prepared by the invention has good slow-release performance (the accumulative release rate of soil SRF is 79.4 percent on day 30), has excellent fertilizer-keeping and water-retaining capacity, and can be used for agriculture and horticulture to improve soil and vegetable productivity.

Drawings

FIG. 1 is a schematic diagram of the preparation process of SRF and its release mechanism in soil.

FIG. 2: FIG. 2(a) is an FTIR spectrum of MOF (Cu), CMC, gelatin, MOF (Cu) @ biochar and CGMB-1;

FIG. 2(b) is an XRD pattern of MOF (Cu) and CGMB-1;

FIG. 2(c) is TG spectrogram of hybrid aerogel (CGMB-1, CGMB-2, CGMB-3, CGMB-4, CGMB-5);

FIG. 2(d) is N of MOF (Cu) @ biochar and CGMB-1₂Adsorption-removal of attached figure.

Fig. 3 is a SEM image of a material, wherein: (a) MOF (Cu), (b) biochar, (c) MOF (Cu) @ biochar, (d) CGMB-1, (e) CGMB-3, and (f) CGMB-5.

FIG. 4 is an XPS spectrum of MOF (Cu) @ biochar and SRF.

FIG. 5: FIG. 5(a) is a graph of the effect of different MOF (Cu) @ biochar contents on hybrid aerogel swelling;

FIG. 5(b) is a graph showing the effect of different pH on the swelling properties of CGMB-2;

FIG. 5(c) is a graph showing the effect of different salt species on the swelling properties of CGMB-2;

FIG. 5(d) is a graph showing the effect of different salt concentrations on the swelling performance of CGMB-2.

FIG. 6 is a graph of water holding ratio of hybrid aerogels under different negative pressure conditions.

FIG. 7 is a comparison of water retention performance for soil with and without SRF.

FIG. 8: FIG. 8a is NH₄Cumulative release profiles of Cl and SRF in soil; FIG. 8b shows ln (W) of SRF in soil_t/W_∞) A relationship diagram to lnt.

FIG. 9 is an EDS spectrum of SRF surface elements before and after soil release.

FIG. 10: FIG. 10(a) is a digital photograph of Allium plants treated with and without SRF;

FIG. 10(b) is a graph showing the growth of the plant length and the root length of seedlings of Allium fistulosum treated with and without SRF.

Detailed Description

The applicant will now further describe the technical solution of the present invention in detail with reference to specific examples. It should be understood that the examples are for the purpose of further illustrating the subject invention and should not be construed in any way as limiting the scope of the invention as claimed.

In the following examples, the starting materials used:

sand and soil: obtaining sandy soil from an Enshi local farm, drying the sandy soil to constant weight, and sieving the sandy soil with a 10-mesh sieve for use;

carboxymethyl cellulose (CMC), copper (II) nitrate trihydrate, terephthalic acid (TPA), gelatin, and glutaraldehyde are all available from Guoyao chemical reagents, Inc.;

n, N-Dimethylformamide (DMF), ammonium chloride and ethanol were all obtained from Fuchen chemical Co., Ltd;

the other solvents were analytical grade and used without any purification.

Example 1

Preparing biochar: collecting dried corn cob from Enshi local farmland, pulverizing corn cob, sieving with 40 mesh sieve, and sieving with N₂Under protection, placing the biomass in a tube furnace, heating to 600 ℃ at the heating rate of 10 ℃/min, keeping the temperature for 2h for pyrolysis, and taking out the biomass after cooling to obtain biocar (biochar).

Example 2

Preparation of mof (cu): taking 0.01mol of Cu (NO)₃)₂·3H₂Dissolving O in 10mL of water, dissolving 0.01mol of TPA in 10mL of DMF, mixing the two solutions, transferring the mixture into a glass reactor, and reacting for 24 hours under the condition of oil bath at the temperature of 120 ℃; and (3) cooling to room temperature, sequentially washing with DMF (dimethyl formamide) and chloroform for 3 times respectively, and then placing the collected solid in a vacuum oven for drying at 40 ℃ for 24 hours to obtain blue powder, namely MOF (Cu).

Example 3

Preparation of mof (cu) @ biochar: 0.1g of biochar (prepared in example 1) was dispersed in 150mL of DMF-water mixture (DMF mixed with water at a volume ratio of 1: 1), sonicated in an ice bath for 1h, and 0.01mol Cu (NO) was taken₃)₂·3H₂Dissolving O in 10mL of water, dissolving 0.01mol of TPA in 10mL of DMF, mixing the three solutions, transferring the mixture into a glass reactor, and reacting for 24 hours under the condition of oil bath at the temperature of 120 ℃; and (3) cooling to room temperature, sequentially washing with DMF and chloroform for 3 times, and then placing the collected solid in a vacuum oven at 40 ℃ for drying for 24 hours to obtain black powder, namely MOF (Cu) @ biochar, also recorded as MOF (Cu) @ biochar.

Example 4

Preparation of CMC/gelatin/mof (cu) @ biochar hybrid aerogel: sequentially dissolving 1g of CMC and 2g of gelatin in 50mL of distilled water at 50 ℃, adding different weights of (0.3, 0.5, 0.7, 0.9 and 1.1g) MOF (Cu) @ biochar (prepared in example 3) into the solution, and carrying out ultrasonic treatment for 30min to obtain uniform systems (respectively marked as system 1, system 2, system 3, system 4 and system 5); transferring the mixture in the uniform system into ice bath, dropwise adding 2mL of 1% (w/v) glutaraldehyde into the system by using an injector, crosslinking the obtained mixture at 50 ℃ for 20min, and freeze-drying at-20 ℃ to obtain CMC/Gelatin/MOF (Cu) @ biochar hybrid aerogel which is respectively named as CGMB-1, CGMB-2, CGMB-3, CGMB-4 and CGMB-5.

Example 5

Preparation of SRF: dissolving 1g CMC and 2g gelatin in 50mL distilled water at 50 deg.C, and adding 0.5g NH₄Cl, after being completely dissolved under stirring at a constant rotating speed, 0.3g of MOF (Cu) @ biochar (prepared in example 3) is added into the mixture, and the mixture is subjected to ultrasonic treatment for 30min to obtain a uniform system; transferring the mixture in the uniform system into ice bath, dropwise adding 2mL of 1% (w/v) glutaraldehyde into the system by using an injector, crosslinking the obtained mixture at 50 ℃ for 20min, and freeze-drying at-20 ℃ to obtain NH coated with CMC/Gelatin/MOF (Cu) @ biochar hybrid aerogel₄And Cl, obtaining the slow release fertilizer, and recording as SRF.

Example 6 the materials prepared in the above examples were characterized:

the apparatus used was as follows:

(1) an infrared spectrometer (Nicolet Avatar360, USA) is used for measuring 4000-400 cm^-1In the wavenumber range of (a), infrared spectroscopic analysis was performed on the sample using a KBR press.

(2) The X-ray diffraction results were recorded on an X-ray diffractometer using Cu ka radiation (40kV and 40mA), with a 2 theta range of 10-80 ° and a scan rate of 10 °/min.

(3) N was determined by the Brunauer-Emmett-Teller (BET) method₂And (5) obtaining the structural parameters of the sample through an adsorption-desorption experiment.

(4) The morphology of the sample was observed with a scanning electron microscope (SEM, Hitachi S-4800, Japan).

(5) Thermogravimetric analysis (TGA, TG209F1, Germany) on N₂Measuring the thermal stability of the sample at a heating rate of 10 ℃/min and a temperature range of 30-600 ℃ in an atmosphereAnd (4) sex.

(6) The binding energy of the product was measured by X-ray photoelectron spectroscopy (model: PHI5600 spectrometer).

Note: all samples were dried in a vacuum oven at 40 ℃ for 24h before measurement.

Infrared spectrum:

as shown in FIG. 2(a), FTIR spectra of MOF (Cu), CMC, gelatin, MOF (Cu) @ biochar and CGMB-1 are shown. FTIR spectra of MOF (Cu) at 1408 and 1630cm^-1The absorption peaks at (a) are due to symmetric stretching and asymmetric stretching of the carboxylic acid groups, respectively. In 1250-600 cm^-1Several peaks of the region are associated with out-of-plane vibration of the TPA carboxylic acid groups. At 1594 and 3004cm^-1The adsorption peak at (a) was assigned to aromatic ring oscillation of ArC-H, C ═ C in the aromatic ring.

In the FTIR spectrum of CMC, at 3375cm^-1The peak at (A) is due to the tensile vibration of the hydroxyl group, CH₂Tensile vibration of (2) occurred at 2930cm^-1To (3). The absorption peak of COO group appears at 1575cm^-1. At 1322cm^-1And 1475cm^-1The absorption peaks at (A) are respectively attributed to OH bending vibration and CH of CMC₂And (4) shearing vibration. In the range of 1000 to 1200cm^-1The broad peak of the region is due to sugar ring absorption by the CMC molecule.

In the FTIR spectra of gelatin, 3300cm and 1650cm^-1The absorption peaks in (b) are assigned to N-H and C ═ O stretched bands, respectively. At 1450cm^-1The absorption adsorption of (b) is due to tensile vibration of the Aldimine (Aldimine) group, confirming the crosslinking of the gelatin.

In the FTIR spectra of MOF (Cu) @ biochar, features related to the parent material MOF (Cu) appeared in the FTIR spectra, indicating successful formation of the hybrid material. Furthermore, in MOF (Cu) @ biochar, at 1502/1408cm^-1And 1578/1630cm^-1The intensity of the absorption peak was changed due to the interaction between the TPA ligands of MOF and the biochar functional groups.

The infrared spectrum of CGMB-1 is a combination of the spectra of MOF (Cu) @ biochar, CMC and gelatin, with characteristic adsorption peaks associated with MOF (Cu) @ biochar, CMC and gelatin.

XRD spectrum:

as shown in FIG. 2(b), XRD patterns of MOF (Cu) and CGMB-1 are shown. The range of 2 theta is 10-80 degrees. In the XRD spectrum of mof (cu), the characteristic peaks of mof (cu) appear at 2 θ of 10.14 °, 12.06 °, 17.12 °, 17.42 °, 20.62 °, 21.92 °, 24.78 °, 29.57 °, 34.4 ° and 42.06 °, which is a good indication that the structure is consistent with the Face Centered Cubic (FCC) cubic crystal structure, confirming the successful synthesis of mof (cu).

In the XRD pattern of CGMB-1, the diffraction peaks of MOF (Cu) are attenuated since MOF (Cu) is surrounded by an amorphous polymer matrix. In conjunction with FTIR spectroscopy, the hybrid aerogels were successfully prepared.

TG spectrum:

as shown in FIG. 2(c), TG spectra of the hybrid aerogels (CGMB-1, CGMB-2, CGMB-3, CGMB-4, CGMB-5) are shown. The weight loss of CGMB-1 comprises two phases. The first stage is at 50-100 ℃, which is probably due to the loss of water molecules absorbed in the aerogel network. The second stage is at 250-400 ℃, and the weight loss is caused by the structural decomposition of the hybrid aerogel. However, the first loss phase was not clearly shown in other CGMB, probably due to the increased content of MOF (Cu) @ biochar, resulting in less water molecules in the gel network. The TG spectrum of the hybrid aerogel demonstrates that an increase in mof (cu) @ biochar content is beneficial to the improvement of the thermal stability of the hybrid aerogel.

N₂Adsorption-desorption diagram:

as shown in FIG. 2(d), N of MOF (Cu) @ biochar and CGMB-1 is shown₂The specific surface area of CGMB-1 is 21.48m as shown in the figure²(ii)/g, higher than MOF (Cu) @ biochar (20.59 m)²/g), indicating that the presence of mof (cu) @ biochar can effectively improve the pore structure of its hybrid aerogel.

SEM atlas:

as shown in FIG. 3, the surface topography of (a) MOF (Cu), (b) biochar, (c) MOF (Cu) @ biochar, (d) CGMB-1, (e) CGMB-3 and (f) CGMB-5 is shown. Fig. 3(a), mof (cu) exhibits cubic crystal aggregation. FIG. 3(b) is rough in surface, indicating that it is suitable for a support. It is clear that mof (cu) particles were supported on the biochar surface, as shown in fig. 3 (c). The combination of the pictures of CGMB-3 and CGMB-5 shows that the MOF (Cu) @ biochar is well embedded in the hydrogel network, which is beneficial to improving the specific surface area of the hybrid aerogel. With the increase of the content of MOF (Cu) @ biochar, the surface of the hybrid aerogel shows the morphology of MOF (Cu) @ biochar, and the MOF (Cu) @ biochar is proved to be adsorbed by the network.

XPS spectra:

as shown in FIG. 4, XPS spectra of MOF (Cu) @ biochar and SRF are shown. FIG. 4(a) shows the full spectrum of MOF (Cu) @ biochar and SRF, and it is clear that C, N, O and Cu are present in MOF (Cu) @ biochar and SRF. For C1s XPS spectra of MOF (Cu) @ biochar and SRF, see FIG. 4(b), three characteristic signals of 284.8, 285.7 and 288.2eV are attributable to-C-, graphitic C (SP)²-C ═ C —) and carbonyl (-C ═ O), which indicates that they have a large number of functional groups. In addition, the peak of SRF at 288.2eV was stronger than the peak of mof (cu) @ biochar, indicating that more carbonyl groups (-C ═ O) were present in SRF. FIG. 4(c) is a Cu 2p spectrum of MOF (Cu) @ biochar and SRF. In the spectrum of MOF (Cu) @ biochar, the peak at 934.6eV is shifted to the lower binding energy 932.3eV in the SRF spectrum. This lower binding energy indicates that mof (cu) crystals are tightly bound to CMC/gelatin. The new peak at 198eV is due to Cl 2p, as compared to MOF (Cu) @ biochar, see FIG. 4 (d). The results show that NH₄The Cl was successfully encapsulated in CMC/gel/mof (cu) @ biochar hybrid aerogel.

Example 7 swelling behavior and water holding properties at different negative pressures of hybrid aerogels:

swelling behavior:

certain mass of hybrid aerogel (CGMB-1, CGMB-2, CGMB-3, CGMB-4 and CGMB-5) is taken and immersed in 100mL of deionized water for 48 hours at room temperature, and the expanded polymer is filtered and weighed. Swelling Ratio (SR): SR ═ M₂-M₁)/M₁Wherein M is₁And M₂Respectively, a dry weight (g) and a swollen weight (g).

As shown in FIG. 5(a), as the content of MOF (Cu) @ biochar [ weight ratio of MOF (Cu) @ biochar to (CMC + gelatin ]) increased from 10% to 36.7%, the swelling ratio of the hybrid aerogel decreased from 7.84g/g to 4.59 g/g.

Study with CGMB-2The effect of different pH, physiological saline and concentration on the swelling of the hybrid aerogel was examined. Solutions of different pH values were prepared with 0.1mol/LHCl and 0.1mol/LNaOH, and different physiological saline concentrations (KCl, CaCl) were prepared₂、FeCl₃) As a culture medium.

As shown in fig. 5(b), the SR of CGMB-2 was 8.7g/g at pH 2.0. The isoelectric point of gelatin is about 4.8, and-NH in gelatin₂Almost protonated by strong acid while inhibiting the ionization of carboxyl groups in gelatin and CMC, resulting in electrostatic repulsion. When the pH was increased to 6, the SR of CGMB-2 reached 9.6 g/g. This is due to partial ionization of the carboxyl groups in gelatin and CMC. -NH in gelatin when pH reaches 10₂The group cannot be ionized and thus the SR decreases.

As shown in FIG. 5(c), the swelling ratio of the mixed aerogel was K⁺>Ca²⁺>Fe³⁺. This phenomenon is caused by their valence states. Higher cation states favor the neutralization of-COO, -NH in the aerogel chain₂and-OH groups form intramolecular and intermolecular complexes. In addition, trivalent cations and divalent cations can be combined with anions by electrostatic attraction, thereby increasing the degree of ionic crosslinking and increasing rigidity.

As shown in FIG. 5(d), FeCl increased with increasing brine concentration₃The water absorption of the solution decreases. This is due to the difference in osmotic pressure of the aerogel matrix and the physiological saline solution. The greater the salt solution concentration, the greater the osmotic pressure, resulting in a significant decrease in the swelling ratio.

Water holding capacity under different negative pressures:

dry hybrid aerogel (CGMB-1, CGMB-2, CGMB-3, CGMB-4 and CGMB-5) is immersed in 250mL of distilled water to reach swelling equilibrium at room temperature. Swollen products were weighted (W) and placed in an oven for 12h at 25 ℃, each different negative pressure, and the weight at the different negative pressure (Wp) was recorded. The water holdup was calculated using the following formula:

the water absorbed and transported by the plants is driven by the negative pressure of transpiration. The limited negative pressure generated by transpiration is lower than-0.1 MPa. Thus, only a part of the water can be absorbed by the plants in the negative pressure range. As shown in fig. 6, the water holding ratio curves for the hybrid aerogels under different negative pressure conditions are shown. As the negative pressure increases from-0.02 MPa to-0.08 MPa, the water absorbed by the hybrid aerogel decreases significantly. In addition, the water holding rate of CGMB-2 is reduced from 23.4% to 13.1% as the negative pressure is increased from-0.02 MPa to-0.08 MPa. The results show that more than 80% of the water in the mixed aerogel is absorbed by the plants.

Example 8 application of SRF:

water retention behavior of soil SRF:

2g of the SRF prepared in example 5 (20 mesh) was mixed well with 100g of sandy soil and the mixture was put into a beaker and infiltrated with running water until an aqueous phase appeared from the soil interstices. The beaker was placed in an oven at 25 ℃. The weight of the beaker was recorded daily. The water retention was calculated by the following formula, where m₁Is the total weight (G) of SRF and sandy soil after operation, m_iIs the weight (G) recorded daily.

Soil without SRF was used as a control.

As shown in FIG. 7, the water retention properties of the soil without SRF on

days

4 and 6 were 32.1% and 14.3%, respectively, while the water retention properties of the soil with 2 wt% SRF were 37.1% and 22.3%, respectively. The results show that the water retention performance of the SRF soil is higher than that of the SRF-free soil. Therefore, the soil SRF can be used for reducing the irrigation frequency and improving the drought resistance of plants.

Slow release behavior of SRF in soil:

1g of the SRF prepared in example 5 (20 mesh sieve) was mixed well with 100g of sandy soil, and the mixture was added to a glass column fitted with a non-woven fabric and a valve, and distilled water was added until saturation of soil water was reached (water layer appeared on the column). At regular intervals, 25mL of the soil drench solution was collected, and absorbance at a wavelength of 697nm was measured by the salicylic acid method (see HJ 536-2009). At the same time, in the release mediumThe medium was supplemented with 25mL of distilled water to maintain a constant volume. Release experiments were performed in triplicate. Adding NH into soil₄Cl as control.

As shown in FIG. 8a, is NH₄Cumulative release profiles of Cl and SRF in soil. The results show that SRF has a lower nitrogen release rate in soil than NH₄And (4) Cl. Over 97% of the nitrogen from NH within 2 days₄Cl is released, this result being due to NH₄Cl is a water-soluble compound fertilizer, and nutrients are quickly exhausted when the Cl is dissolved in a soil solution. However, the cumulative release rates of SRF were 49.2% and 79.4% on

days

10 and 30, respectively. This result is due to the slow swelling of CMC/gelatin/MOF (Cu) @ biochar hybrid aerogels in soil solution. There is a dynamic exchange between the free water in the aerogel and the water in the soil. The nutrients in the swollen aerogel matrix diffuse slowly from the network through dynamic water exchange. It is well known that CMC and MOF (Cu) contain a large number of carboxyl groups. Due to aerogel-COO-and NH₄ ⁺Due to electrostatic interaction, nutrient substances are slowly diffused out of the aerogel network, and the slow release performance can be effectively improved.

In order to research the release mechanism of the SRF in the soil, the data are evaluated by using a Korsmeyer-Peppas equation,

where t is time, K_KPIs the diffusion content and n is the diffusion index. M_t/M_∞Is the release fraction of the fertilizer at time t, and the release mechanism is classified according to the n value. FIG. 8b is ln (W) of SRF in soil_t/W_∞) For the relationship chart of lnt, n-0.5128 illustrates that the release mechanism is consistent with non-Fickian diffusion.

SRF was buried in soil for 5 days with 20mL of water per day to keep the soil moist. The SRF was removed and washed with distilled water. And analyzing SRF surface elements before and after soil release by adopting an EDS spectrum. As shown in FIG. 9, the chlorine content dropped significantly from 0.79 wt% to 0.38 wt%, which was found to be due to NH₄And (4) releasing Cl. NH (NH)₄K, Ca appears in the spectrum of SRF after Cl has been released in the soil² ⁺、Mg²⁺And Fe³⁺The peak of (2). This phenomenon is due to the NH partitioning into the hybrid aerogel₄ ⁺With K in the soil⁺、Ca²⁺And Fe³⁺Exchange of cations therebetween. Thus, the release mechanism of SRF in soil is a concerted effect of (non-fick) diffusion and cation exchange.

Early seedling growth experiment:

100g of dried sandy soil was mixed with 0.5g of SRF (20 mesh sieve) prepared in example 5, and then placed in a container. Scallion seeds are added into the soil, and 20mL of water is irrigated every three days. Seedlings were observed daily for germination and growth. On day 15, groups of equal number of seedlings were randomly collected and investigated for seedling length and fresh biomass. This experiment was performed in triplicate. The onion without SRF treatment was used as a Control (Control).

As shown in FIG. 10, FIG. 10(a) is a digital photograph of Allium plants treated with and without SRF. The plant length and root length of the exposed seedlings of onions increased 50.67% and 73.33%, respectively, after SRF treatment compared to Control soil (Control), see FIG. 10 (b). This finding is due to the ability of SRF to effectively control water and nutrient loss. Thus, SRF can be used in agriculture and horticulture to improve soil and increase vegetable productivity.

Claims

1. A preparation method of a pH/salt sensitive porous 3D structure slow-release nitrogen fertilizer based on MOF (Cu) @ biochar is characterized by comprising the following steps:

(1) preparation of mof (cu) @ biochar: dispersing biochar in a DMF-water mixture, carrying out ultrasonic treatment in an ice bath for 1h, dissolving copper nitrate in water to prepare a solution, dissolving terephthalic acid in DMF to prepare a solution, mixing the three solutions, transferring the mixture to a glass reactor, and carrying out reaction for 24-30 h in an oil bath at 120 ℃; after cooling to room temperature, sequentially washing with DMF and chloroform for at least 3 times, and then placing the collected solid in an oven for vacuum drying at 40-60 ℃ for 12-24 h to obtain black powder, namely MOF (Cu) @ biochar;

(2) preparing a slow-release nitrogen fertilizer: dissolving carboxymethyl cellulose and gelatin in distilled water at 50-65 ℃, and adding NH into the solution₄After Cl is completely dissolved, adding the MOF (Cu) @ biochar obtained in the step (1) into the solution, and carrying out ultrasonic treatment for 15-30 min to obtain a uniform system; transferring the uniform system into an ice bath, dropwise adding a cross-linking agent, cross-linking the obtained mixture at 50 ℃ for 20-30 min, and freeze-drying at the temperature below-20 ℃ to obtain NH coated with CMC/Gelatin/MOF (Cu) @ biochar hybrid aerogel₄Cl to obtain the final product;

in the step (2): the mass ratio of the carboxymethyl cellulose to the gelatin is 1: 2; the added MOF (Cu) @ biochar accounts for 10-36.67% of the total mass of the carboxymethyl cellulose and the gelatin; NH added₄1/6 represents the sum of the mass of the carboxymethyl cellulose and the gelatin; the added cross-linking agent is 1% (w/v) glutaraldehyde aqueous solution, and the ratio of the volume of the cross-linking agent to the total mass of the carboxymethyl cellulose and the gelatin is (0.5-1) mL:1 g;

in the step (1), the preparation method of the biochar comprises the following steps: taking dried corncobs, crushing, sieving with a 40-mesh sieve, placing in a tubular furnace under the protection of inert atmosphere, heating to 600 ℃ at a heating rate of 5-10 ℃/min, and pyrolyzing for 2h to obtain the corn cob core.

2. The production method according to claim 1, wherein in the step (1): the dosage relation of the biochar, copper nitrate and terephthalic acid is 0.1g, 0.01mol and 0.01 mol.

3. The production method according to claim 2, wherein in the step (1), before the three solutions are mixed: after the copper nitrate is dissolved in water, the concentration of the copper nitrate is 1 mol/L; after terephthalic acid was dissolved in DMF, the concentration of terephthalic acid was 1 mol/L.

4. The production method according to claim 1, wherein in the step (1): the dosage relationship of the biochar and the DMF-water mixture is 0.1g: 100-200 mL, and the DMF and the water in the DMF-water mixture are mixed according to the volume ratio of 1: 1.

5. The production method according to claim 1, wherein in the step (2): the added MOF (Cu) @ biochar accounts for 10 percent of the total mass of the carboxymethyl cellulose and the gelatin.

6. The use of the slow-release nitrogen fertilizer obtained by the preparation method of any one of claims 1 to 5 as a fertilizer.

7. The use according to claim 6, characterized in that the slow release nitrogen fertilizer is used as a fertilizer special for vegetable planting, and the spreading amount is as follows: the slow release nitrogen fertilizer in the sandy soil is =100 g: 1-2 g.