Method for preparing blended micromolecular biphenyl tetracarboxylic acid-polyvinylidene fluoride membrane through electrostatic spinning, blended membrane and method for recovering silver ions in water body

Abstract

The invention discloses a method for preparing a blended micromolecular biphenyl tetracarboxylic acid-polyvinylidene fluoride membrane through electrostatic spinning, a blended membrane and a method for recovering silver ions in a water body. The method for preparing the blended small molecular biphenyl tetracarboxylic acid-polyvinylidene fluoride membrane through electrostatic spinning comprises the following steps: S1, carrying out electrostatic spinning on a solution composed of polyvinylidene fluoride, biphenyl tetracarboxylic acid and a solvent, and collecting a fiber membrane; and S2, carrying out heat treatment on the fiber membrane to obtain the blended small molecular biphenyl tetracarboxylic acid-polyvinylidene fluoride membrane. According to the BPTC-PVDF nanofiber matrix membrane prepared by the method disclosed by the invention, adsorbed silver ions can be converted into elemental silver under an illumination condition, so that not only can the silver ions in wastewater be effectively removed, but also silver resources can be recycled, and powerful technical support and a solution are provided for solving global problems such as water resource shortage and environmental pollution.

Description

Technical Field

[0001] The present invention belongs to the field of environmental science and technology, and particularly relates to a method for preparing a blend of small molecule biphenyltetracarboxylic acid and polyvinylidene fluoride membrane by electrospinning, a blend membrane, and a method for recovering silver ions from water bodies. Background Art

[0002] Silver, as a precious noble metal, occupies a crucial position in many industrial fields such as electronics, mechanical manufacturing, and military due to its excellent electrical conductivity, outstanding thermal conductivity, and unparalleled low resistance among metals. However, this extensive application has directly led to the increasing shortage of silver resources. Industrial wastewater, especially wastewater from electroplating and other related processes, often contains a large amount of silver ions. According to the strict regulations of the World Health Organization (WHO), the content of soluble silver in drinking water must be strictly controlled below 100 micrograms per liter. Therefore, effectively enriching and recovering silver ions from industrial wastewater can not only effectively curb the trend of water resource pollution but also achieve the sustainable recycling of silver resources and reduce resource waste.

[0003] Given its significant environmental and economic benefits, treating wastewater containing silver ions (Ag + ) and recovering silver has become a key research direction in the current scientific research field. To address this challenge, currently developed and applied treatment technologies include, but are not limited to, chemical precipitation method, electrolysis method, solvent extraction method, ion exchange method, adsorption method, and membrane separation method, etc. Among them, membrane separation technology has attracted much attention due to its high treatment efficiency, friendly environmental adaptability, and strong metal ion adsorption ability. The adsorption method is widely adopted because of its advantages such as no secondary pollutant generation, simple operation, and significant purification effect when treating low-concentration silver ion solutions.

[0004] To further improve the adsorption performance and selectivity of membrane materials, researchers have actively tried to blend various high-performance adsorbents with basic polymers to prepare composite membranes with excellent properties. These adsorbents include porous materials, montmorillonite, alumina, nanoparticles, etc. Although the composite membranes have improved the performance of membrane materials to a certain extent, there are still many problems. For example, the composite membranes lack specific selectivity and cannot accurately identify and adsorb silver ions in water bodies with multiple heavy metal ions coexisting; the adsorption capacity of some composite membranes is limited, resulting in low reuse efficiency of the membranes; in addition, the raw material preparation process of some composite membranes is complex and cumbersome, making it difficult to meet the requirements of large-scale industrial production.

[0005] In summary, improving the existing blend membranes, introducing new adsorbents with high adsorption performance and specific selectivity, and preparing composite membrane materials with good stability and high selectivity for silver ions have become the current research hotspots. Summary of the Invention

[0006] The object of the present invention is to provide a method for preparing a blend of small molecule biphenyltetracarboxylic acid - polyvinylidene fluoride membrane by electrospinning, the blend membrane, and a method for recovering silver ions from water. Under natural light conditions, silver ions can be reduced to silver metal on the membrane surface, realizing the recycling of silver resources; the preparation method provided by the present invention has easily available raw materials, no toxic by-products, is safe and green, and the electrospinning membrane preparation technology adopted is mature and there are already industrialized equipment, which can achieve large-scale production.

[0007] To achieve the above object, the present invention adopts the following technical solutions:

[0008] In the first aspect, the present invention provides a method for preparing a blend of small molecule biphenyltetracarboxylic acid - polyvinylidene fluoride membrane by electrospinning, comprising the following steps:

[0009] S1. Electrospin a solution composed of polyvinylidene fluoride, biphenyltetracarboxylic acid, and a solvent, and collect the fibrous membrane;

[0010] S2. Heat-treat the fibrous membrane to obtain the blend of small molecule biphenyltetracarboxylic acid - polyvinylidene fluoride membrane.

[0011] In the above preparation method, further, the mass ratio of polyvinylidene fluoride to biphenyltetracarboxylic acid is 12:(0.25 - 1), including but not limited to 12:0.25, 12:0.5, 12:0.75, 12:1. As an example, the average molecular weight of polyvinylidene fluoride is 600,000.

[0012] In the above preparation method, further, the solvent is composed of N,N-dimethylformamide and acetone with a volume ratio of 3:1, and this volume ratio can have better solubility, volatility, and better deposition of small molecule biphenyltetracarboxylic acid on the surface or inside of the fiber;

[0013] The mass percentage content of polyvinylidene fluoride in the solution is 12%;

[0014] The mass percentage content of biphenyltetracarboxylic acid in the solution is 0.25% - 1%, such as 1%, 0.75%, 0.5%, or 0.25%.

[0015] In the above preparation method, further, the method for preparing the solution composed of polyvinylidene fluoride, biphenyltetracarboxylic acid and solvent includes: 1) Mix the polyvinylidene fluoride with the solvent, stir at 800 rpm for 3 h at 80 °C for 2 - 3 h, such as stirring at 800 rpm for 3 h; 2) Add the biphenyltetracarboxylic acid to the mixture obtained in step 1), continue stirring for 8 - 9 h, such as 9 h, and finally perform ultrasonic treatment for 20 - 30 min, such as 30 min. Among them, in step 1), when adding the mixed solvent, it is necessary to adopt a slow and uniform manner to prevent the solvent impact from causing the PVDF powder to fly. After the solvent is added, gently shake and initially stir immediately to effectively prevent the PVDF powder from forming lumps at the bottom of the container; stirring at 800 rpm at 80 °C can promote the rapid and uniform dissolution of PVDF in the mixed solvent to form a homogeneous solution system; in step 2), the continuous stirring is carried out under the same conditions as in step 1) (such as 800 rpm at 80 °C) for 9 h, which can ensure that BPTC can be fully dissolved and uniformly dispersed in the solution, and at the same time promote the interaction between PVDF and BPTC; in addition, in order to further improve the uniformity of the solution, eliminate potential bubbles, and promote the full dispersion of fine particles and avoid agglomeration. Ultrasonic treatment of the entire solution system for 30 min can achieve thorough degassing and high dispersion of the solution, laying a foundation for the subsequent electrospinning to prepare the membrane material step. Based on the above operations, the present invention can obtain a uniform and stable PVDF / DMF / acetone / BPTC mixed solution.

[0016] In the above preparation method, further, the temperature of the electrospinning is 24 - 26 °C, and the relative humidity (RH) is 30 - 40%. Such an environment is conducive to the uniform formation and deposition of fibers;

[0017] The voltage of the electrospinning is set as follows:

[0018] The positive voltage is 17 - 21 kV, and the negative voltage is -2 - 0 kV, such as the positive voltage of 19 kV and the negative voltage of 2 kV; this is the optimal voltage value verified through multiple experiments, which can effectively overcome the liquid surface tension and achieve stable and continuous fiber jetting;

[0019] In the electrospinning step, the distance from the nozzle to the collector is 15 - 18 cm, such as 15 cm, which can ensure that the fibers are fully stretched in the electric field to form an ideal fiber structure;

[0020] In the electrospinning step, the feeding flow rate is 0.0010 - 0.0020 mm / s, and the moving speed of the platform is 1 - 3 mm·s -1 and the translation distance is 30 - 40 mm, such as the feeding flow rate of 0.0020 mm / s and the moving speed of the platform of 1 mm·s -1, the translation distance is 30 mm. The entire electrospinning process continues until the solution feeding is completed to ensure obtaining a fiber membrane with sufficient area and thickness. As an example, the time for the electrospinning is 10 hours.

[0021] In the above preparation method, further, the temperature of the heat treatment is 60 - 80 °C, and the time is 10 - 12 h, such as heat treatment in an 80 °C oven for 10 h. Through the heat treatment, the remaining solvent can be further volatilized and the fibers can be stabilized.

[0022] Based on the above operations, the present invention can obtain a PVDF / BPTC electrospun membrane with an ideal fiber structure.

[0023] In a second aspect, the present invention provides a blend of small molecule biphenyltetracarboxylic acid - polyvinylidene fluoride membrane, which is prepared by the method described in any one of the above.

[0024] In a third aspect, the present invention provides a method for recovering silver ions from industrial wastewater, comprising the following steps:

[0025] Place the blend of small molecule biphenyltetracarboxylic acid - polyvinylidene fluoride membrane in industrial wastewater containing silver ions, and perform oscillation treatment under natural light irradiation to adsorb the silver ions in the industrial wastewater and convert them into zero - valent silver.

[0026] In the above method for recovering silver ions from industrial wastewater, further, the content of silver ions in the industrial wastewater is greater than 0 and ≤ 1000 ppm, such as 1 ppm;

[0027] The ratio of the industrial wastewater to the blend of small molecule biphenyltetracarboxylic acid - polyvinylidene fluoride membrane is 50 mL:(0.04 - 0.1) g, such as 50 mL:0.04 g, 50 mL:0.05 g;

[0028] The conditions for the oscillation treatment are as follows: the temperature is 20 - 30 °C, and the rotation speed is 150 - 200 rpm, such as oscillation at 25 °C and 180 rpm.

[0029] In the above method for recovering silver ions from industrial wastewater, further, the industrial wastewater also contains Be 2+ , Na + , Mg 2+ , Al 3+ , K + , Ca 2+ , Cr 3+ , Mn 2+ , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , As 3+ , Cd2+ , Sb 5+ , Ba 2+ , Tl 3+ , Pb 2+ One or more of the above, such as the concentration is 1ppm.

[0030] In the above method for recovering silver ions from industrial wastewater, the method further comprises: 1) placing the adsorbed blended small molecule biphenyltetracarboxylic acid-polyvinylidene fluoride membrane in an eluent composed of thiourea and nitric acid (e.g., 0.3 mol / L thiourea (TU) + 0.1 mol / L HNO 3 Mixed solution) for immersion treatment (e.g., 15 minutes) to desorb Ag + ; 2) cleaning the desorbed blended small molecule biphenyltetracarboxylic acid-polyvinylidene fluoride membrane for recycling.

[0031] Compared with the prior art, the present invention has the following beneficial technical effects:

[0032] 1. The present invention obtains a BPTC-PVDF nanofiber matrix membrane by an electrospinning method, so that the prepared BPTC can be directly confined on the surface of the nanofiber, which is a good application carrier of the nano-adsorbent and has a wide range of application prospects.

[0033] 2. The BPTC-PVDF nanofiber matrix membrane prepared by the present invention has good water flux itself, realizes full utilization of active sites, enrichment of ions, and accurately adsorbs silver ions through the chelation of carboxyl groups, thereby achieving high selectivity.

[0034] 3. The BPTC-PVDF nanofiber matrix membrane prepared by the present invention has good physical and chemical stability, can be subjected to multiple adsorption-desorption, and can be regenerated by a binary mixed solution of nitric acid and thiourea to achieve the reuse of the material, and has practical application potential.

[0035] 4. The BPTC-PVDF nanofiber matrix membrane prepared by the present invention can convert the adsorbed silver ions into silver element under light conditions. This new composite membrane material can not only effectively remove silver ions in wastewater, but also realize the recycling of silver resources, providing powerful technical support and solutions for solving global problems such as water shortage and environmental pollution.

[0036] 5. The polyvinylidene fluoride used in the method of the present invention has a wide range of sources and is low in price, and has already formed a commercial commodity membrane. BRIEF DESCRIPTION OF THE DRAWINGS

[0037] Figure 1 PVDF / BPTC films with different BPTC concentrations and Fourier transform infrared spectra after adsorption in Examples 1-4 of the present invention and Comparative Example 1.

[0038] Figure 2 Figure showing the surface change of the membrane material after adsorption in the recovery performance test of the examples. (A) Surface of the membrane after adsorption in the dark in conical flask #2. (B) Surface of the membrane after adsorption under light in conical flask #1.

[0039] Figure 3 For Ag in the recovery performance test of the examples 0 XPS diagram.

[0040] Figure 4 SEM images of the membrane fibers of Example 1 and Comparative Example 1 and after adsorption without light or under light. a) 12% PVDF nanofibers of Comparative Example 1; b) 12% PVDF and 1% BPTC blended nanofibers of Example 1; c) 12% PVDF and 1% BPTC blended nanofibers of Example 1 after adsorption without light; d) 12% PVDF and 1% BPTC blended nanofibers of Example 1 after adsorption under light.

[0041] Figure 5 SEM images of single fiber of the membrane fibers of Example 1, Example 4 and Comparative Example 1 and after adsorption without light or under light. a) 12% PVDF of Comparative Example 1; b) 0.25% BPTC and 12% PVDF of Example 4; c) 1% BPTC and 12% PVDF of Example 1 after adsorption without light; d) 1% BPTC and 12% PVDF of Example 1 after adsorption under light. Detailed implementation manners

[0042] The present invention will be further described in detail below in conjunction with the specific implementation manners. The examples given are only for clarifying the present invention, rather than limiting the scope of the present invention. The following examples can be used as a guide for those of ordinary skill in the art to make further improvements, and do not limit the present invention in any way.

[0043] The methods used in the following examples, unless otherwise specified, are all conventional methods, carried out according to the techniques or conditions described in the literature in this field or according to the product specifications. The materials, reagents, etc. used in the following examples, unless otherwise specified, can be obtained from commercial sources.

[0044] The polyvinylidene fluoride used in the following examples is commercial polyvinylidene fluoride powder with an average molecular weight of 600,000.

[0045] The electrospinning machine used in the following examples is Tianjin Yunfan YFSP-GIII.

[0046] Example 1: Preparation of BPTC-PVDF nanofiber matrix membrane

[0047] The specific steps are as follows:

[0048] 1) Weigh PVDF: Precisely weigh 1.8 g (mass percentage in the BPTC-PVDF solution is 12%) of PVDF powder and add it to a 50 ml glass bottle. Measure 13.05 g of the mixed solvent of DMF and acetone (volume ratio 3:1) and add it to the glass bottle. Gently shake and stir to prevent caking. Magnetic stirring: Stir at 800 rpm for 3 h at 80 °C.

[0049] 2) Then add 0.15 g (mass percentage in the BPTC-PVDF solution is 1%) of BPTC to the glass bottle and continue stirring for 9 hours. Then perform ultrasonic treatment for 30 min.

[0050] 3) Pour the casting solution into a 10 ml syringe. Set the parameters of the electrospinning machine: room temperature 25 ± 1 °C, relative humidity 35 ± 5%, positive voltage 19 kV, negative voltage 2 kV, distance from the nozzle to the collector 15 cm, feeding flow rate 0.0020 mm / s, moving speed of the platform 1 mm·s -1 , translation distance 30 mm, continuously electrospin for 10 hours to form a fibrous membrane.

[0051] 4) Then place the prepared fibrous membrane in an oven at 80 °C for heat treatment for 10 h to obtain a blend membrane, denoted as 12% PVDF-1% BPTC blend membrane.

[0052] Example 2

[0053] The preparation steps are the same as those in Example 1, only adjust the addition amount of BPTC in step 2) to 0.1125 g (mass percentage in the BPTC-PVDF solution is 0.75%), adjust the mixed solvent of DMF and acetone (volume ratio 3:1) to 13.0875 g, keep the mass of PVDF unchanged, and the rest is the same, denoted as 12% PVDF-0.75% BPTC blend membrane.

[0054] Example 3

[0055] The preparation steps are the same as those in Example 1, only adjust the addition amount of BPTC in step 2) to 0.075 g (mass percentage in the BPTC-PVDF solution is 0.5%), adjust the mixed solvent of DMF and acetone (volume ratio 3:1) to 13.125 g, keep the mass of PVDF unchanged, and the rest is the same, denoted as 12% PVDF-0.5% BPTC blend membrane.

[0056] Example 4

[0057] The preparation steps are the same as those in Example 1, except that the addition amount of BPTC in step 2) is adjusted to 0.0375 g (mass percentage in the BPTC-PVDF solution is 0.25%), the mixed solvent of DMF and acetone (volume ratio of 3:1) is adjusted to 13.1625 g, the mass of PVDF remains unchanged, and the rest is the same, denoted as 12% PVDF-0.25% BPTC blend film.

[0058] Comparative Example 1

[0059] The preparation steps are the same as those in Example 1, only the step of adding BPTC is omitted, specifically as follows:

[0060] 1) Weigh PVDF: Accurately weigh 1.8 g (mass percentage in the BPTC-PVDF solution is 12%) of PVDF powder and add it to a 50 ml glass bottle. Measure a total of 13.2 g of the mixed solvent of DMF and acetone (volume ratio of 3:1) and add it to the glass bottle.

[0061] 2) Gently shake and stir to prevent caking. Magnetic stirring: Stir at 800 rpm for 3 h at 80 °C.

[0062] 3) Pour the casting solution into a 10 ml syringe. Set the parameters of the electrospinning machine, room temperature 25 ± 1 °C, relative humidity 35 ± 5%, positive voltage 19 kV, negative voltage 2 kV, nozzle to collector 15 cm, feeding flow rate 0.0020 mm / s, moving speed of the platform is 1 mm·s -1 , translation distance is 30 mm, and electrospin for 10 hours to form a fibrous membrane.

[0063] 4) Then place the prepared fibrous membrane in an 80 °C oven for heat treatment for 10 h, denoted as 12% PVDF membrane.

[0064] Adsorption performance test and characterization

[0065] Use the membrane material prepared in Example 1 to conduct a selective adsorption experiment of Ag, and the specific steps are as follows:

[0066] 1) Add 50 mL containing Be 2+ , Na + , Mg 2+ , Al 3+ , K + , Ca 2+ , Cr 3 + , Mn 2+ , Fe 3+ , Co 2+ , Ni 2+ , Cu 2+ , Zn 2+ , As3+ 、Cd 2+ 、Sb 5+ 、Ba 2+ 、Tl 3+ 、Pb 2+ (in the form of nitrate) and aqueous solutions with a concentration of 1 ppm each, and they were named 1#, 2#, 3#, and 4# respectively. Then, 0.04 g of the 12% PVDF-1% BPTC blend membrane prepared in Example 1 was added to the conical flasks of 1# and 2#, and the same amount of the 12% PVDF membrane prepared in Comparative Example 1 was added to the conical flasks of 3# and 4#. These conical flasks were placed in a shaker and oscillated for several hours under the conditions of 25 °C and 180 rpm. Among them, 1# was in the dark condition, 2# was in the light condition, 3# was in the dark condition, and 4# was in the light condition.

[0067] 2) At 8 h respectively, 5 ml of the solution was taken out from each conical flask, and these solutions were subsequently tested on an ICP-MS. The test results showed that the concentrations of silver ions in the four conical flasks were 179 ppb, 174 ppb, 1143 ppb, and 1131 ppb respectively, while the concentrations of other heavy metal ions remained unchanged. Thus, it can be seen that the PVDF-BPTC blend membrane of the present invention can achieve selective adsorption of silver ions.

[0068] The operation steps were the same as those in step 1) above. The 12% PVDF-0.75% BPTC blend membrane prepared in Example 2, the 12% PVDF-0.5% BPTC blend membrane prepared in Example 3, and the 12% PVDF-0.25% BPTC blend membrane prepared in Example 4 were added to the conical flasks for oscillating adsorption (light condition), and then the tests were taken out according to step 2) above respectively. The concentrations of silver ions were 351 ppb, 580 ppb, and 798 ppb respectively.

[0069] As Figure 1 shown are the membranes prepared in Examples 1-4 and Comparative Example 1, and the membrane corresponding to the 2# sample after the above adsorption performance test (i.e., Figure 1 1%-wt% BPTC-after adsorption under light treatment during adsorption) infrared spectrogram: The peaks at 840 and 877 cm -1 are the C-C stretching vibration peaks, the peak at 1072 cm -1 is the C-F stretching vibration peak, the peak at 1172 cm -1 is the C-F bending vibration peak, the peak at 1402 cm -1 is the C-H vibration absorption peak, which is the PVDF polymer chain. The absorption peak at 1715 cm -1 is the C=O stretching vibration of the carboxyl group on BPTC. The coordination bond formed between the carboxyl group and silver after adsorption causes the peak at 1715 cm -1 to shift to 1625 cm-1 The result shows that BPTC has been successfully blended into the PVDF membrane.

[0070] Recycling performance test and characterization

[0071] Only make the following modifications to step 1) in the adsorption performance test, and keep other conditions unchanged:

[0072] 1) Add 50 mL of 100 ppm AgNO 3 solution to three dry 50 mL conical flasks, numbered 1 # , 2 # , 3 # respectively. Add 0.05 g of the 12% PVDF-1% BPTC blend membrane prepared in Example 1 (3# is not added) to flasks 1 # and 2 # . And place flasks 1 # and 3 # in a shaker and oscillate for several hours under the conditions of 25 °C, 180 rpm and light. Place flask 2 # in the dark, and keep other conditions the same as those of 1 # and 3 # .

[0073] The test results show that the silver ion concentrations in flasks 1 # , 2 # and 3 # are 46 ppm, 50 ppm and 112 ppm respectively. Through analysis and calculation, the adsorption capacity for silver ions is 60 mg / g. As Figure 2 shows, Figures (B) and (A) are the surface change diagrams of the membranes under light in flask 1 and in the dark in flask 2 respectively. As Figure 2 can be seen, black spots appear on the membrane surface of flask 1 # , which are confirmed to be zero-valent silver ( Figure 3 ), indicating that silver ions can be converted into zero-valent silver under light conditions.

[0074] The membrane fibers prepared in Examples 1-4 and Comparative Example 1 and their scanning electron microscope images after adsorption in the dark or under light are shown in Figures 4 - 5 .

[0075] Figure 4 a) and Figure 5 a) are the scanning electron microscope image of 12% PVDF nanofibers in Comparative Example 1 and the scanning electron microscope image of a single fiber; Figure 4 b) and Figure 5 b) are the scanning electron microscope images of the blend nanofibers prepared in Example 1 and Example 4 respectively and the scanning electron microscope image of a single fiber; It is found by comparison that blocks appear on the surface of the nanofibers after blending, indicating that small molecules have grown on the nanofibers.

[0076] Figure 4 c) and Figure 5 c) is the SEM image of the blend nanofibers of 12% PVDF and 1% BPTC in Example 1 after adsorption in the dark and the SEM image of a single fiber. Figure 4 d) and Figure 5 d) is the SEM image of the blend nanofibers of 12% PVDF and 1% BPTC in Example 1 after adsorption under light irradiation and the SEM image of a single fiber. It is found by comparison that a large number of silver nanoparticles are deposited on the blend nanofibers after adsorption under light irradiation, further indicating that anions are converted into zero-valent silver under light irradiation conditions.

[0077] Thus, it can be seen that the blend nanofibers of PVDF and BPTC in the present invention can not only adsorb silver ions in industrial wastewater, but also further convert the adsorbed silver ions into zero-valent silver under natural light irradiation, realizing the recycling of silver resources in industrial wastewater.

[0078] Reusability test

[0079] The membrane material after adsorption in the 2# conical flask in the adsorption performance test was used with a mixed solution of 0.3 mol / L thiourea (TU) + 0.1 mol / L HNO 3 as the eluent and soaked at room temperature for 15 minutes to desorb Ag + (desorption efficiency > 99%). After desorption, the membrane was rinsed with deionized water again to prepare for the next round of adsorption.

[0080] Number of cycles: The above adsorption-desorption process was repeated 8 times to evaluate the performance decay of the membrane. The adsorption capacity decay rate only lost 4.15% after 8 cycles.

[0081] Thus, it can be seen that the nanofiber matrix membrane of BPTC-PVDF prepared in the present invention has good physical and chemical stability, can be used for multiple adsorption-desorption, and can be regenerated by a binary mixed solution of nitric acid and thiourea, realizing the reuse of the material and having practical application potential.

[0082] The above has detailed the present invention. For those skilled in the art, within the scope not departing from the purpose and spirit of the present invention, the present invention can be implemented within a relatively wide range under equivalent parameters, concentrations and conditions. Although specific embodiments of the present invention are given, it should be understood that the present invention can be further improved. In short, according to the principle of the present invention, this application intends to include any changes, uses or improvements to the present invention, including changes made by using conventional techniques known in the art that depart from the scope disclosed in this application.

Claims

1. A method for preparing a blended small molecule biphenyltetracarboxylic acid-polyvinylidene fluoride film by electrospinning, characterized in that: The steps include: S1, electrospinning a solution consisting of polyvinylidene fluoride, biphenyltetracarboxylic acid and a solvent, and collecting a fiber membrane; S2. heat-treating the fiber membrane to obtain the blended small molecule biphenyltetracarboxylic acid-polyvinylidene fluoride membrane.

2. The preparation method according to claim 1, characterized in that: The mass ratio of the polyvinylidene fluoride to the biphenyltetracarboxylic acid is 12:(0.25-1).

3. The preparation method according to any one of claims 1-2, characterized in that: The solvent consists of N,N-dimethylformamide and acetone in a volume ratio of 3:1; The mass percentage of the polyvinylidene fluoride in the solution is 12%; The mass percentage of the biphenyltetracarboxylic acid in the solution is 0.25% to 1%.

4. The preparation method according to claim 3, characterized in that: The method for preparing the solution composed of polyvinylidene fluoride, biphenyltetracarboxylic acid and solvent comprises: 1) mixing the polyvinylidene fluoride and the solvent, and stirring at 80°C and 600-1000 rpm for 2-3 hours; 2) adding the biphenyltetracarboxylic acid to the mixed solution obtained in step 1), continuing to stir for 8-9 hours, and finally performing ultrasonic treatment for 20-30 minutes.

5. The preparation method according to any one of claims 1 to 4, characterized in that: The electrospinning conditions are as follows: temperature 24-26° C., relative humidity 30-40%; The voltage of the electrospinning was set as follows: the positive voltage was 17 to 21 kV, and the negative voltage was -2 to 0 kV; In the electrospinning step, the distance from the nozzle to the collecting electrode is 15 to 18 cm; In the electrospinning step, the feed flow rate is 0.0010-0.0020 mm / s, and the moving speed of the platform is 1-3 mm·s -1 , the translation distance is 30 to 40 mm.

6. The preparation method according to any one of claims 1 to 5, characterized in that: The heat treatment temperature is 60-80° C. and the time is 10-12 hours.

7. A blended small molecule biphenyltetracarboxylic acid-polyvinylidene fluoride film, characterized in that: Prepared by the method according to any one of claims 1 to 6.

8. A method for recovering silver ions from industrial wastewater, characterized in that: The steps include: The blended small molecule biphenyltetracarboxylic acid-polyvinylidene fluoride membrane described in claim 7 is placed in industrial wastewater containing silver ions and subjected to oscillation treatment under natural light irradiation to adsorb the silver ions in the industrial wastewater and convert them into zero-valent silver.

9. The method for recovering silver ions from industrial waste water according to claim 8, characterized in that: The content of silver ions in the industrial wastewater is greater than 0 and ≤ 1000 ppm; The ratio of the industrial wastewater to the blended small molecule biphenyltetracarboxylic acid-polyvinylidene fluoride membrane is 50 mL: (0.04-0.1) g; The conditions of the shaking treatment are as follows: the temperature is 20-30° C. and the rotation speed is 150-200 rpm.

10. The method for recovering silver ions from industrial waste water according to claim 8 or 9, characterized in that: The industrial wastewater also contains Be 2+ 、Na + Mg 2+ 、Al 3+ , K + , Ca 2+ Cr 3+ , Mn 2+ , Fe 3+ 、Co 2+ 、Ni 2+ , Cu 2+ 、Zn 2+ 、As 3+ 、Cd 2+ , Sb 5+ , Ba 2+ 、Tl 3+ , Pb 2+ One or more of the following; and / or, The method further comprises: 1) placing the adsorbed blended small molecule biphenyltetracarboxylic acid-polyvinylidene fluoride membrane in an eluent consisting of thiourea and nitric acid for immersion treatment to desorb Ag+; 2) Cleaning the desorbed blended small molecule biphenyltetracarboxylic acid-polyvinylidene fluoride membrane for recycling.

Images