Preparation method and application of polyvinylidene fluoride-containing polymer hydrogel adsorbent

Abstract

The invention discloses a preparation method and application of a polyvinylidene fluoride-containing polymer hydrogel adsorbent. The method comprises the following steps: (1) modifying PVDF powder with a KOH / ethanol solution to obtain an active group containing double bonds, (2) grafting polyacrylic acid (PAA) through free radical polymerization to form a PVDF-g-PAA amphiphilic polymer, and (3) crosslinking with acrylamide (AM) to construct the three-dimensional interpenetrating network hydrogel. The efficient lithium ion adsorbent can be obtained only through three simple steps, the raw materials are low in price, and the method is suitable for large-scale preparation. The adsorbent realizes efficient lithium adsorption, the lithium ion adsorption capacity in a salt lake brine alkaline environment reaches 33.48 mg / g, and the lithium-magnesium selectivity coefficient reaches 45. Experiments show that the adsorption capacity of the adsorbent is still kept at 82% after multiple times of circulation, and the adsorbent has excellent mechanical stability and cyclic regeneration capacity and is particularly suitable for lithium resource extraction of salt lake brine with a high magnesium-lithium ratio.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of lithium adsorption materials, and particularly relates to a preparation method and application of a polyvinylidene fluoride-containing polymer hydrogel adsorbent. Background Art

[0002] With the transformation of the global energy structure towards cleaner energy, lithium resources, as the core material of high-energy density batteries, have become increasingly prominent in their strategic position. As an important carrier of lithium resources, salt lake brine accounts for more than 60% of the global lithium reserves. However, its complex ionic composition (such as high concentrations of Na + , K + , Mg 2+ etc.) and low lithium concentration (usually 0.01 - 0.15%) make the efficient selective extraction of lithium a technical bottleneck. Traditional adsorption materials (such as manganese-based and titanium-based oxides) have certain adsorption capabilities, but generally face problems such as poor selectivity, insufficient cycle stability, and easy agglomeration of nanoparticles, making it difficult to meet industrial demands.

[0003] Currently, the technologies for lithium recovery at home and abroad mainly include: chemical precipitation method, liquid-liquid extraction method, electrochemical reduction method, membrane separation method, adsorption method, etc. Among them, the adsorption method is widely used for the separation of different ions in liquids due to its advantages such as simple method, environmental friendliness, and economic efficiency. During the lithium extraction process, lithium is selectively adsorbed and then desorbed by the solution, thereby separating from coexisting ions. Several types of inorganic metal-based (lithium aluminum layered double hydroxide (LiAl-LDH), lithium titanium oxide-based lithium ion sieve (LTO-LIS), and lithium manganese oxide-based lithium ion sieve (LMO-LIS)) and metal-organic frameworks (MOF) as well as crown ether-based adsorbents have been tested and reported for lithium recovery.

[0004] For example, the Chinese invention patent document with the authorization number CN115475607B discloses a preparation method of a cationic amphiphilic fluorine-containing hydrogel adsorbent. After dissolving 1-vinylimidazole in a solvent, 1,6-dibromohexane is added. After reacting to obtain an intermediate product, it is then reacted with perfluoroalkanoic acid. After filtering and column chromatography purification of the product, a cationic amphiphilic fluorine-containing monomer is obtained. Then, the obtained amphiphilic fluorine-containing monomer is reacted with acrylic acid monomer and N,N'-methylenebisacrylamide, and finally a fluorine-containing hydrogel adsorbent is obtained. However, the process of preparing the adsorbent by this method is relatively cumbersome, and column chromatography treatment is required for the intermediate product, which is not conducive to large-scale production. Moreover, the price of perfluoroalkanoic acid is relatively expensive, and the overall production cost of the adsorbent is relatively high. Summary of the Invention

[0005] The object of the present invention is to provide a simple and convenient technology to meet the actual needs, and to provide a preparation method of a polyvinylidene fluoride-based polymer hydrogel adsorbent for solving the problems of complex adsorption synthesis process and high cost of existing lithium adsorbents. This adsorbent realizes efficient lithium adsorption by means of the specific coordination of the C-F bond of PVDF with Li + and the ionization of the carboxylic acid groups of PAA, and the lithium adsorption capacity of this material for lithium ions in water can reach 33.48 mg / g at room temperature, and its adsorption effect on other metal cations is relatively low. For example, the lithium / magnesium selectivity coefficient for magnesium ions can reach 45, and it has good adsorption performance in an alkaline environment, is very suitable for the alkaline environment of salt lake brine, can be reused, has good sustainability, and at the same time makes it have significant application potential.

[0006] To solve the technical problems of the present invention, the following solutions are proposed: A preparation method of a polyvinylidene fluoride-based polymer hydrogel adsorbent, comprising the following steps:

[0007] (1) Preparation of modified polyvinylidene fluoride (PVDF) powder

[0008] a. First, prepare an aqueous KOH solution with a mass fraction of 5-20%, and then mix it with absolute ethanol, with a volume ratio of 1:0.5-3. Uniformly disperse the polyvinylidene fluoride powder in the above-mentioned KOH water / absolute ethanol mixed solution, and stir magnetically to make it fully dispersed, and then stir at 40-70 °C to obtain a brown solution;

[0009] b. The post-treatment of the reaction solution includes the steps of: centrifuging to take the lower-layer solid, washing with deionized water, ultrasonic dispersion, and centrifuging again. Repeat this step until the solution is washed to neutral, and freeze-dry the obtained solid under vacuum to obtain modified PVDF powder.

[0010] (2) Preparation of PVDF-g-PAA

[0011] a. Add the modified PVDF powder into a mixed solution of water / absolute ethanol (the volume ratio of water and absolute ethanol is 1:0.5-3), ultrasonically disperse it evenly, then add azobisisobutyronitrile and acrylic acid monomer, and carry out free radical polymerization at 60-80 °C under a nitrogen atmosphere;

[0012] b. Dilute the reaction solution with deionized water, then carry out centrifugation treatment on it, and repeatedly wash and centrifuge it with deionized water to wash away the unreacted acrylic acid monomer. Finally, freeze-dry the obtained brown solid under vacuum to obtain dry PVDF-g-PAA powder.

[0013] (3) Preparation of PVDF-g-PAA / PAM polymer hydrogel adsorbent

[0014] a. First, ultrasonically disperse and mix PVDF-g-PAA with ultrapure water evenly, then add N,N′-methylenebisacrylamide, ammonium persulfate, and acrylamide. After the chemicals are completely dissolved, react at 50 - 70 °C under a nitrogen atmosphere;

[0015] b. After the reaction is completed, wash the surface of the PVDF-g-PAA / PAM polymer hydrogel adsorbent with deionized water.

[0016] Preferably, in step (1) a, the mass fraction of the KOH aqueous solution is 10%; in step (1) a, the volume ratio of the KOH aqueous solution to absolute ethanol is 1:1; in step (1) a, the reaction temperature is 60 °C; in step (2) a, the volume ratio of water to absolute ethanol is 1:1; in step (2) a, the reaction temperature is 70 °C; in step (3) a, 1000 mg of PVDF-g-PAA, 5 mL of ultrapure water, 15 mg of N,N′-methylenebisacrylamide, 15 mg of ammonium persulfate, and 1000 mg of acrylamide are used; in step (3) a, the reaction temperature is 60 °C.

[0017] It includes the following steps:

[0018] (1) Preparation of modified polyvinylidene fluoride PVDF powder

[0019] a. First, prepare 20 mL of a 10% KOH aqueous solution, then mix it with 20 mL of absolute ethanol with a volume ratio of 1:1. Disperse 10 g of polyvinylidene fluoride powder evenly in the above KOH water / absolute ethanol mixed solution, and stir magnetically to make it fully dispersed. Then stir at 60 °C for 10 min to obtain a brown solution;

[0020] b. The post-treatment of the reaction solution includes the steps: centrifuge to take the lower-layer solid, wash it with deionized water, ultrasonically disperse it, and centrifuge again. Repeat this step until the solution is washed to neutral. Freeze-dry the obtained solid under vacuum for 24 hours to obtain the modified PVDF powder;

[0021] (2) Preparation of PVDF-g-PAA

[0022] a. Add 1 g of the modified PVDF powder to 20 mL of a water / absolute ethanol mixed solution with a volume ratio of 1:1, ultrasonically disperse it evenly, then add 20 mg of azobisisobutyronitrile and 1 mL of acrylic acid monomer, and react at 70 °C for 12 hours under a nitrogen atmosphere for free radical polymerization;

[0023] b. Dilute the reaction solution with deionized water, then centrifuge it, and repeatedly wash and centrifuge it with deionized water to wash away the unreacted acrylic monomer. Finally, freeze-dry the obtained brown solid under vacuum for 24 hours to obtain dry PVDF-g-PAA powder;

[0024] (3)Preparation of PVDF-g-PAA / PAM polymer hydrogel adsorbent

[0025] a. First, ultrasonically disperse and mix 1000 mg of PVDF-g-PAA powder with 5 mL of ultrapure water until evenly mixed. Then add 15 mg of N,N′-methylenebisacrylamide, 15 mg of ammonium persulfate, and 1 g of acrylamide. After the drugs are completely dissolved, react at 60 °C for 4 hours under a nitrogen atmosphere;

[0026] b. After the reaction is completed, wash the surface of the PVDF-g-PAA / PAM polymer hydrogel adsorbent with deionized water.

[0027] That is, Example 1 is the optimal process parameter, and the adsorption capacity is increased by more than 50% compared with other cross-linking materials (such as 4VP / DMAEMA). This material exhibits excellent performance: the lithium adsorption capacity reaches 33.48 mg / g, and the lithium / magnesium selectivity coefficient is as high as 45, significantly superior to existing manganese-based and titanium-based adsorbents (15 - 22 mg / g). Its three-dimensional network structure endows excellent mechanical properties, and the adsorption capacity remains 82% after 5 cycles, breaking through the bottleneck that traditional adsorbents are prone to agglomeration and inactivation. Compared with other comparative examples and examples, it has unexpected technical effects.

[0028] To solve the technical problems of the present invention, another solution proposed by the present invention is as follows: Application of the polyvinylidene fluoride-containing polymer hydrogel adsorbent in lithium ion adsorption.

[0029] Preferably, place the hydrogel adsorbent in a lithium-containing solution for 20 - 30 hours to achieve the adsorption of lithium ions.

[0030] Preferably, after taking out the hydrogel adsorbent that has completed adsorption in the lithium-containing solution, desorb it, and place it in the next portion of the lithium-containing solution to adsorb lithium ions. Repeat the same operation multiple times to achieve the cyclic adsorption of the hydrogel adsorbent.

[0031] Preferably, the prepared hydrogel adsorbent is placed in a lithium-containing solution with a pH of 10 and a lithium ion concentration of 300 mg / L to adsorb lithium ions. After the adsorption is completed, the hydrogel adsorbent is taken out and placed in a 1 mmol sulfuric acid solution for desorption. Then, the desorbed hydrogel adsorbent is placed in another 300 mg / L lithium-containing solution for the second adsorption. The above operation is repeated five times. The first adsorption capacity of the polyvinylidene fluoride-based hydrogel adsorbent for lithium ions is 33.48 mg / g. After five cycles, the adsorption capacity can still reach 27.45 mg / g.

[0032] The beneficial effects of the present invention are as follows:

[0033] A preparation method of a polyvinylidene fluoride-based hydrogel adsorbent of the present invention has the advantage that polyvinylidene difluoride is used as the backbone, and amphiphilic polymers are formed by grafting polyacrylic acid, and then a three-dimensional interpenetrating network is constructed with polyacrylamide; the C-F bond of PVDF forms specific coordination (hard acid-hard base interaction) with Li + through strong electronegativity to preferentially adsorb lithium ions; the carboxylic acid groups of PAA are ionized under the high pH conditions of salt lake brine, and Li is enriched through electrostatic attraction + , improving the adsorption capacity; the flexible chain of PAM is complementary to the rigid chain of PVDF, enhancing the mechanical properties of the hydrogel. The PAM network expands / contracts to regulate the pore structure and accelerate the diffusion of Li + ; the C-F bond of PVDF is chemically inert to resist corrosive ions such as Cl - , SO 4 2- etc., improving its stability in the brine environment.

[0034] The polyvinylidene fluoride-based hydrogel adsorbent synthesized by the method of the present invention has a simple synthesis method and low cost. This material exhibits excellent lithium ion adsorption performance. It has been verified that the lithium ion adsorption capacity of this material in water can reach 33.48 mg / g at room temperature, showing a very large improvement compared with the lithium ion adsorption amounts of 15 mg / g in the authorized patent No. CN108435142, 17 mg / g in the authorized patent No. CN108126651, 22.05 mg / g in the authorized patent No. CN110898794, etc.; the adsorption effect on other metal cations is relatively low. For example, the lithium-magnesium selectivity coefficient for magnesium ions can reach 45, and it has good adsorption performance in an alkaline environment, being very suitable for the alkaline environment of salt lake brine, and can be reused, having good sustainability, and at the same time having significant application potential.

[0035] Aiming at the problems of complex synthesis process, high cost and poor selectivity of existing adsorbents, this technology constructs a three-dimensional interpenetrating network hydrogel with PVDF as the matrix through a three-step method: First, KOH / ethanol is used to modify PVDF to introduce active double bonds, then free radical polymerization is carried out to graft PAA to form an amphiphilic polymer, and finally it is cross-linked with AM to form a PVDF-g-PAA / PAM composite hydrogel. This material innovatively uses the C-F bond of PVDF to specifically coordinate lithium ions and exhibits excellent performance in an alkaline environment (pH = 10): the lithium adsorption capacity reaches 33.48 mg / g, and the lithium-magnesium selectivity coefficient is as high as 45, significantly superior to existing manganese-based and titanium-based adsorbents (15-22 mg / g). Its three-dimensional network structure endows excellent mechanical properties, and the adsorption capacity remains 82% after 5 cycles, breaking through the bottleneck that traditional adsorbents are prone to agglomeration and inactivation. Experiments confirm that the optimized KOH concentration (10%), reaction temperature (60 °C) and AM dosage (1 g), that is, Example 1 is the best process parameter, and the adsorption amount is increased by more than 50% compared with other cross-linked materials (such as 4VP / DMAEMA). This technology uses cheap and easily available raw materials, and the process is simple and controllable, especially suitable for the extraction of lithium resources from salt lake brines with a high magnesium-lithium ratio, providing new ideas for the development of clean energy materials. Description of the Drawings

[0036] Figure 1 Schematic diagram of the synthesis of the amphiphilic polymer PVDF-g-PAA in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent;

[0037] Figure 2 1H NMR spectrum of PVDF-g-PAA prepared in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent;

[0038] Figure 3 SEM image of the PVDF-g-PAA / PAM polymer hydrogel adsorbent prepared in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent;

[0039] Figure 4 Selective adsorption capacity diagram of the PVDF-g-PAA / PAM polymer hydrogel adsorbent prepared in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent for lithium ions in a mixed solution of monovalent and divalent metals;

[0040] Figure 5 Adsorption capacity diagram of the PVDF-g-PAA / PAM polymer hydrogel adsorbent prepared in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent changing with adsorption time;

[0041] Figure 6Pseudo-first-order kinetic adsorption curve of the PVDF-g-PAA / PAM polymer hydrogel adsorbent prepared in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent;

[0042] Figure 7 Pseudo-second-order kinetic adsorption curve of the PVDF-g-PAA / PAM polymer hydrogel adsorbent prepared in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent;

[0043] Figure 8 Isothermal adsorption curve of the PVDF-g-PAA / PAM polymer hydrogel adsorbent prepared in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent;

[0044] Figure 9 Adsorption capacity diagram of the PVDF-g-PAA / PAM polymer hydrogel adsorbent prepared in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent for lithium ions at different pH values;

[0045] Figure 10 Cyclic adsorption capacity diagram of the PVDF-g-PAA / PAM polymer hydrogel adsorbent prepared in Example 1 of a preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent. Detailed implementation manners

[0046] The technical solutions in the embodiments of the present invention will be further clearly and completely described below with reference to the accompanying drawings in the embodiments of the present invention. Obviously, the described embodiments are only a part of the embodiments of the present invention, rather than all the embodiments.

[0047] The reagents and materials used in the following embodiments, unless otherwise specified, can all be obtained from commercial channels.

[0048] Example 1

[0049] A preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent includes the following steps:

[0050] (1) Prepare modified polyvinylidene fluoride (PVDF) powder

[0051] a. First, prepare 20 mL of a 10% KOH aqueous solution, and then mix it with 20 mL of absolute ethanol, with a volume ratio of 1:1. Uniformly disperse 10 g of polyvinylidene fluoride powder in the above KOH water / absolute ethanol mixed solution, and stir magnetically to fully disperse it, and then stir at 60 °C for 10 min to obtain a brown solution;

[0052] b. The post-treatment of the reaction solution includes the steps of: centrifuging to obtain the lower-layer solid, washing with deionized water, ultrasonically dispersing, centrifuging again, repeating this step until the solution is washed to neutrality, and freeze-drying the obtained solid under vacuum for 24 hours to obtain the modified PVDF powder.

[0053] (2) Preparation of PVDF-g-PAA

[0054] a. Add 1 g of the modified PVDF powder into 20 mL of a mixed solution of water / absolute ethanol (the volume ratio of water to absolute ethanol is 1:1), ultrasonically disperse it evenly, then add 20 mg of azobisisobutyronitrile and 1 mL of acrylic acid monomer, and carry out free radical polymerization at 70 °C for 12 hours under a nitrogen atmosphere;

[0055] b. Dilute the reaction solution with deionized water, then carry out centrifugation treatment on it, and repeatedly wash and centrifuge it with deionized water to wash away the unreacted acrylic acid monomer. Finally, freeze-dry the obtained brown solid under vacuum for 24 hours to obtain the dry PVDF-g-PAA powder.

[0056] (3) Preparation of PVDF-g-PAA / PAM polymer hydrogel adsorbent

[0057] a. First, ultrasonically disperse and mix 1000 mg of PVDF-g-PAA powder with 5 mL of ultrapure water evenly, then add 15 mg of N,N′-methylenebisacrylamide, 15 mg of ammonium persulfate, and 1 g of acrylamide. After the drugs are completely dissolved, react at 60 °C for 4 hours under a nitrogen atmosphere;

[0058] b. After the reaction is completed, wash the surface of the PVDF-g-PAA / PAM polymer hydrogel adsorbent with deionized water.

[0059] In this example, the preparation route of PVDF-g-PAA is as Figure 1 shown. After the synthesis is completed, nuclear magnetic resonance hydrogen spectrum characterization and analysis are carried out on the final product in step (2) b. Dissolve the PVDF-g-PAA prepared in this example in deuterated dimethyl sulfoxide, and then carry out nuclear magnetic resonance hydrogen ( 1 1H NMR) spectrum test, 1 The 1H NMR spectrum is as Figure 2 shown, indicating that PVDF-g-PAA has been successfully synthesized.

[0060] Freeze the PVDF-g-PAA / PAM polymer hydrogel adsorbent prepared in this example at -20 °C, and then place it under vacuum for freeze-drying for 24 hours, and carry out scanning electron microscope (SEM) test. The SEM image is as Figure 3As shown, the hydrogel has a uniform pore distribution and uniform pore size, which is beneficial for adsorbing lithium ions in water.

[0061] The application of a polyvinylidene fluoride-based polymer hydrogel adsorbent in adsorbing lithium ions includes the following steps:

[0062] The hydrogel adsorbent prepared in Example 1 was placed in mixed solutions of lithium ions, sodium ions, potassium ions, magnesium ions, and calcium ions with different concentrations for 24 hours of adsorption; the hydrogel adsorbent prepared in Example 1 was placed in lithium-containing solutions with different pH values for adsorption; the hydrogel adsorbent prepared in Example 1 was placed in a lithium-containing solution with pH = 10 for 24 hours, and the adsorption capacity of lithium ions at different time states was tested, and it was fitted with a pseudo-first-order kinetic model and a pseudo-second-order kinetic model; the hydrogel adsorbent prepared in Example 1 was placed in a solution with pH = 10 and different lithium concentrations. In addition to room temperature of 25 °C, the same operations were performed at 35 °C and 45 °C, and it was fitted with a Langmuir model and a Freundlich model; the hydrogel adsorbent prepared in Example 1 was placed in a lithium-containing solution with a lithium ion concentration of 300 mg / L to adsorb lithium ions. After the adsorption was completed, the hydrogel adsorbent was taken out and placed in a 1 mmol sulfuric acid solution for desorption, and then the desorbed hydrogel adsorbent was placed in another 300 mg / L lithium-containing solution for the second adsorption. The above operations were repeated five times.

[0063] Summary: The adsorption results are as Figures 4 - 10 shown. The results show that the hydrogel has good selectivity for magnesium ions and calcium ions. The influence of sodium ions and potassium ions is slightly higher, but generally it does not prevent the adsorption of lithium ions. The lithium-magnesium selectivity coefficient for magnesium ions can reach 45; the hydrogel adsorbent can reach the adsorption equilibrium at 600 minutes. The fitting results of the pseudo-first-order kinetic model ( Figure 6 ) and the pseudo-second-order kinetic model ( Figure 7 ) are more biased towards the pseudo-second-order kinetic model, indicating that the adsorption of lithium ions by the polyvinylidene fluoride-based polymer hydrogel adsorbent is chemisorption; as the temperature increases, the adsorption capacity of lithium ions increases accordingly. Heating is beneficial for the adsorption of lithium ions, and the fitting result is more biased towards the Langmuir model, indicating that the adsorption is monolayer adsorption; under alkaline conditions, it is more beneficial for the adsorption of lithium ions, and it also makes it more adaptable to the alkaline environment of salt lake brine; the first adsorption amount of lithium ions by the polyvinylidene fluoride-based polymer hydrogel adsorbent is 33.48 mg / g. After five cycles, the adsorption amount can still reach 27.45 mg / g. It shows that the prepared hydrogel adsorbent has good cyclic adsorption performance and good sustainability. Experiments show that the adsorption capacity of the adsorbent still remains 82% after several cycles.

[0064] Example 2

[0065] The preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent is the same as that of Example 1, and the differences are as follows: in step (1)a, the mass fraction of the KOH aqueous solution is 5%, and the volume ratio of water to absolute ethanol is 1:2; in step (2)a, the amount of azobisisobutyronitrile used is 40 mg, the amount of acrylic acid used is 2 mL, and the reaction time is 16 h; in step (3)a, the amount of PVDF-g-PAA used is 500 mg. Other steps are the same as those of Example 1.

[0066] The application of a polyvinylidene fluoride-containing polymer hydrogel adsorbent in adsorbing lithium ions is the same as that of Example 1 except that the gel used is the one prepared in Example 2, and the other steps are not repeated here.

[0067] Summary: A polyvinylidene fluoride-containing polymer hydrogel adsorbent prepared in Example 2 has a good selective adsorption effect on lithium ions, conforms to the pseudo-second-order kinetic model and the Langmuir model, and the adsorption capacity for lithium ions in a lithium-containing solution with pH = 10 and a concentration of 300 mg / L is 22.35 mg / g. After five cycles, the adsorption capacity is 18.11 mg / g. The decrease in the amount of PVDF-g-PAA used leads to a decrease in the adsorption capacity, indicating that the content of fluorine element is closely related to the adsorption capacity.

[0068] Example 3

[0069] The preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent is the same as that of Example 1, and the differences are as follows: in step (1)a, the mass fraction of the KOH aqueous solution is 20%, and the reaction time is 15 min; in step (2)a, the reaction temperature is 80 °C, and the reaction time is 8 h; in step (3)a, the amount of PVDF-g-PAA used is 1500 mg. Other steps are the same as those of Example 1.

[0070] The application of a polyvinylidene fluoride-containing polymer hydrogel adsorbent in adsorbing lithium ions is the same as that of Example 1 except that the gel used is the one prepared in Example 3, and the other steps are not repeated here.

[0071] Summary: A polyvinylidene fluoride-containing polymer hydrogel adsorbent prepared has a good selective adsorption effect on lithium ions, conforms to the pseudo-second-order kinetic model and the Langmuir model, and the adsorption capacity for lithium ions in a lithium-containing solution with pH = 10 and a concentration of 300 mg / L is 38.09 mg / g. After five cycles, the adsorption capacity is 32.76 mg / g. This example has the largest adsorption capacity, but it uses too much PVDF-g-PAA, and the performance improvement is not significant. Considering the economic benefits, the dosage of Example 1 is the best choice.

[0072] Example 4

[0073] The preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent is the same as that of Example 1, and the differences are as follows: in step (1)a, the mass fraction of the KOH aqueous solution is 15%, the reaction temperature is 40 °C, and the reaction time is 30 min; in step (2)a, the reaction temperature is 60 °C and the reaction time is 24 h; in step (3)a, the reaction temperature is 50 °C and the reaction time is 6 h. Other steps are the same as those in Example 1.

[0074] The application of a polyvinylidene fluoride-containing polymer hydrogel adsorbent in adsorbing lithium ions is the same as that of Example 1 except that the gel used is the one prepared in Example 4, and the other steps are not repeated.

[0075] Summary: The prepared polyvinylidene fluoride-containing polymer hydrogel adsorbent has a good selective adsorption effect on lithium ions, conforms to the second-order kinetic model and the Langmuir model, and the adsorption capacity for lithium ions in a lithium-containing solution with pH = 10 and 300 mg / L is 32.58 mg / g. After five cycles, the adsorption capacity is 27.69 mg / g. The dosage of PVDF-g-PAA is the same as that in Example 1, and the change in adsorption capacity is not significant. The reaction temperature and time for forming the gel change slightly, indicating that the gel is easy to prepare, has less strict requirements on the reaction temperature and time, and is more conducive to large-scale production.

[0076] Comparative Example 1

[0077] A preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent includes the following steps:

[0078] The preparation method of the adsorbent is basically the same as that of Example 1, and the difference is that acrylamide is replaced by 4-vinylpyridine (4VP) to prepare a PVDF-g-PAA / P4VP polymer hydrogel adsorbent.

[0079] The application of a polyvinylidene fluoride-containing polymer hydrogel adsorbent in adsorbing lithium ions is basically the same as that of Example 1 and will not be repeated.

[0080] Summary: The prepared polyvinylidene fluoride-containing polymer hydrogel adsorbent has a good selective adsorption effect on lithium ions, conforms to the second-order kinetic model and the Langmuir model, and the adsorption capacity for lithium ions in a lithium-containing solution with pH = 10 and 300 mg / L is 20.32 mg / g. After five cycles, the adsorption capacity is 16.25 mg / g. The hydrogel formed using P4VP has a worse absorption and swelling effect than the hydrogel formed using PAM, which leads to a decrease in the adsorption amount, and the cost of AM is much lower than that of 4VP.

[0081] Comparative Example 2

[0082] A preparation method of a polyvinylidene fluoride-containing polymer hydrogel adsorbent comprises the following steps:

[0083] The preparation method of the adsorbent is basically the same as that of Example 1, except that acrylamide is replaced with dimethylaminoethyl methacrylate (DMAEMA) to prepare a PVDF-g-PAA / PDMAEMA polymer hydrogel adsorbent.

[0084] An application of a polyvinylidene fluoride-containing polymer hydrogel adsorbent in adsorbing lithium ions is basically the same as that of Example 1 and will not be repeated.

[0085] Summary: The prepared polyvinylidene fluoride-containing polymer hydrogel adsorbent has a good selective adsorption effect on lithium ions, conforms to the second-order kinetic model and the Langmuir model, and has an adsorption capacity of 23.41 mg / g for lithium ions in a lithium-containing solution with pH = 10 and a concentration of 300 mg / L. After five cycles, the adsorption capacity is 18.84 mg / g. The hydrogel formed by using PDMAEMA has a worse absorption and swelling effect than the hydrogel formed by PAM, which leads to a decrease in the adsorption amount, and the cost of AM is lower than that of DMAEMA.

[0086] In summary, the present invention discloses a preparation method and application of an efficient selective lithium ion adsorbent, belonging to the technical field of lithium adsorption materials. Aiming at the problems of complex synthesis process, high cost and poor selectivity of existing adsorbents, this technology uses PVDF as the matrix to construct a three-dimensional interpenetrating network hydrogel by a three-step method: firstly, PVDF is modified with KOH / ethanol to introduce active double bonds, then PAA is grafted by free radical polymerization to form an amphiphilic polymer, and finally it is crosslinked with AM to form a PVDF-g-PAA / PAM composite hydrogel. This material innovatively uses the C-F bond of PVDF to specifically coordinate lithium ions and exhibits excellent performance in an alkaline environment (pH = 10): the lithium adsorption capacity reaches 33.48 mg / g, and the lithium-magnesium selectivity coefficient is as high as 45, significantly superior to existing manganese-based and titanium-based adsorbents (15 - 22 mg / g). Its three-dimensional network structure endows excellent mechanical properties, and the adsorption capacity remains 82% after 5 cycles, breaking through the bottleneck that traditional adsorbents are prone to agglomeration and inactivation. Experiments confirm that the optimized KOH concentration (10%), reaction temperature (60 °C) and AM dosage (1 g), that is, Example 1 is the best process parameters, and the adsorption amount is increased by more than 50% compared with other crosslinked materials (such as 4VP / DMAEMA). This technology has cheap and easily available raw materials, a simple and controllable process, and is especially suitable for the extraction of lithium resources from high magnesium-lithium ratio salt lake brines, providing new ideas for the development of clean energy materials.

[0087] The embodiments disclosed in the present invention are intended to exemplarily demonstrate the technical concept and implementation path of the present invention, and do not exhaustively list all possible forms. Those skilled in the art can make adaptive adjustments to the specific implementation structure, material selection or process parameters on the premise of fully understanding the essence of the present invention, including but not limited to using equivalent technical means to replace some feature elements. Any reasonable extension, combination optimization or application scenario expansion based on the core idea of this technical solution, as long as it does not exceed the protection scope defined by the claims, is considered to fall within the legal protection radius of this patent.

Claims

1. A method for preparing a polyvinylidene fluoride-containing polymer hydrogel adsorbent, characterized in that: The steps include: (1) Preparation of modified polyvinylidene fluoride (PVDF) powder a. First, prepare a KOH aqueous solution with a mass fraction of 5-20%, and then mix it with anhydrous ethanol in a volume ratio of 1:0.5-3; evenly disperse the polyvinylidene fluoride powder in the above KOH water / anhydrous ethanol mixed solution, stir it magnetically to fully disperse it, and stir it at 40-70 ° C to obtain a brown solution; b. The post-treatment of the reaction solution comprises the steps of: removing the solid layer by centrifugation, washing with deionized water, ultrasonic dispersion, centrifuging again, repeating the steps until the solution is washed to neutrality, and freeze-drying the obtained solid under vacuum to obtain a modified PVDF powder; (2) Preparation of PVDF-g-PAA a. Add the modified PVDF powder to a mixed solution of water / anhydrous ethanol with a volume ratio of water to anhydrous ethanol of 1:0.5-3, disperse it evenly by ultrasonication, then add azobisisobutyronitrile and acrylic acid monomer, and react at 60-80 °C under a nitrogen atmosphere for free radical polymerization; b. The reaction solution was diluted with deionized water, and then centrifuged and repeatedly washed with deionized water and centrifuged to remove the unreacted acrylic acid monomer. Finally, the obtained brown solid was freeze-dried under vacuum to obtain a dry PVDF-g-PAA powder. (3) Preparation of PVDF-g-PAA / PAM polymer hydrogel adsorbent a. First, PVDF-g-PAA and ultrapure water are ultrasonically dispersed and mixed uniformly, and then N,N′-methylenebisacrylamide, ammonium persulfate, and acrylamide are added. After the drugs are completely dissolved, the reaction is carried out at 50-70 °C under a nitrogen atmosphere; b. After the reaction is completed, the surface of the PVDF-g-PAA / PAM polymer hydrogel adsorbent is washed with deionized water.

2. The method for preparing the polyvinylidene fluoride-containing polymer hydrogel adsorbent according to claim 1, characterized in that: The mass fraction of the KOH aqueous solution in step (1) a is 10%; the volume ratio of the KOH aqueous solution to anhydrous ethanol in step (1) a is 1:1; the reaction temperature in step (1) a is 60 °C; the volume ratio of water to anhydrous ethanol in step (2) a is 1:1; the reaction temperature in step (2) a is 70 °C; in step (3) a, PVDF-g-PAA is 1000 mg, ultrapure water is 5 mL, N,N′-methylenebisacrylamide is 15 mg, ammonium persulfate is 15 mg, and acrylamide is 1000 mg; the reaction temperature in step (3) a is 60 °C.

3. The method for preparing the polyvinylidene fluoride-containing polymer hydrogel adsorbent according to claim 1, characterized in that: The steps include: (1) Preparation of modified polyvinylidene fluoride PVDF powder a. First, prepare 20 mL of a 10% KOH aqueous solution, then mix it with 20 mL of anhydrous ethanol in a volume ratio of 1:1, and evenly disperse 10 g of polyvinylidene fluoride powder in the above KOH water / anhydrous ethanol mixed solution. After magnetic stirring to fully disperse it, stir at 60 ° C for 10 min to obtain a brown solution; b. The post-treatment of the reaction solution comprises the steps of: removing the solid layer by centrifugation, washing with deionized water, ultrasonic dispersion, centrifuging again, repeating the steps until the solution is washed to neutrality, and freeze-drying the obtained solid under vacuum for 24 hours to obtain a modified PVDF powder; (2) Preparation of PVDF-g-PAA a. Add 1 g of modified PVDF powder to 20 mL of water / anhydrous ethanol (water and anhydrous ethanol in a volume ratio of 1:1) mixed solution, disperse evenly by ultrasonication, then add 20 mg of azobisisobutyronitrile and 1 mL of acrylic acid monomer, and react at 70 °C for 12 hours under a nitrogen atmosphere for free radical polymerization; b. The reaction solution was diluted with deionized water, centrifuged, and repeatedly washed with deionized water and centrifuged to remove the unreacted acrylic acid monomer, and finally the obtained brown solid was freeze-dried under vacuum for 24 hours to obtain a dry PVDF-g-PAA powder; (3) Preparation of PVDF-g-PAA / PAM polymer hydrogel adsorbent a. First, 1000 mg of PVDF-g-PAA powder was evenly mixed with 5 mL of ultrapure water by ultrasonic dispersion, and then 15 mg of N,N′-methylenebisacrylamide, 15 mg of ammonium persulfate, and 1 g of acrylamide were added. After the drugs were completely dissolved, the mixture was reacted at 60 °C for 4 hours under a nitrogen atmosphere. b. After the reaction is completed, the surface of the PVDF-g-PAA / PAM polymer hydrogel adsorbent is washed with deionized water.

4. Use of the polyvinylidene fluoride-containing polymer hydrogel adsorbent prepared according to any one of claims 1 to 3 in lithium ion adsorption.

5. The use of the polyvinylidene fluoride polymer hydrogel adsorbent according to claim 4 in lithium ion adsorption, characterized in that: The hydrogel adsorbent is placed in a lithium-containing solution for 20-30 hours to achieve adsorption of lithium ions.

6. The use of the polyvinylidene fluoride polymer hydrogel adsorbent according to claim 4 in lithium ion adsorption, characterized in that: The hydrogel adsorbent that has completed adsorption in the lithium-containing solution is taken out and desorbed, and then placed in the next lithium-containing solution to adsorb lithium ions. The same operation is repeated for multiple cycles to achieve cyclic adsorption of the hydrogel adsorbent.

7. Use of the polyvinylidene fluoride polymer hydrogel adsorbent prepared according to the method of claim 3 in lithium ion adsorption, characterized in that: The prepared hydrogel adsorbent was placed in a lithium-containing solution with a pH of 10 and a lithium ion concentration of 300 mg / L to adsorb lithium ions. After the adsorption was completed, the hydrogel adsorbent was taken out and placed in a 1 mmol sulfuric acid solution for desorption. The desorbed hydrogel adsorbent was then placed in another 300 mg / L lithium-containing solution for a second adsorption. The above operation was repeated for five cycles. The initial adsorption capacity of lithium ions by polyvinylidene fluoride polymer hydrogel adsorbent is 33.48 mg / g. After five cycles, the adsorption capacity can still reach 27.45 mg / g.

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