A thiol-functionalized PVA / SA@ZIF-67 composite hydrogel as well as a preparation method and application thereof

Abstract

This invention discloses a thiol-functionalized polyvinyl alcohol (PVA) / sodium alginate (SA)@ZIF-67 composite hydrogel, its preparation method, and its applications. The preparation method uses PVA and SA as matrices, forming hydrogel spheres through ionic crosslinking. After adsorbing cobalt ions, thiol-functionalized ZIF-67 crystals are grown in situ in a mixed ligand solution containing 2-methylimidazole and 2-mercapto-1-methylimidazole to obtain the PVA / SA@SH-ZIF-67 composite hydrogel. This invention achieves efficient adsorption and integrated "adsorption-reduction-fixation" removal of Cr(VI) by introducing thiol-functionalized ZIF-67 into the hydrogel network, utilizing the strong coordination and chemical reduction effects of thiol groups on hexavalent chromium. This composite hydrogel has advantages such as large adsorption capacity, high removal efficiency, and easy separation and recovery, and has broad application prospects in the field of heavy metal wastewater treatment.

Description

Technical Field

[0001] This invention belongs to the field of functional materials technology, and relates to composite hydrogels, specifically to a thiol-functionalized PVA / SA@ZIF-67 composite hydrogel, its preparation method, and its application. Background Technology

[0002] With the acceleration of industrialization, heavy metal pollution has become increasingly serious. Among them, hexavalent chromium (Cr(VI)) has become a key focus of environmental governance due to its high toxicity, high mobility, and carcinogenicity. Cr(VI) mainly originates from wastewater from industries such as electroplating, leather tanning, printing and dyeing, and metallurgy, posing a serious threat to human health and the ecological environment. Therefore, developing efficient, economical, and environmentally friendly Cr(VI) removal technologies is of significant practical importance.

[0003] Currently, the main methods for removing Cr(VI) from water include chemical reduction precipitation, ion exchange, membrane separation, and adsorption. Among these, adsorption is widely used in the treatment of heavy metal wastewater due to its advantages such as simple operation, low cost, high efficiency, and recyclability. Traditional adsorption materials, such as activated carbon and clay minerals, suffer from limited adsorption capacity, poor selectivity, and difficulty in separation and recovery. Therefore, developing novel adsorption materials with high adsorption capacity, high selectivity, and good regeneration performance has become a research hotspot.

[0004] Metal-organic frameworks (MOFs) exhibit great potential in adsorption due to their high specific surface area, tunable pore structure, and abundant active sites. ZIF-67 is an important member of the MOF family, possessing good chemical and thermal stability. However, powdered ZIF-67 suffers from problems such as easy agglomeration, difficulty in separation, and potential secondary pollution during use, limiting its practical application. Summary of the Invention

[0005] To address the shortcomings of existing technologies, the present invention aims to provide a thiol-functionalized PVA / SA@ZIF-67 composite hydrogel, its preparation method, and its application. By in-situ growing ZIF-67 in a PVA / SA hydrogel network and introducing thiol functionalization modification, a composite hydrogel material with excellent Cr(VI) adsorption performance is prepared, achieving efficient removal of Cr(VI) from water.

[0006] This invention is achieved through the following technical solution: A method for preparing a thiol-functionalized PVA / SA@ZIF-67 composite hydrogel includes the following steps: Step 1: Take 100 parts of deionized water by mass, add 2-5 parts of polyvinyl alcohol, heat to 80-95℃ and stir until completely dissolved, cool and add 2-4 parts of sodium alginate, continue stirring until completely dissolved to obtain a polyvinyl alcohol / sodium alginate mixed solution. Step 2: Drop the mixed solution obtained in Step 1 into a calcium chloride solution with a mass concentration of 2-3% to form gel spheres. After solidification and washing, polyvinyl alcohol / sodium alginate hydrogel spheres are obtained. Step 3: Immerse the hydrogel balls obtained in Step 2 in a methanol solution of cobalt salt with a mass concentration of 10~15 g / L, and stir for 6~12 hours to allow the hydrogel balls to adsorb cobalt ions. Step 4: Take 2-methylimidazole and 2-mercapto-1-methylimidazole in a molar ratio of (1~9): (9~1), add them to anhydrous methanol and stir until dissolved to obtain a mixed ligand solution with a total concentration of 0.1~0.5 mol / L; The hydrogel spheres with adsorbed cobalt ions obtained in step 3 were placed in a mixed ligand solution and stirred for 10-14 hours to generate thiol-functionalized ZIF-67 crystals in situ inside the hydrogel spheres. The product was washed and dried to obtain thiol-functionalized PVA / SA@ZIF-67 composite hydrogel.

[0007] The present invention also has the following technical features: Preferably, the curing process in step two involves immersing the gel balls in a 2-3% calcium chloride solution for 20-28 hours.

[0008] Preferably, the washing in step two involves washing with deionized water 3 to 5 times.

[0009] Preferably, the cobalt salt mentioned in step three is one or more of cobalt nitrate, cobalt chloride, and cobalt acetate.

[0010] Preferably, the washing in step four involves washing with anhydrous methanol 3 to 5 times.

[0011] Preferably, the drying in step four is vacuum drying, with a drying temperature of 50~70℃ and a drying time of 10~14 hours.

[0012] This invention also protects a thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared by the method described above and its application in the removal of hexavalent chromium from water.

[0013] Preferred application methods include: Thiol-functionalized PVA / SA@ZIF-67 composite hydrogel was added to wastewater containing hexavalent chromium, the pH was adjusted to 2-3, and adsorption was carried out by shaking at 25-45℃.

[0014] Compared with the prior art, the present invention has the following technical effects: This invention uses PVA / SA hydrogel as a matrix and forms a three-dimensional network structure through ionic cross-linking, providing an ideal carrier for the in-situ growth of ZIF-67 and effectively avoiding the problems of easy agglomeration and difficulty in separation of powdered ZIF-67. This invention introduces 2-mercapto-1-methylimidazole into the ZIF-67 synthesis process, successfully achieving thiol functionalization modification of ZIF-67. The thiol group has a strong coordination and chemical reduction effect on Cr(VI), which can reduce the highly toxic Cr(VI) to the less toxic Cr(III), realizing integrated removal of "adsorption-reduction-fixation", with large adsorption capacity and high removal efficiency. The composite hydrogel prepared by this invention has a regular spherical morphology and good mechanical strength, which facilitates the separation and recovery after adsorption and avoids secondary pollution. The composite hydrogel prepared by this invention has excellent adsorption performance for Cr(VI), with high adsorption capacity and fast adsorption rate. The adsorption effect is optimal at pH=2, and the adsorption process conforms to the pseudo-second-order kinetic model, which belongs to chemisorption. The adsorption of Cr(VI) by the composite hydrogel prepared in this invention is a spontaneous endothermic process. Increasing the temperature is beneficial to the adsorption. Thermodynamic parameters indicate that the adsorption process is an entropy-driven spontaneous endothermic reaction. Attached Figure Description

[0015] Figure 1 These are photographs of the PVA / SA hydrogel spheres prepared in Example 2 of this invention. Figure 2 This is a photograph of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared in Example 2 of this invention. Figure 3 This is a comparison image of the powdered materials obtained after drying and grinding of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared in Example 2 of this invention before and after adsorption of Cr(VI); Figure 4 This is a graph showing the adsorption performance of thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared in Example 2 of this invention for Cr(VI) under different pH conditions. Figure 5 This is the adsorption kinetics curve of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared in Example 2 of this invention. Detailed Implementation

[0016] The present invention will be further described in detail below with reference to specific embodiments. These descriptions are for explanation purposes only and are not intended to limit the scope of the invention.

[0017] In the following examples: the polyvinyl alcohol (PVA) was manufactured by Tianjin Kemei Chemical Reagent Co., Ltd., batch number 20120113, conforming to standard Q / 12HB 3878-2009, with a degree of alcoholysis of 99.8%~100% and a viscosity of 33.0~41.0 mPa·s for a 4% aqueous solution; sodium alginate (SA) was a commercially available product with a viscosity of 200±20 mPa·s; calcium chloride, cobalt nitrate, 2-methylimidazole, 2-mercapto-1-methylimidazole, etc., were all analytical grade reagents.

[0018] Example 1 This embodiment provides a method for preparing a thiol-functionalized PVA / SA@ZIF-67 composite hydrogel, comprising the following steps: Step 1: Preparation of PVA / SA mixed solution: Take 100 mL of deionized water, add 2.5 g of polyvinyl alcohol, heat to 90℃ and stir until completely dissolved, cool to room temperature and add 2.8 g of sodium alginate, continue stirring for 8 hours until completely dissolved to obtain PVA / SA mixed solution; Step 2: Preparation of PVA / SA hydrogel spheres: Prepare 400 mL of calcium chloride solution with a mass concentration of 2.4%. Add the mixed solution obtained in Step 1 dropwise into the calcium chloride solution using a syringe at a dropping rate of 1 drop / second to form gel spheres. Soak the gel spheres in the calcium chloride solution for 24 hours for solidification. Then wash the gel spheres three times with deionized water to obtain PVA / SA hydrogel spheres. Step 3, adsorption of cobalt ions: Prepare a methanol solution of cobalt nitrate with a concentration of 11.7 g / L. Take 100 mL of the solution and immerse the hydrogel spheres obtained in Step 2 into the solution. Stir at 100 rpm for 10 hours at room temperature. Step 4: In-situ growth of thiol-functionalized ZIF-67: Prepare a mixed ligand solution by dissolving 2-methylimidazole and 2-mercapto-1-methylimidazole in 100 mL of anhydrous methanol at a molar ratio of 1:1. The concentrations of 2-methylimidazole and 2-mercapto-1-methylimidazole are 0.1 mol / L. Place the hydrogel spheres with adsorbed cobalt ions obtained in Step 3 into this mixed ligand solution and stir at 100 rpm for 12 hours at room temperature. Wash the obtained product three times with anhydrous methanol, using 50 mL of methanol each time, and then dry it in a vacuum drying oven at 60℃ for 12 hours to obtain thiol-functionalized PVA / SA@ZIF-67 composite hydrogel.

[0019] Example 2 This embodiment provides a method for preparing a thiol-functionalized PVA / SA@ZIF-67 composite hydrogel, comprising the following steps: Step 1: Preparation of PVA / SA mixed solution: Take 100 mL of deionized water, add 2.48 g of polyvinyl alcohol, heat to 90℃ and stir until completely dissolved, cool and add 2.81 g of sodium alginate, continue stirring for 8 hours until completely dissolved, to obtain polyvinyl alcohol / sodium alginate mixed solution; Step 2: Preparation of PVA / SA hydrogel spheres: Prepare 400 mL of a 2.4% calcium chloride solution. Add the mixed solution obtained in Step 1 dropwise to the calcium chloride solution using a 5 mL syringe at a dropping rate of 1 drop / second to form gel spheres. After stirring at low speed in the calcium chloride solution for 30 minutes, transfer the solution to a freshly prepared 2.4% calcium chloride solution and soak for 24 hours for solidification. Then wash the solution three times with deionized water to obtain PVA / SA hydrogel spheres. Step 3, adsorption of cobalt ions: Prepare a methanol solution of cobalt nitrate with a concentration of 11.7 g / L. Take 100 mL of the solution and immerse the hydrogel spheres obtained in Step 2 into the solution. Stir at 100 rpm for 10 hours at room temperature. Step 4: In-situ growth of thiol-functionalized ZIF-67: Two mixed ligand solutions with different ratios were prepared, with a total ligand concentration of 0.2 mol / L:9:1 (2-methylimidazole 0.18 mol / L, 2-mercapto-1-methylimidazole 0.02 mol / L). The hydrogel spheres with adsorbed cobalt ions obtained in Step 3 were placed in the mixed ligand solutions and stirred at 100 rpm for 12 hours at room temperature. Step 5: Wash the product obtained in Step 4 three times with anhydrous methanol, using 50 mL of methanol each time, and then dry it in a vacuum drying oven at 60℃ for 12 hours to obtain two thiol-functionalized PVA / SA@ZIF-67 composite hydrogels.

[0020] The samples obtained in Example 2 were characterized and tested as follows: Figure 1 These are photographs of the PVA / SA hydrogel spheres prepared in Example 2. Figure 1 As can be seen, the prepared hydrogel spheres are regular spherical in shape, white and transparent in color, with a diameter of about 2~4 mm, smooth surface, and good mechanical strength.

[0021] Figure 2 This is a photograph of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared in Example 2. Figure 2 As can be seen, after adsorbing cobalt ions, the hydrogel spheres turn pink, as... Figure 2 As shown in (a); after reacting with the mixed ligand solution, the hydrogel spheres turned dark purple, as... Figure 2 As shown in (b), this indicates that ZIF-67 was successfully grown in situ inside the hydrogel.

[0022] Figure 3 This is a comparison of the powdered materials obtained after drying and grinding of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared in Example 2 of this invention before and after Cr(VI) adsorption. As shown in the figure, after Cr(VI) adsorption, the color of the powder changes from dark purple to green. This color change indicates that the Cr(VI) adsorbed on the material surface is reduced to Cr(III) by the thiol functional groups, confirming that the removal of Cr(VI) by this material follows a synergistic mechanism of "adsorption-reduction-fixation".

[0023] Figure 4 This is a graph showing the adsorption performance of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared in Example 2 for Cr(VI) under different pH conditions. Figure 4 As can be seen, the adsorption capacity of the three materials (PVA / SA, PVA / SA@ZIF-67, and PVA / SA@SH-ZIF-67) for Cr(VI) decreased with increasing pH, with the highest adsorption capacity at pH=2. Among them, the adsorption capacity of PVA / SA@SH-ZIF-67 was significantly higher than that of the other two materials, reaching 96.5 mg / g at pH=2, indicating that thiol functionalization significantly improved the adsorption performance of the materials for Cr(VI).

[0024] Figure 5 This is an adsorption kinetics curve of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared in Example 2 of this invention. Figure 5 It is evident that the adsorption capacity increases rapidly with time, with an extremely fast initial adsorption rate followed by a slowdown, reaching adsorption equilibrium after 30 minutes. The linear correlation coefficient R² of the pseudo-second-order kinetic model is greater than 0.99, and the calculated theoretical equilibrium adsorption capacity is in high agreement with the experimental value, indicating that the adsorption process is mainly controlled by a chemisorption mechanism.

[0025] Example 3 This embodiment provides a method for preparing a thiol-functionalized PVA / SA@ZIF-67 composite hydrogel, comprising the following steps: Step 1: Preparation of PVA / SA mixed solution: Take 100 mL of deionized water, add 2.0 g of polyvinyl alcohol, heat to 80℃ and stir until completely dissolved, cool to room temperature and add 2.0 g of sodium alginate, continue stirring for 8 hours until completely dissolved to obtain PVA / SA mixed solution; Step 2: Preparation of PVA / SA hydrogel spheres: Prepare 400 mL of calcium chloride solution with a mass concentration of 2.0%. Add the mixed solution obtained in Step 1 dropwise into the calcium chloride solution using a syringe at a dropping rate of 1 drop / second to form gel spheres. Soak the gel spheres in the calcium chloride solution for 20 hours for solidification. Then wash the gel spheres four times with deionized water to obtain PVA / SA hydrogel spheres. Step 3, adsorption of cobalt ions: Prepare a methanol solution of cobalt chloride with a concentration of 10.0 g / L. Take 100 mL of the solution and immerse the hydrogel spheres obtained in Step 2 into the solution. Stir at 120 rpm for 12 hours at room temperature. Step 4: In-situ growth of thiol-functionalized ZIF-67: Prepare a mixed ligand solution by dissolving 2-methylimidazole and 2-mercapto-1-methylimidazole in 100 mL of anhydrous methanol at a molar ratio of 1:9. The concentration of 2-methylimidazole is 0.02 mol / L and the concentration of 2-mercapto-1-methylimidazole is 0.08 mol / L. Place the hydrogel spheres with adsorbed cobalt ions obtained in Step 3 into this mixed ligand solution and stir at 120 rpm for 14 hours at room temperature. Wash the obtained product four times with anhydrous methanol, using 50 mL of methanol each time, and then dry it in a vacuum drying oven at 50 °C for 14 hours to obtain thiol-functionalized PVA / SA@ZIF-67 composite hydrogel.

[0026] This embodiment tests the Cr(VI) adsorption performance of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared in Example 3. The specific method is as follows: 0.0500 g of dried composite hydrogel was added to 25.0 mL of a 50 mg / L Cr(VI) solution. The pH was adjusted to 2.0 with 0.1 mol / L HCl, and the solution was placed in a constant temperature shaker at 25℃ and shaken at 180 rpm for 24 hours. After adsorption, the supernatant was filtered, and the remaining Cr(VI) concentration was determined by diphenylcarbazide spectrophotometry. The results showed that the material could remove more than 95% of Cr(VI), with an adsorption capacity of approximately 23.8 mg / g. After being reused 5 times, the adsorption capacity remained above 85% of the initial adsorption capacity, indicating that the material has good regeneration performance.

[0027] Example 4 This embodiment provides a method for preparing a thiol-functionalized PVA / SA@ZIF-67 composite hydrogel, comprising the following steps: Step 1: Preparation of PVA / SA mixed solution: Take 100 mL of deionized water, add 5.0 g of polyvinyl alcohol, heat to 95℃ and stir until completely dissolved, cool to room temperature and add 4.0 g of sodium alginate, continue stirring for 8 hours until completely dissolved to obtain PVA / SA mixed solution; Step 2: Preparation of PVA / SA hydrogel spheres: Prepare 400 mL of calcium chloride solution with a mass concentration of 3.0%. Add the mixed solution obtained in Step 1 dropwise into the calcium chloride solution using a syringe at a dropping rate of 1 drop / second to form gel spheres. Soak the gel spheres in the calcium chloride solution for 28 hours for solidification. Then wash them 5 times with deionized water to obtain PVA / SA hydrogel spheres. Step 3, adsorption of cobalt ions: Prepare a methanol solution of cobalt acetate with a concentration of 15.0 g / L. Take 100 mL of the solution and immerse the hydrogel spheres obtained in Step 2 into the solution. Stir at 150 rpm for 6 hours at room temperature. Step 4: In-situ growth of thiol-functionalized ZIF-67: Prepare a mixed ligand solution by dissolving 2-methylimidazole and 2-mercapto-1-methylimidazole in 100 mL of anhydrous methanol at a molar ratio of 4:6. The concentration of 2-methylimidazole is 0.2 mol / L and the concentration of 2-mercapto-1-methylimidazole is 0.3 mol / L. Place the hydrogel spheres with adsorbed cobalt ions obtained in Step 3 into this mixed ligand solution and stir at 150 rpm for 10 hours at room temperature. Wash the obtained product 5 times with anhydrous methanol, each time with 50 mL of methanol, and then dry it in a vacuum drying oven at 70℃ for 10 hours to obtain thiol-functionalized PVA / SA@ZIF-67 composite hydrogel.

[0028] The above embodiments are only used to illustrate the technical solutions of the present invention, and are not intended to limit them. Although the present invention has been described in detail with reference to the foregoing embodiments, those skilled in the art should understand that modifications can still be made to the technical solutions described in the foregoing embodiments, or equivalent substitutions can be made to some or all of the technical features therein. Such modifications or substitutions do not cause the essence of the corresponding technical solutions to deviate from the scope of the technical solutions of the present invention.

Claims

1. A method for preparing a thiol-functionalized PVA / SA@ZIF-67 composite hydrogel, characterized in that, Includes the following steps: Step 1: Take 100 parts of deionized water by mass, add 2-5 parts of polyvinyl alcohol, heat to 80-95℃ and stir until completely dissolved, cool and add 2-4 parts of sodium alginate, continue stirring until completely dissolved to obtain a polyvinyl alcohol / sodium alginate mixed solution. Step 2: Drop the mixed solution obtained in Step 1 into a calcium chloride solution with a mass concentration of 2-3% to form gel spheres. After solidification and washing, polyvinyl alcohol / sodium alginate hydrogel spheres are obtained. Step 3: Immerse the hydrogel balls obtained in Step 2 in a methanol solution of cobalt salt with a mass concentration of 10~15 g / L, and stir for 6~12 hours to allow the hydrogel balls to adsorb cobalt ions. Step 4: Take 2-methylimidazole and 2-mercapto-1-methylimidazole in a molar ratio of (1~9): (9~1), add them to anhydrous methanol and stir until dissolved to obtain a mixed ligand solution with a total concentration of 0.1~0.5 mol / L; The hydrogel spheres with adsorbed cobalt ions obtained in step 3 were placed in a mixed ligand solution and stirred for 10-14 hours to generate thiol-functionalized ZIF-67 crystals in situ inside the hydrogel spheres. The product was washed and dried to obtain thiol-functionalized PVA / SA@ZIF-67 composite hydrogel.

2. The preparation method of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel as described in claim 1, characterized in that, The curing process described in step two involves immersing the gel balls in a 2-3% calcium chloride solution for 20-28 hours.

3. The method for preparing the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel as described in claim 1, characterized in that, The washing process described in step two involves washing with deionized water 3 to 5 times.

4. The preparation method of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel as described in claim 1, characterized in that, The cobalt salt mentioned in step three is one or more of cobalt nitrate, cobalt chloride, and cobalt acetate.

5. The method for preparing the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel as described in claim 1, characterized in that, The washing process described in step four involves washing with anhydrous methanol 3 to 5 times.

6. The method for preparing the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel as described in claim 1, characterized in that, The drying process described in step four is vacuum drying, with a drying temperature of 50~70℃ and a drying time of 10~14 hours.

7. A thiol-functionalized PVA / SA@ZIF-67 composite hydrogel prepared by the method described in any one of claims 1 to 6.

8. The application of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel as described in claim 7 in the removal of hexavalent chromium from water.

9. The application of the thiol-functionalized PVA / SA@ZIF-67 composite hydrogel as described in claim 8 in the removal of hexavalent chromium from water, characterized in that the method... include: Thiol-functionalized PVA / SA@ZIF-67 composite hydrogel was added to wastewater containing hexavalent chromium, the pH was adjusted to 2-3, and adsorption was carried out by shaking at 25-45℃.

Images